Insight into the loading temperature of sulfur on sulfur/carbon cathode in lithium-sulfur batteries

How to Make a Cocktail of Palladium Catalysts with Cola and Alcohol: Heteroatom Doping vs. Nanoscale Morphology of Carbon Supports

Sparkling drinks such as cola can be considered an affordable and inexpensive starting material consisting of carbohydrates and sulfur- and nitrogen-containing organic substances in phosphoric acid, which makes them an excellent precursor for the production of heteroatom-doped carbon materials. In this study, heteroatom-doped carbon materials were successfully prepared in a quick and simple manner using direct carbonization of regular cola and diet cola. The low content of carbon in diet cola allowed reaching a higher level of phosphorus in the prepared carbon material, as well as obtaining additional doping with nitrogen and sulfur due to the presence of sweeteners and caffeine. Effects of carbon support doping with phosphorus, nitrogen and sulfur, as well as of changes in textural properties by ball milling, on the catalytic activity of palladium catalysts were investigated in the Suzuki–Miyaura and Mizoroki–Heck reactions. Contributions of the heteroatom doping and specific surface area of the carbon supports to the increased activity of supported catalysts were discussed. Additionally, the possibility of these reactions to proceed in 40% potable ethanol was studied. Moreover, transformation of various palladium particles (complexes and nanoparticles) in the reaction medium was detected by mass spectrometry and transmission electron microscopy, which evidenced the formation of a cocktail of catalysts in a commercial 40% ethanol/water solution.

A Covalent Organic Framework with Extended π-Conjugated Building Units as a Highly Efficient Recipient for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have recently become a research hotspot because of their tempting theoretical capacity and energy density. Nevertheless, the notorious shuttle of polysulfides hinders the advancement of Li-S batteries. Herein, a two-dimensional covalent organic framework (COF) with extended π-conjugated units has been designed, synthesized, and used as sulfur recipients with 88.4 wt % in loading. The COF offers an elaborate platform for sufficient Li-S redox reactions with almost theoretical capacity release (1617 mA h g^-1 at 0.1 C), satisfactory rate capability, and intensively traps polysulfides for a decent Coulombic efficiency (ca. 98.0%) and extremely low capacity decay (0.077% per cycle after 528 cycles at 0.5 C). The structural factors of the COF on the high-performance batteries are revealed by density functional theory calculations to be the high degrees of conjugation and proper interlayer space. This work not only demonstrates the great potential of COFs as highly efficient sulfur recipients but also provides a viable guidance for further design of COF materials to tackle shuttling issues toward active materials in electrochemical energy storage.

Thermal Modulation of MOF and Its Application in Lithium-Sulfur Batteries

The applications of metal-organic frameworks (MOFs) as sulfur hosts focus on pristine MOFs and their carbonization products for establishing high-performance lithium-sulfur batteries (LSBs). However, the mechanism that modulates the specific nanostructures and the compositions at different treatment temperatures still needs further exploration. In this work, we modulate the pyrolysis products of UiO-66-NH₂@rGO (U@rGO) at a predetermined specific temperature by thermogravimetric (TG) analysis and systematically investigate their microstructure and chemical characteristic evolution. The composite processed at 300 °C (U@rGO-P300) results in the rearrangement of an octahedral nanophase into an amorphous material while retaining a large number of functional groups of -NH₂ and COO-, which lead to an additional nanostructure interface and a high Brunauer-Emmett-Teller (BET) surface area of 298.4 m² g^-1. Moreover, the newly created abundant suboxidative Zr³⁺ atom sites serve as the polysulfide anchor and transfer mediator active sites. The assembled LSBs could deliver a capacity of 906.1 and retain 801.7 mA h g^-1 after 300 cycles at 1C with a low fading rate of around 0.05% per cycle and an improved rate performance of 619.1 mA h g^-1 at 4C. The Li⁺ diffusion coefficients are significantly increased by 10-60 times. This work provides a simple route to activate the metal sites in the MOF category with a suboxidative state, leading to intriguing and unanticipated properties.

Critical Role of Anion Donicity in Li2S Deposition and Sulfur Utilization in Li-S Batteries

Lithium-sulfur batteries offer a high theoretical gravimetric energy density and low cost, but the full utilization of the sulfur electrode has been limited by the premature passivation of insulating lithium sulfide (Li₂S). Anion has been one of the major parameters to improve Li-S batteries in addition to solvent, additives, and electrode structures. Here, we reveal the role of anion donicity on the passivation of Li-S battery and its underlying working mechanism. We show that anions with high donicity effectively reduce the charge-transfer resistance during the cycling of Li-S cells and alleviate the Li₂S passivation by transforming the dense film Li₂S to porous three-dimensional flake Li₂S. UV-vis spectroscopy revealed that anions with higher donicity exhibit higher Li₂S₄ solubility, which is consistent with their stronger bonding to Li⁺, as revealed by nuclear magnetic resonance and density functional theory calculations. Our study reveals the role of anion donicity in Li₂S passivation and its underlying mechanism, offering rational design consideration for electrolyte salts in achieving high sulfur utilization and high energy efficiency for Li-S batteries.

