Crystallinity-Engineered Hard Carbon from Upcycled Polyethylene Terephthalate for Enhanced Na Storage

The development of sustainable sodium-ion battery (SIB) anodes is fundamentally constrained by the need to balance three competing requirements: rapid sodium-ion (Na⁺) transport hindered by the large ionic radius of Na⁺, long-term cycling stability, and high energy density in hard carbon materials. In this study, we address this longstanding trilemma through a crystallinity-engineering strategy applied to upcycled polyethylene terephthalate (PET) waste. Ambient H₂SO₄/H₂O₂ pretreatment induces trans-conformational crystallization and incorporates exogenous oxygen atoms, which regulate cross-linking during pyrolysis and direct the formation of a hierarchical porous carbon architecture (HC-OPET). In addition, oxygen doping expands the interlayer spacing to promote localized domain growth, thereby optimizing in-plane charge transport while suppressing graphitization behavior. HC-OPET features expanded pseudographitic domains (d₀₀₂ = 0.41 nm), which lower the Na⁺ intercalation energy barrier via DFT modeling, alongside a hierarchical porous structure. Critically, this architecture combines open mesopores facilitating rapid ionic diffusion with tailored closed micropores serving as stable Na⁺ reservoirs. This synergistic design enables HC-OPET to deliver high reversible capacities of 366 mAh g^-1 (0.1 A g^-1) and 242 mAh g^-1 (2 A g^-1) with excellent cycling stability (76% retention after 2000 cycles at 0.5 A g^-1). By demonstrating the effective chemical conversion of plastic waste into functional energy materials, this work offers a sustainable and scalable route for advancing high-performance SIB anodes.