Double-shelled hollow carbon sphere with microporous outer shell towards high performance lithium-sulfur battery

Customized Structure Design and Functional Mechanism Analysis of Carbon Spheres for Advanced Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are attracting much attention due to their high theoretical energy density and are considered to be the predominant competitors for next-generation energy storage systems. The practical commercial application of LSBs is mainly hindered by the severe "shuttle effect" of the lithium polysulfides (LiPSs) and the serious damage of lithium dendrites. Various carbon materials with different characteristics have played an important role in overcoming the above-mentioned problems. Carbon spheres (CSs) are extensively explored to enhance the performance of LSBs owing to their superior structures. The review presents the state-of-the-art advances of CSs for advanced high-energy LSBs, including their preparation strategies and applications in inhibiting the "shuttle effect" of the LiPSs and protecting lithium anodes. The unique restriction effect of CSs on LiPSs is explained from three working mechanisms: physical confinement, chemical interaction, and catalytic conversion. From the perspective of interfacial engineering and 3D structure designing, the protective effect of CSs on the lithium anode is also analyzed. Not only does this review article contain a summary of CSs in LSBs, but also future directions and prospects are discussed. The systematic discussions and suggested directions can enlighten thoughts in the reasonable design of CSs for LSBs in near future.

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.

Simultaneously enhancing redox kinetics and inhibiting the polysulfide shuttle effect using MOF-derived CoSe hollow sphere structures for advanced Li-S batteries

Lithium-sulfur (Li-S) batteries generally suffer from a serious "shuttle effect" during the charging/discharging process, resulting in the loss of active components and sluggish redox reaction kinetics that hinder the cycle life and rate performance of the battery. To address this, CoSe/C hollow structures (CoSe/C HSs) were prepared via a simple hydrothermal strategy and used as a sulfur host for Li-S batteries. The battery with CoSe/C HSs exhibited a high initial specific discharge capacity of 1405 mA h g-1 with a coulombic efficiency of 99.8% at 0.1C. Additionally, S@CoSe/C HS cathodes with a high sulfur loading of 5.1 mg cm-2 delivered a considerable specific discharge capacity of 1256.1 mA h g-1 and maintained a high capacity of 1120 mA h g-1 after 100 cycles with a capacity decay rate of 0.11% per cycle at 0.1C. The unique raspberry-like structure of CoSe/C HSs prevents polysulfides from escaping the cathode host via both physical containment and the formation of Co-S and Se-Li chemical bonds, and it also enhances the polysulfide redox kinetics. Furthermore, the peculiar raspberry-like structure can withstand volume changes during charging/discharging to better protect the cathode.

Two Competing Reactions of Sulfurized Polyacrylonitrile Produce High-Performance Lithium-Sulfur Batteries

Sulfurized polyacrylonitrile (SPAN) is an attractive cathode candidate for the advanced lithium-sulfur (Li-S) batteries owing to its outstanding cyclic stability. Nevertheless, SPAN suffers from inadequate initial Coulombic efficiency (CE) induced by the sluggish reaction kinetics, which is primarily ascribed to the low Li-ion diffusivity and high electronic resistivity of the Li₂S product. In this work, an optimal trace amount of soluble lithium polysulfide of Li₂S₈ is introduced as a redox mediator for a freestanding fibrous SPAN cathode to enhance the reversible oxidation efficiency of Li₂S. During the delithiation process, the chemical interactions between Li₂S and Li₂S₈ additive facilitate the electrochemical oxidation of Li₂S, resulting in the transformation of not only C-S/S-S bonds in SPAN but also elemental sulfur. Benefiting from the synergistic effect of the two competing reactions, a high initial CE of 82.9% could be achieved at a current density of 200 mA g^-1. Moreover, a superior capacity retention along with a high capacity of 1170 mAh g^-1 up to the 400th cycle is available at 1000 mA g^-1. The study offers a feasible approach for Li-S batteries toward the practical applications of SPAN.

