JP2002216841A - Nonaqueous electrolyte secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary cell having excellent low temperature property, stability for long time, recycle property, and high energy density. SOLUTION: The nonaqueous electrolyte secondary cell comprises a negative electrode at least containing graphite as a partial component of negative electrode material for absorbing and releasing lithium, a positive electrode, negative and positive current collectors, a nonaqueous electrolyte containing a solute and an organic solvent, and separator. A mixture of solvents containing ethylene sulfide and propylene carbonate is used as the organic solvent, and the material, used at the part where the positive electrode current collector and an outside case of positive electrode are contacting with nonaqueous electrolyte, is made of valve metal or an alloy of the same.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high energy density non-aqueous electrolyte secondary battery having excellent low-temperature characteristics, long-term stability, and cycle characteristics.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries having a high energy density have attracted attention as electric appliances become lighter and smaller. In addition, with the expansion of the application field of the lithium secondary battery, improvement in battery characteristics is also demanded. Non-aqueous organic solvents such as carbonates or esters such as ethylene carbonate, propylene carbonate, diethyl carbonate, and γ-butyrolactone have been used as the solvent for the electrolyte solution of such a lithium secondary battery.

[0003] Among them, propylene carbonate is a solvent having a high dielectric constant, dissolves lithium salt-based solute (electrolyte) well, and exhibits high electric conductivity even at a low temperature, so that it has excellent performance as a main solvent of an electrolytic solution. Have. However, when various graphite-based electrode materials are used alone or as a negative electrode by mixing with other negative electrode materials capable of inserting and extracting lithium, propylene carbonate is generally decomposed vigorously on the graphite electrode surface. It is known that it is impossible to smoothly insert and extract lithium into a graphite electrode (7th International)
Symposium on Li Batterie
s, P259, 1995).

At present, ethylene carbonate is less likely to decompose and is widely used as a solvent for an electrolyte of a non-aqueous electrolyte secondary battery using a graphite-based negative electrode. However, ethylene carbonate has a freezing point of 3 times as compared with propylene carbonate.
Since it is as high as 6.4 ° C., it is used as a mixture with a low-viscosity solvent such as dialkyl carbonate such as dimethyl carbonate and diethyl carbonate, dimethoxyethane and dioxolane (“Functional Materials”, Vol. 15, April, p. 48). , 1
995). However, at low temperatures, solidification of the electrolyte solution and low conductivity are often regarded as problems, and it is expected that propylene carbonate is used as a main solvent even in a negative electrode containing a graphite-based negative electrode material. Also, low-viscosity solvents used as mixed solvents in ethylene carbonate systems often have a low boiling point, so when added in large amounts, the vapor pressure in the battery generally increases, and leakage of the solvent is a safety issue. Leaves anxiety in terms of aspects.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and uses propylene carbonate as a main solvent of an electrolyte for a non-aqueous electrolyte secondary battery using a graphite anode. An object of the present invention is to provide a high energy density non-aqueous electrolyte secondary battery having excellent low-temperature characteristics, long-term stability, and cycle characteristics.

[0006]

According to the present invention, a negative electrode and a positive electrode containing graphite as at least one component thereof as a negative electrode material capable of inserting and extracting lithium, a negative electrode current collector and a positive electrode current collector, Non-aqueous electrolyte comprising a solute and an organic solvent, and a non-aqueous electrolyte secondary battery including a separator and an outer can, wherein the organic solvent contains propylene carbonate and ethylene sulfite, and An object of the present invention is to provide a non-aqueous electrolyte secondary battery characterized in that a valve metal or an alloy thereof is used as a material of a portion of a positive electrode current collector and a positive electrode side outer can contacting a non-aqueous electrolyte.

