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Analytical Letters
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Electrochemical Properties of LaNi5-xGax
Alloys Used as the Negative Electrodes of
Ni-MH Batteries
Dam Nhan Ba
c

a c

, Luu Tuan Tai

Tuan & Tran Quang Huy

a b

a

, Nguyen Phuc Duong , Chu Van

d

a

International Training Institute for Material Science (ITIMS) - Hanoi
University of Science and Technology (HUST) , Hanoi , Vietnam
b

Faculty of Physics - Hanoi University of Science, Vietnam National
University (VNU) , Hanoi , Vietnam
c

Hung Yen University of Technology and Education, Khoai Chau ,
Hung Yen , Vietnam
d

National Institute of Hygiene and Epidemiology (NIHE) , Hanoi ,
Vietnam
Accepted author version posted online: 19 Mar 2013.Published
online: 25 Jul 2013.

To cite this article: Dam Nhan Ba , Luu Tuan Tai , Nguyen Phuc Duong , Chu Van Tuan & Tran Quang
Huy (2013) Electrochemical Properties of LaNi5-xGax Alloys Used as the Negative Electrodes of Ni-MH
Batteries, Analytical Letters, 46:12, 1897-1909, DOI: 10.1080/00032719.2013.777920
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Analytical Letters, 46: 1897–1909, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0003-2719 print=1532-236X online
DOI: 10.1080/00032719.2013.777920

Electrochemistry

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ELECTROCHEMICAL PROPERTIES OF LaNi5-XGaX
ALLOYS USED AS THE NEGATIVE ELECTRODES
OF Ni-MH BATTERIES
Dam Nhan Ba,1,3 Luu Tuan Tai,1,2 Nguyen Phuc Duong,1
Chu Van Tuan,3 and Tran Quang Huy4
1

International Training Institute for Material Science (ITIMS) - Hanoi

University of Science and Technology (HUST), Hanoi, Vietnam
2
Faculty of Physics - Hanoi University of Science, Vietnam National
University (VNU), Hanoi, Vietnam
3
Hung Yen University of Technology and Education, Khoai Chau,
Hung Yen, Vietnam
4
National Institute of Hygiene and Epidemiology (NIHE), Hanoi, Vietnam
The effects of the substitution of nickel by gallium on the structures and the electrochemical
properties of LaNi5-xGax (x ¼ 0.1À0.5) alloys were studied systematically. The structure
of the alloy was tested by X-ray diffraction (XRD) measurements. Electrochemical properties and battery parameters were measured by bipotentiostat and battery tester equipment.
The results showed that when gallium is doped into alloys, the lattice of the LaNi5-xGax is
slightly increased but retains the CaCu5 structure. Gallium has a low melting temperature.
When gallium replaces nickel in the LaNi5 alloy, it covers material particles and reduces
oxidation process, which leads to a longer lifetime and makes charge/discharge process
more stable. The shapes of electrochemical impedance spectroscopy measurements of all
the LaNi5-xGax samples were similar, and the value increases as the substitution of Ni by
Ga increases. The cyclic voltammograms of all the LaNi5-xGax samples were similar to
the one of pure LaNi5. For the same Ga-doped concentration and experimental conditions,
the current density Jmax and charge quantity Q of the samples were increased cycle by cycle
of charge/discharge.
Keywords: Cyclic voltammetry; Electrochemical impedance spectroscopy; Electrochemical properties;
LaNi5; Ni-MH batteries

INTRODUCTION
Nickel-metal hydride (Ni-MH) batteries were discovered in the 1970s, and then
launched into the market in the 1990s (Van Vucht, Kuijpers, and Bruning 1970; The
Received 13 December 2012; accepted 2 February 2013.
Address correspondence to Dam Nhan Ba, Department of Basic Sciences, Hung Yen University of

Technology and Education, Khoai Chau, Hung Yen, Vietnam. E-mail: damnhanba@gmail.
com.vn
1897


