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Comparative study on the capacity degradation behavior of Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys
Accepted Manuscript Comparative study on the capacity degradation behavior of Pr5Co19-type singlephase Pr4MgNi19 and La4MgNi19 alloys Yumeng Zhao, Lu Zhang, Yanqiao Ding, Juan Cao, Zeru Jia, Chunping Ma, Yuan Li, Shumin Han PII:
S0925-8388(16)33162-0
DOI:
10.1016/j.jallcom.2016.10.054
Reference:
JALCOM 39221
To appear in:
Journal of Alloys and Compounds
Received Date: 8 August 2016 Revised Date:
30 September 2016
Accepted Date: 6 October 2016
Please cite this article as: Y. Zhao, L. Zhang, Y. Ding, J. Cao, Z. Jia, C. Ma, Y. Li, S. Han, Comparative study on the capacity degradation behavior of Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
Pr4MgNi19
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La4MgNi19
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Comparative study on the capacity degradation behavior of Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys
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Yumeng Zhaoa, b, Lu Zhanga, b, Yanqiao Dinga, b, Juan Caoa, b, Zeru Jiaa, b,Chunping Maa, b, Yuan Lia, b, Shumin Hana, b∗ a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical
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b
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Qinhuangdao 066004, P. R. China
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Engineering, Yanshan University, Qinhuangdao 066004, P. R. China
* Corresponding author: Tel.: +86-335-8074648, Fax: +86-335-8074648. E-mail address: [email protected]. 1
ACCEPTED MANUSCRIPT Abstract Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys are prepared by powder sintering, and the capacity degradation behavior of the two alloys is
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investigated. The Pr4MgNi19 and La4MgNi19 alloy electrodes have similar discharge capacities but exhibit large differences in cycling stability. The capacity loss rate of Pr4MgNi19 alloy electrode is 22.5% at the 200th cycle, while that of La4MgNi19 alloy
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electrode is 43.5%. It is found that the hydroxides on surface of La4MgNi19 alloys
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accumulate disorderly while the hydroxides on surface of Pr4MgNi19 alloys form a compact layer that protects the active material from corrosion. After 200 cycles, the abundance of Pr(OH)3 in Pr4MgNi19 alloy is 8.07 wt.%, which is far less than that of La(OH)3 in La4MgNi19 alloy (24.74 wt.%). Pr4MgNi19 alloys not only have lower
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surface oxidization but also possess higher anti-pulverization ability. The abundance of the Pr5Co19-type phase in Pr4MgNi19 alloy remains 80.88 wt.% after 200 cycles, while that of La4MgNi19 alloy is 32.01 wt.%. The difference between the expansion
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rates of [A2B4] and [AB5] subunits in Pr4MgNi19 alloy is 4.26%, which is smaller than
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that in La4MgNi19 alloy (10.14%). The dense corrosion-resistant protective layer and lower lattice mismatch jointly contribute to the superior cycling stability of Pr4MgNi19 alloy.
Keywords: Metal hydride electrode; Pr5Co19-type single phase; Electrochemical properties; Cycling stability
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1. Introduction
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The development of Ni/MH batteries is facing new challenges and embracing unique opportunities due to the booming development of portable electronic devices and advances in the Hybrid Electric Vehicles (HEV) industry. One critical issue
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required for the advancement of Ni/MH batteries is the search for enhanced negative electrode materials [1–6]. La–Mg–Ni-based alloys are widely studied because of their high discharge capacity of up to 410 mAh g-1 [7]. However, their poor cycling
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stability hampers their use in practical applications [8–10]. It has been reported that
the oxidation and pulverization of alloy particles are the primary reasons for the rapid
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capacity degradation of La–Mg–Ni-based alloys. In addition, the pulverization of La–Mg–Ni-based alloys can be attributed to the inter-molecular strains, which are mainly caused by the difference between the expansion/contraction rates of the [LaNi5] and [LaMgNi4] subunits during the charge/discharge process. It has been observed
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that these strains can be relieved by increasing the [LaNi5]/[LaMgNi4] subunit ratio and thus improving the cycling stability of the alloy electrode [11]. In particular,
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La–Mg–Ni-based alloys contain several phases due to the existence of different superlattice structures. The most common phases in the La–Mg–Ni system are the
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AB3-, A2B7- and A5B19-type phases, which correspond to [LaNi5]/[LaMgNi4] ratios of 1:1, 2:1 and 3:1, respectively. Generally, the A5B19-type La–Mg–Ni-based alloy usually possesses superior cycling stability among the aforementioned superlattice phase alloys [12]. On the other hand, many researchers have found that La–Mg–Ni-based alloys would achieve an improved lifetime by partially replacing La with Pr [13–17]. Zhang
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et al. [15] concluded that the substitution of Pr for La enhanced the cycle stability of
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the La0.75-xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0, 0.1, 0.2, 0.3, 0.4) alloys and the capacity retention rate at the 100th cycle (S100) increased from 73.97 to 93.08% as x increased from 0 to 0.4. Liu et al. [16] also studied the effect of Pr on the cycling stability of
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La–Mg–Ni-based alloys and found that the alloys tended to form the (La,Mg)5Ni19 phase when La was partially substituted by Pr; this could improve the ability of the alloys to resist pulverization.
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To clarify the difference in the effect of Pr and La elements on the structure and electrochemical performance of A5B19-type RE–Mg–Ni-based alloys, we designed
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and prepared Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys by a powder sintering method and studied the phase structures, electrochemical properties and capacity degradation mechanism of the two alloys. 2. Experimental
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Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys were obtained by a step-wise sintering method. The precursors for preparation of the Pr4MgNi19 alloy
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were PrMgNi4 and PrNi5 alloys, while the precursors for preparation of the La4MgNi19 alloy were LaMgNi4 and LaNi5 alloys. All the precursor alloys were
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prepared by induction melting of Pr, Mg, Ni or La elements with a purity of 99.5%. Before the sintering, the samples were pre-processed. The detailed sample pre-processing procedure for the Pr4MgNi19 alloy is as follows: (1) the PrMgNi4 and PrNi5 alloys were ground separately into powders of < 300 mesh and then mixed together with a designated molar ratio of 1.4 (PrMgNi4/PrNi5). (2) The mixtures with a total weight of 2.5 g were cold-pressed into a 10-mm-diameter and 8-mm-thick
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pellet under a pressure of 10 MPa and then the pellet was wrapped in Ni foil. The
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sample pre-processing for La4MgNi19 is similar to that for Pr4MgNi19 alloy. After pre-processing, the sample was sintered. The sintering time (6900 min) and the heat preservation stages while the temperature increased from 873–1073 K were the same
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for both the Pr4MgNi19 and La4MgNi19 alloys. On the other hand, the sintering temperatures were 1223 K and 1173 K for the Pr4MgNi19 and La4MgNi19 alloys,
respectively. The molar ratio of LaMgNi4 and LaNi5 (LaMgNi4/LaNi5) is 0.85 for the
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La4MgNi19 alloy. The chemical compositions of the sintered samples were measured
Supplementary Materials Table S1.
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by an inductively coupled plasma (ICP) system. The results are summarized in
The analyses of the crystallographic properties of the alloy samples were carried out by X-ray diffraction (XRD) on a Rigaku D/Max 2500PC X-ray diffractometer (Cu-Ka radiation). The XRD data were analyzed by the Rietveld method using the
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Rietica program. The phase distribution and compositions of the Pr4MgNi19 and La4MgNi19 alloys were determined by an S-4800 scanning electron microscope (SEM)
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using energy dispersive spectroscopy (EDS). A standard three-electrode system consisting of a MH electrode (the working
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electrode), the sintered NiOOH/Ni(OH)2 electrode (the counter electrode) and a Hg/HgO electrode (the reference electrode) was used for the electrochemical study at 298 K. To prepare the working electrode, the alloy samples were ground into powders of 200–400 meshes and mixed with nickel carbonyl powders in a weight ratio of 1:5 with a total weight of 0.9 g. Then, the mixture was cold-pressed under a pressure of 15 MPa into a 10-mm-diameter and 8-mm-thick pellet, which was later welded to a
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nickel stick.
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Before all the electrochemical tests, the alloy electrodes were first activated by charging at a current density of 60 mA g–1 for 8 h and discharging at the same current density to a cut-off potential of –0.6 V until a maximum discharge capacity was
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reached. The high rate dischargeability (HRD) performance was measured by testing the discharge capacity at various discharge current densities of 300, 600, 900, 1200 and 1500 mA g–1.
