JP2014071948A - Method for producing negative electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve initial charge/discharge efficiency and cycle characteristics of a nonaqueous electrolyte secondary battery when the nonaqueous electrolyte secondary battery is manufactured using a negative electrode active material produced by a method of the present invention.SOLUTION: A method for producing a negative electrode active material comprises: a first step of mixing a lithium compound and SiO(0.8≤X≤1.2) having a surface coated with carbon so that the lithium compound adheres to the surface of the SiO; and a second step of subjecting the SiOhaving a surface on which the lithium compound adheres to heat treatment to form a lithium silicate phase in the SiO.

Description

  The present invention relates to a method for producing a negative electrode active material.

Since the silicon oxide represented by SiO _X has a high specific capacity and a volume expansion coefficient when absorbing lithium during charging is smaller than that of Si, it has been studied to mix with graphite and use it as a negative electrode active material. (See Patent Document 1).
However, the nonaqueous electrolyte secondary battery using the silicon oxide represented by SiO _X as the negative electrode active material has significantly higher initial charge / discharge efficiency and capacity at the beginning of the cycle than when only graphite is used as the negative electrode active material. There is a problem of lowering.
In order to improve the initial charge and discharge efficiency, composite particles having a structure in which silicon oxide is dispersed in a carbon active material and silicon and a lithium silicate phase are included in the silicon oxide have been proposed (Patent Document 2). reference).

JP2011-233245A JP 2007-59213 A

  However, in the proposal described in Patent Document 2, since the silicon oxide dispersed in the carbon active material has a structure in which the silicon oxide is scattered in the carbon matrix, the carbon matrix inhibits lithium diffusion during charging and discharging. To do. For this reason, lithium may not reach the silicon oxide sufficiently, and the actual battery capacity is significantly smaller than the theoretical capacity, and the initial charge / discharge efficiency is reduced.

The present invention includes a first step in which SiO _X (0.8 ≦ X ≦ 1.2) and a lithium compound are mixed to adhere a lithium compound to the surface of SiO _X , and SiO having a lithium compound adhered to the surface. _A second step of heat treating _X to form a lithium silicate phase in the SiO _X.

  In the nonaqueous electrolyte secondary battery using the negative electrode active material manufactured by the method described in the examples of the present invention, the initial charge / discharge efficiency and the cycle characteristics are dramatically improved.

It is a graph showing the XRD measurement result of SiO _X in cell A1, Z.

The present invention includes a first step in which SiO _X (0.8 ≦ X ≦ 1.2) and a lithium compound are mixed to adhere a lithium compound to the surface of SiO _X , and SiO having a lithium compound adhered to the surface. _A second step of heat treating _X to form a lithium silicate phase in the SiO _X.
In the nonaqueous electrolyte secondary battery using the negative electrode active material produced by the above method, the initial charge / discharge efficiency and the cycle characteristics are improved. The reason is shown below.

SiO _X is a fine mixture of Si and SiO _2, and the initial charge reaction when used as a negative electrode active material can be generally expressed by the following formula (1).
4SiO (2Si + 2SiO ₂ ) + 16Li ⁺ + 16e ⁻ → 3Li ₄ Si + Li ₄ SiO ₄ (1)
As in the above formula (1), Li ₄ SiO ₄ is generated during the initial charge, and this Li ₄ SiO ₄ is an irreversible reactant. Therefore, not all Si in SiO _X reacts reversibly, and the theoretical efficiency is lowered. Specifically, when Li ₄ SiO ₄ is generated as an irreversible reactant as in the above formula (1), four of the 16 lithium ions are irreversible, so the theoretical efficiency is 75%. Become.

Therefore, as described above, SiO _{X in} which a lithium silicate phase such as Li ₄ SiO ₄ is formed is used as SiO _X at the time of battery fabrication (before the first charge). With such a configuration, the amount of lithium taken away by the irreversible reactant during the first charge is reduced, so that the first charge / discharge efficiency can be drastically improved. In addition, the volume of the SiO _X particles is increased by forming a lithium silicate phase. Therefore, in the case of using SiO _X as a negative electrode active material, SiO _X having lithium silicate phase, the expansion during charge and discharge than SiO _X having no lithium silicate phase displacement during shrinkage is small. Therefore, if SiO _X having a lithium silicate phase is used, peeling in the negative electrode mixture layer and peeling between the negative electrode mixture layer and the negative electrode current collector can be suppressed, and thus cycle characteristics are improved. In addition, since there is no carbon matrix around SiO _X , lithium diffusion is performed smoothly. Therefore, the actual battery capacity is increased.

