JP2009286669A - Manufacturing method of olivine type compound, and lithium-ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of producing an olivine type compound suitable for a positive electrode active material of a lithium ion secondary battery with high productivity, and a lithium ion secondary battery using the olivine type compound obtained by the method.
SOLUTION: General formula Li _a A _1-x M _x PO ₄ (A is at least one element selected from the group consisting of Fe, Co, Mn, Ni, V, Cr and Cu, M is Li And an olivine-type compound represented by 0.9 ≦ a ≦ 1.2 and 0 ≦ x ≦ 0.2), at least a Li source, an element An olivine type comprising: a step of preparing a solution by dissolving an A source, a P source and a polyvalent carboxylic acid in a solvent; a step of obtaining a precursor by evaporating and drying the solution; and a step of firing the precursor. A method for producing a compound, and a lithium ion secondary battery having the olivine type compound as a positive electrode active material.
[Selection figure] None

Description

  The present invention relates to a method for producing an olivine type compound suitable for a positive electrode active material of a lithium ion secondary battery with high productivity, and a lithium ion secondary battery using the olivine type compound.

  Lithium ion secondary batteries are being rapidly developed as batteries for use in portable electronic devices and hybrid vehicles. In current lithium ion secondary batteries, carbon materials are mainly used as the negative electrode active material in this case, and metal oxides, metal sulfides, various polymers, etc. are used as the positive electrode active material. ing. In particular, lithium composite oxides such as lithium cobaltate, lithium nickelate, and lithium manganate are widely used as positive electrode active materials because they can realize a high-energy density and high-voltage lithium ion secondary battery.

On the other hand, as disclosed in Patent Document 1 and Patent Document 2, a lithium ion secondary battery using a transition metal phosphate compound having an olivine structure as a positive electrode active material has also been proposed. Describes a lithium ion secondary battery using inexpensive lithium iron phosphate (LiFePO ₄ ) as a positive electrode active material. An olivine-type compound such as lithium iron phosphate forms a highly covalent PO ₄ polyanion, so that it is difficult to release O ₂ even at high temperatures and has excellent thermal stability. At present, there is an increasing demand for safety of lithium ion secondary batteries, and the above olivine type compounds are attracting attention as meeting these requirements.

  However, since the olivine type compound as described above is structurally slow in self-diffusion of Li (lithium) ions, a lithium ion secondary battery using the same, for example, a lithium ion secondary battery using lithium cobaltate. However, there is a problem in that it is difficult to take out the capacity even with the output normally used. For this reason, it is necessary to reduce the primary particle size of the olivine type compound particles and to reduce the Li ion diffusion path in the particles.

As a method for producing the olivine-type compound as described above, for example, in Non-Patent Document 1, CoNH ₄ PO ₄ and LiOH are mixed in a dry process, calcined at 400 ° C. for 12 hours, pulverized, and 3 at 700 ° C. A method of performing main firing for a day is disclosed. In such a so-called solid phase reaction method, a reaction occurs at the interface between the raw material particles, and therefore, in order to synthesize a highly uniform olivine type compound, it requires relatively long firing at a relatively high temperature. The obtained olivine type compound is likely to become coarse powder with particle growth. Further, since the firing step becomes long, the productivity is low, and for example, it is not suitable as a method for producing an olivine type compound on an industrial scale. Furthermore, when a part of Co, which is the main element of the olivine type compound, is substituted with other metal elements, the components are easily segregated, and there is a problem in the uniformity of the obtained olivine type compound.

On the other hand, in Non-Patent Document 2, lithium acetate, manganese acetate, ammonium hydrogen phosphate, and glycolic acid are mixed, and the pH is adjusted with concentrated nitric acid to prepare a solution. The solution is evaporated to dryness, A method for producing LiMnPO ₄ which is an olivine type compound by firing is disclosed. This method is a solution method and has an advantage that it is easy to produce an olivine type compound having relatively high uniformity.

