JP5505868B2 - Precursor of positive electrode active material for lithium secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery and a method for producing the same, and further, a precursor for forming the positive electrode active material, a method for producing the same, and a lithium secondary battery using the positive electrode active material It is about.

At present, 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 equipment. 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.

  Furthermore, in addition to these small batteries, industrial large batteries include hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), electric vehicles (EV), power leveling, power storage, etc. 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.
Under such circumstances, LiFePO ₄ that exhibits 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, Mn, Fe, etc.) 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 has a lower electronic conductivity than LiFePO _4, and the obtained capacity is significantly lower than the theoretical value. Therefore, the particles are made finer, and a conductive material such as graphite is further reduced. Attempts have been made to improve conductivity by coating and combining. As a method for producing the lithium metal silicate salt, it is important to obtain fine Li ₂ MSiO ₄ particles and to be able to be combined with a conductive material by a method as simple as possible.

As methods for synthesizing lithium metal silicate reported so far, a solid phase method, a hydrothermal method, a sol-gel method and the like have been reported. In the solid phase method, it is disclosed that lithium silicate and oxalate as a divalent transition metal raw material are mixed in a solvent and fired to obtain a high-purity lithium metal silicate salt (for example, Patent Document 1). 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.
Furthermore, since the temperature required for the reaction is generally high in the solid phase method, the primary particles are likely to be coarsened and agglomerated, and the particle size is increased. Since the Li ₂ MSiO ₄ obtained in this way has coarse particles and low conductivity, it requires a powerful refinement treatment.

Moreover, it is disclosed that a lithium metal silicate of high purity can be obtained by a hydrothermal method as a metal silicate lithium salt that does not use 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 the above-mentioned lithium metal silicate salt that is efficient in a short time, a method comprising a step of mixing, heating, melting and then gradually cooling a compound that is a source of alkali metal, transition metal and silicon 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 seems difficult to atomize.

On the other hand, measures for improving battery characteristics by coating and combining conductive materials have also been made.
A positive electrode active material for a non-aqueous electrolyte secondary battery containing a lithium metal silicate and a carbon component derived from the carbon material is obtained by heat-treating the mixture containing the lithium metal silicate and the carbon material in an inert atmosphere. Producing is proposed (for example, Patent Document 2). According to this proposal, it is said that a high-safety and large-capacity nonaqueous electrolyte secondary battery can be provided. However, the particle diameter of the lithium metal silicate used is several tens of μm, and the effect of atomization is sufficiently obtained. It is hard to say that it can be utilized.

  In addition, an electrode material has been proposed in which a conductive carbon material obtained by thermal decomposition of an organic substance is uniformly deposited on the surface of a lithium metal silicate, and a regular electric field distribution can be obtained on the particle surface ( For example, Patent Document 3). However, in this proposal as well, the process of depositing the carbon material on the particle surface by thermally decomposing organic matter is performed on the synthesized lithium metal silicate salt, taking into account the atomization of the lithium metal silicate salt is not.

  As described above, in the proposals so far, there has been nothing that has been achieved by functionally combining the atomization of lithium metal silicate salt and the coating / compositing of conductive material, but the fine and high-performance metal silicate. It has been difficult to produce lithium salts industrially at low cost.

JP 2001-266882 A JP 2008-218303 A JP 2008-186807 A

A. Nyten, et al. , Electrochemistry Communications, vol. 7, 2005, p. 156 R. Dominko, et al. , Electrochemistry Communications, vol. 8, 2006, p. 217

  An object of the present invention is to provide a lithium secondary battery having both high safety and high capacity and excellent cycle life using a high-performance positive electrode active material for a lithium secondary battery capable of obtaining a fine and high capacity. There is. Furthermore, it is providing the precursor used in order to obtain this positive electrode active material for lithium secondary batteries, its manufacturing method, and its manufacturing method suitable also for industrial production.

In light of the solution of the above problems, the present inventors have intensively studied lithium metal silicate as a positive electrode active material for a lithium secondary battery, and as a result, identified a solution containing transition metal ions and a solution containing silicate ions. It was found that a precursor suitable for a positive electrode active material for a lithium secondary battery can be obtained by crystallizing the coprecipitate while mixing while maintaining the pH of the solution.
Furthermore, the present inventors can obtain a positive electrode active material suitable for a lithium secondary battery by mixing and firing the precursor with a lithium compound and a carbon source, using a carbohydrate as the carbon source, For lithium secondary batteries that can increase the capacity by reducing the metal lithium silicate particles and improving the conductivity by heating after heating at a specific temperature before mixing with carbohydrates and baking. The inventors have found that a positive electrode active material can be obtained, and have completed the present invention.

That is, the precursor of the positive electrode active material for a lithium secondary battery according to the first aspect of the present invention has a general formula M _y SiO ₃ (0.90 ≦ y ≦ 1.10, where M is Mn, Fe, Ni, Co And a solution containing a divalent ion of M element in the general formula and a solution containing a silicate ion (SiO ₃ ²⁻ ). It is an amorphous coprecipitate of M element and silicic acid obtained in this way.

