Status and prospects in sulfur–carbon composites as cathode materials for rechargeable lithium–sulfur batteries

Sulfur stands as a very promising cathode candidate for the next-generation rechargeable batteries due to its high energy density, natural abundance, low cost and environmental friendliness. However, the application of lithium–sulfur batteries suffers from low sulfur utilization and poor cycle life of the sulfur cathode. The problems are mainly ascribed to the electrically insulating nature of sulfur and the discharge products, and to the dissolution of the reaction intermediates of polysulfides. Among various approaches, fabricating sulfur–carbon composite cathodes with sulfur embedded within conductive carbon frameworks has been proven promising. Carbon materials, including nanoporous carbon, carbon nanotubes, graphene nanosheets and some other forms, have excellent conductivity, robust chemistry, good mechanical stability, and great abundance. By constraining sulfur within carbon frameworks, the conductivity of the sulfur electrode can be greatly enhanced, and the dissoluble loss of intermediate sulfur species in the liquid electrolyte can also be restrained due to the sorption properties of carbon, leading to a much improved electrochemical performance. This review summarizes the progresses in the sulfur–carbon composite cathodes for lithium–sulfur batteries in recent years, and introduces the roles and the effectiveness of various carbon structures on the electrochemical properties.     

Disordered mesoporous carbon as polysulfide reservoir for improved cyclic performance of lithium–sulfur batteries

Mesoporous carbon, which was templated by colloidal silica, was added to a sulfur cathode as a functional material to confine polysulfides to improve the cyclic performance of lithium–sulfur batteries. To investigate the effect of the pore size and the pore volume of mesoporous carbon on the absorption characteristics of the Li-polysulfides, mesoporous carbons with various pore sizes and total pore volumes were prepared by varying the size and the amount of colloidal silica templates. The results show that mesoporous carbon-containing sulfur cathode enhanced the cyclic performance of the batteries significantly. Comparable performances were observed regardless of pore size, suggesting that the pore size is not a critical factor affecting the absorption characteristics of the Li-polysulfides. However, the cyclic performance was affected by the total pore volume, suggesting that a certain pore volume is necessary to confine the majority of the soluble Li-polysulfides generated during cycling and to enhance sulfur utilization. The novel results obtained in this study will contribute to the consolidation of S electrochemistry and further development of high-energy lithium–sulfur batteries.     

The electrochemical performance of lithium–sulfur batteries with LiClO4 DOL/DME electrolyte

The electrochemical performance of lithium–sulfur batteries with LiClO₄ DOL/DME as electrolyte was investigated. Impedance and SEM analysis indicated that too high content of DME(Dimethoxy ethane) in electrolyte could raise the interfacial resistance of battery due to the impermeable layer formed on the surface of the sulfur cathode, which led to bad cycle performance, while the increase of DOL(1,3-dioxolane) could change those phenomena. The optimal composition of electrolyte was DME:DOL = 2:1 (v/v). With this electrolyte, the lithium–sulfur battery obtained a high initial discharge capacity of 1,200 mA h g^?1 and still remained 800 mA h g^?1 after 20 cycles.     

Pitfalls in the characterization of sulfur/carbon nanocomposite materials for lithium–sulfur batteries

The preparation of sulfur/carbon composite materials for lithium–sulfur batteries is currently a very active research field. Thereto, nanoporous carbon materials are mixed with or infiltrated by sulfur to provide a close contact between both compounds. The characterization of these often complex and on the nanoscale structured composite materials is usually done by vacuum based methods such as nitrogen physisorption or scanning electron microscopy, for example. In this study we show that results from these measurements can be misinterpreted. The reason is the volatility of sulfur that leads to a rapid migration and continuous redistribution effects, especially at low pressures and/or elevated temperatures. For nitrogen physisorption this means that virtually identical isotherms are found for S₈/C samples, irrespective of their pre-treatment, making it impossible to prove intentional nanostructuring by pore filling. Similar effects are found for scanning electron microscopy studies where sulfur migration and contamination of originally sulfur free samples is evidenced in situ. Further evidence is provided by macroscopic experiments combined with elementary analysis. The results show that characterizing the structure of S₈/C composite materials or electrodes is very challenging. In addition, the observed rapid sulfur redistribution might also have important consequences for the performance of practical lithium-sulfur batteries.     

