Journal of Materials Science & Technology  2020 , 38 (0): 47-55 https://doi.org/10.1016/j.jmst.2019.07.043

Research Article

Effects of external field treatment on the electrochemical behaviors and discharge performance of AZ80 anodes for Mg-air batteries

Xingrui Chena, Shaochen Ninga, Qichi Lea*, Henan Wanga, Qi Zoua, Ruizhen Guoa, Jian Houa, Yonghui Jiaa, Andrej Atrensb, Fuxiao Yua

aKey Lab of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China
bSchool of Mechanical and Mining Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia

Corresponding authors:   ∗Corresponding author.E-mail address: qichil@mail.neu.edu.cn (Q. Le).

Received: 2019-06-20

Revised:  2019-07-18

Accepted:  2019-07-22

Online:  2020-02-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

In this work, the effects of external field treatment on electrochemical behaviors and discharge performance of as-cast Mg-8%Al-0.5%Zn (wt%) anodes for Mg-air batteries are systematically investigated in 3.5 wt% NaCl solution. The external field treatment particularly the variable-frequency ultrasonic field can dramatically refine the α-Mg grains and β phases. The grain size decreases from 1594.8 ± 43 μm (untreated billet) to 140.6 ± 15 μm after variable-frequency ultrasonic field treatment. The values of open circuit potential, electrochemical activity and corrosion resistance of Mg-8%Al-0.5%Zn anode are improved with external field treatment, which should be attributed to refined grains and dispersive β phase. The external field treatment especially the variable-frequency ultrasonic field improves stability and the value of cell voltage, and enhances the discharge performance of the Mg-8%Al-0.5%Zn anode. The variable-frequency ultrasonic field treated anode has the best discharge capacity and anodic efficiency at current density of 45 mA cm-2, with the values of 1417 mA h g-1 and 63.3%, respectively. The ultrasonic field vibration changes the dissolution behavior of α-Mg matrix during the discharge process, showing the oriented surface morphologies.

Keywords: Mg-air batteries ; Magnesium alloy ; Discharge performance ; Electrochemical behaviors ; Ultrasonic field ; Electromagnetic field

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Xingrui Chen, Shaochen Ning, Qichi Le, Henan Wang, Qi Zou, Ruizhen Guo, Jian Hou, Yonghui Jia, Andrej Atrens, Fuxiao Yu. Effects of external field treatment on the electrochemical behaviors and discharge performance of AZ80 anodes for Mg-air batteries[J]. Journal of Materials Science & Technology, 2020, 38(0): 47-55 https://doi.org/10.1016/j.jmst.2019.07.043

1. Introduction

Mg alloy, the lightest metal structure material until now, has caught attentions from manufactures for vehicles, aircrafts, and trains [[1], [2], [3], [4]]. It is also an excellent anodic material for metal-air primary batteries, because of its unique characteristics such as high theoretical voltage, high specific discharge capacity, low toxicity and abundant reserves on earth [[5], [6], [7], [8]]. However, two major problems hinder the development and further application of the Mg anode. One is the high self-corrosion rate of Mg alloys in aqueous electrolyte [9,10], and the other one is the formation of a passive discharge-product film adhered to the surface of anode during discharge [11,12]. These two existed issues can dramatically reduce the discharge voltage, resulting in the low discharge performance.

To solve these problems, researchers mainly add the alloy element to modify the composition of Mg based anodes. For example, Li and Ce can improve the anodic discharge properties of Mg anodes due to weak adhesion of discharge product on the anode surface [13]. Another useful element is Ca, which can improve the self-corrosion behavior and discharge performance of Mg anode [[14], [15], [16]]. The addition of In element can provide a more negative potential and produce higher utilization efficiency than pure magnesium during the half-cell test at the large current density [6]. The different addition level of Li element into Mg alloys can form different phase constitutions, and then cause different discharge performance of Mg anode [17]. While extra element may increase the cost of Mg anode, and some elements such as Ca can increase the hot cracking potential during manufacturing of large-sized billets.

