Lowering oxygen content on the surface of Si/C composite anode material of lithium-ion batteries with calcium carbide

Abstract

The inevitable oxidation of the surface increases the resistance of silicon (Si) as well as impairs its electrochemical performance as the anode of lithium-ion batteries (LIBs). The oxygen content of the Si/C composite surface is effectively reduced by using calcium carbide (CaC2) as a reductant in CaCl2-NaCl molten salt. The Si(O)/C-X (X: synthesis temperature) composite precursors are designed and synthesized by the oxidation of the industrial CaSi2 with the excess CaCO3 in molten CaCl2-NaCl, then the precursors are deoxidized to form the Si/C-X@C-Y (Y: reduction temperature) with lower oxygen content. The oxygen content of the Si/C-650@C-600 composite surface can be reduced by about 30 % compared to that of Si(O)/C-650. The rate performance and specific capacity of the Si/C-X@C-Y anode are significantly better than those of Si(O)/C-X. The Si/C-650@C-600 exhibits excellent rate performance with a capacity of 950.87 mAh·g−1 at 2 A·g−1, which is 74.4 % of its average capacity at 200 mA·g−1. Additionally, the Si/C-650@C-700 composite shows a capacity retention of 84.5 % after 400 cycles at 0.8 A·g−1. Lowering the surface oxygen content of Si(O)/C composites by using CaC2 in the molten salt can significantly improve the cycling stability of Si/C materials by effectively reducing the side-reactions between the Si anode and the electrolyte.