JP2009272153A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having high capacity, superior safety at abnormal overheating, and excellent reliability, with respect to short-circuit. <P>SOLUTION: The lithium secondary battery includes a positive electrode, a negative electrode, and a separator. The separator includes a porous layer (I) of which the main material is a porous thermoplastic resin, and a porous layer (II) mainly containing a filler with a heat-resistant temperature of 150°C or higher. The negative electrode includes a negative electrode mixture layer which contains a compound, containing Si and O in a constituent element (where the atomic ratio "x" of O to Si satisfies 0.5≤x≤1.5), and a conductive material. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery that has a high capacity and is safe even in a high temperature environment.

  Lithium secondary batteries are widely used as power sources for portable devices such as mobile phones and notebook personal computers because of their high energy density. As the performance of portable devices increases, the capacity of lithium secondary batteries tends to increase further, and it is important to ensure safety.

  In current lithium secondary batteries, a polyolefin microporous film having a thickness of about 20 to 30 μm is used as a separator interposed between a positive electrode and a negative electrode. In addition, as separator material, the constituent resin of the separator is melted below the thermal runaway temperature of the battery to close the pores, thereby increasing the internal resistance of the battery and improving the safety of the battery in the event of a short circuit. In order to ensure the so-called shutdown effect, polyethylene having a low melting point may be applied.

  By the way, as such a separator, for example, a uniaxially stretched film or a biaxially stretched film is used for increasing the porosity and improving the strength. Since such a separator is supplied as a single film, a certain strength is required in terms of workability and the like, and this is secured by the stretching. However, with such a stretched film, the degree of crystallinity has increased, and the shutdown temperature has increased to a temperature close to the thermal runaway temperature of the battery. Therefore, it can be said that the margin for ensuring the safety of the battery is sufficient. hard.

  In addition, the film is distorted by the stretching, and there is a problem that when it is exposed to a high temperature, shrinkage occurs due to residual stress. The shrinkage temperature is very close to the melting point, ie the shutdown temperature. For this reason, when using a polyolefin-based microporous membrane separator, if the battery temperature reaches the shutdown temperature in the event of abnormal charging, the current must be immediately reduced to prevent the battery temperature from rising. This is because if the pores are not sufficiently closed and the current cannot be reduced immediately, the battery temperature easily rises to the contraction temperature of the separator, and there is a risk of ignition due to an internal short circuit.

  As a technique for preventing such a short circuit due to thermal contraction of the separator and improving the reliability of the battery, for example, a first separator layer mainly including a resin for ensuring a shutdown function, and a filler having a heat resistant temperature of 150 ° C. or more It has been proposed that an electrochemical element is constituted by a porous separator having a second separator layer containing mainly as a main component (Patent Document 1).

  According to the technique of Patent Document 1, it is possible to provide an electrochemical element such as a lithium secondary battery in which thermal runaway hardly occurs even when abnormal overheating occurs.

  By the way, as a negative electrode material (negative electrode active material) of a lithium secondary battery, a natural or artificial graphite-based carbon material capable of inserting and removing Li ions in addition to Li (lithium) and a Li alloy is applied. Recently, however, there has been a demand for a further increase in capacity of a lithium secondary battery for portable devices that has been reduced in size and multifunction, and as a result, low crystalline carbon, Si (silicon), Sn ( Materials that can accommodate more Li, such as tin, are attracting attention as negative electrode materials (hereinafter also referred to as “high-capacity negative electrode materials”).

As one of such high-capacity negative electrode materials for lithium secondary batteries, SiO _x having a structure in which Si ultrafine particles are dispersed in SiO ₂ has attracted attention (for example, Patent Documents 2 to 4). When this material is used as a negative electrode active material, since Si that reacts with Li is an ultrafine particle, charging and discharging are performed smoothly. On the other hand, the SiO _x particle itself having the above structure has a small surface area, and thus the negative electrode mixture layer The coating property when forming a coating material for forming the electrode and the adhesion of the negative electrode mixture layer to the current collector are also good.

International Publication No. 2007/066768 JP 2004-47404 A Japanese Patent Laid-Open No. 2005-259697 JP 2007-242590 A

  However, lithium secondary batteries using such high-capacity negative electrode materials will have high energy density, and it may be difficult to ensure reliability and safety compared to conventional lithium secondary batteries. There is. In addition, since the high capacity negative electrode material has a large expansion / contraction due to charging / discharging, a short circuit between the negative electrode and the positive electrode tends to occur in a lithium secondary battery using the high capacity negative electrode material due to expansion of the high capacity negative electrode material during charging. For this reason, there is a demand for technical development that can further improve the reliability and safety of the high-capacity lithium secondary battery using the high-capacity negative electrode material.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lithium secondary battery that has a high capacity, safety when abnormally overheated, and good reliability against a short circuit.

The lithium secondary battery of the present invention that has achieved the above object is a lithium secondary battery having a positive electrode, a negative electrode, and a separator, the separator comprising a porous layer (I) mainly composed of a thermoplastic resin, A porous layer (II) mainly containing a filler having a heat resistant temperature of 150 ° C. or higher, and the negative electrode is a compound containing Si and O as constituent elements (however, the atomic ratio x of O to Si is: 0.5 ≦ x ≦ 1.5 (hereinafter, the compound may be abbreviated as “SiO _x ”) and a negative electrode mixture layer containing a conductive material. It is what.

  Except for the porous substrate described later, “heat-resistant temperature is 150 ° C. or higher” in this specification means that deformation such as softening is not observed at least at 150 ° C.

  Further, in the porous layer (I) relating to the separator in the present specification, “mainly comprising a thermoplastic resin” means a solid content ratio in the porous layer (I), which is a thermoplastic resin [hereinafter, resin ( Abbreviated as A)] means 50% by volume or more. Further, in the porous layer (II) relating to the separator in the present specification, “mainly including a filler having a heat resistant temperature of 150 ° C. or higher” means a solid content ratio in the layer (however, it has a porous substrate described later) In this case, it means that the filler having a heat-resistant temperature of 150 ° C. or higher is 50% by volume or higher in terms of the solid content ratio excluding the porous substrate.

  According to the present invention, it is possible to provide a lithium secondary battery that has a high capacity, safety when abnormally overheated, and good reliability against short circuit.

The lithium secondary battery of the present invention includes a porous layer mainly composed of a negative electrode containing at least a compound (SiO _x ) containing Si and O as constituent elements and a conductive material, and a resin (A) that is a thermoplastic resin. A separator having (I) and a porous layer (II) having a porous layer mainly containing a filler having a heat resistant temperature of 150 ° C. or higher.

  The porous layer (I) relating to the separator used in the lithium secondary battery of the present invention is for ensuring a shutdown function. When the temperature of the lithium secondary battery reaches or exceeds the melting point of the resin (A), the resin (A) melts and closes the pores of the separator, thereby shutting down the progress of the electrochemical reaction.

  In addition, the porous layer (II) relating to the separator used in the lithium secondary battery of the present invention has the original function of the separator, mainly the function of preventing a short circuit due to direct contact between the positive electrode and the negative electrode. The function is secured by a filler having a heat resistant temperature of 150 ° C. or higher. That is, in the temperature range where the battery is normally used by the porous layer (II), when the positive electrode and the negative electrode are pressed through the separator to form an electrode body, the positive electrode active material penetrates the separator and the negative electrode. Generation | occurrence | production of the short circuit by contact can be prevented. In addition, when the battery becomes hot, the porous layer (II) can suppress the thermal contraction of the separator and maintain its shape. Therefore, direct contact between the positive and negative electrodes that can occur when the separator is thermally contracted It is also possible to prevent a short circuit due to.

That is, in the lithium secondary battery of the present invention, the capacity is increased by using SiO _x as the negative electrode active material, and the battery has a high energy density. Is suppressed by using the separator having the porous layer (I) and the porous layer (II), and the reliability and safety are improved. Further, as described above, since SiO _x is greatly expanded and contracted by charging and discharging, the negative electrode expands due to the expansion of SiO _x during charging, and a short circuit is likely to occur due to contact with the positive electrode. Since the porous layer (II) has high shape stability not only in the high temperature range but also in the temperature range where the battery is normally used, the short circuit can be prevented.

  The resin (A) in the porous layer (I) has electrical insulation, is stable with respect to the non-aqueous electrolyte of the battery, and is not easily oxidized or reduced in the battery operating voltage range. A chemically stable material is preferred. Specifically, polyethylene (PE), polypropylene (PP), copolymerized polyolefin, polyolefin derivatives (such as chlorinated polyethylene), polyolefin such as polyolefin wax; polyester such as polyethylene terephthalate and copolymerized polyester; petroleum wax; carnauba wax; Etc. Examples of the copolymer polyolefin include an ethylene-vinyl monomer copolymer, more specifically, an ethylene-vinyl acetate copolymer (EVA); an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, Examples thereof include an ethylene-acrylic acid copolymer; an ethylene-propylene copolymer; The structural unit derived from ethylene in the copolymerized polyolefin is desirably 85 mol% or more. Moreover, polycycloolefin etc. can also be used. As the resin (A), the above exemplified resins may be used alone or in combination of two or more. Moreover, the resin (A) may contain various known additives (for example, antioxidants) added to the resin as necessary.

  In addition, it is preferable that the separator of this invention has the property (namely, shutdown function) which the hole obstruct | occludes in 80 degreeC or more and 140 degrees C or less (more preferably 100 degreeC or more). Therefore, as the resin (A), among the resins exemplified above, the melting point, that is, the melting temperature measured using a differential scanning calorimeter (DSC) in accordance with the provisions of JIS K 7121 is 80 ° C. or higher. A thermoplastic resin at 140 ° C. (more preferably 100 ° C. or higher) is more preferable.

  The shape of the resin (A) is not particularly limited. For example, in addition to fine particles, for example, a fibrous material constituting the porous substrate described later is used as a core material, and the surface is adhered or covered. Alternatively, it may be contained in the porous layer (I). Further, the porous layer (I) is in the form of a core-shell structure in which the “filler having a heat resistant temperature of 150 ° C. or higher” in the porous layer (II), which will be described later, is the core and the resin (A) or the resin (B) is the shell. ). Furthermore, a microporous film (porous film) mainly composed of the resin (A) may be used as the porous layer (I).

  When the porous layer (I) is composed of a microporous film mainly composed of the resin (A), the microporous film is a single-layer microporous film mainly composed of PE, or PE and PP. A laminated microporous film in which 2 to 5 layers are laminated is preferable.

  As the microporous membrane, for example, a microporous membrane composed of the exemplified thermoplastic resin used in a conventionally known lithium secondary battery or the like, that is, by a solvent extraction method, a dry or wet stretching method, etc. The produced ion-permeable microporous membrane can be used.

  When the porous layer (I) is formed by using the resin (A) in a form other than the microporous film, the resin (A) includes PE, polyolefin wax, or among the materials exemplified above. EVA having a structural unit derived from ethylene of 85 mol% or more is more preferable.

  When the porous layer (I) is composed of a thermoplastic resin having a melting point of 80 ° C. or more and 140 ° C. or less such as PE and a thermoplastic resin having a melting point exceeding 140 ° C. such as PP. For example, a microporous film formed by mixing PE and a resin having a higher melting point than PE such as PP is used as a porous layer (I), or a porous layer (I) containing PE fine particles and PP fine particles (I ) Or a laminated microporous membrane formed by laminating a PE layer and a layer made of a resin having a higher melting point than PE, such as a PP layer, as the porous layer (I), In the resin (A) constituting the porous layer (I), the resin (for example, PE) having a melting point of 80 ° C. or more and 140 ° C. or less is preferably 30% by mass or more, and more preferably 50% by mass or more. preferable.

