Unraveling Polysulfide's Adsorption and Electrocatalytic Conversion on Metal Oxides for Li-S Batteries

Advances in the Catalytic Mechanism of Metal Oxides for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are one of the promising next-generation energy storage/conversion devices, considering their high energy density and low cost. However, the shuttle of polysulfides hinders the practical application of Li-S batteries, which leads to reduced cycling stability. Although chemical adsorption strategies have made significant progress in improving the cycling stability of Li-S batteries, the poor catalytic conversion ability of the polysulfide host results in an imbalance between chemical adsorption and catalytic conversion. Recent studies have revealed that metal oxides with adjustable electronic structures exhibit good catalytic ability as polysulfide hosts. However, there is currently no systematic review of the catalytic mechanism of metal oxides in Li-S batteries. Herein, the working principle and primary challenge of Li-S batteries are first introduced, followed by a categorization of catalyst design strategies. Furthermore, a comprehensive review of recent advancements in understanding the reaction mechanism of metal oxide catalysts in Li-S batteries is also provided. Finally, personal perspectives on the future development of Li-S batteries enhanced by metal oxide catalysis are offered. It is hoped that this review can provide valuable insights into the catalytic role of metal oxides in accelerating polysulfide conversion for Li-S batteries.

Unlocking Enhanced Redox Dynamics: The Power of a Bifunctional Catalytic Zinc Phosphide Interface in Full Cell and Pouch Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries face significant challenges, such as polysulfide dissolution, sluggish reaction kinetics, and lithium anode corrosion, hindering their practical application. Herein, we report a highly effective approach using a zinc phosphide (ZnP2) bifunctional catalyst to address these issues. The ZnP2 catalyst effectively anchors lithium polysulfides (LiPSs), catalytically reactivates them, and enhances lithium-ion diffusion. Utilizing a ZnP2-modified separator in a Li-S half-cell achieves an impressive initial capacity of 1145.4 mAh g-1, retaining 954 mAh g-1 and 99.8% Coulombic efficiency after 100 cycles, compared to the pristine separator. The underlying reaction mechanisms are thoroughly investigated through post-mortem analyses and density functional theory (DFT) calculations. Moreover, a Li-S full cell with an E/S ratio of 10 μL mg-1 demonstrates stable cycling performance, achieving an initial capacity of 797.5 and 534 mAh g-1 after 100 cycles at 0.1C, with a negative-to-positive mass ratio of 3:1. Additionally, the real-world feasibility of lightweight and flexible Li-S pouch batteries with ZnP2-modified separators is explored, showing a stable performance over 100 cycles at 0.1C with 80% capacity retention. This engineered separator can be integrated with advanced sulfur cathodes to create high-energy-density, stable Li-S batteries for commercial applications.

Tungsten oxide nanowire clusters anchored on porous carbon fibers as a sulfur redox mediator for lithium-sulfur batteries

Addressing the sluggish redox kinetics of sulfur electrodes and mitigating the shuttle effect of intermediate lithium polysulfides (LiPS) are crucial for the advancement of high-energy lithium-sulfur batteries. Here, we introduce a pioneering flexible self-supporting composite scaffold that incorporates tungsten oxide nanowire clusters anchored on core-shell porous carbon fibers (WO3/PCF) for sulfur accommodation. The core of PCF serves as a robust electrode supporting scaffold, whereas the porous shell of PCF provides a 3D interconnected conductive network to accommodate sulfur, restrain polysulfide diffusion and buffer electrode expansion. The WO3 nanowire clusters not only entrap polysulfides but also function as a redox mediator to promote sulfur conversion, thus greatly mitigating the shuttle effect and boosting redox kinetics. The unique core-shell porous structure of PCF and the dual functionality of WO3 for LiPS capture and conversion contribute to the high capacity, exceptional cycling stability, and superior rate capability of the WO3/PCF/S cathode. Impressively, at a sulfur loading of 3.0 mg cm-2, it achieves an initial capacity of 1082 mA h·g-1 at 1 C with an ultralow decay rate of 0.039% over 1000 cycles. Even under a high sulfur loading of 6.1 mg cm-2, it maintains a reversible capacity of 536 mA h·g-1 after 1000 cycles with a decay rate of only 0.043% at 0.5 C.

