JP2003007331A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve charging/discharging cycle characteristics and storage characteristics at a high temperature. SOLUTION: A nonaqueous electrolyte secondary battery comprises a positive electrode 2 containing lithium cobalt complex oxide, a negative electrode 2 containing graphite, and a nonaqueous electrolyte, wherein the nonaqueous electrode contains LiPF6 and imido salt in a mole ratio range of 95:5 or 95:5 to 50:50. Since both LiPF6 and imido salt are used in the electrode and the content ratio of both substances is set to be the above mole ratio range, the aluminum used as a positive current collector is not corroded, so that the charging/discharging cycle characteristics and the high temperature storage characteristics of the nonaqueous electrolyte secondary battery is improved, compared to a case where each electrolyte, the LiPF6 and the imido salt, is used individually.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode containing a lithium cobalt composite oxide, a negative electrode containing graphite, and a non-aqueous electrolyte, and particularly to charge / discharge at high temperature. The present invention relates to improvement of cycle characteristics and high temperature storage characteristics.

[0002]

2. Description of the Related Art In recent years, a video tape recorder integrated with a camera, an audiovisual device such as a liquid crystal television, a notebook computer, a personal digital assistant (P
DA), mobile information devices such as word processors, and mobile communication devices such as mobile phones have been rapidly spread, and reduction in size and weight thereof has been demanded. Research has been actively conducted on batteries, particularly secondary batteries, which are power sources for driving these electronic devices, to reduce the size, weight, and high energy density. Among various secondary batteries, the lithium-based battery is
Compared with conventional secondary batteries, for example, aqueous solution type secondary batteries such as lead batteries and nickel-cadmium batteries, it has a feature of high energy density. For this reason, expectations for lithium-based batteries are very high.

Lithium-based batteries, specifically lithium batteries and lithium-ion batteries, include non-aqueous electrolytes prepared by dissolving LiPF ₆ as an electrolyte in a carbonate-based non-aqueous solvent such as propylene carbonate or diethyl carbonate as the non-aqueous electrolyte. Liquid
It is widely used because it has a relatively high conductivity and is stable in terms of potential.

[0004]

However, the LiP
A battery containing a non-aqueous electrolyte solution in which F ₆ is dissolved is not satisfactory in thermal stability, and there is a problem that the battery has poor charge / discharge cycle characteristics and high temperature storage characteristics at high temperatures.

In recent years, imide salts, specifically LiN (C _n
A non-aqueous electrolyte prepared by dissolving F _{2n + 1} SO ₂ ) ₂ (where n is 1 or more) in a non-aqueous solvent exhibits relatively high conductivity and is excellent in thermal stability. It is expected as a non-aqueous electrolyte.

However, in a battery using a non-aqueous electrolyte solution containing an excess amount of imide salt, the imide salt lowers the oxidation potential of aluminum used as the positive electrode current collector, so that in the aging step, the positive electrode current collector is used. The aluminum foil used as was attacked and aluminum was eluted. As a result, there is a problem in that the positive electrode current collector is cut off and it does not function as a battery. Here, the aging step is a step of charging the battery to a predetermined voltage and determining whether the battery is good or bad based on the voltage change after standing for a certain period of time.

In view of such conventional circumstances, the present invention prevents the elution of aluminum used as a positive electrode current collector while using a non-aqueous electrolyte containing an imide salt,
It is proposed for the purpose of providing a non-aqueous electrolyte secondary battery which is excellent in charge / discharge cycle characteristics at high temperatures and high temperature storage characteristics.

[0008]

In order to achieve the above object, a non-aqueous electrolyte secondary battery according to the present invention comprises a positive electrode containing a lithium cobalt composite oxide, a negative electrode containing graphite, and a non-aqueous electrolyte. An electrolyte is provided, and LiPF ₆ and an imide salt are contained in the non-aqueous electrolyte in a molar ratio of 95: 5 or 95: 5 to 50:50.

Since the non-aqueous electrolyte secondary battery according to the present invention configured as described above uses LiPF ₆ and an imide salt in combination, when each electrolyte is used alone, it is charged at high temperature. It is not restricted by the lower level of discharge cycle characteristics and high temperature storage characteristics. Further, in the non-aqueous electrolyte secondary battery according to the present invention, the content of the imide salt used as the electrolyte is defined in the above range, the oxidation potential of aluminum used as the positive electrode current collector does not decrease, and the aging step At, the aluminum foil is not attacked.

