Improving the Initial Coulombic Efficiency of Phenolic Resin-Derived Hard Carbon Anodes for Sodium-Ion Batteries by Pore-Forming
Hard carbon (HC) is a leading anode candidate for sodium-ion batteries (SIBs) due to its disordered structure and expanded interlayer spacing (3.4–4 Å), which facilitate sodium-ion intercalation. However, the poor initial Coulombic efficiency (ICE) of HC remains a critical barrier to commercial viability. Phenolic resin (PF) precursors offer high carbon yield and good reversible capacity, yet the relationship between PF solid content and ICE is not fully understood. This study investigates four commercial PF-based hard carbons with varying solid contents, then modifies them via pore-forming agents, cross-linking curing, and ball-milling. The optimized U-HC sample, derived from the highest solid-content PF, achieves an ICE of 89.84% and a specific discharge capacity of 354.18 mAh g⁻¹ at 35 mA g⁻¹. Baseline PF-derived HCs typically exhibit ICE values below 82%, as reported for resorcinol-formaldehyde resin (82%) and PTCDA-modified PF (77.9%). The pore-forming strategy enhances ICE beyond 86% across modified samples, with U-HC reaching 89.84%. This improvement is attributed to optimized pore architecture that reduces irreversible sodium trapping and SEI formation. The findings provide a rational design pathway for high-ICE PF-derived hard carbon anodes, addressing a key bottleneck in SIB commercialization.