JP3992529B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery having a high capacity and excellent cycle characteristics.
[0002]
[Prior art]
A battery using an alkali metal as an active material has attracted attention as a high-performance battery having a high energy density. Among them, lithium batteries have a particularly high energy density and are excellent in reliability such as storability, and are already widely used as power sources for small electronic devices as primary batteries. Recently, with the spread of small portable electric devices, the demand for lithium secondary batteries that can be charged and used repeatedly has increased rapidly.
[0003]
As the negative electrode material of the lithium secondary battery, for example, a lithium metal, a lithium alloy, or a carbonaceous material in which lithium is occluded in a carbon material that can occlude / release lithium is used.
[0004]
In non-aqueous electrolyte secondary batteries using lithium metal or lithium alloy as the negative electrode, a battery with a high energy density is obtained, but the dissolution and precipitation of lithium are repeated as the charge / discharge cycle progresses, and the activity deposited at that time Since lithium reacts with the solvent of the electrolytic solution, there is a problem that chargeable / dischargeable lithium is lost and the charge / discharge efficiency of the negative electrode is lowered. Furthermore, since lithium precipitates as dendrites (dendritic crystals), there is a risk that the dendrites penetrate the separator and cause an internal short circuit.
[0005]
Therefore, in place of lithium metal or lithium alloy, carbon materials such as coke or amorphous carbon such as glassy carbon that can be doped / undoped with lithium ions, or natural or artificial graphite are used as the negative electrode material. Things have been done. For example, JP-A-1-204361, JP-A-2-66856, JP-A-4-24831, JP-A-5-17669, and the like disclose that using a carbon material as a negative electrode material, lithium It is described that cycle durability is imparted to the secondary battery.
[0006]
However, the theoretical capacity of a negative electrode using the above carbon material as a negative electrode material is, for example, 372 mAh / g for graphite, which is insufficient for the recent demand for higher capacity of batteries for portable devices. Therefore, recently, negative electrode materials made of silicon (Si), tin (Sn), or the like, which are elements capable of forming an alloy with lithium, have attracted attention. For example, JP-A-7-29602 discloses Li _x A non-aqueous electrolyte secondary battery using Si (0 ≦ x ≦ 5) as a negative electrode active material is described.
[0007]
[Problems to be solved by the invention]
However, the negative electrode material made of an element capable of forming an alloy with lithium can have a higher capacity than the above carbon material, but the expansion / contraction of the negative electrode material due to the charge / discharge cycle is large, As a result, there is a problem that the conductive network in the negative electrode is destroyed and the capacity is remarkably lowered or the internal resistance is increased. Moreover, in the negative electrode produced by the conventional method of applying the negative electrode mixture to the metal foil, the negative electrode material expands greatly in the thickness direction due to the large expansion and contraction of the negative electrode material, and the current collecting performance of the current collector is reduced. Or the negative electrode itself bends or the battery can swells.
[0008]
Here, the reason why the negative electrode material composed of an element capable of forming an alloy with lithium is large in expansion / contraction will be described by taking silicon as an example.
[0009]
Silicon contains 8 Si atoms in its crystallographic unit cell (cubic, space group Fd-3m). Calculated from the lattice constant a = 0.5431 nm, the unit cell volume is 0.1592 nm. ^Three The volume occupied by one Si atom (the value obtained by dividing the unit cell volume by the number of Si atoms in the unit cell) is 0.0199 nm. ^Three It is. When a negative electrode made of silicon is charged to 100 mV or less (lithium is inserted), Li is a compound containing a large amount of lithium. ₁₅ Si _Four Or Li _{twenty one} Si _Five The capacity corresponds to about 4000 mAh / g, but the volume expansion coefficient becomes extremely large. For example, Li _{twenty one} Si _Five The unit cell (cubic crystal, space group F4-3m) contains 83 Si atoms. Calculated from the lattice constant a = 1.8750 nm, the unit cell volume is 6.5918 nm. ^Three The volume occupied by one Si atom is 0.079 nm ^Three It is. This value is 3.95 times the volume of simple silicon. That is, the volume of silicon increases by about 4 times by charging. Furthermore, since the volume difference during charging / discharging is extremely large as described above, a large strain is generated in the silicon particles, the particles are pulverized and spaces are formed between the particles, the conductive network is divided, and the electrochemical reaction occurs. The portion that cannot be involved increases, and the charge / discharge capacity decreases.
