|The team achieved three-times-higher storage capacity through the reversible redox reactions of copper and iron—breaking and reforming copper-fluorine and iron-fluorine bonds while absorbing and releasing lithium. Source: BNL. Click to enlarge.|
A team led by researchers at Brookhaven National Laboratory (BNL) has found that adding copper atoms to iron fluoride—a member of the class of materials called transition metal fluorides that are potential extremely high-capacity cathodes for future Li-ion batteries—produces a group of new fluoride materials that can reversibly store three times as many Li ions as conventional cathode materials. Measurements also indicate that these new materials could yield a cathode that is extremely energy-efficient. Their research is described in an open access paper in the journal Nature Communications.
The capacity of mainstream conventional cathodes (e.g., LiCoO2 or LiFePO4) is low (140–170 mAh g−1) and currently limits the energy density of most commercial cells, the researchers note. Although a number of alternative anodes (such as silicon and tin) show capacities well above 500 mAh g−1, few cathodes have been identified that can the high capacity. However, transition metal fluorides, which contain the element fluorine plus one or more of the transition metals, such as iron and copper, have much higher ion-storage capacities than traditional cathodes.
While a conventional cathode stores ions between the layers of its molecular structure (intercalation) the metal fluorides store them via a reversible electrochemical reaction, called a conversion reaction. During this multi-step process, the metals oxidize (lose electrons) and break from the fluorine atoms. The lithium ions then bond temporarily to the fluorine atoms.
Despite the theoretical promise of metal fluorides, issues related to reversibility, energy efficiency and kinetics have prevented their practical application. As an example, copper fluoride (CuF2) has a very high potential voltage as a cathode material but low electrochemical activity; additionally, its conversion reaction is not reversible. The iron fluorides (FeF2 and FeF3), are reversible but their working voltages are low and they are not sufficiently energy-efficient enough.
Recently, extensive research on metal fluoride cathodes has provided new insights into the mechanisms involved in the conversion reactions and the issues relevant to cycling reversibility and efficiency (for example, hysteresis). Although poor electronic and ionic transport plague many conversion electrodes, recent studies show that the electronic conductivity inFeF2 improves lithiation and approaches that of metallic Fe. The percolating Fe network formed during lithiation provides a facile electronic pathway and the high interfacial area provides abundant pathways for rapid Li+ transport.
In contrast, the conversion reaction in CuF2 involves highly mobile Cu2+ ions, which leads to coarsening and growth of large, isolated Cu particles during lithiation, making reconversion difficult. In addition, a recent study of the CuF2 conversion reaction by Hua et al. clearly showed that the dominant reaction occurring during the 1st charge is the dissolution of Cu into the electrolyte to form an unidentified Cu+ species, resulting in considerable loss of capacity.
An intriguing new concept, derived from these recent findings, is the possibility of substituting Cu into the Fe fluoride system, and thereby forming a ternary solid–solution CuyFe1-yF2. An electrode configured in this way would potentially benefit from the percolating iron network, which may be effective at ‘trapping’ Cu ions allowing them to fully oxidize into Cu2+. The addition of a second cation into a solid–solution is also an effective strategy for tailoring electrochemical properties (thermodynamics and kinetics) and improving electrochemical performance, as already demonstrated in many electrodes. Surprisingly, despite tremendous research on the binary metal fluorides, studies of conversion reactions in the ternary fluorides (involving two transition metal cations) have been largely overlooked.—Wang et al.
The BNL work builds on two other studies recently published in the Journal of the American Chemical Society and Nature Communications, which reveal the merits of FeF2 for use in batteries, particularly for achieving highly reversible lithium conversion reactions.
In this latest work, Feng Wang, a physicist in Brookhaven's Sustainable Energy Technologies Department, and colleagues Sung-Wook Kim, Liping Wang, and Dong Su of Brookhaven Lab; Dong-Hwa Seo and Kisuk Kang of Seoul National University (Korea); and John Vajo, John Wang, and Jason Graetz of HRL Laboratories investigated the synthesis, structural and electrochemical properties of ternary metal fluorides (M1yM21-yFx: M1, M2=Fe, Cu).
The studies were conducted at two DOE Office of Science user facilities, the National Synchrotron Light Source (NSLS, now closed and replaced by the new NSLS-II) and the Center for Functional Nanomaterials.
The group began with FeF2 and then incorporated copper atoms into the iron lattice. They synthesized many samples, containing different ratios of copper and iron, and studied them in operando, simultaneously tracking the samples’ reactivity and structural properties.
The network of iron atoms “traps” the copper atoms and results in a cooperative oxidation reaction, ultimately allowing the copper ions to undergo the reversible redox reaction. Moreover, the reaction is achieved with an extremely low voltage hysteresis. This parameter is a measure of how strongly the reaction draws on the voltage of the system; in short, it is a measure of how energy-efficient the cathode is during the charging process.
