Lithium compound deposition on mesocarbon microbead anode of lithium ion batteries after long-term cycling.

Lithium compound deposition on mesocarbon microbead (MCMB) anode after long-term cycling was studied in LiCoO2/MCMB battery. Lithium compound deposition did not generate on the activated MCMB anode, but it generated unevenly on the long-term cycled anode. Gray deposition composed of dendrites and particles was formed on the lower surface of the MCMB layer first, then on the upper surface. The deposition and MCMB layer peeled off from the current collector, and a bump was formed in the cycled anode. The exfoliation and thick deposition increased the ohmic resistance, film resistance, and charge transfer resistance of the cell and decreased the capacity significantly. Metallic lithium did not exist in either the upper or the lower deposition layer according to the results of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), the discharge curve, and anode potential. The outer region of both the lower and the upper deposition layers consisted of Li2CO3, LiOH, ROCO2Li, and ROLi. The inner region of the etched lower deposition layer mainly consisted of Li2O, LiF, and Li2CO3, and that of the etched upper deposition layer mainly consisted of Li2CO3, ROCO2Li, ROLi, and LiF. Solid electrolyte interphase (SEI) film hindering the intercalation of lithium ions into carbon layers and LiCoO2 cathode providing lithium source for the deposition were the two reasons leading to the formation of lithium compound deposition during long-term cycles. Because SEI film on the lower surface of MCMB layer was thicker than that on the upper surface, lithium compound deposition generated on the lower surface first.