Xingrui Chen ,Yonghui Ji ,Qichi Le,? ,Henn Wng ,Xiong Zhou ,Fuxio Yu ,Andrej Atrens

aKey Lab of Electromagnetic Processing of Materials,Ministry of Education,Northeastern University,314 Mailbox,Shenyang 110819,People’s Republic of China

b School of Mechanical and Mining Engineering,The University of Queensland,St.Lucia,QLD 4072,Australia

Abstract In this work,the discharge properties and electrochemical behaviors of as-cast AZ80-La-Gd anode for Mg-air battery have been investigated and compared with the AZ80 anode.The microstructure evolution,electrochemical behaviors and surface morphologies after discharge have been discussed to connect the discharge properties.The results indicate that the modified AZ80-La-Gd is an outstanding candidate for anode for Mg-air batter,which has high cell voltage,stable discharge curves,good specific capacity and energy,and good anodic efficiency.It exhibits the best anodic efficiency,specific capacity and energy of 76.45%,1703.6 mAh?g?1 and 2186.3 mWh?g?1,respectively,which are 20.24%,18.92% and 25.71% higher than values for AZ80 anode.Such excellent discharge performance is attributed to the Al-RE particles.They refine the Mg17Al12 phase and therefore improve the self-corrosion resistance and desorption ability of AZ80 anode.

Keywords: Mg-air batteries;Magnesium anode;Discharge performance;Electrochemical behaviors;RE compound.

1.Introduction

Environmental issues such as global warming have attacked a lot of attention,which are mainly caused by CO2emission from the burning of fossil fuel [1–4].With the encouragement of governments and the awareness of environmental protection,there is a trend to electric vehicles(EVs)[5–7].Generally,there are two crucial parts in EVs,namely,the electric machine and the electric battery.In recent years,the challenge of EVs mainly comes from the lack of battery with good discharge performance.The Mg is not only the widely used structural material but also the important energy material [8–15].The Mg-air battery is a promising candidate for the future generation of EVs because it uses oxygen from the air as one of the battery’s main reactants,reducing the weight of the battery and freeing up more space devoted to energy storage [16,17].The exchangeable Mg anode ensures the endurance of EVs.The Mg-air battery also has outstanding energy properties than others,such as high theoretical specific capacity(2200 mAh?g?1),discharge voltage(3.03V)[18–21].

The Mg-air battery is designed by a Mg anode,an air cathode,and an electrolyte.The performance of Mg anode usually holds the key to discharge propertied of Mg-air battery.The Mg anode faces two major problems,namely the high self-corrosion rate and adherent discharge products [18].The poor self-corrosion resistance gives rise to extra mass and energy loss which reduces anodic efficiency.Discharge products act as barriers to the direct connection between fresh Mg and electrolyte [22].The chemical composition,phase morphology and distribution strongly affect the discharge performance of Mg-air battery.Therefore,researchers are strugglingto develop advanced Mg anode using several methods such as alloying,deformation and external field treatment [23].Although external processing has the ability to change the grain size and phase distribution,the alloying method is the primary modification for phase composition,which is also an easy method to implement.Many elements such as Al,Li,Pb,Ca and RE are proposed to improve the discharge properties of magnesium anode[24–27].For example,the addition of Ca can refine the grain size and improve the electrochemical activity of anode,promoting the discharge performance[27,28].The RE elements are popular candidates to improve the strength and corrosion resistance of magnesium alloy.Researchers also used RE elements to promote the discharge performance of Mg anode.Liu et al.reported the combinative Ca,Sm and La additions to as-extruded AZ91 anodes could improve the discharge performance of Mg-air batteries[29].The modified AZ31-Gd anode outputs both high specific capacity and energy density than AZ31 [30].

Such studies indicate both La and Gd are useful elements for Mg anode to improve its discharge performance.However,the effects of combination addition of La and Gd in Mg-Al based anodes on the discharge performance of Mg-air battery has not been investigated.In addition,recent researches on the role of La and Gd on Mg corrosion [31–35]have indicated that these elements seem to be good choices to solve the problem of high self-corrosion of the Mg anode.Therefore,this work systematically investigated the discharge performance and electrochemical behaviors of AZ80-La-Gd anode.This work aims to provide the basic data for the design of advanced Mg anode for Mg-air batteries.

