Dual-functional lactic acid/zinc chloride deep eutectic solvent pretreatment: simultaneous production of antioxidative lignin and high-performance hard carbon anodes for sodium-ion batteries

Abstract

The recalcitrance of lignocellulose and the challenge of achieving high-value utilization of overall components simultaneously hinder the sustainable biorefining of biomass. Herein, a lactic acid/zinc chloride (LA/ZnCl₂) deep eutectic solvent (DES) pretreatment was developed to fractionate Eucommia ulmoides wood (EUW) for co-producing antioxidant lignin and high-performance hard carbon anodes. This strategy effectively separated lignin with a high yield of 91.6% and a high purity of 99.8%, which exhibited low molecular weight and enhanced antioxidant activity (radical scavenging index increased from 7.0 to 13.8) due to increased phenolic hydroxyl groups (from 1.77 to 2.29 mmol/g). Simultaneously, appropriately removing amorphous contents in the precursor resulted superior structural properties in the derived hard carbon for assembling high-performance hard carbon anodes in sodium-ion battery (SIB). The optimal material (EUWC90) featured an expanded interlayer spacing (0.388 nm), enhanced graphitization, and well-defined closed pores. Consequently, EUWC90 delivered a high reversible capacity of 296 mAh g−1 at 20 mA g−1, an exceptional initial coulombic efficiency (ICE) of 87.6%, and outstanding cycling stability (90% capacity retention after 1000 cycles at 20 mA g−1). Overall, the dual functional DES pretreatment strategy has the excellent potential in the integrated and high-value utilization of lignocellulose in material applications.