JP2009158496A - Lithium polymer battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the following problems in a lithium polymer battery provided by polymerization after impregnating a precursor of a high polymer solid electrolyte into a graphite negative electrode: an unreactive monomer remains since a radical generated in polymerization is consumed by graphite, so that a polymer battery cannot be produced which is superior in long-term reliability; and liquid leakage occurs in abnormal time since gel is liquefied at the high temperature in a polymer battery using physical cross-linked gel. <P>SOLUTION: The lithium polymer battery includes: a positive electrode; a negative electrode including a negative electrode active material composed of carbon material powder and a binder material; and an electrolyte using chemical cross-linked gel, wherein the carbon material powder is composed of a mixture of at least two or more of composite graphite material powders in which a surface of high crystalline graphite powder (A) is covered with a low crystalline carbon material (B). The graphite material powders have the mutually different covering ratio (an average value of quantity of B/(quantity of A + quantity of B)), and the binder material is a rubber-based binding material. By the lithium polymer battery, the battery superior in high energy density and long-term reliability (a cycle characteristic) can be provided. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium polymer battery. More specifically, the present invention relates to a lithium polymer battery using a negative electrode using a composite graphite material powder in which a low crystalline carbon material is adhered to the surface of a highly crystalline graphite powder as an active material.

  In recent years, instead of using metallic lithium or an alloy thereof for the negative electrode, a matrix material such as a carbon material or a conductive polymer using a lithium ion storage-release process has been developed. As a result, the generation of dendrite that occurs when metallic lithium or an alloy thereof is used does not occur in principle, and the problem of short circuit inside the battery is drastically reduced. In particular, it is known that a carbon material has a lithium occlusion-release potential closer to a lithium folding-dissolution potential than other materials. Above all, the graphite material is a carbon material having a high capacity per unit weight and unit volume because it can theoretically incorporate lithium into the crystal lattice at a ratio of one lithium atom to six carbon atoms. . Further, the graphite material has a flat lithium insertion-extraction potential, is chemically stable, and greatly contributes to the cycle stability of the battery.

For example, there is one using a surface-treated graphite-based carbon material as a negative electrode active material (see Non-Patent Document 1, Patent Documents 1 to 5).
By using an organic electrolyte mainly composed of ethylene carbonate (EC), the graphite-based carbon material can obtain a discharge capacity that is almost equal to the theoretical capacity. In addition, since the charge / discharge potential is slightly higher than the dissolution-folding potential of lithium and is very flat, when a battery is produced using a graphite-based carbon material as a negative electrode active material, A secondary battery with a high flatness of the battery voltage can be realized, and a high capacity of the battery can be achieved.

  However, the graphite-based carbon material has a problem of causing decomposition of the organic electrolyte because of its high crystallinity. For example, propylene carbonate (PC), which is a solvent for organic electrolytes, has a wide potential window, a low freezing point (−70 ° C.), and high chemical stability. Widely used as a solvent. However, when a graphite-based carbon material is used as the negative electrode active material, the decomposition reaction of PC is remarkable, and the negative electrode made of the graphite-based carbon material cannot be charged / discharged only by the presence of 10% PC in the electrolyte. Has been reported (see Non-Patent Document 2).

  As a solution, an additive such as vinylene carbonate is added to the electrolytic solution, and a film is positively formed on the graphite-based carbon material, which is the negative electrode active material. By covering the surface of the crystalline graphite carbon material with a low crystalline carbon material, the high capacity characteristics of the high crystalline graphite carbon material and the selectivity of the electrolyte, which is a characteristic of the low crystalline carbon material, can be achieved. There has been proposed a composite graphite material that has nothing.

In recent years, ion conductive polymers having high ion conductivity have been reported for the purpose of improving leakage resistance, high safety, and long-term storage stability of batteries using liquid electrolytes, and solve the above problems. Various researches are underway as one of the means. As ion-conducting polymers currently under investigation, homopolymers or copolymers of linear polymers, reticulated crosslinked polymers, comb polymers, etc. based on ethylene oxide have been proposed and are almost in practical use.
Batteries using the above ion-conducting polymers are widely described in patent documents and the like, for example, U.S. patents by Armand et al. (See U.S. Patent Document 6) and U.S. patents by North (Patent Documents). 7) and Hooper et al. US Patent (see Patent Document 8).

A feature of these is that an ion conductive polymer containing a solution in which an electrolyte salt is dissolved in a polymer material having a polyether structure is used. Since these ionic conductive polymers have low ionic conductivity at room temperature or lower, it is not possible to realize the small size, light weight, and high energy density required for batteries for driving power sources for mobile electronic devices and memory backup power sources.
Therefore, as a method for further improving the ion conductivity compared to the above-described ion conductive polymer, a monomer, an organic solvent (in particular, a high dielectric constant organic solvent such as EC or PC) and an electrolyte solution composed of an electrolyte salt are mixed. In addition, a gel-like polymer electrolyte (hereinafter referred to as a chemical cross-linking gel) has been proposed in which an electrolytic solution is held in a polymer network by polymerizing monomers to maintain a solid state.

