Voltage Fade Mechanism Study Of Lithium-Manganese-Rich Nickel Manganese Cobalt Oxides

Lithium-manganese-rich nickel manganese cobalt oxides (LMR-NMC) are considered as promising cathode materials for advanced lithium-ion batteries to power electrical vehicles (EVs), owing to their low-cost, long life, and high performance. However, the deployment of this class of materials was hindered because of their intrinsically structural instability, a continuous decrease of their average working potential during cycling (called voltage fade latter) that leads to a rapid fade in the energy density without a significant fade in the reversible capacity. LMR-NMC have a complex structure containing two component, Li2MnO3and LiTMO2(TM=transition metal), leading to difficulty in establishing their structure-property relationship. Therefore, to understand the voltage fade mechanism of LMR-NMC, this work firstly investigated the Li2MnO3and LiCoO2model systems, respectively. Then by study the local structure of LMR-NMC, the mechanism of electrochemical activation, and the spinel-like phase forming after electrochemical activation, the voltage fade mechanism was determined at last. Additionally advanced synchrotron probes were used to get high quality structure data. Detailed discussions regarding all these topics are provided as following:1. Both in situ high-energy X-ray diffraction and in situ X-ray absorption spectroscopy were used to investigate the structural evolution of materials during the solid-state synthesis of Li2MnO3. Combining X-ray absorption spectroscopy and factor analysis techniques, we were able to capture the spectrum and evolution of an intermediate phase (MnO2) that could not be detected by the diffraction technique. Meanwhile, the X-ray diffraction data clearly showed the anisotropic crystallization of Li2MnO3during sintering above600℃.2. In situ high-energy X-ray diffraction was carried out to investigate the structural transformation of lithium cobalt oxide (LiCoO2) during the solid-state synthesis. Two allotropic phases were observed during the synthesis process; Li2Co2O4with a spinel structure was formed within the temperature window between450℃and650℃, beyond which Li2CO2O4was converted to its hexagonal counterpart, layered LiCoO2, through a cation exchange between Li and Co. In electrochemical tests, the Li2Co2O4was estimated to have a very low reversible capacity,～20mAh·g-1, and a high initial irreversible capacity loss of about80 mAh·g-1. An interfacial phase between layered LiCoO2domain and spinel Li2CO2O4domain was also identified by ex situ high-resolution X-ray diffraction.3. In situ synchrotron probes, as well as Monte Carlo simulation, were carried out to investigate the formation and electrochemical activation mechanism of this class of materials. Both the theoretical and experimental results revealed that the potential drop is an intrinsic thermodynamically driven process, relating to the structural instability of highly delithiated Li2MnO3component. It was experimentally illustrated that the potential drop could be kinetically suppressed by a careful control on the domain size and the degree of electrochemical activation of Li2MnO3components in lithium-rich transition metal oxides.4. Li2MnO3is an integrated component in lithium-manganese-rich nickel manganese cobalt oxides, and the conversion of Li2MnO3to spinel-like structure after electrochemical activation has been affiliated to the continuous potential decay of the material. Delithiated LiMnO3and delithiated LiMn2O4were used as model materials to investigate the mechanism of forming the spinel-like structure. In situ high-energy X-ray diffraction technique was used to trace the structural change of materials at elevated temperatures, a procedure to mimic the structural transformation during the normal cycling of batteries. It was also found that the migration of Mn atoms from the octahedral sites to tetrahedral sites is the key step for phase transformation from monoclinic structure to spinel structure.