A comparative study on the oxidation state of lattice oxygen among Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3, LiNi0.5Co0.2Mn0.3O2 and LiCoO2 for the initial charge-discharge

The Li-rich layered oxides are attractive electrode materials due to their high reversible specific capacity (>250 mA h g(-1)); however, the origin of their abnormal capacity is still ambiguous. In order to elucidate this curious anomaly, we compare the lattice oxygen oxidation states among the Li-rich layered oxide Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3 and LiNi0.5Co0.2Mn0.3O2, the two components in Li-rich layered oxides, and the most common layered oxide LiCoO2 before and after initial charge-discharge. For simplicity, we employ chemical treatments of NO2BF4 and LiI acetonitrile solutions to simulate the electrochemical delithiation and lithiation processes. X-ray photoelectron spectroscopy (XPS) studies reveal that part of lattice oxygen in Li1.14Ni0.136Co0.136Mn0.544O2 and Li2MnO3 undergoes a reversible redox process (possibly O2- <-> O-2(2-)), while this does not occur in LiNi0.5Co0.2Mn0.3O2 and LiCoO2. This indicates that the extra capacity of Li-rich layered oxides can be attributed to the reversible redox processes of oxygen in the Li2MnO3 component. Thermogravimetric analysis (TGA) further suggests that the formed O-2(2-) species in the delithiated Li1.14Ni0.136Co0.136Mn0.544O2 can decompose into O-2 at about 210 degrees C. This phenomenon demonstrates a competitive relationship between extra capacity and thermal stability, which presents a big challenge for the practical applications of these materials.