Biochar Synthesized from Post-Consumer Coffee Waste Using Molten Salts for Sodium-Ion Battery Applications

Biochars derived from post-consumer coffee residues were synthesized using NaCl and NaHCO3 as impregnation agents, which were pyrolyzed at 500 and 1000 °C. Structural characterization revealed that NaHCO3 treatment at 1000 °C generated a highly interconnected porous network, with a surface area of 1353.22 m2 g−1, pore volume of 0.83 cm3 g−1, and average pore size of 2.6 nm. These features, confirmed by nitrogen physisorption and SEM, favor Na+ accessibility and insertion. XRD and Raman analyses indicated a predominantly amorphous carbon, with graphitic domains and an interplanar distance of ≈0.34 nm, providing both adsorption capacity and electrical conductivity. Electrochemical evaluation showed that BCNaHCO3-1000°C achieved an initial capacity of 34 mAh g−1, stable for more than 15 cycles, outperforming NaCl-treated biochars. However, despite the favorable morphology, the high surface area may also promote side reactions and irreversible capacity loss, limiting overall efficiency. These findings demonstrate the feasibility of valorizing coffee waste into carbonaceous materials for sodium-ion battery anodes, while highlighting the need for further optimization of porosity, graphitization, and compositional modifications to enhance energy storage performance.