Effect of hydrogen treatment on the discharge ... - Springer Link

Materials Science, Vol. 41, No. 6, 2005

EFFECT OF HYDROGEN TREATMENT ON THE DISCHARGE PROPERTIES OF ELECTRODES MADE OF ZrCrNi ALLOY I. I. Bulyk and Yu. B. Basaraba

UDC 546.8:546.112:541.13

For metal-hydride electrodes based on ZrCrNi alloy, we have established an increase in their activation rate (two charge-discharge cycles) and discharge capacitance after homogenization of the Laves phase with a C14-type structure at the expense of treatment of the alloy by the method of partial disproportionation in hydrogen at 873 K and recombination in vacuum at 1223 K. The discharge capacitance decreases after disproportionation at 1083 K and recombination accompanied by the formation of the Laves phase with a C15-type structure. We have also determined the optimal milling conditions for this alloy in a planetary mill in hydrogen: rotational speed 100 rpm and milling time 30 min.

In recent years, nickel-metal-hydride cells (MHC) have been widely used as secondary power sources since they possess substantial advantages over related nickel-cadmium cells, namely, MHC have a higher capacitance, are ecologically reliable, guarantee high charge-discharge currents, and do not manifest the memory effect. American (Ovonic Battery and Duracell) and Japanese (Panasonic, Sanyo, and Toshiba) companies have organized the full-scale production of MHC for electronic devices. Alloys based on compounds of rare-earth metals and zirconium serve as materials for metal-hydride electrodes. For this purpose, alloys of the AB5 type are applied most often (here, A is the mishmetal, i.e., a mixture of light rare-earth metals La, Ce, Pr, and Nd, and B is nickel partially replaced by other metals [1]). Much attention is given to studying the methods of treatment and the properties of electrodes made of zirconium alloys since they have a high discharge capacitance and are cheaper [2, 3]. One of the problems of using zirconium alloys in MHC is connected with their slow activation. The electrodes are manufactured of powder whose particles are covered with oxide films, which impede hydrogen penetration into the intermetallic compound. In the course of activation (charge-discharge of the electrode), surface oxides are reduced and dissolved or become conducting, and, as a result, hydrogen penetrates into the intermetallic particles and takes part in the electrochemical reaction [3]. The activation properties of zirconium alloys can be improved by different methods, in particular, by mechanical treatment in a planetary mill [4 – 6]. As shown in [4], the milling of Zr (Cr0.4 Ni0.6 ) 2 alloy in a planetary mill in argon decreases the number of charge-discharge cycles necessary for its activation to five to eight, but, at the same time, the maximum discharge capacitance falls from the predicted value 400 to 95–140 mA ⋅ h / g, and, after milling of the mixture alloy–nickel powder, the electrode is activated to 195 mA⋅ h / g for two cycles. A decrease in the maximum discharge capacitance in the second specimen, according to [4], is attributable to the penetration of a certain part of zirconium into the surface layer, enriched with nickel, and to the formation of highly stable zirconium hydrides in the course of electrochemical charge. Similar treatment was applied to ZrCrNi alloy [5], and, as a result, the number of activation cycles decreased from 20 for a cast specimen to 1 for a specimen milled in a planetary mill in argon together with nickel powder. However, the capacitance was reduced from 257 to 205 mA⋅ h / g. The number of activation cycles decreased to 5 when the alloy under study was transformed into the amorphous state by means of milling and crystallization in vacuum with Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 6, pp. 49 – 54, November–December, 2005. Original article submitted November 14, 2005. 764

1068–820X/05/4106–0764

© 2005

Springer Science+Business Media, Inc.

