Surface Effects of Quench Methodology on Lithium Rich Nickel Manganese Oxide Cathode Particles

Abstract

Our investigation found that rapid cooling, through use of a water quench, of layered lithium rich nickel manganese oxide (LLRNMO) cathodes, post 900 °C calcination, resulted in cathode materials with significantly improved electrochemical performance and more homogenous physio-structural properties near the surfaces. Our investigation was conducted on LLRNMO from the Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 compositional series, where x = 0.25. Our material was prepared using a hybrid sol-gel route where the post 900 °C cooling step was varied between the two extremes, rapid quenching through water immersion (sample Wq), and slow natural oven cooling (sample Oc). Surface analysis of these samples was conducted before and after repeated galvanostatic-static constant-current potential limited (GCPL) cycling of coin cells with cathodes made of these active materials to elucidate any electrochemical effects that could arise from the physio-structural differences between the samples. Surface analysis was conducted using high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The rapidly quenched Wq samples were found to have higher specific capacities, higher average discharge voltages, greater cycle stability, and greater voltage stability when compared to the Oc samples. This is consistent with one of our previous investigations.1 Surface studies of the samples revealed that the pristine Oc samples had evidence of phase segregation between the nickel poor bulk phase and a nickel rich surface phase. These two phases were reminiscent of a core/shell architecture. Surface analysis conducted after the course of cycling suggested that the concentration of transition metals homogenized over the course of cycling, resulting in less distinction between the surface and bulk phases, as well as the nucleation of defect phases. These behaviors of the Oc samples are in large contrast to the Wq samples’ where the surface and bulk phases were isostructural with more homogenous concentrations of transition metals. These data suggest that more rapidly quenched samples see increased stabilization of the meta-stable R-3m phase resulting in the reported superior electrochemical properties. S. Burke and J. F. Whitacre, J. Electrochem. Soc., 167, 160518 (2020). Figure 1