Understanding and Improving Cycling Performance of Li 1.2Ni0.15Mn0.55Co0.1O2-Graphite Lithium-Ion Batteries

Owing to their high capacity and energy density, lithium- and manganese-rich transition metal layered oxides (LMR-NMC) are considered promising cathode materials for lithium-ion batteries to power electrical vehicles (EVs) in the future. Especially, Li1.2Ni0.15Mn0.55Co0.1O2, which can also be expressed in the two-component notation as 0.5Li2MnO3∙0.5LiNi0.375Mn0.375Co0.25O2, is drawing a lot of interest worldwide. Therefore, evaluation of this material in practical lithium-ion battery systems is important. In this work, I have conducted research on a lithium-ion battery system bearing Li1.2Ni0.15Mn0.55Co0.1O2/graphite during extended cycling. The data indicate that this system shows significant performance degradation, including severe capacity fade, large impedance rise and voltage fade. A series of comprehensive materials characterization techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), Raman spectroscopy, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS), are applied to analyze the harvested electrodes and electrolyte. The data show that capacity fade is mainly caused by lithium trapping in the continuous solid-electrolyte interphase (SEI) formation on the surface of the graphite-based negative electrode. Impedance rise and voltage fade are coming from the Li1.2Ni0.15Mn0.55Co0.1O2-based positive electrode. Recognizing the origin of performance degradation has enabled us to develop effective strategies to extend the cell life. The strategies, such as using alumina powder as an additive for the positive electrode, applying atomic layer deposition (ALD) alumina coatings onto the whole positive electrode, or applying a pulsed-sonochemical method to coat the oxide particles with alumina, have been used to enhance the performance of full-cells. All of these strategies effectively improve the capacity retention of the full-cells. ALD coating of the whole positive electrode and pulsed-sonochemical coating of the individual oxide particles also successfully mitigate the impedance rise of the cell. However, for all those strategies, there is little-to-no effect on voltage fade, which is a characteristic of this cathode material. In order to understand the effects of voltage fade on the total energy density, a method, which uses a resistance-corrected average voltage, is proposed to track and quantify voltage fade of this LMR-NMC material. In addition to that, the effects of cycling window, temperature and the voltage fade behaviors of different cathode materials at high voltages are discussed and compared. Possible voltage fade mechanisms are also summarized to provide possible strategies to mitigate voltage fade in the future. Detailed discussions regarding all these topics are…