The Hydrotropic Effect of Ionic Liquids in Water-in-Salt Electrolytes

Abstract

The water-in-salt approach has played a pivotal role in extending the electrochemical stability of aqueous electrolytes far beyond the thermodynamic stability limit of water.[1–4] At the molality of the original water-in-salt electrolyte of 21 moles of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) per kilogram of water, the water-to-lithium ratio equals 2.65. Strong cation-water and cation-anion interactions, the formation of a solid-electrolyte interphase on the anode side, and the alignment of TFSI anions in a water-blocking electrochemical double-layer on the cathode surface result in a particularly high oxidative stability.[1,5] However, even at this concentration, a small fraction of free water molecules, which are prone to hydrolysis reactions, remains. Hence, focus was put on further decreasing the water-to-lithium ratio to eliminate the remaining fraction of free water and to improve the quality of the solid-electrolyte interphase, either by partially substituting water with organic solvents[6] or ionic liquids.[7] Interestingly, the presence of ionic liquid boosts the LiTFSI solubility, resulting in a very low water-to-lithium ratio of ≤1.3.[7] Here we report on this enhancement of the LiTFSI solubility by examining ion-ion and ion-water interactions using Raman and NMR spectroscopy.[8] We identify a hydrotropic effect of ionic liquids as origin of the enhanced LiTFSI solubility. The increased Li concentration leads to an improved reductive stability and enables cycling of Li4Ti5O12 (LTO) anodes when coated with a layer of niobium oxide. Furthermore, we exploit the reduced water content of the water-in-salt/ionic liquid hybrid electrolytes and demonstrate compatibility with LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes. For full cells based on the LTO/NMC811 electrode couple, we obtain relatively high Coulombic efficiencies of 99.4% and 99.2% at rates of 1C and C/2, respectively, for such aqueous high-voltage batteries. This cell also displays a very high initial energy density of up to 150 Wh/kg on the active material level owing to the high capacity of NMC811.[1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science 2015, 350, 938–943.[2] Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, A. Yamada, Nat. Energy 2016, 1, 16129.[3] R. S. Kühnel, D. Reber, C. Battaglia, ACS Energy Lett. 2017, 2, 2005–2006.[4] M. Becker, R.-S. Kühnel, C. Battaglia, Chem. Commun. 2019, 55, 12032–12035.[5] J. Vatamanu, O. Borodin, J. Phys. Chem. Lett. 2017, 8, 4362–4367.[6] J. Chen, J. Vatamanu, L. Xing, O. Borodin, H. Chen, X. Guan, X. Liu, K. Xu, W. Li, Adv. Energy Mater. 2020, 10, 1902654.[7] L. Chen, J. Zhang, Q. Li, J. Vatamanu, X. Ji, T. P. Pollard, C. Cui, S. Hou, J. Chen, C. Yang, L. Ma, M. S. Ding, M. Garaga, S. Greenbaum, H.-S. Lee, O. Borodin, K. Xu, C. Wang, ACS Energy Lett. 2020, 5, 968–974.[8] M. Becker, D. Rentsch, D. Reber, A. Aribia, C. Battaglia, R.-S. Kühnel, Angew. Chemie Int. Ed. 2021, accepted manuscript, 10.1002/ange.202103375.