It is preferable that the content of sodium in the metal silicate is less than 1.0 wt%, the precursor has the general formula of the present invention _{Li x M y SiO 4 (1.90} ≦ x ≦ 2.15 0.90 ≦ y ≦ 1.10, wherein M is one or more elements selected from Mn, Fe, Ni, and Co), and a positive electrode active material for a lithium secondary battery comprising a lithium metal silicate salt It is preferable that it is what is used in order to form .

A second invention of the present invention is a method for producing a precursor of a positive electrode active material for a lithium secondary battery. _y SiO ₃ (0.90 ≦ y ≦ 1.10, where M is one or more elements selected from Mn, Fe, Ni, Co) A solution containing divalent ions of M element and silicate ions (SiO ₃ ^2- ) Is adjusted to and maintained in the pH range of 8 to 11, and the coprecipitate is crystallized. After solid-liquid separation, the mixture is washed with water and dried.

Further, as the solution containing silicate ions, it is preferable to use sodium silicate or an aqueous solution thereof.

Furthermore, it is preferable to adjust the pH range using at least one of sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonia.

Moreover, it is preferable to perform the crystallization reaction of the coprecipitate in an inert atmosphere.

ADVANTAGE OF THE INVENTION According to this invention, the precursor suitable as a raw material of the positive electrode active material for lithium secondary batteries can be obtained industrially easily, and it is high performance lithium which can obtain a fine and high capacity | capacitance by using the precursor. A positive electrode active material for a secondary battery can be formed.
Furthermore, a lithium secondary battery equipped with a positive electrode using the positive electrode active material for a lithium secondary battery has high safety and high capacity, is excellent in cycle life, and has extremely high industrial value. is there.

3 is a result of X-ray diffraction analysis of a positive electrode active material obtained in Example 1. FIG. 3 is a result of X-ray diffraction analysis of a positive electrode active material obtained in Example 2. FIG. 3 is a result of X-ray diffraction analysis of a precursor obtained in Example 2. FIG.

The present invention includes a positive electrode active material for a lithium secondary battery, a precursor used as a raw material thereof, and a lithium secondary battery including a positive electrode using the positive electrode active material. Furthermore, the manufacturing method of the positive electrode active material and its precursor is included.
First, after explaining the positive electrode active material for a lithium secondary battery and its precursor, these manufacturing methods will be explained.

[Positive electrode active material for lithium secondary battery]
The positive electrode active material for a lithium secondary battery of the present invention has a general formula Li _x M _y SiO ₄ (1.90 ≦ x ≦ 2.15, 0.90 ≦ y ≦ 1.10, M represents Mn, Fe, Ni, A positive electrode active material composed of a lithium metal silicate represented by one or more elements selected from Co) and composed of lithium metal silicate particles composed of primary particles and secondary particles in which primary particles are aggregated. Yes.
The primary particle size is 10 to 300 nm, the secondary particle size is 0.5 to 10 μm, the specific surface area of the positive electrode active material is 25 to 35 m ² / g, and the carbon content is 3 to 7% by mass. To do.

  The lithium metal silicate used has a large theoretical value of discharge capacity, but because of its low electronic conductivity, the discharge capacity of a battery using the current lithium metal silicate salt as the positive electrode active material is significantly lower than the theoretical value. It has become a thing. Therefore, in the present invention, the primary particles of the lithium metal silicate salt are refined, and the positive electrode active material for the lithium secondary battery contains carbon, thereby improving the electronic conductivity, thereby increasing the high discharge capacity. It has been realized.

  In addition, lithium metal silicate is charged and discharged by insertion and extraction of lithium atoms, but the reaction itself is carried out at the contact surface between the particle surface and the electrolyte, thus facilitating movement of lithium atoms to the particle surface. It is necessary to increase the contact surface with the electrolyte.

  The positive electrode active material of the present invention is produced by reacting a metal silicate and lithium to produce primary particles of the metal silicate lithium salt, and further aggregating the primary particles to form secondary particles. Accordingly, the particles of the positive electrode active material are primary particles and secondary particles in which the primary particles are aggregated. In such a particle form, the contact with the electrolytic solution is mainly performed on the surface of the primary particle. Therefore, when the primary particle size is large, the resistance of lithium atoms moving from the inside of the particle to the surface increases. Therefore, it is necessary to reduce the primary particle size.

  In the present invention, the primary particle diameter is set to 10 to 300 nm to facilitate the movement of lithium atoms from the inside of the particle to the surface, thereby greatly reducing the resistance of the charge / discharge reaction. However, a fine primary particle size is advantageous for the charge / discharge reaction, but when the particle size is less than 10 nm, the bulk density of the obtained positive electrode active material becomes low, so the amount of positive electrode active material per volume is small, and the battery of the same volume There is a problem that the capacity is reduced as compared with the above. On the other hand, if the primary particle diameter exceeds 300 nm, the charge / discharge reaction is hindered, so that sufficient battery capacity cannot be obtained.

  Furthermore, the particle size of the secondary particles is 0.5 to 10 μm. When the particle diameter is less than 0.5 μm, the bulk density of the positive electrode active material obtained in the same manner as the primary particle diameter is lowered, and the capacity is reduced as compared with a battery having the same volume. When the particle diameter exceeds 10 μm, the electrolyte does not penetrate into the secondary particles, the charge / discharge reaction is inhibited, and sufficient battery capacity cannot be obtained.

