JP2006310232A - Battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-capacity battery excelling in battery characteristics such as charge-discharge efficiency or a low-temperature characteristic. <P>SOLUTION: A positive electrode 21 and a negative electrode 22 are stacked by interposing electrolyte layers 24. The negative electrode 22 contains a negative electrode active material storing and releasing lithium. The negative electrode active material is so structured that at least a part of graphite having a real density not smaller than 2.19 g/cm<SP>3</SP>is coated with a polymer compound such as sodium glutamate including a structure represented by C=O or C-O. The electrolyte layer 24 contains a solvent, an electrolyte salt and a polymer compound, and is levigated. The solvent contains 30-70 mass% of propylene carbonate. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a battery using a negative electrode containing graphite and an electrolytic solution containing propylene carbonate.

  In recent years, mobile electronic devices such as mobile phones, PDAs (personal digital assistants) or notebook computers have been energetically reduced in size and weight, and as part of these efforts, Improvement of the energy density of a battery as a driving power source, particularly a secondary battery is strongly desired. As a secondary battery capable of obtaining a high energy density, for example, a lithium ion secondary battery is known.

In this lithium ion secondary battery, for example, a material using a carbon material such as graphite capable of inserting and extracting lithium (Li) in the negative electrode and a carbonate ester such as propylene carbonate in the electrolytic solution has been put into practical use. ing. However, since propylene carbonate is easily decomposed at the negative electrode, it has been studied to suppress the reaction by adding various additives to the electrolytic solution. Examples of the additive include 1,3-dioxol-2-one and 4-vinyl-1,3-dioxolan-2-one (see, for example, Patent Documents 1 to 3). It is considered that a film is formed on the surface to suppress the decomposition reaction of the electrolytic solution.
JP-A-8-45545 Japanese Patent Laid-Open No. 11-185806 JP-A-7-122296

  By the way, it is known that when the content of propylene carbonate in the electrolytic solution is increased, battery characteristics such as low-temperature characteristics and load characteristics are improved. It is necessary to increase the content of the electrolyte, the viscosity of the electrolytic solution increases, and the low-temperature characteristics and load characteristics decrease. Therefore, there has been a problem that the amount of propylene carbonate cannot be increased.

  Further, for example, when graphite having a high true density is used in order to increase the capacity, there is a problem that such graphite cannot be used because propylene carbonate is more easily decomposed.

  Furthermore, 1,3-dioxol-2-one has a reaction potential (reduction potential) close to that of propylene carbonate, and due to kinetic factors, the decomposition reaction of propylene carbonate at the first charge / discharge cannot be sufficiently suppressed, There was a problem that the initial charge / discharge efficiency was lowered.

  Furthermore, it is known to use a low viscosity solvent such as dimethyl carbonate, ethyl methyl carbonate or diethyl carbonate for the purpose of improving low temperature characteristics. There was also a problem that the low-viscosity solvent decomposed during storage and the battery swelled.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a battery that has a high capacity and can improve battery characteristics such as charge / discharge efficiency or low-temperature characteristics.

The battery of the present invention is provided with an electrolyte together with a positive electrode and a negative electrode. The negative electrode has a core part made of graphite having a true density of 2.19 g / cm ³ or more and at least a part of the surface of the core part. A negative electrode active material having a covering portion formed of a polymer compound including a structure represented by C═O or C—O, wherein the electrolyte is 30% by mass or more and 70% by mass of propylene carbonate. % Containing the solvent contained in the range of below.

  According to the battery of the present invention, from a polymer compound including a core portion made of graphite and at least a part of the surface of the core material and having a structure represented by C═O or C—O. Since the negative electrode active material having the coating portion to be used is used, the decomposition reaction of propylene carbonate can be suppressed and the ion conductivity can be improved. Therefore, the capacity can be increased by using graphite having a high true density, the content of propylene carbonate can be increased, and battery characteristics such as excellent initial charge / discharge efficiency or low temperature characteristics can be obtained.

  In addition, the ratio of the covering portion to the core portion (covering portion / core portion) is in the range of 0.1% by mass or more and 5% by mass or less, or 0% of the unsaturated compound cyclic carbonate is added to the electrolyte. When it is contained within the range of 1% by mass or more and 5% by mass or less, the initial charge / discharge efficiency can be further improved and the load characteristics can be improved.

  Furthermore, since it is possible to improve battery characteristics such as initial charge / discharge efficiency or low-temperature characteristics without using a low-viscosity solvent, it is particularly effective when a film-like exterior member is used.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an exploded structural example of a secondary battery according to an embodiment of the present invention. This secondary battery has a configuration in which a wound electrode body 20 to which a positive electrode lead 11 and a negative electrode lead 12 are attached is housed in a film-shaped exterior member 31.

