Enhancing the Structural Stability and Electrochemical Performance of δ-MnO2 Cathodes via Fe3+ Doping for Aqueous Zinc-Ion Batteries

Due to its unique layered structure that facilitates ion intercalation and deintercalation, δ-MnO2 has emerged as a promising cathode material for aqueous zinc-ion batteries (ZIBs). However, its structural collapse and Mn dissolution during prolonged cycling significantly limit its practical application. In this study, we demonstrate that metal ion doping, particularly with Fe3+, can effectively stabilize the δ-MnO2 structure and enhance its electrochemical performance. Through a hydrothermal synthesis approach, δ-MnO2 materials with varying Fe3+ doping ratios are prepared and systematically investigated. Among them, the sample with a Mn:Fe molar ratio of 20:1 exhibits the best performance, maintaining the layered δ-MnO2 phase while significantly increasing Mn3+ content and promoting the formation of oxygen vacancies. At a current density of 0.5 A·g−1, the iron-doped sample exhibited an initial specific capacity of 116.24 mAh·g−1, with a capacity retention rate of 41.7% after 200 cycles. In contrast, the undoped δ-MnO2 showed an initial specific capacity of only 85.15 mAh·g−1, with a capacity retention rate of merely 19.9% after 200 cycles. The results suggest that Fe3+ doping not only suppresses Mn dissolution but also improves structural stability and Zn2+ transport kinetics. This work provides new insights into the development of durable Mn-based cathode materials for aqueous ZIBs.