JP2005093248A - Electrolyte, and battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrolyte and a battery capable of improving cycle characteristics after preservation at high temperature. <P>SOLUTION: A cathode 21, an anode 22 and an electrolyte 24 are housed in a film-shaped sheathing member 30. The electrolyte 24 contains polymeric compound and electrolyte liquid in which the polymeric compound is retained, and gelatinized. The electrolyte liquid contains a solvent containing γ-valerolactone to within a range of not lower than 0.5 wt% and not higher than 10 wt%, and vinyl ethylenecarbonate to within a range of not lower than 0.5 wt% and not higher than 5 wt%. By this, the cycle characteristics, after preservation at a high temperature is improved, without lowering the initial characteristics. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a battery including an electrolyte together with a positive electrode and a negative electrode, and an electrolyte used therefor.

  In recent years, many portable electronic devices such as notebook type portable computers, cellular phones, and camera-integrated VTRs (video tape recorders) have appeared, and their size and weight have been reduced. Accordingly, as a power source for these portable electronic devices, development of a secondary battery that is lightweight and capable of obtaining a high energy density is in progress. As a secondary battery capable of obtaining a high energy density, for example, a lithium ion secondary battery using a material capable of inserting and extracting lithium (Li) such as a carbon material as a negative electrode active material, or a negative electrode A lithium secondary battery using metallic lithium as an active material is known.

  In these lithium ion secondary batteries or lithium secondary batteries, a nonaqueous solvent is used for the electrolyte, unlike lead batteries or nickel cadmium batteries. Therefore, it is necessary to prevent leakage. In order to prevent liquid leakage, it is preferable to use a metal container with high airtightness for the exterior member, but in that case, for example, a thin shape such as a sheet type or a card type, or flexibility should be provided. It is difficult.

  Therefore, instead of the electrolyte solution, a solid electrolyte, specifically, a polymer solid electrolyte in which an electrolyte salt is dispersed in an inorganic compound or polymer compound having ion conductivity, or the electrolyte solution is held in the polymer compound It has been proposed to use a gelled electrolyte.

  If this solid electrolyte is used, there is little fear of a liquid leak, a film etc. can be used for an exterior member, and a battery can be made thinner. In particular, when a laminate film made of a polymer film and a metal foil that can be heat-sealed is used, airtightness can be improved by hot sealing or the like, and the thickness of the battery can be further reduced. In addition, since the laminate film is lighter and cheaper than a metal container, the weight and cost can be reduced.

  However, since the film is thin, it is easily affected by the ambient temperature. Therefore, the battery using the film as the exterior member has a problem that, when stored at a high temperature, the battery characteristics deteriorate due to decomposition of the electrolytic solution, specifically, the solvent or the electrolyte salt. For example, in a notebook type portable computer or the like, the temperature rises due to heat generated by a CPU (Central Processing Unit), and battery characteristics such as cycle characteristics may be deteriorated. In addition, when the portable electronic device is left in a car in the summer, the ambient temperature becomes about 100 ° C. In this case, battery characteristics such as cycle characteristics may be significantly deteriorated. . As described above, since secondary batteries are often used as power sources for portable electronic devices, it is important to solve these problems.

As a battery in which decomposition of the electrolytic solution is suppressed, there is a battery using a lactone compound such as γ-valerolactone and a nitrogen-containing heterocyclic compound such as 1-methyl-2-pyrrolidone as a solvent (Patent Document 1). reference). In addition, there is a solvent using an unsaturated carbonate or a cyclic ester compound as a solvent (see Patent Document 2).
JP 2003-7333 A JP 2003-197259 A

  However, the solvent described in Patent Document 1 is mainly intended to improve the cycle characteristics at room temperature, and there is a problem that it is difficult to sufficiently improve the cycle characteristics after high-temperature storage. In addition, the solvent described in Patent Document 2 has a problem that the capacity of the battery is low because the initial charge / discharge efficiency is low.

  This invention is made | formed in view of this problem, The objective is to provide the electrolyte and battery which can improve the cycling characteristics after high temperature storage.

  The electrolyte according to the present invention contains a solvent containing γ-valerolactone in a range of 0.5 wt% to 10 wt% and vinyl ethylene carbonate in a range of 0.5 wt% to 5 wt%.

  The battery according to the present invention includes an electrolyte together with a positive electrode and a negative electrode, and the electrolyte is 0.5% by weight to 10% by weight of γ-valerolactone and 0.5% by weight to 5% by weight of vinylethylene carbonate. % Of the solvent contained in each range.

