JP2007194202A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which has excellent cycle characteristics in case charge/discharge cycles are conducted under a charging voltage of 4.25 V or more and 6.00 V or less. <P>SOLUTION: The lithium ion secondary battery is composed of a cathode 2 having an active substance and a binder, an anode 3, and an electrolyte as well as a separator 4 positioned between the cathode 2 and the anode 3, and an open circuit voltage per a unit cell in a full charge is 4.25 V or more and 6.00 V or less. For such a lithium ion secondary battery, a binder containing polyacrylonitrile group resin is used to obtain excellent cycle characteristics. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to a positive electrode applicable to a lithium ion secondary battery.

  In recent years, with the widespread use of portable information electronic devices such as mobile phones, video cameras, and notebook computers, the performance, size, and weight of the devices are rapidly developing. The power source used in these devices is a disposable primary battery or a reusable secondary battery. However, these power supplies have good overall balance such as economy, high performance, small size and light weight. Among batteries, demand for lithium ion secondary batteries is growing.

  Recently, in order to achieve higher performance of portable information electronic devices, in particular, (1) higher energy density of lithium ion secondary batteries and (2) improvement of cycle characteristics are required. Hereinafter, (1) increasing the energy density of the lithium ion secondary battery and (2) improving the cycle characteristics will be sequentially described.

(1) High energy density of lithium ion secondary battery For increasing the energy density, it is one of effective methods to use a positive electrode having a high discharge capacity per unit volume. In order to realize such a positive electrode It is known that (a) the selection of the active material and (b) the increase of the charging upper limit voltage are important. However, in recent years, research on increasing the energy by (b) increasing the charging upper limit voltage has been actively conducted. Has been made.

In addition to LiCoO ₂ and the like, LiNiO ₂ and LiMn ₂ O _{4 and the} like are known as positive electrode active materials for lithium ion secondary batteries. LiNiO ₂ has a relatively high capacity of about 190 mAhg ^−1, but it is necessary to lower the discharge cutoff voltage in order to obtain the capacity. However, since the average discharge voltage is low, it can be said that it is not suitable for applications such as notebook computers that require high power. LiMn ₂ O ₄ has a low capacity and is not suitable for increasing the energy density of a lithium ion secondary battery.

For the above reasons, lithium-containing transition metal oxides such as LiCoO ₂ having a high average discharge voltage are desirable for high charge pressure lithium ion secondary batteries intended for notebook computers. In a lithium ion secondary battery using LiCoO ₂ as a positive electrode active material and a carbon material as a negative electrode active material, the end-of-charge voltage is 4.1 V to 4.2 V. Under such charging conditions, the positive electrode is only used about 50% to 60% of the theoretical capacity. Therefore, if the charging voltage can be increased, the capacity of the positive electrode can be utilized at 70% or more of the theoretical capacity, and the capacity and the energy density of the lithium ion secondary battery can be increased. .

  In fact, for example, as described in Patent Document 1, it is known that high energy density is manifested by setting the voltage during charging to 4.30 V or higher.

International Publication No. WO03 / 019731 Pamphlet

(2) Improvement of cycle characteristics In lithium ion secondary batteries, a positive electrode active material such as a lithium-containing transition metal composite oxide, a binder such as a fluorine resin, a positive electrode mixture comprising a conductive agent, etc., is used as an aluminum foil The one coated with is used. For example, Patent Document 2 describes that cycle characteristics can be improved by adding an anti-settling agent containing a copolymer of an α-olefin and an α, β-unsaturated carboxylic acid to the positive electrode mixture.

JP 2004-247292 A

  However, when the inventors set the charging voltage of a conventional lithium ion secondary battery operating at a maximum of 4.2 V to exceed 4.20 V, the amount of discharge that can be taken out per cycle decreases. The problem inherent to this battery system was revealed.

  This may be due to a plurality of factors such as an increase in electron transfer resistance due to a decrease in the contact area of the active material, conductive agent, and current collector, alteration of the electrolyte, and an increase in diffusion resistance due to an increase in the surface coating. Among these factors, the increase in the electron transfer resistance due to the decrease in the contact area of the active material, the conductive agent, and the current collector increases the adhesion of the positive electrode mixture in a high oxidizing atmosphere by increasing the upper limit voltage of charging. The decline is considered to be one of the factors.

  Actually, the inventors of the present invention performed a charge / discharge cycle with a battery using a PVDF (polyvinylidene fluoride) binder, which is a fluororesin, at a charge voltage higher than the upper limit voltage of 4.2 V and the upper limit voltage of 4.2 V. When the battery after the charge / discharge cycle was disassembled and the positive electrode was taken out, peeling of the positive electrode mixture and the current collector was more remarkable in the battery that was charged / discharged at a charge voltage higher than 4.2V. I was able to confirm.

  As described above, in the PVDF binder which is a fluorine-based resin, the adhesion of the positive electrode mixture is lowered, and when the charge / discharge cycle is performed at a charge voltage higher than the upper limit voltage 4.2 V, the cycle characteristics are remarkably deteriorated. It became clear that. In addition, it was found that in the battery that was subjected to a charge / discharge cycle higher than the upper limit voltage of 4.2 V, the cycle characteristics deteriorated remarkably at high temperatures.

  Accordingly, an object of the present invention is to provide a lithium ion secondary battery having good cycle characteristics particularly at high temperatures when a charge / discharge cycle is performed under a charge voltage of 4.25V to 6.00V. is there.

  The inventors of the present application include a positive electrode having at least a positive electrode active material and a binder, a negative electrode, an electrolyte and a separator interposed between the positive electrode and the negative electrode, and an open circuit voltage per unit cell at full charge is 4 It has been found that in a lithium ion secondary battery having a voltage of 0.25 V or more and 6.00 V or less, good cycle characteristics can be obtained particularly at high temperatures by using a binder containing a polyacrylonitrile resin.

That is, in order to solve the above-described problem, the present invention
A positive electrode having at least a positive electrode active material and a binder, a negative electrode, and an electrolyte and a separator interposed between the positive electrode and the negative electrode;
A lithium ion secondary battery having an open circuit voltage per unit cell in a fully charged state of 4.25V to 6.00V,
The positive electrode active material has a general formula: Li _a Co _1-x Me _x O _2-b (wherein Me is V (vanadium), Cu (copper), Zr (zirconium), Zn (zinc), Mg (magnesium)) ), Al (aluminum) and Fe (iron), at least one or more metal elements are selected, where a is 0.9 ≦ a ≦ 1.1, x is 0 ≦ x ≦ 0.3, b takes a value of -0.1 ≦ b ≦ 0.1 lithium expressed in) - cobalt composite oxide, and the general _{formula:. Li a Ni 1-xyz} Co x Mn y Me z O 2-b ( formula Me is at least one or more selected from V (vanadium), Cu (copper), Zr (zirconium), Zn (zinc), Mg (magnesium), Al (aluminum) and Fe (iron). A represents a metal element, a is 0.9 ≦ a ≦ 1.1, x is 0 <x <0.4, and y is 0 <Y <0.4, z is 0 <z <0.3, and b has a value of −0.1 ≦ b ≦ 0.1.) The lithium-cobalt-nickel-manganese oxide represented by Including any
The binder is a lithium ion secondary battery characterized by containing a polyacrylonitrile-based resin.

