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...

Free-standing porous carbon nanofiber interlayers with tunable surface area and pore structure have been studied to enhance the Li–S battery capacity and cycle life.

Two optimized cathode configurations (a porous current collector and an interlayer) are utilized to determine the better architecture for improving the cycle stability and reversibility of lithium-sulfur (Li-S) cells. The electrochemical analysis on the upper-plateau discharge capacity (QH) and the lower-plateau discharge capacity (QL) is introduced for assessing, respectively, the polysulfide retention and the electrochemical reactivity of the cell. The analysis results in line with the expected materials chemistry principles suggest that the interlayer configuration offers stable cell performance for sulfur cathodes. The significance of the interlayer is to block the free migration of the dissolved polysulfides, which is a key factor for immobilizing and continuously utilizing the active material in sulfur cathodes. Accordingly, the carbon mat interlayers provide sulfur cathodes with a high discharge capacity of 864 mA h g(-1) at 1 C rate with a high capacity retention rate of 61% after 400 cycles.

This paper demonstrates a highly favored route for the synthesis of controlled nanostructures at high rate, high yield, and low cost by molten carbonate electrolysis splitting of CO2. We show the wide, portfolio of carbon nanotubes (CNTs) that can be produced by controlling the electrolysis conditions in this one-pot synthesis. For example solid core carbon nanofibers are formed with C-13 isotope CO2, whereas hollow core CNTs are formed with natural abundance CO2 (which contains 99% C-12 and 1% C-13). Shown are the first doped electrosynthesized carbon nanotubes, prepared with added electrolytic LiBO2 for boron doping, and salts for phosphorous, nitrogen or sulfur CNT doping are probed. Boron doping greatly enhances conductivity of the CNTs. Electrolytic CaCO3 produces thin-walled CNTs, while excess electrolytic oxide yields tangled CNTs. Addition of up to 50 mol% Na2CO3 to a Li2CO3 electrolyte, decreases electrolyte costs and improves conditions for intercalation in Na-ion CNT anodes. Addition of BaCO3 increases electrolyte density. Longer electrolysis time leads to proportionally wider diameter CNTs. Synthetic components (steel cathode, nickel anode and inorganic carbonate electrolyte) are available and inexpensive. Advantages include (1) production is limited only by the cost of electrons (electricity) providing a substantial cost reduction compared to conventional CVD and polymer spinning syntheses and (2) the only reactant consumed in the formation of the CNTs is CO2, transforming this greenhouse gas into a stable, valuable product and providing an economic incentive to the removal of anthropogenic CO2 from flue gas or from the atmosphere.

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A novel and simple method of incorporating pseudocapacitive surface functionalities on free-standing carbon nanofibers using common salt (sodium chloride) is presented. The blend of sodium chloride (NaCl) and polyacrylonitrile is electrospun together, followed by pyrolysis and mild acid treatment to obtain functionalized free-standing (binder-free) carbon nanofibers. The synthesized materials have a low surface area of only 24 m 2 g À1 , however the electrochemical studies show a five-fold increase in specific capacitance on incorporation of NaCl compared to that without NaCl. The XPS characterization demonstrates that the presence of NaCl leads to enhanced oxygen on the surface of carbon nanofibers, particularly in the form of carboxyl groups. These carboxyl groups then facilitate the adsorption of sulfur functional groups on acid treatment. A high specific capacitance of 204 F g À1 , areal capacitance of 1.15 F cm À2 , and volumetric capacitance of 63 F cm À3 in 1 M H 2 SO 4 are obtained, which are attributed to the surface functional groups participating in the pseudocapacitive redox reactions. The fabricated nanofibers demonstrate good capacitance retention at high current densities and high cyclability.

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