JP2011100616A - Particle for electrode, negative electrode material for lithium ion secondary battery, and manufacturing method of particle for electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a particle for electrode which is suitable as a negative electrode material for a lithium ion secondary battery and has a high lithium storage and release capacity, and is hardly broken even if continuous charge and discharge are repeated. <P>SOLUTION: The particle for electrode contains a metal particle to form an alloy with lithium, a conductive carbon material, and a binding resin and has a gap in the interior, and is manufactured by a process in which a compound particle consisting of the metal particle which forms an alloy with lithium, the conductive carbon material, and a resin component is prepared by mechanically mixing and pulverizing and a process in which the compound particle is heated on a condition in which only a part of the resin component is decomposed and volatilized. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a particle for an electrode that is suitable as a negative electrode material for a lithium ion secondary battery, has a high lithium storage / release capacity, and is not easily damaged even after continuous charge / discharge. Moreover, it is related with the negative electrode material for lithium ion secondary batteries which uses this particle | grain for electrodes.

Carbon materials made of carbonaceous fired bodies are used for electrode materials such as lithium ion secondary batteries, electric double layer capacitors, and capacitors.
For example, in a lithium ion secondary battery, a carbon material is used as a negative electrode active material, lithium is occluded (intercalated) into the carbon material in an ionic state when the battery is charged, and released as ions (deintercalation) during discharge. It uses a “rocking chair type” battery configuration.

As electronic devices are rapidly becoming smaller or higher in performance, there is a growing demand for higher energy density in lithium ion secondary batteries. However, since graphite constituting the carbon material has a theoretical lithium storage / release capacity limited to 372 mAh / g, a negative electrode material having a larger lithium storage / release capacity is required.

In contrast, a method using a silicon material instead of a carbon material having a low charge / discharge capacity has been studied. However, the silicon material has a large volume change due to charging and discharging, and there is a problem that the electrode material may be damaged by continuous charging and discharging. Therefore, carbon-silicon composite materials have also been studied (for example, Patent Documents 1 to 4). However, even with these carbon-silicon composite materials, the problem of material breakage due to the volume change of the silicon still contained has not been sufficiently solved.

JP 2004-259475 A JP 2003-187798 A JP 2001-160392 A JP-A-2005-123175

An object of the present invention is to provide an electrode particle that is suitable as a negative electrode material for a lithium ion secondary battery, has a high lithium storage / release capacity, and is not easily damaged even when continuous charge / discharge is performed. Moreover, it aims at providing the negative electrode material for lithium ion secondary batteries which uses this particle | grain for electrodes.

The present invention is a particle for an electrode containing metal particles forming an alloy with lithium, a conductive carbon material, and a binding resin, and having voids therein, and mechanically mixing and pulverizing the lithium and alloy. For an electrode manufactured by a step of preparing composite particles composed of metal particles to be formed, a conductive carbon material and a resin component, and a step of heating the composite particles to a condition in which only a part of the resin component is decomposed and volatilized Particles.
The present invention is described in detail below.

The inventors of the present invention have a lithium occlusion / discharge capacity that is higher than that of a carbon material composed of only carbon, including metal particles that form an alloy with lithium, a conductive carbon material, and a binder resin, and electrode particles having voids therein. And it was found that even if continuous charge / discharge was performed, the material was not easily damaged. Also in the electrode particles of the present invention, volume change of the metal particles forming an alloy with lithium occurs when continuous charge / discharge is performed. However, since the electrode particles of the present invention have voids, the stress due to the volume change can be dispersed and absorbed, so that it is considered that the electrode particles are not damaged.

The electrode particles of the present invention contain metal particles that form an alloy with lithium, a conductive carbon material, and a binding resin.
Examples of the metal particles that form an alloy with lithium include metal particles made of silicon, tin, magnesium, titanium, vanadium, cadmium, selenium, iron, cobalt, nickel, manganese, platinum, boron, and the like. Among these, metal particles made of silicon or tin are preferable, and metal particles made of silicon are more preferable because a particularly high lithium storage / release capacity can be exhibited.

