JP5860377B2 - Lithium silicate compound and lithium ion battery using the same - Google Patents

Description

The present invention relates to a lithium silicate compound and a lithium ion battery using the lithium silicate compound, and more specifically, as a positive electrode active material by substituting at least part of a metal element constituting a Li ₂ MnSiO ₄ composition with an Al element. The present invention relates to a lithium silicate compound capable of increasing the capacity of a lithium ion battery 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.

So far, solid phase method, hydrothermal method, sol-gel method and the like are known as methods for synthesizing lithium metal silicate. In the solid phase method, it has been reported that lithium silicate and oxalate as a divalent transition metal raw material are mixed in a solvent and baked 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.
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.
Further, it has been reported that a high-purity metal lithium silicate can be obtained by a hydrothermal method as a metal silicate lithium salt without using an oxalate (for example, Non-Patent Document 2). The crystal structure of the lithium metal silicate is orthorhombic. However, the hydrothermal method requires a pressure vessel, and the amount of product obtained with respect to the size of the apparatus is small, and mass production cannot be performed.

Furthermore, as a method for producing a lithium metal silicate salt that is efficient in a short time, it is proposed to mix, heat, melt, and slowly cool a compound that is a source of alkali metal, transition metal and silicon. (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 also proposed that the raw material metal silicate solution and a solution containing silicate ions were mixed under a specific pH condition to crystallize a coprecipitate, followed by solid-liquid separation, washing with water, and drying. (For example, refer to Patent Document 4).

  However, a conventional lithium silicate compound has a capacity of about 190 mAh / g corresponding to 1.2 electron reaction at most in a high temperature environment, and it is difficult to cause a sufficient battery charge / discharge reaction in a room temperature environment. Therefore, the appearance of a lithium silicate compound excellent in conductivity by a liquid phase method that solves the problem of the crystal structure of the lithium silicate compound is desired.

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

An object of the present invention is to provide a lithium silicate compound capable of increasing the capacity of a lithium ion battery when used as a positive electrode active material by substituting a part of the Li ₂ MnSiO ₄ composition with an Al element, and lithium using the lithium silicate compound It is to provide an ion battery.

As a result of intensive studies to solve the above problems, the inventors mixed an aqueous solution containing a specific amount of Al salt and an Si source solution into an aqueous solution containing an Mn salt and an Li salt, and after drying, The composite oxide obtained by firing in an active atmosphere or a reducing atmosphere has a crystal structure equivalent to that of Li ₂ MnSiO _4, and the maximum intensity peak of the powder XRD measurement pattern measured under conditions using CuKα rays is specified. As a result, the present inventors have found that the present invention has been completed.

That is, according to the first invention of the present invention, a lithium silicate compound in which at least a part of a metal element constituting the Li ₂ MnSiO ₄ composition is substituted with an Al element, the crystal structure is equivalent to that of Li ₂ MnSiO ₄ . There is provided a lithium silicate compound characterized in that the maximum intensity peak of the powder XRD measurement pattern measured under conditions using CuKα rays is a diffraction angle 2θ = 32.5 degrees to 33.0 degrees .

According to a second aspect of the present invention, there is provided a lithium silicate compound characterized in that in the first aspect, a carbon coat layer is formed on the surface.
The third invention of the present invention is a method for producing a lithium silicate compound of the first or second invention,
An aqueous solution containing an Mn salt and an Li salt, and an aqueous solution containing an Al salt, an Si source solution, Li: Mn: Si: Al = 2: y: 1: 1-y (0.9 ≦ y <1) A method for producing a lithium silicate compound, comprising obtaining the lithium silicate compound having a composite oxide as a main component obtained by mixing, drying, and firing in an inert atmosphere or a reducing atmosphere. Is provided.
According to a fourth aspect of the present invention, there is provided the method for producing a lithium silicate compound according to the third aspect , wherein the Si source is a silicon compound that causes dehydration condensation upon drying.
According to a fifth aspect of the present invention, there is provided the method for producing a lithium silicate compound according to the third or fourth aspect , wherein the Si source is a water-soluble silicon compound.
According to a sixth invention of the present invention, in any one of the third to fifth inventions, the aqueous solution further contains an Fe salt, and the content thereof is Li: Mn: Fe: Si: Al = 2. : Xy: y-xy: 1: 1-y (0.75 ≦ x <1, 0.9 ≦ y <1) is provided. A method for producing a lithium silicate compound is provided. Is done.

