Deciphering multistep transformation mechanism of cobalt-vanadium bimetallic sulfides anode in sodium storage

Transition metal sulfides as a high specific capacity sodium -ion batteries (SIBs) anode which have been suffering from successive phase transitions bringing about stress changes, poor electronic conductivity and ineffective ion transport kinetics. Combining self -sacrificial templates and ion -exchange transition reaction mechanisms, herein, cobalt, vanadium bimetallic sulfides hollow nano -cubic boxes with nitrogen -doped nanocarbon coating layer (CoV 2 S 4 @NC) are designed to solve these symbiosis problems. With a combination of in -situ Raman, ex -situ XPS/TEM, and density -functional theory calculations, the synergistically coordination mechanism between cobalt and vanadium binary metal during multi -step conversion processes is responsible for the faster charge transfer kinetics and boosted Na + pseudocapacitance storage capability. Furthermore, the hollow cubic structure combined with the carbon shell layer can release the internal stress during phase transitions and avoid the recurrent growth of SEI film by volume change during the electrochemical process, thus realizing a superior electrode/electrolyte interface. Thus, when used as anode materials for SIBs, the prepared anode materials display excellent electrochemical performance, delivering capacity of 492 mA h g -1 at a current density of 10 A g -1 and high capacity stability with 484 mAh g -1 after 1000 cycles at 5 A g -1 . More importantly, the assembled full cell (Na 3 V 2 (PO 4 ) 3 // CoV 2 S 4 @NC) also shows high capacity of 574 mAh g -1 with 94.7% capacity retention after 85 cycles, suggesting the potential application of bimetallic sulfides in SIBs.