JP6660581B2 - Electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to an electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery having the electrode.

Non-aqueous electrolyte secondary batteries represented by lithium secondary batteries are widely used in portable terminals, electric vehicles, hybrid vehicles, etc. Currently, lithium transition metal oxides such as lithium cobalt oxide are used as positive electrode active materials. However, a carbon material such as graphite is often used for the negative electrode active material.
However, since higher capacity is expected in the future, the introduction of high-capacity materials such as lithium nickelate and lithium-excess type active materials for the positive electrode is being studied. The study of higher capacity is under way. Furthermore, high-rate charging performance is also required.

  To increase the capacity of the negative electrode, a method of partially adding a high-capacity material such as silicon oxide to graphite has been proposed.

  Patent Literature 1 describes in the summary column “[Problem] To provide a non-aqueous secondary battery having high capacity and good battery characteristics. [Solution] A positive electrode includes Ni, Mn and the like as essential constituent elements. The positive electrode mixture layer contains a specific Li-containing transition metal oxide, and the negative electrode is made of a material containing Si and O as constituent elements (the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) and a graphite-containing negative electrode mixture layer, and in the negative electrode mixture layer, when the total of graphite and a material containing Si and O as constituent elements is 100% by mass, Si The problem is solved by a non-aqueous secondary battery in which the ratio of a material containing P and O as constituent elements is 3 to 20% by mass. "

It has also been proposed that a negative electrode having a multilayer structure be used as a negative electrode having high input / output characteristics.
Patent Literature 2 discloses “a lithium ion secondary battery including a negative electrode current collector and a negative electrode mixture layer including crystalline carbon and amorphous carbon as a negative electrode active material disposed on the negative electrode current collector. A negative electrode for a battery, wherein the negative electrode mixture layer is composed of a plurality of layers, and a layer closer to the negative electrode current collector has a higher content of crystalline carbon, and a layer farther from the negative electrode current collector has a higher amorphous carbon content. , The crystalline carbon content of the layer closest to the negative electrode current collector is higher than the amorphous carbon content of the negative electrode current collector. " Is described. Then, as an example, a graphite-carbon mixture using a highly crystalline graphite material having an interplanar spacing (d value) of (002) plane of 0.34 nm or less by X-ray diffraction on both surfaces of a current collector foil as an active material A layer A is formed, and an amorphous carbon material using an amorphous carbon material having an interplanar spacing (d value) of (002) plane of 0.36 nm to 0.38 nm by X-ray diffraction as an active material is formed on the layer A. The negative electrode in which the mixture layer B is formed and the ratio of the coating amounts of the mixture A and the mixture B is 4: 1 is described (paragraphs [0047] to [0060]).

  Patent Literature 3 discloses “a negative electrode current collector plate, a negative electrode plate including a negative electrode active material layer including negative electrode active material particles, and formed on the negative electrode current collector plate, and a separator formed on the negative electrode plate. A positive electrode plate opposed to the negative electrode active material particles, wherein the negative electrode active material particles comprise at least a first particle made of graphite, and a second particle made of amorphous carbon, Negative electrode active material layer, compared to the ratio of the first particles in the entire negative electrode active material particles contained in the negative electrode active material layer, of the negative electrode active material layer in the layer thickness direction of the negative electrode current collector plate side In the region, the ratio of the first particles in the entire negative electrode active material particles contained in the region is higher than the ratio of the second particles in the entire negative electrode active material particles contained in the negative electrode active material layer. The thickness of the negative electrode active material layer At the site of the surface side, it has been described for the invention of the lithium ion secondary battery the ratio of the second particles to the total the negative electrode active material particles contained in the site formed by high. "(Claim 1). As an example, the ratio of the first particles included in the first layer L1 on the copper foil side is 100%, and the ratio of the second particles included in the second layer L2, which is the upper layer of the first layer L1, is 100%. A battery having a certain negative electrode plate is described.

  Patent Document 4 discloses “a positive electrode including a positive electrode current collector and a positive electrode active material layer, a negative electrode including a negative electrode current collector and a negative electrode active material layer, and a porous insulating layer interposed between the positive electrode and the negative electrode. And a non-aqueous electrolyte, wherein the negative electrode active material layer contains graphite particles, and the degree of graphitization of the graphite particles distributed on the surface side of the negative electrode active material layer is distributed on the negative electrode current collector side. Non-aqueous electrolyte secondary battery having a degree of graphitization lower than that of the above. " Further, as an example thereof, a 45 μm first layer using spherical natural graphite as the first graphite particles is formed on the negative electrode current collector, and a part of the surface is formed as the second graphite particles on the first layer. A battery having a negative electrode in which a second layer of the same thickness is formed using amorphous graphite particles is described (paragraphs [0050] to [0052]).

