JP2013145731A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery excellent in long storage stability at high temperature.SOLUTION: A lithium secondary battery comprises a positive electrode containing a positive electrode active material capable of occluding and discharging lithium ion; a negative electrode containing a negative electrode active material capable of occluding and discharging lithium ion; and a nonaqueous electrolyte. The nonaqueous electrolyte contains an electrolyte (1); an ionic compound containing carbonyl group and/or sulfonyl group; and solvent. A residual ratio of the electrolyte (1) in the nonaqueous electrolyte after the nonaqueous electrolyte is preserved for 7 days under conditions of 80°C, is more than or equal to 93%.

Description

  The present invention relates to a lithium secondary battery using a non-aqueous electrolyte.

  In recent years, since lithium secondary batteries have high energy density, they have been used as power supplies for mobile communication devices, power supplies for portable information terminals, etc., and the market has grown rapidly with the spread of various information terminals. Various researches have been conducted for the purpose of ensuring safety, improving cycle characteristics and energy density, and improving high-temperature storage characteristics.

  Among the lithium secondary batteries described above, in the field of lithium ion batteries using a non-aqueous electrolyte, by adding a cyclic carbonate having an unsaturated bond such as vinylene carbonate or vinyl ethylene carbonate to the non-aqueous electrolyte, the negative electrode It is known to improve cycle characteristics by forming a film on top and suppressing decomposition of an electrolyte contained in the nonaqueous electrolyte (Patent Document 1). However, even in such a lithium secondary battery, there is room for improvement in terms of capacity retention rate, cycle characteristics at high temperature, and storage characteristics.

JP-A-11-185806

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to provide a lithium secondary battery having good storage characteristics. The present inventors have found that by using a specific compound in the non-aqueous electrolyte, decomposition of the electrolyte contained in the non-aqueous electrolyte can be suppressed during high-temperature storage, and a high-performance battery can be provided. The present invention has been completed.

The lithium secondary battery of the present invention that has achieved the above object is:
A lithium secondary battery including a positive electrode containing a positive electrode active material capable of inserting and extracting lithium ions, a negative electrode containing a negative electrode active material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte, The electrolytic solution includes an electrolyte (1), an ionic compound having a carbonyl group and / or a sulfonyl group, and a solvent,
It is characterized in that the residual ratio of the electrolyte (1) in the nonaqueous electrolytic solution after being stored at 80 ° C. for 7 days is 93% or more.

The ionic compound having the carbonyl group and / or sulfonyl group is
(XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ represent a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms. , X and X ′ are preferably fluorine atoms.).

  The lithium secondary battery of the present invention achieves a lithium secondary battery that can stably charge and discharge even when stored at high temperature by containing a specific compound in the non-aqueous electrolyte. It is. The lithium secondary battery of the present invention is considered to exhibit high performance even in fields where safety at high temperatures is required, such as in-vehicle (EV, PHEV, HEV) and stationary power supply applications for power storage. It is done.

The lithium secondary battery of the present invention that has solved the above problems is
A positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, a lithium secondary battery comprising a non-aqueous electrolyte, The electrolytic solution includes an electrolyte (1), an ionic compound having a carbonyl group and / or a sulfonyl group, and a solvent,
It is characterized in that the residual ratio of the electrolyte (1) in the nonaqueous electrolytic solution after being stored at 80 ° C. for 7 days is 93% or more.

  The inventors of the present invention have repeatedly studied to develop a higher performance lithium secondary battery. In addition to the electrolyte (1), the nonaqueous electrolyte includes ions having a carbonyl group and / or a sulfonyl group. When the active compound is used, decomposition of the electrolyte (1) can be suppressed even when stored at a high temperature, and a sufficient electrolyte concentration can be maintained. The present invention was completed by finding out that it can be a battery. The present invention will be described below.

