Much of the focus of new battery development has been to improve the performance of lithium-ion batteries. But these batteries have suffered in the past from negative characteristics such as flammability, which is leading researchers to look at other options.

In a previous TLT article, the preparation of lithium-sulfur batteries using the natural material lignin as the source for the sulfur-carbon composite cathode was described (1). Lithium-sulfur batteries have strong potential to be used commercially because they theoretically can hold more than double the energy compared to conventional chemistries. Initial work showed that the lithium-sulfur battery exhibited better durability compared to batteries prepared with pure sulfur or bulk sulfur-carbon cathodes.

Another battery type that has been under evaluation is produced from magnesium (see Figure 3). Yan Yao, associate professor of electrical and computer engineering at the University of Houston in Houston, says, “Magnesium anodes display much higher volumetric capacities and are potentially safer than lithium batteries. The latter is due to magnesium anodes not forming dendrites, which can lead lithium batteries to short circuit.”


Figure 3. The potential for eventually using magnesium batteries has been shown in the development of a new design that uses a minimal amount of a chloride-free electrolyte in a lean electrolyte design. (Figure courtesy of the University of Houston.)

Compared to lithium, Yao pointed out that magnesium is more readily available. 

Yao says, “The challenges are to find a suitable electrolyte and cathodic material for use in magnesium batteries. Most electrolytes currently used contain chloride anions, which are corrosive. Magnesium’s divalent nature leads to strong polarization, which generates high energy barriers as cations of this element migrate through negatively charged intercalation hosts. Many cathodic materials have been evaluated with limited success.”

The focus of magnesium battery electrolyte development has been on nonaqueous materials because of the corrosivity of magnesium in water. Yao indicated that several problems exist with using chloride in the electrolyte. He says, “The presence of chloride in the electrolyte leads to the formation of ion pairs.”

In collaboration with researchers at the Toyota Research Institute of North America, the researchers have now developed a magnesium battery using an organic polymer-based cathode, chloride-free electrolyte and magnesium metal anode that displays high specific energy, power and cycling stability not seen previously.

Lean electrolyte design
A design for a magnesium battery was developed where the cathode is either a polymeric quinone known as poly(1,4-anthraquinone) (P14AQ) or the conjugated redox polymer poly{[N, N’-bis(2-octyldodecyl) -1,4,5,8,-naphthalenedicarboximide-2,6-diyl] -alt-5,5’-(2,2’-bithiophene)} (P(NDI2OD-T2)). The electrolytes of choice were based on bis (trifluoromethane) sulfonimide and closo-carborane anions that were weakly coordinating with magnesium cations. Both materials were mixed with the organic solvent, tetraglyme, to produce the electrolyte. 

Yao says, “P14AQ and P(NDI2OD-T2) were selected because they both displayed promising results in lithium-ion batteries. We found that the use of these cathodes and electrolytes with a magnesium anode generated specific energies up to 243-watt hours per kilogram and specific powers up to 3.4 kilowatts per kilogram when only considering the mass of active materials. The cell with P(NDI2OD-T2) demonstrated good stability when operated for 2,500 cycles, which represents more than 700 hours. Only a 13% drop in battery capacity was found.”

Yao believes this represents the most stable cycling performance for a non-aqueous magnesium battery. 

An attractive feature of this battery is the use a minimal amount of electrolyte or lean electrolyte design. 

The researchers found that when magnesium chloride and magnesium were both used in a hybrid-ion cell design, this complicated the operation of the battery. Magnesium cations and chloride anions are stored in the cathode, but the anode only stores magnesium. This means the electrolyte must not only act as an ion conductor but also as a reservoir for chloride anions. The result is the amount of chloride anions in the electrolyte must match the capacity of the cathode limiting the practical cell energy the battery can produce. 

Three-electrode cells were used in this study so that the researchers could evaluate many combinations of cathodes and electrolytes. Two-electrode coin cells were used in evaluating battery systems with lean electrolyte content. The researchers evaluated the voltage profiles of organic magnesium ion batteries at specific current densities.

The researchers will now be determining how to improve the performance of the organic magnesium batteries. Yao says, “We anticipate preparing electrodes with higher mass loadings that require lower electrolyte demand. The goal is to further boost the voltage generated by the battery. We feel these steps are equally important in developing better cathode materials.”

Additional information on this research can be found in a recent article (2) or by contacting Yao at yyao4@uh.edu. 

REFERENCES
1. Canter, N. (2018), “Preparation of lithium-sulfur batteries from lignosulfonate,” TLT,” 74 (7), pp. 12-13.
2. Duong, H., Liang, Y., Tutusaus, O., Mohtadi, R., Zhang, Y, Hao, F. and Yao, Y. (2018), “Directing Mg-Storage Chemistry in Organic Polymers toward High-Energy Mg Batteries,” Joule, DOI: Available here.