Carbon Sulfur Composites Research Articles

Graphene Sandwiched by Sulfur-Confined Mesoporous Carbon Nanosheets: A Kinetically Stable Cathode for Li-S Batteries.

The practical use of lithium-sulfur batteries for the next-generation energy storage, especially the automobiles, was hindered by low electronic conductivity of sulfur and the resulting poor rate capabilities. Here, we report a sulfur-carbon composite by confining S into a graphene sandwiched in mesoporous carbon nanosheets with a two-dimensional ultrathin morphology, suitable mesopore size and large pore volume, and excellent electronic conductivity. Serving as cathode material for a Li-S battery, the elaborately designed S/C composite leads to "kinetically stable" transmissions of Li ions and electrons, triggering a stable electrochemistry and a record-breaking rate performance. In this way, the S/C composite has been proved a promising cathode material for high-rate Li-S batteries targeted at automobile storage.

Sulfur-Infiltrated Hierarchical Porous Carbon (HPC) Cathodes for Lithium-Sulfur Batteries

Lithium sulfur (Li-S) batteries have been considered as promising candidates for next-generation battery applications because of its high theoretical capacity (1675 mAh/g is 5 times higher than those of traditional cathode materials based on transition metal oxides or phosphates), abundance, safety, low cost, and eco-friendliness. However, the practical implementation of Li-S batteries is greatly hindered by the high resistance of sulfur (5 x 10-30 S cm-1 at 25 oC) and dissolution of soluble long-chain lithium polysulfides (Li2Sn, 4≤n≤8) in the polar organic electrolytes, and volume expansion of sulfur during discharge process, resulting in an insufficient cycle life. To this end, a number of studies have focused mainly on the development of carbon-sulfur composite with the intention to improve the cycle stability. Many types of mesoporous carbon materials have been studied as the electrically conducting host materials for improving the electrical conductivity of sulfur and preventing dissolution of soluble long-chain lithium polysulfides, which are intermediates formed as reaction in both discharge and charge processes in the electrolytes. Those approaches improve the electrical conductivity of sulfur, but are associated with inherent limitation of the dissolution of polysulfides due to the limitation of physical adsorption of the polysulfides by mesopores, which tends to limit the electrochemical performance of carbon-sulfur composite cathode materials. Recently, to overcome these problems, microporous carbon materials have been designed. Although the characterization of sulfur confined in micropores is not completely determined, the micropores have indeed turned out to be efficacious in mitigation of the soluble lithium polysulfides via various mechanisms: (1) strong adsorption of soluble polysulfides, (2) construction of solvent-free environment, and (3) formation of insoluble small S2-4 molecules. Because of these attributes, the microporous carbon-sulfur composites show outstanding cycling performance and rate capability. Additionally, Manthiram et al. reported a novel Li-S cell configuration having carbon interlayers with micropores between the separator and the regular sulfur electrodes. This interlayer works as polysulfide stockroom to maintain the cycle stability. Based on these micropore approaches, we combined sulfur encapsulation and micropores into a single electrode component by developing a hierarchical porous carbon (HPC) structure. In the HPC structure prepared by spray pyrolysis process, the inner meso- and macro- pores are surrounded by outer carbon shell with micropores. Hence, stable cycling was achieved by the outer micropores that shut dissolution of lithium polysulfides down, while most of active sulfur was loaded in the larger inner pores.

The Role of the Carbon Framework in Sulfur-Carbon Composite Cathodes in Li-S Batteries

Lithium-sulfur (Li-S) batteries are considered a promising energy storage solution for today’s society, which faces significant energy and environmental issues. Successful implementation of Li-S batteries requires the development of a robust and high-performance sulfur cathode. Unfortunately, the current approaches focus on the development of sulfur-carbon composites without a fundamental understanding of the reduction mechanism of sulfur and the role of carbon. To develop a clear understanding of the efficacy of carbon/sulfur composites, we investigated the electrochemical behavior of Li-S batteries employing different cathode structures. Our comparative study suggests that nanoscale electrical contact between sulfur and carbon is not essential for improving the electrochemical performance of Li-S batteries or for initiating electrochemical redox reactions of the sulfur cathode.