A Tröger's Base-Derived Covalent Organic Polymer Containing Carbazole Units as a High-Performance Supercapacitor

Porous organic polymers have been received considerable attention due to their heteroatom-containing structures and high surface areas, which can offer high electrochemical performance in energy applications. The majority of reported Tröger's base-functionalized porous organic polymers have been applied as effective candidates for sensing and gas separation/adsorption, while their use as electrode materials in supercapacitors is rare. Here, a novel covalent microporous organic polymer containing carbazole and Tröger's base CzT-CMOP has been successfully synthesized through the one-pot polycondensation of 9-(4-aminophenyl)-carbazole-3,6-diamine (Cz-3NH₂) with dimethoxymethane. The polycondensation reaction's regioselectivity was studied using spectroscopic analyses and electronic structure calculations that confirmed the polycondensation occurred through the second and seventh positions of the carbazole unit rather than the fourth and fifth positions confirmed by first-principles calculations. Our CzT-CMOP exhibited high thermal stability of approximately 463.5 °C and a relatively high Brunauer-Emmett-Teller surface area of 615 m² g^-1 with a nonlocal density functional theory's pore size and volume of 0.48 cm³ g^-1 and 1.66 nm, respectively. In addition, the synthesized CzT-CMOP displayed redox activity due to the existence of a redox-active carbazole in the polymer skeleton. CzT-CMOP revealed high electrochemical performance when used as active-electrode material in a three-electrode supercapacitor with an aqueous electrolyte of 6 M KOH, and it showed specific capacitance of 240 F g^-1 at a current density of 0.5 A g^-1 with excellent stability after 2000 cycles of 97% capacitance retention. Accordingly, such porous organic polymer appears to have a variety of uses in energy-related applications.

Conductive Porous Laminated Vanadium Nitride as Carbon-Free Hosts for High-Loading Sulfur Cathodes in Lithium-Sulfur Batteries

Improving the sulfur loading in cathodes is a significant challenge for practical lithium-sulfur batteries. Although carbonaceous sulfur hosts can achieve higher sulfur content and loading, the low tap densities of carbonaceous materials lead to low volumetric energy densities, restricting practical application. Here, conductive porous laminated vanadium nitride (VN) as a carbon-free sulfur host has been successfully developed to construct high tap density, high sulfur loading, and high energy density sulfur electrodes. The laminated stacking multiscale VN featuring interconnected holes possesses high storage space for sulfur loading, achieving high sulfur loading and utilization. VN@S materials' sulfur content and tap density can achieve 80 wt % and 1.17 g cm^-3, respectively. At the sulfur loading of 1.0 mg cm^-2, the VN@S cathode reaches the reversible capacity of 790 mAh g^-1 at 1 C after 200 cycles and 145.2 mAh g^-1 at 15 C after 500 cycles. Precisely, at a high sulfur loading of 12.6 mg cm^-2, the VN@S cathode delivers a reversible capacity of 518.8 mAh g^-1 (485.6 mAh cm^-3) at 0.1 C after 100 cycles.

Self-assembled N-doped carbon with a tube-in-tube nanostructure for lithium-sulfur batteries

Lithium-sulfur batteries hold broad prospects as the low-cost and high-energy storage system. However, the practical application is limited by the intrinsic insulating nature of sulfur and severe shuttle effect of soluble polysulfide intermediates. Herein, we demonstrate a convenient self-assembly strategy for encapsulating carbon nanotubes in nitrogen-doped hollow carbon shells, to construct a nitrogen-doped tube-in-tube carbon nanostructure (NTTC) as a host material of sulfur. In this peculiar structure, the highly conductive carbon nanotube cores facilitate the electron transfer while the hollow porous structure is capable of accommodating high sulfur content of 70 wt% in the composites. Moreover, the nitrogen doping helps to alleviate the shuttle effect owing to enhanced chemisorption towards polysulfides. Benefiting from these merits, the NTTC/S composite with the high areal mass loading of ~2.5 mg cm^-2 presents a high reversible capacity (1346.9 mAh g^-1 at 0.05 C) and excellent rate capability (533.5 mAh g^-1 at 3C). More impressively, NTTC/S electrode exhibits good cycling stability at a high rate of 2 C corresponding to slight capacity decay of 0.055% per cycle over 500 discharge/charge cycles.

