Salt Diffusivity in a Cu-Cellulose Nanofiber Based, Multi-Component Electrolyte Assessed Using Restricted Diffusion, Molecular Dynamics, and Nuclear Magnetic Resonance

Cu 2+ -doped cellulose nano fiber (Cu-CNF) solid electrolytes were developed for fast charge, high energy density batteries. Cellulose chains in the fibers are opened by Cu 2+ coordination, and a binary salt (LiTFSI) was added with minimal residual solvent left after the drying process. The Li + and TFSI − transport both within and between fibers greatly enhances the ion transport. To advance understanding of salt transport in Cu-CNF electrolytes, we conducted restricted diffusion experiments on two formulations of Cu-CNF samples to obtain the binary salt diffusivity ( D ± ) vs temperature, and measured the influences of test protocols and sample-to-sample variation. In addition, we did pulsed field gradient (PFG) nuclear magnetic resonance (NMR) experiments and used molecular dynamics (MD) simulations to model species motion. Restricted diffusion experiments show the Cu-CNF materials display concentration polarization voltage relaxation, and the resulting salt diffusivity is D ± ≈ 5 × 10 − 12 m 2 s −1 at 45 °C, with E a ≈ 0.5 5 eV . PFG-NMR resulted in values for D L i + and D TFS I − with a similar magnitude to D ± , while MD (focused on intra-fiber transport) gave lower ion diffusivities and activation energies. The multi-component, fiber-based structure of these electrolytes provides new opportunities for transport theory and measurements, with future work focused on studying composition-property relationships to better quantify transport mechanisms.