JP4990447B2 - POLYMER FILM FOR SOLID ELECTROLYTE AND METHOD FOR PRODUCING THE SAME, SOLID ELECTROLYTE FILM, AND LITHIUM ION SECONDARY BATTERY - Google Patents

Description

【００４９】
表１により、基材を線膨張率の大きいＰＥＴから線膨張率の小さい金属（アルミニウム）に代えることで、固体電解質フィルム作製の際の電解液への浸漬時に固体電解質用ポリマーフィルムに皺が発生するのを防止できることが明らかになった。さらに、この効果は固体電解質用ポリマーフィルムを薄くしても維持されることも確認できた。
【００５０】
次に、本発明に係る固体電解質用ポリマーフィルムを用いた電池の特性評価を行った。
【００５１】
（固体電解質用ポリマーフィルムの作製）
[実施例３、４、比較例２]
実施例３、実施例４、比較例２ではそれぞれ実施例１、実施例２、比較例１で得た固体電解質用ポリマーフィルムを用意し、下記のリチウムイオン二次電池を組み立てて、それぞれについて下記の試験を行った。
【００５２】
（正極シートの作製）
正極活物質としての、コバルト酸リチウム粒状物９０重量部、結着剤としてのポリビニリデンフルオライド５重量部、導電剤としての人造黒鉛５重量部およびＮ−メチルピロリドン７０重量部を混合してスラリーとした。このスラリーを正極集電体としての帯状アルミニウム箔（厚み２０μｍ）の両面上に塗布、乾燥し、次いで圧延処理（圧延温度２５℃、圧延率３０％）して、アルミニウム箔の片面あたりの活物質の付着量が２０ｍｇ／ｃｍ²の正極活物質組成物層（厚み７０μｍ）を有する正極シート（幅３０ｍｍ、長さ５０ｍｍ）を作製した。
【００５３】
（負極シートの作製）
黒鉛化カーボンファイバー９５重量部、ポリビニリデンフルオライド５重量部、およびＮ−メチルピロリドン６０重量部を混合してスラリーとした。このスラリーを負極集電体としての帯状銅箔（厚み１４μｍ）の両面に塗布、乾燥し、次いで圧延処理（圧延温度１００℃、圧延率２０％）して、銅箔の片面あたりの活物質の付着量が１０ｍｇ／ｃｍ²の負極活物質組成物層（厚み７５μｍ）を有する帯状負極（幅３２ｍｍ、長さ５２ｍｍ）を作製した。
【００５４】
（電池の組み立て）
前記の正極と負極の間にポリビニリデンフルオライド多孔質フィルムを介在させて、最外層に負極がくるように積層して、これをアルミラミネートフィルム内に電解液と共に密閉し、タブを取り出し、セルとした。なお、ここでのポリビニリデンフルオライド多孔質フィルムは、実施例３、４および比較例２で作製したポリビニリデンフルオライド多孔質フィルムであり、電解液とは、実施例１、２および比較例１で使用した電解液である。
【００５５】
（低温特性試験）
作製したリチウムイオン二次電池について室温で充電を行なった後、これを−２０℃の大気雰囲気中に６時間放置する。なお、充電は、１Ｃ（６００ｍＡ）定電流で電圧が４．２Ｖとなるまで電流を流した後、続いて全充電時間が２．５時間となるまで４．２Ｖ定電圧で電流を流して行なった。次に、この−２０℃の大気雰囲気中で１Ｃ（６００ｍＡｈ）／２．５Ｖカットオフ電圧で放電を行い、その時の放電容量（ｍＡｈ）を求める。また、室温（２０℃）でも同様の条件で充電と放電を行い、放電容量（ｍＡｈ）を求める。さらに、−２０℃下での放電容量を室温下での放電容量で割って放電容量変化率を求めた。
【００５６】
（サイクル特性試験）
１Ｃ（すなわち６００ｍＡｈの定電流）充放電を繰り返し、放電容量に対する放電容量維持率（％）が８０％以下になるサイクル数を測定した。
【００５７】
（レート特性試験）
作製したリチウムイオン二次電池について、２Ｃ充放電を室温（２０℃）下で行い、放電容量の全容量に対する割合を算出した。なお、２Ｃとは、リチウムイオン二次電池の放電容量（６００ｍＡｈ）に対する１２００ｍＡの定電流をいう。
【００５８】
以上の試験の結果を表２にまとめた。
【００５９】
【表２】

【００６０】
【発明の効果】
本発明に係る方法により、皺や歪みのない（あるいは非常に少ない）固体電解質フィルムを提供することができる。これにより、負極と固体電解質フィルムとの接触不均一に起因する電極表面での電解質の析出を防ぐことができ、サイクル寿命が長い電池を作製できる。また、本発明に係る固体電解質フィルムを用いて得られた電池は、レート特性、低温特性においても優れた効果を奏する。この効果は固体電解質フィルムが薄くても維持されるので、固体電解質フィルムの薄層化ひいては電池の小型化をも可能にする。
【図面の簡単な説明】
【図１】固体電解執拗ポリマーフィルムが塗工された基材の断面図である。
【符号の説明】
１ 基材
２ ポリマー[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a polymer film for solid electrolyte and a method for producing the same, a solid electrolyte film, and a lithium ion secondary battery.
