Integrated Design of MnO2 @Carbon Hollow Nanoboxes to Synergistically Encapsulate Polysulfides for Empowering Lithium Sulfur Batteries

Donor-Acceptor Conjugated Microporous Polymer toward Enhanced Redox Kinetics in Lithium-Sulfur Batteries

Conjugated microporous polymers (CMPs) with porous structure and rich polar units are favorable for high-performance lithium-sulfur (Li-S) batteries. However, understanding the role of building blocks in polysulfide catalytic conversion is still limited. In this work, two triazine-based CMPs are constructed by electron-accepting triazine with electron-donating triphenylbenzene (CMP-B) or electron-accepting triphenyltriazine (CMP-T), which can grow on a conductive carbon nanotube (CNT) to serve as separator modifiers for Li-S batteries. CMP-B@CNT features faster ion transportation than the counterpart of CMP-T@CNT. More importantly, compared with acceptor-acceptor (A-A) CMP-T, donor-acceptor (D-A) CMP-B possesses a higher degree of conjugation and a narrower band gap, which are conducive to the electron transfer along the polymer skeleton, thus accelerating the sulfur redox kinetics. Consequently, the CMP-B@CNT functional separator endows Li-S cells with an outstanding initial capacity of 1371 mAh g^-1 at 0.1 C and favorable cycling stability with a capacity degradation rate of 0.048% per cycle at 1 C for 800 cycles. This work provides insight into the rational design of efficient catalysts for advanced Li-S batteries.

Crystal phase engineering of nanoflower-like hollow MoSe2boosting polysulfide conversion for lithium-sulfur batteries

The battery performance of sulfur cathode has obviously depended on the redox reaction kinetics of polysulfides upon cycling. Herein, an effective strategy was proposed to achieve the conversion from 2H (semiconductor phase) to 1T (metal phase) in hollow nano-flowered molybdenum selenide sphere (HFSMS) through crystal phase engineering. The HFSMS with different phase ratio was realized by regulating the proportion of reducing agents. Specifically, the 1T phase content can reach up to 60.8%, and then subsequently decreased to 59.1% with the further increase of the reducing agent. The as-prepared HFSMS with the 1T phase content of 60.8% showed a smallest Tafel slopes (49.99 and 79.65 mV/dec in reduction and oxidation process, respectively), fastest response time and highest response current (520 s, 0.459 mA in Li₂S deposition test), which further exhibited excellent catalytic activity and faster reaction kinetics. This result was verified by electrochemical performance, which manifested as stable cycle life with only 0.112% capacity decay per cycle. It was found that the hollow structure can ensures a rich sulfur storage space, and effectually buffer the volume changes of the active substance. More importantly, the improved performance is attributed to the introduction of the 1T phase, which significantly improves the catalytic activity of MoSe₂with promoting the polysulfide conversion.

Design and synthesis of novel pomegranate-like TiN@MXene microspheres as efficient sulfur hosts for advanced lithium sulfur batteries†

Lithium–sulfur (Li–S) batteries have the characteristics of low cost, environmental protection, and high theoretical energy density, and have broad application prospects in the new generation of electronic products. However, there are some problems that seriously hinder the Li–S batteries from going from the laboratory to the factory, such as poor stability caused by the large volume expansion of sulfur during charging and discharging, sluggish kinetics of the electrochemical reaction resulting from the low conductivity of the active materials, and loss of active materials arising from the dissolution and diffusion of the intermediate product lithium polysulfides (LiPSs). In this paper, the two-dimensional layered material MXene and TiN are firstly combined by spray drying method to prepare pomegranate-like TiN@MXene microspheres with both adsorption capacity and catalytic effect on LiPSs conversion. The interconnected skeleton composed of MXene not only solves the problem of easy stacking of MXene sheets but also ensures the uniform distribution of sulfur. Without affecting the excellent characteristics of MXene itself, the overall conductivity of the composite electrode material is improved. The TiN hollow nanospheres are coated with MXene layers to form a shell, catalyzing the adsorption of LiPSs and accelerating the transformation of high-order LiPSs to Li₂S₂/Li₂S. As a result, the TiN@MXene cathode delivers a high initial discharge capacity of 1436 mA h g⁻¹ at 0.1C, excellent rate performance of 636 mA h g⁻¹ up to 3C, and an ultralong lifespan over 1000 cycles with a small capacity decay of 0.048% per cycle at the current density of 1.0C.

A novel pomegranate-like TiN@MXene microsphere was constructed by a facile spray drying method and synergistically enhanced the conversion of polysulfides in Li–S batteries.

Entropy Enhanced Perovskite Oxide Ceramic for Efficient Electrochemical Reduction of Oxygen to Hydrogen Peroxide

The electrochemical oxygen reduction reaction (ORR) offers a most promising and efficient route to produce hydrogen peroxide (H₂O₂), yet the lack of cost‐effective and high‐performance electrocatalysts have restricted its practical application. Herein, an entropy‐enhancement strategy has been employed to enable the low‐cost perovskite oxide to effectively catalyze the electrosynthesis of H₂O₂. The optimized Pb(NiWMnNbZrTi)_1/6O₃ ceramic is available on a kilogram‐scale and displays commendable ORR activity in alkaline media with high selectivity over 91 % across the wide potential range for H₂O₂ including an outstanding degradation property for organic dyes through the Fenton process. The exceptional performance of this perovskite oxide is attributed to the entropy stabilization‐induced polymorphic transformation assuring the robust structural stability, decreased charge mobility as well as synergistic catalytic effects which we confirm using advanced in situ Raman, transient photovoltage, Rietveld refinement as well as finite elemental analysis.

