JPH10255800A - Negative electrode material for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide negative electrode material which reduces lithium ions remaining in a negative electrode and has high initial discharge efficiency, by composing it with graphite particles adhered to amorphous mineral material solid. SOLUTION: Silicate, phosphate, sulfate and the like of lithium, calcium, sodium, potassium, ion or magnesium, their mixture, silica or boric acid lithium is preferable for amorphous mineral material to adhere to a graphite particle surface. A quantity of the amorphous mineral material is preferably 0.5-10% by weight to the graphite article, and if it exceeds beyond its upper limit it is coated up to a surface between graphite layers where lithium ion is doped and doping of the lithium ion is removed, so that a discharge capacity declines, while enhancement of an initial discharge capacity can not be expected if it is less than its lower limit. A method that the amorphous mineral material is deposited and adhered onto a graphite particle surface, a method that the amorphous material is melted and fused for fitting, and the like are considered. Thereby, negative electrode material with high safety for lithium secondary battery can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a negative electrode material for a lithium secondary battery, and more particularly, to a negative electrode material for a lithium ion secondary battery comprising improved graphite particles.

[0002]

2. Description of the Related Art As electronic devices have become smaller and lighter, there has been a strong demand for the development of secondary batteries having high energy density and high output density. To meet this demand, a lithium secondary battery having a high voltage and a high energy density has been proposed. Such a lithium secondary battery is inferior in quick chargeability, has cycle deterioration due to repeated charging and discharging, and has a risk of fire or explosion due to dendrite deposited on the negative electrode during charging.

[0003] To solve these problems, lithium ion secondary batteries have been developed. The battery uses a carbon material or a graphite material in place of metal lithium conventionally used as a negative electrode, and lithium ions are doped between carbon layers at the time of charging to form a so-called graphite intercalation compound. Lithium ions are dedoped from between the layers, and are combined again with the lithium compound of the positive electrode. The battery enables charge and discharge through such a cycle, and in order to form a battery with a high energy density, the carbon material or the graphite material as the negative electrode can dope and dedope a large amount of lithium ions. It is necessary.

However, in a conventional negative electrode material such as a carbon material, doped lithium ions are not completely undoped, and in particular, 70 to 80% ( Only the initial discharge efficiency). Therefore, a large amount of lithium ions remains in or on the surface of the negative electrode, and lithium as an active material is wasted.
In addition, the large amount of lithium ions remaining in the negative electrode results in wasteful consumption of rare metal, such as cobalt, used for the positive electrode material, which is disadvantageous in terms of price, resources, and energy density.

[0005] In addition, by repeating charge and discharge, lithium ions form dendrites at active points on the surface and edge portions of the graphite particles of the negative electrode, and even in the case of lithium ion batteries, a fire or explosion due to a short circuit inside the battery due to the dendrite may occur. It is pointed out that there is a danger.

[0006]

DISCLOSURE OF THE INVENTION The present invention reduces lithium ions remaining in a negative electrode, has a high initial discharge efficiency,
It is another object of the present invention to provide a highly safe negative electrode material for a lithium secondary battery.

[0007]

Means for Solving the Problems The inventors of the present invention have found that the cause of lithium ions remaining in the negative electrode of the lithium secondary battery or on the surface thereof without undoping is that active points on the graphite particle surface or edge portion or Various investigations were repeated on the assumption that lithium ions were incorporated into the structural defects. As a result, it is possible to suppress lithium ions from remaining on the negative electrode by crushing active points and structural defects that take in lithium ions by attaching an amorphous inorganic substance to these parts, and furthermore, active points at the graphite particle surface, especially at the edge part. It has also been found that if an amorphous inorganic substance is adhered to the carbon black, no dendrite is generated even when the charge / discharge cycle is repeated, and the present invention has been completed.

That is, the present invention is (1) a negative electrode material for a lithium secondary battery comprising graphite particles having an amorphous inorganic substance solid adhered to the surface.

