Effect of Zn Addition on the Microstructure and Discharge Performance of Mg-Al-Mn-Ca Alloys for Magnesium-Air Batteries

Abstract

This study explores the effects of Zn addition through micro-alloying on the microstructure and discharge performance of Mg-Al-Mn-Ca alloy anodes for magnesium-air batteries. The results show that the second-phase particles (d > 1 μm) in a Mg-Al-Mn-Ca alloy promote dynamic recrystallization (DRX) via particle-stimulated nucleation (PSN), resulting in a uniform equiaxed grain structure and fiber texture. In contrast, Zn and Ca co-segregation in a Mg-Al-Mn-Ca-Zn alloy facilitates continuous dynamic recrystallization (CDRX) and, combined with the PSN mechanism, forms a unique structure where three types of grains with different grain boundary densities coexist. The addition of Zn and Ca effectively reduces the c/a axis ratio, promoting texture homogenization. The Mg-Al-Mn-Ca alloy exhibits rough discharge surfaces due to simultaneous discharge at numerous grain boundaries and severe hydrogen evolution corrosion from micro-galvanic effects, inducing the chunk effect (CE). Conversely, the structure where three types of grains with different grain boundary densities coexist in the Mg-Al-Mn-Ca-Zn alloy promotes discharge product detachment through stress cracking, achieving uniform discharge and significantly enhancing discharge performance. The uniform texture reduces hydrogen evolution corrosion, improving anode utilization. This study demonstrates that controlling the microstructure, particularly grain boundary density and grain texture, enables the development of high-performance Mg-Al-Mn-Ca-Zn alloy anodes, especially at higher current densities, offering a new strategy for designing efficient magnesium alloy anode materials.

1. Introduction

In the pursuit of efficient and environmentally friendly energy storage solutions, metal-air batteries have emerged as a promising clean energy source. Compared to the widely commercialized lithium-ion batteries, metal-air batteries offer significantly higher theoretical energy density [1]. Various metals such as magnesium, lithium, zinc, and aluminum can serve as anode materials for metal-air batteries. Among these, magnesium-air batteries have garnered considerable attention due to magnesium’s highly negative standard electrode potential (−2.37 V vs. standard hydrogen electrode (SHE)), high theoretical discharge voltage (3.1 V), and high specific energy density (6.8 kWh·kg⁻¹) [2]. Additionally, magnesium is low-cost, abundantly available, and environmentally benign [2]. These distinctive advantages of magnesium-air batteries render them an exceedingly promising option for a variety of commercial applications, particularly as an innovative and sustainable energy storage solution. Their suitability extends from serving as backup power sources for critical facilities like schools and hospitals to powering an array of everyday devices such as mobile phones, televisions, and vehicles, as well as specialized equipment like underwater instruments [1].

However, the large-scale commercialization of magnesium-air batteries faces several challenges. Notably, the deposition of discharge products on the magnesium anode surface during discharge leads to discharge voltages far below the theoretical value [3], and the low anode utilization efficiency is caused by the chunk effect (CE) [4] and hydrogen evolution corrosion [5,6]. These problems severely limit the performance improvement and commercialization process of magnesium-air batteries.

Alloying is generally considered an effective means of improving anode performance [3,4,7,8]. However, adding large amounts of expensive alloying elements increases the production cost of magnesium alloy anodes, such as rare earth elements, which hinders market promotion [2,5]. Therefore, adopting a micro-alloying strategy (wt.% < 4.5%) and utilizing the synergistic effects of multiple low-cost alloying elements such as Al, Mn, Ca, and Zn to improve magnesium alloy anode performance cost-effectively and efficiently has become an attractive choice. Studies have shown that the Mg-Al-Mn-Ca alloy system exhibits good performance improvements through micro-alloying [3]. For instance, appropriate amounts of Al can refine grains [7], Mn can form particles with impurity elements during the smelting process and precipitate them out [8], and Ca can enhance electrochemical activity [4]. The combined addition of Al and Ca can also promote the shedding of discharge products [3].

