ADELPHI, Md. -- In a joint research effort, scientists from the U.S. Army Research Laboratory have further opened the door to the possibility of a low-cost, safe and higher voltage replacement for lithium-ion batteries.
In today's world, our lives revolve around small digital devices that are readily available whenever we need them, no matter where we are.
Behind this life-changing technology lies the high energy density lithium-ion battery.
Lithium-ion batteries have matured, approaching the ceiling energy density allowed by their chemical nature, using inert frameworks as the intercalation host for Li-ions.
These frameworks ensure good reversibility (long cycle life), but limit the energy density because they don't participate in cell reactions.
Scientists have been looking for alternative chemistries that can store more energy per unit weight, and have set their eyes on the bivalent cation chemistries, such as magnesium and zinc, which carry twice the charge of a single lithium-ion battery.
Magnesium batteries have been viewed as a potential replacement for the lithium-ion battery for more than two decades, due to its high abundance in earth-crust, low cost, safety, non-dendritic nature and most importantly, a volumetric capacity even higher than lithium-metal, thanks to its bivalence.
However, the fundamental scientific barrier to a magnesium-based battery also lies in its bivalence.
Roughly the same size of Li-ion (Li+) but bearing twice as much the charge, the movement of Mg2+ through electrolytes and electrode materials is especially clumsy.
At the interphases separating electrode and electrolyte, it is almost impossible to move an Mg2+ cation across.
Hence, all magnesium electrolytes under exploration thus far must rely on exotic electrolytes that do not form any interphase, which include Grignard reagents or organometallic compounds dissolved in ether solvents.
Unfortunately, these exotic electrolytes are toxic, highly flammable, volatile, extremely corrosive and particularly unstable against oxidation.
These electrolytes restrict the voltages of all designed magnesium batteries to a very low level (~1.5 V), making the potential capacity of magnesium electrode inaccessible.
The scientists said a dilemma exists because of the bivalence of magnesium.
Through a collaborative research effort, scientists at the National Renewable Energy Laboratory, ARL and the University of Maryland have eliminated this "Mg dilemma" caused by incompatibility between magnesium electrode and high voltage electrolytes.
The work, entitled "Mg2+-Conducting Film as Artificial Interphase Enables Reversible Mg Chemistry in Nitrile- and Carbonate Electrolytes," is published in the newest issue of Nature Chemistry, and was led by Dr. Chunmei Ban at NREL and co-authored by Dr. Kang Xu at ARL and Prof. Chunsheng Wang at UMD.
In this work, these scientists report for the first time an artificial interphase tailored on magnesium metal anode surface, which is not only Mg2+ conducting and facilitating reversible magnesium deposition and stripping, but more importantly protects magnesium from reacting with conventional electrolytes.
The artificial interphase is a polymeric film that is pre-loaded with Mg2+, which serves as a protection layer between magnesium metal anode and the electrolytes, but allows fast Mg2+ conduction as shown in the above figure.
With this interphase in place, these exotic and unstable electrolytes are no longer required. Instead, the use of electrolyte solvents that are oxidation-resistant is allowed, and the upper voltage limit of a practical magnesium battery is hence raised to 3.0 V level.
The composite magnesium electrode protected by such artificial interphase was demonstrated in nitrile- and carbonate-based electrolytes using non-corrosive magnesium salts (Mg(TFSI)2 and Mg(ClO4)2), in both symmetric magnesium cell and full magnesium batteries, with marked improvements over the unprotected magnesium electrodes.
Such artificial interphase on magnesium surface opens up a new avenue for magnesium electrolytes design and synthesis that might eventually lead to a high voltage (> 3.0 V) magnesium battery.
"The Army will benefit from the exploration of new battery chemistries that bring higher energy density and higher safety," Xu said. "The bivalent chemistries based on magnesium and zinc promised such merits, but they suffer from poor stability. This work resolved a crucial scientific problem of magnesium chemistry that has been plaguing the community for decades and brought us one step closer to its practical application."
According to Dr. Arthur von Wald Cresce, ARL materials scientist who works closely with Xu, magnesium batteries are a promising path forward for Soldiers and the systems they use in combat.
"Magnesium-ion batteries are a step forward for Soldier batteries in that magnesium has almost double the volumetric energy density compared to lithium," Cresce said. "Magnesium is also more abundant than lithium, its sources are common and its price is much less tied to the politics/business situation internationally. Magnesium-ion batteries are a prime candidate for vehicle electrification and stationary storage, where volumetric efficiency tends to be more important than gravimetric energy density."
Xu and Cresce stated that without the collaborative nature of this research and the resources shared, the results would not have been as effective as they were.
"Collaborative efforts are the primary way researchers use to multiply their effort in a complex system like a battery," Cresce said. "We like to think we could do it all ourselves, but the time required is prohibitive and unrealistic. It's also very important to tap into different points of view and utilize other pieces of equipment that we may not have at ARL. Collaboration is a key way to advance ARL's goals in a real and meaningful way, in a time frame that reflects the rapid movement of innovation in the technology industry.
According to Xu, there is more work to be done to make this battery a useable technology for the battlefield.
"We have developed an efficient artificial interphase in this work," Xu said. "In the next step, we will explore the possibility of forming the same or similar interphases in the "in-situ" manner within a battery. Only that approach could make the large-scale application practical."
While crucial for the Solider of the future, magnesium batteries do have potential applications off of the battlefield.
"Outside of the Department of Defense, the need for batteries for vehicles and stationary storage is just as strong, if not more so," Cresce said. "Volumetrically efficient magnesium-ion batteries should be price-competitive with lithium-ion batteries given the costs of the source materials, which would position them to compete with lithium-ion batteries in cars and trucks."
In addition, Xu and Cresce think that magnesium-ion batteries would also be a key technology to enable microgrid electrical storage, where power generated and not used can be stored for later release into the grid.
Such a storage system could form the basis for local civilian microgrids in the same way they would work for Army forward operating bases.
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The U.S. Army Research Laboratory is part of the U.S. Army Research, Development and Engineering Command, which has the mission to ensure decisive overmatch for unified land operations to empower the Army, the joint warfighter and our nation. RDECOM is a major subordinate command of the U.S. Army Materiel Command.
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