JP2006049157A - Composite polymer electrolyte for lithium ion battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polymer electrolyte composition for a lithium ion battery, having high ion conductivity and improved mechanical properties. <P>SOLUTION: The electrolyte composition is manufactured, in such a way that a monomer composition containing a quaternary ammonium salt fused salt into which a polymerizable functional group is introduced, a lithium salt is polymerized under existence of a polymer reinforcing material, and a polymer obtained is vacuum heat-pressed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a composite polymer electrolyte composition disposed between electrodes in a lithium ion battery.

For the lithium secondary battery, a non-aqueous electrolyte containing a lithium salt is generally used. This solution was prepared by dissolving a lithium salt in an aprotic polar solvent such as carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, lactones such as γ-butyrolactone, and ethers such as tetrahydrofuran. Is.

However, these organic solvents are volatile, flammable, and have safety problems during overcharge, overdischarge, and short circuit. Also, the liquid electrolyte is difficult to handle when sealing the battery in a liquid-tight manner. Even if gelled non-aqueous electrolyte is used, the problems of volatilization of organic solvent and danger of ignition are not solved, and the problem of leakage of electrolyte separated from gel remains.

Recently, a lithium secondary battery using a non-aqueous electrolyte in which a lithium salt is dissolved in a room temperature molten salt containing a quaternary ammonium cation has been proposed. For example, see JP-A-10-92467, JP-A-10-265674, JP-A-11-92467 and JP-A-2002-11230. Room temperature molten salt is safe because it is liquid at room temperature but is non-volatile and non-flammable, but it contains liquid as a gel due to the matrix polymer, and its mechanical properties are insufficient, and the liquid undergoes phase separation. As a result, handling and battery design issues still remain.

There have also been proposals for producing an all solid polymer electrolyte by introducing a vinyl group into an imidazolium salt forming an ion conductive molten salt and polymerizing the monomer. See JP-A-10-83821 and JP-A-2000-11753. However, this polyelectrolyte does not have sufficient mechanical strength.
Japanese Patent Laid-Open No. 10-83821 JP-A-10-92467 JP-A-10-265673 JP-A-11-92467 JP 2000-11754 JP 2002-11230 A

Therefore, there remains a need for safe polyelectrolytes with high ionic conductivity and satisfactory mechanical properties.

The present invention relates to a monomer composition comprising a molten salt monomer having a quaternary ammonium salt structure comprising a quaternary ammonium cation and a fluorine-containing anion and a polymerizable functional group, and a lithium salt. The above needs are met by providing a composite polyelectrolyte composition produced by polymerization in the presence of polyvinylidene fluoride as a chemically inert polymeric reinforcement material and vacuum heat pressing.

There are several methods for combining an electrochemically inactive polymer reinforcing material represented by polyvinylidene fluoride with a polymer of molten salt monomer.

The first method is a method in which a molten salt monomer containing a lithium salt and a polymer reinforcing material are dissolved in an appropriate solvent, and this solution is cast into a film and then polymerized.

The second method is basically the same as the first method, but the first method is that a polymer reinforcing material into which a functional group that becomes a crosslinking point such as a carbon-carbon double bond is introduced in advance is used. The method is different.

In either case, the polymerization can be carried out by heat, light (ultraviolet light) or electron beam irradiation.

The solution containing the molten salt monomer before polymerization can be cast as a film on a non-adhesive substrate such as glass or polyester, and then peeled off after polymerization to be used as an independent film. The film may be applied to the active material surface and polymerized in that state to form a film integrated with the electrode.

Furthermore, it has been found that when the polymer electrolyte film once formed is crushed by a vacuum heat press so as to eliminate voids, the ionic conductivity is dramatically improved.

The polymer electrolyte film of the present invention formed as described above has remarkably improved mechanical properties represented by tensile strength as compared with a film not containing the polymer reinforcing material due to the presence of the polymer reinforcing material. If desired, a small proportion of polyfunctional monomer can be copolymerized with the molten salt monomer to further improve the mechanical properties. As a result of the reinforcement, the composite polymer electrolyte composition of the present invention can be used to assemble a compact high-performance battery having a high energy density.

A quaternary ammonium salt structure composed of a quaternary ammonium cation and a fluorine atom-containing anion and a salt structure of a monomer containing a polymerizable functional group are aliphatic, alicyclic, aromatic, or heterocyclic quaternary ammonium. It is a salt structure composed of a cation and a fluorine atom-containing anion. The “quaternary ammonium cation” here means an onium cation of nitrogen, and includes heterocyclic onium ions such as imidazolium and pyridium. Mention may be made of a salt structure comprising at least one ammonium cation selected from the following ammonium cation group and at least one anion selected from the following anion group.

