Li2S Nanocrystals Confined in Free-Standing Carbon Paper for High Performance Lithium-Sulfur Batteries

Lithium Sulfide Batteries: Addressing the Kinetic Barriers and High First Charge Overpotential

Ever-rising global energy demands and the desperate need for green energy inevitably require next-generation energy storage systems. Lithium-sulfur (Li-S) batteries are a promising candidate as their conversion redox reaction offers superior high energy capacity and lower costs as compared to current intercalation type lithium-ion technology. Li₂S with a prelithiated cathode can, in principle, capture the high capacity while reducing some of the issues in conventional Li-S cells utilizing metallic lithium anodes and elemental sulfur cathodes. However, it also faces its own set of technical issues, including the insulating nature and the notorious shuttling effect that plagues the Li-S system. In addition, the high activation potential also hinders its electrochemical performance. To lower the high conversion barrier, key parameters of charge/ion transfer kinetics have to be considered in improving the reaction kinetics. This Review of lithium sulfide batteries examines the recent progress in this rapidly growing field, beginning with the revisiting of the fundamentals, working principles, and challenges of the Li-S system as well as the Li₂S cathode. The strategies adopted and methods that have been devised to overcome these issues are discussed in detail, by focusing on the synthesis of the nanoparticles, the structuring of the functional matrixes, and the promoting of the reaction kinetics through additives, aiming at providing a broad view of paths that can lead to a market viable Li₂S cathode in the near future.

Biredox-Ionic Anthraquinone-Coupled Ethylviologen Composite Enables Reversible Multielectron Redox Chemistry for Li-Organic Batteries

Organic compounds bearing redox-active ionic pairs as electrode materials for high-performance rechargeable batteries have gained growing attention owing to the properties of synthetic tunability, high theoretical capacity, and low solubility. Herein, an innovative biredox-ionic composite, i.e., ethylviologen dianthraquinone-2-sulfonate (EV-AQ₂ ), affording multiple and reversible active sites as a cathode material in lithium-organic batteries is reported. EV-AQ₂ exhibits a high initial capacity of 199.2 mAh g^-1 at 0.1 C rate, which corresponds to the transfer of two electrons from one redox couple EV²⁺ /EV⁰ and four electrons from two redox-active AQ^- anions. It is notable that EV-AQ₂ shows remarkably improved cyclability compared to the precursors. The capacity retention is 89% and the Coulombic efficiency approaches 100% over 120 cycles at 0.5 C rate. The results offer evidence that AQ^- into the EV²⁺ scaffold with multiple redox sites are promising in developing high-energy-density organic rechargeable batteries.

A Li2S-Based Catholyte/Solid-State-Electrolyte Composite for Electrochemically Stable Lithium-Sulfur Batteries

Li₂S, which features a high theoretical capacity of 1,166 mA·h g^-1, is an attractive cathode material for developing high-energy-density lithium-sulfur batteries. However, pristine Li₂S requires a high activation voltage of 4.0 V, which degrades both the electrolyte and electrode, leading to poor cycling performance. In an effort to reduce the activation overpotential, in this study, we investigate the use of P₂S₅ in an advanced Li₂S-P₂S₅ catholyte and demonstrate a new synthetic approach that enables facile and low-temperature processing. Our findings show the P₂S₅ additive generates two thiophosphates with high ionic conductivities in the catholyte, which improve the activation efficiency and the electrochemical utilization. To further improve this advanced catholyte design, we also investigate two modified Li₂S-P₂S₅ catholytes based on carbon black (to strengthen the conductivity) and dilute polysulfide (Li₂S₆; to amplify the reaction activity). Our analysis indicates that the optimal Li₂S-P₂S₅-Li₂S₆ catholyte attains high ionic conductivity and strong reaction kinetics, achieving a high charge-storage capacity of 700 mA·h g^-1 with a long-term cyclability of 200 cycles.

