Thermal shock reducing amorphous carbon ratio in hard carbon for improved rate capability of sodium ion storage

Coal stands poised to emerge as the primary material for sodium-ion battery anode fabrication, owing to its cost-effectiveness and carbon content. Unlike other feedstocks with well-defined structural formulae, coal-based carbon framework commonly presents a mixed state of high-graphitized crystalline and amorphous carbon structure, greatly hindering sodium-ion rapid transport. To overcome the rate bottleneck of coal-based carbon, herein, the thermal conversion pathway of coal is altered by rapid switching of heating and cooling states, thereby reducing amorphous carbon and high-graphitized crystalline in the obtained carbon structure and endowing sodium-ion storage with improved rate capability. Mechanistically, thermal shock accelerates amorphous carbon depolymerization, accompanied by rapid release of gas-phase products, thereby rearranging crystalline growth and promoting pore connectivity. The obtained carbon with optimized crystalline distribution and porosity connectivity alleviates the diffusion resistance of sodium-ions in hard carbon, enabling a greatly improved rate capability (achieving 161 mAh g−1 at a high rate of 2.0 C). Moreover, the fabricated full-cell exhibits an energy density of 236 Wh kg−1 comparable to commercial hard carbon systems, while reducing energy consumption for anode manufacturing by approximately 80 %. This work lays groundwork for regulating amorphous carbon in coal-based anodes and provides an energy-saving production strategy for high-performance hard carbons.