Chemistry, Local Molybdenum Clustering, and Electrochemistry in the Li 2+ x Mo 1- x O 3 Solid Solutions.

A broad range of cationic nonstoichiometry has been demonstrated for the Li-rich layered rock-salt-type oxide Li 2 MoO 3 , which has generally been considered as a phase with a well-defined chemical composition. Li 2+ x Mo 1- x O 3 (-0.037 ≤ x ≤ 0.124) solid solutions were synthesized via hydrogen reduction of Li 2 MoO 4 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 Mo 3 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 3 clusters compatible with local electroneutrality and chemical composition. The geometry of the triangular clusters with their oxygen environment (Mo 3 O 13 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 Li 2+ x Mo 1- x O 3 in Li cells. According to DFT, all electrochemical capacity in Li 2+ x Mo 1- x O 3 solely originates from the cationic Mo redox process, which proceeds via oxidation of the Mo 3 triangular clusters into bent Mo 3 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 3 chains into Mo 2 dimers and further into individual Mo 6+ cations.