Colloidal Synthesis of Na2Fe2(SO4)3 Nanocrystals as the Cathode Toward High‐Rate Capability and High‐Energy Density Sodium‐ion Batteries

Abstract Alluaudite‐type Na 2+2x Fe 2‐x (SO 4 ) 3 (NFS) with high theoretical energy density is regarded as the promising cathode of sodium‐ion batteries (SIBs), while practical rate and cyclic performances are still hindered by intrinsic poor conductivity. Here, a facile method is developed, collaborating high‐boiling organic solvents assisted colloidal synthesis (HOS‐CS) with sintering for tailoring Na 2 Fe 2 (SO 4 ) 3 nanocrystals decorated by conductive carbon network toward high‐rate‐capability cathode of SIBs. Impressively, the as‐prepared Na 2 Fe 2 (SO 4 ) 3 @MC provides 60.6 and 46.9 mAh g −1 of reversible capacities even at ultrahigh rates of 20 and 30 C, respectively, ranking the superior state among the current NFS‐based cathode. More importantly, Na 2 Fe 2 (SO 4 ) 3 @MC achieves 73% of capacity retention at 20 C after 500 cycles, highlighting its potential for application as a fast chargeable cathode. As a bonus, the full‐cell configuration constructed with Na 2 Fe 2 (SO 4 ) 3 @MC cathode and commercial hard carbon (HC) anode delivers 45.6 mAh g −1 at 10 C and 68.3 mAh g −1 of initial capacity with ≈79.4% of retention after 100 cycles at 2 C. Also, Na 2 Fe 2 (SO 4 ) 3 @MC||HC full cell supplies as high as 140 Wh kg −1 of practical energy density. This work offers a novel approach to prepare NFS cathode for SIBs with both high energy density and fast‐charging ability.