JP5928302B2 - Method for producing positive electrode active material for lithium secondary battery - Google Patents

Description

  The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery, a lithium secondary battery, a positive electrode for a lithium secondary battery, and a positive electrode active material for a lithium secondary battery.

  As a positive electrode active material for a lithium secondary battery, lithium cobaltate has hitherto been the mainstream, and lithium secondary batteries using this have been widely used. However, cobalt, which is a raw material for lithium cobaltate, is low in production and expensive, and alternative materials are being studied. Lithium manganate having a spinel structure listed as an alternative material has a problem in that the discharge capacity is insufficient and manganese is eluted at a high temperature. In addition, lithium nickelate, which can be expected to have a high capacity, has a problem in thermal stability (safety) at high temperatures.

From the viewpoint of thermal stability (safety), a compound having an olivine structure is particularly excellent and is expected as a positive electrode active material for a lithium secondary battery. A compound having an olivine structure (hereinafter sometimes abbreviated as olivine) is represented by the chemical formula LiMPO ₄ (M is a transition metal), has a strong PO bond in the structure, and deoxidizes even at high temperatures. Thermal stability (safety) is high because it is not separated.

  However, olivine has the disadvantage that it is inferior in electronic conductivity compared to lithium cobaltate. For this reason, there is a problem that the discharge capacity cannot be taken out sufficiently when the current value increases. The reason why the electron conductivity of olivine is low is that electrons are localized because the above-mentioned strong PO bond exists.

  In order to improve the electron conductivity for such a problem, a process of coating the surface of olivine with carbon (carbon coating) is required. As a method for producing a carbon-coated compound having an olivine structure, various methods have been proposed for the purpose of improving performance, simplifying the process, etc., based on a process of mixing raw materials and heat-treating.

For example, in Patent Document 1, a lithium compound, an iron compound, and a phosphoric acid compound are mixed to prepare a raw material mixture (A), and the raw material mixture is placed in an inert atmosphere, a reducing atmosphere, or a vacuum atmosphere. In the inert atmosphere, the step (B) of pre-baking to prepare a pre-fired product, the step (C) of preparing a fired product by mixing the pre-fired product with a polymer material, and the fired product in an inert atmosphere And a step (D) of firing in a reducing atmosphere or in a vacuum atmosphere in this order, a method for producing a positive electrode active material for a Li-ion battery is disclosed. According to Patent Document 1, LiFePO ₄ obtained by the above method has a carbon fine particle on the surface, a discharge capacity is very high, and a positive electrode active material for a Li-ion battery with good cycle characteristics, an expensive apparatus and a raw material are used. It is said that it can be easily manufactured without using it.

  In Patent Document 2, iron particles containing 0.5% by mass or more of oxygen are added to an aqueous solution containing phosphoric acid, carboxylic acid and a lithium source, and the components in the aqueous solution and the iron particles are added in an oxidizing atmosphere. Obtained in the above-mentioned precursor production step, the precursor production step for producing a lithium iron phosphate precursor by drying the reaction solution obtained in the synthesis step, In addition, a method for producing lithium iron phosphate is disclosed that includes a primary firing step of firing a lithium iron phosphate precursor in a non-oxidizing atmosphere to obtain lithium iron phosphate. According to Patent Document 2, by controlling the reaction of iron particles, a precursor of lithium iron phosphate uniformly mixed at an atomic level can be prepared, and as a result, it is an important characteristic required for a positive electrode active material. It is said that lithium iron phosphate excellent in high-speed charge / discharge characteristics can be produced stably at low cost.

Patent Document 3 discloses a general formula Li _x MPO ₄ (wherein M is at least one element selected from the group consisting of Co, Ni, Fe and Mn, and 0 <x ≦ 1). A positive electrode active material comprising the olivine-type lithium-containing phosphate compound represented by the above-mentioned condition)), for constituting the positive electrode active material including a lithium source, a phosphate source and an M element source A starting material mixture in an aqueous solvent to prepare a gel-like raw material mixture, heating the raw material mixture in a predetermined temperature range where the starting raw material does not crystallize, and calcining the temporary material A method for producing a positive electrode active material is disclosed which includes a step of adding conductive powder to a calcined product obtained in the firing step, mixing and firing. According to Patent Document 3, it is said that a lithium-containing phosphoric acid compound crystal close to the stoichiometric composition can stably grow, and a positive electrode active material having high crystallinity and excellent conductivity can be provided.

Non-Patent Document 1 and Non-Patent Document 2 describe the structure and physical properties of a compound having an olivine structure. According to Non-Patent Document 1, amorphous Li ₄ P ₂ O ₇ and / or Li ₃ PO ₄ is formed in the Li ₂ O—P ₂ O ₅ system (corresponding to olivine Li and P being excessive). Then it is described. According to Non-Patent Document 2, the crystallization temperature of LiFePO ₄ having an olivine type structure is considered to be around 420 ° C.

JP 2011-210376 International Publication No. 2011/086872 JP 2010-267501 A

Journal of the European Ceramic Society 29 (2009) 1895-1902. Robert Dominko, Marjan Bele, Jean-Michel Goupil, Miran Gaberscek, Darko Hanzel, Iztok Arcon, and Janez Jamnik "Wired Porous Cathode Materials: A Novel Concept for Synthesis of LiFePO4" Chemistry of Materials 19 (2007), pp. 2960-2969.

  While olivine can obtain sufficient electron conductivity for the first time by carbon coating, there is a problem that when carbon or the like is taken into the crystal of olivine, the crystallinity is lowered and the discharge capacity is lowered.

  Patent Document 1 describes a step of firing a raw material mixture and a material to be fired in a non-oxidizing atmosphere such as an inert atmosphere, a reducing atmosphere, or a vacuum atmosphere. In this case, when the raw material mixture is heat-treated in a non-oxidizing atmosphere, a part of the components other than Li, M, P, and O contained in the raw material mixture is taken into the crystal, and the crystallinity of the pre-fired body is reduced, resulting in a capacity. May decrease.