In order to form a lithium silicate phase in SiO _X , as shown in the second step in the method for producing a negative electrode active material, SiO _X having a lithium compound attached to the surface may be heat-treated. The reaction formula when LiOH is used as the lithium compound is shown in the following formula (2). As is clear from the formula (2), it can be seen that SiO ₂ existing in SiO _X reacts with LiOH to produce Li ₄ SiO ₄ .
SiO ₂ + 4LiOH → Li ₄ SiO ₄ + 2H ₂ O (2)
The lithium silicate phase is a compound of Li, Si, and O. Besides Li ₄ SiO ₄ , there are Li ₂ SiO ₃ and Li ₂ Si ₂ O ₅ , depending on the amount of lithium compound added and the processing method. Products may be different. Further, examples of the lithium compound include Li ₂ CO ₃ , LiF, and LiCl in addition to the above LiOH.

When the lithium silicate phase is formed, the heat treatment temperature is preferably higher than the melting point of the lithium compound. Although depending on the type of the lithium compound, when the heat treatment is performed at a temperature lower than the melting point of the lithium compound, the lithium silicate phase may not be sufficiently formed and may remain as the lithium compound. However, the upper limit of the heat treatment temperature is preferably 1400 ° C. or less, and particularly preferably 1100 ° C. or less. When heated at a temperature higher than 1100 ° C., it occurs Si and SiO ₂ of crystallization in SiO _X, the increased loss and particle resistance of the lithium diffusion paths in SiO _X, the charge-discharge capacity decreases significantly. In particular, when heat treatment is performed at a temperature higher than 1400 ° C., such a problem becomes remarkable, and when the SiO _X surface is coated with carbon, SiC is formed, and charge / discharge capacity is further reduced. .

When forming the lithium silicate phase in the SiO _X, it is preferable to form a uniform lithium silicate phase in the SiO _X. For this purpose, it is necessary to uniformly disperse and adhere the lithium compound to the SiO _X surface before heat treatment. As such a method, there is a method in which SiO _X and a lithium compound are uniformly kneaded in a powder state and then heat-treated (hereinafter, this method may be referred to as a dry treatment). In addition, there is a method in which a lithium compound and SiO _X are dispersed in a solvent such as water and dried to precipitate the lithium compound on the surface of SiO _X and then heat treatment is performed (hereinafter, this method is referred to as a wet treatment). is there). In general, the wet process is preferably used rather than the dry process because the lithium compound is more easily dispersed and adhered to the surface of the SiO _X surface. Incidentally, when performing a wet process, as the SiO _X and the lithium compound is dispersed well through a solvent, it may be dispersed with addition of dispersants and surfactants. In this case, since the dispersant and the surfactant are evaporated in the subsequent heat treatment step, there is almost no influence on the battery characteristics.
As the surfactant, it is preferable that the battery performance is not affected, and specifically, nonionic surfactants such as polyoxyethylene alkyl ether and polyoxyethylene alkyl ester that do not hinder ion conduction are preferable. . Examples of the dispersant include CMC (carboxymethylcellulose), sodium polyacrylate, and the like.

Moreover, it is preferable to include the process of removing an unreacted lithium compound after heat processing with a lithium compound. By removing the lithium compound that does not contribute to the charge / discharge reaction, the capacity per weight can be increased. In addition, lithium compounds often show alkalinity in an aqueous solvent. For this reason, when CMC as a dispersing agent (thickening agent) is used at the time of negative electrode slurry preparation, the problem that a thickening effect falls remarkably and CMC will open a ring also arises.
In addition, as a method for removing the unreacted lithium compound, SiO _X after the thermal (chemical conversion) treatment may be washed with water, or may be neutralized using ultrasonic washing or acid / alkali.