[0002]
[Prior art]
  As a battery for an electronic device such as a portable phone or a personal computer, a solid electrolyte battery such as a lithium ion secondary battery that does not cause a so-called liquid leakage is used. The basic structure of a solid electrolyte battery is that a solid electrolyte having both functions of a separator and an electrolyte is interposed between a positive electrode and a negative electrode. In the case of batteries for electronic devices, from the viewpoint of miniaturization and weight reduction, those obtained by impregnating an organic compound polymer with an electrolyte solution to form a gel (these are hereinafter referred to as solid electrolyte films) are known. Yes. These solid electrolyte films are obtained by coating a polymer-containing solution in which the starting polymer is dissolved and dispersed in a solvent onto a plastic substrate such as polyethylene terephthalate (PET) or polypropylene (PP) that has solvent resistance and heat resistance. Then, after solidifying by heating and cooling, the polymer film for solid electrolyte is prepared by peeling from the plastic substrate, and then the solid electrolyte film is obtained by impregnating the film with an electrolytic solution in which a lithium salt is dissolved.
[0003]
  A problem of batteries using such a polymer film for solid electrolyte impregnated with an electrolyte solution as an electrolyte is that the electrolyte component is deposited on the negative electrode surface. For example, in a lithium ion secondary battery using a composite oxide of lithium and a transition metal such as carbon for the negative electrode and cobalt for the positive electrode, metallic lithium may be deposited on the negative electrode surface. When such a precipitate grows, it breaks through the solid electrolyte film and comes into contact with the positive electrode, causing an internal short circuit and shortening the cycle life of the battery. Therefore, it is desired to provide a battery in which the deposition of the electrolyte component is suppressed. Yes.
[0004]
  The reason for the deposition of such electrolyte components has not been known so far, and has been a major obstacle to the development of solid electrolyte batteries. As a result of detailed investigations on the shape and occurrence of precipitates, the present inventors have found for the first time that the non-uniform contact between the electrode and the electrolyte, which has been conventionally overlooked, is the cause of the precipitation of the electrolyte component. . Therefore, in order to prevent the deposition of the electrolyte component, it is necessary to realize uniform contact between the electrode and the electrolyte. For this reason, as a solid electrolyte film, a thing without a distortion and a wrinkle is strongly desired. However, when a solid electrolyte film is manufactured by the above-described method, it is a fact that distortion and wrinkles are likely to occur when the solid electrolyte polymer film peeled off from the substrate is immersed in the electrolyte solution.
[0005]
[Problems to be solved by the invention]
  The present invention provides a method for producing a solid electrolyte film free from distortion or wrinkles (or very little), and using such a solid electrolyte film as a battery electrolyte allows lithium ions to be prevented from depositing electrolyte components. An object is to provide a secondary battery.
[0006]
[Means for Solving the Problems]
  As a result of conducting research on the above-mentioned problems while paying attention to the residual stress of the polymer film for solid electrolyte derived from the expansion and contraction due to heat of the base material when producing the polymer film for solid electrolyte, the following results were obtained. The present invention having features has been completed.
[0007]
(1) Linear expansion coefficient is 5 × 10^-5K^-1Coating a polymer-containing liquid on a substrate made of the following materialsA step of evaporating the solvent of the polymer-containing liquid by heating and then solidifying the polymer by cooling;After the polymer is solidifiedThe polymerPeeling processWhenThe manufacturing method of the polymer film for solid electrolytes containing this.
(2) Linear expansion coefficient is 5 × 10−^FiveK^-1The method for producing a polymer film for a solid electrolyte according to the above (1), wherein the following material is a metal.
(3) The method for producing a polymer film for a solid electrolyte according to (2), wherein the metal is Al, Ni, Fe, or an alloy containing at least one of these metals.
(4) The method for producing a polymer film for a solid electrolyte according to any one of the above (1) to (3), wherein the polymer is a fluoropolymer having vinylidene fluoride as a main unit.
(5) Polymer density is 0.6 to 1.3 g / cm ³ The method for producing a polymer film for a solid electrolyte according to any one of (1) to (4), wherein
(6) (1) to (5The polymer film for solid electrolytes manufactured by the manufacturing method in any one of).
(7) (6A solid electrolyte film obtained by impregnating a polymer film for solid electrolyte described in (1) with an electrolyte solution in which a lithium salt is dissolved.
