JP2007048560A - Battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a battery capable of enhancing energy density and cycle characteristics. <P>SOLUTION: The battery is equipped with a rolled electrode body 10 formed by rolling a positive electrode 13 and a negative electrode 14 through a separator 15 and an electrolyte 16. The open circuit voltage in a full charge state is present in a range of 4.25-6.00 V. A positive active material layer 13B contains a positive active material including Co as a constituting element, and the content of Co in the positive active material layer 13B is 30-60 mass%. Li concentration in the electrolyte 16 is 0.30-0.56 mass%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a battery having an open circuit voltage of 4.25 V or more in a fully charged state per pair of positive electrode and negative electrode.

  In recent years, many portable electronic devices such as a camera-integrated VTR, a mobile phone, and a portable computer have appeared, and their size and weight have been reduced. However, these electronic devices have been miniaturized and multifunctional and high performance has been promoted. As a result, power consumption does not necessarily decrease, and the usage time tends to be longer due to the multifunctional functions. Is the reality. Users desire to be able to use these portable electronic devices for a longer time, and further increase in energy density is desired for lithium ion secondary batteries widely used as power sources.

Conventional lithium ion secondary batteries generally use lithium cobaltate for the positive electrode and a carbon material for the negative electrode, and the operating voltage is in the range of 4.2V to 2.5V. Thus, in a lithium ion secondary battery operating at a maximum of 4.2 V, the positive electrode active material such as lithium cobaltate used for the positive electrode only uses a capacity of about 60% of its theoretical capacity. . For this reason, it is possible in principle to utilize the remaining capacity by further increasing the charging pressure. It is known that high energy density is realized by actually setting the voltage during charging to 4.25 V or more (see Patent Document 1).
International Publication No. WO03 / 0197131 Pamphlet

  However, in a battery in which the charging voltage is set to exceed 4.2 V, the oxidizing atmosphere is particularly strong in the vicinity of the positive electrode surface, so that the positive electrode active material is easily collapsed or eluted, and the electrolyte is liable to deteriorate, resulting in a decrease in charge / discharge efficiency. There was a problem of doing.

  This invention is made | formed in view of this problem, The objective is to provide the battery which can improve an energy density and charging / discharging efficiency.

  A battery according to the present invention includes an electrolyte together with a positive electrode and a negative electrode, and an open circuit voltage in a fully charged state per pair of the positive electrode and the negative electrode is in a range of 4.25 V to 6.00 V. The positive electrode current collector is provided with a positive electrode active material layer containing a positive electrode active material containing cobalt (Co) as a constituent element, and the proportion of cobalt in the positive electrode active material layer is 30% by mass or more and 60% by mass or less. The electrolyte contains a lithium salt and a solvent containing a high dielectric constant solvent having a relative dielectric constant of 20 or more of more than 70% by mass, and the concentration of lithium (Li) in the electrolyte is 0.30. It is within the range of not less than 0.56% by mass.

  According to the battery of the present invention, the open circuit voltage in the fully charged state per pair of positive electrode and negative electrode is 4.25 V or more and 6.00 V or less, and the proportion of cobalt in the positive electrode active material layer is 30% by mass or more and 60% or less. Since it was made to be in the range of mass% or less, a high energy density can be obtained. Moreover, since it was made for the density | concentration of lithium in electrolyte to be in the range of 0.30 mass% or more and 0.56 mass% or less, charging / discharging efficiency can be improved.

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

  FIG. 1 shows a configuration of a secondary battery according to an embodiment of the present invention. This secondary battery uses lithium as an electrode reactant. For example, the secondary battery has a structure in which a wound electrode body 10 to which a positive electrode lead 11 and a negative electrode lead 12 are attached is housed in a film-like exterior member 20. Have.

  The positive electrode lead 11 and the negative electrode lead 12 are led out from the inside of the exterior member 20 to the outside, for example, in the same direction. The positive electrode lead 11 and the negative electrode lead 12 are each made of a metal material such as aluminum (Al), copper (Cu), nickel (Ni), or stainless steel, and each have a thin plate shape or a mesh shape.

