Polysulfides Transformation Research Articles

Regulating Polysulfide Transformation and Deposition Kinetics in Lithium-Sulfur Batteries Based on 3D Conductive Framework.

The polysulfide shuttle effect and slow liquid-solid conversion are supposed to be the main bottlenecks limiting lithium-sulfur battery practicality. Although a great deal of research has been devoted to the nucleation and transformation kinetics of polysulfides, many implicit details cannot be captured. In this work, we design a conducting network, FeNx-NPC, derived from hemin, and induce a 3D nucleation mode. Different from the control group with the 2D nucleation mode, a higher Li2S deposition and earlier nucleation are observed. Here, in situ impedance is applied to further understand the potential relationship between nucleation mode and liquid-solid transformation, and DRT results from impedance data are systematically compared from two aspects: (1) single battery under different voltages and (2) different batteries under the same voltage. It reveals that the 3D nucleation mode ensures more growth sites, on which a covered thin Li2S layer exhibits no charge transfer limitation. What is more, the porous structure with in situ-derived nanotubes favors Li+ faster diffusion. Hence, these advantages allow Li-S cells to deliver high capacity (about 1423 mA h g-1 at 0.1 C), low capacity attenuation (0.029% per cycle at 2 C), and excellent rate performance (620 mA h g-1 at 5 C).

Toward Complete Transformation of Sodium Polysulfides by Regulating the Second‐Shell Coordinating Environment of Atomically Dispersed Fe

AbstractRoom temperature sodium‐sulfur (RT Na‐S) batteries are highly competitive as potential energy storage devices. Nevertheless, their actually achieved reversible capacities are far below the theoretical value due to incomplete transformation of polysulfides. Herein, atomically dispersed Fe‐N/S active center by regulating the second‐shell coordinating environment of Fe single atom is proposed. The Fe−N4S2 coordination structure with enhanced local electronic concentration around the Fermi level is revealed via synchrotron radiation X‐ray absorption spectroscopy (XAS) and theoretical calculations, which can not only significantly promote the transformation kinetics of polysulfides, but induce uniform Na deposition for dendrite‐free Na anode. As a result, the obtained S cathode delivers a high initial reversible capacity of 1590 mAh g−1, nearly the theoretical value. This work opens up a new avenue to facilitate the complete transformation of polysulfides for RT Na‐S batteries.

Toward Complete Transformation of Sodium Polysulfides by Regulating the Second-Shell Coordinating Environment of Atomically Dispersed Fe.

Room temperature sodium-sulfur (RT Na-S) batteries are highly competitive as potential energy storage devices. Nevertheless, their actually achieved reversible capacities are far below the theoretical value due to incomplete transformation of polysulfides. Herein, atomically dispersed Fe-N/S active center by regulating the second-shell coordinating environment of Fe single atom is proposed. The Fe-N4S2 coordination structure with enhanced local electronic concentration around the Fermi level is revealed via synchrotron radiation X-ray absorption spectroscopy (XAS) and theoretical calculations, which can not only significantly promote the transformation kinetics of polysulfides, but induce uniform Na deposition for dendrite-free Na anode. As a result, the obtained S cathode delivers a high initial reversible capacity of 1590 mAh g-1, nearly the theoretical value. This work opens up a new avenue to facilitate the complete transformation of polysulfides for RT Na-S batteries.

Boosting Polysulfide Transformation by NiCo2S4 Hollow Dodecahedra@Nitrogen-Doped Carbon Core/Shell Nanostructures as Cathodes for Lithium–Sulfur Batteries

Boosting Polysulfide Transformation by NiCo₂S₄ Hollow Dodecahedra@Nitrogen-Doped Carbon Core/Shell Nanostructures as Cathodes for Lithium–Sulfur Batteries

Mechanochemical strategy assisted morphology recombination of COFs for promoted kinetics and LiPS transformation in Li–S batteries

A covalent organic frameworks (COFs) material with regular pores and stable structure can be used as the host of lithium-sulfur batteries to improve battery kinetics and polysulfides conversion. Herein, we designed and synthesized two kinds of rod-liked bulk COFs by adjusting different pore sizes (COF-BTD and COF-TFB), unfortunately, the active sites masking and sluggish kinetics have not met our expectations. Generally, the available layered COFs prepared from mechanochemical can expose abundant active sites and favorable kinetics than bulk COFs. Thus, simple mechanical ball milling is applied to activate the above COFs (M-COFs group). It is worth noting that layered R–COF–BTD is directly synthesized from rod-liked precursors by simple morphological reconstruction. A series of characterization methods are used to systematically explore the advantages of the group of M-COFs@S electrodes in the cycling process, including the effects of specific morphology on the kinetics and transformation of polysulfides. Our research provides a feasible plan for the development and selection of the host material of lithium-sulfur batteries.

