简介概要

Microstructure and electrochemical properties of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1（x=0,0.1,0.2,0.3）alloys

来源期刊：Rare Metals2017年第8期

论文作者：Na Zhou Wen-Bo Du Zhao-Hui Wang Ke Liu Shu-Bo Li

文章页码：645 - 650

摘要：The present study aims to improve electrochemical properties of the La-Mg-Ni-based hydrogen storage alloys through partial substitution for La by mischmetal（MM）.The La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1（x=0,0.1,0.2,0.3）alloys were prepared by inductive melting,and their phase structures and electrochemical properties were studied by X-ray diffraction（XRD）,scanning electron microscope（SEM）,energy-dispersive X-ray spectrometry（EDX）and electrochemical tests.Results show that the alloys mainly consist of La₂Ni₇-type phase,La₅Ni₁₉-type phase,LaNi₅-type phase and LaNi₃-type phase.The addition of MM does not change the phase compositions,while it leads to more uniform phase distribution and obviously promotes the formation of La₂Ni₇-type phase which possesses favorable electrochemical properties.Electrochemical studies indicate that the substitution for La by MM could effectively improve the high rate dischargeability（HRD）of the alloy electrode,and the optimal value of HRD₁₅₀₀(HRD at 1500 mA·g^-1)increases from 40.63%（x=0）to 60.55%（x=0.3）.Although the activation properties of the alloy electrodes keep almost unchanged,both the maximum discharge capacity(C_max)and the cycling stability are significantly improved by MM addition.

详情信息展示

稀有金属(英文版) 2017,36(08),645-650

Microstructure and electrochemical properties of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1(x=0,0.1,0.2,0.3)alloys

Na Zhou Wen-Bo Du Pei-Long Zhang Yong-Guo Zhu Zhao-Hui Wang Ke Liu Shu-Bo Li

College of Materials Science and Engineering, Beijing University of Technology

Whole Win Materials Science and Technology Co.,Ltd

收稿日期：27 January 2015

基金：financially supported by State Key Laboratory of Advanced Metals and Materials(No.2011-ZD06);Beijing Municipal Science and Technology Commission (No. Z131100003213019);the RiXin Talents Plan of Beijing University of Technology (2014-RX-L07);Beijing Natural Science Foundation (No.2144043);

Microstructure and electrochemical properties of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1(x=0,0.1,0.2,0.3)alloys

Na Zhou Wen-Bo Du Pei-Long Zhang Yong-Guo Zhu Zhao-Hui Wang Ke Liu Shu-Bo Li

College of Materials Science and Engineering, Beijing University of Technology

Whole Win Materials Science and Technology Co.,Ltd

Abstract：

The present study aims to improve electrochemical properties of the La-Mg-Ni-based hydrogen storage alloys through partial substitution for La by mischmetal(MM).The La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1(x=0,0.1,0.2,0.3)alloys were prepared by inductive melting,and their phase structures and electrochemical properties were studied by X-ray diffraction(XRD),scanning electron microscope(SEM),energy-dispersive X-ray spectrometry(EDX)and electrochemical tests.Results show that the alloys mainly consist of La₂Ni₇-type phase,La₅Ni₁₉-type phase,LaNi₅-type phase and LaNi₃-type phase.The addition of MM does not change the phase compositions,while it leads to more uniform phase distribution and obviously promotes the formation of La₂Ni₇-type phase which possesses favorable electrochemical properties.Electrochemical studies indicate that the substitution for La by MM could effectively improve the high rate dischargeability(HRD)of the alloy electrode,and the optimal value of HRD₁₅₀₀(HRD at 1500 mA·g^-1)increases from 40.63%(x=0)to 60.55%(x=0.3).Although the activation properties of the alloy electrodes keep almost unchanged,both the maximum discharge capacity(C_max)and the cycling stability are significantly improved by MM addition.

Keyword：

Hydrogen storage alloy; Microstructure; Discharge capacity; High rate dischargeability; Cycling stability;

Author： Na Zhou,e-mail:zhouna_0220@126.com; Wen-Bo Du,e-mail:duwb@bjut.edu.cn N.Zhou;

Received： 27 January 2015

1 Introduction

As we know,hydrogen storage alloys can be used not only as solid medium to absorb hydrogen gas but also as negative electrodes in nickel metal hydride (Ni-MH) batteries [ 1, 2] .Several types of hydrogen storage alloys have been studied extensively,including AB_5-type alloys,AB_2-type alloys,AB-type alloys,A₂B-type alloys and V-based solid solution alloys.However,all of the alloys mentioned above have their inherent disadvantage,such as the low energy density of AB_5-type alloys (at present the commercialized materials are mainly LaNi_5-type alloys whose capacity reaches 300-330 mAh·g^-1),the poor activation capability of the AB_2-type Laves phase as well as V-based solid solution electrode alloys and the poor cycle stability of the A₂B-type alloys [ 3, 4, 5, 6] .Therefore,new-type hydrogen storage alloys with good overall electrochemical properties are urgently needed to be developed.

