Structural and Electrochemical Study of the Li-Mn-Ni Oxide System within the Layered Single Phase Region

The phase diagram of the Li-Mn-Ni-oxide pseudoternary system1 was used as a starting point to understand the influence of transition metal composition and lithium content on the structures and electrochemistry of materials with compositions: Li1+x(NiyMn1-y)(1-x)O-2 (y = 0.2, 0.33, 0.4, 0.5, 0.6, and 0.7, 0 = x = 0.34). Mixed transition metal hydroxide precursors were synthesized via coprecipitation using a continuously stirred tank reactor. Powder X-ray diffraction results showed that additional spinel, rocksalt and/or layered phases were observed when the lithium content was less than that required for a single layered phase in agreement with ref 1. Additionally, residual Li2CO3 was detected in the backscattered scanning electron microscopy images for samples with relatively high lithium content. The boundaries of the single phase region were therefore defined and the contour plots of the lattice constants versus composition in the single phase region were generated. Electrochemical measurements showed that phase separation should be avoided and single phase samples should be prepared in order to obtain the highest capacity. The maximum reversible capacity to 4.6 V was found near the middle of the single phase region when 0.5 < y < 0.7, whereas it was at the top (smallest lithium content) of the single phase region when y <= 0.5.