Synergistically Accelerating Adsorption-Electrocataysis of Sulfur Species via Interfacial Built-In Electric Field of SnS2 -MXene Mott-Schottky Heterojunction in Li-S Batteries

Tuning interfacial charge transport via Ti-O-Sn bonds for efficient CO2 conversion

Here, the clever design of forming an ohmic contact between SnS2 and MXene can regulate interfacial electron transport through Ti-O-Sn chemical bonds. This fast directional charge transport kinetics is attributed to the built-in electric field formed by the ohmic contact. As expected, the photoreduction CO2 activity of the optimized SSTC-5 catalyst is 10.4 times that of the original SnS2.

Accelerated Ion-Electron Transport in Bi-Heterostructures Constructed Based on Ohmic Contacts for Efficient Bi-Directional Catalysis of Lithium-Sulfur Batteries

Although lithium-sulfur batteries have satisfactory theoretical specific capacity and energy density, they are difficult to further commercialize due to the shuttle effect of soluble polysulfides and slow sulfur oxidation kinetics. Based on this, in this work, the catalyst MXene-VS4-SnS2 (MVS), a dual heterostructured catalyst with ohmic contacts, is prepared by a one-step hydrothermal method and electrostatic self-adsorption for lithium-sulfur battery cathode materials. Experimental and theoretical results show that the ohmic contact induces spontaneous charge rearrangement, resulting in the formation of a fast charge transfer pathway at the MVS heterojunction interface, which helps to reduce the energy barrier for polysulfide reduction and Li2S oxidation during the discharge/charge process. In addition, the inherent sulfophilicity of VS4 and SnS2 promotes the conversion of S species, while the pleated MXene nanosheets not only provide a highly conductive network for the active sulfur but also retain a rich internal space to maintain the integrity of the cathode structure during the continuous cycling process. As a result, the MVS cathode exhibits excellent electrochemical performance even under high sulfur loading. The integration of excellent performance with a facile synthesis process provides a promising approach for designing highly efficient electrocatalysts suitable for the energy field.

Bi-Functional Materials for Sulfur Cathode and Lithium Metal Anode of Lithium-Sulfur Batteries: Status and Challenges

Over the past decade, the most fundamental challenges faced by the development of lithium-sulfur batteries (LSBs) and their effective solutions have been extensively studied. To further transfer LSBs from the research phase into the industrial phase, strategies to improve the performance of LSBs under practical conditions are comprehensively investigated. These strategies can simultaneously optimize the sulfur cathode and Li-metal anode to account for their interactions under practical conditions, without involving complex preparation or costly processes. Therefore, "two-in-one" strategies, which meet the above requirements because they can simultaneously improve the performance of both electrodes, are widely investigated. However, their development faces several challenges, such as confused design ideas for bi-functional sites and simplex evaluation methods (i. e. evaluating strategies based on their bi-functionality only). To date, as few reviews have focused on these challenges, the modification direction of these strategies is indistinct, hindering further developments in the field. In this review, the advances achieved in "two-in-one" strategies and categorizing them based on their design ideas are summarized. These strategies are then comprehensively evaluated in terms of bi-functionality, large-scale preparation, impact on energy density, and economy. Finally, the challenges still faced by these strategies and some research prospects are discussed.

In Situ Built ZnS/MXene Heterostructure by a Mild Method for Inhibiting Polysulfide Shuttle in Li-S Batteries

With high specific surface area, excellent polysulfide conversion activity, and fast electron/ion transfer at the interface, MXene-derived heterostructures can be employed as catalysts for lithium-sulfur (Li-S) batteries to accelerate sulfur redox kinetics and suppress shuttle effect. However, the preparation of MXene-derived heterostructures often requires high-temperature reactions, which can easily lead to the oxidation of MXene and sacrifice the electrical conductivity. Herein, a catalytic two-dimensional heterostructure (ZnS/MXene) was successfully synthesized via a mild method. The MXene skeleton retains the original nanosheet structure without oxidation. The in situ-grown ZnS nanospheres prevent the restacking of MXene nanosheets, which not only increases the active sites, but also guarantees channels for the fast passage of lithium ions. The interfacial built-in electric field further promotes electron/ion migration, thereby expediting the polysulfide conversion and suppressing the shuttle effect. Consequently, the batteries using ZnS/MXene modified separators exhibit a high initial discharge capacity of 1230 mAh g-1 at 0.1 C and a low decaying rate of 0.082 % per cycle after 500 cycles at 0.5 C. This work offers a reference for the fabrication of MXene-based heterostructure in Li-S batteries.

