Lithium-sulfur batteries: progress and prospects

Development of advanced energy-storage systems for portable devices, electric vehicles, and grid storage must fulfill several requirements: low-cost, long life, acceptable safety, high energy, high power, and environmental benignity. With these requirements, lithium-sulfur (Li-S) batteries promise great potential to be the next-generation high-energy system. However, the practicality of Li-S technology is hindered by technical obstacles, such as short shelf and cycle life and low sulfur content/loading, arising from the shuttling of polysulfide intermediates between the cathode and anode and the poor electronic conductivity of S and the discharge product Li2 S. Much progress has been made during the past five years to circumvent these problems by employing sulfur-carbon or sulfur-polymer composite cathodes, novel cell configurations, and lithium-metal anode stabilization. This Progress Report highlights recent developments with special attention toward innovation in sulfur-encapsulation techniques, development of novel materials, and cell-component design. The scientific understanding and engineering concerns are discussed at the end in every developmental stage. The critical research directions needed and the remaining challenges to be addressed are summarized in the Conclusion.

Nanostructured Metal Oxides and Sulfides for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries with high energy density and long cycle life are considered to be one of the most promising next-generation energy-storage systems beyond routine lithium-ion batteries. Various approaches have been proposed to break down technical barriers in Li-S battery systems. The use of nanostructured metal oxides and sulfides for high sulfur utilization and long life span of Li-S batteries is reviewed here. The relationships between the intrinsic properties of metal oxide/sulfide hosts and electrochemical performances of Li-S batteries are discussed. Nanostructured metal oxides/sulfides hosts used in solid sulfur cathodes, separators/interlayers, lithium-metal-anode protection, and lithium polysulfides batteries are discussed respectively. Prospects for the future developments of Li-S batteries with nanostructured metal oxides/sulfides are also discussed.

Recent Advances in Layered Ti3 C2 Tx MXene for Electrochemical Energy Storage

Ti₃ C₂ T_x , a typical representative among the emerging family of 2D layered transition metal carbides and/or nitrides referred to as MXenes, has exhibited multiple advantages including metallic conductivity, a plastic layer structure, small band gaps, and the hydrophilic nature of its functionalized surface. As a result, this 2D material is intensively investigated for application in the energy storage field. The composition, morphology and texture, surface chemistry, and structural configuration of Ti₃ C₂ T_x directly influence its electrochemical performance, e.g., the use of a well-designed 2D Ti₃ C₂ T_x as a rechargeable battery anode has significantly enhanced battery performance by providing more chemically active interfaces, shortened ion-diffusion lengths, and improved in-plane carrier/charge-transport kinetics. Some recent progresses of Ti₃ C₂ T_x MXene are achieved in energy storage. This Review summarizes recent advances in the synthesis and electrochemical energy storage applications of Ti₃ C₂ T_x MXene including supercapacitors, lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries. The current opportunities and future challenges of Ti₃ C₂ T_x MXene are addressed for energy-storage devices. This Review seeks to provide a rational and in-depth understanding of the relation between the electrochemical performance and the nanostructural/chemical composition of Ti₃ C₂ T_x , which will promote the further development of 2D MXenes in energy-storage applications.

MoS2 /Celgard Separator as Efficient Polysulfide Barrier for Long-Life Lithium-Sulfur Batteries

A high lithium conductive MoS₂ /Celgard composite separator is reported as efficient polysulfides barrier in Li-S batteries. Significantly, thanks to the high density of lithium ions on MoS₂ surface, this composite separator shows high lithium conductivity, fast lithium diffusion, and facile lithium transference. When used in Li-S batteries, the separator is proven to be highly efficient for depressing polysulfides shuttle, leading to high and long cycle stability. With 65% of sulfur loading, the device with MoS₂ /Celgard separator delivers an initial capacity of 808 mAh g^-1 and a substantial capacity of 401 mAh g^-1 after 600 cycles, corresponding to only 0.083% of capacity decay per cycle that is comparable to the best reported result so far. In addition, the Coulombic efficiency remains more than 99.5% during all 600 cycles, disclosing an efficient ionic sieve preventing polysulfides migration to the anode while having negligible influence on Li⁺ ions transfer across the separator. The strategy demonstrated in this work will open the door toward developing efficient separators with flexible 2D materials beyond graphene for energy-storage devices.

