Raman spectroscopy complemented with XRD and TEM for studying structural evolution in initial cycles of...

In this study, we present a detailed investigation of the structural evolution of NMC111 (LiNi_1/3Mn_1/3Co_1/3O₂) cathode material during cycling over an extended potential window, using a combination of in situ and ex situ Raman spectroscopy, ex situ X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HR-TEM). The in situ Raman spectroscopy enabled real-time monitoring of structural changes under operating conditions, while ex situ Raman provided a more detailed post-mortem analysis. We revealed an energy-dependent Raman response that offered further insights into the electronic band structure and phase transitions within the material. We also observed a significant surface layer reconstruction at high potentials, where the NMC111 layered structure transitions into a cubic phase. This surface layer transformation is reversible during the initial stages of cycling but contributes to irreversible degradation with extended cycling, especially at elevated voltages. Ex situ XRD was employed to study the bulk structural evolution of NMC111, confirming that conventional XRD effectively captures large-scale structural changes during cycling, including significant volume variations. Additionally, TEM imaging revealed stress fringes in relithiated grains, indicating cycling-induced stress accumulation from unit cell changes and revealing particle cracking mechanism due to repeated volume fluctuations. This complete analysis provided a complementary view of both surface and bulk modifications, illustrating the advantages of integrating multiple characterization techniques. It offers valuable insights into the mechanisms behind the capacity fading, cracking of the material, and performance loss observed in lithium-ion batteries with NMC cathodes. This comprehensive understanding is essential for improving the design and performance of such batteries, particularly for high-voltage operation.