Lithium-ion batteries power most all smart devices on the market today and also allow us to travel in electric vehicles and use energy storage in our homes.
For some time now, scientists have been looking for ways to improve these batteries so that they will last longer. In energy storage, this is a greater concern for those looking to relieve homeowners of being on the grid. The promise of improved lithium-ion batteries is that it could facilitate the widespread use of wind and solar energy.
Now, researchers at the University of Maryland (UMD), the U.S. Department of Energy’s Brookhaven National Laboratory and the U.S. Army Research Lab have developed a new cathode material that could triple the energy density of lithium-ion battery electrodes.
"Lithium-ion batteries consist of an anode and a cathode," said Xiulin Fan, a scientist at UMD. "Compared to the large capacity of the commercial graphite anodes used in lithium-ion batteries, the capacity of the cathodes is far more limited. Cathode materials are always the bottleneck for further improving the energy density of lithium-ion batteries."
The new cathode material is a modified and engineered form of iron trifluoride (FeF3) that is composed of environmentally benign elements—iron and fluorine. These materials have been proven to offer inherently higher capacities than traditional cathode materials. FeF3 is also capable of transferring multiple electrons through a more complex reaction mechanism, called a conversion reaction.
Despite these advantages, FeF3 has historically not worked well in lithium-ion batteries due to three complications: poor energy efficiency, a slow reaction rate and side reactions that can cause poor cycling life. Scientists added cobalt and oxygen atoms to FeF3 nanorods in order to combat these challenges.
"When lithium ions are inserted into FeF3, the material is converted to iron and lithium fluoride," said Sooyeon Hwang, 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 tests on the technology using a beam of electrons at a resolution of 0.1 nanometers—a technique called transmission electron microscopy (TEM). The TEM experiment enabled researchers to determine the exact size of the nanoparticles in the cathode structure and analyze how the structure changed between different phases of the charge-discharge process. The researchers saw a faster reaction speed for the substituted nanorods.
The full research can be found in the journal Nature Communications.