Abstract :
[en] Fluorophosphates are considered among the most interesting series of cathode material for Li/Na-ion batteries1. Na2FePO4F (space group Pbcn), with its layered structure and two-dimensional pathways for facile Na+/Li+ transport, exhibits minimal structural changes upon reduction/oxidation. The average working voltage is 3.3 V vs. Li/Li+. Intercalation/deintercalation results in a volume change of only 3.7%. However, one of the key drawbacks of Na2FePO4F electrodes is their low intrinsic electronic conductivity.
In order to study the effect of the carbon black and carbon nanotubes on the electrochemical performance of Na2FePO4F cathode material for lithium-ion batteries, Na2FePO4F, Na2FePO4F/CB and Na2FePO4F/CNT were prepared by a spray-drying method with different ratios of CB and CNT (10 and 20%). The crystal and local structure were analyzed by XRD and Mössbauer spectroscopy. The electrochemical properties were studied by galvanostatic cycling in lithium cells. The electrochemical performance is markedly better in the case of Na2FePO4F/CNT (20 wt%), with specific capacities of about 100 mAh/g (Na2FePO4F/CNT) at C/4 rate2 vs. 50 mAh/g for Na2FePO4F/CB. The characterization of Na2FePO4F/CB particles by electron microscopy revealed a carbon-poor surface and a good carbon dispersion for Na2FePO4F/CNT particles attributed to better diffusion of carbon nanotubes in the droplets during drying.
References : 1-N. Eshraghi, S. Caes, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschini, Electrochim. Acta, 228 (2017) 319–324.
2-M. Brisbois, S. Caes, M-T. Sougrati, B. Vertruyen, A. Schrijnemakers, R. Cloots, N. Eshraghi, R-P. Hermann, A. Mahmoud, F. Boschini, Solar Energy Materials & Solar Cells 148 (2015) 67-72.
