Sulfur Encapsulated in Graphitic Carbon Nanocages for High-Rate and Long-Cycle Lithium-Sulfur Batteries

Hierarchical C/SiO x /TiO2 ultrathin nanobelts as anode materials for advanced lithium ion batteries

TiO₂-based nanomaterials are demonstrated to be a promising candidate for next generation lithium ion batteries due to their stable performance and easy preparation. However, their inherent low capacity impedes their wide application compared to commercial carbon nanomaterials. Here we present a unique in situ grafting-graphitization method to achieve a ternary nanocomposite of C/SiO _x /TiO₂ ultrathin nanobelts with a core-shell heterostructure. The obtained ternary nanocomposite integrates the merits of high specific capacity of SiO _x , the excellent mechanical stability of graphite-like carbon and the high reactivity of TiO₂. Cyclic voltammetric curves and cycling performance manifest the optimal ternary nanocomposite and deliver a very high initial specific capacity of ∼1196 mA h g^-1 with both good rate capability (∼200 mA h g^-1 up to 10 C) and especially enhanced cycle stability. Our work demonstrates that building hierarchical core-shell heterostructures is an effective strategy to improve capacity and cycling performance in other composite anodes for electrochemical energy storage materials.

Novel Non-Carbon Sulfur Hosts Based on Strong Chemisorption for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered as promising candidates for energy storage systems owing to their high theoretical capacity and high energy density. The application of Li-S batteries is hindered by several obstacles, however, including the shuttle effect, poor electrical conductivity, and the severe volume expansion of sulfur. The traditional method is to integrate sulfur with carbon materials. But the interaction between polysulfide intermediates and carbon is only weak physical adsorption, which easily leads to the escape of species from the framework (shuttle effect) of the material causing capacity loss. Recently, however, there has been a trend for the introduction of novel non-carbon materials as sulfur hosts based on the strong chemisorption. This review highlights recent research progress on novel non-carbon sulfur hosts based on strong chemisorption, in Li-S batteries. In comparison with carbon-based sulfur hosts, most non-carbon sulfur hosts have been demonstrated to be polar host materials that could efficiently adsorb polysulfide via strong chemisorption, mitigating their dissolution. The intrinsic mechanism associated with the role of non-carbon-based host materials in improving the performance of Li-S batteries is discussed.

Rationally designing S/Ti3C2Tx as a cathode material with an interlayer for high-rate and long-cycle lithium-sulfur batteries

Lithium-sulfur batteries suffer from poor cycling stability and inferior rate capability, mainly caused by low conductivity and lithium polysulfide dissolution. To tackle these problems, this work demonstrates that Ti3C2Tx "clay", synthesized by selectively extracting the Al layers from the Ti3AlC2 phases with a mixture of HCl and LiF, is an effective host material for sulfur cathodes. To further enhance the rate performance and cycling stability of S/Ti3C2Tx composites, a single-walled carbon nanotube thin film was prepared by a simple vacuum filtration method and inserted between the cathode and the separator as an interlayer for Li-S batteries. The S/Ti3C2Tx composite with an interlayer could deliver a high initial discharge capacity of 1458 mA h g-1 at a current density of 0.1 A g-1 and an ultralow capacity decay of 0.04% per cycle at 0.8 A g-1 for over 1500 cycles was achieved. More importantly, a reversible capacity of 608 mA h g-1 was obtained at a high current density of 8.2 A g-1 (≈5C), demonstrating superior rate capability. These results suggest that the S/Ti3C2Tx composite is a promising sulfur cathode material and the introduction of the interlayer will pave the way for the future development and design of high-rate with long-cycle Li-S batteries.

Systematic Exploration of the Role of a Modified Layer on the Separator in the Electrochemistry of Lithium-Sulfur Batteries

As a significant constituent of lithium-sulfur batteries, the separator also exerts a considerable effect on the performance of the sulfur cathode. In our work, the mixture of acetylene black and multiwalled carbon nanotubes is uniformly applied onto the commercial polypropylene membranes to attain the modified separators. While investigating different samples, the underlying influence of the coating layer is systematically scrutinized on the electrochemical behaviors of sulfur cathodes, relying on the extensive electrochemical and structural measurements. During the charge/discharge process, the coating layer can function as the second current collector and drastically contribute to the improved lithium ions diffusion, determining the electrochemical kinetics of sulfur-involved reactions. Moreover, it is found that the thicker the layer, the faster the lithium ions diffuse. It should be noted that the coating layer also plays a role of the second sulfur reservoir to endow the ample active sites for sulfur and polysulfides, which is directly witnessed for the first time. Because of the positiveness and effectiveness of the modified layer on separators, the sulfur cathode can offer the superior cycling and rate performance to that with the original separator.

Blocking Polysulfides and Facilitating Lithium-Ion Transport: Polystyrene Sulfonate@HKUST-1 Membrane for Lithium-Sulfur Batteries

Minimizing the shuttle effect of polysulfides (PS) is crucial for practical applications of lithium-sulfur (Li-S) batteries. However, the trade-off between effective suppression of the shuttle effect and fast redox reaction kinetics is inevitable for separator-based Li-S batteries. Herein, via a self-confined solid-conversion process, we develop a polystyrene sulfonate (PSS)-threaded well-intergrown HKUST-1 (Cu₃(BTC)₂) (BTC: 1,3,5-benzenetricarboxylic acid)-coated Celgard separator (PSS@HKUST-1/Celgard, PHC) for high-performance Li-S batteries. The PHC membrane favors the interception and accommodation of long-chain PS. Notably, enormous sulfonate groups of the three-dimensional PSS networks in PSS@HKUST-1 membrane significantly facilitate lithium-ion transport, which guarantee fast redox kinetics. The PHC separator demonstrates efficient inhibition of PS (i.e., 4 orders of magnitude lower in PS permeation rate) with fast Li⁺ transportation (i.e., 71% higher in ionic conductivity) than the Celgard separator. When applying the PHC membrane in Li-S batteries with conventional sulfur/super P carbon cathode, highly reversible capacity with an average fading rate of 0.05% per cycle is maintained for 500 cycles at 0.5 C, excellent rate performance up to 5 C, and high areal capacity over 7 mA h cm^-2 are also achieved. This work paves a new way for addressing the trade-off between suppressing the PS shuttle effect and fast kinetic reaction for separator-based Li-S batteries.

