镍-金属氢化物电池（Ni-MH）负极材料的优化选择

Structures and Electrochemical Characteristics of Several Kinds of Alloys^[11]

Minshou Zhao,Xinbo Zhang,Yujun Chai’Changying Sun

(Key Laboratory of Rare Earth Chemistry and Physics Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022,P.R.China)

Abstract:The structures and the electrochemical characteristics of La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy,Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy and AB3＜x＜5-type alloy,which are the representative examples of AB3-type alloy,solid solution alloy and non-AB5-type alloy,respectively,have been investigated,and the performances of MH-Ni battery in which AB3＜x＜5 type alloy is used as the negative electrode material are examined at relatively low temperature.

Key words:La0.7-x Cex Mg0.3 Ni2.8 Co0.5 alloy;Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3 alloy;AB3＜x＜5-type alloy;Structure;Characteristics.

INTRODUCTION

Hydrogen is expected to be a promising new energy source to replace conventional fossil fuels for solving the shortage of fossil energy sources and global warming in the near future.In order for hydrogen to become to a viable solution for the energy crisis and the environmental problem,hydrogen storage is one of many important processes.Among different ways to store hydrogen,absorption in solid to form hydride is very attractive,since it allows safe storage at pressure and temperature close to ambient conditions[1].It is the most successful for the metal hydrides that are used as a negative electrode material in many application fields of metal hydrides.The Ni-MH secondary battery has been widely adopted in various portable electronic devices,electric hand tools and hybrid electric vehicles[2,3].To date,almost all commercial Ni-MH batteries are employing AB5-type alloy as the negative electrode material.However,as the electrochemical capacity of the AB5-type alloy is limited to about 372 mAh/g,the energy density of the Ni-MH battery is not competing favorably with lithium ion battery.Therefore,it is necessary to search for a new type alloy with much higher energy density,better rate dischargeability and lower cost,which perhaps may be an alternative for the conventional rareearthbased AB5-type alloy or another choice.

The structures and electrochemical characteristics of several type alloys have been investigated in our group to ascertain for possibility to be used as the novel negative electrode materials in Ni-MH battery or hydrogen storage alloys.The experimental details are the same as described in our previous papers[4-6].

La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)Alloy

Recently,Kadir et al.[7]have reported the discovery of a new type of ternary alloy with the general formula RMg2 Ni9(R=rare earth,Ca,Y)with a PuNi3-type structure.Some of the R-Mg-Ni based ternary alloys can absorb/desorb 1.8—1.87 mass%H and are,thus,regarded as promising candidates for hydrogen storage[8].Chen et al.[9]have studied the structure and the electrochemical characteristics of LaCaMg(Ni,M)9(M=Al,Mn)alloy,and almost at the same time,Kohno et al.[10]have reported that the discharge capacity of La0.7 Mg0.3 Ni2.8 Co0.5 alloy reached 410 mAh/g.Adzic et al.[11]pointed out that the degradation rate of electrochemical capacity due to electrode alloy corrosion was significantly decreased by the presence of Ce.Sakai et al.[12]reported that the replacement of lanthanum by large amounts of cerium can improve the cycle stability of the alloy even in the low content range of cobalt.

On the basis of our previous studies and the belief that the Ce addition may result in some noticeable modifications,the structure and electrochemical characteristics of the La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy have been investigated systematically as a representative example of AB3-type alloys.

Structure characteristics

Figure 1 shows the Rietveld analysis pattern of the La0.6 Ce0.1 Mg0.3 Ni2.8 Co0.5 alloy as a representative example of La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy.It can be found that,except for small amounts of impurity phases,LaNi and La Ni2,the alloy mainly consisted of a La2 MgNi9 phase with a PuNi3-type rhombohedral structure and a LaNi5 phase with a Ca-Cu5-type hexagonal structure.The lattice parameter,cell volume and phase abundance of La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy are presented in Table 1.The results indicate that the cell volume of the La(La,Mg)2 Ni9 phase and the LaNi5 phase in the alloy decrease monotonically with the increase of x,respectively,which can be attributed to the fact that the atomic radius of Ce(1.824Å)is smaller than that of La(1.877Å).The plots of unit cell volume of the La(La,Mg)2 Ni9 and LaNi5 as a function of x are linear,as shown in Fig.2.Figure 3 shows the abundance of the La(La,Mg)2 Ni9 phase and the La Ni5 phase as a function of x.As can be seen in Fig.3 and Table 1,the La(La,Mg)2 Ni9 phase abundance decreases from 76.54%to 53.65%but the LaNi5 phase abundance increases from 21.2%to 43.23%with increasing x.These results may influence the hydrogen storage and electrochemical characteristics of the alloy studied.

