Free-Standing Porous Carbon Nanofiber/Carbon Nanotube Film as Sulfur Immobilizer with High Areal Capacity for Lithium-Sulfur Battery

Carbon Nanofiber-Based Sandwich Free-Standing Cathode for High-Performance Lithium-Sulfur Batteries

Challenges including rapid capacity degradation and reduced Coulombic efficiency due to the shuttle effect have hindered the commercial viability of lithium-sulfur (Li-S) batteries. A novel sandwich-structured electrode with an optimized electrode structure and current collector interface design was presented as a free-standing positive electrode for Li-S batteries. Fabricated via a simple slurry coating process, the electrode embedded multiwalled carbon nanotubes within carbon nanofiber composite films (PCNF/T). Owing to the superior conductivity and reduced weight in comparison to both carbon nanofibers (PCNF) and the conventional aluminum foil current collector (Al), the PCNF/T electrode exhibited diminished polarization and accelerated redox reaction kinetics. Thus, it delivers an initial discharge capacity of 990.23 mA h g-1 at 0.5 C. Even after 400 cycles, while retains a reversible capacity of 707.45 mA h g-1, corresponding to a minimal capacity degradation rate of merely 0.07% per cycle. Notably, the electrode exhibits a capacity retention of 619.81 mA h g-1 after 400 cycles at 1 C, with a capacity decay rate of only 0.08% per cycle. This study presents an innovative approach to developing a new free-standing cathode for high-performance Li-S batteries.

Construction of SnS2-modified multi-hole carbon nanofibers with sulfur encapsulated as free-standing cathode electrode for lithium sulfur battery

Lithium-sulfur (Li-S) batteries exhibit a huge potential in energy storage devices for the thrilling theoretical energy density (2600 Wh kg-1). Nevertheless, the serious shuttle effect rooted in polysulfides and retardative hysteresis reaction kinetics results in inferior cycling and rate performances of Li-S batteries, impeding commercial applications. In order to further promote the energy storage abilities of Li-S batteries, a unique binder-free sulfur carrier consisting of SnS2-modified multi-hole carbon nanofibers (SnS2-MHCNFs) has been constructed, where MHCNFs can offer abundant space to accommodate high-level sulfur and SnS2can promote the adsorption and catalyst capability of polysulfides, synergistically promoting the lithium-ion storage performances of Li-S batteries. After sulfur loading (SnS2-MHCNFs@S), the material was directly applied as a cathode electrode of the Li-S battery. The SnS2-MHCNFs@S electrode maintained a good discharge capacity of 921 mAh g-1after 150 cycles when the current density was 0.1 C (1 C = 1675 mA g-1), outdistancing the MHCNFs@S (629 mAh g-1) and CNFs@S (249 mAh g-1) electrodes. Meanwhile, the SnS2-MHCNFs@S electrode still exhibited a discharge capacity of 444 mAh g-1at 2 C. The good performance of SnS2-MHCNFs@S electrode indicates that combining multihole structure designation and polar material modification are highly effective methods to boost the performances of Li-S batteries.

Aqueous heterocoagulation-driven assembly of graphene oxide and polycation-coated sulfur particles for nanocomposite Li-S battery cathodes

HYPOTHESIS

Reduced graphene oxide (rGO/polycation/sulfur) composites are promising cathode materials for Li-S battery applications because homogeneously dispersed sulfur nano/micro clusters in suitable carbon hosts enable remarkable cycle life for Li-S battery cells. New, benign and economic synthesis methods based only on aqueous colloidal dispersions are demanded for achieving high dispersity grade of sulfur within the carbon host. Colloidal interactions leading to heteroaggregation between carbonaceous lamellae and polycation-modified sulphur nanoparticles at ambient conditions in water are foreseen to afford nanocomposite cathodes, which maintain excellent electrochemical performance.

EXPERIMENTS

Hydrophilic sulfur nanoparticles (SNPs) were coated by low doses of polycation (PDDA) until reaching the isoelectric point (IEP), and in high dose to achieve charge reversal. Streaming potential titrations were performed to reveal appropriate mass ratios of PPDA, SNP and GO. Positively charged SNPs formed stable heteroaggregated structures with GO, and were employed to fabricate rGO/polycation/sulphur cathodes.

FINDINGS

Charge reversal characteristics of SNPs, polycation and GO were characterized quantitatively and mass ratios of PDDA to SNP beyond IEP were found to mediate attractive interactions leading to rapid heteroaggregation between SNPs and GO and also alleviate lithium polysulfide migration. The composite cathode showed an initial discharge capacity of 522 mAhg-1 at 0.2C rate with an excellent capacity retention of 91.4 % and coulombic efficiency of 98.5% after 100 charge-discharge cycles.