  The exterior member 20 is made of, for example, a rectangular aluminum laminated film in which a nylon film, an aluminum foil, and a polyethylene film are bonded together in this order. For example, the exterior member 20 is disposed so that the polyethylene film side and the wound electrode body 10 face each other, and the outer edge portions are in close contact with each other by fusion bonding or an adhesive. An adhesive film 21 is inserted between the exterior member 20 and the positive electrode lead 11 and the negative electrode lead 12 to prevent intrusion of outside air. The adhesion film 21 is made of a material having adhesion to the positive electrode lead 11 and the negative electrode lead 12, for example, a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene.

  The exterior member 20 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 aluminum laminated film.

  FIG. 2 shows a cross-sectional structure taken along line II of the spirally wound electrode body 10 shown in FIG. The wound electrode body 10 is formed by laminating a pair of a positive electrode 13 and a negative electrode 14 via a separator 15 and an electrolyte 16, and the outermost peripheral portion is protected by a protective tape 17.

  The positive electrode 13 has a structure in which, for example, a positive electrode active material layer 13B is provided on both surfaces of a positive electrode current collector 13A having a pair of opposed surfaces. Although not shown, the positive electrode active material layer 13B may be provided only on one surface of the positive electrode current collector 13A. The positive electrode current collector 13A is made of, for example, a metal foil such as an aluminum foil, a nickel foil, or a stainless steel foil. The positive electrode active material layer 13B includes, for example, one or more positive electrode materials capable of occluding and releasing lithium, which is an electrode reactant, as a positive electrode active material. A conductive agent and a binder such as polyvinylidene fluoride are included.

  As a positive electrode material capable of inserting and extracting lithium, for example, a material containing a composite oxide containing lithium and cobalt is preferable, and includes a first positive electrode material having an average composition shown in Chemical Formula 1. Is preferred. This is because a high energy density can be obtained. In Chemical Formula 1, the element M1 and fluorine (F) are not essential constituent elements. This is because the stability of the positive electrode material is improved by containing these elements, but the capacity decreases as the content increases.

(Chemical formula 1)
Li _a Co _1-b M1 _b O _2-c F _d
(In the formula, M1 is manganese (Mn), nickel, magnesium (Mg), aluminum, boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper, zinc (Zn ), Gallium (Ga), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr) and tungsten (W) It represents at least one of them, and the values of a, b, c and d are 0.8 ≦ a ≦ 1.2, 0 ≦ b ≦ 0.2, −0.1 ≦ c ≦ 0.2, 0 ≦. d ≦ 0.1 The composition of lithium varies depending on the state of charge and discharge, and the value of a represents the value in the complete discharge state.

  As the positive electrode material capable of inserting and extracting lithium, for example, a second positive electrode material having an average composition shown in Chemical Formula 2 is also preferable, and may be used by mixing with the first positive electrode material. . This is because the second positive electrode material has a lower capacity than the first positive electrode material, but high stability can be obtained.

(Chemical formula 2)
_{Li p Mn (1-qr)} Ni q M2 r O 2-s F t
(Wherein M2 is at least one selected from the group consisting of cobalt, magnesium, aluminum, boron, titanium, vanadium, chromium, iron, copper, zinc, gallium, yttrium, zirconium, niobium, molybdenum, tin, calcium, strontium and tungsten. The value of p, q, r, s and t is 0.8 ≦ p ≦ 1.2, 0.1 ≦ q ≦ 0.8, 0 ≦ r ≦ 0.5, q + r ≦ 1, -0.1 ≦ s ≦ 0.2 and 0 ≦ t ≦ 0.1 The composition of lithium varies depending on the state of charge and discharge, and the value of p represents the value in the fully discharged state. .)

However, the proportion of cobalt in the positive electrode active material layer 13B is preferably in the range of 30% by mass to 60% by mass. This is because the capacity decreases as the cobalt content decreases. The 60 mass% is a value when the positive electrode active material layer 13B is made of LiCoO ₂ . For example, in the case of using the positive electrode material described above, the ratio of the first positive electrode material in the positive electrode active material is preferably in the range of 50% by mass to 100% by mass, and the second positive electrode in the positive electrode active material The ratio of the material is preferably 50% by mass or less.

Note that as the positive electrode active material, other positive electrode materials capable of inserting and extracting lithium may be used, and in addition to the first positive electrode material and the second positive electrode material, Instead of the positive electrode material, another positive electrode material capable of inserting and extracting lithium may be used. Examples of other positive electrode materials capable of inserting and extracting lithium include the spinel type lithium composite oxide shown in Chemical Formula 3 or the olivine type lithium composite phosphate shown in Chemical Formula 4, _{Further, MnO 2, V 2 O 5} , V 6 O 13, NiS, include inorganic compounds containing no lithium such as MoS.

