JP4483253B2 - Positive electrode material for lithium secondary battery, positive electrode for lithium secondary battery, and lithium secondary battery - Google Patents

Description

  The present invention relates to a novel lithium iron phosphate-based positive electrode material for lithium secondary batteries, a positive electrode for lithium secondary batteries using the same, and a lithium secondary battery. The positive electrode material according to the present invention provides a positive electrode for a secondary battery having a large initial discharge capacity and a high discharge capacity even when discharged at a large current.

  In recent years, with the advance of electronic technology, electronic devices have been dramatically improved in performance, size, and portability. Along with this, research on rechargeable secondary batteries as power sources that can be used conveniently and economically for a long time is underway. Conventionally, as a secondary battery, a lead storage battery, an alkaline storage battery, or a lithium ion secondary battery has been widely used, and among them, the lithium ion secondary battery has been attracting attention as being capable of realizing high output and high energy density. ing.

In a lithium ion secondary battery, as a positive electrode material, a metal chalcogen compound such as TiS ₂ , MoS ₂ , NbSe ₂ or V ₂ O ₅ , or a lithium transition metal composite such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ or LiMn ₂ O _{4 is used.} An oxide or the like is used.
Among these, LiCoO ₂ has been widely put into practical use as a positive electrode material having a potential of about 4 V with respect to the lithium metal potential, has high energy density and high voltage, and is an ideal positive electrode material in various aspects. is there. However, since Co as a resource is unevenly distributed on the earth and is rare, there is a problem that stable supply is difficult and material cost is high. LiNiO ₂ is a preferable positive electrode material because it has a large theoretical capacity, has a high discharge potential, and can further reduce costs. However, since the crystal structure collapses as the charge / discharge cycle progresses, there is a problem that the discharge capacity is reduced and the thermal stability is also poor. LiMn ₂ O ₄ having a positive spinel structure has a high potential equivalent to that of LiCoO ₂ and can provide a high battery capacity, and is easy to synthesize and can be reduced in cost. Therefore, it is promising as a positive electrode material. is there. However, there is a problem in that the capacity deterioration during storage at high temperature is large and Mn is dissolved in the electrolyte, so that the stability or cycle characteristics are not sufficient.

In recent years, it has been proposed to use a phosphate compound of transition metal M having an olivine structure (M is a metal selected from the group consisting of Fe, Mn, Co, and Ni) as a positive electrode material (Patent Document 1). reference). In addition, it has also been proposed to use LiFePO ₄ as a positive electrode material among the phosphate compounds of transition metals M having the olivine structure (see Patent Document 2). This LiFePO ₄ has a large density of 3.6 g / cm ³ , generates a high potential of 3.4 V, and has a large theoretical capacity of 170 mAh / g. Moreover, LiFePO ₄ is preferable as a positive electrode material because it contains one Li atom that can be electrochemically detached in the initial state per one Fe atom.

In a lithium iron phosphate compound (LiFePO ₄ ) having an olivine structure, iron exists in a divalent state instead of a trivalent state, and therefore, charge / discharge reaction is caused by release / occlusion of lithium ions using Fe ²⁺ / Fe ³⁺ redox. Done. Therefore, this lithium iron phosphate compound is produced by mixing a lithium compound, a phosphorus compound and an iron (II) compound, and firing the mixture in a nitrogen atmosphere so that iron (II) is not oxidized to iron (III). (See Patent Document 1).

However, as shown in Comparative Example 1 to be described later, a battery manufactured using a positive electrode material obtained by this method has insufficient rate characteristics.
JP-A-9-134724 Japanese Patent Laid-Open No. 9-171827

  Therefore, an object of the present invention is to provide a lithium iron phosphate compound having an olivine structure, which is a positive electrode material for a lithium secondary battery that gives a secondary battery exhibiting excellent rate characteristics.

In view of the above problems, the present inventor has paid attention to the oxidation number of iron in the lithium iron phosphate compound, that is, the molar ratio represented by Fe (II) / Fe (III), and as a result of repeated studies, it is surprising. In particular, lithium produced using a lithium iron phosphate compound containing a divalent iron and a trivalent iron in a certain ratio rather than a lithium iron phosphate compound substantially free of trivalent iron. The present invention was completed by finding that the secondary battery exhibits excellent rate characteristics.
That is, the present invention relates to a positive electrode material for a lithium secondary battery comprising a lithium iron phosphate compound comprising divalent iron [Fe (II)] and trivalent iron [Fe (III)], wherein Fe (II) / A positive electrode material for a lithium secondary battery, a manufacturing method thereof, and a positive electrode for a lithium secondary battery and a lithium using the positive electrode material for a lithium secondary battery, wherein the molar ratio of Fe (III) is 1 to 50 Secondary battery.

  The positive electrode material of the lithium secondary battery according to the present invention can be produced at low cost, and the lithium secondary battery produced using the positive electrode material exhibits excellent rate characteristics.

The present invention is described in detail below.
The positive electrode material for a lithium secondary battery according to the present invention contains a lithium iron phosphate compound and has a molar ratio Fe (II) of divalent iron [Fe (II)] and trivalent iron [Fe (III)]. ) / Fe (III) is 1 to 50.
Specific examples of the divalent iron lithium iron phosphate compound (hereinafter sometimes referred to as “Fe (II) lithium iron phosphate compound”) include LiFePO ₄ having an olivine structure.

