Sodium-Sulfur Batteries with Unprecedented Capacity, Cycling Stability and Operation Temperature Range Enabled by a CoFe₂O₄ Catalytic Additive Under an External Magnetic Field

The electrochemical performance of room-temperature sodium-sulfur batteries (SSBs) is limited by slow reaction kinetics and sulfur loss in the form of sodium polysulfides (SPSs). Here, it is demonstrated that through electron spin polarization, at no additional energy cost, an external magnetic field (M on) generated by a permanent magnet can significantly improve the SPSs adsorption capacity and reaction dynamics of a ferrimagnetic sulfur host. More specifically, the preparation of a carbon nanofiber/CoFe₂O₄/S (CNF/CoFe₂O₄/S) cathode with unprecedented performance and stability at ambient temperature is detailed when M on. It is experimentally and theoretically demonstrated that the magnetic field polarizes the electrons of Co ions, enhancing the adsorption of SPSs and their catalytic conversion. CNF/CoFe₂O₄/S cathodes with spin polarization provide unprecedented decay rates down to 0.0039% per cycle at 1.0 C for 2700 cycles. The performance of SSBs is further tested, which has 248 mAh g⁻¹ under 1.0 C after 100 cycles when M on. Furthermore, it is evidenced that even when removing the external magnetic field, the magnetic polarization effect persists, opening the door for practical applications. This study not only demonstrates an effective strategy to improve electrochemical performance in SSBs, but also contributes to the enrichments of spin effects in the fields of electrocatalysis.