Long-chain Lithium Polysulfides Research Articles

Direct Observation of Hybridization Between Co 3d and S 2p Electronic Orbits: Moderating Sulfur Covalency to Pre-Activate Sulfur-Redox in Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs) offer high energy density and environmental benefits hampered by the shuttle effect related to sluggish redox reactions of long-chain lithium polysulfides (LiPSs). However, the fashion modification of the d-band center in separators is still ineffective, wherein the mechanism understanding always relies on theoretical calculations. This study visibly probed the evolution of the Co 3d-band center during charge and discharge using advanced inverse photoemission spectroscopy/ultraviolet photoemission spectroscopy (IPES/UPS), which offers reliable evidence and are consistent well with theoretical calculations. This, coupled with in situ Raman and X-ray diffraction (XRD) and electrochemical data, co-evidences a novel pre-activating S redox mechanism in LSBs: LiPSs desert/insert in C-N matrixes within a series of Co@NCNT-based separators. The insight of the S redox pre-activation is discovered that the Co 3d-band center downshifts to hybridized with S 2p orbitals in LiPSs, giving rise to a more pronounced S covalency and thus accelerating the conversion of LiPSs to S₈. Benefiting from these advantages, the optimized LSB possesses a minimal decay rate of 0.0058% after 200 cycles at a high discharge rate of 10 C. This study provides new insights into LSB mechanisms and supports conventional theoretical models of the d-band center's impact on LSB performance.

A Catalytic Disproportionation Reaction of Polysulfides Enabled By a Bio-Carbon Host for Li-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are regarded one of the promising alternatives of conventional lithium-ion (Li-ion) batteries because of its high theoretical energy density (2600 Wh kg-1), abundance resources on earth, and low cost of sulfur. Nevertheless, several significant challenges persist in the practical application of Li-S batteries, with the most urgent being the dissolved shuttle effect of long-chain lithium polysulfide (LiPS) species during cycling [1]. Using porous host material for sulfur cathode is one of the commonly applied strategies to physically prevent the diffusion of LiPS from diffusing toward lithium anode, and therefore inhibit the shuttle effect [2]. However, the weak Van der Waals forces is in sufficient to anchor the dissolved LiPS because the non-polar surface of carbon material is unable to bind polar and ionic polysulfide ions. Introducing heteroatoms into the host materials is a way to enhancing anchoring of PS at cathode side [3]. In our previous research, we reported a novel carbon host (denoted as NC) for sulfur cathode from natural silk [4]. The obtained carbon host presented excellent cycling performance because of its hierarchical porous structures and nitrogen-contained functional group. In addition, we found the carbon has the ability to catalyst the disproportionation reactions of PS, accelerating the conversion of PS ions into solid products of elemental sulfur (S8) and lithium sulfide (Li2S2) serves to minimize the residence time of PS ions in the electrolyte. In this study, we proposed a hypothesis of a catalytic pseudo-8-electron redox reaction of S/NC cathode and analysis the process using high-performance liquid chromatograph (HPLC) analysis and electrochemical characterizations.[1] Zhang, S. S. (2013). Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions. Journal of Power Sources, 231, 153-162.[2] Ye, H., Yin, Y. X., Xin, S., & Guo, Y. G. (2013). Tuning the porous structure of carbon hosts for loading sulfur toward long lifespan cathode materials for Li–S batteries. Journal of Materials Chemistry A, 1(22), 6602-6608.[3] Song, J., Xu, T., Gordin, M. L., Zhu, P., Lv, D., Jiang, Y. B., ... & Wang, D. (2014). Nitrogen‐doped mesoporous carbon promoted chemical adsorption of sulfur and fabrication of high‐areal‐capacity sulfur cathode with exceptional cycling stability for lithium‐sulfur batteries. Advanced functional materials, 24(9), 1243-1250.[4] Qiu, D., Zhang, X., Zheng, D., Ji, W., Ding, T., Qu, H., ... & Qu, D. (2023). High-performance Li-S batteries with a minimum shuttle effect: disproportionation of dissolved polysulfide to elemental sulfur catalyzed by a bifunctional carbon host. ACS Applied Materials & Interfaces, 15(30), 36250-36261

