Sustainable Formic Acid Pretreatment Enables High-Capacity Hard Carbon Anodes for Sodium Ion Batteries and Lignocellulosic Sugar for Biorefineries

Hard carbon is a promising anode material for sodium-ion batteries (SIBs), but the complex structure and poor homogeneity of industrial lignin limit the performance of lignin-derived hard carbon. To address this issue, this study proposes a green and scalable strategy for the sustainable production of high-capacity hard carbon from lignocellulosic biomass, coupled with the coproduction of value-added sugars. First, this study fractionated industrial lignin using the recyclable ethyl acetate solvent to identify the structural characteristics of target-structured lignin suitable for preparing high-performance hard carbon. It was found that this target-structured lignin is featured with high ether linkage (β-O-4) content, high molecular weight, and few reactive functional groups. Based on these findings, low-toxicity and recyclable formic acid was used to chemically treat different lignocellulosic biomasses for the selective extraction of the target-structured lignin. After carbonization, the extracted lignin exhibits a disordered microstructure with enlarged interlayer spacing and abundant closed nanopores. Consequently, it delivers superior sodium storage performance, including a high reversible capacity and excellent cycling stability. Furthermore, the technique enables full utilization of biomass components, exemplified by bamboo powder, achieving utilization efficiencies of 78.6% for lignin, 93.0% for hemicellulose, and 83.8% for cellulose. This work provides an effective and operable approach for the industrial production of lignin-derived hard carbon for SIBs and the sustainable covalorization of lignocellulosic biomass into sugars.