JP 2010-212228 A JP 2012-15051 A International Publication No. 2012/077176 JP 2010-267540 A

In the negative electrode described in Patent Document 1 in which a high-capacity material is simply added to graphite, the current density per unit area of the electrode increases, so that, particularly at the time of high-rate charging, electrodeposition of Li metal occurs to deteriorate cycle performance. There is. Therefore, it is necessary to improve the Li acceptability during charging.
Patent Documents 2 to 4 disclose that the negative electrode active material layer has a multilayer structure, the active material of the current collector side layer is crystalline carbon, and the active material of the surface side (far side from the current collector) is amorphous. It describes that carbon or carbon having a low degree of graphitization is used. Regarding the content of amorphous carbon or carbon having a low degree of graphitization contained in the negative electrode active material layer, Patent Document 2 discloses an example in which the active material is 20%, and Patent Document 3, No. 4 describes an example in which 50% of the active material is used.
Amorphous carbon or carbon with a low degree of graphitization has a high Li-accepting property, but the potential at the end of discharge gradually rises, so that a battery using these quickly reaches the cutoff voltage. For this reason, a sufficient discharge capacity cannot be taken out, and in particular, in the case of a battery combined with a high-capacity positive electrode active material such as lithium nickelate or lithium excess, the substantial discharge capacity is reduced with respect to the design capacity. .
In Patent Documents 2 to 4, no consideration was given to the content of amorphous carbon or carbon having a low degree of graphitization contained in the negative electrode active material layer from the viewpoint of substantial reduction in discharge capacity.

  The problem to be solved by the present invention is to provide an electrode for a non-aqueous electrolyte secondary battery which has improved acceptability for charging without reducing the substantial discharge capacity when used in a battery, and An object of the present invention is to provide a non-aqueous electrolyte secondary battery using an electrode as a negative electrode.

The configuration of the present invention is as follows.
(1) An electrode for a non-aqueous electrolyte secondary battery including a current collector and an active material layer, wherein the active material layer includes a first layer in contact with the current collector, and a surface of the electrode. has at least two layers of a second layer positioned on a side, the first layer and the second layer are both in the content of graphite than 80 mass% in the active material, the first The second layer further contains hardly graphitizable carbon or easily graphitizable carbon as low crystalline carbon , and the content of graphite in the first layer is higher than the content of graphite in the second layer. Electrodes for secondary batteries.
(2) The non-aqueous electrolyte secondary battery according to (1), wherein the content of the low-crystalline carbon contained in the second layer is 20% by mass or less of the active material contained in the second layer. electrode.
(3) The electrode for a non-aqueous electrolyte secondary battery according to (1) or (2), wherein the thickness of the second layer is smaller than the thickness of the first layer.
(4) The non-aqueous liquid according to any one of (1) to (3), wherein the content of the low-crystalline carbon contained in the active material layer is 8% by mass or less of the active material contained in the active material layer. Electrodes for electrolyte secondary batteries.
(5) The electrode for a non-aqueous electrolyte secondary battery according to any one of the above (1) to (4), wherein the capacity density per unit area is 4.0 mAh / cm ² or more.
(6) The electrode for a non-aqueous electrolyte secondary battery according to any one of (1) to (5), wherein the second layer further contains silicon or a silicon compound.
(7) A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is any one of the above-described (1) to (6).

  Advantageous Effects of Invention According to the present invention, it is possible to provide an electrode having improved charge acceptability without substantially reducing the discharge capacity, and a nonaqueous electrolyte secondary battery using the electrode as a negative electrode.

Schematic diagram of the electrode according to the present embodiment Potential profile with respect to state of charge in comparison between Example and Comparative Example in Li charge acceptability evaluation test Potential profile with respect to state of charge in comparison between Example and Comparative Example in Li charge acceptability evaluation test External view of a nonaqueous electrolyte secondary battery according to the present embodiment Schematic diagram of a power storage device formed by collecting nonaqueous electrolyte secondary batteries according to the present embodiment

  As shown in FIG. 1, the electrode of the present embodiment is a nonaqueous electrolyte secondary battery electrode 10 including a current collector 12 and an active material layer 11, and a first layer close to the current collector 12. 14 (hereinafter, referred to as “lower layer”) and a second layer 13 (hereinafter, referred to as “upper layer”) close to the surface, and the lower layer 14 and the upper layer 13 are both main active material materials. Is graphite, the upper layer 13 further contains low crystalline carbon, and the graphite content of the lower layer 14 is higher than the graphite content of the upper layer 13. Specifically, as shown in FIG. 1, lower layer 14 is in contact with current collector 12, and upper layer 13 is located on the surface of electrode 10. According to the above configuration, an electrode having a high capacity, a high rate of high charge acceptance, and an excellent cycle life performance is provided.