Simple method to construct three-dimensional porous carbon for electrochemical energy storage

A 3D porous carbon matrix with a high nitrogen content has been synthesized by employing particles of a nitrogen-enriched superabsorbent polymer (SAP) from the waste diapers of newborn babies. The derived material exhibits an ultrathin layered structure with interconnected pores and a large specific surface area. As it inherits the unique skeleton of the functional polymer from waste diapers, the resulting material (NSAPC-W) has been assessed as an inserting host anode with excellent ultralong cycling performance, as well as steady rate capability for both Li+ and Na+ ions in half cells. Furthermore, the unique structure imparts intimate structural interconnectivity, wide open channels for ion diffusion, and a large accessible surface area, as well as high structural stability, and opens up a wide horizon for electrochemical applications.

Self-Assembled Close-Packed MnO2 Nanoparticles Anchored on a Polyethylene Separator for Lithium-Sulfur Batteries

Separator modification has been proved to be an effective approach for overcoming lithium polysulfide (LiPS) shuttling in lithium-sulfur (Li-S) cells. However, the weight and stability of the modified layer also affect the cycling properties and the energy density of Li-S cells. Here, we initially construct an ultrathin and lightweight MnO₂ layer (380 nm, 0.014 mg cm^-2) on a commercial polyethylene (PE) separator (MnO₂@PE) through a chemical self-assembly method. This MnO₂ layer is tightly anchored onto the PE substrate because of the presence of oxygen-containing groups that form a relatively strong chemical force between the MnO₂ nanoparticles and the PE substrate. In addition, the mechanical strength of the separator is not affected by this modification procedure. Moreover, because of the catalytic effect and compactness of the MnO₂ layer, the MnO₂@PE separator can greatly suppress LiPS shuttling. As a result, the application of this MnO₂@PE separator in Li-S batteries leads to high reversible capacity and superior cycling stability. This strategy provides a novel approach to separator surface modification.

Three-dimensional porous microspheres comprising hollow Fe2O3 nanorods/CNT building blocks with superior electrochemical performance for lithium ion batteries

It is highly desirable to develop anode materials with rational architectures for lithium ion batteries to achieve high-performance electrochemical properties. In this study, three-dimensional porous composite microspheres comprising hollow Fe2O3 nanorods/carbon nanotube (CNT) building blocks are successfully constructed by direct deposition and further thermal transformation of beta-FeOOH nanorods on CNT porous microspheres. The CNT porous microsphere, which is prepared by a spray pyrolysis, provides ample sites for the direct growth of beta-FeOOH nanorods. During the further oxidation process, the beta-FeOOH nanorods are transformed into hollow Fe2O3 nanorods as a result of dehydroxylation and lattice shrinkage, resulting in the formation of hollow Fe2O3 nanorods/CNT porous microspheres. Such a hierarchical structure of composite microspheres not only facilitates electrolyte accessibility but also offers conductive networks for electrons during electrochemical reactions. Accordingly, the electrodes exhibit a high discharge capacity of 1307 mA h g-1 after 300 cycles at a current density of 1 A g-1; this is associated with an excellent capacity retention of 84%, which is calculated from the initial cycle. In addition, the composite delivers a discharge capacity of 703 mA h g-1 at a current density of 15 A g-1.

Three-dimensional sp2 carbon networks prepared by ultrahigh temperature treatment for ultrafast lithium-sulfur batteries

The current challenge in the development of high-performance lithium-sulfur (Li-S) batteries is to facilitate the redox kinetics of sulfur species as well as to suppress the shuttle effect of polysulfides, especially at high current rates. Herein, aiming the application of Li-S at high current rates, we coupled a sp2 carbon configuration consisting of 3D carbon nanotubes/graphene prepared by ultrahigh temperature treatment (2850 °C) with S (2850CNTs-Gra-S) for application in Li-S batteries. The 2850CNTs-Gra as the host material exhibits a nearly perfect sp2 hybridized structure because ultrahigh temperature treatment not only repairs the raw defects in CNTs and graphene, but it also forms new sp2 C-C bonds between them. The 3D sp2 carbon network ensures ultrafast ion/electron transfer and efficient heat dissipation to protect the integrity of the separator when the Li-S battery is running at an ultrahigh rate. Based on these unique advantages, the 2850CNTs-Gra-S cathode shows a high current rate performance. Critically, it still delivers a considerable specific capacity after 1500 cycles even at a current rate of 15C and exhibits an extremely low capacity degradation rate of 0.0087% per cycle.