JP-A-4-115457 JP-A-4-115458 JP-A-4-368778 Japanese Patent Laid-Open No. 5-28996 Japanese Patent Laid-Open No. 5-114421 U.S. Pat. No. 4,303,748 US Pat. No. 4,589,197 US Pat. No. 4,547,440 JP-T 8-507407

J. et al. Electrochm. Soc. , Vol. 137, 2009 (1990) J. et al. Electrochem. Soc. , Vol. 142, 1746 (1995)

Some chemical cross-linking gels have high electrolytic solution holding power and do not melt even at high temperatures (for example, about 100 ° C.), and are effective from the viewpoint of preventing liquid leakage in an abnormal state. However, there are still many problems to be solved, such as poor performance at low temperatures, and the initiator added in the precursor for crosslinking remains in the battery, causing deterioration of battery performance.
On the other hand, a gel electrolyte (hereinafter, referred to as a physical cross-linked gel) in which an electrolytic solution is held by physically cross-linking a hexafluoropropylene-polyvinylidene fluoride copolymer has been proposed (see Patent Document 9).

This type of gel is formed by injecting an electrolytic solution after forming a battery element, melting it once by heating, and forming a gel by cooling, so that a method for producing a lithium ion battery using a liquid electrolytic solution and It has the advantage that it can be manufactured by a similar method.
The physical cross-linking gel is in the form of a gel at room temperature and can be expected to have a certain degree of liquid leakage prevention effect. However, since the gel-like material melts at a high temperature and cannot maintain the gel state, it is not sufficient from the viewpoint of preventing liquid leakage when the battery is abnormal. Under such circumstances, there is a strong demand for a high-performance lithium polymer battery using a chemically cross-linked gel that can be sufficiently expected to prevent liquid leakage.

The object of the present invention is to provide high performance lithium having load characteristics, temperature characteristics, and energy density comparable to those of a lithium ion battery without causing leakage of the electrolyte even when the temperature of the battery rises when the battery is abnormal. It is to provide a polymer battery.
Thus, according to the present invention, a positive electrode, a negative electrode including a negative electrode active material made of carbon material powder and a binder, and an electrolyte using a chemical cross-linking gel, the carbon material powder is a highly crystalline graphite powder (A ) Of the composite graphite material powder of which at least two types of composite graphite material powders are coated with the low crystalline carbon material (B) and have different coverage ratios (average amount of B / (A amount + B amount)) There is provided a lithium polymer battery comprising a mixture, wherein the binder is a rubber-based binder.

According to the present invention, a lithium polymer battery having high energy density and excellent long-term reliability (cycle characteristics) can be obtained by using a negative electrode active material in which at least two types of composite graphite material powders are mixed.
Furthermore, since the lithium polymer battery provided by the present invention uses a chemically crosslinked gel, the gel does not dissolve and liquefy even when the battery is exposed to high temperatures. Therefore, it is possible to obtain a highly reliable battery that is unlikely to swell.
Furthermore, since the binder for the negative electrode of the lithium polymer battery provided by the present invention is a rubber-based binder, it can follow the expansion and contraction of the active material and suppress cycle deterioration of the negative electrode.
The feature of the lithium polymer battery is that it can be thinned and has a free shape, and can be used more effectively by being incorporated in a gap of an electronic device. For usage that cannot be easily replaced by the user, long-term reliability and the difficulty of battery expansion are very important, and this industrial significance is great.

It is sectional drawing which shows an example of the composite graphite material powder of this invention.

  The composite graphite material powder is excellent as a negative electrode active material, but the low crystallinity carbon material on the surface hinders the lubricity conventionally possessed by high crystallinity graphite powder (hereinafter also referred to as graphite powder) and has poor packing properties. In some cases, a high-density negative electrode cannot be obtained. The inventors of the present invention examined the filling property in detail, and as a result, it was found that the filling property greatly depends on the physical properties of the composite graphite material powder such as the coating ratio or the type of the core material.

  Furthermore, from the above viewpoint, the inventors of the present invention can efficiently crosslink when curing the precursor of the chemically crosslinked gel by using two or more kinds of composite graphite material powders having different physical property values as the negative electrode active material. I also found out that it would be done. The reason why the crosslinking is efficiently performed is assumed as follows. The graphite powder consumes radicals generated from the initiator by means of heat, UV, or the like at its active site and inhibits the polymerization reaction. By coating the surface with a low crystalline carbon material, it is considered that the consumption of radicals by the graphite powder is suppressed, and the generated radicals are used efficiently in the polymerization reaction.

  In the present invention, the inclusion of at least two types of composite graphite material powders having different physical property values means that at least two types of composite graphite material powder as a main active material and composite graphite material powder for improving the filling properties thereof. Is included. With such a negative electrode including at least two types of composite graphite material powder, a high-performance secondary battery having a high capacity density and excellent load characteristics can be produced.

  The composite graphite material powder is an aggregate of powder. Strictly speaking, it is difficult to obtain a completely uniform powder aggregate. Therefore, the physical property value of each particle and the physical property value (average value) ). In the present invention, the term “two or more types” means that there are two or more types of composite graphite material powders having different physical properties as the latter aggregate. Here, the physical property values include crystallinity (plane spacing, crystallite size, etc.), specific surface area, particle size distribution, coating ratio, Raman intensity ratio, true density, bulk density, purity, shape, and the like.

In the present invention, it is preferable to use two or more composite graphite material powders having different coating ratios. Specifically, different coating ratios mean that the amount of low crystalline carbon material / (the amount of high crystalline graphite powder + the amount of low crystalline carbon material) (hereinafter referred to as the coating ratio) is different. Means. By making the covering ratios different, a secondary battery with higher capacity density and higher performance can be produced.
The covering ratio means a value as an aggregate of composite graphite material powder, that is, an average value, not a value for each particle. This value is calculated from the weight of the graphite powder in the manufacturing process and the weight change of the finally obtained composite graphite material powder.