Therefore, the non-aqueous electrolyte secondary battery according to the present invention uses an imide salt as an electrolyte, but aluminum is prevented from being eluted, and compared with the case where each electrolyte is used alone, Excellent charge / discharge cycle characteristics at high temperature and high temperature storage characteristics.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A non-aqueous electrolyte secondary battery to which the present invention is applied will be described below in detail with reference to the drawings.

The non-aqueous electrolyte secondary battery 1 is a so-called lithium ion secondary battery, and as shown in FIG. 1, a positive electrode 2 and a negative electrode 3 are laminated with a separator 4 in between and wound in the longitudinal direction. And an outer container 8 including a battery can 6 and a battery lid 7. An electrode body 5 is housed in an outer container 8 composed of a battery can 6 and a battery lid 7, and a nonaqueous electrolytic solution is further injected.

The positive electrode 2 has a positive electrode active material layer formed on both main surfaces of a positive electrode current collector. This positive electrode active material layer is
It is formed by applying a positive electrode mixture containing a positive electrode active material onto a positive electrode current collector and drying it.

As the positive electrode current collector, any conventionally known material can be used, for example, aluminum foil or the like can be used.

The positive electrode active material layer contains a lithium cobalt composite oxide as a positive electrode active material.

The positive electrode active material layer may contain a positive electrode active material of a lithium cobalt composite oxide. Specifically, it is preferable to use a chalcogen compound with an alkali metal-containing transition metal, and of these, in particular, an oxide of an alkali metal and a transition metal. Also, the crystal structure is
Compounds that are layered or spinel type can also be used.

The above compound is represented by the general formula A _x M'M''O ₂
It is represented by. Here, A is one kind selected from Li, Na, and K, and 0.5 ≦ x ≦ 1.1. M'is Fe, Co, Ni, Mn, Cu, Zn, C
At least one is selected from the group consisting of r, V, and Ti. M ″ is Fe, Co, Mn, Cu, Zn,
It is at least one selected from the group consisting of Al, Sn, B, Ga, Cr, V, Ti, Mg, Ca, and Sr.

Further, examples of the positive electrode active material that can be used in combination with the lithium cobalt composite oxide include lithium manganese composite oxide and lithium nickel composite oxide.

Here, the lithium manganese composite oxide and
Is the general formula Li_xMn_2-yM ’_yO _FourCompound represented by
It is a thing. The value of x is in the range of 0.9 ≦ x ≦ 2, for example.
And the value of y should be in the range of 0.01 ≦ y ≦ 0.5.
Is preferred. That is, the set of the first element with respect to manganese
The composition ratio M '/ M is 0.01 / 1.99 or more in molar ratio,
It is preferably within the range of 0.5 / 1.5 or less. Well
M'is Fe, Co, Ni, Cu, Zn, Al, S
n, Cr, V, Ti, Mg, Ca, Sr, B, Ga, I
It is at least one of n, Si, and Ge.

The lithium nickel composite oxide is
Lithium, nickel, and at least one second element selected from the group consisting of metal elements other than nickel and boron;
It is a compound containing oxygen. The lithium-nickel composite oxide has, for example, a layered structure, and the second element exists in a part of the nickel atom site by being replaced with the nickel atom. When the second element is represented by M ″, the lithium nickel composite oxide has a general formula of Li _x Ni _1-z M ′ _z O ₂
Is a compound represented by. The composition ratio of lithium and oxygen does not have to be Li: O = 1: 2. Further, the value of x is appropriately selected, and the value of z is 0.01 ≦
It is preferable that z ≦ 0.5. That is, the composition ratio M ″ / Ni of the second element to nickel is preferably in the range of 0.01 / 0.99 or more and 0.5 / 0.5 or less in terms of molar ratio. In addition, M ″ is Fe, Co, M
n, Cu, Zn, Al, Sn, Cr, V, Ti, Mg,
It is at least one of Ca, Sr, B, Ga, In, Si, or Ge.

As the binder contained in the positive electrode active material layer, any of the conventionally known binders usually used in this type of battery can be used. In addition, in the positive electrode active material layer,
If necessary, a conventionally known additive such as a conductive agent may be added.

The negative electrode 3 has a negative electrode active material layer formed on both main surfaces of a negative electrode current collector. This negative electrode active material layer is
The negative electrode mixture containing the negative electrode active material is applied on the negative electrode current collector and dried.

