Wenfeng Wang ,Xiaoxue Liu ,Lu Zhang,∗ ,Shuang Zhang ,Wei Guo ,Yumeng Zhao ,Hongming zhang,Yuan Li,Shumin Han,∗

aState Key Laboratory of Metastable Materials Science and Technology,Yanshan University,Qinhuangdao 066004,PR China

b School of Environmental and Chemical Engineering,Yanshan University,Qinhuangdao 066004,PR China

Abstract Rare earth–Mg–Ni-based alloys with superlattice structures are new generation negative electrode materials for the nickel metal hydride batteries.Among them,the novel AB4-type superlattice structure alloy is supposed to have superior cycling stability and rate capability.Yet its preparation is hindered by the crucial requirement of temperature and the special composition which is close to the other superlattice structure.Here,we prepare rare earth–Mg–Ni-based alloy and study the phase transformation of alloys to make clear the formation of AB4-type phase.It is found Pr5Co19-type phase is converted from Ce5Co19-type phase and shows good stability at higher temperature compared to the Ce5Co19-type phase in the range of 930–970 °C.Afterwards,with further 5 °C increasing,AB4-type superlattice structure forms at a temperature of 975 °C by consuming Pr5Co19-type phase.In contrast with A5B19-type alloy,AB4-type alloy has superior rate capability owing to the dominant advantages of charge transfer and hydrogen diffusion.Besides,AB4-type alloy shows long lifespan whose capacity retention rates are 89.2% at the 100th cycle and 82.8% at the 200th cycle,respectively.AB4-type alloy delivers 1.53 wt.% hydrogen storage capacity at room temperature and exhibits higher plateau pressure than Pr5Co19-type alloy.The work provides novel AB4-type alloy with preferable electrochemical performance as negative electrode material to inspire the development of nickel metal hydride batteries.

Keywords: Nickel metal hydride batteries;Hydrogen storage alloys;AB4-type superlattice structure;Electrochemical performance;Kinetics properties.

1.Introduction

Hydrogen energy have been being laser-focused owing to the significant role in viable solution aimed to worldwide climate change and atmospheric pollution [1–4].Its storage and utilization drive the research of hydrogen storage materials and rechargeable batteries [5–7].Notably,Mg-containing alloys are attractive candidate for solid-state hydrogen storage and can be further applied in electrochemical systems where the hydrogen atoms occupy the interstitial forming metal hydride(MH)[1,7–10].A vital application of Mg-containing alloy is as the negative electrode material for nickel metal hydride(Ni/MH)batteries which have been developed as mature battery technology applied in power tools,modern military devices and alternative energy generation systems,especially in hydride electric vehicles(HEVs)[8,11,12].Ni/MH batteries have the advantages of high safety,environmental friendliness,and good performance at low temperature [5].Since 1999,they have been widely used in commercialized HEVs made by Toyota,Honda,Ford,etc.[8].

Fig.1.Schematic illustration of a nickel metal hydride battery(a)and radar chart comparison of AB2-type,AB5-type and RE–Mg–Ni-based hydrogen storage alloys(b).

A Ni/MH battery is composed of a Ni(OH)2positive electrode,metal MH negative electrode and an alkaline electrolyte(KOH solution)(Fig.1(a)).The electrochemical performance of Ni/MH batteries strongly depends on the MH negative electrode materials which can reversibly storage hydrogen [5,12].So far,various types of hydrogen storage alloys have been developed as negative electrode materials of Ni/MH batteries,such as AB5-type,AB2-type,AB3-type,A2B7-type,ABtype,A2B-type and Mg-based alloys [7,12–15].Mg-based alloy has high capacity and relatively low cost but undergoes poor hydriding/dehydriding kinetics at room temperatures [12,13].AB5-type alloys were firstly applied as the commercialized negative materials with good stability during hydrogen absorption/desorption cycling [1,14].Nevertheless,it suffers from low discharge capacity [15].AB2-type alloys with typical C14-type and C15-type structures have been developed as the second-generation electrode materials [16,17].In contrast with AB5-type alloy,AB2-type alloys are capable to absorb a larger amount of hydrogen leading to higher discharge capacity,whereas they are less competitive owing to the poor cycling stability caused by hydrogen-induced amorphization [18,19].Notably,the alloys with the combination of [AB5]and [A2B4]subunits are expected to have promising performance.New family of rare earth(RE)–Mg–Ni-based superlattice hydrogen storage alloys composed of various [AB5]and [A2B4]subunits are receiving growing attention in recent years.They can meet the requirement of overall electrochemical performance,showing high discharge capacity,long durability,good rate capability,admirable discharge capacity at low temperature and low self-discharge(Fig.1b)[12,19–24].The RE–Mg–Ni-based alloys can be expressed as ABy(2

