Chemical Regulation of Carbonization Enables Structure-Tailored Hard Carbon Anodes from Recycled Polypropylene Separators

Abstract

The high-value recycling of spent lithium-ion battery separators and the rational design of hard carbon anodes are critical for sustainable sodium-ion batteries. Herein, a sulfonation-induced crosslinking strategy is proposed to regulate the carbonization behavior of recycled polypropylene (PP) separators, enabling their direct conversion into structure-tailored hard carbon via a one-step carbonization process. Sulfonation not only introduces sulfonic functionalities but, more importantly, induces intermolecular crosslinking, which suppresses severe chain scission and volatilization during thermal treatment and transforms the decomposition pathway into a solid-state carbonization process. As a result, the structure-tailored HC is constructed by chemically regulating the carbonization behavior of recycled PP separators, enabling efficient sodium storage with clarified structure–sodium storage correlations. When applied as anodes for SIBs, the PP-derived HC exhibits high reversible capacity of 293.0 mAh g-1 at 0.2 C and superior rate capability of 77.1 mAh g-1 at 10 C. For long-term cyclic performance, the capacity maintained at 222.7 mAh g-1 after 1000 cycles at 1 C with a capacity retention of 89.1%. When coupled with Na3V2(PO4)3 cathode, the full cell can deliver a capacity of 83.0 mAh g-1 after 200 cycles with 80.1% retention. This work demonstrates that chemical regulation of the carbonization pathway provides an effective route for both high-value separator recycling and structure-oriented hard carbon design for sodium-ion batteries.