Binary Hierarchical Porous Graphene/Pyrolytic Carbon Nanocomposite Matrix Loaded with Sulfur as a High-Performance Li-S Battery Cathode

A N,O-codoped hierarchical porous nanocomposite consisting of binary reduced graphene oxide and pyrolytic carbon (rGO/PC) from chitosan is fabricated. The optimized rGO/PC possesses micropores with size distribution concentrated around 1.1 nm and plenty of meso/macropores. The Brunauer-Emmett-Teller specific surface area is 480.8 m² g^-1, and it possesses impressively large pore volume of 2.14 cm³ g^-1. On the basis of the synergistic effects of the following main factors: (i) the confined space effect in the hierarchical porous binary carbonaceous matrix; (ii) the anchor effects by strong chemical bonds with codoped N and O atoms; and (iii) the good flexibility and conductivity of rGO, the rGO/PC/S holding 75 wt % S exhibits high performance as Li-S battery cathode. Specific capacity of 1625 mA h g^-1 can be delivered at 0.1 C (1 C = 1675 mA g^-1), whereas 848 mA h g^-1 can be maintained after 300 cycles at 1 C. Even at high rate of 5 C, 412 mA h g^-1 can be restrained after 1000 cycles.

Porphyrin Organic Framework Hollow Spheres and Their Applications in Lithium-Sulfur Batteries

Organic frameworks represent an emerging family of advanced materials that can be precisely controlled at the atomic level. However, morphology control of organic frameworks remains perplexing and difficult, strongly limiting the advantages of organic frameworks in multiple practical applications. Herein, porphyrin organic framework hollow spheres (POF-HSs) are fabricated by a template method as a proof of concept of organic frameworks with precisely controlled morphology. POF-HS exhibits explicit chemical structures of 2D POF and an expected hollow structure. The morphology of POF-HS is further regulated in terms of void size and shell thickness. Benefited from the polar chemical structures and the hollow spherical morphology, POF-HS sufficiently mitigates the shuttle of polysulfides by taking the dual effects of chemical adsorption and physical confinement and functions as a desirable host material for sulfur cathode to endow lithium-sulfur batteries with high capacity, long cycling life, and excellent rate performance. The accurate synthesis of POF-HSs demonstrates the highly controllable and versatile morphology of organic framework materials beyond precise integration of organic building blocks and represents infinite possibility of offering exotic organic frameworks for chemistry, sustainable energy, and material science.

Synergistically Enhanced Interfacial Interaction to Polysulfide via N,O Dual-Doped Highly Porous Carbon Microrods for Advanced Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have received tremendous attention because of their extremely high theoretical capacity (1672 mA h g^-1) and energy density (2600 W h kg^-1). Nevertheless, the commercialization of Li-S batteries has been blocked by the shuttle effect of lithium polysulfide intermediates, the insulating nature of sulfur, and the volume expansion during cycling. Here, hierarchical porous N,O dual-doped carbon microrods (NOCMs) were developed as sulfur host materials with a large pore volume (1.5 cm³ g^-1) and a high surface area (1147 m² g^-1). The highly porous structure of the NOCMs can act as a physical barrier to lithium polysulfides, while N and O functional groups enhance the interfacial interaction to trap lithium polysulfides, permitting a high loading amount of sulfur (79-90 wt % in the composite). Benefiting from the physical and chemical anchoring effect to prevent shuttling of polysulfides, S@NOCMs composites successfully solve the problems of low sulfur utilization and fast capacity fade and exhibit a stable reversible capacity of 1071 mA h g^-1 after 160 cycles with nearly 100% Coulombic efficiency at 0.2 C. The N,O dual doping treatment to porous carbon microrods paves a way toward rational design of high-performance Li-S cathodes with high energy density.

Synergetic Effects of Multifunctional Composites with More Efficient Polysulfide Immobilization and Ultrahigh Sulfur Content in Lithium-Sulfur Batteries

A high sulfur loading cathode is the most crucial component for lithium-sulfur batteries (LSBs) to obtain considerable energy density for commercialization applications. The major challenges associated with high sulfur loading electrodes are poor material utilization caused via the nonconductivity of the charged product (S) and the discharged product (Li₂S), poor stability arisen from dissolution of lithium polysulfides (LiPSs) into most organic electrolytes and pulverization, and structural damage of the electrode caused by large volumetric expansion. A multifunctional synergistic composite enables ultrahigh sulfur content for advanced LSBs, which comprises the sulfur particle encapsulated with an ion-selective polymer with conductive carbon nanotubes and dispersed around Magnéli phase Ti₄O₇ (MS-3) by the bottom-up method. The ion-selective polymer provides a physical shield and electrostatic repulsion against the shuttling of polysulfides with negative charge, whereas it can permit the transmission of lithium ion (Li⁺) through the polymer membrane, and the carbon nanotubes twined around the sulfur promote electronic conductivity and sulfur utilization as well as strong chemical adsorption of LiPSs by means of Ti₄O₇. Because of this hierarchical construction, the cathode possesses a lofty final sulfur loading of 72% and large sulfur areal mass loading of 3.56 mg cm^-2, which displays the large areal specific capacity of 4.22 mA h cm^-2. In the same time, it can provide excellent cyclic performance with the corresponding capacity attenuation ratio of 0.08% per cycle at 0.5 C after 300 cycles. Especially, while sulfur areal mass loading is sharply enhanced to 5.11 mg cm^-2, the MS-3 composite exhibits a large initial areal capacity of 5.04 mA h cm^-2 and still keeps a high reversible capacity of 696 mA h g^-1 at 300th cycle even at a 1.0 C. The design of high sulfur content cathodes is a viable approach for boosting practical commercialized application of LSBs.