[0007]

[Function] By using a mixed solvent containing propylene carbonate and ethylene sulfite, a considerably stable protective film is formed on a graphite-based electrode prior to occlusion of lithium, minimizing decomposition of the electrolyte. In addition, the smooth insertion and extraction of lithium into and from the graphite-based electrode becomes possible. Further, since the surface of the valve metal is covered with an oxide film, it is possible to prevent the oxidative decomposition reaction of ethylene sulfite at a portion where the valve metal comes into contact with the electrolytic solution in the positive electrode current collector or the positive electrode outer can.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION A nonaqueous electrolyte secondary battery according to the present invention comprises a negative electrode and a positive electrode containing graphite as at least one of its constituents as a negative electrode material capable of inserting and extracting lithium; A positive electrode current collector, a nonaqueous electrolyte solution including a solute and an organic solvent, and a nonaqueous electrolyte secondary battery including a separator and an outer can, including propylene carbonate and ethylene sulfite as the organic solvent. In addition, a valve metal or an alloy thereof is used as a material of a portion of the positive electrode current collector and a part of the positive electrode side can contacting the electrolyte.

Non-aqueous electrolyte: The non-aqueous electrolyte contains a solute, propylene carbonate and ethylene sulphite. The content of propylene carbonate in the mixed solvent of the non-aqueous electrolyte is 40 to 99.95 vol%, preferably 5 to 99.95 vol%.
0 to 99.9 vol%, and that of ethylene sulfite is 0.05 to 60 vol%, preferably 0.1 to
It is used in a range of 50 vol%.

In the above mixed solvent, as a third solvent component other than the above-mentioned propylene carbonate and ethylene sulfite, cyclic carbonates such as ethylene carbonate and butylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate. , Γ-butyrolactone, cyclic esters such as γ-valerolactone, chain esters such as methyl acetate and methyl propionate, cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran and tetrahydropyran, dimethoxyethane, dimethoxymethane and the like. It is possible to use a mixture of a chain ether, a sulfur-containing organic solvent such as sulfolane, diethyl sulfone and the like. These solvents may be used as a mixture of two or more kinds.

As a solute, LiClO is used._Four, LiP
F₆, LiBF_FourAn inorganic lithium salt or L selected from
iCF_ThreeSO_Three, LiN (CF_ThreeSO_Two)_Two , LiN
(CF_ThreeCF_TwoSO_Two)_Two, LiN (CF_ThreeSO_Two)
(C_FourF₉SO_Two), LiC (CF _ThreeSO_Two)_ThreeIncluding
Fluoroorganic lithium salts can be used. these
Two or more solutes may be used as a mixture. Dissolution in electrolyte
The molar concentration of the lithium salt is 0.5-2.0 mol / l
It is desirable to be a title. 0.5 mol / l or less
Below or at 2.0 mol / L or more, the
Air conductivity is low and battery performance deteriorates.
No.

Negative electrode: As a negative electrode material constituting a battery,
Preferably, artificial graphite and purified natural graphite produced by high-temperature heat treatment of graphitic pitch obtained from various raw materials, or materials obtained by subjecting these graphites to various surface treatments including pitch are mainly used. These graphite materials have a lattice plane (002 plane) d value (interlayer distance) of 0.335 to 0.34 nm, more preferably 0.335 to 0.337 nm, obtained by X-ray diffraction by the Gakushin method. Is preferred. These graphite materials have an ash content of 1% by weight or less, more preferably 0.5% by weight or less, most preferably 0.1% by weight or less, and a crystallite size (L) determined by X-ray diffraction according to the Gakushin method.
c) is preferably 30 nm or more. Further, the crystallite size (Lc) is more preferably 50 nm or more,
Those having a thickness of 100 nm or more are most preferable. The median diameter of the graphite material is 1 μm or more and 100 μm or less, preferably 3 μm, as a median diameter by a laser diffraction / scattering method.
m to 50 μm, more preferably 5 μm to 40 μm
m, more preferably 7 μm or more and 30 μm or less.