Recently,La-Mg-Ni-based alloys were reported as one of the most promising candidates owing to their high discharge capacities and low production costs in spite of their poor cycling stabilities [ 2] .Several methods,such as elemental substitution [ 7, 8, 9] ,proper annealing [ 10, 11, 12] ,rapid quenching [ 13] ,composite alloying [ 14] and surface treatment [ 15] ,have been applied to improve the poor cycling stability which hindered their practical application in Ni-MH batteries.The elemental substitution was considered to be an effective method among these methods,and many elements have been confirmed that they could improve the overall electrochemical properties,especially the cycling stability.So far,both A side (commonly Labased) and B side (commonly Ni-based) of the La-Mg-Nibased alloys have been investigated by multi-alloying,and the useful and common elements which are substituted for A side are Ce [ 16] ,Pr [ 17] ,Nd [ 18, 19] ,Zr [ 20] ,etc.,and B side are Co [ 21, 22, 23] ,Cr [ 24] ,Al [ 25, 26] ,Fe [ 27, 28] ,Mn [ 29] ,etc.

Based on these investigations and some preliminary work,the alloys La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 (x=0,0.1,0.2,0.3) were designed by the substitution for La by mischmetal (MM) in order to obtain a kind of La-Mg-Nibased alloy with good overall electrochemical properties in the present investigation.The effects of MM addition on structural evolution and electrochemical properties of the above alloy electrodes were circumstantially studied and discussed.

2 Experimental

La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 alloys were prepared by inductive levitation melting and re-melted two times for homogeneity under argon atmosphere.A kind of Mg-Ni master alloy was used instead of Mg to reduce Mg loss,and MM was composed of 24.71 wt%La,53.26 wt%Ce,5.83 wt%Pr,15.98 wt%Nd and 0.22 wt%impurity.The purities of other constituent metal elements (La,Mg-Ni,Ni,Co,Al) were 99.5%.In addition,excessive Mg-Ni,La and MM were added to compensate for evaporative loss during the melting process.The cast ingots were prepared by pouring the melt into a copper mold cooled by water.The ingots were annealed at 1173 K for 8 h.The chemical compositions of the alloys were examined by X-ray fluorescence (XRF,Magic PW2403) and inductively coupled plasma (ICP,IRIS Intrepid ER/S) analysis.

The phases of the alloys were determined by X-ray diffractometer (XRD,D8ADVANCE) with Cu Kαradiation.The prepared ingots were mechanically crushed and ground into fine powders,and the 75-μm powders were used for XRD analysis.

The microstructures of the alloys were examined by a FEI Quanta 650 scanning electron microscope (SEM).The compositions of the alloys in microregion were examined by an energy-dispersive X-ray analyzer (EDX).The specimens were laid in epoxy resin for polishing before SEM examination.

The hydrogen storage alloy electrodes were prepared by mixing the alloy powder and the nickel powder with a proportion of 1:4 in weight,and then the mixture was coldpressed under a pressure of 30 MPa into a pellet with a diameter of 10 mm and a thickness of 1 mm.The total mass of the alloy electrode was about 0.6 g.A electrode open cell,consisting of a metal hydride electrode and two NiOOH/Ni(OH)2 counter electrodes,was used for testing the electrochemical performances of the experimental alloy electrodes.

The electrolyte was a mixture of 6 mol-L^-1 KOH and17.5 g.L^-1 LiOH·H₂O solution,and the temperature was controlled at 25℃.The electrode pellets were dipped into the solution for 24 h in order to wet fully the electrodes before the electrochemical measurement.The charging/discharging characteristics were measured by LANHE CT2001A battery testing instrument.

3 Results and discussion

3.1 Micros tructure

XRD patterns of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 (x=0,0.1,0.2,0.3) alloys are shown in Fig.1.It is obviously found that all of the alloys have multi-phase structures.They include four major phases,i.e.,La₂Ni_7-type phase,La₅Ni_19-type phase,LaNi_5-type phase and LaNi_3-type phase.In addition,it can also be found that the intensity of diffraction peaks of the La₂Ni_7-type phase is becoming stronger than that of the others after substitution of MM as shown in Fig.1,though the phase compositions does not change by MM.