  Since graphite is a carbon material having high crystallinity and high true density, it has a high filling property and a high capacity. Further, since the discharge potential is low, the energy density is high. Therefore, in the present embodiment, the main active material of the lower layer 14 and the upper layer 13 is graphite, but excluding containing a small amount of other active materials as long as the high capacity of graphite is not impaired. Not. Further, the active material layer of the present embodiment has a multilayer structure having at least the lower layer 14 and the upper layer 13, and one or more intermediate layers between the lower layer 14 and the upper layer 13 as long as the effects of the present embodiment are not impaired. Can be provided.

  In the present embodiment, the main active material of both the lower layer 14 and the upper layer 13 is graphite. “Main” means that the graphite content in the active material contained in the lower layer 14 or the upper layer 13 is 80% by mass or more, preferably 85% by mass or more, more preferably 90% by mass or more. is there.

  By the way, graphite has a high degree of orientation, so that the basal surface is more oriented on the surface of the electrode than on the edge surface, which is the insertion surface of Li ions, on the surface of the electrode. Therefore, the Li ion acceptability is low, and the Li metal is easily deposited during high-rate charging, and the cycle life performance tends to decrease.

On the other hand, low crystalline carbon has a lower energy density because the packing density is lower than graphite. In addition, since the discharge potential gradually rises at the end of discharge, the battery is quickly subjected to the cutoff voltage especially when combined with a high-capacity positive electrode, and a sufficient discharge capacity cannot be obtained. However, since the packing density is low, the electrolyte is easily held, and since the orientation is small, an edge surface, which is the insertion surface of Li ions, is likely to appear on the surface. Therefore, the Li ion insertion / release efficiency is excellent, and electrodeposition of Li metal charged at a high rate can be suppressed. High rate discharge is also advantageously performed.
Examples of the low crystalline carbon include non-graphitizable carbon and graphitizable carbon.
Note that graphite in the present embodiment refers to a carbon material having an interlayer distance d002 of less than 0.34 nm, easily graphitizable carbon having an interlayer distance d002 of 0.34 nm or more and less than 0.36 nm, and a non-graphitizable carbon of 0. It is a carbon material of .36 nm or more.

  In the present embodiment, since the upper layer 13 of the active material layer further contains low-crystalline carbon, in the surface region in contact with the electrolyte, the retention of the electrolyte can be increased, and the efficiency of insertion and release of Li ions can be increased. As a result, the cycle life performance can be improved by suppressing the deposition of Li metal, and the high-rate charging performance can be improved.

  Further, by adding low crystalline carbon to the upper layer 13 and graphite to the lower layer 14 at a higher content than the upper layer 13, the amount of low crystalline carbon added to the entire active material layer can be reduced, A substantial decrease in discharge capacity can be suppressed. When an intermediate layer is present, it is preferable that the intermediate layer also contains graphite at a higher content than the upper layer 13 in order to suppress the amount of low-crystalline carbon and increase the capacity.

The content of the low-crystalline carbon contained in the upper layer 13 is preferably 20% by mass or less in the active material contained in the upper layer 13.
Even if it exceeds 20% by mass, it is of course effective for cycle life performance and high-rate charge acceptability, but in order to maintain the packing density of the active material and maintain the high capacity density, 20% by mass is required. The following is preferred.
The content of low-crystalline carbon contained in the entire active material layer including the upper layer 13 and the lower layer 14 is preferably 8% by mass or less in the active material material contained in the active material layer 11 from the viewpoint of increasing the capacity. preferable.

From the viewpoint of increasing the capacity, the capacity density per unit area is preferably set to 4.0 mAh / cm ² or more.

  In order to increase the efficiency of insertion / release of Li ions in the surface region in contact with the electrolyte, the thickness of the upper layer 13 may be made relatively thin and the concentration of low crystalline carbon in the upper layer 13 may be increased. Therefore, it is preferable that the thickness of the upper layer 13 is smaller than the thickness of the lower layer 14.