  In this invention, by using a binder containing a polyacrylonitrile-based resin, it is possible to improve the decrease in the adhesion of the positive electrode mixture under a high oxidizing atmosphere due to an increase in the upper limit voltage for charging. In a lithium ion secondary battery having an open circuit voltage per cell of 4.25 V to 6.00 V, good cycle characteristics can be obtained particularly at high temperatures.

  According to the present invention, in a lithium ion secondary battery having an open circuit voltage per unit cell in a full charge of 4.25 V or more and 6.00 V or less, good cycle characteristics can be obtained particularly at high temperatures.

(1) First Embodiment (1-1) Configuration of Lithium Ion Secondary Battery Hereinafter, embodiments of the present invention will be described with reference to the drawings. In a first embodiment of the present invention, in a lithium ion secondary battery that includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, and an open circuit voltage per unit cell in a fully charged state is 4.25 V or more and 4.60 V or less, The positive electrode active material layer has a binder containing a polyacrylonitrile-based resin.

  Here, the fully charged state means a final state when the battery is charged with a current value of 0.5 C or less or a constant current-constant voltage method (the constant voltage unit is voltage cut with a current value of 0.1 C or less). C is charging current value (mA) / battery capacity or electrode capacity (mA). The charge potential of the positive electrode in the fully charged state can be measured, for example, by opening a hole through which the electrolyte can enter and exit the battery, immersing the battery in a test cell into which the electrolyte is injected, and using lithium as a reference electrode.

  As the negative electrode active material, for example, when a carbon material having a negative electrode charging potential of 0.1 V (vs. Li / Li +) in a fully charged state is used, the lithium ion secondary according to the first embodiment of the present invention is used. The battery is charged with an end-of-charge voltage of 4.35V or higher.

  FIG. 1 shows a cross-sectional structure of a lithium ion secondary battery according to a first embodiment of the present invention. This battery is called a so-called cylindrical type, and a wound electrode body 20 in which a belt-like positive electrode 2 and a belt-like negative electrode 3 are wound through a separator 4 inside a substantially hollow cylindrical battery can 1. have.

  The battery can 1 is made of, for example, iron (Fe) plated with nickel (Ni), and has one end closed and the other end open. Inside the battery can 1, a pair of insulating plates 5 and 6 are arranged perpendicular to the winding peripheral surface so as to sandwich the winding electrode body 20.

  At the open end of the battery can 1, a battery lid 7, a safety valve mechanism 8 and a thermal resistance element (PTC element: Positive Temperature Coefficient) 9 provided inside the battery lid 7 are interposed via a gasket 10. It is attached by caulking, and the inside of the battery can 1 is sealed. The battery lid 7 is made of, for example, the same material as the battery can 1. The safety valve mechanism 8 is electrically connected to the battery lid 7 via a heat sensitive resistance element 9, and the disk plate 11 is reversed when the internal pressure of the battery becomes a certain level or more due to internal short circuit or external heating. Thus, the electrical connection between the battery lid 7 and the wound electrode body 20 is cut off. The heat-sensitive resistor element 9 limits the current by increasing the resistance value when the temperature rises, and prevents abnormal heat generation due to a large current. The gasket 10 is made of, for example, an insulating material, and asphalt is applied to the surface.

  The wound electrode body 20 is wound around, for example, the center pin 12. A positive electrode lead 13 made of aluminum or the like is connected to the positive electrode 2 of the wound electrode body 20, and a negative electrode lead 14 made of nickel or the like is connected to the negative electrode 3. The positive electrode lead 13 is welded to the safety valve mechanism 8 to be electrically connected to the battery lid 7, and the negative electrode lead 14 is welded to and electrically connected to the battery can 1.

  FIG. 2 is an enlarged view of a part of the spirally wound electrode body 20 shown in FIG. As shown in FIG. 2, the positive electrode 2 has, for example, a structure in which a positive electrode active material layer 2B is provided on both surfaces of a strip-shaped positive electrode current collector 2A. The negative electrode 3 has a structure in which a negative electrode active material layer 3B is provided on both surfaces of a strip-shaped negative electrode current collector 3A. The positive electrode 2 and the negative electrode 3 are opposed to each other with the separator 4 interposed therebetween.

[Positive electrode]
The positive electrode 2 can be obtained by applying a positive electrode mixture obtained by mixing a positive electrode active material, a conductive agent, a binder, and the like to the surface of the current collector 3A. Specifically, the positive electrode 2 is formed by applying a positive electrode mixture slurry composed of a powdered positive electrode active material, a conductive agent, a binder and a solvent or dispersion medium of a binder to a positive electrode current collector 2A such as an aluminum foil. The positive electrode active material layer 2B can be formed on both surfaces of the positive electrode current collector 2A by being processed, dried and press-rolled.

[Positive electrode active material]
As the positive electrode active material, a lithium-containing compound can be used. As the compound containing lithium, for example, lithium oxide, lithium sulfide, or an intercalation compound containing lithium is suitable, and two or more of these may be used in combination. For example, as a cathode active material, the general _{formula: Li p Ni (1-qr} ) Mn q M1 r O in _(2-y) X _{z (wherein,} M1 is nickel (Ni), 2 group except for manganese (Mn) ~ At least one element selected from Group 15 and X represents at least one element selected from Group 16 and Group 17 elements other than oxygen (O), and p, q, r, y, and z are 0 ≦ p ≦ 1. .5, 0 ≦ q ≦ 1.0, 0 ≦ r ≦ 1.0, −0.10 ≦ y ≦ 0.20, and 0 ≦ z ≦ 0.2. It is preferable that lithium composite oxide is included.

  Specifically, examples of the lithium composite oxide include the following.

(Chemical formula 1)
Li _a Co _1-x Me _x O _2-b
(Chemical formula 2)
Li _a Ni _1-x Me _x O _2-b
(Chemical formula 3)
Li _a Mn _1-x Me _x O _2-b
(In the chemical formulas 1 to 3, Me is at least one selected from V (vanadium), Cu (copper), Zr (zirconium), Zn (zinc), Mg (magnesium), Al (aluminum) and Fe (iron). Represents a seed or two or more metal elements, a is 0.9 ≦ a ≦ 1.1, x is 0 ≦ x ≦ 0.3, and b is −0.1 ≦ b ≦ 0.1. .)

(Chemical formula 4)
Li _a Ni _1-xy Co _x Me _y O _2-b
(Chemical formula 5)
Li _a Ni _1-xy Mn _x Me _y O _2-b
(Chemical formula 6)
Li _a Co _1-xy Mn _x Me _y O _2-b
(In the chemical formulas 4 to 6, Me is at least one selected from V (vanadium), Cu (copper), Zr (zirconium), Zn (zinc), Mg (magnesium), Al (aluminum), and Fe (iron). A represents 0.9 ≦ a ≦ 1.1, x represents 0 ≦ x ≦ 0.05, y represents 0 ≦ y ≦ 0.05, b represents −0.1 ≦ b ≦ 0.1.)

(Chemical formula 7)
_{Li a Ni 1-xyz Co x} Mn y Me z O 2-b
(In the chemical formula 7, Me is at least one selected from V (vanadium), Cu (copper), Zr (zirconium), Zn (zinc), Mg (magnesium), Al (aluminum) and Fe (iron) or 2 A represents 0.9 ≦ a ≦ 1.1, x represents 0 <x <0.4, y represents 0 <y <0.4, z represents 0 <z <0.3, b takes a value of −0.1 ≦ b ≦ 0.1.)