(8) (7) A lithium ion secondary battery containing the solid electrolyte film described above.
[0008]
  The lithium ion secondary battery manufactured using the solid electrolyte film obtained from the polymer film for solid electrolyte that suppresses the generation of soot as described above can not only suppress the original deposition of lithium, but also will be described later. In addition, high-rate charge / discharge is possible, and the effect of improving the characteristics at low temperatures was obtained.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
  The manufacturing method of the polymer film for solid electrolytes concerning this invention is demonstrated with reference to FIG. 1 (Schematic diagram of the polymer film for solid electrolytes coated on the base material). The greatest feature of the present invention is that the material of the base material 1 is conventional PET (linear expansion coefficient is 6.5 × 10^-FiveK^-1), PP (linear expansion coefficient is 7-10 × 10^-FiveK^-1), Etc., instead of a material with a large linear expansion coefficient.
[0010]
  Here, the linear expansion coefficient refers to β in the following formula 1, and is a value measured by the method described in JIS K7197.
[0011]
  β = (1 / l₀) ・ (Dl / dθ)
However, the length at Celsius temperatures 0 ° C and θ is 1₀, L.
[0012]
  The linear expansion coefficient of the base material 1 used in the present invention is 5 × 10.^-FiveK^-1Or less, preferably 2.5 × 10^-FiveK^-1It is as follows. Examples of the material of the substrate 1 include metal (whether it is a simple substance or an alloy). These may be pure, may contain impurities, or may be a mixture of a plurality of materials, and only need to have the above-described linear expansion coefficient when used as a base material. From the viewpoint of solvent resistance and the like, it is preferably a metal (including an alloy), more preferably, the metal is any one of Al, Ni, Fe, or an alloy including any of these, preferably aluminum (linear expansion) The rate is 2.4x10^-FiveK^-1), Stainless steel (SUS304, linear expansion coefficient is 1.2 × 10^-FiveK^-1), Nickel (linear expansion coefficient is 1.3 × 10^-FiveK^-1) Is used.
[0013]
  The shape of the substrate 1 is not particularly limited as long as it conforms to the method for producing a polymer film for solid electrolyte described later, but in the present invention for producing a polymer film for solid electrolyte by coating, solidifying and peeling of the polymer, 1 is preferably a long thin plate, for example, having a width of 1 to 5 cm, a length of 1 to 100 cm, and a thickness of 10 to 20 μm.
[0014]
  The polymer film for a solid electrolyte according to the present invention is obtained by applying a polymer-containing liquid as a raw material for a polymer film for a solid electrolyte on the base material 1 made of a material having a small linear expansion as described above, and heating the solvent. The polymer 2 is solidified by evaporating and then cooled, and then the solidified polymer 2 is peeled off (FIG. 1). By using a material having a small linear expansion coefficient as the material of the base material 1, the expansion and contraction of the base material 1 during heating and cooling are reduced, and the residual stress in the polymer 2 is also reduced. For this reason, distortion and wrinkles associated with the release of stress that occur when the electrolyte solution is impregnated with a polymer solidified on a base material made of a material having a large linear expansion coefficient such as PET or PP are not affected by the method according to the present invention. It becomes difficult to occur.
[0015]
  The material of the polymer 2 which is the main component of the polymer film for solid electrolyte used in the present invention is not particularly limited as long as it can be used for the battery electrolyte. Here, what can be used for the electrolyte of the battery means that the polymer itself does not have ionic conductivity, but the ionic conductivity is obtained by permeation of the electrolyte solution into the polymer.
[0016]
  The material of such a polymer 2 is preferably one having a high affinity for the electrolyte solution, such as polystyrene, polybutadiene and copolymers thereof, polyvinylidene fluoride, polyacrylonitrile, polyvinyl pyrrolidone, polyvinylidene carbonate, and the like. .
[0017]
  In the present invention, although the polymer itself does not have ionic conductivity, a polymer that acquires ionic conductivity by permeation of the electrolyte solution into the polymer, particularly a fluoropolymer having vinylidene fluoride as a main unit is particularly preferably used. The fluoropolymer having vinylidene fluoride as a main unit means a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride and other vinyl monomers having a fluorine atom. Even if mixed, it can be used. Examples of other vinyl monomers having a fluorine atom include hexafluoropropylene, chlorotrifluoroethylene, and tetrafluoroethylene. The form of the copolymer may be either random or block. In the case of a copolymer, the proportion of vinylidene fluoride (unit) is preferably 70 mol% or more, particularly preferably 75 mol% or more.