[0010]
On the other hand, Japanese Patent Application Laid-Open No. 2000-272911 describes a lithium battery excellent in charge / discharge characteristics using composite particles in which silicon particles are embedded in graphite and amorphous carbon as a negative electrode. By combining graphite and amorphous carbon with silicon, the expansion of silicon particles can be reduced, and the charge / discharge cycle characteristics can be improved. However, when the utilization rate of silicon that expresses a high capacity of about 1000 mAh / g is high, the charge / discharge cycle characteristics are not sufficient and the practical level is not reached. This is thought to be due to the fact that when the silicon utilization rate is high, the expansion / contraction of silicon increases, and the expansion / contraction of the composite particles increases accordingly, and the conductive network inside the negative electrode is destroyed. .
[0011]
The present invention has been made in order to solve the above-described conventional problems. Even when the charge / discharge cycle is repeated, the expansion / contraction of the electrode does not increase, the conductive network inside the electrode is not destroyed, and the battery capacity is increased. An object of the present invention is to provide a non-aqueous electrolyte secondary battery having a high energy density that does not decrease or increase the internal resistance.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, a non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and forms an alloy with lithium on a current collector having a three-dimensional structure as the negative electrode. A composite comprising a material containing an element that can be processed and a conductive material Particle surface is coated with carbon by vapor phase method An electrode filled with a negative electrode material is used.
[0013]
The surface of a composite particle composed of a material containing an element capable of forming an alloy with lithium and a conductive material is coated with carbon by a vapor phase method, and By using a current collector having a three-dimensional structure, expansion / contraction of the negative electrode does not increase even when the charge / discharge cycle is repeated, and the conductive network inside the negative electrode is not destroyed.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
[0015]
The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. As the negative electrode, an element capable of forming an alloy with lithium is formed on a current collector having a three-dimensional structure. Composite composed of contained material and conductive material Particle surface is coated with carbon by vapor phase method An electrode filled with a negative electrode material is used.
[0016]
Examples of the element capable of forming an alloy with lithium include silicon, silver, gold, zinc, cadmium, aluminum, gallium, indium, thallium, germanium, tin, lead, antimony, and bismuth. In particular, silicon, tin, and aluminum are preferable from the viewpoint of material cost and handling.
[0017]
Further, the material containing an element capable of forming an alloy with lithium may be in any state of crystal, low crystal, and amorphous. As this material, a simple substance of an element capable of forming an alloy with lithium and an alloy or a compound containing the element can be used. For example, a solid solution or an intermetallic compound of silicon, tin, aluminum, silicon oxide (SiO), tin oxide (SnO), silicon, tin, aluminum, and other metals and the like. As the material containing silicon or germanium, for example, a material which has become an n-type or p-type semiconductor by doping with boron or phosphorus and whose electric resistance is greatly reduced may be used.
[0018]
In addition, these materials containing elements that can form alloys with lithium are combined with conductive materials to form composites. And the surface is coated with carbon by the vapor phase method. Must. By this combination, the pulverization of the negative electrode material accompanying the charge / discharge cycle can be suppressed, and the conductive network in the negative electrode material particles when further pulverized can be maintained.
[0019]
The complex is usually in the form of particles, and the average particle diameter is preferably 2 μm or more and 100 μm or less, particularly preferably 2 to 50 μm. When the average particle diameter of the composite particles is 2 μm or more, the material containing the element capable of forming an alloy with lithium constituting the composite particles or the conductive material is 0.5 μm or more. It can be used, granulation and compounding are facilitated, the specific surface area of the composite particles is not excessive, and the production process and battery characteristics are not adversely affected. On the other hand, when the average particle diameter of the composite particles is 50 μm or less, filling into a current collector having a three-dimensional structure is facilitated, which is advantageous for producing an electrode.
