JP2012069457A - Porous layer and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous layer having high film strength, and a lithium ion secondary battery having a good cycle characteristic and excellent short-circuit resistance and using the porous layer as a separator.SOLUTION: The porous layer includes: an oxide inorganic particle (A); and an object formed by reacting a polymer (B) including a reactive functional group and a crosslinking agent (C) that reacts to the reactive functional group.

Description

  The present invention relates to a lithium ion secondary battery in which a porous layer and a porous layer formed on an electrode are applied as a separator.

  In recent years, portable terminals such as notebook personal computers, mobile phones, and PDAs (Personal Digital Assistants) have been widely used. In such portable terminals, more comfortable portability is required, and miniaturization is rapidly progressing. And the lithium ion battery is used abundantly as a secondary battery for the power supply of the said portable terminal.

  In a lithium ion battery, a polyolefin-based microporous film is used as a separator interposed between a positive electrode and a negative electrode. The microporous film is produced by uniaxial stretching or biaxial stretching in order to achieve high porosity and film strength.

  Films produced by this stretching method are often distorted, so that there is a problem that due to this characteristic, shrinkage occurs due to residual stress when exposed to high temperatures. Therefore, for example, when the battery starts to generate heat due to abnormal charging or the like, the separator contracts, and as a result, there is a problem that there is a risk of ignition due to an internal short circuit.

In order to solve this problem, a method has been proposed in which a porous layer containing insulating inorganic particles and a binder resin is applied to a separator to prevent the separator from shrinking even when exposed to high temperatures. (For example, see Patent Documents 1 and 2.)
It is said that the methods described in Patent Documents 1 and 2 can improve the heat resistance of the separator and exhibit high heat resistance by using inorganic particles.

  However, at present, the interaction between the inorganic particles and the binder resin is low, and the porous layer does not have sufficient strength. Therefore, the mechanical strength as a separator is not sufficient, and there is a problem that the separator is destroyed due to the generation of lithium dendride, causing a short circuit.

Japanese Patent No. 3253632 Special table 2008-521964

  The present invention has been made in view of the above-mentioned problems, and the object thereof is a porous layer having high membrane strength, and the porous layer is used as a separator, which has good cycle characteristics and excellent short-circuit resistance. It is to provide a lithium ion secondary battery.

  The above object of the present invention is achieved by the following means.

  1. Contains oxide inorganic particles (A) and a polymer (B) having a reactive functional group and a crosslinker (C) that reacts with the reactive functional group. A porous layer characterized by that.

  2. 2. The porous layer according to 1 above, wherein the reactive functional group is a hydroxyl group.

  3. 3. The porous layer according to 1 or 2 above, wherein the polymer (B) has a weight average molecular weight of 50,000 to 500,000.

  4). 4. The lithium ion secondary battery having a porous layer between a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, wherein the porous layer is porous according to any one of 1 to 3 above. A lithium ion secondary battery characterized by being a porous layer.

  5. 5. The lithium ion secondary battery according to 4, wherein the porous layer is formed by coating on the positive electrode active material layer or the negative electrode active material layer.

  6). In a lithium ion secondary battery having a porous layer between a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, the porous layer includes the positive electrode active material layer or the negative electrode active material layer A lithium ion secondary battery produced by simultaneously applying and drying, and containing the crosslinking agent (C) in a coating solution for the positive electrode active material layer or the negative electrode active material layer.

  According to the present invention, it was possible to provide a porous layer having high film strength and a lithium ion secondary battery using the porous layer as a separator and having good cycle characteristics and excellent short-circuit resistance.

  Hereinafter, embodiments for carrying out the present invention will be described in detail.

  As a result of intensive studies in view of the above problems, the present inventor has found that oxide inorganic particles (A), a polymer (B) having a reactive functional group, and a crosslinking agent (C) that reacts with the reactive functional group. It is possible to realize a porous layer having high film strength by using a porous layer characterized by containing lithium, and by applying this porous layer as a separator of a lithium ion battery, As a result, the present inventors have found that a lithium ion secondary battery that can significantly improve the short-circuit property due to the above and can realize a self-discharge characteristic can be realized.

  Hereinafter, details of the lithium ion secondary battery of the present invention will be described.

<Porous layer>
The porous layer of the present invention contains an oxide inorganic particle (A) and a polymer (B) having a reactive functional group, and further has a crosslinking agent (C) that reacts with the reactive functional group. It is characterized by.

  Hereinafter, the detailed structure of the porous layer of the present invention will be described.

(A: oxide inorganic particles)
The composition of the oxide inorganic particles according to the present invention can be selected from known materials. Specifically, for example, silica (SiO ₂ ), alumina (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), zeolite, titanium oxide (TiO ₂ ), barium titanate (BaTiO ₃ ), strontium titanate (SrTiO ₃ ). ₃ ), calcium titanate (CaTiO ₃ ), aluminum borate, iron oxide, calcium carbonate, barium carbonate, lead oxide, tin oxide, cerium oxide, calcium oxide, trimanganese tetroxide, magnesium oxide, niobium oxide, tantalum oxide, tungsten oxide, antimony oxide, aluminum phosphate, calcium silicate, zirconium silicate, ITO (Sn (tin) containing indium _oxide, an In _{2 O} 3), titanium silicates, FSM16 (mesoporous silica), MCM41 (Mobile Crystall ne Material 41, honeycomb-shaped mesoporous silica (manufactured by Mobil Corp.), montmorillonite, saponite, vermiculite, hydrotalcite, kaolinite, kanemite, ilarite, magadiite, and kenyaite, and these composite oxides are also preferable. Can be used. Among the above oxide inorganic particles, neutral to acidic oxide inorganic particles are effective from the viewpoint of strength. For example, zirconium oxide, monlorillonite, saponite, vermiculite, hydrotalcite, kaolinite, kanemite, oxidation Examples thereof include tin, tungsten oxide, titanium oxide, aluminum phosphate, silica, zinc oxide, and alumina. Particularly preferred oxide inorganic particles in the present invention are alumina or silica, which are easily available industrially.