Additionally, the impact of Zn on the Mg-Al-Mn alloy system has been extensively studied. Li et al. [9] discovered that in Mg-Al-Ca-Mn alloys, the incorporation of Zn leads to the emergence of Zn-Ca clusters. These clusters accumulate at dislocation sites, triggering Lüders deformation and contributing to a stress plateau during the strain process, which characterizes the discontinuous yielding phenomenon. Eom et al. [10] reported that Zn addition provides solid solution strengthening, enhancing the yield strength of Mg-Al-Mn-Ca alloys. However, most existing research focuses on the mechanical properties of Zn in the Mg-Al-Mn-Ca alloy system, while its effects on discharge performance have not been comprehensively analyzed. This gap limits further performance optimization of magnesium-air batteries.

Grain boundary density, grain texture, and other microstructures play crucial roles in the discharge performance of magnesium alloy anodes [6,11,12,13]. Typically, thermomechanical processing, especially hot extrusion, is a common production method for magnesium alloys [14]. Dynamic recrystallization (DRX) occurring during hot extrusion can significantly increase grain boundary density, effectively improving discharge voltage [11,15]. However, as the discharge process continues, discharge products with stronger interfacial cohesion in regions with high grain boundary density are difficult to detach, significantly affecting material exchange between the anode surface and the electrolyte [12]. To address this contradiction, a grain structure with varying boundary densities has become a promising research direction. Although there is some research foundation on the mechanical properties of grain structures with varying boundary densities, exploration of their discharge performance is still insufficient [16].

In terms of preparation methods for grain structures with varying boundary densities, recrystallization refinement methods are highly regarded because they can be produced using traditional hot extrusion processes [17]. For instance, Li et al. [18] successfully prepared structures with different grain boundary densities by promoting continuous dynamic recrystallization (CDRX) during hot extrusion. The elements added during alloying have a significant impact on the formation of grain structures with varying boundary densities. Several studies have indicated that the occurrence of CDRX is more probable in alloys where solute drag, caused by atomic segregation, is present, such as in the cases of Zn, Ca, Gd, and Y [18,19]. Zeng et al. [20] further discovered that the co-segregation of Zn and Ca atoms substantially enhances solute drag. Additionally, coarse second-phase particles (d > 1 μm) formed during alloying can also promote DRX based on the particle-stimulated nucleation (PSN) mechanism [21].

Grain textures such as basal texture and fiber texture can easily form during hot extrusion, influenced by extrusion temperature, extrusion ratio, alloy composition, and recrystallization mechanisms [18,22,23,24]. Fiber textures, in particular, tend to induce hydrogen evolution corrosion, having unexpected adverse effects on anode performance [13]. However, studies by Ding et al. [25] have shown that the combined addition of Zn and Ca can effectively reduce the c/a axis ratio, promoting texture uniformity and weakening fiber textures. This may provide new insights into texture control for the Mg-Al-Mn-Ca alloy system.

Given the potential role of Zn in the Mg-Al-Mn alloy system, this study aims to systematically investigate the specific mechanisms by which Zn addition affects the discharge performance of this alloy. By thoroughly analyzing the impact of Zn on grain boundary density and grain texture, this research provides scientific evidence for optimizing the performance of magnesium-air battery anode materials, thereby advancing the commercialization of magnesium-air battery technology.

2. Materials and Methods

The extruded plates of Mg-Al-Mn-Ca and Mg-Al-Mn-Ca-Zn alloys were used as anodes for magnesium-air batteries. The raw materials were commercial Mg-Al-Mn-Ca and Mg-Al-Mn-Ca-Zn ingots (purchased from Shanxi Yinguang Huasheng Magnesium Co., Limited, Yuncheng, China), with chemical compositions detailed in

Table 1. Chemical compositions of Mg-Al-Mn-Ca and Mg-Al-Mn-Ca-Zn alloys.

Elements	Al	Ca	Mn	Zn	Mg
wt.%	1.20	0.10	0.50	0	Bal.
wt.%	1.20	0.13	0.50	1.30	Bal.

. The magnesium alloy ingots were homogenized at 400 °C for 24 h, then extruded at a ratio of 56:1 and a speed of 35 m·min⁻¹ to obtain the magnesium alloy sheets. The extruded magnesium alloy sheets were cut to produce magnesium alloy anode samples.

