JP5920666B2 - Cathode active material for non-aqueous electrolyte secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery having high capacity and good charge / discharge cycle characteristics, and a method for producing the same.

In recent years, with the widespread use of portable electronic devices such as mobile phones and notebook personal computers, development of non-aqueous electrolyte secondary batteries having high energy density, small size and light weight is strongly desired.
Examples of such secondary batteries include lithium ion secondary batteries.
This lithium ion secondary battery is composed of a negative electrode, a positive electrode, an electrolyte, and the like, and a material capable of desorbing and inserting lithium is used as an active material for the negative electrode and the positive electrode.

  This lithium ion secondary battery is currently under active research and development. Among them, lithium ion secondary batteries using a layered lithium metal composite oxide or a spinel lithium metal composite oxide as the positive electrode material are among them. Since a high voltage of 4V class can be obtained, practical use is progressing as a secondary battery having a high energy density.

Here, as materials proposed for the positive electrode active material, lithium-cobalt composite oxide (LiCoO ₂ ) that is relatively easy to synthesize, lithium-nickel composite oxide (LiNiO ₂ ) using nickel that is cheaper than cobalt. ) And lithium / manganese composite oxide (LiMn ₂ O ₄ ) using manganese.

  On the other hand, in the automobile industry, development of large-sized lithium secondary batteries to be mounted on electric vehicles (EV) and hybrid electric vehicles (HEV), which are expected to reduce greenhouse gas emissions, has been actively conducted. There, there is a demand for higher capacity and higher output of lithium ion secondary batteries, and development of new positive electrode active materials is being promoted.

On the other hand, the capacity of an olivine type lithium phosphate compound represented by the general formula: LiFePO ₄ or the like (see, for example, Patent Documents 1 and 2) is not so large, but is strong (PO ₄ ) ³ due to a covalent bond between phosphorus and oxygen. ^- for constituting the polyanion, charge and discharge volume change is small by, difficult to release oxygen.
For this reason, it is strong against overdischarge and overcharge that cause problems in safety, and has high stability at high temperatures. Furthermore, there is an advantage in cycle life and rapid charge / discharge.

However, since the electrical conductivity is as low as 10 ⁻⁸ S / cm 2 or less, a device on the process is required. Therefore, at present, in most cases, the surface of LiFePO ₄ particles is coated with conductive carbon.
Furthermore, since the diffusibility of Li is low and sufficient performance cannot be obtained as it is, fine particles are formed to a diameter of several tens of nm, the diffusion distance is shortened and the surface area is increased. On the other hand, the finely divided LiFePO ₄ particles are difficult to be filled at high density, and the discharge capacity cannot be sufficiently improved.

Further, a lithium silicate compound represented by Li ₂ FeSiO ₄ has a composition formula capable of two-electron reaction from divalent to tetravalent, and a theoretically predicted discharge capacity is about 330 mAh. / G and very large. In addition, polyanion compounds such as phosphate compounds containing lithium and transition metals such as Fe are inexpensive and have a high capacity, but when Li is removed by one or more electron reactions, the crystal structure collapses, resulting in high temperatures. Thus, elution of transition metals such as Fe is likely to occur, and there is a problem in that the capacity of the lithium secondary battery is reduced by repeated use.

Thus, various studies have been made so far on methods for improving the charge / discharge cycle characteristics of a lithium-silicate compound-based positive electrode material.
For example, in a non-aqueous electrolyte secondary battery using a lithium silicate compound as a positive electrode active material, a low internal resistance and high even after high-temperature storage or high-temperature charge / discharge by incorporating a fluorosilane compound into the non-aqueous electrolyte A technique for maintaining electric capacity has been proposed (see, for example, Patent Document 3).

  However, there is a limit to controlling the deterioration of the crystal structure of the positive electrode active material only with the components of the non-aqueous electrolyte.

JP-A-9-134725 WO2005 / 041327 WO2012 / 066878

  The present invention has been made to solve the above problems, and an object thereof is to provide a nonaqueous electrolyte secondary battery having a high capacity and good charge / discharge cycle characteristics.

