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

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.