Many studies have proved that employing external fields during the solidification process of magnesium alloy is a useful method to refine the grain and phase [[18], [19], [20]]. For example, Fang et al. employed the ultrasonic vibration to treat the Mg-Zn-Y alloy, and found that ultrasonic vibration could significantly refine primary α-Mg grains and the agglomerated Mg-Zn-Y compounds [21]. Li et al. used the electromagnetic vibration to refine the 3xxx aluminum alloy and found good refinement efficiency of intermetallic compounds [22]. Considering the fact that grain size and phase morphology hold the key to the anodic discharge performance of Mg-air batteries, the external fields may have abilities to improve the discharge properties of Mg anodes. Therefore, in this study, the AZ80 Mg anodes casted with different external field treatment were prepared and their electrochemical behaviors and discharge performance in 3.5 wt.% NaCl solution were investigated. This work is aimed at understanding the effects of external field treatment on discharge performance of Mg anode for Mg-air battery. This work can also provide reliable method to improve discharge performance of Mg-air battery except the alloying method.

2. Experimental

2.1. Semi-continuous casting for AZ80 Mg anode

The chemical composition of AZ80 magnesium alloy for Mg anode used in this study is given in Table 1. At first, the commercial pure magnesium, pure aluminum, pure zinc and anhydrous MnCl2 were melted in a resistance furnace at 740 °C with the protection of CO2 + 0.5% SF6 (1:2) atmosphere. As shown in Fig. 1(a), the melt was transferred to the crystallizer at the temperature of 670 °C from the crucible. With the downward movement of the casting machine, the AZ80 billets (φ255 mm) were made (as shown in Fig. 1(b)). During the casting process, the preheated ultrasonic horn was inserted 50 mm under the interface of melting liquid in order to introduce the ultrasonic vibration into the melt. The ultrasonic wave was provided by a self-designed ultrasonic vibration system, which comprised an ultrasonic generator, an ultrasonic transducer, a wave-guide and a 35 mm diameter stainless steel acoustic horn. The ultrasonic generator could produce two kinds of ultrasonic vibration fields, namely the fixed-frequency ultrasonic field (FUF) and variable-frequency ultrasonic field (VUF). The frequency of FUF was fixed at 20 kHz, while the VUF had a 20 kHz center frequency and a 200 Hz changing frequency. The electromagnetic control unit and the induction coil provided the low-frequency electromagnetic field (LEF). The frequency of LEF was 20 Hz and the alternating current was 150 A. Samples for microstructure characterization were ground by the different grades of SiC papers and then polished and etched using a solution of 4.2 g picric acid to reveal their microstructures. The characterizations of microstructure were obtained using an optical microscopy (OM) and a scanning electron microscopy (SEM) with energy-dispersive spectrum (EDS).

Table 1   Chemical composition of AZ80 alloy (wt%).

AlZnMnSiFeNiCuCaMg
8.450.6110.4050.0060.0030.0040.0020.001Bal.

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Fig. 1.   Schematic of the experimental procedure: (a) the semi-continuous casting for the Mg anodes; (b) the AZ80 billet; (c) Mg-air battery test.

2.2. Electrochemical tests

Electrochemical experiments were carried out using a ChenHua CHI660E electrochemistry workstation at room temperature. A classical three-electrode cell was employed using a platinum foil as counter electrode, a saturated calomel electrode (SCE) as reference electrode and the tested sample as working electrode. The electrochemical samples with an area of 1 cm2 were ground on a 2000 grade SiC paper and exposed to 3.5 wt% NaCl aqueous solution at room temperature. The polarization curves were recorded with a scan rate of 1 mV s -1 after being held at solution for 1 h. The electrochemical impedance spectra (EIS) were measured after 1 h immersion to achieve the steady value of open circuit potential (OCP). The scanned frequency range was 100 kHz to 100 mHz, and the voltage amplitude was 5 mV. At least five replicates were tested for each test. After test, the measured EIS were fitted using the ZSimpWin software.

