Calcium, as a promising candidate in multivalent battery technologies, offers the promise of a more than two-fold increase in the volumetric capacity compared to monovalent lithium-ion batteries because of its safe, economic, and nontoxic nature. The understanding of the solvation and charge transfer mechanism at the electrolyte/electrode interface and how different types of cations and anions will affect the calcium solvation at this interface is the key to developing 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 operando electrochemical conditions. Meanwhile, total fluorescence yield (TFY) mode XAS is sensitive to bulk speciation. Calcium bis(trifluoromethanesulfonyl)imide (CaTFSI2) in tetrahydrofuran (THF) is an attractive electrolyte because of its oxidative stability and high solubility in various solvents. Combining operando 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 electrolyte/electrode interface was deciphered, and the influence of a second cation and anion were investigated. These in-situ/operando synchrotron-based spectroscopic and scattering characterization will answer the fundamental questions regarding the solvation and charge transfer at the electrode/electrolyte interface and benefit the future development of multivalent battery development.

Probing calcium solvation at the electrolyte/electrode interphase under operando conditions

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