DSpace at VNU: Electrochemical Properties of LaNi5-xGax Alloys Used as the Negative Electrodes of Ni-MH Batteries

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Analytical Letters

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Alloys Used as the Negative Electrodes of Ni-MH Batteries

Dam Nhan Ba ac , Luu Tuan Tai ab , Nguyen Phuc Duong a , Chu Van Tuan c & Tran Quang Huy 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

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

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ALLOYS USED AS THE NEGATIVE ELECTRODES

OF Ni-MH BATTERIES Dam Nhan Ba,1,3Luu Tuan Tai,1,2Nguyen Phuc Duong,1 Chu Van Tuan,3and 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

4National 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 LaNi 5-x Ga x (x ¼ 0.1
0.5) 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 LaNi 5-x Ga x is slightly increased but retains the CaCu 5 structure Gallium has a low melting temperature When gallium replaces nickel in the LaNi 5 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 LaNi 5-x Ga x samples were similar, and the value increases as the substitution of Ni by

Ga increases The cyclic voltammograms of all the LaNi 5-x Ga x samples were similar to the one of pure LaNi 5 For the same Ga-doped concentration and experimental conditions, the current density J max and charge quantity Q of the samples were increased cycle by cycle

of charge/discharge.

Keywords: Cyclic voltammetry; Electrochemical impedance spectroscopy; Electrochemical properties; LaNi 5 ; 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

Analytical Letters, 46: 1897–1909, 2013

Copyright # Taylor & Francis Group, LLC

ISSN: 0003-2719 print=1532-236X online

DOI: 10.1080/00032719.2013.777920

1897

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 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 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 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 diffusion processes; one is the diffusion of H atom from alloy bulk to alloy surface, and the other 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 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 RT5compounds have been carried out (Meli, Zuettel, and Schlap-bach 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 LaNi5
xMx(M¼ 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 proper-ties of LaNi5-xGaxalloys used for Ni-MH batteries will be reported

MATERIALS AND METHODS

Reagents and MH Electrode Preparation The LaNi5-xGax(x¼ 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=cm2and density 0.25 g=cm2to form a test electrode Before measurements, the MH electrode was modified by immersing it in 1 M LiOH and

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 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-Karadiation, 0.02 per step, 2s per step, 2h¼ 10
100) 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 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 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
0.7 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 E¼
0.9, E ¼
1.0, E ¼
1.1 and E ¼
1.2 (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 vol-tammetry was applied to re-activate charge-discharge for 50 cycles with a rate of

10 mV=s with a voltage range from
1.4 to
0.7 V=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 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

ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1899

Table 1 represents the lattice parameters and cell volume determined for the LaNi5-xGaxsamples 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 the lattice parameter can be explained by smaller in atomic radius of Ni (1.24 A˚ ) than 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 LaNi5structure there exist two distinguished layers of atoms The basal layer (z¼ 0) contains La atoms (1a sites) and Ni atoms (2c sites), and the intermediate layer (z¼ 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

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

Table 1 Lattice parameters of the intermetallic alloys LaNi 5-x Ga x (with x  0.5) Sample a(A ˚ ) c(A ˚ ) c=a V(A ˚ ) 3

LaNi 5 5.0125 3.9838 0.7948 86.684 LaNi 4.9 Ga 0.1 5.0203 4.0151 0.7998 87.637 LaNi 4.8 Ga 0.2 5.0236 4.0196 0.8001 87.850 LaNi 4.7 Ga 0.3 5.0285 4.0241 0.8003 88.120 LaNi 4.6 Ga 0.4 5.0314 4.0290 0.8008 88.329 LaNi 4.5 Ga 0.5 5.0345 4.0389 0.8022 88.655

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 attain-ing the greatest discharge capacity through a chargattain-ing–dischargattain-ing cycle at a con-stant 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(x¼ 0 and 0.3, respectively) electrode alloys The LaNi5alloys dis-play 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 charg-ing–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 stor-age capacity of LaNi5-xGax (x¼ 0
0.5) electrodes as a function of the number of cycles is presented in Figure 3 For LaNi5alloy, 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 tem-perature, when arc molten, Ga will melt first, sneak and cover the LaNi5-xGax par-ticles which then makes LaNi5-xGax crystals smaller and less oxygen in the

Figure 2 Charge and discharge potential curves of alloys: (a) LaNi 5 and (b) LaNi 4.7 Ga 0.3 (Figure available in color online.)

ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1901

charge-discharge process All of this leads to battery’s longer lifetime However, covering LaNi5-xGaxparticles 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 (Drassner and Blazˇina 2003, 2004; Chen et al 2008; Prigent et al 2011, 2012) makes the material’s ability absorption decrease but the lifetime and perfor-mance 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) LaNi and (b) LaNi Ga (Figure available in color online.)

Figure 3 Discharge capacities vs cycle number of LaNi 5-x Ga x (x ¼ 0
0.5) alloys (Figure available in color online.)

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 (x¼ 0 and 0.5) at different polarization potentials (
1.2 to
0.9 V vs SCE) in the whole fre-quency range (105 to 10
2Hz) As shown in this figure, the shapes of the electro-chemical 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 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 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 para-meters 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 para-meters of samples with increasing Ga content substituted for Ni; both parapara-meters

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 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 peripheral layers of electrolytes and the electrode surface is decreased This result is in

Figure 5 The dependence of (a) R ct and (b) C dl in LaNi 5-x Ga x alloys on (x) concentration (Figure available in color online.)

ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1903

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

Cyclic Voltammetry Cyclic voltammetric measurements of the negative electrode were performed in the potential range of
1.4 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 and LaNi4.5Ga4.5compounds 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 (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 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 current density represents good quality of electrode materials with increased charge=dis-charge 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 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 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 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) LaNi 5 and (b) LaNi 4.5 Ga 0.5 (Figure available in color online.)