A nitrogen and sulfur dual-doped carbon derived from polyrhodanine@cellulose for advanced lithium-sulfur batteries

A sulfur electrode exhibiting strong polysulfide chemisorption using a porous N, S dual-doped carbon is reported. The synergistic functionalization from the N and S heteroatoms dramatically modifies the electron density distribution and leads to much stronger polysulfide binding. X-ray photoelectron spectroscopy studies combined with ab initio calculations reveal strong Li(+) -N and Sn (2-) -S interactions. The sulfur electrodes exhibit an ultralow capacity fading of 0.052% per cycle over 1100 cycles.

Mechanism and Kinetics of Li2S Precipitation in Lithium-Sulfur Batteries

The kinetics of Li2 S electrodeposition onto carbon in lithium-sulfur batteries are characterized. Electrodeposition is found to be dominated by a 2D nucleation and growth process with rate constants that depend strongly on the electrolyte solvent. Nucleation is found to require a greater overpotential than growth, which results in a morphology that is dependent on the discharge rate.

Design Principles for Heteroatom-Doped Nanocarbon to Achieve Strong Anchoring of Polysulfides for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have been intensively concerned to fulfill the urgent demands of high capacity energy storage. One of the major unsolved issues is the complex diffusion of lithium polysulfide intermediates, which in combination with the subsequent paradox reactions is known as the shuttle effect. Nanocarbon with homogeneous nonpolar surface served as scaffolding materials in sulfur cathode basically cannot afford a sufficient binding and confining effect to maintain lithium polysulfides within the cathode. Herein, a systematical density functional theory calculation of various heteroatoms-doped nanocarbon materials is conducted to elaborate the mechanism and guide the future screening and rational design of Li-S cathode for better performance. It is proved that the chemical modification using N or O dopant significantly enhances the interaction between the carbon hosts and the polysulfide guests via dipole-dipole electrostatic interaction and thereby effectively prevents shuttle of polysulfides, allowing high capacity and high coulombic efficiency. By contrast, the introduction of B, F, S, P, and Cl monodopants into carbon matrix is unsatisfactory. To achieve the strong-couple effect toward Li2 Sx , the principles for rational design of doped carbon scaffolds in Li-S batteries to achieve a strong electrostatic dipole-dipole interaction are proposed. An implicit volcano plot is obtained to describe the dependence of binding energies on electronegativity of dopants. Moreover, the codoping strategy is predicted to achieve even stronger interfacial interaction to trap lithium polysulfides.

Defect Engineering on Electrode Materials for Rechargeable Batteries

The reasonable design of electrode materials for rechargeable batteries plays an important role in promoting the development of renewable energy technology. With the in-depth understanding of the mechanisms underlying electrode reactions and the rapid development of advanced technology, the performance of batteries has significantly been optimized through the introduction of defect engineering on electrode materials. A large number of coordination unsaturated sites can be exposed by defect construction in electrode materials, which play a crucial role in electrochemical reactions. Herein, recent advances regarding defect engineering in electrode materials for rechargeable batteries are systematically summarized, with a special focus on the application of metal-ion batteries, lithium-sulfur batteries, and metal-air batteries. The defects can not only effectively promote ion diffusion and charge transfer but also provide more storage/adsorption/active sites for guest ions and intermediate species, thus improving the performance of batteries. Moreover, the existing challenges and future development prospects are forecast, and the electrode materials are further optimized through defect engineering to promote the development of the battery industry.

Sulfur and Its Role In Modern Materials Science

Although well-known and studied for centuries, sulfur continues to be at the center of an extensive array of scientific research topics. As one of the most abundant elements in the Universe, a major by-product of oil refinery processes, and as a common reaction site within biological systems, research involving sulfur is both broad in scope and incredibly important to our daily lives. Indeed, there has been renewed interest in sulfur-based reactions in just the past ten years. Sulfur research spans the spectrum of topics within the physical sciences including research on improving energy efficiency, environmentally friendly uses for oil refinery waste products, development of polymers with unique optical and mechanical properties, and materials produced for biological applications. This Review focuses on some of the latest exciting ways in which sulfur and sulfur-based reactions are being utilized to produce materials for application in energy, environmental, and other practical areas.

A flexible sulfur-graphene-polypropylene separator integrated electrode for advanced Li-S batteries

A flexible Li-S battery based on an integrated structure of sulfur and graphene on a separator is developed. The internal graphene current collector offers a continuous conductive pathway, a modified interface with sulfur, and a good barrier to and an effective reservoir for dissolved polysulfides, consequently improving the capacity and cyclic life of the Li-S battery.