Above RE–Mg–Ni-based superlattice structure alloys behave differently in electrochemical and hydrogen storage properties.In case of AB3-type alloy,it shows good activation performance and high discharge capacity.Kohno et al.reported that a La0.7Mg0.3Ni2.8Co0.5alloy with a AB3-type structure delivered a maximum discharge capacity of 410 mAh g–1higher than that of A2B7-type La0.75Mg0.25Ni3.0Co0.5alloy(390 mAh g–1)[26].Nevertheless,the AB3-type alloy exhibits poor durability and rate capability.Actually,with the increasing value ofn,the discharge capacity of ABysuperlatttice structure hydrogen storage alloys decrease because[A2B4]subunits have larger hydrogen absorption ability in contrast to [AB5]subunits.Otherwise,the cycling stability and rate capability improve whennincreases.In terms of cycling stability,it is affected by the pulverization and amorphization of the alloys coming from the lattice strains inside the crystal structure,where the volume mismatch between the subunits leads to strains because of the desynchronized volume expansion and contraction of [AB5]and [A2B4]during hydrogenation/dehydrogenation [27–29].Liu et al.found that a rising ratio of [AB5]/[A2B4]along with the improved accommodation ability of volume mismatch between [AB5]and [A2B4]subunits lead to the increased cycling stability[30].Similar conclusion was derived in the Sm–Mg–Ni system[31].In addition,the stability of hydride weakens with increasing [AB5]/[A2B4],indicated by the increasing hydrogen desorption pressure with the following order:La2MgNi9

Here,to promote the application of AB4-type hydrogen storage alloy as the negative electrode materials of Ni/MH batteries with desired performance,we design an AB4-type alloy with multi-element,La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16alloy,with a respect to reach a superior cycling stability and high rate dischargeability at a reasonable cost.The La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16alloys were prepared by an induction melting method followed by annealing treatment during which the formation mechanism of AB4-type superlattice structure was analyzed.The electrochemical performance and hydrogen storage property of the alloys with different phases were studied.Besides,the kinetics characteristics tests were performed to reveal the effect of AB4-type on rate capability.

2.Experimental

2.1.Preparation of the alloy

The La–Sm–Nd–Mg–Ni–Al-based alloys were prepared by an induction melting method with the protection of argon at 1100 °C.The purities of raw material mixture including La,Sm,Nd,Mg,Ni and Al are 99.9%.The obtained as-cast blocks were annealed at 900,930,960,970,and 975 °C,respectively.The annealing treatment was operated in tube furnace under 0.02 MPa argon atmosphere.The heating process contains two stages with adopted heating rates of 4 °C min–1and 1 °C min–1below and up 600 °C,respectively.All the alloy samples were maintained for 12 h at the target temperature and cooled naturally to room temperature.

2.2.Composition and structure analysis

To analyze the element content of the alloys,the chemical composition test was operated by the inductively coupled plasma(ICP)using an iCAP-6300 Inductively Coupled Plasma-Atomic Emission Spectrometer.X-ray diffraction(XRD)measurement was performed using a Rigaku D/Max-2500/PC X-ray diffraction(Cu Kαradiation)with the alloy powder(<400 mesh).The XRD data was collected within the range of 5–80° with a scanning rate of 2° min–1.The structure refinement was performed by Rietveld method using RIETICA software to get the information of crystal structure of the alloys,during whichRp,RwpandSwere adopted to evaluate the quality of refinement.To observe the morphologies of the alloy,the alloys were polished and then etched with 20%HF solution for 10 s.Scanning electron microscopy(SEM)was conducted by S-3400 with energy dispersive spectrometer(EDS)to analyze the phase composition and the distribution of element.