Synthesis and Electrochemical Energy Storage Applications of Micro/Nanostructured Spherical Materials

Micro/nanostructured spherical materials have been widely explored for electrochemical energy storage due to their exceptional properties, which have also been summarized based on electrode type and material composition. The increased complexity of spherical structures has increased the feasibility of modulating their properties, thereby improving their performance compared with simple spherical structures. This paper comprehensively reviews the synthesis and electrochemical energy storage applications of micro/nanostructured spherical materials. After a brief classification, the concepts and syntheses of micro/nanostructured spherical materials are described in detail, which include hollow, core-shelled, yolk-shelled, double-shelled, and multi-shelled spheres. We then introduce strategies classified into hard-, soft-, and self-templating methods for synthesis of these spherical structures, and also include the concepts of synthetic methodologies. Thereafter, we discuss their applications as electrode materials for lithium-ion batteries and supercapacitors, and sulfur hosts for lithium–sulfur batteries. The superiority of multi-shelled hollow micro/nanospheres for electrochemical energy storage applications is particularly summarized. Subsequently, we conclude this review by presenting the challenges, development, highlights, and future directions of the micro/nanostructured spherical materials for electrochemical energy storage.

Rational Fabrication of Nitrogen and Sulfur Codoped Carbon Nanotubes/MoS2 for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries are more promising and attractive than lithium-ion batteries owing to a higher charge-storage capacity. However, their commercial applications are hindered by an undesirable polysulfide shuttling effect during the cycling procedure. Herein, nitrogen and sulfur codoped carbon nanotubes intertwined with flower-like molybdenum disulfide (NSCNTs/MoS₂ ) were synthesized by using a feasible hydrothermal method and used as effective lithium polysulfides (LiPSs) tamers. The NSCNTs/MoS₂ had a strong hybrid structure with strong interfacial interactions for physical confinement, chemical adsorption, and electrocatalytic conversion of intermediate LiPSs during the charge-discharge process. The NSCNTs intensified the flexibility and constructed a conductive framework for rapid ion/electron transfer, whereas the electrocatalysis of MoS₂ managed the sulfur reaction chemistry in two ways: it chemically immobilized LiPSs through Li-S bonds and kinetically sped up the sulfur redox reactions. Owing to these merits, the Li-S cell with a NSCNTs/MoS₂ host and NSCNTs/MoS₂ -coated separator (NSCNTs/MoS₂ /S-NM) exhibited a high reversible capacity of 814 mAh g^-1 at 1.0 C and long-lasting cycling durability with an ultralow capacity decay of 0.02 % per cycle over 1000 cycles. Accordingly, rationally integrating the concepts of physical immobilization, chemical capture, and electrocatalysis to establish a multifaced cell structure for the architecture of high-performance Li-S batteries is a significant strategy.

TiO2 and Co Nanoparticle-Decorated Carbon Polyhedra as Efficient Sulfur Host for High-Performance Lithium-Sulfur Batteries

Metal organic frameworks (MOFs)-derived porous carbon is proposed as a promising candidate to develop novel, tailorable structures as polysulfides immobilizers for lithium-sulfur batteries because of their high-efficiency electron conductive networks, open ion channels, and abundant central ions that can store a large amount of sulfur and trap the easily soluble polysulfides. However, most central ions in MOFs-derived carbon framework are encapsulated in the carbon matrix so that their exposures as active sites to adsorb polysulfides are limited. To resolve this issue, highly dispersed TiO₂ nanoparticles are anchored into the cobalt-containing carbon polyhedras that are converted from ZIF-67. Such a type of TiO₂ and Co nanoparticles-decorated carbon polyhedras (CCo/TiO₂ ) provide more exposed active sites and much stronger chemical trapping for polysulfides, hence improving the sulfur utilization and enhancing reaction kinetics of sulfur-containing cathode simultaneously. The sulfur-containing carbon polyhedras decorated with TiO₂ nanoparticles (S@CCo/TiO₂ ) show a significantly improved cycling stability and rate capability, and deliver a discharge capacity of 32.9% higher than that of TiO₂ -free S@CCo cathode at 837.5 mA g^-1 after 200 cycles.