Confined Polysulfides in N-Doped 3D-CNTs Network for High Performance Lithium-Sulfur Batteries

Improving the utilization efficiency of active materials and suppressing the dissolution of lithium polysulfides into the electrolyte are very critical for development of high-performance lithium-sulfur batteries. Herein, a novel strategy is proposed to construct a three-dimensional (3D) N-doped carbon nanotubes (CNTs) networks to support lithium polysulfides (3D-NCNT-Li₂S₆) as a binder-free cathode for high-performance lithium-sulfur batteries. The 3D N-doped CNTs networks not only provide a conductive porous 3D architecture for facilitating fast ion and electron transport but also create void spaces and porous channels for accommodating active sulfur. In addition, lithium polysulfides can be effectively confined among the networks through the chemical bond between Li and N. Owing to the synergetic effect of the physical and chemical confinement for the polysulfides dissolution, the 3D-NCNT-Li₂S₆ cathodes exhibit enhanced charge capacity and cyclic stability with lower polarization and faster redox reaction kinetics. With an initial discharge capacity of 924.8 mAh g^-1 at 1 C, the discharge capacity can still maintain 525.1 mAh g^-1 after 200 cycles, which is better than that of its counterparts.

Ultrasmall Li2S-Carbon Nanotube Nanocomposites for High-Rate All-Solid-State Lithium-Sulfur Batteries

Due to the intrinsic poor ionic/electronic conductivities of Li₂S, it is a great challenge to realize high-rate all-solid-state lithium-sulfur batteries (ASSLSBs) with long cyclic performance. Herein, ultrasmall Li₂S (∼15 nm) is evenly deposited on a carbon nanotube (CNT) via a facile liquid-phase method to address these issues. The unique structure of the Li₂S deposited on a CNT composite cathode can improve ionic/electronic conductivities of Li₂S effectively and relieve the generated internal stress/strain during cycling. Specifically, the resultant Li/75%Li₂S-24%P₂S₅-1%P₂O₅/Li₁₀GeP₂S₁₂/Li₂S-53%CNT ASSLSBs show a reversible capacity of 651.4 mAh g^-1 under 1.0C at 60 °C after 300 cycles and even at a much higher cathode load of 5.08 mg cm^-2, a high discharge capacity of 556 mAh g^-1 can still be obtained under 0.1C after 20 cycles. The outstanding electrochemical performances are also attributed to the high diffusion coefficient of Li₂S-53%CNT, which is 39 times that of pristine Li₂S. This work presents an efficient procedure to design cathode materials with high ionic/electronic conductivities and paves the way for the successful commercialization of high-rate ASSLSBs.

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.

A new high-capacity and safe energy storage system: lithium-ion sulfur batteries

Lithium-ion sulfur batteries as a new energy storage system with high capacity and enhanced safety have been emphasized, and their development has been summarized in this review. The lithium-ion sulfur battery applies elemental sulfur or lithium sulfide as the cathode and lithium-metal-free materials as the anode, which can be divided into two main types. One is anode-type, where elemental sulfur is applied as the cathode, and the anode provides lithium ions. The other one is cathode-type, where lithium sulfide as the cathode provides lithium ions, and lithium-metal-free materials (e.g., graphite, silicon/carbon) function as the anode. Recently, some new lithium-ion sulfur battery systems have also been proposed, and are discussed in this review as well. The lithium-ion sulfur batteries not only maintain the advantage of high energy density because of the high capacities of sulfur and lithium sulfide, but also exhibit the improved safety of the batteries due to a non-lithium-metal in the anode. This review paper aims to track the recent progress in the development of lithium-ion sulfur batteries and summarize the challenges and the approaches for improving their electrochemical performances, including the lithiation methods to prepare lithium-metal-free anodes in anode-type lithium-ion sulfur batteries and the lithium sulfide cathode modification approaches in cathode-type lithium-ion sulfur batteries. Furthermore, the challenges and perspectives for future research and commercial applications have also been enumerated.