The Impact of Oxygen Content in O-Doped MoS2 on the Kinetics of Polysulfide Conversion in Li-S Batteries

Polysulfide shuttle and sluggish sulfur redox kinetics remain key challenges in lithium-sulfur batteries. Previous researches have shown that introducing oxygen into transition metal sulfides helps to capture polysulfides and enhance their conversion kinetics. Based on this, further investigations are conducted to explore the impact of oxygen doping levels on the physical-chemical properties and electrocatalytic performance of MoS2. The findings reveal that MoS2 doped with high-content oxygen exhibits enhanced conductivity and polysulfides conversion kinetics compared to MoS2 with low-content oxygen doping, which can be attributed to the alteration of crystal structure from 2H-phase to the 1T-phase, the introduction of increased Li-O interactions, and the effect of defects resulting from high-oxygen doping. Consequently, the lithium-sulfur batteries using high-oxygen doped MoS2 as a catalyst deliver a high discharge capacity of 1015 mAh g-1 at 0.25C and maintain 78.5% capacity after 300 more cycles. Specifically, lithium-sulfur batteries employing paper-based electrodedemonstrate an areal capacity of 3.91 mAh cm-2 at 0.15C, even with sulfur loading of 4.1 mg cm-2 and electrolyte of 6.7 µL mg-1. These results indicate that oxygen doping levels can modify the properties of MoS2, and high-oxygen doped MoS2 shows promise as an efficient catalyst for lithium-sulfur batteries.

Effect of Size Tuning of Hexagonal SnS2 Nanosheet on the Efficiency of Photocatalytic Degradation Processes

Recent research on SnS2 materials aims to enhance their photocatalytic efficiency for water pollution remediation through doping and constructing heterojunctions. These methods face challenges in cost-effectiveness and practical scalability. This study synthesizes hexagonal SnS2 nanosheets of various sizes via a hydrothermal method, assessing their performance in degrading methyl orange (MO) and reducing hexavalent chromium (Cr(VI)). The results show that smaller SnS2 nanosheets exhibit higher photocatalytic efficiency under sunlight. Specifically, 50 mg of small-sized nanosheets degraded 100 ml of MO (10 mgL-1) in 30 min and reduced Cr(VI) (10 mgL-1) in 105 min. The enhanced performance is attributed to: i) an energy bandgap of 2.17 eV suitable for visible light, and ii) more surface sulfur (S) vacancies in smaller nanosheets, which create electronic states near the Fermi level, reducing electron-hole recombination. This study offers a straightforward strategy for improving 2D materials like SnS2.

Construction of an Ohmic Contact Cathode by Two Metal Sulfides for efficient Capture and Catalysis of Polysulfide

The slow reaction kinetics and severe shuttle effect of lithium polysulfide make Li-S battery electrochemical performance difficult to meet the demands of large electronic devices such as electric vehicles. Based on this, an electrocatalyst constructed by metal phase material (MoS2) and semiconductor phase material (SnS2) with ohmic contact is designed for inhibiting the dissolution of lithium polysulfide with improving the reaction kinetics. According to the density-functional theory calculations, it is found that the heterostructured samples with ohmic contacts can effectively reduce the reaction-free energy of lithium polysulfide to accelerate the sulfur redox reaction, in addition to the excellent electron conduction to reduce the overall activation energy. The metallic sulfide can add more sulfophilic sites to promote the capture of polysulfide. Thanks to the ohmic contact design, the carbon nanotube-MoS2-SnS2 achieved a specific capacity of 1437.2 mAh g-1 at 0.1 C current density and 805.5 mAh g-1 after 500 cycles at 1 C current density and is also tested as a pouch cell, which proves to be valuable for practical applications. This work provides a new idea for designing an advanced and efficient polysulfide catalyst based on ohmic contact.

Enhancing Lithium-Sulfur Battery Performance with MXene: Specialized Structures and Innovative Designs

Established in 1962, lithium-sulfur (Li-S) batteries boast a longer history than commonly utilized lithium-ion batteries counterparts such as LiCoO2 (LCO) and LiFePO4 (LFP) series, yet they have been slow to achieve commercialization. This delay, significantly impacting loading capacity and cycle life, stems from the long-criticized low conductivity of the cathode and its byproducts, alongside challenges related to the shuttle effect, and volume expansion. Strategies to improve the electrochemical performance of Li-S batteries involve improving the conductivity of the sulfur cathode, employing an adamantane framework as the sulfur host, and incorporating catalysts to promote the transformation of lithium polysulfides (LiPSs). 2D MXene and its derived materials can achieve almost all of the above functions due to their numerous active sites, external groups, and ease of synthesis and modification. This review comprehensively summarizes the functionalization advantages of MXene-based materials in Li-S batteries, including high-speed ionic conduction, structural diversity, shuttle effect inhibition, dendrite suppression, and catalytic activity from fundamental principles to practical applications. The classification of usage methods is also discussed. Finally, leveraging the research progress of MXene, the potential and prospects for its novel application in the Li-S field are proposed.