Ultrasonic Synthesis and Multi-Scale Model of a Carbon Compartment & Sulfur Composite

In the field of battery technology, the lithium-sulfur battery has been widely pursued due to high energy density and high theoretical capacity that can satisfy viability thresholds for electric vehicles. However, this elusive technology remains in the research sector due to a wealth of engineering challenges resulting from the complex chalcogenide electrochemistry. The most infamous challenge remains the polysulfide shuttle effect, in which lithium polysulfide intermediates are produced as elemental sulfur (S8) is reduced to lithium sulfide (Li2S) during the discharge cycle. Since the higher order polysulfides are soluble in organic electrolyte, battery cycling can result in dissolution of the cathode, dendrite formation upon a lithium metal anode, and introduce impedance via electrode passivation. This can ultimately cause rapid capacity fade and unstable Coulombic efficiency. This work presents a novel carbon-sulfur composite, utilizing a cost-effective, bimodal-porosity carbon substrate and a sonochemical sulfur loading technique to improve mono-dispersity in sulfur distribution. The cavity-rich carbon substrate, appropriately named carbon compartments, is synthesized through a single step heating process of commercial wheat flour. The surface area and bi-modal porosity of the carbon compartments enable high sulfur loading (ca. 70 %-wt.). Optimization of a novel fluorinated-electrolyte demonstrates radical improvement in columbic efficiency and cycling stability of the carbon-sulfur composite. The demonstrated Li-S cathode has shown stable half-cell performance (ca. 750 mAh g-1) and columbic efficiency (> 96 %) for all cycles at a current density of 28 mA g-1. Further analysis using in-depth multi-scale modeling describes the electrochemical performance of the composite. Density functional theory and ab initio molecular dynamics characterize the development and behavior of sulfur-containing compounds at the electrolyte-composite surface. Stochastic modeling of the composite microstructure describes electrochemical and physical consequences of polysulfide precipitation. This multi-scale model provides fundamental insight into the mechanisms concerning the role of carbon morphology in the polysulfide shuttle phenomenon.

Self-Assembled MWCNT Interlayer for High-Performance Lithium-Sulfur Batteries

Rechargeable batteries are one of the prime approaches for electrical energy storage. The current lithium-ion battery technology based on transition-metal oxide cathodes and graphite anode have limited energy storage capacity, so there is immense interest to develop alternate high-capacity cathode and anode materials. Lithium-sulfur (Li-S) batteries are appealing in this regard as they offer a high energy density of 2500 Wh kg-1 when coupled with lithium-metal anode due to the large charge-storage capacity of sulfur.[1] Moreover, sulfur is much more abundant, inexpensive, and non-toxic compared to the transition-metal oxide cathodes. There are several challenges with the Li-S batteries, such as low electrical conductivity of sulfur and the discharge product Li2S and the migration of dissolved polysulfides from the cathode to the anode, resulting in low efficiency and inadequate cycle life.[2] Well-designed carbon-sulfur composites have been extensively explored to overcome these issues.[3] In these strategies, sulfur was entrapped in carbon structures using porous carbon, carbon nanotubes (CNT), and hollow carbon spheres. Carbon backbone serves as the electron conducting path for sulfur and reduce polysulfide migration by trapping the polysulfides during cycling. However, polysulfide migration still occurs and cycle life is inadequate for practical applications. Manthiram group and other groups introduced the use of carbon-paper interlayers in between the sulfur cathode and the polymer separator or carbon-coated membrane and demonstrated high sulfur utilization with better cycle life due to the suppression of polysulfide diffusion to the anode and the serving of the carbon-paper interlayer as a pseudo-upper current collector.[4] We present here the fabrication of an interlayer consisting of MWCNT and the electrolyte in a single fabrication process. Such MWCNT-electrolyte-paper interlayers are much more effective compared to the bare MWCNT-paper interlayer due to the fast ionic and electronic transport and a better interface between the sulfur cathode and the interlayer as well as between the separator and the interlayer.

