Towards Stable Water-in-Salt Electrolytes for Sodium-Ion Batteries

Abstract

Water-in-salt electrolytes have enabled the development of novel high-voltage aqueous lithium-ion batteries due to their high electrochemical stability compared to traditional aqueous electrolytes.[1, 2] The development of equivalent sodium electrolytes is hindered by two factors: Low solubility of suitable sodium salts. For example, NaTFSI has a solubility at room temperature of only 8 mol/kg compared to 21 mol/kg for LiTFSI.[3]A higher salt concentration is needed to reach the same level of electrochemical stability due to the lower charge density of Na+ compared to Li+.[4] In this study, we screen >100 combinations of up to five sodium salts with water to identify highly concentrated solutions that are thermodynamically in the liquid state at ≤25 °C.[5, 6] An additional criterion for a stable water-in-salt electrolyte is a high content of SEI-forming anions. Furthermore, we find that the electrochemical stability of the electrolyte benefits from anions that are only weakly coordinating. In fact, we discovered a strong correlation between the stability of the electrolyte and the position of the anion in the Hofmeister series.[6]Our findings led to the development of multiple 2 V-class sodium-ion batteries with Coulombic efficiencies of up to 99.8% during cycling at a rate of 1C and record-high energy densities for such devices of up to 77 Wh/kg based on the active masses of both anode and cathode.[6][1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science 350 (2015) 938–943.[2] Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, A. Yamada, Nat. Energy 1 (2016) 16129.[3] R.-S. Kühnel, D. Reber, C. Battaglia, ACS Energy Lett. 2 (2017) 2005–2006.[4] D. Reber, R. Figi, R.-S. Kühnel, C. Battaglia, Electrochim. Acta 321 (2019) 134644.[5] D. Reber, R.-S. Kühnel, C. Battaglia, ACS Mater. Lett. 1 (2019) 44–51.[6] D. Reber, M. Becker, R.-S. Kühnel, C. Battaglia, submitted.