Specific examples of trivalent iron lithium iron phosphate compounds (hereinafter sometimes referred to as “Fe (III) lithium iron phosphate compounds”) include Li ₃ Fe ₂ (PO ₄ ) ₃ .
The positive electrode material for a lithium secondary battery according to the present invention may further contain an iron oxide of trivalent iron, and specifically includes Fe ₂ O ₃ .
The positive electrode material for a lithium secondary battery according to the present invention is usually composed of the above-described crystal particles of Fe (II) lithium iron phosphate compound, Fe (III) lithium iron phosphate compound, and iron oxide of trivalent iron. Are mixed. Those in which the respective crystal particles are uniformly mixed, particularly those in which the primary particles are uniformly mixed at the crystal level are preferable.

If the Fe (II) / Fe (III) (molar ratio) in the positive electrode material for a lithium secondary battery is too large, the rate characteristics are liable to deteriorate, so the molar ratio is preferably 20 or less.
The ratio (molar ratio) of Fe (II) lithium iron phosphate compound / Fe (III) lithium iron phosphate compound in the positive electrode material for a lithium secondary battery is usually 3 or more and 150 or less. If this molar ratio is small, the capacity of the produced battery tends to decrease, so the lower limit is preferably 9 or more. On the other hand, if the molar ratio is too large, the rate characteristics are liable to decrease, so the upper limit is preferably 60 or less.

  Further, the ratio (molar ratio) of Fe (II) lithium iron phosphate compound / trivalent iron oxide in the positive electrode material for a lithium secondary battery is usually 6 or more and 300 or less. If this molar ratio is small, the capacity of the produced battery tends to decrease, so the lower limit is preferably 18 or more. On the other hand, if the molar ratio is too large, the rate characteristics are liable to deteriorate, so the upper limit is preferably 120 or less.

  The average primary particle size of the positive electrode material for a lithium secondary battery according to the present invention is usually 0.01 μm or more and 5 μm or less. If the primary particle size is too small, side reactions and the like are likely to occur on the surface, and the cycle characteristics and the like are likely to deteriorate. Therefore, the lower limit is preferably 0.02 μm or more, particularly 0.05 μm or more. On the other hand, if the primary particle size is too large, rate characteristics and capacity are liable to be reduced due to inhibition of lithium diffusion, lack of conductive paths, etc., so the upper limit is preferably 1 μm or less. The average primary particle size can be measured by SEM observation. The average secondary particle size is usually 1 μm or more and 50 μm or less. If the average secondary particle size is too small, the cycle characteristics and safety tend to be lowered, so the lower limit is preferably 4 μm or more. On the other hand, if the average secondary particle size is too large, the internal resistance increases and it becomes difficult to produce a sufficient output. Therefore, the upper limit is preferably 40 μm or less. The average secondary particle size can be measured by a known laser diffraction / scattering particle size distribution measuring apparatus, and examples of the dispersion medium used for the measurement include 0.1 wt% sodium hexametaphosphate aqueous solution.

Moreover, the BET specific surface area of the positive electrode material for lithium secondary batteries according to the present invention is usually 0.1 m ² / g or more and 10 m ² / g or less. If the specific surface area is too small, the primary particle size becomes large and the rate characteristics and capacity are likely to decrease. Therefore, the lower limit is preferably 0.2 m ² / g or more, particularly preferably 0.5 m ² / g or more. On the other hand, if the specific surface area is too large, cycle characteristics and the like may be deteriorated or problems may be caused in coating properties. Therefore, the upper limit is 5 m ² / g or less, 3 m ² / g or less, especially 2 m ² / g or less. Is preferred.

The positive electrode material for a lithium secondary battery according to the present invention comprises four components: (i) a phosphorus compound raw material, (ii) a lithium compound raw material, (iii) a divalent iron compound raw material, and (iv) a trivalent iron compound raw material. After mixing, it can be produced by firing in an atmosphere having an oxygen partial pressure of 1 × 10 ⁻³ atm or less.
Examples of the phosphorus compound raw material include NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , and P ₂ O ₅ . Among these, NH ₄ H ₂ PO ₄ or (NH ₄ ) ₂ HPO _{4 which} is relatively low in hygroscopicity and easy to handle is preferable.

Examples of the lithium compound raw material include inorganic salts of lithium such as Li ₂ CO ₃ , LiNO ₃ and Li ₃ PO ₄ ; organic acid salts of lithium such as fatty acid lithium such as lithium acetate; LiOH, LiOH · H ₂ O and the like Examples thereof include organic lithium compounds such as lithium hydroxide and alkyl lithium. Among these, LiOH or LiOH.H ₂ O is preferable because it has a low melting point and high reactivity with the iron raw material and does not generate harmful substances during the firing step.

  The lithium of the lithium compound raw material is usually used at a ratio of 1.0 mol or more and 1.2 mol or less with respect to 1 mol of phosphorus of the phosphorus compound raw material. If the proportion of the lithium compound is low, unreacted substances remain and the proportion of the active material tends to decrease. Therefore, the lower limit is preferably 1.02 mol or more. On the contrary, if the proportion of the lithium compound is high, a heterogeneous phase is likely to be generated, so the upper limit is preferably 1.15 mol or less.