<Lithium secondary battery>
The lithium secondary battery according to the present invention has a structure in which a positive electrode and a negative electrode are housed in a case via a separator impregnated with the electrolytic solution according to the present invention. The shape of the lithium secondary battery according to the present invention is arbitrary, and may be any of a cylindrical shape, a square shape, a laminate shape, a coin shape, a large size, and the like. Moreover, it is good also as a battery module comprised by connecting each battery in series as a high voltage power supply which requires several 10V-several 100V for mounting in an electric vehicle, a hybrid electric vehicle, etc.

  The lithium secondary battery of the present invention is characterized in that the remaining ratio of the electrolyte (1) in the nonaqueous electrolytic solution after being stored at 80 ° C. for 7 days is 93% or more. The remaining rate of the electrolyte (1) can be calculated, for example, by storing the prepared electrolytic solution in a high temperature state and comparing the concentration of the electrolyte (1) before and after storage. The residual ratio of the electrolyte (1) is preferably 93% or more, more preferably 95% or more. In a more preferred form, the remaining rate of the electrolyte (1) in the nonaqueous electrolytic solution after being stored at 80 ° C. for 7 days in a fully charged state is 93% or more. As an evaluation in a fully charged state, for example, a battery prepared by using an electrolyte solution having a specific composition is subjected to a high-temperature storage test described later, and then the electrolyte solution is taken out. ) And the concentration can be calculated.

Although details regarding the mechanism by which the decomposition of the electrolyte (1) is suppressed by including an ionic compound having a carbonyl group and / or a sulfonyl group in the nonaqueous electrolytic solution are unclear, the present inventors have described the following. I think so. For example, when the electrolyte (1) is LiPF ₆ , it is known to cause a self-decomposition reaction and generate PF ₅ . PF5 further _PF 3 O while react to generate HF and like water, will decompose _PF 2 _{O 2} or the like. In this process, by a compound having a carbonyl group and / or a sulfonyl group is considered to coordinate to PF ₅ coexist, to stabilize the PF _5, it is thought to inhibit the degradation proceed.

<Positive electrode>
The positive electrode is a sheet in which a positive electrode active material composition containing a positive electrode active material, a conductive additive, a binder, a dispersion solvent and the like is supported on a positive electrode current collector, and is usually in a sheet form.

  The method for producing the positive electrode is, for example, a method in which the positive electrode active material composition is applied to the positive electrode current collector by the doctor blade method or the like and then dried; the positive electrode active material composition is kneaded, molded and dried. A method of joining the sheet to the positive electrode current collector via a conductive adhesive, pressing and drying; forming a positive electrode active material composition to which a liquid lubricant has been added on the positive electrode current collector; The method of removing and then extending | stretching to a uniaxial or multiaxial direction etc. is mentioned.

  The material of the positive electrode current collector is not particularly limited, and for example, a conductive metal such as aluminum, an aluminum alloy, SUS, or titanium can be used. Among these, aluminum is preferable from the viewpoint of being easily processed into a thin film and being inexpensive.

  Examples of the conductive assistant include acetylene black, carbon black, graphite, metal powder material, single-phase carbon nanotube, multi-walled carbon nanotube, and vapor grown carbon fiber.

  As binders, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide resins such as polyamideimide; polyolefin resins such as polyethylene and polypropylene A poly (meth) acrylic resin; a polyacrylic acid; a cellulose resin such as carboxymethylcellulose; These binders may be used alone or in combination of two or more. These binders may be dissolved in a solvent or dispersed in a solvent.

  The blending amount of the conductive auxiliary agent or the binder can be appropriately adjusted in consideration of the intended use of the battery (emphasis on output, importance on energy, etc.), ion conductivity, and the like.

  Examples of the solvent used for dispersing the positive electrode material when producing the positive electrode include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetone, ethanol, ethyl acetate, and water.

<Positive electrode active material>
In the lithium secondary battery of the present invention, the positive electrode active material only needs to be able to occlude and release lithium ions, and a positive electrode active material used in a conventionally known lithium secondary battery can be used.