The surface of SiO _X used in the present invention may be coated with carbon. Since SiO _X has low electron conductivity, the electron conductivity can be increased by coating the surface with carbon.
In the case where the surface of SiO _X is coated with carbon, it is preferable to perform carbon coating after forming a lithium silicate phase in SiO _X. Since the carbon coating prevents the diffusion of the lithium compound, the lithium compound may not be uniformly diffused into the SiO _X if the carbon coating is present before the heat treatment. Therefore, the lithium silicate phase is unevenly distributed. In addition, since the amount of lithium diffusion into SiO _X is reduced, there may be a problem that the formed lithium silicate phase is reduced. Furthermore, due to the decrease in the amount of lithium diffused into the SiO _X , unreacted lithium compounds remain on the surface of the carbon coating, and this adheres to the surface of the carbon coating, leading to a problem that the conductivity is lowered. .

When coating with carbon, the thickness of the carbon coating is preferably 1 nm or more and 200 nm or less. If it is less than 1 nm, the conductivity is low and it is difficult to coat uniformly. On the other hand, when the thickness exceeds 200 nm, the carbon coating inhibits lithium diffusion, so that lithium does not reach the SiO _X sufficiently, and the capacity is greatly reduced.

The ratio of the number of moles of the lithium silicate phase to the total number of moles of single particles of SiO _X (0.8 ≦ X ≦ 1.2) is preferably 0.5 mol% or more and 25 mol% or less. When the proportion of the lithium silicate phase is less than 0.5 mol%, the effect of improving the initial charge / discharge efficiency is small. On the other hand, when the proportion of the lithium silicate phase exceeds 25 mol%, the amount of Si that undergoes reversible reaction decreases, and the charge / discharge capacity decreases.

SiO _X used in the present invention may be used alone as a negative electrode active material, or may be used by mixing with a carbon-based active material such as graphite or hard carbon. Since the specific capacity of SiO _X is higher than that of the carbon-based active material, the capacity can be increased as the addition amount increases. However, SiO _X has a larger expansion / contraction rate at the time of charge / discharge than the carbon-based active material, and if the ratio is too large, peeling at the interface between the negative electrode mixture layer and the negative electrode current collector, or the negative electrode active material Since the conductive contact between the particles is reduced, the cycle characteristics may be significantly reduced. Therefore, when SiO _X and a carbon-based active material are mixed and used, the ratio of SiO _{X to} the total amount of the negative electrode active material is preferably 20% by mass or less. On the other hand, if the proportion of SiO _X is too small, the merit of increasing the capacity by adding SiO _X is reduced, so the proportion of SiO _X with respect to the total amount of the negative electrode active material is preferably 1% by mass or more.

The average primary particle diameter of SiO _X used in the present invention is preferably 1 μm or more and 15 μm or less. When the average primary particle diameter of SiO _X is less than 1 μm, the particle surface area becomes too large, the amount of reaction with the electrolytic solution increases, and the capacity may decrease. Further, the amount of expansion and contraction of SiO _X is small, and the influence on the negative electrode mixture layer is small. Therefore, even if a lithium silicate phase is not formed in advance in SiO _X , separation between the negative electrode mixture layer and the negative electrode current collector hardly occurs and the cycle characteristics do not deteriorate so much. On the other hand, when the average primary particle diameter of SiO _X exceeds 15 μm, lithium may not diffuse into the inside of SiO _X during the formation of the lithium silicate phase, and the lithium silicate phase may be formed only on the SiO _X surface. . Since the lithium silicate phase is insulative, when such a structure is used, lithium diffusion is hindered, and lithium cannot be diffused to the vicinity of the center of SiO during charge / discharge, which may result in a decrease in capacity and load characteristics. Therefore, the average primary particle diameter of SiO _X is preferably 1 μm or more and 15 μm or less, and particularly preferably 4 μm or more and 10 μm or less.
In addition, the average primary particle diameter (D ₅₀ ) of SiO _X is the cumulative 50 volume% diameter in the particle size distribution measured by the laser diffraction scattering method.

The positive electrode and the non-aqueous electrolyte can be used without any particular limitation as long as they are used for a non-aqueous electrolyte secondary battery.
Examples of the positive electrode active material include lithium complex oxide containing lithium cobaltate, nickel or manganese, olivine type lithium phosphate represented by lithium iron phosphate (LiFePO ₄ ), and the like. Examples of the lithium composite oxide containing nickel or manganese include lithium composite oxides such as Ni—Co—Mn, Ni—Mn—Al, and Ni—Co—Al. These positive electrode active materials may be used alone or in combination.