[0020]
Further, the content of an element capable of forming an alloy with lithium in the composite is preferably 30% by mass or more and 80% by mass or less. When it is 30% by mass or more, when a capacity of about 1000 mAh / g is expressed, the utilization factor of an element capable of forming an alloy with lithium does not become too high, and an alloy can be formed with lithium. The expansion of the individual material particles containing the element does not increase, and it becomes difficult to pulverize. In addition, when the content is 80% by mass or less, the number of adhesion points between the material containing an element capable of forming an alloy with lithium and the conductive material increases, and thus the construction of the conductive network is facilitated.
[0021]
Examples of the conductive material included in the composite include artificial graphite, natural graphite, earthy graphite, expanded graphite, flake graphite, or a heat-treated product thereof, a carbon material obtained by pyrolyzing an organic substance under various conditions, or A metal material such as copper can be used. In particular, a fibrous or coiled carbon material or metal material is preferable. Since these are flexible thin thread-like shapes, they can effectively follow the expansion and contraction of materials containing elements that can form an alloy with lithium that is bonded or adjacent to them. This is because it can. Examples of the fibrous carbon material that can be used in the present invention include polyacrylonitrile (PAN) -based carbon fiber, pitch-based carbon fiber, and vapor-grown carbon fiber, and any of them may be used.
[0022]
Although the manufacturing method of the composite which consists of a material containing the element which can form an alloy with lithium, and an electroconductive material is not restrict | limited, For example, the method shown next can be used. When silicon is used as a material containing an element capable of forming an alloy with lithium and carbon is used as a conductive material, first, silicon and carbon are granulated, and then a carbon precursor such as an organic substance is used. The target composite can be obtained by a method of carbonizing the carbon precursor by mixing, or a method of granulating silicon and carbon and then coating the surface with a gas phase method. As the granulation method, rolling granulation, compression granulation, sintering granulation, vibration granulation, mixed granulation, pulverization granulation and the like can be suitably used. The void volume occupancy in the composite can be adjusted by controlling the type of mixed material, particle size, mixing ratio, granulation conditions, and the like. As a method for coating carbon by a vapor phase method, a thermal decomposition CVD method in which a hydrocarbon-based gas is thermally decomposed and coated, a PVD method in which vapor deposition is performed by a pseudo arc discharge using a carbon rod, and the like can be suitably used.
[0023]
The composite particles of the present invention obtained as described above have a specific surface area of 10 m. ² / G is preferable. Specific surface area is 10m ² When exceeding / g, depending on the conditions, the composite particles react with the electrolytic solution, and a film is easily formed on the surface of the composite particles. Lithium is taken into the film and lithium that is not involved in charge / discharge increases. This is because the irreversible capacity may increase.
[0024]
In the negative electrode of the nonaqueous electrolyte secondary battery of the present invention, a current collector having a three-dimensional structure is used. The surface was coated with carbon by vapor phase method The negative electrode material containing the composite particles is filled. That is, since the current collector is integrated with the negative electrode material in a state of being three-dimensionally spread in the negative electrode material, the conductive network continues to be maintained even if the composite particles are greatly expanded and contracted. Further, since the average distance between the negative electrode material and the current collector is small, the internal resistance of the negative electrode is small. Therefore, since most of the negative electrode material exhibits its function, a battery having a large capacity can be obtained and can withstand a large current.
[0025]
To the current collector The surface was coated with carbon by vapor phase method When filling the negative electrode material containing the composite particles, the negative electrode material is preferably filled into a current collector together with a binder and pressed. The binder can prevent the negative electrode material from falling off, and by pressing at the time of producing the electrode, a contraction force can be applied to the expansion in the thickness direction of the electrode. Therefore, the expansion of the electrode can be suppressed even if the composite particles have a large expansion / contraction.
[0026]
The current collector having the three-dimensional structure is preferably a sheet or mat made of a foam metal or a fibrous metal sintered body. This is because they have excellent current collecting performance and have a large resistance to expansion / contraction of the negative electrode material.
