Altering Thermal Transformation Pathway to Create Closed Pores in Coal-Derived Hard Carbon and Boosting of Na⁺ Plateau Storage for High-Performance Sodium-Ion Battery and Sodium-Ion Capacitor

Coal features low-cost and high carbon yield and is considered as a promising precursor for carbon anode of sodium-ion batteries (SIBs) and sodium-ion capacitors (SICs). Regulation of microcrystalline state and pore configuration of coal structure during thermal transformation is key to boost Na⁺ storage behavior. Herein, a facile strategy is reported to create abundant closed pores in anthracite-derived carbon that greatly improves Na⁺ plateau storage. An altered thermal transformation pathway of chemical activation followed by high-temperature carbonization is adopted to achieve the conversion of open nanopores and ordered carbon crystallite into closed pores surrounded by short-range carbon structures. The optimized sample delivers a large reversible capacity of 308 mAh g^–1 that is dominantly contributed by the low-voltage plateau process. Experimental results reveal the enhanced pore-filling mechanism in the developed closed pores. Benefitting from the improved plateau behavior, the constructed SIB delivers a high-energy density of 231.2 Wh kg^–1 with an average voltage of 3.22 V; the assembled full-carbon SIC shows high energy and power densities (101.8 Wh kg^–1 and 2.9 kW kg^–1). This work provides a universal thermal transformation approach for designing high-performance carbon anode with closed porosity from low-cost and highly aromatic precursors.