JP2015118874A - Positive electrode material for lithium batteries, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode material which enables the improvement of battery characteristics.SOLUTION: A positive electrode material for lithium batteries comprises: (A) an active material of an olivine structure expressed by the general formula (I), LiMYPO(where M represents at least one transition metal selected from a group consisting of Mn, Fe, Co and Ni; Y is at least one transition metal selected from a group consisting of Mn, Fe, Co, Ni, Mg, Cr, Ti, Al, Zn, Cu and V, but different from M; and 0<x≤1); and (B) a coating film of a carbon material complex formed on at least part of the surface of each particle of the active material. The coating film includes a first carbon material forming a carbon layer deposited on the surface of each active material particle, and a fibrous second carbon material having an average fiber length of 1 μm or less in the proportions of 25:75 to 62.5:37.5 by mass ratio. The carbon material complex coating films formed on at least part of the surfaces of the adjacent active material particles are connected with each other through at least part of the fibrous second carbon material.

Description

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

As a positive electrode material of a lithium ion secondary battery, phosphate LiMPO ₄ (M = Mn, Fe, Co, Ni) (hereinafter referred to as “olivine phosphate”) having an olivine structure has high stability and high It has attracted attention because of its capacity. However, olivine-type phosphate has a drawback that its rate characteristics are very poor because of its extremely low electronic conductivity. Of the olivine-type phosphates, olivine-type LiMnPO ₄ exhibiting a high operating potential has extremely low conductivity, so that it is very difficult to obtain battery characteristics even at a low rate.

  As a technique for increasing the electronic conductivity of olivine-type phosphate, it is proposed in Patent Document 1 to disperse carbon material fine particles in olivine-type phosphate particles. Conductivity of olivine-type phosphate and carbon, etc. Patent Document 2 proposes that a solution or suspension containing an active substance is spray pyrolyzed and then fired. In order to obtain a high-capacity electrochemical device that can be charged / discharged at high speed, it is proposed in Patent Document 3 to form a composite in which a carbon material such as carbon black is coated with olivine-type lithium phosphate. . In Patent Document 4, in order to suppress a decrease in the permeability of the electrolyte due to the densification of the electrodes, the electrolyte penetrates between the electrode active material powders by including carbon fibers in the positive electrode for the lithium battery. It has been proposed to create fine voids to facilitate. Patent Document 5 proposes to disperse a conductive material such as fibrous carbon in the positive electrode in order to improve the binding property at the interface between the current collector and the mixture layer. Patent Document 6 describes that a composite of fibrous carbon and carbon black and an active material are mechanically mixed by a cracking machine. Patent Document 7 describes that after a first conductive material made of a carbon material is attached to the surface of the active material particles, a second conductive material made of a fibrous carbon material is mechanochemically bonded to the first conductive material. Has been. Patent Document 8 describes a mechanochemical reaction between conductive carbon black having a specific surface area, graphite having a specific surface area or activated carbon having a specific surface area, and a lithium manganese phosphate precursor. ing.

JP 2003-36889 A JP 2004-259471 A JP 2005-123107 A JP 2013-65482 A JP 2006-86116 A JP 2011-108522 A JP 2009-43514 A Special table 2011-517053 gazette

However, in the techniques described in Patent Documents 1 and 2, since the carbon material fine particles or the conductive substance are taken into the olivine-type phosphate particles, an electron conduction path is provided between the olivine-type phosphate particles. It cannot be formed sufficiently. Moreover, in the technique described in Patent Document 3, since the carbon material is coated with olivine-type lithium phosphate, sufficient battery characteristics cannot be achieved.
Therefore, the problem to be solved by the present invention is to further improve battery characteristics in a positive electrode material containing olivine-type phosphate as an active material.

According to the present invention,
(A) The following general formula (I):
LiM _x Y _(1-x) PO ₄
(Wherein, M is at least one transition metal selected from the group consisting of Mn, Fe, Co and Ni, Y is a transition metal different from M, and Mn, Fe, Co, Ni, Mg And at least one transition metal selected from the group consisting of Cr, Ti, Al, Zn, Cu and V, and 0 <x ≦ 1)
A particulate active material comprising a lithium transition metal phosphate having an olivine structure represented by:
(B) a carbon material composite film formed on at least a part of the surface of the active material particles;
Including
The coating of the carbon material composite is
A first carbon material forming a carbon layer attached to the surface of the active material particles;
A fibrous second carbon material having an average fiber length of 1 μm or less;
In a mass ratio of first carbon material: second carbon material = 25: 75 to 62.5: 37.5,
Provided is a positive electrode material for a lithium battery in which carbon layers attached to the surfaces of a plurality of adjacent active material particles are connected to each other through at least a part of the fibrous second carbon material.

