Oil products and high-performance carbon materials from biomass–plastic co-liquefaction for sodium-ion capacitors

• LDPE/ST co-liquefaction produced oil with highest HHV of 28.4 MJ/kg. • Solid residue-derived activated carbon served as cathode in a supercapacitor. • LDPE/ST-derived carbon showed high surface area (2512 m 2 /g) and microporosity. • LDPE/ST-based cathode gave highest specific capacity (89 mAh/g) in sodium-ion half-cells. • Full cell with LDPE/ST-based cathode delivered 83.3 Wh/kg with stability over 3000 cycles. The increasing accumulation of agricultural residues and plastic waste is widely recognised as a critical waste management challenge, while simultaneously offering opportunities for integrated resource recovery. This study proposes a zero-waste valorisation pathway that converts these waste streams such as cotton gin trash (CGT), sugarcane trash (ST), and rice husk (RH) together with low-density polyethylene (LDPE) into value-added fuels and materials via supercritical ethanol co-liquefaction. This process not only converted the mixed feedstock into energy-dense oil products (yields ∼23–25 wt%) but also transformed the solid residues (∼ 57 wt%) typically considered wastes into high-surface-area activated carbons. When evaluated as cathodes in sodium-ion capacitors (NICs), the ST/LDPE-derived carbon delivered superior electrochemical performance, achieving a high specific energy of 83.3 Wh/kg with excellent cycling stability. Overall, the proposed approach demonstrates an effective strategy for transforming heterogeneous agricultural and plastic wastes into functional precursors for both sustainable fuel production and energy storage materials, within a circular-economy framework.