  However, even in the production method described in Non-Patent Document 2, the firing conditions are relatively long at 450 to 800 ° C. for 3 hours, and it is necessary to use a relatively large amount of expensive glycolic acid. From the point of view, productivity is still insufficient, and it is not suitable as a method for producing an olivine type compound on an industrial scale.

JP-A-9-134724 JP-A-9-171827 Electrochemical and Solid State Letters, 2002, 5 (10), A234-A237. Electrochemical and Solid State Letters, 2006, 9 (6), A277-A280.

  The present invention has been made in view of the above circumstances, and the object thereof is a method capable of producing an olivine type compound suitable for a positive electrode active material of a lithium ion secondary battery with high productivity, and an olivine type obtained by the method. It is providing the lithium ion secondary battery using a compound.

The method for producing the olivine-type compound of the present invention that has achieved the above-mentioned object has the general formula Li _a A _1-x M _x PO ₄ (A is a group consisting of Fe, Co, Mn, Ni, V, Cr, and Cu). At least one element selected from: M is a metal element other than Li and the element M, and 0.9 ≦ a ≦ 1.2 and 0 ≦ x ≦ 0.2) A method for producing a compound, the step of dissolving at least a Li source, an element A source, a P source and a polyvalent carboxylic acid in a solvent, and a step of obtaining a precursor by evaporating the solution to dryness And a step of firing the precursor.

  Moreover, the lithium ion secondary battery of this invention has the olivine type compound obtained by the manufacturing method of this invention as a positive electrode active material.

  The glycolic acid according to the production method described in Non-Patent Document 2, which is a production method of the olivine type compound, forms a chelate with these compounds in a solution containing the raw material compound of the olivine type compound. Contributes to the formation of a more uniform olivine compound by increasing the uniformity of the precursor obtained by evaporating the solution to dryness (the precursor of the olivine compound. If this is fired, it becomes an olivine compound). it seems to do.

  However, glycolic acid is very expensive, and the production method described in Non-Patent Document 2 uses glycolic acid in a relatively large amount. Therefore, in the production method described in Non-Patent Document 2, the production cost of the olivine type compound becomes very high, and the productivity cannot be increased.

  In the method of the present invention, as described above, polyvalent carboxylic acid is used for preparing the raw material solution of the olivine type compound precursor. This polyvalent carboxylic acid has a high chelate-forming ability, and can form chelates with raw material compounds such as Li source, element A source, and element M source in the raw material solution of the olivine type compound precursor even in a relatively small amount. A precursor with high uniformity can be formed. In addition, an inexpensive polycarboxylic acid can be used, and the production cost of the olivine type compound can be reduced.

  Furthermore, although the reason is unknown, it is possible to obtain an olivine-type compound with high uniformity even when the precursor obtained using a polyvalent carboxylic acid has a very short firing time. And since baking time is short, an olivine type compound with a small particle size can be manufactured efficiently.

  From these facts, according to the method of the present invention, the productivity of the olivine type compound can be greatly increased. For example, the industrial scale production of the olivine type compound suitable for the positive electrode active material of the lithium ion secondary battery can be achieved. It becomes possible.

  And the lithium ion secondary battery which has a favorable battery characteristic can be comprised by using the olivine type compound manufactured by this invention method as a positive electrode active material.

  ADVANTAGE OF THE INVENTION According to this invention, the method which can manufacture the olivine type compound suitable for the positive electrode active material of a lithium ion secondary battery with sufficient productivity, and the lithium ion secondary battery using the olivine type compound obtained by this method are provided. be able to.

The olivine type compound produced by the method of the present invention is represented by the general formula Li _a A _1-x M _x PO ₄ . The element A is Fe, Co, Mn, Ni, V, Cr, or Cu, and may be one of these, or two or more.

  Further, the element M in the general formula relating to the olivine type compound is a metal element other than Li and the element A, and specifically, for example, Mg, Ti, Al, Zr, Nb, Mo, Ba, Sc Y, Ce, Nd, Sm, Ag, In, Sn, Ga, Zn, Ca, Sr, and the like.

