Insight into the Role of Closed-Pore Size on Rate Capability of Hard Carbon for Fast-Charging Sodium-Ion Batteries

The sluggish Na ion diffusion kinetics and Na metal plating in hard carbon negative electrodes restrict the fast-charging of sodium-ion batteries, which is intricately entwined with the crystal structure and pore structure. Here, the pore structures of hard carbon materials are focused on and reveal that the pore size significantly affects the Na-filling potentials during the sodiation process. Specifically, the micro closed pores exhibit higher Na-filling potentials, which reduces risks of Na metal plating at high current densities, thus enabling improved rate performance. As a result, the optimized hard carbon with closed micropores (1.6 nm) achieves an initial capacity exceeding 400 mAh g⁻¹ at 20 mA g⁻¹ and a plateau retention rate of 73.3% at a current density of 500 mA g⁻¹. Paired with P2-type layered oxide positive electrodes, the 2.2 Ah pouch cell shows 10 min charging for ≈90% of the capacity and ≈90% capacity retention after 1500 cycles at a 6C rate. This work establishes a bridge between pore size and rate performance, offering guidance for the design of fast-charging sodium-ion batteries.