  According to the electrolyte and battery of the present invention, since γ-valerolactone is contained at a content of 0.5% by weight or more, the electrolyte is thermally decomposed at a high temperature, or the positive electrode and the electrolyte react to generate gas. Generation | occurrence | production can fully be suppressed. Further, although γ-valerolactone is easily reduced, the content of the electrolyte in the electrolyte is set to 10% by weight or less, so that reductive decomposition of γ-valerolactone at the negative electrode can also be suppressed. Furthermore, although γ-valerolactone is likely to be decomposed at the first charge, the electrolyte contains vinyl ethylene carbonate at a content of 0.5% by weight or more. Generation of gas can be sufficiently suppressed. In addition, although vinyl ethylene carbonate is easily oxidized, the content in the electrolyte is set to 5% by weight or less, so that the vinyl ethylene carbonate can be prevented from being oxidatively decomposed at the positive electrode. Therefore, the cycle characteristics after high temperature storage can be improved without deteriorating the initial characteristics.

  Moreover, since generation | occurrence | production of gas is suppressed, when using the film-shaped exterior member which is easy to be influenced by gas, a big effect can be exhibited.

  In particular, if the positive electrode contains a predetermined amount of lithium carbonate in addition to the positive electrode active material, thermal decomposition of the electrolyte salt at a high temperature can be suppressed, and the characteristics can be further improved.

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

  FIG. 1 shows an exploded view of a secondary battery according to an embodiment of the present invention. In the secondary battery, a battery element 20 to which a positive electrode terminal 11 and a negative electrode terminal 12 are attached is enclosed in a film-shaped exterior member 30.

  The positive electrode terminal 11 and the negative electrode terminal 12 are led out from the inside of the exterior member 30 to the outside, for example, in the same direction. The positive electrode terminal 11 is made of a metal material such as aluminum (Al), and the negative electrode terminal 12 is made of a metal material such as copper (Cu) or nickel (Ni).

  The exterior member 30 is made of, for example, a laminate film in which two or more insulating layers, metal layers, and the like are laminated and bonded together, and the insulating layer is disposed on the inner side. The outer edge portions of the exterior member 30 are in close contact with each other by fusion bonding or an adhesive. The constituent material of the inner insulating layer is not particularly limited as long as it is a material having adhesiveness with respect to the positive electrode terminal 11 and the negative electrode terminal 12, but polyethylene, polypropylene, modified polyethylene, modified polypropylene, copolymers thereof, and the like are used. Polyolefin resins are preferred because they can reduce permeability and are excellent in airtightness. In addition, as a constituent material of the outer insulating layer, for example, nylon is preferable from the viewpoint that strength against tearing and piercing can be increased. As a constituent material of the metal layer, for example, aluminum, stainless steel, nickel, iron, or the like formed into a foil shape or a plate shape can be given.

  The exterior member 30 may be made of a laminated film having another structure, a polymer film such as polypropylene, or a metal film instead of the above-described laminated film.

  An adhesion film 31 is inserted between the exterior member 30 and the positive electrode terminal 11 and the negative electrode terminal 12 to prevent intrusion of outside air. The adhesion film 31 may be any film that has adhesion to the positive electrode terminal 11 and the negative electrode terminal 12, and is made of, for example, a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a copolymer thereof. .

  FIG. 2 shows a cross-sectional structure along the line II-II of the battery element 20 shown in FIG. The battery element 20 is formed by laminating and winding a positive electrode 21 and a negative electrode 22 with a separator 23 and an electrolyte 24, 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 having a pair of opposed surfaces and a positive electrode mixture layer 21B provided on both surfaces or one surface of the positive electrode current collector 21A. The positive electrode current collector 21A has an exposed portion where the positive electrode mixture layer 21B is not provided at one end portion in the longitudinal direction, and the positive electrode terminal 11 is attached to the exposed portion. The positive electrode current collector 21A is made of, for example, a metal material such as aluminum and has a foil shape or a net shape.

The positive electrode mixture layer 21B contains one or more positive electrode materials capable of inserting and extracting lithium as a positive electrode active material, and may be polyvinylidene fluoride or styrene butadiene rubber as necessary. A binder and a conductive agent such as a carbon material may be included. Examples of the positive electrode material capable of inserting and extracting lithium include lithium such as titanium sulfide (TiS ₂ ), molybdenum sulfide (MoS ₂ ), niobium selenide (NbSe ₂ ), and vanadium oxide (V ₂ O ₅ ). Metal sulfides or metal oxides containing no lithium, or lithium composite oxides containing lithium.

