JP2008140760A - Nonaqueous electrolytic solution for secondary battery, and nonaqueous electrolytic solution secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic solution for a secondary battery for a high capacity usage and a high output usage, which has excellent load characteristics and durability and provide a nonaqueous electrolytic solution secondary battery. <P>SOLUTION: The electrolytic solution of a secondary battery contains a solvent and an electrolyte containing lithium salt, and the solvent contains chain carboxylic acid ester expressed as R<SB>1</SB>COOR<SB>2</SB>(wherein R<SB>1</SB>, R<SB>2</SB>are alkyl groups of a carbon number 3 or less) and 4-fluoro ethylene carbonate and a ratio of 4-fluoro ethylene carbonate in a total amount of the solvent is 7 vol.% or more. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to improvement of characteristics of a non-aqueous electrolyte secondary battery, and in particular, can ensure good electrolyte permeability and improve load characteristics and durability in a battery with a high coating amount and high filling density. The present invention relates to a non-aqueous electrolyte for a secondary battery and a non-aqueous electrolyte secondary battery using the same.

  In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and batteries for driving power sources are required to have higher capacities. The non-aqueous electrolyte secondary battery that performs charging / discharging by moving between positive and negative electrodes along with charging / discharging has a high battery voltage, high energy density, and high capacity. It is widely used as a driving power source for such mobile information terminals. Currently, as the non-aqueous electrolyte secondary battery, a battery using a lithium-containing transition metal oxide as a positive electrode active material and a graphite-based carbon material as a negative electrode active material is generally used. However, it is difficult to say that the non-aqueous electrolyte secondary battery having such a structure completely satisfies the demand for long-time driving in recent mobile information terminals, and further increase in capacity is an urgent need. In recent years, non-aqueous electrolyte secondary batteries have been actively developed for use in power tools that require high output, and in automotive applications such as electric vehicles and hybrid vehicles. There is a need for a secondary battery having both high output and high durability.

  Here, in order to increase the capacity of the non-aqueous electrolyte secondary battery, it is possible to improve the depth of use of the positive electrode active material by increasing the end-of-charge voltage, silicon having a higher specific capacity than graphite-based carbon materials, etc. Although the development of alloy-based negative electrodes is effective and some have been put into practical use, it is still possible to efficiently pack active materials into battery cans such as increasing the amount of active material applied or increasing the active material packing density. The current situation depends on technology. However, when such a technique is employed, the distance from the electrode surface layer to the vicinity of the current collector is increased, or the gap in the electrode is decreased, so that the permeability of the electrolytic solution is deteriorated. For this reason, in a battery with a high coating amount and a high filling density, the overvoltage required for the movement of lithium ions increases, and the load characteristics are sacrificed. In addition, when the cycle test is performed, the charge / discharge reaction is repeated in a state where the electrolyte does not uniformly reach the electrode, so that the reaction between the electrode and the electrolyte becomes uneven and the capacity is reduced. In addition, there is a problem that the liquid injection time in the battery assembling process becomes long and the manufacturing cost of the battery is increased.

More specifically, in terms of the type of electrolyte, the current non-aqueous electrolyte secondary battery includes cyclic carbonates such as ethylene carbonate and chain carbonates such as diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate. An electrolyte solution in which a lithium salt such as LiPF ₆ or LiBF ₄ is dissolved in a mixed solvent is generally used, and has succeeded in extracting good charge / discharge characteristics (see Patent Document 1 below). However, it is strongly desired to increase the capacity of the battery, and in the situation where the coating amount and the packing density of the electrode increase year by year, it is difficult to ensure sufficient permeability to the electrode with the above-described electrolyte solution, which is favorable. Battery characteristics cannot be obtained. Under such circumstances, development of a non-aqueous electrolyte that can bring out good battery characteristics in batteries for high capacity applications is indispensable.

  On the other hand, in order to increase the output of the non-aqueous electrolyte secondary battery, load characteristics can be improved by reducing the application amount and packing density of the active material and increasing the amount of the electrolyte in the electrode. ing. However, even if the amount of the electrolyte is increased by reducing the coating amount and filling density, the current value also increases accordingly, so that the electrolyte goes uniformly to the electrodes as in the case of the battery for increasing the capacity described above. A situation that does not pass will occur. In addition, if the application amount and packing density of the active material are reduced, the electrode must be lengthened to obtain a desired battery capacity, and an extra separator is required. For this reason, in order to reduce the manufacturing cost of the battery, it is necessary to prevent the lengthening of the separator by increasing the coating amount and the packing density of the active material even in the battery for high output use. Considering such a situation, it is desired to improve the permeability of the electrolytic solution and improve the load characteristics and durability even in a battery for high output use.

Japanese Patent Laid-Open No. 5-21070

As described above, when an electrolytic solution in which a conventional cyclic carbonate and a chain carbonate are mixed is applied to a battery for high capacity use or high output use, load characteristics and durability are impaired, and sufficient battery characteristics are obtained. It had the problem that it could not be obtained.
Accordingly, the present invention provides a non-aqueous electrolyte for a secondary battery and a non-aqueous electrolyte secondary battery using the same that can obtain good load characteristics and durability in a battery for high capacity use and high output use. The purpose is to do.

In order to achieve the above object, the present invention provides a non-aqueous electrolyte for a secondary battery comprising a solvent and an electrolyte containing a lithium salt. The solvent includes R ₁ COOR ₂ (R ₁ and R ₂ are A chain carboxylic acid ester represented by an alkyl group having 3 or less carbon atoms) and 4-fluoroethylene carbonate, and the ratio of the 4-fluoroethylene carbonate to the total amount of the solvent is 7% by volume or more. It is characterized by that.

According to the present invention, a nonaqueous electrolytic solution containing a chain carboxylic acid ester represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms) and 4-fluoroethylene carbonate in a solvent is used. In the conventional battery, load characteristics and durability can be improved dramatically. This is because the chain carboxylic acid ester represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms) is included, and 4-fluoroethylene carbonate is included. The reason for this is roughly explained.

(1) Reason resulting from the inclusion of a chain carboxylic acid ester represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms) As described above, conventional electrolytic solutions For example, a mixture of a cyclic carbonate such as ethylene carbonate and a chain carbonate is used. In this case, ethylene carbonate is mixed for the purpose of improving the dissociation property of the electrolyte and forming a good film on the surface of the negative electrode active material, and the chain carbonate ester is a solid at room temperature. Is mixed for the purpose of liquefying and reducing the viscosity. However, as described above, it is difficult to ensure the permeability to the electrode with the conventional electrolyte solution in which the cyclic carbonate and the chain carbonate are mixed, as the coating amount and packing density of the electrode increase year by year. It has become. However, even in a battery using a conventional chain carbonate ester electrolyte, dimethyl carbonate having a low molecular weight and low viscosity of 0.59 mPas is included in the electrolyte, and the amount of dimethyl carbonate mixed can be increased. It is possible to ensure the permeability to the electrode. However, since the melting point of the dimethyl carbonate is 3 ° C., there is a problem that the battery characteristics are greatly deteriorated at low temperatures. Therefore, in the present invention, the viscosity of the electrolytic solution can be lowered and the melting point is lower than that of the chain carbonate ester, and is represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms). Attention was focused on the chain carboxylic acid ester.

That is, such a chain carboxylic acid ester has a much lower viscosity than a chain carbonate ester usually used. For example, taking methyl acetate (CH ₃ COOCH ₃ ), which is one of chain carboxylic acid esters, as an example, the viscosity is 0.37 mPas, and a commonly used chain carbonate (for example, diethyl carbonate has a viscosity of Viscosity is much lower than 0.75 mPas). Therefore, by including the chain carboxylic acid ester in the solvent of the electrolytic solution, the viscosity of the electrolytic solution is lowered, so that the permeability to the electrode can be improved as compared with the conventional electrolytic solution.

