ELECTROCHEMICAL OXYGEN INTERCALATION IN LA2NIO4+DELTA CRYSTALS

The electrochemical intercalation of oxygen from an aqueous KOH electr olyte into La2NiO4+delta has been investigated by a potential step met hod at potentials below oxygen evolution. The first reduction cycle oc curs at nearly constant potential for the whole composition range and is characteristic of nucleation and growth behavior. On the first oxid ation and subsequent reduction-oxidation cycles, the results are simil ar to those obtained previously for polycrystalline electrodes and sho w the presence of a two-phase region for 0 less than or equal to delta less than or equal to 0.06 and a single phase with the composition 0. 06 less than or equal to delta less than or equal to 0.13. The P4(2)/n cm phase with delta similar to 0.02 and a narrow range of stoichiometr y, is apparent in d delta/dV versus delta data and in the behavior of the current decay curves. Examination of the crystals at various stage s of cycling show the formation of extensive cracks that apparently in crease in density with cycling. Measurements of the transient currents after potential steps have been used to estimate the diffusion coeffi cient of oxygen in La2NiO4+delta in the single phase region. Data were obtained for both polycrystalline and single crystal electrodes.