The covering ratio of the composite graphite material powder having the larger covering ratio is preferably in the range of 0.03 to 0.25, more preferably 0.1 to 0.25. When the covering ratio is larger than 0.25, the low crystal portion is excessively increased, and the charge / discharge capacity is lowered, which is not preferable. Moreover, since the control effect with respect to decomposition | disassembly of electrolyte solution will become low when it becomes less than 0.03, it is unpreferable.
The coverage ratio of the composite graphite material powder having the smaller coverage ratio is preferably 0.01 to 0.10. More preferably, it is 0.01-0.05. If the material is less than 0.01, it is not sufficient to suppress the above-described polymerization inhibition factors, and a coating ratio greater than 0.1 is not preferable because it is difficult to obtain a negative electrode with excellent performance and high packing density.

  Furthermore, the amount of the composite graphite material powder having a small coating ratio relative to the amount of the composite graphite material powder having a large coating ratio is preferably 50% or less by weight, and more preferably about 10 to 30%. If the amount added is too large, the properties of the material will dominate, and the effect of suppressing the factors that inhibit the polymerization will not be sufficiently obtained, or the material itself will be oriented, so that the penetration of the electrolyte will deteriorate. , Load characteristics, etc. may deteriorate. Further, if the amount to be added is not sufficient, there arises a problem that the effect of improving the filling property cannot be obtained sufficiently.

  Moreover, it is also possible to obtain a battery having a high capacity density and a high performance by changing the kind of graphite powder instead of changing the covering ratio. This is the case where two or more types of graphite powders with different crystallinity evaluations defined by X-ray diffraction, Raman spectroscopy, true density, etc., are used to coat a low crystalline carbon material with an equivalent coating ratio. However, it became clear that the filling properties differ depending on the type of graphite powder. This is because the difference in filling properties is caused by the difference in the fine structure alignment, the shape of the composite graphite material powder derived from the shape of the core material, the balance of particle diameters, etc., which cannot be judged by the evaluation means such as the X-ray diffraction method. Inferred.

The coating ratio when the graphite powder is different is not particularly limited, but in order to further improve the filling property, the coating ratio of the composite graphite material powder having a large coating ratio is preferably 0.03 to 0.25. The covering ratio of the small composite graphite material powder is preferably 0.01 to 0.10, more preferably 0.01 to 0.05.
Further, the amount of the composite graphite material powder having a small coating ratio relative to the amount of the composite graphite material powder having a large coating ratio is preferably 50% or less by weight, and more preferably about 10 to 30%. If the amount added is too large, the properties of the material will dominate, and the effect of suppressing the factors that inhibit the polymerization will not be sufficiently obtained, or the material itself will be oriented, so that the penetration of the electrolyte will deteriorate. , Load characteristics, etc. may deteriorate. Further, if the amount to be added is not sufficient, there arises a problem that the effect of improving the filling property cannot be obtained sufficiently.

In the present invention, good results are obtained when massive artificial graphite is used as the graphite powder of the composite graphite material powder having a large coating ratio and natural graphite powder is used as the graphite powder of the composite graphite material powder having a low coating ratio. ing.
In the present invention, a parameter called a compression ratio is defined as an index indicating the degree of filling property, and the physical properties of the composite graphite material powder are defined. Here, the compression ratio means that 7.5 parts by weight of PVDF (polyvinylidene fluoride) as a binder is added to the target composite graphite material powder and mixed with a solvent. The slurry obtained in this way is applied on a copper foil and means a change in thickness when pressed at a linear pressure of 300 kg / cm. Specifically,
Compression ratio = defined by the thickness of the coating film after pressing / the thickness of the coating film before pressing.

  Furthermore, the coating film density when the coating film is produced is also important. That is, the higher the coating film density before pressing and the smaller the compression ratio, the higher the density of the negative electrode can be obtained. Even if the compression ratio is sufficiently small, if the coating film density before pressing is too small, the orientation of the graphite powder tends to occur when pressing to obtain a high-density negative electrode, resulting in a negative electrode with poor load characteristics. There is a case. Moreover, even if the coating film density before pressing is sufficiently high, a high energy density negative electrode cannot be obtained if the compression ratio is large, which is not preferable.

In the present invention, the coating density before pressing is 0.7 g / cm ³ or more, the compression ratio is 0.4 to 0.7, and the coating density after pressing is 1.5 g / cm ^3. In order to achieve the above, it is preferable to mix two or more kinds of composite graphite material powder. The higher the coating film density after pressing, the higher the capacity of the negative electrode is obtained, but the high density negative electrode obtained when the coating density before pressing is low and the compression ratio is small is May be worse, and is not preferable. Moreover, it is preferable that the coating film density after pressing is suppressed to about 1.8 g / cm ³ . If it is higher than this, since the voids in the negative electrode are not sufficient, the amount of electrolyte in the negative electrode is not sufficient, and battery characteristics such as load characteristics may not be sufficiently obtained.