In view of such a situation, the present inventor is able to suppress elution of transition metals such as Fe at high temperature by phosphoric acid-treating the surface of the lithium silicate compound particles and coating with a phosphate film. I found.
Furthermore, coating the outside of the phosphate coating with carbon leads to a positive electrode active material for a non-aqueous electrolyte secondary battery having high capacity and good charge / discharge cycle characteristics by ensuring conductivity between particles. The present inventors have found that a non-aqueous electrolyte secondary battery having a positive electrode plate made of a substance can be obtained, and have completed the present invention.

The first invention of the present invention has a general formula Li _a M _b SiO _c (where a = 0.02-4, b = 0.5-2, c = 1.4-4, M is a transition element) And at least one selected from the group consisting of Co, Mn, Ni and Fe), wherein the surface of the lithium silicate compound particles is phosphorous. A positive electrode for a non-aqueous electrolyte secondary battery, characterized in that it is a phosphate film coated with (PO ₄ ) ^3- polyanion by covalent bond of oxygen and oxygen, and the carbon film is coated on the phosphate film It is an active material.

The second invention of the present invention has a general formula Li _a M _b SiO _c (where a = 0.02-4, b = 0.5-2, c = 1.4-4, M is a transition element) And a step of forming a phosphate film by phosphoric acid treatment of the particle surface of the lithium silicate compound particles represented by at least one selected from the group consisting of Co, Mn, Ni and Fe), It is a manufacturing method of the positive electrode active material for nonaqueous electrolyte secondary batteries characterized by including the process of coat | covering the particle | grain surface of a phosphate membrane | film | coat with a carbon membrane.

  According to the present invention, the surface of the lithium silicate compound particle is coated with a phosphate film, and the surface is further coated with a carbon film, so that the lithium silicate compound particle in a high temperature environment during charging is obtained. A positive electrode active material for a non-aqueous electrolyte secondary battery having the characteristics of suppressing the elution of transition metals such as Fe and maintaining the conductivity between particles is obtained. By using this positive electrode active material, charging and discharging are performed at a high capacity. A non-aqueous electrolyte secondary battery having good cycle characteristics can be obtained.

It is sectional drawing of the positive electrode active material for nonaqueous electrolyte secondary batteries which concerns on this invention. It is sectional drawing of a 2032 type coin battery produced for evaluation of a positive electrode active material.

The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention has a structure in which the surface of lithium silicate compound particles is treated with phosphoric acid and coated with a phosphate film, and further coated with a carbon film. Yes.
Here, the technical feature of the positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention is that the surface of the lithium silicate compound particles is treated with phosphoric acid and coated with a phosphate film, and further, a carbon film It is in the structure coated with.

The positive electrode active material for a nonaqueous electrolyte secondary battery of the present invention and the method for producing the same will be described in detail.
<1. Cathode active material>
The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention will be specifically described with reference to the drawings.
FIG. 1 is a cross-sectional view specifically showing an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention.
As shown in FIG. 1, the positive electrode active material 11 for a non-aqueous electrolyte secondary battery according to the present invention has a phosphate coating 2 formed by phosphoric acid treatment on the surface of a lithium silicate compound particle 3, and The lithium silicate compound particles having the carbon film 1 formed on the surface of the phosphate film.

[Lithium silicate compound particles]
The lithium silicate compound particle 3 mainly constituting the positive electrode active material of the present invention has a general formula Li _a M _b SiO _c (where a = 0.02-4, b = 0.5-2, c = 1. 4 to 4, M is a transition element, and is at least one selected from the group consisting of Co, Mn, Ni, and Fe).

[Coating with phosphate coating]
The phosphate film 2 of the present invention covers the surface of the lithium silicate compound particle 3 and suppresses elution of transition metals such as Fe in the lithium silicate compound particle in a high temperature environment during charging. If it is.
That is, when lithium silicate compound particles are used as the positive electrode, Li is extracted from the lithium silicate compound particles during charging, so the structure becomes unstable, and at the interface between the silicate compound particles and the electrolyte, Elution of transition metals such as Fe occurs.

