MATERIALS FORUM VOL. 27 (2004) 28 - 32
IMPROVED HIGH-TEMPERATURE PERFORMANCE OF ADVANCED
NICKEL-METAL HYDRIDE BATTERIES THROUGH MATERIALS
DEVELOPMENT
Y. Wang 1, Z. Gong 1, M. Geng 1, J. Yan 2 and D. O. Northwood3
1
2
Peace Bay Power Sources Co. Ltd., Tianjin 300384, China.
Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China.
3
Department of Mechanical, Automotive and Materials Engineering,
University of Windsor, Windsor, ON, Canada N9B 3P4.
ABSTRACT
The high-temperature performance of nickel metal hydride (Ni/MH) batteries has been improved using the addition
of yttrium in the lattice of the spherical nickel hydroxide powder used for the positive electrode. The crystal structure
of yttrium-doped nickel hydroxide powder is similar to that of regular nickel hydroxide powder. The charge
acceptance of the Ni/MH battery at 60oC is about 70% with the addition of yttrium. The measured specific capacity
of yttrium-doped Ni(OH)2 powder is much higher than that of the regular spherical Ni(OH)2 powder at higher
temperatures (50 and 60oC). The yttrium-doped nickel hydroxide active material remarkably improves the
high-temperature charge/discharge efficiency.
1. INTRODUCTION
The need for high energy density storage batteries has
been growing in recent years. The advent of electric
scooters, hybrid electric vehicles, laptop computers,
cellular telephones, PDAs and power tools has made
this need even more urgent. Although conventional
storage batteries such as nickel-cadmium (Ni/Cd) and
lead-acid batteries have been further improved in
design and packaging in the recent years, there is still
a need for improved energy density, power density and
service lifetime. The innate toxicity of cadmium and
lead has also come under scrutiny. The use of metal
hydrides as active negative electrode material in
rechargeable alkaline batteries has been studied for a
long time. Although Ni/MH batteries have superior
specific energy than the other two aqueous electrolyte
systems (Lead-acid and Cd/Ni batteries), they remain
largely inferior to the new rechargeable lithium (Li-ion)
batteries. However, the nickel metal hydride batteries
have superior performance to lithium batteries in
power tool and hybrid electric vehicle applications.
The pasted nickel hydroxide electrodes have been
largely improved using a co-precipitation of zinc and
cobalt in the lattice of the spherical nickel hydroxide
powders. The addition of zinc has effectively reduced
the nickel electrode swelling and largely increased the
Ni-MH battery cycle lifetime. Armstrong 1 reported
that the addition of cobalt into the Ni(OH)2 lattice
increases the oxygen evaluation potential and
improves the Ni(OH)2 active material utilization. Sood
2
also reported that the cobalt addition can be used to
prohibit the γ-NiOOH formation. Other workers have
reported that doping with Zn, Cd, Ba, Mg and Ca
elements in Ni(OH)2 can effectively prevent γ-NiOOH
formation 3. These elemental additions in the lattice of
Ni(OH)2 can also improve the battery charge/discharge
utilization and cycle lifetime. Physically and
chemically Ni, Co and Cd coated Ni(OH)2 powders
have been developed to improve the electrochemical
performances by Panasonic4,5. The high utilization of
cobalt -coated Ni(OH)2 powders had also been
reported and the charge/discharge reversibility and
oxygen evaluation potential of the cobalt-coated nickel
hydroxide electrode have been improved 6,7. The rare
earth elements have also been used to improve the
charge/discharge acceptance of the nickel hydroxide
electrode at higher temperatures. A small amount of
Y2O3, which was mixed into the nickel hydroxide
electrode, was beneficial in increasing the electrode
Ni(OH)2 utilization at higher temperatures 8 . There is
still an urgent requirement for further improvements
for applications in electric vehicles and power tools at
high-temperature conditions.
In this paper, the authors have chemically synthesized
yttrium-doped spherical nickel hydroxide powders.
The yttrium-doped nickel hydroxide electrode
effectively
improves
the
high-temperature
charge/discharge acceptance of advanced Ni-MH
batteries.
2. EXPERIMENTAL
A sodium hydroxide aqueous solution (5mol/l) and
ammonia aqueous solution (13mol/l) were gradually
added into a mixture of nickel sulfate, cobalt sulfate,
zinc sulfate and yttrium chloride aqueous solutions
(mole ratio of Ni:Co:Zn:Y is 100:2.5:5:1.1). The
concentration of NiSO4 aqueous solution was
© Institute of Materials Engineering Australasia Ltd – Materials Forum Volume 27 - Published 2004
28
MATERIALS FORUM VOL. 27 (2004) 28 - 32
1.5~2.0mol/l. The pH (11.5~12) was constantly
adjusted using an ammonia aqueous solution
(NH3:Ni2+=0.5:1.0). The reaction bath was 5L. The
stirring speed was 1500~2000rpm. The total nickel
hydroxide precipitation time is in a range of 9~12hrs.
