JP2003100299A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the decrease in the recovery capacity of a lithium secondary battery when it is reserved at a high temperature, where the decrease in the recovery capacity occurs when BF (boron-fluoride) salt is used for an electrolyte. SOLUTION: The lithium secondary battery comprises a positive electrode containing PVDF (polyvinylidene fluoride) homopolymer synthesized by an emulsion polymerization method and a small amount of niobium, and negative electrode containing styrenebutadiene rubber and carboxymethylcellulose, and an electrolyte containing lithium fluoroborate and lactone.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrode composition for a battery material such as a lithium ion battery.

[0002]

2. Description of the Related Art Due to the remarkable development of mobile devices in recent years,
Demand for batteries used as power sources for portable devices, especially lithium-ion batteries, is rapidly increasing. Further, as the functions of mobile devices increase, higher energy and accompanying improvements in battery characteristics are the goals of technological development.

Among the above, the following items are listed as important technical problems. (1) Improvement of safety (overcharge) (2) Improvement of high temperature storage characteristics (3) Improvement of cycle characteristics

Regarding the high temperature storage characteristics among them, in the case of salts used in battery systems, lithium ion secondary batteries, LiPF ₆ , LiBF ₄ already disclosed, and other imide type salts such as LiClO _4, etc. Thermal stability is believed to be one factor. In recent years, the special table 2000-60834
A novel lithium salt compound as described in JP-A-0 is also proposed and put to practical use.

Other factors are considered to be related to the electrochemical stability of the solvent used in the electrolytic solution and the water content in the solvent, and the addition of additives and various solvents is considered. It is considered. As described above, various methods have been attempted as measures against high temperature storage, but it is difficult to improve the high temperature storage characteristics while maintaining other battery characteristics from the viewpoint of balancing the entire characteristics.

As an example, the case where LiBF ₄ is used as an electrolyte salt will be described. This LiBF ₄ (hereinafter abbreviated as BF) salt has a lower conductivity than LiPF ₆ (hereinafter abbreviated as PF) but has thermal stability, and therefore has a high temperature storage property, for example, the inside of the battery measured by AC after storage. The change in impedance is smaller than when the PF system is used.

However, since LiBF ₄ has a low conductivity,
The normal 1C battery capacity is reduced as compared with the PF battery. For this reason, when BF is used as the electrolyte salt, it is necessary to control the solvent composition of the electrolytic solution in consideration of this point of view, and techniques associated therewith have been disclosed, but the capacity is lower than that of the PF system. Is not settled.

Further, the recovery of the battery capacity after storing the battery is also a problem in practical use. Particularly, in the case of the above-mentioned BF system, the recovery capacity is lower than that of the EC system,
It was an issue that should be improved practically.

[0009]

The object of the present invention is to provide a BF
It is an object of the present invention to provide a lithium secondary battery capable of preventing a decrease in battery capacity recovery after high temperature storage in a battery system using a system salt.

[0010]

That is, the above object is achieved by the following constitution of the present invention. (1) PVDF synthesized on the positive electrode side by emulsion polymerization
A lithium secondary battery comprising a homopolymer and a small amount of niobium, styrene-butadiene rubber and carboxymethylcellulose on the negative electrode side, and further containing a lithium fluoroborate salt and a lactone as an electrolyte. (2) The solvent of the electrolyte further contains a cyclic carbonate, and the volume ratio of cyclic carbonate: lactone is 6:
The lithium ion secondary battery of (1) above, which is 4 to 1: 9, (3) The lithium of (1) or (2) above, wherein the positive electrode contains 0.1 to 0.4% by mass of niobium (Nb). Ion secondary battery. (4) The lithium secondary battery according to any one of (1) to (3) above, wherein the PVDF has a molecular weight of 80,000 or more. (5) The electrolyte is a gelled solid electrolyte, and the solid electrolyte contains the PVDF homopolymer, a lactone, and a lithium fluoroborate-based salt according to any one of the above (1) to (4). Lithium-ion secondary battery. (6) The electrolyte contains 1 to 2 of vinylene carbonate or vinyl ethylene carbonate as an additive.
The lithium ion secondary battery according to any one of (1) to (5) above, which contains mass%.

[0011]

The inventors of the present invention have made various investigations with the goal of optimizing the electrode structure to quickly diffuse lithium ions inside the electrode with respect to the above problems, and in particular, have studied in detail the kind of binder. I went. As a result, they have found that the combination of the binders affects the BF-based battery characteristics.

