Morphology/behavior relationship in reversible electrochemical lithium insertion into graphitic materials

Different types of graphite particles, including synthetic flakes, natural graphite flakes, graphite fibers, round-shaped graphite particles (mesocarbon microbeads), and combinations of graphite flakes and disordered carbon particles were studied as electrode materials in lithium insertion/deinsertion processes. We suggest that the morphology of the graphite particles plays an important role in their stability. One of the key factors is the structure of the facets, perpendicular to the basal planes, through which Li insertion takes place. The smoother these facets are, and which contain fewer crevices, the more stable are the graphite particles in Li insertion reactions. This is attributed to the fact that the surface reactions of the cathodically polarized graphite electrodes in the nonaqueous Li salt solutions involve gas formation, which may have a detrimental effect on the integrity of the graphite particles. Another important factor is the existence of some degree of disorder and distortion in the graphite structure. More distortion in the ordered graphite structure may lead to higher stability. A clue to the understanding of the factors that determine stability or destruction of lithiated graphite electrodes is the pronounced detrimental effect of the presence of propylene carbonate in ethylene carbonate-based solutions, as discussed herein. Another important factor dealt with in this paper is the correlation between the morphology of the graphite particles and their orientation in practical electrodes. The major tools for this study included chronopotentiometry, electron microscopy, and X-ray diffraction.