Single Nickel Atoms on Nitrogen-Doped Graphene Enabling Enhanced Kinetics of Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have arousing interest because of their high theoretical energy density. However, they often suffer from sluggish conversion of lithium polysulfides (LiPS) during the charge/discharge process. Single nickel (Ni) atoms on nitrogen-doped graphene (Ni@NG) with Ni-N₄ structure are prepared and introduced to modify the separators of Li-S batteries. The oxidized Ni sites of the Ni-N₄ structure act as polysulfide traps, efficiently accommodating polysulfide ion electrons by forming strong S_x ^2- ⋅⋅⋅NiN bonding. Additionally, charge transfer between the LiPS and oxidized Ni sites endows the LiPS on Ni@NG with low free energy and decomposition energy barrier in an electrochemical process, accelerating the kinetic conversion of LiPS during the charge/discharge process. Furthermore, the large binding energy of LiPS on Ni@NG also shows its ability to immobilize the LiPS and further suppresses the undesirable shuttle effect. Therefore, a Li-S battery based on a Ni@NG modified separator exhibits excellent rate performance and stable cycling life with only 0.06% capacity decay per cycle. It affords fresh insights for developing single-atom catalysts to accelerate the kinetic conversion of LiPS for highly stable Li-S batteries.

Revisiting the Role of Polysulfides in Lithium-Sulfur Batteries

Intermediate polysulfides (S_n , where n = 2-8) play a critical role in both mechanistic understanding and performance improvement of lithium-sulfur batteries. The rational management of polysulfides is of profound significance for high-efficiency sulfur electrochemistry. Here, the key roles of polysulfides are discussed, with regard to their status, behavior, and their correspondingimpact on the lithium-sulfur system. Two schools of thoughts for polysulfide management are proposed, their advantages and disadvantages are compared, and future developments are discussed.

Lithium Bond Chemistry in Lithium-Sulfur Batteries

The lithium-sulfur (Li-S) battery is a promising high-energy-density storage system. The strong anchoring of intermediates is widely accepted to retard the shuttle of polysulfides in a working battery. However, the understanding of the intrinsic chemistry is still deficient. Inspired by the concept of hydrogen bond, herein we focus on the Li bond chemistry in Li-S batteries through sophisticated quantum chemical calculations, in combination with ⁷ Li nuclear magnetic resonance (NMR) spectroscopy. Identified as Li bond, the strong dipole-dipole interaction between Li polysulfides and Li-S cathode materials originates from the electron-rich donors (e.g., pyridinic nitrogen (pN)), and is enhanced by the inductive and conjugative effect of scaffold materials with π-electrons (e.g., graphene). The chemical shift of Li polysulfides in ⁷ Li NMR spectroscopy, being both theoretically predicted and experimentally verified, is suggested to serve as a quantitative descriptor of Li bond strength. These theoretical insights were further proved by actual electrochemical tests. This work highlights the importance of Li bond chemistry in Li-S cell and provides a deep comprehension, which is helpful to the cathode materials rational design and practical applications of Li-S batteries.

Mesoporous Titanium Nitride-Enabled Highly Stable Lithium-Sulfur Batteries

The TiN-S composite cathode exhibits superior performance because of higher electrical conductivity and the capture of the soluble intermediate species of the electrode reactions by 2-5 nm mesopores and strong N-S surface bonding.

A Cooperative Interface for Highly Efficient Lithium-Sulfur Batteries

A cooperative interface constructed by "lithiophilic" nitrogen-doped graphene frameworks and "sulfiphilic" nickel-iron layered double hydroxides (LDH@NG) is proposed to synergistically afford bifunctional Li and S binding to polysulfides, suppression of polysulfide shuttles, and electrocatalytic activity toward formation of lithium sulfides for high-performance lithium-sulfur batteries. LDH@NG enables high rate capability, long lifespan, and efficient stabilization of both sulfur and lithium electrodes.

Yolk-Shelled C@Fe3 O4 Nanoboxes as Efficient Sulfur Hosts for High-Performance Lithium-Sulfur Batteries

Owing to the high theoretical specific capacity (1675 mA h g^-1 ) and low cost, lithium-sulfur (Li-S) batteries offer advantages for next-generation energy storage. However, the polysulfide dissolution and low electronic conductivity of sulfur cathodes limit the practical application of Li-S batteries. To address such issues, well-designed yolk-shelled carbon@Fe₃ O₄ (YSC@Fe₃ O₄ ) nanoboxes as highly efficient sulfur hosts for Li-S batteries are reported here. With both physical entrapment by carbon shells and strong chemical interaction with Fe₃ O₄ cores, this unique architecture immobilizes the active material and inhibits diffusion of the polysulfide intermediates. Moreover, due to their high conductivity, the carbon shells and the polar Fe₃ O₄ cores facilitate fast electron/ion transport and promote continuous reactivation of the active material during the charge/discharge process, resulting in improved electrochemical utilization and reversibility. With these merits, the S/YSC@Fe₃ O₄ cathodes support high sulfur content (80 wt%) and loading (5.5 mg cm^-2 ) and deliver high specific capacity, excellent rate capacity, and long cycling stability. This work provides a new perspective to design a carbon/metal-oxide-based yolk-shelled framework as a high sulfur-loading host for advanced Li-S batteries with superior electrochemical properties.

