High‐Performance Alluaudite‐Type Na2.54(Fe0.97Mg0.03)1.73(SO4)3 Microspheres Cathode for Sodium‐Ion Batteries with an Ultrahigh Rate Capability and 9000 Cycle Life

Na2+2xFe2-x(SO4)3 (NFSO) is a promising cathode material for sodium-ion batteries (SIBs) due to its low cost and high operating potential (≈3.8 V). However, poor intrinsic electronic conductivity and sluggish kinetics are major drawbacks to its practical application. Herein, magnesium doped Na2+2xFe2-x(SO4)3 microspheres (Na2.54(Fe0.97Mg0.03)1.73(SO4)3) are synthesized via a designed spray-drying process. The optimized cathode material (NFSO@C-Mg0.02) possesses excellent rate performance up to 50C (50.8 mAh g-1 at 50C) and long-cycle stability (capacity retention of 78.3% even after 9000 cycles at 10C). Despite an increase in the mass loading to 10 mg cm-2, the electrode continues to represent a reversible capacity of 72.1 mAh g-1 at 3C. Furthermore, NFSO@C-Mg0.02║HC full cell demonstrates superior cycling stability (80% capacity retention over 8000 cycles at 5C) and high energy density (≈310 Wh kg-1, based on the cathode). In situ X-ray diffraction (XRD) results reveal that the Mg doping strategy successfully mitigates the variation in lattice volume. The density functional theory (DFT) calculations verify that the prominent rate performance is attributed to the enhanced Na+ diffusion kinetics and low ionic-migration energy barrier. This work provides an effective strategy and a fundamental understanding to enhance the electrochemical performance of cathode materials for SIBs.