Ion transport in semi-solid in-salt electrolytes: LiTFSI–H2O as a model system

Abstract

The water-in-salt (WIS) electrolyte represents a newly developed battery electrolyte system with high ionic conductivity that enables cells with a broader electrochemical window and improved cyclability. Several theories have been proposed to explain the co-existence of high conductivity and salt concentration in a WIS model system with lithium bis(trifluoromethanesulfonyl)imide salt — (LiTFSI)–H₂O. However, experimental and theoretical studies focused on salt concentrations below 21 mol kg⁻¹. Here, we have systematically measured the physical and electrochemical properties of the LiTFSI–H₂O binary system in the salt concentration range from 21–55.5 mol kg⁻¹ in the semi-solid regime. Based on the Raman spectroscopy and lithium transference number results, we propose that the majority of the species formed are neutral or negatively charged ion pairs or clusters in higher aggregation states. The transference number of elemental Li measured by pulsed-field gradient nuclear magnetic resonance (PFG)-NMR remains unexpectedly high (around 0.7) when the mole ratio of Li and H₂O reaches 1 : 1. On the other hand, the lithium transference numbers measured by the galvanostatic polarization method for various concentrations are lower than 0.1. We show that charged associates can provide a qualitative explanation for the observed ion transport behavior. We also report the existence of local minima in temperature-dependent conductivity at various concentrations, and details of the phase transition of LiTFSI monohydrate.