About the oxide inorganic particle which concerns on this invention, the oxide inorganic particle which has porosity can also be used. The oxide inorganic particles having porosity are oxide inorganic particles having innumerable fine voids on the surface or inside thereof, and the specific surface area is preferably 500 to 1000 m ² / g. If the specific surface area is 500 m ² / g or more, when the porous layer of the present invention is applied to a separator of a lithium ion secondary battery, the lithium ion conductivity tends to be improved, which is preferable. A specific surface area of 1000 m ² / g or less is preferred because the strength of the porous layer tends to be improved. The specific surface area referred to in the present invention can be measured by a conventionally known mercury intrusion method or gas adsorption method (BET method). As a method for measuring the specific surface area in the present invention, the BET method can be suitably used. The BET method is a method in which a molecule having a known adsorption area (for example, N ₂ ) is adsorbed on the particle surface at the temperature of liquid nitrogen and the specific surface area of the sample is obtained from the adsorbed amount.

  Further, as the pore diameter which is another index of porosity, it is preferable to have a pore diameter in the meso region. The meso region is a region of 2 to 50 nm to which Kelvin's capillary condensation theory can be applied. If it is larger than 2 nm, the ionic conductivity of the solid electrolyte tends to be improved, and if it is smaller than 50 nm, the strength of the solid electrolyte tends to be improved. The pore diameter is determined by analyzing the hysteresis pattern of the adsorption / desorption isotherm obtained by the gas adsorption method with a pore diameter distribution measuring device, or by obtaining the median diameter of the pore distribution, or by observation with a transmission electron microscope (TEM) It can ask for.

  As the oxide inorganic particles, for example, as described in JP-A-7-133105, a nanometer-sized composite having a low refractive index, in which the surface of porous oxide inorganic particles is coated with silica or the like. Oxide fine particles are suitable. In addition, as described in JP-A-2001-233611, low-refractive-index nanometer-sized silica-based fine particles composed of silica and inorganic oxides other than silica and having cavities inside are also suitable. ing.

  The average particle diameter of the oxide inorganic particles according to the present invention is preferably 1 nm or more and 200 nm or less. If the average particle size is 1 nm or more, when the porous layer of the present invention is applied to a separator of a lithium ion secondary battery, the lithium ion conductivity tends to improve. If the average particle size is 200 nm or less, This is preferable because the strength of the porous layer tends to be improved.

  The average particle diameter of the oxide inorganic particles is the number average value of the diameters of the respective oxide inorganic particles, and this value can be obtained by observation with an electron microscope. That is, from observation of the oxide inorganic particles by electron microscope, 200 or more random oxide inorganic particles that can be observed as a circle, an ellipse, or a substantially circle or ellipse are randomly observed, and the particle diameter of each oxide inorganic particle is obtained. It is obtained by obtaining the number average value. Here, the average particle diameter according to the present invention is the minimum distance among the distances between the outer edges of the oxide inorganic particles that can be observed as a circle, an ellipse, or a substantially circle or ellipse, between two parallel lines. Point to. When measuring the average particle diameter, the one that clearly represents the side surfaces of the oxide inorganic particles is not measured.

  As the oxide inorganic particles according to the present invention, particles whose surface is modified with a coupling agent or the like can be used, and the surface modifying group is not particularly limited and can be used.

  Surface modification methods include a dry method in which alkoxysilane, chlorosilane, aluminum alkoxide, titanium alkoxide, and the like are directly sprayed onto the oxide inorganic particles by heating and fixed, or the oxide inorganic particles are dispersed in the solution. In addition, a wet method in which a surface treatment agent is added and the surface is treated may be mentioned, but a wet method capable of dispersing particles more uniformly is preferable. For example, particles treated by a wet method as described in JP-A-2007-264581 are suitable for the present invention because they have high dispersibility. In both the dry method and the wet method, the coupling reaction can be promoted by hydrothermal treatment of the oxide inorganic particles in advance.

  The content of the oxide inorganic particles (A) in the porous layer of the present invention is not particularly limited, but is preferably 10% by mass or more and 95% by mass or less, more preferably 50% by mass or more with respect to the porous layer. 90 mass% or less. If it is 10 mass% or more, the lithium ion conductivity tends to be high, and if it is 95 mass% or less, it is preferable from the viewpoint that the mechanical strength of the porous layer tends to be high.

(B: polymer having a reactive functional group)
The polymer having a reactive functional group according to the present invention is not particularly limited as long as it is a resin that is stable to an electrolytic solution.

  Here, the reactive functional group can be a hydroxyl group, a carbonyl group, an amino group, an isocyanate group, an organic halogen group, an epoxy group, an oxetanyl group, an alkoxide group, a vinyl group, an aldehyde group, etc. It is a hydroxyl group.

  Examples of the polymer having these groups include polyvinyl alcohol, polyacrylic acid (and its salt), polyacrylamide, poly, saccharide, gelatin and the like. In addition, polyethylene, polypropylene, polyester, polyamide, polyimide, polyamideimide, aramid, polystyrene, polyvinylidene fluoride, polytetrafluoroethylene, etc. as a main skeleton, the above-described reactive functional group introduced polymer, or reactive functional group It may be a copolymer with a monomer having Among these, polyvinyl alcohol and polyacrylic acid are preferable, and polyvinyl alcohol having a hydroxyl group that is industrially inexpensive and easy to control the reaction is particularly preferable.

  The molecular weight of the polymer having a reactive functional group according to the present invention is preferably 50,000 or more and 500,000 or less as a weight average molecular weight in terms of styrene by GPC measurement. If it is 50,000 or more, it tends to react with the crosslinking agent to form a high-strength porous layer, and if it is 500,000 or less, the viscosity of the coating solution for the porous layer tends to be suppressed. To preferred.

  The content of the polymer having a reactive functional group according to the present invention is not particularly limited, but is preferably 1.0% by mass or more and 80% by mass or less, more preferably 3% with respect to 100% by mass of the porous layer. It is 0.0 mass% or more and 60 mass% or less. If it is 1.0 mass% or more, the strength of the porous layer is improved, and if it is 80 mass% or less, it is preferable from the viewpoint of improving the ionic conductivity of the electrolytic solution impregnated in the porous layer.