Among these, lithium composite oxides are preferable because there are those that can obtain a high voltage and a high energy density. Examples of such a lithium composite oxide include those represented by the chemical formula Li _x MIO ₂ or Li _y MIIPO ₄ . In the formula, MI and MII represent one or more transition metals, and at least one of cobalt (Co), nickel, manganese (Mn), iron (Fe), aluminum, vanadium (V), and titanium (Ti). In particular, it is more preferable that at least one of cobalt, nickel and manganese is included. The values of x and y vary depending on the charge / discharge state of the battery, and are generally 0.05 ≦ x ≦ 1.10 and 0.05 ≦ y ≦ 1.10. Specific examples of the lithium composite oxide represented by the chemical formula Li _x MIO ₂ include LiCoO ₂ , LiNiO ₂ , LiNi _z Co _1-z O ₂ (0 <z <1), or LiMn ₂ O _4. .

The positive electrode mixture layer 21B preferably contains lithium carbonate (Li ₂ CO ₃ ). It is because it can suppress that the below-mentioned electrolyte salt thermally decomposes when exposed to a high temperature state. The amount of lithium carbonate is preferably in the range of 0.2% by weight or less with respect to the positive electrode active material. If there is too much lithium carbonate, excess lithium carbonate decomposes at a high temperature to generate carbon dioxide, and this carbon dioxide accumulates between the positive electrode 21 and the negative electrode 22, which may increase battery resistance. It is. Moreover, it is preferable that the amount of lithium carbonate is 0.05 weight% or more with respect to a positive electrode active material. This is because when the amount of lithium carbonate is too small, it is difficult to sufficiently suppress the thermal decomposition of the electrolyte salt.

For example, in the case of producing LiCoO ₂ as a positive electrode active material, lithium carbonate is mixed with lithium carbonate and cobalt carbonate (CoCO ₃ ) as starting materials, and this mixture is baked to form LiCoO _2. Can be obtained. In this case, the lithium carbonate content in LiCoO ₂ can be adjusted by adjusting the mixing ratio of lithium carbonate and cobalt carbonate. In addition, lithium carbonate may be included in the positive electrode active material in the process of synthesizing the positive electrode active material as described above. However, by adding to the positive electrode active material, lithium carbonate is included in the positive electrode mixture layer 21B. Also good.

  The negative electrode 22 includes, for example, a negative electrode current collector 22A having a pair of opposed surfaces, and a negative electrode mixture layer 22B provided on both surfaces or one surface of the negative electrode current collector 22A. The negative electrode current collector 22A has an exposed portion without being provided with the negative electrode mixture layer 22B at one end portion in the longitudinal direction, and the negative electrode terminal 12 is attached to the exposed portion. The negative electrode current collector 22A is made of a metal material such as copper, nickel, or stainless steel, and has a foil shape or a net shape.

  The negative electrode mixture layer 22B includes, for example, any one or more of a negative electrode material capable of inserting and extracting lithium and metallic lithium as a negative electrode active material, and polyvinylidene fluoride as necessary. Alternatively, a binder such as styrene butadiene rubber and a conductive agent such as a carbon material may be included. Examples of the negative electrode material capable of inserting and extracting lithium include a carbon material. Examples of the carbon material include a low crystalline carbon material obtained by firing at a relatively low temperature of 2000 ° C. or lower, or artificial graphite obtained by firing a raw material that is easily crystallized at a high temperature around 3000 ° C. And highly crystalline carbon materials. Specific examples include non-graphitizable carbon, pyrolytic carbons, cokes, graphites, glassy carbon fibers, organic polymer compound fired bodies, carbon fibers, activated carbon or carbon blacks. Among these, coke includes pitch coke, needle coke, petroleum coke, and the like, and the organic polymer compound fired body is obtained by firing and carbonizing a phenol resin or a furan resin at an appropriate temperature. . Moreover, as a negative electrode material capable of inserting and extracting lithium, for example, a polymer compound such as polyacetylene or polypyrrole, or an oxide such as iron oxide, ruthenium oxide, molybdenum oxide, tungsten oxide, or titanium oxide, Alternatively, a nitride obtained by replacing oxygen in these oxides with nitrogen is also included.

