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Batteries 2017, 3(4), 34; doi:10.3390/batteries3040034

Article
Comparison among Constituent Phases in Superlattice Metal Hydride Alloys for Battery Applications
1
Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA
2
BASF/Battery Materials—Ovonic, 2983 Waterview Drive, Rochester Hills, MI 48309, USA
3
Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Andreas Jossen
Received: 14 September 2017 / Accepted: 18 October 2017 / Published: 31 October 2017

Abstract

:
The effects of seven constituent phases—CeNi3, NdNi3, Nd2Ni7, Pr2Ni7, Sm5Ni19, Nd5Co19, and CaCu5—on the gaseous phase and electrochemical characteristics of a superlattice metal hydride alloy made by induction melting with a composition of Sm14La5.7Mg4.0Ni73Al3.3 were studied through a series of annealing experiments. With an increase in annealing temperature, the abundance of non-superlattice CaCu5 phase first decreases and then increases, which is opposite to the phase abundance evolution of Nd2Ni7—the phase with the best electrochemical performance. The optimal annealing condition for the composition in this study is 920 °C for 5 h. Extensive correlation studies reveal that the A2B7 phase demonstrates higher gaseous phase hydrogen storage and electrochemical discharge capacities and better battery performance in high-rate dischargeability, charge retention, and cycle life. Moreover, the hexagonal stacking structure is found to be more favorable than the rhombohedral structure.
Keywords:
metal hydride (MH); nickel/metal hydride (Ni/MH) battery; hydrogen absorbing alloy; electrochemistry; superlattice alloy

1. Introduction

Misch metal (Mm, a mixture of more than one rare earth element)-based superlattice metal hydride (MH) alloys are very important for today’s nickel/metal hydride (Ni/MH) batteries, because of their higher hydrogen storage (H-storage) capacity, better high-rate dischargeability (HRD) capability, superior low-temperature and charge retention performances, and improved cycle stability [1,2,3,4,5,6,7,8]. The three main components in the superlattice alloy can be classified by chemical stoichiometry—or more precisely, the B/A ratio—and they are AB3, A2B7, and A5B19. In each component, there are two different types of structures—hexagonal and rhombohedral, depending on the stacking sequence for the A2B4 slabs (illustrated in pink in Figure 1). Each A2B4 slab shifts on the ab-plane by 1/3 a and 1/3 b from its neighbor. Studies on superlattice alloys began with the structure [9] and gaseous phase (GP) H-storage characteristics of the single rare earth element (RE)-based AB3 alloys [10,11,12] and soon shifted to the Mm-based A2B7 chemistry for battery applications [13,14,15,16]. Some researchers continued to work on RE-based AB3 chemistry for its basic electrochemical (EC) properties [17,18,19,20,21,22]. In the meantime, RE-based A5B19 was also highly promoted for battery applications [23,24,25,26,27]. Several comparative works on the EC performances of various superlattice phases were previously reported and are summarized as follows. In a (LaMg)Nix (x = 3, 3.5, and 3.8) system, the capacity decreased, and both HRD and cycle stability increased with the increase of x from 3 to 3.5 and finally 3.8 [28]. In a (LaY)(NiMnAl)x (x = 3, 3.5, and 3.8) system, both capacity and cycle stability reached the maximum at x = 3.5, and HRD increased with the increase in x [29]. The EC properties of the Mm-based ABx (where A is Mm, B is a combination of several transition metals and Al, and x = 2, 3, 3.5, 3.8, and 5) were compared, and A2B7 showed the best overall performance [30,31], which prompted many investigations in the A2B7 superlattice alloys [32,33,34,35,36,37]. However, no data were provided to support the comparative work. Therefore, in this work we detail the correlations between the constituent phase abundances and various properties.
Annealing has been used to effectively alter the phase components in the AB [38,39], AB2 [40,41,42,43], AB3 [44,45,46,47], A2B7 [48,49,50,51], A5B19 [52], AB5 [53,54,55], A8B21 [56,57], and body-centered-cubic (bcc) [58] MH alloys. According to first-principle calculations on superlattice MH alloy systems, the preferable phase abundance can be influenced by the starting composition and annealing condition [59,60]. For example, the A2B7 phase is more stable than the mixture of the AB3 and AB5 phases in the La3MgNi14 alloy, however, the opposite is true in the La2CeMgNi14 alloy [60]. Therefore, while annealing increased the A2B7 phase abundance in the La-based superlattice alloy [61], it promoted phase segregation in the LaPrNd-based alloy [62]. Moreover, phase segregation is more prominent in the Mm-based (A is more than one RE element) superlattice alloys than in the RE-based (A is only one RE element) alloys. In this work, annealing is applied to engineer the abundances of constituent components in a SmLa-based superlattice alloy, and the correlations of phase abundances with the GP and EC properties are reported.