Examples of the divalent iron compound raw material include FeC ₂ O ₄ .2H ₂ O, Fe ₃ (PO ₄ ) ₂ .8H ₂ O, Fe (CH ₃ COO) _{2 and the} like. Of these, FeC ₂ O ₄ .2H ₂ O is preferred because it is highly reactive and does not generate a gas that adversely affects the surroundings.
The iron of the divalent iron compound raw material is usually used at a ratio of 0.5 mol or more and 0.98 mol or less with respect to 1 mol of phosphorus of the phosphorus compound raw material. When the ratio of the divalent iron compound is low, the generation ratio of LiFePO _{4 that} is the main body of charge / discharge decreases, and the performance of the manufactured lithium secondary battery is likely to deteriorate. Therefore, the lower limit is preferably 0.75 mol or more. On the contrary, if the ratio of the divalent iron compound raw material is high, the rate characteristics of the produced battery are likely to be lowered, so that the upper limit is preferably 0.95 mol or less.

Examples of the trivalent iron compound raw material include Fe ₂ O ₃ , FeOOH, FePO ₄ .2H ₂ O, Fe (CH ₃ COO) ₃ , Fe ₂ (C ₂ O ₄ ) ₃ .5H ₂ O, and the like. . Among these, Fe ₂ (C ₂ O ₄ ) ₃ · 5H ₂ O is preferable because it is highly reactive and does not generate a gas that adversely affects the surroundings.
The iron of the trivalent iron compound raw material is usually used at a ratio of 0.02 mol or more and 0.5 mol or less with respect to 1 mol of phosphorus of the phosphorus compound raw material. If the ratio of the trivalent iron compound is low, the rate characteristics of the produced battery are likely to deteriorate, so the lower limit is preferably 0.05 mol or more. On the contrary, if the ratio of the trivalent iron compound raw material is high, the generation ratio of LiFePO _{4 which} is the main body of charge / discharge decreases, and the performance of the manufactured lithium secondary battery is likely to be lowered. The following is preferred.

The atmosphere of mixing the four components described above is arbitrary, but in this stage, an atmosphere mainly composed of an inert gas such as nitrogen, argon, helium, etc., mainly nitrogen or argon is used so that the divalent iron raw material is not oxidized. It is preferable to perform the mixing in an atmosphere of
If the oxygen partial pressure at the time of firing is low, the trivalent iron raw material is reduced to divalent, and the proportion of trivalent iron becomes small, so that the rate characteristics of the manufactured battery are likely to deteriorate. Therefore, the lower limit of the oxygen partial pressure is preferably 5 × 10 ⁻⁶ atm or more, particularly preferably 10 ⁻⁵ atm or more. On the contrary, if the oxygen partial pressure is high, iron is oxidized from divalent to trivalent and the production rate of LiFePO _{4 that} is the main charge and discharge is reduced, and the performance of the manufactured lithium secondary battery is likely to deteriorate. Is preferably 5 × 10 ⁻⁴ atm or less, particularly preferably 1 × 10 ⁻⁴ atm or less.

To perform firing at this oxygen partial pressure, a normal inert gas such as helium, argon or nitrogen is mainly used, and a reducing gas such as hydrogen or carbon monoxide or an oxidizing gas such as air or oxygen is appropriately used. What was mixed and adjusted may be used. Of the inert gases, nitrogen or argon is preferred, hydrogen as the reducing gas, and air as the oxidizing gas.
Firing is usually performed at a temperature of 400 ° C. or higher and 800 ° C. or lower. If the firing temperature is low, the reaction may be incomplete or a product with poor crystallinity may be obtained. Therefore, firing at 500 ° C. or higher is preferable. On the contrary, if the firing temperature is high, the crystal particles are coarsened and the specific surface area is lowered, which may adversely affect the battery performance.

The firing time is usually 1 hour or more and 100 hours or less. If the firing time is short, the reaction may be incomplete, and it is preferable to fire for 5 hours or more, particularly 10 hours or more. On the other hand, since it is not practical that the firing time is long, it is preferable to fire for 75 hours or less, particularly 50 hours or less.
In order to carry out the reaction efficiently and uniformly, the firing may be repeated twice or more with a pulverization / disintegration step in between.

As described above, a part of divalent iron is oxidized to trivalent depending on the oxygen partial pressure in the firing atmosphere. Therefore, the positive electrode material for a lithium secondary battery according to the present invention includes (i) a phosphorus compound raw material, (ii ) After mixing the three components of the lithium compound raw material and (iii) the divalent iron compound raw material, oxygen is added to the inert gas so that the oxygen partial pressure is 1 × 10 ⁻⁴ atm or more and 5 × 10 ⁻³ atm or less. It can also be produced by firing in a mixed atmosphere.

  The lithium of the lithium compound raw material is usually used at a ratio of 1.0 mol or more and 1.2 mol or less with respect to 1 mol of phosphorus of the phosphorus compound raw material. If the proportion of the lithium compound is low, unreacted substances remain and the proportion of the active material tends to decrease. Therefore, the lower limit is preferably 1.02 mol or more. On the contrary, if the proportion of the lithium compound is high, a heterogeneous phase is likely to be generated, so the upper limit is preferably 1.15 mol or less.