A facile synthetic approach to nanostructured Li2S cathodes for rechargeable solid-state Li-S batteries

Li-S solid state batteries, employing Li₂S as a pre-lithiated cathode, present a promising low cost, high capacity and safer alternative to their liquid electrolyte counterparts, where dissolution of intermediate polysulfide species can result in loss of active material and a subsequent decrease in ionic conductivity. A nanostructured Li₂S material would afford greater flexibility in optimising the cathode composite for more harmonious electrode-electrolyte interactions, yet facile routes to such nanoscale materials are limited. Here, we report a facile and scalable microwave approach to directly synthesize nanostructured Li₂S from a glyme solution containing lithium polysulfides. As-synthesized Li₂S presents an ideal architecture for the construction of free-standing cathodes for all-solid-state Li-S batteries.

Lowering the charge overpotential of Li2S via the inductive effect of phenyl diselenide in Li-S batteries

We find that phenyl diselenide (PDSe) can lower the charge overpotential of Li₂S via an inductive effect. The Se-Se bond of PDSe can break when it is in contact with Li₂S. The attraction between Se and Li weakens the Li-S bonds and facilitates the oxidation of Li₂S in lithium batteries.

A binder-free electrode architecture design for lithium-sulfur batteries: a review

Lithium-sulfur batteries (LSBs) are considered to be one of the most promising next-generation electrochemical power sources to replace commercial lithium-ion batteries because of their high energy density. However, practical application of LSBs is hindered by two critical drawbacks: "redox shuttle reactions" of dissolved polysulfides at the cathode side and Li dendrites at the Li anode side. Therefore, various approaches have been proposed to break down technical barriers in LSB systems. The overall device performance of LSBs depends on not only the development of host materials but also the superior architecture design of electrodes. Among these architectures, binder-free electrodes are verified to be one of the most effective structural designs for high-performance LSBs. Therefore, it is urgent to review recent advances in binder-free electrodes for promoting the fundamental and technical advancements of LSBs. Herein, recently emergent studies using various binder-free architectures in sulfur cathodes and lithium metal anodes are reviewed. These binder-free electrodes, with well-interconnected structures and abundant structural space, can provide a continuous pathway for fast/uniform electron transport/distribution, load sufficient active materials for ensuring high energy density, and afford large electrochemically active surface areas where electrons and Li ions can come into contact with the active materials for fast conversion reactions, thus leading to suitable applications for LSBs. Subsequently, the advantages and challenges of binder-free architectures are discussed from several recently emergent studies using binder-free structured sulfur cathodes or Li metal anodes. The future prospects of LSBs with binder-free electrode structure designs are also discussed.

Trace ethanol as an efficient electrolyte additive to reduce the activation voltage of the Li2S cathode in lithium-ion–sulfur batteries

Trace ethanol as a cheap and efficient electrolyte additive to reduce the activation voltage of the Li₂S cathode in lithium-ion–sulfur batteries by converting a solid–solid reaction into a solid–liquid reaction.

Conducting polymer-coated MIL-101/S composite with scale-like shell structure for improving Li–S batteries

Lithium–sulfur batteries are regarded as a promising energy storage system. However, they are plagued by rapid capacity decay, low coulombic efficiency, a severe shuttle effect and low sulfur loading in cathodes. To address these problems, effective carriers are highly demanded to encapsulate sulfur in order to extend the cycle life. Herein, we introduced a doped-PEDOT:PSS-coated MIL-101/S multi-core–shell structured composite. The unique structure of MIL-101, large specific area and conductive shell ensure high dispersion of sulfur in the composite and minimize the loss of polysulfides to the electrolyte. The doped-PEDOT:PSS-coated sulfur electrodes exhibited an increase in initial capacity and an improvement in rate characteristics. After 192 cycles at the current density of 0.1C, a doped-PEDOT:PSS-coated MIL-101/S electrode maintained a capacity of 606.62 mA h g⁻¹, while the MIL-101/S@PEDOT:PSS electrode delivered a capacity of 456.69 mA h g⁻¹. The EIS measurement revealed that the surface modification with the conducting polymer provided a lower resistance to the sulfur electrode, which resulted in better electrochemical behaviors in Li–S battery applications. Test results indicate that the MIL-101/S@doped-PEDOT:PSS is a promising host material for the sulfur cathode in the lithium–sulfur battery applications.