Promoting the Radical-Mediated Pathway and Li₂S Conversion By N-Doped Carbon-Embedded Cobalt-Doped Titanium Nitride Electrocatalyst with High Donor-Number Solvent for Lean Electrolyte Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are recognized as some of the most promising candidates for next-generation batteries, owing to their high specific capacity, cost-effectiveness, and lightweight properties. Despite these advantages, the dissolution of the sulfur leads to the shuttling effect of long-chain lithium polysulfides (Li2Sx, 4≤x≤8). Moreover, the lean electrolyte system experiences a reduction in ionic transport due to the formation of ion-pairing and clusters of polysulfides. Therefore, the development of electrolytes that can stabilize the solvation structure of lithium-polysulfide and the introduction of electrocatalysts to suppress the shuttling effect are required for lean electrolyte lithium-sulfur batteries.High donor-number solvents can stabilize the Li+-solvated structure by effectively shielding the Li+ ion’s effective nuclear charge. This Li+-solvated structure, in coordination with high donor-number solvents, can effectively interact with tri-sulfur radicals and long-chain polysulfides, which possess relatively delocalized negative charges. As a result, a high donicity electrolyte can enhance sulfur utilization by increasing the solubility of polysulfides. However, the poor compatibility of high donor-number solvents with Li metal presents a challenge, making their excessive use impractical. Therefore, optimizing the amount of high donor-number solvent is essential. Through careful optimization, it is possible to achieve high donicity electrolyte conditions that are compatible with Li metal. The 1 vol% N,N-Dimethylacetamide (DMA) in 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) co-solvent (DMA:DME:DOL=1:49.5:49.5 vol%) electrolyte achieved a higher discharge capacity after 20 cycles compared to both a low donicity (DME/DOL, 1:1 vol%) and a high donicity (DMA/DOL, 1:1 vol%) electrolyte. Electrolyte modification alone cannot completely suppress the shuttling effect, as the radical-mediated pathway activated by high donor-number solvent leads to the presence of tri-sulfur radicals. Recent research has investigated the ferromagnetic catalysts to enhance the adsorption of polysulfides through the Lorentz force and facilitate the conversion of Li2S with spin polarization in lithium-sulfur batteries. Introducing ferromagnetism through cobalt-doping into N-doped carbon embedded titanium nitride nanowires electrocatalysts could improve the adsorption of polysulfides and tri-sulfur radicals, as well as their conversion to Li2S and promote radical-mediated pathways with high donor-number solvents in lean electrolyte lithium-sulfur batteries. Consequently, N-doped carbon-embedded cobalt-doped titanium nitride nanowires on separator within the 1 vol% DMA electrolyte achieves a high discharge capacity of 454.8 mA h g−1 after 120 cycles with sulfur loading of 3.6 mg cm−2 and E/S ratio of 10 μL mg−1.

Synergistic Interaction of Strongly Polar Zinc Selenide and Highly Conductive Carbon Nanoframeworks Accelerates Redox Kinetics of Polysulfides.

Lithium-sulfur batteries (LSBs) have become strong competitors in secondary battery systems because of their superior theoretical capacity and energy density. However, due to the serious shuttle effect of soluble long-chain lithium polysulfides (LiPSs) and the slow solid-solid reaction kinetics, LSBs face some specific challenges, such as a short cycle life and low rate performance. The introduction of selenide/carbon composites derived from zeolite imidazolate frameworks (ZIFs) into separator coatings is a direct and effective solution to the aforementioned problems. Here, a zinc selenide/carbon catalyst material (ZnSe@C) was constructed and employed to modify commercial polypropylene (PP) separators to accelerate the conversion of intermediates. The highly polar ZnSe effectively fixes the active material on the cathode side by transferring electrons between elements with LiPSs and improves the utilization rate of sulfur. Concurrently, the highly conductive carbon nanoskeleton generated following the pyrolysis of ZIF-8 ensures the rapid transfer of charges during the catalytic reaction. The prepared ZnSe@C has a large specific surface area (250.07 m2 g-1) and mesoporous ratio (78.03%), which not only enhances adsorption and catalysis but also promotes the penetration of the electrolyte and the transport of Li+. Based on this, ZnSe@C/PP separator cells exhibit a low average capacity decay rate of 0.051% per cycle after 500 cycles at 1 C.

