JP2003007336A - Non-aqueous electrolyte secondary battery and non-aqueous electrolyte used therefor - Google Patents

Abstract

(57) [Problem] To provide a non-aqueous electrolyte secondary battery excellent in charge / discharge efficiency and storage characteristics. SOLUTION: A lithium salt is dissolved in a non-aqueous solvent,
An electrolytic solution containing a fluorinated nitrile compound is used.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to a non-aqueous electrolyte secondary battery that improves charge / discharge efficiency by using a specific non-aqueous electrolyte and has excellent charge / discharge efficiency and storage characteristics even at high temperatures.

[0002]

2. Description of the Related Art With the recent lightening and downsizing of electric products, the development of lithium secondary batteries having a high energy density has been desired more than ever, and the application fields of lithium secondary batteries are expanding. Accordingly, improvement of battery characteristics is also demanded. Currently, the positive electrode has LiCoO ₂ , LiMnO ₂ , Li
In addition to metallic oxide salts such as NiO ₂ and metallic lithium for the negative electrode,
Carbon materials such as coke, artificial graphite and natural graphite, Sn,
A non-aqueous electrolyte secondary battery using a compound capable of inserting and extracting lithium ions such as a metal oxide material such as Si has been proposed.

However, in these lithium secondary batteries, it is known that the solvent of the electrolytic solution is decomposed on the positive electrode and / or the negative electrode, which causes the deterioration of charge / discharge efficiency and storage characteristics. Has become. For example,
In a non-aqueous electrolyte secondary battery in which graphite is used alone or as a negative electrode by mixing with another material capable of inserting and extracting lithium, propylene carbonate, which is generally preferred in lithium primary batteries, is the main solvent. When the electrolyte solution is used, the decomposition reaction of the solvent progresses vigorously on the surface of graphite, and smooth insertion and release of lithium into graphite becomes impossible.

On the other hand, since ethylene carbonate is less likely to be decomposed like this, it is often used as the main solvent of the electrolytic solution of the non-aqueous electrolytic solution secondary battery. However, even if ethylene carbonate is used as the main solvent, there is a problem that the charge / discharge efficiency is lowered because the electrolytic solution is decomposed little by little on the electrode surface during the charge / discharge process.

[0005]

In a non-aqueous electrolyte secondary battery, a high energy density battery having a high degree of charge / discharge efficiency, which has a minimum decomposition of the electrolyte solution, and has excellent storage characteristics even at high temperatures, should be used. In order to realize it, it has been proposed to form a lithium ion permeable and stable protective film on the electrode, and an electrolytic solution containing a material that forms such a protective film is desired. The present invention seeks to meet these needs.

[0006]

Means for Solving the Problems The present inventors have used an electrolyte containing a fluorinated nitrile compound as an electrolyte of a non-aqueous electrolyte secondary battery so that the surface of the electrode can be charged from the initial charge. In addition, a lithium ion permeable and stable film is efficiently formed, and excessive decomposition of the electrolyte solution is suppressed. Therefore, it was found that charge and discharge efficiency and storage characteristics can be improved, and the present invention has been completed. It was

That is, the present invention relates to a negative electrode containing metallic lithium, a lithium alloy, or a material capable of inserting and extracting lithium, a positive electrode containing a material capable of inserting and extracting lithium, and a non-aqueous solvent. A non-aqueous electrolyte secondary battery containing an electrolyte solution in which a lithium salt is dissolved is characterized in that an electrolyte solution containing a fluorinated nitrile compound is used.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Cyclic carbonates, chain carbonates, lactone compounds, chain esters, cyclic ethers, chain ethers, sulfur-containing organic solvents are the main components of the non-aqueous solvent of the electrolytic solution used in the present invention. Among the known solvents for non-aqueous electrolytes for secondary batteries,
It can be appropriately selected and used. These solvents may be used alone or in combination of two or more. Suitable solvents include cyclic carbonates having 3 to 9 carbon atoms,
Examples thereof include lactone compounds, chain carbonates, chain carboxylic acid esters and chain ethers. Some preferred specific examples of these solvents are shown below.

