Synergistic effects of porosity and sulfur doping on hard carbon for superior sodium-ion storage

Hard carbon is a promising commercial anode for sodium-ion batteries, however, achieving high capacity, fast Na⁺ kinetics, and long-term stability remains a critical challenge. To tackle this, we develop a controllable synthesis of a sulfur-rich hard carbon with hierarchical porosity (denoted as MSC). The interconnected porous network ensures efficient electrolyte penetration and rapid Na⁺ diffusion, while the uniformly incorporated sulfur species widen the interlayer spacing and engage in reversible Na-S reactions, contributing extra capacity. This synergy endows the MSC anode with a high reversible capacity of 545.5 mAh g^-1 at 0.1 A g^-1 and a remarkable rate capability of 163.5 mAh g^-1 at a high current density of 5.0 A g^-1. More importantly, it exhibits exceptional long-term stability, retaining 99% of its capacity after 5000 cycles at 5.0 A g^-1, demonstrating its outstanding reaction reversibility and structural stability. This study highlights that the synergistic regulation of porosity and heteroatom doping effectively enhances both kinetics and cycling durability, providing a viable design strategy for high-performance hard carbon anodes.