  Among these lithium composite oxides, the lithium-cobalt composite oxide represented by Chemical Formula 1 and the lithium-cobalt-nickel-manganese composite oxide represented by Chemical Formula 7 are preferable.

  The lithium-cobalt composite oxide represented by Chemical Formula 1 has an R-3m rhombohedral structure. Moreover, for lithium-cobalt composite oxide, in order to improve charge / discharge cycle durability and discharge characteristics, calcium (Ca), magnesium (Mg), titanium (Ti), tantalum (Ta), niobium (Nb), Metals such as zirconium (Zr), hafnium (Hf), and aluminum (Al) are added in an atomic ratio of 0.001 to 5% with respect to cobalt (Co). For example, lithium-cobalt-aluminum composite oxide, lithium- A cobalt-aluminum-magnesium composite oxide may be used.

  The structure of the lithium-cobalt-nickel-manganese composite oxide represented by Chemical Formula 7 is preferably an R-3m rhombohedral structure. In the chemical formula 7, it is not preferable that (1-xyz) is less than 0.20 because it is difficult to obtain a stable R-3m rhombohedral structure. Further, in the chemical formula 7, if (1-xyz) exceeds 0.60, safety is not preferable. In the chemical formula 7, y is particularly preferably 0.25 to 0.55. In chemical formula 7, a ≦ 0.9 ≦ a ≦ 1.2 is adopted because a is a capacity expression.

  By adding any atom of Fe, Cr, and Al to the lithium-nickel-cobalt-manganese composite oxide represented by Chemical Formula 7, the charge / discharge cycle durability, safety, capacity, and the like can be improved. . The addition amount z of Me atoms is 0 to 0.2, preferably 0.01 to 0.18, and more preferably 0.05 to 0.16.

  Furthermore, it is preferable to use a mixture of the lithium-cobalt composite oxide represented by Chemical Formula 1 and the lithium-cobalt-nickel-manganese composite oxide represented by Chemical Formula 7 as the positive electrode active material. This is because a higher charge capacity can be used and a high filling property can be obtained, so that a high discharge capacity per unit volume tends to be obtained. Moreover, it is because the battery performance which the balance of capacity | capacitance and safety improved can be expressed rather than the case where each single lithium transition metal complex oxide used for mixing is used. In addition, the battery performance is superior in capacity, safety and charge / discharge cycle stability compared to the case of using a positive electrode active material composed of a single lithium transition metal composite oxide and having the same content of the transition metal element used for mixing. It is because it can obtain.

  The reason why the mixture of the lithium-cobalt composite oxide represented by Chemical Formula 1 and the lithium-cobalt-nickel-manganese composite oxide represented by Chemical Formula 7 is superior to that of a single substance is not clear. The lithium-nickel-cobalt-manganese composite oxide thus produced is considered to have a synergistic effect by mixing because of its particularly high safety and relatively good capacity development.

The mixture powder of the lithium-cobalt composite oxide represented by Chemical Formula 1 and the lithium-cobalt-nickel-manganese composite oxide represented by Chemical Formula 7 is obtained when only the powder is press-filled at a pressure of 1 t / cm ² . The powder press density is preferably 3.0 g / cm ³ or more, and more preferably 3.20 g / cm ³ . By using such a mixture, the volume per volume can be increased when the mixture is made into a slurry and coated, dried, and pressed on the current collector aluminum foil.

Such a powder press density of 3.0 g / cm ³ or more is achieved by optimizing the particle size distribution of the mixture powder. That is, the particle size distribution is wide, the volume fraction of small particle size is 20% to 50%, and the particle size distribution of large particle size is narrowed to increase the density of the powder press. It is done.

BET specific surface area of the positive electrode active material is preferably 0.05m ² /g~10.0m ² / g, more preferably 0.1m ² /g~5.0m ² / g. By setting it as such a range, the reactivity of the positive electrode active material and electrolyte solution in a high electric potential can be suppressed.

  As a method for producing the lithium composite oxide, a known production method can be used. Specifically, for example, a method in which a lithium compound and a metal compound are mixed and heat-treated, a lithium compound and a metal in solution A wet method obtained by reacting a compound can be used.

[Conductive agent]
As the conductive agent, a carbon-based conductive material such as acetylene black, graphite, or ketjen black can be used.

[binder]
As the binder, one containing a polyacrylonitrile resin can be used. Examples of the polyacrylonitrile resin include compounds having a nitrile group such as poly (meth) acrylonitrile, for example, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, 2- Alkyl (meth) acrylates such as ethylhexyl (meth) acrylate, lauryl (meth) acrylate, cyclohexyl (meth) acrylate, cycloalkyl (meth) acrylate, 2-hydroxylethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, etc. Aminoalkyl such as hydroxyalkyl (meth) acrylate, aminomethyl (meth) acrylate, N-methylaminomethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate (Meth) acrylate, methacrylic acid, acrylic acid, styrene monomers such as styrene vinyl toluene, α-methyl styrene, vinyl derivatives such as vinyl chloride, vinylidene chloride, vinyl acetate, isopropenyl acetate, maleic acid, fumaric acid, etc. What copolymerized unsaturated dibasic acid etc. is mentioned. These may be used alone or in combination of two or more.

Moreover, as a binder, it is desirable to include the compound which has an ester bond represented by the following formula (I). The monomer represented by the formula (I) is not particularly limited, but R ₁ is preferably H or CH ₃ . R ₂ is preferably a divalent hydrocarbon group. Here, the divalent hydrocarbon group is suitably, for example, an alkylene group having 4 to 100 carbon atoms, an alkylene group having 6 to 50 carbon atoms, or an alkylene group having 8 to 15 carbon atoms. The alkylene group may be linear or branched. The alkylene group may be partially substituted with halogen such as fluorine, chlorine, bromine, iodine, nitrogen, phosphorus, aromatic ring, C3-C10 cycloalkane, or the like. In formula (I), R ₂ is suitably an alkylene group having 2 to 6 carbon atoms, preferably an alkylene group having 2 to 4 carbon atoms. In addition, R ₂ is 2 to 20, preferably 2 to 10, more preferably 2 to 5 C 2-6 alkylene groups linked via an ether bond and / or an ester bond. It may be a group. Here, the alkylene group may be substituted with halogen such as fluorine, chlorine, bromine and iodine, nitrogen, phosphorus, aromatic ring, C3-C10 cycloalkane and the like. Specific examples include triethylene glycol monoacrylate such as monoethylene glycol monomethacrylate.

式（Ｉ）

Formula (I)

  Moreover, as polyacrylonitrile-type resin, what has at least 1 type of functional group chosen from an alcoholic hydroxyl group, a carboxyl group, or a nitrile group in a molecule | numerator is preferable.

  Further, the binder is preferably a polyacrylonitrile resin containing a carboxyl group, and more preferably a mixture of a polyacrylonitrile resin containing a carboxyl group and a vinylidene fluoride polymer.

  The ratio of the polyacrylonitrile copolymer resin having a carboxyl group and the vinylidene fluoride polymer is, by volume, vinylidene fluoride polymer: polyacrylonitrile copolymer resin having a carboxyl group = 1: 99 to 99: 1. More preferably, the volume ratio of vinylidene fluoride polymer: polyacrylonitrile copolymer resin having a carboxyl group is 20:80 to 80:20.