Cyclic carbonates: ethylene carbonate, propylene carbonate, butylene carbonate,
Vinylene carbonate, vinyl ethylene carbonate.
Among these, ethylene carbonate and propylene carbonate are more preferable. Lactone compound: γ-butyrolactone, γ-valerolactone, δ-valerolactone. Among them, γ-butyrolactone is more preferable.

Chain carbonate: dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, di-n-butyl carbonate,
Diisobutyl carbonate, di-t-butyl carbonate, n-butyl isobutyl carbonate, n-butyl-
t-butyl carbonate, isobutyl-t-butyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, n-butyl methyl carbonate,
Isobutyl methyl carbonate, t-butyl methyl carbonate, ethyl-n-propyl carbonate, n-butyl ethyl carbonate, isobutyl ethyl carbonate, t-butyl ethyl carbonate, n-butyl-n-
Propyl carbonate, isobutyl-n-propyl carbonate, t-butyl-n-propyl carbonate, n
-Butyl isopropyl carbonate, isobutyl isopropyl carbonate, t-butyl isopropyl carbonate. Among these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are more preferable.

Chain carboxylic acid ester: methyl acetate, ethyl acetate, acetic acid-n-propyl, acetic acid-isopropyl,
-N-butyl acetate, isobutyl acetate, -t-butyl acetate, methyl propionate, ethyl propionate, -n-propyl propionate, isopropyl propionate,
-N-butyl propionate, isobutyl propionate,
T-Butyl propionate. Among these, ethyl acetate, methyl propionate and ethyl propionate are more preferable.

Chain ether: dimethoxymethane, 1,2
-Dimethoxyethane, diethoxymethane, 1,2-diethoxyethane, ethoxymethoxymethane, 1-ethoxy-2-methoxyethane. Among these, dimethoxyethane and diethoxyethane are more preferable. Cyclic ether; tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,3-dioxane, 1,
4-dioxane. Sulfur-containing organic solvent; tetramethylene sulfone, 3-methyltetramethylene sulfone, tetramethylene sulfoxide, 1,3-propane sultone, 1,3,2-dioxathiolane-2,2-dioxide. Particularly preferred as the non-aqueous solvent of the electrolytic solution used in the present invention is a lactone compound having 3 to 9 carbon atoms or a cyclic carbonate occupying 20% by volume or more, and including this, a lactone compound having 3 to 9 carbon atoms. , A cyclic carbonate, a chain carbonate, a chain ether, and a chain carboxylic acid ester are mixed solvents which account for 70% by volume or more.

As a solute of the electrolytic solution used in the present invention
Is a lithium salt. Non-water as lithium salt
Although it is known that it can be used as a solute in the electrolyte,
Among them, it can be appropriately selected and used. How many
As an example, Inorganic lithium salt: LiPF₆, LiAsF₆, LiB
F_Four, LiAlF_FourInorganic fluoride salts such as LiClO_Four,
LiBrO_Four, LiIO_FourPerhalogenates such as Organolithium salt: LiCF₃SO₃Organic sulfonic acid such as
Salt, LiN (CF₃SO₂)₂, LiN (C₂F_FiveS
O₂)₂, LiN (CF₃SO₂) (C_FourF₉SO₂) Etc.
-Fluoroalkyl sulfonic acid imide salt, LiC (CF
₃SO₂)₃Perfluoroalkyl sulfonic acid methide, etc.
Salt, LiPF (CF₃)_Five, LiPF₂(CF₃)_Four, Li
PF₃(CF₃)₃, LiPF₂(C₂F_Five)_Four, LiPF
₃(C₂F_Five)₃, LiPF (n-C₃F₇)_Five, LiPF
₂(N-C₃F₇)_Four, LiPF₃(N-C₃F₇)₃, LiP
F (iso-C₃F₇)_Five, LiPF₂(Iso-C₃F₇)
_Four, LiPF₃(Iso-C₃F₇)₃, LiB (C
F₃)_Four, LiBF (CF₃)₃, LiBF₂(CF₃)₂,
LiBF₃(CF₃), LiB (C₂F_Five)_Four, LiBF
(C₂F_Five)₃, LiBF₂(C₂F_Five)₂, LiBF₃(C₂
F_Five), LiB (n-C₃F₇)_Four, LiBF (n-C
₃F₇)₃, LiBF₂(N-C₃F₇)₂, LiBF₃(N-
C₃F₇), LiB (iso-C₃F₇)_Four, LiBF (i
so-C₃F₇)₃, LiBF₂(Iso-C₃F ₇)₂, L
iBF₃(Iso-C₃F₇) Etc.
Inorganic fluoride salt fluoro substituted with fluoroalkyl group
Fluorine-containing organic compounds of phosphate and perfluoroalkyl
Lithium salt.

