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 (N$_{07}$$^{+}$), bearing seven ether oxygen atoms, can effectively displace the bis(trifluoromethanesulfonyl)imide anion (TFSI$^{-}$) from the Ca$^{2+}$ 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 an organic-rich, but inorganic-poor solid electrolyte interphase layer, which enables Ca$^{+2}$ ion diffusion rather than passivating the Ca metal electrode. Finally, as a proof-of-concept, a prototype Ca/V$_{2}$O$_{5}$ cell using the optimized IL-based electrolyte ([Ca(BH$_{4}$)$_{2}$]$_{0.05}$ [N$_{07}$TFSI]$_{0.95}$) is demonstrated for the first time, exhibiting a remarkable initial discharge capacity of 332 mA h g$^{-1}$ and reversible capacity of 244 mA h g$^{-1}$.