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

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 CoS2 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 CoS2 and Na2S, but also a solid-liquid phase conversion process between active materials and soluble sodium polysulfide (Na2Sn, 2 < n < 8). More importantly, it is first revealed that the long-term stability and the reversibility of CoS2 anode are mainly due to the solid-liquid conversion behavior, which makes bulk CoS2 gradually develop into a stable porous structure with fast Na+ transport kinetics and small stress/strain during cycling. Consequently, the CoS2 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).

Automated summary

The study reveals the positive impact of ether-based electrolytes on CoS2 anodes, leading to the development of a stable porous structure for excellent electrochemical performance.