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To illustrate the capacity degradation mechanisms of Pr5Co19-type single-phase
Pr4MgNi19 and La4MgNi19 alloys during the charge/discharge process after long-time
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working cycles, alloy electrodes without nickel carbonyl powder were made for testing. The preparation of these special electrodes is similar to that of the normal electrodes except that the special electrodes are only made from 0.4 g of the alloy powder and do not contain nickel carbonyl powder. The alloy powders of 200–400
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meshes with a weight of 0.4 g were cold-pressed into a 10-mm-diameter and 1-mm-thick pellet under a pressure of 15 MPa. The pellet was wrapped with a piece
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of foaming nickel metal mesh and then welded to a nickel stick. Electrode particles of Pr4MgNi19 and La4MgNi19 Pr5Co19-type single-phase alloys after 50, 100 and 200
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charge/discharge cycles were separated from the foaming nickel metal mesh and measured using XRD and SEM. 3. Results and discussion 3.1 Microstructures XRD patterns of the Pr4MgNi19 and La4MgNi19 alloys are shown in Fig. 1. The two XRD patterns are both dominated by the Pr5Co19-type structure. No other
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superlattice structure is observed and all the peaks in the 3–25° regions are the
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characteristic diffraction peaks of the Pr5Co19-type phase [12, 18]. In spite of the high similarity in the XRD patterns, some diffraction peaks of the Pr4MgNi19 alloy shift right compared to the peaks of the La4MgNi19 alloy in the (109) (110), (201), (118),
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(0,0,16), (1,1,16), (219), (300), (2,0,17), (308) and (220) diffraction directions. Rietveld refinements of the XRD patterns are presented in Fig. 2. The lattice
parameters, unit cell volumes, atomic coordinates and occupation numbers of the two
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alloy crystal structures determined by Rietveld refinement are listed in Table 1. The
Lattice parameters and unit cell volume of the Pr5Co19-type structure in the Pr4MgNi19
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alloy are both smaller than those of the Pr5Co19-type structure in the La4MgNi19 alloy. In addition, the Mg occupation ratios in the corresponding Pr and La (4f2) sites are 0.606 and 0.588, respectively.
The SEM images of the Pr4MgNi19 and La4MgNi19 alloys are presented in Fig.
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3(a) and (b). For both alloys, the scanned areas show a uniform color, which indicates that the two alloys consist of a single-phase structure. The compositions in the marked
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areas determined by EDS (shown in Table 2) suggest that area A and B are both A5B19 phase with compositions of Pr4.03Mg0.97Ni19.00 and La3.98Mg1.02Ni19.00, respectively,
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which agree well with the normal compositions of the alloys. 3.2 Electrochemical properties The activation and cycling stability curves of the Pr4MgNi19 and La4MgNi19
alloy electrodes are exhibited in Fig. 4(a) and (b). The high rate dischargeability (HRD) curves of the alloy electrodes are presented in Fig. S1. The values of relevant electrochemical properties are summarized in Table S2. The maximum discharge
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capacities and the HRD1500 (the high rate dischargeability at 1500 mA g–1) of the two
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alloy electrodes are similar; the discharge capacities are 334 mAh g–1 for Pr4MgNi19 and 338 mAh g–1 for La4MgNi19, and the HRD1500 values are 61.5% and 60.4% for the Pr4MgNi19 and La4MgNi19 alloy electrodes, respectively. However, the Pr4MgNi19
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and La4MgNi19 alloy electrodes exhibit very different activation properties and cycling stability.
The La4MgNi19 alloy electrode reaches its maximum discharge capacity at the
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second cycle while the Pr4MgNi19 alloy electrode needs 10 cycles for activation. The large difference in activation may be attributed to the different oxide films on the
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surface of the two alloy particles. During the activation process, the oxide film is removed and the activity of the alloy particles is recovered. It is presumed that the oxide film on the surface of Pr4MgNi19 alloy particles has stronger stability than that of La4MgNi19 alloy particles.
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It is noteworthy to observe that after 100 cycles, the capacity degradation of the Pr4MgNi19 alloy electrode decelerates remarkably compared with that of the
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La4MgNi19 alloy electrode. After the first 100 cycles, the capacity retention rate of the Pr4MgNi19 alloy electrode falls to 86.1%, and after 100 subsequent cycles, the
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capacity retention rate declines by only 8.6%. However, for the La4MgNi19 alloy electrode, the capacity retention rates after the first 100 cycles and 100 subsequent cycles decrease by 22% and 21.5%, respectively, and the decrease in the rates shows no signs of slowing. The capacity degradation mechanisms of Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys were studied to determine the reason for the different behaviors of the Pr4MgNi19 and La4MgNi19 alloy electrodes during
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the charge/discharge cycles.
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3.3 Cycling behaviors and mechanism Fig. 5 presents the SEM images of the Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys, which show the changes of the surface morphology after different
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charge/discharge cycles. It is observed that a few white objects and some cracks appear on the surface of the Pr4MgNi19 and La4MgNi19 alloys after 10 cycles.
However, the white objects on the surface of the two alloys have different shapes.
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Those attached on the surface of the Pr4MgNi19 alloy are flaky, while those on the
surface of the La4MgNi19 alloy are acicular. As the charge/discharge cycles increase,
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the amount of the white objects grows. The white materials are considered as hydroxides that form as a result of the corrosion of the active materials, such as Pr, La and Mg [9]. When the charge/discharge loop increases to 100 cycles, the morphologies of the two alloy particles are increasingly different (Fig. 5 (a3) and (b3)).
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The hydroxides on the surface of the La4MgNi19 alloy accumulate and agglomerate with disorganized states. Contrastingly, the hydroxides on the surface of the
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Pr4MgNi19 alloy are evenly distributed on the alloy particles and form regular flakes; the hydroxides here form a film-like layer on the surface. The contrast becomes
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clearer after 200 cycles. The hydroxide layer of the Pr4MgNi19 alloy grows more compact and homogeneous, which restrains the further corrosion and oxidation of the alloy particles. For the La4MgNi19 alloy, the amount of hydroxides increases significantly, but the hydroxides are still unevenly and randomly distributed. Meanwhile, many cracks form on surface of the La4MgNi19 alloy particles. It is presumed that the compact hydroxide film reduces the capacity degradation rate and
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contributes largely to the remarkable cycling stability of the Pr4MgNi19 alloy [19].
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The XRD patterns of the Pr4MgNi19 and La4MgNi19 alloys before and after 50 and 100 charge/discharge cycles are shown in Fig. 6. Both the alloys retain their original XRD patterns except for a relative decrease in the peak intensity after 50
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charge/discharge cycles. However, the amorphization rate of the La4MgNi19 alloy rises dramatically after subsequent cycles. After 100 cycles, most of the characteristic peaks of the Pr5Co19-type structure in the La4MgNi19 alloy vanish. On the other hand,
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the XRD pattern of the Pr4MgNi19 alloy is still similar to the original peak pattern
oxidation/corrosion [20].
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except for the presence of some small hydroxide peaks, which are products of
Fig. 7 shows the comparison between the XRD patterns of the Pr4MgNi19 and La4MgNi19 alloy powders before and after 200 cycles. The peak patterns of the Pr4MgNi19 alloy powders after 200 cycles exhibit slight changes compared with the
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original peak patterns; relative reductions in the peak intensities are observed. For the La4MgNi19 alloy, the peak patterns after 200 cycles collapse severely and most of the
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characteristic peaks of the Pr5Co19-type structure disappear. This indicates that the La4MgNi19 alloy powders have serious amorphous problems during the
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charge/discharge cycles. Therefore, the hydrogen storage capacity of the collapsed crystal in the La4MgNi19 alloy declines sharply. The full widths at half maxima (FWHM) intensities are also shown in Fig. 7. FWHM is used to characterize the grain size and is calculated from the main diffraction peaks of the alloys. The wider the main peak is, the larger the FWHM value is. According to the Scherrer equation, D=Kλ /(βcos θ), larger FWHM values denote smaller grain sizes. The FWHM of the
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Pr4MgNi19 alloy powder is 0.218, which is much smaller than that of the La4MgNi19
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alloy (0.965). The results show that pulverization and amorphization are more pronounced after 200 cycles in the La4MgNi19 alloy than in the Pr4MgNi19 alloy. As shown in Fig. 6 (c) and (d), the characteristic hydroxide peaks, Pr(OH)3 and
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La(OH)3, appear in the XRD patterns of the Pr4MgNi19 and La4MgNi19 alloys, respectively. The Rietveld refinements of the XRD patterns of the Pr4MgNi19 and
La4MgNi19 alloys after 50, 100 and 200 charge/discharge cycles are presented in Fig.
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S2, and the lattice constants and phase abundances are listed in Table 3. Both the
Pr4MgNi19 and La4MgNi19 alloys gradually decompose during the charge/discharge
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cycles [21]. However, the degree of decomposition of the Pr4MgNi19 alloy is much lower than that of the La4MgNi19 alloy. Noticeably, the (002), (201), (112) and (202) peaks of the CaCu5 phase are observed in the XRD pattern of the La4MgNi19 alloy powder after 50 cycles, while none of the peaks are observed in the XRD pattern of
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the Pr4MgNi19 alloy. This indicates that the Pr5Co19-type phase in the La4MgNi19 alloy decomposes more easily than that in the Pr4MgNi19 alloy. The decrystallization of the
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alloys is closely related with the degree of decomposition. It is presumed that decomposition of the alloys would occur, when the decrystallization resulted from the
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mismatch between [AB5] and [A2B4] subunits in expansion/contraction aggravates to a certain extent. After 200 cycles, the abundance of the Pr5Co19-type phase in the Pr4MgNi19 alloy is 80.88 wt.% while that of the La4MgNi19 alloy is only 32.01 wt.%. The main phase of La4MgNi19 alloy becomes the LaNi5 phase with an amorphous structure. The degree of decomposition of the Pr5Co19-type phase in the Pr4MgNi19 alloy is much lower than that of the Pr5Co19-type phase in the La4MgNi19 alloy. The
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results are in accordance with the decrystallization results. The hydroxides are also
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included in the Rietveld refinements. After 200 cycles, the abundance of Pr(OH)3 in the Pr4MgNi19 alloy is 8.07 wt.%, and the abundance of La(OH)3 in the La4MgNi19 alloy is 24.74 wt.%, as shown in Table 3. The abundances of Mg(OH)2 in the
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Pr4MgNi19 and La4MgNi19 alloys are 5.04 wt.% and 10.17 wt.%, respectively. The results show that more oxidization occurs in the La4MgNi19 alloy.
