Separator-free Li-S thin-film battery with spin-coated S/CNT/SP cathode and PEO/PVDF/LTFSI/LLZO composite electrolyte

The advancement of miniaturized energy storage systems is essential for the next generation of electronics. Lithium-sulfur (Li-S) microbatteries are able to offer exceptional theoretical capacity and energy density for microdevices. However, their practical implementation is hindered by challenges in material stability and electrode design. In this study, we introduced a spin-coated sulfur-carbon nanotube-Super P (S/CNT/SP) cathode integrated with a spin-coated polyethylene oxide (PEO)/polyvinylidene fluoride (PVDF)/lithium lanthanum zirconium oxide (LLZO) composite electrolyte. The spin-coating technique ensured the formation of uniform electrode and electrolyte thin films, which could work without a separator. The polymer-ceramic composite electrolyte with nanopores effectively suppressed polysulfide dissolution, improved ionic conductivity, and stabilized the electrode-electrolyte interface. Electrochemical evaluation revealed that the quasi-solid-state Li-S battery achieved near-theoretical capacity with enhanced cycling stability, retaining approximately 1000 mA h g-1 (60% of its initial capacity) after 150 cycles across various C-rates. In a pouch-cell configuration, the cell retained 64% of its initial capacity over 60 cycles. These findings underscore the potential of spin-coating and composite quasi-solid electrolytes in enabling high-performance, safe, and compact Li-S battery technologies for next-generation energy storage applications.

Advanced Li-S Battery Configuration Featuring Sulfur-Coated Separator and Interwoven rGO/CNT Fabric Current Collector

The development of lithium-sulfur batteries (LSBs) marks a crucial milestone in advancing energy storage solutions essential for sustainable energy transitions. With high theoretical specific capacity, cost-effectiveness, and reduced ecological footprint, LSBs promise to enhance electric vehicle ranges, extend portable electronics' operational times, and stabilize grids integrated with renewable energy. However, challenges like complex processing, electrode instability, and poor cycling stability hinder their commercialization. This study introduces a novel battery design that addresses these issues by coating sulfur directly onto the separator instead of the current collector, demonstrating that active sulfur can be effectively utilized without being incorporated into the electrode structure. Using an interwoven substrate made from carbon nanotube (CNT) fabric adorned with reduced graphene oxide (rGO), this setup enhances manufacturing scalability, supports optimal sulfur utilization, and improves battery performance. The rGO decoration provides multiple highly conductive polysulfide trapping sites, enhancing active material reutilization, while the flexibility and mechanical strength of CNT fabric contribute to electrode integrity. This combination boosts electrical conductivity and polysulfide-capturing capability, effectively managing migrating sulfur species during charge-discharge cycles and mitigating sulfur loss and polysulfide shuttling. The results demonstrate superior cycling stability and efficiency, highlighting the potential of this approach in advancing LSB technology.

Progress on critical cell fabrication parameters and designs for advanced lithium-sulfur batteries

Since 1990, commercial lithium-ion batteries have made significant strides, approaching their theoretical performance limits, albeit with escalating costs. To address these challenges, attention has shifted toward lithium-sulfur batteries, which offer higher theoretical energy densities and cost-effectiveness. However, lithium-sulfur cells face challenges such as active-material loss, excessive electrolyte usage, and rapid degradation of lithium-metal anodes. To overcome these issues, research has focused on optimizing cell configurations and fabrication parameters while exploring novel electrolytes and electrode materials. This feature article delves into the intrinsic material challenges and extrinsic engineering issues in current lithium-sulfur research and explores the development of advanced lithium-sulfur cells with crucial progress on high-loading sulfur cathodes, lean-electrolyte cells, and solid-state electrolytes. Moreover, it outlines the fundamental principles, structures, performances, and developmental trajectories indicated in research articles published after 2020, highlighting future research directions aimed at resolving key challenges for the practical application of lithium-sulfur cells.

Advanced Polymers in Cathodes and Electrolytes for Lithium-Sulfur Batteries: Progress and Prospects

Lithium-sulfur (Li-S) batteries, which store energy through reversible redox reactions with multiple electron transfers, are seen as one of the promising energy storage systems of the future due to their outstanding advantages. However, the shuttle effect, volume expansion, low conductivity of sulfur cathodes, and uncontrollable dendrite phenomenon of the lithium anodes have hindered the further application of Li-S batteries. In order to solve the problems and clarify the electrochemical reaction mechanism, various types of materials, such as metal compounds and carbon materials, are used in Li-S batteries. Polymers, as a class of inexpensive, lightweight, and electrochemically stable materials, enable the construction of low-cost, high-specific capacity Li-S batteries. Moreover, polymers can be multifunctionalized by obtaining rich structures through molecular design, allowing them to be applied not only in cathodes, but also in binders and solid-state electrolytes to optimize electrochemical performance from multiple perspectives. The most widely used areas related to polymer applications in Li-S batteries, including cathodes and electrolytes, are selected for a comprehensive overview, and the relevant mechanisms of polymer action in different components are discussed. Finally, the prospects for the practical application of polymers in Li-S batteries are presented in terms of advanced characterization and mechanistic analysis.