The iron of the divalent iron compound raw material is usually used in an equimolar amount with respect to phosphorus of the phosphorus compound raw material.
The atmosphere of mixing the above three components is arbitrary, and may be performed in an air atmosphere. However, in order to prevent the divalent compound raw material from being oxidized to trivalent in the mixing stage, nitrogen, argon, helium, etc. It is preferable to carry out in an atmosphere mainly composed of an inert gas, particularly an atmosphere mainly composed of nitrogen or argon.

If the oxygen partial pressure at the time of firing is low, the ratio of the trivalent iron compound in the product is too small, and the rate characteristics of the produced battery are likely to deteriorate. Therefore, the lower limit is 2 × 10 ⁻⁴ atm or more, especially 3 × 10 ⁻⁴ atm or more is preferable. On the other hand, if the oxygen partial pressure is high, the oxidation of iron proceeds too much, and the production rate of LiFePO _{4 that} is the main body of charge / discharge decreases, and the performance of the manufactured lithium secondary battery is likely to deteriorate. 10 ^-3 atm or less, particularly preferably 1 × ¹⁰ -3 atm.

The firing temperature and firing time may be the same as in the production method using the four components described above. Moreover, in order to perform reaction efficiently and uniformly, you may perform baking twice or more across a grinding | pulverization / disintegration process.
(I) Phosphorus compound raw material, (ii) Lithium compound raw material, (iii) Divalent iron compound raw material, and (iv) Trivalent iron compound raw material, the manufacturing method which mixes four components, or (i) Phosphorus compound In the production method in which the raw material, (ii) the lithium compound raw material, and (iii) the divalent iron compound raw material are mixed, after mixing each raw material compound, the volatile matter contained in each raw material compound is removed. Alternatively, heat treatment may be performed in advance in order to remove components generated when the raw material compound is thermally decomposed.

  The heat treatment is usually performed at 80 ° C. or more and less than 400 ° C. When the temperature of the heat treatment is low, removal of volatile components and thermal decomposition components becomes insufficient, and there is a case where a product having an adverse effect on firing and inferior crystallinity may be obtained. On the other hand, if the temperature is too high, the firing reaction starts without removing the volatile components and thermal decomposition components, which may adversely affect the battery performance.

  The heat treatment is usually performed for 10 minutes or more and 20 hours or less. If the treatment time is short, the treatment may be incomplete, so it is preferable to carry out the treatment for 30 minutes or longer, particularly 1 hour or longer. On the contrary, it is not practical that the processing time is long, so it is preferable to perform the treatment for 10 hours or less. In addition, in order to perform reaction efficiently and uniformly, you may repeat heat processing 2 times or more on both sides of a grinding | pulverization / disintegration process.

Production of a positive electrode for a lithium secondary battery using the positive electrode material for a lithium secondary battery according to the present invention as a positive electrode active material can be carried out by a conventional method. If desired, other active materials can be used in combination with this positive electrode material.
The positive electrode active material layer is usually made by pressing a positive electrode material, a binder, and a conductive material and a thickener, etc., which are used as necessary, in a dry form into a sheet shape, and then pressure-bonding it to the positive electrode current collector. Alternatively, a material obtained by dissolving or dispersing these materials in a liquid medium to form a slurry can be applied to a positive electrode current collector and dried.
The positive electrode material for a lithium secondary battery is usually used in the positive electrode active material layer so as to be 10 wt% or more and 99.9 wt% or less. If this ratio is low, the capacity may be insufficient. Therefore, it is preferably used so as to be 30% by weight or more, particularly 50% by weight or more. On the other hand, if this ratio is high, the strength of the positive electrode may be insufficient, so that it is preferably used so as to be 99% by weight or less.

  Any binder can be used. In the case of the coating method, any material that is stable with respect to the liquid medium used at the time of electrode production may be used. For example, resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, and nitrocellulose. Rubbery polymers such as SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), fluorine rubber, isoprene rubber, butadiene rubber, ethylene / propylene rubber; styrene / butadiene / styrene block copolymer and hydrogenation thereof , Thermoplastic elastomeric polymers such as EPDM (ethylene-propylene-diene terpolymer), styrene / ethylene / butadiene / ethylene copolymer, styrene / isoprene styrene block copolymer and hydrogenated products thereof; Soft resinous polymers such as tactic-1,2-polybutadiene, polyvinyl acetate, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer; polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyfluoride Fluorine polymers such as vinylidene and polytetrafluoroethylene / ethylene copolymers; polymer compositions having ion conductivity of alkali metal ions (particularly lithium ions), and the like. In addition, these may be used individually by 1 type and may use 2 or more types together by arbitrary combinations and a ratio.

  The ratio of the binder in the positive electrode active material layer is usually 0.1% by weight or more and 80% by weight or less. If the ratio of the binder is low, the positive electrode active material cannot be sufficiently retained and the positive electrode has insufficient mechanical strength, which may deteriorate battery performance such as cycle characteristics. In particular, it is preferably used so as to be 5% by weight or more. On the other hand, if this ratio is high, the battery capacity and conductivity may be lowered. Therefore, it is preferable to use 60% by weight or less, further 40% by weight or less, and particularly 10% by weight or less.

