JP6013125B2 - Method for producing lithium silicate compound and method for producing lithium ion battery - Google Patents

Description

  The present invention relates to a method for producing a lithium silicate compound, a lithium silicate compound to be obtained, and a lithium ion battery using the same, and more specifically, a high-purity lithium silicate compound having a carbon film uniformly formed on the surface, The present invention relates to a method for producing a lithium silicate compound that can be produced in a short time, the obtained lithium silicate compound, and a lithium ion battery using the same.

Currently, lithium secondary batteries are lightweight and have high energy density, so they are widely used in small batteries such as mobile phones, laptop computers, and other IT devices. With the development and popularization of IT devices, The demand is growing on a global scale.
In these small batteries, a positive electrode active material made of a layered rock salt compound such as LiCoO ₂ , LiCoNiMnO ₂ , LiNiAlO ₂ is mainly used.
In addition to these small batteries, industrial large batteries include hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), electric vehicles (EV), power leveling, and power storage. The demand is expected to expand in various fields, and research and development are actively conducted.

In such a demand environment, high safety, long life, high output, and low price are required for the positive electrode material as a challenge for full-scale commercialization of large industrial batteries.
And LiFePO ₄ which shows high safety and excellent cycle performance and can be manufactured at a low price is attracting attention as an alternative positive electrode material such as LiCoO ₂ or LiMn ₂ O ₄ . Since this LiFePO ₄ has a strong skeleton of phosphoric acid, it is a safe material with a high discharge capacity and a good cycle life. However, the actual capacity of LiFePO ₄ has reached a theoretical value of 170 mAh / g, and it is difficult to further increase the capacity.

Thus, lithium silicate metal (Li ₂ MSiO ₄ : M is a transition metal such as Mn, Fe) has been proposed as a material that is expected to further increase the capacity while maintaining high safety (for example, Patent Document 1). When this lithium metal silicate is used as the positive electrode active material, it contains 2 moles of Li per mole of transition metal (M), so the theoretical capacity reaches 330 mAh / g. However, Li ₂ MSiO ₄ still has lower electronic conductivity than LiFePO _4, and the obtained capacity is much lower than the theoretical value, so that the particles are made finer and further coated with a conductive material such as graphite. -Attempts have been made to improve electrical conductivity by compounding. In order to produce the lithium metal silicate, it is important that fine Li ₂ MSiO ₄ particles can be obtained by a method as simple as possible and can be combined with a conductive material.

So far, solid-phase methods, hydrothermal methods and the like have been reported as methods for synthesizing metal lithium silicates.
In the solid-phase method, it has been proposed to mix lithium silicate and oxalate as a divalent transition metal raw material in a solvent and fire (for example, Non-Patent Document 1). Thereby, a high purity metal silicate lithium salt can be obtained. However, since oxalate is expensive and has strong toxicity, it is not preferable in terms of human body and environment, and a method for synthesizing a large amount has not been established.
In addition, since the temperature required for the reaction is generally high in the solid phase method, the primary particles are coarsened, tend to aggregate and grow, and the particle size becomes large. In addition, the Li ₂ MSiO ₄ obtained in this way needs to be strongly refined because the particles are coarse and have low conductivity.
On the other hand, the hydrothermal method is characterized in that a high-purity lithium metal silicate can be obtained without using an oxalate (for example, Non-Patent Document 2). However, the hydrothermal method requires twice as much lithium as the product at the time of synthesis, and there are many problems when synthesizing a large amount industrially.

Furthermore, as a method for producing a lithium metal silicate salt that is efficient in a short time, a method of mixing, heating and melting a compound that is a source of alkali metal, transition metal, and silicon, and then slowly cooling it has been proposed. (For example, patent document 2). In this proposal, since the raw material is heated to a molten state, the obtained lithium metal silicate becomes coarse particles, and it is said that it is difficult to atomize.
As described above, although the solid phase method has an advantage that a high-purity lithium metal silicate can be obtained, since the particle size of the starting material is generally about several μm, Lithium silicate compound as a battery material because there is a problem that the reaction requires a long time, the doping is extremely difficult depending on the type of element, and lithium defects due to the high reaction temperature. It is not suitable for the synthesis of.
Since the lithium silicate compound is inferior in conductivity to the characteristics of its crystal structure, it is indispensable to impart conductivity by carbon coating in order to develop battery characteristics. From the viewpoint of synthesis efficiency and energy saving, the synthesis of the lithium silicate compound At the same time, there is a craving for a method of carbon coating all at once.
However, in lithium silicate by solid-phase reaction, since the particle size of the starting material is generally about several μm, there is a problem in ion diffusivity and a long time is required for the reaction. For this purpose, a high temperature is required, and it is extremely difficult to perform carbon coating using an organic substance by simultaneous treatment.

