JP4640544B2 - Gphrato copolymer having poly (quaternary ammonium salt) side chain and electrolyte membrane - Google Patents

Description

  The present invention relates to a graft copolymer having a quaternary ammonium salt polymer chain in the side chain and an electrolyte membrane using the graft copolymer.

  For example, a non-aqueous electrolyte containing a lithium salt is generally used for a lithium ion battery. This solution was obtained 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, short circuit, and the like. Also, the liquid electrolyte is difficult to handle when sealing the battery in a liquid-tight manner. Even if the gelled non-aqueous electrolyte is used, the problem of volatilization of the organic solvent and the danger of ignition are not solved, and the problem that the electrolyte separated from the gel leaks still remains.

  Recently, a lithium ion 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-10-265673, JP-A-11-92467, and JP-A-2002-11230. Normal temperature molten salt is safe because it is liquid at normal temperature but is non-volatile and non-flammable, but it contains liquid even when gelled by matrix polymer, and its mechanical properties are insufficient, and the liquid is phase separated. As such, handling issues and battery design issues remain.

  There has also been proposed a method 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 polymer electrolyte also does not have sufficient mechanical strength.

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

  In recent years, electronic devices, in particular portable electronic devices, are becoming lighter and smaller in size, and the electrolyte material of a lithium ion battery as a power source for such devices has ideally the same ionic conductivity as a liquid electrolyte. It is desirable to have the same mechanical strength and moldability as general thermoplastics and to function as a reliable separator when placed between electrodes as a film.

  The present invention provides a graft copolymer having a polymer chain having a repeating unit having a quaternary ammonium room temperature molten salt (ionic liquid) structure as a side chain. This graft copolymer has a plurality of poly (quaternary ammonium salt) side chains graft-polymerized to a homo- or copolymer of a halogen-substituted ethylenically unsaturated hydrocarbon monomer by an atom transfer radical polymerization method.

  The present invention also provides an electrolyte membrane for a lithium ion battery obtained by doping the graft copolymer of the present invention with a lithium salt.

Preferred embodiment

The atom transfer radical polymerization method (ATRP) is a living polymerization method using a transition metal complex salt as a catalyst as described in WO96 / 30421, WO97 / 1824, WO98 / 01480, WO99 / 54368, WO02 / 22712. It is known as a polymerization method for obtaining a polymer having a narrow molecular weight distribution represented by Mw / Mn. Other vinyl monomers can also be graft copolymerized using ATRP to a halogen-substituted ethylenically unsaturated hydrocarbon polymer such as polyvinylidene fluoride. See WO02 / 22712. In the case of the present invention, the halogenated ethylenically unsaturated hydrocarbon polymer (trunk polymer) is activated by the transition metal complex salt to produce a macroinitiator having free radicals (initiation reaction). Next, the produced macroinitiator reacts with a quaternary ammonium salt monomer having a vinyl group (growth reaction), and ends with a termination reaction. By repeating these reactions, a quaternary ammonium salt polymer chain is graft-copolymerized to the backbone polymer. When the halogen has regularly bonded to the main chain of the backbone polymer, the resulting graft copolymer has a comb structure.
In the polymerization method, after grafting a monomer having a vinyl group and a functional group such as a hydroxyl group, an epoxy group, or an isocyanate group, the molten ammonium salt can be introduced into the side chain using the functional group. Examples of such monomers include 2-hydroxyethyl methacrylate, glycidyl methacrylate, 2-methacryloyloxyethyl isocyanate, and the like.

  The central transition metal species and ligand species of complex salts that can be used as catalysts in ATRP are described in detail in the above-cited PCT International Publication, and graft copolymerization with ATRP is disclosed in detail in WO 02/22712. In general, the initiator of ATRP is a halogen-containing compound, and the metal species of the complex is a transition metal belonging to Group 7 to Group 11, such as copper, iron, nickel, ruthenium, rhodium, palladium, rhenium. This polymerization is called atom transfer radical polymerization because the halogen atom at the growth end moves each time the monomer is inserted. However, it is not known to graft copolymerize a quaternary ammonium salt monomer and to use the resulting graft copolymer as an electrolyte membrane material that also serves as a separator for a lithium ion battery. Therefore, here, the trunk polymer and the quaternary ammonium salt monomer to be graft copolymerized will be mainly described in detail.