[0027]
Hereinafter, the negative electrode of the present invention will be further described with reference to the case where silicon is used as a material containing an element capable of forming an alloy with lithium and carbon is used as a conductive material (silicon / carbon composite material). To do.
[0028]
The negative electrode is, for example, a mixture of a silicon / carbon composite material and a binder made of a fluororesin to form a slurry, and this slurry is applied to a foam metal sheet or a fibrous metal sintered body mat. It can be obtained by drying. Subsequently, it compresses with a press etc. and adjusts thickness and porosity. In addition, a fluororesin and a crosslinking agent are dissolved in an organic solvent such as toluene and xylene, and a powder of silicon / carbon composite material is mixed with the fluororesin and a cross-linking agent to form a slurry. It may be applied to the mat of the bonded body, dried at 50 to 100 ° C. to remove the solvent, and compressed and cured with a press or the like while heating at 100 to 180 ° C.
[0029]
In the negative electrode, since the foamed metal or fibrous metal sintered body and the binder bind the negative electrode material, even if the silicon / carbon composite material repeatedly expands and contracts as the charge / discharge cycle progresses, The contact between the particles of the silicon / carbon composite material is maintained, the increase in the internal resistance of the negative electrode is suppressed, and the initial capacity of the battery can be maintained without collapsing the conductive network.
[0030]
Moreover, since this negative electrode can be compressed with a press or the like, preferably at a pressure of 9.8 to 980 MPa, and the porosity thereof can be adjusted, the capacity per volume of the negative electrode can be increased. In addition, an appropriate amount of gap can be secured in the negative electrode so that the electrolyte can be easily impregnated in the negative electrode, and a necessary path for lithium ion diffusion is ensured. The utilization rate of substances is high. Moreover, even if the composite particles expand during charging, the voids can make up for the particle expansion volume integral, so that the expansion of the electrode can be suppressed.
[0031]
As described above, the adjustment of the thickness and porosity of the negative electrode is performed by compressing a foamed metal or fibrous metal sintered body filled with a silicon / carbon composite material containing a binder with a press or the like. The thickness is preferably 0.1 mm or more, and more preferably 0.15 mm or more. When the thickness is less than 0.1 mm, the amount of the electrode material supported is small and the battery capacity becomes small. On the other hand, if it is too thick, the practicality is inferior, for example, it is difficult to adjust the porosity by compression. In this case, if the pressing is performed while heating to a temperature equal to or higher than the melting point of the binder, the strength of the negative electrode is increased and the battery characteristics are remarkably improved. In addition, when the porosity is small, impregnation with the electrolytic solution becomes difficult, the ionic conductivity through the electrolytic solution decreases, the function of the negative electrode material is limited and the battery capacity decreases, so the porosity is 20 to 50%, Preferably it is 25 to 40%. The void ratio is expressed by the volume occupied by the void / apparent volume × 100, and the volume occupied by the void is measured by a mercury intrusion method.
[0032]
The foam metal is preferably a sponge-like porous body having continuous openings. As a result, the internal resistance is reduced, and the conductive network is maintained even when the charge / discharge cycle is repeated, so that an increase in the internal resistance can be prevented and the expansion of the electrode can be suppressed. Moreover, it is preferable that the aperture diameter of a foam-like metal is 10 micrometers-1.0 mm. When the aperture diameter is 10 μm or more, the mixture of the silicon / carbon composite material and the binder can be easily filled into the aperture, and when the aperture diameter is 1.0 mm or less, the foamed metal and the negative electrode which are current collectors The average distance between the silicon / carbon composite material that occludes lithium, which is a material, does not increase, and it is easy to maintain a conductive network that accompanies a charge / discharge cycle, reducing the capacity and increasing the internal resistance of the electrode. No longer cause.
[0033]
Moreover, it is preferable that the porosity of a foam-like metal is 70 to 99%. When the opening ratio is 70% or more, a large amount of negative electrode material composed of a silicon / carbon composite material and a binder that can be filled in the opening can be filled, and a sufficient battery capacity can be secured. Further, when the porosity is 99% or less, the strength of the foam metal is not reduced, and the force to bind the negative electrode material can be maintained.
