Cations controlled growth of β-MnO < inf> 2 crystals with tunable facets for electrochemical energy storage

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© 2018 Elsevier Ltd 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 β-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 β-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 β-MnO2 from bipyramid prism ({100}+{111} facets) to an octahedron structure ({111} facets). The K+ cations controlled the morphology of β-MnO2 by affecting the formation of α-K0.5Mn4O8 intermediate phase and the subsequent phase transition. Utilizing the β-MnO2 crystals as the cathode for Li-ion batteries showed that highly exposed {111} facets offered β-MnO2 crystal better rate performance, with ~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 metal-air batteries and fuel cells.

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Nano Energy