Novel Protection Strategies for Sulfur and Silicon in Advanced Lithium-Sulfur Batteries

In our daily life the demand for electrochemical power sources with high energy density is becoming increasingly important. This kind of resources is fundamental for our future as it may represent a viable alternative to fossil fuels and gases offering a solution to environmental issues as well as energy crises. Until now Lithium-ion batteries (LIBs) have been the dominant power sources in the commercial energy storage devices field. Though representing a tremendous improvement with respect to previous systems like Lead-acid batteries, However, LIBs still cannot represent a feasible alternative where gravimetric energy density, lower than 300 Wh.kg-1, is at stake. In the transportation field, f. i., it is not yet possible to buy an electric vehicle having performance comparable to that of an internal combustion vehicle of the same cost. Among various types of new “beyond Lithium-ion” batteries, the Lithium-sulfur battery holds great potential due to its high theoretical specific capacity of 1672 mAh.g-1 and specific energy of 2600 Wh. Kg-1. Nevertheless, the development of Lithium-sulfur battery has met several challenges for its typical low sulfur utilization, low rate capacity and poor cycle life, mainly due to the electronic and ionic insulating nature of sulfur, the solubility of reductive polysulfides in organic solvents based electrolytes and the large expansion of sulfur upon cycling. Therefore different approaches need to be explored to solve these issues. Short lifetime of Lithium-sulfur batteries is linked, upon other factors, to the use of Li metal anode, which is known to cause safety hazard and internal short-circuit due to the formation of lithium dendrites, which may as well result in thermal runaway. Moreover this material is very expensive. One of the promising solutions is to use lithiated Si instead of Li metal. The theoretical capacity (4200 mAh.g-1) of silicon as anode is higher than the one of Li metal (3860 mAh.g-1), making it ideal to couple with the high capacity sulfur cathode in order to maintain a comparable energy density despite the lower discharge voltage obtained with silicon. However this approach raises another issue, silicon undergoes large volume expansion during lithiation risking compromising the anode structure. Therefore a silicon core is enclosed in a silica shell which is then partially removed by acidic treatment, thus creating empty spaces around the core to allow an effective lithiation. In order to make the whole structure conductive, an external shell is then created around it (carbon, metal,…). In a parallel study, again regarding the anode, the synthesis of nanostructured materials such as, Graphene-Silicon composites to make cages that accommodate silicon’s volume changes during charging and discharging, is being carried out. Silicon nanoparticles are treated with acid to achieve fresh Si surface. Then, Si nanoparticles are reacted with amine group terminated long chain alkenes through addition reaction. Si nanoparticles bearing amine functional groups are dispersed with graphene oxide in mild acidic conditions. In this strategy, the interaction between positively charged nanoparticles and graphene oxide allow the uniform wrapping of the particles and long carbon chains will provide the room for large volume and morphological changes. The graphene oxides are further reduced to graphene to provide conductive pathways for electronic and ionic conductivities. Regarding the cathode, a sustainable solution is suggested to prepare conductive carbon matrices from bio-based materials. Template-assisted hydrothermal carbonization of dextrose and/or β-cyclodextrin is carried out to prepare carbon nanospheres or nanosponges to host Sulfur (S). S is incorporated inside carbon nanosponges via solvent impregnation and thermal diffusion. Then, as prepared carbon-sulfur composite is encapsulated with conductive polymers like polyaniline/polypyrrole. This dual protection strategy in conductive matrices provide conductive network for S and effectively help to suppress the polysulfide dissolution.