2.3. Mg-air battery tests

The discharge performance of the as-cast AZ80 alloys without and with different external fields treatment used as anodes for Mg-air batteries were tested by the NEWARE battery testing system at current densities of 1.5, 6, 15, 30, 45 and 60 mA cm-2 for 300 min. A self-designed battery testing system was employed, as shown in Fig. 1c. The catalyst of air cathode was MnO2/C, and the electrolyte was 3.5 wt% NaCl solution. The reaction area of the anode and cathode was 4 and 9 cm2, respectively. After tests, using chromic acid (200 g L-1) to remove the discharge products, and then the discharge capacity and anodic efficiency were calculated by the mass loss method, as shown in Eqs. (1) and (2):

Discharge capacity (mAh g-1)=$\frac{I×t×A}{ΔW}$×1000 (1)

Anodice fficiency (%)=$\frac{3.6×I×t×A×M}{2F×ΔW}$×100% (2)

where I is the current density (A cm-2), t the discharge time (h), A the surface area (cm2), ΔW the mass loss during discharge process (g), M the molar mass (g mol-1) and F the Faraday constant (96,485 C mol-1). The surface morphologies after the discharge were observed using SEM. At least three replicates were carried out for each test to ensure good reproducibility.

3. Results and discussion

3.1. Microstructures

Fig. 2 shows the microstructure of as-cast AZ80 anodic candidates casted with different methods. There are developed dendrites and coarse grains existed in the DC AZ80 billet. The average grain size of DC sample is 1594.8 ± 43.0 μm (Fig. 2(f)). The FUF and LEF can refine the grain of AZ80 billet to some degree, while some coarse grains are still observed, showing the average grain size of 929.2 ± 38.0 μm and 1014.6 ± 31.0 μm, respectively. The VUF treatment presents the excellent ability to refine the α-Mg grain. The grains become tiny and globular, with the average grain size of 140.6 ± 15.0 μm.

Fig. 2.   Microstructures of as-cast AZ80 alloys by different methods: (a) traditional DC method; (b) with FUF treatment; (c) with LEF treatment; (d) with VUF treatment; (e) statistic of grain size.

When the ultrasonic weave propagates in the melt, the liquid molecules are suffered a cyclic alternating sound field. The cavitation bubble is formed and enlarged during the negative acoustic pressure period. The bubble will absorb heat from the melt and produce local supercooling on the surface of the bubble during this process, and consequently, leading to the nucleation on bubble’s surface. When the cavitation bubble collapses in positive period, high temperature, pressure and strong impact force is released in the melt, which can fragment the initial crystal nuclei and dendrites. These two mechanisms can increase the number of nuclei. With the help of acoustic streaming, these nuclei are transported to the whole melt. According to our previous study [20], the VUF can produce higher acoustic pressure, which enhances the cavitation-enhanced heterogeneous nucleation and dendrite fragmentation effects, resulting in the better grain refinement efficiency. The LEF can accelerate the heat transfer during the semi-continuous casting process. The vibrating forces can also break the initial solidified grains formed in the mush zone. Therefore, the α-Mg grains are refined by the treatment of LEF.

Fig. 3 shows the SEM images of as-cast AZ80 billets without and with different external field treatment. There are two kinds of phases in AZ80 alloy, namely the Mg17Al12 (the β phase) and Al-Mn phase. The β phase (63.62 at.% Mg-36.38 at.% Al) is the main component in AZ80 alloy because of the high content of Al element. It is obvious that the β phase and Al-Mn phase (59.07 at.% Al-40.93 at.% Mn) have different morphologies. The Al-Mn phase shows the square-like form. While the β phase shows three morphologies, namely the laminar form, the acicular form and the dot-like form. It notes that the external field treatment also affects the morphology and distribution of intermetallic compound. In DC billet, the laminar β phase is dominant in the Mg matrix with a few dot-like β phase. According to the statistic of area fraction of β phase, the similar relative frequency can be observed for each area fraction, with the highest frequency of 0.117 located in the section from 0.45% to 0.50%. With FUF treatment, chunks of laminar β phase disappear, which are replaced by acicular β phases. The AZ80 billet with LEF treatment shows similar circumstance. The highest peak of relative frequency of FUF and LEF shifts to left to 0.10%-0.15% and 0.25%-0.30%, respectively. The VUF treatment makes the β phase become smaller than other external fields. The dot-like β phase dominates the matrix, showing the highest peak of relative frequency of 0.572 located in 0-0.05%. Considering the eutectic characteristic of β phase, such changes are attributed to interaction between grain refinement and the effects of external fields including acoustic cavitation, acoustic streaming and electromagnetic oscillation [18,23,24].

