A case study of β- and δ-MnO2 with different crystallographic forms on ion-storage in rechargeable aqueous zinc ion battery

beta-MnO2 and delta-MnO2 submicrospheres with different crystallographic forms were parallelly synthesized via a facile way using MnCO3 submicrospheres as manganese source and template. Fine observation enclosed that the beta-MnO2 and delta-MnO2 submicrospheres were composed of nanoparticles and nanosheets respectively. The performances of these samples as cathode for aqueous zinc ion batteries were investigated: the prepared beta-MnO2 and delta-MnO2 submicrospheres exhibit an initial capacity of 100 and 126 mAh/g and preserve about 99% and 81% of their original capacity over 100 cycles at a current density of 100 mA/g, respectively. It is observed from the galvanostatic charge/discharge profiles that the majority of excess capacity for delta-MnO2 than beta-MnO2 is contributed by the first discharge step referred to H+ insertion process, as delta-MnO2 has lower charge transfer resistance and larger diffusion coefficient than beta-MnO2. We ascribe this to the larger layer space of delta-MnO2 which would benefit for the easily insertion/ extraction of hydronium ions. For contrast, the delta-MnO2 encounters a sluggish transportation of hydronium ion along with the breaking of H-H2O bond during ion inserting. (C) 2019 Elsevier Ltd. All rights reserved.