A Critical Artifact in Aqueous Zinc-Ion Batteries: Charging under Aerial Oxidation Distorts Discharged-Cathode Characterization

• • Air-drying of discharged NVO cathodes causes spontaneous aerial oxidation, leading to a self-charging effect that recovers ~83% of capacity, distorting post-mortem characterization and overestimating actual capacity. • • Ex situ XPS of discharged NVO shows only V4+/V5+ signals, implying a theoretical capacity of 245.5 mAh g−1, yet measured capacity is ~334.5 mAh g−1 at 0.2 A g−1, highlighting a critical discrepancy between valence-state analysis and electrochemical performance. • • Electrochemical re-oxidation (EO-NVO) yields a higher average vanadium valence state (V4+/V5+ mix) resembling V2O5, with sharper V–V coordination peaks and reduced Debye-Waller factor, indicating a more ordered structure and superior kinetics compared to aerial oxidation (AO-NVO). • • The artifact is generalizable but varies by material: Mn-oxide cathodes recover only ~8.3% capacity due to passivating surface byproducts, underscoring the need for controlled oxidation protocols in cathode characterization.