Fig. 3.   SEM images of as-cast AZ80 anodes with different field treatment: (a) traditional DC method; (b) with FUF treatment; (c) with LEF treatment; (d) with VUF treatment; (e) statistic of area fraction of β phase.

3.2. Electrochemical analysis

Fig. 4(a) shows the open circuit potential in 3600 s of as-cast AZ80 alloys without and with different external field treatment in 3.5 wt.% NaCl solution. All the four samples represent similar tendency. The potential rapidly increases to maximum at first and decreases gradually to the stable value after a sudden fall. While the difference is that those samples spend different time to reach the stable potential. The VUF samples use the shortest time (1247s) compared with the 3099 s value of DC samples. This result is attribute to the obvious difference of microstructure. The VUF has more homogeneous grain and β phases than DC. The average values of OCP of investigated AZ80 alloys in last 200 s of the whole OCP test are given in Table 2. It is found that the value of OCP varies with different external field treatment. The VUF treated AZ80 alloy has the lowest potential (-1.612 V). Fig. 4(b) presents polarization curves of as-cast AZ80 alloys treated by different external fields. The corrosion potential of DC anode is -1.463 V (vs. SCE). The corrosion potential shifts negatively with external field treatment, showing the value of -1.477 V, -1.475 V, and -1.490 V after FUF, LEF and VUF treatment, respectively. This result means that external field treatment can increase the electrochemical activity of AZ80 anode. The cathodic branch of polarization curve is driven with hydrogen evolution reaction [14]. Linear cathodic areas are obviously observed, and the cathodic current density deceases with the external field treatment. The self-corrosion current density is calculated using Tafel extrapolation, as show in Table 2. The self-corrosion current density of DC anode is 58.8 μA cm-2, which decreases to 16.6 μA cm-2 with VUF treatment, suggesting the kinetic of cathodic reaction tends to be slower with external field treatment. Therefore, it seems that the external field treatment especially the VUF treatment can improve the self-corrosion rate of AZ80 alloys. The refined microstructure should respond to this results, and detailed mechanism is discussed behind.

Fig. 4.   Open circuit potential (a) and polarization curves (b) of investigated as-cast AZ80 magnesium alloys treated by different external fields in 3.5 wt% NaCl solution.

Table 2   Fitting results of the polarization curves.

AnodeEOCP (VSCE)Icorr (A cm-2)Ecorr (VSCE)
DC-1.5925.88×10-5-1.463
FUF-1.5993.02×10-5-1.477
LEF-1.5974.78×10-5-1.475
VUF-1.6121.66×10-5-1.490

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Fig. 5 shows the EIS results of AZ80 anode without and with different external field treatment at OCP in 3.5 wt% NaCl solution. Similar form of Nyquist plots (Fig. 5(a)) are observed for four anodes. There are two semi-circles existed at high frequency and low frequency, respectively. At high frequency, a charge transfer resistance in parallel with the double layer capacity responds to the first semi-circle. The second semi-circle locates in the fourth quadrant, namely the low frequency inductance loop. This loop is result in the chemical reaction of Mg+ and H2O where the corrosion products films is broken [25,26]. Fig. 5(b) shows the equivalent circuit for all investigated AZ80 anodes. The Rs is the the solution resistance. The Rct, charge transfer resistance, and the CPEdl, the capacitor of double layer due to non-ideal capacitive behaviors, are employed to respond the first semi-circle. The RL and L respond the low frequency inductance loop, associated with the resistance of Mg+ reaction and the inductance of Mg+ reaction on the breaking area of partial protective film [27]. Fig. 5(c) presents values of charge transfer resistance (Rct) of four anodes. The DC AZ80 anode has the lowest value of Rct (715.5 Ω cm2), which increases to 1056.0, 936.9 and 1710.0 Ω cm2 after FUF, LEF and VUF treatment. Besides, the external field treatment also increases the value of induction (L). For instance, the value of L increases from 712.1 H (DC) to 2275 H with VUF treatment.