Architecture and Performance of the Novel Sulfur Host Material Based on Ti2O3 Microspheres for Lithium-Sulfur Batteries

Lithium-sulfur batteries are considered as promising next-generation green secondary batteries. Irrespective of the enhancement of the cycling stability or the suppression of polysulfide species shuttle, although much progress has recently been achieved, improving the conductivity of host materials and capturing the sulfide species as far as possible are still hot topics in the research of lithium-sulfur batteries nowadays. Here, we put forward a novel sulfur host architecture based on Ti₂O₃ microspheres fabricated by magnesiothermic reduction. The Ti₂O₃ microspheres possess both high electronic conductivity and excellent ability of anchoring lithium polysulfide species. The high electronic conductivity endowed by a narrow band gap can adequately activate insulative sulfur and reduce the battery resistance so that high specific capacity and excellent rate capability can be achieved, while the polar Ti₂O₃ could afford abundant polar active sites for the absorption of polysulfides for high capacity retention. As a result, Ti₂O₃ microspheres are applied in the research of lithium-sulfur batteries; excellent electrochemical performance has been revealed. The initial specific capacity is 1245 mAh g^-1 at 0.2C, with 91.57% capacity retention after 180 cycles. Even with a high areal loading of 3.6 mg cm^-2, an initial capacity of 665 mAh g^-1 at 0.5C and a good capacity retention of 70.98% after 300 cycles could be achieved. Apparently, the preparation and application of Ti₂O₃ microspheres can not only further extend the application field of the Ti-based compound but also boost the electrochemical performance of lithium-sulfur batteries.

Uniform Mesoporous MnO2 Nanospheres as a Surface Chemical Adsorption and Physical Confinement Polysulfide Mediator for Lithium-Sulfur Batteries

The actual implementation of lithium-sulfur batteries is hindered by inferior cyclic stability and poor coulombic efficiency stemming from the notorious shuttling of soluble polysulfide intermediates. Herein, uniform mesoporous MnO₂ nanospheres were prepared using a facile self-assembly and room-temperature reaction method. As a sulfur carrier of sulfur cathodes, the versatile architecture of MnO₂ not only provides powerful chemical adsorption to anchor polysulfide intermediates on the large polar surface area but also restrains them within the nanopores by physical confinement. The mesoporous MnO₂-stabilized sulfur cathode demonstrates a high initial reversible capacity of 1349.3 mA h g^-1 and a capacity fading rate of 0.073% at 1.0 C over 500 cycles. Furthermore, a reversible areal capacity of 2.5 mA h cm^-2 was achieved with stable cycling performance at a sulfur content of 80.7%. Our work offers a facile method to build efficient sulfur cathodes for high performance lithium-sulfur batteries.

Systematic Exploration of the Role of a Modified Layer on the Separator in the Electrochemistry of Lithium-Sulfur Batteries

As a significant constituent of lithium-sulfur batteries, the separator also exerts a considerable effect on the performance of the sulfur cathode. In our work, the mixture of acetylene black and multiwalled carbon nanotubes is uniformly applied onto the commercial polypropylene membranes to attain the modified separators. While investigating different samples, the underlying influence of the coating layer is systematically scrutinized on the electrochemical behaviors of sulfur cathodes, relying on the extensive electrochemical and structural measurements. During the charge/discharge process, the coating layer can function as the second current collector and drastically contribute to the improved lithium ions diffusion, determining the electrochemical kinetics of sulfur-involved reactions. Moreover, it is found that the thicker the layer, the faster the lithium ions diffuse. It should be noted that the coating layer also plays a role of the second sulfur reservoir to endow the ample active sites for sulfur and polysulfides, which is directly witnessed for the first time. Because of the positiveness and effectiveness of the modified layer on separators, the sulfur cathode can offer the superior cycling and rate performance to that with the original separator.

