Reversible Lithium Storage in Carbon Encapsulated Hollow Silicon Nanospheres

One of the major challenges associated with the application of silicon as anode material for Li-ion batteries is the huge volume expansion/shrinkage during the lithiation/delithiation process. This volume change can result in the formation of crack in the particles and degrade the capacity. To solve this problem, we report a facile synthesis method which maintains the structural integrity of the material during cycling. Polystyrene beads was used as the templating agent and coated with silica through the sol-gel process. Then, the core was removed in the stepwise heating. Subsequently, the obtained shell was reduced to silicon via the magnesiothermic reduction. The final step is the coating of hollow Si spheres with carbon in the autoclave. SEM, TEM, XRD, FTIR, TGA and Raman were used to investigate the microstructure of the core-shell nanoparticles. Carbon encapsulated hollow spheres were mixed with carbon black, polyacrylic acid (PAA) and N-Methyl-2-pyrrolidone (NMP) to form a slurry. Copper foil as the current collector was covered with the slurry and dried at 120 ºC overnight. Half-cells were fabricated using a Li foil coupled with the carbon encapsulated Si hollow spheres in CR2032 coin cell cases. Charge and discharge were performed in the voltage window of 0.005-1.5 V with different current densities. The highest discharge capacity was about 2000 mAh/g at the first cycle. Cyclic voltammetry (CV) test confirms the reversibility of the reactions in both charge and discharge sections. Impedance spectroscopy measurements verify the decrease in impedance of the battery after cycling.