JP2015103404A - Negative electrode active material layer for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a novel and improved negative electrode active material layer for nonaqueous electrolyte secondary batteries which enables the formation of an aqueous negative electrode and the suppression of expansion and shrinkage of a negative electrode active material; and a secondary battery arranged by such a negative electrode active material layer.SOLUTION: To solve the problem, a negative electrode active material layer for nonaqueous electrolyte secondary batteries is provided according to an aspect of the present invention, which comprises: a water-soluble polymer; a negative electrode active material including a silicon-based active material and a carbon-based active material; a first fluorine-based polymer; and microparticles of a fluorine-based polymer for binding the negative electrode active material. The negative electrode active material layer includes 1-10 mass% of the fluorine-based polymer microparticles to the total mass thereof.

Description

  The present invention relates to a negative electrode active material layer for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

  Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are widely used as power sources for portable devices such as notebook PCs and mobile phones. -Because of its high capacity, great expectations are placed on its development. As a negative electrode material (negative electrode active material) of such a non-aqueous electrolyte secondary battery, in addition to lithium metal and lithium alloy, a graphitic carbon material such as natural graphite or artificial graphite that can desorb and insert Li ions. Etc. are used.

  Recently, a further increase in capacity has been desired for batteries for portable devices that have become smaller and more multifunctional, and in response to this, carbon-based active materials (for example, graphitic carbon materials) that are widely used as negative electrode active materials. ) New negative electrode active materials have been investigated. As a new negative electrode active material, a tin (Sn) alloy, a silicon (Si) alloy, a silicon (Si) oxide, a lithium (Li) nitride, and the like have been attracting attention. Discharge cycle characteristics are inferior to graphitic carbon materials.

  The carbon-based active material has a layered structure, and Li is inserted / extracted between the layers at the time of charge / discharge. In contrast, the new negative electrode material has a more complicated structure than the carbon-based active material, and has a large amount of Li insertion / desorption per unit mass during charge / discharge. For this reason, the new negative electrode material has a large expansion / contraction associated with charge / discharge, and as a result, in a charge / discharge cycle in which expansion / contraction is repeated, the structure of the electrode is broken and the electronic conductivity is lowered. For this reason, it is thought that charging / discharging cycling characteristics worsen compared with a graphitic carbon material.

  By the way, due to the rapid spread of tablet terminals and smartphones in recent years, not only high capacity but also thinning of nonaqueous electrolyte secondary batteries is required. For this reason, an aluminum laminated film is often used as the exterior material of the nonaqueous electrolyte secondary battery. This is because a battery using an aluminum laminate film as an exterior material has a high degree of freedom in battery size (Size) design and can be easily reduced in thickness. However, the problem of expansion and contraction of the negative electrode is particularly noticeable in a battery using an aluminum laminate for the exterior body. This is because an exterior body made of an aluminum laminate has low rigidity, so that expansion of the negative electrode cannot be suppressed and is easily deformed by expansion of the negative electrode.

  Further, in the recent negative electrode manufacturing of non-aqueous electrolyte secondary batteries, it is possible to produce a negative electrode using an aqueous slurry (slurry) (aqueous binder) due to environmental considerations and cost reduction during manufacturing. It has been requested. Here, the water-based slurry is obtained by dispersing a water-soluble polymer, a latex polymer, and a negative electrode active material in water. A negative electrode is produced by applying an aqueous slurry to a current collector and drying. A negative electrode produced using an aqueous slurry is also referred to as an aqueous negative electrode.

Thus, Patent Document 1 proposes a technique for suppressing the expansion of the water-based negative electrode during charge / discharge. Specifically, in the technology disclosed in Patent Document 1, a water-based slurry in which water-soluble adhesive resin, fluororubber composed of a fluorinated resin copolymer, graphitic carbon material, and SiO _x are dispersed in water is prepared. A negative electrode is prepared using this aqueous slurry.

JP 2013-145669 A

  However, in the technique of Patent Document 1, since the fluororubber has a large swelling ratio with the nonaqueous electrolytic solution inside the battery, sufficient mechanical strength cannot be obtained. Therefore, the technique disclosed in Patent Document 1 cannot sufficiently suppress the negative electrode expansion during charging / discharging.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is a new and improved structure that constitutes a water-based negative electrode and can suppress expansion and contraction of the negative electrode active material. An object of the present invention is to provide a negative electrode active material layer for a non-aqueous electrolyte secondary battery and a secondary battery using the same.

  In order to solve the above problems, according to one aspect of the present invention, a negative electrode containing a water-soluble polymer, a negative electrode active material containing a silicon-based active material and a carbon-based active material, and a first fluorine-based polymer, Fluorine-based polymer fine particles that bind active materials to each other, and the fluorine-based polymer fine particles are contained in the negative electrode active material layer in an amount of 1 to 10% by mass with respect to the total mass of the negative electrode active material layer. A negative electrode active material layer for a non-aqueous electrolyte secondary battery is provided.

  As described above, according to this aspect, the fluorine-based polymer fine particles firmly bind the negative electrode active materials to each other, thereby suppressing the expansion and contraction of the silicon-based active materials and connecting the negative electrode active materials to each other. Can be maintained. In addition, the negative electrode active material layer is formed by applying an aqueous slurry in which a material constituting the negative electrode active material layer is dispersed in water to a current collector and drying the current collector, and thus forms an aqueous negative electrode.

  Here, the first fluorine-based polymer may contain polyvinylidene fluoride.

  According to this viewpoint, the negative electrode active materials are more firmly bound to each other.

  The fine particles of the fluorine-based polymer may be prepared by emulsion polymerization of a monomer constituting the first fluorine-based polymer.

