JP2007188878A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery of which cycle property in the case of repeating intermittent cycles is more heightened than before. <P>SOLUTION: A cathode activator particle contains lithium complex oxide, The lithium complex oxide is expressed by Li<SB>x</SB>M<SB>1-y</SB>L<SB>y</SB>O<SB>2</SB>(0.85≤x≤1.25, 0≤y≤0.50, M is at least one kind selected from a group composed of Ni and Co, L is at least one kind selected from a group composed of alkali earth element, transition metal element excluding Ni and Co, rare earth element, IIIb group element, and IVb group element). Oxide of molybdenum expressed by Li<SB>a</SB>MoO<SB>b</SB>(1≤a≤4, 1≤b≤6 ) exists on the surface of the activator particle. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery having excellent life characteristics.

  Lithium ion secondary batteries, which are representative of nonaqueous electrolyte secondary batteries, are characterized by high electromotive force and high energy density, increasing demand as the main power source for mobile communication devices and portable electronic devices. is doing.

In the development of a lithium ion secondary battery, increasing its reliability has become an important technical issue. Lithium composite oxides such as Li _x CoO ₂ and Li _x NiO ₂ (x varies depending on charging / discharging of the battery) include Co ⁴⁺ and Ni ⁴⁺ that are highly reactive and are in an expensive state during charging. As a result, the decomposition reaction of the electrolyte solution involving the lithium composite oxide is promoted under a high temperature environment. As a result, gas is generated in the battery, and it becomes difficult to suppress heat generation during a short circuit, or sufficient cycle characteristics and high temperature storage characteristics may not be obtained.

  Therefore, from the viewpoint of improving the reliability of the lithium ion secondary battery, a specific metal oxide is formed on the surface layer portion of the positive electrode active material particles, and the decomposition reaction of the electrolyte solution involving the lithium composite oxide is suppressed. It has been proposed (Patent Documents 1 to 5).

It has also been proposed to stabilize a crystal structure and improve cycle characteristics and high-temperature storage characteristics by dissolving different elements in a specific lithium composite oxide (Patent Documents 6 to 8).
Japanese Patent Laid-Open No. 9-35715 JP 11-317230 A Japanese Patent Laid-Open No. 11-16666 JP 2001-196063 A JP 2003-173775 A JP 2004-111076 A Japanese Patent Laid-Open No. 11-40154 JP 2002-15740 A

As described above, many proposals have been made to suppress gas generation, suppress heat generation during a short circuit, or improve cycle characteristics and high-temperature storage characteristics. However, these techniques have the following points to be improved.
Many of lithium ion secondary batteries are used for various portable devices. Various portable devices are not always used immediately after charging a battery. In many cases, the battery is maintained in a charged state for a long time and then discharged. However, in general, the cycle life characteristics of a battery are generally evaluated under conditions different from such actual use conditions.

  For example, a general cycle life test is performed under conditions with a short rest time after charging (for example, a rest time of 30 minutes). If the evaluation is performed under such conditions, the cycle life characteristics can be improved to some extent by the above-described techniques.

  However, assuming an actual use condition, when repeating an intermittent cycle (a charge / discharge cycle in which a rest time after charging is set long), for example, when a cycle life test is performed for a rest time of 720 minutes, As a result, it has been found that sufficient life characteristics cannot be obtained. That is, the problem of improvement of intermittent cycle characteristics remains in the conventional lithium ion secondary battery.

  In view of the above, an object of the present invention is to improve intermittent cycle characteristics in a lithium ion secondary battery including a lithium composite oxide containing nickel or cobalt as a main component as a positive electrode active material.

The present invention has a chargeable / dischargeable positive electrode, a chargeable / dischargeable negative electrode, and a non-aqueous electrolyte. The positive electrode includes active material particles, the active material particles include lithium composite oxide, and the lithium composite oxide. Is Li _x M _1-y L _y O ₂ (where 0.85 ≦ x ≦ 1.25, 0 ≦ y ≦ 0.5, M is at least one selected from the group consisting of Ni and Co, L is represented by an alkaline earth element, at least one selected from the group consisting of transition metal elements other than Ni and Co, rare earth elements, IIIb group elements, and IVb group elements). , Li _a MoO _b (where 1 ≦ a ≦ 4, 1 ≦ b ≦ 6).

  In the case of 0 <y, L preferably includes at least one selected from the group consisting of Al, Mn, Ti, Mg, Zr, Nb, Y, Ca, In, and Sn.

  The present invention includes a case where L is distributed more on the surface layer side than in the active material particles.

The amount of molybdenum oxide represented by Li _a MoO _b (where 1 ≦ a ≦ 4, 1 ≦ b ≦ 6) is based on the lithium composite oxide represented by Li _x M _1-y L _y O _2. Therefore, 2 mol% or less is preferable.

The average particle diameter of the active material particles is preferably 10 μm or more.
When further improvement in intermittent cycle characteristics is desired, it is preferable that the non-aqueous electrolyte contains at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, fluorobenzene and phosphazene.

Usually, the molybdenum oxide (Li _a MoO _b ) present in the surface layer portion of the active material particles is a lithium composite oxide represented by Li _x M _1-y L _y O ₂ (hereinafter also referred to as lithium composite oxide ML). ) Has a different crystal structure. The crystal structure of the lithium composite oxide is usually a layer structure (for example, R3m), and oxygen has a cubic close-packed arrangement. On the other hand, Li _a MoO _b has _a composition such as Li ₄ MoO ₅ , Li ₆ Mo ₂ O ₇ , LiMoO ₂ , Li ₂ MoO ₃ , and Li ₂ MoO ₄ .

