Predominant growth orientation of Li1.2(Mn0.4Co0.4)O2cathode materials produced by the NaOH compound molten salt method and their enhanced electrochemical performance

Abstract

Li-rich layered cathode materials Li1.2(Mn0.4Co0.4)O2 with excellent crystal structure and enhanced electrochemical performance were synthesized by a facile compound molten salt method with different molar ratios of mineralizer NaOH to transition metals R (0, 2.5, 5 and 10). The effects of the molar ratio of NaOH to R on the morphology, selective orientation growth, and electrochemical properties of the as-prepared material were investigated through X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), and galvanostatic charge–discharge tests. With the introduction of NaOH into the molten salt reaction system, the crystals grew along the [0001] crystal axis predominantly and the particle morphology changed from hexagonal tablets to columns, which resulted in enhanced electrochemical performance by facilitating Li ion migration. The initial capacity increased from 220 mA h g−1 (R = 0) to 258 mA h g−1 (R = 5), and the capacity retention improved from 70.0% (R = 0) to 89.9% (R = 5) at a current density of 0.1 C after 50 cycles. Furthermore, by using high resolution TEM (HRTEM) and electron energy loss spectroscopy (EELS), the crystal local structure variation and Mn ion valence reduction (Mn4+ to Mn3+) were investigated, which are relevant to the capacity loss after charge–discharge cycling. Our work demonstrated that the prepared particle crystal structure was improved in NaOH flux, and the additional formation of a spinel-like structure was remarkably suppressed during cycling, which contributed to the improved electrochemical properties.