Boosting Polysulfide Redox Kinetics by Temperature-Induced Metal-Insulator Transition Effect of Tungsten-Doped Vanadium Dioxide for High-Temperature Lithium-Sulfur Batteries

The practical application of Li-S batteries is still severely restricted by poor cyclic performance caused by the intrinsic polysulfides shuttle effect, which is even more severe under the high-temperature condition owing to the inevitable increase of polysulfides' solubility and diffusion rate. Herein, tungsten-doped vanadium dioxide (W-VO2 ) micro-flowers are employed with first-order metal-insulator phase transition (MIT) property as a robust and multifunctional modification layer to hamper the shuttle effect and simultaneously improve the thermotolerance of the common separator. Tungsten doping significantly reduces the transition temperature from 68 to 35 °C of vanadium dioxide, which renders the W-VO2 easier to turn from the insulating monoclinic phase into the metallic rutile phase. The systematic experiments and theoretical analysis demonstrate that the temperature-induced in-suit MIT property endows the W-VO2 catalyst with strong chemisorption against polysulfides, low energy barrier for liquid-to-solid conversion, and outstanding diffusion kinetics of Li-ion under high temperatures. Benefiting from these advantages, the Li-S batteries with W-VO2 modified separator exhibit significantly improved rate and long-term cyclic performance under 50 °C. Remarkably, even at an elevated temperature (80 °C), they still exhibit superior electrochemical performance. This work opens a rewarding avenue to use phase-changing materials for high-temperature Li-S batteries.

Single-Atomic Dispersion of Fe and Co Supported on Reduced Graphene Oxide for High-Performance Lithium-Sulfur Batteries

High theoretical energy density and low cost make lithium-sulfur (LSB) batteries a promising system for next-generation energy storage. LSB performance largely depends on efficient reversible conversion of elemental sulfur to Li2S. Here, well-designed sulfur host materials including Fe or Co single atoms embedded on N-doped reduced graphene oxide (MNC/G with M = Fe or Co) are proposed to tackle the LSB challenges and enhance the electrochemical performance. Using a combination of Mössbauer spectroscopy and high-resolution scanning electron microscopy, the atomic dispersion of Co and Fe was revealed up to relatively high mass loadings. After optimization of the electrolyte/sulfur (E/S) ratio, FeNC/G shows the most promising cycle performance combining a constant high discharge capacity at low E/S values with the lowest polarization. In particular, the material FeNC/G@S with a high sulfur loading (9.4 mg cm-2) delivers a high area capacity of 7.7 mAh cm-2 under lean electrolyte conditions (6 mL g-1).

Development of Kovacs model for electrical conductivity of carbon nanofiber-polymer systems

This study develops a model for electrical conductivity of polymer carbon nanofiber (CNF) nanocomposites (PCNFs), which includes two steps. In the first step, Kovacs model is developed to consider the CNF, interphase and tunneling regions as dissimilar zones in the system. In the second step, simple equations are expressed to estimate the resistances of interphase and tunnels, the volume fraction of CNF and percolation onset. Although some earlier models were proposed to predict the electrical conductivity of PCNFs, developing of Kovacs model causes a better understanding of the effects of main factors on the nanocomposite conductivity. The developed model is supported by logical influences of all factors on the conductivity and by experimented conductivity of several samples. The calculations show good accordance to the experimented data and all factors rationally manage the conductivity of PCNFs. The highest conductivity of PCNF is gained as 0.019 S/m at the lowest ranges of polymer tunnel resistivity (ρ = 500 Ω m) and tunneling distance (d = 2 nm), whereas the highest levels of these factors (ρ > 3000 Ω m and d > 6 nm) cannot cause a conductive sample. Also, high CNF volume fraction, poor waviness, long and thin CNF, low "k", thick interphase, high CNF conduction, high percentage of percolated CNFs, low percolation onset and high interphase conductivity cause an outstanding conductivity in PCNF.