  The backbone polymer that can be used in the present invention is a homo- or copolymer of a halogenated ethylenically unsaturated hydrocarbon monomer. Typical examples of such monomers are vinyl chloride, vinylidene chloride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene. Polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyhexafluoropropylene and copolymers thereof are preferred. The molecular weight of the backbone polymer is arbitrary, but it is preferable that the number average molecular weight (Mn) is at least 50,000 to 500,000 in order to give satisfactory mechanical strength.

  The quaternary ammonium salt monomer is a salt of an ammonium cation having an ethylenically unsaturated group (including vinyl and allyl groups) and a fluorine atom-containing anion.

Typical examples of ammonium cation species include 1-vinyl-3-alkylimidazolium cation, 4-vinyl-1-alkylpyridinium cation, 1-alkyl-3-allylimidazolium cation, 1- (4-vinyloxyethyl)- 3-alkyl imidazolium cation, 1-vinyl imidazolium cation, 1-allyl imidazolium cation, N-allyl benzimidazolium cation, N, N-diallylalkyl ammonium cation, vinyl benzyl alkyl ammonium cation, acryloylaminoalkyl ammonium cation and Includes N- (meth) acryloyloxyalkyl-N, N, N-trialkylammonium cations.

  Typical examples of fluorine-containing anion species are bis [(trifluoromethyl) sulfonyl] amide anion, 2,2,2-trifluoro-N- (trifluoromethylsulfonyl) acetamide anion, bis [(pentafluoroethyl) sulfonyl]. Including amide anion, bis (fluorosulfonyl) amide anion, tetrafluoroborate anion, and trifluoromethanesulfonate anion.

  A second copolymer is formed by graft-copolymerizing an ethylenically unsaturated monomer having a first reactive functional group to a trunk polymer, and forming a linking group by reacting with the first reactive functional group on the side chain of the resulting graft polymer. A molten ammonium salt-type graft copolymer can also be produced by reacting a molten ammonium salt having a reactive functional group.

  Non-limiting examples of the combination of the first reactive functional group and the second reactive functional group forming the linking group are an isocyanate group and a hydroxyl group or an amino group, and a glycidyl group and a carboxyl group. Other combinations without reaction byproducts obvious to those skilled in the art are of course within the scope of the present invention.

  The ethylenically unsaturated monomer having the first reactive functional group is preferably a (meth) acrylic acid ester. Examples are 2-hydroxyethyl (meth) acrylate, glycidyl (meth) acrylate, 2-cyanatoethyl (meth) acrylate and the like. The second reactive functional group is bonded to one of the hydrocarbon groups bonded to nitrogen of the quaternary ammonium salt. When the first reactive functional group is a hydroxyl group, it is an isocyanate group, and when it is a glycidyl group, In the case of a carboxyl group or an isocyanate group, it is a hydroxyl group or an amino group. For example, N- (3-carboxypropyl) -N, N, N-trimethylammonium salt and hydroxyethyltrimethylammonium salt are preferable. The anion species of these ammonium salts are the fluorine atom-containing anions mentioned above, preferably bis (trifluoromethylsulfonyl) imide anions. When the first or second reactive functional group is an isocyanate group, it is preferably blocked with a known blocking agent. Although any blocking agent that regenerates isocyanate groups by heating can be used, oximes such as methyl ethyl ketoxime, the reaction product of methyl ethyl ketone and hydroxylamine, are preferred.

  The conditions for the graft polymerization reaction by ATRP are described in WO02 / 22712 cited above. One advantage of graft copolymerization with ATRP is that the molecular weight of the main chain of the graft copolymer is always greater than or equal to the molecular weight of the main chain of the backbone polymer. In other words, there is no depolymerization of the trunk polymer. Accordingly, the molecular weight of the entire graft copolymer increases by the molecular weight of the entire grafted quaternary ammonium salt polymer side chain. In order to make the graft copolymer doped with lithium salt have practical ionic conductivity, the molecular weight of the entire graft copolymer should be at least 1.5 times, especially 2 to 5 times the molecular weight of the trunk polymer. Is preferred.

  The use of the graft copolymer of the present invention is typically a material for an electrolyte membrane that also serves as a separator disposed between a negative electrode and a positive electrode in a lithium ion battery, but is not limited thereto. For example, it is also suitable for materials such as proton conductive membranes for fuel cells and electrolyte membranes for dye-sensitized wet solar cells. Here, the production of an electrolyte membrane for a lithium ion battery will be described as an example.

