Construction of flexible lignin/polyacrylonitrile-based carbon nanofibers for dual-carbon sodium-ion capacitors

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 m2 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° bending state. This work provides a new sight to prepare flexible lignin-based carbon materials, which widens the road for sustainable application of lignin.