  In Patent Document 2, a precursor is prepared by adding inexpensive iron particles to an aqueous solution containing phosphoric acid, carboxylic acid and a lithium source, and preparing a reaction liquid in an oxidizing atmosphere, and drying the obtained reaction liquid. And a primary firing step of firing the obtained precursor in a non-oxidizing atmosphere. In this case, the carboxylic acid dissolved in the aqueous solution becomes a part of the precursor when dried, and is taken into the temporary fired body in the primary firing step of firing in a non-oxidizing atmosphere, and the crystallinity of the temporarily fired body is reduced. May reduce the capacity.

  In Patent Document 3, a starting material including a lithium source, a phosphoric acid source, and an M element source (M is at least one element selected from the group consisting of Co, Ni, Fe, and Mn) in an aqueous solvent. The step of adjusting the gel-like raw material mixture by mixing, the pre-baking step of heating the raw material mixture at a temperature at which it does not crystallize, and the step of adding and mixing conductive powder to the pre-fired body are described. ing. In this case, since the calcination process is performed at a temperature that does not crystallize, the calcination body does not form olivine. Therefore, at the stage where the conductive powder is added, mixed and fired, olivine is crystallized, and the conductive powder is taken into the olivine, resulting in a decrease in crystallinity and a decrease in capacity.

  Accordingly, an object of the present invention is to suppress a decrease in crystallinity of a compound having an olivine type structure in a method for producing a positive electrode active material for a lithium secondary battery using a highly safe compound having an olivine type structure. An object of the present invention is to provide a method for producing a positive electrode active material for a lithium secondary battery that can achieve both higher capacity (150 Ah / kg or more) and higher rate characteristics (80% or more).

In order to achieve the above object, one embodiment of the present invention provides
A step of mixing a raw material of a positive electrode active material for a lithium secondary battery represented by the following (Chemical Formula 1) and an organic substance not containing a metal element;
A step of calcining a mixture of the raw material of the positive electrode active material and the organic substance at a temperature of 420 ° C. to 600 ° C. in an oxidizing atmosphere;
A step of mixing a carbon compound not containing a metal element into the pre-fired body obtained by the pre-baking step;
A step of subjecting the calcined body mixed with the carbon compound to a temperature equal to or higher than the calcining temperature and a calcining in a reducing atmosphere or an inert atmosphere,
In Fe in the positive electrode active material for a lithium secondary battery represented by the following (Chemical Formula 1) obtained after the main firing step, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ ) Is 0.01 ≦ (III / II) ≦ 0.3. A method for producing a positive electrode active material for a lithium secondary battery is provided.
LiMP _1-x A _x O ₄ (Chemical Formula 1)
(M is a metal element, includes Fe, and includes at least one of Mn, Co, and Ni, A includes at least one selected from B, Si, Ti, and V, and 0 ≦ x ≦ 0 .25.)

  According to the present invention, in a positive electrode active material for a lithium secondary battery using a highly safe compound having an olivine structure, a decrease in crystallinity of the compound having an olivine structure is suppressed, and the capacity is higher than before. A method for producing a positive electrode active material for a lithium secondary battery that can achieve both (150 Ah / kg or more) and high rate characteristics (80% or more) can be provided.

It is a flowchart which shows the manufacturing method of the positive electrode active material for lithium secondary batteries which concerns on this invention. It is a cross-sectional schematic diagram which shows an example of the lithium secondary battery (18650 type lithium ion secondary battery) with which this invention is applied. 2 is a diagram showing an X-ray diffraction profile of a positive electrode active material for a lithium secondary battery of Example 1. FIG. It is a graph which shows the relationship between the monopolar discharge capacity and rate characteristic of the positive electrode active material of Examples 1-15, Comparative Examples 1, 2, and Reference Examples 1 and 2.

As described above, the method for producing a positive electrode active material for a lithium secondary battery according to the present invention includes:
A step of mixing a raw material of the positive electrode active material for a lithium secondary battery represented by (Chemical Formula 1) and an organic substance not containing a metal element (raw material mixing step);
A step of pre-baking the mixture of the raw material of the positive electrode active material and the organic substance in a temperature of 420 ° C. to 600 ° C. in an oxidizing atmosphere (pre-baking step);
A step of mixing a carbon compound not containing a metal element (carbon source mixing step) into the pre-fired body obtained by the pre-baking step;
And a step of subjecting the temporary fired body mixed with the carbon compound to a temperature equal to or higher than the preliminary firing temperature and a reducing atmosphere or an inert atmosphere (main firing step).

The method for producing a positive electrode active material for a lithium secondary battery of the present invention has the following two features. That is, (1) It is generated by decomposition of the organic substance by mixing an organic substance not containing a metal element in the raw material mixing step and baking in an oxidizing atmosphere at a temperature of 420 ° C. to 600 ° C. in the preliminary baking step. Gas suppresses the generation of harmful foreign phases. Here, the harmful foreign phase is a metal oxide other than a compound having an olivine structure among metal oxides containing a transition metal generated from a raw metal element. Specifically, Fe ₂ O ₃ , LiFeP ₂ O ₇ , Li ₃ Fe ₂ (PO ₄ ) ₃ , Mn ₂ O ₃ , MnO ₂ , Mn ₂ P ₂ O _{7 and the} like. (2) Since the preliminary firing step is an oxidizing atmosphere, the organic matter disappears in the temporary firing step and does not remain in the crystal structure of the positive electrode active material.

  According to the above (1) and (2), a compound having a highly crystalline olivine structure can be obtained, and a lithium secondary battery having high capacity and high rate characteristics can be provided. Details will be described later.

In the method for producing a positive electrode active material for a lithium secondary battery according to the present invention, the present invention can be improved or changed as follows.
(I) In the positive electrode active material, the proportion of Fe in M is 10 mol% to 50 mol%.
(Ii) The positive electrode active material includes amorphous Li ₄ P ₂ O ₇ and / or Li ₃ PO ₄ .
(Iii) The organic substance not containing the metal element is carboxylic acid.
(Iv) The carboxylic acid is one or more selected from the group consisting of acetic acid, citric acid and malic acid.
(V) The organic substance not containing the metal element is sugar.
(Vi) The sugar is one or more selected from the group consisting of sucrose, glucose, starch, cellulose, and dextrin.
(Vii) The temperature of the said pre-baking is 430 degreeC or more and 500 degrees C or less.
(Viii) The oxidizing atmosphere of the pre-baking step has an oxygen concentration of 1% or more.