On the other hand, since the charge / discharge reaction is performed at the contact surface between the particle surface and the electrolytic solution, it is not sufficient to make the primary particle diameter fine. If the primary particle size is made finer, aggregation tends to occur, and the actual contact surface with the electrolytic solution may decrease. For this reason, in this invention, the specific surface area of the positive electrode active material was used as a parameter | index of a contact surface with electrolyte solution, and the contact surface with electrolyte solution was ensured by enlarging the specific surface area. That is, by setting the specific surface area to 25 to 35 m ² / g, the charge / discharge reaction is easily performed, and a sufficient battery capacity is obtained. When the specific surface area is less than 25 m ² / g, the resistance of the charge / discharge reaction increases and the battery capacity decreases. On the other hand, when the specific surface area exceeds 35 m ² / g, the bulk density is lowered and the battery capacity is also lowered.

The positive electrode active material of the present invention contains 3 to 7% by mass of carbon.
Even if the primary particle size is reduced and the specific surface area is increased, the carbon content is limited, if the conductivity of the lithium metal silicate particles is low, the electron transfer associated with the charge / discharge reaction is hindered, and the battery capacity is sufficient. Cannot be obtained. Therefore, since carbon does not dissolve in the metal silicate lithium salt, the contained carbon is the primary metal silicate lithium salt as a simple carbon or a carbon-containing material (hereinafter collectively referred to as a carbonaceous material). It exists outside the particle. Since carbon is generally conductive, the presence of this carbonaceous material improves electron transfer and increases battery capacity.

When the carbon content is less than 3% by mass, the amount of carbon that contributes to the improvement of electron transfer is small, and a sufficient battery capacity cannot be obtained. On the other hand, when the carbon content exceeds 7% by mass, the amount of the lithium metal silicate in the same mass decreases, and the battery capacity decreases.
In particular, when a carbonaceous material having conductivity is present on the primary particle surface of the metal silicate lithium salt, the effect of improving the movement of electrons associated with the battery reaction occurring on the primary particle surface is great. Therefore, it is preferable that at least a part of the surface of the lithium metal silicate particles is coated with a carbonaceous material.

  Such a carbonaceous material is not particularly limited. However, in the production method described later as a preferred embodiment of the present invention, since the lithium metal silicate is mixed with carbohydrate and heat-treated, the carbonaceous material is carbohydrate. It is preferable that the product is produced by thermal decomposition because of simplification of production.

Then the positive electrode active material is represented by the general formula _{Li x M y SiO 4 (1.90} ≦ x ≦ 2.15,0.90 ≦ y ≦ 1.10, M is selected Mn, Fe, Ni, from Co 1 It consists of a metal silicate lithium salt represented by an element of more than species.

When x is less than 1.90, the crystallinity of the obtained metal silicate lithium salt is lowered, and the capacity of the battery obtained when used as the positive electrode of the battery is lowered. On the other hand, if x exceeds 2.15, the excess lithium promotes the sintering of the metal silicate lithium salt particles during firing at the time of manufacture. The capacity of is reduced.
The M element may be a transition metal, but Mn, Fe, Ni, and Co may be mentioned as industrially produced transition metals, and may be one or more elements selected from these. In particular, Mn and Fe are resource-rich and inexpensive, and are preferable from the viewpoint of cost.

[Precursor of positive electrode active material for lithium secondary battery]
The precursor of the positive electrode active material for a lithium secondary battery of the present invention has a general formula M _y SiO ₃ (0.90 ≦ y ≦ 1.10, where M is one or more selected from Mn, Fe, Ni, Co) Element M) obtained by mixing a solution containing a divalent ion of M element in a general formula and a solution containing a silicate ion (SiO ₃ ²⁻ ) and silicic acid It is characterized by being an amorphous coprecipitate.
Since this coprecipitate is an amorphous metal silicate salt in which M and silicon are uniformly distributed, the reaction with lithium is easy and the reaction temperature can be lowered. Being amorphous can be confirmed by the absence of a diffraction peak in X-ray diffraction analysis. For this reason, it is possible to lower the firing temperature for producing the positive electrode active material of the present invention, and a positive electrode active material having a fine primary particle diameter and a large specific surface area can be obtained.

On the other hand, when sodium is contained in the positive electrode active material for a lithium secondary battery, battery characteristics deteriorate. For this reason, it is necessary to reduce the amount of sodium, which can be achieved by reducing the amount of sodium contained in the precursor of the positive electrode active material for a lithium secondary battery. Therefore, the sodium content in the silicate metal salt is preferably 1.0% by mass or less.
By using this precursor, a positive electrode active material having a fine primary particle diameter and a large specific surface area can be easily obtained. Therefore, it is suitable as a precursor for the positive electrode active material for a lithium secondary battery of the present invention.

[Method for producing precursor of positive electrode active material for lithium secondary battery]
The method for producing a precursor of a positive electrode active material for a lithium secondary battery according to the present invention has a general formula M _y SiO ₃ (0.90 ≦ y ≦ 1.10, and the M element is selected from Mn, Fe, Ni, and Co. A method for producing a metal salt of silicate represented by one or more elements), a solution containing a divalent ion of M element in the general formula and a solution containing a silicate ion (SiO ₃ ²⁻ ), The coprecipitate is crystallized by mixing while adjusting and maintaining the pH in the range of 8 to 11, and after solid-liquid separation, it is produced by washing with water and drying.

