Unraveling the Degradation Mechanisms of Vanadium Oxide Cathode-Based Aqueous Zinc-Ion Batteries
Authors: Sim Kiyeon , Eom Kwang Sup 
Published: December 2024

The growing environmental problems are driving a great demand for renewable energy generation coupled with new energy storage devices. While lithium-ion batteries (LIBs) have gained great attention due to their higher energy densities, power densities in recent years, limitations such as the low abundance of lithium in Earth’s crust, high cost, unsafety, and environmental pollution problems caused by the flammable and toxic organic electrolyte hinder their applicability in large-scale energy storage systems (ESSs). Consequently, aqueous rechargeable ion batteries have emerged as a promising alternative owing to high ionic conductivity, eco-friendliness, and non-flammable electrolytes.
Particularly, aqueous Zn-ion batteries (AZIBs) have attracted great attention as energy storage devices for ESS due to cost-efficiency, low redox potential (-0.76 V vs. SHE), and the remarkable theoretical volumetric capacity (5854 mAh cm-3) of the Zn metal anodes (ZMAs). However, their poor cycling stability at low current densities (<1C) has been largely overlooked in the past few years, as previous literature has mainly highlighted the superior cycling stability but low-capacity performance of AZIBs at high currents (>5C). Unfortunately, the unstable cycling behavior of AZIBs at moderate rates hinders their practical implementation for grid-scale ESSs.
Among the cathode materials for AZIBs, layered vanadium oxide (VOs) has been extensively studied due to its high specific capacities due to wide redox range of V (from V5+ to V3+) and adjustable large interlayer distance. This unique structure mitigates the high Columbic interaction of Zn2+, promoting facile Zn2+ (de)intercalation. [2] However, in aqueous electrolytes, V dissolution occurs due to contact between VOs and H2O.[3] The dissolved V ions might contaminate the electrolyte and deposit on the ZMA surface, which would bring additional negative effects. Therefore, previous research speculates that the inevitable chemical dissolution of V is more severe at low currents due to increased reaction time, consequently resulting in aggravated capacity fading.
However, until now, there is still a lack of understanding of the large gap between low-rate and high-rate cycling performances. Therefore, further investigation into the influence of current densities on VOs cathode degradation and its subsequent impact on the overall electrochemical performance of the full cell is warranted.
Herein, we elucidate the underlying degradation mechanism of VO-based AZIBs focused on the electrochemical degradation during charge/discharge process. Specifically, we present a case study using the Zn//hydrated VO (VOH) battery configuration employing a 2 M ZnSO4 aqueous electrolyte, a setup commonly utilized in various studies. Through meticulous observation, we characterize the changes in ZMA surface and VO cathode structure at different depth of discharge/charge, number of cycles, and current densities using galvanostatic tests. It is intriguing to find that the product formed on the ZMA surface after charge is composite composed of Zn, V, and O. Moreover, at lower current densities, the more product layer irreversibly precipitates on ZMA surfaces after cycling. These findings inform that the capacity fading of VO-based AZIBs is closely related to the electrochemical dissolution of V and its subsequent deposition onto the ZMA surface through galvanic corrosion between V ions and ZMAs, which is exacerbated at low current densities. This comprehensive investigation into the degradation mechanisms of VO-based AZIBs aims to provide valuable insights for the development of this promising energy storage.

Figure 1