Among these, LiPF ₆ , LiBF ₄ ,
_{LiCF 3 SO 3, LiN (CF} 3 SO 2) 2, LiN (C 2
_{F 5 SO 2) 2, LiN} (CF 3 SO 2) (C 4 F 9 SO 2),
LiPF ₃ (C ₂ F ₅ ) ₃ , LiBF ₂ (CF ₃ ) ₂ and LiB
F ₂ (C ₂ F ₅ ) ₂ is more preferable. Note that these solutes are 2
You may use it in mixture of 2 or more types. The lithium salt molar concentration of the solute in the electrolytic solution is preferably 0.5 to 3 mol / liter. If the concentration is too low, the electrical conductivity of the electrolytic solution is too low, which is not preferable. Conversely, even if the concentration is too high, the viscosity increases and the electrical conductivity decreases, and precipitation at low temperatures easily occurs. However, the battery performance is deteriorated, which is not preferable.

In the electrolytic solution used in the present invention, in addition to the above-mentioned solvent and solute, at least one hydrogen atom of a fluorinated nitrile compound, that is, a hydrocarbon compound substituted with a cyano group is further substituted with a fluorine atom. Containing a compound of the indicated structure. Other substituents may be present on this compound. Examples of the nitrile compound substituted with a fluorine atom suitable for use in the present invention include the following.

Chain nitrile compound; fluoroacetonitrile, difluoroacetonitrile, trifluoroacetonitrile, α-fluoropropionitrile, β-fluoropropionitrile, α, α-difluoropropionitrile, α, β-difluoropropionitrile, β, β-
Difluoropropionitrile, α, α, β-trifluoropropionitrile, α, β, β-trifluoropropionitrile, β, β, β-trifluoropropionitrile, α, α, β, β-tetrafluoropropionitrile Nitrile, α, β, β, β-tetrafluoropropionitrile, pentafluoropropionitrile

Alicyclic nitrile compound; 2-fluorocyclohexanecarbonitrile, 3-fluorocyclohexanecarbonitrile, 4-fluorocyclohexanecarbonitrile, 2,3-difluorocyclohexanecarbonitrile, 2,4-difluorocyclohexanecarbonitrile, 2, 5-difluorocyclohexanecarbonitrile, 2,6-difluorocyclohexanecarbonitrile. Aromatic nitrile compound; 2-fluorobenzonitrile,
3-fluorobenzonitrile, 4-fluorobenzonitrile, 2,3-difluorobenzonitrile, 2,4-difluorobenzonitrile, 2,5-difluorobenzonitrile, 2,6-difluorobenzonitrile.

In general, the nitrile compound tends to be unstable when the degree of fluorination is large, and is difficult to manufacture. Further, when it is perfluorinated, the solubility is liable to be lowered.
It is preferable to use monofluoronitrile compounds such as -fluoropropionitrile, β-fluoropropionitrile, 2-fluorobenzonitrile, 3-fluorobenzonitrile and 4-fluorobenzonitrile. Among them, fluoroacetonitrile, 2-fluorobenzonitrile, 3-fluorobenzonitrile and 4-fluorobenzonitrile are more preferable.

Two or more kinds of fluorinated nitrile compounds may be used in combination, and for example, isomer mixtures which are difficult to separate can be used without separation. The fluorinated nitrile compound preferably constitutes 0.01 to 10% by weight of the electrolytic solution, more preferably 0.1 to 5% by weight. If the content is too small, a sufficient coating cannot be formed on the electrode. On the contrary, if the content is too large, the surplus for forming the coating adversely affects the battery characteristics. In the electrolytic solution,
In addition to the carbonate, ester or ether, the solute, and the fluorinated nitrile compound which are the main components of the solvent, various commonly used compounds may be contained.