The above results suggest that the degree of lattice distortion of the
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Pr–Mg–Ni-based Pr5Co19-type phase is lower than that of the La–Mg–Ni-based
Pr5Co19-type phase. It is known that the mismatch between [AB5] and [A2B4] subunits
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in expansion/contraction during the charge/discharge process is the main factor that results in lattice strains and leads to the pulverization of alloys [5]. Fig. 8 (a) and (b) show the subunit volumes of the Pr5Co19-type phase in the Pr4MgNi19 and La4MgNi19 alloys after different charge/discharge cycles and the differences between the
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expansion rates of [A2B4] and [AB5] subunits. Table 4 shows the values and expansion rates of the [A2B4] and [AB5] subunit volumes after different
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charge/discharge cycles. The expansion rates of the subunits are calculated from the difference between the volumes of [AB5] or [A2B4] subunits after charge/discharge
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cycles and the volume of [AB5] or [A2B4] subunits before the cycles. The expansion rate of [AB5] subunits after 50 charge/discharge cycles is equal to the change value (∆V[AB5]) of the [AB5] subunit after 50 charge/discharge cycles divided by the original volume (Vº[AB5]). As the number of cycles increase, the [A2B4] and [AB5] subunit volumes increase for both the Pr4MgNi19 and La4MgNi19 alloys. However, the expansion rates of [A2B4] and [AB5] subunits are different. For both Pr4MgNi19 and
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La4MgNi19 alloys, the expansion rates of the [AB5] subunit are always lower than
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those of the [A2B4] subunit, which results in a mismatch between [A2B4] and [AB5] subunits. For example, the expansion rate of the [AB5] subunit for the Pr4MgNi19 alloy after 100 cycles is 2.67% and that of the [A2B4] subunit is 4.63%. Moreover, the
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degree of mismatch between [A2B4] and [AB5] subunits becomes larger as the loop number increases, and it is different for the Pr4MgNi19 and La4MgNi19 alloys. The differences between the expansion rates of [A2B4] and [AB5] subunits in the
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Pr4MgNi19 alloys are always smaller than those in the La4MgNi19 alloys. After 50
charge/discharge cycles, the differences between the expansion rates of [A2B4] and
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[AB5] subunits in the Pr4MgNi19 and La4MgNi19 alloy are 0.64% and 2.49%, respectively; after 100 cycles, the differences are 1.96% and 4.52%, respectively. Furthermore, the differences increase to 4.26% and 10.14%, respectively, after 200 cycles. The mismatch between [A2B4] and [AB5] subunits results in lattice strains [11,
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22], and the lattice strains would accelerate the speed of amorphization. Therefore, the Pr4MgNi19 alloy retains its XRD patterns better than the La4MgNi19 alloy during the
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charge/discharge process and the Pr5Co19-type single-phase Pr4MgNi19 alloy possesses superior cycle life compared with the Pr5Co19-type single-phase La4MgNi19
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alloy [23, 24].
To further compare the degree of pulverization of the two alloy electrodes, the
general trends in the variation of the mean particle diameters of the two alloy powders are shown in Fig. 8 (c). The mean particle diameters of both alloys decrease sharply during activation in the first 10 cycles, and the reducing trends slow in the following cycles. The mean particle diameters of the two alloys after activation are similar.
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However, the mean particle diameter of the Pr4MgNi19 alloy powders remains
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relatively larger than that of La4MgNi19 alloy powders during charge/discharge cycles. Larger alloy powders are not easily corroded by alkaline electrolytes [16]. This result indicates that the degree of pulverization of the Pr4MgNi19 alloy powder is lower than
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that of La4MgNi19 alloy powder, and it is a primary reason for why the Pr4MgNi19 alloy exhibits excellent cycling stability. 4. Conclusions
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Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys are obtained by a step-wise powder sintering method. Electrochemical measurements show that the
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discharge capacities of the two alloy electrodes are always the same, while their cycling stabilities are relatively different. The maximum discharge capacities of the Pr4MgNi19 and La4MgNi19 alloy electrodes are 334 mAh g–1 and 338 mAh g–1, respectively. At the 200th cycle, the capacity retention rates of the Pr4MgNi19 and
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La4MgNi19 alloy electrodes are 77.5% and 56.5%, respectively. From SEM analysis, we observe that the hydroxides assembled on the surface of the Pr4MgNi19 alloy
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particles show a regular and compact structure, while those assembled on the surface of the La4MgNi19 alloy particles accumulate in a disordered state. Rietveld
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refinements show that after 200 charge/discharge cycles, the abundance of Pr(OH)3 in the Pr4MgNi19 alloy is 8.07 wt.% and that of La(OH)3 in the La4MgNi19 alloy is 24.74 wt.%. In addition to the lower surface oxidization, Pr4MgNi19 alloys also possess higher anti-pulverization ability. The abundance of the Pr5Co19-type phase in the Pr4MgNi19 alloy after 200 cycles remains at 80.88 wt.%, while that of the La4MgNi19 alloy is only 32.01 wt.%. Meanwhile, the differences between the expansion rates of
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[A2B4] and [AB5] subunits of the Pr4MgNi19 and La4MgNi19 alloys are 4.26% and
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10.14%, respectively.
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ACCEPTED MANUSCRIPT Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NOs. 51571173 and 21303157), and the Natural Science Foundation of
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Hebei Province (NOs. B2014203114).
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La–Mg–Ni-based hydrogen storage alloys, J. Power Sources 300 (2015) 77–86. [12] Y.M. Zhao, S.M. Han, Y. Li, J.J. Liu, L. Zhang, S.Q. Yang, D.D. Ke, Characterization and improvement of electrochemical properties of Pr5Co19-type
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single-phase La0.84Mg0.16Ni3.80 alloy, Electrochim. Acta 152 (2015) 265–273.
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[13] T.T. Zhai, T. Yang, Z.M. Yuan, S. Xu, W.G. Bu, Y. Qi, Y.H. Zhang, Influences of hydrogen-induced amorphization and annealing treatment on gaseous hydrogen storage properties of La1-xPrxMgNi3.6Co0.4 (x = 0 – 0.4) alloys, J. Alloy. Compd. 639 (2015) 15–20.
[14] H.G. Pan, S. Shuai, J. Shen, J.J. Tan, J.L. Deng, M.X. Gao, Effect of the substitution of PR for LA on the microstructure and electrochemical properties of La0.7−xPrxMg0.3Ni2.45Co0.75Mn0.1Al0.2 (x = 0.0 – 0.3) hydrogen storage electrode alloys, 18
ACCEPTED MANUSCRIPT Int. J. Hydrogen Energy 32 (2007) 2949–2956. [15] Y.H. Zhang, H.P. Ren, B.W. Li, S.H. Guo, Q.C. Wang, X.L. Wang, Structures and electrochemical hydrogen storage behaviours of La0.75-xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0
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– 0.4) alloys prepared by melt spinning, Int. J. Hydrogen Energy 34 (2009) 6335–6342.
[16] J.J. Liu, S.M. Han, Y. Li, S.Q. Yang, X.C. Chen, C. Wu, C.P. Ma, Effect of Pr on
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phase structure and cycling stability of La–Mg–Ni-based alloys with A2B7- and
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A5B19-type superlattice structures, Electrochim. Acta 184 (2015) 257–263. [17] E.P. Banczek, L.M.C. Zarpelon, R.N. Faria, I. Costa, Corrosion resistance and microstructure characterization of rare-earth-transition metal–aluminum–magnesium alloys, J. Alloy. Compd. 479 (2009) 342–347.
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[18] Q.A. Zhang, M.H. Fang, T.Z. Si, F. Fang, D.L. Sun, L.Z. Ouyang, M. Zhu, Phase stability, structural transition, and hydrogen absorption-desorption features of the polymorphic La4MgNi19 compound, J. Phys. Chem. C 114 (2010) 11686–11692.
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[19] J.J. Liu, S.M. Han, Y. Li, S.Q. Yang, L. Zhang, Y.M. Zhao, Effect of Al
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incorporation on the degradation in discharge capacity and electrochemical kinetics of La–Mg–Ni-based alloys with A2B7-type super-stacking structure, J. Alloy. Compd. 619 (2015) 778–787.
[20] L. Zhang, Y. Li, X. Zhao, J.J. Liu, D.D. Ke, W.K. Du, S.Q. Yang, S.M. Han, Phase transformation and cycling characteristic of Ce2Ni7-type single-phase La0.78Mg0.22Ni3.45 metal hydride alloy. J. Mater. Chem. A 3 (2015) 13679–13690. [21] R.F. Li, P.Z. Xu, Y.M. Zhao, J. Wan, X.F. Liu, R.H. Yu, The microstructures and 19
ACCEPTED MANUSCRIPT electrochemical performances of La0.6Gd0.2Mg0.2Ni3.0Co0.5-xAlx (x = 0 – 0.5) hydrogen storage alloys as negative electrodes for nickel/metal hydride secondary batteries, J. Power Sources 270 (2014) 21–27.
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[22] W.K. Du, L. Zhang, Y. Li, Y.M. Zhao, S.Q. Yang, B.Z. Liu, S.M. Han, Phase Structure, Electrochemical Properties and Cyclic Characteristic of a
Rhombohedral-Type Single-Phase Nd2MgNi9 Hydrogen Storage Alloy, J.
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Electrochem Soc. 163 (2016) A1–A10.