Design Principles for Optimum Performance of Porous Carbons in Lithium–Sulfur Batteries

A series of experiments is presented that establishes for the first time the role of some of the key design parameters of porous carbons including surface area, pore volume, and pore size on battery performance. A series of hierarchical porous carbons is used as a model system with an open, 3D, interconnected porous framework and highly controlled porosity. Specifically, carbons with surface areas ranging from ≈500–2800 m2 g−1, pore volume from ≈0.6–5 cm3 g−1, and pore size from micropores (≈1 nm) to large mesopores (≈30 nm) are synthesized and tested. At high sulfur loadings (≈80 wt% S), pore volume is more important than surface area with respect to sulfur utilization. Mesopore size, in the range tested, does not affect the sulfur utilization. No relationship between porosity and long‐term cycle life is observed. All systems fail after 200–300 cycles, which is likely due to the consumption of the LiNO3 additive over cycling. Moreover, cryo‐scanning transmission electron microscopy imaging of these carbon–sulfur composites combined with X‐ray diffraction (XRD) provides further insights into the effect of initial sulfur distribution on sulfur utilization while also revealing the inadequacy of the indirect characterization techniques alone in reliably predicting distribution of sulfur within porous carbon matrices.

Aspergillus flavus Conidia-derived Carbon/Sulfur Composite as a Cathode Material for High Performance Lithium-Sulfur Battery.

A novel approach was developed to prepare porous carbon materials with an extremely high surface area of 2459.6 m2g−1 by using Aspergillus flavus conidia as precursors. The porous carbon serves as a superior cathode material to anchor sulfur due to its uniform and tortuous morphology, enabling high capacity and good cycle lifetime in lithium sulfur-batteries. Under a current rate of 0.2 C, the carbon-sulfur composites with 56.7 wt% sulfur loading deliver an initial capacity of 1625 mAh g−1, which is almost equal to the theoretical capacity of sulfur. The good performance may be ascribed to excellent electronic networks constructed by the high-surface-area carbon species. Moreover, the semi-closed architecture of derived carbons can effectively retard the polysulfides dissolution during charge/discharge, resulting in a capacity of 940 mAh g−1 after 120 charge/discharge cycles.

Wet Chemistry Synthesis of Multidimensional Nanocarbon-Sulfur Hybrid Materials with Ultrahigh Sulfur Loading for Lithium-Sulfur Batteries.

An optimized nanocarbon-sulfur cathode material with ultrahigh sulfur loading of up to 90 wt % is realized in the form of sulfur nanolayer-coated three-dimensional (3D) conducting network. This 3D nanocarbon-sulfur network combines three different nanocarbons, as follows: zero-dimensional carbon nanoparticle, one-dimensional carbon nanotube, and two-dimensional graphene. This 3D nanocarbon-sulfur network is synthesized by using a method based on soluble chemistry of elemental sulfur and three types of nanocarbons in well-chosen solvents. The resultant sulfur-carbon material shows a high specific capacity of 1115 mA h g(-1) at 0.02C and good rate performance of 551 mA h g(-1) at 1C based on the mass of sulfur-carbon composite. Good battery performance can be attributed to the homogeneous compositing of sulfur with the 3D hierarchical hybrid nanocarbon networks at nanometer scale, which provides efficient multidimensional transport pathways for electrons and ions. Wet chemical method developed here provides an easy and cost-effective way to prepare sulfur-carbon cathode materials with high sulfur loading for application in high-energy Li-S batteries.

Facile assembly of partly graphene-enveloped sulfur composites in double-solvent for lithium–sulfur batteries

A series of graphene-enveloped sulfur (G/S) composites with different sulfur particle sizes and high sulfur contents of 82wt% are synthesized using a double solvent approach for lithium–sulfur batteries. This double solvent approach is facile, low cost, and effective in preparing graphene–sulfur composites for large-scale production. Scanning electron microscopy and energy dispersive X-ray spectroscopy mapping were used to verify the structure and composition of the G/S composites. The graphene-coating layers not only provide a conductive carbon matrix surrounding the sulfur particles but also prevent the soluble polysulfide intermediates dissolving into the electrolyte. The sizes of sulfur particles can be controlled by adjusting the ratios of double solvent (CS2/alcohol) and may further affect the electrochemical behavior. When used as cathode materials for lithium–sulfur batteries, the G/S-200 composite with the smallest sulfur particle size shows an initial discharge capacity of 1068.5mAhg−1 and retains 729.5mAhg−1 after 100 cycles at 0.1C. This novel double solvent method is a promising approach to obtain other carbon sulfur composites.

