PVP-Assisted Synthesis of Uniform Carbon Coated Li2S/CB for High-Performance Lithium-Sulfur Batteries

New Scalable Sulfur Cathode Containing Specifically Designed Polysulfide Adsorbing Materials

Because of its considerable theoretical specific capacity and energy density, lithium-sulfur battery technology holds great potential to replace lithium-ion battery technology. However, a versatile, low-cost, and easily scalable bulk synthesis method is essential for translating bench-level development to large-scale production. This paper reports the design and synthesis of a new scalable sulfur cathode, S@CNT/PANI/PPyNT/TiO2 (BTX). The rationally chosen cathode components suppress the migration of polysulfide intermediates via chemical interactions, enhance redox kinetics, and provide electrical conductivity to sulfur, rendering outstanding long-term cycling performance and strong initial specific capacity in terms of electrochemical performance. This cathode's cell demonstrated an initial specific capacity of 740 mA h g-1 at 0.2 C (with a capacity decay rate of 0.08% per cycle after 450 cycles).

Nanostructured Li2S Cathodes for Silicon-Sulfur Batteries

Lithium-sulfur batteries are regarded as an advantageous option for meeting the growing demand for high-energy-density storage, but their commercialization relies on solving the current limitations of both sulfur cathodes and lithium metal anodes. In this scenario, the implementation of lithium sulfide (Li2S) cathodes compatible with alternative anode materials such as silicon has the potential to alleviate the safety concerns associated with lithium metal. In this direction, here, we report a sulfur cathode based on Li2S nanocrystals grown on a catalytic host consisting of CoFeP nanoparticles supported on tubular carbon nitride. Nanosized Li2S is incorporated into the host by a scalable liquid infiltration-evaporation method. Theoretical calculations and experimental results demonstrate that the CoFeP-CN composite can boost the polysulfide adsorption/conversion reaction kinetics and strongly reduce the initial overpotential activation barrier by stretching the Li-S bonds of Li2S. Besides, the ultrasmall size of the Li2S particles in the Li2S-CoFeP-CN composite cathode facilitates the initial activation. Overall, the Li2S-CoFeP-CN electrodes exhibit a low activation barrier of 2.56 V, a high initial capacity of 991 mA h gLi2S-1, and outstanding cyclability with a small fading rate of 0.029% per cycle over 800 cycles. Moreover, Si/Li2S full cells are assembled using the nanostructured Li2S-CoFeP-CN cathode and a prelithiated anode based on graphite-supported silicon nanowires. These Si/Li2S cells demonstrate high initial discharge capacities above 900 mA h gLi2S-1 and good cyclability with a capacity fading rate of 0.28% per cycle over 150 cycles.

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.

A New Graphitic Nitride and Reduced Graphene Oxide-Based Sulfur Cathode for High-Capacity Lithium-Sulfur Cells

Lithium-sulfur (Li-S) batteries can provide at least three times higher energy density than lithium-ion (Li-Ion) batteries. However, Li-S batteries suffer from a phenomenon called the polysulfide shuttle (PSS) that prevents the commercialization of these batteries. The PSS has several undesirable effects, such as depletion of active materials from the cathode, deleterious reactions between the lithium anode and electrolyte soluble lithium polysulfides, resulting in unfavorable coulombic efficiency, and poor cycle life of the battery. In this study, a new sulfur cathode composed of graphitic nitride as the polysulfide absorbing material and reduced graphene oxide as the conductive carbon host has been synthesized to rectify the problems associated with the PSS effect. This composite cathode design effectively retains lithium polysulfide intermediates within the cathode structure. The S@RGO/GN cathode displayed excellent capacity retention compared to similar RGO-based sulfur cathodes published by other groups by delivering an initial specific capacity of 1415 mA h g−1 at 0.2 C. In addition, the long-term cycling stability was outstanding (capacity decay at the rate of only 0.2% per cycle after 150 cycles).

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.

Li2 S-Based Li-Ion Sulfur Batteries: Progress and Prospects

The great demand for high-energy-density batteries has driven intensive research on the Li-S battery due to its high theoretical energy density. Consequently, considerable progress in Li-S batteries is achieved, although the lithium anode material is still challenging in terms of lithium dendrites and its unstable interface with electrolyte, impeding the practical application of the Li-S battery. Li₂ S-based Li-ion sulfur batteries (LISBs), which employ lithium-metal-free anodes, are a convenient and effective way to avoid the use of lithium metal for the realization of practical Li-S batteries. Over the past decade, studies on LISBs are carried out to optimize their performance. Herein, the research progress and challenges of LISBs are reviewed. Several important aspects of LISBs, including their working principle, the physicochemical properties of Li₂ S, Li₂ S cathode material composites, LISBs full batteries, and electrolyte for Li₂ S cathode, are extensively discussed. In particular, the activation barrier in the initial charge process is fundamentally analyzed and the mechanism is discussed in detail, based on previous reports. Finally, perspectives on the future direction of the research of LISBs are proposed.