Furthermore, according to the present invention,
(I) The following general formula (I):
LiM _x Y _(1-x) PO ₄
(Wherein, M is at least one transition metal selected from the group consisting of Mn, Fe, Co and Ni, Y is a transition metal different from M, and Mn, Fe, Co, Ni, Mg And at least one transition metal selected from the group consisting of Cr, Ti, Al, Zn, Cu, and 0 <x ≦ 1)
A particulate active material comprising a lithium transition metal phosphate having an olivine structure represented by:
A first carbon material precursor in the form of tabular grains, spherical grains, or mixtures thereof;
A fibrous second carbon material precursor;
A first carbon material precursor: second carbon material precursor = mechanically milling at a mass ratio of 25:75 to 62.5: 37.5 to obtain a positive electrode material precursor; and (ii) the positive electrode Calcining a material precursor in an inert atmosphere to obtain a positive electrode material in which a coating of a carbon material composite is formed on at least a part of the surface of the particles of the active material, wherein the carbon material composite The coating is
A first carbon material attached to the surface of the active material particles to form a carbon layer;
A fibrous second carbon material having an average fiber length of 1 μm or less;
In a mass ratio of first carbon material: second carbon material = 25: 75 to 62.5: 37.5,
Carbon layers attached to the surfaces of a plurality of adjacent active material particles are connected to each other through at least a part of the fibrous second carbon material;
A method for producing a positive electrode material for a lithium battery is provided.

FIG. 1 is a schematic enlarged cross-sectional view of the positive electrode material of the present invention. FIG. 2 is a graph showing the battery characteristics of Examples 1 to 6 and Comparative Examples 1 to 4. FIGS. 3A and 3B show scanning electron microscope (SEM) images before and after ball milling of the positive electrode material of Example 1, respectively.

Hereinafter, the positive electrode material and the manufacturing method thereof of the present invention will be described in detail.
As described above, the positive electrode material of the present invention is selected from the group consisting of (A) general formula (I): LiM _x Y _(1-x) PO ₄ (wherein M is Mn, Fe, Co, and Ni). Is at least one transition metal, and Y is a transition metal different from M, and is at least one selected from the group consisting of Mn, Fe, Co, Ni, Mg, Cr, Ti, Al, Zn, Cu, and V A particulate active material comprising a lithium transition metal phosphate having an olivine structure represented by 0 <x ≦ 1), and (B) at least the surface of the particles of the active material And a carbon material composite film formed in part. The film of the carbon material composite includes a first carbon material forming a carbon layer attached to the surface of the active material particles and a fibrous second carbon material having an average fiber length of 1 μm or less. Carbon material: Second carbon material = Included in a mass ratio of 25:75 to 62.5: 37.5.

In the positive electrode material of the present invention, the particulate active material (A) composed of a lithium transition metal phosphate having an olivine structure preferably contains manganese (Mn), more preferably LiMn _x Fe _{(1-x )} PO ₄ (where 0 <x ≦ 1). The active material comprising a lithium transition metal phosphate having an olivine structure can be prepared according to a known method or can be commercially available. The active material (A) preferably has an average particle size of 1 μm or less. In the positive electrode material of the present invention, the average particle diameter of the active material is a particle diameter d50 corresponding to a 50% cumulative value based on the number in the particle size distribution of the equivalent circle diameter obtained by image analysis with a scanning electron microscope (SEM). Means. Since the particulate active material having such a small average particle size has a very high specific surface area, it is possible to secure a surface on which lithium ion insertion / extraction reaction occurs. The positive electrode material of the present invention is preferably an active material having a total mass of 100 parts by mass of the first carbon material and the second carbon material contained in the positive electrode material, preferably 50 to 95 parts by mass, more preferably 70 to 95 parts by mass. (A) is included. If the amount of the active material (A) in the positive electrode material is too small, the function as the positive electrode cannot be sufficiently exerted. On the other hand, if the amount of the active material (A) is too large, molding may be difficult. There is.

