While many perovskites remain crystalline during the oxygen evolution reaction (OER) in alkaline media, some highly active perovskites become amorphous. We studied the local structure changes of perovskites LaCoO3, Ba0.5Sr0.5Co0.8Fe0.2O3‑δ, and SrCo0.8Fe0.2O3‑δ before and after OER by X-ray absorption spectroscopy. No change in either local structure or OER activity was observed for LaCoO3, while considerably enhanced OER activities and the conversion of the local structure from corner-sharing octahedra to edge-sharing octahedra were noted for Ba0.5Sr0.5Co0.8Fe0.2O3‑δ and SrCo0.8Fe0.2O3‑δ as a result of the OER. Possible processes responsible for the structural change and enhanced OER activities are discussed.

AlPO 4 "-coated LiCoO 2 was shown to exhibit markedly improved capacity retention and reduced impedance growth relative to bare "LiCoO 2 " upon cycling to 4.7 V. Scanning electron microscopy imaging showed that the surfaces of the cycled bare "LiCoO 2 " particles remained very smooth whereas there were many newly formed patches distributed on the surfaces of the cycled coated particles. X-ray powder diffraction analyses revealed that select peak broadening was observed for cycled bare electrodes suggesting that structural damage to Li x CoO 2 was introduced upon cycling. In contrast, no apparent structural changes to Li x CoO 2 were found for cycled coated electrodes. Pristine and cycled bare and "AlPO 4 "-coated LiCoO 2 electrodes were studied by X-ray photoelectron spectroscopy. No significant change was detected in the surface chemistry of Co for cycled bare electrodes, but surface LiF and Li x PF y O z components were found to considerably increase during cycling, which led to partial surface coverage of Li x CoO 2 . A very small amount of Co-containing oxyfluoride species was detected on the cycled bare electrodes while considerable amounts of Co-containing and Al-containing fluorides and/or oxyfluorides and species such as PF x (OH) y were found on the cycled coated electrodes, which completely covered the surfaces of the Li x CoO 2 particles. The mechanism responsible for the enhanced cycling stability and reduced impedance of coated relative to bare electrodes is discussed in detail.

This paper describes a new technique to quantitatively extract the number of unoccupied d states in a material utilizing measurements of the L X-ray absorption edge spectra. A correlation between the area under each of the L2 and L3 X-ray absorption edges and d-band vacancies in platinum-containing materials which exhibit white lines is given for the first time. The technique is demonstrated with a platinum catalyst supported on silica as an example. The quantity determined is the fractional change of the d-band occupancy for the sample from that of bulk platinum.

Understanding the thermal stability of major reaction products, Li2O2 (space group P63/mmc) and Li2O (space group Fm3¯m) is critical to improve the safety characteristics of Li-air batteries. The changes in the crystal structure and surface chemistry of Li2O2 and Li2O were examined as a function of temperature via in situ X-ray diffraction (XRD) and in situ X-ray photoelectron spectroscopy (XPS). Significant decreases in the lattice parameters and the c/a ratio of Li2O2 were found at 280°C and higher. These structural changes can be attributed to the transformation of Li2O2 to Li2O2-δ, which is supported by density functional theory calculations. Upon further heating to 700°C, a lithium-deficient Li2-δO phase appeared at 300°C and gradually became stoichiometric upon further heating to ∼550°C. XPS measurements of Li2O2 revealed that Li2O appeared on the surface starting at 250°C, which is in agreement with the onset temperature of phase transformation as detected by XRD. In addition, the growth of Li2CO3 on the surface was found at 250°C, which can be attributed to chemical reactions between Li2O2/Li2O and carbon-containing species (e.g. hydrocarbons) present in the XPS chamber. This finding highlights the challenges of developing stable carbon-based oxygen electrode for Li-air batteries.

AlPO 4 "-coated LiCoO 2 was shown to exhibit markedly improved capacity retention relative to bare LiCoO 2 upon cycling to 4.7 V. Scanning and transmission electron microscopy imaging showed that the coating thickness of "AlPO 4 "-coated LiCoO 2 varied from ∼10 to ∼100 nm. Energy-dispersive X-ray mapping revealed that the coating was not single-phase "AlPO 4 ", rather consisting of P-rich thick regions (∼100 nm) and Al-rich thin regions (∼10 nm). Detailed X-ray photoelectron spectroscopy (XPS) studies of the "AlPO 4 "-coated LiCoO 2 in comparison to bare LiCoO 2 and various reference compounds such as Li 2 CO 3 , Li 3 PO 4 , and AlPO 4 indicate that (1) AlPO 4 is absent on the surface; (2) the surface consisted of Li 3 PO 4 and heavily Al substituted LiAl y Co 1-y O 2 , which may result from AlPO 4 nanoparticles reacting with bare LiCoO 2 during the coating heat treatment at 700°C; and (3) the amount of surface Li 2 CO 3 is markedly reduced in the coated sample relative to the bare LiCoO 2 . The existence of Li 3 PO 4 in "AlPO 4 "-coated LiCoO 2 was confirmed with X-ray powder diffraction. The coating microstructure of "AlPO 4 "-coated LiCoO 2 is proposed, and the mechanisms of enhancement in the cycling and thermal characteristics by particle surface microstructure are discussed in detail.