[0018]
  In addition, the fluoropolymer having vinylidene fluoride as a main unit includes a carboxyl group (—COOH) and a sulfonic acid group (—SOOH).₂OH), carboxylate group (—COOR), carbamoyl group (—CONH)₂) Or phosphate group (-PO (OH)₂Or a vinyl monomer polymer having a functional group composed of, for example, a substituent R in the carboxylic acid ester group (—COOR) having 1 to 1 carbon atoms such as a methyl group, an ethyl group, and a butyl group. 4 lower alkyl groups are preferred.). It is preferable to use a fluoropolymer in the form of a polymer grafted with a polymer containing such a functional group because the adhesion of the polymer film to the positive electrode or negative electrode of the battery is improved.
[0019]
  As the vinyl monomer having a functional group, a monomer composed of a compound having 4 or less carbon atoms in the portion excluding the functional group is suitable. As a carboxyl group-containing monomer, one having one carboxyl group such as acrylic acid, methacrylic acid, crotonic acid, vinyl acetic acid, and allyl acetic acid, and one having two or more carboxyl groups such as itaconic acid and maleic acid can be used. It is. As the sulfonic acid group-containing monomer, styrene sulfonic acid, vinyl sulfonic acid and the like are suitable. As the carboxylic acid ester group-containing monomer, methyl acrylate, butyl acrylate and the like are suitable. As the carbamoyl group-containing monomer, acrylamide or the like is preferable. As the phosphate group-containing monomer, triphenyl phosphate, tricresyl phosphate and the like are suitable. The degree of grafting is preferably from 2 mol% to 20 mol%, more preferably from 3 mol% to 12 mol%, particularly preferably from 5 mol% to 10 mol% of the final weight.
[0020]
  The polymer used in the present invention is preferably porous from the viewpoint of electrolyte retention, and the density is 0.60 to 1.3 g / cm.^Three, Preferably 0.70 to 0.80 g / cm^ThreeIt is good to do. The density of the polymer is 0.60 g / cm^ThreeIf it is less than the above, there is a concern that it becomes difficult to handle when assembling the battery due to a decrease in mechanical strength, and the density is 1.30 g / cm.^ThreeThis is because if it is larger, it is difficult to obtain a sufficient rate characteristic and low temperature characteristic improvement effect.
[0021]
  Production of a polymer film for a solid electrolyte made of a polymer of such a material comprises a step of coating a polymer-containing liquid on the above-mentioned substrate, solidifying it by heating and cooling, and then peeling it from the substrate. Hereinafter, each step will be described.
[0022]
  The solvent for preparing the polymer-containing liquid is not particularly limited as long as it dissolves and disperses the polymer, and may be a single substance or a mixture of two or more substances. For example, octanol, 1-pentanol, 3-methyl-1-butanol, 2-methyl-1-butanol, 2-pentanol, 3-pentanol, 2-methyl-2-butanol, 3-methyl-2-butanol Etc.) and a non-foaming solvent (for example, tetrahydrofuran (THF), dimethylacetamide, dimethylformamide, etc.) are preferably combined. This is because the above-mentioned pores are formed in the polymer that solidifies upon evaporation of the solvent. Among these combinations, a mixture of dimethylacetamide and octanol is preferably used.
[0023]
  The polymer-containing liquid used in the present invention is obtained by dissolving or dispersing a polymer raw material in the above-described solvent. What is necessary is just to adjust the density | concentration of the polymer in a containing liquid so that it may become a viscosity suitable for the construction method to apply. The dissolution in preparing the polymer-containing liquid can take any method such as stirring. When mixing, bubbles are mixed, or dissolution and dispersion are insufficient and the concentration of the polymer raw material is non-uniform, which causes variation in the thickness of the polymer film for solid electrolyte. It is desirable to leave it for about an hour.
[0024]
  In the present invention, this polymer-containing liquid is applied onto the above-described base material having a small linear expansion coefficient. The coating method is not particularly limited, but for example, direct coating using a device such as a blade knife type coating machine, indirect coating using a device such as a reverse roll, etc., the thickness accuracy at the time of coating is high. It is preferable at a high point. As for the thickness of the coating, it is desirable that the thickness of the polymer film for solid electrolyte after drying is 1 μm to 50 μm, more preferably 10 μm to 30 μm.
[0025]
  After coating the polymer-containing liquid on the substrate, the polymer-containing liquid is dried by heating in a batch-type or continuous hot-air oven. The temperature and heating time at this time may be determined so that the polymer has a desired density according to the solvent. As an example of drying conditions, a polymer-containing liquid consisting of 85 parts by weight of dimethylacetamide, 15 parts by weight of octanol, and 15 parts by weight of polyvinylidene fluoride is passed through a drying furnace at 70 to 100 ° C. and a furnace length of 10 m at 0.1 m / mim. However, the present invention is not limited to this condition.
[0026]
  As described above, there is a concern about the thermal expansion of the substrate coated with the polymer-containing liquid during the heating for drying. However, since the manufacturing method according to the present invention uses a base material made of a material having a low coefficient of linear expansion, the thermal expansion of the base material becomes extremely small. For this reason, the stress with respect to a polymer also becomes very small, and generation | occurrence | production of the distortion and wrinkles of a solid electrolyte film can be prevented.
