Influence of Anions on the Electrolyte/Electrode Interfacial Dynamics Probed By in-Situ/Operando Soft X-Ray Absorption Spectroscopy

Abstract

Since the demonstration of room temperature plating and stripping of calcium with high capacities and low polarization using Ca(BH4)2 as the electrolyte, there is considerable interest in the study of electrochemical processes in calcium batteries in addition to the conventional magnesium batteries. The greater polarizability of Ca2+ over Mg2+represents greater configurational flexibility, facilitating ionic cluster formation and rapid delivery of Ca2+ ions to the electrode interface. With such motivation in calcium batteries, a critical question to address is the probing of the solvation and charge transfer mechanism at the electrode/electrolyte interface. In this work, the solvation and charge transfer mechanisms of two calcium electrolytes, Ca(BH4)2 and calcium hexafluoroisopropoxyborate (Ca(BHFIP)2), with different types of anions, are compared under operando conditions. It has been reported that when BH4 - is substituted by the more weakly coordinated BHFIP-, the electrolyte ionic conductivity will increase by an order of magnitude, which demonstrates the strong coordination of BH4 - to Ca2+ in THF. To probe the Ca2+ coordination chemistry difference at this electrolyte/electrode interface as a function of the type of anions, total electron yield (TEY) mode soft X-ray absorption spectra (XAS) sensitive to the interfacial speciation have been employed under operando electrochemical conditions. It was observed that the Ca(BHFIP)2 in THF shows a small shoulder on the right side of the L2-edge and a pre-edge peak on the left side of the L2-edge peak. A less prominent pre-edge peak is also observed on the left side of the L3-edge peak. These signatures become more prominent when the potential is swept negatively and are not reversible when the potential is swept positively in a cyclic voltammetry cycle. On the other hand, the Ca(BH4)2 in THF is more reversible, and no shoulder peak on the right side of the L2-edge peak was observed. The TEY spectra indicate their differences in the Ca2+coordination chemistry at the electrode/electrolyte interface as a result of the interacted anions and it will guide future development of electrolytes for calcium batteries. The detailed solvation structure of the calcium is to be resolved with the DFT calculations. Such operando soft XAS studies of the interfacial dynamics reveal the transient processes in the solvation at the electrolyte/electrode interface and will pave the way for the development of calcium electrolytes in batteries.