Moreover, the present invention can add the following improvements and changes to the lithium secondary battery.
(Ix) The positive electrode active material includes amorphous Li ₄ P ₂ O ₇ and / or Li ₃ PO ₄ .

  Embodiments according to the present invention will be described below. However, the present invention is not limited to the embodiment taken up here, and can be appropriately combined and improved without departing from the scope of the invention.

(Method for producing positive electrode active material for lithium secondary battery)
FIG. 1 is a flowchart showing a method for producing a positive electrode active material for a lithium secondary battery according to the present invention. Hereafter, the manufacturing method of the positive electrode active material for lithium secondary batteries which concerns on this invention along FIG. 1 is demonstrated.
(1) Raw Material Mixing Step First, the positive electrode active material raw material represented by (Chemical Formula 1) and an organic substance not containing a metal element are mixed. Here, the organic substance is defined as one that is mixed in the raw material mixing step. As the positive electrode active material raw material, carbonate, hydroxide, sulfate, acetate, chloride, oxalate, citrate, nitrate, or the like can be used. Among these, it is preferable to use carbonates, hydroxides, and oxalates from the viewpoint of suppressing the generation of harmful gases and the like.

As the organic substance, an organic substance that does not contain a metal element is used. Such an organic substance is decomposed in a pre-baking step described later, and a non-oxidizing gas (for example, CO, CO ₂ or the like) is generated by the decomposition. By generating such non-oxidizing gas, other than compounds having an olivine structure that can be generated by oxidation of transition metal generated from the metal element of the raw material, specifically Fe ₂ O ₃ , LiFeP ₂ O ₇ , Li ₃ Fe ₂ (PO ₄ ) ₃ , Mn ₂ O ₃ , MnO ₂ , Mn ₂ P ₂ O _7, etc. A compound having a highly olivine type structure can be obtained. From such a viewpoint, the organic material is preferably contained in an amount of 1 to 40% by weight with respect to the positive electrode active material raw material. If it is less than 1% by weight, it is not possible to sufficiently suppress the occurrence of a harmful foreign phase of the metal oxide generated from the positive electrode active material raw material. On the other hand, if it exceeds 40% by weight, the non-oxidizing gas generated by the decomposition becomes excessive, and only unnecessary gas is generated.

As the organic substance not containing a metal element, carboxylic acid, sugar and the like are preferable. Specifically, the carboxylic acid is preferably at least one selected from the group consisting of acetic acid, citric acid, and malic acid. Specifically, the sugar is preferably one or more selected from the group consisting of sucrose, glucose, starch, cellulose, and dextrin.
The mixing means of the positive electrode active material raw material and the organic substance not containing a metal element is not particularly limited, and a known method such as a solid phase method, a liquid phase method, a coprecipitation method, or a complex method can be used. Among these, it is more preferable to grind and mix to 1 μm or less using a bead mill or the like by a solid phase method.
(2) Temporary calcination step The mixture prepared above is calcined at a temperature of 420 ° C. to 600 ° C. in an oxidizing atmosphere. In this specification, “420 ° C. to 600 ° C.” means 420 ° C. or more and 600 ° C. or less. According to Non-Patent Document 2 described above, since the crystallization temperature of LiFePO ₄ having an olivine structure is 420 ° C., the pre-baking is performed at 420 ° C. or higher. Below 420 ° C., LiMP _1-x A _x O ₄ (M is a metal element, contains Fe, and contains at least one of Mn, Co, and Ni, and A is selected from B, Si, Ti, and V) In this case, there is a possibility that the carbon source does not crystallize, so that the carbon source may be mixed into the inside of the crystal during the main baking and the crystallinity may be lowered. Moreover, temporary baking is performed at 600 degrees C or less. If the temperature exceeds 600 ° C., the crystal grains become coarse, the lithium diffusion distance increases, and the capacity may decrease. More preferably, it is 430 degreeC or more and 500 degrees C or less.

  The calcining time for the pre-firing is preferably 2 hours to 30 hours. If it is less than 2 hours, crystallization does not proceed sufficiently and the crystallinity is not improved. On the other hand, even if it exceeds 30 hours, the crystal growth at the holding temperature has progressed sufficiently, so that it is hardly possible to further improve the crystallinity and only the productivity is lowered. More preferably, it is 4 to 15 hours.

The atmosphere at the time of temporary baking is an oxidizing atmosphere. When pre-baking is performed in an oxidizing atmosphere, the organic matter disappears due to combustion, so that it does not remain in the olivine type crystal, and a decrease in crystallinity can be prevented. In addition, the organic substance generates a non-oxidizing gas (for example, CO, CO _2, etc.) near the temperature at which olivine crystallizes when fired in an oxidizing atmosphere, and the non-oxidizing gas is harmful to a metal oxide or the like. Suppresses the generation of other heterogeneous phases. With these effects, a positive electrode active material having a highly crystalline olivine structure can be obtained.

In the present invention, the oxidizing atmosphere is preferably an atmosphere having an oxygen concentration of 1% or more. More preferably, the oxygen concentration is 1% or more and 50% or less. When the oxygen concentration is less than 1%, a part of the organic matter contained in the raw material mixed powder remains in the temporarily fired body, and the crystallinity is lowered. On the other hand, when the oxygen concentration exceeds 50%, the effect of the non-oxidizing gas generated from the organic substance at a temperature in the vicinity of olivine crystallization is reduced, and a harmful foreign phase such as a metal oxide is generated. Capacity will drop.
(3) Carbon source mixing process The carbon compound which does not contain a metal element as a carbon source is mixed with the temporary baking body obtained above. Here, the carbon compound is defined to be mixed in the carbon source mixing step. As a carbon source, if it is a carbon compound which does not contain a metal element, there will be no limitation in particular, The organic substance which does not contain the metal element used at the raw material mixing process can also be used. More preferred are sucrose and dextrin with a high carbon content. The amount added is preferably such that the carbon content after the main calcination is 0.5% by mass to 10% by mass with respect to the positive electrode active material, and preferably 1% by mass to 5% by mass. More preferred.