  The olivine-type compound may not contain the element M, but when it is contained, the amount x of the element M in the general formula is 0.005 or more in order to effectively exert its action. Is more preferable and 0.01 or more is more preferable. However, if the amount of the element M is too large, the capacity may be reduced, so the upper limit is 0.2, preferably 0.15 or less.

  In addition, when the Li content a in the general formula is in the range of 0.9 to 1.2, the compound having an olivine structure can be easily synthesized.

  The method of the present invention for producing the above olivine type compound mainly has three steps. The first step is a solution preparation step of preparing a solution (mixed solution) by dissolving a Li source, an element A source, a P source, a polyvalent carboxylic acid, and an element M source used as necessary in a solvent. .

  In the solution preparation step, which is the first step, a mixed solution is prepared by dissolving a raw material containing each element constituting the olivine type compound in a solvent. Examples of the Li-containing compound serving as the Li source, the element A-containing compound serving as the element A source, and the element M-containing compound serving as the element M source include oxides, hydroxides, oxyhydroxides, carbonates, and basics. Examples thereof include carbonates, nitrates, chlorides, sulfates, organic acid salts (for example, carboxylates such as citrates and malates).

Examples of the Li-containing compound serving as the Li source, the element A-containing compound serving as the element A source, and the element M-containing compound serving as the element M source include carbonates, basic carbonates, and hydroxides among the various compounds exemplified above. Oxides or carboxylates are preferred. For example, when using acetic acid salt and nitrate, since toxic gases, such as acetic acid and NO _x during the firing, which will be described later occurs, the industrial production requires large gas processing facilities, also the firing furnace The inside may corrode. Further, when acetate or nitrate is used, acetate ions and nitrate ions are likely to remain in a precursor (described later in detail) obtained by evaporating and drying the mixed solution. For this reason, the precursor tends to be a mixture of the remaining salt derived from these ions and a separately formed metal carboxylate, but since these have different decomposition temperatures, local segregation of metal ions occurs. It is easy to control the firing conditions in the firing step.

  In contrast, Li-containing compounds as Li sources, Element A-containing compounds as Element A sources, and Element M-containing compounds as Element M sources include carbonates, basic carbonates, hydroxides, oxides or carboxyls. When an acid salt is used, the above-described problems can be avoided, and a more uniform olivine type compound can be obtained while simplifying the production equipment.

  P-containing compounds that serve as P sources include phosphorus alone (red phosphorus, etc.), acids (phosphoric acid, etc.), P-containing organic compounds (trimethyl phosphate, triethyl phosphate, etc.), oxides (phosphorus pentoxide, etc.), ammonium, etc. And salts (diammonium hydrogen phosphate, ammonium dihydrogen phosphate, etc.).

As the Li, element A source, element M source, and P source, the various compounds exemplified above may be used alone or in combination of two or more. Moreover, the need to use a compound containing each element separately without, for example, as LiH ₂ PO _4, Li, element A, be a compound containing two or more elements selected from the elements M and P Good. In short, each element for constituting the olivine type compound may be contained in a necessary amount in the mixed solution.

  Examples of the polyvalent carboxylic acid include citric acid, tartaric acid, malic acid, succinic acid, malonic acid, oxalic acid and the like, and compounds having 2 to 7 carbon atoms are preferably used.

  Examples of the solvent for dissolving the various raw materials include water; alcohols such as methanol and ethanol; glycols such as ethylene glycol; and the like.

  The method for preparing the mixed solution is not particularly limited. For example, the various raw materials may be added to the solvent and dissolved while stirring. Moreover, when dissolving the various raw materials in a solvent, heating may be performed.

  In the second step of the method of the present invention, the mixed solution obtained in the solution preparation step, which is the first step, is heated, and the solvent is volatilized and evaporated to dryness. It is a process of obtaining a body. Examples of means for drying the mixed solution include various means such as an evaporator, a hot plate, a water bath, an oil bath, and a mantle heater. The heating conditions are not particularly limited, and a heating temperature and a heating time that can remove the solvent and do not cause side reactions such as decomposition of the precursor may be selected.