That is, PVDF is used as a binder on the positive electrode side.
System, SBR type rubber material as a binder on the negative electrode side, and C
By using MC, the recovery capacity after high temperature storage is greatly improved compared to the conventional one. With respect to this effect, it is noted that the impedance on the negative electrode side significantly increases after the normal storage.
As seen with the use of salt, the results of this study revealed that the increase in impedance was also small, which is considered to be related to the improvement in recovery capacity.

Further, the lithium battery of the present invention can be applied not only to a battery using a conventional electrolytic solution and a separator, but also to a battery using a gel-based solid electrolyte, which has been attracting attention in recent years. This battery is different from the organic solid electrolyte in which lithium ions are conducted in the polymer medium, which has been studied in the past, and can discharge a large current.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium secondary battery of the present invention comprises a PVDF homopolymer synthesized by an emulsion polymerization method and a trace amount of niobium (Nb) on the positive electrode side, and styrene-butadiene rubber and carboxymethylcellulose on the negative electrode side. Lithium fluoroborate (BF) in the electrolyte
It contains a system salt and a lactone.

That is, PVDF (polyvinylidene fluoride) is used on the positive electrode side as a binder for the electrode, a rubber latex material such as styrene-butadiene rubber (SBR) is used on the negative electrode side, and carboxymethyl cellulose (CMC) is added as an additive component. use. Further, a lactone such as γ-butyrolactone is used as a constituent component of the gelled solid electrolyte component. Further, as the electrolyte solvent, preferably cyclic carbonate, particularly ethylene carbonate (EC) is used.

As a result, even when a BF-based salt is used as the electrolyte salt, PVDF synthesized by the emulsion polymerization method and SBR and CMC are used as the binder, and the lactone is contained in the electrolytic solution, so that BF can be obtained. Despite the use of the system salt, the reduction in recovery capacity after high temperature storage can be greatly reduced compared to the conventional case.

According to the present invention, this polyvinylidene fluoride homopolymer (hereinafter referred to as PVDF) is used as a positive electrode side binder, styrene butadiene rubber (SBR) as a negative electrode side binder, and carboxymethyl cellulose (CMC) as an additive component. ), And only when γ-butyrolactone is present in the electrolytic solution, lithium fluoroborate type (hereinafter BF type)
It is possible to suppress the phenomenon of capacity decrease.

When PVDF produced by another synthesis method is used, or when SBR and CMC are not combined, no such effect is observed. Therefore, it is an extremely effective means for increasing the energy density of the electrode.

The mechanism is not clear at this time, but there is an active site inherent in PVDF, which reduces the resistance due to the interaction of the salt with the BF system inside the electrode, and the difference in the crystallinity of the resin. It is settled that the resulting swelling property is improved and the diffusion of lithium is facilitated, and as a result, the decrease in battery capacity is halved compared to the conventional one. Further, by using SBR and CMC in combination, ester groups derived from CMC are present on the surface of the negative electrode, which act when the negative electrode coating film is formed at the time of charging, and are usually easily consumed at the time of forming the coating film. It is presumed that the decomposition of the lactone is prevented and a more dense film is formed.

The point of using the emulsion polymerization method is disclosed in JP-A-8-250127. The emulsion polymerization method is, for example, in the presence of an iodine compound or a bromine compound, preferably a diiodine compound, in a water medium under substantially oxygen-free conditions, and a perhaloolefin, if necessary, a monomer that provides a curing site. There may be mentioned a method of carrying out emulsion polymerization in the presence of a radical initiator while stirring under pressure.

One advantage of using the homopolymers obtained by emulsion polymerization is that the polymers are of very high purity, that is to say trace impurities, ie impurities in the ppb (parts per million) range. can get.

The crystallinity of the homopolymer obtained by this emulsion polymerization method is 30% or more, especially about 35 to 55%. The molecular weight thereof is preferably 80,000 or more, and particularly preferably 100,000 to 140,000.

The electrode is preferably an electrode active material and a binder,
A composition with a conductive additive is used if necessary.

A carbon material is used as an active material for the negative electrode. If necessary, a conductive auxiliary agent is added to the negative electrode, styrene-butadiene rubber (SBR) is used as a binder, and carboxymethyl cellulose (CMC) is used as an additive.

Styrene-butadiene rubber (SBR) used as a binder has good adhesiveness to the negative electrode current collector, and maintains stable adhesive force even at high temperatures. Carboxymethyl cellulose (CMC) contained as an additive is
Used to perform thickening treatment on carbon materials. A good coating solution can be obtained by mixing with SBR after this treatment.