  Further, such a chain carboxylic acid ester has a very low melting point. For example, methyl acetate and methyl propionate have melting points of −98 ° C. and −88 ° C., respectively, which are much lower than dimethyl carbonate (melting point: 3 ° C.) and diethyl carbonate (melting point: −43 ° C.). For this reason, even if the mixing amount of these chain carboxylic acid esters is increased in order to reduce the viscosity of the electrolytic solution, unlike dimethyl carbonate, the characteristics at low temperature are not sacrificed.

In the chain carboxylic acid ester represented by R ₁ COOR ₂ , R ₁ and R ₂ are limited to alkyl groups having 3 or less carbon atoms because R ₁ and R ₂ are alkyl groups having 4 or more carbon atoms. If so, the viscosity of the chain carboxylic acid ester is increased, and the effects of the present invention cannot be sufficiently exhibited.

(2) Reasons resulting from the inclusion of 4-fluoroethylene carbonate The reasons resulting from the inclusion of 4-fluoroethylene carbonate are described in JP-A-5-74487 in order to facilitate understanding thereof. This will be described in comparison with the inventions described in Japanese Patent Laid-Open No. 5-74490, Japanese Patent Laid-Open No. 8-195221, and Japanese Patent Laid-Open No. 2004-319212.

  In these publications, a method for improving load characteristics and low-temperature characteristics by mixing a chain carboxylic acid ester with an electrolytic solution is proposed. In this case, the chain carboxylic acid ester generally has higher reactivity with the graphite-based negative electrode active material than the cyclic carbonate, and therefore has ethylene carbonate or a C═C unsaturated bond in order to suppress the reaction. The combined use with a cyclic carbonate was indispensable (the above invention is also such an invention). However, in such a configuration, as will be described in detail later, even when the decomposition reaction of the chain carboxylic acid ester can be suppressed at the initial stage of the charge / discharge test, Capacity degradation due to decomposition of the carboxylic acid ester is observed. In JP-A-5-74487 and JP-A-5-74490, there is a description that good cycle characteristics can be obtained by mixing ethylene carbonate and methyl propionate in a solvent. As a result of our investigation, Ethylene carbonate is insufficient to suppress the decomposition of the chain carboxylic acid ester, and even when a cyclic carbonate having a C═C unsaturated bond is further added, there is still a problem in cycle characteristics. Presumably, the new surface is exposed due to the volume change of the negative electrode active material due to charge and discharge, and the additive in the electrolyte is continuously consumed, resulting in the depletion of the additive and the decomposition of the chain carboxylic acid ester. it is conceivable that. In addition, the conventional cyclic carbonate having a C═C unsaturated bond has a problem that if the addition amount is too large, the surface film of the negative electrode becomes thick, causing an increase in resistance and gas generation. Thus, even if chain carboxylic acid ester was mixed, sufficient battery characteristics could not be obtained with the composition of the conventional electrolytic solution.

Therefore, as a result of examining a solvent that functions as a surface film forming agent for the negative electrode and can increase the amount of mixing for the purpose of suppressing the reaction between the chain carboxylic acid ester and the negative electrode active material, It has been found that ethylene carbonate is very effective, and the reaction between the chain carboxylic acid ester and the negative electrode active material can be suppressed by mixing 7% by volume or more with respect to the total amount of the solvent.
This is because when 4-fluoroethylene carbonate is mixed with a solvent and 4-fluoroethylene carbonate forms a film at a potential nobler than the decomposition potential of the chain carboxylic acid ester, the decomposition reaction of the chain carboxylic acid ester This is because it is suppressed. In addition, it was confirmed that even when 40% by volume or more of 4-fluoroethylene carbonate was mixed with respect to the entire solvent, no significant deterioration in battery characteristics occurred. The reason for this is not clear, but it is considered that the increase in resistance due to the increase in the thickness of the negative electrode surface film is suppressed. From the above, since 4-fluoroethylene carbonate can be used as a solvent (that is, not used as an additive as in the prior art), there is no problem of additive depletion associated with charge / discharge cycles, and good durability. It is possible to secure the sex.

In Japanese Patent Application Laid-Open No. 2004-241339, Li is occluded and released at 4.5 V or more by mixing a fluorine-substituted carbonate (4-fluoroethylene carbonate) and a chain carboxylic acid ester (methyl propionate). Although it is described that the cycle characteristics are improved in the secondary battery using LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material, both the fluorine-substituted carbonate ester and the chain carboxylate ester are mixed in a small amount, and further in a fully charged state. The comparative example describes that when LiMn ₂ O ₄ having a positive electrode potential of less than 4.5 V is used as the positive electrode active material, no improvement in cycle characteristics is observed.
On the other hand, in the present invention, a chain carboxylic acid ester is added for the purpose of improving cycle characteristics by lowering the viscosity of the electrolyte and homogenizing the charge / discharge reaction. In order to sufficiently suppress decomposition on the negative electrode, the ratio of 4-fluoroethylene carbonate to the total amount of the solvent is regulated to 7% by volume or more.
Therefore, in the invention described in the above publication, cycle characteristics of a battery using a positive electrode active material having a positive electrode potential of less than 4.5 V in a fully charged state such as commonly used LiCoO ₂ or LiMn ₂ O _4. (In addition, there is no description in the above publication regarding the technique for improving the cycle characteristics of a battery using such a positive electrode active material), the electrolytic solution of the present invention was used. In this case, it is greatly different in that the cycle characteristics of a battery using the positive electrode active material can be dramatically improved.

(3) Conclusion From the above, by using a mixture of a chain carboxylic acid ester and 4-fluoroethylene carbonate and regulating the amount of 4-fluoroethylene carbonate mixed, the decomposition reaction of the chain carboxylic acid ester can be carried out. It is suppressed and the merit of lowering the viscosity of the electrolytic solution possessed by the chain carboxylic acid ester can be maximized. As a result, good electrolyte permeability can be ensured even in batteries with high capacity and high output, and a nonaqueous electrolyte secondary battery having both high capacity, high output, and high durability can be obtained.

Here, as chain carboxylic acid ester represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms) in the present invention, methyl acetate [CH ₃ COOCH ₃ ], ethyl acetate [CH ₃ COOC ₂ H ₅ ], n-propyl acetate [CH ₃ COOCH ₂ CH ₂ CH ₃ ], i-propyl acetate [CH ₃ COOCH (CH ₃ ) CH ₃ ], methyl propionate [C ₂ H ₅ COOCH ₃ ], Ethyl propionate [C ₂ H ₅ COOC ₂ H ₅ ], n-propyl propionate [C ₂ H ₅ COOCH ₂ CH ₂ CH ₃ ], i-propyl propionate [C ₂ H ₅ COOCH (CH ₃ ) CH ₃ ] N-methyl butyrate [CH ₃ CH ₂ CH ₂ COOCH ₃ ], ethyl n-butyrate [CH ₃ CH ₂ CH ₂ COOC ₂ H ₅ ], n-propyl butyrate [CH ₃ CH ₂ CH ₂ COOCH ₂ CH ₂ CH ₃ ] I-propyl n-butyrate [CH ₃ CH ₂ CH ₂ COOCH (CH ₃ ) CH ₃ ], methyl i-butyrate [CH ₃ (CH ₃ ) CHCOOOCH ₃ ], ethyl i-butyrate [CH ₃ (CH ₃ ) CHCOOC ₂ H ₅ ], n-propyl i-butyrate [CH ₃ (CH ₃ ) CHCOOCH ₂ CH ₂ CH ₃ ], i-propyl i-butyrate [CH ₃ (CH ₃ ) CHCOOCH (CH ₃ ) CH ₃ ] and the like Can be mentioned.

