Vanadium Dioxide-Graphene Composite with Ultrafast Anchoring Behavior of Polysulfides for Lithium-Sulfur Batteries

Locally boosted Li2S nucleation on VO2 by loading carbon quantum dots for soft-packaged lithium-sulfur pouch cells

VO2 affords ultrafast polysulfide adsorption on account of its oxidation potential, which matches the sulfur working window (1.7-2.8 V). Nevertheless, its nonconductive surface limits direct sulfur conversion. Herein, we gently load carbon quantum dots on VO2 to increase direct Li2S nucleation by enhanced electron conductivity. As a result, the soft-packaged lithium-sulfur pouch cell yields a capacity retention of 88.8% at 0.5C after 100 cycles and a decay rate of 0.17% per cycle over 200 cycles at 2C. The cell energy density of the multilayer cell is up to 386.1 W h kg-1.

In situ composition of Thienothiophene-based covalent organic framework on carbon nanotube as a host for high performance Li-S batteries

Lithium-sulfur batteries (LSBs) is a promising secondary battery system with high energy density and environment-friendly characteristics, however, the severe "shuttle effect" and poor conductivity usually lead to short service life and low initial capacity. Carbon Nanotubes (CNTs) with excellent conductivity and large quantity of cavities are promising host materials, whereas, the weak interaction between CNTs and polysulfides usually leads to serious shuttle effect in charge/discharge processes. Herein, thienothiophene-based covalent organic framework is uniformly wrapped on the outer surface of CNTs to form a nanocomposite TT-BOST@CNT. It is observed that the coexistence of the electron-rich S, O and the electron-deficient B atoms enables the effective adsorption of both Li⁺ and S_x^2- in lithium polysulfides (LiPSs). Studies reveal that the B, O and S atoms endow the nanocomposite with good catalysis ability, whereby, conversion of the insoluble long-chain polysulfides to the soluble short-chain polysulfides is accelerated. Consequently, the TT-BOST@CNT/S cathode displays outstanding electrochemical performance, with a high discharge specific capacity of 1545 mAh g^-1 at 0.2 C and a small attenuation rate of 0.035% per cycle in 1000 cycles at 1 C.

Carbon Coated Metal-Based Composite Electrode Materials for Lithium Sulfur Batteries: A Review

Lithium-sulfur battery is one of the most promising secondary battery systems due to their high energy density and low material cost. During the past decade, great progress has been achieved in promoting the performances of Li-S batteries by addressing the challenges at the laboratory-level model systems. With growing attention paid to the application of Li-S batteries, new challenges at practical cell scales emerge as the bottleneck. However, challenges remain for the commercialization of lithium-sulfur batteries. The current review mainly focused on metal-based catalysts decorated-carbon materials for enhanced lithium sulfur battery performance. Firstly, the synthesis methods of various carbon-sulfur composites are discussed, as well as the influence of different material structures on the electrochemical performance. Secondly, a variety of catalysts, including metal atoms, metal oxides, sulfides, phosphides, nitrides, and carbide-decorated carbon nanomaterials, are systematically introduced to determine how lithium can be enhanced by suppressing polysulfides and promoting redox conversion reactions. Also, analyzed the multi-step electrochemical reaction mechanism of the battery during the charging and discharging process, and provide a feasible path for the practical application of high energy density lithium-sulfur batteries.

Advanced Nanostructured MXene-Based Materials for High Energy Density Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) are one of the most promising candidates for next-generation high-energy-density energy storage systems, but their commercialization is hindered by the poor cycling stability due to the insulativity of sulfur and the reaction end products, and the migration of lithium polysulfide. MXenes are a type of emerging two-dimensional material and have shown excellent electrochemical properties in LSBs due to their high conductivity and large specific surface area. Herein, several synthetic strategies developed for MXenes since their discovery are summarized alongside discussion of the excellent properties of MXenes for LSBs. Recent advances in MXene-based materials as cathodes for LSBs as well as interlayers are also reviewed. Finally, the future development strategy and prospect of MXene-based materials in high-energy-density LSBs are put forward.