A solid-state electrolyte for electrochemical lithium-sulfur cells

Post-lithium-ion batteries are designed to achieve high energy density and high safety by modifying their active material and cell configuration. In terms of the active material, lithium-sulfur batteries have the highest charge-storage capacity and high active-material utilization because of the use of a conversion-type sulfur cathode, which involves conversion between solid-state sulfur, liquid-state polysulfides, and solid-state sulfides. In terms of the configuration, solid-state batteries ensure high safety by using a solid-state electrolyte in between the two electrodes. Herein, we use a lithium lanthanum titanate (LLTO) solid-state electrolyte in the lithium-sulfur cell with a polysulfide catholyte electrode. The LLTO, which replaces the conventional liquid electrolyte, is a solid-state electrolyte that offers smooth lithium-ion diffusion and prevents the loss of polysulfides, while the highly active polysulfide electrode, which replaces the solid-state sulfur cathode, improves the reaction kinetics and the active-material utilization. The material and electrochemical analyses confirm the stabilized electrodes exhibit long-lasting lithium stripping/plating stability and limited polysulfide diffusion. Moreover, the morphologically and electrochemically smooth interface between the solid-state electrolyte and catholyte enables fast charge transfer in the cell, which demonstrates a high charge-storage capacity of 1429 mA h g-1, high rate performance, and high electrochemical efficiency.

Metal-organic-framework derived NiS2/C hollow structures for enhanced polysulfide redox kinetics in lithium-sulfur batteries

To cope with the shuttling of soluble lithium polysulfides in lithium-sulfur batteries, confinement tactics, such as trapping of sulfur within porous carbon structures, have been extensively studied. Although performance has improved a bit, the slow polysulfide conversion inducing fast capacity decay remains a big challenge. Herein, a NiS2/carbon (NiS2/C) composite with NiS2 nanoparticles embedded in a thin layer of carbon over the surface of micro-sized hollow structures has been prepared from Ni-metal-organic frameworks. These unique structures can physically entrap sulfur species and also influence their redox conversion kinetics. By improving the reaction kinetics of polysulfides, the NiS2/carbon@sulfur (NiS2/C@S) composite cathode with a suppressed shuttle effect shows a high columbic efficiency and decent rate performance. An initial capacity of 900 mAh g-1 at the rate of 1 C (1 C = 1675 mA g-1) and a low-capacity decline rate of 0.132% per cycle after 500 cycles are obtained, suggesting that this work provides a rational design of a sulfur cathode.

Recent advances and promise of MXene-based composites as electrode materials for sodium-ion and potassium-ion batteries

With the increasing demand for sustainable energy and concerns about the scarcity of lithium resources, sodium and potassium ion batteries have emerged as promising alternative energy storage technologies. MXene, as a novel two-dimensional material, possesses exceptional electrical conductivity, high surface area, and tunable structural features that make it an ideal candidate for high-performance electrode materials. However, its limited theoretical capacity hinders its widespread application. To overcome this limitation, MXene has been combined with other materials through synergistic effects between different components to enhance the overall electrochemical performance and expand its application in sodium/potassium ion batteries. Recently, substantial advancements have been realized in the exploration of MXene-based composites as energy storage materials, encompassing their synthesis, design, and the comprehension of charge storage mechanisms. This paper aims to propose a comprehensive summary of the latest developments in MXene-based composites as electrode materials for sodium ion batteries and potassium ion batteries, with a particular emphasis on the enhanced physicochemical properties resulting from composite formation. Moreover, the challenges faced by MXene materials in sodium ion batteries and potassium ion batteries are thoroughly discussed, and future research directions to further advance this field are proposed.

Recent Configurational Advances for Solid-State Lithium Batteries Featuring Conversion-Type Cathodes

Solid-state lithium metal batteries offer superior energy density, longer lifespan, and enhanced safety compared to traditional liquid-electrolyte batteries. Their development has the potential to revolutionize battery technology, including the creation of electric vehicles with extended ranges and smaller more efficient portable devices. The employment of metallic lithium as the negative electrode allows the use of Li-free positive electrode materials, expanding the range of cathode choices and increasing the diversity of solid-state battery design options. In this review, we present recent developments in the configuration of solid-state lithium batteries with conversion-type cathodes, which cannot be paired with conventional graphite or advanced silicon anodes due to the lack of active lithium. Recent advancements in electrode and cell configuration have resulted in significant improvements in solid-state batteries with chalcogen, chalcogenide, and halide cathodes, including improved energy density, better rate capability, longer cycle life, and other notable benefits. To fully leverage the benefits of lithium metal anodes in solid-state batteries, high-capacity conversion-type cathodes are necessary. While challenges remain in optimizing the interface between solid-state electrolytes and conversion-type cathodes, this area of research presents significant opportunities for the development of improved battery systems and will require continued efforts to overcome these challenges.