  In order to cause the crystallization reaction, a solution containing divalent ions of M element (hereinafter sometimes referred to as M salt solution) and a solution containing silicate ions (hereinafter referred to as silicate solution). And a solution containing silicate ions (hereinafter sometimes referred to as a silicate solution) may be mixed to adjust the pH. A salt solution may be added dropwise and mixed, and the pH of the resulting mixture may be adjusted. However, since this M salt solution shows acidity and the silicate solution shows alkalinity, when directly mixed, the reaction may occur in a state where pH control is insufficient and coprecipitation may not occur. High accuracy is required. For this reason, in order to easily control the pH, first, a reaction solution which is a solvent such as water whose pH is adjusted to 8 to 11 with an alkali is prepared, and the pH of the reaction solution is maintained in the above range. , A solution containing M element divalent ions (hereinafter sometimes referred to as M salt solution) and a solution containing silicate ions (hereinafter sometimes referred to as silicate solution) are dropped. Thus, it is preferable to obtain an amorphous coprecipitate of M element and silicic acid, that is, an amorphous metal silicate. The coprecipitate can be easily crystallized by adjusting the pH of the reaction solution.

  Here, when the pH of the reaction solution is less than 8, the M element is not sufficiently precipitated, resulting in a composition shift in the resulting metal silicate salt. Moreover, when pH exceeds 11, the alkali component will mix in the precipitated silicate metal salt, and the impurity contained in the precursor obtained will increase. In particular, as will be described later, when sodium hydroxide is used as the alkali, the amount of sodium as an impurity increases, and the lithium secondary battery using the positive electrode active material for a lithium secondary battery finally obtained The characteristics will deteriorate.

  The molar ratio of M element divalent ions and silicate ions (M / silicate ratio) contained in this reaction solution is 0.9 to 0.9, centered on 1 which is the stoichiometry of the metal silicate lithium salt. Although it can be in the range of 1.1, it is preferably 0.95 to 1.05. When the pH of the reaction solution is adjusted to the above range, all the M element ions and silicate ions contained in the reaction solution are crystallized as a coprecipitate. This is consistent with the M / silicic acid ratio. Further, the M / silicic acid ratio does not change in the production of the positive electrode active material for a lithium secondary battery, which is a subsequent process. Therefore, by setting the M / silicic acid ratio in the reaction solution in the range of 0.9 to 1.1, a lithium metal silicate having a desired M / silicate ratio, that is, a positive electrode for a lithium secondary battery. An active material can be obtained.

  Next, a water-soluble divalent M salt can be widely used as the M salt, but an inorganic salt is preferable, and a sulfuric acid first salt having no impurities contained in the coprecipitate is particularly preferable. . Moreover, it is preferable that the density | concentration of M salt in M salt solution shall be 0.5-2 mol / L. An M salt concentration of less than 0.5 mol / L is not preferable because the amount of liquid during the reaction increases so that the productivity decreases. If the M salt concentration exceeds 2 mol / L, the M salt may reprecipitate in the M salt solution, which may cause fluctuations in the M / silicic acid ratio and equipment failure, which is not preferable.

  Although a water-soluble silicate can be used, it is preferable to use sodium silicate. Although there are few water-soluble silicates and many of them are expensive, sodium silicate is readily available and inexpensive. The inventors of the present invention crystallized in the above pH range, so that the obtained coprecipitate, that is, the range in which the battery characteristics when sodium in the metal silicate is used as a positive electrode active material is not adversely affected. It was found that an amorphous metal silicate having a fine particle size and M element and silicon distributed uniformly can be obtained.

Since the sodium silicate is a liquid, it can be used alone. Therefore, when sodium silicate is used for the silicate solution, it contains sodium silicate alone. However, sodium silicate is preferably used as an aqueous solution in order to facilitate supply to the reaction solution. When sodium silicate is used as an aqueous solution, the concentration of sodium silicate in the silicate solution is preferably 0.5 to 2 mol / L in consideration of productivity and ease of supply to the reaction solution. .
If the concentration of sodium silicate is less than 0.5 mol / L, the amount of liquid in the reaction becomes too large, and the productivity is lowered. When the concentration of sodium silicate exceeds 2 mol / L, the viscosity of the solution increases, and the ease of supply may decrease. In addition, when using silicates other than sodium silicate, it is preferable to use in the same concentration range as sodium silicate.
The M salt solution and the silicate solution are both aqueous solutions, but the M salt and silicate are preferably dissolved in pure water such as distilled water and ion exchange water.

For pH adjustment of the reaction solution, one or more selected from sodium hydroxide, lithium hydroxide, and ammonia can be used.
Sodium hydroxide is an inexpensive alkali source, and when it is kept at pH 9 or less, there is little residual sodium. Even when lithium hydroxide is used, lithium does not basically precipitate. However, even if a small amount of lithium remains in the precursor, there is no problem because it is mixed with the lithium compound in the subsequent step. Since ammonia contains a large amount of nitrogen in the wastewater, there is a need for wastewater treatment and the like. Furthermore, since lithium hydroxide and ammonia are expensive, they are disadvantageous in terms of cost. For the above reasons, sodium hydroxide is particularly preferably used for the pH adjustment.

