A team at Toyota Research Institute of North America (TRINA) reports a critical advance in the the development of electrolytes for magnesium (Mg) batteries in the journal Angewandte Chemie. The researchers, led by Dr. Rana Mohtadi, Principal Scientist at TRINA, developed an electrolyte based on a simple-type magnesium monocarborane salt (MMC) that is compatible with Mg metal (> 99 % coulombic efficiency); possesses high anodic stability (3.8 V vs. Mg); and is non-corrosive. By contrast, state-of-the-art Mg electrolyte systems are complex, halogen-based, and corrosive.
The properties of the new electrolyte, coupled with its “inert and benign character”, make MMC-based electrolytes well-suited for future Mg batteries, the scientists said. The development of this non-corrosive electrolyte enabled the first demonstration of a high voltage coin cell battery, previously prohibited using all known systems. “This achievement is a turning point in the research and development of Mg electrolytes that has deep implications on realizing practical rechargeable Mg batteries,” the scientists wrote.
TRINA researchers have been investigating magnesium batteries for a number of years (e.g., earlier post). Because magnesium is divalent, it can displace double the charge per ion (i.e., Mg2+ rather than Li+). As an element, magnesium is much more abundant than lithium, and more stable. Magnesium-ion batteries theoretically could offer good electrochemical performance, while being safer and less expensive than Li-ion batteries. However, Mg-ion batteries have suffered from a number of limitations, among them being anode/electrolyte incompatibility.
Currently, the prospect of attaining energy densities beyond those offered by current lithium-ion batteries is driving interest in rechargeable magnesium batteries. Mg metal offers high volumetric capacity (3833 mAh cm-3 vs. 2036 mAh cm-3 for Li metal) while being non-dendritic and abundant in the earth crust (fifth most abundant element). Since Aurbach et al. demonstrated the first and only rechargeable Mg battery prototype, challenges toward realizing Mg batteries still remain. These stem from the absence of practical electrolytes and high capacity/high voltage cathodes. For instance, the field demands electrolytes capable of operating at high voltages whilst being compatible with Mg metal and all other battery components.—Tutusaus et al.
Early work suggested that salts uses in Li-ion batteries would fail in Mg system because they passivate the Mg metal surface. Alternative strategies that successfully resulted in complex solutions with high anodic stability; however these corrosive solutions had reduced compatibility with other battery components such as steel casings and current collectors.
In 2014, Mohtadi and her TRINA colleagues demonstrated that Mg(BH4)2 was a highly competent electrolyte for magnesium-battery applications. This electrolyte was the first non-organomagnesium electrolyte compatible with Mg metal and provided excellent electro-chemical performance.
In tandem with further research into this system, the TRINA team sought to pursue potential routes to materials with enhanced oxidative stability as compared to Mg(BH4)2’s observed oxidation onset potential of 1.7 V (vs. Mg on Pt).
High oxidative stability is crucial for the development of Mg batteries for operation with future high-voltage cathodes. Although organomagnesium compounds with oxidative stability above 3 V (vs. Mg) have been reported, most have significantly reduced oxidation onset potentials when deposition and stripping occurs on non-noble-metal surfaces. This reduced stability has been linked to corrosion of the metal by the electrolyte solution and constitutes a major hurdle to the utilization of current high-voltage electrolytes, as non-noble metals are the most appropriate materials for the construction of battery-casing and current-collector materials. Notably, Mg(BH402 displayed enhanced stability on these metals, as corrosion of the electrode material did not occur. We therefore sought to identify an electrolyte candidate which would retain the attractive properties of Mg(BH4)2 but offer enhanced oxidative stability. The identification of such an electrolyte would effectively overcome a major hurdle towards the development of Mg batteries with high energy densities.—Tutusaus et al.
Pursuing a new design concept involving boron cluster anions, they found that monocarborane CB11H12- produced the first halogen-free, simple-type Mg salt that is compatible with Mg metal and displays an oxidative stability surpassing that of ether solvents.
In the short term, tangible impact of MMC electrolyte on accelerating the research and development of high-voltage cathodes is expected by enabling the examination of a variety of cathodes in coin cells. Finally, as the anodic stability of MMC surpasses that of ethers, discovering new Mg-compatible and oxidatively stable solvents emerges as one of the new avenues for electrolyte research that can widen the portfolio of accessible high-voltage Mg cathodes.—Tutusaus et al.
Oscar Tutusaus, Rana Mohtadi, Timothy S. Arthur, Fuminori Mizuno, Emily G. Nelson, and Yulia V. Sevryugina (2015) “An Efficient Halogen-Free Electrolyte for Use in Rechargeable Magnesium Batteries”Angewandte Chemie International Edition doi: 10.1002/anie.201412202
Tyler J. Carter, Rana Mohtadi, Timothy S. Arthur, Fuminori Mizuno, Ruigang Zhang, Soichi Shirai, and Jeff W. Kamp (2014) “Boron Clusters as Highly Stable Magnesium-Battery Electrolytes” Angewandte Chemie International Edition Volume 53, Issue 12 Pages: 3173–3177 doi: 10.1002/anie.201310317