Li diffusion and migration are influenced differently by co-solvents in polymer electrolytes based on poly(ɛ-caprolactone) and poly(ethylene oxide)

Abstract

In polymer electrolytes, poly(ɛ-caprolactone) (PCL) provides a lower lithium cation coordination strength as compared to the classically employed poly(ethylene oxide) (PEO), leading to enhanced lithium transference. Similar to PEO, however, PCL suffers from poor ionic conductivity, motivating plasticization with low-molecular weight co-solvents for improvement. To facilitate the choice of suitable co-solvents, we investigate here 15-crown-5 (15C5), tetra- and diglyme (G4, G2), dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF), sulfolane (SL), glycerol (GL) as well as propylene and vinylene carbonate (PC, VC) in terms of their effects on ion transport in PCL-based electrolytes with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a conducting salt ([Li+]/[monomer] = 0.25). Diffusion coefficients by pulsed-field gradient NMR and Raman analysis show a distinct difference between strongly and weakly Li-coordinating co-solvents, interestingly with a trend opposed to that for previously studied co-solvents in PEO-based electrolytes, which is explained by the competitive coordination of polymer and co-solvent to Li ions. Surprisingly, the strong influence of co-solvent coordination on diffusion does not result in an altered lithium transference number, T+, as obtained from ion mobilities determined by electrophoretic NMR. Ion migration in an electric field is far less dependent on the nature of the co-solvent, and even more, T+ is invariant between both polymers. We attribute this to the coupling of the species’ hydrodynamic fluxes, which is dominating migration in an electric field, rendering T+ independent of polymer coordination and only weakly dependent on co-solvent coordination. Thus, our key conclusion is that while diffusive properties of ions are largely controlled by the interaction strength of polymer and co-solvent with Li+ ions, these mutual interactions hardly influence the migration in an electric field. Instead of coordination properties, it is hydrodynamic fluxes which couple the drift velocities during migration in an electric field, and ultimately dominate lithium transference. These findings also question diffusion coefficients as an indicator for Li transference.