E FFECT OF HYDROGEN TREATMENT ON THE DISCHARGE P ROPERTIES OF ELECTRODES MADE

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the formation of crystallites smaller than 100 nm. In this case, the capacitance falls as well (from 257 to 238 mA ⋅ h / g), which is explained by the formation of the Zr9 Ni11 phase [6]. To accelerate the activation of ZrCrNi zirconium alloy, we first proposed the process of hydrogenation, disproportionation, desorption, and recombination (HDDR), which forms a given phase-structural state in the alloy [7]. In addition, we recommend to optimize the conditions of powdering of this alloy in a planetary mill. Experimental Procedure We produced ZrCrNi alloy by fusion of the mixture of the initial components with purity not lower than 99.5 mass % in an electric arc furnace in argon atmosphere. We carried out HDDR using differential thermal analysis in the course of hydrogenation and disproportionation (HD) and measuring the hydrogen pressure in the chamber during desorption and recombination (DR) [8]. The heating rate in hydrogen and vacuum was equal to 5 K / min, the initial hydrogen pressure was 5 MPa, and the temperature was up to 1223 K. We milled hydride of the alloy together with nickel powder of dispersiveness less than 1 μm, for binding, in the proportion 2 : 1 (mass %) in a planetary mill. Some electrodes were produced by means of separate milling and mixing with nickel powder. The rotational speed of the mill was 100 to 400 rpm, and the milling time was 15 to 30 min. We manufactured the electrodes by pressing the mixture of alloy and nickel powders in a nickel gauze under a pressure of up to 700 MPa. Six-molar aqueous KOH solution was used as an electrolyte, and NiOOH served as an auxiliary electrode. The electrodes were charged with a current of 0.1C for 15 h (here, C is the specific discharge capacitance of the material of metal-hydride electrodes, i.e., the discharge capacitance of a unit mass) and discharged with a current of 0.2C up to reaching a voltage of 0.9 V between the metal-hydride and nickel electrodes. The pause between charge and discharge and between the cycles was 30 min. We carried out X-ray phase analysis of the materials under study by taking powder diffractograms on a HZG-4A diffractometer (CuKα-radiation). The phases and lattice constants were determined with the use of the CSD program package [9]. The electrochemical properties of the materials were investigated on a charge-discharge facility, developed and manufactured at the Physicomechanical Institute [10], by taking the charge-discharge curves and calculating the discharge capacitance. Experimental Results We have established the dependence of discharge capacitance of the electrodes on the method of their powdering and phase-structural state formed by the hydrogen treatment of the material. Properties of the Electrodes after Milling of the Alloy in a Planetary Mill in Hydrogen under Different Conditions Mode No. 1. The time of milling of the alloy was equal to 15 min (Table 1), and the alloy–nickel mixture was prepared by mixing the powders in a planetary mill for 10 min with ν = 200 rpm. The electrode capacitance grows slowly with increase in the number of charge-discharge cycles and reaches 172 mA⋅ h / g after 36 cycles (Fig. 1). The diffractogram of this mixture is shown in Fig. 2a. Mode No. 2. The electrode was manufactured of the alloy–nickel mixture after milling of the alloy for 20 min with ν = 300 rpm together with nickel powder. Charge activation was performed at 353 K [11]. The maximum capacitance 240 mA ⋅ h / g was reached already in the first cycle (Fig. 1).

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Fig. 1. Dependence of the capacitance of metal-hydride electrodes made of ZrCrNi alloy on the rotational speed of the ball mill ( ν, rpm), treatment time ( τ, min), and the number of charge-discharge cycles. Conditions of treatment: (1) ν = 400, τ = 15, (2) ν = 300, τ = 20, (3) ν = 100, τ = 30, (4) levigation in a mortar.

Table 1. Conditions of the Powdering of ZrCrNi Alloy in a Planetary Mill and Properties of Electrodes Manufactured of It

Modes of treatment

Milling conditions

Alloy–Ni, mass %

Activation

Idisch , mA / g

Cmax , mA ⋅ h / g

ν, rpm

τ, min

No. 1

400

15

2: 1

36

25

172

No. 2

300

20

2: 1

1

3

25

240

No. 3

100

30

2: 1

4

50

228

No. 4

Levigation in a mortar

—

1: 2

7

50

332

N, cycle

3

No. 5

1

100

30

2: 1

2

50

293

No. 6

2

100

30

2: 1

6

50

246

Comments: 1. HDDR under 5 MPa, HD at 873 K, DR at 1223 K. 2. HDDR under 5 MPa, HD at 1083 K, DR at 1223 K. 3. Charge at 353 K, Ich = 50 mA ⋅ h / g, τ = 8 h [11]; Idisch is the discharge current and C max is the maximum discharge capacitance.

Mode No. 3. The electrode was made of the alloy–nickel mixture after milling of the alloy, loaded to the mill together with nickel powder, for 30 min with ν = 100 rpm. The number of activation cycles was 4, and the maximum capacitance was 228 mA ⋅ h / g. Mode No. 4. The alloy was reduced to powder by levigation in a mortar and mixed with nickel powder (Table 1). The electrode was activated by hot charge (353 K), and the discharge current was equal to 50 mA / g. After 30 cycles, the electrode reached the maximum capacitance 332 mA⋅ h / g. As is seen in Fig. 2b, the reflections of the main phase of the composite obtained under mode No. 4 are similar to those in the diffractogram of the initial specimen.


