By improving a water-in-salt battery prototype, researchers believe they are well on their way to developing new high energy batteries.
A team from the University of Maryland (UMD) and the U.S. Army Research Lab (ARL) have created a new type of chemical transformation of the cathode that yields, for the first time ever, a reversible solid salt layer in a water-based battery.
“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,” Chongyin Yang, an assistant research scientist in the UMD department of chemical & biomolecular engineering and co-first author of the study, said in a statement.
In what has been a multi-year pursuit of a new high-energy battery alternative, the researchers first created a new cathode in 2015 that did not have a transition metal and operated at an average potential of 4.2 volts. This battery also delivered unprecedented energy density with good cycling stability.
While progress has been made using water-in-salt electrolytes, the limited lithium intercalation capacities of less than 200 milliampere-hours per gram of typical transition-metal-oxide cathodes preclude higher energy densities. Partial or exclusive anionic redox reactions may achieve higher capacity, but at the expense of reversibility.
The researchers leveraged the reversible halogens intercalation in graphite structures due to a super-concentrated aqueous electrolyte to generate the increased energy density that could end up higher than non-aqueous lithium-ion batteries, which was previously thought to be impossible.
The super concentrated solution, combined with a graphite anode’s ability to automatically build and re-form a protective layer within the battery, was found to enable a stable and long-lasting battery with exceptionally high energy.
“This new ‘Conversion-Intercalation’ chemistry inherits the high energy of conversion-reaction and the excellent reversibility from intercalation of graphite,” Ji Chen, co-first author of the paper and a research associate in the department of chemical & biomolecular engineering, said in a statement.
Ultimately, the energy output of the new battery is 25 percent higher than the energy density of an ordinary lithium-ion battery used in most cell phones. The new cathode also holds 240 milliamps per gram for an hour of operation, which is twice the energy storage of a typical cathode used in cell phones and laptops.
The battery is now being tested. It is currently the size of a small button, but more research is needed to scale the prototype up into a practical battery that can be manufactured. If the researchers are able to scale up their water-in-salt battery, it could be particularly useful for applications that involve large energies at the kilowatt or megawatt levels. They also can be used in applications where battery safety and toxicity are important, such as the non-flammable batteries used in airplanes, naval vessels and spaceships.
The study was published in Nature.