As a method for efficiently mixing the calcined body and the carbon compound, it is preferable to apply a mechanical pressure using a ball mill or a bead mill.
(4) Main firing step Main firing is performed on the mixture of the temporarily fired body and the carbon compound obtained above, and the compound having an olivine structure is coated with carbon. The main baking step is performed in an inert atmosphere or a reducing atmosphere (non-oxidizing atmosphere) in order to prevent oxidation of metal elements in the crystal structure and to perform carbon coating. Specifically, nitrogen, argon or hydrogen atmosphere is preferable.

  The main baking is performed at a temperature equal to or higher than the temporary baking temperature. In order to carbonize the organic matter and improve the conductivity, the firing temperature is desirably 600 ° C. or higher. In addition, it is desirable that the main baking is performed at a temperature lower than the temperature at which the positive electrode active material is thermally decomposed. A desirable range of the main firing temperature is 600 ° C to 800 ° C. If it is less than 600 degreeC, a carbon source cannot fully be carbonized and electroconductivity cannot be provided. On the other hand, when the temperature exceeds 800 ° C., the particles become coarse and the capacity of the lithium secondary battery decreases. More preferably, it is 650 degreeC-750 degreeC. After the main firing, it may be gradually cooled while the atmosphere is controlled, or may be rapidly cooled using liquid nitrogen or the like.

  As described above, when the method for producing a positive electrode active material according to the present invention is used, a compound having a high crystallinity and a carbon-coated olivine structure can be obtained.

According to the above-described method for producing a positive electrode active material for a lithium secondary battery according to the present invention, in Fe, the molar ratio (III / II) of the Fe ³⁺ ratio (III) to the Fe ²⁺ ratio (II) is 0.00. A positive electrode active material satisfying 01 ≦ (III / II) ≦ 0.3 is obtained. This will be described in detail later.

(Lithium secondary battery)
A configuration of a lithium secondary battery to which the present invention is applied will be described. FIG. 2 is a schematic cross-sectional view showing an example of a lithium secondary battery (18650 type lithium ion secondary battery) to which the present invention is applied. As shown in FIG. 2, in the lithium secondary battery 10, the positive electrode 1 and the negative electrode 2 are wound in a state where the separator 3 is sandwiched so that they are not in direct contact with each other to form an electrode group. The structure of the electrode group is not limited to winding in a cylindrical shape, a flat shape, or the like, and may be a laminate of strip electrodes.

  A positive electrode lead 7 is attached to the positive electrode 1, and a negative electrode lead 5 is attached to the negative electrode 2. The leads 5 and 7 can take any shape such as a wire shape, a foil shape, and a plate shape. A structure and material that can reduce electrical loss and ensure chemical stability are selected.

  The electrode group is accommodated in the battery can 4, so that the inserted electrode group is not in direct contact with the battery can 4 by the insulating plate 4 installed on the top of the battery can 4 and the insulating plate 9 installed on the bottom. It has become. Further, a non-aqueous electrolyte (not shown) is injected into the battery can 4. As the shape of the battery can 4, a shape (for example, a cylindrical shape, a flat columnar shape, a rectangular column, or the like) that matches the shape of the electrode group is usually selected. As the insulating plate 9, any material (for example, thermosetting resin, glass hermetic seal, etc.) that does not react with the non-aqueous electrolyte and has excellent airtightness is suitable.

  The material of the battery can 4 is selected from materials that are corrosion resistant to the non-aqueous electrolyte, such as aluminum, stainless steel, and nickel-plated steel. The sealing lid 6 can be attached to the battery can 4 by other methods such as caulking and adhesion in addition to welding.

  Between the battery can 4 and the sealing lid portion 6, a packing (seal material) 8 that prevents leakage of the electrolyte and separates the positive electrode 1 of the positive electrode and the negative electrode 2 of the negative electrode is formed. The packing 8 is made of an electrically insulating material such as rubber.

  The positive electrode 2 constituting the lithium secondary battery 10 is formed by applying and drying a positive electrode mixture slurry containing a positive electrode active material on one side or both sides of a positive electrode current collector, and then compression-molding using a roll press machine or the like. It is produced by cutting into a predetermined size. For the positive electrode current collector, an aluminum foil having a thickness of 5 to 25 μm, a copper foil having a thickness of 10 μm, an aluminum perforated foil having a thickness of 10 to 100 μm and a pore diameter of 0.1 to 10 mm, an expanded metal, and an aluminum foam plate Etc. are used. In addition to aluminum and copper, stainless steel, titanium, and the like are also applicable.

  Similarly, the negative electrode 2 constituting the lithium ion secondary battery is formed by applying and drying a negative electrode mixture slurry containing a negative electrode active material on one side or both sides of a negative electrode current collector, and then compression molding using a roll press or the like. Then, it is produced by cutting into a predetermined size. For the negative electrode current collector, a copper foil having a thickness of 5 to 20 μm, a copper perforated foil having a thickness of 10 to 100 μm and a hole diameter of 0.1 to 10 mm, an expanded metal, a foamed copper plate, and the like are used. In addition, stainless steel, titanium, nickel, and the like are also applicable.

  There is no particular limitation on the method for applying the positive electrode mixture slurry and the negative electrode mixture slurry, and conventional methods (for example, a doctor blade method, a dipping method, a spray method, etc.) can be used.

  As the positive electrode active material used for the positive electrode 1, the positive electrode active material manufactured by using the manufacturing method of the present invention described above is used. A positive electrode mixture slurry is prepared by mixing a binder, a thickener, a conductive material (for example, acetylene black, Denka black (registered trademark), graphite powder, etc.), a solvent, and the like as necessary with the positive electrode active material. The Since olivine has a high specific surface area, it is desirable that the specific surface area of the conductive material is large in order to form a conductive network. Specifically, acetylene black or Denka Black (registered trademark) is desirable. In the present invention, the positive electrode active material olivine is coated with carbon, and the coated carbon also functions as a conductive material. The binder is for ensuring adhesion with the current collector, and a general binder such as PVDF (polyvinylidene fluoride) or polyacrylonitrile can be used. The type of the binder is not particularly limited as long as it has sufficient binding properties.

