JP2009295465A - Positive electrode active material for lithium secondary battery and manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode material for lithium secondary battery which is inexpensive and has a high capacity. <P>SOLUTION: The positive electrode active material for lithium secondary battery is represented by the general formula Li<SB>x</SB>FePO<SB>4</SB>(0.95≤x≤1.05), and satisfies D/G of 0.5 or more and F/G of 1.5 or less, wherein D is the intensity of peak present in 1325-1365 cm<SP>-1</SP>based on Raman spectroscopy, G is the intensity of peak present in 1560-1610 cm<SP>-1</SP>, and F is the intensity of peak present in 900-1200 cm<SP>-1</SP>. The method for manufacturing the positive electrode active material for lithium secondary battery represented by the above general formula includes adding tetraethylene glycol and Li source to FePO<SB>4</SB>followed by baking. A lithium secondary battery using the positive electrode active material for lithium secondary battery is also provided. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

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

  Lithium secondary batteries are excellent in energy density and output density, and are effective in reducing the size and weight. Therefore, the demand for lithium secondary batteries as a power source for portable devices such as notebook computers, mobile phones, and handy video cameras is growing rapidly. Show. Lithium secondary batteries are also attracting attention as power sources for electric vehicles and power load leveling.

  Conventionally, lithium cobaltate, lithium nickelate, cobalt-nickel composite oxides, and the like have been used as positive electrode active materials for lithium secondary batteries. However, since rare metals such as cobalt and nickel are used for these positive electrodes, it is a cause of increasing the cost of the lithium secondary battery, and a cheaper positive electrode active material has been demanded.

Therefore, in recent years, lithium iron phosphate (LiFePO ₄ ) using iron, which is an inexpensive and abundant element, has attracted attention as a positive electrode active material, but LiFePO ₄ was used as a positive electrode active material because of its low conductivity. In this case, there is a problem that the capacity of the battery is lowered particularly in discharging with a large current.

In order to solve this problem, an attempt has been made to improve battery performance by improving conductivity by attaching or coating a conductive material such as carbon on the surface of LiFePO ₄ .

For example, in Patent Document 1 and Patent Document 2, it is possible to produce LiFePO ₄ having high conductivity by adding a carbon material to a raw material and performing firing after pulverization and mixing. However, in the method of Patent Document 1, since the carbon content per unit mass is 3% by mass or more, there is a case where sufficient capacity cannot be ensured by limiting the amount of the active material per unit mass. .

  In both Patent Document 1 and Patent Document 2, attention is paid to the crystallinity of the carbon material existing on the surface of the active material, but attention is not paid to the ratio of the active material to the carbon material on the surface of the active material.

On the other hand, in Patent Document 3, high-conductivity LiFePO ₄ can be produced by almost completely covering the surface of the positive electrode active material with the amount of carbon. However, since the carbon coating step is provided separately from the firing step, the manufacturing cost may increase.

Japanese Patent Laid-Open No. 2002-075364 JP 2002-110162 A JP 2006-302671 A

  This invention is made | formed in view of this background art, The subject is providing the positive electrode active material for lithium secondary batteries which has a cheap and high capacity | capacitance.

[Positive electrode active material for lithium secondary battery]
As a result of intensive studies to solve the above problems, the present inventor, in the Raman spectrum of lithium iron phosphate as a positive electrode active material, the carbon attached to the surface has a certain range of amorphousness, In addition, the inventors have found that good battery characteristics can be realized when a certain amount or more of carbon adheres to the surface of the active material, and have completed the present invention.

[Positive electrode active material for lithium secondary battery]
That is, the present invention is represented by the general formula Li _x FePO ₄ (0.95 ≦ x ≦ 1.05), and the peak intensity existing at 135-1365 cm ⁻¹ in Raman spectroscopy is D, and 1560-1610 cm ^{−. When} the intensity of the peak existing in ¹ is G and the intensity of the peak existing in 900 to 1200 cm ⁻¹ is F, D / G is 0.5 or more and F / G is 1.5 or less. The present invention provides a positive electrode active material for a lithium secondary battery.

Moreover, this invention provides the positive electrode active material whose C density | concentration in an active material is 0.6 to 1.5 mass% in said positive electrode active material. Moreover, this invention provides said positive electrode active material whose specific surface area by BET method is 7-30 m < ² > / g. Moreover, this invention provides said positive electrode active material whose primary particle size is 200 nm or less.

