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

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 3Na 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//Na2NiFeMnO6 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.