Improving initial coulombic efficiency of wood-waste hard carbon: key factors controlling first-cycle irreversibility and practical mitigation strategies

Abstract

Low initial coulombic efficiency (ICE) remains a major barrier for biomass-derived hard carbon anodes in sodium-ion batteries (SIBs), where first-cycle Na loss is dominated by interphase formation and irreversible trapping. Here, we improve and rationalize ICE in wood-waste (WW)-derived hard carbon by combining intrinsic and extrinsic strategies within one framework. Intrinsically, metal organic framework (MOF)-assisted catalytic processing increases local structural order, raising ICE from 26% (WW) to 41% with a first-charge capacity of 191 mAh g⁻ 1 . Extrinsically, the catalysttreated carbon is evaluated in carbonate and ionic-liquid electrolytes, including a low-salt NaFSI-EMIFSI (10 mol% NaFSI) benchmark and NaFSI-C 3 MPYRFSI (50:50 mol%). While NaPF 6 -EC/PC delivers a slightly higher ICE (42%), the ionic-liquid electrolytes provide higher reversible capacities (up to 195 mAh g⁻ 1 ) and improved retention. Finally, direct-contact pre-sodiation increases ICE to 75% at an optimized contact time of 15 min (approximately threefold higher than the untreated WW carbon), whereas longer contact lowers reversible capacity. Impedance analysis and rate/cycling tests confirm improved interfacial kinetics and stability. Overall, this combined approach mitigates firstcycle losses and places the ICE improvement among the larger gains reported for biomass-derived hard carbons.