  In the solution preparation step, a polyvalent carboxylic acid having excellent chelating ability is added to the mixed solvent, so that in this evaporation / drying step, each component element when the mixed solution is evaporated to dryness can be obtained. Separation can be prevented and a more uniform state can be maintained, and it becomes possible to lower the firing temperature and shorten the firing time when firing the precursor in the third step described later.

  Carboxylic acid tends to form a complex with a metal ion in a solution, but in a carboxylic acid having one carboxyl group in the molecule, one metal ion is coordinated to one molecule of the carboxylic acid, whereas a polyvalent carboxylic acid In this case, since a complex containing two or more metal ions per molecule is formed, for example, the distribution of Li and element A in the precursor can be made more uniform. If it is a precursor in such a state, since each element is in a state of being close when pyrolyzing by firing, synthesis of an olivine type compound by firing at a lower temperature and in a shorter time becomes possible.

  The 3rd process in this invention method is a baking process which bakes the precursor obtained by the said evaporation-drying process which is a 2nd process.

  Since the precursor obtained in the second step is usually obtained in the form of powder or lump, the device used in this firing step is a device usually used for firing inorganic powder, such as a box. A furnace, a tubular furnace, a tunnel furnace, a rotary kiln, etc. can be used.

  When the firing process is divided into three steps of temperature rise, constant temperature hold, and temperature drop, the second constant temperature hold portion is not necessarily limited to one time. For example, temperature rise, constant temperature hold, temperature rise, constant temperature Holding, and so on, if necessary, may be subjected to two or more steps, and a pulverization step that means eliminating aggregation to such an extent that the secondary particles are not destroyed, or primary particles or Further, the step of raising the temperature, maintaining a constant temperature, and lowering the temperature may be repeated twice or more under the same condition or different conditions with a pulverization step meaning to crush to a fine powder.

  In the temperature raising step, the temperature in the furnace is usually raised in the range of 1 to 20 ° C./min. If the heating rate is too fast, the furnace temperature cannot follow the set temperature, and if it is too slow, it is industrially disadvantageous.

  The firing temperature is preferably 300 ° C. or higher, more preferably 400 ° C. or higher, preferably 1200 ° C. or lower, more preferably 1000 ° C. or lower, and still more preferably 600 ° C. or lower. Moreover, the time (baking time) held after raising the temperature to the firing temperature is preferably 0.1 hours or more, more preferably 0.5 hours or more, preferably 50 hours or less, more preferably 20 hours or less. More preferably, it is less than 3 hours, particularly preferably 1 hour or less. If the firing time is too long, sintering between the powders proceeds and subsequent crushing becomes difficult. If the firing time is too short, powder with high crystallinity cannot be obtained or unreacted substances remain.

The environment (atmosphere) at the time of firing may be selected from among O ₂ , N ₂ , Ar, H ₂ , the atmosphere, or a mixed gas thereof, in which an appropriate gas exists depending on the properties of the olivine type compound to be synthesized. . For example, when LiFePO ₄ is synthesized, since Fe ²⁺ is easily oxidized to Fe ³⁺ , it is preferably fired in an Ar atmosphere or a mixed gas atmosphere of Ar and H ₂ . On the other hand, LiCoPO ₄ can be synthesized by firing in the air.

  In the method of the present invention, as described above, the precursor obtained through the first step and the second step is in a state where the elements for constituting the olivine type compound are in close proximity by the action of the polyvalent carboxylic acid. Therefore, for example, the olivine type compound can be synthesized satisfactorily even if the firing temperature is lowered to 600 ° C. or lower and the firing time is shortened to less than 3 hours (more preferably 1 hour or less). Therefore, according to the method of the present invention, the productivity of the olivine type compound can be increased.

  The olivine type compound obtained by the method of the present invention is suitable for a positive electrode active material for a lithium ion secondary battery.

  The lithium ion secondary battery of the present invention only needs to have the olivine type compound obtained by the method of the present invention as a positive electrode active material, and the other configurations and structures are the same as those of conventionally known lithium ion secondary batteries. The configuration and structure can be adopted.