  The negative electrode active material used for the negative electrode 2 is not particularly limited as long as the material can occlude and release lithium ions. Examples thereof include artificial graphite, natural graphite, amorphous carbon, non-graphitizable carbons, activated carbon, coke, pyrolytic carbon, metal oxide, metal nitride, lithium metal, or lithium metal alloy. Any one of these or a mixture of two or more of them can be used. Among these, since amorphous carbon is a material having a small volume change rate during insertion and extraction of lithium ions, it is preferable to include amorphous carbon as the negative electrode active material because the cycle characteristics of charge and discharge are enhanced. . A negative electrode mixture slurry is prepared by mixing a negative electrode active material with a binder, a thickener, a conductive material, a solvent, and the like as necessary.

Examples of materials that can insert lithium ions (Li ⁺ ) or can form lithium compounds include metals such as aluminum, tin, silicon, indium, gallium, and magnesium, alloys containing these elements, tin, silicon, and the like. And metal oxides containing Furthermore, composite materials of these metals, alloys, metal oxides, and graphite-based or amorphous carbon materials can be mentioned.

  As the negative electrode conductive material, a conductive polymer material (for example, polyacene, polyparaphenylene, polyaniline, polyacetylene, etc.) can be used in addition to the conductive material of the positive electrode active material described above.

  There are no particular limitations on the binder, thickener and solvent used in the mixture slurry, and the same ones as before can be used.

  The separator 3 is preferably a porous body (for example, a pore diameter of 0.01 to 10 μm and a porosity of 20 to 90%) because it is necessary to transmit lithium ions during charging and discharging of the secondary battery. Examples of the material of the separator 3 include a polyolefin polymer sheet (for example, polyethylene and polypropylene), a multilayer structure sheet in which a polyolefin polymer sheet and a fluorine polymer sheet (for example, tetrafluoropolyethylene) are welded, Or a glass fiber sheet can be used conveniently. Further, a mixture of ceramics and binder may be formed in a thin layer on the surface of the separator 3.

As the electrolyte, lithium salts such as LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , and LiN (SO ₂ F) ₂ can be used alone or in combination. As the solvent for dissolving the lithium salt, a chain carbonate, a cyclic carbonate, a cyclic ester, a nitrile compound, or the like can be used. Specific examples include ethylene carbonate, propylene carbonate, diethyl carbonate, dimethoxyethane, γ-butyrolactone, n-methylpyrrolidine, and acetonitrile.

  The electrolyte is preferably contained in the solvent in an amount of 0.8 to 1.5M.

  In addition, a polymer gel electrolyte or a solid electrolyte can also be used as the electrolyte. When a solid polymer electrolyte (polymer electrolyte) is used, an ion conductive polymer such as ethylene oxide, acrylonitrile, polyvinylidene fluoride, methyl methacrylate, and hexafluoropropylene polyethylene oxide can be preferably used. When these solid polymer electrolytes are used, the separator 3 can be omitted.

  Furthermore, a compound having a carboxylic anhydride group, a compound having a sulfur element (S) such as propane sultone, and a compound having boron (B) may be mixed. The purpose of adding these compounds is to suppress the reductive decomposition reaction of the electrolytic solution on the surface of the negative electrode 2, to prevent reduction deposition of the metal element such as Mn eluted from the positive electrode 1 on the negative electrode 2, and to the ionic conductivity of the electrolytic solution. Improvement of the electrolyte, flame resistance of the electrolyte, and the like. The compound to be mixed may be selected according to the purpose of addition.

(Positive electrode for lithium secondary battery and positive electrode active material for lithium secondary battery)
The positive electrode for a lithium secondary battery and the positive electrode active material for a lithium secondary battery according to the present invention are manufactured using the above-described method for manufacturing a positive electrode active material for a lithium secondary battery according to the present invention.

In the positive electrode active material for a lithium secondary battery represented by (Chemical Formula 1) according to the present invention, the molar ratio (III / II) of the Fe ³⁺ ratio (III) to the Fe ²⁺ ratio (II) is Fe. 0.01 ≦ (III / II) ≦ 0.3. These values depend on the firing conditions (temperature and atmosphere) of pre-baking and main baking. In the present invention, since pre-baking is performed in the atmosphere, (III / II) is 0.01 or more. In the case of 0.01 or more, there is an effect that Fe ³⁺ becomes a starting point of the reaction and lithium diffusibility is improved. Further, (III / II) is 0.3 or less under the pre-firing and main-firing conditions of the present invention. When the ratio is larger than 0.3, that is, when the proportion of Fe ³⁺ is larger, the amount of Li ions required for reduction from Fe ³⁺ to Fe ²⁺ is too large, and the electrolyte concentration in the electrolytic solution is increased, resulting in lithium diffusibility. Decreases and the capacity decreases.

Further, when the proportion of Fe ³⁺ becomes larger, harmful foreign phases (such as Fe ₂ O ₃ ) are likely to be generated, which may lead to a decrease in rate characteristics. The range of (III / II) is more preferably 0.06 ≦ (III / II) ≦ 0.17. The valence of Fe can be confirmed by Mossbauer spectroscopy, XAFS measurement, or the like.

  In the compound represented by (Chemical Formula 1), the proportion of Fe is preferably 10 mol% or more. In M in (Chemical Formula 1), the higher the proportion of Fe, the lower the resistance, and the higher the proportion of Mn, Ni and Co, the higher the average voltage. As the average voltage increases, the energy density (capacity × voltage) increases. However, if the proportion of Mn, Ni or Co exceeds 90%, the resistance is too high to obtain a capacity, and the energy density is also reduced.

  When about 10 mol% of Fe is added as M, the resistance is reduced and the capacity is obtained, so that a high energy density is obtained. However, in a region where there is too much Fe, the resistance becomes too low and a high capacity can be obtained, but the effect of lowering the average voltage is higher than the effect of increasing the capacity, and the energy density is lowered. As mentioned above, it is preferable that the ratio of Fe is 10 mol%-50 mol%.

In the positive electrode active material, it is preferable that Li and P are contained at a ratio of M or more. In such a case, even if only a single phase of olivine can be confirmed by confirming the crystal structure using X-rays, Li and P contained excessively are amorphous Li ₄ P ₂ O ₇ and / or Li ₃ PO _4. It is considered that these compounds contribute to improving the conductivity of Li ions (Non-patent Document 1). The presence of these amorphous compounds can be confirmed by MAS-NMR method, FT-IR method or the like.

