Refined Structural Analysis Using Synchrotron XRD and XAFS for Cobalt Oxyhydroxides as Additive for Nickel Hydroxide Electrode

Cobalt oxides such as CoO and Co(OH)2 are used as additives of nickel hydroxide electrode for the nickelmetal hydride (Ni-MH) batteries. The Cobalt oxides are converted to cobalt oxyhydroxide (CoOOH) by the electrochemical oxidation during the charging, providing a good electrical network between the nickel hydroxide particles [1]. Nickel hydroxide was also coated with cobalt hydroxide [2]. The cobalt hydroxide was oxidized to the CoOOH by the heat treatment above 80 C under NaOH solution and air atmosphere [2, 3]. In this work, the relationship between the structure and the electrical conductivity for the CoOOH was investigated. The structural analysis for the CoOOH has been done by using a high-energy synchrotron X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The CoOOH was prepared from α-cobalt hydroxide by the heat treatment in the temperature range of 80-160 C under NaOH solution and air atmosphere. The XRD and XAFS measurements were conducted at the BL19B2 and BL01B1 beam line, the synchrotron radiation facility SPring-8, Japan. Figure 1 compares the observed XRD patterns and calculated ones on the basis of the Rietveld refinements for the CoOOH heat-treated at 120 C. We have succeeded to refine the structural dates by two phases with large and small c lattice constants. The Rwp values of the pattern are below 10 %, showing the good agreement between the observed and calculated patterns. The CoOOH with large and small c lattice constants is called a L phase and a S one, respectively. Figure 2 shows phase abundance derived from the Rietveld refinements for the CoOOH heat-treated at various temperatures. With increasing of treatment temperature, the phase abundance for the L phases was increased, whereas one for the S phases was decreased. Above 140 C, some decomposed oxide such as Co3O4 and CoO appeared. Figure 3 shows the edge shifts derived from the XAFS and the cobalt oxidation state for the CoOOH heat-treated at various temperatures. The average oxidation state of cobalt ions could be estimated to be 2.84 at 80 C and about 3.2 above 100 C. Above 100 C, the cobalt ions for the CoOOH were oxidized to the higher oxidation state. This result is in good agreement with the fact that the resistivity for the CoOOH was rapidly decreased by the treatment temperature above 100 C. The structural refinement for the CoOOH has been done successfully on the basis of two phase models (the L and the S phases). The increase in the electrical conductivity for the CoOOH would be explained by the increase in cobalt oxidation state. There are some tendencies that the phase abundance for the L phase was increased with the increase in the cobalt oxidation state.