2. Experimental Setup

First, Eutectix (Troy, MI, USA) prepared a 250 kg ingot using the conventional induction melting method [31]. Five 2 kg ingot pieces were annealed in 1 atm atmosphere of Ar for 5 h at different temperatures. Each ingot was hydrided and then crushed and ground into the size of −200 mesh. Chemical compositions of ingots before and after annealing were verified with a Varian Liberty 100 inductively coupled plasma-optical emission spectrometer (ICP-OES, Agilent Technologies, Santa Clara, CA, USA). A Philips X’Pert Pro X-ray diffractometer (XRD, Amsterdam, The Netherlands) and a JEOL-JSM6320F scanning electron microscope (SEM, Tokyo, Japan) with energy dispersive spectroscopy (EDS) was used to conduct microstructure analysis. GP H-storage characteristics were evaluated with a Suzuki-Shokan multi-channel pressure-concentration-temperature system (PCT, Tokyo, Japan). Negative electrodes were fabricated by compacting the alloy powder onto an expanded nickel substrate through a roll mill without any binder. Half-cell measurements were performed using a CTE MCL2 Mini cell testing system (Chen Tech Electric MFG. Co., Ltd., New Taipei, Taiwan) with a partially pre-charged Ni(OH)2 positive electrode and a 30% KOH electrolyte. The electrode was first charged with a current density of 100 mA·g−1 for 4 h and then discharged with a current density of 400 mA·g−1 until a cutoff voltage of 0.9 V was reached. More discharges at smaller current densities (300, 200, 100, 50, and 5 mA·g−1) with the same cutoff voltage were performed afterward with a 2 min rest in between. The sum of capacities from six discharge stages was used as the full discharge capacity. The HRD was defined as the ratio of the capacity obtained at the highest rate (400 mA·g−1) vs. full discharge capacity. A Solartron 1250 Frequency Response Analyzer (Solartron Analytical, Leicester, UK) with a sine wave amplitude of 10 mV and a frequency range of 0.5 mHz to 10 kHz was used for the AC impedance measurement. A Digital Measurement Systems Model 880 vibrating sample magnetometer (MicroSense, Lowell, MA, USA) was used to measure the magnetic susceptibility of the alloy powder surface after activation, which was performed by immersing the powder in 30 wt % KOH solution at 100 °C for 4 h.
For the sealed cell testing, a C-size cylindrical cell was chosen. While the negative electrode was fabricated by dry compacting the alloy powder onto nickel mesh current collectors, the counter positive electrode was fabricated by pasting a mixture of 89% standard AP50 [63] with the composition of Ni0.91Co0.045Zn0.045(OH)2 (BASF—Ovonic, Rochester Hills, MI, USA), 5 wt % Co powder, and 6% CoO powder onto nickel foam substrates. Scimat 700/79 acrylic acid grafted polypropylene/polyethylene separators were used (Freudenberg Group, Weinheim, Germany). A 1.5 to 1.7 negative-to-positive capacity ratio cell design was used to maintain a good balance between the over-charge and over-discharge reservoirs [64]. A 30 wt % KOH solution with LiOH (1.5 wt %) additive was used as the electrolyte. Formation was performed with a six-cycle process using a Maccor Battery Cycler (Maccor, Tulsa, OK, USA). Details of cell testing can be found in an earlier publication [65].

3. Results and Discussion

The design compositions for this study are presented in Table 1. Sm was chosen as the main RE element because it is relatively inexpensive (like La and Ce) and less oxidable (for a comparison to other RE elements, see Table 7 in [31]). However, the metal–hydrogen bond strength of the Sm-based superlattice MH alloy is too weak (Sm2Ni7 has a discharge capacity of 170 mAh·g−1 [66]), and thus adding La is necessary to increase the storage capacity [67]. LaSm-based superlattice MH alloys with the La/Sm ratio above 1 were previously reported. While the AB5 phase cannot be removed completely by annealing in those alloys, the AB5 phase abundance still decreased with the increase in Sm-content [27,68,69,70,71,72]. In this experiment, a La/Sm ratio of 0.4 was adopted to attempt to suppress the AB5 phase abundance. Two common constituent elements used in the AB5 MH alloy, Mn and Co, are excluded in this study for better charge retention and cycle stability [1,73]. Al is included to prevent the hydrogenation-induced-amorphization in the superlattice MH alloys [36].
Five pieces of ingot from the induction melting were annealed at 880, 900, 920, 940, and 960 °C for 5 h in an Ar environment. Compositions of the as-cast (alloy A0) and annealed ingots (alloys A1 to A5) were measured by ICP, and the results are summarized in Table 1. ICP results reveal that the annealed ingots have similar Mg-content, but are slightly rich in La and lean in Ni.

3.1. Microsctructure Analysis

XRD patterns of alloys in this study are presented in Figure 2. These patterns show the typical multi-phase superlattice structures. By using the Jade 9.0 software (MDI, Livermore, CA, USA), we were able to deconvolute each pattern into its constituent phases, and the results are summarized in Table 2. One example of such deconvolution is plotted in Figure 3. There are two stacking structures for each stoichiometry (AB3, A2B7, and A5B19)—hexagonal (H) and rhombohedral (R). Most of the phase transformations in the superlattice phases are through the peritectic reaction and are very sensitive to the annealing conditions (temperature and duration). From Table 2, it is clear that annealing initially suppresses the unwanted non-superlattice AB5 phase, which decreases both the capacity and HRD [72,73]. However, further increases in annealing temperature promotes the formation of AB5. Evolutions in phase stoichiometry (x value in ABx) and stacking structure (hexagonal vs. rhombohedral) with different annealing temperatures are plotted in Figure 4. Annealing first increases the A2B7 abundance at the expense the AB3 abundance, but further increases in annealing temperature decreases the A2B7 abundance. In comparison, the changes in A5B19 abundance are less obvious. For the stacking structure evolution, annealing first increases the hexagonal structure and then slowly decreases it as the annealing temperature increases (Figure 4b).
Representative SEM backscattering electron image (BEI) micrographs from alloys A0 (as-cast) and A3 (annealed at 920 °C) are compared in Figure 5. While obvious phase segregation can be observed in alloy A0 (Figure 5a), alloy A3 appears to be more uniform (Figure 5b). EDS was used to measure chemical compositions of several spots in each micrograph, and the results are summarized in Table 3. In alloy A0 (Figure 5a), undissolved La (spot 1), the SmNi (spot 2), AB3 (spot 4), AB2 (spot 6) and AB5 (spot 7) phases can be identified. The main phase (spot 5) is a mixture of the AB3, A2B7, and A5B19 phases. In the annealed alloy A3, only occasional undissolved La and Sm (spot 1) and the AB2 phase (spot 4) can be identified, and the majority is composed of a fine mixture of the AB3, A2B7, and A5B17 phases (Figure 5b).