To investigate the microstructure, the observed surface of the magnesium alloy samples was ground with SiC paper from #400 to #4000, polished with 0.5 µm diamond paste, and etched with a solution of 24 mL ethanol, 2 mL acetic acid, and 2 mL nitric acid. The samples’ metallographic morphology was analyzed using an optical microscope (OM, ZEISS Axio Vert.A1, Oberkochen, Germany). The microstructure and chemical composition of the second-phase particles were examined using a scanning electron microscope (SEM, HITACHI, SU5000, Chiyoda City, Japan) equipped with an energy dispersive X-ray spectrometer (EDS). Electron backscatter diffraction (EBSD) detectors (Oxford Instruments, Abingdon, UK) were used to analyze the samples’ microstructure and texture characteristics.

Half-cell discharge tests were conducted in 3.5 wt.% NaCl solution using an electrochemical workstation (CHI660E) at current densities of 2.5 mA·cm⁻² and 5 mA·cm⁻² for 1 h. The schematic diagram of the half-cell test setup is shown in Figure 1. Electrochemical properties of magnesium-based alloys were tested using the conventional three-electrode system, with a saturated calomel electrode as the reference electrode, a bright platinum plate electrode as the counter electrode, and the magnesium alloy anode sample as the working electrode. All samples were welded with copper wires and embedded in epoxy resin, exposing a 10 mm × 10 mm surface for testing. Before testing, the samples’ surfaces were ground with SiC paper to #4000, ultrasonically cleaned, and rapidly dried.

Magnesium-air battery discharge performance tests were conducted in 3.5 wt.% NaCl solution using a constant current discharge device (LAND) at current densities of 2.5 mA·cm⁻² and 5 mA·cm⁻² for 10 h. The magnesium alloy samples served as the anode, and a commercial air electrode with manganese dioxide as a catalyst was used as the cathode (purchased from Changzhou You Teke New Energy Technology Co., Ltd., Changzhou, China). Before testing, the magnesium alloy anode samples were sanded with SiC paper to #4000, ultrasonically cleaned, and rapidly dried. After discharge, the samples were cleaned with chromic acid solution to remove discharge products, ultrasonically cleaned, and dried. The morphology of the discharged samples was observed using SEM. The samples’ weight before and after the discharge test was measured to determine weight loss and calculate anode utilization efficiency using Equation (1) [4]:

$$\mathrm{\eta }={\displaystyle \frac{W_{theo}}{\Delta W}}\times 100\%,$$

(1)

where η is the anode utilization efficiency (%) and ∆W is the actual mass consumption (g), which is the difference between the mass before and after the reaction, with the reaction products removed. W_theo is the theoretical mass consumption (g), calculated using Equation (2):

$$W_{theo}={\displaystyle \frac{\mathrm{I}\times \mathrm{t}}{F\times {\displaystyle \sum (x_i\times n_i/m_i)}}},$$

(2)

here, I is the applied current (A); t is the discharge time (h); F is Faraday’s constant (26.8 Ah·mol⁻¹); and x_i, n_i, and m_i represent the mass fraction of each element in the alloy, the number of electrons exchanged in the solution, and the atomic mass (g·mol⁻¹), respectively.

3. Results

3.1. Microstructure

Figure 2 illustrates the optical microscope (OM) images of Mg-Al-Mn-Ca and Mg-Al-Mn-Ca-Zn alloys. The Mg-Al-Mn-Ca alloy displays a typical equiaxed grain morphology, indicative of significant dynamic recrystallization (DRX) during hot extrusion [26]. Conversely, the Mg-Al-Mn-Ca-Zn alloy exhibits three distinct grain characteristics: large, elongated grains (blue circles); slightly coarse equiaxed grains (red circles); and fine equiaxed grains (yellow circles), with an increasing grain boundary density. This variation suggests that the addition of Zn substantially influences the DRX process, resulting in a unique structure where three types of grains with different grain boundary densities coexist.

Figure 3 presents the scanning electron microscope (SEM) images of the alloys. The Mg-Al-Mn-Ca alloy shows clear equiaxed grains, while the Mg-Al-Mn-Ca-Zn alloy displays fine equiaxed grains (yellow circles) and large grains (blue circle). Both alloys contain numerous second-phase particles, appearing as white bright spots, uniformly distributed within the grains and at grain boundaries. The second-phase particles are primarily fine and dispersed, with occasional larger aggregates.

