Discharge performance of the magnesium anodes with different phase constitutions for Mg-air batteries

doi:10.1016/j.jpowsour.2018.06.085

Journal of Power Sources

Volume 396, 31 August 2018, Pages 667-674

https://doi.org/10.1016/j.jpowsour.2018.06.085 Get rights and content

Highlights

•
Discharge performance of Mg anode with different phase structure were investigated.
•
The warm rolled Mg-Li alloy still has high discharge performance at 40 mA/cm².
•
The dense matrix precipitates make the β-Li dissolve uniformly.

Abstract

The discharge performance of the wrought Mg-3Al-1Zn (AZ31), Mg-8Li-3Al-1Zn-0.2Y (LAZ831), and Mg-11Li-3Al-1Zn-0.2Y (LAZ1131) anodes (wt.%) for Mg-air primary batteries is investigated using an aqueous electrolyte (3.5 w% NaCl-solution). The AZ31, LAZ831 and LAZ1131 anodes are mainly composed of the equiaxed α-Mg (hcp) grains, the α-Mg (hcp) + β-Li (bcc) fibers, and the single β-Li (bcc) phase, respectively. The phase constitution has a strong influence on the discharge performance of Mg based anodes. The active β-Li phase provides high and stable cell voltage. Furthermore, the fine and dense active particles in the matrix contribute to a uniform corrosion and good corrosion resistance. In addition, the active β-Li peels away in the form of particle clusters. The LAZ1131 anode has the best discharge performance at a large current density of 40 mA cm⁻². The highest discharge capacity and anodic efficiency are 1478 mAh·g⁻¹ and 62.1%, respectively. It suggests that the LAZ1131 anode is a good excellent candidate for high performance Mg-air batteries.

Introduction

Metal–air primary batteries are attractive candidates for the development of energy storage systems [1]. It reduces the weight and frees up space for the batteries by taking oxygen in air as the reactants [2,3]. Great effort has been continuously spared to prepare cathode catalysts with low over-potential, good durability, and high oxygen reduction reaction (ORR) activity [[4], [5], [6], [7]]. However, it is also vital to select adaptive anode materials for metal–air primary batteries.

Magnesium is one of the attractive metal anodes, due to its high theoretical voltage (3.09 V), faradic capacity (2200 mAh·g⁻¹), energy density (6800 Wh·kg⁻¹), low toxicity and abundant reserves on earth [8,9]. However, Mg-air primary batteries can never approach these theoretical values (low coulombic efficiency), due to their high self-discharge rates in the aqueous electrolyte [10]. In addition, the insoluble discharge product masking the anode surface strongly decreases the shelf life and performance of the batteries [3,11]. To overcome these problems, one of the effective strategies is the additions of specific alloying element (Al, Pb, Sn, In, Ga, Hg, Li and rare earths (RE), etc.) to the magnesium anodes [[11], [12], [13], [14], [15], [16], [17]]. It can accelerate the breakdown of discharge product and improve the battery performance.

Hereinto, Mg-Li based anodes have received much attention owing to their negative standard electrode potentials and large Faradic capacities [[18], [19], [20]]. Adding over 5 wt.% Li element brings active β-Li phase (body centered cubic, BCC) into the α-Mg matrix (hexagonal close packed, HCP) [21]. It strongly enhances the discharge activity. Ma et al. prepared a totally BCC structured Mg-Li-Al-Ce anode (14 wt.% Li) with outstanding discharge performance at small current densities (≤10 mA/cm²) [18]. Lin et al. also studied the effects of Al content on the discharge activity of Mg-Li based anodes with BCC structure (Li content > 11 wt.%) [22]. More recently, Wang et al. investigated the discharge and corrosion behaviors of the HCP + BCC structured Mg-Li-Al based anodes (8 wt.% Li) [23]. In addition, minor addition of RE (Ce and Y) was able to improve the discharge performance of the Mg-Li based anodes [19,20,24].

The studies above mainly focused on the discharge performance of the as-cast Mg-Li based anodes with either HCP + BCC or single BCC structure. Nevertheless, few researches focused on the effects of the phase structures (HCP, HCP + BCC and BCC) on the discharge performance of Mg based anodes. Furthermore, the wrought Mg based anodes were suggested to have stronger discharge performance over the as-cast anodes [25,26]. The wrought Mg-Li based anodes are significant for the high performance Mg-air batteries, which has not been fully considered yet.

This work prepared three wrought Mg based anodes with different phase structures (α-Mg, α-Mg+β-Li and β-Li) via ingot casting and warm rolling, aimed at investigating the effects of phase constitutions on the discharge performance of wrought Mg based anodes. Meanwhile, the corrosion behaviors are also discussed to help connect the microstructures to the battery performance. This work can also provide a scalable processing route for high performance Mg based anodes for the metal-air primary batteries.

Section snippets

Preparation of materials

The commercial pure magnesium, pure lithium, pure aluminum, pure zinc (99.9 wt.%, the same below) and Mg-25%Y master alloy were taken as raw materials to prepare Mg-3Al-1Zn, Mg-8Li-3Al-1Zn-0.2Y and Mg-11Li-3Al-1Zn-0.2Y anodes (wt.%, named as AZ31, LAZ831 and LAZ1131, respectively). The addition amount of Li was selected to deliberately produce Mg based anodes with α-Mg + β-Li and β-Li phase structures, according to the Mg-Li binary phase diagram [21]. The minor Y was added to improve the

Microstructures

Fig. 2 shows the XRD patterns and microstructures of the investigated anodes. The as-rolled AZ31 anode consists of the primary α-Mg phase (HCP) and minor Mg₁₇Al₁₂ phase, according to the XRD pattern (Fig. 2a). Furthermore, the AZ31 anode is full of the dynamically recrystallized (DRXed) α-Mg grains (32 μm), as shown in Fig. 2b.

The LAZ831 anode is mainly composed of α-Mg and β-Li phase, according to the XRD pattern (Fig. 2c). In addition, the AlLi, MgAlLi₂, and Al₂Y phase are also detected. Fig.

Conclusions

The discharge performance of different phase-structured Mg anodes (α, α+β and β) used as anodes for Mg air batteries are investigated by electrochemical tests and constant current discharge tests in 3.5 mass% NaCl solution. In addition, microstructure analysis is used to help understand the microstructural response to discharging behaviors. The results are summarized as follows:

(1)
The three investigated wrought Mg anodes have totally different microstructure features and phase constitutions. The

Acknowledgements

The authors are very grateful to the Natural Science Foundation of China (Grant No. 51704020, 51674025, and 51434005), Beijing Natural Science Foundation (2184110) and Fundamental Research Funds for the Central Universities (FRF-TP-17-035A1, FRF-TP-17-010A) for funding support.

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Discharge performance of the magnesium anodes with different phase constitutions for Mg-air batteries

Highlights

Abstract

Introduction

Section snippets

Preparation of materials

Microstructures

Conclusions

Acknowledgements

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

J. Mater. Sci. Technol.

Mater. Des.

Electrochim. Acta

Electrochim. Acta

T Nonferr Metal Soc

T Nonferr Metal Soc

J. Power Sources

J. Power Sources

J. Power Sources

Corrosion Sci.

Corrosion Sci.

J. Power Sources

J. Alloy. Comp.

Mater. Lett.

J. Power Sources

Ultrason. Sonochem.

Mater. Des.

Ultrason. Sonochem.

Ultrason. Sonochem.