JP4074418B2 - Thin film type lithium secondary battery - Google Patents

Description

【００５３】
【発明の効果】
本発明によれば、ゲル状電解質を使用した電池要素を可撓性を有するケースに収納したリチウム二次電池に適したスペーサーを使用することによって、容量やレート特性等の電池特性に優れ、さらには安全性が高く、エネルギー密度が高いリチウム二次電池を提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery, and more particularly to an improvement in a spacer of a lithium secondary battery in which a battery element using a gel electrolyte is enclosed in a flexible case.
[0002]
[Prior art]
Since the lithium secondary battery uses a non-aqueous electrolyte solution in addition to the positive electrode and the negative electrode, it is necessary to prevent moisture from entering and leaking. Therefore, these battery elements are normally enclosed in a case having rigidity such as a metal can. However, when a case having such rigidity is used, there is a problem in that there is a limit in improving the energy density of the lithium secondary battery, and the degree of freedom in shape is small.
[0003]
In recent years, in order to solve such problems, attempts have been made to use a gel electrolyte in which a non-aqueous electrolyte solution is held by a polymer and non-fluidized. In such a lithium secondary battery, since the electrolyte does not have fluidity, it is much easier to hold the electrolyte than a lithium secondary battery using a conventional electrolytic solution, and as a result, the battery element is laminated. It can also be stored in a flexible case such as a film. Therefore, the energy density and the degree of freedom of battery shape can be improved.
[0004]
[Problems to be solved by the invention]
In the case of a thin-film lithium secondary battery, there is a problem that a short-circuiting phenomenon (mild short) is likely to occur although the risk of short-circuiting is small because the electrolyte does not have fluidity. In particular, when the battery element is housed in a flexible case, the lithium secondary battery has a large degree of freedom in shape, so it is often used by being deformed into various shapes depending on its use. In such applications, the above phenomenon appears remarkably. Therefore, a method for improving the mechanical strength of the lithium secondary battery, in particular, the mechanical strength of the electrolyte layer is also conceivable. This causes a problem of impairing battery performance.
[0005]
In addition, since the electrolyte is non-fluid, the ion conductivity tends to be inevitably insufficient as compared with a lithium secondary battery using a conventional liquid electrolyte.
On the other hand, in a general battery, a spacer made of a polyolefin material made of a porous film is provided between a positive electrode and a negative electrode. However, according to the study of the present inventor, in the case of a conventional liquid lithium secondary battery and a lithium secondary battery housed in a flexible case using a gel electrolyte, the required characteristics of the spacer to be used I found that there was a difference.
[0006]
First, in a lithium secondary battery using a conventional liquid electrolyte, a spacer having a somewhat thick film thickness and a low porosity is provided in order to provide the spacer with a shutdown performance (function to prevent a short circuit due to a gap being filled at a high temperature). I used it. However, in the case of the above-described gel electrolyte, the shutdown function is not essentially important because the gel electrolyte itself also serves as a spacer.
[0007]
In addition, in a lithium secondary battery using a conventional liquid electrolyte, the electrolyte solution has sufficient ionic conductivity, so there is no need to consider the influence on the ionic conduction of the spacer. When used, since the ionic conductivity of itself tends to be low, it is necessary to consider the influence of the spacer.
In particular, in the case of the lithium secondary battery using the flexible case described above, the battery is not only leaked when the solvent of the electrolyte solution volatilizes, but the case is deformed, as compared with the conventional metal case. There is a problem that the entire external shape is easily damaged. That is, in such a case, it is considered better to avoid using a solvent having a low boiling point (high volatility) as a solvent for the electrolytic solution. Therefore, a non-aqueous solvent having a high boiling point is used. In this case, however, the gel membrane tends to be insufficiently filled with the porous membrane, resulting in insufficient rate characteristics. Therefore, the selection of the spacer in such a case is also an important factor.
[0008]
Furthermore, in the case of a gel electrolyte, since it has self-supporting properties, the strength of the spacer necessary for preventing a short circuit is also different from that of a conventional liquid lithium secondary battery. A perspective is needed.
In short, the spacer of the lithium secondary battery when using a flexible case is sufficient even though it is necessary to select the spacer based on a completely different concept from the conventional liquid lithium secondary battery. However, the current situation has not been studied.
