JP2003217584A - Manufacturing method of positive electrode active material for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To compose an olivine system positive electrode in a very short time. <P>SOLUTION: In the manufacture of the general formula LiMPO<SB>4</SB>(provided that, M is at least one kind of metal element selected from transition metals), a mixture of at least a lithium compound, a transition metal M compound, and a phosphorus compound is fired by high-frequency heating. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an improved lithium-containing transition metal phosphate compound used as a positive electrode active material of a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, with the progress of portable and cordless devices, expectations for a non-aqueous electrolyte secondary battery having a small size, a light weight and a high energy density are increasing. The active material for the non-aqueous electrolyte secondary battery includes LiCo
O ₂ , LiNi _0.8 Co _0.2 O ₂ , LiMn ₂ O ₄
A composite oxide of lithium and a transition metal such as is known.

Generally, a positive electrode active material used in a non-aqueous electrolyte secondary battery is composed of a composite oxide in which a transition metal such as cobalt, nickel and manganese is solid-dissolved in lithium which is a main active material. Electrode characteristics such as electric capacity, reversibility, operating voltage, and safety are different depending on the type of transition metal used, and there is a problem of cost in the complex oxide using cobalt and nickel.

[0004] Conventionally, new positive electrode materials satisfying the requirements of high capacity, high safety, high charge / discharge cycle durability, and low price have been actively developed. Among them, olivine-based positive electrodes such as LiFePO ₄ and LiMn _0.6 have recently been used as positive electrode materials having high capacity and high safety.
Fe _0.4 PO ₄ , LiCoPO ₄ , LiNiPO _{4 and the} like have been actively researched, and using these as a positive electrode active material, a negative electrode active material such as a carbon material capable of inserting and extracting lithium can be used. Development of high-voltage, high-energy-density non-aqueous electrolyte secondary batteries by combining them is in progress.

Among them, LiFePO ₄ , LiMn
_{Since 0.4} Fe _0.6 PO ₄ uses an inexpensive element, there is a possibility that a positive electrode material with high capacity, high safety and low cost can be produced. LiCoPO ₄ has high voltage, high energy density and high safety. It has received particular attention as a positive electrode material.

[0006]

However, in order to synthesize these positive electrode materials, for example, a lithium compound, an iron compound, a manganese compound, and NH ₄ H ₂ PO ₄ are mixed and solidified at 550 to 800 ° C. 1 in the inert gas by the phase method
It was necessary to bake for 0 to several tens of hours (AKP
adhi et. al. , Journal of T
he Electrochemical Societ
y, vol 144, 1188-1194 (1997),
A. Yamada et. al. , Journal o
f The Electrochemical Soci
ety, vol148, A960-A967 (200
See 1)).

[0007]

As a result of intensive studies, the present inventors have found that an olivine type positive electrode can be synthesized in an extremely short time when high frequency heating is used for the reaction, and have completed the present invention. . That is, the present invention has the general formula Li
In producing MPO ₄ (where M is at least one metal element selected from transition metals), at least a lithium compound, a transition metal M compound and a phosphorus compound are mixed,
It is characterized by firing by high frequency heating.

Further, as another feature of the present invention, some preferred embodiments described below are included. The first is that the transition metal M contains at least Fe, and the second
Is that the metal powder is added to the mixture, and thirdly, the oxygen partial pressure during firing by high frequency heating is 20.
% Or less.

[0009]

DETAILED DESCRIPTION OF THE INVENTION LiMP produced by the present invention
In O ₄ , M is at least 1 selected from transition metals
From the viewpoint of battery performance, Fe, Co, Ni, and Mn are mentioned, which mean the metallic elements of the species. Of these, Fe and Mn are particularly preferable because they are inexpensive elements.

