Researchers from the University of Houston and the Toyota Research Institute of America have discovered a promising new version of high-energy magnesium batteries, with potential applications ranging from electric vehicles to battery storage for renewable energy systems.

The battery, reported in the journal Joule, is the first to operate with limited electrolytes while using an organic electrode, a change the researchers said allows it to store and discharge far more energy than earlier magnesium batteries. They used a chloride-free electrolyte, another change from the traditional electrolyte used by magnesium batteries, which enabled the discovery.

Magnesium batteries could offer high energy density and safety due to the non-dendritic Mg metal anode. However, Mg²⁺ ingress into and diffusion within cathode materials are kinetically sluggish. It is therefore intriguing that recently organic cathodes were shown to deliver high energy and power even at room temperature.

Herein we reveal that previous organic cathodes likely all operated on a MgCl-storage chemistry sustained by a large amount of electrolyte that significantly reduces cell energy. We then demonstrate Mg batteries featuring a Mg²⁺-storage chemistry using quinone polymer cathodes, chloride-free electrolytes, and a Mg metal anode.

Under lean electrolyte conditions, the Mg²⁺-storing organic cathodes deliver the same energy while using ~10% of the amount of electrolyte needed for the MgCl-based counterparts. The observed specific energy (up to 243 Whr kg^-1), power (up to 3.4 kW kg^-1), and cycling stability (up to 87% at 2,500 cycles) of Mg-storage cells consolidate organic polymers as promising cathodes for high-energy Mg batteries.

—Dong et al.

Schematic illustrations of the working mechanism of organic cathodes in (left) chloride-containing and (right) chloride-free electrolytes. Dong et al.

Yan Yao, associate professor of electrical and computer engineering at UH, said the researchers were able to confirm that chloride in the commonly used electrolyte contributes to sluggish performance.

Scientists have spent decades searching for a high-energy magnesium battery, hoping to take advantage of the natural advantages that magnesium has over lithium, the element used in standard lithium ion batteries. Magnesium is far more common and therefore less expensive, and it’s not prone to breaches in its internal structure that can cause lithium batteries to explode and catch fire.

But magnesium batteries won't be commercially competitive until they can store and discharge large amounts of energy. Yao said previous cathode and electrolyte materials have been a stumbling block.

Other researchers on the project include first authors Hui Dong, a doctoral student at UH, and Yanliang Leonard Liang, research assistant professor at UH; Oscar Tutusaus and Rana Mohtadi, both with the Toyota Research Institute of North America; and UH doctoral students Ye Zhang and Fang Hao.

Liang noted that until now, the best cathode for magnesium batteries has been a Chevrel phase molybdenum sulfide, developed almost 20 years ago. It has neither the power nor the energy storage capacity to compete with lithium batteries, he said.

But recent reports suggest organic cathode materials can provide high storage capacity at room temperature. Dong said both organic polymer cathodes tested provided higher voltage than the Chevrel phase cathode.

Yao said future research will focus on further improving the specific capacity and voltage for the batteries in order to compete against lithium batteries.

Resources

• Dong et al. (2018) “Directing Mg-Storage Chemistry in Organic Polymers toward High-Energy Mg Batteries,” Joule doi: 10.1016/j.joule.2018.11.022