On the Practicability of the Solid‐State Electrochemical Pre‐Sodiation Technique on Hard Carbon Anodes for Sodium‐Ion Batteries

Abstract Hard carbon (HC) with low cost and high specific capacity is considered the appropriate anode material for sodium‐ion batteries (SIBs), but the low initial coulombic efficiency (ICE) caused by solid electrolyte interface (SEI) formation and inherent active defects impede its practical battery application. Here, the practicability of solid‐state electrochemical (SSE) pre‐sodiation technique for hard carbon anode is assessed to conquer such challenges. The uniformly pre‐sodiated HC (Pre‐HC) can be fabricated through the SSE reaction between the HC and preloaded sodium metal film without introducing a liquid electrolyte. After being immerged in the electrolyte, a thin artificial SEI with abundant inorganic species is formed on the surface of Pre‐HC due to the spontaneous chemical reaction, and the ICE of HC is improved from 76.0% to 107.9%. Full cell paired with Na 3 V 2 (PO 4 ) 3 cathode exhibits a high ICE of 94.0% with 70% of energy density augment (from 126.5 to 214.4 Wh kg −1 ) after anode pre‐sodiation. Pre‐HC anode still retains the pre‐sodiation capacity of 671.1 mAh g Na −1 after being stored in the dry air for 2 h. This work demonstrates the practical applicability of this SSE pre‐sodiation strategy to enhance the energy density of SIBs.