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

We report the electrochemistry of a hitherto unexplored Na2MoS4 phase as conversion electrode material for Na- and Li-cation reversible storage. The material adopts an amorphous coordination polymer structure, with mixed Mo and S valences. Ex-situ XPS and in-situ XRD analysis reveal a complex interplay between Mo and S redox, while excluding the formation of free sulfur, lithium sulfide or other crystalline phases. Na2MoS4 behaves as a mixed ionic-electronic conductor, with electronic conductivity of 6.1 × 10 -4 S cm -1 , that allowed the use of this material without any conductive carbon in an electrochemical cell. A reversible capacity of 500 mAh g -1 is attained corresponding to 5-electron redox exchange, with the end-member reaching from Na 5MoS4 . This study not only emphasizes on the excellent charge storage performances of Na2MoS4 for Li or Na batteries, but also enriches the emerging library and knowledge of sulfide phases with mixed anionic and cationic redox.