A Novel Conceptual Mg Battery with High Rate Capability

Lithium ion batteries (LIBs) are quickly becoming the mainstream power sources for environmentally friendly vehicles such as hybrid vehicles (HV), plug-in hybrid vehicles (PHV) and electric vehicles (EV) due to their high energy density. On the other hand, a battery system with even higher energy density is required for the long-range PHV and EV applications, and therefore, post lithium ion batteries (PLIB) such as Li-sulfur batteries and Li-air batteries have been getting more attention in recent years. Rechargeable magnesium batteries are also a candidate for the PLIB thank to the natural abundance of magnesium and the absence of dendrite formation when magnesium metal is used as the anode. In addition, the magnesium batteries are expected to have high energy density because of its divalent nature. However, there is not much progress on the development of novel cathodes since an innovation of Chevrel phase materials such as Mg_xMo₃S₄. The difficulty lies in the strong polarization character of the small and divalent Mg²⁺ and consequently the intercalation and diffusion of Mg²⁺ ions is somewhat difficult and complicated.

Here, we report an interesting approach to find another redox reaction on the cathode side that replaces the classic intercalation reaction and balances the electron transfer.  The charge transfer is achieved through the simultaneous transport of dual ions in the electrolyte during the electrochemical cycling. Reversibility on the interesting chemistry was confirmed by multiple techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, Scanning electron microscopy and so on. Figure 1 shows one of the conceptual Mg batteries comprising a Mg metal anode, a AgCl cathode and a Grignard-based electrolyte.  As can be seen, fairly flat discharge-charge curves are repeatedly observed.  Also, even at a rate of 10 C, the cell works very well and maintains over 100 mAh/g which is over 50 % of the capacity observed at 0.1 C. The significant change in cathode material design gave us a new finding of an impressive Mg battery with high rate capability.