Nickel Foam Coated by Ni Nanoparticle-Decorated 3D Nanocarbons as a Freestanding Host for High-Performance Lithium-Sulfur Batteries

Nanocarbons (NCs) consisting of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) were coated on the surface of nickel foam (NF) via a chemical vapor deposition method. The CNFs formed conductive networks on NF, while the CNTs grew perpendicular to the surface of the CNFs, accompanied with the formation of Ni nanoparticles (Ni NPs) at the end of CNTs. The unique Ni-NCs-coated NF with a porous structure was applied as the three-dimensional (3D) current collector of lithium-sulfur (Li-S) batteries, which provided enough space to accommodate the electrode materials inside itself. Therefore, the 3D interconnected conductive framework of the coated NF collector merged in the electrode materials shortened the path of electron transport, and the generated Ni NPs could adsorb lithium polysulfides (LiPSs) and effectively accelerated the conversion kinetics of LiPSs as well, thereby suppressing the "shuttle effect". Moreover, the rigid framework of NF would also constrain the movement of the electrode compositions, which benefited the stability of the Li-S batteries. As a matter of fact, the Li-S battery based on the Ni-NCs-coated NF collector delivered an initial discharge capacity as high as 1472 mAh g^-1 at 0.1C and outstanding high rate capability at 3C (802 mAh g^-1). Additionally, low decay rates of 0.067 and 0.08% at 0.2C (300 cycles) and 0.5C (500 cycles) have been obtained, respectively. Overall, our prepared Ni-NCs-coated NF collector is promising for the application in high-performance Li-S batteries.

CuCo2S4 Nanoparticles Embedded in Carbon Nanotube Networks as Sulfur Hosts for High Performance Lithium-Sulfur Batteries

Rational design of sulfur hosts for lithium-sulfur (Li-S) batteries is essential to address the shuttle effect and accelerate reaction kinetics. Herein, the composites of bimetallic sulfide CuCo₂S₄ loaded on carbon nanotubes (CNTs) are prepared by hydrothermal method. By regulating the loading of CuCo₂S₄ nanoparticles, it is found that when Cu²⁺ and CNT are prepared in a 10:1 ratio, the CuCo₂S₄ nanoparticles loaded on the CNT are relatively uniformly distributed, avoiding the occurrence of agglomeration, which improves the electrical conductivity and number of active sites. Through a series of electrochemical performance tests, the S/CuCo₂S₄-1/CNT presents a discharge specific capacity of 1021 mAh g^-1 at 0.2 C after 100 cycles, showing good cycling stability. Even at 1 C, the S/CuCo₂S₄-1/CNT cathode delivers a discharge capacity of 627 mAh g^-1 after 500 cycles. This study offers a promising strategy for the design of bimetallic sulfide-based sulfur hosts in Li-S batteries.

Two-dimensional MXenes: recent emerging applications

MXenes, are a rapidly growing family of two-dimensional materials exhibiting outstanding electronic, optical, mechanical, and thermal properties with versatile transition metal and surface chemistries. A wide range of transition metals and surface termination groups facilitate the properties of MXenes to be easily tuneable. Due to the physically strong and environmentally stable nature of MXenes, they have already had a strong presence in different fields, for instance energy storage, electrocatalysis, water purification, and chemical sensing. Some of the newly discovered applications of MXenes showed very promising results, however, they have not been covered in any review article. Therefore, in this review we comprehensively review the recent advancements of MXenes in various potential fields including energy conversion and storage, wearable flexible electronic devices, chemical detection, and biomedical engineering. We have also presented some of the most exciting prospects by combining MXenes with other materials and forming mixed dimensional high performance heterostructures based novel electronic devices.

Design of advanced separators for high performance Li-S batteries using natural minerals with 1D to 3D microstructures

Lithium-sulfur (Li-S) batteries are of great interest due to their high energy density. However, polysulfides shuttle and low S loading severely impede their practical applications. Here, we report design of advanced separators for Li-S batteries using natural minerals with 1D to 3D microstructures. Four natural minerals with different microstructures including 1D halloysite nanotubes, 1D attapulgite nanorods, 2D Li⁺-montmorillonite (Mmt) nanosheets and 3D porous diatomite were used together with carbon black (CB) for preparation of the mineral/CB-Celgard separators. The Si-OH groups of the minerals act as Lewis acid sites, which could effectively absorb polysulfides by forming LiO and OS bonds with polysulfides. Among all the separators, the Mmt/CB-Celgard separator endowed the Li-S battery with the highest upper plateau discharge capacity (369 mA h g^-1), initial reversible capacity (1496 mA h g^-1 at 0.1 C), rate performance and cycling stability (666 mA h g^-1 after 500 cycles at 1.0 C with 0.046% capacity decay per cycle). The Mmt/CB-Celgard separator also enabled stable cycling of the Li-S battery with high S loading (8.3 mg cm^-2) cathode. This work will provide inspiration for future development of advanced separators for high-energy-density Li-S batteries.

