Changes in the stage structure of Li-intercalated graphite electrode at elevated temperatures

To examine the phase transition upon deintercalation of Li from graphite at elevated temperatures, structural changes in C6Lix (x = 0.284, 0.532, 0.666, 0.739, 0.812) samples during heating were evaluated by in situ X-ray diffraction (XRD) analysis. The diffraction patterns of the sample, in which the stage-1 and the stage-2 structures coexist, drastically converted to the profile of a Li-defect stage-1 structure at temperatures above 200 degrees C without Li intercalation and/or deintercalation. Meanwhile, the reflection peak corresponding to the stage-1 structure in the XRD patterns of C6Li0.812 continuously shifted to higher angles at temperature above 330 degrees C, which implied deintercalation of Li through reaction with the binder and the formation of the Li-defect stage-1 structure. Li deintercalated from C6Lix while maintaining the same stage structure, likely via a solid-state reaction at temperatures above 330 degrees C. The lattice misfit calculated from the difference in interlayer spacing between stage-1 and stage-2 in C6Li0.666 drastically decreased from 4.58% at 50 degrees C to 1.43% at 350 degrees C. These kinetics for Li graphite intercalation compounds at elevated temperatures are formulated for the first time and should be helpful in understanding the Li intercalation/deintercalation mechanism of the graphite electrodes used in Li-ion batteries.