  Each of the positive electrode lead 11 and the negative electrode lead 12 has, for example, a strip shape, and is led out from the inside of the exterior member 31 to the outside, for example, in the same direction. The positive electrode lead 11 is made of a metal material such as aluminum (Al), and the negative electrode lead 12 is made of a metal material such as nickel (Ni).

  The exterior member 31 is made of, for example, a rectangular laminate film in which a nylon film, an aluminum foil, and a polypropylene film are bonded together in this order. The exterior member 31 is disposed, for example, so that the polypropylene film side and the wound electrode body 20 face each other, and the outer edge portions are in close contact with each other by fusion or an adhesive.

  An adhesion film 32 between the exterior member 31 and the positive electrode lead 11 and the negative electrode lead 12 for improving the adhesion between the positive electrode lead 11 and the negative electrode lead 12 and the inside of the exterior member 31 and preventing the intrusion of outside air. Has been inserted. The adhesion film 32 is made of a material having adhesion to the positive electrode lead 11 and the negative electrode lead 12. For example, when the positive electrode lead 11 and the negative electrode lead 12 are made of the metal materials described above, polyethylene, polypropylene, It is preferably composed of a polyolefin resin such as modified polyethylene or modified polypropylene.

  FIG. 2 shows a cross-sectional structure taken along line II-II of the spirally wound electrode body 20 shown in FIG. The wound electrode body 20 is obtained by stacking and winding a positive electrode 21 and a negative electrode 22 with a separator 23 and an electrolyte layer 24 interposed therebetween, and the outermost peripheral portion is protected by a protective tape 25.

  The positive electrode 21 includes, for example, a positive electrode current collector 21A and a positive electrode active material layer 21B provided on both surfaces or one surface of the positive electrode current collector 21A. In the positive electrode current collector 21A, for example, there is an exposed portion without providing the positive electrode active material layer 21B at one end portion in the longitudinal direction, and the positive electrode lead 11 is attached to this exposed portion. The positive electrode current collector 21A is made of, for example, a metal material such as aluminum.

The positive electrode active material layer 21B includes, for example, one or more positive electrode active materials capable of inserting and extracting lithium as an electrode reactant. As the positive electrode active material capable of occluding and releasing lithium, for example, lithium-containing compounds such as lithium oxide, lithium phosphorous oxide, lithium sulfide, or an intercalation compound containing lithium are suitable. You may mix and use. In particular, in order to increase the energy density, a lithium composite oxide or lithium phosphorus oxide represented by the general formula Li _x MIO ₂ or Li _y MIIPO ₄ is preferable. In the formula, MI and MII represent one or more transition metals such as cobalt (Co), nickel, manganese (Mn), iron (Fe), aluminum, vanadium (V), titanium (Ti), and zirconium. At least one of (Zr) is preferred. The values of x and y vary depending on the charge / discharge state of the battery, and are usually values in the range of 0.05 ≦ x ≦ 1.10 and 0.05 ≦ y ≦ 1.10. Specific examples of the lithium composite oxide represented by Li _x MIO ₂ include LiCoO ₂ , LiNiO ₂ , LiNi _0.5 Co _0.5 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , or LiMn ₂ O having a spinel crystal structure. ₄ and so on. Specific examples of the lithium phosphorus oxide represented by Li _y MIIPO ₄ include LiFePO ₄ and LiFe _0.5 Mn _0.5 PO ₄ .

  The positive electrode active material layer 21B also contains, for example, a conductive agent, and may further contain a binder as necessary. Examples of the conductive agent include carbon materials such as graphite, carbon black, and ketjen black, and one kind or a mixture of two or more kinds is used. In addition to the carbon material, a metal material or a conductive polymer material may be used as long as it is a conductive material. Examples of the binder include synthetic rubber such as styrene butadiene rubber, fluorine rubber or ethylene propylene diene rubber, or a polymer material such as polyvinylidene fluoride, and one or two or more kinds are used in combination. It is done.

  The negative electrode 22 includes, for example, a negative electrode current collector 22A and a negative electrode active material layer 22B provided on both surfaces or one surface of the negative electrode current collector 22A, as with the positive electrode 21. The negative electrode current collector 22A has, for example, an exposed portion where the negative electrode active material layer 22B is not provided at one end in the longitudinal direction, and the negative electrode lead 12 is attached to the exposed portion. The anode current collector 22A is made of a metal material such as copper (Cu), for example.

  The negative electrode active material layer 22B includes a negative electrode active material capable of occluding and releasing lithium as an electrode reactant, and, for example, a binder similar to the positive electrode active material layer 21B is used as necessary. May be included.

