Al-doped α-MnO₂ coated by lignin for high-performance rechargeable aqueous zinc-ion batteries

Zn/MnO₂ batteries, one of the most widely studied rechargeable aqueous zinc-ion batteries, suffer from poor cyclability because the structure of MnO₂ is labile with cycling. Herein, the structural stability of α-MnO₂ is enhanced by simultaneous Al³⁺ doping and lignin coating during the formation of α-MnO₂ crystals in a hydrothermal process. Al³⁺ enters the [MnO₆] octahedron accompanied by producing oxygen vacancies, and lignin further stabilizes the doped Al³⁺ via strong interaction in the prepared material, Al-doped α-MnO₂ coated by lignin (L + Al@α-MnO₂). Meanwhile, the conductivity of L + Al@α-MnO₂ improves due to Al³⁺ doping, and the surface area of L + Al@α-MnO₂ increases because of the production of nanorod structures after Al³⁺ doping and lignin coating. Compared with the reference α-MnO₂ cathode, the L + Al@α-MnO₂ cathode achieves superior performance with durably high reversible capacity (∼180 mA h g^-1 at 1.5 A g^-1) and good cycle stability. In addition, ex situ X-ray diffraction characterization of the cathode at different voltages in the first cycle is employed to study the related mechanism on improving battery performance. This study may provide ideas of designing advanced cathode materials for other aqueous metal-ion batteries.