Integrating CoP/Co heterojunction into nitrogen-doped carbon polyhedrons as electrocatalysts to promote polysulfides conversion in lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have garnered extensive research interest as one of the most promising energy storage devices due to their ultra-high theoretical energy density. However, the sluggish reaction kinetics, abominable shuttling effect and inferior cycling stability severely restrict its practical application. Herein, a multifunctional CoP/Co@NC/CNT heterostructure host material was elaborately designed and synthesized by integrating CoP/Co heterojunction, N-doped carbon hollow polyhedrons (NC) and carbon nanotubes (CNTs). Specifically, the CoP/Co heterojunction can reconfigure the local electronic structure, resulting in a synergistic effect that enhances adsorption capacity and catalytic activity compared to CoP and Co alone. Furthermore, the CNTs-grafted NC not only provides multi-dimensional pathways for rapid electron transport and ion diffusion, but also physically restricts the diffusion of polysulfides during charge-discharge processes. Owing to these advantages, the battery assembled with the CoP/Co@NC/CNT/S cathode yields an impressive discharge specific capacity of 1479.9 mAh g-1 at 0.1C, and excellent capacity retention of 793.7 mAh g-1 over 500 cycles at 2C (∼85.5 % of initial capacity). The rational integration of multifunctional heterostructures could provide an effective strategy for designing high-efficiency nanocomposite electrocatalysts to promote sulfur redox kinetics in Li-S batteries.

Tailoring Cathode-Electrolyte Interface for High-Power and Stable Lithium-Sulfur Batteries

Global interest in lithium-sulfur batteries as one of the most promising energy storage technologies has been sparked by their low sulfur cathode cost, high gravimetric, volumetric energy densities, abundant resources, and environmental friendliness. However, their practical application is significantly impeded by several serious issues that arise at the cathode-electrolyte interface, such as interface structure degradation including the uneven deposition of Li2S, unstable cathode-electrolyte interphase (CEI) layer and intermediate polysulfide shuttle effect. Thus, an optimized cathode-electrolyte interface along with optimized electrodes is required for overall improvement. Herein, we comprehensively outline the challenges and corresponding strategies, including electrolyte optimization to create a dense CEI layer, regulating the Li2S deposition pattern, and inhibiting the shuttle effect with regard to the solid-liquid-solid pathway, the transformation from solid-liquid-solid to solid-solid pathway, and solid-solid pathway at the cathode-electrolyte interface. In order to spur more perceptive research and hasten the widespread use of lithium-sulfur batteries, viewpoints on designing a stable interface with a deep comprehension are also put forth.

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 5.1 µL mg-1 and a sulfur loading of 2.3 mg cm-2. Even when the sulfur loading is increased to 10.02 mg 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.

Synergistic Regulation of Bidirectional Conversion of LiPSs and Li2S Using Anthraquinone as a Redox Mediator

Lithium-sulfur (Li-S) batteries are strong contenders as energy storage options in the next-generation, primarily because of their potential for delivering high energy densities. Nonetheless, their widespread commercialization faces several obstacles, including sluggish sulfur redox kinetics, the insulating properties of the Li2S discharge product, and significant reaction energy barriers. In this work, anthraquinone (AQ) was introduced as a redox mediator and incorporated onto Co-doped carbon materials through π-π interactions. The results showed that synergistic effect between AQ and Co atoms facilitated the bidirectional conversion of lithium polysulfides (LiPSs) and Li2S. During charging, AQ lowered the reaction energy barrier for Li2S oxidation and thereby enhanced the reversibility of sulfur redox reactions. Density functional theory (DFT) calculations showed that AQ-Li2Sx exhibits a lower energy for the lowest unoccupied molecular orbital (LUMO) and a higher energy for the highest occupied molecular orbital (HOMO). Experimental results demonstrated that an impressive initial discharge specific capacity of 1290 mAh g-1 was achieved by the fabricated S@AQ/Co-N-C electrode at 0.1 C. After 600 cycles at 1 C, it retained 64% of this capacity and exhibited a minimal 0.06% capacity decay rate per cycle.

