Understanding Ca Electrodeposition and Speciation Processes in Nonaqueous Electrolytes for Next-Generation Ca-Ion Batteries

Electrochemical and analytical techniques were utilized to study Ca electrodeposition in nonaqueous electrolytes. Linear sweep voltammograms obtained at Au and Pt ultramicroelectrodes (UMEs) exhibit an inverse dependence between current density and scan rate, indicative of the presence of a chemical reaction step in a chemical–electrochemical (CE) deposition process. However, the magnitude of change in current density as a function of scan rate is larger at the Au UME than at the Pt UME. COMSOL simulation reveals that the chemical reaction step rate (kc) obtained at the Pt UME is ∼10 times faster than that at the Au UME. Field desorption ionization mass spectrometry (MS) suggests that dehydrogenation of the borohydride anions by the metal substrate is the chemical reaction step. Pt is more efficient at abstracting hydride from borohydride ions than Au, leading to larger kc. Raman spectroscopy and electrospray ionization MS data show that Ca2+ ions are strongly coordinated with tetrahydrofuran and weakly interacting with BH4– anions. Electron microscopy shows that the surface morphology of Ca electrodeposition is different between Au and Pt, with Au exhibiting a smooth deposit, while a patchier deposit is seen on Pt.