Correlating the Sodium Storage Mechanism and Enhancing the Initial Coulombic Efficiency of Biomass‐Derived Hard Carbon in Sodium‐Ion Batteries

Traditional graphite‐based anode materials perform poorly in sodium‐ion battery (SIB) due to their insufficient interlayer spacing. Hard carbon has received much attention as anode material for SIBs. Biomass materials are a perfect source of hard carbon precursors due to their inherent benefits and ability to be renewed. Herein, silicon/hard carbon (Si/HC‐X) composite is synthesized from Citrus limon leaves as a useful anode material for SIBs. The intrinsic doping of silicon in carbon matrixes increases interlayer distance as well as defects, which promotes mass transportability and Na ⁺ adsorption capacity. The optimized Si/HC‐1100 electrode shows ≈261 mAh g ⁻¹ of reversible specific capacity with ≈79.8% initial Coulombic efficiency (ICE). Additionally, the composite exhibits ≈87.4% capacity retention at 200 mA g ⁻¹ after 200 cycles. The plateau capacity increases from Si/HC‐900 to Si/HC‐1300 as the micro‐ and nanopores formation increases with increasing calcination temperature. The expanded‐interlayer distance contributes to the plateau capacities, whereas slope capacity arises due to adsorption of Na ⁺ on defects and open pores. The galvanostatic intermittent titration technique, ex‐situ XRD and Raman analysis suggest “adsorption‐intercalation‐pore filling” mechanism, where intercalation contributes more to the plateau capacity of Si/HC‐1100. Moreover, the full‐cell (Si/HC‐1100||Na ₃ V ₂ (PO ₄ ) ₃ ) achieves a maximum 207 mAhg ⁻¹ reversible capacity from the anode side and remarkable cycle stability.