Deciphering anomalous zinc ion storage in intermediate-state MnO2 during layer-to-tunnel structural transition†

MnO₂ materials have attracted intensive attention as cathode materials for aqueous zinc ion batteries (AZIBs) owing to their outstanding structural diversity, decent capacity and competitive cost. Although various types of MnO₂ have been adopted, none of them completely meet practical demands owing to structural collapse during cycling. Herein, intermediate-state MnO₂ (IS-MnO₂) undergoing a transition from a layered to a tunnel structure is reported, which exhibits significant improvements in rate and cycling performance compared with purely layered or tunnel MnO₂. Systemic structural analysis reveals the presence of abundant two-phase transition regions within IS-MnO₂, which results in a distorted lattice and deformed [MnO₆] octahedron unit within the two-phase transition region as well as a reduced average valence state of Mn ions. The deformation of [MnO₆] reduces the geometric symmetry of the ligand field and thereby eliminates the 3d orbital degeneracy of the center Mn ion, which effectively avoids the Jahn–Teller effect of Mn³⁺ and enhances cycling stability. Additionally, low-valence Mn leads to a decrease in electrostatic repulsion during ion insertion/extraction, thus efficiently improving rate performance. This work presents a high-performance cathode for AZIBs and provides new avenues to eliminate the Jahn–Teller effect of Mn³⁺.