JP2007226974A - Lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery having a high ion conductivity and a mechanical strength, with superior cycle characteristics, safety, and reliability. <P>SOLUTION: An electrolyte film material is formed by making an independent or copolymer of halogen substituted ethylenic unsaturated hydrocarbon monomer as a trunk polymer, and by graft copolymerization of a fourth class ammonium salt type room-temperature molten salt monomer having ethylenic unsaturated group by an atomic movement radical polymerization method; and an appropriate amount of lithium salt and, as necessary, a non-polymerizing fourth class ammonium salt type room-temperature molten salt or other additive is added to the graft copolymer produced, and a film is formed on the surface of an active material such as Sn and Si alloy, thereby, a safe lithium-ion battery suppressing expansion and shrinkage and having a stable performance is manufactured. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a lithium ion secondary battery that is used in a portable device and requires a further increase in battery capacity from the market, and has improved cycle characteristics in particular while placing importance on safety.

With the progress of miniaturization of portable devices, more and more functions have been provided, and the demand for safety beyond the increase in energy density has been demanded from the market for lithium batteries in general.

At present, lithium ion secondary batteries generally use lithium cobalt oxide, lithium manganate, lithium nickelate, etc. as lithium metal oxides that store and release lithium ions in the positive electrode, and carbon that store and release lithium ions in the negative electrode. Materials, lithium metals, lithium alloys, and the like are used, and an electrolyte in which a lithium salt is dissolved in an organic solvent that is liquid at room temperature is used as an electrolyte. Examples of the organic solvent used in the electrolytic solution include carbonates such as ethylene carbonate and propylene carbonate, lactones such as γ-butyrolactone, and ethers such as tetrahydrofuran.

However, the organic solvent is volatile and easily flammable.
And there was a problem with safety when a short circuit occurred. Moreover, the use of the gelled non-aqueous electrolyte does not solve the problem of volatilization of organic solvents and the danger of ignition, and the problem that the electrolyte separated from the gel leaks still remains. Therefore, lithium secondary batteries excellent in safety that do not contain a combustible substance such as an organic solvent as a main component have been proposed. These proposals contain a room temperature molten salt having a quaternary ammonium organic cation called an ionic liquid as electrolyte and a lithium salt. For example, see JP-A-10-92467, JP-A-10-265673, JP-A-11-92467, and JP-A-2002-11230. Although room temperature molten salt is liquid at room temperature, it is non-flammable and non-flammable, so safety is high.However, even if gelled in combination with a matrix polymer, it contains liquid and its mechanical properties are reduced. As phase separation may occur, handling problems and challenges to be overcome in battery design remain.

On the other hand, ionic liquids can act as an electric double layer when cations and anions apply an electric field to inhibit the migration of lithium ions, and room temperature molten salts are liquids. Basically, it is difficult to make a thin, compact, high-performance battery with high energy density due to problems such as handling during manufacturing, as well as the battery structure, ensuring electrical insulation between the electrodes. There was a technical problem.

In addition, as lithium ion secondary batteries are required to have higher capacities, lithium nickel oxide has been studied for positive electrodes and Sn alloys and Si alloys have been studied for negative electrodes in place of conventional positive and negative electrode active material materials. ing. However, since it is a novel material, the conventional electrolyte and separator structure has a problem that the battery performance cannot be sufficiently improved and high safety cannot be maintained.

In order to solve these problems, new compositions such as ionic liquids have been synthesized and compounded in various directions, but sufficient safety and battery characteristics are still required for lithium secondary batteries. Nothing with the intended characteristics is made.

The present invention has a polymer composite electrolyte having a safe, high ionic conductivity and excellent mechanical properties suitable for a new active material, that is, a quaternary ammonium room temperature molten salt (ionic liquid) structure. Provided is a graft copolymer having a polymer chain having a repeating unit as a side chain. This graft copolymer has a plurality of poly (quaternary ammonium salt) side chains graft-polymerized to a halogen-substituted ethylenically unsaturated hydrocarbon monomer alone or to a copolymer by an atom transfer radical polymerization method. Is intended to solve the above-mentioned problems.