Fig. 5.   Electrochemical impedance spectra of as-cast AZ80 alloys with different external field treatment in 3.5 wt% NaCl solution: (a) Nyquist plots; (b) equivalent circuit; (c) charge transfer resistance and induction.

According to EIS and polarization curves results, the external field treatment can reduce the self-corrosion current density and increase the charge transfer resistance of AZ80 magnesium anodes, suggesting that the external field treatment can improve the self-corrosion resistance of AZ80 alloy. It is worth to note that the β phase holds the key to corrosion process of AZ80 alloys [28]. The β phase is much nobler than Mg matrix [29], which acts as the micro-cathode during the corrosion process. Thus, the β phase can accelerate the local corrosion rate until it falls off from the matrix. According to the SEM images, the external fields especially the VUF can significantly change the morphology and distribution of β phase. Tiny and uniform β phase are obtained after treatment, which means that they can easily fall off from the matrix, and consequently, side effects of the micro galvanic couples are stopped.

3.3. Discharge performance of Mg-air batteries with different anodes

Fig. 6 shows the discharge curves of assembled Mg-air batteries with different anodes at relatively low and high current density in 3.5 wt% NaCl solution for 5 h. The average discharge potential during discharge tests are given in Table 3. It is obvious that AZ80 anodes with external field treatment have higher cell voltage than untreated DC anode at both low and high current density, and the VUF treated anode has the highest cell voltage. Fig. 6(a) shows the discharge curves of Mg-air batteries at low current density (1.5 mA cm-2). At the initial stage of discharge process, the cell voltages decrease rapidly because of the accumulation of discharge products on the surface of anodes. Then four anodes show different tendencies. Anodes with external field treatment decrease slightly over time, while the untreated DC anode presents a slightly increased trend. Similar growth trend is also observed at the current density of 15 mA cm-2. However, no downward tendency of cell voltage can be seen in the case of treated anodes at this current density. When the current density increases to 30 mA cm-2, AZ80 anodes with external field treatment still remain stable cell voltages in spite of some tiny fluctuation. However, the voltage of the DC anode decreases during the whole discharge process, and the decreases rate (ΔV/t) increases from 6.09 mV h-1 to 18.12 mV h-1 after 3.1 h. With the further augment of current density (to 60 mA cm-2), external field treated anodes also have stable discharge voltage, and the same decreasing trend is observed for DC anode.

Fig. 6.   Discharge curves of AZ80 anodes casted with different field treatment in 3.5 wt% NaCl solution at current density of (a) 1.5 mA cm-2, (b) 15 mA cm-2, (c) 30 mA cm-2 and (d) 60 mA cm-2.

Table 3   Average discharge potential during the discharge tests.

Current (mA cm-2)DCFUFLEFVUF
1.51.4011.4321.4241.471
151.2491.2761.2791.330
301.1371.1651.1761.210
600.9541.0291.0221.088