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[23] T.T. Zhai, T. Yang, Z.M. Yuan, S. Xu, W.G. Bu, Y. Qi, Y.H. Zhang, Influences of hydrogen-induced amorphization and annealing treatment on gaseous hydrogen storage properties of La1-xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys, J. Alloy. Compd. 639 (2015) 15–20.
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[24] K. Young, T. Ouchi, B. Huang, Effects of various annealing conditions on (Nd,
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Mg, Zr)(Ni, Al, Co)3.74 metal hydride alloys, J. Power Sources 248 (2014) 147–153.
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ACCEPTED MANUSCRIPT Table 1 – Structural parameters of the Pr4MgNi19 and La4MgNi19 alloys. Sites
Pr1 2c Pr2 4f Pr3 4f Mg 4f Ni1 2a Ni2 2b Ni3 2d Ni4 4e Ni5 4f Ni6 12k Ni7 12k a = 0.4986 nm; c = 3.2120 nm; V/f.u. = 0.6915 nm3; La1 2c La4MgNi19 Pr5Co19-type La2 4f La3 4f Mg 4f Ni1 2a Ni2 2b Ni3 2d Ni4 4e Ni5 4f Ni6 12k Ni7 12k a = 0.5032 nm; c = 3.2223 nm; V/f.u. = 0.7066 nm3;
y
z
Occupancy
1/3 1/3 1/3 1/3 0 0 1/3 0 1/3 0.8350 0.8350
2/3 2/3 2/3 2/3 0 0 2/3 0 2/3 2x 2x
1/4 0.12790 0.01940 0.01940 0 1/4 3/4 0.12500 0.87390 0.06200 0.18930
1 1 0.394 0.606 1 1 1 1 1 1 1
1/4 0.12951 0.01687 0.01687 0 1/4 3/4 0.12759 0.87576 0.06549 0.18998
1 1 0.412 0.588 1 1 1 1 1 1 1
1/3 1/3 1/3 1/3 0 0 1/3 0 1/3 0.8350 0.8350
2/3 2/3 2/3 2/3 0 0 2/3 0 2/3 2x 2x
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Pr4MgNi19 Pr5Co19-type
x
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Atoms
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Phases
ACCEPTED MANUSCRIPT Table 2 – Chemical composition of the Pr4MgNi19 and La4MgNi19 alloys obtained by EDS. Pr(at.%)
La(at.%)
Mg(at.%)
Ni(at.%)
Chemical composition
A
16.80
---
4.00
79.20
Pr4.03Mg0.97Ni19.00
B
---
16.58
4.05
79.37
La3.98Mg1.02Ni19.00
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ACCEPTED MANUSCRIPT Table 3 – Lattice constants and phase abundances of the cycled alloys from Rietveld refinement.
Cycle number
Str. type
a (nm)
c (nm)
V (nm3)
Mass fraction (wt.%)
Pr4MgNi19
50
Pr5Co19 Pr(OH)3
0.4986 0.6457
3.2282 0.3784
0.6932 0.1366
97.77 2.23
100
Pr5Co19 CaCu5 Pr(OH)3 Pr5Co19 CaCu5 Pr(OH)3 Mg(OH)2 Pr5Co19 CaCu5 La(OH)3 Pr5Co19 CaCu5 La(OH)3 Pr5Co19 CaCu5 La(OH)3 Mg(OH)2
0.4987 0.4982 0.6445 0.4992 0.4991 0.6438 0.3142 0.5029 0.5014 0.6577 0.5047 0.4961 0.6541 0.5068 0.4984 0.6559 0.3340
3.2368 0.4028 0.3773 3.4449 0.4027 0.3773 0.4766 3.2291 0.3976 0.3552 3.2760 0.4064 0.3851 3.2771 0.4093 0.3869 0.4562
0.6971 0.0866 0.1357 0.7068 0.0869 0.1354 0.0407 0.7071 0.0866 0.1331 0.7103 0.0866 0.1427 0.7198 0.0881 0.1441 0.0441
88.66 5.19 6.15 80.88 6.01 8.07 5.04 86.25 10.81 2.94 59.85 19.39 20.76 32.01 33.08 24.74 10.17
50
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La4MgNi19
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100
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ACCEPTED MANUSCRIPT Table 4 – Comparison of subunit volume of the Pr5Co19-type phase in Pr4MgNi19 and La4MgNi19 alloys 3
0
Vº[AB5] (nm ) Vº[A2B4] (nm3) V[AB5] (nm3) ∆V[AB5]/Vº[AB5] (%) V[A2B4] (nm3) ∆V[A2B4]/Vº[A2B4] (%) V[AB5] (nm3) ∆V[AB5]/Vº[AB5] (%) V[A2B4] (nm3) ∆V[A2B4]/Vº[A2B4] (%) V[AB5] (nm3) ∆V[AB5]/Vº[AB5] (%) V[A2B4] (nm3) ∆V[A2B4]/Vº[A2B4] (%)
50
100
La4MgNi19
83.95 85.75 84.69 0.88% 87.05 1.52% 86.19 2.67% 89.72 4.63% 91.19 8.62% 96.79 12.88%
85.78 87.62 87.50 2.00% 91.55 4.49% 90.49 5.49% 96.39 10.01% 97.29 13.42% 108.26 23.56%
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Pr4MgNi19
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Subunit volume
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Charge/discharge cycles
ACCEPTED MANUSCRIPT Figure captions Fig. 1 X-ray diffraction patterns of the alloys in different angle region: (a) range of 10–80°; (b) range of 3–25°.
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Fig. 2 Rietveld refinements of the XRD patterns for the alloys: (a) Pr4MgNi19 alloy; (b) La4MgNi19 alloy.
Fig. 3 SEM images of the alloys: (a) Pr4MgNi19 alloy; (b) La4MgNi19 alloy.
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Fig. 4 Discharge capacity vs. cycle number curves of the alloy electrodes: (a) activation
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curves; (b) cycling stability curves
Fig. 5 SEM images of the alloys: (a1)-(a4) Pr4MgNi19 alloys after 10, 50, 100 and 200 charge/discharge cycles; (b1)-(b4) La4MgNi19 alloys after 10, 50, 100 and 200 charge/discharge cycles.
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Fig. 6 XRD patterns of the alloys: (a) the original Pr4MgNi19 alloy and alloys after 50 and 100 charge/discharge cycles; (b) the original La4MgNi19 alloy and alloys after 50 and 100
range of 10–60°.
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charge/discharge cycles; (c) 50 cycles in the 2θ range of 28–80°; (d) 100 cycles in the 2θ
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Fig. 7 Comparison of the XRD patterns between the Pr4MgNi19 and La4MgNi19 alloy powders before and after 200 cycles. Fig. 8 (a) subunit volumes of the Pr5Co19-type phase in Pr4MgNi19 and La4MgNi19 alloys after different charge/discharge cycles; (b) differences between expansion rates of [A2B4] and [AB5]; (c) mean particle diameters of the Pr4MgNi19 and La4MgNi19 alloy powders; (d) phase abundances of Pr5Co19-type phase and hydroxides in Pr4MgNi19 and La4MgNi19 alloys after different charge/discharge cycles.
La4MgNi19
20
30
40
50
60
2θ/ degree
[2 2 0]
[3 0 8] 70
80
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10
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[3 0 0]
Pr4MgNi19
Pr5Co19
[0 0 16] [2 1 9] [2 0 17]
[1 0 8] [1 0 9]
Intensity/a.u.
[1 1 0]
[0 0 16]
(a)
[2 0 1] [1 1 8]
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Pr5Co19
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Intensity/a.u.
(b)
Pr4MgNi19
La4MgNi19
0
4
8
12
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2θ/ degree
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Fig. 1
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20
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Pr4MgNi19
(a)
Rp = 4.5
La4MgNi19
Rp = 5.3
(b)
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S = 1.50
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S = 1.84
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Fig. 2
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(a)
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A
(b)
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B
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Fig. 3
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340
(a)
300
Pr4MgNi19
280
La 4MgNi19
260 240 220 200 180 0
3
6
9
12
Cycle number 110
90
Pr4MgNi19
80 70
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Capacity retention/%
15
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(b)
100
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Discharge capacity/mAh g
-1
320
La4MgNi19
60 50 40 30 0
40
80
120
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Cycle number
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Fig. 4
160
200
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(b1)
(a2)
(b2)
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(a1)
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(b3)
(a4)
(b4)
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(a3)
Fig. 5
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(b) La4MgNi19
before cycling after 50 cycles
after 100 cycles
20
after 100 cycles
30
40
50
70
80
Intensity/a.u.
La4MgNi19
40
50
2θ/degree
60
70
80
Fig. 6
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30
40
50
2θ/degree
60
70
80
Pr(OH)3 La(OH)3 Mg(OH)2 LaNi5
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30
20
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Pr4MgNi19
10
(d)
Pr(OH)3 La(OH)3 LaNi5
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Intensity/a.u.
60
2θ/degree
(c)
after 50 cycles
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10
before cycling
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Intensity/a.u.
Intensity/a.u.
(a) Pr4MgNi19
20
30
40
2θ/degree
Pr4MgNi19
La4MgNi19
50
60
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Pr4MgNi19
10000
before cycling after 200 cycles
6000 4000 2000
40
42
44
46
48
5000
La4MgNi19
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Intensity/a.u.