  A negative electrode active material capable of inserting and extracting lithium is a material having a core material made of graphite and a covering portion made of a polymer compound formed so as to cover at least a part of the surface of the core material. Contains.

Graphite, true density is at 2.19 g / cm ³ or more, more preferably equal to 2.2 g / cm ³ or more. This is because the capacity can be increased. Further, the graphite may be natural graphite or artificial graphite.

  The polymer compound constituting the coating portion has a structure represented by C═O or C—O, and may have both of these structures. It is because ion conductivity improves by having these structures. The polymer compound includes a salt.

  Specific examples of such polymer compounds include propylene glycol alginate, glycine, alanine, valine, leucine, isoleucine, serine, threonine, cysteine, cystine, methionine, lysine, arginine, phenylalanine, tyrosine, histidine, Tryptophan, proline, oxyproline, aminobutyric acid, aminobenzoic acid, aspartic acid, sodium aspartate, glutamic acid, sodium glutamate, or hydroxyalkyl starch such as acetate starch, phosphate starch, carboxymethyl starch or hydroxyethyl starch, or pullulan or Viscous polysaccharides such as dextrin, or carboxymethylcellulose, methylcellulose, hydroxyethylcellulose or hydroxypro Water-soluble cellulose derivatives such as cellulose, or water-soluble cellulose derivatives such as carboxymethylcellulose sodium salt or carboxymethylcellulose ammonium salt, or polyuronides such as pectinic acid or alginic acid, or polyuronide salts such as sodium alginate or potassium alginate, or There are water-soluble synthetic resins such as water-soluble acrylic resins, water-soluble epoxy resins, water-soluble polyester resins, and water-soluble polyamide resins.

  The ratio of the covering portion to the core portion (covering portion / core portion) is preferably in the range of 0.1% by mass or more and 5% by mass or less. This is because the initial charge / discharge efficiency or the load characteristics can be improved within this range.

  Such a negative electrode active material can be produced, for example, by introducing graphite particles serving as the core into an aqueous solution in which these polymer compounds are dissolved, followed by dispersion treatment, and filtration and drying. For example, it can be produced by spraying an aqueous solution in which a polymer compound is dissolved onto graphite particles serving as a core and drying. At that time, the coating amount of the polymer compound can be adjusted by adjusting the concentration of the aqueous solution.

  As a negative electrode active material capable of inserting and extracting lithium, in addition to these negative electrode active materials, other negative electrode active materials may be mixed and used.

  The separator 23 is made of, for example, a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramic, and has a structure in which two or more kinds of porous films are laminated. May be. Among these, a porous film made of polyolefin is preferable because it is excellent in short-circuit preventing effect and can improve the safety of the battery due to the shutdown effect. In particular, polyethylene is preferable as a material constituting the separator 23 because a shutdown effect can be obtained within a range of 100 ° C. or higher and 160 ° C. or lower and the electrochemical stability is excellent. Polypropylene is also preferable, and any other resin having chemical stability can be used by copolymerizing or blending with polyethylene or polypropylene.

  The electrolyte layer 24 includes, for example, an electrolytic solution and a polymer compound that holds the electrolytic solution, and has a so-called gel shape. The electrolytic solution contains, for example, a solvent such as a nonaqueous solvent and an electrolyte salt dissolved in the solvent.

  As the solvent, propylene carbonate is contained, and the content thereof is in the range of 30% by mass or more and 70% by mass or less with respect to the whole solvent. This is because the above-described negative electrode active material is used, so that even when the content of propylene carbonate is 30% by mass or more, the decomposition reaction is suppressed, and the low temperature characteristics or the initial charge / discharge efficiency can be improved. However, even if the content of propylene carbonate in the solvent is too much, the initial charge / discharge efficiency is lowered, and therefore it is preferably 70% by mass or less.

  As the solvent, in addition to propylene carbonate, another solvent may be mixed and used. Examples of other solvents include ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, γ-valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, anisole, acetate, butyrate, propionate, fluorobenzene, etc. The nonaqueous solvent of these is mentioned. As the other solvent, one kind may be used alone, or two or more kinds may be mixed and used.

Examples of the electrolyte salt include LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiAlCl ₄ , LiSi ₂ F ₆ , LiCl, LiBr, or a lithium salt can be used. Also good.

  The content of the electrolyte salt is preferably in the range of 0.5 mol / kg or more and 3.0 mol / kg or less with respect to the solvent. Outside this range, there is a risk that sufficient battery characteristics may not be obtained due to an extreme decrease in ionic conductivity.