The particle diameter of the metal particles forming the alloy with lithium is not particularly limited, but a preferable upper limit of the average particle diameter is 1 μm. When the particle diameter of the metal particles forming the alloy with lithium exceeds 1 μm, the lithium occlusion / release capacity may be lowered.

The minimum with preferable content of the metal particle which forms an alloy with the said lithium in the particle | grains for electrodes of this invention is 5 weight%. When the content of the metal particles forming the alloy with lithium is less than 5% by weight, a high lithium storage / release capacity may not be exhibited. The minimum with more preferable content of the metal particle which forms an alloy with the said lithium is 10 weight%.
The upper limit of the content of the metal particles that form an alloy with lithium is not particularly limited. The higher the amount of metal particles that form an alloy with lithium, the higher the lithium storage / release capacity. However, if the content of the metal particles forming an alloy with lithium is too large, the conductivity of the resulting electrode particles may be insufficient. The upper limit with preferable content of the metal particle which forms an alloy with the said lithium is 80 weight%.

The said conductive carbon material has a role which improves the electroconductivity of the particle | grains for electrodes of this invention.
The conductive carbon material is preferably at least one selected from the group consisting of graphite, carbon black, carbon nanotubes, graphene and fullerene, for example.

The minimum with preferable content of the said electroconductive carbon material in the particle | grains for electrodes of this invention is 10 weight%, and a preferable upper limit is 90 weight%. When the content of the conductive carbon material is less than 10% by weight, the resulting electrode particles may have insufficient conductivity. When the content exceeds 90% by weight, the metal particles form an alloy with lithium. In some cases, the lithium content decreases, and a high lithium storage / release capacity cannot be exhibited. The minimum with more preferable content of the said conductive carbon material is 20 weight%, and a more preferable upper limit is 80 weight%.

The binding resin serves as a binder that binds the metal particles forming an alloy with lithium and the conductive carbon material, and maintains the particle shape of the electrode particles of the present invention. The binding resin can also serve to disperse and absorb the stress caused by the volume change when the metal particles forming the alloy with lithium change in volume during continuous charge and discharge.
The binder resin is a resin component remaining after the step of heating the composite particles to a condition in which only a part of the resin component is decomposed and volatilized in the method for producing electrode particles of the present invention described later.

The preferable lower limit of the content of the binder resin in the electrode particles of the present invention is 5% by weight, and the preferable upper limit is 50% by weight. When the content of the binding resin is less than 5% by weight, the particle shape of the electrode particles may not be maintained. When the content exceeds 50% by weight, the metal particles that form an alloy with the lithium and the conductive carbon material In some cases, the lithium content decreases, and a high lithium storage / release capacity cannot be exhibited. A more preferable lower limit of the content of the binding resin is 10% by weight, and a more preferable upper limit is 20% by weight.

The particles for electrodes of the present invention have a preferable lower limit of the average particle diameter of 10 nm and a preferable upper limit of 1 mm. When the average particle size of the electrode particles is less than 10 nm, the particle shape may not be maintained. When the average particle size exceeds 1 mm, the electrode particles cannot be formed into a desired shape or size when formed into a negative electrode material for a lithium ion secondary battery. Sometimes. The minimum with a preferable average particle diameter of the particle | grains for electrodes of this invention is 1000 nm, and a preferable upper limit is 500 micrometers.

In the electrode particles of the present invention, the preferable lower limit of the porosity is 10%, and the preferable upper limit is 90%. When the porosity of the electrode particles of the present invention is less than 10%, the stress due to the volume change cannot be sufficiently dispersed and absorbed when the volume of the metal particles forming the alloy with lithium during continuous charge / discharge changes. The material may be damaged, and if it exceeds 90%, the particle shape may not be maintained. The more preferable lower limit of the porosity of the electrode particles of the present invention is 20%, and the more preferable upper limit is 80%.
The porosity can be calculated by the Archimedes method, for example, from the specific gravity measured with a pycnometer true density measuring instrument or the like.