In the present embodiment, in order to compensate for a decrease in capacity caused by containing low-crystalline carbon in the upper layer 13, silicon or a silicon compound as an active material having a higher capacity than the carbon material can be contained in the active material layer 11. .
As silicon or a silicon compound, for example, a silicon oxide particle containing Si and O as constituent elements described in Patent Document 1 (atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) is used as carbon. A silicon oxide-carbon composite material coated with a conductive material such as
Since silicon or a silicon compound has a large volume change due to charge and discharge, if it is present on the current collector side, the adhesion to the current collector is reduced due to expansion and contraction, and the active material layer 11 is likely to fall off. Therefore, it is preferable that the upper layer 13 on the front side contains silicon or a silicon compound. When the upper layer 13 contains silicon or a silicon compound, it is considered that expansion due to insertion of Li ions during charging causes an increase in porosity on the surface side, and an effect of suppressing the deposition of Li metal is increased. In addition, since the lower layer 14 can buffer the volume change, it is considered that the adhesion between the current collector 12 and the active material layer 11 is easily maintained.
From the viewpoint of cycle life performance, it is preferable that the degree of expansion of the active material is not too large. Therefore, the content of silicon or silicon compound in the active material layer 11 is preferably 5% by mass or less, and more preferably 3% by mass. Or less, more preferably 2% by mass or less.

The electrode according to the present embodiment has a structure schematically shown in FIG. 1, and can be manufactured by the following method.
The active material and binder for each of the lower layer 14 and the upper layer 13 and, if necessary, a thickener, a conductive agent, a filler and the like are dispersed in an organic solvent such as N-methylpyrrolidone and toluene or water. Thus, a paste for the lower layer 14 and a paste for the upper layer 13 are prepared.
A paste for the lower layer 14 is applied on both sides or one side of the current collector 12 such as a Cu foil and dried, and then a paste for the upper layer 13 is applied thereon, dried, and roll-pressed.

  Examples of the binder include thermoplastic resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, and polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, and styrene-butadiene rubber (SBR). ) And polymers having rubber elasticity such as fluororubber can be used as one kind or as a mixture of two or more kinds.

  Cellulose-based resins such as carboxymethylcellulose (CMC) and hydroxypropylcellulose, polyvinyl alcohol, and polyacrylic acid can be used as the thickener.

  Examples of the conductive agent include conductive materials such as carbon black, acetylene black, Ketjen black, carbon whiskers, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, and conductive ceramic material. It can be included as a seed or a mixture thereof.

  The filler is not limited as long as it does not adversely affect battery performance. Usually, olefin polymers such as polypropylene and polyethylene, amorphous silica, alumina, zeolite, glass, carbon and the like are used.

  Regarding the application method, for example, it is preferable to apply to an arbitrary thickness and an arbitrary shape using a means such as a roller coating such as an applicator roll, a screen coating, a doctor blade method, a spin coating, and a bar coater.

  An intermediate layer paste containing an active material having a graphite content intermediate between the lower layer 14 and the upper layer 13 is separately prepared, and after the lower layer 14 is formed and before the upper layer 13 is formed, the intermediate paste is applied and dried. By doing so, one or more intermediate layers in which graphite is the main active material and the content of graphite is between the lower layer 14 and the upper layer 13 may be formed.

An undercoat layer having at least a conductive material and a binder may be provided between the current collector 12 and the active material layer 11, that is, between the current collector 12 and the lower layer 14. By providing the undercoat layer, the adhesion between the current collector 12 and the active material layer 11 is improved.
An overcoat layer having at least inorganic particles and a binder may be provided on the surface of the active material layer 11, that is, on the surface of the upper layer 13 on the side different from the lower layer 14. By providing the overcoat layer, effects such as an improvement in ion conductivity and a decrease in the possibility of a short circuit can be obtained.

The content of the low-crystalline carbon contained in the upper layer 13 or the active material layer 11 can be determined as follows.
The non-aqueous electrolyte secondary battery in the discharged state is disassembled, the negative electrode is taken out, washed with dimethyl carbonate, and then dried in vacuum. Cut negative electrode cross-section a cross section polisher or the like, by measuring the Raman scattering spectrum, mapping the peak intensity ratio of G band and 1360 cm ^-1 vicinity of D band near 1580 cm ^-1. Particles having an R value (R = I ₁₃₆₀ / I ₁₅₈₀ ) of less than 0.8 expressed as a peak intensity ratio between the G band and the D band are regarded as graphite, and particles having an R value of 0.8 or more are regarded as low crystalline carbon. Then, the area ratio of the region of each carbon material in the lower layer is calculated.

  The mass ratio of graphite and low crystalline carbon in the upper and lower layers is calculated by multiplying the area ratio of graphite and low crystalline carbon by the value of the true density of each substance.