Said subject is solved by forming the composite composition of an ionic liquid and a host monomer as a film | membrane on the active material surface of a negative electrode and a positive electrode.
In this composite composition, a halogen-substituted ethylenically unsaturated hydrocarbon monomer alone or a copolymer is used as a backbone polymer, and a reactive or polymerizable ionic liquid is graft-copolymerized by, for example, an atom transfer radical polymerization method. Is. The resulting graft copolymer is a comb-type graft copolymer having a plurality of poly (quaternary ammonium salt) side chains, and the graft ratio is preferably 3% or more.
In a preferred embodiment, the negative electrode is selected from metallic lithium, an Si-based alloy, or an Sn-based alloy, and the positive electrode is formed from lithium nickelate containing an element selected from cobalt, manganese, or aluminum.
In a preferred embodiment, a conductive material is added to the composite composition, a film is formed on the active material surfaces of the negative electrode and the positive electrode, the active materials and the conductive assistant are bound together, Useful for connection. The composite composition to which the conductive material is added is preferably interposed not only between the electrode active materials but also on the electrode surface, and can also be combined with a separator.

The quaternary ammonium cation having an ethylenically unsaturated group as the ionic liquid 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 (DAA) ), N-alkylpiperidinium cation (MPP), vinylbenzylalkylammonium cation (VBTMA), acryloylaminoalkylammonium cation (AAPEDMA) and N- (meth) acryloyloxyalkyl-N, N, N-trialkylammonium cation ( AOEEDMA). Preferred fluorine-containing anions as a counter ion are bis [(trifluoromethyl) sulfonyl] amide anion, 2,2,2-trifluoro-N- (trifluoromethylsulfonyl) acetamide anion, bis [(pentafluoroethyl) [Sulphonyl] amide anion, bis (fluorosulfonyl) amide anion, tetrafluoroborate anion, and trifluoromethanesulfonate anion.

The composite composition of the ionic liquid and the host monomer according to the present invention and the composite composition added with the additive are formed into a film on the surface of the active material particles of the electrode, the electrode surface and the separator, and lithium ions are contained in the active material particles. It is considered that the cycle characteristics are stabilized by forming a dense SEI (passivation passivation) film on the particle surface during the electrochemical reaction at the time of occlusion and release and suppressing decomposition of the electrolyte. In addition, when the composite composition has a function as a binder for the electrode, the particles and the conductive auxiliary agent against the movement of the negative electrode particles with a very large volume change (3 to 4 times) accompanying the insertion and extraction of lithium. The binder stretches so as to follow the volume change so that the contact with the wire does not break. Further, since the composite composition film is formed on the active material surface, the electrode surface and / or the separator, it is highly safe against an abnormal destruction phenomenon of the battery. In particular, when lithium nickelate is used for the positive electrode, it is necessary to take measures for safety. In addition, the polymer composite electrolyte has excellent electrical conductivity, mechanical properties, and workability, and can be used for lithium ion secondary batteries, etc. without using electrolytes or separators, taking advantage of the properties of this electrolyte. Can provide lithium-ion secondary batteries that meet demands for safety and downsizing.

In order to form a composite composition of an ionic liquid and a host monomer on the surface of the active material of the negative electrode or the positive electrode, for example, the main chain of the backbone polymer prepared by the atom transfer radical polymerization method (ATRP) is regularly arranged. A comb-shaped graft copolymer composition having bonded halogen is impregnated on the surface of the positive electrode and the negative electrode prepared in advance by a method such as coating, or kneaded together with each electrode composition on the surface of the current collector Can be integrated by coating.

The composite composition of the present invention is doped with a lithium salt composed of a lithium cation and a fluorine atom-containing anion as an electrolyte, and examples thereof include the following salts.
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, and R is an alkyl group or aryl] are selected from the group consisting of The doping amount of a lithium salt, typically (CF ₃ SO ₂ ) NLi (abbreviated as TFSI), is generally 0.05 to 0.8, preferably 0. It is in the range of 1 to 0.7, most preferably 0.15 to 0.5.

As the host polymer of the present invention, oxidation / reduction resistance, solvent resistance, low water absorption, flame resistance and other electrochemical / chemical stability, heat resistance, cold resistance and other temperature characteristics, as well as mechanical properties It is a polymer with excellent properties (strong elongation, flexibility) and excellent polymer moldability. 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 (PHFP) 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.

When the Tg of the graft copolymer is higher than the operating temperature of the lithium ion secondary battery, or when it is desired to increase the ionic conductivity of the membrane while maintaining a balance with the mechanical strength, the graft copolymer is plasticized or gelled. Can be 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.

When the composite composition is formed on the particle surface of the electrode, the film thickness is 1 μm or less, preferably several tens of nm or less.

When the composite composition is formed on the surface of the electrode, the film thickness is 100 μm or less, preferably 25 μm or less. If it becomes thicker than this, the existing separator cannot be removed, the battery thickness cannot be reduced, and the volumetric energy efficiency of the battery cannot be improved.