The temperature was 50oC. After drying at 80oC, a
spherical (Ni,Co,Zn,Y)(OH)2 powder was obtained.
The diameter of the spherical nickel hydroxide is
about 10µm. In comparison with the yttrium-doped
nickel hydroxide, the regular nickel hydroxide was
co-precipitated in a sodium hydroxide aqueous
solution (5mol/l) and ammonia aqueous solution
(13mol/l), which were gradually added into a mixture
of nickel sulfate, cobalt sulfate, and zinc sulfate
aqueous solutions (mole ratio of Ni:Co:Zn is
100:2.5:5).
The cobalt sulfate and the sodium hydroxide were
used to react with the spherical nickel hydroxide
powder in another reaction vessel to precipitate cobalt
hydroxide on the surface of the hydroxide powder.
While monitoring the pH, the drop additions of
sodium hydroxide were stopped when the reaction
reached pH=9~10. The reaction temperature was
controlled at 50oC. The Co(OH)2-coated nickel
hydroxide powder was continuously treated in a hot
alkaline solution. The content of cobalt hydroxide at
the surface of nickel hydroxide powder is 7wt.%. The
Co(OH)2-coated nickel hydroxide powder was used to
paste the positive electrode of the nickel metal hydride
batteries.
middle of two metal hydride electrodes. These
electrode plates were separated using polyolefin
separator and two stainless steel sheets were used to
fix these electrodes to minimize the electrode swelling.
A Hg/HgO electrode was used as a reference electrode.
A Luggin capillary tube, which connects to the
reference electrode and working electrode, was placed
close to the working electrode in order to minimize the
ohmic drop across the electrolyte solution. The
emphasis of these charge/discharge tests of the cells
was on the electrochemical stability of the nickel
hydroxide electrode. Thus the capacity of the negative
electrode plates was designed to be three times higher
than that of the positive electrode. The electrolyte in
the cells was a 6M KOH aqueous solution.
The experimental cells were set up in a
temperature-controlled water bath. The cells were
tested at temperatures ranging from 0~70oC. The
charge/discharge tests of the experimental cells were
conducted using a LAND Battery Testing Equipment
(CT2001A). The experimental cells were charged at a
current of 0.2C for 7.5hrs and discharged at a current
of 0.2C to an end-of-discharge-potential of 0.1 V vs.
Hg/HgO. The commercial AAA-size Ni-MH batteries
were manufactured in Peace Bay Power Sources
Group Co. Ltd. The battery positive electrodes were
made from the laboratory-synthesized regular nickel
hydroxide and yttrium-doped nickel hydroxide
powders.
3. RESULTS AND DISCUSSION
The spherical nickel hydroxide powders were
characterized
by
powder
X-ray
diffraction
measurements (CuKαλ=1.1518Å). The regular and
yttrium-doped nickel hydroxide powders were mixed
with cobalt oxide (CoO) powder in a weight ratio of
10:1 together with a small amount (1wt.%) of
polytetrafluoroethylene (PTFE) aqueous solution as a
binder and then the mixture was pressed into a nickel
foam. After drying out at 60oC for 1 hour, the pasted
nickel hydroxide plate was roll-pressed. The thickness
of the nickel electrode plate is 0.70mm. A set-up of an
experimental cell was in a glass apparatus with three
electrode plates. The positive electrode was placed in a
Figures 1 and 2 show X-ray diffraction patterns of
yttrium-doped nickel hydroxide and regular nickel
hydroxide powders. The X-ray diffraction patterns
show that the crystal structure of the yttrium-doped
nickel hydroxide powder is close to the regular nickel
hydroxide powder. The lattice parameters (shown in
Table 1) were calculated from the X-ray-diffraction
patterns. It can be seen that the values of lattice
constants of the yttrium-doped nickel hydroxide
powder are close to that of regular nickel hydroxide
powders, because of similar atomic radii of nickel and
yttrium.