  The electrolytic solution also preferably contains a cyclic carbonate of an unsaturated compound as an additive. This is because a stable film can be formed on the electrode surface, and the decomposition reaction of the solvent can be suppressed. Examples of the cyclic carbonate of the unsaturated compound include 1,3-dioxol-2-one, 4-vinyl-1,3-dioxolan-2-one, and derivatives thereof. In addition, although the cyclic carbonate of the unsaturated compound remaining without participating in the film formation also functions as a solvent, the content of propylene carbonate in the solvent described above is a solvent excluding the cyclic carbonate of the unsaturated compound. Calculate from

  The content of the cyclic carbonate of such an unsaturated compound is preferably within a range of 0.1% by mass or more and 5% by mass or less with respect to the entire electrolytic solution. The initial charge / discharge efficiency can be further improved, and the load characteristics can be improved.

  The polymer compound used for the electrolyte layer 24 may be any polymer compound that absorbs a solvent and gels. For example, a fluorine-based polymer compound such as polyvinylidene fluoride or a copolymer of vinylidene fluoride and hexafluoropropylene. And ether polymers such as polyethylene oxide or a crosslinked product containing polyethylene oxide, and those containing polyacrylonitrile, polymethacrylate or polyacrylate as a repeating unit. In particular, a fluorine-based polymer compound is desirable from the viewpoint of redox stability. Any one of these polymer compounds may be used alone, or two or more thereof may be mixed and used.

  For example, the secondary battery can be manufactured as follows.

  First, for example, a positive electrode active material, a binder, and a conductive agent are mixed to prepare a positive electrode mixture, and dispersed in a solvent such as N-methyl-2-pyrrolidone to prepare a positive electrode mixture slurry. Next, this positive electrode mixture slurry is applied to both surfaces or one surface of the positive electrode current collector 21A, dried, and compression-molded to form the positive electrode active material layer 21B, thereby producing the positive electrode 21. Subsequently, for example, the positive electrode lead 11 is joined to the positive electrode current collector 21A by, for example, ultrasonic welding or spot welding. After that, a precursor solution containing an electrolytic solution, a polymer compound, and a mixed solvent is prepared and applied on the positive electrode active material layer 21B, that is, on both sides or one side of the positive electrode 21, and the mixed solvent is volatilized to obtain an electrolyte. Layer 24 is formed.

  Further, for example, the negative electrode active material and the binder described above are mixed to prepare a negative electrode mixture, and dispersed in a solvent such as N-methyl-2-pyrrolidone to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to both surfaces or one surface of the negative electrode current collector 22A, dried, and compression molded to form the negative electrode active material layer 22B, whereby the negative electrode 22 is fabricated. Subsequently, the negative electrode lead 12 is joined to the negative electrode current collector 22A by, for example, ultrasonic welding or spot welding, and the electrolyte is formed on the negative electrode active material layer 22B, that is, on both sides or one side of the negative electrode 22 in the same manner as the positive electrode 21. Layer 24 is formed.

  After that, the positive electrode 21 and the negative electrode 22 on which the electrolyte layer 24 is formed are laminated and wound via the separator 23, and the protective tape 25 is adhered to the outermost peripheral portion to form the wound electrode body 20. Finally, for example, the wound electrode body 20 is sandwiched between the exterior members 31, and the outer edges of the exterior members 31 are in close contact with each other by thermal fusion or the like and sealed. At that time, an adhesion film 32 is inserted between the positive electrode lead 11 and the negative electrode lead 12 and the exterior member 31. Thereby, the secondary battery shown in FIGS. 1 and 2 is completed.

  Moreover, you may produce the above-mentioned secondary battery as follows. First, the positive electrode 21 and the negative electrode 22 are prepared as described above, and after the positive electrode lead 11 and the negative electrode lead 12 are attached to the positive electrode 21 and the negative electrode 22, the positive electrode 21 and the negative electrode 22 are stacked via the separator 23 and wound. Then, the protective tape 25 is bonded to the outermost peripheral portion to form a wound body that is a precursor of the wound electrode body 20. Next, the wound body is sandwiched between the exterior members 31, and the outer peripheral edge portion excluding one side is heat-sealed to form a bag shape, and is stored inside the exterior member 31. Subsequently, an electrolyte composition including an electrolytic solution, a monomer that is a raw material of the polymer compound, a polymerization initiator, and other materials such as a polymerization inhibitor as necessary is prepared, and the interior of the exterior member 31 is prepared. Inject.

  After injecting the electrolyte composition, the opening of the exterior member 31 is heat-sealed in a vacuum atmosphere and sealed. Next, heat is applied to polymerize the monomer to obtain a polymer compound, thereby forming the gel electrolyte layer 24, and assembling the secondary battery shown in FIGS.

  In the secondary battery, when charged, for example, lithium ions are extracted from the positive electrode 21 and inserted in the negative electrode 22 through the electrolytic solution. When discharging is performed, for example, lithium ions are released from the negative electrode 22 and inserted in the positive electrode 21 through the electrolytic solution. Here, since the negative electrode active material described above is contained in the negative electrode 22, the capacity is improved and the decomposition reaction is suppressed even if the content of propylene carbonate is increased.