The solvent and solute of the nonaqueous electrolyte solution are not particularly limited as long as they can be used for the nonaqueous electrolyte secondary battery.
Solutes of the non-aqueous electrolyte include LiBF ₄ , LiPF ₆ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _6-x (C _n F _{2n + 1} ) _x [wherein 1 <x <6, n = 1 or 2], or a lithium salt having an oxalato complex as an anion can also be used. As a lithium salt having this oxalato complex as an anion, in addition to LiBOB [lithium-bisoxalate borate], a lithium salt having an anion in which C ₂ O ₄ ²⁻ is coordinated to the central atom, for example, Li [M (C ₂ O ₄ ) _x R _y ] (wherein M is a transition metal, an element selected from groups IIIb, IVb, and Vb of the periodic table, R is selected from a halogen, an alkyl group, and a halogen-substituted alkyl group) Group, x is a positive integer, and y is 0 or a positive integer). Specifically, there are Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ], and the like. However, it is most preferable to use LiBOB in order to form a stable film on the surface of the negative electrode even in a high temperature environment.

  In addition, the said solute may be used not only independently but in mixture of 2 or more types. The concentration of the solute is not particularly limited, but is preferably 0.8 to 1.8 mol per liter of the electrolyte. Furthermore, in applications that require discharging with a large current, the concentration of the solute is desirably 1.0 to 1.6 mol per liter of the electrolyte.

  On the other hand, as the solvent of the non-aqueous electrolyte, carbonate solvents such as ethylene carbonate, propylene carbonate, γ-butyl lactone, diethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and a part of hydrogen in these solvents are F. Substituted carbonate solvents are preferably used. As the solvent, it is preferable to use a combination of a cyclic carbonate and a chain carbonate.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications without departing from the scope of the present invention. It is a thing.
<First embodiment>

Example 1
(Production of negative electrode)
SiO _X (X = 0.93, average primary particle size: 5.0 μm) whose surface was coated with carbon was prepared. The coating was performed using the CVD method, and the ratio of carbon to SiO _X was 10% by mass. 1 mol of SiO _X and 0.2 mol of LiOH were mixed in a powder state (the ratio of LiOH to SiO _X is 20 mol%), and LiOH was adhered to the surface of SiO _X. Next, heat treatment was performed in an Ar atmosphere at 800 ° C. for 10 hours to produce SiO _X in which a lithium silicate phase was formed. When SiO _X after this heat treatment was analyzed by XRD (the radiation source was CuKα), as shown in FIG. 1, the peaks of Li ₄ SiO ₄ and Li ₂ SiO ₃ which are lithium silicates were confirmed. Further, the number of moles of lithium silicate phase to the total number of moles of SiO _X (hereinafter, the ratio of the lithium silicate phase in the SiO _X, may be referred to as) was 5 mol%.

SiO _X in which the lithium silicate phase is formed and PAN (polyacrylonitrile) as a binder are mixed at a mass ratio of 95: 5, and NMP (N-methyl-2-pyrrolidone as a dilution solvent) is further mixed. ) Was added. This was stirred using a mixer (Primics, Robomix) to prepare a negative electrode mixture slurry.
The negative electrode mixture slurry was applied on one surface of a copper foil so that the mass per 1 m ^{2 of the} negative electrode mixture layer was 25 g / m ² . Next, this was dried at 105 ° C. in the atmosphere and rolled to prepare a negative electrode. The filling density of the negative electrode mixture layer was 1.50 g / ml.

(Preparation of non-aqueous electrolyte)
To a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ ) was added at 1.0 mol / liter. This was added to prepare a non-aqueous electrolyte.

[Assembling the battery]
In an inert atmosphere, an electrode body was produced using the above negative electrode with a Ni tab attached to the outer periphery, a lithium metal foil, and a polyethylene separator disposed between the negative electrode and the lithium metal foil. This electrode body was put into a battery casing made of an aluminum laminate, and a non-aqueous electrolyte was injected into the battery casing, and then the battery casing was sealed to produce a battery.
The battery thus produced is hereinafter referred to as battery A1.

(Example 2)
When mixing and heat-treating the lithium source and SiO _X , Li ₂ CO ₃ was used instead of LiOH as the lithium source (the ratio of Li ₂ CO ₃ to SiO _X was 10 mol%), A battery was fabricated in the same manner as in Example 1 of the first example. Incidentally, the SiO _X after heat treatment, was analyzed by XRD, the peak of the _Li 4 SiO ₄ and _Li 2 SiO ₃ is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%.
The battery thus produced is hereinafter referred to as battery A2.

