Modelling Ca2+ Mobility and Dynamics in Solvent-Free Molten Salt Electrolytes for Calcium Batteries

Abstract

Multivalent elements based batteries using earth-abundant minerals is a route with promises, not only from a performance but also from a supply and cost point of view; this is not the last true for calcium (Ca) batteries since Ca is the fifth most abundant element in the Earth's crust [1,2,3]. There is, however, no functional electrolyte available yet for Ca batteries, calling for early modelling to be made to map the influence of different Ca-salts and salt concentrations on electrolyte physicochemical properties such as density, viscosity, ionic conductivity, charge carrier dynamics and diffusivity [3, 4]. One particular route little trodden is molten salt electrolytes (MSEs), materials that consist of ions with different valences in direct contact with each other due to the complete absence of any solvent. In practice, MSEs are multicomponent salt mixtures with reduced melting points with respect to the ingoing components and furthermore they also expand the operating temperature range upwards as compared to conventional liquid electrolytes. Moreover, MSEs have high ionic conductivities due to their intrinsically high ionic content and ionic mobility. Additionally, MSEs exhibit wide electrochemical stability windows, non-flammability, and non-toxicity [5, 6]. We have investigated MSEs composed of Ca(FSI)2, LiFSI, NaFSI, and KFSI, by molecular dynamics (MD) simulations. Apart from the basic properties, we focus on the relative dynamical properties of the different cations in the MSEs in order to obtain a fundamental understanding of the systems’ behaviour, both globally and locally, but for natural reasons with special emphasis on the Ca2+ ions.