A comparative study on the characteristics of La0.7Mg0.3(Ni0.85Co0.15)(3.5) and La0.68Ce0.18Pr0.14Ni3.7Co0.6Al0.25Mn0.45 alloys for Ni/MH battery

摘要

The discharge characteristics of an AB(3.5) type alloy, La0.7Mg0.3(Ni0.85Co0.15)3.5, and an AB(5) type alloy, La0.68Ce0.18Pr0.14Ni3.7Co0.6Al0.25Mn0.45, were evaluated for use in a nickel metal hydride battery at low temperatures (lower than -30 degrees C). It was found that the capacity of the two alloys significantly decreased when the operation temperature was lower than -30 degrees C, but the capacity of the La0.7Mg0.3(Ni0.85Co0.15)(3.5) alloy was much better than that of the La0.68Ce0.18Pr0.14Ni3.7Co0.6Al0.25Mn0.45 alloy. From a comparison of exchange current density, charge-transfer resistance and capability of H diffusion of electrochemical impedance spectroscope measurement at different state of charge of the alloys, it was inferred that the rate-determining step in the discharge process of the negative electrode at low temperatures was the H mass-transfer reaction, and La0.7Mg0.3(Ni0.85Co0.15)(3.5) alloy has larger H diffusion coefficient than La0.68Ce0.18Pr0.14Ni3.7Co0.6Al0.25Mn0.45 alloy. X-ray powder diffraction results and Rietveld analyses also confirmed that the La0.7Mg0.3(Ni(0.85)Co(0.)15)(3.5) alloy has a smaller cell volume than that of the La0.68Ce0.18Pr0.14Ni3.7Co0.6Al0.25Mn0.45 alloy, and a two phase boundary structure of the La0.7Mg0.3(Ni0.85Co0.15)(3.5) alloy is adapt hydrogen to diffuse in alloy bulk AB(3.5) alloy may be a promising candidate material for nickel-metal hydride batteries with wide range operating temperature regime.