(Chemical formula 3)
Li _v Mn _2-w M3 _w O _x F _y
(In the formula, M3 represents at least one selected from the group consisting of cobalt, nickel, magnesium, aluminum, boron, titanium, vanadium, chromium, iron, copper, zinc, molybdenum, tin, calcium, strontium, and tungsten. v, w, x and y are values in the range of 0.9 ≦ v ≦ 1.1, 0 ≦ w ≦ 0.6, 3.7 ≦ x ≦ 4.1, 0 ≦ y ≦ 0.1. (Note that the composition of lithium varies depending on the state of charge and discharge, and the value of v represents the value in a fully discharged state.)

(Chemical formula 4)
Li _z M4PO ₄
(Wherein M4 represents at least one member selected from the group consisting of cobalt, manganese, iron, nickel, magnesium, aluminum, boron, titanium, vanadium, niobium, copper, zinc, molybdenum, calcium, strontium, tungsten and zirconium. Z is a value in the range of 0.9 ≦ z ≦ 1.1, wherein the composition of lithium varies depending on the state of charge and discharge, and the value of z represents the value in the fully discharged state.

  The negative electrode 14 has, for example, a structure in which a negative electrode active material layer 14B is provided on both surfaces of a negative electrode current collector 14A having a pair of opposed surfaces. Although not shown, the negative electrode active material layer 14B may be provided only on one surface of the negative electrode current collector 14A. The negative electrode current collector 14A is made of, for example, a metal foil such as a copper foil, a nickel foil, or a stainless steel foil having good electrochemical stability, electrical conductivity, and mechanical strength. In particular, copper foil is most preferable because it has high electrical conductivity.

  The negative electrode active material layer 22B includes one or more negative electrode materials capable of inserting and extracting lithium as the negative electrode active material, and the positive electrode active material layer 21B as necessary. It is comprised including the binder similar to.

  Examples of the negative electrode material capable of inserting and extracting lithium include non-graphitizable carbon, graphitizable carbon, graphite, pyrolytic carbons, cokes, glassy carbons, and fired organic polymer compounds , Carbon materials such as carbon fiber or activated carbon. Among these, examples of coke include pitch coke, needle coke, and petroleum coke. An organic polymer compound fired body is a carbonized material obtained by firing a polymer material such as a phenol resin or a furan resin at an appropriate temperature, and part of it is non-graphitizable carbon or graphitizable carbon. Some are classified as: These carbon materials are preferable because the change in crystal structure that occurs during charge and discharge is very small, a high charge and discharge capacity can be obtained, and good cycle characteristics can be obtained. In particular, graphite is preferable because it has a high electrochemical equivalent and can provide a high energy density. Further, non-graphitizable carbon is preferable because excellent characteristics can be obtained. Furthermore, those having a low charge / discharge potential, specifically, those having a charge / discharge potential close to that of lithium metal are preferable because a high energy density of the battery can be easily realized.

  Examples of the negative electrode material capable of inserting and extracting lithium include materials capable of inserting and extracting lithium and containing at least one of a metal element and a metalloid element as a constituent element. . This is because a high energy density can be obtained by using such a material. In particular, the use with a carbon material is more preferable because a high energy density can be obtained and excellent cycle characteristics can be obtained. The negative electrode material may be a single element, alloy or compound of a metal element or metalloid element, or may have at least a part of one or more of these phases. In the present invention, alloys include those containing one or more metal elements and one or more metalloid elements in addition to those composed of two or more metal elements. Moreover, the nonmetallic element may be included. 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 constituting the negative electrode material include magnesium, boron, aluminum, gallium, indium (In), silicon (Si), germanium (Ge), tin, lead (Pb), bismuth (Bi), Examples thereof include cadmium (Cd), silver (Ag), zinc, hafnium (Hf), zirconium, yttrium, palladium (Pd), and platinum (Pt). These may be crystalline or amorphous.

  Among these, as the negative electrode material, a material containing a 4B group metal element or a semimetal element in the short-period type periodic table as a constituent element is preferable, and at least one of silicon and tin is particularly preferable as a constituent element. This is because silicon and tin have a large ability to occlude and release lithium, and a high energy density can be obtained.