This electrolyte membrane must contain a lithium salt as a supporting salt. Although any lithium salt can be used, typical examples of preferred lithium salts are LiBF ₄ , LiPF ₆ , CnH _{2n + 1} CO ₂ Li, (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₃ CLi, (CF ₃ SO ₂ —N—COCF ₃ ) Li, and (R—SO ₂ —N—SO ₂ CF ₃ ) Li [wherein n is an integer of 1 to 4, R is an alkyl group Or an aryl group]. The dope amount of the lithium salt is generally 0.05 to 0.8, preferably 0.1 to 0.7, and most preferably 0.15 to 0.5, when the graft copolymer is 1 by weight ratio. It is a range.

  If the Tg of the graft copolymer is higher than the operating temperature of the battery, or if it is desired to increase the ionic conductivity of the membrane while maintaining a balance with the mechanical strength, the graft copolymer may be plasticized or gelled. it can. The plasticizer that can be used for this purpose may be a solvent of a non-aqueous liquid electrolyte generally used in lithium ion batteries, but is preferably an ionic liquid, that is, an ammonium salt-based room temperature molten salt having no polymerizable functional group. It is. A number of such ionic liquids are known, but the preferred typical example is that the ammonium cation species is N-methyl-N-propylpyridinium cation, N-methoxyethyl-N-methyl-N, N-diethylammonium cation, Or a quaternary ammonium salt compound selected from N, N, N-triethyl-N-methoxyethylammonium cations and an anion species selected from bis [(trifluoromethyl) sulfonyl] amide anions or tetrafluoroborate anions. . Since plasticization with an ionic liquid also affects the mechanical strength of the graft copolymer membrane, the amount must be determined so that the ionic conductivity and mechanical strength are optimally balanced. When the weight ratio of the graft copolymer is 1, the ratio of the ionic liquid is generally 1.0 or less, preferably 1.0 to 0.5.

  The electrolyte membrane is prepared by dissolving a graft copolymer in a solvent such as N-methyl-2pyrrolidone (NMP), dissolving a lithium salt to be doped therein and an ionic liquid when used, and dissolving the obtained solution. It can be produced by casting on a substrate such as a glass plate or polyester film, drying the solvent and then peeling the film. Thereafter, it is preferable to heat press under vacuum in order to make the membrane compact. The electrolyte membrane produced in this way is believed to have sufficient performance to function as a reliable separator, but if necessary, electrochemically inert reinforcements such as silica whiskers and short glass fibers. The material may be suspended in the above solution to reinforce the membrane.

  The obtained electrolyte membrane is sandwiched between a known negative electrode and a positive electrode used in a lithium ion battery. It is preferable to apply or impregnate the above solution also on the negative electrode and the positive electrode active material, and similarly to dry the solvent and then heat-press it under vacuum to make it compact.

  The following examples illustrate the invention. In these examples, polyvinylidene fluoride (PVDF) is used as the backbone polymer, copper (I) chloride is used as the transition metal salt, and 4,4′-dimethyl-2,2′-dipyridyl (BPY) is used as the ligand of the complex salt. Although used, it will be appreciated that other backbone polymers, transition metal salts and ligands disclosed in the prior art can be used to synthesize the graft copolymers of the present invention. In addition, the meaning of the symbol used in the Example is as follows.

PVDF: polyvinylidene fluoride NMP: N-methyl-2-pyrrolidone BPY: 4,4′-dimethyl-2,2′-bipyridyl MOETMA • TFSI: N-methacryloyloxyethyl-N, N, N-trimethylammonium bis ( Trifluoromethylsulfonyl) imide salt HTFSI: bis (trifluoromethylsulfonyl) imidic acid

  In a three-necked flask equipped with a stirrer, 2 g of PVDF and 30 g of NMP were added and dissolved, and then 28 g of MOETMA · TFSI (corresponding to 2 mol per 1 PVDF repeating unit) was added and dissolved by stirring. Next, a solution of 0.2 g of BPY and 0.04 g of copper chloride (CuCl) in 10 g of NMP was added to this solution, and argon gas was blown into the solution while stirring. And it was made to react at 90 degreeC for 20 hours in a water bath. After completion of the reaction, 500 ml of 40% ethanol aqueous solution was added to the reaction solution and stirred, and the precipitate was separated by filtration, redissolved in 30 g of NMP, and again added with 500 ml of 40% ethanol aqueous solution to precipitate the graft copolymer and filtered. Thereafter, it was vacuum dried at 70 ° C. to obtain 8.8 g of a graft copolymer. Infrared spectrum analysis confirmed that 33 mol% of MOETMA · TFSI was grafted.