[0027]
  After evaporation of the solvent, the solution is cooled to near room temperature by cooling or the like, and then the solidified polymer film for solid electrolyte is peeled off from the substrate. The peeling method is arbitrary as long as it does not cause scratches or tears on the polymer film for solid electrolyte. Examples of the peeling method include mechanical peeling with a hoisting machine and immersion in a liquid for peeling, but the invention is not limited to these. Moreover, although the polymer film for solid electrolytes is cut into a predetermined size and used, the polymer film for solid electrolytes may be cut after peeling or may be cut together with the substrate before peeling.
[0028]
  Next, the impregnation of the solid electrolyte polymer film into the electrolyte solution will be described in the order of the electrolyte constituting the electrolyte solution, the solvent, and the solid electrolyte polymer film impregnation method.
[0029]
  Any electrolyte known in the art can be used as the electrolyte. When the battery is a lithium ion secondary battery, LiClO_Four, LiBF_Four, LiPF₆, LiAsF₆LiAlCl_Four, Li (CF_ThreeSO₂)₂One or more selected from N and the like are preferably used. As the solvent, the solvent dissolves the electrolyte and penetrates into the polymer film for the solid electrolyte, and includes ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, sulfolane, and γ-butyrolactone. 1,2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane, 2-methyltetrahydrofuran, diethyl ether and the like, and one or a mixture of two or more of these is used. The electrolyte concentration in the electrolyte solution is preferably 0.1 mol / l to 2 mol / l, more preferably 0.5 mol / l to 1.5 mol / l. If the electrolyte concentration is less than 0.1 mol / l, the battery capacity cannot be sufficiently obtained due to the decrease in ionic conductivity, and the high rate characteristics are remarkably deteriorated. On the other hand, when the concentration exceeds 2 mol / l, there is a problem in that the viscosity is remarkably increased and the high rate characteristics and the low temperature characteristics are deteriorated.
[0030]
  In addition, when using a mixed solvent for a compatible solvent, in particular, at least one selected from diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) is included, and ethylene carbonate (EC), propylene carbonate (PC), and A mixture containing dimethyl carbonate (DMC) is preferred. The mixing ratio of each component constituting the mixture is preferably 25% by volume to 50% by volume and at least 30% by volume to 35% by volume in at least one selected from diethyl carbonate and ethyl methyl carbonate. More preferred. In ethylene carbonate, the mixing ratio is preferably 4% by volume to 20% by volume, and more preferably 6% by volume to 18% by volume. In propylene carbonate, the mixing ratio is preferably 3% by volume to 17% by volume, and more preferably 5% by volume to 15% by volume. Moreover, in dimethyl carbonate, it is preferable that a mixing ratio exceeds 40 volume% and is 60 volume% or less, and it is more preferable that it is 45 volume%-55 volume%.
[0031]
  In at least one selected from diethyl carbonate and ethyl methyl carbonate, if the mixing ratio is less than 25% by volume, the freezing point of the solution in which the salt (electrolyte) is dissolved increases, and particularly at a low temperature of −20 ° C. or lower. In addition, the internal resistance of the battery increases, and the charge / discharge cycle characteristics and the low temperature characteristics tend to decrease. On the other hand, when the mixing ratio exceeds 50% by volume, the viscosity of the solution in which the salt (electrolyte) is dissolved increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease.
[0032]
  In ethylene carbonate, when the mixing ratio is less than 4% by volume, a stable film is hardly formed on the surface of the negative electrode plate, and the cycle characteristics tend to deteriorate. When the mixing ratio exceeds 20% by volume, salt (electrolyte) ) Is increased, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease.
[0033]
  In propylene carbonate, if the mixing ratio is less than 3% by volume, the effect of suppressing an increase in impedance associated with the charge / discharge cycle is reduced, and the cycle characteristics tend to be reduced. If the mixing ratio exceeds 17% by volume, The viscosity of the solution in which the salt (electrolyte) is dissolved increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease.
[0034]
  In dimethyl carbonate, if the mixing ratio is 40% by volume or less, the viscosity of the solution in which the salt (electrolyte) is dissolved increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease, When the mixing ratio exceeds 60% by volume, the freezing point of the solution in which the salt (electrolyte) is dissolved rises, and the internal resistance of the battery increases especially at a low temperature of −20 ° C. or lower, and the cycle characteristics and low temperature characteristics are improved. It tends to decrease.