  Examples of the negative electrode material capable of inserting and extracting lithium further include simple elements, alloys or compounds of metal elements or metalloid elements capable of forming an alloy with lithium. In addition to the alloy composed of two or more metal elements, the alloy includes an alloy composed of one or more metal elements and one or more metalloid elements. There are structures in which a solid solution, a eutectic (eutectic mixture), an intermetallic compound, or two or more of them coexist.

Examples of metal elements or metalloid elements capable of forming an alloy with lithium include magnesium (Mg), boron (B), arsenic (As), aluminum, gallium (Ga), indium (In), silicon (Si), Germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), cadmium (Cd), silver (Ag), zinc (Zn), hafnium (Hf), zirconium (Zr), Examples thereof include yttrium (Y), palladium (Pd), and platinum (Pt). These alloys or compounds, such as those represented by the chemical formula Ma _s Mb _t. In this chemical formula, Ma represents at least one of a metal element and a metalloid element capable of forming an alloy with lithium, and Mb represents at least one of elements other than Ma. The values of s and t are s> 0 and t ≧ 0, respectively.

  Among them, a simple substance, alloy or compound of a group 14 metal element or metalloid element in the long-period periodic table is preferable, and silicon or tin, or an alloy or compound thereof is particularly preferable. These may be crystalline or amorphous.

Specific examples of such alloys or compounds include LiAl, AlSb, CuMgSb, SiB ₄ , SiB ₆ , Mg ₂ Si, Mg ₂ Sn, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi _{2. , CaSi 2, CrSi 2, Cu} 5 Si, FeSi 2, MnSi 2, NbSi 2, TaSi 2, VSi 2, WSi 2, ZnSi 2, SiC, Si 3 N 4, Si 2 N 2 O, SiO v (0 < _{v ≦ 2), SnO w (} 0 <w ≦ 2), there is such SnSiO _3, LiSiO or LiSnO.

  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.

  The electrolyte 24 is in a gel form in which an electrolytic solution is held in a polymer compound. A gel electrolyte is preferable because it can obtain high ionic conductivity and can prevent battery leakage or swelling at high temperatures.

  As the polymer compound, for example, any one may be used as long as it absorbs the electrolytic solution and gels, and one kind may be used alone, or a plurality kinds may be mixed and used. For example, a fluorine polymer compound such as polyvinylidene fluoride or a copolymer of vinylidene fluoride and hexafluoropropylene, an ether polymer compound such as polyethylene oxide or a crosslinked product containing the same, or polyacrylonitrile It is done.

  The electrolytic solution includes, for example, a solvent such as a nonaqueous solvent and an electrolyte salt dissolved in the solvent. As long as the non-aqueous solvent dissolves and dissociates the electrolyte salt, the non-aqueous solvent may contain one kind alone, or may contain plural kinds mixed. For example, cyclic carbonates such as propylene carbonate or ethylene carbonate, chain carbonates such as diethyl carbonate or dimethyl carbonate, carboxylic acid esters such as methyl propionate or methyl butyrate, 2-methyltetrahydrofuran or dimethoxy Examples include ethers such as ethane, butyrolactones such as γ-butyrolactone, sulfolanes, and compounds in which these hydrogens are substituted with halogens. Among these, it is preferable to include a compound having a relatively high dielectric constant.

  The non-aqueous solvent also contains γ-valerolactone shown in Chemical Formula 1 and vinyl ethylene carbonate shown in Chemical Formula 2. The γ-valerolactone is for suppressing the thermal decomposition of the electrolyte 24 at a high temperature or the generation of gas due to the reaction between the positive electrode 21 and the electrolyte 24. However, γ-valerolactone has the property of degrading the initial charge / discharge efficiency and the like because it decomposes and generates gas during the initial charge. Vinyl ethylene carbonate is for suppressing the decomposition of γ-valerolactone. That is, in this embodiment, by including both γ-valerolactone and vinyl ethylene carbonate, cycle characteristics after high-temperature storage can be improved without reducing initial characteristics such as initial charge / discharge efficiency. It is like that.

  The content of γ-valerolactone in the solvent is preferably in the range of 0.5 wt% to 10 wt%. When the amount of γ-valerolactone is small, it is difficult to obtain the effect of γ-valerolactone sufficiently. When the amount is too large, γ-valerolactone is reduced and decomposed on the surface of the negative electrode mixture layer 22B to deteriorate the characteristics. This is because there is a fear.

