Al-doped α-MnO 2 coated by lignin for high-performance rechargeable aqueous zinc-ion batteries.

Zn/MnO 2 batteries, one of the most widely studied rechargeable aqueous zinc-ion batteries, suffer from poor cyclability because the structure of MnO 2 is labile with cycling. Herein, the structural stability of α-MnO 2 is enhanced by simultaneous Al 3+ doping and lignin coating during the formation of α-MnO 2 crystals in a hydrothermal process. Al 3+ enters the [MnO 6 ] octahedron accompanied by producing oxygen vacancies, and lignin further stabilizes the doped Al 3+ via strong interaction in the prepared material, Al-doped α-MnO 2 coated by lignin (L + Al@α-MnO 2 ). Meanwhile, the conductivity of L + Al@α-MnO 2 improves due to Al 3+ doping, and the surface area of L + Al@α-MnO 2 increases because of the production of nanorod structures after Al 3+ doping and lignin coating. Compared with the reference α-MnO 2 cathode, the L + Al@α-MnO 2 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.