Garnet solid-state electrolyte with benzenedithiolate catholyte for rechargeable lithium batteries

A solid-state electrolyte, Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO), is used to block the crossover of lithium 1,2-benzenedithiolate (LBDT) in the catholyte to the lithium metal anode in a rechargeable lithium battery. The cell maintains a relatively stable cycle life (65.6% capacity retention after 100 cycles). The density functional theory simulation confirms that LBDT has strong interaction with LLZTO, which is helpful for reducing the interfacial resistance.

La2MoO6 as an Effective Catalyst for the Cathode Reactions of Lithium-Sulfur Batteries

Lithium-sulfur batteries with high theoretical energy density have emerged as one of the most promising next-generation rechargeable batteries, while their discharge capacity and cycle stability are challenges mainly due to the shuttle effect of polysulfide intermediates. Employing an effective catalyst for the conversion of polysulfides in cathode reactions can promote the reaction kinetics to restrain the shuttle of polysulfides. Here, for the first time, La₂MoO₆ (LMO) as a catalyst is introduced into sulfur cathodes. To investigate the effect of La₂MoO₆, we prepare two different structures of La₂MoO₆/carbon nanofiber composites. One is carbon nanofiber-supported crystalline La₂MoO₆ nanoparticles (LMO@CNFs) and the other is amorphous La₂MoO₆ nanoparticles embedded in carbon nanofibers (LMO-in-CNFs). For sulfur electrodes with ∼73 wt % sulfur loading, LMO@CNFs/S and LMO-in-CNFs/S deliver initial gravimetric capacities of 1493.4 and 1246.7 mA h g^-1, respectively, at a 0.1C rate, obviously higher than that of the control sample CNFs/S. Moreover, LMO@CNFs/S shows much better rate performance than LMO-in-CNFs/S, indicating strongly that La₂MoO₆ is a highly effective catalyst to promote kinetic conversion of polysulfides.

An integrated flexible film as cathode for High-Performance Lithium-Sulfur battery

Poor cycling stability and low volumetric capacity of sulfur cathode prevents practical application of Lithium-sulfur (Li-S) batteries. Herein, we demonstrate a strategy to address the two drawbacks of sulfur cathode by synthesizing a compact and flexible film cathode with bilayer structure using a two-step vacuum filtration method. Two layers make up the sulfur cathode, active layer (sulfur-acethlene black (SC) spheres) and barrier layer (three dimensional MnO₂-graphene oxide-multi-walled carbon nanotubes (MnO₂-GO-CNTs) composites), which are integrated together by reduced graphene oxide (rGO) through self-binding. The rGO sheets provide an electrical conductive framework and a stable architecture to accommodate volume changes of sulfur. SC spheres stacked orderly between the rGO layers facilitate fast Li⁺ storage and energy release. Polar MnO₂-GO-CNTs composites with large specific surface area have not only afforded efficient sites for chemically binding polysulfides, but also provided fast electron transfer for accelerating polysulfides redox reaction. Consequently, the integrated film cathode exhibits an unprecedented cycling stability of ~0.0279% capacity decay per cycle over more than 600 cycles at 1C and high volumetric capacity of 1021.9 Ah L^-1 at 2C. Meanwhile, a foldable Li-S battery based on this flexible cathode is fabricated and shows excellent mechanical and electrochemical properties. The integrated flexible sulfur cathode of this study sheds light on the design strategies for application in flexible high volumetric capacity system.

Module-Designed Carbon-Coated Separators for High-Loading, High-Sulfur-Utilization Cathodes in Lithium-Sulfur Batteries

Lithium–sulfur batteries have great potential as next-generation energy-storage devices because of their high theoretical charge-storage capacity and the low cost of the sulfur cathode. To accelerate the development of lithium–sulfur technology, it is necessary to address the intrinsic material and extrinsic technological challenges brought about by the insulating active solid-state materials and the soluble active liquid-state materials. Herein, we report a systematic investigation of module-designed carbon-coated separators, where the carbon coating layer on the polypropylene membrane decreases the irreversible loss of dissolved polysulfides and increases the reaction kinetics of the high-loading sulfur cathode. Eight different conductive carbon coatings were considered to investigate how the materials’ characteristics contribute to the lithium–sulfur cell’s cathode performance. The cell with a nonporous-carbon-coated separator delivered an optimized peak capacity of 1112 mA∙h g⁻¹ at a cycling rate of C/10 and retained a high reversible capacity of 710 mA∙h g⁻¹ after 200 cycles under lean-electrolyte conditions. Moreover, we demonstrate the practical high specific capacity of the cathode and its commercial potential, achieving high sulfur loading and content of 4.0 mg cm⁻² and 70 wt%, respectively, and attaining high areal and gravimetric capacities of 4.45 mA∙h cm⁻² and 778 mA∙h g⁻¹, respectively.

