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 (Na₂S_n, 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.