Materials Near the Layered Boundary

Abstract

The structural changes taking place during regular cooling of materials with metallic compositions near that of LiNi_0.5Mn_0.5O₂ were studied after heating under various oxygen partial pressures and were related to features in the phase diagram determined previously. Materials made with 5 $\%$ excess lithium (at point B₉ in Fig. 9.1) were found to stay single phase even when the regular cooling rate of 5–10 °C/min was used. These single-phase layered materials showed good electrochemical performance with a reversible capacity of 180 mAh/g for the quenched sample and 170 mAh/g for the regular cooled sample when cycled up to 4.8 V. By contrast, under the same cycling conditions, samples without excess lithium (at point A₉ in Fig. 9.1) made in 2 $\%$ oxygen had a capacity of 140 mAh/g when quenched and only 90 mAh/g when regular cooled. The changes in the XRD pattern for this A₉ sample were small with peak broadening only seen at high angle, consistent with the first sign of phase separation into layered–layered nano-composites. The dramatic loss in capacity seen in the sample made in 2 $\%$ oxygen at point A₉ can be attributed to nickel clustering on the lithium layer such that lithium islands form, many of which would be surrounded by the clustered nickel. This behavior was expected from the phase diagram where phase separation into a nickel rich and nickel poor phase was demonstrated in Chap. 6, and this clustering of nickel was also seen in a Monte Carlo simulation performed in this chapter.