A Critical Artifact in Aqueous Zinc-Ion Batteries: Charging under Aerial Oxidation Distorts Discharged-Cathode Characterization
Aqueous zinc-ion batteries (AZIBs) are promising for safe, low-cost energy storage, but accurate cathode characterization is essential for understanding their electrochemical behavior. This study identifies a critical artifact: routine air-drying of deeply discharged cathodes triggers spontaneous aerial oxidation, which distorts post-mortem analysis. Using NH4V4O10 (NVO) as a model cathode, we show that ex situ X-ray photoelectron spectroscopy (XPS) of discharged electrodes reveals only V4+/V5+ signals, with no detectable V3+, implying a theoretical capacity of only 245.5 mAh g−1, yet experimentally measured capacity reaches ~334.5 mAh g−1 at 0.2 A g−1. This discrepancy arises because air exposure during sample preparation oxidizes the reduced vanadium states, leading to a self-charging effect that recovers ~83% of capacity. Electrochemical re-oxidation (EO-NVO) is superior to aerial oxidation (AO-NVO), producing a stable, long-range ordered bulk structure with efficient Zn2+ transport channels, whereas aerial oxidation induces only superficial changes and structural disorder. These findings resolve a key analytical inconsistency and reveal a novel capacity-contribution pathway, with direct implications for accurate material assessment and advanced battery design.