  Moreover, it is preferable that content of the vinyl ethylene carbonate in a solvent exists in the range of 0.5 weight% or more and 10 weight% or less. If the amount of vinyl ethylene carbonate is small, it is difficult to sufficiently suppress the decomposition of γ-valerolactone, and if it is too large, the vinyl ethylene carbonate oxidatively decomposes on the surface of the positive electrode mixture layer 21B and the battery characteristics such as cycle characteristics deteriorate. It is because there is a possibility of doing.

Examples of the electrolyte salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiN (SO ₂ CF ₃ ) ₂ , LiN ( Examples thereof include lithium salts such as SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ , LiCl, or LiBr. In particular, LiPF ₆ or LiBF ₄ is preferable from the viewpoint of oxidation stability. Any one of the lithium salts may be used, or a plurality of lithium salts may be mixed and used.

The concentration of the electrolyte salt is not particularly limited, it is preferred for the solvent, in the range of 0.4 mol / dm ³ or more 1.5 mol / dm ³ or less. It is because the ion conductivity of electrolyte solution can be made high by setting it as this range.

  In the secondary battery, when charged, for example, lithium ions are released from the positive electrode mixture layer 21 </ b> B and inserted into the negative electrode mixture layer 22 </ b> B through the electrolyte 24. When discharging is performed, for example, lithium ions are released from the negative electrode mixture layer 22 </ b> B and inserted into the positive electrode mixture layer 21 </ b> B through the electrolyte 24. Here, since γ-valerolactone is contained in a predetermined amount, even when stored at a high temperature, the electrolyte 24 is thermally decomposed, or the positive electrode 21 and the electrolyte 24 react to generate gas. Sufficiently suppressed. Further, since vinylethylene carbonate is contained in a predetermined amount, generation of gas due to decomposition of γ-valerolactone, which is easily reduced during the initial charge, at the negative electrode 22 is sufficiently suppressed. Therefore, excellent cycle characteristics can be obtained even after high temperature storage without deterioration of initial characteristics such as initial charge / discharge efficiency.

  The secondary battery having such a configuration can be manufactured, for example, as follows.

  First, a positive electrode active material, a binder and, if necessary, a conductive agent are mixed to prepare a positive electrode mixture, and dispersed in a solvent such as 1-methyl-2-pyrrolidone to prepare a positive electrode mixture coating liquid To do. Subsequently, this positive electrode mixture coating solution is applied to both surfaces or one surface of the positive electrode current collector 21A, dried, and compression molded to form the positive electrode mixture layer 21B, thereby producing the positive electrode 21.

  Further, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and dispersed in a solvent such as 1-methyl-2-pyrrolidone to prepare a negative electrode mixture coating solution. Subsequently, the negative electrode mixture coating liquid is applied to both surfaces or one surface of the negative electrode current collector 22A, dried, and compression molded to form the negative electrode mixture layer 22B, whereby the negative electrode 22 is manufactured.

  Further, for example, a high dielectric constant solvent, γ-valerolactone, and vinyl ethylene carbonate are mixed in a predetermined amount, and an electrolyte salt is dissolved in the mixed solvent to prepare an electrolytic solution. Subsequently, the electrolytic solution, the polymer compound, and a mixed solvent such as dimethyl carbonate are mixed and stirred to prepare a sol electrolyte solution.

  After that, a sol electrolyte solution is applied to each of the positive electrode 21 and the negative electrode 22, and the mixed solvent is volatilized to form the gel electrolyte 24. Next, the positive electrode terminal 11 is attached to the end portion of the positive electrode current collector 21A, and the negative electrode terminal 12 is attached to the end portion of the negative electrode current collector 22A. Subsequently, the positive electrode 21 and the negative electrode 22 on which the electrolyte 24 is formed are stacked and wound with a separator 23 interposed therebetween, and a protective tape 25 is adhered to the outermost peripheral portion to form the battery element 20.

  Finally, for example, the battery element 20 is sandwiched between the exterior members 30, and the outer edges of the exterior members 30 are brought into close contact with each other by thermal fusion or the like. At that time, an adhesion film 31 is inserted between the positive electrode terminal 11 and the negative electrode terminal 12 and the exterior member 30. Thereby, the secondary battery shown in FIGS. 1 and 2 is completed.