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According to the discharge curves above, it is evident that the external field treatment particularly the VUF treatment can not only improve the cell voltage of Mg-air batteries but also stabilize the discharge curves during the discharge tests. The internal resistance of batteries, electrode polarization and discharge product commonly hold the key to the cell voltage of Mg-air batteries [30]. Based on the OCP results, the external fields treated anodes have more negative OCP value than untreated anode. Therefore, although the modified anodes have higher charger transfer resistance (Fig. 5(c)), they still show higher cell voltage. While it is worth to note that the difference of OCP value between modified and unmodified AZ80 anodes varies from 5 mV to 20 mV, which is smaller than the difference of cell voltage (23 mV to 134 mV), suggesting that other reasons need to be considered. Nyquist plots present that all the AZ80 anodes have low frequency inductance loops. The desorption process of surface products because of the gas release during the discharg proecss should respond these loops [17]. Fig. 5(c) shows values of induction of different anodes. It is found that the moidfied AZ80 anodes have larger value of induction, implying the stronger abilites of desorption ability. Besides, the refined microstructure is also beneficial to the discharge performance. It is clear that external field treatment can refine the α-Mg grains. It has been reported that the fine-grained microstucture can improve the discharge properties of Mg-air battery [31,32]. Refined grian size means the higher density of grain boundaries, which can dramatically promote the dischrge activity [33]. The contribution of morphology and distribution of β phase should also be discussed. The Mg17Al12 is strong galvanic cathode and automatically catalyzed under anodic polarization [28]. Mg is dissolved because of the galvanic corrosion. With the increase of time, the Mg17Al12 phase will fall off due to the disappearance of supporting Mg. In order to improve the discharge properties, the insignificant self-corrosion should be avoided. The external field treatment particularly the VUF makes the β phase become tiny and homogeneous. The refined β phase can fall off easily, stopping the adverse effect of of Mg17Al12. On the other hand, the refined and disperse β phase is also beneficial to the even formation of surface product on the anodic surface during discharge process.

Fig. 7 shows the discharge properties such as discharge capacity, average cell voltage and anodic efficiency of assembled Mg-air batteries with different anodes. The average cell voltage shows a downward trend with the increase of discharge current density, while the discharge capacity and anodic efficiency present adverse tendency before the current density of 45 mA cm-2. It also notes that anodes with external field treatment have higher cell voltage, discharge capacity and anodic efficiency. For example, the discharge capacity and anodic efficiency of DC anode at 15 mA cm-2 are 1072.9 mA h g-1 and 47.9%, respectively, while the values for VUF treated anode increase to 1225 mA h g-1 and 54.7%, respectively. The DC anode reachs to maximum discharge capacity and anodic efficiency at 45 mA cm-2, with values of 1306 mA h g-1 and 58.3%. The modified AZ80 anodes also reach the best discharge properties at this current density, with the values of discharge capacity of 1417 mA h g-1 and the anodic efficiency of 63.3% for VUF treated anode.

Fig. 7.   Discharge properties of assembled Mg-air batteries with different anodes: (a) discharge capacity and average cell voltage; (b) anodic efficiency.

With the further increase of current density (60 mA cm-2), the discharge properties of all anodes are decreased, while the VUF anode still remain relatively good discharge capacity and anodic efficiency, with the values of 1329 mA h g-1 and 59.3%, respectively. Such good performance should be attribute to the disperse β phase and the refined grain because of the external field treatment. Tiny and disperse β phase can stimulate the discharge smoothly and reduce the self-corrosion rate especially at high current densities. The refined grains bring a large quantity of grain boundaries, which usually have higher energy and can be disintegrated preferentially [34]. Thus, the external field treated anodes have more stable discharge process and better discharge performance. Moreover, the enhancement of desorption ability of discharge products due to the external field treatment is also a reason to improve the discharge properties of AZ80 anode. Discharge products usually covers the surface of anode, hindering the contact between the fresh metal and the electrolyte. According to the EIS result (shown in Fig. 5), low frequency inductance loops are observed. The fitting results of the equivalent circuit show that the external field treatment can increase the value of inductance. The existence of low frequency inductance loop means that there is ion exchange on the surface of anode which is not covered by film or products. Therefore, the value of inductance reflects the ability of desorption. In this case, the external field treatment rises the inductance value and improve the desorption ability. Thus, more fresh metal partipates in the electrode reaction, promoting the discharge properties.