(Ammonium cation group) pyrrolium cation, pyridinium cation, imidazolium cation, pyrazolium cation, benzimidazolium cation, indolium cation, carbazolium cation, quinolinium cation, pyrrolidinium cation, piperidinium cation, Piperazinium cation, alkylammonium cation (including those substituted with a hydrocarbon group having 1 to 30 carbon atoms, hydroxyalkyl, alkoxyalkyl). Any of them includes N and / or a ring having a hydrocarbon group having 1 to 10 carbon atoms, a hydroxyalkyl group, or an alkoxyalkyl group.

(Anion group) BF ₄ , PF ₆ , C _n F _2n-1 CO ₂ (where n is an integer of 1 to 4), C _n F _2n-1 SO ₃ (where n is an integer of 1 to 4), (FSO _{2) 2 N, (CF 3} SO 2) 2 N, (C 2 F 5 SO 2) 2 N, (CF 3 SO 2) 3 C, CF 3 -SO 2 -N-COCF 3), R-SO 2 _{-N-SO 2 CF 3 Li (} R is an aliphatic group), and _{ArSO 2 -N-SO 2 CF 3} (Ar is an aromatic group).

The above ammonium cation and anion species are excellent in heat resistance, reduction resistance or oxidation resistance, have a wide electrochemical window, and are preferable for use in batteries and capacitors.

Polymerizable functional groups in monomers include carbon-carbon unsaturated groups such as vinyl, acrylic, methacrylic and allyl groups, cyclic alkoxide groups such as epoxy and oxetane groups, isocyanate groups, hydroxyl groups and carboxyl groups. Can be illustrated.

Particularly preferred ammonium cation species include 1-vinyl-3-alkylimidazolium cation, 4-vinyl-1-alkylpyridinium cation, 1-alkyl-3-allylimidazolium cation, 1- (4-vinylbenzyl) -3. -Alkyl imidazolium cation, 1- (vinyloxyethyl) -3-alkyl imidazolium cation, 1-vinyl imidazolium cation, 1-allyl imidazolium cation, N-allylbenzimidazolium cation, diallyl-dialkylammonium cation, etc. I can list them. However, alkyl is a C1-C10 alkyl group.

Particularly preferred anionic species include bis {(trifluoromethyl) sulfonyl} amide anion, 2,2,2-trifluoro-N- (trifluoromethylsulfonyl) acetamide anion, bis {(pentafluoroethyl) sulfonyl} amide anion, Bis {(fluoro) sulfonyl} amide anion, tetrafluoroborate anion, trifluoromethanesulfonate anion, and the like.

Particularly preferable monomers include 1-vinyl-3-alkylimidazolium bis {(trifluoromethyl) sulfonyl} amide (wherein alkyl is C1 to C10), 1-vinyl-3-alkylimidazolium tetrafluoroborate ( However, alkyl is C1-C10), 4-vinyl-1-alkylpyridinium bis {(trifluoromethyl) sulfonyl} amide (wherein alkyl is C1-C10), 4-vinyl-1-alkylpyridinium tetrafluoroborate (however, , Alkyl is C1-C10), 1- (4-vinylbenzyl) -3-alkylimidazolium bis {(trifluoromethyl) sulfonyl} amide (wherein alkyl is C1-C10), 1- (4-vinylbenzyl) -3-alkylimidazolium tetrafluoroborate (however, Alkyl is C1-C10), 1-glycidyl-3-alkyl-imidazolium bis {(trifluoromethyl) sulfonyl} amide (wherein alkyl is C1-C10), 1-glycidyl-3-alkyl-imidazolium tetrafluoroborate (However, alkyl is C1-C10), N-vinylcarbazolium tetrafluoroborate, etc. can be illustrated.

The charge transfer ion source of the lithium ion battery is a lithium salt. In the present invention, a lithium salt composed of the following lithium cation and fluorine atom-containing anion can be preferably used.

The lithium salt is LiBF ₄ , LiPF ₆ , C _n F _2n-1 CO ₂ Li (where n is an integer of 1 to 4), C _n F _2n-1 SO ₃ Li (where n is an integer of 1 to 4). , (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ , NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) 3 CLi, (CF ₃ —SO ₂ —N—COCF ₃ ) Li, (R—SO ₂ —N—SO ₂ CF ₃ ) Li (R is an aliphatic group), and (ArSO ₂ —N—SO ₂ CF ₃ ) Li (Ar is an aromatic group).

