Construction of the hierarchical porous biochar with an ultrahigh specific surface area for application in high-performance lithium-ion capacitor cathode

Biochar with a highly accessible specific surface area can display a higher performance when it is used as the cathode of lithium-ion capacitors. Facing the complex composition and diversity of biomass precursors, there is a lack of a universally applicable method to construct hierarchical porous biochar controllably. In this work, a multi-stage activation strategy combining the feature of different activation methods is proposed for this target. To confirm the porous characteristic in prepared samples, N adsorption–desorption and transmission electron microscope were used. As the optimal sample, BC-P3K4S had the highest specific surface area of 3583.3 m g. Evaluated as the electrode for a lithium-ion capacitor, BC-P3K4S displayed a capacity of 139.1 mAh g at 0.1 A g. After coupling it with pre-lithiated hard carbon, the full device exhibited a high energy density of 129.3 W h kg at 153 W kg. The work outlined herein offers some insights into the preparation of hierarchical porous biochar from complex biomass by multistep activation method. Graphical Abstract: [Figure not available: see fulltext.].