[Method for producing positive electrode active material for lithium secondary battery]
In addition, as a means for realizing the above-described problems of the present invention, a positive electrode active material for a lithium secondary battery may be manufactured by using iron phosphate, that is, FePO ₄ as a precursor, and using tetraethylene glycol as a carbon source. I found. Furthermore, as a means for realizing the present invention, it has been found that the above-described positive electrode active material for a lithium secondary battery can be produced by using iron phosphate, that is, FePO ₄ as a precursor and using tetraethylene glycol as a carbon source.

That is, the present invention relates to a method for producing a positive electrode active material for a lithium secondary battery represented by a general formula Li _x FePO ₄ (0.95 ≦ x ≦ 1.05), wherein tetraethylene glycol and a Li source are added to FePO _4. It is another object of the present invention to provide a production method characterized by adding and baking.

In addition, the present invention provides the above production method, wherein FePO ₄ is subjected to a separate heat treatment before adding tetraethylene glycol and a Li source and firing. Moreover, this invention provides said manufacturing method whose baking temperature is 850 degrees C or less.

[Lithium secondary battery]
Moreover, this invention provides the lithium secondary battery using the positive electrode active material produced by the said positive electrode active material and the said manufacturing method.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material for lithium secondary batteries which has a cheap and high capacity | capacitance can be provided.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. However, the description of the constituent elements described below is an example (representative example) of an embodiment of the present invention. It is not limited to the specific contents of.

[Positive electrode active material for lithium secondary battery]
The positive electrode active material for lithium secondary batteries of the present invention (hereinafter sometimes simply referred to as “positive electrode active material”) is represented by the general formula Li _x FePO ₄ (0.95 ≦ x ≦ 1.05), the intensity of the peaks present in 1325～1365Cm ^-1 in Raman spectroscopy is D, the intensity of the peaks present in 1560～1610Cm ^-1 and G, when the intensity of the peaks present in 900～1200Cm ^-1 was F Furthermore, D / G is characterized by being 0.5 or more and F / G being 1.5 or less.

<Peak intensity ratio in Raman spectroscopy>
Hereafter, the measuring method of the Raman spectrum in this invention is shown.
(Measurement method of Raman spectrum)
For Raman spectroscopic measurement, a Raman spectroscope (for example, a Raman spectroscope manufactured by JASCO Corporation) is used, and a powder sample is set in a measurement cell, and measurement is performed while irradiating the sample surface in the cell with argon ion laser light. While rotating in a plane perpendicular to the laser beam. The Raman measurement conditions here are as follows.
Argon ion laser wavelength: 514.5nm
Laser power on sample: about 0.5mW
Resolution: 8cm ^-1
Measurement range: 600 cm ^{−1 to} 2100 cm ⁻¹
Smoothing processing: Simple average, 5 points of convolution

Here, the peak intensity D, that is, the intensity of the peak existing at 1325 to 1365 cm ⁻¹ is derived from amorphous carbon, and the peak intensity G, that is, the peak intensity G present at 1560 to 1610 cm ⁻¹ is crystalline. Derived from qualitative carbon. Therefore, the ratio D / G is equivalent to the higher the amorphousness of carbon, the higher the value.

In the present invention, when the intensity of the peak existing at 1325 to 1365 cm ⁻¹ is D and the intensity of the peak existing at 1560 to 1610 cm ⁻¹ is G, it is essential that D / G is 0.5 or more. However, it is preferably 0.7 or more.

The peak intensity F, that is, the peak intensity F existing at 900 to 1200 cm ⁻¹ is derived from LiFePO ₄ . Therefore, F / G, which is the ratio to carbon, is equivalent to the fact that the higher the value, the lower the proportion of carbon on the surface of the positive electrode active material.

In the present invention, when the intensity of the peak existing at 900 to 1200 cm ⁻¹ is F, it is essential that F / G is 1.5 or less, preferably 1.0 or less, more preferably 0. .7 or less. In addition, if F / G is too low, the surface is almost carbon only, which may hinder the movement of lithium ions on the surface of the active material. Therefore, F / G is preferably 0.1 or more.

<C concentration>
In the positive electrode active material of the present invention, the C concentration in the positive electrode active material is preferably 0.6% by mass or more and 1.5% by mass or less. In the present invention, the “C concentration” refers to the content% by mass of carbon with respect to the whole positive electrode active material. The C concentration in the active material is derived from the amount of added carbon raw material, and if the C concentration is too high, the capacity may decrease due to a relative decrease in the amount of active material that contributes to charge and discharge. On the other hand, if the C concentration is too low, the capacity may decrease due to a shortage of carbon contributing to conductivity. The C concentration in the active material in the present invention is preferably 0.6% by mass or more and 1.5% by mass or less, more preferably 0.7% by mass or more and 1.4% by mass or less, and still more preferably 0.8% by mass. % To 1.3% by mass.