  The atmosphere during the crystallization reaction is preferably a non-oxidizing atmosphere, particularly an inert atmosphere, in order to prevent oxidation of the M element. Here, the inert atmosphere only needs to reduce oxygen to such an extent that oxidation of the M element can be prevented. When the crystallization reaction is performed in an oxidizing atmosphere, the M element may be oxidized from divalent, and a metal silicate having a desired composition ratio may not be obtained.

  The reaction apparatus to be used is not particularly limited, but it is preferable to use a reaction vessel equipped with a stirring blade or the like in order to make the reaction uniform. Moreover, it is preferable to have a structure in which an airtightness can be maintained and an inert gas can be introduced by attaching an upper lid to the reaction tank. By introducing the inert gas, oxidation of the M element can be sufficiently prevented.

  The formed coprecipitate is washed from the viewpoint of reducing impurities. In particular, when the pH is adjusted using sodium hydroxide, it is preferable to sufficiently reduce sodium by washing with water.

Next, the precipitate of the present invention is obtained by drying the washed precipitate.
The atmosphere at the time of drying is not particularly limited, and is preferably an inert atmosphere, a reducing atmosphere, a vacuum, or a reduced pressure atmosphere in order to prevent oxidation. The drying temperature is not particularly limited, but is preferably 50 to 200 ° C. As the drying apparatus, it is preferable to use a normal drying apparatus capable of controlling the atmosphere.

[Method for producing positive electrode active material for lithium secondary battery]
The method for producing a positive electrode active material for a lithium secondary battery according to the present invention has a general formula Li _x M _y SiO ₄ (1.90 ≦ x ≦ 2.15, 0.90 ≦ y ≦ 1.10, M is Mn, Fe , One or more elements selected from Ni and Co), and a method for producing a positive electrode active material for a lithium secondary battery, comprising a lithium compound and a carbon source. After mixing, firing is performed at 450 ° C. to 750 ° C. in a non-oxidizing atmosphere.

  In the production method of the present invention, a precursor, a lithium compound and a carbon source are mixed before firing. The carbon source becomes a conductive carbon material during firing, and coats the metal silicate lithium salt particles constituting the positive electrode active material for a lithium secondary battery. Thereby, the movement of electrons accompanying the battery reaction occurring on the surface of the metal silicate lithium salt particles is improved. Furthermore, the carbon source or the carbon material produced during firing also has an effect of suppressing the grain growth of the metal silicate lithium salt particles. Since this effect and the precursor used are fine, the resulting metal silicate lithium salt particles can be made fine.

  As the carbon source to be used, a carbohydrate that becomes a carbon material having conductivity during firing, or conductive carbon such as acetylene black and ketjen black can be used. In particular, carbohydrates are preferred because of their low toxicity.

Carbohydrate having a melting point of 250 ° C. or lower as a carbon source, after mixing with a precursor, a lithium compound and a carbon source, and in a non-oxidizing atmosphere before firing, above the melting point of the carbohydrate used and not higher than 300 ° C. It is particularly effective to perform the heat treatment at the temperature. By heating the carbohydrate in this temperature range, the molten carbohydrate penetrates between the primary or secondary particles of the precursor and coats the particles evenly. As a result, a sufficient grain growth suppressing effect is obtained, and a conductive carbon material coating layer is formed on the surface of the sintered metal lithium silicate particles.
When the melting point of this carbohydrate exceeds 250 ° C. or the heat treatment temperature exceeds 300 ° C., the formation reaction of the metal lithium silicate starts before the carbohydrate permeates, so that the above-described grain growth suppression effect is sufficient. May not be available. Furthermore, the formation of the coating layer of the carbon material may be insufficient.

  The carbohydrate to be used is preferably at least one of monosaccharides and disaccharides, and sucrose and glucose are particularly preferable. Monosaccharides and disaccharides are preferable because they melt at 300 ° C. or lower and penetrate into the surface of the precursor particles in the heat treatment.

  As the lithium compound, lithium compounds that are usually used in the production of positive electrode active materials for lithium secondary batteries such as lithium hydroxide, lithium carbonate, and lithium acetate can be used. In particular, lithium carbonate that is inexpensive and easy to handle should be used. Is preferred.

The mixing is not limited as long as the precursor, the lithium compound, and the carbon source can be uniformly mixed, and a mixer such as a shaker mixer, a ball mill, a planetary mill, a vibration mill, or a bead mill can be used. The said mixing can be performed on the conditions performed normally.
Furthermore, in the production method of the present invention, it is preferable to grind the precursor used before firing in order to obtain fine metal lithium silicate particles. The pulverization may be performed either before or after mixing, but it is preferable to use various mills listed as a mixer at the time of mixing because sufficient mixing can be performed simultaneously with the pulverization. For the pulverization, a dry or wet mill using alumina or zirconia spheres can be used.

  The carbon source may be mixed with a precursor and a lithium compound before pulverization and pulverized as a mixture thereof. However, when using at least one of carbohydrates, particularly monosaccharides and disaccharides, the carbon source dissolves during heating. In order to easily penetrate into the particle surface, it may be mixed after the pulverization. When a wet mill is used for the pulverization, the carbohydrate is preferably mixed after the pulverization.

