Layer symmetry and interlayer engineering of birnessites towards high-performance rechargeable aqueous Zn-MnO2 batteries

Rechargeable aqueous Zn-MnO2 batteries are promising candidates for large-scale energy storage systems, yet still plagued by the phase transition and structural collapse issues of MnO2 cathodes during cycling. Interlayer intercalation for the layered MnO2 turns out to be a viable alternative and become the mainstream structure design strategy. However, the characteristics of Mn octahedral layers are generally neglected. Herein, for the first time we elucidate apart from interlayer ions, how layer symmetry of birnessites exerts on the electrochemical performance. The Mn(II) ions stabilized hexagonal birnessite exhibits elevated charge storage performance than its monoclinic precursor, attributing to the layer cation vacancies generated after symmetry transformation, interlayer Mn(II) ions and nanosized morphology. A high specific capacity of 279 mAh g-1 at 1 C is achieved, as well as an outstanding long-term cycling stability with 97% retention over 8000 cycles. The reaction mechanism is comprehensively illustrated. This work previews a new gateway for the design of high-performance layered cathode materials by synergistic manipulation of the crystal structure of the layers and the interlayer environment.