Mn in misch-metal based superlattice metal hydride alloy – Part 2 Ni/MH battery performance and failure mechanism

• The battery performances of misch-metal superlattice alloys were compared to those from AB5.
• Mn-free superlattice alloy shows better low temperature and high power performance when compared to AB5.
• Mn in the superlattice improves cell voltage and high-rate dischargeability.
• The main degradation mode for misch-metal based superlattice alloy is pulverization and loss in electrolyte.
• MnxO debris was found in the separator and separator/positive electrode interface.

The performance and failure mode of Ni/MH batteries made from a series of Mn-modified A2B7 superlattice and a commercially available AB5 metal hydride alloys were studied and reported. Cells with the Mn-free A2B7 alloy generally show improved low-temperature, higher peak power, and similar charge-retention behavior over those with a conventional AB5 alloy. As Mn-additive amount increased, cell voltage and high-rate capacity improved, low temperature, charge retention, and cycle life first improved, but then deteriorated, and peak power and high temperature voltage stand deteriorated. Analysis of battery performance test results show the use of a superlattice alloy containing 2.3% Mn as the best overall alloy composition. Failure analysis of the highly cycled AB5 alloy containing cells indicate a balanced degradation in negative, positive, separator, and a moderate loss of electrolyte. Same analysis on cells containing the various superlattice alloys suffered from a high degree of pulverization and oxidation of its negative electrode (with the 9.3% Mn content experiencing the worst amount of pulverization/oxidation) and a high degree of electrolyte loss.