<Specific surface area by BET method>
In the present invention, specific surface area by the BET method: (hereinafter sometimes abbreviated as "specific surface area") is preferably not more than ^7m 2 / g or more ^{30 m} 2 / g, more preferably ^8m 2 / g or more ^{20 m} 2 / g or less. When the specific surface area is smaller than the above range, the battery performance is lowered due to the reduction of the electrode reaction area. When the specific surface area is larger than the above range, the powder has a low bulk density, which may cause difficulty in handling the powder.

<Primary particle size>
In the present invention, the primary particle size of the positive electrode active material is preferably 200 nm or less, and when the primary particle size is larger than 200 m, the specific surface area is decreased, and the lithium diffusion distance in the primary particles is increased, thereby reducing the battery performance. There is a case.

  As the “primary particle size” of the positive electrode active material in the present invention, 20 primary particles are arbitrarily selected in a transmission electron microscope (hereinafter abbreviated as “TEM”) photograph, and each primary particle is selected. It is the number average diameter obtained by approximating with a sphere and measuring its diameter and taking the arithmetic mean. An example of a TEM photograph used for the measurement is shown in FIGS.

[Method for producing positive electrode active material for lithium secondary battery]
The method for producing a positive electrode active material in the present invention are the compounds of formula _{Li x FePO 4 (0.95 ≦ x} ≦ 1.05) The method for producing a positive electrode active material for a lithium secondary battery represented by, tetraethylene the FePO ₄ It is characterized in that it is fired by adding glycol and Li sources.

There is no particular limitation on the Li raw material, and specific examples, anhydrous _{LiOH, LiOH · H 2 O,} Li 2 CO 2, Li 2 O, Li 2 C 2 O 4, lithium nitrate, various Li compounds such as lithium sulfate Two or more kinds of Li raw materials may be used at the same time.

There is no particular limitation on the Fe raw material, specific examples, iron nitrate, iron sulfate, iron citrate, iron acetate, iron oxides and the like _{(FeO, Fe} 2 O 2, _Fe 2 _{O 4,} etc.), with 2 More than one type of Fe raw material may be used simultaneously. Among these, a water-soluble Fe raw material is preferably used from the viewpoint of easy reaction.

The phosphorus raw material is not particularly limited, and specific examples include Na ₂ HPO ₄ · 12H ₂ O, NaH ₂ PO ₄ · 12H ₂ O, H ₂ PO ₄ , P ₂ O ₅ , metaphosphoric acid, polyphosphoric acid, and the like. Two or more types of phosphorus raw materials may be used simultaneously.

For the synthesis of FePO _{4 as} a precursor, a general method for synthesizing an inorganic compound can be used. Specific examples include a coprecipitation method, a solid phase reaction method, a sol-gel method, and the like, which can be appropriately selected depending on the raw material to be used.

  When pulverization is required for the raw material, the precursor and the fired product, various general pulverization methods can be used. The pulverization may be either dry or wet, and specific examples include a ball mill, a jet mill, and a bead mill.

FePO ₄ used as a precursor is preferably subjected to a heat treatment before adding the Li raw material and tetraethylene glycol. In this case, the heat treatment temperature is preferably 400 ° C. or higher, more preferably 500 ° C. or higher.

  For firing, for example, a box furnace, a tubular furnace, a tunnel furnace, a rotary kiln or the like can be used. The firing step is usually divided into three steps: temperature increase, maximum temperature retention, and temperature decrease. The second maximum temperature holding step is not necessarily limited to one time, and two or more steps may be included depending on the purpose, which means that aggregation is eliminated to the extent that the secondary particles are not destroyed. The temperature raising, maximum temperature holding, and temperature lowering steps may be repeated two or more times with a pulverization step or a pulverization step that means crushing to primary particles or even fine powder.

  In the temperature raising step, the temperature in the furnace is usually raised at a temperature raising rate of 1 ° C./min to 10 ° C./min. Even if this rate of temperature rise is too slow, it takes time and is industrially disadvantageous, but if it is too fast, the furnace temperature may not follow the set temperature depending on the furnace.

