Layered manganese dioxide nanoflowers with Cu²⁺and Bi³⁺ intercalation as high-performance cathode for aqueous...

For aqueous zinc ion batteries (AZIBs), birnessite MnO₂ (δ-MnO₂) has been intensively used as one of the most potential cathode materials due to its layered structure, which is conducive to reversible insertion/extraction of zinc ions. However, δ-MnO₂ has not been attained for zinc ion storage performance because of its inferior conductivity as well as the undesirable structural degradation upon charge/discharge cycling. Herein, we have designed two kinds of cathode materials of Cu_0.06MnO₂·1.7H₂O (CuMO) and Bi_0.09MnO₂·1.5H₂O (BiMO) with nanoflower structure for the first time by a facile one-step hydrothermal method, which will be applied for high-performance AZIBs.The pre-intercalated metal ions and water molecules serve as pillars to sustain the layered structures, improving the stability of these materials. Particularly, the CuMO may experience a replacement reaction except the zinc ion insertion/extraction to form metallic Cu during the cycling process, which can enhance the diffusion rate of Zn²⁺, thus resulting in an excellent electronic conductivity and exhibiting remarkable specific capacities. Furthermore, a pseudo-capacitance that is derived from the surface-adsorbed Cu²⁺and Bi³⁺ also contributes to the improved electrochemical performances. The reversible capacity of CuMO is estimated as 350 mAh g^-1 at 0.5 A g^-1, which is much higher than that of pure δ-MnO₂ (190 mAh g^-1 at 0.5 A g^-1). However, BiMO demonstrates long-term cycling stability, maintaining a capacity of 114.5 mAh g^-1 even after 1100 charged-discharged cycles at 1 A g^-1. The capacity retention is found to be as high as 98.6%, which is much higher than that of pure δ-MnO₂ (53.8%). This can contribute to the development of high-performance AZIBs and the application of metal ion pre-intercalation methods in other areas.