Renewables are poised to expand by 50 percent in the next five years, according to the International Energy Agency. Much of that wind and solar power will need to be stored. But a growing electric-vehicle market might not leave enough lithium and cobalt for lithium-ion grid batteries.
Some battery researchers are taking a fresh look at lithium's long-ignored cousin, potassium, for grid storage. Potassium is abundant, inexpensive, and could in theory enable a higher-power battery. However, efforts have lagged behind research on lithium and sodium batteries.
But potassium could catch up quickly, says Shinichi Komaba, who leads potassium-ion battery research at the Tokyo University of Science: “Although potassium-battery development has just been going on for five years, I believe that it is already competitive with sodium-ion batteries and expect it to be comparable and superior to lithium-ion."
People have historically shied away from potassium because the metal is highly reactive and dangerous to handle. What's more, finding electrode materials to hold the much heftier potassium ions is difficult.
Yet a flurry of reports in the past five years detail promising candidates for the cathode. Among the leaders are iron-based compounds with a crystalline structure similar to Prussian blue particles, which have wide open spaces for potassium ions to fill. A group from the University of Texas at Austin led by John Goodenough, coinventor of the lithium-ion battery and a winner of the 2019 Nobel Prize in Chemistry, has reported Prussian blue cathodes with an exceptionally high energy density of 510 watt-hours per kilogram, comparable to that of today's lithium batteries.
But Prussian blue isn't perfect. “The problem is, we don't know how water content in the material affects energy density," says Haegyeom Kim, a materials scientist at Lawrence Berkeley National Laboratory. “Another issue is that it's difficult to control its chemical composition."
Kim is placing bets on polyanionic compounds, which are made by combining potassium with any number of elements plucked from the periodic table. Potassium vanadium fluorophosphate seems to hold special promise. Kim and his colleagues have developed a cathode with the compounds that has an energy density of 450 Wh/kg.
Other researchers are looking at organic compounds for cathodes. These cost less than inorganic compounds, and their chemical bonds can stretch to take up potassium ions more easily.
While Goodenough is giving potassium a chance, his fellow lithium-battery inventor and Nobel Prize winner M. Stanley Whittingham, professor of chemistry at Binghamton University, in New York, isn't sold. “It's a scientific curiosity," he says. “There's no startup looking at potassium batteries."
Potassium, says Whittingham, is not a practical technology because of its heft and volatility. Potassium also melts at a lower temperature than lithium or sodium, which can trigger reactions that lead to thermal runaway.
Those are valid concerns, says Vilas Pol, a professor of chemical engineering at Purdue University, in West Lafayette, Ind. But he points out that in a battery, potassium ions shuttle back and forth, not reactive potassium metal. Special binders on the electrode can tame the heat-producing reactions.
Developing the right electrolyte will be key to battery life and safety, says Komaba, of the Tokyo University of Science. Conventional electrolytes contain flammable solvents that, when combined with potassium's reactivity, could be dangerous. Selecting the right solvents, potassium salts, salt concentration, and additives can prevent fires.
Komaba's group has made electrolytes using potassium-fluoride salts, superconcentrated electrolytes that have fewer solvents than traditional mixes, and ionic liquid electrolytes that don't use solvents. In January, materials scientist Zaiping Guo and her team from the University of Wollongong, Australia, reported a nonflammable electrolyte for potassium batteries. They added a flame retardant to the solvent.
Potassium enthusiasts point out that the technology is still at an early stage. It's never going to match the high energy density of lithium, or be suitable for electric cars. Yet for immense grid batteries, cheap potassium might have an upper hand. “Potassium-ion [batteries] could have worked earlier, but there was no need for [them]," says Pol. “Lithium isn't enough now."
In the end, the sum will have to be as good as its parts. Most research has focused on the materials that go into the electrodes and the electrolyte. Put it all together in a battery cell and the energy density drops after just 100 charging cycles or so; practical batteries will need to withstand several hundred.
“It will take time to figure out the exact combination of electrolyte, cathode, and anode," Pol says. “It might take another 15 years from now to get to the market."
This article appears in the March 2020 print issue as “Potassium Batteries Show Promise."