Cobalt-Doped Vanadium Nitride Yolk-Shell Nanospheres @ Carbon with Physical and Chemical Synergistic Effects for Advanced Li-S Batteries

Lithium-sulfur (Li-S) battery has been attracting increasing attention because of its high energy density and the presence of abundance of sulfur. However, its commercialization is still restricted owing to the low conductivity of sulfur, large volume expansion, and a severe polysulfide-shuttle effect. To address these problems, here, we have reported for the first time a simple template-free solvothermal method combined with a subsequent calcination method to prepare cobalt-doped vanadium nitride (VN) yolk-shell nanospheres, encapsulated in a thin layer of a nitrogen-doped carbon (Co-VN@C) composite as an ideal sulfur host. Benefiting from the unique structural advantages and the synergistic effect of conductive VN, cobalt, and nitrogen-doped carbon (NC), the obtained composite could not only facilitate the kinetics of polysulfide conversion as a functional catalyst but also physically confine and chemically absorb the polysulfides effectively. With these advantages, the batteries present a high initial discharge capacity of 1379.2 mAh g^-1 at 0.1 C (1 C is defined as 1675 mA g^-1), good rate performance, and excellent cycling performances (∼715 mAh g^-1 at 0.5 C after 200 cycles and ∼600 mAh g^-1 at 1 C after 300 cycles, respectively), even with a high areal sulfur loading of 4.07 mg cm^-2 (∼830 mAh g^-1 at 0.2 C after 100 cycles). These results demonstrate that the rationally designed multifunctional sulfur host material Co-VN@C has great potential for application in Li-S batteries.

High-Level Heteroatom Doped Two-Dimensional Carbon Architectures for Highly Efficient Lithium-Ion Storage

In this work, high-level heteroatom doped two-dimensional hierarchical carbon architectures (H-2D-HCA) are developed for highly efficient Li-ion storage applications. The achieved H-2D-HCA possesses a hierarchical 2D morphology consisting of tiny carbon nanosheets vertically grown on carbon nanoplates and containing a hierarchical porosity with multiscale pore size. More importantly, the H-2D-HCA shows abundant heteroatom functionality, with sulfur (S) doping of 0.9% and nitrogen (N) doping of as high as 15.5%, in which the electrochemically active N accounts for 84% of total N heteroatoms. In addition, the H-2D-HCA also has an expanded interlayer distance of 0.368 nm. When used as lithium-ion battery anodes, it shows excellent Li-ion storage performance. Even at a high current density of 5 A g^-1, it still delivers a high discharge capacity of 329 mA h g^-1 after 1,000 cycles. First principle calculations verifies that such unique microstructure characteristics and high-level heteroatom doping nature can enhance Li adsorption stability, electronic conductivity and Li diffusion mobility of carbon nanomaterials. Therefore, the H-2D-HCA could be promising candidates for next-generation LIB anodes.

A Universal Method to Engineer Metal Oxide-Metal-Carbon Interface for Highly Efficient Oxygen Reduction

Oxygen is the most abundant element in the Earth's crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes and energy converting/storage systems. Developing techniques toward high-efficiency ORR remains highly desired and a challenge. Here, we report a N-doped carbon (NC) encapsulated CeO₂/Co interfacial hollow structure (CeO₂-Co-NC) via a generalized strategy for largely increased oxygen species adsorption and improved ORR activities. First, the metallic Co nanoparticles not only provide high conductivity but also serve as electron donors to largely create oxygen vacancies in CeO₂. Second, the outer carbon layer can effectively protect cobalt from oxidation and dissociation in alkaline media and as well imparts its higher ORR activity. In the meanwhile, the electronic interactions between CeO₂ and Co in the CeO₂/Co interface are unveiled theoretically by density functional theory calculations to justify the increased oxygen absorption for ORR activity improvement. The reported CeO₂-Co-NC hollow nanospheres not only exhibit decent ORR performance with a high onset potential (922 mV vs RHE), half-wave potential (797 mV vs RHE), and small Tafel slope (60 mV dec^-1) comparable to those of the state-of-the-art Pt/C catalysts but also possess long-term stability with a negative shift of only 7 mV of the half-wave potential after 2000 cycles and strong tolerance against methanol. This work represents a solid step toward high-efficient oxygen reduction.

Hierarchical N-Rich Carbon Sponge with Excellent Cycling Performance for Lithium-Sulfur Battery at High Rates

Lithium-sulfur batteries (LSBs) are receiving extensive attention because of their high theoretical energy density. However, practical applications of LSBs are still hindered by their rapid capacity decay and short cycle life, especially at high rates. Herein, a highly N-doped (≈13.42 at %) hierarchical carbon sponge (HNCS) with strong chemical adsorption for lithium polysulfide is fabricated through a simple sol-gel route followed by carbonization. Upon using the HNCS as the sulfur host material in the cathode and an HNCS-coated separator, the battery delivers an excellent cycling stability with high specific capacities of 424 and 326 mA h g^-1 and low capacity fading rates of 0.033 % and 0.030 % per cycle after 1000 cycles under high rates of 5 and 10 C, respectively, which are superior to those of other reported carbonaceous materials. These impressive cycling performances indicate that such a battery could promote the practical application prospects of LSBs.

Green Synthesis of Hierarchically Porous Carbon Nanotubes as Advanced Materials for High-Efficient Energy Storage

Hierarchically porous carbon nanomaterials with well-defined architecture can afford a promising platform for effectively addressing energy and environmental concerns. Herein, a totally green and straightforward synthesis strategy for the fabrication of hierarchically porous carbon nanotubes (HPCNTs) by a simple carbonization treatment without any assistance of soft/hard templates and activation procedures is demonstrated. A high specific surface area of 1419 m² g^-1 and hierarchical micro-/meso-/macroporosity can be achieved for the HPCNTs. The unique porous architecture enables the HPCNTs serving as excellent electrode/host materials for high-performance supercapacitors and Li-sulfur batteries. The design strategy may pave a new avenue for the rational synthesis of hierarchically porous carbon nanostructures for high-efficient energy storage applications.

