Investigations in lithium stoichiometry and secondary phase content in lithium manganese spinel cathode materials
Sun, Ruoheng; Eichel, Rüdiger-Albert (Thesis advisor); Simon, Ulrich (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2019
Lithium-ion battery (LIB) is one of the potential solutions in near future for stationary storage of renewable energy and power supply for electric vehicles. LIB technology has led to a massive advance of its performance over the last few decades. Its development is recently accelerated by automobile industry in order to adapt to the requirements of the electric cars. Large quantities of research have focused on cathode materials because they work as the reservoir of lithium and limit the energy density of LIB. Spinel-type lithium manganese oxides, especially Li1+xMn2-xO4 with 0 < x < 1/3 have been commercialized as cathode materials. The stoichiometry factor x, with respect to the atomic ratio between Mn4+ and Mn3+, has an impact on the electrochemical performance of this material. Furthermore, trace amounts of Li2MnO3 usually occur as a secondary phase in synthesized Li1+xMn2-xO4 in the common high temperature of solid-state synthesis, affecting the overall Li-Mn stoichiometry in the spinel phase and thereby the electrochemical performance. This PhD work focuses on investigating the relationship between secondary-phase Li2MnO3, stoichiometry and electrochemistry of Li1+xMn2-xO4.The formation of secondary-phase Li2MnO3 in trace amounts during the synthesis of lithium manganese spinel has a considerable influence on the stoichiometry of the spinel host. The detection and quantitative analysis of trace amounts of Li2MnO3 in spinel host is challenging. In Chapter 3, an efficient methodology is developed to analyze quantitatively trace amounts of Li2MnO3 formed in lithium manganese spinel hosts by exploiting the different line shapes between Li2MnO3 and lithium manganese spinel in Electron Paramagnetic Resonance (EPR) spectrum. The superior sensitivity of EPR enables a detected Li2MnO3 mass-fraction less than 10-2 wt.%. A successful separation of the EPR signals belonging to Li2MnO3 and lithium manganese spinel is achieved. According to the quantified Li2MnO3 mass-fraction, the changes in the stoichiometry of lithium manganese spinel materials are determined. The result indicates that even Li2MnO3 exists in trace amount, its impact on the stoichiometry factor x in Li1+xMn2-xO4 should be considered. By resolving the EPR spectra of Li1+xMn2-xO4 from this methodology, a narrowed EPR linewidth along the increase of stoichiometry factor x is identified due to the enhanced exchange interaction inside the spinel lattice. In Chapter 4, study is focused on the reaction between Li1+xMn2-xO4 and secondary phase Li2MnO3 along with heat treatments at temperatures between 773 K and 873 K. At moderately high temperature, Secondary-phase Li2MnO3 can react with the spinel phase during heat treatment. This reaction appears to be clear that two educts namely LMO spinel and Li2MnO3 produce a new spinel phase with higher Li-content. But the mechanism and the kinetics of this reaction are challenging to be investigated. The physical and chemical processes during the reaction have not yet been clarified. A strategy is established to monitor experimentally the change in Li2MnO3 amount in the synthesized LMO spinel materials by implementing Electron Paramagnetic Resonance (EPR) spectroscopy. The observation of the reaction process is succeeded by recording the subtle decrease in Li2MnO3 at different temperatures along varied reaction times. It is shown that the kinetic behavior could be assigned to either pseudofirst-order or second-order depending on the microstructure of Li2MnO3 as well as the Li-content of the educt spinel phase. The product spinel phase which is transformed originally from Li2MnO3 phase has been visualized by SEM, presenting a distinct morphology. At last, the reaction mechanism has been described in detail including lithium diffusion, oxygen exchange and rearrangement of atom positions. It addresses an interpretation of how monoclinic Li2MnO3 does integrate into lithium manganese spinel hosts. According to the studies in previous chapters, the exact stoichiometries of synthesized LMO spinels have been determined, where the stoichiometry factor x varies from 0.000 to 0.182. Those synthesized LMO spinel materials are casted into electrodes and assembled into half cells. The electrochemical data from Chapter 5 provides a comparison of varied electrochemical performances along with different stoichiometries of the samples. The obtained experimental capacities of synthesized Li1+xMn2-xO4 with x > 0 coincide quantitatively with their theoretical capacities after correcting the stoichiometry factors x according to Li2MnO3 mass-fractions from EPR analysis. Moreover, ex-situ EPR present a systematic change of linewidths with different states of charge. The linewidths decrease during the solid-solution reaction region and increase during the two-phase reaction region.