Oxidation state of vanadium in amorphous MnV2O6 formed during discharge–charge cycle and the improvement of its synthesis condition

Crystalline powders of anhydrous MnV₂O₆ were successfully synthesized at a temperature below 200 °C under autogenous hydrothermal condition. MnV₂O₆ powders, which were synthesized using high concentration solutions, more than 0.1 mol/L, gave a relatively high reversible capacity of 600 mA h/g and interesting cyclic performance, reversible capacity increasing to more than an initial charge capacity after the 3rd or 4th cycle of charge–discharge. The change in oxidation state of V ion on the course of the 1st discharge–charge process was investigated by different techniques, such as XAFS and XPS. During discharging, V⁵⁺ was found to change gradually to V⁴⁺, but it returned back completely to V⁵⁺ on charging process. The thinner particles of MnV₂O₆ crystals with rod-like morphology were synthesized at 135 °C, and much better anodic performance was achieved, much smaller irreversible capacity as about 300 mA h/g, stable reversible capacity as 600 mA h/g, and 100% Coulombic efficiency.