Regulating Microcosmic Crystal Structure of Hpmc Coated on Separator: Break Through Safety of Libs with High Electrochemical Performances

Abstract

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 hydroxyl propyl methyl cellulose (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 indicating 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 and does not affect energy density of batteries. As the result, the 60 °C, Nature and Freeze separator in Li (50 µm)// LiFePO4 (LFP) cell display a high initial capacity of 134.61 mAh g-1,144.63 mAh g-1 and 137.82 mAh g-1 at 1 C, respectively and stable cycling performance over 400 cycles. Notably, the capacity retention rate remains high at 93.56%, 97.97%, and 98.94% even after 200 cycles, respectively. This work clearly illustrates the microstructure of HPMC as a coating and the reasons for totally solving the risk of thermal runaway of high-performance batteries, which is beneficial to the practical application in various high energy density devices. More impressively, we first achieved robust cycling performance of LIBs assembled in atmospheric environment for more than 1000 cycles, and the milestone discovery will undoubtedly create a new research direction for LIBs.

Keywords: Lithium-ion battery, Functional separator coating, Hydroxypropyl methylcellulose, Structural color, Thermal runaway risks