Investigation into Mechanical Degradation and Fatigue of High-Ni NCM Cathode Material: A Long-Term Cycling Study of Full Cells

Nickel-rich NCMs [Li1+x(Ni1-y-z-CoyMnz)(1)O--x(2)] are among the most promising cathode materials for use in high-energy lithium-ion batteries. While the cathode composition can vary depending on the application, graphite remains the material of choice at the anode side. In this article, we study the degradation of Li1+x(Ni0.85Co0.1Mn0.05) O-1-x(2) (NCM851005) in practical graphite-based full cells over hundreds of cycles by a combination of operando X-ray diffraction, electrochemical impedance spectroscopy, and advanced electron microscopy and correlate the results with data from galvanostatic charge/discharge measurements at 1 C rate and 45 degrees C. In addition, half-cells were assembled from the cycled positive and negative electrodes for better understanding of the fatigue behavior. Electrochemical testing revealed virtually linear capacity decay and impedance growth with cycling, which can be attributed predominantly to NCM851005 degradation. Although electron microscopy indicated that severe fracture of the NCM851005 secondary particles occurred, the capacity fading is found to be due not only to mechanical degradation and/or side reactions but also to continuous surface reconstruction on the primary particle level from layered to rock-salt-like structure, thus building up a kinetic barrier.