For the positive electrode, it is preferable to use a positive electrode active material such as an oxide capable of intercalating / deintercalating lithium ions. Furthermore, the positive electrode preferably contains a trace amount of niobium (Nb) in addition to the active material.
As the binder, the PVDF homopolymer synthesized by the emulsion polymerization method is used.

By using such an electrode, a lithium secondary battery having good characteristics can be obtained.

The carbon material used as the electrode active material may be appropriately selected from, for example, mesocarbon microbeads (MCMB), natural or artificial graphite, resin-fired carbon material, carbon black, carbon fiber and the like. These are used as powders except for carbon fibers. Among these, carbon fiber is preferable. Graphite is also preferable, and its average particle diameter is preferably 1 to 30 μm, particularly 5 to 25 μm. If the average particle size is too small, the charge / discharge cycle life tends to be short, and the capacity variation (individual difference) tends to increase. If the average particle size is too large, the variation in capacity becomes extremely large and the average capacity becomes small. When the average particle diameter is large, the variation in capacity is considered to be due to the variation in the contact between graphite and the current collector and the contact between graphite.

Lithium ion is an intercalating day
Intercalatable oxides include lithium
Complex oxides are preferred, for example LiCoO 2.₂, LiM
n₂O _Four, LiNiO₂, LiV₂O_FourAnd so on.
The average particle size of these oxide powders is about 1-40 μm
Is preferred.

The positive electrode contains a trace amount of niobium (Nb) element in addition to the active material. Niobium (N
The content of b) is preferably 0.1-0.4% by weight, in particular 0.12-0.35% by weight. If the niobium content is too high, the capacity will be reduced, and if it is too low, the capacity will be recovered after high temperature storage and the cycle characteristics thereafter will be deteriorated.

A conductive auxiliary agent is added to the electrode, if necessary. Examples of the conductive aid include graphite, carbon black, carbon fibers, metals such as nickel, aluminum, copper, silver and the like, and graphite and carbon black are particularly preferable.

In the negative electrode, the electrode composition is a weight ratio of active material: conductive additive: SBR: CMC = 85 to 95: 2 to 8: 2.
The range of 5: 1 to 2 is preferable, and in the positive electrode, the weight ratio of active material: conductive additive: PVDF homopolymer = 85 to 92: 5.
The range of -9: 3-6 is preferable.

To manufacture the electrode, first, an active material, a binder and, if necessary, a conductive auxiliary agent are dispersed in a binder solution to prepare a coating solution.

Then, the electrode coating liquid is applied to the current collector. The means for applying is not particularly limited and may be appropriately determined depending on the material and shape of the current collector. Generally, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, a screen printing method and the like are used.
Then, if necessary, rolling treatment is performed by a flat plate press, a calendar roll, or the like.

The current collector may be appropriately selected from ordinary current collectors depending on the shape of the device used by the battery, the method of arranging the current collector in the case, and the like. Generally, aluminum or the like is used for the positive electrode and copper, nickel or the like is used for the negative electrode.
A metal foil, a metal mesh, or the like is usually used as the current collector. Although the metal mesh has a smaller contact resistance with the electrode than the metal foil, the metal foil can also obtain a sufficiently small contact resistance.

Then, the solvent is evaporated to produce an electrode. The coating thickness is preferably about 50 to 300 μm.

<Lithium Secondary Battery> The structure of the lithium secondary battery is not particularly limited, but is usually composed of a positive electrode, a negative electrode and a separator, and is applied to a laminated battery, a cylindrical battery or the like.

Such a positive electrode, a separator and a negative electrode are laminated in this order and pressure-bonded to obtain a battery body.

The electrolytic solution with which the separator is impregnated generally comprises an electrolyte salt and a solvent. Examples of the electrolyte salt include L
iBF ₄ , LiPF ₆ , LiAsF ₆ , LiSO ₃ CF
Lithium salts such as ₃ , LiClO ₄ and LiN (SO ₂ CF ₃ ) ₂ can be mentioned, but LiBF ₄ is used in the present invention.
Lithium fluoroborate is used.

The solvent for the electrolytic solution may be any solvent in which a lactone such as γ-butyrolactone is always present. When using a mixed solvent of lactone and another solvent, a separator,
The other solvent is not particularly limited as long as it has good compatibility with the electrolyte salt, but a polar organic solvent that does not decompose even at a high operating voltage in a lithium battery or the like is preferable. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate, carbonates such as ethylmethyl carbonate, tetrahydrofuran (abbreviation THF), cyclic ethers such as 2-methyltetrahydrofuran, Examples thereof include cyclic ethers such as 1,3-dioxolane and 4-methyldioxolane, and sulfolane.

