Bohr Magnetons: Microscopic Disruptors of Layered MnO₂ Structures

Abstract Doping of metal ions into MnO 2 can introduce impurity energy levels, thereby improving its electrical conductivity and enhancing the potassium storage performance. However, the unpredictable phase transition from layered δ‐MnO 2 to tunnel α‐MnO 2 trigger under certain synthesis conditions significantly reduces its specific capacity. In this work, eight transition metal ions with different Bohr magnetons ( μ B ) are doped into δ‐MnO 2 . Paramagnetic metal (PM) ions (Fe 3+ , Cr 3+ , Co 2+ , Ni 2+ , Cu 2+ ) tend to occupy the interlayer K + sites in δ‐MnO 2 and their unpaired d‐orbital electrons can shield the Coulomb force between the interlayer K + and O in the [MnO 6 ] octahedra, leading to loss of K + . Except for Cu 2+ , these PM ions also possess the ability to substitute Mn to form stable PM‐oxide octahedra. Thus, unsupported δ‐MnO 2 undergoes a collapse phase transition to generate α‐MnO 2 . On the other hand, diamagnetic metal (DM) ions (Mo 6+ , V 5+ , W 6+ ) doping causes lattice distortion in δ‐MnO 2 without phase transition. The resulting O defects help form more active sites for surface Faraday reactions and enhance potassium storage capacity. This work provides a detailed explanation of the phase transition mechanism in δ‐MnO 2 , which is of guiding significance for the synthesis strategies of metal‐doped δ‐MnO 2 .