Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes

Abstract

The emergence of magnesium and calcium batteries as potential beyond Li ion energy storage technologies has generated significant interest into the fundamental aspects of alkaline earth metal cation coordination in multivalent electrolytes and the impact of coordination on application-critical electrolyte properties such as solubility, transport, and electrochemical stability. Understanding these details in calcium electrolytes is of immediate importance due to recent, unprecedented demonstrations of reversible calcium metal electrodeposition in a limited number of ethereal solvent-based systems. In this work, we provide insight connecting Ca 2+ coordination tendencies to important calcium battery electrolyte properties. Our results demonstrate a clear solvent:Ca 2+ coordination strength trend across a series of cyclic ether and linear glyme solvents that controls the extent of ion association in solutions of "weakly" coordinating salts. We apply understanding gained from these results to rationalize relative anion:Ca 2+ coordination tendencies and attendant Ca 2+ coordination structures using two oxidatively stable anions of particular interest for current battery electrolytes. Armed with this understanding of solvent and anion interactions with Ca 2+ , we demonstrate and interpret differences in electrochemical calcium deposition behavior across several electrolyte exemplars with varying solvent and anion coordination strengths. Our findings demonstrate that solvents exhibiting especially strong coordination to Ca 2+ , such as triglyme, can inhibit reversible calcium deposition despite effective elimination of anion:Ca 2+ coordination while solvents exhibiting more modest coordination strength, such as 1,2-dimethoxyethane, may enable deposition provided anion:Ca 2+ coordination is substantially limited. These results reveal that the strength of coordination of both anion and solvent should be considered in the design of electrolytes for calcium batteries.

Monocarborane cluster as a stable fluorine-free calcium battery electrolyte

“…Ca electrolytes were prepared by dissolving CMC into the weakly coordinating solvents, which enabled the dissociation of Ca salts 37 – 39 . As the weakly coordinating solvents, THF, DME, and a mixture of DME/THF (1:1, v/v) were selected, and the solubilities of CMC were evaluated by ICP-OES using saturated solutions.…”

“…S8 ). Considering the influence of the type of ether solvent and anion coordination on Ca, the incompetency of these electrolytes arises due to the coordination of Ca with diglyme and triglyme being stronger than that of Ca with DME and THF 37 .

Figure 2 ( a ) Cyclic voltammograms of Ca plating/stripping after conditioning cycles at 20 mV s −1 with a three electrode setup using Au as the working electrode and Ca as the reference and counter electrodes at room temperature.

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Rationalizing Calcium Electrodeposition Behavior by Quantifying Ethereal Solvation Effects on Ca2+ Coordination in Well-Dissociated Electrolytes

“…14,15 As a result, the large size of Ca 2+ can act to drive anionic association within the electrolytic solution, which can inhibit rapid transport of ions or induce anion reduction at the electrode surface. 10,16,17 In fact, our most recent work combined spectroscopic analysis with MD simulations to characterize ion association tendencies in ethereal-CaTFSI2 electrolytes in which we found the solvent:Ca 2+ coordination strength to increase as 2-MeTHF < THF < G1 < G2 < G3 with clear evidence of Ca 2+ -TFSI contact ion-pairs even with G2 solvation. 10 These factors all reveal the critical importance of the Ca 2+ solvation environment for electrochemical deposition and stripping of Ca metal.…”

“…9 However, similar measurements using triglyme (G3), a longer linear ethereal solvent, provided negligible success. 10 While many of these electrolyte compositions provide promising steps towards reversible Ca 2+ electrochemistry, each exhibits susceptibility to decomposition leading to the formation CaF2 layers and organic byproducts on the electrode surface that inhibit calcium diffusion. [8][9][10] For example, a comparison of the compositions of calcium deposits by energy dispersive X-ray analysis revealed that using THF as the supporting solvent leads to greater relative F content while using G1 or G2 leads to greater relative O content.…”

“…10 While many of these electrolyte compositions provide promising steps towards reversible Ca 2+ electrochemistry, each exhibits susceptibility to decomposition leading to the formation CaF2 layers and organic byproducts on the electrode surface that inhibit calcium diffusion. [8][9][10] For example, a comparison of the compositions of calcium deposits by energy dispersive X-ray analysis revealed that using THF as the supporting solvent leads to greater relative F content while using G1 or G2 leads to greater relative O content. 9 These differences suggest that the solvent and specifically the local coordination around the cationic species plays an equally important role in the reversibility of the calcium deposition process.…”