3.2. Gaseous Phase Hydrogen Storage

In this study, PCT analysis was used to examine hydrogen absorption/desorption characteristics of alloys, and the isotherms measured at 30 and 45 °C are plotted in Figure 6. While no obvious plateau is observed in the isotherms of alloy A0, isotherms of the annealed alloys show well-defined plateau regions. GP H-storage properties obtained from the PCT analysis are summarized in Table 4. Both maximum and reversible capacities first increase and then decrease with an increase in annealing temperature, and the annealed alloys have higher storage capacities compared to the as-cast alloy A0. The desorption pressure at 0.75 wt % H-storage increases monotonically with an increase in annealing temperature. For the annealed alloys, the slope factor (ratio of the H-storage capacity between 0.02 and 0.5 MPa to the total reversible capacity) first decreases (more slanted, less uniform in composition [74]) and then increases (flatter and more uniform in composition) as the annealing temperature increases, but all are higher than that of the as-cast alloy A0. PCT hysteresis, defined as
PCT   hysteresis = ln ( absorption   pressure   at   0.75   wt   %   H storage   desorption   pressure   at   0.75   wt   %   H storage   )
of alloys A4 and A5 are relatively smaller, which suggest a better resistance to pulverization during charge/discharge cycling [75]. Changes in enthalpy (ΔH) and entropy (ΔS) were calculated by using the desorption pressures (P) at 0.75 wt % H-storage measured at different temperatures in the Van’t Hoff equation,
ΔG = ΔHTΔS = T lnP,
where T and are the absolute temperature and ideal gas constant, respectively. Calculation results are listed in the last two rows of Table 4. Both ΔH and ΔS of the annealed alloys are lower than those of the as-cast alloy A0. Among the annealed alloys, both ΔH and ΔS increase (except for alloy A5) with an increase in annealing temperature. The lowest ΔS value obtained from alloy A5 is the closest to the ΔS value for H2 gas (−135 J·mol H21 K1), indicating the highest degree of order of hydrogen in the MH alloy [76].

3.3. Electrochemical and Magnetic Susceptibility Measurements

Before the half-cell capacity measurement, the compacted negative electrode went through a 4 h activation process in 30 wt % KOH at 100 °C. The second cycle capacities obtained at different discharge currents for each alloy are plotted in Figure 7a together with those for a standard and commercially available AB5 alloy (Alloy B with a chemical composition of La10.5Ce4.3Pr0.5Nd1.4Ni60Co12.7Mn5.9Al4.7 supplied by Eutectix). Discharge capacity decreases with the increase in discharge rate in a roughly linear manner. The slope of capacity vs. discharge rate for the superlattice alloys (dashed red line) is lower than the slope for the AB5 alloy (solid green line), which confirms the superiority of superlattice alloys in HRD, as previously reported [30]. EC test results of alloys in this study are summarized in Table 5. Annealing improves the capacity substantially. Both the capacity and half-cell HRD (the ratio between the capacities obtained at 400 and 5 mA·g−1) increase first and then decrease with the increase in annealing temperature and peak at alloy A2 (Figure 7b). EC discharge capacities of alloys A1 to A3 (323 to 326 mAh·g−1) are very close to the maximum H-storage capacities in GP (1.18 to 1.21 wt %, which is equivalent to 316 to 324 mAh·g−1).
Evolution in half-cell HRD was further investigated by the bulk hydrogen diffusion constant (D) and surface reaction current (Io), and the results are listed in Table 5. Details of both measurements were previously reported [33]. While the D value remains approximately the same, the Io value peaks at alloy A2. Therefore, we conclude the half-cell HRD in this series of annealed superlattice alloys is related more to the surface catalytic ability, which agrees with our previous findings from the Co-substituted Mm-based superlattice MH alloys [33]. With the −40 °C AC impedance measurement, both charge-transfer resistance (R) and double-layer capacitance (C) were obtained, and are summarized in Table 5. With an increase in annealing temperature, both R and RC product (a measure of surface catalytic ability [77]) first decrease and then increase; this trend is similar to that observed in HRD. Moreover, alloys A2, A3, and A4 demonstrate the best low-temperature performance (the lowest Rs), which is dominated by the surface catalytic ability (the lowest RC products).
Magnetic susceptibility was measured to study the evolution in surface metallic Ni with the annealing temperature. Details of this measurement and the connection between magnetic susceptibility and HRD were previously published [78,79]. Saturated magnetic susceptibility (MS, closely related to the surface catalytic ability) first decreases and then increases with an increase in annealing temperature, suggesting that the surface’s catalytic ability first decreases and then increases. This result is contradictory to the conclusion drawn from the RC product result. Therefore, the lowest Rs and RC products observed in alloys A2, A3, and A4 do not correlate with the amount of surface metallic nickel. This discrepancy is rare, but there is example when the surface oxide microstructure, rather than the metallic nickel, played an important role in affecting the HRD and low temperature performances [80]. Further study of the surface oxide microstructure by transmission microscope is necessary to explain the source of highly catalytic surface of alloys A2, A3, and A4. Lastly, the applied field strength corresponding to half of MS (H1/2) of these alloys are similar, indicating the size of metallic nickel inclusion in the surface oxide remains constant despite the change in annealing condition.