Particle size control of cathode components for high-performance all-solid-state lithium–sulfur batteries

Understanding the size effect of each component on battery performance is essential for designing high-performance Li₂S/S cathode for all-solid-state Li–S batteries. However, the size effects of different components are always coupled because ball-milling, an indispensable process to synthesize reversible cathode, simultaneously and uncontrollably reduces the particle size of all the components. Here, a liquid-phase method, without ball-milling, is developed to synthesize the Li₂S composite cathode, so that the particle size of the active material Li₂S and the solid electrolyte Li₃PS₄ (LPS) can be independently controlled at nano- or microscale. This helps reveal that compositing Li₂S and the conductive agent at nanoscale is essential for enhancing the reaction kinetics, whereas the nanoscale particle size and homogenous distribution of LPS is important for accommodating the large volume change of the cathode. By reducing the particle size of Li₂S to 9.4 nm and that of LPS to 44 nm, the liquid-phase-synthesized composite cathode exhibits reversible capacity and 100% utilization of Li₂S under 0.1 C rate.     

Sucrose derived microporous–mesoporous carbon for advanced lithium–sulfur batteries

A microporous–mesoporous carbon has been successfully prepared via carbonization of sucrose followed by heat treatment process. The obtained porous carbon possesses abundant micropores and mesopores, which can effectively increase the sulfur loading. The composite exhibited a remarkable initial capacity of 1185 mAh g^?1 at 0.2 A g^?1 and maintained at 488 mAh g^?1 after 200 cycles, when employed for lithium?sulfur batteries. Moreover, the composite displayed enhanced rate capabilities of 1124, 914 and 572 mAh g^?1 at 0.2, 0.5 and 1.0 A g^?1. The outstanding electrochemical capabilities and facile low?cost preparation make the new microporous–mesoporous carbon as an excellent candidate for lithium sulfur batteries.     

Large-scale synthesis of ordered mesoporous carbon fiber and its application as cathode material for lithium–sulfur batteries

A novel type of one-dimensional ordered mesoporous carbon fiber has been prepared via the electrospinning technique by using resol as the carbon source and triblock copolymer Pluronic F127 as the template. Sulfur is then encapsulated in this ordered mesoporous carbon fibers by a simple thermal treatment. The interwoven fibrous nanostructure has favorably mechanical stability and can provide an effective conductive network for sulfur and polysulfides during cycling. The ordered mesopores can also restrain the diffusion of long-chain polysulfides. The resulting ordered mesoporous carbon fiber sulfur (OMCF-S) composite with 63% S exhibits high reversible capacity, good capacity retention and enhanced rate capacity when used as cathode in rechargeable lithium–sulfur batteries. The resulting OMCF-S electrode maintains a stable discharge capacity of 690 mAh/g at 0.3 C, even after 300 cycles.     

Cathode materials based on carbon nanotubes for high-energy-density lithium–sulfur batteries

The rational integration of conductive nanocarbon scaffolds and insulative sulfur is an efficient method to build composite cathodes for high-energy-density lithium–sulfur batteries. The full demonstration of the high-energy-density electrodes is a key issue towards full utilization of sulfur in a lithium–sulfur cell. Herein, carbon nanotubes (CNTs) that possess robust mechanical properties, excellent electrical conductivities, and hierarchical porous structures were employed to fabricate carbon/sulfur composite cathode. A family of electrodes with areal sulfur loading densities ranging from 0.32 to 4.77 mg cm⁻² were fabricated to reveal the relationship between sulfur loading density and their electrochemical behavior. At a low sulfur loading amount of 0.32 mg cm⁻², a high sulfur utilization of 77% can be achieved for the initial discharge capacity of 1288 mAh g_S⁻¹, while the specific capacity based on the whole electrode was quite low as 84 mAh g_{C/S+binder+Al}⁻¹ at 0.2 C. Moderate increase in the areal sulfur loading to 2.02 mg cm⁻² greatly improved the initial discharge capacity based on the whole electrode (280 mAh g_{C/S+binder+Al}⁻¹) without the sacrifice of sulfur utilization. When sulfur loading amount further increased to 3.77 mg cm⁻², a high initial areal discharge capacity of 3.21 mAh cm⁻² (864 mAh g_S⁻¹) was achieved on the composite cathode.     

Sulfur infiltrated mesoporous graphene–silica composite as a polysulfide retaining cathode material for lithium–sulfur batteries

The lithium–sulfur (Li–S) system is an attractive candidate to replace the current state-of-the-art lithium-ion battery due to the promising theoretical charge capacity of 1675 mA h/g and energy density of 2500 Wh/kg; however, the dissolution of intermediate polysulfides into the organic liquid electrolyte during cycling hinders its practical realization. We report the synthesis of mesoporous graphene–silica composite (m-GS) as a supporting material of sulfur for Li–S batteries. The ordered porous silica structure was synthesized parallel to functionalized graphene sheets (FGSs) through the ternary cooperative assembly of the graphene, silica, and block copolymer precursors. The well-defined, unique mesoporous structure integrates the electronic conductivity of graphene and the dual functions of silica as a structure building block and in situ polysulfide ab-/ad-sorbing agent to give a Li–S battery that has both good retention ability of polysulfides and good rate capability.     