  The P site may be substituted with one or more elements selected from the group consisting of B, Si, Ti, and V. The amount of substitution is preferably 0.25 mol% or less, and more preferably 0.10 mol% or less with respect to Li. If the amount of substitution exceeds 0.25 mol%, the olivine crystal structure becomes unstable.

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. The present invention is not limited to these examples.
(Preparation of lithium secondary battery of Example 1)
As raw materials, lithium carbonate (Li ₂ CO ₃ ), manganese carbonate (MnCO ₃ ), iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), phosphoric acid (H ₃ PO ₄ ), organic substances not containing metal elements Citric acid (C ₆ H ₈ O ₇ ) was used and weighed so that the molar ratio was Li: Mn: Fe: P = 1.1: 0.8: 0.2: 1.1. Citric acid was weighed so as to be 18% by mass of the total raw material powder. These were pulverized and mixed with a dry bead mill and then calcined. The calcination atmosphere was air, the calcination temperature was 440 ° C., and the calcination time was 8 hours.

  Sucrose was added as a carbon source to the obtained calcined product. The addition amount was 14% by mass of the positive electrode active material. This was pulverized and mixed using a ball mill and then fired. The main baking atmosphere was a nitrogen stream, the main baking temperature was 700 ° C., and the main baking time was 10 hours. The positive electrode active material was obtained by the above process.

Then, the positive electrode was created using the positive electrode active material obtained above. Hereinafter, a method for producing the electrode will be described. A positive electrode active material, a carbon-based conductive material, and a binder previously dissolved in N-methyl-2-pyrodinone (NMP) were mixed in a ratio of 82.5: 10: 7.5, expressed in mass percent, respectively. A slurry was prepared. The uniformly mixed slurry was applied onto an aluminum foil current collector having a thickness of 20 μm. Then, it dried at 120 degreeC and compression-molded so that the electrode density might be set to 2.0 g / cm < ³ > with a press. After compression molding, a positive electrode for a test battery was manufactured by punching into a disk shape having a diameter of 15 mm using a punched metal fitting.

Subsequently, a test battery was produced using the positive electrode obtained above and metallic lithium as the negative electrode. As the electrolytic solution, a solution containing 1.2 mol of LiPF ₆ as an electrolyte and a mixed solution of EC (ethylene carbonate) and ethyl methyl carbonate (EMC) as a solvent was used.
(Test and evaluation)
(A) XRD measurement (crystal phase identification)
Powder X-ray diffraction measurement (XRD measurement) was performed by the following procedure, and the crystal phase of the carbon-coated positive electrode active material obtained above was identified. An automatic X-ray diffractometer (manufactured by Rigaku Corporation, model: RINT-UltimaIII) was used as the measuring apparatus. The measurement conditions are a concentration method, using CuK _α- ray as X-ray, X-ray output is 48 kV × 40 mA, scanning range is 2θ = 15 to 50 deg, diverging slit is DS = 1 deg, scattering slit is SS = 1 deg, The light receiving slit was RS = 0.3 mm, the monochromator slit was 0.6 mm, the step width was 0.02 ° by the step scanning method, and the measurement time per step was 1 second.

  About the diffraction pattern obtained by the measurement, the crystal phase was identified using ICSD (Inorganic Crystal Structure Database).

The X-ray diffraction profile of the positive electrode active material for a lithium secondary battery of Example 1 is shown in FIG.
(B) Element weight ratio measurement (evaluation of positive electrode active material composition)
The weight ratio of elements of the positive electrode active material was measured as follows using a high-frequency inductively coupled plasma emission spectroscopy (hereinafter abbreviated as “ICP”) analyzer (manufactured by Hitachi, Ltd., model: P-4000).

First, 5 g of a positive electrode active material and 2 ml of nitric acid were added to 45 ml of ion exchange water in a beaker, and the mixture was stirred for 30 minutes with a stirrer (stirrer). After standing for 5 minutes, the filtrate filtered with filter paper was sprayed into a high-frequency atmosphere together with argon gas, and the light intensity specific to each excited element was measured to calculate the weight ratio of the elements. The composition of the positive electrode active material of Example 1 is shown in Table 1 described later.
(C) Fe valence measurement (calculation of molar ratio (III / II) of Fe ³⁺ ratio (III) and Fe ²⁺ ratio (II))
The valence of Fe was ^measured using a Mossbauer spectrometer, and the molar ratio (III / II) of the ratio (III) of Fe ³⁺ and the ratio (II) of Fe ²⁺ was calculated. The calculated values are also shown in Table 1.
(D) Carbon content measurement The carbon content of the positive electrode active material was measured as follows using a solid carbon analyzer (manufactured by Horiba, Ltd., model: EMIA-110). A 100 mg sample and a combustor were added to an air-baked magnetic crucible and burned in a high-frequency heating furnace in an oxygen stream. CO ₂ and CO gas in the combustion gas were quantified, and the weight of carbon was calculated. The measurement results are also shown in Table 1.
(E) Charge / discharge test (capacity evaluation)
About the test battery produced above, the following charging / discharging test was implemented and the capacity | capacitance was evaluated. Charged at a constant current / constant voltage up to 4.5V with a charging rate of 0.1C (speed of 100% charging in 10 hours). After reaching 4.5V, the current value decays to 0.03C Constant voltage charging was performed until Thereafter, the battery was discharged at a constant current of 0.1 C up to 2 V, and the discharge capacity at that time was defined as the capacity. The results are also shown in Table 1.
(F) Rate characteristic evaluation After repeating the above charge / discharge cycle for 3 cycles, the rate characteristic was evaluated under the following conditions. Rate characteristics obtained by dividing the capacity when a constant-current charge and constant-voltage charge test battery is discharged at a current value of 5C by the constant current discharge of 5C as with the capacity measurement (%) It was. The results are also shown in Table 1.

