The Improved Electrochemistry of Single-Phase Layered Li-Mn-Ni-O Materials over That of Layered-Layered Nano-Composites

The Li-Mn-Ni-O system has received much attention for potential positive electrode materials in lithium ion batteries.  In recent work [1-4], the entire phase diagram has been mapped out. Using the phase diagram as a guide, it is possible to select compositions near the boundary of the layered region.  These materials can either be single phase layered if prepared by quenching from high temperature or layered-layered nano-composites if cooled more slowly.  Work presented here will compare the electrochemistry of materials made under various synthesis conditions at such compositions.

Here, two compositions near LiNi_0.5Mn_0.5O₂are studied under various oxygen partial pressures and cooling rates.  Figure 1 (top) shows the phase diagram with the two compositions studied, A and B (B is slightly lithium rich compared to A).  XRD patterns and peak width analysis will be used to show that the materials made at B are single-phase while the only material made at A that was single-phase was made in 2% oxygen and quenched.  The samples made in air showed peak broadening attributed to phase separation and the non-quenched sample in 2% oxygen showed the smallest signs of phase separation (high angle peak broadening only).

Figure 1 (bottom) shows that the capacities of the single-phase materials are all higher than those that show phase separation and the cycling stability is comparable.  The poor performance of the layered-layered composites is attributed to the compositions of the two phases: they are not Li₂MnO₃ and LiNi_0.5Mn_0.5O₂ as promoted in the literature [5].  Instead, the end-members both contain some nickel and one contains a high proportion of nickel on the lithium layer (as high as 30% depending on synthesis conditions).  Interestingly, the sample that showed the smallest sign of phase separation in the XRD (2% O₂, RC) had the largest drop in capacity (140 to 100 mAh/g) compared to the quenched sample. 

The dramatic decrease in capacity in the sample showing the first signs of forming a layered-layered nano-composite suggests that layered-layered nano-composities should be avoided in the Li-Mn-Ni-O system.  The approach often used in research experiments of adding a small amount of excess lithium then serves to help keep the material single phase which improves the electrochemistry.

Figure 1: Top: a partial Li-Mn-Ni-O phase diagram showing how the upper layered boundary moves with temperature, synthesis condition (Q is quench, RC is regular cooling at a rate of 5°C/min) and atmosphere (air versus 2% O₂).  For all conditions, the lower layered boundary is the curved solid line joining Li₂MnO₃ to LiNiO₂.  The red lines indicate the a lattice parameter contour plots (the corresponding cones will be shown also) while the blue dotted line is a rocksalt to layered phase transition.  Bottom: capacity vs. cycle number for materials made at compositions A and B in the top panel.

References:

[1] E. McCalla and J.R. Dahn, Solid State Ionics 242, 1 (2013).

[2] E. McCalla, A.W. Rowe, R. Shunmugasundaram, and J.R. Dahn, Chem. Mater. 25, 989 (2013).

[3] E. McCalla, A.W. Rowe, C.R. Brown, L.R.P. Hacquebard and J.R. Dahn, J. Electrochem. Soc. 160, A1134 (2013).

[4] E. McCalla, A.W. Rowe, J. Camardese and J.R. Dahn, Chem. Mater. 25, 2716 (2013).

[5] M. M. Thackeray, C. S. Johnson, J. T. Vaughey and S. A. Hackney, J. Mater. Chem. 15, 2257 (2005).