JP5239109B2 - Lithium ion secondary battery - Google Patents

Description

Technical field to which the invention belongs

  The present invention relates to an electrode composition for battery materials such as lithium ion batteries.

  Due to the remarkable development of portable devices in recent years, the demand for batteries used as power sources for portable devices, particularly lithium ion batteries, is rapidly increasing. Furthermore, with the increase in functions of mobile devices, higher energy and improved battery characteristics are becoming the goals of technological development.

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

Among these, for high-temperature storage characteristics, for salts used in battery systems, lithium ion secondary batteries, such as LiPF ₆ and LiBF ₄ already disclosed, and other imide-based salts such as LiClO ₄ Stability is considered a factor. In recent years, a novel lithium salt compound as described in JP 2000-608340 A has also been proposed and put into practical use.

  In addition, other factors are considered to be related to the electrochemical stability of the solvent used in the electrolyte and the water content in the solvent, and the use of additives and various solvents is considered. Yes. As described above, various methods have been tried as a countermeasure 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, a case where LiBF ₄ is used as an electrolyte salt will be described. This LiBF ₄ (hereinafter abbreviated as BF) salt has lower conductivity than LiPF ₆ (hereinafter abbreviated as PF) but is thermally stable. Therefore, it has a high temperature storage characteristic, for example, an internal measurement of the battery by AC measurement after storage. The change in impedance is smaller than when the PF system is used.

However, since LiBF ₄ has low conductivity, the normal 1C battery capacity is reduced compared to the PF battery. For this reason, when BF is used as an electrolyte salt, it is necessary to control the electrolyte solvent composition in consideration of such a viewpoint, and the accompanying technology has been disclosed, but the capacity is reduced as compared with the PF system. Is not solved.

  Furthermore, recovery of the battery capacity after storing the battery is also a problem that becomes a practical problem. In particular, in the case of the BF system described above, the recovery capacity is lower than that of the EC system, which has been a problem to be improved in practice.

Problems to be solved by the invention

  An object of the present invention is to provide a lithium secondary battery capable of preventing a reduction in battery capacity recovery after high-temperature storage in a battery system using a BF salt.

Means for solving the problem

[Means for Solving the Problems]
That is, the above object is achieved by the following configuration of the present invention.
(1) PVDF homopolymer synthesized on the positive electrode side by an emulsion polymerization method;
It has styrene butadiene rubber and carboxymethyl cellulose on the negative electrode side,
Further lithium ion secondary battery containing the salt with γ- Buchiro lactone lithium fluoride boron acid in the electrolyte.
(2) The solvent of the electrolyte further contains a cyclic carbonate,
Cyclic carbonate: .gamma. Buchiro volume ratio of lactone, 6: 4 to 1: 9 in a lithium ion secondary battery (1).
(3) The lithium ion secondary battery according to (1) or (2), wherein the molecular weight of the PVDF is 80,000 or more.
(4) The electrolyte is a gelled solid electrolyte,
The PVDF homopolymer to the solid electrolyte, .gamma. Buchiro lactones, either the lithium ion secondary battery (1) to which contains a salt of lithium boron fluoride acid (3).
(5) The lithium ion secondary battery according to any one of (1) to (4), wherein the electrolyte contains 1-2% by mass of either vinylene carbonate or vinyl ethylene carbonate as an additive.

Action

  The inventors of the present invention have made various studies with the goal of optimizing the electrode configuration and allowing lithium ions to diffuse quickly inside the electrode, particularly the types of binders. . As a result, the present inventors have found that the combination of binders affects the BF battery characteristics.

  That is, the recovery capacity after high-temperature storage is greatly improved by using PVDF as the binder on the positive electrode side, SBR rubber material as the binder on the negative electrode side, and CMC. With regard to this action, the impedance on the negative electrode side usually increases significantly after storage, but it has been found that the increase in impedance is small in the results of this study, which improves the recovery capacity. It is thought that it is tied to.

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

DETAILED DESCRIPTION OF THE INVENTION
The lithium ion secondary battery of the present invention has a PVDF homopolymer synthesized by an emulsion polymerization method on the positive electrode side, styrene butadiene rubber and carboxymethyl cellulose on the negative electrode side, and further contains lithium fluorinated boronate ( those containing the BF) system of salt and γ- Buchiro lactone.

That is, as the electrode binder, PVDF (polyvinylidene fluoride) is used on the positive electrode side, styrene butadiene rubber (SBR ) is used on the negative electrode side , and carboxymethyl cellulose (CMC) is used as an additional component. Furthermore, as the gelling solid electrolyte components gamma - butyrolactone is used as a constituent component. Further, as the electrolyte solvent, a cyclic carbonate, particularly ethylene carbonate (EC) is preferably used.