In order to obtain particularly good load characteristics and durability, a chain carboxylic acid ester having 5 or less carbon atoms is preferred. Specifically, methyl acetate [CH ₃ COOCH ₃ ], ethyl acetate [CH ₃ COOC ₂ H ₅ ], N-propyl acetate [CH ₃ COOCH ₂ CH ₂ CH ₃ ], i-propyl acetate [CH ₃ COOCH (CH ₃ ) CH ₃ ], methyl propionate [C ₂ H ₅ COOCH ₃ ], ethyl propionate [C ₂ H ₅ COOC ₂ H ₅ ], methyl n-butyrate [CH ₃ CH ₂ CH ₂ COOCH ₃ ], and methyl i-butyrate [CH ₃ (CH ₃ ) CHCOOCH ₃ ] are preferred. Among them, methyl acetate [CH ₃ COOCH ₃ ], ethyl acetate [CH ₃ COOC ₂ H ₅ ] and methyl propionate [C ₂ H ₅ COOCH ₃ ] having low viscosity are preferable.

Specifically, the viscosity of methyl acetate [CH ₃ COOCH ₃ ] is 0.37 mPas as described above, and ethyl acetate [CH ₃ COOC ₂ H ₅ ] and methyl propionate [C ₂ H ₅ COOCH ₃ ]. The viscosities are 0.44 mPas and 0.43 mPas, respectively, and the viscosities are higher than those of commonly used chain carbonates (diethyl carbonate: 0.75 mPas, ethyl methyl carbonate: 0.65 mPas, dimethyl carbonate: 0.59 mPas). Very low. Therefore, by including methyl acetate or the like in the solvent of the electrolytic solution, the viscosity of the electrolytic solution can be reduced, so that the permeability to the electrode can be improved.

Further, among the methyl acetate [CH ₃ COOCH ₃ ], ethyl acetate [CH ₃ COOC ₂ H ₅ ], and methyl propionate [C ₂ H ₅ COOCH ₃ ], methyl propionate [C ₂ H ₅ COOCH ₃ ] is Most preferred. This, as described above, although methyl propionate [C ₂ H ₅ COOCH _3] is viscosity slightly higher than methyl acetate [CH ₃ COOCH _3], than methyl acetate [CH ₃ COOCH ₃ The reaction of the negative electrode This is due to the low nature.
Of course, the above-mentioned chain carboxylic acid esters may be used not only alone but also in combination.

Further, the ratio of the chain carboxylic acid ester to the total amount of the solvent is preferably 20% by volume or more, and particularly preferably 40% by volume or more.
This is because if the content of the chain carboxylic acid ester is below these ranges, the electrolyte solution viscosity becomes high and the electrolyte solution permeability becomes insufficient, so that good load characteristics may not be obtained. Because.

Furthermore, it is preferable that the ratio of 4-fluoroethylene carbonate to the total amount of the solvent is 10 to 50% by volume, particularly 20 to 40% by volume.
If the content of 4-fluoroethylene carbonate is below these ranges, a sufficient film may not be formed on the negative electrode surface, and good durability may not be obtained. On the other hand, when the content of 4-fluoroethylene carbonate exceeds these ranges, the content of the chain carboxylic acid ester is relatively lowered, so that the viscosity of the electrolytic solution is increased and the permeability of the electrolytic solution is insufficient. As a result, good load characteristics may not be obtained.

In addition, it is preferable that vinylene carbonate or vinyl ethylene carbonate is added to the solvent.
Thus, if vinylene carbonate or vinyl ethylene carbonate, which is a kind of cyclic carbonate having a C═C unsaturated bond, is added as a film forming agent for the negative electrode, a good film can be formed on the negative electrode, This is preferable because it decomposes at a potential nobler than the decomposition potential of the chain carboxylic acid ester.

  However, the cyclic carbonate having a C═C unsaturated bond added as a film forming agent for the negative electrode is not limited to vinylene carbonate or vinyl ethylene carbonate, but 4,5-dimethyl vinylene carbonate, 4,5- Diethyl vinylene carbonate, 4,5-dipropyl vinylene carbonate, 4-ethyl-5-methyl vinylene carbonate, 4-ethyl-5-propyl vinylene carbonate, 4-methyl-5-propyl vinylene carbonate, divinyl ethylene carbonate, etc. Also good. However, if vinylene carbonate or vinyl ethylene carbonate is used, a good film can be formed on the negative electrode, and these are preferably used.

Moreover, in order to achieve the said objective, it is the nonaqueous electrolyte secondary battery which consists of the positive electrode containing a positive electrode active material, a negative electrode, a separator, and the nonaqueous electrolyte solution for secondary batteries mentioned above.
As the positive electrode active material in the present invention, a lithium-containing transition metal oxide having a layered structure or a spinel structure can be used, and among them, a lithium-containing transition metal oxide having a layered structure from the viewpoint of increasing energy density. In particular, lithium cobalt oxide, lithium composite oxide of cobalt-nickel-manganese, and lithium composite oxide of aluminum-nickel-cobalt are preferable.
Moreover, these positive electrode active materials may be used alone or in combination with other positive electrode active materials. Furthermore, when producing a positive electrode, these positive electrode active materials are kneaded with a conductive agent such as acetylene black and carbon black and a binder such as PTFE (polytetrafluoroethylene) and PVdF (polyvinylidene fluoride), and used as a positive electrode mixture. be able to.

Moreover, as for the positive electrode active material in this invention, it is desirable that the positive electrode potential in a fully charged bear is less than 4.5V with respect to lithium metal.
The positive electrode active material having a layered structure typified by lithium cobaltate is generally charged to about 4.3 V with respect to lithium metal, but the present invention is not limited to this voltage, Charging can be performed until the voltage is 4.3 V or higher, specifically, less than 4.5 V. Here, the reason why the potential of the positive electrode in the fully charged state is regulated to less than 4.5 V with respect to the lithium metal is as follows. The chain carboxylic acid ester has high reactivity with the negative electrode, but the reaction between the chain carboxylic acid ester and the negative electrode active material can be suppressed by mixing 4-fluoroethylene carbonate with the electrolytic solution. . However, when the positive electrode potential is 4.5 V or more, the chain carboxylic acid ester reacts with the positive electrode active material, which causes a disadvantage that gas is generated when the battery is stored at a high temperature. In addition, when the positive electrode configuration is such that the lithium metal is charged to around 4.5 V, and a graphite-based material is used as the negative electrode active material, the battery voltage is about 4.4 V.

It is desirable that the positive electrode active material contains at least lithium cobaltate in which aluminum or magnesium is dissolved, and zirconium is fixed to the surface of the lithium cobaltate.
The reason for such a structure is as follows. That is, when lithium cobaltate is used as the positive electrode active material, the crystal structure becomes unstable as the charging depth increases. Thus, aluminum or magnesium is dissolved in the positive electrode active material (inside the crystal) to reduce crystal distortion in the positive electrode. However, although these elements greatly contribute to the stabilization of the crystal structure, the initial charge / discharge efficiency is lowered and the discharge operating voltage is lowered. Therefore, in order to alleviate such a problem, zirconium is fixed to the lithium cobalt oxide surface.

(Other matters)
(1) As a solvent for the non-aqueous electrolyte in the present invention, in addition to the chain carboxylate ester and 4-fluoroethylene carbonate, a solvent conventionally used in non-aqueous electrolyte secondary batteries is mixed and used. can do. Examples of such solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, and 2,3-butylene carbonate, cyclic esters such as γ-butyrolactone and propane sultone, diethyl carbonate, ethyl methyl carbonate, Chain carbonates such as dimethyl carbonate, chain ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether and methyl ethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, acetonitrile Etc. are exemplified.

(2) Moreover, as the electrolyte of the non-aqueous electrolyte in the present invention, an electrolyte that has been conventionally used in a non-aqueous electrolyte secondary battery can be used. Such electrolytes include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiClO ₄ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC ( CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiB (C ₂ O ₄ ) ₂ , Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ] and the like. Among them, LiPF ₆ is preferable because it has a good electrical conductivity, and LiBF ₄ is preferable because it can also form a better film in the non-aqueous electrolyte containing 4-fluoroethylene carbonate. . However, if the amount of LiBF ₄ mixed is too large, a negative electrode film is formed excessively, so that the discharge capacity of the battery is reduced. From such a viewpoint, it is preferable to use a mixture of LiPF ₆ and LiBF _4, and in particular, LiPF ₆ is 0.4 to 1.6 mol / l and LiBF ₄ is 0.05 to 0 in the non-aqueous electrolyte. It is preferable to make it contain in the ratio of 6 mol / l.