  The vinylidene fluoride polymer includes a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride, and modified products thereof. The vinylidene fluoride polymer preferably has an intrinsic viscosity in the range of 1.7 dl / g to 20 dl / g, and more preferably 2.0 dl / g to 15 dl / g.

  The ratio of the binder in the positive electrode mixture is 1% by mass to 7% by mass, preferably 2% by mass to 5% by mass. If it is 1% by mass or less, the binding property cannot be maintained, and the active material may not be immobilized on the current collector. If the content is 7% by mass or more, a binder having no electron conductivity and ionic conductivity may cover the positive electrode active material, which may prevent rapid charge / discharge.

[Negative electrode]
The negative electrode 3 can be obtained by applying a negative electrode mixture obtained by mixing a negative electrode active material, a conductive agent, a binder, and the like to the surface of the negative electrode current collector 3A to provide a negative electrode active material layer 3B.

[Negative electrode active material]
As the negative electrode active material, for example, carbon materials, metal compounds, oxides, sulfides, lithium nitrides such as LiN ₃ , lithium metals, metals that form alloys with lithium, or polymer materials can be used.

  As the carbon material, for example, non-graphitizable carbon, graphitizable carbon, graphite, pyrolytic carbons, cokes, glassy carbons, organic polymer compound fired bodies, carbon fibers or activated carbon can be used. . 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:

  Examples of the polymer material include polyacetylene and polypyrrole. Among such negative electrode materials which can be doped / undoped with lithium, those having a charge / discharge potential relatively close to lithium metal are preferable. This is because the lower the charge / discharge potential of the negative electrode, the easier it is to increase the energy density of the battery. Among these, a carbon material is preferable in that a change in crystal structure that occurs during charge / discharge is very small, a high charge / discharge capacity can be obtained, and good cycle characteristics can be obtained. In particular, graphite is preferable because it has a large electrochemical equivalent and can obtain a high energy density. Furthermore, non-graphitizable carbon is preferable because excellent cycle characteristics can be obtained.

  In addition, as the negative electrode material capable of doping / de-doping lithium, lithium metal alone, a metal element or metalloid element capable of forming an alloy with lithium, an alloy, or a compound can be used. These are preferable because a high energy density can be obtained, and in particular, when used together with a carbon material, a high energy density can be obtained and excellent cycle characteristics can be obtained, and therefore, it is more preferable.

  Note that in this specification, alloys include those composed of one or more metal elements and one or more metalloid elements in addition to those composed of two or more metal elements. The structures include solid solutions, eutectics (eutectic mixtures), intermetallic compounds, or those in which two or more of them coexist.

  Examples of such metal elements or metalloid elements include tin (Sn), lead (Pb), aluminum, indium (In), silicon (Si), zinc (Zn), antimony (Sb), bismuth (Bi), and cadmium. (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y) or hafnium (Hf). be able to.

These alloys or compounds, for example, may include those represented by the chemical formula Ma _S Mb _t Li _u or a chemical formula Ma _p Mc _q Md _r,. In these chemical formulas, Ma represents at least one of a metal element and a metalloid element capable of forming an alloy with lithium, Mb represents at least one of a metal element and a metalloid element other than lithium and Ma, Mc represents at least one of nonmetallic elements, and Md represents at least one of metallic elements and metalloid elements other than Ma. The values of s, t, u, p, q, and r are s> 0, t ≧ 0, u ≧ 0, p> 0, q> 0, and r ≧ 0, respectively.

  Among these, a simple substance, alloy or compound of the group 4B metal element or metalloid element in the short-period type periodic table is preferable, and silicon, tin, or an alloy or compound thereof is particularly preferable. These may be crystalline or amorphous.

[Conductive agent]
The conductive agent for the negative electrode is not particularly limited as long as it is an electron conductive material. For example, graphite such as artificial graphite and expanded graphite, carbon such as acetylene black, ketjen black, channel black, and furnace black Examples thereof include conductive fibers such as blacks, carbon fibers and metal fibers, metal powders such as copper and nickel, and organic conductive materials such as polyphenylene derivatives, and these can be used alone or as a mixture. Among these conductive agents, acetylene black, ketjen black, and carbon fiber are particularly preferable. Although the addition amount of a electrically conductive agent is not specifically limited, 0.1 mass part-30 mass parts are preferable with respect to 100 mass parts of negative electrode active materials, Furthermore, 0.5 mass part-10 mass parts are more preferable.

[binder]
As the negative electrode binder, for example, polytetrafluoroethylene, polyvinylidene fluoride, or the like is preferably used.

  The negative electrode current collector 3A is not particularly limited as long as it is an electronic conductor that does not cause a chemical change in the configured battery. Examples of such a material include stainless steel, nickel, copper, Titanium etc. can be mentioned. Moreover, copper is preferable as such a material. Although thickness is not specifically limited, Preferably it is the range of 1 micrometer-100 micrometers, More preferably, it is the range of 5 micrometers-30 micrometers.

[Electrolyte]
As the electrolytic solution, a nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a nonaqueous solvent can be used. As the non-aqueous solvent, for example, it is preferable that at least one of ethylene carbonate and propylene carbonate is included from the viewpoint of improving cycle characteristics. Moreover, from the point which can improve cycling characteristics more, what mixed and contains ethylene carbonate and propylene carbonate, for example is preferable.

  Furthermore, the non-aqueous solvent preferably contains at least one of chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, or methyl propyl carbonate from the viewpoint that the cycle characteristics can be further improved. .

  Furthermore, the non-aqueous solvent preferably contains at least one of 2,4-difluoroanisole and vinylene carbonate. This is because 2,4-difluoroanisole can improve the discharge capacity. This is because vinylene carbonate can further improve cycle characteristics. In particular, it is more preferable that they are mixed and contained because both the discharge capacity and the cycle characteristics can be improved.

  Further, examples of the non-aqueous solvent include butylene carbonate, γ-butyrolactone, γ-valerolactone, those obtained by substituting some or all of hydrogen groups of these compounds with fluorine groups, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl Tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, methyl acetate, methyl propionate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropironitrile, N, N-dimethylformamide N-methylpyrrolidinone, N-methyloxazolidinone, N, N-dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, dimethyl sulfoxide, or trimethyl phosphate may be included. Yes.

  Furthermore, depending on the electrode to be combined, the reversibility of the electrode reaction may be improved by using a material in which some or all of the hydrogen atoms of the substances contained in the nonaqueous solvent group described above are substituted with fluorine atoms. Therefore, these substances can be appropriately used as the non-aqueous solvent.

Examples of the lithium salt that is an electrolyte include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , 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, or LiBr can be used. Moreover, any 1 type or 2 types or more of these can be mixed and used.

Among these, as the lithium salt, it is preferable to use LiPF ₆ because high ion conductivity can be obtained and cycle characteristics can be improved. Further, when charged with a high charging voltage, for example, although aluminum is a positive electrode current collector is easily dissolved in the presence of LiPF _6, by the LiPF ₆ is decomposed, to form a film on the aluminum surface Thus, dissolution of aluminum can be suppressed.

[Separator]
As the separator 4, an insulating microporous film having a large ion permeability and a predetermined mechanical strength can be used. Moreover, what has a function which obstruct | occludes a hole above a fixed temperature and raises resistance is preferable. Specifically, for example, a sheet, a nonwoven fabric or a woven fabric made of an olefin polymer such as polypropylene and polyethylene having resistance to organic solvents and hydrophobicity or glass fiber can be used.

