A (110) Facet-Dominated Vanadium Dioxide Enabling Bidirectional Electrocatalysis for Lithium-Sulfur Batteries

Realization of Enhanced Interfacial Lithium-Ion Transfer in Composite Polymer Electrolytes via Grafting Oligo-PEG Molecular Brushes on Silica-Coated Nanofibers for All-Solid-State Lithium Metal Batteries

Polyether-based polymer electrolytes are attractive but still challenging for high-energy-density solid-state lithium metal batteries due to their limited Li-ion conductivity at room temperature. Herein, an oligomeric polyethylene glycol methyl ether methacrylate (PEGMEM)-modified silica-coated polyimide fibrous scaffold (PINF@PEGMEM-SiO2) was introduced in polyethylene glycol dimethyl ether (PEGDME) to enhance the Li-ion transportation at room temperature. PINF@PEGMEM-SiO2 was developed to build a continuous and interconnected interface for continuous Li-ion transportation in bulk. The carbonyl groups (C═O) of PEGMEM on SiO2 can promote the dissociation of lithium salts and enhance the migration of free Li ions at the interface. The same -C-C-O- unit contained in both PEGMEM and PEGDME ensures the compatibility of PEGMEM at the interface and PEGDME in the bulk. The prepared PEGDME-based polymer electrolyte exhibits a high ionic conductivity of 1.14 × 10-4 S cm-1 at 25 °C and an improved Li-ion transference number of 0.41. Furthermore, LiFePO4/Li and LiNi0.8Co0.1Mn0.1O2/Li cells with excellent cyclability and rate capability at ambient temperature are obtained.

Constructing Orbital Coupling-Modulated Homogeneous Dual-Atom Fe-Fe Sites for Boosting Bidirectional Conversion of Polysulfides

Homogeneous dual-atom catalysts (HDACs) have garnered significant attention for their potential to overcome the shuttling effect and sluggish reaction kinetics in lithium-sulfur (Li-S) batteries. However, modulating the electron structure of metal atomic orbitals for HDACs to dictate the catalytic activity toward polysulfides has remained meaningful but unexplored so far. Herein, an interfacial cladding strategy is developed to obtain a new type of dual-atom iron matrix with a unique FeN2P1-FeN2P1 coordination structure (Fe2@NCP). The 3d orbital electrons of the Fe centers are redistributed by incorporating phosphorus atoms into the first coordination sphere. The theoretical calculations disclose that the strong coupling between the Fe d orbital and the S p orbital exhibits an enhanced Fe-S bond and improved reactivity toward polysulfides. Moreover, the Fe2@NCP catalyst achieves robust adsorption ability toward Li2Sn (1 ≤ n ≤ 8) and significantly boosts bidirectional sulfur redox reaction kinetics by lowering the Li2S deposition/decomposition energy barriers. Consequently, the assembled Li-S batteries present a high retention ratio of 77.3% after 500 cycles at 1C. Furthermore, the Li-S pouch cell also exhibits good performance at 0.1C (80.2% retention over 100 cycles) for practical application with a sulfur loading of 4.0 mg/cm2. The outcome of this study will facilitate the design of homogeneous dual-atom catalysts for Li-S batteries.

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.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

Boosting Polysulfide Redox Kinetics by Temperature-Induced Metal-Insulator Transition Effect of Tungsten-Doped Vanadium Dioxide for High-Temperature Lithium-Sulfur Batteries

The practical application of Li-S batteries is still severely restricted by poor cyclic performance caused by the intrinsic polysulfides shuttle effect, which is even more severe under the high-temperature condition owing to the inevitable increase of polysulfides' solubility and diffusion rate. Herein, tungsten-doped vanadium dioxide (W-VO2 ) micro-flowers are employed with first-order metal-insulator phase transition (MIT) property as a robust and multifunctional modification layer to hamper the shuttle effect and simultaneously improve the thermotolerance of the common separator. Tungsten doping significantly reduces the transition temperature from 68 to 35 °C of vanadium dioxide, which renders the W-VO2 easier to turn from the insulating monoclinic phase into the metallic rutile phase. The systematic experiments and theoretical analysis demonstrate that the temperature-induced in-suit MIT property endows the W-VO2 catalyst with strong chemisorption against polysulfides, low energy barrier for liquid-to-solid conversion, and outstanding diffusion kinetics of Li-ion under high temperatures. Benefiting from these advantages, the Li-S batteries with W-VO2 modified separator exhibit significantly improved rate and long-term cyclic performance under 50 °C. Remarkably, even at an elevated temperature (80 °C), they still exhibit superior electrochemical performance. This work opens a rewarding avenue to use phase-changing materials for high-temperature Li-S batteries.