Construction of Free-Standing N-doped carbonaceous interlayers encapsulated with Co nanoparticles for Lithium–Sulfur batteries

Lithium sulfur (Li–S) batteries are suffered from the polysulfide shuttling and the sluggish redox kinetics of sulfur, impacting on their commercialization. In this work, free-standing carbonaceous materials anchored with N-doped carbon nanotubes embedded with Co nanoparticles (NCN-Co) were synthesized through the carbonization of non-woven fabrics and used as interlayers for Li-S batteries. The NCN-Co interlayers could provide plenty of active chemisorption sites and storage space for the adsorption and catalytic transformation of polysulfides. The free-standing hierarchical structure could act as a physical barrier of polysulfides. N-doping together with the encapsulated Co nanoparticles could improve the catalytic activities, enhancing the redox kinetics of polysulfides. Benefiting from the synergistic effect of the NCN-Co interlayers, the electrochemical performance of Li-S batteries was significantly improved. An initial discharge specific capacity of 1092 mAh g−1 was achieved at 2C and high-capacity retention rate was demonstrated even after discharged/charged for over 600 cycles. This work provides a promising route for the preparation of interlayer materials for Li-S batteries.

Revealing the Multifunctional Electrocatalysis of Indium‐Modulated Phthalocyanine for High‐Performance Lithium‐Sulfur Batteries

The sluggish kinetics of complicated multiphase conversions and the severe shuttling effect of lithium polysulfides (LiPSs) significantly hinder the applications of Li‐S battery, which is one of the most promising candidates for the next‐generation energy storage system. Herein, a bifunctional electrocatalyst, indium phthalocyanine self‐assembled with carbon nanotubes (InPc@CNT) composite material, is proposed to promote the conversion kinetics of both reduction and oxidation processes, demonstrating a bidirectional catalytic effect on both nucleation and dissolution of Li2S species. The theoretical calculation shows that the unique electronic configuration of InPc@CNT is conducive to trapping soluble polysulfides in the reduction process, as well as the modulation of electron transfer dynamics also endows the dissolution of Li2S in the oxidation reaction, which will accelerate the effectiveness of catalytic conversion and facilitate sulfur utilization. Moreover, the InPc@CNT modified separator displays lower overpotential for polysulfide transformation, alleviating polarization of electrode during cycling. The integrated spectroscopy analysis, HRTEM, and electrochemical study reveal that the InPc@CNT acts as an efficient multifunctional catalytic center to satisfy the requirements of accelerating charging and discharging processes. Therefore, the Li–S battery with InPc@CNT‐modified separator obtains a discharge‐specific capacity of 1415 mAh g−1 at a high rate of 0.5 C. Additionally, the 2 Ah Li–S pouch cells deliver 315 Wh kg−1 and achieved 80% capacity retention after 50 cycles at 0.1 C with a high sulfur loading of 10 mg cm−2. Our study provides a practical method to introduce bifunctional electrocatalysts for boosting the electrochemical properties of Li–S batteries.

Mesoporous hydroxyl vanadium oxide/nitrogen-doped graphene enabled fast polysulfide conversion kinetics for high-performance lithium-sulfur batteries

Transition metal vanadium (V)-based oxides with various morphologies have been used in lithium-sulfur (Li-S) batteries to restrain the polysulfide shuttling because of their strong chemical interaction with polysulfides and the high catalytic activity toward the polysulfide transformation. Herein, hollow vanadium oxide spheres (VOOH) with several integrated merits are prepared and used to construct an interlayer between separator and cathode. The mesopore-abundant thin shells of VOOH are beneficial for the fast ion transport. Combined with nitrogen (N)-doped graphene sheets (NG), the as-obtained VOOH/NG interlayer allows fast electron/ion transport, finally enhancing polysulfide transformation kinetics. The Li-S batteries with the VOOH/NG exhibit a high initial discharge capacity (1441.1 mAh g−1 at 0.1C), low capacity decay rate (0.075% per cycle at 1C after 600 cycles), good rate capability, and high areal discharge capacity exceeding that of commerical lithium ion batteries. In addition, the real application of such VOOH-based interlayer is further demonstrated by the successful assembly and good performance of the corresponding pouch cells.