An Ultrahigh Capacity Graphite/Li2S Battery with Holey-Li2S Nanoarchitectures

The pairing of high-capacity Li2S cathode (1166 mAh g-1) and lithium-free anode (LFA) provides an unparalleled potential in developing safe and energy-dense next-generation secondary batteries. However, the low utilization of the Li2S cathode and the lack of electrolytes compatible to both electrodes are impeding the development. Here, a novel graphite/Li2S battery system, which features a self-assembled, holey-Li2S nanoarchitecture and a stable solid electrolyte interface (SEI) on the graphite electrode, is reported. The holey structure on Li2S is beneficial in decomposing Li2S at the first charging process due to the enhanced Li ion extraction and transfer from the Li2S to the electrolyte. In addition, the concentrated dioxolane (DOL)-rich electrolyte designed lowers the irreversible capacity loss for SEI formation. By using the combined strategies, the graphite/holey-Li2S battery delivers an ultrahigh discharge capacity of 810 mAh g-1 at 0.1 C (based on the mass of Li2S) and of 714 mAh g-1 at 0.2 C. Moreover, it exhibits a reversible capacity of 300 mAh g-1 after a record lifecycle of 600 cycles at 1 C. These results suggest the great potential of the designed LFA/holey-Li2S batteries for practical use.

Directly Formed Alucone on Lithium Metal for High-Performance Li Batteries and Li-S Batteries with High Sulfur Mass Loading

Lithium metal is considered the "holy grail" of next-generation battery anodes. However, severe parasitic reactions at the lithium-electrolyte interface deplete the liquid electrolyte and the uncontrolled formation of high surface area and dendritic lithium during cycling causes rapid capacity fading and battery failure. Engineering a dendrite-free lithium metal anode is therefore critical for the development of long-life batteries using lithium anodes. In this study, we deposit a conformal, organic/inorganic hybrid coating, for the first time, directly on lithium metal using molecular layer deposition (MLD) to alleviate these problems. This hybrid organic/inorganic film with high cross-linking structure can stabilize lithium against dendrite growth and minimize side reactions, as indicated by scanning electron microscopy. We discovered that the alucone coating yielded several times longer cycle life at high current rates compared to the uncoated lithium and achieved a steady Coulombic efficiency of 99.5%, demonstrating that the highly cross-linking structured material with great mechanical properties and good flexibility can effectively suppress dendrite formation. The protected Li was further evaluated in lithium-sulfur (Li-S) batteries with a high sulfur mass loading of ∼5 mg/cm². After 140 cycles at a high current rate of ∼1 mA/cm², alucone-coated Li-S batteries delivered a capacity of 657.7 mAh/g, 39.5% better than that of a bare lithium-sulfur battery. These findings suggest that flexible coating with high cross-linking structure by MLD is effective to enable lithium protection and offers a very promising avenue for improved performance in the real applications of 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.

Anatase TiO2 as a Cheap and Sustainable Buffering Filler for Silicon Nanoparticles in Lithium-Ion Battery Anodes

The design of effective supporting matrices to efficiently cycle Si nanoparticles is often difficult to achieve and requires complex preparation strategies. In this work, we present a simple synthesis of low-cost and environmentally benign aAnatase TiO₂ nanoparticles as buffering filler for Si nanoparticles (Si@TiO₂ ). The average anatase TiO₂ crystallite size was approximately 5 nm. A complete structural, morphological, and electrochemical characterization was performed. Electrochemical test results show very good specific capacity values of up to 1000 mAh g^-1 and cycling at several specific currents, ranging from 500 to 2000 mA g^-1 , demonstrating a very good tolerance to high cycling rates. Postmortem morphological analysis shows very good electrode integrity after 100 cycles at 500 mA g^-1 specific current.