Versatile Separators Toward Advanced Lithium-Sulfur Batteries: Status, Recent Progress, Challenges and Perspective

Lithium-sulfur batteries (LSBs) have recently gained extensive attention due to their high energy density, low cost, and environmental friendliness. However, serious shuttle effect and uncontrolled growth of lithium dendrites restrict them from further commercial applications. As "the third electrode", functional separators are of equal significance as both anodes and cathodes in LSBs. The challenges mentioned above are effectively addressed with rational design and optimization in separators, thereby enhancing their reversible capacities and cycle stability. The review discusses the status/operation mechanism of functional separators, then primarily focuses on recent research progress in versatile separators with purposeful modifications for LSBs, and summarizes the methods and characteristics of separator modification, including heterojunction engineering, single atoms, quantum dots, and defect engineering. From the perspective of the anodes, distinct methods to inhibit the growth of lithium dendrites by modifying the separator are discussed. Modifying the separators with flame retardant materials or choosing a solid electrolyte is expected to improve the safety of LSBs. Besides, in-situ techniques and theoretical simulation calculations are proposed to advance LSBs. Finally, future challenges and prospects of separator modifications for next-generation LSBs are highlighted. We believe that the review will be enormously essential to the practical development of advanced LSBs.

Preparation of a lithium-sulfur battery diaphragm catalyst and its battery performance

Lithium-sulfur batteries (LSBs) with metal lithium as the anode and elemental sulfur as the cathode active materials have attracted extensive attention due to their high theoretical specific capacity (1675 mA h g-1), high theoretical energy density (2600 W h kg-1), low cost, and environmental friendliness. However, the discharge intermediate lithium polysulfide undergoes a shuttle side reaction between the two electrodes, resulting in low utilization of the active substances. This limits the capacity and cycle life of LSBs and further delays their commercial development. However, the number of active sites and electron transport capacity of such catalysts still do not meet the practical development needs of lithium-sulfur batteries. In view of these issues, this paper focuses on a zinc-cobalt compound catalyst, modifying it through heteroatom doping, bimetallic synergistic effect and heterogeneous structure design to enhance the performance of LSBs as a separator modification material. A carbon shell-supported boron-doped ZnS/CoS2 heterojunction catalytic material (B-ZnS/CoS2@CS) was prepared, and its performance in lithium-sulfur batteries was evaluated. A carbon substrate (CS) was prepared by pyrolysis of sodium citrate, and the boron-doped ZnS/CoS2 heterojunction catalyst was formed on the CS using a one-step solvothermal method. The unique heterogeneous interface provides numerous active sites for the adsorption and catalysis of polysulfides. The uniformly doped, electron-deficient boron further enhances the Lewis acidity of the ZnS/CoS2 heterojunction, while also regulating electron transport. The B-ZnS/CoS2@CS catalyst effectively inhibits the diffusion of LiPS anions by utilizing additional lone-pair electrons. The lithium-sulfur battery using the catalyst-modified separator achieves a high specific capacity of 1241 mA h g-1 at a current density of 0.2C and retains a specific capacity of 384.2 mA h g-1 at 6.0C. In summary, B-ZnS/CoS2@CS heterojunction catalysts were prepared through boron doping modification. They can promote the conversion of polysulfides and effectively inhibit the shuttle effect. The findings provide valuable insights for the future modification and preparation of lithium-sulfur battery catalysts.

Advances in Functional Organosulfur-Based Mediators for Regulating Performance of Lithium Metal Batteries

Rechargeable lithium metal batteries (LMBs) are promising next-generation energy storage systems due to their high theoretical energy density. However, their practical applications are hindered by lithium dendrite growth and various intricate issues associated with the cathodes. These challenges can be mitigated by using organosulfur-based mediators (OSMs), which offer the advantages of abundance, tailorable structures, and unique functional adaptability. These features enable the rational design of targeted functionalities, enhance the interfacial stability of the lithium anode and cathode, and accelerate the redox kinetics of electrodes via alternative reaction pathways, thereby effectively improving the performance of LMBs. Unlike the extensively explored field of organosulfur cathode materials, OSMs have garnered little attention. This review systematically summarizes recent advancements in OSMs for various LMB systems, including lithium-sulfur, lithium-selenium, lithium-oxygen, lithium-intercalation cathode batteries, and other LMB systems. It briefly elucidates the operating principles of these LMB systems, the regulatory mechanisms of the corresponding OSMs, and the fundamentals of OSMs activity. Ultimately, strategic optimizations are proposed for designing novel OSMs, advanced mechanism investigation, expanded applications, and the development of safe battery systems, thereby providing directions to narrow the gap between rational modulation of organosulfur compounds and their practical implementation in batteries.