Carbon Wrapped Sulfur Cathode Materials for Rechargeable Batteries

Introduction: Currently available rechargeable Li-ion batteries based on spinal, layer and olivine crystal structures of Li (Mn, Ni, Co) Ox and related material systems, shows lower capacity and poor cycling characteristics. In addition, LiCoO2 is expensive and environmentally unfriendly. Due to high volume and gravimetric energy density, rechargeable lithium batteries have become the dominant power source for portable electronic devices including cell phones and laptops. However, the energy and power densities of rechargeable lithium batteries require significant improvement in order to power electric vehicles1. Lower specific capacities of cathode materials (~150mAh/g for layered oxides and ~170mAh/g for LiFePO4) compared to those of the anode (370mAh/g for graphite and 4200mAh/g for Si) have been a limiting factor to the energy density of batteries. It is highly desirable to develop and optimize high capacity cathode materials for rechargeable lithium batteries. Sulfur is a promising cathode material with a theoretical specific capacity of 1672mAh/g,2 ~ 5 times higher than those of traditional cathode materials based on transition metal oxides or phosphates. Sulfur also possesses other advantages such as low cost and environmental friendly. Experimental: Here we present a rational design and synthesis of a novel carbon-sulfur composite material at low temperature. Sulfur particles are synthesized and wrapped by carbon black a simple assembly process. The phase formation behavior of the synthesized powder was investigated using X-ray diffraction, using Cu Kα radiation. The morphology of the synthesized powder was investigated using a scanning electron microscopy (SEM), and transmission electron microscopy (TEM, Carl Zeiss Leo Omega 922 at 200 KeV). Result: The XRD patterns (Fig1.) of all samples could be indexed to the orthorhombic space group based on the phase –pure ordered structure. Electrochemical properties will be study using cyclic Voltammetry, Charge-Discharge Curves, and Electrochemical Impedance spectroscopy (EIS). These measurements are still in progress. Fig1. X ray diffraction patterns of Sulfur-carbon composite (insert morphology image).

A Facile Synthesis of High-Surface-Area Sulfur-Carbon Composites for Li/S Batteries.

Small-grained elemental sulfur is precipitated from sodium thiosulfate (Na2 S2 O3 ) in a carbon-containing oxalic acid (HOOC-COOH) solution through a novel spray precipitation method. Surface area analysis, elemental mapping, and transmission electron micrographs revealed that the spray-precipitated sulfur particles feature 11 times higher surface area compared to conventional precipitated sulfur, with homogeneous distribution in the carbon. Moreover, the scanning electron micrographs show that these high-surface-area sulfur particles are firmly adhered to and covered by carbon. This precipitated S-C composite exhibits high discharge capacity with about 75 % capacity retention. The initial discharge capacity was further improved to 1444 mA h g(-1) by inserting a free-standing single-walled carbon nanotube layer in between the cathode and the separator. Moreover, with the help of the fixed capacity charging technique, 91.6 % capacity retention was achieved.

Carbon–Sulfur Composites from Cylindrical and Gyroidal Mesoporous Carbons with Tunable Properties in Lithium–Sulfur Batteries

Sulfur is a high-potential candidate for next generation electrical energy storage due to its abundance, low cost, and nontoxicity. The electrically insulating property of sulfur and lithium sulfide and the solubility of polysulfides in the electrolyte, however, pose great challenges for the realization of long-lasting lithium–sulfur batteries for commercial applications. In recent years, much attention has been focused on mesoporous carbon–sulfur composites as cathode material. Fundamental studies on the performance of such electrodes correlated to systematic variations of structural and porosity characteristics of the carbons remain elusive, however. In this work a variety of block copolymer (BCP) derived mesoporous carbons were studied with uniform and tunable pore sizes as sulfur hosts. Morphologies included hexagonally packed cylinders and co-continuous gyroids, with one- and three-dimensional porosity, respectively. Dependence of the cyclability of carbon–sulfur composites was tested on mesopore siz...