Specifically, lithium cobalt acid, lithium nickel acid, lithium manganese _{acid, LiNi 1-x-y Co} x Mn y O 2 or _{LiNi 1-x-y Co x} Al y O 2 (0 ≦ x ≦ 1,0 ≦ y ≦ 1) transition metal oxides such as ternary oxides, compounds having an olivine structure such as LiAPO ₄ (A = Fe, Mn, Ni, Co), and solid solutions incorporating a plurality of transition metals A material (a solid solution of an electrochemically inactive layered Li ₂ MnO ₃ and an electrochemically active layered LiMO (transition metals such as M = Co and Ni)) can be used. These may be used alone or in combination. Moreover, these positive electrode active materials may be coat | covered with electroconductive materials, such as carbon.

<Negative electrode>
The negative electrode is a sheet in which a negative electrode active material composition containing a negative electrode active material, a dispersion solvent, a binder, and a conductive auxiliary agent as necessary is supported on a negative electrode current collector, and is usually in the form of a sheet.

  As a material of the negative electrode current collector, a conductive metal such as copper, iron, nickel, silver, and stainless steel SUS (SUS stainless steel) can be used. However, copper is preferable from the viewpoint of easy processing into a thin film.

  As a negative electrode active material, the negative electrode active material used with a conventionally well-known lithium secondary battery can be used, What is necessary is just what can occlude / release lithium ion. Specifically, graphite materials such as artificial graphite and natural graphite, mesophase fired bodies made from coal / petroleum pitch, carbon materials such as non-graphitizable carbon, silicon Si-based negative electrode materials such as Si, Si alloy, and SiO Sn-based negative electrode materials such as Sn alloy, and lithium alloys such as lithium metal and lithium-aluminum alloy can be used.

In addition, the manufacturing method of a negative electrode can employ | adopt the method similar to the manufacturing method of a positive electrode. As the conductive auxiliary agent, binder, and material dispersion solvent that can be used for the negative electrode, the same ones as those used for the positive electrode can be used.
<Separator>
The separator is disposed so as to separate the positive and negative electrodes. The separator is not particularly limited, and a conventionally known separator can be used. For example, a porous sheet (for example, a polyolefin-based microporous separator or a cellulose-based separator) made of a polymer that absorbs and retains a nonaqueous electrolytic solution, a nonwoven fabric separator, a porous metal body, and the like can be given. Among these, a polyolefin microporous separator having a property of being chemically stable with respect to an organic solvent is preferable.

  Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

  Examples of the material of the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass, and the like, depending on the mechanical strength required for the nonaqueous electrolyte layer or the like. Used by mixing.

<Non-aqueous electrolyte>
[Electrolyte (1)]
The electrolyte (1) is a substance that functions as a main electrolyte in the nonaqueous electrolytic solution of the lithium secondary battery of the present invention. Since the present invention has a gist in that the compound (2) is used in addition to the electrolyte (1), the electrolyte (1) is not particularly limited, and is used as an electrolyte in electrolytes of various power storage devices. Any conventionally known electrolyte can be used. The electrolyte (1) is preferably one that has a large dissociation constant in the electrolyte and generates an anion that is difficult to solvate with the solvent described below. Specific examples include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCl, and LiI. One or more compounds selected from these groups, and the like are included in the present invention. It is used as the electrolyte (1) according to the above. Among these, LiPF ₆ and LiBF ₄ are preferable, and LiPF ₆ is more preferable. The electrolyte (1) may be used alone or in combination of two or more.

  Moreover, it is preferable that the density | concentration of the electrolyte (1) in a non-aqueous electrolyte is 0.1 mol / L or more and below a saturation concentration. More preferably, it is 0.1 mol / L to 2.5 mol / L, more preferably 0.3 mol / L to 2 mol / L, still more preferably 0.4 mol / L to 1.5 mol / L, and most preferably Is 0.5 mol / L to 1.2 mol / L. If the concentration of the electrolyte (1) is too high, the viscosity of the non-aqueous electrolyte solution becomes high, the conductivity is lowered, and the battery performance may not be sufficiently exhibited.

