Biochars from various biomass types as precursors for hard carbon anodes in sodium-ion batteries

Last years have shown an explosion of interest about biochars. Until now, the applications targeted have mostly been in the fields of energy or agronomy. But high-added value applications in the field of electrochemistry, such as hard carbon precursor in anodes for sodium ion batteries, become more and more mentioned as promising uses to investigate. The present study aims at evaluating the feasibility of using biochars from various biomass types for this application and drawing links between biomass properties and the resulting hard carbon electrochemical performance. To achieve this goal, hard carbons were obtained by high-temperature slow pyrolysis of four woody and agricultural biomasses. These hard carbons were prepared as anodes, characterized regarding structural and textural properties and finally tested during successive cycles to assess electrode performances and correlate them with biomass composition. Whatever biomass, the hard carbon could be used in cells and thus biomass appears as a promising feedstock for this application. However, there are apparent differences between biomasses concerning coulombic efficiency and to some extent, cycle stability. Hence resinous wood appears to be the most suitable precursor, while wheat straw would be the least one, probably because of its too high surface area or inorganic element content. Based on these observations, hypotheses were suggested about the influence of lignin and hemicelluloses composition as well as of inorganic elements. To our knowledge, this study is pioneering in proposing a systematic study approach to understand the influence of different biomass properties in a hard carbon electrode for sodium-ion batteries.