JP2013030292A - Olivine-type positive electrode active material, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material having high conductivity.SOLUTION: The positive electrode active material comprises a complex composed of an olivine-type compound represented by the formula (1): Lix(MyMyMy)PO, where 0<x≤2, each of y, y, and yis equal to or greater than 0 and equal to or smaller than 1.2, the sum total of y, y, and yis equal to or greater than 0.8 and equal to or smaller than 1.2, Mrepresents a 3d transition metal element, and Mand Meach represent a metal element, and a carbon allotrope.

Description

  The present invention relates to an olivine-based positive electrode active material used for a lithium ion secondary battery or the like and a method for producing the same.

  Lithium ion secondary batteries are excellent in energy density and output density, and are effective in reducing the size and weight. Therefore, the demand for lithium-ion secondary batteries as a power source for portable devices such as notebook computers, mobile phones, and handy video cameras is growing rapidly. In addition, it has been studied as a power source for hybrid vehicles and electric vehicles that have rapidly gained market in recent years.

  Conventionally, lithium cobaltate, lithium nickelate, cobalt-nickel composite oxide, and the like have been used as positive electrode active materials for lithium ion secondary batteries. However, since rare metals such as cobalt and nickel are used for these positive electrodes, there are problems such as an increase in cost of the lithium ion secondary battery and poor oxidation stability.

Therefore, in recent years, a positive electrode active material (olivine-based positive electrode active material) mainly composed of lithium iron phosphate (LiFePO ₄ ) having an olivine type crystal structure, which is a safe and relatively inexpensive phosphate, has been attracting attention. ing.
LiFePO ₄ uses a divalent iron salt such as iron phosphate Fe ₃ (PO ₄ ) ₂ or iron acetate Fe (CH ₃ COO) ₂ as an Fe source as a synthesis raw material, and is 800 ° C. in a reducing environment. It is synthesized by firing at a relatively high temperature for a long time (see Patent Document 1).

Moreover, it is synthesized by adding Fe powder as a reducing agent in the precursor in the mixing step (Patent Documents 2 and 3), adding polysaccharides and firing for a long time in a hydrogen atmosphere (Patent Documents). 4). This is because divalent Fe is sensitive to a small amount of oxygen contained in the synthesis atmosphere, and is easily oxidized and becomes trivalent. Therefore, trivalent iron compounds are likely to be mixed in the product, and single phase This is because it is difficult to obtain LiFePO ₄ .

JP-A-9-134725 International Publication No. WO00 / 60679 JP 2010-212250 A JP 2003-203628 A

The positive electrode active material mainly composed of lithium iron phosphate has low conductivity, and has a problem that the capacity of the battery is low particularly when discharging at a large current.
An object of the present invention is to provide a positive electrode active material having high conductivity.

