Modified MnO2-Based Cathode for Zinc-Ion Batteries Using Facile Processing and Easily Available Materials
Authors: Wang Junru , Ferreira Isabel Maria Mercês , Vandeginste Veerle 
Published: February 2024

With the fast development of diverse electronics, the demand for safe energy storage systems with high energy density and high stability has increased rapidly. So far, lithium-ion batteries (LIBs) have dominated the market, from small smart electronics to electric vehicles. Nevertheless, LIBs have several limitations, such as high cost, limited raw material resources, high flammability, and harsh environmental impact. Therefore, novel rechargeable batteries which can mitigate these shortcomings must be explored. Aqueous zinc-ion batteries (ZIBs) have shown a promising future. Zinc with high theoretical volumetric capacity (5855 mAh cm-3) and high stability in aqueous electrolytes should enable high-performing, safe, and low-cost batteries. Additionally, the simple assembly of ZIBs in the ambient environment results in low capital costs. Despite these attractive merits, developing cathode materials with high capacity, rate performance, and stability in order to build commercial high-performing ZIBs, remains a great challenge.
Cathode materials directly influence the electrochemical performance of ZIBs. Manganese oxide (MnO2) has been investigated as a promising cathode in ZIBs research due to its good specific capacity, low cost and high safety. However, the low ionic conductivity, slow diffusion kinetics, and low stability of MnO2 severely deteriorate the electrochemical performance and cycling stability of zinc-ion batteries. Recently, a series of studies have revealed the high effectiveness of coating on improving MnO2 cathode stability, including polymer coatings, carbon-based material coatings, artificial cathode-electrolyte interfaces, and metal oxide coatings.[1] Nevertheless, the improvement of other parameters, such as the rate performance, could be further improved for practical usage. In this research, we develop a novel modification method combining coating and doping strategies to induce MnO2 cathode with high capacity, high rate performance and low capacity decay after long-term operation. First, we utilize a facile one-step hydrothermal synthesis for Ag-doped α-MnO2. Different amounts of AgNO3 are added into reactors with KMnO4 and MnSO4 (n(Ag):n(Mn)=1:100-4:100) to investigate the influence of doping on capacity and rate performance. Subsequently, the doped tunnel-type MnO2 cathodes are mixed with precursor solutions of Al2O3 and then thoroughly stirred. Finally, an annealing process is conducted to achieve a nanoscale Al2O3 coating on the tunnel type MnO2. The coating configurations influence the electrochemical performance of cathodes;[2] thus, we test precursor solutions with different concentrations (1-3 wt% Al2O3) and annealing temperatures (500-700 °C) to investigate the influence of coating thickness and annealing treatment.
With the assistance of scanning and transmission electron microscopes, the nanoscale Al2O3 layer coating on the doped active material is verified. The stable Al2O3 coating protects active materials from dissolution and structural collapse, improving cycling stability and resulting in high capacity retention after long-term operation. Moreover, a significant improvement in rate performance compared to unmodified MnO2 is verified through electrochemical tests. The X-ray photoelectron spectroscopy (XPS) and EPR spectra showed that the doped Ag+ ions lead to oxygen vacancies due to the formation of Ag-O-Mn.[3] The abundant vacancies act as active sites, significantly improving conductivity and ion insertion rates. The ex-situ X-ray diffractometer (XRD) and electrochemical tests are used to investigate the ion insertion mechanism during charging/discharging operations. The Ag-MnO2 @ Al2O3 cathode is synthesized through a facile methodology with environment-friendly materials. The modified cathode exhibits promising performance, especially regarding cycling stability and rate performance, which will be a crucial step for developing high-performing cathodes in aqueous zinc-ion batteries.
Reference:
[1] Shi, W., Lee, W. S. V., & Xue, J. (2021). Recent development of Mn‐based oxides as zinc‐ion battery cathode. ChemSusChem, 14(7), 1634-1658.
[2] Zhou, A., Xu, J., Dai, X., Yang, B., Lu, Y., Wang, L., ... & Li, J. (2016). Improved high-voltage and high-temperature electrochemical performances of LiCoO2 cathode by electrode sputter-coating with Li3PO4. Journal of Power Sources, 322, 10-16.
[3] Pu, X., Li, X., Wang, L., Maleki Kheimeh Sari, H., Li, J., Xi, Y., ... & Wu, Y. (2022). Enriching Oxygen Vacancy Defects via Ag–O–Mn Bonds for Enhanced Diffusion Kinetics of δ-MnO2 in Zinc-Ion Batteries. ACS Applied Materials & Interfaces, 14(18), 21159-21172.

Figure 1