Cations controlled growth of β-MnO2 crystals with tunable facets for electrochemical energy storage

Engineering crystal facets to enhance their functionalities often require complex processing routes to suppress the growth of surfaces with the lowest thermodynamic energies. Herein, we report a unique method to control the morphologies of beta-MnO2 crystals with different occupancy of {100}/{111} facets through the effect of K+ cations. Combining aberration-corrected scanning transmission electron microscopy (STEM), ultramicrotomy, and dynamic functional theory (DFT) simulation, we clarified that the beta-MnO2 crystals were formed through a direct solid-state phase transition process. Increasing the concentration of K+ cations in the precursor gradually changed the morphology of beta-MnO2 from bipyramid prism ({100}+{111} facets) to an octahedron structure ({111} facets). The K+ cations controlled the morphology of beta-MnO2 by affecting the formation of a-K0.5Mn4O8 intermediate phase and the subsequent phase transition. Utilizing the beta-MnO2 crystals as the cathode for Li-ion batteries showed that highly exposed {111} facets offered beta-MnO2 crystal better rate performance, with similar to 70% capacity retention when the charge-discharge rate increased from 20 mA/g to 200 mA/g. Our work revealed a new mechanism to tune the morphology of this earth-abundant metal oxide crystal, which could be used to adjust its electrochemical performance for different applications, such as supercapacitors and catalysts for metalair batteries and fuel cells.