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Research Webzine of the KAIST College of Engineering since 2014

Spring 2025 Vol. 24
Engineering

Atomic-Scale Unveiling of Hydrated Zn-Ion Insertion for Aqueous Rechargeable Batteries

July 26, 2023   hit 194

Atomic-Scale Unveiling of Hydrated Zn-Ion Insertion for Aqueous Rechargeable Batteries

 

Using atomic-column-resolved scanning transmission electron microscopy, the simultaneous intercalation of both H2O and Zn is directly visualized during discharge of Zn ions into a V2O5 cathode with an aqueous electrolyte. In particular, when further Zn insertion proceeds, multiple intermediate phases are clearly imaged at atomic scale.

 

Article | Spring 2022

 

 

Although Li-ion batteries have been widely since 1991 used as major rechargeable power sources for many applications, both a safety issue, particularly recent incidents of explosions of portable electronic devices, and a long-standing cost issue related to the utilization of Li have been consistently raised. This has led researchers to seek an intercalation chemistry alternative to Li, while avoiding flammable organic electrolytes. Various aqueous batteries operated using earth-abundant redox cations have been suggested over the last two decades. Among them, Zn-ion-based aqueous rechargeable batteries have attracted a surge of attention because metallic Zn, with relatively high capacity, can be used as an anode with water-based nonflammable aqueous solutions as electrolyte. This configuration has important advantages, but it is generally accepted that the lack of in-depth understanding of Zn intercalation in metal-oxide cathodes under an aqueous environment is a serious hurdle in research on Zn-ion aqueous batteries.

Using polycrystalline V2O5 thin films with an aqueous electrolyte, Prof. Sung-Yoon Chung’s group in the Department of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology (KAIST) has successfully determined that most Zn ions do not occupy large interstitial cavities, where Li and Mg are usually inserted during electrochemical intercalation with conventional non-aqueous electrolytes; rather, unusually, Zn ions locate at pyramidal interstitial sites, in addition to direct visualization of the presence of water molecules in the V2O5 lattice.

Moreover, as Zn intercalation proceeds, multiple topotactic phase transformations are clearly observed at nanoscale. To achieve atomic-scale precise site identification of both Zn ions and water molecules, as well as nanoscale probing of structural phase transformation, scanning transmission electron microscopy with atomic resolution was intensively used in both high-angle annular dark-field and annular bright-field modes. Ab initio density-functional theory calculations were also carried out to theoretically support the unusual Zn occupation during intercalation. More importantly, as Zn insertion proceeds, the presence of multiple intermediate topotactic phases, which cannot be identified by macroscopic powder diffraction, is directly reveled at atomic scale, as shown in Figure 1. The remarkable reversible capacity and cyclability of V2O5 thus appear to have a strong correlation with the topotactically smooth transformation between the charged and discharged phases via intermediate transient states. The findings in this study suggest that the availability for facile multiphase transitions during charge/discharge may be an important condition for cathode materials with high capacity and reversibility in aqueous rechargeable batteries, where the crystal structure of discharged cathodes comprehensively differs from the initial crystalline state.

This work was published in Nature Communications (vol. 12, 4599 (2021)).

Additional links for more information:

https://www.nature.com/articles/s41467-021-24700-w

https://sites.google.com/site/atomicscaledefects/home

Figure 1. Topotactic multiphase transformations are demonstrated. As denoted by red rectangles in each projection, boundary regions of grains in both b- and c-axis projections were scrutinized. A larger amount of Zn is detected when the location for observation approaches the grain boundary.