The Effect of Shell Thickness, Sintering Temperature and Interdiffusion on the Electrochemical Properties of Lithium-Rich Core-Shell Cathodes
Authors: Li Jing , Doig Renny , Liu Hanshuo , Botton Gianluigi , Dahn Jeff R 
Published: February 2020

Core-shell (CS) 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.1  However, diffusion of the cations between the core and shell phases occurs during sintering 2.

 In this work, the effect of the initial shell thickness, sintering temperature and the interdiffusion in a ternary system on the electrochemical performance of CS cathodes was studied.  CS precursors with (Ni0.6Mn0.2Co0.2)(OH)2 as the core and 10 mol% (CS10), 20 mol% (CS20) or 33 mol% (CS33) (Ni0.2Mn0.6Co0.2)(OH)2 shell were first synthesized.  Lithiated samples were then prepared by sintering the precursor and LiOH with three different lithium contents (average Li/TM of 1.02, 1.04 and 1.06) for each shell content at 850 or 900oC for 10 h. The samples were labeled as CS10 (20, 33) - 850 (900) - 1 (2,3), which indicate the initial shell content, sintering temperature and lithium content, respectively.  For example, CS20-900-3 indicates a sample with 20 mol.% shell, sintered at 900oC with a Li/TM ratio of 1.06.

Figure 1 shows a SEM image and energy dispersive spectroscopy (EDS) mapping results of  CS33-850-2.  Figures 1c, 1d and 1e show that the Mn-rich shell was maintained, while the Ni and Co content at the surface is lower than that in the core (less bright) after sintering.  Figures 1a and 1b clearly show that the core and shell have two different morphologies where the core was sintered to a polycrystalline monolith nearly free of interior voids (besides some big pores), whereas the shell was composed of spiky flakes with pores in between.  This could be improved by adjusting the synthesis approach in the future.

 In order to further examine the interdiffusion phenomena in spherical CS particles, a focused ion-beam (FIB) was used to cut a thin slice (~100 nm) through the center of a randomly selected particle.  Figure 2a shows a scanning transimission electron microscope (STEM) image of the prepared slice.  The yellow line shows the path where EDS point analysis was performed.  Figure 2b shows the measured concentration profiles with symbols, calculated profiles with solid lines and simulated initial concentration profiles with dashed lines respectively.  Ni moved from the core to the shell and the Ni content on the surface changed from ~21% to ~30% during sintering, while Mn moved from the shell into the bulk, and the Mn content on the surface changed from ~57% to ~55%.  Surprisingly, Co moved into the core from the shell, even though the initial Co content in the core and shell was the same, in order to compensate for the increase of Ni content in the surface.  This is because the interdiffusion between Ni/Co is much faster than Ni/Mn as discussed in Ref. 2.  This suggests that the present of Co in the shell can accelerate the diffusion of Ni from the core to the shell.

Samples CS20-850-3 and CS33-900-3 were selected from 24 synthesized samples for testing in full cell coin cells with graphite as the counter electrode using two different electrolytes, in comparison to a commercial material (Umicore coated NMC622) designed for high voltages.  The control electrolyte was 1M LiPF6in 3:7 v:v ethylene carbonate (EC): diethylcarbonate (DEC).  PES211 electrolyte is the control electrolyte plus 2% prop-1-ene-1,3-sultone + 1% Methylene methane disulfonate + 1% tri(trimethylsilyl) phosphite (PES).  The cells were tested between 2.8 and 4.6 V with a rate of C/5 followed by one cycle of C/20 in every 20 cycles.  Figure 3 shows the capacity of the cells as a function of cycle number.  It is seen that cells with PES 211 have slightly higher capacity than the control cells.  Figure 3a shows that the cells have similar capacity retention with control electrolyte, while Figure 3b shows that CS20-850-3 has slightly better capacity retention (~90% after 100 cycles ) comparing to CS33-900-3 and coated NMC622,

References

(1)      Li, J.; Camardese, J.; Shunmugasundaram, R.; Glazier, S.; Lu, Z.; Dahn, J. R. Chem. Mater.
2015, 27, 3366–3377.

(2)      Li, J.; Doig, R.; Camardese, J.; Plucknett, K.; Dahn, J. R.;Chem. Mater. 2015, 27 (22), 7765–7773.

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Figure 1