2.Experimental

2.1.Casting method and material observation

Table 1 lists the chemical composition of two AZ80-La-Gd and AZ80(set as the control group).Ingots were made by the semi-continuous casting method.The casting system was the same as our previous work[36]with a ring of 120 in diameter.The pure Mg(99.95%),pure Al(99.99%),pure Zn(99.95%),anhydrous MnCl2,Mg-30% wt.% Gd and Mg-25% wt.% La master alloys were melted in an iron crucible with the heating of a resistance furnace.The melt was then transported to the casting system at 680°C with the protection of CO2+0.5%SF6atmosphere.With the downward movement of the casing machine,ingots were cast.Samples for microstructure observation were cut from the ingots as longitude direction.They were ground,polished,and etched using the mixture of 4.2g picric acid,glacial acetic acid,and alcohol.Microstructure observation was carried out in optical microscopy(OLYMPUS BX53)and scanning electron microscopy(SEM,Zeiss ULTRA55)with EDS.Phase identification was done using X-ray diffraction(XRD).

Table 1 Chemical composition of experimental alloys(wt.%).

Table 2 Fitting results of the polarization curves.

2.2.Electrochemical measurement

The ChenHua CHI660E electrochemistry workstation was employed to measure the electrochemical properties of two investigated alloys,using a platinum foil,the saturated calomel electrode(SCE)and the magnesium alloys as the counter electrode,reference electrode and working electrode,respectively.The tested samples with the dimension of 10mm×10mm×6mm were ground with a 3000 grade SiC paper and then put into 3.5wt.% NaCl aqueous solution at room temperature(25°C).The polarization curves were recorded with a scan rate of 1mV s?1.The electrochemical impedance(EIS)was measured with the voltage amplitude of 5mV and the frequency range from 100 kHz to 0.1 Hz.At least three replicates were tested to ensure repeatability.The EIS patterns were finally fitted by ZSimpWin software

2.3.Mg-air battery tests

The NEWARE battery testing system was used to record the discharge curves of investigated anodes during the 10 h discharge process at room temperature.The anodes were set in a self-designed battery system,as described in Ref [23].The cathode was the commercial MnO2/C catalyst.The electrolyte was the 3.5wt.% NaCl solution.The tested current densities were 2.5,5,10,20,40 and 80mA cm?2,respectively.The reaction areas of the anode and cathode were 2 and 9 cm2,respectively.The anodes were washed in the chromic acid solution containing 180g/l CrO3after the discharge process to remove the surface products.Discharge surfaces were then observed by SEM.The discharge capacity and anodic efficiency were calculated using the mass loss method.At least three replicated were tested for reproducibility.

3.Results and discussion

3.1.Microstructures evolution

Fig.1 shows the OM images of as-cast AZ80 and AZ80-La-Gd.There are developed dendrites in both two alloys.The addition of La and Gd reduces the average grain size of AZ80 from 712±30 μm to 489±25 μm.SEM images present the distribution and morphology of compounds in Mg matrix,as shown in Fig.2a and Fig.2b.There are coarse bar-like particles in AZ80 alloy with some dot-like phases.These particles are identified as theβ-phase(Mg17Al12)associated with the XRD results in Fig.2c.Typical eutectic feature(seen in the partial enlarged image of Fig.2a)further confirms the identification ofβ-phase.In the case of modified AZ80-La-Gd,the number and the size ofβ-phase are both reduced.Some needle-like particles appear in the Mg matrix.According to the XRD results,the modified alloy contains another two phases except forβ-phase,which are Al11La3and Gd2Al.The element distribution sweep by EDS is carried out to identify the phase composition,as shown in the partial enlarged image of Fig.2b.All the bright particles contain Al element.The La element is detected in the needle-like particles,while the Gd element only distributes in some points.Thus,the needle-like particles are Al11La3and the tiny dot-like particles are Al2Gd.The Al2Gd particles also refine theα-Mg grains by particle-stimulated nucleation(PSN)mechanism.They act as nucleation sites during the solidification and promote heterogeneous nucleation [37].

Fig.1.Optical microscopy metallographs of as-cast(a)AZ80 alloy and(b)AZ80-La-Gd alloy.

Fig.2.The SEM images of as-cast(a)AZ80 alloy and(b)AZ80-La-Gd alloy;(c)XRD patterns of investigated alloys.

Fig.3.Polarization curves(a)and linear sweep voltammetry of the polarization curves(b)of investigated anodes in 3.5wt.% NaCl solution.