[0009]
[Means for Solving the Problems]
The present invention has been made in order to solve the above-described problems. The positive electrode and the negative electrode are laminated in a flat plate shape with a spacer made of a porous film, and only a high-boiling-point solvent is used as a solvent in the voids of the porous film. On the premise of a lithium secondary battery in which a battery element filled with a gel-like electrolyte is sealed and accommodated in a flexible case, the lithium secondary battery By using a membrane with a thickness of 10-25 μm, a porosity of 45-75%, and an average pore diameter of 0.2 μm or less as the porous membrane, both the mechanical strength of the lithium secondary battery and the battery performance are achieved. This is a lithium secondary battery.
[0010]
That is, the gist of the present invention is that a positive electrode and a negative electrode are laminated in a flat plate shape through a spacer made of a porous film, and an electrolyte solution obtained by dissolving a lithium salt in a non-aqueous solvent having a boiling point of 150 ° C. or higher is held by a polymer. In a lithium secondary battery in which a battery element in which a gel electrolyte is filled in a void of the porous membrane is sealed and accommodated in a flexible case, the porous membrane has a thickness of 10 The present invention resides in a lithium secondary battery characterized by using a film having −25 μm, a porosity of 45 to 75%, and an average pore diameter of 0.2 μm or less.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Examples of the porous membrane material used as a spacer in the present invention include polyolefins such as polyethylene and polypropylene, polyolefins in which some or all of these hydrogen atoms are substituted with fluorine atoms, polyacrylonitrile, polyaramid, and the like. These resins are mentioned. Preferred are polyolefins and fluorine-substituted polyolefins, and specific examples include polyethylene, polypropylene, and those in which some or all of these hydrogen atoms are substituted with fluorine atoms. Particularly preferred are polyolefins such as polyethylene and polypropylene, polytetrafluoroethylene (PTFE), and polyfluorinated pyrylidene from the viewpoint of chemical stability with respect to the gel used as the electrolyte and stability against applied voltage. Of course, these copolymers and mixtures can also be used.
[0012]
The thickness of the porous membrane is usually 9-28 μm, particularly 10-25 μm, preferably 15 μm or more as the lower limit and 20 μm or less as the upper limit. If the film thickness is too small, self-discharge due to a mild short phenomenon is likely to occur. If the film thickness is too large, not only battery characteristics such as rate characteristics become insufficient, but also the volume energy density tends to decrease.
[0013]
The porosity of the porous membrane is usually 45-90%, in particular 45-75%. A preferable lower limit is 55% or more, and a preferable upper limit is 72% or less. If the porosity is too small, the membrane resistance increases and the rate characteristics tend to deteriorate. In particular, the capacity decreases when used at a high rate. On the other hand, if the porosity is too high, the mechanical strength of the film decreases, and as a result, short-circuiting is likely to occur particularly during use that involves a change in shape.
[0014]
The average pore diameter of the porous membrane is preferably 0.2 μm or less, particularly 0.18 μm or less, and further 0.15 μm or less. If it is too large, a short circuit is likely to occur. Further, if the pore diameter is too small, the membrane resistance increases and the battery performance such as the rate characteristic tends to be lowered. Therefore, it is usually 0.01 μm or more, preferably 0.07 μm or more.
The porous membrane usually has a withstand voltage of 0.3 kV or higher, preferably 0.5 kV or higher. “Having withstand voltage of XkV” means that when a voltage of XkV or more is applied across the porous membrane, a current of 100 mA or more does not flow between the electrodes. If the withstand voltage is too low, if the resistance partially increases for some reason when charging the battery, the temperature may rise abnormally as a result. Further, it tends to be difficult to effectively prevent self-discharge. In view of practical availability, the withstand voltage is preferably 100 kV or less, particularly 10 kV or less.
[0015]
In order to prevent short circuit more effectively, the pin penetration strength when the porous membrane is locally pressurized is preferably 200 gf or more, particularly 230 gf or more, and more preferably 300 gf or more. However, since it is not practical that the pin penetration strength is too large, it is usually 10000 gf or less, preferably 2000 gf or less. Further, the strain generated with respect to a tensile force of 0.1 kg / cm in a certain direction is preferably 1% or less. As a result, the short circuit can be effectively prevented, and the positional accuracy of the porous film can be easily maintained at the time of stacking the spacers in battery manufacturing, and the yield can be improved. However, since it is difficult to obtain a product having the above-mentioned distortion, it is usually 0.01% or more, preferably 0.1% or more.
[0016]
Furthermore, since the spacer is often heated during the formation of the gel-like electric reforming during battery production or battery use, the heat shrinkage rate at 100 ° C. of the porous film is 2% or less per direction, particularly 1.5% or less. Is preferred. If the thermal shrinkage is too large, a short circuit is likely to occur and the rate characteristics tend to deteriorate. On the other hand, those having too small thermal shrinkage are difficult to obtain practically, so they are usually 0.01% or more, preferably 0.1% or more.