The composite graphite material powder in the present invention is coated with the low crystalline carbon material 2 on the surface of the high crystalline graphite powder 1 as shown in FIG. 1 by means of a gas phase method, a liquid phase method, a solid phase method, or the like. Can be obtained.
Graphite powder used for the core material is natural graphite, particulate (scalar or lump, fiber, whisker, spherical, crushed, etc.) artificial graphite, or mesocarbon microbeads, mesophase pitch powder, isotropic pitch One or more of graphitized products such as powder can be used.

Here, the graphite powder as the core material is more preferably an average interplanar spacing (d002) of (002) plane of 0.335 to 0.340 nm and a crystallite thickness in the (002) plane direction by X-ray wide angle diffraction method. (Lc) is 10 nm or more (more preferably, 40 nm or more), (110) plane crystallite thickness (La) is 10 nm or more (more preferably, 50 nm or more), and a peak near 1580 cm ⁻¹ by argon laser Raman. It is preferable that the peak intensity ratio (hereinafter referred to as R value) near 1360 cm ^{−1 with} respect to the intensity ratio is 0.5 or less (more preferably 0.4 or less). When the average interplanar spacing is larger than 0.340 nm, or when Lc and La are smaller than 10 nm, or when the R value exceeds 0.5, the crystallinity of the graphite powder is not sufficient, and a composite graphite material powder is produced. In this case, the capacity of the low potential portion (0 to 300 mV on the basis of the Li potential) close to the dissolution and precipitation of lithium is not sufficient, which is not preferable.

The particle size distribution of the graphite powder as the core material is preferably about 0.1 to 150 μm. The particle size of the composite graphite material powder whose surface of the graphite powder is coated with the low crystalline carbon material substantially depends on the particle size of the graphite powder that is the core material. The particle size of the film is also almost specified. When the particle size of the core material is smaller than 0.1 μm, the risk of causing an internal short circuit through the pores of the battery separator increases, whereas when larger than 150 μm, the uniformity of the negative electrode Further, since the packing density of the active material, the handling property in the process of producing the negative electrode, and the like are deteriorated, neither is preferable.
The particle size here is an average value of the powder, and a value indicating a peak in the particle size distribution measured by a laser diffraction particle size distribution meter is defined as the particle size.

In the method of forming a low crystalline carbon material on the surface of the graphite powder, the vapor phase method is a method of spraying gaseous raw material or liquid raw material or transporting it into the reaction system by a method such as bubbling, In this method, carbon is formed from the gas phase on the surface of the graphite powder by pyrolysis of the raw material. Although it changes with raw materials as a thermal decomposition temperature, it can carry out in the temperature range of about 450-1500 degreeC.
Examples of the raw material include aliphatic saturated hydrocarbons such as methane, ethane, and propane, aliphatic unsaturated hydrocarbons such as propylene, and aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene, and perylene. In addition, an inert gas such as argon or nitrogen can be used as a carrier gas as appropriate, and a method of adding hydrogen to suppress the generation of soot in the gas phase is also conceivable.

  In the method of forming a low crystalline carbon material on the surface of the graphite powder, the liquid phase method is a method in which a carbon precursor is carbonized via a liquid phase, attached to the surface of the graphite, and fired. To form carbon on the surface. Raw materials include aromatic hydrocarbons such as naphthalene, phenanthrene, acenaphthylene, anthracene, triphenylene, pyrene, chrysene, and perylene, tars or pitches obtained by polycondensation of these under heat and pressure, or aromatic hydrocarbons thereof. Tar, pitch, asphalt, and oils mainly composed of a mixture of the above are available, regardless of whether they are petroleum-based or coal-based.

  Prior to firing, graphite powder coated with a carbon precursor may be subjected to a washing step. By adding a washing step, low molecular components of the carbon precursor can be removed, the carbonization rate from the carbon precursor can be improved, and particles can be prevented from being fused or aggregated during firing. An effect is obtained. Here, examples of the organic solvent used for washing include toluene, quinoline, acetone, hexane, benzene, xylene, methylnaphthalene, alcohols, coal oil, and petroleum oil. Among these, toluene, quinoline, acetone, benzene, xylene, methanol, coal-based light oil / middle oil, petroleum-based light oil / middle oil, and the like are more preferable.

  In the method of forming a low crystalline carbon material on the surface of the above graphite powder, the solid phase method is a method in which a raw material that is carbonized via a solid phase is attached to the surface of the graphite, and these are fired. This is a method of forming carbon on the surface. In general, the carbonization of the resin proceeds via a solid phase, but in order to attach such a resin to the surface of the graphite powder, the resin is dissolved in a solvent, heated to a temperature higher than the melting point, etc. The method of mixing by the method described in description of the said liquid phase method, and making it adhere to the surface is mentioned. It is also possible to mix the resin and graphite powder by mixing them and holding them near the melting point during firing.

  Specific raw materials include: polyamideimide resin; polyamide resin; conjugated resins such as polyacetylene, poly (p-phenylene), poly (p-phenylene vinylene), phenol resin; furfuryl alcohol resin; cellulose; polyacrylonitrile, poly Acrylic resins such as (α-halogenated acrylonitrile); halogenated vinyl resins such as polyvinyl chloride, polyvinylidene chloride, and chlorinated polyvinyl chloride; and the like. As the firing conditions, the firing method and firing atmosphere described in the above liquid layer method can be applied.