Therefore, the elution of transition metals such as Fe is suppressed by coating the surface of the lithium silicate compound particles with a strong (PO ₄ ) ^3- polyanion by covalent bonding of phosphorus and oxygen.
Furthermore, it is only necessary that the lithium ion is easily diffused with a structure that is continuously connected to the surface of the lithium silicate compound particles and hardly peels off.

[Coating with carbon film]
The coating with the carbon film 1 covers the surface of the lithium silicate compound particles coated with the phosphate film 2 to improve the electron conductivity.
As the carbon source, organic acids such as acetic acid and citric acid, sugars such as cellulose, glucose, sucrose, lactose and maltose, polymers such as polyvinyl alcohol, and the like can be used. Although the mass ratio of the carbon material to the lithium / manganese composite oxide particles coated with the phosphate film is not limited, the lower the mass ratio of the carbon material, the more the positive electrode active material amount can be reduced.

<2. Method for producing positive electrode active material>
[Lithium silicate compound particles]
The method for producing lithium silicate compound particles is not particularly limited as long as the transition metal can be solid-dissolved in Li. Various methods such as a molten carbonate method, a solid phase reaction method, and a hydrothermal synthesis method may be used. Synthesize.
For example, in a hydrothermal synthesis method, a Li source mixed with a predetermined ratio, a transition metal source, and amorphous SiO ₂ are dispersed in distilled water, placed in a pressure vessel, and subjected to hydrothermal treatment to produce lithium silicate compound particles. Etc.

[Method of coating with phosphate film]
As a coating method using a phosphate film, a method of grinding in an organic solvent in the presence of phosphoric acid can be used.
According to this method, by adding phosphoric acid when the lithium silicate compound particles are pulverized by an attritor or the like, even if a new surface is generated on the aggregated particles by pulverization, it reacts instantaneously with phosphoric acid in the solvent, A stable phosphate film is formed on the particle surface. Thereafter, even if the pulverized lithium-manganese composite oxide particles are aggregated, the contact surface is already stabilized, and an incomplete region of coating does not occur due to crushing.

Furthermore, the thickness of the phosphate film necessary for protecting the lithium-silicate compound particle surface is usually 5 to 100 nm on average.
If the average thickness of this phosphate film is less than 5 nm, sufficient suppression of elution of transition metals such as Fe cannot be obtained, and if it exceeds 100 nm, battery characteristics deteriorate.

There is no restriction | limiting in particular as phosphoric acid used for formation of such a phosphate membrane | film | coat, Commercially available normal phosphoric acid, for example, 85% concentration phosphoric acid aqueous solution can be used.
The method for adding the phosphoric acid is not particularly limited. For example, when the lithium-manganese composite oxide particles are pulverized with an attritor or the like, phosphoric acid is added to an organic solvent used as a solvent. Phosphoric acid only needs to finally have a desired phosphoric acid concentration, and may be added all at once before the start of pulverization or gradually during pulverization.

  The organic solvent is not particularly limited, and usually alcohols such as ethanol or isopropyl alcohol, ketones, lower hydrocarbons, aromatics, or a mixture thereof is used.

The amount of phosphoric acid added is generally unrelated because it is related to the particle size, surface area, and the like of the pulverized lithium / manganese composite oxide particles. More than 0.1 mol / kg and less than 2 mol / kg, More preferably, it is 0.15-1.5 mol / kg, More preferably, it is 0.2-0.4 mol / kg.
That is, when the amount is less than 0.1 mol / kg, the surface treatment of the lithium / nickel composite oxide particles is not sufficiently performed, so that elution suppression of transition metals such as Fe is not sufficient.

  Furthermore, it is preferable to heat-treat the lithium / manganese composite oxide particles coated with the phosphate film obtained as described above at 100 ° C. or higher in an inert gas or in a vacuum. When the heat treatment is performed at less than 100 ° C., drying does not proceed sufficiently and formation of a stable surface film is inhibited.

