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.     

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.     

Grafting polymeric sulfur onto carbon nanotubes as highly-active cathode for lithium–sulfur batteries

Lithium–sulfur(Li–S)batteries are being explored as promising advanced energy storage systems due to their ultra-high energy density.However,fast capacity fading and low coulombic efficiency,resulting from the dissolution of polysulfides,remain a serious challenge.Compared to weak physical adsorptions or barriers,chemical confinement based on strong chemical interaction is a more effective approach to address the shuttle issue.Herein,we devise a novel polymeric sulfur/carbon nanotube composite for Li–S battery,for which 2,5-dithiobiurea is chosen as the stabilizer of long-chain sulfur.This offers chemical bonds which bridge the polymeric sulfur and carbon nanotubes.The obtained composite can deliver an ultra-high reversible capacity of 1076.2 m Ah g~(-1)(based on the entire composite)at the rate of 0.1 C with an exceptional initial Coulombic efficiency of 96.2%,as well as remarkable cycle performance.This performance is mainly attributed to the reaction reversibility of the obtained polymeric sulfur-based composite during the discharge/charge process.This was confirmed by density functional theory calculations for the first time.     

Lithium–sulfur redox: challenges and opportunities

Mechanistic and kinetic insights into the lithium–sulfur (Li–S) redox processes are essential to fundamentally increase the utilization of active material and further realize the practical applications of Li–S batteries. In this article, recent advances of in situ/operando characterizations of Li–S reaction processes and mechanism are presented, revealing the multistep transformations of S species. Interfacial visualization, from the whole interface to nanometer scale, provides specific evidence of sulfur distribution, polysulfide diffusion, and lithium sulfide precipitation. Moreover, the development of efficient electrocatalysts to improve the reaction kinetics are additionally presented and discussed. Although the understanding of the mechanism of the Li–S redox processes has improved in recent times, additional efforts are required for the scale-up production and practical applications of Li–S batteries.     

Synthesis and application of single-atom catalysts in sulfur cathode for high-performance lithium–sulfur batteries

Lithium–sulfur(Li-S) batteries are regarded as one of the most promising energy storage devices because of their low cost, high energy density, and environmental friendliness. However, Li-S batteries suffer from sluggish reaction kinetics and serious “shuttle effect” of lithium polysulfides(LiPSs), which causes rapid decay of battery capacity and prevent their practical application. To address these problems, introducing single-atom catalysts(SACs) is an effective method to improve the electroch...     

Emerging energy chemistry in lithium–sulfur pouch cells

正On pursuing high-energy-density energy storage systems beyond the current lithium-ion battery technique, lithium–sulfur(Li–S) batteries have attracted worldwide attention due to their ultrahigh theoretical energy density up to 500 Wh kg~(-1)[1]. The unique Li–S chemistry based on the conversion reactions between solid sulfur, dissolved lithium polysulfides, and solid lithium sulfide affords thermodynamic advantages including high cathode specific capacity and low anode potential [2]. However,     

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 %.     

Facile preparation of visible-light-sensitive sulfur–nitrogen-codoped titanium dioxide

S–N-codoped TiO₂ powders have been synthesized through a facile one-step sol–gel method by using tetrabutyltitanate and thiourea as precursors. The S–N-codoped TiO₂ treated at 500 °C showed the highest photocatalytic activity for degrading methylene blue under visible light irradiation. XRD, XPS and UV–vis studies revealed that the high visible-light photocatalytic activity of the doped TiO₂ may originate from the synergetic effect of sulfur and nitrogen codoping into TiO₂.     

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.     

Iron–sulfur clusters and their electronic and magnetic properties

Iron–sulfur clusters of diverse nuclearities constitute the active sites of a large and prominent family of metalloproteins which play essential roles in all living organisms, such as in electron transfer chains, reduction catalysis, photosynthesis, the respiratory chain and nitrogen fixation. This review is devoted to the presentation of the current state of understanding of their electronic and magnetic properties, which is here derived from their Mössbauer, EPR and ENDOR spectroscopic properties. These techniques constitute fine tools for characterization and provide knowledge of the different oxidation states of these proteins, although our interest here will be mainly centered on the [4Fe–4S*]ⁿ⁺ clusters (with n=1–3). A qualitative physical model involving the competing magnetic interactions in these clusters is discussed. Moreover, this article contains new developments on two more specialized subjects:

• 1.some quantitative consequences of an already published theory of the g-tensors of [4Fe–4S*]ⁿ⁺ clusters (n=1,3) will be derived in Section 3;
• 2.a model permitting the rationalization, from very simple ingredients and formulae, of the redox potentials of a whole set of known synthetic redox clusters (with 1, 2, 3, 4 and 6 iron atoms) will be presented in the final Section 6.

