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Electrochemical Properties of LaNi5-xGaxAlloys Used as the Negative Electrodes of

Ni-MH BatteriesDam Nhan Ba

ac, Luu Tuan Tai

ab, Nguyen Phuc Duong

a, Chu Van

Tuanc

& Tran Quang Huyd

aInternational Training Institute for Material Science (ITIMS) - Hanoi

University of Science and Technology (HUST) , Hanoi , VietnambFaculty of Physics - Hanoi University of Science, Vietnam National

University (VNU) , Hanoi , VietnamcHung Yen University of Technology and Education, Khoai Chau ,

Hung Yen , VietnamdNational 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 QuangHuy (2013) Electrochemical Properties of LaNi5-xGaxAlloys Used as the Negative Electrodes of Ni-MH

Batteries, Analytical Letters, 46:12, 1897-1909, DOI: 10.1080/00032719.2013.777920

To link to this article: http://dx.doi.org/10.1080/00032719.2013.777920

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Electrochemistry

ELECTROCHEMICAL PROPERTIES OF LaNi5-XGaXALLOYS USED AS THE NEGATIVE ELECTRODESOF 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

1International Training Institute for Material Science (ITIMS) - Hanoi

University of Science and Technology (HUST), Hanoi, Vietnam2Faculty of Physics - Hanoi University of Science, Vietnam National

University (VNU), Hanoi, Vietnam3Hung Yen University of Technology and Education, Khoai Chau,

Hung Yen, Vietnam4National 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 0.10.5) alloys were studied systematically. The structure

of the alloy was tested by X-ray diffraction (XRD) measurements. Electrochemical proper-

ties 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-xGaxis

slightly increased but retains the CaCu5structure. 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-xGaxsamples 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 Jmaxand 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 thenlaunched 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

Analytical Letters, 46: 18971909, 2013

Copyright # Taylor & Francis Group, LLC

ISSN: 0003-2719 print=1532-236X onlineDOI: 10.1080/00032719.2013.777920

1897


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Economist 2008). These devices have become a clean alternative to the traditionaltechnology 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 advan-tage 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 ahydrogen-poor (0.1 H per metal atom) solidified solution and the hydrogen-richhydride (0.61 H per metal atom) in these compounds. Hydrogen was stored inthe crystal lattice of material, and then this material became a clean energy reservetank with minimal pollution to the environment (Linden and Reddy 2001). This fea-ture has found many applications in science and engineering. One of these applica-tions is the negative electrode for Ni-MH rechargeable batteries (Cuevas et al. 2001;Daniel and Besenhard 2011). The alloy discharge reaction involves two diffusionprocesses; one is the diffusion of H atom from alloy bulk to alloy surface, and theother is the diffusion of OH from solution bulk to alloy surface. This former pro-

cess has been thoroughly investigated (Feng et al. 2000; Kadir, Sakai, and Uehara2000; Kohno et al. 2000). Ni-MH batteries are largely used and their productionincreases rapidly from year to year, and research and development works on thesebatteries continue to grow (Klebanoff 2012). Especially, in order to improve thequality and to decrease the cost of Ni-MH batteries, many studies on the optimalcomposition in RT5 compounds have been carried out (Meli, Zuettel, and Schlap-bach 1992; Luo et al. 1997; Talaganis, Esquivel, and Meyer 2011). Long-term cyclingleads to severe degradation of the material (Boonstra, Lippits, and Bernards 1989;Park and Lee 1987). To overcome this problem, substitutions have been performedon the Ni positions which leads to pseudo-binary compounds LaNi5xMx(MMn,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 proper-ties of LaNi5-xGaxalloys used for Ni-MH batteries will be reported.

MATERIALS AND METHODS

Reagents and MH Electrode Preparation

The LaNi5-xGax(x 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) ofpurity at least 99.9% were weighted according to their compositions. A slight excessof La was added to compensate the weight loss during the arc-melting process. Theingots 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 bypulverizing the as-melted compounds in an agate mortar during 30 minutes.

For the electrochemical measurements, negative electrodes were prepared bymixing LaNi5-xGax powder with nickel and cooper powders at 70:28:2 ratio ofweight and then this mixture well with a small amount of 2% polyvinyl alcohol.The mixture was scrubbed into porous foamed nickel substrates and finally pressedat 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

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6 M KOH solution for 810 h to the accelerated dissociation of H2 on the oxidesurface by the presence of Li in the surface region.

Microstructure MeasurementsThe 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-K

aradiation, 0.02 per step, 2s per step, 2h 10100). The obtained

powder XRD patterns were analyzed by means of a Rietveld refinement procedureusing Xpert High Score Plus in order to determine the type of structure and thelattice parameters (Rietveld 1969; Pecharsky and Vitalij 2009).

