High Power Hydrides for Hybrid Electric Vehicle Batteries

Abstract: Alkaline and hydrogen peroxide surface treatments have been developed to activate the surface of metal hydride alloy. AB5 type alloy is covered with a porous surface layer containing needle shaped rare earth hydroxides after etching by a potassium hydroxide solution. TEM studies show in addition the presence of a denser surface oxide layer with embedded Ni containing clusters covering the bulk alloy. After a hot KOH solution treatment, AB2 alloy shows fast activation and high discharge capacity above the expected from the gas-phase storage in the alloy. This was attributed to the formation of rough surface of alloy, which may stabilize hydrogen bubbles allowing pressures above 1 atm to be reached locally in the surface. Compare to alkaline treatment, hydrogen peroxide surface treatment can give similar improvement of discharge capacity at high c-rates for AB5 type alloy, but better performance at -20°C, presumed due to the thicker oxidize layer which creates more catalytic Ni clusters and contains more channels and larger pores for electrolyte to penetrate freely.Corrosion not only reduces the metal hydride capacity, but also consumes water in the electrolyte resulting in increased internal resistance and evolves hydrogen causing an unbalance between anode and cathode. In this thesis, a controlled addition of oxygen was added to the internal gas space of a cycled Nilar NiMH battery to rebalance the electrodes and replenish the electrolyte – as the added oxygen reacts with hydrogen that was formed during the corrosion process. Thus, the two most detrimental factors in cell ageing can be mitigated. To fully restore the electrolyte content as well as electrode balance, both oxygen and hydrogen are needed to compensate for the loss to hydroxide ions OH- formed in the corrosion process. A proper optimization of the gas additions combined with a cell design including an excess amount of MH-alloy to compensate for the corrosion can substantially increase the cycle life of NiMH batteries.Mechanical as well as chemical treatments were tried to recycle hydrogen storage alloys from spent NiMH batteries. SEM and XRD studies indicate that the corrosion layer can be partly removed by sonication and ball milling methods, and fully removed by acid treatment. But the alkaline treatment on the other hand causes an even thicker corrosion layer. In electrochemical half-cell tests, the alloy shows faster activation and better discharge capacity after ball milling, sonication or acid treatments. The removal of corrosion products and smaller particle fragments give an increase in capacity. Sonication and ball milling also lead to a break-up of agglomerated particles, and thereby improving the high-rate properties of the alloy by increasing the access to highly active surface sites.