As long as LiMPO ₄ can be formed,
The combination of elements is not particularly limited, but LiFePO ₄ and LiMn _x Fe, which are composed of low-cost elements and which are known to exhibit battery performance in solid-phase synthesis, are known.
_1-x PO ₄ (especially LiMn _{0 .4} Fe _0.6 PO ₄₎
In manufacturing the present invention, in the present invention, at least a lithium compound, a transition metal M compound and a phosphorus compound are mixed and fired by high frequency heating. Since LiCoPO ₄ has a high discharge voltage, it is useful as a positive electrode material for high energy density batteries.

As the lithium compound, LiOH.H2
_{O, Li 2 CO 3, CH} 3 COOLi , and the like.
Examples of the transition metal compound include oxides, hydroxides, organic acid salts, carbonates, oxalates and the like. Examples of phosphorus compounds include NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , and Li _3.
Examples include PO ₄ , Fe ₃ (PO ₄ ) ₂ · H ₂ O and the like.
Li ₃ PO ₄ has the roles of both a phosphorus compound and a lithium compound. Fe ₃ (PO ₄ ) ₂ · H ₂ O has the roles of both a phosphorus compound and a transition metal compound.

In the present invention, it is particularly preferable to add a small amount of metal powder in addition to the lithium compound, the transition metal M compound and the phosphorus compound because the olivine positive electrode will be rapidly synthesized. This is considered to be because the addition of the metal powder causes the high frequency to be absorbed by the metal powder, and the high frequency heating to proceed uniformly and efficiently. Examples of the metal powder include metal iron powder, metal manganese powder, metal cobalt powder and the like. The amount of the metal powder added is preferably 0.1 to 20 mol%, particularly preferably 1 to 10 mol%, in terms of lithium atom ratio.

In the present invention, the reaction atmosphere is preferably composed mainly of an inert gas. In order to keep the reaction atmosphere in an inert gas atmosphere or a reducing atmosphere, it is sometimes preferable to add carbon powder. Examples of the carbon powder include activated carbon, carbon black, black smoke and the like. When high-frequency heating is performed in the presence of carbon powder, it reacts with oxygen gas in the system, which has the effect of reducing the concentration of oxidizing gas.

If the content of oxidizing gas is large, undesired products such as Fe ₂ O ₃ and Li ₃ Fe ₂ (PO ₄ ) ₃ will increase, which is not preferable. The oxygen concentration in the reaction atmosphere is preferably 20 mol% or less, particularly preferably 2 mol% or less.

The high frequency irradiation is expressed by the oscillation frequency and the high frequency output of the high frequency output device. In the present invention, high frequency in high frequency heating means so-called 1 MH
1000M from the high frequency range represented by z to 20MHz
It means up to the microwave region represented by Hz to 1000 KMHz. For example, an oscillation frequency of 1 MHz to 3000
In the case of MHz and high-frequency output of 600 W, the irradiation time required for heating is several minutes to several tens of minutes. As a high frequency output device,
You can use a household microwave oven without any problems.

A positive electrode mixture is formed by mixing the LiMPO ₄ powder obtained by the present invention with a carbon-based conductive material such as acetylene black, graphite or Ketchen black and a binder. The binding material is polyvinylidene fluoride, polytetrafluoroethylene, polyamide,
Carboxymethyl cellulose, acrylic resin, etc. are used.

A slurry comprising a LiMPO ₄ powder obtained by the present invention, a conductive material, a binder, and a solvent or a dispersion medium for the binder is applied to a positive electrode current collector such as an aluminum foil, dried, and press-rolled to make the positive electrode active. On the positive electrode current collector foil, a material layer is formed on the positive electrode current collector, or a kneading sheet composed of LiMPO ₄ powder, a conductive material, a binder, and a solvent or dispersion medium of the binder is formed in advance and then dried. The desired positive electrode body can be obtained by placing it on the substrate.

In a lithium battery using LiMPO ₄ produced according to the present invention as a positive electrode active material, a carbonate ester is preferable as the solvent of the electrolyte solution. The carbonic acid ester may be cyclic or linear. Examples of the cyclic carbonic acid ester include propylene carbonate and ethylene carbonate. Examples of the chain carbonic acid ester include dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, methylisopropyl carbonate and the like.

