Preparation of yolk-shell sulfur/carbon nanocomposite via an organic solvent route for lithium–sulfur batteries

A yolk-shell sulfur/carbon (S/C) composite for the cathode of lithium–sulfur batteries was successfully prepared by an accessible method with tetrahydrofuran as solvent. The as-prepared composites are characterized by thermal gravimetric, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption and desorption. In this composite, sulfur particle is encapsulated in the carbon shell even entering into the micropores of carbon Bp2000. The electrochemical performance of the S/C composites is evaluated. The results indicate that the S/C composite with 50 wt% sulfur content shows good reversibility, excellent rate capability, and slow degradation. It delivers an initial capacity of 784.4 mAh g^?1 (based on sulfur weight) and preserves at 598.3 mAh g^?1 after 195 cycles at 1C. It achieves a high-capacity retention of 76.27 % from the 5th to 200th cycle, and as high as 91.19 % during the latter 150 cycles. The improvement is mainly attributed to the favorable structure of the S/C composite, in which the carbon cannot only facilitate transport of electrons and Li⁺ ions but also trap polysulfides and retard the shuttle effect during charge/discharge process.     

Synthesis of spherical porous carbon by spray pyrolysis and its application in Li-S batteries

A spherical porous carbon (SPC) with high specific surface area is prepared by spray pyrolysis at 800 °C followed by removing silica template. The prepared SPC is employed as a conductive matrix in the sulfur cathode (S-SPC) for lithium–sulfur secondary batteries. The BET surface area of the prepared SPC sample is as high as 1,133 m² g^?1 and the total pore volume is 2.75 cm³ g^?1. The electrochemical evaluations including charge–discharge tests, cyclic voltammograms (CV), and electrochemical impedance spectrum suggest that the prepared S-SPC composite presents superior electrochemical stability when compared to the S-SP cathode. The as-prepared S-SPC composite shows improved cycle performance. The reversible discharge capacity is about 637 mAh g^?1 after 50 cycles, which is much better than that of the as-prepared sulfur–Super P carbon black composite. It may be attributed to the high porosity and excellent conductive structure of the SPC.     

A comparison of sulfur loading method on the electrochemical performance of porous carbon/sulfur cathode material for lithium–sulfur battery

To get a high sulfur loaded porous carbon/sulfur cathode material with an excellent performance, we investigated four different sulfur loading treatments. The samples were analyzed by the Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD) patterns, thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM). We proved that it is more effective to introduce the sulfur into the pores of porous carbon at 300 °C than at 155 °C. Especially, the porous carbon/sulfur composite heated in a sealed reactor at 300 °C for 8 h presents a fine sulfur load with sulfur content of 78 wt.% and exhibits an excellent electrochemical performance. The discharge capacity is 760, 727, 744, 713, and 575 mAh g^?1 of sulfur at a current density of 80, 160, 320, 800, and 1,600 mA g^?1 based on the sulfur/carbon composite, respectively. What is more, there is almost no decay at the current density of 800 mA g^?1 for 50 cycles and coulombic efficiency remains over 95 %.     

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.     

Mesoporous carbon/sulfur composite with N-doping and tunable pore size for high-performance Li-S batteries

Mesoporous carbons (MCs) were used as the matrixes to load sulfur for lithium sulfur (Li-S) batteries, and pore sizes were tuned by heat treatment at different high temperatures. The cathode material shows the highest discharge capacity of 1158.2 mAh g^?1 at the pore size of 4.1 nm among as-prepared nitrogen-free materials with different sizes. Meanwhile, the nitrogen doping of mesoporous carbon helps to inhibit the diffusion of polysulfide species via an enhanced surface adsorption. The carbon/sulfur containing N (4.56%) shows a high initial discharge capacity of 1315.8 mAh g^?1 and retains about 939 mAh g^?1 after 100 cycles at 0.2 C. The improved electrochemical performance is ascribed to the proper pore size, surface chemical property, and conductivity of the N-doped carbon material.     

