Correlation of Structure and Performance of Hard Carbons as Anodes for Sodium Ion Batteries

Hard carbons are the material of choice as negative electrode in sodium ion batteries. Despite being extensively studied, there is still debate regarding the mechanisms responsible for storage in low- and high-potential regions. This work presents a comprehensive approach to elucidate the involved storage mechanisms when Na ions insert into such disordered structures. Synchrotron X-ray total scattering experiments were performed to access quantitative information on atomic ordering in these materials at the nanoscale. Results prove that hard carbons undergo an atomic rearrangement as the graphene layers cross-link at intermediate temperatures (1200-1600 degrees C), resulting in an increase of the average interplanar distance up to 1400 degrees C, followed by a progressive decrease. This increase correlates with the positive trend in the reversible capacity of biomass-derived carbons when processed up to 1200-1600 degrees C due to an increased capacity at low potential (<= 0.1 V vs Na/Na+). A decrease in achievable sloping capacity with increasing heat-treatment temperature arises from larger crystalline domains and a lower concentration of defects. The observed correlation between structural parameters and electrochemical properties clearly supports that the main storage of Na ions into a hard-carbon structure is based on an adsorption-intercalation mechanism.