In the present invention, at least γ is added to the solvent of the electrolytic solution.
-Contains a lactone such as butyrolactone. Further, in combination with this lactone such as γ-butyrolactone, a cyclic carbonate such as EC is preferable in the above solvent. Further, the volume ratio of cyclic carbonate and lactone is
Converted to ethylene carbonate and γ-butyrolactone,
Preferably 6/4 to 1/9, more preferably 3/7 to 1
It is preferably / 9, particularly preferably 1/3 to 3/17.

The concentration of the electrolyte salt when considering that the solvent and the electrolyte salt constitute the electrolyte solution is preferably 0.3 to 5 mol.
l / l. Usually, the highest ionic conductivity is shown around 0.8 to 2.0 mol / l.

As the solid electrolyte or separator sheet forming the separator, it is preferable to use the above-mentioned polyvinylidene fluoride homopolymer, especially one produced by the emulsion polymerization method.

The microporous membrane for solid electrolyte used in the present invention is preferably formed by the following wet phase separation method.

The wet phase separation method is a method in which phase separation is performed in a solution in film formation by a solution casting method. That is, a polymer to be a microporous film is dissolved in a solvent in which the polymer can be dissolved, and the obtained film-forming stock solution is uniformly applied on a support such as a metal or plastic film to form a film. After that, a film-forming undiluted solution cast into a film is introduced into a solution called a coagulation bath to cause phase separation to obtain a microporous film. The coating solution for film formation may be applied in a coagulation bath.

An adhesive for improving the adhesiveness between the microporous film and the electrode may be used. Specifically, UNISTOL (manufactured by Mitsui Chemicals, Inc.), SBR (manufactured by Zeon Corporation),
Examples thereof include polyolefin adhesives such as Aquatex (manufactured by Chuo Rika Co., Ltd.) and Adcoat (manufactured by Morton Co.), and among others, Aquatex is preferred.

The adhesive is dissolved or dispersed in water or an organic solvent such as toluene, and then applied or placed on the microporous membrane by spraying, coating or the like.

The porosity of the microporous membrane is 50% or more, preferably 50 to 90%, more preferably 70 to 80%.
The pore diameter is 0.02 μm or more and 2 μm or less, preferably 0.02 μm or more and 1 μm or less, and more preferably 0.
04 μm or more and 0.8 μm or less, particularly preferably 0.1
μm or more and 0.8 μm or less, more preferably 0.1 μm
It is m or more and 0.6 μm or less. The thickness of the microporous film is preferably 20 to 80 μm, more preferably 25 to 45 μm.
m.

The melting point of the microporous membrane is preferably 150 ° C. or higher, particularly 160 to 170 ° C., and the heat of fusion is preferably 30 J.
/ G or more, particularly preferably 40 to 60 J / g of material.

Other gel type polymers may be used for the separator. For example, (1) polyalkylene oxide such as polyethylene oxide and polypropylene oxide,
(2) a copolymer of ethylene oxide and acrylate,
(3) Copolymer of ethylene oxide and glycyl ether, (4) Copolymer of ethylene oxide, glycyl ether and allyl glycyl ether, (5) Polyacrylate (6) Polyacrylonitrile (7) Polyvinylidene fluoride, Fluorinated Fluorinated vinylidene-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene chloride copolymer, vinylidene fluoride-hexafluoropropylene fluororubber, vinylidene fluoride "tetrafluoroethylene-hexafluoropropylene fluororubber, etc." Polymers and the like can be mentioned.

The gel polymer may be mixed with the electrolytic solution or may be applied to the separator. Further, the gel polymer may be crosslinked by ultraviolet rays, EB, heat, etc. by adding an initiator.

The thickness of the solid electrolyte is 5 to 100 μm,
Further, it is preferably 5 to 60 μm, and particularly preferably 10 to 40 μm. Since the solid electrolyte of the present invention has high strength,
The film thickness can be reduced. The solid electrolyte of the present invention can be made into a thin film as compared with a conventional gel electrolyte which could not be practically made 60 μm or less, and further, a separator (usually 25 μm) used in a solution type lithium ion battery. Can be thinner than Therefore, one of the advantages of using the solid electrolyte can be reduced in thickness and area, that is, in the form of a sheet.

