Cathode Material For Lithium-sulfur Batteries Research Articles

Graphene/Sulfur Nanocomposite for High Performance Lithium-Sulfur Batteries

The graphene/sulfur nanocomposite has been synthesized by heating a mixture of graphene sheets and elemental sulfur. The morphology, structure and electrochemical performance of graphene/sulfur nanocomposite as cathode material for lithium-sulfur batteries were systematically investigated by field-emission scanning electron microscope, X-ray diffraction and a variety of electrochemical testing techniques. The graphene/sulfur nanocomposite cathodes display a high reversible capacity of 800-1200 mAh g-1, and stable cycling for more than 100 deep cycles at 0.1 C. The graphene sheets have good conductivity and an extremely high surface area, and provide a robust electron transport network. The graphene network also accommodates the volume change of the electrode during the Li-S electrochemical reaction.

Sulfur-infiltrated graphene-based layered porous carbon cathodes for high-performance lithium-sulfur batteries.

Because of advantages such as excellent electronic conductivity, high theoretical specific surface area, and good mechanical flexibility, graphene is receiving increasing attention as an additive to improve the conductivity of sulfur cathodes in lithium-sulfur (Li-S) batteries. However, graphene is not an effective substrate material to confine the polysulfides in cathodes and stable the cycling. Here, we designed and synthesized a graphene-based layered porous carbon material for the impregnation of sulfur as cathode for Li-S battery. In this composite, a thin layer of porous carbon uniformly covers both surfaces of the graphene and sulfur is highly dispersed in its pores. The high specific surface area and pore volume of the porous carbon layers not only can achieve a high sulfur loading in highly dispersed amorphous state, but also can act as polysulfide reservoirs to alleviate the shuttle effect. When used as the cathode material in Li-S batteries, with the help of the thin porous carbon layers, the as-prepared materials demonstrate a better electrochemical performance and cycle stability compared with those of graphene/sulfur composites.

Dual Protection of Sulfur by Carbon Nanospheres and Graphene Sheets for Lithium–Sulfur Batteries

Well-confined elemental sulfur was implanted into a stacked block of carbon nanospheres and graphene sheets through a simple solution process to create a new type of composite cathode material for lithium-sulfur batteries. Transmission electron microscopy and elemental mapping analysis confirm that the as-prepared composite material consists of graphene-wrapped carbon nanospheres with sulfur uniformly distributed in between, where the carbon nanospheres act as the sulfur carriers. With this structural design, the graphene contributes to direct coverage of sulfur to inhibit the mobility of polysulfides, whereas the carbon nanospheres undertake the role of carrying the sulfur into the carbon network. This composite achieves a high loading of sulfur (64.2 wt %) and gives a stable electrochemical performance with a maximum discharge capacity of 1394 mAh g(-1) at a current rate of 0.1 C as well as excellent rate capability at 1 C and 2 C. The improved electrochemical properties of this composite material are attributed to the dual functions of the carbon components, which effectively restrain the sulfur inside the carbon nano-network for use in lithium-sulfur rechargeable batteries.

One step synthesis of sulfur–carbon nanosheet hybrids via a solid solvothermal reaction for lithium sulfur batteries

Sulfur–carbon nanosheet hybrids (SCNHs) are simply synthesized by a single-step solid solvothermal reaction as the cathode material for lithium sulfur (LiS) batteries. Sulfur is homogeneously embedded into the carbon nanosheet by an in situ hybridization process, which enhances the electron and ion transportation due to the higher electronic conductivity and morphology, respectively. The solid solvothermal reaction provides insight into the unique chemical-state of the sulfur content in the sulfur–carbon hybrids due to the small difference in electronegativity between sulfur and carbon contents. This characteristic effect on the production of electrolyte-dissoluble lithium sulfide in the electrochemical reaction can enhance the cycle performance of the hybrid as a cathode material in LiS batteries. The SCNHs show improved electrochemical performance of 810 mA h g−1 at 100 mA g−1 and excellent Coulombic efficiency close to 100% at a high rate.

Preparation of three-dimensional hybrid nanostructure-encapsulated sulfur cathode for high-rate lithium sulfur batteries

A three-dimensional hybrid nanostructure incorporating the merits of the MWCNTs webs (MWCNTs-W) and the reduced graphene oxide (RGO) is designed to improve the high-rate cycling performance of the lithium-sulfur batteries. Owing to the excellent Li+ ion and electronic transport properties of the MWCNTs-W and the RGO, this unique structure can provide a three-dimensional conductive network and promote rapid charge-transfer reaction at the cathode. Furthermore, because of the rough surface and porous structure of the MWCNTs after activation with KOH, and the special adsorption ability of the RGO, the soluble polysulfide intermediates can be effectively trapped in the cathode. Therefore, when evaluating the electrochemical properties of the RGO@MWCNTs-W/S composite as the cathode material for lithium-sulfur batteries, it exhibits an excellent cyclical stability and high rate performance. In particular, even at an ultrahigh rate (5 C), a discharge capacity as high as 620 mAh g−1 is still retained for the RGO@MWCNTs-W/S composite with 68.93 wt% sulfur after 200 cycles, and the average coulombic efficiency is 96%.

