Solvent-free fabrication of TPU-reinforced PE/carbon composites for high-performance positive temperature coefficient materials in lithium-ion battery safety

Positive temperature coefficient (PTC) materials are pivotal for safeguarding lithium iron phosphate batteries, yet their industrial application is hindered by critical drawbacks: excessive film thickness, high internal resistance, and poor solvent sustainability. Addressing these challenges, this study innovatively develops a solvent-free thermal rolling process to fabricate an asymmetric expansion polymer film, specifically thermoplastic polyurethane (TPU) reinforced polyethylene (PE)/carbon composites, which significantly enhances the PTC effect. The core mechanism lies in the asymmetric thermal expansion of TPU and PE: this unique behavior disrupts the conductive carbon network, triggering a sharp PTC transition at around 120 °C. Further validation via an electrochemical-thermal coupling model confirms that the abrupt resistance increase effectively terminates battery discharge, a key safety function. The optimized composite (1% TPU-1% C@PE) exhibits exceptional performance: it features an ultrathin structure (8.5 μm) with low resistance (78 Ω) at room temperature, while reaching a high-resistance state (10 380 Ω) at 120 °C, achieving an impressive resistance ratio of 133.1. Notably, this performance is achieved without compromising the battery's cycling stability. The open circuit voltage of the 1% TPU-1% C@PE battery can quickly drop to 2.5 V within 570 s in an environment of 160 °C, due to the PTC effect. This work not only overcomes long-standing industrial bottlenecks but also provides a practical, scalable strategy, paving the way for advanced safety in lithium battery technology.