The lithium composite oxide ML represented by Li _x M _1-y L _y O ₂ may contain Mo as L, but L is in a solid solution state in the lithium composite oxide ML. Therefore, Mo as L can be distinguished from Mo of Li _a MoO _b by various analysis methods. Analysis methods include elemental mapping with EPMA (Electron Probe Micro-Analysis), analysis of chemical bonding state with XPS (X-ray Photoelectron Spectroscopy), and SIMS (secondary ion mass spectrometry). : Secondary Ionization Mass Spectroscopy).

Li _x M _1-y L _y O ₂ (where 0.85 ≦ x ≦ 1.25, 0 ≦ y ≦ 0.5, M is at least one selected from the group consisting of Ni and Co, and L is , At least one selected from the group consisting of alkaline earth elements, transition metal elements other than Ni and Co, rare earth elements, IIIb group elements, and IVb group elements) When the molybdenum oxide represented by Li _a MoO _b (where 1 ≦ a ≦ 4, 1 ≦ b ≦ 6) is applied to the surface layer portion, the intermittent cycle characteristics are drastically improved.

  Further, when the lithium composite oxide ML includes at least one selected from the group consisting of Al, Mn, Ti, Mg, Zr, Nb, Y, Ca, In, and Sn as L, the intermittent cycle characteristics are Further improvement.

The reason why the intermittent cycle characteristics are dramatically improved is known only from a phenomenological viewpoint, but at present, the following knowledge is obtained from the AC impedance analysis of the battery during the intermittent cycle.
(1) When the molybdenum oxide represented by Li _a MoO _b is not present in the surface layer portion of the active material particles, the activation energy required for insertion and desorption of lithium ions from the active material particles is <i> during intermittent cycle It increases in proportion to the number of cycles, and also increases in proportion to the charge / discharge rest time in the <ii> intermittent cycle. As a result of various experiments, it has been found that the activation energy is related to the phenomenon of solvation and desolvation of lithium ions.

(2) When molybdenum oxide represented by Li _a MoO _b is present in the surface layer portion of the active material particles, the activation energy required for insertion and desorption of lithium ions from the active material particles is <i> during intermittent cycle However, it is not proportional to the charge / discharge rest time, and the increase in activation energy is suppressed.

  Therefore, it is considered that the molybdenum oxide present in the surface layer part of the active material particles has an effect of suppressing an increase in activation energy, and the activation energy is related to the phenomenon of solvation and desolvation of lithium ions. To do.

  When the lithium composite oxide contains at least one selected from the group consisting of Al, Mn, Ti, Mg, Zr, Nb, Y, Ca, In and Sn as L, the increase in activation energy is further increased. It has been found to be suppressed.

Below, the positive electrode which concerns on this invention is demonstrated. The positive electrode contains the following active material particles.
The active material particles include a lithium composite oxide ML, and the lithium composite oxide ML is Li _x M _1-y L _y O ₂ (where 0.85 ≦ x ≦ 1.25, 0 ≦ y ≦ 0.5). , M is at least one selected from the group consisting of Ni and Co, and L is selected from the group consisting of alkaline earth elements, transition metal elements other than Ni and Co, rare earth elements, IIIb group elements and IVb group elements At least one kind). Molybdenum oxide represented by Li _a MoO _b (where 1 ≦ a ≦ 4, 1 ≦ b ≦ 6) exists in the surface layer portion of the active material particles.

  L is a group consisting of Al, Mn, Ti, Mg, Zr, Nb, Y, Ca, In, and Sn from the viewpoint of further improving the intermittent cycle characteristics and stabilizing the crystal structure of the lithium composite oxide. And at least one selected from the group consisting of Al, Mn, Ti, Mg, Zr, Nb, and Y is particularly preferable. L may be contained alone in the lithium composite oxide ML, or a plurality of types may be contained.

  In the lithium composite oxide ML, usually, a plurality of primary particles are aggregated to form secondary particles. The average particle size of the primary particles is generally 0.1 to 3 μm, but is not particularly limited. The average particle diameter of the active material particles composed of the secondary particles of the lithium composite oxide is not particularly limited, but is preferably 1 to 30 μm, and particularly preferably 10 to 30 μm. The average particle diameter can be measured by, for example, a wet laser particle size distribution measuring device manufactured by Microtrack. In this case, the 50% value (median value: D50) on the volume basis can be regarded as the average particle diameter of the active material particles.

In Li _x M _1-y L _y O ₂ , the range of x representing the Li content is increased or decreased by charging / discharging of the battery. The range of x in the complete discharge state (initial state) is preferably 0.85 ≦ x ≦ 1.25, but more preferably 0.93 ≦ x ≦ 1.1.

  The range of y representing the L content may be 0 ≦ y ≦ 0.5, but considering the balance between the thermal stability and the capacity of the lithium composite oxide ML, 0.005 ≦ y ≦ 0. 35 is preferable, and 0.01 ≦ y ≦ 0.1 is more preferable. When 0.50 <y, there is no merit of using an active material mainly composed of Ni or Co. For example, a specific high capacity cannot be realized.