  Further, the low crystalline carbon material obtained by the above method is more preferably a crystallite in the (002) plane direction having an average (002) plane spacing (d002) of more than 340 nm by the X-ray wide angle diffraction method. The thickness (Lc) is smaller than 40 nm (more preferably smaller than 10 nm), the crystallite thickness (La) in the (110) plane direction is smaller than 50 nm (more preferably smaller than 10 nm), and the R value is 0.4. It is preferably larger (more preferably larger than 0.5).

  A negative electrode is formed by mixing 1 to 30 parts by weight of a binder with respect to 100 parts by weight of the active material. As the binder, fluorine-based polymers such as polytetrafluoroethylene and polyvinylidene fluoride, polyolefin-based polymers such as polyethylene and polypropylene, synthetic rubbers and the like can be used, but are not limited thereto. If the binder is more than 30 parts by weight, the resistance or polarization of the negative electrode increases and the discharge capacity decreases, making it impossible to produce a practical lithium polymer battery. On the other hand, when the amount of the binder is less than 1 part by weight, the binding ability is lost, which is not practical. In the preparation of the negative electrode, heat treatment can be performed at a temperature around the melting point of the binder in order to increase the binding property.

  In order to obtain a negative electrode having the compression ratio, it is preferable to use synthetic rubbers as the binder among the binders. A negative electrode having the above compression ratio and having a physical property of a coating film density after pressing of 1.5 g / cc or more may easily undergo cycle deterioration due to repeated expansion and contraction of the negative electrode accompanying charging and discharging. Since the rubber-based binder has strong binding ability and rubber elasticity, it is considered that it can follow the expansion and contraction of the active material and suppress cycle deterioration of the negative electrode.

  The polymerizable monomer used as the raw material for the chemically crosslinked gel in the present invention is not particularly limited as long as it is a compound having an affinity for an electrolyte solvent solution and having a polymerizable functional group. For example, the thing which has a polyether structure and an unsaturated double bond group, the polymeric monomer which gives polymers, such as oligoester acrylate, polyester, polyimine, polythioether, polysulfane, is used individually or in combination of 2 or more types. In addition, those having a polyether structure and an unsaturated double bond group are preferred from the viewpoint of affinity with a solvent. Examples of the polyether structural unit include ethylene oxide, propylene oxide, butylene oxide, glycidyl ethers and the like, and these are used alone or in combination of two or more. In the case of a combination of two or more, the form can be selected as appropriate regardless of whether it is a block or random. Among them, the use of a polymerizable monomer composed of a polyfunctional monomer and a monofunctional monomer as an acrylate-based material is preferable because it gives a gel having strength and elasticity to follow the volume change of the negative electrode.

  As the acrylate monomer, one obtained by esterifying the terminal hydroxyl group of polyether polyol with acrylic acid can be suitably used. The polyfunctional monomer is a polyhydric alcohol such as ethylene glycol, glycerin, or trimethylolpropane as an initiator, and the end of polyether polyol obtained by addition polymerization of ethylene oxide (EO) alone or EO and propylene oxide (PO). It is obtained by esterifying a hydroxyl group with acrylic acid. The monofunctional monomer is a terminal end of a polyether polyol obtained by addition polymerization of ethylene oxide (EO) alone or EO and propylene oxide (PO) with a monohydric alcohol such as methanol, ethanol or propanol as an initiator. It is obtained by esterifying a hydroxyl group with acrylic acid.

Here, the polyfunctional monomer shows an important role in the liquid retention of the electrolyte solution of the gel electrolyte, and the average molecular weight is more preferably in the range of 5,000 to 10,000. When a polyfunctional monomer within the above range is employed, it can be easily dissolved in the electrolyte when adjusting the precursor, and has excellent liquid retention when preparing the gel electrolyte.
On the other hand, the monofunctional monomer having a smaller average molecular weight can improve the flexibility of the gel electrolyte, and those having an average molecular weight of about 200 to 3,000 are preferably used.

  Examples of the electrolyte used for the electrolyte include cyclic carbonates such as PC, EC, and butylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate, γ-butyrolactone, and γ-valero. Lactones such as lactone, furans such as tetrahydrofuran and 2-methyltetrahydrofuran, ethers such as diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, dioxane, dimethyl sulfoxide, sulfolane, Examples thereof include methyl sulfolane, acetonitrile, methyl formate, methyl acetate, and the like.

As an electrolyte salt, lithium perchlorate (LiClO _4), lithium borofluoride (LiBF _4), Rinfu' lithium (LiPF _6), 6 fluoride arsenate lithium (LiAsF _6), 6 lithium fluoride antimonate (LiSbF _6), Examples thereof include lithium salts such as lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium trifluoroacetate (LiCF ₃ COO), lithium trifluoromethanesulfonate (LiN (CF ₃ SO ₂ ) ₂ ), and the like. The above can be mixed and used.
The electrolyte is obtained by preparing an electrolytic solution by dissolving an electrolyte salt in the solvent selected above, mixing with the polymerizable monomer, and polymerizing.

The ratio of the polymerizable monomer to the electrolytic solution is preferably from 70:30 to 99: 1 of the electrolytic solution: monomer from the viewpoint of performance comparable to the electrolytic solution and the retention of the electrolytic solution that does not cause liquid leakage. Preferably it is 80: 20-97: 3.
Moreover, when mixing and using a monofunctional monomer and a polyfunctional monomer, it is preferable that the mixing ratio of a monomer mixes in the range of polyfunctional monomer: monofunctional monomer 4: 6-9: 1.