New Insight into the Working Mechanism of Lithium-Sulfur Batteries under a Wide Temperature Range

Sweet potato-derived carbon with a unique solid core/porous layer core/shell structure is used as a conductive substrate for gradually immobilizing sulfur to construct a cathode for Li-S batteries. The first discharge specific capacity of the Li-S batteries with the C-10K@2S composite cathode at 0.1C is around 1645 mAh g^-1, which is very close to the theoretical specific capacity of active sulfur. Especially, after 175 cycles at 0.5C, the maintained specific discharge capacities of the C-10K@2S cathode at -20, 0, 25, and 40 °C are about 184.9, 687.2, 795.5, and 758.3 mAh g^-1, respectively, and the cathode is superior to most of the classical carbon form matrices. Working mechanisms of the cathodes under different temperatures are confirmed based on X-ray photoelectron spectroscopy (XPS) and in situ X-ray diffraction (XRD) characterizations. Distinctively, during the discharge stage, the widely proposed two-step cathodic reactions occur simultaneously rather than sequentially. In addition, the largely accelerated phase conversion efficiency of the cathode at a higher temperature (from room temperature to 40 °C) contributes to its enhanced charge/discharge specific capacity, while the byproduct Li₂S₂O₇ or Li₃N irreversibly formed during the cycles limits its application performance at 0 °C. These conclusions would be very significant and useful for designing cathodes for Li-S batteries with excellent wide working temperature performance.

Advanced Current Collectors with Carbon Nanofoams for Electrochemically Stable Lithium-Sulfur Cells

An inexpensive sulfur cathode with the highest possible charge storage capacity is attractive for the design of lithium-ion batteries with a high energy density and low cost. To promote existing lithium–sulfur battery technologies in the current energy storage market, it is critical to increase the electrochemical stability of the conversion-type sulfur cathode. Here, we present the adoption of a carbon nanofoam as an advanced current collector for the lithium–sulfur battery cathode. The carbon nanofoam has a conductive and tortuous network, which improves the conductivity of the sulfur cathode and reduces the loss of active material. The carbon nanofoam cathode thus enables the development of a high-loading sulfur cathode (4.8 mg cm⁻²) with a high discharge capacity that approaches 500 mA·h g⁻¹ at the C/10 rate and an excellent cycle stability that achieves 90% capacity retention over 100 cycles. After adopting such an optimal cathode configuration, we superficially coat the carbon nanofoam with graphene and molybdenum disulfide (MoS₂) to amplify the fast charge transfer and strong polysulfide-trapping capabilities, respectively. The highest charge storage capacity realized by the graphene-coated carbon nanofoam is 672 mA·h g⁻¹ at the C/10 rate. The MoS₂-coated carbon nanofoam features high electrochemical utilization attaining the high discharge capacity of 633 mA·h g⁻¹ at the C/10 rate and stable cyclability featuring a capacity retention approaching 90%.

Mn-N-C Nanostructure Derived from MnO2-x/PANI as Highly Performing Cathode Additive in Li-S Battery

Highly dispersed Mn metallic nanoparticles (15.87 nm on average) on a nitrogen-doped porous carbon matrix were prepared by thermal treatment of MnO2-x/polyaniline (PANI), which was derived from the in situ polymerization of aniline monomers initiated by γ-MnO2 nanosheets. Owing to the large surface area (1287 m2/g), abundant active sites, nitrogen dopants and highly dispersed Mn sites on graphitic carbon, an impressive specific capacity of 1319.4 mAh g−1 with an admirable rate performance was delivered in a Li-S battery. After 220 cycles at 1 C, 80.6% of the original capacity was retained, exhibiting a good cycling stability.

Electrosynthesis of 1,4-bis(diphenylphosphanyl) tetrasulfide via sulfur radical addition as cathode material for rechargeable lithium battery

Organic electrodes are promising as next generation energy storage materials originating from their enormous chemical diversity and electrochemical specificity. Although organic synthesis methods have been extended to a broad range, facile and selective methods are still needed to expose the corners of chemical space. Herein, we report the organopolysulfide, 1,4-bis(diphenylphosphanyl)tetrasulfide, which is synthesized by electrochemical oxidation of diphenyl dithiophosphinic acid featuring the cleavage of a P-S single bond and a sulfur radical addition reaction. Density functional theory proves that the external electric field triggers the intramolecular rearrangement of diphenyl dithiophosphinic acid through dehydrogenation and sulfur migration along the P-S bond axis. Impressively, the Li/bis(diphenylphosphanyl)tetrasulfide cell exhibits the high discharge voltage of 2.9 V and stable cycling performance of 500 cycles with the capacity retention of 74.8%. Detailed characterizations confirm the reversible lithiation/delithiation process. This work demonstrates that electrochemical synthesis offers the approach for the preparation of advanced functional materials.