  The proportion of the electrolyte (1) in the nonaqueous electrolytic solution is preferably 30 mol% or more in the total amount of the electrolyte contained in the nonaqueous electrolytic solution of the present invention. More preferably, it is 50 mol% or more. The upper limit may be 99 mol% or less.

[Ionic compound having carbonyl group and / or sulfonyl group]
The nonaqueous electrolytic solution of the present invention contains an ionic compound having a carbonyl group and / or a sulfonyl group. Similar to the electrolyte (1), an ionic compound having a carbonyl group and / or a sulfonyl group dissociates an anion and a cation in the electrolytic solution, and thus can serve as a source of charge carriers. By using an ionic compound having a specific amount of a carbonyl group and / or a sulfonyl group with respect to the electrolyte (1), the present inventors have a function on the electrode as described later in addition to the function as a charge carrier. It has been found that the electrolyte (1) is hardly decomposed when stored in a fully charged state in order to act on the coating and further stabilize PF ₅ and suppress decomposition as described above. Furthermore, by including an ionic compound having a carbonyl group and / or a sulfonyl group, as described above, it is possible to suppress the decomposition of the electrolyte (1) and to make a battery that can withstand high-temperature storage. The ionic compound having a carbonyl group and / or a sulfonyl group is not particularly limited as long as it has a carbonyl group and / or a sulfonyl group in a part of the compound, but lithium borate having a cyclic structure, boric acid Examples thereof include lithium, lithium phosphate having a cyclic structure, and a compound (2) represented by the general formula (2) described later. Among them, the compound represented by the general formula (2) (2) is preferred because a high capacity to stabilize the PF _5.

  As lithium borate having a cyclic structure, lithium difluoro (oxalato) borate, lithium tetrafluoro (oxalato) phosphate, lithium bis (oxalato) borate, (2,2-difluoromalonatooxalato-O, O ′ ) Lithium borate, [bis (3,3,3-trifluoromethyl) glycolatooxalato-O, O ′] lithium borate, (2-trifluoromethylpropionatooxalato-O, O ′) boric acid Lithium, (3,3,3-trifluoromethylpropionatooxalato-O, O ') lithium borate, fluorotrifluoromethyl [oxalato-O, O'] lithium borate, fluoropentafluoromethyl [oxalato-O , O ′] lithium borate, fluorotrifluoromethyl [malonate-O, O ′] lithium borate, fluoro Lifluoromethyl [difluoromalonate-O, O ′] lithium borate, fluorotrifluoromethyl [3,3,3-trifluoro-2-oxide-2-trifluoromethylpropinate (2-)-O, O '] Lithium borate, fluorotrifluoromethyl [methylene disulfonate-O, O'] lithium borate, fluorotrifluoromethyl [sulfoacetate-O, O '] lithium borate and the like.

  Lithium borate compounds include tetrakis (trifluoroacetate) lithium borate, tetrakis (difluoroacetate) lithium borate, tetrakis (fluoroacetate) lithium borate, tetrakis (chlorodifluoroacetate) lithium borate, tetrakis (trichloroacetate) Lithium borate, tetrakis (dichloroacetate) lithium borate, tetrakis (pentafluoropropanoate) lithium borate, tetrakis (3-chlorotetrafluoropropanoate) lithium borate, tetrakis (heptafluorobutanoate) boric acid Examples include lithium and tetrakis (2,2-bis-trifluoromethylbutanoate) lithium borate.

  Examples of the lithium phosphate having a cyclic structure include fluorotrispentafluoroethyl [oxalato-O, O ′] lithium phosphate, fluorotrispentafluoroethyl [malonate-O, O ′] lithium phosphate, fluorotrispentafluoroethyl [ And difluoromalonate-O, O ′] lithium phosphate.