  In the coating (B) of the carbon material composite, the first carbon material forms a carbon layer at the interface with the active material particles, and the carbon layer adheres to the surface of the active material particles. In order to secure an electron conduction path, the thickness of the first carbon material deposited on the active material particles is preferably as small as possible, and is preferably 50 nm or less. The fibrous second carbon material has an average fiber length of 1 μm or less, preferably 0.1 μm to 1 μm, more preferably 0.3 to 1 μm. The average fiber length of the second carbon material means a length d50 corresponding to a 50% cumulative value based on the number distribution in the length distribution obtained by image analysis of SEM.

  A good electron conduction network is formed in the positive electrode material of the present invention by the carbon layer formed of the first carbon material and the fibrous second carbon material. In the positive electrode material of the present invention, the carbon layers attached to the surfaces of the plurality of adjacent active material particles are connected to each other through at least a part of the fibrous second carbon material. A large number of electron conduction paths are ensured between the particles, and it is considered that a positive electrode material exhibiting a high battery capacity can be obtained. As the average fiber length of the second carbon material becomes longer, the carbon layers deposited on the surfaces of a plurality of adjacent active material particles per unit length or unit mass of the second carbon material are connected to each other by the second carbon material. It is believed that the number of electron conduction paths will decrease and the number of electron conduction paths will decrease accordingly. Therefore, if the average fiber length of the second carbon material is too long, it is difficult to ensure a sufficient electron conduction path by the second carbon material. An amount of the second carbon material of less than 37.5% by mass based on the total mass of the first carbon material and the second carbon material is not sufficient to increase the number of electron conduction paths. When the ratio of the second carbon material exceeds 75% by mass based on the total mass of the first carbon material and the second carbon material, the ratio of the first carbon material that can adhere to the surface of the active material particles and form a carbon layer It becomes difficult to sufficiently increase the conductivity between the active material particles. In addition to the increase in the electron conduction path in the positive electrode material, the higher the electron conductivity of the first carbon material, the easier the electrons flow through the carbon layer on the active material particles. It is thought that can be obtained. The positive electrode material of the present invention preferably has a density of 1.3 to 2.7 g / cc. If the density of the positive electrode material is too low, the electron conduction path decreases, and if the density of the positive electrode material is too high, the diffusibility of lithium ions is considered to decrease.

  In FIG. 1, the typical expanded sectional view of the positive electrode material of this invention is shown. In FIG. 1, a positive electrode material 10 includes active material particles 12, and a coating of a carbon material composite including a first carbon material 14 and a fibrous second carbon material 16 on at least a part of the surface of the active material particles 12. Is formed. The first carbon material forms a carbon layer attached to the surface of the active material particles. The carbon layers attached to the surfaces of the adjacent active material particles 12 are connected to each other through a fibrous second carbon material. As shown in FIG. 1, carbon layers on adjacent active material particles may be in contact with each other.

The positive electrode material of the present invention comprises (i) a particulate active material comprising a lithium transition metal phosphate having an olivine structure represented by the general formula (I),
A first carbon material precursor in the form of tabular grains, spherical grains, or mixtures thereof;
A fibrous second carbon material precursor;
And (ii) firing the positive electrode material precursor in an inert atmosphere to form a carbon material composite on at least a part of the surfaces of the active material particles. Obtaining a positive electrode material having a coating formed thereon;
It can manufacture by the method containing.
In the positive electrode material obtained by the above production method, the coating of the carbon material composite is
A first carbon material attached to the surface of the active material particles to form a carbon layer;
A fibrous second carbon material having an average fiber length of 1 μm or less;
In a mass ratio of first carbon material: second carbon material = 25: 75 to 62.5: 37.5,
The coatings of the carbon material composite formed on at least a part of the surfaces of a plurality of adjacent active material particles are connected to each other via at least a part of the fibrous second carbon material.

