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 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 (k c) 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 k c. 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.

Current Understanding of Nonaqueous Electrolytes for Calcium‐Based Batteries

“…Thus, the authors concluded that Ca 2+ is predominantly solvated by THF via the ethereal oxygen and is weakly coordinated, if at all, to BH4 -. [63] Overall, the findings point towards a surface-chemical role of the anion. Interestingly, the conductivities of Ca(BH4)2/THF electrolytes were significantly lower than their carbonate-based counterparts (Fig.…”

“…Recently, Ta et al proposed an explanation for the functional role of BH4in the successful Ca(BH 4 ) 2 /THF system. [63] Linear sweep voltammetry with ultramicroelectrodes was performed on both Au and Pt working electrodes. The authors found that Ca deposition occurs through a chemicalelectrochemical mechanism, in which chemical H abstraction from BH4in the bulk electrolyte (and not THF) and surface H adsorption precedes subsequent reduction of Ca 2+ .…”

Dawn of Calcium Batteries

“…And for the first time, a Ca(BH 4 ) 2 electrolyte in THF was demonstrated for reversible calcium deposition and stripping at room temperature with low polarization (<0.1 V) and high coulombic efficiencies (≈95 %) by Bruce and Wang et al . and further confirmed by Gewirth et al . However, the reductive nature and insufficient anodic stability (ca.…”

High Performance Voltage Tailorable Room-temperature Ca-metal Batteries

“…Except for the self-characters such as electropositivity, ionic radius, and charges etc., it's generally accepted that metal plating/stripping potentials are dependent on the solvation/desolvation properties, which are highly in uenced by the ion concentration, type of solvents, substrates, and temperatures. [7][8][31][32] In the case of Ca-metal, the situations seem more complicated due to anion corrosion and large internal resistance resulted from the extremely insulative passivation layers. [4][5]7 Anyway, we found that the polarization potentials for Ca deposition/dissolution could be largely lowered through the increase of Ca 2+ concentration by adding Ca(BF 4 ) 2 .…”