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Economist 2008). These devices have become a clean alternative to the traditional
technology of Ni=Cd (Linden and Reddy 2001). In low-weight electronic devices,
Ni-MH batteries have been used to replace Ni=Cd ones because of their green advantage as well as a higher energy capacity. According to Daniel and Besenhard (2011),
hydride formation takes place by means of a discrete phase transition between a
hydrogen-poor (0.1 H per metal atom) solidified solution and the hydrogen-rich
hydride (0.6–1 H per metal atom) in these compounds. Hydrogen was stored in
the crystal lattice of material, and then this material became a clean energy reserve
tank with minimal pollution to the environment (Linden and Reddy 2001). This feature has found many applications in science and engineering. One of these applications is the negative electrode for Ni-MH rechargeable batteries (Cuevas et al. 2001;
Daniel and Besenhard 2011). The alloy discharge reaction involves two diffusion
processes; one is the diffusion of H atom from alloy bulk to alloy surface, and the
other is the diffusion of OH À from solution bulk to alloy surface. This former process has been thoroughly investigated (Feng et al. 2000; Kadir, Sakai, and Uehara
2000; Kohno et al. 2000). Ni-MH batteries are largely used and their production
increases rapidly from year to year, and research and development works on these
batteries continue to grow (Klebanoff 2012). Especially, in order to improve the
quality and to decrease the cost of Ni-MH batteries, many studies on the optimal
composition in RT5 compounds have been carried out (Meli, Zuettel, and Schlapbach 1992; Luo et al. 1997; Talagan˜is, Esquivel, and Meyer 2011). Long-term cycling
leads to severe degradation of the material (Boonstra, Lippits, and Bernards 1989;
Park and Lee 1987). To overcome this problem, substitutions have been performed
on the Ni positions which leads to pseudo-binary compounds LaNi5ÀxMx (M ¼ Mn,

Fe, Co, Ni, Al, Sn, Ge, Si) with improved resistance towards degradation (Bowman
et al. 2002; Li et al. 2008; Shahgaldi et al. 2012; Dongliang et al. 2012; Prigent,
Joubert, and Gupta 2012).
In this work, the effects of substitution of Ni by Ga on electrochemical properties of LaNi5-xGax alloys used for Ni-MH batteries will be reported.

MATERIALS AND METHODS
Reagents and MH Electrode Preparation
The LaNi5-xGax (x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples were prepared by the arc
melting method under an argon atmosphere. The starting materials (La, Ni, Ga) of
purity at least 99.9% were weighted according to their compositions. A slight excess
of La was added to compensate the weight loss during the arc-melting process. The
ingots were turned over and re-melted several times to attain good homogeneity.
Powder samples with an average particle size of about 50 mm were obtained by
pulverizing the as-melted compounds in an agate mortar during 30 minutes.
For the electrochemical measurements, negative electrodes were prepared by
mixing LaNi5-xGax powder with nickel and cooper powders at 70:28:2 ratio of
weight and then this mixture well with a small amount of 2% polyvinyl alcohol.
The mixture was scrubbed into porous foamed nickel substrates and finally pressed
at a pressure of 6 ton=cm2 and density 0.25 g=cm2 to form a test electrode. Before
measurements, the MH electrode was modified by immersing it in 1 M LiOH and


ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES

1899

6 M KOH solution for 8–10 h to the accelerated dissociation of H2 on the oxide
surface by the presence of Li in the surface region.
Microstructure Measurements