As a result of intensive studies, the present inventors have found that a positive electrode active material obtained by mixing a carbon allotrope with a raw material of an olivine compound and firing it has high conductivity, and completed the present invention.
According to the present invention, the following positive electrode active materials and the like are provided.
1. A positive electrode active material in which an olivine compound represented by the following formula (1) and a carbon allotrope form a complex.
Lix (M ₁ y ₁ M ₂ y ₂ M ₃ y ₃ ) PO ₄ (1)
(In the formula, x is 0 <x ≦ 2.
y ₁ , y ₂ and y ₃ are each 0 or more and 1.2 or less, and the sum of y ₁ , y ₂ and y ₃ is 0.8 or more and 1.2 or less.
M ₁ is a 3d transition metal element, and M ₂ and M ₃ are each a metal element. )
2. 2. The positive electrode active material according to 1, wherein the carbon allotrope is a carbon nanotube.
3. 3. The positive electrode active material according to 1 or 2, wherein the carbon allotrope content is 3% by weight or less.
4). The positive electrode active material according to any one of 1 to 3, wherein the olivine-based compound is a single phase.
5. The positive electrode active material according to any one of 1 to 4, wherein the electric conductivity is 1.0 × 10 ⁻² S / cm or more.
6). The positive electrode active material according to any one of 1 to 5, wherein the carbon allotrope has a structure penetrating the outer shell of particles formed by the olivine-based compound.
7). The positive electrode active material according to any one of 1 to 6, wherein M ₁ is Fe, and y ₂ and y ₃ are both 0.
8). The manufacturing method of a positive electrode active material which has the process 1 which mixes the raw material of the olivine type compound represented by following formula (1), and a carbon allotrope, and the process 2 which heats the mixture obtained at the said process 1. .
Lix (M ₁ y ₁ M ₂ y ₂ M ₃ y ₃ ) PO ₄ (1)
(In the formula, x is 0 <x ≦ 2.
y ₁ , y ₂ and y ₃ are each 0 or more and 1.2 or less, and the sum of y ₁ , y ₂ and y ₃ is 0.8 or more and 1.2 or less.
M ₁ is a 3d transition metal element, and M ₂ and M ₃ are each a metal element. )
9. 9. The method for producing a positive electrode active material according to 8, wherein the carbon allotrope is a carbon nanotube (CNT).
10. The method for producing a positive electrode active material according to 8 or 9, wherein the carbon allotrope content is 3% by weight or less.
11. The manufacturing method of the positive electrode active material in any one of 8-10 which implements the said process 1 and the process 2 under a normal pressure.
12 In the said process 1, in addition to the said raw material and the said carbon allotrope, the manufacturing method of the positive electrode active material in any one of 8-11 which mixes water further.
13. The manufacturing method of the positive electrode active material in any one of 8-12 which makes the heating temperature of the said process 2 into 400 to 900 degreeC.
14 The positive electrode active material according to any one of 8 to 13, wherein the raw material of the compound represented by the formula (1) is a raw material in which M ₁ is Fe and y ₂ and y ₃ are both 0. Manufacturing method.
15. The manufacturing method of the positive electrode active material in any one of 8-14 whose said olivine type compound is a single phase.

  According to the present invention, a highly conductive positive electrode active material using an olivine compound can be provided.

2 is an electron micrograph of a positive electrode active material produced in Example 1. 2 is an X-ray diffraction chart of a positive electrode active material produced in Example 1. FIG.

The positive electrode active material of the present invention contains an olivine compound represented by the following formula (1) and a carbon allotrope, which form a composite.
Lix (M ₁ y ₁ M ₂ y ₂ M ₃ y ₃ ) PO ₄ (1)

In the formula (1), x is 0 <x ≦ 2.
y ₁ , y ₂ and y ₃ are each 0 or more and 1.2 or less, and the sum of y ₁ , y ₂ and y ₃ is 0.8 or more and 1.2 or less. Preferably, both y ₂ and y ₃ are 0.

M ₁ is a 3d transition metal element. Examples of the 3d transition metal include Fe, Ti, V, Cr, Mn, Co, Ni, and Cu. M ₁ is preferably Fe, Mn, Co, or Ni, and particularly preferably Fe.
M ₂ and M ₃ are each a metal element. Examples of the metal element include the 3d transition metal described above. M ₂ and M ₃ are each one of Mn, Co, and Ni, for example.
As a combination of M ₁ , M ₂ and M ₃ , for example, a combination in which M ₁ is Fe and M ₂ is Mn (y ₃ is 0), M ₁ is Fe and M ₂ is Co (y ₃ is 0). A combination in which M ₁ is Fe and M ₂ is Ni (y ₃ is 0), a combination in which M ₁ is Fe, M ₂ is Mn and M ₃ is Co, and M ₁ is Fe A combination in which M ₂ is Mn and M ₃ is Ni, and a combination in which M ₁ is Fe, M ₂ is Co, and M ₃ is Ni.

The elemental composition ratio (atomic ratio) of the olivine compound can be measured by an induction plasma emission analyzer (ICP-AES).
Whether it is an olivine compound can be confirmed by X-ray diffraction measurement. In the case of an olivine compound, a peak derived from the olivine crystal structure is observed. The identification of the crystal structure is based on a comparison with an X-ray diffraction chart (JCPDS No. 40-1499) of LiFePO ₄ .

