Characterization of Low Cobalt Cathode Degradation Using Distribution of Relaxation Times Analysis
Authors: Flannagin Megan , Gonzalez Malabet Hernando , Nelson George J. 
Published: November 2022

High-capacity cathode materials with reduced or eliminated cobalt usage are being pursued for lithium-ion batteries (LIB) due to environmental and humanitarian issues in the cobalt supply chain. Consequently, increased nickel content and reduced cobalt content results in faster cell degradation. A connection between morphological and chemical changes within the cathode microstructure, charge cut-off voltage, and operating temperature has been developed through a combination of cycling conditions, X-ray Diffraction (XRD), and Distribution of Relaxation Times (DRT) analysis.
Low cobalt half-cell batteries were fabricated with Li(Ni0.8Mn0.1Co0.1)O2 (NMC811) active material and tested under high cut-off voltage (4.3V), low cut-off voltage (4.0V), high temperature (60°C), and ambient temperature (25°C). XRD analysis of high cut-off voltage samples show a presence of NiOx crystalline structure, a low diffusivity rock-salt phase. To further understand the electrochemical changes due to the appearance of the rock-salt phase, DRT analysis was conducted on the electrochemical impedance spectra taken after formation of the solid electrolyte interphase (SEI) and after cycling. DRT analysis is becoming a preferred method of analysis due to the fact that electrochemical processes, like solid-state diffusion, cathode electrolyte interphase (CEI), and SEI formation, show up at characteristic frequencies, allowing for interpretation of changes that are often not visible within typical impedance plots. Prior to cycling, the primary contribution was initially found to be from the CEI and SEI. At low cut-off voltage cycling conditions, ambient temperature cycling showed that the contribution shifted from the CEI to diffusion resistance, but changes within the SEI and CEI were still detected. High temperature cycling conditions within the low cut-off voltage samples showed that the diffusion resistance again increased and the contributions from the CEI and SEI decreased. At high cut-off voltage cycling conditions, ambient temperature cycling showed that diffusion was becoming the main contribution but changes due to the CEI were still evident. High temperature cycling conditions in the high cut-off voltage samples showed that the mechanism was completely taken over by diffusion, due to the lithium transport being limited by the rock-salt phase or electrolyte degradation.
XRD patterns confirmed that the rock-salt phase was beginning to form within the low cut-off voltage high temperature cycling condition, but in much lower intensity than that of the high cut-off voltage condition. This observation clearly shows that the diffusion mechanism, which hinders intercalation and deintercalation, is driven by the formation of the non-conductive rock-salt phase. While primarily influenced by cut-off voltage, elevated temperature may also contribute to this degradation mechanism. The combination of different electrochemical and microstructural characterization techniques supports this observation and demonstrate that DRT analysis is an effective method to better understand the driving mechanisms behind battery degradation.