Effect of excess lithium on LiNi0.5Mn0.5O2+δ and its electrochemistry as lithium insertion material

Layered Li–Ni–Mn–O compounds were successfully synthesized by an emulsion drying method, in which the emulsion makes it possible to intermix cations in atomic scale and therefore leads to homogeneous and fine oxide products. The as-synthesized powders were crystallized into a well-ordered α-NaFeO2 (R3̄ m) structure. The calculated lattice parameters varied by changing the lithium amounts. The lattice parameters of Li1.13Ni0.5Mn0.5O2+z showed smaller a- and c-axes comparing to those of the LiNi0.5Mn0.5O2, whereas the c/a ratio remained constant for Li1+xNi0.5Mn0.5O2+z (0≤x≤0.13) compounds. Comparing to LiNi0.5Mn0.5O2 and Li1+xNi0.5Mn0.5O2+z electrodes, the delivered capacity for the Li1.13Ni0.5Mn0.5O2+z was much higher that that of the LiNi0.5Mn0.5O2. The lithium excess Li1+xNi0.5Mn0.5O2+z sample exhibited very stable cyclability with high capacity of about 175 mA h (g oxide)−1 after the 50th cycle at 25 °C (I=20 mA g−1) and 50 °C (75.5 mA g−1), which indicates that structural stability of the lithium excess Li1+xNi0.5Mn0.5O2+z is superior to that of the stoichiometric LiNi0.5Mn0.5O2.