• SO2Cl2 serves as a safe and efficient chlorine generator, eliminating the need for hazardous Cl2 gas in the synthesis of metal chloride-intercalated graphite.
• The BiCl3-GICs synthesized at 200 °C for 20 h exhibit a large interlayer spacing of 1.26 nm and a high BiCl3 loading of 42%, enabling high specific capacity and excellent rate performance in sodium-ion batteries.
• In-situ Raman spectroscopy reveals weakened electronic interaction between graphite and intercalated BiCl3 during first discharge, which is favorable for sodium storage.
• The method is broadly applicable to other metal chlorides, offering a scalable and safer route for advanced energy storage materials.