Magnetic Studies of Conversion Processes in Iron (Oxy)Fluoride Based Electrodes

Fundamental studies of reaction mechanisms in Li-ion batteries are driven by a demand for high energy density and safe electrodes capable of fast lithiation/delithiation reactions. While the current intercalation systems are mostly limited to one Li per red-ox center, conversion systems do not have this limitation. Conversion electrodes are usually nanostructured, which facilitates phase transformations, but makes their characterization quite challenging. In this work we use magnetic methods to investigate conversion and reconversion mechanisms in FeF₂, FeO_yF_2-y and FeF₃-based electrodes, which form nano-network of metallic Fe upon Li insertion. Bulk Fe is ferromagnetic at room temperature, while small (< 6-8 nm) nanoparticles become superparamagnetic, meaning that the magnetic moment of the whole particle can flip under the influence of thermal energy. The amount of Fe can be determined from the saturation magnetization (M_s), and the particle size can be estimated from the blocking temperature T_b below which the magnetic moments cannot flip. The crystallinity and presence of defects can be estimated from their effects on the magnetic ordering transition, i.e. antiferromagnetic ordering of FeF₂ at 82 K. Magnetization studies have confirmed the initial intercalation processes upon lithiation of FeO_0.7F_1.3 and FeF₃ electrodes, followed by the conversion reaction accompanied by linear magnetization increase due to metallic iron formation. FeF₂ system reveals the presence of metallic Fe already at 10% lithiation, meaning that intercalation is minimal, if any, in this system. Upon delithiation, FeO_0.7F_1.3 shows the best reversibility, i.e. M_s dependences on Li content are essentially the same upon lithiation and delithiation. The studies of superparamagnetic behavior in lithiated samples reveal that small Fe nanoparticles (2-5 nm) observed in the TEM studies are magnetically interacting, forming an interconnecting network over the whole initial (oxy)fluoride crystallite, i.e. 10-15 nm. Evolution of the magnetic properties over long cycling (2^nd, 10^th and 20^th cycles) will be reported and correlated with the electrochemical cycling stability. The differences between these three systems will be discussed, involving the results obtained by other techniques (TEM, PDF, XAS, electrochemistry). This research is supported as part of the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0001294.