[Compound (2)]
The compound (2) of the present invention is a compound (2) represented by the general formula (2); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ .
In the general formula (2), X and X ′ represent a fluorine atom, an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms, and at least one of X and X ′ is a fluorine atom. . The alkyl group having 1 to 6 carbon atoms is preferably a linear alkyl group, and examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include those in which some or all of the hydrogen atoms of the alkyl group are substituted with fluorine atoms, such as a fluoromethyl group, a difluoromethyl group, and a trifluoromethyl group. , Fluoroethyl group, difluoroethyl group, trifluoroethyl group, pentafluoroethyl group and the like. Among the alkyl groups or fluoroalkyl groups, a fluorine atom, a trifluoromethyl group, and a pentafluoroethyl group are preferable. Preferred compounds (2) include lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (methylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) ) Imide, lithium (fluorosulfonyl) (ethylsulfonyl) imide, and more preferably lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethyl) Sulfonyl) imide, more preferably lithium bis (fluorosulfonyl) imide.

The concentration of the compound (2) in the nonaqueous electrolytic solution of the present invention is preferably in the range of 0.01 mol / L to 2.0 mol / L, more preferably 0.05 mol / L to 1.0 mol / L. More preferably, it is 0.1 mol / L to 0.5 mol / L. When the concentration of the compound (2) is 0.01 mol / L or less, the effect on the film on the electrode and the stabilization of PF ₅ are not sufficient, and the desired battery performance may not be obtained. When the concentration of the compound (2) is 2.0 mol / L or more, the viscosity of the nonaqueous electrolytic solution is increased, the conductivity is lowered, and the battery performance may not be sufficiently exhibited.

The compounding amount of the compound (2) in the nonaqueous electrolytic solution of the present invention is such that the compound (2) is 50 mol% or less and 0.1 mol% when the total of the electrolyte (1) and the compound (2) is 100 mol%. The above is preferable. More preferably, they are 0.5 mol% or more and 30 mol% or less, More preferably, they are 1 mol% or more and 20 mol% or less. If the compounding amount of the compound (2) is too small, the stabilization of PF ₅ and the effect on the coating on the electrode are not sufficient, and the desired battery performance may not be obtained.
As compound (2), a commercially available product may be used, or a compound synthesized by a conventionally known method may be used.

[solvent]
In the non-aqueous electrolyte of the present invention, various non-aqueous solvents conventionally used in non-aqueous electrolytes can be used as the solvent for dissolving the electrolytes. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate; tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1 Ethers such as 1,2-dimethoxyethane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane; γ-butyrolactone, γ-valerolactone, α-methyl-γ-butyrolactone, etc. Lactones of the above; chain carboxylic acid esters such as methyl propionate and methyl butyrate can be used. Among these non-aqueous solvents, carbonate solvents such as cyclic carbonates and chain carbonates are preferably used because they are difficult to decompose upon application of voltage and are stable. In addition, the said non-aqueous solvent may be used independently and may mix and use 2 or more types.

[Additive]
The non-aqueous electrolyte of the lithium secondary battery of the present invention may contain additives in addition to the electrolyte (1), the compound (2) and the solvent in order to improve cycle characteristics and safety. As additives, cyclic carbonate compounds having an unsaturated bond such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), methyl vinylene carbonate (MVC), ethyl vinylene carbonate (EVC); fluoroethylene carbonate, trifluoropropylene carbonate Carbonate compounds such as phenylethylene carbonate and erythritan carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentane Carboxylic anhydrides such as tetracarboxylic dianhydride and phenyl succinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methane Sulfur-containing compounds such as methyl sulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1, Nitrogen-containing compounds such as 3-dimethyl-2-imidazolidinone and N-methylsuccinimide; phosphates such as monofluorophosphate and difluorophosphate; hydrocarbon compounds such as heptane, octane and cycloheptane Can be given. When these additives are used in the non-aqueous electrolyte, the concentration is preferably 0.1% by weight to 5% by weight.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Electrolyte adjustment example 1
Electrolyte (1) lithium hexafluorophosphate as (LiPF _6, manufactured by Kishida Chemical Co., LBG grade) 1.52 g (10 mmol), lithium bis (fluorosulfonyl) as the compound (2) imide (LiFSI) 0.38 g (2 mmol ) Was measured in a 10 mL volumetric flask, and the volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (both manufactured by Kishida Chemical Co., Ltd., LBG grade) was 3/7 (EC / EMC). An electrolyte was prepared by measuring the volume with a solvent, and this was designated as electrolyte 1.

