Precisely Regulating the Graphitic Layers of Hard Carbon via Oxygen Release to Elucidate the Formation Mechanism of Closed Pores

Hard carbon (HC), recognized as the most promising anode material for sodium-ion batteries (SIBs), exhibits multiple forms of sodium storage (adsorption on graphitic layers, insertion between graphitic sheets, and filling in closed pores). Low initial coulombic efficiency (ICE) and low plateau region capacity are the main issues with HC, and it is necessary to understand the evolution laws of graphitic layers and closed pores. Here, we regulate the structure of graphitic layers by deliberately changing the oxygen content in HC materials and reveal the mechanism of formation of closed pores. Additionally, the density functional theory (DFT) calculation results provide crucial evidence for the promotion of interlayer spacing increase by the introduction of oxygen atoms into the graphitic layers. The improved sample with the largest number of closed pores (5.16 × 10¹⁹ g^-1) displays the largest plateau capacity (218.8 mAh g^-1), proportion (76.0%), ICE (84.8%), and excellent kinetic performance (D_Na⁺, _slope = 1.26 × 10^-9 cm² s^-1, D_Na⁺, _plateau = 3.72 × 10^-10 cm² s^-1). This work reveals the structural regulation mechanism of HC by adjusting the oxygen content and introduces a strategy to develop HC for high-performance SIBs.