     

Edge-functionalized acetylene black anchoring sulfur for high-performance Li–S batteries

To date, most of the research on electrodes for lithium sulfur batteries has been focused on the nanostructured sulfur cathodes and achieves significant success. However, from the viewpoint of manufacturers, the nanostructured sulfur cathodes are not so promising, because of the low volumetric energy density and high cost. In this work, we obtained the low-cost, scalable, eco-friendly mass production of edge-functionalized acetylene black-sulfur(FAB-S) composites by high-energy ball-milling technique for lithium sulfur batteries. The as-prepared FAB-S composite can deliver a high initial discharge capacity of 1304 mAh/g and still remain a reversible capacity of 814 mAh/g after 200 cycles at a charge-discharge rate of 0.2 C in the voltage range of 1.7–2.7 V. The observed excellent electrochemical properties demonstrate that the cathodes obtained by the facile high-energy ball-milling method as the cathode for rechargeable Li-S batteries are of great potential because it used the sole conductive additive acetylene black(AB).Such improved properties could be attributed to the partially exfoliation of AB, which not only keeps the AB's inherent advantage, but also increases the specific surface area and forms chemical bonds between carbon and sulfur, resulting in the accumulation of the polysulfides intermediate through both the physical and chemical routes.     

Hierarchically porous nitrogen-doped carbon as cathode for lithium–sulfur batteries

Porous nitrogen-doped carbon is an especially promising material energy storage due to its excellent conductivity, stable physicochemical properties, easy processability, controllable porosity and low price.Herein, we reported a novel well-designed hierarchically porous nitrogen-doped carbon(HPNC) via a combination of salt template(ZnCl_2) and hard template(SiO_2) as sulfur host for lithium–sulfur batteries. The low-melting ZnCl_2 is boiled off and leaves behind micropores and small size mesopores during pyrolysis process, while the silica spheres are removed by acid leaching to generate interconnected 3D network of macropores. The HPNC-S electrode exhibits an initial specific capacity of 1355 mAh g~(–1) at 0.1 C(1 C = 1675 m Ah g~(–1)), a high-rate capability of 623 m Ah g~(–1) at 2 C, and a small decay of 0.13% per cycle over 300 cycles at 0.2 C. This excellent rate capability and remarkable long-term cyclability of the HPNC-S electrode are attributed to its hierarchical porous structures for confining the soluble lithium polysulfide as well as the nitrogen doping for high absorbability of lithium polysulfide.     

Synthesis and structural characterization of iron–sulfur complexes with hydrophilic nitrogen–phosphorus ligands

Reaction of the parent complex (μ-PDT)Fe₂-(CO)₆ (A) (PDT = 1,3-SCH₂CH₂CH₂S^2?) with the bidentate N/P ligand [(Ph₂P)₂N(C₆H₄Cl-p)] in the presence of Me₃NO as decarbonylating agent produced an unexpected iron–sulfur complex [(μ-PDT)Fe₂(CO)₅{PPh₂(NHC₆H₄Cl-1,4)}] (1). Extending this chemistry further, two similar complexes [(μ-PDT)Fe₂(CO)₅{PPh₂(NHC₆H₄NO₂-1,4)}] (2) and [(μ-PDT)Fe₂(CO)₅{PPh₂(NHC₆H₄CO₂Et-1,4)}] (3) could be prepared from the simple substitution reactions of the precursor A with the monodentate N/P ligands Ph₂P(NHC₆H₄NO₂-1,4) and Ph₂P(NHC₆H₄CO₂Et-1,4), respectively. These new complexes, which can be considered as active site models of [FeFe] hydrogenases, have been characterized by elemental analysis, FTIR, and NMR (¹H, ¹³C, ³¹P) spectroscopies, as well as by X-ray crystallography for complex 1.     

Insight into demand-driven preparation of single-atomic mediators for lithium–sulfur batteries

Lithium-sulfur(Li-S) batteries have attracted considerable attention as one of the most appealing energy storage systems.Strenuous efforts have been devoted to tackling the tremendous challenges,mainly pertaining to the severe shuttle effect,sluggish redox kinetics and lithium dendritic growth.Single-atomic mediators as promising candidates exhibit impressive performance in addressing these intractable issues.Related research often utilizes a trial-and-error approach,proposing solutions to fabri...     

Computational identification of a global carbon–sulfur triply bonded isomer SCBO

Searching for stable sulfur–carbon triply bonded molecules has been of great interest from both the fundamental and applied viewpoints. The known polyatomic sulfur–carbon triply bonded molecules are usually not the global minima. Here, we report a potential energy surface investigation of a tetra-atomic molecule [S,C,B,O] in both doublet and quartet states. The B3LYP and M06-2X methodologies with 6-311+G(3df,2p) and aug-cc-pVTZ basis sets were applied for geometrical optimization and CCSD(T)/aug-cc-pVTZ for single-point energy calculations. The thermodynamically most stable isomer is the linear SCBO 01 (0.0 kcal/mol). Kinetically, SCBO 01 is separated from the other isomers and fragments by the rather high barriers of at least 44.7 kcal/mol. In particular, isomer SCBO 01 contains a typical carbon–sulfur triple bond based on the systematic analysis from the structure, vibrational frequency, molecular orbital, Wiberg bond index, and adiabatic bond dissociation energy. In addition, there exists a second low-lying isomer, i.e., linear SBCO 02 (7.3 kcal/mol) with S≡B triple bonding, whose kinetic stability is governed by its fragmentation to ²SB+¹CO (30.4 kcal/mol). The remaining isomers are either kinetically unstable with low conversion barriers or energetically very high lying. We propose that the simple two-body association between SC and BO, SB and CO pairs can preferentially lead to the formation and stabilization of SCBO 01 and SBCO 02, respectively. The isomer SCBO 01, which is the global structure and extraordinarily stable against both isomerization and fragmentation, strongly deserves future laboratory studies.     

