Chemical design and data-driven innovation: a cross-scale review of flame-retardant electrolytes for safe high-energy-density batteries

High-energy-density lithium batteries, essential for electric vehicles and large-scale energy storage, face intensified safety risks, with thermal runaway remaining a critical barrier to deployment. Flame-retardant electrolytes offer a direct materials-level route to mitigate fire hazards, yet practical implementation requires concurrently meeting stringent demands on ionic transport, interfacial compatibility, and mechanical/thermal robustness. In this cross-scale review, we integrate “mechanism–assessment–design–computation” to guide the development of high-safety flame-retardant electrolytes for solid-state batteries. We first compare thermal runaway pathways in polymer- and inorganic-electrolyte cells from both abuse-triggered initiation and internally driven propagation, and establish a multidimensional safety assessment framework encompassing flame retardancy, thermal stability, mechanical integrity, and chemical/electrochemical stability. We then summarize chemical design strategies for two major classes of solid flame-retardant electrolytes: (i) polymer-based composite electrolytes enabled by flame-retardant additives—including inorganic fillers, halogen/phosphorus/nitrogen/silicon chemistries, and MOFs/derivatives—and (ii) intrinsically flame-retardant polymer electrolytes (in situ copolymerized and gel-like systems) together with inorganic solid electrolytes (oxides, sulfides, and halides). For each class, we elucidate composition–structure–property relationships, gas- and condensed-phase flame-inhibition mechanisms, and performance regulation strategies, with emphasis on interface engineering and thermodynamics–kinetics coupling to reconcile stability with fast ion transport. Finally, we highlight how quantum chemical calculations and (ab initio/classical) molecular dynamics reveal atomistic decomposition and transport, how finite-element and fluid-dynamics simulations enable cell-level safety evaluation, and how materials genomics and AI accelerate multi-objective screening, reverse design, and optimization. We also distill remaining bottlenecks—conductivity–flammability trade-offs in polymers, interfacial impedance and air/moisture sensitivity in inorganic solids—and outline scalable, cost-effective pathways toward practical implementation. This review consolidates current knowledge and provides actionable principles for translating safer electrolytes toward practical high-energy batteries.