JP4604505B2 - Method for producing lithium difluorophosphate, non-aqueous electrolyte and non-aqueous electrolyte secondary battery using the same - Google Patents

Description

  The present invention relates to a method for producing lithium difluorophosphate, a non-aqueous electrolyte for a secondary battery, and a non-aqueous electrolyte secondary battery. In particular, a method for producing lithium difluorophosphate that can be advantageously produced industrially, a non-aqueous electrolyte solution for a secondary battery containing lithium difluorophosphate produced by this method, and the non-aqueous electrolyte solution were used. The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, with the downsizing of electronic devices, it is desired to further increase the capacity of high-capacity secondary batteries. For this reason, lithium ion secondary batteries with higher energy density are attracting attention as compared to nickel / cadmium batteries and nickel / hydrogen batteries.

Lithium secondary batteries include cyclic carbonates such as ethylene carbonate and propylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, cyclic esters such as γ-butyrolactone and γ-valerolactone, methyl acetate, propion Chain esters such as methyl acid, cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, and tetrahydropyran, chain ethers such as dimethoxyethane and dimethoxymethane, and sulfur-containing organic solvents such as sulfolane and diethylsulfone. non-aqueous _{solvent, LiPF 6, LiBF 4, LiClO} 4, LiCF 3 SO 3, LiAsF 6, LiN (CF 3 SO 2) 2, LiCF 3 (CF 2) 3 dissolved solute of SO ₃ such as formed by a non-aqueous Electrolyte used That.

  In such a secondary battery using a non-aqueous electrolyte solution, the reactivity varies depending on the composition of the non-aqueous electrolyte solution. Therefore, the battery characteristics vary greatly depending on the non-aqueous electrolyte solution. In particular, it is difficult to improve the cycle characteristics and storage characteristics of the secondary battery because decomposition and side reactions of the non-aqueous electrolyte affect the cycle characteristics and storage characteristics of the secondary battery. In contrast, conventionally, attempts have been made to improve cycle characteristics and storage characteristics by mixing various additives into a nonaqueous electrolytic solution.

Under these circumstances, Patent Document 1 uses a nonaqueous electrolytic solution containing at least one additive selected from lithium monofluorophosphate (Li ₂ PO ₃ F) and lithium difluorophosphate (LiPO ₂ F ₂ ). By reacting this additive with lithium, a film is formed at the interface between the positive electrode and the negative electrode, thereby suppressing the decomposition of the non-aqueous electrolyte caused by the contact between the electrolyte, the positive electrode active material, and the negative electrode active material. Describes that self-discharge is suppressed and the storage characteristics after charging are improved.

Incidentally, as a method for producing a lithium difluorophosphate, such as P ₂ O ₃ F ₄ being prepared by reacting a metal salt is described in Non-Patent Document 1.

Japanese Patent Laid-Open No. 11-67270 J. et al. Fluorine Chem. (1988), 38 (3), p. 297-302

However, in the conventional method for producing difluorophosphate described in Non-Patent Document 1, the raw material P ₂ O ₃ F ₄ is difficult to obtain and very expensive, and separation and purification of by-products are necessary. For these reasons, it was extremely disadvantageous for application on an industrial scale as a method for producing lithium difluorophosphate as an additive for a non-aqueous electrolyte.

  The present invention was devised in view of the above problems, and a production method for industrially advantageously producing lithium difluorophosphate from an inexpensive raw material that is easily available, and difluorophosphoric acid produced by this production method It aims at providing the non-aqueous electrolyte solution for secondary batteries containing lithium, and the non-aqueous electrolyte secondary battery using this non-aqueous electrolyte solution.

  As a result of intensive studies to solve the above problems, the present inventors have found that lithium hexafluorophosphate, which is widely used as an electrolyte lithium salt for non-aqueous electrolytes, and silicon dioxide that is industrially available and very inexpensive. It was found that by reacting and in a non-aqueous solvent, lithium difluorophosphate can be produced very advantageously industrially, and the present invention was completed.

That is, the gist of the present invention resides in a method for producing lithium difluorophosphate, characterized in that lithium hexafluorophosphate and silicon dioxide are reacted in a non-aqueous solvent (claim 1).
At this time, the non-aqueous solvent is one or more solvents selected from the group consisting of cyclic carbonates, chain carbonates, cyclic esters, chain esters, cyclic ethers, chain ethers, and sulfur-containing organic solvents. It is preferable that it is present (claim 2).

Another gist of the present invention is a method for producing a nonaqueous electrolytic solution for a secondary battery comprising at least lithium hexafluorophosphate and lithium difluorophosphate in a nonaqueous solvent, the difluorophosphorus A method for producing a non-aqueous electrolyte solution for a secondary battery, characterized in that at least a part of lithium acid is obtained by reacting lithium hexafluorophosphate and silicon dioxide in a non-aqueous solvent (claim 3). .
At this time, it is preferable to use a reaction product obtained by reacting lithium hexafluorophosphate and silicon dioxide in a non-aqueous solvent as the non-aqueous electrolyte for a secondary battery.
Furthermore, it is preferable that the concentration of lithium difluorophosphate in the nonaqueous electrolytic solution for secondary battery is 1 × 10 ⁻³ mol / kg or more and 0.5 mol / kg or less.

According to the method for producing lithium hexafluorophosphate of the present invention, lithium difluorophosphate can be industrially advantageously produced from an inexpensive and easily available material.
In addition, according to the non-aqueous electrolyte for secondary batteries and the non-aqueous electrolyte secondary battery of the present invention, non-aqueous electrolysis for secondary batteries containing lithium hexafluorophosphate useful as an additive for improving high-temperature cycle characteristics. Liquid and non-aqueous electrolyte secondary batteries can be easily obtained and are industrially advantageous.

