The chiral nematic liquid crystal of hydroxypropyl methylcellulose coated on separator: Break through safety of LIBs with high electrochemical performances.

The commercial polypropylene (PP) separator of lithium-ion batteries (LIBs) suffers from abominable thermal runaway, which seriously impedes their wide application in electric vehicles, portable electronic devices, energy storage, and other fields. To resolve this obstacle, herein, we for the first time report the phenomenon of hydroxypropyl methylcellulose (HPMC) crystallizing on the PP separator via natural drying to form structural color, which comprehensively breaks through the safety of LIBs. In-situ thermal monitoring indicates that the chiral nematic liquid crystal phase (CLC) with structural color formed by HPMC under natural drying can uniform the temperature distribution during battery operation. The most important achievement, benefiting from the preeminent thermal stability of CLC special structure, is that the pouch cell assembled with this separator exhibits a lower temperature under nail penetration tests with Φ5 mm and Φ8 mm nail, even without any risk of thermal runaway. The superior cycling stability of the pouch cells under various commercial cathode materials indicates the HPMC coating exists stably in commercial energy storage systems. More impressively, we first achieved robust cycling performance of LIBs assembled in an atmospheric environment for more than 1000 cycles, and the milestone discovery will undoubtedly create a new research direction for LIBs.