Ferrocene‐Promoted Long‐Cycle Lithium–Sulfur Batteries

Confining lithium polysulfide intermediates is one of the most effective ways to alleviate the capacity fade of sulfur‐cathode materials in lithium–sulfur (Li–S) batteries. To develop long‐cycle Li–S batteries, there is an urgent need for material structures with effective polysulfide binding capability and well‐defined surface sites; thereby improving cycling stability and allowing study of molecular‐level interactions. This challenge was addressed by introducing an organometallic molecular compound, ferrocene, as a new polysulfide‐confining agent. With ferrocene molecules covalently anchored on graphene oxide, sulfur electrode materials with capacity decay as low as 0.014 % per cycle were realized, among the best of cycling stabilities reported to date. With combined spectroscopic studies and theoretical calculations, it was determined that effective polysulfide binding originates from favorable cation–π interactions between Li⁺ of lithium polysulfides and the negatively charged cyclopentadienyl ligands of ferrocene.     

Developing A “Polysulfide‐Phobic” Strategy to Restrain Shuttle Effect in Lithium–Sulfur Batteries

Inspired by hydrophobic interface, a novel design of “polysulfide‐phobic” interface was proposed and developed to restrain shuttle effect in lithium–sulfur batteries. Two‐dimensional VOPO₄ sheets with adequate active sites were employed to immobilize the polysulfides through the formation of a V?S bond. Moreover, owing to the intrinsic Coulomb repulsion between polysulfide anions, the surface anchored with polysulfides can be further evolved into a “polysulfide‐phobic” interface, which was demonstrated by the advanced time/space‐resolved operando Raman evidences. In particular, by introducing the “polysulfide‐phobic” surface design into separator fabrication, the lithium–sulfur battery performed a superior long‐term cycling stability. This work expands a novel strategy to build a “polysulfide‐phobic” surface by “self‐defense” mechanism for suppressing polysulfides shuttle, which provides new insights and opportunities to develop advanced lithium–sulfur batteries.     

Pomegranate‐Structured Silica/Sulfur Composite Cathodes for High‐Performance Lithium–Sulfur Batteries

Porous materials have many structural advantages for energy storage and conversion devices such as rechargeable batteries, supercapacitors, and fuel cells. When applied as a host material in lithium‐sulfur batteries, porous silica materials with a pomegranate‐like architecture can not only act as a buffer matrix for accommodating a large volume change of sulfur, but also suppress the polysulfide shuttle effect. The porous silica/sulfur composite cathodes exhibit excellent electrochemical performances including a high specific capacity of 1450 mA h g^?1, a reversible capacity of 82.9 % after 100 cycles at a rate of C/2 (1 C=1672 mA g^?1) and an extended cyclability over 300 cycles at 1 C‐rate. Furthermore, the high polysulfide adsorption property of porous silica has been proven by ex‐situ analyses, showing a relationship between the surface area of silica and polysulfide adsorption ability. In particular, the modified porous silica/sulfur composite cathode, which is treated by a deep‐lithiation process in the first discharge step, exhibits a highly reversible capacity of 94.5 % at 1C‐rate after 300 cycles owing to a formation of lithiated‐silica frames and stable solid‐electrolyte‐interphase layers.     

Fast,Reversible Lithium Storage with a Sulfur/Long‐Chain‐Polysulfide Redox Couple

The cathodic reactions in Li–S batteries can be divided into two steps. Firstly, elemental sulfur is transformed into long‐chain polysulfides (S₈?Li₂S₄), which are highly soluble in the electrolyte. Next, long‐chain polysulfides undergo nucleation reaction and convert into solid‐state Li₂S₂ and Li₂S (Li₂S₄?Li₂S) by slow processes. As a result, the second‐step of the electrochemical reaction hinders the high‐rate application of Li–S batteries. In this report, the kinetics of the sulfur/long‐chain‐polysulfide redox couple (theoretical capacity=419 mA h g^?1) are experimentally demonstrated to be very fast in the Li–S system. A Li–S cell with a blended carbon interlayer retains excellent cycle stability and possesses a high percentage of active material utilization over 250 cycles at high C rates. The meso‐/micropores in the interlayer are responsible for accommodating the shuttling polysulfides and offering sufficient electrolyte accessibility. Therefore, utilizing the sulfur/long‐chain polysulfide redox couple with an efficient interlayer configuration in Li–S batteries may be a promising choice for high‐power applications.     

