Aluminum-based coatings are commonly used in lithium-ion batteries to modify the surface of LiCoO2 particles, to limit cobalt dissolution in the electrolyte at high voltage. It was shown that the ...formation of a LiCo1−x Al x O2 solid solution occurs at the interface between the coating and the core material. In this paper, we investigated the surface properties of LiCo1−x Al x O2 materials by X-ray photoelectron spectroscopy. We explored the surface acid−base properties of these materials by adsorption of gaseous probe molecules (NH3 and SO2) followed by XPS analyses. We showed that the basic character of the LiCo1−x Al x O2 surface strongly decreases when x increases, which makes these materials less reactive than LiCoO2 toward acidic species (such as HF) that are present in LiPF6-based electrolytes. This is a possible explanation for the efficiency of Al-based coatings to protect LiCoO2 against cobalt dissolution in the electrolyte.
Zinc/cobalt aluminates with spinel-type structure were prepared by a polymeric route, leading to a pure phase with controlled grain size. The prepared pigments were characterized by powder X-ray ...diffraction Rietveld analyses in order to determine structural features, scanning electron microscopy for morphological investigation, helium pycnometry and (27)Al MAS NMR in order to highlight the occurrence of defects inside the structure, and UV-visible-near-IR spectroscopy to identify electronic transitions responsible for the compounds' color. The green-blue coloration of these pigments is known to be dependent on the sample thermal history. Here, for the first time, the Zn(1-x)Co(x)Al(2)O(4) color is newly interpreted. The pigment is green once synthesized at low temperature (i.e., with diminution of the pigment grain size); this variation was attributed to the appearance of a new absorption band located at about 500 nm, linked to a complex network feature involving Co ions in octahedral sites as well as oxygen and cationic vacancies. Hence, this work shows the possibility of easily getting a nonstoichiometric network with an abnormal cationic distribution from "chimie douce" processes with moderate synthesis temperature, and so various colorations for the same composition.
Zinc/cobalt aluminates with spinel-type structure were prepared by a polymeric route, leading to a pure phase with controlled grain size. The prepared pigments were characterized by powder X-ray ...diffraction Rietveld analyses in order to determine structural features, scanning electron microscopy for morphological investigation, helium pycnometry and 27Al MAS NMR in order to highlight the occurrence of defects inside the structure, and UV−visible−near-IR spectroscopy to identify electronic transitions responsible for the compounds' color. The green-blue coloration of these pigments is known to be dependent on the sample thermal history. Here, for the first time, the Zn1−x Co x Al2O4 color is newly interpreted. The pigment is green once synthesized at low temperature (i.e., with diminution of the pigment grain size); this variation was attributed to the appearance of a new absorption band located at about 500 nm, linked to a complex network feature involving Co ions in octahedral sites as well as oxygen and cationic vacancies. Hence, this work shows the possibility of easily getting a nonstoichiometric network with an abnormal cationic distribution from “chimie douce” processes with moderate synthesis temperature, and so various colorations for the same composition.
The 6,7Li MAS NMR spectra of lithium ions in paramagnetic host materials are extremely sensitive to number and nature of the paramagnetic cations in the Li local environments and large shifts (Fermi ...contact shifts) are often observed. The work presented in this paper aims to provide a rational basis for the interpretation of the 6,7Li NMR shifts, as a function of the lithium local environment and electronic configuration of the transition metal ions. We focus on the layered rocksalts often found for LiMO2 compounds and on materials that are isostructural with the K2NiF4 structure. In order to understand the spin-density transfer mechanism from the transition metal ion to the lithium nucleus, which gives rise to the hyperfine shifts observed by NMR, we have performed density functional theory (DFT) calculations in the generalized gradient approximation. For each compound, we calculate the spin densities values on the transition metal, oxygen and lithium ions and map the spin density in the M-O-Li plane. Predictions of the calculations are in good agreement with several experimental results. We show that DFT calculations are a useful tool with which to interpret the observed paramagnetic shifts in layered oxides and to understand the major spin-density transfer processes. This information should help us to predict the magnitudes and signs of the Li hyperfine shifts for different Li local environments and t2g vs eg electrons in other compounds.
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