  As a positive electrode, what has the positive mix layer containing the olivine type compound obtained by this invention method on the single side | surface or both surfaces of a collector is mentioned, for example.

  The positive electrode mixture layer in the positive electrode is formed by layering a positive electrode mixture having at least an olivine type compound obtained by the method of the present invention on the surface of the current collector. The positive electrode mixture layer preferably contains a binder and a conductive additive. By including a conductive additive and a binder in the positive electrode mixture layer, the electronic conductivity and strength of the positive electrode can be increased.

  As the conductive additive for the positive electrode, any inorganic material or organic material can be used as long as it is chemically stable in the battery. For example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black (trade name), channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fiber and metal fiber; aluminum Metal powder such as powder; carbon fluoride; zinc oxide; conductive whisker made of potassium titanate; conductive metal oxide such as titanium oxide; organic conductive material such as polyphenylene derivative; These conductive assistants may be used alone or in combination of two or more. Among these, carbon black is particularly preferable. Since carbon black has a small average particle diameter of 0.01 to 1 μm, it can be filled in the gaps between the positive electrode active material particles, and a space not originally involved in the battery capacity can be used, so without reducing the amount of the positive electrode active material particles This is because electron conductivity can be imparted.

As the binder for the positive electrode, any thermoplastic resin or thermosetting resin can be used as long as it is chemically stable in the battery. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin) , Polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrif Oroechiren copolymer (ECTFE), vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene copolymer, vinylidene fluoride - perfluoromethyl vinyl ether - tetrafluoroethylene copolymer, ethylene - acrylic acid copolymer or its Na ⁺ Ionic cross-linked product, ethylene-methacrylic acid copolymer or its Na ⁺ ionic cross-linked product, ethylene-methyl acrylate copolymer or its Na ⁺ ionic cross-linked product, ethylene-methyl methacrylate copolymer or its Na ⁺ ionic cross-linked product Etc. These binders may be used alone or in combination of two or more. Among these, PVDF and PTFE are particularly preferable. This is because the binding force can be exerted in a small amount.

  In the positive electrode, for example, a positive electrode mixture in which a conductive additive or a binder is appropriately added to an olivine type compound which is a positive electrode active material is dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) (binding). The adhesive may be dissolved in a solvent) and is made by forming a slurry-like or paste-like composition and applying the composition to a current collector to form a strip-shaped molded body (positive electrode mixture layer). The However, the manufacturing method of the positive electrode is not limited to the above method, and may be manufactured by other methods. The thickness of the positive electrode mixture layer formed on the current collector surface is usually 20 to 100 μm per one side of the current collector.

  The material for the current collector of the positive electrode is not particularly limited as long as it is an electron conductor that is chemically stable in the constructed battery. For example, in addition to aluminum or aluminum alloy, stainless steel, nickel, titanium, carbon, conductive resin, etc., a composite material in which a carbon layer or a titanium layer is formed on the surface of aluminum, aluminum alloy, or stainless steel can be used. . Among these, aluminum or an aluminum alloy is particularly preferable. This is because they are lightweight and have high electron conductivity. For the current collector, for example, a foil, a film, a sheet, a net, a punching sheet, a lath body, a porous body, a foamed body, a fiber group molded body, or the like made of the above material is used. In addition, the surface of the current collector can be roughened by surface treatment. Although the thickness of a collector is not specifically limited, Usually, it is 1-500 micrometers.

  In the positive electrode mixture layer, the positive electrode active material is contained in an amount of 80% by mass to 98% by mass with respect to the total mass of the positive electrode active material, the binder, and the conductive additive, and the binder is contained in an amount of 1% by mass to 10% by mass. It is preferable to include 1% by mass or more and 19% by mass or less of a conductive aid. This is because, within this range, the content of the binder that does not directly participate in the electrode reaction is small, so that the capacity of the electrode can be increased.