(Invited) Understanding Electrochemical Reaction Mechanisms of Sulfur in All-Solid-State Batteries through Operando and Theoretical Studies

All-solid-state lithium-sulfur batteries (ASLSBs) have been considered a promising next-generation energy storage technology due to their remarkable safety and high energy density. In ASLSBs, the electrochemical pathways are intrinsically different from conventional Li-S batteries using liquid electrolytes. However, the mechanism still lacks clear identification and deep understanding. Herein, for the first time, we investigated the chemistries and explored the electrochemical reaction mechanism in ASLSBs through coupling operando Raman spectroscopy and ex-situ X-ray absorption spectroscopy. We proved that no long-chain lithium polysulfides (Li2Sn, 4≤n≤8) were formed during the redox reactions, but a short-chain polysulfide (Li2S2) intermediate phase formation was identified in the conversion between active material S8 and reduction product Li2S. This evidence explains the electrochemical behavior differences of Li-S batteries when using liquid and solid electrolytes. Moreover, there are partial S8 and Li2S2 remaining at the cathode even at fully lithiation stage, suggesting the sluggish reaction kinetics in the ASLSBs. This study revealed the generation of Li2S2 intermediates in ASLSBs, inspiring future research to further improve the performance of ASLSBs through completing the conversions and promoting reaction kinetics in ASLSBs.

First-Principles Investigation of Phosphorus-Doped Graphitic Carbon Nitride as Anchoring Material for the Lithium-Sulfur Battery.

The utilization of lithium-sulfur battery is hindered by various challenges, including the "shuttle effect", limited sulfur utilization, and the sluggish conversion kinetics of lithium polysulfides (LiPSs). In the present work, a theoretical design for the viability of graphitic carbon nitride (g-C3N4) and phosphorus-doping graphitic carbon nitride substrates (P-g-C3N4) as promising host materials in a Li-S battery was conducted utilizing first-principles calculations. The PDOS shows that when the P atom is introduced, the 2p of the N atom is affected by the 2p orbital of the P atom, which increases the energy band of phosphorus-doping substrates. The energy bands of PC and Pi are 0.12 eV and 0.20 eV, respectively. When the lithium polysulfides are adsorbed on four substrates, the overall adsorption energy of PC is 48-77% higher than that of graphitic carbon nitride, in which the charge transfer of long-chain lithium polysulfides increase by more than 1.5-fold. It is found that there are powerful Li-N bonds between lithium polysulfides and P-g-C3N4 substrates. Compared with the graphitic carbon nitride monolayer, the anchoring effect of the LiPSs@P-g-C3N4 substrate is enhanced, which is beneficial for inhibiting the shuttle of high-order lithium polysulfides. Furthermore, the catalytic performance of the P-g-C3N4 substrate is assessed in terms of the S8 reduction pathway and the decomposition of Li2S; the decomposition energy barrier of the P-g-C3N4 substrate decrease by 10% to 18%. The calculated results show that P-g-C3N4 can promote the reduction of S8 molecules and Li-S bond cleavage within Li2S, thus improving the utilization of sulfur-active substances and the ability of rapid reaction kinetics. Therefore, the P-g-C3N4 substrates are a promising high-performance lithium-sulfur battery anchoring material.

Mirror Plane Effect of Magnetoplumbite-Type Oxide Restraining Long-Chain Polysulfides Disproportionation for High Loading Lithium Sulfur Batteries.

