Investigation into Anion Effects on the Bulk and Interfacial Solvation Structure for Multivalent Ion Batteries

Abstract

Zinc ion batteries (ZIB) are attractive because of the relative abundance of Zn as a core earth element, a higher volumetric energy density compared to lithium metal and it is cost effective. Despite this promise, a general lack of fundamental understanding of the electrode-electrolyte interface in ZIB, and multivalent (MV) ion technologies more broadly, hinders their wide scale deployment. In particular, the solvation structure of aqueous and non-aqueous electrolytes plays a critical role in promoting the necessary intermolecular charge transfer interactions to drive reversible plating/stripping at the metal anode. Here we examine the dynamics of the bulk and interfacial solvation structure to uncover differences in the bulk vs. the interfacial speciation as a function of anion chemistry in hopes to develop new design principles for new electrolyte development in ZIB and MV technologies. Utilizing Raman spectroscopy to probe the bulk solvation structures of Zn-based electrolytes containing one anion such as SO4 2-, bis-trifluoromethane sulfonimide (TFSI-) and trifluoromethanesulfonate (triflate, OTf-), we find three solvation environments present; a main feature corresponding to the “free” anion breathing mode and, in some cases, small amounts of mono- or bidentate contact ion pairs (CIPs). In some cases, introduction of co-anions (i.e., halides, X = Cl-, Br-, I-) results in significant extra intensity on the low frequency side of the overall Raman peak envelope that does not correspond with solvation structures observed in the single anion electrolytes. First principles density functional theory (DFT) calculations indicate the additional vibrational mode corresponds to a mixed anion CIP, suggesting anion-anion interactions can induce nominally dissociated anions to coordinate the cation in ways they otherwise would not. Raman and DFT analysis of Mg-based electrolytes indicate similar mixed anion CIP structures are formed, indicating similar solvation effects are active in other MV systems. Preliminary measurements utilizing shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERs) will also be discussed as a means of developing in situ understanding of the evolution of interfacial solvation and surface chemistry during the stripping/plating process. Overall, these results point to new possibilities in controlling the multivalent cation solvation structures present in the bulk and at the interface. This will aid in the development of highly reversible electrolytes that are stable against both divalent metal anodes and high voltage cathodes.