Amorphous and nanocrystalline hydrogen storage alloy materials for nickel-metal hydride batteries

229 Pages·2011·3.96 MB·English

University of Wollongong
Research Online
University of Wollongong Thesis Collection University of Wollongong Thesis Collections
2000
Amorphous and nanocrystalline hydrogen storage
alloy materials for nickel-metal hydride batteries
Sun Li
University of Wollongong
Recommended Citation
Li, Sun, Amorphous and nanocrystalline hydrogen storage alloy materials for nickel-metal hydride batteries, Doctor of Philosophy
thesis, Department of Materials Engineering, University of Wollongong, 2000. http://ro.uow.edu.au/theses/1483
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AMORPHOUS AND NANOCRYSTALLINE
HYDROGEN STORAGE ALLOY MATERIALS
FOR NICKEL-METAL HYDRIDE BATTERIES
A thesis submitted in fulfilment of the requirements
for the award of the degree
DOCTOR OF PHILOSOPHY
from
UNIVERSITY OF WOLLONGONG
by
LI SUN, B.E.
Department of Materials Engineering
August 2000
Declaration
1
DECLARATION
/ hereby declare that the work presented in this thesis is original and
was carried out by the candidate at the Department of Materials
Engineering, the University of Wollongong, New South Wales,
Australia; and has not been submitted for a degree to any other
university or institution.
Li Sun
Department of Materials Engineering, University of Wollongong, NSW 2522, Australia
Abstract ii
ABSTRACT
Rechargeable nickel-metal hydride (Ni-MH) batteries employing hydrogen storage
alloys as their negative electrode materials have been some of the most promising
power sources for many important applications. For most commercial Ni-MH
batteries, their negative electrodes are of rare-earth system alloys, which have low
theoretical discharge capacities (e.g., LaNi alloy has a theoretical discharge capacity
5
of only about 370 mAh/g). The capacities of the negative alloy electrodes restrict the
further improvement of the capacities of nickel-metal hydride batteries.
Magnesium-based hydrogen storage alloys are promising energy conversion and
storage materials because they possess very high theoretical hydrogen storage
capacity, eg., the theoretical discharge capacity of Mg2Ni alloy is approximately 1000
mAh/g. Also magnesium is abundant in nature, light in weight and relatively low in
cost. FeTi-type hydrogen storage alloys are also prime candidates for hydrogen
storage systems and have high theoretical discharge capacities of about 500 mAh/g.
FeTi-type alloys are also the cheapest of the promising hydrogen conversion and
storage alloys. However, for a long period, it has been thought that Mg-based and
FeTi type hydrogen storage alloys were unsuitable for Ni-MH negative electrodes
Department of Materials Engineering, University of Wollongong, NSW 2522, Australia
Abstract iii
because of their slow hydriding/dehydriding kinetics at ambient temperature and their
susceptibility to corrosion in alkaline solution.
Amorphous and nanocrystalline metals and alloys have the features of new alloy
compositions and new atomic configurations, which are different from those of
crystalline alloys. These features enable various kinds of characteristics to be
achieved such as excellent mechanical properties, useful physical properties and
unique chemical properties, which have not been obtained for conventional
crystalline alloys. Thus, there appear to be some possibilities to achieve high
discharge capacities for amorphous and nanocrystalline Mg-based and FeTi-type
alloys.
The purposes of this study are to exploit the electrode properties of novel amorphous
and nanocrystalline Mg-based and FeTi-type hydrogen storage alloys as high
performance and low cost negative materials for the Ni-MH batteries. For these
purposes, amorphous and nanocrystalline Mg-based and FeTi-type hydrogen storage
alloys were prepared by mechanical alloying and milling methods using a planetary
ball mill. Their structures were determined by X-ray diffraction, differential thermal
analysis and transmission electron microscopy. Their electrode behaviour was
investigated in a 6 M KOH solution at room temperature.
Department of Materials Engineering, University of Wollongong, NSW 2522, Australia
Abstract iv
A nanocrystalline FeTi alloy with an average grain size of about 8 nm can be
obtained by mechanically milling a crystalline FeTi alloy for 40 hours. Alternatively,
an amorphous FeTi alloy can be obtained by mechanically alloying mixtures of Fe
and Ti powders for 40 hours. Crystalline FeTi alloy has an extremely low discharge
capacity of only 3 mAh/g at a discharge current density of 50 mA/g, which is
negligibly small compared with its theoretical discharge capacity. Therefore,
crystalline FeTi alloy is not suitable for use as a rechargeable anode material in an
alkaline solution at room temperature. The nanocrystalline FeTi alloy has an
improved discharge capacity of 54 mAh/g at the first discharge, which is much higher
than that of crystalline FeTi alloy, but is only around 10 % of the calculated
theoretical electrochemical capacity of FeTi alloy. Due to its good discharge capacity
during the first twenty cycles, it may be possible to improve the electrochemical
discharge capacity of FeTi-type alloy with good cycle life by achieving a
nanocrystalline structure. The amorphous FeTi alloy achieves its highest discharge
capacity of 220 mAh/g at the first charge-discharge cycle at a discharge current
density of 50 mA/g. This discharge capacity is almost comparable with the capacities
of commercial alloys used in Ni-MH batteries. Although the discharge capacity of
the amorphous FeTi alloy decreases rapidly with prolonged charge-discharge cycles,
it still has a discharge capacity of 78 mAh/g at the twentieth cycle. Thus, it is an
effective method in achieving high discharge capacity for the FeTi-type alloy to form
an amorphous or nanocrystalline structure.