  In the first embodiment of the present invention, the open circuit voltage at full charge is 4.25 V or more. From this point, the surface of the separator 4 in contact with the electrode is preferably polypropylene. Specifically, as the separator 4, a separator having a three-layer structure in which polypropylene, polyethylene, and polypropylene are sequentially laminated can be used. For example, when the separator 4 in contact with the electrode is a mixture of polyethylene and polypropylene, the proportion of polypropylene is preferably larger than that of polyethylene.

  Furthermore, the pore diameter of the separator 4 is preferably in a range in which the positive electrode active material, the negative electrode active material, the conductive agent, the binder, and the like detached from the positive electrode 2 or the negative electrode 3 do not permeate. Specifically, the pore diameter of the separator 4 is preferably 0.01 μm to 1 μm, for example.

  Furthermore, the thickness of the separator 4 is preferably 10 μm to 300 μm, and more preferably 15 μm to 30 μm. Furthermore, the porosity of the separator 4 is determined according to the permeability of electrons and ions, the material, and the film thickness. The porosity of the separator 4 is preferably 30% to 80%, and more preferably 35% to 50%.

(1-2) Method for Manufacturing Lithium Ion Secondary Battery Next, a method for manufacturing a lithium ion secondary battery according to the first embodiment of the present invention will be described. Hereinafter, a cylindrical lithium ion secondary battery will be described as an example, and a method for manufacturing a lithium ion secondary battery will be described.

  The positive electrode 2 is produced as described below. First, for example, a positive electrode active material, a conductive agent, and a binder are mixed to prepare a positive electrode mixture, and the positive electrode mixture is dispersed in a solvent such as 1-methyl-2-pyrrolidone to form a positive electrode mixture slurry. And

  Next, the positive electrode mixture slurry is applied to a positive electrode current collector 2A having a conductive layer, and the solvent is dried. Then, the positive electrode active material layer 2B is formed by compression molding with a roll press or the like. Is made.

  The negative electrode 3 is produced as described below. First, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and this negative electrode mixture is dispersed in a solvent such as 1-methyl-2-pyrrolidone to obtain a negative electrode mixture slurry. Next, after applying this negative electrode mixture slurry to the negative electrode current collector 3A and drying the solvent, the negative electrode active material layer 3B is formed by compression molding with a roll press or the like, and the negative electrode 3 is produced.

  Next, the positive electrode lead 13 is attached to the positive electrode current collector 2A by, for example, welding, and the negative electrode lead 14 is attached to the negative electrode current collector 3A, for example, by welding. Next, the positive electrode 2 and the negative electrode 3 are wound through the separator 4, and the tip portion of the positive electrode lead 13 is welded to the safety valve mechanism 8, and the tip portion of the negative electrode lead 14 is welded to the battery can 1. The positive electrode 2 and the negative electrode 3 are sandwiched between a pair of insulating plates 5 and 6 and stored in the battery can 1.

  Next, an electrolytic solution is injected into the battery can 1 and the separator 4 is impregnated with the electrolytic solution. Next, the battery lid 7, the safety valve mechanism 8 and the heat sensitive resistance element 9 are fixed to the opening end of the battery can 1 by caulking through the gasket 10. As described above, the lithium ion secondary battery according to the first embodiment of the present invention is manufactured.

  According to the first embodiment of the present invention, in a lithium ion secondary battery that performs a charge / discharge cycle under a high charge voltage of 4.25 V or more and 6.00 V or less, a battery containing a polyacrylonitrile resin is used as a binder. Thus, good cycle characteristics can be obtained. In addition, it is possible to suppress the deterioration of the cycle characteristics that occurs remarkably, particularly at a temperature higher than room temperature.

(2) Second Embodiment (2-1) Configuration of Lithium Ion Secondary Battery FIG. 3 shows the structure of a lithium ion secondary battery according to the second embodiment of the present invention. As shown in FIG. 3, this lithium ion secondary battery is sealed by housing the battery element 30 in an exterior material 37 made of a moisture-proof laminate film and welding the periphery of the battery element 30. The battery element 30 is provided with a positive electrode lead 32 and a negative electrode lead 33, and these leads are sandwiched between outer packaging materials 37 and pulled out to the outside. A resin piece 34 and a resin piece 35 are coated on both surfaces of the positive electrode lead 32 and the negative electrode lead 33 in order to improve the adhesion to the exterior material 37.

[Exterior material]
The packaging material 37 has, for example, a stacked structure in which an adhesive layer, a metal layer, and a surface protective layer are sequentially stacked. The adhesive layer is made of a polymer film. Examples of the material constituting the polymer film include polypropylene (PP), polyethylene (PE), non-axially oriented polypropylene (CPP), linear low density polyethylene (LLDPE), low A density polyethylene (LDPE) is mentioned. The metal layer is made of a metal foil, and an example of a material constituting the metal foil is aluminum (Al). Moreover, it is also possible to use metals other than aluminum as a material which comprises metal foil. Examples of the material constituting the surface protective layer include nylon (Ny) and polyethylene terephthalate (PET). The surface on the adhesive layer side is a storage surface on the side where the battery element 30 is stored.

[Battery element]
For example, as shown in FIG. 4, the battery element 30 includes a strip-shaped negative electrode 43 provided with a gel electrolyte layer 45 on both sides, a separator 44, and a strip-shaped positive electrode 42 provided with a gel electrolyte layer 45 on both sides. The battery element 30 is a wound type battery element 30 that is formed by laminating the separator 44 and wound in the longitudinal direction.

  The positive electrode 42 includes a strip-like positive electrode current collector 42A and a positive electrode active material layer 42B formed on both surfaces of the positive electrode current collector 42A. The positive electrode current collector 42A is a metal foil made of, for example, aluminum (Al).

  One end of the positive electrode 42 in the longitudinal direction is provided with a positive electrode lead 32 connected by, for example, spot welding or ultrasonic welding. As a material of the positive electrode lead 32, for example, a metal such as aluminum can be used.

  The negative electrode 43 includes a strip-shaped negative electrode current collector 43A and a negative electrode active material layer 43B formed on both surfaces of the negative electrode current collector 43A. The negative electrode current collector 43A is made of, for example, a metal foil such as a copper (Cu) foil, a nickel foil, or a stainless steel foil.

  Similarly to the positive electrode 42, the negative electrode lead 33 connected by, for example, spot welding or ultrasonic welding is also provided at one end portion of the negative electrode 43 in the longitudinal direction. As a material of the negative electrode lead 33, for example, copper (Cu), nickel (Ni) or the like can be used.

  Since things other than the gel electrolyte layer 45 are the same as those in the first embodiment, the gel electrolyte layer 45 will be described below.

  The gel electrolyte layer 45 includes an electrolytic solution and a polymer compound serving as a holding body that holds the electrolytic solution, and has a so-called gel shape. The gel electrolyte layer 45 is preferable because it can obtain high ionic conductivity and prevent battery leakage. The configuration of the electrolytic solution is the same as that of the first embodiment. Examples of the polymer compound include polyacrylonitrile, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polytetrafluoroethylene, polyhexafluoropropylene, polyethylene oxide, polypropylene oxide, polyphosphazene, and polysiloxane. , Polyvinyl acetate, polyvinyl alcohol, polymethyl methacrylate, polyacrylic acid, polymethacrylic acid, styrene-butadiene rubber, nitrile-butadiene rubber, polystyrene or polycarbonate. In particular, from the viewpoint of electrochemical stability, polyacrylonitrile, polyvinylidene fluoride, polyhexafluoropropylene, or polyethylene oxide is preferable.