Mesoporous TiO<Subscript>2</Subscript> nanosheet with a large amount of exposed {001} facets as sulfur host for high-performance lithium–sulfur batteries

Li–S battery is an attractive electrochemical energy storage system because of its high energy density. However, its commercialization has been greatly affected by the poor cycle life and low rate performance, which is attributed to the dissolution of polysulfides and their shuttle effects. In this study, titanium dioxide particles with a large amount of exposed {001} facets (TDPEF) were prepared by alcohol-thermal method. The as-prepared TDPEF achieved a relatively high specific surface area of 92 m² g^?1 and a pore volume of 0.27 cm g^?1. Sulfur was mixed with the TDPEF to form TDPEF/S composite by a melt diffusion process. The TDPEF/S composite exhibits much excellent discharge capacity retention of 80 % after 100 cycles compared with pure sulfur at a high current rate of 0.5 C, and it still has a discharge capacity as high as 530 mAh g^?1 even at the current rate of 4 C.     

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.     

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.     

Nickel foam as interlayer to improve the performance of lithium–sulfur battery

In this report, a porous, electronically conductive nickel foam foil (NFF), which is rolled for smooth surface, is introduced as an interlayer placed between the sulfur electrode and the separator to suppress the loss of active material and self-discharge behavior in lithium–sulfur (Li–S) systems. The electrodes are characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge test. The cell with the rolled NFF interlayer shows superior performance in terms of capacity utilization, reversibility, and enhanced rate capability. It exhibits reversible discharge capacity of 604 mAh g^?1 after 80 cycles at 0.2 C, which is much higher than that of pristine sulfur without NFF (424 mAh g^?1). The improvement on electrochemical performance is attributed to the 3D architecture of nickel foam foil as lithium–sulfur batteries interlayer, which can provide a good conductive network with structural stability and the porous architecture accommodating the migrating polysulfide to reduce the shuttling phenomenon during the charge–discharge processes.     

Nanofibers of V<Subscript>2</Subscript>O<Subscript>5</Subscript>/C@MWCNTs as the cathode material for lithium-ion batteries

Vanadium pentoxide (V₂O₅) nanofibers (NFs) with a thin carbon layer of 3–5 nm, which wrapped on V₂O₅ nanoparticles, and integrated multiwalled carbon nanotubes (MWCNTs) have been fabricated via simple electrospinning followed by carbonization process and post-sintering treatment. The obtained composite displays a NF structure with V₂O₅ nanoparticles connected to each other, and good electrochemical performance: delivering initial capacity of 320 mAh g^?1 (between 2.0 and 4.0 V vs. Li/Li⁺), good cycling stability (223 mAh g^?1 after 50 cycles), and good rate performance (~?150 mAh g^?1 at 2 A g^?1). This can attribute to the carbon wrapped on the V₂O₅ nanoparticles which can not only enhance the electric conductivity to decrease the impendence of the cathode materials but also maintain the structural stability to protect the nanostructure from the corruption of electrolyte and the strain stress due to the Li-ion intercalation/deintercalation during the charge/discharge process. And, the added MWCNTs play the role of framework of the unique V₂O₅ coated by carbon layer and composited with MWCNT NFs (V₂O₅/C@MWCNT NFs) to ensure the material is more stable.     

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.     

NiO/CNTs derived from metal-organic frameworks as superior anode material for lithium-ion batteries

In this work, porous NiO microspheres interconnected by carbon nanotubes (NiO/CNTs) were successfully fabricated by the pyrolysis of nickel metal-organic framework precursors with CNTs and evaluated as anode materials for lithium-ion batteries (LIBs). The structures, morphologies, and electrochemical performances of the samples were characterized by X-ray diffraction, N₂ adsorption-desorption, field emission scanning electron microscopy, cyclic voltammetry, galvanostatic charge/discharge tests, and electrochemical impedance spectroscopy, respectively. The results show that the introduction of CNTs can improve the lithium-ion storage performance of NiO/CNT composites. Especially, NiO/CNTs-10 exhibits the highest reversible capacity of 812 mAh g^?1 at 100 mA g^?1 after 100 cycles. Even cycled at 2 A g^?1, it still maintains a stable capacity of 502 mAh g^?1 after 300 cycles. The excellent electrochemical performance of NiO/CNT composites should be attributed to the formation of 3D conductive network structure with porous NiO microspheres linked by CNTs, which benefits the electron transfer ability and the buffering of the volume expansion during the cycling process.     