Studies on electrochemical properties of poly (ethylene oxide)-based gel polymer electrolytes with the effect of chitosan for lithium–sulfur batteries

Composite gel polymer electrolytes (CGPE), with different proportions of poly (ethylene oxide), plasticizers namely 1,3-dioxolane (DIOX)/tetraethelyneglygol dimethylether (TEGDME) and a lithium salt (LITFSI) with the addition of filler chitosan were prepared using the solution casting technique in an argon atmosphere. The membranes were subjected to various characterization techniques such as TG/DTA, FTIR and an ac impedance analysis. A lithium symmetric cell (Li/CGPE/Li) was assembled and the interfacial stability of the polymer electrolyte with a lithium anode was measured. The electrochemical stability and the transport properties of the high conducting sample were also measured. TG/DTA shows the thermal stability of the high conducting sample. The optimal value of the plasticizers was to be found in the ionic conductivity point of view.     

High-loading polysulfide/cement cathode for the manufacture of dynamically and statically stable lean-electrolyte lithium–sulfur batteries

In order to design lithium-sulfur cells for practical viability in the high-density energy storage, the exploitation of the effective contribution of the large amount of active-material mass to the high specific capacity at a lean-electrolyte condition must be considered. However, this is limited by the insulating nature of solid-state sulfur/sulfide, and the diffusion loss of the liquid-state polysulfides. In this study, Portland cement is adopted as a reinforcement material to design a high-loading polysulfide/cement cathode with sulfur loading and content of 8.64 mg cm⁻² and 60 wt%, respectively. The nonporous cement exhibits a high polysulfide-trapping capability and low electrolyte consumption, which enable the high-sulfur-loading cathode to achieve record low electrolyte-to-sulfur ratios of 7–3 μL mg⁻¹, with high gravimetric capacity and areal capacity of the cathode (i.e., 768 mA h g⁻¹ and 11.06 mA h cm⁻², respectively) and dynamically/statically electrochemical stability with 90% capacity retention after 100 cycles and a 1-month rest.     

A hierarchical carbon fiber/sulfur composite as cathode material for Li–S batteries

A novel hierarchical structure carbon/sulfur composite is presented based on carbon fiber matrices, which are synthesized by electrospinning. The fibers are constituted with hollow graphitized carbon spheres formed using catalytic Ni nano-particles as hard templates. Sulfur is loaded to the carbon substrates via thermal vaporization. The structure and composition of the hierarchical carbon fiber/S composite are characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and nitrogen adsorption isotherms. The electrochemical performance is evaluated by cyclic voltammetry and galvanostatic charge–discharge. The results exhibit an initial discharge capacity of 845 mA h g⁻¹ at 0.25 C (420 mA g⁻¹), with a retention of 77% after 100 cycles. A discharge capacity of 533 mA h g⁻¹ is still attainable when the rate is up to 1.0 C. The good cycling performance and rate capability are contributed to the uniform dispersion of sulfur, the conductive network of carbon fibers and hollow graphitized carbon spheres.     

Electrospun nanofiber polyacrylonitrile coated separators to suppress the shuttle effect for long-life lithium–sulfur battery

Polymeric coating on the separator with effective polysulfides diffusion inhibition can provide intimate contact between intermediate polysulfides and conductive layer of separator for high-energy lithium–sulfur (Li–S) batteries. Herein, polyacrylonitrile–poly(1,5-diaminoanthraquinone) (PAN/PDAAQ) and PAN-potassium functionalized graphene (PAN/K-FGF) nanofibers are synthesized via electrospinning method and act as effective separators for Li–S batteries to minimize polysulfides diffusion toward the anode. PAN/K-FGF coated separator shows capacity retention of 768 mAh g⁻¹ after 100 cycles at 1C. The capacity maintains at 419 mAh g⁻¹ after 500 cycles. PAN/PDAAQ nanofibers are coated on glass fiber separator functions as physical and chemical barrier for polysulfides diffusion. Therefore, the cell with PAN/PDAAQ coating on the separator demonstrates capacity retention of 881 mAh g⁻¹ after 100 cycles at 1C and small capacity decay rate of 0.11% per cycle resulted in 800 cycles at 1C. PAN/PDAAQ could define as an ideal separator material for Li–S batteries. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48606.     

