Secondary-battery porous membrane composition, secondary-battery porous membrane and secondary battery

Abstract

A composition for a porous membrane of a secondary battery including a non-conductive particle and a water-soluble polymer, wherein the water-soluble polymer contains 80% by weight or more of a (meth)acrylamide monomer unit, and the water-soluble polymer has a storage modulus at 150 C. of 2.010.sup.5 Pa or more; and a porous membrane manufactured therefrom and a secondary battery including the same.

Claims

1. A composition for a porous membrane of a secondary battery, comprising a non-conductive particle and a water-soluble polymer, wherein the water-soluble polymer contains 80% by weight or more of a (meth)acrylamide monomer unit, and the water-soluble polymer has a storage modulus at 150 C. of 2.010.sup.5 Pa or more.

2. The composition for a porous membrane of a secondary battery according to claim 1, wherein a glass transition temperature obtained by measuring dynamic viscoelasticity of the water-soluble polymer is 150 to 200 C.

3. The composition for a porous membrane of a secondary battery according to claim 1, wherein the water-soluble polymer has a weight average molecular weight of 2.010.sup.5 to 1.0010.sup.6.

4. The composition for a porous membrane of a secondary battery according to claim 1, wherein the water-soluble polymer contains an acid group-containing monomer unit.

5. A porous membrane for a secondary battery, obtained by forming a layer of the composition for a porous membrane of a secondary battery according to claim 1, and drying the formed layer.

6. A secondary battery comprising the porous membrane for a secondary battery according to claim 5.

Description

EXAMPLES

(1) Hereinafter, the present invention will be specifically described by illustrating Examples. However, the present invention is not limited to the following Examples, which can be optionally modified within the scope not departing from the claims of the present invention and their equivalents for implementation. Unless otherwise stated, % and parts indicating quantity in the following description are based on weight.

(2) In Examples and Comparative Examples, measurement of the molecular weight, storage modulus, and glass transition temperature of the water-soluble polymer (A); measurement of the pH and viscosity of the composition for porous membranes; measurement of the particle size of the non-conductive particle; evaluation of the application property, moisture content, and thermal shrinkage resistance of the separator; and evaluation of the high-temperature cycle property in the secondary battery, the volume change amount of the cell, and the halogen trapping amount were performed as described below.

Molecular Weight Measurement Method

(3) An aqueous solution of the water-soluble polymer (A) obtained in each of Examples and Comparative Examples was diluted to a concentration of 0.5% by weight, and thereafter added with caustic soda until pH 10 to 12 was achieved. Then, the resultant product was immersed in hot water bath at 80 C. or higher for one hour. After that, it was diluted with an eluent (described below) to 0.025% by weight. Thus, a sample was prepared. This sample was analyzed by gel permeation chromatography under the following conditions to obtain the weight average molecular weight of the water-soluble polymer (A).

(4) GPC device body: manufactured by Tosoh Corporation

(5) Column: one PWXL and two GMPWXLs that are Guard Column manufactured by Tosoh Corporation (temperature: 40 C.)

(6) Eluent: 0.5 mol/l acetic acid buffer (aqueous solution of 0.5 mol/l acetic acid (manufactured by Wako Pure Chemical Industries, Ltd.)+0.5 mol/l sodium acetate (manufactured Kishida Chemical Co., Ltd.), pH: approximately 4.2)

(7) Flow rate: 0.8 ml/min

(8) Detector: TDA MODEL 301 manufactured by Viscotech Co., Ltd. (concentration detector and 90 light scattering detector and viscosity detector (temperature: 40 C.)), RALLS method

pH Measurement Method

(9) A desktop pH meter (F-51 manufactured by HORIBA) was calibrated with a pH standard solution (pH 4, pH 7, and pH 9). Then, this calibrated pH meter was used to measure the pH of the slurry of the composition for porous membranes.

Storage Modulus and Glass Transition Temperature

(10) An aqueous solution of the water-soluble polymer (A) obtained in each of Examples and Comparative Examples was dried at room temperature to obtain a film with a thickness of 0.5 mm. The film was punched into a circular shape with a diameter of 8 mm to be used as a sample piece. Using the following apparatus, the sample piece was distorted with a frequency of 1 Hz, and measured for dynamic viscoelasticity while increasing the temperature at the following temperature increasing rate. On the basis of this measurement result, the storage modulus and glass transition temperature were obtained.