Three-dimensional carbon architectures with O doping and rich defects for catalytic conversion of polysulfides

To deal with the notorious shuttle behavior and sluggish conversion of lithium polysulfides (LiPSs), heteroatoms doping and defects creating are practical strategies for improving capture and catalytic conversion of LiPSs. In this work, O doped porous carbon materials (OPC) with a 3D hierarchical structure, consisting of 2–4 μm carbon sheets decorated with macrospores of 0.2–0.4 μm, was fabricated with MgO template. It is found that the increasing the carbonization temperature and the amount of MgO will make OPC rich in oxygen functional groups and defect sites. Electrochemical measures show that the OPC12–800 achieves reversible capacity (an initial discharge specific capacity of 1448.4 mAh g−1 at current density of 0.1 C) and cycling performance (717.7 mAh g−1 at 2 C over 200 cycles). The excellent electrochemical performance is attributed to the hierarchical porous structure, abundant C–O/C=O and defects, which effectively adsorbs polysulfides and promote faster redox reaction of LiPSs. This study provides an alternative to improve the performance of carbon materials as host of Li–S batteries by regulating the types of oxygen-containing functional groups and defects on carbon surface.

Novel functional separator with self-assembled MnO2 layer via a simple and fast method in lithium-sulfur battery

Modifying separator with metal oxides has been considered as a strong strategy to inhibit the shuttling of soluble polysulfide in the lithium-sulfur battery (Li-S battery). Manganesedioxide (MnO₂), one kind of transition metal oxide, is widely applied to decorate the PP (Polypropylene) separator. However, the fabrication by physical coating is always multistep and complicated. Here, we design a simple and fast method to chemically decorate separator. Based on the oxidizing property of acidic KMnO₄ solution, the PP separator was oxidized and an ultrathin self-assembled MnO₂ layer was directly constructed on one side of separator, by immersing in acidic KMnO₄ solution for only 1 h. The self-assembled MnO₂ layer has the synergistic effect of adsorption and catalytic conversion on polysulfides, which can effectively inhibit the shuttle effect. It can also help battery to maintain excellent electrochemical kinetics in the electrochemical cycle and maintain the effective recycling of active substances. As a result, the shuttling of polysulfide is greatly prohibited by this novel functional separator, and cycling stability is outstandingly improved, with a low-capacity decaying of 0.058% after 500 cycles at 0.5C. The rapid and simple modification method proposed in this study has a certain reference value for the future large-scale application of lithium-sulfur battery.

Enhanced Catalysis of LIS3· Radical-to-Polysulfide Interconversion via Increased Sulfur Vacancies in Lithium–Sulfur Batteries

The practical application of lithium–sulfur (Li–S) batteries is seriously hindered by severe lithium polysulfide (LiPS) shuttling and sluggish electrochemical conversions. Herein, the Co₉S₈/MoS₂ heterojunction as a model cathode host material is employed to discuss the performance improvement strategy and elucidate the catalytic mechanism. The introduction of sulfur vacancies can harmonize the chemisorption of the heterojunction component. Also, sulfur vacancies induce the generation of radicals, which participate in a liquidus disproportionated reaction to reduce the accumulation of liquid LiPSs. To assess the conversion efficiency from liquid LiPSs to solid Li₂S, a new descriptor calculated from basic cyclic voltammetry curves, nucleation transformation ratio, is proposed.

Sulfur vacancies can harmonize chemisorption and generate amounts of radicals to boost the conversions of LiPSs.

Lithium-Sulfur Battery Cathode Design: Tailoring Metal-Based Nanostructures for Robust Polysulfide Adsorption and Catalytic Conversion

Lithium-sulfur (Li-S) batteries have a high specific energy capacity and density of 1675 mAh g^-1 and 2670 Wh kg^-1 , respectively, rendering them among the most promising successors for lithium-ion batteries. However, there are myriads of obstacles in the practical application and commercialization of Li-S batteries, including the low conductivity of sulfur and its discharge products (Li₂ S/Li₂ S₂ ), volume expansion of sulfur electrode, and the polysulfide shuttle effect. Hence, immense attention has been devoted to rectifying these issues, of which the application of metal-based compounds (i.e., transition metal, metal phosphides, sulfides, oxides, carbides, nitrides, phosphosulfides, MXenes, hydroxides, and metal-organic frameworks) as sulfur hosts is profiled as a fascinating strategy to hinder the polysulfide shuttle effect stemming from the polar-polar interactions between the metal compounds and polysulfides. This review encompasses the fundamental electrochemical principles of Li-S batteries and insights into the interactions between the metal-based compounds and the polysulfides, with emphasis on the intimate structure-activity relationship corroborated with theoretical calculations. Additionally, the integration of conductive carbon-based materials to ameliorate the existing adsorptive abilities of the metal-based compound is systematically discussed. Lastly, the challenges and prospects toward the smart design of catalysts for the future development of practical Li-S batteries are presented.

