Efficacy of Stabilizing Calcium Battery Electrolytes through Salt-Directed Coordination Change

a) Raman-derived TFSI- coordination trends. b) Solvent-dependent Ca deposition response for mixed-salt electrolytes. c) Raman spectra for Ca deposition active and inactive Ca-BH4 coordination structures. d) Cartoon depiction of critical multimer formation transition driven by excess BH4.

Scientific Achievement

The factors determining the extent to which salt anions can be stabilized during Ca plating by eliminating their coordination with Ca2+ through co-salt addition were elucidated. Contrary to expectation, the exemplar bis(trifluoromethylsulfonyl)imide (TFSI-) anion is unstable whether in the coordinated or free state. Instead, the type of Ca2+ coordination structure formed with the co-salt anion determines whether Ca deposition can be achieved.

Significance and Impact

This work teaches us that eliminating TFSI- coordination is insufficient to prevent TFSI- decomposition at Ca anodes. Interphase design strategies must be employed to cycle Ca at high coulombic efficiency.

Research Details

The influence of adding the strongly coordinating BH4- anion to Ca(TFSI)2 solutions was characterized in four ethereal solvents.

Raman spectroscopy and ionic conductivity measurements indicated that the degree of TFSI- coordination displacement is determined by the solvent’s coordination strength.

Electrochemical Ca deposition trends revealed that the degree of TFSI- coordination displacement is not correlated with the reductive stability of TFSI-.

Assessment of the total coordination environment (TFSI-, solvent, BH4-) revealed that multimer Ca-BH4 coordination structures (shared BH4- ligands) are necessary for stabilizing TFSI-, whether TFSI- coordination is present or not.

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DOI: 10.1021/acs.jpcc.2c02587

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