Designing Microemulsion Electrolytes with Widened Electrochemical Stability Window for Highly Reversible Aqueous Zinc Ion Batteries

Abstract Aqueous zinc‐ion batteries hold great promise due to their inherent safety and low cost. However, challenges such as the narrow electrochemical stability window and water‐related side reactions need to be addressed to achieve higher energy densities and longer stability. In this work, a microemulsion system is introduced into the electrolyte design. The oil‐in‐water microemulsion electrolytes can expand the electrochemical stability window to 3.52 V. The oil‐rich layer formed by the microemulsion at the electrode/electrolyte interface effectively alleviates the water‐splitting reaction with suppressed corrosion current density. This effect enables stable Zn||Zn symmetric cell operation over 4800 h at 1 mA cm −2 , and significantly improves the performance of Zn||Cu half cells and Zn||NaV₃O₈ full cells. Notably, Zn electrodes treated in the microemulsion electrolytes for the first three cycles can be re‐assembled and cycled well in new aqueous batteries with no oil additives. This is analogous to graphite solid‐electrolyte interphases (SEIs) formation in specially formulated electrolytes in the formation cycle and then switched to a differently formulated electrolytes for cycling. The work paves an attractive path to address the electrochemical stability window issues in AZIBs and provides a practical approach for the rational design of reliable electrolytes for high‐performance aqueous batteries.