Rechargeable zinc ion batteries (ZIBs) featuring high abundance, environmental benignity, cost effectiveness, and intrinsic safety are regarded as high-potential grid energy storage systems, but the developments of high-performance cathodes with large capacity, high energy density, and long-term cyclability remain a huge challenge, owing to sluggish Zn²⁺ intercalation kinetics with bivalent charges in the cathodes. Herein, we report novel NH₄V₃O₈·1.9H₂O nanobelts as advanced cathode materials in aqueous and quasi-solid-state (QSS) ZIBs. When examined in aqueous ZIBs, these cathode materials enable ultrafast Zn²⁺ diffusion and highly reversible processes, exhibiting superior electrochemical performances with a high discharge capacity of 463 mA h g⁻¹ at 0.1 A g⁻¹, excellent rate capability (183 mA h g⁻¹ even at 10 A g⁻¹), and impressive cycling stability with a capacity retention of 81% after 2000 cycles, retaining a decent discharge capacity of 166 mA h g⁻¹ at 10 A g⁻¹. Moreover, the NH₄V₃O₈·1.9H₂O electrode can deliver a high energy density of 332 W h kg⁻¹ at a power density of 72 W kg⁻¹ and retain an energy density of 101 W h kg⁻¹ at a high power density of 5519 W kg⁻¹. In addition, the QSS flexible Zn/NH₄V₃O₈·1.9H₂O battery is investigated, showing durable cycling performance and stable electrochemical properties under various bending states. This study shows that the NH₄V₃O₈·1.9H₂O nanobelt cathode with high energy density and long cycle life is a potential candidate for grid energy storage systems, and it sheds light on the rational design of novel cathodes for practical rechargeable ZIBs.