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

MnO 2 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 MnO 2 have been adopted, none of them completely meet practical demands owing to structural collapse during cycling. Herein, intermediate-state MnO 2 (IS-MnO 2 ) 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 MnO 2 . Systemic structural analysis reveals the presence of abundant two-phase transition regions within IS-MnO 2 , which results in a distorted lattice and deformed [MnO 6 ] octahedron unit within the two-phase transition region as well as a reduced average valence state of Mn ions. The deformation of [MnO 6 ] 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 Mn 3+ 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 Mn 3+ . A novel intermediate-state MnO 2 structure formed during the phase transition from a layered to a tunnel structure reduces the symmetry of [MnO 6 ] octahedral units, eliminates ground-state degeneracy and suppresses the Jahn-Teller effect.