A facile solid-state approach is employed to synthesize a novel magnetoplumbite-type oxide of NdMgAl11O19, which integrates spinel-stacking layers (MgAl2O4) with Nd-O6 mirror plane structures. The resulting NdMgAl11O19 exhibits remarkable catalytic activity and conversion efficiency during the sulfur reduction reaction (SRR) in lithium-sulfur batteries. By employing the 2D projection mapping technique of in situ confocal Raman spectroscopy and electrochemical technique, it is discovered that the exposed mirror plane structure of Nd-O6 can effectively suppress the undesiring disproportionation reaction (S8 2-→S6 2-+1/4 S8) of long-chain lithium polysulfides at the initial stages of sulfur reduction, thereby promoting the positive process of sulfur to lithium sulfide. This not only mitigates the issue of sulfur shuttle loss but also significantly improve the kinetics of the conversion process. Leveraging these advantages, the NdMgAl11O19/S cathode delivered an impressive initial capacity of up to 1398 mAh g-1 at an electrolyte/sulfur (E/S) ratio of 5.1µL mg-1 and a sulfur loading of 2.3mg cm-2. Even when the sulfur loading is increased to 10.02mg cm-2, the cathode retained a reversible areal capacity of 10.01 mAh cm-2 after 200 cycles. This mirror engineering strategy provides valuable and universal insights into enhancing the efficiency of cathodes in Li-S battery.

Effect of the anionic composition of sulfolane based electrolytes on the performances of lithium-sulfur batteries

In lithium-sulfur batteries, cell design, specifically electrolyte design, has a key impact on the battery performance. The effect of lithium salt anion donor number (DN) (DN[PF6]− = 2.5, DN[N(SO2CF3)2]− = 5.4, DN[ClO4]− = 8.4, DN[SO3CF3]− = 16.9, and DN[NO3]− = 21.1) on the patterns of lithium-sulfur batteries and lithium metal electrode performances with sulfolane-based electrolytes is investigated. An increase in DN of lithium salt anions leads to an increase in the depth and rate of electrochemical reduction of sulfur and long-chain lithium polysulfides and to a decrease in those for medium- and short-chain lithium polysulfides. DN of lithium salt anions has weak effect on the discharge capacity of lithium-sulfur batteries and the Coulomb efficiency during cycling, with the exception of LiSO3CF3 and LiNO3. An increase in DN of lithium salt anions leads to an increase in the cycling duration of lithium metal anodes and to a decrease in the presence of lithium polysulfides. In sulfolane solutions of LiNO3 and LiSO3CF3, lithium polysulfides do not affect the cycling duration of lithium metal anodes.

Two-dimensional phosphorus carbides (β-PC) as highly efficient metal-free electrocatalysts for lithium-sulfur batteries: a first-principles study.

Li-S batteries are considered as the next-generation batteries due to their exceptional theoretical capacity. However, their practical application is hampered by the shuttling effects of lithium polysulfides (LiPSs) and the sluggish Li2S decomposition, particularly the slow conversion from Li2S2 to Li2S. Addressing these challenges, the quest for effective catalysts that can accelerate the conversion of LiPSs and enhance the performance of Li-S batteries is crucial. In this study, we explored the electrocatalytic activity of two-dimensional phosphorus carbides (β0-PC and β1-PC) in Li-S batteries based on first-principles calculations. Our findings reveal that these materials demonstrate optimal binding strengths (ranging from 1.09 to 1.83 eV) with long-chain LiPSs, effectively preventing them from dissolving into the electrolyte. Additionally, they show remarkable catalytic activity during the sulfur redox reaction (SRR), with ΔG being only 0.37 eV for β0-PC and 0.13 eV for β1-PC. The low energy barrier induced by β-PC enhances ion migration barrier and significantly expedites the charge/discharge cycles of Li-S batteries. Furthermore, we investigated the conversion dynamics of Li2S2 to Li2S, employing the computational lithium electrode (CLE) model. The excellent performance in these aspects underscores the potential of these materials as electrocatalysts for Li-S batteries, paving the way for advanced high-efficiency energy storage solutions.