[0034]
The fiber diameter (diameter) of the fibrous metal sintered body is preferably 1 to 50 μm for the same reason as in the case of the pore diameter of the foam metal. As the fibrous metal sintered body, a short fiber or long fiber sintered body is used. For the same reason as in the case of the foam metal, it is preferable to use a porosity of 50 to 95%.
[0035]
The material of the foam metal or fibrous metal sintered body used in the negative electrode of the present invention is preferably a metal having corrosion resistance to lithium such as nickel, copper, nickel-copper alloy, nickel-iron-chromium alloy. Is done.
[0036]
A composite comprising a material containing an element capable of forming an alloy with lithium and a conductive material Anode material with particles coated with carbon by vapor phase method Can be mixed with a binder to form a negative electrode mixture, but a negative electrode conductive material may be further mixed. The negative electrode conductive material for producing the mixture is not particularly limited as long as it is an electron conductive material that does not cause a chemical change in the constituted nonaqueous electrolyte secondary 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 powder, nickel powder, aluminum powder, silver powder, etc.), Metal fibers or conductive materials such as polyphenylene derivatives described in JP-A-59-20971 can be used. These conductive materials can be used alone, but a plurality of conductive materials can be mixed and used.
[0037]
As the binder, either a thermoplastic resin or a thermosetting resin may be used. As the binder, starch, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, polyvinyl chloride, polyvinyl pyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, ethylene -Propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, butadiene rubber, polybutadiene, fluororubber, polyethylene oxide, and other polysaccharides, thermoplastic resins, rubber elastic polymers, etc., and their modified products Among these, at least one kind or a mixture thereof can be used. In particular, it is preferable to use a fluororesin that is not soluble in the solvent of the electrolytic solution and is stable under conditions in which the nonaqueous electrolyte secondary battery functions electrochemically. The fluororesin is excellent in heat resistance and chemical resistance. When the fluororesin is used as a binder, the effect of maintaining contact between the negative electrode material particles and preventing the negative electrode material from falling off the current collector is improved. The fluororesin used for the binder is preferably one that is dispersed or soluble in an organic solvent, such as polytetrafluoroethylene or polyvinylidene fluoride. In this case, it is preferable to disperse or dissolve the binder in an organic solvent, mix this with the negative electrode material to form a slurry, and fill the current collector with this slurry. In addition, as a fluororesin, what is used with a hardening | curing agent (crosslinking agent etc.) can also be used preferably.
[0038]
For the positive electrode of the nonaqueous electrolyte secondary battery of the present invention, an electrode formed by a conventional coating method can be used. Further, a foamed metal or fibrous metal sintered body mainly composed of aluminum, titanium, and stainless steel (SUS316 or SUS316L) is filled with a mixture of a positive electrode material capable of inserting and extracting lithium and a conductive material together with a binder. Further, a material having a thickness of 0.1 mm or more and a porosity of 20 to 50% may be used.
[0039]
Examples of the positive electrode material capable of occluding / releasing lithium include, for example, 4 genera, 5 genera, 6 genera, 7 genera, 8 genera, 9 genera, 10 genera, 11 genera, 12 genera, 13 genera and 14 in the periodic table. Oxides mainly composed of metals belonging to the genus, chalcogenides such as composite oxides and sulfides, and oxyhalides mainly composed of these metals are used. In addition, conductive polymer materials such as polyaniline, polypyrrole, polythiophene, polyacene, polyparaphenylene, or derivatives thereof can also be used as the positive electrode material.
[0040]
By using a positive electrode material with a high operating potential and a large capacity for occluding and releasing lithium, the energy density of the battery can be increased, so that the chemical formula is LiCoO ₂ LiNiO ₂ LiMnO ₂ Or LiMn ₂ O _Four It is preferable to use a spinel type lithium manganese oxide represented by the above as a positive electrode material.
[0041]
The particle diameter of the positive electrode material powder is preferably 1 to 80 μm so that an electrode can be easily produced, lithium can be smoothly inserted and extracted, and is not so bulky.
