In Situ X-ray Diffraction Study of Layered Li-Ni-Mn-Co Oxides: Effect of Particle Size and Structural Stability of Core-Shell Materials

Lithium-rich Li[LixM1-x]O-2 (M = Ni, Mn, Co) materials have been claimed to be two phase by some researchers and to be one phase by others when all the available lithium is extracted electrochemically. To clear up this confusion, the Li-rich samples [Li[Li-0.12(Ni0.5Mn0.5)(0.88)]O-2 and Li[Li-0.23(Ni0.2Mn0.8)(0.77)]O-2 with different particle sizes were synthesized for in situ X-ray diffraction experiments. In situ X-ray diffraction measurements revealed two-phase behavior of 10 mu m particles and one-phase behavior for samples with submicrometer particles. The phase separation in samples with large particles agrees with literature proposals of oxygen release from a surface layer and the observation of distinct surface and bulk phases. The small particle samples are so small that they are entirely composed of the surface phase found in the large particle samples. These results strongly suggest that the size of particles can significantly affect the structural evolution testing and electrochemical performance of the Li- and Mn-rich materials. It is proposed that the surface phase continuously grows during charge discharge cycling, which leads to voltage fade in large particle samples. Meanwhile, in situ X-ray diffraction measurements were also performed for the layered Li Ni Mn Co oxides with varying nickel contents, including NMC811 (LiNi0.8Mn0.1Co0.1O2), NMC442 (LiNi0.42Mn0.42Co0.16O2), [Li[Li-0.12(Ni0.5Mn0.5)(0.88)]O-2, and Li[Li-0.23(Ni0.2Mn0.8)(0.77)]O-2. Samples. with higher nickel content showed much faster contraction of unit cell volume as a function of cell voltage, which suggests that the core shell structures with a nickel-rich core (e.g., NMC811) and a Mn-rich shell (e.g., Li1.23Ni0.154Mn0.616O2) should not crack during charge discharge cycling.