1200
800
1000
600
intensity
intensity
800
600
400
400
200
200
0
0
0
10
20
30
40
50
60
70
80
90
0
10
Figure 1. XRD pattern of regular
spherical Ni(OH)2 powder
20
30
40
50
60
70
80
90
2 theta
2theta
Figure 2. XRD pattern of yttrium-doped
spherical Ni(OH)2 powder
© Institute of Materials Engineering Australasia Ltd – Materials Forum Volume 27 - Published 2004
29
MATERIALS FORUM VOL. 27 (2004) 28 - 32
Table 1. Lattice parameters of regular spherical Ni(OH)2 and yttrium-doped spherical Ni(OH)2 powders
Yttrium-doped
Ni(OH)2
Regular
Ni(OH)2
a(nm)
b(nm)
c(nm)
Cell Volume(nm3)
0.3128831
0.3128831
0.4624570
39.21×10－3
0.3125538
0.3125538
0.4628678
39.16×10－3
Table 2. Discharge capacity of the regular Ni(OH)2, yttrium-doped Ni(OH)2 and β-Co(OH)2-coated
regular Ni(OH)2 for first 6-cycles.
Number of Cycles
1
2
3
4
5
6
Regular Ni(OH)2
(mAh/g)
259.1
261.7
263.1
265.9
266.0
266.2
yttrium-doped Ni(OH)2
(mAh/g)
220.1
230.5
236.8
242.2
248.5
252.7
242.5
255.2
260.1
262.2
262.1
261.9
β-Co(OH)2-coated
Ni(OH)2 (mAh/g)
Table 3. Specific discharge capacity of regular spherical Ni(OH)2 and yttrium-doped spherical Ni(OH)2 at 0.2C
discharge current
Temperature
Discharge capacity of
yttrium-doped Ni(OH)2 (mAh/g)
Discharge capacity of
yttrium-doped Ni(OH)2 (%)
Discharge capacity of regular
Ni(OH)2 (mAh/g)
Discharge capacity of regular
Ni(OH)2 (%)
20oC
30oC
40oC
50oC
60oC
70oC
258.8
261.0
261.6
256.6
178.1
174.5
100
100.9
101.1
99.1
68.8
67.4
266.2
261.1
253.0
231.2
129.2
109.1
100
98.1
95.0
86.9
48.5
40.9
0.50
0.6
0.45
0.40
60¡æ0.2Ccharge/discharge curves
20¡æ0.2Ccharge/discharge curves
0.4
Voltage£¨V)
Voltage£¨V£©
0.5
0.3
0.35
0.30
0.25
0.20
0.2
0.15
0.1
0.10
0
50
100
150
200
250
300
350
400
450
500
Capacity£¨mAh/g)
Figure 3. Charge/discharge curves of the
regular spherical Ni(OH)2 and the
yttrium-doped spherical Ni(OH)2 at 20oC
0.05
0
100
200
300
400
Capacity£¨mAh/g)
Figure 4. Charge/discharge curves of the
regular spherical Ni(OH)2 and the spherical
Ni(OH)2 doped with yttrium at 60oC
© Institute of Materials Engineering Australasia Ltd – Materials Forum Volume 27 - Published 2004
30
MATERIALS FORUM VOL. 27 (2004) 28 - 32
The specific discharge capacity of regular spherical
Ni(OH)2 and yttrium-doped spherical Ni(OH)2 at a
0.2C discharge current and different temperatures are
given in Table 3. It can be seen that the specific
capacity of the regular nickel hydroxide is a little
higher than that of the yttrium-doped nickel hydroxide
at 20oC (shown in Figure 3). However, the specific
capacity of the yttrium-doped nickel hydroxide active
material is higher than that of the regular nickel
hydroxide at 50oC and 60oC. Figure 4 shows the
discharge curves of the regular nickel hydroxide
powder and yttrium-doped nickel hydroxide powder.
The ratio of the discharge capacity of yttrium-doped
nickel hydroxide at 60oC and 20oC is 68.8%. The
ratio of the discharge capacity of regular nickel
hydroxide powder at 60oC and 20oC is only 48.5%.
The ratio of the discharge capacity of regular Ni(OH)2
and yttrium-doped Ni(OH)2 at 60 oC is 72.5%. The
charge/discharge capacity utilization of yttrium-doped
nickel hydroxide powder is much higher than that of
the regular nickel hydroxide at 60oC. Thus, the
high-temperature charge/discharge capacity utilization
is improved by doping yttrium into the spherical nickel
hydroxide powder. The discharge capacity of
β-Co(OH)2-coated regular Ni(OH)2 powder is
262mAh/g. The nickel hydroxide powder contains
7wt.% cobalt hydroxide. The specific capacity, based
on the real nickel hydroxide content in the
Co(OH)2-coated powder, is 281.7mAh/g. The capacity
utilization of the nickel hydroxide powder after
coating with cobalt hydroxide is over 97% (the
theoretical specific capacity of the nickel hydroxide is
289mAh/g).