  When M contains Co, the atomic ratio a of Co to the total of M and L is preferably 0.05 ≦ a ≦ 0.5, and 0.05 ≦ a ≦ 0.25. Further preferred.

  When M contains Ni, the atomic ratio b of Ni with respect to the sum of M and L is preferably 0.25 ≦ b ≦ 0.9, and preferably 0.30 ≦ b ≦ 0.85. Further preferred.

  When L contains Al, the atomic ratio c of Al with respect to the sum of M and L is preferably 0.005 ≦ c ≦ 0.1, and 0.01 ≦ c ≦ 0.08. Further preferred.

  When L contains Mn, the atomic ratio d of Mn to the sum of M and L is preferably 0.005 ≦ d ≦ 0.5, and 0.01 ≦ d ≦ 0.35. Further preferred.

  When L contains Ti, the atomic ratio e of Ti with respect to the sum of M and L is preferably 0.005 ≦ e ≦ 0.35, and 0.01 ≦ e ≦ 0.1. Further preferred.

The lithium composite oxide represented by Li _x M _1-y L _y O ₂ can be synthesized by firing a raw material having a predetermined metal element ratio in an oxidizing atmosphere. The raw material includes lithium, M, and optionally L. The raw materials include oxides, hydroxides, oxyhydroxides, carbonates, nitrates, organic complex salts and the like of each metal element. These may be used alone or in combination of two or more.

  From the viewpoint of facilitating the synthesis of the lithium composite oxide ML, the raw material preferably contains a solid solution containing a plurality of metal elements. The solid solution containing a plurality of metal elements can be formed in any of oxides, hydroxides, oxyhydroxides, carbonates, nitrates, sulfates, organic complex salts, and the like. For example, a solid solution containing Ni and Co, a solid solution containing Ni, Co, and Al, a solid solution containing Ni, Co, and Mn, and a solid solution containing Ni, Co, and Ti can be used.

The firing temperature of the raw material and the oxygen partial pressure in the oxidizing atmosphere depend on the composition, amount, synthesis apparatus, and the like of the raw material, but those skilled in the art can appropriately select appropriate conditions.
In addition, elements other than Li, M, and L may be mixed as impurities in amounts within the range normally contained in industrial raw materials, but do not significantly affect the effects of the present invention.

Molybdenum oxide (Li _a MoO _b ) contained in the surface layer portion of the active material particles is usually deposited on, adhered to, or supported on the surface of the lithium composite oxide ML.

The amount of molybdenum oxide (Li _a MoO _b ) contained in the active material particles is preferably 2 mol% or less with respect to the lithium composite oxide ML, and is 0.1 mol% or more and 1.5 mol% or less. More preferably. That is, the amount of Mo contained in molybdenum oxide (Li _a MoO _b ) is 2 mol with respect to the sum of M and L contained in lithium composite oxide ML (Li _x M _1-y L _y O ₂ ). % Or less, more preferably 0.1 mol% or more and 1.5 mol% or less. When the amount of molybdenum oxide (Li _a MoO _b ) exceeds 2 mol%, the surface layer portion of the active material particles becomes a resistance layer, and the overvoltage increases, so that the cycle characteristics begin to deteriorate. On the other hand, if the amount of molybdenum oxide (Li _a MoO _b ) is less than 0.1 mol%, the effect of improving the intermittent cycle characteristics may not be sufficiently obtained.

  Mo in the molybdenum oxide in the surface layer portion may diffuse into the lithium composite oxide, and the concentration of L in the lithium composite oxide ML may be higher in the vicinity of the surface layer portion than in the active material particles. That is, Mo in the surface layer portion may change to L constituting the lithium composite oxide ML. However, since the amount of Mo diffused from the surface layer portion into the lithium composite oxide ML is very small, it can be ignored. Even if this is ignored, the effect of the present invention is hardly affected.

  In the case of an active material in which primary particles are aggregated to form secondary particles, molybdenum oxide may exist only on the surface of the primary particles, or may exist only on the surface of the secondary particles. It may be present on the surface of both particles and secondary particles. In any case, the effects of the present invention are similarly exhibited.

Next, an example of a method for manufacturing the positive electrode will be described.
(I) 1st step First, the hydroxide used as the raw material of lithium complex oxide ML is prepared. The method for preparing the hydroxide is not particularly limited. For example, a method of preparing a co-precipitated hydroxide by preparing an aqueous solution of a raw material salt mixture containing M and L in a predetermined molar ratio and adding an alkali thereto.

  Then, a predetermined amount of a lithium compound is added to the obtained coprecipitated hydroxide, and a mixture (first mixture) of the coprecipitated hydroxide and the lithium compound is baked in an oxidizing atmosphere for about 10 hours, for example. A composite oxide ML is obtained. Firing of the mixture of the coprecipitated hydroxide and the lithium compound is, for example, 650 to 750 ° C., and the pressure of the oxidizing atmosphere is preferably 10 kPa to 50 kPa. A firing temperature, an oxygen partial pressure in an oxidizing atmosphere, and the like are appropriately selected according to the composition, amount, synthesis apparatus, and the like of the first mixture.

