Synthesis–Structure–Property Relations in Layered, “Li-excess” Oxides Electrode Materials Li [ Li1 / 3 − 2x / 3NixMn2 / 3 − x / 3 ] O2 (x = 1 / 3 , 1/4, and 1/5)

Abstract

Relations between synthesis conditions, detailed crystal structures, and electrochemical properties of the Li-excess layered oxides are studied by X-ray diffraction, scanning electron microscopy (EELS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electron energy-loss spectrometry, combined with electrochemical property measurements including potentiostatic intermittent titration technique (PITT). Optimal synthesis conditions are obtained for stoichiometric samples sintered at in air followed by furnace cooling. The materials exhibit capacities of , 230, and 200 mAh/g within a voltage range of 2–4.8 V on discharge for , 1/4 and 1/3, respectively. Diffraction data of electrochemically cycled electrode materials show an expanded lattice ratio and changing Li/Ni interlayer mixing indicating peculiar cation migration in the structures. High resolution TEM images and XPS spectra show obvious differences in the surface characteristics of the samples synthesized with stoichiometric and excess amount of LiOH, suggesting that surface characteristics is one of the contributing factors to the difference in electrochemical properties. Our results suggest that the first cycle irreversible capacity is affected by both the bulk and surface characteristics of pristine materials, which is strongly influenced by precursor chemistry. The PITT results suggest that cation rearrangement during the charge/discharge has a significant impact on the lithium chemical diffusivity.