On the other hand, according to the seventh invention of the present invention, there is provided a lithium ion battery using the lithium silicate compound of the first or second invention as a positive electrode active material.

The lithium silicate compound of the present invention is obtained by substituting a part of the Li · Mn · Si composite oxide with Al, has a crystal structure equivalent to Li ₂ MnSiO _4, and is measured under the conditions using CuKα rays. Since it has a novel crystal structure in which the maximum intensity peak of the powder XRD measurement pattern is a diffraction angle 2θ = 32.5 degrees to 33.0 degrees, by using it as a positive electrode active material, high safety and A lithium secondary battery having high capacity and excellent cycle life can be easily manufactured.
Furthermore, this lithium silicate compound has a carbon coating on the surface, and is a material that achieves both high reactivity and easy handling of the powder.

It is a chart which shows the result of having analyzed the lithium silicate compound of this invention by X-ray diffraction.

  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. [Lithium silicate compound]
The lithium silicate compound of the present invention is a lithium silicate compound in which at least a part of a metal element constituting the Li ₂ MnSiO ₄ composition is substituted with an Al element, the crystal structure is equivalent to Li ₂ MnSiO ₄ , and CuKα ray The maximum intensity peak of the powder XRD measurement pattern measured under the conditions using is a diffraction angle 2θ = 32.5 degrees to 33.0 degrees.

The lithium silicate compound of the present invention has a structure in which at least a part of a metal element in Li ₂ MnSiO ₄ composition serving as a crystal skeleton, that is, Li, Mn, or Si is substituted with an Al element. In particular, since Si ⁴⁺ ions and Al ³⁺ ions have similar covalent bonds and ionic radii, they can be substituted with lithium silicate compounds, but such lithium silicate compounds have been known so far. Not.

In the present invention, as described in detail later, when a lithium silicate compound is synthesized using a silicon source that undergoes dehydration condensation at the time of drying, such as water-soluble propylene glycol-modified silane (PGMS), concentrated Al ³⁺ Since ions tend to easily participate in the condensation reaction, it has become possible to efficiently obtain a structure in which Si element in SiO ₄ is Al-isomorphously substituted with Al element. Li and Mn may take a structure substituted with Al element because the ionic radius is relatively close.

According to the powder XRD measurement pattern measured under conditions using CuKα rays, the lithium silicate compound of the present invention mainly has a crystal structure equivalent to Li ₂ MnSiO ₄ whose maximum intensity peak is the space group Pmn21, that is, orthorhombic crystal The diffraction angle is 2θ = 32.5 degrees to 33.0 degrees.

Here, mainly means that the crystal structure equivalent to Li ₂ MnSiO ₄ , that is, orthorhombic is 80% or more. The lithium silicate compound of the present invention may contain a small amount of crystal structure other than orthorhombic crystal, that is, monoclinic crystal. Monoclinic crystals may be generated due to the synthesis conditions of the lithium silicate compound, such as an increase in the firing temperature. However, considering the capacity in the case of a battery, the orthorhombic crystal is preferably 90% or more.

In the lithium silicate compound of the present invention, the reason why the capacity is increased when Al is doped is that the lattice volume is increased by replacing Si in SiO ₄ which is the crystal skeleton of the lithium silicate compound with an Al element, This is thought to be because the lithium movement route becomes wider. Further, when Mn is substituted with Al element, it is considered that vacancy of Li site is formed in order to balance the charge of the whole material, and ion conductivity is improved. On the other hand, when Li is substituted with an Al element, ionic conductivity can be increased.
Here, the Al doping amount is 10 mol% or less. If it exceeds 5 mol%, the capacity of the battery is reduced because Li not formed in the crystal structure forms a different phase in order to maintain the charge balance. A more preferable Al doping amount is 5 mol% or less.