(C: Crosslinking agent)
Crosslinking agents are roughly classified into inorganic and organic types, but in the present invention, sufficient effects can be obtained by either, and these crosslinking agents may be used alone or in combination.

  The inorganic crosslinking agent used in the present invention may be any one that reacts with a reactive functional group. For example, an acid having a boron atom (boric acid, orthoboric acid, diboric acid, metaboric acid, tetraboric acid, Pentaboric acid, etc.) or salts thereof, zinc salts (such as zinc sulfate), copper salts (such as copper sulfate), zirconium salts (such as zirconyl nitrate, zirconyl acetate), aluminum salts (such as aluminum sulfate salts), titanium salts (lactic acid) Titanium salt etc.). Among these, preferred inorganic crosslinking agents are acids having boron atoms or salts thereof, aluminum salts, and zirconium salts, and particularly preferred are zirconium salts.

  The organic crosslinking agent used in the present invention may be any one that reacts with a reactive functional group. For example, aldehyde crosslinking agents (formalin, glyoxal, dialdehyde starch, polyacrolein, N-methylolurea, N- Methylol melamine, N-hydroxylmethylphthalimide, etc.), active vinyl cross-linking agent (bisvinylsulfonylmethylmethane, tetrakisvinylsulfonylmethylmethane, N, N, N-trisacryloyl-1,3,5-hexahydrotriazine, etc.), Examples thereof include an epoxy-based crosslinking agent and a polyisocyanate-based crosslinking agent. Among these, a polyisocyanate crosslinking agent, an epoxy crosslinking agent, and an aldehyde crosslinking agent are preferable, and a polyisocyanate crosslinking agent and an epoxy crosslinking agent exhibiting a good reaction with a hydroxyl group are particularly preferable.

  Epoxy crosslinking agents are compounds having at least two glycidyl groups in the molecule. For example, many crosslinking agents are commercially available from Nagase Chemtech under the trade name Denacol.

  A polyisocyanate-based crosslinking agent is a compound having at least two isocyanate groups in the molecule and has high reactivity with a hydroxyl group or the like.

  The main polyisocyanate crosslinking agents include, for example, toluylene diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, and their modified products and prepolymers, polyfunctional aromatic isocyanates, aromatic polyisocyanates, polyfunctional aliphatics. Examples include isocyanate, block-type polyisocyanate, and polyisocyanate prepolymer. The details of these polyiasocyanate-based crosslinking agents are described in, for example, the crosslinking agent handbook (published by Taiseisha, published in October 1981).

  Both the inorganic crosslinking agent and the organic crosslinking agent are preferably used in a range of approximately 1% by mass to 50% by mass with respect to the polymer, more preferably 2-40% by mass. When the content is 1% by mass or more, the strength of the porous layer tends to be improved as curing proceeds, and when the content is 40% by mass or less, the porosity is improved when adapted to a lithium ion battery. It is preferable because lithium ion conductivity in the material tends to be improved.

  The method for supplying these cross-linking agents to the porous layer is not particularly limited. For example, the cross-linking agent may be added to the coating liquid for forming the porous layer. You may coat and supply. The timing for overcoating the cross-linking agent can be any time, either simultaneously with or immediately after the application of the coating liquid for forming the porous layer. In addition, a method of applying the crosslinking agent, a method of supplying by a spray, or a method of immersing in a crosslinking agent solution may be used.

(Method for forming porous layer)
Next, a method for forming the porous layer of the present invention will be described.

  The method for forming the porous layer of the present invention is not particularly limited, but a wet coating method is preferred. Specifically, first, the inorganic oxide particles (A) according to the present invention, the polymer (B) having a reactive functional group, the crosslinking agent (C) that reacts with the reactive functional group, water, Or an organic solvent is mixed, slurry is prepared and it is set as a coating liquid. Thereafter, the coating liquid is applied on a resin film such as polyethylene terephthalate and dried, and then the formed porous layer is peeled off from the resin film.

  In addition, when used in a lithium ion secondary battery, the coating solution is applied to a positive electrode in which a positive electrode active material is applied on a current collector or a negative electrode in which a negative electrode active material is applied to a current collector. Thus, a porous layer integrated with the electrode can be formed.

  Water or an organic solvent that can be used in the wet coating method is a solvent that can uniformly disperse the oxide inorganic particles, a solvent that can stably dissolve or disperse the polymer used, and a solution that dissolves or disperses the crosslinking agent. Any solvent that can be uniformly dispersed may be used. Specifically, water, methanol, ethanol, 2-propanol, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, N-dimethylformaldehyde and the like can be used.

  Various additives may be added to stabilize the coating solution or dispersion. Even if these additives are materials that can be removed by heating, when the porous layer of the present invention is applied to a lithium ion secondary battery, it is a material that stably exists at high temperatures and high voltages and does not inhibit battery reaction. If so, it may remain in the battery.

  The coating method using the coating solution is not particularly limited as long as it can be applied uniformly to a desired thickness. For example, screen printing, bar coater method, roll coater method, reverse coater method, gravure printing method, doctor blade method, die coater method, etc. can be mentioned, and the application type is necessary for continuous application, intermittent application, stripe application, etc. It can be used properly according to your needs.

  The drying method after coating is not particularly limited as long as the organic solvent containing the coating solution can be removed. Various methods such as hot air drying, infrared drying, far infrared drying, microwave drying, electron beam drying, and vacuum drying are available. A method can be selected as appropriate.

  The drying temperature is not particularly limited, but is preferably 50 to 400 ° C, more preferably 80 to 200 ° C.

  The reaction between the reactive functional group present in the polymer according to the present invention and the crosslinking agent is preferably thermal crosslinking by heat at the time of drying, but UV crosslinking by irradiating UV after coating should also be used. I can do it.

  The reaction temperature during the crosslinking reaction is preferably 50 to 400 ° C., more preferably 80 to 200 ° C., similarly to the drying temperature.

  The thickness of the separator having the porous layer of the present invention formed by the above method is not particularly limited, but is preferably 1 μm to 200 μm, and more preferably 5 μm to 30 μm. If it is the said range, the short circuit between positive and negative electrodes can be prevented, and a battery characteristic can be improved.