3.4. Sealed Cell Performance

Five annealed alloys (alloys A1 to A5) were incorporated into cylindrical C-size cells (20 cells for each alloy), and each cell has a nominal capacity of 5.0 Ah. After a standard formation process [65], cells were distributed to various tests, and the results are discussed in the following sections.

3.4.1. High-Rate Performance

Four different discharge rates (C/5, C/2, C, and 2C) were used to obtain the room temperature (RT) discharge voltage curves, and the results from cells made with alloys A1 and A3 are shown in Figure 8. As the discharge rate increases, voltage is suppressed by the internal resistivity in the cell [65], and the discharge capacity obtained at the fixed cutoff voltage (0.8 V) decreases. Cells made with alloy A3 show a smaller voltage reduction with the increase in discharge rate, which indicates a lower cell internal resistance compared to the internal resistance in the cell made with alloy A1. The normalized discharge capacity obtained at a 2C rate for cells made with the five annealed alloys are listed in Table 6. The data displays an initial increasing and later decreasing trend with an increase in alloy annealing temperature. Cells made with alloy A3 yield the best RT high-rate (2C) performance in this test.

3.4.2. Low-Temperature Performance

Low-temperature performance was evaluated at −10 °C and a 1C discharge rate, and the results are summarized in Table 6. Like the trend in high-rate performance, a trend of initial increase followed by decrease is observed for the low-temperature performance with an increase in alloy annealing temperature. The cell made with alloy A3 shows the best low-temperature result, which can be attributed to its surface catalytic ability being the best (lowest RC product in Table 5).

3.4.3. Charge Retention

RT charge retention test results are shown in Figure 9a. All cells (except for the one made with alloy A4) exhibit similar charge retention characteristics. Three cells made with alloy A4 were tested, and all show inferior charge retention performance compared to the cells made with the other annealed alloys. 45 °C voltage stand test results are plotted in Figure 9b and summarized in Table 6. In this test, the cells made with alloy A2 demonstrate the best result.

3.4.4. Peak Power

Peak power was measured at the 20th cycle and RT, and the results are compared in Table 6. Peak power follows the same trend as those in high-rate and low-temperature performances. Cells made with alloy A3 show the best balance among peak power, best high-rate, and low-temperature performances.

3.4.5. Cycle Life

The RT cycle life performance was evaluated in two different configurations: a regular configuration charged at a C/2 rate and discharged at a C/2 rate, and an accelerated configuration charged at a C rate and discharged at a C rate. The results are plotted in Figure 10 and summarized in Table 6. The number of cycles before reaching a capacity of 3 Ah first increases and then decreases with an increase in alloy annealing temperature. Cells with alloy A3 show the best cycle stability in both C/2-C/2 and C-C cycling tests. As reported previously, the main degradation mechanism for superlattice alloy free of Mn and Co is the combination of particle pulverization and surface oxidation [34,35].