2. [Method for producing lithium silicate compound]
In the lithium silicate compound of the present invention, an Si source solution is mixed with an aqueous solution containing an Mn salt and an Li salt and an aqueous solution containing an Al salt so that Li: Mn: Si: Al has a specific molar ratio. After drying, it is produced by firing in an inert 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 is preferable from the viewpoint of improving battery characteristics, and iron acetate, iron nitrate, iron citrate, and the like can be used.
In the present invention, aluminum acetate, aluminum nitrate or the like is used as the Al salt in addition to the raw metal component. Of these, it is particularly preferable to use an acetate that is inexpensive and easy to handle. By using the Al salt, the lattice constant of the lithium silicate compound is increased, and in particular, from the viewpoint of lithium diffusibility, increasing the lattice volume is effective for increasing the capacity.

  Further, in the present invention, the Si source is not particularly limited. For example, water-soluble silicon such as propylene glycol modified silane (PGMS), polyethylene glycol modified silane (PEGMS), glycerin modified silane (GMS), tetraethyl ortho Examples thereof include silicate and tetramethylorthosilicate. These may be used alone or in combination. In addition, an ethanol solution of tetraethoxysilane dissolved in an arbitrary concentration, fumed silica, and the like can be given.

As the water-soluble silicon compound, propylene glycol modified silane (PGMS) is preferable. 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.
Tetraethylorthosilicate and tetramethylorthosilicate are silicon compounds that similarly undergo dehydration condensation upon drying, and during the polycondensation of silane, Al ³⁺ ions tend to be added to the condensation reaction. It is easy to obtain a structure in which Si element in SiO ₄ is Al-isomorphously substituted with Al element. Furthermore, it is more preferable to use a water-soluble silicon compound (for example, propylene glycol modified silane (PGMS), etc.) because it can be handled in an aqueous solution.

This is because the propylene glycol-modified silane (PGMS) has a property of being dissolved in water at an arbitrary ratio and having high stability. 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 material metal components Next, a solution of a water-soluble silicon compound as a Si source in an aqueous solution containing a Mn salt, a Li salt and an Al salt has a metal element ratio of Li: Mn: Si: Al = 2: y: 1: 1-y (0.9 ≦ y <1). The metal element ratio is preferably Li: Mn: Si: Al = 2: y: 1: 1-y (0.95 ≦ y <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 is Li: Mn: Fe: Si: Al = 2: xy: y-xy: 1: 1-y (0.75 ≦ x <1, 0.9 ≦ y <1). By setting the metal element ratio within this range, an active material having excellent capacity characteristics and cycle characteristics can be produced. Here, preferable ranges of x and y are 0.80 ≦ x <1 and 0.95 ≦ y <1.
First, various raw material salts are blended in a predetermined ratio and mixed. As for the mixing method, each raw material may be mixed uniformly, and pulverized and mixed with a reika machine or a ball mill. Here, it is preferable to use a water-soluble salt as a raw material because it is uniformly mixed in the solution. Furthermore, it is preferable because Al ³⁺ ions are easily taken in during polycondensation of silicate.

Moreover, you may mix a carbon source simultaneously with various raw material salts. The carbon source is carbon-coated on each lithium silicate compound particle by a heat treatment performed thereafter.
As a carbon source, an organic acid having a carboxyl group and a hydroxyl group in the molecule or having two or more carboxyl groups, for example, citric acid [CH ₂ —COOH (HO—C—COOH) CH ₂ —COOH], tartaric acid [COOH] (CHOH) ₂ COOH], malonic acid [HOOC—CH ₂ —COOH], 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.
As the carbon source, a water-soluble organic compound having a melting point of 250 ° C. or lower is preferable, and citric acid is particularly preferable.

The mixing of the aqueous solution of Mn salt, Li salt, and Al salt and the Si source solution is sufficient if the raw material metal components can be mixed uniformly, and a normal 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 is precipitated in the solid phase as manganese oxide depending on the pH of the aqueous solution and 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.

Next, the obtained mixed powder or gel of the raw material is dried and then carbon-coated by heat treatment. Although there is no restriction | limiting in particular as a drying method, The method in which particle | grains do not aggregate is preferable, for example, it is preferable to freeze-dry. When lyophilization is employed, a solidified product in which the raw materials are uniformly mixed can be easily obtained without forming an insoluble Mn organic acid complex.
Freeze-drying is preferably carried out under reduced pressure conditions of −30 to 0 ° C. and 4 to 48 hours after the aqueous dispersion is frozen before becoming a gel state. More preferably, after freezing, it is carried out under reduced pressure conditions of -30 to -10 ° C and 4 to 36 hours. If the aqueous dispersion is not frozen until it is in a gel state, the organic acid and Mn react with each other to produce an aggregate of the organic acid Mn salt, which is not preferable. For freeze-drying, a freeze-drying device having a normal cooling / depressurizing function can be used.

