Effect of Silicate Addition on ZnO Formation during Zn Discharging Process
Authors: Wang Tanyanyu , Kunimoto Masahiro , Homma Takayuki 
Published: August 2024

Rechargeable zinc batteries have been attracting extensive attention as promising candidates for next-generation energy storage, owing to their intrinsic safety, mature recycling processes, low material cost and high theoretical energy density.[1] Nevertheless, there are still several issues holding back the development of zinc batteries, such as the capacity loss due to Zn passivation and poor cyclability caused by Zn dendrite formation. [2] In the past few years, efforts have been made to improve the performance of zinc batteries, among which electrolyte modification with additives is the most widely used. [3] For further development of zinc batteries towards practical application, it is also essential to deepen the understanding of chemical state changes in a working Zn electrode. In our previous work, Zn dissolution−passivation behavior with ZnO formation during Zn discharging process has been systematically elucidated via in-situ analysis. [4] Herein this work, through comparative studies with silicate addition in the electrolyte, the correlation between ZnO formation and the discharging performance of Zn anodes is further clarified, shedding new insights into further optimization of zinc batteries.
Firstly, various electrolyte additives were evaluated during Zn discharging at a current density of 40 mA/cm2. As a result, potassium silicate addition with an optimum concentration of 100 mM effectively enhanced the discharge capacity of Zn anode. Effects of silicate addition on Zn discharging process were then further investigated. After 500 s of discharging, ZnO layer with a thickness around 7 um is formed on Zn anodes, which is attributed to the dehydration of supersaturated zincate ions. [4] It is observed that as-formed ZnO petals turned from sharp to round with silicate addition. Moreover, a porous ZnO layer was formed with silicate addition, while a compact one was obtained without any additives. Accordingly, it is assumed that silicate may form adsorption or coordination with ZnO particles, leading to morphology and porosity changes in ZnO.
Further analyses were carried out to verify whether the chemical state of ZnO changes with silicate addition. The TEM-EDS mapping result and XPS depth profile indicate that Si is uniformly distributed within the ZnO layer. Moreover, the presence of Si-O bond is confirmed, along with both Zn 2p 2/3 and O 1s peak shifts towards higher binding energy side. Accordingly, the coordination between silicate and ZnO via the formation of Si-O-Zn bridge is suggested, which is also supported by the DFT calculation result, as a stable existence of silicate on ZnO can be achieved.
On the other hand, it is noticed via XRD and HRTEM characterization that the crystallite size of ZnO became smaller with silicate addition. Moreover, ZnO formed with silicate addition presented lower crystallinity with higher defect contents, as evidenced by Raman spectra. Given the above, it is considered that silicate modulates the electronic structure of ZnO molecules via the Si-O-Zn bonding, thereby suppresses ZnO crystal growth and particle aggregation, which eventually contributes to a loose ZnO structure that can maintain the activity of Zn anodes. With the discussion of ZnO formation herein, we hope this work may open a new avenue for developing high-performance zinc batteries.
Acknowledgements
This study was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) Projects, RISING3 [JPNP21006], commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.
[1] P. Ruan et al., Angew. Chem., 2022, 134, e202200598
[2] W. Shang et al., Energy Storage Mater., 2020, 31, 44–57
[3] S. Guo et al., Energy Storage Mater., 2021, 34, 545–562
[4] T. Wang et al., Energy Environ. Mater., 2023, 0, e12481