The lithium secondary battery of the present invention contains a non-aqueous solvent and a lithium salt as an electrolyte. Therefore, as a reference electrolytic solution, a mixed solvent of propylene carbonate and ethylene carbonate in a volume ratio of 1: 1 is mixed with LiClO. _Four As a membrane resistance value in a state of being impregnated with a solution in which is dissolved at a concentration of 1 mol / L, 1Ω or less, particularly 0.9Ω or less is preferable. As a result, a high capacity can be realized when charging is performed under a high rate condition. If the film resistance is too small, self-discharge tends to occur. Therefore, the film resistance value is usually 0.1Ω or more, preferably 0.4Ω or more.
[0017]
The surface tension of the porous membrane is usually 50 dyne / cm or more, preferably 60 dyne / cm or more, particularly preferably 70 dyne / cm or more. As a result, it becomes easy to sufficiently fill the voids in the porous membrane with the gel electrolyte, and it is possible to improve productivity and improve rate characteristics. In order to obtain a film having such a surface tension, it is usually preferable to subject the porous film to surface modification treatment such as corona discharge treatment, plasma treatment, and fluorine gas treatment. However, since it is difficult to obtain a film having an excessively large surface tension, it is usually 1000 dyne / cm or less, preferably 500 dyne / cm or less.
[0018]
The number average molecular weight of the porous membrane is usually 10,000 or more, preferably 100,000 or more, and usually 10 million or less, preferably 3 million or less. If the molecular weight is too small, the mechanical strength becomes insufficient and a short circuit tends to occur. On the other hand, if the molecular weight is too large, it is difficult to fill the electrolyte in the voids of the porous membrane, which tends to reduce battery production efficiency and battery performance such as rate characteristics. Furthermore, if the molecular weight is too large, film formation may be difficult in a method of stretching after mixing a plasticizer described later.
[0019]
The porous membrane as described above can be manufactured, for example, as follows. A plasticizer as a heterogeneous dispersion medium is mixed with a resin having a number average molecular weight of about 10,000 to 10,000,000, preferably 100,000 to 3,000,000, and then kneaded to form a sheet. Furthermore, a desired porous film can be obtained by performing a process of extracting a plasticizer with a solvent and a process of stretching in a longitudinal or lateral direction at a predetermined magnification or both. In the present invention, the porous membrane is preferably produced by uniaxial or biaxial stretching. In the case of a nonwoven fabric that has been frequently used in the past, short circuits are likely to occur, which is not preferable.
[0020]
In the present invention, as the electrolyte, a gel electrolyte is used in which an electrolytic solution obtained by dissolving a lithium salt in a non-aqueous solvent is held by a polymer. The electrolyte exists between the positive electrode and the negative electrode including the voids of the porous film, and an electrolyte layer is formed as a whole.
The ratio of the polymer in the gel electrolyte is usually 0.1 to 80% by weight, preferably 1 to 50% by weight. When the ratio of the polymer is too low, it is difficult to hold the electrolytic solution and leakage occurs, and when it is too high, the ionic conductivity tends to decrease and the battery characteristics tend to deteriorate. Although the ratio of the polymer with respect to a solvent is suitably selected according to molecular weight, it is 0.1-50 weight% normally, Preferably it is 1-30 weight%. When the proportion of the polymer is too small, it is difficult to form a gel, and the retention of the electrolytic solution is lowered, and there is a tendency that problems of flow and liquid leakage occur. When the proportion of the polymer is too large, the viscosity becomes too high and handling becomes difficult, and the ion conductivity is lowered due to the decrease in the concentration of the electrolytic solution, and the battery characteristics such as the rate characteristics tend to be lowered.
[0021]
As a lithium salt, specifically, LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF _Four LiClO _Four , LiI, LiBr, LiCl, LiAlCl, LiHF ₂ , LiSCN, LiSO _Three CF ₂ Etc. Of these, LiPF in particular ₆ LiClO _Four Is preferred. The content of these lithium salts in the electrolytic solution (total amount of lithium salt and solvent) is generally 0.5 to 2.5 mol / l.
[0022]
Specific examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate and propylene carbonate, acyclic carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, and glyme such as tetrahydrofuran, 2-methyltetrahydrofuran and dimethoxyethane. , Lactones such as γ-butyl lactone, sulfur compounds such as sulfolane, nitriles such as acetonitrile, or a mixture of two or more thereof. Of these, one or more mixed solutions selected from cyclic carbonates, acyclic carbonates and lactones are particularly suitable. Moreover, what substituted some hydrogen atoms of these molecules by halogen etc. can be used.