3.5. Performance Correlation with Individual Phase

Correlations between the seven constituent phase abundances (CeNi3, NdNi3, Nd2Ni7, Pr2Ni7, Sm5Ni19, Nd5Co19, and CaCu5) and seven GP, eight half-cell EC, and five sealed cell properties were studied by the linear regression method, and the resulting correlation factors (R2) are summarized in Table 7. Although these phase abundances were obtained from the XRD analysis of the un-activated alloys, the XRD performed on the activated alloys only showed additional minute rare-earth oxide (hydride) peaks and no changes in the main phase peaks were found [81]. For the maximum H-storage capacity, Nd2Ni7 (Figure 11a) and Pr2Ni7 are considered beneficial, whereas NdNi3 and Nd5Co19 are detrimental. Reversible H-storage capacity does not correlate well with any phase. NdNi3, Nd5Co19 (Figure 11b), and CaCu5 (Figure 11c) are effective in increasing the PCT plateau pressure; however, Nd2Ni7 and Pr2Ni7 have an opposite effect. CeNi3 increases the flatness of PCT isotherm, but CaCu5 decreases it. While CeNi3 reduces the PCT hysteresis, Nd2Ni7 and Pr2Ni7 increase it (Figure 11d). The only strong influence on the change in enthalpy is from CaCu5, which contributes to a much less negative ∆H value (weaker metal-hydrogen bond strength). NdNi3 and Nd5Co19 may also increase ∆H. None of the phases have a significant correlation with the change in entropy (which is essentially the change of entropy between hydrogen gas and hydrogen in an ordered solid).
For the EC properties, Nd2Ni7 (Figure 11a) and Pr2Ni7 make positive contributions to the high-rate and low-rate discharge capacities, while NdNi3, Nd5Co19 (Figure 11b) and CaCu5 have an opposite effect. Nd2Ni7 and Sm5Ni19 increase HRD, but NdNi3, Nd5Co19, and CaCu5 decrease it. The increase in HRD comes from different sources: improvement in surface reaction for Nd2Ni7 (indicated by its positive correlation with Io) and enhancement of bulk diffusion for Sm5Ni19 (indicated by its positive correlation with D). For the low-temperature characteristics determined by the AC impedance measured at −40 °C, Sm5Ni19 decreases the charge-transfer resistance by increasing the surface’s catalytic ability (indicated by its negative correlation with the RC product), but Pr2Ni7 increases it by reducing the surface’s reactive area (indicated by its negative correlation with C). Although NdNi3, Nd5Co19, and CaCu5 (Figure 11c) can increase the surface reactive area, they have a marginal effect on the charge-transfer resistance, due to their relatively low catalytic abilities.
Judging from the ratio between the number of significant correlations (R2 ≥ 0.60) found and number of properties (7/7 for the GP properties, 14/8 for the half-cell EC properties, and 2/5 for the sealed cell properties), correlations with the sealed cell properties are the weakest. There are two explanations for the difficulty in setting up correlations for the sealed cell properties: manual assembly operation and limited cell number. While the former adds inconsistency, the latter limits the accuracy of sampling. Therefore, further confirmation on a larger scale (hundreds of cells) is needed. Nevertheless, correlations obtained from the 100 cells (20 for each annealed alloy) in this study are shown in the bottom five rows of Table 7. Only CeNi3 shows a significant and detrimental effect on the high-rate performance. For the low-temperature performance, only Sm5Ni19 exhibits a positive influence. Both Nd2Ni7 and Pr2Ni7 show a positive impact on the charge retention performance, but Sm5Ni19 influences it negatively. Among all phases, Sm5Ni19 most effectively increases the peak power. Only NdNi3 contributes to the increase in cycle stability, whereas CeNi3 (Figure 11d) and CaCu5 deteriorate it.
As the exact chemical composition in each superlattice phase cannot be quantified by SEM–EDS, the correlations found in this session may come from the non-uniform distribution of A-site and B-site elements in these phases, but this cannot be verified.

3.6. Performance Correlation with Phase Stoichiometry

In earlier superlattice MH alloy studies of various phases’ contributions to the EC performances, phases are grouped according to the phase stoichiometry, such as AB3 (CeNi3 and NdNi3), A2B7 (Nd2Ni7 and Pr2Ni7), and A5B19 (Sm5Ni19 and Nd5Co19) [28,29,30]. In this study, we correlated the properties with the phase stoichiometry, and the resulted R2s are summarized and shown in the first three columns of Table 8. AB3 decreases the GP and EC capacities (Figure 12a), increases the plateau pressure, decreases HRD because of the reductions in D and Io, which also contribute to an inferior high-rate performance. Moreover, although AB3 decreases the PCT hysteresis, it still deteriorates the cycle stability. A2B7 increases the GP and EC capacities (Figure 12b), lowers the equilibrium pressure, increases HRD because of the enhanced D and Io, and improves both the charge retention and cycle life. A5B19 increases the GP reversible H-storage, improves the −40 °C performance via the increase in surface catalytic ability, increases the peak power, and deteriorates the charge retention. By comparing these correlations, A2B7 appears to be the most desirable stoichiometry for battery applications among all stoichiometries for the superlattice MH alloys. This conclusion is in complete agreement with previous reports [29,30,31].

3.7. Performance Correlation with Phase Structure

Another way to classify the various phases in the superlattice MH alloys is by structure symmetry. Stacking of the A2B4 slabs in the CeNi3, Nd2Ni7, and Sm5Ni19 phases is the same as that in the C14 crystal structure and is considered as the hexagonal group, and the rhombohedral group containing the NdNi3, Pr2Ni7, and Nd5Co19 phases shares the same A2B4 stacking as the C15 structure. The C14- and C15-based MH alloys behave differently in the EC environment, and we have reported the comparison previously [82] and concluded that the C14-predominated alloy is more suitable for high-capacity and long-cycle life applications, whereas the C15-predominated alloy is preferable in applications requiring easy activation and good high-rate and low-temperature performances. Therefore, it will be interesting to determine whether similar conclusions can be drawn from the superlattice MH alloy study.
R2s obtained by correlating the total abundances of the hexagonal and rhombohedral phase groups with various properties are listed in the last two columns in Table 8. Compared to those in the rhombohedral group, the hexagonal group shows higher GP and EC capacities (Figure 13a), a lower equilibrium pressure (Figure 13a), a flatter PCT isotherm, a better HRD (Figure 13b), and a reduced surface reactive area at −40 °C (Figure 13b), but without affecting the charge-transfer resistance. Based on the results, a hexagonal phase will be more desirable for battery applications. Furthermore, combining the preferences in stoichiometry and structure yields the champion of this study—the Nd2Ni7 phase.

