A Computational Study on the Ca2+ Solvation, Coordination Environment, and Mobility in Electrolytes for Calcium Ion Batteries

Abstract

Calcium (ion) batteries are promising next-generation energy storage systems, owing to their numerous benefits in terms of performance metrics, low-cost, mineral abundance, and economic sustainability. A central and critical area to the advancement of the technology is the development of suitable eletrolytes that allow for good salt solubility, ion mobility, electrochemical stability, and reversible redox activity. At this time, the study of different solvent-salt combinations is very limited. Here, we present a computational study on the coordination environment, solvation energetics, and diffusivity of calcium ions over a range of pertinent ionic liquids, cyclic and acylic alkyl carbonates, and specific alkyl nitriles and alkyl formamides, using the salts calcium bis(trifluoromethylsulfonyl)imide (Ca(TFSI)2) and calcium perchlorate (Ca(ClO4)2). Key findings are that several solvents from different solvent classes present comparable solvation environments and mobilities. Ca(TFSI)2 is prefered over Ca(ClO4)2 owing to the former’s mix coordination of Ca2+ to O and N atoms. Ionic liquids with alkyl sulfonate anions provide better coordation over TFSI, which leads to greater diffusivity. Binary organic mixtures (carbonates) provide the best solvation of Ca2+, however, single organic solvents also provide good solvation, such as EC, THF and DMF, as well as some acyclic carbonates. Ion pairing with the salt anion is always present, but can be mitigated through solvent selection, which also correlates to greater mobility; however, there are examples in which strong ion pairing is not significantly adverse to diffusivity. The solvent incorporate into the solvation structure with binary organic mixtures correlates well with the solvation capabilities of the individual solvents. Finally, we show that ionic liquids (specifically alkyl imidazole (cation) alkyl sulfonate (anion) ionic liquids) do not decompose when coordinating at a Ca metal interface, which indicates its promising stability. Overall, this study contributes further generalized understanding of the correlation between solvent and salt and the resultant Ca2+ complexes and Ca2+ mobility in a range of electrolytes, and reveals a range of possible solvents suitable for exploration in calcium (ion) batteries.

Vinylimidazole–based Polymer Electrolytes with Superior Conductivity and Promising Electrochemical Performance for Calcium Batteries

“…From liquid to polymer, the SN increased for 0.1 M and 0.5 M salt concentrations, but decreased slightly for the 1.0 M salt concentration. The SNs of 6-8 observed here are in good agreement with previous results, 5,16 whereas the higher SN for 0.1 M salt concentration is an artefact of the dilute salt concentration and overestimation of solvation by FTIR. Apparent coordination numbers of a similar magnitude have been reported in the literature.…”

“…conductivity and difficulties with achieving high redox rates in electrochemical cells. 14 This is fundamentally associated, in part, to the calcium ion's combined large ionic radius and greater charge density resulting in strong coordination to oxygen, as compared to other cations 15,16 in addition to the large hopping distance between neighboring coordinating sites for a PEO chain. 17 Incorporating polarizable units in the polymer backbone and softening cation-polymer interactions are some of the potential strategies to overcome challenges encountered with ether-based multivalent polymer electrolytes.…”