Structural characterization of layered LixNi0.5Mn0.5O2 (0 < x ≤ 2) oxide electrodes for Li batteries

X-ray diffraction and X-ray absorption spectroscopy experiments were used to study chemical and electrochemical Li insertion and extraction reactions of LiNi0.5Mn0.5O2. These results, along with galvanostatic cycling data, suggest that LiNi0.5Mn0.5O2 layered electrodes in lithium batteries operate predominantly off two-electron redox couples, Ni4+/Ni2+, between approximately 4.5 and 1.25 V and Mn4+/Mn2+ between 1.25 and 1.0 V versus metallic Li, respectively. The retention of a stable layered framework structure and the apparent absence of Jahn-Teller ions Ni3+ and Mn3+ in the high- or low-voltage region is believed to be responsible for the excellent structural and electrochemical stability of these electrodes. The LiNi0.5Mn0.5O2 layered oxide reversibly reacts chemically or electrochemically with Li to form an air-sensitive, dilithium. compound, Li2Ni0.5Mn0.5O2, with a hexagonal structure analogous to Li2MnO2. The cycling behavior of Li/LiNi0.5Mn0.5O2 cells over a large voltage window (4.6-1.0 V) and with very slow rates shows that rechargeable capacities >500 mA(.)h/g can be obtained.