Efficient upcycling of polypropylene-based waste disposable masks into hard carbons for anodes in sodium ion batteries

Abstract We demonstrate the conversion of disposable polypropylene (PP) masks into non-graphitizable carbon powders that can be applied as anode materials in sodium ion batteries. Sulfuric acid treatment and subsequent pyrolysis of the masks resulted in polyaromatic and carbon structures, respectively. Fourier-transform infrared, Raman, and X-ray photoelectron spectroscopies revealed that a longer sulfuric acid treatment time results in a higher carbon yield (up to 50%), indicating that the infusible structures generated during sulfonation played a critical role in the development of the resulting carbon. In addition, we confirmed the detailed mechanism by NMR analysis, which indicated that sulfonation induced not only simple cross-linking but also polyaromatic hydrocarbons, contributing to distinct D and G bands in the Raman spectra. However, even heat-treatment at a high temperature of 2400 °C could not facilitate a graphitic structure, implying that PP is intrinsically non-graphitizable. Finally, we used mask-derived carbon as an anode material of sodium ion batteries. The prepared hard carbon anode showed a high reversible capacity of ∼340 mA h/g at a current rate of 0.01 A/g, and ∼53% of the capacity was maintained at 100 times higher current rate, suggesting the superior rate capability. In addition, the assembled full cell achieved a reversible capacity of ∼110 mA h/g with a high energy density of ∼352 Wh/kg, validating the feasibility of its application as an anode material of sodium ion batteries. The solid-to-solid conversion of PP-based masks to carbons could contribute to the upcycling technology as one of the potentially affordable waste plastic management techniques.