A preferred polymer reinforcing material is a fluorine-based polymer, and particularly preferred is polyvinylidene fluoride, a copolymer thereof, and a modified polymer thereof. The molecular weight is 2000 to 2000000 as a number average molecular weight, preferably 3000 to 100000, particularly preferably 5000 to 50000.

Further, it was found that one of the conditions of the vacuum heat press effective for dramatically improving the ionic conductivity of the once formed polymer electrolyte film was 90 ° C. and 21 Kg / cm ² .

According to the present invention, the molten salt monomer, lithium salt, and polymer reinforcing material are dissolved in a suitable solvent such as dimethylacetamide and the solution is cast on a substrate such as glass or polyester film. It is possible to obtain a thin film of a polymer electrolyte composition which is coated and peeled after polymerization to form a composite.

In the case of an electrolyte composition forming a polymer salt of a molten salt monomer and a polymer blend of a reinforcing material, the proportion of the polymeric reinforcing material should be such that an optimal balance between mechanical properties and ionic conductivity is obtained. It is determined.

The weight ratio of reinforcing material to molten salt monomer is generally between 0.1 and 0.8, particularly preferably between 0.35 and 0.65. For a combination of a specific molten salt monomer and a specific polymeric reinforcement material, the optimal balance between mechanical properties and ionic conductivity can be determined experimentally.

Similarly, in the case of blend type electrolyte compositions, the optimal balance between lithium salt and mechanical properties depends on the ratio of lithium salt to molten salt monomer. This ratio is generally in the range from 0.05 to 0.8, preferably from 0.1 to 0.7, particularly preferably from 0.15 to 0.5, by weight.

Further, it was found that one of the conditions of the vacuum heat press effective for dramatically improving the ionic conductivity of the once formed polymer electrolyte film was 90 ° C. and 21 Kg / cm ² .

The optimum ratio of the combination of a specific molten salt monomer and a specific lithium salt can be easily determined experimentally.

The molten salt monomer can be homopolymerized or copolymerized with a monomer that can be copolymerized therewith. One of the preferred embodiments is the formation of a graft cross-linked polymer using a polymer reinforcing material having a functional group that reacts with the molten salt monomer. As the monomer to be copolymerized, two or more types of molten salt monomers may be used, a monomer that does not contain a salt structure, or a polyfunctional monomer having a plurality of polymerizable functional groups. May be.

The main polymerization reaction is usually performed by adding a monomer thermal polymerization initiator and a curing agent and heating to 40 ° C to 200 ° C. When the polymerizable functional group is a carbon-carbon unsaturated group, the thermal polymerization initiator may be benzoyl peroxide, dicumyl peroxide, di-t-butyl peroxide, 1,1-bis (t-butylperoxy). ) cyclohexane, peroxides such as cumene hydroperoxide, 2,2 ^- azobisisobutyronitrile, ^2,2, - azobis (2,4-dimethylvaleronitrile) azobis compounds such as, inorganic, such as ammonium persulfate A system initiator etc. can be mentioned.

The amount of the polymerization initiator used is usually 0.1 to 10%, preferably 1 to 5%, based on the total weight of the polymerizable monomers. When the polymerizable functional group is an epoxy group, amines, acid anhydrides and carboxylic acids can be used as curing agents, and alkylimidazole derivatives can be used as reaction catalysts.

In order to polymerize, radiation such as ultraviolet rays (using a photopolymerization initiator) or an electron beam can be irradiated. Electron beam polymerization is a preferred embodiment because it can be expected to undergo a crosslinking reaction of the polymer reinforcing material itself and a graft reaction of the monomer to the reinforcing material. The irradiation amount is 0.1 to 50 Mrad, preferably 1 to 20 Mrad.

Examples of polyfunctional monomers containing two or more polymerizable functional groups copolymerizable with molten salt are divinylbenzene, diallyl phthalate, ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (Meth) acrylate, trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, triallyl isocyanurate, triallyl cyanurate, diallyl-dimethylammonium bis-{(trifluoromethyl) sulfonyl} amide, diallyl- Examples include dimethylammonium tetrafluoroborate and 2,2-bis (glycidyloxy) phenylpropane. These polyfunctional monomers can be used in an amount of 0.5 to 10 mol% of the molten salt monomer.

In the polymer electrolyte composition of the present invention, in the case of a polymer blend, the molten salt monomer containing the lithium salt and the reinforcing material polymer are phase-separated microscopically to impart mechanical properties to the respective ionic conductivity. It is considered that the function is performed.