Electrochemical Measurements

Electrochemical measurements were performed in a three electrode system con-sisting 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 purposeof the LiOH addition into the 6 M KOH electrolyte is to increase electrochemicalactivity of the MH electrode (Uchida et al. 1999; Uchida 1999; Cui, Luo, andChuang 2000; Izawa et al. 2003; Mohamad et al. 2003). In charge-discharge capacitymeasurements, the electrodes were connected to a potential device called aBi-Potentiostat 366A. The electrodes were fully charged (the over-charged ratiowas approximately 30%50%) at a current density of 50 mA=g, and then dischargedat the same current density to a cut-off potential of0.7 V (versus SCE). The datawere transmitted to a computer containing the software for treatment and display ofresults by graphical and data files. Electrochemical impedance spectroscopy (EIS)and cyclic voltammetry (CV) measurements were performed by using an Autolab4.9 system. Electrochemical impedance spectroscopy was performed on samples withvarious polarization rates E0.9, E1.0, E1.1 and E1.2 (V=SCE); thepower AC voltage was a sinusoidal amplitude of 5 mV, and frequencies ranged from1 MHz to 5 mHz. Measurement data were analyzed by FRA software. The cyclic vol-tammetry was applied to re-activate charge-discharge for 50 cycles with a rate of10mV=s with a voltage range from 1.4 to 0.7V=SCE across all of the electrodes.The current density Jmaxand 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 lat-tice parameters of synthesized materials. Figure 1 shows the XRD patterns of theLaNi5-xGax (x 0.5) system. The data confirmed that all the samples were singlephase, 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.

ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1899


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Table 1 represents the lattice parameters and cell volume determined for theLaNi5-xGax samples with x 0.5 by using the Rietveld refinement analysis. It canbe seen that with the increase of Ga content in the alloys, the lattice parameter a, cand the cell volume V increased linearly as function of content, x. The increase ofthe lattice parameter can be explained by smaller in atomic radius of Ni (1.24 A) thanthat of Ga (1.35 A). The value ofc=aalso increased withx, clearly indicates which of

the two available crystallographic positions in the crystal structure are involved in thesubstitution process of Ga for Ni. It is well known that in the LaNi 5structure thereexist two distinguished layers of atoms. The basal layer (z 0) contains La atoms (1asites) and Ni atoms (2c sites), and the intermediate layer (z 1=2) contains only Niatoms (3 g sites). The observed increase ofc=a suggests that replacement of Ni withGa takes place preferentially within the intermediate layer rather than within thebasal or both available layers. The results obtained are in good agreement withexperimental data for Sn, Ga, Pd, and Rh found in previous literature (Shuanget al. 1999; Bowman et al. 2002; Prigent et al. 2012; Cero n-Hurtado and Esquivel2012). This indicates that in the latter system the basal or both available nickelcrystallographic positions are involved in the substitution process.

Figure 1. The XRD patterns at room temperature of the intermetallic alloys LaNi5-xGax(with x 00.5).

(Figure available in color online.)

Table 1. Lattice parameters of the intermetallic alloys LaNi5-xGax (with x 0.5)

Sample a(A) c(A) c=a V(A)3

LaNi5 5.0125 3.9838 0.7948 86.684

LaNi4.9Ga0.1 5.0203 4.0151 0.7998 87.637

LaNi4.8Ga0.2 5.0236 4.0196 0.8001 87.850

LaNi4.7Ga0.3 5.0285 4.0241 0.8003 88.120

LaNi4.6Ga0.4 5.0314 4.0290 0.8008 88.329

LaNi4.5Ga0.5 5.0345 4.0389 0.8022 88.655



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Galvanostatic Charge-Discharge at Constant Current

When hydrogen storage electrode is first charged, the stored hydrogen in thealloy is released gradually after absorption. The process in which the freshly formed

hydride electrodes are continuously charged and discharged in order to obtain themaximum electrochemical capacity is called activation. The activation capabilitywas characterized by the number of chargingdischarging cycles required for attain-ing the greatest discharge capacity through a chargingdischarging cycle at a con-stant current density 50 mA=g. The fewer the number of chargingdischargingcycles, the better the activation performance. This is important for practical use ofNi-MH batteries. Figure 2 (Fig. 2a and Fig. 2b) shows the activation capabilitiesof the LaNi5-xGax(x 0 and 0.3, respectively) electrode alloys. The LaNi5alloys dis-play excellent activation performances and can attain their maximum dischargecapacities after 57 chargingdischarging cycles. For the substitution of Ni by Ga,the activation of LaNi4.5Ga0.5 alloys needs a bigger number of cycles. However,

the chargingdischarging curve of LaNi5 is unstable, as the chargingdischargingcycle could not repeat even in the 10 cycle. LaNi5 samples that were Ga-dopedhad better and more stable chargingdischarging cycles. Only a few initial charg-ingdischarging cycles of materials were more stable and durable, and can serve asan electrode of a battery.

