Investigating the Origin of the Enhanced Sodium Storage Capacity of Transition Metal Sulfide Anodes in Ether‐Based Electrolytes

Abstract Although ether‐based electrolytes have gradually been identified as a vital factor to achieving the excellent electrochemical performance observed in transition metal sulfide (TMS) anodes in sodium‐ion batteries (SIBs), there is still a lack of a fundamental understanding about the origin of the positive effect of ether‐based electrolytes on TMS anodes. Herein, a microspherical CoS 2 anode has been taken as a representative of TMS. It has been demonstrated that the sodiation process involves not only a traditional conversion reaction taking place between solid‐state CoS 2 and Na 2 S, but also a solid–liquid phase conversion process between active materials and soluble sodium polysulfide (Na 2 S n , 2 < n < 8). More importantly, it is first revealed that the long‐term stability and the reversibility of CoS 2 anode are mainly due to the solid–liquid conversion behavior, which makes bulk CoS 2 gradually develop into a stable porous structure with fast Na + transport kinetics and small stress/strain during cycling. Consequently, the CoS 2 electrode delivers remarkable long‐cycle life with an ultrahigh capacity retention rate of 94.8% even after 1500 cycles at 2 A g −1 (only 2.13 mAh g −1 fading per 100 cycles) and high volumetric capacity of 949 mAh cm −3 at a high active material loading of 3.3 mg cm −2 .