(Example 3)
When mixing and heat-treating the lithium source and SiO _X , LiCl was used instead of LiOH as the lithium source (the ratio of LiCl to SiO _X was 20 mol%). A battery was produced in the same manner as in Example 1. Incidentally, the SiO _X after heat treatment, was analyzed by XRD, the peak of the _Li 4 SiO ₄ and _Li 2 SiO ₃ is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%.
The battery thus produced is hereinafter referred to as battery A3.

Example 4
When the heat treatment was performed by mixing the lithium source and SiO _X , LiF was used instead of LiOH as the lithium source (the ratio of LiF to SiO _X was 20 mol%). A battery was produced in the same manner as in Example 1. Incidentally, the SiO _X after heat treatment, was analyzed by XRD, the peak of the _Li 4 SiO ₄ and _Li 2 SiO ₃ is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%.
The battery thus produced is hereinafter referred to as battery A4.

(Comparative example)
Example of the first example except that LiOH and SiO _X were not mixed and heat treatment was not performed (that is, untreated SiO _X was used as SiO _X as the negative electrode active material). A battery was produced in the same manner as in Example 1. When this SiO _X was analyzed by XRD, a lithium silicate phase was not confirmed as shown in FIG.
The battery thus produced is hereinafter referred to as battery Z.

(Experiment)
The batteries A1 to A4, Z were charged / discharged under the following conditions, and the initial charge / discharge efficiency represented by the following formula (3) and the capacity retention rate at the 10th cycle represented by the following formula (4) were examined. The results are shown in Table 1.
(Charging / discharging conditions)
Constant current charging was performed until the voltage became 0 V with a current of 0.2 It (4 mA), and then constant current charging was performed until the voltage became 0 V with a current of 0.05 It (1 mA). Next, after resting for 10 minutes, constant current discharge was performed until the voltage became 1.0 V at a current of 0.2 It (4 mA).

[Calculation formula for initial charge / discharge efficiency]
Initial charge / discharge efficiency (%) = (discharge capacity at the first cycle / charge capacity at the first cycle) × 100 (3)
[Calculation formula of capacity maintenance ratio at 10th cycle]
Capacity maintenance ratio (%) at 10th cycle = (discharge capacity at 10th cycle / discharge capacity at 1st cycle) × 100 (4)

Batteries A1 to A4 using SiO _X having a lithium silicate phase inside have improved initial charge / discharge efficiency and cycle characteristics as compared to battery Z using SiO _X having no lithium silicate phase inside. I understand. This is because if SiO _X before charge / discharge has a lithium silicate phase in advance, the amount of lithium taken away by Li ₄ SiO ₄ generated at the time of initial charge is small, and the amount of lithium that can be involved in charge / discharge increases. Because it does. In addition, SiO _X having a lithium silicate phase inside has a smaller degree of expansion during charging although the charge amount is the same as SiO _X having no lithium silicate phase inside. Therefore, it is considered that the difference in expansion and contraction during charge / discharge is reduced, and peeling at the negative electrode mixture layer is suppressed.
As the lithium compound used in the heat treatment is not limited to _{LiOH, Li 2 CO 3, LiCl} , or LiF to express same effect it was confirmed. Moreover, it can be estimated that even if it is lithium compounds other than these, the same effect is expressed.

<Second embodiment>
(Example 1)
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 2 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, a peak of Li ₂ SiO ₃ which is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 0.5 mol%.
The battery thus produced is hereinafter referred to as battery B1.

(Example 2)
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 50 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 12.5 mol%.
The battery thus produced is hereinafter referred to as battery B2.

(Example 3)
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 80 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 20 mol%.
The battery thus produced is hereinafter referred to as battery B3.

Example 4
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 100 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 25 mol%.
The battery thus produced is hereinafter referred to as battery B4.

(Experiment)
The batteries B1 to B4 were charged / discharged under the same conditions as those shown in the experiment of the first embodiment, and the initial charge / discharge efficiency shown by the above formula (3) and the 10 shown by the above formula (4) were used. The capacity retention rate at the cycle was examined, and the results are shown in Table 2. Table 2 also shows the results of the batteries A1 and Z.