Examples of the carbon material that can be used as the first carbon material precursor in the form of tabular particles, spherical particles, or a mixture thereof in the above method for producing a positive electrode material include graphite and spherical hollow carbon fine particles (for example, hollow shell-shaped particles). Ketjen black, which is a carbon black having a structure). When graphite is used as the first carbon material precursor, the fine particles of graphite preferably have an average particle size in the range of 0.1 μm to 100 μm. The spherical hollow carbon fine particles (for example, ketjen black) preferably have an average particle size in the range of 10 nm to 50 nm, more preferably 15 nm to 30 nm. The average particle diameter of the first carbon material precursor means a particle diameter d50 corresponding to a 50% cumulative value based on the number in the particle size distribution of the equivalent area circle diameter obtained by image analysis with a scanning electron microscope (SEM). . As the spherical hollow carbon fine particles, ketjen black is preferable. Both graphite and ketjen black are preferable because they form a carbon layer in close contact with the surface of the active material particles by mechanical milling. Both graphite and ketjen black are generally known to have high conductivity. However, ketjen black usually has lower crystallinity (or degree of graphitization) than graphite, and therefore graphite. Lower powder resistance. When used in the positive electrode material of the present invention, the graphite preferably has a powder resistance of 1 × 10 ⁻³ Ω · cm or less when measured at a temperature of 25 ° C., and the ketjen black has a temperature of 25 ° C. It is preferable to have a powder resistance of 1 × 10 ⁻² Ω · cm or less. As the first carbon material precursor, graphite is more preferable. The higher the conductivity of the first carbon material, the easier it is for electrons to flow through the carbon layer formed on the active material particles. However, in order to improve the battery capacity of the positive electrode material, the carbon material composite itself has a higher electron. In addition to having conductivity, it is considered necessary that a large number of electron conduction paths exist between a plurality of adjacent active material particles.

  Examples of the carbon material that can be used as the fibrous second carbon material precursor include carbon nanofibers, carbon nanotubes, vapor grown carbon fibers (VGCF), and the like. The fibrous second carbon material precursor has an average length of 10 μm or less, preferably 0.1 μm to 10 μm, more preferably 0.2 μm to 2 μm. The average length of the second carbon material precursor means a length d50 corresponding to a 50% cumulative value on a number basis in the length distribution obtained by image analysis of SEM. The fibrous second carbon material precursor preferably has a fiber diameter of 50 nm to 300 nm.

  Simultaneously compounding the first carbon material precursor and the second carbon material precursor by mechanically milling the active material particles, the first carbon material precursor and the second carbon material precursor in the step (i). A film of the carbon material composite precursor including the first carbon material precursor and the second carbon material precursor is formed on at least a part of the surface of the active material particles. In step (i), before mechanically milling the active material particles, the first carbon material precursor, and the second carbon material precursor, the first carbon material precursor and the second carbon material precursor are mixed in advance. Also good. The mechanical milling in the step (i) can be performed using a mechanical milling device such as a ball mill at a temperature within a range of 25 to 300 ° C. in an air atmosphere. Mechanical milling is a mechanical sufficient to form a coating of a carbon material composite precursor on at least a part of the surface of the active material particles at the same time that the first carbon material precursor and the second carbon material precursor are combined. It is carried out with a certain amount of energy and milling time. For example, when mechanical milling is performed using a ball mill, the ball mill time is preferably 10 hours or longer and 40 hours or shorter. When the ball mill time is less than 10 hours, the first carbon material precursor and the second carbon material precursor are combined, and at the same time, the first carbon material precursor and the second carbon material precursor are formed on at least a part of the surface of the active material particles. It is insufficient to form a carbon material composite precursor film containing a body, and when the time exceeds 40 hours, the rate of carbon material composite precursor film formation on the surface of the active material particles is saturated. When the fibrous second carbon material precursor has an average fiber length exceeding 1 μm, the first carbon material precursor and the second carbon material precursor are combined and at least on the surface of the active material particles In addition to forming a coating of the carbon material composite precursor in part, in the positive electrode material obtained after mechanical milling, mechanical energy sufficient to make the average fiber length of the second carbon material precursor less than 1 μm Perform mechanical milling with quantity and milling time.

  After forming the positive electrode material precursor in which the coating of the carbon material composite precursor is formed on at least a part of the surface of the active material particles by the step (i), the positive electrode material precursor is deactivated in the step (ii). By baking in an atmosphere (for example, in an argon gas atmosphere), a positive electrode material for a lithium battery can be obtained. The firing temperature in step (ii) is preferably in the range of 500 ° C. to 900 ° C., more preferably in the range of 600 ° C. to 800 ° C., and the firing time is preferably 0.5 hours to 5 hours, more preferably. 1 to 3 hours. If the firing temperature is too high, or if the firing time is too long, the particles grow and the specific surface area decreases, so the capacity decreases.

