Self-Formation of Mg Metal / Electrolyte Interface in Rechargeable Mg Batteries
Authors: Mizuno Fuminori , Singh Nikhilendra , Arthur Timothy S. , Tutusaus Oscar , Takechi Kensuke , Mohtadi Rana 
Published: February 2020

Rechargeable magnesium batteries have recently gained more and more attention for a possible candidate of post lithium-ion batteries, because of i) potentially high energy density ((3.0V x 2e- for Mg) / (4.0V x 1e- for Li) = 1.5), ii) low cost (high Clarke number; Mg (1.93) vs. Li(0.006)) and iii) intrinsic safety (no dendrite growth for Mg, while needle-like dendrite for Li). In 2000, a great success of Mg battery was achieved by Prof. Aurbach in the prototype system comprising Mg metal anode, Chevrel phase Mo6S8 cathode and Grignard based electrolyte [1]. Guided by this discovery, intensive researches on anode, cathode and electrolyte materials have accelerated all over the world, to maximize the advantages of the Mg battery system.
A major challenge of the rechargeable Mg batteries is that there is still lack of practical cathode materials. Chevrel phase Mo6T8 (T = S, Se) compounds are only well-known materials showing high reversibility over hundreds of cycles at reasonably high rates [2]. Unfortunately, the Chevrel phase showed low operation voltage (1.2 V vs. Mg) and low capacity (122 mAh/g), and therefore lots of cathode candidates have been extensively investigated to improve the capacity as well as the operating voltage, aiming a higher energy density battery system. Some candidates beyond the Chevrel phase have been found to date, however still on an exploratory level. The main issue is sluggish diffusion of Mg2+ in the host structure, retarding findings of potential cathode materials. On the other hand, we found that some cathode materials have not been monitored properly because of high impedance caused by a Mg metal anode / electrolyte interface. Figure 1 shows the Nyquist plots of a symmetrical Mg/Mg cell with PhMgCl-AlCl3 (APC) electrolyte as a function of holding time under open circuit voltage. Inset figure shows the Nyquist plot of the same symmetrical cell under a bias voltage of 150 mV after holding for 12 h. As can be seen, the interfacial resistance between Mg metal and electrolyte was getting bigger and bigger with holding time even under open circuit voltage. However, once a bias voltage was applied for the cell, the observed impedance drastically decreased [3]. As a result, such huge impedance potentially veiled any electrochemical responses for the entire cell, which may regard as no possibility of cathode materials. The interfacial issue on a Mg metal anode has been pointed out by Prof. Aurbach et al [4,5]. Here, we newly found that the huge impedance was switchable under open circuit and bias potentials in the currently-developed Mg battery electrolytes, and then strongly depended on electrochemical states [6]. Thus, mechanistic understanding of the interface between Mg metal and electrolyte is significantly important to find out future cathode materials as well as to develop highly stable electrolytes which are compatible with Mg deposition and dissolution.
In this presentation, we will present our findings of the Mg metal / electrolyte interface. Then, we will discuss a new idea of self-formed Mg metal / electrolyte interface, which is one of the approaches to decrease the huge impedance on the Mg metal anode.
References
[1] D. Aurbach et al., Nature, 407 (2000) 724.
[2] D. Aurbach et al., Adv. Mater., 19 (2007) 4260.
[3] F. Mizuno et al., Frontiers in Energy Research, (2014) doi: 10.3389/fenrg.2014.00046.
[4] D. Aurbach et al., Electrochem. Solid-State Lett., 148 (2001) A1004.
[5] D. Aurbach et al., J. Electrochem. Soc., 3 (2000) 31.
[6] F. Mizuno et al., submitted for publication (2015).

Figure 1