Structural Study of the Li-Mn-Ni Oxide Pseudoternary System of Interest for Positive Electrodes of Li-Ion Batteries

Solution-based combinatorial samples of lithium manganese nickel oxide were synthesized and studied by X-ray diffraction in order to map out the entire pseudoternary system. The samples were made by heating to 800 degrees C in an oxygen atmosphere, and two cooling methods were tested: quenching and a slower cooling rate. This article focuses on the areas of the Gibbs triangle that lie between the layered and spinet structures as well as the single-phase layered region. The layered region is much larger than previously reported in the literature, and lattice parameters throughout the solid solution are reported. The complex coexistence region includes a two-phase region as well as two 3-phase regions. Supporting evidence demonstrating the 3-phase regions and careful identification of their corners is included here. The 3-phase regions also undergo a transformation during slow cooling that can be attributed to high entropy phases no longer being favored. In order to confirm that the findings for the small combinatorial samples are relevant to bulk samples, we present XRD patterns from a few bulk samples synthesized in oxygen as well as some heated in air. The results show that though the boundaries in the phase diagram move with synthesis conditions, the main features of the coexistence regions remain the same.