CoS2@C catalyzes polysulfide conversion to promote the rate and cycling performances of lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have been considered one of the most promising candidates for next-generation energy storage devices due to their high theoretical energy density and low cost. Nonetheless, the practical application of Li-S batteries is still inhibited by their lithium polysulfide (LiPS) shuttling and sluggish redox kinetics, which cause rapid capacity decay and inferior rate performance. Hence, anchoring LiPSs and catalyzing their conversion reactions are imperative to enhance the performance of Li-S batteries. In this work, one-dimensional (1D) porous carbon-encapsulated CoS2 (CoS2@C) fiber structures were prepared through a simple two-step hydrothermal reaction and they exhibited a robust LiPS trapping ability and rapid catalytic conversion of LiPSs. The formed three-dimensional (3D) architecture (CoS2@C/MWCNT) facilitates the physical adsorption of LiPSs and rapid ion transport. The electrode exhibited a high initial capacity of 1329.5 mA h g-1 at a current density of 0.1 C and a reversible capacity of 1060.6 mA h g-1 after 100 cycles, with an 80% capacity retention rate. Meanwhile, the decay rate of the electrode is 0.048% per cycle at 1 C and after 500 cycles. With a sulfur loading of 3 mg cm-2, the capacity retention rate is approximately 83.7% after 80 cycles.

Full Potential Catalysis of Co0.4Ni1.6P-V/CNT with Phosphorus Vacancies for Li2S1-2 Deposition/Decomposition and S8/Li2Sn (3 ≤ n ≤ 8) Conversion in Li-S Batteries

The slow kinetics of polysulfide conversions hinders the commercial progress of Li-S batteries. The introduction of high-efficiency catalysts accelerates heterogeneous reactions and enhances the utilization of S. The full potential of the Co0.4Ni1.6P-V/CNT-modified separator catalyzes the all-process reactions of the S electrode and increases the rates and cycling lives of the batteries. The two-site synergistic effect of Co0.4Ni1.6P-V/CNT regulates the catalytic activity, and the phosphorus vacancies enrich the active sites. The higher electron density at the Co and Ni double sites increases chemisorption of the Co0.4Ni1.6P-V/CNT on Li2Sn (1 ≤ n ≤ 4), stretches and breaks the Li-S and Ni-S bonds during Li2S decomposition, and reduces the energy barrier for Li2S decomposition. The cyclic voltammograms of the asymmetric batteries demonstrated that Co0.4Ni1.6P-V/CNT also catalyzed the Li2Sn ⇌ S8 (3 ≤ n ≤ 8) reaction, realizing the full catalytic potential of the Li-S batteries. Increased Li+ diffusion/migration in the Co0.4Ni1.6P-V/CNT-modified separator ensured fast electrochemical reactions. The excellent catalytic effect of Co0.4Ni1.6P-V/CNT provided smaller polarization and superior rate performance, which led to high discharge specific capacities of 1511.9, 1172.6, 1006.0, 881.0, and 785.7 mA h g-1 at current densities of 0.1, 0.2, 0.5, 1, and 2 mA cm-2 with sulfur loadings of 7.98 mg cm-2, respectively. This approach involving simple crystal modulation and introduction of defects provides a new way to achieve the full catalytic potential of Li-S batteries.

Single-Atomic Dispersion of Fe and Co Supported on Reduced Graphene Oxide for High-Performance Lithium-Sulfur Batteries

High theoretical energy density and low cost make lithium-sulfur (LSB) batteries a promising system for next-generation energy storage. LSB performance largely depends on efficient reversible conversion of elemental sulfur to Li2S. Here, well-designed sulfur host materials including Fe or Co single atoms embedded on N-doped reduced graphene oxide (MNC/G with M = Fe or Co) are proposed to tackle the LSB challenges and enhance the electrochemical performance. Using a combination of Mössbauer spectroscopy and high-resolution scanning electron microscopy, the atomic dispersion of Co and Fe was revealed up to relatively high mass loadings. After optimization of the electrolyte/sulfur (E/S) ratio, FeNC/G shows the most promising cycle performance combining a constant high discharge capacity at low E/S values with the lowest polarization. In particular, the material FeNC/G@S with a high sulfur loading (9.4 mg cm-2) delivers a high area capacity of 7.7 mAh cm-2 under lean electrolyte conditions (6 mL g-1).

