Effect of the Activation Agent on Carbons Derived from Exhausted Olive Pomace as Sulfur Hosts in Sustainable Lithium–Sulfur Batteries

Alpeorujo, a highly abundant byproduct of olive oil extraction, poses serious environmental concerns due to its massive accumulation. In parallel, the urgent global transition toward greener, more efficient, and sustainable energy storage technologies remains a critical challenge. This work addresses both issues simultaneously by pioneering the valorisation of alpeorujo through its transformation into activated carbons (ACs) for application as cathode matrices in lithium–sulfur (Li–S) batteries. A straightforward, one-step calcination route, aligned with green chemistry principles, was employed using KOH, ZnCl2, and H3PO4 as chemical activating agents (AA). The resulting ACs, with a remarkable high carbon content above 85%, exhibited diverse and well-developed porosities and were simply combined with sulfur (AC@S) via mechanical grinding, enabling 70% sulfur infiltration into the porous carbon network. The ACs displayed distinctive textural properties depending on the AA used. Notably, all ACs contained considerable amounts of nitrogen, acting as a self-doped heteroatom that enhances electrochemical functionality without additional treatments. Electrochemical testing revealed excellent performance, particularly for the AC obtained from H3PO4, which delivered specific capacities of 1100 mAh/g. Near-ideal Coulombic efficiency (∼100%) and a significantly lower decay rate (around 0.07%/cycle) were maintained over ultralong-term cycling. Furthermore, all ACs demonstrated strong polysulfide adsorption, with the AC obtained from H3PO4 achieving the best results, thereby confirming its outstanding electrochemical behavior. These results underscore the potential of alpeorujo as a sustainable precursor for high-performance carbon materials, bridging waste valorisation and green energy storage solutions.