(2-1) Method for Manufacturing Lithium Ion Secondary Battery Next, a method for manufacturing a lithium ion secondary battery according to the second embodiment of the present invention will be described. First, a precursor solution containing a solvent, an electrolyte salt, a polymer compound, and a mixed solvent is applied to each of the positive electrode 42 and the negative electrode 43, and the mixed solvent is volatilized to form the gel electrolyte layer 45. In addition, the positive electrode lead 32 is attached to the end of the positive electrode current collector 42A in advance by welding, and the negative electrode lead 33 is attached to the end of the negative electrode current collector 43A by welding.

  Next, the positive electrode 42 and the negative electrode 43 on which the gel electrolyte layer 45 is formed are laminated via a separator 44 to form a laminated body, and then the laminated body is wound in the longitudinal direction thereof to form a wound battery element 30. Form.

  Next, a recess 36 is formed by deep drawing the exterior material 37 made of a laminate film, the battery element 30 is inserted into the recess 36, and an unprocessed portion of the exterior material 37 is folded back to the upper portion of the recess 36. The outer peripheral part of is thermally welded and sealed. As described above, the lithium ion secondary battery according to the second embodiment of the present invention is manufactured.

  Hereinafter, the present invention will be described in more detail with reference to examples. Note that the present invention is not limited to these examples.

  Table 1 shows the compositions of the positive electrodes used in Examples 1 to 15 and Comparative Examples 1 to 10 and the drying conditions for producing the positive electrodes. Table 2 shows the intrinsic viscosities of PVDF1 and PVDF2. Table 3 shows the negative electrodes used in Examples 1 to 15 and Comparative Examples 1 to 10. Table 4 shows the separators used in Examples 1 to 15 and Comparative Examples 1 to 10. Hereinafter, Examples 1 to 15 and Comparative Examples 1 to 10 will be described with reference to Tables 1 to 4.

1. Production of Positive Electrode Production of positive electrodes used in Examples 1 to 15 and Comparative Examples 1 to 10 will be described with reference to Table 1.

First, lithium-cobalt-aluminum-magnesium composite oxide (I) [LiCo _0.8 Al _0.1 Mg _0.1 O ₂ ] (hereinafter appropriately referred to as active material 1), lithium-nickel-cobalt-manganese used for the production of the positive electrode Complex oxide (II) [LiNi _0.5 Co _0.2 Mn _0.3 O ₄ ] (hereinafter appropriately referred to as active material 2), active material 3, PAN (polyacrylonitrile) resin 1, PAN resin 2, PVDF1, PVDF2 To do.

<Synthesis 1> (Synthesis of Active Material 1)
The coprecipitated hydroxide represented by LiOH and Co _0.98 Al _0.01 Mg _0.01 (OH) ₂ was mixed in a mortar so that the molar ratio of Li: transition metals was 1: 1. The mixture was heat-treated in an air atmosphere at 800 ° C. for 12 hours and then pulverized to obtain a lithium-cobalt composite oxide (A) having a BET specific surface area of 0.44 m ² / g and an average particle size of 6.2 μm [LiCo _0.8 Al _0.1 Mg. _0.1 O ₂ ] and a BET specific surface area of 0.20 m ² / g and an average particle size of 16.7 μm of lithium-cobalt-aluminum-magnesium composite oxide (B) [LiCo _0.8 Al _0.1 Mg _0.1 O ₂ ] were obtained. The active material 1 was obtained by mixing (A) and (B) 1: 1. When the active material 1 was analyzed by X-ray diffraction using CuKα, it was confirmed to be an R-3 rhombohedral layered rock salt structure.

<Synthesis 2> (Synthesis of Active Material 2)
The coprecipitated hydroxide represented by LiOH and Ni _0.5 Co _0.2 Mn _0.3 (OH) ₄ was mixed in a mortar so that the molar ratio of Li: transition metal total was 1: 1. This mixture was heat treated at 1000 ° C. for 20 hours in an air atmosphere and then pulverized to obtain an active material 2 having an average particle size of 11.5 μm. The BET specific surface area of the active material 2 was 0.38 m ² / g.

<Synthesis 3> (Synthesis of Active Material 3)
As the active material 3, lithium iron phosphorus oxide (LiFePO ₄ ), which is a lithium phosphorus oxide, was produced under the following conditions. Lithium phosphate and iron (II) phosphate octahydrate are mixed so that the element ratio of lithium and iron is 1: 1, so that the ketjen black powder becomes 10% of the total fired product obtained after firing. To obtain a mixed sample. This mixed sample was put into an alumina container and milled with a planetary ball mill under the conditions of a sample / alumina ball weight ratio of 60%, a rotation speed of 300 rpm, and an operation time of 10 hours. Thereafter, firing was performed at 600 ° C. for 5 hours in an electric furnace in a nitrogen atmosphere in a ceramic crucible to obtain a fired product of LiFePO ₄ containing a carbon material.

<Synthesis of binder>
<PAN resin 1>
1804 g of purified water is charged into a 3 liter separable flask equipped with a stirrer, thermometer, cooling pipe and nitrogen gas introduction pipe, and the temperature is raised to 75 ° C. while stirring under a nitrogen gas flow rate of 200 ml / min. After that, the ventilation of nitrogen gas was stopped. Next, an aqueous solution in which 2.29 g of polymerization initiator potassium persulfate was dissolved in 76 g of purified water was added, and immediately, 174.2 g of acrylonitrile as a nitrile group-containing monomer, a monomer represented by the formula (I) 2-acryloyloxyethyl succinic acid (Kyoeisha Chemical Co., Ltd., trade name: Light Acrylate HOA-MS) 26.1 g (percentage of 0.037 mole per mole of acrylonitrile) and α of chain transfer agent -A liquid mixture of 0.174 g of methylstyrene dimer was added dropwise over 2.5 hours while maintaining the temperature of the system at 73 ± 2 ° C. After completion of the dropwise addition, the reaction was allowed to proceed for an additional 30 minutes at the same temperature. Subsequently, an aqueous solution in which 0.58 g of potassium persulfate was dissolved in 21.3 g of purified water was added to the obtained reaction product in a suspended state, the temperature was raised to 83 ° C., and the temperature of the system was changed to 83 ± 3 ° C. The reaction was allowed to proceed for 3 hours. Then, after cooling to 40 degreeC over 1 hour, stirring was stopped and it stood to cool to room temperature overnight, and the reaction liquid which the binder resin composition of this invention precipitated was obtained. The reaction solution was subjected to suction filtration, and the collected wet precipitate was washed three times with 1800 g of purified water, dried at 80 ° C. for 24 hours, isolated and purified, and the binder resin composition of the present invention was obtained. . The yield was 93%, and the acid value was 38 KOH mg / g (theoretical value: 34 KOH mg / g).

<PAN resin 2>
A polyacrylonitrile resin containing no ester group in the molecule was used.

<PVDF1, PVDF2>
Further, PVDF1 and PVDF2 having different intrinsic viscosities shown in Table 2 were used as PVDF binders. The intrinsic viscosity was measured by the method described below.