Fine crystalline and fine lithium metal silicate by firing a mixture obtained by mixing a precursor, a lithium compound and a carbon source in a non-oxidizing atmosphere at 450 to 750 ° C., preferably 550 to 700 ° C. A salt (Li _x M _y SiO ₄ ) can be obtained.
In the production method of the present invention, a mixture containing a carbon source is fired, and depending on the carbon source to be used, graphitization proceeds at 600 ° C. or higher. Composite metal lithium silicate fine particles can be obtained.

The atmosphere during firing is preferably an inert atmosphere such as nitrogen or argon or a reducing atmosphere, including heat treatment, in order to suppress oxidation of the M element and the carbon source in the metal silicate salt. More preferably. The reducing atmosphere is preferably an inert gas containing hydrogen, particularly a nitrogen atmosphere containing hydrogen, and the hydrogen content is preferably 1 to 2% by volume.
The firing time is not particularly limited, and is a time for obtaining a metal silicate lithium salt sufficiently crystallized in the above temperature range. For example, it is preferably 1 to 20 hours. If it is less than 1 hour, the crystallinity of the metal silicate lithium salt may not be sufficient, and if it exceeds 20 hours, the sintering of the metal silicate lithium salt particles may progress and fine particles may not be obtained. The heat treatment time may be a time for the used carbohydrate to melt and penetrate into the precursor, and is preferably 1 to 5 hours, for example.

  Moreover, in the said manufacturing method, the said mixture can be calcined by hold | maintaining at 400-550 degreeC for 1 to 10 hours in non-oxidizing atmosphere before baking. Since the crystallinity of the lithium metal silicate can be improved by calcination, when sufficient crystallinity cannot be obtained when the firing time is shortened to obtain fine particles. It is valid. In addition, after performing calcination, when mixing carbohydrate, it is preferable to mix carbohydrate after grinding | pulverization. Since the sintering of the particles proceeds to some extent even during calcination, if the pulverization is not performed, the carbohydrates do not sufficiently penetrate between the particles in the heat treatment, and the resulting lithium metal silicate particles become coarse. There is.

  The firing furnace may be any furnace that can control the atmosphere, but an electric furnace that does not generate gas is preferable. A stationary furnace such as a tubular furnace or a muffle furnace, a pusher furnace, a roller hearth kiln, a rotary kiln, or a fluidized bed furnace A continuous furnace can be used. A similar furnace can be used for heat treatment and calcination.

The positive electrode active material obtained by the above production method is composed of fine metal lithium silicate particles, but may be slightly sintered by firing. In such a case, it is possible to easily form fine metal lithium silicate particles by pulverization or classification, or a combination thereof as necessary. The crushing and classification are not particularly limited, and can be performed by usual methods and conditions.
According to the production method of the present invention, it is not necessary to use a high-pressure vessel such as a hydrothermal synthesizer, and since it can be synthesized using a raw material that is inexpensive and has low toxicity, a lithium metal silicate salt is industrially inexpensively produced. Can be manufactured.

[Lithium secondary battery]
The lithium secondary battery according to the present invention is composed of the same components as those of a general lithium secondary battery, such as a positive electrode, a negative electrode, and a non-aqueous 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 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 total solid content (meaning excluding the solvent) of the positive electrode mixture is 100% by mass, the positive electrode active material is 60 to 95% by mass, and the conductive material is 1 to 5 like the positive electrode of a general lithium secondary battery. It is desirable that 20% by mass and the binder be 1 to 20% by 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 selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butanesultone, phosphorus compounds such as triethyl phosphate and trioctyl phosphate, or a mixture of two or more. Can be used. 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 configured as described above can have various shapes 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.

  The lithium secondary battery of the present invention includes a positive electrode using the positive electrode active material for a lithium secondary battery of the present invention as a positive electrode material, and is charged and discharged at a potential of 3.0 to 4.5 V. A lithium secondary battery that is extremely safer than conventional lithium metal composite oxides and also has a high capacity can be industrially realized.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, the analysis method of the metal of lithium metal silicate used in the Examples, the measurement method of X-ray diffraction and specific surface area, and the evaluation method of battery capacity are as follows.

(1) Elemental analysis:
Analysis was performed by ICP emission analysis using an ICP emission analyzer (Varian, 725ES).

(2) 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.

(3) Measurement of specific surface area:
Using a BET method measuring machine (Kantasorb QS-10 manufactured by Yuasa Ionics Co., Ltd.), the BET method was performed by nitrogen gas adsorption.

(4) Battery capacity evaluation:
About the obtained positive electrode active material, the coin type battery was produced in the following procedures, and the charge / discharge capacity of the battery was measured and evaluated. 33% by mass of acetylene black as a conductive material, 17% by mass of polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) solution as a binder are added to and mixed with the positive electrode active material, and the above positive electrode active material is 50% by mass-conductive. A mixture of 33% by mass of the material and 17% by mass of PVDF was obtained. This mixture was applied on an aluminum foil, dried at 80 ° C., punched to an electrode size of 11 mm, and pressed at a pressing pressure of 98 MPa (1.0 tonf / cm ² ) to produce an electrode. The electrode as a positive electrode, metallic Li as an anode in a glove box, with an equal volume mixture (volume ratio of ethylene carbonate containing electrolyte LiClO ₄ 1 mol / L as an electrolyte (EC) and diethyl carbonate (DEC) EC / A C2023 coin battery was prepared using DMC = 1/1). The battery was charged / discharged by charging 0.2 mA / cm ² , 4.5 V, resting 12 hours, discharging 0. It implemented on the conditions of ² mA / cm < ² >, 1.5V, 50 degreeC, and used the discharge capacity of the 2nd cycle as an evaluation value.