Figure 5 shows the charge/discharge curves of regular
spherical Ni(OH)2 and yttrium-doped spherical
Ni(OH)2 powders at 60oC and at 1C (250mA/g)
charge/discharge current. The discharge capacity of
regular Ni(OH)2 and yttrium-doped spherical Ni(OH)2
at 60oC is 225.7mAh/g and 244.6mAh/g,
respectively. Thus, the yttrium-doped spherical
Ni(OH)2 electrode can release a high charge/discharge
capacity at higher temperatures for a larger discharge
current. The capacity utilization of the yttrium-doped
nickel at 60oC and 1C discharge current is about 20%
higher than that of the regular nickel hydroxide at
60oC. The yttrium-doped nickel hydroxide as a
positive electrode active material is beneficial for
high-temperature battery applications.
AAA-size Ni-MH batteries (1800mAh) were
manufactured using the regular nickel hydroxide and
yttrium-doped nickel hydroxide powder. The nickel
hydroxide electrode is made using the nickel foam
filled with a mixture of nickel hydroxide powder,
cobalt monoxide and a little polymer material
(100:10:0.5). The metal hydride electrode is made
using metal hydride powder pressed onto a punched
metal substrate. The weight of nickel hydroxide is
designed to be 9.0g and the weight of metal hydride
electrode 10.0g. The measured capacity of AAA-sized
Ni-MH batteries at room temperature is 1800mAh.
0.60
0.55
0.50
0.45
Voltage£¨V£©
As shown in Table 2, the discharge capacity of the
regular nickel hydroxide electrode reaches a value of
266mAh/g after five cycles. The discharge capacity
of the yttrium-doped spherical nickel hydroxide
electrode reaches a value of 248.5mAh/g after five
cycles. The regular nickel hydroxide electrode is easily
is activated and achieved a higher specific capacity in
comparison with yttrium-doped spherical Ni(OH)2
electrode.
The
specific
capacity
of
the
β-Co(OH)2-coated Ni(OH)2 powder including 7wt.%
cobalt hydroxide in β-Co(OH)2-coated Ni(OH)2
powder reaches 262mAh/g after four cycles.
60¡æ1Ccharge/discharge curves
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
50
100
150
200
250
300
350
Capacity£¨mAh/g)
Figure 5. 1C (250mA/g) charge/discharge curves
of the common spherical Ni(OH)2 and the
spherical Ni(OH)2 doped with yttrium at 60℃
Figure 6. Capacity utilization of AAA-size
Ni-MH batteries at different temperatures
(Charge: 0.2C, 7hrs; discharge: 0.2C, 1.0V
end of discharge voltage
As shown in Figure 6, the capacity utilization of the
AAA-size battery using the regular nickel hydroxide
and yttrium-doped nickel hydroxide electrodes is set to
be 100% at 20oC. The discharge capacity of the AAA
batteries is measured at different temperatures. Thus,
the discharge performance of the positive electrodes
using yttrium-doped or regular nickel hydroxide
powders remains at the same discharge utilization in a
temperature range from 0oC to 30oC. However, at
temperatures above 40oC (especially at 50 and 60oC),
the discharge capacity of the battery using
yttrium-doped nickel hydroxide powder is higher than
that of the battery using regular nickel hydroxide
© Institute of Materials Engineering Australasia Ltd – Materials Forum Volume 27 - Published 2004
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MATERIALS FORUM VOL. 27 (2004) 28 - 32
powder. As the oxygen evaluation potential drops at
higher temperatures, the oxidation potential and the
oxygen evaluation potential overlap, and the charge
efficiency of the positive active material declines.
However, the nickel hydroxide powder doped with
yttrium in the lattice can prevent the decrease in
oxygen evaluation potential and thus improve
high-temperature charge efficiency.
comparison with the regular Ni(OH)2 powders. The
improvement of the high-temperature performance of
the Ni-MH battery is beneficial in power-use
equipment.
The spherical Ni(OH)2 powder doped with yttrium in
the nickel hydroxide lattice has a superior
high-temperature charge/discharge performance. The
addition of yttrium in the nickel hydroxide lattice
improves the high temperature performance for the
traction-use Ni-MH batteries, especially for hybrid
electric vehicle applications.
2.
4. CONCLUSIONS
6.
The spherical Ni(OH)2 powder doped with yttrium in
the lattice has the superior high-temperature
charge/discharge performances. The charge/discharge
utilization of the yttrium-doped spherical Ni(OH)2
powder has been improved at higher temperatures in
7.
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3.
4.
5.
8.
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Cshitai, m. et al., J. Appl. Electrochem., 16, 403
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Mitsyu, K., Paste-type Nickel cathodes for
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Tetsuyoshi, G., Manufacture of Nickel cathode for
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Wang, X.Y., Yan, J., Yuan, H.T., J. Power Sources,
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32