Uncovering the Salt-Controlled Porosity Regulation in Coal-Derived Hard Carbons for Sodium Energy Storage

Coal is a promising precursor of hard carbon (HC) anodes for sodium-ion batteries (SIBs), by virtue of resource abundance, low cost, and high product yield. However, the concomitant inorganic salt is usually recognized as impurities and plays an obscure and even contradictory effect on the regulation of pore structure in HCs. Herein, a two-step pyrolysis procedure to the representative salty coal is performed, in which the acid washing program is selectively inserted. It is illuminated that salt acts as a template or activating agent for the generation of open pores at low temperatures but inhibits the closure of pores during the following high-temperature carbonization. The optimized HC delivers a reversible capacity of 322.4 mAh g^-1, a high plateau capacity of 192 mAh g^-1, and an initial coulombic efficiency of 80%, outperforming to most coal-based HCs. Assembled with an NVPOF cathode, the full-cell exhibits a high energy density of 284.7 Wh kg^-1. This work not only provides a systematic understanding of salt-dependent pore structure modulation but also practices a simple, cost-effective, and potentially scalable technique for the production of coal-based HCs.