In Situ Exsolution-Prepared Solid-Solution-Type Sulfides with Intracrystal Polarization for Efficient and Selective Absorption of Low-Frequency Electromagnetic Wave

The excellent dielectric properties and tunable structural design of metal sulfides have attracted considerable interest in realizing electromagnetic wave (EMW) absorption. However, compared with traditional monometallic and bimetallic sulfides that are extensively studied, the unique physical characteristics of solid-solution-type sulfides in response to EMW have not been revealed yet. Herein, a unique method for preparing high-purity solid-solution-type sulfides is proposed based on solid-phase in situ exsolution of different metal ions from hybrid precursors. Utilizing CoAl-LDH/MIL-88A composite as a precursor, Fe0.8Co0.2S single-phase nanoparticles are uniformly in situ formed on an amorphous substrate (denoted as CoAl), forming CoAl/Fe0.8Co0.2S heterostructure. Combing with density functional theory (DFT) calculations and wave absorption simulations, it is revealed that Fe0.8Co0.2S solid solution has stronger intracrystal polarization and electronic conductivity than traditional monometallic and bimetallic sulfides, which lead to higher dielectric properties in EM field. Therefore, CoAl/Fe0.8Co0.2S heterostructure exhibits significantly enhanced EMW absorption ability in the low-frequency region (2-6 GHz) and can achieve frequency screening by selectively absorbing EMW of specific frequency. This work not only provides a unique method for preparing high-purity solid-solution-type sulfides but also fundamentally reveals the physical essence of their excellent EMW absorption performance.

Constructing Built-In Electric Fields with Semiconductor Junctions and Schottky Junctions Based on Mo-MXene/Mo-Metal Sulfides for Electromagnetic Response

The exploration of novel multivariate heterostructures has emerged as a pivotal strategy for developing high-performance electromagnetic wave (EMW) absorption materials. However, the loss mechanism in traditional heterostructures is relatively simple, guided by empirical observations, and is not monotonous. In this work, we presented a novel semiconductor-semiconductor-metal heterostructure system, Mo-MXene/Mo-metal sulfides (metal = Sn, Fe, Mn, Co, Ni, Zn, and Cu), including semiconductor junctions and Mott-Schottky junctions. By skillfully combining these distinct functional components (Mo-MXene, MoS2, metal sulfides), we can engineer a multiple heterogeneous interface with superior absorption capabilities, broad effective absorption bandwidths, and ultrathin matching thickness. The successful establishment of semiconductor-semiconductor-metal heterostructures gives rise to a built-in electric field that intensifies electron transfer, as confirmed by density functional theory, which collaborates with multiple dielectric polarization mechanisms to substantially amplify EMW absorption. We detailed a successful synthesis of a series of Mo-MXene/Mo-metal sulfides featuring both semiconductor-semiconductor and semiconductor-metal interfaces. The achievements were most pronounced in Mo-MXene/Mo-Sn sulfide, which achieved remarkable reflection loss values of - 70.6 dB at a matching thickness of only 1.885 mm. Radar cross-section calculations indicate that these MXene/Mo-metal sulfides have tremendous potential in practical military stealth technology. This work marks a departure from conventional component design limitations and presents a novel pathway for the creation of advanced MXene-based composites with potent EMW absorption capabilities.

Triple Effect of "Conductivity-Adsorption-Catalysis" Enables MXene@FeCoNiP to be Sulfur Hosts for Lithium-Sulfur Batteries

The weak chemical immobilization ability and poor catalytic effect of MXene inhibit its application in lithium-sulfur (Li-S) batteries. Herein, a novel MXene@FeCoNiP composite is rationally developed and utilized as a sulfur host for Li-S batteries. In this well-designed MXene-based nanostructure, the introduction of FeCoNiP in the interlayer of MXene nanosheets can not only effectively inhibit the restacking of MXene nanosheets but also act as an accelerator for the adsorption and catalysis of polysulfides to restrain the shuttling effect and facilitate the transformation of polysulfides. The existence of two-dimensional MXene nanosheets provides more active sites and improves the conductivity, which is beneficial for accelerating the reaction kinetics. Thus, the as-prepared MXene@FeCoNiP composites achieve an outstanding performance for Li-S batteries. This work provides an opportunity to construct an ideal sulfur host with the triple effect of "conductivity-adsorption-catalysis".