  According to this viewpoint, fine particles of a fluorine-based polymer having a very small particle diameter are easily produced.

  Further, the fine particles of the fluorine-based polymer may be prepared by pulverizing coarse particles synthesized by suspension polymerization of the monomer constituting the first fluorine-based polymer.

  According to this viewpoint, fine particles of a fluorine-based polymer having a very small particle diameter are easily produced.

  The water-soluble polymer may contain a cellulose polymer.

  According to this viewpoint, the binding force between the negative electrode active materials is further improved.

  The cellulose polymer may be a metal salt of carboxymethyl cellulose.

  According to this viewpoint, the binding force between the negative electrode active materials is further improved.

The silicon-based active material may contain a silicon oxide represented by a composition formula SiO _x (0.5 ≦ x ≦ 1.5).

  According to this viewpoint, battery characteristics such as the discharge capacity of the lithium ion secondary battery 10 are improved.

  According to another aspect of the present invention, the front and back surfaces of the separator include a nonaqueous electrolytic solution, a separator, the negative electrode active material layer, and a second fluoropolymer that holds the nonaqueous electrolytic solution in a gel state. A non-aqueous electrolyte secondary battery comprising: a negative electrode-side binder layer formed on a surface facing the negative electrode active material layer.

  According to this viewpoint, the separator and the negative electrode active material layer are firmly bound by the fine particles of the fluorine-based polymer distributed on the outermost surface of the negative electrode active material layer being fused to the negative electrode side binder layer. Therefore, the nonaqueous electrolyte secondary battery can improve the binding force between the aqueous negative electrode and the separator. As a result, the non-aqueous electrolyte secondary battery has improved buckling strength.

  Here, the fine particles of the fluorine-based polymer may be fused to the outermost surface of the negative electrode active material layer.

  According to this viewpoint, the negative electrode active material layer and the separator are more firmly bound.

  Further, the first fluorine-based polymer and the second fluorine-based polymer may be the same type of fluorine-based polymer.

  According to this viewpoint, the negative electrode active material layer and the separator are more firmly bound.

  In addition, the first fluorine-based polymer and the second fluorine-based polymer may contain polyvinylidene fluoride.

  According to this viewpoint, the negative electrode active material layer and the separator are more firmly bound.

  Moreover, you may provide the exterior body comprised with the aluminum laminate film.

  According to this viewpoint, the rigidity of the outer package is lowered, but the fluorine-based polymer fine particles can suppress the expansion and contraction of the silicon-based active material, and the negative electrode active material layer and the separator are firmly bonded. Therefore, an increase in the thickness of the nonaqueous electrolyte secondary battery is suppressed, and the buckling resistance is improved.

  As described above, the negative electrode active material layer according to the present invention constitutes a water-based negative electrode and can suppress expansion and contraction of the negative electrode active material.

It is a perspective view which shows the external appearance structure of the nonaqueous electrolyte secondary battery which concerns on embodiment of this invention. It is a sectional side view which shows roughly the internal structure of a nonaqueous electrolyte secondary battery. It is a graph which shows the correspondence of the load (test force) applied to the indenter which presses a lithium ion (Lithium ion) secondary battery, and the displacement of an indenter. It is a SEM photograph which shows a mode that the fine particle (PVDF microparticles | fine-particles) of the fluorine-type polymer adhered to the surface of the negative electrode active material (graphite). It is a SEM photograph which shows a mode that the microparticles | fine-particles (PVDF microparticles | fine-particles) of a fluorine-type polymer fuse | melted on the surface of the negative electrode active material (graphite).

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

<1. Configuration of lithium ion secondary battery>
The nonaqueous electrolyte secondary battery according to the present invention can be applied to all kinds of nonaqueous electrolyte secondary batteries. Therefore, in this embodiment, an example in which the present invention is applied to a lithium ion secondary battery will be described.

  As shown in FIG.1 and FIG.2, the lithium ion secondary battery 10 which concerns on this embodiment is the positive electrode 20, the negative electrode 30, the separator layer 40, the exterior body 100, the positive electrode collector tab 110, and a negative electrode. And a current collector tab 120.

  The separator layer 40 includes a separator 40a, a positive electrode side binder layer 40b, a negative electrode side binder layer 40c, and a nonaqueous electrolytic solution. The separator 40a is not particularly limited, and any separator may be used as long as it is used as a separator for a lithium ion secondary battery. As the separator 40a, it is preferable to use a porous film or a non-woven fabric exhibiting excellent high rate discharge performance alone or in combination. Examples of the resin constituting the separator 40a include polyolefin resins represented by polyethylene, polypropylene, and the like, polyethylene terephthalate, and polybutylene terephthalate. Polyester resin, PVDF, vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene ( tetrafluoroety ene) copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride -Ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene (Hexafluoropropylene) copolymer, vinylidene fluoride-ethylene (e hylene) - can be exemplified tetrafluoroethylene (tetrafluoroethylene) copolymers.

  The positive electrode side binder layer 40b is formed on the surface facing the positive electrode 20 among the front and back surfaces of the separator 40a. The positive electrode side binder layer 40b becomes a gel by being swollen by the non-aqueous electrolyte. That is, the positive electrode side binder layer 40b holds the non-aqueous electrolyte in a gel form. Furthermore, the positive electrode side binder layer 40b binds the separator layer 40 and the positive electrode 20 (more specifically, the positive electrode active material layer 22). Specifically, the positive electrode binder layer 40b includes a fluorine-based polymer (second fluorine-based polymer) that is swollen by a non-aqueous electrolyte and becomes a gel. Preferably, the positive electrode side binder layer 40b is made of such a fluorine-based polymer. Examples of the fluorine-based polymer constituting the positive electrode side binder layer 40b include PVDF, a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), and the like. If the mass ratio of HFP to VDF is too large, the fluorine-based polymer is dissolved in the non-aqueous electrolyte. Therefore, the mass ratio between VDF and HFP is that the fluorine-based polymer is not dissolved in the non-aqueous electrolyte. It may be adjusted by the degree. Other examples of the fluorine-based polymer constituting the positive electrode side binder layer 40b include a copolymer P (VDF-TFE) of vinylidene fluoride and tetrafluoroethylene, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene. Copolymer P (VDF-TFE-HFP) and the like. The positive electrode side binder layer 40b may be omitted.