As the negative electrode current collector, any conventionally known material can be used, for example, copper foil or the like can be used.

The negative electrode active material layer contains graphite as a negative electrode active material.

The negative electrode active material layer may contain a conventionally known negative electrode active material in combination with graphite. For example, a negative electrode material capable of occluding / releasing lithium, specifically, a metal or semiconductor capable of forming an alloy or compound with lithium, and an alloy or compound thereof can be used. For example, a compound represented by the general formula DsEtLiu can be used. is there. However, D represents at least one of a metal element or a semiconductor capable of forming an alloy or a compound with lithium. Further, E represents at least one kind of metal element or semiconductor element other than lithium and D. Also, s,
t and u are s> 0, t ≧ 0, and u ≧ 0, respectively.

Particularly, the metal or semiconductor capable of forming an alloy or compound with lithium is preferably a metal element or semiconductor element of Group 4B, particularly preferably silicon or tin, and most preferably silicon. These alloys or compounds are also preferable, and specifically, SiB ₄ ,
SiB ₆ , Mg ₂ Si, Mg ₂ SaANi ₂ Si, Ti
Si ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaS
i ₂ , CrSi ₂ , Cu ₅ Si, FeSi ₂ , Mbri
₂ , NbSi ₂ , TaSi ₂ , VSi ₂ , WSi ₂ or Zbri ₂ can be used.

Furthermore, as the negative electrode material capable of inserting and extracting lithium, a carbon material, a metal oxide, a polymer material or the like can be used. Examples of the carbon material include non-graphitizable carbon, artificial graphite, cokes, graphites,
Glassy carbons, organic polymer compound fired bodies, carbon fibers,
Activated carbon, carbon black or the like can be used. Among these, cokes include pitch coke, needle coke, petroleum coke, and the like, and an organic polymer compound fired body is carbonized by firing a polymer material such as phenol or furan at an appropriate temperature. I say what I did. Also,
Iron oxide, ruthenium oxide, molybdenum oxide, tin oxide or the like can be used as the metal oxide, and polyacetylene or polypyrrole or the like can be used as the polymer material.

As the binder contained in the negative electrode active material layer, any of the conventionally known binders usually used in batteries of this type can be used. In addition, a conventionally known additive may be added to the negative electrode active material layer, if necessary.

As the separator 4, any conventionally known material can be used, for example, a microporous polypropylene film or the like can be used.

The non-aqueous electrolytic solution is prepared by dissolving an electrolyte salt in a non-aqueous solvent. And, in this non-aqueous electrolyte, LiPF ₆ and an imide salt,
It is contained in a molar ratio of 95: 5 or 95: 5 to 50:50.

By using LiPF ₆ and an imide salt in combination as an electrolyte, the non-aqueous electrolyte secondary battery 1 has a high charge / discharge cycle characteristic and a high temperature storage characteristic when each electrolyte is used alone. Not restricted to lower levels.
Moreover, since the content of the imide salt used as the electrolyte is defined in the above range, the oxidation potential of aluminum used as the positive electrode current collector does not decrease, and the aluminum foil is not corroded in the aging step. Here, the aging step is a step of charging the battery to a predetermined voltage and determining whether the battery is good or bad based on the voltage change after standing for a certain period of time.

The molar ratio of LiPF ₆ to the imide salt is 95:
When it exceeds 5, the desired effect cannot be obtained because the charge / discharge cycle characteristics under high temperature and the high temperature storage characteristic are restricted to the lower level. On the other hand, when the molar ratio of LiPF ₆ to the imide salt is less than 50:50 or 50:50, the oxidation potential of aluminum used as the positive electrode current collector decreases, so the aluminum foil is corroded in the aging step. The non-aqueous electrolyte secondary battery 1 does not function as a battery.

Therefore, the non-aqueous electrolyte secondary battery 1 has L
The molar ratio of iPF ₆ and imide salt is 95: 5 or 9
By containing the nonaqueous electrolytic solution contained in the range of 5: 5 to 50:50, the elution of aluminum is prevented even if the imide salt is contained as the electrolyte. Also,
Compared to using individual electrolytes alone, at high temperatures,
Specifically, it has excellent charge / discharge cycle characteristics and high-temperature storage characteristics under a temperature environment of about 40 ° C to 60 ° C.

Specific examples of the imide salt include LiN (C
_n F _{2n + 1} SO ₂ ) ₂ (where n is 1 or more)
Is used, and more preferably LiN (CF ₃ SO ₂ ) ₂ is used.

