Impact of the biomass precursor composition in the hard carbon properties and performance for application in a Na-ion battery

Biobased hard carbon is gaining attention as anode for Na-ion batteries. However, biomass composition influence on hard carbon properties and performance is rarely addressed. Here a systematic study was led on 25 biomass precursors. Woody and agricultural samples with high lignin contents (>25 weight-moisture-free, wmf%) exhibited promising yields (>20 wmf%). Samples with low to moderate ash content (<5 wmf%) delivered hard carbons with high C purity, the turbostratic structure required, and therefore reversible capacities up to 314 mAh/g. These materials exhibited low N2 (<14 m2/g) and CO2 (<66 m2/g) specific surface area (SSA), thus low irreversible loss, with initial coulombic efficiencies (ICE) up to 87%. Grass samples presented higher polysaccharides (>70 wmf%) and extractives (25 wmf%) contents, leading to lower hard carbon yields, ultramicroporosity formation and the highest CO2 SSA (> 199 m2/g). Most grass samples had high ash contents (6–15 wmf%), rich in Si and Ca. SiC whiskers were observed over hard carbon surface, responsible for high N2 SSA (20–97 m2/g), and consequently, lower ICE (<74%). Localized graphitic domains were identified originating from Ca and Si catalytic effect for graphitization. Limited turbostratic domains and C purity in these samples induced low reversible capacities (<254 mAh/g).