Transport Limitations in Binary Electrolytes: EC-Free Solvents and NaPF6 Vs. LiPF6 Salts

Abstract

For numerical simulations of battery systems, the ion-transport model for concentrated electrolyte solutions introduced by Newman and Thomas-Alyea1is frequently used and depends on three ion transport parameters: the conductivity, the transference number and the binary diffusion coefficient. In addition, the thermodynamic factor which is derived from the mean molar activity coefficient is required for the correct description of the thermodynamic behavior of a binary electrolyte solution. A vast spectrum of physico-chemical parameters can be found in the literature (e.g., Valoen and Reimers2), but since the investigated electrolytes and the experimental techniques differ from study to study, it is difficult to find a consistent parameter set for a given electrolyte. In addition, some of the parameters are often fitted to match experimental performance data and may thus be fitting parameters rather than intrinsic physico-chemical parameters with predictive capability. In our work, a new method to determine the thermodynamic factor3is combined with con-centration cell experiments to determine trans-ference numbers. Using common polarization cell experiments, we additionally determine diffusion coefficients while conductivities are measured using turn-key equipment. Our main objective is to analyze the limitations of new, emerging battery chemistries, e.g., EC-free electrolytes4 for lithium ion batteries or electrolytes for sodium batteries,5 and compare their electro-chemical transport properties to a common solution, e.g., LiPF6 in EC:EMC (3:7 w:w). Exemplarily shown is the comparison of the ionic conductivity for a NaPF6 and a LiPF6electrolyte in the same solvent, EC:DEC (1:1 v:v). To compare the obtained transport parameters and to depict their impact on the cell performance, we incorporate the obtained functional descriptions in an arbitrarily chosen graphite-NMC cell model. We will show which fraction of the cell overpotential is caused by the electrolyte and analyze its effect on the rate performance of the cell. Our research will help to understand the limitation caused by concentration overpotentials in current lithium ion battery systems. Adaption of our transport parameters in Newman type battery models will enhance numerical predictions. Analysis of the type of solvent and salt will help to understand the sources for ionic transport limitations and thereby helps improving future battery electrolyte solutions. Figure 1: Conductivity of NaPF6 and LiPF6 from 0.1 M to 2 M in EC:DEC (1:1 vol.) at 25°C.