Analyzing the Phase Change Behavior in NMC Material by Ex-Situ Studies

Lithium-ion batteries (LIBs) are gaining popularity in the transportation sector, particularly with hybrid and electric vehicles. Among them, lithiated nickel manganese cobalt oxide (NMC) is a prominent cathode material due to its high energy density, thermal stability, and extended cycle life.¹ Furthermore, the ability of NMC material to bear prolonged charge and discharge cycles makes it an ideal candidate in automotive sector. However, phase change phenomena in NMC during battery operation can significantly affect its electrochemical performance and durability. X-ray diffraction (XRD) and Raman technique are the two useful methods which help to identify the structural and phase transformations in cathode materials. In this study, we focused on comparing the phase differences during the operation of lithium-ion batteries using NMC materials. Ex-situ study is a very useful approach as the same electrode can be used for different characterizations without time constrains. During ex-situ XRD, we can detect a phase change behavior in bulk of the NMC electrodes. Also, the potential-dependent analysis of the phase composition in NMC111 (LiNi_0.3Mn_0.3Co_0.3O₂),NMC622 (LiNi_0.6Mn_0.2Co_0.2O₂) and NMC811 (LiNi_0.8Mn_0.1Co_0.1O₂) was performed. Ex-situ Raman mapping gives us information on local structural changes in addition with the degree of disorder, presence of defects or impurities and oxygen vacancies during lithiation and delithiation process.²
Electrochemical tests were carried to simulate the actual cell operation and hence the conventional method of charging, constant current constant voltage (CCCV) technique, compared to constant current (CC), was used. In CCCV, initially the battery is charged up to a particular voltage and then the battery is charged with constant voltage until the current reaches a cut-off current value or a particular set time is reached.³ Further to understand how and when the phase change begins, constant voltage was applied at different time intervals after charging the cell to a particular voltage. All the electrochemical tests were carried by both higher and lower C-rates and the electrodes were disassembled for ex-situ studies. Thus by combining XRD and Raman results, we could identify the phase change behavior, structural dynamics and electrochemical behavior in NMC materials. This work was funded by the National Center for Science in Poland through the Sonata BIS 11 (No. UMO-2021/42/E/ST5/00390).

References:
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