Abstract: Silicon/carbon (Si/C) anode is considered as a promising anode material for high-density lithium ion batteries. In the present work, a novel strategy to in-situ synthesize Si/C composites by one-step reduction of CO₂ with Mg₂Si was proposed. The effects of volume ratio of Ar∶CO₂ mix gas and the reaction temperature on the microstructure and electrochemical properties of the synthesized Si/C were investigated. It is found that crystalline Si and amorphous C are interwoved each other in the Si/C particles with the size of about several nanometers. As anode materials for lithium ion batteries, the Si/C composites synthesized at 700℃ under the Ar∶CO₂ gas ratio of 7∶1 exhibit a high capacity of 1134 mA·h/g over 500 cycles at a current density of 0.2 A/g. This study provides an avenue to synthesize Si/C energy storage materials from greenhouse CO₂ gas with the merits of facile operation procedure, easy large-scale production and potential commercial applications.