Rationally Regulating Closed Pore Structures by Pitch Coating to Boost Sodium Storage Performance of Hard Carbon in Low-voltage Platforms

Hard carbon (HC) materials with rich closed pore structures and nano-scaled soft carbon coating layer have emerged as promising anode in sodium-ion batteries (SIBs). However, it still remains a tremendous challenge to precisely regulate closed pore structures and soft carbon coating thicknesses for achieving excellent electrochemical performance in SIBs at low-voltage platforms. Herein, PCHC-10 with abundant and suitable-sized closed pore size (0.45 nm) and nano-scaled soft carbon coating layer has been accurately designed by chemical crosslink reaction between the pre-oxidized phenolic resin and a small addition of pitch to form ester-based bond. As anode, PCHC-10 delivered large reversible capacity of 359.8 mAh g⁻¹ within 0.001–2.5 V, and high capacity of 242.8 mAh g⁻¹ in low voltage platforms (≤0.15 V). Besides, PCHC-10 anode exhibits 91.4% capacity retention for 100 cycles, and Na₃V₂(PO₄)₃//PCHC-10 full cell has superior rate performance and high energy density of 231.2 Wh kg⁻¹. Furthermore, the detailed electrochemical storage behaviors and theoretical calculations revealed that the HC owning closed pore-size of 0.45 nm has the strongest Na⁺ storage abilities in low-voltage platforms. This work presents a novel insight for constructing HC with suitable-sized closed pore structures and soft coating layer to boost Na⁺ storage capability in low-voltage platforms.