  When the resin (A) is in the form of fine particles, the particle diameter may be smaller than the thickness of the porous layer (I), but the average particle diameter is 1/100 to 1/3 of the thickness of the porous layer (I). It is preferable to have. Specifically, the average particle size of the resin (A) is preferably 0.1 to 20 μm. When the particle size of the resin (A) is too small, the gap between the particles becomes small, the ion conduction path becomes long, and the battery characteristics may deteriorate. On the other hand, if the particle size is too large, the thickness of the porous layer (I) increases, which is not preferable because the energy density of the battery is reduced. In addition, the average particle diameter of the fine particles [resin (A), filler described later, plate-like particles described later, fine particles of secondary particle structure described later] referred to in this specification is, for example, a laser scattering particle size distribution meter (for example, HORIBA In the case of a resin (A) or resin (B), a filler, plate-like particle, or secondary particle structure described later is added to a medium that does not swell these resins (for example, water). In the case of these fine particles, it can be defined as the number average particle diameter measured by dispersing these fine particles in a medium in which they are not dissolved.

  Further, the porous layer (I) may contain a filler or the like in order to improve the strength and the like within a range that does not impair the action of imparting the shutdown function to the separator. Examples of the filler that can be used for the porous layer (I) include the same fillers that can be used for the porous layer (II) described later (a filler having a heat resistant temperature of 150 ° C. or higher).

  The particle size of the filler is an average particle size, for example, preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 10 μm or less, more preferably 1 μm or less.

  The content of the resin (A) in the separator is preferably as follows, for example, in order to more easily obtain the shutdown effect. The volume of the resin (A) in all the constituent components of the separator is preferably 10% by volume or more, and more preferably 20% by volume or more. Further, the volume of the resin (A) is 50% by volume or more in all the constituents of the porous layer (I) (in the case of using the porous base described later, in all the constituents excluding the porous base). Preferably, it is 70% by volume or more, more preferably 80% by volume or more. Furthermore, the porosity of the porous layer (II) obtained by the method described later is 20 to 60%, and the volume of the resin (A) is 50% or more of the pore volume of the porous layer (II). Preferably there is.

  On the other hand, the volume of the resin (A) in all the constituent components of the separator is preferably 80% by volume or less, more preferably 40% by volume or less, from the viewpoint of securing the shape stability at high temperatures of the separator. .

  The filler related to the porous layer (II) has a heat-resistant temperature of 150 ° C. or higher, is stable with respect to the non-aqueous electrolyte of the battery, and is electrochemically stable that is not easily oxidized or reduced in the battery operating voltage range. Any organic particles or inorganic particles may be used as long as they are fine, but fine particles are preferable from the viewpoint of dispersion, and inorganic fine particles are more preferably used from the viewpoint of stability.

Specific examples of the constituent material of the inorganic particles include inorganic oxides such as iron oxide, Al ₂ O ₃ (alumina), SiO ₂ (silica), TiO ₂ , BaTiO ₂ , ZrO ₂ ; aluminum nitride, silicon nitride, etc. Inorganic nitrides of the above; poorly soluble ionic crystals such as calcium fluoride, barium fluoride and barium sulfate; covalent bonds such as silicon and diamond; clays such as montmorillonite; Here, the inorganic oxide may be boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, or other mineral resource-derived substances or artificial products thereof. In addition, the surface of a conductive material exemplified by metal, SnO ₂ , conductive oxide such as tin-indium oxide (ITO), carbonaceous material such as carbon black, graphite, etc., is a material having electrical insulation ( For example, the particle | grains which gave the electrical insulation property by coat | covering with the said inorganic oxide etc. may be sufficient. As the inorganic particles, when the battery is configured so that the porous layer (II) faces the positive electrode, its high-temperature storability and charge / discharge cycle characteristics can be improved (details will be described later). Inorganic oxide particles (fine particles) are preferable, and alumina, silica and boehmite are particularly preferably used.

  Organic particles (organic powder) include crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensate, etc. Examples thereof include various crosslinked polymer particles and heat-resistant polymer particles such as polysulfone, polyacrylonitrile, polyaramid, polyacetal, and thermoplastic polyimide. The organic resin (polymer) constituting these organic particles is a mixture, modified body, derivative, or copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) of the materials exemplified above. Polymer) or a crosslinked product (in the case of the above-mentioned heat-resistant polymer).

  The average particle diameter of the filler relating to the porous layer (II) is, for example, preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 15 μm or less, more preferably 5 μm or less.

  The amount of filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) is the total volume of the constituent components of the porous layer (II) [However, when using the porous substrate described later, the porous substrate During the entire volume of components excluding. The same applies to the content of each component of the porous layer (II). ], Preferably 70% by volume or more, more preferably 80% by volume or more, and still more preferably 90% by volume or more. By making the filler in the porous layer (II) high as described above, it is possible to better suppress the occurrence of a short circuit due to direct contact between the positive electrode and the negative electrode when the battery becomes high temperature. In particular, in the case of a separator having a configuration in which the porous layer (I) and the porous layer (II) are integrated, the thermal contraction of the entire separator can be satisfactorily suppressed.

  Moreover, the porous layer (II) binds fillers having a heat resistant temperature of 150 ° C. or more, or binds the porous layer (I) and the porous layer (II) as necessary. Therefore, it is preferable to contain an organic binder. From such a viewpoint, the preferred upper limit of the filler amount having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) is, for example, a constituent component of the porous layer (II) Is 99% by volume. If the amount of the filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) is less than 70% by volume, for example, it is necessary to increase the amount of the organic binder in the porous layer (II). The pores of the porous layer (II) are easily filled with an organic binder, and the function as a separator may be reduced. There is a possibility that the effect of suppressing the heat shrinkage may be reduced due to the excessively large interval.

  In the separator of the present invention, it is preferable that at least one of the porous layer (I) and the porous layer (II) contains plate-like particles. When the porous layer (II) contains plate-like particles, the plate-like particles can also serve as a filler having a heat resistant temperature of 150 ° C. or higher.

  When at least one of the porous layer (I) and the porous layer (II) contains plate-like particles, a path between the positive electrode and the negative electrode in the separator, that is, a so-called curvature is increased. Therefore, in a battery in which at least one of the porous layer (I) and the porous layer (II) is configured using a separator containing plate-like particles, even when dendrite is generated, the dendrite reaches the positive electrode from the negative electrode. It becomes difficult and the reliability with respect to a dendrite short can be improved more. In addition, when plate-like particles are contained in the porous layer (II), the separator between the plate-like particles can be used even in a separator in which the porous layer (I) and the porous layer (II) are integrated. The force that the porous layer (I) contracts due to the collision can be more effectively suppressed.

Examples of the plate-like particles include various commercially available products, such as “Sun Lovely (trade name)” (SiO ₂ ) manufactured by Asahi Glass S Tech Co., Ltd., and pulverized products of “NST-B1 (trade name)” manufactured by Ishihara Sangyo Co., Ltd. TiO ₂ ), Sakai Chemical Industry's plate-like barium sulfate “H series (trade name)”, “HL series (trade name)”, Hayashi Kasei Co., Ltd. “micron white (trade name)” (talc), Hayashi Kasei "Bengel (trade name)" (bentonite), Kawai Lime "BMM (trade name)" and "BMT (trade name)" (Boehmite), Kawai Lime "Cerasur BMT-B (trade name)" [Alumina (Al ₂ O ₃ )], “Seraph (trade name)” (alumina) manufactured by Kinsei Matec Co., Ltd., “Yodogawa Mica Z-20 (trade name)” (sericite) manufactured by Yodogawa Mining Co., Ltd., etc. are available. is there. In addition, SiO ₂ , Al ₂ O ₃ , ZrO, and CeO ₂ can be produced by the method disclosed in Japanese Patent Laid-Open No. 2003-206475.

  As the form of the plate-like particles, the aspect ratio (ratio between the maximum length of the plate-like particles and the thickness of the plate-like particles) is preferably 5 or more, more preferably 10 or more, and preferably 100 or less. Preferably it is 50 or less. The aspect ratio of the plate-like particles can be obtained, for example, by analyzing an image taken with a scanning electron microscope (SEM).

  The average particle size of the plate-like particles may be smaller than the thickness of the porous layer (I) or the porous layer (II) containing the plate-like particles, while the porous layer (I) containing the plate-like particles Or it is preferable to set it as 1/100 or more of the thickness of porous layer (II). More specifically, the number average particle diameter measured by the above-described measurement method is, for example, preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 15 μm or less, more preferably 5 μm or less. It is.

  The presence form of the plate-like particles in the porous layer (I) or the porous layer (II) is preferably such that the flat plate surface is substantially parallel to the surface of the separator, more specifically, the surface of the separator. Regarding the plate-like particles in the vicinity, the average angle between the flat plate surface and the separator surface is preferably 30 ° or less [most preferably, the average angle is 0 °, that is, the plate-like flat plate surface in the vicinity of the separator surface. Is parallel to the surface of the separator]. Here, “near the surface” refers to a range of about 10% from the surface of the separator to the entire thickness. By increasing the orientation of the plate-like particles so that the presence form of the plate-like particles is in the state as described above, it is possible to exert the above action by the plate-like particles more strongly.

It is also preferable that at least one of the porous layer (I) and the porous layer (II) contains fine particles having a secondary particle structure in which primary particles are aggregated. When the porous layer (I) or the porous layer (II) contains the fine particles having the secondary particle structure, the same effects as those obtained when the plate-like particles are used can be obtained. Examples of the fine particles having the secondary particle structure include “Boehmite C06 (trade name)”, “Boehmite C20 (trade name)” (boehmite) manufactured by Daimei Chemical Co., Ltd., “ED-1 (trade name) manufactured by Yonesho Lime Industry Co., Ltd. ) ”(CaCO ₃ ), J. et al. M.M. Examples include “Zeolex 94HP (trade name)” (clay) manufactured by Huber.

  The average particle diameter of the fine particles having the secondary particle structure is a number average particle diameter measured by the above-described method, and is preferably 0.01 μm or more, more preferably 0.1 μm or more, and preferably 15 μm or less. Preferably it is 5 micrometers or less.

  In order to more effectively exert the effect of containing at least one of the porous layer (I) and the porous layer (II) with plate-like particles or fine particles having the secondary particle structure, The content of the fine particles having the secondary particle structure is 25% in the total volume of all the constituent components of the separator (however, when using the porous substrate described later, in the total volume of all the constituent components excluding the porous substrate), 25 % Or more, preferably 40% or more, and more preferably 70% or more.

  The plate-like particles and the fine particles having the secondary particle structure are more preferably contained in the porous layer (II). In the porous layer (II), a filler having a heat-resistant temperature of 150 ° C. or more is added to the plate-like particles or More preferably, the fine particles have a secondary particle structure.

  The porous layer (I) and the porous layer (II) according to the separator of the present invention preferably contain an organic binder for ensuring the shape stability of the separator. [In the case where the porous layer (I) is composed of a microporous film mainly composed of the resin (A), the porous layer (I) contains an organic binder.) It does not have to be. ]. Examples of the organic binder include EVA (with a structural unit derived from vinyl acetate of 20 to 35 mol%), ethylene-acrylic acid copolymer such as ethylene-ethyl acrylate copolymer, fluorine-based rubber, and styrene-butadiene rubber (SBR). , Carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), crosslinked acrylic resin, polyurethane, epoxy resin, etc. A heat-resistant binder having a heat-resistant temperature of 5 mm is preferably used. As the organic binder, those exemplified above may be used singly or in combination of two or more.