  In a three-necked flask equipped with a stirrer, 2 g of PVDF and 30 g of NMP were added and dissolved, and then 6.5 g of MOETMA · HCl (corresponding to 1 mol per unit of PVDF) was added and dissolved. Next, a solution of 0.2 g of BPY and 0.04 g of copper chloride (CuCl) in 10 g of NMP was added to this solution, and argon gas was blown into the solution while stirring. And it was made to react at 90 degreeC for 20 hours in a water bath. After completion of the reaction, the reaction product was purified and dried in the same manner as in Example 1 to obtain 3.0 g of a graft copolymer. This was redissolved in 30 g of NMP, 2 g of 70% HTFSI aqueous solution was added, and the mixture was stirred at 50 ° C. for 1 hour, and 500 ml of 40% ethanol aqueous solution was added to the reaction solution to precipitate the product. Vacuum drying at 4 ° C. for 4 hours yielded 4.1 g of an anion exchanged graft copolymer. Infrared spectrum analysis confirmed that 12 mol% of MOETMA · TFSI was grafted.

Introduction of molten ammonium base by reaction of isocyanate group and hydroxyl group In a three-necked flask equipped with a stirrer, 2 g of PVDF and 30 g of NMP were added and dissolved. In addition, it was dissolved. Next, a solution of 0.2 g of BPY and 0.04 g of copper chloride (CuCl) in 10 g of NMP was added to this solution, and argon gas was blown into the solution while stirring. And it was made to react at 90 degreeC for 20 hours in a water bath. The reaction product was purified and dried in the same manner as in Example 1 to obtain 4.2 g of a graft copolymer. This polymer was dissolved in 40 g of NMP, 3.5 g of hydroxyethyltrimethylammonium · TFSI was added, and the mixture was reacted at 145 ° C. for 1 hour to introduce a molten ansenium salt into the side chain. After completion of the reaction, purification is carried out in the same manner as in Example 1 to obtain a graft copolymer having a side chain into which a molten ammonium base has been introduced.

Introduction of molten ammonium base by reaction of epoxy group and carboxyl group In a three-necked flask equipped with a stirrer, 2 g of PVDF and 30 g of NMP were added and dissolved, then 6.7 g of glycidyl methacrylate (1.5 mol per unit of PVDF) and 0.2 g of BPY in 10 g of NMP and A solution of copper chloride (CuCl) 0.04 g was added and stirred, and argon gas was blown in. And it was made to react at 90 degreeC for 20 hours in a water bath. The reaction product was purified and dried in the same manner as in Example 1 to obtain 2.9 g of a graft copolymer. This polymer was dissolved in 40 g of NMP, 2.7 g of N- (2-carboxypropyl) -N, N, N-trimethylammonium · TFSI was added, and the mixture was reacted at 80 ° C. for 1 hour to introduce a molten ansenium salt into the side chain. After completion of the reaction, purification is carried out in the same manner as in Example 1 to obtain a graft copolymer having a side chain into which a molten ammonium base has been introduced.

Molding of electrolyte membrane A graft copolymer having a PVDF: MOETMA · TFSI weight ratio of 17: 3 was produced according to the procedure of Example 1. This graft copolymer was dissolved in dimethylacetamide, LiTFSI was added to obtain a solution having a graft copolymer: lithium salt weight ratio of 7: 3, and the concentration was adjusted to 20% by weight with dimethylacetamide. This solution was applied to a glass plate using an applicator whose scale was adjusted to 100 μm, and dried at 110 ° C. for 10 minutes under vacuum to obtain an electrolyte membrane for a lithium ion battery having a thickness of 30 μm. The electrolyte membrane had an ionic conductivity of 26 × 10 ⁻³ S / cm and a tensile strength of 13 MPa.

  The thickness of the electrolyte membrane can be changed in the range of 10 μm to 100 μm by adjusting the coating amount (applicator scale).

