A Mg²⁺-Regulated Hydrated Vanadium Oxide Positive Electrode for Aqueous Mg-Ion Batteries

Aqueous Mg-ion batteries (AMIBs) have emerged as promising candidates for grid-level energy storage systems, thanks to their exceptional safety characteristics, cost-effectiveness, and abundant Mg resources. However, AMIBs confront great challenges, such as the shortage of high-performance electrodes and the sluggish Mg²⁺ diffusion in the electrodes. In this work, a Mg²⁺-regulated bilayered vanadium oxide (MgVOnH) positive electrode, holding a large interplanar spacing of ∼13.4 Å, was investigated in 0.8 m Mg(TFSI)₂-85% poly(ethylene glycol) (PEG)-15% H₂O and 0.8 m Mg(TFSI)₂-65% PEG-20% dimethyl sulfoxide (DMSO)-15% H₂O (20% DMSO-containing) electrolytes. MgVOnH delivers a first discharge capacity of 268 mAh g^-1 at 50 mA g^-1, obtaining 81% capacity retention after 100 cycles (against a second discharge capacity of 249 mAh g^-1) in a DMSO-free electrolyte, whereas MgVOnH exhibits much better rate capability and high capacity at 500 and 1000 mA g^-1 in the DMSO-containing electrolyte, respectively. Particularly, MgVOnH shows a first discharge capacity of 106 mAh g^-1 at 1000 mA g^-1, maintaining 80/65% of its capacity after 920/2000 cycles. Furthermore, the electrochemical reaction mechanism and reversibility of MgVOnH during Mg²⁺ (de)intercalation are systematically explored through ex situ techniques. This work helps us to understand the mechanisms, and this can guide us in achieving a better design for high-performance positive electrodes for AMIBs.