Development of a Simple Analysis Method for Determining Ion Transport Properties in Electrolytes

Abstract

Quantifying the transport properties of ions in electrolytes is important to evaluate the performance of lithium ion batteries. For example, the limiting current in an electrolyte is governed by the ion transport. The transport properties of ions are mainly characterized by diffusion coefficient (D) and transference number (t). D and t were usually obtained from nuclear magnetic resonance (NMR) [1] and electrochemical experiments [2]. In an NMR measurement, the self-diffusion coefficient of each ion can precisely be determined. However, this technique can be used only for NMR active species and it is sometimes difficult to use due to the high cost. In contrast, electrochemical methods are advantageous in regards to the relative simplicity of the setup and the wide applicability to sample species. To the best of our knowledge, there is no single-step electrochemical measurement method to obtain D and t simultaneously. Thus, in this study, we developed this method. An equivalent electric circuit model of an experimental setup was proposed and the time course of ionic current was obtained by solving the time-dependent one-dimensional diffusion equation. The solution resistance and inverse electromotive force were considered in the boundary conditions. The blocking characteristics was also considered on the surface of a metal lithium electrode only for the anions. Initially, the ion concentration was uniform throughout the electrolyte. The time course of ionic current was calculated from the time evolution of ion concentration profiles in the electrolyte. Electrochemical measurement was also conducted by applying a constant voltage of 10 mV to the parallel-plate cell consisting of Li metal electrodes and 5 mM-electrolyte (LiClO4 salt was dissolved in dehydrated propylene carbonate; PC) at 25 °C. The diffusion coefficient (D) and transference number (t) of the electrolyte were obtained by fitting the analytical solution of the time course of ionic current to the transient current measured under potentiostatic condition. The fitting range was 0.08–1.0 s. From the curve fitting, D = 5.02 × 10-6 cm2 s-1 and t_ = 0.67 (anionic transference number) were obtained. Regarding D, the measured value of 2.6 × 10-6 cm2 s-1 was reported for 1 M LiClO4–PC electrolyte [3]. Although the electrolyte concentrations were not identical, these two values were comparable to each other. However, as the electrolyte concentration increased, the analytical solution of the time course of ionic current poorly reproduced the measured one. This suggests that the effect of strong interactions between ions should also be considered on the transport model at high electrolyte concentrations [4].