Iron Substitution in Lithium-Overstoichiometric “Li1.1CoO2”: Combined 57Fe Mössbauer and 7Li NMR Spectroscopies Studies

Iron substitution was attempted by direct solid-state synthesis in stoichiometric LiCoO 2 and lithium-overstoichiometric “Li 1.1 CoO 2 ”. Iron substitution was not obtained in stoichiometric LiCo 0.98 Fe 0.02 O 2 samples, consistent with the fact that the size of Fe 3+ ions is significantly larger than that of Co 3+ ions in the octahedral site. In contrast, up to 8 atom % iron could be substituted in the lithium-overstoichiometric “Li 1.1 CoO 2 ” samples with an actual composition of Li 1.04 Co 0.96 O 1.96, 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 Co 3+ charge-compensated by an oxygen vacancy in the cobalt layers, which creates two adjacent square-based pyramids containing intermediate-spin (IS) Co 3+ ([Li] interslab [Co 3+(LS) 1 - 3 t Co 3+(IS) 2 t Li t ] slab [O 2 - t ], with t = 0.04 and 0.08 IS Co 3+ and LS = low spin). 7 Li MAS NMR showed that the signals associated with intermediate-spin Co 3+ 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 Mössbauer parameters, isomer shift 0.249 mm·s -1 and quadrupolar splitting 0.4 mm·s -1, revealed that high-spin Fe 3+ 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 “Li 1.1 CoO 2 ”.