The Dual-Site Adsorption and High Redox Activity Enabled by Hybrid Organic-Inorganic Vanadyl Ethylene Glycolate for High-Rate and Long-Durability Lithium-Sulfur Batteries

Transition metal oxides (TMOs) have attracted considerable attention owing to their strong anchoring ability and natural abundance. However, their single-site adsorption toward sulfur (S) species significantly lowers the possibility of S species reacting with Li⁺ in the electrolyte and increases the reaction barrier. This study investigates molecular modification by coupling the TMO structure with Li⁺ conductive polymer ligands, and vanadyl ethylene glycolate (VEG) is successfully synthesized by introducing organic ligands into the VO_x crystal structure. In addition to the strong interaction between the VO_x and lithium polysulfides via the V-S bond, the groups in the VEG polymer ligands can reversibly couple/decouple with Li⁺ in the electrolyte. Such dual-site adsorption enables a smooth dynamic adsorption-diffusion process. Accordingly, the VEG-based Li-S cells exhibit excellent rate reversibility, cyclic stability, and a long cycle life without the addition of conducting agents. Encouragingly, the VEG-based cells also exhibit close and excellent capacity decays of 0.081%, 0.078%, and 0.095% at 0, 25, and 50 °C (1 C for 200 cycles), respectively. This work provides a novel approach for developing advanced catalysts that can realize Li-S batteries with long-term durability, fast charge-discharge properties, and applications in a wide temperature range.

Origin and Acceleration of Insoluble Li2 S2 -Li2 S Reduction Catalysis in Ferromagnetic Atoms-based Lithium-Sulfur Battery Cathodes

Accelerating insoluble Li2 S2 -Li2 S reduction catalysis to mitigate the shuttle effect has emerged as an innovative paradigm for high-efficient lithium-sulfur battery cathodes, such as single-atom catalysts by offering high-density active sites to realize in situ reaction with solid Li2 S2 . However, the profound origin of diverse single-atom species on solid-solid sulfur reduction catalysis and modulation principles remains ambiguous. Here we disclose the fundamental origin of Li2 S2 -Li2 S reduction catalysis in ferromagnetic elements-based single-atom materials to be from their spin density and magnetic moments. The experimental and theoretical studies disclose that the Fe-N4 -based cathodes exhibit the fastest deposition kinetics of Li2 S (226 mAh g-1 ) and the lowest thermodynamic energy barriers (0.56 eV). We believe that the accelerated Li2 S2 -Li2 S reduction catalysis enabled via spin polarization of ferromagnetic atoms provides practical opportunities towards long-life batteries.

Advances in the Development of Single-Atom Catalysts for High-Energy-Density Lithium-Sulfur Batteries

Although lithium-sulfur (Li-S) batteries are promising next-generation energy-storage systems, their practical applications are limited by the growth of Li dendrites and lithium polysulfide shuttling. These problems can be mitigated through the use of single-atom catalysts (SACs), which exhibit the advantages of maximal atom utilization efficiency (≈100%) and unique catalytic properties, thus effectively enhancing the performance of electrode materials in energy-storage devices. This review systematically summarizes the recent progress in SACs intended for use in Li-metal anodes, S cathodes, and separators, briefly introducing the operating principles of Li-S batteries, the action mechanisms of the corresponding SACs, and the fundamentals of SACs activity, and then comprehensively describes the main strategies for SACs synthesis. Subsequently, the applications of SACs and the principles of SACs operation in reinforced Li-S batteries as well as other metal-S batteries are individually illustrated, and the major challenges of SACs usage in Li-S batteries as well as future development directions are presented.

The Catalyst Design for Lithium-Sulfur Batteries: Roles and Routes

Lithium-sulfur battery is a promising candidate for next-generation high energy density batteries due to its ultrahigh theoretical energy density. However, it suffers from low sulfur utilization, fast capacity decay, and the notorious "shuttle effect" of lithium polysulfides (LiPSs) due to the sluggish reaction kinetics, which severely restrict its practical applications. Using the electrocatalyst can accelerate the redox reactions between sulfur, LiPSs and Li₂ S and suppress the shuttling of LiPSs, and thus, it is a promising strategy to solve the above problems, enabling the battery with high energy density and long cycling stability. In this personal account, we discuss the catalyst design for lithium-sulfur batteries according to the sulfur reduction reaction (SRR) and sulfur evolution reaction (SER) in the discharging and charging processes. The catalytic effects for each step in SRR and SER are highlighted and the homogenous catalysts, the selective catalysts, and the bidirectional catalysts are discussed, which can help guide the rational design of the catalysts and practical applications of lithium-sulfur batteries.