Metastable marcasite NiSe2 nanodendrites on carbon fiber clothes to suppress polysulfide shuttling for high-performance lithium-sulfur batteries

The incorporation of catalytic components is a promising strategy to promote redox reaction kinetics and suppress polysulfide shuttling for high-performance lithium-sulfur batteries (LSBs). In this work, metastable marcasite NiSe₂ nanodendrites grown on carbon fiber clothes (m-NiSe₂/CFC) were synthesized to improve chemical adsorption and electrocatalytic activity towards lithium polysulfides. The multifunctional m-NiSe₂/CFC film was utilized as both the interlayer and the three-dimensional (3D) current collector in LSBs. In comparison with the stable pyrite NiSe₂ nanodendrite-covered CFC (p-NiSe₂/CFC) counterpart, the m-NiSe₂/CFC film exhibits even stronger chemisorption, higher catalytic activity and faster reaction kinetics, thereby resulting in significantly improved lithium storage performance. The Al@S/rGO@m-NiSe₂/CFC cell has a high reversible capacity of 1646 mA h g^-1 at 0.2C, a high Q_L/Q_H ratio of 3.00 at 0.2C, a high rate capability of 900 mA h g^-1 at 4C, and an outstanding cyclic stability exhibiting a low capacity decay of 0.028% per cycle for 600 cycles at 4C. Moreover, a symmetrically sandwiched cathode of m-NiSe₂/CFC@S/rGO@m-NiSe₂/CFC was designed for high sulfur loading LSBs (4.5 mg cm^-2) with superior electrochemical performance of 3.73 mA h cm^-2 after 100 cycles at 1C rate. Our work opens up a new opportunity to enhance the electrochemical performance of LSBs by phase engineering of NiSe₂ catalysts in sandwiched structural cathodes.

Graphene-Based Materials for Flexible Lithium-Sulfur Batteries

The increasing demand for wearable electronic devices necessitates flexible batteries with high stability and desirable energy density. Flexible lithium-sulfur batteries (FLSBs) have been increasingly studied due to their high theoretical energy density through the multielectron chemistry of low-cost sulfur. However, the implementation of FLSBs is challenged by several obstacles, including their low practical energy density, short life, and poor flexibility. Various graphene-based materials have been applied to address these issues. Graphene, with good conductivity and flexibility, exhibits synergistic effects with other active/catalytic/flexible materials to form multifunctional graphene-based materials, which play a pivotal role in FLSBs. This review summarizes the recent progress of graphene-based materials that have been used as various FLSB components, including cathodes, interlayers, and anodes. Particular attention is focused on the precise nanostructures, graphene efficacy, interfacial effects, and battery layout for realizing FLSBs with good flexibility, energy density, and cycling stability.

Carbon coating on metal oxide materials for electrochemical energy storage

During the past decades, nano-structured metal oxide electrode materials have received growing attention due to their low development cost and high theoretical specific capacity, accordingly, quite a lot of metal oxide electrode materials are being used in electrochemical energy storage devices. However, the further development was limited by the relatively low electrical conductivity and the volume expansion during electrochemical reactions. Thus, many approaches have been proposed to obtain high-efficiency metal oxide electrode materials, such as designing nanomaterials with ideal morphology and high specific surface area, optimizing with carbon-based materials (such as graphene and glucose) to prepare nanocomposites, combining with conductive substrates to enhance the conductivity of electrodes, etc. Owning to the advantages of low cost and high chemical stability of carbon materials, core-shell structure formed by carbon-coated metal oxides is considered to be a promising solution to solve these problems. Therefore, this review mainly focuses on recent research advances in the field of carbon-coated metal oxides for energy storage, summarizing the advantages and disadvantages of common metal oxides and different types of carbon sources, and proposing methods to optimize the material properties in terms of structure and morphology, carbon layer thickness, coating method, specific surface area and pore size distribution, as well as improving electrical conductivity. In addition, the double or multi-layer coating strategy is also a reflection of the continuous development of carbon coating method. Hopefully, this rereview may provide a new direction for the renewal and development of future energy storage electrode materials.

Advances in Lithium-Sulfur Batteries: From Academic Research to Commercial Viability

Lithium-ion batteries, which have revolutionized portable electronics over the past three decades, were eventually recognized with the 2019 Nobel Prize in chemistry. As the energy density of current lithium-ion batteries is approaching its limit, developing new battery technologies beyond lithium-ion chemistry is significant for next-generation high energy storage. Lithium-sulfur (Li-S) batteries, which rely on the reversible redox reactions between lithium and sulfur, appears to be a promising energy storage system to take over from the conventional lithium-ion batteries for next-generation energy storage owing to their overwhelming energy density compared to the existing lithium-ion batteries today. Over the past 60 years, especially the past decade, significant academic and commercial progress has been made on Li-S batteries. From the concept of the sulfur cathode first proposed in the 1960s to the current commercial Li-S batteries used in unmanned aircraft, the story of Li-S batteries is full of breakthroughs and back tracing steps. Herein, the development and advancement of Li-S batteries in terms of sulfur-based composite cathode design, separator modification, binder improvement, electrolyte optimization, and lithium metal protection is summarized. An outlook on the future directions and prospects for Li-S batteries is also offered.

