factors, lithium-sulfur (Li-S) batteries are one of the most promising candidates for next-generation rechargeable batteries. Typical Li-S cells are assembled with an Li metal as the anode and active sulfur as the cathode with a separator and an electrolyte in between. Under cell discharging, lithium ions from the Li-metal anode travel across the cell and react with active sulfur to form Li polysulfi des (Li 2 S x , x = 4-8) in the active-sulfur cathode. Subsequently, the polysulfi de intermediates convert to the discharge product, lithium sulfi de (Li 2 S). Upon cell charging, lithium ions are plated back onto the anode while the Li 2 S converts back to S 8 . The overall electrochemical reaction (16Li + S 8 = 8Li 2 S) involves two electrons per sulfur. [ 2 ] Sulfur cathodes based on the S 0 ↔ S 2− electrochemical conversion have a high theoretical capacity (1672 mA h g −1 ). This cathode capacity is an order of magnitude higher than that of Li-insertion compound oxide cathodes used in the current Li-ion technology. [ 3 ] Coupled with the operating voltage of 2.15 V versus Li + /Li 0 , the theoretical energy density of Li-S batteries reaches 2600 W h kg −1 , a value which is 3-5 times higher than that of current commercial Li-ion batteries. [ 2,3 ] Further advantages of Li-S batteries include the relatively low cost of sulfur owing to its natural abundance and the relatively low ecological impact of sulfur due to its environmentally benign nature. [ 2,3 ] However, despite these promising attributes, the prototypical Li-S battery suffers from (i) the insulating nature of the active material and (ii) the diffusion of soluble polysulfi de intermediates. [ 3a-c , 4 ] The fi rst scientifi c challenge, the low electronic and ionic conductivity of the active material, causes poor redox accessibility and hence leads to low electrochemical utilization. [ 4d,e ] The second scientifi c challenge, the polysulfi de diffusion, results from the dissolution of the highly soluble polysulfi des into the liquid electrolyte currently used in Li-S batteries. Without effective constraints, the dissolved polysulfi des diffuse out readily from the cathode, penetrating through the separator and reacting detrimentally with the Limetal anode. [ 3c , 5 ] This is the main cause for the irreversible loss of the active material and for the unfavorable polysulfi de shuttle As a primary component in lithium-sulfur (Li-S) batteries, the separator may require a custom design in order to facilitate electrochemical stability and reversibility. Here, a custom separator with an activated carbon nanofi ber (ACNF)-fi lter coated onto a polypropylene membrane is presented. The entire confi guration is comprised of the ACNF fi lter arranged adjacent to the sulfur cathode so that it can fi lter out the freely migrating polysulfi des and suppress the severe polysulfi de diffusion. Four differently optimized ACNF-fi lter-coated separators have been developed with tunable micropores as an investigation into the electrochemical and engineering design param...

Lithium sulfide (Li2S) is considered a highly attractive cathode for establishing high‐energy‐density rechargeable batteries, especially due to its high charge‐storage capacity and compatibility with lithium‐metal‐free anodes. Although various approaches have recently been pursued with Li2S to obtain high performance, formidable challenges still remain with cell design (e.g., low Li2S loading, insufficient Li2S content, and an excess electrolyte) to realize high areal, gravimetric, and volumetric capacities. This study demonstrates a shell‐shaped carbon architecture for holding pure Li2S, offering innovation in cell‐design parameters and gains in electrochemical characteristics. The Li2S core–carbon shell electrode encapsulates the redox products within the conductive shell so as to facilitate facile accessibility to electrons and ions. The fast redox‐reaction kinetics enables the cells to attain the highest Li2S loading of 8 mg cm−2 and the lowest electrolyte/Li2S ratio of 9/1, which is the best cell‐design specifications ever reported with Li2S cathodes so far. Benefiting from the excellent cell‐design criterion, the core–shell cathodes exhibit stable cyclability from slow to fast cycle rates and, for the first time, simultaneously achieve superior performance metrics with areal, gravimetric, and volumetric capacities.

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Lithium-sulfur (Li-S) cells have a strong edge to become an inexpensive, high-capacity rechargeable battery system. However, currently, several prohibitive challenges occur within the sulfur core, especially the polysulfide-diffusion problem. To address these scientific issues, we present here a boron-doped multiwalled carbon nanotube coated separator (B-CNT-coated separator). The B-CNT-coated separator creates a polysulfide trap between the pure sulfur cathode and the polymeric separator as a "polysulfide-trapping interface," stabilizing the active material and allowing the dissolved polysulfides to activate the bulk sulfur cores. Therefore, the dissolved polysulfides change from causing fast capacity fade to assisting with the activation of bulk sulfur clusters in pure sulfur cathodes. Moreover, the heteroatom-doped polysulfide-trapping interface is currently one of the missing pieces of carbon-coated separators, which might inspire further studies in its effect and battery chemistry. Li-S cells employing B-CNT-coated separators (i) exhibit improved cyclability at various cycling rates from 0.2C to 1.0C rate and (ii) attain a high capacity retention rate of 60% with a low capacity fade rate of 0.04% cycle(-1) after 500 cycles. We believe that our B-CNT-coated separator could light up a new research area for integrating heteroatom-doped carbon into the flexible, lightweight, carbon-coated separator.

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