Regulating chiral nematic liquid crystal of hydroxypropyl methylcellulose coating on separator for High-Safety Lithium-Ion batteries

The conventional commercial polypropylene separator (PP) struggles to inhibit the thermal runaway triggered by dendrite short circuits, and its safety cannot meet the needs of the development of high-energy-density batteries. Herein, an easily commercializable design strategy was employed to induce the aqueous solution of natural polymer hydroxypropylmethylcellulose (HPMC) to self-assemble on the surface of the PP separator by Na2SO4, MnSO4, and Al-2(SO4)(3) to form the chiral nematic liquid crystal (CLC) with excellent performance, and finally the thermally stable separators (H-Na@PP, H-Mn@PP, and H-Al@PP) were obtained. Theoretical calculations and experiments demonstrate that the CLC induced by Na+, Mn2+, and Al3+ can interact with the electrolyte solvent to form a desolvation structure of Li+, which reduces the migration barrier of Li+ through the separator and accelerate the Li+ transport. Furthermore, the ordered CLC structure can ensure uniform electric field and Li+ flux. Hence, Li//LFP, Li (50 mu m)//LFP, and Li//NCM811cells are assembled using these modified separators, featuring remarkable cycling stability and high Coulombic efficiency. As the result, H-Na@PP, H-Mn@PP, and H-Al@PP separators in Li//LFP cell display a high initial capacity of 141.2 mAh/g, 146.7 mAh/g and 130.7 mAh/g at 1C, respectively and stable cycling performance over 1000 cycles. Notably, the capacity retention rate remains high at 87 %, 69 %, and 59 % even after 700 cycles, respectively, which are higher than the 21 % capacity retention rate of PP separator. Meanwhile, the pouch cells equipped with these separators deliver exceptional electrochemical performance and show a lower temperature distribution without thermal runaway behavior under the Phi 3 mm nail penetration test, indicating its feasibility for high-safety energy storage systems.