Figure 4 demonstrates the energy dispersive X-ray spectrometer (EDS) analysis of the second-phase particles. In the Mg-Al-Mn-Ca alloy, the Al element shows a brightening phenomenon, likely indicating Mg-Al phase particles. In the Mg-Al-Mn-Ca-Zn alloy, significant brightening in both Al and Mn elements suggests Al-Mn phase particles. No other distinct second-phase particles were detected.

To investigate the dynamic recrystallization (DRX) behavior of the two alloys, grains were categorized based on grain orientation spread (GOS) into three groups: (1) grains with GOS values less than 2° were considered fully dynamic recrystallized (DRX) grains, (2) grains with GOS values greater than 7° were classified as deformed (DEF) grains, and (3) grains with GOS values between 2° and 7° were defined as dynamically recovered (DRV) grains. Additionally, a 15° threshold was set for high-angle grain boundaries (HAGBs, black lines) and 2~15° for low-angle grain boundaries (LAGBs, gray lines) [19,27].

Figure 5 illustrates the inverse pole figure (IPF) maps, grain size statistics, and grain orientation spread (GOS) maps. The Mg-Al-Mn-Ca alloy displays a typical equiaxed grain structure, whereas the Mg-Al-Mn-Ca-Zn alloy shows a unique structure where three types of grains with different grain boundary densities coexist with the OM observations. The grain size distribution analysis indicates that the Mg-Al-Mn-Ca alloys are predominantly within the range of 0 to 25 μm and exhibit a relatively uniform distribution, whereas the Mg-Al-Mn-Ca-Zn alloy shows unimodal prominence with fine grains around 5 μm comprising 81.65% of the total, resulting in an average grain size of 7.95 μm.

In the GOS maps, the Mg-Al-Mn-Ca alloy predominantly consists of DRX grains, with minimal DRV grains and no noticeable DEF. In contrast, the Mg-Al-Mn-Ca-Zn alloy features significant large-sized DEF grains. In light of the substantial amount of second-phase particles observed in Figure 4a, the DRX in the Mg-Al-Mn-Ca alloy may be due to the numerous coarse second-phase particles (d > 1 μm) providing active sites for particle-stimulated nucleation (PSN) [21]. The Mg-Al-Mn-Ca-Zn alloy (Figure 5d) shows a color gradient at the edges of DEF (white arrows), characteristic of lattice rotation and large numbers of fine equiaxed grains, suggesting the occurrence of continuous dynamic recrystallization (CDRX) [18]. It is potentially promoted by the co-segregation of Zn and Ca which enhance solute drag [20]. Concurrently, in the Mg-Al-Mn-Ca-Zn alloy, a large number of uniformly dispersed, relatively coarse second-phase particles facilitate the DRX process through the PSN mechanism, thereby leading to the formation of moderately coarse DRX grains [21].

Figure 6 depicts the pole figures corresponding to the IPF of the two alloys presented in Figure 5a,d. The Mg-Al-Mn-Ca alloy shows a prominent fiber texture with weaker basal texture strength and with a slight deviation in the basal texture pole toward the ED direction, a phenomenon related to the randomness of recrystallized grain orientation under the PSN mechanism [28]. The Mg-Al-Mn-Ca-Zn alloy exhibits spread texture characteristics due to the combined Zn and Ca addition, reducing the c/a axis ratio [26]. Concurrently, the CDRX mechanism, which is fostered by the co-segregation of Zn and Ca, is closely related to dislocation slip behavior. The critical resolved shear stress (CRSS) for basal <a> dislocation slip is the lowest, which facilitates the easiest activation of basal <a> dislocation slip [29,30]. Consequently, this leads to an enhancement in the basal texture pole intensity of the formed CDRX grains, while simultaneously reducing the tilt angle towards the ED.

