Comparison of ionic transport properties of non-aqueous lithium and sodium hexafluorophosphate electrolytes

To bridge the gap between current lithium-ion battery technology and alternative cell chemistries such as, e.g., sodium-ion batteries, the majority of the research in this field focuses on the improvement of the cell's energy density by the development of new active materials for reversible storage of sodium ions. On the other hand, the power density, which is determined by the ionic transport and thermodynamic parameters in the electrolyte, namely the conductivity, the thermodynamic factor, the transference number, and the diffusion coefficient, is attracting little attention. In this contribution, we determine these electrolyte properties for 0.1 M to 2 M LiPF6 and NaPF6 in a mixture of ethylene carbonate and diethyl carbonate (EC:DEC (1:1 v:v)) and use them in 1D simulations to show their impact on the theoretical discharge rate performance of the lithium and sodium cell chemistry. We show that the increased cation size of sodium and its corresponding weaker solvent interactions are beneficial for high power applications and that the improved ionic transport properties would allow for a substantial increase of either the (dis)charge currents or the electrode areal loading, compared to the well-established lithium system.