2.3.Electrochemical and kinetics properties

The electrochemical properties and kinetics characteristics were studied by half-cell system and three-electrode system,respectively.The counter electrode was Ni(OH)2/NiOOH electrode.The working electrode was made by the mixture of alloy powder(200–400 mesh)and carbonyl nickel in a weight ratio of 1:5.Hg/HgO electrode was adopted as the reference electrode for the three-electrode system.The electrolyte is KOH(6 M)solution.To make the electrodes fully activated,the system was charged/discharged at a current density of 70 mA g–1with a cut-off voltage of 1.0 V in the half-cell.After activation,the cycling stability test was conducted with a charge current density of 750 mA g–1for 1.5 h and a discharge current density of 70 mA g–1.The discharge capacity of the system was recorded every 20 cycles.The high rate dischargeability was measured at various discharge current densities including 350,700,1050 and 1400 mA g–1,respectively.The electrochemical pressure-composition(P-C)test was performed as our previous study [35].The kinetics property research consisting of linear polarization,anode polarization and potential-step measurement were operated by ZF-9 potentiostat.The testing potential range of linear polarization and anode polarization is−7 to +7 mV and 0–1500 mV,respectively,vs.open circuit potential.Being fully charged,the potential-step measurement was operated with a potential step of 500 mV.Derived from the kinetics measurements,the exchange current density(I0),limiting current density(IL)and hydrogen diffusion coefficient(D)were obtained [36].

2.3.Hydrogen storage and thermodynamics properties

The hydrogen storage property of the alloy was researched by a Sieverts-type apparatus manufacture(Suzuki Shokan,Japan).The alloy was held in a stainless-steel reactor for hydrogenation/dehydrogenation several times to be fully activated.The activation was performed under 5.0 MPa H2atmosphere at room temperature.Pressure-compositiontemperature(P-C-T)curves were obtained at the temperatures of 40,60 and 80 °C,respectively.The enthalpy change(∆H)and entropy change(∆S)were calculated based on the Van’t Hoff equation to evaluate the thermodynamics characteristics.

3.Results and discussion

3.1.Composition and crystal structure

The La–Sm–Nd–Mg–Ni–Al-based alloys were prepared by an induction melting method.The obtained as-cast alloy was annealed with increasing annealing temperatures as the phase peritectic reaction happened at various temperatures.ICP results show that the chemical formula of the as-cast alloy is La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16and the annealed samples are similar indicating the effective seal during annealing treatment.Fig.2 shows the XRD patterns of the as-cast alloy and annealed alloys which exhibit the transition of phase structures.The as-cast alloy has multiphase composition including LaNi5-type and MgCu4Sn-type none-superlattice structures,and a Ce5Co19-type superlattice structure(Fig.2a and c).After annealing at 900 °C,the LaNi5-type and MgCu4Sn-type structures remain with reduced phase contents of 18.9 wt.%and 5.6 wt.%,respectively(Table 1).A new phase named Ce2Ni7-type superlattice appears resulting from the peritectic reaction of LaNi5and MgCu4Sn.At the meantime,the content of Ce5Co19-type superlattice structure grows to 62.1 wt.%.The increasement of Ce5Co19can be attributed to the transformation of LaNi5and Ce2Ni7[37].Keep increasing the annealing temperature to 930 °C,we observe the disappearance of LaNi5and Ce2Ni7which all transform to Ce5Co19(42.1 wt.%)through peritectic reactions.The phase abundance of the Ce5Co19increases to 78.8 wt.%.Notably,the characteristic peak located atca.32.2° indicates the appearance of a Pr5Co19-type structure.In the range of 930–970 °C,the relative intensity for the(109)(2θ=32.2°)gets stronger revealing the increased content of Pr5Co19(JCSD 42–1198)(Fig.2a),while the peak intensity of(1013)(2θ=31.6°)and(0018)(2θ=32.9°)corresponding to the Ce5Co19(JCSD 26–1084)weaken with the increasing temperature.Indeed,Ce5Co19-type structure is stable at relatively low temperature,compared with Pr5Co19-type structure.When the temperature goes to 970°C,total transformation accomplishes and the Pr5Co19-type single-phase alloy is achieved(Fig.2a-c and Fig.3).The alloy annealed at 975 °C consists of Pr5Co19(6.2 wt.%),Ce2Ni7(15.3 wt.%)and AB4(78.5 wt.%)phases.New phase transformation occurs with just 5 °C increasing,where the Pr5Co19-type phase partially decomposes at a high temperature leading to the reappearance of Ce2Ni7,and then the undecomposed Pr5Co19rapidly transforms to the new AB4-type superlattice structure.As shown in Fig.2a,the characteristic peaks atca.29.0°,31.3° and 32.4° belong to the AB4-type structure.Refinement result shows the lattice parameters of the thAB4-type phase are 5.0220 ˚A and 60.485 ˚A,corresponding toaandc,respectively(Table 1).

