High energy density asymmetric supercapacitors enabled by La-induced defective MnO2 and biomass-derived activated carbon

•La-doping enhances MnO2 increases redox active surface area.•La-doped MnO2 and empty palm bunch-derived AC achieved high capacitance in a three-electrode system.•Asymmetric supercapacitor showed outstanding energy density and cycle stability. This study reports a high energy density asymmetric supercapacitor enabled by La-induced defective MnO2 and biomass-derived activated carbon. Our findings reveal that the incorporation of La ions enhances the conductivity of α-MnO2, resulting from a significant increase in lattice defects that provide a greater redox active surface area. In the three-electrode system, the synthesized La-doped MnO2 and activated carbon from empty palm bunches (AC-EPB) achieved a maximum capacity of 202 F g−1 and 178 F g−1 at 0.5 A g−1, respectively. In the assembled La0.2Mn0.8O2//AC-EPB asymmetric supercapacitor, we achieved an outstanding energy density of 32.8 Wh Kg−1 at a power density of 4500 Wh Kg−1 while maintaining 97.2% of initial capacity after 5,000 cycles. This research provides new insights into the preparation of defective pseudocapacitive materials by rare-earth metal for energy storage applications.