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

Abstract

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 Li_1.14Ni_0.136Co_0.136Mn_0.544O₂, Li₂MnO₃ and LiNi_0.5Co_0.2Mn_0.3O₂, 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 Li_1.14Ni_0.136Co_0.136Mn_0.544O₂ and Li₂MnO₃ undergoes a reversible redox process (possibly O²⁻ ↔ O₂²⁻), while this does not occur in LiNi_0.5Co_0.2Mn_0.3O₂ 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 O₂²⁻ species in the delithiated Li_1.14Ni_0.136Co_0.136Mn_0.544O₂ can decompose into O₂ at about 210 °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.