The material of the negative electrode is not particularly limited as long as it can occlude and release lithium in addition to metallic lithium and lithium alloys. For example, thermal decomposition products of organic substances under various thermal decomposition conditions, carbon materials such as artificial graphite and natural graphite,
Examples thereof include metal oxide materials.

Of these, carbon materials, especially artificial graphite produced by high-temperature heat treatment of graphitizable pitch obtained from various raw materials, purified natural graphite, or graphite materials obtained by subjecting these to various surface treatments containing pitch. Is preferably used. Among them, the lattice plane (0
02 surface) has a d value (interlayer distance) of 0.335 to 0.34n
m, particularly 0.335 to 0.337 nm is preferably used. It is preferable to use a graphite material having a low ash content, and usually, a graphite material having an ash content of 1% by weight or less is used. The ash content is 0.5% by weight or less, particularly 0.1% by weight or less, and the crystal size (L
It is preferable to use one having c) of 30 nm or more. Further, the crystallite size (Lc) is preferably 50 nm or more, and most preferably 100 nm or more. As the graphite material, it is preferable to use one having a median diameter of 1 μm to 100 μm, particularly 3 μm to 50 μm as measured by the laser diffraction / scattering method. Median diameter is 5μm-40μ
It is most preferable to use a resin having a thickness of m, particularly 7 μm to 30 μm. BET specific surface area of graphite material is usually 0.5 m ²
/ G to 25.0 m ² / g, preferably 0.7 m ² / g
It is g-20.0 m < ² > / g. BET specific surface area is 1.
0m ² /g~15.0m ² / g, especially 1.5m ² / g~1
It is most preferable to use the one having 0.0 m ² / g. Further, the graphite material has a peak P _A (peak intensity I _A ) in the range of 1580 to 1620 cm ⁻¹ and 1350 to ¹ in a Raman spectrum analysis using argon ion laser light.
It has a peak P _B (peak intensity I _B ) in the range of 370 cm ⁻¹ and its intensity ratio R = I _B / I _A is 0 to 0.5,
The FWHM of the peak in the range of 1580 to 1620 cm ^-1 is 2
6 cm ^-1 or less, a half value width of the peak in the range of 1350 ^-1 is more preferable if 25 cm ^-1 or less.

It is also possible to mix these carbon materials with other negative electrode materials capable of inserting and extracting lithium. As the negative electrode material capable of inserting and extracting lithium other than the carbon material, Ag, Zn, Al, Ga, In, Si, G
e, Sn, Pb, P, Sb, Bi, Cu, Ni, Sr,
Examples include alloys of metals such as Ba and Li, metal oxide materials such as oxides of these metals, and lithium metals. Among them, Sn oxides, Si oxides, Al oxides, Sn, S
It is preferable to use a lithium alloy of i, Al, metallic lithium, or the like. Two or more kinds of these negative electrode materials may be mixed and used.

A negative electrode may be manufactured using these negative electrode materials by a conventional method. For example, a negative electrode is manufactured by adding a binder, a thickener, a conductive material, a solvent and the like to a negative electrode material to form a slurry, applying the slurry to a substrate of a current collector, and drying the slurry. You can Alternatively, a negative electrode material to which a binder or the like is added may be roll-formed as it is to form a sheet electrode, or a pellet electrode may be formed by compression molding.

Binders, thickeners, conductive materials, etc. used in the production of electrodes are stable to the solvent used in the production of electrodes and stable to the electrolyte and other materials used in batteries. There is no particular limitation as long as it exists. As the binder, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, isoprene rubber, butadiene rubber, etc. are usually used.

As the thickener, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, etc. are usually used. As the conductive material, a metal material such as copper or nickel, or a conductive carbon material such as graphite or carbon black is used. Metals such as copper, nickel, and stainless are used for the negative electrode current collector, and among these, copper foil is preferable from the viewpoint of easy processing into a thin film and cost.

As the material of the positive electrode constituting the battery of the present invention, lithium transition metal composite oxide materials such as lithium cobalt oxide, lithium nickel oxide and lithium manganese oxide are usually used. The positive electrode can also be manufactured according to the above-described manufacturing method of the negative electrode.

