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

Abstract

Electrochemical and analytical techniques were utilized to study Ca electrodeposition in nonaqueous electrolyte. Linear sweep voltammograms obtained at Au and Pt ultramicroelectrodes (UME) 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 mechanism. 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 suggests that dehydrogenation of the borohydride anions by the metal substrate is the preceding chemical reaction. Pt is more efficient at abstracting hydride from borohydride ions than Au, leading to the larger kc. Raman spectroscopy and electrospray ionization mass spectrometry data show Ca2+ ions are strongly coordinated to THF and weakly interacting with BH4− anions. Electron microscopy shows the surface morphology of Ca electrodeposition are different between Au and Pt, with Au exhibiting a smooth deposit, while a patchier deposit is seen on Pt.