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The crystalline structure and the phase impurity of the samples at room
temperature were examined on a D=Max-2500=PC X-ray powder diffractometer
(using Cu-Ka radiation, 0.02 per step, 2s per step, 2h ¼ 10 À100 ). The obtained
powder XRD patterns were analyzed by means of a Rietveld refinement procedure
using X’pert High Score Plus in order to determine the type of structure and the
lattice parameters (Rietveld 1969; Pecharsky and Vitalij 2009).
Electrochemical Measurements
Electrochemical measurements were performed in a three electrode system consisting of the working electrode (WE) as the prepared sample, a counter electrode
(CE) of platinum, and a reference electrode (saturated calomel electrode, SCE,
Hg=Hg2Cl2, calomel). The electrolyte was 1 M LiOH and 6 M KOH. The purpose
of the LiOH addition into the 6 M KOH electrolyte is to increase electrochemical
activity of the MH electrode (Uchida et al. 1999; Uchida 1999; Cui, Luo, and
Chuang 2000; Izawa et al. 2003; Mohamad et al. 2003). In charge-discharge capacity
measurements, the electrodes were connected to a potential device called a
Bi-Potentiostat 366A. The electrodes were fully charged (the over-charged ratio
was approximately 30%–50%) at a current density of 50 mA=g, and then discharged
at the same current density to a cut-off potential of À0.7 V (versus SCE). The data
were transmitted to a computer containing the software for treatment and display of
results by graphical and data files. Electrochemical impedance spectroscopy (EIS)
and cyclic voltammetry (CV) measurements were performed by using an Autolab
4.9 system. Electrochemical impedance spectroscopy was performed on samples with
various polarization rates E ¼ À0.9, E ¼ À1.0, E ¼ À1.1 and E ¼ À1.2 (V=SCE); the
power AC voltage was a sinusoidal amplitude of 5 mV, and frequencies ranged from
1 MHz to 5 mHz. Measurement data were analyzed by FRA software. The cyclic voltammetry was applied to re-activate charge-discharge for 50 cycles with a rate of
10 mV=s with a voltage range from À1.4 to À0.7 V=SCE across all of the electrodes.
The current density Jmax and charge quantity Q of all samples were calculated by the
GRES software.
RESULTS AND DISCUSSION

Crytal Structure Analysis
X-ray diffraction (XRD) was used to investigate the crystal structure and lattice parameters of synthesized materials. Figure 1 shows the XRD patterns of the
LaNi5-xGax (x 0.5) system. The data confirmed that all the samples were single
phase, and crystallized in the hexagonal CaCu5-type structure, the same structure,
as does the prototype LaNi5, and no secondary phase was detected within 1% error
of measurements.


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D. N. BA ET AL.

Figure 1. The XRD patterns at room temperature of the intermetallic alloys LaNi5-xGax (with x ¼ 0À0.5).
(Figure available in color online.)

Table 1 represents the lattice parameters and cell volume determined for the
LaNi5-xGax samples with x 0.5 by using the Rietveld refinement analysis. It can
be seen that with the increase of Ga content in the alloys, the lattice parameter a, c
and the cell volume V increased linearly as function of content, x. The increase of
˚ ) than
the lattice parameter can be explained by smaller in atomic radius of Ni (1.24 A
˚
that of Ga (1.35 A). The value of c=a also increased with x, clearly indicates which of
the two available crystallographic positions in the crystal structure are involved in the
substitution process of Ga for Ni. It is well known that in the LaNi5 structure there
exist two distinguished layers of atoms. The basal layer (z ¼ 0) contains La atoms (1a
sites) and Ni atoms (2c sites), and the intermediate layer (z ¼ 1=2) contains only Ni
atoms (3 g sites). The observed increase of c=a suggests that replacement of Ni with

Ga takes place preferentially within the intermediate layer rather than within the
basal or both available layers. The results obtained are in good agreement with
experimental data for Sn, Ga, Pd, and Rh found in previous literature (Shuang
et al. 1999; Bowman et al. 2002; Prigent et al. 2012; Cero´n-Hurtado and Esquivel
2012). This indicates that in the latter system the basal or both available nickel
crystallographic positions are involved in the substitution process.

