Recently, the amount of CO2 gas contained in the atmosphere has increased, which increases the occurrence of the greenhouse effect causing global warming. Effects of the air pollution due to substances including CO2, NOx, hydrocarbon and the like emitted from automobiles used as transportation means negatively affects the health of people. From the viewpoint of the increase in prices for energy such as crude oil and environmental protection, much expectation has been placed on a smart grid which is a system that optimizes the balance of need for electric power by network management of electric power in a hybrid vehicle, which combines an electric motor and an engine operated by electricity stored in a power storage device, an electric vehicle, and an electric power generation facility, which have high energy efficiency.
Further, even in the information communication field, an information terminal such as a smart phone has rapidly infiltrated into the society due to the ease of exchanging information and sending messages. Under these circumstances, in order to enhance the performance of a smart phone, a hybrid vehicle, an electric vehicle, a smart grid and the like and reduce production costs, it has been expected to develop a power storage device such as a capacitor or a secondary battery, which combines a high electric power density and a high energy density, and a long service life.
Among currently commercially available devices as the power storage device, a device having the highest energy density is a lithium ion secondary battery using carbon such as graphite in the negative electrode and compounds of lithium and a transition metal in the positive electrode. However, since the negative electrode is composed of a carbon material in the “lithium ion battery”, only up to ⅙ of lithium atoms per carbon atom may be theoretically intercalated. For this reason, it is difficult to achieve a new high capacitance battery, and there is a need for a new material for the negative electrode for achieving high capacity. In addition, even though the “lithium ion battery” has high energy density, and thus is expected to be a power source for a hybrid vehicle or an electric vehicle, there is a problem in that the “lithium ion battery” cannot release a sufficient amount of electricity due to high internal resistance of the battery during a rapid discharge, that is, has small output density. For that reason, there is need for the development of a power storage device having high output density and high energy density.
In order to satisfy these demands, studies have been conducted on tin or silicon and an alloy thereof, which may store and release a larger amount of lithium ions than graphite. Tin or silicon may store electrochemically a larger amount of lithium ions, but expands in volume by about 4 times and causes pulverization when expansion and contraction occur due to repeating charge and discharge, thereby causing deterioration in performance of the battery. In order to prevent the aforementioned pulverization, attempts have been conducted for extending the service life of the negative electrode of the battery by grinding silicon or silicon alloys into particulates.
As a method of making the silicon material a particulate, there is a mechanical grinding method, and as a device which may grind the silicon material into a particle size of sub-micron or less, there is a wet beads mill which is a kind of media mill. Grinding by the wet beads mill has problems of (1) reducing the content of oxygen in silicon powder which is a raw material, (2) suppressing oxidation during the grinding, (3) suppressing ground particles from re-aggregating, and (4) suppressing particles from aggregating when the silicon material is ground and then dried.
Patent Documents 1, 2, 3, 4 and 5 disclose methods of using beads mill in grinding silicon or silicon alloys.
Patent Document 1 discloses that (i) Si particles having an average particle diameter (D50) of 0.05 to 5 μm are prepared by a wet grinding using a beads mill, (ii) as a solvent to be used, toluene, xylene, mesitylene, methyl naphthalene, creosote oil and the like which are inert to Si are used, and (iii) a wet-mixing heat treatment is performed by adding ground Si particles and a carbon material or a precursor thereof. However, there are problems in that when Si particles are mixed with a carbon material or a precursor thereof before the heat treatment, an oxygen source has not completely been removed, and accordingly, Si particles are oxidized, Si particles having a particle diameter of 0.05 to 5 μm aggregate in a heat treatment, and accordingly, the charging/discharging life cycle of the battery is not long, and the like.
Patent Document 2 discloses a beads mill as a wet grinding device which forms an alloy from a mixture of Si powder and transition metal by a mechanical alloying method, and grinds the alloy into particles having an average particle diameter (D50) of 0.50 to 20 μm. Patent Document 2 also discloses that it is possible to use a non-protic solvent such as hexane, acetone, and n-butyl acetate, and a protic solvent such as water, methyl alcohol, ethyl alcohol, 1-propyl alcohol, 2-propyl alcohol, 1-butyl alcohol, and 2-butyl alcohol as a dispersion medium used in the wet grinding. However, Patent Document 2 does not disclose a method of grinding the silicon powder into particles having an average particle diameter (D50) of less than 0.5 μm.
Patent Documents 3, 4, and 5 disclose that a Si—Sn—Cu alloy powder is ground into particles having an average particle diameter of up to 0.28 μm by using a beads mill grinding using zirconia beads as beads and isopropyl alcohol as a medium. However, there are problems in that due to pulverization, contact resistance between particles is increased, and charging/discharging efficiency deteriorates, and characteristics expected by using particulates have not been exhibited. Furthermore, as an attempt to increase the electrochemical reaction efficiency by increasing conductivity even in the negative electrode, an attempt to increase the conductivity by adding carbon nanotube and carbon nanofiber in small amounts has also been conducted, but carbon nanotube and carbon nanofiber aggregate with each other, so that it is difficult to efficiently disperse carbon nanotube and carbon nanofiber, the coasts are high, and it is difficult to increase the content of carbon nanotube and carbon nanofiber in the negative electrode.