4. Conclusions

Constituent phase abundances of an SmLa-based superlattice alloy were engineered by changing the annealing temperature. Various gaseous phase, half-cell electrochemical, and sealed cell properties and performances were correlated to the individual phase, phase stoichiometry, and type of structure. Many significant correlations were identified; however, because of the complication of phase structure and limited sampling size used in this study, all correlations need further confirmation. In general, an A2B7 stoichiometry and a hexagonal structure are more favorable for battery applications. Among the six superlattice phases in this study, Nd2Ni7 is the most desirable. As a result, establishment of a single target of maximizing Nd2Ni7 phase abundance simplifies the composition/process optimization task. In this study, an annealing condition of 920 °C for 5 h for the superlattice Sm14La5.7Mg4.0Ni73Al3.3 MH alloy renders the highest Nd2Ni7 phase abundance and the best overall electrochemical performance.

Acknowledgments

The authors would like to thank the following individuals from BASF—Ovonic for their help: Su Cronogue, Baoquan Huang, Diana F. Wong, S. Chang, Chaolan Hu, Benjamin Reichman, Nathan English, Sui-ling Chen, Cheryl Setterington, David Pawlik, Allen Chan, and Ryan J. Blankenship.

Author Contributions

Kwo-Hsiung Young designed the experiments and analyzed the results. Taihei Ouchi prepared the alloy samples and performed the PCT and XRD analyses. Jean Nei prepared the electrode samples and conducted the magnetic measurements. Yu-Ling Lien organized the data and prepared the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

MmMisch metal
MH Metal hydride alloy
Ni/MHNickel/metal hydride
H-storageHydrogen-storage
HRDHigh-rate dischargeability
GPGaseous phase
RERare earth
ECElectrochemical
bccBody-centered-cubic
ICP-OESInductively coupled plasma-optical emission spectrometer
XRDX-ray diffractometer
SEMScanning electron microscope
EDSEnergy dispersive spectroscopy
PCTPressure-concentration-temperature
BEIBackscattering electron image
ΔHChange in enthalpy or heat of hydride formation
ΔSChange in entropy
PDesorption pressure
TTemperature
RIdeal gas constant
DBulk hydrogen diffusion coefficient
IoSurface exchange current
RSurface charge-transfer resistance
CSurface double-layer capacitance
MSSaturated magnetic susceptibility
H1/2Applied field strength corresponding to half of MS
RTRoom temperature
R2Correlation factor