All tests were performed at room temperature (25 ° C.).
(Preparation of lithium secondary battery of Example 2)
Synthesis was performed in the same manner as in Example 1 except that the atmosphere at the time of calcination was changed from the air to a mixed air stream of air + nitrogen (flow rate ratio 1: 1) to obtain LiMn _0.8 Fe _0.2 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 3)
The raw material was synthesized in the same manner as in Example 1 except that Li: Mn: Fe: P = 1.1: 0.85: 0.15: 1.1, and LiMn _0.85 Fe _0.15 PO ₄ was obtained. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 4)
A LiMn _0.8 Fe _0.2 PO ₄ was obtained by synthesizing in the same manner as in Example 1 except that the organic substance not containing a metal element added in the raw material mixing step was changed from citric acid to acetic acid (CH ₃ COOH). . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Production of lithium secondary battery of Example 5)
It was synthesized in the same manner as in Example 1 except that the organic substance not containing a metal element added at the time of raw material mixing was changed from citric acid to malic acid (C ₄ H ₆ O ₅ ), and LiMn _0.8 Fe _0.2 PO ₄ Got. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 6)
A LiMn _0.8 Fe _0.2 PO ₄ was synthesized in the same manner as in Example 1 except that the organic substance not containing a metal element added at the time of raw material mixing was changed from citric acid to sucrose (C ₁₂ H ₂₂ O ₁₁ ). Obtained. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 7)
Synthesis was performed in the same manner as in Example 1 except that the organic substance not containing a metal element added at the time of raw material mixing was changed from citric acid to dextrin (C ₆ H ₁₀ O ₅ ), and LiMn _0.8 Fe _0.2 PO ₄ was synthesized. Obtained. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Production of lithium secondary battery of Example 8)
Synthesis was performed in the same manner as in Example 1 except that the calcination temperature was changed from 440 ° C. to 420 ° C. to obtain LiMn _0.8 Fe _0.2 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 9)
A LiMn _0.8 Fe _0.2 PO ₄ was obtained in the same manner as in Example 1 except that the calcination temperature was changed from 440 ° C. to 600 ° C. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 10)
Other than adding magnesium hydroxide (Mg (OH) ₂ ) as a raw material and weighing to make Li: Mn: Fe: Mg: P = 1.1: 0.88: 0.10: 0.02: 1.1 Was synthesized in the same manner as in Example 1 to obtain LiMn _0.88 Fe _0.10 Mg _0.02 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 11)
Except for adding fine particle titanium oxide (TiO ₂ ) as a raw material and weighing it to be Li: Mn: Fe: P: Ti = 1.1: 0.8: 0.2: 1.07: 0.04: In the same manner as in Example 1, LiMn _0.8 Fe _0.2 P _0.96 Ti _0.04 PO ₄ was obtained. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 12)
Weigh so that Li: Mn: Fe: P: V = 1.1: 0.8: 0.2: 1.07: 0.04 by adding fine particle vanadium pentoxide (V ₂ O ₅ ) as a raw material. Except that, synthesis was performed in the same manner as in Example 1 to obtain LiMn _0.8 Fe _0.2 P _0.96 V _0.04 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Production of lithium secondary battery of Example 13)
The raw material was synthesized in the same manner as in Example 1 except that Li: Mn: Fe: P = 1.1: 0.5: 0.5: 1.1, and LiMn _0.5 Fe _0.5 PO ₄ was obtained. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 14)
Example except that cobalt carbonate (CoCO ₃ ) was added as a raw material and weighed so that Li: Mn: Fe: Co: P = 1.1: 0.8: 0.15: 0.05: 1.1 1 was obtained in the same manner as in Example 1 to obtain LiMn _0.8 Fe _0.15 Co _0.05 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of lithium secondary battery of Example 15)
Other than adding nickel carbonate (NiCO ₃ .H ₂ O) as a raw material and weighing to make Li: Mn: Fe: Ni: P = 1.1: 0.8: 0.15: 0.05: 1.1 Was synthesized in the same manner as in Example 1 to obtain LiMn _0.8 Fe _0.15 Ni _0.05 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the evaluation results of capacity and rate characteristics are also shown in Table 1.
(Preparation of the lithium secondary battery of Reference Example 1)
Synthesis was performed in the same manner as in Example 1 except that the calcination temperature was 350 ° C., to obtain LiFe _0.2 Mn _0.8 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the capacity and the rate characteristics were also evaluated in the same manner as in Example 1. The results are listed in Table 2.
(Preparation of lithium secondary battery of Reference Example 2)
Synthesis was performed in the same manner as in Example 1 except that the atmosphere during the main firing was air, and LiFe _0.2 Mn _0.8 PO ₄ was obtained. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the capacity and the rate characteristics were also evaluated in the same manner as in Example 1. The results are also shown in Table 2.

In the present specification, the reference example is the same as the present invention, in which a positive electrode active material is synthesized by adding an organic substance that does not contain a metal element and firing it in an oxidizing atmosphere. The temperature is lower than the crystallization temperature of olivine, or the atmosphere during the main baking is not a reducing atmosphere or an inert atmosphere. Therefore, although the reference example is not known per se, it is described in order to show the importance of the preliminary baking temperature and the atmosphere during the main baking of the present invention.
(Preparation of lithium secondary battery of Comparative Example 1)
Synthesis was performed in the same manner as in Example 1 except that the atmosphere at the time of calcination was changed from air to a nitrogen stream to obtain LiFe _0.2 Mn _0.8 PO ₄ . XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the capacity and the rate characteristics were also evaluated in the same manner as in Example 1. The results are listed in Table 2.
(Preparation of lithium secondary battery of Comparative Example 2)
Synthesis was performed in the same manner as in Example 1 except that sucrose added as a carbon source to the calcined product was obtained, and LiFe _0.2 Mn _0.8 PO ₄ was obtained. XRD measurement, element weight ratio measurement, Fe valence measurement, carbon content measurement, charge / discharge test, and rate characteristic evaluation were similarly performed. The composition of the positive electrode active material, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ (III / II), the capacity and the rate characteristics were also evaluated in the same manner as in Example 1. The results are listed in Table 2.

  As shown in Table 1 and Table 2, the lithium secondary battery manufactured using the method for manufacturing a positive electrode active material for a lithium secondary battery according to the present invention has a high capacity (150 Ah / kg or more) and a high rate characteristic ( 80% or more) could be achieved at the same time. On the other hand, both the capacity and rate characteristics of the comparative example and the reference example showed lower values than the example.

