Competitive Solvation‐Driven Interface Stabilization for Protic Deep Eutectic Solid Electrolyte in Sodium‐Metal Batteries

Solid‐state sodium batteries (SSSBs) are promising for safe and high‐energy storage, while their development is hindered by the low ionic conductivity of solid electrolytes and severe interfacial side reactions, especially when protic deep eutectic electrolytes (DEEs) are employed to enhance conductivity. The active hydrogen in typical DEEs (e.g., N‐methylacetamide, NMA) readily reacts with the sodium metal negative electrode, leading to rapid performance decay. Herein, we propose a solvation reconstruction strategy to address this issue by incorporating polar carbonate ester into a composite solid electrolyte (CSE) based on NaTFSI‐NMA DEE. Carbonate ester molecules preferentially enter the solvation structure of Na + , replacing NMA from the primary solvation sheath layer, thereby inhibiting its interfacial by‐reaction with the Na negative electrode. The optimized electrolyte (PNDC) exhibits a high ionic conductivity of 2.82 mS·cm −1 , a Na + transference number of 0.77, a low activation energy of 0.12 eV, and a wide electrochemical window of 4.8 V. The assembled sodium metal cell can operate stably for 2500 cycles at 5 C. Moreover, it demonstrates excellent safety performance. This work presents a rational solvation engineering approach to overcome the interfacial challenges of protic DEEs, offering a safe and high‐performance electrolyte for fast‐charging SSSBs.