JP5809200B2 - Silicon-based negative electrode active material - Google Patents

Description

  The present invention relates to a silicon-based negative electrode active material that can exhibit good battery characteristics.

  Conventionally, a highly conductive substance is used as a positive electrode material or a negative electrode material in order to improve battery performance. In recent years, next-generation batteries such as lithium ion batteries have been increasingly used. As a negative electrode material in such batteries, carbon-based materials are mainly used, but for example, silicon-based materials described in Non-Patent Document 1 are used. The use of negative electrode materials is also being studied.

Jae-hun Kim et al, Journal of Electroanalytical Chemistry 661 (2011), p245-249.

  However, silicon-based negative electrode materials have a large volume change during charge / discharge, and as the charge / discharge is repeated, the particle structure collapses and the cycle life is likely to decrease, so that the desired battery characteristics may not be sufficiently exhibited. .

  Therefore, the subject of this invention is providing the silicon type negative electrode active material which can express the outstanding battery physical property as a negative electrode material of a lithium ion battery.

  Accordingly, the present inventors have made various studies and found that a silicon-based negative electrode active material obtained by performing a specific treatment exhibits good cycle characteristics and can exhibit excellent battery properties. It came to complete.

  That is, the present invention provides a silicon-based negative electrode active material obtained by mixing a silicon compound and conductive carbon and then mixing them while adding compressive force and shearing force.

  Since the silicon-based negative electrode active material of the present invention is a particle in which a silicon compound and conductive carbon are dispersed extremely uniformly and the voids are reduced, the volume change during charge / discharge is effectively suppressed. The cycle characteristics can be improved.

2 is a SEM image showing particles of a silicon-based negative electrode active material obtained in Example 1.

Hereinafter, the present invention will be described in detail.
The silicon-based negative electrode active material of the present invention can be obtained by mixing a silicon compound and conductive carbon and then mixing them while applying compressive force and shearing force. Through this treatment, the silicon compound and the conductive carbon are uniformly agglomerated while being uniformly dispersed to form particles (hereinafter also referred to as “composite particles”), thereby reducing the voids. Particles can be obtained. Therefore, when used as a negative electrode material, it is considered that the volume change at the time of charging and discharging is effectively suppressed, and the cycle characteristics of the obtained battery can be effectively improved. Alternatively, a conductive carbon layer can be formed by adhering to the surface of particles (hereinafter also referred to as “primary particles”) exhibited by a silicon compound while deforming or extending the conductive carbon. It is preferable to use an airtight container provided with an impeller that rotates at a peripheral speed of 25 to 40 m / s for the process of mixing while applying a compressive force and a shearing force. By introducing a silicon compound and conductive carbon into such a container and operating the container, it is possible to perform a mixing process while applying a compressive force and a shearing force. In a closed container equipped with such an impeller, the silicon compound and conductive carbon are uniformly mixed by the rotation of the impeller, and a shearing force is also applied while a compressive force is applied between the impeller and the inner wall of the container. . The peripheral speed of the impeller is preferably 25 to 40 m / s, more preferably 27 to 35 m / s, from the viewpoint of effectively improving the cycle characteristics by reducing the voids of the obtained composite particles.

  In addition, from the viewpoint of increasing the uniformity of the obtained composite particles, and from the viewpoint of shortening the processing time in the closed container equipped with the impeller, before introducing the silicon compound and the conductive carbon into the closed container, These may be mixed.

Examples of the apparatus provided with a closed container that can be mixed while applying such compressive force and shearing force include a high-speed shear mill, a blade-type kneader, and the like. (Manufactured by Hosokawa Micron Corporation) can be preferably used. By using such an apparatus, it is possible to easily perform a mixing process while applying a predetermined compressive force and shearing force, and the silicon-based negative electrode active material of the present invention can be obtained only by performing such a process. .
As the processing conditions for the mixing, the processing temperature is preferably 5 to 80 ° C., more preferably 10 to 50 ° C., and the processing time is preferably 5 to 90 minutes, more preferably 10 to 60 minutes. The treatment atmosphere is not particularly limited, but is preferably an inert gas atmosphere or a reducing gas atmosphere.

  The mass ratio of the silicon compound and the conductive carbon charged into the closed container is preferably 97: 3 to 85:15 from the viewpoint of effectively suppressing the volume change and improving the cycle characteristics. Preferably it is 95: 5-88: 12, More preferably, it is 93: 7-90: 10.

  In addition, you may bake the obtained composite particle from a viewpoint of improving cycling characteristics more. The firing conditions are 400 ° C. or higher, preferably 400 to 800 ° C. for 10 minutes to 3 hours, preferably 0.5 to 1.5 hours under an inert gas atmosphere or reducing conditions.