[0023]
The effect of the present invention is particularly remarkable when only a solvent having a boiling point of 150 ° C. or higher is used. Moreover, the range of practical boiling points is 300 degrees C or less. Specifically, ethylene carbonate (boiling point 243 ° C.), propylene carbonate (boiling point 240 ° C.), γ-butyrolactone (boiling point 204 ° C.) and the like are preferable. Of course, these mixed solvents may also be used, and are preferable in terms of battery characteristics. Since a high boiling point solvent has low volatility, there is an advantage that even when it is housed in a flexible case, safety is high and deformation is small. On the other hand, by using the porous film, it is possible to reduce difficulty in filling the spacer, which is a difficulty of the high boiling point solvent. In particular, when a porous membrane having a weight average molecular weight in the range of 100,000 to 3,000,000 is used, the above effect is particularly remarkable and sufficient mechanical strength is exhibited, which is preferable.
[0024]
Polymers constituting the gel electrolyte include those produced by polycondensation such as polyester, polyamide, polycarbonate and polyimide, those produced by polyaddition such as polyurethane and polyurea, and acrylic derivatives such as polymethyl methacrylate. There are polymers, polyvinyl acetate, and those produced by addition polymerization such as polyvinyl chloride such as polyvinyl chloride. In the present invention, it is preferable to polymerize after impregnating the spacer. It is desirable to use a polymer produced by addition polymerization that is easy and does not generate by-products during polymerization. Examples of such a polymer include poly (meth) acrylate-based polymers, and are preferable in terms of battery characteristics such as battery capacity, rate characteristics, and mechanical strength. In particular, a poly (meth) acrylate polymer having an ethylene glycol unit is a preferred polymer.
[0025]
As a monomer component of the polymer, acrylic acid, methyl acrylate, ethyl acrylate, ethoxyethyl acrylate, methoxyethyl acrylate, ethoxyethoxyethyl acrylate, polyethylene glycol monoacrylate, ethoxyethyl methacrylate, methoxyethyl methacrylate, ethoxyethoxyethyl methacrylate, Polyethylene glycol monomethacrylate, N, N-diethylaminoethyl acrylate, N, N-dimethylaminoethyl acrylate, glycidyl acrylate, allyl acrylate, 2-methoxyethoxyethyl acrylate, 2-ethoxyethoxyethyl acrylate, acrylonitrile, N-vinylpyrrolidone, diethylene glycol Diacrylate, triethylene glycol dia (Meth) acrylic monomers such as relate, tetraethylene glycol diacrylate, polyethylene glycol diacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, polyethylene glycol dimethacrylate can be used. Those that are preferable from the standpoint of safety may be used alone or in combination.
[0026]
In the present invention, preferably, the gel electrolyte is formed by filling the voids of the porous membrane in a state where the above-mentioned monomer is contained in the electrolytic solution, and then polymerizing the monomer. As a method for polymerizing these monomers, there are methods using heat, ultraviolet rays, electron beams and the like. In the case of polymerization by heat, a polymerization initiator that reacts with heat can be added to the electrolytic solution to be impregnated in order to effectively advance the reaction. Usable thermal polymerization initiators include azo compounds such as azobisisobutyronitrile, dimethyl 2,2′-azobisinbutyrate, benzoyl peroxide, cumene hydroperoxide, t-butylperoxy-2-ethylhexanoate. Peroxides such as acid can be used, and those preferable from the viewpoint of reactivity, polarity, safety, etc. may be used alone or in combination.
[0027]
Various positive electrodes and negative electrodes that can be used in the lithium secondary battery of the present invention can be used.
Examples of the positive electrode active material used for the positive electrode include various inorganic compounds such as transition metal oxides, composite oxides of lithium and transition metals, and transition metal sulfides. Here, Fe, Co, Ni, Mn, or the like is used as the transition metal. Specifically, MnO, V ₂ O _Five , V ₆ O ₁₃ TiO ₂ Transition metal oxide powders such as lithium nickel oxide, lithium cobaltate, lithium manganate and other complex oxide powders of TiS, TiS ₂ , FeS, MoS ₂ And transition metal sulfide powders. These compounds may be partially element-substituted in order to improve the characteristics. Organic compounds such as polyaniline, polypyrrole, polyacene, disulfide compounds, polysulfide compounds, and N-fluoropyridinium salts can also be used. You may mix and use these inorganic compounds and organic compounds. The particle diameter of the active material of these positive electrodes is usually 1 to 30 μm, preferably 1 to 10 μm. If the particle size is too large or too small, battery characteristics such as rate characteristics and cycle characteristics tend to deteriorate.