  Hereinafter, embodiments of the present invention will be described in detail. However, the description of the constituent elements described below is an example (representative example) of an embodiment of the present invention, and the present invention is not limited to the following embodiments. It can be implemented with arbitrary modifications.

1. Method for Producing Lithium Difluorophosphate The method for producing lithium difluorophosphate (LiPO ₂ F ₂ ) of the present invention comprises lithium hexafluorophosphate (LiPF ₆ ) and silicon dioxide (SiO ₂ ) in a non-aqueous solvent. It is made to react.

1-1. Kind of non-aqueous solvent There is no restriction | limiting about the non-aqueous solvent used for the manufacturing method of lithium difluorophosphate of this invention, Arbitrary non-aqueous solvents can be used. This non-aqueous solvent is a reaction solvent for the formation reaction of lithium difluorophosphate. Usually, for example, cyclic carbonates such as ethylene carbonate and propylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; γ-butyrolactone Cyclic esters such as methyl acetate and methyl propionate; cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran and tetrahydropyran; chain ethers such as dimethoxyethane and dimethoxymethane; and sulfolane, A sulfur-containing organic solvent such as diethyl sulfone is used. Furthermore, as will be described later, when the reaction product obtained by the reaction is used for non-aqueous electrolyte applications, among these, cyclic carbonates such as ethylene carbonate and propylene carbonate; chains such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate A carbonate is preferred as the non-aqueous solvent. From the viewpoint of reaction rate, chain carbonates such as ethylene carbonate and propylene carbonate are particularly preferable.
In addition, these non-aqueous solvents may be used individually by 1 type, and may use 2 or more types together by arbitrary combinations and a ratio.

1-2. Charge ratio of lithium hexafluorophosphate and silicon dioxide The charge ratio of lithium hexafluorophosphate and silicon dioxide to be subjected to the reaction is not particularly limited and can be any ratio. However, from the viewpoint of efficiently synthesizing lithium difluorophosphate, the molar ratio of silicon dioxide to lithium hexafluorophosphate (SiO ₂ / LiPF ₆ ) is usually 1 × 10 ⁻³ or more, preferably 3 × 10 ⁻ It should be ³ or more, usually 1 or less, preferably 0.8 or less. This is because if the lower limit of the above range is not reached, the yield may not be sufficiently secured, and if the upper limit is exceeded, side reactions may proceed.

In particular, when the reaction product obtained by the reaction is supplied to a non-aqueous electrolyte for a secondary battery as a lithium difluorophosphate source, that is, lithium difluorophosphate is obtained from the reaction product and the lithium difluorophosphate is added to the non-aqueous electrolyte. When used as at least part of lithium difluorophosphate, which is an additive for non-aqueous electrolyte for secondary batteries, the charging rate (SiO ₂ / PF ₆ ) is usually 1 × 10 ⁻³ or more, preferably in molar ratio. Is advantageously 5 × 10 ⁻³ or more, and usually 0.8 or less, preferably 0.5 or less.
Furthermore, when the reaction product solution is used as it is as a non-aqueous electrolyte solution for a secondary battery, that is, when the reaction product solution is used as a non-aqueous electrolyte solution for a secondary battery without adjusting the components of the reaction product solution, The charging rate (SiO ₂ / PF ₆ ) is usually 1 × 10 ⁻³ or more, preferably 5 × 10 ⁻³ or more, and usually 0.3 or less, preferably 0.2 or less. This is because if the lower limit of the above range is not reached, the cycle characteristics may not be sufficiently improved, and if the upper limit is exceeded, the battery capacity may be reduced.

1-3. Concentration of lithium hexafluorophosphate to be subjected to reaction The concentration of lithium hexafluorophosphate to be subjected to reaction in the non-aqueous solvent is not particularly limited and may be any concentration, but is usually 0 with respect to the non-aqueous solvent. .3 mol / L or more, preferably 0.5 mol / L or more, and usually 2.5 mol / L or less, preferably 2.0 mol / L or less. By being above the lower limit of this range, the reaction rate can be prevented from decreasing, and by making it below the upper limit, the progress of side reactions can be suppressed.

In particular, when the reaction product solution obtained by the reaction is supplied as a lithium difluorophosphate to a non-aqueous electrolyte for a secondary battery, the concentration of lithium hexafluorophosphate in the non-aqueous solvent is usually 0.5 mol / L or more, preferably 0.7 mol / L or more, and usually usually 2.0 mol / L or less, preferably 1.6 mol / L or less. This is because the closer to a suitable concentration as a non-aqueous electrolyte, the easier it is to handle.
Further, when the reaction product solution is used as it is as a non-aqueous electrolyte for a secondary battery, the concentration of lithium hexafluorophosphate in the non-aqueous solvent is usually 0.7 mol / L or more, preferably 0.8 mol / L. In addition, it is usually 2.0 mol / L or less, preferably 1.6 mol / L or less. By setting it within this range, it is possible to eliminate the possibility that the conductivity of the non-aqueous electrolyte for secondary batteries is lowered.

1-4. The concentration of silicon dioxide subjected to the reaction The concentration of silicon dioxide subjected to the reaction in the non-aqueous solvent is not particularly limited and may be any concentration, but is usually 1 × 10 ⁻³ mol / min with respect to the non-aqueous solvent. kg or more, preferably 3 × 10 ⁻³ mol / kg or more, usually 2 mol / kg or less, preferably 1 mol / kg or less. A sufficient amount of difluorophosphate can be obtained when the amount is not less than the lower limit of the above range, and the progress of side reactions can be suppressed by being not more than the upper limit.