Lean-electrolyte lithium-sulfur electrochemical cells with high-loading carbon nanotube/nanofiber-polysulfide cathodes

A carbon nanotube/nanofiber (CNT/CNF) composite is applied as a cathode substrate to develop a high-loading polysulfide cathode (8.64 mg cm^-2, 68 wt% sulfur). The high-loading CNT/CNF-polysulfide cathode demonstrates high energy densities (11.4-15.7 mW h cm^-2) and long-term cyclability (200 cycles) with low electrolyte-to-sulfur (E/S) ratios of 4-7 μL mg^-1.

Incorporation ZnS quantum dots into carbon nanotubes for high-performance lithium-sulfur batteries

Constructing sulfur hosts with high electronic conductivity, large void space, strong chemisorption, and rapid redox kinetics is critically important for their practical applications in lithium-sulfur batteries (LSBs). Herein, by coupling ZnS quantum dots (QDs) with carbon nanotubes (CNTs), one multifunctional sulfur host CNT/ZnS-QDs is designed via a facile one-step hydrothermal method. SEM and TEM analyses reveal that small ZnS-QDs (<5 nm) are uniformly anchored on the CNT surface as well as encapsulated into CNT channels. This special architecture ensures sulfur direct contacting with highly conductive CNTs; meanwhile, the catalytic effect of anchored ZnS-QDs improves the chemisorption and confinement to polysulfides. Benefiting from these merits, when used as sulfur hosts, this special architecture manifests a high specific capacity, superior rate capability, and long-term cycling stability. The ZnS-QDs dependent electrochemical performance is also evaluated by adjusting the mass ratio of ZnS-QDs, and the host of CNT/ZnS-QDs 27% owns the optimal cell performance. The specific capacity decreases from 1051 mAh g^-1 at 0.2 C to 544 mAh g^-1 at 2.0 C, showing rate capability much higher than CNT/S and other CNT/ZnS-QDs/S samples. After 150 cycles, the cyclic capacity at 0.5 C exhibits a slow reduction from 1051 mAh g^-1 to 771 mAh g^-1, showing a high retention of 73.4% with a coulombic efficiency of over 99%. The electrochemical impedance spectroscopy analyses demonstrate that this special architecture juggles high conductivity and excellent confinement of polysulfides, which can significantly suppress the notorious shuttle effect and accelerate the redox kinetics. The strategy in this study provides a feasible approach to design efficient sulfur hosts for realizing practically usable LSBs.

Spontaneously Splitting Copper Nanowires into Quantum Dots on Graphdiyne for Suppressing Lithium Dendrites

As an emerging carbon allotrope, the controllable growth of graphdiyne has been an important means to explore its unique scientific properties and applications. In this work, the effect of the crystal structure of copper (Cu) on the growth of graphdiyne is systematically studied. It is found that the crystal boundaries are the origin of the reaction activity. The polycrystalline Cu nanowire with many crystal boundaries is spontaneously split into Cu quantum dots (about 3 nm) by the grown graphdiyne. These Cu quantum dots are uniformly dispersed on the graphdiyne, and they block the long-range ordered growth of the graphdiyne. These Cu quantum dots in situ supported on graphdiyne demonstrate high efficiency in inhibiting the growth of lithium dendrites in lithium metal batteries. Based on this interesting finding, the Cu quantum dots anchored on the all-carbon graphdiyne can be prepared on a large scale, and unique applications of Cu quantum dots in electrochemical fields can be implemented.

Highly Graphitized Porous Carbon-FeNi3 Fabricated from Oleic Acid for Advanced Lithium-Sulfur Batteries

Improving the electrical conductivity of sulfur, suppressing shuttle/dissolution of polysulfide, and enhancing reaction kinetics in Li-S batteries are essential for practical applications. Here, for the first time, we have used inexpensive oleic acid as a single carbon source, and have added commercial SiO₂ as a template to form a porous structure, whereas introducing Fe(NO₃ )₃ and Ni(NO₃ )₂ as catalysts to increase the degree of graphitization. Moreover, the dual metal salts Fe(NO₃ )₃ and Ni(NO₃ )₂ can also form FeNi₃ alloy, and our results show that FeNi₃ nanoparticles accelerate the kinetic conversion reactions of polysulfide. By virtue of the well-developed porous structure and high degree of graphitization, the highly graphitized porous carbon-FeNi₃ (GPC-FeNi₃ ) has high conductivity to ensure fast charge transfer, and the hierarchically porous structure facilitates ion diffusion and traps polysulfide. Thus, a GPC-FeNi₃ /S cathode displays excellent electrochemical performance. At current rates of 0.2 and 1 C, a cathode of the GPC-FeNi₃ /S composite with a sulfur content of 70 % delivers high initial discharge capacities of 1108 and 880 mA h g^-1 , respectively, and retains reversible specific capacities of 850 mA h g^-1 after 200 cycles at 0.2 C and 625 mA h g^-1 after 400 cycles at 1 C.