Advances and challenges in tuning the reversibility & cyclability of room temperature sodium–sulfur and potassium–sulfur batteries with catalytic materials

The high theoretical energy density of room temperature sodium-sulfur and potassium-sulfur batteries (Na–S; 1274 Wh/kg, K–S; 914 Wh/kg; based on the mass of sulfur) due to the multi-electron transfer associated with the unique conversion chemistry of S and the natural abundance of Na, K, and S raw materials make them ideal candidates for large-scale energy storage applications beyond Li batteries. However, achieving good reversibility, cyclability, and active material utilization in Na–S and K–S batteries demands alleviation of the complex polysulfide dissolution and the shuttle phenomena during cycling. Rational employment of catalytic materials is beneficial to address these issues by facilitating effective polysulfide transformation and thereby accelerating the sluggish reaction kinetics. This review focuses on the roles and evolution of catalytic materials in polysulfide adsorption, catalytic conversion, and redox mediation in facilitating high-performing Na–S and K–S batteries. Specifically, the advances in tuning the reversibility and cyclability of Na-S and K–S batteries strategically with catalytic material-incorporated S-host cathodes, separators, and interlayers and the interaction of various catalytic materials with the polysulfide species are discussed in the light of advanced characterization techniques. Lastly, the challenges and the plausible strategies for future research are elucidated.

A tandem electrocatalyst with dense heterointerfaces enabling the stepwise conversion of polysulfide in lithium-sulfur batteries

The complicated sulfur phase transformation and diversity kinetics of lithium polysulfide intermediates hinder the practical applications of Li-S batteries. Herein, we report that Mo-Ni bimetal sulfide nanosheets with dense heterointerfaces and defects can decouple the complicated reactions into well-controlled multi-steps in tandem. Specifically, the MoS2 in Mo-Ni tandem catalyst is more effective in catalyzing the phase transformation from Li2S8 to Li2S4, while NiS2 delivers higher catalytic activity toward both the Li2S4 to Li2S conversion and the Li2S dissociation steps, as indicated by density functional theory calculations and comprehensive experiments. This integrated system tends to selectively propel the stepwise conversion of sulfur and thereby enhancing the battery performance. As a result, the sulfur loaded cathode delivers a high initial capacity of 1481 mAh g−1 at 0.2 C, and a low capacity decay rate of 0.0236% per cycle over 1000 cycles at 2 C. This study demonstrates the feasibility of using tandem catalysis for regulating the smooth solid-liquid-solid multiphase transformation of polysulfides.

Defect-Rich Single Atom Catalyst Enhanced Polysulfide Conversion Kinetics to Upgrade Performance of Li-S Batteries.

Lithium-sulfur (Li-S) batteries have attracted considerable attention owing to their extremely high energy densities. However, the application of Li-S batteries has been limited by low sulfur utilization, poor cycle stability, and low rate capability. Accelerating the rapid transformation of polysulfides is an effective approach for addressing these obstacles. In this study, a defect-rich single-atom catalytic material (Fe-N4/DCS)is designed. The abundantly defective environment is favorable for the uniform dispersion and stable existence of single-atom Fe, which not only improves the utilization of single-atom Fe but also efficiently adsorbs polysulfides and catalyzes the rapid transformation of polysulfides. To fully exploit the catalytic activity, catalytic materials are used to modify the routine separator (Fe-N4 /DCS/PP). Density functional theory and in situ Raman spectroscopy are used to demonstrate that Fe-N4 /DCS can effectively inhibit the shuttling of polysulfides and accelerate the redox reaction. Consequently, the Li-S battery with the modified separator achieves an ultralong cycle life (a capacity decay rate of only 0.03% per cycle at a current of 2 C after 800 cycles), and an excellent rate capability (894 mAh g-1 at 3 C). Even at a high sulfur loading of 5.51mg cm-2 at 0.2 C, the reversible areal capacity still reaches 5.4 mAh cm-2 .