(11) Apparatus: product name MCR300 manufactured by Anton Paar

(12) Set temperature range: 25 C. to 200 C.

(13) Set temperature increasing rate: 10 C./min

(14) Measurement frequency: 1 Hz

Viscosity

(15) The viscosity of the slurry of the composition for porous membranes was measured at 25 C. and at a revolution of 60 rpm using a B-type viscometer.

Evaluation of Application Property

(16) The state of the porous membrane of the separator (the separator substrate and the porous membrane formed on one surface of the separator substrate) obtained in each of Examples and Comparative Examples was observed from the porous membrane side while light irradiation was performed from the separator substrate side surface. Thus, presence or absence of a streak and application unevenness was evaluated.

Measurement Method of Particle Sizes of Non-conductive Particle and Particulate Binder

(17) The non-conductive particle or the particulate binder was ultrasonically dispersed with an aqueous solution of sodium hexametaphosphate, and then analyzed by a laser diffraction particle size distribution analyzer (SALD-7100 manufactured by Shimadzu Corporation). Thus, particle size D50 was obtained.

Measurement of Moisture Content

(18) The separator obtained in each of Examples and Comparative Examples was cut out into a size of 10 cm in width10 cm in length as a test piece. This test piece was left to stand at a temperature of 25 C. and a dew-point temperature of 60 C. for 24 hours. After that, the moisture content of the test piece was measured using a coulometric titration-type moisture meter by the Karl Fischer method (JIS K-0068(2001) moisture vaporization method, vaporization temperature: 150 C.). This was adopted as the moisture content of the porous membrane, and evaluated in accordance with the following criteria.

(19) A: Moisture content of porous membrane is less than 200 ppm.

(20) B: Moisture content of porous membrane is equal to or more than 200 ppm and less than 300 ppm.

(21) C: Moisture content of porous membrane is equal to or more than 300 ppm and less than 400 ppm.

(22) D: Moisture content of porous membrane is equal to or more than 400 ppm.

Thermal Shrinkage Resistance

(23) The separator obtained in each of Examples and Comparative Examples was cut into a square of 12 cm in width12 cm in length, and a square having edges each having a length of 10 cm was drawn inside the cut square to obtain a test piece. The test piece was placed in a constant temperature bath at 130 C., and left to stand for one hour for heat treatment. After the heat treatment, the area of the square drawn inside the test piece was measured, and a change in area before and after the heat treatment was obtained as thermal shrinkage ratio. Then, the obtained thermal shrinkage ratio was evaluated in accordance with the following criteria. Smaller thermal shrinkage indicates that the separator has better thermal shrinkage resistance.

(24) A: Thermal shrinkage is less than 1%.

(25) B: Thermal shrinkage is equal to or more than 1% and less than 5%.

(26) C: Thermal shrinkage is equal to or more than 5% and less than 10%.

(27) D: Thermal shrinkage is equal to or more than 10%.

High-Temperature Cycle Property

(28) A charge and discharge process of charging up to 4.2 V and discharging down to 3V by a constant current method at 0.2 C under the atmosphere at 60 C. was repeated 50 times (=50 cycles) on 10 batteries obtained in each of Examples and Comparative Examples, and the electric capacity was measured. The average value of the measurement results for 10 batteries was adopted as a measured value. The ratio of the electric capacity after completion of 200 cycles relative to the electric capacity after completion of 5 cycles was calculated in percentage to obtain a charge and discharge capacity retention rate. This was adopted as the evaluation base for high-temperature cycle property, and evaluated in accordance with the following criteria. A higher value indicates that high-temperature cycle property is better.

(29) A: Charge and discharge capacity retention rate is equal to or more than 80%.

(30) B: Charge and discharge capacity retention rate is equal to or more than 70% and less than 80%.

(31) C: Charge and discharge capacity retention rate is equal to or more than 60% and less than 70%.

(32) D: Charge and discharge capacity retention rate is less than 60%.

Measurement Method of Change in Volume of Cell

(33) The battery obtained in each of Examples and Comparative Examples was left to stand in the environment at 25 C. for 24 hours. After that, a charge and discharge operation of charging up to 4.35 V at 0.1 C and discharging down to 2.75 V at 0.1 C under the environment at 25 C. was performed. After that, the battery was immersed in liquid paraffin, and measured for its volume V0.

(34) Furthermore, a cycle of charging up to 4.35 V at 0.1 C and discharging down to 2.75 V at 0.1 C under the environment at 60 C. was repeated 1000 times. After that, the battery was immersed in liquid paraffin, and measured for its volume V1.

