Magnesium rechargeable batteries (MRBs) are emerging as an attractive candidate for energy storage in terms of safety, energy density, and scalability because magnesium metal has ideal properties as a battery anode: high capacity, low redox potential, dendrite-free deposition, and earth-abundant resources. Since the first MRB prototyped by Aurbach et al., significant progress has been made in cathodes, electrolytes, and anodes. One critical challenge for MRBs is the development of Mg storage cathodes with higher capacity and operating voltage than Chevrel phase Mo₆S₈ cathodes, which operate at ca. 1 V vs Mg/Mg²⁺ with capacity of ca. 100 mAh g^–1.

… In this work, we report a MRB based on a MgCl⁺ intercalation cathode, a Mg anode, and a standard chloride-based electrolyte. Moving from the divalent Mg²⁺ to the monovalent MgCl⁺ as the charge carrier makes Mg^- ions similar to one-electron-transfer alkaline metal ions where (1) only low-energy desolvation (Ea~0.8 eV) but not high-energy Mg−Cl scission (E_a> 3 eV) is necessary before intercalation and (2) the polarization strength ofthe ion, and hence the ion diffusion energy barrier, is low. The new battery chemistry illustrated using interlayer-expanded titanium disulfide (TiS₂) cathode as an example demonstrates 1 and 1.7 MgCl⁺ intercalation per formula of TiS₂ at 25 and 60 °C, respectively, corresponding to high reversible capacities of up to 400 mAh g⁻¹ based on the mass of TiS₂.

—Yoo et al.

The work was first conceived by Yan Yao, associate professor of electrical and computer engineering at the University of Houston and postdoctoral fellow Hyun Deog Yoo in 2014; the project spanned several years and involved scientists from three universities and three national laboratories, working both experimentally and theoretically.

Magnesium ion is known to be hard to insert into a host. First of all, it is very difficult to break magnesium-chloride bonds. More than that, magnesium ions produced in that way move extremely slowly in the host. That altogether lowers the battery’s efficiency.

—Hyun Deog Yoo, first author

The new battery stores energy by inserting magnesium monochloride into a host, such as titanium disulfide. By retaining the magnesium-chloride bond, Yao said, the cathode demonstrated much faster diffusion than traditional magnesium versions.

Voltage of the new battery remains low at about one volt. That compares to three to four volts for lithium batteries. The high voltage, coupled with their high energy density, has made lithium ion batteries the standard. But lithium is expensive and can develop dendrite growths, which can cause the batteries to catch fire. An earth-abundant resource, magnesium is cheaper—and does not form dendrites.

Until now, however, it has been held back by the need for a better cathode and more efficient electrolytes.

The key, Yoo said, is to expand the titanium disulfide to allow magnesium chloride to be inserted—a four-step process called intercalation—rather than breaking the magnesium-chloride bonds and inserting the magnesium alone. Retaining the magnesium-chloride bond doubled the charge the cathode could store.

The magnesium monochloride molecules are too large to be inserted into the titanium disulfide using conventional methods. Building upon their earlier work, the researchers created an open nanostructure by expanding the gaps in the titanium disulfide by 300 percent, using organic “pillars.”

The increased opening—from 0.57 nanometers to 1.8 nanometers—allowed for the magnesium chloride to be inserted.

We hope this is a general strategy. Inserting various polyatomic ions in higher voltage hosts, we eventually aim to create higher-energy batteries at a lower price, especially for electric vehicles.

—Hyun Deog Yoo

In addition to Yao and Yoo, authors on the paper include Yanliang Liang, Hui Dong, Yifei Li, Qiang Ru, Yan Jing and Qinyou An, all of UH; Junhao Lin and Sokrates T. Pantelides of Vanderbilt University and the Oak Ridge National Laboratory; Wu Zhou of Oak Ridge National Laboratory; Hua Wang and Xiaofeng Qian of Texas A&M University; Yisheng Liu and Jinghua Guo of Lawrence Berkeley National Laboratory; Lu Ma, Tianpin Wu and Jun Lu of Argonne National Laboratory.

Resources

• Hyun Deog Yoo, Yanliang Liang, Hui Dong, Junhao Lin, Hua Wang, Yisheng Liu, Lu Ma, Tianpin Wu, Yifei Li, Qiang Ru, Yan Jing, Qinyou An, Wu Zhou, Jinghua Guo, Jun Lu, Sokrates T. Pantelides, Xiaofeng Qian & Yan Yao (2017) “Fast kinetics of magnesium monochloride cations in interlayer-expanded titanium disulfide for magnesium rechargeable batteries” Nature Communications 8, Article number: 339 doi: 10.1038/s41467-017-00431-9