Towards optimized Li-ion storage performance: Insight on the oxygen species evolution of hard carbon by H₂ reduction

With high capacity and good rate performance, hard carbon is a promising anode material for lithium ion batteries. However, they are suffering from low coulombic efficiency and poor cycling stability, due to the residue oxygen functional groups in the as-prepared material. Thus, the mild thermal reduction in H₂ atmosphere is employed to further remove the oxygen while maintaining the pristine morphology. However, the chemical evolution of oxygen species in the reduction process, as well as their effect on the lithium ion storage performance is still unclear. In this contribution, the structural evolution of oxygen functional groups from 300 to 700 °C is studied by XPS and TPR-MS, and the mechanism is further proposed combining with DFT calculation. The electrochemical results verify that the adsorption-desorption between Li-ions and quinone/C–O groups is irreversible while the behavior of C[dbnd]O is reversible, which has great effect on the specific capacity, columbic efficiency and cycling stability of LIB. This work will provide chemical fundamental for designing high-performance hard carbon anode materials for advanced batteries.