  The temperature at the maximum temperature holding, that is, the firing temperature is preferably 850 ° C. or lower, more preferably 700 ° C. or lower. When the firing temperature exceeds 850 ° C., the growth of particles progresses, the specific surface area decreases, the lithium diffusion distance in the primary particles increases, and the battery performance may decrease.

  Although the holding time in the maximum temperature holding step varies depending on the temperature, it is usually 30 minutes or longer, preferably 3 hours or longer, more preferably 6 hours or longer, usually 50 hours or shorter, preferably 25 within the above-mentioned temperature range. It is not more than time, more preferably not more than 20 hours. If the firing time is too short, it becomes difficult to obtain a positive electrode active material with good crystallinity, and it is not practical to be too long. If the firing time is too long, then crushing may be necessary, or crushing may be difficult.

  In the temperature lowering step, the temperature in the furnace is normally decreased at a temperature decreasing rate of 0.1 ° C./min to 10 ° C./min. If the rate of temperature decrease is too slow, it takes time and is industrially disadvantageous. However, if the rate of temperature decrease is too fast, the uniformity of the target product may be lost or the deterioration of the container may be accelerated.

  As the atmosphere during firing, a reducing atmosphere that does not produce iron trivalent compounds can be used. Specific examples include hydrogen, argon, nitrogen, and the like, and these can also be used in combination.

[Configuration of positive electrode]
The lithium secondary battery of the present invention is formed by forming a positive electrode active material layer containing a powder of a positive electrode active material for a lithium secondary battery of the present invention and a binder on a current collector.

  The positive electrode active material layer is usually formed by mixing a positive electrode material, a binder, and a conductive material and a thickener, which are used if necessary, in a dry form into a sheet shape, and then pressing the positive electrode current collector on the positive electrode current collector. Alternatively, these materials are dissolved or dispersed in a liquid medium to form a slurry, which is applied to the positive electrode current collector and dried.

  As the material for the positive electrode current collector, metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, and carbon materials such as carbon cloth and carbon paper are usually used. Of these, metal materials are preferable, and aluminum is particularly preferable. As for the shape, in the case of a metal material, a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, an expanded metal, a punch metal, a foam metal, etc., and in the case of a carbon material, a carbon plate, a carbon thin film, a carbon cylinder Etc. Among these, metal thin films are preferable because they are currently used in industrialized products. In addition, you may form a thin film suitably in mesh shape.

  When a thin film is used as the positive electrode current collector, its thickness is arbitrary, but it is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and usually 100 mm or less, preferably 1 mm or less, more preferably 50 μm or less. The range of is preferable. If it is thinner than the above range, the strength required for the current collector may be insufficient. On the other hand, if it is thicker than the above range, the handleability may be impaired.

  The binder used in the production of the positive electrode active material layer is not particularly limited, and in the case of a coating method, any material that is stable with respect to the liquid medium used during electrode production may be used. Specific examples include polyethylene, Resin polymers such as polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, nitrocellulose, SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), fluorine rubber, isoprene rubber, butadiene rubber, ethylene・ Rubber polymers such as propylene rubber, styrene / butadiene / styrene block copolymer and hydrogenated products thereof, EPDM (ethylene-propylene-diene terpolymer), styrene / ethylene / butadiene / ethylene copolymer, Styrene / isoprene styrene bromide Such as thermoplastic elastomeric polymers such as copolymer and hydrogenated products thereof, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer, etc. Fluorine polymers such as soft resin-like polymers, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene / ethylene copolymers, ion conductivity of alkali metal ions (especially lithium ions) And a polymer composition having the same. In addition, these substances 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 mass or more, preferably 1% by mass or more, more preferably 5% by mass or more, and usually 80% by mass or less, preferably 60% by mass or less. More preferably, it is 40 mass% or less, Most preferably, it is 10 mass% or less. If the proportion of the binder is too 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. It may lead to a decrease in capacity and conductivity.

  The positive electrode active material layer usually contains a conductive material in order to increase conductivity. There are no particular restrictions on the type, but specific examples include metal materials such as copper and nickel, graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon such as needle coke. The carbon material etc. of this can be mentioned. In addition, these substances may be used individually by 1 type, and may use 2 or more types together by arbitrary combinations and a ratio. The proportion of the conductive material in the positive electrode active material layer is usually 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 1% by mass or more, and usually 50% by mass or less, preferably 30%. It is at most 15% by mass, more preferably at most 15% by mass. If the proportion of the conductive material is too low, the conductivity may be insufficient, and conversely if it is too high, the battery capacity may be reduced.

