Design And Electrochemical Studies On High Performance Si-Based Composite Anode For Lithium Ion Batteries

With the rapid development of new industrial technology, such as renewable energy storage and EV applications, there are growing demands for the next-generation lithium ion batteries with high energy density as well as high power performance. Search for electrode materials with large capacity and long cycle life is a key for developing higher performance lithium-ion batteries. Given that the low theoretical capacity (372 mAh g-1) of currently commercialized graphite anode materials can not satisfy the demand of high performance battery systems, it is urgent to develop new anode materials with high capacity.In view of small volume change and excellent cyclablity of carbon and large lithium insertion of silicon, they were adopted as matrix and main active material respectively. New types of Si/C composites were prepared by one step of high-temperature sintering and magnesiothermic reduction method and the electrochemical performance was investigated in our work.A novel flexible current collector, carbon fiber foil (CFF), was presented to replace traditional Cu foil. The Si/C/CFF electrode was obtained by one step of high-temperature sintering of Si, PVC and CFF. The composites can not only buffer the volume change of silicon during lithium insertion but also provide effective electric connection networks surrounding Si particles. Due to the above reasons, the Si/C/CFF electrode demonstrated that, unlike Si/C on Cu foil, Si/C on CFF can undergo several tens of lithiathion/delithiation cycles and give a high capacity of 977mAh g-1 at 0.5C, thus confirming that carbon paper is a suitable current collector-substrate which is able to improve the cycling performance of silicon.We also report a novel lotus-root-like mesoporous Si based on a magnesiothermic reduction method. The mesopores in the range of 20 50 nm are formed between the primary silicon grains. Furthermore, a chemical vapor deposition (CVD) method is employed to coat the mesoporous Si with a very thin carbon layer using acetylene as carbon source. The mp-Si@C composite shows a stable capacity of ca. 1500 mAh g-1 for 100 cycles at 1C and excellent rate performance even up to 15C. The improved capability of mp-Si@C electrode can be ascribed to the following reasons: 1) the mesoporous structure offers sufficient void space to absorb the huge volume expansions of silicon; 2) the surface carbon layer maintains the conducting network of the electrode during repeated cycles; 3) the open structure of the porous particles is favorable for fast transport of Li+ and gives rise to high rate performance.