SnO2/SnS heterojunction anchoring on CMK-3 mesoporous network improves the reversibility of conversion reaction for lithium/sodium ions storage

Tin oxide-based (SnO2) materials show high theoretical capacity for lithium and sodium storage benefiting from a double-reaction mechanism of conversion and alloying reactions. However, due to the limitation of the reaction thermodynamics and kinetics, the conversion reaction process of SnO2usually shows irreversibility, resulting in serious capacity decay and hindering the further application of the SnO2anode. Herein, SnO2/SnS heterojunction was anchored on the surface and inside of CMK-3 byinsitusynthesis method, forming a stable 3D structural material (SnO2/SnS@CMK-3). The electrochemical properties of SnO2/SnS@CMK-3 composite show high capacity and reversible conversion reaction, which was attributed to the synergistic effect of CMK-3 and SnO2/SnS heterojunction. To further investigate the influence of the heterojunction on the reversibility of the conversion reaction, the Gibbs free energy (ΔG) was calculated using density functional theory. The results show that SnO2/SnS heterojunction has a closer to zero ΔGfor lithium/sodium ion batteries compared to SnO2, indicating that the heterojunction enhances the reversibility of the conversion reaction in chemical reaction thermodynamics. Our work provides insights into the reversibility of the conversion reaction of SnO2-based materials, which is essential for improving their electrochemical performance.

Palygorskite-Derived Ternary Fluoride with 2D Ion Transport Channels for Ampere Hour-Scale Li-S Pouch Cell with High Energy Density

Although various excellent electrocatalysts/adsorbents have made notable progress as sulfur cathode hosts on the lithium-sulfur (Li-S) coin-cell level, high energy density (WG ) of the practical Li-S pouch cells is still limited by inefficient Li-ion transport in the thick sulfur cathode under low electrolyte/sulfur (E/S) and negative/positive (N/P) ratios, which aggravates the shuttle effect and sluggish redox kinetics. Here a new ternary fluoride MgAlF5 ·2H2 O with ultrafast ion conduction-strong polysulfides capture integration is developed. MgAlF5 ·2H2 O has an inverse Weberite-type crystal framework, in which the corner-sharing [AlF6 ]-[MgF4 (H2 O)2 ] octahedra units extend to form two-dimensional Li-ion transport channels along the [100] and [010] directions, respectively. Applied as the cathode sulfur host, the MgAlF5 ·2H2 O lithiated by LiTFSI (lithium salt in Li-S electrolyte) acts as a fast ionic conductor to ensure efficient Li-ion transport to accelerate the redox kinetics under high S loadings and low E/S and N/P. Meanwhile, the strong polar MgAlF5 ·2H2 O captures polysulfides by chemisorption to suppress the shuttle effect. Therefore, a 1.97 A h-level Li-S pouch cell achieves a high WG of 386 Wh kg-1 . This work develops a new-type ionic conductor, and provides unique insights and new hosts for designing practical Li-S pouch cells.

Efficient Catalysis of Ultrathin Two-Dimensional Fe2 O3 -CoP Heterostructure Nanosheets for Polysulfide Redox Reactions

The "shuttle effect" and slow redox reactions of Li-S batteries limit their practical application. To solve these problems, a judicious catalyst design for improved battery cycle life and rate performance is essential. Herein, this issue is addressed by modifying the Li-S battery separator using a 2D Fe2 O3 -CoP heterostructure that combines the dual functions of polar Fe2 O3 and high-conductivity CoP. The synthesized ultrathin nanostructure exposes well-dispersed active sites and shortens the ion diffusion paths. Theoretical calculations, electrochemical tests, and in situ Raman spectroscopy measurements reveal that the heterostructure facilitates the inhibition of polysulfide shuttling and enhances the electrode kinetics. A sulfur cathode constructed using the Fe2 O3 -CoP-based separator provides an astonishing capacity of 1346 mAh g-1 at 0.2 C and a high capacity retention of ≈84.5%. Even at a high sulfur loading of 5.42 mg cm-2 , it shows an area capacity of 5.90 mAh cm-2 . This study provides useful insights into the design of new catalytic materials for Li-S batteries.

