Preparation of core-shell Si/NiSi2/carbon composite and its application to lithium secondary batteries

Abstract

A core-shell-typed active (Si)/inactive (NiSi2)/carbon composite was prepared by heat-treatment on a mixture of Si powder and nickel phthalocyanine. Uniform reaction for the inactive phase formation and carbon coating was possible since nickel phthalocyanine is sublimed on Si surface, which is further decomposed to generate metallic nickel that diffuses into Si matrix to produce a core-shell Si/NiSi2 composite. The phthalocyanine moiety is converted to carbon so as to cover the Si/NiSi2 composite particles. An improved cycle performance was observed with the Si/NiSi2/carbon composite electrodes. This favorable feature is indebted to a retardation of crystalline Li15Si4 phase formation, which is possible by the presence of inactive (NiSi2) shell layer that likely blocks Li+ diffusion into Si matrix. The rate capability for alloying/de-alloying reaction of Si core, however, becomes poorer due to the blocking action of the inactive phase.