Other separator constituent materials include one or more kinds of polyolefins such as polyethylene and polypropylene (in the case of two or more kinds, a laminated product of two or more layers of film), polyethylene terephthalate, etc. Examples include polyesters, thermoplastic fluororesins such as ethylene-tetrafluoroethylene copolymer, and celluloses. Sheet form is JIS-P81
The air permeability measured by the method specified in 17 is 5 to 2000 seconds /
There are microporous film, woven cloth, non-woven cloth, etc. having a thickness of about 100 cc and a thickness of about 5 to 100 μm.

Vinylene carbonate and vinyl ethylene carbonate are preferably added to the electrolyte as additives.
Contains about 2% by mass. By including such an additive, the initial cycle characteristics are improved and the long-term cycle characteristics are remarkably improved. In addition, the impedance is lowered and the electric characteristics are improved.

The outer bag is composed of a laminate film in which a polyolefin resin layer such as polypropylene or polyethylene as a heat-adhesive resin layer or a heat-resistant polyester resin layer is laminated on both surfaces of a metal layer such as aluminum. . The outer bag is formed in a bag shape in which one side is opened by previously heat-bonding two laminated films to each other on the heat-adhesive resin layers on the end faces of the three sides to form a first seal portion. Alternatively, a single laminated film may be folded back and the end faces of both sides may be heat-bonded to form a seal portion to form a bag shape.

The laminate film has a laminated structure of a thermoadhesive resin layer / polyester resin layer / metal foil / polyester resin layer from the interior side in order to secure insulation between the metal foil forming the laminate film and the lead-out terminal. It is preferable to use a laminated film. By using such a laminated film, the polyester resin layer having a high melting point remains without being melted at the time of heat bonding, so that a separation distance between the lead-out terminal and the metal foil of the outer bag can be secured and insulation can be secured. Therefore, the thickness of the polyester resin layer of the laminated film is preferably about 5 to 100 μm.

[0057]

EXAMPLES The present invention will be described below with reference to examples.

Example 1 As the electrolyte membrane, a microporous membrane was prepared using the following materials, and this was used as a solid electrolyte.

A mixed solution of 40 parts by weight of dimethylacetamide and 40 parts by weight of dioxane was added to polyvinylidene fluoride [Kynar, manufactured by Elf Atchem (Atofina)].
741] 20 parts by weight was melted and cast on a glass plate to a film thickness of 200 μm using a doctor blade method.

Immediately after casting, it was immersed in a coagulation bath consisting of 80 parts by weight of dioxane and 20 parts by weight of water for 10 minutes to coagulate it, washed in running water for 30 minutes, then dried at 60 ° C. for 1 hour, and then thickened. A 50 μm thick polyvinylidene fluoride homopolymer was obtained.

The microporous membrane obtained had a porosity of 70% and a pore diameter of 0.2 μm.

In order to impart adhesiveness to the surface of the microporous film, a polyolefin material may be deposited by spraying or the like.

Then, an electrolyte solution (abbreviated as EL) 2M LiBF ₄ / EC + γ-butyrolactone (EC / γ-butyrolactone = 2/8 (volume ratio)) was added to the solid electrolyte sheet.
Was impregnated with 1% by mass of vinylene carbonate to obtain a solid electrolyte.

Further, a battery was produced using the obtained solid electrolyte.

LiCoO ₂ was used as the positive electrode active material, acetylene black was used as the conduction aid, and polyvinylidene fluoride homopolymer synthesized by the emulsion polymerization method was used as the binder.

The weight ratio was LiCoO ₂ : acetylene black: PVDF = 87: 8: 5, and n-methylpyrrolidone (NMP) was added to NMP: PVDF.
= 9: 1 (weight ratio), and these were mixed at room temperature to obtain a positive electrode slurry.

Mesocarbon microbeads (MCMB) were used as the negative electrode active material, acetylene black was used as the conduction aid, SBR was used as the binder, and CMC was used as the dispersant.

By weight ratio MCMB: acetylene black:
SBR: CMC = 90: 5.5: 3: 1.5 was weighed, and pure water was further added so that pure water: CMC = 98: 2 (weight ratio), and MCMB and acetylene black were added. Was added to form a paste. Then, pure water is purified water:
The dispersion liquid adjusted so that SBR = 1: 1 (weight ratio) was added to the above paste to prepare a negative electrode slurry.

The obtained positive electrode slurry and negative electrode slurry were applied on an aluminum foil and a copper foil, respectively, by a doctor blade method, and NMP or pure water was evaporated and dried at 100 ° C. to 130 ° C. to form a sheet. . The sheet-formed positive electrode and negative electrode were each processed into a predetermined thickness by roll pressing.