The carbon allotropes include those having a crystal structure and those having no crystal structure, and any of them can be used.
Specific examples include carbon nanotubes (CNT), acetylene black, ketjen black, and graphite. Carbon nanotubes are preferred from the standpoint that extremely high conductivity can be imparted in a small amount, and that single-phase olivine crystals can be easily obtained. Here, the carbon nanotube is a general term for substances in which a six-membered ring network (graphene sheet) composed of carbon atoms has a single-layer or multi-layer coaxial tube, and there is a limit to the value of the diameter of the coaxial tube. Absent. The carbon nanotube may be either a single wall nanotube or a multi-wall nanotube. Multiwall nanotubes are preferred because of their availability.
As the carbon allotrope, those commercially available on the industry can be used.

  In the positive electrode active material of the present invention, the above-described olivine compound and carbon allotrope form a complex. The form of the complex is not particularly limited, and the olivine-based compound and the carbon allotrope may be in contact or bonded partially or entirely. For example, a form in which a carbon allotrope covers a part or the whole of the outer shell of the olivine compound particles, a form in which a part of the carbon allotrope is inserted in the outer shell of the olivine compound particles, and the like.

  In the composite of the present invention, an olivine compound and a carbon allotrope are prepared separately and then mixed by a mixing means such as a ball mill, or a raw material of the olivine compound and a carbon allotrope are prepared separately. In addition, a composite obtained by mixing by a mixing means such as a ball mill and then chemically reacting the raw material of the olivine compound is included. As the complex of the present invention, the latter complex is preferable compared to the former complex. That is, a composite obtained by separately preparing an olivine compound raw material and a carbon allotrope, mixing them by a mixing means such as a ball mill, and then chemically reacting the olivine compound raw material is preferable. In this case, it is inferred that the raw material of the olivine-based compound forms an olivine-based compound in a state in which part or all of the carbon allotrope particles and fibers are incorporated by chemical reaction.

  In the present invention, a form in which a part of the carbon allotrope is inserted into the outer shell of the olivine-based compound particle (a form in which the carbon allotrope penetrates the outer shell of the olivine particle) is preferable. By having this form, the conductivity can be improved up to the inside of the particle, so that the charge / discharge reaction can be performed efficiently.

In the positive electrode active material of the present invention, the carbon allotrope content is preferably 3% by weight or less. If it exceeds 3% by weight, the capacity per unit of the positive electrode active material may be small.
On the other hand, the carbon allotrope content is preferably 0.01% by weight or more. If the content is less than 0.01% by weight, the effect of adding the carbon allotrope may not be exhibited.
The carbon allotrope content in the positive electrode active material can be measured by a high frequency combustion method.

  In the present invention, in particular, a composite obtained by firing a mixture in which carbon nanotubes are used as a carbon allotrope and carbon nanotubes are mixed with a raw material of an olivine compound is preferable. Thereby, the composite_body | complex which has the form which the carbon nanotube penetrated the olivine particle outer shell is obtained.

  Further, by firing a mixture of carbon nanotubes mixed with a raw material of the olivine compound, single-phase olivine-type crystal particles can be obtained. The reason why carbon nanotubes promote single-phase particle growth of olivine-type phosphate is not always clear, but the crystal lattice size of olivine-type phosphate depends on the carbon atom bond distance, bond angle, etc. It seems that this is consistent.

  In addition, by using carbon nanotubes, a positive electrode active material having extremely high conductivity is obtained despite a small amount. The improvement in electrical conductivity is due to the contact between the carbon nanotubes. Since the carbon nanotube has a high aspect ratio, it is presumed that the conductivity can be improved with a small amount.

In the present invention, the olivine-based compound is preferably composed of a crystalline phase having an olivine structure, that is, a single phase (single phase). By being a single phase, the capacity per unit is increased. The single phase is confirmed by X-ray diffraction measurement. For example, when the olivine compound is LiFePO ₄ , the peak intensity derived from LiFePO ₄ observed at a position where 2θ is 29.7 to 29.8 ° (JCPDS database: No. 40-1499), It is assumed that the olivine compound is a single phase when the peak intensity due to a compound other than LiFePO ₄ is 5% or less.

The electrical conductivity of the positive electrode active material of the present invention is preferably 1.0 × 10 ⁻² S / cm or more. Thereby, it can use suitably as a positive electrode active material of a lithium ion secondary battery.

The positive electrode active material of the present invention can be produced, for example, by mixing and compounding the above-mentioned carbon allotrope with an olivine compound synthesized by a known method.
However, since the electrical conductivity is greatly improved, it is preferably produced by the production method of the present invention.