As a result, even when a BF type salt as an electrolyte salt, and PVDF synthesized by emulsion polymerization as a binder, using the SBR and CMC, by containing the electrolyte γ- Buchiro lactone, BF In spite of the use of the salt of the system, the reduction in the recovery capacity after high temperature storage can be greatly reduced as compared with the conventional case.

According to the present invention, this polyvinylidene fluoride homopolymer (hereinafter PVDF) is used as a positive electrode side binder, styrene butadiene rubber (SBR) is used as a negative electrode side binder, and carboxymethyl cellulose (CMC) is used as an additive component. Only when γ - butyrolactone is present in the electrolytic solution, the capacity decreasing phenomenon in the lithium fluoroboronate system (hereinafter referred to as BF system) can be suppressed.

  When PVDF obtained by another synthesis method is used, and when the combination of SBR and CMC is not used, such an effect is not seen at all. Therefore, it is an extremely effective means for increasing the energy density of the electrode.

Although the mechanism is not clear at present, there is an active site inherent in PVDF, and the resistance is reduced by the interaction of salt with the BF system inside the electrode, and the swelling is caused by the difference in crystallinity of the resin. Therefore, the diffusion of lithium is facilitated, and as a result, the decrease in battery capacity is halved compared to the prior art. Further, by using SBR and CMC in combination, CMC-derived ester groups and the like are present on the negative electrode surface, and they act when the negative electrode film is formed during charging, and are usually easily consumed when forming the film. It prevents the degradation of γ- Buchiro lactone is estimated that the formation of a more dense film have been made.

  The point of using the emulsion polymerization method is disclosed in JP-A-8-250127. The emulsion polymerization method is, for example, a monomer that provides a perhaloolefin, and optionally a cure site, in the presence of an iodine or bromine compound, preferably a diiodine compound, substantially in the absence of oxygen. And emulsion polymerization in the presence of a radical initiator while stirring under pressure.

  One advantage of using a homopolymer obtained by emulsion polymerization is that a very high purity polymer is obtained, that is, a polymer with trace amounts of impurities, ie, impurities in the ppb (parts per million) range.

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

  The electrode preferably uses a composition of an electrode active material and a binder, and if necessary, a conductive additive.

  For the negative electrode, a carbon material is used as an active material. If necessary, a conductive additive 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 adhesion to the negative electrode current collector, and maintains stable adhesion even at high temperatures. Carboxymethylcellulose (CMC) contained as an additive is used for thickening the carbon material. A good coating solution can be obtained by mixing with SBR after this treatment.

For the positive electrode, it has preferred that the lithium ions is used a positive electrode active material such as intercalating and deintercalating possible oxides. Also, as the binder, using PVDF homopolymer synthesized by the emulsion polymerization method.

  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 the average particle size is preferably 1 to 30 μm, particularly preferably 5 to 25 μm. When the average particle size is too small, the charge / discharge cycle life is shortened and the capacity variation (individual difference) tends to increase. When the average particle diameter is too large, the variation in capacity becomes remarkably large and the average capacity becomes small. The reason why the variation in capacity occurs when the average particle size is large is thought to be because the contact between graphite and the current collector or the contact between graphites varies.

The oxide capable of intercalating and deintercalating lithium ions is preferably a composite oxide containing lithium, and examples thereof include LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , and LiV ₂ O ₄ . The average particle diameter of these oxide powders is preferably about 1 to 40 μm.

  If necessary, a conductive additive is added to the electrode. Preferred examples of the conductive aid include metals such as graphite, carbon black, carbon fiber, nickel, aluminum, copper, and silver, and graphite and carbon black are particularly preferable.

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

  In producing the electrode, first, an active material, a binder, and if necessary, a conductive assistant are dispersed in a binder solution to prepare a coating solution.

  And this electrode coating liquid is apply | coated to a collector. The means for applying is not particularly limited, and may be appropriately determined according to the material and shape of the current collector. In general, 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, a rolling process is performed using a flat plate press, a calendar roll, or the like.

  The current collector may be appropriately selected from ordinary current collectors according to 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. In addition, a metal foil, a metal mesh, etc. are normally used for a collector. The metal mesh has a smaller contact resistance with the electrode than the metal foil, but a sufficiently small contact resistance can be obtained even with the metal foil.

  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 stacked battery, a cylindrical battery, and the like.

  Such a positive electrode, a separator, and a negative electrode are laminated in this order, and pressed to form a battery body.

