Cathode For Lithium-sulfur Batteries Research Articles

Polyglycerol-functionalized microporous carbon/sulfur cathode for Li-S battery

Lithium–sulfur (Li-S) batteries are promising next-generation rechargeable devices that have low cost and high energy densities. However, the commercialization of Li-S batteries is challenging because lithium polysulfide (Li2Sn), which forms during the charge–discharge process, dissolves in the electrolyte. To suppress Li2Sn dissolution, we propose a method of modifying microporous carbon with polyglycerol, in which only microporous carbon materials and monomers are mixed and heated. Using a sulfur cathode with polyglycerol-treated microporous carbon as a host material for the sulfur, the capacity retention (discharge capacity at the 50th cycle divided by the discharge capacity at the 2nd cycle) improved from 57.6 to 75.2%. We also used Fourier-transform infrared spectroscopy to reveal that polyglycerol treatment suppresses the dissolution of Li2Sn from the cathode. Finally, polyglycerol treatment is shown to improve the cycle performance of Li-S batteries.

Suppressing Polysulfides Shuttling and Promoting Sulfur Utilization via Transition Metal and Nitrogen Co-Doping on Graphdiyne Cathodes of Lithium-Sulfur Batteries: A First-Principles Modeling

While lithium-sulfur (Li-S) batteries are considered the next-generation energy storage devices, several issues inhibit their commercialization, including poor conductivity, shuttling of lithium polysulfides (LiPSs), and sluggish decomposition of small LiPSs. Here, we illustrate through first-principles modeling that doping graphdiyne (GDY) with transition metal (Co, Fe) and nitrogen (TM-N) effectively suppresses the shuttle effect and enhances sulfur utilization by promoting dissociation of small LiPSs. We find that the TM-N doping provides additional electrons to the substrate, which could improve the redox activity of the substrates. The additional electrons also improve charge transfer between the substrate and adsorbate that enhances the LiPS adsorption. Electronic structure analysis reveals that the strong interactions originate from the formation of TM-S bonds and electrostatic attractions between Li and N. Such strong adsorptions prevail in the interaction between the LiPSs and electrolyte molecules; hence, the doped GDY could effectively prevent the dissolution of LiPSs and inhibit the shuttle effect. Moreover, the doped GDY substrates exhibit small barriers of Li–S and Li–N bond breaking that facilitate the decomposition of small LiPSs. This implies that TM-N-doped GDY promotes kinetics of LiPS transformation, which increases sulfur utilization and diminishes capacity fading upon charge–discharge cycles. From the computational viewpoints, the TM-N-doped GDY substrates are considered promising cathodes of Li-S batteries. This work serves as a useful guide to rationally design high-performance two-dimensional (2D) carbon-based cathode materials for Li-S batteries.

The Fig-Like hierarchical Double-Shelled hollow TiN particles as sulfur host for Lithium-Sulfur batteries

Lithium-sulfur battery(LSB)’s commercial manufacturing has been mainly retarded by the “shuttle effect” and low electrical conductivity of polysulfides (LiPSs). Designing a cathode with hollow and hierarchically porous structures was expected to solve the above problems. Herein, a kind of TiN particles with the hierarchical hollow double-shelled structures was synthesized and applied to cathodes of LSB. The Fig-like hollow TiO2 particles (FHTiO2s) were firstly synthesized by the hard-template method. Subsequently, the Fig-like hierarchical hollow double-shelled TiN particles (FHTiNs) were synthesized by the template-free sequential transformation and separation approach (STSA). It was verified that the heating temperature and time were key parameters. Special Fig-like double-shell hollow structures could greatly increase the loading of S, and the excellent initial capacity of FHTiNs cathodes was up to 1159 mAh/g. On the one hand, the Fig-like framework in internal cavity and double-shell structures could promote the ultrahigh specific surface area, and the adsorption to LiPSs was improved by increasing active sites; On the other hand, the shuttle effect of LiPSs was weakened by the fig-like framework and double-shell structures, which slowed down the massive dissolution of sulfur in the electrolyte. As a result, the pleasant rate performance of FHTiNs cathodes was up to 400 mAh/g at 5C. This novel structures and synthesis method provided a new strategy for the designing of LSB cathodes.