(35) The volume change amount V of a battery cell before and after the 1000 cycles of charging and discharging was calculated by the formula V=(V1V0)/V0100(%), and evaluated in accordance with the following criteria. A smaller value of this volume change amount V indicates that an ability of suppressing gas generation is better.

(36) A: Value of V is less than 22%.

(37) B: Value of V is equal to or more than 22% and less than 24%.

(38) C: Value of V is equal to or more than 24% and less than 26%.

(39) D: Value of V is equal to or more than 26%.

Halogen Trapping Amount

(40) After the high-temperature cycle test, the electrolytic solution was taken out of the battery, and the electrolytic solution was measured for its fluorine ion concentration and chloride ion concentration by an inductively coupled plasma atomic emission spectroscopic analyzer (ICP-AES). The total amount of these ion concentrations was adopted as the halogen concentration, and evaluated in accordance with the following criteria. It is considered that the smaller the halogen concentration in the electrolytic solution is, the higher the ability of trapping halogen inside the battery is, thereby contributing to longer battery life and reduced gas generation.

(41) A: Halogen concentration in electrolytic solution is less than 100 ppm.

(42) B: Halogen concentration in electrolytic solution is equal to or more than 100 ppm or less than 120 ppm.

(43) C: Halogen concentration in electrolytic solution is equal to or more than 120 ppm or less than 140 ppm.

(44) D: Halogen concentration in electrolytic solution is equal to or more than 140 ppm.

Example 1

1-1. Manufacture of Water-Soluble Polymer (A)

(45) In a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas introduction pipe, a monomer composition consisting of 89.5 parts of acrylamide, 9 parts of methacrylic acid, and 1.5 parts of dimethylacrylamide, as well as 365 parts of ion exchanged water and 5 parts of isopropyl alcohol were charged. Then, oxygen in the reaction system was removed with nitrogen gas. Next, under stirring, 7 parts of a 5% aqueous solution of ammonium persulfate and 3 parts of a 5% aqueous solution of sodium bisulfite were poured as a polymerization initiator in the flask. After that, the obtained mixture was increased in temperature from room temperature to 80 C., and thermally maintained for three hours. Then, 162 parts of ion exchanged water was added, and the pH was adjusted to 5 with 48% caustic soda. Thus, there was obtained an aqueous solution of the water-soluble polymer (A) having a solid content of 15.2%, a viscosity (25 C.) of 3050 mPa.Math.s, and a weight average molecular weight of 361100.

(46) The obtained water-soluble polymer (A) was measured for its weight average molecular weight. Furthermore, the dynamic viscoelasticity was measured, and the storage modulus and glass transition temperature were obtained.

1-2. Manufacture of Particulate Binder

(47) Into a reaction container equipped with a stirrer, 70 parts of ion exchanged water, 0.15 parts of sodium lauryl sulfate (manufactured by Kao Chemical, product name: Aimard (registered trademark) 2F) as an emulsifier, and 0.5 parts of ammonium peroxodisulfate as a polymerization initiator were supplied. Then, the gas phase part was substituted with nitrogen gas, and the temperature was increased to 60 C.

(48) On the other hand, in another container, 50 parts of ion exchanged water, and 0.5 parts of sodium dodecylbenzenesulfonate as an dispersant, as well as, as a monomer composition, 94.8 parts of n-butyl acrylate, 1 part of methacrylic acid, 1.2 parts of N-methylolacrylamide, 2 parts of acrylonitrile, and 1 part of allyl glycidyl ether were mixed. Thus, a monomer mixture was obtained. This monomer mixture was continuously added in the reaction container for four hours for performing polymerization. While the monomer mixture was added, the reaction was performed at 60 C. After the addition of the monomer mixture was completed, stirring was further continued at 70 C. for three hours to terminate the reaction. Thus, an aqueous dispersion liquid containing the particulate binder was manufactured.

(49) The obtained particulate binder had a volume average particle size D50 of 0.37 m and a glass transition temperature of 45 C.