(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 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.

  By firing the carbon-coated mixture at 500 to 900 ° C., preferably 600 to 800 ° C. in a non-oxidizing atmosphere, a fine lithium silicate compound with good crystallinity can be obtained. When the firing temperature is less than 500 ° C., the crystallinity of the material is low, and when used as a positive electrode active material for a lithium secondary battery, the capacity decreases. On the other hand, when the temperature is higher than 900 ° C., transpiration of lithium becomes remarkable, and it becomes difficult to obtain a material having a desired composition ratio.

In order to suppress oxidation of Al, Fe element and carbon source in the lithium silicate compound, the firing atmosphere is preferably an inert atmosphere such as nitrogen or argon or a reducing atmosphere including a heat treatment. It is more preferable to use medium. The reducing atmosphere is preferably an inert gas containing hydrogen, particularly a nitrogen atmosphere containing hydrogen, and the hydrogen content is preferably 2 to 4%.
The firing time is not particularly limited, and is a time for obtaining a lithium silicate compound that is 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 lithium silicate compound may not be sufficient, and if it exceeds 20 hours, sintering of the lithium silicate compound particles proceeds and fine particles may not be obtained.

Further, by replacing a part of the Mn salt with an additive element (M) salt such as an Fe salt, the general formula Li ₂ Mn _xy Al _1-y M _y-xy SiO ₄ (M is Fe, Co, Ca, Zn) , A transition metal such as Mg or an alkaline earth metal, and a lithium silicate compound represented by 0.75 ≦ x <1, 0.9 ≦ y <1) can be synthesized, and an increase in charge / discharge capacity, Effects such as improved cycle performance can be obtained. 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 in the form of particles having an average primary particle size of 1000 nm, and if necessary, is pulverized to form particles having a size of 300 nm or less. However, if PGMS is used as the Si source, the carbon contained in PGMS inhibits the sintering, so that there is no need for a separate pulverization and fine particle process.

Further, when PGMS is used as the Si source, the carbon remains about 0.5 to 2.0% by mass even after the completion of 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.

If the material contains 0.5 to 2.0% by mass of carbon, the conventional carbon supporting step can be omitted. Further, 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 as described above with a raw material before firing and firing in an inert atmosphere, a conductive carbon as a material after firing, etc. The method of making it adhere mechanically can be used.
Since lithium silicate compound particles usually have poor conductivity, the carbon silicate improves the conductivity of the lithium silicate compound particles and can exhibit battery characteristics.

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 PROMRD), the obtained positive electrode active material was measured by powder X-ray diffraction using Cu-Kα rays.
(2) 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. .

(3) Battery evaluation After the obtained lithium silicate compound was crushed in a mortar for 10 minutes, acetylene black / fibrous graphite as a conductive additive, 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 room temperature and 60 ° C. As the charge / discharge device, HJ-SM8 (manufactured by Hokuto Denko) was used.

Example 1
First, water-soluble silicon propylene glycol modified silane (PGMS) 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. Further, 100 μl of hydrochloric acid was added to the mixed solution and stirred at room temperature for 1 hour. Distilled water was returned to room temperature in this stirring liquid, and diluted with a 100 ml volumetric flask to prepare 1M water-soluble silicon.
Next, 15 ml of this water-soluble silicon, 15 ml of 2M lithium acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution, 13.5 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution, aluminum nitrate nonahydrate ( Kanto Chemical Co., Ltd., special grade) 1.5 mmol and hydrochloric acid (Kanto Chemical Co., Ltd., special grade) 160 μl were mixed and stirred at room temperature for 30 minutes.
Thereafter, the obtained aqueous solution was frozen at −30 ° C. and freeze-dried while reducing the pressure. The obtained dried gel was baked at 700 ° C. in a nitrogen gas atmosphere for 5 hours. The primary particle diameter of the powder was an average of 500 nm. The XRD measurement result of the powder after baking is shown in FIG. Moreover, the maximum intensity peak of the powder XRD measurement pattern measured under the conditions using CuKα rays was a diffraction angle 2θ = 32.9 degrees.
As a result, fine particles of a lithium silicate compound having a structure in which a part of a metal element constituting the Li ₂ MnSiO ₄ composition is substituted with an Al element and having a crystal structure equivalent to Li ₂ MnSiO ₄ which is the space group Pmn21 are obtained. It was confirmed that Moreover, 1.0 mass% carbon was contained when the amount of carbon was measured. Further, the battery was constructed by the above-described method, and the characteristics were evaluated. Table 1 shows the measurement results such as conductivity.