  The positive electrode active material layer usually contains a conductive material in order to increase conductivity. Any conductive agent can be used, for example, metal materials such as copper and nickel; graphite such as natural graphite and artificial graphite; graphite black such as acetylene black; and amorphous carbon such as needle coke. And carbon materials. In addition, these may be used individually by 1 type and may use 2 or more types together by arbitrary combinations and ratios.

  The proportion of the conductive material in the positive electrode active material is usually 0.01% by weight or more and 50% by weight or less. If the proportion of the conductive material is low, the conductivity may be insufficient. Therefore, the conductive agent is preferably used in an amount of 0.1% by weight or more, particularly 1% by weight or more. On the contrary, if this ratio is high, the battery capacity may be lowered, so that it is preferably used so that it is 30 wt% or less, particularly 15 wt% or less.

  The solvent for forming the slurry may be any solvent as long as it can dissolve or disperse the positive electrode material, the binder, and the thickener and conductive material used as necessary. Either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water and an aqueous alcohol solution. Examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran (THF). ), Toluene, acetone, dimethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, hexane and the like. When an aqueous solvent is used, a dispersant is added together with the thickener, and the slurry is made into a slurry using a latex such as SBR. These solvents may be used alone or in combination of two or more in any combination and ratio.

The thickness of the positive electrode active material layer is usually about 10 to 200 μm. The positive electrode active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the positive electrode active material.
Examples of the material of the positive electrode current collector include metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; and carbon materials such as carbon cloth and carbon paper. Among these, a metal material, particularly aluminum, is preferable. Examples of the shape of the current collector using a metal material include metal foil, metal thin film, expanded metal, punch metal, and foam metal. In addition, examples of the shape of the current collector using a carbon material include a carbon plate and a carbon thin film. Of these, metal thin films are preferred because they are currently used in industrialized products. In addition, you may form a thin film suitably in mesh shape. The thickness of the thin film is arbitrary, but is usually 1 μm or more and 100 mm or less. If the thin film is thin, the strength of the current collector may be insufficient. Therefore, the thickness of the thin film is preferably 3 μm or more, particularly 5 μm or more. On the contrary, if the thin film is thick, the handleability is deteriorated, and therefore a thickness of 1 mm or less, particularly 50 μm or less is preferable.

  A lithium secondary battery according to the present invention is a lithium secondary battery comprising a negative electrode capable of inserting and extracting lithium, a non-aqueous electrolyte containing a lithium salt as an electrolyte, and a positive electrode capable of inserting and extracting lithium, The positive electrode for a lithium secondary battery is used as the positive electrode. Furthermore, you may provide the separator holding a nonaqueous electrolyte between a positive electrode and a negative electrode. In order to effectively prevent a short circuit due to contact between the positive electrode and the negative electrode, it is preferable to interpose a separator in this way.

The negative electrode is usually configured by providing a negative electrode active material layer on a negative electrode current collector, as in the case of the positive electrode.
As a material of the negative electrode current collector, a metal material such as copper, nickel, stainless steel, or nickel-plated steel; a carbon material such as carbon cloth or carbon paper is used. Examples of the shape of the negative electrode current collector using a metal material include a metal foil and a metal thin film. In addition, examples of the shape of the negative electrode current collector using a carbon material include a carbon plate, a carbon thin film, and a carbon cylinder. Among these, metal thin films are preferred because they are currently used in industrialized products. In addition, you may form a thin film suitably in mesh shape. When a metal thin film is used as the negative electrode current collector, the preferred thickness is the same as that of the positive electrode current collector.

As the negative electrode active material, any material can be used as long as it can electrochemically occlude and release lithium ions, and a carbon material that can occlude and release lithium is usually used from the viewpoint of high safety.
Arbitrary things can be used as a carbon material, For example, graphite (graphite), such as artificial graphite and natural graphite, the thermal decomposition thing of organic substance on various thermal decomposition conditions, etc. are mentioned. Examples of pyrolysis products of organic matter include coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, or carbide obtained by oxidizing these pitches, needle coke, pitch coke, phenol resin, crystalline cellulose, etc. And carbon materials obtained by partially graphitizing these, furnace black, acetylene black, pitch-based carbon fibers, and the like. Above all, it is artificial graphite, refined natural graphite produced by subjecting graphite, especially easy-graphite pitch obtained from various raw materials to high-temperature heat treatment, or graphite material containing pitch in these graphite, and various surface treatments. What has been applied is preferred. One of these carbon materials may be used alone, or two or more thereof may be used in combination.

When a graphite material is used as the negative electrode active material, the d value (interlayer distance) of the lattice plane (002 plane) determined by X-ray diffraction by the Gakushin method is usually 0.335 nm or more and 0.34 nm or less, preferably 0.8. What is 337 nm or less is preferable.
The ash content of the graphite material is usually 1% by weight or less with respect to the weight of the graphite material. It is preferably 0.5% by weight or less, particularly preferably 0.1% by weight or less.

Further, the crystallite size (Lc) of the graphite material determined by X-ray diffraction by the Gakushin method is usually 30 nm or more, preferably 50 nm or more, particularly preferably 100 nm or more.
Further, the median diameter of the graphite material determined by the laser diffraction / scattering method is usually 1 μm or more and 100 μm or less. The lower limit of the median diameter is preferably 3 μm or more, more preferably 5 μm or more, and particularly preferably 7 μm or more. Further, the upper limit is preferably 50 μm or less, more preferably 40 μm or less, and particularly preferably 30 μm or less.