Regulating the Molecular Interactions in Polymer Binder for High-Performance Lithium-Sulfur Batteries

Polymer binders have been shown to efficiently conquer the notorious lithium polysulfide (LiPS) shuttle effects in lithium-sulfur (Li-S) batteries for years, but more study is needed. Herein, a water dispersible and molecular interaction regulated polymer binder (PNAVS) for Li-S batteries was elaborately designed by co-polymerizing N-acryloyl glycinamide and 3-(1-vinyl-3-imidazolio)propanesulfonate. We demonstrate that by modulating the multiple interactions between the functional groups through copolymerization the binder was able to coordinate the LiPSs with higher binding energy for shuttle effect alleviation and cycling performance improvement. In addition, the Li⁺ diffusion coefficient is also optimized in the PNAVS binder, which facilitates acceleration of the redox kinetics during cycling. Consequently, the PNAVS binder renders the Li-S battery with an ultrastable open circuit voltage for more than 3000 h. Even with a high sulfur loading of 11.7 mg cm^-2, the battery can still exhibit excellent areal capacity of 12.21 mA h cm^-2. As proof of concept, a pouch cell was also demonstrated with the stable cycling performance for 110 cycles. The binder engineering strategy in this work will propel the practical applications of high-performance batteries.

Rational construction of a CNTs@VO2 nanosheets modified separator for enhancing the performance of lithium-sulfur batteries

Although lithium-sulfur (Li-S) batteries possess great potential to become the next generation of energy storage technology due to their fivefold higher energy density than commercial lithium-ion batteries, their practical application is still hindered by their poor cycling stability, especially resulting from the disturbing shuttle effect of soluble intermediates. In this study, vanadium dioxide (VO₂) nanosheets were successfully grown onto CNTs to form CNTs@VO₂ through hydrothermal and calcining processes. The hollow structure of the high conductive CNTs offers internal space and mesopores to accommodate the electrolyte combined with the polar metal oxide VO₂ nanosheets providing the chemical anchoring. The hollow binary core-shell host acting as the nanoreactor that serves as the modifier of the separator results in the intensive physical and chemical dual adsorption of lithium polysulfide species (LiPSs), promoting the conversion of long-chain LiPSs to alleviate the shuttle effect significantly and boosting the performance. In addition, the CNTs enhance the electronic conductivity and the electrolyte infiltration of the separator. Notably, the modified separator demonstrates a high initial discharge capacity of 1397 mA h g^-1 at 0.2C and retains a stable cycling ability with a reversible capacity of 965 mA h g^-1 over 200 cycles at 1C. Even for the high sulfur loading of 7.4 mg cm^-2, it can deliver a high areal capacity of 5.4 mA h cm^-2 at 0.5C.

A flame-retardant polyimide interlayer with polysulfide lithium traps and fast redox conversion towards safety and high sulfur utilization Li-S batteries

In recent years and following the progress made in lithium-ion battery technology, substantial efforts have been devoted to developing practical lithium-sulfur (Li-S) batteries for next-generation commercial energy storage devices. The practical application of Li-S batteries is still limited by dramatically reduced capacities, cycling instabilities, and safety issues arising from flammable components. In this study, we designed and fabricated a flame-retardant, multifunctional interlayer which integrated electroconductive networks, lithium polysulfide (LiPS) traps and catalysts to significantly elevate the electrochemical performance and safety of pristine Li-S batteries. The LiPS adsorptive polymer polyimide (PI) constrains polysulfides to the cathode region and effectively suppresses the shuttle effect. Coralloid PI/multiwalled carbon nanotube (MCNT) compounds provide plentiful reaction sites for active materials. The catalytic Ni on the metal skeleton surface notably promotes Li⁺ diffusion, lowers the redox overpotential and accelerates LiPS conversion, which improves the redox kinetics associated with sulfur-related species and significantly elevates sulfur utilization. At different current densities of 0.2 C and 0.5 C, impressive initial discharge capacities of 1275.3 mA h g^-1 and 1190.9 mA h g^-1 are attainable respectively, with high capacity retentions of 80.3% and 78.6% over 600 cycles. Besides, the multifunctional interlayer can also act as a flame-retardant layer to promote the safety of Li-S batteries by inhibiting the spread of fire. This study provides a feasible and prospective strategy that adopts a multifunctional interlayer to develop Li-S batteries with higher capacities, longer cycling lives and safer working conditions.