Table 1. Lattice parameters of the intermetallic alloys LaNi5-xGax (with x
Sample
LaNi5
LaNi4.9Ga0.1
LaNi4.8Ga0.2
LaNi4.7Ga0.3
LaNi4.6Ga0.4
LaNi4.5Ga0.5

0.5)

˚)
a(A

˚)
c(A

c=a

˚ )3
V(A

5.0125

5.0203
5.0236
5.0285
5.0314
5.0345

3.9838
4.0151
4.0196
4.0241
4.0290
4.0389

0.7948
0.7998
0.8001
0.8003
0.8008
0.8022

86.684
87.637
87.850
88.120
88.329
88.655


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Galvanostatic Charge-Discharge at Constant Current
When hydrogen storage electrode is first charged, the stored hydrogen in the
alloy is released gradually after absorption. The process in which the freshly formed
hydride electrodes are continuously charged and discharged in order to obtain the
maximum electrochemical capacity is called activation. The activation capability
was characterized by the number of charging–discharging cycles required for attaining the greatest discharge capacity through a charging–discharging cycle at a constant current density 50 mA=g. The fewer the number of charging–discharging
cycles, the better the activation performance. This is important for practical use of
Ni-MH batteries. Figure 2 (Fig. 2a and Fig. 2b) shows the activation capabilities
of the LaNi5-xGax (x ¼ 0 and 0.3, respectively) electrode alloys. The LaNi5 alloys display excellent activation performances and can attain their maximum discharge
capacities after 5–7 charging–discharging cycles. For the substitution of Ni by Ga,
the activation of LaNi4.5Ga0.5 alloys needs a bigger number of cycles. However,
the charging–discharging curve of LaNi5 is unstable, as the charging–discharging
cycle could not repeat even in the 10 cycle. LaNi5 samples that were Ga-doped
had better and more stable charging–discharging cycles. Only a few initial charging–discharging cycles of materials were more stable and durable, and can serve as
an electrode of a battery.
The effect of the substitution of Ni by Ga on the course of the hydrogen storage capacity of LaNi5-xGax (x ¼ 0À0.5) electrodes as a function of the number of
cycles is presented in Figure 3. For LaNi5 alloy, there is a fast increase in capacity
in the first few cycles; the highest capacity Cmax of electrode was observed at the
7th cycle. All the Ga-doped electrodes reach their highest capacity Cmax near at
the same time after about the 10th cycles; from the 12th cycle on the discharge
capacity is almost saturated. Compared with the alloy original LaNi5, Ga-doped
alloys had a slightly lower capacity but prolonged lifetime and a more stable
charge-discharge process. This can be explained by, since Ga has a low melting temperature, when arc molten, Ga will melt first, sneak and cover the LaNi5-xGax particles which then makes LaNi5-xGax crystals smaller and less oxygen in the

Figure 2. Charge and discharge potential curves of alloys: (a) LaNi5 and (b) LaNi4.7Ga0.3. (Figure
available in color online.)

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Figure 3. Discharge capacities vs. cycle number of LaNi5-xGax (x ¼ 0À0.5) alloys. (Figure available in
color online.)

charge-discharge process. All of this leads to battery’s longer lifetime. However,
covering LaNi5-xGax particles also reduces the battery’s capacity. This is in good
agreement with the results obtained previously. The substitution of Ni by Mn, Cu,
Sn, and In (Drasner and Blazˇina 2003, 2004; Chen et al. 2008; Prigent et al. 2011,
2012) makes the material’s ability absorption decrease but the lifetime and performance of the batteries is increased enough to be used as negative electrode for
Ni-MH rechargeable batteries.

Figure 4. The electrochemical impedance spectra (EIS) of electrodes with various different polarization
potentials: (a) LaNi5 and (b) LaNi4.5Ga0.5. (Figure available in color online.)


ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES

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Electrochemical Impedance Spectroscopy
The electrochemical impedance spectroscopy (EIS) is an effective method