Solid Carbon Spheres with Interconnected Open Pore Channels Enabling High-Efficient Polysulfide Conversion for High-Rate Lithium-Sulfur Batteries.

Hollow carbon spheres or core-sheath porous carbon spheres have been widely used in the S cathode of lithium-sulfur batteries. However, the sphere shells or the pore walls may block the free transport of active species to a certain extent and may have a negative influence on the effective accommodation of elemental sulfur. Herein, solid but porous carbon spheres (PNCS) with large porosity and high specific surface area are developed, which enable high sulfur loading and ample cathode/electrolyte contact area, and the interconnected open pore channels significantly shorten the ion/electron transport pathways. Together with high-conducting nitrogen-doped graphene (NG), facilitated polysulfide conversion kinetics is realized in the as-assembled Li-S batteries, which deliver a high initial discharge capacity of 1445 mAh g-1 at 0.2 C, excellent rate capability of 872 mAh g-1 at 4 C, and low capacity decay of 0.047% per cycle for 500 cycles at 1 C. Even under high sulfur loading of 5.5 mg cm-2 and low electrolyte/sulfur (E/S) ratio of 5 μL mg-1, the Li-S batteries still display high specific capacities of 896 mAh g-1 and 4.96 mAh cm-2. The real application of PNCS/NG is also demonstrated by the corresponding Li-S pouch cells showing high discharging capacity and stable open circuit voltage. This work exhibits the promising application of the solid carbon spheres as the S host for effectively addressing the polysulfide shuttle and propelling the development of high-performance Li-S batteries.

Hybridization of Mesoporous Carbon and Iron Oxide for Better Mitigation of Polysulfide Shuttling in Li-S Batteries

With a higher theoretical capacity, lithium-sulfur (Li-S) batteries have been considered as promising candidates for next-generation batteries. Due to the non-conducting nature of sulfur, however, lithium-sulfur batteries tend to exhibit poor performance at a high current rate (C-rate).Here we demonstrate that Fe3O4, synthesized from precursor iron (III) acetylacetonate (AAI), and mesoporous carbon material, Ketjen Black (KB), can be synergistically combined to enhance the electrochemical performance of lithium-sulfur batteries substantially. Instead of adding commercial magnetite particles into Li-S cathodes, iron oxides are synthesized and imbedded into KB from a precursor, AAI, through thermal treatments in air and Ar. The sulfur, then, is incorporated into iron oxides/KB hybrids by melting at an elevated temperature. With air-controlled electrospray as the processing method, sulfur embedded iron oxides/KB, poly(acrylic acid), and rGO sheets are mixed and directly deposited onto the carbon-coated aluminum collector, to employ as the Li-S battery cathode. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) studies confirm that Fe3O4 and iron carbide (Fe3C) are synthesized from the precursor, AAI, embedded in mesoporous carbon materials, and reducing the charge transfer resistance of batteries. The rate-capability tests show that systems with KB/Fe3O4 can achieve enhanced performance compared to batteries without iron oxides, especially at high C-rates. With 14 wt % of solid materials as iron oxides and iron carbide, batteries exhibit 750 mAh/g at 2C discharge/charge rates, which is 83% higher compared to systems without iron oxides. However, incorporated via 250 °C for 0.5 hr in air and 850 °C for 2 hr in Ar, the composite of KB/Fe3O4/Fe3C induces irreversible side reactions during the initial charging process, which causes a huge difference in capacity between the first and the second cycle. To achieve an optimal status with improved rate capability and acceptable initial charging time, we modify the thermal treatments and thus increase the proportion of Fe3O4 in the mixture. According to the postmortem analysis, cathodes with iron oxides can interact with soluble polysulfides strongly and alleviate the polysulfide shuttle effect significantly, compared with those without Fe3O4. Such enhanced rate capability by KB/Fe3O4 (synthesized) over KB only or KB/Fe3O4 (commercial) systems suggests that the incorporation of iron oxides can play an important role in improving the electrical conductivity of cathode and mitigating polysulfide shuttle effect. Figure 1

