Probing Ca²⁺ solvation dynamics at the electrolyte/electrode interface

Calcium is considered a promising candidate in multivalent beyond lithium-ion batteries because of its safe, economic, and nontoxic nature. It offers a two-fold increase in volumetric capacity compared to monovalent lithium-ion batteries. A systematic understanding of the solvation mechanism and charge transfer at the electrolyte/electrode interface and factors that will affect the solvation is critical in the development of novel calcium batteries. To probe the Ca2+ coordination at the electrolyte/electrode interface as a function of the type of anions, total electron yield (TEY) mode soft X-ray absorption spectroscopy (XAS) sensitive to the interfacial speciation has been employed under in-situ and operando conditions. Meanwhile, total fluorescence yield (TFY) mode XAS is sensitive to bulk speciation. Calcium bis(trifluoromethanesulfonyl)imide (CaTFSI2) in tetrahydrofuran (THF) was selected as an attractive electrolyte because of its oxidative stability and high solubility in various solvents. Combining calcium L-edge soft XAS with resonant soft X-ray scattering (RSoXS) through a patterned interface, the solvation and desolvation dynamics of calcium at the interface were investigated, and the influence of a second cation and anion was investigated. These in-situ/operando synchrotron-based spectroscopic and scattering characterization provide a key to the fundamental mechanisms regarding the solvation and charge transfer at the electrode/electrolyte interface and will benefit the future development of novel energy storage devices.