1-3. Manufacture of Slurry of Composition for Porous Membranes

(50) 100 parts of barium sulfate (volume average particle size: 0.5 m, specific surface area: 5.5 g/m.sup.2), 0.5 parts of ammonium polycarboxylate (dispersant, manufactured by Toagosei Co., Ltd., trade name Aron A-6114), and water were mixed. The amount of water was adjusted such that the solid content concentration became 50%. The mixture was treated using a media-less disperser to disperse barium sulfate. To the obtained dispersion liquid, 1.5 parts (in terms of the solid content) of the aqueous solution of the water-soluble polymer (A) having a solid content of 15.2% obtained in step (1-1) was added and mixed. The added water-soluble polymer (A) dissolved in the mixture. Next, 5 parts (in terms of the solid content) of the particulate binder obtained in step (1-2), and 0.2 parts of a wetting agent (manufactured by San Nopco Limited, trade name SN WET 366) were added. Furthermore, water was mixed such that the solid content concentration became 40%. Thus, a slurry of the composition for porous membranes was manufactured.

(51) The obtained slurry of the composition for porous membranes was measured for its pH and viscosity.

1-4. Manufacture of Separator

(52) A single-layer polyethylene separator substrate having a width of 250 mm, a length of 1000 m, and a thickness of 12 m manufactured by a wet method was prepared. The slurry for a porous membrane obtained in step (1-3) was applied onto one surface of the separator substrate using a gravure coater at a speed of 20 m/min such that the thickness of the dried coat became 2.5 Next, the separator substrate with the coat was dried in a drying furnace at 50 C., and then wound. Thus, a separator including the separator substrate and the porous membrane formed on one surface of the separator substrate was prepared.

(53) The obtained separator was evaluated for its application property, moisture content, and thermal shrinkage resistance.

1-5. Manufacture of Positive Electrode

(54) 100 parts of LiCoO.sub.2 (volume average particle size D50: 12 m) as a positive electrode active material, 2 parts of acetylene black (manufactured by Denki Kagaku Kogyo Kabushiki Kaisha, HS-100) as a conductive material, 2 parts in terms of the solid content of PVDF (polyvinylidene fluoride, manufactured by Kureha Corporation, #7208) as a binder for positive electrode active material layers, and NMP (N-methylpyrrolidone) were mixed, such that the total solid content concentration became 70%. These were mixed in a planetary mixer to obtain a slurry composition for positive electrodes.

(55) The obtained slurry composition for positive electrodes was applied onto aluminum foil having a thickness of 20 m as a current collector using a comma coater such that the film thickness of the dried coat became approximately 150 m, and then dried. This drying was performed by conveying the aluminum foil in an oven at 60 C. for two minutes at a speed of 0.5 m/min. After that, the positive electrode raw material was rolled with a roll press. Thus, a positive electrode including a positive electrode active material layer with a thickness of 95 m was obtained.

1-6. Manufacture of Negative Electrode

(56) In a 5 MPa pressure resistant container equipped with a stirrer, 33.5 parts of 1,3-butadiene, 3.5 parts of itaconic acid, 62 parts of styrene, 1 part of 2-hydroxyethyl acrylate, 0.4 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion exchanged water, and 0.5 parts of potassium peroxodisulfate as a polymerization initiator were placed. The mixture was sufficiently stirred, and then warmed to 50 C. for initiating polymerization. When the polymerization conversion ratio reached 96%, the mixture was cooled to terminate the reaction. Thus, a mixture containing a binder for negative electrode active material layers (SBR) was obtained. A 5% aqueous sodium hydroxide solution was added to the mixture containing the binder for negative electrode active material layers to adjust to pH 8. After that, unreacted monomers were removed through distillation by heating under reduced pressure. After that, the obtained product was cooled to 30 C. or lower. Thus, an aqueous dispersion liquid containing a desired binder for negative electrode active material layers was obtained.

(57) A mixture of 100 parts of artificial graphite (volume average particle size D50: 15.6 m) and 1 part in terms of the solid content of a 2% aqueous solution of sodium salt of carboxymethyl cellulose (manufactured by Nippon Paper Industries Co., Ltd., MAC350HC) as a thickener was prepared to a solid content concentration of 68% with ion exchanged water, and mixed at 25 C. for 60 minutes. The mixture was further adjusted to a solid content concentration of 62% with ion exchanged water, and mixed at 25 C. for 15 minutes. The aforementioned binder for negative electrode active material layers (SBR) in an amount of 1.5 parts in terms of the solid content and ion exchanged water were poured into the mixture, and the final solid content concentration was adjusted to 52%. Then, the obtained product was further mixed for 10 minutes. This mixture was subjected to defoaming treatment under reduced pressure. Thus, a slurry composition for negative electrodes having favorable fluidity was prepared.

