Researchers at DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Argonne National Laboratory were already working on a magnesium battery (earlier post), which offers higher energy density than lithium, but were stymied by the dearth of good options for a liquid electrolyte, most of which tend to be corrosive against other parts of the battery.

Magnesium offers a number of advantages over lithium: it is safer and earth-abundant, and doubles the total charge per ion, delivering larger theoretical volumetric capacity compared with a typical lithium-ion battery. Further, in Mg batteries (MB) the anode is energy-dense Mg metal (~ 3,830  Ah l⁻¹)—surpassing the theoretical volumetric energy density of graphite anodes (~ 700 Ah  l⁻¹) and even that of lithium metal (2,062  Ah  l^–1).

Magnesium is such a new technology, it doesn’t have any good liquid electrolytes. We thought, why not leapfrog and make a solid-state electrolyte?

—Gerbrand Ceder, a Berkeley Lab Senior Faculty Scientist

However, poor mobility of Mg²⁺ (and other multivalent cations) prevents the development of a broad spectrum of cathode materials, as are available to lithium- and sodium-ion battery technologies. Poor Mg transport also limits the use of solid barrier coatings to protect electrodes from reaction with the liquid electrolyte, or the development of full solid-state MBs, which would alleviate many of today’s problems caused by liquid electrolytes, the researchers noted.

In this study we show that high Mg²⁺ mobility in solids can be achieved by judicious tuning of crystal structure and chemistry. By combining ab initio calculations, synchrotron X-ray diffraction (XRD), electrochemical impedance spectroscopy and solid-state nuclear magnetic resonance (SS-NMR), we demonstrate facile Mg²⁺ conduction at room temperature. Experimentally, we demonstrate the discovery of the first generation of crystalline solids, i.e., spinel MgX₂Z₄, with X = (In, Y, Sc) and Z = (S, Se), which possess high Mg²⁺ cation mobility at room temperature. In addition, we propose practical design rules to identify fast multivalent-ion solid conductors. Our theoretical calculations and electrochemical experiments suggest that sulfide and selenide spinels can potentially integrate with current state-of-the-art Mg cathodes, e.g., spinel-MgTi₂S₄ and Chevrel-Mo₆S₈.

—Canepa et al.

The research team also included scientists at MIT, who provided computational resources, and Argonne, who provided key experimental confirmation of the magnesium scandium selenide spinel material to document its structure and function.

Co-author Baris Key, a research chemist at Argonne, conducted nuclear magnetic resonance (NMR) spectroscopy experiments. These tests were among the first steps to experimentally prove that magnesium ions could move through the material as rapidly as the theoretical studies had predicted.

NMR is akin to magnetic resonance imaging (MRI), which is routinely used in medical settings, where it shows hydrogen atoms of water in human muscles, nerves, fatty tissue, and other biological substances. But researchers can also tune NMR frequency to detect other elements, including the lithium or magnesium ions that are found in battery materials.

The NMR data from the magnesium scandium selenide material, however, involved material of unknown structure with complex properties, making them challenging to interpret.

Canepa noted the challenges of testing materials that are so new.

Protocols are basically non-existent. These findings were only possible by combining a multi-technique approach (solid-state NMR and synchrotron measurements at Argonne) in addition to conventional electrochemical characterization.

—Pieremanuele Canepa, lead author

The team plans to do further work to use the conductor in a battery. Additionally, the research identified two related fundamental phenomena that could significantly affect the development of magnesium solid electrolytes in the near future, namely, the role of anti-site defects and the interplay of electronic and magnesium conductivity, both published recently in Chemistry of Materials.

There are enormous efforts in industry to make a solid-state battery. It’s the holy grail because you would have the ultimate safe battery. But we still have work to do. This material shows a small amount of electron leakage, which has to be removed before it can be used in a battery.

—Gerbrand Ceder

Funding for the project was provided by the DOE Office of Science through the Joint Center for Energy Storage Research, a Department of Energy Innovation Hub. The Advanced Photon Source, a DOE Office of Science User Facility at Argonne, added vital data to the study regarding the structure of the solid conductor. The National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility at Berkeley Lab, provided computing resources. Other co-authors on the paper are Juchaun Li of Berkeley Lab, William Richards and Yan Wang of MIT, and Tan Shi and Yaosen Tian of UC Berkeley.

Resources

• Pieremanuele Canepa, Shou-Hang Bo, Gopalakrishnan Sai Gautam, Baris Key, William D. Richards, Tan Shi, Yaosen Tian, Yan Wang, Juchuan Li & Gerbrand Ceder (2017) “High magnesium mobility in ternary spinel chalcogenides,” Nature Communications 8, Article number: 1759 doi: 10.1038/s41467-017-01772-1