(Ii) Second Step A raw material of molybdenum oxide (Li _a MoO _b ) is added to the obtained lithium composite oxide ML. For example, if lithium composite oxide ML is dispersed in an aqueous solution in which a salt containing molybdenum is dissolved, stirred and dried, a composite of lithium composite oxide ML and a raw material of molybdenum oxide (hereinafter referred to as composite) The product MLMo).

  Examples of the salt containing molybdenum include disodium molybdate dihydrate and hexaammonium hexamolybdate tetrahydrate. When the lithium composite oxide ML is put into an aqueous solution in which a salt containing molybdenum is dissolved and stirred, the temperature in the liquid is not particularly limited. However, it is preferable to control to 20-40 degreeC from a viewpoint of workability | operativity or manufacturing cost, for example. The stirring time is not particularly limited, but for example, stirring for 3 hours is sufficient. The method for removing the liquid component is not particularly limited, but it is sufficient to dry the composite MLMo for about 2 hours at a temperature of about 100 ° C., for example.

(Iii) Third Step A lithium compound as a raw material for Li _a MoO _b is further added to the obtained composite MLMo, and the mixture (second mixture) of the composite MLMo and the lithium compound is added for a long time, for example, 24 hours. The baking is preferably performed in an oxidizing atmosphere for 30 to 48 hours. The firing temperature of the mixture (second mixture) of the composite MLMo and the lithium compound is, for example, 650 to 750 ° C., and the pressure in the oxidizing atmosphere is preferably 10 kPa to 50 kPa. By such a long firing, a molybdenum oxide phase different from the lithium composite oxide ML is deposited on the surface of the lithium composite oxide ML, and active material particles are obtained. The firing temperature, the oxygen partial pressure in the oxidizing atmosphere, and the like are appropriately selected according to the composition, amount, synthesis apparatus, and the like of the second mixture. The molybdenum oxide deposited on the surface of the lithium composite oxide ML has a composition represented by the general formula Li _a MoO _b (1 ≦ a ≦ 4, 1 ≦ b ≦ 6).

(Iv) Fourth Step A positive electrode is formed using the active material particles obtained in the third step. The method for producing the positive electrode is not particularly limited, but generally, a positive electrode mixture containing active material particles and a binder is first supported on a belt-like positive electrode core material (positive electrode current collector). In addition, the positive electrode mixture may contain an additive such as a conductive material as an optional component. The positive electrode mixture can be supported on the core material by dispersing the positive electrode mixture in a liquid component, preparing a paste, applying the paste to the core material, and drying. Next, the positive electrode mixture supported on the positive electrode core material is rolled with a roller.

  Either a thermoplastic resin or a thermosetting resin may be used as the binder contained in the positive electrode mixture, but a thermoplastic resin is preferable. Examples of the thermoplastic resin that can be used as the binder include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). , Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE) , Polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer Examples thereof include a copolymer, an ethylene-methyl acrylate copolymer, and an ethylene-methyl methacrylate copolymer. These may be used alone or in combination of two or more. Moreover, these may be a crosslinked body by Na ions or the like.

  The conductive material included in the positive electrode mixture may be any electron conductive material that is chemically stable in the battery. For example, graphite such as natural graphite (such as flake graphite), artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and conductive such as carbon fiber and metal fiber Use conductive fibers, metal powders such as aluminum, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, organic conductive materials such as polyphenylene derivatives, carbon fluoride, etc. Can do. These may be used alone or in combination of two or more. The addition amount of the conductive material is not particularly limited, but is preferably 1 to 50% by weight, more preferably 1 to 30% by weight, and particularly preferably 2 to 15% by weight with respect to the active material particles contained in the positive electrode mixture. .

  The positive electrode core material (positive electrode current collector) may be any electronic conductor that is chemically stable in the battery. For example, a foil or sheet made of aluminum, stainless steel, nickel, titanium, carbon, conductive resin, or the like can be used, and an aluminum foil, an aluminum alloy foil, or the like is preferable. A carbon or titanium layer or an oxide layer can be formed on the surface of the foil or sheet. Further, unevenness can be imparted to the surface of the foil or the sheet, and a net, a punching sheet, a lath body, a porous body, a foamed body, a fiber group molded body, and the like can also be used. Although the thickness of a positive electrode core material is not specifically limited, For example, it exists in the range of 1-500 micrometers.

  Hereinafter, components other than the positive electrode of the lithium ion secondary battery of the present invention will be described. However, the following description does not limit the present invention.

  As the chargeable / dischargeable negative electrode, for example, a negative electrode core material containing a negative electrode active material and a binder and a negative electrode mixture containing a conductive material and a thickener as optional components can be used. Such a negative electrode can be produced in the same manner as the positive electrode.

  The negative electrode active material only needs to be a metal made of lithium or a material that can occlude and release lithium electrochemically. For example, graphites, non-graphitizable carbon materials, lithium alloys, metal oxides, and the like can be used. The lithium alloy is particularly preferably an alloy containing at least one selected from the group consisting of silicon, tin, aluminum, zinc and magnesium. Moreover, as a metal oxide, the oxide containing silicon and the oxide containing tin are preferable, and when it hybridizes with a carbon material, it is still more preferable. Although the average particle diameter of a negative electrode active material is not specifically limited, It is preferable that it is 1-30 micrometers.

  For example, the same material as the binder that can be included in the positive electrode mixture can be used as the binder included in the negative electrode mixture.