Table 1 Phase composition and lattice parameters of as-cast alloy and annealed samples of RE–Mg–Ni-based alloys.

Table 2 High rate discharge performance and kinetics properties of La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16 as-cast alloy and annealed samples at 900,930,970 and 975 °C.

Scanning electron microscopy(SEM)images show the phase composition of micrograph of the as-cast alloy and annealed samples,which is identified based on the EDS results of selected area.The as-cast alloy is composed of three phases which are A5B19(A),MgCu4Sn(B)and LaNi5(C)(Fig.4a).Combined with the XRD and refinement results,the A5B19phase in here is the Ce5Co19-type phase.For the alloy annealed at 900 °C,the new Ce2Ni7-type phase is classified as D(Fig.4b).Nevertheless,we cannot separate the Ce5Co19-type and Pr5Co19-type structures of samples annealed at 930 and 960 °C since they are allotrope with same composition(Fig.4c and d).We observe that the alloy annealed at 970 °C has uniform color on the alloy surface indicating the obtained single phase(Fig.4e),which is identified by the evenly distributed La,Sm,Nd,Mg,Ni and Al elements from the EDX maps(Fig.4g-l).With regards to the sample annealed at 975 °C,three types of phases with various colors are observed.The A,D and E regions are corresponding to the A5B19-type,Ce2Ni7-type and AB4-type phases,respectively.

Fig.2.XRD patterns of La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16 alloys in the 2 theta range of 10–80°(a)and partial range of 34–46°(b).The phase abundance of as-cast alloy and annealed alloys at 900,930,960,970 and 975 °C,respectively(c).

Fig.3.Rietveld refinement patterns of as-cast alloy(a)and samples annealed at 900 °C(b),930 °C(c),960 °C(d),970 °C(e)and 975 °C(f). Rp, Rwp and S are used to check the fitting quality of refinement.Vertical bars below the pattern show the positions of all possible reflection peaks of the LaNi5,Ce5Co19 and MgCu4Sn phases for(a),LaNi5,Ce5Co19,MgCu4Sn and Ce2Ni7 phases for(b),Ce5Co19 and Pr5Co19 phases for(c),Pr5Co19 and Ce5Co19 for phases(d),Pr5Co19 phase for(e),and Pr5Co19, AB4,and Ce2Ni7 phases for(f).

3.2.Hydrogen storage characteristics

Hydrogen storage alloys are required to absorb/desorb hydrogen at appropriate temperature conditions.The pressurecomposition-temperature(P-C-T)curves show the transformation between H-dissolved solid solution phase(αphase)and full hydride(βphase)(Fig.5a).Only one plateau is observed in theP-C-Tcurves.The maximum hydrogen storage capacity of the alloy is 1.53 wt.% at room temperature.The hydrogen absorption/desorption mid-point pressure of plateau increases with the growing temperature owing to the weaken stabilization of hydride at a higher temperature.The hydrogen absorption platform pressure of the AB4-type alloy at 40 °C is 0.026 MPa and it goes to 0.096 MPa when the temperature increases to 80 °C(Fig.5a).The hysteresis can be used to evaluate the resistance ability of pulverization in the process of hydrogen absorption and desorption [38].It is calculated based on the mid-point pressure of hydrogen absorption/desorption plateau.The hysteresis value of AB4-type La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16alloy at 40 °C is 0.21,indicating good resistance to the pulverization.We calculated the enthalpy change(∆H)and entropy change(∆S)using Van’t Hoff equation based on the pressure-composition isotherms to analyze the thermodynamics properties of the alloy(Fig.5b).The absolute values of ∆Sare 65.7 and 59.7 J(K mol)–1H2,corresponding to hydrogenation and dehydrogenation,respectively.The calculated ∆H(absolute value)of hydrogenation is 23.6 kJ mol–1H2,higher than that of the AB4-type ternary La–Mg–Ni-based alloy(22.0 kJ mol–1H2)[39].Besides,we measured the PCT curve at 80 °C of the A5B19-type single-phase alloy annealed at 970 °C as shown in Fig.5c.It is observed that the hydrogen absorption midpoint pressure of the AB4-type alloy is 0.104 MPa higher than that of the A5B19-type alloy(0.082 MPa),indicating less stable hydride of the AB4-type alloy which is beneficial to the hydrogen diffusion.In consideration of the hydrogen storage capacity,the A5B19-type alloy exhibits superior performance.The hydrogen storage capacity of the A5B19-type alloy is 1.42 wt.% while that of the AB4-type alloy is 1.35 wt.%at 80 °C.