3.4. Surface morphologies after the discharge

Fig. 8 shows the surface morphologies of Mg-air batteries with different AZ80 anodes after the discharge in 3.5 wt% NaCl solution at 1.5 mA cm-2 for 5 h with removing of discharge products. As shown in Fig. 8(a), discharge grooves grow along dendrites, displaying fish bone like surface morphologies in Mg matrix. This result responds to the developed dendrites of DC sample in microstructure observation in Fig. 2(a). Besides, some big and deep holes are observed as well after discharge with the existence of undissolved Mg matrix, suggesting the uneven dissolving effect during the discharge process. Fig. 8(b) shows the enlarged SEM image associated with holes, where small holes are seen on the wall. Skeletal α-Mg matrix is observed in depleted zone. The FUF anode shows different surface morphologies after discharge. No dendritic grooves can be found. Discharge holes are detected as well, while the size of the hole becomes small. It worth to note that the dissolution behavior of Mg matrix of FUF anode changes completely. Skeletal ruins disappear and plate-like structures are observed, as shown in Fig. 8(d). In the case of LEF anode, skeletal structures appear same as the DC anode. Some Al-Mn particles (Mg-37.1 at.% Al-32.7 at.% Mn by EDS) distribute in the ridge of ruins. Besides, dense small holes are observed in a small area, which may due to the release of hydrogen gas. As for VUF anode, no big discharge holes can be observed, and some equirotal pits appear after discharge. This result is attributed to the significant grain refinement after VUF treatment. Some smaller pits distribute on the wall of big pits. However, there are other two surface morphologies inside some big pits. Fig. 8(h) presents the fiber-like surface morphology with same orientations. Fig. 8(i) shows some tiny grooves distributed with same directions.

Fig. 8.   Surface morphologies of (a, b) DC, (c, d) FUF, (e, f) LEF, (g-i) VUF AZ80 anodes after the discharge in 3.5 wt% NaCl solution at 1.5 mA cm-2 for 5 h after removing of discharge products.

It is interesting to note that the ultrasonic treatment has the ability to change the dissolution behavior of Mg matrix during the discharge process. According to our previous study, the ultrasonic vibration can change the melt structure, which means that it can change the short-range order of metal materials [35]. The change of structure responds to the change of dissolution behavior of AZ80 anodes, because the oriented surface morphologies are observed in small region. Moreover, the VUF has stronger ability than FUF according to the microstructure evolution (Fig. 2), resulting in the complicated change of dissolution behavior. While further studies should be carried out to completely understand this phenomenon.

4. Conclusions

In this work, the electrochemical behaviors and discharge performance of AZ80 anodes with and without external field treatment for Mg-air batteries were investigated in 3.5 wt% NaCl solution. The evolution of microstructure due to external field treatment were employed to understand the discharge behaviors of different anodes. The following conclusions are given:

(1) The external field treatment can refine the α-Mg grains and phases of AZ80 billet. The grain size reduces from 1594.8 ± 43.0 μm (DC) to 140.6 ± 15.0 μm after VUF treatment, and the β phase also become tiny and dispersive.

(2) The external field treatment especially the VUF increases the OCP value, electrochemical activity and corrosion resistance of AZ80 anode. The refined grains and dispersive β phase should respond to such improvements.

(3) The AZ80 anode with external field treatment has higher and stable cell voltage and better discharge performance than DC anode. The VUF treated anode has the best discharge capacity and anodic efficiency at current density of 45 mA cm-2, with the values of 1417 mA h g-1 and 63.3%, respectively.

(4) Ultraonic field treatment changes the dissolution behavior of α-Mg matrix during the discharge process, showing the different surface morphologies after discharge. The short-range order due to ultraonic vibration may cause this phenomenon.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (No. 2016YFB0301104), the National Natural Science Foundation of China (No. 51974082) and the Programme of Introducing Talents of Discipline Innovation to Universities 2.0 (the 111 Project of China 2.0, No. BP0719037). The raw materials were provided by State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization.


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