Insights into Electrochemical Processes of Hollow Octahedral Co 3 Se 4 @rGO for High-Rate Sodium Ion Storage.

Sodium ion batteries (SIBs), as an alternative and promising energy storage system, have attracted considerable attention due to the abundant reserves and low cost of sodium. However, it remains a great challenge to achieve high capacity and rate capability required for practical applications. Herein, hollow octahedral Co 3 Se 4 particles encapsulated in reduced graphene oxide (Co 3 Se 4 @rGO) were designed and synthesized and exhibited excellent electrochemical performances as anodes of SIBs, especially rate capability. Sodiation/desodiation processes and involved mechanisms were investigated by using in situ TEM and in situ XRD. During sodiation, a crystalline Na 2 Se layer with numerous amorphous fine Co nanoparticles dispersed on it was observed to appear on the surface of the original Co 3 Se 4 @rGO particles, and the movable Co-Na 2 Se composites further migrated to the rGO network with high electron/ion dual conductivity, resulting in ultrafast sodium storage kinetics and remarkable rate performance of the Co 3 Se 4 @rGO anode evidenced by delivering a discharge capacity of 229.3 mAh g -1 at a large current density of 50 A g -1 . Our findings reveal the fundamental mechanism behind the enhanced performance of the Co 3 Se 4 @rGO anode and offer valuable insights into the rational design of electrode materials for high-performance SIBs.