When the composite composition is integrated with the separator, specifically, a fiber knitted fabric or nonwoven fabric, a porous film or sheets are used, and the thickness is preferably 25 μm or less. In addition, when employ | adopting as a large sized battery, a film thickness may be about 100 micrometers.

In some cases, a composition containing a solvent can be formed into a sheet or a thin film by applying a melt film formation method, a solution casting method, a solution coating method, or the like. For example, the graft copolymer is dissolved in a solvent such as N-methyl-2pyrrolidone (NMP), the lithium salt to be doped and the ionic liquid, if used, are dissolved, and the resulting solution is added to a glass plate or polyester. It can be manufactured by casting on a substrate such as a film, drying the solvent, and then peeling the film.

The peeled graft copolymer film is sandwiched between the negative electrode and the positive electrode and stored in the battery container, and then sealed to complete the battery.

The composite composition of the present invention may contain other low-molecular to high-molecular organic compounds and inorganic compounds as additives, such as solvents, various stabilizers, lubricants, and crosslinking agents.
For example, when used as a separator, silicon oxide fine powder such as Aerosil is used to increase mechanical strength, or carbon fine powder such as ketjen black or acetylene black or graphite is used to impart electron conductivity. Can be added.

By adding a substance selected from fluorinated carboxylic acid and / or unsubstituted carboxylic acid derivatives such as cyclohexane and fluoroacetic acid to the composite composition at a volume ratio of 20% or less, the polymer is softened. In addition to modifying the surface of the active material, the composition binds an electrode component such as an active material to the surface of the current collector, acting as a binder and a flexible support for constructing an electron conduction network Can be made. That is, the flexibility of the coating is increased, and the volume change accompanying lithium occlusion and release of the negative electrode alloy is followed. By maintaining the electrical conductivity, the resistance of the negative electrode is increased, and the capacity decreasing tendency is suppressed. However, even if it exceeds 20 vol%, there is a tendency that there is no further effect.

In addition, by adding a fluorine compound such as fluoroacetic acid, the coating formed on the surface of the negative electrode active material particles suppresses the decomposition of the electrolyte and suppresses deterioration of the cycle characteristics of the battery due to gas generation and increased internal resistance. To do. This is because a derivative of unsubstituted carboxylic acid such as ethylene diformate forms a thin reaction film of nano-order or less on the alloy particle surface, thereby reducing direct contact with the electrolyte and suppressing decomposition. The cycle characteristics of the battery are stabilized. In addition, you may form the thin membrane | film | coat (for example, lithium carbonate) which has the inorganic fine hole on the particle | grain surface. For example, a thin layer of sulfide, carbonate, nitride, carbide, phosphide, fluoride, boride and the like can be mentioned.

In addition, lithium nickelate has low thermal stability, so it contains cobalt, aluminum, manganese, etc. to improve thermal stability, but it cannot be said that thermal stability is still sufficient, and it can cause a hard short circuit inside the battery. Has a problem in a lithium ion secondary battery using a non-aqueous organic solvent. On the other hand, although the thermal stability is improved to some extent in the gel polymer battery, it is a problem as the battery capacity increases. On the other hand, the above-described prescription of the present invention is highly effective, and the thermal stability even at 150 ° C. or higher. Was recognized.

The film according to the present invention can be made into a separator. That is, a mixture of electrode materials such as active material particles, conductive assistants, diluents and / or composite compositions is applied to a current collector surface such as aluminum foil or copper foil, and then dried and / or thermally polymerized. After the electrode is formed, the composite composition is impregnated into the electrode by adding a fine powder of silicon oxide such as Aerosil (trade name) and diluting with a solvent such as DMAC to the electrode surface and impregnating the electrode. A film having a thickness of about 10 μm can be formed on the surface. In order to increase the mechanical strength, the content of the host polymer is set higher than that in the electrode. This configuration has the advantage that the battery thickness can be made thinner than the conventional polymer resin film of 15 μm to 25 μm.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to the examples.

2 g of PVDF and 30 g of NMP were added and dissolved, then 28 g of MOETMA · TFSI (corresponding to 2 mol per 1 PVDF repeating unit) was added and dissolved by stirring, and then BPY (2,2′-bipyridyl) in 10 g of NMP was added to this solution. .2 g and a solution of copper chloride (CuCl) 0.04 g were added, and argon gas was blown into the mixture while stirring, followed by reaction at 90 ° C. 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. After that, it was vacuum dried at 70 ° C. The obtained graft copolymer was grafted with MOETMA · TFSI at 33 mol% by infrared spectrum analysis. This composite composition was dissolved in a DMAc solvent, and 1M equivalent of LiTFSI was doped to prepare an electrolyte solution, which was applied to and impregnated on the negative electrode surface made of Sn alloy particles. Next, it was dried to form a film on the electrode surface and the active material particle surface. If the grafting rate is 3% or less, the migration of lithium ions is hindered and the theoretical capacity of the battery cannot be extracted, so 3% or more is preferable. More preferably, it is 15 to 25 wt.%, And exhibits optimum characteristics.

