The construction of flexible lignin-based carbon materials is usually impeded by the formation of abounding rigid benzene rings during carbonization. To improve the flexibility, the chemical structure of lignin/polyacrylonitrile (PAN)-based electrospun nanofibers was regulated by tuning cooling process of pre-oxidation. More non-crosslinked branches of lignin with single bonds remained after pre-oxidation, which is beneficial for the formation of flexible lignin/PAN-based carbon nanofibers (CNFs). Assembled as half cells versus sodium metal, the flexible lignin/PAN-based CNFs exhibit a characteristic of hard carbon with a high reversible capacity (207 mA h g(-1) at 0.2 A g(-1)) and a superior cycling life (125% capacity retention after 10,000 cycles at 1.0 A g(-1)), and the activated lignin/PAN-based CNFs display a large specific surface area of 1390 m(2) g(-1) and good rate performance (43 mA h g(-1) at 2.0 A g(-1)), which can be used directly as the anode and cathode of sodium-ion capacitors (SICs), respectively. The assembled SIC shows excellent cycling performance (100% capacity retention after 6,000 cycles at 1.0 A g(-1)) and a good energy density with a good power density (68 W h kg(-1) @ 172 W kg(-1), 40 W h kg(-1) @ 2000 W kg(-1)). The flexible SIC can supply energy under diverse bending conditions and retain 80% of specific capacity after 1,500 cycles under a 180 degrees bending state. This work provides a new sight to prepare flexible lignin-based carbon materials, which widens the road for sustainable application of lignin. [GRAPHICS] .
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