Layered nickel-rich lithium transition-metal oxides (${\mathrm{Li}\mathrm{Ni}}_x{\mathrm{Mn}}_y{\mathrm{Co}}_{1-x-y}{\mathrm{O}}_2$; where x ≥ 0.8), with single-crystalline morphology, are promising future high-energy-density $\mathrm{Li}$-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. Compared to their polycrystalline counterparts, there is a need to study single-crystalline $\mathrm{Ni}$-rich cathodes using operando x-ray methods in uncompromised machine-manufactured industrylike full cells to understand their bulk degradation mechanisms as a function of different electrochemical cycling protocols. This can help us identify factors to improve their long-term performance. Here, through in-house operando x-ray studies of pilot-line-built ${\mathrm{Li}\mathrm{Ni}}_{0.8}{\mathrm{Mn}}_{0.1}{\mathrm{Co}}_{0.1}{\mathrm{O}}_2$–graphite A7 pouch cells, it is shown that their electrochemical-capacity fade under harsh conditions (2.5–4.4 V and 40 °C for 100 cycles at a C/3 rate) primarily stems from the high-voltage reconstruction of the cathode surface from a layered to a cubic (rock-salt) phase that impedes the ${\mathrm{Li}}^{+}$ kinetics and increases cell impedance. Postmortem electron and x-ray microscopy show that these cathodes can withstand severe anisotropic structural changes and show no cracking when cycled under such conditions. Comparing these results to those from commercial $\mathrm{Li}$-ion cells with surface-modified single-crystalline $\mathrm{Ni}$-rich cathodes, it is identified that cathode surface passivation can mitigate this type of degradation and prolong cycle life. In addition to furthering our understanding of degradation in single-crystalline $\mathrm{Ni}$-rich cathodes, this work also accentuates the need for practically relevant and reproducible fundamental investigations of $\mathrm{Li}$-ion cells and presents a methodology for achieving this.

• Received 23 June 2023
• Revised 5 December 2023
• Accepted 18 December 2023

DOI:https://doi.org/10.1103/PRXEnergy.3.013004

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Published by the American Physical Society