  On the other hand, lithium silicate synthesis by a liquid phase method is performed by drying and heating an aqueous solution or aqueous dispersion of a starting material. The present applicant has also proposed that a solution containing a raw metal salt and a solution containing silicate ions are mixed under a specific pH condition to crystallize a coprecipitate, and after solid-liquid separation, washed with water and dried (for example, , See Patent Document 5). Here, it is described that the obtained positive electrode active material precursor is mixed with a saccharide such as sucrose and glucose and a carbohydrate such as a disaccharide and carbon coated. As a result, the conductivity is improved, but the number of manufacturing steps is increased, and further improvement is required.

For this reason, in the process of complexing metal ions in the aqueous solution of the starting material, a method of simultaneously adding a carbon source, heating and drying, and further performing carbon coating along with lithium silicate synthesis at the time of firing has been proposed. A method of adding a metal salt into an aqueous solution or dispersion of water is reported (Non-patent Document 3). The use of citric acid has already been proposed in Patent Document 4 relating to the production of a lithium battery active material of LiMn ₂ O ₄ , but it can be a good carbon coating material by thermal decomposition at a temperature of 400 ° C. or higher. It has been confirmed.

  However, in the case of lithium silicate synthesis, since an aqueous solution containing Mn is dried by heating, it is difficult to obtain a uniform precursor because Mn forms an aggregate of citric acid and a hardly soluble compound at that time. As a result, the purity and battery characteristics of the obtained lithium silicate compound are insufficient, and a practical lithium battery has not been obtained.

JP 2001-266882 A JP 2008-218303 A JP 2008-186807 A JP-A-9-50811 JP 2012-18832 A

A. Nyten, et al. , Electrochemistry Communications, vol. 7, 2005, p. 156 R. Dominko, et al. , Electrochemistry Communications, vol. 8, 2006, p. 217 Y. B. Xu, et al. , J. et al. Power Sources, vol. 170, 2006, p. 570

  An object of the present invention is to provide a method capable of producing a lithium silicate compound suitable as a positive electrode material for a lithium battery in a short time and without subjecting the lithium silicate compound to carbon coating separately, and a lithium ion battery using the material Is to provide.

  As a result of intensive studies on the above-mentioned problems, the present inventors have prepared a water-soluble solution represented by propylene glycol-modified silane as a Si source when preparing a reaction solution containing a Mn salt, a Li salt, and a Si source. When a silicon compound is used, the aqueous solution or water dispersion containing the metal salt is heat-dried and then heat-treated at a high temperature in an inert atmosphere or a reducing atmosphere to form a lithium silicate compound having a carbon film formed on the surface. As a result, it has been found that it exhibits excellent characteristics as a positive electrode material for a lithium battery, and has completed the present invention.

That is, according to the first invention of the present invention, a solution of a water-soluble silicon compound as a Si source is added to an aqueous solution of Mn salt and Li salt with a metal element ratio (Li: Mn: Si) of 1.8 to 2.2. : 0.9 to 1.1: After mixing so as to be 0.9 to 1.1, the obtained aqueous solution or dispersion of metal salt is dried and subsequently heated in an inert atmosphere or a reducing atmosphere. Treatment to synthesize a lithium silicate compound with a carbon film formed on the surface ,
The water-soluble silicon compound of the Si source is at least one selected from propylene glycol modified silane (PGMS), polyethylene glycol modified silane (PEGMS), and glycerin modified silane (GMS).
A method for producing a lithium silicate compound is provided.

