Structural Characterization of Layered Li_xNi_0.5Mn_0.5O₂ (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 LiNi 0.5 Mn 0.5 O 2 . These results, along with galvanostatic cycling data, suggest that LiNi 0.5 Mn 0.5 O 2 layered electrodes in lithium batteries operate predominantly off two-electron redox couples, Ni 4+ /Ni 2+, between approximately 4.5 and 1.25 V and Mn 4+ /Mn 2+ 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 Ni 3+ and Mn 3+ in the high- or low-voltage region is believed to be responsible for the excellent structural and electrochemical stability of these electrodes. The LiNi 0.5 Mn 0.5 O 2 layered oxide reversibly reacts chemically or electrochemically with Li to form an air-sensitive, dilithium compound, Li 2 Ni 0.5 Mn 0.5 O 2, with a hexagonal structure analogous to Li 2 MnO 2 . The cycling behavior of Li/LiNi 0.5 Mn 0.5 O 2 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.