Electrolyte adjustment example 2
An electrolyte solution was prepared in the same manner as in the electrolyte solution adjustment example 1 except that 1.82 g (12 mmol) of lithium hexafluorophosphate (LiPF ₆ , LBG grade, manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte (1). Was used as electrolytic solution 2.

<Battery creation>
One commercially available positive electrode sheet (active material: lithium cobaltate, 3 mAh / cm ² ) having an area of the positive electrode active material layer of 12 cm ² (3 × 4 cm) and an area of the negative electrode active material layer of 13.44 cm ² (3. 2 × 4.2 cm) of a commercially available negative electrode sheet (active material: graphite, 3.2 mAh / cm ² ) was laminated so as to face each other, and one polyolefin separator was sandwiched therebetween. The positive and negative electrode sheets were sandwiched between two aluminum laminate films, and the aluminum laminate film was filled with an electrolytic solution and sealed in a vacuum state.

Using a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.), charge / discharge was performed once at 30 ° C. at a charge / discharge rate of 0.2 C (constant current mode) and 3.5 V to 4.2 V Thereafter, the laminate cell was opened and then sealed again in a vacuum state. Under the same conditions, charging / discharging was repeated 5 times to complete a laminated lithium battery.
Example 1
Two laminated lithium ion batteries were prepared using the electrolytic solution 1, and one of them was subjected to a high-temperature storage test. Each battery was processed as follows and measured for ¹⁹ F-NMR.
Experimental example 2
The same operation as in Example 1 was performed except that the electrolytic solution 2 was used, and a high-temperature storage test and ¹⁹ F-NMR measurement were performed.

<High temperature storage test>
The laminate type lithium battery thus prepared was charged and discharged at 30 ° C. at a charge / discharge rate of 0.2 C (constant current mode). The discharge capacity at this time is defined as the initial discharge capacity (A). The laminated lithium battery was charged to 4.2 V at 30 ° C. at a charge / discharge rate of 0.2 C (constant current mode), and stored at 80 ° C. for 1 week. Then, discharge was performed at 30 ° C. and 0.2C. Let the discharge capacity at this time be the discharge capacity (B) after high-temperature storage. The capacity retention rate before and after high temperature storage was calculated by the following formula, and the results are shown in Table 1.
Capacity retention rate = discharge capacity after high temperature storage (B) / initial discharge capacity (A) × 100

<Remaining rate evaluation>
The residual ratio of LiPF ₆ was evaluated as follows.
First, the electrolytic solution 1 was diluted with deuterated methanol to which difluoroanisole was added as an internal standard, and ¹⁹ F-NMR measurement was performed. When the concentrations of LiPF ₆ , LiPF ₂ O ₂ and LiFSI were calculated based on 120.6 ppm of difluoroanisole, the results were as shown in Table 2 below.

[ ¹⁹ F-NMR measurement conditions]
Device name: ECA400 (manufactured by JEOL)
Measurement nucleus: ¹⁹ F
Observation frequency: 376 MHz
Solvent: Deuterated methanol Internal standard: 2,4-Difluoroanisole (DFA)
The assignment of NMR was as follows.
LiPF ₆ ... 71.4ppm, 73.3ppm
LiPF ₂ O ₂ ... 81.2 ppm, 83.74 ppm
Difluoroanisole ... 120.6ppm, 130.3ppm
LiFSI ... 52.8ppm
Since the concentration of preparation was 1.0 mol / L for LiPF _{6 and} 0.2 mol / L for LiFSI, the conversion factor for LiPF ₆ was 1 / 0.905 = 1.105, and the conversion factor for LiFSI was 0.2 / 0.138 = 1.449. ^The concentration calculated by ¹⁹ F-NMR measurement was calculated by multiplying the concentration (mol / kg) calculated from ¹⁹ F-NMR measurement by the above conversion factor.

