ELECTROCHEMICAL PROPERTIES OF THE ZrNiMnCrV ALLOY DEPENDING ON QUANTITATIVE PHASE COMPOSITION 

Abstract

The ZrNi_1.2Mn_0.5Cr_0.2V_0.1 alloy was qualified using the hydrogen desorption isotherms in a gas atmosphere, equilibrium curves of hydrogen desorption in a 30% KOH solution, and  X-ray diffraction data. The data on the equilibrium pressure of the alloy in a gas atmosphere at 20 °C, equal to ~1 atm (101325 Pa), indicated that the ZrNi_1.2Mn_0.5Cr_0.2V_0.1 alloy could be used in Ni–MH batteries, and the equilibrium hydrogen desorption curve obtained by the electrochemical method indicated that insignificant self-discharge could occur. The alloy samples crystallized at different cooling rates differ in quantitative phase composition. Electrodes made of the sample with a greater amount of the Zr₇Ni₁₀ phase activated faster and those with a greater amount of the C15 and C14 phases have higher maximum achieved discharge capacity. After being exposed to air for 14 days, the Zr₇Ni₁₀ phase decreased by 7 vol.% and the total content of the C15 and C14 phases increased by 7 vol.% in the sample that had a greater amount of the Zr₇Ni₁₀ phase (~ 24 vol.%). No changes in quantitative phase composition were found in the sample with a smaller amount of the Zr₇Ni₁₀ phase (~12 vol.%). Electrodes prepared from the sample with a lower content of Zr₇Ni₁₀ phase showed better cyclic stability both before and after a pause in the cycle. The loss of the maximum achieved discharge capacity of these electrodes for 100 cycles with a pause in the cycle for 14 days (after the 80^th cycle) and for 200 cycles with a pause in the cycle for 45 days (after the 150^th cycle) was only 2 and 16%, respectively, and that of electrodes with a higher content of the Zr₇Ni₁₀ phase was 30 and 52%, respectively. Thus, the electrodes from the alloy sample that showed a certain stability of the quantitative phase composition, according to X-ray diffraction, had better hydrogenation–dehydrogenation cyclic stability when exposed to air as powder for 14 days. Taking into account some similarities in hydrogenation-dehydrogenation and exposure to air of the zirconium alloys (oxidation of alloy components, significant increase in nickel concentration on the surface, etc.), it is logically assumed that better cyclic stability of the electrodes is due to more stable quantitative phase composition of the surface. The relationship between the electrochemical properties of the ZrNi_1.2Mn_0.5Cr_0.2V_0.1 alloy and the cooling rate in crystallization (quantitative phase composition) allowed the product of materials with predicted functional properties.