The content of silicon or a silicon compound contained in the upper layer 13 or the active material layer 11 can be determined as follows.
The non-aqueous electrolyte secondary battery in the discharged state is disassembled, the negative electrode is taken out, washed with dimethyl carbonate, and then dried in vacuum. The cross section of the negative electrode is cut out with a cross section polisher or the like, and EPMA measurement is performed. Si and C are mapped, and the area ratio of graphite and the area ratio of silicon or a silicon compound are calculated from the peak intensity ratio of Si and C.
By multiplying the area ratio of graphite and the area ratio of silicon or silicon compound by the value of the true density of each material, the content of silicon or silicon compound in the upper and lower layers is calculated.

The upper layer and the lower layer are distinguished based on the ratio of low crystalline carbon and graphite. That is, the portion near the surface of the negative electrode where the proportion of low crystalline carbon is high is the upper layer, and the portion near the current collector where the proportion of graphite is high is the lower layer.
Further, the boundary between the upper layer and the lower layer may partially enter each other, or the content of low crystalline carbon may continuously change in the thickness direction of the electrode. In such a case, assuming that the thickness of the active material layer is 100%, the upper layer is 20% from the side closer to the surface of the electrode in the thickness direction, and the lower layer is 20% from the side closer to the current collector in the thickness direction.

  In addition, the present embodiment relates to a non-aqueous electrolyte secondary battery including a positive electrode, the negative electrode serving as the above-described electrode, and a non-aqueous electrolyte. Hereinafter, the nonaqueous electrolyte secondary battery of the present embodiment will be described in detail.

(Positive electrode)
The positive electrode active material is not limited. For _example, Li x _MO y composite oxide (M is at least representative of a kind of transition metal) represented by _{(LiCoO 2, LiNiO 2, LiNi} y Mn z Co (1-y-z) O 2, Li 1 + α _{(Ni y Mn z Co (1} -y-z)) 1-α O 2, LiMn 2 O 4 , _{etc.), Li w M x (XO} y) z (X represents for example P, Si, B, and V) (LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , Li ₂ CoPO ₄ F, etc.). The elements or polyanions in these compounds may be partially substituted with other elements or anionic species.
Above all, the present embodiment uses, for example, a lithium nickel oxide described in Patent Document 1 or a high-capacity positive electrode active material using a lithium-excess lithium transition metal composite oxide described in WO2012 / 091015. In this case, the substantial discharge capacity can be increased.
The positive electrode is prepared by applying and drying the paste containing the positive electrode active material, the binder and other necessary components on a current collector such as an Al foil in the same manner as the negative electrode. can do.

(Non-aqueous electrolyte)
The non-aqueous electrolyte used for the non-aqueous electrolyte secondary battery according to the present embodiment is not limited, and those generally proposed for use in a lithium secondary battery or the like can be used. Examples of the non-aqueous solvent used for the non-aqueous electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, cyclic carbonates such as vinylene carbonate; γ-butyrolactone, cyclic esters such as γ-valerolactone; dimethyl carbonate; Chain carbonates such as diethyl carbonate and ethyl methyl carbonate; chain esters such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxy Ethers such as ethane, 1,4-dibutoxyethane and methyldiglyme; nitriles such as acetonitrile and benzonitrile; dioxolane or derivatives thereof; ethylene sulfide, sulfolane, sultone or derivatives thereof. Examples of the conductor include a single conductor or a mixture of two or more conductors, but are not limited thereto.

The electrolyte salt used for the non-aqueous electrolyte is not limited. For example, lithium (Li) such as LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN ), An inorganic ion salt containing one kind of sodium (Na) or potassium (K), LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , (CH ₃ ) ₄ NBr, (C ₂ H) _{5) 4 NClO 4, (C} 2 H 5) 4 NI, (C 3 H 7) 4 NBr, (n-C 4 H 9) 4 NClO 4, (n-C 4 H _{) 4 NI, (C 2 H} 5) 4 N-maleate, (C 2 H 5) 4 N-benzoate, (C 2 H 5) 4 N-phthalate, lithium stearyl sulfonate, lithium octyl sulfonate, dodecylbenzene sulfonate Organic ionic salts such as lithium oxide and the like can be mentioned, and these ionic compounds can be used alone or in combination of two or more.

Further, by using a mixture of LiPF ₆ or LiBF ₄ and a lithium salt having a perfluoroalkyl group such as LiN (C ₂ F ₅ SO ₂ ) ₂ , the viscosity of the electrolyte can be reduced. It is more preferable that the performance can be enhanced and the self-discharge can be suppressed.

  Further, a room temperature molten salt or an ionic liquid may be used as the non-aqueous electrolyte.

  The concentration of the electrolyte salt in the non-aqueous electrolyte is preferably from 0.1 mol / L to 5 mol / L, more preferably from 0.5 mol / L to 2 mol / L in order to reliably obtain a non-aqueous electrolyte battery having high battery performance. 0.5 mol / L.