  Thus, in this embodiment, since the electrolyte contains γ-valerolactone in a content of 0.5 wt% or more, the electrolyte 24 is thermally decomposed at a high temperature, or the positive electrode 21 and the electrolyte 24 react with each other. Thus, the generation of gas can be sufficiently suppressed. In addition, although γ-valerolactone is easily reduced, the content in the electrolyte 24 is set to 10% by weight or less, so that the reductive decomposition of γ-valerolactone at the negative electrode 22 can also be suppressed. Furthermore, although γ-valerolactone is likely to be decomposed at the first charge, the electrolyte contains vinyl ethylene carbonate at a content of 0.5% by weight or more. The generation of gas can also be sufficiently suppressed. In addition, although vinyl ethylene carbonate is easily oxidized, the content of the electrolyte 24 in the electrolyte 24 is set to 5% by weight or less, so that the oxidative decomposition of the vinyl ethylene carbonate at the positive electrode 21 can also be suppressed. Therefore, the cycle characteristics after high temperature storage can be improved without deteriorating the initial characteristics.

  Moreover, since generation | occurrence | production of gas is suppressed, when the film-shaped exterior member 30 which is easy to be influenced by gas is used, a big effect can be exhibited.

  In particular, if the positive electrode 21 contains a predetermined amount of lithium carbonate in addition to the positive electrode active material, it is possible to suppress the thermal decomposition of the electrolyte salt at a high temperature, and the characteristics can be further improved.

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

(Examples 1-12)
First, lithium carbonate and cobalt carbonate were mixed, fired in air at 900 ° C. for about 5 hours, and then pulverized to produce LiCoO ₂ having an average particle diameter of 8 μm. At that time, in Examples 2 to 12, the mixing ratio of lithium carbonate and cobalt carbonate was adjusted, and the lithium carbonate content was 0.01% by weight, 0.25% by weight, 0.05% by weight with respect to LiCoO ₂ . % Or 0.2% by weight. In Example 1, LiCoO ₂ produced so that the lithium carbonate content was 0.05% by weight was immersed in ion-exchanged water for 24 hours, and then lithium carbonate was removed by filtration. The LiCoO ₂ of Examples 1 to 12 produced, was determined by neutralization titration of lithium carbonate amount. The results are shown in Table 1. In Example 1, since it was below the detection limit, in Table 1, the amount of lithium carbonate was described as zero.

Next, using the produced LiCoO ₂ as a positive electrode active material, 91 parts by weight of LiCoO _2, 3 parts by weight of polyvinylidene fluoride, and 6 parts by weight of scaly graphite as a conductive agent, 1-methyl-2 as a solvent -Kneaded with a planetary mixer using pyrrolidone to prepare a positive electrode mixture coating solution. Subsequently, the positive electrode mixture coating solution was uniformly applied to both surfaces of the positive electrode current collector 21A made of a strip-shaped aluminum foil having a thickness of 20 μm using a die coater, dried in a reduced pressure state at 100 ° C. for 24 hours, and then rolled. The positive electrode mixture layer 21B was formed by compression molding. After that, the positive electrode 21 was manufactured by cutting into a width of 118 mm and a length of 640 mm.

Further, the negative electrode active material has a (002) plane spacing of 0.337 nm, a (002) plane C-axis crystallite thickness of 50 nm, a true density by a pycnometer method of 2.23 g / cm ³ , and a specific surface area by a BET method. 1.6 m ² / g, an average particle diameter of 33 .mu.m, and the graphite 90 wt% of the bulk density of 0.98 g / cm ^3, polyvinylidene fluoride 10 wt% and a binder is the solvent 1-methyl - 2-Pyrrolidone was used to knead with a planetary mixer to prepare a negative electrode mixture coating solution. Subsequently, the negative electrode mixture coating solution was uniformly applied to both surfaces of the negative electrode current collector 22A made of a strip-shaped copper foil having a thickness of 10 μm using a die coater, dried at 120 ° C. for 24 hours in a reduced pressure state, and then a roll press machine. The negative electrode mixture layer 22B was formed by compression molding. After that, it was cut into a width of 120 mm and a length of 800 mm to produce a negative electrode 22.

In addition, an electrolyte was prepared by dissolving LiPF ₆ at a concentration of 0.8 mol / kg in a solvent in which ethylene carbonate, propylene carbonate, γ-valerolactone, and vinyl ethylene carbonate were mixed. At that time, the contents of ethylene carbonate, propylene carbonate, γ-valerolactone and vinyl ethylene carbonate in the solvent were changed as shown in Table 1 in Examples 1-12. In Table 1, EC represents ethylene carbonate, PC represents propylene carbonate, GVL represents γ-valerolactone, and VEC represents vinyl ethylene carbonate. Next, polyvinylidene fluoride copolymerized with 7% by weight of hexafluoropropylene, the prepared electrolyte, and dimethyl carbonate as a mixed solvent were mixed and stirred to prepare a sol electrolyte solution.

