Mixed Cation Water-in-salt electrolytes: Concentration study of Ion solvation, local order and nano-heterogeneity

Solvent-in-salt (SIS) electrolytes differ substantially from the traditional electrolytes: in such systems amounts of the dissolved salt is equivalent or more by weight/volume of that of the solvent. As a result, SIS systems feature a number of unique properties and provide electrochemical performance advantages that could not be attained by usage of the traditional electrolytes. One of the notable examples of SIS electrolytes are highly concentrated “water-in-salt”, WIS, systems which were demonstrated to be highly attractive electrolytes for aqueous batteries since they can offer significantly extended stability window, improved coulombic efficiency and cycling stability.^1,2,3,4 Yet, while it is clear that such an extreme increase in salt concentration would result in highly modified interactions of the ionic and molecular components of the electrolyte a detailed our understanding is still lacking.

Here we present on multi-technique study of the two distinct WIS systems based on i) lithium salts of organic imides and ii) potassium acetate. Using a combination of the X-Ray Absorption Spectroscopy, wide angle X-Ray scattering and molecular dynamics simulations we reveal effects of the solution concentration and introduction of the multivalent cations such as Zn²⁺ on the solvation structure of the ionic species, local order within electrolytes and nano-heterogeneity within the studied WIS systems.

References
1. L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang and K. Xu, Science, 2015, 350, 938
2. Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama and A. Yamada, Nat. Energy, 2016, 1, 16129
3. MR Lukatskaya, JI Feldblyum, DG Mackanic, F Lissel, DL Michels, Y Cui, Z. Bao, Energy Environ. Sci., 2018, 11, 2876-2883
4. F. Wang, O. Borodin, T. Gao, X. Fan, W. Sun, F. Han, A. Faraone, J. A. Dura, K. Xu, C. Wang, Nat. Materials, 2018, 17, 543–549