[Method of coating with carbon film]
For example, a lithium silicate compound particle coated with a phosphate film is impregnated in a cellulose acetate solution dissolved in acetone. After drying, the temperature is raised and held in an argon atmosphere in a heating furnace. Subsequently, it cools in steps in argon atmosphere. By this step, the surface of the lithium silicate compound particles coated with the phosphate film can be further coated with the carbon film.

<3. Production of secondary battery>
Using the positive electrode active material for a nonaqueous electrolyte secondary battery, for example, a positive electrode is produced as follows.
First, a powdered positive electrode active material, a conductive material, and a binder are mixed, and, if necessary, a target solvent such as activated carbon and viscosity adjustment is added and kneaded to prepare a positive electrode mixture paste.
Each mixing ratio in the positive electrode mixture paste is also an important factor for determining the performance of the secondary battery. When the total solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, Similar to the positive electrode of the non-aqueous electrolyte secondary battery, the positive electrode active material content is 50 to 95 parts by mass, the conductive material content is 1 to 30 parts by mass, and the binder content is 1 to 20 parts by mass. Is desirable.

The obtained positive electrode mixture paste is applied to the surface of a current collector made of, for example, an aluminum foil and dried to disperse the solvent. If necessary, pressurization may be performed by a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode can be produced.
The produced sheet-like positive electrode can be cut into an appropriate size or the like according to the intended battery and used for battery production. However, the manufacturing method of the positive electrode is not limited to the above-described examples, and may be other methods.

Examples of the conductive agent to be used include graphite (natural graphite, artificial graphite, expanded graphite, etc.), carbon black materials such as acetylene black and ketjen black.
The binder plays a role of anchoring the active material particles. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine rubber, ethylene propylene diene rubber, styrene butadiene, cellulosic resin, polyacrylic. An acid or the like can be used.

  Furthermore, if necessary, a positive electrode active material, a conductive material, and activated carbon are dispersed, and a solvent that dissolves the binder is added to the positive electrode mixture. Specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent. In addition, activated carbon can be added to the positive electrode mixture in order to increase the electric double layer capacity.

Next, components other than the positive electrode used in the secondary battery of the present invention will be described.
However, the secondary battery of the present invention is characterized in that the positive electrode active material is used, and other components are not particularly limited.

As the negative electrode, for example, metallic lithium, lithium alloy, or the like, a negative electrode mixture made by mixing a binder with a negative electrode active material capable of occluding and releasing lithium ions, and adding a suitable solvent to form a paste, For example, a metal foil current collector such as a metal foil is applied and dried, and is compressed to increase the electrode density as necessary.
As the negative electrode active material constituting this negative electrode, for example, natural graphite, artificial graphite, a fired organic compound such as phenol resin, and a powdery carbon material such as coke can be used.
As the negative electrode binder, as in the positive electrode, fluorine-containing resins such as polyvinylidene fluoride can be used. As the solvent for dispersing these active materials and the binder, organic solvents such as N-methyl-2-pyrrolidone are used. Can be used.

The separator to be used is disposed by being sandwiched between the positive electrode and the negative electrode.
This separator separates a positive electrode and a negative electrode and retains an electrolyte. For example, a thin film of polyethylene, polypropylene or the like and a film having many minute holes can be used.

  The non-aqueous electrolytic solution used is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate, tetrahydrofuran, and 2-methyltetrahydrofuran. At least one selected from ether compounds such as dimethoxyethane, sulfur compounds such as ethyl methyl sulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate can be used.

As the supporting salt, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , or a composite salt thereof can be used.
Furthermore, the non-aqueous electrolyte solution may contain a radical scavenger, a surfactant, a flame retardant, and the like.

The shape of the lithium secondary battery according to the present invention composed of the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte can be various, such as a cylindrical type and a laminated type.
In any case, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, and the electrode body is impregnated with the non-aqueous electrolyte solution. A current collector lead or the like is connected between the positive electrode current collector and the positive electrode terminal that communicates with the outside, and between the negative electrode current collector and the negative electrode terminal that communicates with the outside. The battery having the above structure can be sealed in a battery case to complete the battery.

