A polymer-embedded molecular fuse for safer high-nickel lithium batteries

Abstract Thermal runaway in high-energy lithium batteries is driven by autocatalytic radical reactions and flammable gas accumulation, posing a critical safety bottleneck for next-generation energy storage. Here we report a cathode-integrated polymeric molecular fuse enabling thermally responsive safety control without compromising electrochemical performance. By tuning inductive and hyperconjugative effects of phosphonate side chains, the dissociation energy of sacrificial P–C bonds is regulated, yielding an ∼10°C shift in activation temperature, as supported by density functional theory and in situ variable-temperature Raman spectroscopy. Upon activation, P-centered radicals suppress exothermic chain reactions, achieving a 76% reduction in heat release and delaying thermal runaway in LiNi0.8Co0.1Mn0.1O2 cathodes and 0.6 Ah Li-metal pouch cells. Notably, the activation threshold exceeds typical cathode processing temperatures, preventing premature radical generation during fabrication while enabling on-demand safety activation. This work establishes a substituent-regulated molecular fuse that couples fabrication-compatible stability with triggerable intrinsic safety, transforming battery thermal management from passive suppression to controllable molecular-level reaction pathways.