The manufacturing method of this invention has the process 1 which mixes the raw material of the olivine type compound represented by Formula (1), and a carbon allotrope, and the process 2 which heats the mixture obtained at the process 1. FIG.
In the above step 1, as a raw material for the olivine-based compound, a compound containing any one of the constituent elements (Li, M ₁ , M ₂ , M ₃ , P) of the compound represented by the formula (1), or two kinds The mixture of the above compounds is mentioned.

As a compound containing lithium, lithium carbonate is preferable.
Examples of the compound containing M ₁ include low-valent salts such as oxalate, acetate, phosphate and nitrate of M ₁ (for example, divalent for Fe and trivalent for Co). It is done. When Fe is used as the 3d transition metal, iron oxalate is preferred.
Examples of the compound containing M ₂ or M ₃ include salts such as oxalate, acetate, phosphate and nitrate.
As the compound containing phosphorus element, ammonium hydrogen phosphate is preferable.
The raw material compound is preferably in a powder form.
For the carbon allotrope, the carbon nanotubes described above are used.

  The mixing method of the above raw material and carbon allotrope is not particularly limited. For example, mixing may be performed in a mortar, or a mixing method using a known apparatus such as an attritor, jet mill, ball mill, planetary mill, or ultrasonic wave.

In Step 1, it is preferable to further mix a solvent in addition to the raw material of the olivine compound and the carbon allotrope. The solvent can be selected from water, alcohol, and ether, but water is preferable from the viewpoint of safety. The amount of the solvent is preferably 1000% by weight or less. More preferably, it is 500% by weight or less. The lower limit of the solvent is
10 wt% is preferable, and 20 wt% is particularly preferable. If the amount of the solvent is too small, the amount of the impurity phase increases because the reaction of the precursor is not sufficient.

  In step 1, a dispersant may be further mixed. Examples of the dispersant include polyethylene glycol (PEG), BYC2010, BYK2012, BYK2015 manufactured by BYK Chemie Japan Co., Ltd., which is used as a dispersant for carbon and the commercially available carbon.

The mixture obtained in step 1 is heated with.
The heating method is not particularly limited, and a general electric furnace, a microwave heating furnace, or the like can be used.
The heating temperature is preferably 400 to 900 ° C. If it is 400 degreeC or more, reaction will occur efficiently, and if it is 900 degrees C or less, the target object can be obtained, without decomposing | disassembling a carbon nanotube and wasting energy.
The lower limit of the heating temperature is preferably 450 ° C., more preferably 500 ° C., and the upper limit is preferably 850 ° C., more preferably 800 ° C.

  The heating time is not particularly limited. In the production method of the present invention, firing in a short time is possible. For example, it is 0.1 to 5 hours. This is because if the heating time is 0.1 hour or longer, the synthesis of olivine-type phosphate crystals can proceed efficiently, and if it is 5 hours or shorter, the particles will not become too large due to grain growth. . The lower limit of the heating time is preferably 10 minutes, more preferably 30 minutes, and the upper limit is preferably 4 hours, more preferably 3 hours.

  In order to obtain single-phase olivine-type phosphate particles, a low-oxygen atmosphere is preferable, and thus the heating atmosphere is preferably an inert gas atmosphere. In an inert gas atmosphere, the oxygen concentration is preferably 1% by volume or less. Here, examples of the inert gas include nitrogen gas, helium, neon, argon, and krypton.

It is also preferable that the heating atmosphere be a hydrogen gas atmosphere. By using a hydrogen gas atmosphere, hydrogen promotes the reaction, so that the heating temperature can be lowered. However, impurities may occur when firing at a low temperature.
In addition, hydrogen gas atmosphere means what contained hydrogen gas in air | atmosphere or inert gas, and what contains 0.5-2 volume% of hydrogen gas is preferable.

  Regarding step 2, it is possible to promote the reaction by heat treatment under pressure, but a device for pressurization is separately required, and the manufacturing cost increases, so a device for pressurization is not provided. It is preferable to carry out under the environment, that is, under normal pressure. The same applies to step 1. Under normal pressure in the present application means a state where the inside of the apparatus is not pressurized using a pressurizing apparatus. For example, the pressure that is about the increase in the internal pressure of the apparatus due to heating for firing is in the range under normal pressure.

