Nonignorable Influence of Oxygen in Hard Carbon for Sodium Ion Storage

As an effective method, heteroatom doping is widely used to improve the electrochemical performance of carbon materials. However, the influence of oxygen-containing functional groups in carbon materials is often neglected. Therefore, we use buckwheat hulls as the precursor to prepare oxygen-doped hard carbon by simple carbonization. The buckwheat hull at a pyrolysis temperature of 1100 °C has the highest reversible capacity of 400 mA h g^-1 at 50 mA g^-1, and the capacity can maintain 96% of the initial capacity after 3000 cycles at 2A g^-1. These results confirm that the natural pore structure and proper interlayer spacing of the BPC-1100 contribute to the transport and insertion of sodium ions. In addition, the first principle proves that the role of oxygen atoms cannot be ignored in the storage of sodium ions. In particular, the improvement of the C-O bond is helpful to improve the adsorption capacity of hard carbon to sodium ions and enhance the reversible capacitance.