(3) The current high capacity-oriented non-aqueous electrolyte secondary battery uses lithium cobalt oxide as the positive electrode active material, the content of the positive electrode active material in the electrode is 92% by mass, and the packing density is 3.5 g / cc. The coating amount is designed to be 400 g / 10 cm ² or more on both sides. The battery design suitable for using the electrolytic solution of the present invention is that the diffusion of the electrolytic solution becomes insufficient in such a high coating amount, high filling density type battery, and the load characteristics and durability are deteriorated. It is. The coating amount and packing density at which the effect becomes significant depend on the active material, conductive agent, binder, and content thereof, and are difficult to generalize. When the content per mass of the substance is x and the true density is y, the coating amount of the positive electrode is 60 xyg / cm ² or more on both sides except the mass of the current collector, and the packing density of the positive electrode is 0.60 xyg. The effect is high in a design of more than / cc. In particular, in a design in which the coating amount of the positive electrode is 70 xyg / cm ² or more on both sides and the packing density of the positive electrode is 0.70 xyg / cc or more, load characteristics and durability can be greatly improved.

This is when the positive electrode active material is a lithium cobaltate having a layered structure (true density: 5.00 g / cc) and the content in the positive electrode is 95% by mass (x = 0.95, y = 5.00). ), The effect is high in a design in which the coating amount of the positive electrode is 285 g / 10 cm ² or more on both sides and the packing density is 2.85 g / cc or more. In particular, in a design in which the coating amount of the positive electrode is 333 g / 10 cm ² or more on both sides and the packing density is 3.33 g / cc or more, load characteristics and durability can be greatly improved.

(4) The negative electrode active material in the present invention can be used without particular limitation as long as it is a material capable of occluding and releasing lithium. For example, lithium materials such as metallic lithium, lithium-aluminum alloy, lithium-lead alloy, lithium-silicon alloy, lithium-tin alloy, carbon materials such as graphite, coke, organic fired body, SnO ₂ , SnO, TiO _2, etc. A metal oxide whose base potential is lower than that of the positive electrode active material can be given. Among these, in the nonaqueous electrolytic solution containing 4-fluoroethylene carbonate, a graphite-based carbon material is preferably used from the viewpoint that a high-quality film can be formed on the surface.
The negative electrode material can be kneaded with a binder such as SBR (styrene butadiene rubber), PTFE (polytetrafluoroethylene), PVdF (polyvinylidene fluoride), and used as a mixture.

As in the present invention, a non-aqueous electrolyte containing a chain carboxylic acid ester represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms) and 4-fluoroethylene carbonate in a solvent And by regulating the ratio of 4-fluoroethylene carbonate to the total amount of the solvent, the capacity and output of the non-aqueous electrolyte secondary battery can be increased, and the durability has been dramatically improved. There is an excellent effect that can be made.

  Hereinafter, the present invention will be described in more detail. However, the present invention is not limited to the following best modes, and can be appropriately modified and implemented without departing from the scope of the present invention.

(Preparation of positive electrode)
First, lithium cobalt oxide as a positive electrode active material (Al and Mg are each dissolved in an amount of 1.0 mol%, and zirconium is present on the surface of the lithium cobalt oxide, and the proportion of zirconium is 0.05 mol%) And carbon as a conductive agent and PVdF (polyvinylidene fluoride) as a binder were adjusted to a mass ratio of 95: 2.5: 2.5, and then NMP (N-methyl-2 -Pyrrolidone) Kneaded in a solution to prepare a positive electrode slurry. This positive electrode slurry was applied to both sides of an aluminum foil as a current collector so as to have a ratio of 520 g / 10 cm ² , dried, and then rolled so that the positive electrode packing density was 3.7 g / cc to produce a positive electrode. .

(Preparation of negative electrode)
Graphite as the negative electrode active material, SBR (styrene butadiene rubber) as the binder, and CMC (carboxymethyl cellulose) as the thickener were adjusted to a mass ratio of 97.5: 1.5: 1. Thereafter, a negative electrode slurry was prepared by mixing in an aqueous solution. This negative electrode slurry was applied to both sides of a copper foil as a current collector so as to have a ratio of 220 g / 10 cm ² , dried, and then rolled to a negative electrode filling density of 1.7 g / cc to prepare a negative electrode. .

(Preparation of electrolyte)
4-Fluoroethylene carbonate (FEC) and methyl acetate [CH ₃ COOCH ₃ ] were mixed so that the volume ratio was 20:80, and LiPF ₆ as an electrolyte (lithium salt) was added to this solvent at 1 mol / l. A non-aqueous electrolyte was prepared by dissolving at a ratio of

(Production of battery)
After winding the positive electrode and the negative electrode so as to face each other through a polyethylene separator, a wound electrode body is produced, and then the wound electrode body is electrolyzed in a glove box under an inert gas atmosphere. A non-aqueous electrolyte secondary battery was produced by enclosing the battery together with a liquid in a cylindrical 18650 size battery can.

[First embodiment]
(Example 1)
As Example 1, the non-aqueous electrolyte secondary battery shown in the best mode was used.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as the present invention battery A1.

(Example 2)
Except that vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) as additives were added at a ratio of 2% by mass with respect to the total mass of the solvent and the electrolyte, respectively, as in Example 1. A non-aqueous electrolyte secondary battery was produced.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as the present invention battery A2.

(Example 3)
Except for using 4-fluoroethylene carbonate (FEC), ethylene carbonate (EC), and methyl acetate [CH ₃ COOCH ₃ ] mixed in a volume ratio of 10:10:80 as a solvent, A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as the present invention battery A3.

Example 4
The same as in Example 1 except that 4-fluoroethylene carbonate (FEC) and methyl acetate [CH ₃ COOCH ₃ ] were mixed at a volume ratio of 40:60 as a solvent. A water electrolyte secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A4.

(Example 5)
Except for using 4-fluoroethylene carbonate (FEC), ethylene carbonate (EC) and methyl acetate [CH ₃ COOCH ₃ ] mixed in a volume ratio of 20:20:60 as a solvent, A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A5.

(Example 6)
Except for using 4-fluoroethylene carbonate (FEC), propylene carbonate (PC) and methyl acetate [CH ₃ COOCH ₃ ] mixed in a volume ratio of 20:20:60 as a solvent, A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1.
The non-aqueous electrolyte secondary battery produced in this manner is hereinafter referred to as the present invention battery A6.

(Example 7)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the ratio of LiPF ₆ as the electrolyte was 0.5 mol / l.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A7.

(Example 8)
A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 except that the ratio of LiPF ₆ as the electrolyte was 1.5 mol / l.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A8.

Example 9
LiPF ₆ and LiBF ₄ are used as the electrolyte, the ratio of LiPF ₆ is 0.9 mol / l, and the ratio of LiBF ₄ is 0.00. A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the concentration was 1 mol / l.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A9.

(Example 10)
Nonaqueous electrolysis was performed in the same manner as in Example 1 except that LiPF ₆ and LiBF ₄ were used as the electrolyte, the ratio of LiPF ₆ was 0.8 mol / l, and the ratio of LiBF ₄ was 0.2 mol / l. A liquid secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A10.

(Example 11)
Nonaqueous electrolysis in the same manner as in Example 1 except that LiPF ₆ and LiBF ₄ were used as the electrolyte, the ratio of LiPF ₆ was 0.5 mol / l, and the ratio of LiBF ₄ was 0.5 mol / l. A liquid secondary battery was produced.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as the present invention battery A11.