  The negative electrode side binder layer 40c is formed on the surface facing the negative electrode 30 among the front and back surfaces of the separator 40a. The negative electrode side binder layer 40c is gelled by swelling with the non-aqueous electrolyte. That is, the negative electrode side binder layer 40c holds the non-aqueous electrolyte in a gel form. Furthermore, the negative electrode side binder layer 40c binds the separator layer 40 and the negative electrode 30 (more specifically, the negative electrode active material layer 32). Specifically, the negative electrode side binder layer 40c includes a fluorine-based polymer (second fluorine-based polymer) that is swollen by a non-aqueous electrolyte and becomes a gel. Preferably, the negative electrode side binder layer 40c is made of such a fluorine-based polymer. Examples of the fluorine-based polymer constituting the negative electrode side binder layer 40c include PVDF, a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), and the like. If the mass ratio of HFP to VDF is too large, the fluorine-based polymer is dissolved in the non-aqueous electrolyte. Therefore, the mass ratio between VDF and HFP is that the fluorine-based polymer is not dissolved in the non-aqueous electrolyte. It may be adjusted by the degree. Other examples of the fluorine-based polymer constituting the negative electrode side binder layer 40c include a copolymer P (VDF-TFE) of vinylidene fluoride and tetrafluoroethylene, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene. Copolymer P (VDF-TFE-HFP) and the like.

  As the non-aqueous electrolyte, the same non-aqueous electrolyte as conventionally used for lithium secondary batteries can be used without any particular limitation. The nonaqueous electrolytic solution has a composition in which an electrolyte salt is contained in a nonaqueous solvent. Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, vinylene carbonate (vinyl carbonate), and the like. ); Cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate chain carbonates such as ethyl carbonate; chain esters such as methyl formate, methyl acetate, butyric acid methyl; tetrahydrofuran (tetrahydrofuran) or derivatives thereof; 1,3- Ethers such as dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyl diglyme; Nitriles such as acetonitrile and benzonitrile e) class; dioxolane or a derivative thereof; ethylene sulfide, sulfolane, sultone, or a derivative thereof alone or a mixture of two or more thereof, etc. It is not limited to.

Examples of the electrolyte salt include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiPF _6-x (C _n F _{2n + 1} ) _x [where 1 <x <6, n = 1or2], LiSCN, LiBr, Inorganic ions containing one kind of lithium (Li), sodium (Na) or potassium (K) such as LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN Salt, 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 9) 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 (octyl sulfonic acid), lithium dodecylbenzene sulfonate (lithium dodecylbenzene sulfonate) organic ion salts such as dodecyl benzene sulphonic acid) and the like, and these ionic compounds can be used alone or in admixture of two or more. The concentration of the electrolyte salt may be the same as that of the nonaqueous electrolytic solution used in the conventional lithium secondary battery, and is not particularly limited. In this embodiment, a nonaqueous electrolytic solution containing an appropriate lithium compound (electrolyte salt) at a concentration of about 0.8 to 1.5 mol / L can be used.

  The positive electrode 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode current collector 21 may be any conductor as long as it is a conductor, for example, aluminum (aluminum), stainless steel (stainless steel), nickel plated steel (nickel plating steel), or the like. The positive electrode current collector 21 is connected to the positive electrode current collector tab 110.

The positive electrode active material layer 22 includes at least a positive electrode active material, and may further include a conductive agent and a positive electrode binder. The positive electrode active material is, for example, a solid solution oxide containing lithium, but is not particularly limited as long as the material can electrochemically occlude and release lithium ions. The solid solution oxide is, for example, Li _a Mn _x Co _y Ni _z O ₂ (1.150 ≦ a ≦ 1.430, 0.45 ≦ x ≦ 0.6, 0.10 ≦ y ≦ 0.15,. 20 ≦ z ≦ 0.28), LiMn _x Co _y Ni _z O ₂ (0.3 ≦ x ≦ 0.85, 0.10 ≦ y ≦ 0.3, 0.10 ≦ z ≦ 0.3), LiMn _1.5 Ni _0.5 O ₄ . More specifically, the solid solution oxide is, for example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1. It may be a lithium-containing transition metal oxide such as O ₂ or a metal oxide not containing lithium such as MnO ₂ .

  The conductive agent is, for example, carbon black such as ketjen black, acetylene black, natural graphite, artificial graphite, etc., and particularly if it is for increasing the conductivity of the positive electrode. Not limited.

  The positive electrode binder is not particularly limited as long as it can bind the positive electrode active material and the conductive agent onto the current collector 21, but is preferably composed of a fluorine-based polymer. Examples of the fluorine-based polymer include PVDF, PTFE, P (VDF-HFP), P (VDF-TFE), a copolymer of vinylidene fluoride and chlorotrifluoroethylene, P (VDF-CTFE), and P (VDF- TFE-HFP) and the like. Other examples of the resin constituting the positive electrode binder include ethylene propylene diene terpolymer, styrene butadiene rubber (SBR), acrylonitrile butadiene rubber, polyvinyl acetate ( Examples thereof include polyvinyl acetate, polymethyl methacrylate, polyethylene, and nitrocellulose.

