Vegetal-Extracted Polyphenols as a Natural Hard Carbon Anode Source for Na-Ion Batteries

Hard carbons are promising anode materials for Na-ion batteries that can be produced using a wide variety of synthetic or natural precursors. This work focuses on the development of hard carbons from natural polyphenols derived from different vegetal extracts. Herein, five natural tannin-based polyphenols (catechu, chestnut, myrobalan, and two mimosa extracts) were used to synthesize hard carbons through a single pyrolysis process at 1500 °C. The precursors lead to a high carbon yield (35–44%) and the obtained hard carbons have disordered structures with a large interlayer spacing (d002 between 3.55 and 3.67 Å) and acertain amount of inorganic compounds (< 6 wt %). N2 and CO2 physisorption assays revealed the presence of a low volume of meso-, micro-, and ultramicropores and very low specific surface areas (SSAs) (N2-SSA < 7 m2·g–1 and CO2-SSA < 24 m2·g–1). The electrochemical performance showed a high initial Coulombic efficiency (iCE > 84%), which reached 100% after a few cycles, as well as good cycling stability. Myrobalan- and mimosa-based hard carbons exhibited reversible capacities of approximately 304 mAh·g–1 when cycled at C/10 (C = 372 mA·g–1), whereas catechu- and chestnut-derived hard carbons exhibited reversible capacities of 280 mAh·g–1, due to the presence of impurities, localized graphitic domains, and slightly lower d002 values. In addition, the electrochemical behavior of myrobalan- and mimosa-based hard carbons is stable at higher current densities (C), while the capacity decreases for the other materials. The best performance was achieved for materials with low impurity levels, more disordered structures, and low specific surface areas (i.e., myrobalan-derived hard carbon).