(Example 12)
Other than using LiPF ₆ and LiB (C ₂ O ₄ ) _{2 as} the electrolyte, the ratio of LiPF ₆ being 0.9 mol / l, and the ratio of LiB (C ₂ O ₄ ) ₂ being 0.1 mol / l Produced a nonaqueous electrolyte secondary battery in the same manner as in Example 1.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as “invention battery A12”.

(Example 13)
Example 1 was used except that 4-fluoroethylene carbonate (FEC) and ethyl acetate [CH ₃ COOC ₂ H ₅ ] were mixed in a volume ratio of 20:80 as a solvent. Thus, a non-aqueous electrolyte secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A13.

(Example 14)
A mixture of 4-fluoroethylene carbonate (FEC) and ethyl acetate [CH ₃ COOC ₂ H ₅ ] in a volume ratio of 20:80 is used as a solvent, and LiPF ₆ and LiBF ₄ are used as an electrolyte. used, and, 1.0 mol proportion of LiPF ₆ / l, except that the proportion of LiBF ₄ was 0.2 mol / l, to prepare a non-aqueous electrolyte secondary battery in the same manner as in example 1.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A14.

(Examples 15 to 19)
The volume ratio of 4-fluoroethylene carbonate (FEC) and methyl propionate [C ₂ H ₅ COOCH ₃ ] is 10:90, 20:80, 30:70, 40:60, and 50:50, respectively. A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the mixture was used as a solvent.
The non-aqueous electrolyte secondary batteries produced in this manner are hereinafter referred to as invention batteries A15, A16, A17, A18, and A19, respectively.

(Example 20)
A mixture of 4-fluoroethylene carbonate (FEC) and methyl propionate [C ₂ H ₅ COOCH ₃ ] in a volume ratio of 20:80 is used as a solvent, and LiPF ₆ and LiBF ₄ are used as electrolytes. A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 except that the ratio of LiPF ₆ was 1.0 mol / l and the ratio of LiBF ₄ was 0.2 mol / l.
The non-aqueous electrolyte secondary battery produced in this manner is hereinafter referred to as the present invention battery A20.

(Example 21)
Except that 4-fluoroethylene carbonate (FEC) and n-propyl acetate [CH ₃ COOCH ₂ CH ₂ CH ₃ ] mixed at a volume ratio of 20:80 were used as solvents. In the same manner as in Example 1, a nonaqueous electrolyte secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A21.

(Example 22)
Implementation was performed except that 4-fluoroethylene carbonate (FEC) and i-propyl acetate [CH ₃ COOCH (CH ₃ ) CH ₃ ] were mixed at a volume ratio of 20:80 as a solvent. A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A22.

(Example 23)
Example 1 except that 4-fluoroethylene carbonate (FEC) and ethyl propionate [C ₂ H ₅ COOC ₂ H ₅ ] mixed at a volume ratio of 20:80 were used as the solvent. In the same manner, a non-aqueous electrolyte secondary battery was produced.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as the present invention battery A23.

(Example 24)
Example, except that 4-fluoroethylene carbonate (FEC) and methyl n-butyrate [CH ₃ CH ₂ CH ₂ COOCH ₃ ] were mixed so that the volume ratio was 20:80. In the same manner as in Example 1, a nonaqueous electrolyte secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as the present invention battery A24.

(Examples 25-27)
4-Fluoroethylene carbonate (FEC), dimethyl carbonate (DMC) and methyl propionate [C ₂ H ₅ COOCH ₃ ] in volume ratios of 20:20:60, 20:40:40, 20:60: A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that a mixture of 20 was used as a solvent.
The non-aqueous electrolyte secondary batteries produced in this way are hereinafter referred to as invention batteries A25, A26, and A27, respectively.

(Comparative Example 1)
What mixed ethylene carbonate (EC) and ethyl methyl carbonate (EMC) so that it might become 30:70 by volume ratio was used as a solvent, and as an additive with respect to the total mass of this solvent and electrolyte, A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that vinylene carbonate (VC) was added at a ratio of 2% by mass.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as a comparative battery Z1.

(Comparative Example 2)
What mixed ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) so that it might become 35: 5: 60 by volume ratio was used as a solvent, and with respect to the total mass of a solvent and electrolyte. A non-aqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 1 except that vinylene carbonate (VC) as an additive was added at a ratio of 3% by mass.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as a comparative battery Z2.

(Comparative Example 3)
Methyl acetate [CH ₃ COOCH ₃ ] alone is used as a solvent, vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) are used as additives, and addition of additives to the total mass of the solvent and electrolyte A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 1 except that the amount was 2% by mass.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as a comparative battery Z3.

(Comparative Example 4)
Methyl acetate [CH ₃ COOCH ₃ ] alone is used as a solvent, vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) are used as additives, and addition of additives to the total mass of the solvent and electrolyte A non-aqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 1 except that the amount was 4% by mass.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as a comparative battery Z4.

(Comparative Example 5)
A mixture of ethylene carbonate (EC) and methyl acetate [CH ₃ COOCH ₃ ] in a volume ratio of 20:80 is used as a solvent, and vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) are used as additives. A non-aqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 1 except that the additive amount of the additive with respect to the total mass of the solvent and the electrolyte was 2 mass%.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as comparative battery Z5.

(Comparative Example 6)
A mixture of ethylene carbonate (EC) and ethyl acetate [CH ₃ COOC ₂ H ₅ ] so as to have a volume ratio of 20:80 is used as a solvent, and vinylene carbonate (VC) and vinyl ethylene carbonate ( VEC) and a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 1, except that the additive amount of the additive with respect to the total mass of the solvent and the electrolyte was 2% by mass.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as comparative battery Z6.

(Comparative Example 7)
A mixture of ethylene carbonate (EC) and methyl propionate [C ₂ H ₅ COOCH ₃ ] in a volume ratio of 20:80 is used as a solvent, and vinylene carbonate (VC) and vinyl ethylene carbonate are used as additives. (VEC) was used, and a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 1 except that the additive amount of the additive with respect to the total mass of the solvent and the electrolyte was 2% by mass. .
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as a comparative battery Z7.

(Comparative Example 8)
Except that 4-fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) mixed at a volume ratio of 20:80 were used as a solvent and no additive was added. A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as comparative battery Z8.

(Comparative Example 9)
A mixture of 4-fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) so as to have a volume ratio of 20:80 is used as a solvent, LiPF ₆ and LiBF ₄ are used as an electrolyte, and The nonaqueous electrolyte secondary battery is the same as Comparative Example 1 except that the ratio of LiPF ₆ is 1.0 mol / l, the ratio of LiBF ₄ is 0.2 mol / l, and no additive is added. Was made.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as a comparative battery Z9.

(Comparative Example 10)
Similar to Comparative Example 1 except that 4-fluoroethylene carbonate (FEC) and methyl propionate [C ₂ H ₅ COOCH ₃ ] were mixed as a solvent in a volume ratio of 5:95. Thus, a non-aqueous electrolyte secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as comparative battery Z10.

(Experiment 1)
Since the said invention battery A1-A27 and comparative battery Z1-Z10 were charged / discharged on the following conditions and the load characteristic of each battery was investigated, the result is shown in Table 1-Table 4. In Tables 1 to 4, the discharge capacity when the comparative battery Z1 is discharged with a current of 0.2 It is shown as 100.

[Charging / discharging conditions]
-Charging conditions Conditions for each battery to be constant current charged at a current of 0.2 It until the battery voltage reaches 4.2 V, and then charged at a constant voltage of 4.2 V until the current value becomes 0.02 It. .
-Discharge condition The condition that the battery voltage is discharged to 2.75 V at each current of 0.2 It and 2.0 It. Then, during this discharge, the discharge capacity at 0.2 It and the discharge capacity at 2.0 It were measured.
In addition, the temperature of charging / discharging is 25 degreeC.