  In the production method of the present invention, the production process can be shortened as compared with the case where a positive electrode active material is produced by mixing with a carbon allotrope after obtaining a powder of an olivine compound having low conductivity.

  In the present invention, it is preferable to use a carbon nanotube as the carbon allotrope. Thereby, single-phase olivine type crystal particles are obtained. In addition, by using carbon nanotubes, a positive electrode active material having extremely high conductivity is obtained despite a small amount.

  In the present invention, the mixed material of step 1 may be dried or calcined before step 2. The calcining temperature is preferably 300 ° C to 900 ° C, and the time is preferably 1 second to 12 hours. In this case, it is also preferable to use a spray drying method or a spray pyrolysis method. At this time, it is also preferable to adjust the amount of the solvent so that the viscosity is suitable for spraying.

Example 1
Lithium carbonate (Wako Pure Chemical Industries, Ltd .: purity 99%) 2.227 g, iron oxalate dihydrate 10.842 g (Wako Pure Chemical Industries, Ltd .: purity 90%), ammonium dihydrogen phosphate ( 6.931 g of Wako Pure Chemical Industries, Ltd. (purity: 99%) was weighed and mixed in a mortar for 5 minutes at room temperature in the atmosphere. 4 g of water was gradually added dropwise thereto, and further mixed for 10 minutes.
To this mixture, 0.4 g of carbon nanotubes (CNT; BAYTUBES C 70 P manufactured by Bayer) was added and further mixed at 220 rpm for 2 hours in a planetary ball mill (manufactured by Fritsch Japan).
Next, the obtained mixed powder was dried in the air at 90 ° C. for 3 hours. After drying, this mixed powder is put into an alumina boat, this alumina boat is inserted into a tubular furnace, nitrogen gas is flowed at a flow rate of 0.5 liter / min as a heating atmosphere, and the temperature is raised to 300 ° C. Baked.
The calcined sample was crushed in a mortar and then returned to the alumina boat, and heated by flowing nitrogen gas at a flow rate of 0.5 liter / min. The heating temperature was 700 ° C. and the heating time was 180 minutes. The temperature is raised from room temperature to 700 ° C. over about 20 minutes, held at 700 ° C. for 180 minutes, and then the heating is stopped. Immediately after, the alumina boat is taken out from the heating furnace and cooled to 80 ° C. with nitrogen flowing. A positive electrode active material that was black powder was obtained.
The content of carbon nanotubes, which are carbon allotropes in the positive electrode active material, was 1.9% by weight. The carbon allotrope content was measured by a high frequency combustion method.

Table 1 shows the electrical conductivity, X-ray diffraction measurement results, and carbon allotrope content of this black powder.
The measurement method is as follows.
(1) Electrical conductivity Measured by a four-probe method while applying pressure using a particle resistance measurement system (manufactured by Dia Instruments Co., Ltd.), and the electrical conductivity at 9.81 MPa was determined from the pressure-electric conductivity graph. Obtained.
(2) X-ray diffraction measurement Using a Smart lab (2080A102, manufactured by Rigaku Corporation), X-ray diffraction of a powder sample was measured under the following conditions.
Measurement conditions: X-ray Cu-Ka line, wavelength 1 = 1.5406 mm, output 200 kV, 45 mA
Optical system: reflection method, DS, SS, RS slit 1.0 mm
Measurement range: 2θ = 5 ° -80 °, step interval 0.02 °
Scanning speed: 2θ / θ continuous scan 20 ° / min

CNT1: BAYTUBES C 70 P manufactured by Bayer
CNT2: Flow Tube 7000 manufactured by Cano
X-ray diffraction results: due to compounds other than LiFePO _{4 with} respect to the peak intensity derived from LiFePO ₄ observed at a position where 2θ is 29.7 to 29.8 ° (JCPDS database: No. 40-1499) A case where the peak intensity was 5% or less was marked as ◯, and a case where the peak intensity exceeded 5%.

