Lithium ion batteries have become the energy storage medium of choice for everything from cell phones to electric vehicles and large-scale support of power grids. But lithium isn’t the only lightweight metal that can be used in batteries. As a result, researchers around the world are examining other candidates for possible commercialization.
Magnesium is of particular interest. Potentially, batteries made from magnesium metal could have higher energy density, greater stability, and lower cost than today’s lithium ion cells. At least those are the predictions of scientists at the Department of Energy’s National Renewable Energy Laboratory (NREL), who have detailed in Nature Chemistry their recent results with rechargeable magnesium metal batteries.
Chunmei Ban, a scientist in NREL’s Materials Science department and a co-author of the Nature paper, stated the following in an NREL press release: “The dominant lithium ion battery technology is approaching the maximum amount of energy that can be stored per unit volume, so there is an urgent need to explore new battery chemistries that can provide more energy at lower cost.”
Magnesium has several other advantages over lithium in battery applications. Each magnesium atom releases two electrons during the battery discharge phase, compared to one electron for lithium. This gives it the potential to deliver nearly twice the electrical energy that is possible from a lithium cell. In addition, magnesium does not grow dendrites on the metal surface during the battery charging phase. The spiky dendrite crystals that grow on a lithium metal surface can cause short-circuiting of the battery. The lack of dendrite growth should make magnesium batteries easier to handle and safer. Lastly, magnesium is more common and readily available than lithium. According to the USGS, it is the eighth most abundant element and can be commercially extracted from mineral deposits or seawater.
In battery applications, however, magnesium has had some problems. This is where the NREL scientists have concentrated their efforts. The biggest problem is that magnesium reacts with conventional carbonate electrolytes to create a passivating layer on the surface of the metal that acts as a barrier and prevents magnesium ions from reaching the magnesium metal during charging. NREL researchers needed to find a solid interphase layer, which they could use to coat the magnesium metal to prevent the reaction with the electrolyte—but that would still allow the transfer of magnesium ions to and from the metal surface.
|On the left side, without a coating, the magnesium forms a layer that prevents ions from passing to and from the metal. On the right side, the artificial layer produced by the NREL researchers allows magnesium ions to pass from the metal to the electrolyte. (Image source: John Frenzl/NREL)|
They chose polyacrylonitrile, a synthetic, semicrystalline, organic polymer resin. It is used, in fiber form, to make products as varied as outdoor awnings, sails for yachts, reinforcement for concrete, and as a starting point for strands of carbon fiber. The organic polymer is mixed with magnesium-ion salt and coats the magnesium metal to protect it from the electrolyte. A prototype battery cell made with the coated magnesium metal has shown great promise and enabled reversible charging and discharging—something never demonstrated before, according to NREL.
As with nearly every breakthrough in battery research, the NREL announcement about its magnesium battery must be looked at as only the first step in a long road to commercialization. Still, it is a beginning and will hopefully inspire others to examine magnesium as a possible eventual replacement for lithium.
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.