Batteries A1, B1 to B4 using SiO _X having a lithium silicate phase inside have higher initial charge / discharge efficiency and cycle characteristics than battery Z using SiO _X having no lithium silicate phase inside. Was also found to be good. Moreover, when comparing batteries A1 and B1 to B4, it was found that the higher the ratio of the lithium silicate phase in SiO _X , the higher the initial charge / discharge efficiency and the better the cycle characteristics. Furthermore, in the batteries B2 to B4 in which the ratio of the lithium silicate phase in SiO _X is 12.5 mol% or more, the initial charge / discharge efficiency exceeding the theoretical charge / discharge efficiency (75%) when SiO _X is used as the negative electrode active material. It was confirmed that

From the above, it is desirable that the proportion of the lithium silicate phase in SiO _X is 0.5 mol% or more and 25 mol% or less. When the proportion of the lithium silicate phase in SiO _X is less than 0.5 mol%, the effect of forming the lithium silicate phase is reduced, and when the proportion exceeds 25 mol%, the charge / discharge capacity decreases.

<Third embodiment>
Example 1
As SiO _X (SiO _X before heat treatment) as a raw material, _SiO X (x = 0.93, a carbon coating amount of 10 mass%) Average primary particle diameter of 1.0μm Except for using, the first A battery was fabricated in the same manner as in Example 1 of the example. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%.
The battery thus produced is hereinafter referred to as battery C1.

(Example 2)
As SiO _X (SiO _X before heat treatment) as a raw material, _SiO X (x = 0.93, a carbon coating amount of 10 mass%) Average primary particle size of 15.0μm Except for using, the first A battery was fabricated in the same manner as in Example 1 of the example. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%.
The battery thus produced is hereinafter referred to as battery C2.

(Experiment)
The batteries C1 and C2 were charged / discharged under the same conditions as those shown in the experiment of the first example, and the initial charge / discharge efficiency shown by the above formula (3) and the 10 shown by the above formula (4). The capacity retention rate at the cycle was examined, and the results are shown in Table 3. Table 3 also shows the results of the batteries A1 and Z.

Cell A1, C1, C2 using a SiO _X having an internal lithium silicate phase, than the batteries Z using the SiO _X having no internal lithium silicate phase, high initial charge and discharge efficiency, cycle characteristics Was also found to be good. Therefore, the average primary particle diameter of SiO _X is preferably 1 μm or more and 15 μm or less. In addition, when the average primary particle diameter of SiO _X is less than 1 μm, the particle surface area is large, so that a side reaction of the electrolytic solution easily occurs. On the other hand, when the average primary particle diameter of SiO _X exceeds 15 μm, lithium does not diffuse to the inside of the SiO _X during the chemical conversion treatment, and many lithium silicate phases are formed on the surface of the SiO _X. May cause a drop.

<Fourth embodiment>
Example 1
The SiO _X after heat treatment, washed with pure water until pH of the filtrate reached 8.0, and filtered, except that removal of the lithium compound unreacted from the surface of the SiO _X after the heat treatment, the first A battery was fabricated in the same manner as in Example 1 of the example.
The battery thus produced is hereinafter referred to as battery D1.

(Example 2)
A battery was fabricated in the same manner as in Example 1 of the first example except that the following treatment was performed before the heat treatment.
When mixing SiO _X and LiOH, a predetermined amount of SiO _X and a nonionic surfactant (trade name: SN Wet 980, polyether-based surfactant manufactured by San Nopco Co., Ltd.) are added to a solution in which LiOH is previously dissolved in water. Agent) was added and dispersed. In addition, the addition amount of the nonionic surfactant was 1 mass% with respect to the total amount of solid content. Next, the dispersion was dried in a thermostatic bath set at a temperature of 110 ° C., water as a solvent was removed, and heat treatment was performed.
The battery thus produced is hereinafter referred to as battery D2.

Example 3
The SiO _X after heat treatment, washed with pure water until pH of the filtrate reached 8.0, and filtered, except that to remove unreacted lithium compound from the surface of the SiO _X after the heat treatment, the fourth embodiment A battery was fabricated in the same manner as in Example 2.
The battery thus produced is hereinafter referred to as battery D3.

(Experiment)
The batteries D1 to D3 were charged / discharged under the same conditions as those shown in the experiment of the first embodiment, and the initial charge / discharge efficiency shown by the above formula (3) and the 10 shown by the above formula (4). The capacity retention rate at the cycle was examined, and the results are shown in Table 4. Table 4 also shows the results of battery A1.

