Investigation of Potential Window of Aqueous Electrolyte Solution in Aqueous Rechargeable Lithium Batteries

Abstract

Introduction Conventional lithium-ion batteries (LIBs) consist of organic electrolyte solution. Therefore, because of flammability of organic electrolyte solution, the safety concern remains especially on large size LIBs. To overcome this concern, the promising approach is to use aqueous electrolyte solution. Aqueous rechargeable lithium battery (ARLB) was first reported by Dahn’s group in 19941. However, its average voltage was only 1.5 V because of narrow potential window of aqueous electrolyte solution. Therefore, in order to improve energy density of ARLBs, development of aqueous electrolyte solution with large potential window is required and various works have been reported2. We reported that the saturated disodium propane-1,3-disulfonate (PDSS) aqueous solution showed 2.4 V potential window, and LiNi0.5Mn1.5O4 was successfully worked in this solution3. However, the reason why the potential window broadens is not clear. In this study, we focused on the salt type and concentration, and studied about its relationship to potential window of aqueous electrolyte solution. Experimental The potential window was measured by cyclicvoltammetry. The three-electrode cell was constructed with Pt-plate, Pt-wire as a working and a counter electrode respectively, and Ag/AgCl as a reference electrode. The aqueous salt solutions were prepared with neutral pH salts, Na2SO4, NaClO4, PDSS, NaNO3, LiNO3, LiTFSA, from 0.5 moldm−3 to saturated concentration. Prior to each measurement, solutions were thoroughly deaerated with argon gas for at least 1 hour. Sweep rate was 1 mV s−1. Results and Discussion Potential window (electrolyte-stable potential width) dependence on water concentration is shown in Figure 1. In order to compare the salt type, we focused on water concentration, because the potential window width is determined by water electrolysis reaction. The potential window in neutral pH salt solution did not depend on salt type but water concentration. They were almost linearly increased with water concentration decrease, i.e. salt concentration increase. This is probably attributed to reaction kinetics of water. Reaction rate of water electrolysis got slow with water concentration decrease, and consequently, potential window was extended. In addition, the potential window in neutral pH solution was obviously larger than that in acid solution, difference of which was ca. 0.5 V. However, this extension was not observed in the solution with buffer capacity, even though its pH was neutral. This is probably because the local pH in vicinity of electrode changed drastically with water electrolysis in neutral pH solution without buffer capacity. Therefore reaction potential of water electrolysis changed with this local pH change, and consequently, potential window was extended. References 1 W. Li, J. R. Dahn and D. S. Wainwright, Science., 264, 1115, (1994). 2 L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang and K. Xu, Science., 350, 938, (2015). 3 K. Miyazaki, T. Shimada, S. Ito, Y. Yokoyama, T. Fukutsuka and T. Abe, Chem. Commun., 52, 4979, (2016). Figure 1