Further, from Tables 1 and 2, the lithium secondary batteries manufactured using the method for manufacturing a positive electrode active material for a lithium secondary battery according to the present invention are all Fe ³⁺ ratio (III) and Fe ²⁺ ratio (II). The molar ratio (III / II) was in the range of 0.01 ≦ (III / II) ≦ 0.3. On the other hand, the molar ratio (III / II) of the ratio (III) of Fe ³⁺ and the ratio (II) of Fe ²⁺ in Comparative Examples 1 and 2 was out of this range.

In FIG. 3, as a result of the qualitative analysis of the XRD profile of Example 1, it was confirmed that Example 1 was LiMn _0.8 Fe _0.2 PO ₄ having an orthorhombic olivine structure.

  In more detail, Example 1, a reference example, and a comparative example are demonstrated. In Reference Example 1, the calcination temperature is lower than that of the present invention. For this reason, when olivine is crystallized during the main firing, a large amount of organic substances and the like are taken into the crystal, and the crystallinity is lowered. As a result, the capacity and rate characteristics are considered to be lowered.

In Reference Example 2, the atmosphere during the main firing is an oxidizing atmosphere, which is different from the provisions of the present invention. For this reason, it is considered that the metal element was oxidized during the main firing, the olivine structure could not be maintained, and the capacity and rate characteristics were significantly reduced. Further, since the main calcination was performed in an oxidizing atmosphere, Fe was further oxidized, and the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ was within the range defined by the present invention. It is thought that it was greatly deviated from.

In Comparative Example 1, the atmosphere at the time of temporary firing is a conventional nitrogen atmosphere, which is different from the provisions of the present invention. For this reason, it is considered that the organic matter added before the pre-firing does not disappear at the time of pre-firing, the carbon source remains excessively even after the main firing, and the capacity is reduced. Further, since both the pre-firing and the main firing are performed in an inert atmosphere, Fe is not oxidized, and the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ is 0. It is thought that it became.

In Comparative Example 2, no carbon source was added during the main firing. For this reason, it is considered that the positive electrode active material was not coated with carbon and the conductivity was not improved, so that the capacity and rate characteristics were lowered. In addition, since the main calcination is performed without adding a carbon source, Fe is more oxidized as compared with the case where it is added, and the molar ratio (III) of the ratio of Fe ³⁺ (III) to the ratio of Fe ²⁺ (II) (III / II) is considered to be 0.5.

  The carbon contents of the positive electrode active materials of Examples 1 to 15 were 4% by mass in Examples 1 to 3 and Examples 5 to 15, and 3% by mass in Example 4. Further, the carbon content of Reference Example 1 was 6% by mass, and no carbon was detected in Reference Example 2. The carbon content of Comparative Example 1 was 12% by mass, and no carbon was detected in Comparative Example 2.

  FIG. 4 is a graph showing the relationship between the single electrode discharge capacity and rate characteristics of the positive electrode active materials of Examples 1 to 15, Comparative Examples 1 and 2, and Reference Examples 1 and 2. As shown in FIG. 4, it can be seen that the positive electrode active material for a lithium secondary battery according to the present invention is superior to the comparative example and the reference example in both capacity and rate characteristics.

  From the above results, the positive electrode active material for a lithium secondary battery according to the present invention uses a highly safe polyanion compound, and has a higher capacity than a lithium secondary battery using a conventional polyanion positive electrode active material ( It has been shown that 150 Ah / kg and higher) and high rate characteristics (80% and higher) can be achieved simultaneously.

  DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Negative electrode, 3 ... Separator, 4 ... Battery can, 5 ... Negative electrode lead, 6 ... Sealing lid part, 7 ... Positive electrode lead, 8 ... Packing, 9 ... Insulating plate, 10 ... Lithium secondary battery.

Claims (9)

A step of mixing a raw material of a positive electrode active material for a lithium secondary battery represented by the following (Chemical Formula 1) and an organic substance not containing a metal element;
A step of calcining a mixture of the raw material of the positive electrode active material and the organic substance at a temperature of 420 ° C. to 600 ° C. in an oxidizing atmosphere;
A step of mixing a carbon compound not containing a metal element into the pre-fired body obtained by the pre-baking step;
A step of subjecting the calcined body mixed with the carbon compound to a temperature equal to or higher than the calcining temperature and a calcining in a reducing atmosphere or an inert atmosphere,
In Fe in the positive electrode active material for a lithium secondary battery represented by the following (Chemical Formula 1) obtained after the main firing step, the molar ratio (III / II) of the ratio (III) of Fe ^{3+ to} the ratio (II) of Fe ²⁺ ) Is 0.01 ≦ (III / II) ≦ 0.3. A method for producing a positive electrode active material for a lithium secondary battery.
LiMP _1-x A _x O ₄ (Chemical Formula 1)
(M is a metal element, includes Fe, and includes at least one of Mn, Co, and Ni, A includes at least one selected from B, Si, Ti, and V, and 0 ≦ x ≦ 0 .25.)   2. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the proportion of Fe in M in the positive electrode active material is 10 mol% to 50 mol%. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the positive electrode active material includes amorphous Li ₄ P ₂ O ₇ and / or Li ₃ PO _4. .   The method for producing a positive electrode active material for a lithium secondary battery according to any one of claims 1 to 3, wherein the organic substance not containing the metal element is a carboxylic acid.   The method for producing a positive electrode active material for a lithium secondary battery according to claim 4, wherein the carboxylic acid is one or more selected from the group consisting of acetic acid, citric acid, and malic acid.   The method for producing a positive electrode active material for a lithium secondary battery according to any one of claims 1 to 5, wherein the organic substance not containing the metal element is sugar.   The method for producing a positive electrode active material for a lithium secondary battery according to claim 6, wherein the sugar is one or more selected from the group consisting of sucrose, glucose, starch, cellulose, and dextrin.   The method for producing a positive electrode active material for a lithium secondary battery according to any one of claims 1 to 7, wherein a temperature of the pre-baking is not less than 430 ° C and not more than 500 ° C.   The method for producing a positive electrode active material for a lithium secondary battery according to any one of claims 1 to 8, wherein an oxidizing atmosphere in the pre-baking step has an oxygen concentration of 1% or more.

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