Understanding Degradation Mechanisms of Layered Transition Metal Oxide Cathodes through Multiscale Characterization
Authors: Wen Jianguo , Zhou Tao , Yu Lei , Liu Tongchao , Amine Khalil 
Published: December 2024

Co has been extensively used in lithium-ion battery (LIB) cathodes with LiTMO2 structure (TM: transition metals) due to its high specific capacity and stability. However, the scarcity and costliness of Co have prompted researchers to explore alternatives utilizing more abundant and cost-effective elements like Mn and Ni. Despite these efforts towards Co reduction or elimination, there remains a critical knowledge gap concerning the specific functions of cobalt in cathode capacity and structural stability. In this study, we employed advanced characterization techniques including transmission electron microscopy (TEM) and X-ray diffraction (XRD) to investigate the roles of cobalt in Co-free Ni-rich cathodes, as well as to elucidate degradation mechanisms in Li- and Mn-rich cathodes.

To understand the Co roles, Co-rich LiNi0.6Co0.4O2, (NC64) and Mn-substituted Co-free LiNi0.6Mn0.4O2 (NM64) were used [1]. TEM observations show that both NC64 and NM64 surface structures change to a rock-salt phase after prolonged cycling, suggesting surface phase transformation was independent of the Co and Mn content. However, the surface structures for NC64 and NM64 during the cycling exhibit clear difference. The subsurface structure of NC64 completely transited to a Co3O4-type spinel-like structure. Such an irreversible phase transition is related to oxygen release, which triggers TM migration to form a spinel-like phase. In contrast, the structure stability of NM64 remained a typical layered structure after 100 cycles, indicating the Co reduction and Mn substitution improved the stability of the structure and oxygen of Ni-rich cathodes. Electron energy-loss spectroscopy (EELS) line scans from surface to subsurface of NM64 sample shows the O K-edge pre-peak only decreased near the surface, and the L edges of Ni and Mn also consistently shift towards lower energy loss near the surface. These results suggest that oxygen release only occurred on surface areas of the NM64 particles. In contrast, oxygen released not only occurred on the surface, but also extended into bulk NC64, because the O K-edge pre-peak of NC64 exhibited a relatively low intensity in the whole scan area of around 40 nm. In conclusion, Co is detrimental to structural/ morphological stability and oxygen reversibility. Co reduction and Mn increase could greatly alleviate these challenges in Ni-rich cathodes.

HRTEM observations indicate that Li- and Mn-rich cathodes consist of nanoscale Li2MnO3 and layered domains. To understand the voltage fade mechanism in Li- and Mn-rich cathodes [2], XRD, TEM, 3D rotation electron diffraction (3D-rED), and EELS were performed on delithiated samples to investigate lattice displacement and nanostructure evolution. XRD and TEM studies show that inhomogeneous electrochemical activities and structural difference between Li2MnO3 and layered domains result in nanoscale strain. The accumulated strain severely affects the structural stability of the composite Li- and Mn-rich cathode, which triggers the decomposition of Li2MnO3 domains, oxygen release and TM migration. The activation of Li2MnO3 and oxygen release in turn release the lattice strain at high voltages.

References

[1] Liu, Tongchao, et al. Nature Energy 6.3 (2021): 277-286.

[2] Liu, Tongchao, et al. Nature 606.7913 (2022): 305-312.

[3] Acknowledgements: Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.