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Lamellar phases with nominal composition Li(Ni 1Ϫy Fe y )O 2 (y Յ 0.30) were synthesized and characterized by Rietveld refinement of their X-ray diffraction (XRD) patterns. These materials exhibited the formula Li 1Ϫz (Ni 1Ϫy Fe y ) 1ϩz O 2 with 0.06 Յ z Յ 0.08 and were used as positive electrodes in lithium batteries. Their electrochemical performances decreased with increasing iron content. The Li x (Ni 0.90 Fe 0.10 ) 1.06 O 2 phases were characterized by XRD and Mössbauer spectroscopy. A solid solution appeared in the entire deintercalation domain 0.28 Յ x Յ 0.94, and Rietveld refinement of the XRD patterns allowed us to characterize the variation of structural parameters upon lithium deintercalation. 57 Fe Mössbauer spectroscopy showed that nickel and iron ions were oxidized simultaneously. The fraction of high-spin Fe 4ϩ was related to the strong ligand field resulting from the presence of the prevailing Ni 3ϩ and Ni 4ϩ ions which lead to a lattice contraction. The behavior upon deintercalation of the Li(Ni 0.90 Fe 0.10 )O 2 phase was compared to that of two-dimensional LiFeO 2 .

The electrochemical cycling of a hydrated, iron‐substituted nickel hydroxide (20 atomic percent of iron substituted for nickel) in a Ni//Cd cell was studied in situ by 57normalFe Mössbauer spectroscopy. The variations of the iron oxidation state (and spin configuration) were followed throughout the discharge and correlated to the changes of the overall distribution of cationic oxidation states, as well as to the structural modifications. The charge starting from an α‐phase leads to a pure γ oxyhydroxide containing NiIII,NiIV,FeIV , and average valence Fefalse〈3.5+false〉 ions. During the discharge, the evolution of the various cationic populations deduced from the Mössbauer data provides evidence of a γ‐type solid solution domain, in which the NiIV ions are reduced to the trivalent state rather than the iron ions, followed by γ + α biphased domain.

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