Understanding the Electrolyte‐Hard Carbon Interphase Synergy in Sodium‐Ion Batteries: From Mechanistic Insights to Design Strategies

ABSTRACT Hard carbon is the most practical anode for sodium‐ion batteries, yet its deployment is hindered by an unstable, non‐uniform solid electrolyte interphase (SEI) that causes irreversible Na loss, high interfacial resistance, and capacity fading. This review discusses SEI formation as a dynamic coupling between electrolyte chemistry and the local carbon interface, and discuss the interactions of solvation structure, anion chemistry, concentration, and additives with hard carbon surface groups, dopants, pores, and defects to govern SEI properties. Based on this understanding, we propose a perspective framework for electrolyte‐hard carbon interphase synergy, which includes four coupled design directions: (i) solvation‐pore coupling to balance desolvation, pore accessibility, and confined sodium storage; (ii) surface‐ and additive‐guided electrolyte decomposition to regulate SEI composition; (iii) pore‐defect‐electrolyte regulation to enable selective Na + transport while suppressing intrapore SEI growth; (iv) constructing chemically and mechanically robust interphases through inorganic‐organic hybrid architectures. A key gap remains the missing in operando link between interfacial electric fields and SEI evolution, hindering rational design of self‐limiting passivation. We outline future directions toward chemically robust, ultrathin, and ionically conductive interphases for high‐performance sodium‐ion batteries.