  The conductive material included in the negative electrode mixture may be any electron conductive material that is chemically stable in the battery. For example, graphite such as natural graphite (such as flake graphite), artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and conductive such as carbon fiber and metal fiber For example, conductive fibers, metal powders such as copper and nickel, and organic conductive materials such as polyphenylene derivatives can be used. These may be used alone or in combination of two or more. Although the addition amount of a electrically conductive material is not specifically limited, 1-30 weight% is preferable with respect to the active material particle contained in a negative mix, and 1-10 weight% is still more preferable.

  The negative electrode core material (negative electrode current collector) may be anything as long as it is an electron conductor that is chemically stable in the battery. For example, a foil or sheet made of stainless steel, nickel, copper, titanium, carbon, conductive resin, or the like can be used, and copper foil or copper alloy foil is preferable. On the surface of the foil or sheet, a layer of carbon, titanium, nickel or the like can be provided, or an oxide layer can be formed. Further, unevenness can be imparted to the surface of the foil or the sheet, and a net, a punching sheet, a lath body, a porous body, a foamed body, a fiber group molded body, and the like can also be used. Although the thickness of a negative electrode core material is not specifically limited, For example, it exists in the range of 1-500 micrometers.

For the non-aqueous electrolyte, a non-aqueous solvent in which a lithium salt is dissolved is preferably used.
Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), cyclic carbonates such as butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl. Chain carbonates such as methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate and ethyl propionate, γ-butyrolactone, γ-valerolactone, etc. Lactones, chain ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), and ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, Dimethyl sulfoxide, 1,3 -Dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propyl nitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolide Non, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, dimethyl sulfoxide, and N-methyl-2-pyrrolidone can be used. These may be used alone, but it is preferable to use a mixture of two or more. Of these, a mixed solvent of a cyclic carbonate and a chain carbonate or a mixed solvent of a cyclic carbonate, a chain carbonate, and an aliphatic carboxylic acid ester is preferable.

Examples of the lithium salt dissolved in the non-aqueous solvent include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCl, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , Examples include LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, lithium chloroborane, lithium tetraphenylborate, lithium imide salt, and the like. These may be used alone or in combination of two or more. It is preferable to use at least LiPF ₆ . The amount of lithium salt dissolved in the non-aqueous solvent is not particularly limited, but the lithium salt concentration is preferably 0.2 to 2 mol / L, and more preferably 0.5 to 1.5 mol / L.

  Various additives can be added to the non-aqueous electrolyte for the purpose of improving the charge / discharge characteristics of the battery. As the additive, for example, at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, phosphazene and fluorobenzene is preferably used. An appropriate amount of these additives is 0.5 to 10% by weight of the non-aqueous electrolyte.

  Various other additives such as triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, pyridine, hexaphosphoric triamide, nitrobenzene derivatives, crown ethers, quaternary ammonium salts, ethylene glycol dialkyl ether, etc. Can be used.

It is necessary to interpose a separator between the positive electrode and the negative electrode.
As the separator, a microporous thin film having a large ion permeability, a predetermined mechanical strength, and an insulating property is preferably used. The microporous thin film preferably has a function of closing the pores at a certain temperature or higher and increasing the resistance. The material of the microporous thin film is preferably a polyolefin such as polypropylene or polyethylene having excellent organic solvent resistance and hydrophobicity. In addition, a sheet made of glass fiber or the like, a nonwoven fabric, a woven fabric, or the like is also used. The pore diameter of the separator is, for example, 0.01 to 1 μm. The thickness of the separator is generally 10 to 300 μm. Moreover, the porosity of the separator is generally 30 to 80%.

  A non-aqueous electrolyte and a polymer electrolyte made of a polymer material that holds the non-aqueous electrolyte can also be used as a separator integrated with a positive electrode or a negative electrode. The polymer material is not particularly limited as long as it can hold the nonaqueous electrolytic solution, but a copolymer of vinylidene fluoride and hexafluoropropylene is particularly preferable.

[Example 1]
<< Example Battery A1 >>
(I) Synthesis of Active Material Particles 3.2 kg of a mixture of nickel sulfate and cobalt sulfate mixed so that the molar ratio of Ni atoms to Co atoms is 80:20 is dissolved in 10 L of water to obtain a raw material solution. Got. 400 g of sodium hydroxide was added to the raw material solution to form a precipitate. The precipitate was sufficiently washed with water and dried to obtain a coprecipitated hydroxide.

A predetermined amount of lithium carbonate was added to 3 kg of the obtained Ni—Co coprecipitated hydroxide, and calcined at a temperature of 750 ° C. for 12 hours in an atmosphere having an oxygen partial pressure of 0.5 atm. Through the steps so far, lithium composite oxide ML (LiNi _0.8 Co _0.2 O ₂ ) containing Ni and Co as M and not containing L was obtained.

A solution in which disodium molybdate dihydrate was dissolved in ion-exchanged water was prepared. In 3 L of this solution, 3 kg of the obtained lithium composite oxide ML (LiNi _0.8 Co _0.2 O ₂ ) was dispersed and stirred at 25 ° C. for 3 hours. Then, water was removed and dried at 100 ° C. for 2 hours. The dissolution amount of disodium molybdate dihydrate in the solution was 0.1 mol% with respect to the lithium composite oxide ML.

