Mitigating Thermal Runaway in Lithium Metal Batteries With Flame‐Retardant Magnesium Hydroxide Nanocomposite Gel Electrolytes

ABSTRACT Lithium‐ion batteries are the preeminent energy storage technology in consumer electronics, electric vehicles, and grid‐level applications. Nevertheless, they present significant safety hazards including thermal runaway that results in catastrophic failure due to the flammability of carbonate‐based electrolytes. The pursuit of enhanced energy density through nickel‐rich cathodes and lithium‐metal anodes further exacerbates these concerns. To address these challenges, this study reports a nanocomposite gel electrolyte (NGE) incorporating flame‐retardant Mg(OH) 2 (magnesium hydroxide, MH) nanoplatelets. NGEs using MH nanoplatelets provide robust mechanical properties (>20 MPa storage modulus) with high room‐temperature ionic conductivity (≈1 mS cm −1 ), while remaining compatible with scalable slurry‐based fabrication. Additionally, MH nanoplatelets enhance thermal safety due to endothermic decomposition at elevated temperatures that mitigates thermal runway, release of water vapor as a combustion suppressant, and formation of MgO char that inhibits flame propagation and maintains electrode separation. Moreover, MH NGEs can be configured into bilayer electrolytes, expanding the electrochemical stability window for higher energy densities. Notably, an MH‐based bilayer gel electrolyte incorporating diglyme and succinonitrile layers significantly improves cycle life in LFP|Li and NCA|Li cells compared to either layer alone. These results highlight the potential of MH NGEs as safe and scalable electrolytes for next‐generation lithium energy storage technologies.