Reasonably Introduced ZnIn2S4@C to Mediate Polysulfide Redox for Long-Life Lithium-Sulfur Batteries

In consideration of the inferior rate performance and low sulfur utilization of lithium-sulfur batteries (LSBs), an effective strategy via combining polar materials with the conductive carbon sulfur host is widely applied. Herein, metal organic framework-derived in situ-developed ZnIn₂S₄@C is innovatively synthesized to mediate lithium polysulfide (LPS) conversion based on high electron conductivity and strong chemical interactions for advanced LSBs. Polar ZnIn₂S₄ possesses strong chemisorption in keeping with the DFT calculation results and catalytic for LPSs, ensuring a high sulfur utilization. Meanwhile, the hollow non-polar carbon frame possessing hierarchical pores not only provides internal space to contain active species but also accommodates efficient electronic transferring and diffusion of lithium ions in the process of cycling. The above advantages make the electrode possess promising stability and good rate performances, achieving long-term and high-rate cycling. Thus, under a sulfur loading of 1.5 mg cm^-2, after 500 cycles, at 2 and 5 C, the as-prepared ZnIn₂S₄@C@S delivers reversible capacities of 734 mA h g^-1 (75.7% of the initial capacity with a dropping rate of 0.015% per cycle) and 504 mA h g^-1 (68.5% of the primal capacity with a dropping rate of 0.029% per cycle), respectively. Even at a high sulfur loading of 5.0 mg cm^-2, at 5 C, 65.6% of the initial capacity can be maintained with a low fading rate of 0.430% per cycle after 500 loops with a high Coulombic efficiency of around 99.8%.

Hollow C@SnS2/SnS nanocomposites: High efficient oxygen evolution reaction catalysts

Using structural phase transitions to enhance electrochemical properties without has received wide attention due to its large active area and excellent electron transport capacity. In this work, hollow C@SnS₂/SnS nanocomposites were successfully synthesized from hollow C@SnS₂ by controlling the temperature and time of the phase transitions. It is found that this hollow C@SnS₂/SnS nanocomposite can serve as electrocatalyst, showing excellent oxygen evolution reaction performance. The Sn (IV) heterostructure easily accepts electrons in water and plays a crucial role in the oxygen evolution reaction. Meanwhile, the low-valence Sn (II) can maintain a stable structure in the electrochemical reaction and thus exhibits good electrochemical performance with an overpotential of 380 mV, at the current density of 10 mA cm^-2, and a low Tafel slope of 63 mV dec^-1, which is far lower than that of pure SnS or SnS₂.

Fabrication of NiO–carbon nanotube/sulfur composites for lithium-sulfur battery application

The practical applications of lithium–sulfur batteries are still a great challenge due to the polysulfide shuttle and capacity decay. Herein, we report a NiO–carbon nanotube/sulfur (NiO–CNT/S) composite by hydrothermal and thermal treatments. This hybrid combines the high conductivity of CNTs and double adsorption of CNTs and NiO (physical and chemical adsorption) to improve the electrochemical performance for the sulfur electrodes. Compared with CNT/S and NiO/S, the developed NiO–CNT/S composites present a preferable initial reversible discharge capacity (1072 mA h g⁻¹) and is maintained at 609 mA h g⁻¹ after 160 cycles at 0.1C.

The NiO–CNT/S composites combine the high conductivity of CNT and double adsorption of CNT and NiO (physical adsorption and chemical adsorption) to enhance electrochemical performance for sulfur electrode materials.

Construction of reduced graphene oxide wrapped yolk-shell vanadium dioxide sphere hybrid host for high-performance lithium-sulfur batteries

Owing to the considerable theoretical energy density, lithium-sulfur batteries have been deemed as a competitive candidate for the next-generation energy storage devices. However, its commercialization still depends on the moderation of the shuttle effect and the conductivity improvement of the sulfur cathode. Herein, a novel reduced graphene oxide (rGO) wrapped yolk-shell vanadium dioxide (VO2) sphere hybrid host (rGO/VO2) is reported to simultaneously tackle these barriers. In particular, the polar VO2 sphere can chemically anchor and catalyze the conversion of polysulfides effectively both on the yolk and the shell surfaces. Meanwhile, the highly conductive 3D porous rGO network not only allows sufficient penetration of electrolyte and provides efficient transport pathways for lithium ions and electrons, but also buffers the volume variation during the lithiation process. Besides, the dissolution of the polysulfides can also be alleviated by physical confinement via the interconnected carbon network. Benefiting from these synergistic features, such designed rGO/VO2/S cathode delivers outstanding cycle stability (718.6 mA h g-1 initially, and 516.1 mA h g-1 over 400 cycles at 1C) with a fading rate of 0.07% per cycle. Even at 3C, a capacity of 639.7 mA h g-1 is reached. This proposed unique structure could provide novel insights into high-energy batteries.