Nitrogen-doped porous carbon nanofiber webs/sulfur composites as cathode materials for lithium-sulfur batteries

Nitrogen-doped porous carbon nanofiber webs-sulfur composites (N-CNFWs/S) were synthesized for the first time with sulfur (S) encapsulated into nitrogen-doped porous carbon nanofiber webs (N-CNFWs) via a modified oxidative template route, carbonization-activation and thermal treatment. The composites were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), X-ray powder diffraction (XRD), and thermogravimetry (TG) measurements. The results show that sulfur is well dispersed and immobilized homogeneously in the micropores of nitrogen-doped porous carbon nanofiber webs (N-CNFWs) with high electrical conductivity, surface area and large pore volume. The electrochemical tests show that the N-CNFWs/S composites with 60wt. % of S have a high initial discharge capacity of 1564mA h g−1, a good cycling stability at the current density of 175mAg−1, and excellent rate capability (reversible discharging capacity of above 400mA h g−1 at 1600mAg−1).

Tailoring Porosity in Carbon Nanospheres for Lithium–Sulfur Battery Cathodes

Porous hollow carbon spheres with different tailored pore structures have been designed as conducting frameworks for lithium-sulfur battery cathode materials that exhibit stable cycling capacity. By deliberately creating shell porosity and utilizing the interior void volume of the carbon spheres, sufficient space for sulfur storage as well as electrolyte pathways is guaranteed. The effect of different approaches to develop shell porosity is examined and compared in this study. The most highly optimized sulfur-porous carbon nanosphere composite, created using pore-formers to tailor shell porosity, exhibits excellent cycling performance and rate capability. Sulfur is primarily confined in 4-5 nm mesopores in the carbon shell and inner lining of the shells, which is beneficial for enhancing charge transfer and accommodating volume expansion of sulfur during redox cycling. Little capacity degradation (∼0.1% /cycle) is observed over 100 cycles for the optimized material.

Preparation of Mesoporous Carbon/Sulfur Composite Loaded with ZnS and Its Property for Lithium-sulfur Batteries

With the aim to obtain high performance cathode materials for lithium-sulfur batteries, composite of mesoporous carbon (MC) loaded with ZnS (ZnS/MC) was prepared through ultrasonic dispersion and heat treatment. The as-prepared composite was blended with elemental sulfur by heating, then cathode materials (ZnS/MC/S) was obtained. The ZnS/MC composite was characterized by XRD, SEM, EDS and N2 adsorption-desorption isotherms. The results showed that ZnS particle was uniformly loaded to the surface of MC through ultrasonic dispersion when content of ZnS was less than 17wt%. ZnS nanoparticles aggregated to form sphalerite ZnS by heat treatment or increasing content of ZnS. Cyclic voltammetry test showed that ZnS accelerated the oxidation process of polysulfides. Charge/ discharge tests indicated that the initial discharge specific capacity was 1354.6 mAh/g, while the coulombic efficiency of the first charge and discharge was 98.7%. The capacity still remained 650 mAh/g after 50 cycles.

Porous Nitrogen‐Doped Carbon Nanotubes Derived from Tubular Polypyrrole for Energy‐Storage Applications

Porous nitrogen-doped carbon nanotubes (PNCNTs) with a high specific surface area (1765 m(2) g(-1)) and a large pore volume (1.28 cm(3) g(-1)) have been synthesized from a tubular polypyrrole (T-PPY). The inner diameter and wall thickness of the PNCNTs are about 55 nm and 22 nm, respectively. This material shows extremely promising properties for both supercapacitors and for encapsulating sulfur as a superior cathode material for high-performance lithium-sulfur (Li-S) batteries. At a current density of 0.5 A g(-1), PNCNT presents a high specific capacitance of 210 F g(-1), as well as excellent cycling stability at a current density of 2 A g(-1). When the S/PNCNT composite was tested as the cathode material for Li-S batteries, the initial discharge capacity was 1341 mA h g(-1) at a current rate of 1 C and, even after 50 cycles at the same rate, the high reversible capacity was retained at 933 mA h g(-1). The promising electrochemical energy-storage performance of the PNCNTs can be attributed to their excellent conductivity, large surface area, nitrogen doping, and unique pore-size distribution.