The solid electrolyte, the positive electrode and the negative electrode thus obtained were cut into a predetermined size, the respective sheets were laminated and thermally laminated at 130 to 160 ° C. afterwards,
The positive electrode was thermally laminated with an aluminum grid previously coated with a conductive adhesive as a current collector, and the negative electrode was thermally laminated with a copper grid previously coated with a conductive adhesive as a current collector. Then, this was impregnated with 2M LiBF ₄ / EC + γ-butyrolactone (EC / γ-butyrolactone = 2/8 (volume ratio)) to which 1% by mass of vinylene carbonate was added as an electrolytic solution, and then the mixture was sealed in an aluminum laminate pack, A lithium secondary battery was produced.

The capacity of the produced sample after charging was measured. The battery capacity was based on the capacity when a PF-based battery was manufactured.

Example 2 Electrons were prepared and measured in the same manner as in Example 1, except that the composition of the electrolytic solution was changed to the following. Electrolyte composition EC / γ-butyrolactone = 3/7 (volume ratio)

<Comparative Example 1> A PF battery was prepared in the same manner as in Example 1 except that the following electrolytic solution composition was used. Electrolyte solution composition EC / DEC = 3/7 (volume ratio) Salt: 1M LiPF ₆

<Comparative Example 2> For both the positive and negative electrodes as a binder,
A battery was prepared in the same manner as in Example 1 except that PVDF KF1000 obtained by the suspension polymerization method and a homopolymer were used, and the capacity was measured.

<Comparative Example 3> A battery was prepared in the same manner as in Comparative Example 1 except that the electrolytic solution had the following composition. Electrolyte composition EC / DEC = 4/6 (volume ratio)

Table 1 shows the results of the initial capacity and the recovery capacity after storage at 60 ° C. for the above batteries.

[0077]

[Table 1]

As is clear from Table 1, in the case of Examples 1 and 2, the initial capacity is equal to or more than that of Comparative Examples 1 and 2, and the recovery capacity is improved to a level close to 100%. I understand.

This uses γ-butyrolactone and PVDF polymer as the gelled solid electrolyte constituents, and also uses PVDF on the positive electrode side shown in Example 1 as the electrode binder.
The point is that SBR and CMC are used for the polymer and the negative electrode side. This is an effect that is exhibited only when the gelled solid electrolyte component and the binder coexist.

Although the gel-type solid electrolyte is used in this example, if PVDF, SBR and γ-butyrolactone coexist, the same effect can be obtained in the conventional solution-type battery. Further, it was confirmed that substantially the same effect was obtained when vinyl ethylene carbonate was used instead of vinylene carbonate in Example 1.

[0081]

As described above, according to the present invention, it is possible to suppress a decrease in recovery capacity during high temperature storage that occurs when a BF salt is used.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Satoshi Maruyama             1-13-1, Nihonbashi, Chuo-ku, Tokyo             -In DC Inc. F-term (reference) 5H029 AJ03 AK03 AL06 AM03 AM04                       AM07 AM16 EJ12 HJ01 HJ07                       HJ11                 5H050 AA08 AA10 BA17 CA07 CA08                       CA09 CB07 DA11 DA13 DA18                       EA22 EA23 HA01 HA07 HA11

Claims (6)

[Claims] 1. A P synthesized on the positive electrode side by an emulsion polymerization method.
A lithium secondary battery containing VDF homopolymer and a trace amount of niobium, styrene-butadiene rubber and carboxymethylcellulose on the negative electrode side, and further containing a lithium fluoroborate salt and a lactone as an electrolyte. 2. The solvent of the electrolyte further contains a cyclic carbonate, and the volume ratio of cyclic carbonate: lactone is 6: 4 to 1:
9. The lithium-ion secondary battery according to claim 1, wherein 3. The positive electrode contains niobium (Nb) in an amount of 0.1 to 0.1.
The lithium ion secondary battery according to claim 1, which contains 0.4% by mass. 4. The lithium secondary battery according to claim 1, wherein the PVDF has a molecular weight of 80,000 or more. 5. The electrolyte is a gelled solid electrolyte, and the solid electrolyte contains the PVDF homopolymer, a lactone, and a lithium fluoroborate-based salt.
4. The lithium ion secondary battery according to any one of 4 above. 6. The lithium ion secondary battery according to claim 1, wherein the electrolyte contains 1 to 2 mass% of either vinylene carbonate or vinyl ethylene carbonate as an additive.