Two-Dimensional Black Phosphorus Nanomaterials: Emerging Advances in Electrochemical Energy Storage Science

Two-dimensional black phosphorus (2D BP), well known as phosphorene, has triggered tremendous attention since the first discovery in 2014. The unique puckered monolayer structure endows 2D BP intriguing properties, which facilitate its potential applications in various fields, such as catalyst, energy storage, sensor, etc. Owing to the large surface area, good electric conductivity, and high theoretical specific capacity, 2D BP has been widely studied as electrode materials and significantly enhanced the performance of energy storage devices. With the rapid development of energy storage devices based on 2D BP, a timely review on this topic is in demand to further extend the application of 2D BP in energy storage. In this review, recent advances in experimental and theoretical development of 2D BP are presented along with its structures, properties, and synthetic methods. Particularly, their emerging applications in electrochemical energy storage, including Li-/K-/Mg-/Na-ion, Li-S batteries, and supercapacitors, are systematically summarized with milestones as well as the challenges. Benefited from the fast-growing dynamic investigation of 2D BP, some possible improvements and constructive perspectives are provided to guide the design of 2D BP-based energy storage devices with high performance.

Enhanced Polysulfide Regulation via Porous Catalytic V2O3/V8C7 Heterostructures Derived from Metal-Organic Frameworks toward High-Performance Li-S Batteries

The development of Li-S batteries is largely impeded by the complicated shuttle effect of lithium polysulfides (LiPSs) and sluggish reaction kinetics. In addition, the low mass loading/utilization of sulfur is another key factor that makes Li-S batteries difficult to commercialize. Here, a porous catalytic V₂O₃/V₈C₇@carbon composite derived from MIL-47 (V) featuring heterostructures is reported to be an efficient polysulfide regulator in Li-S batteries, achieving a substantial increase in sulfur loading while still effectively suppressing the shuttle effect and enhancing kinetics. Systematic mechanism analyses suggest that the LiPSs strongly adsorbed on the V₂O₃ surface can be rapidly transferred to the V₈C₇ surface through the built-in interface for subsequent reversible conversion by an efficient catalytic effect, realizing enhanced regulation of LiPSs from capture to conversion. In addition, the porous structure provides sufficient sulfur storage space, enabling the heterostructures to exert full efficacy with a high sulfur loading. Thus, this S-V₂O₃/V₈C₇@carbon@graphene cathode exhibits prominent rate performance (587.6 mAh g^-1 at 5 C) and a long lifespan (1000 cycles, 0.017% decay per cycle). It can still deliver superior electrochemical performance even with a sulfur loading of 8.1 mg cm^-2. These heterostructures can be further applied in pouch cells and produce stable output at different folding angles (0-180°). More crucially, the cells could retain 4.3 mAh cm^-2 even after 150 cycles, which is higher than that of commercial lithium-ion batteries (LIBs). This strategy for solving the shuttle effect under high sulfur loading provides a promising solution for the further development of high-performance Li-S batteries.

Multifunctional NiCo2O4 nanosheet-assembled hollow nanoflowers as a highly efficient sulfur host for lithium-sulfur batteries

Herein, a simple approach was developed to fabricate novel NiCo₂O₄ hollow nanoflowers (NCOHFs), which were explored as a sulfur carrier material for lithium-sulfur (Li-S) batteries. Remarkably, the capacity of the NCOHF/S composite electrode was ∼666.8 mA h g^-1 in the fourth cycle, which was maintained at ∼432.9 mA h g^-1 in the 400th cycle under a current density of 1.0C, with a low decay rate of 0.087% per cycle. The overall outstanding properties of the NCOHF/S composite are attributed to the successful new strategy in structural design via in situ and ex situ procedures. Firstly, NCOHF with bifunctional catalytic activity and chemical adsorption can efficiently promote the redox reactions of lithium polysulfides (LiPSs) and suppress the diffusion of polysulfides. Secondly, NCOHF with inherently high electronic conductivity acts as a conduit to accelerate the transport of electrons and ions. Thirdly, the flower-like NiCo₂O₄ nanosheets are anchored tightly to the conductive carbon and binder during the Li⁺ insertion and extraction processes, which can effectively suppress the aggregation of the NCOHF/S composite during cycling. Finally, the hollow space inside the NCOHF/S composite provides sufficient free space for the expansion of encapsulated pure sulfur.

2 D Materials for Inhibiting the Shuttle Effect in Advanced Lithium-Sulfur Batteries

A lithium-sulfur battery is a new energy storage device with low cost, high energy storage density, and environmental protection. It is an important tool for future battery systems. However, owing to the shuttle of polysulfides, insulation performance of sulfur, and volume change, capacity decay and cycle instability result, which limits the future application of lithium-sulfur batteries. 2 D materials, such as graphene, carbide, nitride, sulfide, oxide, and their aggregates, provide high surface area to improve sulfur utilization and cycle performance. In this Minireview, various 2 D materials are summarized that use physical confinement and chemical interactions to inhibit the shuttle of polysulfides. We outline the controlled spacing of 2 D materials, abundant active sites, and large transverse size separators and interlayers. The effects on the interlayer and separator based on 2 D materials at the lithium anode prevent polysulfide dissolution are also reviewed. Finally, the challenges and prospects of 2 D materials for lithium-sulfur batteries are discussed.

