Lithium-Rich Core-Shell Cathodes with Low Irreversible Capacity and Mitigated Voltage Fade

Lithium-rich layered Ni-Mn-Co oxide materials have been intensely studied in the last decade.  Mn-rich materials have serious voltage fade issues and the Ni-rich materials have poor thermal stability and readily oxidize the organic carbonate-based electrolyte.¹  Core-shell (CS) strategies that use Ni-rich material as the core and Mn-rich materials as the shell can balance the pros and cons of these materials in a hybrid system.²  The lithium-rich CS materials synthesized through co-precipitation method introduced here showed much improved overall electrochemical performance compared to the core-only (Ni-rich) and shell-only (Mn-rich) samples.

In this work, CS samples with 67 mol% of core with the composition Li_1+x(Ni_0.67Mn_0.33)_1-xO₂  (C) and  33 mol% of shell with the compositions Li_1+x(Ni_0.2Mn_0.6Co_0.2)_1-xO₂  (S1) or Li_1+x(Ni_0.4Mn_0.5Co_0.1)_1-xO₂  (S2) with varied lithium content were synthesized and  studied.

Figure 1 shows the energy dispersive spectroscopy results of the CS precursor and lithiated samples. Figure 1 shows that there was diffusion of transition metals between the core and shell phases after sintering at 900^oC compared to the prepared hydroxide precursors.  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.

The CS samples with optimal lithium content showed low irreversible capacity (IRC), as well as high capacity and excellent capacity retention. Sample CS2-3 had a reversible capacity of ~218 mAh/g with 12.3% (~30 mAh/g) IRC and 98% capacity retention after 40 cycles to 4.6 V at 30^oC at a rate of ~C/20. 

Figure 2 shows the average discharge potential (a) and the difference between the average charge and discharge potentials, delta V (b), of cells with the core-only (C), CS and shell-only (S) samples. Figure 2 shows that cells of the CS samples have stable impedance (delta V is stable) as well as a very stable average voltage as compared to cells of the core-only and shell-only samples. Ultra-high precision coulometry (UHPC) measurements confirmed that the CS samples with optimal lithium content had distinctively better columbic efficiency.

Apparently, 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.

Reference

1. E.-J. Lee, H.-J. Noh, C. S. Yoon, and Y.-K. Sun, J. Power Sources, 273, 663–669 (2015).

2. J. Camardese, J. Li, D. W. Abarbanel, a. T. B. Wright, and J. R. Dahn, J. Electrochem. Soc., 162, A269–A277 (2014).