An uncommon oxygen absorption/desorption behavior is reported for the cation-stoichiometric cobalt oxide, YBaCo 4 O 7 , structurally composed of two kinds of layers of corner-sharing CoO 4 tetrahedra. We have found that YBaCo 4 O 7+δ absorbs and desorbs oxygen up to δ ≈ 1.5 in a narrow temperature range below 400 °C. The oxygen uptake/release process is highly reversible, being controlled by both temperature and oxygen partial pressure. Such a large low-temperature oxygen-content tunability is of great promise in regard to applications related to, for example, oxygen storage. Materials with similar characteristics are, to a large degree, lacking today.

Evolution of the O-K and Co-L edge XANES spectra for SrCoO3-δ upon increasing oxygen content from 2.50 to 2.82 reveals that superoxide (O2 −) related features are overwhelmingly strong in the surface region of highly oxygen-deficient samples. Moreover, should it be always present in the bulk, octahedrally coordinated cobalt is unobservable in the surface region of highly oxygen-deficient samples. On the other hand, the surface region hosts square-pyramidally coordinated cobalt even in brownmillerite SrCoO2.50. Observations in the present work support an oxygen-intercalation mechanism in which (i) O2 is first absorbed on the surface as O2 −, (ii) then reductively split into O x- associated with square-pyramidally coordinated cobalt, and (iii) finally in the bulk eventually reoxidized to O z- (0 < z < x) being attached to octahedrally coordinated cobalt. The fact that only minute chemical shifts of the Co-L edge branches are seen indicates that electron transfer mostly takes place between oxygen atoms.

Two different Li4Ti5O12 materials were investigated: smaller primary particle size forming large secondary particle aggregates (LTO-SP, surface area 22 m 2 /g) and larger primary particle size with less secondary particle aggregates (LTO-LP, surface area 7 m 2 /g). Both samples were synthesized using the same high temperature solid state synthesis but different end processing, resulting in same crystalline structure but different particle morphology. At 0.1C measured discharge capacities were close to the theoretical capacity of Li4Ti5O12 (175 mAh/g) and similar capacities were obtained at low C-rates and room temperature for both LTO-SP and LTO-LP. However, higher capacities were obtained with LTO-SP at high C-rates and -20 °C indicating beneficial effect of small particle size and large surface area. Shapes of the charge/discharge curves were different for LTO-SP and LTO-LP and this is attributed to the large surface area of LTO-SP which affects the electrochemical performance because of different reaction potentials at surface sites vs. bulk.

Extraordinary oxygen ordering and mobility have been found in an oxygen-nonstoichiometric mixed-valence cobalt oxide, YBaCo(4)O(8.5). The excess oxygen atoms appear to be incorporated in an orderly way into the YBaCo(4)O(7) parent lattice with different configurations as the oxygen content varies. Intense electron-beam irradiation was used to observe recurrent oxygen migration in the lattice, causing reversible structural modulation transitions. The oxygen migration can be attributed mainly to the electron-beam heating effect. This study highlights the high degree of freedom of surplus oxygen within the investigated structure and advances our understanding of the oxygen diffusion process in related transition-metal oxide systems.