Do Solvent Exchange Rate Drives Electrochemical Performance of Multivalent Ions?

Solvation structures of metal ions have been considered as one of the key components that influence battery performances. Our work here reports that the solvation dynamics, i.e., how fast the solvents and anions exchange between the free and coordinating states, matters more in multivalent electrolytes due to the stronger interaction between the multivalent cations and the solvents/anions compared to monovalent cations. Based on our previous work on magnesium bis(trifluoromethanesulfonyl)imide (MgTFSI₂) in 1,2-dimethoxyethane (DME), where Mg²⁺ ions form stable fully solvated clusters [Mg(DME)₃]²⁺ at ≥ 0.1 M, we prepare a variety of mixed-solvent electrolytes by maintaining a Mg : DME molar ratio of 1:3 while varying cosolvents ranging from tetrahydrofuran (THF), 2-methyltetrahydrofuran (MeTHF), bis(2,2,2-trifluoroethyl) ether (BTFE), propylene carbonate (PC) to acetonitrile (ACN), trimethylsilyl imidazole (TMSI) and 2-Methoxyethylamine (MEA). Multinuclear (¹H, ¹³C, ¹⁹F, and ¹⁷O) NMR studies of these MgTFSI₂-3DME-cosolvents reveal a wide range of solvation structures with a fraction of DME, cosolvent or TFSI^- forms stable first solvation sheath with Mg²⁺. The residence time of coordinating DME or cosolvent can be estimated from ¹H-¹H 2D EXSY NMR, and the coordination power of the cosolvents can be evaluated accordingly and correlated to the physicochemical properties of the cosolvents (coordination asymmetry, donor number, relative permittivity, viscosity, electrostriction, etc.). Furthermore, cyclic voltammograms of these cosolvent electrolytes demonstrate that a shorter-lived solvation shell with faster exchange between coordinating and free solvent/anions can facilitate cation transport, reduce desolvation energy and promote fast charge transfer at the interface, and therefore leads to better reversible magnesium platting and stripping.