Hierarchical carbon nanocages confining high-loading sulfur for high-rate lithium–sulfur batteries

Lithium–sulfur batteries are hindered by the low utilization of sulfur, short cycle life and poor rate capability which are severe challenges today. Herein we report a new kind of carbon–sulfur composites by infusing sulfur into the novel hierarchical carbon nanocages (hCNC) with high pore volume, network geometry and good conductivity. The designed S@hCNC composite with a high sulfur loading of 79.8wt% presents the large capacity, high-rate capability and long cycle life, which could shorten the charging time for mobile devices from hours to minutes. The excellent performance derives from the unique mesostructure of hCNC that enables the encapsulation of high-loading sulfur inside the carbon nanocages to alleviate polysulfide dissolution, meanwhile much enhance the electron conduction and Li-ion diffusion.

A systematic approach to high and stable discharge capacity for scaling up the lithium–sulfur battery

A systematic approach to improving the performance of the Li–S battery is presented, based on applying high energy ball milling to create a porous sulfur–carbon composite, insertion of a free-standing layer, and adoption of a new charging method. Surface area analysis and field emission scanning electron microscope imaging show that the ball-milled sulfur powder has a porous structure and very high specific surface area. A vacuum-filtrated single-walled carbon nanotube free-standing layer is inserted in between the sulfur cathode and the separator. It is believed that high-surface-area porous sulfur will help to increase the conductivity of the elemental sulfur due to better adhesion between the conducting carbon and the sulfur, while the free-standing layer will sequester longer chain polysulfides, which are responsible for the well-known shuttling phenomenon. By the combination of these methods, we have achieved excellent capacity and cycle life. Finally, a new charging method which will largely prevent the formation of longer chain polysulfides is also applied to increase the capacity retention. It is believed that with the combination of ball milling, the free-standing layer, and the new charging method, it is possible to commercialize the Li–S battery with better capacity and cycle life.

Minimizing Polysulfide Shuttles in Lithium Sulfur Batteries by Introducing Immobile Lithium Ions into Carbon–Sulfur Nanocomposites

AbstractA facile synthesis of a carbon–sulfur nanocomposite, of which the capacity retention ratio is substantially enhanced by almost 100 %, is presented for polysulfide shuttle reduction in lithium sulfur batteries through the introduction of immobile lithium‐ion sites into the composite. A commercial Nafion ionomer, which can reduce thiosulfate sodium to sulfur in situ, owing to its acidity, provides immobile lithium ions after lithium exchange in the nanocomposite to attract the negatively charged polysulfide species for reduction during discharge. The capacity decay of the second cycle discharge, which is about 25 % of the value for a conventional carbon–sulfur composite, is reduced to about 3 % by introducing Nafion into the nanocomposite, and the capacity retention of this novel carbon–Nafion–sulfur nanocomposite is improved by more than 100 % during long‐term battery operation.

Mesoporous carbon-sulfur composite as cathode for lithium-sulfur battery

A novel carbon-sulfur composite, mesoporous carbon-sulfur, was developed as cathode of lithium/sulfur battery. The mesoporous carbon was prepared with sucrose as carbon precursor and calcium carbonate nanoparticles as pore producer. The sulfur was encapsulated in the mesoporous carbon via a simple chemical deposition strategy and a subsequent low-temperature thermal treatment process. The morphology and structure of the resulting composite were characterized with scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller measurement, and thermogravimetric analysis. Its electrochemical performances as cathode of lithium/sulfur battery were evaluated with cyclic voltammetry and charge-discharge test. The prepared carbon showed a specific surface area of 598 m2 g−1 with a bimodal pore distribution centered at 3.6 and 40 nm. The resulting mesoporous carbon-sulfur delivered an initial capacity of 1,380 mAh g−1 at 0.02 C and maintained a capacity of 750 mAh g−1 after 100 cycles at 0.1 C.

