According to the researchers, the highly concentrated “water-in-bisalt” solution widens the electrochemical stability window of the electrolyte and enables graphite as a cathode material in a practical aqueous system—something previously thought of as impossible. This helps stabilize the electrolyte at high voltages, allowing the graphite to electrochemically oxidize before the aqueous electrolyte.
The new chemistry is better for the environment because it uses cathodes made of highly abundant carbon-based materials like natural graphite
“The concentration of salt ions is so incredibly high, it’s almost like water is not there anymore. So, the aqueous electrolyte doesn’t decompose in voltages where it normally would, allowing the use of graphite,” Ismael Rodríguez Pérez, lead author of the study and a materials scientist at the Pacific Northwest National Laboratory, said in a media statement. “That is the most amazing outcome in this.”
Rodríguez Pérez said that the battery showed promising performance during testing and that at approximately 2.3 to 2.5 volts, it achieved one of the highest operating potentials of any aqueous battery.
The researcher also said that, besides improving battery performance, the new chemistry is better for the environment because cathodes made of highly abundant carbon-based materials, like natural graphite, are less costly and extracted in a more sustainable manner than nickel and cobalt.
Using an aqueous electrolyte also makes DIBs safer as they are non-flammable compared to commercial Li-on batteries, which use non-aqueous electrolytes exclusively.
(Graphic by Cortland Johnson, courtesy of the PNNL).
Rodríguez Pérez and his team explain in their paper that, until now, the use of graphite as a cathode has been limited by the narrow electrochemical stability of water, which caps out at 1.23 volts. The electrochemical stability window is the potential range between which the electrolyte is neither oxidized nor reduced, and an important measuring stick for the efficiency of an electrolyte in contact with an electrode. Graphite would require a much wider stability window. And this is what this new cell chemistry does.
Despite these advantages, DIBs still perform at only about a third of the capacity of Li-ion batteries, which still have one of the highest energy densities of any comparable system. This is why Li-ion batteries are favoured for mobile applications such as smartphones and electric cars.
For Rodríguez Pérez, however, the way to make DIBs compete with their lithium counterparts is by making three times bigger, achieving high voltage and, thus, turning them into suitable candidates for grid energy storage applications.