The above carbonic acid esters may be used alone or in admixture of two or more. Moreover, you may use it, mixing with another solvent. Further, depending on the material of the negative electrode active material, the combined use of a chain carbonic acid ester and a cyclic carbonic acid ester may improve the discharge characteristics, cycle durability, and charge / discharge efficiency.

Further, vinylidene fluoride-hexafluoropropylene copolymer (for example, Kainer manufactured by Atchem Co.) and vinylidene fluoride-perfluoropropyl vinyl ether copolymer are added to these organic solvents, and the following solutes are added. It may be a gel polymer electrolyte.

[0021] As the _solute, ClO 4 _{-, CF} 3 S
O ₃ −, BF ₄ −, PF ₆ −, AsF ₆ −, SbF
_{6 -, CF 3 CO 2 -} , it is preferable to use the (CF ₃ SO ₂₎ 1 or more either 2 N- such a lithium salt having an anion.

In the above electrolyte solution or polymer electrolyte, it is preferable to add an electrolyte consisting of a lithium salt to the above solvent or solvent-containing polymer at a concentration of 0.2 to 2.0 mol / L. If it deviates from this range, the ionic conductivity will decrease, and the electric conductivity of the electrolyte will decrease. More preferably, 0.5 to 1.5 mol / L is selected. Porous polyethylene or porous polypropylene film is used for the separator.

The negative electrode active material stores lithium ions,
A releasable material is used. The material forming the negative electrode active material is not particularly limited, and examples thereof include lithium metal, lithium alloys, carbon materials, oxides mainly composed of metals of Groups 14 and 15 of the periodic table, carbon compounds, silicon carbide compounds, silicon oxide compounds, sulfides. Examples include titanium and boron carbide compounds.

As the carbon material, those obtained by thermally decomposing organic matter under various thermal decomposition conditions, artificial graphite, natural graphite, soil graphite, expanded graphite, flake graphite and the like can be used. Further, as the oxide, a compound mainly containing tin oxide can be used. Copper foil, nickel foil, or the like is used as the negative electrode current collector.

The positive electrode and the negative electrode are preferably obtained by kneading the active material with an organic solvent to form a slurry, and applying the slurry to a metal foil current collector, drying and pressing. The shape of the lithium battery using the positive electrode active material obtained by the present invention is not particularly limited. A sheet shape (so-called film shape), a folding shape, a winding type bottomed cylindrical shape, a button shape, etc. are appropriately selected according to the application.

[0026]

EXAMPLES Next, specific examples 1 and 2 of the present invention will be described, but the present invention is not limited to these examples.

Example 1 Lithium carbonate and NH ₄ H ₂ P
O ₄ and Fe (CH ₃ CHOCOO) ₂ and metallic iron powder are used as Li, P, Fe derived from iron lactate and Fe derived from metallic iron in an atomic ratio of about 1: 1: 1: 0.05, respectively. , And wet-mixed for 1 hour while dropping ethanol. After the mixture was dried, it was compacted at 98 MPa. The molded pellet was placed in an alumina crucible, the crucible was replaced with argon gas, and the lid was placed in a household microwave oven (oscillation frequency 2450 MHz, high-frequency output 600 W) and irradiated with high frequency for 10 minutes. Then, the inside of the crucible was replaced with argon gas again, and the high frequency was further irradiated for 3 minutes. Pellet after irradiation is crushed and X-rayed by Cu-Kα ray
The spectrum obtained by performing line diffraction is shown in FIG. From this spectrum, it can be seen that the powder is mainly composed of LiFePO ₄ . The powder thus obtained, acetylene black, and polytetrafluoroethylene powder were mixed with each other.
The mixture was mixed at a weight ratio of 0/16/4, kneaded while adding toluene and dried to prepare a positive electrode plate having a thickness of 150 μm. 25 μm thick porous polyethylene was used for the separator, and 300 μm thick metallic lithium foil was used for the negative electrode.
Nickel foil is used for the negative electrode current collector, and the electrolyte is 1 mL
Two coin cell 2030 types were assembled in an argon glove box using iPF ₆ / EC + DEC (1: 1). Regarding one coin cell, in a temperature atmosphere of 25 ° C., a constant-current charge of 10 mA / g of positive electrode active material to 4.3 V was performed, and then a constant-current discharge of 10 mA / g of positive electrode active material to 2.0 V / g was performed. 20 tests
The capacity retention rate was calculated from the ratio of the discharge capacity after the first charge / discharge and the discharge capacity after the 20th charge / discharge.
As a result, the initial discharge capacity was 100 mAh / g and the capacity retention rate was 102%. The voltage of the flat portion of the discharge curve was about 3.4V. For the other coin cell, 60
In a temperature atmosphere of ℃, 1g of the positive electrode active material was charged with a constant current of 10mA to 4.3V, and then 1g of the positive electrode active material was discharged with a constant current to 2.0V at 10mA. The capacity retention rate was obtained from the ratio of the discharge capacity after the 20th charge / discharge and the discharge capacity after the 20th charge / discharge. As a result, the initial discharge capacity was 115 mAh.
/ G, the capacity retention rate was 103%. The voltage of the flat portion of the discharge curve was about 3.4V.

