Converting Environmental Waste into High‑Value Carbon Electrodes for Sodium‑Ion and Li–S Batteries

This thesis, carried out in collaboration among four institutions (the Italian Institute of Technology, the University of Genoa, Ca’ Foscari University of Venice, and Hasselt University), addresses sustainable secondary battery materials by focusing on the conversion of a waste biomass source, Sargassum, into functional carbon through controlled carbonization treatments within a waste‑to‑energy approach. Because conventional battery components and manufacturing practices can contribute to environmental burdens, the development of lower‑impact material choices and processing routes remains necessary. In this work, the sustainability problem is addressed through the use of a single waste biomass source, Sargassum, and a straightforward processing route consisting of pyrolysis followed by physical activation to produce electrode-grade carbon for electrochemical storage applications. The Sargassum‑derived carbon was engineered using CO₂ activation to develop the required porosity and enhanced interplanar distance without relying on hazardous activating agents. The electrochemical behavior of the resulting carbon was first investigated as an anode material for sodium‑ion batteries (SIBs) in half‑cell configuration. Subsequently, the same Sargassum‑derived carbon was also evaluated in a second application as a sulfur host in lithium–sulfur (Li–S) cathodes, where the carbon framework is expected to support electronic transport and contribute to sulfur utilization. Overall, the thesis provides a coherent assessment of a single biomass‑derived carbon material across two battery chemistries, and the results are discussed in the context of relevant literature to support the present work. High performance hard carbon was successfully obtained from biomass Sargassum through a simple one‑step pyrolysis at 700 °C in an inert atmosphere, followed by activation at 700 °C under the same conditions, a route that is relatively simple and industrially applicable. From a sustainability perspective, the use of acid pre‑ and post‑treatments is, in principle, beneficial for both economic and environmental sustainability. By investigating the structural and morphological behavior of the active material, ABC displays suitable interlayer spacing and a good surface area, enabling insertion together with interfacial adsorption of sodium. When used as an anode for SIBs, ABC shows good electrochemical performance, delivering a specific capacity of 137 mAh g⁻¹ at 30 mA g⁻¹ with a remarkable 78% ICE and promising capacity retention; these performances are superior or comparable to other hard carbons reported in the literature. The electrode also exhibits fast reaction kinetics, while interfacial stability arises from the cumulative contribution of graphitic domains, defects, and surface pores. According to the experimental observations, surface‑controlled reactions dominate sodium storage in the sloping region, whereas diffusion‑controlled intercalation becomes predominant on approaching the low‑voltage plateau. Taken together, these results represent a relevant contribution to the in‑depth understanding and optimization of sodium‑storage mechanisms in carbonaceous hosts and to the development of high performing, sustainable anodes for SIBs, while also indicating that further optimizations, such as improvement of the ICE of hard carbon, are still needed for practical applications. A simple route to synthesize a sulfur–activated biochar (ABC20S80) electrode material for Li-S batteries were also explored. The solvent evaporation method used here leads elemental sulfur directly into the ABC matrix, and the same procedure allows easy control of the sulfur to carbon ratio. The resulting sulfur electrodes, tested in lithium metal half cells, show stable long-term cycling. At C/10, the cathode delivered an initial specific capacity of about 982 mAh g⁻¹ and retained around 624 mAh g⁻¹ after 124 cycles, while at 1C the initial specific capacity was about 567 mAh g⁻¹ and decreased to around 150 mAh g⁻¹ at the 95th cycle. The linear average capacity fade per cycle was about 0.30% per cycle at C/10 and 0.35% per cycle at 1C, indicating better capacity retention at the lower C-rate. This performance is linked to morphology, particularly the pores and graphitic domains of turbostratic structure, which physically confine soluble polysulfides and improve the electrode’s conductivity. Given the straightforward preparation and tunable composition, the ABC20S80 composite is a practical cathode choice for Li–S batteries.