Facile preparation of hierarchically porous carbons from metal-organic gels and their application in energy storage

Porous carbon materials have numerous applications due to their thermal and chemical stability, high surface area and low densities. However, conventional preparing porous carbon through zeolite or silica templates casting has been criticized by the costly and/or toxic procedure. Creating three-dimensional (3D) carbon products is another challenge. Here, we report a facile way to prepare porous carbons from metal-organic gel (MOG) template, an extended metal-organic framework (MOF) structure. We surprisingly found that the carbon products inherit the highly porous nature of MOF and combine with gel's integrated character, which results in hierarchical porous architectures with ultrahigh surface areas and quite large pore volumes. They exhibit considerable hydrogen uptake and excellent electrochemical performance as cathode material for lithium-sulfur battery. This work provides a general method to fast and clean synthesis of porous carbon materials and opens new avenues for the application of metal-organic gel in energy storage.

Preparation and electrochemical performance of sulfur-alumina cathode material for lithium-sulfur batteries

Nano-sized sulfur particles exhibiting good adhesion with conducting acetylene black and alumina composite materials were synthesized by means of an evaporated solvent and a concentrated crystallization method for use as the cathodes of lithium-sulfur batteries. The composites were characterized and examined by X-ray diffraction, environmental scanning electron microscopy and electrochemical methods, such as cyclic voltammetry, electrical impedance spectroscopy and charge–discharge tests. Micron-sized flaky alumina was employed as an adsorbent for the cathode material. The initial discharge capacity of the cathode with the added alumina was 1171mAhg−1, and the remaining capacity was 585mAhg−1 after 50 cycles at 0.25mAcm−2. Compared with bare sulfur electrodes, the electrodes containing alumina showed an obviously superior cycle performance, confirming that alumina can contribute to reducing the dissolution of polysulfides into electrolytes during the sulfur charge–discharge process.

Sulfur@graphene oxide core–shell particles as a rechargeable lithium–sulfur battery cathode material with high cycling stability and capacity

Sulfur@GO core–shell composites were prepared by the self-assembly of sulfur particles stabilized by a cationic surfactant and anionic graphene oxide nanosheets through electrostatic interaction. Due to the effective entrapment of polysulfide intermediates by the GO shell, the composites exhibit high cycling stability with 81% capacity maintenance over 210 cycles as the cathode for Li–S batteries.

One-step synthesis of a sulfur-impregnated graphene cathode for lithium–sulfur batteries

A practical route is introduced for synthesizing a sulfur-impregnated graphene composite as a promising cathode material for lithium-sulfur batteries. Sulfur particles with a size of a few microns are successfully grown in the interior spaces between randomly dispersed graphene sheets through a heterogeneous crystal growth mechanism. The proposed route not only enables the control of the particle size of active sulfur but also affords quantitative yields of composite powder in large quantities. We investigate the potential use of the sulfur-impregnated graphene composite as a cathode material owing to its advantages of confining active sulfur, preventing the dissolution of soluble polysulfides, and providing sufficient electrical conduction. A high discharge capacity of 1237 mA h g(-1) during the first cycle and a good cyclic retention of 67% after 50 cycles are attained in a voltage range of 1.8-2.6 V vs. Li/Li(+). These results emphasize the importance of tailoring cathode materials for improving the electrochemical properties of lithium-sulfur batteries. Our results provide a basis for further investigations on advanced lithium batteries.

Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium–Sulfur Battery Cathode Material with High Capacity and Cycling Stability

We report the synthesis of a graphene-sulfur composite material by wrapping poly(ethylene glycol) (PEG) coated submicrometer sulfur particles with mildly oxidized graphene oxide sheets decorated by carbon black nanoparticles. The PEG and graphene coating layers are important to accommodating volume expansion of the coated sulfur particles during discharge, trapping soluble polysulfide intermediates, and rendering the sulfur particles electrically conducting. The resulting graphene-sulfur composite showed high and stable specific capacities up to ∼600 mAh/g over more than 100 cycles, representing a promising cathode material for rechargeable lithium batteries with high energy density.

Sandwich-type functionalized graphene sheet-sulfur nanocomposite for rechargeable lithium batteries

A functionalized graphene sheet-sulfur (FGSS) nanocomposite was synthesized as the cathode material for lithium-sulfur batteries. The structure has a layer of functionalized graphene sheets/stacks (FGS) and a layer of sulfur nanoparticles creating a three-dimensional sandwich-type architecture. This unique FGSS nanoscale layered composite has a high loading (70 wt%) of active material (S), a high tap density of ∼0.92 g cm(-3), and a reversible capacity of ∼505 mAh g(-1) (∼464 mAh cm(-3)) at a current density of 1680 mA g(-1) (1C). When coated with a thin layer of cation exchange Nafion film, the migration of dissolved polysulfide anions from the FGSS nanocomposite was effectively reduced, leading to a good cycling stability of 75% capacity retention over 100 cycles. This sandwich-structured composite conceptually provides a new strategy for designing electrodes in energy storage applications.