Co-intercalation of solvated Mg2+ in amine-chain-expanded VOPO4 cathodes with fast kinetics under high-voltage condition

Abstract

Layered cathode materials represent promising candidates for rechargeable magnesium batteries (RMBs). Among them, hydrated vanadyl phosphate (VOPO₄∙2H₂O) has gained traction due to its relatively high redox potential. However, its limited interlayer spacing leads to sluggish Mg²⁺ diffusion kinetics and low specific capacity. We address this issue by systematically studying various organic molecules as pre-insertion agents. We introduce theoretical descriptors, including molecular orbital energy levels and adsorption energy of organic molecules on VOPO₄ material, to guide the selection of applicable interlayer-expanding agents. Our di-n-butylamine (PD) pre-inserted VOPO₄ (PD-VOPO₄) cathode exhibited an expanded interlayer spacing from 0.746 nm to 1.42 nm and simultaneously delivered superior stability. It delivered an enhanced specific capacity of 118.5 mAh∙g⁻¹ with a high discharge potential of 2.74 V (vs. Mg²⁺/Mg) at 50 mA∙g⁻¹, and retained 81.2% of its capacity over 200 cycles. Theoretical calculations and electrochemical characterizations demonstrated that the PD-VOPO₄ cathode exhibited faster Mg²⁺ migration kinetics and a higher intercalation amount, while ex-situ characterization measurements revealed the co-intercalation mechanism of solvated Mg²⁺. This work offers new insights into the development of high-voltage, stable, and high-capacity layered cathode materials for RMBs.