Polysulfide Redox Reactions Research Articles

Cost-Effective Membrane and Advanced Electrode for Stable Polysulfide-Ferricyanide Flow Battery

Based on inexpensive, safe, and environmentally friendly active redox species, neutral polysulfide-ferrocyanide redox flow batteries (PFRFBs) have attracted much attention for large-scale energy storage. However, the development of PFRFBs is undermined by the expensive commercial membrane materials as well as the sluggish polysulfide redox reactions. This work attempts to solve these critical problems by combining the economical membrane with the highly catalytic electrode. In specific, K + -exchanged sulfonated polyether ether ketone (SPEEK-K) membranes have been investigated in PFRFBs to replace the costly Nafion membrane. SPEEK-K with optimized degree of sulfonation enables the PFRFB high average coulombic efficiency of 99.80% and superior energy efficiency of 90.42% at a current density of 20 mA cm -2 . Meanwhile, to overcome the kinetic limitations of polysulfide redox reactions, a CuS-modified carbon felt electrode is demonstrated with excellent catalytic performance, enabling the PFRFB higher and more stable energy efficiency over cycling. The combination of the cost-effective membrane with the catalytic electrode in one cell leads to a capacity retention of 99.54% after 1180 cycles and an outstanding power density (up to 223 mW cm -2 ). The significant enhancements of electrochemical performance at reduced capital cost will make the PFRFB more promising for large-scale energy storage systems.

Graphene-supported polyoxometalate entrapped in a MIL-88A network with highly efficient conversion of polysulfides in Li-S batteries.

It has been demonstrated that polyoxometalates (POMs) have strong anchoring abilities with efficient catalysis of lithium polysulfides (LiPSs). However, the severe aggregation that buries the effective active sites of POMs along with poor electrical conductivity limits the practical application of POMs in lithium sulfur batteries (LSBs). In our strategy, we utilized reduced graphene oxide (rGO) to support a POM catalyst entrapped in a MIL-88A(FeCo) network with a hollow shell skeleton as the sulfur host material. H4PW11VO40 (PW11V) with optimal vanadium atom implantation ensures the ruggedness and integrity of the hollow structure, which is conducive to achieving high sulfur loading as well as accommodating the volume change of the sulfur cathode during the charging and discharging process. Importantly, PW11V can capture polysulfides through firm chemical adsorption and accelerate redox reactions of LiPS conversion by effective electrochemical catalysis. Furthermore, the satisfactory electrical conductivity of rGO provides access for electrons to reach the interface of PW11V and polysulfides and trigger Li-S conversion reactions. Thus, the constructed PW11V-based sulfur cathode exhibited a superior specific capacity of 905 mA h g-1 after 100 cycles under 0.2 C and long cycling life with a capacity recession rate of 0.046% for each cycle upon 500 cycles under 3 C. This research reveals the effect of vanadium atom substitution of POMs on the cycling performance of a sulfur cathode and affords insight for developing high-performance LSBs.

Manganese–nickel bimetallic oxide electrocatalyzing redox reactions of lithium polysulfides in lithium–sulfur batteries

MnOx–NiOx/C composite bidirectionally catalyzes LiPS reduction and Li2S oxidation via constructing a thiosulfate-mediating reaction path beyond the common route. These intensified Li2Sx (x ≤ 4) redox reactions promote long-term cycling.

Intermolecular Adsorption-Pairing Synergy for Accelerated Polysulfide Redox Reactions Towards Lithium-Sulfur Battery with High Stability

Intermolecular Adsorption-Pairing Synergy for Accelerated Polysulfide Redox Reactions Towards Lithium-Sulfur Battery with High Stability

Co4 N-Decorated 3D Wood-Derived Carbon Host Enables Enhanced Cathodic Electrocatalysis and Homogeneous Lithium Deposition for Lithium-Sulfur Full Cells.

