Unraveling the Steric Effect of Trialkyl Phosphates on the Solvation Sheath and Solid Electrolyte Interphase in High-Efficiency Magnesium Electrolytes

Designing Mg-compatible and chloride-free electrolytes is important to achieving high-voltage and long-life Mg metal batteries. In electrolytes based on Mg(OTf)₂/diglyme: triethyl phosphate that exhibit near 100% Coulombic efficiency for Mg plating/stripping at high current densities and capacities, the TXP (X = alkyl group) plays a key role. Herein, we unveil steric hindrance as the fundamental origin of this role in optimizing the Mg²⁺ solvation structure and forming electrolyte-derived solid electrolyte interphases. The steric hindrance was tuned by systematically changing the alkyl chain length of the phosphate (TXP) from one to five. Its impact on altering bulk electrolyte solvation and interfacial SEI chemistry was studied by combining both spectroscopy techniques and theoretical modeling methods. We demonstrate that low steric hindrance leads to easy decomposition of phosphate solvents, whereas high steric hindrance fails to interrupt Mg²⁺-triflate/diglyme interactions. TEP with medium hindrance shows optimized behavior in both scenarios, which is the origin of its superior electrochemical performance. Our finding provides a new solvent design principle at the molecular level for chloride-free Mg²⁺ electrolytes.