Bulk Silicon/Graphite Alloy Anode Material for High Energy Density Li-Ion Batteries

Abstract

Silicon anodes are an optimum alternative to traditional graphite anodes because of their higher lithium ion storage capacity. However, the massive first cycle capacity loss and large fade during cycling have hindered its commercialization. Here we present a solution for these issues in the form of commercial grade silicon/graphite alloy. A silicon-carbon alloy structure with carbon-coated silicon nanoparticles gives an ability to buffer volume changes and retain the silicon amorphous phase. The material achieved a first cycle reversible capacity of over 480 mAh/g when cycled between 0.01-1.5V at a c-rate of 0.05C, with first cycle efficiency of close to 95%. The capacity of the silicon alloy was ~30 mAh/g even at 10C after cycling, and recovered to 480 mAh/g at 0.05C. We performed further studies using galvanostatic intermittent titration (GITT) technique which showed the diffusion coefficient to be 10-9 to 10-12 cm2/sec. Nyquist plots were obtained using electrochemical impedance spectroscopy (EIS) at every 5%SOC. 95x64 mm pouch cells (2Ah) with the silicon alloy coupled with high capacity NMC cathode had impressive long term cycling, over 500 cycles, at a high c-rate (1C/1C), and stable c-rate performance at 10C. The cells were disassembled after cycling to study the SEI formation, electrode expansion and coating stability. Physical measurement of electrodes and SEM analysis of active materials showed that even after long term cycling, the coating was intact without minor cracks or paste delamination.