Activated carbon aerogels with high bimodal porosity for lithium/sulfur batteries

Activated carbon aerogels (ACAs) with high bimodal porosity were obtained for lithium/sulfur batteries by potassium hydroxide (KOH) activation. Then sulfur–activated carbon aerogels (S–ACAs) composites were synthesized by chemical deposition strategy. The S–ACAs composites were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy, and N₂ adsorption/desorption measurements. It is found that the activated carbon aerogels treated by KOH activation presents a porous structure, and sulfur is embedded into the pores of the ACAs network-like matrix after a chemical deposition process. The Li/S–ACAs (with 69.1 wt% active material) composite cathode exhibits discharge capacities of 1,493 mAh g^?1 in the first cycle and 528 mAh g^?1 after 100 cycles at a higher rate of C/5 (335 mA g^?1). The S–ACAs composite cathode exhibits better electrochemical reversibility, higher active material utilization, and less severe polysulfide shuttle than S–CAs composite cathode because of high bimodal porosity structure of the ACAs matrix.     

Hollow Carbon Nanofibers Filled with MnO2 Nanosheets as Efficient Sulfur Hosts for Lithium–Sulfur Batteries

Lithium–sulfur batteries have been investigated as promising electrochemical‐energy storage systems owing to their high theoretical energy density. Sulfur‐based cathodes must not only be highly conductive to enhance the utilization of sulfur, but also effectively confine polysulfides to mitigate their dissolution. A new physical and chemical entrapment strategy is based on a highly efficient sulfur host, namely hollow carbon nanofibers (HCFs) filled with MnO₂ nanosheets. Benefiting from both the HCFs and birnessite‐type MnO₂ nanosheets, the MnO₂@HCF hybrid host not only facilitates electron and ion transfer during the redox reactions, but also efficiently prevents polysulfide dissolution. With a high sulfur content of 71 wt % in the composite and an areal sulfur mass loading of 3.5 mg cm^?2 in the electrode, the MnO₂@HCF/S electrode delivered a specific capacity of 1161 mAh g^?1 (4.1 mAh cm^?2) at 0.05 C and maintained a stable cycling performance at 0.5 C over 300 cycles.     

A Four‐Electron Sulfur Electrode Hosting a Cu2+/Cu+ Redox Charge Carrier

The elemental sulfur electrode with Cu²⁺ as the charge carrier gives a four‐electron sulfur electrode reaction through the sequential conversion of S?CuS?Cu₂S. The Cu‐S redox‐ion electrode delivers a high specific capacity of 3044 mAh g^?1 based on the sulfur mass or 609 mAh g^?1 based on the mass of Cu₂S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu‐S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g^?1. The remarkable utility of this Cu‐S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion‐exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half‐cell specific energy of 547 Wh kg^?1 based on the mass of the Cu₂S/carbon composite cathode, and stable cycling over 110 cycles.     

Synthesis and electrochemical performance of sulfur–carbon composite cathode for lithium–sulfur batteries

In this paper, porous carbon was synthesized by an activation method, with phenolic resin as carbon source and nanometer calcium carbonate as activating agent. Sulfur–porous carbon composite material was prepared by thermally treating a mixture of sublimed sulfur and porous carbon. Morphology and electrochemical performance of the carbon and sulfur–carbon composite cathode were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectra (EIS), and galvanostatic charge–discharge test. The composite containing 39 wt.% sulfur obtained an initial discharge capacity of about 1,130 mA?h g^?1 under the current density of 80 mA?g^?1 and presented a long electrochemical stability up to 100 cycles.     

