Symmetry-Breaking Induced In Situ Electrochemical Oxidation of MnO to MnO2: A Space Group-Dependent Phase Evolution...

The electrochemical oxidation of MnO to MnO2 is a fundamental solid-state phase transition process, intimately associated with the evolution of crystal symmetry and space group. From the perspective of space group evolution, this study systematically reveals the transformation mechanism of MnO to MnO2 during in situ electrochemical oxidation, as well as the mode by which this oxidation process diffuses from the surface to the interior of MnO. The results demonstrate that the electrochemical oxidation of MnO (space group: Fm-3m) to MnO2 is induced by symmetry breaking, which is followed by the evolution of the parent space group Fm-3m into its subgroups. Such symmetry breaking is forcibly triggered by the Jahn-Teller effect during the oxidation process. During the oxidation process, MnO undergoes a gradual reduction in crystal symmetry, with its parent space group Fm-3m transforming into its subgroups I41/amd, I4/m, and P42/mnm, which correspond to Zn2Mn2O4 (space group: I41/amd), α-MnO2 (space group: I4/m), and β-MnO2 (space group: P42/mnm), respectively. This phase transition is achieved through mild atomic displacement and local lattice relaxation rather than drastic structural reconstruction. Importantly, this group-subgroup relationship ensures the topological feasibility of the phase transition, enabling the electrochemical oxidation of MnO to MnO2. During this process, MnO undergoes nonmechanical fragmentation, continuously exposing fresh MnO surfaces and thereby facilitating the inward diffusion of the oxidation process into the bulk of MnO. Benefiting from this synergistic mechanism, the MnO-based electrode exhibits a reversible capacity of 60 mAh g-1 even at an ultrahigh current density of 25 A g-1, along with stable cycling over 1000 cycles. This work provides profound insights into the fundamental mechanism of symmetry-breaking-induced solid-state phase transitions in transition metal oxides and offers a perspective for the rational design of high-performance oxide materials through crystal symmetry regulation.