Defective Hard Carbon Anode for Na-Ion Batteries

Hard carbon as an anode is critical for the near-future commercialization of Na-ion batteries. However, where Na ions are located at different states of charge with respect to the local structures of hard carbon remains a topic that is under debate. Recently, some groups, including ours, have suggested a structure–property correlation that assigns the slope capacity in galvanostatic charge/discharge curves to the binding of Na ions to structural defects of hard carbon. To test this correlation, herein, we prepared a highly defective hard carbon by microwaving a carbon that was obtained by pyrolysis of cellulose at 650 °C. After this microwave treatment for just 6 s, the reversible capacity of the hard carbon increased from 204 to 308 mAh/g, which is significantly higher than that of hard carbon annealed at 1100 °C for 7 h (274 mAh/g). The microwave treatment not only is energy-efficient but also retains a high extent of the structural vacancies in hard carbon, as demonstrated by neutron total scattering and the associated pair distribution function results. Indeed, such a defective structure exhibits a slope capacity much higher than that of the conventional hard carbon. This work serves as one of the first examples of rationally designed hard carbon guided by the new Na-ion storage mechanism. Furthermore, microwave heating represents a promising strategy for fine-tuning the structures of hard carbon for Na-ion batteries.