A Multifunctional Secondary Based on Heterogeneous Co-MnO@NC for Depth-Induced Deposition and Conversion of Polysulfides in Li─S Batteries

The conductive carbon-based interlayer, as the secondary current collector in the self-dissolving battery system, can effectively capture escaping cathode active materials, inducing deep release of remaining capacity. In the multi-step reactions of Li─S batteries, the environmental tolerance of the conductive carbon-based interlayer to polysulfides determines the inhibition of shuttle effects. Here, a modified metal-organic framework (Mn-ZIF67) is utilized to obtain nitrogen-doped carbon-coated heterogeneous Co-MnO (Co-MnO@NC) with dual catalytic center for the functional interlayer materials. The synergistic coupling mechanism of NC and Co-MnO achieves rapid deposition and conversion of free polysulfide and fragmented active sulfur on the secondary current collector, reducing capacity loss in the cathode. The Li─S battery with Co-MnO@NC/PP separator maintains an initial capacity of 1050 mAh g-1 (3C) and excellent cycle stability (0.056% capacity decay rate). Under extreme testing conditions (S load = 5.82 mg cm-2, E/S = 9.1 µL mg-1), a reversible capacity of 501.36 mAh g-1 is observed after 200 cycles at 0.2 C, showing good further practical reliability. This work demonstrates the advancement application of Co-MnO@NC bimetallic heterojunctions catalysts in the secondary current collector for high-performance Li─S batteries, thereby providing guidance for the development of interlayer in various dissolution systems.

Coordination Engineering of Fe-Centered Catalysts for Superior Li-S Battery Performance

Iron-nitrogen functionalized graphene has emerged as a promising cathode host for rechargeable lithium-sulfur batteries (RLSBs) due to its affordability and enhanced battery performance. To optimize its catalytical efficiency, we propose a novel approach involving coordination engineering. Our investigation spans a plethora of catalysts with varied coordination environments, focusing on elements B, C, N and O. We revealed that Fe-C4 and Fe-B2C2-h are particularly effective for promoting Li2S oxidation, whereas Fe-N4 excels in catalyzing the sulfur reduction reaction (SRR). Importantly, our study identified specific descriptors - namely, the Integrated Crystal Orbital Hamilton Population (ICOHP) and the bond length between Fe and S in Li2S adsorbed state - as the most effective predictive descriptors for Li2S oxidation barriers. Meanwhile, Li2S adsorption energy emerges as a reliable descriptor for assessing the SRR barrier. These identified descriptors are expected to be instrumental in rapidly identifying promising cathode hosts across various metal-centered systems with diverse coordination environments. Our findings not only offer valuable insights into the role of coordination environment, but also present an effective path for rapidly identifying high performance catalysts for RLSBs, enabling the acceleration of advanced RLSBs development.

Developing a Multifunctional Cathode for Photoassisted Lithium-Sulfur Battery

Integration of solar cell and secondary battery cannot only promote solar energy application but also improve the electrochemical performance of battery. Lithium-sulfur battery (LSB) is an ideal candidate for photoassisted batteries owing to its high theoretical capacity. Unfortunately, the researches related the combination of solar energy and LSB are relatively lacking. Herein, a freestanding photoelectrode is developed for photoassisted lithium-sulfur battery (PALSB) by constructing a heterogeneous structured Au@N-TiO2 on carbon cloths (Au@N-TiO2/CC), which combines multiple advantages. The Au@N-TiO2/CC photoelectrode can produce the photoelectrons to facilitate sulfur reduction during discharge process, while generating holes to accelerate sulfur evolution during charge process, improving the kinetics of electrochemical reactions. Meanwhile, Au@N-TiO2/CC can work as an electrocatalyst to promote the conversion of intermediate polysulfides during charge/discharge process, mitigating induced side reactions. Benefiting from the synergistic effect of electrocatalysis and photocatalysis, PALSB assembled with an Au@N-TiO2/CC photoelectrode obtains ultrahigh specific capacity, excellent rate performance, and outstanding cycling performance. What is more, the Au@N-TiO2/CC assembled PALSB can be directly charged under light illumination. This work not only expands the application of solar energy but also provides a new insight to develop advanced LSBs.