[0042]
The positive electrode is produced, for example, as follows. That is, an organic solvent is added to a mixture of a positive electrode material powder, a conductive material, and a fluororesin as a binder to form a slurry, which is then applied onto a metal foil, or a sheet of foamed metal or a fibrous metal firing. It is applied to the mat of the bonded body and dried to remove the organic solvent. Subsequently, it compresses with a press etc. and adjusts the thickness and porosity of a positive electrode.
[0043]
In addition, the thing similar to what was used in the case of the above-mentioned negative electrode can be used suitably for the binder for positive electrodes.
[0044]
In addition, the nonaqueous electrolyte used in the present invention can be either a nonaqueous liquid electrolyte or a polymer electrolyte. However, since a liquid electrolyte generally called an electrolyte is often used, hereinafter, an “electrolyte” is referred to for this liquid electrolyte. It explains with the expression. That is, the nonaqueous electrolytic solution is composed of an organic solvent and a lithium salt dissolved in the organic solvent. Examples of the organic solvent include propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3 -Dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether , 1,3-propane sultone, etc., a solvent obtained by mixing one or more aprotic organic solvents It is possible to have. Examples of the lithium salt dissolved in the organic solvent include LiClO. _Four , LiBF ₆ , LiPF ₆ , LiCF _Three SO _Three , LiCF _Three CO ₂ , LiAsF ₆ , LiSbF ₆ , LiB _Ten Cl _Ten , Lower aliphatic lithium carboxylate, LiAlCl _Four One or more salts such as LiCl, LiBr, LiI, lithium chloroborane, and lithium tetraphenylborate can be used. Among them, a mixed solvent of ethylene carbonate or propylene carbonate, 1,2-dimethoxyethane and / or diethyl carbonate and / or methyl ethyl carbonate, LiClO _Four , LiBF ₆ , LiPF ₆ And / or LiCF _Three SO _Three An electrolytic solution in which is dissolved is preferable.
[0045]
The amount of these nonaqueous electrolytes used in the battery is not particularly limited, but the required amount can be adjusted according to the amount of active material and the size of the battery. The concentration of the lithium salt that is the supporting electrolyte is not particularly limited, but the electrolyte solution 1 dm ^Three 0.2 to 3.0 mol per unit is preferable. Within this concentration range, the ionic conductivity does not decrease and the lithium salt does not precipitate.
[0046]
As the separator, a microporous film, a nonwoven fabric, or the like is used. Examples of the material include polyolefins such as polyethylene and polypropylene, and tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA) for heat resistance. ), Polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polybutylene terephthalate (PBT), and the like.
[0047]
The shape of the nonaqueous electrolyte secondary battery of the present invention may be any of a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, a square type, a large type used for an electric vehicle, and the like.
[0048]
【Example】
EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited to these Examples.
[0049]
Example 1
A composite comprising a material containing an element capable of forming an alloy with lithium and a conductive material was produced as follows.
[0050]
First, a silicon powder having a particle diameter of 1 μm, a vapor grown carbon fiber (VGCF) having a length of 5 μm and a diameter of 0.2 μm, and graphite having a particle diameter of 2 μm are mixed with a mass of silicon: VGCF: graphite = 60: 10: 30. The mixture was mixed at a ratio and tumbled and granulated using a planetary ball mill. As a result, composite particles having an average particle diameter of 15 μm were obtained. Subsequently, the surface of the composite particle was coated with carbon at a temperature of 1000 ° C. by a chemical vapor deposition method (CVD) using benzene as a carbon source. The amount of coated carbon was determined from the change in mass before and after coating. The composition of the composite particles after coating was a mass ratio of silicon: VGCF: graphite: coated carbon = 56: 9: 28: 7. The average particle size of the composite particles was about 15 μm.
