Interfacial Engineering of Fe₃O₄/Biochar via CMC Bridging for Durable Lithium/Sodium-Ion Storage

The development of cost-effective and durable anode materials derived from renewable biomass is essential for practical lithium- and sodium-ion batteries (LIBs/SIBs). However, conventional strategies for integrating high-capacity metal oxides with biomass carbon often suffer from weak oxide-carbon adhesion, nanoparticle aggregation, and large volume changes during cycling. To overcome these limitations, we present an interfacial engineering approach in which carboxymethyl cellulose (CMC) serves as a molecular bridge to anchor Fe₃O₄ nanoparticles in situ onto ginkgo branch-derived biochar (GBC). The abundant hydroxyl and carboxyl groups of CMC simultaneously guide uniform Fe₃O₄ nucleation and create a molecular bridge that mediates interfacial chemical interactions that facilitate charge transfer while accommodating volume strain. Structural and compositional analyses confirm the formation of CMC/Fe₃O₄/GBC with well-dispersed nanocrystals, a hierarchically porous architecture, and magnetic responsiveness. As an anode, CMC/Fe₃O₄/GBC delivers 256 mAh/g at 2000 mA/g in SIBs, substantially higher than that of bare GBC and unmodified Fe₃O₄/GBC. Comparable durability is observed in LIBs, where the composite retains 297 mAh/g after 2000 cycles at the same rate. Kinetic studies using cyclic voltammetry, galvanostatic intermittent titration technique, and electrochemical impedance spectroscopy reveal that CMC bridging enhances pseudocapacitive contribution and accelerates ion diffusion. Post-cycling X-ray photoelectron spectroscopy and transmission electron microscopy further confirm the preserved electrode integrity. These results establish a scalable interfacial modification strategy for biomass-derived magnetic composites toward durable alkali-ion storage.