  Examples of the tin alloy include, as the second constituent element other than tin, silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony (Sb), and chromium. The thing containing at least 1 sort (s) of the group which consists of is mentioned. As an alloy of silicon, for example, as a second constituent element other than silicon, among the group consisting of tin, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony and chromium The thing containing at least 1 sort (s) of these is mentioned.

  Examples of the tin compound or silicon compound include those containing oxygen or carbon (C), and may contain the second constituent element described above in addition to tin or silicon.

Examples of the negative electrode material capable of inserting and extracting lithium further include other metal compounds or polymer materials. Examples of other metal compounds include oxides such as MnO ₂ , V ₂ O ₅ , V ₆ O ₁₃ ), sulfides such as NiS and MoS, and lithium nitrides such as LiN _3. Examples include polyacetylene, polyaniline, and polypyrrole.

  In this secondary battery, by adjusting the amounts of the positive electrode active material and the negative electrode active material, the open circuit voltage (that is, the battery voltage) at the time of full charge is in the range of 4.25V to 6.00V. So that high energy density can be obtained. For example, when the open circuit voltage at the time of full charge is 4.25 V or more, the amount of lithium released per unit mass increases even with the same positive electrode active material as compared to a 4.20 V battery. Accordingly, the amount of the negative electrode active material is adjusted.

  The separator 15 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.

  The electrolyte 16 includes an electrolytic solution and a polymer compound that holds the electrolytic solution, and has a so-called gel shape. The electrolytic solution contains a solvent and a lithium salt.

  The solvent contains more than 70% by mass of a high dielectric constant solvent having a relative dielectric constant of 20 or more. This is because high ionic conductivity can be obtained. Examples of the high dielectric constant solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate or vinylene carbonate, 4-fluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3. A cyclic carbonate derivative having a halogen atom, such as -dioxolan-2-one, 4,5-difluoro-1,3-dioxolan-2-one or 4-fluoromethyl-1,3-dioxolan-2-one, or Lactones such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone.

  The high dielectric constant solvent may be used alone or in combination of two or more. For example, it is preferable to use a mixture of ethylene carbonate and propylene carbonate because high ion conductivity can be obtained and excellent low-temperature characteristics and load characteristics can be obtained. In addition, it is preferable to include vinylene carbonate because the decomposition reaction of the solvent in the electrode can be suppressed.

  The solvent may be composed of only a high dielectric constant solvent, but a low dielectric constant solvent having a relative dielectric constant smaller than 20 may be mixed and used within a range of 30% by mass or less. This is because the low dielectric constant solvent has a low viscosity and may improve low-temperature characteristics and the like, but if the content increases, the battery tends to swell. Examples of the low dielectric constant solvent include chain carbonate esters such as dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate, 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran Or ethers, such as 2-methyltetrahydrofuran, or methyl acetate is mentioned.

Lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiClO ₃ , LiBrO ₃ , LiIO ₃ , LiNO ₃ , LiCH ₃ COO, LiB (C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ , LiCl, LiBr, LiI, difluoro [oxolato-O, O ′] lithium borate, or lithium bisoxalate Examples include borate. A lithium salt may be used individually by 1 type, and 2 or more types may be mixed and used for it.

  The lithium concentration in the electrolyte 16 is preferably in the range of 0.30 mass% to 0.56 mass%. This concentration is a ratio of lithium to the total mass of the electrolyte 16 including the polymer compound. When the battery voltage is higher than 4.20 V, the positive electrode 13 and the electrolyte 16 are deteriorated. In particular, when the proportion of cobalt present in the positive electrode active material layer 13B is large, the charge / discharge efficiency is significantly reduced. It is because the fall of charging / discharging efficiency can be suppressed by carrying out in this range.

  The polymer compound preferably contains, for example, a polymer containing vinylidene fluoride as a component. This is because the oxidation stability is high and the oxidative decomposition reaction in the vicinity of the positive electrode 13 can be suppressed even when the battery voltage is increased. This polymer may be polyvinylidene fluoride or a copolymer containing vinylidene fluoride as a component, and one kind may be used alone, or two or more kinds may be mixed and used. Moreover, in addition to the polymer containing vinylidene fluoride as a component, one or more other polymer compounds may be mixed and used.

