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X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used to investigate the electrochemical reactions of NiCo 2 O 4 in lithium and sodium test cells. Nanosized particles of NiCo 2 O 4 were prepared by the thermal decomposition of a mixed oxalate precursor and used as active electrode material. Ni K and Co K XANES results give evidence of the successive steps in the reduction mechanism of the oxide during the first cell discharge. In a first step NiCo 2 O 4 reacts with lithium and sodium and the reduction of both Ni 3+ and Co 3+ to the 2+ oxidation state is shown by a peak shift to lower energy values. As a result, sodium oxide or lithium oxide and the transition metal monoxides are formed. Second, metallic products are formed, in which the average coordination numbers of the nearest neighbors for Ni and Co derived from the EXAFS data are 4 and 5, respectively, associated with ca. 1 nm particle size. These novel ultrafine particles are stabilized in the Li 2 O matrix of the discharged electrodes. Reoxidation of the metallic products up to a divalent state of Ni and Co in the form of monoxides is achieved during the charge process of the electrochemical cells.

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We report the results of an extended x-ray absorption fine structure (EXAFS) study of a sample of ZrO2 prepared by high-energy ball milling. X-ray diffraction showed that the sample contained nanocrystals that were predominantly monoclinic with a particle size of 15 nm. The EXAFS for the sample was strongly attenuated in comparison to that for bulk monoclinic ZrO2. This has been interpreted as the ball-milled sample containing a large level of disorder whose possible origins are discussed. In contrast, our previous EXAFS studies of nanocrystalline oxides prepared by sol–gel methods have shown that these samples contain well-ordered crystallites with grain boundaries similar to those in bulk materials. It is concluded that ball-milled samples are very different from oxide nanocrystals produced by other techniques.