Aqueous synthesis of three sodium vanadium oxides as cathode materials for zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are gaining attention due to their high safety, low cost, and abundant materials though finding a suitable cathode material remains challenging. Vanadium pentoxide (V2O5) stands out for its high capacity but is prone to structural collapse due to the insertion and extraction of voluminous hydrated zinc ions and water molecules during cycling. Introduction of Na+ ions into V2O5 is known to improve structural stability. In this work, we report the synthesis of β-Na0.33V2O5 and Na1.16V3O8 via an aqueous route involving stirring V2O5 powder in an aqueous NaCl solution, followed by heat treatment. The formation of the β-Na0.33V2O5 tunnel phase and the Na1.16V3O8 layered phase were followed by high-temperature X-ray diffraction. Samples of nanocrystalline barnesite, pure β-Na0.33V2O5, and pure Na1.16V3O8 were then compared in terms of morphology, specific surface area, and electrochemical performance as cathode materials in zinc-ion batteries. The tunnel structure of β-Na0.33V2O5 ensured good stability, with a consistent discharge capacity of about 300 mAh g−1 at 0.1 A g−1 but did not allow easy insertion of zinc ions at high current density. Conversely, the layered structure of the nanocrystalline barnesite and Na1.16V3O8 phases led to easier zinc ion insertion into the structure. Among the three materials, nanocrystalline barnesite exhibited the highest specific capacity (initially 319 mAh g−1 at 0.1 A g−1) and displayed superior stability over 2000 cycles, attributed to sodium ions and water serving as stabilizing pillars for the structure.