Figure 1 Rietveld profile refinement of XRD pattern of La0.6 Ce0.1 Mg0.3 Ni2.8 Co0.5 alloy.Phase 1,La(La,Mg)2 Ni9;phase 2,LaNi5;phase 3,LaNi;phase 4,LaNi2

Table 1　Characteristics of alloy phase in La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy

Figure 2 Variation of the unit cell volume of the La(La,Mg)2 Ni9 and LaNi5 with x

Figure 3 Phase abundance of the La(La,Mg)2 Ni9 phase and the LaNi5 phase in La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloys

P-C isotherms

The electrochemically measured PC-T curves for hydrogen desorption in the La0.7-x Cex Mg0.3 Ni2.8 Co0.5 alloy at 298 K are presented in Fig.4.With increasing x,the plateau pressure of the alloy increases monotonically and the plateau region becomes narrower and steeper,which leads to the decrease of the hydrogen storage capacity(H/M)from 0.952 to 0.177,which is in good agreement with that reported by Liu et al.[13].The change of the P-C-T curve profile can be mainly attributed to the reduction of the unit cell volume and the relative change of phase abundance of La(La,Mg)2 Ni9 phase and La Ni5 phase.The increase of the plateau pressure of the La0.7-x Cex Mg0.3 Ni2.8 Co0.5 alloy can be mainly ascribed to two factors.On the one hand,a unit cell of the AB3 compound contains 1/3 AB5 and 2/3 AB2 structure[7]and,thus,the plateau pressure of the La(La,Mg)2 Ni9 phase is essentially similar to that of La Ni5 phase.In fact,the plateau pressure of the La(La,Mg)2 Ni9 phase is slightly lower than that of the La Ni5 phase.With increasing Ce content,as mentioned above,the abundance of La(La,Mg)2 Ni9 phase decreases,whereas the phase abundance of the La Ni5 phase increases,which thereby slightly increases the plateau pressure of the La0.7-x Cex Mg0.3 Ni2.8 Co0.5 alloy.On the other hand,the cell volumes of both the La(La,Mg)2 Ni9 phase and the LaNi5 phase decrease with increasing Ce content,as shown in Table 1 and Fig.2,which will also inevitably increase the plateau pressure of the alloy.Precheron-Guegan et al.[14]pointed out that the hydrogen absorbed in an alloy enters and stays in the interstitial cavities of crystal lattice of the alloy.The alloy with bigger cell volumes and hence bigger interstitial cavities generally has larger hydrogen storage capacity.The cell volumes of both La(La,Mg)2 Ni9 phase and LaNi5 phase decrease with the increasing Ce content,which inevitably lowers the hydrogen storage capacity.These results indicate that the change of hydrogen storage capacity for the alloy results from the relative variation of the phase abundance of the La(La,Mg)2 Ni9 phase and LaNi5 phase.

Figure 4 Electrochemical desorption P-C-T curve for La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy electrodes at 298 K

Maximum discharge capacity and cycle stability

The maximum discharge capacity of La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy decreases from 367.5 mAh/g(x=0.1)to 68.3 mAh/g(x=0.5)with increasing Ce content,as shown in Table 2,which is in good agreement with the results of P-C-T curves.The capacity retention rate is also listed in Table 2.The capacity retention rate(S70)can be gradually improved with increasing Ce content.

Table 2　Performance of La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy electrode

High-rate dischargeability(HRD)

Figure 5 shows the effect of the discharge current density on the discharge capacity of the La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy electrode.The HRD of the alloy electrode increases first and then decreases as x increases.Taking a discharge current density of 1200mA/g as an example,the HRD of the alloy electrode increases from 55.4%(x=0.1)to 67.5%(x=0.3)and then decreases to 52.1%(x=0.5),as shown in Table 2.Figure 6 shows the linear polarization curves of the alloy electrode at 50%DOD and at 298 K.The polarization resistance Rp of the alloy electrode decreases from 94.8 mΩ(x=0.1)to 68.7 mΩ(x=0.5);accordingly the exchange current density I0 of the alloy electrode increases from 270.9 to 373.6mA/g when x increases from 0.1 to 0.5,as shown in Table 3.Pan[15]pointed out that for La-Mg-Ni-Co system alloys,the LaNi5 phase works not only as an hydrogen reservoir,but also as a catalyst to activate the La(La,Mg)2 Ni9 phase to absorb/desorb hydrogen reversibly in the alkaline electrolyte[14].The La Ni5 phase abundance increases with increasing Ce content,as mentioned above and,hence,increases the I0 of the alloy electrode.The hydrogen diffusion coefficient D in the alloy bulk is also listed in Table 3 and decreases from 15.17×10-10 cm2/s(x=0.1)to 7.32×10-10 cm2/s(x=0.5),which can be attributed to the reduction of cell volume of the alloy with increasing Ce content.The I0 monotonically increases with increasing x,whereas D decreases linearly.Overall,the combined effect of the two factors will result in an optimal value of x for HRD.The optimal value of Ce content for the alloy is 0.3 from our work.