As is apparent from Tables 1 to 4, the present invention batteries A1 to A27 containing a chain carboxylic acid ester and 4-fluoroethylene carbonate (FEC) as the solvent of the electrolytic solution, and ethylene carbonate (EC) as the solvent of the electrolytic solution. ), Ethyl methyl carbonate (EMC), EC, propylene carbonate (PC), and dimethyl carbonate (DMC) (composed of cyclic carbonate and chain carbonate as in the conventional electrolyte) Z1, Z2 When comparing the comparative batteries Z8 and Z9 (comprising fluorine-substituted carbonate ester and chain carbonate ester) made of FEC and EMC, there is almost no difference between them when discharged at a current of 0.2 It. However, when discharged at a current of 2.0 It, the batteries A1 to A27 of the present invention are comparative batteries Z1, Z2, Z8, Discharge capacity compared to 9 is found to be equal to or greater. This is because the batteries A1 to A27 of the present invention contain a low-viscosity chain carboxylic acid ester in the solvent of the electrolytic solution, so that comparative batteries Z1, Z2, Compared to Z8 and Z9, it is considered that the electrolyte easily penetrates to the vicinity of the current collector, and the overvoltage required for lithium ion diffusion is reduced.
Since the comparative batteries Z3 to Z7 contain a chain carboxylic acid ester in the solvent of the electrolytic solution, the discharge capacity when discharged at a current of 2.0 It is larger than that of the comparative batteries Z1 and Z2. It is recognized that This is considered to be due to the same reason as described above.

In addition, the batteries A1, A13, A16, A21 to A24 of the present invention (these batteries are the same in that the lithium salt is LiPF ₆ alone, the solvent contains FEC, and no additive is added). , Only the type of the chain carboxylic acid ester of the solvent is different), and when discharged at a current of 2.0 It, the batteries A1, A13, A16 of the present invention have a discharge capacity compared to the batteries A21-A24 of the present invention. It is recognized that it is slightly larger. This is because methyl acetate (MA), ethyl acetate (EA), and methyl propionate (MP), which are chain carboxylates of the present invention batteries A1, A13, A16, are chain carboxyl groups of the present invention batteries A21 to A24. The viscosity is low compared with the acid esters n-propyl acetate (n-PA), i-propyl acetate (i-PA), ethyl propionate (EP), and methyl n-butyrate (n-MB). Therefore, it is considered that the viscosity of the electrolytic solution can be further reduced, and as a result, the permeability of the electrolytic solution to the electrode can be further improved.

In addition, the present invention batteries A15 to A19 and comparative battery Z10 (these batteries are the same in that the lithium salt is LiPF ₆ alone, the solvent contains FEC and MP, and no additive is added. In other words, the present invention batteries A15 to A19 in which the ratio of FEC to the total amount of the solvent (hereinafter sometimes simply referred to as the ratio of FEC) is 10 to 50% by volume are compared. It can be seen that the discharge capacity is larger than that of the comparative battery Z10 having a FEC ratio of 5% by volume. Therefore, in order to obtain good discharge load characteristics, it is considered that FEC needs to be mixed by 7% by volume or more, and particularly preferably 10% by volume or more.
On the other hand, the battery A19 of the present invention having an FEC ratio of 50% by volume has a slightly lower discharge load characteristic than the batteries A15 to A18 of the present invention having an FEC ratio of 10 to 40% by volume. Although not shown in Table 4, it was also found that the discharge load characteristics are further deteriorated when the FEC ratio exceeds 50% by volume. Therefore, the upper limit of the FEC ratio is preferably regulated to 50% by volume or less, particularly 40% by volume or less. This is because FEC has a higher viscosity than the chain carboxylic acid ester, and when the proportion of FEC is too large, the viscosity of the electrolytic solution increases.

(Experiment 2)
Since charging / discharging of the present invention batteries A1 to A27 and comparative batteries Z1 to Z10 was repeatedly performed under the following conditions and the durability (capacity maintenance ratio) of each battery was examined, the results are shown in Tables 5 to 8.

[Charging / discharging conditions]
(I) First cycle / charging conditions After each battery is charged at a constant current of 0.2 It until the battery voltage reaches 4.2 V, the current value becomes 0.02 It at a voltage of 4.2 V The condition of constant voltage charging.
-Discharge condition The condition of discharging until the battery voltage becomes 2.75 V with a current of 0.2 It. Then, initial discharge capacity D ₁ measured at the discharge.
In addition, the temperature of charging / discharging is 25 degreeC.

(I) 2nd cycle and after ・ Charging conditions After charging each battery at a current of 1.0 It until the battery voltage reaches 4.2 V, the current value becomes 0.02 It at a voltage of 4.2 V The condition of constant voltage charging.
-Discharge condition The condition of discharging until the battery voltage becomes 2.75 V at a current of 1.0 It. Then, (in this experiment, n = 100 and 200) n during the discharge the discharge capacity was measured D _n after the cycle.
In addition, the temperature of charging / discharging is 25 degreeC.

Then, from the discharge capacity D _n after n cycles (in this experiment, n = 100 and 200) and the initial discharge capacity D ₁ , the capacity retention rate (%) after n cycles was determined by the following equation (1). However, the evaluation of the battery whose capacity maintenance ratio fell below 70% during the cycle test was stopped at that time.
Capacity maintenance rate (%) = (D _n / D ₁ ) × 100 (1)
In addition, about this invention battery A1, A10, A13, A14, A16, A20-A23, and the comparison battery Z1, the test was implemented to 500 cycles.

[Consideration up to 200 cycles]
As is apparent from Tables 5 to 8, the capacity retention rate of the batteries A1 to A27 of the present invention containing a chain carboxylic acid ester and FEC as the solvent of the electrolytic solution, and the solvent of the electrolytic solution is EC and EMC or EC. When comparing the capacity maintenance rate of comparison batteries Z1 and Z2 made of PC, DMC, and comparison batteries Z8 and Z9 made of FEC and EMC (made of fluorine-substituted carbonate ester and chain carbonate ester), It is recognized that the batteries A1 to A27 of the present invention have an improved capacity maintenance rate compared to the comparative batteries Z1, Z2, Z8, and Z9. In comparative batteries Z1, Z2, Z8, and Z9 that do not use a chain carboxylic acid ester as a solvent for the electrolytic solution, the amount of active material applied to the electrode plate is increased or the packing density of the active material on the electrode plate is increased. Under such circumstances, the permeability of the electrolyte is insufficient, and lithium ions do not easily diffuse, so the reaction becomes uneven and the deterioration of the battery is promoted. On the other hand, in the batteries A1 to A27 of the present invention containing the chain carboxylic acid ester and FEC in the solvent of the electrolytic solution, the application amount of the active material of the electrode plate is increased, or the packing density of the active material of the electrode plate is This is probably because the electrolyte has good permeability even under high conditions, and lithium ions can diffuse sufficiently, so that reaction disproportionation is suppressed and deterioration of the battery is suppressed.

  In addition, comparative batteries Z3 and Z4 in which the solvent of the electrolytic solution is made of a chain carboxylic acid ester and an additive is added thereto, and comparative batteries Z5 to Z7 in which the solvent of the electrolytic solution is made of a chain carboxylic acid ester and EC. Then, it is recognized that the capacity | capacitance maintenance factor like this invention battery A1-A27 which added the FEC to this in the solvent of electrolyte solution contains chain | strand-shaped carboxylic acid ester cannot be exhibited. From the result of the above-mentioned experiment 1, although the load characteristics are improved if the solvent of the electrolytic solution contains the chain carboxylic acid ester as in the comparative batteries Z3 to Z7, the chain carboxylic acid ester is simply added to the solvent of the electrolytic solution. It is considered that the deterioration of the battery occurs early because the reaction suppression between the negative electrode active material and the chain carboxylic acid ester becomes insufficient only by including.

