The team modified and engineered a form of iron trifluoride (FeF3) which has inherently higher capacities than traditional cathode materials by transferring three electrons during operation rather than one, hence the tripling.
“Cathode materials are always the bottleneck for further improving the energy density of lithium-ion batteries,” said Xiulin Fan, a scientist at UMD and one of the lead authors of the paper in Nature Communications. This is why many other teams are looking at new materials such as manganese, sulfur and carbon at the cathode
Despite FeF3‘s potential to increase cathode capacity, the compound has not historically worked well in lithium-ion batteries due to three complications with its conversion reaction: poor energy efficiency (hysteresis), a slow reaction rate, and side reactions that can cause poor cycling life. To overcome these challenges, the scientists added cobalt and oxygen atoms to FeF3 nanorods through chemical substitution. This allowed the scientists to manipulate the reaction pathway and make it more reversible.
“When lithium ions are inserted into FeF3, the material is converted to iron and lithium fluoride,” said Sooyeon Hwang, a co-author of the paper and a scientist at Brookhaven’s Center for Functional Nanomaterials (CFN). “However, the reaction is not fully reversible. After substituting with cobalt and oxygen, the main framework of the cathode material is better maintained and the reaction becomes more reversible.”
The team conducted multiple experiments at CFN and the National Synchrotron Light Source II at Brookhaven to examine the nanoparticles in the cathode and saw a faster reaction speed for the substituted nanorods.
“TEM is a powerful tool for characterizing materials at very small length scales, and it is also able to investigate the reaction process in real time,” said Dong Su, a scientist at CFN and a co-corresponding author of the study. “However, we can only see a very limited area of the sample using TEM. We needed to rely on the synchrotron techniques at NSLS-II to understand how the whole battery functions.”
“We also performed advanced computational approaches based on density functional theory to decipher the reaction mechanism at an atomic scale,” said Xiao Ji, a scientist at UMD and co-author of the paper. “This revealed that chemical substitution shifted the reaction to a highly reversible state by reducing the particle size of iron and stabilizing the rocksalt phase.”
The team say this research strategy could be applied to other high energy conversion materials and other battery technologies.