26-electrons redox-active polyoxovanadate clusters for aqueous zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are attaining increasing attention for their high safety and low cost. Despite significant progresses in realizing high-performance cathode material for ZIBs, simultaneously endowing them with high capacity and fast-charging capability, the long-term cycling stability remains a major unsolved challenge. In this work, a polyoxovanadate cluster of (NH₄)₈[V₁₉O₄₁(OH)₉]·11H₂O (NOV) is defined as a cathode material for ZIBs that contains mixed-valence vanadium sites (V⁴⁺ and V⁵⁺). A maximum of 26 electrons can be accommodated in one [V₁₉O₄₁(OH)₉]⁸⁻ {V₁₉O₅₀} cluster, contributing to the high theoretical specific capacity of 328 mA·h·g⁻¹. The Ti₃C₂T_x MXene nanosheets are incorporated into NOV with the help of ionic liquid (IL) linkers to restrain the dissolution of vanadium species and facilitate electron transport across the electrode. The interfacial bonding, anion exchange, and electrostatic interactions among NOV and MXene are provided by IL liquid. The nanohybrid of NOV-IL-MXene endows excellent contact between MXene and NOV, thereby enhanced charge transfer is observed at interface. Subsequently, the as-synthesized NOV-IL-MXene cathodes exhibit high discharge capacity of 413 mA·h·g⁻¹ at 0.2 A·g⁻¹ even at high mass loading of 5.2 mg·cm⁻², remarkable rate performance of 182 mA·h·g⁻¹ at 10 A·g⁻¹, and impressive cycling stability of 94% capacity retention after 2000 cycles. This work opens up new opportunities to develop advanced polyoxovanadate hybrid cathodes for low-cost and high-performance aqueous ZIBs.