  By using a positive electrode active material for a non-aqueous electrolyte secondary battery in which the surface of the lithium silicate compound particles of the present invention is phosphoric-treated and coated with a phosphate film, and further coated with a carbon film, High capacity due to silicate compound particles is obtained, and elution of transition metals such as Fe in lithium silicate compound particles under high temperature environment during charging is suppressed by phosphate coating, and charge / discharge cycle characteristics at high capacity Can be obtained.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example at all.

<Preparation of positive electrode active material>
(1). Preparation of Lithium-silicate Compound Particles LiOH.H ₂ O, FeCl ₂ .4H ₂ O, and amorphous SiO ₂ in a nitrogen atmosphere so that the molar ratio of Li: Fe: Si is 2: 1: 1 In addition, it is added to ion-exchanged water, stirred, mixed, put into a pressure vessel made of Teflon (registered trademark), hydrothermally synthesized at a reaction temperature of 150 ° C. and a reaction time of 72 hours, and having a composition of Li ₂ FeSiO _4. -Obtained silicate compound particles.

(2). Coating with phosphate film Next, 1 kg of lithium silicate compound particles were pulverized in 1.5 kg of isopropanol for 2 hours at an rpm of 200 rpm using an attritor in which the inside of the container was replaced with nitrogen, and the average particle size was 3 μm. Lithium silicate compound particles were prepared.
During or after the grinding, a predetermined amount of 85% orthophosphoric acid aqueous solution was added to and mixed with the lithium silicate compound particles according to the description in Table 1.
Thereafter, the lithium silicate compound particles were dried in vacuum at 120 ° C. for 4 hours to obtain lithium silicate compound particles coated with a phosphate film.

(3). Coating with Carbon Lithium silicate compound particles coated with a phosphate film are impregnated in a solution of cellulose acetate (acetyl group content: 39.7 mass%, weight average molecular weight Mw 50000) dissolved in acetone.
The amount of cellulose acetate added is 5% by weight of the lithium silicate compound particles coated with the treated phosphate coating.

  Utilization of the carbon precursor as a solution allows complete distribution onto the lithium silicate compound particles coated with the phosphate coating. After drying, the temperature is raised to 700 ° C. at 6 ° C./min in an argon atmosphere in a heating furnace and held for 1 hr. Subsequently, it cools in steps in argon atmosphere.

Through the steps 1 to 3, the surface of the lithium silicate compound particles coated with the phosphate film could be further coated with the carbon film.
FIG. 1 is a cross-sectional view of lithium silicate compound particles 11a which are positive electrode active materials for a nonaqueous electrolyte secondary battery according to Example 1 coated with the carbon film 1 and the phosphate film 2. 3a is a lithium silicate compound particle before coating with phosphate and carbon.
This sample contains 1% by weight of carbon, which corresponds to a carbonization efficiency of cellulose acetate of 20%.

[Measurement of phosphate film thickness]
The phosphate film thickness of the obtained lithium silicate compound particles was monitored by XPS for P and O spectra.
From the P profile of the film, the position where the maximum strength was reduced to 50% was defined as the interface position between the film and the base, and the sputtering time L (sec) from the surface to the interface position was read. This L was multiplied by a sputtering rate of 5 nm / min in the standard sample SiO ₂ to obtain a SiO ₂ equivalent film thickness.
Table 1 shows the results.

[Battery fabrication]
60 parts by mass of the positive electrode active material powder was mixed with 30 parts by mass of acetylene black (manufactured by Denki Kogyo Co., Ltd.) and 10 parts by mass of PTFE (manufactured by Daikin Kogyo Co., Ltd.). A pellet was prepared and used as a positive electrode.
Lithium metal was used as the negative electrode, and an equivalent mixed solution of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) using 1M LiPF ₆ as a supporting salt (manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the electrolyte.
Using these, a 2032 type coin battery was manufactured in a glove box of Ar atmosphere in which the dew point was controlled at −80 ° C.