  Among the organic binders exemplified above, highly flexible binders such as EVA, ethylene-acrylic acid copolymer, fluorine-based rubber, and SBR are preferable. Specific examples of such highly flexible organic binders include Mitsui DuPont Polychemical's “Evaflex Series (EVA)”, Nihon Unicar's EVA, Mitsui DuPont Polychemical's “Evaflex-EAA Series (Ethylene). -Acrylic acid copolymer) ", Nippon Unicar's EEA, Daikin Industries'" Daiel Latex Series (Fluororubber) ", JSR's" TRD-2001 (SBR) ", Nippon Zeon's" EM-400B " (SBR) ".

  When the organic binder is used for the porous layer (I) or the porous layer (II), it is dissolved or dispersed in a solvent of a composition for forming these porous layers described later. What is necessary is just to use with the form of the made emulsion.

  Moreover, in order to ensure the shape stability and flexibility of the separator, a fibrous material or the like may be mixed with the filler or the resin (A). As a fibrous material, the heat-resistant temperature is 150 ° C. or more, it has electrical insulation, is electrochemically stable, and is used in the manufacture of non-aqueous electrolytes and separators described in detail below. As long as the solvent to be used is stable, the material is not particularly limited. The “fibrous material” in the present specification means an aspect ratio [length in the longitudinal direction / width in the direction perpendicular to the longitudinal direction (diameter)] of 4 or more. The ratio is preferably 10 or more.

  Specific constituent materials of the fibrous material include, for example, cellulose and its modified products [CMC, hydroxypropyl cellulose (HPC), etc.], polyolefin [PP, propylene copolymer, etc.], polyester [polyethylene terephthalate (PET), etc. , Polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), etc.], polyacrylonitrile (PAN), aramid, polyamideimide, polyimide and other resins; glass, alumina, zirconia, silica and other inorganic oxides; A fibrous material may be formed by using two or more of these constituent materials in combination. The fibrous material may contain various known additives (for example, an antioxidant in the case of a resin) as necessary.

  The separator used in the battery of the present invention is handled when the entire separator is used as an independent membrane, or when the porous layer (I) (other than the microporous membrane) and the porous layer (II) are used as an independent membrane. A porous substrate can be used in order to enhance the properties. The porous substrate has a heat resistant temperature of 150 ° C. or more formed by forming a sheet-like material such as a woven fabric or a nonwoven fabric (including paper), and a commercially available nonwoven fabric or the like is used as the substrate. Can do. In the separator of this aspect, it is preferable to contain fine particles of the filler and resin (A) in the voids of the porous substrate, but in order to bind the porous substrate and fine particles of the filler and resin (A), The organic binder can also be used.

  The “heat resistance” of the porous substrate means that a substantial dimensional change due to softening or the like does not occur, and the change in the length of the object, that is, the porous substrate with respect to the length at room temperature. The heat resistance is evaluated based on whether or not the upper limit temperature (heat resistance temperature) at which the shrinkage ratio (shrinkage ratio) can be maintained at 5% or less is sufficiently higher than the shutdown temperature of the separator. In order to increase the safety of the lithium secondary battery after shutdown, the porous substrate preferably has a heat resistance temperature that is 20 ° C. or more higher than the shutdown temperature, and more specifically, the heat resistance temperature of the porous substrate is It is preferable that it is 150 degreeC or more, and it is more preferable that it is 180 degreeC or more.

  The porous substrate can be used for the porous layer (I) mainly composed of the resin (A), or can be used for the porous layer (II) mainly including the filler. A porous substrate can also be used for both the porous layer (I) and the porous layer (II). In that case, the porous layer (I) and the porous layer (II) may be integrated by sharing a single porous substrate. For each of the porous layer (I) and the porous layer (II), It may have another porous substrate.

  When the separator is formed using a porous substrate, all of the resin (A) (in the case of fine particles) and the filler, further preferably used plate-like particles, fine particles of the secondary particle structure, etc. Alternatively, it is preferable that a part is present in the voids of the porous substrate. By setting it as such a form, effects, such as resin (A), the said filler, a plate-shaped particle, the fine particle of the said secondary particle structure, can be exhibited more effectively.

  The diameter of the fibrous material (including the fibrous material constituting the porous substrate and other fibrous materials) may be equal to or less than the thickness of the porous layer (I) or the porous layer (II). It is preferable that it is 0.01-5 micrometers. If the diameter of the fibrous material is too large, the entanglement between the fibrous materials is insufficient. For example, when a porous substrate is formed by forming a sheet-like material, the strength becomes small and handling becomes difficult. There is. On the other hand, when the diameter of the fibrous material is too small, the pores of the separator become too small and the ion permeability tends to be lowered, and the load characteristics of the lithium secondary battery may be lowered.

  The content of the fibrous material in the separator of the present invention is, for example, 10% by volume or more, more preferably 20% by volume or more, and 90% by volume or less, more preferably 80% by volume or less in all the constituent components. It is desirable. The state of the presence of the fibrous material in the separator is, for example, that the angle of the long axis (long axis) with respect to the separator surface is preferably 30 ° or less on average, and more preferably 20 ° or less.

  When the fibrous material is used as the porous substrate, the content of other components is adjusted so that the proportion of the porous substrate is 10% by volume or more and 90% by volume or less in all the constituent components of the separator. It is desirable to adjust.

  The thickness of the separator in the lithium secondary battery of the present invention is preferably 6 μm or more, and more preferably 10 μm or more, from the viewpoint of more reliably separating the positive electrode and the negative electrode. On the other hand, if the thickness of the separator is too large, the energy density of the lithium secondary battery may be lowered. Therefore, the thickness is preferably 50 μm or less, and more preferably 30 μm or less.

  When the thickness of the porous layer (I) constituting the separator is A (μm) and the thickness of the porous layer (II) is B (μm), the ratio A / B between A and B is 10 or less. Preferably, it is 5 or less, more preferably 1/8 or more, and even more preferably 1/5 or more. In the separator according to the lithium secondary battery of the present invention, even if the thickness ratio of the porous layer (I) is increased and the porous layer (II) is thinned, the separator is thermally contracted while ensuring a good shutdown function. Generation | occurrence | production of a short circuit can be suppressed highly. In the separator, when there are a plurality of porous layers (I), the thickness A is the total thickness, and when there are a plurality of porous layers (II), the thickness B is the total thickness. .

  In terms of specific values, the thickness of the porous layer (I) [when the separator has a plurality of porous layers (I), the total thickness] is preferably 5 μm or more, It is preferable that it is 30 micrometers or less. The thickness of the porous layer (II) [when the separator has a plurality of porous layers (II), the total thickness] is preferably 1 μm or more, more preferably 2 μm or more, and 4 μm. More preferably, it is 20 μm or less, more preferably 10 μm or less, and even more preferably 6 μm or less. If the porous layer (I) is too thin, the shutdown function may be weakened. If it is too thick, the effect of improving the energy density of the battery due to the use of a negative electrode material described later may be reduced. For example, in the configuration in which the porous layer (I) and the porous layer (II) are integrated, the action of suppressing the thermal contraction of the entire separator may be reduced. Further, if the porous layer (II) is too thin, the effect of suppressing the occurrence of a short circuit due to thermal contraction of the separator may be reduced, and if it is too thick, the thickness of the entire separator is increased.

Further, the porosity of the separator is preferably 30% or more in a dried state in order to secure the amount of the nonaqueous electrolyte solution and to improve the ion permeability. On the other hand, from the viewpoint of securing separator strength and preventing internal short circuit, the separator porosity is preferably 70% or less in a dry state. The porosity of the separator: P (%) can be calculated by obtaining the sum of each component i from the thickness of the separator, the mass per area, and the density of the constituent components using the following equation (1).
P = 100− (Σa _i / ρ _i ) × (m / t) (1)
Here, in the above formula, a _i : ratio of component i expressed by mass%, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of separator (g / cm ² ), t: The thickness (cm) of the separator.

In the formula (10), m is the mass per unit area (g / cm ² ) of the porous layer (I), and t is the thickness (cm) of the porous layer (I). The porosity: P (%) of the porous layer (I) can also be obtained using the equation (1). The porosity of the porous layer (I) determined by this method is preferably 30 to 70%.

Furthermore, in the formula (1), m is the mass per unit area (g / cm ² ) of the porous layer (II), and t is the thickness (cm) of the porous layer (II), The porosity: P (%) of the porous layer (II) can also be obtained using the formula (1). The porosity of the porous layer (II) obtained by this method is preferably 20 to 60%.

Further, the separator according to the lithium secondary battery of the present invention is measured by a method according to JIS P 8117, and the Gurley value is indicated by the number of seconds that 100 ml of air permeates through the membrane under a pressure of 0.879 g / mm ^2. However, it is desirable that it is 10 to 300 sec. If the air permeability is too high, the ion permeability is reduced, whereas if it is too low, the strength of the separator may be reduced. Further, the strength of the separator is desirably 50 g or more in terms of piercing strength using a needle having a diameter of 1 mm. If the piercing strength is too small, a short circuit may occur due to the piercing of the separator when lithium dendrite crystals are generated. By employ | adopting the said structure, it can be set as the separator which has the said air permeability and piercing strength.

  The average pore size of the separator is preferably 0.01 μm or more, more preferably 0.05 μm or more, preferably 1 μm or less, more preferably 0.5 μm or less. The average pore size of the porous layer (I) is preferably 0.01 to 0.5 μm, and the average pore size of the porous layer (II) is preferably 0.05 to 1 μm. The average pore diameter of the separator, the porous layer (I), and the porous layer (II) can be determined from pore distribution measurement using a mercury porosimeter.

  The shutdown characteristic of the lithium secondary battery of the present invention having the separator having the above-described configuration can be obtained, for example, by the temperature change of the internal resistance of the battery. Specifically, it can be measured by installing a lithium secondary battery in a thermostat, increasing the temperature from room temperature at a rate of 1 ° C. per minute, and determining the temperature at which the internal resistance of the lithium secondary battery increases. Is possible. In this case, the internal resistance of the lithium secondary battery at 150 ° C. is preferably 5 times or more of room temperature, more preferably 10 times or more. By using the separator having the above-described configuration, such characteristics are obtained. Can be secured.

  The separator used in the lithium secondary battery of the present invention preferably has a thermal shrinkage rate at 150 ° C. of 5% or less. With the separator having such characteristics, even when the inside of the lithium secondary battery reaches about 150 ° C., the separator hardly contracts, so that a short circuit due to contact between the positive and negative electrodes can be more reliably prevented, The safety of the lithium secondary battery can be further increased. By employ | adopting the said structure, it can be set as the separator which has the above thermal contraction rates.

  When the porous layer (I) and the porous layer (II) are integrated, the heat shrinkage here refers to the shrinkage rate of the integrated separator as a whole, and the porous layer (I) and the porous layer (I) When the layer (II) is independent, the value of the smaller shrinkage rate is indicated. As will be described later, the porous layer (I) and / or the porous layer (II) can be integrated with the electrode. In that case, the measurement was performed in an integrated state with the electrode. Refers to heat shrinkage.

