Phase transition in HTB-FeF3/rGO cathode material: x-ray spectral diagnostics and ab inition modelling
In our study we pursue the goal to determine structural changes which take place inside the full Li-ion cell and to look at those processes in situ during cycle. Experiment was carried on the B station of BM01 (SNBL) beamline at ESRF, Grenoble, as a mixed XAFS/XRD experiment. Samples were prepared with synthesis of iron fluoride nanoparticles inside reduced graphene oxide sheets which increases conductivity. The material offers a stable discharge energy of 600-700 Wh/kg over 100 cycles, which is higher than the widely applied cathode materials (300-500 Wh/kg). Each sample we cycled with 20 mAh/g in the 1,2-4,2 V range, while measuring Fe K-edge XAFS spectra in transmission mode and XRD patterns with 15 minutes interval. For measurements we used self-made test cells with glassy carbon windows connected to Gamry potentiostats responsible for cycling and data acquisition.
Results of the x-ray studies were associated with cycling data to obtain structure-charge state dependency. HTB structure of the as-prepared material has open intercalation channels as a result, full electrochemical reaction can be separated into initial intercalation of one Li- anion per formula unit and following conversion reaction involving two more Li-, which gives us 3LiF/Fe mixture and a complete three electron transition. To prove this we performed principal component analysis (PCA) on the series of XAFS experimental spectra. We have used FitIt software to mathematically decompose the series of the Fe K-edge spectra at different voltages into independent sub-spectra. It was found that all spectra for discharge process can be reproduced as a combination of three components. First component corresponds to HTB structure, second to the intercalated structure with Fe2+ charge state and the third one corresponds to metallic Fe. We have observed that pure iron nanoparticles form after HTB conversion to intercalated phase. We have performed a set of ab initio calculations using evolutionary algorythms (USPEX software) within pseudopotential DFT approximation (VASP 5.2) to analyse phase transition in FeF3 material upon intecalation and conversion regimes.
Figure 1. In situ XAS/XRD experiment of HTB-FeF3/rGO composite. Top: Voltage profile during charge/discharge of the cell with grey circles indicating points where ex situ Mossbauer spectra were collected. Middle: PCA concentration profile of Fe, Fe2+ and Fe3+ components obtained from simultaneous decomposition of 260 Fe K-edge XANES spectra. Bottom: XRD contour plot (lambda=0.51 Å) showing evolution of HTB-FeF3, LiF and Fe phases during charge/discharge.
The good cycle performance of the material was attributed to the microstructure, which consists of FeF3particles embedded into a matrix of graphene oxide. The close contact of hygroscopic FeF3particles with carbon permits the preparation of high quality electrodes without the need
of moisture protection in ambient air and maintains electronic as well as Li ionic conductivity during prolonged cycling. The detailed reaction mechanism was investigated using a combination of in situ methods. Upon discharge, Li is inserted fast into the HTB-FeF3 tunnel structure followed by a slow conversion reaction to LiF/Fe nanoparticles. Upon charge, LiF/Fe is slowly converted back to rutile FeF2 and amorphous FeF3 phases without reformation of the HTB-FeF3 framework.
 - A. Pohl, M. Faraz, A. Schroder, M. Baunach, W. Schabelb, A. Guda, V. Shapovalov, A. Soldatov, et.al., Development of a water based process for stable conversion cathodes on the basis of FeF3, Journal of Power Sources 313 (2016) 213-222