  Thus, according to the secondary battery according to the present embodiment, it is represented by C═O or C—O formed so as to cover the core made of graphite and at least a part of the surface of the core. Since the negative electrode active material having a structure-containing polymer compound is used, the decomposition reaction of propylene carbonate can be suppressed and the ion conductivity can be improved. Therefore, the capacity can be increased using graphite having a high true density, the content of propylene carbonate can be increased, and battery characteristics such as excellent initial charge / discharge efficiency or low temperature characteristics can be obtained.

  In addition, the ratio of the covering portion to the core portion (covering portion / core portion) is in the range of 0.1% by mass or more and 5% by mass or less, or 0% of the unsaturated compound cyclic carbonate is added to the electrolyte. When it is contained within the range of 1% by mass or more and 5% by mass or less, the initial charge / discharge efficiency can be further improved and the load characteristics can be improved.

  Furthermore, battery characteristics such as initial charge / discharge efficiency or low temperature characteristics can be improved without using a low-viscosity solvent, which is particularly effective when the film-like exterior member 31 is used.

  Further, specific embodiments of the present invention will be described in detail.

(Examples 1-1 to 1-3)
First, lithium cobalt oxide (LiCoO ₂ ) as a positive electrode active material, graphite as a conductive agent, and polyvinylidene fluoride as a binder are mixed to prepare a positive electrode mixture, and this positive electrode mixture is a solvent N— After being dispersed in methyl-2-pyrrolidone to form a positive electrode mixture slurry, it is uniformly applied to a positive electrode current collector 21A made of aluminum foil, dried, and compression molded with a roll press to form a positive electrode active material layer 21B Thus, a positive electrode 21 was produced. After that, the positive electrode lead 11 was attached to the positive electrode current collector 21A.

In addition, graphite serving as a core part was introduced into an aqueous solution in which a polymer compound serving as a coating part was dissolved, and after being subjected to a dispersion treatment, filtration and drying were performed to prepare a negative electrode active material having a core part and a coating part. In that case, the density | concentration of aqueous solution was adjusted so that the ratio of the coating | coated part with respect to a nucleus part might be 1 mass%. The graphite was natural graphite with a true density of 2.26 g / cm ³ , and the polymer compound was sodium glutamate. The obtained negative electrode active material and polyvinylidene fluoride as a binder are mixed to prepare a negative electrode mixture, and the negative electrode mixture is dispersed in N-methyl-2-pyrrolidone as a solvent to form a negative electrode mixture. After forming the slurry, the negative electrode current collector 22A made of copper foil was uniformly applied and dried, and compression molding was performed with a roll press to form the negative electrode active material layer 22B, thereby preparing the negative electrode 22. After that, the negative electrode lead 12 was attached to the negative electrode current collector 22A.

Subsequently, LiPF ₆ as an electrolyte salt is dissolved in a solvent in which ethylene carbonate, propylene carbonate and diethyl carbonate, or ethylene carbonate and propylene carbonate are mixed as a solvent so as to be 1 mol / kg, and 1,3-dioxole- An electrolyte was prepared by mixing 2-one. At that time, the content of propylene carbonate was 30% by mass, 50% by mass or 70% by mass. Specifically, the ratio by mass ratio of each solvent is set to ethylene carbonate: propylene carbonate: dimethyl carbonate = 50: 30: 20 in Example 1-1, and ethylene carbonate: propylene carbonate = 50: 50 in Example 1-2. 50, and in Example 1-3, ethylene carbonate: propylene carbonate = 30: 70. Further, the content of 1,3-dioxol-2-one in the electrolytic solution was set to 1% by mass.

  Next, the obtained electrolyte solution was held in a copolymer of hexafluoropropylene and vinylidene fluoride, which is a polymer compound, to form a gel electrolyte layer 24 on each of the positive electrode 21 and the negative electrode 22. The ratio of hexafluoropropylene in the copolymer was 6.9% by mass.

  After that, the positive electrode 21 and the negative electrode 22 each formed with the electrolyte layer 24 were laminated via a separator 23 made of a polyethylene film, and wound to produce a wound electrode body 20.

  The obtained wound electrode body 20 was sandwiched between exterior members 31 made of a laminate film, and sealed under reduced pressure to produce the secondary battery shown in FIGS.