The silicon compound is preferably primary particles obtained by pulverizing raw materials in advance.
Examples of silicon compounds that can be used include silicates containing silicon monoxide, silicon dioxide, magnesium and the like. These may be used alone or in combination of two or more. Of these, silicon monoxide is preferable from the viewpoint of improving cycle characteristics. A commercial product of Aldrich Chemical Co. can also be used.

  Moreover, as a method of pulverizing the raw material to obtain primary particles of the silicon compound, for example, the method described in Journal of Electrochemical Chemistry 661 ((2011), p245-249) can be used.

  Specifically, for example, a commercially available product (325 mesh, 99%) of Aldrich Chemical may be first mixed and pulverized as it is, or mixed and pulverized using an alcohol such as water or ethanol as a solvent. For the mixing / pulverization treatment, for example, a ball mill or a bead mill can be used. Subsequently, the silicon compound used as a raw material can be obtained by drying.

  The average particle diameter X of the silicon compound is preferably 20 to 200 nm, more preferably 20 to 150 nm, and still more preferably 20 from the viewpoint of improving uniformity as a composite particle to improve the cycle specification. ~ 100 nm. The average particle diameter X means a value measured by a dynamic light scattering particle size analyzer (Nanotrack UPA-EX150, manufactured by Nikkiso Co., Ltd.) after uniformly dispersing the sample with a solvent.

  Carbon black is preferable as the conductive carbon, and specific examples include acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. Of these, acetylene black and ketjen black are preferable from the viewpoint of improving the cycle specification while imparting good conductivity. Further, as the shape of these conductive carbons, from the viewpoint of further improving the physical properties of the battery obtained by coating at least a part of the surface of the primary particles of the silicon compound with a layer made of conductive carbon, It preferably exhibits a shape including voids.

  In addition, the conductive carbon used in the present invention preferably has an average particle size Y that is equal to or less than the average particle size X of the primary particles of the silicon compound. When the conductive carbon has such an average particle size, the conductive carbon is efficiently arranged in the gap between the silicon compound particles and the particles, and a highly uniform composite particle with reduced voids is obtained. And volume change can be effectively suppressed.

  The ratio (X / Y) of the average particle diameter X possessed by the silicon compound and the average particle diameter Y possessed by the conductive carbon is such that it is more efficiently disposed in the gap between the silicon compound particles and the cycle characteristics. From the viewpoint of improving, it is preferably 1 to 20, and more preferably 1.5 to 10. Moreover, the average particle diameter Y which electroconductive carbon has becomes like this. Preferably it is 10-100 nm, More preferably, it is 10-50 nm. In addition, this average particle diameter Y means the value measured by the method similar to the average particle diameter X of the said silicon compound.

  Further, the silicon compound and the conductive carbon in the silicon-based negative electrode active material of the present invention are the primary particles of the silicon compound from the viewpoint of enhancing the uniformity and dispersibility thereof and improving the cycle characteristics more effectively. It is preferable that at least a part of the surface is covered with a layer made of conductive carbon. The layer made of conductive carbon may cover at least a part of the surface of the primary particles, or may cover almost the entire surface of the primary particles. Thereby, it is possible to suppress aggregation of the primary particles of the silicon compound and the conductive carbon, and a silicon-based negative electrode active material that is a composite particle in which the conductive carbon is more densely and uniformly dispersed is obtained. It is possible to increase the conductivity while effectively suppressing the volume change and improving the cycle characteristics.

  The thickness of the layer made of conductive carbon is preferably 0.1 to 5.0 nm, more preferably 0.5 to 3.0 nm.

  Moreover, the average particle diameter Z which the silicon-type negative electrode active material of this invention has is 5-50 micrometers from a viewpoint which aims at weight reduction, maintaining the battery physical property which was excellent in the lithium ion battery obtained, Preferably it is 5-5. It is 30 micrometers, More preferably, it is 5-20 micrometers. Thus, since the silicon-based negative electrode active material of the present invention is a fine composite particle containing a uniformly dispersed silicon compound and conductive carbon, by using this to form a negative electrode, A lithium ion battery excellent in cycle characteristics can be obtained by effectively suppressing volume change. In addition, this average particle diameter Z means the value measured by the method similar to the said average particle diameter X and Y.

  The method for producing a lithium ion battery using the silicon-based negative electrode active material of the present invention thus obtained is not particularly limited, and any known method can be used. For example, such a silicon-based negative electrode active material is mixed with additives such as a binder and a solvent to obtain a coating liquid. At this time, if necessary, a conductive additive may be further added and mixed. The binder is not particularly limited, and any known agent can be used. Specific examples include polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, and ethylene propylene diene polymer. Moreover, it does not specifically limit as this conductive support agent, Any well-known agent can be used. Specific examples include acetylene black, ketjen black, natural graphite, artificial graphite, and fibrous carbon. Next, this coating solution is applied onto a negative electrode current collector such as an aluminum foil and dried to form a negative electrode.