Lithium carbonate was added to the lithium composite oxide ML (LiNi _0.8 Co _0.2 O ₂ ) supporting Mo thus obtained so that the molar ratio was Mo / Li = 2/1, and the oxygen partial pressure was 0.2. Firing was performed at a temperature of 750 ° C. for 24 hours in an atmosphere of atmospheric pressure. As a result, active material particles (average particle size 12 μm) comprising lithium composite oxide ML (LiNi _0.8 Co _0.2 O ₂ ) containing Ni and Co as M and not containing L and containing molybdenum oxide in the surface layer portion are obtained. It was.

When the surface layer portion of the obtained active material particles was analyzed by XRD, XPS, EPMA, ICP emission analysis, etc., it was found that the surface layer portion contained molybdenum oxide represented by Li ₄ MoO ₅ .

(Ii) Production of Positive Electrode 1 kg of the obtained active material particles was added to 0.5 kg of PVDF # 1320 (N-methyl-2-pyrrolidone (NMP) solution having a solid content of 12% by weight), acetylene black, manufactured by Kureha Chemical Co., Ltd. A positive electrode mixture paste was prepared by stirring in a double-arm kneader together with 40 g and an appropriate amount of NMP. This paste was applied to both sides of an aluminum foil having a thickness of 20 μm, dried, and rolled to a total thickness of 160 μm. Thereafter, the obtained electrode plate was slit to a width that could be inserted into a cylindrical battery case of 18650 to obtain a positive electrode.

(Iii) Production of Negative Electrode 3 kg of artificial graphite, 200 g of BM-400B (dispersion of modified styrene-butadiene rubber having a solid content of 40% by weight), 50 g of carboxymethylcellulose (CMC), and an appropriate amount of water manufactured by Nippon Zeon Co., Ltd. In addition, the mixture was stirred with a double-arm kneader to prepare a negative electrode mixture paste. This paste was applied to both sides of a 12 μm thick copper foil, dried, and rolled to a total thickness of 160 μm. Then, the obtained electrode plate was slit to a width that can be inserted into a cylindrical 18650 battery case to obtain a negative electrode.

(Iv) Assembly of Battery As shown in FIG. 1, the positive electrode 5 and the negative electrode 6 were wound through a separator 7 to constitute a spiral electrode plate group. As the separator 7, a composite film of polyethylene and polypropylene (2300 manufactured by Celgard Co., Ltd., thickness 25 μm) was used.

  A positive electrode lead 5a and a negative electrode lead 6a made of nickel were attached to the positive electrode 5 and the negative electrode 6, respectively. An upper insulating plate 8 a was disposed on the upper surface of the electrode plate group, and a lower insulating plate 8 b was disposed on the lower surface, inserted into the battery case 1, and 5 g of nonaqueous electrolyte was injected into the battery case 1.

A solvent mixture of ethylene carbonate and methyl ethyl carbonate having a volume ratio of 10:30 was used as the solvent for the nonaqueous electrolytic solution. To this mixed solvent, 2% by weight of vinylene carbonate, 2% by weight of vinyl ethylene carbonate, 5% by weight of fluorobenzene, and 5% by weight of phosphazene were added. A solution obtained by dissolving LiPF ₆ at a concentration of 1.5 mol / L in the obtained mixed solution was used as a non-aqueous electrolyte.

  Thereafter, the sealing plate 2 provided with the insulating gasket 3 around it and the positive electrode lead 5 a were made conductive, and the opening of the battery case 1 was sealed with the sealing plate 2. Thus, a cylindrical 18650 lithium secondary battery was completed. This is designated as Example Battery A1.

<< Example Battery A2 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate and cobalt sulfate mixed so that the molar ratio of Ni atom to Co atom is 50:50 is used. A battery A2 was produced in the same manner as the battery A1, except that the amount of disodium molybdate dihydrate was 2 mol% with respect to the lithium composite oxide ML.

<< Example Battery A3 >>
Battery A1 except that a mixture of nickel sulfate, cobalt sulfate, and niobium nitrate mixed so that the molar ratio of Ni atoms, Co atoms, and Nb atoms was 80: 15: 5 was used in the synthesis of the coprecipitated hydroxide. A battery A3 was produced in the same manner as described above.

<< Example Battery A4 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate, cobalt sulfate, and niobium nitrate mixed so that the molar ratio of Ni atom, Co atom, and Nb atom is 35:15:50 is used. In the synthesis, a battery A4 was produced in the same manner as the battery A1, except that the amount of disodium molybdate dihydrate dissolved in ion-exchanged water was 2 mol% with respect to the lithium composite oxide ML.

<< Example Battery A5 >>
In the synthesis of the coprecipitated hydroxide, a battery other than using a mixture of nickel sulfate, cobalt sulfate, and manganese sulfate mixed so that the molar ratio of Ni atom, Co atom, and Mn atom is 80: 15: 5. A battery A5 was produced in the same manner as A1.

<< Example Battery A6 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate, cobalt sulfate, and manganese sulfate mixed so that the molar ratio of Ni atom, Co atom, and Mn atom is 35:15:50 is used. In the synthesis, a battery A6 was produced in the same manner as the battery A1, except that the amount of disodium molybdate dihydrate dissolved in ion-exchanged water was 2 mol% with respect to the lithium composite oxide ML.