Fig.4.Scanning electron microscopy(SEM)images of La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16 alloys for the as-cast sample and annealed samples at 900,930,960,970 and 975 °C(a-f).EDS mapping of La,Sm,Nd,Mg,Ni and Al(g-i)elements for the alloy annealed at 970 °C.

3.3.Electrochemical characteristics

The electrochemical measurement shows the alloy electrodes have good activation properties that the samples are activated rapidly at the 1stcycle(Fig.6a).The maximum electrochemical discharge capacity of the as-cast alloy is 347.9 mAh g–1.The maximum discharge capacity of annealed alloy all increase which is in the range of 356.4 -368.4 mAh g–1.The increasement of maximum discharge capacity is related with the improvement of the homogeneity in distribution of alloy components.The low capacity of it is consistent with the short plateau region in the pressure-composition(PC)isotherms(Fig.6b).The electrochemical desorption(PC)curve of the as-cast alloy electrodes has two plateaus which areca.0.03 MPa and 0.30 MPa,respectively.The second plateau is related to the CaCu5-type structure with high hydrogen absorption/desorption pressure.In addition,the CaCu5-type has low electrochemical hydrogen storage capacities making less contribution to the capacity of as-cast alloy electrode.Compared with the alloy annealed at 900 °C,the alloy annealed at 930 °C has lower desorption pressure owing to the increasing Ce5Co19-type phase(Fig.6b).The electrochemical desorption pressure of AB4-type alloy electrode(∼0.10 MPa)is higher than Pr5Co19-type alloy electrode(∼0.09 MPa)which is in consistent of the result of gas-solid pressure-composition-temperature(P-C-T)curves.

Fig.5.Pressure-composition-temperature(PCT)curves(a)and Van’t Hoff plots(b)of the AB4-type La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16 alloy annealed at 975 °C.PCT curves measured at 80 °C of alloy samples annealed at 970 °C and 975 °C,respectively(c).

The cycling durability of alloys is strongly related to the phase composition.It is observed the discharge capacity of the as-cast alloy reduces quickly.The poor cycling stability of it associates with its multiphase content.As previous study reports,multiphase alloy bears large pressure of lattice strains among the phase boundaries resulting from the unsynchronized volume change during hydrogen absorption/desorption[9,40].Thus,the pulverization leading to capacity degradation is severer in multiphase alloy than the single-phase alloy.The calculated lattice strain of the as-cast alloy is 0.253%,much larger than that of the annealed alloy which is in the range of 0.089–0.153%(Fig.6c).The results indicate the as-cast alloy has less stable structure resulting in the short cycling life.In contrast with the as-cast alloy electrode,the cycling stabilities of the alloy electrodes are enhanced.The capacity retention rates of the annealed samples after cycling 100 cycles are 88.1%,89.1% and 89.3%,corresponding to annealed at 900,930 and 970 °C(Fig.6a).It is observed the cycling stability is improved with the increasing content of Pr5Co19-type phase.The higher capacity retention rate of Pr5Co19-type single phase alloy is related to its stable microstructure with less lattice strain of 0.089%(Fig.6c).For the alloy annealed at 975 °C with main AB4-type phase,its cycling life is 89.2%at the 100thcycle and 82.8% at the 200thcycle.Although the alloy annealed at 975 °C is composed of multiphase,it has good cycling stability owing to the existence of AB4-type superlattice structure.It is known that the superlattice structure is staked by [AB5]and [A2B4]subunits,while the stabilities of two types of subunits are different that the [AB5]-type subunit has higher stability than and Laves-type [A2B4]subunit [14].Previous study showed that the structural stability is enhanced by growing ratio of [AB5]/[A2B4][30].Hence,the AB4-type phase has superior structural stability leading to the promoted cycling durability.Nevertheless,the capacity retention rate of AB4-type alloy(82.8%)is smaller than the Pr5Co19-type single phase alloy(83.5%)(Fig.6a).The reason is that the structural stability of AB4-type multiphase alloy annealed at 975 °C is negatively affected by the remaining Pr5Co19-type and extra A2B7-type phases.It is revealed by the higher value of lattice strain(0.118%)(Fig.6c).