The electrode particles of the present invention are prepared by mechanically mixing and pulverizing to prepare composite particles comprising metal particles that form an alloy with lithium, a conductive carbon material, and a resin component; And a process of heating to a condition where only a part of the components decompose and volatilize. Manufacturing by such a manufacturing method eliminates the need for carbonization by high-temperature firing, and allows the internal voids to be controlled without using a hollow agent or the like.
Such a method for producing electrode particles is also one aspect of the present invention.

The method for producing particles for an electrode according to the present invention involves mechanically mixing and pulverizing to produce composite particles in which metal particles forming an alloy with lithium and a conductive carbon material are dispersed in a resin component. Only a part of the resin component is decomposed and volatilized by heating.
A relatively uniform void can be formed inside the composite particles by decomposing and volatilizing a part of the resin component from the composite particles. On the other hand, a resin component that does not decompose and volatilize remains under the heating condition, and the remaining resin component plays a role as the binding resin.

The resin component may be only one kind of resin.
When the thermogravimetric measurement of the thermal decomposability of the resin is performed by using a balance (thermobalance) equipped with a heating device under the isothermal condition, the volatility (%) varies greatly depending on the heating temperature. For example, in the case of polystyrene, the volatilization rate is almost 0%, that is, hardly decomposes and volatilizes even if it is isothermally heated at a temperature of about 320 ° C. for 30 minutes. On the other hand, when heated isothermally at a temperature of about 360 ° C. for 30 minutes, the volatilization rate is almost 100%, that is, almost all is decomposed and volatilized. Therefore, when polystyrene is used as the resin component, if the heating time is adjusted in a temperature range of about 320 to 360 ° C., a condition in which only a part thereof is decomposed and volatilized can be set.

The resin component may be a copolymer composed of a plurality of monomers, preferably a block copolymer composed of a plurality of blocks, or a mixture of a plurality of resins. In such a case, it is easy to set conditions under which only a part of them decomposes and volatilizes.
For example, when the resin component is a block copolymer comprising a block having t-butyl acrylate as a structural unit and a block having a styrene monomer as a structural unit, most of the blocks having the styrene monomer as a structural unit Can be decomposed and volatilized by heating under the condition that most of the block containing t-butyl acrylate as a constituent unit is not decomposed and volatilized. .
Similarly, for example, when the resin component is a mixture of t-butyl polyacrylate and polystyrene, most of the polystyrene is decomposed and volatilized, and most of the polyt-butyl acrylate is not decomposed and volatilized. By heating, only a part of the resin component in the composite particles can be decomposed and volatilized.

In the method for producing electrode particles of the present invention, the step of preparing composite particles comprising metal particles, an electrically conductive carbon material, and a resin component that form an alloy with lithium by mechanically mixing and pulverizing is specifically, Examples thereof include a granulation method and a melt kneading method.
As the apparatus used for the mixing and pulverization, for example, a planetary ball mill, a theta composer, a jet mill and the like are suitable.

The electrode particles of the present invention have a high lithium storage / release capacity and are not easily damaged even when continuous charge / discharge is performed.
The electrode particles of the present invention can be suitably used for electrode materials, particularly negative electrode materials for lithium ion secondary batteries. Moreover, it can use suitably also for the electrode material for electric double layer capacitors, and the electrode material for capacitors.
The negative electrode material for lithium ion secondary batteries containing the electrode particles of the present invention and a binder resin is also one aspect of the present invention.

The binder resin serves as a binder for bonding the electrode particles of the present invention, and plays a role of forming into an arbitrary shape. However, if a binder resin is added in a large amount, the conductivity of the obtained negative electrode material for a lithium ion secondary battery may be lowered.
Examples of the binder resin include fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, and styrene butadiene rubber.

The negative electrode material for a lithium ion secondary battery of the present invention preferably further contains at least one conductive aid selected from the group consisting of graphite, carbon black, carbon nanotubes, graphene, and fullerene. By mix | blending the said conductive support agent, the electroconductivity of the negative electrode material for lithium ion secondary batteries of this invention can be improved more.