  After that, this electrolyte solution was applied to the surfaces of the positive electrode 21 and the negative electrode 22, and dimethyl carbonate was volatilized to form an electrolyte 24. Next, the strip-like positive electrode terminal 11 was attached to the positive electrode current collector 21A, and the strip-like negative electrode terminal 12 was attached to the negative electrode current collector 22A. Subsequently, the positive electrode 21 and the negative electrode 22 on which the electrolyte 24 was formed were stacked via a separator 23 and wound, and a protective tape 25 was adhered to the outermost peripheral portion to form the battery element 20.

  After that, the battery element 20 was sandwiched between a three-layer exterior member 30 in which an aluminum foil having a thickness of 50 μm was sandwiched between polyolefin films having a thickness of 30 μm, and the outer edges of the exterior member 30 were heat-sealed and sealed. At that time, an adhesion film 31 made of propylene resin was inserted between the positive electrode terminal 11 and the negative electrode terminal 12 and the exterior member 30. The secondary battery of Examples 1-12 was obtained by the above process.

  Further, as Comparative Examples 1 to 12 with respect to Examples 1 to 12, except that the amount of lithium carbonate and the composition of the solvent with respect to the positive electrode active material were changed as shown in Table 1, others were the same as in Examples 1 to 12. A secondary battery was manufactured. In Table 1, GBL represents γ-butyrolactone, and VC represents vinylene carbonate.

  About the produced secondary battery of Examples 1-12 and Comparative Examples 1-12, in 23 degreeC atmosphere, the electric current value 0.2C, the upper limit voltage 4.2V, and the constant current constant voltage charge of the charge time 10 hours were performed. Thereafter, constant current discharge was performed under the conditions of a current value of 0.5 C and a final voltage of 3 V. Then, from the obtained initial charge capacity and initial discharge capacity, the initial charge / discharge efficiency was calculated as (initial discharge capacity) / (initial charge capacity) × 100 (%). The results are shown in Table 1. In addition, 0.2 C is a current value that discharges the rated capacity of the battery in 5 hours, and 0.5 C is a current value that discharges the rated capacity of the battery in 2 hours.

  For the fabricated secondary batteries of Examples 1-12 and Comparative Examples 1-12, first, in a 23 ° C. atmosphere, a constant current constant voltage with a current value of 0.5 C, an upper limit voltage of 4.2 V, and a charging time of 5 hours. After charging, it was stored in an atmosphere at 60 ° C. for 30 days. After that, constant current discharge with a current value of 1 C and a final voltage of 3 V was performed in an atmosphere at 23 ° C. Next, after performing constant current and constant voltage charging with a current value of 1 C, an upper limit voltage of 4.2 V, and a charging time of 3 hours in a 23 ° C. atmosphere, a constant current value of 1 C and a final voltage of 3 V are fixed in the 23 ° C. atmosphere. A current discharge was performed. This charging / discharging was repeated 250 cycles. The capacity retention rate after high-temperature storage was calculated as the ratio of the discharge capacity at the 250th cycle to the discharge capacity at the 3rd cycle. The results are shown in Table 1.

  As shown in Table 1, in Examples 1 to 10, the capacity retention rate after high-temperature storage was better than Comparative Examples 1 to 12. Further, the initial charge / discharge efficiency was better than those of Comparative Examples 3, 4, 6, 7, and 9-12.

  In Comparative Examples 1, 2, 5, and 9, the capacity retention rate after high-temperature storage deteriorated because, in Comparative Example 1, γ-butyrolactone is weak in both oxidation and reduction, so it decomposes during high-temperature storage or charge / discharge cycles. In Comparative Example 2, the solvent does not contain γ-valerolactone, and in Comparative Examples 5 and 9, the amount of γ-valerolactone is small. This is probably because it was difficult to suppress thermal decomposition. Further, the characteristics deteriorated in Comparative Examples 3, 4, 6 to 8, 10, and 12. In Comparative Examples 3 and 4, reductive decomposition of γ-valerolactone in the negative electrode 22 during the initial charge can be suppressed. In Comparative Examples 6 and 10, the amount of γ-valerolactone was too much, and it was considered that excessive γ-valerolactone was reduced on the negative electrode during high-temperature storage, resulting in an increase in battery resistance. No. 11 has an insufficient reduction suppressing effect, and Comparative Examples 8 and 12 have too much vinyl ethylene carbonate, and excessive vinyl ethylene carbonate was oxidized on the positive electrode during high-temperature storage, resulting in an increase in battery resistance. it is conceivable that.