FIG. 1 shows an electron micrograph of the positive electrode active material produced in Example 1. FIG. 2 shows an X-ray diffraction chart.
This positive electrode active material consisted only of the crystal phase of LiFePO ₄ and showed high electrical conductivity. In addition, it is clear from FIG. 1 that the CNTs responsible for electrical conductivity are not only attached to the crystal surface but also penetrated into the crystal, and it becomes clear that they can be used as a positive electrode material for a Li secondary battery. It was.

Example 2
A black powder was prepared and evaluated in the same manner as in Example 1 except that 0.4 g of polyethylene glycol was added during mixing of the raw materials. The results are shown in Table 1.
This powder consisted only of the crystal phase of LiFePO ₄ and showed high electrical conductivity. Moreover, it was confirmed from SEM observation that the CNT responsible for electrical conductivity not only adhered to the crystal surface but also penetrated the inside of the crystal.

Example 3
2.227 g of lithium carbonate (Wako Pure Chemical Industries, Ltd .: purity 99%), 10.842 g of iron oxalate dihydrate and 6.931 g of ammonium dihydrogen phosphate were weighed, and 5 in a mortar at room temperature in the atmosphere. Mixed for minutes. 4 g of water was gradually added dropwise thereto, and further mixed for 10 minutes. To this mixture, 0.1 g of CNT (Flow Tube 7000 manufactured by Cano) was added, and further mixed at 220 rpm for 2 hours using a planetary ball mill (manufactured by Fritsch Japan).
Next, the obtained mixed powder was dried in the air at 90 ° C. for 3 hours. After drying, this mixed powder was put into an alumina boat, this alumina boat was inserted into a tubular furnace, and calcination was performed by flowing nitrogen gas at a flow rate of 0.5 liter / min as a heating atmosphere. The baking was performed by raising the temperature from room temperature to 700 ° C. over about 20 minutes, holding at 700 ° C. for 30 minutes, then stopping the heating, taking out the alumina boat, and cooling to obtain a black powder.
The black powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 4
2.227 g of lithium carbonate (Wako Pure Chemical Industries, Ltd .: purity 99%), 10.842 g of iron oxalate dihydrate and 6.931 g of ammonium dihydrogen phosphate were weighed, and 5 in a mortar at room temperature in the atmosphere. Mixed for minutes. The mixture was further mixed for 10 minutes while gradually adding 40 g of water thereto. To this mixture, 0.6 g of CNT (FloTube ™ 7000 manufactured by Cnano) was added, and further mixed at 220 rpm for 2 hours using a planetary ball mill (manufactured by Fritsch Japan).
Next, the obtained mixed powder was dried in the air at 90 ° C. for 3 hours. After drying, this mixed powder was put into an alumina boat, this alumina boat was inserted into a tubular furnace, and calcination was performed by flowing nitrogen gas at a flow rate of 0.5 liter / min as a heating atmosphere. The baking was performed by raising the temperature from room temperature to 700 ° C. over about 20 minutes, holding at 700 ° C. for 30 minutes, then stopping the heating, taking out the alumina boat, and cooling to obtain a black powder.
The black powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

Comparative Example 1
The procedure was the same as Example 1 except that no carbon nanotube was added. As a result, a brown powder was obtained.
The brown powder was evaluated in the same manner as in Example 1. The results are shown in Table 1. In this powder, not only the crystal phase of LiFePO ₄ but also peaks of Li ₄ P ₂ O ₇ , Li ₃ Fe ₂ (PO ₄ ) ₃ , Fe ₂ O _{3 and the} like were confirmed. In addition, the electrical conductivity was not measurable (10 ⁻⁸ S / cm or less), and it became clear that it could not be used as a positive electrode material.

Comparative Example 2
The procedure was the same as in Example 2 except that no carbon nanotube was added. As a result, a reddish brown powder was obtained.
The brown powder was evaluated in the same manner as in Example 1. The results are shown in Table 1. Similar to Comparative Example 1, this powder confirmed not only the crystal phase of LiFePO ₄ but also peaks of Li ₄ P ₂ O ₇ , Li ₃ Fe ₂ (PO ₄ ) ₃ , Fe ₂ O _{3 and the} like. In addition, the electrical conductivity was not measurable (10 ⁻⁸ S / cm or less), and it became clear that it could not be used as a positive electrode material.

