Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
Aqueous zinc-ion batteries (AZIBs) are a leading candidate for next-generation energy storage due to their intrinsic safety, low cost, and environmental benignity. However, accurate assessment of cathode materials is hampered by a pervasive artifact: routine air-drying of discharged cathodes induces spontaneous aerial oxidation, which distorts post-mortem characterization. This phenomenon leads to a self-charging effect that can recover up to 83% of capacity, thereby masking true electrochemical performance and creating inconsistencies between valence-state analyses (e.g., XPS) and measured capacities. For instance, NVO cathodes show only V4+/V5+ signals in XPS, implying a theoretical capacity of 245.5 mAh g−1, yet experimentally they deliver ~334.5 mAh g−1 at 0.2 A g−1—a discrepancy that has puzzled researchers.
This study systematically investigates the impact of aerial oxidation on discharged NVO cathodes, comparing it with electrochemical re-oxidation. By reassembling air-dried cathodes into fresh cells, we demonstrate that open-circuit voltage (OCV) recovers to 1.0 V from a cutoff of 0.4 V, confirming oxidation during drying. Structural analyses reveal that electrochemical oxidation (EO-NVO) produces a stable, long-range ordered bulk structure with efficient Zn2+ transport channels, whereas aerial oxidation (AO-NVO) induces only superficial changes and pervasive lattice distortion. These findings resolve a key analytical discrepancy and provide a novel pathway for capacity contribution, with direct implications for accurate material assessment and advanced battery design.
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ZHU Xinyu, SHANG Ziyun, LI Chi, HUANG Huijie, WANG Qingbo, WANG Shiyu, ZHONG Hongxia, WANG Hai (2026). A Critical Artifact in Aqueous Zinc-Ion Batteries: Charging under Aerial Oxidation Distorts Discharged-Cathode Characterization. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4196-x
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Frequently Asked Questions
What is the quantitative impact of aerial oxidation on the measured capacity of NVO cathodes, and how does this affect the reliability of ex situ characterization?
Aerial oxidation of discharged NVO cathodes leads to a self-charging effect that recovers ~83% of capacity, as evidenced by OCV recovery from 0.4 V to 1.0 V. This distorts post-mortem XPS analysis, which shows only V4+/V5+ signals, implying a theoretical capacity of 245.5 mAh g−1, while actual measured capacity is ~334.5 mAh g−1 at 0.2 A g−1. This discrepancy underscores the need for controlled oxidation protocols to avoid overestimating capacity and misinterpreting valence states.
How does electrochemical re-oxidation (EO-NVO) compare with aerial oxidation (AO-NVO) in terms of structural order and electrochemical performance?
EO-NVO exhibits a higher average vanadium valence state (V4+/V5+ mix) resembling V2O5, with sharper V–V coordination peaks and a reduced Debye-Waller factor in EXAFS, indicating a more ordered crystalline structure with fewer defects. In contrast, AO-NVO shows pervasive lattice distortion and dislocations, with a predominance of V4+, indicating superficial oxidation and trapped Zn2+ in a defective host. Consequently, EO-NVO provides efficient two-dimensional channels for rapid Zn2+ transport, leading to superior electrochemical kinetics and overall performance.
Is the aerial oxidation artifact specific to vanadium-based cathodes, or does it affect other cathode materials in AZIBs?
The artifact is generalizable but varies in magnitude. For Mn-oxide cathodes, the recovered capacity is only ~8.3%, limited by passivating surface byproducts. This indicates that while the phenomenon is universal, its impact depends on the material's surface chemistry and the formation of passivation layers. Therefore, researchers must account for this artifact when characterizing any discharged cathode in AZIBs.
What are the practical implications of this artifact for the development of high-capacity AZIB cathodes?
This artifact can lead to overestimation of cathode capacity and misinterpretation of charge storage mechanisms, potentially misdirecting research efforts. By identifying and controlling aerial oxidation, researchers can achieve more accurate assessments of material performance. Moreover, electrochemical re-oxidation offers a superior method for achieving full de-zincation and structural stability, which could be exploited in battery design to enhance cycling stability and rate capability.
How can researchers avoid or mitigate the aerial oxidation artifact during ex situ characterization?
To avoid aerial oxidation, cathodes should be transferred from the cell to the characterization chamber under an inert atmosphere (e.g., argon) without exposure to air. Alternatively, electrochemical re-oxidation can be performed in situ to achieve a controlled, fully oxidized state. These practices ensure that post-mortem analyses reflect the true state of the electrode, enabling accurate determination of valence states and structural evolution.
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