Valence-controlled manganese oxide by solvent-assisted permanganate reduction for advanced aqueous zinc-ion batteries

Abstract

Manganese oxide-based cathodes (MnO_x) play a pivotal role in advancing aqueous Zinc-Ion batteries (AZIB) due to their high theoretical capacity, low cost and environmental friendliness. However, given the MnO_x's diverse structural, valence and textural properties, it is challenging to pinpoint the ideal manganese oxide material type and optimize simple and tunable synthesis routes to achieve great capacity retention and rate capability properties. In this work, we develop for the first time a synthesis method controlling the crystallization pathways and valence properties of MnO_x materials through permanganate reduction using different reducing agents (Ethanol and Propanal) followed by heat treatment under 500 °C, thus achieving the synthesis of multivalent E500-MnO_x (MnO₂) and Trivalent P500-MnO_x (Mn₂O₃). As cathode materials in AZIB, P500-MnO_x reached a specific capacity of 315 mAh·g⁻¹ without any apparent capacity decay, while E500-MnO_x showed a lower specific capacity of 150 mAh·g⁻¹ at 100 mA·g⁻¹ with a capacity retention of only 65 %. Through Ex-Situ XRD and SEM imaging, P500-MnO_x exhibited a reversible cycling mechanism compared to its E500-MnO_x counterpart which had preoccupied insertion sites.