(Separator)
As the separator, it is preferable to use a porous film or a nonwoven fabric exhibiting excellent high-rate discharge performance alone or in combination. Examples of the material constituting the separator for a non-aqueous electrolyte battery include polyolefin resins represented by polyethylene, polypropylene, etc., polyester resins represented by polyethylene terephthalate, polybutylene terephthalate, etc., polyvinylidene fluoride, vinylidene fluoride-hexa. Fluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, fluorine Vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride - tetrafluoroethylene - hexafluoropropylene copolymer, vinylidene fluoride - ethylene - can be mentioned tetrafluoroethylene copolymer.

  The porosity of the separator is preferably 98% by volume or less from the viewpoint of strength. From the viewpoint of charge and discharge performance, the porosity is preferably 20% by volume or more.

  Further, as the separator, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, polyvinylidene fluoride and the like and an electrolyte may be used. It is preferable to use the non-aqueous electrolyte in a gel state as described above, since it has an effect of preventing liquid leakage.

  Furthermore, it is preferable to use a polymer gel in combination with the above-described porous membrane, nonwoven fabric, or the like as the separator because the liquid retention of the electrolyte is improved. For example, by forming a film having a thickness of several μm or less coated with a lipophilic polymer on the surface and the micropore wall surface of the polyethylene microporous membrane, and holding an electrolyte in the micropores of the film, Gels.

  Examples of the solvent-philic polymer include, in addition to polyvinylidene fluoride, a crosslinked polymer of an acrylate monomer having an ethylene oxide group or an ester group, an epoxy monomer, a monomer having an isocyanate group, or the like. The monomer can be subjected to a cross-linking reaction by irradiation with an electron beam (EB) or heating or ultraviolet (UV) irradiation with the addition of a radical initiator.

(Configuration of non-aqueous electrolyte secondary battery)
The configuration of the nonaqueous electrolyte secondary battery according to this embodiment is not particularly limited, and examples thereof include a cylindrical battery, a square battery, and a flat battery in which a positive electrode, a negative electrode, and a separator are wound in a roll shape. It is listed as.
FIG. 4 shows an example of a prismatic battery. An electrode group 2 composed of a positive electrode and a negative electrode wound around a separator is housed in a rectangular battery container 3, and the positive electrode terminal 4 is connected via a positive electrode lead 4 ′, and the negative electrode terminal 5 is connected via a negative electrode lead 5 ′. It is led out of the battery container.

(Configuration of power storage device)
The non-aqueous electrolyte secondary battery of the present embodiment is particularly suitable for use as a power source for vehicles such as electric vehicles (EV), hybrid vehicles (HEV) and plug-in hybrid vehicles (PHEV). Can be mounted as a power storage device (battery module) configured as a group.
FIG. 5 shows an example of the power storage device 30 in which the power storage units 20 in which the non-aqueous electrolyte secondary batteries 1 are assembled are further assembled.

(Example 1)
<Preparation of electrode>
Spherulite artificial graphite, natural graphite, and flaky graphite were mixed at a mass ratio of 40:40:20 to obtain a negative electrode active material for a lower layer. This negative electrode active material, carboxymethylcellulose (CMC), and styrene-butadiene copolymer (SBR) were contained at a solid content mass ratio of 97: 1: 2, and a coating liquid for a lower layer was prepared using water as a dispersion medium. did. An active material layer (lower layer) on the side in contact with the current collector was prepared by applying a lower layer coating liquid to a 10-μm-thick copper foil current collector and drying it.
Next, a negative electrode active material obtained by mixing spherulite artificial graphite, natural graphite, flaky graphite, and non-graphitizable carbon at a mass ratio of 36: 36: 18: 10, carboxymethyl cellulose (CMC), styrene butadiene copolymer ( SBR) at a solid content mass ratio of 97: 1: 2, and a coating liquid for an upper layer containing water as a dispersion medium was prepared. The coating liquid was applied on the lower layer and dried. Here, the lower layer graphite content is 100% by mass of the active material, the upper layer graphite content and non-graphitizable carbon content are 90% by mass and 10% by mass, respectively. The content of the non-graphitizable carbon contained in the active material layer was 3.8% by mass.
The coating amount per unit area after drying, 8 mg / cm ² in the lower layer was made to be 5 mg / cm ² in the upper layer. This laminate was roll-pressed so that the electrode porosity was 30% by volume. This was processed into a shape in which a portion to which a current collecting terminal could be welded was attached to a coated portion having an area of 3 cm × 4 cm, to produce a test electrode according to Example 1.

