Nickel-doped δ-MnO2 abundant in oxygen vacancies as a cathode material for aqueous Zn-ion batteries with superior performance

Manganese oxide is widely considered as a prominent cathode material for Zn-ion batteries (ZIBs) because of its merits including low cost, high voltage, non-toxicity and excellent stability. However, its sluggish reaction kinetics and structural instability limit its practical application in ZIBs. In this paper, a one-step method for preparing nickel-doped δ-MnO₂ (Ni-δ-MnO₂) at room temperature is introduced. This method not only addresses and surpasses the intrinsic deficiencies of δ-MnO₂, but also reduces the preparation cost and energy consumption of heteroatom-doped δ-MnO₂. The introduction of Ni by doping modified the crystallinity of δ-MnO₂ and introduced extra oxygen vacancies, leading to enhanced conductivity, an expanded specific surface area, and a greater total pore volume. These changes provide additional electroactive sites for Zn²⁺/H⁺ ion storage and facilitate smoother ion insertion/extraction processes, leading to remarkable electrochemical behavior. The Ni-δ-MnO₂ cathode demonstrates a significant specific capacity of 401.6 mA h g^-1 (0.1 A g^-1), achieves a high energy density of up to 540.2 W h kg^-1 (136.0 W kg^-1), and exhibits excellent cyclability with a capacity retention as high as 75.5% after 1000 cycles. The mechanism of charge storage in the Ni-δ-MnO₂ cathode with H⁺/Zn²⁺ co-intercalation/deintercalation is elucidated by ex situ characterization studies. The facile preparation and superior performance offer a potential avenue for mass production of high-performance ZIB cathode materials.