Toward High-Performance Lithium-Sulfur Batteries: Upcycling of LDPE Plastic into Sulfonated Carbon Scaffold via Microwave-Promoted Sulfonation

Lithium-sulfur batteries were intensively explored during the last few decades as next-generation batteries owing to their high energy density (2600 Wh kg^-1) and effective cost benefit. However, systemic challenges, mainly associated with polysulfide shuttling effect and low Coulombic efficiency, plague the practical utilization of sulfur cathode electrodes in the battery market. To address the aforementioned issues, many approaches have been investigated by tailoring the surface characteristics and porosities of carbon scaffold. In this study, we first present an effective strategy of preparing porous sulfonated carbon (PSC) from low-density polyethylene (LDPE) plastic via microwave-promoted sulfonation. Microwave process not only boosts the sulfonation reaction of LDPE but also induces huge amounts of pores within the sulfonated LDPE plastic. When a PSC layer was utilized as an interlayer in lithium-sulfur batteries, the sulfur cathode delivered an improved capacity of 776 mAh g^-1 at 0.5C and an excellent cycle retention of 79% over 200 cycles. These are mainly attributed to two materialistic benefits of PSC: (a) porous structure with high surface area and (b) negatively charged conductive scaffold. These two characteristics not only facilitate the improved electrochemical kinetics but also effectively block the diffusion of polysulfides via Coulomb interaction.

Synergistically Enhanced Interfacial Interaction to Polysulfide via N,O Dual-Doped Highly Porous Carbon Microrods for Advanced Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have received tremendous attention because of their extremely high theoretical capacity (1672 mA h g^-1) and energy density (2600 W h kg^-1). Nevertheless, the commercialization of Li-S batteries has been blocked by the shuttle effect of lithium polysulfide intermediates, the insulating nature of sulfur, and the volume expansion during cycling. Here, hierarchical porous N,O dual-doped carbon microrods (NOCMs) were developed as sulfur host materials with a large pore volume (1.5 cm³ g^-1) and a high surface area (1147 m² g^-1). The highly porous structure of the NOCMs can act as a physical barrier to lithium polysulfides, while N and O functional groups enhance the interfacial interaction to trap lithium polysulfides, permitting a high loading amount of sulfur (79-90 wt % in the composite). Benefiting from the physical and chemical anchoring effect to prevent shuttling of polysulfides, S@NOCMs composites successfully solve the problems of low sulfur utilization and fast capacity fade and exhibit a stable reversible capacity of 1071 mA h g^-1 after 160 cycles with nearly 100% Coulombic efficiency at 0.2 C. The N,O dual doping treatment to porous carbon microrods paves a way toward rational design of high-performance Li-S cathodes with high energy density.

Fluorinated, Sulfur-Rich, Covalent Triazine Frameworks for Enhanced Confinement of Polysulfides in Lithium-Sulfur Batteries

Lithium-sulfur battery represents a promising class of energy storage technology owing to its high theoretical energy density and low cost. However, the insulating nature, shuttling of soluble polysulfides and volumetric expansion of sulfur electrodes seriously give rise to the rapid capacity fading and low utilization. In this work, these issues are significantly alleviated by both physically and chemically restricting sulfur species in fluorinated porous triazine-based frameworks (FCTF-S). One-step trimerization of perfluorinated aromatic nitrile monomers with elemental sulfur allows the simultaneous formation of fluorinated triazine-based frameworks, covalent attachment of sulfur and its homogeneous distribution within the pores. The incorporation of electronegative fluorine in frameworks provides a strong anchoring effect to suppress the dissolution and accelerate the conversion of polysulfides. Together with covalent chemical binding and physical nanopore-confinement effects, the FCTF-S demonstrates superior electrochemical performances, as compared to those of the sulfur-rich covalent triazine-based framework without fluorine (CTF-S) and porous carbon delivering only physical confinement. Our approach demonstrates the potential of regulating lithium-sulfur battery performances at a molecular scale promoted by the porous organic polymers with a flexible design.

Unusual Mesoporous Carbonaceous Matrix Loading with Sulfur as the Cathode of Lithium Sulfur Battery with Exceptionally Stable High Rate Performance

Unusual three-dimensional mesoporous carbon/reduced graphene oxide (MP-C/rGO) matrix possessing graphene nanolayer pore walls built up by three to five graphene monosheets and some carbon particles with the sizes of about 5 nm located between the graphene nanolayers was prepared by facile freeze-drying and then carbonization of the poly(vinyl alcohol) and graphene oxide mixture. The mesoporous carbonaceous MP-C/rGO sample has a high specific surface area of 661.6 m² g^-1, large specific pore volume of 1.54 m³ g^-1, and focused pore size distribution of 2-10 nm. About 64 wt % sulfur could be held in the pores of the MP-C/rGO matrix. As the cathode of a Li-S battery, the MP-C/rGO/S composite showed excellent electrochemical property including a high initial specific capacity of 919 mA h g^-1 at 1 C with the capacity retention ratio of 63.3% and the Coulombic efficiency above 90% after 500 cycles. Meanwhile, the initial specific capacity of 602 mA h g^-1 at 5 C and remaining capacity of 391 mA h g^-1 after 500 cycles with an outstanding Coulombic efficiency of 97% indicate its exceptionally stable rate performance.