<Preparation of cell>
Metal lithium was used as the counter electrode in order to grasp the behavior of the negative electrode alone. What processed so that it might become the same area as the coating part of a negative electrode plate, and stuck on the nickel foil collector was used as the counter electrode plate.
A polyethylene microporous membrane was used for the separator.
As an electrolytic solution, LiPF ₆ was dissolved in a solvent in which ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed at a volume ratio of 6: 7: 7 so as to be 1 mol / l.
A negative electrode plate and a counter electrode plate (metal lithium) are opposed to each other via a separator, and each current collecting terminal is exposed to the outside, and is housed inside a bag-shaped aluminum laminate film, and an electrolyte is injected. After that, it was hermetically sealed.

<Confirm capacity>
Using the above-mentioned laminate cell, the charging conditions were a constant current constant voltage charging of 0.1 CA (setting potential 0.01 V (Li ⁺ / Li), a cutoff current of 0.02 CA), and a discharging condition was a constant current of 0.1 CA. Initial charging / discharging was repeated for two cycles of charging / discharging as discharge (cutoff potential 0.6 V (Li ⁺ / Li)), and the discharge capacity at the second cycle was confirmed. In all cycles, a 10 minute pause was set after charging and discharging. Note that the cutoff potential was set assuming application to an actual cell.

<Charge acceptance evaluation>
A constant current charge is performed at a current density of 6.5 mA / cm ² without regulating the voltage range, and the potential (Li ⁺ / Li) with respect to the state of charge (hereinafter referred to as “SOC”) of the test electrode. I got a profile. As the charging proceeds, the overvoltage to the test electrode increases. However, when Li electrodeposition occurs, a current flows through the electrodeposited portion, thereby reducing the overvoltage. The SOC at the inflection point at which the current ratio sharply increased and the overvoltage began to decrease was defined as the Li electrodeposited SOC during charging, and the charge acceptability was evaluated.

(Example 2)
Example 1 was the same as Example 1 except that the upper layer was produced using an upper layer paste containing an active material having a mass ratio of spherulite artificial graphite, natural graphite, flaky graphite, and non-graphitizable carbon of 32: 32: 16: 20. Similarly, an electrode according to Example 2 was manufactured. Here, the lower layer graphite content was 100% by mass of the active material, the upper layer graphite content and the non-graphitizable carbon content were 80% by mass and 20% by mass, respectively. The content of the non-graphitizable carbon contained in the active material layer is 7.7% by mass.

(Example 3)
Spherulite artificial graphite, natural graphite, flaky graphite, non-graphitizable carbon, and silicon monoxide (hereinafter referred to as “SiO”) in a mass ratio of 35.28: 35.28: 17.64: 9.8 by mass ratio. The electrode according to Example 3 was produced in the same manner as in Example 1 except that the upper layer was produced using the paste for the upper layer containing the active material material of No. 2: 2. Here, the lower layer graphite content is 100% by mass of the active material, the upper layer graphite content and the non-graphitizable carbon content are 88.2% by mass and 9.8%, respectively. The content of the non-graphitizable carbon contained in the lower active material layer is 3.8% by mass.

(Comparative Example 1)
The lower layer paste prepared in Example 1 and containing spherulite artificial graphite, natural graphite, and flaky graphite at a weight ratio of 40:40:20 was applied in an amount of 13 mg / cm ² per unit area after drying. Thus, an electrode according to Comparative Example 1 was produced in the same manner as in Example 1, except that a single active material layer was produced by coating on a Cu foil and drying.

(Comparative Example 2)
The upper layer paste containing the spherulite artificial graphite, natural graphite, flaky graphite, and non-graphitizable carbon in a mass ratio of 36: 36: 18: 10 prepared in Example 1 was dried per unit area. An electrode according to Comparative Example 2 was applied in the same manner as in Example 1 except that a single active material layer was prepared by coating on a Cu foil and drying so that the coating amount was 13 mg / cm ^2. Was prepared.

(Comparative Example 3)
The upper layer paste containing spherulite artificial graphite, natural graphite, flaky graphite, and non-graphitizable carbon in a mass ratio of 32: 32: 16: 20, prepared in Example 2, was dried per unit area. An electrode according to Comparative Example 3 was prepared in the same manner as in Example 1 except that a single active material layer was prepared by coating on a Cu foil and drying so as to have a coating amount of 13 mg / cm ^2. Was prepared.

(Comparative Example 4)
Spherulite artificial graphite, natural graphite, flaky graphite, non-graphitizable carbon, and silicon monoxide produced in Example 3 were in a mass ratio of 35.28: 35.28: 17.64: 9.8: 2. Except that the upper layer paste contained in was coated on a Cu foil and dried to prepare a single-layer active material layer so that the coating amount per unit area after drying was 13 mg / cm ² . An electrode according to Comparative Example 4 was produced in the same manner as in Example 1.

