UC Berkeley

Most of the battery electrolyte candidates for multivalent-ion systems (e.g. Mg, Ca) present interesting physicochemical properties. In these are included anomalous conductivity, where molar conductivity increases with salt concentration. This phenomenon is related to the electrochemical performance and electrolyte viability. In this thesis, this phenomenon is explained via understanding and quantification of the strong inter-ionic interactions in solution. To this end, computational methods including classical molecular dynamics and electronic structure methods are combined with dielectric relaxation spectroscopy measurements. Two cases studies with ether based electrolytes, one with MgTFSI2 salt and one with Ca(BH4)2 are undertaken. In both cases, the anomalous conductivity originates from changes in salt speciation, i.e. changes in the population of distinct chemical species formed by the ions in the solution. A larger fraction of ionic salt species are promoted at higher salt concentration at the expense of larger, associated clusters. Moreover, it is found that the increase in dielectric constant with increasing salt concentration, due to a significant fraction of polar associated salt species, is likely one of the thermodynamic driving forces for this effect. The findings here are important for the rational design of multivalent electrolytes for electrochemical applications, and help inform salt and solvent selection for advanced electrolyte formulations.