As the electrolyte, any conventionally known lithium salt or the like can be used in combination with LiPF ₆ and an imide salt. For example, LiPF ₆ , LiAsF ₆ , LiBF ₄ , L
iClO _4, LiCF ₃ SO _3, can be used _LiC 4 _F 9 SO ₃ and the like. These electrolyte salts may be used alone or in combination of two or more.

Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, γ-butyl lactone and γ.
-Cyclic ester compounds such as valerolactone, ether compounds such as diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxane, chain ester compounds such as methyl acetate and methyl propylene oxide, dimethyl carbonate, diethyl carbonate and ethyl Chain carbonate such as methyl carbonate, 2,
4-difluoroanisole, 2,6-difluoroanisole, 4-bromoveratrol and the like can be used. Also,
These non-aqueous solvents may be used alone or in combination of two or more.

The non-aqueous electrolyte secondary battery 1 having the above-described structure is an imide salt, specifically, LiN (C _n F _{2n + 1).}
A non-aqueous electrolytic solution in which SO ₂ ) ₂ (where n is 1 or more) is dissolved in a non-aqueous solvent is used.
The molar ratio of PF ₆ and imide salt is 95: 5 or 95: 5.
Since it is used together as a range of up to 50:50, the aluminum foil is not attacked in the aging step,
Compared to the case where each electrolyte is used alone, it has excellent charge-discharge cycle characteristics at high temperatures and high-temperature storage characteristics.

According to the inventors, in the non-aqueous electrolyte secondary battery 1, the imide salt is considered to promote the corrosion of the oxide film of the non-conductor covering the aluminum foil, and the imide salt is excessive. When present in, it is believed to eventually attack the aluminum foil.

Although the non-aqueous electrolyte battery using the non-aqueous electrolyte prepared by dissolving the electrolyte in the non-aqueous solvent has been described above, the present invention is not limited to this, and the non-aqueous solvent is contained in the matrix polymer. It is also applicable to a solid electrolyte battery using a gel electrolyte in which is impregnated with and an electrolyte is dispersed.

As the polymer material used for the gel electrolyte, for example, polyacrylonitrile and a copolymer of polyacrylonitrile can be used. Examples of the copolymerization monomer (vinyl-based monomer) include vinyl acetate, methyl methacrylate, butyl methacrylate, methyl acrylate, butyl acrylate, itaconic acid, hydrogenated methyl acrylate, hydrogenated ethyl acrylate, acrylamide, vinyl chloride, fluorine. Vinylidene chloride, vinylidene chloride, etc. can be used. Further, acrylonitrile butadiene rubber, acrylonitrile butadiene styrene resin, acrylonitrile chlorinated polyethylene propylene diene styrene resin, acrylonitrile vinyl chloride resin, acrylonitrile methacrylate resin, acrylonitrile acrylate resin and the like can be used.

As the polymer material used for the gel electrolyte, for example, polyethylene oxide or a copolymer of polyethylene oxide can be used.
As the copolymerization monomer, polypropylene oxide, methyl methacrylate, butyl methacrylate, methyl acrylate, butyl acrylate, or the like can be used.

Furthermore, as the polymer material used for the gel electrolyte, for example, polyvinylidene fluoride and a copolymer of polyvinylidene fluoride can be used. As the copolymerization monomer, for example, hexafluoropropylene, tetrafluoroethylene or the like can be used.

As the non-aqueous solvent used for the gel electrolyte, ethylene carbonate, propylene carbonate,
Cyclic ester compounds such as butylene carbonate, vinylene carbonate, γ-butyl lactone and γ-valerolactone, diethoxyethane, tetrahydrofuran, 2-
Ether compounds such as methyltetrahydrofuran and 1,3-dioxane, chain ester compounds such as methyl acetate and methyl propylene oxide, chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, 2,4-difluoroanisole, 2, 6-
Difluoroanisole, 4-bromoveratrol and the like can be used. Moreover, these non-aqueous solvents may be used alone or in combination of two or more kinds.

In the gel electrolyte, when polyvinylidene fluoride is used as the polymer material, a gel electrolyte composed of a multi-component polymer in which polyhexafluoropropylene, polytetrafluoroethylene, etc. are copolymerized is used. Are preferably formed. Further, the gel electrolyte is more preferably formed using a polymer material made of a copolymer of polyvinylidene fluoride and polyhexafluoropropylene. This makes it possible to form a gel electrolyte having higher mechanical strength.