The sluggish kinetics of sulfur conversion in the cathode and the nonuniform deposition of lithium metal at the anode result in severe capacity decay and poor cycle life for lithium-sulfur (Li-S) batteries. Resolving these deficiencies is the most direct route toward achieving practical cells of this chemistry. Herein, a vertically aligned wood-derived carbon plate decorated with Co4 N nanoparticles host (Co4 N/WCP) is proposed that can serve as a host for both the sulfur cathode and the metallic lithium anode. This Co4 N/WCP electrode host drastically enhances the reaction kinetics in the sulfur cathode and homogenizes the electric field at the anode for the uniform lithium plating. Density functional theory calculations confirm the experimental observations that Co4 N/WCP provides a lower energy barrier for the polysulfide redox reaction in the cathode and a low adsorption energy for lithium deposition at the anode. Employing the Co4 N/WCP host at both electrodes in a S@Co4 N/WCP||Li@Co4 N/WCP full cell delivers a specific capacity of 807.9mAhg-1 after 500 cycles at a 1C rate. Additional experiments are performed with high areal sulfur loading of 4mgcm-2 to demonstrate the viability of this strategy for producing practical Li-S cells.

Concurrent Polyvalent Interaction and Electrocatalysis to Improve Lithium‐Sulfur Battery Performance

AbstractBatteries with improved efficiency are desired. Li−S batteries are attractive due to their high specific capacity and energy density. However, sluggish sulfur redox reaction and polysulfide dissolution are significant challenges in Li−S batteries. In this work, we report graphene with doped layer to electrocatalyze the sluggish sulfur redox reaction. The doped layer comprises heteroatoms such as either N or N and S. The doped layer also comprises cations of Ni. We have chosen a “doped layer on graphene” over “doped graphene” to avoid defects in the basal plane of graphene. We found the doped layer comprising graphene (DLC−G) to electrocatalyze the polysulfide redox reaction. However, the interaction between the doped layer and polysulfide is still weak, hence the dissolution is not suppressed. To circumvent the polysulfide dissolution, graphene with cationic layer was prepared. We found that cations in the layer electrostatically attract the polysulfides due to the polyvalent interaction. Thus, the dissolution is suppressed. While using this material in the Li−S batteries, the specific capacity, energy density and power density were found to be 1345 mAh g−1, 782 Wh kg−1 and 4437 W kg−1, respectively.

Synergistic Adsorption-Catalytic Sites TiN/Ta2O5 with Multidimensional Carbon Structure to Enable High-Performance Li-S Batteries.

Lithium-sulfur (Li-S) batteries are deemed to be one of the most optimal solutions for the next generation of high-energy-density and low-cost energy storage systems. However, the low volumetric energy density and short cycle life are a bottleneck for their commercial application. To achieve high energy density for lithium-sulfur batteries, the concept of synergistic adsorptive–catalytic sites is proposed. Base on this concept, the TiN@C/S/Ta2O5 sulfur electrode with about 90 wt% sulfur content is prepared. TiN contributes its high intrinsic electron conductivity to improve the redox reaction of polysulfides, while Ta2O5 provides strong adsorption capability toward lithium polysulfides (LiPSs). Moreover, the multidimensional carbon structure facilitates the infiltration of electrolytes and the motion of ions and electrons throughout the framework. As a result, the coin Li-S cells with TiN@C/S/Ta2O5 cathode exhibit superior cycle stability with a decent capacity retention of 56.1% over 300 cycles and low capacity fading rate of 0.192% per cycle at 0.5 C. Furthermore, the pouch cells at sulfur loading of 5.3 mg cm−2 deliver a high areal capacity of 5.8 mAh cm−2 at low electrolyte/sulfur ratio (E/S, 3.3 μL mg−1), implying a high sulfur utilization even under high sulfur loading and lean electrolyte operation.

Utilizing the Built-in Electric Field of p-n Junctions to Spatially Propel the Stepwise Polysulfide Conversion in Lithium-Sulfur Batteries.