The effect of the substitution of Ni by Ga on the course of the hydrogen stor-age capacity of LaNi5-xGax (x 00.5) electrodes as a function of the number ofcycles is presented in Figure 3. For LaNi5 alloy, there is a fast increase in capacityin the first few cycles; the highest capacity Cmax of electrode was observed at the7th 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 dischargecapacity is almost saturated. Compared with the alloy original LaNi5, Ga-dopedalloys had a slightly lower capacity but prolonged lifetime and a more stablecharge-discharge process. This can be explained by, since Ga has a low melting tem-perature, when arc molten, Ga will melt first, sneak and cover the LaNi 5-xGaxpar-ticles 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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charge-discharge process. All of this leads to batterys longer lifetime. However,covering LaNi5-xGax particles also reduces the batterys capacity. This is in goodagreement with the results obtained previously. The substitution of Ni by Mn, Cu,Sn, and In (Drassner and Blazina 2003, 2004; Chen et al. 2008; Prigent et al. 2011,2012) makes the materials ability absorption decrease but the lifetime and perfor-mance of the batteries is increased enough to be used as negative electrode forNi-MH rechargeable batteries.

Figure 4. The electrochemical impedance spectra (EIS) of electrodes with various different polarization

potentials: (a) LaNi5and (b) LaNi4.5Ga0.5. (Figure available in color online.)

Figure 3. Discharge capacities vs. cycle number of LaNi5-xGax (x 00.5) alloys. (Figure available in

color online.)



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Electrochemical Impedance Spectroscopy

The electrochemical impedance spectroscopy (EIS) is an effective methodcharacterizing the electrochemical performance of MH electrode. Figure 4 shows

typical Nyquist impedance spectra of electrode material LaNi5-xGax (x 0 and0.5) at different polarization potentials (1.2 to 0.9 V vs SCE) in the whole fre-quency range (105 to 102 Hz). As shown in this figure, the shapes of the electro-chemical impedance spectra are similar, with only one semicircle, and no apparentlinear response appears in the low frequency region for these electrodes. It is similarto the case with substitution of Ni by Ge and Sn, as reported by Witham (1997). Ithas been suggested that the loop in the impedance spectra is a characteristic of thecharge transfer reaction. The diameter of the loop increases apparently with increas-ing the Ga concentration in the alloys. On the other hand, the diameters of semicir-cles are smaller when the polarization potential increases. The diameter of semicirclecorresponds 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 thepreliminary charge transfer resistance Rct and double layer capacitance Cdl para-meters of the electrode material by FRA software and used the equivalent circuitmethod. Figure 5 shows that when the same voltage was applied to the samplesand the increased Ga content was substituted for Ni, the charge transfer resistanceof the material electrodes increased (Figure 5a), and, inversely, the double layercapacitance was decreased (Figure 5b). A similar increase was reported by Panet al. (1999). The obtained results suggest that there is a variation in lattice para-

meters of samples with increasing Ga content substituted for Ni; both parametersa and c increased with the increasing Ga-doped proportion. This change in crystalstructure makes the conductivity and charge transfer more difficult. In addition,the decrease of Cdlalso shows that the density of conductive ions in the charge dou-ble layer is smaller and it leads to the possibility of charge exchange at the peripherallayers 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




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agreement with the previous studies. The doped Ga increases the materialsimpedance but the lifetime and performance of the batteries is increased enoughto be used as negative electrode for Ni-MH rechargeable batteries.

Cyclic Voltammetry

Cyclic voltammetric measurements of the negative electrode were performed inthe potential range of1.4 to 0.7 V at sweep rate of 5 mV=s. The cyclic voltamme-try 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 andLaNi4.5Ga4.5compounds have similar formats. The cyclic voltammetry are continu-ous, with no wave or peak expression of side effects during the test from the begin-ning to the end of cycle. It was in good agreement with some reference data (Ananthet 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 sol-ution. For the same charge potential value and experiment conditions, current den-sity increases with each cycle in all the samples. The increase of charge currentdensity represents good quality of electrode materials with increased charge=dis-charge cycle performance.