[0035]
  In addition to the above solvents, tetraethylene glycol dimethyl ether, N-methyl-pyrrolidone (1-methyl-2-pyrrolidone), ethylene glycol for the purpose of preventing crystallization of the solution at the battery operating temperature (especially when used at low temperatures). It is preferable to use a plasticizer such as dimethyl ether or diethylene glycol dimethyl ether. By adding the plasticizer, crystallization of the electrolyte from the electrolyte solution infiltrated into the polymer is further prevented, and the present invention can be more effectively realized.
[0036]
  The method for impregnating the electrolyte solution with the polymer film for solid electrolyte is not particularly limited. The battery described later may be assembled after impregnating the polymer film for solid electrolyte in the electrolyte solution. However, from the viewpoint of ease of processing, considering that the polymer film gels after impregnation, It is preferable to assemble the battery using the polymer film for solid electrolyte and then impregnate the electrolyte solution.
[0037]
  The solid electrolyte film of the present invention can be applied to various conventionally known lithium ion secondary batteries. For example, between the positive electrode (sheet) and the negative electrode (sheet) formed into a belt-like body, a solid electrolyte film having a belt state of approximately the same size as these is interposed, and these three members are wound in a spiral shape along the length direction. A wound body is formed by turning, and the wound body is accommodated in an exterior material (a so-called wound type), or between a rectangular positive electrode (sheet) and a negative electrode (sheet). Examples include a form (so-called laminated type) in which a laminated structure is formed by repeating one or more units (cells) sandwiched between rectangular solid electrolyte films, and the laminated structure is accommodated in an exterior material. It is done. Moreover, as one variation of the laminated type, a laminated structure formed by laminating a positive electrode bagging body in which a rectangular positive electrode (sheet) is surrounded by a bag-shaped solid electrolyte film and a rectangular negative electrode (sheet) is laminated. The form (what is called a bagging type) accommodated in the exterior material can be mentioned. In such a packaged type, since the positive electrode sheet is completely surrounded by the bag-shaped solid electrolyte film, there is no physical contact between the positive electrode (sheet) and the negative electrode (sheet), and the positive electrode sheet and the negative electrode sheet are completely insulated. There is an advantage that you can. In addition, as a method of forming the solid electrolyte film into a bag shape, various methods that have been widely performed in the art can be given. For example, a method of heating the outer peripheral portion of a solid electrolyte film bent at an appropriate size using an apparatus such as a heat press, a heat sealer, or a vacuum hot press to fix the outer peripheral portion, and the outer peripheral portion using an apparatus such as a press roll. And a method of pressurizing the outer peripheral portion while heating using a hot press, a hot roll press, a vacuum press, or the like.
[0038]
  In the lithium ion secondary battery using the solid electrolyte film of the present invention, a known positive electrode (sheet) and negative electrode (sheet) for a lithium ion secondary battery are used as the positive electrode (sheet) and the negative electrode (sheet), respectively. can do. As a preferable example, for example, a positive electrode active material composition or a negative electrode active material composition obtained by mixing a positive electrode active material or a negative electrode active material with a conductive material or a binder is applied onto a current collector, and then dried and rolled. Thus, a positive electrode active material layer or a negative electrode active material layer is formed.
[0039]
  As the positive electrode active material, Li-Co-based composite oxide, Li-Mn-based composite oxide, Li-Ni-based composite oxide, etc. can be used. Among them, it is chemically stable, easy to handle, and high Lithium cobalt oxide (LiCoO₂) Is preferably used. The conductive material may be artificial or natural graphite, or a granular carbon material such as carbon black such as ketjen black, acetylene black, oil furnace black, or extrudable furnace black (granular means scaly, spherical, pseudo A spherical shape, a lump shape, a whisker shape, and the like are included and are not particularly limited). Moreover, as a binder in a positive electrode (sheet), polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, an ethylene-propylene-diene polymer, or the like can be used. In the positive electrode active material composition, 3 to 15 parts by weight of the conductive material and 1 to 10 parts by weight of the binder are blended with respect to 100 parts by weight of the total amount of the positive electrode active material, the conductive material and the binder. It is preferable.
[0040]
  As the current collector, a foil or a perforated foil formed of a conductive metal can be used, and the thickness may be about 5 to 100 μm. As the material of the positive electrode current collector, copper, nickel, silver, stainless steel, or the like is used.
[0041]
  The positive electrode (sheet) and the negative electrode (sheet) can be produced according to a known method. For example, the positive electrode (sheet) is obtained by mixing and processing a positive electrode active material, a conductive material and a binder, dispersing in an organic solvent such as N-methylpyrrolidone, and applying and drying on a positive electrode current collector. It can be obtained by pressurizing and cutting into an appropriate shape.