Improving the cycling stability of lithium-sulfur batteries by hollow dual-shell coating

Herein, a novel hybrid S@MnO2@C nanosphere, comprising sulfur nanoparticles encapsulated by a MnO2@C hollow dual-shell, is reported. Benefiting from a conductive C outer layer, the S@MnO2@C hybrid nanosphere provided highly efficient pathways for fast electron/ion transfer and sufficient free space for the expansion of the encapsulated sulfur nanoparticles. Moreover, the dual-shell composed of a MnO2 inner layer and a C outer layer coating on S not only improved the efficacious encapsulation of sulfur, but also significantly suppressed the dissolution of polysulfides during cycling. As a result, the S@MnO2@C electrode shows high capacity, high coulombic efficiency and excellent cycling stability. The S@MnO2@C cathode delivered a discharge capacity of 593 mA h g-1 in the fourth cycle and was able to maintain 573 mA h g-1 after 100 charge-discharge cycles at 1.0C, corresponding to a capacity retention of 96.6%.

In situ sulfur loading in graphene-like nano-cell by template-free method for Li-S batteries

Carbon nanomaterials with 3D structures as sulfur hosts have been widely developed in lithium-sulfur batteries because of their high specific surface area, high conductivity and structural stability. However, sulfur, loaded by melting-diffusion method, is usually attached to the outside surface of carbon host, resulting in weak adsorption to expose polysulfide. Herein, we report a template-free method for synthesizing graphene-like nano-cell (GLC) with high in situ sulfur loading (S@GLC). The GLC is expected to provide physical adsorption by enclosed graphene cell architecture and chemical adsorption by pyridinic N-doping and oxygen functional group. With these merits, the S@GLC cathode owned high sulfur content (72%) and also, it exhibited a reversible specific capacity of 1253 mA h g^-1 at 0.2C, excellent rate performance, and long cycling stability (502 mA h g^-1 after 400 cycles at 1C).

How to make inert boron nitride nanosheets active for the immobilization of polysulfides for lithium-sulfur batteries: a computational study

The search for effective anchoring nanomaterials for the immobilization of soluble lithium polysulfide (Li₂S_n) species to suppress their shuttling effect has been a key scientific issue for the large-scale practical application of lithium-sulfur (Li-S) batteries. In this work, by means of comprehensive density functional theory (DFT) computations, we systematically investigated the potential of a series of doped and defective boron nitride (BN) nanosheets as chemical immobilizers for the soluble Li₂S_n species. Our results revealed that the introduction of dopants and defects can enhance the binding strength of Li₂S_n species with BN nanosheets due to the strong LiN or SB interaction. In particular, the doped BN nanosheets that can moderately interact with Li₂S_n species are shown to exhibit outstanding anchoring effects for Li-S batteries because they can keep a balance between the binding strength and integrity of Li₂S_n species. Therefore, by carefully controlling the type of dopants, the inert BN nanosheet can be converted to quite a promising electrode material with high efficiency for Li-S batteries.

Spherical Macroporous Carbon Nanotube Particles with Ultrahigh Sulfur Loading for Lithium-Sulfur Battery Cathodes

A carbon host capable of effective and uniform sulfur loading is the key for lithium-sulfur batteries (LSBs). Despite the application of porous carbon materials of various morphologies, the carbon hosts capable of uniformly impregnating highly active sulfur is still challenging. To address this issue, we demonstrate a hierarchical pore-structured CNT particle host containing spherical macropores of several hundred nanometers. The macropore CNT particles (M-CNTPs) are prepared by drying the aerosol droplets in which CNTs and polymer particles are dispersed. The spherical macropore greatly improves the penetration of sulfur into the carbon host in the melt diffusion of sulfur. In addition, the formation of macropores greatly develops the volume of the micropore between CNT strands. As a result, we uniformly impregnate 70 wt % sulfur without sulfur residue. The S-M-CNTP cathode shows a highly reversible capacity of 1343 mA h g^-1 at a current density of 0.2 C even at a high sulfur content of 70 wt %. Upon a 10-fold current density increase, a high capacity retention of 74% is observed. These cathodes have a higher sulfur content than those of conventional CNT hosts but nevertheless exhibit excellent performance. Our CNTPs and pore control technology will advance the commercialization of CNT hosts for LSBs.

3D interconnected porous carbon nanosheets/carbon nanotubes as a polysulfide reservoir for high performance lithium-sulfur batteries

Carbon materials have attracted considerable attention as the hosts for lithium-sulfur batteries, especially the 3D structural carbon matrix. Herein, novel 3D interconnected porous carbon nanosheets/carbon nanotubes (denoted as PC/CNT) as a polysulfide reservoir are synthesized by a simple one-pot pyrolysis method. In the designed hybrid carbon matrix, porous carbon nanosheets exhibit hierarchical porous structures for high sulfur loading and effectively strengthen the physical confinement to trap soluble polysulfides, while carbon nanotubes provide a highly robust conductive pathway which can facilitate electron transport and maintain structural integrity. Moreover, the 3D interconnected structure combining 1D carbon nanotubes and 2D porous carbon nanosheets is beneficial for rapid electrical/ionic transport and favorable electrolyte infiltration. As a result, the S-PC/CNT composite exhibits outstanding electrochemical performance, with a high active-sulfur utilization, high specific capacity (1485.4, 1300.3 and 1138 mA h g^-1 at 0.5, 1 and 2 C, respectively), superior cycling stability (only 0.1% capacity decay per cycle over 400 cycles at 2 C) and excellent rate capability (the reversible capacity of 749 mA h g^-1 even at 4 C).

