Atomically Coupled 2D MnO2/MXene Superlattices for Ultrastable and Fast Aqueous Zinc-Ion Batteries

The delta manganese dioxide (δ-MnO2) has sparked a great deal of scientific research for application as the cathode in aqueous zinc-ion batteries (AZIBs) owing to its characteristic layered structure. However, further development and commercial application of the δ-MnO2 cathode are hindered by the low rate performance and poor cycling stability, which are derived from its inherently poor electrical conductivity and structural instability during the charge/discharge process. Herein, we report the fabrication of the 2D MnO2/MXene superlattice by the solution-phase assembly of unilamellar MnO2 and Ti3C2Tx MXene nanosheets, where the unilamellar MnO2 nanosheet is separated and stabilized between unilamellar MXene nanosheets. The MXene nanosheets can not only serve as structural stabilizers to isolate the MnO2 nanosheets and prevent them from aggregating but also act as conductive contributors to strengthen the electrical conductivity, thus maintaining the overall structural stability and realizing the rapid electron transport. Additionally, the regular stacking with a repeating periodicity of the 2D MnO2/MXene can lead to highly exposed active sites, promoting ion diffusion. As a consequence, the large specific capacity of 315.1 mAh g-1 at 0.2 A g-1, prominent rate performance of 149.8 mAh g-1 at 5 A g-1, and excellent long-term cycling stability after 5000 cycles with 88.1% capacity retention are obtained for the MnO2/MXene cathode in AZIBs. Meanwhile, the superior H+/Zn2+ diffusion kinetics and desirable pseudocapacitive behaviors are elucidated by electrochemical measurements and density functional theory computations. This study provides an advanced perspective for the innovation of manganese oxide-based cathode materials in AZIBs.