In-Situ Synchrotron XANES Study of Iron Oxyfluoride As Cathode Material in Lithium Ion Batteries
Authors: Yang Fan , Liu Yadong , Sun Chengjun , Li Zhe-Fei , Liu Qi , Ren Yang , Xie Jian 
Published: February 2020

Iron oxyfluoride (FeOF) was recently found to be a conversion type cathode material for lithium ion batteries because of its high theoretical capacity[1] (885mAh). Rutile structure FeOF was both environmental friendly and economic which is promising to be put into market. During the charge and discharge process, the valence of iron changes from 3+ to 0 which means that it can deliver 3 electrons. Although there were a lot of work done by researchers on FeOF cathode, the cyclability of this cathode was still too poor[2]. In-situ pair distribution function (PDF) was performed on FeOF batteries by Wiadereck et.al[3]. However, more valance and local structure evolution was yet to be revealed. In order to increase the cycle life of FeOF, it is very important to clearly elucidate the failure mechanism by clearly understand the atom environment in real-time during the cycling.

Synchrotron X-ray near-edge structure (XANES) could be very helpful to illustrate the local structure and state of charge of the element we are interested in[4]. With the help of XANES, we can apply in-situ characterization of FeOF cathode to better elucidate real-time local structure and valence change at different state of charge (SOC) and depth of discharge (DOD) in order to better illustrate the mechanism of the iron ion evolution and FeOF failure mechanism. In this study, FeOF was prepared in our lab and mixed with carbon black, PVDF and NMP to form a uniform slurry. The cathode was prepared by coating the slurry on aluminum foil. The in-situ test coin cell was assembled by FeOF cathode, Celgard separator and lithium foil anode and was sealed in our home-made coin cell shell. K-edge of Fe was measured during the in-situ characterization to observe the valence change and local structure evolution during the discharge and charge process between the voltage range of 4V and 1V.

The in-situ XANES spectrum was plotted in Fig.1. From 0% DOD to around 50% DOD, the K-edge shifted to low energy direction and then shifted back to high energy direction. This phenomenon indicated that there were two different mechanism during discharge process. To better analyze the in-situ XANES data, the contour plot and the charge/discharge profile were shown in Fig.2. The XANES and charge/discharge profile correlated with each other very well. At the beginning of discharge process, the K-edge shifted towards low energy direction and the intensity increased gradually, which means this is a Li+intercalation process. At the end of the discharge process, the K-edge shifted back to the high energy direction and there was a sharp intensity change which corresponding to the conversion process. Similar conversion and deintercalation process could be observed at the beginning and end of the charge process respectively.  

In conclusion, by applying in-situ XANES technic, we clearly visualize the valence and local structure evolution mechanism during real working condition and got our preliminary conclusion that the charge/discharge process of FeOF battery contains two typical process. Li+ intercalation occurs at high voltage range and conversion process occurs at lower voltage range.

Reference

[1] A. Kitajou, H. Komatsu, R. Nagano, S. Okada, Synthesis of FeOF using roll-quenching method and the cathode properties for lithium-ion battery, Journal of Power Sources, 243 (2013) 494-498.

[2] N. Pereira, F. Badway, M. Wartelsky, S. Gunn, G.G. Amatucci, Iron Oxyfluorides as High Capacity Cathode Materials for Lithium Batteries, Journal of The Electrochemical Society, 156 (2009) A407-A416.

[3] K.M. Wiaderek, O.J. Borkiewicz, E. Castillo-Martínez, R. Robert, N. Pereira, G.G. Amatucci, C.P. Grey, P.J. Chupas, K.W. Chapman, Comprehensive Insights into the Structural and Chemical Changes in Mixed-Anion FeOF Electrodes by Using Operando PDF and NMR Spectroscopy, Journal of the American Chemical Society, 135 (2013) 4070-4078.

[4] A.N. Mansour, P.H. Smith, W.M. Baker, M. Balasubramanian, J. McBreen, In situ XAS investigation of the oxidation state and local structure of vanadium in discharged and charged V2O5 aerogel cathodes, Electrochimica Acta, 47 (2002) 3151-3161.

Figure 1