Linear superposition behavior of blended salt organic carbonate-based electrolyte formulations

The design of an electrolyte to improve the performance of the resulting battery chemistry depends on many factors. Greater variability is achieved by using a mixture of conducting salts rather than a single salt. Experimentally, the ionic conductivity for different anion pairs is shown to change linearly as one electrolyte component is gradually replaced by the other. Using molecular dynamics simulations with a polarizable force field, the change in structural and transport properties is analyzed and discussed for the specific case of lithium hexafluorophosphate (LiPF₆) and lithium bis(fluorosulfonyl)imide (LiFSI). In addition to a quantitative reproduction of the experimental ionic conductivities, insight into the dependence of the microscopic mechanisms on the conductivity upon salt substitution is presented by comparing the blended salt electrolyte with the same number of anions with the two limiting single salt electrolytes. It is found that the structure of the blended salt electrolyte, as characterized by lithium nearest neighbor shells, can be well predicted from the structure of the two single salt electrolytes under the assumption of random mixing. Furthermore, the structural and dynamical properties of the lithium anion pairs are basically insensitive to the salt composition, i.e. they exhibit mixing invariance. It is argued that the validity of random mixing and mixing invariance, together with the hydrodynamic effects of cross correlations between like ions, justify the linear composition dependence of the conductivity. Additionally, distinct differences are identified in how lithium interacts with either PF₆^- or FSI^- anions.