Rational design of 3DOM porous TiO2 matrix decorated with Co nanoparticles as novel sulfur hosts for high-performance lithium-sulfur batteries

In this work, we devise a hard-templating strategy to realize the controllable synthesis of three-dimensional ordered porous TiO2 matrix decorated with Co nanoparticles as sulfur hosts for Li/S batteries. Specifically, TiO2 matrix with unique 3D ordered porous structure not only can facilitate rapid lithium ion and electron transport, but also provide high affinity for lithium polysulfides. Besides, the Co nanoparticles embedded in TiO2 matrix possesses inherent metallic conductivity and enables boosted kinetics of polysulfide transformation to mitigate the shuttle effect. As expected, the S/3DOM Co-TiO2 perform exceptionally first discharge capacity of 929.6 mAh g−1 at 0.2 C, and improved rate discharge capacity of 517.5 mAh g−1 at 3 C, as well as an enhanced cycling performance.

Melamine-Sacrificed Pyrolytic Synthesis of Spiderweb-like Nanocages Encapsulated with Catalytic Co Atoms as Cathode for Advanced Li-S Batteries

Due to the high theoretical capacity of 1675 mAh g−1 of sulfur, lithium-sulfur (Li-S) batteries can reach a high energy density of 2600 Wh kg−1, which has shown fascinating potential in recent decades. Herein, we report the spiderweb-like nanocage (Co/Mel) as a novel sulfur host with a melamine-sacrificed pyrolysis method. The incorporation of embedded cobalt nanoparticles (Co NPs) in the tips of carbon nanotubes (CNTs) can catalyze polysulfide transformation kinetics. In addition, the nanocages form a conductive three-dimensional spiderweb-like network that facilitates electrolytic penetration and electronic/ionic transportation. Moreover, the porous internal nano-cavities not only improve sulfur loading levels but also provide buffer space for volume expansion during charging and discharging. As a result, the hollow Co/Mel polyhedra with a high content of sulfur (75.5 wt%) displays outstanding electrochemical performance with an initial discharge-specific capacity of 1425.2 mAh g−1 at 0.1 C and a low decay rate of only 0.028% after 1000 cycles at 1 C.

Creating Edge Sites within the 2D Metal‐Organic Framework Boosts Redox Kinetics in Lithium–Sulfur Batteries

AbstractLithium–sulfur batteries have received extensive interest owing to their exceptionally high energy density. Nonetheless, their practical implementation is still impeded by the shuttle effect of polysulfides and sluggish conversion kinetics. Considering that, a porous 2D defective zeolitic imidazolate framework‐7 (ZIF‐7) with abundant active edges is rationally designed as multifunctional sulfur carriers for Li–S batteries. The 2D ZIF‐7 enables uniform distribution of sulfur and rapid Li‐ion diffusion, while rich edges facilitate sufficient exposure to active sites capturing and catalyzing polysulfides. In addition, the nitrogen defects on edge sites can further accelerate the transformation of polysulfides and decrease the energy barrier of Li2S decomposition. Consequently, the Li–S batteries demonstrate surprisingly practical prospects with a stable capacity of 676.9 mAh g−1 over 500 cycles at 1 C (capacity retention rate = 72.3%). When assembled into a pouch cell at 2.3 mg cm−2, it still exhibits a high capacity of 901.1 mAh g−1 after 100 cycles at 0.1 C. This work offers a rational structural design strategy to tackle the challenges of the sulfur cathode.