  The silicon-based negative electrode active material of the present invention is useful in that it exhibits a very excellent discharge capacity as a negative electrode of a lithium ion battery. A lithium battery to which such a negative electrode can be applied is not particularly limited as long as it has a positive electrode, a negative electrode, an electrolytic solution, and a separator as essential components.

  Here, as for the positive electrode, as long as lithium ions can be released during charging and occluded during discharging, the material configuration is not particularly limited, and a known material configuration can be used. For example, it is preferable to suitably use various olivine compounds obtained by hydrothermal reaction of the raw materials.

  The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used for an electrolyte solution of a lithium ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones An oxolane compound or the like can be used.

The type of the supporting salt is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and organic salt derivatives It is preferable that it is at least 1 sort of.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.
[Example 1]
As the primary particles of the silicon compound, SiO (manufactured by Alrdich Chemical) was used, and 93.0 g of this was mixed with 7.0 g of Ketjen Black (manufactured by Lion Corporation, average particle size of 30 nm) to obtain a mixture, The obtained mixture was put into a fine particle composite apparatus Nobilta (manufactured by Hosokawa Micron Corporation) and mixed at 25 to 35 ° C. for 30 minutes to obtain composite particles A. The average particle size of the obtained composite particles A was 20 μm.
The SEM image of the obtained composite particle A is shown in FIG.

[Comparative Example 1]
Using the same silicon compound as in Example 1, 2.63 g of this, Ketjen Black (manufactured by Lion, average particle size 30 nm) 0.09 g, and a dispersion stabilizer (carboxymethylcellulose, manufactured by Daicel Finechem) were added. 30 g of water was put into a zirconia pot of a planetary ball mill (planet type ball mill P-5, manufactured by Fritsch) together with zirconia balls, mixed for 240 minutes at 25 to 70 ° C., and dried to obtain composite particles B . The average particle size of the obtained composite particles B was 2 μm.

[Test Example 1]
Using the composite particles obtained in Example 1 and Comparative Example 1, a negative electrode of a lithium ion secondary battery was produced. The composite obtained in Example 1 and Comparative Example 1, ketjen black (conductive agent), and polyvinylidene fluoride (binding agent) were mixed at a weight ratio of 70:15:15, and this was mixed with N-methyl. -2-Pyrrolidone was added and sufficiently kneaded to prepare a negative electrode slurry. The negative electrode slurry was applied to a current collector made of a copper foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a negative electrode.

Next, a coin-type lithium ion secondary battery was constructed using the negative electrode. Lithium foil was used for the positive electrode. As the electrolytic solution, a solution obtained by dissolving LIPF ₆ at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 1 was used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium secondary battery (CR-2032).

A charge / discharge test at a constant current density was performed using the manufactured lithium ion secondary battery, and a charge / discharge capacity was measured. The charging conditions at this time were constant current and constant voltage charging with a current of 0.1 CA (360 mA / g) and a voltage of 3.0 V, and the discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 0.005 V. All temperatures were 30 ° C. This charge / discharge test was performed from 2 cycles to 100 cycles, the charge / discharge capacities of the second and 100th cycles were measured, and the capacity retention rate (%) was determined according to the following formula (A).
Capacity maintenance rate (%)
= (100th cycle charge / discharge capacity / second cycle charge / discharge capacity) x 100 (A)
Table 1 shows the calculation results of the capacity retention rate.

  From the above results, the composite particle A obtained in Example 1 has a very high uniformity and reduced voids compared to the composite particle B obtained in Comparative Example 1, and thus the volume change. It can be seen that excellent cycle characteristics are exhibited by effectively suppressing the above. On the other hand, although the composite particle B obtained in Comparative Example 1 is a fine particle, it does not contain a sufficient amount of conductive carbon, and because there are many voids, the volume change cannot be sufficiently suppressed, It is thought that the cycle characteristics were degraded.

Claims (2)

  After mixing silicon monoxide and carbon black at a mass ratio of 93: 7 to 90:10, a process of mixing while further applying compressive force and shearing force is performed at a temperature of 10 to 50 ° C. and a time of 10 to 60 minutes. The manufacturing method of the silicon-type negative electrode active material performed in an airtight container provided with the impeller rotated with a peripheral speed of 25-40 m / s. The method for producing a silicon-based negative electrode active material according to claim 1 , wherein the silicon compound is pulverized in advance before mixing silicon monoxide and carbon black.