0 38
FWHM = 0.218
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Intensity/Counts
8000
4000 Intensity/Counts
before cycling
3000
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after 200 cycles
FWHM = 0.965
2000 1000
0 38
20
30
40
50
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2θ/degree
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Fig. 7
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10
60
40
70
42
44
80
46
48
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(a)
3
V(AB5) ---Pr4MgNi19
-3
105
V(A2B4) ---La4MgNi19
100
mismatch in Pr4MgNi19 mismatch in La4MgNi19
8
V(AB5) ---La4MgNi19
☒
95
4
☒
90
85
0
80 60
50
100
150
Cycle number
0
50
100
150
200
150
200
Cycle number
(c)
100
(d)
Pr4MgNi19
50
La4MgNi19
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Phase abundance/%
40
30
20
10
60
Pr(OH)3 ---Pr4MgNi19 La(OH)3---La4MgNi19
40
20
Pr5Co19
---Pr4MgNi19
Pr5Co19
---La4MgNi19
50
100
150
200
Cycle number
Fig. 8
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0
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0
0
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Mean particle size/µm
200
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0
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Subunit volume/10 nm
(b)
V(A2B4)---Pr4MgNi19
V[AB5]- V[A2B4]
110
0
50
100
Cycle number
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Highlights > Single-phase Pr5Co19-type Pr4MgNi19 and La4MgNi19 alloys are obtained > Decomposition processes of Pr5Co19-type phase are observed
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> Volume expansibility of [A2B4] and [AB5] subunits in two alloys show large
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difference
S0925-8388(16)33162-0
DOI:
10.1016/j.jallcom.2016.10.054
Reference:
JALCOM 39221
To appear in:
Journal of Alloys and Compounds
Received Date: 8 August 2016 Revised Date:
30 September 2016
Accepted Date: 6 October 2016
Please cite this article as: Y. Zhao, L. Zhang, Y. Ding, J. Cao, Z. Jia, C. Ma, Y. Li, S. Han, Comparative study on the capacity degradation behavior of Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
Pr4MgNi19
A 2B 4
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Intensity/a.u.
A 2B 4
AB5
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AB5
La4MgNi19
AB5
AB5
10
20
30
40
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A 2B 4
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Comparative study on the capacity degradation behavior of Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys
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Yumeng Zhaoa, b, Lu Zhanga, b, Yanqiao Dinga, b, Juan Caoa, b, Zeru Jiaa, b,Chunping Maa, b, Yuan Lia, b, Shumin Hana, b∗ a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical
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b
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Qinhuangdao 066004, P. R. China
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Engineering, Yanshan University, Qinhuangdao 066004, P. R. China
* Corresponding author: Tel.: +86-335-8074648, Fax: +86-335-8074648. E-mail address: [email protected]. 1
ACCEPTED MANUSCRIPT Abstract Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys are prepared by powder sintering, and the capacity degradation behavior of the two alloys is
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investigated. The Pr4MgNi19 and La4MgNi19 alloy electrodes have similar discharge capacities but exhibit large differences in cycling stability. The capacity loss rate of Pr4MgNi19 alloy electrode is 22.5% at the 200th cycle, while that of La4MgNi19 alloy
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electrode is 43.5%. It is found that the hydroxides on surface of La4MgNi19 alloys
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accumulate disorderly while the hydroxides on surface of Pr4MgNi19 alloys form a compact layer that protects the active material from corrosion. After 200 cycles, the abundance of Pr(OH)3 in Pr4MgNi19 alloy is 8.07 wt.%, which is far less than that of La(OH)3 in La4MgNi19 alloy (24.74 wt.%). Pr4MgNi19 alloys not only have lower
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surface oxidization but also possess higher anti-pulverization ability. The abundance of the Pr5Co19-type phase in Pr4MgNi19 alloy remains 80.88 wt.% after 200 cycles, while that of La4MgNi19 alloy is 32.01 wt.%. The difference between the expansion
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rates of [A2B4] and [AB5] subunits in Pr4MgNi19 alloy is 4.26%, which is smaller than
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that in La4MgNi19 alloy (10.14%). The dense corrosion-resistant protective layer and lower lattice mismatch jointly contribute to the superior cycling stability of Pr4MgNi19 alloy.
Keywords: Metal hydride electrode; Pr5Co19-type single phase; Electrochemical properties; Cycling stability
2
1. Introduction
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The development of Ni/MH batteries is facing new challenges and embracing unique opportunities due to the booming development of portable electronic devices and advances in the Hybrid Electric Vehicles (HEV) industry. One critical issue
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required for the advancement of Ni/MH batteries is the search for enhanced negative electrode materials [1–6]. La–Mg–Ni-based alloys are widely studied because of their high discharge capacity of up to 410 mAh g-1 [7]. However, their poor cycling
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stability hampers their use in practical applications [8–10]. It has been reported that
the oxidation and pulverization of alloy particles are the primary reasons for the rapid
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capacity degradation of La–Mg–Ni-based alloys. In addition, the pulverization of La–Mg–Ni-based alloys can be attributed to the inter-molecular strains, which are mainly caused by the difference between the expansion/contraction rates of the [LaNi5] and [LaMgNi4] subunits during the charge/discharge process. It has been observed
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that these strains can be relieved by increasing the [LaNi5]/[LaMgNi4] subunit ratio and thus improving the cycling stability of the alloy electrode [11]. In particular,
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La–Mg–Ni-based alloys contain several phases due to the existence of different superlattice structures. The most common phases in the La–Mg–Ni system are the
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AB3-, A2B7- and A5B19-type phases, which correspond to [LaNi5]/[LaMgNi4] ratios of 1:1, 2:1 and 3:1, respectively. Generally, the A5B19-type La–Mg–Ni-based alloy usually possesses superior cycling stability among the aforementioned superlattice phase alloys [12]. On the other hand, many researchers have found that La–Mg–Ni-based alloys would achieve an improved lifetime by partially replacing La with Pr [13–17]. Zhang
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et al. [15] concluded that the substitution of Pr for La enhanced the cycle stability of
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the La0.75-xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0, 0.1, 0.2, 0.3, 0.4) alloys and the capacity retention rate at the 100th cycle (S100) increased from 73.97 to 93.08% as x increased from 0 to 0.4. Liu et al. [16] also studied the effect of Pr on the cycling stability of
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La–Mg–Ni-based alloys and found that the alloys tended to form the (La,Mg)5Ni19 phase when La was partially substituted by Pr; this could improve the ability of the alloys to resist pulverization.
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To clarify the difference in the effect of Pr and La elements on the structure and electrochemical performance of A5B19-type RE–Mg–Ni-based alloys, we designed
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and prepared Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys by a powder sintering method and studied the phase structures, electrochemical properties and capacity degradation mechanism of the two alloys. 2. Experimental
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Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys were obtained by a step-wise sintering method. The precursors for preparation of the Pr4MgNi19 alloy
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were PrMgNi4 and PrNi5 alloys, while the precursors for preparation of the La4MgNi19 alloy were LaMgNi4 and LaNi5 alloys. All the precursor alloys were
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prepared by induction melting of Pr, Mg, Ni or La elements with a purity of 99.5%. Before the sintering, the samples were pre-processed. The detailed sample pre-processing procedure for the Pr4MgNi19 alloy is as follows: (1) the PrMgNi4 and PrNi5 alloys were ground separately into powders of < 300 mesh and then mixed together with a designated molar ratio of 1.4 (PrMgNi4/PrNi5). (2) The mixtures with a total weight of 2.5 g were cold-pressed into a 10-mm-diameter and 8-mm-thick
4
pellet under a pressure of 10 MPa and then the pellet was wrapped in Ni foil. The
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sample pre-processing for La4MgNi19 is similar to that for Pr4MgNi19 alloy. After pre-processing, the sample was sintered. The sintering time (6900 min) and the heat preservation stages while the temperature increased from 873–1073 K were the same
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for both the Pr4MgNi19 and La4MgNi19 alloys. On the other hand, the sintering temperatures were 1223 K and 1173 K for the Pr4MgNi19 and La4MgNi19 alloys,
respectively. The molar ratio of LaMgNi4 and LaNi5 (LaMgNi4/LaNi5) is 0.85 for the
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La4MgNi19 alloy. The chemical compositions of the sintered samples were measured
Supplementary Materials Table S1.
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by an inductively coupled plasma (ICP) system. The results are summarized in
The analyses of the crystallographic properties of the alloy samples were carried out by X-ray diffraction (XRD) on a Rigaku D/Max 2500PC X-ray diffractometer (Cu-Ka radiation). The XRD data were analyzed by the Rietveld method using the
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Rietica program. The phase distribution and compositions of the Pr4MgNi19 and La4MgNi19 alloys were determined by an S-4800 scanning electron microscope (SEM)
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using energy dispersive spectroscopy (EDS). A standard three-electrode system consisting of a MH electrode (the working
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electrode), the sintered NiOOH/Ni(OH)2 electrode (the counter electrode) and a Hg/HgO electrode (the reference electrode) was used for the electrochemical study at 298 K. To prepare the working electrode, the alloy samples were ground into powders of 200–400 meshes and mixed with nickel carbonyl powders in a weight ratio of 1:5 with a total weight of 0.9 g. Then, the mixture was cold-pressed under a pressure of 15 MPa into a 10-mm-diameter and 8-mm-thick pellet, which was later welded to a
5
nickel stick.
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Before all the electrochemical tests, the alloy electrodes were first activated by charging at a current density of 60 mA g–1 for 8 h and discharging at the same current density to a cut-off potential of –0.6 V until a maximum discharge capacity was
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reached. The high rate dischargeability (HRD) performance was measured by testing the discharge capacity at various discharge current densities of 300, 600, 900, 1200 and 1500 mA g–1.