Figure 2 shows the backscattered electron (BSE) images of the alloys observed by SEM.EDX was used to confirm the phase compositions marked by A,B,C and D in Fig.2.EDX results indicate that the phases corresponding to A,B,C and D are LaNi_5-type phase,La₂Ni_7-type phase,LaNi_3-type phase and La₅Ni_19-type phase,respectively,which are in agreement with the results of XRD.

As shown in Fig.2,the addition of MM changes the phase morphology and distribution,although it does not change the phase composition and structures of the alloys.It can be seen that the phases in the alloys without MM addition (Fig.2a,c) are larger and they display poor distribution homogeneity,whereas the alloy with MM addition (Fig.2b,d) shows smaller and more homogeneous phases.This result confirms that MM addition can modify the phase morphologies which are also the important factor to enhance the cycling stability and other electrochemical properties of the electrode.By combining XRD and SEM results shown in Figs.1 and 2,it can be found that the amount of main La₂Ni_7-type phase increases with the increase of MM addition.The contents of La₅Ni_19-type phase,LaNi_5-type phase and LaNi_3-type phase which are the main phases for the alloy without MM addition decrease with the increase of MM addition.It suggests that MM addition promotes the formation of La₂Ni_7-type phase.

Fig.1 XRD patterns of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 (x=0,0.1,0.2,0.3) alloys

Fig.2 BSE images of SEM images of La_0.8-xMM_xMg_0.2Ni_3.1-Co_a3Al_o.1 alloys:a,c x=0;b,d x=0.3.A,LaNi_5-type phase;B,La₂Ni_7-type phase;C,LaNi_3-type phase;D,La₅Ni_19-type phase

3.2 Electrochemical properties

3.2.1 Activation properties and discharge properties

The activation properties and the maximum discharge capacities (C_max) of the electrodes were measured at a charge-discharge current density of 60 mA·g^-1.As we know,the activation capability is characterized by the number of charge-discharge cycles required for attaining the greatest discharge capacity through a charge-discharge cycle at a constant current density.The fewer the number of charge-discharge cycles is,the better the activation performance is.

Figure 3 and Table 1 show the activation properties (N^*)and CCmax of La_0.8xMM_xM_0.2Ni_3.1Co_0.3Al_0.1 (x=0-0.3)alloy electrodes.From Table 1 and Fig.3,it can be found that all the alloy electrodes are fully activated within two cycles,which is ascribed to the multi-phase compositions of the alloys [ 12] .In the mean time,it indicates that all the alloys display excellent activation performances.

Figure 3b shows the discharge capacities of the La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_o.1 alloys as a function of MM content.It indicates that C_max of the alloys increases from 341.8to 3 8 1.2 mAh·g^-1 with MM substitution increasing from 0 to0.3.In Ref. [ 30] ,it was reported C_max of La_0.8Mg_0.2Ni_3.4x-yCo_0.3Mn_xAl_y alloys was 350 mAh·g^-1.Compared with the results shown in Ref. [ 30] ,it is evident that MM addition is effective to increase C_max of the alloy electrodes,and the optimal value of C_max is obtained as x=0.3 in the present investigation.Wang et al. [ 31] reported that the formation of La₂Ni_7-type phase could play a dominant role in discharge capacity of hydrogen storage alloys.From analysis of Fig.2,it is known that MM addition promotes the increase of La₂Ni_7-type phase abundance.Therefore,MM addition is beneficial to enhance discharge capacity of the alloys.

Fig.3 Activation curves of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 alloy electrodes:a discharge capacity versus cycle number and b C_max versus MM content (x)

下载原图

Table 1 Electrochemical characteristics of La_0.8-xM[M_xMg_0.2Ni_3.1Co_0.3Al_0.1 (x=0-0.3) alloy electrodes

Fig.4 HRD versus I_d of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 alloy electrodes

3.2.2 High rate dischargeability (HRD)

HRD of the alloys with various contents of the substitution by MM in the cases of the discharge current density of 300,900 and 1500 mA·g^-1 is shown in Fig.4.HRD_d,a comprehensive index representing kinetic properties of hydrogen storage alloy electrodes,is defined as the following equation:

where HRD_d is high rate dischargeability at current density ofI_d,C_d is discharge capacity of an alloy electrode discharged underI_d to a cutoff potential of 0.8 V versus counter electrode,C₆₀ is residual discharge capacity of the same alloy electrode discharged to 0.8 V at 60 mA.g^-1again after its preceding discharge atI_d,andI_d is discharge current density,i.e.,300,900 and 1500 mA·g^-1.