As the negative electrode, for example, a negative electrode mixture having a negative electrode active material and a binder or the like formed into a layer shape (negative electrode mixture layer) on one side or both sides of a current collector can be used. Examples of the negative electrode active material for the negative electrode include graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, Si, and Sn such as Sn. An alloyable metal or an alloy thereof is used. An oxide such as metallic lithium, lithium-aluminum alloy, or Li ₄ Ti ₅ O ₁₂ can also be used.

As the binder for the negative electrode, various binders exemplified as the binder for the positive electrode can be used. Among them, styrene butadiene rubber, PVDF, ethylene-acrylic acid copolymer or its Na ⁺ ion crosslinked body, ethylene -A methacrylic acid copolymer or its Na ⁺ ion crosslinked product, an ethylene-methyl acrylate copolymer or its Na ⁺ ion crosslinked product, an ethylene-methyl methacrylate copolymer or its Na ⁺ ion crosslinked product are particularly preferred.

  Although it is not necessary to add a conductive support agent to the negative electrode mixture layer, it may be added. As the conductive aid for the negative electrode, various conductive aids exemplified as the conductive aid for the positive electrode can be used.

  The negative electrode is prepared by, for example, dispersing a negative electrode mixture in which a binder (further necessary, a conductive auxiliary agent) or the like is appropriately added to the negative electrode active material in a solvent such as water or NMP (the binder is added to the solvent). (It may be dissolved) It is prepared by forming a slurry-like or paste-like composition and applying the composition to a current collector to form a band-shaped molded body (negative electrode mixture layer). However, the manufacturing method of the negative electrode is not limited to the above method, and may be manufactured by other methods. The thickness of the negative electrode mixture layer formed on the current collector surface is usually 20 to 100 μm per one surface of the current collector.

  The material of the current collector of the negative electrode is not particularly limited as long as it is an electron conductor that is chemically stable in the constructed battery. For example, in addition to copper or copper alloy, stainless steel, nickel, titanium, carbon, conductive resin, etc., a composite material in which a carbon layer or a titanium layer is formed on the surface of copper, copper alloy, or stainless steel can be used. . Among these, copper or a copper alloy is particularly preferable. This is because they are not alloyed with lithium and have high electron conductivity. For the current collector of the negative electrode, for example, a foil, a film, a sheet, a net, a punching sheet, a lath body, a porous body, a foamed body, a molded body of a fiber group, etc. made of the above-described materials, etc. Is used. In addition, the surface of the current collector can be roughened by surface treatment. Although the thickness of a collector is not specifically limited, Usually, it is 1-500 micrometers.

  As the non-aqueous electrolyte, a non-aqueous electrolytic solution in which an electrolyte salt is dissolved in a solvent, or a gelled product thereof can be used. Examples of the solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), γ-butyrolactone, 1, 2 -Dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, Sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, diethyl ether, 1,3-propane sultone, etc. One aprotic organic solvent mixed solvent or a mixture of two or more can be used. In these, the mixed solvent of EC, MEC, and DEC is preferable, and it is especially preferable that this mixed solvent contains 15 volume% or more and 80 volume% or less of DEC with respect to the whole volume of a mixed solvent. This is because within this range, the stability of the solvent during high-voltage charging can be enhanced while maintaining the low-temperature characteristics and charge / discharge cycle characteristics of the battery.

As the electrolyte salt related to the non-aqueous electrolyte, a salt of a fluorine-containing compound such as lithium perchlorate, lithium organic boron, trifluoromethanesulfonate, imide salt, or the like is preferably used. Specific examples of the electrolyte salt, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F2 _{n + 1} SO ₃ (n ≧ 2), LiN (Rf ₃ OSO ₂ ) ₂ [where Rf Represents a fluoroalkyl group. These may be used alone or in combination of two or more. Among these, LiPF ₆ and LiBF ₄ are particularly preferable because of good charge / discharge characteristics. This is because these fluorine-containing organic lithium salts have a large anionic property and easily ion-separate, so that they are easily dissolved in the solvent.