  The “150 ° C. heat shrinkage rate” means that the separator or the porous layer (I) and the porous layer (II) (when integrated with the electrode, in an integrated state with the electrode) , The temperature is raised to 150 ° C. and left for 3 hours, then taken out, and the dimensions required by comparing with the dimensions of the separator or porous layer (I) and porous layer (II) before being put in the thermostatic bath The percentage of decrease is expressed as a percentage.

  As a manufacturing method of the separator of the present invention, for example, the following methods (a) to (f) can be employed. In the production method (a), a porous layer (I) -forming composition containing a resin (A) (such as a liquid composition such as a slurry) or a porous layer containing a filler (II) ) A manufacturing method in which a composition for formation (a liquid composition such as a slurry) is applied, dried at a predetermined temperature, and then the other composition is applied and then dried at a predetermined temperature. Specifically, the porous substrate in this case has a woven fabric composed of at least one kind of fibrous material containing each of the exemplified materials as a constituent component, or a structure in which these fibrous materials are entangled with each other. Examples thereof include a porous sheet such as a nonwoven fabric. More specifically, non-woven fabrics such as paper, PP non-woven fabric, polyester non-woven fabric (PET non-woven fabric, PEN non-woven fabric, PBT non-woven fabric, etc.) and PAN non-woven fabric can be exemplified.

  In addition to the resin (A) (for example, fine particles), the composition for forming the porous layer (I) may be a filler (plate-like particles or fine particles having the secondary particle structure) as necessary. And an organic binder, and these are dispersed in a solvent (including a dispersion medium, the same applies hereinafter). The organic binder can be dissolved in a solvent. The solvent used in the composition for forming the porous layer (I) is not particularly limited as long as it can uniformly disperse the resin (A), the filler, and the like, and can uniformly dissolve or disperse the organic binder. In general, organic solvents such as aromatic hydrocarbons such as tetrahydrofuran, furans such as tetrahydrofuran, and ketones such as methyl ethyl ketone and methyl isobutyl ketone are preferably used. In addition, for the purpose of controlling the interfacial tension, alcohols (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these solvents. In addition, when the organic binder is water-soluble or used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.) are appropriately added. It is also possible to control the interfacial tension.

  The composition for forming the porous layer (II) includes the filler (which may be plate-like particles or fine particles having the secondary particle structure), and if necessary, the resin (A) (for example, fine particles) In addition, an organic binder or the like is contained, and these are dispersed in a solvent. As the solvent, the same solvents as those exemplified for the composition for forming the porous layer (I) can be used, and the composition for forming the porous layer (I) is appropriately used as a component for controlling the interfacial tension. You may add the said various components illustrated regarding the thing.

  In the composition for forming the porous layer (I) and the composition for forming the porous layer (II), the solid content containing the resin (A), the filler and the organic binder may be, for example, 10 to 80% by mass. preferable.

  When the pore diameter of the porous substrate is relatively large, for example, when it is 5 μm or more, this tends to cause a short circuit of the battery. Therefore, in this case, the resin (A), the filler, and further, all or part of the plate-like particles and the fine particles having the secondary particle structure may be present in the voids of the porous substrate. preferable. In order to allow the resin (A), filler, plate-like particles, fine particles having the secondary particle structure, etc. to be present in the voids of the porous substrate, for example, a porous layer forming composition containing them is added to the porous substrate. After coating, a process such as passing through a certain gap, removing excess composition, and drying may be used.

  In addition, in order to increase the orientation of the plate-like particles contained in the separator and to make the function work more effectively, the porous substrate-forming composition containing the plate-like particles was applied to and impregnated on the porous substrate. Thereafter, a method of applying a shear or a magnetic field to the composition may be used. For example, as described above, after applying the porous layer forming composition containing plate-like particles to the porous substrate, the composition can be shared by passing through a certain gap.

  Further, in order to more effectively exert the functions of the respective constituents such as the resin (A), the filler, and further the plate-like particles and the fine particles having the secondary particle structure, the constituents are unevenly distributed, It is good also as a form which the said structure gathered in the layer form in parallel or substantially parallel to the film surface. In order to achieve such a form, for example, using two heads and rolls of a die coater or reverse roll coater, separate compositions from both the front and back sides of the porous substrate, for example, a composition for forming a porous layer (I) The method of apply | coating an object and the composition for porous layer (II) separately, and drying can be employ | adopted.

  In the separator production method (b), the porous layer (I) forming composition or the porous layer (II) forming composition is applied to a porous substrate, and the other composition is dried before the applied composition is dried. It is a method of applying an object and drying it.

  In the separator production method (c), a porous layer (I) -forming composition is applied to a porous substrate and dried to form a porous layer (I) mainly composed of a resin (A). A porous layer (II) -forming composition is applied to the porous substrate and dried to form a porous layer (II) containing a filler as a main component. This is a method of using a separator. In this case, the porous layer (I) and the porous layer (II) may be integrated with each other, and each of them is an independent structure, and is integrated in a state of being overlapped in the battery by assembling the battery. It may function as a separator.

  In the separator production method (d), the porous layer (I) -forming composition and the porous layer (II) -forming composition further contain a fibrous material as necessary, and this is used as a film or metal foil. It is a method of coating on a substrate, etc., drying at a predetermined temperature, and then peeling from the substrate. Also in the method (d), as in the method (c), the porous layer (I) mainly composed of the resin (A) and the porous layer (II) mainly composed of a filler are configured as independent from each other. Or it is good also as an integrated structure. When the porous layer (I) and the porous layer (II) are integrated, the same porous layer is formed and dried as in the method (a). It may be formed, or one porous layer forming composition may be applied, and the other porous layer forming composition may be applied before drying, or both porous layer forming compositions may be applied. A so-called simultaneous multi-layer coating method in which the two are simultaneously applied may be used.

  Moreover, the separator and the electrode were integrated by forming the porous layer (I) or the porous layer (II) on the surface of at least one of the positive electrode and the negative electrode constituting the battery by the method (d). It is good also as a structure. In this case, both the porous layers can be formed on at least one of the positive electrode and the negative electrode, the porous layer (I) is formed on one of the positive electrode and the negative electrode, and the porous layer (II) is formed on the other electrode. It is also possible to form the electrode.

  In addition, when manufacturing a separator by any one of manufacturing methods (a) to (d), in order to integrate the porous layer (I) and the porous layer (II), as described above, In addition to the method of applying the other porous layer forming composition to the surface of the porous layer formed by drying the coated film, or drying the coated layer, etc. Alternatively, a method in which the porous layer (I) and the porous layer (II) formed independently of each other are superposed and bonded together by a roll press or the like can be employed.

  Moreover, when manufacturing a separator by any one of manufacturing methods (a) to (d), the porous layer (I) or the porous layer (II) according to the separator and the electrode (positive electrode or negative electrode) are integrated. In addition to the method for forming the porous layer (I) or the porous layer (II) directly on the electrode surface, as described above, the porous layer forming composition is applied to the electrode surface and dried. Alternatively, the porous film (I) or the porous film (II) formed independently, or the separator having the porous layer (I) and the porous layer (II), and the electrode are overlapped with each other by a roll press or the like. A method of bonding the two can also be employed.

  In the separator production method (e), the porous layer (II) -forming composition is applied to the same porous substrate as described in the production method (a), and then dried at a predetermined temperature. In this method, the porous layer (II) is formed, and this is overlapped with the microporous film for forming the porous layer (I) produced by the above-described method to form one separator. In this case as well, the porous layer (I) and the porous layer (II) may be integrated, or they are independent films, and are stacked in the battery by assembling the lithium secondary battery. It may function as an integrated separator in the state.

  Also in this case, in the porous layer (II), for example, by the same method as described above, the fine particles such as the filler are present, or all or part of the porous substrate is present in the voids of the porous substrate. It is preferable that the structure be

  In order to integrate the porous layer (I) and the porous layer (II), for example, as in the case described above, the porous layer (I) and the porous layer (II) are overlapped, and a roll press, etc. The method of sticking both together can be adopted.

  In the separator production method (f), the porous layer (II) -forming composition further contains a fibrous material as necessary, and this is applied onto a substrate such as a film or a metal foil, and is subjected to a predetermined temperature. After drying with the above method, a porous film that peels from the substrate to form a porous layer (II) is formed, and this porous film is superposed on the microporous film that constitutes the porous layer (I). This is a method of using two separators.

  In the production method (f), as in the production method (e), the porous layer (I) composed of a microporous film mainly composed of the resin (A) and the porous layer (II) mainly composed of a filler Each may be an independent configuration or may be an integrated configuration. In order to integrate the porous layer (I) and the porous layer (II), in addition to the method of laminating the individually formed porous layer (II) and porous layer (I) by a roll press or the like, Instead of using the substrate, the composition for forming the porous layer (II) is applied to the surface of the porous layer (I), dried, and the porous layer (II) is directly applied to the surface of the porous layer (I). ) May be employed.

  In the case of employing any of the manufacturing methods (e) and (f), the porous layer (I) may be integrated with at least one of the positive electrode and the negative electrode.

  Note that the porous layer (I) and the porous layer (II) do not have to be one each, and a plurality of layers may be present in the separator. For example, a configuration in which the porous layer (I) is disposed on both sides of the porous layer (II) or a configuration in which the porous layer (II) is disposed on both sides of the porous layer (I) may be employed. However, increasing the number of layers may increase the thickness of the separator and increase the internal resistance of the battery or decrease the energy density. Therefore, it is not preferable to increase the number of layers, and the porous layer in the separator is not preferable. The total number of layers (I) and porous layer (II) is preferably 5 or less.

  In addition, as described above, the porous layer (I) and the porous layer (II) are integrated as independent components in addition to constituting a separator as an independent film, and lithium secondary batteries are assembled. At the stage, the battery is superposed in the lithium secondary battery, and can function as a separator interposed between the positive electrode and the negative electrode. Furthermore, the porous layer (I) and the porous layer (II) do not need to be in contact with each other, and another layer, for example, a fibrous material layer constituting the porous substrate is interposed therebetween. May be.

For the negative electrode according to the lithium secondary battery of the present invention, a negative electrode having a negative electrode mixture layer containing at least SiO _{x as} a negative electrode material and a conductive material is used.

  A compound containing Si (silicon) and O (oxygen) as constituent elements used for the negative electrode material (however, the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) is a microcrystal of Si. Alternatively, an amorphous phase may be included, and in this case, the atomic ratio between Si and O is a ratio including Si microcrystals or amorphous phase Si.

That is, the compounds are in the SiO ₂ matrix of amorphous Si (e.g., microcrystalline Si) is include the dispersed structure, the SiO ₂ of the amorphous, dispersed therein It is sufficient that the atomic ratio x satisfies 0.5 ≦ x ≦ 1.5 in combination with Si. For example, in the case of a compound in which Si is dispersed in an amorphous SiO ₂ matrix and the molar ratio of SiO ₂ to Si is 1: 1, x = 1, so that the structural formula is represented by SiO. The In the case of a compound having such a structure, for example, in X-ray diffraction analysis, a peak due to the presence of Si (microcrystalline Si) may not be observed, but when observed with a transmission electron microscope, the presence of fine Si Can be confirmed.

The compound SiO _x is preferably a composite that is combined with a conductive material such as a carbon material. For example, the surface of the SiO _x is preferably covered with a conductive material (such as a carbon material). As described above, since SiO _x has poor conductivity, when using it as a negative electrode active material, from the viewpoint of securing good battery characteristics, a conductive material (conductive aid) is used, and SiO _x in the negative electrode is used. It is necessary to form an excellent conductive network by mixing and dispersing the material and the conductive material well. In the case of a composite in which SiO _x is combined with a conductive material, for example, a conductive network in the negative electrode is formed better than when a material obtained by simply mixing SiO _x and a conductive material is used. The

As a composite of SiO _x and a conductive material, as described above, the surface of SiO _x is covered with a conductive material (preferably a carbon material), and SiO _x and a conductive material (preferably a carbon material). And the like.