According to a second invention of the present invention, there is provided a method for producing a lithium silicate compound, characterized in that, in the first invention, the pH of the aqueous solution or dispersion of the metal salt is adjusted to 6 or less. The
According to the third invention of the present invention, in the first invention, the mixed liquid is further mixed with one or more aqueous solutions selected from Fe salt, Co salt, Ca salt, Zn salt, or Mg salt. And a method for producing a lithium silicate compound characterized by being dissolved.
According to a fourth invention of the present invention, in the first invention, the drying treatment is performed after the aqueous solution or the aqueous dispersion is made into a gel state in advance. A method is provided.
Furthermore, according to a fifth invention of the present invention, in the first invention, the heat treatment is performed at a temperature of 500 to 900 ° C. for 1 to 24 hours in an atmosphere containing at least one of nitrogen and hydrogen. A method for producing a lithium silicate compound is provided.

On the other hand, according to the sixth invention of the present invention, in any one of the first to fifth inventions, a carbon coating is formed on the surface, the primary particle diameter is 1000 nm or less, and the amount of carbon supported is 0.5-2. Lithium silicate compound characterized by obtaining a lithium silicate compound having 0% by mass and having an electrical conductivity (powder resistance measurement at 63.66 MPa pressurization) of 1.0 × 10 ⁻⁷ S / cm or more A manufacturing method is provided.
On the other hand, according to the seventh aspect of the present invention, a method of manufacturing a lithium-ion battery, characterized in Rukoto using lithium silicate compound produced in any one of the first to sixth as the positive electrode active material is provided Is done.

According to the present invention, a high-purity lithium silicate compound having excellent characteristics as a positive electrode material for a lithium battery can be synthesized, and at that time, a carbon film is uniformly formed on the surface. There is an advantage that it can be manufactured easily and in a short time. In the present invention, since the raw material can be handled in an aqueous system and no oxalate is used, the lithium silicate compound can be produced safely.
According to this production method, a lithium silicate compound having a particle size of 10 to 1000 nm coated with carbon on the surface and a composite of a lithium silicate compound and carbon having excellent conductivity can be obtained. It is possible to achieve both ease of handling.
Furthermore, by using it as a positive electrode active material, a lithium secondary battery having both high safety and high capacity and excellent cycle life can be easily produced.

It is a chart which shows the result of having analyzed the lithium silicate compound of this invention by X-ray diffraction. It is the photograph which image | photographed the structure of the lithium silicate compound of this invention with the scanning electron microscope (SEM). It is the photograph which image | photographed the structure of the conventional lithium silicate compound obtained using the fumed silica as Si source with the scanning electron microscope (SEM).

  Below, the lithium silicate compound of this invention used as a positive electrode active material for lithium secondary batteries, its manufacturing method, etc. are demonstrated in detail.

1. [Method for producing lithium silicate compound]
In the method for producing a lithium silicate compound of the present invention (hereinafter also referred to as the present synthesis method), a metal element ratio (Li: Mn: Si) is obtained by adding a solution of a water-soluble silicon compound as an Si source to an aqueous solution of Mn salt and Li salt. 1.8 to 2.2: 0.9 to 1.1: After mixing so as to be 0.9 to 1.1, the obtained aqueous solution or dispersion of metal salt is dried and subsequently inactive. A lithium silicate compound having a carbon film formed on the surface is synthesized by heat treatment in an atmosphere or a reducing atmosphere.

(1) Types of raw material metal component and organic acid There are no particular restrictions on the Mn salt and Li salt used as the raw material metal component as long as they are normally used in the production of a positive electrode active material for a lithium secondary battery, Water-soluble ones are preferred. The Mn salt and Li salt are preferably dissolved in pure water such as distilled water and ion exchange water.

Manganese compounds such as manganese acetate and manganese nitrate can be used as the Mn salt, and lithium compounds such as lithium hydroxide, lithium acetate, and lithium nitrate can be used as the Li salt. For any metal salt, it is particularly preferable to use an acetate that is inexpensive and easy to handle.
Further, a part of the Mn salt can be substituted with a transition metal salt such as Fe salt, Co salt, Ca salt, Zn salt, Mg salt, or alkaline earth metal salt. Among these, Fe salt and Co salt are preferable from the viewpoint of improving battery characteristics, and iron acetate, iron citrate, iron nitrate, cobalt acetate, cobalt citrate, cobalt nitrate, and the like can be used.