  The liquid medium for forming the slurry is a positive electrode active material powder, a binder, and a solvent that can dissolve or disperse the conductive material and the thickener used as necessary. There is no particular limitation on the type, and either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water, alcohol and the like, and examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N , N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran (THF), toluene, acetone, dimethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, hexane, etc. Can be mentioned. In particular, when an aqueous solvent is used, a dispersant is added together with the thickener, and a slurry such as SBR is slurried. In addition, these solvents may be used individually by 1 type, and may use 2 or more types together by arbitrary combinations and a ratio.

  The content ratio of the active material powder of the present invention as the positive electrode material in the positive electrode active material layer is usually 10% by mass or more, preferably 30% by mass or more, more preferably 50% by mass or more, and usually 99.9% by mass. % Or less, preferably 99% by mass or less. If the proportion of the active material powder in the positive electrode active material layer is too large, the strength of the positive electrode tends to be insufficient, and if it is too small, the capacity may be insufficient.

  The thickness of the positive electrode active material layer is usually about 10 μm or more and 200 μm or less. 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.

[Lithium secondary battery]
The lithium secondary battery of the present invention includes the above-described positive electrode for a lithium secondary battery of the present invention capable of occluding and releasing lithium, a negative electrode capable of occluding and releasing lithium, and a non-aqueous electrolyte using a lithium salt as an electrolytic salt, Is provided. Furthermore, a separator that holds a non-aqueous electrolyte may be provided between the positive electrode and the negative electrode. In order to effectively prevent a short circuit due to contact between the positive electrode and the negative electrode, it is desirable to interpose a separator in this way.

  The negative electrode is usually formed by forming a negative electrode active material layer on a negative electrode current collector, similarly to the positive electrode.

  As a material of the negative electrode current collector, a metal material such as copper, nickel, stainless steel, nickel-plated steel, or a carbon material such as carbon cloth or carbon paper is used. Among these, in the case of a metal material, a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, etc., and in the case of a carbon material, a carbon plate, a carbon thin film, a carbon cylinder, etc. are mentioned. Among these, metal thin films are preferable 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 range is the same as the range described above for the positive electrode current collector.

  The negative electrode active material layer includes a negative electrode active material. The negative electrode active material is not particularly limited as long as it can electrochemically occlude and release lithium ions, but it can usually occlude and release lithium from the viewpoint of high safety. A carbon material is used.

  Although there is no restriction | limiting in particular as a carbon material, The graphite (graphite), such as artificial graphite and natural graphite, and the thermal decomposition thing of organic substance on various thermal decomposition conditions 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. Among them, graphite is preferable, and particularly preferable is artificial graphite, purified natural graphite, or graphite material containing pitch in these graphites manufactured by subjecting easy-graphite pitch obtained from various raw materials to high-temperature heat treatment. Therefore, those subjected to various surface treatments are mainly used. 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 usually 0.34 nm or less, preferably Is preferably 0.337 nm or less. Moreover, it is preferable that the ash content of graphite material is 1 mass% or less normally with respect to the mass of graphite material, especially 0.5 mass% or less, especially 0.1 mass% or less. Moreover, it is preferable that the crystallite size (Lc) of the graphite material calculated | required by the X-ray diffraction by the Gakushin method is 30 nm or more normally, especially 50 nm or more, especially 100 nm or more.

  The median diameter of the graphite material determined by the laser diffraction / scattering method is usually 1 μm or more, especially 3 μm or more, more preferably 5 μm or more, especially 7 μm or more, and usually 100 μm or less, especially 50 μm or less, more preferably 40 μm or less, especially 40 μm or less. It is preferable that it is 30 micrometers or less.

Moreover, the BET specific surface area of the graphite material is usually 0.5 m ² / g or more, preferably 0.7 m ² / g or more, more preferably 1.0 m ² / g or more, and further preferably 1.5 m ² / g. or more, and usually 25.0 m ² / g or less, preferably 20.0 m ² / g or less, more preferably 15.0 m ² / g or less, further preferably at most 10.0 m ² / g.

Also, when performing Raman spectroscopy using argon laser light for graphite material, and strength _{I A} of the peak _{P A} is detected in the range of 1580～1620Cm ^-1, it is detected in the range of 1350 ^-1 that intensity ratio _I a / _{I B} of the intensity _{I B} of a peak _{P B} is what is preferably 0 to 0.5. Further, the half width of the peak _{P A} is preferably 26cm ^-1 or less, 25 cm ^-1 or less is more preferable.