Suppression of long-chain lithium polysulfide formation through a selenium-doped linear sulfur copolymer cathode for high-performance lithium–organosulfur batteries

A novel organosulfur cathode material for practical Li–S batteries was developed, featuring a selenium-doped linear sulfur chain bonded to a trithiocyanuric acid copolymer (SexS-TTCA).

Nafion-Based Quasi-Solid-State Polymer Electrolyte for Lithium-Sulfur Batteries

Lithium-sulfur batteries are considered a strong contender for next-generation high-energy battery systems given their high gravimetric energy density. However, this battery chemistry is still facing challenges such as the polysulfide shuttle effect caused by the high solubility of long-chain lithium polysulfides (Li2Sn) resulting in active material loss and compromised cycle life. Solid-state polymer electrolytes (SPEs) represent a promising pathway to address this issue by preventing the formation and transport of these undesirable polysulfide intermediates.1,2 Moreover, the application of flexible polymer electrolytes promotes safe high energy batteries for smart textiles.In this study, single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs) based on lithiated Nafion membrane have been investigated, utilizing the chemical and thermal stability of this kind of ionomers. Among others, a mixture of ethylene carbonate (EC) and propylene carbonate (PC) was tested as organic aprotic swelling solvent offering advantageous characteristics such as great chemical stability and high ion mobility while avoiding phase separation issues and undesired leaching.Sulfur-infiltrated ultramicroporous carbon composite cathodes were utilized in combination with the investigated SLIC-SPEs, facilitating a quasi-solid-state lithium-sulfur full cell with carbonate-solvent compatibility and high active material utilization.3 Electrochemical performance of the full cell was investigated elaborately and compared with a reference system using solely the liquid EC:PC solvent. While rapid capacity fading can be observed for the liquid electrolyte cell, an enduring high capacity is found in the lithiated-Nafion system. For better understanding of these effects, cycling analysis was accompanied by implementing impedance spectroscopy upon galvanostatic cycling. The electrochemical characterization indicates the formation of a stable film and high degree of reversibility during cycling of the lithiated-Nafion cell whereas an increased film resistance due to sulfur cross-over and electrolyte decomposition was observed within the liquid electrolyte system.(1) Z. Li, W. Lu, N. Zhang, Q. Pan, Y. Chen, G. Xu, D. Zeng, Y. Zhang, W. Cai, M. Yang, J. Mater. Chem. A 2018, 6 (29), 14330-14338.(2) X. Hao, H. Wenren, X. Wang, X. Xia, J. Tu, J. Colloid Interface sci. 2020, 558, 145-154,(3) M. Nojabaee, B. Sievert, M. Schwan, J. Schettler, F. Warth, N. Wagner, B. Milow, K. A. Friedrich, J. Mater. Chem. A 2021, 9 (10), 6508-6519,

(Digital Presentation) Electrocatalytic and Conductive Vanadium Oxide on Carbonized Bacterial Cellulose Aerogel for the Sulfur Cathode in Li-S Batteries

Li-S chemistry is regarded as promising next-generation battery technology. However, its complex redox process and long-chain lithium polysulfides shuttling are hindering the development of practical Li-S battery technology. To address these problems, many transition-metal-oxide-based catalysts have been investigated to chemically bind soluble lithium polysulfides and accelerate their redox kinetics. However, the intrinsic poor electrical conductivities of these oxides restrict their catalytic performance, consequently limiting the sulfur utilization and the rate performance of Li-S batteries. Herein, we report a freestanding electrocatalytic sulfur host consisting of hydrogen-treated VO2 nanoparticles (H-VO2) anchored on nitrogen-doped carbonized bacterial cellulose aerogels (N-CBC). The hydrogen treatment enables the formation and stabilization of the rutile VO2(R) phase with metallic conductivity at room temperature, significantly enhancing its catalytic capability compared to the as-synthesized insulative VO2(M) phase. We characterized the electrocatalytic performance of this unique H-VO2@N-CBC structure using several methods and found that the activation energy between S8 and polysulfides and the one between polysulfides and Li2S are largely reduced by 28.2 and 43.3 kJ/mol, respectively. Accordingly, the Li-S battery performance is greatly improved, with a high initial specific capacity of 1347 mAh g−1 at 0.1 C and remarkable cycling stability with a capacity of 758 mAh g-1 after 300 cycles at 1 C. This work provides an effective strategy for designing and fabricating conductive oxide catalysts to promote the electrochemical performance of Li-S batteries.