[0028]
Examples of the negative electrode active material used for the negative electrode include carbon-based active materials such as graphite and coke. These carbon-based active materials can be used even in the form of a metal, a salt thereof, a mixture with an oxide, or a coating. Also, oxides such as silicon, tin, zinc, manganese, iron, nickel, sulfates, metal lithium, lithium alloys such as Li—Al, Li—Bi—Cd, Li—Sn—Cd, and lithium transition metal nitrides Things, silicon, etc. can also be used. The particle size of the active material of these negative electrodes is usually 1 to 50 μm, preferably 15 to 30 μm. If it is too large or too small, battery characteristics such as initial efficiency, rate characteristics, and cycle characteristics tend to deteriorate.
[0029]
Each of the positive electrode and the negative electrode may contain a binder. In the case of the binder with respect to 100 parts by weight of the active material, the amount is preferably 0.1 to 30 parts by weight, more preferably 1 to 15 parts by weight. When the amount of the binder is too small, it is difficult to form a strong active material layer. If the amount of the binder is too large, not only will there be an adverse effect on the energy density and cycle characteristics, but if the active material layer contains an electrolyte component, the amount of voids in the active material layer will decrease, making it difficult to impregnate the electrolyte component. . When a binder is used, voids can be formed in these layers, and the gel electrolyte can be filled in the voids, which is preferable in terms of battery characteristics.
[0030]
Examples of the binder include alkane polymers such as polyethylene, polypropylene and poly-1,1-dimethylethylene, unsaturated polymers such as polybutadiene and polyisoprene, polystyrene, polymethylstyrene, polyvinylpyridine, and poly-N-vinylpyrrolidone. Polymer having a ring, polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyacrylic acid, polymethacrylic acid, polyacrylamide and other acrylic derivative polymers, polyvinyl fluoride , Fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide, polyvinyl acetate and polyvinyl alcohol Alkenyl alcohol polymers, polyvinyl chloride, halogen-containing polymers such as polyvinylidene chloride, various resins such as a conductive polymer such as polyaniline can be used. Further, a mixture such as the above-mentioned polymer, a modified product, a derivative, a random copolymer, an alternating copolymer, a graft copolymer, a block copolymer, and the like can be used. In addition, inorganic compounds such as silicate and glass can be used. However, in order to achieve the object of the present invention, it is not preferable to use a resin that can be easily dissolved in the electrolytic solution. The weight average molecular weight of the resin is preferably 10,000 to 1,000,000, more preferably 20,000 to 300,000. If it is too low, the strength of the coating film is lowered, which is not preferable. If it is too high, the viscosity becomes high and it becomes difficult to form an active material layer.
[0031]
The positive electrode and the negative electrode may each contain additives that exhibit various functions such as conductive materials and reinforcing materials, powders, fillers, and the like as necessary. The conductive material is not particularly limited as long as an appropriate amount can be mixed with the above active material to impart conductivity, but usually carbon powder such as acetylene black, carbon black, graphite, various metal fibers, foil, etc. Is mentioned. As the reinforcing material, various inorganic, organic spherical and fibrous fillers can be used.
[0032]
As an electrode base material, foils such as metals and alloys such as aluminum foil and copper foil are generally used. The thickness is usually 1 to 50 μm, preferably 1 to 30 μm. If it is too thin, the mechanical strength becomes weak, which causes a problem in production. If it is too thick, the capacity of the battery as a whole tends to decrease.
In the lithium secondary battery of the present invention, the positive electrode, the negative electrode, and the electrolyte layer are preferably laminated in a flat plate and accommodated in a case. In this case, the thickness of the positive electrode and the negative electrode is usually 1 μm or more, preferably 10 μm or more, and usually 500 μm or less, preferably 200 μm or less. Even if it is too thick or thin, battery performance such as capacity and rate characteristics tends to decrease. The thickness of the electrolyte layer is usually 1 μm or more, preferably 5 μm or more, and usually 500 μm or less, preferably 200 μm or less, particularly preferably 100 μm or less, and most preferably 50 μm or less. If it is too thick, the capacity tends to decrease, and if it is too thin, the insulating property tends to decrease.