Integrating sulfur cathodes with effective catalysts to accelerate polysulfide conversion is a suitable way for overcoming the serious shuttling and sluggish conversion of polysulfides in lithium-sulfur batteries. However, because of the sharp differences in the redox reaction kinetics and complicated phase transformation of sulfur, a single-component catalyst cannot consistently accelerate the entire redox process. Herein, hierarchical and defect-rich Co3 O4 /TiO2 p-n junctions (p-Co3 O4 /n-TiO2 -HPs) are fabricated to implement the sequential catalysis of S8(solid) → Li2 S4(liquid) → Li2 S(solid) . Co3 O4 sheets physiochemically immobilize the pristine sulfur and ensure the rapid reduction of S8 to Li2 S4 , while TiO2 dots realize the effective precipitation of Li2 S, bridged by the directional migration of polysulfides from p-type Co3 O4 to n-type TiO2 attributed to the interfacial built-in electric field. As a result, the sulfur cathode coupled with p-Co3 O4 /n-TiO2 -HPs delivers long-term cycling stability with a low capacity decay of 0.07% per cycle after 500 cycles at 10 C. This study demonstrates the synergistic effect of the built-in electric field and heterostructures in spatially enhancing the stepwise conversion of polysulfides, which provides novel insights into the interfacial architecture for rationally regulating the polysulfide redox reactions.

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.

Embedding Cobalt Atom Clusters in CNT-Wired MoS2 Tube-in-Tube Nanostructures with Enhanced Sulfur Immobilization and Catalyzation for Li-S Batteries.

Lithium-sulfur batteries are one of the most promising next-generation energy storage systems. The efficient interconversion between sulfur/lithium polysulfides and lithium sulfide is a performance-determining factor for lithium-sulfur batteries. Herein, a novel strategy to synthesize a unique tube-in-tube CNT-wired sulfur-deficient MoS2 nanostructure embedding cobalt atom clusters as an efficient polysulfide regulator is successfully conducted in Li-S batteries. It is confirmed that encapsulating MWCNTs into hollow porous sulfur-deficient MoS2 nanotubes embedded with metal cobalt clusters not only can accelerate electron transport and confine the dissolution of lithium polysulfide by physical/chemical adsorption, but also can catalyze the kinetics of polysulfide redox reactions. Based on DFT calculations, in situ spectroscopic techniques, and various electrochemical studies, catalytic effects of CNT/MoS2 -Co nanocomposite in Li-S battery are deeply investigated for the first time. The CNT/MoS2 -Co composite cathode exhibits a very remarkable rate capability (641 mAh g-1 at 5.0 C) and excellent cycling stability (capacity decay rate of 0.050% per cycle at 5.0 C) even at high sulfur mass loading of 3.6mg cm-2 . More crucially, CNT/MoS2 -Co tube-in-tube nanostructures present a superior specific capacity of 650 mAh g-1 in a Li-S pouch cell at 0.2 C (4.0mg cm-2 ).

Intrinsic defects promote rapid conversion of polysulfides on carbon surface to achieve high rate performance

Polysulfides can be captured rapidly by defects, which possess strong catalytic redox abilities. Species of defects have important influence on rate performance of metal-sulfur electrodes. However, catalytic reaction mechanisms have not been explored in depth. This paper focuses on comparing differences between intrinsic defect carbon materials (IDC) and external introduced defect carbon materials (EIDC) in promoting polysulfide redox abilities and exploring catalytic redox reaction mechanism deeply. Results show that IDC have stronger catalytic redox reaction abilities than EIDC, which is mainly due to stable C–S bonds on surface of IDC. After cycling, C–S bond contents in IDC electrodes maintain more than 70 %. During cycling process, C–S bonds can quickly adsorb polysulfides and promote their conversion into final products, thereby achieving high rate performance. Weak catalytic redox abilities of EIDC electrodes is mainly due to loss of C–S bonds. When the current density is 5C, capacities of IDC electrodes is twice as much as that of EIDC electrodes. Good rate performance of IDC electrodes reflects important roles of stable C–S bonds in promoting redox reaction of polysulfides, which has important research significance for preparation of carbon-based composite electrodes.

