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+2xFe_2‐x(SO₄)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 Na₂Fe₂(SO₄)₃ nanocrystals decorated by conductive carbon network toward high‐rate‐capability cathode of SIBs. Impressively, the as‐prepared Na₂Fe₂(SO₄)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, Na₂Fe₂(SO₄)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 Na₂Fe₂(SO₄)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, Na₂Fe₂(SO₄)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.