In particular, when the reaction product obtained by the reaction is supplied to a non-aqueous electrolyte for a secondary battery as a lithium difluorophosphate source, the concentration of silicon dioxide in the non-aqueous solvent is usually higher than that of the non-aqueous solvent. 1 × 10 ⁻³ mol / kg or more, preferably 5 × 10 ⁻³ mol / kg or more, usually 0.6 mol / kg or less, preferably 0.5 mol / kg or less. Outside this range, it is difficult to obtain the effect of improving the cycle characteristics.
Furthermore, when the reaction product solution is used as it is as a non-aqueous electrolyte for a secondary battery, it is usually 1 × 10 ⁻³ mol / kg or more, preferably 5 × 10 ⁻³ mol / kg or more with respect to the non-aqueous solvent. In addition, it is usually 0.5 mol / kg or less, preferably 0.3 mol / kg or less. By setting it to the lower limit of this range or more, it becomes easy to obtain an additive effect when used as a non-aqueous electrolyte for a secondary battery, and by making the upper limit or less, the progress of side reactions can be suppressed.

1-5. Other components used for reaction In the method for producing lithium difluorophosphate of the present invention, components other than lithium hexafluorophosphate and silicon dioxide may be present in the non-aqueous solvent. For example, although the action is not clear, when a small amount of water or hydrogen fluoride is present in the non-aqueous solvent, the formation reaction of lithium difluorophosphate proceeds rapidly. Therefore, for example, when the reaction product solution is used as a non-aqueous electrolyte solution for a secondary battery, a minute amount of water that does not affect the battery performance, specifically, the concentration in the non-aqueous electrolyte solution for a secondary battery As such, about 10 to 200 ppm of water may coexist in the reaction system. In addition, in this specification, ppm represents the ratio on the basis of mass.

1-6. Reaction conditions Reaction conditions such as reaction temperature and reaction time are not particularly limited, and an optimum one may be selected according to the situation, but is preferably as follows.
The reaction temperature is not particularly limited as long as the formation reaction of lithium difluorophosphate proceeds. Usually, the reaction proceeds faster under a temperature condition higher than room temperature. Specifically, the reaction temperature is usually 20 ° C. or higher, preferably 30 ° C. or higher, more preferably 40 ° C. or higher, and usually 80 ° C. or lower, preferably 70 ° C. or lower. By setting it to the lower limit or more of this range, the reaction can be surely progressed, and by setting it to the upper limit or less, the vaporization of the solvent can be prevented and the decomposition of lithium hexafluorophosphate can be prevented. However, it is desirable to ensure a sufficient reaction time when the reaction temperature is low.

  Also, the reaction time is not particularly limited as long as the lithium difluorophosphate production reaction proceeds, but usually it is sufficient to take the time until the target amount of lithium difluorophosphate is produced. Specifically, it is usually 2 hours or longer, preferably 6 hours or longer. As a guideline, it usually takes 24 hours or more if the temperature condition is 30 ° C., and 12 hours or more if the temperature condition is 40 ° C. By setting the amount to be equal to or more than the lower limit of the above range, the reaction can be completed with certainty and a target amount of lithium difluorophosphate can be obtained. The upper limit of the reaction time is not particularly defined, but the efficiency is poor if it is too long for several days from the viewpoint of productivity.

1-7. Reaction product solution The reaction product solution thus obtained, that is, the reaction product solution obtained by reacting lithium hexafluorophosphate and silicon dioxide in a non-aqueous solvent is usually an unreacted hexafluorophosphate. Lithium acid and silicon dioxide and lithium difluorophosphate produced by the reaction are contained in a non-aqueous solvent.

When isolating lithium difluorophosphate from the reaction product liquid, the isolation method is not particularly limited, and can be isolated by any method as long as lithium difluorophosphate is not decomposed. Specific isolation methods include distillation, recrystallization and the like.
However, the step of isolating lithium difluorophosphate from the reaction product solution can be omitted depending on the purpose. For example, as will be described later, it can be omitted when the reaction product liquid is supplied to the non-aqueous electrolyte for secondary batteries as a lithium difluorophosphate source. The ability to omit the isolation step in this way is very advantageous industrially.

  In addition to the target lithium difluorophosphate, the reaction product solution contains unreacted lithium hexafluorophosphate and a non-aqueous solvent. The lithium hexafluorophosphate is a non-aqueous electrolyte solution for secondary batteries. It is a substance used as an electrolyte lithium salt. Therefore, for example, when preparing a nonaqueous electrolyte solution for a secondary battery comprising at least lithium hexafluorophosphate as an electrolyte lithium salt in a nonaqueous solvent and further containing lithium difluorophosphate as an additive. The reaction product solution can be used as a part or all of the non-aqueous electrolyte solution by selecting a non-aqueous solvent as a reaction solvent that does not interfere with the use of the non-aqueous solvent for the electrolyte solution. Can be used as the lithium difluorophosphate source of the non-aqueous electrolyte for secondary batteries. That is, lithium difluorophosphate in the reaction product solution can be used as an additive for the non-aqueous electrolyte solution.

When the reaction product solution is used as part or all of the non-aqueous electrolyte solution, the concentration of each component in the reaction product solution is preferably as follows.
The content concentration of lithium difluorophosphate in the reaction product solution is usually 1 × 10 ⁻³ mol / kg or more, preferably 5 × 10 ⁻³ mol / kg or more, more preferably 1 × 10 6 with respect to the reaction product solution. ^-2 mol / kg or more, and usually 0.7 mol / kg or less, preferably 0.6 mol / kg or less.
The content concentration of lithium hexafluorophosphate remaining in the reaction product liquid is usually 0.2 mol / L or more, preferably 0.3 mol / L or more, and usually 2 mol / L or less with respect to the reaction product liquid. , Preferably it is 1.6 mol / L or less.

