Failure Mechanisms of Nickel/Metal Hydride Batteries with Cobalt-Substituted Superlattice Hydrogen-Absorbing Alloy Anodes at 50 °C
The incorporation of a small amount of Co in the A2B7 superlattice hydrogen absorbing alloy (HAA) can benefit its electrochemical cycle life performance at both room temperature (RT) and 50 °C. The electrochemical properties of the Co-substituted A2B7 and the failure mechanisms of cells using such alloys cycled at RT have been reported previously. In this paper, the failure mechanisms of the same alloys cycled at 50 °C are reported. Compared to that at RT, the trend of the cycle life at 50 °C versus the Co content in the Co-substituted A2B7 HAAs is similar, but the cycle life is significantly shorter. Failure analysis of the cells at 50 °C was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and inductively coupled plasma (ICP) analysis. It was found that the elevated temperature accelerates electrolyte dry-out and the deterioration (both pulverization and oxidation) of the A2B7 negative electrode, which are major causes of cell failure when cycling at 50 °C. Cells from HAA with higher Co-content also showed micro-shortage in the separator from the debris of the corrosion of the negative electrode.
Investigation into the deterioration in storage characteristics of nickel-metal hydride batteries during cycling
Abstract The deterioration mechanism of the storage characteristics of nickel-metal hydride batteries for hybrid electric vehicles (HEVs) has been investigated. The deterioration in the storage characteristics during charge–discharge cycling is caused by the microscopic short circuit in the separator which is influenced by the deposits from the positive and negative electrode elements such as zinc, cobalt and manganese. Based on the elucidated deterioration mechanism, the deterioration in the storage characteristics of nickel-metal hydride batteries was suppressed by employing a hydrogen-absorbing alloy with higher oxidation resistance for the negative electrode.
Effects of annealing on Zr8Ni19X2 (X = Ni, Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf): Hydrogen storage and electrochemical properties
Abstract The effects of 8-h annealing at 960 °C on the gaseous phase hydrogen storage and electrochemical properties of partial-Ni substituted Zr 8 Ni 21 alloys were studied. The substituting elements included Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf. Only the main phase of the annealed Sn-substitution remained Zr 8 Ni 21 -structured while those of other substitutions turned into Zr 7 Ni 10 or Zr 2 Ni 7 . The observed trend in maximum gaseous phase hydrogen storage capacity followed the increasing order of B/A ratio of the main phase as orthogonal Zr 7 Ni 10  > tetragonal Zr 7 Ni 10  > Zr 8 Ni 21  > Zr 2 Ni 7 . After annealing, due to the increase in abundance of the main phase, the maximum gaseous phase hydrogen storage capacities of alloys with higher capacities before annealing increased while others decreased. The full discharge capacity also improved in the same increasing order of B/A ratio in the main phase. Hf-substitution showed the highest electrochemical discharge capacity at 200 mAh g −1 . After annealing, all alloys with the same main phase as the as-cast alloys showed degradation in full electrochemical capacity due to the reduction in both number and abundance of the catalytic secondary phases. All supplements assisted in improving surface exchange current from the base binary Zr 8 Ni 21 alloy. Except for La- and Hf-substitutions, annealing reduced the surface exchange current density. The bulk hydrogen diffusion coefficient decreased with most of the supplements except for V- and Sn-substitutions. All supplements, except for Sc, showed improvement in the bulk diffusion after annealing. Furthermore, the maximum gaseous phase hydrogen storage capacity showed a strong correlation to the full electrochemical discharge capacity. Among all alloys in this study, the as-cast Hf-substituted Zr 8 Ni 21 alloy demonstrated the best overall gaseous hydrogen storage and electrochemical properties.