It is desired that graphite particles used as a negative electrode material of a lithium secondary battery have as few structural defects and active sites as possible from the viewpoint that the amount of lithium remaining in the negative electrode is reduced and the generation of dendrite can be suppressed. I have. On the other hand, in order to obtain a high charge / discharge capacity, fine particles in which lithium ions are more easily diffused are desired. However, such fine particles have a contradictory problem that they have many structural defects and active points. The present invention reduces the lithium remaining on the negative electrode irrelevant to the battery performance as much as possible by selectively attaching and crushing an amorphous inorganic substance to active points and structural defects on the surface of the fine graphite particles having a large charge / discharge capacity. Let me
A high initial discharge efficiency is obtained, and the generation of dendrite is also suppressed. Thereby, a lithium secondary battery having high charge / discharge capacity and high safety can be provided.

In a preferred embodiment, (2) the amorphous inorganic substance is lithium, sodium, calcium, potassium,
The negative electrode material according to the above (1), which is silicate, phosphate, sulfate, nitrate or borate of iron or magnesium, or a mixture thereof, or silica, alumina or magnesia; The negative electrode material according to the above (1) or (2), which is silica or lithium borate, (4)
0.5 to 10% by weight of amorphous inorganic substance based on graphite particles
The negative electrode material as described in any one of (1) to (3) above, and (5) an amorphous inorganic substance in an amount of 0.1 to 100% with respect to graphite particles.
The negative electrode material according to any one of the above (1) to (3), which is contained in an amount of 5 to 5% by weight, and (6) the graphite particles in which the graphite particles are pulverized natural graphite or expanded graphite. The negative electrode material according to any one of (1) to (7), wherein the amorphous inorganic substance is deposited on the surface of the graphite particles to thereby form the negative electrode material for the lithium secondary battery according to any one of (1) to (6) above. (8) The method for producing a negative electrode material as described in any one of (1) to (6) above, wherein (8) the inorganic substance is melted on the surface of the graphite particles and the amorphous inorganic substance is fused to the surface. Of producing a negative electrode material for a lithium secondary battery.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the amorphous inorganic substance adhered to the surface of graphite particles is preferably silicate, phosphate, sulfate of various metals such as lithium, sodium, calcium, potassium, iron or magnesium. , Nitrates or borates, or mixtures thereof, or various oxides such as silica, alumina, and magnesia. Among them, silica or lithium borate is particularly preferably used. It is presumed that these amorphous inorganic substances are such that oxygen atoms contained in the substances are bonded to the graphite particles in some form and adhere to the surface of the graphite particles.

The upper limit of the amount of the amorphous inorganic substance is preferably 10% by weight, particularly preferably 5% by weight, and the lower limit is preferably 0.5% by weight, based on the graphite particles. If the upper limit is exceeded, the lithium ion is coated even on the graphite interlayer that is doped and dedoped, and the charge / discharge capacity is reduced. As the amount of lithium increases, improvement in the initial discharge efficiency cannot be expected. The charge / discharge capacity gradually decreases as the amount of the attached amorphous inorganic substance increases. However, in the range of the present invention, the rate of decrease in charge capacity is greater than the rate of decrease in discharge capacity, so it is considered that the initial discharge efficiency can be maintained high.

The method for attaching the amorphous inorganic substance to the surface of the graphite particles is not particularly limited. For example, a method in which an amorphous inorganic substance is deposited on graphite particles and adhered thereto, or a method in which an inorganic substance is melted on the surface of graphite particles and the amorphous inorganic substance is fused and adhered to the surface, etc. . Preferably, graphite particles and a substance that forms the above-mentioned amorphous inorganic substance coexist in a liquid medium, and are subjected to hydrolysis or neutralization treatment, etc., to thereby form a desired amorphous inorganic substance on the graphite particles. Can be deposited. Alternatively or preferably, a substance forming the above-mentioned amorphous inorganic substance is attached to the graphite particles, and then heated and melted to fuse and adhere the desired amorphous inorganic substance onto the graphite particles. it can. Which method you choose,
It is determined by the type of the above-mentioned amorphous inorganic substance adhered to the graphite particle surface.