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To illustrate the capacity degradation mechanisms of Pr5Co19-type single-phase
Pr4MgNi19 and La4MgNi19 alloys during the charge/discharge process after long-time
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working cycles, alloy electrodes without nickel carbonyl powder were made for testing. The preparation of these special electrodes is similar to that of the normal electrodes except that the special electrodes are only made from 0.4 g of the alloy powder and do not contain nickel carbonyl powder. The alloy powders of 200–400
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meshes with a weight of 0.4 g were cold-pressed into a 10-mm-diameter and 1-mm-thick pellet under a pressure of 15 MPa. The pellet was wrapped with a piece
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of foaming nickel metal mesh and then welded to a nickel stick. Electrode particles of Pr4MgNi19 and La4MgNi19 Pr5Co19-type single-phase alloys after 50, 100 and 200
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charge/discharge cycles were separated from the foaming nickel metal mesh and measured using XRD and SEM. 3. Results and discussion 3.1 Microstructures XRD patterns of the Pr4MgNi19 and La4MgNi19 alloys are shown in Fig. 1. The two XRD patterns are both dominated by the Pr5Co19-type structure. No other
6
superlattice structure is observed and all the peaks in the 3–25° regions are the
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characteristic diffraction peaks of the Pr5Co19-type phase [12, 18]. In spite of the high similarity in the XRD patterns, some diffraction peaks of the Pr4MgNi19 alloy shift right compared to the peaks of the La4MgNi19 alloy in the (109) (110), (201), (118),
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(0,0,16), (1,1,16), (219), (300), (2,0,17), (308) and (220) diffraction directions. Rietveld refinements of the XRD patterns are presented in Fig. 2. The lattice
parameters, unit cell volumes, atomic coordinates and occupation numbers of the two
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alloy crystal structures determined by Rietveld refinement are listed in Table 1. The
Lattice parameters and unit cell volume of the Pr5Co19-type structure in the Pr4MgNi19
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alloy are both smaller than those of the Pr5Co19-type structure in the La4MgNi19 alloy. In addition, the Mg occupation ratios in the corresponding Pr and La (4f2) sites are 0.606 and 0.588, respectively.
The SEM images of the Pr4MgNi19 and La4MgNi19 alloys are presented in Fig.
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3(a) and (b). For both alloys, the scanned areas show a uniform color, which indicates that the two alloys consist of a single-phase structure. The compositions in the marked
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areas determined by EDS (shown in Table 2) suggest that area A and B are both A5B19 phase with compositions of Pr4.03Mg0.97Ni19.00 and La3.98Mg1.02Ni19.00, respectively,
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which agree well with the normal compositions of the alloys. 3.2 Electrochemical properties The activation and cycling stability curves of the Pr4MgNi19 and La4MgNi19
alloy electrodes are exhibited in Fig. 4(a) and (b). The high rate dischargeability (HRD) curves of the alloy electrodes are presented in Fig. S1. The values of relevant electrochemical properties are summarized in Table S2. The maximum discharge
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capacities and the HRD1500 (the high rate dischargeability at 1500 mA g–1) of the two
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alloy electrodes are similar; the discharge capacities are 334 mAh g–1 for Pr4MgNi19 and 338 mAh g–1 for La4MgNi19, and the HRD1500 values are 61.5% and 60.4% for the Pr4MgNi19 and La4MgNi19 alloy electrodes, respectively. However, the Pr4MgNi19
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and La4MgNi19 alloy electrodes exhibit very different activation properties and cycling stability.
The La4MgNi19 alloy electrode reaches its maximum discharge capacity at the
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second cycle while the Pr4MgNi19 alloy electrode needs 10 cycles for activation. The large difference in activation may be attributed to the different oxide films on the
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surface of the two alloy particles. During the activation process, the oxide film is removed and the activity of the alloy particles is recovered. It is presumed that the oxide film on the surface of Pr4MgNi19 alloy particles has stronger stability than that of La4MgNi19 alloy particles.
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It is noteworthy to observe that after 100 cycles, the capacity degradation of the Pr4MgNi19 alloy electrode decelerates remarkably compared with that of the
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La4MgNi19 alloy electrode. After the first 100 cycles, the capacity retention rate of the Pr4MgNi19 alloy electrode falls to 86.1%, and after 100 subsequent cycles, the
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capacity retention rate declines by only 8.6%. However, for the La4MgNi19 alloy electrode, the capacity retention rates after the first 100 cycles and 100 subsequent cycles decrease by 22% and 21.5%, respectively, and the decrease in the rates shows no signs of slowing. The capacity degradation mechanisms of Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys were studied to determine the reason for the different behaviors of the Pr4MgNi19 and La4MgNi19 alloy electrodes during
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the charge/discharge cycles.
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3.3 Cycling behaviors and mechanism Fig. 5 presents the SEM images of the Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys, which show the changes of the surface morphology after different
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charge/discharge cycles. It is observed that a few white objects and some cracks appear on the surface of the Pr4MgNi19 and La4MgNi19 alloys after 10 cycles.
However, the white objects on the surface of the two alloys have different shapes.
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Those attached on the surface of the Pr4MgNi19 alloy are flaky, while those on the
surface of the La4MgNi19 alloy are acicular. As the charge/discharge cycles increase,
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the amount of the white objects grows. The white materials are considered as hydroxides that form as a result of the corrosion of the active materials, such as Pr, La and Mg [9]. When the charge/discharge loop increases to 100 cycles, the morphologies of the two alloy particles are increasingly different (Fig. 5 (a3) and (b3)).
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The hydroxides on the surface of the La4MgNi19 alloy accumulate and agglomerate with disorganized states. Contrastingly, the hydroxides on the surface of the
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Pr4MgNi19 alloy are evenly distributed on the alloy particles and form regular flakes; the hydroxides here form a film-like layer on the surface. The contrast becomes
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clearer after 200 cycles. The hydroxide layer of the Pr4MgNi19 alloy grows more compact and homogeneous, which restrains the further corrosion and oxidation of the alloy particles. For the La4MgNi19 alloy, the amount of hydroxides increases significantly, but the hydroxides are still unevenly and randomly distributed. Meanwhile, many cracks form on surface of the La4MgNi19 alloy particles. It is presumed that the compact hydroxide film reduces the capacity degradation rate and
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contributes largely to the remarkable cycling stability of the Pr4MgNi19 alloy [19].
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The XRD patterns of the Pr4MgNi19 and La4MgNi19 alloys before and after 50 and 100 charge/discharge cycles are shown in Fig. 6. Both the alloys retain their original XRD patterns except for a relative decrease in the peak intensity after 50
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charge/discharge cycles. However, the amorphization rate of the La4MgNi19 alloy rises dramatically after subsequent cycles. After 100 cycles, most of the characteristic peaks of the Pr5Co19-type structure in the La4MgNi19 alloy vanish. On the other hand,
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the XRD pattern of the Pr4MgNi19 alloy is still similar to the original peak pattern
oxidation/corrosion [20].
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except for the presence of some small hydroxide peaks, which are products of
Fig. 7 shows the comparison between the XRD patterns of the Pr4MgNi19 and La4MgNi19 alloy powders before and after 200 cycles. The peak patterns of the Pr4MgNi19 alloy powders after 200 cycles exhibit slight changes compared with the
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original peak patterns; relative reductions in the peak intensities are observed. For the La4MgNi19 alloy, the peak patterns after 200 cycles collapse severely and most of the
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characteristic peaks of the Pr5Co19-type structure disappear. This indicates that the La4MgNi19 alloy powders have serious amorphous problems during the
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charge/discharge cycles. Therefore, the hydrogen storage capacity of the collapsed crystal in the La4MgNi19 alloy declines sharply. The full widths at half maxima (FWHM) intensities are also shown in Fig. 7. FWHM is used to characterize the grain size and is calculated from the main diffraction peaks of the alloys. The wider the main peak is, the larger the FWHM value is. According to the Scherrer equation, D=Kλ /(βcos θ), larger FWHM values denote smaller grain sizes. The FWHM of the
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Pr4MgNi19 alloy powder is 0.218, which is much smaller than that of the La4MgNi19
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alloy (0.965). The results show that pulverization and amorphization are more pronounced after 200 cycles in the La4MgNi19 alloy than in the Pr4MgNi19 alloy. As shown in Fig. 6 (c) and (d), the characteristic hydroxide peaks, Pr(OH)3 and
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La(OH)3, appear in the XRD patterns of the Pr4MgNi19 and La4MgNi19 alloys, respectively. The Rietveld refinements of the XRD patterns of the Pr4MgNi19 and
La4MgNi19 alloys after 50, 100 and 200 charge/discharge cycles are presented in Fig.
