PNNL

Abstract

Most multivalent secondary batteries have employed electrolytes composed of cyclic ether solvents such as tetrahydrofuran or linear glycol ether solvents (glymes) such as 1,2-dimethoxyethane. A robust understanding of multivalent cation solvation tendencies in these classes of solvents provides insight into corresponding struc-ture-property relationships which, in turn, promotes the design and discovery of improved electrolytes. This study utilizes pulsed-field gradient (PFG) NMR in conjunction with Raman spectroscopy and ionic conductivity measurements to elucidate the preferential inter-actions between multivalent cations, anions, and solvent molecules along with their correlated ion dynamics. These investigations in-corporate two representative divalent cations (Ca2+ and Zn2+) as well as two ethereal solvent representatives from both the cyclic ether and glyme structural classes. The results reveal that anions coordinate more readily with divalent cations in cyclic ethers than in glymes. Further, the coordination of the anions with Ca2+ i.e. con-tact-ion pair (CIP) formation is more pronounced than with Zn2+ in a glyme solvent of limited chain length (G1), providing insight into cation size effects that are important for translating solvation behav-ior across various multivalent electrolytes. Importantly, we find that specific anion coordination is more strongly controlled by solvent structure than by salt concentration in the practical range of 0.1 M to 0.5 M. However, simply reducing these inner-sphere interionic interactions by changing solvent structure does not necessarily de-correlate ionic motion. Instead, concentration-dependent changes in molar ionic conductivity suggest that second-shell interactions, i.e. solvent separated ion pair (SSIP) are prevalent in these electrolytes, and that the solution dielectric constant, which is increased by the presence of dipolar ion pairs, is critical for controlling these interac-tions.