As shown in FIG. 2, the produced 2032 type coin battery has a separator 5 impregnated with an electrolyte disposed between a positive electrode 6 as an evaluation electrode and a negative electrode 4 made of lithium metal, The whole is covered with a negative electrode can 8 from the negative electrode side, and a positive electrode can 9 from the positive electrode side.
A gasket 7 is disposed between the positive electrode can 9 and the negative electrode can 8 to prevent the positive electrode can 9 and the negative electrode can 8 from being short-circuited and to shield the inside of the 2032 type coin battery 10 from the outside.

[Discharge capacity evaluation]
The produced battery was allowed to stand for about 24 hours, and after the OCV was stabilized, the discharge capacity was measured in an environment of 60 ° C.
The discharge capacity was evaluated by conducting a charge / discharge test with a current density with respect to the positive electrode of 0.5 mA and a cut-off voltage of 1.5 to 4.6 V.
Table 1 shows the initial discharge capacity, the measurement results of the discharge capacity at the 20th cycle, and the capacity retention rate.

Except for the thickness of the phosphate film being 22 nm, lithium-silicate compound particles were prepared in the same manner as in Example 1, and a 2032 type coin provided with a positive electrode plate using the lithium-silicate compound particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured.
The results are shown in Table 1.

Except for the thickness of the phosphate film being 69 nm, lithium-silicate compound particles were prepared in the same manner as in Example 1, and a 2032 type coin provided with a positive electrode plate using the lithium-silicate compound particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured.
The results are shown in Table 1.

(Comparative Example 1)
A coin battery was manufactured in the same manner as in Example 1 except that the coating step with the phosphate film was omitted.
The initial discharge capacity of the produced battery was measured in the same manner as in Example 1.
Table 1 shows the measurement results of the initial discharge capacity.

In a nitrogen atmosphere, LiOH.H ₂ O, MnCl ₂ .4H ₂ O, and amorphous SiO ₂ are added to ion-exchanged water and stirred so that the molar ratio of Li: Mn: Si is 2: 1: 1. After mixing, the mixture was put into a pressure vessel made of Teflon (registered trademark), and hydrothermal synthesis was performed at a reaction temperature of 150 ° C. and a reaction time of 72 hours to obtain lithium silicate compound particles having a composition of Li ₂ MnSiO ₄ . Except for the above, lithium silicate compound particles coated with a phosphate coating and a carbon coating were produced in the same manner as in Example 1.

FIG. 1 shows a cross-sectional view of lithium silicate compound particles 11b which are the positive electrode active material for a nonaqueous electrolyte secondary battery according to Example 4 coated with the carbon film 1 and the phosphate film 2. 3b is a lithium silicate compound particle before coating with phosphate and carbon.
This sample contains 1% by weight of carbon, which corresponds to a carbonization efficiency of cellulose acetate of 20%.

A coin battery using these as a positive electrode active material was produced. The initial discharge capacity of the produced battery was measured in the same manner as in Example 1 except that the cut-off voltage was 1.5 to 4.6 V.
Table 2 shows the measurement results of the initial discharge capacity.

  Except for the thickness of the phosphate film being 22 nm, lithium-silicate compound particles were produced in the same manner as in Example 4, and a 2032 type coin provided with a positive electrode plate using the lithium-silicate compound particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured. The results are shown in Table 2.

  A 2032 type coin having a positive electrode plate using lithium-silicate compound particles prepared in the same manner as in Example 4 except that the thickness of the phosphate film was 69 nm. A battery was prepared, and its phosphate film thickness and battery characteristics were measured. The results are shown in Table 2.

(Comparative Example 2)
A coin battery was manufactured in the same manner as in Example 4 except that the coating step with the phosphate film was omitted.
The initial discharge capacity of the fabricated battery was measured by the same method as in Example 4.
Table 2 shows the measurement results of the initial discharge capacity.