A metal such as aluminum, titanium, tantalum or an alloy thereof is used for the positive electrode current collector. Among these, aluminum or its alloy is particularly preferable in terms of energy density because it is lightweight. As the separator for separating the negative electrode and the positive electrode, it is preferable to select one that is stable to the electrolytic solution and has excellent liquid retention, and a porous sheet or nonwoven fabric made of polyolefin such as polyethylene or polypropylene is used. Is preferred.

The non-aqueous electrolyte secondary battery according to the present invention can be manufactured by a conventional method except that an electrolyte containing a fluorinated nitrile compound is used. The shape of the battery
The sheet electrode and the separator may have a spiral shape, a cylinder type having an inside-out structure in which the pellet electrode and the separator are combined, a coin type in which the pellet electrode and the separator are laminated, and the like.

[0029]

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples as long as the gist thereof is not exceeded. In addition, a positive electrode, a negative electrode, and a coin-type battery using the same,
The evaluation was performed as follows.

Preparation of positive electrode; LiCo as positive electrode active material
The O ₂ 85% by weight of carbon black 6 wt% and polyvinylidene fluoride KF-1000 (Kureha Chemical Co., Ltd., trade name) 9 wt% were added and mixed, further to N- methyl -
2-Pyrrolidone was added to make a slurry. This was uniformly applied onto an aluminum foil having a thickness of 20 μm, which is a positive electrode current collector, dried, and punched into a disk shape having a diameter of 12.5 mm to obtain a positive electrode.

Preparation of negative electrode; lattice plane (0
The d value of the 02 plane is 0.336 nm, and the crystallite size (L
c) is 264 nm, ash content is 0.04% by weight, median diameter by laser diffraction / scattering method is 17 μm, BET specific surface area is 8.9 m ² / g, and Raman spectrum analysis using argon ion laser light is 1580- 162
Peak P _A (peak intensity I _A ) and 13 in the range of 0 cm ⁻¹
It has a peak P _B (peak intensity I _B ) in the range of 50 to 1370 cm ⁻¹ and its intensity ratio R = I _B / I _A is 0.15.
, And the artificial graphite powder KS-44 half-width of the peak in the range of 1580～1620Cm ^-1 is 22.2Cm ^-1
(Timcal, trade name) 94% by weight, styrene-butadiene rubber dispersed in distilled water 6% by weight in solid content
Was added. Mix this with a disperser,
The slurry was used as a negative electrode current collector with a thickness of 18μ.
m evenly coated on copper foil, dried and then 12.5 mm in diameter
It was punched into a disk shape to obtain a negative electrode.

Preparation of coin type cell: The positive electrode was housed in a stainless steel can which also functions as a positive electrode conductor, and the negative electrode was placed on the positive electrode via a polyethylene separator impregnated with an electrolytic solution. The can body and the sealing plate which also serves as the negative electrode conductor were caulked and sealed via an insulating gasket to produce a coin cell.

Evaluation of coin-type cell: 0.5 m at 25 ° C. at a charge end voltage of 4.2 V and a discharge end voltage of 2.5 V
A charge / discharge test was performed at a constant current A, and a value obtained by dividing the discharge capacity at the second cycle by the charge capacity at the second cycle was defined as the charge / discharge efficiency at the second cycle. Further, after being charged under the same conditions after 4 cycles, after being stored in a charged state at 85 ° C. for 72 hours and then discharged, the value obtained by dividing the discharge capacity at this time by the charge capacity at the 4th cycle was defined as the storage characteristic.

Example 1 Fluoroacetonitrile was dissolved in a mixed solution of ethylene carbonate: diethyl carbonate = 1: 1 in a volume ratio of 2% by weight. LiPF _{6 in an amount} of 1 mol /
An electrolytic solution was prepared by dissolving so as to have a volume of 1 liter. LiPF ₆ was used after being sufficiently dried in a dry argon atmosphere.

Example 2 α-Fluoropropionitrile was dissolved in a mixed solution of ethylene carbonate: diethyl carbonate = 1: 1 in a volume ratio of 2% by weight. Add 1 LiPF ₆ to this
An electrolytic solution was prepared by dissolving so as to have a mol / liter.

Example 3 Volume ratio of ethylene carbonate: diethyl carbonate
2: 1 of 2-fluorobenzonitrile to a mixed solution of = 1: 1
It dissolved so that it might be a weight%. LiPF ₆1 mo
The electrolytic solution was prepared by dissolving so as to have a volume of 1 / liter.

