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University of Maryland (UMD) and US Army Research Lab (ARL) researchers developed an approach that improved the capacity of batteries to have high energy through enhancing their water-in-salt battery with a new kind of chemical transformation of the cathode that results in a reversible solid salt layer.

A previous study by the team published in 2015 in the journal Science showed their discoveries of the water-in-salt electrolytes. Their current research added a new cathode to this old discovery which does not contain a transition metal. Their findings on the current research were published in the journal Nature that demonstrated that the cathode works at an average potential of 4.2 volts with higher cycling stability and energy density compared with non-aqueous Li-ion batteries.

"The University of Maryland and ARL research has produced the most creative new battery chemistry I have seen in at least 10 years," said Prof. Jeffrey Dahn of Dalhousie University in Canada, an expert in the field not affiliated with the research. "However, it remains to be seen if a practical device with long lifetime can be created."

The team was able to produce an "impossible" energy density powered by the intercalation of reversible halogens in graphite structures through a super-concentrated aqueous electrolyte. "The researchers found that the superconcentrated solution of the water-in-salt battery, combined with graphite anode's ability to automatically build and re-form a protective layer within the battery, gave a stable and long-lasting battery with high energy," according to Phys.

"This new cathode chemistry happens to be operating ideally in our previously-developed 'water-in-salt' aqueous electrolyte, which makes it even more unique-it combines high energy density of non-aqueous systems with high safety of aqueous systems," said a co-first author of the paper, Chongyin Yang, an assistant research scientist in the UMD Department of chemical & biomolecular engineering.

"This new 'Conversion-Intercalation' chemistry inherits the high energy of conversion-reaction and the excellent reversibility from intercalation of graphite," said Ji Chen, co-first author of the paper and a research associate in the department of chemical & biomolecular engineering.

The research is led by ChBE Professor Chunsheng Wang, with ARL Fellow Kang Xu, and ARL scientists Olege Borodin. This team was able to test the battery in the size of a small button which is common in research labs. The research needs a long process in scaling up the battery into a practical and manufacturable one.

The water-based battery has an energy output which is 25% higher than that of an ordinary cell phone battery. "The new cathode is able to hold 240 milliamps per gram for an hour of operation, roughly twice that of a typical cathode currently found in cell phones and laptops," according to Phys.

Various applications can be used in the water-in-salt battery. It involves large energies at kilowatt or megawatt levels and where battery safety and toxicity are of prime importance.