Synergistic Effect between Monodisperse Fe3O4 Nanoparticles and Nitrogen-Doped Carbon Nanosheets to Promote Polysulfide Conversion in Lithium-Sulfur Batteries

Effective fabrication of electrocatalysts active in anchoring and converting lithium polysulfides is critical for the manufacturing of high-performance lithium-sulfur batteries (LSBs). In this study, original Fe₃O₄ nanospheres with diameters close to 12 nm were finely dispersed over a porous nitrogen-doped carbon matrix by the freeze-drying method to produce a three-dimensional composite material (nano-Fe₃O₄/PNC) suitable for application as a sulfur host in LSBs. Nano-Fe₃O₄/PNC loaded with sulfur (S@nano-Fe₃O₄/PNC) was used as a cathode in a Li-S cell, whose initial discharge specific capacity reached 1256 mA h g^-1 at a 0.1 C rate. After 100 charge-discharge cycles at a 0.2 C rate, the reversible capacity of S@nano-Fe₃O₄/PNC remained at 745 mA h g^-1, demonstrating a capacity retention rate of 70%. Importantly, a high Coulombic efficiency of more than 99% was achieved, indicating effective inhibition of the polysulfides' "shuttle effect" by nano-Fe₃O₄/PNC. The use of electrolytes containing lithium nitrate further reduces the "shuttle effect" of polysulfides. This study demonstrates the synergistic effect between metal oxide nanoparticles and N-doped carbon, which plays an important role in promoting the adsorption and conversion of polysulfides in LSBs.

Covalent organic framework wrapped by graphene oxide as an efficient sulfur host for high performance lithium-sulfur batteries

The practical application of lithium-sulfur battery is seriously limited by the loss of active substances and the deterioration of cycle stability caused by the 'shuttle effect' of lithium polysulfides (LiPSs). In this work, graphene oxide (GO) coated covalent organic framework (COF) compound materials were synthesized as sulfur host material in spray-drying process. The polar groups on COF can efficiently adsorb LiPSs through lithiophilic interaction, which can reduce the 'shuttle effect' caused by soluble LiPSs. Besides, GO in the outer layer can wrap discrete sulfur to reduce the loss of active substances, which further improves the cycle stability of the cathode. The COF@GO/S cathode exhibits a high initial specific capacity of 848.4 mAh g^-1and retains a capacity of 601.1 mAh g^-1after 500 cycles at 1 C counting with a low capacity fading of 0.058% per cycle.

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.

Structural and Surfacial Modification of Carbon Nanofoam as an Interlayer for Electrochemically Stable Lithium-Sulfur Cells

Electrochemical lithium-sulfur batteries engage the attention of researchers due to their high-capacity sulfur cathodes, which meet the increasing energy-density needs of next-generation energy-storage systems. We present here the design, modification, and investigation of a carbon nanofoam as the interlayer in a lithium-sulfur cell to enable its high-loading sulfur cathode to attain high electrochemical utilization, efficiency, and stability. The carbon-nanofoam interlayer features a porous and tortuous carbon network that accelerates the charge transfer while decelerating the polysulfide diffusion. The improved cell demonstrates a high electrochemical utilization of over 80% and an enhanced stability of 200 cycles. With such a high-performance cell configuration, we investigate how the battery chemistry is affected by an additional polysulfide-trapping MoS2 layer and an additional electron-transferring graphene layer on the interlayer. Our results confirm that the cell-configuration modification brings major benefits to the development of a high-loading sulfur cathode for excellent electrochemical performances. We further demonstrate a high-loading cathode with the carbon-nanofoam interlayer, which attains a high sulfur loading of 8 mg cm-2, an excellent areal capacity of 8.7 mAh cm-2, and a superior energy density of 18.7 mWh cm-2 at a low electrolyte-to-sulfur ratio of 10 µL mg-1.

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.

Construction of all-carbon micro/nanoscale interconnected sulfur host for high-rate and ultra-stable lithium-sulfur batteries: Role of oxygen-containing functional groups

Carbon nanotubes (CNTs) are often used to settle down the sluggish reaction kinetics in lithium-sulfur batteries (LSBs). However, the self-aggregation of CNTs often makes them fail to effectively inhibit the shuttling effect of soluble lithium polysulfide (LiPS) intermediates. Herein, a type of ultra-stable carbon micro/nano-scale interconnected "carbon cages" has been designed by incorporating polar acid-treated carbon fibers (ACF) into three-dimensional (3D) CNT frameworks during vacuum filtration processes. Results show that the ACF-CNT composite frameworks possess a reinforced-concrete-like structure, in which the ACFs can well work as the main mechanical supporting frames for the composite electrodes, and the oxygen-containing functional groups (OFGs) formed on them as cross linker between ACFs and CNTs. Benefitted from this design, the ACF-CNT/S cathodes deliver an excellent rate capability (retain 72.6% at 4C). More impressively, the ACF-CNT/S cathodes also show an ultrahigh cycling stability (capacity decay rate of 0.001% per cycle over 350 cycles at 2C). And further optimization suggests that the suitable treatment on CFs could balance the chemical adsorption (OFGs) and physical confinement (carbon cages), leading to fast and durable electrochemical reaction dynamics. In addition, the assembled soft-pack LSBs further show a high dynamic bending stability.

