Iron substitution in lithium-overstoichiometric "Li1.1CoO2":: Combined 57Fe Mossbauer and 7Li NMR spectroscopies studies

Iron substitution was attempted by direct solid-state synthesis in stoichiometric LiCoO2 and lithium-overstoichiometric "Li1.1CoO2". Iron substitution was not obtained in stoichiometric LiCo0.98Fe0.02O2 samples, consistent with the fact that the size of Fe3+ ions is significantly larger than that of Co3+ ions in the octahedral site. In contrast, up to 8 atom % iron could be substituted in the lithium-overstoichiometric "Li1.1CoO2" samples with an actual composition of Li1.04Co0.96O1.96, which could be rationalized by considering the structural defect model proposed previously by some of us. In the defect model, lithium-overstoichiometric samples consist of excess Li+ replacing Co3+ charge-compensated by an oxygen vacancy in the cobalt layers, which creates two adjacent square-based pyramids containing intermediate-spin (IS) Co3+ ([Li](interslab) [Co1-3t3+(LS)Co2t3+(IS)Lit](slab)[O2-t], with t = 0.04 and 0.08 IS Co3+ and LS = low spin). Li-7 MAS NMR showed that the signals associated with intermediate-spin Co3+ decreased but were not completely suppressed upon iron substitution, even for the 8 atom % Fe-substituted sample, with no new signal appearing in iron-substituted lithium-overstoichiometric samples. Moreover, the values of Mossbauer parameters, isomer shift 0.249 mm(.)s(-1) and quadrupolar splitting 0.4 mm(.)s(-1), revealed that high-spin Fe3+ was present in the square-pyramidal sites in the 2 atom % iron-substituted lithium-overstoichiometric sample. These results lent further support for the nature of the defects proposed previously for lithium-overstoichiometric "Li1.1CoO2".