Example 4 Vinylene carbonate and fluoroacetonitrile were dissolved in a mixed solution of ethylene carbonate: diethyl carbonate = 1: 1 in a volume ratio of 2% by weight each. LiPF ₆ was dissolved in this to a concentration of 1 mol / liter to prepare an electrolytic solution.

Example 5 Propylene carbonate was mixed with 2 parts of fluoroacetonitrile.
1% LiPF ₆ was added to the mixed solution which was dissolved so as to become the weight%.
An electrolytic solution was prepared by dissolving so as to have a mol / liter.

Example 6 Fluoroacetonitrile was dissolved in a mixed solution of ethylene carbonate: diethyl carbonate = 1: 1 in a volume ratio of 2% by weight. 1 mol of LiBF ₄
An electrolytic solution was prepared by dissolving so as to have a volume of 1 liter.

Example 7 LiBF ₄ was dissolved in a mixed solution of γ-butyrolactone dissolved in 2% by weight of fluoroacetonitrile at 1 mol / liter to prepare an electrolytic solution.

Example 8 LiPF ₆ was dissolved in a mixed solution of γ-butyrolactone in which 2% by weight of fluoroacetonitrile was dissolved to prepare an electrolytic solution of 1 mol / liter.

Comparative Example 1 An electrolytic solution was prepared by dissolving LiPF ₆ in a mixed solution of ethylene carbonate: diethyl carbonate = 1: 1 in a volume ratio of 1 mol / liter.

Comparative Example 2 LiPF ₆ was dissolved in propylene carbonate to a concentration of 1 mol / liter to prepare an electrolytic solution.

Comparative Example 3 An electrolytic solution was prepared by dissolving LiBF _{4 in} γ-butyrolactone at 1 mol / liter.

[0045]

[Table 1]

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Makoto Ue             3-3-1 Chuo 8-chome, Ami Town, Inashiki District, Ibaraki Prefecture             Mitsubishi Chemical Corporation Tsukuba Research Center F-term (reference) 5H029 AJ02 AJ04 AK03 AL07 AM03                       AM04 AM05 AM07 HJ01 HJ13                 5H050 AA08 BA17 CA08 CB07 HA01                       HA13

Claims (7)

[Claims] 1. A negative electrode comprising metallic lithium, a lithium alloy, or a material capable of inserting and extracting lithium.
A non-aqueous electrolyte secondary battery comprising a positive electrode containing a material capable of inserting and extracting lithium, and an electrolytic solution obtained by dissolving a lithium salt in a non-aqueous solvent, wherein the secondary battery is fluorinated in the electrolytic solution. A non-aqueous electrolyte secondary battery comprising a nitrile compound. 2. The fluorinated nitrile compound is fluoroacetonitrile, α-fluoropropionitrile,
The non-aqueous electrolyte secondary battery according to claim 1, which is selected from the group consisting of and β-fluoropropionitrile. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the fluorinated nitrile compound in the electrolyte occupies 0.01 to 10% by weight of the electrolyte. 4. The lactone compound or cyclic carbonate having 3 to 9 carbon atoms occupies 20% by volume or more in the non-aqueous solvent, and the lactone compound, cyclic carbonate or chain carbonate having 3 to 9 carbon atoms including the lactone compound or cyclic carbonate, 7 selected from the group consisting of chain ethers and chain carboxylic acid esters
The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, which is a mixed solvent occupying 0% by volume or more. 5. The lithium salt is LiPF₆, LiBF_Four,
LiCF₃SO₃, LiN (CF₃SO₂)₂, LiN (C₂
F_FiveSO₂)₂, LiN (CF₃SO₂) (C_FourF₉SO₂),
LiPF₃(C₂F_Five)₃, LiBF₂(CF₃)₂, And L
iBF₃(C₂F _Five)₂Is selected from the group consisting of
The method according to any one of claims 1 to 4, characterized in that
Non-aqueous electrolyte secondary battery. 6. A negative electrode material capable of inserting and extracting lithium is a carbon material having a d-value of a lattice plane (002 plane) of 0.335 to 0.34 nm in X-ray diffraction. The non-aqueous electrolyte secondary battery according to claim 1. 7. A non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery according to claim 1.