The electrolyte solution structure for acetonitrile (AN)-lithium salt mixtures has been examined for highly dissociated salts. Phase diagrams are reported for (AN) n -LiN(SO 2 CF 3 ) 2 (LiTFSI) and -LiPF 6 electrolytes. Single crystal structures and Raman spectroscopy have been utilized to provide information regarding the solvate species present in the solid-state and liquid phases, as well as the average solvation number variation with salt concentration. Molecular dynamics (MD) simulations of the mixtures have been correlated with the experimental data to provide additional insight into the molecular-level interactions. Quantum chemistry (QC) calculations were performed on (AN) n -Li-(anion) m clusters to validate the ability of the developed many-body polarizable force field (used for the simulations) to accurately describe cluster stability (ionic association). The combination of these techniques provides tremendous insight into the solution structure within these electrolyte mixtures.

A systematic study of electrolytes has been conducted to explore how solution structure dictates electrolyte properties. Specifically, the transport properties (viscosity, conductivity and molar conductivity) of acetonitrile-lithium salt mixtures, (AN)n-LiX, are reported for electrolytes with LiPF6, LiTFSI (i.e., LiN(SO2CF3)2), LiClO4, LiBF4 and LiCF3CO2. These salts have widely varying ion solvation/ionic association behavior which is directly reflected in the transport properties of the AN solutions. Information about the solution structure has been utilized, in concert with molecular dynamic (MD) simulations, to provide mechanistic explanations for the variability noted in the transport properties of the electrolyte mixtures.

Solution structure is the key determinant for electrolyte properties, but little is known about the ion solvate structures present in liquids. A detailed exploration of this topic is begun here utilizing acetonitrile (AN) due to the simplicity of this solvent's interactions with Li + cations. Phase diagrams have been prepared for (AN) n -LiClO 4 and -LiBF 4 mixtures (salts with intermediate ionic association). The solvate species present in the solid and liquid phases have been analyzed utilizing single crystal solvate structures and Raman spectroscopy to determine how the anion identity influences the solvate species equilibrium (i.e., ionic association and solvation number). The phase behavior and solvation interactions of these mixtures are compared with those for (AN) n -LiCF 3 SO 3 , -LiNO 3 and -LiCF 3 CO 2 mixtures (salts which are highly associated). Quantum chemical calculations for the (AN) n -LiBF 4 and -LiClO 4 solvates have been performed to aid in the analysis. Results from MD simulations for (AN) n -LiBF 4 and -LiClO 4 mixtures have been compared to the experimental work to explore both the insight gained and limitations of the experimental work and simulations for electrolyte characterization.

The lithium–sulfur battery has long been seen as a potential next generation battery chemistry for electric vehicles owing to the high theoretical specific energy and low cost of sulfur. However, even state-of-the-art lithium–sulfur batteries suffer from short lifetimes due to the migration of highly soluble polysulfide intermediates and exhibit less than desired energy density due to the required excess electrolyte. The use of sparingly solvating electrolytes in lithium–sulfur batteries is a promising approach to decouple electrolyte quantity from reaction mechanism, thus creating a pathway toward high energy density that deviates from the current catholyte approach. Herein, we demonstrate that sparingly solvating electrolytes based on compact, polar molecules with a 2:1 ratio of a functional group to lithium salt can fundamentally redirect the lithium–sulfur reaction pathway by inhibiting the traditional mechanism that is based on fully solvated intermediates. In contrast to the standard catholyte sulfur electrochemistry, sparingly solvating electrolytes promote intermediate- and short-chain polysulfide formation during the first third of discharge, before disproportionation results in crystalline lithium sulfide and a restricted fraction of soluble polysulfides which are further reduced during the remaining discharge. Moreover, operation at intermediate temperatures ca. 50 °C allows for minimal overpotentials and high utilization of sulfur at practical rates. This discovery opens the door to a new wave of scientific inquiry based on modifying the electrolyte local structure to tune and control the reaction pathway of many precipitation–dissolution chemistries, lithium–sulfur and beyond.