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

Abstract

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.

Operando Monitoring the Insulator–Metal Transition of LiCoO2

“…In agreement with most literature studies, we observe the E g /A 1g intensity ratio to change from E g /A 1g > 1 under a 632 nm excitation, 14,34,35 to E g /A 1g < 1 under 514 nm, 36−41 532 nm 14,42−44 and even 785 nm 45 excitations. Note that our DFTcalculated Raman activities exhibit the same E g /A 1g < 1 intensity ratio as the experimental spectrum under 532 nm excitation (Figure 2).…”

Operando Monitoring the Insulator-Metal Transition of LiCoO2

“…A Raman spectrum might be resonance-enhanced when the energy of the incident radiation coincides with the energy required to promote an electronic transition. In agreement with most literature studies, we observe the Eg/A1g intensity ratio to change from Eg/A1g > 1 under a 632 nm excitation, 14,34,35 to Eg/A1g < 1 under 514 nm [36][37][38][39][40][41] , 532 nm 14,[42][43][44] and even 785 nm 45 excitations. Note that our DFT-calculated Raman activities exhibit the same Eg/A1g < 1 intensity ratio as the experimental spectrum under 532 nm excitation ( Figure 2).…”

Raising the Intrinsic Safety of Layered Oxide Cathodes by Surface Re‐Lithiation with LLZTO Garnet‐Type Solid Electrolytes

“…[ 47 ] This spinel phase was generated from the charged LiCoO 2 after surface oxygen release and surface structural reconstruction. [ 48 ] Additionally, a Raman spectrum containing the E g and A 1g vibrational mode peaks of the layered structure and the A 1g vibrational mode peak of the spinel structure was observed on the surface of 4.5 V charged LiCoO 2 electrodes, corresponding to a mixed phase of layered and spinel structures. According to the characteristic Raman spectra of these three phases, two‐dimension Raman mapping was performed to determine the distribution and content of these phases in the surface region of the 4.5 V charged LiCoO 2 electrodes.…”