Abstract:The responses of one AB 5 , two AB 2 , four A 2 B 7 , and one C14-related body-centered-cubic (BCC) metal hydrides to an alkaline-etch (45% KOH at 110 • C for 2 h) were studied by internal resistance, X-ray diffraction, scanning electron microscope, inductively coupled plasma, and AC impedance measurements. Results show that while the etched rare earth-based AB 5 and A 2 B 7 alloys surfaces are covered with hydroxide/oxide (weight gain), the transition metal-based AB 2 and BCC-C14 alloys surfaces are corroded and leach into electrolyte (weight loss). The C14-predominated AB 2 , La-only A 2 B 7 , and Sm-based A 2 B 7 showed the most reduction in the internal resistance with the alkaline-etch process. Etched A 2 B 7 alloys with high La-contents exhibited the lowest internal resistance and are suggested for use in the high-power application of nickel/metal hydride batteries.
High-power cylindrical nickel metal/hydride batteries using a misch metal-based Al-free superlattice alloy with a composition of La 11.3 Pr 1.7 Nd 5.1 Mg 4.5 Ni 63.6 Co 13.6 Zr 0.2 were fabricated and evaluated against those using a standard AB 5 metal hydride alloy. At room temperature, cells made with the superlattice alloy showed a 40% lower internal resistance and a 59% lower surface charge-transfer resistance compared to cells made with the AB 5 alloy. At a low temperature (−10 • C), cells made with the superlattice alloy demonstrated an 18% lower internal resistance and a 60% lower surface charge-transfer resistance compared to cells made with the AB 5 alloy. Cells made with the superlattice alloy exhibited a better charge retention at −10 • C. A cycle life comparison in a regular cell configuration indicated that the Al-free superlattice alloy contributes to a shorter cycle life as a result of the pulverization from the lattice expansion of the main phase.
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