  A positive electrode material manufactured as described above, and if necessary, a conductive material and a binder are mixed in a solvent to prepare a slurry-like composition, applied onto the positive electrode current collector, dried, and pressed. By doing so, a positive electrode in which a positive electrode mixture layer is formed on the positive electrode current collector can be obtained. A battery can be produced by arranging the obtained positive electrode so as to face the negative electrode with a separator interposed between the positive electrode and the electrolytic solution.

  Examples of the positive electrode current collector include, but are not limited to, aluminum, nickel, titanium, stainless steel, and the like having a foil shape, a plate shape, a net shape, and the like. The shape of the positive electrode current collector is appropriately selected according to the shape of the battery to be produced. Examples of the conductive material include acetylene black, graphite, nanotube, vapor grown carbon fiber (VGCF), and the like. Examples of the binder include fluorine-based binders such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), and rubber-based binders such as styrene butadiene rubber (SBR) and butadiene rubber (BR). It is not limited to.

  As an example of the solvent that can be used for forming the slurry, one or two or more kinds of solvents conventionally used in the production of non-aqueous electrolyte secondary batteries can be used, and amide solvents (for example, N-methyl) can be used. -2-pyrrolidone (NMP), N, N-dimethylformamide, N, N-dimethylacetamide), alcohol solvents (eg 2-propanol, ethanol, methanol), ketone solvents (eg acetone), ester solvents (eg Methyl acetate, ethyl acetate), amine solvents (for example, diethyltriamine), nitrile solvents (for example, acetonitrile), and the like.

  The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode can have the same configuration as a conventional negative electrode for a lithium battery, and its production method is not particularly limited. The negative electrode mixture layer includes at least a negative electrode active material (for example, carbon materials such as natural graphite, artificial graphite, hard carbon, and soft carbon, metal oxides such as titanium oxide, vanadium oxide, and lithium silicon composite oxide). A conductive material, a binder, and the like may be included as necessary. The negative electrode mixture layer is prepared, for example, by mixing a negative electrode active material and, if necessary, a conductive material and a binder in a solvent to prepare a slurry-like composition, and a negative electrode current collector (for example, a foil shape, a plate) To obtain a negative electrode in which a negative electrode mixture layer is formed on a negative electrode current collector by coating, drying, and pressing on copper, nickel, titanium, stainless steel, etc. having a shape such as a net or a net. it can.

[Example 1]
<Preparation of positive electrode material>
1. Synthesis of positive electrode active material A positive electrode active material was synthesized using a sol-gel method. Lithium acetate dihydrate (Nacalai Tesque), ammonium dihydrogen phosphate (Nacalai Tesque), manganese acetate tetrahydrate (Nacalai Tesque), iron acetate (Aldrich) 1L pure while adjusting with concentrated nitric acid so that the pH is kept at 1.5 or less at a molar ratio of Li: Mn: Fe: P = 1: 0.8: 0.2: 1. Dissolved in water. Thereafter, glycolic acid (manufactured by Nacalai Tesque Co., Ltd.) having a molar amount five times that of the target compound LiMn _0.8 Fe _0.2 PO ₄ is used as a chelating agent for particle growth inhibition. added. The obtained sol-like solution was immersed in an oil bath at 80 ° C. and stirred for about 20 hours to evaporate water, thereby obtaining a gel-like active material precursor. Next, the obtained precursor was put into a drying furnace at 80 ° C. and further dried for 24 hours. Thereafter, the precursor was temporarily calcined at 200 ° C. in an argon atmosphere. The obtained powder was pulverized in a mortar and then calcined at 600 ° C. for 1 hour in an argon atmosphere to obtain a positive electrode active material LiMn _0.8 Fe _0.2 PO ₄ .
2. Coating of positive electrode active material with carbon material composite Active material obtained as described above, Ketjen Black (manufactured by Ketjen Black International Co., Ltd.) and vapor grown carbon fiber (VGCF) (Showa Denko Co., Ltd.) VGCF-H, average fiber length: 5 μm) was previously mixed in a mortar for 15 minutes, and then charged into a ball mill pot. Then, a ball having a mass 30 times that of the active material was added to the planetary ball mill device P- 7 (manufactured by Fritsch) was used and mechanical milling was performed at a temperature of 25 ° C. and 300 rpm for 25 hours. Based on the total mass of KB (first carbon material) and VGCF (second carbon material), the mass% of VGCF (second carbon material) was 37.5 mass%.

