Chemistry, Local Molybdenum Clustering, and Electrochemistry in the Li_2+xMo_1–xO₃ Solid Solutions

Abstract

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.

Effects of Ru doping on the structural stability and electrochemical properties of Li2MoO3 cathode materials for Li-ion batteries

“…At the same current density, the discharge capacity of the LMO sample after 100 cycles was only 76.5 mA h g −1 , and the capacity retention rate was only 30.5%. According to previous literature, [13][14][15][16][17][18]25,47,48 it is found that the cycling performance of pristine LMO is actually rather poor, and the reasons for this phenomenon are very complex. For Li-rich cathode materials, the irreversible phase transformation/structural rearrangement, the dissolution of the TM ions into the electrolyte, and the surface side reactions were proposed to be…”

First-Principles Investigation on Delithiation Mechanisms in a Li-Rich Monoclinic Li2MoO3 Cathode Material for Li-Ion Batteries

“…Even though various experimental analyses evidenced ordered arrangements of Mo and Li in Li 2 MoO 3 , the exact ordering is still unknown. ,, Therefore, Savina et al argued that the α-NaFeO 2 structural type for Li 2 MoO 3 is an oversimplified model. They have observed a quasi-ordered Mo 3 O 13 cluster arrangement and symmetry lowering to a monoclinic structure with a C 2/ m space group . Moreover, in recent work by Kim et al, the C 2/ m phase of Li 2 MoO 3 is identified as a nearly stable structure with a slight energy difference from the rhombohedral structure with space group R 3̅ m …”

“…The Li 2 MoO 3 has an α-NaFeO 2 type structure with an R3̅ m space group, while others have a honeycomb interlayer Li/M ordering in their ground state. 12,13 Multiple studies discuss the disordered nature of Li 2 Mo 0.90 Co 0.10 O 3 , 25 etc. The study by Ma et al 10 has proved that Li 2 MoO 3 is an attractive replacement for Li 2 MnO 3 to use as a cathode in LIBs.…”

“…The Li 2 MoO 3 has an α-NaFeO 2 type structure with an R 3̅ m space group, while others have a honeycomb interlayer Li/M ordering in their ground state. , Multiple studies discuss the disordered nature of Li 2 MoO 3 , in which many assert that the formation of Mo 3 O 13 clusters is due to the triangular Mo–Mo bonds, and this cluster is arranged between Li in LiMo 2 layers. , Goodenough et al explained that the lower magnetic moment of R 3̅ m -Li 2 MoO 3 is due to the delocalized nature of electrons from the 4d orbital of Mo and also suggested that the anomalously low electrical conductivity of Li 2 MoO 3 is associated with the formation of Mo 3 O 13 clusters. A completely random arrangement of the Mo 3 O 13 cluster is assumed in many studies, but such models failed to preserve local charge neutrality due to the formation of the Li-rich and Mo-rich regions . A recent study by Savina et al reveals that the ordered arrangements of Mo 3 O 13 clusters in between Li ions cannot maintain the stoichiometry of Li 2 MoO 3 .…”

“…A completely random arrangement of the Mo 3 O 13 cluster is assumed in many studies, but such models failed to preserve local charge neutrality due to the formation of the Li-rich and Mo-rich regions . A recent study by Savina et al reveals that the ordered arrangements of Mo 3 O 13 clusters in between Li ions cannot maintain the stoichiometry of Li 2 MoO 3 . Even though various experimental analyses evidenced ordered arrangements of Mo and Li in Li 2 MoO 3 , the exact ordering is still unknown. ,, Therefore, Savina et al argued that the α-NaFeO 2 structural type for Li 2 MoO 3 is an oversimplified model.…”