  The positive electrode binder is preferably the same type as the resin constituting the positive electrode binder layer 40b. For example, when the positive electrode binder layer 40b is made of PVDF, the positive electrode binder is preferably made of PVDF. When both the positive electrode side binder layer 40b and the positive electrode binder are made of PVDF, the binding force between the positive electrode active material layer 22 and the positive electrode side binder layer 40b is particularly improved. As a result, the buckling resistance of the lithium ion secondary battery 10 is improved.

  The positive electrode active material layer 22 is formed by, for example, dry-mixing a positive electrode active material, a conductive agent, and a positive electrode binder to form a positive electrode mixture, and dispersing the positive electrode mixture in a suitable organic solvent. An agent slurry is formed, this positive electrode mixture slurry is applied onto the positive electrode current collector 21, dried and rolled.

  The negative electrode 30 includes a negative electrode current collector 31 and a negative electrode active material layer 32. The negative electrode current collector 31 may be any conductor as long as it is a conductor, and is made of, for example, aluminum, stainless steel, nickel-plated steel, or the like. The negative electrode current collector 31 is connected to the negative electrode current collector tab 120.

  The negative electrode active material layer 32 includes a negative electrode active material, a water-soluble polymer, and fine particles of a fluorine-based polymer. The negative electrode active material includes a silicon-based active material and a carbon-based active material.

The silicon-based active material is a material that contains silicon (atom) and can electrochemically occlude and release lithium ions. Examples of the silicon active material include fine particles of silicon alone, fine particles of silicon oxide, and alloys based on silicon. The oxide of silicon is represented by SiO _x (0.5 ≦ x ≦ 1.5). An alloy containing silicon as a basic material is an alloy in which the mass% of silicon relative to the total mass of the alloy is the largest among all the metal elements constituting the alloy, and examples thereof include a Si—Al—Fe alloy.

  On the other hand, the carbon-based active material is a material that contains carbon (atom) and can electrochemically occlude and release lithium ions. Examples of the carbon-based active material include graphite active materials (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, and the like).

  In addition, the mass ratio of the silicon-based active material and the carbon-based active material in the negative electrode active material is not particularly limited. Further, the mass% of the silicon-based active material with respect to the total mass of the negative electrode active material layer is not particularly limited. That is, the mass ratio and mass% that the negative electrode active material layer containing a silicon-based active material can take are sufficient. As an example, the silicon-based active material may be about 3% by mass with respect to the total mass of the negative electrode active material layer.

  The water-soluble polymer serves as a thickener. Examples of the water-soluble polymer include cellulose polymers, polyacrylic acid polymers, polyvinyl alcohol, polyethylene oxide, and the like. Examples of the cellulosic polymer include metal salts of carboxymethyl cellulose (CMC), metal salts of cellulose derivatives such as methyl cellulose, ethyl cellulose, and hydroxyalkyl cellulose. . Therefore, the negative electrode 30 is a so-called aqueous negative electrode.

  The fluorine polymer fine particles are fine particles containing a fluorine polymer (first fluorine polymer) and serve as a binder for the negative electrode active material layer 32. The fine particles of the fluorine-based polymer are preferably composed of a fluorine-based polymer. The fine particles of the fluorine-based polymer are dispersed in the negative electrode active material layer 32 and bind the negative electrode active materials to each other. The fine particles of the fluorine-based polymer are fused to the outermost surface of the negative electrode active material layer 32, that is, the surface in contact with the negative electrode side binder layer 40c. The finest particles of the fluorine-based polymer on the outermost surface are also fused (compatible) with the negative electrode side binder layer 40c, thereby firmly bonding the negative electrode side binder layer 40c and the negative electrode active material layer 32 together. The reason why the fluorine-based polymer is used as fine particles is as follows. That is, in this embodiment, since the negative electrode is a water-based negative electrode, water is used as a solvent in which the negative electrode mixture is dispersed. However, fluoropolymers, especially PVDF, are not soluble in water. One method for solving this problem is to prepare a solvent for dissolving PVDF, but this method requires a separate solvent. Therefore, in this embodiment, the fluorine-based polymer is used as fine particles. Thereby, the fine particles of the fluorine-based polymer are dispersed in water to form a latex, so that they are dispersed (uniformly) in the negative electrode active material layer.

  The fluorine polymer constituting the fine particles of the fluorine polymer has crystallinity. Examples of such a fluorine-based polymer include PVDF and a copolymer containing PVDF. That is, in this embodiment, the fluorine-based polymer constituting the fluorine-based polymer fine particles does not include an amorphous polymer, for example, fluorine rubber. Examples of the copolymer containing PVDF include a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). If the mass ratio of HFP to VDF is too large, the fluoropolymer fine particles are dissolved in the non-aqueous electrolyte. Therefore, the mass ratio of VDF and HFP is the same as that of the fluoropolymer fine particles. It may be adjusted to such an extent that it does not dissolve in the liquid. Other examples of the fluorine-based polymer constituting the fluorine-based polymer particles include a copolymer of vinylidene fluoride (VDF) and tetrafluoroethylene (TFE).

  The fluoropolymer constituting the fluoropolymer fine particles is preferably of the same type as the resin constituting the negative electrode binder layer 40c. For example, when the negative electrode binder layer 40c is made of PVDF, the fluorine-based polymer fine particles are preferably made of PVDF. When both the negative electrode side binder layer 40c and the fluoropolymer fine particles are made of PVDF, the binding force between the negative electrode active material layer 32 and the negative electrode side binder layer 40c is particularly improved. As a result, the buckling resistance of the lithium ion secondary battery 10 is improved.