  As a method for crosslinking the polymer solid electrolyte, a method using light energy such as ultraviolet rays, electron beams, or visible light, or a method using heating can be used. It is also important to use a polymerization initiator if necessary. Particularly in the crosslinking method by ultraviolet rays or heating, it is preferable to add a polymerization initiator of several percent or less. As the polymerization initiator, photopolymerization initiators such as trimethylsilylbenzophenone, benzoin, 2-methylbenzoin, 4-methoxybenzophenone, benzoin methyl ether anthraquinone, benzyldimethyl ketal, benzoyl peroxide, methyl ethyl ketone peroxide, α, α′- A polymerization initiator such as azobisisobutyronitrile may be added.

  In addition, the wavelength of ultraviolet polymerization and ultraviolet light is suitably 250 to 360 nm. Even when an initiator is used, according to the present invention, it is possible to obtain a good polymer with a small amount of initiator. Residual polymerization initiators and by-products from the polymerization initiators may adversely affect battery characteristics, and are preferably kept to the minimum necessary. Although it depends on the type of the initiator, the amount of the initiator is usually preferably suppressed to 3000 ppm or less with respect to the precursor composed of the polymerizable monomer and the electrolytic solution.

As the positive electrode in the lithium polymer secondary battery of the present invention, for example, an oxide containing lithium can be used as the positive electrode active material. Specific examples include LiCoO ₂ , LiNiO ₂ , LiFeO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , or materials obtained by substituting a part of their transition metals, and the like. In some cases, it is formed by mixing a solid electrolyte or the like.
The mixing ratio can be 5 to 50 parts by weight of the conductive material and 1 to 30 parts by weight of the binder with respect to 100 parts by weight of the active material. As the conductive material, carbons such as carbon black (acetylene black, thermal black, channel black, etc.), graphite powder, metal powder, and the like can be used, but are not limited thereto.

As the binder, fluorine-based polymers such as polytetrafluoroethylene and polyvinylidene fluoride, polyolefin-based polymers such as polyethylene and polypropylene, synthetic rubbers and the like can be used, but are not limited thereto.
When the conductive material is less than 5 parts by weight or the binder is more than 30 parts by weight, the resistance or polarization of the positive electrode increases and the discharge capacity decreases, so that a practical lithium polymer battery cannot be produced. If the conductive material is more than 50 parts by weight (the weight part varies depending on the type of conductive material to be mixed), the amount of the active material contained in the positive electrode is reduced, so that the discharge capacity as the positive electrode is reduced. When the binder is less than 1 part by weight, the binding ability is lost. When the binder is more than 30 parts by weight, the amount of the active material contained in the positive electrode is reduced as in the case of the conductive material. As described above, since the resistance or polarization of the positive electrode increases and the discharge capacity decreases, it is not practical. In the production of the positive electrode, it is preferable to perform a heat treatment at a temperature around the melting point of each binder in order to improve the binding property.

Example 1 and Comparative Example 1
In order to see the effect of the solid polymer electrolyte on the curing of the precursor, a curing experiment of the precursor was performed in a state where the graphite material powder was mixed.
Preparation of precursor A 1: 1 mixed solvent of EC and γ-butyrolactone in which 1 mol / liter of LiBF ₄ was dissolved was used as an electrolytic solution. A tetrafunctional acrylate monomer having an average molecular weight of 7500 to 9000 comprising a copolymer of ethylene oxide and propylene oxide and a monofunctional acrylate monomer having an average molecular weight of 200 to 300 is converted to a monomer having a weight ratio of 7: 3. Was mixed with the electrolytic solution so that the ratio of the ratio was 95: 5. A thermal polymerization initiator was added to the solution in an amount of 200 ppm based on the total weight to prepare a precursor. The thermal polymerization initiator used is t-butyl peroxyneodecanate.

Both were mixed so that the weight ratio of the graphite powder and the precursor was 1:10, and kept at 80 ° C. for 24 hours to confirm the curing state of the precursor.
The amount of low crystalline carbon material in the composite graphite material powder coated with low crystalline carbon material with artificial graphite (KS25) made by Timcal as the core as graphite powder / (amount of high crystalline graphite powder + low crystallinity) A curing experiment was carried out using a carbon material whose average value was changed (Example 1). Further, Comparative Example 1 was defined as a case where it was not coated with a low crystalline carbon material. The results are shown in Table 1. In Table 1, ◎ means that it is sufficiently cured and has no liquid part, ○ means that the gel is slightly soft but no liquid part remains, and × does not sufficiently cure, It means a situation where a liquid part is observed.

From Table 1, the larger the coating ratio, the better the curing situation.
Similarly, when the amount of the initiator was changed and the experiment was conducted, the tendency of the degree of curing was the same even when the amount was changed, and the larger the covering ratio, the better the curing.
In addition, when the initiator was increased to about 5000 ppm with respect to the total amount of the precursor, a certain degree of curing was obtained with only graphite powder. Therefore, it was found that the composite graphite material powder was cured regardless of the initiator concentration, and in particular, the effect was greater than the case of using only the core material in the region where the initiator was 5000 ppm or less.