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Figure 1. Stacking sequences along the c-axis direction of various hexagonal and rhombohedral structures available for the superlattice metal hydride alloys.
Figure 1. Stacking sequences along the c-axis direction of various hexagonal and rhombohedral structures available for the superlattice metal hydride alloys.
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Figure 2. XRD patterns using Cu-Kα as the radiation source for alloys (a) A0, (b) A1, (c) A2, (d) A3, (e) A4, and (f) A5.
Figure 2. XRD patterns using Cu-Kα as the radiation source for alloys (a) A0, (b) A1, (c) A2, (d) A3, (e) A4, and (f) A5.
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Figure 3. Phase deconvolution performed by Jade 9 software on XRD pattern from alloy A0. The red and black lines are the raw data and fitting curve, respectively. The residue of fitting is plotted in the top of the figure.
Figure 3. Phase deconvolution performed by Jade 9 software on XRD pattern from alloy A0. The red and black lines are the raw data and fitting curve, respectively. The residue of fitting is plotted in the top of the figure.
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Figure 4. Phase abundance distributions classified by (a) stoichiometry and (b) structure family.
Figure 4. Phase abundance distributions classified by (a) stoichiometry and (b) structure family.
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Figure 5. SEM backscattering electron image (BEI) micrographs from alloys (a) A0 and (b) A3. Compositions of the numbered areas were analyzed by EDS, and the results are shown in Table 3.
Figure 5. SEM backscattering electron image (BEI) micrographs from alloys (a) A0 and (b) A3. Compositions of the numbered areas were analyzed by EDS, and the results are shown in Table 3.
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Figure 6. PCT isotherms measured at (a,b) 30 and (c,d) 45 °C for alloys A0–A5. Open and solid symbols represent absorption and desorption curves, respectively.
Figure 6. PCT isotherms measured at (a,b) 30 and (c,d) 45 °C for alloys A0–A5. Open and solid symbols represent absorption and desorption curves, respectively.
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Figure 7. Electrochemical discharge capacities as functions of (a) discharge rate and (b) annealing temperature. Alloy B is a commercially available La-rich AB5 MH alloy.
Figure 7. Electrochemical discharge capacities as functions of (a) discharge rate and (b) annealing temperature. Alloy B is a commercially available La-rich AB5 MH alloy.
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Figure 8. Room temperature discharge voltage curves at different discharge rates (C/5, C/2, C, and 2C) for cells made with alloys (a) A1 and (b) A3.
Figure 8. Room temperature discharge voltage curves at different discharge rates (C/5, C/2, C, and 2C) for cells made with alloys (a) A1 and (b) A3.
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Figure 9. (a) Room temperature charge retentions and (b) 45 °C voltage stands for cells made with alloys A1 to A5.
Figure 9. (a) Room temperature charge retentions and (b) 45 °C voltage stands for cells made with alloys A1 to A5.
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Figure 10. Room temperature cycle life from cells made with alloys A1 to A5 (a) in the configuration of a C/2 rate charge, a C/2 rate discharge, and a −∆V cutoff voltage of 3 mV and (b) in the configuration of a C rate charge, a C rate discharge, and a −∆V cutoff voltage of 5 mV.
Figure 10. Room temperature cycle life from cells made with alloys A1 to A5 (a) in the configuration of a C/2 rate charge, a C/2 rate discharge, and a −∆V cutoff voltage of 3 mV and (b) in the configuration of a C rate charge, a C rate discharge, and a −∆V cutoff voltage of 5 mV.
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Figure 11. Examples of correlations between the constituent phase abundances and several gaseous phase/electrochemical properties: (a) Nd2Ni7 (b) Nd5Co19 (c) CaCu5 (d) CeNi3.
Figure 11. Examples of correlations between the constituent phase abundances and several gaseous phase/electrochemical properties: (a) Nd2Ni7 (b) Nd5Co19 (c) CaCu5 (d) CeNi3.
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Figure 12. Examples of correlations between the abundances of phases with (a) an AB3 and (b) an A2B7 stoichiometry and several gaseous phase/electrochemical properties.
Figure 12. Examples of correlations between the abundances of phases with (a) an AB3 and (b) an A2B7 stoichiometry and several gaseous phase/electrochemical properties.
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Figure 13. Plots of (a) PCT H2 equilibrium pressure and half-cell electrochemical discharge capacity and (b) half-cell high-rate dischargeability and capacitance measured at −40 °C vs. the hexagonal superlattice structure abundance.
Figure 13. Plots of (a) PCT H2 equilibrium pressure and half-cell electrochemical discharge capacity and (b) half-cell high-rate dischargeability and capacitance measured at −40 °C vs. the hexagonal superlattice structure abundance.
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Table 1. Designed composition and ICP results in at%. B/A is the ratio of the B-atom (Ni and Al) to the A-atom (La, Sm, and Mg).
Table 1. Designed composition and ICP results in at%. B/A is the ratio of the B-atom (Ni and Al) to the A-atom (La, Sm, and Mg).
AlloyAnnealing TemperatureSourceLaSmMgNiAlB/A
--Design5.714.04.073.03.33.2
A0-ICP5.714.03.973.03.43.2
A1880 °CICP6.014.54.272.13.23.0
A2900 °CICP5.813.94.072.93.43.2
A3920 °CICP5.813.94.073.03.33.2
A4940 °CICP5.814.34.072.73.23.1
A5960 °CICP5.814.54.072.53.23.1
Table 2. Abundances (in wt %) of the constituent phases obtained from the XRD data for hexagonal (H) and rhombohedral (R) systems.
Table 2. Abundances (in wt %) of the constituent phases obtained from the XRD data for hexagonal (H) and rhombohedral (R) systems.
AlloyCeNi3 (H)NdNi3 (R)Nd2Ni7 (H)Pr2Ni7 (R)Sm5Ni19 (H)Nd5Co19 (R)CaCu5 (H)