1 mol (278 g) of iron sulfate heptahydrate (special grade manufactured by Wako Co., Ltd .: purity 99.5 wt%) was dissolved in 1 L of distilled water to prepare an M salt solution. 1 mol (180 g) of sodium silicate (manufactured by Kanto Chemical Co., Ltd., deer grade 1) as Si was dissolved in 1 L of distilled water to prepare a silicate solution. A 25% aqueous sodium hydroxide solution was used as the pH adjustment solution.
1 L of pure water was put into a 5 L separable flask equipped with a stirrer and stirred for 30 minutes while the inside was replaced with nitrogen. Next, the M salt solution and the silicate solution were respectively added at a rate of 10 ml / min while controlling the pH of the pH adjustment solution to 8.9 to 9.1 using a pH controller. After completion of the dropwise addition, stirring was continued for 30 minutes while the inside of the separable flask was replaced with nitrogen, and the crystallization reaction was allowed to proceed completely. The coprecipitate formed by this crystallization was recovered by filtration through suction filtration. The coprecipitate was washed with water and then dried at 80 ° C. in a vacuum dryer to obtain a precursor of a positive electrode active material for a lithium secondary battery. As a result of X-ray diffraction measurement of the obtained precursor, a diffraction peak was not obtained, and it was confirmed that the precursor was amorphous of iron and silicon.

30 g of the obtained precursor, 14.1 g of lithium carbonate (Kanto Chemical Co., Ltd .: 99.0%) and 70 ml of ethanol were placed in a 250 ml zirconia pot with an internal volume of 650 g of 1 mm diameter zirconia balls, and planets It was mixed and ground for 30 minutes at 300 rpm by a ball mill (manufactured by Fritsch Japan). Thereafter, the zirconia balls were sieved and ethanol was removed by vacuum drying to obtain a mixture of the precursor and lithium carbonate (primary mixture).
30 g of the obtained primary mixture and 4.9 g of sucrose were placed in a 250 ml zirconia pot having an internal volume of 350 g of zirconia balls having a diameter of 5 mm, and mixed at 200 rpm for 10 minutes by a planetary ball mill. Zirconia balls were sieved to produce a mixture of precursor, lithium carbonate and sucrose.

The mixture was kept at 190 ° C. for 2 hours at a heating rate of 10 ° C./min while purging the inside of the furnace with 1 L / min of nitrogen containing 2% by volume using an electric furnace, and then 5% at 650 ° C. The positive electrode active material for lithium secondary batteries was formed by baking for a time. The composition ratio of lithium: iron: silicon of the positive electrode active material is 2.01: 0.99: 1.00, the carbon content is 4.8% by mass, and the sodium content is 0.9% by mass. Met. Further, it was identified as lithium iron silicate by X-ray diffraction (FIG. 1). When this positive electrode active material was observed with a scanning microscope (SEM), the primary particle size was 50 to 250 nm. The specific surface area determined by the BET method was 30.9 m ² / g. The evaluation results of the battery capacity are shown in Table 1.

In the same manner as in Example 1, except that a solution prepared by dissolving 174 g of manganese sulfate n hydrate (manufactured by Chuo Denko: 99.9 wt%) in 1 L of distilled water as M salt solution was used. An active material was obtained and evaluated.
The composition ratio of lithium: manganese: silicon of the obtained positive electrode active material was 2.02: 0.98: 1.00, the carbon content was 4.5 mass%, and the sodium content was 0.8. It was mass%. Further, it was identified as lithium manganese silicate by X-ray diffraction (FIG. 2). As a result of X-ray diffraction measurement of the precursor as in Example 1, it was confirmed that the obtained precursor was amorphous of manganese and silicon (FIG. 3).
When this positive electrode active material was observed by SEM, the primary particle size was 50 to 200 nm. The specific surface area determined by the BET method was 33.3 m ² / g. The evaluation results of the battery capacity are shown in Table 1.

Except that 139 g of iron sulfate heptahydrate and 87 g of manganese sulfate n hydrate (0.5 mol each of Fe and Mn) were dissolved in 1 L of distilled water, the same method as in Example 1 was used as the M salt solution. A positive electrode active material was obtained and evaluated.
The composition ratio of lithium: iron: manganese: silicon of the obtained positive electrode active material was 2.01: 0.49: 0.51: 1.00, the carbon content was 4.6% by mass, and sodium. The content was 0.8% by mass. Further, it was identified as lithium iron silicate and lithium manganese silicate by X-ray diffraction. As a result of X-ray diffraction measurement of the precursor as in Example 1, it was confirmed that the obtained precursor was amorphous of iron, manganese and silicon. When this positive electrode active material was observed by SEM, the primary particle diameter was 50 to 250 nm. The specific surface area determined by the BET method was 31.7 m ² / g. The evaluation results of the battery capacity are shown in Table 1.