characterizing the electrochemical performance of MH electrode. Figure 4 shows
typical Nyquist impedance spectra of electrode material LaNi5-xGax (x ¼ 0 and
0.5) at different polarization potentials (À1.2 to À0.9 V vs SCE) in the whole frequency range (105 to 10À2 Hz). As shown in this figure, the shapes of the electrochemical impedance spectra are similar, with only one semicircle, and no apparent
linear response appears in the low frequency region for these electrodes. It is similar
to the case with substitution of Ni by Ge and Sn, as reported by Witham (1997). It
has been suggested that the loop in the impedance spectra is a characteristic of the
charge transfer reaction. The diameter of the loop increases apparently with increasing the Ga concentration in the alloys. On the other hand, the diameters of semicircles are smaller when the polarization potential increases. The diameter of semicircle
corresponds to the charge transfer resistance, Rct. It means charge transfer reaction
is realized at high applied polarization.
In order to see more clearly the influence of Ga content substituted for Ni on
the electrochemical impedance spectrum of alloy electrodes, we have calculated the
preliminary charge transfer resistance Rct and double layer capacitance Cdl parameters of the electrode material by FRA software and used the equivalent circuit
method. Figure 5 shows that when the same voltage was applied to the samples
and the increased Ga content was substituted for Ni, the charge transfer resistance
of the material electrodes increased (Figure 5a), and, inversely, the double layer
capacitance was decreased (Figure 5b). A similar increase was reported by Pan
et al. (1999). The obtained results suggest that there is a variation in lattice parameters of samples with increasing Ga content substituted for Ni; both parameters
a and c increased with the increasing Ga-doped proportion. This change in crystal
structure makes the conductivity and charge transfer more difficult. In addition,
the decrease of Cdl also shows that the density of conductive ions in the charge double layer is smaller and it leads to the possibility of charge exchange at the peripheral
layers of electrolytes and the electrode surface is decreased. This result is in

Figure 5. The dependence of (a) Rct and (b) Cdl in LaNi5-xGax alloys on (x) concentration. (Figure
available in color online.)


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D. N. BA ET AL.

agreement with the previous studies. The doped Ga increases the material’s
impedance but the lifetime and performance of the batteries is increased enough
to be used as negative electrode for Ni-MH rechargeable batteries.

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Cyclic Voltammetry
Cyclic voltammetric measurements of the negative electrode were performed in
the potential range of À1.4 to À0.7 V at sweep rate of 5 mV=s. The cyclic voltammetry curves of the MH electrodes are illustrated in Figure 6. As shown in this figure,
we can see that charge and discharge cyclic characteristics of the LaNi5 and
LaNi4.5Ga4.5 compounds have similar formats. The cyclic voltammetry are continuous, with no wave or peak expression of side effects during the test from the beginning to the end of cycle. It was in good agreement with some reference data (Ananth
et al. 2009; van Druten et al. 2000). This suggests that the samples are clean, have
high structural uniformity, and contain no impurities in electrolytic dissociation solution. For the same charge potential value and experiment conditions, current density increases with each cycle in all the samples. The increase of charge current
density represents good quality of electrode materials with increased charge=discharge cycle performance.
To see more clearly the influence of Ga-doped concentration on the
charge-discharge process, the GRES software was used to calculate the current density (Jmax) and charge quantity (Q) of each sample. Figure 7 shows the activity capability of electrodes through charge=discharge cycles characterized by the maximal
discharge current density Jpmax (Figure 7a) and the maximal charge current density
Jnmax(Figure 7b). During the hydrogen storage process of negative electrode, these
current densities increase when the number of charge=discharge cycles increases.
The increase of the maximal current densities shows well the increase of activity of
materials due to the increasing number of cycles. The increased rate of the discharge
current density is higher than that of the charge current. For initial cycles, the maximal current density Jmax is very low and then increases rapidly to the increase of the

Figure 6. Cyclic voltammetry (CV) curves of the alloy electrodes: (a) LaNi5 and (b) LaNi4.5Ga0.5. (Figure
available in color online.)


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Figure 7. Variation of the charge density Jcmax (a) and discharge density Jdmax (b) as a function of number
of cycles. (Figure available in color online.)

charge=discharge cycles. This explains why it is necessary to activate a few cycles
before the use starts. This fast increase of current densities for initial cycles is
explained mainly by the necessity of adsorbing process on the electrode surface to
ease the charge=discharge process. When the number of charge=discharge cycles
increases, the hydrogen uncovered area of electrodes decreases which leads to the
decrease of current densities. From cycle 20 the increase rate reduces which means
the electrodes are more stable. This increase is mainly due to the diffusion of hydrogen atoms into material particles.
The change of the charge quantity Q of electrode materials at the same
scanning rate is illustrated in Figure 8. The results show that for the same Ga-doped

Figure 8. Variation of the charge quantity Qc and Qd as a function of number of cycles. (Figure available
in color online.)