Defect-Rich Multishelled Fe-Doped Co3O4 Hollow Microspheres with Multiple Spatial Confinements to Facilitate Catalytic Conversion of Polysulfides for High-Performance Li-S Batteries

Over the past decade, lithium-sulfur (Li-S) batteries have been thought of as promising alternatives for the new generation of battery systems. Although the Li-S batteries possess high-theoretical energy density (2600 Wh kg^-1) and capacity (1675 mAh g^-1), the problems of poor electron and ion conduction, volumetric expansion, and sulfur immobilization greatly impede the wide applicability of Li-S batteries. Herein, a defect-rich multishelled Co₃O₄ microsphere structure doped with Fe was synthesized via a one-step hydrothermal method and subsequent thermal treatment. The unique multishelled structure provides multiple spatial confinements for lithium polysulfides trapping and buffering the volume variation during cycling. Moreover, the rich oxygen defect designed by controlled Fe doping can provide numerous catalytic sites for polysulfide redox reactions. Attributed to the synergistic effect of structural design and oxygen-defect fabrication, the sulfur composite electrode delivers a notable cycle performance, presenting a much lower capacity fading of 0.017% per cycle over 1000 cycles at 1 C and an excellent rate capability of 571.3 mAh g^-1 at 5 C. This work proposes a potential approach for designing a transition metal oxide-based multishelled hollow structure combined with oxygen defect, which also offers a new perspective on high-performance Li-S batteries.

Graphene-nanoscroll-based Integrated and self-standing electrode with a sandwich structure for lithium sulfur batteries

A sandwich-like and self-standing electrode integrating a current collector, active material and interlayer provides multifunctional polysulfide-trapping ability in lithium sulfur batteries.

Shape-controlled MnO2 as a sulfur host for high performance lithium–sulfur batteries

The three-dimensional porous network structure self-assembled from birnessite-type MnO₂ flakes and urchin-like structure composed of MnO₂ nanotubes was fabricated by a convenient one-step hydrothermal method as the sulfur scaffold for high performance lithium–sulfur batteries.

Polysulfide Trapping in Carbon Nanofiber Cloth/S Cathode with a Bifunctional Separator for High-Performance Li-S Batteries

The development of carbon nanofiber (CNF) cloth-based cathodes is essential for the fabrication of high energy density and flexible Li-S batteries. Surface modification is generally used to improve the electrochemical performance of CNF cloth/S cathodes. However, this strategy creates some problems such as structure collapse, complex fabrication steps, and poor consistency. Herein, a β-MnO₂ nanowire/graphene-modified separator is used to improve the performance of CNF cloth/S cathodes without changing their structure. β-MnO₂ can facilitate chemical bonding with polysulfides, whereas graphene can decrease the inner resistance and trap polysulfide by physical shielding. The cathode with the bifunctional separator displayed a high discharge capacity of 529.9 mAh g^-1 with a low capacity decay of 0.051 % per cycle after 500 cycles at 1 C, which is 3 times higher compared with a bare separator. Even with a high sulfur loading of 9.0 mg cm^-2 , a high areal capacity of 3.8 mAh cm^-2 was delivered over 100 cycles.

Membrane-Free Hybrid Capacitive Deionization System Based on Redox Reaction for High-Efficiency NaCl Removal

Capacitive deionization (CDI) is a promising technology for desalination due to its advantages of low driven energy and environmental friendliness. However, the ion removal capacity (IRC) of CDI is insufficient for practical application because such a capacity is limited by the available surface area of the carbon electrode for ion absorption. Thus, the development of a novel desalination technology with high IRC and low cost is vital. Here, a membrane-free hybrid capacitive deionization system (HCDI) with hollow carbon@MnO₂ (HC@MnO₂) to capture sodium via redox reaction and hollow carbon sphere with net positive surface charges (PHC) for chloride adsorption is introduced. The as-obtained HC@MnO₂ with unique structure and high conductivity can improve the utilization of MnO₂ pseudocapacitive electrodes. Meanwhile, the PHC can selectively adsorb Cl^- and prevent the adsorption of Na⁺ due to electrostatic repulsion. As expected, the membrane-free HCDI system demonstrates excellent desalination performance. The system's IRC and maximum removal rate are 30.7 mg g^-1 and 7.8 mg g^-1 min^-1, respectively. Moreover, the proposed system has a low cost because of the absence of expensive ion exchange membranes (IEM), which is suitable for practical application. The excellent performance of this HCDI makes it a promising desalination technology for future use.

