High Voltage Stability and Enhanced Electrochemical Performances of LiCoO2 by CeF3 Coating

Surface coating of LiCoO2 remained one of the efficient methods to enhance its electrochemical and thermal performances, especially at high cut-off potential. For several years, REMINEX research center has worked on the development of cathode materials for lithium batteries. Recently, a patent was published on a new cathode materials based on Ce doped-LiCoO2. Based on previous works and our know-how on cerium chemistry, we developed LiCoO2 coating using Ce-based compounds. In this work, CeF3 coated LiCoO2 was synthesized via solid state reaction of cobalt oxide and lithium carbonates followed by precipitation of CeF3 in presence of lithium cobalt oxide powder. The morphology and structure of the modified cathode material were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show the disappearance of the Hexagonal-Monoclinic transition around 4.1V by coating. This transition contributes indeed in the structural instability of bare-LiCoO2. the fluoride compound CeF3 is successfully coated on the surface of LiCoO2 cathode particles with an average layer thickness about 40 nm. The electrochemical tests show that the CeF3-coating layer significantly enhances the cycling performance of LiCoO2 cathode material, even at high cut-off potential. While the bare LiCoO2 cathode displays fast fading at 4.6 V vs. Li/Li cutoff potential, the surface-modified electrode exhibits the great capacity of 160 mAh/g with an excellent capacity retention on several cycles. The thermal stability of coated LiCoO2 was also improved by the surface modification. We concluded that the electrochemical and the thermal enhancement at high potential are ascribed to the presence of CeF3 coating layer which prevents the side reaction during the charge discharge process, alleviate the attack by the acidic electrolyte and reduce the damage of electrode structure.