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concentration, the values of both charge quantity Qd and Qc increased by the
number of cycles. This indicates that the amount of hydrogen atoms absorbed in
the electrode material increased cycle by cycle. In general, an oxidation reaction of
hydrogen on the MH electrode surface consists of two processes. The first one is a

charge transferring process at the interface of electrode=electrolyte and the other
one is a diffusion process of hydrogen atoms from the inside of the electrode to
its surface. If the hydrogen atom diffusion is much faster than that of the rate of
charge transfer process, the charge quantity Q will rapidly raise up with increasing
numbers of cycles. The curves in Figure 8 indicate that in initial cycles, the charge
quantity Q increases strongly which shows its oxidation reaction is controlled by
hydrogen atom diffusion. From cycle 20, charge quantity Q increases more slowly
with increasing number of cycles, suggesting that the oxidation reaction is controlled
mainly by charge transfer process on the electrode surface or, at least, by mixture of
diffusion and charge transfer processes.
To evaluate the performance of the electrode materials, we have calculated the
performance between the discharge quantity Qd and charge quantity Qc according to
the following formula:
HcÀd ¼

Qd
:100%
Qc

Calculation results are shown in Figure 9. For Ga-doped LaNi5 cases, the performance of batteries was higher than for LaNi5. In initial cycles, the performance of
all the electrodes has achieved more than 50% rate, and the performance increased
when the number of activated cycles increases. From cycle 20 onward, the performance of the material increases more slowly. After 50 cyclic voltammetric cycles,

Figure 9. Variation of charge-discharge performance Hd-c as a function of number of cycles. (Figure
available in color online.)


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ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES

1907

the performance of the electrode has reached 90% rate. With Ga replaced for Ni, the
performance of electrodes was higher. It shows that the Ga-doped LaNi5 alloy
enhances the performance and stability of the electrodes.
We can see in Figure 7 and 8, compared with the original material LaNi5, when
we substitute a part Ni by Ga, the current density and charge quantity of the materials decreased. This is consistent with the results of measuring the discharge capacity
of the materials shown in Figures 2 and 3. As the doping concentration increased,
the capacity of the materials decreased. However, when increasing the doping concentration, prolonged lifetime and charge-discharge performance of materials were
higher (Figure 9). The initial cycle, the current density and charge quantity and
capacity were small and increased strongly with the number of cycles, but after some
period of training, their value increased slowly and gradually stabilize. The electrochemical properties of LaNi5-xGax showed that the material can be used as the negative electrode in Ni-MH rechargeable batteries.

CONCLUSIONS
We have investigated the crystallographic and the electrochemical properties of
the intermetallic compounds LaNi5-xGax (x 0.5). The XRD patterns provide evidence of the single phase and crystallization in the hexagonal CaCu5-type structure
in all the samples. The lattice parameters calculated by the Rietveld method, both a
and c, slightly increased with increasing Ga concentration. Comparing with the original LaNi5 alloy, Ga-doped alloys had a slightly lower capacity but prolonged lifetime, a more stable charge-discharge process, and better performance. The
Nyquist plots of electrodes LaNi5-xGax in the vicinity of equilibrium potential
(À1.2 to À0.9 V vs. SCE) were similar. They have only a semicircle shape, like pure
LaNi5. This demonstrates that, after doping, the electric conductivity and charge
transfer of negative electrodes were not changed. The cyclic voltammetry was studied
in the different polarized potential, and the results indicate that voltammograms
were similar. For the same Ga-doped concentration and experimental conditions,
the maximal current density Jmax and charge quantity Q increased with each cycle
in all samples.

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