Deciphering Anomalous Zinc Ions Storage in Intermediate State MnO2 of Layer-to-Tunnel Transition

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