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In order to satisfy the escalating energy demands, it is inevitable to improve the energy density of current Li-ion batteries. As the development of high-capacity cathode materials is of paramount significance compared to anode materials, here we have designed for the first time a unique synergistic hybrid cathode material with enhanced specific capacity, incorporating cost-effective iron sulfide (FeS) nanoparticles in a sulfurized polyacrylonitrile (SPAN) nanofiber matrix through a rational in situ synthesis strategy. Previous reports on FeS cathodes are scarce and consist of an amorphous carbon matrix to accommodate the volume changes encountered during the cycling process. However, this inactive buffering matrix eventually increases the weight of the cell, reducing the overall energy density. By the rational design of this hybrid composite cathode, we ensure that the presence of covalently bonded sulfur in SPAN guarantees high sulfur utilization, while effectively buffering the volume changes in FeS. Meanwhile, FeS can compensate for the conductivity issues in the SPAN, thereby realizing a synergistically driven dual-active cathode material improving the overall energy density of the composite. Simultaneous in situ generation of FeS nanoparticles within the SPAN fiber matrix was carried out via electrospinning followed by a one-step heating procedure. The developed hybrid cathode material displays enhanced lithium-ion storage, retaining 688.6 mA h g(FeS@SPAN composite) –1 at the end of 500 cycles at 1 A g–1 even within a narrow voltage range of 1–3.0 V. A high discharge energy density > 900 W h kg(FeS@SPAN composite) –1, much higher than the theoretical energy density of the commercial LiCoO2 cathode, was also achieved, revealing the promising prospects of this hybrid cathode material for high energy density applications.

Phone: þ82 55 772 1668, Fax: þ82 55 772 1670During discharge of lithium sulfur (Li-S) battery with a liquid electrolyte system, sulfur is first reduced to Li 2 S 8 , which is dissolved into the organic electrolyte and this serves as the liquid cathode. In solution, lithium polysulfides undergo a series of chemical reactions and their concentration varies during cell reaction. The amount of sulfur and electrolytes in the system plays an important role in determining the cell performance. In this work, the effect of sulfur loading in cathode and the amount of electrolyte on the energy density and cycle performance of Li-S battery has been investigated. Cathodes with sulfur loading of 0.99, 2.98, and 6.80 mg_S cm À2 were prepared. Precisely controlled amount of electrolyte was added with varied electrolyte/sulfur (E/S) ratios of 1.67, 5, 10, 20, and 40 ml/mg_S. The surface morphology of fresh and cycled sulfur cathodes was characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

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