Operando X-ray and Post-mortem Investigations of Electrochemical Degradation in Single-crystalline LiNi0.8Mn0.1Co0.1O2–Graphite pouch cells

Layered nickel-rich lithium transition metal oxides (LiNixMnyCo1−x−yO2; where x ≥ 0.8), with single-crystalline morphology, are promising future high-energy-density Li-ion battery cathodes due to their ability to mitigate particle-cracking-induced degradation. This is due to the absence of grain boundaries in these materials, which prevents the build-up of bulk crystallographic strain during electrochemical cycling. However, compared to their polycrystalline counterparts, there is a need to study single-crystalline Ni-rich cathodes using operando X-ray methods in uncompromised machine-manufactured industry-like full cells to understand their degradation mechanism. This can, in turn, help us identify factors to improve their long-term performance. Here, through in-house operando X-ray studies of pilot-line-built LiNi0.8Mn0.1Co0.1O2–Graphite A7 pouch cells, it is shown that their electrochemical degradation under harsh conditions (2.5–4.4 V at 40 °C for 100 cycles) primarily stems from the decreasing abundance of electrochemically active Li+ species as a result of electrode surface layer growth. Post-mortem electron imaging and tomography show that these cathodes can withstand severe anisotropic structural changes and show no cracking, even when cycled under such harsh conditions. These results are then benchmarked and contrasted with those from commercial Li-ion cells incorporating surface-modified Ni-rich cathodes, enabling us to identify the advantages of cathode surface passivation in prolonging cycle life. In addition to furthering our understanding of degradation in single-crystalline Ni-rich cathodes, this work also accentuates the need for practically relevant and reproducible fundamental investigations of Li-ion cells and presents a methodology for achieving this.