In particular, when the reaction product solution is used as it is as a non-aqueous electrolyte solution, that is, when the reaction product solution is used as a non-aqueous electrolyte solution without adjusting the components, the concentration of each component in the reaction product solution is: Preferably it is as follows.
The content concentration of lithium difluorophosphate in the reaction product solution is usually 1 × 10 ⁻³ mol / kg or more, preferably 5 × 10 ⁻³ mol / kg or more, more preferably 1 × 10 6 with respect to the reaction product solution. ^-2 mol / kg or more, and usually 0.5 mol / kg or less, preferably 0.3 mol / kg or less.
The content concentration of lithium hexafluorophosphate remaining in the reaction product liquid is usually 0.65 mol / L or more, preferably 0.7 mol / L or more, and usually 2 mol / L or less with respect to the reaction product liquid. , Preferably it is 1.6 mol / L or less.

  Furthermore, the unreacted silicon dioxide present in the reaction product liquid can be removed by a filtration operation. Although a precipitate may be generated by a side reaction, it is preferably removed at the same time by a filtration operation. In particular, when the reaction product solution is supplied to a non-aqueous electrolyte for a secondary battery, it is preferable to undergo a filtration operation.

Therefore, in the method for producing lithium difluorophosphate of the present invention, when the reaction product solution is supplied to the non-aqueous electrolyte for a secondary battery as a lithium difluorophosphate source, the concentration of each component in the reaction product solution is It is preferable to adjust the amounts of lithium hexafluorophosphate and silicon dioxide (concentration in the non-aqueous solvent) to be used for the reaction so that each of them falls within the above range.
Further, if necessary, the non-aqueous solvent is removed from the reaction product solution by an operation such as distillation to concentrate the reaction product solution, or conversely, the reaction product solution is diluted with a non-aqueous solvent to obtain difluorophosphoric acid. It is preferable to adjust the concentration of each component such as lithium.
Furthermore, it is also preferable to adjust the concentration of each component by appropriately adding each component such as lithium hexafluorophosphate.

  Therefore, in the non-aqueous electrolyte for secondary battery of the present invention, lithium difluorophosphate is obtained from the reaction product solution, and the lithium difluorophosphate is used as an additive for the non-aqueous electrolyte for secondary battery. The supply of the reaction product solution as a lithium difluorophosphate source in the non-aqueous electrolyte for secondary batteries is used in addition to supplying the obtained reaction product solution as it is, and appropriately using a non-aqueous solvent. Can be concentrated by operations such as distillation, extraction or the like, or conversely diluted with a non-aqueous solvent to adjust the concentration of lithium difluorophosphate, etc., or appropriately add each component such as lithium hexafluorophosphate, It includes supplying after adjusting the component concentration.

1-8. By the way, when using lithium difluorophosphate for a non-aqueous electrolyte secondary battery, silicon dioxide is present in the battery can, for example, in the positive electrode plate or the negative electrode plate, and the lithium difluorophosphate containing lithium hexafluorophosphate is contained. Even if the non-aqueous electrolyte for a secondary battery is infiltrated, the above generation reaction does not easily proceed. This is because water and hydrogen fluoride, which are considered to catalyze the reaction, are typically lithium transition metal composite oxides as positive electrode materials, carbonaceous materials typical as negative electrode materials, metallic lithium, lithium alloys, Si This is presumed to be trapped by Sn or the like.

2. Non-aqueous electrolyte for secondary battery The non-aqueous electrolyte for secondary battery of the present invention is a non-aqueous electrolyte for secondary battery comprising at least lithium hexafluorophosphate and lithium difluorophosphate in a non-aqueous solvent. It is a liquid, and at least a part of lithium difluorophosphate is obtained by reacting lithium hexafluorophosphate and silicon dioxide in a non-aqueous solvent.

  Here, lithium difluorophosphate can be isolated from the reaction product obtained by the above-described method for producing lithium difluorophosphate of the present invention and used as an additive for a non-aqueous electrolyte for a secondary battery. Further, as described above, by supplying the reaction product liquid as a lithium difluorophosphate source, the lithium difluorophosphate in the reaction liquid is used as an additive in the non-aqueous electrolyte solution for secondary batteries of the present invention. It may be. If the reaction product solution is supplied as a lithium difluorophosphate source, steps such as separation and purification can be omitted, which is extremely industrially advantageous.

2-1. Nonaqueous solvent The kind of the nonaqueous solvent of the nonaqueous electrolyte solution for secondary batteries of this invention is not specifically limited, Arbitrary nonaqueous solvents can be used. Specific examples thereof include cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, cyclic esters such as γ-butyrolactone and γ-valerolactone, methyl acetate, Examples include chain esters such as methyl propionate, cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran and tetrahydropyran, chain ethers such as dimethoxyethane and dimethoxymethane, and sulfur-containing organic solvents such as sulfolane and diethylsulfone. These non-aqueous solvents may be used alone or in combination of two or more in any combination and ratio.

  Among the above examples, a mixed non-aqueous solvent in which a cyclic carbonate and a chain dialkyl carbonate are mixed is preferable from the viewpoint of improving overall battery performance such as charge / discharge characteristics and battery life. The mixed non-aqueous solvent contains a cyclic carbonate and a chain dialkyl carbonate in an amount of 20% by volume or more of the whole non-aqueous solvent, and the total volume thereof is 70% by volume or more of the whole non-aqueous solvent. preferable.

  As the cyclic carbonate used in the mixed non-aqueous solvent, an alkylene carbonate having 2 to 4 carbon atoms in the alkylene group is preferable. Specific examples thereof include ethylene carbonate, propylene carbonate, butylene carbonate and the like. Of these, ethylene carbonate and propylene carbonate are preferable.

Moreover, as chain | strand-shaped dialkyl carbonate used for said mixed non-aqueous solvent, the C1-C4 dialkyl carbonate of an alkyl group is preferable. Specific examples thereof include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate and the like. Of these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferable.
Each of these cyclic carbonates and dialkyl carbonates may be used independently, or a plurality of types may be used in any combination and ratio.