Figure 5 HRD of the La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy electrode at 298 K

Table 3　Electrochemical parameter of La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy electrode

Figure 6 Linear polarization curve for La0.7-x Cex Mg0.3 Ni2.8 Co0.5(x=0.1-0.5)alloy electrodes at 298 K

Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)Alloy

Vanadium and vanadium-based solid solution alloy have attracted considerable interest due to their higher hydrogen storage capacity.Takahashi et al.[16]reported that V3 TiNi0.56 and V3 TiNi0.56 Hf0.24 alloy electrode had higher discharge capacity than other hydrogen storage alloys,and Lei et al.further studied the effect of Mn and Cr on the performance of V3 TiNi0.56 Hf0.24 alloy electrode[17].Iwakura et al.[18]studied the properties of TiV2.1 Ni0.3 alloy and found that the discharge capacity of the alloy electrode reached 540 mAh/g.Furthermore,Lei et al.[19]discussed the effect of Ni on the properties of Ti V2.1 and pointed out that TiV2.1 Ni0.5 had high discharge capacity and HRD capability.Ti-V-Cr and Ti-V-Mn are demonstrated to absorb/desorb an appreciable amount of hydrogen,and the effective hydrogen storage capacity is about 2.2 mass%,as reported by Akiba and Iba[20].However,there are only a few papers on the electrochemical characteristics of Ti-V-Cr alloy.Ti0.25 V0.35 Cr0.1 Ni0.3 alloy only delivered an electrochemical capacity of 240 mAh/g at 313 K.

Figure 7 XRD pattern of Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy

Zirconium is used to substitute partial titanium to improve the property of the Ti-V-Cr-Ni alloy electrode,and the structures and electrochemical characteristics of Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05—0.15)alloy are discussed as a representative example of solid solution alloy.(www.xing528.com)

Alloy structure

When titanium is partially substituted by zirconium,the C14 Laves phase appears.The XRD patterns of Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy are shown in Fig.7.The alloy is mainly composed of Vbased solid solution phase with a bodycentered cubic(BCC)structure and a C14 Laves phase with hexagonal structure.When x=0.05,the peaks of the Laves phase are not discernible.The peaks of the C14 Laves phase appear and shift to the lower angle evidently,and the volume fraction and the lattice parameter of C14 Laves phase increase with increasing x,as shown in Table 4.Moreover,the lattice parameter of V-based solid solution changes slightly when x increases from 0.05 to 0.10 and decreases as x increases further.It is probable that some zirconium solutes enter the solid solution and cause the lattice swelling due to the partial substitution of zirconium for titanium,while the majority of zirconium forms a C14 Laves phase with other elements.When the content of zirconium is greater than 0.15,more elements dissolve in the C14 Laves phase and,thus,change the element species in the solid solution alloy.

Table 4　Composition and lattice parameters of Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy

Electrochemical property

Figure 8 shows the dependence of discharge capacity on cycle number for Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy at 313 K.It can be seen that the discharge capacity increases with changing x from 0.05 to 0.15 after the electrode is activated,and the cyclic stability of the alloy electrode is good.In KOH electrolyte solution,the single solid solution phase has no electrochemical discharge capability because it does not have electrochemical catalytic activity.However,C14 Laves phase can act as the electrochemical catalyst and micro-current collector and make the solid solution alloy reversibly absorb/desorb hydrogen.There is a C14 Laves phase in Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy,as mentioned above,and the content of C14 Laves phase increases as x increases.The contents of solid solution and C14 Laves phase in the alloys are evaluated semi-quantitatively from the peak intensity in XRD patterns.When the mol ratio of the solid solution and C14 Laves phase is less than 3 and greater than 1,the discharge capacity only changes a little,which implies a correlation between the discharge capacity and the fraction of the phase in the alloy.In the meantime the presence of chromium also limits the corrosion of vanadium and leads to cyclic stability to be improved[21].All the measurements of the discharge capacity are conducted by charging at ambient temperature and discharging at various temperatures.The discharge capacity changes less at 303—323 K and evidently increases up to 333 K,when the maximum discharge capacity of 350 mAh/g is obtained,as shown in Fig.9.However,the discharge capacity decreases when temperature is above 333 K.

