Batteries containing magnesium ions (Mg²⁺) would, theoretically, have a higher energy density than current lithium-ion batteries on the market because they would enable the use of a dense and light Mg metal anode. However, a significant factor in the lack of functional devices is the challenge associated with the need to move the doubly charged ions through the structure of the solid materials that must be used as a cathode to complete a functional device.

Research from the Joint Center for Energy Storage Research (JCESR) in the USA has used theoretical a​nd experimental methods to investigate the movement of this ion, to try to understand what prevents these materials from forming viable battery materials. They found that the ionic movement itself that was not limiting the function of the cathode, but rather that the material left behind once the ions had been removed was highly unstable. This change in thinking renews the prospect of finding suitable materials capable of high density of energy storage in a battery.

The materials investigated in this study were the metal oxides MgMn₂O₄ and MgCr₂O₄. These oxides take a 'spinel' structure, a structure predicted to provide the favourable combination of capacity, voltage of operation and ionic mobility when used as a battery cathode.

After making the metal oxides, the researchers used a combination of powder diffraction, ²⁵Mg variable temperature solid-state nuclear magnetic resonance (VT ss-NMR), and muon spin relaxation (μSR) studies on EMU to investigate the barrier to movement of the Mg²⁺ ions.

Using muons to measure magnesium motion has not been done before, as it is more challenging than measuring lithium or sodium motion. To measure ionic motion, the muon relies on an ion with a nuclear moment moving past. For both lithium and sodium, there is 100% abundance of an isotope with a nuclear moment, so every ion that moves past is counted. However, with magnesium, the abundance is only 10%, so there is less signal to detect.

“Contrary to existing assumptions, the barriers to movement were not as large as expected," explains Jordi Cabana, Associate Professor at the University of Illinois, Chicago; “In fact, the voltage needed to move the ions is sufficiently low enough for the material to function as an electrode, renewing the impetus to investigate these materials as potential cathode materials for batteries."

Further information

The full paper can be found at: https://doi.org/10.1021/acs.chemmater.9b02450

More science highlights from EMU can be found here.