<< Example Battery A7 >>
In the synthesis of a coprecipitated hydroxide, nickel sulfate, cobalt sulfate, and titanium sulfate (Ti (SO ₄ ) ₂ ) mixed so that the molar ratio of Ni atom, Co atom, and Ti atom is 80: 15: 5. A battery A7 was produced in the same manner as the battery A1, except that the mixture was used.

<< Example Battery A8 >>
In the synthesis of coprecipitated hydroxide, nickel sulfate, cobalt sulfate, and titanium sulfate (Ti (SO ₄ ) ₂ ) mixed so that the molar ratio of Ni atom, Co atom, and Ti atom is 35:15:50. In the synthesis of the positive electrode active material using the mixture, the amount of disodium molybdate dihydrate dissolved in ion-exchanged water was set to 2 mol% with respect to the lithium composite oxide ML in the same manner as in the battery A1, A battery A8 was produced.

<< Example Battery A9 >>
In the synthesis of the coprecipitated hydroxide, a battery other than using a mixture of nickel sulfate, cobalt sulfate, and magnesium sulfate mixed so that the molar ratio of Ni atom, Co atom, and Mg atom is 80: 15: 5. A battery A9 was produced in the same manner as A1.

<< Example Battery A10 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate, cobalt sulfate, and magnesium sulfate mixed so that the molar ratio of Ni atom, Co atom, and Mg atom is 35:15:50 is used. In the synthesis, a battery A10 was produced in the same manner as the battery A1, except that the amount of disodium molybdate dihydrate dissolved in ion-exchanged water was 2 mol% with respect to the lithium composite oxide ML.

<< Example Battery A11 >>
Except for using a mixture of nickel sulfate, cobalt sulfate, and zirconium sulfate mixed so that the molar ratio of Ni atom, Co atom, and Zr atom is 80: 15: 5 in the synthesis of the coprecipitated hydroxide. A battery A11 was produced in the same manner as A1.

<< Example Battery A12 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate, cobalt sulfate, and zirconium sulfate mixed so that the molar ratio of Ni atom, Co atom, and Zr atom is 35:15:50 is used. In the synthesis, a battery A12 was produced in the same manner as the battery A1, except that the amount of disodium molybdate dihydrate dissolved in ion-exchanged water was 2 mol% with respect to the lithium composite oxide ML.

<< Example Battery A13 >>
In the synthesis of the coprecipitated hydroxide, a battery other than using a mixture of nickel sulfate, cobalt sulfate, and aluminum sulfate mixed so that the molar ratio of Ni atom, Co atom, and Al atom is 80: 15: 5. A battery A13 was produced in the same manner as A1.

<< Example Battery A14 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate, cobalt sulfate, and aluminum sulfate mixed so that the molar ratio of Ni atom, Co atom, and Al atom is 35:15:50 is used. In the synthesis, a battery A14 was produced in the same manner as the battery A1, except that the amount of disodium molybdate dihydrate dissolved in ion-exchanged water was 2 mol% with respect to the lithium composite oxide ML.

<< Example Battery A15 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate, cobalt sulfate, and yttrium nitrate hexahydrate mixed so that the molar ratio of Ni atom, Co atom, and Y atom was 80: 15: 5 was used. A battery A15 was made in the same manner as the battery A1, except that.

<< Example Battery A16 >>
In the synthesis of the coprecipitated hydroxide, a mixture of nickel sulfate, cobalt sulfate, and yttrium nitrate hexahydrate mixed so that the molar ratio of Ni atom, Co atom, and Y atom is 35:15:50 is used. In the synthesis of the positive electrode active material, the battery A16 was prepared in the same manner as the battery A1, except that the amount of disodium molybdate dihydrate dissolved in ion-exchanged water was 2 mol% with respect to the lithium composite oxide ML. Produced.

[Example 2]
<< Example Battery B1-16 >>
In the synthesis of the positive electrode active material, the amount of lithium carbonate added to the lithium composite oxide ML supporting Mo is changed so that the molar ratio is Mo / Li = 8/3, and the oxygen partial pressure in the subsequent firing is set to 0. Batteries B1 to B16 were produced in the same manner as the batteries A1 to A16 of Example 1 except that the pressure was changed to 0.06 atm and the firing temperature was changed to 500 ° C.
When the surface layer part of the obtained active material particles was analyzed by XRD, XPS, EPMA, ICP emission analysis, etc., the surface layer part was composed of molybdenum oxide represented by Li ₆ Mo ₂ O ₇ (that is, Li ₃ MoO _3.5 ). It was found to contain.

[Example 3]
<< Example Battery C1-16 >>
In the synthesis of the positive electrode active material, the amount of lithium carbonate added to the lithium composite oxide ML supporting Mo is changed so that the molar ratio is Mo / Li = 1/1, and the oxygen partial pressure in the subsequent firing is set to 0. Batteries C1 to C16 were produced in the same manner as the batteries A1 to A16 of Example 1 except that the pressure was changed to 0.01 atm.
When the surface layer portion of the obtained active material particles was analyzed by XRD, XPS, EPMA, ICP emission analysis or the like, it was found that the surface layer portion contained molybdenum oxide represented by LiMoO ₂ .

