Reinventing the magnesium battery
“The worldwide push to advance renewable energy is limited by the availability of energy storage vectors,” says Texas A&M University chemist Sarbajit Banerjee. “Currently, lithium-ion technology dominates; however, the safety and long-term supply of lithium remain serious concerns. By contrast, magnesium is much more abundant than lithium, has a higher melting point, forms smooth surfaces when recharging, and has the potential to deliver more than a five-fold increase in energy density if an appropriate cathode can be identified.”
In their research, the team focused on a redesigned form of an old Li-ion cathode material – vanadium pentoxide – which they proved is capable of reversibly inserting magnesium ions.
“We’ve essentially reconfigured the atoms to provide a different pathway for magnesium ions to travel along, thereby obtaining a viable cathode material in which they can readily be inserted and extracted during discharging and charging of the battery,” says Banerjee.
The researchers achieved this by limiting the location of the magnesium ions to relatively “uncomfortable” atomic positions by design, based on the way the vanadium pentoxide is made – a property known as metastability. This metastability helps prevent the magnesium ions from getting trapped within the material and promotes complete harvesting of their charge-storing capacity with negligible degradation of the material after many charge-recharge cycles.
The researchers were able to observe the process by which such ions move in and out of other materials within batteries – called intercalation – by using of one of the world’s most powerful soft X-ray microscopes at the Canadian Light Source in tandem with one of the world’s highest-resolution aberration-corrected transmission electron microscopes, housed at the University of Illinois at Chicago (UIC). This enabled them to directly observe and prove magnesium-ion intercalation into their novel vanadium pentoxide material.
Previously, magnesium-ion technology, while promising, has been difficult to use to make viable magnesium batteries. Mainly, says Banerjee, the magnesium ions get waylaid as they are traversing through the paths within the cathode material, resulting in “sluggish” movement.
“In many structures, some of these interactions are very favorable, meaning that the magnesium is quite happy to sit and stay a while in those specific sites,” says Texas A&M chemistry graduate student and NASA Space Technology Research Fellow Justin Andrews, first author of a paper on the research. “In our material, the magnesium is ‘frustrated’ as it moves through the lattice, because it encounters many less-than-optimal environments. In this sense, it is more than happy to just keep moving right along, leading to an improvement in capacity and diffusion.”
This latest research, says Andrews, marks an important turning point and represents a significant advance toward solving the cathode problem of magnesium batteries. However, say the researchers, there are still several other fundamental problems to overcome before magnesium batteries become a reality.
For more, see “Reversible Mg-Ion Insertion in a Metastable One-Dimensional Polymorph of V2O5.”
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