Atomistic Transport Mechanisms in Lithium Salt-Doped Ionic Covalent Organic Framework Electrolytes

Abstract

Ionic covalent organic frameworks (iCOFs) have garnered significant attention as potential single-ion conductive solid-state electrolytes, where researchers have made substantial efforts in designing iCOF-based composites, aiming to improve their intrinsic low conductivity. One successful case is to fill iCOF channels with lithium salts, such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). However, the ion transport mechanisms in these composite electrolytes are still largely unknown, hindering their further improvement. Here molecular dynamics simulations were employed to systematically predict the ion diffusivity in iCOF (e. g., TpPa-SO₃Li COF)-LiTFSI composite electrolytes with varying LiTFSI compositions at different temperatures. A positive correlation was observed between Li⁺ diffusivity and LiTFSI:iCOF ratio, which was also verified by our experiments. Interestingly, the Li⁺ diffusion energy barrier obtained by the Arrhenius equation exhibited nearly no dependency on the LiTFSI concentration, indicating the importance of temperature-insensitive microstructural-related factors. Radial distribution functions revealed that with a higher LiTFSI proportion, the coordination number of SO₃⁻ decreases, while that of TFSI⁻ increases, suggesting a competition between these two species in the Li⁺ solvation shell. Furthermore, configurational entropy and bond orientational order parameter calculations examined the degree of disorder in the Li⁺ solvation structure. These results should improve our mechanistic understanding of iCOF-based electrolytes.