Controllable phase transition of Al3+ induced manganese dioxide from β- to δ-type and their Al-doped δ-type manganese dioxide for high-performance asymmetric supercapacitors

• Simple hydrothermal synthesis achieves controllable phase transition of MnO2 from β to δ via Al3+ doping. • Al3+ doping reduces crystallinity and increases specific surface area, providing more active sites for electrochemical reactions. • The optimized MnO2-3 electrode delivers high specific capacitance and rate performance; an asymmetric supercapacitor reaches 18.4 W·h/kg energy density and retains 90% capacitance after 20,000 cycles. • The study provides practical guidance for designing high-performance, low-cost MnO2-based supercapacitor electrodes.