Self-assembled reduced graphene oxide/sulfur composite encapsulated by polyaniline for enhanced electrochemistry performance

Reduced graphene oxide/sulfur/polyaniline (referred to RGO/S/PANI) composite was self-assembled through in situ synthesis and used to investigate the electrochemical properties of lithium/sulfur cells. The RGO/S/PANI composite possessed 809.3/801.9 mAh g^?1 of initial charge/discharge capacities, higher than 588.3/588.2 mAh g^?1 for reduced graphene oxide/sulfur (referred to RGO/S) and 681.4/669.9 mAh g^?1 for sulfur/polyaniline (referred to S/PANI) at similar conditions. The RGO/S/PANI composite obtained 400 mAh g^?1 at 2 C and good reversible capacities of 605.5 and 600.8 mAh g^?1 at 100th charge/discharge cycle at 0.1 C, in comparison with low electrochemical performance of RGO/S and S/PANI. The improved properties could be attributed to the collaboration of RGO and PANI. Co-generation of RGO and sulfur acted as seeds for their depositions, stimulated their uniform distributions, and restricted the agglomeration of sulfur particles in situ synthesis. Polyaniline coated RGO/S and stabilized the nanostructure of RGO/S/PANI in repeated charge/discharge cycles. In addition, RGO and PANI provided many electron channels to enhance sulfur conductivity and sufficient void space for sulfur swelling during charge/discharge cycles.     

Hybrids of MnO<Subscript>2</Subscript> nanoparticles anchored on graphene sheets as efficient sulfur hosts for high-performance lithium sulfur batteries

Lithium–sulfur (Li–S) battery is considered as a promising option for electrochemical energy storage applications because of its low-cost and high theoretical capacity. However, the practical application of Li–S battery is still hindered due to the poor electrical conductivity of S cathode and the high dissolution/shuttling of polysulfides in electrolyte. Herein, we report a novel physical and chemical entrapment strategy to address these two problems by designing a sulfur–MnO₂@graphene (S–MnO₂@GN) ternary hybrid material structure. The MnO₂ particles with size of ~ 10 nm are anchored tightly on the wrinkled and twisted GN sheets to form a highly efficient sulfur host. Benefiting from the synergistic effects of GN and MnO₂ in both improving the electronic conductivity and hindering polysulfides by physical and chemical adsorptions, this unique S–MnO₂@GN composite exhibits excellent electrochemical performances. Reversible specific capacities of 1416, 1114, and 421 mA h g^?1 are achieved at rates of 0.1, 0.2, and 3.2 C, respectively. After a 100 cycle stability test, S–MnO₂@GN composite cathode could still maintain a reversible capacity of 825 mA h g^?1.     

Sulfur‐Impregnated Core–Shell Hierarchical Porous Carbon for Lithium–Sulfur Batteries

Core–shell hierarchical porous carbon spheres (HPCs) were synthesized by a facile hydrothermal method and used as host to incorporate sulfur. The microstructure, morphology, and specific surface areas of the resultant samples have been systematically characterized. The results indicate that most of sulfur is well dispersed over the core area of HPCs after the impregnation of sulfur. Meanwhile, the shell of HPCs with void pores is serving as a retard against the dissolution of lithium polysulfides. This structure can enhance the transport of electron and lithium ions as well as alleviate the stress caused by volume change during the charge–discharge process. The as‐prepared HPC‐sulfur (HPC‐S) composite with 65.3 wt % sulfur delivers a high specific capacity of 1397.9 mA h g^?1 at a current density of 335 mA g^?1 (0.2 C) as a cathode material for lithium–sulfur (Li‐S) batteries, and the discharge capacity of the electrode could still reach 753.2 mA h g^?1 at 6700 mA g^?1 (4 C). Moreover, the composite electrode exhibited an excellent cycling capacity of 830.5 mA h g^?1 after 200 cycles.     

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.     

Hierarchical SnO2 with double carbon coating composites as anode materials for lithium ion batteries

Hierarchical SnO₂ with double carbon coating (polypyrrole-derived carbon and reduced graphene oxide in order) composites have been successfully synthesized as anode materials for lithium ion batteries. The composites were characterized and examined by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, cyclic voltammetry, and galvanostatic discharge/charge tests. Such a novel nanostructure can not only provide a high conductivity but also prevent aggregation of SnO₂ nanoparticles, leading to the improvement of the cycling performance. Comparing with pure hierarchical SnO₂ and polypyrrole-derived carbon-coated hierarchical SnO₂, hierarchical SnO₂ with double carbon coating composite exhibits higher lithium storage capacities and better cycling performance, 554.8 mAh g^?1 after 50 cycles at a current density of 250 mA g^?1. In addition, the rate performance of hierarchical SnO₂ with double carbon coating composite is also very well. For all the improved performances, this double carbon coating architecture may provide some references for other electrode materials of lithium ion batteries.     

