Chelating Effects of Polyphenolic Biomolecules to Improve β-MnO₂ Cathode Performance for Aqueous Rechargeable Zinc-Ion Batteries

Aqueous rechargeable zinc-ion batteries (ARZBs) are promising energy storage systems (ESSs) due to lots of advantages, such as high safety, high capacity, abundant resources, and low cost. However, the tunnel-structured Mn-based cathode materials such as α, β, and γ-MnO₂, which is widely used as the cathode of ARZBs, contain a phase transition in which Mn²⁺ ions are eluted during the discharge reaction of Zn²⁺ insertion, resulting in decreasing cycle life and rate capability of the ARZBs. Here, in order to enhance the cycle life and rate capability of ARZBs by retaining eluted Mn²⁺ ions around the β-MnO₂ cathode during the discharge process, tannic acid (TA), a type of polyphenolic biomolecule containing rich -OH groups, is introduced as a coating material. This provides a chelating effect with the eluted Mn²⁺ ions and hydroxyl groups on the surface of the β-MnO₂ cathode. This study clearly shows that the TA coating improves the performance of the cathode material by using a range of analytical methods. Owing to the chelating effects of TA, TA-coated β-MnO₂ cathode shows a high discharge capacity of 268.2 mAh g^-1 at the current of 100 mA g^-1 and 86.8% of high capacity retention after 50 cycles. This study provides the coating agents with chelating effects to develop Zn//MnO₂ battery chemistry and further improve large ESSs through high electrochemical performance.