Alkoxy-functionalized Ionic Liquid Electrolytes: Understanding Ionic Coordination of Calcium Ion Speciation for the Rational Design of Calcium Electrolytes

There is growing interest in the rational design of electrolytes for multivalent-ion batteries by tuning the molecular-level interactions of solvate species present in the electrolytes. Herein, we report our effort to control Ca-ion speciation in ionic liquid (IL) based electrolytes through the design of alkoxy-functionalized cations. Quantitative analysis reveals that the alkoxy-functionalized ammonium cation, bearing seven ether oxygen atoms, can effectively displace the bis(trifluoromethanesulfonyl)imide anion from the Ca2+ ion coordination sphere, facilitating the reversible Ca deposition/stripping process. More importantly, post-analysis of Ca deposits surface chemistry and density functional theory calculations of Ca-ion speciation indicate the formation of a polymeric solid electrolyte interphase layer, which enables Ca2+ ion diffusion rather than passivating the Ca metal electrode. Finally, as a proof-of-concept, a prototype Ca/V2O5 cell using the optimized IL-based electrolyte is demonstrated for the first time, exhibiting a remarkable initial discharge capacity of 332 mAh g−1 and reversible capacity of 244 mAh g−1.