A thermoresponsive electrolyte additive for high-energy, long-cycling, and safe lithium batteries

Simultaneously achieving both high energy density and intrinsic safety in lithium-based batteries remains a fundamental challenge. Here, we resolve this dilemma by designing a thermoresponsive additive that improves the electrochemical performance of the batteries at normal service temperatures, while activating safety intervention under high temperatures to avert thermal runaway. At lower temperatures below polymerization, 3-phenyl-7-(trifluoromethyl)-3,4-dihydro-2H-1,3-benzoxazine (mCF₃-BA) contributes to forming a highly stable solid electrolyte interphase (SEI)/cathode electrolyte interphase (CEI) during initial charge/discharge cycles, as evidenced by the 98.1% capacity retention at the 100^th cycle for a 500 mAh Li||LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) pouch cell and 80% at the 800^th cycle for a 1,800 mAh Si@Gr450 (mixture of Si and graphite with a designated capacity of 450 mAh g⁻¹)||NCM811 pouch cell. At elevated temperatures upon thermal abuse, the rapid polymerization of mCF₃-BA creates an insulating thermoset network that physically prevents electrode contact and chemical crosstalk. Such a mechanism elevates the thermal runaway threshold temperature (T₂) by 34.5°C and 43.9°C for 500 mAh Li||NCM811 and 1,800 mAh Si@Gr450||NCM811 lithium batteries, respectively.