Rare Earth Single-Atom Catalysis for High-Performance Li-S Full Battery with Ultrahigh Capacity

Lithium-sulfur (Li-S) batteries have many advantages but still face problems such as retarded polysulfides redox kinetics and Li dendrite growth. Most reported single atom catalysts (SACs) for Li-S batteries are based on d-band transition metals whose d orbital constitutes active valence band, which is inclined to occur catalyst passivation. SACs based on 4f inner valence orbital of rare earth metals are challenging for their great difficulty to be activated. In this work, we design and synthesize the first rare earth metal Sm SACs which has electron-rich 4f inner orbital to promote catalytic conversion of polysulfides and uniform deposition of Li. Sm SACs enhance the catalysis by the activated 4f orbital through an f-d-p orbital hybridization. Using Sm-N3C3 modified separators, the half cells deliver a high capacity over 600 mAh g-1 and a retention rate of 84.3 % after 2000 cycles. The fabricated Sm-N3C3-Li|Sm-N3C3@PP|S/CNTs full batteries can provide an ultra-stable cycling performance of a retention rate of 80.6 % at 0.2 C after 100 cycles, one of the best full Li-S batteries. This work provides a new perspective for the development of rare earth metal single atom catalysis in electrochemical reactions of Li-S batteries and other electrochemical systems for next-generation energy storage.

Kelvin Probe Force Microscopy and Electrochemical Atomic Force Microscopy Investigations of Lithium Nucleation and Growth: Influence of the Electrode Surface Potential

Lithium metal is promising for high-capacity batteries because of its high theoretical specific capacity of 3860 mAh g-1 and low redox potential of -3.04 V versus the standard hydrogen electrode. However, it encounters challenges, such as dendrite formation, which poses risks of short circuits and safety hazards. This study examines Li deposition using electrochemical atomic force microscopy (EC-AFM) and Kelvin probe force microscopy (KPFM). KPFM provides insights into local surface potential, while EC-AFM captures the surface response evolution to electrochemical reactions. We selectively removed metallic coatings from current collectors to compare lithium deposition on coated and exposed copper surfaces. Observations from the Ag-coated Cu (Ag/Cu), Pt-coated Cu (Pt/Cu), and Au-coated Cu (Au/Cu) samples revealed variations in lithium deposition. Ag/Cu and Au/Cu exhibited two-dimensional growth, whereas Pt/Cu exhibited three-dimensional growth, highlighting the impact of electrode materials on morphology. These insights advance the development of safer lithium metal batteries.

In Situ Growth Strategy to Construct "Four-In-One" Separators with Functionalized Polyphosphazene Coatings for Safe and Stable Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are facing many challenges, such as the inadequate conductivity of sulfur, the shuttle effect caused by lithium polysulfide (LiPSs), lithium dendrites, and the flammability, which have hindered their commercial applications. Herein, a "four-in-one" functionalized coating is fabricated on the surface of polypropylene (PP) separator by using a novel flame-retardant namely InC-HCTB to meet these challenges. InC-HCTB is obtained by cultivating polyphosphazene on the surface of carbon nanotubes with an in situ growth strategy. First, this unique architecture fosters an enhanced conductive network, bolstering the bidirectional enhancement of both ionic and electronic conductivities. Furthermore, InC-HCTB effectively inhibits the shuttle effect of LiPSs. LSBs exhibit a remarkable capacity of 1170.7 mA h g-1 at 0.2 C, and the capacity degradation is a mere 0.0436% over 800 cycles at 1 C. Third, InC-HCTB coating serves as an ion migration network, hindering the growth of lithium dendrites. More importantly, InC-HCTB exhibits notable flame retardancy. The radical trapping action in the gas phase and the protective effect of the shielded char layer in the condensed phase are simulated and verified. This facile in situ growth strategy constructs a "four-in-one" functional separator coating, rendering InC-HCTB a promising additive for the large-scale production of safe and stable LSBs.