3.2.Electrochemical tests

Fig.3a exhibits the polarization curves of two investigate alloys.The corrosion potential(Ecorr)of the original AZ80 alloy is?1.299V(vs.SCE),which shifts negatively to?1.362V in case of AZ80-La-Gd,implying the modified alloy has higher electrochemical activity.The more negative potential makes the alloy respond to the current actively during the discharge process as an anode.The cathodic branch of polarization of AZ80 alloy shifts downward with the addition of La and Gd.The cathode branch of polarization curves refers to the hydrogen reaction [27].This shift represents the weakening of cathodic kinetics.Table 2 listed the self-corrosion current density(Icorr)of two investigated alloys,using Tafel extrapolation.It should be noticed that both two alloys have low levels ofIcorrvalue compared with other Mg-Al-Zn alloys [30,38].TheIcorrvalue of AZ80 alloy is 7.98×10?6A cm?2,which reduces to 5.34×10?6A cm?2in case of modified AZ80-La-Gd alloy.Although the value of corrosion current densities cannot represent the corrosion rate accurately of many Mg alloy,it reflects corrosion resistance qualitatively [39].Thus,the modified alloy has higher corrosion resistance.Fig.3b presents the linear sweep voltammetry of the anodic branch of polarization curves.The potential raises sharply with the tiny increase of current density at the beginning in both two alloys.The increasing rate of potential of AZ80 alloy slows down to 12.33V A?1cm2at the current range between 0.2mA to 6.18mA,which is further reduced to 3.35V A?1cm2and keeps this level.The increasing rate of modified AZ80-La-Gd alloy decreases its increasing rate to 3.07V A?1cm2at 2.57mA and keeps this value with the further increase of current density.These current-potential curves can help understand the discharge performance of anodes,which will be discussed in detail in Section 3.3 with the results of discharge curves.

The EIS results of AZ80 and modified AZ80-La-Gd alloys are shown in Fig.4.One large and other smaller semicircles are observed in Fig.4a.The EIS patterns of both two alloys have impedance located in the fourth-quadrant.Therefore,each EIS pattern is made of two semi-circles.The phase angle vs.frequency in the Bode plot in Fig.4c confirms this proposal.Thus,the EIS patterns of these two alloys contain a high-frequency capacitive loop and a low-frequency inductive loop.Fig.4b shows the Bode plot in terms of the modulus value of impedance vs.frequency.The modulus of two alloys increases from high frequency to the peak value and reduces at low frequency.The modulus of modified AZ80-La-Gd is higher than the AZ80,suggesting the higher corrosion resistance,which agrees with theIcorrvalue in Table 2.Fig.4d shows the EIS pattern of two investigated alloys at potential 150mV more positive than OCPs(OCP+150mV).The EIS patterns significantly shrink at a more positive potential.

Table 3 Electrochemical parameters of the fitted equivalent circuits.

Based on the analysis above,a proposed equivalent circuit is given in Fig.5,which includes a capacitor,an inductor and three electric resistances.TheRsrepresents the solution resistance between the Luggin capillary and the specimen.TheRtis the charge transfer resistance.TheCPEdlpresents the double-layer structure as a capacitor which has two parameters,Y0andn,describing the properties of the capacitor [40].The value ofnvaries from 0 to 1,representing the tendency between the pure resistance and capacitor [41].The inductor andRLare employed to present the EIS spectra located in the forth quadrant.

Fig.4.Electrochemical impedance spectra of investigated alloys in 3.5wt.% NaCl solution:(a)Nyquist plots;(b)Bode plots of impedance modulus vs.frequency and(c)Bode plots of phase angle vs.frequency;(d)EIS patterns of investigated alloys at the potential 150mV more positive than OCPs;(e)charge transfer resistance and the low frequency inductive reactance of the investigated alloys.

Table 3 exhibits the fitting results based on the equivalent circuit.TheRtof AZ80 alloy is 631.5Ωcm2,which increases to 1317Ωcm2with the addition of La and Gd elements.The charge transfer resistance generally has a positive relationship with the self-corrosion resistance[42,43].Therefore,the modified AZ80 alloy has a higher self-corrosion resistance,which agrees with the results ofIcorrvalue in Table 2.It is also worth noting that theLvalue of AZ80-La-Gd is higher than the original AZ80 alloy at both OCP and OCP+150mV(seen in Fig.4e),suggesting the better desorption ability which is discussed with discharge curves and morphologies in behind.

Table 4 Average discharge potential during the discharge tests.