An example of a method by hydrolysis will be described. A silane compound, such as methoxysilane or ethoxysilane, is dissolved in a solvent that dissolves both water and the silane compound, such as methanol and ethanol. Separately, graphite particles are added to aqueous ammonia and sufficiently stirred and mixed to prepare a slurry, and then the silane compound solution is added dropwise to the stirred slurry. The temperature at the time of addition is preferably room temperature to 70 ° C. After completion of the addition, stirring is preferably continued for a further 0.5 to 3 hours, and the obtained slurry is filtered and then dried to obtain graphite particles to which silica has adhered.

Next, an example of a method by neutralization will be described. The pH is adjusted to 10 to 11 by adding an alkali to a water slurry in which graphite particles are dispersed. Separately, a solution of silicates such as sodium silicate, potassium silicate, calcium silicate and the like, and an acid solution such as sulfuric acid, nitric acid, hydrochloric acid and the like are prepared. next,
While maintaining the pH in the above graphite particle slurry,
The silicate solution and the acid solution are simultaneously added dropwise. The temperature at the time of addition is preferably 70 to 100 ° C. After completion of the addition, preferably after further stirring for 2 to 3 hours,
The resulting slurry is filtered and dried to obtain graphite particles to which silica is attached, after neutralizing by adding an acid.

As another method, there can be mentioned a method in which an inorganic substance is melted on the surface of graphite particles to fuse an amorphous inorganic substance. For example, after mixing a hydroxide such as lithium hydroxide, sodium hydroxide, potassium hydroxide and magnesium hydroxide and an acid such as boric acid or silicic acid with water, the mixture is impregnated on the graphite surface. Alternatively, after the above-mentioned hydroxide and acid are mixed, graphite is further added and pulverized and mixed until the mixture becomes sufficiently uniform so that the hydroxide and acid are attached to the graphite surface. Next, in the former method, the slurry after the treatment is filtered,
In the latter method, the resulting solid is dried, and then heated at a temperature equal to or higher than the melting points of the hydroxide and the acid, preferably for 0.5 to 3 hours, preferably in an inert gas atmosphere. A glassy thin film is deposited on graphite.

The graphite particles used in the present invention are preferably those obtained by pulverizing natural graphite or artificial graphite by a dry method or a wet method using a jet mill, turbo mill, micro mill, ball mill, ultrasonic wave or the like. Further, as the graphite particles, those obtained by pulverizing expanded graphite treated by a known method in the same manner as described above can also be used.

The graphite particles preferably have an average particle diameter of 50 μm or less and particles having a particle diameter of 100 μm or less.
5% by volume or more, particularly preferably an average particle size of 2
Particles having a particle diameter of 0 μm or less and 50 μm or less are 95% by volume or more. If the particle size exceeds the above range, the diffusion of lithium ions will be slow, and the high-speed charge / discharge characteristics will decrease,
In addition, a high discharge capacity cannot be obtained, which is not preferable.
Further, as the particle diameter decreases, active points and structural defects on the surface of the graphite particles increase. However, in the present invention, there is no problem because an amorphous inorganic substance is attached to the active points and the like.

The method for producing a negative electrode of a lithium secondary battery using the graphite particles having the amorphous inorganic substance adhered to the surface obtained as described above is not particularly limited. For example, after adding a binder and a solvent to the graphite particles and sufficiently kneading the mixture, a method of pressure-bonding to a current collector such as a metal mesh or a method of coating a slurry comprising the graphite particles, the binder and the solvent in a metal plate shape is used. Can be manufactured. Examples of the binder include various pitches, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), ethylene propylene diene polymer (E
A known material such as PDM) can be used. Among them, PVDF and EPDM are particularly preferred. Although the cathode material of the lithium secondary battery is not particularly limited, LiCoO ₂ ,
Lithium-containing oxides such as LiNiO ₂ and LiMn ₂ O ₄ are preferred. The powdered positive electrode material, after adding a conductive material, a solvent, etc. as necessary in addition to the binder and kneading sufficiently,
It can be manufactured by molding with a current collector.

There is no particular limitation on the separator.
Known materials can be appropriately used.