  Furthermore, the mixed nonaqueous solvent may contain a solvent other than cyclic carbonate and dialkyl carbonate as long as the battery performance of the produced nonaqueous electrolyte secondary battery is not deteriorated. The ratio of solvents other than cyclic carbonate and dialkyl carbonate in the mixed non-aqueous solvent is usually 30% by weight or less, preferably 10% by weight or less.

2-2. Electrolyte lithium salt The nonaqueous electrolyte solution for secondary batteries of the present invention contains lithium hexafluorophosphate, which functions as an electrolyte lithium salt in the nonaqueous electrolyte solution for secondary batteries.
Furthermore, the non-aqueous electrolyte for secondary batteries of the present invention may contain a lithium salt other than lithium hexafluorophosphate as an electrolyte lithium salt. There is no particular limitation on the electrolyte lithium salt other than lithium hexafluorophosphate, _{Examples, LiClO 4, LiBF 4, LiAsF} 6, LiSbF inorganic lithium salt such as _{6, LiCF 3 SO 3, LiN} (CF 3 SO 2 ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), and LiC (CF ₃ SO ₂ ) ₃ . Of these, LiClO ₄ and LiBF ₄ are preferable. In addition, these may be used individually by 1 type and may use 2 or more types together by arbitrary combinations and ratios.

2-3. Composition of non-aqueous electrolyte for secondary battery There is no particular limitation on the concentration of electrolyte lithium salt in the non-aqueous electrolyte for secondary battery, but from the viewpoint of electrical conductivity and viscosity, the non-aqueous electrolyte for secondary battery On the other hand, it is usually 0.5 mol / L or more, preferably 0.7 mol / L or more, and usually 2 mol / L or less, preferably 1.5 mol / L or less. When two or more electrolyte lithium salts are used in combination, it is desirable that the total concentration of the electrolyte lithium salts used be within the above range.

Further, the concentration of lithium difluorophosphate in the nonaqueous electrolyte for secondary batteries is not particularly limited, but is usually 1 × 10 ⁻³ mol / kg or more with respect to the nonaqueous electrolyte for secondary batteries, preferably 3 × 10 ⁻³ mol / kg or more, more preferably 1 × 10 ⁻² mol / kg or more, and usually 0.5 mol / kg or less, preferably 0.3 mol / kg or less, more preferably 0.2 mol / kg. It is desirable that By making it more than the lower limit of this range, it is easy to obtain the additive effect, and by making it less than the upper limit, an increase in viscosity can be prevented.

  Furthermore, in the non-aqueous electrolyte for secondary batteries of the present invention, any additive can be used in any appropriate amount. Examples of such additives include overcharge inhibitors such as cyclohexylbenzene, biphenyl, and difluoroanisole, negative electrode film forming agents such as vinylene carbonate, vinylethylene carbonate, and succinic anhydride, and positive electrode protective agents such as propane sultone. Can be mentioned.

2-4. Others As described above, a lithium difluorophosphate-containing solution provided as a reaction product liquid of lithium hexafluorophosphate and silicon dioxide can be used for the preparation of the non-aqueous electrolyte solution for a secondary battery of the present invention. At that time, not only use the reaction product liquid itself as a non-aqueous electrolyte for secondary batteries, but also increase or decrease the amount of non-aqueous solvent or add electrolyte lithium salt and additives as appropriate. Is possible. For example, depending on the charged amount, the amount of lithium hexafluorophosphate in the reaction product liquid may decrease, but this can be added later to optimize the concentration. In addition, the reaction product liquid can also be used as a supply source of the non-aqueous electrolyte solution for the secondary battery, and in this case, the composition of the non-aqueous solvent serving as the reaction medium is the solvent of the non-aqueous electrolyte solution for the secondary battery. It is easy to handle if it matches the composition.

3. Non-aqueous electrolyte secondary battery The non-aqueous electrolyte secondary battery of the present invention comprises the above-described non-aqueous electrolyte for a secondary battery of the present invention, and a positive electrode and a negative electrode capable of inserting and extracting lithium. Is done. In addition, a separator is appropriately used in the non-aqueous electrolyte secondary battery of the present invention.

3-1. Positive electrode The positive electrode is usually configured by providing a positive electrode active material layer on a positive electrode current collector.
As the material of the positive electrode current collector, known materials can be arbitrarily used, but usually metal materials such as aluminum, stainless steel, nickel plating, titanium and tantalum, and carbon materials such as carbon cloth and carbon paper are used. . Of these, metal materials are preferable, and aluminum is particularly preferable.

In addition, as the shape of the positive electrode current collector, in the case of a metal material, for example, a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, an expanded metal, a punch metal, a foam metal, and the like can be mentioned. For example, a carbon plate, a carbon thin film, a carbon cylinder, etc. are mentioned.
As the positive electrode current collector, among the above examples, a metal thin film is preferable because it is currently used in industrialized products. In addition, you may form a thin film suitably in mesh shape.

  When a thin film is used as the positive electrode current collector, the thickness is arbitrary, but is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and usually 1 mm or less, preferably 100 μm or less, more preferably 50 μm. The following ranges are preferred. If the thickness is less than the above range, the strength required for the positive electrode current collector may be insufficient. On the other hand, if it is thicker than the above range, the handleability may be impaired.

The positive electrode active material layer includes a positive electrode active material. The positive electrode active material is not limited as long as it can occlude and release lithium ions. Preferable examples include lithium transition metal composite oxides.
Specific examples of the lithium-transition metal composite oxide, lithium cobalt complex oxides such as LiCoO _2, lithium-nickel composite oxide such as LiNiO _2, include lithium-manganese composite oxides such as LiMnO _2. In these lithium transition metal composite oxides, some of the main transition metal atoms are Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, etc. Replacing with other metals is preferable because it can be stabilized. Any one of these positive electrode active materials may be used alone, or two or more thereof may be used in any combination and ratio.

