Influence of the Electrolyte Alkaline Ions on the Efficiency of the Na[sub 0.6]CoO[sub 2] Conductive Additive in Ni–MH Cells

Abstract

The influence of the electrolyte alkaline ions, potassium, lithium, and sodium, on the efficiency of the Na 0.6 CoO 2 phase, as a conductive additive in the positive nickel oxide electrode, is analyzed in the present paper. Aging tests of the Na 0.6 CoO 2 phase and of the ␥-Co phases, deriving from it, in the three electrolyte components, KOH, NaOH, and LiOH, have shown the presence of topotactic exchange/͑de͒intercalation reactions of the alkaline ions, occurring within the materials. Electrochemical tests, performed in the various electrolytes, underline an unfavorable effect of lithium ions. The formation of a semiconductive Li x CoO 2 phase has been identified as responsible for the loss of efficiency of the conductive material, when lithium ions are present in the electrolyte.

Effect of deep discharge on the electrochemical behavior of cobalt oxides and oxyhydroxides used as conductive additives in Ni-MH cells

“…The distance that corresponds to this wide peak (around 6.67 Å) is intermediate between the distances that are observed for the (0 0 3) lines of Na 0.6 CoO 2 (d(0 0 3) = 5.51 Å) and ␥-Co (d(0 0 3) = 6.85 Å). The presence of such an interstratified phase is in accordance with the results obtained during the specific study of the evolution of Na 0.6 CoO 2 in KOH solution [17]. This work highlighted, thanks to ageing tests, a spontaneous transformation of Na 0.6 CoO 2 into ␥-Co, with a mechanism of exchange of alkaline ions in interslab space.…”

“…Table 1 Comparison of chemical compositions, average oxidation states of cobalt, conductivity and activation energy values at room temperature, for the studied conductive additives: Na0.6CoO2, the material obtained by ageing in KOH, ␥-Co obtained by electrochemical oxidation and ␥-Co obtained by chemical oxidation from the Na0.6CoO2 phase. Co type phase [17]; the semi-conductor character of the ␤(III)-Co component is the reason for the difference with the other studied ␥-Co phases. The metallic character of Na 0.6 CoO 2 and the ␥-Co phases is due to an overlapping of the cobalt t 2g orbitals, across the shared edges of the CoO 6 octahedra, in CoO 2 slabs.…”

“…Finally, the third method that is used to obtain a ␥-Co phase consists in ageing the Na 0.6 CoO 2 phase, during one month, in 8 M KOH electrolyte. Such treatment leads to spontaneous transformation of Na 0.6 CoO 2 into a mixture of a majority ␥-Co type phase and of a ␤(III) type phase, as described in a previous paper [17].…”

“…Table 1 summarises the chemical compositions and the average oxidation degree of cobalt in these synthesized four materials. Comparison of the chemical compositions of the three ␥-Co derivatives materials with the composition of the Na 0.6 CoO 2 precursor phase shows the exchange of a majority part of the sodium ions by potassium ions, which occurs simultaneously with insertion of water molecules [17].…”

“…Tronel et al have shown that the Na 0.6 CoO 2 phase is transformed, after few cycles within the electrode, into a ␥-type hydrated cobalt oxyhydroxide [16]. This latter phase, formed in situ, is a promising candidate because it exhibits a slow reduction kinetics in concentrated alkaline electrolyte [17].…”