[0033]
As a case to be used, a flexible one is used. A flexible case means a case having shape variability such as flexibility and bendability, and the material is plastic, polymer film, metal film, rubber, thin metal plate, metal layer and resin layer. The laminated film etc. which have are mentioned. Specific examples of the case include a bag made of a polymer film such as a plastic bag, a vacuum packaging bag or vacuum pack made of a polymer film, a vacuum packaging bag or vacuum made of a laminate material of a metal foil and a polymer film. Examples include a pack, a can made of plastic, and a case where the periphery is fixed by welding, bonding, fitting, etc. sandwiched between plastic plates. Among these, a vacuum packaging bag or vacuum pack made of a polymer film or a vacuum packaging bag or vacuum pack made of a laminate material of a metal foil and a polymer film is preferable in terms of airtightness and shape changeability. These cases do not have the same weight and rigidity as metal cans, and have flexibility, flexibility, flexibility, etc., so that there is the freedom of shape that can be bent after storage of the battery and the weight can be reduced. have. Of course, for the convenience of battery installation, etc., it is possible to enclose the battery in a shape-variable case, shape it into a preferred shape, and then store multiple cases in a rigid outer case if necessary. is there.
[0034]
In addition, it is preferable that the battery element is sealed in the above-mentioned case in a reduced pressure state from the viewpoints of downsizing of the device and contact of the battery element. In this case, the difference from the atmospheric pressure is a force for pressing the battery element.
A particularly preferable case is a case made of a laminate film in which both surfaces of a metal layer are covered with a resin layer, and it is particularly preferable that the battery element is sealed and housed in a reduced pressure state.
[0035]
【Example】
<Manufacture of positive electrode>
On an aluminum current collector with a thickness of 20 μm, LiCoO ₂ (Average particle size 5 μm: manufactured by Nippon Kagaku Kogyo Co., Ltd.) A solution in which 5 parts by weight of polyvinylidene fluoride (PVdF) and 5 parts by weight of acetylene black were mixed as a binder with respect to 90 parts by weight was dried and then positive electrode An active material layer was obtained.
[0036]
<Manufacture of negative electrode>
A solution in which 10 parts by weight of polyvinylidene fluoride (PVdF) is mixed with 90 parts by weight of mesocarbon particles ((MCMB) average particle diameter 6 μm: manufactured by Osaka Gas Chemical) on a copper collector with a thickness of 20 μm is used. After coating, this was dried to obtain a negative electrode active material layer.
[0037]
<Manufacture of electrolyte solution>
LiClO _Four Is dissolved in a mixed solvent of propylene carbonate and ethylene carbonate in a volume ratio of 1: 1 (concentration: 1 mol / L) to 93 parts by weight of polyethylene glycol diacrylate (average number of polyethylene glycol units = approximately 4, manufactured by Toagosei Co., Ltd.). 4.67 parts by weight of Aronix M-240) and trimethylolpropane ethylene oxide-modified triacrylate (average number of polyethylene glycol units = approximately 2, Aronix M-370 manufactured by Toagosei Co., Ltd.) were added, and a polymerization initiator (Trignox23 C-70) was added. : Kayaku Akzo) 0.1 parts by weight was added to obtain an electrolyte solution.
[0038]
<Production of battery>
A battery in which the electrolyte solution is applied and impregnated into the positive electrode active material layer, the negative electrode active material layer, and a predetermined porous film and laminated in a flat plate shape, and heated at 90 ° C. for 5 minutes to gel the electrolyte. The element was made. The lithium secondary battery was obtained by vacuum-sealing this while making the terminal of a positive electrode negative electrode project in the flexible lamination bag which consists of aluminum / polyethylene.
[0039]
<Battery characteristics evaluation>
(1) Rate evaluation: LiCoO ₂ The rate of discharge was set to 120 mAh / g per hour, and the rate of discharge was set to 1 C from the ratio of this to the amount of active material of the positive electrode. After being charged / discharged at (1/24) C, the battery was charged at 0.25C and discharged at a rate of 1C and 1.5C. The ratio of the discharge capacity at 1C and 1.5C to the discharge capacity at (1/24) C was calculated, and each was used as the discharge capacity retention rate.
(2) Short-circuit occurrence rate: (1/24) The probability of occurrence of a battery in which a potential drop of 0.2 V or more occurred at a potential of 4.0 V or less during the voltage increase process due to C charging (number of test points 3) was determined. It was. (3) Self-discharge occurrence rate: The probability of occurrence of a battery that has dropped to 2.5 V or less after a lapse of 240 hours when a 3.0 V voltage battery is exposed to room temperature in an open circuit (number of test points: 5), and self-discharge is generated. Rate.