We were surprised that the measured hysteresis is so low. In fact, it is the lowest reported yet in any of the metal fluorides, indicating the potential for achieving high-energy efficiency in cathodes made with them. And in a broader sense, this work shows that the addition of a second positive ion may provide a new avenue for tailoring key electrochemical properties of conversion-type electrodes.—Feng Wang
A patent related to this work was filed in January of this year, titled “High-Energy Cathodes for Lithium Rechargeable Batteries.” The patent positions this metal fluoride as a low-cost upgrade for the cathodes in existing Li-ion batteries and is aimed at possible commercial applications. As in the journal paper, the patent application describes how the material was characterized and tested. But it also provides additional details into how the material is synthesized and the steps to fashion it into a working cathode.
Measurements of cathode performance. The researchers synthesized solid solutions of the ternary metal fluorides via mechanochemical reactions. These samples were made into test cells and their electrochemical behavior measured as the cell was discharged and charged. The measurements show that the system’s electrochemical properties are guided by the cooperative oxidation and reduction (redox) that occurs when the copper and iron are sitting on the same lattice.
For example, the measurements do not show the voltage dip during iron conversion that is exhibited by pure FeF2, indicating that iron conversion in the samples occurs with less energy. Measurements taken during the redox reaction of the copper atoms reveal peaks that show up cycle after cycle, indicating the reversibility of that reaction, unlike in pure CuF2.
The group achieved further insight into the redox reactions, and corroboration of the electrochemical measurements, using in-operando x-ray absorption spectroscopy techniques at NSLS. X-ray beams were aimed at the samples as they charged and discharged. As they passed through the sample, some of the x-rays were absorbed. These absorption patterns give the scientists a way to see what was happening in the cell in real time. The techniques are element-specific, meaning they are tuned to return information about a single element, such as copper.
The x-ray data show that, on discharge, as lithium ions enter the cathode, the copper conversion occurs first, followed by the iron conversion at lower voltages. The copper-iron and iron-fluoride bonds break, yielding to the lithium ions, while metallic copper-copper and iron-iron bonds form between the freed metal atoms. Upon charging, the copper-iron bonds reform, as evident by a strong peak in the x-ray absorption data that is nearly identical in position and shape to the original material—another hallmark of good reversibility.
Further x-ray data was taken to learn more about what happens to the copper atoms after the first discharge and charge cycle, and into the second discharge. The researchers note an issue with copper ions dissolving, which leads to a breakdown of cell performance. They suggest possible mitigation methods, such as surface coatings to stabilize the electrode at high potentials or barrier layers to prevent copper ion crossover. These fixes may be explored in future studies.
Wang and his team plan to continue investigating this new type of copper-based fluoride for battery applications at Brookhaven’s new synchrotron, National Synchrotron Light Source II, the world’s brightest synchrotron light source.
Down the road, we plan to closely examine how they degrade after repeatedly absorbing and releasing lithium, in order to find remedies for this behavior. The new NSLS-II XPD beamline, designed for in-situ and operando studies of materials, is the ideal tool for imaging the full local and global structure of our samples during cycling, in real time and under real-world reaction conditions.—Feng Wang
This research was initiated as part of the Northeastern Center for Chemical Energy Storage, one of the Energy Frontier Research Centers (EFRC) funded by the US Department of Energy’s Office of Science, under Award Number DE-SC0001294; and also partially supported by another EFRC center, the Center on Nanostructuring for Efficient Energy Conversion, under award number DE-SC0001060.
The in-operando studies were supported by the DOE Office of Energy Efficiency and Renewable Energy under the Batteries for Advanced Transportation Technologies (BATT) Program (being incorporated into the new Advanced Battery Materials Research program) under contract number DE-AC02-98CH10886 (recently changed to DE-SC0012704). Other sources of support include the Human Resources Development program (20124010203320) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Korea government Ministry of Trade, Industry and Energy.
Feng Wang, Sung-Wook Kim, Dong-Hwa Seo, Kisuk Kang, Liping Wang, Dong Su, John J. Vajo, John Wang & Jason Graetz (2015) “Ternary metal fluorides as high-energy cathodes with low cycling hysteresis” Nature Communications 6, Article number: 6668 doi: 10.1038/ncomms7668
Feng Wang, Hui-Chia Yu, Min-Hua Chen, Lijun Wu, Nathalie Pereira, Katsuyo Thornton, Anton Van der Ven, Yimei Zhu, Glenn G. Amatucci& Jason Graetz (2012) “Tracking lithium transport and electrochemical reactions in nanoparticles,” Nature Communications 3, Article number: 1201 doi: 10.1038/ncomms2185
Feng Wang, Rosa Robert, Natasha A. Chernova, Nathalie Pereira, Fredrick Omenya, Fadwa Badway, Xiao Hua, Michael Ruotolo, Ruigang Zhang, Lijun Wu, Vyacheslav Volkov, Dong Su, Baris Key, M. Stanley Whittingham, Clare P. Grey, Glenn G. Amatucci, Yimei Zhu, and Jason Graetz (2011) “Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes,” Journal of the American Chemical Society 133 (46), 18828-18836 doi: 10.1021/ja206268a