Mono- Vs. Di-Valent Solvated Cations Studied By Raman Spectroscopy

Abstract

Raman spectroscopy is a powerful tool to study molecular level interactions in e.g. battery electrolytes. As cations and solvents interact, this affects the ion-pair formation and the nature of the solvation shells, which both are key factors affecting solution equilibria and ion transport. While Raman spectroscopy has been extensively used to study lithium-ion battery (LIB) electrolytes, studies on alternative battery technology electrolytes are scarce. Most concern sodium-ion batteries (SIB) and the interpretations are eased by the chemical similarity of Li and Na — both being monovalent. For divalent battery technologies such as Ca and Mg the differences in cation solvation can be expected to be larger, due to the fundamental differences between mono- and divalent ions, and prevents the full application of LIB electrolyte know-how and hence calls for further studies. The recently unveiled calcium battery technology [1] is today limited in scope of electrolytes, basically two compositions dominate [1, 2]. The lack of variety and understanding considerably slows down the Ca battery development, why more in depth analysis of the Ca2+ solvation is called for, to highlight issues hindering fast cation transport within the electrolytes. Here monovalent electrolytes composed of NaPF6 or NaTFSI dissolved in mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC), i.e. EC:DMC (1:1) were investigated for concentrations ranging from 0.3–1.0 M. They were selected based on the similarities with common LIB electrolytes either commercialized or extensively studied at the laboratory scale [3]. Divalent electrolytes composed of Ca(TFSI)2 and Ca(BF4)2 dissolved in a mixture of ethylene carbonate (EC) and propylene carbonate (PC), i.e. EC:PC (1:1) were chosen for their compatibility with available Ca electrodes [4, 5]. Both Raman spectroscopy and density functional theory calculations were employed to understand quantitatively how the speciation and solvation differ between these monovalent and divalent electrolytes, and ultimately how it could affect ion transport within both electrolytes and electrodes. In addition to the ion conductivities also the viscosities were measured to enable us to apply the fractional Walden rule [6] and qualitatively estimate the ion-pairing as function of both temperature and salt concentration. We observe that the solvation shell configurations change with the salt concentration for both multivalent and divalent formulations; the sizes of the divalent cationic complexes are nearly twice as large as the monovalent but the solvent coordination order is similar; EC tends to predominantly coordinate the cations, followed by PC or DMC. Finally, the Walden plots indicate that formation of contact ion-pairs (CIP) and/or aggregates (AGG) may be more likely for the BF4 based electrolytes, independent of the nature of the cation.