Pt-NbC Composite as a Bifunctional Catalyst for Redox Transformation of Polysulfides in High-Rate-Performing Lithium–Sulfur Batteries

Accelerating the redox reaction of polysulfides via catalysis is an effective way to suppress the shuttling effect in lithium-sulfur (Li-S) cells. However, recent studies have mainly focused on the singular function of the catalyst, i.e., either oxidation or reduction of polysulfides. As such, the goal of rapid cycling of sulfur species remains to be highly desired. Herein, a Pt-carbide composite as a bifunctional catalyst was developed to simultaneously accelerate both the reduction of soluble polysulfides and the oxidation of insoluble Li2S/Li2S2. Typically, a Pt-NbC composite was synthesized by growing Pt nanoparticles on the surface of NbC, and the resultant intimate interface in the hybrid is a key component for the bifunctional catalysis. During the reduction process, polysulfides could be grabbed on the surface of NbC via strong adsorption, and then these trapped polysulfides could be catalytically converted by Pt nanoparticles. During the oxidation process, both NbC and Pt exhibited catalytic activities for the dissolution of Li2S. This process could lead to the renewal of the surface of the catalyst. By combining the sulfur cathode with a Pt-NbC-CNT (Pt-NbC anchored on a carbon nanotube)-coated separator, the cell was able to demonstrate a high initial capacity of 1382 mAh g-1 at a current density of 0.2C. Furthermore, the cell was able to achieve an exceptional rate capability of 795 mAh g-1 at 5C, and it was also able to show significantly inhibited self-discharge behavior. Thus, this work explores the catalyst design and the mechanism of a bifunctional catalyst for the performance enhancement in Li-S cells.

Polypyrrole-enveloped Prussian blue nanocubes with multi-metal synergistic adsorption toward lithium polysulfides: high-performance lithium-sulfur batteries

The development of lithium-sulfur (Li-S) batteries is largely hindered by the shuttling and poor electronic conductivity of polysulfide. To solve these two problems simultaneously, we report a new strategy that combines the advantages of Prussian blue analogues (PBAs) with the conductive characteristic of polypyrrole (ppy) to construct FeCoNi PBA-S-ppy composite. The open metal centers in PBAs act as Lewis acid sites with good affinity to the negatively charged polysulfide anions, while the open framework structure allows effective storage of sulfur and confinement of lithium polysulfides (LiPS). Herein, the polysulfide redox reaction mechanism and kinetics are revealed by using in situ X-ray diffraction. The adsorption ability test and electrochemical performance assessment together with density functional theory calculations reveal that the FeCoNi PBA can achieve enhanced LiPS absorption ability and better cycling performance than the FeNi and FeCo PBAs. This can be attributed to the multi-metal synergistic adsorption toward LiPS within FeCoNi PBA, which can act as an effective chemical-anchor to capture LiPS. After combining the FeCoNi PBA with ppy, a physical barrier formed, and the improvement of the conductivity resulted in remarkable electrochemical performance. We believe that PBAs can play a beneficial role in guaranteeing more stable and efficient Li-S batteries.

