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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-K_0.5Mn₄O₈ intermediate phase and the subsequent phase …