The BET specific surface area of the graphite material is usually 0.5 m ² / g or more and 25.0 m ² / g or less. The lower limit of the BET specific surface area is preferably 0.7 m ² / g or more, more preferably 1.0 m ² / g or more, and particularly preferably 1.5 m ² / g or more. Moreover, as an upper limit, 20.0 m < ² > / g or less, Furthermore 15.0 m < ² > / g or less, Especially 10.0 m < ² > / g or less are preferable.
When performing Raman spectrum analysis using an argon laser beam on the graphite material, the intensity _{I A} of the peak _{P A} is detected in the range of 1580～1620Cm ^-1, peaks detected in a range of 1350 ^-1 intensity ratio _I a / _{I B} of the intensity _{I B} of P _B are 0 or more, is preferable 0.5 or less. Further, the half width of the peak _{P A} is 26cm ^-1 or less, it is preferred in particular 25 cm ^-1 or less. In addition to the various carbon materials described above, other materials capable of inserting and extracting lithium can be used as the negative electrode active material. Specific examples of the negative electrode active material other than the carbon material include metal oxides such as tin oxide and silicon oxide, and lithium alloys such as lithium alone and lithium aluminum alloys. One of these materials other than the carbon material may be used alone, or two or more thereof may be used in combination. Moreover, you may use in combination with the above-mentioned carbon material.

  As in the case of the positive electrode active material layer, the negative electrode active material layer is usually formed by slurrying the above-described negative electrode active material, a binder, and, if necessary, a thickener and a conductive material with a solvent. It can manufacture by apply | coating to a collector and drying. As the solvent, binder, thickener, conductive material and the like forming the slurry, the same materials as those described above for the positive electrode active material can be used.

As the electrolytic solution, for example, a known organic electrolytic solution, a polymer solid electrolyte, a gel electrolyte, an inorganic solid electrolyte, or the like can be used. Among them, an organic electrolytic solution is preferable.
The organic electrolytic solution is prepared by dissolving an electrolyte in an organic solvent.
As the organic solvent, any one commonly used in an electrolytic solution can be used. For example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, Amines, esters, amides, phosphate ester compounds and the like can be used. For example, dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, vinylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 4-methyl-2-pentanone, 1,2-dimethoxyethane, 1,2-di Ethoxyethane, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane , A single solvent such as dimethylformamide, dimethyl sulfoxide, trimethyl phosphate, triethyl phosphate, or a mixed solvent of two or more.

  The organic solvent preferably includes a high dielectric constant solvent in order to dissociate the electrolyte. Here, the high dielectric constant solvent means a compound having a relative dielectric constant of 20 or more at 25 ° C. Examples of the high dielectric constant solvent include ethylene carbonate, propylene carbonate, and compounds obtained by substituting those hydrogen atoms with other elements such as halogen or alkyl groups. The proportion of the high dielectric constant solvent in the electrolytic solution is preferably 20% by weight or more, more preferably 30% by weight or more, and most preferably 40% by weight or more.

As the electrolyte, any conventionally known electrolyte can be used. For example, LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiN (SO ₂ CF ₃ ) ₂ , LiN ( Examples thereof include lithium salts such as SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , and LiN (SO ₃ CF ₃ ) _2, and polymer solid electrolytes. Any one of these may be used alone, or two or more may be used in any combination and ratio. Further, any ratio of an additive that forms a good film that enables efficient charge and discharge of lithium ions on the negative electrode surface, such as gas such as CO ₂ , N ₂ O, CO, and SO ₂ and polysulfide S _x ²⁻ And may be added to the above alone or in a mixed solvent.

The lithium salt is usually contained in the electrolyte so as to be 0.5 mol / L or more and 1.5 mol / L or less. Outside this range, the conductivity may decrease and adversely affect battery characteristics. Therefore, the lower limit is preferably 0.75 mol / L or more and the upper limit is 1.25 mol / L or less.
As the polymer solid electrolyte, any conventionally known crystalline / amorphous inorganic substance can be used. Examples of the crystalline inorganic solid electrolyte include LiI, Li ₃ N, Li _{1 + x} M _x Ti _2-x (PO ₄ ) ₃ (M = Al, Sc, Y, La), Li _0.5-3x RE _{0. .5 + x} TiO ₃ (RE = La, Pr, Nd, Sm) and the like. Examples of the amorphous inorganic solid electrolyte include oxide glasses such as 4.9LiI-34.1Li ₂ O-61B ₂ O ₅ and 33.3Li ₂ O-66.7SiO ₂ . Any one of these may be used alone, or two or more may be used in any combination and ratio.