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

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

Zn-Ion Batteries: Boosting the Rate Capability and Low-temperature Performance by Combining Structure and Morphology Engineering

Prussian blue analogues (PBAs) have been considered as one kind of the most promising cathode materials for Zn-ion batteries (ZIBs) due to their low cost, high performance, high safety, and high abundance. However, owing to the low conductivity and single electron reaction, it is a great challenge to obtain a PBA cathode material with high reversible capacity, high rate capability, and good temperature adaptability. Here, a cathode material, K_1.14(VO)_3.33[Fe(CN)₆]₂·6.8H₂O (KVHCF), with a multielectron reaction and double conductive carbon framework (DCCF) is designed and synthesized by combining structure and morphology engineering. With the multielectron reaction and high electronic conductivity simultaneously, the KVHCF@DCCF cathode material delivers a high specific capacity (180 mAh·g^-1 @ 400 mA·g^-1) and the best rate performance (116 mAh·g^-1 @ 8000 mA·g^-1) of the reported PBAs. Moreover, KVHCF@DCCF presents a high specific capacity of 132 mAh·g^-1 @ 400 mA·g^-1 at 0 °C. Even at -10 °C, it still delivers specific capacities of 127 mAh·g^-1 @ 40 mA·g^-1 and 80 mAh·g^-1 @ 400 mA·g^-1 with a retention of 86% after 700 cycles. In situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS) are carried out to investigate the charge-discharge electrochemical reaction mechanism.

Graphene-Based Nanomaterials as the Cathode for Lithium-Sulfur Batteries

The global energy crisis and environmental problems are becoming increasingly serious. It is now urgent to vigorously develop an efficient energy storage system. Lithium-sulfur batteries (LSBs) are considered to be one of the most promising candidates for next-generation energy storage systems due to their high energy density. Sulfur is abundant on Earth, low-cost, and environmentally friendly, which is consistent with the characteristics of new clean energy. Although LSBs possess numerous advantages, they still suffer from numerous problems such as the dissolution and diffusion of sulfur intermediate products during the discharge process, the expansion of the electrode volume, and so on, which severely limit their further development. Graphene is a two-dimensional crystal material with a single atomic layer thickness and honeycomb bonding structure formed by sp2 hybridization of carbon atoms. Since its discovery in 2004, graphene has attracted worldwide attention due to its excellent physical and chemical properties. Herein, this review summarizes the latest developments in graphene frameworks, heteroatom-modified graphene, and graphene composite frameworks in sulfur cathodes. Moreover, the challenges and future development of graphene-based sulfur cathodes are also discussed.

Hydrated vanadium pentoxide/reduced graphene oxide composite cathode material for high-rate lithium ion batteries

As well-known, hydrated vanadium pentoxide (V₂O₅·nH₂O) has a larger layer spacing than orthogonal V₂O₅, which could offer more active sites to accommodate lithium ions, ensuring a high specific capacity. However, the exploration of V₂O₅·nH₂O cathode is limited by its inherently low conductivity and slow electrochemical kinetics, leading to a significant decrease in capability. Herein, we prepared V₂O₅·nH₂O/reduced graphene oxide (rGO) composite with low rGO content (8 wt%) via a simple yet effective dual electrostatic assembly strategy. When used as the cathode material for lithium-ion batteries (LIBs), V₂O₅·nH₂O/rGO manifests a high reversible capacity of 268 mAh g^-1 at 100 mA g^-1 and especially an excellent rate capability (196 mAh g^-1 at 1000 mA g^-1 and 129 mA h g^-1 at 2000 mA g^-1), surpassing those of the V₂O₅/carbon composites reported in the literatures. Notably, the remarkable performance should be referable to the synergetic effects between one-dimensional V₂O₅·nH₂O nanobelts and two-dimensional rGO nanosheets, which provide a short transport pathway and enhanced electrical conductivity. This strategy opens a new opportunity for designing high-performance cathode material with excellent rate performance for advanced LIBs.

