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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. [ABSTRACT FROM AUTHOR]

Electrolytes play a crucial role in facilitating the ionic movement between cathode and anode, which is essential for the flow of electric current during the charging and discharging process of the rechargeable batteries. In particular, electrolyte additives are considered as effective and economical approaches into the advancements of the battery technologies in both the conventional non‐aqueous and burgeoning aqueous electrolyte systems. Herein, a systematic and comprehensive review of the electrolyte additives is reported for the interfacial engineering of Li and Zn metal anodes in the non‐aqueous and aqueous electrolytes, respectively. The types of electrolyte additives and their corresponding functionalities for the protection of these two metal anodes are discussed along with the electrochemical features of solid electrolyte interphase (SEI) derived from electrolyte additives. The recent progress on electrolyte additives for these two battery systems are also addressed from the perspectives of electrode, electrolyte, and the associated SEI. Finally, the outlook and perspective on the current issues and future directions in the field of electrolyte additive engineering are presented for next‐generation battery technologies beyond the conventional Li‐ion batteries. [ABSTRACT FROM AUTHOR]