  Further, in the coating method according to the present invention, it is more preferable to laminate the positive electrode active material layer or the negative electrode active material layer at the same time. Details will be described below.

(Simultaneous multilayer coating)
The simultaneous multilayer coating in the present invention is, for example, a method in which an electrode active material layer is applied on a current collector with a coater constituted by a single head (slot), and before the electrode active material layer is dried, it is applied to another coating machine. There is a method of stacking by moving a current collector coated with an electrode active material and applying a porous layer on the electrode active material layer (two-coater two-head method). In addition, a system in which two single heads as described above are installed adjacent to each other, and an electrode active material layer and a porous layer are coated on a current collector with each head using the same coater (one coater, two head system) ) And a method of applying an electrode active material layer and a porous layer onto a current collector in one step by an extrusion die composed of a single head having a plurality of flow paths (one coater / one head method). Can be mentioned. By extending these methods, the electrode active material layer and the multi-layered porous layer can be simultaneously stacked.

  By simultaneous layering, a slightly mixed phase is formed at the interface between the layers, and this mixed portion improves the adhesion between the layers, and at the same time increases the resistance to internal short circuits. Moreover, productivity is improved by layering simultaneously, and the laminated body of an electrode-porous layer can be manufactured at low cost.

  Moreover, in this invention, it is preferable to add a crosslinking agent to the coating liquid of a positive electrode active material layer or a negative electrode active material layer, and to laminate | stack simultaneously. By adding the electrode active material to the coating liquid, it is possible to prevent the crosslinking from proceeding in the porous layer coating liquid, and the pot life tends to be improved.

  Examples of the coating method according to the present invention include various methods as described in Edward Cohen, Edgar Gutoff's “MODERN COATING AND DRYING TECHNOLOGY”, such as a dip coating method, a blade coating method, an air knife coating method, Wire bar coating, gravure coating, reverse coating, reverse roll coating, extrusion coating, slide bead coating, curtain coating, and the like are known. In these coating methods, in order to obtain a uniform dry film thickness in the width direction of the support, the coating is performed while paying attention to the coating film thickness accuracy and uniformity during coating.

  In recent years, a coating apparatus having a flow-regulating die is capable of simultaneous multi-layer coating, and has been widely used as a coating apparatus for photographic photosensitive materials and magnetic recording materials due to its characteristics. A preferable example thereof is Russell, etc. Is a method using a multilayer slide bead coating apparatus proposed in US Pat. No. 2,761,791. This type of coater creates a coating liquid reservoir called a bead between the tip of the coating device (also simply referred to as “lip”) and a flexible support (also referred to as “web”) that travels, and through this bead. Application is performed. In the present invention, it is preferable to use these apparatuses when performing simultaneous multilayer coating.

  These coatings may be performed on each side or on both sides simultaneously. Furthermore, the application may be continuous, intermittent, or a combination thereof.

  A method for producing a positive electrode active material, a negative electrode active material, an electrode mixture, a current collector, and a secondary battery when the porous layer of the present invention is used as a separator for a lithium ion secondary battery will be described in detail below. To do.

(Positive electrode and positive electrode active material)
There is no restriction | limiting in particular in a positive electrode, What stuck the positive electrode active material to the electrical power collector can be utilized.

  Examples of the positive electrode active material include inorganic active materials, organic active materials, and composites thereof, but inorganic active materials or composites of inorganic active materials and organic active materials are particularly high in energy density. This is preferable.

Examples of the inorganic active material include Li _0.2 MnO ₂ , Li ₄ Mn ₅ O ₁₂ , V ₂ O ₅ , LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiFePO ₄ , LiCo _0.5 Ni _0.5 Mn _0.5 O ₂ , Li _1.2 (Fe _0.5 Mn _0.5 ) _0.8 O ₂ , Li _1.2 (Fe _0.4 Mn _0.4 Ti _0.2 ) _0.8 O ₂ , Li _{1 + x} (Ni _0.5 Mn _0.5 ) _1-x O ₂ , LiNi _0.5 Mn _1.5 O ₄ , Li ₂ MnO ₃ , Li _0.76 Mn _0.51 Ti _0.49 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , Fe ₂ O ₃ , etc., metal oxides such as LiFePO ₄ , LiCoPO ₄ , LiMnPO ₄ , Li ₂ MPO ₄ F (M = Fe, Mn), LiMn _{0. 875} Fe _{0 125 PO 4, Li 2 FeSiO 4} , Li 2-x MSi 1-x P x O 4 (M = Fe, Mn), LiMBO 3 (M = Fe, Mn) phosphoric acids such as silicic acid, boric acid-based In the chemical formula, x is preferably in the range of 0-1. Further, fluorine-based compounds such as FeF ₃ , Li ₃ FeF ₆ , and Li ₂ TiF ₆ , metal sulfides such as Li ₂ FeS ₂ , TiS ₂ , MoS ₂ , and FeS, and composite oxides of these compounds and lithium can be given.

  Examples of the organic active material include conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyparaphenylene, organic disulfide compounds, and organic sulfur compounds DMcT (2,5-dimercapto-1,3,4-thiadiazole). Benzoquinone compound PDBM (poly 2,5-dihydroxy-1,4-benzoquinone-3,6-methylene), carbon disulfide, sulfur-based positive electrode materials such as active sulfur, and organic radical compounds.

  Moreover, it is preferable from the point which extends the lifetime of a battery that the surface of a positive electrode active material is coat | covered with the inorganic oxide. As a method of coating the surface of the positive electrode active material with an inorganic oxide, a method of coating the surface of the positive electrode active material is preferable, and as a method of coating, for example, a method of coating using a surface modifying apparatus such as a hybridizer Etc.

Examples of the inorganic oxide covering the surface of the positive electrode active material include, for example, magnesium oxide, silicon oxide, alumina, zirconia, titanium oxide and other IIA to VA groups, transition metals, IIIB, IVB oxide, barium titanate, Examples thereof include calcium titanate, lead titanate, γ-LiAlO ₂ and LiTiO ₃ , and silicon oxide is particularly preferable.

