Decoding aromatic ring effects on crosslinking chemistry and closed-pore engineering in hard carbons for sodium-ion batteries

Hard carbons (HCs) derived from pitch show promise as low-cost anodes for sodium-ion batteries (SIBs) due to their high carbon yield. However, pitch's complex polyaromatic structure makes its modification and HC microstructure control difficult. This study selects bicyclic naphthalene (Nap), tricyclic phenanthrene (Phe) and tetracyclic pyrene (Pyr) as model precursors to investigate how the aromatic ring number impacts HCs' cross-linking mechanisms, microstructure, and electrochemical performance. HCs based on Nap, Phe and Pyr are prepared via combined molecular and oxidative cross-linking. After molecular cross-linking, polycyclic aromatic resin's layer size and aromaticity grow with precursor ring number, R-Pyr having the largest aromatic layers and strongest π-π interactions. During oxidative cross-linking, these layers enhance –C(O)O group retention and 3D stacking, leading to closed pores with smaller size, larger volume, and thicker graphite microcrystals. Consequently, Pyr-based HCs (Pyr-HC) exhibit superior electrochemical performance, with a reversible capacity of 328.20 mAh g −1 at 0.1 A g −1 . In full-cell tests, the Pyr-HC || NVP reaches 184.8 mAh g −1 at 1 A g −1 and an energy density of 147.2 Wh kg −1 , retaining 74.78 % of its capacity after 200 cycles. This work highlights the importance of precursor structure for HC optimization and screening pitch precursors.