A Bifunctional Perovskite Promoter for Polysulfide Regulation toward Stable Lithium-Sulfur Batteries

Lithium-sulfur (LiS) batteries are strongly considered as the next-generation rechargeable cells. However, both the shuttle of lithium polysulfides (LiPSs) and sluggish kinetics in random deposition of lithium sulfides (Li₂ S) significantly degrade the capacity, rate performance, and cycling life of LiS cells. Herein, bifunctional Ba_0.5 Sr_0.5 Co_0.8 Fe_0.2 O_3-δ perovskite nanoparticles (PrNPs) are proposed as a promoter to immobilize LiPSs and guide the deposition of Li₂ S in a LiS cell. The oxygen vacancy in PrNPs increases the metal reactivity to anchor LiPSs, and co-existence of lithiophilic (O) and sulfiphilic (Sr) sites in PrNP favor the dual-bonding (LiO and SrS bonds) to anchor LiPSs. The high catalytic nature of PrNP facilitates the kinetics of LiPS redox reaction. The PrNP with intrinsic LiPS affinity serves as nucleation sites for Li₂ S deposition and guides its uniform propagation. Therefore, the bifunctional LiPS promoter in LiS cell yields high rate performance and ultralow capacity decay rate of 0.062% (a quarter of pristine LiS cells). The proposed strategy to immobilize LiPSs, promotes the conversion of LiPS, and regulates deposition of Li₂ S by an emerging perovskite promoter and is also expected to be applied in other energy conversion and storage devices based on multi-electron redox reactions.

Ionothermal synthesis of graphene-based microporous carbon for lithium–sulfur batteries

Graphene-based microporous carbon with a high conductivity and diverse porous structure was designed via an ionothermal method for lithium–sulfur batteries.

Bimetallic sulfide nanoparticles confined by dual-carbon nanostructures as anodes for lithium-/sodium-ion batteries

Bimetallic sulfide ((Ni_0.3Co_0.7)₉S₈) nanoparticles confined by dual-carbon nanostructures are prepared, and deliver high electrochemical performances as anodes for Li⁺/Na⁺ batteries.

Clustered-Microcapsule-Shaped Microporous Carbon-Coated Sulfur Composite Synthesized via in Situ Oxidation

Hollow materials as sulfur hosts have been intensively investigated to address the poor cycling stabilities of Li-S batteries. Herein, we report an enhanced hollow framework to improve the applicability of the sulfur confinement. A clustered-microcapsule-shaped microporous carbon coated sulfur (CM-S@MPC) composite is prepared from the clustered zinc sulfide precursor, through an in situ oxidation process. The high specific surface area and the in situ preparation guarantee the uniform distribution of sulfur inside the carbon microcapsule, even under a higher sulfur content of 83 wt %. In addition, the interconnected frame constructed by the stacking of carbon microcapsules also mitigates the lithium polysulfide loss by setting interlayered hurdles on their pathway along the outward diffusion. Hence, these enable a full demonstration of excellent cycling stability, compared to the control sample obtained via physical sulfur infiltration. The outstanding decay rate of 0.039% per cycle is achieved during 700 cycles at 1 C, even under high sulfur loading.

Toward Theoretically Cycling-Stable Lithium-Sulfur Battery Using a Foldable and Compositionally Heterogeneous Cathode

Rechargeable lithium-sulfur (Li-S) batteries have been expected for new-generation electrical energy storages, which are attributed to their high theoretical energy density, cost effectiveness, and eco-friendliness. But Li-S batteries still have some problems for practical application, such as low sulfur utilization and dissatisfactory capacity retention. Herein, we designed and fabricated a foldable and compositionally heterogeneous three-dimensional sulfur cathode with integrated sandwich structure. The electrical conductivity of the cathode is facilitated by three different dimension carbons, in which short-distance and long-distance pathways for electrons are provided by zero-dimensional ketjen black (KB), one-dimensional activated carbon fiber (ACF) and two-dimensional graphene (G). The resultant three-dimensional sulfur cathode (T-AKG/KB@S) with an areal sulfur loading of 2 mg cm^-2 exhibits a high initial specific capacity, superior rate performance and a reversible discharge capacity of up to 726 mAh g^-1 at 3.6 mA cm^-2 with an inappreciable capacity fading rate of 0.0044% per cycle after 500 cycles. Moreover, the cathode with a high areal sulfur loading of 8 mg cm^-2 also delivers a reversible discharge capacity of 938 mAh g^-1 at 0.71 mA cm^-2 with a capacity fading rate of 0.15% per cycle and a Coulombic efficiency of almost 100% after 50 cycles.

More Reliable Lithium-Sulfur Batteries: Status, Solutions and Prospects

Lithium-sulfur (Li-S) batteries have attracted tremendous interest because of their high theoretical energy density and cost effectiveness. The target of Li-S battery research is to produce batteries with a high useful energy density that at least outperforms state-of-the-art lithium-ion batteries. However, due to an intrinsic gap between fundamental research and practical applications, the outstanding electrochemical results obtained in most Li-S battery studies indeed correspond to low useful energy densities and are not really suitable for practical requirements. The Li-S battery is a complex device and its useful energy density is determined by a number of design parameters, most of which are often ignored, leading to the failure to meet commercial requirements. The purpose of this review is to discuss how to pave the way for reliable Li-S batteries. First, the current research status of Li-S batteries is briefly reviewed based on statistical information obtained from literature. This includes an analysis of how the various parameters influence the useful energy density and a summary of existing problems in the current Li-S battery research. Possible solutions and some concerns regarding the construction of reliable Li-S batteries are comprehensively discussed. Finally, insights are offered on the future directions and prospects in Li-S battery field.

