Molten Salt Based High Entropy Electrolytes for Calcium Batteries

Abstract

Calcium (Ca) is in principle a very attractive metal anode for rechargeable batteries due to the large abundance of calcium in the Earth’s crust, its low electrochemical potential, and its low cost1. Until rather recently2, however, stable plating and stripping of Ca and hence rechargeability of Ca-batteries was deemed impossible, due to the stable phases formed at the electrolyte/electrode interface, completely blocking Ca2+ transfer3.Since this pivotal breakthrough, Ca-batteries have attracted more interest, but they are still in their very infancy and new battery designs – such as using novel electrolyte concepts – are needed to achieve stable cycling of Ca metal anodes, and this preferably at room temperature.Here we report on the use of molten salt electrolytes, i.e. binary and multi-component systems of inorganic cations and anions, and more specifically on their physico-chemical and electrochemical properties as a function of composition with special emphasis on the compositional entropy. By using completely solvent-free electrolytes issues associated with organic electrolytes and solvents, such as the blocking of Ca2+ transfer and electrolyte flammability3, are avoided. Furthermore, they should/do not induce any large polarization, as opposed to ionic liquid-based electrolytes4.In more detail, using the Ca(FSI)2 salt in combination with the analogous Li-, Na-, and KFSI salts in equimolar compositions, all with melting temperatures (significantly) above 100 °C, we can create electrolytes melting at ca. 60-75 °C for the ternary systems, which is further reduced to ca. 55 °C for the quaternary system. These lowered melting temperatures are achieved by increased entropy of mixing, reducing the Gibbs free energy. This can for example be illustrated by Raman spectroscopy via comparing the spectra of “as cast” electrolytes with the spectra for the ones “aged” for one week (Figure 1). Here the FSI anion band at 772 cm-1 (νsS-N-S)5 shows that the electrolytes partially (re-)crystallize/micro-separate for the ternary systems, but stay more stable for the quaternary system, which we, at least partially, attribute to an entropy effect. Reference s M. E. Arroyo-De Dompablo, A. Ponrouch, P. Johansson, and M. R. Palacín, Chem . Rev., 120, 6331–6357 (2020).A. Ponrouch, C. Frontera, F. Bardé, and M. R. Palacín, Nat. Mater., 15, 169–172 (2016).D. Aurbach, R. Skaletsky, and Y. Gofer, J . Electrochem Soc., 138, 3536–3545 (1991).Q. Pang et al., Nature, 608, 704–711 (2022).K. Matsumoto, T. Oka, T. Nohira, and R. Hagiwara, Inorg . Chem ., 52, 568–576 (2013). Figure 1