Figure 8 Discharge capacity vs.cycle number for Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy at 313 K

Figure 9 Discharge capacity of Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy as a function of discharge current density

Figure 10 gives the relation between the discharge capacity and the discharge current density.The discharge capacity increases with increasing x,which is perhaps related to the content of C14 Laves phase.The Laves phase accelerates the cracking of the alloy and creates more fresh surface area,which will affect the rate capability[22].

The electrochemical impedance spectra of the alloy electrode at a DOD of 50%are shown in Fig.11.It is demonstrated that the spectra of all metal hydrides consist of two semicircles at high frequency and a straight line at low frequency.The first semicircle looks like a straight line,which may be related to the porosity effect of the alloy electrode[23].The radius of the second semicircle decreases as x changes from 0.05 to 0.08,and then changes slightly.The fitted results by least square method are shown in Fig.12.It is evident that the electrolyte resistance R1,the contact resistance R2 and R3 are almost independent of x.However,the charge-transfer resistance R4 decreases markedly as x changes from 0.05 to 0.08 and then decreases slightly,which corresponds to the result mentioned above.These results indicate that the electrochemical activity is close to the chargetransfer resistance on the alloy surface.From the above results,it can be understood that the partial substitution of zirconium for titanium changes the charge-transfer resistance,decreases the electrochemical reaction resistance and enhances the discharge capacity and rate capability.

Figure 10 Dependence of discharge capacity of Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy on temperature

Figure 11 Electrochemical impedance spectra of Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)alloy electrode

Figure 12.Resistive component for Ti0.25-x Zrx V0.35 Cr0.1 Ni0.3(x=0.05-0.15)metal hydride

Characteristics of AB3＜x＜5-Type Alloy Electrode and MH-Ni Battery at Relatively Low Temperature

The commercial MH-Ni battery with AB5-type alloy electrode is required to be used at the span of temperature between-18 and 40℃because discharge capacity of the battery decreases dramatically below-20℃,and is almost zero at-40℃.The experimental results have indicated that the influence of the negative electrode material on performances of the MH-Ni battery is the most critical at low temperatures.

The performance of the battery at low temperature should be improved,especially when used in places,which include more than the third of land area over the world where the temperature could decrease down to-40℃in winter.It is urgent and significant to find a valuable negative electrode material for MH-Ni battery used at relatively low temperature.

The electrochemical characteristics of hydrogen storage alloy have been investigated at low temperatures[24-26],but the discharge characteristics have not been dealt yet at-40℃.

Influence of temperature on discharge capacity

The discharge capacity of any alloy electrode would decrease with decreasing temperature.The discharge capacity of AB3＜x＜5-type as a function of temperature is listed in Table 5 and that of commercial AB5 type is listed for comparison.The discharge capacity of AB3＜x＜5-type alloy is higher than that of AB5-type alloy at low temperature,especially at-40℃,although the situation is inverse at ambient temperature.The former belongs to the CaCu5 structure and is made of single phase,and the latter is constituted of multiphase and is non-stoichiometric,as mentioned above.The influences of the structure and the non-stoichiometry of the alloy on electrochemical characteristics are two critical factors at low temperature.

Table 5　Discharge capacity as a function of temperature

Id=65mA/g.

Table 6　Measured discharge capacity of D-type battery sample(in Ah/g)

Manufacture of a D-type battery sample

A D-type battery sample is fabricated,in which AB3＜x＜5-type alloy is used as electrode material.The measured capacity is listed in Table 6.It is evaluated that the discharge capacity of the D-type battery sample at-40℃is about 75%of that at ambient temperature.The results indicated that the uniformity of the battery sample is fairly good,but not desirable enough.

Essential consideration for synthesis of low-temperature electrode alloy

In general,the position of a plateau in the P-C-T plot for hydrogen storage alloy would move down with decreasing temperature,and desorbing hydrogen is an endothermic reaction.Therefore,desorbing hydrogen and the diffusion of hydrogen in alloy bulk will be very difficult at very low temperatures and equilibrium hydrogen pressure is very low.Besides,the information in the literature and our results indicated that non-stoichiometeic compound is appropriate for improving performances of metal hydride electrode at low temperature.All situations mentioned above should be essentially considered for the synthesis of alloys.The following hypotheses have been proposed preliminarily.

(1)Hydrogen desorption is the ratelimiting step,although charge transfer on the alloy electrode surface and diffusion of hydrogen in the hydride bulk are two important factors for discharge process.

(2)Composition and structure of the alloy should be chosen to be beneficial to hydrogen diffusion in the alloy and enhancing the plateau pressure of hydrogen desorption.

(3)Effect of the alloy electrode corrosion in alkali solution on electrochemical performances of the alloy electrode is an important factor,but not a very key factor yet.

Of course,these hypotheses should be examined in the future.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China(Grant No.20171042).