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A facile one-step pyrolysis and activation synthesis method is utilized to convert a common biomass of willow catkin into interconnected porous carbon nanosheets (PCNs), and then followed by effective nitrogen and sulfur co-doping. Owing to the unique hollow and multilayered structure of willow catkin fiber, the pore structure of obtained carbons can be controlled by adjusting the mass ratio of raw material to alkali. As a result, the nitrogen and sulfur co-doped PCNs demonstrate a high specific capacitance of 298 F g⁻¹ at 0.5 A g⁻¹ and 233 F g⁻¹ at 50 A g⁻¹, revealing excellent rate performance. In addition, the electrode demonstrates superb cycling stability with only 2% capacitance loss after 10,000 cycles. Furthermore, the assembled symmetric cell with a wide voltage range of 1.8 V yields a remarkable specific energy of 21.0 Wh kg⁻¹ at 180 W kg⁻¹. These exciting results exhibit a green and low-cost design of …