(58) The obtained slurry composition for negative electrodes was applied onto copper foil having a thickness of 20 m as a current collector using a comma coater such that the film thickness of the dried coat became approximately 150 m, and then dried. This drying was performed by conveying the copper foil in an oven at 60 C. for two minutes at a speed of 0.5 m/min. After that, the negative electrode raw material was rolled with a roll press. Thus, a negative electrode including a negative electrode active material layer with a thickness of 100 was obtained.

1-7. Manufacture of Lithium Ion Secondary Battery

(59) As an exterior of a battery, an aluminum exterior package was prepared. The positive electrode obtained in step (1-5) was cut out into a square of 4.6 cm4.6 cm to obtain a rectangular positive electrode. The separator obtained in step (1-4) was cut out into a square of 5.2 cm5.2 cm to obtain a rectangular separator. Furthermore, the negative electrode obtained in step (1-6) was cut out into a square of 5 cm5 cm to obtain a rectangular negative electrode. The rectangular positive electrode was disposed in the aluminum exterior package such that its surface on the current collector side came in contact with the exterior package. The rectangular separator was disposed on the surface on the positive electrode active material layer side of the rectangular positive electrode such that the surface on the porous membrane side came in contact with the rectangular positive electrode. Furthermore, the rectangular negative electrode was disposed on the separator such that the surface on the negative electrode active material layer side faced the separator. An electrolytic solution (solvent: EC/DEC/VC=68.5/30/1.5 in volume ratio, electrolyte: LiPF.sub.6 having a concentration of 1 M) was injected such that air was not left. Furthermore, for sealing the opening of the aluminum package, heat sealing at 150 C. was performed to close the opening of the aluminum exterior package. Thus, a lithium ion secondary battery was manufactured.

(60) This lithium ion secondary battery was evaluated for high-temperature cycle property, a volume change of a cell, and a halogen trapping amount.

Example 2

(61) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the water-soluble polymer (A) in step (1-1), the monomer composition was changed to include 97 parts of acrylamide, 2 parts of methallylsulfonic acid soda, and 1.0 part of dimethylacrylamide. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Example 3

(62) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the water-soluble polymer (A) in step (1-1), the monomer composition was changed to include 83 parts of acrylamide, 9 parts of methallylsulfonic acid soda, 1.0 part of dimethylacrylamide, and 7 parts of dimethylaminoethyl acrylate. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Example 4

(63) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the water-soluble polymer (A) in step (1-1), the monomer composition was changed to include 85 parts of acrylamide, 10 parts of methacrylic acid, and 5 parts of dimethylaminoethyl acrylate. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Example 5

(64) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the water-soluble polymer (A) in step (1-1), the monomer composition was changed to include 88.5 parts of acrylamide and 2.5 parts of dimethylacrylamide. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Example 6

(65) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the water-soluble polymer (A) in step (1-1), the monomer composition was changed to include 90.5 parts of acrylamide and 0.5 parts of dimethylacrylamide. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Example 7

(66) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the water-soluble polymer (A) in step (1-1), the monomer composition was changed to include 89.0 parts of acrylamide, 9 parts of methacrylic acid, and 2.0 parts of dimethylacrylamide. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Example 8

(67) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the slurry of the composition for porous membranes in step (1-3), alumina (specific surface area: 5.0 g/m.sup.2, volume average particle size: 0.55 m) was used in place of barium sulfate. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Comparative Example 1

(68) A lithium ion secondary battery was manufactured in the same manner as in Example 1, except that, in the manufacture of the water-soluble polymer (A) in step (1-1), the monomer composition was changed to include 75 parts of acrylamide, 9.8 parts of methacrylic acid, 0.2 parts of dimethylacrylamide, and 15 parts of dimethylaminoethyl acrylate. Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

Comparative Example 2

(69) A lithium ion secondary battery was manufactured in the same manner as in steps (1-2) to (1-7) of Example 1, except that, in the manufacture of the slurry of the composition for porous membranes in step (1-3), sodium carboxymethyl cellulose salt (product name Daicel D1220, manufactured by Daicel FineChem Ltd., etherification degree: 0.8 to 1.0) was used in place of an aqueous solution of the water-soluble polymer (A) obtained in step (1-1). Then, the manufactured lithium ion secondary battery and its components were measured and evaluated in the same manner as in Example 1.

(70) The evaluation results of the Examples and Comparative Examples are shown in Table 1 to Table 2.