Multifunctional SnO2 QDs/MXene Heterostructures as Laminar Interlayers for Improved Polysulfide Conversion and Lithium Plating Behavior

Poor cycling stability in lithium-sulfur (Li-S) batteries necessitates advanced electrode/electrolyte design and innovative interlayer architectures. Heterogeneous catalysis has emerged as a promising approach, leveraging the adsorption and catalytic performance on lithium polysulfides (LiPSs) to inhibit LiPSs shuttling and improve redox kinetics. In this study, we report an ultrathin and laminar SnO2@MXene heterostructure interlayer (SnO2@MX), where SnO2 quantum dots (QDs) are uniformly distributed across the MXene layer. The combined structure of SnO2 QDs and MXene, along with the creation of numerous active boundary sites with coordination electron environments, plays a critical role in manipulating the catalytic kinetics of sulfur species. The Li-S cell with the SnO2@MX-modified separator not only demonstrates superior electrochemical performance compared to cells with a bare separator but also induces homogeneous Li deposition during cycling. As a result, an areal capacity of 7.6 mAh cm-2 under a sulfur loading of 7.5 mg cm-2 and a high stability over 500 cycles are achieved. Our work demonstrates a feasible strategy of utilizing a laminar separator interlayer for advanced Li-S batteries awaiting commercialization and may shed light on the understanding of heterostructure catalysis with enhanced reaction kinetics.

Cathode Electrolyte Interphase Engineering for Prussian Blue Analogues in Lithium-Ion Batteries

The increasing use of low-cost lithium iron phosphate cathodes in low-end electric vehicles has sparked interest in Prussian blue analogues (PBAs) for lithium-ion batteries. A major challenge with iron hexacyanoferrate (FeHCFe), particularly in lithium-ion systems, is its slow kinetics in organic electrolytes and valence state inactivation in aqueous ones. We have addressed these issues by developing a polymeric cathode electrolyte interphase (CEI) layer through a ring-opening reaction of ethylene carbonate triggered by OH- radicals from structural water. This facile approach considerably mitigates the sluggish electrochemical kinetics typically observed in organic electrolytes. As a result, FeHCFe has achieved a specific capacity of 125 mAh g-1 with a stable lifetime over 500 cycles, thanks to the effective activation of Fe low-spin states and the structural integrity of the CEI layers. These advancements shed light on the potential of PBAs to be viable, durable, and efficient cathode materials for commercial use.

Perspectives on Advanced Lithium-Sulfur Batteries for Electric Vehicles and Grid-Scale Energy Storage

Intensive increases in electrical energy storage are being driven by electric vehicles (EVs), smart grids, intermittent renewable energy, and decarbonization of the energy economy. Advanced lithium-sulfur batteries (LSBs) are among the most promising candidates, especially for EVs and grid-scale energy storage applications. In this topical review, the recent progress and perspectives of practical LSBs are reviewed and discussed; the challenges and solutions for these LSBs are analyzed and proposed for future practical and large-scale energy storage applications. Major challenges for the shuttle effect, reaction kinetics, and anodes are specifically addressed, and solutions are provided on the basis of recent progress in electrodes, electrolytes, binders, interlayers, conductivity, electrocatalysis, artificial SEI layers, etc. The characterization strategies (including in situ ones) and practical parameters (e.g., cost-effectiveness, battery management/modeling, environmental adaptability) are assessed for crucial automotive/stationary large-scale energy storage applications (i.e., EVs and grid energy storage). This topical review will give insights into the future development of promising Li-S batteries toward practical applications, including EVs and grid storage.

Undercoordination Chemistry of Sulfur Electrocatalyst in Lithium-Sulfur Batteries

Undercoordination chemistry is an effective strategy to modulate the geometry-governed electronic structure and thereby regulate the activity of sulfur electrocatalysts. Efficient sulfur electrocatalysis is requisite to overcome the sluggish kinetics in lithium-sulfur (Li-S) batteries aroused by multi-electron transfer and multi-phase conversions. Recent advances unveil the great promise of undercoordination chemistry in facilitating and stabilizing sulfur electrochemistry, yet a related review with systematicness and perspectives is still missing. Herein, it is carefully combed through the recent progress of undercoordination chemistry in sulfur electrocatalysis. The typical material structures and operational strategies are elaborated, while the underlying working mechanism is also detailly introduced and generalized into polysulfide adsorption behaviors, polysulfide conversion kinetics, electron/ion transport, and dynamic reconstruction. Moreover, perspectives on the future development of undercoordination chemistry are further proposed.

