Atomic-Scale Origin of Spatially Heterogeneous Insertion in Hollandite-Type MnO2

Abstract Hollandite-type α-MnO2 is a prototypical tunnel-structured host for rechargeable batteries, yet the atomic-scale mechanisms governing multivalent-ion insertion remain poorly understood. Here, we reveal how guest-ion species dictate electrochemical reaction pathways through distinct occupation modes within the one-dimensional tunnels of α-MnO2. By combining electrochemical analyses, atomic-resolution scanning transmission electron microscopy, and soft X-ray absorption spectroscopy, we directly correlate reaction kinetics with guest-ion configurations and the accompanying structural and electronic responses of the host framework. Electrochemical measurements reveal substantial diffusion-controlled contributions for both Na+ and Mg2+, whereas Ca2+ exhibits predominantly surface-controlled behavior. Atomic-resolution imaging demonstrates that, despite their comparable macroscopic kinetic contributions, Na+ and Mg2+ exhibit distinctly different microscopic insertion modes: Na+ homogeneously occupies the tunnel-center sites throughout the particles, whereas Mg2+ occupies both tunnel-center and off-center sites, with pronounced near-surface enrichment accompanied by local tunnel distortion. In contrast, the Ca2+ reaction is largely confined to the near-surface region, with no clearly detectable occupation of the internal tunnel sites. Soft X-ray absorption spectroscopy further reveals spatially heterogeneous Mn reduction, with more pronounced reduction near the structurally less constrained particle surface. These results establish a direct relationship between guest-ion occupation modes, electrochemical reaction pathways, and the resulting structural evolution, providing an atomic-scale design principle for engineering tunnel-type host materials for multivalent-ion insertion.