Further, it was found that the ionic conductivity was improved by about 24% when the pores were eliminated by vacuum heat pressing the once formed polymer electrolyte film at 90 ° C. and 21 kg / cm ² .

The composite polymer electrolyte composition of the present invention is sandwiched between opposing electrodes of a lithium ion battery.

The lithium-ion battery, a negative electrode, typically comprising an active material layer made of a carbon material that the lithium ion occluding and releasing is _{graphite, LiCoO 2, LiFeO, LiNi n} Co 1-n O 2 (n <1) , A positive electrode having an active material layer made of a composite metal oxide containing lithium that absorbs and releases lithium ions typified by LiMn ₂ O ₄ is used.

The invention is illustrated by the following non-limiting examples.

All parts and percentages in the examples are on a weight basis unless otherwise specified. The measurement in an Example was performed with the following method.

Ionic conductivity: Electrode area: 0.95cm ² between the platinum electrodes, measure the membrane resistance by AC impedance method (0.1V, frequency 1Hz-10MHz) at room temperature and 65% RH, and determine the ionic conductivity. Calculated.

Tensile strength: A tensile tester Tensilon RT1350 manufactured by A & D, 23 ° C., 5 cm / min. Measured with

Moreover, the compound synthesize | combined in the Example was identified by IR spectrum and NMR spectrum.

Synthesis of 1-methyl-3- (4-vinylbenzyl) imidazolium bis {(trifluoromethyl) sulfonyl} amide [abbreviated as MVBI / TFSI]

While dissolving 37.0 g (0.45 mol) of 1-methylimidazole in 200 ml of 1,1,1-trichloroethane and stirring at room temperature, 68.7 g (0.45 mol) of p-chloromethylstyrene was added to 100 ml of 1,1,1-trichloroethane. A solution dissolved in 1,1-trichloroethane was added dropwise over 1 hour, and the reaction was further continued for 10 hours at 65 ° C. with stirring. The product was separated, washed twice with 100 ml each of 1,1,1-trichloroethane, dried at 65 ° C. and 0.1 mm for 2 hours, and pale yellow solid 1-methyl-3- (4-vinylbenzyl) ) 52.8 g (50%) of imidazolium chloride [MVBI · Cl] was obtained. Next, 31.9 g (0.1 mol) of potassium bis {(trifluoromethyl) sulfonyl} amide (KTFSI) was dissolved in 100 ml of water at 70 ° C. and stirred at 50 ° C., and the MVBI · Cl obtained above was dissolved. A solution obtained by dissolving 23.4 g (0.1 mol) in 50 ml of water was dropped and mixed in 15 minutes. After further metathesis reaction for 2 hours with vigorous stirring at 50 ° C., the produced oil layer was separated. The product was washed twice with 50 ml of water each and then dried at 60 ° C. and 0.1 mmHg for 2 hours to give 1-methyl-3- (4-vinylbenzyl) imidazolium bis {(trifluoromethyl) sulfonyl} amide. [It abbreviates as MVBI * TFSI] 40.8g (yield 85%) was obtained.

Synthesis of polyvinylidene fluoride modified polymer (abbreviated as DBF) containing a carbon-carbon double bond

Atofina [Co., Ltd., 15 g of polyvinylidene fluoride (Kynar 461) and 85 g of N-methylpyrrolidone-2 [NMP] were placed in a 300 ml three-necked flask equipped with a stirrer and dissolved at 90 ° C. While stirring at the same temperature, 2.37 g of triethylamine was added dropwise in about 10 minutes. Furthermore, it was made to react for 30 minutes, stirring at the same temperature. After cooling, it was added to 300 ml of water under stirring and reprecipitated. The precipitated polymer was immersed and washed twice with 500 ml of water and filtered, and vacuum dried at 60 ° C. for 10 hours.

The recovered polymer was found to have about 8 mol% of double bonds introduced by NMR spectrum analysis.

8.4 g of 1-methyl-3- (4-vinylbenzyl) imidazolium bis {(trifluoromethyl) sulfonyl} amide [abbreviated as MVBI · TFSI] obtained in Example 1, obtained in Example 2 A solution in which 10.0 g of polyvinylidene fluoride-modified polymer [DBF] and 0.17 g of benzoyl peroxide were dissolved in 80 g of dimethylacetamide was prepared. In this solution, 4.0 g of lithium bis {(trifluoromethyl) sulfonyl} amide [abbreviated as LiTFSI] was dissolved to prepare an electrolyte precursor solution. This solution was coated on a 100 μm polyester film (T type manufactured by Toray Industries, Inc.), heated with a hot air dryer at 130 ° C. for 30 minutes, and simultaneously with drying, a polymerization reaction was performed. The coating film was peeled off from the polyester film to obtain a transparent film having a film thickness of 25 μm.

