X-ray diffraction, 57Fe Mössbauer and step potential electrochemical spectroscopy study of LiFeyCo1−yO2 compounds

With the aim of finding new cathode materials for lithium-ion batteries, the synthesis of layered solid solutions with LiFeyCo1−yO2 stoichiometries has been studied in this work. X-ray single phase products were obtained by a ceramic procedure for 0≤y≤0.2. The unit cell dimensions of the powdered solids increase with the iron content. The Rietveld analysis of a sample with y=0.1 using anisotropic thermal parameters led to RBRAGG=3.37. The hexagonal unit cell parameters of this solid were a=2.8271(1) Å and c=14.1266(7) Å. The site occupancy used in the Rietveld procedure was: (FeT0.0086)6c[Li0.9868FeO0.0046]3b[Li0.0132Co0.9000FeO0.0868]3aO2 according to the intensity of the signals observed in the Mössbauer spectrum. This consists of one intense (87%) quadrupole split signal with isomer shift of ca. 0.316(3) mm/s is ascribable to Fe(III) replacing cobalt in the CoO2 layers. Two weaker quadrupole signals result from small amounts of iron in octahedral 3b and pseudotetrahedral 6c sites of the LiO2 layers. The presence of pseudotetrahedral iron puts obstacles to the lithium ion diffusivity. In consequence, the electrochemical spectra evidence an increased cell polarization as increases. The lithium extraction at the end of the first charge decreases with iron content, with a maximum of 0.6 Li per formula for y=0.1. The introduction of nickel in the composition of these solids may be useful to improve the electrochemical performance of the solid solutions. Ternary systems show an improved electrochemical behaviour.