Enhanced Li Transference in Polymer Electrolytes: Why Anion Size Affects Cation Mobility
Simon Buyting, Monika Schönhoff
Abstract
High Resolution Image Download MS PowerPoint Slide The observation of increasing lithium-ion transference numbers upon increasing anion size has been frequently reported in the literature. While in dilute systems this can be explained by reduced migration of anions with a larger Stokes radius, an abnormally strong increase in lithium transference upon increasing anion size is sometimes observed in concentrated or solvent-free polymer electrolytes. To investigate its origin, lithium salts with the anions trifluoromethanesulfonate (OTf), bis(fluorosulfonylimide) (FSI), bis(trifluorosulfonylimide) (TFSI), nonafluoro-1-butanesulfonate (ONf), and bis(pentafluoroethylsulfonyl)imide (BETI) are studied in dimethyl sulfoxide (DMSO)-plasticized poly(ethylene oxide) (PEO)-based polymer electrolytes. Structural and transport parameters for electrolytes with varying anion size are determined by impedance spectroscopy, Raman spectroscopy, and pulsed field gradient (PFG)-NMR diffusion, while drift velocities of all individual constituents in an electric field are determined by electrophoretic NMR (eNMR). Interestingly, for large anion sizes, high drift velocities of the uncharged components PEO and DMSO are observed, in line with high lithium-ion transference. While arguments based on the migration velocity of single species, as expected by their charge and coordination, fail to explain the observed transport behavior, we show that an overall constraint, which couples the transport of all species, is responsible for the unexpected positive drift velocity of neutral species. This is rationalized in the framework of local volume conservation, yielding a coupling of hydrodynamic fluxes of distinct constituents, which induce significant negative ion–ion correlations. In particular, in the case of very large anions, the required compensation of their volume flux causes a beneficial acceleration of lithium cations and neutral species. Up to 60% of the increase in lithium-ion transference number can be attributed to hydrodynamics, a contribution strongly influenced by the degree of salt dissociation. Understanding the impact of hydrodynamic coupling of electrolyte constituents’ migration in an electric field allows a compositional optimization toward boosting lithium transference.