(71) TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Non-conductive Barium Barium Barium Barium Barium Barium particle type sulfate sulfate sulfate sulfate sulfate sulfate Specific surface 5.5 5.5 5.5 5.5 5.5 5.5 area Particle size 0.5 m 0.5 m 0.5 m 0.5 m 0.5 m 0.5 m Acrylamide 89.5 wt % 97 wt % 83 wt % 85 wt % 88.5 wt % 90.5 wt % amount Acid group unit MAA SAS(D) SAS(D) MAA MAA MAA type Acid group unit 9 wt % 2 wt % 9 wt % 10 wt % 9 wt % 9 wt % amount DMAA amount 1.5 wt % 1.0 wt % 1.0 wt % 0 wt % 2.5 wt % 0.5 wt % DMAEA amount 7 wt % 5 wt % Storage modulus 6.6 10.sup.5 3.3 10.sup.5 5.6 10.sup.5 2.5 10.sup.5 6.3 10.sup.5 5.5 10.sup.5 Tg 168 162 156 160 168 168 Molecular weight 361100 250000 387500 312000 9805000 200000 Viscosity 25 23 30 24 70 10 Application Favorable Favorable Favorable Favorable A few A few property streaks streaks Moisture content in A A A A A A porous membrane Thermal shrinkage A B B B B B resistance High-temperature A A A A B B cycle Cell volume A A A A B B Halogen trapping A A A A A A amount

(72) TABLE-US-00002 TABLE 2 Compar- Comparative ative Example 7 Example 8 Example 1 Example 2 Non-conductive Barium Alumina Barium Barium particle type sulfate sulfate sulfate Specific surface 5.5 5.0 5.5 5.5 area Particle size 0.5 m 0.55 m 0.5 m 0.5 m Acrylamide 89.0 wt % 89.5 wt % 75 wt % CMC amount (D1220) Acid group unit MAA MAA MAA type Acid group unit 9 wt % 9 wt % 9.8 wt % amount DMAA amount 2.0 wt % 1.5 wt % 0.2 wt % DMAEA amount 15 wt % Storage modulus 6.50 10.sup.5 6.6 10.sup.5 1.4 10.sup.5 6.75 10.sup.5 Tg 167 168 140 Molecular weight 576000 361100 332500 Viscosity 38 30 26 82 Application Favorable Favorable Favorable Streaks property Moisture A C A D content in porous membrane Thermal A A C A shrinkage resistance High-temperature A B B B cycle Cell volume A C B D Halogen trapping A B A C amount

(73) Abbreviations in the tables mean as follows.

(74) Specific surface area: specific surface area of non-conductive particle, unit g/m.sup.2.

(75) Particle size: particle size of non-conductive particle, unit m.

(76) Acrylamide amount: ratio of acrylamide in monomer composition for preparing water-soluble polymer (A), unit % by weight.

(77) Acid group unit type: type of acid group-containing monomer in monomer composition for preparing water-soluble polymer (A). MAA: methacrylic acid. SAS(D): methallylsulfonic acid soda.

(78) Acid group unit amount: ratio of acid group-containing monomer in monomer composition for preparing water-soluble polymer (A), unit % by weight.

(79) DMAA amount: ratio of dimethylacrylamide in monomer composition for preparing water-soluble polymer (A), unit % by weight.

(80) DMAEA amount: ratio of dimethylaminoethyl acrylate in monomer composition for preparing water-soluble polymer (A), unit % by weight.

(81) Storage modulus: storage modulus of water-soluble polymer (A), unit Pa.

(82) Tg: glass transition temperature of water-soluble polymer (A), unit C.

(83) Molecular weight: molecular weight of water-soluble polymer (A).

(84) Viscosity: viscosity of composition for porous membranes, unit mPa.Math.s.

(85) As seen from the results in Table 1 to Table 2, in Examples using the composition for porous membranes having the specific water-soluble polymer (A), application property of the composition for porous membranes was favorable, the moisture content in the porous membrane was low, thermal shrinkage resistance of the porous membrane was favorable, high-temperature cycle property in the obtained battery was favorable, the change in volume of the cell was small, and many halogen atoms in the electrolytic solution were trapped. Although the moisture content of the porous membrane in Example 8 was high compared to other Examples and the change in volume of the cell was large, it is considered that this may be due to the use of alumina as the non-conductive particle. However, the use of the water-soluble polymer (A) allowed the moisture content to become low compared to Comparative Example 3 including barium sulfate and carboxymethyl cellulose.