[0042]
  In the present invention, as a battery exterior material, a metal can such as a cylindrical can, a rectangular tube can, a button can, or a sheet-shaped exterior material such as a laminate film is used. The laminate film is preferably one in which a thermoplastic resin laminate layer such as polyester or polypropylene is formed on at least one surface of a metal foil such as copper or aluminum. Examples of the metal foil include aluminum, copper, and iron. Examples of the resin include thermoplastic resins such as polyester and polypropylene. If it has such a thermoplastic resin layer, it can be sealed simply by sheathing it on the wound body or laminated structure and thermally welding the periphery thereof, and the battery can be manufactured easily. The thickness of the metal foil is preferably 10 to 100 μm, more preferably 20 to 70 μm, the thickness of the resin layer is preferably 5 to 100 μm, more preferably 10 to 50 μm, and the total thickness is preferably It is 20-200 micrometers, More preferably, it is 40-150 micrometers.
[0043]
  In the battery according to the present invention, as various constituent members not described above such as a battery can lid, a safety structure, and an electrode terminal (lead terminal in a sheet battery), existing ones and those developed in the future are used. be able to.
[0044]
【Example】
  [Example 1]
  30 g of polyvinylidene fluoride was dissolved in 170 g of dimethylacetamide as a raw material for the polymer film for solid electrolyte. To this solution, 30 g of octanol was gradually mixed over 1 hour and allowed to stand for 12 hours or more. An aluminum foil (rolled aluminum foil, thickness 20 μm) was prepared as a base material for coating. The solution was applied to the substrate by direct coating using a doctor blade, and the solvent was volatilized by passing through a drying oven (120 ° C., oven length 10 m) at 0.1 m / mim. By this operation, the density is 0.8 g / cm.^ThreeThe following polymer film for solid electrolyte was obtained. By adjusting the distance between the blades according to the viscosity of the solution, a polymer film for solid electrolyte of 20 μm and a polymer film for solid electrolyte of 30 μm were obtained.
[0045]
  The polymer film for solid electrolyte was cut into 10 cm × 10 cm together with the substrate, and after cutting, the substrate was impregnated with dimethoxyethane and peeled off. Prepare a tray (15 cmW x 15 cmD x 5 cmH) containing 100 cc of an electrolytic solution (solution of LiPF6 dissolved in a 1: 1 (volume ratio) mixed solvent of ethylene carbonate and ethylmethyl carbonate at a concentration of 1.0 mol / l). The polymer film for solid electrolyte peeled from the substrate was immersed. At this time, it was investigated whether or not wrinkles occurred. In the investigation, the presence or absence of shrinkage of the polymer film for solid electrolyte due to the occurrence of wrinkles was visually determined.
[0046]
  [Example 2]
  A polymer for solid electrolyte derived from generation of wrinkles in the same manner as in Example 1 except that an aluminum foil having a thickness of 14 μm was used as a base material for coating a polymer-containing liquid as a raw material for the polymer film for solid electrolyte The film was examined for shrinkage.
[0047]
  [Comparative Example 1]
  For solid electrolyte derived from generation of wrinkles by the same operation as in Example 1 except that a polyethylene terephthalate film having a thickness of 50 μm was used as a base material for coating a polymer-containing liquid as a raw material for the polymer film for solid electrolyte The presence or absence of shrinkage of the polymer film was investigated. The results of Example 1, Example 2, and Comparative Example are summarized in Table 1.
[0048]
[Table 1]

[0049]
  According to Table 1, by changing the base material from PET having a high coefficient of linear expansion to a metal (aluminum) having a low coefficient of linear expansion, wrinkles are generated in the polymer film for the solid electrolyte when immersed in the electrolyte during the production of the solid electrolyte film It has become clear that this can be prevented. Furthermore, it was confirmed that this effect was maintained even when the polymer film for solid electrolyte was thinned.
[0050]
  Next, the characteristics of the battery using the polymer film for solid electrolyte according to the present invention were evaluated.
[0051]
  (Preparation of polymer film for solid electrolyte)
[Examples 3 and 4, Comparative Example 2]
  In Example 3, Example 4, and Comparative Example 2, the polymer films for solid electrolyte obtained in Example 1, Example 2, and Comparative Example 1 were prepared, and the following lithium ion secondary batteries were assembled. The test was conducted.
[0052]
  (Preparation of positive electrode sheet)
  A slurry obtained by mixing 90 parts by weight of lithium cobaltate particulates as a positive electrode active material, 5 parts by weight of polyvinylidene fluoride as a binder, 5 parts by weight of artificial graphite as a conductive agent, and 70 parts by weight of N-methylpyrrolidone It was. The slurry is applied on both sides of a strip-shaped aluminum foil (thickness 20 μm) as a positive electrode current collector, dried, and then rolled (rolling temperature 25 ° C., rolling rate 30%) to obtain an active material per one side of the aluminum foil. Of 20mg / cm²A positive electrode sheet (width 30 mm, length 50 mm) having a positive electrode active material composition layer (thickness 70 μm) was prepared.