3.2. Discharge Performance

Figure 7 shows the half-cell discharge curves of the alloys in 3.5 wt.% NaCl solution. The Mg-Al-Mn-Ca-Zn alloy exhibits a more negative and stable discharge voltage at both current densities (2.5 mA·cm⁻² and 5 mA·cm⁻²). In half-cell magnesium-air battery discharge performance tests, the more negative the discharge potential of the material, the higher the discharge voltage it exhibits as an anode material. At 2.5 mA·cm⁻², the discharge potentials are approximately −1.80 V and −1.85 V for Mg-Al-Mn-Ca and Mg-Al-Mn-Ca-Zn alloys, respectively. At 5 mA·cm⁻², the difference expands to about 0.1 V, indicating superior discharge performance of the Mg-Al-Mn-Ca-Zn alloy at higher current densities.

Figure 8 shows the discharge curves of the alloys in a magnesium-air battery setup. As shown in the figure, the discharge voltage of both alloys rapidly decreases in the initial stage, mainly due to the rapid deposition of discharge products reducing the effective contact area between the anode and electrolyte [31]. At 2.5 mA·cm⁻², the Mg-Al-Mn-Ca-Zn alloy maintains a higher discharge voltage (approximately 1.35 V) compared to the Mg-Al-Mn-Ca alloy (1.33 V). At 5 mA·cm⁻², the Mg-Al-Mn-Ca-Zn alloy’s discharge voltage remains around 1.30 V, while the Mg-Al-Mn-Ca alloy’s drops to about 1.23 V. It was observed that at a current density of 5 mA·cm⁻², the Mg-Al-Mn-Ca-Zn alloy maintained a relatively high discharge voltage. As depicted in Figure 9, this study compares the stable discharge voltages of the Mg-Al-Mn-Ca-Zn alloy and the Mg-Al-Mn-Ca alloy at current densities of 2.5 mA·cm⁻² and 5 mA·cm⁻² with those of the ZK60 and ZW101 alloys from Tong et al. [5]. The ZW101 alloy, which contains the rare earth element Y, exhibited a higher stable discharge voltage. However, upon increasing the current density to 5 mA·cm⁻², the voltage of the ZW101 alloy declined more significantly compared to the Mg-Al-Mn-Ca-Zn alloy in this study, thereby further substantiating the superior performance of the Mg-Al-Mn-Ca-Zn alloy at higher current densities.

Figure 10 shows the SEM images of the discharge morphology after 10 h at 2.5 mA·cm⁻². The Mg-Al-Mn-Ca alloy surface exhibits extreme surface roughness with numerous pits (yellow arrows) and small holes (green arrows), likely due to localized discharge and hydrogen gas evolution [6]. In contrast, the Mg-Al-Mn-Ca-Zn alloy surface is relatively smooth, indicating a more uniform discharge process and weaker self-corrosion effects, leading to higher discharge activity and efficiency [32].

Figure 11 shows the anode utilization efficiency of the alloys. The Mg-Al-Mn-Ca-Zn alloy exhibits higher efficiency, correlating with its superior discharge performance observed in both half-cell and magnesium-air battery discharge performance tests.

4. Discussion

4.1. Influence of Grain Boundary Density Characteristics on Discharge

The grain boundary density plays a crucial role in the discharge behavior of magnesium alloy anodes. Grain boundaries act as high-energy defect regions, increasing discharge voltage and serving as preferential discharge sites, thereby enhancing local discharge activity [11,13,33]. However, the formation of discharge products at these sites can reduce the effective contact area between the anode and electrolyte due to strong interfacial cohesion of the products, making them difficult to detach [12].

In this study, the Mg-Al-Mn-Ca alloy exhibited a uniform equiaxed grain structure, where discharge predominantly occurred at grain boundaries. This leads to a deep extension of the discharge path along the grain boundaries and partial diffusion into the grains (Figure 12a). The extensive discharge at numerous grain boundaries causes rapid accumulation of discharge products, as evidenced by the densely distributed discharge pits observed in Figure 10a. The uniform grain structure results in uniform interfacial cohesion of discharge products, without significant local stress differences to promote the detachment of these products [12].

