V₃O₇·H₂O as a Cathode Material for Aqueous Mg²⁺/Na⁺ Hybrid Electrochemical Cells

To improve safety and sustainability aspects of today's energy storage solutions, aqueous post-lithium-ion electrochemical cells are promising concepts. In this context, we synthesize hydrated vanadium oxide (V₃O₇·H₂O) via a hydrothermal process and investigate its electrochemical properties as a cathode in aqueous post-lithium-ion hybrid cells. This active material exhibits an excellent initial specific capacity of 404 mAh·g^-1 at a practical current density of 100 mA·g^-1 and a promising stability of 123 mAh·g^-1 at a current density of 1 A·g^-1 after 500 cycles upon Mg²⁺ and Na⁺ cointercalation. The insertion characteristics of this hybrid cell are investigated by cyclovoltammetry, operando XRD, and post-mortem transmission electron microscopy measurements. The results suggest that Na⁺ is inserted preferentially at lower potentials and that its effect fades quickly, while the Mg²⁺ contribution enables stable cycling, resulting in a competitive capacity retention of 73% after 500 cycles. In addition, we find that the charge transfer resistances are about one hundred times smaller in aqueous than in pure organic electrolyte as measured by electrochemical impedance spectroscopy. These results demonstrate that V₃O₇·H₂O is a suitable cathode material for aqueous Mg²⁺/Na⁺ hybrid cells, enabling a performance that is comparable to cathodes in less sustainable and less safe organic electrolytes with respect to capacity and stability and is superior in transport-related properties.