The In Situ Polyelectrolyte With Liquid Metal Filler for High Rate and High Stability Quasi‐Solid‐State Sodium Battery

Solid‐state sodium metal batteries garner significant attention due to their low cost, high safety, and remarkable energy density. However, interface instability hinders their further development. Herein, an in situ polymerized electrolyte is developed incorporating a liquid metal alloy (LM) that simultaneously achieves high ionic conductivity and self‐healing interfacial stability. This in situ polymerized electrolyte achieves a remarkable ionic conductivity of 2.7 × 10 −3 S cm −1 at room temperature. Additionally, the electrolyte demonstrates self‐healing capabilities because of the special electric field‐induced motion and liquid fluidity of the LM. The resulting battery enhances cycling stability. Symmetric cells exhibit 1,900 h cycling stability and a limiting current density of 1.91 mA cm −2 at room temperature, demonstrating exceptional long‐term reliability. This rational design strategy achieves a significant breakthrough in quasi‐solid‐state sodium metal battery technology while providing a practical route toward commercializing high‐energy‐density energy storage systems.