The ionic conductivity of this film was 2.1 × 10 ⁻³ S / cm at 20 ° C., and the tensile strength was 11 MPa.

Furthermore, when this once formed polymer electrolyte film was vacuum heat pressed at 90 ° C. and 21 Kg / cm ² , the ionic conductivity was improved by about 24% while maintaining the tensile strength of the film.

In the same manner as in Example 1, triethyl- (4-vinylbenzyl) ammonium chloride was synthesized from triethylamine and p-chloromethylstyrene and further reacted with KTFSI to obtain triethyl- (4-vinylbenzyl) ammonium bis { (Trifluoromethyl) sulfonyl} amide [abbreviated as TEVBA · TFSI] was synthesized. Next, an electrolyte precursor solution obtained by dissolving 7.0 g of TEVBA · TFSI obtained above, 13.0 g of polyvinylidene fluoride resin (Kynar 461 manufactured by Atofina), 0.14 g of benzoyl peroxide, and 7.0 g of LiTFSI in 80 g of dimethylacetamide was prepared. It was adjusted. This solution was applied onto a 3 mm glass plate, heated at 130 ° C. for 30 minutes with the glass plate, and dried and polymerized. The coating film was peeled off from the glass plate to obtain a film with a thickness of 30 μm.

The ionic conductivity of this film was 3.0 × 10 ⁻⁴ S / cm at 20 ° C., and the tensile strength was 6 MPa.

Furthermore, when this once formed polymer electrolyte film was vacuum heat pressed at 90 ° C. and 21 Kg / cm ² , the ionic conductivity was improved by about 24% while maintaining the tensile strength of the film.

Claims (7)

Lithium ions were added to a monomer composition containing a quaternary ammonium salt structure comprising a quaternary ammonium cation and a fluorine-containing anion and a molten salt monomer having a polymerizable functional group, and electrochemically added. A composite polymer electrolyte composition for a lithium ion battery produced by polymerization in the presence of polyvinylidene fluoride as an inert polymer reinforcing material and vacuum heat pressing. The composite polymer electrolyte composition according to claim 1, wherein the monomer composition contains a polyfunctional monomer that can be copolymerized with the molten salt monomer. The molten salt monomer is 1-vinyl-3-alkylimidazolium cation, 4-vinyl-1-alkylpyridinium cation, 1-alkyl-3-allylimidazolium cation, 1- (4-vinylbenzyl) -3. -Alkyl imidazolium cation, 1- (vinyloxyethyl) -3-alkyl imidazolium cation, 1-vinyl imidazolium cation, 1-allyl imidazolium cation, N-allylbenzimidazolium cation, and quaternary diallyldialkylammonium A quaternary ammonium cation selected from the group consisting of bis [(trifluoromethyl) sulfonylamide anion, 2,2,2-trifluoro-N- (trifluoromethylsulfonyl) acetamide anion, bis [(pentafluoro Ethyl) sulfo 2. The composite polymer electrolyte composition according to claim 1, which is a salt with an anion selected from the group consisting of [Lu] amide anion, bis [(fluoro) sulfonyl] amide anion, tetrafluoroborate anion, and trifluoromethanesulfonate anion. . The composite polymer electrolyte composition according to claim 1, wherein the polyvinylidene fluoride is polyvinylidene fluoride containing a carbon-carbon unsaturated double bond. The composite polymer electrolyte composition according to claim 1, wherein the polyvinylidene fluoride forms a polymer blend with a polymer of the molten salt monomer. The lithium salt is LiBF ₄ , LiPF ₆ , C _n F _2n-1 CO ₂ Li (where n is an integer of 1 to 4), C _n F _2n-1 SO ₃ Li (where n is an integer of 1 to 4). , (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ , NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) 3 CLi, (CF ₃ —SO ₂ —N—COCF ₃ ) The composite polymer electrolyte composition according to claim 1, which is selected from the group consisting of Li and (R—SO ₂ —N—SO ₂ CF ₃ ) Li (where R is an alkyl group or an aryl group). The composite polymer electrolyte composition according to any one of claims 1 to 6, wherein the vacuum heat press is performed under conditions of 90 ° C and 21 kg / cm ² .