Preformation of Insoluble Solid‐Electrolyte Interphase for Highly Reversible Na‐Ion Batteries

Abstract A stable solid‐electrolyte interphase (SEI) is crucial for cycling reversibility of Na‐ion batteries by mitigating continuous side reactions. So far, the severe SEI dissolution leads to low Coulombic efficiency (CE) and short cycle life. Meanwhile, the quantified relationship between SEI components and their solubility remains unclear. In this work, we establish the direct correlation between SEI components and SEI solubility, and quantify that the solubility of organic‐rich SEI is 3.26 times of inorganic‐rich SEI. We further propose a feasible strategy to preform inorganic‐rich insoluble SEI and demonstrate a practical hard carbon (HC)||NaMn 0.33 Fe 0.33 Ni 0.33 O 2 full cell in a commercial electrolyte of 1 M NaPF 6 in propylene carbonate (PC) with 80.0 % capacity retention for 900 cycles, and achieve a record‐high average CE of 99.95 % for a practical Na‐ion full cell. This study provides an effective strategy of preforming insoluble SEI to suppress its dissolution towards highly reversible Na‐ion batteries.