Polyolefin‐Based Janus Separator for Rechargeable Sodium Batteries

Rechargeable sodium batteries are a promising technology for low‐cost energy storage. However, the undesirable drawbacks originating from the use of glass fiber membrane separators have long been overlooked. A versatile grafting–filtering strategy was developed to controllably tune commercial polyolefin separators for sodium batteries. The as‐developed Janus separators contain a single–ion‐conducting polymer‐grafted side and a functional low‐dimensional material coated side. When employed in room‐temperature sodium–sulfur batteries, the poly(1‐[3‐(methacryloyloxy)propylsulfonyl]‐1‐(trifluoromethanesulfonyl)imide sodium)‐grafted side effectively enhances the electrolyte wettability, and inhibits polysulfide diffusion and sodium dendrite growth. Moreover, a titanium‐deficient nitrogen‐containing MXene‐coated side electrocatalytically improved the polysulfide conversion kinetics. The as‐developed batteries demonstrate high capacity and extended cycling life with lean electrolyte loading.     

Double‐Shelled Nanocages with Cobalt Hydroxide Inner Shell and Layered Double Hydroxides Outer Shell as High‐Efficiency Polysulfide Mediator for Lithium–Sulfur Batteries

Lithium–sulfur (Li‐S) batteries have been considered as a promising candidate for next‐generation electrochemical energy‐storage technologies because of their overwhelming advantages in energy density. Suppression of the polysulfide dissolution while maintaining a high sulfur utilization is the main challenge for Li–S batteries. Here, we have designed and synthesized double‐shelled nanocages with two shells of cobalt hydroxide and layered double hydroxides (CH@LDH) as a conceptually new sulfur host for Li–S batteries. Specifically, the hollow CH@LDH polyhedra with complex shell structures not only maximize the advantages of hollow nanostructures for encapsulating a high content of sulfur (75 wt %), but also provide sufficient self‐functionalized surfaces for chemically bonding with polysulfides to suppress their outward dissolution. When evaluated as cathode material for Li–S batteries, the CH@LDH/S composite shows a significantly improved electrochemical performance.     

A Selection Rule for Hydrofluoroether Electrolyte Cosolvent: Establishing a Linear Free‐Energy Relationship in Lithium–Sulfur Batteries

Hydrofluoroethers (HFEs) have been adopted widely as electrolyte cosolvents for battery systems because of their unique low solvating behavior. The electrolyte is currently utilized in lithium‐ion, lithium–sulfur, lithium–air, and sodium‐ion batteries. By evaluating the relative solvating power of different HFEs with distinct structural features, and considering the shuttle factor displayed by electrolytes that employ HFE cosolvents, we have established the quantitative structure–activity relationship between the organic structure and the electrochemical performance of the HFEs. Moreover, we have established the linear free‐energy relationship between the structural properties of the electrolyte cosolvents and the polysulfide shuttle effect in lithium–sulfur batteries. These findings provide valuable mechanistic insight into the polysulfide shuttle effect in lithium–sulfur batteries, and are instructive when it comes to selecting the most suitable HFE electrolyte cosolvent for different battery systems.     

Sulfur Cathodes Based on Conductive MXene Nanosheets for High‐Performance Lithium–Sulfur Batteries

Lithium–sulfur batteries are amongst the most promising candidates to satisfy emerging energy‐storage demands. Suppression of the polysulfide shuttle while maintaining high sulfur content is the main challenge that faces their practical development. Here, we report that 2D early‐transition‐metal carbide conductive MXene phases—reported to be impressive supercapacitor materials—also perform as excellent sulfur battery hosts owing to their inherently high underlying metallic conductivity and self‐functionalized surfaces. We show that 70 wt % S/Ti₂C composites exhibit stable long‐term cycling performance because of strong interaction of the polysulfide species with the surface Ti atoms, demonstrated by X‐ray photoelectron spectroscopy studies. The cathodes show excellent cycling performance with specific capacity close to 1200 mA h g^?1 at a five‐hour charge/discharge (C/5) current rate. Capacity retention of 80 % is achieved over 400 cycles at a two‐hour charge/discharge (C/2) current rate.     