  Furthermore, in the present invention, a water-soluble silicon compound is used as the Si source. Examples of the water-soluble silicon compound include propylene glycol modified silane (PGMS), polyethylene glycol modified silane (PEGMS), glycerin modified silane (GMS) and the like. These may be used alone or in combination. Propylene glycol-modified silane (PGMS) has a functional group bonded to silicon, that is, propylene glycol has a hydrophilic OH group in the molecule, so it dissolves in water at an arbitrary ratio and has high stability. It is a preferable water-soluble silicon compound in terms of having it.

This propylene glycol modified silane (PGMS) comprises, for example, a condensate of a specific silicon compound and a water-soluble compound having at least one hydroxyl group and / or amino group in the molecule, as described in JP-A-2010-7032. It can manufacture by making an acidic substance contact after making it contact.
Propylene glycol-modified silane (PGMS) is produced, for example, by transesterifying the ethoxy group of tetraethoxysilane (TEOS) with propylene glycol (PG) as shown in the following reaction formula (1).

[Chemical 1]
Si (OEt) ₄ + 4C ₃ H ₆ (OH) ₂ → Si (OC ₃ H ₆ OH) ₄ + 4EtOH (1)

  TEOS before the reaction is hydrophobic, but since three or more of the four ethoxy groups undergo transesterification with PG, the product PGMS has at least three or more PGs in the molecule, and is water-soluble. It becomes sex.

(2) Mixing and dissolution of raw metal components Next, a solution of a water-soluble silicon compound, which is a Si source, in an aqueous solution of Mn salt and Li salt has a metal element ratio (Li: Mn: Si) of 1.8-2. 2: 0.9-1.1: Mix to 0.9-1.1.
When a part of the Mn salt is replaced with a transition metal salt (M salt) such as an Fe salt, the metal element ratio (Li: Mn: M: Si) is 1.8 to 2.2: 0. .68 to 1.1: 0 to 0.28: 0.9 to 1.1. By setting the metal element ratio within this range, an active material having excellent capacity characteristics and cycle characteristics can be produced.

The mixing of the aqueous solution of Mn salt and Li salt and the solution of the Si source is not limited as long as the raw metal components can be mixed uniformly, and an ordinary mixer can be used. These components are easily dissolved by stirring at room temperature to 80 ° C. for 1 to 120 minutes. Preference is given to stirring at room temperature for 10 to 60 minutes.
The Mn salt precipitates in the solid phase as manganese oxide depending on the pH of the aqueous solution and the degree of ORP. In order to prevent this, since the Mn salt is stable in an acidic solution, it is preferable to adjust the pH to 6 or less by adding an acid to the mixed solution. More preferably, the pH is adjusted to 4 or less. If the pH exceeds 6, the Mn salt is not sufficiently stabilized and may precipitate as a solid phase. The acid used for adjusting the pH is not limited by type as long as it reacts with an element contained in a solution such as Mn salt and does not precipitate a solid, and hydrochloric acid, nitric acid, or the like can be used.

(3) Drying Next, the aqueous solution or water dispersion in which the raw materials are mixed is dried and solidified. At that time, it is preferable to hydrolyze and polycondensate the water-soluble silicon compound so as to form a gel because a gel body in which various raw material elements are uniformly mixed at the atomic level is obtained. Drying is not particularly limited depending on the heating temperature, atmosphere, type of drying apparatus, and the like, as long as moisture can be evaporated from the aqueous solution. In order to dry efficiently, it is preferable to heat at 60 to 200 ° C. in an atmosphere of air, inert gas, or vacuum.

(4) Heat treatment The obtained solidified product and / or gel body is then heat treated at a temperature of 500 to 900 ° C. for 1 to 24 hours in an atmosphere containing at least one of nitrogen and hydrogen. The inventive lithium silicate compound is obtained.

The solidified product and / or gel body is more preferably heat-treated at 600 ° C to 800 ° C. When the firing temperature is less than 500 ° C. and the heating time is less than 1 hour, the crystallinity of the material is low, and when used as a positive electrode active material for a lithium secondary battery, the capacity decreases. Further, when the temperature is higher than 900 ° C. or the heating time exceeds 24 hours, transpiration of lithium becomes remarkable, and it becomes difficult to obtain a material having a target composition ratio.
When a mixed gas of nitrogen and hydrogen is used, the hydrogen content is preferably 1 to 2% by volume. Further, part or all of nitrogen may be replaced with an inert gas such as argon.