<Preparation of positive electrode>
The positive electrode material produced as described above, acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were changed to N-methyl-2-pyrrolidone (manufactured by Nacalai Tesque Co., Ltd.). A slurry was obtained by dispersing in pyrrolidone (manufactured by Nacalai Tesque). The obtained slurry was applied onto an aluminum foil (thickness: 15 μm) by a doctor blade method, dried at 80 ° C. for 30 minutes, and then pressed by a roll press so that the electrode density was 2 g / cc. The electrode was obtained by vacuum drying with a punching machine and punched into a circle. The mass ratio of the positive electrode active material coated with the carbon material composite, acetylene black, and PVDF was 75: 20: 5.
<Production of battery>
The positive electrode and the negative electrode (lithium metal) produced as described above are arranged so as to face each other through a separator (polypropylene (PP) -polyethylene (PE) laminated porous film), and accommodated in a SUS coin-type case together with the electrolyte. Then, a lithium secondary battery was produced. The electrolytic solution is obtained by dissolving electrolyte LiPF ₆ at a ratio of 1 mol / liter in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 3: 4. It was prepared by.

[Example 2]
In the production of the positive electrode material, the amount of the carbon material composite with respect to the active material is not changed, and the amount of VGCF (second carbon material) based on the total mass of KB (first carbon material) and VGCF (second carbon material). A battery was produced in the same manner as in Example 1 except that the content of was changed to 62.5% by mass.

[Example 3]
In the production of the positive electrode material, the amount of the carbon material composite with respect to the active material is not changed, and the amount of VGCF is changed to 75% by mass based on the total mass of KB (first carbon material) and VGCF (second carbon material). A battery was fabricated in the same manner as in Example 1 except that.

[Example 4]
In the production of the positive electrode material, graphite (Gr) (manufactured by Nacalai Tesque) was used instead of KB as the first carbon material without changing the amount of the carbon material composite with respect to the active material. A battery was fabricated in the same manner as in Example 1 except that the amount of VGCF (second carbon material) was changed to 37.5% by mass based on the total mass of the material) and VGCF (second carbon material).

[Example 5]
The amount of VGCF (second carbon material) based on the total mass of graphite (first carbon material) and VGCF (second carbon material) without changing the amount of the carbon material composite with respect to the active material in the production of the positive electrode material A battery was fabricated in the same manner as in Example 4 except that the content was changed to 62.5% by mass.

[Example 6]
The amount of VGCF (second carbon material) based on the total mass of graphite (first carbon material) and VGCF (second carbon material) without changing the amount of the carbon material composite with respect to the active material in the production of the positive electrode material A battery was fabricated in the same manner as in Example 4 except that the content was changed to 75% by mass.

[Comparative Example 1]
A battery was produced in the same manner as in Example 1 except that the positive electrode was produced without mechanically milling the positive electrode active material, ketjen black and VGCF with a planetary ball mill apparatus.

[Comparative Example 2]
A battery was produced in the same manner as in Example 1 except that in the production of the positive electrode material, all of VGCF was replaced with KB.

[Comparative Example 3]
A battery was produced in the same manner as in Example 1 except that in the production of the positive electrode material, all KB was replaced with VGCF.

<Evaluation of battery capacity>
The batteries of Examples 1 to 6 and Comparative Examples 1 to 4 are charged in a constant current / constant voltage mode with an upper limit of 4.8 V at a current rate of 0.1 C and an actual capacity of 150 mAh / g at a temperature of 25 ° C. Thereafter, the battery was discharged to 3 V and the discharge capacity was measured. The measurement results are shown in Table 1 and FIG.

  Furthermore, the powder resistance of graphite (Gr), ketjen black (KB), and VGCF used for the production of the positive electrode material was measured by a four-point probe method. The apparatus used was a powder resistance system (Mitsubishi Chemical Analytech Co., Ltd.). After the powder was compacted at 20 MPa, the electrical resistance was measured. The measurement results are shown in Table 2 below.