[Example 4]
<< Example Battery D1-16 >>
In the synthesis of the positive electrode active material, the amount of lithium carbonate added to the lithium composite oxide ML supporting Mo is changed so that the molar ratio is Mo / Li = 4/1, and the oxygen partial pressure in the subsequent firing is set to 0. Batteries D1 to 16 were produced in the same manner as the batteries A1 to A16 of Example 1 except that the pressure was changed to 0.06 atm.
When the surface layer portion of the obtained active material particles was analyzed by XRD, XPS, EPMA, ICP emission analysis, etc., it was found that the surface layer portion contained molybdenum oxide represented by Li ₂ MoO ₃ .

[Example 5]
<< Example Battery E1-16 >>
In the synthesis of the positive electrode active material, the amount of lithium carbonate added to the lithium composite oxide ML supporting Mo is changed so that the molar ratio is Mo / Li = 4/1, and the oxygen partial pressure in the subsequent firing is set to 0. Batteries E1 to 16 were produced in the same manner as the batteries A1 to A16 of Example 1 except that the pressure was changed to 5 atm.
When the surface layer portion of the obtained active material particles was analyzed by XRD, XPS, EPMA, ICP emission analysis, etc., it was found that the surface layer portion contained molybdenum oxide represented by Li ₂ MoO ₄ .

[Comparative Example 1]
<< Comparative Battery R1-16 >>
In the synthesis of the positive electrode active material, the same procedure as in the batteries A1 to A16 of Example 1 was performed, except that the lithium composite oxide ML was not immersed in an aqueous solution of disodium molybdate dihydrate to carry Mo. Thus, batteries R1 to R16 were produced.

[Evaluation]
(Discharge characteristics)
Each battery was conditioned and discharged twice and then stored for 2 days in a 40 ° C. environment. Thereafter, the following two patterns of cycle tests were performed for each battery. However, the design capacity of the battery is 1 CmAh.

First pattern (normal cycle test)
(1) Constant current charging (45 ° C.): 0.7 CmA (end voltage 4.2 V)
(2) Constant voltage charging (45 ° C.): 4.2 V (end current 0.05 CmA)
(3) Charging rest (45 ° C): 30 minutes (4) Constant current discharge (45 ° C): 1 CmA (end voltage 3 V)
(5) Discharge rest (45 ° C): 30 minutes

Second pattern (intermittent cycle test)
(1) Constant current charging (45 ° C.): 0.7 CmA (end voltage 4.2 V)
(2) Constant voltage charging (45 ° C.): 4.2 V (end current 0.05 CmA)
(3) Charging rest (45 ° C): 720 minutes (4) Constant current discharge (45 ° C): 1 CmA (end voltage 3V)
(5) Discharge rest (45 ° C): 720 minutes

  The discharge capacity after 500 cycles obtained by the first and second patterns is shown in Tables 1-6.

  Although lithium composite oxide ML synthesized using various raw materials instead of the coprecipitated hydroxide was also evaluated, these descriptions are omitted.

  INDUSTRIAL APPLICABILITY The present invention is useful in a lithium ion secondary battery including a lithium composite oxide containing nickel or cobalt as a main component as a positive electrode active material, and has conventionally exhibited cycle characteristics under conditions closer to actual use conditions (for example, intermittent cycles). Can be further increased.

  The shape of the lithium ion secondary battery of the present invention is not particularly limited, and may be any shape such as a coin shape, a button shape, a sheet shape, a cylindrical shape, a flat shape, and a square shape. Further, the form of the electrode plate group including the positive electrode, the negative electrode, and the separator may be a wound type or a laminated type. The size of the battery may be small for a small portable device or large for an electric vehicle. Therefore, the lithium ion secondary battery of the present invention can be used, for example, as a power source for a portable information terminal, a portable electronic device, a small household power storage device, a motorcycle, an electric vehicle, a hybrid electric vehicle, and the like. However, the application is not particularly limited.

It is a longitudinal cross-sectional view of the cylindrical lithium ion secondary battery which concerns on the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation gasket 5 Positive electrode 5a Positive electrode lead 6 Negative electrode 6a Negative electrode lead 7 Separator 8a Upper insulating plate 8b Lower insulating plate

Claims (6)

A chargeable / dischargeable positive electrode, a chargeable / dischargeable negative electrode, and a non-aqueous electrolyte;
The positive electrode includes active material particles,
The active material particles include a lithium composite oxide,
The lithium composite oxide is Li _x M _1-y L _y O ₂ (where 0.85 ≦ x ≦ 1.25, 0 ≦ y ≦ 0.5, M is selected from the group consisting of Ni and Co) And at least one selected from the group consisting of alkaline earth elements, transition metal elements other than Ni and Co, rare earth elements, IIIb group elements, and IVb group elements)
A lithium ion secondary battery in which molybdenum oxide represented by Li _a MoO _b (where 1 ≦ a ≦ 4, 1 ≦ b ≦ 6) is present in a surface layer portion of the active material particles.   The lithium ion secondary battery according to claim 1, wherein L includes at least one selected from the group consisting of Al, Mn, Ti, Mg, Zr, Nb, Y, Ca, In, and Sn.   The lithium ion secondary battery according to claim 1, wherein L is distributed more on a surface layer side than in the active material particles.   The lithium ion secondary battery according to claim 1, wherein an amount of the molybdenum oxide is 2 mol% or less with respect to the lithium composite oxide.   The lithium ion secondary battery according to claim 1, wherein the average particle diameter of the active material particles is 10 μm or more.   The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte includes at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, fluorobenzene, and phosphazene.

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