Si-based nanocomposites derived from layered CaSi2: influence of synthesis conditions on the composition and anode performance in Li ion batteries

Abstract

Various Si-based nanocomposites consisting of nanoflake-like Ca_xSi₂ particles and transition metal silicide (MSi_y) particles were synthesized by a solid-state exfoliation reaction using layered CaSi₂ and transition metal (M) chlorides (M: Ni, Fe or Mn). CaCl₂ was found to form in all samples irrespective of the chloride used. Based on the shape and composition of the nanocomposites, it is thought that Ca_xSi₂ particles were formed by extraction of Ca from the layered CaSi₂. The molar ratio of CaSi₂ and NiCl₂ and the synthesis temperature were systematically varied to investigate their influence on the characteristics of the reaction products, e.g. crystalline phases, microstructure, anode capacity as lithium-ion batteries and electrical conductivity. Nanocomposites with various mixtures of Ca_xSi₂ particles (Li storage sites) and NiSi_y particles (conducting media) were formed. The synthesized samples showed a wide range of electrical properties depending on the composition. For example, the samples exhibited an anode capacity and an electrical conductivity of 1020 mA h g⁻¹ and 6 × 10⁻⁸ S cm⁻¹, or 479 mA h g⁻¹ and 2 × 10⁻⁵ S cm⁻¹, respectively. Because of its simplicity, the solid-state exfoliation reaction using layered CaSi₂ and MCl₂ provides a facile and scalable method to synthesize Si-based nanocomposite anode materials for lithium ion batteries.