Impact of biomass inorganic impurities on hard carbon properties and performance in Na-ion batteries

Biomass waste recently emerged as efficient precursors for hard carbon anode preparation for Na-ion batteries. Despite their very complex microstructure (organic/inorganic) there is a lack of knowledge about their impact on carbon formation. In this paper, the influence of inorganic impurities of three raw local biomass wastes (asparagus, grape and potato) on the hard carbon properties and on their electrochemical performance is investigated, by performing a washing step either before or after the thermal treatment (TT) at 1300 °C. When washing was done after the TT (with HCl), most of crystalline inorganic impurities (K, Ca, Si, Mg - based compounds) could be significantly removed. This triggered the increase of ultramicroporosity along with mesoporosity formation and graphite interlayer space (d002) contraction. Such observations are less pronounced on grape derived carbon due to the inorganic's catalytic induced local graphitization during pyrolysis. The oxygen content in the pristine carbons was high, owing to the presence of inorganic metal oxides and carbonates, and could be diminished after washing along with the amount of defects. Thus, the carbon content and the electronic conductivity of the materials were enhanced. The electrochemical performance improvement after washing was limited since the positive effect brought by impurities removal was negatively compensated by the changes occurring in the materials, particularly the increase in the specific surface area. Diffrently, the washing done before the TT (with water) induced only fewer changes on the materials porosity and structure and slightly improved capacity (from 215 to 230 mAh g−1). Furthermore, higher pyrolysis temperature (1400 °C) on washed HCs afforded a better reversible capacity up to 280 mAh g−1. This comprehensive study opens the door for green and mild synthesis approach to be further explored for sustainable fabrication of hard carbon for Na-ion batteries.