Ex situ and operando study of LiCoO2 thin films by Raman spectroscopy: Thermal and electrochemical properties

Lithiated transition metal oxides are active materials used as positive electrode in lithium ion cells thanks to their high capacity and air stability when. Amongst lithiated oxides, LiCoO2 is the most commonly and practically used due to its high-specific energy and its excellent cycle life. This work focuses on phase transformation of LiCoO2 thin films during the annealing step and during the cycling life. The formation of unwanted products is highlighted by an innovative method: in situ Raman analysis. During the annealing used to transform the almost amorphous as-deposited LiCoO2 into the rhombohedral structure useful for electrochemical cycling, the formation of Co3O4 has been observed. The second studied phase transformation is due to the electrochemical cycling and highlights the kinetics of the structure change. The study was done by comparing ex situ and operando results and highlights the main advantages of operando analysis: more data in a shorter time and the ability to assess the time dependence of phenomena.