  As a copolymer containing vinylidene fluoride as a component, as other components, for example, monoester of unsaturated dibasic acid such as hexafluoropropylene and monomethylmaleic acid ester, halogenated ethylene such as ethylene trifluoride chloride, Examples thereof include cyclic carbonates of unsaturated compounds such as vinylene carbonate, or epoxy group-containing acrylic vinyl monomers. The other components may be one type or two or more types.

  Among these, as this polymer, a copolymer containing vinylidene fluoride and hexafluoropropylene as components is preferable. This is because the adhesiveness and impregnation with the electrode are high, and excellent battery characteristics can be obtained. In particular, these block copolymers are preferable because higher properties can be obtained. The copolymerization amount of hexafluoropropylene in this copolymer is preferably 7% by mass or less. This is because if the copolymerization amount of hexafluoropropylene is too large, the crystallinity of the base polymer is changed, and the mechanical strength and the electrolyte holding capacity are lowered.

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

  First, for example, the positive electrode active material layer 13B is formed on the positive electrode current collector 13A to produce the positive electrode 13. The positive electrode active material layer 13B is prepared, for example, by mixing a positive electrode material capable of inserting and extracting lithium, a conductive agent, and a binder to prepare a positive electrode mixture. By dispersing in a solvent such as methyl-2-pyrrolidone to obtain a paste-like positive electrode mixture slurry, applying this positive electrode mixture slurry to the positive electrode current collector 13A, drying the solvent, and then compression molding with a roll press or the like Form.

  Further, for example, the negative electrode active material layer 14B is formed on the negative electrode current collector 14A to produce the negative electrode 14. The negative electrode active material layer 14B may be formed by, for example, any one of a vapor phase method, a liquid phase method, a firing method, and coating, or a combination of two or more thereof. As the vapor phase method, for example, a physical deposition method or a chemical deposition method can be used. Specifically, a vacuum deposition method, a sputtering method, an ion plating method, a laser ablation method, a thermal CVD (Chemical Vapor Deposition) A chemical vapor deposition method) or a plasma CVD method can be used. As the liquid phase method, a known method such as electrolytic plating or electroless plating can be used. As for the firing method, a known method can be used. For example, an atmosphere firing method, a reaction firing method, or a hot press firing method can be used. In the case of application, it can be formed in the same manner as the positive electrode 13.

  Next, a precursor solution containing an electrolytic solution, a polymer compound, and a mixed solvent is applied to each of the positive electrode 13 and the negative electrode 14, and the mixed solvent is volatilized to form the electrolyte 16. After that, the positive electrode lead 11 is attached to the positive electrode current collector 13A, and the negative electrode lead 12 is attached to the negative electrode current collector 14A. Subsequently, the positive electrode 13 and the negative electrode 14 on which the electrolyte 16 is formed are laminated via a separator 15 to form a laminated body, and then the laminated body is wound in the longitudinal direction, and the protective tape 17 is bonded to the outermost peripheral portion. Thus, the wound electrode body 10 is formed. Finally, for example, the wound electrode body 10 is sandwiched between the exterior members 20, and the outer edge portions of the exterior member 20 are brought into close contact with each other by thermal fusion or the like. At that time, an adhesion film 21 is inserted between the positive electrode lead 11 and the negative electrode lead 12 and the exterior member 20. Thereby, the secondary battery shown in FIGS. 1 and 2 is obtained.

  In the secondary battery, when charged, lithium ions are released from the positive electrode active material layer 13B and inserted in the negative electrode active material layer 14B through the electrolyte 16. Next, when discharging is performed, lithium ions are released from the negative electrode active material layer 14 </ b> B and inserted into the positive electrode active material layer 13 </ b> B through the electrolyte 16. In the present embodiment, the open circuit voltage during full charge is as high as 4.25 V or higher, and the vicinity of the positive electrode 13 is a strong oxidizing atmosphere, but the lithium concentration in the electrolyte 16 is within an appropriate range. Therefore, deterioration of the positive electrode 13 and the electrolyte 16 is suppressed, and high cycle characteristics are obtained.

  As described above, in the present embodiment, the open circuit voltage in the fully charged state per pair of the positive electrode 13 and the negative electrode 14 is set to 4.25 V or more and 6.00 V or less, and the proportion of cobalt present in the positive electrode active material layer 13B is set to 30. Since it was made into the range of the mass% or more and 60 mass% or less, a high energy density can be obtained. In addition, since the lithium concentration in the electrolyte 16 is in the range of 0.30 mass% or more and 0.56 mass% or less, the charge / discharge efficiency can be improved, and high cycle characteristics can be obtained.

