Local structure and entropic stabilization of Ca‐based molten salt electrolytes

Abstract

Here molten salt electrolytes (MSEs) and specifically their physico‐chemical properties as a function of composition are reported on, with a special emphasis on the compositional entropy and targeting calcium battery application. By using MSEs, several problematic issues associated with organic electrolytes, such as the blocking of Ca2+ transfer at the electrode/electrolyte interfaces and electrolyte flammability, are avoided. Ca(FSI)2 salt in combination with the analogous Li‐, Na‐, and KFSI salts are used in equimolar compositions to first create several ternary MSEs, melting at ca. 60‐75 ◦C, a melting temperature which is further reduced to ca. 55 ◦C for the unique quaternary MSE. This is ascribed to an increased entropy of mixing, which also contributes to an improved stability vs. (re‐)crystallization, as shown by Raman spectroscopy. Furthermore, molecular dynamics simulations of the quaternary MSE alongside density functional theory calculations targeting the ion‐ion interactions are used to elucidate the local structure in more detail, demonstrating that both the ionic radii and valence influence the coordination and solvation of the cations. These MSEs are stepping‐stones towards completely solvent‐free, semi‐solid, and ideally room‐temperature Ca‐conducting electrolytes.