The Crystallinity Effect of Manganese Dioxide for Durability of Rechargeable Zn-Air Battery

Abstract

Zn-air battery is of great interest in rechargeable batteries due to its low cost, environment-friendly and theoretically high energy density which is about 1086 Wh kg-1 [1]. The activity and durability of cathode are the bottleneck of the battery performance, which attribute to the sluggish kinetics of the oxygen reduction reaction (ORR) during discharging and oxygen evolution reaction (OER) during charging. Manganese dioxide (MnO2), an abundant and valence-varied transitional metal oxide, has great potential as bifunctional air-cathode catalyst in Zn-air battery. Despite extensive research efforts have been devoted to MnO2 for Zn-air battery, few disclose the relationship between crystallinity and battery durability. Herein, we present the synthesis and characterization of MnO2 samples prepared in a simple beaker (BK) and samples synthesized via the hydrothermal method (HT). The ORR studies revealed respectively for BK-sample and HT-sample an electron number of 3.67-3.95 and 3.68-3.84 in the potential range of 0.4-0.8 V. Afterwards, the samples were evaluated for their performance in a homemade Zn-Air battery. The sample BK delivered a peak power density (42.8 mW cm-2 at an applied current density of 71.0 mA cm-2) on a par with the sample HT (41.6 mW cm-2 at 68.2 mA cm-2). Interestingly, even though both of the sample showed similar discharge and charge voltage during the initial few cycles, the as-prepared sample HT showed more stable cycling performance after long-term discharge-charge cycling, especially for the charging process. After 120th cycle at current density of 2 mA cm-2, the voltage gap of sample HT (1.12 V) is much lower than that of sample BK (1.32 V). The electrochemical impedance spectra (EIS) recorded before and after the cycling measurement exhibited comparable charge transfer resistance for the two as-prepared samples whereas smaller charge transfer resistance of sample HT after the cycle test. The superior cycling durability in sample HT might be ascribed to the better crystallinity of MnO2 and less trend in collapse from the aspect of structure.