The heat treatment furnace may be any furnace that can control the atmosphere, but an electric furnace that does not generate gas is preferable. The continuous furnace can be used.
At this time, the decomposition product of the water-soluble silicon compound becomes a carbon source, and a carbon coat layer can be formed on the surface of the lithium silicate compound particles to impart conductivity. As described above, in the present invention, since the carbon coating is simultaneously performed when the solidified product and / or the gel body is fired, the manufacturing process can be simplified.

2. [Lithium silicate compound]
The lithium silicate compound of the present invention (hereinafter also referred to as material) is obtained as an aggregate of lithium silicate compounds having a carbon film formed on the surface. In the present invention, a lithium silicate compound represented by the general formula Li ₂ MnSiO ₄ can be synthesized by using a predetermined amount of the above-described raw materials.

Further, by substituting a part of the Mn salt with an additive element (M) salt such as an Fe salt, the general formula Li ₂ MnxM (1-x) SiO ₄ (M is Fe or Co, Ca, Zn, Mg, etc.). A lithium silicate compound represented by 0.75 ≦ x ≦ 1.0) that is a transition metal or alkaline earth metal can be synthesized, and effects such as an increase in charge / discharge capacity and an improvement in cycle performance can be obtained. Can do. The addition amount of the additive element (M) is preferably 25% or less with respect to the number of moles of Mn. If it exceeds 25%, the crystal structure of Li ₂ MnSiO ₄ as a base material cannot be maintained, and the material contains a large amount of by-products, which is not preferable.

  The material obtained by this synthesis method is fine particles having a size of 1000 nm or less immediately after synthesis, and does not require a separate pulverization step. This is because PGMS is used as the Si source, and carbon contained in PGMS inhibits sintering.

Further, this carbon remains about 0.5 to 2.0% by mass even after the completion of the firing. Residual carbon exists on the surface of the particles, and when used as a lithium secondary battery material, assists electronic conduction and improves battery characteristics. As a result, the conductivity measured by the powder resistance at the time of pressurizing 63.66 MPa shows conductivity of 1.0 × 10 ⁻⁷ S / cm or more. It is also possible to synthesize a carbon-coated lithium silicate compound having an electrical conductivity of 1.0 × 10 ⁻⁶ S / cm or more by optimizing the production conditions.
In contrast, materials obtained by methods other than the present synthesis method require not only a separate carbon loading process to obtain equivalent conductivity, but also a similar amount of carbon by the carbon loading process. Even if it is supported, the conductivity of the carbon coat is poor because the uniformity of the carbon coat is poor.

In this synthesis method, since the material contains 0.5 to 2.0 mass% of carbon, the conventional carbon supporting step can be omitted. However, if necessary, more carbon may be supported to increase the conductivity auxiliary effect. Regardless of the method of carbon coating, a method of precipitating carbon by mixing an organic compound with a raw material before firing and firing in an inert atmosphere, mechanically adding conductive carbon to the fired material with a ball mill or the like The method of making it adhere can be used. The material can carry up to 20% by mass of carbon, and preferably up to 15% by mass.
As the organic compound, an organic acid having both a carboxyl group and a hydroxyl group in the molecule or having two or more carboxyl groups can be used. Examples of the organic acid include citric acid [CH ₂ —COOH (HO—C—COOH) CH ₂ —COOH], tartaric acid [COOH (CHOH) ₂ COOH], malonic acid [HOOC—CH ₂ —COOH], and lactic acid. [CH ₃ CH (OH) COOH], glycolic acid and the like can be exemplified. These organic acids such as citric acid can hold and coordinate more than one lithium and transition metal in one molecule, the composition of lithium salt and transition metal salt is always stable, and drying and thermal decomposition reactions Can increase the efficiency.

3. [Lithium secondary battery]
The lithium secondary battery of the present invention uses the above lithium silicate compound as a positive electrode active material. A lithium ion battery is composed of the same components as a general lithium secondary battery, such as a positive electrode, a negative electrode, and a nonaqueous electrolyte.

  Hereinafter, embodiments of the lithium secondary battery of the present invention will be described in detail by dividing the components, uses, and the like, but the following embodiments are merely examples, and the lithium secondary battery of the present invention is the present invention. The embodiments described in the specification can be implemented in various modified and improved forms based on the knowledge of those skilled in the art.