  The positive electrode active material layer is prepared by applying a slurry obtained by slurrying the above-described positive electrode active material, a binder (binder), and, if necessary, a thickener and a conductive material with a solvent, onto a positive electrode current collector, and drying. Can be manufactured. Further, the positive electrode active material described above may be roll-formed to form a sheet electrode, or may be formed into a pellet electrode by compression molding. Hereinafter, a case where the slurry is applied to the positive electrode current collector and dried will be described.

  The binder is not particularly limited as long as it is a material that is stable with respect to the non-aqueous solvent used in the electrolytic solution and the solvent used during electrode production. Specific examples include polyethylene, polypropylene, polyethylene terephthalate, Resin polymers such as methyl methacrylate, aromatic polyamide, cellulose, nitrocellulose, rubbers such as SBR (styrene butadiene rubber), isoprene rubber, butadiene rubber, fluoro rubber, NBR (acrylonitrile-butadiene rubber), ethylene propylene rubber -Like polymer, styrene-butadiene-styrene block copolymer and its hydrogenated product, EPDM (ethylene-propylene-diene terpolymer), styrene-ethylene-butadiene-styrene copolymer, styrene-isoprene-styrene block Copolymer Thermoplastic elastomeric polymers such as hydrogenated products thereof, soft resinous polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer , Fluorinated polymers such as polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene / ethylene copolymer, and polymer compositions having ion conductivity of alkali metal ions (particularly lithium ions) Etc. These substances may be used alone or in combination of two or more in any combination and ratio.

  The ratio of the binder in the positive electrode active material layer is usually 0.1% by weight or more, preferably 1% by weight or more, more preferably 5% by weight or more, and usually 80% by weight or less, preferably 60% by weight or less. More preferably, it is 40% by weight or less, and most preferably 10% by weight or less. If the proportion of the binder is too low, the positive electrode active material cannot be sufficiently retained and the positive electrode has insufficient mechanical strength, which may deteriorate battery performance such as cycle characteristics. There is a risk of reducing the capacity and conductivity.

The thickener is used for the purpose of adjusting the slurry viscosity. Although there is no restriction | limiting in particular in the kind, As a specific example, carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein etc. are mentioned. These substances may be used alone or in combination of two or more in any combination and ratio.
The proportion of the thickener in the positive electrode active material layer is usually 0.1% by weight or more, preferably 0.2% by weight or more, more preferably 0.3% by weight or more, and usually 15% by weight or less. Preferably it is 10 weight% or less, More preferably, it is 5 weight% or less. If the proportion of the thickener is too low, the viscosity may be low and thus application may be difficult. Conversely, if it is too high, battery characteristics may be deteriorated.

The positive electrode active material layer usually contains a conductive material in order to increase conductivity. There are no particular restrictions on the type, but specific examples include metal materials such as copper and nickel, graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon such as needle coke. And carbon materials. These substances may be used alone or in combination of two or more in any combination and ratio.
The proportion of the conductive material in the positive electrode active material layer is usually 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 1% by weight or more, and usually 50% by weight or less, preferably 30%. % By weight or less, more preferably 15% by weight or less. If the proportion of the conductive material is too low, the conductivity may be insufficient, and conversely if it is too high, the battery capacity may be reduced.

  As the solvent for forming the slurry, any kind of solvent can be used as long as it can dissolve or disperse the positive electrode active material, the binder, and the thickener and conductive material used as necessary. There is no particular limitation on the above, and either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water and alcohol. Examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N -N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran (THF), toluene, acetone, dimethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, hexane, etc. Is mentioned. In addition, these solvents may be used individually by 1 type, and may use 2 or more types together by arbitrary combinations and a ratio.

By applying and drying a slurry prepared by dispersing or dissolving the above-described positive electrode active material, binder, and thickener and conductive material used as necessary in a solvent to a positive electrode current collector, the positive electrode active material Form a layer.
The proportion of the positive electrode active material layer in the positive electrode is usually 10% by weight or more, preferably 30% by weight or more, more preferably 50% by weight or more, and usually 99.9% by weight or less, preferably 99% by weight or less. The thickness of the positive electrode active material layer is usually about 10 to 200 μm.
The positive electrode active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the positive electrode active material.

3-2. Negative electrode The negative electrode is usually configured by providing a negative electrode active material layer on a negative electrode current collector, as in the case of the positive electrode.
As a material of the negative electrode current collector, a known material can be arbitrarily used. For example, a metal material such as copper, nickel, stainless steel, nickel-plated steel, or the like is used. Among these, copper is particularly preferable from the viewpoint of ease of processing and cost.

  Examples of the shape of the negative electrode current collector include a metal foil, a metal cylinder, a metal coil, a metal plate, and a metal thin film. Among these, metal thin films are preferable because they are currently used in industrialized products. In addition, you may form a thin film suitably in mesh shape. When a metal thin film is used as the negative electrode current collector, the preferred thickness range is the same as the range described above for the positive electrode current collector.

  The negative electrode active material layer includes a negative electrode active material. The negative electrode active material is not particularly limited as long as it can occlude and release lithium ions, but it is usually a carbonaceous material that can occlude and release lithium in terms of safety and capacity. Alternatively, a metal material such as Sn or Si capable of forming an alloy with lithium is used.

  Although there is no restriction | limiting in particular as said carbon material, Artificial graphite, graphite (graphite), such as natural graphite, and the thermal decomposition thing of organic substance on various thermal decomposition conditions are mentioned. Examples of pyrolysis products of organic matter include coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, or carbide obtained by oxidizing these pitches, needle coke, pitch coke, phenol resin, crystalline cellulose, etc. And carbon materials obtained by partially graphitizing these, furnace black, acetylene black, pitch-based carbon fibers, and the like. Among them, graphite is preferable, and particularly preferable is artificial graphite, purified natural graphite, or graphite material containing pitch in these graphites, which is manufactured by subjecting easy-graphite pitch obtained from various raw materials to high-temperature heat treatment. Therefore, those subjected to various surface treatments are mainly used. One of these carbon materials may be used alone, or two or more thereof may be used in combination.

