Porous Silicon-Cobalt Silicide-Carbon Composites for Reversible Lithium Storage

摘要

With increasing use of portable electronic devices and electric vehicles, demand for development of high-energy density rechargeable batteries is rising. To meet the market demand for long-lasting rechargeable batteries, energy density of Li-ion batteries (LIBs) should be increased. The energy density of batteries is determined by the capacities of electrodes. Carbonaceous materials have been used as negative electrodes (anodes) in commercial LIBs because of their low operating potential and reasonable cycle life. However, the theoretical capacity of graphite anodes is relatively low, which is 372 mAh g~(-1) for the fully intercalated LiC_6 phase. To increase the capacity, different types of materials using alloying or conversion reaction should be developed. Li-alloy-based materials can store several Li ions with an atom by electrochemical alloying reactions and therefore, high capacity can be obtained. In particular, Si has a high theoretical capacity of 3580 mAh g~(-1) for Li_(15)Si_4 and low operating potential. However, serious capacity fading usually occurs because of huge volume changes during cycling and their low electric conductivity. To address this problem, many strategies have been developed through changes in materials design. For example, incorporation of carbon is the most famous method to solve the capacity fading of Li-alloy-based materials upon cycling. The existence of carbon in Si matrix can buffer the volume expansion of Si materials with increased electric conductivity, thus simultaneously improving both cycling and rate performance. Porous structure can be another strategy to relieve the volume expansion by using the internal pores. However, the process is generally time and energy consuming. In this study, we report a simple preparation process of porous Si-CoSi_2-C composites as anode materials for LIBs. The porous composite was synthesized through cheap starting materials and cost-effective methods. Si, magnesium silicide, and cobalt oxide were mixed using high energy mechanical milling (HEMM). Then, a carbon material was incorporated by HEMM. To produce porous structure, MgO formed by HEMM was removed by a wet etching process. As a result, porous Si-CoSi_2-C composites were prepared. These composites exhibited a reversible capacity of 1100 mAh g~(-1) and good capacity retention after 100 cycles. The porous structure and the amorphous carbon matrix alleviated the volume change to allow the enhanced electrochemical performance.