Modulating Salt Dissociation and Solvent Immobilization Through Dipole Interactions in Polymer Electrolytes for Lithium Metal Batteries

Poly(vinylidene fluoride) (VDF)-based solid polymer electrolytes (SPEs) show great potential for application in solid-state lithium batteries. However, their poor ion transport capabilities and uncontrolled electrode/electrolyte interfacial reactions induced by residual solvents limit their overall electrochemical performance. To address these challenges, a LiTFSI-replaced dual-functional cationic covalent organic framework (COF) is strategically designed, denoted COF-LiTFSI, as organic fillers into poly(vinylidene fluoride-co-hexafluoropropylene) (PVHF)-based solid polymer electrolyte (SPE) to regulate Li⁺ transport and electrode/electrolyte interface stability. The introduction of dipole interactions between cationic moieties embedded in the COF-LiTFSI framework and Li salt significantly enhances dissociation, thereby improving Li⁺ transport and lowering the activation energy (E_a), while the strong dipole interaction between residual N-methylpyrrolidone (NMP) and the COF-LiTFSI framework immobilizes the NMP molecule, markedly enhancing the electrochemical stability of the PVHF-COF-CPE with a Li metal anode. Consequently, the optimized PVHF-COF-CPE achieves a high room temperature ionic conductivity of 0.63 mS cm^-1. Furthermore, the Li/Li, Li/LFP (LiFePO₄), and Li/NCM811(LiNi_0.8Co_0.1Mn_0.1O₂) cells equipped with PVHF-COF-CPE achieve remarkably stable cycling. This work presents a novel strategy to regulate ion transport and interface stability in VDF-based SPEs by leveraging dipole interactions, enabling high-performance solid-state lithium batteries.