(A) Positive electrode A positive electrode is formed from the positive electrode compound material containing the positive electrode active material using the lithium silicate compound of this invention, the electrically conductive material, and the binder. Specifically, a powdered positive electrode active material and a conductive material are mixed, a binder is added thereto, and if necessary, a solvent for viscosity adjustment is further added to adjust the positive electrode mixture paste, For example, the positive electrode mixture paste is applied to the surface of a current collector made of aluminum foil, dried, and pressurized as necessary to produce a sheet-like positive electrode.
The conductive material is for ensuring the electrical conductivity of the positive electrode, and for example, a material obtained by mixing one or more carbon material powders such as carbon black, acetylene black, and graphite can be used. .
The binder plays a role of anchoring the active material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluorine rubber, a thermoplastic resin such as polypropylene and polyethylene, and other suitable materials. Can be used. If necessary, an organic solvent such as N-methyl-2-pyrrolidone is used as a solvent to be added to the positive electrode mixture, that is, as a solvent for dispersing the active material, conductive material, activated carbon, and dissolving the binder. it can. Activated carbon can also be added to increase the electric double layer capacity.
Such a positive electrode active material, a conductive material, and a binder are mixed, and if necessary, activated carbon and a solvent are added and kneaded to prepare a positive electrode mixture paste.
Each mixing ratio in the positive electrode mixture can also be an important factor that determines the performance of the lithium ion secondary battery. When the solid content of the positive electrode mixture is 100% by mass, the positive electrode active material is 60 to 95% by mass, the conductive material is 1 to 20% by mass, and the binder is the same as the positive electrode of a general lithium secondary battery. It is desirable to set it as 1-20 mass%.
For example, on the surface of a metal foil current collector such as aluminum, the above-mentioned positively mixed positive electrode mixture paste is applied and dried to disperse the solvent, and if necessary, a roll press to increase the electrode density thereafter The positive electrode can be formed into a sheet shape by compressing with the above. The sheet-like positive electrode can be cut into an appropriate size according to the intended battery and used for battery production.

(B) Negative electrode For the negative electrode, metallic lithium, lithium alloy, or the like, and a negative electrode mixture made by mixing a binder with a negative electrode active material capable of inserting and extracting lithium ions and adding a suitable solvent to form a paste. , And applied to the surface of a current collector of a metal foil such as copper, dried, and compressed to increase the electrode density as necessary.

  At this time, as the negative electrode active material, for example, a fired organic compound such as natural graphite, artificial graphite, or a phenol resin, or a powdery carbon material such as coke can be used. In this case, the negative electrode binder is a fluorine-containing resin such as polyvinylidene fluoride as in the positive electrode, and the negative electrode active material and the binder are organic solvents such as N-methyl-2-pyrrolidone. A solvent can be used.

(C) Separator A separator is sandwiched and loaded between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode and retains the electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

(D) Nonaqueous electrolyte The nonaqueous electrolyte 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, ethyl methyl carbonate, and dipropyl carbonate, and tetrahydrofuran, 2- One kind selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butanesultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate are used alone or in admixture of two or more. be able to. As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiASF ₆ , LiN (CF ₃ SO ₂ ) ₂ , and complex salts thereof can be used. Further, the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

The lithium secondary battery of the present invention is configured as described above, and the shape thereof can be various, such as a cylindrical type and a stacked type. Even if any shape is adopted, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, and between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal, A current collecting lead or the like is used for connection, the electrode body is impregnated with the nonaqueous electrolyte, and the battery case is sealed to complete the battery.
In the lithium secondary battery of the present invention, a positive electrode using the positive electrode active material containing the lithium silicate compound as a positive electrode material is provided, and charging / discharging is performed at a potential of 2.0 to 4.5 V. Therefore, it is possible to industrially realize a lithium secondary battery that is extremely safer than the lithium metal composite oxide and that has a higher capacity.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further more concretely, this invention is not limited only by an Example. In addition, the physical property of the obtained lithium silicate compound and the characteristic of the battery manufactured using this were measured in the following way.