Construction of hierarchical yolk-shell structured Mn3O4@NC as efficient sulfur hosts for Li–S batteries

The high theoretical capacity and abundant reserves of sulfur makes Li–S batteries a promising candidate for future energy-storage devices. However, the low electrical conductivity of sulfur and severe polysulfides dissolution and migration hinder it practical application. To address the problems, we design a hierarchical yolk-shell structure with polar metal oxide Mn₃O₄ yolks and N-doped carbon shells as sulfur host. The N-doped carbon shell enhances the conductivity and provide physical confinement to polysulfides while the Mn₃O₄ yolks have strong chemical bonding effect with polysulfides. Besides, the sufficient void space in yolk-shell structure can ensure a high sulfur loading content (80%) as well as accommodate severe volume change of sulfur during lithiation. Benefiting from these merits, the yolk-shell Mn₃O₄@NC/S electrode exhibit a high capacity of 581 mAh g^-1 at 1 C and enhanced cycling stability with a capacity retain of 84% over 300 cycles at 0.5 C, which is superior to yolk-shell Mn₃O₄@NC/S with more Mn₃O₄ residual and N-doped carbon shells/S without Mn₃O₄ inside.     

Core–shell CNT–Ni–Si nanowires as a high performance anode material for lithium ion batteries

Core–shell carbon nanotube (CNT)–Si heterogeneous nanowires have been identified as one of the most promising candidates for future anode materials in lithium ion batteries. However, stress in these nanostructures, is the long-existing bottleneck, rendering severe fading of the capacities and even failure of the batteries. We prove that the interfaces between CNT cores and Si shells play a critical role in the stress engineering. With rationally engineered interfaces, our core–shell nanowires with CNT–Ni–Si structure are able to offer excellent capacity retention and rate performance. Introduction of the rough Ni interfacial layer and utilization of the CNT cores lead to reinforced stability of the structure, well accommodated stress and enhanced charge transfer, which are responsible for the improved performance. This core–shell CNT–Ni–Si nanostructure provides a simple but effective pathway towards realization of long lifetime and high performance lithium ion batteries.     

Hydrothermal synthesis of a monoclinic VO2 nanotube–graphene hybrid for use as cathode material in lithium ion batteries

The hydrothermal reaction of a mixture of a colloidal dispersion of graphite oxide and ammonium vanadate yielded a hybrid made of graphene and a nanotubular metastable monoclinic polymorph of VO₂, known as VO₂(B). The formation of VO₂(B) nanotubes is accompanied by the reduction of graphite oxide. Initially the partially scrolled graphite oxide layers act as templates for the crystallization of VO₂(B) in the tubular morphology. This is followed by the reduction of graphite oxide to graphene resulting in a hybrid in which VO₂(B) nanotubes are dispersed in graphene. Electron microscopic studies of the hybrid reveal that the VO₂(B) nanotubes are wrapped by and trapped between graphene sheets. The hybrid shows potential to be a high capacity cathode material for lithium ion batteries. It exhibits a high capacity (～450 mAh/g) and cycling stability. The high capacity of the hybrid is attributed to the interaction between the graphene sheets and the VO₂(B) tubes which improves the charge-transfer. The graphene matrix prevents the aggregation of the VO₂(B) nanotubes leading to high cycling stability.     

Preparation and electrochemical performance of SiCN–CNTs composite anode material for lithium ion batteries

The composite of silicon carbonitride (SiCN) and carbon nanotubes (CNTs) was synthesized by sintering the mixture of polysilylethylenediamine-derived amorphous SiCN and multi-walled CNTs at a temperature of 1,000 °C for 1 h in argon. The as-prepared SiCN–CNTs material, which was used as anode active substance in a lithium ion battery, showed excellent electrochemical performance. Charge–discharge tests showed the SiCN–CNTs anode provided a high initial specific discharge capacity of 1176.6 mA h g⁻¹ and a steady specific discharge capacity of 450–400 mA h g⁻¹ after 30 charge–discharge cycles at 0.2 mA cm⁻². Both of the abovementioned values are higher than that of pure polymer-derived SiCN, CNTs, and commercial graphite at the same charge–discharge condition. It was deduced that the CNTs in the composite not only improved the electronic conductivity and offered channels and sites for the immigrating and intercalating of Li⁺ but also stabilized the structure of the composite.