Anisotropic Lattice Strain and Mechanical Degradation of High- and Low-Nickel NCM Cathode Materials for Li-Ion Batteries

In the near future, the targets for lithium-ion batteries concerning specific energy and cost can advantageously be met by introducing layered LiNi_xCo_yMn_zO₂ (NCM) cathode materials with a high Ni content (x ≥ 0.6). Increasing the Ni content allows for the utilization of more lithium at a given cell voltage, thereby improving the specific capacity but at the expense of cycle life. Here, the capacity-fading mechanisms of both typical low-Ni NCM (x = 0.33, NCM111) and high-Ni NCM (x = 0.8, NCM811) cathodes are investigated and compared from crystallographic and microstructural viewpoints. In situ X-ray diffraction reveals that the unit cells undergo different volumetric changes of around 1.2 and 5.1% for NCM111 and NCM811, respectively, when cycled between 3.0 and 4.3 V vs Li/Li⁺. Volume changes for NCM811 are largest for x(Li) < 0.5 because of the severe decrease in interlayer lattice parameter c from 14.467(1) to 14.030(1) Å. In agreement, in situ light microscopy reveals that delithiation leads to different volume contractions of the secondary particles of (3.3 ± 2.4) and (7.8 ± 1.5)% for NCM111 and NCM811, respectively. And postmortem cross-sectional scanning electron microscopy analysis indicates more significant microcracking in the case of NCM811. Overall, the results establish that the accelerated aging of NCM811 is related to the disintegration of secondary particles caused by intergranular fracture, which is driven by mechanical stress at the interfaces between the primary crystallites.