(1) X-ray diffraction:
Using a powder X-ray diffractometer (manufactured by PANALYTICAL, X'Pert PRO MRD), the obtained positive electrode active material was measured by powder X-ray diffraction using Cu-Kα rays.
(2) Particle size:
The obtained lithium silicate compound was subjected to observation of the obtained lithium silicate compound using a scanning electron microscope (JSMOL, JSM-7001F), and the primary particle size was measured. (3) Carbon content:
The amount of carbon formed on the surface of the lithium silicate compound was measured by a high frequency combustion infrared absorption method using a carbon analyzer (CS-600) manufactured by LECO. In addition, the positive electrode active material 0.2 was burned in an oxygen stream in a high-frequency furnace using a copper and iron chip as an auxiliary combustor, and the carbon that became CO _{2 at} that time was measured by the amount of infrared energy. .
(4) Conductivity:
The conductivity of the obtained lithium silicate compound was measured with a 4-probe probe using a powder resistance measurement system (Mitsubishi Chemical Analytech, MCP-PD51 type), pressurizing 3 g of sample to 63.66 MPa.

(5) Battery evaluation After the obtained lithium silicate compound was crushed in a mortar for 10 minutes, acetylene black / fibrous graphite as a conductive aid, PVDF as a binder, and NMP as a solvent were mixed at a predetermined ratio. The electrode paste was obtained by performing a dispersion treatment. This paste was applied onto an Al substrate with a doctor blade with a target weight of 2 mg / cm ² to obtain an evaluation electrode. The electrode mixture part was designed to have a weight ratio of synthetic sample / conductive aid / binder = 80/10/10 (mass%).
The battery characteristics of the obtained electrode for evaluation are as follows: Li metal as a counter electrode, polyolefin microporous membrane as a separator, and a non-aqueous electrolyte solution in which LiPF ₆ is dissolved in an EC / EMC / DMC carbonate mixed solvent as an electrolyte solution. Evaluation was performed using the 2032 type coin cell used.
Evaluation of the synthetic powder by the produced 2032 type coin cell was performed as follows. Sample of synthetic sample when CC-CV charge (4.5V, 3hours or 1 / 200C cut) and CC discharge (2.0Vcut) are repeated 5 cycles at 1 / 20C rate calculated from the theoretical capacity of lithium silicate compound The change in capacity was measured at 60 ° C. As the charge / discharge device, HJ-SM8 (manufactured by Hokuto Denko) was used.

Example 1
First, propylene glycol modified silane (PGMS) which is water-soluble silicon was produced as follows. Tetraethoxysilane: TEOS (manufactured by Kanto Chemical Co., Ltd.) and propylene glycol (99% manufactured by Kanto Chemical Co., Ltd.) were weighed 22.4 ml and 29.3 ml, respectively, and mixed at 80 ° C. for 1 hour. Furthermore, 100 μl of hydrochloric acid (Kanto Chemical Co., Ltd., special grade) was added to the mixed solution and stirred at room temperature for 1 hour. This stirring liquid was returned to normal temperature, distilled water was added, and it diluted with the 100 ml volumetric flask, and produced 1M water-soluble silicon.
Next, 15 ml of this water-soluble silicon was mixed with 15 ml of 2M lithium acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution and 15 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution, and the pH of the solution was further adjusted. For the purpose of adjustment, a multi-element mixed aqueous solution to which 160 μl of hydrochloric acid (Kanto Chemical Co., Ltd., special grade) was added was stirred at room temperature for 30 minutes and then stirred at 60 ° C. for 30 minutes. Then, the gel was obtained by leaving still.
The gel thus obtained was heat-dried at 120 ° C. under atmospheric conditions, and then baked at 700 ° C. in a nitrogen gas atmosphere for 5 hours.
FIG. 1 shows the XRD of the powder after firing, and FIG. 2 shows the SEM observation results. It was confirmed that Li ₂ MnSiO ₄ fine particles having a primary particle diameter of 100 nm were obtained. When the amount of carbon was measured, 0.9% by mass of carbon was contained. Further, the battery was constructed by the above-described method, and the characteristics were evaluated. Table 1 shows the measurement results such as conductivity.

(Example 2)
15 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution of Example 1 was added to 13.5 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution and iron (III) citrate n hydrate ( A sample was prepared in the same procedure as in Example 1 except that the iron aqueous solution (1M) prepared using Kanto Chemical Co., Ltd., 1st grade was changed to 1.5 ml, and the battery was evaluated. The measurement results are shown in Table 1.

