Role of Li2MnO3 in Li-Rich Composite Cathode through Defect Reaction, Doping and Electrochemical Testing

Abstract

Li-rich layered oxide, 0.5Li2MnO3‧0.5Li(Ni1/3Mn1/3Co1/3)O2, exhibits interestingly high capacity among several cathode systems. In this study, the structure and electrochemical behaviors of Li-rich 0.5Li2MnO3‧0.5Li(Ni1/3Mn1/3Co1/3)O2 composite were investigated. To circumvent the complication of the combination of ternary transition metal and Li excess system, Li2MnO3 was fabricated and studied as cathode material alone.H2 treatment and tungsten doping on Li2MnO3 are adopted for the understanding of its electrochemical behavior and resulted defect structure. The objective of this study was to find a promising solution for improvement of Li-rich cathode materials based on the understanding of role of Li2MnO3 in Li-rich system. H2-treated oxygen-deficient Li2MnO3 initially shows high capacity resulted from oxygen oxidation (release) and the formation of VO ․․. On the other hand, such high concentration of oxygen vacancies is detrimental to its structural stability. Furthermore, the enhancement of electrochemical performance may be contributed to better electronic conductivity, expansion of lattice, and structure stability at surface influenced by tungsten doping. However, the poor cycling performance of Li2MnO3 was still observed due to the instability of surface structure resulted in the hindrance of lithium intercalation after first cycle. Such electrochemical behavior of Li2MnO3 is illustrated based on defect chemistry consideration. The electrochemical properties of Li2MnO3 were characterized by X-ray diffraction(XRD), X-ray photoemission Spectroscopy(XPS), DC conductivity measurement, Electrochemical impedance spectroscopy(EIS), scanning/transmission electron microscopy(STEM), and charge-discharge measurement.