Enhanced Sodium Storage Performance of Few-Layer Graphene-Encapsulated Hard Carbon Fiber Composite Electrodes

Hard carbon anodes are promising for sodium-ion batteries due to their low cost and high reversible capacity. However, the long-term Na+ (de)intercalating process destroys the structure of the two-phase interface between the electrode and electrolyte, impairing cycling stability. In this paper, a few-layer graphene (FLG)-coated hard carbon fiber composite is constructed. A uniform graphene encapsulation is confirmed by synchrotron small-angle X-ray scattering and transmission electron microscopy technologies. Post-cycling observation reveals FLG participation in forming a hybrid solid electrolyte interphase (SEI). At a proper concentration, the FLG with a small specific surface area and pore size characteristics is well matched in the SEI. The FLG-integrated SEI not only mitigates volume expansion but also enhances ion conductivity through its oxygen-rich functional groups. As a result, the composite structure maintains 98.2% capacity retention after 100 cycles and reaches 164 mAh g−1 at 1000 mA g−1, compared to 97 mAh g−1 for the pristine hard carbon. This work demonstrates that FLG coating simultaneously stabilizes the interfacial chemistry and improves the ion transport, offering a practical pathway to advance hard carbon anodes for high-performance sodium-ion batteries.