Long-life lithium-sulfur batteries with high areal capacity based on coaxial CNTs@TiN-TiO2 sponge

Rational design of heterostructures opens up new opportunities as an ideal catalyst system for lithium polysulfides conversion in lithium-sulfur battery. However, its traditional fabrication process is complex, which makes it difficult to reasonably control the content and distribution of each component. In this work, to rationally design the heterostructure, the atomic layer deposition is utilized to hybridize the TiO₂-TiN heterostructure with the three-dimensional carbon nanotube sponge. Through optimizing the deposited thickness of TiO₂ and TiN layers and adopting the annealing post-treatment, the derived coaxial sponge with uniform TiN-TiO₂ heterostructure exhibits the best catalytic ability. The corresponding lithium-sulfur battery shows enhanced electrochemical performance with high specific capacity of 1289 mAh g⁻¹ at 1 C and capacity retention of 85% after 500 cycles at 2 C. Furthermore, benefiting from the highly porous structure and interconnected conductive pathways from the sponge, its areal capacity reaches up to 21.5 mAh cm⁻².

It is challenging to optimize catalytic heterostructures for lithium sulfur (Li-S) batteries. Here, authors prepare nanometer-scale TiN-TiO₂ heterostructures via atomic layer deposition on carbon nanotube sponge to realize stable Li-S batteries with high areal capacity and improved rate capability.

Strong adsorption, catalysis and lithiophilic modulation of carbon nitride for lithium/sulfur battery

Lithium/sulfur (Li/S) batteries have emerged as one of the most promising next-generation energy storage systems with advantages of high theoretical energy density, low cost and environmental friendliness. However, problems regarding to severe shuttle effect of soluble polysulfide, poor electronic/ionic conductor of solid charged/discharged products (S₈ and Li₂S), and fatal swell of volume along with the growth of Li dendrites greatly deteriorate the sulfur utilization and capacity retention during extended charge-discharge cycles. With advantages of high nitrogen content, lithiophilic modulation and tunable charge density and charge transfer, carbon nitride (g-C₃N₄) has played a positive role in restricting the shuttle effects and dendrite formation. This minireview mainly discusses these research achievements of g-C₃N₄ in Li/S batteries, aiming to provide a basic understanding and direct guidance for further research and development of functionalized g-C₃N₄ materials in electrical energy storage. The two-dimensional (2D) structure of g-C₃N₄ with abundant hierarchical pores improves its accommodation capacity for sulfur by effectively confining the lithium polysulfides (LiPSs) into the pores, and provides favorable channels for ion diffusion. The rich nitrogen and carbon defects further offer more active sites for strongly adsorbing LiPSs and bridge electron transfer pathway at atomic scale for catalytic reactions to accelerate redox kinetics of Li/S conversion chemistry. Moreover, the features of lithiophilic wettability, high adsorption energy and densely distributed lithiophilic N of g-C₃N₄ provide a large number of adhesive sites for lithium cation (Li⁺) and disperse the nucleation sites to enable uniform nucleation and deposition of Li on the anode surface and to suppress formation and growth of Li dendrites. Finally, the g-C₃N₄ also effectively regulates the wettability between Li anode and solid inorganic electrolyte, and reduces the crystallinity of solid polymer electrolyte to enhance the Li⁺ migration ability and ionic conductivity.

Strategy of Enhancing the Volumetric Energy Density for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries hold the promise of the next generation energy storage system beyond state-of-the-art lithium-ion batteries. Despite the attractive gravimetric energy density (W_G ), the volumetric energy density (W_V ) still remains a great challenge for the practical application, based on the primary requirement of Small and Light for Li-S batteries. This review highlights the importance of cathode density, sulfur content, electroactivity in achieving high energy densities. In the first part, key factors are analyzed in a model on negative/positive ratio, cathode design, and electrolyte/sulfur ratio, orientated toward energy densities of 700 Wh L^-1 /500 Wh kg^-1 . Subsequently, recent progresses on enhancing W_V for coin/pouch cells are reviewed primarily on cathode. Especially, the "Three High One Low" (THOL) (high sulfur fraction, high sulfur loading, high density host, and low electrolyte quantity) is proposed as a feasible strategy for achieving high W_V , taking high W_G into consideration simultaneously. Meanwhile, host materials with desired catalytic activity should be paid more attention for fabricating high performance cathode. In the last part, key engineering technologies on manipulating the cathode porosity/density are discussed, including calendering and dry electrode coating. Finally, a future outlook is provided for enhancing both W_V and W_G of the Li-S batteries.