[0053]
  (Preparation of negative electrode sheet)
  A slurry was prepared by mixing 95 parts by weight of graphitized carbon fiber, 5 parts by weight of polyvinylidene fluoride, and 60 parts by weight of N-methylpyrrolidone. This slurry was applied to both sides of a strip-like copper foil (thickness 14 μm) as a negative electrode current collector, dried, and then subjected to a rolling treatment (rolling temperature 100 ° C., rolling rate 20%) to obtain an active material per one side of the copper foil. Adhering amount is 10mg / cm²A negative electrode active material composition layer (thickness 75 μm) having a strip-like negative electrode (width 32 mm, length 52 mm) was prepared.
[0054]
  (Battery assembly)
  A polyvinylidene fluoride porous film is interposed between the positive electrode and the negative electrode, laminated so that the negative electrode comes to the outermost layer, this is sealed together with the electrolyte in the aluminum laminate film, the tab is taken out, and the cell It was. In addition, the polyvinylidene fluoride porous film here is the polyvinylidene fluoride porous film produced in Examples 3 and 4 and Comparative Example 2. The electrolytic solutions are Examples 1 and 2 and Comparative Example 1. It is the electrolyte solution used in 1.
[0055]
  (Low temperature characteristic test)
  The prepared lithium ion secondary battery is charged at room temperature, and then left in an atmosphere of −20 ° C. for 6 hours. Charging is performed by flowing current at a constant current of 1 C (600 mA) until the voltage reaches 4.2 V, and then flowing current at a constant voltage of 4.2 V until the total charging time reaches 2.5 hours. It was. Next, discharging is performed at 1C (600 mAh) /2.5 V cut-off voltage in the air atmosphere at −20 ° C., and the discharge capacity (mAh) at that time is obtained. Further, charging and discharging are performed under the same conditions at room temperature (20 ° C.), and the discharge capacity (mAh) is obtained. Furthermore, the discharge capacity change rate was calculated by dividing the discharge capacity at −20 ° C. by the discharge capacity at room temperature.
[0056]
  (Cycle characteristic test)
  1C (ie, a constant current of 600 mAh) charge / discharge was repeated, and the number of cycles at which the discharge capacity retention rate (%) with respect to the discharge capacity was 80% or less was measured.
[0057]
  (Rate characteristics test)
  About the produced lithium ion secondary battery, 2C charging / discharging was performed at room temperature (20 degreeC), and the ratio with respect to the total capacity | capacitance of discharge capacity was computed. 2C means a constant current of 1200 mA with respect to the discharge capacity (600 mAh) of the lithium ion secondary battery.
[0058]
  The results of the above tests are summarized in Table 2.
[0059]
[Table 2]

[0060]
【The invention's effect】
  The method according to the present invention can provide a solid electrolyte film free from wrinkles and distortion (or very little). Thereby, precipitation of the electrolyte on the electrode surface resulting from contact nonuniformity of a negative electrode and a solid electrolyte film can be prevented, and a battery with a long cycle life can be produced. In addition, the battery obtained using the solid electrolyte film according to the present invention exhibits excellent effects in rate characteristics and low temperature characteristics. Since this effect is maintained even if the solid electrolyte film is thin, it is possible to reduce the thickness of the solid electrolyte film and hence the size of the battery.
[Brief description of the drawings]
1 is a cross-sectional view of a substrate coated with a solid electrolytic polymer film.
[Explanation of symbols]
  1 Base material
  2 Polymer

Claims (8)

A step of applying a polymer-containing liquid on a base material made of a material having a linear expansion coefficient of 5 × 10 ⁻⁵ K ⁻¹ or less ;
A step of evaporating the solvent of the polymer-containing liquid by heating and then solidifying the polymer by cooling;
And a step of peeling the polymer from the substrate after solidification of the polymer, method for producing a polymer film for solid electrolyte. The method for producing a polymer film for a solid electrolyte according to claim 1 , wherein the material having a linear expansion coefficient of 5 × 10 ⁻⁵ K ⁻¹ or less is a metal. The method for producing a polymer film for a solid electrolyte according to claim 2 , wherein the metal is Al, Ni, Fe, or an alloy containing at least one of these metals. The method for producing a polymer film for a solid electrolyte according to any one of claims 1 to 3, wherein the polymer is a fluoropolymer having vinylidene fluoride as a main unit. The density of the polymer is 0.6 to 1.3 g / cm ³ The method for producing a polymer film for a solid electrolyte according to any one of claims 1 to 4, wherein the polymer film is for a solid electrolyte. Polymer films for solid electrolytes produced by the production method according to any one of claims 1 to 5. Wherein the polymer film for solid electrolyte, the solid electrolyte film obtained by impregnating an electrolyte lithium salt is dissolved according to claim 6. Lithium ion secondary batteries containing the solid electrolyte film according to claim 7.