Role of the Solvation Shell Structure and Dynamics on K-Ion and Li-Ion Transport in Mixed Carbonate Electrolytes

Ethylene and diethyl carbonates (EC : DEC) have been attractive solvents for Li-ion electrolytes because these can exhibit a good ionic conductivity and stable electrochemical performance. However, in the contemporary K-ion electrolytes, KPF₆ presents a limited solubility of ∼0.6 M in EC : DEC (volume ratio 1 : 1), which restricts its performance. Here, the molecular basis for these distinct solvation behaviors is clarified by combining experimental attenuated total reflectance-infrared spectroscopy and electrochemical impedance spectroscopy measurements with molecular dynamics simulations, revealing the distinct roles of EC and DEC solvents on the solubility, ionic conductivity, and solvation shell structures of KPF₆ and LiPF₆ salts in EC : DEC mixtures. In turn, these insights have enabled the formulation of a new K-ion electrolyte with EC : DEC volume ratio of 1.5 that exhibits significantly increased KPF₆ solubility (∼0.9 M) and ionic conductivity, reaching 13.99 mS cm⁻¹ at 25 °C.