  Furthermore, specific examples of the present invention will be described in detail.

(Examples 1-1-1 to 1-3-5)
The secondary battery shown in FIGS. First, as a positive electrode active material, a first positive electrode material having an average composition represented by LiCo _0.95 Al _0.03 Mg _0.02 O ₂ and a second positive electrode material having an average composition represented by LiNi _0.5 Co _0.2 Mn _0.3 O ₂ In Examples 1-1-1 to 1-1-5, only the first positive electrode material was used, and in Examples 1-2-1 to 1-2-5, the first positive electrode material was 80% by mass. In Example 1-3-1 to 1-3-5, 50% by mass of the first positive electrode material and 50% by mass of the second positive electrode material are mixed. Used.

  Next, 92% by mass of the positive electrode active material, artificial graphite powder as a conductive agent, and polyvinylidene fluoride as a binder were mixed and dispersed in N-methyl-2-pyrrolidone as a solvent to obtain a positive electrode mixture slurry. . Subsequently, the positive electrode mixture slurry is uniformly applied to both sides of the positive electrode current collector 13A made of a strip-shaped aluminum foil having a thickness of 20 μm, dried, and compression-molded with a roll press to form the positive electrode active material layer 13B. A positive electrode 13 was produced. The proportion of cobalt present in the positive electrode active material layer 13B is as shown in Table 1, respectively.

  Further, spherical graphite powder is prepared as a negative electrode active material, 90% by mass of this spherical graphite powder and polyvinylidene fluoride as a binder are mixed and dispersed in N-methyl-2-pyrrolidone as a solvent to form a negative electrode. A mixture slurry was obtained. Next, this negative electrode mixture slurry was uniformly applied to both surfaces of a negative electrode current collector 14A made of a strip-shaped copper foil having a thickness of 10 μm, and subjected to hot press molding to form a negative electrode active material layer 14B, whereby a negative electrode 14 was produced. With respect to the positive electrode 13 and the negative electrode 14, the specified charging voltage is 4.40 V, and the theoretical lithium release amount per unit area of the positive electrode 13 and the theoretical lithium occlusion per unit area of the opposing negative electrode 14 at the specified charging voltage. The coating amount of the positive electrode active material and the negative electrode active material was adjusted so that the ratio to the amount was positive electrode / negative electrode = 0.95.

Subsequently, LiPF ₆ is dissolved in a solvent obtained by mixing 49.5% by mass of ethylene carbonate, 49.5% by mass of propylene carbonate, and 1% by mass of vinylene carbonate, and an electrolyte solution is prepared. A block copolymer of vinylidene fluoride and hexafluoropropylene having a weight average molecular weight of about 600,000 was mixed and dissolved using a mixed solvent to prepare a precursor solution. At that time, the amount of LiPF ₆ added was adjusted, and the concentration of lithium in the electrolyte 16 was changed as shown in Table 1. Thereafter, the precursor solution was applied to both surfaces of the positive electrode 13 and the negative electrode 14, and the mixed solvent was volatilized to form the electrolytes 16, respectively. Next, the positive electrode lead 11 was attached to the positive electrode current collector 13A, and the negative electrode lead 12 was attached to the negative electrode current collector 14A.

  Subsequently, the positive electrode 13 and the negative electrode 14 on which the electrolyte 16 was formed were laminated via a separator 15 made of a microporous polyolefin film and wound to produce a wound electrode body 10. After that, the wound electrode body 10 was sandwiched between exterior members 20 made of an aluminum laminate film, and the peripheral portion thereof was brought into close contact and sealed to obtain a secondary battery.

  As Comparative Examples 1-1-1, 1-1-2, 1-2-1, 1-2-2, 1-3-1, 1-3-3 for the present example, the concentration of lithium in the electrolyte is shown. A secondary battery was fabricated in the same manner as in this example except for the change as shown in FIG. Further, as Comparative Examples 1-4-1 to 1-5-7, except that the ratio of the first positive electrode material and the second positive electrode material and the concentration of lithium in the electrolyte were changed as shown in Table 1. Other than that, a secondary battery was fabricated in the same manner as in this example. Furthermore, as Comparative Examples 1-6-1 to 1-10-7, the specified charging voltage was 4.20 V, the ratio of the first positive electrode material and the second positive electrode material and the concentration of lithium in the electrolyte were shown in Table 2. A secondary battery was fabricated in the same manner as in this example, except that the changes were made as described above.

