Toyota working with Brookhaven National Lab on investigation of cathodes for Mg-ion batteries

3 November 2014

As noted in a 2012 paper, Toyota researchers are interested in the potential of rechargeable magnesium-ion (Mg-ion) batteries as a possible post-Li-ion solution. (Earlier post.) To probe molecular structures and track the rapid chemical reactions in these promising batteries, Ruigang Zhang, a Toyota Motor Corporation scientist specializing in energy storage technology and his colleagues are working with the Center for Functional Nanomaterials (CFN) at the US Department of Energy’s Brookhaven National Laboratory.

Magnesium is divalent—i.e., it can thereby displace double the charge per ion ( Mg²⁺ rather than Li⁺). As those ions move back and forth from electrodes during each charge/discharge cycle, the nanometer structure of the battery material degrades and transforms. The degradation rates and patterns—whether uniform or asymmetrical—must be probed in a variety of conditions to understand the underlying mechanisms. Once pinpointed, scientists can then design new atomic architectures or customized compounds that overcome these obstacles to extend battery lifetimes and optimize performance.

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Issues related to cost, power, energy density, and durability of Li-ion batteries have slowed their implementation in large-scale applications, such as electric and hybrid vehicles. A rechargeable magnesium (Mg) battery system is one interesting candidate that offers much greater earth abundance than lithium and higher storage capacity—but the necessary research remains a challenge. Solving the magnesium challenges may open the door for other multivalent batteries such as cadmium or aluminum, thereby shedding light on the next generation of battery technology.

The Toyota researchers plan to target the specific chemistry of a promising magnesium cathode based on fullerenes. The compound offers consistent energy output, a rapid cycling rate, and extremely low voltage hysteresis (i.e., it stays relatively intact even after many cycles of charge and discharge).

(The Toyota team presented some of their work on and observations on challenges with Mg-ion cathodes at the 226^th meeting of the Electrochemical Society in October.)

Despite all that, the performance and ease of battery integration still need work, and scientists need a full understanding of structural evolution, crystallization mechanisms, and other factors influencing the electrochemical reactions.

Unfortunately, our preliminary x-ray diffraction (XRD) results indicated material amorphization—a loss of crystalline structure—during operation that makes it challenging to follow the structural evolution. We now plan to use the advanced electron microscope facilities at CFN for local structural studies, particularly to track the reaction as it occurs rather than just before or after.

Zhang’s team will work with Feng Wang of Brookhaven Lab’s Sustainable Energy Technologies Department (who is leading the collaboration with Zhang’s team) to use CFN’s high-resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) techniques to identify morphologies and chemical elements as they emerge or transform. In these techniques, a focused beam of electrons strikes and interacts with the material’s atomic structure and then carries that information into highly sensitive detectors.

Wang will work alongside CFN’s electron microscopy staff and other experts across Brookhaven Lab to facilitate Toyota’s research and pioneer new technologies and techniques.

Down the road, we also plan to use Brookhaven’s powerful new x-ray light source facility, the National Synchrotron Light Source II, to investigate battery properties and reaction evolutions in real time under real-world reaction conditions.

The world-leading NSLS-II, scheduled to begin operations in 2015, will achieve single-nanometer resolution and enable unprecedented in operando energy research.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy (DOE). CFN and NSLS-II are both DOE Office of Science User Facilities. Toyota researchers gained access to the facilities on a competitive peer-reviewed basis as users undertaking open, non-proprietary research.

The Office of Science is the single largest supporter of basic research in the physical sciences in the United States.

Resources

• Fuminori Mizuno, Timothy S. Arthur, Ruigang Zhang and Chen Ling () “Beyond Chevrel Phase Mo₆S₈ : High Energy Density Mg Battery Cathodes” (2014 ECS, Nº 108)