As the non-aqueous solvent used as the electrolyte solvent, a known aprotic solvent having a low dielectric constant that dissolves a lithium salt is preferable. For example, ethylene carbonate, propylene carbonate, diethylene carbonate, acetonitrile, propionitrile, tetrahydrofuran,
γ-butyrolactone, 2-methyltetrahydrofuran,
1,3-dioxolan, 4-methyl-1,3-dioxolan, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, sulfolane, methylsulfolane, nitromethane, N, N-dimethylformamide,
Solvents such as dimethyl sulfoxide can be used alone or in combination of two or more.

The lithium salt used as the electrolyte is L
iClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ ,
LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ S
O ₃ Li, CF ₃ SO ₃ Li and the like are preferable, and these salts may be used alone or as a mixture of two or more.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[0024]

【Example】

[0025]

Example 1 Natural graphite produced in Madagascar was pulverized by a dry pulverizer. The average particle size of the pulverized graphite particles was 16 μm (ash content: 0.65% by weight). 50 g of the graphite particles, 90 g of water and 10 g of methanol were placed in a 2 liter flask equipped with a stirrer, and stirred for about 10 minutes to sufficiently wet the graphite particles. Further, after adding 400 g of water and sufficiently stirring, a 5% aqueous sodium hydroxide solution was added dropwise to obtain a graphite slurry whose pH was adjusted to 10. Separately, 3.3 g of sodium silicate is weighed into a beaker and diluted with water to obtain 20 g.
0 g and 200 g of 0.25% diluted sulfuric acid were prepared.

The above graphite slurry is stirred at 90 ° C.
The aqueous solution of sodium silicate and diluted sulfuric acid were simultaneously added dropwise in approximately the same amount over 4 hours. After the addition,
After continuing stirring for 1 hour, dilute sulfuric acid was added to
Was set to 7. The slurry was filtered, washed thoroughly with water until the water-soluble salt was removed, and dried to obtain 49.7 g of graphite particles having silica attached. As a result of the analysis,
The ash content of the obtained graphite particles was 1.67% by weight, indicating that about 1% by weight of silica was attached.

Next, the graphite particles to which the silica was adhered were subjected to a performance test as a negative electrode material for a lithium secondary battery. A non-aqueous solvent battery using graphite with silica attached as a negative electrode and lithium cobalt oxide as a positive electrode is charged and discharged,
The capacity of doping and undoping of lithium ions to the negative electrode was measured.

Here, the negative electrode was manufactured by the following method. Polyvinylidene fluoride (PVDF) as a binder is added to 90 parts by weight of the graphite particles to which the silica is attached.
After adding 10 parts by weight and a small amount of N-methylpyrrolidone as a solvent and mixing well to form a paste, the mixture was pressure-formed on a circular stainless steel mesh (2.5 cm ² ) at 3.6 ton / cm ² , The molded product was manufactured by vacuum drying at 200 ° C. for 2 hours.

The positive electrode was manufactured by the following method. After 90 parts by weight of lithium cobaltate and 10 parts by weight of artificial graphite were mixed, 10 parts by weight of PVDF and a small amount of N-methylpyrrolidone were further added and mixed well to form a paste. Then
It was manufactured by pressing and drying on a circular stainless steel plate having the same area as the negative electrode in the same manner as the negative electrode.

As an electrolytic solvent, a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio 1: 2) was used. LiPF ₆ was used as the electrolyte, and its concentration was 1.
It was adjusted to 0 mol / l.

A porous polypropylene non-woven fabric was used as a separator, a glass fiber filter was impregnated with an electrolyte, and a coin cell was prepared in an argon atmosphere.

In the charge / discharge test, the current density during charge and discharge was 0.4 mA / cm ² . Initial charge is 387
mAh / g. The discharge was cut at 1.5 V to perform a cycle test. The discharge amount in the first cycle is 358 mA
h / g, and the initial discharge efficiency (discharge / charge amount × 100)
%) Was 92.5%. The test was performed for 200 cycles, but no decrease in the discharge amount was observed until the 200th cycle. After the charge / discharge test, the battery was disassembled in an argon stream, and the negative electrode was observed under a microscope. As a result, no formation of metallic lithium dendrites was observed on the graphite surface.