CoFe2O4 nanoparticles loaded N-doped carbon nanofibers networks as electrocatalyst for enhancing redox kinetics in Li-S batteries

Lithium sulfur (Li-S) batteries have been paid more attention to meet the demand of high capacity energy storage. However, most substrates applied to electrodes, which have both high conductivity and full coverage of adsorption-catalysis synergies, are difficult to achieve. Herein, the combination of electrospinning and hydrothermal method is developed to fabricate the composite membrane of ferri-based spinel CoFe2O4 (CFO) loaded nitrogen doped carbon nanofibers (CFONC) applied to positive current collector with Li2S6 catholyte and binder-free of Li-S batteries. Benefiting from the improved catalytic performances in redox reaction of lithium polysulfides due to the abundant active sites which originate from CFO, the CFONC composite with S loading of 4.74 mg exhibits an initial specific discharge capacity of 1096 mAh g−1 at 0.2 C and a high specific discharge capacity of 681 mAh g−1 after 500 cycles with a capacity decay as small as 0.076% per cycle. Even with S loading of 7.11 mg, the cell of CFONC delivers a high initial capacity of 6.1 mAh and maintains 4.8 mAh after 300 cycles. The results show that the efficient chemical anchoring polysulfides and catalyzing redox reaction by multifunctional CFONC composites is a feasible strategy for the large-scale application of lithium sulfur batteries with high performance in the future.

MXene Nanoflakes Confined in Multichannel Carbon Nanofibers as Electrocatalysts for Lithium–Sulfur Batteries

Abstract Lithium–sulfur (Li–S) batteries have been research hotspots because of their significant advantages in high-energy density and low cost. However, the notorious shuttle effect results in poor electrochemical performance, which is a serious obstacle for their practical application. The delicate design of sulfur hosts is a very important strategy to suppress the shuttle effect. Herein, MXene nanoflakes confined within multichannel carbon nanofibers (MXene@MCNF) have been successfully synthesized as robust electrocatalysts for Li–S batteries based on a simple electrospun method followed by a carbonization process. This unique structure effectively prevents the restacking of MXene nanoflakes, which is conducive to improve the electrocatalytic activity of MXene for propelling the redox reaction of polysulfides owing to the abundant exposure of surface active sites. Moreover, the multichannel hollow structure can inhibit the outward dissolution of polysulfides via the physical confinement caused by their abundant pore structures and alleviate the huge volume change of sulfur cathode. Benefiting from these aforementioned advantages, MXene@MCNF-sulfur (MXene@MCNF-S) cathode delivers a high capacity of 1177 mA h/g at 0.2 C and excellent cycling stability after 200 cycles at 2.0 C.

Tunable Electrocatalytic Behavior of Sodiated MoS2 Active Sites toward Efficient Sulfur Redox Reactions in Room-Temperature Na-S Batteries.

Room-temperature (RT) sodium-sulfur (Na-S) batteries hold great promise for large-scale energy storage due to the advantages of high energy density, low cost, and resource abundance. The research progress on RT Na-S batteries, however, has been greatly hindered by the sluggish kinetics of the sulfur redox reactions. Herein, an elaborate multifunctional architecture, consisting of N-doped carbon skeletons and tunable MoS2 sulfiphilic sites, is fabricated via a simple one-pot reaction followed by in situ sulfurization. Beyond the physical confinement and chemical binding of polarized N-doped carbonaceous microflowers, the MoS2 active sites play a key role in catalyzing polysulfide redox reactions, especially the conversion from long-chain Na2 Sn (4 ≤ n≤ 8) to short-chain Na2 S2 and Na2 S. Significantly, the electrocatalytic activity of MoS2 can be tunable via adjusting the discharge depth. It is remarkable that the sodiated MoS2 exhibits much stronger binding energy and electrocatalytic behavior compared to MoS2 sites, effectively enhancing the formation of the final Na2 S product. Consequently, the S cathode achieves superior electrochemical performance in RT Na-S batteries, delivering a high capacity of 774.2 mAh g-1 after 800 cycles at 0.2 A g-1 , and an ultrahigh capacity retention with a capacity decay rate of only 0.0055% per cycle over 2800 cycles.