Highly electrocatalytic active amorphous Al2O3 in porous carbon assembled on carbon cloth as an independent multifunctional interlayer for advanced lithium-sulfur batteries

To realize practical applications of lithium-sulfur batteries, rational strategies have to be applied to overcome obstacles such as the shuttle effect and sluggish reaction kinetics of soluble long-chain lithium polysulfides (LPS). In this work, highly electrocatalytic active amorphous Al2O3 uniformly dispersed in porous carbon (Al2O3/C) anchored on carbon cloth (CC) was successfully fabricated. The resultant flexible Al2O3/C@CC was used as an independent interlayer for Li-S batteries, demonstrating strong physical confinement and chemical binding ability for LPS, as well as high catalytic activity for LPS conversion kinetics due to a synergistic effect of 3D conductive network, amorphous nature of Al2O3/C, porous structure and abundant active sites. The cells with such interlayer exhibit high specific capacity, superior cycling stability and rate capability, 903 mAh/g at 0.2C after 100 cycles and 810 mAh/g at 1.0C after 300 cycles can be achieved, moreover, 741 mAh/g can still be retained at a high rate of up to 5.0C. By contrast, Al-MOF grown on carbon cloth (Al-MOF@CC) exhibits limited performance improvement due to poor electrical conductivity, low structure stability, as well as limited ion diffusion rate and weak Lewis acid-base interactions due to lack of open channels.

Desolvation Synergy of Multiple H/Li-Bonds on an Iron-Dextran-Based Catalyst Stimulates Lithium-Sulfur Cascade Catalysis.

Traditional lithium-sulfur battery catalysts are still facing substantial challenges in solving sulfur redox reactions, which involve multistep electron transfer and multiphase transformations. Here, inspired by the combination of iron dextran (INFeD) and ascorbic acid (VC) as a blood tonic for the treatment of anemia, a highly efficient VC@INFeD catalyst is developed in the sulfur cathode, accomplishing the desolvation and enrichment of high-concentration solvated lithium polysulfides at the cathode/electrolyte interface with the assistance of multiple H/Li-bonds and resolving subsequent sulfur transformations through gradient catalysis sites where the INFeD promotes long-chain lithium polysulfide conversions and VC accelerates short-chain lithium polysulfide conversions. Comprehensive characterizations reveal that the VC@INFeD can substantially reduce the energy barrier of each sulfur redox step, inhibit shuttle effects, and endow the lithium-sulfur battery with high sulfur utilization and superior cycling stability even under a high sulfur loading (5.2mg cm-2 ) and lean electrolyte (electrolyte/sulfur ratio, ≈7µL mg-1 ) condition.

Carbonized Bacterial Cellulose Decorated with ZIF-8 Derived Iron Single-Atom Catalyst Nanocages As a Cathode Host for Lithium-Sulfur Batteries

