Comparison of Layered Li(Li0.2Rh0.8)O2and LiRhO2upon Li Removal: Stabilizing Effect of Li Substitution

Phase transformations upon delithiation in layered oxides with the NaCrS2 structure type are widely studied for numerous combinations of 3d transition metals because of the application of LiCoO2 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(Li0.2Rh0.8)O2, 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 LiRhO2. The latter compound undergoes a transformation from the layered NaCrS2 to the tunnel-like rutile-ramsdellite intergrowth structure of the γ-MnO2 type. 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 Lix≈0(Li0.2Rh0.8)O2 with the Rh4.75+ intermediate valence state at 4.8 V, in contrast to Lix≈0RhO2 with Rh4+ 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)O2 layers impede the shortening of O-O distances needed for transformation to the γ-MnO2 type because of a higher negative charge on O anions connected to Li cations and the stronger electrostatic repulsion between them.