Gradient-Structured and Robust Solid Electrolyte Interphase In Situ Formed by Hydrated Eutectic Electrolytes for High-Performance Zinc Metal Batteries

The mechanically and electrochemically stable and ionically conducting solid electrolyte interphase (SEI) is important for the stabilization of metal anodes. Since SEIs are originally absent in aqueous zinc metal batteries (AZMBs), it is very challenging to suppress water-induced side reactions and dendrite growth of Zn metal anodes (ZMAs). Herein, a gradient-structured and robust solid gradient SEI, consisting of B,O-inner and F,O-exterior layer, in situ formed by hydrated eutectic electrolyte for the homogeneous and reversible Zn deposition, is demonstrated. Moreover, the molar ratio of acetamide to Zn salt is modulated to prohibit the water activity and the hydrolysis of BF₄⁻ as well as to achieve high ionic conductivity owing to the regulation of the solvation sheath of Zn²⁺. Consequently, the eutectic electrolyte allows Zn||Zn symmetric cells to achieve a cycling lifespan of over 4400 h at 0.5 mA cm⁻² as well as Zn||PANI full cells to deliver a high capacity retention of 73.2% over 4000 cycles at 1 A g⁻¹ and to demonstrate the stable operation at low temperatures. This work provides the rational design for the hydrated eutectic electrolyte and the corresponding gradient SEIs for dendrite-free and stable Zn anodes even under harsh conditions.