A preferable lower limit of the blending amount of the conductive assistant is 1% by weight, and a preferable upper limit is 90% by weight. If the blending amount of the conductive aid is less than 1% by weight, a sufficient conductivity improving effect may not be obtained, and if it exceeds 90% by weight, the lithium storage capacity may be lowered.
In addition, when the said conductive support agent is mix | blended to some extent, the role of the binder which couple | bonds the particle | grains for electrodes can also be exhibited. When the said conductive support agent exhibits the role of a binder, the compounding quantity of binder resin can be reduced and higher electroconductivity can be exhibited.

The method for producing the negative electrode material for a lithium ion secondary battery of the present invention includes, for example, a method of molding after mixing the electrode particles of the present invention, a conductive additive and a binder resin to obtain a mixture.
The mixture may contain an organic solvent so that it can be easily molded.
The organic solvent is not particularly limited as long as it is a poor solvent for the binder resin contained in the electrode particles and can dissolve the binder resin. For example, N-methylpyrrolidone, N, N-dimethyl And formamide.

ADVANTAGE OF THE INVENTION According to this invention, it is suitable as a negative electrode material for lithium ion secondary batteries, has a high lithium occlusion-release capacity | capacitance, and can provide the particle | grains for electrodes which are hard to be damaged even if it performs continuous charge / discharge. Moreover, the negative electrode material for lithium ion secondary batteries which uses this particle | grain for electrodes can be provided.

Hereinafter, embodiments of the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

Example 1
(1) Preparation of composite particles Stainless steel container (volume 80 mL), stainless steel balls (diameter 10 mm), 10 parts by weight of poly (t-butyl acrylate), 90 parts by weight of polystyrene, silicon particles (Aldrich silicon nanopowder, average) 50 parts by weight of particle size (100 nm) and 40 parts by weight of graphite particles (manufactured by SEC Carbon Co., SNO-3) are charged, purged with argon gas, sealed, and a planetary ball mill (P-6, manufactured by German Fritsch). Then, after mechanical alloying treatment at 400 rpm for 10 hours, pulverization was performed to obtain composite particles containing silicon particles and graphite particles.

(2) Production of Electrode Particles The obtained composite particles containing silicon particles and graphite particles were heated at 370 ° C. for 2 hours in a nitrogen atmosphere to obtain electrode particles.

(3) Production of negative electrode material for lithium ion secondary battery 10 parts by weight of carbon black (manufactured by Mitsubishi Chemical Corporation, # 3230B) and 10 parts by weight of polyvinylidene fluoride as a binder resin with respect to 100 parts by weight of the obtained electrode particles Then, N-methylpyrrolidone was mixed as an organic solvent to prepare a mixed solution. The obtained mixed solution was applied to one side of a 18 μm-thick Cu foil, dried, and then pressure-formed with a press roll to obtain a sheet-like negative electrode material for a lithium ion secondary battery.

(Examples 2 to 4)
Composite particles, electrode particles, and negative electrode material for lithium ion secondary batteries were the same as in Example 1 except that the mixing ratio of the binding resin, the amount of silicon particles, and the type and amount of the conductive carbon material were changed as shown in Table 1. Got. In addition, the multi-wall carbon nanotube by Showa Denko KK was used for the carbon nanotube as a conductive carbon material.

(Comparative Example 1)
10 parts by weight of carbon black (manufactured by Mitsubishi Chemical Corporation, # 3230B), 10 parts by weight of polyvinylidene fluoride as a binder resin, and N as an organic solvent with respect to 100 parts by weight of graphite particles (manufactured by Wako Pure Chemical Industries, average particle size 20 μm) -Methylpyrrolidone was mixed to prepare a mixed solution.
The obtained mixed solution was applied to one side of a 18 μm-thick Cu foil, dried, and then pressure-formed with a press roll to obtain a sheet-like negative electrode material for a lithium ion secondary battery.

(Comparative Example 2)
Stainless steel container (volume 80 mL), stainless steel balls (diameter 10 mm), polyacrylic acid t-butyl 30 parts by weight, silicon particles (Aldrich silicon nanopowder, average particle diameter 100 nm) 50 parts by weight, carbon nanotube (Showa 20 parts by weight of Denko Co., Ltd. (multi-walled carbon nanotubes) were charged, purged with argon gas, sealed, and subjected to mechanical alloying at 400 rpm for 10 hours with a planetary ball mill (P-6, manufactured by German Fritsch). Thereafter, pulverization was performed to obtain composite particles containing silicon particles and carbon nanotubes.
Electrode particles and a negative electrode material for a lithium ion secondary battery were obtained in the same manner as in Example 1 except that the obtained composite particles containing silicon particles and carbon nanotubes were used.