(Negative electrode and negative electrode active material)
There is no restriction | limiting in particular about a negative electrode, What adhered the negative electrode active material to the electrical power collector can be utilized. Powders such as graphite and tin alloys can be applied as pastes together with binders such as styrene butadiene rubber and polyvinylidene fluoride on the current collector, dried, and press-molded. . A silicon-based thin film negative electrode or the like in which a silicon-based thin film having a thickness of 3 to 5 μm is directly formed on a current collector by physical vapor deposition (such as sputtering or vacuum deposition) can also be used.

  In the case of a lithium metal negative electrode, a lithium foil having a thickness of 10 to 30 μm attached on a copper foil is suitable. From the viewpoint of increasing the capacity, it is preferable to be composed of a silicon-based thin film negative electrode or a lithium metal negative electrode.

  The negative electrode active material is not particularly limited, and a known negative electrode active material can be used. Examples of the negative electrode active material in the present invention include graphite, a tin alloy and a binder, a silicon thin film, and a lithium foil.

  The negative electrode active material of graphite or tin alloy and a binder can be obtained by drying a paste mixed with powder such as graphite or tin alloy and a binder such as styrene butadiene rubber or polyvinylidene fluoride. As the negative electrode active material of the lithium foil, a current collector obtained by bonding a lithium foil having a thickness of 10 to 30 μm can be used. It is preferable to use a negative electrode active material composed of a silicon-based thin film negative electrode or a lithium metal negative electrode because the capacity can be increased and an electrode mixture is not essential.

(Electrode mixture)
Examples of the electrode mixture used in the present invention include those to which a lithium salt, an aprotic organic solvent, or the like is added in addition to a conductive agent, a binder, a filler, and the like.

  As the conductive agent, any material may be used as long as it is an electron conductive material that does not cause a chemical change in the constituted lithium ion secondary battery. Usually, natural graphite (eg, scaly graphite, scaly graphite, earthy graphite, etc.), artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber or metal powder (eg, copper, nickel, aluminum, silver (special Etc.), etc.), metal fibers or polyphenylene derivatives (described in JP-A-59-20971), or a mixture of two or more conductive materials. it can.

  Among these, the combined use of graphite and acetylene black is particularly preferable. As addition amount of the said electrically conductive agent, 1.0-50 mass% is preferable with respect to electrode mixture total mass, and 2.0-30 mass% is more preferable. In the case of carbon or graphite, 2.0 to 15% by mass is particularly preferable.

  In the present invention, a binder can be used to hold the electrode mixture. Examples of such a binder include polysaccharides, thermoplastic resins, and polymers having rubber elasticity. Among them, for example, starch, carboxymethylcellulose, cellulose, diacetylcellulose, methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, Water-soluble polymers such as sodium alginate, polyacrylic acid, sodium polyacrylate, polyvinyl phenol, polyvinyl methyl ether, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylonitrile, polyacrylamide, polyhydroxy (meth) acrylate, styrene-maleic acid copolymer , Polyvinyl chloride, polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, vinyl Redene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, polyvinyl acetal resin, methyl methacrylate, 2-ethylhexyl acrylate, etc. ) (Meth) acrylic ester copolymer containing acrylic ester, (meth) acrylic ester-acrylonitrile copolymer, polyvinyl ester copolymer containing vinyl ester such as vinyl acetate, styrene-butadiene copolymer , Styrene-butadiene-acrylic copolymer, acrylonitrile-butadiene copolymer, polybutadiene, neoprene rubber, fluororubber, polyethylene oxide, polyester polyurethane resin, Examples include ether polyurethane resin, polycarbonate polyurethane resin, polyester resin, phenol resin, epoxy resin emulsion (latex) or suspension. Among them, polyacrylate ester latex, carboxymethyl cellulose, polytetrafluoroethylene, polyvinylidene fluoride Is preferred.

  The said binder can be used individually by 1 type or in mixture of 2 or more types. For this reason, the addition amount of the binder is preferably 1.0 to 30% by mass and more preferably 2.0 to 10% by mass with respect to the total mass of the electrode mixture. If it exists in the said range, since the retention strength and cohesion force of an electrode mixture increase, it is preferable.

  The filler can be used without particular limitation as long as it is a fibrous material that does not cause a chemical change in the lithium ion secondary battery of the present invention. Usually, olefin polymers such as polypropylene and polyethylene, fibers such as glass and carbon are used. Although the addition amount of a filler is not specifically limited, 0-30 mass% is preferable with respect to electrode mixture total mass.

(Current collector)
As the current collector for the positive electrode and the negative electrode, an electron conductor that does not cause a chemical change in the lithium ion secondary battery of the present invention is used.

  As the current collector of the positive electrode, in addition to aluminum, stainless steel, nickel, titanium, etc., the surface of aluminum or stainless steel is preferably treated with carbon, nickel, titanium, or silver. Among them, aluminum and aluminum alloys are preferable. More preferred.

  As the negative electrode current collector, copper, stainless steel, nickel, and titanium are preferable, and copper or a copper alloy is more preferable.

  As the shape of the current collector according to the present invention, a film sheet is usually used, but a porous body, a foam, a molded body of a fiber group, and the like can also be used. Although it does not specifically limit as thickness of the said electrical power collector, 1-500 micrometers is preferable. Moreover, it is also preferable to give an uneven structure to the surface of the current collector by surface treatment.

(Electrolyte solvent)
In the lithium ion secondary battery of the present invention, the solvent used in the electrolyte solution has excellent ionic conductivity, such as one having a low viscosity and improving ionic conductivity, or one having a high dielectric constant and effective carrier concentration. A compound that can be expressed is desirable.

  Examples of such solvents include carbonate compounds such as ethylene carbonate, propylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, heterocyclic compounds such as 3-methyl-2-oxazolidinone, and ethers such as dioxane and diethyl ether. Compounds, chain ethers such as ethylene glycol dialkyl ether, propylene glycol dialkyl ether, polyethylene glycol dialkyl ether, polypropylene glycol dialkyl ether, nitrile compounds such as acetonitrile, glutaronitrile, methoxyacetonitrile, propionitrile, benzonitrile, carboxylic acid Esters such as esters, phosphate esters and phosphonate esters, dimethyls Fokishido, and the like aprotic polar substances such as sulfolane.

