Mixed Anionic and Cationic Redox Chemistry in a Tetrathiomolybdate Amorphous Coordination Framework.

We report the electrochemistry of a hitherto unexplored Na2 MoS4 phase as a conversion electrode material for Na- and Li-ion batteries. The material adopts an amorphous coordination polymer structure with mixed Mo and S valences. XPS and XRD analysis reveal a complex interplay between Mo and S redox chemistry, while excluding the formation of free sulfur, lithium sulfide, or other crystalline phases. Na2 MoS4 behaves as a mixed ionic-electronic conductor, with electronic conductivity of 6.1×10-4  S cm-1 , that permits carbon-free application in an electrochemical cell. A reversible capacity of up to 500 mAh g-1 was attained, corresponding to a five-electron redox exchange, with species ranging from <Na<1 MoS4 > (highest oxidized state) to <Na>5 MoS4 > (lowest oxidized state). This study emphasizes the excellent charge-storage performances of Na2 MoS4 for Li- or Na-ion batteries, and enriches the emerging library and knowledge of sulfide phases with mixed anionic and cationic redox properties.