Smart Electrochemical Energy Storage Devices with Self-Protection and Self-Adaptation Abilities

Currently, with booming development and worldwide usage of rechargeable electrochemical energy storage devices, their safety issues, operation stability, service life, and user experience are garnering special attention. Smart and intelligent energy storage devices with self-protection and self-adaptation abilities aiming to address these challenges are being developed with great urgency. In this Progress Report, we highlight recent achievements in the field of smart energy storage systems that could early-detect incoming internal short circuits and self-protect against thermal runaway. Moreover, intelligent devices that are able to take actions and self-adapt in response to external mechanical disruption or deformation, i.e., exhibiting self-healing or shape-memory behaviors, are discussed. Finally, insights into the future development of smart rechargeable energy storage devices are provided.

A Microribbon Hybrid Structure of CoOx-MoC Encapsulated in N-Doped Carbon Nanowire Derived from MOF as Efficient Oxygen Evolution Electrocatalysts

Developing highly efficient electrocatalysts for oxygen evolution is vital for renewable and sustainable energy production and storage. Herein, nitrogen-doped carbon encapsulated CoOx-MoC heterostructures are reported for the first time as high performance oxygen evolution electrocatalysts. The composition can be tuned by the addition of a Mo source to form a nanowire-assembled hierarchically porous microstructure, which can enlarge the specific surface area, thus exposing more active sites, facilitating mass transport and charge transfer. Moreover, it is demonstrated that the formation of CoOx-MoC heterostructures and the resulting synergistic effect between MoC and Co facilitate the reaction kinetics, leading to significantly improved oxygen evolution reaction (OER) activity with an onset overpotential of merely 290 mV, and a low overpotential of 330 mV to afford a current density of 10 mA cm^-2 . The well-constructed microarchitecture contributes to superior long term stability electrochemical behaviors. This work provides a facile strategy via composition tuning and structure optimization for the development of next-generation nonprecious metal-based OER electrocatalysts.

Fluorinated, Sulfur-Rich, Covalent Triazine Frameworks for Enhanced Confinement of Polysulfides in Lithium-Sulfur Batteries

Lithium-sulfur battery represents a promising class of energy storage technology owing to its high theoretical energy density and low cost. However, the insulating nature, shuttling of soluble polysulfides and volumetric expansion of sulfur electrodes seriously give rise to the rapid capacity fading and low utilization. In this work, these issues are significantly alleviated by both physically and chemically restricting sulfur species in fluorinated porous triazine-based frameworks (FCTF-S). One-step trimerization of perfluorinated aromatic nitrile monomers with elemental sulfur allows the simultaneous formation of fluorinated triazine-based frameworks, covalent attachment of sulfur and its homogeneous distribution within the pores. The incorporation of electronegative fluorine in frameworks provides a strong anchoring effect to suppress the dissolution and accelerate the conversion of polysulfides. Together with covalent chemical binding and physical nanopore-confinement effects, the FCTF-S demonstrates superior electrochemical performances, as compared to those of the sulfur-rich covalent triazine-based framework without fluorine (CTF-S) and porous carbon delivering only physical confinement. Our approach demonstrates the potential of regulating lithium-sulfur battery performances at a molecular scale promoted by the porous organic polymers with a flexible design.

Bis(aryl) Tetrasulfides as Cathode Materials for Rechargeable Lithium Batteries

An organotetrasulfide consists of a linear chain of four sulfur atoms that could accept up to 6 e^- in reduction reactions, thus providing a promising high-capacity electrode material. Herein, we study three bis(aryl) tetrasulfides as cathode materials in lithium batteries. Each tetrasulfide exhibits two major voltage regions in the discharge. The high voltage slope region is governed by the formation of persulfides and thiolates, and the low voltage plateau region is due to the formation of Li₂ S₂ /Li₂ S. Based on theoretical calculations and spectroscopic analysis, three reduction reaction processes are revealed, and the discharge products are identified. Lithium half cells with tetrasulfide catholytes deliver high specific capacities over 200 cycles. The effects of the functional groups on the electrochemical characteristics of tetrasulfides are investigated, which provides guidance for developing optimum aryl polysulfides as cathode materials for high energy lithium batteries.

Sulfur in Hyper-cross-linked Porous Polymer as Cathode in Lithium-Sulfur Batteries with Enhanced Electrochemical Properties

Sulfur was impregnated into hyper-cross-linked porous polymer (HCP) with a high specific area and unique porous structure. Compared to its inorganic or carbon counterparts, the HCP has a relatively high specific surface area of 1980 m² g^-1 with a total pore volume of 2.61 cm³ g^-1, resulting in sulfur content in HCP/S of as high as 80 wt %. As a benefit of the unique HCP structure, the HCP/S composite exhibits a high initial discharge specific capacity (1333 mA h g^-1 at 0.2 C), high-rate property, and good cycling stability (658 mA h g^-1 after 120 cycles at 0.5 C and 604 mA h g^-1 after 80 cycles at 1 C). Furthermore, the capacity of cells loses less than 1% after the first 20 charge/discharge cycles, while the HCP/S cathode can be cycled with an excellent Coulombic efficiency of above 94% after 120 cycles. Compared with pristine sulfur, the superior electrochemical performance of HCP/S composite is related to the cross-linked porous framework. Such structure could provide short ionic/electronic conduction pathways and suppress the polysulfide shuttle during the discharge process.

