Inorganic salt-assisted hydrothermal synthesis of composite vanadium oxides for stabilization of aqueous zinc ion batteries with low current density cycling

Vanadium-based materials have been widely investigated as cathode materials for aqueous zinc-ion batteries because of their multiple valences, large interlayer spacing and open framework. However, their sluggish reaction kinetics, poor structural stability and cycling stability at low rates still limit their further development. At present, the improvement of vanadium-based materials mostly involves complex processes or expensive materials that are difficult to synthesize on a large scale. In this work, by introducing an inorganic salt (NH4)2HPO4 as an additive and adjusting the reaction temperature of hydrothermal synthesis, a composite vanadium oxide (NVO(2 1 0)-2P) with the coexistence of NH4V4O10 and V5O12·6H2O was successfully synthesized. Compared with the surfactant-assisted process, the inorganic salt-assisted hydrothermal synthesis has the advantages of being greener and more environmentally friendly. As a cathode material for aqueous zinc ion batteries, the obtained NVO(2 1 0)-2P shows excellent cycle stability at low current density (88.1 % retention over 250 cycles at 0.3 A/g and 80 % after 600 cycles at 0.5 A/g). The excellent electrochemical performance is attributed to the well-structured nanosheets synthesized using (NH4)2HPO4 as an additive. At the same time, the generated V5O12·6H2O provides a large interlayer distance, reduces the structural water molecules of electrostatic interaction and indirectly forms a heterogeneous layered structure with NH4V4O10, which reduces the distribution density of NH4+ and avoids irreversible deamination.