Except for using the electrodes according to Examples 2 and 3, and Comparative Examples 1 to 4, each laminated cell was prepared in the same manner as in Example 1, and the capacity confirmation and the charge acceptability evaluation were performed under the same conditions as in Example 1. went.
FIGS. 2 (Examples 1 and 2 and Comparative Example 1) and FIG. 3 (Example 3 and Comparative Examples 1 and 4) show the profile of the potential (Li ⁺ / Li) with respect to the SOC acquired for the evaluation of the charge receiving chargeability. Shown in
Table 1 below shows, for the laminate cells according to Examples 1 to 3 and Comparative Examples 1 to 4, the active material included in the lower layer and the upper layer, and the non-graphitizable carbon in the active material included in the active material layer. The content ratio (HC ratio), the Li deposited SOC during charging (Li deposited SOC), the discharge capacity, and the capacity density per unit area (capacity density) are shown. In addition, about Comparative Examples 1-4 in which an active material layer is a single layer, the active material is described in the column of the lower layer.

In Comparative Example 1, although the discharge capacity was large, the layer on the surface side was also graphite, so that the Li deposited SOC was low and the charge acceptability was poor.
In Comparative Examples 2 and 3, since the non-graphitizable carbon is also present on the surface side, the charge acceptability is improved. However, since the ratio of the non-graphitizable carbon contained in the active material layer was high, the discharge capacity was reduced, and particularly in Comparative Example 3, it was significantly reduced.
Comparative Example 4 includes SiO, which is a high-capacity active material, and thus has the same discharge capacity as Example 1, but has a low degree of improvement in charge acceptability.

On the other hand, Example 1 has the same discharge capacity as Comparative Example 1, which is a single layer having the same active material as the lower layer, and has greatly improved charge acceptability. Further, the charge acceptability is comparable to that of Comparative Example 2, which is a single layer having the same active material as the upper layer, and the discharge capacity is improved.
In Example 2, since the non-graphitizable carbon ratio was higher than that of Example 1, the discharge capacity was lower than that of Example 1, but the charge acceptability was improved. If the ratio exceeds the non-graphitizable carbon ratio in Example 2, it is difficult to suppress a decrease in discharge capacity. Therefore, the non-graphitizable carbon ratio in the active material is preferably 8% by mass or less.
In the third embodiment, SiO, which is a high-capacity active material, is further included in the upper layer, so that a higher discharge capacity can be obtained while maintaining the same level of charge acceptability as in the first embodiment.

According to the present embodiment, it is possible to provide a non-aqueous electrolyte secondary battery electrode having a high capacity and excellent charge acceptability, and a non-aqueous electrolyte secondary battery having the electrode as a negative electrode.
Therefore, it is expected to be used as a power source for a portable terminal requiring a higher capacity and a higher rate charging performance, or a power source for an electric vehicle, a hybrid vehicle and the like.

DESCRIPTION OF SYMBOLS 1 Non-aqueous electrolyte secondary battery 2 Electrode group 3 Battery container 4 Positive terminal 4 'Positive lead 5 Negative terminal 5' Negative lead 10 Electrode 11 Active material layer 12 Current collector 13 Upper layer 14 Lower layer 20 Power storage unit 30 Power storage device

Claims (7)

An electrode for a non-aqueous electrolyte secondary battery including a current collector and an active material layer,
The active material layer has at least two layers, a first layer on the side in contact with the current collector, and a second layer located on the surface side of the electrode,
Both the first layer and the second layer have a graphite content of 80% by mass or more in the active material,
The second layer further includes hardly graphitizable carbon or easily graphitizable carbon as low crystalline carbon,
An electrode for a non-aqueous electrolyte secondary battery, wherein the graphite content of the first layer is higher than the graphite content of the second layer.   2. The non-aqueous electrolyte according to claim 1, wherein the content of the low-crystalline carbon contained in the second layer is 20% by mass or less of the active material contained in the second layer. 3. Electrodes for secondary batteries.   The electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a thickness of the second layer is smaller than a thickness of the first layer.   The non-crystalline carbon according to any one of claims 1 to 3, wherein the content of the low-crystalline carbon contained in the active material layer is 8% by mass or less of the active material contained in the active material layer. Electrodes for water electrolyte secondary batteries. The electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein a capacity density per unit area is 4.0 mAh / cm ² or more.   The electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the second layer further contains silicon or a silicon compound.   A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is the electrode according to any one of claims 1 to 6. .