Lithium–sulfur batteries are regarded as a promising energy storage system.

A review of flexible lithium–sulfur and analogous alkali metal–chalcogen rechargeable batteries

This review summarizes recent progress in flexible Li–S and analogous alkali metal–chalcogen batteries, including flexible chalcogen cathodes, flexible alkali metal anodes, flexible solid-state electrolytes, and flexible battery prototypes.

Analytical Multimode Scanning and Transmission Electron Imaging and Tomography of Multiscale Structural Architectures of Sulfur Copolymer-Based Composite Cathodes for Next-Generation High-Energy Density Li-S Batteries

Poly[sulfur-random-(1,3-diisopropenylbenzene)] copolymers synthesized via inverse vulcanization represent an emerging class of electrochemically active polymers recently used in cathodes for Li-S batteries, capable of realizing enhanced capacity retention (1,005 mAh/g at 100 cycles) and lifetimes of over 500 cycles. The composite cathodes are organized in complex hierarchical three-dimensional (3D) architectures, which contain several components and are challenging to understand and characterize using any single technique. Here, multimode analytical scanning and transmission electron microscopies and energy-dispersive X-ray/electron energy-loss spectroscopies coupled with multivariate statistical analysis and tomography were applied to explore origins of the cathode-enhanced capacity retention. The surface topography, morphology, bonding, and compositions of the cathodes created by combining sulfur copolymers with varying 1,3-diisopropenylbenzene content and conductive carbons have been investigated at multiple scales in relation to the electrochemical performance and physico-mechanical stability. We demonstrate that replacing the elemental sulfur with organosulfur copolymers improves the compositional homogeneity and compatibility between carbons and sulfur-containing domains down to sub-5 nm length scales resulting in (a) intimate wetting of nanocarbons by the copolymers at interfaces; (b) the creation of 3D percolation networks of conductive pathways involving graphitic-like outer shells of aggregated carbons;

An Overview and Future Perspectives of Aluminum Batteries

A critical overview of the latest developments in the aluminum battery technologies is reported. The substitution of lithium with alternative metal anodes characterized by lower cost and higher abundance is nowadays one of the most widely explored paths to reduce the cost of electrochemical storage systems and enable long-term sustainability. Aluminum based secondary batteries could be a viable alternative to the present Li-ion technology because of their high volumetric capacity (8040 mAh cm(-3) for Al vs 2046 mAh cm(-3) for Li). Additionally, the low cost aluminum makes these batteries appealing for large-scale electrical energy storage. Here, we describe the evolution of the various aluminum systems, starting from those based on aqueous electrolytes to, in more details, those based on non-aqueous electrolytes. Particular attention has been dedicated to the latest development of electrolytic media characterized by low reactivity towards other cell components. The attention is then focused on electrode materials enabling the reversible aluminum intercalation-deintercalation process. Finally, we touch on the topic of high-capacity aluminum-sulfur batteries, attempting to forecast their chances to reach the status of practical energy storage systems.

Infiltrated Porous Polymer Sheets as Free-Standing Flexible Lithium-Sulfur Battery Electrodes

Free-standing, high-capacity Li2 S electrodes with capacity loadings in the range from 1.5 to 3.8 mA h cm(-2) are produced by using infiltration of active materials into porous carbonized biomass sheets. The proposed electrode design can be effectively utilized for the low-cost fabrication of flexible lithium batteries with high specific energy.

A binder-free sulfur/carbon composite electrode prepared by a sulfur sublimation method for Li–S batteries

A binder-free sulfur/carbon composite electrode was prepared by a sulfur sublimation method. Sulfur nanoparticles fill large pores in a carbon paper substrate and the composite electrode shows a long cycle life of over 200 cycles in Li–S batteries.