  As Comparative Examples 1-1 to 1-3 with respect to Examples 1-1 to 1-3, except that the content of propylene carbonate in the solvent was 10% by mass, 20% by mass, or 80% by mass, the other examples Secondary batteries were fabricated in the same manner as 1-1 to 1-3. In that case, the ratio by the mass ratio of each solvent is ethylene carbonate: propylene carbonate: dimethyl carbonate = 50: 10: 40 in Comparative Example 1-1, and ethylene carbonate: propylene carbonate: dimethyl carbonate = 50 in Comparative Example 1-2. : 20:30, and in Comparative Example 1-3, ethylene carbonate: propylene carbonate = 20: 80.

Further, as Comparative Examples 1-4 to 1-9, except that natural graphite having a true density of 2.26 g / cm ³ in which a coating portion made of a polymer compound was not formed was used as the negative electrode active material, Secondary batteries were fabricated in the same manner as in Examples 1-1 to 1-3. At that time, the content of propylene carbonate in the solvent was changed in the range of 10% by mass to 80% by mass. Specifically, the ratio by mass ratio of each solvent was set to ethylene carbonate: propylene carbonate: dimethyl carbonate = 50: 10: 40 in Comparative Example 1-4, and ethylene carbonate: propylene carbonate: dimethyl carbonate in Comparative Example 1-5. = 50: 20: 30, in Comparative Example 1-6, ethylene carbonate: propylene carbonate: dimethyl carbonate = 50: 30: 20, in Comparative Example 1-7, ethylene carbonate: propylene carbonate = 50: 50, Comparative Example 1 −8 was ethylene carbonate: propylene carbonate = 30: 70, and Comparative Example 1-9 was ethylene carbonate: propylene carbonate = 20: 80.

  For the fabricated secondary batteries of Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-9, the initial charge / discharge efficiency and the low temperature characteristics were examined. The results are shown in Table 1 and FIG.

  The initial charge / discharge efficiency is determined by performing constant current constant voltage charge at 0.2 C at 23 ° C. up to an upper limit of 4.2 V for a total charge time of 7 hours, and then constant current discharge at 1 C at 23 ° C. with a final voltage of 3. It was calculated from the ratio of the discharge capacity to the charge capacity at this time, that is, (discharge capacity at 23 ° C., 1C / charge capacity at 23 ° C., 0.2C) × 100 (%). 1C and 0.2C are current values at which the theoretical capacity can be discharged in 1 hour and 5 hours, respectively.

  The low temperature characteristic is that constant current constant voltage charging at 1 C at -10 ° C. is performed up to an upper limit of 4.2 V with a total charging time of 2.5 hours, then constant current discharging at 1 C at −10 ° C. is performed at a final voltage of 3. The discharge capacity at −10 ° C. with respect to the discharge capacity at 23 ° C., that is, (−10 ° C., discharge capacity at 1 C / discharge capacity at 23 ° C., 1 C) × 100 (%) was obtained.

  As can be seen from Table 1 and FIG. 3, Example 1 in which graphite having a coating portion made of sodium glutamate was used as the negative electrode active material and the content of propylene carbonate in the solvent was in the range of 30 mass% to 70 mass%. According to -1 to 1-3, the low temperature characteristics are improved as compared with Comparative Examples 1-1, 1-2, 1-4, and 1-5 in which the content of propylene carbonate is less than 30% by mass. The initial charge / discharge efficiency was improved as compared with Comparative Examples 1-3 and 1-9 in which the content was more than 80% by mass. In addition, the initial charge / discharge efficiency was improved as compared with Comparative Examples 1-6 to 1-8 using graphite having no coating portion made of sodium glutamate.

  That is, it has a core part made of graphite and a covering part made of a polymer compound formed so as to cover at least a part of the surface of the core part and having a structure represented by C═O or C—O. It has been found that the use of a negative electrode active material and propylene carbonate can improve the initial charge / discharge efficiency and low temperature characteristics.

(Examples 2-1 to 2-7)
Except that the polymer compound constituting the coating was sodium alginate, aspartic acid, glutamic acid, sodium aspartate, sodium glutamate, carboxymethylcellulose ammonium salt or carboxymethylcellulose sodium salt, the others were the same as in Example 1-2. A secondary battery was produced.

  For the fabricated secondary batteries of Examples 2-1 to 2-7, the initial charge and discharge efficiency was examined in the same manner as in Examples 1-1 to 1-3. The results are shown in Table 2 together with the results of Example 1-2 and Comparative Example 1-7.

  As can be seen from Table 2, the same results as in Example 1-2 were obtained. That is, it has been found that the initial charge / discharge efficiency can be improved even if the coating portion is made of another polymer compound containing a structure represented by C═O or C—O.

(Examples 3-1 to 3-6)
A secondary battery was fabricated in the same manner as in Example 1-2 except that the ratio of the covering portion to the core portion (covering portion / core portion) was changed in the range of 0.05% by mass to 10% by mass. did. The core part is made of natural graphite having a true density of 2.26 g / cm ³ , and the covering part is made of sodium glutamate.

