Mechanistic Insights into Sodium Plating in Hard Carbon Anodes: Electrolyte Design Principles for Practical Medium Voltage Sodium-Ion Full Batteries

Hard carbon (HC) is a leading anode for sodium-ion batteries (SIBs), yet its practical application is hindered by Na plating stemming from its multistage Na-storage mechanism, which generates quasi-metallic Na clusters near the deposition potential, triggering uncontrolled metal deposition. Despite numerous advances in electrolyte design that improve cycling stability, the electrolyte dependence of Na plating is poorly understood. Herein, the Na plating behavior of HC in practical pouch-type full cells is systematically investigated, establishing electrolyte design principles that highlight the necessity of addressing Na plating/stripping reversibility alongside Na+ insertion/extraction. Na plating is found to be intrinsic and unavoidable under realistic operating conditions, including fast charging, prolonged cycling, and low-temperature cycling. Comparative analysis of ester-based (EC/DEC) and ether-based (G2) electrolytes reveals that the G2 electrolyte enables highly reversible Na plating/stripping, attributed to its lower desolvation barrier, faster interfacial kinetics, and the formation of an inorganic-rich solid electrolyte interphase (SEI). These findings underscore the importance of jointly enhancing the Na plating reversibility and SEI robustness for next-generation HC-based SIBs. Notably, ether-based formulations are validated as suitable for coupling low-voltage cathode systems, mitigating N/P ratio constraints, and unlocking higher energy densities.