Long Cycle Life Organic Polysulfide Catholyte for Rechargeable Lithium Batteries

Organic compounds with active sites for lithiation can be used as electrode materials for lithium batteries. Their tunable structures allow a variety of materials to be made and investigated. Herein, a spectrum of dipyridyl polysulfides (Py2S x , 3 ≤ x ≤ 8) is prepared in electrolyte by a one-pot synthesis method from dipyridyl disulfide (Py2S2) and elemental sulfur. It renders up to seven dipyridyl polysulfides (i.e., Py2S3, Py2S4, Py2S5, Py2S6, Py2S7, and Py2S8) which show fully reversible electrochemical behavior in lithium batteries. In the discharge, the initial lithiation occurs at 2.45 V leading to the breakage of Sα-Sβ bonds in Py2S x and formation of lithium 2-pyridinethiolate, in which lithium is coordinated in between N and S atoms. The left sulfur species act as elemental sulfur, showing two voltage plateaus at 2.3 and 2.1 V. The molecular dynamics simulations show the attraction between pyridyl groups and lithium polysulfides/sulfide via N···Li···S bonds, which enable good retention of soluble discharge products within electrodes and stable cycling performance. In the recharge, low-order Py2S x (e.g., Py2S3, Py2S4, and Py2S5) remain as the charged products. The mixture catholyte exhibits superlong cycle life at 1C rate with 1200 cycles and 70.5% capacity retention.

Enhanced Chemical Immobilization and Catalytic Conversion of Polysulfide Intermediates Using Metallic Mo Nanoclusters for High-Performance Li-S Batteries

Rechargeable lithium-sulfur batteries have attracted tremendous scientific attention owing to their high energy density. However, their practical application is greatly hindered by the notorious shuttling of soluble lithium polysulfide (LPS) intermediates with sluggish redox reactions and uncontrolled precipitation behavior. Herein, we report a semiliquid cathode composed of an active LPS solution/carbon nanofiber (CNF) composite layer, capped with a carbon nanotube (CNT) thin film decorated with metallic Mo nanoclusters that regulate the electrochemical redox reactions of LPS. The trace amount (0.05 mg cm^-2) of metallic Mo on the CNT film provides sufficient capturing centers for the chemical immobilization of LPS. Together with physical blocking of LPS by the compact CNT film, free diffusion of LPS is significantly restrained and the self-discharge behavior of the Li-S cell is thus effectively suppressed. Importantly, the metallic Mo nanoclusters enable fast catalytic conversion of LPS and regular deposition of lithium sulfide. As a result, the engineered cathode exhibits a high active sulfur utilization (1401 mAh g^-1 at 0.1 C), stable cycling (500 cycles at 1 C with 0.06% decay per cycle), high rate performance (694 mAh g^-1 at 5 C), and low self-discharge rate (3% after 72 h of rest). Moreover, a high reversible areal capacity of 4.75 mAh cm^-2 is maintained after 100 cycles at 0.2 C for a cathode with a high sulfur loading of 7.64 mg cm^-2. This work provides significant insight into the structural and materials design of an advanced sulfur-based cathode that effectively regulates the electrochemical reactions of sulfur species in high-energy Li-S batteries.

Cobalt atoms dispersed on hierarchical carbon nitride support as the cathode electrocatalyst for high-performance lithium-polysulfide batteries

Lithium-sulfur batteries are promising candidates for next-generation energy storage but are confronted with several challenges. One of the possible solutions is to design proper cathode electrocatalysts to accelerate the redox interconversion of solvated polysulfide intermediates. Herein, we report cobalt atoms dispersed on hierarchical carbon nitride support as an effective cathode electrocatalyst for lithium-polysulfide batteries. The electrocatalyst material is prepared from the simple reaction between melamine and cyanuric acid in the presence of Co²⁺, followed by the Ar annealing. The product has a unique hierarchical structure consisting of many thin and porous C₃N₄ nanosheets finely dispersed with Co atoms. The atomic dispersion of Co species is confirmed by X-ray absorption experiments. Electrochemical measurements reveal that it can promote the interconversion of polysulfides. As a result, batteries using this cathode electrocatalyst achieve large capacity (∼1400 mAh/g at 1.6 mA/cm²), good rate performance (∼800 mAh/g at 12.8 mA/cm²) and impressive cycling stability under different current densities and different sulfur loadings.

