A novel approach to ligand-exchange rates applied to lithium-ion battery and sodium-ion battery electrolytes

A novel approach based on analyzing the forces and velocities of solvents and anions to compute ligand-exchange rates is here presented and applied to lithium-ion battery (LIB) and sodium-ion battery (SIB) electrolytes. By using ab initio molecular dynamics generated data, we find the ligand-exchange rates to increase as functions of electrolyte salt concentration and to be higher in SIB electrolytes as compared to LIB electrolytes. This indicates both that Na+ transport will be more non-vehicular in nature and have improved kinetics vs Li+, and that increasing the salt concentration is beneficial. The systems studied were basically the first cation solvation shells of Li/NaPF6 in propylene carbonate and acetonitrile using three solvent to salt ratios. Overall, the solvation shells are solvent rich at low salt concentrations, and as functions of concentration, the solvents are replaced by anions. As the SIB electrolytes display higher cation coordination and solvation numbers, we also expect an earlier onset of highly concentrated electrolyte behavior for SIB than LIB electrolytes. These observations should all have an impact on the design of electrolytes for optimal bulk properties, but also be useful with respect to interfacial dynamics.