Combination of Phosphorus Doping and Gas Treatment to Enhance the Capacity and Initial Coulombic Efficiencies of Hard Carbon Anode for Sodium Ion Batteries

Phosphorus doping shows importance for enhancing the electrochemical capacity of hard carbon (HC) anodes for sodium ion batteries although the initial Coulombic efficiency (ICE) decreases because of factors such as increased specific surface area by the doping. For this study, we fabricated HC having high capacity and higher ICE by combining doping of a small amount of phosphorus and subsequent surface coating treatment by organic gas. The phosphorus‐doped HC synthesized with a less than 0.1 molar ratio of phosphorus increases the capacity most efficiently. Furthermore, C 2 H 2 gas treatment increased the capacity and ICE. The C 2 H 2 ‐treated P‐doped HC shows 405 mAh g −1 capacity and 93% ICE. X‐ray photoelectron spectroscopy and hard X‐ray photoelectron spectroscopy and P NMR analyses show that oxidized phosphorus sites are distributed mainly on the HC surface or in the near‐surface region, whereas phosphorus incorporated within the carbon layer is distributed throughout the particle interior. Density functional theory calculations revealed that both P—C and P=O conditions lead to stable Na adsorption, and P=O sites are favorable compared to P—C site. The carbon layer formed by C 2 H 2 gas treatment enhances reversible sodium storage capacity by coating the phosphorus site surface.