Lithium−sulfur (Li/S) technology holds great promise for efficient, safe, and economic next-generation batteries. However, commercialization is limited by some issues, which are related to the fast degradation of Li/S cells and poor rate capability. Existing strategies addressing these issues are often unsuitable for commercialization because of their complexity and lack of scalability. This Letter presents a simple, cheap, and scalable synthesis of a sulfur-based cathode material from commercially available poly(methyl methacrylate)/poly-(acrylonitrile) (PMMA/PAN) fibers. Thermal conversion of PMMA/PAN fibers with elemental sulfur yields sulfurized poly(acrylonitrile) (SPAN) with up to 46 wt % covalently bound sulfur. The fibrous morphology with cylindrical macropores helps to form electronic conduction networks in the cathode and provides directed diffusion pathways for ions. Consequently, these Li/SPAN cells show low internal resistances, high initial capacities up to 1672 mAh•g −1 sulfur , high rate capabilities up to 8C, and excellent cycle stabilities over 1200 cycles. In addition, structure and postmortem analysis allow the correlation of electrochemical performance with SPAN's chemical structure.

Lithium–sulfur (Li/S) batteries are among the most promising next‐generation energy storage systems because of their high theoretical specific energy of ≈2600 Wh kg−1. However, conventional Li/S batteries require high amounts of redox‐inactive liquid electrolytes, which do not contribute to cell capacity. Thus, the practical specific energy of Li/S batteries is often relatively poor (<500 Wh kg−1) and barely competitive with Li‐ion batteries. Herein, a new hybrid Li/S battery that contains both a liquid and a solid cathode, i.e., dimethyl trisulfide (DMTS) and fibrous sulfurized poly(acrylonitrile) (SPAN) as active materials is presented. These Li/DMTS/SPAN cells exhibit high capacity (formally up to 7100 mA h gsulfur of cathode−1), high areal capacity up to 4.3 mA h cm−2, high rate capability up to 8 C, and excellent cycle stability (>700 cycles). In addition, both the working and aging mechanism are elucidated by NMR, Raman, X‐ray photoelectron and electronic impedance spectroscopy, X‐ray powder diffraction, cyclic voltammetry, and postmortem analysis.

Two different Li/S cathodes are compared in terms of capacity (mA . h . g sulfur −1 ) and intermediates during discharge and charge. One cathode material is based on fibrous SPAN, a sulfur-containing material obtained via the thermal conversion of poly(acrylonitrile), PAN, in the presence of sulfur. In this material, sulfur is covalently bound to the polymeric backbone. The second cathode material is based on porous activated carbon fibers (ACFs) with elemental sulfur embedded inside the ACFs' micropores. Cyclic voltammetry clearly indicates different discharge and charge chemistry of the two materials. While S-containing ACFs show the expected redoxchemistry of sulfur, SPAN does not form long-chain polysulfides during discharge; instead, sulfide is chopped off the polymer-bound sulfur chains to directly form Li 2 S. The high reversibility of this process accounts for both the high cycle stability and capacity of SPAN-based cathode materials.

Carbonate-based electrolytes were used in combination with sulfurized poly(acrylonitrile)-(SPAN) based lithium-sulfur-cells. High specific capacities of 780 mA•h•g sulfur -1 after 1050 cycles were reached. A linear correlation between the viscosity of linear symmetric carbonates as solvents in the electrolytes and specific capacity was found. Also, ethylene carbonate and fluorinated ethylene carbonate were compared in terms of cell performance in the cell. Ethylene carbonate-based cells gave the highest discharge capacities of 990 mA•h•g -1 after 600 cycles.