Reversible MnO to Mn₃O₄ Oxidation in Manganese Oxide Nanoparticles

AbstractManganese is an attractive element for sustainable solutions. It is largely available in the earth's crust, making it ideal for cost‐effective and large‐scale applications. Especially MnO nanoparticles have recently received attention for applications in battery technology. However, manganese has many oxidation states that are energetically very similar, indicating that they may easily transform from one to the other. Herein, the reversible oxidation of MnO nanoparticles to Mn3O4 studied with in situ transmission electron microscopy is shown. The oxygen sublattices of MnO and Mn3O4 are found to be perfectly aligned, and an atomic mechanism where the transformation is facilitated by the migration of Mn cations on the shared O sublattice is proposed. Even when protected with an amorphous carbon layer, MnO particles are highly unstable and oxidize to Mn3O4 in ethanol. The poor stability of MnO lacks discussion in many battery‐related works, and strategies aimed at avoiding this should be developed.