  The particle diameter of the fine particles of the fluorine-based polymer (the diameter when the fluorine-based fine particles are regarded as a sphere) is not particularly limited, and may be any value as long as it can be dispersed in the negative electrode active material layer 32. Good. For example, the average particle diameter (arithmetic average value of particle diameter) of the fluoropolymer fine particles may be around 150 nm. The average particle diameter of the fluorine-based polymer fine particles is measured by, for example, laser diffraction (laser diffractometry). Specifically, the particle size distribution of the fluorine-based fine particles may be measured by a laser diffraction method, and the arithmetic average value of the particle sizes may be calculated based on this particle size distribution. Here, the average particle diameter is an arithmetic average value of the particle diameter before melting.

  The fine particles of the fluorine-based polymer are produced (synthesized) by, for example, emulsion polymerization of a monomer (for example, VDF) constituting the fluorine-based polymer. The fluorine-based fine particles may be produced by suspension polymerization of monomers constituting the fluorine-based polymer and pulverizing coarse particles obtained thereby.

  The fluorine polymer fine particles are contained in the negative electrode active material layer 32 in an amount of 1 to 10% by mass with respect to the total mass of the negative electrode active material layer 32. When the content of the fluoropolymer fine particles is less than 1% by mass, the binding force between the negative electrode active material layer 32 and the negative electrode side binder layer 40c is not sufficient. In addition, when the content of the fluorine-based polymer fine particles exceeds 10% by mass, the negative electrode active material density in the negative electrode active material layer 32 decreases, and the energy density of the lithium ion secondary battery 10 decreases.

  In the present embodiment, the negative electrode active material layer 32 contains crystalline fluoropolymer fine particles. The fine particles of the crystalline fluoropolymer are less likely to swell by the electrolytic solution (specifically, the swelling rate is lower than that of fluororubber). Here, the swelling rate of a certain resin can be obtained, for example, by dividing the volume of the resin after immersion of the electrolyte by the volume of the resin before immersion of the electrolyte. Accordingly, the fine particles of the fluorine-based polymer can firmly bind the negative electrode active materials to each other even after being swollen with the electrolytic solution.

  Therefore, the fluorine-based polymer can suppress the expansion and contraction of the silicon-based active material during the charge / discharge cycle, and thus can suppress the volume change (expansion) of the lithium ion secondary battery 10 during the charge / discharge. That is, the thickness change of the lithium ion secondary battery 10 is reduced. Furthermore, the fine particles of the fluorine-based polymer can follow the expansion and contraction of the silicon-based active material and maintain the connection between the negative electrode active materials containing the silicon-based active material even if the silicon-based active material expands and contracts. . Therefore, the fluorine-based polymer fine particles can prevent structural destruction of the negative electrode active material layer 32 and can maintain a high electronic conductivity (conductivity) in the negative electrode active material layer 32. As a result, the cycle life of the lithium ion secondary battery 10 is improved.

  The negative electrode active material layer is produced, for example, by the following method. That is, a negative electrode mixture slurry (aqueous slurry) is formed by dispersing fine particles of a negative electrode active material, a water-soluble polymer, and a fluorine-based polymer in water, and this negative electrode mixture slurry is coated on a current collector. And dry. In the negative electrode mixture slurry, the fluorine-based polymer is dispersed in the negative electrode active material layer 32 and attached to the surface of the negative electrode active material. Next, the dried negative electrode mixture is rolled together with the current collector 31. An SEM photograph of the negative electrode mixture after rolling is shown in FIG. In this example, PVDF fine particles 32b, which are fine particles of a fluorine-based polymer, are attached to graphite 32a, which is a negative electrode active material.

  The rolled coating film is then heated at a temperature equal to or higher than the melting point of the fluoropolymer fine particles. Thereby, the fine particles of the fluorine-based polymer are fused to the surface of the negative electrode active material, and the negative electrode active materials are firmly bonded to each other. Further, the fluorine-based polymer fine particles are fused to the outermost surface of the negative electrode active material layer 32. For example, when the fluoropolymer fine particles are composed of PVDF, the melting point of PVDF is about 165 ° C., and thus the rolled coating film may be heated at about 170 ° C. An example in which the fluorine-based fine particles are fused to the surface of the negative electrode active material is shown in FIG. In FIG. 5, PVDF fine particles 32b, which are fluorine-based fine particles, are fused on graphite 32a, which is a negative electrode active material.

  The outer package 100 is made of an aluminum laminate film and incorporates the configuration shown in FIG. 2, that is, the positive electrode 20, the negative electrode 30, and the separator layer 40. The positive electrode current collector tab 110 is connected to the positive electrode current collector 21 and protrudes outside the exterior body 100. The negative electrode current collector tab 120 is connected to the negative electrode current collector 31 and protrudes outside the exterior body 100.

  The lithium ion secondary battery 10 may be a laminate in which the positive electrode 20, the negative electrode 30, and the separator layer 40 are sequentially laminated, and is a winding in which a sheet composed of the positive electrode 20, the negative electrode 30, and the separator layer 40 is wound. A rotating element may be used. The lithium ion secondary battery 10 may have a structure other than these.

<2. Manufacturing method of lithium ion secondary battery>
Next, a method for manufacturing the lithium ion secondary battery 10 will be described. The positive electrode 20 is produced as follows. First, a positive electrode mixture is prepared by mixing a positive electrode active material, a conductive agent, and a positive electrode binder in a desired ratio. Next, a positive electrode mixture slurry is formed by dispersing the positive electrode mixture in an organic solvent (for example, N-methyl-2-pyrrolidone). Next, the positive electrode mixture slurry is formed (for example, coated) on the current collector 21 and dried. The coating method is not particularly limited. Examples of the coating method include a knife coater method and a gravure coater method. The following coating steps are also performed by the same method. Next, the dried coating film and the positive electrode current collector are rolled by a press machine. Thereby, the positive electrode active material layer 22 is produced. Here, the thickness of the positive electrode active material layer 22 is not particularly limited as long as the positive electrode active material layer of the lithium ion secondary battery has a thickness. Next, the positive electrode current collector tab 110 is welded to the positive electrode current collector.