Li-S chemistry is regarded as promising next-generation battery technology. However, its complex redox process and long-chain lithium polysulfides shuttling are hindering the development of practical Li-S battery technology. Various electrocatalysts are being investigated to address the problem by expediting reaction kinetics. Here we report Fe single metal atom catalyst (Fe-SAC) used in the sulfur cathode of Li-S batteries.Fe-SAC is synthesized using a modified metal-organic framework (MOF)-based approach, in which the low vapor pressure Fe element substitutes Zn during pyrolysis of ZIF-8 to attain Fe-N4 moieties by being coordinated with four substitutional nitrogen atoms in the graphene lattice. A freestanding 3D structure with Fe-SAC-functionalized carbon nanocages linked by a carbon nanofiber network is applied as the sulfur cathode scaffold. Here the nanocages are derived from Fe-doped ZIF with Fe-N4 SAC moieties embedded on the inside and outside surfaces. The carbon nanofiber network is derived from the carbonization of bacterial cellulose.We report several methods for assessing Fe-SAC catalytic function on sulfide redox reactions for the prepared electrode in cells with lithium polysulfides electrolytes. Electrochemical studies compare the performance of the nanostructured electrodes with or without Fe-SAC to confirm the catalytic effects in improving the Li-S battery performance. Accordingly, the prepared composite cathode delivers a high initial capacity of 1338 mAh/g at 0.1C and excellent rate capability at 2C with a reversible capacity of 840 mAh/g.

Polysulfide Speciation in Li–S Battery Electrolyte via In-Operando Optical Imaging and Ex-Situ UV-vis Spectra Analysis

Lithium sulfur (Li–S) batteries have received significant attention as one of the energy storage systems with excellent prospects for emerging applications due to their high energy density and low-cost. However, there are fundamental challenges impeding the commercialization of Li–S batteries. Notorious among those challenges is the “polysulfide shuttle” consisting of the dissolution into the electrolyte solvent and subsequent crossover to the anode of long-chain lithium polysulfides. Sparingly solvating electrolytes have been exploited as an approach to reduce the dissolution of polysulfides and thereby the shuttle effect. Using an optical in operando lithium-sulfur cell and ex situ UV–vis spectroscopy, we elucidate the speciation of polysulfides in fully and sparingly solvating electrolytes for Li–S batteries. Extensive literature meta-analysis reveals that the most unambiguous effect of sparingly solvating solvent is in improving the coulombic efficiency of sulfur-cells. Experimental optical imaging and UV–vis characterization elucidate a shift towards shorter-chain polysulfides in electrolytes with increasing lithium-salt concentration (more sparingly solvating). The shift to shorter-chain polysulfides corresponds to a reduction of polysulfide species participating in shuttling which corroborate the increased coulombic efficiency in sparingly-solvating electrolytes.

Oxygen heteroatom enhanced sulfur-rich polymers synthesized by inverse vulcanization for high-performance lithium-sulfur batteries

Sulfur-rich polymers, synthesized through inverse vulcanization, with tunable heteroatom containing crosslinkers were investigated as the S-containing cathode active material within Li–S cells. The polymers possess up to 73% active sulfur while, importantly, not containing any unreacted, crystalline sulfur. Li–S-polymer cells achieve specific capacities of 1206 mAh gs−1 for an active material polymer containing 80 wt% sulfur, with 10 wt% of a structural crosslinker (dicyclopentadiene), and 10 wt% of an oxygen-containing crosslinker (ethylene glycol dimethacrylate). The high capacity achieved within this composition is associated with of this sample, in a higher average sulfur rank within the polymer structure. Furthermore, S-polymers with heteroatomic crosslinkers exhibited lower capacity fade (0.093% per cycle) compared with polymers without functionalized heteroatomic crosslinkers (0.165% per cycle) and elemental sulfur (0.312% per cycle). X-ray photoelectron spectroscopy showed the inherent binding between the ester group carbonyl oxygen within the crosslinker to the long-chain lithium polysulfides, which may inhibit dissolution of these intermediates.

Uniformly distributed 1T/2H-MoS2 nanosheets integrated by melamine foam-templated 3D graphene aerogels as efficient polysulfides trappers and catalysts in lithium-sulfur batteries