  In addition to the above-mentioned various carbon materials, it can also be used as a negative electrode active material of other materials capable of inserting and extracting lithium. Specific examples of the negative electrode active material other than the carbon material include metal oxides such as tin oxide and silicon oxide, lithium simplex and lithium alloys such as lithium aluminum alloy. 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 prepared by slurrying the above-described negative electrode active material, a binder, and optionally a conductive material and a thickener in a liquid medium. It can manufacture by apply | coating to a negative electrode electrical power collector, and drying. As the liquid medium, the binder, the thickener, the conductive material, and the like that form the slurry, the same materials as those described above for the positive electrode active material layer can be used.

  As the electrolyte, for example, known organic electrolytes, polymer solid electrolytes, gel electrolytes, inorganic solid electrolytes, and the like can be used. Among them, organic electrolytes are preferable. The organic electrolytic solution is configured by dissolving a solute (electrolyte) in an organic solvent.

  Here, the type of the organic solvent is not particularly limited. For example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, esters, amides. A phosphoric acid ester compound or the like can be used. Typical examples are 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-diethoxyethane, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, benzonitrile, butyronitrile, valeronitrile 1,2-dichloroethane, dimethylformamide, dimethyl sulfoxide, trimethyl phosphate, triethyl phosphate and the like, and these can be used alone or in combination of two or more.

  The organic solvent described above preferably contains a high dielectric constant solvent in order to dissociate the electrolytic salt. Here, the high dielectric constant solvent means a compound having a relative dielectric constant of 20 or more at 25 ° C. Among the high dielectric constant solvents, it is preferable that ethylene carbonate, propylene carbonate, and compounds in which hydrogen atoms thereof are substituted with other elements such as halogen or alkyl groups are contained in the electrolytic solution. The proportion of the high dielectric constant solvent in the electrolytic solution is preferably 20% by mass or more, more preferably 30% by mass or more, and most preferably 40% by mass or more. If the content of the high dielectric constant solvent is less than the above range, desired battery characteristics may not be obtained.

The type of lithium salt that is an electrolytic salt is not particularly limited, and any conventionally known solute can be used. Specific examples include 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 (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) _{2 and the} like. Any one of these electrolytic salts may be used alone, or two or more thereof may be used in any combination and ratio.

  The lithium salt of the electrolytic salt is usually contained in the electrolytic solution so as to have a concentration of 0.5 mol / L to 1.5 mol / L. Even if the lithium salt concentration in the electrolytic solution is less than 0.5 mol / L or more than 1.5 mol / L, the electrical conductivity may be lowered, and the battery characteristics may be adversely affected. The lower limit is preferably 0.75 mol / L or more and the upper limit is 1.25 mol / L or less.

In the electrolyte solution, an additive that forms a good film that enables efficient charge and discharge of lithium ions on the surface of the negative electrode, such as gas such as CO ₂ , N ₂ O, CO, SO ₂ and polysulfide S _x ² , May be added at an arbitrary ratio.

Even when a polymer solid electrolyte is used, the type thereof is not particularly limited, and any known crystalline / amorphous inorganic substance can be used as the solid electrolyte. Examples of the crystalline inorganic solid electrolyte include LiI, Li ₃ N, Li _{1 + x} J _x Ti _2-x (PO ₄ ) ₃ (J = 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 the above-described organic electrolyte is used as the electrolyte, a separator is 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 not particularly limited, 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. Preferable examples include microporous films, sheets, nonwoven fabrics and the like 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. In particular, from the viewpoint of chemical and electrochemical stability, which is an important factor for separators, polyolefin polymers are preferable. From the viewpoint of self-occluding temperature, which is one of the purposes of use of separators in batteries, polyethylene is preferred. Is particularly desirable.

  When using a separator made of polyethylene, it is preferable to use ultra-high molecular weight polyethylene from the viewpoint of maintaining high-temperature shape, and the lower limit of the molecular weight is preferably 500,000, more preferably 1,000,000, and most preferably 1,500,000. On the other hand, the upper limit of the molecular weight is preferably 5 million, more preferably 4 million, and most preferably 3 million. This is because if the molecular weight is too large, the fluidity becomes too low, and the pores of the separator may not close when heated.

  The lithium secondary battery of the present invention is produced by assembling the above-described positive electrode for a lithium secondary battery of the present invention, a negative electrode, an electrolyte, and a separator used as necessary into an appropriate shape. Furthermore, other components such as an outer case can be used as necessary.

