Exploration of high‐performance cathode materials for rechargeable aqueous Zn ion batteries (ZIBs) is highly desirable. The potential of molybdenum trioxide (MoO₃) in other electrochemical energy storage devices has been revealed but held understudied in ZIBs. Herein, a demonstration of orthorhombic MoO₃ as an ultrahigh‐capacity cathode material in ZIBs is presented. The energy storage mechanism of the MoO₃ nanowires based on Zn²⁺ intercalation/deintercalation and its electrochemical instability mechanism are particularly investigated and elucidated. The severe capacity decay of the MoO₃ nanowires during charging/discharging cycles arises from the dissolution and the structural collapse of MoO₃ in aqueous electrolyte. To this end, an effective strategy to stabilize MoO₃ nanowires by using a quasi‐solid‐state poly(vinyl alcohol)(PVA)/ZnCl₂ gel electrolyte to replace the aqueous electrolyte is developed. The capacity retention of the assembled ZIBs after 400 charge/discharge cycles at 6.0 A g⁻¹ is significantly boosted, from 27.1% (in aqueous electrolyte) to 70.4% (in gel electrolyte). More remarkably, the stabilized quasi‐solid‐state ZIBs achieve an attracting areal capacity of 2.65 mAh cm⁻² and a gravimetric capacity of 241.3 mAh g⁻¹ at 0.4 A g⁻¹, outperforming most of recently reported ZIBs.

The electrolyte and interphase chemistry plays a significant role in anode-free sodium metal batteries. Progresses in electrolyte optimization and interphase engineering are herein reviewed, focusing on their collective performance improvement.