  The negative electrode 30 is produced as follows. First, a negative electrode mixture is prepared by mixing negative electrode active material, water-soluble polymer, and fluorine-based polymer fine particles in a desired ratio. Next, a negative electrode mixture slurry is formed by dispersing the negative electrode mixture in water. Next, the negative electrode mixture slurry is formed (for example, coated) on the current collector 31 and dried. Next, the dried coating film is rolled together with the current collector 31. Next, the dried coating film is heated at a temperature equal to or higher than the melting point of the fluoropolymer fine particles. Thereby, the negative electrode active material layer 32 is formed. Thereby, the negative electrode 30 is produced. Next, the negative electrode current collector tab 120 is welded to the negative electrode current collector 31.

  On the other hand, a fluorine polymer solution (for example, PVDF dissolved in N-methyl-2-pyrrolidone) is applied to both surfaces of the separator 40a. Next, the fluoropolymer is solidified by a method such as bathing the separator 40a, and then the fluoropolymer is dried. Thereby, the positive electrode side binder layer 40b and the negative electrode side binder layer 40c (or only the negative electrode side binder layer 40c) are formed in the separator 40a.

  Next, an electrode structure sheet is produced by sandwiching the separator on which the binder layer is formed between the positive electrode and the negative electrode. Next, the electrode structure is processed into a laminate shape. For example, after winding an electrode structure sheet, it is crushed to produce a wound element. Then, this winding element is inserted into the outer package 100. Here, the positive electrode current collector tab 110 and the negative electrode current collector tab 120 are projected outside the exterior body 100. Subsequently, parts other than the liquid injection part of the exterior body 100 are heat-welded. Next, the pores in the separator are impregnated with the electrolytic solution by injecting the electrolytic solution having the above composition into the outer package 100 from the liquid injection part. Next, the exterior body 100 is sealed and the exterior body 100 is heated. Thereby, the fine particles of the fluorine-based polymer and the negative electrode side binder layer 40c absorb the electrolyte and swell, and are fused to each other. Further, the positive electrode side binder layer 40 b also absorbs the electrolyte and swells, and is fused to the binder of the positive electrode active material layer 22. Thereafter, the outer package 100 is cooled. Thereby, each fusion | melting part solidifies and the coupling | bonding of the separator layer 40, the positive electrode active material layer 22, and the negative electrode active material layer 32 becomes strong. Through the above steps, a lithium ion secondary battery is manufactured.

[Production of positive electrode]
Next, (lithium cobaltate (LiCoO ₂ )) as the positive electrode active material, carbon black powder as the positive electrode conductive agent, and polyvinylidene fluoride as the positive electrode binder (binder), active material: conductive agent: A positive electrode mixture was prepared by dry mixing so that the mass ratio (weight ratio) of the binder was 96: 2: 2. Subsequently, the positive electrode mixture slurry was obtained by dispersing the positive electrode mixture in N-methyl-2-pyrrolidone as a dispersion medium.

  Next, this positive electrode mixture slurry was applied to both surfaces of a 13 μm thick aluminum foil as a positive electrode current collector, dried, and then rolled. Thereby, a positive electrode active material layer was formed on the current collector. That is, a positive electrode was produced. The packing density and thickness of the positive electrode active material layer were 3.95 g / cc and 140 μm at the portion where the positive electrode active material layer was formed on both sides of the current collector.

[Manufacture of negative electrode]
A mixture of carbon-based active material (artificial graphite) and silicon-based active material (SiO _x ) (x = 1.05) at a mass ratio (weight ratio) of 97: 3, CMC-Na (carboxymethylcellulose sodium), A negative electrode mixture was prepared by mixing fine particles of polyvinylidene fluoride (PVDF) at a mass ratio of 98: 1: 1. Subsequently, the negative electrode mixture slurry was obtained by dispersing the negative electrode mixture in water as a solvent.

  Next, this negative electrode mixture slurry was applied to both sides of a copper foil having a thickness of 8 μm as a negative electrode current collector, dried, and then rolled. Next, the negative electrode mixture after rolling was heated at 170 ° C., which is a temperature higher than the melting point of PVDF. As a result, the PVDF fine particles were fused to the surfaces of the carbon-based active material and the silicon-based active material. Through the above steps, a negative electrode active material layer was formed on the current collector. That is, a negative electrode was produced. The packing density and thickness of the negative electrode active material layer on the current collector were 1.75 g / cc and 137 μm at the portion where the negative electrode active material layer was formed on both sides of the current collector.

[Manufacture of separators]
Polyvinylidene fluoride (KF9300; manufactured by Kureha) was added to 5% by weight with respect to N-methyl-2-pyrrolidone, and stirred to completely dissolve. Next, it is applied on both sides of a 9 μm thick polyethylene microporous membrane (separator) using a dip coater, solidified in a water bath, and then dried to make polyvinylidene fluoride (PVDF) on both sides. A separator provided with a layer was obtained. The film thickness after drying was 12 μm.