Further, by using the composite in which the surface of SiO _x is coated with a conductive material (preferably a carbon material) in combination with a conductive material (such as a carbon material), an even better conductive network can be obtained in the negative electrode. Therefore, it is possible to realize a lithium secondary battery with higher capacity and excellent battery characteristics (for example, charge / discharge cycle characteristics). The complex of the SiO _x and a conductive material coated with a conductive material, for example, like granules the mixture was further granulated with SiO _x and a conductive material coated with a conductive material .

As the SiO _x whose surface is coated with a conductive material, a complex with SiO _x and a conductive material resistivity value is less than (e.g. granulate), preferably between SiO _x and the carbon material Those in which the surface of the composite is further coated with a carbon material can be preferably used. In a state where SiO _x and the conductive material are dispersed in the granulated body, a better conductive network can be formed. Therefore, in a lithium secondary battery having a negative electrode containing this as a negative electrode material, heavy load discharge characteristics The battery characteristics such as can be further improved.

Preferred examples of the conductive material that can be used for forming a composite with SiO _x include carbon materials such as graphite, low crystalline carbon, carbon nanotube, and vapor grown carbon fiber.

Details of the conductive material include a fibrous or coiled carbon material, a fibrous or coiled metal, carbon black (including acetylene black and ketjen black), artificial graphite, graphitizable carbon and non-graphitizable. At least one material selected from the group consisting of carbon is preferred. A fibrous or coiled carbon material or a fibrous or coiled metal is preferable in that it easily forms a conductive network and has a large surface area. Carbon black (including acetylene black and ketjen black), artificial graphite, graphitizable carbon, and non-graphitizable carbon have high electrical conductivity and high liquid retention. _{Even if the x} particles expand and contract, it is preferable in that it has a property of easily maintaining contact with the particles.

Among the conductive materials exemplified above, a fibrous carbon material is particularly preferable as a material used when the composite with SiO _x is a granulated body. Fibrous carbon material can follow the expansion and contraction of SiO _x with the charging and discharging of the battery due to the high shape is thin threadlike flexibility, also because bulk density is large, many and SiO _x particles It is because it can have a junction. Examples of the fibrous carbon include polyacrylonitrile (PAN) -based carbon fiber, pitch-based carbon fiber, vapor-grown carbon fiber, and carbon nanotube, and any of these may be used.

In addition, a fibrous carbon material and a fibrous metal can also be formed on the surface of SiO _x particles by, for example, a vapor phase method.

The specific resistance value of SiO _x is usually 10 ³ to 10 ⁷ kΩcm, whereas the specific resistance value of the exemplified conductive material is usually 10 ^{−5 to} 10 kΩcm.

  The negative electrode material according to the present invention may further include a material layer (a material layer containing non-graphitizable carbon) that covers the carbon material coating layer on the particle surface.

The negative electrode material according to the present invention, when the complex of the SiO _x and a conductive material, the ratio between the SiO _x and a conductive material, from the viewpoint of satisfactorily exhibiting the effect by compounding the conductive material, SiO x: relative to ₁₀₀ parts by weight, the conductive material is preferably at least 5 parts by weight, and more preferably 10 parts by mass or more. Moreover, in the negative electrode material, if the ratio of the conductive material to be combined with SiO _x is too large, the amount of SiO _x in the negative electrode mixture layer may be reduced, and the effect of increasing the capacity may be reduced. , SiO _x : The conductive material is preferably 50 parts by mass or less and more preferably 40 parts by mass or less with respect to 100 parts by mass of SiO _x .

  The negative electrode material according to the present invention can be obtained, for example, by the following method.

First, a manufacturing method in the case of combining SiO _x will be described. A dispersion liquid in which SiO _x is dispersed in a dispersion medium is prepared, and sprayed and dried to produce composite particles including a plurality of particles. For example, ethanol or the like can be used as the dispersion medium. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. In addition to the above method, similar composite particles can be produced also by a granulation method by a mechanical method using a vibration type or planetary type ball mill or rod mill.

Incidentally, the SiO _x, in the case of manufacturing a granulated body with small conductive material resistivity value than SiO _x is adding the conductive material into the dispersions SiO _x are dispersed in a dispersion medium, Using this dispersion, composite particles (granulated body) may be formed by the same technique as that for combining SiO _x . Further, by granulation process according to the similar mechanical method, it is possible to produce a granular material of SiO _x and a conductive material.

Next, when the surface of SiO _x particles (SiO _x composite particles or a granulated body of SiO _x and a conductive material) is coated with a carbon material to form a composite, for example, SiO _x particles and hydrocarbons are used. The system gas is heated in the gas phase, and carbon generated by thermal decomposition of the hydrocarbon system gas is deposited on the surface of the particles. As described above, according to the vapor deposition (CVD) method, the hydrocarbon-based gas spreads to every corner of the composite particle, and the surface of the particle and the pores in the surface are thin and contain a conductive carbon material. Since a uniform film (carbon material coating layer) can be formed, the SiO _x particles can be imparted with good conductivity with a small amount of carbon material.

In the production of SiO _x coated with a carbon material, the processing temperature (atmosphere temperature) of the vapor deposition (CVD) method varies depending on the type of hydrocarbon gas, but usually 600 to 1200 ° C. is appropriate. Among these, the temperature is preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the treatment temperature, the less the remaining impurities, and the formation of a coating layer containing carbon having high conductivity.

  As the liquid source of the hydrocarbon-based gas, toluene, benzene, xylene, mesitylene and the like can be used, but toluene that is easy to handle is particularly preferable. A hydrocarbon-based gas can be obtained by vaporizing them (for example, bubbling with nitrogen gas). Moreover, methane gas, acetylene gas, etc. can also be used.

In addition, after covering the surface of SiO _x particles (SiO _x composite particles, or a granulated body of SiO _x and a conductive material) with a carbon material by a vapor deposition (CVD) method, petroleum pitch, coal-based After attaching at least one organic compound selected from the group consisting of pitch, thermosetting resin, and a condensate of naphthalene sulfonate and aldehydes to a coating layer containing a carbon material, the organic compound is The adhered particles may be fired.

Specifically, a dispersion liquid in which a SiO _x particle (SiO _x composite particle or a granulated body of SiO _x and a conductive material) coated with a carbon material and the organic compound are dispersed in a dispersion medium is prepared. The dispersion is sprayed and dried to form particles coated with the organic compound, and the particles coated with the organic compound are fired.

  An isotropic pitch can be used as the pitch, and a phenol resin, a furan resin, a furfural resin, or the like can be used as the thermosetting resin. As the condensate of naphthalene sulfonate and aldehydes, naphthalene sulfonic acid formaldehyde condensate can be used.

As a dispersion medium for dispersing the SiO _x particles coated with the carbon material and the organic compound, for example, water or alcohols (ethanol or the like) can be used. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. The firing temperature is usually 600 to 1200 ° C., preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the processing temperature, the less the remaining impurities, and the formation of a coating layer containing a high-quality carbon material with high conductivity. However, the processing temperature needs to be lower than the melting point of SiO _x .

The negative electrode according to the present invention includes a suitable solvent (a negative electrode mixture) containing a mixture of the negative electrode material (SiO _x or a composite of SiO _x and a conductive material) and a binder (binder). A paste-like or slurry-like negative electrode mixture-containing composition obtained by adding and dispersing the dispersion medium) is applied to a current collector, and the solvent (dispersion medium) is removed by drying or the like to obtain a predetermined thickness. And a negative electrode mixture layer having a density can be obtained. In addition, the negative electrode which concerns on this invention is not restricted to what was obtained by the said manufacturing method, The thing manufactured by the other manufacturing method may be used.

The binder used for the negative electrode mixture layer is preferably at least one of polyimide, polyamideimide and polyamide. These binders are separated from negative electrode materials in the negative electrode mixture layer, negative electrode material (a composite of SiO _x particles or SiO _x and a conductive material), and a composite of SiO _x and a conductive material. Since the power to bind the conductive material contained alone as a conductive aid to the mixture layer is strong, even if expansion and contraction of SiO _x occurs due to repeated charging and discharging of the battery, these contacts are maintained. The conductive network in the negative electrode mixture layer can be favorably retained.

  Examples of the polyimide include various known polyimides, and any of thermoplastic polyimide and thermosetting polyimide can be used. In the case of a thermosetting polyimide, either a condensation type polyimide or an addition type polyimide may be used. More specifically, for example, “Semicofine (trade name)” manufactured by Toray Industries, “PIX series (trade name)” manufactured by Hitachi Chemical DuPont Microsystems, “HCI series (trade name)” manufactured by Hitachi Chemical, Ube Commercial products such as “U-Varnish (trade name)” manufactured by Kosan Co., Ltd. can be used. For reasons such as good electron mobility, those having an aromatic ring in the molecular chain, that is, aromatic polyimide are more preferred. Only one type of polyimide may be used, or two or more types may be used in combination.

  Examples of the polyamideimide include various known polyamideimides. More specifically, for example, commercially available products such as “HPC series (trade name)” manufactured by Hitachi Chemical Co., Ltd. and “Viromax (trade name)” manufactured by Toyobo Co., Ltd. can be used. In addition, also in a polyamideimide, for the same reason as a polyimide, what has an aromatic ring in a molecular chain, ie, an aromatic polyamideimide, is more preferable. Polyamideimide may use only 1 type and may use 2 or more types together.

  As polyamide, various polyamides, such as nylon 66, nylon 6, aromatic polyamide (nylon MXD6 etc.), can be used, for example. In addition, also in a polyamide, the thing which has an aromatic ring in a molecular chain, ie, an aromatic polyamide, is more preferable for the same reason as a polyimide etc. Only one type of polyamide may be used, or two or more types of polyamide may be used in combination.

  In addition to the above, polysaccharides such as starch, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose and their modified products; polyvinyl chloride, polyvinyl pyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, Thermoplastic resins such as polyethylene and polypropylene, and modified products thereof; ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, butadiene rubber, polybutadiene, fluorine rubber, polyethylene oxide, etc. A polymer or a modified product thereof may be used as a binder.

  The various binders described above may be used alone or in combination of two or more.

  A conductive material may be further added to the negative electrode mixture as a conductive auxiliary. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery. Usually, natural graphite (such as scaly graphite, scaly graphite, earthy graphite), artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, metal powder (copper, nickel, aluminum, silver, etc.), metal fiber, One or more materials such as polyphenylene derivatives (described in JP-A-59-20971) can be used.

In the negative electrode mixture layer related to the negative electrode, from the viewpoint of increasing the capacity of the battery, the content of the negative electrode material (SiO _x or a composite of SiO _x and a conductive material) is preferably 60% by mass or more. 70% by mass or more is more preferable. However, if the amount of the negative electrode material in the negative electrode mixture layer is too large, for example, the amount of the binder decreases, and the action of the binder (for example, the action of suppressing the destruction of the conductive network in the negative electrode due to the volume change of SiO _x ) ) May be small, the content of the negative electrode material is preferably 99% by mass or less, and more preferably 98% by mass or less.

