Optical Microscopy of Electrically-Driven Lithium Intercalation Dynamics in Graphite*

Using optical microscopy we observed lithium intercalation and deintercalation in a micro-scale graphite electrode. Graphite was mechanically exfoliated and transferred via wet processing techniques to a pre-patterned nickel electrode. Then the nickel electrode was encapsulated with insulating Al₂O₃ using atomic layer deposition to minimize the nickel background contribution to the electrochemical currents. The counter and reference electrodes were lithium foil, the electrolyte was 1M LiClO₄ in EC:DMC, and the experiments were conducted in an argon atmosphere at room temperature. We tracked the graphite’s color, which is directly related to its lithium content, as it evolved through grey, blue, red, and gold during multiple intercalation cycles. These color changes reveal that the intermediate lithium carbide structures adopted by the electrode during intercalation do not appear in reverse sequence during deintercalation. In other words, the deintercalation pathway is not the time-reverse of the intercalation pathway.