Chemistry, Local Molybdenum Clustering, and Electrochemistry in the LiMoO Solid Solutions.

A broad range of cationic nonstoichiometry has been demonstrated for the Li-rich layered rock-salt-type oxide LiMoO, which has generally been considered as a phase with a well-defined chemical composition. LiMoO (-0.037 ≤ ≤ 0.124) solid solutions were synthesized via hydrogen reduction of LiMoO in the temperature range of 650-1100 °C, with 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 Mo 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 Mo clusters compatible with local electroneutrality and chemical composition. The geometry of the triangular clusters with their oxygen environment (MoO 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 LiMoO in Li cells. According to DFT, all electrochemical capacity in LiMoO solely originates from the cationic Mo redox process, which proceeds via oxidation of the Mo triangular clusters into bent Mo 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 Mo chains into Mo dimers and further into individual Mo cations.