Construction of frustrated Lewis pairs at N and Mo2C double sites boosts efficient electrocatalysts for Li-S batteries

The polysulfide shuttle effect and uncontrollable Li-dendrite growth on Li anode are the major reasons limiting the commercial development of Li-S batteries. Transition metal compounds on the carbon vector on the transition of polysulfides have been widely reported. However, there are few reports on the principle of simultaneous transformation and the effect of double active site spacing. Herein, we are based on the mechanism of the anchoring effect of doped elements on the carbon vector. Molybdenum carbide nanoparticles are mounted onto N-doped carbon nanosheets and coated on commercial separators (Mo2C-NCNS) for modification to satisfy the synchronization of S cathode and Li anode. The uniformly dispersed N element promotes the dispersion, homogenization and micro size regulation of Mo2C nanoparticles by anchoring Mo atoms. At the same time, the Mo2C site adjacent to N forms a spatially frustrated Lewis pairs with it. The reasonable spacing between them promotes the uniform deposition of lithium ions, the rapid transformation of polysulfides and the synchronous synergy between them. Characterization by XPS showed that using carbon as a bridge site further enhanced the strong metal-vector interaction between C3N4 and Mo2C, so as to improve the reaction kinetics. Thus, it has an initial specific capacity of 1186 mAh g−1 at 1C, and after 1000 cycles, 400 mAh g−1 remained, with a decay rate of only 0.06% per cycle. At a current density of 1 mA/cm2, the symmetric cell with Mo2C-NCNS modified separator exhibits steady cyclability over 1500 h, with a small overpotential of 76 mV.

Niobium Diboride Nanoparticles Accelerating Polysulfide Conversion and Directing Li2S Nucleation Enabled High Areal Capacity Lithium-Sulfur Batteries.

The shuttle effect of polysulfides and Li2S sluggish nucleation are the major problems hampering the further development of lithium-sulfur batteries. The reasonable design for sulfur host materials with catalytic function has been an effective strategy for promoting polysulfide conversion. Compared with other types of transition metal compounds, transition metal borides with high conductivity and catalytic capability are more suitable as sulfur host materials. Herein, a niobium diboride (NbB2) nanoparticle with abundant and high-efficiency catalytic sites has been synthesized by facile solid-phase reaction. The NbB2 with both high conductivity and catalytic nature could regulate 3D-nucleation and growth of Li2S, decrease the reaction energy barrier, and accelerate the transformation of polysulfides. Thus, the NbB2 cathode could retain a high capacity of 1014 mAh g-1 after 100 cycles. In addition, the high initial specific capacities of 703/609 mAh g-1 are also achieved at 5 C/10 C and could run for 1000/1300 cycles within a low decay rate of 0.057%/0.051%. Even with a high sulfur loading up to 16.5 mg cm-2, an initial areal capacity of 17 mAh cm-2 could be achieved at 0.1 C. This work demonstrates a successful method for enhancing the kinetics of polysulfide conversion and directing Li2S nucleation.

Lithiated 3, 6-dioxa-1, 8-octane dithiol as redox mediator to manipulate polysulfides conversion for high-performance lithium-sulfur batteries

• Lithiated 3, 6-Dioxa-1, 8-Octane Dithiol (DODL) can serve as a redox medium. • DODL can promote the bidirectional conversion of polysulfides. • S 8 can dissolve into the electrolyte with DODL to activate the dead Li 2 S. Limited by sluggish sulfur redox kinetics of lithium-sulfur (Li-S) batteries, polysulfides conversion heavily depends on the surface of conductive substrate, which will result in electrode passivation and accumulation of dead sulfur. Here, the lithiated 3, 6-dioxa-1, 8-octane dithiol (DODL) is proposed as a redox mediator to promote the bidirectional polysulfides transformation in actual operation. DODL generated by lithiation reaction has weaker acidity and is friendly to the lithium anode. During discharging, DODL can chemically reduce long-chain polysulfides, lessening the dependence of polysulfide conversion on conductive substrate and mediate 3D deposition of Li 2 S. Moreover, acceleration of sulfur redox kinetics can reduce the possibility of side reactions between polysulfides and lithium anode. During charging, the addition of DODL is found to decrease the voltage barrier of Li 2 S to polysulfides, enhancing the reversibility of sulfur redox. In particular, S 8 can dissolve in the electrolyte with DODL, which can alleviate the irreversible accumulation of dead Li 2 S according to comproprtionation reaction. Attributing to these properties, the Li-S cells with DODL display excellent stability of long cycles at 0.5 C, 1 C and 2 C. Under the harsh environment of low electrolyte/sulfur ratio and high sulfur loading, the cells with DODL also perform well. The proposal of DODL demonstrates the importance of manipulating polysulfide conversion path in Li-S batteries and stimulates the research enthusiasm of developing novel redox medias.