The BET specific surface area of the graphite material is 0.5 m
^Two/ G or more 25.0m^Two/ G or less, preferably
0.7m^Two/ G or more 10.0m^Two/ G or less, more preferred
1.0m^Two/ G or more 7.0m^Two/ G or less, more preferred
Or 1.5m^Two/ G or more 5.0m^Two/ G or less.
Raman spectroscopy using argon ion laser light
1580-1620 cm in torr analysis^-1In the range
Ark P_A(Peak intensity I _A) And 1350-1370
cm^-1Peak P in the range_B(Peak intensity I_B) Intensity ratio
R = I_B/ I_AIs 0 or more and 0.5 or less, 1580 to 162
0cm^-1The half width of the peak in the range is 26 cm^-1Below, 1
580-1620cm^-1The half width of the peak in the range is 25
cm^-1The following is more preferred. Lithium is added to these graphite materials.
It is also possible to mix negative electrode materials that can absorb and release
Wear. A negative electrode material that can store and release lithium other than graphite
Non-graphitic carbon such as non-graphitizable carbon or low-temperature calcined carbon
Elemental materials, metal oxide materials such as tin oxide and silicon oxide,
Examples include lithium metal as well as various lithium alloys.
The shape of the negative electrode, together with the binder and conductive agent
After mixing, the sheet electrode and
Pellet electrodes that have been formed can be used.

Negative electrode current collector: As the material of the negative electrode current collector, metals such as copper, nickel, and stainless steel are used, and among these, copper foil is preferable in terms of easy processing into a thin film and cost. Separator: As a separator constituting a battery, a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene is used.

Positive electrode: As a positive electrode material constituting a battery,
Materials that can occlude and release lithium, such as lithium transition metal composite oxide materials such as lithium cobalt oxide and lithium nickel oxide, can be used. As the shape of the positive electrode, a sheet electrode applied to a current collector after mixing with a binder and a conductive agent as necessary, and a pellet electrode subjected to press molding can be used.

Positive electrode current collector: As the material of the positive electrode current collector, a valve metal such as aluminum, titanium, and tantalum or an alloy thereof is used. Among these valve metals, aluminum or its alloy is particularly preferable in terms of energy density because of its light weight. It is not preferable to use a metal material other than the valve metal, for example, stainless steel, because the oxidative decomposition reaction of ethylene sulfite proceeds and the cycle characteristics deteriorate.

Outer can: The outer can of the battery is preferably made of stainless steel, but the portion electrically connected to the positive electrode and in contact with the electrolyte must be made of valve metal such as aluminum. As a method of protecting with a method, there is a method of protecting with plating or foil. Further, aluminum or an aluminum alloy may be used as the outer can material. The outer can referred to here includes a lead wire housed inside the battery, a safety valve that operates when the internal pressure inside the battery rises, and the like.

Battery: The shape of the battery can be a cylinder type in which a sheet electrode and a separator are formed in a spiral shape, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, and a coin type in which a pellet electrode and a separator are laminated. is there. FIG. 1 shows a sectional view of a coin-type non-aqueous electrolyte battery.
In the figure, 1 is a positive electrode, 2 is a negative electrode, 3 is a positive electrode can, 4 is a sealing plate,
5 is a separator, 6 is an aluminum foil, 7 is a gasket, 8 is a positive electrode current collector, and 9 is a negative electrode current collector. The non-aqueous electrolyte is generally impregnated in the separator.

[0019]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the present invention is limited thereto. (Examples 1 and 2) LiCoO as positive electrode active material
₂ (85 parts by weight), carbon black (6 parts by weight) and polyvinylidene fluoride (9 parts by weight) were added and mixed, and dispersed with N-methyl-2-pyrrolidone to form a slurry. A 20 μm-thick aluminum foil was uniformly coated, dried, and punched into a predetermined shape to obtain a positive electrode.

