Chemistry, Local Molybdenum Clustering, and Electrochemistry in the Li₂₊Mo_1-O₃ Solid Solutions

A broad range of cationic nonstoichiometry has been demonstrated for the Li-rich layered rock-salt-type oxide Li₂MoO₃, which has generally been considered as a phase with a well-defined chemical composition. Li₂₊Mo_1-O₃ (-0.037 ≤ x ≤ 0.124) solid solutions were synthesized via hydrogen reduction of Li₂MoO₄ in the temperature range of 650-1100 °C, with x decreasing with the increase of the reduction temperature. The solid solutions adopt a monoclinically distorted O3-type layered average structure and demonstrate a robust local ordering of the Li cations and Mo₃ triangular clusters within the mixed Li/Mo cationic layers. The local structure was scrutinized in detail by electron diffraction and aberration-corrected scanning transmission electron microcopy (STEM), resulting in an ordering model comprising a uniform distribution of the Mo₃ clusters compatible with local electroneutrality and chemical composition. The geometry of the triangular clusters with their oxygen environment (Mo₃O₁₃ groups) has been directly visualized using differential phase contrast STEM imaging. The established local structure was used as input for density functional theory (DFT)-based calculations; they support the proposed atomic arrangement and provide a plausible explanation for the staircase galvanostatic charge profiles upon electrochemical Li⁺ extraction from Li₂₊Mo_1-O₃ in Li cells. According to DFT, all electrochemical capacity in Li₂₊Mo_1-O₃ solely originates from the cationic Mo redox process, which proceeds via oxidation of the Mo₃ triangular clusters into bent Mo₃ chains where the electronic capacity of the clusters depends on the initial chemical composition and Mo oxidation state defining the width of the first charge low-voltage plateau. Further oxidation at the high-voltage plateau proceeds through decomposition of the Mo₃ chains into Mo₂ dimers and further into individual Mo⁶⁺ cations.