According to the results above,the modified AZ80-La-Gd alloy has higher self-corrosion resistance than the original AZ80 alloy.The phase composition of AZ80 alloy isα-Mg andβ-Mg17Al12compound.Theβphase dominates the corrosion process of the AZ80 alloy.The Mg17Al12compound has higher electrode potential than Mg matrix in both neutral and alkaline solution [44],which builds the galvanic couples with Mg matrix and accelerates the corrosion process of AZ80 alloy [45].With the addition of La and Gd elements to the AZ80 alloy,the phase composition and morphology ofβphase are changed dramatically.As shown in Fig.2b,the coarseβphases are replaced by small-sized dots and short rods because of the addition of La and Gd.Thus,the adverse influence ofβphases is weakened.In addition,the RE elements also help form more protective corrosion product film to improve the corrosion resistance of modified alloy [31,46].

Fig.5.The equivalent circuit for the investigated Mg based anodes fitted based on the EIS.

3.3.Discharge performance of Mg-air batteries with LPSO contained anodes

Fig.6 presents the discharge curves of assembled Mgair batteries with AZ80 and AZ80-La-Gd anodes at different current density in 3.5wt.% NaCl solution for 10h.Table 4 shows the average discharge cell voltages.The average cell voltages of modified AZ80-La-Gd are higher than the original AZ80 anode at every tested current density.For example,the average cell voltage of AZ80 and AZ80-La-Gd anode at 2.5mA cm?2is 1.375V and 1.397V,respectively.At low current density,both two anodes show stable discharge voltages without evident turbulence and drop.However,the discharge curve of AZ80 anode at 80mA cm?2shows a dramatic drop from 0.911V to 0.449V with a decrease rate of 0.0462V h?1.In contrast,the modified AZ80-La-Gd anode still remains stable cell voltages even at large current density(80mA cm?2).A slight voltage recovery from 0.875V to 0.982V is observed in the first 2.5h,and then the cell voltage reduces slightly to 0.879V in the next 7.5h with the decrease rate of only 0.0146V h?1.

The above results show that the modified AZ80-La-Gd anode has a higher and more stable cell voltage during the discharge process.In general,the internal resistance of battery,electrode polarization and discharge products usually hold the key to the discharge performance of Mg-air battery[47].The cell voltage of AZ80 and AZ80-La-Gd at 0 current density(the no-load condition,seen in Table 4)are 1.765V and 1.809V,respectively.Although the modified anode has higher transfer resistance(seen in Table 3),namely higher initial resistance of the battery,the AZ80-La-Gd anode can keep higher cell voltage during the discharge process.The linear sweep voltammetry of the anodic branch of polarization curves also provides the evidence for the better discharge performance of modified AZ80-La-Gd alloy.During the anodic polarization,the potential increases with the augment of current density,which agrees with the voltage decrease due to the increase of discharge current.The potential-current respond curve of modified AZ80 alloy locates under the original AZ80 anode despite the variation of current density,suggesting the higher potential modulus of AZ80-La-Gd anode at every certain current density.

Fig.7 summarizes the discharge properties of investigated Mg-air batteries with original and modified AZ80 anodes in 3.5wt.% NaCl solution.,including average cell voltage,discharge capacity,specific energy and anodic efficiency.Continuously decreasing trends of average cell voltage are observed in two anodes with the augment of current density.There is a considerate drop of voltage between no-load condition and 2.5mA cm?2current density.This phenomenon is attributed to the formed discharge products during the discharge process,which hinders the direct contact between fresh Mg and electrolyte.The discharge capacity of the AZ80 anode rises to the peak value of 1432.6 mAh·g?1at 40mA cm?2,while the modified AZ80-La-Gd anode reaches its peak discharge capacity at 20mA cm?2,with the value of 1703.6 mAh·g?1.The anodic efficiency shows the same tendency as the discharge capacity,displaying peak figures of 63.58% and 76.45% for AZ80 and AZ80-La-Gd anodes,respectively.Although it is difficult to directly compare these values reported in the literature and obtained in this work because of the different chemical composition and battery testing systems,the anodic efficiency of modified AZ80-La-Gd is high [29,30,48].The specific energy of modified AZ80-La-Gd anode is higher than the original AZ80 anode at each tested current density.The peak specific energy output of modified AZ80-La-Gd is 2186.3 mWh·g?1,which is 25.7% higher than the original AZ80 anode(1739.3 mWh·g?1).

The magnesium anode is proposed to become Mg2+and provide electric energy without extra energy loss during the discharge process in the ideal circumstance.However,the presence of hydrogen evolution reaction(HER)and negative differential effect(NDE)usually cause extra mass loss except for energy transformation.