Applications, prospects and challenges of metal borides in lithium sulfur batteries

Although the lithium-sulfur (Li-S) battery has a theoretical capacity of up to 1675 mA h g-1, its practical application is limited owing to some problems, such as the shuttle effect of soluble lithium polysulfides (LiPSs) and the growth of Li dendrites. It has been verified that some transition metal compounds exhibit strong polarity, good chemical adsorption and high electrocatalytic activities, which are beneficial for the rapid conversion of intermediate product in order to effectively inhibit the "shuttle effect". Remarkably, being different from other metal compounds, it is a significant characteristic that both metal and boron atoms of transition metal borides (TMBs) can bind to LiPSs, which have shown great potential in recent years. Here, for the first time, almost all existing studies on TMBs employed in Li-S cells are comprehensively summarized. We firstly clarify special structures and electronic features of metal borides to show their great potential, and then existing strategies to improve the electrochemical properties of TMBs are summarized and discussed in the focus sections, such as carbon-matrix construction, morphology control, heteroatomic doping, heterostructure formation, phase engineering, preparation techniques. Finally, the remaining challenges and perspectives are proposed to point out a direction for realizing high-energy and long-life Li-S batteries.

Hybrid Ascharite/Reduced Graphene Oxide with Polysulfide Adsorption Host for Advanced Lithium-Sulfur Batteries

Balancing the adsorption of lithium-polysulfide intermediates on polar host material surfaces and the effect of their electronic conductivity in the subsequent oxidation and reduction kinetics of electrochemical reactions is necessary and remains a challenge. Herein, we have evaluated the role of polarity and conductivity in preparing a series of ascharite/reduced graphene oxide (RGO) aerogels by dispersing strong polar ascharite nanowires of varying mass into the conductive RGO matrix. When severed as Li-S battery cathodes, the optimized S@ascharite/RGO cathode with a sulfur content of 73.8 wt % demonstrates excellent rate performance and cycle stability accompanied by a high-capacity retention for 500 cycles at 1.0 C. Interesting advantages including the enhanced adsorption ability by the formation of the Mg-S and Li bonds, the continuous and quick electron/ion transportations assembled conductive RGO framework, and the effective deposition of Li2S are combined in the ascharite/RGO aerogel hosts. The electrochemical results further demonstrate that the polarity of ascharite components for the S cathode plays a dominant role in the improvement of electrochemical performance, but the absence of a conductive substrate leads to serious capacity attenuation, especially the rate performance. The balanced design protocol provides a universal method for the synthesis of multiple S hosts for high-performance LSBs.

Selective Reduction of Multivariate Metal-Organic Frameworks for Advanced Electrocatalytic Cathodes in High Areal Capacity and Long-Life Lithium-Sulfur Batteries

Lithium-sulfur batteries hold great promise as next-generation high-energy-density batteries. However, their performance has been limited by the low cycling stability and sulfur utilization. Herein, we demonstrate that a selective reduction of the multivariate metal-organic framework, MTV-MOF-74 (Co, Ni, Fe), transforms the framework into a porous carbon decorated with bimetallic CoNi alloy and Fe3O4 nanoparticles capable of entrapping soluble lithium polysulfides while synergistically facilitating their rapid conversion into Li2S. Electrochemical studies on coin cells containing 89 wt % sulfur loading revealed a reversible capacity of 1439.8 mA h g-1 at 0.05 C and prolonged cycling stability for 1000 cycles at 1 C/1060.2 mA h g-1 with a decay rate of 0.018% per cycle. At a high areal sulfur loading of 6.9 mg cm-2 and lean electrolyte/sulfur ratio (4.5 μL:1.0 mg), the battery based on the 89S@CoNiFe3O4/PC cathode provides a high areal capacity of 6.7 mA h cm-2. The battery exhibits an outstanding power density of 849 W kg-1 at 5 C and delivers a specific energy of 216 W h kg-1 at 2 C, corresponding to a specific power of 433 W kg-1. Density functional theory shows that the observed results are due to the strong interaction between the CoNi alloy and Fe3O4, facilitated by charge transfer between the polysulfides and the substrate.

Ultrathin MgB2 nanosheet-modified polypropylene separator for high-efficiency lithium-sulfur batteries

The separator is an important component in lithium-sulfur (Li-S) batteries. However, the conventional polypropylene (PP) separators have the problem of easy shuttling of lithium polysulfide (LiPSs). Herein, ultrathin magnesium boride (MgB2) nanosheets were prepared by ultrasonic-assisted exfoliation technology, and were suction-filtered onto a separator to serve as a separator modification layer. The introduction of a microporous structure into MgB2 nanosheets after ultrasonic peeling increases the specific surface area and pore volume, with more adsorption sites, which can fully utilize the surface adsorption/catalytic performance of MgB2 for LiPSs and accommodate the volume expansion of lithium sulfide (Li2S). Therefore, MgB2@PP as a separator significantly improves the sulfur utilization and cycle stability in Li-S batteries. When the MgB2@PP separator is used, the reversible specific capacity of the assembled Li-S battery at 0.1 C (current rate) is 1184 mAh/g, and the specific capacity at 2 C is 732 mAh/g. After 500 cycles at 2 C, it remains at 497 mAh/g.

