Researchers Demonstrate a Transformative Step to Boost the Energy Density of Lithium-Ion Batteries

According to a new study, soldiers who carry batteries weighing 15 to 25 pounds could now carry batteries that weigh significantly less but come with the same level of energy and offer better safety.

An Army illustration shows a molecular structure of the fully charged cathode developed in this work. (Image credit: Army illustration)

Now, in the recent issue of the journal Nature, a research team at the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory—the Army’s corporate research laboratory called ARL—and the University of Maryland (UMD) showed a revolutionary step in battery technology with the detection of a novel cathode chemistry.

Entirely free of transition metal and providing unparalleled high capacity by reversibly preserving Li-ion at high potential (approximately 4.2 V), the latest finding presents new opportunities to considerably boost the energy density of lithium-ion batteries and, at the same time, preserve safety because of the electrolyte’s aqueous nature, stated Dr Kang Xu, an ARL fellow and senior research chemist.

Such a high energy, safe and potentially flexible new battery will likely give the Soldiers what they need on the battlefield: reliable high energy source with robust tolerance against abuse. It is expected to significantly enhance the mobility and lethality of the Soldier while unburdening logistics requirements.

Dr Kang Xu, ARL Fellow and Senior Research Chemist, U.S. Army Research Laboratory

Based on their earlier findings of the intrinsically safe “water-in-salt electrolytes, or WiSE in short, as well as the method to stabilize graphite anodes in WiSE, the researchers’ advancement of the new cathode chemistry additionally extends the available energy for aqueous batteries to a level that was not achieved before.

The authors exploited the reversible halogen conversion and intercalation within a graphite structure facilitated by a super-concentrated aqueous electrolyte and ultimately showed the complete aqueous Li-ion batteries with outstanding cycling stability and an estimated energy density of 460 Wh/Kg (total mass of anode and cathode), which is similar or even higher than sophisticated Li-ion batteries utilizing flammable non-aqueous electrolytes and transition metal oxide cathodes.

Headed by Chunsheng Wang, R.F. and F.R. Wright Distinguished Chair Professor in UMD’s Department of Chemical & Biomolecular Engineering and Department of Chemistry and Biochemistry; Oleg Borodin, ARL scientist, and Kang Xu, ARL Fellow, the researchers successfully created the new battery into a testable stage using button cell configuration that is usually utilized as a test vehicle in research laboratories, and defined in details the conversion–intercalation chemistry that contributes to the improved energy density. According to Kang, more studies are required to scale it up into a viable large-scale battery.

This new cathode chemistry happens to be operating ideally in our previously-developed 'water-in-salt' aqueous electrolyte in Science in 2015, which makes it even more unique—it combines both high energy density of non-aqueous systems and high safety of aqueous systems.

Chongyin Yang, Study First Author and Assistant Research Scientist, Department of Chemical and Biomolecular Engineering, College Park

The energy output of water-based battery reported in this work is comparable to ones based on flammable organic liquids other than water, but is much safer. It gets about 25% extra the energy density of an ordinary cell phone battery. The new cathode is able to hold, per gram, 240 milliamps for an hour of operation, whereas the kind widely used cathode in cell phones, laptops, and tools (LiCoO2), provides only 120-140 milliamps each hour per gram.

Chunsheng Wang R.F. and F.R. Wright Distinguished Chair Professor, Department of Chemical and Biomolecular Engineering; Department of Chemistry and Biochemistry, University of Maryland

This novel aqueous battery chemistry, in addition to serving as portable batteries for soldiers, could also be utilized in those applications, where large energies at megawatt or kilowatt levels are involved, or where toxicity and safety of batteries are major concerns, including non-flammable batteries for spaceships, naval vessels, or airplanes, or in civilian applications for large-scale grid storage, electric vehicles, and portable electronics.

The paper by the University of Maryland and the Army team is the most creative new battery chemistry I have seen in at least 10 years. The fact that the LiCl and LiBr reversibly convert and form halogen intercalated graphite is truly incredible. The team has demonstrated encouraging reversibility for 150 cycles and have shown that high energy densities should be attainable in 4-volt cells that contain no transition metals and no non-aqueous solvents. It remains to be seen if a practical long-lived commercial cell can be developed, but I am very excited by this research.

Jeff Dahn, Professor, Dalhousie University, Canada

A leader in battery technology, Professor Dahn is one of the inventors of the lithium-ion battery.

Professor Gleb Yushin of Georgia Tech and CTO of SILA nanotechnology, who was not part of the study, observed that “In their paper C. Wang et al. demonstrated an absolutely remarkable progress in their development of nonflammable aqueous Li-ion batteries by simultaneously increasing cell voltage and utilizing cobalt-free and nickel-free cathodes. In contrast to traditional intercalation cathodes based on rare, expensive and rather toxic transition metals, such as cobalt and nickel, researchers demonstrated excellent cycle stability in a graphite-salt composite cathode coupled with a pure graphite anode. Their innovative solution enables the use of cheaper and environmentally safer graphite as a higher gravimetric capacity cathode that operates at a higher average voltage than state of the art. In yet another contrast to traditional Li-ion where Li-ions do all the work, the new cells utilize both Li cations and halogen anions for charge storage. Overall, this work reports on multiple key milestones for aqueous ion batteries and provides a major leap towards their commercially viable use in stationary storage and possibly even electric transportation applications.”

This work is mainly about a brand-new concept of Li-ion cathode chemistry—using the redox reactions of halogens (Br and Cl in this case) to store charges, and using their intercalation nature to stabilize their strong oxidizing products inside the interlayer of graphite, forming dense-packed graphite intercalation compounds. This new ‘Conversion-Intercalation’ chemistry inherits the high energy of conversion-reaction and the excellent reversibility from topotactic intercalation.

Chongyin Yang, Study First Author and Assistant Research Scientist, Department of Chemical and Biomolecular Engineering, College Park

Yang is also a scientist at Argonne’s Advanced Photon Source.

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