[0045]

EXAMPLES A non-aqueous electrolyte secondary battery to which the present invention is applied will be described in detail below based on specific experimental results.

Example 1 [Method for producing positive electrode] LiCoO ₂ was used as a positive electrode active material.
1 wt% and 6 wt% graphite as a conductive agent,
Polyvinylidene fluoride (PVdF) as a binder was mixed with 3% by weight and then dispersed in N-methyl-2-pyrrolidone to prepare a slurry-like positive electrode mixture.

Next, this positive electrode mixture was applied to both sides of an aluminum foil serving as a positive electrode current collector, and the coating film was dried and compression-molded to form a positive electrode active material layer. And
A positive electrode was obtained by cutting into a predetermined size. In addition,
The thickness of the aluminum foil is 20 μm.

[Manufacturing Method of Negative Electrode] 90% by weight of graphite as a negative electrode active material and 10% by weight of polyvinylidene fluoride (PVdF) as a binder were mixed, and then N-
It was dispersed in methyl-2-pyrrolidone to prepare a slurry-like negative electrode mixture.

Next, this negative electrode mixture was applied to both surfaces of a copper foil which was a negative electrode current collector, and the coating film was dried and compression-molded to form a negative electrode active material layer. Then, the negative electrode was obtained by cutting into a predetermined size. The thickness of the copper foil is 10 μm.

[Preparation Method of Non-Aqueous Electrolyte Solution] LiPF ₆ as an electrolyte was added to a mixed solvent prepared by mixing ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 3.
6 mol / l, LiN (CF ₃ SO ₂ ) ₂ 0.4 mo
A non-aqueous electrolytic solution was prepared by dissolving it so that the concentration became 1 / l.

[Battery Assembling Method] First, a separator, a positive electrode, a separator, a negative electrode, and a separator were laminated in this order, and this was wound in the longitudinal direction to obtain a wound electrode body. next,
The electrode body was housed in a battery can having a nickel plating inside. Insulating plates are arranged on both upper and lower surfaces of the electrode body, and the positive electrode lead made of aluminum is led out from the positive electrode current collector to the battery lid, and the negative electrode lead made of nickel is led out from the negative electrode current collector to the negative electrode can. Welded. Next, the non-aqueous electrolyte was injected into the battery can. Next, the battery can was fixed by caulking the battery can through an insulating sealing gasket having asphalt coated on the surface, and the airtightness inside the battery was maintained. Through the above steps, a cylindrical non-aqueous electrolyte battery having a diameter of 18 mm and a height of 65 mm was produced.

Examples 2 to 5, Comparative Examples 1 to 4 In preparing non-aqueous electrolytes, LiPF ₆ and LiN (C
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that F ₃ SO ₂ ) ₂ was added in the ratio shown in Table 1 below.

[0053]

[Table 1]

For Examples 1 to 5 and Comparative Examples 1 to 4 produced as described above, first, 0.50
A was initially charged at a constant current and a constant voltage of 4.25V. Next, each non-aqueous electrolyte secondary battery was disassembled, and it was confirmed whether or not a cut portion of the positive electrode current collector was generated due to elution of aluminum.

A charging / discharging cycle test was carried out in a thermostatic chamber at 45 ° C. for Examples 1 to 5 and Comparative Examples 1 to 4, and the capacity retention was measured to cycle under an environment of 45 ° C. The properties and high temperature storage properties were evaluated.

In the charge / discharge cycle test, the charging condition was 1.0 A, a constant current and constant voltage of 4.2 V, and the discharging condition was 0.
A charging / discharging cycle with a constant current of 70 A and a final voltage of 2.5 V was repeated. Then, the initial discharge capacity, the discharge capacity after 300 cycles, and the discharge capacity after 500 cycles were measured, and the ratio of the discharge capacity after 300 cycles to the initial discharge capacity was obtained, and the capacity retention rate after 300 cycles (unit: %). Further, the ratio of the discharge capacity after 500 cycles to the initial discharge capacity was obtained, and this was taken as the capacity retention rate (unit:%) after 500 cycles.

The above measurement results are shown in Table 2 below. Also,
The measurement results of the charge / discharge cycle test are shown in FIG. In addition,
In FIG. 2, the vertical axis represents the capacity retention rate (unit:%), and the horizontal axis represents the number of cycles (unit: times).