Department of Materials Engineering, University of Wollongong, NSW 2522, Australia
Abstract
v
In order to determine the effect of the preparation method for Mg-based alloys on
their discharge capacity, the crystalline Mg-based alloys used were obtained in two
ways: sintering elemental powders and induction melting. At a discharge current
density of 50 mA/g, crystalline Mg2Ni alloys prepared by sintering Mg and Ni
powders have extremely low discharge capacities of only 15 or 18 mAh/g. Induction
melted crystalline Mg2Ni alloy also has an extremely low discharge capacity of 18
mAh/g. The discharge capacity of sintered crystalline Mg Nio.6Coo.4 alloy is still
2
very poor (about 20 mAh/g), demonstrating that cobalt addition does not alter the
discharge capacity of crystalline Mg-Ni alloys. All the discharge capacities are
negligibly small in comparison with their theoretical discharge capacity. Thus, the
crystalline Mg Ni-type alloy is not suitable for use as a rechargeable anode material
2
in an alkaline solution at room temperature.
A nanocrystalline Mg Ni alloy with an average grain size of about 13 nm can be
2
fabricated by mechanical milling induction melted (TM) crystalline Mg Ni alloy. The
2
initial discharge capacity of the nanocrystalline Mg Ni alloy is about 111 mAh/g,
2
which is much higher than that of the TM crystalline Mg Ni alloy (only 18 mAh/g).
2
Like the nanocrystalline FeTi alloy, the discharge capacity of nanocrystalline Mg Ni
2
alloy is only 10 % of the calculated theoretical electrochemical capacity of Mg Ni
2
alloy.
Department of Materials Engineering, University of Wollongong, NSW 2522, Australia
Abstract vi
All the mechanically alloyed sintered Mg-Ni and Mg-Ni-Co alloy electrodes studied
have completely or mainly amorphous structures, showing very high initial discharge
capacities (around 300 mAh/g) compared with their crystalline counterparts. The
addition of a small amount of Co to amorphous Mg-Ni alloy can increase its initial
discharge capacity and cycle life. Increasing the Co addition in the amorphous Mg-
Ni alloy results in a lower initial discharge capacity but a slightly better cycle life.
All the results indicate that the amorphous structure is a key factor in order to obtain
high initial discharge capacities for Mg-based alloy electrodes. In the mechanical
alloying process, the ball milling parameters have significant effects on the discharge
capacities of the Mg-based alloy electrodes. Increasing the ball milling time and the
ratio of ball to sample weight are effective methods to further improve the discharge
capacity for Mg-based alloy electrodes.
Assuming that the composition of amorphous alloys can be obtained in a certain
range, non-stoichiometric miiform amorphous MgNi V alloys (x = 1, 1.28; y = 0,
x y
0.1, 0.2, 0.4) have been firstly synthesised by mechanically alloying induction
melting Mg Ni alloy, Ni and V powders based on a stoichiometric amorphous MgNi
2
alloy component. The results indicate that non-stoichiometric amorphous Mg-based
alloys can be obtained by either increasing the Ni content or adding a range of
vanadium or both, through the mechanical alloying method. The non-stoichiometric
Mg-based alloy electrodes studied have shown improved initial discharge capacities
(more than 400 mAh/g) compared with the stoichiometric amorphous MgNi alloy
Department of Materials Engineering, University of Wollongong, NSW 2522, Australia
Abstract vii
(about 330 mAh/g). These results describe a novel method of achieving better Mg-
based alloy electrodes with high discharge capacities. The method enables a larger
composition range to be achieved with a range of different elemental additions.
Finally, it is demonstrated that the initial discharge capacity of amorphous Mg-based
alloy electrodes is higher than that of rare-earth system and zirconium-based Laves
phase alloys in commercial batteries. Thus, Mg-based hydrogen storage alloys are
very promising negative materials for the Ni-MH battery. Also the results have
shown that FeTi-type alloys having improved discharge capacity and good cycle Me
can be obtained by achieving an amorphous or nanocrystalline structure.
Department of Materials Engineering, University of Wollongong, NSW 2522, Australia