Two-dimensional imine-based covalent–organic-framework derived nitrogen-doped porous carbon nanosheets for high-performance lithium–sulfur batteries

COF-derived nitrogen-doped porous carbon nanosheets with S loading achieve high capacities and good long-cycling performance for lithium–sulfur batteries.

Key issues facing electrospun carbon nanofibers in energy applications: on-going approaches and challenges

Electrospun carbon nanofibers (CNFs), with one-dimensional (1D) morphology, tunable size, mechanical flexibility, and functionalities by themselves and those that can be added onto them, have witnessed the intensive development and extensive applications in energy storage and conversion, such as supercapacitors, batteries, and fuel cells. However, conventional solid CNFs often suffer from a rather poor electrical conductivity and low specific surface area, compared with the graphene and carbon nanotube counterparts. A well-engineered porous structure in CNFs increases their surface areas and reactivity, but there is a delicate balance between the level and type of pores and mechanical robustness. In addition, CNFs by themselves often show unsatisfactory electrochemical performance in energy storage and conversion, where, to endow them with high and durable activity, one effective approach is to dope CNFs with certain heteroatoms. Up to now, various activation strategies have been proposed and some of them have demonstrated great success in addressing these key issues. In this review, we focus on the recent advances in the issue-oriented schemes for activating the electrospun CNFs in terms of enhancing the conductivity, modulating pore configuration, doping with heteroatoms, and reinforcing mechanical strength, in close reference to their applications in supercapacitors. The basic scientific principles involved in these activation processes and their effectiveness in boosting the electrochemical performance of CNFs are examined. Finally, some of the on-going challenges and future perspectives in engineering CNFs for better performance are highlighted.

Ultrafine nanosulfur particles sandwiched in little oxygen-functionalized graphene layers as cathodes for high rate and long-life lithium-sulfur batteries

Although lithium-sulfur batteries are one of the promising candidates for next-generation energy storage systems, the practical applications are still hampered by the poor cycle life, which can be attributed to the insulating properties of sulfur and the shuttle effect of electrochemical intermediate polysulfides. To address these problems, we synthesize sandwich-like composites which consist of ultrafine nanosulfur particles enveloped by little oxygen-functionalized graphene layers (F-GS@S). In this structure, the little oxygen-functionalized graphene backbone can not only accelerate the redox kinetics of sulfur species, but also eliminate the shuttle effect of polysulfides by strong chemical interaction. Moreover, the sandwich confinement structures can further inhibit the dissolution of polysulfides by physical restraint and accommodate the volume contraction/expansion of sulfur during cycling. As a result, the F-GS@S composites used as cathodes for lithium-sulfur batteries display a superior rate capability with the high capacities of 1208 mAh g^-1 at 0.1 C and 601.7 mAh g^-1 at 2 C and high cycling stability with a capacity retention of 70.5% after 500 cycles at 2 C. In situ characterizations and real-time monitoring experiments during the charge-discharge process are carried out to elucidate the reaction mechanism of the F-GS@S composites as cathodes for high rate and long-life lithium-sulfur batteries.

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.

A wet-processed, binder-free sulfur cathode integrated with a dual-functional separator for flexible Li-S batteries

Developing flexible, robust and lightweight sulfur cathodes by rationally designing their structures and configurations through a viable and scalable strategy is a critical enabler for fulfilling flexible lithium-sulfur (Li-S) batteries. However, besides the requirements for cathode flexibility, intrinsic limitations from the shuttling of lithium polysulfides and the growth of Li dendrites have restricted the widespread implementations of Li-S batteries. Here, we report a wet-processed strategy by dissolving and recrystallizing S in a suitable solvent to fabricate a flexible, binder-free S cathode. Integrating the resulting S cathode with a dual-functional separator has demonstrated to be able to suppress both the shuttle effect and growth of dendritic Li. The wet-processed strategy not only enables the fabrication of flexible and binder-free S-nanomat cathodes, but also facilitates the deposition of the cathodes on the separators. Meanwhile, a dual-functional separator is fabricated by vapor-phase polymerization of polypyrrole (PPy) coating on both surfaces of the commercial separator, which leads to the reduction of the shuttle effect and the suppression of the growth of dendritic Li simultaneously. As a result, by integrating the S-nanomat and the dual-functional separator, the cathode exhibits exceptional mechanical properties and electrochemical performance. Li-S pouch cells are further demonstrated to show stable cycling performance in the bending state, indicating the feasibility of the integrated S cathode for flexible Li-S batteries.

