Safety concerns related to the abuse operation and thermal runaway are impeding the large-scale employment of high-energy-density rechargeable lithium batteries. Here, we report that by incorporating phosphorus-contained functional groups into a hydrocarbon-based polymer, a smart risk-responding polymer is prepared for effective mitigation of battery thermal runaway. At room temperature, the polymer is (electro)chemically compatible with electrodes, ensuring the stable battery operation. Upon thermal accumulation, the phosphorus-containing radicals spontaneously dissociate from the polymer skeleton and scavenge hydrogen and hydroxyl radicals to terminate the exothermic chain reaction, suppressing thermal generation at an early stage. With the smart risk-responding strategy, we demonstrate extending the time before thermal runaway for a 1.8-Ah Li-ion pouch cell by 100% (~9 hours) compared with common cells, creating a critical time window for safety management. The temperature-triggered automatic safety-responding strategy will improve high-energy-density battery tolerance against thermal abuse risk and pave the way to safer rechargeable batteries.