Porous Coconut Shell Carbon Offering High Retention and Deep Lithiation of Sulfur for Lithium-Sulfur Batteries

Retaining soluble polysulfides in the sulfur cathodes and allowing for deep redox are essential to develop high-performance lithium-sulfur batteries. The versatile textures and physicochemical characteristics of abundant biomass offer a great opportunity to prepare biochar materials that can enhance the performance of Li-S batteries in sustainable mode. Here, we exploit micro-/mesoporous coconut shell carbon (CSC) with high specific surface areas as a sulfur host for Li-S batteries. The sulfur-infiltrated CSC materials show superior discharge-charge capacity, cycling stability, and high rate capability. High discharge capacities of 1599 and 1500 mA h g^-1 were achieved at current rates of 0.5 and 2.0 C, respectively. A high reversible capacity of 517 mA h g^-1 was retained at 2.0 C even after 400 cycles. The results demonstrate a high retention and a deep lithiation of the CSC-confined sulfur. The success of this strategy provides insights into seeking high-performance biochar materials for Li-S batteries from abundant bioresources.

Integration of Graphene, Nano Sulfur, and Conducting Polymer into Compact, Flexible Lithium-Sulfur Battery Cathodes with Ultrahigh Volumetric Capacity and Superior Cycling Stability for Foldable Devices

Lithium-sulfur batteries, as one of the most promising next-generation batteries, attract tremendous attentions due to their high energy density and low cost. However, their practical application is hindered by their short cycling life and low volumetric capacity. Herein, compact, flexible, and free-standing films with a sandwich structure are designed simply by vacuum filtration, in which nanosulfur is homogenously coated by graphene and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). This unique hierarchical structure not only provides a highly conductive network and intimate contacts between nanosulfur and graphene/PEDOT:PSS for effective charge transportation, but also offers synergistic physical restriction and chemical confinement of dissoluble intermediate lithium polysulfides during electrochemical processes. Therefore, these conductive compact films, used directly as cathodes, show the highest reversible volumetric capacity of 1432 Ah L^-1 at 0.1 C and 1038 Ah L^-1 at 1 C, and excellent cycling stability with a minimal decay rate of 0.04% per cycle over 500 cycles at 1 C. Meanwhile, remarkable rate performance with a high capacity of 701 mAh g^-1 at 4 C is also achieved. Soft-packaged batteries based on this flexible cathode are further fabricated and demonstrate excellent mechanical and electrochemical properties with little capacity decay under folded state, highlighting the practical application of our deliberately designed electrode in a flexible power system.

Unusual Mesoporous Carbonaceous Matrix Loading with Sulfur as the Cathode of Lithium Sulfur Battery with Exceptionally Stable High Rate Performance

Unusual three-dimensional mesoporous carbon/reduced graphene oxide (MP-C/rGO) matrix possessing graphene nanolayer pore walls built up by three to five graphene monosheets and some carbon particles with the sizes of about 5 nm located between the graphene nanolayers was prepared by facile freeze-drying and then carbonization of the poly(vinyl alcohol) and graphene oxide mixture. The mesoporous carbonaceous MP-C/rGO sample has a high specific surface area of 661.6 m² g^-1, large specific pore volume of 1.54 m³ g^-1, and focused pore size distribution of 2-10 nm. About 64 wt % sulfur could be held in the pores of the MP-C/rGO matrix. As the cathode of a Li-S battery, the MP-C/rGO/S composite showed excellent electrochemical property including a high initial specific capacity of 919 mA h g^-1 at 1 C with the capacity retention ratio of 63.3% and the Coulombic efficiency above 90% after 500 cycles. Meanwhile, the initial specific capacity of 602 mA h g^-1 at 5 C and remaining capacity of 391 mA h g^-1 after 500 cycles with an outstanding Coulombic efficiency of 97% indicate its exceptionally stable rate performance.

Sandwich-Type NbS2@S@I-Doped Graphene for High-Sulfur-Loaded, Ultrahigh-Rate, and Long-Life Lithium-Sulfur Batteries

Lithium-sulfur batteries practically suffer from short cycling life, low sulfur utilization, and safety concerns, particularly at ultrahigh rates and high sulfur loading. To address these problems, we have designed and synthesized a ternary NbS₂@S@IG composite consisting of sandwich-type NbS₂@S enveloped by iodine-doped graphene (IG). The sandwich-type structure provides an interconnected conductive network and plane-to-point intimate contact between layered NbS₂ (or IG) and sulfur particles, enabling sulfur species to be efficiently entrapped and utilized at ultrahigh rates, while the structural integrity is well maintained. NbS₂@S@IG exhibits prominent high-power charge/discharge performances. Reversible capacities of 195, 107, and 74 mA h g^-1 (1.05 mg cm^-2) have been achieved after 2000 cycles at ultrahigh rates of 20, 30, and 40 C, respectively, and the corresponding average decay rates per cycle are 0.022%, 0.031% and 0.033%, respectively. When the area sulfur loading is increased to 3.25 mg cm^-2, the electrode still maintains a high discharge capacity of 405 mAh g^-1 after 600 cycles at 1 C. Three half-cells in series assembled with NbS₂@S@IG can drive 60 indicators of LED modules after only 18 s of charging. The instantaneous current and power of the device reach 196.9 A g^-1 and 1369.7 W g^-1, respectively.

Quantitative Analysis of Electrochemical and Electrode Stability with Low Self-Discharge Lithium-Sulfur Batteries

The viability of employing high-capacity sulfur cathodes in building high-energy-density lithium-sulfur batteries is limited by rapid self-discharge, short shelf life, and severe structural degradation during cell resting (static instability). Unfortunately, the static instability has largely been ignored in the literature. We present in this letter a long-term self-discharge study by quantitatively analyzing the control lithium-sulfur batteries with a conventional cathode configuration, which provides meaningful insights into the cathode failure mechanisms during resting. Utilizing the understanding obtained with the control cells, we design and present low self-discharge (LSD) lithium-sulfur batteries for investigating the long-term self-discharge effect and electrode stability.