Electrochemical Characterization of Melt Spun AB5 Type Alloys for Metal Hydride Batteries

Abstract

This thesis is part of a larger research project where two metal hydride forming AB5 type alloys have been investigated. A slightly non-stoichiometric alloy with mischmetal on A-site and nickel, cobalt, manganese and aluminium on B-site, La0.60Ce0.29Pr0.04Nd0.07Ni3.37Co0.79Mn0.25Al0.74, has been characterized. The composition of this material, which will be referred to as Mm(NiCoMnAl)5.15, is close to the normal battery composition. The other alloy characterized is LaNi5 based, where nickel is partly substituted with tin, LaNi4.76Sn0.24. This material will later be referred to as La(NiSn)5. These materials were produced by melt spinning to vary the cooling rate during solidification. The main purpose of the study has been to characterize the electrochemical properties related to battery performance. The production as well as the metallurgical and structural characterization of the materials were performed in another part of the project.

For Mm(NiCoMnAl)5.15 the unit cell volume was dependent on the cooling rate before heat treatment, while the unit cell volume was almost independent of the cooling rate for La(NiSn)5. For both alloy compositions, the electrochemical properties seemed to change with varying cooling rate. The desorption equilibrium potential, the discharge capacity when discharging at a low current and the deterioration rate were found to be reduced with decreasing unit cell volume and increased with increasing unit cell volume, before heat treatment of Mm(NiCoMnAl)5.15. The self discharge rate was observed to be inversely proportional to the unit cell volume for this material. For not heat treated La(NiSn)5, produced at different cooling rates, the desorption equilibrium potential decreased when the self discharge rate and the discharge capacity increased after cycling for 300 cycles. The deterioration rate decreased when the desorption equilibrium potential was reduced for La(NiSn)5. The electrochemical parameters both before and after heat treatment of La(NiSn)5, did not seem to be related to the structural parameters, because the unit cell volume was almost independent of the cooling rate.

After heat treatment of both melt spun Mm(NiCoMnAl)5.15 and La(NiSn)5, the unit cell volume and the electrochemical properties seemed to be almost independent of the cooling rate, within the experimental error.

La(NiSn)5 showed an effect of storage of the materials. For Mm(NiCoMnAl)5.15 this effect was weaker. The unit cell volume and the electrochemical parameters after aging were less affected when the cooling rate was varied.