Sulfur impregnation in polypyrrole-modified MnO2 nanotubes: efficient polysulfide adsorption for improved lithium-sulfur battery performance

Rechargeable lithium-sulfur batteries have emerged as a viable technology for next generation electrochemical energy storage, and the sulfur cathode plays a critical role in determining the device performance. In this study, we prepared functional composites based on polypyrrole-coated MnO₂ nanotubes as a highly efficient sulfur host (sulfur mass loading 63.5%). The hollow interior of the MnO₂ nanotubes not only allowed for accommodation of volumetric changes of sulfur particles during the cycling process, but also confined the diffusion of lithium polysulfides by physical restriction and chemical adsorption, which minimized the loss of polysulfide species. In addition, the polypyrrole outer layer effectively enhanced the electrical conductivity of the cathode to facilitate ion and electron transport. The as-prepared MnO₂-PPy-S composite delivered an initial specific capacity of 1469 mA h g^-1 and maintained an extremely stable cycling performance, with a small capacity decay of merely 0.07% per cycle at 0.2C within 500 cycles, a high average coulombic efficiency of 95.7% and an excellent rate capability at 470 mA h g^-1 at the current density of 3C.

Triple-Layered Carbon-SiO2 Composite Membrane for High Energy Density and Long Cycling Li-S Batteries

Here we report a highly scalable yet flexible triple-layer structured porous C/SiO₂ membrane via a facile phase inversion method for advancing Li-sulfur battery technology. As a multifunctional current-collector-free cathode, the conductive dense layer of the C/SiO₂ membrane offers hierarchical macropores as an ideal sulfur host to alleviate the volume expansion of sulfur species and facilitate ion/electrolyte transport for fast kinetics, as well as spongelike pores to enable high sulfur loading. The triple-layer structured membrane cathode enables the filling of most sulfur species in the macropores and additional loading of a thin sulfur slurry on the membrane surface, which facilitates ion/electrolyte transport with faster kinetics than the conventional S/C slurry-based cathode. Furthermore, density functional theory simulations and visual adsorption measurements confirm the critical role of the doped SiO₂ nanoparticles (∼10 nm) in the asymmetric C membrane in suppressing the shuttle effect of polysulfides via chemisorption and electrocatalysis. The rationally designed C/SiO₂ membrane cathodes demonstrate long-term cycling stability of 300 cycles at a high sulfur loading of 2.8 mg cm^-2 with a sulfur content of ∼75%. This scalable yet flexible self-supporting cathode design presents a useful strategy for realizing practical applications of high-performance Li-S batteries.

Mesoporous N-doped graphene prepared by a soft-template method with high performance in Li-S batteries

Lithium sulfur (Li-S) batteries, which have high theoretical capacity, are promising candidates for energy storage systems; however, several obstacles, including insulating nature and volumetric expansion of sulfur as well as shuttle effects of polysulfides, impede the commercialization of these batteries. Herein, we utilized mesoporous N-doped graphene (NGM) as a sulfur host, which was synthesized using a type of triblock copolymer, Pluronic F127 (PF127), as a soft template. PF127 not only acted as a spacer to prevent the graphene layers from restacking during the hydrothermal reaction, but also resulted in the formation of high-density wrinkles on the surface of the graphene sheets after annealing. The crumpled NGM with a nitrogen doping content of 4.80 at% had the high specific area and total pore volume of up to 958.72 m2 g-1 and 2.39 cm3 g-1, respectively, providing space for the accommodation and uniform distribution of sulfur in the cathode. Therefore, the sulfur content was increased to 87.2 wt% in the graphene sulfur composite, achieving the discharge capacity of 492.2 mA h g-1TE at 1.08 A g-1TE with a low capacity decay rate of 0.12% per cycle after 400 cycles. Thus, this study provides an effective strategy for enhancing the specific surface area and pore volume of graphene to develop Li-S batteries with high performance.

Uniform Mesoporous MnO2 Nanospheres as a Surface Chemical Adsorption and Physical Confinement Polysulfide Mediator for Lithium-Sulfur Batteries

The actual implementation of lithium-sulfur batteries is hindered by inferior cyclic stability and poor coulombic efficiency stemming from the notorious shuttling of soluble polysulfide intermediates. Herein, uniform mesoporous MnO₂ nanospheres were prepared using a facile self-assembly and room-temperature reaction method. As a sulfur carrier of sulfur cathodes, the versatile architecture of MnO₂ not only provides powerful chemical adsorption to anchor polysulfide intermediates on the large polar surface area but also restrains them within the nanopores by physical confinement. The mesoporous MnO₂-stabilized sulfur cathode demonstrates a high initial reversible capacity of 1349.3 mA h g^-1 and a capacity fading rate of 0.073% at 1.0 C over 500 cycles. Furthermore, a reversible areal capacity of 2.5 mA h cm^-2 was achieved with stable cycling performance at a sulfur content of 80.7%. Our work offers a facile method to build efficient sulfur cathodes for high performance lithium-sulfur batteries.

Recent Advances in Hollow Porous Carbon Materials for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered as one of the most potential next-generation rechargeable batteries due to their high theoretical energy density. However, some critical issues, such as low capacity, poor cycling stability, and safety concerns, must be solved before Li-S batteries can be used practically. During the past decade, tremendous efforts have been devoted to the design and synthesis of electrode materials. Benefiting from their tunable structural parameters, hollow porous carbon materials (HPCM) remarkably enhance the performances of both sulfur cathodes and lithium anodes, promoting the development of high-performance Li-S batteries. Here, together with the templated synthesis of HPCM, recent progresses of Li-S batteries based on HPCM are reviewed. Several important issues in Li-S batteries, including sulfur loading, polysulfide entrapping, and Li metal protection, are discussed, followed by a summary on recent research on HPCM-based sulfur cathodes, modified separators, and lithium anodes. After the discussion on emerging technical obstacles toward high-energy Li-S batteries, prospects for the future directions of HPCM research in the field of Li-S batteries are also proposed.

