With the growing trend towards miniaturization and portability of electronic or communication devices, there have been strong demands for high-performance secondary batteries as their driving power, and development of technology for light and high-performance batteries. Lithium-ion batteries, which represent high electromotive force and high energy density, are being used as the most suitable batteries for these requirements. Therefore, the lithium-ion batteries are expected to play a great role and are extending the range of applications not only to eco-friendly transportation areas such as hybrid cars and electric cars but also to the power storage and medical or space industries. As large high-performance secondary batteries are required, studies have been done on securing safety as well as improving the energy density of lithium-ion batteries. Since carbon such as graphite is used as anode materials of lithium-ion batteries and has a low electrochemical reduction potential of about 0.1 V, the structure is very stable during charging and discharging, which is advantageous in making high-efficiency lithium-ion batteries. However, graphite is limited to a capacity of 372 mA h g-1 per weight and a capacity of 818 mA h cc-1 per volume in terms of maximum theoretical capacity since only one lithium-ion can be inserted between six carbon atoms in its structure. Therefore, the development of high-capacity lithium-ion batteries requires the development of new anode material with a greater capacity than graphite.
Silicon is one of the high-capacity anode materials of lithium-ion batteries with an electrochemical reduction potential of about 0.4 V and a theoretical capacity of about 4200 mA h g-1 by lithium and reaction. Silicon, however, has the shortcoming of the very fast capacity reduction during charging and discharging. As structural collapse occurs due to cracks which are caused by silicon-lithium alloys that bring nearly 300% volume expansion during the process of charging and discharging electrode active materials are finely pulverized by charging and discharging, weakening the properties of electricity generation. Thus, there are fatal problems for commercialization of silicon anode material such as pulverization of anode materials caused by the significant volume change, the matter of contact with the current collector, and the formation of an unstable interface layer between the breakdown of active material and electrolyte. In general, metal anode forms dendrite during charging and discharging, which limits the reversible life of the cell. In addition, internal short circuits due to dendrite growth and reactivity to moisture in some metals may pose a fire hazard, which can raise a major problem to the stability of the battery. To solve this problem, a lot of research has been conducted to improve stability of the battery in the charging and discharging cycle by controlling reaction velocity through contact reaction area and concentration control of silicon and lithium-ion by using refinement of surface and thin film coating of silicon, metal alloy and dispersion, and inert material such as silicon and carbon deposition with low reactivity. In this paper, research covers the development of high-capacity and high-profile anode material with the improvement competence of lithium-ion transmission by the structural changes of silicon and coating the interface which is in contact with silicon to control volumetric expansion.
In the first phase, the acceptance was controlled in the powdery state with the dry process for amorphization and atomization of chip-type silicon. In the second phase, the surface of silicon particle was heat-dissolved and carbon-coated using hydrocarbon gas to increase the adhesion of the particulate matter and facilitate synthesis of carbon nano fiber. In the third phase, the grown carbon nano fiber on the surface of pyrolytic carbon-coated silicon particles formed a network that served as a buffer against volumetric expansion and electrical conductivity for securing cycle characteristics. In order to show high retention of charging and discharging according to the high density in electrode manufacture, in the fourth phase, silicon particles with grown carbon nano fiber were evenly distributed by carbon coatings to reduce specific surface area and the surface treatment was made with control of particle size and shape. In the fifth stage, the life characteristics were improved by mixing nano-based metals to improve the conductivity between particles. It is a direct development process to control volume expansion of silicon from stage 1 to stage 4 while commercialization stage 5 improves life characteristics and the final stage 6 secures anode capacity by combining with carbon.
The initial charging and discharging capacity of silicon anode material with controlled particle size is 3631 mA h g-1 and 3250 mA h g-1 and coulomb efficiency is 89%, which is very good compared to conventional commercial products of 2472 mA h g-1 and 74% efficiency. This is because the widen distance between the sides of polycrystalline silicon through particle size controlled has made it easier for lithium-ions to move, which can be seen from the analysis results of XRD(x-ray diffraction). The specimen coated with a carbon layer on the surface of a nano-level silicon particle showed an good initial discharge capacity (3473 mA h g-1) and initial efficiency (86.2%) that were superior to capacity of silicon and commercial SiO anode active material, since coated carbon improved electrical conductivity, controlling the volume expansion of silicon. The initial discharge capacity of Si/PC/CNF/PC samples was 1512 mA h g-1, lower than the initial discharge capacity of Si/PC/CNF samples which is 2112 mA h g-1. The ratio of silicon among the total content of the particles was decreased resulting in lower initial discharge capacity while causing the initial coulomb efficiency and capacity retention rate of 94%, which is 3 percentage point higher than that of commercial products. If the combined capacity (564 mA h g-1) represents features of maintaining much higher capacity than the existing product and improving the life characteristics effectively further in terms of capacity retention, it would be sufficient for future use as a battery material.