  In addition, the content of the binder in the negative electrode mixture layer is preferably 1% by mass or more, and more preferably 2% by mass or more, from the viewpoint of more effectively exerting the action due to the use of the binder. However, if the amount of the binder in the negative electrode mixture layer is too large, for example, the amount of the negative electrode material decreases and the effect of increasing the capacity may be reduced. Therefore, the binder content is 30% by mass. Or less, more preferably 20% by mass or less.

  In addition, when using polyimide, polyamideimide, or polyamide and a binder other than these as a binder of a negative mix layer, content of the polyimide, polyamideimide, or polyamide in a negative mix layer is preferably 1 mass. % Or more, more preferably 2% by mass or more, and it is desirable to adjust so as to satisfy the preferable binder amount. By making the amount of polyimide, polyamideimide or polyamide in the negative electrode mixture layer as described above, the effect of using these can be more effectively exhibited.

Further, in the negative electrode mixture layer, from the viewpoint of a higher capacity batteries, a conductive material (conductive material contained in the complex with SiO _x, and as a complex with SiO _x as required However, the total amount of the conductive material used alone is preferably 50% by mass or less, and more preferably 40% by mass or less. Further, from the viewpoint of satisfactorily forming a conductive network in the negative electrode mixture layer, the total amount of the conductive material in the negative electrode mixture layer is preferably 5% by mass or more, and preferably 10% by mass or more. More preferred.

  The thickness of the negative electrode mixture layer is preferably 10 to 100 μm, for example.

Further, the negative electrode according to the lithium secondary battery of the present invention has a porous layer (hereinafter sometimes referred to as “coat layer”) containing an insulating material that does not react with Li on the surface of the negative electrode mixture layer. Things can also be used. In FIG. 1, the cross-sectional schematic diagram of an example of the negative electrode which has a coating layer is shown. The negative electrode 1 is configured by laminating a coat layer 2 for improving the strength of the negative electrode 1 on the surface of the negative electrode mixture layer 3 containing a negative electrode material containing SiO _x . Reference numeral 4 denotes a current collector.

When a negative electrode having a coating layer on the surface of the negative electrode mixture layer as described above is used, and when at least one of polyimide, polyamideimide and polyamide is used as the binder of the negative electrode mixture layer, Can suppress the bending of the negative electrode accompanying the volume change of SiO _x and the swelling in the thickness direction of the electrode body including the negative electrode (such as a wound structure electrode body constituted by a negative electrode, a positive electrode and a separator, or an electrode body having a laminated structure). . Therefore, it is possible to prevent the occurrence of battery swelling due to the bending of the negative electrode and the swelling of the electrode body including the negative electrode.

  The coat layer according to the negative electrode is a layer (porous layer) that contains an insulating material that does not react with Li and has pores that allow a non-aqueous electrolyte to pass through.

  Examples of the insulating material that does not react with Li for constituting the coat layer include various inorganic fine particles and organic fine particles. As the inorganic fine particles, chalcogenites (oxides, sulfides, etc.), nitrides, carbides, silicides, etc. of metal elements or nonmetal elements are preferable.

The metal element or non-metal element chalcogenite is preferably an oxide, and more preferably an oxide that is not easily reduced. Examples of such oxides include, for example, MgO, CaO, SrO, BaO, ZrO ₂ , ZnO, B ₂ O ₃ , Al ₂ O ₃ , boehmite typified by AlOOH, Ga ₂ O ₃ , In ₂ O ₃ , Examples thereof include SiO ₂ , As ₄ O ₆ , and Sb ₂ O ₅ . Among these, boehmite typified by ZnO, Al ₂ O ₃ , and AlOOH, Ga ₂ O ₃ , SiO ₂ , and ZrO ₂ are particularly preferable. These oxides may be single or complex oxides.

  Examples of the nitride of the metal element or nonmetal element include aluminum nitride (AlN) and BN, and examples of the carbide and silicide of the metal element or nonmetal element include SiC, which have high insulation and chemical properties. It is preferable in that it is stable.

  Among the insulating materials that do not react with Li for constituting the coat layer, the organic fine particles include, for example, fine particles of fluororesin such as polytetrafluoroethylene (PTFE), crosslinked latex, etc. Those which do not form a film or decompose by flowing at a temperature are preferred.

  The particle size of the insulating material that does not react with Li is, for example, preferably 0.1 μm or more, more preferably 0.2 μm or more, preferably 10 μm or less, more preferably 5 μm or less.

The coat layer may include an electron conductive material. The electron conductive material is not an essential component of the coat layer. However, as will be described later, when Li is introduced in advance into SiO _{x of the} negative electrode, the electron conductive material is contained in the coat layer.

  Examples of the electron conductive material that can be used for the coating layer include carbon materials such as carbon particles and carbon fibers; metal materials such as metal particles and metal fibers; and metal oxides. Among these, carbon particles and metal particles having low reactivity with Li are preferable.

  As the carbon material, for example, a known carbon material that is used as a conductive additive in an electrode constituting a battery can be used. Specifically, carbon such as carbon black (thermal black, furnace black, channel black, lamp black, ketjen black, acetylene black, etc.), graphite (natural graphite such as flake graphite, earth graphite, and artificial graphite) Examples include particles and carbon fibers.

  Among the carbon materials described above, the combined use of carbon black and graphite is particularly preferable from the viewpoint of dispersibility with a binder described later. Further, as the carbon black, ketjen black and acetylene black are particularly preferable.

  The particle size of the carbon particles is, for example, preferably 0.01 μm or more, more preferably 0.02 μm or more, preferably 10 μm or less, more preferably 5 μm or less.

  Among the electron conductive materials constituting the coat layer, the metal particles and the metal fibers are preferably composed of a metal element that has low reactivity with Li and is difficult to form an alloy. Specific metal elements constituting the metal particles and metal fibers include, for example, Ti, Fe, Ni, Cu, Mo, Ta, and W.

  In the case of metal particles, the shape is not particularly limited, and may be any shape such as a lump shape, a needle shape, a column shape, or a plate shape. Further, the metal particles and the metal fibers are preferably those whose surfaces are not oxidized so much, and those that are excessively oxidized are preferably subjected to heat treatment in a reducing atmosphere in advance and then subjected to coating layer formation. .

  The particle size of the metal particles is, for example, preferably 0.02 μm or more, more preferably 0.1 μm or more, preferably 10 μm or less, more preferably 5 μm or less.

As an insulating material that does not react with Li, aluminum oxide (Al ₂ O ₃ , boehmite typified by AlOOH) is particularly preferable.

  In forming the coat layer, it is preferable to use a binder for the purpose of binding the insulating material that does not react with Li. As the binder, for example, various materials exemplified as the binder for the negative electrode mixture layer can be used. When the binder of the coating layer and the binder of the negative electrode mixture layer are of the same type (for example, both the binder of the coating layer and the binder of the negative electrode mixture layer include at least one of polyimide, polyamideimide, and polyamide). Use of one type) is preferable because adhesion between the negative electrode mixture layer and the coating layer is improved.

  When a binder is used for forming the coat layer, the content of the binder in the coat layer is preferably 2% by mass or more, more preferably 4% by mass or more, and preferably 60% by mass or less, more preferably 50% by mass. % Or less.

  When the coating layer contains an electron conductive material, when the total of the insulating material that does not react with Li and the material having electron conductivity is 100% by mass, the material having electron conductivity. The ratio of, for example, is preferably 2.5% by mass or more, more preferably 5% by mass or more, and 96% by mass or less, more preferably 95% by mass. In other words, the insulating property that does not react with Li The ratio of the material is, for example, preferably 4% by mass or more, more preferably 5% by mass or more, and 97.5% by mass or less, more preferably 95% by mass or less.

  The thickness of the coat layer is, for example, preferably 2 μm or more, more preferably 3 μm or more, preferably 10 μm or less, more preferably 8 μm or less. If the coating layer has such a thickness, expansion and bending of the negative electrode mixture layer can be more efficiently suppressed, and higher battery capacity and improved battery characteristics can be achieved better. That is, for example, if the thickness of the coating layer becomes too thin relative to the surface roughness of the negative electrode mixture layer, it becomes difficult to cover the entire surface of the negative electrode mixture layer without pinholes, and the coating layer is formed. On the other hand, if the coat layer is too thick, it may lead to a reduction in battery capacity.

  In addition, since the affinity between the negative electrode and the non-aqueous electrolyte is improved by providing the coat layer, there is an effect that the non-aqueous electrolyte can be easily introduced into the battery.

  For the coating layer, for example, an appropriate solvent (dispersion medium) is sufficiently added to a mixture containing an insulating material that does not react with Li, a material having electronic conductivity used as necessary, and a binder. A paste-like or slurry-like composition (coating material) obtained by kneading can be applied to the surface of the negative electrode mixture layer, and the solvent (dispersion medium) can be removed by drying or the like to form a predetermined thickness. . In addition, you may form a coat layer by methods other than the above. For example, after the composition for forming the negative electrode mixture layer is applied to the surface of the current collector, the composition for forming the coat layer is applied and dried before the coating film is completely dried. The layer and the coat layer may be formed simultaneously. Furthermore, in addition to the sequential method of sequentially applying the composition for forming the negative electrode mixture layer and the composition for forming the coat layer as described above, the application of the composition for forming the negative electrode mixture layer and the formation of the coat layer The negative electrode mixture layer and the coat layer may be formed simultaneously by a simultaneous application method in which the composition for coating is simultaneously applied.

Since SiO _x is irreversible capacity is relatively large in accordance with the anode material, in the negative electrode according to the present invention, it is also preferable to keep introducing advance Li, further higher capacity in this case is possible.

As a method for introducing Li into the negative electrode, for example, a Li-containing layer is formed on the surface opposite to the negative electrode mixture layer side of the negative electrode coating layer (a coating layer containing a material having electronic conductivity). A method of introducing Li from this Li-containing layer to SiO _x in the negative electrode mixture layer is preferable.

The introduction of Li to SiO _x there is a risk that the bending of the negative electrode caused by the volume change of the SiO _x. However, if a coating layer is formed on the negative electrode, in an environment where the non-aqueous electrolyte of the battery is present (for example, inside the battery), Li in the Li-containing layer is electrochemically added to SiO _x in the negative electrode mixture layer. Although introduced, under the environment where the non-aqueous electrolyte does not exist, the reaction of introducing Li into SiO _x hardly occurs. Thus, when employing the above-described Li introduction method, the coating layer related to the negative electrode also has a function of supplying Li in the Li-containing layer to the negative electrode mixture layer via the non-aqueous electrolyte. Thus, the reactivity of SiO _x and Li can be controlled to suppress the bending of the negative electrode accompanying the introduction of Li.

The Li-containing layer for introducing Li into the negative electrode is preferably formed by a general vapor phase method (vapor phase deposition method) such as resistance heating or sputtering (that is, a vapor deposition film). If the Li-containing layer is directly formed on the coating layer as a vapor deposition film by the vapor phase method, it is easy to form a uniform layer over the entire surface of the coating layer in a desired thickness. It can be introduced without excess or deficiency with respect to the irreversible capacity of _x .

  When the Li-containing layer is formed by a vapor phase method, the vapor deposition source and the coat layer related to the negative electrode may be opposed to each other in a vacuum chamber, and vapor deposition may be performed until the layer has a predetermined thickness.

  The Li-containing layer may be composed only of Li, for example, Li-Al, Li-Al-Mn, Li-Al-Mg, Li-Al-Sn, Li-Al-In, Li-Al-Cd. Or a Li alloy such as When the Li-containing layer is composed of a Li alloy, the content ratio of Li in the Li-containing layer is preferably, for example, 50 to 90 mol%.