Dual‐Confined Sulfur Nanoparticles Encapsulated in Hollow TiO2 Spheres Wrapped with Graphene for Lithium–Sulfur Batteries

Lithium–sulfur (Li?S) batteries are attractive owing to their higher energy density and lower cost compared with the universally used lithium‐ion batteries (LIBs), but there are some problems that stop their practical use, such as low utilization and rapid capacity‐fading of the sulfur cathode, which is mainly caused by the shuttle effect, and the uncontrollable deposition of lithium sulfide species. Herein, we report the design and fabrication of dual‐confined sulfur nanoparticles that were encapsulated inside hollow TiO₂ spheres; the encapsulated nanoparticles were prepared by a facile hydrolysis process combined with acid etching, followed by “wrapping” with graphene (G?TiO₂@S). In this unique composite architecture, the hollow TiO₂ spheres acted as effective sulfur carriers by confining the polysulfides and buffering volume changes during the charge‐discharge processes by means of physical force from the hollow spheres and chemical binding between TiO₂ and the polysulfides. Moreover, the graphene‐wrapped skin provided an effective 3D conductive network to improve the electronic conductivity of the sulfur cathode and, at the same time, to further suppress the dissolution of the polysulfides. As results, the G?TiO₂@S hybrids exhibited a high and stable discharge capacity of up to 853.4 mA h g^?1 over 200 cycles at 0.5 C (1 C=1675 mA g^?1) and an excellent rate capability of 675 mA h g^?1 at a current rate of 2 C; thus, G?TiO₂@S holds great promise as a cathode material for Li?S batteries.     

Nafion/Titanium Dioxide‐Coated Lithium Anode for Stable Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries have shown great potential as high energy‐storage devices. However, the stability of the Li metal anode is still a major concern. This is due to the formation of lithium dendrites and severe side reactions with polysulfide intermediates. We herein develop an anode protection method by coating a Nafion/TiO₂ composite layer on the Li anode to solve these problems. In this architecture, Nafion suppresses the growth of Li dendrites, protects the Li anode, and prevents side reactions between polysulfides and the Li anode. Moreover, doped TiO₂ further improves the ionic conductivity and mechanical properties of the Nafion membrane. Li–S batteries with a Nafion/TiO₂‐coated Li anode exhibit better cycling stability (776 mA h g^?1 after 100 cycles at 0.2 C, 1 C=1672 mA g^?1) and higher rate performance (787 mA h g^?1 at 2 C) than those with a pristine Li anode. This work provides an alternative way to construct stable Li anodes for high‐performance Li–S batteries.     

A Facile Synthesis of High‐Surface‐Area Sulfur–Carbon Composites for Li/S Batteries

Small‐grained elemental sulfur is precipitated from sodium thiosulfate (Na₂S₂O₃) 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.     

Biomass‐Derived Porous Carbon with Micropores and Small Mesopores for High‐Performance Lithium–Sulfur Batteries

Biomass‐derived porous carbon BPC‐700, incorporating micropores and small mesopores, was prepared through pyrolysis of banana peel followed by activation with KOH. A high specific BET surface area (2741 m² g^?1), large specific pore volume (1.23 cm³ g^?1), and well‐controlled pore size distribution (0.6–5.0 nm) were obtained and up to 65 wt % sulfur content could be loaded into the pores of the BPC‐700 sample. When the resultant C/S composite, BPC‐700‐S65, was used as the cathode of a Li–S battery, a large initial discharge capacity (ca. 1200 mAh g^?1) was obtained, indicating a good sulfur utilization rate. An excellent discharge capacity (590 mAh g^?1) was also achieved for BPC‐700‐S65 at the high current rate of 4 C (12.72 mA cm^?2), showing its extremely high rate capability. A reversible capacity of about 570 mAh g^?1 was achieved for BPC‐700‐S65 after 500 cycles at 1 C (3.18 mA cm^?2), indicating an outstanding cycling stability.     