Co/CoS2 Heterojunction Embedded in N, S-Doped Hollow Nanocage for Enhanced Polysulfides Conversion in High-Performance Lithium-Sulfur Batteries

Modulating the electronic configuration of the substrate to achieve the optimal chemisorption toward polysulfides (LiPSs) for boosting polysulfide conversion is a promising way to the efficient Li-S batteries but filled with challenges. Herein, a Co/CoS2 heterostructure is elaborately built to tuning d-orbital electronic structure of CoS2 for a high-performance electrocatalyst. Theoretical simulations first evidence that Co metal as the electron donator can form a built-in electric field with CoS2 and downshift the d-band center, leading to the well-optimized adsorption strength for lithium polysulfides on CoS2 , thus contributing a favorable way for expediting the redox reaction kinetics of LiPSs. As verification of prediction, a Co/CoS2 heterostructure implanted in porous hollow N, S co-doped carbon nanocage (Co/CoS2 @NSC) is designed to realize the electronic configuration regulation and promote the electrochemical performance. Consequently, the batteries assembled with Co/CoS2 @NSC cathode display an outstanding specific capacity and an admirable cycling property as well as a salient property of 8.25 mAh cm-2 under 8.18 mg cm-2 . The DFT calculation also reveals the synergistic effect of N, S co-doping for enhancing polysulfide adsorption as well as the detriment of excessive sulfur doping.

High-Energy Aqueous/Organic Hybrid Batteries Enabled by Cu2+ Redox Charge Carriers

Lithium||sulfur (Li||S) batteries are considered as one of the promising next-generation batteries due to the high theoretical capacity and low cost of S cathodes, as well as the low redox potential of Li metal anodes (-3.04 V vs. standard hydrogen electrode). However, the S reduction reaction from S to Li2 S leads to limited discharge voltage and capacity, largely hindering the energy density of Li||S batteries. Herein, high-energy Li||S hybrid batteries were designed via an electrolyte decoupling strategy. In cathodes, S electrodes undergo the solid-solid conversion reaction from S to Cu2 S with four-electron transfer in a Cu2+ -based aqueous electrolyte. Such an energy storage mechanism contributes to enhanced electrochemical performance of S electrodes, including high discharge potential and capacity, superior rate performance and stable cycling behavior. As a result, the assembled Li||S hybrid batteries exhibit a high discharge voltage of 3.4 V and satisfactory capacity of 2.3 Ah g-1 , contributing to incredible energy density. This work provides an opportunity for the construction of high-energy Li||S batteries.

Interfacial Engineering of Ru Nanocluster-Modified TiO2 Nanotube-Assisted Regulation of Lithium Polysulfide Reactions

The inhibition of lithium polysulfide (LiPS) diffusion and the acceleration of reaction kinetics are two major challenges for the practical application of lithium-sulfur (Li-S) batteries. Herein, through an interface engineering strategy, a multifunctional sulfur host based on Ru nanocluster-modified TiO2 nanotubes (TiO2-Ru) was designed. The TiO2-Ru interface field effect, combined with the hollow nanotube structure and the strong chemical action of TiO2, enhanced the LiPS trapping ability and inhibited the "shuttle effect". Furthermore, the high catalytic activity of Ru nanoclusters reduced the energy barrier of multistep LiPS reactions, thus speeding up the electrode kinetics. As a result, the TiO2-Ru-based composite sulfur cathode delivered excellent electrochemical performance, including an extremely low capacity loss of ∼0.015% per cycle and an increased areal capacity of ∼6.1 mAh cm-2 at 4.8 mg cm-2. This work contributes to a better sulfur cathode design from insights into morphology and phase interface engineering.

Local Built-In Field at the Sub-nanometric Heterointerface Mediates Cascade Electrochemical Conversion of Lithium-sulfur Batteries

Heterogeneous catalytic mediators have been proposed to play a vital role in enhancing the multiorder reaction and nucleation kinetics in multielectron sulfur electrochemistry. However, the predictive design of heterogeneous catalysts is still challenging, owing to the lack of in-depth understanding of interfacial electronic states and electron transfer on cascade reaction in Li-S batteries. Here, a heterogeneous catalytic mediator based on monodispersed titanium carbide sub-nanoclusters embedded in titanium dioxide nanobelts is reported. The tunable catalytic and anchoring effects of the resulting catalyst are achieved by the redistribution of localized electrons caused by the abundant built-in fields in heterointerfaces. Subsequently, the resulting sulfur cathodes deliver an areal capacity of 5.6 mAh cm-2 and excellent stability at 1 C under sulfur loading of 8.0 mg cm-2 . The catalytic mechanism especially on enhancing the multiorder reaction kinetic of polysulfides is further demonstrated via operando time-resolved Raman spectroscopy during the reduction process in conjunction with theoretical analysis.