Year

1997

Degree Name

Doctor of Philosophy

Department

University of Wollongong. Department of Materials Engineering

Abstract

Nickel-metal hydride batteries using hydrogen storage alloys as the negative electrode materials have received much attention because of their high energy density, superior charge-discharge characteristics and freedom from poisonous materials. Most recent research in this area is focused on the development of superior negative electrode materials, mainly on rare-earth (mischmetal) systems and Ti-Zr-V Lavesphase type multicomponent hydrogen storage alloys.

Magnesium-based alloys have been used as hydrogen storage alloys since the 1970s. These alloys are considered to be the most promising materials for hydrogen-storage because of their high hydrogen-storage capacity, light weight, abundance of the constituents in the earth's crust, and low-cost compared with alternative systems. Of all the magnesium-based alloys, Mg2Ni is the most remarkable due to its relative highcapacity and favourable kinetics. Nevertheless, magnesium-based hydrogen storage alloys have never been used as electrode materials in Ni-MH secondary batteries because of their sluggish hydriding-dehydriding kinetics and poor corrosion resistance in alkaline aqueous solution.

Numerous investigations have been conducted in order to improve the kinetics of hydriding and dehydriding of magnesium-based alloys at ambient temperature. Nevertheless, all of these investigations have been concentrated only on the hydriding and dehydriding characteristics of magnesium-based alloys in the gas phase. As yet, no work has been directed towards the electrochemical performance of magnesium based alloys in an alkaline solution.

The aims of this study are to exploit the feasibility of utilizing magnesium-based hydrogen storage alloys as low cost and high performance anode materials for the secondary battery. For this purpose, Mg2Ni alloy is made by using a vacuum powder-metallurgical method. The electrode behaviour of this alloy in 6M KOH solution are investigated at room temperature. The experimental discharge capacityof Mg2Ni alloy electrode is only 8 mAh/g, which is almost negligible compared to theoretical electrochemical capacity (999 mAh/g). The reasons for this are discussed.

Effect of alloy composition on the charge and discharge performance of Mg2Ni-type alloy electrodes is investigated, using Mg1.95M0.05Ni (M = Al, Ca, La, Ti and V)ternary alloys and Mgi.9Al0.iNii.xYx (JC = 0 ~ 0.3) quaternary alloys. The ternary have the same phase structure as the Mg2Ni binary does, but the unit cell volume Mg1.95Mo.05Ni alloys increases linearly with the increasing radius of substituting elements. The discharge capacity improves with the ternary substituents studied the order Ca < La < Ti < V < Al. The addition of small amounts of aluminium orvanadium significantly improves the electrode capacity, which makes the specificcapacity more than double (from 8 mAh/g for Mg2Ni to 18 mAh/g for Mg1.95Alo.5Ni and 17.4 mAh/g for Mg1.95V0.5Ni, respectively). Titanium substitution seems also to increase effectively the discharge capacity, while Ca and La have almost no positive effects on the electrode capacity. The value of ;'o increases with increasing radius substituted metals in the same order as the discharge capacity, which suggests that charge transfer is the rate-determining step for the dehydriding reaction of Mg1.95M0.05Ni alloys at this test condition. The Mgi.pAlo.iNi1-xYx (x = 0 ~ 0.3) quaternary alloy electrodes exhibit a higher electrochemical capacity and better discharge rate performance than Mg2Ni. The discharge capacity and discharge rate characteristics of the electrodes are substantially improved with increasing x value Mg1-9Alo.1Nii-xYx. For the Mg1.9Al0.3Nio.7Y0.3 electrode, a discharge capacity of mAh/g at room temperature has been achieved. However, an increase in the yttrium content leads to a decrease of cycle life of the electrodes.

The discharge process of Mg2Ni-type hydrogen storage alloy electrodes is characterized by ac impedance spectroscopy at various depths of discharge (DOD) in 6M KOH solution at room-temperature. Comparative measurements also are performed on a LaNi5 electrode. An equivalent circuit has been developed by simulating experimental impedance dispersion. The rate-determining step of discharge process for the magnesium-based hydrogen storage alloy electrodes is dependent on both the alloy compositions and depth of discharge. The unmodified Mg2Ni electrode has a high charge-transfer resistance and mass-transfer resistance compared to LaNi5 electrode. Additions of yttrium and aluminium in Mg2Ni reduce considerably both resistances, thereby producing a remarkable improvement in the discharge capacity and rate-dischargeability.

In order to obtain further improvments in the electrochemical performance of Mg2Ni type alloy electrodes at ambient temperature, the multicomponent Mg2Ni-based alloy(Mg1.9Yo.1Nio.9Alo.1 ) powder is modified by an ultrasound treatment in an alkaline solution and microencapsulation with Ni-P alloy coating by using a low-temperature electroless plating method. The electrode characteristics such as electrochemical capacity, high-rate dischargeability and cycle life are examined and compared with those of the electrode fabricated from the unmodified alloy powder. The surface modification with ultrasound treatment shows a remarkable increase of electrode capacity and high-rate discharge capability but has little influence on the cycle life.The electrode fabricated from the micro encapsulated alloy powder has a higher discharge capacity, better high-rate discharge capability and longer cycle life as A specific discharge capacity of 220 mAh/g was achieved for Ni-P coated Mg1.9Yo.1Nio.9Alo.1 at room temperature. The electrochemical performances of Mg2Ni-type alloy electrodes with and without surface modification are characterized using dc polarization and ac impedance technique, and their phase compositions and microstructures are characterised by X-ray diffraction and scanning electronmicroscopy. The surface microencapsulation of alloy powder are effective inimproving the electrode discharge performance, but seems to be ineffective inpreventing disintegration of the Mg2Ni-based alloy powder.

A new composite alloy Mg2Ni-x wt.%Ti2Ni has been successfully synthesised usinga novel "particle inlaying" method. Scanning electron microscopy and energy dispersive spectroscopy reveal that very fine Ti2Ni particles were inlaid onto the surface of Mg2Ni particles by mechanical treatment and sintering. X-ray diffraction analysis indicates that the composite alloys are composed of primary alloys Mg2Ni,Ti2Ni and new phases TiNi, Ti-Mg formed in the composite procedure. The electrodecharacteristics of Mg2Ni - x wt.%Ti2Ni alloys in an alkaline solution are investigatedand compared with those of Mg2Ni. The discharge capacity of the alloy electrode is effectively improved from 8 mAh/g for Mg2Ni to 165 mAh/g for Mg2Ni - 40 wt.%Ti2Ni at ambient temperature, which is almost comparable with that for the Ti2Ni electrode (170 mAh/g). It is believed that the fine Ti2Ni particles inlaidon the surface of Mg2Ni particles play a two fold role: firstly, they hydride-dehydrideas hydrogen storage materials themselves; secondly, they provide active sites and pathways for Mg2Ni hydriding-dehydriding. This is supported by analysis of dischargebehaviour and electrochemical impedance spectra studies.

It is concluded that the magnesium-based hydrogen storage alloys are promising negative electrode materials for Ni-MH secondary batteries and may provide further improvements in capacity and cycling performance. The development and application of magnesium-based hydrogen storage alloy electrode materials will breathe new life into the Ni-MH battery and electric vehicle industries.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.