Ultrahigh‐Capacity Sodium Storage in Long‐range Ordered Expanded Flake Graphite: Insights into the Underlying Mechanism

Abstract The narrow interlayer spacing of natural graphite, commonly used in lithium‐ion batteries, restricts the effective intercalation of Na⁺ and leads to sluggish diffusion kinetics. Herein, expanded flake graphite (EFG) with long‐range ordered structure and enlarged interlayer spacing is synthesized by stepwise oxidation and high‐temperature microstructure control technology using large flake graphite. Electrochemical tests and density functional theory (DFT) calculations validate that the long‐range ordered structure is more favorable to reduce the migration energy barrier of Na ⁺ compared to the expanded interlayer spacing, which effectively enhances the sodium storage performance. In situ XRD, in situ Raman, and ^{2 3} Na MAS NMR reveal sodium storage behaviors dominated by pseudocapacitance through adsorption‐intercalation‐pore filling. Surprisingly, the synthesized EFG‐600 °C 1h delivers an ultrahigh capacity and excellent cycle performance of 518.0 mAh g ⁻¹ at 200 mA g ⁻¹ after 300 cycles. Furthermore, the full cell EFG‐600 °C 1h//Na ₂ NiFeMnO ₆ delivers a high energy density of 154.4 Wh kg ⁻¹ at 2000 mA g ⁻¹ after 1200 cycles, outperforming all previously reported expanded graphite. This work elucidates the sodium storage behavior in EFG with long‐range ordered structure, advancing its application as a high‐performance SIBs anode.