Gels dried under supercritical and ambient conditions: a comparative study and their subsequent conversion to silica–carbon composite aerogels

In present work, we have prepared gels with various compositions of methyltrimethoxysilane—3-(2,3-epoxypropoxy) propyltrimethoxysilane (MTMS-GPTMS) using a two-step acid base sol–gel process. To make a comparative study between the two common drying routes, we prepared gels under supercritical and also under ambient conditions. The density of the supercritically dried hybrid aerogels lies between 0.18 and 0.31 gcm^?3, while the density of the ambient dried ones ranges between 0.35 and 0.42 gcm^?3. The surface area of MTMS-0.25 GPTMS aerogel dried under supercritical conditions, has been found to be 464 m² g^?1 with a pore volume and average pore diameter of 1.24 cm³ g^?1 and 11 nm respectively. The same composition dried under ambient conditions is found to have similar properties i.e. a BET surface area of 439 m² g^?1, pore volume of 1.22 cm³ g^?1 and average pore diameter of 11 nm. The aerogels were later pyrolyzed yielding silica/carbon composite aerogels. The pyrolized aerogels possessed a surface area as high as 207 m² g^?1 with a total pore volume of 0.98 cm³ g^?1. The pyrolysed aerogels were also calcined to yield carbon free materials.     

Amorphous iron phosphate/carbonized polyaniline nanorods composite as cathode material in sodium-ion batteries

As-prepared polyaniline (PANI) nanorods have been used to synthesize an iron phosphate/polyaniline (FePO₄/PANI) composition through the microemulsion technique. After sintering at 460 °C under a nitrogen protective atmosphere, the PANI carbonized, yielding the amorphous iron phosphate/carbonized polyaniline nanorods (FePO₄/CPNRs) composite, which acts as the cathode material in sodium-ion batteries (SIBs). The electrochemical performance of FePO₄/CPNRs composite shows an initial discharge specific capacity of 140.2 mAh g^?1, with the discharge specific capacity being maintained at 134.4 mAh g^?1 after the 120th cycle, up to 87.9 % of the theoretical capacity (154.1 mAh g^?1 for NaFePO₄), as well as an excellent rate capability in sodium-ion batteries. Compared with pure FePO₄, the electrochemical performance has been greatly improved. On the one hand, using the CPNRs as conductive medium significantly improves electronic transport. On the other hand, the FePO₄ sphere of nanoscale particles, which has a large specific surface area, can promote an active material/electrolyte interface reaction and improve the speed of sodiation and desodiation during the charge and discharge process. The amorphous FePO₄/CPNRs composite shows outstanding electrochemical performance as competitive cathode material in SIBs.     

Natural graphite enhanced the electrochemical performance of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> cathode material for lithium ion batteries

Natural graphite treated by mechanical activation can be directly applied to the preparation of Li₃V₂(PO₄)₃. The carbon-coated Li₃V₂(PO₄)₃ with monoclinic structure was successfully synthesized by using natural graphite as carbon source and reducing agent. The amount of activated graphite is optimized by X-ray diffraction, scanning electron microscope, transmission electron microscope, Raman spectrum, galvanostatic charge/discharge measurements, cyclic voltammetry, and electrochemical impedance spectroscopy tests. Our results show that Li₃V₂(PO₄)₃ (LVP)-10G exhibits the highest initial discharge capacity of 189 mAh g^?1 at 0.1 C and 162.9 mAh g^?1 at 1 C in the voltage range of 3.0–4.8 V. Therefore, natural graphite is a promising carbon source for LVP cathode material in lithium ion batteries.     

Porous carbon nanotubes improved sulfur composite cathode for lithium-sulfur battery

Porous multi-walled carbon nanotubes (PCNTs) with multiple mesopores structure are synthesized through activation of multi-walled carbon nanotubes (MWCNTs) by potassium hydroxide. The potassium hydroxide activation process results in a significantly enhanced specific surface area with numerous small pores. The as-obtained PCNTs are employed as the conductive matrix for sulfur in the sulfur cathode. Compared with the composite sulfur cathode based on the original MWCNTs, the sulfur-PCNTs cathode shows a significantly improved cycle performance and columbic efficiency. The reversible capacity is 530 mAh?g^?1 and columbic efficiency is 90 % after 100 cycles at a current density of 100 mA?g^?1. The improvement in the electrochemical performance for S-PCNT is mainly attributed to the enlarged surface area and the porous structure of the unique mesopores carbon nanotube host, which cannot only facilitate transport of electrons and Li⁺ ions, but also trap polysulfides, retard the shuttle effect during charge/discharge process.     

