Diffusion in the Layered Li-Ni-Mn-Co Oxide Based Core-Shell/Gradient Materials during Sintering Simulated with Pellets in Contact
Authors: Li Jing , Plucknett Kiven , Dahn Jeff R 
Published: February 2020

Core-shell structured positive electrode materials based on layered Li-Ni-Mn-Co oxide could be the next generation of positive electrode materials for high energy density lithium-ion batteries. This is because a high energy core material with poor stability against the electrolyte can be protected by a thin layer of a stable shell material.
In our previous report1,2, Li and Mn-rich materials were used as the protecting shell, and Ni-rich materials were used as the core. It was shown that the Mn-rich shell can effectively protect the Ni-rich core from reactions with the electrolyte while the Ni-rich core renders a high and stable average voltage.2However, it was also found that diffusion of the cations between the core and shell phases occurs during sintering.
Figure 1 shows energy dispersive X-ray spectroscopy (EDS) results of the core-shell precursor and lithiated samples. It is demonstrated that a Mn-rich shell was still maintained whereas the Co, which was only in the shell in the precursor, was approximately homogeneous throughout the particles after sintering at 900oC compared to the prepared hydroxide precursors.
This diffusion phenomena in core-shell/gradient materials has not been previously examined in detail, to our knowledge, despite the large amount of work on the nickel rich NMC (LiNixMnyCozO2) (without excess lithium) gradient cathode material, which has been published and cited.3–5In this work, a series of experiments to measure the interdiffusivity of the cations were designed. Pellets of core material and shell material were placed in contact and heated to induce cation diffusion over long, measurable distances. Based on the analyzed composition profiles, interdiffusion constants can be extracted.
Li1.09(Ni0.8Mn0.2)0.81O2, Li1.0
5(Co0.9Mn0.1)0.8
5O2, LiNi0.8Co0.2O2 and LiCoO2 powders were produced at 900oC in oxygen for 10 h, while Li2MnO3 was made in air. Powder a was first compressed to 15 MPa for 5 minutes, then powder b was added to the mould on top of the pellet a. The powders were compressed again to 50 MPa for 10 mins to ensure a good contact at the interface. The pellets were heated to 900oC for 10 h and then cooled slowly. The heated pellets were purposely fractured in to smaller pieces, which were subsequently embedded into Crystal BondTM, and eventually polished to a mirror surface finish for scanning electron microscopy (SEM) and EDS experiments.
Figure 2 shows the back scattered SEM image of the cross-section of a pellet that had LiCoO2 powder on the left side and LiNi0.8Co0.2O2 on the right side. Small pores are observed at the interface.
Figure 3a shows the Ni and Mn atomic concentration profile against position for the Li1.09(Ni0.8Mn0.2)0.81O2/Li2MnO3
pellet measured using EDS line scans. The symbols show the original data points. The concentration of Ni decreased from 80% to about 0% in about 2 µm while the concentration of Mn increased from 20% to about 100% over the same distance.
Figure 3b shows the Ni and Co atomic concentration profiles versus position for the LiCoO2/LiNi0.8Co0.2O2
pellet measured using EDS. Figure 3b shows that the atomic concentration of Co decreased from 100% to about 20% in about 20 µm, which is about 10 times larger than that for the Ni and Mn shown in Figure 3a, indicating the interdiffusion between Ni3+ and Co3+ is much larger than that between Ni3+ and Mn4+. This result agrees well with the trends observed in the core-shell samples shown in Figure 1.
Interdiffusion constants of Ni, Co and Mn in various couples as a function of temperature will be extracted and will be reported in the presentation.

References

1. J. Camardese, J. Li, D. W. Abarbanel, A. T. B. Wright, and J. R. Dahn, J. Electrochem. Soc., 162, A269–A277 (2014).
2. J. Li, J. Camardese, R. Shunmugasundaram, S. Glazier, Z. Lu, J. R. Dahn Chem. Mater. (2015). Article ASAP( April.8 2015)

3. E.-J. Lee, H.-J. Noh, C. S. Yoon, and Y.-K. Sun, J. Power Sources, 273, 663–669 (2015).
4. J. Ju, E. Lee, C. S. Yoon, S. Myung, and Y. Sun, J. Phys. Chem. C, 118, 175–182 (2014).
5. H. Noh, S. Myung, Y. J. Lee, and Y. Sun, Chem. Mater., 26, 5973–5979 (2014).

Figure 1