Preparation and electrochemical performance of expanded graphites as anode materials for a lithium-ion battery

Expanded flake graphites (EFGs) with different expansion volumes were prepared by an electrophoresis intercalation method using natural flake graphite as the raw material, concentrated HNO₃ and acetic anhydride as intercalates and KMnO₄ as oxidant. Results indicate that the density of structural defects, and the surface area and d₀₀₂ increase after the intercalation. EFG-0.7 obtained with a KMnO₄ mass percentage of 0.7 wt% has a capacity of 521 mAh/g when used as the anode of a lithium ion battery, which is significantly higher than the theoretical capacity of natural graphite. EFG-0.7 retains more than 99% of its capacity after 30 cycles at 0.2 C and its capacity stabilizes at 188 mAh/g after 50 cycles at 1.0 C. All the EFGs exhibit excellent performance in high-rate charge-discharge. The resistance of the solid-electrolyte interface film and the charge transfer resistance of the EFGs are smaller than natural flake graphite as revealed by electrochemical impedance spectroscopy. The improved electrochemical performance of the EFGs can be ascribed to their increased surface area and d₀₀₂, and the larger number of structural defects.