Calcium Solvation Dynamics Probed By in-Situ/Operando Soft X-Ray Absorption Spectroscopy

Calcium is considered a promising candidate in multivalent battery technologies due to its safe, economic, and nontoxic nature. It has received considerable attention since the recent work showing that calcium can be plated and stripped at room temperature with good stabilities and capacities. It offers the promise of a more than two-fold increase in the volumetric capacity compared to the monovalent lithium-ion batteries. Additionally, because of the higher mobility of Ca2+ compared to that of Mg2+ because of the lower charge density of Ca2+, the interest in calcium batteries has been growing rapidly. 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 develop 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 used under operando electrochemical conditions. Meanwhile, total fluorescence yield (TFY) mode XAS is sensitive to bulk speciation. By comparing the operando TEY and TFY mode XAS from 0.5 M calcium bis(trifluoromethanesulfonyl)imide (CaTFSI2) in THF, the Ca L3,2-edge TEY-XAS intensity increased at positive potentials while the Ca L3,2-edge TFY-XAS intensity decreases at these positive potentials. Such a comparison clearly shows the solvation and desolvation dynamics of calcium at the electrolyte/electrode interface, which will benefit the future development of next-generation multivalent batteries.