Nitrogen-doped hierarchical porous carbon derived from metal–organic aerogel for high performance lithium–sulfur batteries

Nitrogen-doped three-dimensional(3 D) porous carbon materials have numerous applications due to their highly porous structures, abundant structural nitrogen heteroatom decoration and low densities. Herein,nitrogen doped hierarchical 3 D porous carbons(NHPC) were prepared via a novel metal–organic aerogel(MOA), using hexamethylenetetramine(HMT), 1,3,5-benzenetricarboxylic acid and copper(II) as starting materials. The morphology, porous structure of the building blocks in the NHPC can be tuned readily using different amount of HMT, which makes elongation of the pristine octahedron of HKUST-1 to give rise to different aspect ratio rod-like structures. The as-prepared NHPC with rod-like carbons exhibit high performance in lithium sulfur battery due to the rational ion transfer pathways, high N-doped doping and hierarchical porous structures. As a result, the initial specific capacity of 1341 m A h/g at rate of 0.5 C(1 C = 1675 m A h/g) and high-rate capability of 354 m A h/g at 5 C was achieved. The decay over 500 cycles is 0.08% per cycle at 1 C, highlighting the long-cycle Li–S batteries.     

Post lithium–sulfur battery era:challenges and opportunities towards practical application

Lithium–sulfur(Li–S) batteries have been regarded as a promising next-generation energy storage system owing to the high theoretical energy density and natural abundance of sulfur. Abundant fundamental researches have pushed the flourishing development on electrochemical behaviors in recent 20 years. It is time to evolve into post-Li–S battery era with the pursuit towards practical application. During the landmark leap, numerous new challenges appear under harsh conditions, such as high sulfur l...     

Nano-wire networks of sulfur–polypyrrole composite cathode materials for rechargeable lithium batteries

Polypyrrole (PPy) nanowire was synthesized through a surfactant mediated approach. The sulfur–polypyrrole (S–PPy) composite materials were prepared by heating the mixture of element sulfur and polypyrrole nanowire. The materials were characterized by FTIR, SEM. PPy with special morphology serves as conductive additive, distribution agent and absorbing agents, which effectively enhanced the electrochemical performance of sulfur. The initial discharge capacity of the active materials was 1222 mA h g⁻¹ the remaining capacity is 570 mA h g⁻¹ after 20th cycles.     

Edge sulfurized graphene nanoplatelets via vacuum mechano-chemical reaction for lithium–sulfur batteries

Lithium–sulfur batteries have great potential for high energy applications due to their high capacities,low cost and eco-friendliness. However, the particularly rapid capacity decay owing to the dissolution and diffusion of polysulfide intermediate into the electrolyte still hamper their practical applications.And the reported preparation procedures to sulfur based cathode materials are often complex, and hence are rather difficult to produce at large scale. Here, we report a simple mechano-chemical sulfurization methodology in vacuum environment applying ball-milling method combined both the chemical and physical interaction for the one-pot synthesis of edge-sulfurized grapheme nanoplatelets with 3D porous foam structure as cathode materials. The optimal sample of 70%S–Gn Ps-48 h(ball-milled 48 h) obtains 13.2 wt% sulfur that chemically bonded onto the edge of Gn Ps. And the assembled batteries exhibit high initial discharge capacities of 1089 mAh/g at 0.1 C and 950 mAh/g at 0.5 C, and retain a stable discharge capacity of 776 mAh/g after 250 cycles at 0.5 C with a high Coulombic efficiency of over 98%. The excellent performance is mainly attributed to the mechano-chemical interaction between sulfur and grapheme nanoplatelets. This definitely triggers the currently extensive research in lithium–sulfur battery area.     

Photoelectron spectroscopic study of sulfur—sulfur interactions in macrocyclic thioethers

He I photoelectron spectra of 1,4,7-trithiacyclononane, 1,4,7-trithiacyclodecane, 1,4,7,10-tetrathiacyclododecane, 1,4,8,11-tetrathiacyclotetradecane, 1,5,9,13-tetrathiacyclohexadecane and 1,5,9,13-tetrathiacyclohexadecane-3,11-diol were measured and used to obtain information on sulphur—sulphur lone pair interactions. Comparisons with spectra of oxygen macrocyclic analogs allowed several general trends in electronic structures of these ligands to be established.