Investigation of the Chemisorption-Catalysis Behavior of Sulfur Species on the Electrocatalysts Designed by Co-regulation Strategy of Anions and Cations

Li-S batteries possess high energy density and have been one of the most promising energy storage systems. For sulfur cathodes, the electrochemical performance is still seriously hindered by the polysulfide shuttling and sluggish conversion kinetics. It has been demonstrated to be one effective strategy to address the above issues via designing electrocatalysts with robust affinity and catalytic capacity towards polysulfides. However, it is still a great challenge to rapidly and economically discover high-performance electrocatalysts. Herein, using density functional theory calculation, we studied the chemisorption-catalysis behavior of sulfur species on a series of electrocatalysts (MCo2 X4 , M=Co, Zn, Cu, Ni, Fe, and Mn, X=O, S, and Se) to assess the effect of the anions and cations co-regulation on their electronic structure, chemisorption behavior, and catalytic property. FeCo2 Se4 and CuCo2 Se4 combined appropriate chemisorption with superior electronic conductivity and sulfur reduction catalytic capacity have been predicted as novel electrocatalysts for high-performance Li-S batteries. This study gives theoretical guidance for rapid discovery of high-efficient electrocatalyst to boost the electrochemical performance of sulfur cathodes.

Enhancing Lithium-Sulfur Battery Performance by MXene, Graphene, and Ionic Liquids: A DFT Investigation

The efficacy of lithium-sulfur (Li-S) batteries crucially hinges on the sulfur immobilization process, representing a pivotal avenue for bolstering their operational efficiency and durability. This dissertation primarily tackles the formidable challenge posed by the high solubility of polysulfides in electrolyte solutions. Quantum chemical computations were leveraged to scrutinize the interactions of MXene materials, graphene (Gr) oxide, and ionic liquids with polysulfides, yielding pivotal binding energy metrics. Comparative assessments were conducted with the objective of pinpointing MXene materials, with a specific focus on d-Ti3C2 materials, evincing augmented binding energies with polysulfides and ionic liquids demonstrating diminished binding energies. Moreover, a diverse array of Gr oxide materials was evaluated for their adsorption capabilities. Scrutiny of the computational outcomes unveiled an augmentation in the solubility of selectively screened d-Ti3C2 MXene and ionic liquids-vis à vis one or more of the five polysulfides. Therefore, the analysis encompasses an in-depth comparative assessment of the stability of polysulfide adsorption by d-Ti3C2 MXene materials, Gr oxide materials, and ionic liquids across diverse ranges.

Ultrathin Carbon-Shell-Encapsulated Cobalt Nanoparticles with Balanced Activity and Stability for Lithium-Sulfur Batteries

High-performance metal-based catalysts are pursued to improve the sluggish reaction kinetics in lithium-sulfur batteries. However, it is challenging to achieve high catalytic activity and stability simultaneously due to the inevitable passivation of the highly active metal nanoparticles by lithium polysulfides (LiPSs). Herein, we show a design with well-balanced activity and stability to solve the above problem, that is, the cobalt (Co) nanoparticles (NPs) encapsulated with ultrathin carbon shells prepared by the one-step pyrolysis of ZIF-67. With an ultrathin carbon coating (∼1 nm), the direct exposure of Co NPs to LiPSs is avoided, but it allows the fast electron transfer from the highly active Co NPs to LiPSs for their conversion to the solid products, ensuring the efficient suppression of shuttling in long cycling. As a result, the sulfur cathode with such a catalyst exhibited good cycling stability (0.073% capacity fading over 500 cycles) and high sulfur utilization (638 mAh g^-1 after 180 cycles under a high sulfur mass loading of 7.37 mg cm^-2 and a low electrolyte/sulfur ratio of 5 μL mg^-1). This work provides insights into the rational design of a protection layer on a metal-based catalyst to engineer both high catalytic activity and stability toward high-energy and long-life Li-S batteries.