  The shape of the lithium secondary battery of the present invention is not particularly limited, and can be appropriately selected from various commonly employed shapes according to the application. Examples of commonly used shapes include a cylinder type with a sheet electrode and separator spiral, an inside-out cylinder type with a combination of pellet electrode and separator, a coin type with a stack of pellet electrode and separator, etc. Can be mentioned. The method for assembling the battery is not particularly limited, and can be appropriately selected from various commonly used methods according to the shape of the target battery.

  The general embodiment of the lithium secondary battery of the present invention has been described above. However, the lithium secondary battery of the present invention is not limited to the above-described embodiment, and various modifications are possible as long as the gist thereof is not exceeded. Can be implemented.

Production Example 1
[Production of FePO ₄ ]
Fe a _{(NO 2) 3 · 9H 2} O and a solution of 0.1 mol / L was dissolved in water _to a solution of Na ₂ HPO 4 · 12H ₂ O for dissolved as well in water 0.1 mol / L did. These two solutions were mixed, and the resulting precipitate was filtered, washed and dried, and then heat-treated in air at 600 ° C. for 6 hours to obtain FePO ₄ .

[Production of positive electrode active material]
LiOH.H ₂ O was added to the obtained FePO _{4 so as} to have a predetermined composition, and further tetraethylene glycol was added so as to be 10% by mass, and then well pulverized and mixed in an agate mortar. Thereafter, 12 hours calcining in a mixed 600 ° C. in a gas of argon and hydrogen, to obtain a cathode active material containing LiFePO _4.

[Production of evaluation battery]
The obtained positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder were weighed so as to be 90: 5: 5, and then mixed in N-methylpyrrolidone as a solvent to obtain a slurry. The slurry was applied on an aluminum (Al) foil, dried, and punched out to a predetermined size to produce a positive electrode.

The CR 2032 type coin cell was assembled using the positive electrode, Li foil as the negative electrode, “electrolyte solution in which LiPF ₆ was dissolved at 1 mol / L in a solvent of ethylene carbonate: diethyl carbonate = 1: 1 (volume ratio)” and a separator. A battery for evaluation was obtained.

[Battery performance evaluation]
The prepared battery for evaluation was charged at 25 ° C. to 4.0 V at a constant current, and then discharged at a constant current to 2.8 V, and the charge / discharge capacity was calculated from the amount of current and the mass of the active material. The current value at this time was set to a value of 17 mA per 1 g of active material.

Production Example 2
A positive electrode active material containing LiFePO ₄ was obtained in the same manner as in Production Example 1 except that the firing temperature in Production Example 1 was 700 ° C.

Production Example 3
A positive electrode active material containing LiFePO ₄ was obtained in the same manner as in Production Example 1 except that the firing temperature in Production Example 1 was 600 ° C.

Production Example 4
A positive electrode active material containing LiFePO ₄ was obtained in the same manner as in Production Example 1 except that the firing temperature in Production Example 1 was 900 ° C.

  Table 1 shows the values of D / G and F / G calculated from the Raman spectrum measurement of the positive electrode active materials obtained in Production Examples 1 to 4. In addition, Table 1 shows initial charge / discharge capacities of lithium secondary batteries manufactured using the positive electrode active materials obtained in Production Examples 1 to 4.

  As is apparent from Table 1, in Production Examples 1 to 3 in which D / G is 0.5 or more and F / G is 1.5 or less, the initial discharge capacity is 140 mAh / g or more, which shows good battery performance. I understood that. On the other hand, in Production Example 4, the initial discharge capacity was as low as 50 mAh / g.

  Table 2 shows values of firing temperature, charge / discharge capacity, specific surface area, and primary particle size in Production Examples 1 to 3 and Production Example 4. In addition, FIGS. 3 and 4 show examples of TEM photographs of Production Example 1 and Production Example 4 used for calculating the primary particle size.

As is apparent from Table 2, in Production Examples 1 to 3 where the firing temperature is 850 ° C. or less, the primary particle diameter is 200 nm or less and the specific surface area is 7 m ² / g or more. As shown, in the production example 4 in which the primary particle diameter was 200 nm or more and the specific surface area was less than 7 m ² / g, the battery performance was deteriorated.

Production Example 5
In the production of the positive electrode active material containing LiFePO ₄ of Production Example 1, as in Production Example 1, except that sucrose was used instead of tetraethylene glycol and lithium carbonate was used instead of LiOH · H ₂ O. A positive electrode active material containing LiFePO ₄ was obtained.

