Optimizing Interplanar Spacing, Oxygen Vacancies and Micromorphology via Lithium-Ion Pre-Insertion into Ammonium Vanadate Nanosheets for Advanced Cathodes in Aqueous Zinc-Ion Batteries.

Ammonium vanadates, featuring an N─H···O hydrogen bond network structure between NH and V─O layers, have become popular cathode materials for aqueous zinc-ion batteries (AZIBs). Their appeal lies in their multi-electron transfer, high specific capacity, and facile synthesis. However, a major drawback arises as Zn ions tend to form bonds with electronegative oxygen atoms between V─O layers during cycling, leading to irreversible structural collapse. Herein, Li pre-insertion into the intermediate layer of NHVO is proposed to enhance the electrochemical activity of ammonium vanadate cathodes for AZIBs, which extends the interlayer distance of NHVO to 9.8 Å and offers large interlaminar channels for Zn (de)intercalation. Moreover, Li intercalation weakens the crystallinity, transforms the micromorphology from non-nanostructured strips to ultrathin nanosheets, and increases the level of oxygen defects, thus exposing more active sites for ion and electron transport, facilitating electrolyte penetration, and improving electrochemical kinetics of electrode. In addition, the introduction of Li significantly reduces the bandgap by 0.18 eV, enhancing electron transfer in redox reactions. Leveraging these unique advantages, the Li pre-intercalated NHVO cathode exhibits a high reversible capacity of 486.1 mAh g at 0.5 A g and an impressive capacity retention rate of 72% after 5,000 cycles at 5 A g.