Encapsulating Sulfur into Hierarchically Ordered Porous Carbon as a High‐Performance Cathode for Lithium–Sulfur Batteries

A three‐dimensional (3D) hierarchical carbon–sulfur nanocomposite that is useful as a high‐performance cathode for rechargeable lithium–sulfur batteries is reported. The 3D hierarchically ordered porous carbon (HOPC) with mesoporous walls and interconnected macropores was prepared by in situ self‐assembly of colloidal polymer and silica spheres with sucrose as the carbon source. The obtained porous carbon possesses a large specific surface area and pore volume with narrow mesopore size distribution, and acts as a host and conducting framework to contain highly dispersed elemental sulfur. Electrochemical tests reveal that the HOPC/S nanocomposite with well‐defined nanostructure delivers a high initial specific capacity up to 1193 mAh g^?1 and a stable capacity of 884 mAh g^?1 after 50 cycles at 0.1 C. In addition, the HOPC/S nanocomposite exhibits high reversible capacity at high rates. The excellent electrochemical performance is attributed exclusively to the beneficial integration of the mesopores for the electrochemical reaction and macropores for ion transport. The mesoporous walls of the HOPC act as solvent‐restricted reactors for the redox reaction of sulfur and aid in suppressing the diffusion of polysulfide species into the electrolyte. The “open” ordered interconnected macropores and windows facilitate transportation of electrolyte and solvated lithium ions during the charge/discharge process. These results show that nanostructured carbon with hierarchical pore distribution could be a promising scaffold for encapsulating sulfur to approach high specific capacity and energy density with long cycling performance.     

Electrochemical capacitance performance of polypyrrole–titania nanotube hybrid

In this study, the polypyrrole–titania nanotube hybrid has been synthesized for an electrochemical supercapacitor application. The highly ordered and independent titania nanotube array is fabricated by an electro-oxidation of titanium sheet through an electrochemical anodization process in an aqueous solution containing ammonium fluoride, phosphoric acid and ethylene glycol. The polypyrrole–titania nanotube hybrid is then prepared by electrodepositing the conducting polypyrrole into well-aligned titania nanotubes through a normal pulse voltammetry deposition process in an organic acetonitrile solution containing pyrrole monomer and lithium perchlorate. The morphology and microstructure of polypyrrole–titania nanotube hybrid are characterized by scanning electron microscopy, infrared spectroscopy and Raman spectroscopy. The electrochemical capacitance performance is determined by cyclic voltammetry and charge/discharge measurement. It indicates that the polypyrrole film can been uniformly deposited on both surfaces of titania nanotube walls, demonstrating a heterogeneous coaxial nanotube structure. The specific capacitance of polypyrrole–titania nanotube hybrid is determined to be 179?F?g^?1 based on the polypyrrole mass. The specific energy and specific power are 7.8?Wh?kg^?1 and 2.8?kW?kg^?1 at a constant charge/discharge current of 1.85?mA?cm^?2, respectively. The retained specific capacitance still keeps 85% of the initial capacity even after 200 cycle numbers. This result demonstrates the satisfying stability and durability of PPy–TiO₂ nanotube hybrid electrode in a cyclic charge/discharge process. Such a composite electrode material with highly ordered and coaxial nanotube hybrid structure can contribute high energy storage for supercapacitor applications.     

Synthesis and electrochemical performance of TiO2–sulfur composite cathode materials for lithium–sulfur batteries

Titania–sulfur (TiO₂–S) composite cathode materials were synthesized for lithium–sulfur batteries. The composites were characterized and examined by X-ray diffraction, nitrogen adsorption/desorption measurements, scanning electron microscopy, and electrochemical methods, such as cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. It is found that the mesoporous TiO₂ and sulfur particles are uniformly distributed in the composite after a melt-diffusion process. When evaluating the electrochemical properties of as-prepared TiO₂–S composite as cathode materials in lithium–sulfur batteries, it exhibits much improved cyclical stability and high rate performance. The results showed that an initial discharge specific capacity of 1,460 mAh/g at 0.2 C and capacity retention ratio of 46.6 % over 100 cycles of composite cathode, which are higher than that of pristine sulfur. The improvements of electrochemical performances were due to the good dispersion of sulfur in the pores of TiO₂ particles and the excellent adsorbing effect on polysulfides of TiO₂.     

