The environmentally friendly preparation of sulfur-free expanded graphite (EG) remains a great challenge. A large amount of strong acids and strong oxidants were used in the preparation of expandable graphite, resulting in environmental pollution problems. Herein, a novel and green preparation method for sulfur-free expanded graphite (EG) has been developed. Sulfur-free EG was fabricated using natural flake graphite (NFG), sodium perchlorate (NaClO₄), and oxalic acid (C₂H₂O₄) as raw materials. The electrochemical oxidation method was applied to prepare sulfur-free expandable graphite, and microwave irradiation technology was adopted to prepare sulfur-free EG. The optimum weight ratio was as follows: NFG:NaClO₄:C₂H₂O₄ = 1.5:10:2. The anodic oxidation occurred under a constant current density of 47 mA/cm², and the intercalation reaction lasted for 60 min at ambient temperature. The largest expanded volume (EV) of sulfur-free EG was 290 mL/g. The FTIR spectrum proved that the ClO4⁻ and COO⁻ groups had entered the graphite layers. The SEM images showed that the graphite layers were completely opened, and the pore structure was clearly observed. The sulfur-free EG prepared by the method does not cause environmental pollution and has broad application prospects.

A certain amount of sulfur-free expandable graphite was weighed, and the EV was measured using a 150 mL graduated cylinder after expansion. The EV of the EG was calculated by the following equation: EV = V/M (mL/g). The higher expansion volume indicates that the more expanded graphite is obtained.

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