  About the produced secondary battery of the Example and the comparative example, charging / discharging was performed, and the discharge capacity maintenance ratio of the 200th cycle and the 400th cycle with respect to the discharge capacity of the 2nd cycle and the discharge capacity of the 1st cycle was investigated. At that time, at 23 ° C., constant current charging was performed until the battery voltage reached a specified value at a current value at which the theoretical capacity could be discharged in 2 hours, and then constant voltage charging was performed for 5 hours at the specified constant voltage. Fully charged. The specified voltage value is 4.40 V in Examples and Comparative Examples 1-1-1 to 1-5-7, and 4.20 V in Comparative Examples 1-6-1 to 1-10-7. Discharge was performed at 23 ° C. until the battery voltage reached 3.0 V at a current value that could discharge the theoretical capacity in 2 hours, and a complete discharge state was obtained. The obtained results are shown in Tables 1 and 2 and FIGS. The discharge capacity is expressed as a relative value with the value of Comparative Example 1-10-1 as 1.00.

  As shown in Tables 1 and 2 and FIGS. 3 and 5, a higher discharge capacity was obtained when the charging voltage was increased and the proportion of cobalt present in the positive electrode active material layer 13B was increased. In addition, as shown in Tables 1 and 2 and FIGS. 4 and 6, Examples 1-1-1 to 1-3-5 and Comparative Examples 1-1-1 to 1-5 in which the charging voltage was 4.40V were used. In −7, when the cobalt content in the positive electrode active material layer 13B was increased to 32% by mass or more, a high capacity retention rate could be obtained only when the lithium concentration in the electrolyte 16 was within a predetermined range. In contrast, when the proportion of cobalt present in the positive electrode active material layer 13B was low, a high capacity retention rate was obtained regardless of the lithium concentration in the electrolyte. On the other hand, in Comparative Examples 1-6-1 to 1-10-7 in which the charging voltage was 4.20 V, even if the abundance ratio of cobalt in the positive electrode active material layer was changed or the lithium concentration in the electrolyte was changed, the capacity There was no significant difference in maintenance rates.

  That is, if the open circuit voltage in the fully charged state is higher than 4.20 V and the proportion of cobalt in the positive electrode active material layer 13B is in the range of 30% by mass to 60% by mass, high energy is obtained. The density can be obtained, and if the lithium concentration in the electrolyte 16 is in the range of 0.30% by mass or more and 0.56% by mass or less, the charge / discharge efficiency can be improved and high cycle characteristics can be obtained. I understood.

(Examples 2-1-1 to 2-3-5)
Except that the specified charging voltage was 4.50 V, 4.35 V, or 4.25 V and the concentration of lithium in the electrolyte 16 was changed as shown in Table 3, the other examples were 1-1-1. A secondary battery was produced in the same manner as in 1-1-5. As Comparative Examples 2-1-1 to 2-3-2 with respect to this example, except that the charging voltage and the lithium concentration in the electrolyte were changed as shown in Table 3, the other examples were the same as in this example. A secondary battery was manufactured.

  Also in Examples 2-1-1 to 2-3-5 and Comparative Examples 2-1-1 to 2-3-2, charging and discharging were performed in the same manner as in Examples 1-1-1 to 1-1-5. The discharge capacity at the second cycle and the discharge capacity maintenance ratio at the 200th cycle and the 400th cycle with respect to the discharge capacity at the first cycle were examined. The results obtained are shown in Table 3 together with the results of Examples 1-1-1 to 1-1-5 and Comparative examples 1-1-1, 1-1-2, 1-6-1 to 1-6-7. And shown in FIG.

  As shown in Table 3 and FIG. 7, as the charging voltage was increased, the discharge capacity was improved, and the difference in the discharge capacity retention ratio depending on the lithium concentration tended to increase. That is, it was found that a high effect can be obtained when the open circuit voltage in the fully charged state is set to 4.25 V or more.

  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 embodiments and examples described above, the case where the electrolyte 16 contains a polymer compound has been described, but the electrolytic solution may be used as it is as a liquid electrolyte.