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

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

Vanadium based carbide-oxide heterogeneous V2O5@V2C nanotube arrays for high-rate and long-life lithium-sulfur batteries

Due to their ultra-high theoretical energy density, low cost, and environmental friendliness, lithium-sulfur batteries have become a potentially strong competitor for next-generation energy storage devices. The search for a host material that can effectively anchor sulfur to a cathode to solve the adverse effects of the shuttle effect on batteries has become a research hotspot in the academic world. Here, we propose a three-dimensional heterostructure of V₂O₅ nanotube arrays vertically grown on V₂C-MXenes as a sulfur-supporting host material for the cathode of lithium-sulfur batteries. Through first-principles calculations, we found that V₂O₅@V₂C exhibits an extreme adsorption capacity for polysulfides. Besides, thanks to the excellent catalytic performance of V₂O₅ for oxidation reactions, the conversion reaction potential of polysulfides to Li₂S and Li₂S₂ is significantly reduced, and the shuttle effect of lithium-sulfur batteries is effectively suppressed. Also, the larger specific surface area and tubular structure of the composite host material can increase the sulfur loading of the cathode while ensuring the stability of the electrode structure. The V₂O₅@V₂C/S electrode exhibits higher initial capacity (1173 mA h g^-1 at 0.2C), longer cycle life (1000 cycles with 0.047% decay per period), and higher sulfur loading (8.4 mg cm^-2). We believe that this work can provide a reference for the design of cathode host materials for lithium-sulfur batteries with long cycle life.

Crepe Cake Structured Layered Double Hydroxide/Sulfur/Graphene as a Positive Electrode Material for Li-S Batteries

Solving the polysulfide shuttle problem is one of the core challenges for the industrialization of lithium-sulfur batteries. In this work, a triphasic composite of LDH/sulfur/rGO (LDH: layered double hydroxide, rGO: reduced graphene oxide) with a crepe cake like structure is designed and fabricated as a positive electrode material for lithium-sulfur batteries. Sulfur nanoparticles are embedded in the interlayer space of the composite and thus are well protected physically via three-dimensional wrapping and chemically via strong interaction of LDH nanoflakes with lithium polysulfides, such as ionic bonds and S···H hydrogen bonds. In addition, the flexible lamellar structure of the composite with soft graphene layers can tolerate the volume expansion of sulfur during lithiation as well as facilitate ionic permeability and electron transport, which is favorable for the redox reactions of polysulfide. The present work sheds light on the future development and industrialization of lithium-sulfur batteries.

Cobalt(II) Tetraaminophthalocyanine-modified Multiwall Carbon Nanotubes as an Efficient Sulfur Redox Catalyst for Lithium-Sulfur Batteries

An efficient Li-S redox catalyst consisting of MWCNTs covalently modified by cobalt(II) tetraaminophthalocyanines (TaPcCo-MWCNTs) is developed. Effective lithium polysulfide (LiPS) capturing is enabled by the lithiophilic N-containing phthalocyanine rings and the sulfiphilic Co central atoms. This adsorption geometry utilizes the Co unoccupied d-orbitals as electron super-exchange highways. Elevated kinetics of LiPSs reactions in the liquid phase as well as liquid-solid transitions were revealed by electrochemical measurements and density functional theory calculations. Uniform deposition of Li₂ S films was also observed, which preserves cathode integrity and sulfur utilization during cell cycling. The catalyzed sulfur redox is also significantly facilitated by the fast electron and Li-ion transport to and from the reaction sites through the conductive MWCNT skeletons and the lithiophilic substituent amino groups on TaPcCo. With 6 wt % addition of TaPcCo-MWCNT in the cathode coatings, high sulfur utilization is achieved with areal sulfur loadings of up to 7 mg cm^-2 . Stable long-term cycling is achieved at 1 C at a sulfur loading of 5 mg cm^-2 , with an initial areal capacity of 4.4 mAh cm^-2 retention of 3.5 mAh cm^-2 after 500 cycles. Considering the high structural diversity of phthalocyanines macromolecules, this study provides opportunities for a new class of Li-S catalysts.

