A comparative study on the characteristics of la_0.7Mg _0.3(Ni_0.85Co_0.15)_3.5 and La0 _.68Ce_0.18Pr_0.14Ni_3.7Co _0.6Al_0.25Mn_0.45 alloys for Ni/MH battery

The discharge characteristics of an AB_3.5 type alloy, La _0.7Mg_0.3(Ni_0.85Co_0.15) _3.5, and an AB₅ type alloy, La_0.68Ce _0.18Pr_0.14Ni_3.7Co_0.6Al _0.25Mn_0.45, were evaluated for use in a nickel metal hydride battery at low temperatures (lower than -30°C). It was found that the capacity of the two alloys significantly decreased when the operation temperature was lower than -30°C but the capacity of the La _0.7Mg_0.3(Ni_0.85Co_0.15) _3.5alloy was much better than that of the La_0.68Ce _0.18Pr_0.14Ni_3.7Co_0.6Al _0.25Mn_0.45 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 La_0.7Mg_0.3Ni_0.85Co _0.15)_3.5 alloy has larger H diffusion coefficient than La_0.68Ce_0.18Pr_0.14Ni_3.7Co _0.6Al_0.25Mn_0.45 alloy. X-ray powder diffraction results and Rietveld analyses also confirmed that the La_0.7Mg _0.3(Ni_0.85Co_0.15)_3.5 alloy has a smaller cell volume than that of the La_0.68Ce_0.18Pr _0.14Ni_3.7Co_0.6Al_0.25Mn _0.45alloy, and a two phase boundary structure of the La _0.7Mg_0.3(Ni_0.85Co_0.15) _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.