Example 2
(Preparation of positive electrode)
Lithium cobaltate (LiCoO ₂ ) was used as the positive electrode active material. A paste was prepared by dispersing a mixture of the positive electrode active material and acetylene black in a binder solution in which polyvinylidene fluoride as a binder was once dissolved in a solvent N-methyl-2-pyrrolidone in a mortar.
The paste thus obtained was applied onto an aluminum foil current collector, which was temporarily dried at 60 ° C., heat treated at 150 ° C. and pressed. The negative electrode size was 3.5 × 3 cm (coated portion 3 × 3 cm), and an aluminum foil (50 μm) was welded to the uncoated portion. Furthermore, what was dried under reduced pressure at 180 ° C. for water removal was used as a positive electrode for testing. The coating film density was 2.9 g / cm ³ .

(Preparation of negative electrode)
Composite graphite material powder (particle size 12 μm, d (002) = 0.337 nm, R value =) using artificial graphite (KS25) manufactured by Timcal Co. as a negative electrode active material as a core material and having a surface coated with a low crystalline carbon material 0.4, amount of low crystalline carbon material / (amount of high crystalline graphite powder + amount of low crystalline carbon material) = 0.18) and natural graphite (made in Madagascar) as a core material Composite graphite material powder whose surface is coated with low crystalline carbon (particle size 16 μm, d (002) = 0.336 nm, R value = 0.21, amount of low crystalline carbon material / (high crystalline graphite powder (Amount + amount of low crystalline carbon material) = 0.05) was used as a negative electrode active material and a mixture of polyvinylidene fluoride as a binder was dissolved in a solvent N-methyl-2-pyrrolidone in a mortar. Disperse in a solution and make a paste, 20 μm And coating a copper foil, which preliminarily dried at 60 ° C., and pressed after heat treatment at 240 ° C.. The negative electrode size was 3.5 × 3 cm (coated portion 3 × 3 cm), and a nickel foil (50 μm) lead was welded to the uncoated portion. Furthermore, what was vacuum-dried at 150 degreeC for water removal was used as a negative electrode. The coating film density was 1.58 g / cm ³ .

(Precursor preparation)
A 1: 1 mixed solvent of EC and γ-butyrolactone in which 1.8 mol / liter of LiBF ₄ was dissolved was used as an electrolytic solution. A tetrafunctional acrylate monomer having an average molecular weight of 7500 to 9000 comprising a copolymer of ethylene oxide and propylene oxide and a monofunctional acrylate monomer having an average molecular weight of 200 to 300 is converted to a monomer having a weight ratio of 7: 3. Was mixed with the electrolyte solution so that the ratio of the ratio was 97: 3. A thermal polymerization initiator was added to the solution in an amount of 200 ppm based on the total weight to prepare a precursor.

(Production of battery)
A polyester non-woven fabric was used as a separator, and the electrodes obtained as described above were stacked so as to face each other and inserted into a bag of aluminum laminate processed into a bag shape. The above precursor was put therein, and heat sealing was performed under reduced pressure. Next, it was held at 80 ° C. for 4 hours, and thermal polymerization was performed to produce a polymer battery. The obtained battery was repeatedly charged and discharged under the conditions of a current value of 4 mA, a charge of 4.1 V-CCCV (constant current voltage), and a discharge of 2.75 V-CC (constant current), and cycle characteristics were measured. Table 2 shows the materials used, mixing ratios, etc. Table 3 shows the coating density (before pressing), compression ratio, coating density (after pressing), volumetric energy density per negative electrode active material layer, and capacity retention during cycling. Shown in

Comparative Example 2
Composite graphite material powder (particle size 12 μm, d (002) = 0.337 nm, R value) using artificial graphite (KS25) manufactured by Timcal as a core material as a negative electrode active material and having a surface coated with a low crystalline carbon material = 0.4, amount of low crystalline carbon material / (amount of high crystalline graphite powder + amount of low crystalline carbon material) = 0.18) was used in the same way as in Example 2 except that Was made. The coating density of the obtained negative electrode was 1.46 g / cm ³ . A battery was prepared in the same manner as in Example 2 except that the negative electrode thus obtained was used, and a charge / discharge test was performed. Table 2 shows the materials used, mixing ratios, etc. Table 3 shows the coating density (before pressing), compression ratio, coating density (after pressing), volumetric energy density per negative electrode active material layer, and capacity retention during cycling. Shown in

Comparative Example 3
Composite graphite material powder (particle size 12 μm, d (002) = 0.337 nm, R value) using artificial graphite (KS25) manufactured by Timcal Co. as a negative electrode active material as a negative electrode active material and having a surface coated with a low crystalline carbon material = 0.4, amount of low crystalline carbon material / (amount of high crystalline graphite powder + amount of low crystalline carbon material) = 0.18), natural graphite (made in Madagascar, particle size 14 μm) , D (002) = 0.3358 nm, R value = 0.1) was used in the same manner as in Example 2 except that a mixture with 20 parts by weight was used. The coating density of the obtained negative electrode was 1.65 g / cm ³ . A battery was prepared in the same manner as in Example 2 except that the negative electrode thus obtained was used, and a charge / discharge test was performed. Table 2 shows the materials used, mixing ratios, etc. Table 3 shows the coating density (before pressing), compression ratio, coating density (after pressing), volumetric energy density per negative electrode active material layer, and capacity retention during cycling. Shown in

Examples 3-5, Comparative Examples 4-5
A negative electrode and a battery were produced in the same manner as in Example 2 except that the graphite material powder shown in Table 2 was used as the carbon material as the active material. Table 3 shows the results (coating density (before pressing), compression ratio, coating density (after pressing), volumetric energy density per negative electrode active material layer, capacity retention rate during cycling). In Table 2, MCMB is an abbreviation for mesocarbon microbeads, which are pitch-based spherical carbon materials.