3D hybrid of Co9S8 and N-doped carbon hollow spheres as effective hosts for Li-S batteries

Lithium-sulfur (Li-S) batteries as a new generation of high energy batteries, with low cost and environmentally friendly, have a broad application prospects. While the poor conductivity of sulfur, the volume effect and 'shuttle effect' during charge and discharge, and slow redox kinetics of polysulfide intermediates still limit the practical application. To solve these problems, we synthesize a valid 3D hybrid material (Co₉S₈@N-CHS) of nanosized Co₉S₈ evenly distributed on N-doped carbon hollow spheres with strong chemical coupling by in situ carbonization of Co(NO₃)₂@resorcinol/formaldehyde and sulfidation. It presents a high electronic conductivity, absorbing chemical adsorption capability to polysulfides and can catalyze the sulfur redox processes. Compared with S/AC and S/N-CHS electrodes, S/Co₉S₈@N-CHS electrodes achieve an excellent initial discharge specific capacity of 1337 mAh g^-1 at 0.1 C and a long cycle life with an ultralow capacity decay of 0.027% per cycle over 1000 cycles at 1.0 C and the coulombic efficiency is above 99%. Consequently, it is an effective sulfur host material for high performance Li-S batteries.

Bifunctional semi-closed YF3-doped 1D carbon nanofibers with 3D porous network structure including fluorinating interphases and polysulfide confinement for lithium-sulfur batteries

In this study, semi-closed YF₃-doped 1D carbon nanofibers with 3D porous networks (SC-YF₃-doped 3D in 1D CNFs) are fabricated for the first time via electro-blown spinning technology. The internal 3D porous networks not only offer a stable 3D electrode structure to accommodate the volume expansion, but also enable a high sulfur loading (80%). More importantly, the external semi-enclosed carbon layer maintains outstanding conductivity and further blocks polysulfide diffusion, which significantly breaks the limitation of a traditional carbon matrix. On the other hand, the YF₃ nanoparticles are beneficial for forming more uniform fluorinating electrode interphases, achieving the excellent synergistic effect of chemical and physical adsorption to polysulfide. Therefore, the assembled Li-S batteries exhibit a high reversible discharge capacity of 954.2 mA h g^-1 with a decay of merely 0.043% per cycle after 600 cycles at 1C rate. Moreover, the discharge capacity decay can be as low as 0.029% per cycle during 800 cycles at a high current density of 2C rate. Even at a high rate of 5C, the cells still possess a favorable capacity of 636.5 mA h g^-1 while steadily operating for 700 cycles with a capacity decay rate of merely 0.056%, implying the great potential of this stable semi-closed cathode structure for industrialization.

Hierarchical porous Fe/N doped carbon nanofibers as host materials for high sulfur loading Li-S batteries

Improving the sulfur utilization and cycling stability especially under high sulfur loading (>2 mg cm-2) is challenging due to the poor conductivity of sulfur and high soluble nature of polysulfides. Herein, we develop novel self-supporting carbon nanofibers with hierarchical porous structures and Fe/N absorption/nucleation centers (Fe/N-HPCNF) as high performance sulfur hosts via a facile co-spinning method. The highly interior porous carbon fiber structure provides good electrolyte infiltration, stable conductive networks and sufficient surfaces for fast Li+/electron transport and sulfur redox while maintaining high sulfur area loading. In addition, the abundant Fe/N heteroatoms evenly dispersed in the fiber strongly restrain polysulfide diffusion through a chemisorption effect and meanwhile regulate homogeneous sulfur nucleation, enhancing the stability of cathodes. Consequently, S@Fe/N-HPCNF cathodes realize a high initial specific capacity of 1273 mA h g-1 (areal capacity: 4.5 mA h cm-2) and long cycle life over 500 cycles with a high sulfur loading of 3.5 mg cm-2. A stable capacity of 6.6 mA h cm-2 is achieved even under 9 mg cm-2 sulfur. What's more, a pouch cell prototype with an ultrahigh sulfur area density (54 mg cm-2) was assembled and successfully lighted 10 yellow light-emitting diodes (LED), demonstrating the convenient scale-up of our S@Fe/N-HPCNF cathodes.