  Among these, carbonate compounds such as ethylene carbonate and propylene carbonate, heterocyclic compounds such as 3-methyl-2-oxazolidinone, nitrile compounds such as acetonitrile, glutarodinitrile, methoxyacetonitrile, propionitrile, and benzonitrile, and esters are particularly preferred. preferable. These may be used alone or in combination of two or more.

  From the viewpoint of improving durability due to volatility, the electrolyte solvent preferably has a boiling point of 200 ° C. or higher at normal pressure (1 atm), more preferably 250 ° C. or higher, and more preferably 270 ° C. or higher. preferable.

(Ionic liquid)
In the present invention, the lithium ion battery to which the separator according to the present invention is applied can contain an ionic liquid in the electrolytic solution.

  The ionic liquid in the present invention is a compound formed by an onium cation selected from ammonium, phosphonium, and iodonium and an anion, and a compound that exhibits a liquid state in an environment of 0 ° C. or higher and 200 ° C. or lower is used.

  The onium cation is preferably ammonium, and is selected from aliphatic, alicyclic, aromatic, and heterocyclic quaternary ammonium cations. Typically, imidazolium, pyridinium, piperidinium, thiazolium, pyrrolium, pyrazolium, benzimidazolo Examples thereof include lithium, indolium, carbazolium, quinolinium, pyrrolidinium, piperazinium, and alkylammonium. Each may have a substituent.

  The anion moiety is preferably an anion containing a fluorine atom, and typical anions include an imide anion, a borate anion, and a phosphate anion.

  Preferred examples of the cation group in the present invention include 1-ethyl-3-methylimidazolium (EMI), N, N-diethyl-N-methyl N- (2-methoxyethyl) ammonium (DEME), N-methyl- N-propyl pyrrolidinium (P13), N-methyl-N-pyrrolpirpiperidinium (PP13), N-ethyl-N-butyl pyrrolidinium (P24), etc. may be used alone or in combination, and the battery operating voltage range As long as it has a stable structure, the structure is not particularly limited.

Preferred anion groups in the present invention include, for example, bis (fluorosulfonyl) imide (FSI), (fluorosulfonyl) (trifluoromethylsulfonyl) imide (FTI), bis (trifluoromethylsulfonyl) imide (TFSI), bis ( Pentafluoroethylsulfonyl) imide (BETI), (trifluoromethanesulfonyl) trifluoroacetamide (TSAC), tetrafluoroboric acid (BF4), trifluoromethyltrifluoroboric acid (CF ₃ BF ₃ ), pentafluoroethyl trifluoroboron Acid (CF ₃ CF ₂ BF ₃ ), hexafluorophosphoric acid ester (PF ₆ ), dicyanamide, etc. may be used alone or in combination, especially if they have a stable structure within the battery operating voltage range. Limiting the structure No.

  In the present invention, a typical example of the ionic liquid is a combination of the above cation group and anion group, and can be used at any mixing ratio.

  In the present invention, the content of the ionic liquid is not particularly limited, but is preferably 10% by mass or more and 100% by mass or less in the electrolyte solution solvent.

(Supporting electrolyte salt)
In the present invention, a supporting electrolyte salt can be used from the viewpoint of improving the ionic conductivity. As the supporting electrolyte salt, any salt can be used, but preferably a salt of a metal ion belonging to Group Ia or IIa of the periodic table is used. As the metal ions belonging to Group Ia or Group IIa of the periodic table, ions of lithium, sodium and potassium are preferable. As anions of metal ion salts, halide ions (I ⁻ , Cl ⁻ , Br ⁻ etc.), SCN ⁻ , BF ₄ ⁻ , PF ₆ ⁻ , ClO ₄ ⁻ , SbF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (FSO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) (FSO ₂ ) N ⁻ , (CF ₃ CF ₂ SO ₂ ) ₂ N ⁻ , Ph ₄ B ⁻ , (C ₂ H ₄ O ₂ ) _{2 ^{B -, (CF 3 SO 2}} ) 3 C -, CF 3 COO -, CF 3 SO 3 -, C 6 F 5 SO 3 - , and the like. Among them, BF ₄ ⁻ , PF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (FSO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) (FSO ₂ ) N ⁻ , (CF ₃ CF ₂ SO ₂ ) _{2 ^{N -, (CF 3 SO 2}} ) 3 C -, CF 3 SO 3 - is more preferable.

Typical supporting electrolyte salts include LiCF ₃ SO ₃ , LiPF ₆ , LiClO ₄ , LiI, LiBF ₄ , LiCF ₃ CO ₂ , LiSCN, LiN (CF ₃ SO ₂ ) ₂ , Li (FSO ₂ ) ₂ N ⁻ , Examples thereof include Li (CF ₃ SO ₂ ) (FSO ₂ ) N, NaI, NaCF ₃ SO ₃ , NaClO ₄ , NaBF ₄ , NaAsF ₆ , KCF ₃ SO ₃ , KSCN, KPF ₆ , KClO ₄ , and KAsF ₆ . More preferred is the above Li salt. These may be used alone or in combination.

  The blending amount of the supporting electrolyte salt in the electrolytic solution is preferably 5 to 40% by mass, and particularly preferably 10 to 30% by mass.

(Production of secondary battery)
Here, production of a lithium ion secondary battery using the porous layer of the present invention as a separator will be described. The lithium ion secondary battery of the present invention can be applied to any shape such as a sheet, a corner, and a cylinder. The electrode mixture of the positive electrode active material and the negative electrode active material is mainly used after being applied (coated), dried and compressed on the current collector.

  Preferred examples of the electrode mixture coating method include a reverse roll method, a direct roll method, a blade method, a knife method, an extrusion method, a curtain method, a gravure method, a bar method, a dip method, and a squeeze method. . Among these, a blade method, a knife method, and an extrusion method are preferable.

  Moreover, it is preferable that application | coating is implemented at the speed | rate of 0.1-100 m / min. Under the present circumstances, the surface state of a favorable application layer can be obtained by selecting the said application | coating method according to the solution physical property and drying property of an electrode mixture. The application may be performed one side at a time or both sides simultaneously. Furthermore, the application may be continuous, intermittent, or a combination thereof.

