Tuning phase evolution of β-MnO2 during microwave hydrothermal synthesis for high-performance aqueous Zn ion battery

Abstract Mild aqueous Zn–MnO2 battery attracts lots of attention in energy storage filed due to its low cost, high safety and environmental friendliness. To achieve high-performance in battery, phase evolution processes of MnO2 during synthesis and electrochemical reactions need to be understood. Herein, the phase evolution during microwave hydrothermal and correlated battery performance of β-MnO2 are studied. The results demonstrate a phase evolution mechanism from an initial mixture of vernadite, nsutite, and pyrolusite (β-MnO2) to a final single β-MnO2 phase, along with enhanced structure stability, increased Mn valence, and decreased BET surface area. It is found that only when microwave hydrothermal time (MHT) ≥ 120 min, β-MnO2 showing both high capacity and excellent cycling performance can be obtained. β-MnO2 prepared under a MHT of 120 min shows a high reversible capacity of 288 mA h g−1 with a median voltage of 1.36 V vs. Zn/Zn2+, and high capacity retentions of 91.8% after 200 cycles at 0.5C and 84.3% after 1000 cycles at 4C, respectively. In addition, the formation of inactive ZnMn2O4 during cycling is observed, which contributes to the capacity fading of β-MnO2 after long-term cycling. This research makes a step forward to the practical application of Zn–MnO2 batteries, and contributes to the large-scale energy storage field.