  When an organic electrolyte is used as the electrolyte, a separator is usually interposed between the positive electrode and the negative electrode in order to prevent a short circuit between the electrodes. The material and shape of the separator are arbitrary, but those that are stable with respect to the organic electrolyte used, have excellent liquid retention properties, and can reliably prevent short-circuiting between electrodes are preferable. Examples of the separator include microporous films, sheets, and nonwoven fabrics made of various polymer materials. Specific examples of the polymer material include polyolefin polymers such as nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, and polybutene. Of these, polyolefin polymers are preferred from the viewpoint of chemical and electrochemical stability, which is an important factor of separators, and polyethylene is preferred from the viewpoint of self-occluding temperature, which is one of the purposes of use of separators in batteries. Is preferred. As the polyethylene, ultra-high molecular weight polyethylene is preferable from the viewpoint of maintaining high-temperature shape, and the molecular weight is preferably 500,000 to 5,000,000. If the molecular weight is too large, the fluidity is too low and the pores of the separator may not be clogged during heating. Therefore, the lower limit of the molecular weight is preferably 1,000,000 or more, particularly 1.5 million or more. Conversely, the upper limit of the molecular weight is preferably 4 million or less, particularly 3 million or less.

The lithium secondary battery according to the present invention is manufactured by assembling the positive electrode, the negative electrode, the electrolyte, and the separator used as necessary into an appropriate shape. Furthermore, you may use other components, such as an exterior case, as needed.
The shape of the lithium secondary battery according to the present invention is arbitrary, and can be appropriately selected from various commonly employed shapes according to the application. For example, a cylinder type in which a sheet electrode and a separator are spirally formed, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, a coin type in which a pellet electrode and a separator are stacked, and the like can be given. Moreover, the method of assembling the battery is arbitrary, and can be appropriately selected from various commonly used methods according to the shape of the target battery.

As mentioned above, although general embodiment of the lithium secondary battery which concerns on this invention was described, the lithium secondary battery which concerns on this invention is not restrict | limited to the said embodiment, As long as the summary is not exceeded, it is various. It is possible to carry out with the modification of.
The use of the lithium secondary battery according to the present invention is not particularly limited, and can be used for various known uses. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. , Electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, lighting equipment, toys, game machines, watches, strobes, camera power load leveling, electric bicycles, electric scooters, electric cars And the like.

EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not restrict | limited by a following example, unless the summary is exceeded.
In addition, Fe (II) / Fe (III) (molar ratio) was measured by the following method.
<Fe (II) / Fe (III) fractional determination by o-phenanthroline coloring method>
The contact with the atmosphere was cut off, and 10 mg of the sample was dissolved in 10 mL of hydrochloric acid sufficiently substituted with nitrogen by irradiating with ultrasonic waves for 30 minutes. 0.5 mL of this lysate was fractionated, and B solution (coloring solution ^* : 10 mL + H ₂ O: 89 mL) that had been sufficiently purged with nitrogen in advance was added and allowed to stand for 1 hour. Water was added to this solution to 100 mL, and the absorbance at 508 nm was measured to quantify the Fe (II) concentration. Was taken of the same solution 0.5 ml, A solution (hydroxylamine hydrochloride 10% aqueous _{solution: 1mL + H 2 O: 70mL} ) After addition was allowed to stand for 15 minutes, color development solution ^*: was added to 10mL and allowed to stand for 1 hour . Water was added to this solution to make 100 mL, and the total Fe concentration was quantified by measuring the absorbance at 508 nm. The Fe (III) concentration was obtained by subtracting the Fe (II) concentration from the total Fe concentration, and the Fe (II) / Fe (III) fraction was determined.
* Coloring solution: 0.05% o-phenanthroline + 1.2M NaOAc + 0.1M NaFaq.

Example 1
LiOH.H ₂ O, FeC ₂ O ₄ .2H ₂ O, and (NH ₄ ) ₂ HPO ₄ have an air atmosphere such that Li / Fe / PO ₄ = 1.05 / 1/1 (molar ratio). Under mixing using a mortar, the obtained mixture was heated at 350 ° C. for 3 hours under an argon gas atmosphere with an oxygen partial pressure of 1.4 × 10 ⁻³ atm. Next, this was crushed and heated at 675 ° C. for 24 hours in an argon gas atmosphere with an oxygen partial pressure of 1.4 × 10 ⁻³ atm. After being allowed to cool to room temperature, it was crushed and again heated in an argon gas atmosphere with an oxygen partial pressure of oxygen partial pressure of 1.4 × 10 ⁻³ atm at 675 ° C. for 24 hours to obtain LiFePO ₄ , Li ₃ Fe ₂ (PO ₄ ). _Thus , a positive electrode active material having a mixed phase composed of ₃ and Fe ₂ O ₃ was obtained. The obtained positive electrode active material had an Fe (II) / Fe (III) (molar ratio) of 1.2 and a BET specific surface area of 1.56 m ² / g.

(Comparative Example 1)
Example 1 is different from Example 1 except that a mixed gas (oxygen partial pressure 1.5 × 10 ⁻⁶ atm) of argon gas / hydrogen gas = 96/4 (volume ratio) is used instead of argon gas. Similarly, a positive electrode active material having a single phase of LiFePO ₄ having an olivine structure was obtained. The obtained positive electrode active material had an Fe (II) / Fe (III) (molar ratio) of 58 and a BET specific surface area of 1.67 m ² / g.