  The thickness, length and width of the coating layer are determined by the shape and size of the battery, but the thickness of the coating layer on one side is preferably 1 to 2000 μm in a compressed state after drying.

  As a method for drying and dehydrating the electrode sheet coated product, a method in which hot air, vacuum, infrared rays, far infrared rays, electron beams and low-humidity air are used alone or in combination can be used. The drying temperature is preferably 80 to 350 ° C, more preferably 100 to 250 ° C. The water content is preferably 2000 ppm or less for the entire battery, and preferably 500 ppm or less for each of the positive electrode mixture, the negative electrode mixture and the electrolyte.

  As a sheet pressing method, a generally adopted method can be used, but a calendar pressing method is particularly preferable. Although a press pressure is not specifically limited, 0.05-3 t / cm is preferable. The press speed of the calendar press method is preferably 0.1 to 50 m / min, and the press temperature is preferably room temperature to 200 ° C. The ratio of the negative electrode sheet width to the positive electrode sheet is preferably 0.9 to 1.1, particularly preferably 0.95 to 1.0. The content ratio of the positive electrode active material and the negative electrode active material varies depending on the compound type and the electrode mixture formulation.

  The separator composed of the porous layer of the present invention is formed on a resin film such as polyethylene terephthalate as described above, and is formed into a film by peeling, so that it can be overlapped between the positive electrode plate and the negative electrode plate. Alternatively, it may be formed by coating directly on each negative electrode plate, and the positive electrodes are stacked to form a battery.

  Although the form of the lithium ion secondary battery of this invention is not specifically limited, It can enclose in various battery cells, such as a coin, a sheet | seat, and a cylinder.

  The use of the lithium ion secondary battery of the present invention is not particularly limited. For example, as an electronic device, a notebook computer, a pen input personal computer, a mobile personal computer, an electronic book player, a mobile phone, a cordless phone, a pager, a handy terminal , Portable fax machine, portable copy, portable printer, headphone stereo, video movie, LCD TV, handy cleaner, portable CD, mini-disc, electric shaver, transceiver, electronic notebook, calculator, memory card, portable tape recorder, radio, backup power supply, Examples include memory cards.

  Other consumer products include automobiles, electric vehicles, motors, lighting equipment, toys, game equipment, road conditioners, watches, strobes, cameras, medical equipment (such as pacemakers, hearing aids, and shoulder grinders). Furthermore, it can be used for various military purposes and space. Moreover, it can also combine with a solar cell.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. In addition, although the display of "part" or "%" is used in an Example, unless otherwise indicated, "part by mass" or "mass%" is represented.

Example 1
<Oxide inorganic particles>
The following were prepared as oxide inorganic particles.
-Alu-C: Nippon Aerosil Co., Ltd. non-porous alumina Average particle size 13 nm
<Polymer having reactive functional group>
The following resins were prepared as polymers having reactive functional groups.
・ Polyvinyl alcohol 88% saponification weight average molecular weight 40,000 (Kuraray Poval, Kuraray Co., Ltd.)
・ Polyvinyl alcohol Saponification degree 88% Weight average molecular weight 55,000 (Kuraray Poval, Kuraray Co., Ltd.)
・ Polyvinyl alcohol Saponification degree 88% Weight average molecular weight 200,000 (Kuraray Poval, Kuraray Co., Ltd.)
・ Polyvinyl alcohol Saponification degree 88% Weight average molecular weight 480,000 (Kuraray Poval, Kuraray Co., Ltd.)
・ Polyvinyl alcohol Saponification degree 88% Weight average molecular weight 520,000 (Kuraray Poval, Kuraray Co., Ltd.)
・ Polyacrylic acid Weight average molecular weight 200,000 (Aron, manufactured by Toagosei Co., Ltd.)
・ Polyacrylic acid Weight average molecular weight 550,000 (Aron, manufactured by Toagosei Co., Ltd.)
-Polyacrylic acid Weight average molecular weight 1 million (Aron, manufactured by Toagosei Co., Ltd.)
For comparison, the following resin was prepared as a polymer having no reactive functional group.
・ Polyethylene oxide Weight average molecular weight 400,000 (manufactured by Aldrich)
<Crosslinking agent>
The following compounds were prepared as crosslinking agents.
・ Isocyanate compound (Takenate WD720, water-based isocyanate crosslinking agent, manufactured by Mitsui Chemicals, Inc.)
・ Epoxy compound (Denacol EX-313, water-based epoxy crosslinking agent, manufactured by Nagase ChemteX Corporation)
・ Glyoxal (manufactured by Nippon Synthetic Chemical)
・ Zirconyl acetate (Daiichi Rare Element Chemical Co., Ltd.)
・ Titanium ammonium lactate (Matsumoto Fine Chemical Co., Ltd.)
(Porous layer slurry production)
13.5 g of the oxide inorganic particles (Alu-C) was added to 40 g of water, 1.5 g of acetic acid was further added, and the mixture was dispersed and mixed with a homogenizer at 5000 rpm for 30 minutes. Further, 4 g of the polymer described in Table 1 was added, 1 g of the crosslinking agent described in Table 1 was added, and finally 40 g of water was added and stirred, whereby the porous layer having a nonvolatile content ratio of 20% was added. A slurry was obtained. In addition, about Examples 16-21, the slurry was produced, without adding a crosslinking agent.

(Preparation of negative electrode slurry)
45 g of graphite as a negative electrode active material, 4 g of BM-400B (an aqueous dispersion containing 40% modified styrene butadiene rubber manufactured by Nippon Zeon Co., Ltd.) and 1 g of carboxymethylcellulose as a viscosity modifier are added to 50 g of water, and a homogenizer. Was used to knead to obtain a negative electrode slurry. In addition, about Examples 16-21, 1 g of each crosslinking agent in a table | surface was added to the negative electrode slurry.

(Preparation of negative electrode sheet)
The negative electrode slurry produced as described above was applied to a copper foil having a thickness of 15 μm using an extrusion coater and dried at 120 ° C. for 2 minutes. After drying, it was compression molded by a calendar press. When the cross section was observed with a scanning electron microscope after compression, the film thickness of the negative electrode active material layer was 50 μm. This negative electrode sheet was used in Examples 1 to 15 and 22 to 26.

