As the size and scale of the electric vehicle (EV) market grows, there is concern about the supplies and future costs of certain raw materials. Of particular concern is nickel, which is also used in high-temperature metals and stainless steels, and cobalt, which is primarily sources from the Democratic Republic of the Congo, a politically unstable region of Africa. Current commercial lithium ion batteries use both nickel and cobalt in their positive electrodes (cathodes). Finding a replacement for either or both of these elements has proven difficult.
Now, researchers at Georgia Tech have reported on a new lithium ion battery chemistry that does away with both of the problem materials. According to a Georgia Tech news release, the “…new cathode and electrolyte system that replaces expensive metals and traditional liquid electrolyte with lower cost transition metal fluorides and a solid polymer electrolyte.”
A flexible polymer solid electrolyte is one key to the viability of a iron fluoride cathode in lithium ion batteries. (Image source: Georgia Tech)
A New Type of Cathode
The team fabricated a new type of cathode from iron fluoride active material and a solid polymer electrolyte nanocomposite. According to information provided in the news release, iron fluorides have more than double the lithium capacity of traditional cobalt- or nickel-based cathodes. Significantly, iron is 300 times cheaper than cobalt and 150 times cheaper than nickel.
In the production of the cathode, the researchers developed a way to infiltrate a solid polymer electrolyte into a prefabricated iron fluoride electrode. They then hot pressed the entire structure to increase density and reduce any voids.
“Electrodes made from transition metal fluorides have long shown stability problems and rapid failure, leading to significant skepticism about their ability to be used in next generation batteries,” said Gleb Yushin, a professor in Georgia Tech’s School of Materials Science and Engineering. “But we’ve shown that when used with a solid polymer electrolyte, the metal fluorides show remarkable stability – even at higher temperatures – which could eventually lead to safer, lighter and cheaper lithium-ion batteries,” he added.
Solid Polymer Nanocomposite
The polymer-based electrolyte has an ability to flex and accommodate the swelling of the iron fluoride while cycling. It also forms a very stable and flexible interphase with iron fluoride. Swelling and extensive side reactions have been key problems with using iron fluoride in previous battery designs.
According to the news release, “The researchers then tested several variations of the new solid-state batteries to analyze their performance over more than 300 cycles of charging and discharging at elevated temperature of 122 degrees Fahrenheit, noting that they outperformed previous designs using metal fluoride even when these were kept cool at room temperatures.”
The key to the enhanced battery performance was the solid polymer electrolyte. From the news release, “In previous attempts to use metal fluorides, it was believed that metallic ions migrated to the surface of the cathode and eventually dissolved into the liquid electrolyte, causing a capacity loss, particularly at elevated temperatures. In addition, metal fluorides catalyzed massive decomposition of liquid electrolytes when cells were operating above 100 degrees Fahrenheit. However, at the connection between the solid electrolyte and the cathode, such dissolving doesn’t take place and the solid electrolyte remains remarkably stable, preventing such degradations.”
“Cathodes made from iron fluoride have enormous potential because of their high capacity, low material costs and very broad availability of iron,” Yushin said. “But the volume changes during cycling as well as parasitic side reactions with liquid electrolytes and other degradation issues have limited their use previously. Using a solid electrolyte with elastic properties solves many of these problems.”
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.