Upcycling waste thermoplastics into sulfur-doped hard carbon via sulfonation-assisted crosslinking for sodium/potassium-ion storage

The upcycling of waste thermoplastics into high-performance carbon anodes presents a promising solution for next-generation sodium-ion (SIBs) and potassium-ion batteries (PIBs). However, direct valorization is fundamentally restricted by the precursors’ intrinsic fusibility, typically leading to low carbon yields. Addressing this, we propose a universal sulfonation-assisted crosslinking strategy to transform polyethylene, polystyrene, and polyvinyl chloride into sulfur-doped hard carbons. This pretreatment at 180 °C effectively converts fusible linear chains into thermostable crosslinked networks, preserving the carbon skeleton for subsequent pyrolysis. Among the derived carbons, the PVC-derived anode (SPVC-1200) demonstrates favorable electrochemical performance, delivering high reversible capacities of 393.8 mAh g⁻¹ for SIBs and 312.6 mAh g⁻¹ for PIBs. Notably, it exhibits robust long-term durability, maintaining 66% capacity retention after 700 cycles in SIBs. The enhanced storage capability is attributed to a tailored disordered structure derived from the precursor chemistry. Specifically, the in-situ doping of stable thiophene-like sulfur induces a pillaring effect, maximally expanding the interlayer spacing ( d ₀₀₂ = 0.372 nm) to lower the diffusion barriers of bulky Na⁺/K⁺ ions. Simultaneously, the specific dehydrochlorination pathway of PVC fosters a dense, non-porous surface (specific surface area ≈ 1 m² g⁻¹), which effectively suppresses parasitic side reactions and ensures high initial Coulombic efficiency. Furthermore, surface-abundant C=O and S=O functional groups provide extrinsic pseudocapacitive contributions for enhanced rate capability. This work validates a scalable and versatile route for converting mixed plastic waste into functional energy storage materials.