Fig.6.Discharge capacity curves vs.different cycle numbers(a),electrochemical pressure-composition(P-C)curves(b),lattice strains(c)and discharge curves at−40 °C(d)of the La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16 as-cast alloy and annealed samples at 900,930,970,and 975 °C.

As shown in Fig.6d,the La–Sm–Nd–Mg–Ni–Al-based alloy electrodes could give a discharge capacity over 100.5 mAh g–1even at low temperature of−40 °C.Among alloy electrodes with various phase compositions,the AB4-type alloy has superior performance that it delivers 138.5 mAh g–1at−40 °C with a discharge voltage of 0.8 V,indicating the high tolerance to the low temperature.

3.4.Rate performance and kinetics properties

The Ni/MH batteries require high performance of rate capability which has close connection with phase composition of the alloy electrode.It is found the rate performance of superlattice structure alloys is arranged in the order as following:AB3-typeCe5Co19-type>Pr5Co19-type.The results indicate alloy electrodes mainly consisting of A5B19-type superlattice structure have poorer performance than AB4-type alloy.The results indicate the charge transfer rate of AB4-type alloy is faster than the other annealed alloys.The limiting density(IL)and hydrogen diffusion coefficient(D)are used to evaluate the hydrogen diffusion in alloy bulk(Fig.7c and d).In terms ofIL,the AB4-type alloy electrode has highest value(3686.66 mA g–1)in comparison with other annealed alloys with main A5B19-type phase(Table 2).Hydrogen diffusion coefficient values are 1.43,1.59,1.44 and 1.65×10−10cm2s–1,respectively,corresponding to the samples annealed at 900,930,970 and 975 °C.It is noted the AB4-type alloy annealed at 975 °C have quicker hydrogen diffusion rate than the other annealed samples which is in consistent with its less stable hydride.Hence,in combination of the charge transfer and hydrogen diffusion,the AB4-type alloy electrode has superior rate capability than A5B19-type superlattice structure alloy.

Fig.7.High rate dischargeability performance(a),linear polarization curves(b),anode polarization curves(c)and semi-logarithmic curves of anode current vs.time(d)of La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16 as-cast alloy and annealed samples at 900,930,970,and 975 °C.

4.Conclusion

In this paper,we prepared a AB4-type multi-element La0.64Sm0.11Nd0.07Mg0.18Ni3.54Al0.16RE–Mg–Ni-based hydrogen storage alloy and studied its formation during the annealing treatment process.It was found a Pr5Co19-type single phase alloy formed at 970 °C transformed by a Ce5Co19-type phase in a annealing temperature range of 930–970 °C.Further increasing the temperature,the new characteristic peaks located atca.29.0°,31.3° and 32.4° were observed indicating that the AB4-type phase formed resulting from the peritectic reaction of the Pr5Co19-type phase at higher annealing temperature of 975 °C.The AB4-type alloy delivers 1.53 wt.% hydrogen storage capacity at room temperature and has higher mid-point pressure than that of the Pr5Co19-type alloy at 80 °C.The capacity retention rate of the AB4-type alloy is 89.2% after 100 cycles and remained 82.8% even after 200 cycles.The long cycling durability of the AB4-type RE–Mg–Ni-based alloy is beneficial from the structure stability with less microstructure lattice strains.The rate performance of RE–Mg–Ni-based superlattice structure alloy was strongly improved by the AB4-type structure.The HRD value of AB4-type alloy was 34.1% at the discharge current density of 1400 mA g–1,higher than the A5B19-type alloy.The superior rate performance of AB4-type RE–Mg–Ni-based alloy is attributed to the faster charge transfer and hydrogen diffusion rate.The work is aimed to promote the development of hydrogen storage alloys with characteristic properties.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Hebei Province(Nos.E2019203414,E2020203081 and E2019203161),the National Natural Science Foundation of China(Nos.51701175 and 51971197),the Innovation Fund for the Graduate Students of Hebei Province(No.CXZZBS2020062)and the Doctoral Fund of Yanshan University(No.BL19031).