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S2, and the lattice constants and phase abundances are listed in Table 3. Both the
Pr4MgNi19 and La4MgNi19 alloys gradually decompose during the charge/discharge
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cycles [21]. However, the degree of decomposition of the Pr4MgNi19 alloy is much lower than that of the La4MgNi19 alloy. Noticeably, the (002), (201), (112) and (202) peaks of the CaCu5 phase are observed in the XRD pattern of the La4MgNi19 alloy powder after 50 cycles, while none of the peaks are observed in the XRD pattern of
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the Pr4MgNi19 alloy. This indicates that the Pr5Co19-type phase in the La4MgNi19 alloy decomposes more easily than that in the Pr4MgNi19 alloy. The decrystallization of the
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alloys is closely related with the degree of decomposition. It is presumed that decomposition of the alloys would occur, when the decrystallization resulted from the
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mismatch between [AB5] and [A2B4] subunits in expansion/contraction aggravates to a certain extent. After 200 cycles, the abundance of the Pr5Co19-type phase in the Pr4MgNi19 alloy is 80.88 wt.% while that of the La4MgNi19 alloy is only 32.01 wt.%. The main phase of La4MgNi19 alloy becomes the LaNi5 phase with an amorphous structure. The degree of decomposition of the Pr5Co19-type phase in the Pr4MgNi19 alloy is much lower than that of the Pr5Co19-type phase in the La4MgNi19 alloy. The
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results are in accordance with the decrystallization results. The hydroxides are also
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included in the Rietveld refinements. After 200 cycles, the abundance of Pr(OH)3 in the Pr4MgNi19 alloy is 8.07 wt.%, and the abundance of La(OH)3 in the La4MgNi19 alloy is 24.74 wt.%, as shown in Table 3. The abundances of Mg(OH)2 in the
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Pr4MgNi19 and La4MgNi19 alloys are 5.04 wt.% and 10.17 wt.%, respectively. The results show that more oxidization occurs in the La4MgNi19 alloy.
The above results suggest that the degree of lattice distortion of the
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Pr–Mg–Ni-based Pr5Co19-type phase is lower than that of the La–Mg–Ni-based
Pr5Co19-type phase. It is known that the mismatch between [AB5] and [A2B4] subunits
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in expansion/contraction during the charge/discharge process is the main factor that results in lattice strains and leads to the pulverization of alloys [5]. Fig. 8 (a) and (b) show the subunit volumes of the Pr5Co19-type phase in the Pr4MgNi19 and La4MgNi19 alloys after different charge/discharge cycles and the differences between the
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expansion rates of [A2B4] and [AB5] subunits. Table 4 shows the values and expansion rates of the [A2B4] and [AB5] subunit volumes after different
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charge/discharge cycles. The expansion rates of the subunits are calculated from the difference between the volumes of [AB5] or [A2B4] subunits after charge/discharge
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cycles and the volume of [AB5] or [A2B4] subunits before the cycles. The expansion rate of [AB5] subunits after 50 charge/discharge cycles is equal to the change value (∆V[AB5]) of the [AB5] subunit after 50 charge/discharge cycles divided by the original volume (Vº[AB5]). As the number of cycles increase, the [A2B4] and [AB5] subunit volumes increase for both the Pr4MgNi19 and La4MgNi19 alloys. However, the expansion rates of [A2B4] and [AB5] subunits are different. For both Pr4MgNi19 and
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La4MgNi19 alloys, the expansion rates of the [AB5] subunit are always lower than
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those of the [A2B4] subunit, which results in a mismatch between [A2B4] and [AB5] subunits. For example, the expansion rate of the [AB5] subunit for the Pr4MgNi19 alloy after 100 cycles is 2.67% and that of the [A2B4] subunit is 4.63%. Moreover, the
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degree of mismatch between [A2B4] and [AB5] subunits becomes larger as the loop number increases, and it is different for the Pr4MgNi19 and La4MgNi19 alloys. The differences between the expansion rates of [A2B4] and [AB5] subunits in the
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Pr4MgNi19 alloys are always smaller than those in the La4MgNi19 alloys. After 50
charge/discharge cycles, the differences between the expansion rates of [A2B4] and
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[AB5] subunits in the Pr4MgNi19 and La4MgNi19 alloy are 0.64% and 2.49%, respectively; after 100 cycles, the differences are 1.96% and 4.52%, respectively. Furthermore, the differences increase to 4.26% and 10.14%, respectively, after 200 cycles. The mismatch between [A2B4] and [AB5] subunits results in lattice strains [11,
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22], and the lattice strains would accelerate the speed of amorphization. Therefore, the Pr4MgNi19 alloy retains its XRD patterns better than the La4MgNi19 alloy during the
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charge/discharge process and the Pr5Co19-type single-phase Pr4MgNi19 alloy possesses superior cycle life compared with the Pr5Co19-type single-phase La4MgNi19
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alloy [23, 24].
To further compare the degree of pulverization of the two alloy electrodes, the
general trends in the variation of the mean particle diameters of the two alloy powders are shown in Fig. 8 (c). The mean particle diameters of both alloys decrease sharply during activation in the first 10 cycles, and the reducing trends slow in the following cycles. The mean particle diameters of the two alloys after activation are similar.
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However, the mean particle diameter of the Pr4MgNi19 alloy powders remains
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relatively larger than that of La4MgNi19 alloy powders during charge/discharge cycles. Larger alloy powders are not easily corroded by alkaline electrolytes [16]. This result indicates that the degree of pulverization of the Pr4MgNi19 alloy powder is lower than
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that of La4MgNi19 alloy powder, and it is a primary reason for why the Pr4MgNi19 alloy exhibits excellent cycling stability. 4. Conclusions
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Pr5Co19-type single-phase Pr4MgNi19 and La4MgNi19 alloys are obtained by a step-wise powder sintering method. Electrochemical measurements show that the
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discharge capacities of the two alloy electrodes are always the same, while their cycling stabilities are relatively different. The maximum discharge capacities of the Pr4MgNi19 and La4MgNi19 alloy electrodes are 334 mAh g–1 and 338 mAh g–1, respectively. At the 200th cycle, the capacity retention rates of the Pr4MgNi19 and
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La4MgNi19 alloy electrodes are 77.5% and 56.5%, respectively. From SEM analysis, we observe that the hydroxides assembled on the surface of the Pr4MgNi19 alloy
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particles show a regular and compact structure, while those assembled on the surface of the La4MgNi19 alloy particles accumulate in a disordered state. Rietveld
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refinements show that after 200 charge/discharge cycles, the abundance of Pr(OH)3 in the Pr4MgNi19 alloy is 8.07 wt.% and that of La(OH)3 in the La4MgNi19 alloy is 24.74 wt.%. In addition to the lower surface oxidization, Pr4MgNi19 alloys also possess higher anti-pulverization ability. The abundance of the Pr5Co19-type phase in the Pr4MgNi19 alloy after 200 cycles remains at 80.88 wt.%, while that of the La4MgNi19 alloy is only 32.01 wt.%. Meanwhile, the differences between the expansion rates of
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[A2B4] and [AB5] subunits of the Pr4MgNi19 and La4MgNi19 alloys are 4.26% and
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10.14%, respectively.
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ACCEPTED MANUSCRIPT Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NOs. 51571173 and 21303157), and the Natural Science Foundation of
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Hebei Province (NOs. B2014203114).
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ACCEPTED MANUSCRIPT electrochemical performances of La0.6Gd0.2Mg0.2Ni3.0Co0.5-xAlx (x = 0 – 0.5) hydrogen storage alloys as negative electrodes for nickel/metal hydride secondary batteries, J. Power Sources 270 (2014) 21–27.
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[22] W.K. Du, L. Zhang, Y. Li, Y.M. Zhao, S.Q. Yang, B.Z. Liu, S.M. Han, Phase Structure, Electrochemical Properties and Cyclic Characteristic of a
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[23] T.T. Zhai, T. Yang, Z.M. Yuan, S. Xu, W.G. Bu, Y. Qi, Y.H. Zhang, Influences of hydrogen-induced amorphization and annealing treatment on gaseous hydrogen storage properties of La1-xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys, J. Alloy. Compd. 639 (2015) 15–20.
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[24] K. Young, T. Ouchi, B. Huang, Effects of various annealing conditions on (Nd,
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Mg, Zr)(Ni, Al, Co)3.74 metal hydride alloys, J. Power Sources 248 (2014) 147–153.
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ACCEPTED MANUSCRIPT Table 1 – Structural parameters of the Pr4MgNi19 and La4MgNi19 alloys. Sites
Pr1 2c Pr2 4f Pr3 4f Mg 4f Ni1 2a Ni2 2b Ni3 2d Ni4 4e Ni5 4f Ni6 12k Ni7 12k a = 0.4986 nm; c = 3.2120 nm; V/f.u. = 0.6915 nm3; La1 2c La4MgNi19 Pr5Co19-type La2 4f La3 4f Mg 4f Ni1 2a Ni2 2b Ni3 2d Ni4 4e Ni5 4f Ni6 12k Ni7 12k a = 0.5032 nm; c = 3.2223 nm; V/f.u. = 0.7066 nm3;
y
z
Occupancy
1/3 1/3 1/3 1/3 0 0 1/3 0 1/3 0.8350 0.8350
2/3 2/3 2/3 2/3 0 0 2/3 0 2/3 2x 2x
1/4 0.12790 0.01940 0.01940 0 1/4 3/4 0.12500 0.87390 0.06200 0.18930
1 1 0.394 0.606 1 1 1 1 1 1 1
1/4 0.12951 0.01687 0.01687 0 1/4 3/4 0.12759 0.87576 0.06549 0.18998
1 1 0.412 0.588 1 1 1 1 1 1 1
1/3 1/3 1/3 1/3 0 0 1/3 0 1/3 0.8350 0.8350
2/3 2/3 2/3 2/3 0 0 2/3 0 2/3 2x 2x
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Pr4MgNi19 Pr5Co19-type
x
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Atoms
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Phases
ACCEPTED MANUSCRIPT Table 2 – Chemical composition of the Pr4MgNi19 and La4MgNi19 alloys obtained by EDS. Pr(at.%)
La(at.%)
Mg(at.%)
Ni(at.%)
Chemical composition
A
16.80
---
4.00
79.20
Pr4.03Mg0.97Ni19.00
B
---
16.58
4.05
79.37
La3.98Mg1.02Ni19.00
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Maps
ACCEPTED MANUSCRIPT Table 3 – Lattice constants and phase abundances of the cycled alloys from Rietveld refinement.