A high-areal-capacity lithium–sulfur cathode achieved by a boron-doped carbon–sulfur aerogel with consecutive core–shell structures

A boron-doped carbon–sulfur (BCS) aerogel with consecutive “core–shell” structures achieves a high specific capacity of 1326 mA h g⁻¹, a high areal capacity of 13.5 mA h cm⁻², and a long-term cycling stability.

Towards establishing standard performance metrics for batteries, supercapacitors and beyond

Electrochemical energy storage (EES) materials and devices should be evaluated against clear and rigorous metrics to realize the true promises as well as the limitations of these fast-moving technologies.

Inhibiting polysulfide shuttling using dual-functional nanowire/nanotube modified layers for highly stable lithium–sulfur batteries

Dual-functional MnO₂ nanowire/CNT modified layers were prepared to inhibit the polysulfide shuttle effect utilizing their strong adsorption capability and high conductivity.

Construction of ultrathin MnO2 decorated graphene/carbon nanotube nanocomposites as efficient sulfur hosts for high-performance lithium–sulfur batteries

Lithium–sulfur batteries are attracting significant attention due to their high theoretical specific capacity and low cost. However, their applications are hindered by the poor conductivity of sulfur and capacity fading caused by the shuttle effect. Here, ultrathin manganese dioxide decorated graphene/carbon nanotube nanocomposites are designed as sulfur hosts to suppress the shuttle effect and improve the adsorption efficiency of polysulfides. The graphene/carbon nanotube hybrids, with extraordinary conductivity and large surface area, function as excellent channels for electron transfer and lithium ion diffusion. The ultrathin manganese dioxide nanosheets enable efficient chemical interaction with polysulfides and promote the redox kinetics of polysulfides. As a result, an ultrathin manganese dioxide decorated graphene/carbon nanotube sulfur composite with high sulfur content (81.8 wt%) delivers a high initial specific capacity of 1015.1 mA h g⁻¹ at a current density of 0.1C, high coulombic efficiency approaching 100% and high capacity retention of 84.1% after 100 cycles. The nanocomposites developed in this work have promising applications in high-performance lithium–sulfur batteries.

Ultrathin MnO₂ nanosheets and nano size sulfur particles distributed uniformly on the surface of G/CNT hybrids, which exhibit high rate performance and long-term cycling performance.

Fully integrated hierarchical double-shelled Co9S8@CNT nanostructures with unprecedented performance for Li-S batteries

The insulating and dissoluble features of sulfur and polysulfide intermediates significantly hinder the cycling performance and coulombic efficiency of Li-S batteries. We herein design fully integrated hierarchical double-shelled Co₉S₈@CNT hollow nanospheres as a sulfur host, where each shell owns a tri-layer sandwich structure and six functional layers are constructed in total. Uniquely, the hierarchical structures integrate the beneficial functions of electrical conductivity, ion diffusion, polysulfide immobilization and polysulfide redox kinetics. The Co₉S₈@CNT/S delivers a reversible specific capacity of 1415 mA h g^-1 at 0.2C, which is very close to the theoretical capacity of sulfur. Moreover, even at a high rate of 10C, an unprecedented capacity of 676.7 mA h g^-1 can still be achieved, which represents the best rate capability among the ever-reported sulfur cathodes with similar sulfur content. More importantly, the Co₉S₈@CNT/S also displays a high coulombic efficiency of nearly 100% and an excellent cycling performance for up to 1000 cycles with a capacity fading rate of only 0.0448% per cycle. Even at the loading amount of 5.5 mg cm^-2, the areal capacity can still reach 4.3 mA h cm^-2. The concept to rationally integrate distinct components into fully functional units could provide valuable insights for the development of Li-S batteries and beyond.

A Flexible All-in-One Lithium-Sulfur Battery

The recent boom in flexible and wearable electronic devices has increased the demand for flexible energy storage devices. The flexible lithium-sulfur (Li-S) battery is considered to be a promising candidate due to its high energy density and low cost. Herein, a flexible Li-S battery was fabricated based on an all-in-one integrated configuration, where a multiwalled carbon nanotubes/sulfur (MWCNTs/S) cathode, MWCNTs/manganese dioxide (MnO₂) interlayer, polypropylene (PP) separator, and Li anode were integrated together by combining blade coating with vacuum evaporation methods. Each component of the all-in-one structure can be seamlessly connected with the neighboring layers. Such an optimal interfacial connection can effectively enhance electron- and/or load-transfer capacity by avoiding the relative displacement or detachment between two neighboring components at bending strain. Therefore, the flexible all-in-one Li-S batteries display fast electrochemical kinetics and have stable electrochemical performance under different bending states.