Formation and oxidation of nanosized metal particles by electrochemical reaction of Li and Na with NiCo2O4: X-ray absorption spectroscopic study

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 NiCo2O4 in lithium and sodium test cells. Nanosized particles of NiCo2O4 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 NiCo2O4 reacts with lithium and sodium and the reduction of both Ni3+ and Co3+ 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 Li2O 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.