• MF-GA-MoS 2 composite acts as a new separator-modified material. • 1T/2H-MoS 2 nanosheets were integrated onto the 3D graphene aerogel using melamine foam as template. • GA framework acts a conductive network and physical barrier of LiPSs. • 1T/2H-MoS 2 nanosheets can produce strong chemossorption to LiPSs and promote their conversion to Li 2 S 2 /Li 2 S. Lithium-sulfur (Li-S) batteries are regarded as one of the next-generation batteries because of their low cost and higher energy density than Li-ion batteries. However, the rapid capacity fading due to the shuttling effect of long-chain lithium polysulfides (LiPSs) between electrodes and sluggish redox kinetics remain a big hindrance in practical applications. Herein, three-dimensional nitrogen-doped graphene aerogel (GA) with uniformly distributed and defect-rich 1T/2H-MoS 2 nanosheets has been developed by adding melamine foam (MF) as the hard template. The resultant MF-GA-MoS 2 composites have strong electron transfer capabilities and rich catalytic activity sites that can adsorb and catalyze the conversion of LiPSs to Li 2 S 2 /Li 2 S, thus reducing the shuttle effect to improve the performance of Li-S batteries. Li-S batteries with the defect-rich MF-GA-MoS 2 composites modified separator exhibit amazing initial discharge capacity of 1323 mAh g −1 at 0.2C, high ratio capacity of 718 mAh g −1 at 1C after 400 cycles, and low decay rate of 0.0485% per lap after 800 cycles at 3C. This work provides a novel strategy for the development of separator modified materials with high adsorption and catalytic properties in Li-S batteries.

Simultaneously Regulating the Band Gap and Catalytic Activity of TiO2 via Embedded MoO2 Based on Interlaced CNT Carrier for Li-S Battery to Achieve Efficient Adsorption

To tackle the issues of lithium sulfur battery (Li-S), a strategy of simultaneous regulating the band gap and catalytic activity of TiO2 via embedded MoO2 based on interlaced carbon nanotubes carrier as substrate material to achieve efficient adsorption has been proposed. This substrate material is controlled to be nanosized with abundant catalytically active sites and widely-distributed pore through the cross-linked porous conductive skeleton, further promoting the electrolyte penetration and charge transfer. In addition, the introduction of MoO2 tailors the Ti electronic states, so the substrate material renders high adsorption energy of −1.47 eV for Li2S6 by theoretical calculation. Furthermore, the cathode exhibits high conversion efficiency from long-chain lithium polysulfide to Li2S (Qlow/Qhigh (the conversion capacity of Li2S) is 2.91 at 0.1C) and excellent sulfur utilization and fast sulfur reaction kinetics. The cathode also exhibits a low-capacity fade and excellent cycling performance.

Effect of surface bonding PEI with electrospun carbon nanofiber on the dissolution of polysulfide intermediates for lithium-sulfur batteries

It is essential to examine the behavior of the dissolution and migration of long-chain lithium polysulfide (LiPS) intermediates (Li2Sx, 4 ≤ x ≤ 8) in the sulfur cathode to promote the performance of Li-S batteries. In this work, PEI functionalized electrospun carbon nanofibers (CNFs-PEI) were prepared to host sulfur to form carbon nanofibers polyethylenimine/sulfur (CNFs-PEI/S) composites for high performance Li-S batteries. The as-prepared CNFs-PEI/S cathode with a sulfur content of 67% delivers an initial discharge capacity of 1279 mAh g−1 at 0.1 C and retains a cycling stability for 80 cycles with a low capacity fading rate of 0.26% per cycle. Further, compared with CNFs/S, the CNFs-PEI/S revealed an enhanced rate capability and cyclic stability primarily attributed to the formation of the electrostatic interaction between amino groups in the PEI chains and LiPS intermediates, leading to suppression of LiPS dissolution and restraint of the shuttle effect during the charge-discharge cycling process. An in situ rotating ring-disk electrode (RRDE) collection experiment is designed to further examine the LiPS dissolution extent from a sulfur electrode during cycling. A maximum slope of ring current change with time (voltage) occurred at the major LiPS intermediate dissolution step over a voltage range between 2.4 and 2.0 V during disk negative scan (reduction). The CNFs-PEI/S showed a lower maximum slope value, which can be attributed to a lower dissolution of LiPS intermediates from the sulfur electrode. The present study also demonstrated the proposed simple RRDE experiment is a useful method to evaluate the dissolution properties of LiPS intermediates from various sulfur electrode materials.