  The material and shape of the separator are not particularly limited, and those having insulating properties, high ion permeability, low electrical resistance, and high liquid retention are preferable. Usually, a separator having a thickness of 10 to 300 μm and a porosity of 30 to 80% is used. Further, the pore diameter of the separator is preferably such that the active material, the conductive auxiliary agent, the binder, and the like detached from the electrode do not pass through, for example, 0.01 to 1 μm.

  It is preferable that the separator has a shutdown function in which the separator is softened or melted according to heat generated by an internal short circuit (100 to 140 ° C.), whereby the pores of the separator are closed to cut off the current. This is because the safety of the battery can be further improved. Specifically, for example, it is preferable to use a microporous film or nonwoven fabric made of polyolefin such as polyethylene, polypropylene, or polymethylpentene as a separator because a shutdown function can be provided. Further, by using a separator having a multilayer structure constituted by laminating a plurality of microporous films and non-woven fabrics of the above materials or by laminating a plurality of microporous films or non-woven fabrics, in a high temperature environment. The reliability of the battery when used can be further increased.

  As a battery case for storing these battery parts, a metal square case, a metal cylindrical case, a laminate case made of a laminate film, and the like are preferably used.

  Since the lithium ion secondary battery of the present invention uses the olivine type compound obtained by the method of the present invention as the positive electrode active material, it has a high energy density and can be used at a high voltage. The lithium ion secondary battery of the present invention is preferably used in various applications to which a conventionally known lithium ion secondary battery is applied, including power supply applications such as portable electronic devices and hybrid vehicles, taking advantage of these characteristics. Can do.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

Example 1
<Synthesis of olivine-type compounds>
Lithium carbonate, cobalt nitrate hexahydrate, 85 mass% phosphoric acid, and citric acid monohydrate in a total amount of 16.9 g in a molar ratio of Li: Co: P: citric acid monohydrate = 1: 1: 1 : Weighed out to be 1.1. These were put into 40 g of ion-exchanged water and dissolved with stirring to obtain a uniform mixed solution.

  The mixed solution was placed in a petri dish and heated on a hot plate heated to 120 ° C. The viscosity increased as the solvent evaporated, and a flaky solid precursor was obtained after 2 hours. The precursor thus obtained was put in an alumina crucible and fired at 600 ° C. for 1 hour while flowing air at 3 L / min to obtain a powder (olivine type compound). At the time of firing, the rate of temperature rise up to the firing temperature and the rate of temperature drop from the firing temperature were 10 ° C./min.

When the powder obtained as described above was analyzed with an X-ray diffraction (XRD) apparatus, it could be identified as LiCoPO ₄ and no impurities were observed. The profile of the XRD spectrum obtained by the analysis is shown in FIG.

<Preparation of positive electrode>
The above compound (positive electrode active material), PVDF as a binder, and acetylene black as a conductive additive are mixed at a mass ratio of 80: 5: 15 to obtain a positive electrode mixture, To prepare a positive electrode mixture-containing slurry. This slurry was applied to one side of an Al foil serving as a current collector, dried, pressed with a rolling roll, and further dried to produce an electrode. This electrode was punched out with a size of 11 mmφ to obtain a positive electrode.

Using the positive electrode, a metallic lithium foil as a counter electrode, a microporous polyethylene film as a separator, and a 1M LiPF ₆ EC / DEC solution [a mixed solvent of ethylene carbonate, (EC) and diethyl carbonate (DEC) ( A lithium ion secondary battery (discharge capacity measuring cell) was prepared using each of the solutions obtained by dissolving 1 mol / l of LiPF ₆ in the volume ratio EC: EMC = 1: 2).

<Evaluation of initial discharge capacity>
The lithium ion secondary battery was charged at a temperature of 25 ° C. at a constant current density of 17 mA / g until the positive electrode voltage with respect to the Li metal reached 5.0 V, and then charged at a constant voltage of 5.0 V. The constant current-constant voltage charging was performed for 25 hours. After the charging is completed, the lithium ion secondary battery is allowed to stand for 10 minutes, and then discharged at a constant current of 17 mA / g to a final voltage of 3.0 V. The discharge capacity per gram of the positive electrode active material at this time is defined as the initial discharge capacity, The ratio of the initial discharge capacity to the capacity was defined as charging efficiency.