Example 2 LiFePO ₄ was synthesized in the same manner as in Example 1 except that iron acetate was used in place of iron lactate and no metallic iron was added. The X-ray diffraction spectrum is shown in FIG. Further, when the battery performance of the coin cell assembled in the same manner as in Example 1 using the positive electrode plate of LiFePO ₄ obtained in this Example 2 was evaluated, the initial discharge capacity at 25 ° C. was 9
The capacity retention rate after charging / discharging 20 times at 5 mAh / g was 75%. The voltage of the flat portion of the discharge curve was about 3.4V. The initial discharge capacity at 60 ° C is 126 mAh.
/ G, the capacity retention rate after 20 times of charge and discharge was 69%.
The voltage of the flat portion of the discharge curve was about 3.4V.

[0029]

As described above, according to the present invention,
General formula LiMP as positive electrode active material for lithium secondary battery
In producing O ₄ (however, M is at least one metal element selected from transition metals), at least a mixture of a lithium compound, a transition metal M compound and a phosphorus compound is fired by high frequency heating. An olivine-based positive electrode can be synthesized in a short time.

[Brief description of drawings]

FIG. 1 is an X-ray diffraction spectrum diagram of the substance obtained in Example 1 of the present invention.

FIG. 2 is an X-ray diffraction spectrum diagram of the substance obtained in Example 2 of the present invention.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor, number science             3-10 Chigasaki, Chigasaki City, Kanagawa Prefecture             Seimi Chemical Co., Ltd. (72) Inventor Megumi Yukawa             3-10 Chigasaki, Chigasaki City, Kanagawa Prefecture             Seimi Chemical Co., Ltd. F term (reference) 5H029 AJ14 AK03 AL03 AL06 AL07                       AL12 AM03 AM05 AM07 CJ02                       CJ08 CJ28 DJ16 EJ04 EJ12                       HJ15                 5H050 AA19 BA16 BA17 CA07 CB02                       CB05 CB07 CB08 CB12 EA10                       EA24 GA02 GA10 GA27 HA15

Claims (4)

[Claims] 1. In producing a general formula LiMPO ₄ (wherein M is at least one metal element selected from transition metals), at least a lithium compound, a transition metal M compound and a phosphorus compound are mixed and heated by high frequency heating. A method for producing a positive electrode active material for a lithium secondary battery, which comprises firing. 2. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the transition metal M contains at least Fe. 3. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein a metal powder is added to the mixture. 4. The oxygen partial pressure during firing by high frequency heating is 2.
It is 0% or less, Claim 1, 2 or 3 characterized by the above-mentioned.
The method for producing a positive electrode active material for a lithium secondary battery according to 1.