3D Graphene-Foam-Reduced-Graphene-Oxide Hybrid Nested Hierarchical Networks for High-Performance Li-S Batteries

A 3D graphene-foam-reduced-graphene-oxide hybrid nested hierarchical network is synthesized to achieve high sulfur loading and content simultaneously, which solves the "double low" issues of Li-S batteries. The obtained Li-S cathodes show a high areal capacity two times larger than that of commercial lithium-ion batteries, and a good cycling performance comparable to those at low sulfur loading.

Entrapment of Polysulfides by a Black-Phosphorus-Modified Separator for Lithium-Sulfur Batteries

A bifunctional separator modified by black-phosphorus nanoflakes is prepared to overcome the challenges associated with the polysulfide diffusion in lithium-sulfur batteries. It brings the benefits of the entrapment of various sulfur species via the strong binding energy and re-activation of the trapped sulfur species due to its high electron conductivity as well as Li-ion diffusivity.

3D Interconnected Electrode Materials with Ultrahigh Areal Sulfur Loading for Li-S Batteries

Sulfur electrodes based on a 3D integrated hollow carbon fiber foam (HCFF) are synthesized with high sulfur loadings of 6.2-21.2 mg cm(-2) . Benefiting from the high electrolyte absorbability of the HCFF and the multiple conductive channels, the obtained electrode demonstrates excellent cycling stability and a high areal capacity of 23.32 mAh cm(-2) , showing great promise in commercially viable Li-S batteries.

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.

Lithium/Sulfide All-Solid-State Batteries using Sulfide Electrolytes

All-solid-state lithium batteries (ASSLBs) are considered as the next generation electrochemical energy storage devices because of their high safety and energy density, simple packaging, and wide operable temperature range. The critical component in ASSLBs is the solid-state electrolyte. Among all solid-state electrolytes, the sulfide electrolytes have the highest ionic conductivity and favorable interface compatibility with sulfur-based cathodes. The ionic conductivity of sulfide electrolytes is comparable with or even higher than that of the commercial organic liquid electrolytes. However, several critical challenges for sulfide electrolytes still remain to be solved, including their narrow electrochemical stability window, the unstable interface between the electrolyte and the electrodes, as well as lithium dendrite formation in the electrolytes. Herein, the emerging sulfide electrolytes and preparation methods are reviewed. In particular, the required properties of the sulfide electrolytes, such as the electrochemical stabilities of the electrolytes and the compatible electrode/electrolyte interfaces are highlighted. The opportunities for sulfide-based ASSLBs are also discussed.

Bidirectional Catalysts for Liquid-Solid Redox Conversion in Lithium-Sulfur Batteries

Accelerated conversion by catalysis is a promising way to inhibit shuttling of soluble polysulfides in lithium-sulfur (Li-S) batteries, but most of the reported catalysts work only for one direction sulfur reaction (reduction or oxidation), which is still not a root solution since fast cycled use of sulfur species is not finally realized. A bidirectional catalyst design, oxide-sulfide heterostructure, is proposed to accelerate both reduction of soluble polysulfides and oxidation of insoluble discharge products (e.g., Li₂ S), indicating a fundamental way for improving both the cycling stability and sulfur utilization. Typically, a TiO₂ -Ni₃ S₂ heterostructure is prepared by in situ growing TiO₂ nanoparticles on Ni₃ S₂ surface and the intimately bonded interfaces are the key for bidirectional catalysis. For reduction, TiO₂ traps while Ni₃ S₂ catalytically converts polysulfides. For oxidation, TiO₂ and Ni₃ S₂ both show catalytic activity for Li₂ S dissolution, refreshing the catalyst surface. The produced sulfur cathode with TiO₂ -Ni₃ S₂ delivers a low capacity decay of 0.038% per cycle for 900 cycles at 0.5C and specially, with a sulfur loading of 3.9 mg cm^-2 , achieves a high capacity retention of 65% over 500 cycles at 0.3C. This work unlocks how a bidirectional catalyst works for boosting Li-S batteries approaching practical uses.