A collaboration led by scientists at the University of Maryland (UMD), the US Department of Energy’s (DOE) Brookhaven National Laboratory, and the US Army Research Lab have developedand studied a new cathode material—a modified and engineered form of iron trifluoride (FeF3)—that could triple the energy density of lithium-ion battery electrodes. Their open-access paper is published in Nature Communications.
The materials normally used in lithium-ion batteries are based on intercalation chemistry. While efficient, this type of chemical reaction only transfers a single electron, so the cathode capacity is limited, explains Enyuan Hu, a chemist at Brookhaven and one of the lead authors of the paper. However, some compounds like FeF3 are capable of transferring multiple electrons through a more complex reaction mechanism, called a conversion reaction.
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 team added cobalt and oxygen atoms to FeF3 nanorods through a process called chemical substitution. This allowed the scientists to manipulate the reaction pathway and make it more reversible.
Substituting the cathode material with oxygen and cobalt prevents lithium from breaking chemical bonds and preserves the material’s structure.
Iron fluoride, an intercalation-conversion cathode for lithium-ion batteries, promises a high theoretical energy density of 1922 Wh kg–1. However, poor electrochemical reversibility due to repeated breaking/reformation of metal fluoride bonds poses a grand challenge for its practical application. Here we report that both a high reversibility over 1000 cycles and a high capacity of 420 mAh g−1 can be realized by concerted doping of cobalt and oxygen into iron fluoride.
In the doped nanorods, an energy density of ~1000 Wh kg−1 with a decay rate of 0.03% per cycle is achieved. The anion’s and cation’s co-substitutions thermodynamically reduce conversion reaction potential and shift the reaction from less-reversible intercalation-conversion reaction in iron fluoride to a highly reversible intercalation-extrusion reaction in doped material. The co-substitution strategy to tune the thermodynamic features of the reactions could be extended to other high energy conversion materials for improved performance.—Fan et al.
When lithium ions are inserted into FeF3, the material is converted to iron and lithium fluoride. 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.—Sooyeon Hwang, a co-author of the paper and a scientist at Brookhaven’s Center for Functional Nanomaterials (CFN)
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 transmission electron microscopy (TEM) to look at the FeF3 nanorods at a resolution of 0.1 nanometers. 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.
While TEM is a powerful tool for characterizing materials at very small length scales, and can also investigate the reaction process in real time, TEM can only see a very limited area of the sample, said Dong Su, a scientist at CFN and a co-corresponding author of the study.
We needed to rely on the synchrotron techniques at NSLS-II to understand how the whole battery functions.—Dong Su
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 visualize 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. The PDF analysis on the discharged cathodes clearly revealed that the chemical substitution promotes electrochemical reversibility.—Jianming Bai, a co-author of the paper and a scientist at NSLS-II
Combining highly advanced imaging and microscopy techniques at CFN and NSLS-II was a critical step for assessing the functionality of the cathode material.
We also performed advanced computational approaches based on density functional theory to decipher the reaction mechanism at an atomic scale. 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.—Xiao Ji, a scientist at UMD and co-author of the paper
Scientists at UMD 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.
This study was supported by the US Army Research Laboratory and DOE’s Office of Energy Efficiency and Renewable Energy. Operations at CFN and NSLS-II are supported by DOE’s Office of Science.
Xiulin Fan, Enyuan Hu, Xiao Ji, Yizhou Zhu, Fudong Han, Sooyeon Hwang, Jue Liu, Seongmin Bak, Zhaohui Ma, Tao Gao, Sz-Chian Liou, Jianming Bai, Xiao-Qing Yang, Yifei Mo, Kang Xu, Dong Su & Chunsheng Wang (2018) “High energy-density and reversibility of iron fluoride cathode enabled via an intercalation-extrusion reaction” Nature Communications volume 9, Article number: 2324 doi: 10.1038/s41467-018-04476-2