Measurements of Transport Properties of Lithium Salt in Block Copolymer Electrolyte

Abstract

High charging rate of batteries is an important technical challenge to improve performance of electric vehicles. The battery charging is a function of the redox reactions at the electrode surfaces and transport of ions in the electrolyte. Limiting current density in a battery, the maximum current density at steady state, determines the charging rate. It is a function of the transference number and the diffusion coefficient. The measurement of the transference number and the diffusion coefficient of solid electrolytes has been restricted because of the complexity of experiments. Pulsed-field gradient nuclear magnetic resonance is a powerful method to measure the self-diffusion coefficients of the ions, which can be used to calculate the salt diffusion coefficient and the transference number, but it is only valid for dilute solutions.1, 2 Accurate measurement of the transport parameters for concentrated electrolyte has been conducted combining restricted diffusion, concentration cell and constant current experiments.3 The limitations of this approach are inherently large error from the independent measurements and long experiment time. In this study, the diffusion coefficient and transference number of lithium salt in polystyrene-poly(ethylene oxide) (SEO) electrolyte has been measured by spectroscopic and electrochemical methods with a simple experimental setup. A one-dimensional numerical diffusion model was used to determine the salt diffusion coefficient as a function of concentration. A non-monotonic concentration dependence was observed, initially decreasing with increasing salt concentration, then increasing after a minimum at 1.3 M. The transference number was obtained by measuring the concentration change near the electrodes after an applied current. This concentration change can be determined using calibrated cell potential measurements or custom spectroelectroscopic measurements. A steady-state current experiment was also conducted for comparison. This is a conventional measurement of the transference number in dilute limit.4 Deviations between these two measurements are an indication of non-ideality that can be the result of interactions between the ions and the EO segments and/or the existence of ion pairs and multiplets, which are expected in concentrated systems. The spectroelectroscopic experiment would provide more accurate measurement of the transport properties for concentrated solid electrolyte with reduced errors and practical information for the improvement of battery performance. References Pesko, D. M.; Timachova, K.; Bhattacharya, R.; Smith, M. C.; Villaluenga, I.; Newman, J.; Balsara, N. P., Negative Transference Numbers in Poly(ethylene oxide)-Based Electrolytes. 2017; Vol. 164, pp E3569-E3575.Timachova, K.; Watanabe, H.; Balsara, N. P. Macromolecules 2015, 48, (21), 7882-7888.Ma, Y. P.; Doyle, M.; Fuller, T. F.; Doeff, M. M.; Dejonghe, L. C.; Newman, J. Journal of the Electrochemical Society 1995, 142, (6), 1859-1868.Bruce, P. G.; Vincent, C. A. Journal of electroanalytical chemistry and interfacial electrochemistry 1987, 225, (1-2), 1-17.