Accelerated polysulfide conversion on hierarchical porous vanadium-nitrogen-carbon for advanced lithium-sulfur batteries

With high theoretical specific density, low cost, and non-toxicity, Li-S batteries are regarded as a promising candidate for next-generation energy storage systems. However, the shuttling of soluble Li polysulfides (LiPSs) results in self-discharge and rapid capacity degradation. Herein, nitrogen-doped hierarchical porous carbon with embedded highly dispersed vanadium (v)-N_x sites (V-N-C) is developed as a high-performance Li-S battery cathode for the first time. The metal-organic polymer supramolecule structure formed by the electrostatic/hydrogen bond interaction of chitosan-VO₃^- strongly stabilizes V to generate a high density of V-N_x/C sites. During the discharge/charge process, the unique V-N_x/C active sites can serve as efficient catalysts to accelerate the redox kinetics of LiPSs, while the hierarchical porous carbon structure of V-N-C benefits the diffusion/transfer of Li⁺/e^- and suppresses the shuttling of LiPSs. As a result, the S/V-N-C composite delivers a high specific capacity of 1111.2 mA h g^-1 at 0.5C and maintains 573.6 mA h g^-1 at 5C with a low capacity decay rate of 0.087% per cycle (over 500 cycles at 1C). The rate performance of the developed V-N-C cathode in Li-S batteries is superior to that of most of the reported M-N-C and carbon material/metal compound composite electrodes.

Synthesis of cobalt-doped V2O3 with a hierarchical yolk–shell structure for high-performance lithium-ion batteries

Co-V₂O₃-24 yolk–shell nanospheres were synthesized via a solvothermal treatment and subsequent calcination. The electrochemical performance of Co-V₂O₃-24 is greatly improved because of Co-doping and the novel hierarchical yolk–shell structure.

VO2(p)-V2C(MXene) Grid Structure as a Lithium Polysulfide Catalytic Host for High-Performance Li-S Battery

Extensive efforts have been devoted to improving the cycling stability and reversibility of lithium-sulfur batteries. However, unsolved challenges and difficulties still remain in suppressing the shuttle effect, improving the conductivity and structural stability of sulfur cathodes. Here, we report a three-dimensional (3D) grid heterostructure VO₂(p) (paramontroseite-VO₂) nanorod cluster growing on the surface of two-dimensional V₂C (MXene) nanosheets as a high-performance catalytic host for sulfur cathodes. The results of first-principles calculation demonstrate that VO₂(p) nanorods can synergize with V₂C to enhance the adsorption capacity of host for lithium polysulfides in this host structure and reduce the redox reaction barrier in the conversion of polysulfides to short-chain sulfides. In addition, the high specific surface area and structural stability of the host material can increase the redox reaction kinetics and cyclic reversibility of the electrode. The VO₂(p)-V₂C/S cathode exhibits outstanding electrochemical performance and excellent reversible discharge capacity (1250 mAh·g^-1 at 0.2C), long-term cycling stability (69.1% retention at 2C after 500 cycles), and high sulfur loading cycling capacity (initial areal capacity of 9.3 mAh·cm^-2 at 0.2C for 200 cycles). Our research provides a valuable reference for the design of high-performance cathode structures with high sulfur loading.

In situ optical spectroscopy characterization for optimal design of lithium-sulfur batteries

The lithium-sulfur (Li-S) battery is one of the most promising high-energy-density secondary battery systems. However, it suffers from issues arising from its extremely complicated "solid-liquid-solid" reaction routes. In recent years, enormous advances have been made in optimizing Li-S batteries via the rational design of compositions and architectures. Nevertheless, a comprehensive and in-depth understanding of the practical reaction mechanisms of Li-S systems and their effect on the electrochemical performance is still lacking. Very recently, several important in situ optical spectroscopic techniques, including Raman, infrared and ultraviolet-visible spectroscopies, have been developed to monitor the real-time variations of the battery states, and a bridge linking the macroscopic electrochemical performance and microscopic architectures of the components has been set up, thus playing a critical role in scientifically guiding further optimal design of Li-S batteries. In this tutorial review, we provide a systematic summary of the state-of-the-art innovations in the characterization and optimal design of Li-S batteries with the aid of these in situ optical spectroscopic techniques, to guide a beginner to construct in situ optical spectroscopy electrochemical cells, and develop strategies for preventing long-chain polysulfide formation, dissolution and migration, thus alleviating the shuttle effect in Li-S batteries and improving the cell performances significantly.