  For the fabricated secondary batteries of Examples 3-1 to 3-6, the initial charge and discharge efficiency was examined in the same manner as in Examples 1-1 to 1-3. The results are shown in Table 3 and FIG. 4 together with the results of Example 1-2 and Comparative Example 1-7.

  The load characteristics of the secondary batteries of Examples 1-2, 3-1 to 3-6 and Comparative Example 1-7 were examined as follows. First, after performing constant current constant voltage charge of 1 C at 23 ° C. up to an upper limit of 4.2 V with a total charge time of 2.5 hours, constant current discharge of 0.2 C at 23 ° C. is performed to a final voltage of 3.0 V, The discharge capacity at this time (discharge capacity at 23 ° C. and 0.2 C) was determined. In addition, 1C constant current and constant voltage charge at 23 ° C. was performed up to an upper limit of 4.2V, and the total charge time was 2.5 hours, and then 3C constant current discharge was performed at 23 ° C. to a final voltage of 3.0V. Discharge capacity (discharge capacity at 23 ° C. and 3 C) was determined. The load characteristic is the ratio of (discharge capacity at 23 ° C., 3C) to (discharge capacity at 23 ° C., 0.2C), ie, [(discharge capacity at 23 ° C., 3C) / (discharge at 23 ° C., 0.2C) Capacity)] × 100 (%). These results are also shown in Table 3 and FIG. Note that 3C is a current value at which the theoretical capacity can be discharged in 1/3 hour.

  As can be seen from Table 3 and FIG. 4, as the ratio of the covering portion increased, the initial charge / discharge efficiency decreased and the load characteristics increased.

  That is, it was found that the ratio of the covering portion to the core portion is preferably within the range of 0.1% by mass to 5% by mass.

(Examples 4-1 to 4-8)
As Example 4-1, a secondary battery was fabricated in the same manner as Example 1-2, except that 1,3-dioxol-2-one was not used. Further, as Examples 4-2 to 4-8, 4-vinyl-1,3-dioxolan-2-one was used instead of 1,3-dioxol-2-one, and 4-vinyl-1, A secondary battery was made in the same manner as Example 1-2 except that the content of 3-dioxolan-2-one was changed within the range of 0.05% by mass to 10% by mass.

  For the fabricated secondary batteries of Examples 4-1 to 4-8, the initial charge and discharge efficiency was examined in the same manner as in Examples 1-1 to 1-3, and in the same manner as in Examples 3-1 to 3-6. The load characteristics were investigated. These results are shown in Table 4 and FIG.

  As can be seen from Table 4 and FIG. 5, as the content of 4-vinyl-1,3-dioxolan-2-one increased, the initial charge / discharge efficiency increased and the load characteristics decreased.

  That is, it is preferable that the electrolytic solution contains a cyclic carbonate of an unsaturated compound. In particular, the content of the cyclic carbonate of the unsaturated compound in the electrolytic solution is in the range of 0.1% by mass to 5% by mass. It turned out that it is more preferable if it is inside.

(Examples 5-1 and 5-2)
A secondary battery was fabricated in the same manner as in Example 1-2, except that the true density of natural graphite serving as the core was 2.19 g / cm ³ or 2.2 g / cm ³ .

As Comparative Examples 5-1 and 5-2 with respect to Examples 5-1 and 5-2, the true density in which the coating portion made of the polymer compound is not formed is 2.19 g / cm ³ or 2.2 g / cm ³ A secondary battery was fabricated in the same manner as in Examples 5-1 and 5-2, except that natural graphite was used as the negative electrode active material.

  For the fabricated secondary batteries of Examples 5-1 and 5-2 and Comparative Examples 5-1 and 5-2, the initial charge and discharge efficiency was determined in the same manner as in Examples 1-1 to 1-3. The results are shown in Table 5 and FIG. 6 together with the results of Example 1-2 and Comparative Example 1-7. In addition, the energy density was determined. These results are also shown in Table 5 and FIG.

  In addition, energy density charged / discharged similarly to the time of calculating | requiring initial stage charge / discharge efficiency, and calculated | required the energy per unit volume (average discharge voltage x discharge capacity).

  As can be seen from Table 5 and FIG. 6, in Examples 1-2, 5-1 and 5-2 using natural graphite with a covering portion formed, if the true density of natural graphite increases, the same initial charge / discharge Energy density improved while maintaining efficiency. On the other hand, in Comparative Examples 1-7, 5-1 and 5-2 using natural graphite with no coating portion formed, the initial charge / discharge efficiency and energy density decreased as the true density of natural graphite increased. .

