Boosting the Interfacial Stability of Na _{2+2 <i>x</i>} Fe _{2– <i>x</i>} (SO ₄ ) ₃ Cathodes via Graphene Oxide Incorporation toward High-Performance Sodium-Ion Batteries

Polyanionic Na2+xFe2–x(SO4)3 (NFS) cathode materials, with abundant Na–Fe–S–O elements and a high operating voltage of 3.8 V (vs Na+/Na), have garnered significant attention for low-cost and high-energy-density sodium-ion batteries (SIBs). However, NFS materials still suffer from low electronic conductivity and thus unsatisfactory cycling performance issues. Graphene oxide (GO) incorporation has been regarded as an effective way to improve the electronic conductivity and performance of NFS, but its influence on the interfacial stability of NFS cathodes remains unclear, especially gas evolution and thermal stability. Herein, we incorporated GO into NFS materials and found that the electronic conductivity and cycling performance of NFS cathodes were notably improved. For instance, the capacity retention of NFS cathodes increases from 71.9 to 90.6% after 1000 cycles at 10 C. More importantly, we clarified that the performance enhancement of NFS cathodes with GO incorporation is not only due to improved electronic conductivity and Na+ transport kinetics in the lattice but also highly dependent on the increased interfacial stability with alleviated side reactions, less gas release, and improved thermal stability during cycling. This work provides valuable insights into the causes of capacity decay of NFS cathodes and highlights the importance of interfacial stability during cycling toward high-performance SIBs.