The HER(as shown in Eq.(1))occurs on the surface of magnesium anode and produces Mg(OH)2,which not only prevents the contact between fresh Mg and electrolyte but also causes extra mass loss.The NDE amplifies this adverse effect in the discharge process.In the case of the original AZ80 anode,the self-corrosion resistance is deteriorated due to coarse Mg17Al12particles which accelerates the HER,and consequently,the anodic efficiency and specific energy are quite poor.In addition,the existence of HER also produces a net corrosion potential,leading to the great deviation of experimental cell voltage(0.71–1.76V)from the theoretical potential(3.09V)[49].The electrochemical tests indicate that the addition of La and Gd reduces the corrosion rate and restrict HER(weak cathodic kinetics in polarization curves).In response,more Mg participates in energy output,resulting in the promotion of discharge capacity and anodic efficiency.In addition,the modified AZ80-La-Gd anode has small sizedgrain than AZ80.The refined grains bring a large number of grain boundaries,which usually have higher energy and can be disintegrated preferentially [50].This mechanism also improves discharge performance.

Fig.6.Discharge curves of investigated(a)AZ80 and(b)AZ80-La-Gd anodes in 3.5wt.% NaCl solution at different current density for 10h.

Fig.7.Discharge performance of investigated Mg-air batteries with different anodes in 3.5wt.% NaCl solution:(a)average cell voltage and discharge capacity;(b)anodic efficiency and specific energy.

3.4.Discharge surface morphologies

Fig.8 presents surface morphologies of AZ80 and AZ80-La-Gd anodes after 10h discharge in 3.5wt.% NaCl electrolyte at 2.5mA cm?2current density.Large and deep discharge pits with many coralliform ridges are observed on the surface of the original AZ80 anode after the discharge process.This structure acts as a porous container for Mg(OH)2,preventing the contact between Mg matrix and electrolyte.The complicated structure also increases the difficulty of the detachment of discharge products,resulting in the rough and fibrous surface on the bottom of discharge pits(seen in Fig.8b).In contrast,the discharge pits of modified AZ80-La-Gd are shallower and smaller.The surface is also smoother than the original AZ80 anode(as shown in 8d).The smooth surface makes discharge products fall off from Mg matrix easily.In addition,some small grooves are seen,which is associated with the release of hydrogen gas,and therefore,discharge products can be detached from Mg matrix due to the shock of gas bubbles.These two mechanisms give the modified AZ80-La-Gd anode good desorption ability during the discharge process.They also respond to the higherLvalue of EIS results(seen in Table 3).The inductor is employed to describe the impedance located in the fourth quadrant,which reflects the inductance of Mg2+reaction on the breaking area of the partial protective film[31].The higher value of inductance presents the larger area without the cover of discharge products,namely better desorption ability.Thus,the discharge curve of the modified AZ80-La-Gd anode is stable even at large current density.The voltage recovery(seen in Fig.6b)also responds to the good desorption ability of AZ80-La-Gd anode.Some Al11La3particles are on the ridges of the discharge surface,suggesting that they are weak cathodes and had small lightly adverse effects,unlike the Mg17Al12particles.

Fig.8.Surface morphologies of(a)-(b)AZ80 and(c)-(d)AZ80-La-Gd anodes after the discharge in 3.5wt.% NaCl solution at 2.5mA cm?2 for 10h without discharge products.

4.Conclusion

In this work,the discharge properties and electrochemical behaviors of modified as-cast AZ80-La-Gd anode for Mg-air battery have been investigated.The needle-like Al11La3and dot-like Al2Gd particles are detected as well as the Mg17Al12phase.The modified AZ80-La-Gd anode has high cell voltage,stable discharge curves,good specific capacity and energy and good anodic efficiency.It can output the best specific capacity and energy of 1703.6 mAh·g?1and 2186.3 mWh·g?1,respectively with the anodic efficiency of 76.45%.Such outstanding discharge performance is attributed to the Al-RE particles.They refine the Mg17Al12phase and therefore improve the self-corrosion resistance and desorption ability of AZ80 anode.In contrast,the original AZ80 anode exhibits poor peak specific capacity and energy of 1432.6 mAh·g?1and 1739.3 mWh·g?1,respectively with the highest anodic efficiency of 63.58%.The modified AZ80-La-Gd can be regarded as a good anode for the Mg-air battery.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China(Grant No.51974082),and the Programme of Introducing Talents of Discipline Innovation to Universities 2.0(the 111 Project of China 2.0,No.BP0719037).Special thanks are due to the instrumental or data analysis from Analytical and Testing Center,Northeastern University.