In-situ and Ex-situ Observations of Lithium De-intercalation from LiCoO2: Atomic Force Microscopy and Transmission Electron Microscopy Studies

摘要

Lithium cobalt dioxide is the most commonly used material for positive electrodes in lithium rechargeable batteries. During lithium de- intercalation from this material, LixCoO2 undergoes a number of phase transitions, which have been studied by bulk techniques and first-principles calculations. The objective of this work was to examine the effect of charge and discharge on individual LiCoO2 crystals. Atomic Force Microscopy and Transmission Electron Microscopy were used to observe the effects of lithium ion de-intercalation from LixCoO2 in individual crystals. This work was the first study to use in-situ Atomic Force Microscopy to probe the dynamic evolution of LixCoO2 crystal morphology as lithium ions are de-intercalated from the material. The overall crystal morphology did not seem to evolve very much during the first charge and discharge cycle; however, evidence was found for expansion and contraction of crystal steps during cycling. The evolution of layer spacing was quantified from AFM data and was found to be in agreement with X-ray diffraction studies by other researchers. Furthermore, rounded bumps were observed on the surface of most of the crystals, and it is speculated that these bumps are Li2CO3 impurities, formed on the surface of LiCoO2 after storage in air. Lastly, the formation of a surface layer on LixCoO2 crystals and the gold foil substrate was observed when the cell was over-discharged to approximately 2.5V.