[0058]

[Table 2]

From Table 2, LiPF ₆ and LiN (CF ₃ S
O ₂₎ molar ratio of ₂ 95: 5 or 95: 5 to 50: 5
In Examples 1 to 5 in which the range is 0, the generation of the cut portion of the positive electrode current collector is not confirmed, so that it is clear that aluminum is not eluted.

Further, from Table 2 and FIG. 2, since the capacity retention rate after 300 cycles and the capacity retention rate after 500 cycles under the environment of 45 ° C. both exceeded 80%, Examples 1 to 1 It can be seen that No. 5 has excellent cycle characteristics in a high temperature environment. Furthermore, Examples 1 to 1
Comparing the capacity retention rate after 300 cycles and the capacity retention rate after 500 cycles of Example 5, since the decrease in the capacity retention rate with the lapse of charge / discharge cycles is not so large, Examples 1 to 5 It can be seen that is excellent in high temperature storage characteristics.

On the other hand, from Table 2, LiPF ₆ and Li
Comparative Example 1 in which the molar ratio with N (CF ₃ SO ₂ ) ₂ was 10:90, and Comparative Example 2 in which the molar ratio was 50:50,
It can be seen that the aluminum foil is attacked and aluminum is eluted, and a cut portion is generated in the positive electrode current collector, so that it does not function as a battery. This is due to the fact that LiN (C
This is because F ₃ SO ₂ ) ₂ is excessively contained, so that the oxidation potential of aluminum used as the positive electrode current collector is lowered.

Further, from Table 2 and FIG. 2, LiPF ₆ and L
In Comparative Example 3 in which the molar ratio with iN (CF ₃ SO ₂ ) ₂ was 99: 1, and in Comparative Example 4 in which the molar ratio was 100: 1, that is, containing only LiPF ₆ as the electrolyte, the positive electrode current collector was cut. No generation of parts was confirmed, but since the capacity retention rate after 300 cycles and the capacity retention rate after 500 cycles in an environment of 45 ° C. were both less than 80%, cycle characteristics in a high temperature environment required for a practical battery. It turns out that it does not have. Further, Comparative Example 3 and Comparative Example 4-3
Comparing the capacity retention rate after 00 cycles and the capacity retention rate after 500 cycles, the decrease in the capacity retention rate with the lapse of the charging / discharging cycle is very large and the high temperature storage characteristics required for practical batteries are not obtained. I understand.

Therefore, the non-aqueous electrolyte secondary battery uses LiPF ₆ and LiN (CF ₃ SO ₂ ) in the non-aqueous electrolyte.
₂ is a molar ratio of 95: 5 or 95: 5 to 50:50.
Since the aluminum is prevented from being eluted, the charge and discharge cycle characteristics at high temperature and the storage characteristics at high temperature are very excellent as compared with the case where each electrolyte is used alone.

[0064]

As is apparent from the above description, in the non-aqueous electrolyte secondary battery according to the present invention, LiPF ₆ and the imide salt are in a molar ratio of 95: 5 or 95: 5 to 50:50. Since it has a non-aqueous electrolyte contained in, even if an imide salt is used as the electrolyte, there is no elution of aluminum, and compared to the case of using each electrolyte alone, the charge / discharge cycle at high temperature The characteristics and high temperature storage characteristics are greatly improved.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration example of a non-aqueous electrolyte secondary battery.

FIG. 2 is a characteristic diagram showing measurement results of a charge / discharge cycle test.

[Explanation of symbols]

1 non-aqueous electrolyte secondary battery, 2 positive electrode, 3 negative electrode, 4 separator, 5 electrode body, 6 battery can, 7 battery lid, 8
Outer container

Claims (4)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode containing a lithium cobalt composite oxide, a negative electrode containing graphite, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte contains LiPF ₆ and an imide salt. Is contained in a molar ratio of 95: 5 or 95: 5 to 50:50 in a non-aqueous electrolyte secondary battery. 2. The imide salt is LiN (C _n F
The non-aqueous electrolyte secondary battery according to claim 1, which is _{2n + 1} SO ₂ ) ₂ (where n is 1 or more). 3. The imide salt is LiN (CF ₃ S
The non-aqueous electrolyte secondary battery according to claim _2, which is O ₂ ) ₂ . 4. The non-aqueous electrolyte secondary battery according to claim 1, further comprising an electrode body in which the positive electrode and the negative electrode are laminated with a separator interposed therebetween and further wound in a longitudinal direction.