<Measurement of intrinsic viscosity of PVDF binder>
80 mg of a powdery sample was dissolved in 20 ml of N, N-dimethylformamide, and the intrinsic viscosity was determined by the following formula using an Ubbelohde viscometer in a thermostatic bath at 30 ° C.
ηi = (1 / C) · ln (η / η ₀ )
Here, η is the viscosity of the polymer solution, η ₀ is the viscosity of the solvent N, N-dimethylformamide alone, and C is 0.4 (g / dl).

  Next, an active material 1 having an average particle diameter of 11.4 μm produced in Synthesis 1 as a positive electrode active material, an active material 2 having an average particle diameter of 10.0 μm produced in Synthesis 2, and an active material 3 produced in Synthesis 3 Ketjen black (KB), which is a conductive agent, and a binder were mixed in the weight ratios shown in Table 1, respectively, and N-methyl-pyrrolidone was added thereto and kneaded to prepare a positive electrode mixture.

Next, the prepared positive electrode mixture slurry was adjusted on one side of an 20 μm-thick aluminum current collector so that the coating amount of the mixture was 22 g / cm ³ and dried at 80 ° C. Next, it was punched into a 15φ circle and pressed at 2000 kPa with a roll press.

  Next, in order to completely remove volatile components such as residual solvent and adsorbed moisture, the positive electrodes of Examples 1 to 15 and Comparative Examples 1 to 10 were produced by vacuum drying under the drying conditions shown in Table 1. .

2. Production of Negative Electrode As shown in Table 3, different negative electrodes were produced according to Examples 1 to 15 and Comparative Examples 1 to 10. <Negative electrode 1>, <Negative electrode 2> and <Negative electrode 3> were produced as described below.

<Negative electrode 1>
Granular artificial graphite negative electrode powder (BET specific surface area 0.58 m ² / g), polyvinylidene fluoride, and vapor-grown carbon fiber (manufactured by Showa Denko: VGCF) were 96.5% by mass, 2.5% by mass, and 1.% by mass, respectively. 0% by mass was mixed. Next, this mixture was dispersed in N-methyl-pyrrolidone to form a slurry, and the volume density of the negative electrode mixture was 1.85 g / cm ³ on a current collector of 12 μm electrolytic copper foil. After application and drying, N-methyl-pyrrolidone was removed.

  Next, a roll press was performed, and a negative electrode was obtained by punching into a 16φ circle. The roll press conditions were 130 ° C. and 140 kg. The initial charge / discharge efficiency of the negative electrode was 94%, and the discharge capacity of the negative electrode was 347 mAh / g.

<Negative electrode 2>
A thin film made of silicon was formed on a copper foil having a thickness of 15 μm by a sputtering method. The thickness was confirmed by SEM (Scanning Electron Microscope) and was 5.0 μm.

<Negative electrode 3>
A predetermined amount of Cu and Sn was charged into a high-frequency melting furnace, and after melting, sprayed in an Ar atmosphere to obtain a powder material having a composition of Cu 55 wt% and Sn 45 wt%. Next, the obtained powder material was subjected to a mechanical milling process using a dry attritor apparatus (MA1D manufactured by Mitsui Kinzoku). In the mechanical milling process, 1 kg of Cu and Sn powder materials were put in a powder tank under an Ar atmosphere, and further, a hard Cr steel ball having a diameter of 9 mm as a milling mue was put under 18.0 kg in an Ar atmosphere. Then, the rotational speed of the agitating arm was set to 200 rpm, and the mechanical milling process was performed for 1 hour.

  After completion of the reaction, the powder was taken out from the tank, and coarse powder was removed through a 200-mesh sieve to obtain a negative electrode active material. Furthermore, the negative electrode active material was mixed to 50% by mass, artificial graphite as a conductive agent to 45% by mass, and the binder to be 5% by mass of polyvinylidene fluoride to prepare a negative electrode mixture. -Dispersed in methyl-pyrrolidone to form a slurry was applied to both sides of a 12 μm electrolytic copper foil current collector, and after drying, N-methyl-pyrrolidone was removed. Next, a roll press was performed, and a negative electrode was obtained by punching into a 16φ circle. The roll press conditions were 130 ° C. and 140 kg. The initial charge / discharge efficiency of the negative electrode was 87%, and the discharge capacity of the negative electrode was 551 mAh / g.

3. Preparation of Nonaqueous Electrolyte In Examples 1 to 15 and Comparative Examples 1 to 10, the electrolytes contained 10% by mass of propylene carbonate, 15% by mass of ethylene carbonate, 5% by mass of methyl ethyl carbonate, and 69% by mass of dimethyl carbonate. % And 1% by mass of vinylene carbonate were mixed with LiPF _{6 so as} to have a molar concentration of 1.5 mol / kg.

4). Assembling the batteries The coin cells of Examples 1 to 15 and Comparative Examples 1 to 10 were assembled as described below.

  First, the electrode obtained by the produced positive electrode and negative electrode and the separator shown in Table 4 having a thickness of 20 μm were combined, and then the nonaqueous electrolyte obtained by the electrolyte preparation was added, and Examples 1 to 15 were compared. Example 1 The coin cell of Comparative Example 10 was produced.

5). Battery performance evaluation Next, the battery performance evaluation demonstrated below was performed with respect to the produced coin cell of Examples 1 to 15 and Comparative Examples 1 to 10.

<Battery performance evaluation>
In the produced lithium ion secondary battery, the charge / discharge cycle was measured at a high temperature of 60 ° C. Charging was performed by a constant current constant voltage method. Specifically, charging was performed at 1 mA from the first cycle to the third cycle until the charging voltage shown in Table 5 was reached, and constant voltage charging was performed at that voltage until charging current decreased to 0.01 mA.

  An open circuit time of 10 minutes was provided between charging and discharging, and 1 mA discharging was performed. Discharging was completed for one cycle when the voltage reached 3.0V. After 3 cycles, charge / discharge is performed at 2 mA, rotated 60 cycles, initial discharge capacity (mAh / g), initial charge / discharge efficiency (%), 60th capacity retention rate (%) (60th cycle discharge capacity) ) / (Discharge capacity at the third cycle) (%).

  Table 5 shows the results of battery performance evaluation performed on Examples 1 to 15 and Comparative Examples 1 to 10.

6). Study results
Examination of Effect of PAN Resin As shown in Table 5, the capacity retention rate at 4.35 V in Examples 1 to 5 is higher than the capacity retention ratios in Comparative Examples 1 and 2. In Example 1, PAN-based resin 1 is used alone as a binder. In Examples 2 to 5, PAN-based resin 1 and PVDF resin are used as binders. Therefore, it can be seen that a battery containing a PAN resin as a binder has a better capacity retention rate at 4.35 V than a battery containing a PVDF resin alone.

  As shown in Table 5, it can be seen that there is almost no difference between the capacity retention rate at 4.20 V in Comparative Example 3 and the capacity retention rate at 4.20 V in Comparative Examples 4 to 5. In Comparative Example 3, PAN-based resin 1 is used as a binder. Comparative Example 4 uses PVDF1 as a binder. In Comparative Example 5, PVDF2 is used as a binder. Therefore, it can be seen that the PAN-based resin has a particularly good capacity retention rate under a charging voltage higher than 4.20V.

