Homologous carbons prepared by changing the thermal conversion path of lignite as anode and cathode for high-performance full-carbon sodium-ion capacitor

Sodium-ion capacitor (SIC) is a type of energy storage device simultaneously owning high energy and power density, in which carbon materials serve as both anode and cathode. Kinetics and capacity mismatch between anode and cathode limit the performance of SICs because of their structural differences. Here, full-carbon SIC is constructed with homologous carbon-based anode and cathode only by alternating thermal conversion pathway of lignite. By carbonizing lignite under suitable temperature, aromatic fragments in lignite can be developed to short-range graphite-like crystallites via pyrolysis and condensation reactions, producing hard carbon (HC) anode with high reversible capacity (291.4 mA h g⁻¹ at 0.03 A g⁻¹). By introducing KOH into carbonization, intense etching reactions between KOH and carbonaceous skeleton convert lignite to porous activated carbon (AC) with high capacitance (175.5 F g⁻¹ at 0.2 A g⁻¹), which can be used as capacitor-type cathode. Using the prepared homologous HC anode and AC cathode, full-carbon SIC with high energy and power density (172.3 W h kg⁻¹ and 5221.6 W kg⁻¹) as well as long cycling lifespan (86.3 % capacity retention after 4000 cycles) are constructed. This work presents a cost-effective method for preparing high-performance homologous carbon-based anode and cathode of full-carbon SICs using single raw material.