Synthesis of layered Li1.2+x[Ni0.25Mn0.75]0.8−xO2 materials (0 ≤ x ≤ 4/55) via a new simple microwave heating method and their electrochemical properties

Li1.2+x[Ni0.25Mn0.75]0.8−xO2 (0 ≤ x ≤ 4/55) was prepared by a new simple microwave heating method and the effect of extra Li+ content on electrochemistry of Li1.2Ni0.2Mn0.6O2 (x = 0) was firstly revealed. X-ray diffraction identified that they had layered α-NaFeO2 structure (space group R-3m). Linear variation of lattice constant as a function of x value supported the formation of solid solution, that is, extra Li+ is possibly incorporated in structure of layered Li1.2Ni0.2Mn0.6O2 (x = 0), accompanying oxidization of Ni2+ to Ni3+ to form Li1.2+x[Ni0.25Mn0.75]0.8−xO2 (0 ≤ x ≤ 4/55). This was confirmed by X-ray photoelectron spectroscopy that Ni3+ appeared and increased in content with increasing x value. Charge–discharge tests showed that Li1.2+x[Ni0.25Mn0.75]0.8−xO2 (0 ≤ x ≤ 4/55) truly displayed different electrochemical properties (different initial charge–discharge plots, capacities and cycleability). Li1.2Ni0.2Mn0.6O2 (x = 0) in this work delivered the highest discharge capacity of 219 mAh g−1 between 4.8 and 2.0 V. Increasing Li content (x value in Li1.2+x[Ni0.25Mn0.75]0.8−xO2) reduced charge–discharge capacities, but significantly enhancing cycleability.