Simultaneous formation of nitrogen and sulfur-doped carbon nanotubes-mesoporous carbon and its electrocatalytic activity for oxygen reduction reaction

Nitrogen and sulfur dual doped-carbon nanotubes-mesoporous carbon (D-CNTs-MPC) composite is prepared simultaneously and is used in alkaline media as an electrocatalyst for oxygen reduction reaction (ORR). D-CNTs-MPC is synthesized by casting method using nano-CaCO3 as a template, and binuclear cobalt phthalocyanine hexasulfonate as a carbon, nitrogen and sulfur precursor as well as the catalyst for growth of CNTs. D-CNTs-MPC possesses short CNTs adhering to loosely packed carbon with mesopores. Moreover, nitrogen and sulfur are doped into the carbon framework without addition of other heteroatom-containing precursor. The electrochemical behavior shows that D-CNTs-MPC is an active, methanol-tolerant and stable electrocatalyst for ORR.

Improved Electrochemical Performances of Mesoporous Carbon-Sulfur Composites with Lithium Polysulfides Added Electrolyte

Lithium-Sulfur batteries have been intensively studied as a power source of the electric transportation and energy storage system for renewable energy due to their high theoretical capacity of 1675 mAh g-1 and relatively inexpensive raw materials than commercial lithium ion batteries.1 To introduce lithium sulfur batteries to commercial market, there are some drawbacks to be solved. The major problems of lithium sulfur batteries are low active material utilization, severe capacity decay and low coulombic efficiency. Several successive approaches have been adopted to develop practical lithium sulfur batteries either by selecting electrolyte more adequate than conventional carbonate solutions, such as 1,3 dioxolane (DOL), dimethyl ether (DME), and tetraethylene glycol dimethyl ether (TEGDME), or by the addition of soluble polysulfides.2–7 On sulfur cathode side, modification of morphology of active materials have been studied by mixing conductive carbons with sulfur particles, encapsulating sulfur into porous carbon matrix and conductive polymer coating on the surface of active materials. These approaches have led to the development of lithium–sulfur battery with improved cycle life and rate capability. However these progresses of lithium sulfur batteries have not satisfied requirements of commercialization. These lithium sulfur batteries are still suffered from the problem of the solubility of the polysulfides. Recentely, Tarascon and co-workers8 and Zhang and Read reported noticeable achievement by addition soluble lithium polysulfide into the electrolyte.7 In this work, we studied the electrochemical and physical effect of the Li2S8 polysulfide additive diminishing the dissolution of the sulfur cathode and small participation in redox reaction as a dissolved cathode.7, 8 We demonstrated intentionally dissolved Li2S8 polysulfides in the electrolyte suppress polysulfide dissolution from the sulfur cathode by controlling its concentration according to Le Chatelier’s principle. Li2S8 polysulfide additives was deposited on the cathode as sulfur on charging to compensate eventual capacity losses that result from partial cathode dissolution during discharge.2– 8 Fig. 1 shows the cycling performance and efficiency of a mesoporous carbon-sulfur electrode in a lithium cell using the TEGDME–LiCF3SO3 electrolyte with addition of 5 wt% Li2S8. The cell cycled at 20 ℃ shows a small initial capacity decay, that is, from 1650 to 1500 mAh g-1s, followed by a stable trend with an acceptable, minor capacity decay, that is, from 1500 to about 1200 mAh g-1 Sover 50 cycles, with a coulombic efficiency approaching 95%.

Infiltrating sulfur into a highly porous carbon sphere as cathode material for lithium–sulfur batteries

Sulfur composite material with a highly porous carbon sphere as the conducting container was prepared. The highly porous carbon sphere was easily synthesized with resorcinol–formaldehyde precursor as the carbon source. The morphology of the carbon was observed with field emission scanning electron microscope and transmission electron microscope, which showed a well-defined spherical shape. Brunauer–Emmett–Teller analysis indicated that it possesses a high specific surface area of 1563m2g−1 and a total pore volume of 2.66cm3g−1 with a bimodal pore size distribution, which allow high sulfur loading and easy transportation of lithium ions. Sulfur carbon composites with varied sulfur contents were prepared by melt–diffusion method and lithium sulfur cells with the sulfur composites showed improved cyclablity and long-term cycle life.