Cycle number
Str. type
a (nm)
c (nm)
V (nm3)
Mass fraction (wt.%)
Pr4MgNi19
50
Pr5Co19 Pr(OH)3
0.4986 0.6457
3.2282 0.3784
0.6932 0.1366
97.77 2.23
100
Pr5Co19 CaCu5 Pr(OH)3 Pr5Co19 CaCu5 Pr(OH)3 Mg(OH)2 Pr5Co19 CaCu5 La(OH)3 Pr5Co19 CaCu5 La(OH)3 Pr5Co19 CaCu5 La(OH)3 Mg(OH)2
0.4987 0.4982 0.6445 0.4992 0.4991 0.6438 0.3142 0.5029 0.5014 0.6577 0.5047 0.4961 0.6541 0.5068 0.4984 0.6559 0.3340
3.2368 0.4028 0.3773 3.4449 0.4027 0.3773 0.4766 3.2291 0.3976 0.3552 3.2760 0.4064 0.3851 3.2771 0.4093 0.3869 0.4562
0.6971 0.0866 0.1357 0.7068 0.0869 0.1354 0.0407 0.7071 0.0866 0.1331 0.7103 0.0866 0.1427 0.7198 0.0881 0.1441 0.0441
88.66 5.19 6.15 80.88 6.01 8.07 5.04 86.25 10.81 2.94 59.85 19.39 20.76 32.01 33.08 24.74 10.17
50
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La4MgNi19
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100
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ACCEPTED MANUSCRIPT Table 4 – Comparison of subunit volume of the Pr5Co19-type phase in Pr4MgNi19 and La4MgNi19 alloys 3
0
Vº[AB5] (nm ) Vº[A2B4] (nm3) V[AB5] (nm3) ∆V[AB5]/Vº[AB5] (%) V[A2B4] (nm3) ∆V[A2B4]/Vº[A2B4] (%) V[AB5] (nm3) ∆V[AB5]/Vº[AB5] (%) V[A2B4] (nm3) ∆V[A2B4]/Vº[A2B4] (%) V[AB5] (nm3) ∆V[AB5]/Vº[AB5] (%) V[A2B4] (nm3) ∆V[A2B4]/Vº[A2B4] (%)
50
100
La4MgNi19
83.95 85.75 84.69 0.88% 87.05 1.52% 86.19 2.67% 89.72 4.63% 91.19 8.62% 96.79 12.88%
85.78 87.62 87.50 2.00% 91.55 4.49% 90.49 5.49% 96.39 10.01% 97.29 13.42% 108.26 23.56%
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200
Pr4MgNi19
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Subunit volume
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Charge/discharge cycles
ACCEPTED MANUSCRIPT Figure captions Fig. 1 X-ray diffraction patterns of the alloys in different angle region: (a) range of 10–80°; (b) range of 3–25°.
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Fig. 2 Rietveld refinements of the XRD patterns for the alloys: (a) Pr4MgNi19 alloy; (b) La4MgNi19 alloy.
Fig. 3 SEM images of the alloys: (a) Pr4MgNi19 alloy; (b) La4MgNi19 alloy.
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Fig. 4 Discharge capacity vs. cycle number curves of the alloy electrodes: (a) activation
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curves; (b) cycling stability curves
Fig. 5 SEM images of the alloys: (a1)-(a4) Pr4MgNi19 alloys after 10, 50, 100 and 200 charge/discharge cycles; (b1)-(b4) La4MgNi19 alloys after 10, 50, 100 and 200 charge/discharge cycles.
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Fig. 6 XRD patterns of the alloys: (a) the original Pr4MgNi19 alloy and alloys after 50 and 100 charge/discharge cycles; (b) the original La4MgNi19 alloy and alloys after 50 and 100
range of 10–60°.
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charge/discharge cycles; (c) 50 cycles in the 2θ range of 28–80°; (d) 100 cycles in the 2θ
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Fig. 7 Comparison of the XRD patterns between the Pr4MgNi19 and La4MgNi19 alloy powders before and after 200 cycles. Fig. 8 (a) subunit volumes of the Pr5Co19-type phase in Pr4MgNi19 and La4MgNi19 alloys after different charge/discharge cycles; (b) differences between expansion rates of [A2B4] and [AB5]; (c) mean particle diameters of the Pr4MgNi19 and La4MgNi19 alloy powders; (d) phase abundances of Pr5Co19-type phase and hydroxides in Pr4MgNi19 and La4MgNi19 alloys after different charge/discharge cycles.
La4MgNi19
20
30
40
50
60
2θ/ degree
[2 2 0]
[3 0 8] 70
80
SC
10
RI PT
[3 0 0]
Pr4MgNi19
Pr5Co19
[0 0 16] [2 1 9] [2 0 17]
[1 0 8] [1 0 9]
Intensity/a.u.
[1 1 0]
[0 0 16]
(a)
[2 0 1] [1 1 8]
ACCEPTED MANUSCRIPT
Pr5Co19
M AN U
Intensity/a.u.
(b)
Pr4MgNi19
La4MgNi19
0
4
8
12
TE D
2θ/ degree
AC C
EP
Fig. 1
16
20
24
ACCEPTED MANUSCRIPT
Pr4MgNi19
(a)
Rp = 4.5
La4MgNi19
Rp = 5.3
(b)
M AN U
SC
S = 1.50
RI PT
S = 1.84
AC C
EP
TE D
Fig. 2
ACCEPTED MANUSCRIPT
(a)
RI PT
A
(b)
M AN U
SC
B
AC C
EP
TE D
Fig. 3
ACCEPTED MANUSCRIPT
340
(a)
300
Pr4MgNi19
280
La 4MgNi19
260 240 220 200 180 0
3
6
9
12
Cycle number 110
90
Pr4MgNi19
80 70
M AN U
Capacity retention/%
15
SC
(b)
100
RI PT
Discharge capacity/mAh g
-1
320
La4MgNi19
60 50 40 30 0
40
80
120
TE D
Cycle number
AC C
EP
Fig. 4
160
200
ACCEPTED MANUSCRIPT
(b1)
(a2)
(b2)
AC C
EP
TE D
M AN U
SC
RI PT
(a1)
ACCEPTED MANUSCRIPT
(b3)
(a4)
(b4)
AC C
EP
TE D
M AN U
SC
RI PT
(a3)
Fig. 5
ACCEPTED MANUSCRIPT
(b) La4MgNi19
before cycling after 50 cycles
after 100 cycles
20
after 100 cycles
30
40
50
70
80
Intensity/a.u.
La4MgNi19
40
50
2θ/degree
60
70
80
Fig. 6
EP
30
40
50
2θ/degree
60
70
80
Pr(OH)3 La(OH)3 Mg(OH)2 LaNi5
TE D
30
20
M AN U
Pr4MgNi19
10
(d)
Pr(OH)3 La(OH)3 LaNi5
AC C
Intensity/a.u.
60
2θ/degree
(c)
after 50 cycles
SC
10
before cycling
RI PT
Intensity/a.u.
Intensity/a.u.
(a) Pr4MgNi19
20
30
40
2θ/degree
Pr4MgNi19
La4MgNi19
50
60
ACCEPTED MANUSCRIPT
Pr4MgNi19
10000
before cycling after 200 cycles
6000 4000 2000
40
42
44
46
48
5000
La4MgNi19
SC
Intensity/a.u.
0 38
FWHM = 0.218
RI PT
Intensity/Counts
8000
4000 Intensity/Counts
before cycling
3000
M AN U
after 200 cycles
FWHM = 0.965
2000 1000
0 38
20
30
40
50
TE D
2θ/degree
EP
Fig. 7
AC C
10
60
40
70
42
44
80
46
48
ACCEPTED MANUSCRIPT 12
(a)
3
V(AB5) ---Pr4MgNi19
-3
105
V(A2B4) ---La4MgNi19
100
mismatch in Pr4MgNi19 mismatch in La4MgNi19
8
V(AB5) ---La4MgNi19
☒
95
4
☒
90
85
0
80 60
50
100
150
Cycle number
0
50
100
150
200
150
200
Cycle number
(c)
100
(d)
Pr4MgNi19
50
La4MgNi19
M AN U
80
Phase abundance/%
40
30
20
10
60
Pr(OH)3 ---Pr4MgNi19 La(OH)3---La4MgNi19
40
20
Pr5Co19
---Pr4MgNi19
Pr5Co19
---La4MgNi19
50
100
150
200
Cycle number
Fig. 8
EP
0
TE D
0
0
AC C
Mean particle size/µm
200
SC
0
RI PT
Subunit volume/10 nm
(b)
V(A2B4)---Pr4MgNi19
V[AB5]- V[A2B4]
110
0
50
100
Cycle number
ACCEPTED MANUSCRIPT
Highlights > Single-phase Pr5Co19-type Pr4MgNi19 and La4MgNi19 alloys are obtained > Decomposition processes of Pr5Co19-type phase are observed
RI PT
> Volume expansibility of [A2B4] and [AB5] subunits in two alloys show large
AC C
EP
TE D
M AN U
SC
difference