Low‐Concentration Hydrated Eutectic Electrolyte Design for High‐Performance Aqueous K‐Ion Batteries

ABSTRACT Aqueous K‐ion batteries (AKIBs) hold promising potential for large‐scale energy storage due to high safety, environmental friendliness, and low cost. However, traditional aqueous electrolytes are limited by a narrow electrochemical stability window (ESW), while water‐in‐salt electrolytes face challenges such as high cost, low ionic conductivity, and precipitation at low temperatures, which significantly hinder their applications. Herein, we report a class of low‐concentration hydrated eutectic electrolyte (2.45 m KFSI‐Bds‐H 2 O) featuring a wide ESW (3.7 V), reduced viscosity (13.4 mPa·s), improved ionic conductivity (17.5 mS cm −1 ), and excellent non‐flammability characteristics to construct high‐performance AKIBs. In the designed electrolyte, Bds serves dual roles in regulating hydrogen‐bond and solvation structures, wherein it not only disrupts the strong hydrogen‐bond network of H 2 O to suppress its decomposition side reactions, but also reconstructs the K + solvation structure. The AKIB delivers a high energy density of 100 Wh kg −1 and superior capacity retention of 75.1% after 2000 cycles, and can perform stably across a broad temperature window of ‐20°C to 60°C. This work offers a promising strategy for developing low‐cost, high‐safety, and long‐life aqueous batteries.