  From Table 1 and FIG. 2, it can be seen that a positive electrode material containing a flat or spherical first carbon material and a fibrous second carbon material in a specific ratio within the scope of the present invention exhibits a high discharge capacity. . Further, from the results shown in Table 1 and FIG. 2 and the measurement results of the powder resistance of the carbon material shown in Table 2, the positive electrode material is a graphite having a low powder resistance as the first carbon material, that is, a highly conductive graphite. It can be seen that the discharge capacity increases when it contains. From these results, the higher the conductivity of the first carbon material, the more the electrons pass through the carbon material composite on adjacent active material particles connected via the fibrous second carbon material constituting the carbon material composite. Therefore, it is considered that a positive electrode material having a high battery capacity can be obtained. In addition, compared with the battery of Comparative Example 1 including the positive electrode material prepared by simply mixing VGCF and the carbon material, the batteries of Examples 1 and 4 including the positive electrode material prepared by the ball mill as the positive electrode material were: The battery capacity increased significantly.

  Next, for the positive electrode material of Example 1, the surface of each sample collected before and after ball milling was observed with a scanning electron microscope. The acceleration voltage was 20 kV and the current value was 10 μA. Prior to observation, an Au layer was deposited on the observation surface of the positive electrode material. SEM images are shown in FIGS. 3 (a) and 3 (b). FIG. 3A shows an SEM image of a sample collected before ball milling, and FIG. 3B shows an SEM image of a sample collected after ball milling. From comparison between FIG. 3 (a) and FIG. 3 (b), VGCF is finely divided by ball milling, and the active material particles are coated with a carbon material composite of ketjen black and finely divided VGCF. You can see that By finely dividing VGCF, the number of carbon layers on the surface of a plurality of adjacent active material particles per unit length or unit mass of VGCF is increased, and the number of electron conduction paths is increased. It is thought to increase.

  The positive electrode material for a lithium battery of the present invention makes it possible to obtain a lithium battery having high stability, high capacity, and high battery characteristics.

DESCRIPTION OF SYMBOLS 10 Positive electrode material 12 Active material particle 14 1st carbon material 16 Fibrous 2nd carbon material

Claims (3)

(A) The following general formula (I):
LiM _x Y _(1-x) PO ₄
(Wherein, M is at least one transition metal selected from the group consisting of Mn, Fe, Co and Ni, Y is a transition metal different from M, and Mn, Fe, Co, Ni, Mg And at least one transition metal selected from the group consisting of Cr, Ti, Al, Zn, Cu and V, and 0 <x ≦ 1)
A particulate active material comprising a lithium transition metal phosphate having an olivine structure represented by:
(B) a carbon material composite film formed on at least a part of the surface of the active material particles;
Including
The coating of the carbon material composite is
A first carbon material forming a carbon layer attached to the surface of the active material particles;
A fibrous second carbon material having an average fiber length of 1 μm or less;
In a mass ratio of first carbon material: second carbon material = 25: 75 to 62.5: 37.5,
A positive electrode material for a lithium battery, wherein carbon layers attached to the surfaces of a plurality of adjacent active material particles are connected to each other through at least a part of the fibrous second carbon material.   The positive electrode material according to claim 1, wherein M in the general formula (I) is Mn. (I) The following general formula (I):
LiM _x Y _(1-x) PO ₄
(Wherein, M is at least one transition metal selected from the group consisting of Mn, Fe, Co and Ni, Y is a transition metal different from M, and Mn, Fe, Co, Ni, Mg And at least one transition metal selected from the group consisting of Cr, Ti, Al, Zn, Cu and V, and 0 <x ≦ 1)
A particulate active material comprising a lithium transition metal phosphate having an olivine structure represented by:
A first carbon material precursor in the form of tabular grains, spherical grains, or mixtures thereof;
A fibrous second carbon material precursor;
A first carbon material precursor: second carbon material precursor = mechanically milling at a mass ratio of 25:75 to 62.5: 37.5 to obtain a positive electrode material precursor; and (ii) the positive electrode Calcining a material precursor in an inert atmosphere to obtain a positive electrode material in which a coating of a carbon material composite is formed on at least a part of the surface of the particles of the active material, wherein the carbon material composite The coating is
A first carbon material attached to the surface of the active material particles to form a carbon layer;
A fibrous second carbon material having an average fiber length of 1 μm or less;
In a mass ratio of first carbon material: second carbon material = 25: 75 to 62.5: 37.5,
A coating of carbon material composites formed on at least a part of the surface of a plurality of adjacent active material particles is connected to each other through at least a part of the fibrous second carbon material;
The manufacturing method of the positive electrode material for lithium batteries containing.