[Manufacture of secondary batteries]
After superposing the positive electrode, the negative electrode, and the separator produced as described above, a flat wound battery element was produced by winding and crushing in the longitudinal direction. This battery element was inserted from the inside into an exterior material made of a laminate film having a thickness of 120 μm consisting of three layers of polypropylene / aluminum / nylon (nylon), and the electrode terminals were taken out of the exterior material by thermal fusion. Lithium hexafluorophosphate is dissolved at a rate of 1.2 mol / L in a solvent in which ethylene carbonate and diethyl carbonate are mixed in a mass ratio of ethylene carbonate: diethyl carbonate = 3: 7 in the exterior member containing the battery element. The electrolyte solution was injected, and the remaining one side of the exterior material was heat-sealed under reduced pressure and sealed under reduced pressure. By heating this between metal plates at 80 ° C. for 3 minutes, a lithium ion secondary battery having a size of 2 mm thickness × 30 mm width × 30 mm height was obtained.

[Examples 2 to 4]
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the negative electrode mixture composition was changed according to the composition shown in Table 1 to produce a negative electrode.

[Comparative Examples 1 to 4]
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the negative electrode mixture composition and the separator were changed according to the composition of Table 1 to produce a negative electrode.

[First discharge capacity]
About each secondary battery of an Example and a comparative example, it charged to 4.35V with a constant current of 56 mA at 25 degreeC, and then performed constant voltage charge to the charging current of 14 mA. Thereafter, constant current discharge was performed at 56 mA to a final voltage of 2.75V. The amount of electricity discharged at this time was defined as the initial discharge capacity.

[Battery thickness increase rate]
The battery thickness before charging and at the time of initial charging was measured by sandwiching between two parallel plates to which a load of 300 g was added. For each of the secondary batteries of Examples and Comparative Examples, the battery thickness increase rate was calculated by (battery thickness at the time of initial charge) / (battery thickness before charge) × 100. Here, the battery thickness is a dimension of the battery in the stacking direction of the electrodes.

[Capacity maintenance rate of 25 ° C charge / discharge test]
About each secondary battery of an Example and a comparative example, after measuring said first time discharge capacity, it charged to 280mA4.35V in a 25 degreeC thermostat, and performed constant voltage charge to electric current 14mA subsequently. Thereafter, constant current discharge was performed at 280 mA up to 2.75V. After repeating the above charge / discharge operation 100 times, the discharge capacity after the 25 ° C. charge / discharge test was measured by the same operation as the initial discharge capacity. The capacity retention rate was calculated by (discharge capacity after 25 ° C. charge / discharge test) / (initial discharge capacity). Table 1 compares the composition and evaluation results of each secondary battery.

[Bending test]
About each secondary battery of an Example and a comparative example, after measuring said initial discharge capacity, the buckling strength (buckling resistance) was measured using the Shimadzu Corporation tabletop precision universal testing machine AGS-X. Specifically, the nonaqueous electrolyte secondary battery was placed on a jig having a gap of 15 mm, and an indenter having a gap diameter of 2 mmφ and a width of 30 mm was arranged in parallel with the winding element. Then, the load applied when pushing downward with an indenter at 5 mm / min was measured, and the maximum value of the load was regarded as the buckling point of the nonaqueous electrolyte secondary battery, and was defined as the buckling strength. The graph shown in FIG. 3 shows an example of the correlation between the displacement of the indenter and the load (test force) applied to the indenter. According to this graph, the nonaqueous electrolyte secondary battery is buckled with a load of 3100 mN / mm. Therefore, this nonaqueous electrolyte secondary battery has a buckling strength of 3100 mN / mm.

  In Table 1, the PVDF layer means the binder layer described above. The PVDF layer “present” means that the PVDF layer is formed on both sides of the separator, and “no” means that the PVDF layer is not formed on any side of the separator. “←” means the same as the left column. SBR means styrene butadiene rubber.

  Therefore, in Example 1, the PVDF layer is formed on both surfaces of the separator, and the PVDF fine particles are included in the negative electrode active material layer. In Examples 2 to 5, the content of the PVDF fine particles is changed from that in Example 1.

  On the other hand, in Comparative Example 1, 1% by mass of styrene butadiene rubber is used in place of the PVDF fine particles as a binder for the negative electrode active material layer. In Comparative Example 2, a PVDF layer is not formed on the separator, and PVDF fine particles are not used. Therefore, Comparative Example 2 has the same configuration as that of a conventional lithium ion secondary battery. Although the comparative example 3 uses PVDF microparticles | fine-particles, the content is less than 1 mass%.

[Contrast]
Comparing Comparative Examples 1 to 3 and Example 1, Example 1 has a larger capacity retention rate and a smaller thickness increase rate than Comparative Examples 1 to 3. For this reason, in Example 1, the fluorine-based polymer fine particles firmly bind the negative electrode active materials to each other, thereby suppressing the expansion and shrinkage of the silicon-based active material and maintaining the connection between the negative electrode active materials. It is mentioned. Further, in Example 1, the buckling strength is greatly improved as compared with Comparative Examples 1 to 3. The reason for this is that the negative electrode active material layer and the separator layer are firmly bonded by the fusion of the fluorine-based polymer fine particles to the negative electrode binder layer.

  Moreover, according to Examples 2-5, if content of the fine particle of a fluorine-type polymer is in the range of 1-10 mass%, buckling strength and thickness increase rate will fall, so that content increases. I understand. That is, according to Examples 1 to 5, when the binder layer is formed and the content of the fluorine-based fine particles is 1 to 10% by mass, the buckling strength, the thickness increase rate, and the capacity retention rate are further increased. It turns out that it becomes favorable.

