Chelating Effects of Polyphenolic Biomolecules to Improve β-MnO 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-MnO, which is widely used as the cathode of ARZBs, contain a phase transition in which Mn ions are eluted during the discharge reaction of Zn 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 Mn ions around the β-MnO 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 Mn ions and hydroxyl groups on the surface of the β-MnO 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-MnO cathode shows a high discharge capacity of 268.2 mAh g at the current of 100 mA g and 86.8% of high capacity retention after 50 cycles. This study provides the coating agents with chelating effects to develop Zn//MnO battery chemistry and further improve large ESSs through high electrochemical performance.