(Comparative Example 1)
In place of 13.5 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution and 1.5 mmol of aluminum nitrate nonahydrate (special grade, manufactured by Kanto Chemical Co., Ltd.) used in Example 1, 1M manganese acetate (Wako Pure Chemical Industries, Ltd.) A sample was prepared in the same manner as in Example 1 except that 15 ml of an aqueous solution was used and aluminum nitrate nonahydrate was not used. Table 1 shows the XRD measurement results and the battery evaluation results.

(Example 2)
13.5 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution used in Example 1 was added to 12 ml of 1M manganese acetate (Wako Pure Chemical Industries, Ltd., special grade) aqueous solution, 0.5M iron citrate (Kanto Chemical). Example 1 except that 160 ml of hydrochloric acid (Kanto Chemical Co., Ltd., special grade) was changed to 15 ml of 1 M citric acid ((Wako Pure Chemical Industries, Ltd., special grade) aqueous solution. Samples were prepared in the same manner as in Table 2. Table 2 shows XRD measurement results and battery evaluation results.

(Comparative Example 2)
In the same manner as in Example 1, 1.5 mmol of aluminum nitrate nonahydrate (manufactured by Kanto Chemical Co., Ltd., special grade) was changed to 3.0 ml of 0.5 M iron citrate (Kanto Chemical Co., Ltd., first grade) aqueous solution. A sample was prepared. Table 2 shows the XRD measurement results and the battery evaluation results.

"Evaluation"
From Table 1 showing the above results, in Examples 1 and 2, according to the present invention, an Si source was mixed with an aqueous solution containing at least Mn salt, Al salt and Li salt so as to have a specific metal element ratio, and after drying, By firing the solidified product, a lithium silicate compound having a specific crystal structure is obtained. Thereby, when it uses as a positive electrode active material of a lithium ion battery, it turns out that not only room temperature capacity but high temperature capacity can be improved.
On the other hand, in Comparative Examples 1 and 2, since an Al salt was not used as a metal component, 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 lithium silicate compound in which at least a part of a metal element constituting the composition of Li ₂ MnSiO ₄ is substituted with an Al element, the crystal structure is equivalent to that of Li ₂ MnSiO ₄ , and the measurement is performed under the condition using CuKα rays. A lithium silicate compound, wherein the maximum intensity peak of the powder XRD measurement pattern is a diffraction angle 2θ = 32.5 degrees to 33.0 degrees. 2. The lithium silicate compound according to claim 1, wherein a carbon coat layer is formed on the surface. A method for producing a lithium silicate compound according to claim 1 or 2,
An aqueous solution containing an Mn salt and an Li salt, and an aqueous solution containing an Al salt, an Si source solution, Li: Mn: Si: Al = 2: y: 1: 1-y (0.9 ≦ y <1) A method for producing a lithium silicate compound, comprising obtaining the lithium silicate compound having a composite oxide as a main component obtained by mixing, drying, and firing in an inert atmosphere or a reducing atmosphere. . 4. The method for producing a lithium silicate compound according to claim 3 , wherein the Si source is a silicon compound that undergoes dehydration condensation upon drying. The method for producing a lithium silicate compound according to claim 3 or 4 , wherein the Si source is a water-soluble silicon compound. The aqueous solution further contains an Fe salt, and the content thereof is Li: Mn: Fe: Si: Al = 2: xy: y-xy: 1: 1-y (0.75 ≦ x <1, 0.9 6. The method for producing a lithium silicate compound according to claim 3 , wherein a molar ratio of ≦ y <1) is satisfied. A lithium battery using the lithium silicate compound according to claim 1 or 2 as a positive electrode active material.

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