When a graphite material is used as the negative electrode active material, the d value (interlayer distance) of the lattice plane (002 plane) determined by X-ray diffraction by the Gakushin method is usually 0.335 nm or more, and usually 0.34 nm or less, preferably Is preferably 0.337 nm or less.
Further, the ash content of the graphite material is usually 1% by weight or less, particularly 0.5% by weight or less, and particularly preferably 0.1% by weight or less, based on the weight of the graphite material.

Further, the crystallite size (Lc) of the graphite material determined by X-ray diffraction by the Gakushin method is usually 30 nm or more, preferably 50 nm or more, and particularly preferably 100 nm or more.
Further, the median diameter of the graphite material obtained by the laser diffraction / confusion method is usually 1 μm or more, especially 3 μm or more, more preferably 5 μm or more, particularly 7 μm or more, and usually 100 μm or less, especially 50 μm or less, more preferably 40 μm or less, particularly 40 μm or less. It is preferable that it is 30 micrometers or less.

Further, the BET specific surface area of the graphite material is usually 0.5 m ² / g or more, preferably 0.7 m ² / g or more, more preferably 1.0 m ² / g or more, and further preferably 1.5 m ² / g. or more, and usually 25.0 m ² / g or less, preferably 20.0 m ² / g, more preferably 15.0 m ² / g or less, still more preferably 10.0 m ² / g or less.
Further, in the case of performing the Raman spectrum analysis using argon ion laser light for graphite material, and the peak intensity I _A of the peak P _A is detected in the range of 1580～1620Cm ^-1, in the range of 1350 ^-1 intensity ratio I _a / I _B between a peak intensity I _B of a peak P _B detected is what is preferably 0 to 0.5.
Further, the half value width is preferably 26cm ^-1 or less of the peak P _A, 25 cm ^-1 or less is more preferable.

  In addition to the various carbon materials described above, other materials capable of inserting and extracting lithium can be used as the negative electrode active material. Specific examples of the negative electrode active material other than the carbon material include metal oxides such as tin oxide and silicon oxide, and lithium alloys such as lithium alone and lithium aluminum alloys. One of these materials other than the carbon material may be used alone, or two or more thereof may be used in combination. Moreover, you may use in combination with the above-mentioned carbon material.

  As in the case of the positive electrode active material layer, the negative electrode active material layer is usually a negative electrode current collector obtained by slurrying the above-described negative electrode active material, a binder, and, if necessary, a thickener and a conductive material with a solvent. It can be manufactured by applying to the body and drying. As the solvent, binder, thickener, conductive material and the like forming the slurry, the same materials as those described above for the positive electrode active material can be used.

3-3. Separator A separator is usually interposed between the positive electrode and the negative electrode in order to prevent a short circuit between the electrodes. The material and shape of the separator are not particularly limited, but those that are stable with respect to the non-aqueous electrolyte described above, have excellent liquid retention properties, and can reliably prevent short-circuiting between electrodes are preferable. Preferable examples include microporous films, sheets, nonwoven fabrics and the like made of various polymer materials. Specific examples of the polymer material include polyolefin polymers such as nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polyethylene, polypropylene, and polybutene. In particular, from the viewpoint of chemical and electrochemical stability, which is an important factor of the separator, a polyolefin-based polymer is preferable.

3-4. Assembly of secondary battery The non-aqueous electrolyte secondary battery of the present invention assembles the above-described non-aqueous electrolyte for a secondary battery, a positive electrode, a negative electrode, and a separator used as necessary into an appropriate shape. It is manufactured by. Furthermore, other components such as an outer case can be used as necessary.

  The shape of the non-aqueous electrolyte secondary battery of the present invention is not particularly limited, and can be appropriately selected from various shapes generally employed according to the application. Examples of commonly used shapes include a cylinder type with a sheet electrode and separator in a spiral shape, a cylinder type with an inside-out structure combining a pellet electrode and a separator, and a coin type with stacked pellet electrodes and a separator. Can be mentioned. The method for assembling the battery is not particularly limited, and can be appropriately selected from various commonly used methods according to the shape of the target battery.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, comparative examples, and reference examples. However, the present invention is not limited to these examples unless it exceeds the gist.

(1) Production of difluorophosphate (1-1) Example 1
In a mixed solvent (nonaqueous solvent) with a volume ratio of 3: 3: 4 of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) purified in a dry argon atmosphere, a concentration of 1 mol / L is sufficient. The dried lithium hexafluorophosphate (LiPF ₆ ) was dissolved. Further, silicon dioxide (SiO ₂ ) was mixed with this mixed solution at a rate of 0.05 mol / kg and reacted at 50 ° C. for 72 hours.
Thereafter, the reaction product solution was filtered, and the filtrate was measured by ion chromatography. The amount of PO ₂ F ₂ anion detected was 0.049 mol / kg. Thereby, the production | generation of lithium difluorophosphate was confirmed by the manufacturing method of this invention.

(1-2) Comparative Example 1
The same operation as in Example 1 was performed except that SiO ₂ was not used. The filtrate was measured by ion chromatography, but no PO ₂ F ₂ anion was detected. This confirmed that lithium difluorophosphate was not produced by this method.

(2) Production of non-aqueous electrolyte secondary battery (2-1) Example 2
A non-aqueous electrolyte secondary battery was produced by the following method and evaluated. The results are shown in Table 1.

