ABSTRACT
Unlike the interface between two immiscible electrolyte solutions (ITIES) formed between water and polar solvents, molecular understanding of the liquid-liquid interface formed for aqueous biphasic systems (ABSs) is relatively limited and mostly relies on surface tension measurements and thermodynamic models. Here, high-resolution Raman imaging is used to provide spatial and chemical resolution of the interface of lithium chloride - lithium bis(trifluoromethanesulfonyl)imide - water (LiCl-LiTFSI-water) and HCl-LiTFSI-water, prototypical salt-salt ABSs found in a range of electrochemical applications. The concentration profiles of both TFSI anions and water are found to be sigmoidal thus not showing any signs of a positive adsorption for both salts and solvent. More striking, however, is the length at which the concentration profiles extend, ranging from 11 to 2 µm with increasing concentrations, compared to a few nanometers for ITIES. We thus reveal that unlike ITIES, salt-salt ABSs do not have a molecularly sharp interface but rather form an interphase with a gradual change of environment from one phase to the other. This knowledge represents a major stepping-stone in the understanding of aqueous interfaces, key for mastering ion or electron transfer dynamics in a wide range of biological and technological settings including novel battery technologies such as membraneless redox flow and dual-ion batteries.
ABSTRACT
Learning how to tailor Ca2+ speciation and electroactivity is of central importance to engineer next-generation battery electrolytes. Using an exemplar dual-salt electrolyte, Ca(BH4)2 + Ca(TFSI)2 in THF, this work examines how to modulate a critical parameter proposed to govern electroactivity, the BH4 -/Ca2+ ratio. Introduction of a more-dissociating source of Ca2+ via Ca(TFSI)2 drives re-speciation of strongly ion-paired Ca(BH4)2, confirmed by ionic conductivity, Raman spectroscopy, and reaction microcalorimetry measurements, generating larger populations of charged species and enhancing plating currents. Ca plating is possible when [Ca(TFSI)2] < [Ca(BH4)2] and thus BH4 -/Ca2+ >1, but a dramatic shut-down of plating activity occurs when [Ca(TFSI)2] > [Ca(BH4)2] (BH4 -/Ca2+ <1), directly evidencing the significance of coordination-shell chemistry on plating activity. Ca(BH4)2 + TBABH4 in THF, which enables enrichment of BH4 - concentrations compared to Ca2+, is also examined; ionic conductivity and plating currents also increase compared to Ca(BH4)2/THF, with the latter related in part to a decrease in solution resistance. These findings delineate future directions to modulate Ca2+ coordination towards achieving both high plating activity and reversibility.
