Electrospinning with sulfur powder to prepare CNF@G-Fe₉S₁₀ nanofibers with controllable particles distribution for stable potassium-ion storage

As anode materials of electrochemical energy storage system, metal sulfides with high theoretical capacities suffer from issues of materials smashing and deactivation due to huge volume change, resulting in the inferior cycle stability. In this paper, a new strategy of adding sulfur powder into the electrospinning precursor instead of employing sulfur powder during the sulfurizing treatment is proposed to prepare Fe₉S₁₀ composites (CNF@G-Fe₉S₁₀-1). In those composites, most of Fe₉S₁₀ particles are embedded in the graphene-carbon fibers with multiple protection. As anodes for potassium-ion batteries, CNF@G-Fe₉S₁₀-1 display higher rate capacities and more excellent stability (103.2 mAh·g⁻¹ at 1000 mA·g⁻¹ after 892 cycles) than Fe₉S₁₀ composites synthesized by the traditional method. In addition, as anodes for potassium-ion hybrid capacitors, they also deliver high capacities of 102.8 mAh·g⁻¹ at 1000 mA·g⁻¹ after 100 cycles. The morphology characterization evidences indicate that the surface and integrity of CNF@G-Fe₉S₁₀-1 are more smooth and complete than the Fe₉S₁₀ composites fabricated using a common method without sulfur power in electrospinning precursor. The excellent stability and high capacity of CNF@G-Fe₉S₁₀-1 can be attributed to nearly full-wrapped structure of Fe₉S₁₀ in the carbon matrix arising from the new strategy. Owing to the formation of the structure, Fe₉S₁₀ particles are protected from the pulverization, and the structure stability of hybrid carbon fibers is enhanced. This study may provide a new strategy for the controllable synthesis of metal sulfide-CNFs and their application for high stability energy storage.