In contrast, the Mg-Al-Mn-Ca-Zn alloy displayed a unique structure where three types of grains with different grain boundary densities coexist. Regions with high grain boundary densities act as preferential discharge sites. The strong interfacial cohesion of discharge products in these regions impedes further discharge, causing the discharge path to extend into surrounding low-density regions, thus preventing localized discharge trends (Figure 12b). The difference in interfacial cohesion across regions creates local stress differences, leading to stress cracking that facilitates the detachment of discharge products [12,34]. This process increases the effective contact area between the anode and electrolyte, promoting uniform discharge and enhancing discharge activity, as confirmed by the smoother discharge morphology in Figure 10d. Consequently, the Mg-Al-Mn-Ca-Zn alloy consistently exhibited finer discharge performance compared to the Mg-Al-Mn-Ca alloy in the tests shown in Figure 7 and Figure 8.

4.2. Influence of Grain Texture on Discharge

Grain texture significantly impacts the discharge performance of magnesium alloys. Previous studies indicate that grains with prismatic orientation achieve higher discharge voltages [6]. However, achieving single-oriented grains through conventional hot extrusion is challenging, resulting in mixed grain orientations, as observed in the fiber texture of the Mg-Al-Mn-Ca alloy in this study. The pole figure of this alloy (Figure 6a) shows a mixture of prismatic and basal-oriented grains, forming micro-galvanic couples with a high cathode/anode ratio, which exacerbates hydrogen evolution corrosion. As depicted in Figure 12a, the micro-galvanic effect formed between the prismatic-oriented grains (green) and basal-oriented grains (red) leads to the formation of hydrogen evolution pores on the prismatic-oriented grains [6,13,35]. This corrosion process is evidenced by the presence of small holes, indicative of hydrogen evolution pores. Furthermore, excessive penetration and interconnection of hydrogen evolution pores may lead to the chunk effect (CE), which is characterized by the detachment of a large unreacted alloy matrix, as illustrated in Figure 12a. Consequently, the combined effects of the aforementioned detrimental phenomena reduce the anodic utilization efficiency of the Mg-Al-Mn-Ca alloy [6].

In contrast, the Mg-Al-Mn-Ca-Zn alloy exhibited a more uniform texture orientation with smaller orientation differences, significantly reducing self-corrosion. The discharge morphology of this alloy revealed almost no small holes, indicating minimal hydrogen evolution corrosion (Figure 10d). Consequently, the Mg-Al-Mn-Ca-Zn alloy demonstrated better anode utilization efficiency, as shown in Figure 11. These findings suggest that controlling texture orientation in anodes can significantly improve discharge efficiency, suppress self-corrosion, enhance anode utilization efficiency, and ultimately improve overall discharge performance.

5. Conclusions

This study investigated the effects of Zn addition through a micro-alloying strategy to Mg-Al-Mn-Ca alloy anodes on their microstructure and discharge performance. Based on the research results, the following conclusions can be drawn:

• Microstructure Evolution:

The second-phase particles (d > 1 μm) in the Mg-Al-Mn-Ca alloy promote dynamic recrystallization (DRX) through the particle-stimulated nucleation (PSN) mechanism, forming a uniform equiaxed grain structure and fiber texture.

In contrast, the co-segregation of Zn and Ca in the Mg-Al-Mn-Ca-Zn alloy facilitates the continuous dynamic recrystallization (CDRX) process. Combined with the PSN mechanism, this results in a unique structure where three types of grains with different grain boundary densities coexist. The addition of Zn and Ca effectively reduces the c/a axis ratio, promoting texture homogenization.

2.

Discharge Characteristics:

The uniform equiaxed grain structure in the Mg-Al-Mn-Ca alloy leads to simultaneous discharge at numerous grain boundaries, resulting in a rough discharge surface and difficulty in detachment of discharge products. The presence of grains with different orientations in the fiber texture causes severe hydrogen evolution corrosion based on the micro-galvanic effect and induces the chunk effect (CE) to some extent.

In contrast, the unique structure where three types of grains with different grain boundary densities coexist in the Mg-Al-Mn-Ca-Zn alloy promotes the detachment of discharge products through stress cracking generated between regions with different grain boundary densities. This achieves uniform discharge and significantly enhances discharge performance. Additionally, the uniform texture reduces hydrogen evolution corrosion, thereby improving anode utilization.

In conclusion, by controlling the alloy’s microstructure, particularly the grain boundary density and grain texture, we successfully developed Mg-Al-Mn-Ca-Zn alloy anode materials with superior discharge performance, especially at higher current densities. This provides a new strategy for designing high-performance magnesium alloy anode materials.