Innovative synthesis and sodium storage enhancement of closed-pore hard carbon for sodium-ion batteries

Hard carbon with abundant closed-pore structures holds significant promise as an anode material for sodium-ion batteries. In this work, a one-step process was pioneered to produce porous carbon with abundant open-pore structures from walnut shells. Subsequently, small aromatic compounds derived from the pyrolysis of polystyrene were deposited into the pores of the porous carbon, forming hard carbon material with abundant closed-pore structures. The resulting hard carbon anode (WS-PS-1200) demonstrated a high capacity of 385 mAh g⁻¹ at 50 mA g⁻¹, with a corresponding plateau capacity of 225 mAh g⁻¹. It also exhibited an impressive initial Coulombic efficiency (ICE) of 88 % and excellent rate performance, compared to an ICE of only 57.5 % in the anode obtained by direct carbonization. By utilizing 3D time-of-flight secondary-ion mass spectrometry (3D TOF-SIMS) and depth-profiling X-ray photoelectron spectroscopies (XPS) characterization methods to analyze the solid electrolyte interface (SEI), the results indicate that reducing the open-pore structure can minimize the decomposition of the electrolyte, leading to an SEI composition that tends towards inorganic phases. To verify the practical applicability of WS-PS-1200, it was assembled into a full cell with Na₃V₂(PO₄)₃, achieving a capacity of 305 mAh g⁻¹ (0.03 A g⁻¹) and excellent rate performance. Moreover, the assembled all-carbon sodium-ion hybrid capacitor exhibits an energy density of 101 Wh kg⁻¹. This study not only introduces a new strategy for preparing hard carbon with closed pores but also successfully converts waste polystyrene and walnut shells into high-value materials, offering an innovative method for synthesizing hybrid capacitor electrode materials.