As the negative electrode active material, the d value of the lattice plane (002 plane) in X-ray diffraction is 0.336 nm, the crystallite size (Lc) is 100 nm or more (264 nm), the ash content is 0.04 wt%, _A peak P _{A in} the range of 1580 to 1620 cm ^{−1 in} Raman spectrum analysis using an argon ion laser beam with a median diameter of 17 μm, a BET specific surface area of 8.9 m ² / g by a scattering method, and
Intensity ratio (peak intensity I _A) and 1350 ^-1 ranging peak P _B (peak intensity I _B) R = I _B
/ I _A is artificial graphite powder K half-width of a peak in the range of 0.15,1580～1620Cm ^-1 is 22.2Cm ^-1
S-44 (manufactured by Timcal Co., trade name) (94 parts by weight) was mixed with polyvinylidene fluoride (6 parts by weight), and dispersed in N-methyl-2-pyrrolidone to form a slurry, which was used as a negative electrode current collector. A coating was uniformly applied on a copper foil having a thickness of 18 μm, dried, and punched into a predetermined shape to obtain a negative electrode.

As for the electrolytic solution, lithium hexafluorophosphate (Li) was sufficiently dried in a dry argon atmosphere.
PF ₆ ) as a solute and ethylene sulfite (E
It was prepared by dissolving LiPF ₆ at a ratio of 1 mol / liter in a solution in which S) and propylene carbonate (PC) were mixed with the composition shown in Table 1. Using these positive electrode, negative electrode and electrolyte, a coin-type non-aqueous electrolyte battery as shown in FIG. 1 was prepared in a dry argon atmosphere.

Referring to FIG. 1, the positive electrode 1 and the negative electrode 2 are respectively housed in a stainless steel positive electrode can 3 and a sealing plate 4 and are made of a polypropylene microporous film impregnated with each electrolytic solution. The layers are laminated with the separator 5 interposed therebetween. At this time, the inside of the positive electrode can 3 was used by covering the inside of the positive electrode can 3 with an aluminum foil 6 in advance in order to use the material of the liquid contact portion on the positive electrode side as a valve metal. Subsequently, the positive electrode can 3 and the sealing plate 4 were caulked and sealed via the gasket 7 to complete a coin-type battery.

Comparative Example 1 A coin-type battery was prepared in the same manner as in Example 1 except that the inside of the positive electrode can was not covered with aluminum foil. (Comparative Example 2) 1 LiPF ₆ in propylene carbonate
A coin-type battery was prepared in the same manner as in Example 1 except that an electrolytic solution dissolved at a mole / liter ratio was used.

Example 3 Ethylene sulfite, propylene carbonate and diethyl carbonate (DEC)
(10:45:45 vol%) was used in the same manner as in Example 1 except that an electrolyte obtained by dissolving LiPF ₆ at a ratio of 1 mol / liter was used to prepare a coin-type battery. The batteries of Examples 1, 2, and 3 and Comparative Examples 1 and 2 were
At a constant current of 0.5 mA at 4.2 ° C.
A charge / discharge test was performed at V and a discharge end voltage of 2.5 V. Table 1 shows the charge capacity and discharge capacity per negative electrode weight in the first cycle in each battery.

[0025]

[Table 1] Note) ES: ethylene sulfite PC: propylene carbonate

FIG. 2 shows the change in the discharge capacity according to the charge / discharge cycle of Example 1 and Comparative Example 1. As is clear from the results of Example 1 and Comparative Example 1, when the electrolyte contains ethylene sulfite, and when the material of the liquid contact portion on the positive electrode side is stainless steel, the oxidative decomposition reaction of ethylene sulfite is performed. However, it is not possible to obtain a sufficient discharge capacity by progressing, but by using aluminum as the material of the liquid contact portion on the positive electrode side, the oxidative decomposition of ethylene sulfite was suppressed, The cycle characteristics are significantly improved. Further, from Examples 1 and 2 and Comparative Example 2, in the case of the electrolyte solution of propylene carbonate alone, propylene carbonate is decomposed on the surface of the carbon material of the negative electrode and a discharge capacity cannot be obtained, but ethylene sulfite is added to propylene carbonate. By doing so, the discharge capacity and cycle characteristics are significantly improved.