Ultramicroporous Carbon through an Activation-Free Approach for Li-S and Na-S Batteries in Carbonate-Based Electrolyte

We report an activation-free approach for fabricating ultramicroporous carbon as an accommodation of sulfur molecules for Li-S and Na-S batteries applications in carbonate-based electrolyte. Because of the high specific surface area of 967 m² g^-1, as well as 51.8% of the pore volume is contributed by ultramicropore with pore size less than 0.7 nm, sulfur cathode exhibits superior electrochemical behavior in carbonate-based electrolyte with a capacity of 507.9 mA h g^-1 after 500 cycles at 2 C in Li-S batteries and 392 mA h g^-1 after 200 cycles at 1 C in Na-S batteries, respectively.

Immobilizing Polysulfides with MXene-Functionalized Separators for Stable Lithium-Sulfur Batteries

Lithium-sulfur batteries have attracted increasing attention as one of the most promising candidates for next-generation energy storage systems. However, the poor cycling performance and the low utilization of sulfur greatly hinder its practical applications. Here we report the improved performance of lithium-sulfur batteries by coating Ti₃C₂T_x MXene nanosheets (where T stands for the surface termination, such as -O, -OH, and/or -F) on commercial "Celgard" membrane. In favor of the ultrathin two-dimensional structure, the Ti₃C₂T_x MXene can form a uniform coating layer with a minimum mass loading of 0.1 mg cm^-2 and a thickness of only 522 nm. Owing to the improved electric conductivity and the effective trapping of polysulfides, the lithium-sulfur batteries with MXene-functionalized separators exhibit superior performance including high specific capacities and cycling stability.

Ternary Porous Sulfur/Dual-Carbon Architectures for Lithium/Sulfur Batteries Obtained Continuously and on a Large Scale via an Industry-Oriented Spray-Pyrolysis/Sublimation Method

Ternary composites with porous sulfur/dual-carbon architectures have been synthesized by a single-step spray-pyrolysis/sublimation technique, which is an industry-oriented method that features continuous fabrication of products with highly developed porous structures without the need for any further treatments. A double suspension of commercial sulfur and carbon scaffolding particles was dispersed in ethanol/water solution and sprayed at 180 °C using a spray pyrolysis system. In the resultant composites, the sulfur particles were subjected to an ultrashort sublimation process, leading to the development of a highly porous surface, and were meanwhile coated with amorphous carbon, obtained through the pyrolysis of the ethanol, which acts as an adhesive interface to bind together the porous sulfur with the scaffolding carbon particles, to form a ternary composite architecture. This material has an effective conducting-carbon/sulfur-based matrix and interconnected open pores to reduce the diffusion paths of lithium ions, buffer the sulfur volumetric expansion, and absorb electrolyte and polysulfides. Because of the unique chemistry and the structure, the composites show stable cycling performance for 200 cycles and good rate capability of 520 mAh g(-1) at 2 C. This advanced spray-pyrolysis/sublimation method is easy to scale up and shows great potential for commercialization of lithium/sulfur batteries.

Synergistically Enhanced Polysulfide Chemisorption Using a Flexible Hybrid Separator with N and S Dual-Doped Mesoporous Carbon Coating for Advanced Lithium-Sulfur Batteries

Because of the outstanding high theoretical specific energy density of 2600 Wh kg(-1), the lithium-sulfur (Li-S) battery is regarded as a promising candidate for post lithium-ion battery systems eligible to meet the forthcoming market requirements. However, its commercialization on large scale is thwarted by fast capacity fading caused by the Achilles' heel of Li-S systems: the polysulfide shuttle. Here, we merge the physical features of carbon-coated separators and the unique chemical properties of N and S codoped mesoporous carbon to create a functional hybrid separator with superior polysulfide affinity and electrochemical benefits. DFT calculations revealed that carbon materials with N and S codoping possess a strong binding energy to high-order polysulfide species, which is essential to keep the active material in the cathode side. As a result of the synergistic effect of N, S dual-doping, an advanced Li-S cell with high specific capacity and ultralow capacity degradation of 0.041% per cycle is achieved. Pushing our simple-designed and scalable cathode to a highly increased sulfur loading of 5.4 mg cm(-2), the Li-S cell with the functional hybrid separator can deliver a remarkable areal capacity of 5.9 mAh cm(-2), which is highly favorable for practical applications.