Thiuram Vulcanization Accelerators as Polysulfide Scavengers To Suppress Shuttle Effects for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered to be one of the most promising alternatives for next-generation high energy-density storage systems. Nevertheless, the notorious "shuttle effect" and sluggish kinetic conversion in actual operation seriously hamper its practical application. Herein, inspired by the action mechanism of vulcanization accelerators, dipentamethylenethiuram tetrasulfide (DPTT) is employed as a novel electrolyte additive. Just like a scavenger, DPTT sweeps lithium polysulfide by a spontaneous instant chemical reaction between them, and the latter is quickly converted to Li₂S₂, along with the generation of elemental S, which will be reduced to polysulfide again. This is beneficial for relieving the accumulation and shuttling of polysulfide in the electrolyte. Therefore, Li-S batteries with DPTT-containing electrolyte exhibit enhanced capacity retention and improved rate performance. With 4 wt % DPTT additive and 3.03 mg cm^-2 S loading, the cell delivers a high initial capacity of 1227.6 mA h g^-1 and excellent capacity retention of 914.7 mA h g^-1 after 250 cycles at 0.5 C. This study provides a fresh insight into suppressing the shuttle effect and realizing high-performance Li-S batteries.

Enhanced Sulfur Redox and Polysulfide Regulation via Porous VN-Modified Separator for Li-S Batteries

Lithium-sulfur (Li-S) batteries have now emerged as the next-generation rechargeable energy storage system because of the high energy density and theoretical capacity. However, the notorious "lithium polysulfide (LiPS) shuttle" and sluggish kinetics in sulfur redox have posted great threat to their practical applications. Herein, we develop a VN-modified separator as an effective promoter to regulate the LiPSs and accelerate the electrochemical kinetics of Li-S batteries. Benefiting from the dense packing structure and polar surface of porous VN, the VN-modified separator favorably synergizes bifunctionality of physical confinement and chemical entrapment toward LiPSs while affording smooth lithium-ion migration. In addition, the superb electrical conductivity of VN also propels the LiPS conversion. With these advantages, thus-integrated batteries with VN-modified separator exhibit an average capacity decay of 0.077% per cycle at 1 C for 800 cycles. A reasonable areal capacity of 4.2 mAh cm^-2 is achieved even with a high sulfur mass loading of 3.8 mg cm^-2 at 0.2 C. The present work offers a rational strategy to regulate the LiPS behavior and guide the sulfur redox kinetics toward effective and long-life Li-S batteries.

Nitrogen Doped Carbon Nanosheets Encapsulated in situ Generated Sulfur Enable High Capacity and Superior Rate Cathode for Li-S Batteries

Lithium-sulfur batteries (LSBs), with large specific capacity (1,675 mAh g⁻¹), are regarded as the most likely alternative to the traditional Lithium-ion batteries. However, the intrinsical insulation and dramatic volume change of sulfur, as well as serious shuttle effect of polysulfides hinder their practical implementation. Herein, we develop three-dimensional micron flowers assembled by nitrogen doped carbon (NC) nanosheets with sulfur encapsulated (S@NC-NSs) as a promising cathode for Li-S to overcome the forementioned obstacles. The in situ generated S layer adheres to the inner surface of the hollow and micro-porous NC shell with fruitful O/N containing groups endowing both efficient physical trapping and chemical anchoring of polysulfides. Meanwhile, such a novel carbon shell helps to bear dramatic volume change and provides a fast way for electron transfer during cycling. Consequently, the S@NC-NSs demonstrate a high capacity (1,238 mAh g⁻¹ at 0.2 C; 1.0 C = 1,675 mA g⁻¹), superior rate performance with a capacity retention of 57.8% when the current density increases 25 times from 0.2 to 5.0 C, as well as outstanding cycling performance with an ultralow capacity fading of only 0.064% after 200 cycles at a high current density of 5.0 C.