Preferential Coordination As a Means to Investigate Ca2+ Speciation and SEI Formation
Authors: Melemed Aaron Max , Gallant Betar M. 
Published: September 2023

Given concerns about the low earth abundance of Li in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) have received attention due to their ~2000-fold higher concentration in the earth’s crust along with attractive theoretical metrics if Ca is used as the metallic anode. When most common battery electrolytes come into contact with Ca, however, a passivating solid electrolyte interphase (SEI) forms that prevents Ca2+ transport.1 At present, only a small number of nonaqueous electrolytes have been identified that form sufficiently Ca2+-conductive SEIs for Ca electrodeposition, limiting the design space for Ca battery development. Thus, learning how to tailor Ca SEI formation is of central importance to identify handles for next-generation battery design. Several methods of modifying SEI to improve Ca2+ transport have been reported, including synthesizing an ex situ artificial layer,2 or the introduction of boron-containing electrolyte additives that form beneficial SEI compounds.3 Ca electrolyte modification faces a unique challenge, however, as the large size and divalent nature of Ca2+ leads to large coordination environments (up to 7 solvating molecules) in solution.4 Recent studies have begun to identify that co-reduction of these coordinating molecules has a significant impact on Ca electrochemistry. For example, strong coordination between Ca2+ and the anion TFSI- has been demonstrated to deactivate Ca plating behavior via TFSI- decomposition.5,6 However, competition between coordinating molecules can be exploited to favor specific Ca2+ speciation (i.e. BH4
- displaces bidentate TFSI---Ca2+ in favor of BH4
---Ca2+ interactions) and unlock Ca plating behavior.6 Further mechanistic understanding is needed to identify the specific connections between Ca2+ coordination, SEI formation, and anode performance (beyond just a binary toggling of plating behavior). To address this, we have systematically introduced chelating glyme-based solvents into a baseline electrolyte, investigating the subsequent changes in SEI composition and electrochemical behavior.
This study investigates the preferential coordination of Ca2+ by glymes in the baseline electrolyte 1 M Ca(BH4)2 in tetrahydrofuran (THF). THF is systematically replaced by 1,2-dimethoxyethane (G1) or bis(2-methoxyethyl) ether (G2) over a range of Gx/Ca2+ molar ratios. Microcalorimetry measurements demonstrate that the introduction of both G1 and G2 to the electrolyte is enthalpically favorable, as each glyme displaces THF from the Ca2+ coordination sphere and modulates Ca2+--BH4
- speciation, as indicated by NMR and Raman spectroscopy. Addition of a small amount of each glyme (1:1 Gx/Ca2+) increases electrolyte ionic conductivity by nearly half; however, glyme addition systematically increases the overpotentials of both Ca plating and stripping while decreasing Coulombic efficiency. The presence of glymes increases the amount of carbon present in both chemically- and electrochemically-formed SEI as indicated through XPS. We further find increased evidence of fragmentation of strongly Ca2+-coordinating molecules (G2, BH4
-) within the SEI formed during Ca deposition as compared to formed chemically. Calcium-based olefins as well as ionic phases (CaCO3, CaC2) are detected in electrochemically-formed SEI through titration gas chromatography. This work reveals factors that modulate both Ca2+ coordination and subsequent SEI formation, highlighting preferential coordination as a novel approach for Ca-based electrolyte design.
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