In Situ Mg/MgO-Embedded Mesoporous Carbon Derived from Magnesium 1,4-Benzenedicarboxylate Metal Organic Framework as Sustainable Li-S Battery Cathode Support

Development of advanced carbon cathode support with the ability to accommodate high sulfur (S) content as well as effective confinement of the sulfur species during charge-discharge is of great importance for sustenance of Li-S battery. A facile poly(vinylpyrrolidone)-assisted solvothermal method is reported here to prepare Mg-1,4-benzenedicarboxylate metal organic framework (MOF) from which mesoporous carbon is derived by thermal treatment, where the hexagonal sheetlike morphology of the parent MOF is retained. Existence of abundant pores of size 4 and 9 nm extended in three dimensions with zigzag mazelike channels helps trapping of S in the carbon matrix through capillary effect, resulting in high S loading. When tested as a cathode for lithium-sulfur battery, a reversible specific capacity of 1184 mAh g^-1 could be achieved at 0.02 C. As evidenced by X-ray photoelectron spectroscopy, in situ generated Mg in the carbon structure enhances the conductivity, whereas MgO provides support to S immobilization through chemical interactions between Mg and sulfur species for surface polarity compensation, restricting the dissolution of polysulfide into the electrolyte, the main cause for the "shuttle phenomenon" and consequent capacity fading. The developed cathode shows good electrochemical stability with reversible capacities of 602 and 328 mAh g^-1 at 0.5 and 1.0 C, respectively, with retentions of 64 and 67% after 200 cycles. The simple MOF-derived strategy adopted here would help design new carbon materials for Li-S cathode support.

Stabilizing the Performance of High-Capacity Sulfur Composite Electrodes by a New Gel Polymer Electrolyte Configuration

Increased pollution and the resulting increase in global warming are drawing attention to boosting the use of renewable energy sources such as solar or wind. However, the production of energy from most renewable sources is intermittent and thus relies on the availability of electrical energy-storage systems with high capacity and at competitive cost. Lithium-sulfur batteries are among the most promising technologies in this respect due to a very high theoretical energy density (1675 mAh g^-1 ) and that the active material, sulfur, is abundant and inexpensive. However, a so far limited practical energy density, life time, and the scaleup of materials and production processes prevent their introduction into commercial applications. In this work, we report on a simple strategy to address these issues by using a new gel polymer electrolyte (GPE) that enables stable performance close to the theoretical capacity of a low cost sulfur-carbon composite with high loading of active material, that is, 70 % sulfur. We show that the GPE prevents sulfur dissolution and reduces migration of polysulfide species to the anode. This functional mechanism of the GPE membranes is revealed by investigating both its morphology and the Li-anode/GPE interface at various states of discharge/charge using Raman spectroscopy.

Scalable Synthesis of Honeycomb-like Ordered Mesoporous Carbon Nanosheets and Their Application in Lithium-Sulfur Batteries

There is a growing need to improve the electrical conductivity of the cathode and to suppress the rapid capacity decay during cycling in lithium-sulfur (Li-S) batteries. This can be achieved by developing facile methods for the synthesis of novel nanostructured carbon materials that can function as effective cathode hosts. In this Article, we report the scalable synthesis of ordered mesoporous carbon nanosheets (OMCNS) via the etching of self-assembled iron oxide/carbon hybrid nanosheets (IO-C NS), which serve as an advanced sulfur host for Li-S batteries. The obtained two-dimensional (2D) nanosheets have close-packed uniform cubic mesopores of ∼20 nm side length, and the gap between the pores is ∼4 nm, which resembles the honeycomb structure consisting of an ordered array of hexagonal pores. We loaded OMCNS with sulfur by a simple melting infusion process and evaluated the performance of the resulting OMCNS-sulfur composites as the cathode material. As a result, the sulfur-loaded OMCNS hybrid (OMCNS-S) electrode infiltrated with 70 wt % sulfur delivers a high and stable reversible capacity of 505.7 mA h g^-1 after 500 cycles at 0.5 C-rate with excellent capacity retention (a decay of 0.081% per cycle) and excellent rate capability (580.6 mA h g^-1 at a high current density of 2 C). The improved electrochemical properties could be attributed to the fact that the uniform cubic mesopores offer sufficient space for the volume expansion of sulfur inside them and therefore trap the polysulfides during the charging-discharging process. Therefore, these unique structured carbon nanosheets can be promising candidates for other energy-storage applications.

New insights into the electrode mechanism of lithium sulfur batteries via air-free post-test analysis

Schematic models of ball-milled Li₂S cathodes and carbon-coated Li₂S (Li₂S@C) cathodes during charge/discharge cycles.