Wolverton's team, in collaboration with researchers at Argonne National Laboratory, has successfully developed a rechargeable lithium-iron oxide battery that can cycle more lithium ions than a conventional lithium cobalt oxide battery.
The result is a larger capacity battery, which will allow battery-powered electric vehicles and smartphones to work for longer periods of time.
Wolverton, a professor of materials science and engineering at Northwestern University's McCormick School of Engineering, said: "We are very excited about the calculations for the battery, but without experimental confirmation, many skeptics will doubt its scientific validity. In fact, its effects are indeed very significant."
This research, supported by the U.S. Department of Energy's Energy Frontier Research Center program, was recently published in the journal *Nature Energy*. Zhenpeng Yao, a PhD student in Wolverton's lab, and Chun Zhan, a postdoctoral researcher in Argonne, are the first authors of the paper. Argonne led the experimental portion of the research, while Wolverton and Yao were responsible for computational development.
Lithium-ion batteries work by shuttling lithium ions back and forth between the anode and cathode. When the battery is charged, the ions move to the anode. The cathode is formed from a compound consisting of a transition metal, lithium ions, and oxygen. The transition metal is typically cobalt, which effectively stores and releases electrical energy as lithium ions move from the anode to the cathode and back. The capacity of the cathode is then limited by the number of electrons in the transition metal.
Lithium cobalt oxide batteries have been commercially available for 20 years, but researchers have long been searching for larger-capacity, cheaper alternatives. Wolverton's team enhanced conventional lithium cobalt oxide batteries using two strategies: replacing cobalt with iron to force oxygen to participate in the reaction process.
If oxygen can also store and release electrical energy, the battery will have a much larger storage capacity, allowing for the use of increasingly more lithium. Although other research teams have tried this approach in the past, only a few have succeeded.
“The problem before was that if you tried to get oxygen involved in the reaction, the compound would become unstable,” Yao said. “Oxygen would be released from the battery, making the reaction irreversible.”
Wolverton and Yao discovered a formula through calculations that made the reaction reversible. They first substituted iron for cobalt, which was advantageous because it is one of the cheapest elements on the periodic table. Through calculations, they found the correct balance of lithium, iron, and oxygen ions so that iron and oxygen could simultaneously drive the reversible reaction, preventing the escape of oxygen.
Wolverton said, "Because we get electrons from metal and oxygen, and the metal we're using is iron, our battery not only has a very interesting chemical composition, but it also has the potential to make cheaper batteries."
Another important aspect is that a fully rechargeable battery starts with four lithium ions, not just one. Current reactions are capable of reversibly utilizing one of these lithium ions, primarily increasing the capacity of existing batteries. However, the possibility of driving the entire reaction using oxygen and iron is indeed enticing.
“Each metal has four lithium ions, and that will change everything,” Wolverton said. “That means your phone could last eight times longer, or your car could go eight times farther. If electric cars can compete with gasoline-powered cars in terms of range and cost, that will change the world.”
Wolverton has filed a provisional patent for the battery with Northwestern University's Office of Innovation and Venture Capital. Looking ahead, Wolverton and his team plan to discover other compounds to prove that this strategy can work.