Enhanced Performance of a Lithium–Sulfur Battery Using a Carbonate‐Based Electrolyte

The lithium–sulfur battery is regarded as one of the most promising candidates for lithium–metal batteries with high energy density. However, dendrite Li formation and low cycle efficiency of the Li anode as well as unstable sulfur based cathode still hinder its practical application. Herein a novel electrolyte (1 m LiODFB/EC‐DMC‐FEC) is designed not only to address the above problems of Li anode but also to match sulfur cathode perfectly, leading to extraordinary electrochemical performances. Using this electrolyte, lithium|lithium cells can cycle stably for above 2000 hours and the average Coulumbic efficiency reaches 98.8 %. Moreover, the Li–S battery delivers a reversible capacity of about 1400 mAh g^?1_sulfur with retention of 89 % for 1100 cycles at 1 C, and a capacity above 1100 mAh g^?1_sulfur at 10 C. The more advantages of this cell system are its outstanding cycle stability at 60 °C and no self‐discharge phenomena.     

Silver nanowires-copper sulfide core/shell nanostructure for electrochemical supercapacitors

This paper reports the synthesis of a new hybrid core (silver nanowires (AgNWs))/shell (copper sulfide (Cu₂S)) nanostructure using simple and inexpensive drop casting and successive ionic layer adsorption and reaction (SILAR) methods. The effects of the thickness of the Cu₂S shell on the Ag NW core on the electrochemical properties of the nanocomposite were studied by varying the number of SILAR cycles from one to four. The structure and microstructure of the prepared composite nanostructure electrode materials were characterized by X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. The AgNW/Cu₂S exhibited a high specific capacitance of 603 Fg^?1 (stainless steel substrate) and 707 Fg^?1 (Ni foam substrate) at a scan rate of 10 mVs^?1 with an energy density of 10.01 Whkg^?1 and 25.33 Whkg^?1 at an applied current of 0.2 mA. This study provides a simple and cost-effective strategy for the fabrication of nanostructured electrodes for supercapacitor applications.     

百香果皮基原位氮掺杂多孔碳/硫复合正极的制备及储锂性能

以生物质百香果皮为碳源,KHCO₃为活化剂,采用同步活化碳化方法制备原位氮掺杂的分级多孔碳材料,将其与单质硫复合制得多孔碳/硫正极材料。通过X射线衍射（XRD）、X射线光电子能谱（XPS）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）等表征技术对制备材料的物相组成、微观形貌、比表面积及孔结构进行研究分析。同时,利用紫外可见吸收光谱研究了多孔碳对多硫化物的吸附作用,用恒电流充放电测试了不同硫含量（60%~80%）的多孔碳/硫复合正极材料的电化学性能。结果表明,制得的多孔碳材料为无定型,具有1 093 m²·g^-1的高比表面积和0.63 cm³·g^-1的孔容;丰富的多孔结构和原位氮掺杂对多硫化物的物理化学协同吸附作用,有效降低了锂硫电池的“穿梭效应”,提高了电池的放电比容量和循环性能。硫含量为60%的多孔碳/硫复合材料,在0.05C和0.2C倍率下可释放1 057.7和763.4 mAh·g^-1的高初始放电比容量,在1C的高倍率下循环300次后的保持率为75%。     

Cyclability of sulfur/dehydrogenated polyacrylonitrile composite cathode in lithium–sulfur batteries

Sulfur/dehydrogenated polyacrylonitrile composite has been studied as cathode material for lithium–sulfur rechargeable batteries. Nonetheless, capacity fading has been a challenge for the commercialization of batteries. In this study, characterization techniques of scanning electron microscopy, energy dispersive X-ray spectroscopy, elemental analysis, cyclic voltammetry, and electrochemical impedance spectroscopy are used to investigate the change of cathode properties with charge–discharge cycles. Elemental analysis reveals that sulfur accumulates on the surface of the composite at the end of charge, and the sulfur formation decreases with cycle number. Scanning electron microscopy observations indicate that cathode surface morphology changes significantly after several cycles. By modeling the electrochemical impedance spectra of the cell in different discharge states, we suggest that capacity fading arises mainly from the formation and accumulation of irreversible Li₂S (and Li₂S₂) on the cathode surface.