(Comparative Example 1)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the amount of lithium carbonate mixed was 13.0 g. The composition ratio of lithium: iron: silicon of the obtained positive electrode active material was 1.86: 0.99: 1.00, the carbon content was 4.4 mass%, and the sodium content was 0.8. It was mass%. A weak peak identified as lithium iron silicate was detected by X-ray diffraction. When this positive electrode active material was observed by SEM, the primary particle diameter was 50 to 250 nm. The specific surface area determined by the BET method was 27.8 m ² / g. The evaluation results of the battery capacity are shown in Table 1.

(Comparative Example 2)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the amount of lithium carbonate mixed was 15.5 g. The composition ratio of lithium: iron: silicon of the obtained positive electrode active material was 2.19: 0.98: 1.00, the carbon content was 4.8% by mass, and the sodium content was 0.9. It was mass%. Further, it was identified as lithium iron silicate by X-ray diffraction. When this positive electrode active material was observed with an SEM, spherical particles having a primary particle diameter of 50 to 250 nm and plate-like particles having several hundreds of nm were observed. The specific surface area determined by the BET method was 21.8 m ² / g. The evaluation results of the battery capacity are shown in Table 1.

(Comparative Example 3)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the firing temperature was 800 ° C. The composition ratio of lithium: iron: silicon of the obtained positive electrode active material was 2.01: 0.98: 1.00, the carbon content was 1.6% by mass, and the sodium content was 0.9. It was mass%. Further, it was identified as lithium iron silicate by X-ray diffraction. When this positive electrode active material was observed by SEM, the primary particle size was several μm. The specific surface area determined by the BET method was 12.3 m ² / g. The evaluation results of the battery capacity are shown in Table 1.

(Comparative Example 4)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the firing temperature was 400 ° C. The composition ratio of lithium: iron: silicon of the obtained positive electrode active material was 2.01: 0.98: 1.00. Moreover, it was identified as iron oxide and lithium silicate by X-ray diffraction, and lithium iron silicate was not obtained.

(Comparative Example 5)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the pH during the crystallization reaction was controlled to 6. The composition ratio of lithium: iron: silicon of the obtained positive electrode active material was 2.01: 0.85: 1.00. Iron could not be sufficiently coprecipitated and lithium iron silicate could not be obtained.

(Comparative Example 6)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the pH during the crystallization reaction was controlled at 12. The composition ratio of the obtained positive electrode active material lithium: iron: silicon was 2.01: 0.99: 1.00. Further, when X-ray diffraction was performed, peaks derived from lithium iron silicate and sodium silicate were detected. The sodium content in the positive electrode active material was as high as 4.5% by mass, and sodium silicate was detected as a heterogeneous phase.

In Examples 1 and 2, a theoretical capacity of one-electron reaction (166 mAh / g) was obtained, and in Example 3, which is a composite system of iron and manganese, a capacity of 184 Ah / g higher than that was obtained.
On the other hand, it can be seen that Comparative Examples 1 and 2 in which the composition ratio of lithium is not within the range of the present invention and Comparative Example 3 having a high firing temperature have a significantly low battery capacity. Further, in Comparative Example 4 where the firing temperature was low and in Comparative Examples 5 and 6 where the pH during crystallization was not within the range of the present invention, a lithium metal silicate could not be obtained.

Claims (7)

A metal silicate represented by the general formula M _y SiO ₃ (0.90 ≦ y ≦ 1.10, M is one or more elements selected from Mn, Fe, Ni, Co), Lithium characterized in that it is an amorphous coprecipitate of M element and silicic acid obtained by mixing a solution containing divalent ions of M element and a solution containing silicate ions (SiO ₃ ²⁻ ). A precursor of a positive electrode active material for a secondary battery. 2. The precursor of a positive electrode active material for a lithium secondary battery according to claim 1 , wherein a sodium content in the metal silicate is 1.0% by mass or less. Table by the general formula _{Li x M y SiO 4 (1.90} ≦ x ≦ 2.15,0.90 ≦ y ≦ 1.10, M is Mn, 1 or more elements selected Fe, Ni, from Co) precursor of the positive electrode active material for a lithium secondary battery according to claim 1 or 2 characterized in that it is a precursor for forming a positive electrode active material for a lithium secondary battery comprising a metal silicate lithium salt . It is a manufacturing method of the precursor of the positive electrode active material for lithium secondary batteries of any one of Claims 1-3 ,
In the above general formula of the metal silicate represented by the general formula M _y SiO ₃ (0.90 ≦ y ≦ 1.10, M is one or more elements selected from Mn, Fe, Ni, Co) The solution containing the divalent ions of the element and the solution containing the silicate ions (SiO ₃ ²⁻ ) are mixed while maintaining and adjusting the pH in the range of pH 8 to 11, and the coprecipitate is crystallized. , Washing with water, and drying, a method for producing a precursor of a positive electrode active material for a lithium secondary battery . The method for producing a precursor for a positive electrode active material for a lithium secondary battery according to claim 4 , wherein the solution containing silicate ions is sodium silicate or an aqueous solution thereof. 6. The positive electrode active material precursor for a lithium secondary battery according to claim 4 , wherein the pH adjustment is performed using at least one of sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonia. Manufacturing method. The method for producing a precursor of a positive electrode active material for a lithium secondary battery according to any one of claims 4 to 6 , wherein the crystallization reaction of the coprecipitate is performed in an inert atmosphere.

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