Identifying the Origins of Microstructural Defects Such as Cracking within Ni-Rich NMC811 Cathode Particles for Lithium-Ion Batteries

The next generation of automotive lithium-ion batteries may employ NMC811 materials; however, defective particles are of significant interest due to their links to performance loss. Here, it is demonstrated that even before operation, on average, one-third of NMC811 particles experience some form of defect, increasing in severity near the separator interface. It is determined that defective particles can be detected and quantified using low resolution imaging, presenting a significant improvement for material statistics. Fluorescence and diffraction data reveal that the variation of Mn content within the NMC particles may correlate to crystallographic disordering, indicating that the mobility and dissolution of Mn may be a key aspect of degradation during initial cycling. This, however, does not appear to correlate with the severity of particle cracking, which when analyzed at high spatial resolutions, reveals cracking structures similar to lower Ni content NMC, suggesting that the disconnection and separation of neighboring primary particles may be due to electrochemical expansion/contraction, exacerbated by other factors such as grain orientation that are inherent in such polycrystalline materials. These findings can guide research directions toward mitigating degradation at each respective length-scale: electrode sheets, secondary and primary particles, and individual crystals, ultimately leading to improved automotive ranges and lifetimes.