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.     

Metal-free boron-doped carbon microspheres as excellent cathode catalyst for rechargeable Li–O<Subscript>2</Subscript> battery

Rechargeable Li–O₂ batteries are attracting more and more interest due to their high energy density. Meanwhile, the replacement of high-cost and scarce precious-metal catalysts has attracted more and more attention. Currently, many academic researchers have paid attention to find highly efficient metal-free catalysts as air cathode material. Herein, the boron-doped carbon microspheres (B-CMs) were prepared through a novel and facile static calcination method and showed high electrocatalytic activity as a cathode material. The battery with a B-CM cathode delivered a high initial discharge capacity of 13,757.2 mAh g^?1 and outstanding coulombic efficiency of 90.1 % at 100 mA g^?1. In addition, stable cyclability (151 cycles with stable discharge voltage of ~2.60 V with a cutoff capacity of 1000 mAh g^?1 at 200 mA g^?1) has been exhibited. These performances are due to three main points: boron carbide compound changed the surface area of the CMs and formed the mesopore architectures as well as the large surface area of 683.738 m² g^?1; the reduce of boron atom can slow down the oxidation of the CMs during the cyclings; and finally, the electron-deficient boron atom introduction greatly facilitated Li⁺ diffusion and electrolyte immersion and enhanced the oxygen reduction and evolution reaction kinetics.     

Sulfur Encapsulated in a TiO2‐Anchored Hollow Carbon Nanofiber Hybrid Nanostructure for Lithium–Sulfur Batteries

A hollow carbon nanofiber hybrid nanostructure anchored with titanium dioxide (HCNF@TiO₂) was prepared as a matrix for effective trapping of sulfur and polysulfides as a cathode material for Li–S batteries. The synthesized composites were characterized and examined by X‐ray diffraction, nitrogen adsorption–desorption measurements, field‐emission scanning electron microscopy, scanning transmission electron microscopy, and electrochemical methods such as galvanostatic charge/discharge, rate performance, and electrochemical impedance spectroscopy tests. The obtained HCNF@TiO₂–S composite showed a clear core–shell structure with TiO₂ nanoparticles coating the surface of the HCNF and sulfur homogeneously distributed in the coating layer. The HCNF@TiO₂–S composite exhibited much better electrochemical performance than the HCNF–S composite, which delivered an initial discharge capacity of 1040 mA h g^?1 and maintained 650 mAh g^?1 after 200 cycles at a 0.5 C rate. The improvements of electrochemical performances might be attributed to the unique hybrid nanostructure of HCNF@TiO₂ and good dispersion of sulfur in the HCNF@TiO₂–S composite.     

Influence of the microstructure and its stability on the electrochemical properties of EMD produced from a range of precursors

The influence of the microstructure and the stable crystal structure on the electrochemical properties of the electrolytic manganese dioxide (EMD) produced from manganese cake (EMD_MC), low-grade manganese ore (EMD_LMO), and synthetic manganese sulfate solutions (EMD_SMS) is reported. X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry/differential thermal analysis, field emission scanning electron microscopy (FESEM), and chemical analyses were used to determine the structural and chemical characteristics of the EMD samples. The charge–discharge profile was studied in 9 M KOH using a galvanostatic charge–discharge unit. All the samples were found to contain predominantly γ-phase MnO₂, which is electrochemically active for energy storage applications. FESEM images show that preparation method significantly influences surface morphology, shape, and size of the EMD particles. In almost all cases, nanoparticles were obtained, with spindle-shaped nanoparticles for EMD_MC, platy nanoparticles in the case of EMD_LMO, and anisotropic growth of tetra-branched star-like nanoparticles of EMD_SMS. These nanoparticles arrange themselves in a near net-like fashion, resulting in porosity of the flakes of EMD during electrochemical deposition. Thermal studies showed loss of structural water and formation of lower manganese oxides. The EMD_MC showed superior discharge capacity of ~280 mAh g^?1 as compared to EMD_LMO (275 mAh g^?1) and EMD_SMS (245 mAh g^?1).