Scientists have pioneered a method to enable the reversible chemistry of magnesium metal in the noncorrosive carbonate-based electrolytes and tested the concept in a prototype cell. The technology possesses potential advantages over lithium-ion batteries—notably, higher energy density, greater stability, and lower cost.
Magnesium (Mg) batteries theoretically contain almost twice as much energy per volume as lithium-ion batteries. But previous research encountered an obstacle: chemical reactions of the conventional carbonate electrolyte created a barrier on the surface of magnesium that prevented the battery from recharging. The magnesium ions could flow in a reverse direction through a highly corrosive liquid electrolyte, but that barred the possibility of a successful high-voltage magnesium battery.
In seeking to overcome these roadblocks, the researchers developed an artificial solid-electrolyte interphase from polyacrylonitrile and magnesium-ion salt that protected the surface of the magnesium anode. This protected anode demonstrated markedly improved performance.
The scientists assembled prototype cells to prove the robustness of the artificial interphase and found promising results: the cell with the protected anode enabled reversible Mg chemistry in carbonate electrolyte, which has never been demonstrated before. The cell with this protected Mg anode also delivered more energy than the prototype without the protection and continued to do so during repeated cycles. Furthermore, the group has demonstrated the rechargeability of the magnesium-metal battery, which provides an unprecedented avenue for simultaneously addressing the anode/electrolyte incompatibility and the limitations on ions leaving the cathode.
In addition to being more readily available than lithium, magnesium has other potential advantages over the more established battery technology. First, magnesium releases two electrons to lithium’s one, thus giving it the potential to deliver nearly twice as much energy as lithium. And second, magnesium-metal batteries do not experience the growth of dendrites, which are crystals that can cause short circuits and consequently dangerous overheating and even fire, making potential magnesium batteries much safer than lithium-ion batteries.
Commentary from Goatguy
The problem with rechargeable batteries is that there’s a tradeoff between the ionic material involved with storing the power and the cost of the stuff. Lithium, as it is, is not particularly abundant. Luckily, it gets concentrated in certain ancient inland salt seas, and their crystalline underground brines. Luckily. And luckily we have identified no less than 100 years supply of lithium around the world. But it still remains a relatively expensive material. Hard to handle, legendarily flammable, air-unstable.
But magnesium. Well… now that is attractive. Carrying 2 times the electrons as lithium per MOLE. 1.74 g/cc (compared to lithium 0.534 g/cc). atomic mass of 24.3 g/mol (lithium 6.9 g/mol), on the surface it wouldn’t look like a “win”.
But DANG it is abundant.
We make construction cement out of it. (Dolomite type)
If magnesium is 2x more electron abundant, the math works out to:
1/2 Mg = 1 Li (moles)
1/2 24.3 g/mol = 12.2 g Mg … = … 6.9 g Li … or about 2x the mass for same energy.
12.2 g / 1.74 g/cc = 6.98 cc/unit. for magnesium.
6.9 g / 0.534 g/cc = 12.9 cc/unit. for lithium!!!
Ah: now we’re getting somewhere. Magnesium only takes about half as much VOLUME to store the same number of electrons as lithium. Yes, lithium’s low density is attractive for a host of reasons. But for transportation batteries (not laptop batteries!) being dense AND high-energy storing isn’t terribly negative. Volume efficiency is important.
The electronegativity (0.98 vs 1.38) of lithium and magnesium is important: it determines the voltage of the half-cells in a rechargeable battery. The anode chemistry delivers the other half-cell of the voltage equation. All in all one can expect magnesium cells to be somewhat lower output voltage than lithium’s 3.3 V. Maybe 2.9 V. If however somehow magnesium can be fabricated into tough, non-toxic, safe “18650” type cells, and if the cells deliver similar or superior volumetric storage capacity to lithium, then we ought to see a period of significant price-per-kilowatt-hour improvement as they come online. Perhaps they are even better than lithium at making so-called prismatic cells. Much larger than the little 18650 battery format. Much larger reduces the overhead to “packaging” that cells have.
Magnesium-based batteries possess potential advantages over their lithium counterparts. However, reversible Mg chemistry requires a thermodynamically stable electrolyte at low potential, which is usually achieved with corrosive components and at the expense of stability against oxidation. In lithium-ion batteries the conflict between the cathodic and anodic stabilities of the electrolytes is resolved by forming an anode interphase that shields the electrolyte from being reduced. This strategy cannot be applied to Mg batteries because divalent Mg2+ cannot penetrate such interphases. Here, we engineer an artificial Mg2+-conductive interphase on the Mg anode surface, which successfully decouples the anodic and cathodic requirements for electrolytes and demonstrate highly reversible Mg chemistry in oxidation-resistant electrolytes. The artificial interphase enables the reversible cycling of a Mg/V2O5 full-cell in the water-containing, carbonate-based electrolyte. This approach provides a new avenue not only for Mg but also for other multivalent-cation batteries facing the same problems, taking a step towards their use in energy-storage applications.