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Phase transformations upon delithiation in layered oxides with the NaCrS 2 structure type are widely studied for numerous combinations of 3d transition metals because of the application of LiCoO 2 and its derivatives as cathode materials in rechargeable Li-ion batteries. However, complete replacement of 3d by 4d transition metals still yields phenomena never seen in compounds containing 3d metals only. In the present work, the structural evolution of Li-rich O3-Li(Li 0.2 Rh 0.8 )O 2 , having a mixed occupancy of 20% Li and 80% Rh in the metal-O slabs, was studied during electrochemical Li removal and insertion and compared with the isostructural stoichiometric LiRhO 2 . The latter compound undergoes a transformation from the layered NaCrS 2 o the tunnel-like rutile-ramsdellite intergrowth structure of the γ-MnO 2 ype. Partial replacement of Rh by Li, in contrast, completely prevents this transition, resulting in a reversible cell expansion and shrinkage within the layered structure upon (de)lithiation. Moreover, no anomalously short Rh-O and O-O distances were observed in Li  x ≈0 (Li 0.2 Rh 0.8 )O 2 with the Rh 4.75+ intermediate valence state at 4.8 V, in contrast to Li  x ≈0 RhO 2 with Rh 4+ at 4.2 V, as confirmed by operando synchrotron X-ray diffraction and extended X-ray absorption fine structure studies. We believe that the difference in the Li-O and Rh-O covalency is responsible for the observed structural stabilization. The longer and more ionic Li-O bonds in the (Li,Rh)O 2 layers impede the shortening of O-O distances needed for transformation to the γ-MnO 2 ype because of a higher negative charge on O anions connected to Li cations and the stronger electrostatic repulsion between them.

Comparison of Layered Li(Li0.2Rh0.8)O2 and LiRhO2 upon Li Removal: Stabilizing Effect of Li Substitution

Comparison of Layered Li(Li_0.2Rh_0.8)O₂ and LiRhO₂ upon Li Removal: Stabilizing Effect of Li Substitution