Current Status and Future Prospects of Metal-Sulfur Batteries

Lithium-sulfur batteries are a major focus of academic and industrial energy-storage research due to their high theoretical energy density and the use of low-cost materials. The high energy density results from the conversion mechanism that lithium-sulfur cells utilize. The sulfur cathode, being naturally abundant and environmentally friendly, makes lithium-sulfur batteries a potential next-generation energy-storage technology. The current state of the research indicates that lithium-sulfur cells are now at the point of transitioning from laboratory-scale devices to a more practical energy-storage application. Based on similar electrochemical conversion reactions, the low-cost sulfur cathode can be coupled with a wide range of metallic anodes, such as sodium, potassium, magnesium, calcium, and aluminum. These new "metal-sulfur" systems exhibit great potential in either lowering the production cost or producing high energy density. Inspired by the rapid development of lithium-sulfur batteries and the prospect of metal-sulfur cells, here, over 450 research articles are summarized to analyze the research progress and explore the electrochemical characteristics, cell-assembly parameters, cell-testing conditions, and materials design. In addition to highlighting the current research progress, the possible future areas of research which are needed to bring conversion-type lithium-sulfur and other metal-sulfur batteries into the market are also discussed.

Freestanding Carbon Nanotube Film for Flexible Straplike Lithium/Sulfur Batteries

Flexible lithium/sulfur (Li/S) batteries are promising to meet the emerging power demand for flexible electronic devices. The key challenge for a flexible Li/S battery is to design a cathode with excellent electrochemical performance and mechanical flexibility. In this work, a flexible strap-like Li/S battery based on a S@carbon nanotube/Pt@carbon nanotube hybrid film cathode was designed. It delivers a specific capacity of 1145 mAh g^-1 at the first cycle and retains a specific capacity of 822 mAh g^-1 after 100 cycles. Moreover, the flexible Li/S battery retains stabile specific capacity and Coulombic efficiency even under severe bending conditions. As a demonstration of practical applications, an LED array is shown stably powered by the flexible Li/S battery under flattened and bent states. We also use the strap-like flexible Li/S battery as a real strap for a watch, which at the same time provides a reliable power supply to the watch.

Molten-Salt-Assisted Synthesis of Hierarchical Porous MnO@Biocarbon Composites as Promising Electrode Materials for Supercapacitors and Lithium-Ion Batteries

Biomass-derived carbon composites (e.g., metal oxide/biocarbon) have been used as promising electrode materials for energy storage devices owing to their natural abundance and simple preparation process. However, low loading content/inhomogeneous distribution of metal oxides and inefficient cracking of biocarbon (BC) are intractable obstacles that impede the efficient utilization of biomass. In this work, hierarchical porous MnO/BC composites were prepared by a facile molten-salt-assisted strategy based on the superior salt-water absorption ability of agaric. The addition of NaCl induces a liquid reaction medium by formation of a molten salt mixture at high temperature to effectively realize the activation and cracking of the bulk carbon, and it also acts as a recyclable sacrificial template to form mesopores and macropores in the as-prepared hierarchical porous MnO/BC composites. The highly porous and uniform BC framework effectively enhances ion diffusion and electron-transfer ability, serves as a protective layer to prevent fracturing and agglomeration of MnO, and thus enables superior rate performance and cycling stability of the MnO/BC composite for both supercapacitor electrodes (94 % capacity retention at 20 mA cm^-2 after 5000 cycles) and lithium-ion battery anodes (783 mA h g^-1 after 1000 cycles). Notably, considering the simple and low-cost preparation process, this work opens a promising avenue for the large-scale production of advanced metal oxide/BC hybrid electrode materials for electrochemical energy storage.