A biomass-based cathode for long-life lithium-sulfur batteries

With the advantages of high conductivity and low cost, porous carbons have been considered as the most attractive host materials of sulfur cathodes in lithium-sulfur batteries (LSBs). However, LSBs always suffer short cycle life due to the “shuttle effect” of lithium polysulfide species (polysulfides), which are intermediate products during the charge/discharge processes. The weak interaction between carbon and polysulfides results in the dissolution of polysulfides from the cathodes, loss of active material of sulfur and eventually fast capacity fading. To overcome these drawbacks, we employed a biomass-derived carbon as the host material in sulfur cathodes. Results from X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen sorption reveals that this biomass-derived product is amorphous carbon and is composed of both large (>10 nm) and small (<5 nm) pores at an appropriate ratio. Using as hosts of cathodes in LSBs, the biomass-derived carbons deliver a high reversible capacity of >800 mAh/g and retain >80% of initial capacity after 200 cycles. Especially, the activated carbons exhibited an unprecedently high durability with 80% capacity retention after 400 cycles. The promising LSB performance can be ascribed to the unique porous architecture of biomass-derived carbons. The small pores in biomass-derived carbons provide more sites to anchor sulfur and polysulfides, while large pores provide channels for fast transport of ions. This is corroborated by the results from the electrochemical impedance spectroscopy (EIS), the thermogravimetric analysis (TGA) and absorption measurements.

Comprehensive Characterization of Multi-Phase Sulfurized Polyacrylonitrile Cathodes for Lithium-Sulfur Batteries

Sulfurized polyacrylonitrile (SPAN) is considered one of the most promising cathode materials to overcome the operational challenges that plague lithium-sulfur (Li-S) batteries. However, material properties and electrochemical performance implications of SPAN prepared under different synthesis conditions are not yet fully investigated. In this study, we show the impacts of different synthesis conditions on the formation of sulfur to PAN bonds and redox reaction mechanisms of multi-phase SPAN via comprehensive material and electrochemical characterizations. In-situ Raman analysis was first applied to study the multi-phase SPAN-based Li-S cells. We found that both elemental sulfur and chemically bonded sulfur are present under the synthesis condition of 300 °C/3 h along with unreacted PAN. The incompletely sulfurized, multi-phase SPAN exhibited an unusually rapid capacity degradation in the resultant Li-S cells, which is attributed to polysulfide formation and continuously growing interfacial impedance in the Li-S cells. On the other hand, SPAN samples prepared under the synthesis condition of 350 °C/3 h are found completely sulfurized with chemically bonded sulfur to the PAN backbone without the presence of free elemental sulfur. Complete sulfurization of SPAN led to exceptionally stable cycle performance due to excellent reversible redox processes of chemically bonded sulfur with Li+ in the Li-S cells.

Microporous activated carbon derived from azulmic acid precursor with high sulfur loading and its application to lithium-sulfur battery cathode

We have previously reported that an effective activation method for azulmic acid carbon (AZC: AZC is a carbide of azulmic acid, a nitrogen-containing organic material) as a precursor can produce microporous activated carbon. By improving the activation method of AZC, we succeeded in producing an AZC-based activated carbon that can support a larger amount of sulfur than that of previously reported one while keeping its microporous property. We have found that the present AZC activated carbon has a BET specific surface area of 2633 m2 g−1 and a pore volume of 1.286 cc g−1 and can contain up to 70 wt.% sulfur when an optimum alkaline activator and activation temperature are used. The cathode of this activated carbon can be stably charged and discharged in a lithium-sulfur battery with a glyme-based electrolyte. The present highly sulfur-filled microporous AZC is a promising sulfur host material for lithium sulfur batteries.