In a nitrogen atmosphere, LiOH.H ₂ O, FeCl ₂ .4H ₂ O, MnCl ₂ .4H ₂ O, and amorphous SiO ₂ are mixed at a molar ratio of Li: Fe: Mn: Si of 2: 0.5: 0. The mixture was stirred and mixed in ion-exchanged water so as to be 5: 1, then placed in a pressure vessel made of Teflon (registered trademark), hydrothermally synthesized at a reaction temperature of 150 ° C. and a reaction time of 72 hours, and Li ₂ Fe Lithium silicate compound particles coated with a phosphate film and a carbon film in the same manner as in Example 1 except that lithium silicate compound particles having a composition of _0.5 Mn _0.5 SiO ₄ were obtained. Produced.

FIG. 1 is a cross-sectional view of lithium silicate compound particles 11c, which serve as a positive electrode active material for a nonaqueous electrolyte secondary battery according to Example 7 coated with the carbon film 1 and the phosphate film 2. 3c is a lithium silicate compound particle before coating with phosphate and carbon.
This sample contains 1% by weight of carbon, which corresponds to a carbonization efficiency of cellulose acetate of 20%.

A coin battery using these as a positive electrode active material was produced. The initial discharge capacity of the produced battery was measured in the same manner as in Example 7 except that the cut-off voltage was 1.5 to 4.6 V.
Table 3 shows the measurement results of the initial discharge capacity.

A 2032 type coin provided with a lithium silicate compound particle and a positive electrode plate using the lithium silicate compound particle in the same manner as in Example 7 except that the thickness of the phosphate film was 22 nm. A battery was prepared, and its phosphate film thickness and battery characteristics were measured.
The results are shown in Table 3.

Except that the thickness of the phosphate film was 69 nm, lithium-silicate compound particles were prepared in the same manner as in Example 7, and a 2032 type coin provided with a positive electrode plate using the lithium-silicate compound particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured.
The results are shown in Table 3.
(Comparative Example 3)

A coin battery was manufactured in the same manner as in Example 7, except that the coating step with the phosphate film was omitted.
The initial discharge capacity of the produced battery was measured by the same method as in Example 7.
Table 3 shows the measurement results of the initial discharge capacity.

  As is clear from Table 1, Table 2, and Table 3, by forming a phosphate film and a carbon film in order on the surface of the lithium silicate compound particles, the safety of the lithium silicate compound particles can be improved. It can be seen that it is possible to improve the charge / discharge cycle characteristics in a high temperature environment while maintaining.

1 Carbon film 2 Phosphate film 3 Lithium / silicate compound particle 3a Lithium / silicate compound particle 3b according to Example 1 Lithium / silicate compound particle 3c according to Example 4 Lithium / silicate compound particle 3c according to Example 7 Silicate compound particles 4 Lithium metal negative electrode 5 Separator (electrolyte impregnation)
6 Positive electrode (Evaluation electrode)
7 Gasket 8 Negative electrode can 9 Positive electrode can 10 2032 type coin battery 11 Cathode active material for non-aqueous electrolyte secondary battery according to the present invention (lithium silicate compound particles coated in the order of phosphate film and carbon film)
11a Lithium / silicate compound particles coated in order of phosphate film and carbon film according to Example 1 11b Lithium / silicate compound particles coated in order of phosphate film and carbon film according to Example 4 11c Lithium silicate compound particles coated in the order of phosphate film and carbon film according to Example 7

Claims (2)

General formula Li _a M _b SiO _c (where a = 0.02-4, b = 0.5-2, c = 1.4-4, M is a transition element, from Co, Mn, Ni and Fe Lithium silicate compound particles represented by at least one selected from the group consisting of:
The lithium silicate compound particle surface is a phosphate film coated with (PO ₄ ) ^3- polyanion by a covalent bond of phosphorus and oxygen, and a carbon film is coated on the phosphate film. A positive electrode active material for a non-aqueous electrolyte secondary battery. General formula Li _a M _b SiO _c (where a = 0.02-4, b = 0.5-2, c = 1.4-4, M is a transition element, from Co, Mn, Ni and Fe A step of forming a phosphate film by phosphoric acid treatment of the particle surface of the lithium silicate compound particles represented by at least one selected from the group consisting of:
The manufacturing method of the positive electrode active material for nonaqueous electrolyte secondary batteries characterized by including the process of coat | covering the particle | grain surface of the said phosphate film with a carbon film.

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