Structure and electrochemical behaviors of spherical Li1+xNi0.5Mn0.5O2+δ synthesized by rheological phase reaction method

Spherical layered Li1+xNi0.5Mn0.5O2+δ (x = 0, 0.1 and 0.2) materials are synthesized by rheological phase reaction method. X-ray diffraction patterns reveal that the prepared materials have the α-NaFeO2 structure with R-3m space group. The materials with different lithium contents show distinct differences in structure, particle morphology, electrochemical property and safety characteristics. When x = 0.1 and 0.2, excess lithium resides in the transition metal layer to form Li2MnO3-like structure. With lithium content increasing from 1.0 to 1.2, the initial discharge capacity and Coulombic efficiency are decreased, while capacity maintenance is improved. Mn ions are electrochemically activated in the charge-discharge process and has a contribution to the capacity when cycling in a voltage window of 2.5-4.5 V for x = 0.2, leading to a high discharge capacity of near 200 mAh g−1 and the excellent cycling performance (with no capacity decay after 100 cycles) at 20 mA g−1. Electrochemical impedance spectroscopy shows that the appropriate excess of lithium can decrease charge transfer resistance obviously in the initial several cycles. The differential scanning calorimetric indicates that the generated heat of Li1+xNi0.5Mn0.5O2+δ is increased with the increase of x.