Example 2
Lithium carbonate, cobalt hydroxide, 85% phosphoric acid, and citric acid monohydrate are used as raw materials in a molar ratio of Li: Co: P: citric acid monohydrate = 1: 1: 1: 1.1 Weighed out and used. These were put into 40 g of ion-exchanged water and kept at 80 ° C. with a water bath while stirring to dissolve the raw materials to obtain a uniform mixed solution. This solution was placed in a petri dish, and the following procedure was performed in the same manner as in Example 1 to obtain an olivine type compound. When the powder thus obtained was analyzed by XRD, it could be identified as LiCoPO ₄ and no impurities were seen.

  A lithium ion secondary battery was produced in the same manner as in Example 1 except that the above compound was used, and its initial discharge capacity was evaluated.

Comparative Example 1
An olivine type compound was synthesized in the same manner as in Example 1 except that citric acid monohydrate was changed to glycolic acid. When the powder thus obtained was analyzed by XRD, the main phase could be identified as LiCoPO ₄ , but impurities were observed. A magnified portion of the XRD spectrum of this compound is shown in FIG. 2 together with the XRD spectrum of the compound of Example 1.

  A lithium ion secondary battery was produced in the same manner as in Example 1 except that the above compound was used, and its initial discharge capacity was evaluated.

  Table 1 shows the measurement results of the initial discharge capacity in the lithium ion secondary batteries of Examples 1 and 2 and Comparative Example 1.

  As shown in FIG. 1 and FIG. 2, the olivine type compound obtained in Example 1 does not show a peak due to impurities as observed in the compound obtained in Comparative Example 1 in the XRD profile. Although not shown, as described above, in the XRD profile of the olivine-type compound obtained in Example 2, as in the compound according to Example 1, no peak due to impurities was observed. From these facts, it can be seen that a more uniform olivine type compound can be obtained by a production method using a polyvalent carboxylic acid even in a short baking time of 1 hour. For this reason, as shown in Table 1, the lithium ion secondary battery configured using the compounds of Examples 1 and 2 had a large discharge capacity and a high charging efficiency.

2 is a profile showing an XRD spectrum of an olivine type compound produced in Example 1. FIG. It is the figure which expanded the principal part of the XRD spectrum of the olivine type compound manufactured in Example 1 and Example 2. FIG.

Claims (8)

General formula Li _a A _1-x M _x PO ₄ (A is at least one element selected from the group consisting of Fe, Co, Mn, Ni, V, Cr and Cu, and M is other than Li and element M) A metal element of 0.9 ≦ a ≦ 1.2 and 0 ≦ x ≦ 0.2),
A step of preparing a solution by dissolving at least a Li source, an element A source, a P source and a polyvalent carboxylic acid in a solvent;
Evaporating the solution to dryness to obtain a precursor;
And a step of firing the precursor. A method for producing an olivine-type compound.   The method for producing an olivine-type compound according to claim 1, wherein the polycarboxylic acid has 2 to 7 carbon atoms.   The method for producing an olivine-type compound according to claim 2, wherein the polyvalent carboxylic acid is citric acid.   The method for producing an olivine-type compound according to any one of claims 1 to 3, wherein the Li source and the element A source are carbonate, basic carbonate, hydroxide, oxide, or carboxylate.   The method for producing an olivine-type compound according to claim 4, wherein the element M source is carbonate, basic carbonate, hydroxide, oxide, or carboxylate.   The method for producing an olivine type compound according to any one of claims 1 to 5, wherein the firing temperature of the precursor in the firing step is 600 ° C or lower.   The method for producing an olivine type compound according to any one of claims 1 to 6, wherein the firing time of the precursor in the firing step is less than 3 hours.   A lithium ion secondary battery comprising, as a positive electrode active material, an olivine type compound produced by the production method according to claim 1.

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