Effective Trapping of Lithium Polysulfides Using a Functionalized Carbon Nanotube-Coated Separator for Lithium-Sulfur Cells with Enhanced Cycling Stability

Fluorinated Multi-Walled Carbon Nanotubes Coated Separator Mitigates Polysulfide Shuttle in Lithium-Sulfur Batteries

Li-S batteries still suffer from two of the major challenges: polysulfide shuttle and low inherent conductivity of sulfur. Here, we report a facile way to develop a bifunctional separator coated with fluorinated multiwalled carbon nanotubes. Mild fluorination does not affect the inherent graphitic structure of carbon nanotubes as shown by transmission electron microscopy. Fluorinated carbon nanotubes show an improved capacity retention by trapping/repelling lithium polysulfides at the cathode, while simultaneously acting as the "second current collector". Moreover, reduced charge-transfer resistance and enhanced electrochemical performance at the cathode-separator interface result in a high gravimetric capacity of around 670 mAh g^-1 at 4C. Unique chemical interactions between fluorine and carbon at the separator and the polysulfides, studied using DFT calculations, establish a new direction of utilizing highly electronegative fluorine moieties and absorption-based porous carbons for mitigation of polysulfide shuttle in Li-S batteries.

Effects of Lithium Polysulfides on the Formation of Solid Electrolyte Interfaces in Silicon Anodes

Lithium-ion batteries are one of the most important energy storage devices of the future and pave the way for a greener society. In this context, the demand for batteries with high energy density is increasing significantly and is reaching the limits of the technology currently in use. Therefore, intensive research is being conducted to utilize a new class of materials for energy storage. The most promising alternatives to today's nickel-based cathode and graphite anode materials are silicon and sulfur. Both silicon and sulfur are abundant and cheap and possess extremely high theoretical specific capacities of 4200 mAh/gSi and 1675 mAh/gS, respectively. One of the biggest challenges with sulfur-based batteries is the polysulfide shuttle effect, which occurs with sulfur cathodes, leading to an insulating passivation layer, especially on the commonly used lithium metal anodes. Therefore, to replace lithium metal anodes with silicon, it is of major importance to understand the reactivity of polysulfides with silicon. To investigate the effect of lithium polysulfides on the performance of the anodes in the critical formation cycles, mesoporous silicon anodes were galvanostatically cycled in electrolytes containing different concentrations of polysulfides. In this process, the anodes were analyzed after one, five and ten cycles by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine the composition of the SEI. Higher concentrations of polysulfides in the electrolyte result in more inorganic, oxide-containing species in the SEI. Silicon anodes with lower amounts of surface oxide show little or negative effect on the performance in the presence of polysulfides, while anodes with large amounts of surface oxide show higher impedance during cycling, an effect that is enhanced with increasing polysulfide content.

Aramid Fibers Modulated Polyethylene Separator as Efficient Polysulfide Barrier for High-Performance Lithium-Sulfur Batteries

The separators with high absorbability of polysulfides are essential for improving the electrochemical performance of lithium–sulfur (Li–S) batteries. Herein, the aramid fibers coated polyethylene (AF-PE) films are designed by roller coating, the high polarity of AFs can strongly increase the binding force at AF/PE interfaces to guarantee the good stability of the hybrid film. As confirmed by the microscopic analysis, the AF-PE-6 film with the nanoporous structure exhibits the highest air permeability by the optimal coating content of AFs. The high absorbability of polysulfides for AF-PE-6 film can effectively hinder the migration of polysulfides and alleviate the shuttle effect of the Li–S battery. AF-PE-6 cell shows the specific capacity of 661 mAh g⁻¹ at 0.1 C. After 200 charge/discharge cycles, the reversible specific capacity is 542 mAh g⁻¹ with the capacitance retention of 82%, implying the excellent stability of AF-PE-6. The enhanced cell performance is attributed to the porous architecture of the aramid layer for trapping the dissolved sulfur-containing species and facilitating the charge transfer, as confirmed by SEM and EDS after 200 cycles. This work provides a facile way to construct the aramid fiber-coated separator for the inhibition of polysulfides in the Li–S battery.

