Degradation mechanisms of high-energy bipolar nickel metal hydride battery with AB5 and A2B7 alloys

•Failure modes of NiMH battery with AB5 and A2B7 alloys were reported.•Stable gamma-NiOOH was formed with the incorporation of Al from negative electrode.•The formation of gamma-NiOOH caused capacity degradation in AB5-containing battery.•Pulverization of the main A2B7 phase is the main failure mode for A2B7-containing battery.

The failure modes of nickel/metal hydride batteries made from conventional AB5 and La-only Mg-containing A2B7 metal hydride alloys were studied at the end of 70% state-of-charge cycle life testing (reaching below 3.1 Ah capacity in a 5.5 Ah design cell) by the combination of inductively coupled plasma, scanning electron microscope, X-ray energy dispersive spectroscopy and composition mapping, and X-ray diffraction analysis. The capacity degradation of cell with AB5 alloy was found to have been caused by the combination of metal hydride alloy oxidation and non-reversible γ-NiOOH phase formation, which was promoted by the incorporation of Al leached out from the oxidation product of the Al-containing negative electrode. In the cell with Al-free A2B7 alloy, the end of cycle life was the result of pulverization in the main A2B7 phase.