Insight into MoS2–MoN Heterostructure to Accelerate Polysulfide Conversion toward High‐Energy‐Density Lithium–Sulfur Batteries

AbstractLithium–sulfur batteries are deemed as optimal energy devices for the next generation of high‐energy‐density energy storage. However, several problems such as low energy density and short cycle life hinder their application in industry. Here, MoS2–MoN heterostructure nanosheets grown on carbon nanotube arrays as free‐standing cathodes are reported. In this heterostructure, MoN works as a promoter to provide coupled electrons to accelerate the redox reaction of polysulfides while the MoS2, with a two‐dimensional layered structure, provides smooth Li+ diffusion pathways. Through their respective advantages, both MoN and MoS2 could mutually boost the process of “adsorption‐diffusion‐conversion” of polysulfides, which have a synergy enhancement effect to restrain the lithium polysulfides from shuttling. The designed cathodes show excellent long‐term cycling performances of 1000 cycles at 1C with a low decay rate of 0.039% per cycle and a high rate capability up to 6C. A high initial areal capacity of 13.3 mAh cm−2 is also achieved under a low electrolyte volume/sulfur loading (E/S) ratio of 6.3 mL g−1. This strategy of promoting polysulfide conversion by heterostructure MoS2–MoN as presented in this work can provide a more structured design strategy for future advanced Li–S energy storage systems.

Bio-inspired construction of electrocatalyst decorated hierarchical porous carbon nanoreactors with enhanced mass transfer ability towards rapid polysulfide redox reactions

Li-S batteries are considered as a highly promising candidate for the next-generation energy storage system, attributing to their tremendous energy density. However, the two-dimensional island nucleation-growth process of lithium sulfide leads to a thick insulating film covering the electrode, inducing slow electrons transfer and mass-transfer of ions and liquid sulfur species in working Li-S cells. Here, we demonstrate a bio-inspired strategy of constructing ant-nest-like hierarchical porous ultrathin carbon nanosheet networks with the implants of metallic nanoparticles electrocatalysts (HPC-MEC) as efficient nanoreactors enabling rapid mass transfer, via a simple and green NaCl template. Such nanoreactors with a large active surface area could effectively anchor polysulfides for mitigating the shuttle effect, facilitating uniformly thin Li2S film, and promoting the mass transfer for fast sulfur species conversions. This helps contribute to a continuously high sulfur utilization in Li-S batteries with the HPC-MEC reactors. As a typical exhibition, cobalt embedded hierarchical porous carbon (HPC-Co) could realize to deliver a remarkably high specific capacity of 1,540.6 mAh·g−1, an excellent rate performance of 878.8 mAh·g−1 at 2 C, and high area capacity of 11.6 mAh·cm−2 at a high sulfur load of 10 mg·cm−2 and low electrolyte/sulfur ratio of 5 µL·mg−1.

Improved Redox Reaction of Lithium Polysulfides on the Interfacial Boundary of Polar CoC2 O4 as a Polysulfide Catenator for a High-Capacity Lithium-Sulfur Battery.

Invited for this month's cover is the group of Jaeyoung Lee at the Gwangju Institute of Science and Technology. The cover shows how the cobalt oxalates faced on the conductive carbon can act as an electrocatalyst to facilitate the redox reaction of lithium polysulfide at the cathode interface in lithium-sulfur batteries. The facilitated electrochemical redox reaction of lithium polysulfides was proved by a series of catenation reactions formed on the interfacial boundary area. The electrochemical performance was enhanced in terms of the specific capacity and long-term cycle performance from the facilitated electrochemical activity. The Full Paper itself is available at 10.1002/cssc.202002140.

Front Cover: Improved Redox Reaction of Lithium Polysulfides on the Interfacial Boundary of Polar CoC2O4 as a Polysulfide Catenator for a High‐Capacity Lithium‐Sulfur Battery (ChemSusChem 3/2021)

Front Cover: Improved Redox Reaction of Lithium Polysulfides on the Interfacial Boundary of Polar CoC₂O₄ as a Polysulfide Catenator for a High‐Capacity Lithium‐Sulfur Battery (ChemSusChem 3/2021)