Recent Advancements in Selenium-Based Cathode Materials for Lithium Batteries: A Mini-Review

Selenium (Se)-based cathode materials have garnered considerable interest for lithium-ion batteries due to their numerous advantages, including low cost, high volumetric capacity (3268 mAh cm−3), high density (4.82 g cm−3), ability to be cycled to high voltage (4.2 V) without failure, and environmental friendliness. However, they have low electrical conductivity, low coulombic efficiency, and polyselenide solubility in electrolytes (shuttle effect). These factors have an adverse effect on the electrochemical performance of Li-Se batteries, rendering them unsuitable for real-world use. In this study, we briefly examined numerous approaches to overcoming these obstacles, including selecting an adequate electrolyte, the composition of Se with carbonaceous materials, and the usage of metal selenide base electrodes. Furthermore, we examined the effect of introducing interlayers between the cathode and the separator. Finally, the remaining hurdles and potential study prospects in this expanding field are proposed to inspire further insightful work.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Polyaniline-Encapsulated Hollow Co-Fe Prussian Blue Analogue Nanocubes Modified on a Polypropylene Separator To Improve the Performance of Lithium-Sulfur Batteries

Recent studies of lithium-sulfur (Li-S) batteries have identified that a modified separator plays a critical role in challenging the capacity fading and shuttle effect of lithium polysulfides (LiPSs). Herein, we report a polyaniline-encapsulated hollow Co-Fe Prussian blue analogue (CFP@PANI) for separator modification. The open frame-like hollow CFP was synthesized via oriented attachment (OA). To improve the catalytic effect and electrical conductivity, PANI was coated on the synthesized CFP. The resulting CFP@PANI was applied on the conventional polypropylene (PP) separator (CFP@PANI-PP) with vacuum filtration. With a ketjen black/sulfur (KB/S) cathode with 66% of the sulfur load, the CFP@PANI-PP exhibited an initial capacity of 723.1 mAh g^-1 at a current density of 1 A g^-1. Furthermore, the CFP@PANI-PP showed stable cycling performance with 83.5% capacity retention after 100 cycles at 1 A g^-1. During the 100 cycles, each cycle maintained high coulombic efficiency above 99.5%, which indicates that the CFP@PANI-PP could inhibit LiPS migration to the anode side without a Li⁺ transport disturbance across the separator. Overall, the CFP@PANI-PP efficiently suppressed LiPSs, resulting in enhanced electrochemical performance. The current study provides useful insight into designing a nanostructure for separator modification of Li-S batteries.

In-Situ Synthesis of N, O, P-Doped Hierarchical Porous Carbon from Poly-bis(phenoxy)phosphazene for Polysulfide-Trapping Interlayer in Lithium-Sulfur Batteries

The shuttling of polysulfides is the most detrimental contribution to degrading the capacity and cycle stability of lithium-sulfur (Li-S) batteries. Adding a carbon interlayer to prevent the polysulfides from migrating is feasible, and a rational design of the structures and surface properties of the carbon layer is essential to increasing its effectiveness. Herein, we report a hierarchical porous carbon (HPC) created by carbonization of bis(phenoxy)phosphazene and in-situ doping of triple heteroatoms into the carbon lattice to fabricate an effective polysulfide-trapping interlayer. The generated carbon integrates the advantages of a hierarchical porous structure, a high specific area and rich dopants (N, O and P), to yield chemisorption and physical confinement for polysulfides and fast ion-transport synergistically. The HPC interlayer significantly improves the electrochemical performance of Li-S batteries, including an exceptional discharge capacity of 1509 mA h/g at 0.06 C and a high capacity retention of 83.7 % after 250 cycles at 0.3 C. This work thus proposes a facile in-situ synthesis of heteroatom-doped carbon with rational porous structures for suppressing the shuttle effect.

Applications of Carbon in Rechargeable Electrochemical Power Sources: A Review

Rechargeable power sources are an essential element of large-scale energy systems based on renewable energy sources. One of the major challenges in rechargeable battery research is the development of electrode materials with good performance and low cost. Carbon-based materials have a wide range of properties, high electrical conductivity, and overall stability during cycling, making them suitable materials for batteries, including stationary and large-scale systems. This review summarizes the latest progress on materials based on elemental carbon for modern rechargeable electrochemical power sources, such as commonly used lead–acid and lithium-ion batteries. Use of carbon in promising technologies (lithium–sulfur, sodium-ion batteries, and supercapacitors) is also described. Carbon is a key element leading to more efficient energy storage in these power sources. The applications, modifications, possible bio-sources, and basic properties of carbon materials, as well as recent developments, are described in detail. Carbon materials presented in the review include nanomaterials (e.g., nanotubes, graphene) and composite materials with metals and their compounds.