The electrolytic solution impregnated in the separator generally comprises an electrolyte salt and a solvent. Examples of the electrolyte salt include lithium salts such as LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiSO ₃ CF ₃ , LiClO ₄ , and LiN (SO ₂ CF ₃ ) _2, but in the present invention, LiBF ₄ or the like can be used. Boron fluoride lithium is used.

As the solvent of the electrolyte solution, it is sufficient that γ- butyrolactone always exists. If a mixed solvent of γ- Buchiro lactone with other solvents, separators, other solvents as long as compatibility with the electrolyte salt is good is not particularly limited, degradation at high operating voltage in a lithium battery or the like A polar organic solvent in which no occurrence occurs is preferred. For example, carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate, ethyl methyl carbonate, cyclic ethers such as tetrahydrofuran (abbreviated THF), 2-methyltetrahydrofuran, Examples thereof include cyclic ethers such as 1,3-dioxolane and 4-methyldioxolane, sulfolane and the like.

In the present invention contains at least γ- butyrolactone in the solvent of the electrolyte solution. Further, in combination with the γ- butyrolactone, cyclic carbonates EC like in the above solvent is preferred. The volume ratio of the cyclic carbonate and γ- Buchiro lactone, ethylene carbonate and gamma - in terms of butyrolactone, preferably 6 / 4-1 / 9, more preferably 3 / 7-1 / 9, in particular 1 / It is preferably 3 to 3/17.

  The concentration of the electrolyte salt when it is considered that the electrolytic solution is composed of the solvent and the electrolyte salt is preferably 0.3 to 5 mol / l. Usually, the highest ion 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, particularly one produced by an emulsion polymerization method.

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

  The wet phase separation method is a method of performing phase separation 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 onto a support such as a metal or plastic film to form a film. Thereafter, a film-forming stock solution cast into a film is introduced into a solution called a coagulation bath, and phase separation occurs to obtain a microporous film. The film forming stock solution may be applied in a coagulation bath.

  You may use the adhesive agent for improving the adhesiveness of the said microporous film and an electrode. Specific examples include polyolefin adhesives such as Unistall (Mitsui Chemicals), SBR (Nippon Zeon), Aquatex (Chuo Rika), Adcoat (Morton), etc. Of these, Aquatex is preferred.

  The adhesive is dissolved or dispersed in water or an organic solvent such as toluene, and is adhered and disposed on the microporous film 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, 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, and further preferably Is 0.1 μm or more and 0.6 μm or less. The thickness of the microporous membrane is preferably 20 to 80 μm, more preferably 25 to 45 μm.

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

Other gel type polymers may be used for the separator. For example,
(1) Polyalkylene oxides such as polyethylene oxide and polypropylene oxide,
(2) a copolymer of ethylene oxide and acrylate,
(3) a copolymer of ethylene oxide and glycyl ether,
(4) a copolymer of ethylene oxide, glycyl ether and allyl glycyl ether,
(5) Polyacrylate (6) Polyacrylonitrile (7) Polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trichloroethylene copolymer, vinylidene fluoride-hexafluoropropylene fluorine Examples thereof include fluoropolymers such as rubber and vinylidene fluoride “tetrafluoroethylene-hexafluoropropylene fluororubber”.

  The gel polymer may be mixed with the electrolytic solution or applied to the separator. Furthermore, the gel polymer may be cross-linked by ultraviolet rays, EB, heat or the like by adding an initiator.

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

  Other separator constituent materials include one or more of polyolefins such as polyethylene and polypropylene (in the case of two or more, there are two or more laminated films), polyesters such as polyethylene terephthalate, There are thermoplastic fluororesins such as ethylene-tetrafluoroethylene copolymer, celluloses and the like. The form of the sheet includes a microporous membrane film, a woven fabric, a non-woven fabric, etc. having an air permeability measured by the method specified in JIS-P8117 of about 5 to 2000 sec / 100 cc and a thickness of about 5 to 100 μm.

  The electrolyte preferably contains about 1 to 2% by mass of vinylene carbonate and vinyl ethylene carbonate as additives. By containing such an additive, the initial cycle characteristics are improved, and the long-term cycle characteristics are also remarkably improved. In addition, the impedance is lowered and the electrical characteristics are also improved.

  The exterior bag is composed of a laminated 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 exterior bag is formed in a bag shape in which two laminated films are bonded in advance to each other by thermally bonding the heat-adhesive resin layers on the end surfaces of the three sides to form a first seal portion. Alternatively, a single laminate film may be folded and the end faces of both sides may be thermally bonded to form a seal portion to form a bag.

  As a laminate film, a laminate film having a laminated structure of a heat-adhesive resin layer / polyester resin layer / metal foil / polyester resin layer from the interior side is used to ensure insulation between the metal foil constituting the laminate film and the lead-out terminal. It is preferable to use it. By using such a laminate film, the polyester resin layer having a high melting point remains undissolved at the time of thermal 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 laminate film is preferably about 5 to 100 μm.