Synthesis and electrochemical analysis of S/ZIF-67@rGO composite cathodes for lithium-sulfur batteries

Synthesis and electrochemical analysis of S/ZIF-67@rGO composite cathodes for lithium-sulfur batteries

Lithiated carboxylated nitrile butadiene rubber with strong polysulfide immobilization ability as a binder for improving lithium-sulfur battery performance

We report the first use of lithiated carboxylated nitrile butadiene rubber (XNBR-Li) as a functional binder for the sulfur cathode of lithium sulfur batteries (LSBs). XNBR-Li has strong adhesion to sulfur and carbon black particles, forming a uniformly dispersed and robust sulfur cathode structure. Furthermore, due to the presence of the -COOLi groups, XNBR-Li has shown a strong ability to trap lithium polysulfides (LPS), which greatly helps to suppress the shuttle effect of LPS in LSBs. The cyclic voltammetry and electrochemical impedance spectroscopy data indicate that the use of XNBR-Li as the binder can accelerate Li+ diffusion kinetics in the sulfur cathode. As a result, the XNBR-Li-based cathode delivers a high capacity of 1140 mAh g −1 at 0.2 C, high rate performance with a capacity of 729 mAh g−1 at a high rate of 2 C, and excellent long-term cycling stability with a slow capacity decay rate of 0.04% per cycle at 0.5 C over 500 cycles.

Ultraviolet In Situ Polymerized Binders with Polysulfide-Trapping Properties for Long-Cycle-Life Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs) represent a promising energy storage system due to the high theoretical energy density of the cathode; however, the high temperature and long-time drying required for electrode production result in high energy consumption and low efficiency. Ultraviolet (UV)-curing technology is an effective strategy to solve the abovementioned problems. However, carbon black and other conductive agents used in the production of the battery industry show strong absorption of UV light; thus, a single photoinitiator cannot absorb enough light intensity to realize initiation, limiting its application in the battery industry. In this work, the concept of full-band absorption is introduced into the manufacturing process of the LSB cathode to solve the abovementioned problems. The full-band absorption of photoinitiators in the UV band is successfully realized by combining the photoinitiators 2-benzyl-2-dimethylamino-1-(4-morpholinylphenyl)-1-butanone, 2-isopropyl thioxanthone, and bis (2,4,6-trimethyl benzoyl)-phenoxyphosphine. An ultraviolet in situ polymerized polyurethane acrylate (PUA) binder is successfully prepared by the combination of photoinitiators. PUA is used as the binder of LSBs and exhibits an excellent long-cycle performance of 1500 cycles with a low decay rate of 0.04% per cycle at 0.5 C. Thus, UV-curing technology provides a new prospect and possibility of industrialization for battery manufacturing.

Advanced Nanostructured MXene-Based Materials for High Energy Density Lithium-Sulfur Batteries.

Lithium–sulfur batteries (LSBs) are one of the most promising candidates for next-generation high-energy-density energy storage systems, but their commercialization is hindered by the poor cycling stability due to the insulativity of sulfur and the reaction end products, and the migration of lithium polysulfide. MXenes are a type of emerging two-dimensional material and have shown excellent electrochemical properties in LSBs due to their high conductivity and large specific surface area. Herein, several synthetic strategies developed for MXenes since their discovery are summarized alongside discussion of the excellent properties of MXenes for LSBs. Recent advances in MXene-based materials as cathodes for LSBs as well as interlayers are also reviewed. Finally, the future development strategy and prospect of MXene-based materials in high-energy-density LSBs are put forward.

Composite cathodes and modified separators based on corn-silk-based porous carbon for high performance lithium–sulfur batteries

Corn by-products have been widely studied as a precursor of biomass carbon in energy storage systems. Without further doping or coating processes corn silk can achieve a high electrochemical performance when used as an anode of sodium and potassium ion batteries. Nonetheless, there is no report about corn silk used as a cathode for lithium-sulfur batteries. In this work, corn silk was made into porous carbon and used as sulfur scaffolds and modified materials for commercial separator in lithium-sulfur batteries. Corn silk possesses a natural pipe shape which grants the porous carbon a high specific surface area. With a simple dissolution crystallization method approximately 80% of the sulfur could be easily accommodated onto the porous carbon. The hierarchic pores not only limited the particle size of the sulfur to nanometers, but also buffered the volume expansion of sulfur and absorbed it tightly. Furthermore, with an ultralight porous carbon modified separator, significant improvement was achieved in the utilization of sulfur, cycling stability and alleviation of the polysulfide shuttling efficiently. The batteries delivered a specific capacity of 1290 mAh g-1 in the first discharge and 746 mAh g-1 after 100 cycles. The good electrochemical performance can be ascribed to the rational design of battery configuration.