An Aligned and Laminated Nanostructured Carbon Hybrid Cathode for High‐Performance Lithium–Sulfur Batteries

An aligned and laminated sulfur‐absorbed mesoporous carbon/carbon nanotube (CNT) hybrid cathode has been developed for lithium–sulfur batteries with high performance. The mesoporous carbon acts as sulfur host and suppresses the diffusion of polysulfide, while the CNT network anchors the sulfur‐absorbed mesoporous carbon particles, providing pathways for rapid electron transport, alleviating polysulfide migration and enabling a high flexibility. The resulting lithium–sulfur battery delivers a high capacity of 1226 mAh g⁻¹ and achieves a capacity retention of 75 % after 100 cycles at 0.1 C. Moreover, a high capacity of nearly 900 mAh g⁻¹ is obtained for 20 mg cm⁻², which is the highest sulfur load to the best of our knowledge. More importantly, the aligned and laminated hybrid cathode endows the battery with high flexibility and its electrochemical performances are well maintained under bending and after being folded for 500 times.     

X‐ray Absorption Near‐Edge Structure and Nuclear Magnetic Resonance Study of the Lithium–Sulfur Battery and its Components

Understanding the mechanism(s) of polysulfide formation and knowledge about the interactions of sulfur and polysulfides with a host matrix and electrolyte are essential for the development of long‐cycle‐life lithium–sulfur (Li–S) batteries. To achieve this goal, new analytical tools need to be developed. Herein, sulfur K‐edge X‐ray absorption near‐edge structure (XANES) and ^6,7Li magic‐angle spinning (MAS) NMR studies on a Li–S battery and its sulfur components are reported. The characterization of different stoichiometric mixtures of sulfur and lithium compounds (polysulfides), synthesized through a chemical route with all‐sulfur‐based components in the Li–S battery (sulfur and electrolyte), enables the understanding of changes in the batteries measured in postmortem mode and in operando mode. A detailed XANES analysis is performed on different battery components (cathode composite and separator). The relative amounts of each sulfur compound in the cathode and separator are determined precisely, according to the linear combination fit of the XANES spectra, by using reference compounds. Complementary information about the lithium species within the cathode are obtained by using ⁷Li MAS NMR spectroscopy. The setup for the in operando XANES measurements can be viewed as a valuable analytical tool that can aid the understanding of the sulfur environment in Li–S batteries.     

Aromatic amine‐terminated polysulfide oligomer: Synthesis and application in self‐healable polyurea

The dynamic chemistry of disulfide bonds has emerged as one of the most powerful tools used for the fabrication of organic compounds and self‐healing materials. In this article, a novel aromatic amine‐terminated polysulfide oligomer is first synthesized from thiol‐terminated polysulfide oligomer and bis(4‐aminophenyl) disulfide via disulfide metathesis mechanism. The resulting oligomer is confirmed by FTIR and ¹H NMR spectra and then successfully applied in constructing self‐healable polyurea material (A‐LP23‐I), which combines the advantages of higher strength of polyureas and excellent self‐healing ability of polysulfide‐based materials. After subjecting to a temperature of 75 °C for 48 h, both the tensile strength and ultimate elongation of A‐LP23‐I restore to more than 90% of the original values (3.32 MPa and 396%). This study demonstrates a novel strategy for synthesizing aromatic amine‐terminated oligomer. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1460–1466     

A Supramolecular Capsule for Reversible Polysulfide Storage/Delivery in Lithium‐Sulfur Batteries

Supramolecular materials, in which small organic molecules are assembled into regular structures by non‐covalent interactions, attract tremendous interests because of their highly tunable functional groups and porous structure. Supramolecular adsorbents are expected to fully expose their abundant adsorptive sites in a dynamic framework. In this contribution, we introduced cucurbit[6]uril as a supramolecular capsule for reversible storage/delivery of mobile polysulfides in lithium‐sulfur (Li‐S) batteries to control undesirable polysulfide shuttle. The Li‐S battery equipped with the supramolecular capsules retains a high Coulombic efficiency and shows a large increase in capacity from 300 to 900 mAh g⁻¹ at a sulfur loading of 4.2 mg cm⁻². The implementation of supramolecular capsules offers insights into intricate multi‐electron‐conversion reactions and manifests as an effective and efficient strategy to enhance Li‐S batteries and analogous applications that involve complex transport phenomena and intermediate manipulation.     