  It turns out that the battery D1 which performed the water washing after heat processing improved the first time charge / discharge efficiency and cycling characteristics rather than the battery A1 which did not wash with water. Washing with water as in the battery D1 can remove the lithium compound that is an unreacted substance during the heat treatment, so that the surface resistance of the negative electrode active material particles decreases. Therefore, it is considered that a sufficient conductive path is formed between the negative electrode active material particles during discharge.

In addition, when mixing SiO _X before the heat treatment and the lithium compound, the battery D2, which has been wet-treated using a surfactant in advance, is more than the battery A1 simply dry-mixing the SiO _X and the lithium compound before the heat treatment. It can be seen that the initial charge / discharge efficiency and the cycle characteristics were improved. When a surfactant is added and wet-kneaded as in the battery D1, fine LiOH is uniformly deposited on the SiO _X surface. For this reason, it is considered that a more uniform lithium silicate phase was formed during the heat treatment.

Further, the battery D3 that has been subjected to the wet treatment using the surfactant and the water washing treatment after the chemical conversion treatment has improved initial charge / discharge efficiency and cycle characteristics compared to the batteries D1 and D2 that have been subjected to only one treatment. You can see that Therefore, the characteristics can be further improved by combining the two processes.
Incidentally, from the above experimental results, it was found that preferable to uniformly arrange the LiOH to SiO _X surface, in such a state, not limited to the above wet process, a dry process Can also be achieved.

<Fifth embodiment>
(Example)
As SiO _X (SiO _X before heat treatment) as a raw material, _SiO X (X = 0.93, the average primary particle diameter 5.0 .mu.m) which is not carbon coating with use of, after the heat treatment, by a CVD method, SiO _A battery was fabricated in the same manner as in Example 1 of the first example except that the surface of _X was coated with carbon (the ratio of carbon to SiO _X was 10% by mass). When SiO _X after the heat treatment was analyzed by XRD, a peak of Li ₂ SiO ₃ which is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%.
The battery thus produced is hereinafter referred to as battery E.

(Experiment)
The battery E was charged / discharged under the same conditions as those shown in the experiment of the first example, and the initial charge / discharge efficiency shown by the above formula (3) and the 10th cycle shown by the above formula (4). Table 5 shows the results. Table 5 also describes the results of battery A1.

It can be seen that the battery E with the carbon coating after the heat treatment has improved initial charge / discharge efficiency and cycle characteristics than the battery A1 with the carbon coating before the heat treatment. After the heat treatment, the unreacted lithium compound adheres to the surface of the SiO _X particles, so that the conductivity between the particles decreases. Therefore, if the carbon coating is performed after the chemical conversion treatment, the resistance of the particle surface is lowered, and the conductive path between the particles is sufficiently formed. In addition, it is thought that the initial charge / discharge efficiency and the cycle characteristics are further improved because the particle surface resistance is further reduced by washing with water after the chemical conversion treatment and then coating with carbon.

  The present invention can be applied to a drive power source of a mobile information terminal such as a mobile phone, a notebook personal computer, and a PDA, for example, in applications that require a particularly high capacity. In addition, it can be expected to be used in high output applications that require continuous driving at high temperatures, and in applications where the battery operating environment is severe, such as electric vehicles and power tools.

Claims (6)

A first step of mixing SiO _X (0.8 ≦ X ≦ 1.2) and a lithium compound to adhere the lithium compound to the surface of SiO _X ;
A second step of heat-treating SiO _X with a lithium compound adhered to the surface to form a lithium silicate phase in the SiO _X ;
A method for producing a negative electrode active material comprising:   The method for producing a negative electrode active material according to claim 1, wherein in the second step, the temperature of the heat treatment is equal to or higher than the melting point of the lithium compound. 3. The method for producing a negative electrode active material according to claim 1, wherein, in the first step, the lithium compound and SiO _X are dispersed in a solvent and then dried to precipitate the lithium compound on the SiO _X surface. The method for producing a negative electrode active material according to claim 3, wherein in the first step, when the lithium compound and SiO _X are dispersed in a solvent, a dispersant and / or a surfactant is added to the solvent. In the first step, as SiO _X, using the SiO _X, which is a carbon coating on the surface, method of preparing a negative active material according to any one of claims 1-4. Above after the second step, a third step of the carbon coating on the surface of the SiO _X lithium silicate phases are formed, a manufacturing method of the negative electrode active material according to any one of claims 1-4.

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