Study of Diffusion Kinetics and Microstructure for FeF3-CF (Carbon-Fiber) 3D Composite Electrodes

Recently, we investigated the role of electrode architecture on the capacity retention and hysteresis of iron (II and III) fluoride conversion compound.¹ The unique electrode architecture consists of nanometer sized iron fluoride particles (~ 25-50nm) coated with multilayer graphitic platelets and bound to an electronic backbone comprising of an interconnected network of carbon fibers (5-9 µm diameter).  The carbon coated fluoride particles are bound to the carbon fiber through a carbonaceous binder derived from petroleum pitch.² This unique combination of reduced particle size and electrode architecture significantly improves the local electronic conductivity enhancing the electrochemical performance such as better capacity retention and C-rate performance. Further, we observe a significant reduction of voltage hysteresis to ~ 0.9V compared to 1.5 V for normal FeF₃ slurry electrodes³. Electrochemical cycling at elevated temperature (60^oC), improves the reaction transport kinetics yielding almost theoretical specific capacity (700 mAhg^-1) with reasonably good cycle life with a small concomitant reduction in the hysteresis (Figure 1). The estimated overall energy density based on the active mass of FeF₃ in the electrode is about 1650 Wh kg^-1covering both the intercalation and conversion window (1.5–4.5 V). The overall performance of this 3D-structured cathode is impressive, and may provide a new cathode/electrode concept for next generation high energy density batteries.

Figure 1. (a). Voltage vs. capacity profiles of a Li/FeF₃ cell (on carbon fiber) cycled between 1.5 V and 4.5 V (C/50 rate) at 60^oC. (b) Capacity plotted as a function of cycle number for the Li/FeF₃ cell. Capacity is evaluated based only on FeF₃ active material. 

Although initial cycle life looks promising, we notice capacity fade after 50 cycles clearly suggesting that further investigation are needed to understand the diffusion kinetics and/or structural stability of various intermediate conversion phases.  We will present detailed investigation  of various reaction phases and the kinetics of FeF₃ and FeF₂ electrodes at room temperature, 45 and 60 °C based on potentiostatic intermittent titration (PITT), electrochemical impedance spectroscopy (EIS), electron microscopy (S/TEM) and X-ray diffraction (XRD).

Acknowledgement

This research is sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy.

References:

1. S. K. Martha, J. Nanda, H. Zhou, J. C. Idrobo, N. J. Dudney, S. Pannala, S. Dai, J. Wang, and P. V. Braun, RSC Advances. (Accepted 2013).
2. S. K. Martha, N. J. Dudney, J. O. Kiggans and J. Nanda, J. Electrochem. Soc., 159, A1652-A1658 (2012).

3. P. G. Bruce, B. Scrosati, and J-M. Tarascon, Angew. Chem. Int. Ed., 47, 2930-2946 (2008).