Synergistic capture and conversion of polysulfides in cathode composites with multidimensional framework structures

The application of lithium–sulfur batteries are seriously hindered by their poor cycle stability and low sulfur utilization due to their inevitable polysulfide shuttle effect and slow reaction kinetics. Here, Mo2C nanorods that were surface-decorated with metallic–organic framework-derived nitrogen-doped carbon and ultrasmall cobalt nanoparticles (NC–Co@Mo2C) were used as the materials for lithium–sulfur battery cathodes. The prepared NC–Co@Mo2C@S composites had the specific capacity of 1073 mAh·g−1 (0.2 C) and the retained 806 mAh·g−1 after 200 cycles, thus showing excellent discharge specific capacity and cycling stability. The Mo2C nanorods can adsorb lithium polysulfides (LiPSs) through the formation of MoS bonds. Cobalt nanoparticles electrocatalytically accelerated the redox kinetic conversion of LiPSs. Nitrogen doping can effectively reduce the energy potential barrier. The interconnected multidimensional backbone of NC–Co@Mo2C composites contributed to electrolyte permeation, fast electron/Li+ transport, and sufficient volume change buffering. Therefore, the synergistic effect of the adsorption ability of Mo2C nanorods and the catalytic ability of cobalt nanoparticles can effectively improve the sulfur fixation ability of the composites and greatly suppress the shuttle effect.

Ultralong cycle life enabled by in situ growth of CoMo1−xP/Mo heterostructure for lithium-sulfur batteries

Lithium-sulfur batteries (Li-S batteries) are considered as promising new-generation electrochemical energy storage devices due to their extremely high theoretical energy density (2600 Wh kg−1) and theoretical specific capacity (1675 mAh g−1). However, numerous problems such as poor conductivity and the shuttle effect during discharge–charge process limit the practical application of lithium-sulfur batteries. In this work, porous tubular CoMo1−xP/Mo constructed by in situ growth of metal Mo was designed as the sulfur host for lithium-sulfur batteries. The introduction of Mo modulated the electronic structure of CoMoP to improve the conductivity of cathode and facilitate the redox kinetics, as well as the CoMo1−xP/Mo heterostructure was beneficial to inhibit the shuttle effect through the interaction with lithium polysulfides, which improved cycling stability. As a result, CoMo1−xP/Mo/S cathode had a low-capacity decay rate of only 0.029% per cycle after 2000 cycles at 0.5 C. This work provided a new perspective for the further design of high-performance lithium-sulfur battery cathode materials.

Preparation of Porous Yolk–Shell S@Poly(vinyl alcohol) (PVA) Particles for a Lithium–Sulfur Battery Cathode with High Cycling and Rate Performances via a Self-Emulsification Process

Preparation of Porous Yolk–Shell S@Poly(vinyl alcohol) (PVA) Particles for a Lithium–Sulfur Battery Cathode with High Cycling and Rate Performances via a Self-Emulsification Process

Boron nitride wrapped N-doped carbon nanosheet as a host for advanced lithium-sulfur battery

Sandwich structure of boron nitride (BN) nanocrystals wrapped with uniform interconnected N-doped carbon nanosheets (N-CNs) is prepared as a lithium-sulfur battery cathode material to overcome the sluggish redox reaction and severe “shuttle effect” of lithium polysulfides (LiPSs). The BN@N-CNs-2/S presents outstanding rate capacity of 528 mAh g−1 at 2 A g−1, with extended cycle life and sluggish capacity decrease rate of 0.026% per cycle at 2 A g−1 for more than 1000 cycles. Such high performance is a result of the unique sandwich structure of BN nanocrystals evenly compounded on N-CNs, which leads to uniform distribution of active sulfur species and effectively alleviates volume expansion in the cycling process. The BN@N-CNs sandwich structure possesses the key advantages of high conductivity of N-CNs, strong chemisorption and catalytic effect for LiPSs, which is demonstrated by theoretical calculations and cyclic voltammetry curves of the symmetrical cells.

