A Mo5N6 electrocatalyst for efficient Na2S electrodeposition in room-temperature sodium-sulfur batteries

Metal sulfides electrodeposition in sulfur cathodes mitigates the shuttle effect of polysulfides to achieve high Coulombic efficiency in secondary metal-sulfur batteries. However, fundamental understanding of metal sulfides electrodeposition and kinetics mechanism remains limited. Here using room-temperature sodium-sulfur cells as a model system, we report a Mo 5 N 6 cathode material that enables efficient Na 2 S electrodeposition to achieve an initial discharge capacity of 512 mAh g −1 at a specific current of 1 675 mA g −1 , and a final discharge capacity of 186 mAh g −1 after 10,000 cycles. Combined analyses from synchrotron-based spectroscopic characterizations, electrochemical kinetics measurements and density functional theory computations confirm that the high d -band position results in a low Na 2 S 2 dissociation free energy for Mo 5 N 6 . This promotes Na 2 S electrodeposition, and thereby favours long-term cell cycling performance. Incomplete conversion of sodium polysulfides represents a significant issue in room-temperature sodium-sulfur batteries. Here, the authors propose Mo 5 N 6 as an electrocatalyst for efficient Na 2 S electrodeposition and improved cell cycling performances.