[0051]
The negative electrode was produced as follows. First, 90 parts by mass of the composite particles, 5 parts by mass of carbon powder as a conductive material, and 5 parts by mass of polyvinylidene fluoride as a binder were mixed and dispersed in N-methyl-2-pyrrolidone to prepare a slurry. . The obtained slurry was applied to a nickel foam sheet having a thickness of 0.5 mm (aperture ratio 98%, average pore diameter 0.2 mm) and dried by heating at 100 ° C. This sheet was punched into a circle with a diameter of 16 mm, pressed with a press, and the thickness was compressed to 0.2 mm to obtain a negative electrode. In order to completely remove the water contained in the negative electrode, it was dried by holding at 120 ° C. for 24 hours under a reduced pressure of 13 Pa. The porosity of this negative electrode was 37%.
[0052]
Next, the positive electrode was produced as follows. First, LiMnO ₂ 100 parts by weight of the powder, 5 parts by weight of carbon black as the conductive material, 5 parts by weight of flake graphite as the conductive material, and 0.7 parts by weight of polytetrafluoroethylene as the binder are mixed, and after drying, the diameter is 16 mm. It was press-molded into a 0.1 mm thick pellet and heat dried at 250 ° C. to obtain a positive electrode.
[0053]
The separator is made of a nonwoven fabric made of polypropylene and a microporous film made of polypropylene, and the electrolytic solution is 1 mol / dm in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1: 1. ^Three LiPF so that the concentration of ₆ What was dissolved was used.
[0054]
A coin-type non-aqueous electrolyte secondary battery as shown in FIG. 1 was produced using the negative electrode, positive electrode, separator, and electrolytic solution. As shown in FIG. 1, the opening end of the metal outer can 4 also serving as the positive electrode terminal is tightened inward, so that the metal outer can 4 and the sealing plate 5 and gasket 6 also serving as the negative electrode terminal The separator 3 impregnated with the electrolyte is sealed. The impregnation of the electrode with the electrolyte and the sealing of the battery were performed in a glove box having a dry air atmosphere with a dew point of minus 50 ° C.
[0055]
Using the coin-type non-aqueous electrolyte secondary battery, the charge / discharge cycle characteristics were examined under the following conditions. That is, the charge has a current density of 0.5 mA / cm. ² The battery was charged at a constant voltage until the charge voltage reached 120 mV and the current density reached 1/10. Discharge current density 0.5mA / cm ² The discharge end voltage was 1.5V. As a result, the discharge capacity at the second cycle and the capacity retention at the 50th cycle were 1000 mAh / g and 95%, respectively. The discharge capacity was calculated per 1 g of composite particles. The capacity retention rate at the 50th cycle was calculated by dividing the discharge capacity at the 50th cycle by the discharge capacity at the 2nd cycle.
[0056]
Further, the negative electrode and metallic lithium were combined, and a model cell was produced using the same electrolytic solution and separator as described above. Using this model cell, the thickness change of the negative electrode was examined. As a result, the thickness of the negative electrode when charged to 1000 mAh / g under the same conditions as described above was 0.25 mm, and the coefficient of expansion of the negative electrode thickness was 125% when calculated from the thickness of the negative electrode before charging 0.2 mm. .
[0057]
(Example 2)
Instead of the nickel foam sheet, the same procedure as in Example 1 was used except that a fibrous nickel sintered mat (opening ratio 91%, fiber diameter 20 μm) having a thickness of 0.5 mm before pressing was used. Coin-type non-aqueous electrolyte secondary batteries and model cells were prepared, and charge / discharge cycle characteristics and negative electrode thickness changes were similarly examined.
[0058]
The coin-type nonaqueous electrolyte secondary battery had a discharge capacity at the second cycle and a capacity retention at the 50th cycle of 1000 mAh / g and 85%, respectively. Moreover, the thickness change before and after charging of the negative electrode using this model cell is 0.3 mm when the negative electrode is charged to 1000 mAh / g, and the thickness of the negative electrode is calculated from the thickness 0.2 mm of the negative electrode before charging. The expansion coefficient of was 150%.
[0059]
(Example 3)
Instead of the nickel foam sheet, a negative electrode (thickness 0.3 mm, void) prepared using a copper foam sheet (opening ratio 96%, average pore diameter 0.2 mm) having a thickness of 0.6 mm before pressing. A coin-type non-aqueous electrolyte secondary battery and a model cell were produced in the same manner as in Example 1 except that the ratio was 34%), and the charge / discharge cycle characteristics and the thickness change of the negative electrode were similarly examined.