[0040]
<Measurement of film properties>
(4) Withstand voltage: A dielectric breakdown voltage at which a voltage of 0 mA was detected by applying a voltage from 0 V with a porous membrane sandwiched between electrodes was defined as a withstand voltage.
(5) Pin puncture strength: A porous membrane is fixed on a circular support base of 25 mmφ, and a rod with a thickness of 1 mmφ and a tip of 0.5 R is inserted into the center at 2 cm / min, and the load on the rod when the membrane breaks. Was the pin stab strength.
(6) Mechanical strain stress: The tensile strain at the point where the stress was 0.1 kgf / cm 2 in the tensile test of the porous film (width 10 mm, chuck 30 mm speed 2 cm / min) was obtained and used as the mechanical strain stress.
(7) Heat Shrinkage: The heat shrinkage rate is the one with the higher shrinkage ratio in either the longitudinal or the transverse direction after shrinking the 100 mm square porous membrane at 100 ° C. for 5 minutes.
(8) Film resistance: LiClO _Four Was impregnated into a porous membrane at a concentration of 1 mol / L in a mixed solvent of propylene carbonate and ethylene carbonate in a volume ratio of 1: 1, and the value of AC impedance at a frequency of 100 KHz was taken as the membrane resistance value.
(9) Surface tension: The surface tension was determined with a surface tension standard reagent (COROTEC).
(10) Impregnation time: A 2 mmφ droplet of the electrolyte solution before gelation was dropped from one side of the porous membrane, and the time until the dripped portion was completely impregnated was measured and defined as the impregnation time.
[0041]
Example 1:
Corona discharge treatment in the atmosphere (discharge amount 60W / m) on a porous film with a film thickness of 14μm, porosity of 60%, and average pore diameter of 0.1μm produced by biaxial stretching of polyethylene with a number average molecular weight of 800,000 ² / Min) was used, and a lithium secondary battery was produced and evaluated by the above method. The results are shown in Table-1.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this secondary battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0042]
Example 2:
As a spacer, a porous film with a film thickness of 18 μm, porosity of 71%, and average pore diameter of 0.1 μm produced by biaxial stretching of polyethylene with a number average molecular weight of 800,000 was subjected to corona discharge treatment (discharge amount 60 W / m ² / Min) A battery was fabricated and evaluated under the same conditions as in Example 1 except for using the same. The results are shown in Table-1.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0043]
Example 3:
As a spacer, a porous membrane with a surface hydrophilization treatment with a film thickness of 24μm, porosity of 55%, and average pore diameter of 0.1 μm produced by biaxial stretching of polytetrafluoroethylene (PTFE) with a number average molecular weight of 800,000 is used. A battery was produced and evaluated in the same manner as in Example 1 except that. The results are shown in Table-1.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0044]
Comparative Example 1:
A battery was fabricated in the same manner as in Example 1 except that a porous film having a film thickness of 25 μm, a porosity of 40%, and an average pore diameter of 0.1 μm produced by biaxial stretching of polyethylene having a number average molecular weight of 800,000 was used as the spacer. Made and evaluated. The results are shown in Table-1. Although the rate of occurrence of voltage drop due to short circuit and self-discharge during charging was 0%, the discharge capacity maintenance rate was as low as 60%, and the capacity at high rate was low.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0045]
Comparative Example 2:
As a spacer, a corona discharge treatment was applied to a porous membrane (Mitsubishi Chemical Corporation; trade name Clearsep) with a film thickness of 8μm, porosity of 70%, and average pore diameter of 0.12μm produced by biaxial stretching of polyethylene with a number average molecular weight of 800,000. (Discharge amount 60W / m ² / Min) A battery was produced and evaluated in the same manner as in Example 1 except that the battery with the same value was used. The results are shown in Table-1. Although the discharge capacity maintenance rate was 98% and the occurrence rate of short circuit during charging was 0%, voltage drop due to self-discharge occurred at one point out of 5 test points, and the withstand voltage stability was insufficient. there were.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0046]
Comparative Example 3:
Example 1 except that a porous membrane prepared by biaxial stretching of PTFE having a number average molecular weight of 800,000, a porosity of 45%, and an average pore diameter of 0.5 μm was used as a spacer. A battery was prepared and evaluated in the same manner as described above. The results are shown in Table-1. The short-circuit occurrence rate was 100% (number of test points 3), and the insulation was insufficient.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0047]
Comparative Example 4:
A battery was prepared and evaluated in the same manner as in Example 1 except that a polyethylene non-woven fabric having a film thickness of 30 μm, a porosity of 66%, and a hydrophilic surface treated with fluorine / oxygen gas was used as the spacer. The results are shown in Table-1. The short-circuit occurrence rate was 100% (number of test points 3), and the insulation was insufficient.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0048]
Comparative Example 5:
As a spacer, a porous film produced by biaxial stretching of polyethylene having a number average molecular weight of 800,000, having a film thickness of 22 μm, a porosity of 38%, and an average pore diameter of 0.15 μm was subjected to corona discharge treatment (discharge amount 60 W / m ² / Min) A battery was produced and evaluated in the same manner as in Example 1 except that the battery with the same value was used. The results are shown in Table-1. Although the short-circuit occurrence rate and the self-discharge occurrence rate were 0%, the discharge capacity maintenance rate was as low as 65%, and the capacity at a high rate was low.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0049]
Example 4:
As a spacer, a porous film with a film thickness of 16 μm, porosity of 45%, and average pore diameter of 0.05 μm produced by biaxial stretching of ethylene with a number average molecular weight of 800,000 was subjected to corona discharge treatment (discharge amount: 60 W / m ² / Min) A battery was produced and evaluated in the same manner as in Example 1 except that the product was used. The results are shown in Table-1.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0050]
Example 5:
As a spacer, a porous film produced by biaxial stretching of ethylene having a number average molecular weight of 800,000 and having a film thickness of 12 μm, a porosity of 60%, and an average pore diameter of 0.1 μm is subjected to corona discharge treatment (discharge amount 60 W / m ² / Min) A battery was produced and evaluated in the same manner as in Example 1 except that the product was used. The results are shown in Table-1.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0051]
Example 6:
As a spacer, a porous film produced by biaxial stretching of ethylene having a number average molecular weight of 800,000, having a film thickness of 23 μm, a porosity of 61%, and an average pore diameter of 0.1 μm was subjected to corona discharge treatment (discharge amount 60 W / m ² / Min) A battery was produced and evaluated in the same manner as in Example 1 except that the product was used. The results are shown in Table-1.
Moreover, when the film | membrane physical property of the porous film | membrane of the spacer used for this battery was measured on the conditions of said (4)-(10), it was a physical-property value as shown in Table-1.
[0052]
[Table 1]

[0053]
【The invention's effect】
According to the present invention, by using a spacer suitable for a lithium secondary battery in which a battery element using a gel electrolyte is housed in a flexible case, the battery characteristics such as capacity and rate characteristics are excellent. Can provide a lithium secondary battery with high safety and high energy density.

Claims (9)

A gel electrolyte in which a positive electrode and a negative electrode are laminated in a flat plate shape through a spacer made of a porous film, and an electrolytic solution obtained by dissolving a lithium salt in a non-aqueous solvent having a boiling point of 150 ° C. or higher is held by a polymer In the lithium secondary battery in which the battery element filled in the void of the porous film is sealed and accommodated in a flexible case, the porous film has a film thickness of 10-25 μm, a porosity of 45- A lithium secondary battery using a 75% membrane having an average pore diameter of 0.2 μm or less. The lithium secondary battery according to claim 1, wherein the porous film has a withstand voltage of 0.5 kV or more. The lithium secondary battery according to claim 1 or 2, wherein the pin penetration strength of the porous membrane is 200 gf or more. The lithium secondary battery according to any one of claims 1 to 3, wherein a strain generated with respect to a tensile force of 0.1 kg / cm in a certain direction is 1% or less. The lithium secondary battery according to any one of claims 1 to 4, wherein the porous film has a thermal shrinkage at 100 ° C of 2% or less per direction. As a porous membrane, a porous membrane having a membrane resistance value of 0.4-1Ω in a state in which a mixed solvent of propylene carbonate and ethylene carbonate in a volume ratio of 1: 1 is impregnated with a solution obtained by dissolving LiClO ₄ at a concentration of 1 mol / L. The lithium secondary battery according to claim 1, wherein a conductive film is used. The lithium secondary battery according to any one of claims 1 to 6, wherein the porous membrane has a surface tension of 70 dyne / cm 2 or more. The lithium secondary battery according to any one of claims 1 to 7, wherein the number average molecular weight of the porous membrane is 100,000 or more and 3 million or less. The lithium secondary battery according to any one of claims 1 to 8, wherein the non-aqueous solvent has a boiling point of 300 ° C or lower.