Researchers have synthesized a new cathode material from iron fluoride that surpasses the capacity limits of traditional lithium-ion batteries…
Lithium-ion batteries are a common type of battery found in home electronics, smart phones and Tesla cars. They also represent a promising solution for grid-scale energy storage.
Increasing the energy density of lithium-ion batteries could help the development of advanced technologies with long-lasting batteries, as well as the widespread use of wind and solar energy.
Now, researchers have made significant progress toward achieving that goal.
Scientists led by a group from University of Maryland (UMD), the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, and the U.S. Army Research Lab have developed and studied a new cathode material that may triple the energy density of lithium-ion battery electrodes.
The team’s research was published recently in the journal Nature Communications.
“Lithium-ion batteries consist of an anode and a cathode,” said Xiulin Fan, a scientist at UMD and one of the lead authors of the study. “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 scientists synthesized a new cathode material, a modified and engineered form of iron trifluoride (FeF3). Moreover, the material is composed of cost-effective and environmentally benign elements—iron and fluorine.
There has been a lot of interest in using chemical compounds like FeF3 in lithium-ion batteries because they offer inherently higher capacities than traditional cathode materials.
“The materials normally used in lithium-ion batteries are based on intercalation chemistry,” said Enyuan Hu, a chemist at Brookhaven and one of the lead authors of the paper. “This type of chemical reaction is very efficient; however, it only transfers a single electron, so the cathode capacity is limited. Some compounds like FeF3 are capable of transferring multiple electrons through a more complex reaction mechanism, called a conversion reaction.”
To date 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. Yet the potential was recognized.
The scientists added cobalt and oxygen atoms to FeF3 nanorods through a process called chemical substitution to help overcome the issues. The cobalt and oxygen allowed the scientists to manipulate the reaction pathway and make it more “reversible.”
To investigate the reaction pathway, the scientists conducted multiple experiments at CFN and the National Synchrotron Light Source II (NSLS-II)—two DOE Office of Science User Facilities at Brookhaven.
First at CFN, the researchers used a powerful beam of electrons to look at the FeF3 nanorods at a resolution of 0.1 nanometers—a technique called transmission electron microscopy (TEM). The TEM experiment enabled the 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. They 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.”
At NSLS-II’s X-ray Powder Diffraction (XPD) beamline, scientists directed ultra-bright x-rays through the cathode material. By analyzing how the light scattered, the scientists could “see” additional information about the material’s structure.
“At XPD, we conducted pair distribution function (PDF) measurements, which are capable of detecting local iron orderings over a large volume,” said Jianming Bai, a co-author of the paper and a scientist at NSLS-II. “The PDF analysis on the discharged cathodes clearly revealed that the chemical substitution promotes electrochemical reversibility.”
“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 approach 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 scientists say this research strategy could be applied to other high energy conversion materials, and future studies may use the approach to improve other battery systems.