New Insights into the Anchoring Mechanism of Polysulfides inside Nanoporous Covalent Organic Frameworks for Lithium-Sulfur Batteries

The application prospects of lithium-sulfur (Li-S) batteries are constrained by many challenges, especially the shuttle effect of lithium polysulfides (Li₂S _x). Recently, microporous covalent organic framework (COF) materials have been used to anchor electrodes in Li-S batteries, because of their preferable characteristics, such as self-design ability, suitable pore size, and various active groups. To identify the ideal anchoring materials that can effectively restrain the shuttle of Li₂S _x species, the anchoring mechanism between COF materials and Li₂S _x species should be investigated in depth. Therefore, we systematically investigated the anchoring mechanism between specific COF nanomaterials (consisting of boron and oxygen atoms and benzene group) and Li₂S _x ( x = 1, 2, 4, 6, or 8) species on the surface and inside the pore using density functional theory methods with van der Waals interactions. The detailed analysis of the adsorption energy, difference charge density, charge transfer, and atomic density of states can be used to determine that the COF nanomaterials, with the structure of boroxine connecting to benzene groups and boroxine groups not constructed at the corner of the structure, can effectively anchor the Li₂S _x series. Accordingly, this study provides the theoretical basis for the molecular-scale design of ideal anchoring materials, which can be useful to improve the performance of the Li-S batteries.

Biomimetic Root-like TiN/C@S Nanofiber as a Freestanding Cathode with High Sulfur Loading for Lithium-Sulfur Batteries

It is a tough issue to achieve high electrochemical performance and high sulfur loading simultaneously, which is of important significance for practical Li-S batteries applications. Inspired by the transportation system of the plant root in nature, a biomimetic root-like carbon/titanium nitride (TiN/C) composite nanofiber is designed as a freestanding current collector for the high sulfur loading cathode. Like the plant root which absorbs water and oxygen from soil and transfers them to the trunk and branches, the root-like TiN/C matrix provides high-efficiency polysulfide, electron, and electrolyte transfer for the redox reactions via its three-dimensional-porous interconnected structure. In the meantime, TiN can not only anchor the polysulfides via the polar Ti-S and N-S bond but also further facilitate the redox reaction because of its high catalytic effect. With 4 mg cm^-2 sulfur loading, the TiN/C@S cathode delivers a high initial discharge capacity of 983 mA h g^-1 at 0.2 C current density; after 300 charge/discharge cycles, the discharge capacity remains 685 mA h g^-1, corresponding to a capacity decay rate of ∼0.1%. Even when the sulfur loading is increased to 10.5 mg cm^-2, the cell still delivers a high capacity of 790 mA h g^-1 and a decent cycle life. We believe that this novel biomimetic root-like structure can provide some inspiration for the rational structure design of the high-energy lithium-sulfur batteries and other composite electrode materials.

CoO/Co-Activated Porous Carbon Cloth Cathode for High Performance Li-S Batteries

Li-S batteries are one of the most promising candidates for the next generation of batteries because of their high theoretical specific capacity of 1675 mA h g^-1 . However, the dissolution of polysulfide causes severe capacity fading during cycling. Herein, a 3 D CoO/Co-activated free-standing porous carbon fiber cloth (CoO/Co@PCF) is designed to suppress polysulfide diffusion from the cathode. The metallic Co and polar CoO can anchor polysulfide through Co-S ionic bonding and the pores of the carbon cloth can provide abundant active sites for electrochemical reactions. Specifically, sulfur in the cathode is loaded by sulfur vapor deposition, which is scalable for fabrication and adjustable to the sulfur-loading amount. The CoO/Co@PCF cathode with sulfur loading of 3 mg cm^-2 displays high discharge capacities of 1214.2 and 684.3 mA h g^-1 at 0.1 and 2 C, respectively. When the sulfur loading is increased to 5.4 mg cm^-2 , low capacity fading of 0.075 % per cycle over 100 cycles at 0.5 C is still observed.

Structural Design of Lithium–Sulfur Batteries: From Fundamental Research to Practical Application

Abstract

Lithium–sulfur (Li–S) batteries have been considered as one of the most promising energy storage devices that have the potential to deliver energy densities that supersede that of state-of-the-art lithium ion batteries. Due to their high theoretical energy density and cost-effectiveness, Li–S batteries have received great attention and have made great progress in the last few years. However, the insurmountable gap between fundamental research and practical application is still a major stumbling block that has hindered the commercialization of Li–S batteries. This review provides insight from an engineering point of view to discuss the reasonable structural design and parameters for the application of Li–S batteries. Firstly, a systematic analysis of various parameters (sulfur loading, electrolyte/sulfur (E/S) ratio, discharge capacity, discharge voltage, Li excess percentage, sulfur content, etc.) that influence the gravimetric energy density, volumetric energy density and cost is investigated. Through comparing and analyzing the statistical information collected from recent Li–S publications to find the shortcomings of Li–S technology, we supply potential strategies aimed at addressing the major issues that are still needed to be overcome. Finally, potential future directions and prospects in the engineering of Li–S batteries are discussed.

Graphical Abstract