Claims (15)

A graft copolymer having a poly (quaternary ammonium cation) and a fluorine atom-containing anion in the side chain graft-copolymerized by a atom transfer radical polymerization method to a trunk pomer comprising a halogen-substituted ethylenically unsaturated hydrocarbon monomer alone or as a copolymer. Polymer. It is a molten salt of a quaternary ammonium cation having an ethylenically unsaturated group and a fluorine atom-containing anion by an atom transfer radical polymerization method into a trunk pomer comprising a halogen-substituted ethylenically unsaturated hydrocarbon monomer alone or as a copolymer. A graft copolymer obtained by graft copolymerizing monomers and having poly (quaternary ammonium cation) and a fluorine atom-containing anion in the side chain. The graft copolymer according to claim 1 or 2, wherein the trunk polymer is polyvinylidene fluoride, poly (tetrafluoroethylene), poly (hexafluoropropylene), or a copolymer thereof.   The quaternary ammonium cation having an ethylenically unsaturated group includes 1-vinyl-3-alkylimidazolium cation, 4-vinyl-1-alkylpyridinium cation, 1-alkyl-3-allylimidazolium cation, 1- (4 -Vinyloxyethyl) -3-alkylimidazolium cation, 1-vinylimidazolium cation, 1-allylimidazolium cation, N-allylbenzimidazolium cation, N, N-diallylalkylammonium cation, N-alkylpiperidinium Claims selected from the group consisting of cations, vinylbenzylalkylammonium cations, acryloylaminoalkylammonium cations and N- (meth) acryloyloxyalkyl-N, N, N-trialkylammonium cations. Graft copolymer according to any one of to 3.   The fluorine-containing anion includes a bis [(trifluoromethyl) sulfonyl] amide anion, 2,2,2-trifluoro-N- (trifluoromethylsulfonyl) acetamide anion, bis [(pentafluoroethyl) sulfonyl] amide anion, The graft copolymer according to any one of claims 1 to 4, which is selected from the group consisting of a bis (fluorosulfonyl) amide anion, a tetrafluoroborate anion, and a trifluoromethanesulfonate anion. The introduction of the ammonium base involves graft copolymerization of an ethylenically unsaturated monomer having a first reactive functional group selected from an isocyanate group and a glycidyl group into the trunk polymer, and the first reaction of the side chain after the graft copolymerization. 6. The reaction according to claim 1, which is performed by reacting a molten ammonium salt having a second reactive functional group selected from a hydroxyl group, an amino group, and a carboxyl group, which forms a linking group by reaction with the functional group. The graft copolymer described in 1.   The combination of the first reactive functional group and the second reactive functional group is selected from a combination of an isocyanate group and a hydroxyl group or an amino group, or a combination of a glycidyl group and a carboxyl group. Graft copolymer.   The ethylenically unsaturated monomer having the first reactive functional group is glycidyl (meth) acrylate, and the molten ammonium salt having the second reactive functional group is N- (3-carboxypropyl) -N, N, The graft copolymer according to claim 6, which is a salt of N-trimethylammonium cation and bis (trifluoromethylsulfonyl) imide anion.   The ethylenically unsaturated monomer having the first reactive functional group is a block of 2-isocyanatoethyl (meth) acrylate, and the molten ammonium salt having the second reactive functional group is a hydroxyethyltrimethylammonium cation and bis. The graft copolymer according to claim 6, which is a salt with (trifluoromethylsulfonyl) imide and an anion. The electrolyte membrane for lithium ion batteries which doped the lithium salt to the graft copolymer in any one of Claims 1-9 . The lithium salt includes LiBF ₄ , LiPF ₆ , CnF _{2n + 1} CO ₂ Li, (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ − n-COCF ₃₎ Li, and _{(R-SO 2 -N-SO} 2 CF 3) Li [wherein n is an integer of 1 to 4, R represents an alkyl group or an aryl] claim 10 selected from the group consisting of Electrolyte membrane.   The electrolyte membrane according to claim 10 or 11, which is plasticized with a non-polymerizable molten quaternary ammonium salt compound. In the non-polymerizable quaternary ammonium salt compound, the ammonium cation species is N-methyl-N-propylpipedinium cation, N-methoxyethyl-N-methyl-N, N-diethylammonium cation and N, N, N- The electrolyte membrane according to claim 12, which is a salt compound containing a cation selected from triethyl-N-methoxyethylammonium cation , a bis [(trifluoromethyl) sulfonyl] amide anion, and an anion selected from a tetrafluoroborate anion.   The electrolyte membrane according to claim 10, wherein a weight ratio of the lithium salt to the graft copolymer is 0.1: 1 to 1: 1.   The electrolyte membrane according to claim 10, wherein a weight ratio of the non-polymerizable quaternary ammonium salt compound to the graft copolymer is 0.1: 1 to 1: 1.