[0033]

Embodiment 2 Ethoxysilane [Si (OC
H ₃ ) H ₃ ] was added to prepare 207 g of an ethoxysilane solution. Separately, 2 in 497 g of ethanol
55 g of 8% aqueous ammonia was added to prepare an ammonia solution. Next, 15 g of the same graphite particles as in Example 1 were added to the ammonia solution and stirred for 1 hour to disperse the graphite particles to form a slurry.

While stirring the slurry, the above ethoxysilane solution was added dropwise, and after the dropping, the stirring was further continued for 30 minutes. Thereafter, the mixture was filtered and dried to obtain 15.9 g of lead particles to which silica had adhered. As a result of the analysis, it was found that the ash content of the obtained graphite particles was 1.59% by weight, and about 1% by weight of silica was attached.

Next, a battery was prepared in the same manner as in Example 1 using the obtained graphite particles to which silica was attached.

The charge / discharge test was performed in the same manner as in Example 1. The initial charge amount was 385 mAh / g. The discharge amount in the first cycle was 350 mAh / g, and the initial discharge efficiency was 90.9%. Further, no decrease in the discharge amount at 200 cycles was observed. As a result of microscopic observation, formation of metallic lithium dendrites was not observed on the graphite surface of the negative electrode.

[0037]

Example 3 2.4 g of lithium hydroxide and 3.0 g of orthoboric acid were mixed and pulverized. Next, this was added at 2% by weight based on 100 g of the graphite particles of Example 1,
Further, 100 g of water was added, and the mixture was kneaded well and impregnated on graphite particles. After drying the obtained kneaded material, it was heated at 800 ° C. for 1 hour under a nitrogen stream to sufficiently melt lithium hydroxide and orthoboric acid, and then cooled to form a glass film of lithium borate on the surface of the graphite particles. I let it adhere. Using the graphite particles, a battery was prepared in the same manner as in Example 1.

The charge / discharge test was performed in the same manner as in Example 1. The initial charge amount was 397 mA / g. The discharge amount in the first cycle was 366 mAh / g, and the initial discharge efficiency was 92.2%. Further, no decrease in the discharge amount at 200 cycles was observed. As a result of microscopic observation, formation of metallic lithium dendrites was not observed on the graphite surface of the negative electrode.

[0039]

Comparative Example 1 A battery was prepared in the same manner as in Example 1 except that the graphite particles of Example 1 were used and silica was not attached thereto.

The charge / discharge test was performed in the same manner as in Example 1. The initial charge amount was 437 mAh / g. The discharge amount in the first cycle was 344 mAh / g, and the initial discharge efficiency was 78.7%. After 200 cycles, when the negative electrode was observed in the same manner as in Example 1, formation of metallic lithium dendrite was observed on the graphite edge surface.

[0041]

According to the present invention, there is provided a negative electrode material for a lithium secondary battery which has a high initial efficiency and a high safety by reducing lithium ions remaining in the negative electrode.

Claims (7)

[Claims] 1. A negative electrode material for a lithium secondary battery comprising graphite particles having an amorphous inorganic substance solid adhered to the surface. 2. The amorphous inorganic material is lithium, sodium, calcium, potassium, iron or magnesium silicate, phosphate, sulfate, nitrate or borate, or a mixture thereof, or silica, alumina or magnesia. The negative electrode material according to claim 1. 3. The negative electrode material according to claim 1, wherein the amorphous inorganic substance is silica or lithium borate. 4. An amorphous inorganic substance is added to graphite particles in an amount of 0.1 to 0.1%.
The negative electrode material according to any one of claims 1 to 3, which is contained in an amount of 5 to 10% by weight. 5. The negative electrode material according to claim 1, wherein the graphite particles are pulverized products of natural graphite or expanded graphite. 6. The method for producing a negative electrode material for a lithium secondary battery according to claim 1, wherein an amorphous inorganic substance is deposited on the surface of the graphite particles. 7. An inorganic substance is melted on the surface of graphite particles,
The method for producing a negative electrode material for a lithium secondary battery according to claim 1, wherein an amorphous inorganic substance is fused to the surface.