Comparison of thermal and chemical purification methods for graphite from spent lithium-ion batteries

• Spent graphite was purified by chemical, thermal, and combination methods. • Cathode metals were removed from spent graphite using mild acid-leaching conditions. • Aluminum removal required alkaline or thermal conditions. • Combination methods produced graphite with the highest purity and the least defects. Graphite is a key anode material in lithium-ion batteries (LIBs), and its demand is rising with the increasing need for LIBs. Even though spent LIBs contain 12–21 wt% graphite, the material is not currently recovered. Therefore, there is a need for sustainable graphite recycling and purification processes. This study compared purification technologies (hydrometallurgical, thermal, and a combination) to two spent graphite (SG) samples from industrial LIB black masses with initial purities of 69.2 wt% (SG1) and 89.9 wt% (SG2). Both samples contained impurities like lithium, nickel, manganese, cobalt, and aluminum. Sulfuric acid efficiently removed cathode material metals (97.2–98.9 % from SG1) under mild conditions (temperature: 50 °C, time: 30 min, sulfuric acid: 0.5 mol/L, L/S ratio: 10 mL/g) with excess hydrogen peroxide. However, one of the main impurities, aluminum, was not removed. The removal (96.0 % from SG1) required alkaline conditions (temperature: 100 °C, time: 240 min, sodium hydroxide: 10 mol/L, L/S ratio: 20 mL/g). For comparison, thermal treatment (2400 °C, 15 min) was applied by induction annealing in an argon atmosphere to untreated, acid–treated, and acid–alkali–treated samples. Thermal and combination treatments offered the benefit of structural repair in addition to metallic and organic impurity removal, producing graphite with carbon content up to 99.1 wt%. These graphite purification steps improved the electrochemical performance and increased the cyclability of the cells. This study provides valuable information on the removal of impurities from black mass-derived SG, which is essential in the development of SG recovery processes.