Unveiling the Mechanisms of NMC622 Degradation during Cycling Using Ex Situ SEM, XRD, and Raman Studies
Authors: Glaszczka Alicja , Manippady Sai Rashmi , Winkowska-Struzik Magdalena , Struzik Michał , Buchberger Dominika , Czerwinski Andrzej 
Published: February 2024

Lithium-ion cells that employ nickel-rich NMC materials are prevalent because of their high capacity and ability to function at significantly elevated potentials vs Li+/Li. Nonetheless, these cells encounter issues such as a decline in capacity or thermal resistance, and various researchers are currently exploring the underlying causes of these phenomena. Zhang1 has compiled these findings, pointing out the most significant ones: cation mixing, present of residual lithium compounds, oxygen evolution and phase transition to rocksalt. Understanding the phenomena occurring during lithiation and delithiation of NMC can help in selecting the appropriate method to improve its properties. To understand this, scientists use various methods. Through a scanning electron microscopy (SEM), Cheng et al.2 observed grain cracking in the electrode material intensifying with successive cycles. They compared the NMC811 after 14 cycles for a cutoff potential of 4.1 vs. Li+/Li and 4.7 vs. Li+/Li. G anty et al.3 used in situ XRD studies to monitor changes in NMC811 during charge and discharge cycling, with particular emphasis on changes in the a and c unit cell parameters and the appearance of the H2 hexagonal phase during charging (significantly reduced lithium content in the structure). Also, Raman spectroscopy can be used to make comparisons of the changes occurring in the active material during its lithiation and delithiation. Flores et al.4 study layered active materials using Raman spectroscopy and was able to see significant changes in the structure, such as showing that the oxidation of Ni2+/Ni3+ ↔ Ni3+/Ni4+ is gradual.
However, in situ techniques have limitations in combining various methods to test the same sample. To facilitate the use of several techniques, ex situ testing can be employed. In this research, nickel-rich NMC samples were analyzed using a combination of ex situ XRD, SEM, and Raman spectroscopy. This approach provides more information on the phenomena occurring during cell operation, allowing for better understanding of the problems that need to be addressed. SEM images can identify the presence of structural irregularities that can affect the material's electrochemical performance, including defects and cracks. XRD provides overall information on phases present in the bulk sample, their grain size, and crystallographic parameters, and allows for thorough Raman analysis. Ex situ Raman spectroscopy enables experiments to be carried out at a specific location, increases the reliability of tests, and provides detailed information on phase transitions and the causes of capacity drop during the cell operation. Together, these techniques provide valuable insights into the structural changes that occur in NMC622 material during lithiation and delithiation processes. By analyzing the crystal structure, and surface morphology of the cathode material before and after cycling, these techniques can help identify the mechanisms of degradation and suggest strategies for improving battery performance and lifespan. A detailed analysis will be presented during the conference. This work was funded by the National Center for Science in Poland through the Sonata BIS 11 programme (No. UMO-2021/42/E/ST5/00390).
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