A molecular firewall: Spatially heterostructured polymers as intelligent, reversible thermal switches for lithium-ion batteries

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Thermal runaway remains a critical safety challenge for lithium-ion batteries (LIBs), and existing thermal protection strategies usually exhibit irreversible responses, limiting their practical reliability. Herein, we design a spatially heterostructured conducting polymer poly(3-butylthiophene- co -3-tetradecylthiophene) (PBTDT), which exhibits a highly reversible positive temperature coefficient (PTC) effect. Synthesized by chemical oxidative copolymerization of long- and short-side-chain thiophene monomers, PBTDT possesses spatially differentiated rigid and flexible domains: the short-side-chain rigid domains suppress irreversible thermal motion of the long side chains at elevated temperatures, while the long-side-chain flexible domains retain sufficient free volume for anion re-doping upon cooling, enabling reversible resistive switching. The polymer shows a sharp resistivity increase at ~100 °C and maintains a recoverable PTC response over eight thermal cycles. When applied as a surface coating layer on the current collector, the resulting LiFePO 4 -PBTDT cathode delivers a room-temperature discharge capacity of 157 mA h g −1 and autonomously shuts down electrochemical activity at 100 °C, with the capacity dropping to only 1.4 mA h g −1 ; this shutdown-recovery behavior remains highly reversible over five thermal cycles. Under adiabatic short-circuit conditions, LiCoO 2 -PBTDT cells reach a maximum temperature of merely 147 °C, in stark contrast to 290 °C for bare LiCoO 2 cells, effectively mitigating thermal runaway hazards. This molecular firewall strategy provides a promising design concept for self-actuating, reversible safety protection to enhance the intrinsic safety of lithium-ion batteries.