In a glove box in an inert gas (Ar) atmosphere, the corners of the laminate cell were cut and the electrolyte solution of each battery was collected. About 60 mg of the recovered electrolyte solution was dissolved in deuterated methanol (about 0.6 mL), ¹⁹ F-NMR measurement was performed under the above measurement conditions, and the residual rate was calculated. The results are shown in Table 3.

From the results shown in Table 3, it can be seen that, compared with Experimental Example 2 using only LiPF ₆ as the electrolyte, in Experimental Example 1 in which LiFSI is used together, the decrease in the concentration of LiPF ₆ is suppressed before and after the high-temperature storage test. . It has also been found that the production of LiPF ₂ O ₂ produced by the decomposition of LiPF ₆ is suppressed to about half when LiFSI coexists. Moreover, as shown in Table 1, it can be seen that Experimental Example 1 in which LiFSI and LiPF ₆ are used together has a better capacity retention rate after high-temperature storage. This is considered to be because the decomposition of LiPF ₆ is suppressed when LiFSI coexists with LiPF ₆ as described above.

In the electrolyte solution of the present invention, it has been found that the LiFSI coexists to suppress decomposition of LiPF ₆ which is the main electrolyte. Generally, it is known that when LiPF ₆ is decomposed, it reacts with a small amount of water in the battery to generate hydrogen fluoride. Hydrogen fluoride, which is a strong acid, is thought to degrade the active material of the positive electrode or negative electrode and reduce performance, and this tendency is expected to be significant in batteries using manganese-based composite oxides that are weak against acids. . Furthermore, hydrogen fluoride also corrodes the aluminum foil which is a current collector, and deteriorates the performance. By adopting the configuration of the electrolytic solution of the present invention, it is possible to supply a battery that can be stably driven while preventing deterioration of battery performance. Moreover, the decomposition suppression function of LiPF ₆ by LiFSI is not limited to the electrolyte solution that is incorporated in the battery and is in a charge / discharge state, and it is considered that the same effect is exhibited even in the electrolyte solution before being incorporated in the battery. It is done.

  As described above, it has been found that inclusion of LiFSI in the nonaqueous electrolytic solution functions as a main electrolyte decomposition inhibitor in a severe environment of full charge and high temperature storage. Accordingly, by using the compound, it is possible to prevent a decrease in battery capacity due to a decrease in electrolyte concentration, and to provide a battery that can be stably driven at a high temperature for a long time.

Claims (5)

A positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a lithium secondary battery comprising a non-aqueous electrolyte,
The non-aqueous electrolyte includes an electrolyte (1), an ionic compound having a carbonyl group and / or a sulfonyl group, and a solvent,
A lithium secondary battery, wherein the remaining ratio of the electrolyte (1) in the nonaqueous electrolytic solution after being stored at 80 ° C. for 7 days is 93% or more. The ionic compound having the carbonyl group and / or the sulfonyl group is represented by the general formula (2); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are fluorine atoms, carbon atoms 1 to 6). 2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is a compound represented by the formula (1): a C 1-6 fluoroalkyl group, wherein at least one of X and X ′ is a fluorine atom. The lithium secondary battery according to claim 1 or 2 the electrolyte (1) is LiPF _6.   The lithium secondary battery according to any one of claims 1 to 3, wherein the compound represented by the general formula (2) is lithium bis (fluorosulfonyl) imide.   The lithium secondary battery according to claim 1, wherein the solvent includes a carbonate-based solvent.