Hard carbons derived from green phenolic resins for Na-ion batteries

Hard carbons have become recently one of the most promising classes of anode materials for sodium ion batteries (NIBs) owing to their high specific capacity and good cycling stability. Among the precursors used to prepare hard carbon, phenolic resins are of great interest due to their high carbon yield, however, their toxicity must be overcome. In this paper, we propose a green, simple and scalable procedure to obtain phenolic resins which by pyrolysis at high temperature (>1000 °C) result in eco-friendly hard carbons with low surface area, disordered structure and high carbon yield. The influence of several synthesis parameters (type of solvent, thermopolymerization/annealing temperature and gas flow) was studied to determine the impact on both phenolic resin and hard carbon characteristics. The synthesis time (12 h-3 days) was found to depend on the used solvent whereas the carbon yield (25–35%) on the cross-linking degree which could be controlled by adjusting both thermopolymerization temperature and atmosphere. The structure of the hard carbons mainly changed with the carbonization temperature (1100–1700 °C) while the texture of the material was sensitive to most of the studied parameters. Stable reversible capacity up to 270 mAhg−1 and 100% coulombic efficiency (CE) after few cycles are obtained, demonstrating the potential for Na-ion applications.