Combinatorial Studies of Compositions Containing Layered Phases in the Li–Mn–Ni–O System

Abstract

The phase diagrams obtained for quenched samples show that in both three-phase regions all samples contain some of the N and S phases, while during slow cooling they convert to structures containing at least some R and M phases. Though the thermodynamics of this are complex, a qualitative understanding can be achieved by considering the results of the Rietveld refinement. The R and M phases were collectively more ordered than the N and S phases (the N phase in particular showed considerable disorder in the hexagonal layers). This would suggest that the R and M phases have lower combined internal energies and lower combined entropies than the combination of the N and S phases. Since the structure of the three-phase regions at high temperature is driven by entropy, the N and S phases are present in all points in the regions. Upon slow cooling, a temperature is reached where internal energy becomes more important and there is still sufficient thermal energy for atomic transport such that the R and M phases begin to appear in all samples. Further study is required to have a better understanding of the thermodynamics and kinetics of these phase transformations.