That is, even if the true density of graphite is 2.19 g / cm ³ or more, if it has a coating portion made of a polymer compound containing a structure represented by C═O or C—O, the initial charge / discharge efficiency is improved. It was found that the energy density can be improved while suppressing the decrease.

  Although the present invention has been described with reference to the embodiments and examples, the present invention is not limited to the above embodiments and examples, and various modifications can be made. For example, in the above embodiments and examples, a secondary battery having a winding structure has been specifically described, but the present invention uses an exterior member such as a coin type, a sheet type, a button type, or a square type. The present invention can be similarly applied to secondary batteries having other shapes, or secondary batteries having a stacked structure in which a plurality of positive and negative electrodes are stacked.

  In the above embodiments and examples, the case where lithium is used as the electrode reactant has been described, but other group 1 elements in the long-period periodic table such as sodium (Na) or potassium (K), or magnesium Alternatively, the present invention can be applied to the case where a Group 2 element in a long-period periodic table such as calcium (Ca), another light metal such as aluminum, lithium, or an alloy thereof is used, and similar effects are obtained. Can be obtained. At that time, a positive electrode active material or a solvent that can occlude and release the electrode reactant is selected according to the electrode reactant.

  Further, although cases have been described with the above embodiment and examples where a gel electrolyte in which an electrolytic solution is held in a polymer compound is used, other electrolytes may be used instead of these electrolytes. . Examples of other electrolytes include a liquid electrolyte only, a mixture of an ion conductive solid electrolyte and an electrolyte, or a mixture of a solid electrolyte and a gel electrolyte.

  As the solid electrolyte, for example, a polymer solid electrolyte in which an electrolyte salt is dispersed in a polymer compound having ion conductivity, or an inorganic solid electrolyte made of ion conductive glass or ionic crystals can be used. In this case, as the polymer compound, for example, an ether polymer compound such as polyethylene oxide or a crosslinked product containing polyethylene oxide, an ester polymer compound such as polymethacrylate or polyacrylate, or a mixture thereof, or in the molecule It can be used after being copolymerized. As the inorganic solid electrolyte, lithium nitride, lithium iodide, or the like can be used.

  Furthermore, in the above-described embodiments and examples, the case where a film is used for the exterior member 31 has been described. However, the present invention uses, for example, a cylindrical shape, a square shape, a coin shape, or a button shape using a metal container for the exterior member. The present invention can also be applied to a secondary battery, and in that case, the same effect can be obtained. In addition, the present invention can be applied not only to secondary batteries but also to primary batteries.

It is a disassembled perspective view showing the structure of the secondary battery which concerns on one embodiment of this invention. It is sectional drawing showing the structure along the II-II line of the winding electrode body shown in FIG. FIG. 2 shows an example of a characteristic diagram showing the relationship between the content of propylene carbonate, the initial charge / discharge efficiency, and the low temperature characteristics. It shows an example of a characteristic diagram showing the relationship between the ratio of the covering portion, the initial charge / discharge efficiency, and the load characteristic. It shows an example of a characteristic diagram showing the relationship between the content of 4-vinyl-1,3-dioxolan-2-one, the initial charge / discharge efficiency, and the load characteristics. It shows an example of a characteristic diagram showing the relationship between the true density of graphite, the initial charge / discharge efficiency, and the energy density.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Positive electrode lead, 12 ... Negative electrode lead, 20 ... Winding electrode body, 21 ... Positive electrode, 21A ... Positive electrode collector, 21B ... Positive electrode active material layer, 22 ... Negative electrode, 22A ... Negative electrode collector, 22B ... Negative electrode active Material layer 23 ... Separator 24 ... Electrolyte layer 25 ... Protective tape 31 ... Exterior member 32 ... Adhesion film

Claims (6)

A battery comprising an electrolyte together with a positive electrode and a negative electrode,
The negative electrode is formed so as to cover a core part made of graphite having a true density of 2.19 g / cm ³ or more and at least a part of the surface of the core part, and is represented by C═O or C—O. A negative electrode active material having a coating portion made of a polymer compound containing a structure;
The battery includes a solvent containing propylene carbonate in a range of 30% by mass to 70% by mass.   2. The battery according to claim 1, wherein a ratio of the covering portion to the core portion (covering portion / core portion) is in a range of 0.1 mass% to 5 mass%.   The battery according to claim 1, wherein the electrolyte contains a cyclic carbonate of an unsaturated compound.   The battery according to claim 3, wherein the electrolyte contains an electrolytic solution containing the cyclic carbonate in a range of 0.1 mass% to 5 mass%.   The battery according to claim 1, wherein the electrolyte includes an electrolytic solution and a polymer compound. The battery according to claim 1, wherein the positive electrode, the negative electrode, and the electrolyte are housed in a film-shaped exterior member.

Images