Mild pretreatment synthesis of coal-based phosphorus-doped hard carbon with extended plateau capacity as anodes for sodium-ion batteries

Hard carbon as one of the most promising anodes for sodium-ion batteries (SIBs) has received much attention in terms of its high low-voltage plateau capacity. Precursors play an important role in synthesizing hard carbon with low cost and high performance. It is interesting to convert industrial coal to hard carbon with a high yield for energy storage owing to environmental and energy concerns. However, some small molecular organics and inorganics impurities in coal seriously affect the properties of hard carbon by the direct pyrolysis method. Herein, coal-based phosphorus-doped hard carbon is synthesized through a mild pretreatment method of using NaNO₃ and H₃PO₄. This mild pretreatment can not only purify the coal to obtain the high reversible capacity of 284.4 mAh g⁻¹, but also achieve homogeneous phosphorus doping simultaneously. Interestingly, the low-voltage plateau capacity increases during continuous cycling, while the high-voltage sloping capacity does not change obviously, due to the enlargement of the interlayer spacing induced by continuous electrochemical activation of P-doped micron-sized hard carbon material could provide more Na⁺ intercalation sites. The proportion of plateau capacity of this coal-based hard carbon increases to 54.31% after 500 cycles at 100 mA g⁻¹, which is 13% larger than that of unpretreated coal-based hard carbon.