Cation-anion synergistic interactions in ion-regulated cholesteric liquid crystal Hydroxypropyl methylcellulose coatings for lithium metal batteries

The nonpolar surface of the pristine polypropylene (PP) separator is unable to restrict the reverse migration of PF₆⁻ and attract Li⁺, resulting in sluggish lithium ion transport kinetics and dendrite growth. Herein, three metal ions (Na⁺, Li⁺, K⁺) are employed to regulate cholesteric liquid crystal (CLC) structures of Hydroxypropyl methylcellulose (HPMC), and then integrate into PP separators as functional layers (PP@HPNa, PP@HPLi, and PP@HPK). The structural characterization and theoretical calculations have also demonstrated that the helical pitch of HPMC is largest under K⁺ regulation. Furthermore, it has been demonstrated that the PP@HPK separator has a strong affinity for PF₆⁻ and Li⁺, which promotes the reduction of PF₆⁻ at the electrode interface and accelerates Li⁺ transport. Consequently, the PP@HPNa, PP@HPLi, and PP@HPK separators exhibit high Li⁺ transference numbers of 0.74, 0.73, and 0.74, respectively. Notably, the highest LiF content (30.3%) in solid electrolyte interphase (SEI) is observed when using PP@HPK separators, which can be attributed to more efficient PF₆⁻ entrapment within the enlarged helical channels. When applied in full cells with LiFePO₄ and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathodes, the cell with PP@HPK separator demonstrates noteworthy rate capability and prolonged cycling stability, sustaining stable operation for up to 1530 cycles at 1C and 560 cycles at 0.5C, respectively. This work highlights cation-regulated cholesteric HPMC coatings as an effective strategy for designing high-performance separators for LMBs.