Natural Lepidolite Enables Fast Polysulfide Redox for High‐Rate Lithium Sulfur Batteries

AbstractLithium sulfur (Li–S) batteries possess energy capacities well beyond those of current Li‐ion technologies, but are plagued by severe diffusion loss of sulfur intermediates and sluggish sulfur reduction kinetics. Here, the design is reported of a lepidolite‐modified polypropylene (C‐Lepidolite@PP) separator to suppress polysulfide shuttling and accelerate the transformation of polysulfides. Because the electrons of S atoms transfer to the 3p antibonding orbits of Si atoms, lepidolite effectively confines polysulfides by forming strong Si–S bonds to weaken the S–S bonds of polysulfides. The ultralow lithium‐ion diffusion barrier (0.081 eV) of lepidolite allows for free lithium‐ion migration and thus significantly facilitates polysulfide conversion from liquid Li2S8 to solid Li2S to enable fast polysulfide redox for high‐rate current operation. With the C‐Lepidolite@PP design, Li–S batteries deliver an ultrahigh rate capability of 703 mAh g−1 at 7 C and a high areal capacity of 7.53 mAh cm−2 with a sulfur loading of 6.5 mg cm−2. Moreover, the 15‐d self‐discharge rate is reduced by ≈85% with the C‐Lepidolite@PP separator, which provides promise for the practical use of Li–S batteries.

Synergistic action of Pt and Nb2O5 ultrafine nanoparticles for bidirectional conversion of polysulfides in high-performance lithium-sulfur cells

The shuttling effect in Lithium-sulfur (Li-S) cells can be effectively minimized by increasing the polysulfide redox reaction rate via catalysis. However, previous research has focused on this reaction in a single direction (i.e., either oxidation or reduction of polysulfides), but the acceleration of sulfur species cycling relies on advances in both directions. Herein, a Pt-Nb2O5 mixed catalyst was synthesized by dispersing Pt nanoparticles on the surface of Nb2O5 ultrafine nanoparticles to synergistically enhance bidirectional catalysis. During reduction, Nb2O5 served as a trap while Pt catalytically converted the polysulfides. During oxidation, Nb2O5 and Pt both exhibited enhanced catalytic activity for Li2S dissolution to refresh the catalytic surfaces. Consequently, a sulfur cathode with Pt-Nb2O5 separator achieved an initial discharge capacity as high as 1283 mA h g−1, which was degraded minimally to 979 mA h g−1 after 100 cycles (0.2C). This outstanding cycling stability was further demonstrated, where a discharge capacity of 915 mAh g−1 and average capacity fading rate of 0.093% were achieved after 500 cycles (0.5C). In addition, minimal self-discharge was observed. This work demonstrates the use of two compounds to achieve synergistic action for the bidirectional catalytic transformation of polysulfides for high-efficiency Li-S cells.

Direct insight into sulfiphilicity-lithiophilicity design of bifunctional heteroatom-doped graphene mediator toward durable Li-S batteries

The practical applications of lithium-sulfur (Li-S) battery have been greatly hindered by the severe polysulfide shuttle at the cathode and rampant lithium dendrite growth at the anode. One of the effective solutions deals with concurrent management of both electrodes. Nevertheless, this direction remains in a nascent stage due to a lack of material selection and mechanism exploration. Herein, we devise a temperature-mediated direct chemical vapor deposition strategy to realize the controllable synthesis of three-dimensional boron/nitrogen dual-doped graphene (BNG) particulated architectures, which is employed as a light-weighted and multi-functional mediator for both electrodes in Li-S batteries. Benefiting from the “sulfiphilic” and “lithiophilic” features, the BNG modified separator not only enables boosted kinetics of polysulfide transformation to mitigate the shuttle effect but also endows uniform lithium deposition to suppress the dendritic growth. Theoretical calculations in combination with electro-kinetic tests and operando Raman analysis further elucidate the favorable sulfur and lithium electrochemistry of BNG at a molecular level. This work offers direct insight into the mediator design via controllable synthesis of graphene materials to tackle the fundamental challenges of Li-S batteries.