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

Abstract

We report the electrochemistry of a hitherto unexplored Na2MoS4 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. Na2MoS4 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<1MoS4> (highest oxidized state) to <Na>5MoS4> (lowest oxidized state). This study emphasizes the excellent charge‐storage performances of Na2MoS4 for Li‐ or Na‐ion batteries, and enriches the emerging library and knowledge of sulfide phases with mixed anionic and cationic redox properties.

Na2FePO4F/Biocarbon Nanocomposite Hollow Microspheres Derived from Biological Cell Template as High‐Performance Cathode Material for Sodium‐Ion Batteries

“…Recently, Zhu et al report the electrochemistry of a hitherto unexplored Na 2 MoS 4 phase behaved as a mixed ionic-electronic conductor as a conversion electrode material that permits carbon-free application in SIBs. [4] These materials can be sorted as layered transition metal oxide compounds (TMOs), [5] polyanionic compounds, [6] Prussian blue analogs (PBAs), [7] and organic cathodes. [8] Phosphates materials, such as NaFePO 4 , Na 3 Fe 2 (PO 4 ) 3 Na 3 V 2 (PO 4 ) 3 , and Na 2 MPO 4 F (M = Fe, Mn, Co, and Ni), have obtained the most attention for their outstanding stability and high operating voltage generated by effect of the phosphate polyanion.…”

New Cathode Materials in the Fe‐PO4‐F Chemical Space for High‐Performance Sodium‐Ion Storage

“…Metastable phases and materials for energy applications remain sporadically studied yet seem to hold significant promises. [ 12a ] The poor exploration comes from the fact that it is usually considered to work with stable thermodynamic phases as these are also much easier to prepare. It is also typically assumed that the thermodynamically stable phases may also be the best ones with the best electrochemical performances, as is indeed the case for most positive or negative battery electrode materials.…”

“…However, many metastable (or transient) phases have been shown already to outperform their stable counterparts. [ 12a ] Furthermore essential is the richness of the multiple possibilities here, hence the multitude of the reaction channels a process can follow before reaching its lowest energy equilibrium might be endless (Figure 1B ). Our work analyses only a few of these, available in the simple chemical, yet complex compositional and phases diagram of Na‐Fe‐PO 4 ‐F.…”

“…[ 11 ] On the contrary, metastable phases are kinetically trapped phases with positive free energy above the equilibrium state, with many of these known to exhibit superior properties than their corresponding stable phases. [ 5 , 12 ] Although with moderate interest and applicability so far, these have also started gaining interest in the battery materials field. [ 5 , 13 ] Tatsumisago et al have recently summarized the state‐of‐the‐art progress on glassy and metastable crystalline materials as solid electrolytes and electrode materials, aiming at emphasizing the attention on the underexplored potential of metastable materials for battery applications.…”

N‐doped carbon nanotube sponges and their excellent lithium storage performances

“…Lithium‐ion batteries (LIBs) exhibit plenty of advantages, including high gravimetric/volumetric capacities, power density, long lifespan and start attaining practically appealing cost metrics. [ 1–3 ] Ever since Sony Co. commercialized the world's first LIBs with a tailor‐made carbon negative electrode, [ 4 ] great efforts have been devoted to exploring novel energy storage materials. [ 5–20 ] Graphite is commonplace among commercial LIBs because of low redox potential, good stability and electrical conductivity.…”