Evolution of Structure and Lithium Dynamics in LiNi0.8Mn0.1Co0.1O2 (NMC811) Cathodes during Electrochemical Cycling

The nickel-rich layered oxide LiNi0.8Mn0.1Co0.1O2 (NMC811) is a promising future cathode material for lithium-ion batteries in electric vehicles due to its high specific energy density. However, it exhibits fast voltage and capacity fading. In this article, we combine electrochemistry, operando synchrotron X-ray diffraction (XRD), and ex situ solid-state NMR spectroscopy to provide new insights into the structural changes and lithium dynamics of NMC811 during electrochemical charge and discharge, which are essential for a better understanding of its fast degradation. The evolution of the interlayer spacing is tracked by XRD, showing that it gradually increases upon delithiation before collapsing at high state-of-charge (SOC). Importantly, no two-phase O3 → O1 transition is observed at high SOC, demonstrating that this cannot be a major cause of degradation. A strong increase of Li dynamics accompanies the increase of the interlayer spacing, which is shown by 7Li NMR and electrochemical characterization. A...