The thickness of the Li-containing layer is, for example, preferably 2 μm or more, more preferably 4 μm or more, preferably 10 μm or less, more preferably 8 μm or less. By forming the Li-containing layer with such a thickness, Li can be introduced more or less with respect to the irreversible capacity of SiO _x . That is, if the Li-containing layer is too thin, the amount of Li with respect to the amount of SiO _x present in the negative electrode mixture layer decreases, and the capacity improvement effect by introducing Li into the negative electrode in advance may be reduced. On the other hand, if the Li-containing layer is too thick, the amount of Li may be excessive, and the amount of vapor deposition increases, resulting in a decrease in productivity.

  As the current collector for the negative electrode, a foil made of copper or nickel, a punching metal, a net, an expanded metal, or the like can be used, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is preferably 5 μm.

  The lead portion on the negative electrode side is usually provided by leaving the exposed portion of the current collector without forming the negative electrode mixture layer on a part of the current collector and forming the lead portion at the time of preparing the negative electrode. However, the lead portion is not necessarily integrated with the current collector from the beginning, and may be provided by connecting a copper foil or the like to the current collector later.

The positive electrode according to the lithium secondary battery of the present invention is not particularly limited as long as it is a positive electrode used in a conventionally known lithium secondary battery, that is, a positive electrode containing an active material capable of occluding and releasing Li ions. For example, as an active material, a lithium-containing transition metal oxide having a layered structure represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc.), LiMn It is possible to use spinel lithium manganese oxide in which ₂ O ₄ or a part of the element is substituted with another element, or an olivine type compound represented by LiMPO ₄ (M: Co, Ni, Mn, Fe, etc.) It is. Specific examples of the lithium-containing transition metal oxide having a layered structure include LiCoO ₂ and LiNi _1-x Co _xy Al _y O ₂ (0.1 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0. 2) and other oxides containing at least Co, Ni and Mn (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _5/12 Ni _5/12 Co _1/6 O ₂ , LiMn _{3 / 5} Ni _1/5 Co _1/5 O ₂ etc.).

  A carbon material such as carbon black is used as the conductive aid, and a fluororesin such as polyvinylidene fluoride (PVDF) is used as the binder. The positive electrode mixture is a mixture of these materials and an active material. The agent layer is formed on the surface of the current collector, for example. The thickness of the positive electrode mixture layer is preferably 50 to 200 μm, for example.

  Further, as the positive electrode current collector, a metal foil such as aluminum, a punching metal, a net, an expanded metal, or the like can be used, but usually an aluminum foil having a thickness of 10 to 30 μm is preferably used.

  Similarly to the lead portion on the negative electrode side, the lead portion on the positive electrode side usually leaves an exposed portion of the current collector without forming a positive electrode mixture layer on a part of the current collector during the preparation of the positive electrode, and the lead portion Is provided. However, the lead portion is not necessarily integrated with the current collector from the beginning, and may be provided by connecting an aluminum foil or the like to the current collector later.

  The electrode can be used in the form of a laminate electrode group in which the positive electrode and the negative electrode are laminated via the separator, or a wound electrode group in which the electrode is wound. In the battery of the present invention, it is preferable that the porous layer (I) of the separator faces at least the negative electrode, and the electrode group as described above has the porous layer (I) of the separator facing the negative electrode. It is recommended to form as follows.

  Although the detailed reason is unknown, when the separator is arranged so that the porous layer (I) mainly composed of the resin (A) faces at least the negative electrode, the shutdown is performed more than when the separator is arranged on the positive electrode side. When it occurs, the ratio of the resin (A) melted from the porous layer (I) to be absorbed by the electrode mixture layer decreases, and the melted resin (A) closes the pores of the separator. Since it is used more effectively, the effect of shutdown is better.

  In addition, for example, when a lithium secondary battery has a mechanism that lowers the internal pressure of the battery by discharging the gas inside the battery to the outside when the internal pressure of the battery increases due to temperature rise, There is a possibility that the internal non-aqueous electrolyte volatilizes and the electrode is directly exposed to air. When the battery is in a charged state, when the negative electrode and air (oxygen or moisture) come into contact, Li ions occluded in the negative electrode, lithium deposited on the negative electrode surface, and air react to generate heat. And sometimes it catches fire. Moreover, the temperature of the battery rises due to this heat generation, causing a thermal runaway reaction of the positive electrode active material, and as a result, the battery may ignite.

  However, in the case of a battery configured such that the porous layer (I) mainly composed of the resin (A) faces the negative electrode, the resin (A) that is the main body of the porous layer (I) melts at a high temperature. Since the negative electrode surface is covered, the reaction between the negative electrode and air accompanying the operation of the mechanism for discharging the gas inside the battery to the outside can be suppressed. Therefore, there is no fear of heat generation due to the operation of the mechanism for discharging the gas inside the battery to the outside, and the battery can be kept safer.

  Moreover, when a material excellent in oxidation resistance (for example, an inorganic oxide) is used as a filler having a heat resistant temperature of 150 ° C. or higher used for the porous layer (II), the porous layer (II) is directed to the positive electrode side. Therefore, it is possible to suppress the oxidation of the separator by the positive electrode, and the battery can be excellent in storage characteristics at high temperature and charge / discharge cycle characteristics. Therefore, the porous layer (II) is directed to the positive electrode side. More preferably. For example, in the case of a separator having a porous layer (I) mainly composed of a resin (A) or a plurality of porous layers (II), the negative electrode side is the porous layer (I) and the positive electrode side is the porous layer (II). More preferably, the separator is configured so that

  The positive electrode having the positive electrode mixture layer and the negative electrode having the negative electrode mixture layer as described above are, for example, a positive electrode mixture obtained by dispersing the positive electrode mixture in a solvent such as N-methyl-2-pyrrolidone (NMP). Prepared by applying a composition for forming an agent layer (slurry, etc.) or a composition for forming a negative electrode layer (slurry, etc.) in which a negative electrode mixture is dispersed in a solvent such as NMP to the surface of the current collector and drying it. Is done. In this case, for example, an electrode and a separator prepared by applying a composition for forming a mixture layer of these electrodes to the surface of a current collector and applying the composition for forming a porous layer before the composition is dried A lithium secondary battery can also be constituted by using an integrated product with [porous layer (I) or porous layer (II)].

  The lithium secondary battery of the present invention is configured, for example, by enclosing the above electrode group and a non-aqueous electrolyte in an outer package according to a conventional method. As the form of the battery, similarly to the conventionally known lithium secondary battery, a cylindrical battery using a cylindrical (cylindrical or rectangular) outer can, or a flat (round or rectangular flat in plan view) ) And a soft package battery using a laminated film on which a metal is deposited as an exterior body. The outer can can be made of steel or aluminum.

  The lithium secondary battery of the present invention preferably has a mechanism for discharging the gas inside the battery to the outside when the temperature rises. As such a mechanism, a conventionally known mechanism can be used. In other words, in batteries that use metal cans such as steel cans and aluminum cans as outer cans, metal cleavage vents that crack at constant pressure, resin vents that break at constant pressure, rubber that opens the lid at constant pressure A vent made of metal or the like can be used, and among them, a metal cleavage vent is preferably used.

  On the other hand, in a soft package battery, since the sealing portion is sealed by heat sealing of resin, it is difficult to make a structure that can withstand such high temperature and high pressure when the temperature and internal pressure rise in the first place. Even if a mechanism is not provided, the gas inside the battery can be discharged to the outside when the temperature rises. That is, in the soft package battery, the sealing portion (heat fusion portion) of the outer package acts as a mechanism for discharging the gas inside the battery to the outside. In the case of a soft package battery, the gas inside the battery can be discharged to the outside when the temperature rises even by a method such as narrowing the width of the sealing portion only at a specific location (that is, The specific place acts as a mechanism for discharging the gas inside the battery to the outside).

As the non-aqueous electrolyte, for example, a solution in which a lithium salt is dissolved in an organic solvent is used. The lithium salt is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and hardly causes side reactions such as decomposition in a voltage range used as a battery. For example, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ and other inorganic lithium salts, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] and the like can be used. .

  The organic solvent used for the non-aqueous electrolyte is not particularly limited as long as it dissolves the lithium salt and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, chain esters such as methyl propionate, cyclic esters such as γ-butyrolactone, Chain ethers such as dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme, cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran, nitriles such as acetonitrile, propionitrile and methoxypropionitrile And sulfites such as ethylene glycol sulfite. These may be used as a mixture of two or more. Kill. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate. In addition, vinylene carbonates, 1,3-propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, and fluorobenzene are used for the purpose of improving safety, charge / discharge cycleability, and high-temperature storage properties of these non-aqueous electrolytes. An additive such as t-butylbenzene may be added as appropriate.

  The concentration of the lithium salt in the non-aqueous electrolyte is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  Since the lithium secondary battery of the present invention has a high capacity and is excellent in reliability and safety, taking advantage of these characteristics, a power source for a small and multi-functional portable device is used, as well as a conventionally known lithium secondary battery. It can be preferably used in the same applications as those in which batteries are used.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention. Further, the volume content of each component in the porous layer (I) and the porous layer (II) shown in each example excludes the porous substrate when a porous substrate (nonwoven fabric) is used. Volume content in all components. Furthermore, melting | fusing point (melting temperature) of resin (A) shown in each Example is the value measured using DSC according to the prescription | regulation of JISK7121.

Example 1
(Preparation of negative electrode)
SiO (average particle size 5.0 μm) is heated to about 1000 ° C. in a boiling bed reactor, and a mixed gas of 25 ° C. composed of methane and nitrogen gas is brought into contact with the heated particles, and CVD processing is performed at 1000 ° C. for 60 minutes. Went. Thus, carbon (hereinafter also referred to as “CVD carbon”) generated by thermal decomposition of the mixed gas was deposited on the surface of SiO to form a coating layer, thereby obtaining a negative electrode material. When the composition of the negative electrode material was calculated from the mass change before and after the coating layer was formed, it was SiO: CVD carbon = 85: 15 (mass ratio). Negative electrode material: 80 parts by mass (content in the total solid content, the same shall apply hereinafter), graphite: 10 parts by mass, ketjen black (average particle size 0.05 μm) as a conductive additive: 2 parts by mass, binder As a polyamideimide: 8 parts by mass and dehydrated N-methylpyrrolidone (NMP) were mixed to prepare a negative electrode mixture-containing slurry. This negative electrode mixture-containing paste was intermittently applied on both sides of a 10 μm thick current collector made of copper foil so that the coating length was 500 mm on the front surface and 440 mm on the back surface, dried at 100 ° C., and then subjected to calendar treatment. The thickness of the negative electrode mixture layer is adjusted so that the total thickness becomes 60 μm, and further heat-treated at 160 ° C. for 15 hours using a far-infrared heater, cut to a width of 45 mm, a length of 510 mm, A negative electrode having a width of 45 mm was produced. A tab was welded to the exposed portion of the copper foil of the produced negative electrode to form a lead portion.

(Preparation of positive electrode)
95 parts by mass of LiCoO _{2 as a} positive electrode active material, 2.5 parts by mass of Ketjen black as a conductive auxiliary agent, and 2.5 parts by mass of PVDF as a binder are mixed so as to be uniform using NMP as a solvent. Thus, a positive electrode mixture-containing paste was prepared. This paste is intermittently applied to both sides of an aluminum foil having a thickness of 15 μm as a current collector so that the coating length is 500 mm on the front surface and 425 mm on the back surface, dried, and then calendered to a total thickness of 150 μm. Thus, the thickness of the positive electrode mixture layer was adjusted, and the positive electrode mixture layer was cut to have a width of 43 mm to produce a positive electrode having a length of 520 mm and a width of 43 mm. Further, a tab was welded to the exposed portion of the aluminum foil of the positive electrode to form a lead portion.