<Production and evaluation of battery>
75 parts by weight of the positive electrode active material of Example 1 or Comparative Example 1, 20 parts by weight of acetylene black, and 5 parts by weight of polytetrafluoroethylene powder were thoroughly mixed in a mortar, and a thin sheet was used as a whole with a 9 mmφ punch. Punched out to a weight of about 8 mg. This was pressure-bonded to an aluminum expanded metal to obtain a 9 mmφ positive electrode. For a coin cell having this positive electrode as a test electrode and a lithium metal as a counter electrode, a charge upper limit voltage is 4.35 V, a discharge lower limit voltage is 2.0 V, and a constant current constant voltage charge of 0.2 mA / cm ^{2 in} the first cycle. Constant current discharge is performed at a constant current / constant voltage charge / constant current discharge of 0.2 mA / cm ^{2 in} the second cycle, followed by a constant current charge of 0.5 mA / cm ^{2 at} the third to 10th cycles, 0.2 mA. / ^{cm 2,} were tested in each discharge ^{0.5mA / cm 2, 1mA / cm} 2, 3mA / cm 2, 5mA / cm 2, 7mA / cm 2, 9mA / cm 2, 11mA / cm 2. The initial discharge capacity at 0.2 mA / cm ² in the first cycle of this time (mAh / g), and 10 th cycle high-rate discharge capacity at 11 mA / cm ² a (mAh / g) was measured. Table 1 shows the obtained results and the ratio of initial discharge capacity / high rate discharge capacity.

  Table 1 shows that the lithium secondary battery produced using the positive electrode material for a lithium secondary battery according to the present invention has improved rate characteristics. Although it is not clear why the rate characteristics are improved by using a positive electrode material having a Fe (II) / Fe (III) molar ratio of 1 to 50, lithium iron phosphorus composed of divalent iron and trivalent iron By taking the particle form in which the acid compound is mixed at the primary particle crystal level, the particle form is different from the case where only the divalent iron lithium iron phosphate compound exists, and this particle form also has a high current density. This is presumed to be because it is easy to respond.

Claims (8)

In a positive electrode material for a lithium secondary battery including a lithium iron phosphate compound composed of divalent iron [Fe (II)] and trivalent iron [Fe (III)], a lithium iron phosphate compound of divalent iron LiFePO ₄ having an olivine structure, the lithium iron phosphate compound of trivalent iron is Li ₃ Fe ₂ (PO ₄ ) ₃ , and Fe ₂ O ₃ is used as the iron oxide of trivalent iron. A positive electrode material for a lithium secondary battery, wherein the molar ratio of Fe (II) / Fe (III) is 1 to 50. BET specific surface area is 0.5-3 m < ² > / g, The positive electrode material for lithium secondary batteries of Claim 1 or 2 characterized by the above-mentioned.   Lithium iron phosphoric acid compound composed of divalent iron [Fe (II)] and trivalent iron [Fe (III)]
Lithium secondary battery positive electrode material containing lithium, a lithium iron phosphate compound of divalent iron is LiFePO having an olivine structure ₄ Lithium iron phosphate compound of trivalent iron is Li ₃ Fe ₂ (PO ₄ ) ₃ And the molar ratio of Fe (II) / Fe (III) is 1-50.
, BET specific surface area is 0.5-3m ² / G for a positive electrode material for a lithium secondary battery. 1.02 mol or more and 1.2 mol or less of lithium of the lithium compound material, and 0.5 mol or more and 0.98 mol or less of the iron of the divalent iron compound material with respect to 1 mol of phosphorus of the phosphorus compound material, and 3 The iron of the valence iron compound raw material is mixed in an amount of 0.02 mol to 0.5 mol and calcined in an atmosphere having an oxygen partial pressure of 1 × 10 ⁻³ atm or less. The manufacturing method of the lithium secondary battery positive electrode material in any one of these. 1 × 10 ⁻⁴ atm after mixing at a ratio of 1.02 mol to 1.2 mol of lithium of the lithium compound raw material and 1 mol of iron of the divalent iron compound raw material to 1 mol of phosphorus of the phosphorus compound raw material The method for producing a positive electrode material for a lithium secondary battery according to any one of claims 1 to 3, wherein firing is performed in an atmosphere controlled to an oxygen partial pressure of 5 × 10 ^-3 atm or less.   A positive electrode for a lithium secondary battery comprising a positive electrode active material layer containing the positive electrode material for a lithium secondary battery according to any one of claims 1 to 3 and a binder on a current collector.   A lithium secondary battery comprising a negative electrode capable of inserting and extracting lithium, a non-aqueous electrolyte containing a lithium salt as an electrolyte, and a positive electrode capable of inserting and extracting lithium, wherein the positive electrode is the positive electrode according to claim 6. A lithium secondary battery characterized by that. In a positive electrode material for a lithium secondary battery including a lithium iron phosphate compound composed of divalent iron [Fe (II)] and trivalent iron [Fe (III)], the mole of Fe (II) / Fe (III) Ratio is 1
It is a manufacturing method of the positive electrode material for lithium secondary batteries characterized by being -50, Comprising: 1.02 mol or more and 1.2 mol or less of lithium of a lithium compound raw material with respect to 1 mol of phosphorus of a phosphorus compound raw material After mixing 0.5 mol or more and 0.98 mol or less of iron of the divalent iron compound raw material and 0.02 mol or more and 0.5 mol or less of iron of the trivalent iron compound raw material, 1 × 10 ^{− A} method for producing a positive electrode material for a lithium secondary battery, comprising firing in an atmosphere having an oxygen partial pressure of ³ atm or less.