  As the above results show, according to the example, a lithium polymer battery having a high capacity density and excellent cycle characteristics can be obtained. When the composite graphite material powder is used alone (Comparative Examples 2 and 5), the capacity density of the electrode is low, and when natural graphite is used as a filling aid (Comparative Example 3), sufficient cycle characteristics are obtained. There wasn't. This is because when the composite graphite is used alone, the pressability is low, and therefore the density does not increase. When natural graphite is used as a filling aid, the natural graphite is obtained from the results of Example 1. It is assumed that the polymerization of the precursor is inhibited by graphite, and the unreacted monomer remaining in the battery has an adverse effect.

Example 6
It produced similarly to Example 2 except having used the composite graphite material powder shown in Table 4 for the positive electrode and the negative electrode.
(Adjusting the precursor for the positive electrode)
A 1: 2: 1 mixed solvent of EC, γ-butyrolactone, and PC in which 2.5 mol / liter LiBF ₄ was dissolved was used as an electrolyte. A tetrafunctional acrylate monomer having an average molecular weight of 7500 to 9000 comprising a copolymer of ethylene oxide and propylene oxide and a monofunctional acrylate monomer having an average molecular weight of 200 to 300 is a monomer having a weight ratio of 9: 1. Was mixed with the electrolyte solution so that the ratio of the ratio was 97: 3. 2000 ppm of UV initiator was added to the solution with respect to the total weight, and it was set as the precursor.

(Adjusting the precursor for the negative electrode)
A 1: 2: 1 mixed solvent of ethylene carbonate (EC), γ-butyrolactone, and propylene carbonate (PC) in which 1 mol / liter of LiBF ₄ was dissolved was used as an electrolytic solution. A tetrafunctional acrylate monomer having an average molecular weight of 7500 to 9000 comprising a copolymer of ethylene oxide and propylene oxide and a monofunctional acrylate monomer having an average molecular weight of 200 to 300 is a monomer having a weight ratio of 8: 2, electrolyte solution and monomer Was mixed with the electrolytic solution so that the ratio of the ratio was 95: 5. 2000 ppm of UV initiator was added to the solution with respect to the total weight, and it was set as the precursor.

(Production of battery)
A polyester non-woven fabric is laminated on the positive electrode as a separator, impregnated with a precursor for the positive electrode, then sandwiched between quartz plates, irradiated with 30 mW / cm ² UV light for 30 s, and the separator, the positive electrode and the electrolyte layer are integrated. did.
The negative electrode was impregnated with a precursor for the negative electrode, and was similarly irradiated with UV light, so that the negative electrode and the electrolyte layer were integrated. The obtained electrode was bonded so that the active material layers were opposed to each other, inserted into a bag-shaped aluminum laminate, and heat sealed under reduced pressure to produce a lithium polymer battery. The obtained battery was repeatedly charged and discharged under the conditions of a current value of 4 mA, a charge of 4.1 V-CCCV, and a discharge of 2.75 V-CC, and cycle characteristics were measured. Table 4 shows the materials used, mixing ratios, etc. Table 5 shows the coating density (before pressing), compression ratio, coating density (after pressing), volumetric energy density per negative electrode active material layer, and capacity retention during cycling. Shown in

Example 7
A lithium polymer battery was prepared and evaluated in the same manner as in Example 6 except that SBR (styrene butadiene rubber): 2 parts and CMC-NH4: 2 parts were used as the negative electrode binder, and water was used as the dispersion medium. Went. Table 4 shows the mixing ratio and the like, and Table 5 shows the coating density (before pressing), the compression ratio, the coating density (after pressing), the volume energy density per negative electrode active material layer, and the capacity retention during cycling.
Comparative Example 6
A polymer battery was produced in the same manner as in Example 6 except that the composite graphite material powder shown in Table 4 was used. The results are shown in Table 5.

  From the above results, it can be seen that only the composite graphite material powder is different and the cycle characteristics are greatly different. This is presumably because in the composite graphite material powder of the comparative example, radicals generated by the initiator were consumed by the graphite powder, and a large amount of unreacted monomer remained in the negative electrode. Furthermore, it can be seen that when SBR (styrene butadiene rubber) is used as the binder for the negative electrode, better performance can be obtained.

1 High crystalline graphite powder 2 Low crystalline carbon material

Claims (3)

  A positive electrode, a negative electrode including a negative electrode active material made of carbon material powder and a binder, and an electrolyte using a chemically cross-linked gel, wherein the surface of the high crystalline graphite powder (A) has low crystallinity The binder is a mixture of at least two types of composite graphite material powders coated with the carbon material (B) and having different coating ratios (B amount / (A amount + B amount) average value). A lithium polymer battery characterized in that the material is a rubber-based binder.   The lithium polymer battery according to claim 1, wherein the negative electrode includes 1 to 30 parts by weight of the binder with respect to 100 parts by weight of the negative electrode active material. Coating said negative electrode, said negative electrode active material and the coating film after the pressing obtained coating film was pressed comprising said binder has a density of less than 1.5 g / cm ³ or more 1.8 g / cm ³ The lithium polymer battery according to claim 1, comprising a film.

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