  As described above, according to the present embodiment, the negative electrode active material layer 32 includes a water-soluble polymer, a negative electrode active material including a silicon-based active material and a carbon-based active material, and a fluorine-based polymer, Fluorine-based polymer fine particles that bind to each other. The fine particles of the fluorine-based polymer can firmly bind the negative electrode active materials to each other, thereby suppressing the expansion and contraction of the silicon-based active material and maintaining the connection between the negative electrode active materials. Moreover, since the negative electrode active material layer 32 is produced by coating and drying an aqueous slurry on a current collector, it constitutes an aqueous negative electrode.

  Further, the fluorine polymer constituting the fine particles of the fluorine polymer may contain polyvinylidene fluoride. In this case, the negative electrode active materials are more firmly bound to each other.

  Further, the fluorine-based fine particles may be prepared by emulsion polymerization of the monomer constituting the first fluorine-based polymer. In this case, the fluorine-based fine particles having a very small particle diameter are easily produced. The

  Furthermore, the fine particles of the fluorine-based polymer may be prepared by pulverizing coarse particles synthesized by suspension polymerization of the monomer constituting the fluorine-based polymer. In this case, the particle size is very small. Fluorine-based fine particles are easily produced.

  In addition, the water-soluble polymer may include a cellulosic polymer, and in this case, the binding force between the negative electrode active materials is further improved.

  Further, the cellulose polymer may be a metal salt of carboxymethyl cellulose, and in this case, the binding force between the negative electrode active materials is further improved.

The silicon-based active material may contain a silicon oxide represented by a composition formula SiO _x (0.5 ≦ x ≦ 1.5). In this case, the discharge capacity of the lithium ion secondary battery 10 and the like The battery characteristics are improved.

  In addition, the lithium ion secondary battery 10 may include a negative electrode side binder layer 40c. In this case, the fluorine polymer fine particles distributed on the outermost surface of the negative electrode active material layer 32 are fused to the negative electrode side binder layer 40c. By attaching, the negative electrode side binder layer 40c, that is, the separator layer 40 and the negative electrode active material layer 32 are firmly bound. Therefore, the lithium ion secondary battery 10 can improve the binding force between the negative electrode 30 that is a water-based negative electrode and the separator layer 40. As a result, the buckling strength of the lithium ion secondary battery 10 is improved.

  Further, the fluorine polymer constituting the fine particles of the fluorine polymer and the fluorine polymer constituting the negative electrode side binder layer 40c may be of the same type. In this case, the negative electrode 30 and the separator layer 40 are further provided. Tightly bind.

  Furthermore, both the fluorine-based polymer fine particles and the negative electrode side binder layer 40c may be made of PVDF. In this case, the negative electrode 30 and the separator layer 40 are more firmly bound.

  Further, the lithium ion secondary battery 10 may include an exterior body 100 made of an aluminum laminate film. In this case, the rigidity of the outer package 100 is lowered, but the fluorine-based polymer fine particles can suppress the expansion and contraction of the silicon-based active material, and the negative electrode active material layer 32 and the separator layer 40 are firmly bonded. . Therefore, an increase in the thickness of the lithium ion secondary battery 10 is suppressed, and the buckling resistance is improved.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 20 Positive electrode 21 Current collector 22 Positive electrode active material layer 30 Negative electrode 31 Current collector 32 Negative electrode active material layer

Claims (12)

A water-soluble polymer,
A negative electrode active material comprising a silicon-based active material and a carbon-based active material;
A fluorine-containing polymer fine particle containing a first fluorine-based polymer and binding the negative electrode active materials to each other;
The negative electrode active material layer for a non-aqueous electrolyte secondary battery, wherein the fluorine-based polymer fine particles are included in the negative electrode active material layer in an amount of 1 to 10% by mass with respect to the total mass of the negative electrode active material layer. .   The negative electrode active material layer for a non-aqueous electrolyte secondary battery according to claim 1, wherein the first fluorine-based polymer includes polyvinylidene fluoride.   3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the fine particles of the fluorine-based polymer are prepared by emulsion polymerization of a monomer constituting the first fluorine-based polymer. Negative electrode active material layer.   The fine particle of the fluorine-based polymer is produced by pulverizing coarse particles synthesized by suspension polymerization of a monomer constituting the first fluorine-based polymer. Or the negative electrode active material layer for nonaqueous electrolyte secondary batteries of 2.   5. The negative electrode active material layer for a non-aqueous electrolyte secondary battery according to claim 1, wherein the water-soluble polymer includes a cellulose-based polymer.   6. The negative electrode active material layer for a non-aqueous electrolyte secondary battery according to claim 5, wherein the cellulosic polymer is a metal salt of carboxymethyl cellulose. 7. The silicon-based active material according to claim 1, wherein the silicon-based active material includes a silicon oxide represented by a composition formula SiO _x (0.5 ≦ x ≦ 1.5). A negative electrode active material layer for a non-aqueous electrolyte secondary battery. A non-aqueous electrolyte,
A separator;
The negative electrode active material layer according to any one of claims 1 to 7,
A second fluorine-based polymer that holds the non-aqueous electrolyte in a gel state, and a negative electrode-side binder layer formed on a surface facing the negative electrode active material layer of the front and rear surfaces of the separator. A non-aqueous electrolyte secondary battery characterized by the above.   9. The nonaqueous electrolyte secondary battery according to claim 8, wherein the fluorine polymer fine particles are fused to the outermost surface of the negative electrode active material layer.   10. The nonaqueous electrolyte secondary battery according to claim 8, wherein the first fluorine-based polymer and the second fluorine-based polymer are the same type of fluorine-based polymer. 11.   The nonaqueous electrolyte secondary battery according to claim 10, wherein the first fluorine-based polymer and the second fluorine-based polymer include polyvinylidene fluoride. The nonaqueous electrolyte secondary battery according to claim 8, comprising an exterior body made of an aluminum laminate film.