Novel Zr- based AB2 Laves type Alloys as Advanced Anodes for High Energy - High Power Metal Hydride Batteries

Abstract

Metal hydrides have been studied during the last five decades as efficient materials for various energy storage and conversion systems. Among applications, the use of metal hydrides as electrode materials is the most developed one with a huge and expanding world market of commercial rechargeable alkaline Nickel Metal Hydride (Ni-MH) batteries. A typical application is a battery pack used in majority of the hybrid electric vehicles. Cheaper, safer, and simpler than their alternative - Li ion batteries - the Ni-MH batteries are used to store electricity produced by renewable energies, mitigating their intermittent in space and time nature.

To improve the performance of the Ni-MH batteries, the focus of R&D is in optimizing the performance of the main component of the battery - metal hydride anode. This work was aimed to develop advanced anodes of the Ni-MH batteries by achieving high electrochemical capacity, high rate dischargeability, fast activation, excellent cyclic stability and cost efficiency.

Zr- and Ti-based Laves type AB2 alloys - C14 Ti12Zr21.5V10Cr7.5Mn8.1Co8Ni32.2Al0.4Sn0.3 and C15 Ti0.15Zr0.85La0.03V0.12Mn0.7Fe0.12Ni1.2 - were selected for in-depth studies in the present work. Their chemical composition, phase-structural and microstructural states were probed and optimized. The alloys of C15 type contain 7 components (A=Zr, Ti, La; B=V, Mn, Fe, Ni) with variable compositions including both understoichiometric (B/A<2) and overstoichiometric (B/A>2) alloys, AB1.90, AB1.95, AB2.00 and AB2.07.

Nano-structuring of the alloys was performed by using rapid solidification technique at different wheel rotation speeds (5 Hz, 16.5 Hz, 33 Hz,100 Hz) while homogenized materials were prepared by annealing of the arc melted alloys at 950⁰C.

Characterisation of the kinetics and thermodynamics of the metal-H2 interactions was performed by PCT studies while their electrochemical performance was characterised by charge-discharge measurements performed at various current densities, cycling measurements, studies of activation behaviours and electrochemical impedance measurements.

Improvement of the activation behaviours of the alloys was achieved by using small additions of lanthanum metal as a catalyzer of H absorption and also by treatment of the metal hydride alloys in a hot 6 M KOH solution.

Based on the obtained results, advanced anodes of Zr- based multicomponent AB2±x Laves type alloys were successfully developed and show improved performance required for the high energy - high power metal hydride batteries. Indeed, these performance characteristics include high discharge capacity approaching 500 mAh/g at a current density of 1 C, fast activation, good cycle stability with appr. 50 % capacity retention after 500 cycles, good rate performance maintaining ~60 % capacity after 500 cycles while offering an affordable price >75 % lower than that for the AB5 alloys, thus eliminating use of critical raw materials, rare earth metals and cobalt.

Results of the work were published in 4 papers in high impact journals including J. Power Sources, J. Alloys and Compounds and International Journal of Hydrogen Energy.