Three‐Dimensional Hierarchical Constructs of MOF‐on‐Reduced Graphene Oxide for Lithium–Sulfur Batteries

A three‐dimensional (3D) hierarchical MOF‐on‐reduced graphene oxide (MOF‐on‐rGO) compartment was successfully synthesized through an in situ reduced and combined process. The unique properties of the MOF‐on‐rGO compartment combining the polarity and porous features of MOFs with the high conductivity of rGO make it an ideal candidate as a sulfur host in lithium–sulfur (Li‐S) batteries. A high initial discharge capacity of 1250 mAh g^?1 at a current density of 0.1 C (1.0 C=1675 mAh g^?1) was reached using the MOF‐on‐rGO based electrode. At the rate of 1.0 C, a high specific capacity of 601 mAh g^?1 was still maintained after 400 discharge–charge cycles, which could be ascribed to the synergistic effect between MOFs and rGO. Both the hierarchical structures of rGO and the polar pore environment of MOF retard the diffusion and migration of soluble polysulfide, contributing to a stable cycling performance. Moreover, the spongy‐layered rGO can buffer the volume expansion and contraction changes, thus supplying stable structures for Li‐S batteries.     

Selective Pre-concentration and Solid Phase Extraction of Mercury（Ⅱ） from Natural Water by Silica Gel-loaded （E）-N-（1-Thien-2＇-ylethylidene）-1,2-phenylenediamine Phase

Silica gel-loaded （E）-N-（1-thien-2＇-ylethylidene）-1,2-phenylenediamine （TEPDA） phase was synthesized based on physical adsorption approaches. The stability of a chemically modified TEPDA especially in concentrated hydrochloric acid that was then used as a recycling and preconcentration reagent allowed the further uses of silica gel-loaded immobilized TEPDA phase. The application of this silica gel-loaded phase to sorption of a series of metal ions was performed by using different controlling factors such as the pH of the metal ion solution and the equilibration shaking time by the static technique. This difference was interpreted on the basis of selectivity incorporated in these sulfur containing silica gel-loaded TEPDA phases. Hg（Ⅱ） was found to exhibit the highest affinity towards extraction by these silica gel-loaded TEPDA phases. The pronounced selectivity was also confirmed by the determined distribution coefficients （Kd） of all the metal ions, showing the highest value reported for mercury（Ⅱ） extraction by the silica gel immobilized TEPDA phase. The potential applications of the silica gel immobilized TEPDA phase to selective extraction of mercury（Ⅱ） from aqueous solution were successfully accomplished and preconcentration of low concentration of Hg（Ⅱ） （30 pg·mL^-1） from natural tap water with a preconcentration factor of 200 for Hg（Ⅱ） off-line analysis was conducted by cold vapor atomic absorption analysis.     

A Strategy for Configuration of an Integrated Flexible Sulfur Cathode for High‐Performance Lithium–Sulfur Batteries

Lithium–sulfur batteries are regarded as promising candidates for energy storage devices owing to their high theoretical energy density. The practical application is hindered, however, by low sulfur utilization and unsatisfactory capacity retention. Herein, we present a strategy for configuration of the sulfur cathode, which is composed of an integrated carbon/sulfur/carbon sandwich structure on polypropylene separator that is produced using the simple doctor‐blade technique. The integrated electrode exhibits excellent flexibility and high mechanical strength. The upper and bottom carbon layers of the sandwich‐structured electrode not only work as double current collectors, which effectively improve the conductivity of the electrode, but also serve as good barriers to suppress the diffusion of the polysulfide and buffer the volume expansion of the active materials, leading to suppression of the shuttle effect and low self‐discharge behavior.     