Using sp2 N atom anchoring effect to prepare ultrafine vanadium nitride particles on porous nitrogen-doped carbon as cathode for lithium-sulfur battery

Porous carbon-supported transition metals and their compounds have attracted much attention as sulfur host materials for cathodes of lithium-sulfur batteries, due to their high chemisorption capacity and ability to catalyze the conversion of polysulfides. However, actual activity of these materials is not very high because of low specific surface areas of transition metal compounds synthesized at high temperatures. In this study, ultra-fine vanadium nitride particles with an average particle size of ca. 4 nm (VN/M/NC) are successfully grown on the surface of nitrogen-doped three-dimensional carbon using sp2 nitrogen atoms, resulting from melamine pyrolysis in the presence of ammonium metavanadate, as anchor points to lock vanadium atoms in the VN/M/NC material. When used as a cathode for lithium-sulfur battery, VN/M/NC demonstrates initial discharge specific capacity of 1080 mAh g−1 at 0.2 C, and retains a discharge capacity of 475 mAh g−1 at a high rate of 2 C. With capacity attenuation of only 0.037% per cycle after 500 cycles at 1 C, the newly obtained VN/M/NC can be a promising cathode material for lithium-sulfur batteries.

Porous Dithiine-Linked Covalent Organic Framework as a Dynamic Platform for Covalent Polysulfide Anchoring in Lithium-Sulfur Battery Cathodes.

Dithiine linkage formation via a dynamic and self-correcting nucleophilic aromatic substitution reaction enables the de novo synthesis of a porous thianthrene-based two-dimensional covalent organic framework (COF). For the first time, this organo-sulfur moiety is integrated as a structural building block into a crystalline layered COF. The structure of the new material deviates from the typical planar interlayer π-stacking of the COF to form undulated layers caused by bending along the C-S-C bridge, without loss of aromaticity and crystallinity of the overall COF structure. Comprehensive experimental and theoretical investigations of the COF and a model compound, featuring the thianthrene moiety, suggest partial delocalization of sulfur lone pair electrons over the aromatic backbone of the COF decreasing the band gap and promoting redox activity. Postsynthetic sulfurization allows for direct covalent attachment of polysulfides to the carbon backbone of the framework to afford a molecular-designed cathode material for lithium-sulfur (Li-S) batteries with a minimized polysulfide shuttle. The fabricated coin cell delivers nearly 77% of the initial capacity even after 500 charge-discharge cycles at 500 mA/g current density. This novel sulfur linkage in COF chemistry is an ideal structural motif for designing model materials for studying advanced electrode materials for Li-S batteries on a molecular level.

Unraveling the morphological evolution mechanism of solid sulfur species in lithium-sulfur batteries with operando light microscopy

Solid-liquid phase conversion between various sulfur species in lithium-sulfur (Li-S) batteries is a fundamental reaction of the sulfur cathode. Disclosing the morphological evolution of solid sulfur species upon cycling is of great significance to achieving high energy densities. However, an in-depth investigation of the internal reaction is still lacking. In this work, the evolution process of solid sulfur species on carbon substrates is systematically studied by using an operando light microscope combined with in situ electrochemical impedance spectra technology. The observation of phenomena such as bulk solid sulfur species can form and dissolve independently of the conductive substrates and the transformation of supercooled liquid sulfur to crystalline sulfur. Based on the phenomena mentioned above, a possible mechanism was proposed in which the dissolution reaction of solid sulfur species is a spatially free reaction that involves isotropic physical dissolution, diffusion of molecules, and finally the electrochemical reaction. Correspondingly, the formation of solid sulfur species tends to be a form of crystallization in a saturated solution rather than electrodeposition, as is commonly believed. Our findings offer new insights into the reaction of sulfur cathodes and provide new opportunities to design advanced sulfur cathodes for Li-S batteries.