  Table 3 shows the C concentration values of the positive electrode active materials obtained in Production Examples 1 to 5. Table 3 also shows the initial charge / discharge capacities of the lithium secondary batteries produced using the positive electrode active materials obtained in Production Examples 1 to 5.

  As can be seen from Table 3, in Production Examples 1 to 3, good battery performance is shown, but in Production Example 4 in which the C concentration is less than 0.6, battery performance is lowered due to insufficient conductivity due to low carbon. It is thought that. In addition, in Production Example 5 in which the C concentration is higher than 1.5, it is considered that the battery performance is lowered due to the reduction of the reaction area due to excessive carbon.

Production Example 6
In the production of the positive electrode active material containing LiFePO ₄ in Production Example 1, a positive electrode active material containing LiFePO ₄ was obtained in the same manner as in Production Example 1 except that firing was performed without adding tetraethylene glycol to FePO _4. It was.

  Table 4 shows the relationship between the carbon source and battery performance in Production Example 1, Production Example 5 and Production Example 6.

  As shown in Table 4, those with tetraethylene glycol added showed the best performance, and those without sucrose addition or carbon source addition had low battery performance.

Production Example 7
In the production of the positive electrode active material containing FePO ₄ of Production Example 1, a positive electrode active material containing LiFePO ₄ was obtained in the same manner as Production Example 1 except that no heat treatment was performed after drying.

Table 5 shows the difference in battery performance between Production Example 3 and Production Example 7 depending on the presence or absence of heat treatment of the raw material FePO ₄ .

As is clear from Table 5, in Production Example 7 in which the heat treatment of FePO ₄ was not performed, the discharge capacity was reduced, indicating that the heat treatment of FePO ₄ is effective in improving the battery performance.

  The use of the positive electrode active material for a lithium secondary battery of the present invention and the lithium secondary battery using the active material 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, cameras, automobile power sources, and the like.

2 is a Raman spectrum of a positive electrode active material containing FePO ₄ obtained in Production Example 1. ₄ is a Raman spectrum of a positive electrode active material containing FePO ₄ obtained in Production Example 4. ₄ is a TEM photograph of a positive electrode active material containing FePO ₄ obtained in Production Example 1. ₄ is a TEM photograph of a positive electrode active material containing FePO ₄ obtained in Production Example 4. Is a diagram illustrating the battery performance of the positive electrode active material containing FePO ₄ obtained in Preparation Example 1-4.

Claims (12)

It is represented by the general formula Li _x FePO ₄ (0.95 ≦ x ≦ 1.05), and the peak intensity existing at 1325 to 1365 cm ⁻¹ in the Raman spectroscopy is D, and the peak existing at 1560 to 1610 cm ⁻¹ Lithium secondary, characterized in that D / G is 0.5 or more and F / G is 1.5 or less, where G is strength and F is peak intensity existing at 900 to 1200 cm ^−1. Positive electrode active material for batteries.   The positive electrode active material for a lithium secondary battery according to claim 1, wherein the C concentration in the active material is 0.6 mass% or more and 1.5 mass% or less. 3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the specific surface area by the BET method is 7 to 30 m ² / g.   The positive electrode active material for a lithium secondary battery according to any one of claims 1 to 3, wherein the primary particle size is 200 nm or less. In a method for producing a positive electrode active material for a lithium secondary battery represented by a general formula Li _x FePO ₄ (0.95 ≦ x ≦ 1.05), tetraethylene glycol and a Li source are added to FePO ₄ and fired. The manufacturing method characterized by the above-mentioned. The manufacturing method according to claim 5, wherein the heat treatment is separately performed on FePO ₄ before adding tetraethylene glycol and a Li source and firing.   The method according to claim 5 or 6, wherein the firing temperature is 850 ° C or lower. When manufacturing method of positive active material for a lithium secondary battery according to any one of claims 1 to 4, the manufacturing method and performing firing with the addition of tetraethylene glycol and Li sources FePO _4. The manufacturing method according to claim 8, wherein the FePO _{4 is} subjected to a separate heat treatment before adding tetraethylene glycol and a Li source and firing.   The manufacturing method according to claim 8 or 9, wherein the firing temperature is 850 ° C or lower.   The lithium secondary battery using the positive electrode active material for lithium secondary batteries in any one of Claim 1 to 4.   The lithium secondary battery using the positive electrode active material manufactured by the manufacturing method in any one of Claim 5 to 10.