[0060]
The coin-type non-aqueous electrolyte secondary battery had a discharge capacity at the second cycle and a capacity retention at the 50th cycle of 1000 mAh / g and 92%, respectively. Moreover, the thickness change before and after charging of the negative electrode using this model cell is 0.42 mm when the negative electrode is charged to 1000 mAh / g, and the thickness of the negative electrode is calculated from the thickness 0.3 mm of the negative electrode before charging. The expansion rate of was 140%.
[0061]
(Comparative Example 1)
A coin-type nonaqueous electrolyte was used in the same manner as in Example 1 except that a negative electrode (thickness 0.07 mm, porosity 28%) produced using a 10 μm thick copper foil was used instead of the nickel foam sheet. Secondary batteries and model cells were prepared, and charge / discharge cycle characteristics and negative electrode thickness changes were similarly examined.
[0062]
The coin-type nonaqueous electrolyte secondary battery had a discharge capacity at the second cycle and a capacity retention at the 50th cycle of 1000 mAh / g and 60%, respectively. Moreover, the thickness change before and after charging of the negative electrode using this model cell is 0.16 mm when the negative electrode is charged to 1000 mAh / g, and the thickness of the negative electrode is calculated from the thickness of the negative electrode 0.07 mm before charging. The expansion coefficient of was 229%.
[0063]
(Comparative Example 2)
Instead of the composite particles used in Example 1, a negative electrode (thickness 0.2 mm, porosity 36%) prepared using silicon powder having a particle diameter of 5 μm was used, and the same procedure as in Example 1 was performed. Coin-type non-aqueous electrolyte secondary batteries and model cells were prepared, and charge / discharge cycle characteristics and negative electrode thickness changes were similarly examined.
[0064]
The coin-type nonaqueous electrolyte secondary battery had a discharge capacity at the second cycle and a capacity retention at the 50th cycle of 1500 mAh / g and 10%, respectively. Moreover, the thickness change before and after charging of the negative electrode using this model cell is 0.33 mm when the negative electrode is charged to 1000 mAh / g, and the thickness of the negative electrode is calculated from the thickness 0.2 mm of the negative electrode before charging. The expansion coefficient of was 165%.
[0065]
【The invention's effect】
As described above, in the present invention, the current collector having a three-dimensional structure is composed of a composite containing a material containing an element capable of forming an alloy with lithium and a conductive material. Particle surface is coated with carbon by vapor phase method By making the negative electrode filled with the negative electrode material, the expansion / contraction of the electrode does not increase even when the charge / discharge cycle is repeated, the conductive network inside the electrode is not destroyed, the battery capacity is reduced, and the internal resistance is reduced. A non-aqueous electrolyte secondary battery having a high energy density that does not increase can be provided.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a coin-type non-aqueous electrolyte secondary battery of the present invention.
[Explanation of symbols]
1 Positive electrode
2 Negative electrode
3 Separator
4 Metal outer can
5 Sealing plate
6 Gasket

Claims (8)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains an element capable of forming an alloy with lithium in a current collector having a three-dimensional structure A non-aqueous electrolyte secondary battery using an electrode filled with a negative electrode material in which a surface of a composite particle composed of a material to be conductive and a conductive material is coated with carbon by a vapor phase method .   The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is obtained by filling the negative electrode material together with a binder into the current collector and pressing the negative electrode material.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode has a thickness of 0.1 to 10 mm and a porosity of 20 to 50%.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the current collector is composed of a foam metal or a fibrous metal sintered body.   The non-aqueous electrolyte secondary battery according to claim 4, wherein the foamed metal or the fibrous metal sintered body is made of a metal containing at least one selected from the group consisting of nickel and copper.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the element capable of forming an alloy with lithium is silicon.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the composite contains an element capable of forming an alloy with lithium in an amount of 30 to 80% by mass. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the material containing an element capable of forming an alloy with lithium is any one of silicon or tin, an alloy or a compound containing these elements .

Images