Example 3
15 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution of Example 1 was mixed with 13.5 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution and cobalt (II) acetate tetrahydrate (Kanto). A sample was prepared in the same procedure as in Example 1 except that the cobalt aqueous solution (1M) prepared using Chemical Co., Ltd. was changed to 1.5 ml, and battery evaluation was performed. The measurement results are shown in Table 1.

(Comparative Example 1)
Lithium carbonate (FMC) 16.9 g, fumed silica (Aerosil) 13.8 g, manganese carbonate n hydrate (Wako Pure Chemicals, special grade) 30.0 g was dispersed in ethanol and ground with a planetary ball mill.
The pulverized mixture was fired in a nitrogen atmosphere at 700 ° C. to obtain a sample. The SEM image of the sample after baking is shown in FIG. The primary particles were sintered to form huge primary particles of 1000 nm or more.
The battery evaluation of this sample was performed. The measurement results are shown in Table 1.

(Comparative Example 2)
A sample was obtained in the same manner as in Comparative Example 1 except that 1.4 g of sucrose (Kanto Chemical Co., Inc., deer grade 1) was mixed with the mixture before firing in Comparative Example 1.
The calcined sample contained about 1.2% carbon. The measurement results are shown in Table 1.

"Evaluation"
In the examples of Table 1 showing the above results, water-soluble PGMS as a Si source is mixed with an aqueous solution containing at least Mn salt and Li salt so as to have a specific metal element ratio in the method of the present invention. After drying, the solidified product was baked to form a specific amount of carbon film on the surface of the lithium silicate compound. Thereby, it turns out that the outstanding electroconductivity is obtained and it can use as a positive electrode active material of a lithium ion battery.
On the other hand, in Comparative Example 1, since conventional fumed silica was used as the Si source, a carbon film could not be formed on the surface of the lithium silicate compound. In Comparative Example 2, since saccharide sucrose was used as the carbon source, a carbon film could be formed on the surface of the lithium silicate compound, but sufficient characteristics as a battery could not be obtained.

  The lithium silicate compound of the present invention can be used as a positive electrode active material of a lithium ion battery. The lithium ion battery is not only used for small batteries such as mobile phones, notebook computers, and other IT devices, but also for hybrid vehicles (HEV). It can also be used as a large battery for industrial use such as plug-in hybrid vehicles (PHEV), electric vehicles (EV), power leveling, and power storage, and its industrial value is extremely high.

Claims (7)

A solution of a water-soluble silicon compound as a Si source in an aqueous solution of Mn salt and Li salt has a metal element ratio (Li: Mn: Si) of 1.8 to 2.2: 0.9 to 1.1: 0.9 to After mixing to 1.1, the obtained metal salt aqueous solution or aqueous dispersion was dried, and subsequently heat-treated in an inert or reducing atmosphere to form a lithium film with a carbon film formed on the surface. Synthesize silicate compounds,
The water-soluble silicon compound of the Si source is at least one selected from propylene glycol modified silane (PGMS), polyethylene glycol modified silane (PEGMS), and glycerin modified silane (GMS).
A method for producing a lithium silicate compound characterized by the above.   The method for producing a lithium silicate compound according to claim 1, wherein the pH of the aqueous solution or dispersion of the metal salt is adjusted to 6 or less.   2. The lithium silicate compound according to claim 1, wherein at least one aqueous solution selected from an Fe salt, a Co salt, a Ca salt, a Zn salt, or an Mg salt is further mixed and dissolved in the mixed solution. Method.   2. The method for producing a lithium silicate compound according to claim 1, wherein the drying treatment is performed in advance after the aqueous solution or water dispersion is in a gel state.   2. The method for producing a lithium silicate compound according to claim 1, wherein the heat treatment is performed at a temperature of 500 to 900 ° C. for 1 to 24 hours in an atmosphere containing at least one of nitrogen and hydrogen. A carbon film is formed on the surface, the primary particle diameter is 1000 nm or less, the amount of carbon supported is 0.5 to 2.0 mass%, and the conductivity (powder resistance measurement at 63.66 MPa pressurization) is 1. The method for producing a lithium silicate compound according to claim 1 , wherein the lithium silicate compound is 0.0 × 10 ⁻⁷ S / cm or more. Method of manufacturing a lithium-ion battery, characterized in Rukoto using lithium silicate compound produced by the production method according to any one of claims 1 to 6 as a positive electrode active material.

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