Hysteresis and current dependence of the graphite anode color in a lithium-ion cell and analysis of lithium plating at the cell edge

While charging a lithium-ion cell the color of the graphite anode changes as a function of the degree of lithiation. The color of the graphite anode is described in the literature starting from black, through red, to gold. Our ex-situ observations of the graphite anode on opened cells reveal that color differences exist at the same state of charge depending on the charge and discharge direction. This color hysteresis also remains after any length of waiting time. A current dependency of the color can also be seen, with colors which do not correspond to the average state of charge of the graphite anode. Closer investigations show that these colors are homogeneously distributed throughout the anode and do not vary with the layer depth. During charging, high states of charge and during discharging low states of charge exist on the surface of the particles compared with the average state of charge of the graphite anode. This inhomogeneity between particle surface and core subsequently equilibrates with a time constant of approx. 13 min at room temperature. The precise color analysis of the graphite anode helps to explain another effect: the frequently occurring lithium plating on the edge of the graphite anode for cells with an anode overhang. This overhang causes overcharging of the edge area of the cathode. Lateral inhomogeneities of the lithium-ion distribution are formed in the cathode at each cycle. When charging the cell with high current, lithium plating is therefore formed first directly opposite the cathode edge. Minimizing the overhang area can substantially reduce the susceptibility to lithium plating.