Synthesis and characterization of saturated and unsaturated polysulfide polymers and their thermal degradation processes investigated by flash pyrolysis–gas chromatography/mass spectrometry

The synthesis of polysulfide polymers with unsaturated and saturated units in the backbone and their characterization by Fourier transform infrared, NMR, gel permeation chromatography, thermogravimetric analysis, and differential scanning calorimetry are reported. This is the first report on an analysis of the thermal degradation of an unsaturated polysulfide polymer [poly(2‐butene sulfide)] carried out by pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). A unique phenomenon of exothermic degradation has been detected by differential thermal analysis and has been attributed to the energetics of the unsaturated polysulfide linkage during degradation. The thermal degradation products studied by Py–GC/MS indicate that the formation of sulfur‐containing products is more favored than the formation of non‐sulfur‐containing products. Furthermore, a comparative study of the thermal degradation of unsaturated and saturated polysulfide polymers has been conducted with thermogravimetry and Py–GC/MS analyses. These analyses have shown that the mechanisms of degradation of these polymers are different, and the lower number of pyrolysis products indicates a selective cleavage of the polymer during degradation in the saturated polysulfide polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 638–649, 2005     

Beyond the Polysulfide Shuttle and Lithium Dendrite Formation: Addressing the Sluggish Sulfur Redox Kinetics for Practical High‐Energy Li‐S Batteries

Electrolyte modulation simultaneously suppresses polysulfide the shuttle effect and lithium dendrite formation of lithium–sulfur (Li‐S) batteries. However, the sluggish S redox kinetics, especially under high S loading and lean electrolyte operation, has been ignored, which dramatically limits the cycle life and energy density of practical Li‐S pouch cells. Herein, we demonstrate that a rational combination of selenium doping, core–shell hollow host structure, and fluorinated ether electrolytes enables ultrastable Li stripping/plating and essentially no polysulfide shuttle as well as fast redox kinetics. Thus, high areal capacity (>4 mAh cm^?2) with excellent cycle stability and Coulombic efficiency were both demonstrated in Li metal anode and thick S cathode (4.5 mg cm^?2) with a low electrolyte/sulfur ratio (10 μL mg^?1). This research further demonstrates a durable Li‐Se/S pouch cell with high specific capacity, validating the potential practical applications.     

Water‐Promoted Kinetic Separation of trans‐ and cis‐Limonene Oxides

The efficient hydrolytic kinetic separation of trans/cis‐(R)‐(+)‐limonene oxides was realized in a 1:1 mixed solvent of water and 1,4‐dioxane without additional catalyst. Optically pure trans‐(R)‐(+)‐limonene oxide was recovered in high yield (77%).     

Coordination complexes of 2‐thienyl‐ and 2‐furyl‐mercurials

The reactions of di(2‐thienyl)mercury, 2‐thienylmercury chloride and 2‐furylmercury chloride with a variety of nitrogen‐ and phosphorus‐containing ligands have been studied. The presence of the electron‐withdrawing heteroatoms results in these mercurials being stronger acceptors than the corresponding phenylmercury compounds. The complexes have been characterized by elemental analysis, melting points, infrared, and ¹⁹⁹Hg NMR spectroscopy. 2,9‐Dimethyl‐ and 3,4,7,8‐tetramethyl‐phenanthroline form 1:1 chelate complexes, as does 1,2‐bis(diphenylphosphino)ethane, whereas ethylenediamine and 2,2′‐bipyridyl do not form complexes. Though non‐chelating ligands such as 2,4′‐ and 4,4′‐bipyridyl do not form complexes, bis(diphenylphosphino)methane forms 1:2 complexes in which the ligand bridges two mercury atoms. Monodentate ligands, such as triphenylphosphine, cause disproportionation of the organomercury chloride. 2‐Thienylmercury chloride forms a 4:1 complex with 4,4′‐dipyridyl disulfide in which it is believed that a molecule of the organomercurial is coordinated to both of the nitrogen and both of the sulfur atoms. Copyright © 2004 John Wiley & Sons, Ltd.     

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.