The growing accumulation of plastic waste is a serious environmental problem, and converting this waste into useful materials is an attractive solution. In this work, waste polypropylene (PP) was converted into hard carbon anode materials for lithium-ion batteries. When heated, PP decomposes into volatile hydrocarbons and leaves almost no solid residue, so it cannot be carbonized on its own. The waste PP was therefore first treated with concentrated sulfuric acid, which cross-links the polymer chains into a rigid, infusible network. This suppresses volatilization and promotes the formation of a thermally stable carbonaceous residue. The cross-linked material was then carbonized under argon at 700, 800, 900 and 1000 °C, and 22 to 24% of the original PP mass was recovered as carbon powder, depending on the carbonization temperature. X-ray photoelectron spectroscopy showed that the carbon surfaces retain 6.3–7.5 at% oxygen but almost no sulfur (≤0.2 at%), so sulfur is essentially absent from the final carbons. The carbonization temperature influenced the structure of the carbon and its performance in a battery. Among all samples, PP-900 delivered the highest reversible capacity at 0.1 A g⁻¹, reaching 373.1 mAh g⁻¹ at the 100th cycle. The first-cycle Coulombic efficiency was low for all samples (30–47%) and represents the main limitation of these materials. These results show that acid cross-linking followed by carbonization is a feasible route for converting PP waste into carbon anode material, and identify the low first-cycle efficiency and the rate performance at high current density as the principal issues to be addressed in further work.

Polyethylene, the largest component of plastic waste, decomposes on heating almost entirely to volatiles rather than to carbon. Here, one post-consumer low-density polyethylene (LDPE) film and one high-density polyethylene (HDPE) bottle were converted into disordered carbons by sulfonation-assisted carbonization. Sulfonation raised the carbon residue at 900 °C from 1.21 to 32.63 wt.% for LDPE and from 0.06 to 8.82 wt.% for HDPE. The two carbons are indistinguishable by X-ray diffraction and high-resolution transmission electron microscopy (interlayer spacing 0.365–0.366 nm, crystallite thickness ≈1.4 nm), have the same Raman disorder ratio (1.01–1.02) and retain little sulfur (≤0.51 at.%), but differ in specific surface area (53.7 and 32.5 m² g⁻¹). As lithium-ion anodes, the LDPE- and HDPE-derived carbons deliver 338 and 302 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹, with capacity still rising, and 155 and 130 mA h g⁻¹ at 2 A g⁻¹. The larger first-cycle loss and higher capacity of the LDPE-derived carbon correlate with its larger surface area, consistent with the precursor acting through its response to the sulfonation pretreatment rather than through carbonization. These results are a proof of concept; extension to other and mixed waste streams requires further study.