(Preparation of porous layer)
In Examples 1 to 11, the porous slurry was applied on a 100 μm thick polyethylene terephthalate film. Specifically, the porous layer slurry was applied onto a polyethylene terephthalate film with a wire bar and dried at 110 ° C. for 2 minutes. The dried film was peeled from the polyethylene terephthalate film to obtain a porous layer. This porous layer had a thickness of 10 μm.

  About Examples 12-15, 22-26, the porous layer was apply | coated on the active material layer of the said negative electrode sheet. Specifically, the porous layer slurry was applied onto the negative electrode active material layer with a wire bar and dried at 110 ° C. for 2 minutes. When the cross section of the dried coating film was observed with a scanning electron microscope, the porous layer was 10 μm.

  About Examples 16-21, the negative electrode and the porous layer were simultaneously applied in multiple layers and dried. Specifically, using a multilayer coating die coater in which a two-system flow path is formed inside the die, the negative electrode active material layer and the porous layer are simultaneously applied from the copper foil surface, and dried at 120 ° C. for 4 minutes. did. After drying, compression molding was performed with a roll press. The resulting negative electrode with a porous layer was observed for a cross section with a scanning electron microscope. As a result, the negative electrode active material layer was 50 μm and the porous layer was 10 μm.

(Preparation of positive electrode slurry)
43 g of the positive electrode active material LiCoO ₂ , ₂ g of scaly graphite, 2 g of acetylene black, and 3 g of PVDF (PolyVinylline DiFluoride) as a binder were added to 50 g of N-methylpyrrolidone, and kneaded to obtain a positive electrode slurry.

(Preparation of positive electrode sheet)
The positive electrode slurry produced as described above was coated on an aluminum foil having a thickness of 15 μm using an extrusion type coater, dried at 150 ° C. for 5 minutes, and then compression molded by a calendar press. When the cross section of the resulting sheet was observed with a scanning electron microscope, the positive electrode active material layer was 60 μm. An aluminum lead plate was welded to the end of the positive electrode sheet thus produced to prepare a positive electrode sheet having a thickness of 95 μm, a width of 54 mm × a length of 49 mm. Thereafter, it was dried at 150 ° C. for 30 minutes in dry air having a dew point of −40 ° C. or less.

(Production of laminated battery)
About Examples 1-11, the negative electrode sheet was cut into width 55mm x length 50mm, and the lead board made from nickel was welded to the edge part. Thereafter, the porous layer was cut into the same size and superposed with Examples 1 to 11, and dried at 150 ° C. for 30 minutes in dry air having a dew point of −40 ° C. or lower.

  In Examples 12 to 26, since the porous layer was already formed on the negative electrode active material layer, it was cut into a width of 55 mm and a length of 50 mm. Then, it was dried at 150 ° C. for 30 minutes in dry air having a dew point of −40 ° C. or less.

  The negative electrode sheet with a porous layer and the positive electrode sheet obtained as described above were superposed so that the active material layers face each other, and further heated at 80 ° C. under reduced pressure for 3 hours. Thereafter, the electrode laminate was put into an exterior material made of a laminate film of polyethylene (50 μm) -aluminum-deposited polyethylene terephthalate (50 μm) heat-sealed at three edges, and LBG-94923 (EC: DEC) manufactured by Kishida Chemical Co., Ltd. was used as the electrolyte. = 3: 7, LiPF6 1.3 mol / L) was injected. Laminated batteries No. 1 to 26 were obtained by sealing the last one edge after injection by heat-sealing under vacuum.

(Laminated battery cycle test (discharge capacity maintenance rate))
About the obtained laminated battery, it charged to 4.2V with the electric current (0.2C) of the rate which charge ends in 5 hours with respect to theoretical capacity | capacitance using the charging / discharging measuring apparatus made from a measuring instrument center, and 10 minutes Then, the battery was discharged to 3 V at the same current value (0.2 C). This charge-pause-discharge cycle was repeated 500 times, and the discharge capacity retention rate (%) at the 500th time was measured with respect to the first discharge capacity, and the cycle characteristics were evaluated. Similar cycle characteristics were also evaluated at a current (4C) at a rate at which charging was completed in 15 minutes with respect to the theoretical capacity. The higher the discharge capacity retention rate, the higher the strength of the porous layer that can withstand the lithium dendride generated inside the battery.

(Short-resistance evaluation of laminate battery (short-circuit occurrence rate))
500 batteries were manufactured and charged from 2 V to 4.3 V at a current (0.2 C) at a rate at which charging was completed in 5 hours with respect to the theoretical capacity. Thereafter, the battery was left in a 60 ° C. environment for 20 days, and the battery voltage was measured again. A battery in which the battery voltage drop after standing in a 50 ° C. environment was 0.5 V or more was regarded as a short battery. The percentage of short circuits among 500 batteries produced was defined as the short circuit occurrence rate.

  As is clear from the results shown in Table 1, it can be seen that the lithium ion secondary battery using the porous layer of the present invention as a separator has better cycle characteristics and excellent short-circuit resistance than the comparative example. .

Claims (6)

  Contains oxide inorganic particles (A) and a polymer (B) having a reactive functional group and a crosslinker (C) that reacts with the reactive functional group. A porous layer characterized by that.   The porous layer according to claim 1, wherein the reactive functional group is a hydroxyl group.   The porous layer according to claim 1 or 2, wherein the polymer (B) has a weight average molecular weight of 50,000 to 500,000.   The lithium ion secondary battery which has a porous layer between the positive electrode which has a positive electrode active material layer, and the negative electrode which has a negative electrode active material layer, This porous layer of any one of Claim 1 to 3 A lithium ion secondary battery, which is a porous layer.   The lithium ion secondary battery according to claim 4, wherein the porous layer is formed by coating on the positive electrode active material layer or the negative electrode active material layer.   In a lithium ion secondary battery having a porous layer between a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, the porous layer includes the positive electrode active material layer or the negative electrode active material layer A lithium ion secondary battery produced by simultaneously applying and drying, and containing the crosslinking agent (C) in a coating solution for the positive electrode active material layer or the negative electrode active material layer.