Example 5
A black powder was prepared and evaluated in the same manner as in Example 3 except that 0.02 g of carbon nanotubes (BAYTUBES C 70 P manufactured by Bayer) was used. The results are shown in Table 1.
This powder was a single phase of LiFePO ₄ only as in Example 1.

Example 6
A black powder was prepared and evaluated in the same manner as in Example 2 except that 0.4 g of acetylene black was used instead of the carbon nanotube. The results are shown in Table 1.
This powder was a single phase of LiFePO ₄ only as in Example 1.

Example 7
Single-phase olivine (LiFePO ₄ ) was added with the same amount of carbon nanotubes as in Example 1 and mixed to obtain a powder. The electric conductivity was 1.1 × 10 ⁻² S / cm.

Example 8
After adding 1% by weight of carbon nanotubes to single-phase olivine and mixing, powder was obtained. The electric conductivity was 2.7 × 10 ⁻⁷ S / cm.

  In Examples 1-8, higher conductivity was obtained than in Comparative Examples 1-2. Moreover, even if Examples 1-4 were compared with Examples 5-8, especially high electroconductivity was acquired.

  The positive electrode active material of the present invention is suitable as a positive electrode material for a lithium ion secondary battery. The obtained lithium ion secondary battery can be used as a power source for portable devices such as notebook computers, mobile phones, and handy video cameras, and as a power source for hybrid vehicles and electric vehicles.

Claims (15)

An olivine compound represented by the following formula (1);
A positive electrode active material in which a carbon allotrope forms a composite.
Lix (M ₁ y ₁ M ₂ y ₂ M ₃ y ₃ ) PO ₄ (1)
(In the formula, x is 0 <x ≦ 2.
y ₁ , y ₂ and y ₃ are each 0 or more and 1.2 or less, and the sum of y ₁ , y ₂ and y ₃ is 0.8 or more and 1.2 or less.
M ₁ is a 3d transition metal element, and M ₂ and M ₃ are each a metal element. )   The positive electrode active material according to claim 1, wherein the carbon allotrope is a carbon nanotube.   The positive electrode active material according to claim 1 or 2, wherein a content of the carbon allotrope is 3% by weight or less.   The positive electrode active material in any one of Claims 1-3 whose said olivine type compound is a single phase. The positive electrode active material in any one of Claims 1-4 whose electrical conductivity is 1.0 * 10 ^<-2 > S / cm or more.   The positive electrode active material according to claim 1, wherein the carbon allotrope has a structure that penetrates the outer shell of particles formed by the olivine-based compound. The positive electrode active material according to claim 1, wherein M ₁ is Fe, and y ₂ and y ₃ are both 0. Step 1 of mixing the raw material of the olivine compound represented by the following formula (1) and the carbon allotrope,
A method of producing a positive electrode active material, comprising: a step 2 of heating the mixture obtained in the step 1.
Lix (M ₁ y ₁ M ₂ y ₂ M ₃ y ₃ ) PO ₄ (1)
(In the formula, x is 0 <x ≦ 2.
y ₁ , y ₂ and y ₃ are each 0 or more and 1.2 or less, and the sum of y ₁ , y ₂ and y ₃ is 0.8 or more and 1.2 or less.
M ₁ is a 3d transition metal element, and M ₂ and M ₃ are each a metal element. )   The method for producing a positive electrode active material according to claim 8, wherein the carbon allotrope is a carbon nanotube (CNT).   The method for producing a positive electrode active material according to claim 8 or 9, wherein the carbon allotrope content is 3 wt% or less.   The manufacturing method of the positive electrode active material in any one of Claims 8-10 which implements the said process 1 and the process 2 under a normal pressure.   The manufacturing method of the positive electrode active material in any one of Claims 8-11 which mixes water further in the said process 1 in addition to the said raw material and the said carbon allotrope.   The manufacturing method of the positive electrode active material in any one of Claims 8-12 which sets the heating temperature of the said process 2 to 400 to 900 degreeC. The positive electrode according to claim 8, wherein a raw material in which M ₁ is Fe and y ₂ and y ₃ are both 0 is used as a raw material for the compound represented by formula (1). A method for producing an active material.   The manufacturing method of the positive electrode active material in any one of Claims 8-14 whose said olivine type compound is a single phase.