Dynamic Multistage Coupling of FeS2/S Enables Ultrahigh Reversible Na–S Batteries

Abstract The fundamental challenges that coexisted around sulfur cathode energy storage systems, are the severe polysulfide dissolution and low reactivity resulting in poor reversibility and short cycle life, specifically, in inexpensive sodium ion batteries. Herein, the solution‐processed synthesis of ultra‐high intimate contacted FeS 2 /S architecture is reported and evolution of the dynamic multistage coupling between the FeS 2 and S in sodium–sulfur batteries is revealed. Atomic visualization and in situ spectroscopy conclude that: Na x FeS 2 (0 <x ≤1) effectively captures sodium polysulfides and promotes the conversion of S 8 to Na 2 S 4 to Na 2 S 2 /Na 2 S; simultaneously, the presence of Na 2 S 2 /Na 2 S traps the continuous growth of iron grains during continuously discharging to 0.4 V, thereby boosting the reversibility and high capacity. Moreover, the density functional theory further analyses the unique coupling effect of Na 2 S x with different intermediate states of FeS 2 . The electrode with unique structure and dynamic coupling exhibits outstanding cycle reversibility and extremely long life, which delivers a reversible capacity of 860 mAh g −1 after 1000 cycles with no capacity decay at 0.5 A g −1 . Even under a practical areal capacity of 4 mAh cm −2 , it still shows pretty‐well cycling stability.