  From the results of comparative batteries Z3 to Z7 that contain a chain carboxylic acid ester in the electrolyte solvent, the chain carboxylic acid ester is highly reactive with the negative electrode active material. It can be seen that the inclusion of an acid ester is a factor that lowers rather than improves durability. On the other hand, if FEC is used in addition to the chain carboxylic acid ester as the solvent for the electrolyte solution as in the present invention, the FEC suppresses the reaction between the chain carboxylic acid ester and the negative electrode active material, and the electrolyte solution Since the merit of the chain carboxylic acid ester that lowers the viscosity is maximized, it is possible to achieve both excellent load characteristics and durability.

  However, even when FEC is used in addition to the chain carboxylic acid ester as the solvent of the electrolytic solution, the cycle characteristics are improved when the ratio of FEC to the total amount of the solvent is 5% by volume as in the comparative battery Z10. It is not possible. On the other hand, in the present invention batteries A15 to A19 in which the same solvent as the comparative battery Z10 is used but the ratio of FEC to the total amount of the solvent is 10% by volume or more, it is recognized that the cycle characteristics are dramatically improved. This is because the ratio of FEC is too small in the comparative battery Z10, and thus the reaction between the chain carboxylic acid ester and the negative electrode active material cannot be sufficiently suppressed. On the other hand, in this invention battery A15-A19, since the ratio of FEC is sufficient, it is thought that it originates in the reaction of chain | strand-shaped carboxylic acid ester and a negative electrode active material being able to fully be suppressed. . Therefore, it is considered that the ratio of FEC to the total amount of the solvent needs to be regulated to 7% by volume or more. In particular, from the experimental results of the present invention batteries A15 to A19, the ratio of FEC to the total amount of the solvent is 10 to 50 volumes. %, Preferably 20 to 40% by volume.

Further, Table 1 and Table 2, when comparing the present invention cell A1 and the present invention battery A9～A11, present battery A9～A11 mixed with LiBF ₄ is not mixed with LiBF ₄ The discharge capacity is reduced as compared with the battery A1 of the present invention. This is considered to be caused by the fact that LiBF ₄ is involved in the formation of the negative electrode film during initial charging in the present invention batteries A9 to A11 in which LiBF ₄ is mixed. However, when comparing the cycle characteristics, as shown in Table 5 and Table 6, the present invention battery A9~A11 mixed with LiBF _4, rather than the present invention cell A1 which is not a mixture of LiBF _4, capacity retention It can be seen that the rate is further improved. Probably, LiBF ₄ together with FEC is involved in the formation of the negative electrode film, so that a better film is formed than when LiBF _{4 is} not added, and the decomposition of the chain carboxylic acid ester is further suppressed. From such a viewpoint, it is more preferable to use LiBF ₄ in a mixture in the electrolyte solution of the present invention containing a chain carboxylic acid ester and FEC. This is a comparison between the present invention battery A13 and the present invention battery A14 and a comparison between the present invention battery A16 and the present invention battery A20 (however, in this case, the present invention battery A20 is compared with the present invention battery A16). The capacity retention rate at 100 cycles is inferior, but the capacity retention rate at 200 cycles is excellent). Further, as is apparent from the results of the battery A12 of the present invention, it is understood that the lithium salt capable of obtaining such an effect is not limited to LiBF ₄ but may be LiB (C ₂ O ₄ ) _2. .
In addition, it was confirmed that the decomposition of the chain carboxylic acid ester cannot be suppressed with the electrolytic solution using only LiBF ₄ without using FEC. Therefore, it goes without saying that mixing of FEC is essential.

[Consideration up to 500 cycles]
・ Considerations on types of solvent The batteries A1, A13, A16, A21 to A23 of the present invention are the same in that FEC is contained in the solvent and 1 mol / l of LiPF ₆ is added, but the solvent type except FEC is MP. The present invention battery A16 has improved cycle characteristics as compared with the present invention batteries A1, A13, A21 to A23, in which the solvent types except FEC are MA, EA, n-PA, i-PA, and EP, respectively. It is recognized that In order to investigate the reason for this experimental result, the electrical conductivity at room temperature of the electrolyte used in each battery was measured. The results are shown in Table 9.

As is clear from Table 9 above, as a solvent other than FEC, the electrolytic solution using MA having the lowest viscosity has the highest conductivity, followed by the electrolytic solution using EA and the electrolytic solution using MP. ing. From this, diffusion of lithium ions should be superior to that of MP in MA and EA. However, it is recognized that the battery A16 of the present invention using MP as a solvent other than FEC is specifically superior in cycle characteristics.
The reason is not clear, but among chain carboxylic esters, MP has the lowest reactivity with the negative electrode. Therefore, when a cycle test over a long period of time is performed, unlike the order of conductivity shown in Table 9 above, a battery using an electrolyte containing MP is considered to be excellent. Of course, from the result of the comparative battery Z7, it is clear that even if MP is contained in the electrolytic solution, excellent cycle characteristics cannot be obtained unless FEC is mixed.

-Consideration about additive In this invention battery A1 and A13 using electrolyte solution containing MA and EA with high electrical conductivity, the capacity | capacitance fall considered to be attributable to the reactivity with a negative electrode was recognized at the end of a cycle. In particular, in the battery A13 of the present invention using EA, a rapid capacity reduction was observed at the end of the cycle. However, in the batteries A10 and A14 of the present invention in which LiBF ₄ is added to the electrolyte solution of these batteries, it is recognized that the decrease in the capacity retention rate is suppressed even when the charge / discharge cycle is repeated. It was recognized that the battery A14 of the present invention using the electrolytic solution containing it was remarkable. Furthermore, when comparing the electrolytic solution and present battery A16 using the present battery A20 containing MP, present battery A20 was added LiBF ₄ are compared with the present battery A16 without the addition of LiBF ₄ It was confirmed that the decrease in capacity maintenance rate was suppressed. This is considered to be due to the fact that the negative electrode film is firmly formed by adding LiBF ₄ to the electrolytic solution, and as a result, the reactivity between the chain carboxylic acid ester and the negative electrode is further suppressed.
From the results of comparative batteries Z8 and Z9 in Table 8, even when LiBF ₄ was mixed with the conventional electrolyte, the capacity retention rate was not improved, and an electrolyte containing FEC and a chain carboxylic acid ester was used. This is considered to be a phenomenon peculiar to the battery.

As described in detail above, non-aqueous electrolysis containing a chain carboxylic acid ester represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms) and 4-fluoroethylene carbonate in a solvent. By regulating the ratio of the 4-fluoroethylene carbonate to the total amount of the solvent using a liquid, sufficient electrolyte permeability is ensured even in a high coating amount, high packing density type battery, high capacity, A non-aqueous electrolyte secondary battery having both high output and high durability can be obtained.

(Experiment 3)
Table 10 shows the results obtained by examining the electrical conductivity at 25 ° C. and the electrical conductivity at −20 ° C. (value after being left in a thermostatic bath set at −20 ° C. for 2 hours). In Table 10, electrolytic solutions b1 to b4 are electrolytic solutions used in the present invention, and electrolytic solutions y1 and y2 are conventionally used electrolytic solutions.

  As is clear from Table 10, the electrolyte y1 containing DMC that does not contain a chain carboxylic acid ester but has a high melting point has an extremely low electrical conductivity at low temperatures, whereas whether or not it contains DMC. Regardless, it can be seen that the electrolytes b1 to b4 containing the chain carboxylic acid ester exhibit high conductivity even at low temperatures. From this result, it can be seen that in order to reduce the viscosity of the electrolytic solution over a wide temperature range and obtain high conductivity, the electrolytic solution needs to contain a chain carboxylic acid ester. In addition, when the electrolytic solutions b1 to b4 are compared, in order to obtain higher conductivity, the ratio of the chain carboxylic acid ester to the total amount of the solvent is desirably 20% by volume or more, particularly 40% by volume or more. It is desirable to be.