[Production of positive electrode]
90% by weight of lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, 5% by weight of acetylene black as a conductive material, and 5% by weight of polyvinylidene fluoride (PVdF) as a binder, an N-methylpyrrolidone solvent After mixing in a slurry to form a slurry, it was applied to both sides of a 20 μm aluminum foil, dried, and rolled to a thickness of 80 μm with a press machine and cut into a width of 52 mm and a length of 830 mm to obtain a positive electrode. However, both the front and back sides are provided with a 50 mm uncoated portion in the length direction, and the length of the active material layer is 780 mm.

[Production of negative electrode]
The d-value of the lattice plane (002 plane) in X-ray diffraction is 0.336 nm, the crystallite size (Lc) is 100 nm or more (264 nm), the ash content is 0.04 wt%, the median diameter by laser diffraction / scattering method is 17 μm, BET law specific surface area of 8.9 m ² / g, a peak P _a (peak intensity I _a) in the range of 1580～1620Cm ^-1 in the Raman spectrum analysis using argon ion laser light and the peak P in the range of 1350 ^-1 _B intensity ratio (peak intensity _{I B) R = I B /} I a is 0.15, artificial graphite powder KS-44 half-width is 22.2Cm ^-1 peak P _a (Timcal Co., Ltd., trade name) To 94 parts by weight, styrene-butadiene rubber (SBR) dispersed with distilled water was added to a solid content of 6 parts by weight and mixed with a disperser to form a slurry. Was uniformly coated on the both surfaces of a copper foil having a thickness of 18μm is the anode current collector, dried, further width those rolled to 85μm in press 56 mm, cut to length 850 mm, and the negative electrode. However, both the front and back sides are provided with a 30 mm uncoated portion in the length direction.

[Preparation of electrolyte]
The filtrate of the reaction product obtained in Example 1 was used as a non-aqueous electrolyte.

[Battery assembly]
The positive electrode and the negative electrode were wound with a separator made of a porous polyethylene sheet and sealed in a battery can as an electrode group. Thereafter, 5 mL of the electrolyte solution was injected into a battery can loaded with an electrode group, and the electrode solution was sufficiently infiltrated, followed by caulking to produce an 18650 type cylindrical battery.

[Battery evaluation]
A new battery that had not undergone the actual charge / discharge cycle was subjected to initial charge / discharge for 5 cycles at 25 ° C. At this time, the discharge capacity was defined as the initial capacity 0.2C at the fifth cycle (the rated capacity due to the discharge capacity at the hour rate is 1 C, and the same applies hereinafter).

Thereafter, a cycle test was performed in a high temperature environment of 60 ° C., which is regarded as an actual use upper limit temperature of the lithium secondary battery. A charge / discharge cycle in which charging was performed at a constant current of 2 C to a discharge end voltage of 3.0 V after charging with a constant current constant voltage method of 2 C to a charging upper limit voltage of 4.1 V was defined as one cycle, and this cycle was repeated up to 500 cycles.
The battery after the end of the cycle test was charged and discharged for 3 cycles under a 25 ° C. environment, and the 0.2 C discharge capacity of the third cycle was defined as the post-endurance capacity.

(2-2) Comparative Example 2
Instead of using the reaction filtrate obtained in Example 1 as the non-aqueous electrolyte, the same procedure as in Example 2 was used except that the solution of Comparative Example 1 (lithium hexafluorophosphate concentration 1 mol / L) was used. A secondary battery was prepared and evaluated in the same manner. The results are shown in Table 1.

(2-3) Reference Example 1
Instead of using the reaction filtrate obtained in Example 2 as a non-aqueous electrolyte, the volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) purified in a dry argon atmosphere was 3: 3. : Lithium hexafluorophosphate (LiPF ₆ ) that was sufficiently dried was dissolved in a mixed solvent of 4 at a concentration of 1 mol / L, and lithium difluorophosphate prepared according to the method described in Non-Patent Document 1 was further reduced to 0 A secondary battery was produced in the same manner as in Example 2 except that a solution added at a concentration of 0.05 mol / kg was used, and evaluation was performed in the same manner. The results are shown in Table 1.

  As is clear from Table 1, the non-aqueous electrolyte for secondary batteries of the present invention is effective for improving the high-temperature cycle characteristics. The effect is not inferior to that of the case of using lithium difluorophosphate of Reference Example 1.

    The use of the lithium secondary battery of the present invention is not particularly limited, and can be used for various known uses. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. Electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, lighting equipment, toys, game machines, watches, strobes, cameras, etc. In particular, since the lithium secondary battery of the present invention can always obtain a high low temperature rate characteristic before and after storage under a high temperature environment, it is particularly effective when used in an environment with a severe temperature difference. Is obtained.

Claims (5)

  A method for producing lithium difluorophosphate, comprising reacting lithium hexafluorophosphate and silicon dioxide in a non-aqueous solvent.   The non-aqueous solvent is one or more solvents selected from the group consisting of cyclic carbonates, chain carbonates, cyclic esters, chain esters, cyclic ethers, chain ethers, and sulfur-containing organic solvents. The method for producing lithium difluorophosphate according to claim 1, wherein the method is characterized in that: A method for producing a non-aqueous electrolyte for a secondary battery comprising at least lithium hexafluorophosphate and lithium difluorophosphate in a non-aqueous solvent,
The at least a portion of the lithium difluorophosphate, the manufacturing method of the nonaqueous electrolyte solution for a secondary battery, characterized by obtained by reacting a lithium hexafluorophosphate with silicon dioxide in a non-aqueous solvent. 4. The method for producing a non-aqueous electrolyte solution for a secondary battery according to claim 3, wherein a reaction product obtained by reacting lithium hexafluorophosphate and silicon dioxide in a non-aqueous solvent is used . The concentration of lithium difluorophosphate in the non-aqueous electrolyte for a secondary battery is 1 × 10 ⁻³ mol / kg or more and 0.5 mol / kg or less, according to claim 3 or 4. The manufacturing method of the nonaqueous electrolyte solution for secondary batteries of description .