(Preparation of separator)
An organic binder SBR emulsion (solid content ratio 40 mass%): 100 g and water: 4000 g were placed in a container and stirred at room temperature until evenly dispersed. To this dispersion, plate boehmite powder (plate shape, average particle size 1 μm, aspect ratio 10), which is a filler having a heat resistant temperature of 150 ° C. or higher, was added in 4000 times, and the mixture was stirred with a disper at 2800 rpm for 5 hours. A uniform slurry [slurry for forming porous layer (II)] was prepared. On a porous film made of polyethylene [porous layer (I): thickness 16 μm, porosity 40%, average pore diameter 0.02 μm, melting point 135 ° C.], the slurry is applied by a micro gravure coater, dried and porous. By forming the quality layer (II), a separator having a thickness of 22 μm was obtained. The volume ratio of the filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) of this separator was 97% by volume, and the porosity of the porous layer (II) was 48%.

(Battery assembly)
The positive electrode, the negative electrode, and the separator obtained as described above are stacked with the resin porous membrane [porous layer (I)] facing toward the negative electrode, and wound in a spiral shape to produce a wound electrode group. did. The obtained wound body electrode group is crushed into a flat shape, put into an aluminum outer can with a thickness of 6 mm, a height of 50 mm, and a width of 34 mm, and an electrolytic solution (ethylene carbonate and diethylene carbonate are mixed in a volume ratio of 1: 2). After injecting LiPF ₆ dissolved in the solvent at a concentration of 1.0 mol / l), sealing was performed to prepare a lithium secondary battery. In addition, this battery is provided with the cleavage vent for discharging | emitting the gas inside a battery outside when an internal pressure rises to the upper part of a can.

Example 2
Lithium secondary in the same manner as in Example 1 except that the filler having a heat-resistant temperature of 150 ° C. or higher used for the production of the separator was replaced with boehmite in the form of secondary particles in which primary particles were aggregated (average particle size 0.6 μm). A battery was produced. The total thickness of the separator used in this lithium secondary battery was 22 μm, the volume ratio of the filler in the porous layer (II) of the separator was 97% by volume, and the porosity of the porous layer (II) was 44%. It was.

Example 3
A lithium secondary battery was produced in the same manner as in Example 1, except that the filler having a heat-resistant temperature of 150 ° C. or higher used for the production of the separator was changed to granular alumina (average particle diameter 0.4 μm). The total thickness of the separator used in this lithium secondary battery was 20 μm, the volume content of the filler in the porous layer (II) of the separator was 96% by volume, and the porosity of the porous layer (II) was 55 %Met.

Example 4
The porous membrane (I) used for the separator preparation was replaced with a microporous membrane having a three-layer structure of PE / PP / PE (thickness 16 μm, porosity 43%, average pore diameter 0.008 μm, melting point 135 ° C.). A lithium secondary battery was produced in the same manner as Example 1 except for the above. The total thickness of the separator used in this lithium secondary battery was 22 μm, the volume content of the filler in the porous layer (II) of the separator was 97% by volume, and the porosity of the porous layer (II) was 48 %Met.

Example 5
A non-woven fabric made of PET (thickness 12 μm, basis weight 8 g / m ² ) is used as a base material, and this is passed through the same slurry for forming the porous layer (II) as prepared in Example 1 and dried. Thus, a porous layer (II) was produced. The volume content of the filler in the porous layer (II) was 97% by volume, and the porosity of the porous layer (II) was 33%. Further, the same PE microporous film as used in Example 1 was used as the porous layer (I), and the porous layer (I) and the porous layer (II) were stacked without being integrated. A lithium secondary battery was produced in the same manner as in Example 1 except that the separators were used together.

Example 6
A slurry for forming a coating layer was prepared by mixing 94% by mass of α-alumina (average particle size 1 μm) (content in the total solid content, hereinafter the same), 6% by mass of PVDF, and dehydrated NMP.

  Using a blade coater, the same negative electrode mixture-containing slurry as that prepared in Example 1 was used as the lower layer, and the slurry for forming the coating layer as the upper layer. A negative electrode having a negative electrode mixture layer and a coating layer on both sides was prepared in the same manner as in Example 1, except that the coating length was the same as the coating length of the negative electrode mixture-containing slurry. In the negative electrode, the thickness of the negative electrode mixture layer per side of the current collector is 25 μm, and the thickness of the coat layer is 5 μm. Further, the negative electrode has strong adhesion between the negative electrode mixture layer and the current collector, and adhesion between the negative electrode mixture layer and the coat layer, and the negative electrode mixture layer becomes a current collector even by cutting or bending. The coating layer was not peeled from the negative electrode mixture layer.

  A lithium secondary battery was produced in the same manner as in Example 1 except that the negative electrode was used.

Comparative Example 1
A lithium secondary battery was produced in the same manner as in Example 1 except that a PE microporous membrane (thickness 22 μm, porosity 49%, average pore diameter 0.09 μm, melting point 135 ° C.) was used as the separator.

Comparative Example 2
A non-woven fabric made of PET (thickness 15 μm, basis weight 12 g / m ² ) is used as a base material, and this is passed through the same slurry for forming a porous layer (II) as prepared in Example 1, and then applied and dried. Thus, a porous layer (II) was produced. A lithium secondary battery was produced in the same manner as in Example 1 except that only this porous membrane (II) was used as a separator.

Comparative Example 3
A lithium secondary battery was prepared in the same manner as in Example 1, except that graphite was used as the negative electrode active material, and the thickness of the mixture layer of the positive electrode and the negative electrode was adjusted so that the capacity ratio between the positive electrode and the negative electrode was the same as in Example 1. Produced.

  About the separator used for the lithium secondary battery of Examples 1-6 and Comparative Examples 1-3, it was left to stand in a 150 degreeC thermostat for 3 hours, and the thermal contraction rate was measured. The measurement of the heat shrinkage rate was performed as follows.

  A separator test piece cut out to 4 cm × 4 cm was sandwiched between two glass plates with a thickness of 5 mm fixed with clips, left in a thermostatic bath at 150 ° C. for 3 hours, taken out, and the length of each test piece was measured. Measured, and the rate of decrease in length compared to the length before the test was taken as the shrinkage rate. For the separator of Example 5, the heat shrinkage rate of the porous layer (II) with less heat shrinkage was taken as the heat shrinkage rate of the separator. These measurement results are shown in Table 1.

  As shown in Table 1, the thermal shrinkage rate at 150 ° C. of the separators used in the lithium secondary batteries of Examples 1 to 6 is 1% or less.

  Next, about the lithium secondary battery of Examples 1-6 and Comparative Examples 1-3, charging / discharging was performed on the following conditions and the discharge capacity was calculated | required. The charging was constant current-constant voltage charging in which constant current charging was performed until the battery voltage reached 4.2 V at a current value of 0.2 C, and then constant voltage charging at 4.2 V was performed. The total charging time until the end of charging was 15 hours. When the battery after charging was discharged at a discharge current of 0.2 C until the battery voltage became 2.5 V, both the batteries of Examples 1 to 6 and the batteries of Comparative Examples 1 to 3 were charged during charging. It was confirmed that the generation of dendrite was suppressed and the battery operated well.

  Moreover, the following evaluation was performed about the lithium secondary battery of Examples 1-6 and Comparative Examples 1-3. First, the shutdown temperature of the separator used for each battery was determined by the following method. The discharged battery was placed in a thermostatic bath, heated from 30 ° C. to 150 ° C. at a rate of 5 ° C. per minute, and heated, and the temperature change of the internal resistance of the battery was determined. The temperature at which the resistance value increased to 5 times or more the value at 30 ° C. was taken as the shutdown temperature.

  In addition, with respect to a battery different from the one for which the shutdown temperature was measured, constant current charging was performed until the battery voltage reached 4.25 V at a current value of 0.2 C, and then constant voltage charging at 4.25 V. The constant current-constant voltage charging is performed. The total charging time until the end of charging was 15 hours. The battery charged under the above conditions was heated from 30 ° C. to 150 ° C. at a rate of 5 ° C. per minute and then allowed to stand at 150 ° C. for 3 hours to measure the surface temperature of the battery and the battery voltage.

Further, an external short-circuit test was conducted on a battery different from that used for each of the tests described above, by short-circuiting the positive and negative electrodes through a resistance of 100 mΩ, and the maximum temperature on the battery surface was measured. The evaluation results are shown in Table 2.

  As shown in Table 2, it was clarified that the batteries of Examples 1 to 6 were shut down in a temperature range appropriate for ensuring the safety of the battery at a high temperature. Further, in the batteries of Examples 1 to 6, no abnormality such as an increase in the surface temperature of the battery was observed even when held at 150 ° C. for 3 hours.

  In contrast, the battery of Comparative Example 1 was maintained at 150 ° C., and the surface temperature increased after 50 minutes. Moreover, although the battery of Comparative Example 2 was not abnormal when held at 150 ° C., it was confirmed that the surface temperature of the battery increased abnormally in the external short circuit test. This is considered because the shutdown characteristic is not given to the separator. On the other hand, the battery of Comparative Example 3 had no problem in holding at 150 ° C. and in the external short-circuit test, but the capacity decreased by 30% or more compared to other batteries.

It is a cross-sectional schematic diagram which shows an example of the negative electrode which has the porous layer containing the insulating material which does not react with Li in the negative electrode mixture layer surface among the negative electrodes which concern on the lithium secondary battery of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode 2 Porous layer (coat layer) containing the insulating material which does not react with Li
3 Negative electrode mixture layer 4 Current collector

Claims (9)

A lithium secondary battery having a positive electrode, a negative electrode and a separator,
The separator has a porous layer (I) mainly composed of a thermoplastic resin, and a porous layer (II) mainly composed of a filler having a heat resistant temperature of 150 ° C. or higher.
The negative electrode is a negative electrode mixture layer containing a compound containing Si and O as constituent elements (provided that the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) and a conductive material. A lithium secondary battery comprising:   The lithium secondary battery according to claim 1, wherein at least one of the porous layer (I) and the porous layer (II) contains plate-like particles.   The lithium secondary battery according to claim 1 or 2, wherein at least one of the porous layer (I) and the porous layer (II) contains particles having a secondary particle structure in which primary particles are aggregated.   The lithium secondary according to any one of claims 1 to 3, wherein the filler having a heat resistant temperature of 150 ° C or higher in the porous layer (II) is at least one particle selected from the group consisting of alumina, silica and boehmite. battery.   The lithium secondary battery according to any one of claims 1 to 4, wherein the porous layer (I) contains a polyolefin having a melting point of 100 to 140 ° C.   The lithium secondary battery according to any one of claims 1 to 5, wherein the negative electrode mixture layer contains a carbon material as a conductive material.   The lithium secondary battery according to claim 6, wherein a compound containing Si and O as constituent elements and a carbon material form a composite.   The lithium secondary battery according to claim 7, wherein a surface of the composite is further coated with a carbon material.   The lithium secondary battery according to any one of claims 6 to 8, wherein the carbon material contained in the negative electrode mixture layer is produced by thermal decomposition when a hydrocarbon gas is heated in a gas phase.

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