  On the other hand, in Examples 1 to 10, since the contents of γ-valerolactone and vinyl ethylene carbonate in the solvent are appropriate, the decomposition of the electrolyte 24 and the generation of gas during the initial charge and during high-temperature storage are suppressed. It is thought that the characteristics were improved.

  That is, if the electrolyte 24 contains a solvent containing γ-valerolactone in a range of 0.5 wt% to 10 wt% and vinyl ethylene carbonate in a range of 0.5 wt% to 5 wt%, respectively. It was found that the cycle characteristics after high-temperature storage can be improved without deteriorating the initial characteristics.

  Further, as can be seen by comparing Examples 10 to 12, according to Example 10 in which the amount of lithium carbonate is 0.5 wt% or more and 0.2 wt% or less with respect to the positive electrode active material, less than 0.05 wt% Compared with Example 11 and Example 12 with more than 0.2% by weight, the capacity retention rate after high-temperature storage was good.

Thus, Examples 11 and 12 had a lower capacity retention rate after high-temperature storage than Example 10, because in Example 11, there was too little lithium carbonate, and LiPF ₆ was thermally decomposed during high-temperature storage and fluorinated. This is probably because hydrogen (HF) was generated, and the battery element 20 or the electrolyte 24 was slightly deteriorated by the hydrogen fluoride. In Example 12, there was too much lithium carbonate, and excessive carbon dioxide during high-temperature storage. It is considered that this is because lithium decomposed and gas such as carbon dioxide was generated, and this gas stayed between the positive electrode 21 and the negative electrode 22 to slightly increase the battery resistance. On the other hand, in Example 10, since the amount of lithium carbonate was appropriate, the generation of hydrogen fluoride during storage at high temperature and the generation of gases such as carbon dioxide were sufficiently suppressed, and the capacity retention rate after storage at high temperature was good. Conceivable.

  That is, it was found that the high temperature storage characteristics can be further improved if the positive electrode 21 contains lithium carbonate in the range of 0.05 wt% to 0.2 wt% with respect to the positive electrode active material.

  Although the present invention has been described with reference to the embodiments and examples, the present invention is not limited to the embodiments and examples, and various modifications can be made. For example, in the above-described embodiments and examples, the case where the gel electrolyte 24 is used has been described, but other electrolytes may be used. Examples of other electrolytes include an electrolyte, an inorganic solid electrolyte such as lithium nitride or lithium iodide, or a mixture of two or more of an electrolyte, a gel electrolyte, and an inorganic solid electrolyte.

  In the above-described embodiment and examples, the positive electrode 21 and the negative electrode 22 are stacked and wound. However, the positive electrode and the negative electrode may be folded or folded in a so-called bellows shape.

  Further, in the above-described embodiments and examples, the case where a film is used for the exterior member 30 has been described. However, the present invention may be a cylindrical type, a square type, a coin type or a button type 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.

1 is an exploded perspective view of a secondary battery according to an embodiment of the present invention. It is sectional drawing along the II-II line of the battery element shown in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Positive electrode terminal, 12 ... Negative electrode terminal, 20 ... Battery element, 21 ... Positive electrode, 21A ... Positive electrode collector, 21B ... Positive electrode mixture layer, 22 ... Negative electrode, 22A ... Negative electrode collector, 22B ... Negative electrode mixture layer , 23 ... separator, 24 ... electrolyte, 25 ... protective tape, 30 ... exterior member, 31 ... adhesion film.

Claims (4)

  An electrolyte comprising a solvent containing γ-valerolactone in a range of 0.5 wt% to 10 wt% and vinyl ethylene carbonate in a range of 0.5 wt% to 5 wt%. A battery comprising an electrolyte together with a positive electrode and a negative electrode,
The battery contains a solvent containing γ-valerolactone in a range of 0.5 wt% to 10 wt% and vinyl ethylene carbonate in a range of 0.5 wt% to 5 wt%, respectively. .   The battery according to claim 2, wherein the positive electrode contains lithium carbonate in a range of 0.2 wt% or less with respect to the positive electrode active material in addition to the positive electrode active material. 3. The battery according to claim 2, further comprising a film-shaped exterior member that houses the positive electrode, the negative electrode, and the electrolyte.

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