[0027]

According to the present invention, propylene carbonate and ethylene sulfite are selected as the organic solvent for the electrolyte of the non-aqueous electrolyte secondary battery, and the material of the liquid contact portion between the cathode current collector and the electrolyte of the cathode side outer can is selected. By selecting a valve metal or its alloy, even when a graphite-based negative electrode is used, the problem of decomposition of the electrolyte in the electrolyte containing propylene carbonate is suppressed, and a high capacity is obtained, and the cycle characteristics and low-temperature characteristics are improved. An excellent battery can be manufactured, which can contribute to miniaturization and high performance of a non-aqueous electrolyte secondary battery.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a structure of a coin-type battery.

FIG. 2 is a diagram showing the relationship between charge / discharge cycles and battery capacity of non-aqueous electrolyte batteries of Example 1 and Comparative Example 1 of the present invention.

[Explanation of symbols]

 Reference Signs List 1 positive electrode 2 negative electrode 3 positive electrode can 4 sealing plate 5 separator 6 aluminum foil 7 gasket 8 positive electrode current collector 9 negative electrode current collector

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01M 4/58 H01M 4/58 (72) Inventor: Minoru Furuta 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture No. Mitsubishi Tsukuba Research Laboratories (72) Inventor Mark Deshamp 3-1, Chuo, Ami-cho, Inashiki-gun, Ibaraki Pref. 5H011 AA03 CC06 DD15 FF03 GG02 5H014 AA02 AA04 AA06 BB06 EE01 EE05 EE08 EE10 HH00 5H029 AJ03 AJ04 AJ05 AK03 AL02 AL06 AL07 AL12 AM02 AM03 AM04 AM05 AM07 BJ02 BJ03 BJ14 HJ14 DJ01

Claims (9)

[Claims] 1. A negative electrode and a positive electrode containing graphite as at least one component thereof as a negative electrode material capable of inserting and extracting lithium, a negative electrode current collector and a positive electrode current collector, and a solute and an organic solvent. In a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte solution, a separator and an outer can, the organic solvent contains propylene carbonate and ethylene sulfite, and the positive electrode current collector and the positive electrode side outer can Wherein the material of the liquid contact portion with the non-aqueous electrolyte is a valve metal or an alloy thereof. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the valve metal and its alloy are aluminum and an aluminum alloy. 3. The method according to claim 1, wherein the negative electrode material is an electrode obtained by mixing graphite alone or a non-graphite carbon capable of inserting and extracting graphite and lithium, lithium or a lithium alloy, and a metal oxide. The non-aqueous electrolyte secondary battery according to claim 1. 4. A graphite-based negative electrode capable of inserting and extracting lithium has a lattice plane (002 plane) having a d-value of 0.1 in X-ray diffraction.
The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is made of a carbon material having a thickness of 335 to 0.34 nm. 5. A graphite-based negative electrode capable of inserting and extracting lithium has a lattice plane (002 plane) having a d-value of 0.1 in X-ray diffraction.
The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is made of a carbon material having a thickness of 335 to 0.337 nm. 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode capable of inserting and extracting lithium is made of a lithium transition metal composite oxide material. 7. The method according to claim 1, wherein the solute is LiClO ₄ , LiPF ₆ , L
Inorganic lithium salt selected from iBF ₄ or LiCF ₃
SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃
CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F _9
2. The non-aqueous electrolyte secondary battery according to claim 1, which is an organic lithium salt selected from SO ₂ ) and LiC (CF ₃ SO ₂ ) _{3. 3} . 8. The content of ethylene sulfite in the organic solvent is 0.05 vol% to 60 vol%, and the content of propylene carbonate is 99.95 to 40 vol.
The non-aqueous electrolyte secondary battery according to claim 1, wherein the amount is 1%. 9. The solute concentration in the non-aqueous electrolytic solution is from 0.5 to
2. The composition according to claim 1, wherein the amount is 2.0 mol / liter.
The non-aqueous electrolyte secondary battery according to 1.