Expanded graphite (EG) is a promising anode material for sodium-ion batteries (NIBs) due to its large interlayer spacing and long-range-ordered layered structure. EG is synthesized through oxidation and partial reduction of graphite, resulting in an interlayer distance of 4.3 Å, which allows for efficient sodium ion (Na⁺) insertion and extraction. In situ transmission electron microscopy (TEM) confirmed that Na⁺ can be reversibly inserted into and extracted from EG. Galvanostatic studies showed that EG can deliver a high reversible capacity of 284 mAh g⁻¹ at 20 mA g⁻¹, maintain 184 mAh g⁻¹ at 100 mA g⁻¹, and retain 73.92% of its capacity after 2,000 cycles. Graphite, the most common anode for lithium-ion batteries, has limited Na⁺ storage capacity due to its small interlayer spacing. EG, however, retains a long-range-ordered structure with a larger interlayer distance, enabling efficient Na⁺ intercalation. The interlayer spacing of EG can be controlled through oxidation and reduction processes, making it a promising anode material for NIBs. The study also investigated the effects of oxygen content on Na⁺ storage capacity. EG-1h showed a high reversible capacity of ~300 mAh g⁻¹, while EG-5h had a lower capacity due to reduced interlayer spacing. Cyclic voltammetry and in situ TEM confirmed that Na⁺ insertion and extraction in EG is primarily due to intercalation, with a reversible and stable mechanism. EG exhibits excellent cycling stability, with a low capacity decay rate of ~0.013% per cycle after 2,000 cycles. The in situ TEM observations revealed that the microstructure of EG changes during sodiation and desodiation, with reversible microstructural evolution. The layered structure of EG provides a constant transport space for Na⁺ ions, contributing to its high performance. The findings suggest that EG is a promising anode material for NIBs, with potential for high reversible capacity and long cycle life. The study highlights the importance of optimizing interlayer spacing and oxygen content in carbon materials for efficient Na⁺ storage in NIBs.Expanded graphite (EG) is a promising anode material for sodium-ion batteries (NIBs) due to its large interlayer spacing and long-range-ordered layered structure. EG is synthesized through oxidation and partial reduction of graphite, resulting in an interlayer distance of 4.3 Å, which allows for efficient sodium ion (Na⁺) insertion and extraction. In situ transmission electron microscopy (TEM) confirmed that Na⁺ can be reversibly inserted into and extracted from EG. Galvanostatic studies showed that EG can deliver a high reversible capacity of 284 mAh g⁻¹ at 20 mA g⁻¹, maintain 184 mAh g⁻¹ at 100 mA g⁻¹, and retain 73.92% of its capacity after 2,000 cycles. Graphite, the most common anode for lithium-ion batteries, has limited Na⁺ storage capacity due to its small interlayer spacing. EG, however, retains a long-range-ordered structure with a larger interlayer distance, enabling efficient Na⁺ intercalation. The interlayer spacing of EG can be controlled through oxidation and reduction processes, making it a promising anode material for NIBs. The study also investigated the effects of oxygen content on Na⁺ storage capacity. EG-1h showed a high reversible capacity of ~300 mAh g⁻¹, while EG-5h had a lower capacity due to reduced interlayer spacing. Cyclic voltammetry and in situ TEM confirmed that Na⁺ insertion and extraction in EG is primarily due to intercalation, with a reversible and stable mechanism. EG exhibits excellent cycling stability, with a low capacity decay rate of ~0.013% per cycle after 2,000 cycles. The in situ TEM observations revealed that the microstructure of EG changes during sodiation and desodiation, with reversible microstructural evolution. The layered structure of EG provides a constant transport space for Na⁺ ions, contributing to its high performance. The findings suggest that EG is a promising anode material for NIBs, with potential for high reversible capacity and long cycle life. The study highlights the importance of optimizing interlayer spacing and oxygen content in carbon materials for efficient Na⁺ storage in NIBs.