Porous hollow carbon@sulfur composites for high-power lithium-sulfur batteries.

Among cathode materials for secondary lithium batteries, elemental sulfur has the highest theoretical capacity, 1672 mAh g 1 against lithium, which is at least ten times greater than that of commercially used transition-metal phosphates and oxides. As a cathode, sulfur hosts two lithium ions non-topotactically, supporting the electrochemical redox reaction 16 Li + S8Q8Li2S. [1] Other advantages of using sulfur as the cathode material for batteries are its low cost and widespread availability; its intrinsic protection mechanism from overcharging, which enhances battery safety; a wide operating temperature range; and the potential for a long life cycle. 3] Sulfur has consequently been studied extensively as a cathode material and is considered a promising candidate for electric and hybrid electric vehicles. Despite this promise, implementation of Li–S secondary battery systems for high power applications has been problematic. Hindered by the inherent poor electrical conductivity of sulfur (5 10 30 Scm 1 at 25 8C) and shuttling of higher-order polysulfides during charging, a commercially viable Li–S cell is yet to be realized. Sulfur s low electrical conductivity limits active material utilization as a result of poor electrochemical contacts within the material. Shuttling is a cyclic process in which long-chain lithium polysulfides, (Li2Sn, 2< n< 8), generated at the cathode during charging, dissolve into the electrolyte and migrate to the anode where they react with the lithium electrode in a parasitic fashion to generate lower-order polysulfides, which diffuse back to the sulfur cathode and regenerate the higher forms of polysulfide. This shuttling process is driven by the concentration gradient of polysulfide and there are literature reports which suggest that it provides a potential benefit for overcharge protection in Li–S batteries. However, left unchecked, it leads to decreased utilization of the overall active material mass during discharge, triggers current leakage, poor cycleabilty, and reduced columbic efficiency of the battery. Over the last three decades, methods for preventing polysulfide dissolution and shuttling in Li–S secondary batteries have been intensively investigated by research teams world-wide. One line of study focuses on tailoring the electrolyte to restrain polysulfide dissolution. Electrolytes based on THF, THF/toluene and dioxolane, for example, have been reported to facilitate utilization of sulfur at low discharge currents (0.01 mAcm ), which corresponds to a two-month discharge. An electrolyte formulation based on lithium triflate in tetraglyme has been reported to be effective in controlling shuttling in Li–S batteries, but with an unacceptably low efficiency of 48 %. Room temperature ionic liquids have recently attracted attention as electrolytes for Li–S batteries due to their nonflammability, nonvolatility, wide electrochemical window, and thermal and chemical stabilities. Unfortunately, high interfacial impedance at the Li metal electrode limits the rate capability and long-term cycle life of the Li–S batteries. Persistent efforts to safeguard the lithium anode and to reduce mobility of the polysulfide anions in the electrolyte have also proven largely ineffective in enhancing the cycle life and capacity fading in Li–S batteries. An alternative method utilizes composite sulfur powder coated with conducting polymers to prevent shuttling. This approach has attracted significant interest as certain properties of conducting polymers, including their morphology and electrochemical stability, have been shown to produce stable composites, which yield capacities ranging from 500 and 800 mAh 1 after 50 cycles at low (100 mAg ) current rates. Even under these conditions, however, lithium polysulfide dissolution persists and other cathode configurations including organic sulfides like DMcT (2,5-dimercapto1,3,4-thiadiazole), electropolymerized conductive polyaniline, CuS, and FeS2 have been actively studied with some success. The specific capacity (280 mAh g ) of the most promising of these compounds is nonetheless significantly lower than that of elemental sulfur. Metal oxide adsorbents such as silicates, aluminum oxide, vanadium oxide, and metal chalcogenides have been proposed as inhibitors for polysulfide dissolution, but only with limited success. Carbonaceous sorbents with controlled morphology and structural stability have been more successful, with the most notable example coming from the elegant, recent work of Nazar et al. , which utilize highly ordered, mesoporous carbon capable of sequestering sulfur and polysulfides. At a low discharge rate of 168 mAg 1 (0.1 C), these authors reported that capacities in excess of 800 mAh g 1 can be maintained in the pristine carbon material for 20 cycles, with some fading. Addition of a thin polymer coating on the carbon leads to improved electrochemical behavior, but extended cycle life and rate capability data were not reported. Here we report a facile, scalable approach for synthesizing mesoporous hollow carbon capsules that encapsulate and sequester elemental sulfur in its interior and porous shell. The [*] N. Jayaprakash, J. Shen, S. S. Moganty, A. Corona, Prof. L. A. Archer School of Chemical and Biomolecular Engineering Cornell University, Ithaca, NY 14853-5201 (USA) E-mail: laa25@cornell.edu