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767

Table 2. Phase Composition and Crystallographic Characteristics of ZrCrNi Alloy at Different Stages of Manufacture of the Electrodes

Mode of treatment

Lattice parameters, nm Stage of treatment

Milled hydride No. 1

a

c

0.5306(1)

0.8667(4)

C14-type structure

0.5289(2)

0.8664(6)

Ni

0.35266(2)

—

C14-type structure

0.5010(2)

0.8208(5)

Ni

0.35246(4)

—

C14-type structure

0.5003(1)

0.8208(3)

C14-type structure

0.5246(4)

0.860(2)

ε-ZrHx

0.3505(4)

0.450(1)

Cr

0.2897(2)

—

C14-type structure

0.5016(1)

0.8216(2)

0.5012(1)

0.8209(3)

0.35238(3)

—

C14-type structure Cr, traces

Hydride with nickel

No. 4

Phase

After tests

Initial

Zr7 Ni10 , traces Zr9 Ni11 , traces Cr, traces

Disproportionated

No. 5 Recombined

Zr7 Ni10 , traces Zr9 Ni11 , traces Cr, traces

C14-type structure Mixture with nickel

Zr7 Ni10 , traces Cr, traces Ni

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Table 1 (continued)

Mode of treatment

Lattice parameters, nm Stage of treatment

Phase a

c

0.5003(2)

0.8224(4)

0.3509(2)

0.4488(6)

Cr

0.28849(4)

—

C15-type structure

0.7092(2)

—

0.7092(1)

—

0.35239(5)

—

C14-type structure Initial

Zr7 Ni10 , traces Cr, traces

ε-ZrHx Disproportionated

No. 6 Recombined

Zr2 Ni7 , traces

C14-type structure, traces Zr9 Ni11 , traces Cr, traces

C15-type structure Mixture with nickel

C14-type structure, traces ZrNi, traces Ni

Properties of the Electrodes after Thermal Treatment in Hydrogen and Milling in a Planetary Mill in Hydrogen Modes of Treatment No. 5 and 6. We subjected ZrCrNi alloy to thermal treatment, i.e., heated it to 873 and 1083 K in hydrogen ( PH2 = 5 MPa ) and to 1223 K in vacuum. The choice of the heating temperature in hydrogen was based on the data [7], where is shown that, in this case, the alloy disproportionates partially or completely. In hydrogen ( PH2 = 5 MPa) at 873 K, the initial alloy containing the Laves phase with a C14-type structure and Zr7Ni10 , Zr9Ni11 , and Cr admixtures (Fig. 2c) disproportionates partially into zirconium hydride and chromium (Fig. 2d). Based on the phase with a C14-type structure, hydride is formed [7], and, in vacuum, the initial C14-type structure is restored, the reflections of the foreign phases Zr7Ni10 and Zr9Ni11 become more pronounced, and the structure is homogenized (Fig. 2e). After milling of the alloy ( ν = 100 rpm, 30 min ) and its mixing with nickel-binder, we additionally recorded the reflections of nickel in the diffractogram (Fig. 2f). After


OF

ZrCrNi ALLOY

(a)

(b)

(c)

(d)

(e)

(r)

(g)

(h)

(i)

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Fig. 2. Diffractograms of the alloy–nickel mixture obtained under different conditions: (a) ν = 400 rpm, ν = 15 min, (b) levigated in a mortar, ZrCrNi alloy after different stages of treatment: (c) initial alloy, (d) after heating in hydrogen to 873 K, (e) after heating in vacuum to 1223 K, (f) after treatment in hydrogen–vacuum, milling in a planetary mill, and mixing with nickel, (g) after heating in hydrogen to 1083 K, (h) after heating in vacuum to 1223 K, (i) after treatment in hydrogen– vacuum, milling in a planetary mill, and mixing with nickel.

heating in hydrogen to 1083 K, ZrCrNi alloy disproportionates completely (Fig. 2g), and, in vacuum, it recombines with the formation of a mixture of the Laves phases with a C15-type (the dominant phase) and a C14-type structures and chromium and Zr9 Ni11 traces (Fig. 2h). The diffractograms of the recombined alloys show that, after partial disproportionation, the alloy is homogenized, and the completely disproportionated alloy changes the type of structure of the main phase from C14 to C15. The conditions of powdering of the alloys are presented in Table 1, and Fig. 3 illustrates the dependence of discharge capacitance of the electrodes on the number of cycles in the course of activation and on the discharge current. The electrode made of the homogenized alloy with a C14-type structure after the second cycle reaches a capacitance of 260 mA ⋅ h / g, i.e., about 90% of the maximum (293 mA ⋅ h / g). If the electrode alloy is based on the phase with a C15-type structure, it reaches 90% of the maximum capacitance on the eighth cycle, and the maximum capacitance is equal to 256 mA⋅ h / g as the

770

I. I. BULYK

(a)

AND

YU. B. BASARABA

(b)