(Evaluation)
The negative electrode materials for lithium ion secondary batteries obtained in the examples and comparative examples were evaluated as follows.
The results are shown in Table 1.

(1) Production of Lithium Ion Secondary Battery A coin type model cell was produced using the carbon materials obtained in Examples and Comparative Examples as negative electrode materials for lithium ion secondary batteries.
That is, a negative electrode material for a lithium ion secondary battery and a counter electrode lithium metal having a diameter of 16 mm were laminated via a separator. After impregnating the separator with the electrolytic solution, these were caulked with an upper can and a lower can through a gasket. The upper can and the lower can were brought into contact with the negative electrode and the counter electrode lithium, respectively.
As the separator, a polyethylene microporous membrane having a thickness of 25 μm and a diameter of 24 mm was used. As the electrolyte, a mixed solvent of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 2 was used, and LiPF ₆ was used as the electrolyte at a concentration of 1 mol. A solution dissolved so as to be / L was used.

(2) Discharge capacity and initial charge / discharge efficiency Charge / discharge conditions are as follows: voltage and current are 0 for 8 hours, then voltage is reduced to 0.002 V at a current equivalent to 0.2 C, and held for 3 hours to charge. did. After resting for 10 minutes, the battery was discharged at a current of 0.2 C until the voltage reached 1.2V. After resting for 10 minutes, this discharging was repeated. The charge / discharge capacity was determined from the amount of electricity applied during that time.
Further, the initial charge / discharge efficiency was calculated from the following formula. In this test, the process of occluding lithium in the carbon material was charged and the process of detaching was defined as discharge.
Initial charge / discharge efficiency (%) = (first cycle discharge capacity / first cycle charge capacity) × 100

(3) Cycle characteristics The above cycle was repeated 10 times, and the cycle characteristics were calculated using the following formula.
Cycle characteristics = (discharge capacity in the tenth cycle / discharge capacity in the first cycle) × 100

ADVANTAGE OF THE INVENTION According to this invention, it is suitable as a negative electrode material for lithium ion secondary batteries, has a high lithium occlusion-release capacity | capacitance, and can provide the particle | grains for electrodes which are hard to be damaged even if it performs continuous charging / discharging. Moreover, the negative electrode material for lithium ion secondary batteries which uses this particle | grain for electrodes can be provided.

Claims (6)

A particle for an electrode containing metal particles forming an alloy with lithium, a conductive carbon material and a binding resin, and having voids therein,
A step of preparing composite particles composed of metal particles that form an alloy with lithium, a conductive carbon material, and a resin component by mechanically mixing and pulverizing, and only a part of the resin component decomposes and volatilizes the composite particles. A particle for an electrode, which is produced by a step of heating to a condition to be performed. 2. The electrode particle according to claim 1, wherein the metal particles forming an alloy with lithium are metal particles made of silicon. 3. The electrode particle according to claim 1, wherein the conductive carbon material is at least one selected from the group consisting of graphite, carbon black, carbon nanotube, graphene, and fullerene. A negative electrode material for a lithium ion secondary battery, comprising the electrode particles according to claim 1, 2 or 3 and a binder resin. 5. The negative electrode material for a lithium ion secondary battery according to claim 4, wherein at least one conductive aid selected from the group consisting of graphite, carbon black, carbon nanotubes, graphene, and fullerene is blended. A method for producing electrode particles containing metal particles that form an alloy with lithium, a conductive carbon material, and a binding resin, and having voids therein,
At least a step of preparing composite particles composed of metal particles that form an alloy with lithium, a conductive carbon material, and a resin component by mechanically mixing and pulverizing, and only a part of the resin component is used for the composite particles. And a step of heating to a condition for decomposing and volatilizing.