A08.437.114.9--18.621.0
A120.88.650.410.0-10.2-
A2-17.948.67.013.912.6-
A3-18.552.44.811.912.5-
A424.320.817.5-20.913.23.3
A524.327.715.9-12.515.54.2
Table 3. Summary of EDS results from several selective spots in the SEM–BEI micrographs of alloys A0 and A3 shown in Figure 5a,b.
Table 3. Summary of EDS results from several selective spots in the SEM–BEI micrographs of alloys A0 and A3 shown in Figure 5a,b.
SampleLocationLaSmMgNiAlB/APhase
A03a-190.85.11.51.90.70.03La
3a-22.542.97.347.00.30.90AB
3a-39.313.65.265.76.22.56AB2/AB3
3a-44.115.35.471.63.63.03AB3
3a-53.415.04.473.63.63.39AB3/A2B7/A5B19
3a-67.214.313.563.71.21.85AB2
3a-73.514.00.678.33.64.52AB5
A33b-127.961.02.77.31.10.09La/Sm
3b-25.114.45.171.14.33.07AB3/A2B7
3b-33.113.94.075.23.83.76A2B7/A5B19
3b-44.814.515.664.01.01.86AB2
Table 4. Summary of gaseous phase properties measured at 30 °C.
Table 4. Summary of gaseous phase properties measured at 30 °C.
Gaseous Phase PropertiesUnitA0A1A2A3A4A5
Capacity at 2 MPa wt %1.111.181.211.211.181.08
Reversible capacity wt %0.950.981.171.171.131.00
Desorption pressure MPa0.160.0290.0350.0370.0710.091
Slope factor %697876727779
Hysteresis 0.170.160.220.170.080.09
–∆HkJ·mol H2−125.838.835.233.631.839.3
–∆SJ·mol H2−1·K−189118107102102129
Table 5. Summary of electrochemical half-cells.
Table 5. Summary of electrochemical half-cells.
Electrochemical and Magnetics PropertiesUnitA0A1A2A3A4A5
High-rate discharge capacitymAh·g1249288295291283266
Full discharge capacitymAh·g1285323326323315300
Half-cell HRD%87.589.290.490.189.888.7
Diffusion coefficient, D1010 cm2·s14.04.04.24.44.24.1
Surface reaction current, IomA·g124.224.033.123.821.917.6
Charge-transfer resistance at −40 °C, RΩ·g4.98.84.03.73.45.3
Double-layer capacitance at −40 °C, CF·g−11.60.861.020.881.121.29
RC product at −40 °Cs7.77.64.13.33.86.8
Total saturated magnetic susceptibility, MSemu·g11.451.360.960.601.051.12
Applied field where M.S. = ½ MS, H1/2kOe0.110.100.110.120.100.10
Table 6. Summary of C-cell test results (RT stands for room temperature).
Table 6. Summary of C-cell test results (RT stands for room temperature).
C-Cell ResultsUnitA1A2A3A4A5
2C at RT capacity/0.2C at RT capacity%8790939087
1C at −10 °C capacity/0.2C at RT capacity%9294959491
14-day charge retention %85.384.984.876.185.3
28-day 45 °C voltage stand V1.2131.2221.2181.2181.220
Peak power at RT (20th cycle)W·kg−1183198206200194
0.5C/0.5C cycle life (before reaching 3 Ah)Number of cycles220255365340210
C/C cycle life (before reaching 3 Ah)Number of cycles110185205130120
Table 7. Correlation factors (R2) between the constituent phase abundances and various gaseous phase, half-cell, and sealed cell properties. Plus (+) and minus (−) signs after the numbers indicate positive and negative correlations, respectively. Significant correlations with R2 ≥ 0.6 are highlighted in red. EC full capacity is the discharge capacity measured with a 5 mA·g−1 current density.
Table 7. Correlation factors (R2) between the constituent phase abundances and various gaseous phase, half-cell, and sealed cell properties. Plus (+) and minus (−) signs after the numbers indicate positive and negative correlations, respectively. Significant correlations with R2 ≥ 0.6 are highlighted in red. EC full capacity is the discharge capacity measured with a 5 mA·g−1 current density.
PropertiesCeNi3NdNi3Nd2Ni7Pr2Ni7Sm5Ni19Nd5Co19CaCu5
GP maximum capacity0.26−0.51−0.63+0.42+0.07+0.55−0.38−
GP reversible capacity0.24−0.14−0.21+0.03+0.57+0.20−0.37−
Equilibrium pressure0.02+0.89+0.68−0.58−0.09−0.92+0.92+
PCT slope factor0.42+0.31−0.000.05+0.14+0.31−0.46−
PCT hysteresis0.73−0.03−0.46+0.39+0.17−0.02−0.00
H0.12−0.43+0.16−0.24−0.02−0.42+0.61+
S0.28−0.16+0.01−0.05−0.01−0.15+0.33+
EC high-rate capacity0.06−0.79−0.65+0.49+0.17+0.84−0.84−
EC full capacity0.05−0.84−0.69+0.56+0.13+0.89−0.84−
HRD0.09−0.52−0.43+0.22+0.42+0.59−0.76−
Diffusion constant, D0.25−0.05−0.16+0.000.42+0.09−0.27−
Exchange Current, Io0.52−0.08−0.33+0.31+0.000.07−0.02−
−40 °C resistivity, R0.15+0.17−0.06+0.32+0.53−0.11−0.01−
−40 °C capacitance, C0.02+0.91+0.71−0.59−0.05+0.94+0.85+
RC product0.17+0.08+0.11−0.000.65−0.13+0.29+
High rate0.54−0.000.14+0.000.20+0.000.15−
Low temperature0.50−0.02−0.18+0.01+0.19+0.07−0.23−
Charge retention0.17−0.03−0.31+0.30+0.48−0.01−0.21−
Peak power0.29−0.25+0.000.23−0.56+0.17+0.00
Cycle life0.90−0.000.31+0.02+0.07+0.000.30−
Table 8. Correlation factors (R2) between the phase stoichiometry/structure and various gaseous phase, half-cell, and sealed cell properties and. Plus (+) and minus (−) signs after the numbers indicate positive and negative correlations, respectively. Significant correlations with R2 ≥ 0.6 are highlighted in red. EC full capacity is the discharge capacity measured with a 5 mA·g−1 current density.
Table 8. Correlation factors (R2) between the phase stoichiometry/structure and various gaseous phase, half-cell, and sealed cell properties and. Plus (+) and minus (−) signs after the numbers indicate positive and negative correlations, respectively. Significant correlations with R2 ≥ 0.6 are highlighted in red. EC full capacity is the discharge capacity measured with a 5 mA·g−1 current density.
PropertiesAB3A2B7A5B19HexagonalRhombohedral
GP maximum capacity0.75−0.60+0.000.44+0.46−
GP reversible capacity0.40−0.17+0.36+0.29+0.21−
Equilibrium pressure0.52+0.68−0.000.93−0.85+
PCT slope factor0.02+0.01+0.03+0.45+0.41−
PCT hysteresis0.61−0.46+0.21−0.000.00
H0.03+0.17−0.01+0.54−0.43+
S0.02−0.01−0.000.26−0.18+
EC high-rate capacity0.60−0.63+0.01+0.84+0.78−
EC full capacity0.61−0.68+0.000.86+0.81−
HRD0.49−0.40+0.14+0.70+0.59−
Diffusion constant, D0.29−0.11+0.29+0.18+0.11−
Exchange Current, Io0.56−0.34+0.01−0.02+0.01−
−40 °C resistivity, R0.000.10+0.72−0.04+0.08−
−40 °C capacitance, C0.53+0.71−0.01+0.90−0.88+
RC product0.26+0.08−0.46−0.23−0.16+
High rate0.34−0.09+0.13+0.02+0.00
Low temperature0.42−0.13+0.10+0.11+0.09−
Charge retention0.17−0.32+0.38−0.000.04+
Peak power0.05−0.02−0.53+0.11−0.16+
Cycle life0.58−0.24+0.05+0.000.03+
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