Fig. 3. Dependence of the capacitance of the electrodes after their treatment in hydrogen on the number of cycles in the course of activation (a) and on the discharge current (b): HD at 873 K (1) and 1083 K (2).

discharge current constitutes 30 mA / g (Fig. 3a). The electrode capacitance falls after treatment according to mode No. 5 if the discharge current exceeds 50 mA / g and, after treatment by mode No. 6, decreases with increase in the current from 25 mA / g (Fig. 3b). Milling in a planetary mill is an efficient method of powdering materials; however, for the manufacture of electrodes, it has certain disadvantages connected with a decrease in the maximum discharge capacitance, which was established earlier [4 – 6] and corroborated by our data. The worsening of discharge properties of the materials is attributable to the stresses appearing after milling. Broadened reflections and halo in the diffractogram of the milled alloy (Fig. 2a) are evidence of this, whereas, after manual levigation of the alloy in a mortar, the stresses are lower, and the electrode made of this alloy has a higher discharge capacitance. On the other hand, decreasing the rotational speed of the mill to ν = 100 rpm and shortening the milling time to 30 min, which is equivalent to the reduction of intensity of the mechanical action on the alloy, we can achieve a higher discharge capacitance than in [5], where the milling time was 180 min. Hydrogen treatment (heating to 873 K in hydrogen and to 1223 K in vacuum) homogenizes ZrCrNi, and the milling of hydride of the alloy in a planetary mill prevents its oxidation. As a result, we obtain a high ability of the electrodes to activation and an increase in their discharge capacitance as compared with the data [5]. The treatment of ZrCrNi alloy at 1083 K in hydrogen and at 1223 K in vacuum transforms the type of structure of the initial phase from C14 to C15. As a result, the discharge capacitance of the electrode falls. CONCLUSIONS By decreasing the rotational speed of the planetary mill and reducing the time of milling of the hydride of ZrCrNi alloy, one can obtain optimal and efficient powdering and high discharge characteristics of electrodes manufactured of it. The homogenization of ZrCrNi alloy by means of partial disproportionation and recombination with subsequent milling of its hydride in a planetary mill improves the activation properties of the electrodes. REFERENCES 1. www.cobasys.com, www.duracell.com. 2. J. Y. Lee and S. R. Kim, Zirconium-Based Hydrogen Storage Alloy Usable for Negative Electrodes for Secondary Battery, US Patent No. 5591394, C22C 016/00, C22C 030/00 (1997).


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3. O. A. Petrii, S. Ya. Vasina, and I. I. Korobov, “Electrochemistry of hydride-forming intermetallic compounds and alloys,” Usp. Khim., No. 3, 195–210 (1996). 4. D. Sun, M. Latroche, and A. Percheron-Guegan, “Activation behaviour of mechanically Ni-coated Zr-based Laves phase hydride electrode,” J. Alloys Compounds, 257, 302–305 (1997). 5. C. B. Jung, J. H. Kim, and K. S. Lee, “Activation behaviour of ZrCrNi mechanically milled with nickel,” J. Alloys Compounds, 267, 265–269 (1998). 6. C. B. Jung and K. S. Lee, “The effect of the treatment on the electrode characteristics of the ball-milled Zr–Cr–Ni,” J. Alloys Compounds, 274, 254–259 (1998). 7. I. I. Bulyk, Yu. B. Basaraba, and A. M. Trostianchyn, “Effect of hydrogen on the phase-structure transformations in ZrCrNi alloy,” J. Alloys Compounds, 376, 95–104 (2004). 8. I. I. Bulyk, R. V. Denys, V. V. Panasyuk, et al., “HDDR process and hydrogen-sorption properties of didymium–aluminum– iron–boron alloy ( Dd12.3 Al1.2 Fe79.4 B6 ),” Fiz.-Khim. Mekh. Mater., 37, No. 4, 15–20 (2001). 9. L. G. Akselrud, Yu. N. Grin, and P. Yu. Zavalii, “CSD–universal program package for single crystal or powder structure data treatment,” in: Collected Abstr. of the 12 th European Crystallographic Meeting [in Russian], Moscow (1989), p. 155. 10. I. I. Bulyk, A. B. Lozinskii, V. V. Koshevoi, et al., “A multichannel automatized complex for investigations of the charge-discharge characteristics of nickel-metal-hydride cells,” Prikl. Radioélektron., 1, No. 2, 231–234 (2002). 11. J. Y. Lee, D.-M. Kim, K.-Y. Lee, et al., Method of Activation Treatment of Ni/MH Secondary Battery by Hot-Charging, US Patent No. 5874824, H02J 7/00 (1999).