Fire-Safe Hierarchical Solid Polymer Electrolytes Enabled by Dual-Functional Microcapsules for Lithium Metal Batteries.

Lithium metal batteries offer high energy density but suffer from severe safety risks associated with dendrite growth and flammable electrolytes, which can lead to thermal runaway and combustion. Although solid-state electrolytes improve thermal stability, they primarily rely on intrinsic nonflammability and remain ineffective in suppressing combustion once thermal runaway is triggered. Here, we report a dual-functional fire-safe microcapsule-enabled strategy to construct hierarchical solid polymer electrolytes (HSPEs) with integrated fire-suppression capability. Fire-extinguishing agents, perfluoro(2-methyl-3-pentanone), are encapsulated within ammonium hydrogen phytate-modified urea-formaldehyde-based resin shells and incorporated into an ethylene-vinyl acetate matrix. Upon thermal stimulation, the microcapsules undergo rapid structural rupture, releasing perfluoro(2-methyl-3-pentanone) to suppress combustion, while the ammonium hydrogen phytate-derived shell forms a protective char layer that retards heat and mass transfer. This dual-functional design enables thermally triggered fire suppression within the electrolyte framework. Importantly, the HSPEs maintain desirable electrochemical properties, including a room-temperature ionic conductivity of 0.64 mS cm-1 and an electrochemical stability window of up to 4.6 V. Furthermore, pouch cells based on HSPEs effectively resist ignition under thermal abuse conditions. This work provides a viable approach to integrating fire-suppression functionality into solid-state electrolytes, offering new insights into the design of safer lithium metal batteries.