Revisiting the positive roles of liquid polysulfides in alkali metal-sulfur electrochemistry: from electrolyte additives to active catholyte

Polysulfide dissolution and shuttling in liquid organic electrolytes are considered as the most challenging detrimental effects of an Li-S cell, which is one of the most promising next-generation high-energy-density batteries. Therefore, considerable efforts have been devoted to confining solid sulfur or sulfide so as to avoid the formation and diffusion of dissolved polysulfides. However, the positive roles played by the liquid polysulfides in Li-S electrochemistry should not be overlooked. Polysulfide dissolution can promote the cell kinetics and sulfur utilization; as electrolyte additives, polysulfides can help stabilize the Li metal anode, redistribute the active mass in the cathode and act as extra back-up active sulfur sources. After being applied directly as active catholytes, a novel Li-polysulfide redox flow battery (Li-PS RFB) and an Li-polysulfide battery (Li-PS battery) have been developed. This review revisited these beneficial effects of polysulfides and provided a summary of the recent progress on Li-PS RFB and Li-PS batteries, especially with a more comprehensive emphasis on the latter. Furthermore, dissolved polysulfides applied as active catholytes in Na-S and K-S systems and as catholytes or anolytes in aqueous batteries were also briefly discussed. Hopefully, the Li-S electrochemistry can be better understood so as to overcome challenging issues in the way of the practical commercialization of the Li-S batteries.

Boosting High-Rate Li-S Batteries by an MOF-Derived Catalytic Electrode with a Layer-by-Layer Structure

Rechargeable high-energy lithium-sulfur batteries suffer from rapid capacity decay and poor rate capability due to intrinsically intermediate polysulfides' shuttle effect and sluggish redox kinetics. To tackle these problems simultaneously, a layer-by-layer electrode structure is designed, each layer of which consists of ultrafine CoS2-nanoparticle-embedded porous carbon evenly grown on both sides of reduced graphene oxide (rGO). The CoS2 nanoparticles derived from metal-organic frameworks (MOFs) have an average size of ≈10 nm and can facilitate the conversion between Li2S6 and Li2S2/Li2S in the liquid electrolyte by a catalytic effect, leading to improved polysulfide redox kinetics. In addition, the interconnected conductive frameworks with hierarchical pore structure afford fast ion and electron transport and provide sufficient space to confine polysulfides. As a result, the layer-by-layer electrodes exhibit good rate capabilities with 1180.7 and 700 mAh g-1 at 1.0 and 5.0 C, respectively, and maintain an impressive cycling stability with a low capacity decay of 0.033% per cycle within ultralong 1000 cycles at 5.0 C. Even with a high sulfur loading of 3.0 mg cm-2, the electrodes still show high rate performance and stable cycling stability over 300 cycles.

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.

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.

Stable Conversion Chemistry-Based Lithium Metal Batteries Enabled by Hierarchical Multifunctional Polymer Electrolytes with Near-Single Ion Conduction

The low Coulombic efficiency and serious safety issues resulting from uncontrollable dendrite growth have severely impeded the practical applications of lithium (Li) metal anodes. Herein we report a stable quasi-solid-state Li metal battery by employing a hierarchical multifunctional polymer electrolyte (HMPE). This hybrid electrolyte was fabricated via in situ copolymerizing lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethanesulfonyl)imide (LiMTFSI) and pentaerythritol tetraacrylate (PETEA) monomers in traditional liquid electrolyte, which is absorbed in a poly(3,3-dimethylacrylic acid lithium) (PDAALi)-coated glass fiber membrane. The well-designed HMPE simultaneously exhibits high ionic conductivity (2.24×10^-3  S cm^-1 at 25 °C), near-single ion conducting behavior (Li ion transference number of 0.75), good mechanical strength and remarkable suppression for Li dendrite growth. More intriguingly, the cation permselective HMPE efficiently prevents the migration of negatively charged iodine (I) species, which provides the as-developed Li-I batteries with high capacity and long cycling stability.

Moss-Derived Mesoporous Carbon as Bi-Functional Electrode Materials for Lithium⁻Sulfur Batteries and Supercapacitors

In this work, we reported a moss-derived biomass porous carbon (MPC) as a bi-functional electrode material for both the lithium–sulfur battery and the supercapacitor. The MPC was prepared from a high-temperature calcination procedure using the moss as the carbonaceous precursor. Using NaOH, the MPC was activated to give a mesoporous structure with a high specific surface area (1057.1 m² g⁻¹) and large pore volume (0.72 cm³ g⁻¹). When it was used as the cathode material in lithium–sulfur batteries, the MPC material realized a sulfur loading and exhibited a remarkably improved electrochemical performance, i.e., a high discharge capacity of 1070 mAh g⁻¹ at 0.1 C. This activated MPC also worked well as a capacitive electrode in supercapacitors, demonstrating a high specific capacitance of 332 F g⁻¹ (scan rate of 1.0 A g⁻¹) and a high capacity retention > 97% in a long-term cycle of 1000 charge/discharges. This work demonstrated a facile method for the utilization of activated waste biomass material for future clean energy applications.

Polyphenyl polysulfide: a new polymer cathode material for Li–S batteries

We report a new class of polyphenyl polysulfides synthesized by condensation reactions between 4,4′-thiobisbenzenethiol (TBBT) and sulfur with four different molar ratios in a toluene/carbon disulfide mixture at room temperature.

Applications of 2D MXenes in energy conversion and storage systems

This article provides a comprehensive review of MXene materials and their energy-related applications.

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.