To see more clearly the influence of Ga-doped concentration on thecharge-discharge process, the GRES software was used to calculate the current den-sity (Jmax) and charge quantity (Q) of each sample. Figure 7 shows the activity capa-bility of electrodes through charge=discharge cycles characterized by the maximaldischarge current density Jpmax(Figure 7a) and the maximal charge current density

Jnmax(Figure 7b). During the hydrogen storage process of negative electrode, thesecurrent densities increase when the number of charge=discharge cycles increases.The increase of the maximal current densities shows well the increase of activity ofmaterials due to the increasing number of cycles. The increased rate of the dischargecurrent density is higher than that of the charge current. For initial cycles, the maxi-mal current density Jmaxis very low and then increases rapidly to the increase of the

Figure 6. Cyclic voltammetry (CV) curves of the alloy electrodes: (a) LaNi5and (b) LaNi4.5Ga0.5. (Figure




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charge=discharge cycles. This explains why it is necessary to activate a few cyclesbefore the use starts. This fast increase of current densities for initial cycles isexplained mainly by the necessity of adsorbing process on the electrode surface toease the charge=discharge process. When the number of charge=discharge cyclesincreases, the hydrogen uncovered area of electrodes decreases which leads to thedecrease of current densities. From cycle 20 the increase rate reduces which meansthe electrodes are more stable. This increase is mainly due to the diffusion of hydro-gen atoms into material particles.

The change of the charge quantity Q of electrode materials at the samescanning rate is illustrated in Figure 8. The results show that for the same Ga-doped

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.)

Figure 8. Variation of the charge quantity Qcand Qdas 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 thenumber of cycles. This indicates that the amount of hydrogen atoms absorbed inthe electrode material increased cycle by cycle. In general, an oxidation reaction ofhydrogen 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 otherone is a diffusion process of hydrogen atoms from the inside of the electrode toits surface. If the hydrogen atom diffusion is much faster than that of the rate ofcharge transfer process, the charge quantity Q will rapidly raise up with increasingnumbers of cycles. The curves in Figure 8 indicate that in initial cycles, the chargequantity Q increases strongly which shows its oxidation reaction is controlled byhydrogen atom diffusion. From cycle 20, charge quantity Q increases more slowlywith increasing number of cycles, suggesting that the oxidation reaction is controlledmainly by charge transfer process on the electrode surface or, at least, by mixture ofdiffusion and charge transfer processes.

To evaluate the performance of the electrode materials, we have calculated theperformance between the discharge quantity Qdand charge quantity Qcaccording tothe following formula:

HcdQd

Qc:100%

Calculation results are shown in Figure 9. For Ga-doped LaNi5cases, the per-formance of batteries was higher than for LaNi5. In initial cycles, the performance ofall the electrodes has achieved more than 50% rate, and the performance increasedwhen the number of activated cycles increases. From cycle 20 onward, the perfor-

mance 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




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the performance of the electrode has reached 90% rate. With Ga replaced for Ni, theperformance of electrodes was higher. It shows that the Ga-doped LaNi5 alloyenhances 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 materi-als decreased. This is consistent with the results of measuring the discharge capacityof the materials shown in Figures 2 and 3. As the doping concentration increased,the capacity of the materials decreased. However, when increasing the doping con-centration, prolonged lifetime and charge-discharge performance of materials werehigher (Figure 9). The initial cycle, the current density and charge quantity andcapacity were small and increased strongly with the number of cycles, but after someperiod of training, their value increased slowly and gradually stabilize. The electro-chemical properties of LaNi5-xGaxshowed that the material can be used as the nega-tive electrode in Ni-MH rechargeable batteries.

CONCLUSIONS

We have investigated the crystallographic and the electrochemical properties ofthe intermetallic compounds LaNi5-xGax (x 0.5). The XRD patterns provide evi-dence of the single phase and crystallization in the hexagonal CaCu5-type structurein all the samples. The lattice parameters calculated by the Rietveld method, both aandc, slightly increased with increasing Ga concentration. Comparing with the orig-inal LaNi5alloy, Ga-doped alloys had a slightly lower capacity but prolonged life-time, a more stable charge-discharge process, and better performance. The

Nyquist plots of electrodes LaNi5-xGax in the vicinity of equilibrium potential(1.2 to 0.9 V vs. SCE) were similar. They have only a semicircle shape, like pureLaNi5. This demonstrates that, after doping, the electric conductivity and chargetransfer of negative electrodes were not changed. The cyclic voltammetry was studiedin the different polarized potential, and the results indicate that voltammogramswere similar. For the same Ga-doped concentration and experimental conditions,the maximal current density Jmax and charge quantity Q increased with each cyclein all samples.

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