  Hereinafter, the present invention will be described using examples.

<Example 1>
As the electrolyte membrane, a microporous membrane was prepared using the following materials, and a solid electrolyte was obtained using this.

  In a mixed solution consisting of 40 parts by weight of dimethylacetamide and 40 parts by weight of dioxane, 20 parts by weight of polyvinylidene fluoride (manufactured by Elf Atchem, Inc., Kynar 741) is dissolved, and glass is formed to a film thickness of 200 μm using a doctor blade method. Cast on a plate.

  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, solidified, washed for 30 minutes in running water, dried at 60 ° C. for 1 hour, and 50 μm thick. A microporous membrane made of a polyvinylidene fluoride homopolymer was obtained.

  The resulting microporous membrane had a porosity of 70% and a pore size of 0.2 μm.

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

Then, this solid electrolyte sheet is impregnated with an electrolytic solution (abbreviated as EL) 2M LiBF ₄ / EC + γ - butyrolactone (EC / γ - butyrolactone = 2/8 (volume ratio)) added with 1% by mass of vinylene carbonate. A solid electrolyte was obtained.

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

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

Weigh so that LiCoO ₂ : acetylene black: PVDF = 87: 8: 5 by weight ratio, and add n-methylpyrrolidone (NMP) to NMP: PVDF = 9: 1 (weight ratio). Were mixed at room temperature to obtain a positive electrode slurry.

  Further, mesocarbon microbeads (MCMB) were used as the negative electrode active material, acetylene black was used as the conductive auxiliary agent, SBR was used as the binder, and CMC was used as the dispersant.

  Weigh so that MCMB: acetylene black: SBR: CMC = 90: 5.5: 3: 1.5 by weight ratio, and pure water: pure water: CMC = 98: 2 (weight ratio) In addition, MCMB and acetylene black were added to make a paste. Next, a dispersion prepared by adding pure water to pure water: SBR = 1: 1 (weight ratio) was added to the paste to prepare a negative electrode slurry.

  And the obtained slurry for positive electrodes and the slurry for negative electrodes were apply | coated on aluminum foil and copper foil, respectively by the doctor blade method, NMP or pure water was evaporated and dried at 100 to 130 degreeC, and it was set as the sheet. Each of the positive electrode and the negative electrode formed into a sheet was processed to a predetermined thickness by a roll press.

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

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

<Example 2> Electrons were prepared and measured in the same manner as in Example 1 except that the electrolyte composition was changed as follows.
Electrolyte composition EC / γ - butyrolactone = 3/7 (volume ratio)

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

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

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

  The results of the initial capacity and the recovery capacity after storage at 60 ° C. are summarized in Table 1.

  As is apparent from Table 1, in Examples 1 and 2, the initial capacity is equal to or greater than that in Comparative Examples 1 and 2, and the recovery capacity is improved to a level close to 100%.

This is because γ - butyrolactone and PVDF polymer are used as the gelled solid electrolyte constituent, and PVDF polymer is used on the positive electrode side and SBR and CMC are used on the negative electrode side as the electrode binder shown in Example 1. . This is an effect that appears only when the gelled solid electrolyte constituent and the binder coexist.

In this example, a gel-based solid electrolyte was used. However, if PVDF, SBR and γ - butyrolactone coexist, the same effect can be obtained in a conventional solution battery. In Example 1, it was confirmed that substantially the same effect was obtained when vinylethylene carbonate was used instead of vinylene carbonate.

Effect of the invention

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

Claims (5)

PVDF homopolymer synthesized by the emulsion polymerization method on the positive electrode side,
It has styrene butadiene rubber and carboxymethyl cellulose on the negative electrode side,
Further lithium ion secondary battery containing the salt with γ- Buchiro lactone lithium fluoride boron acid in the electrolyte.
Further containing a cyclic carbonate in the solvent of the electrolyte,
Cyclic carbonate: .gamma. Buchiro volume ratio of lactone, 6: 4 to 1: 9 The lithium ion secondary battery of claim 1 is.
The lithium ion secondary battery according to claim 1 or 2, wherein the PVDF has a molecular weight of 80,000 or more.
The electrolyte is a gelled solid electrolyte,
The PVDF homopolymer to the solid electrolyte, .gamma. Buchiro lactones, either the lithium ion secondary battery of claim 1, containing a salt of lithium fluoride boron acid.
  The lithium ion secondary battery according to any one of claims 1 to 4, wherein the electrolyte contains 1-2% by mass of vinylene carbonate or vinyl ethylene carbonate as an additive.