It can be seen that the HRD_d of the alloys with MM substitution are all better than that of non-substitution of MM.This reveals that MM can improve HRD.According to Table 1,as x increases from 0 to 0.3,HRD₃₀₀ increases from 82.00%(x=0) to 96.28%(x=0.3),HRD₉₀₀increases from 50.93%(x=0) to 79.21%(x=0.3),and HRD₁₅₀₀ increases from 40.63%(x=0) to 60.55%(x=0.3).It is obvious that the optimal value of x is 0.3.HRD is a dynamic factor of hydrogen absorbing/desorbing of the alloy electrode,which is influenced mainly by the electrochemical reaction kinetics on the alloy powder surface and the diffusion rate of hydrogen in the bulk of the alloy [ 32] .The smaller phases shown in Fig.2,resulted from the MM substitution,enhance the diffusion capability of hydrogen in the alloy,which consequentially leads to an optimum MM content for the HRD_d of the alloys.

3.2.3 Cycling stability

The capacity retention (S_n) is introduced for accurately evaluating the cycling stability of the alloy after n cycles.It is defined as S_n=C_n/C_max×100%,where C_n is the discharge capacity of the nth cycle at a current density of300 mA·g^-1.The cycling stability of the electrode alloy is a decisive factor of the life of the Ni-MH battery,which is very beneficial to its practical application.With the cycling,the alloy particles were pulverized to smaller ones.The higher pulverization resistance of the smaller particles due to their higher rigidity is an important factor for the reduction of the capacity decline rate with cycling [ 33] .

Figures 5 and 6 show the relationship between discharge capacity and cycle number of the La_0.8-xMM_xMg_0.2Ni_3.1-Co_0.3Al_0.1 alloy electrodes.It can be found that the capacity retention of the alloys first increases and then decreases with increase of MM content.In detail,when MM content rises from 0 to 0.3,the capacity retention increases from72.67%(x=0) to 82.88%(x=0.1),and then drops to73.43%(x=0.3) after 100 cycles.Moreover,it mounts up from 59.86%(x=0) to 72.07%(x=0.1),and then declines to 60.07%after 200 cycles.The results indicate that the substitution of MM for La leads to a significant increase of the capacity retention,and the amount of MM should be appropriate.

As shown in Fig.2,the MM addition improves the phase distribution and makes phases become smaller than those in original alloy,causing an obvious decrease of the decay rates of the discharge capacities of alloys.Moreover,MM addition promotes the formation of La₂Ni₇-type phase,and it has been proved that the electrochemical properties of La₂Ni₇-type alloy electrodes are superior to that of LaNi₃-type alloy electrodes,especially for the cycling stability [ 34] .Obviously,it is expected that the alloy with La₂Ni₇-type phase as main phase developed in the present investigation will exhibit better cycling stability.

Fig.5 Evolution of a discharge capacity with cycle number and b capacity retention ratio after 100 cycles (S₁₀₀) with MM content (x) of La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 alloy electrodes

Fig.6 Evolution of a discharge capacity with cycle number and b capacity retention ratio after 200 cycles (S₂₀₀) with MM content (x) of La_0.8xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1 alloy electrodes

4 Conclusion

The micros true tural evolution and electrochemical properties of the annealed La_0.8-xMM_xMg_0.2Ni_3.1Co_0.3Al_0.1(x=0-0.3) alloys were investigated.The results show that the alloys consist mainly of La₂Ni_7-type phase,La₅Ni_19-type phase,LaNi_5-type phase and LaNi_3-type phase,and La₂Ni_7-type phase is always the main phase.The substitution for La by MM leads to uniform phase distribution and refinement and promotes the formation of La₂Ni_7-type phase.

All of the alloy electrodes can be easily activated by two cycles.The substitution for La by MM has a significant improvement on the electrochemical performance of the alloys.The C_max and HRD abilities of the alloys obviously increase with the increase of MM content in the given range and reach the maximum values of C_max=381.2 mAh·g^-1and HRD₁₅₀₀=60.55%,respectively.The cycling stability first increases and then decreases with the increase in MM content,so a proper MM content is necessary for improving the cycling stability of the alloys.