Temperature-Dependent Vapor Infiltration of Sulfur into Highly Porous Hierarchical Three-Dimensional Conductive Carbon Networks for Lithium Ion Battery Applications

Hierarchical, conductive, porous, three-dimensional (3D) carbon networks based on carbon nanotubes are used as a scaffold material for the incorporation of sulfur in the vapor phase to produce carbon nanotube tube/sulfur (CNTT/S) composites for application in lithium ion batteries (LIBs) as a cathode material. The high conductivity of the carbon nanotube-based scaffold material, in combination with vapor infiltration of sulfur, allows for improved utilization of insulating sulfur as the active material in the cathode. When sulfur is evenly distributed throughout the network via vapor infiltration, the carbon scaffold material confines the sulfur, allowing the sulfur to become electrochemically active in the context of an LIB. The electrochemical performance of the sulfur cathode was further investigated as a function of the temperature used for the vapor infiltration of sulfur into the carbon scaffolds (155, 175, and 200 °C) in order to determine the ideal infiltration temperature to maximize sulfur loading and minimize the polysulfide shuttle effect. In addition, the nature of the incorporation of sulfur at the interfaces within the 3D carbon network at the different vapor infiltration temperatures will be investigated via Raman, scanning electron microscopy/energy dispersive X-ray, and X-ray photoelectron spectroscopy. The resulting CNTT/S composites, infiltrated at each temperature, were incorporated into a half-cell using Li metal as a counter electrode and a 0.7 M LiTFSI electrolyte in ether solvents and characterized electrochemically using cyclic voltammetry measurements. The results indicate that the CNTT matrix infiltrated with sulfur at the highest temperature (200 °C) had improved incorporation of sulfur into the carbon network, the best electrochemical performance, and the highest sulfur loading, 8.4 mg/cm2, compared to the CNTT matrices infiltrated at 155 and 175 °C, with sulfur loadings of 4.8 and 6.3 mg/cm2, respectively.

Cinnamon-Derived Hierarchically Porous Carbon as an Effective Lithium Polysulfide Reservoir in Lithium-Sulfur Batteries

Lithium-sulfur batteries are attractive candidates for next generation high energy applications, but more research works are needed to overcome their current challenges, namely: (a) the poor electronic conductivity of sulfur, and (b) the dissolution and migration of long-chain polysulfides. Inspired by eco-friendly and bio-derived materials, we synthesized highly porous carbon from cinnamon sticks. The bio-carbon had an ultra-high surface area and large pore volume, which serves the dual functions of making sulfur particles highly conductive and acting as a polysulfide reservoir. Sulfur was predominantly impregnated into pores of the carbon, and the inter-connected hierarchical pore structure facilitated a faster ionic transport. The strong carbon framework maintained structural integrity upon volume expansion, and the unoccupied pores served as polysulfide trapping sites, thereby retaining the polysulfide within the cathode and preventing sulfur loss. These mechanisms contributed to the superior performance of the lithium-sulfur cell, which delivered a discharge capacity of 1020 mAh g^-1 at a 0.2C rate. Furthermore, the cell exhibited improved kinetics, with an excellent cycling stability for 150 cycles with a very low capacity decay of 0.10% per cycle. This strategy of combining all types of pores (micro, meso and macro) with a high pore volume and ultra-high surface area had a synergistic effect on improving the performance of the sulfur cathode.

Tin sulfide modified separator as an efficient polysulfide trapper for stable cycling performance in Li-S batteries

