Extended flat voltage profile of hard carbon synthesized using a two-step carbonization approach as an anode in sodium ion batteries

Hard carbon is a promising anode material for sodium ion batteries (NIBs). In this study, a two-step carbonization approach is developed to enhance the electrochemical performance of lignocellulose biomass-derived hard carbon. The first step comprises slow low-temperature pyrolysis of fir wood that produces an amorphous carbon in which hexagonal planes are embedded in the amorphous carbon region to some extent. The second step comprises high-temperature carbonization at 1300 °C, which yields a hard carbon with a high degree of graphitization, an increased layer-plane length, and a low micropore volume. Two-step carbonized hard carbon delivers a large reversible capacity of 276 mAh g−1 at 50 mA g−1 after 100 cycles and high rate capacities of 108 mAh g−1 at 1.0 A g−1 and 76.3 mAh g−1 at 2.5 A g−1. The low-voltage plateau capacity below 0.1 V is 194 mAh g−1. The results of these experiments indicate that the exceptional electrochemical performance of two-step carbonized hard carbon arises from the effective suppression of micropore formation and a good balance between the degree of graphitization and number of defect sites. High-voltage adsorption of Na+ ions in micropores inhibits Na+-ion diffusion into the graphitic region of micropore-enriched hard carbon.