  Further, in the above-described embodiments and examples, the secondary battery having the winding structure in which the positive electrode 13 and the negative electrode 14 are stacked and wound has been described. However, the present invention folds or stacks the positive electrode and the negative electrode. The present invention can be similarly applied to a secondary battery having the above structure. In addition, a can exterior member may be used instead of a film-like exterior member, and can be applied to a so-called coin-type, button-type, cylindrical-type, or square-type secondary battery. Moreover, not only a secondary battery but a primary battery is applicable.

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 along the II line of the winding electrode body shown in FIG. It is a characteristic view showing the relationship between the abundance ratio of cobalt in the positive electrode active material layer, the lithium concentration in the electrolyte, and the discharge capacity when the charging voltage is 4.40V. It is a characteristic view showing the relationship between the abundance ratio of cobalt in the positive electrode active material layer, the lithium concentration in the electrolyte, and the discharge capacity retention rate when the charging voltage is 4.40V. It is a characteristic view showing the relationship between the abundance ratio of cobalt in the positive electrode active material layer, the lithium concentration in the electrolyte, and the discharge capacity when the charging voltage is 4.20V. It is a characteristic view showing the relationship between the abundance ratio of cobalt in the positive electrode active material layer, the lithium concentration in the electrolyte, and the discharge capacity retention rate when the charging voltage is 4.20V. It is a characteristic view showing the relationship between a charging voltage, the lithium concentration in an electrolyte, and a discharge capacity maintenance factor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Positive electrode lead, 12 ... Negative electrode lead, 13 ... Positive electrode, 13A ... Positive electrode collector, 13B ... Positive electrode active material layer, 14 ... Negative electrode, 14A ... Negative electrode collector, 14B ... Negative electrode active material layer, 15 ... Separator, 16 ... electrolyte, 17 ... protective tape, 20 ... exterior member, 21 ... adhesive film.

Claims (6)

A battery comprising an electrolyte together with a positive electrode and a negative electrode,
The open circuit voltage in a fully charged state per pair of positive electrode and negative electrode is in the range of 4.25V to 6.00V,
The positive electrode is provided with a positive electrode active material layer containing a positive electrode active material containing cobalt (Co) as a constituent element on a positive electrode current collector,
The proportion of cobalt present in the positive electrode active material layer is in the range of 30% by mass to 60% by mass,
The electrolyte contains a lithium salt and a solvent containing a high dielectric constant solvent having a relative dielectric constant of 20 or more in excess of 70% by mass;
The battery is characterized in that the concentration of lithium (Li) in the electrolyte is in the range of 0.30 mass% to 0.56 mass%. 2. The battery according to claim 1, wherein the positive electrode active material includes a positive electrode material having an average composition shown in Chemical Formula 1 within a range of 50 mass% to 100 mass%.
(Chemical formula 1)
Li _a Co _1-b M1 _b O _2-c F _d
(In the formula, M1 is manganese (Mn), nickel (Ni), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe). , Copper (Cu), zinc (Zn), gallium (Ga), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr) And at least one member selected from the group consisting of tungsten (W), and the values of a, b, c and d are 0.8 ≦ a ≦ 1.2, 0 ≦ b ≦ 0.2, −0.1. ≦ c ≦ 0.2 and 0 ≦ d ≦ 0.1. The battery according to claim 2, wherein the positive electrode active material further contains a positive electrode material having an average composition shown in Chemical formula 2 within a range of 50 mass% or less.
(Chemical formula 2)
_{Li p Mn (1-qr)} Ni q M2 r O 2-s F t
(Wherein M2 is at least one selected from the group consisting of cobalt, magnesium, aluminum, boron, titanium, vanadium, chromium, iron, copper, zinc, gallium, yttrium, zirconium, niobium, molybdenum, tin, calcium, strontium and tungsten. The value of p, q, r, s and t is 0.8 ≦ p ≦ 1.2, 0.1 ≦ q ≦ 0.8, 0 ≦ r ≦ 0.5, q + r ≦ 1, -0.1 ≦ s ≦ 0.2 and 0 ≦ t ≦ 0.1.) The battery according to claim 1, wherein the electrolyte contains a polymer containing vinylidene fluoride as a component. The battery according to claim 1, wherein the electrolyte contains vinylene carbonate. The battery according to claim 1, wherein the positive electrode, the negative electrode, and the electrolyte are housed in a film-shaped exterior member.

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