Layered Li_<i>x</i>MoO₂ Phases with Different Composition for Electrochemical Application: Structural Considerations

Abstract

Different modifications of Li x MoO 2 (1) without metallic Mo–Mo bonds ( R -3 m, a = 2.858(2) Å, c = 15.701(13) Å), (2) with endless Mo chains with metallic bonds ( C 2/ m, a = 10.614(3) Å, b = 2.878(1) Å, c = 4.955(2) Å, β = 99.37(3)°), and (3) with Mo 4 O 16 clusters ( C 2/ m, a = 9.905(6) Å, b = 5.739(3) Å, c = 5.472(4) Å, β = 107.43(7)°) were synthesized by high-temperature solid-state reaction. Increasing lithium content supports the formation of the rhombohedral form. In situ structural investigations during electrochemical Li extraction and insertion between 2.4–3.5 V on Li 1.0 MoO 2 material containing rhombohedral and monoclinic modifications with endless molybdenum chains revealed the instability of the monoclinic Li 0.9 MoO 2 form with decreasing Li content against transformation into two rhombohedral phases Li x MoO 2 and Li x ≈0 MoO 2 . In contrast, the rhombohedral Li 1.0 (Mo 0.9 Li 0.1 )O 2 form showed a topotactical single-phase behavior during Li extraction and insertion. Further oxidation of Li x MoO 2 at voltages above 4.5 V led to insertion of large anions such as PF 6 – or BF 4 – into the crystal structure. The Li-free rhombohedral form is stable in Ar up to 570 K and transforms irreversibly at higher temperature into a rutile-like structure.