Lithium-sulfur batteries (Li-S) are considered the most promising systems for next-generation energy storage devices due to their high theoretical energy density and relatively low cost. However, the practical applications of Li-S batteries are hindered by the poor electronic conductivity of sulfur and capacity degradation resulting from the shuttle effect of lithium polysulfides (LiPSs). Herein, we demonstrate use of a tin-sulfide (SnS₂) modified separator to facilitate the redox reaction involving LiPS intermediates and realize improved electrochemical performance in a Li-S battery. Density functional theory (DFT) calculations revealed that SnS₂ exhibits a strong affinity with LiPSs and induces a rapid conversion of trapped polysulfides. As a result, Li-S batteries with a SnS₂-modified separator exhibited an enhanced specific capacity of 1300 mA h g^-1 at 0.2C (corresponding to a high areal capacity of 4.03 mA h cm^-2), which was maintained at 1040 mA h g^-1 after 150 cycles. Furthermore, an excellent rate capability is achieved with a capacity of 700 mA h g^-1 (2.17 mA h cm^-2) at 5C. Additionally, the modified separator exhibited excellent cycling performance up to 500 cycles at 2C, with a low capacity decay rate of 0.0710% per cycle. The excellent performance of the sulfur electrode is mainly attributed to the incorporation of the SnS₂ coating layer on the separator, which effectively confines polysulfides via both chemical and physical interaction and rapidly improves lithium ion diffusion. Moreover, the SnS₂ coating layer greatly improves sulfur utilization and efficiently accelerates the kinetic conversion of trapped polysulfides.

Graphitic carbon nitride as polysulfide anchor and barrier for improved lithium-sulfur batteries

The poor conductivity of sulfur and the shuttle effect of soluble polysulfides have considerably hindered the practical application of lithium-sulfur (Li-S) batteries. Here, we have fabricated a three-dimensional graphitic carbon nitride/reduced graphene oxide (GCN@rGO) network as the sulfur host in Li-S batteries, where the bifunctional GCN strongly binds polysulfides through a chemical interaction and catalyzes the redox reactions of polysulfides. Additionally, GCN coating is also applied to different membranes and when these GCN-coated-membranes (GCMs) are used as separators, they are found to effectively act as the polysulfide barrier to suppress the diffusion of polysulfide intermediates to the Li anode and thus ameliorate the shuttle effect. As a result, the Li-S battery assembled from the GCN@rGO/S cathode and GCM separator exhibited a high initial specific capacity of 1000.6 mAh g^-1 at 0.1 C and 87% capacity retention with 0.066% decay per cycle over 200 charge-discharge cycles at 0.5 C.

Enhanced Electrochemical Kinetics and Polysulfide Traps of Indium Nitride for Highly Stable Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are strongly considered as promising energy storage devices due to their high capacity and large theoretical energy density. However, the shuttle of polysulfides and their sluggish kinetic conversion in electrochemical processes seriously reduce the utilization of active sulfur, leading to a rapid capacity fading. Herein we introduced indium nitride (InN) nanowires into Li-S batteries through separator modification. Both the indium cation and electron-rich nitrogen atom of InN served as the polysulfide traps through strong chemical affinity. Meanwhile, the rapid electron transfer on the surface of InN accelerated the conversion of polysulfides in a working battery. The bifunction of InN nanowires effectively suppressed the shuttle effect. Therefore, Li-S batteries with InN-modified separators exhibit excellent rate performance and high stable cycling life with only 0.015% capacity decay per cycle after 1000 cycles, which affords fresh insights into the energy chemistry of high-stable Li-S batteries.

A multi-shelled CoP nanosphere modified separator for highly efficient Li-S batteries

Lithium-sulfur batteries are considered to be one of the most promising energy-storage systems because of their high theoretical energy density, as well as low cost, nontoxicity and natural abundance of sulfur. However, their poor cycling stability mostly originates from the shuttling of polysulfides which hinders their future practical applications. Here, multi-shelled CoP nanospheres are designed as a coated separator material for Li-S batteries for the first time. Conductive CoP can efficiently anchor polysulfides not only owing to its polar character but also its partial natural surface oxidation feature as evidenced by XPS results, which further activates Co sites for chemically trapping polysulfides via strong Co-S bonding. Furthermore, the unique multi-shelled structure can capture polysulfides to alleviate the "shuttle effect". Consequently, the battery using a CoP coated separator exhibits outstanding cycling stability with a capacity degradation of 0.078% per cycle over 500 cycles at a current density of 1 C and excellent rate performance (725 mA h g-1 at 5 C). It is also worth noting that a high areal capacity of 3.2 mA h cm-2 can be achieved even with a sulfur loading of 3.24 mg cm-2. Our approach demonstrates the convenient fabrication and application potential for a multi-shelled CoP nanosphere modified separator for highly efficient Li-S batteries.