Using the negative electrode obtained in Example 1 and using a metal lithium foil as a counter electrode, a flat cell was prepared, and a constant current charge / discharge cycle test of 0.2 C at room temperature was performed. As a result, cycle characteristics as shown in FIG. 1 were obtained. It was. The conventional example is a case where an ionic liquid is simply added, and even when an organic solvent is used, it is similar to the case where the negative electrode alloy surface modification and the occlusion and release of lithium are lost due to lithium occlusion and the cycle characteristics are poor. . On the other hand, according to the present invention, although a capacity decrease with some number of cycles was observed, it was much better than the conventional one. The discharge cutoff voltage is 1.5V, and the charge is 3mV.

A solution obtained by adding 10 vol% of a monomer obtained by converting the proton of acrylamido-2-methylpropanesulfonic acid to lithium to the solution obtained in Example 1 was added to a positive electrode using lithium nickelate containing cobalt and aluminum, and Sn—Co—. After impregnating the negative electrode made of Si-Bi alloy particles and graphite particles and a porous PVdF separator into a lithium ion secondary battery container that has been wound (or laminated), it is dried in a vacuum state at about 130 ° C. A film was formed on the separator. Next, the battery was vacuum impregnated with an electrolytic solution obtained by adding 5 vol% of ethylene diformate and 10 vol% of EC / DEC liquid to 1M MPP-LiTFSI liquid, and then the battery was hermetically sealed. When this battery was subjected to a constant current charge / discharge cycle test of 0.2C at room temperature (charging at constant current was 4.2V and discharging was stopped at 2.5V), cycle characteristics as shown in FIG. 2 were obtained. It was. As can be seen from this figure, in the present invention, the reason why the cycle change is small is considered that the electron conduction network is maintained due to the change in volume of the negative electrode due to the cycle, and that the decomposition of the electrolyte is suppressed. The conventional battery uses each electrode using a normal binder, and the electrolyte is made of a non-aqueous solvent, for example, using 1M LiPF6 + EC / DEC solution.

Further, a safety test was conducted on the lithium ion secondary battery according to the present invention produced in Example 3. The test items are an overcharge test, a heating test, a crush test, a nail penetration test, and a short circuit test. The results are shown in the table below. Although the results of the conventional batteries are not shown in the table, there were batteries that ignited with nail penetration and did not ignite in other tests, but the battery temperature exceeded 80 ° C. However, as shown in the table, the battery of the present invention had no smoke / ignition / rupture in any of the tests. In many cases, the battery temperature was within 60 ° C.

The cycle characteristic of the negative electrode capacity change in the battery according to the present invention and the conventional battery is shown. The cycle characteristic of the battery capacity change of this invention battery and a conventional battery is shown.

Claims (9)

A lithium ion secondary battery, wherein a composite composition of an ionic liquid and a host monomer is formed as a film on the active material surface of a negative electrode or a positive electrode. 2. The lithium ion secondary battery according to claim 1, wherein the negative electrode active material is selected from metallic lithium, Si-based alloy, and Sn-based alloy. The lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode active material is made of lithium nickelate containing an element selected from cobalt, manganese, and aluminum. Grafts having a plurality of poly (quaternary ammonium salt) side chains graft-copolymerized by the atom transfer radical polymerization method to a trunk polymer composed of the halogen-substituted ethylenically unsaturated hydrocarbon monomer alone or a copolymer. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the lithium ion secondary battery is a copolymer. The lithium ion secondary battery according to claim 4, wherein the ionic liquid is reactive and has a grafting rate of 3% or more. The conductive composition is added to the composite composition, the film is formed, the active materials and the conductive auxiliary agent are bound, and the electrical connection with the current collector is performed. The lithium ion secondary battery according to any one of 5. The fluorinated carboxylic acid such as ionic liquid, cyclohexane or fluoroacetic acid and / or a derivative of unsubstituted carboxylic acid is added to the composite composition in an amount of 20 vol% or less. The lithium ion secondary battery as described. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the film is mainly interposed on the electrode surface of the negative electrode and / or the positive electrode. The lithium ion secondary battery according to claim 6, wherein the film is present as a composite with a separator.

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