[Second Embodiment]
(Example 1)
[Production of positive electrode]
The same positive electrode slurry as in the best mode was applied to both sides of an aluminum foil as a current collector so as to have a ratio of 360 g / 10 cm ² , and after drying, the positive electrode packing density was 3.6 g / cc. Rolled to produce a positive electrode.

[Production of negative electrode]
The same negative electrode slurry as in the best mode was applied to both sides of a copper foil as a current collector so as to have a ratio of 160 g / 10 cm ² , and after drying, the negative electrode filling density was 1.6 g / cc. The negative electrode was produced by rolling.

[Preparation of electrolyte]
4-Fluoroethylene carbonate (FEC) and methyl propionate [C ₂ H ₅ COOCH ₃ ] were mixed so that the volume ratio was 20:80, and LiPF ₆ as an electrolyte was added to this solvent at 1 mol / l. A non-aqueous electrolyte was prepared by dissolving at a ratio.

[Production of battery]
The positive electrode and the negative electrode produced by the above method were cut out to a predetermined size, wound up so as to face each other through a polyethylene separator, and crushed flat into a substantially plate shape. Next, the substantially plate-shaped winding body is inserted into a bag-shaped exterior body made of a laminate material produced by laminating PET and aluminum, and then an electrolyte is injected into the exterior body. A non-aqueous electrolyte secondary battery was produced by fusing the opening of the body.
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as the present invention battery C1.

(Example 2)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the coating amount of the positive electrode slurry was 290 g / 10 cm ² .
The non-aqueous electrolyte secondary battery thus produced is hereinafter referred to as the present invention battery C2.

(Comparative Example 1)
Nonaqueous electrolysis in the same manner as in Example 1 except that 4-fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) mixed at a volume ratio of 20:80 were used as a solvent. A liquid secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as comparative battery X1.

(Comparative Example 2)
Nonaqueous electrolysis in the same manner as in Example 2 except that 4-fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) mixed at a volume ratio of 20:80 were used as the solvent. A liquid secondary battery was produced.
The non-aqueous electrolyte secondary battery produced in this way is hereinafter referred to as comparative battery X2.

(Experiment)
The present invention batteries C1 and C2 and comparative batteries X1 and X2 were charged / discharged under the following conditions, then stored under the following conditions, and examined for high-temperature storage characteristics. The results are shown in Table 11.
a. First charging condition Each battery was charged with a current of 1.0 It at a predetermined end-of-charge voltage (for a battery with a positive electrode coating amount of 360 g / 10 cm ² , the end-of-charge voltage was 4.2 V [the positive electrode potential was about 4.3 V], For a battery of 290 g / 10 cm ^2, a constant current charge is performed until the end-of-charge voltage is 4.4 V (positive electrode potential is about 4.5 V)), and then the current value is at a predetermined voltage (4.2 V or 4.4 V). The condition of constant voltage charging until 0.05 It.
b. Discharge condition A condition that discharge is performed at each current of 1.0 It until the end-of-discharge voltage becomes 2.75V.

c. Second charging condition Conditions similar to the first charging condition.
d. Disassembly of the battery After the second charge, each battery was disassembled, and only the positive electrode was taken out and sealed again in the laminate outer package.
e. Storage conditions Conditions in which the positive electrode is sealed in an exterior body and stored at 60 ° C. for 10 days.

(result)
As apparent from Table 11 above, when the charging voltage of the battery was changed from 4.2 V to 4.4 V, an increase in the amount of gas generated was observed in both the comparative battery and the battery of the present invention. However, when the battery C1 of the present invention having a charge end voltage of 4.2 V and the comparative battery X1 are compared, the battery C1 of the present invention has a smaller increase in thickness than the comparative battery X1, whereas the charge end of charge is smaller. When the present battery C2 having a voltage of 4.4 V and the comparative battery X2 were compared, it was found that the present invention battery C2 had a larger thickness increase than the comparative battery X2.
This is considered to be because when the positive electrode potential is 4.5 V (charge end voltage is 4.4 V) or more, the chain carboxylic acid ester is decomposed at the positive electrode, so that the amount of gas generated increases. From this result, it was confirmed that in the battery of the present invention, it is preferable to regulate the potential of the positive electrode in a fully charged state to less than 4.5V.

  The present invention can be applied to a driving power source of a mobile information terminal such as a mobile phone, a notebook computer, and a PDA, and also to a driving power source of an automobile such as an electric tool, an electric vehicle, and a hybrid vehicle.

Claims (15)

In a non-aqueous electrolyte for a secondary battery comprising a solvent and an electrolyte containing a lithium salt,
The solvent includes a chain carboxylic acid ester represented by R ₁ COOR ₂ (R ₁ and R ₂ are alkyl groups having 3 or less carbon atoms) and 4-fluoroethylene carbonate, and the total amount of the solvent. The non-aqueous electrolyte for secondary batteries, wherein the ratio of 4-fluoroethylene carbonate to 7 vol% or more is 7% by volume or more. The chain carboxylic acid ester is methyl acetate [CH ₃ COOCH ₃ ], ethyl acetate [CH ₃ COOC ₂ H ₅ ], methyl propionate [C ₂ H ₅ COOCH ₃ ], n-propyl acetate [CH ₃ COOCH ₂ CH ₂ CH ₃ ], i-propyl acetate [CH ₃ COOCH (CH ₃ ) CH ₃ ], ethyl propionate [C ₂ H ₅ COOC ₂ H ₅ ], methyl n-butyrate [CH ₃ CH ₂ CH ₂ COOCH ₃ ], And a non-aqueous electrolyte for a secondary battery according to claim 1, which is at least one selected from the group consisting of methyl i-butyrate [CH ₃ (CH ₃ ) CHCOOCH ₃ ]. The chain carboxylic acid ester is at least one selected from the group consisting of methyl acetate [CH ₃ COOCH ₃ ], ethyl acetate [CH ₃ COOC ₂ H ₅ ], and methyl propionate [C ₂ H ₅ COOCH ₃ ]. The non-aqueous electrolyte for secondary batteries according to claim 1. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the chain carboxylic acid ester contains methyl propionate [C ₂ H ₅ COOCH ₃ ].   The nonaqueous electrolytic solution for a secondary battery according to any one of claims 1 to 4, wherein a ratio of the chain carboxylic acid ester to the total amount of the solvent is 20% by volume or more.   The nonaqueous electrolytic solution for a secondary battery according to claim 5, wherein a ratio of the chain carboxylic acid ester to the total amount of the solvent is 40% by volume or more.   The nonaqueous electrolytic solution for a secondary battery according to any one of claims 1 to 6, wherein a ratio of the 4-fluoroethylene carbonate to a total amount of the solvent is 10 to 50% by volume.   The nonaqueous electrolytic solution for a secondary battery according to claim 7, wherein the ratio of the 4-fluoroethylene carbonate to the total amount of the solvent is 20 to 40% by volume. The nonaqueous electrolytic solution for a secondary battery according to claim 1, wherein LiBF ₄ is contained in the nonaqueous electrolytic solution. The nonaqueous electrolytic solution for a secondary battery according to claim 9, wherein the concentration of LiBF ₄ is in the range of 0.05 to 0.6 mol / l.   The nonaqueous electrolytic solution for a secondary battery according to any one of claims 1 to 10, wherein vinylene carbonate is added to the solvent.   The nonaqueous electrolytic solution for a secondary battery according to claim 1, wherein vinyl ethylene carbonate is added to the solvent.   A non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode, a separator, and the non-aqueous electrolyte for a secondary battery according to claim 1.   The non-aqueous electrolyte secondary battery according to claim 13, wherein a positive electrode potential in a fully charged bear is less than 4.5 V with respect to lithium metal.   15. The non-positive electrode according to claim 13 or 14, wherein the positive electrode active material contains lithium cobalt oxide in which aluminum and / or magnesium are dissolved, and zirconium is fixed to the surface of the lithium cobalt oxide. Water electrolyte secondary battery.