Examination of composition of binder As shown in Table 5, the capacity retention rate at 4.35 V in Examples 2 to 5 is higher than the capacity retention rate at 4.35 V in Example 1. In Example 1, a PAN-based resin is used as a binder. In Examples 2 to 5, a mixture of PAN-based resin and PVDF is used as a binder. Therefore, as a binder, a PAN-based resin and PVDF mixed and used can obtain a better capacity retention rate at a charging voltage of 4.35 V than when a PAN-based resin is used alone. I understand that.

As shown in Table 5, the capacity retention rate at 4.35 V in Example 6 is higher than the capacity retention rate at 4.35 V in Example 1. In Example 6, the drying conditions were performed at 150 ° C. for 12 hours. In Example 1, the drying condition was 120 ° C. for 12 hours. Therefore, it can be seen that a better capacity retention rate can be obtained when the positive electrode is produced by drying at 150 ° C. for 12 hours.

As shown in Table 5 for charging voltage, Examples 7 to 9 show the capacity retention ratios as a result of charging and discharging with charging cutoff voltages of 4.25 V, 4.45 V and 4.55 V. The capacity maintenance rate of the result of having performed the charge of the battery using PVDF1 with the charge cut-off voltage of 4.25V, 4.45V, and 4.55V in the comparative example 6-the comparative example 8 is shown. It can be seen that Examples 7 to 9 have better capacity retention ratios than Comparative Examples 6 to 8.

Examination of composition of active material As shown in Table 5, the initial discharge capacity at 4.35 V of Example 11 is larger than Example 10. In Example 10, the active material 1 and the active material 2 are used in a weight ratio of 1: 1 as the positive electrode active material. Example 11 uses the active material 2 independently as a positive electrode active material.

As shown in Table 5 of the examination of the separator , the capacity retention rate at 4.35 V in Example 12 is smaller than the capacity retention rate at 4.35 V in Example 3. In Example 3, the surface of the laminated separator is polypropylene, and its air permeability is about 500 s. In Example 12, polyethylene is used as a separator having an air permeability of about 500 s, as in Example 3. Therefore, it can be seen that when the charge upper limit voltage is 4.35 V, a better capacity retention ratio can be obtained by using a laminated separator whose separator surface is polypropylene.

Negative electrode investigation Examples 13 to 14 use negative electrode active materials other than the negative electrode active material made of carbon. As shown in Table 5, it can be seen that a material using a negative electrode active material made of carbon can obtain a better capacity retention rate.

Examination of PAN-based resin As shown in Table 5, the capacity retention rate at 4.35 V in Example 15 is higher than the capacity retention rates in Comparative Example 1 and Comparative Example 2. On the other hand, the capacity maintenance rate at 4.35 V in Example 15 is lower than the capacity maintenance rate in Example 1.

  Example 15 uses a polymer binder of PAN alone as a binder. Therefore, it can be seen that the polymer of PAN alone has better battery characteristics than that using PVDF as the binder. On the other hand, it is understood that better battery characteristics can be obtained by using a PAN-based resin containing an ester group as a binder than a polymer of PAN alone.

  In Comparative Example 9, PVDF1 was used for the olivine positive electrode. In Comparative Example 10, the PAN-based resin 1 is used for the olivine positive electrode. It can be seen that there was no change in the capacity retention rate in both Comparative Example 9 and Comparative Example 10.

  The present invention is not limited to the above-described embodiments of the present invention, and various modifications and applications are possible without departing from the spirit of the present invention. For example, the lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator that separates the positive electrode and the negative electrode. The shape is not particularly limited. It may be a cylindrical shape, a square shape, a coin shape, a button shape, or the like.

  In addition, the lithium ion secondary battery of the present invention is configured to improve safety by providing a current interruption mechanism that interrupts current in the battery in response to an increase in the internal pressure of the battery, for example, when an abnormality occurs during overcharging or the like It can also be.

1 is a schematic cross-sectional view of a lithium ion secondary battery according to a first embodiment of the present invention. FIG. 2 is an enlarged sectional view of a part of a wound electrode body shown in FIG. 1. It is the schematic which shows the structure of the lithium ion secondary battery by 2nd Embodiment of this invention. FIG. 4 is an enlarged cross-sectional view of a part of the battery element shown in FIG. 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Battery can 2 ... Positive electrode 2A ... Positive electrode collector 2B ... Positive electrode active material layer 3A ... Negative electrode collector 3B ... Negative electrode active material layer 3 ... Negative electrode 4. .. Separator 5, 6 ... Insulating plate 7 ... Battery cover 8 ... Safety valve mechanism 9 ... Heat sensitive resistance element 10 ... Gasket 11 ... Disc plate 12 ... Center pin 13 .. Positive electrode lead 14 ... Negative electrode lead 20 ... Winding electrode body 30 ... Battery element 32 ... Positive electrode lead 33 ... Negative electrode lead 34, 35 ... Resin piece 35 ... Negative electrode lead 36 ... Recess 37 ... Exterior material 42 ... Positive electrode 42A ... Positive electrode current collector 42B ... Positive electrode active material layer 43 ... Negative electrode 43A ... Negative electrode current collector 43B ... Negative electrode Active material layer 44 ... Separator 45 ... Gel electrolyte layer

Claims (8)

A positive electrode having at least a positive electrode active material and a binder, a negative electrode, and an electrolyte and a separator interposed between the positive electrode and the negative electrode;
A lithium ion secondary battery having an open circuit voltage per unit cell in a fully charged state of 4.25V to 6.00V,
The positive electrode active material has a general formula: Li _a Co _1-x Me _x O _2-b (wherein Me is V (vanadium), Cu (copper), Zr (zirconium), Zn (zinc), Mg ( Magnesium), Al (aluminum) and Fe (iron) are at least one or two or more metal elements, a is 0.9 ≦ a ≦ 1.1, x is 0 ≦ x ≦ 0.3, b has a value of -0.1 ≦ b ≦ 0.1 lithium expressed in) - cobalt composite oxide, and the general _{formula:. Li a Ni 1-xyz} Co x Mn y Me z O 2-b ( In the formula, Me is at least one or more selected from V (vanadium), Cu (copper), Zr (zirconium), Zn (zinc), Mg (magnesium), Al (aluminum) and Fe (iron). A is 0.9 ≦ a ≦ 1.1, x is 0 <x <0.4, y 0 <y <0.4, z takes 0 <z <0.3, and b takes a value of −0.1 ≦ b ≦ 0.1.) Lithium-cobalt-nickel-manganese oxidation Including any of the objects,
The lithium ion secondary battery, wherein the binder contains a polyacrylonitrile resin. In claim 1,
The lithium-ion secondary battery, wherein the open circuit voltage is 4.25V to 4.55V. In claim 1,
The lithium ion secondary battery, wherein the binder further contains a vinylidene fluoride polymer. In claim 3,
The lithium ion secondary battery according to the above binder, wherein a weight ratio of the vinylidene fluoride polymer to the polyacrylonitrile resin is 1% or more and 99% or less. In claim 1,
The lithium ion secondary battery, wherein the polyacrylonitrile-based resin is a copolymer of acrylonitrile and another monomer, a homopolymer of acrylonitrile, or a modified product of the copolymer. In claim 1,
The polyacrylonitrile-based resin has an ester group, and is a lithium ion secondary battery. In claim 1,
The lithium ion secondary battery, wherein at least polypropylene is contained on a surface of the separator in contact with the positive electrode and the negative electrode. In claim 1,
The lithium ion secondary battery, wherein the electrolyte further contains vinylene carbonate.

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