Microcrystalline regulation of bituminous coal derived hard carbon by pre-oxidation strategy for improved sodium-ion storage

Middle metamorphic coal has the advantages of high earth abundance and easily-adjusted aromatic units and thus is a feasible raw material for the development of functional carbon materials. For the preparation of high-performance Na⁺ storage hard carbon, the key is to inhibit the long-range graphitization of internal microcrystalline in coal structure during thermal conversion process. Herein, we introduce a simple gas/liquid-phase pre-oxidation procedure to graft oxygen-containing cross bonds into bituminous coal, by which the growth of long-range graphitization process during high-temperature carbonization can be suppressed and thereby enable the preparation of hard carbon with short-range microcrystalline distribution and enlarged interlayer distance of 0.38 ∼ 0.385 nm. Benefiting the regulated microcrystalline environment, the fabricated hard carbon anode delivers a high reversible capacity of 274.2 mAh g⁻¹, which is 24 % higher than the carbon anode prepared without pre-oxidation treatment. Electrochemical kinetics analyses further reveal that the improved Na⁺ storage capacity mainly stems from the enhanced Na⁺ diffusion and intercalation into microcrystalline interlayers at the low voltage region, which is of significance for the construction of high energy–density full cell for practical applications.