Breakage of the dense structure of coal precursors increases the plateau capacity of hard carbon for sodium storage†

Abstract

Hard carbon is considered the most commercially viable anode material for sodium ion batteries due to its excellent sodium storage properties. However, the production cost of hard carbon is high, so optimizing the electrochemical performance of coal-derived hard carbon is adopted. However, due to the dense structure of coal, it is difficult to prepare closed pores inside the coal-derived hard carbon, which is not conducive to increasing capacity. Therefore, we propose Zn₂(OH)₂CO₃ assisted ball milling pretreatment followed by carbonization to generate closed pores in coal-derived hard carbon. The reason for the formation of closed pores is that the uniform pores on the coal surface generated by the wear and etching of Zn₂(OH)₂CO₃ are repaired at high temperatures. Via mechanism characterization, we verified that the plateau capacity is related to the filling of sodium ions in closed pores. Therefore, the as-prepared coal-derived hard carbon delivers a high capacity of 325.3 mA h g⁻¹ (plateau capacity accounting for 45.1%) at a current density of 0.03 A g⁻¹ with a capacity retention rate of 83.5% over 500 cycles. This work has demonstrated that reasonable pore design is an effective strategy to improve the electrochemical sodium storage performance of coal-derived hard carbon, providing an effective approach for the high value-added utilization of coal.