MXene-Based Co, N-Codoped Porous Carbon Nanosheets Regulating Polysulfides for High-Performance Lithium-Sulfur Batteries

Active MXene-Based Electrode Interface Chemistry for High Performance Li-S Battery: Design Strategies and Prospects

Lithium-sulfur (Li-S) battery with high capacity and energy density is a promising next-generation energy storage device. However, the shuttle effect of polysulfides causes the low utilization of sulfur and the side reactions at the electrode interface. The electrode/electrolyte interface determines the chemical activity of electrode and electrochemical reversibility as well as the cycling stability of battery. Therefore, the ideal electrode interface in Li-S battery depends on the sulfur loading, the fast ion diffusion, the effective utilization of active intermediates, and the uniform deposition of lithium ion on anode. MXene with two dimension layer structure, good conductivity, and abundant terminal groups can serve as the active interface carrier layer to load sulfur, anchor polysulfides, and accelerate ion transfer. This review summarizes three strategies of active MXene-based electrode interfaces including sulfur host interface, functional separator interface, and lithium anode interface based on the electrochemical principles and challenges of Li-S battery. In addition, the interfacial regulation and application of MXene-based materials focus on the electrochemical activity and reversibility of polysulfides in electrochemical process are also presented. Finally, the further prospective and challenges of MXene in Li-S battery are also discussed.

MOF-derived Carbon-Based Materials for Energy-Related Applications

New carbon-based materials (CMs) are recommended as attractively active materials due to their diverse nanostructures and unique electron transport pathways, demonstrating great potential for highly efficient energy storage applications, electrocatalysis, and beyond. Among these newly reported CMs, metal-organic framework (MOF)-derived CMs have achieved impressive development momentum based on their high specific surface areas, tunable porosity, and flexible structural-functional integration. However, obstacles regarding the integrity of porous structures, the complexity of preparation processes, and the precise control of active components hinder the regulation of precise interface engineering in CMs. In this context, this review systematically summarizes the latest advances in tailored types, processing strategies, and energy-related applications of MOF-derived CMs and focuses on the structure-activity relationship of metal-free carbon, metal-doped carbon, and metallide-doped carbon. Particularly, the intrinsic correlation and evolutionary behavior between the synergistic interaction of micro/nanostructures and active species with electrochemical performances are emphasized. Finally, unique insights and perspectives on the latest relevant research are presented, and the future development prospects and challenges of MOF-derived CMs are discussed, providing valuable guidance to boost high-performance electrochemical electrodes for a broader range of application fields.

An MXene-supported NH2CNT and BiOCl composite as a sulfur reservoir for Li-S batteries with high energy density

To solve the intractable challenges of Li-S batteries, we synthesized MXene-NH2CNT-BiOCl-x to be used as a sulfur host. The M-N-B-10%-S electrode exhibited superior electrochemical performance. In situ XRD measurement confirmed that the M-N-B-10%-S electrode displayed good cycle stability.

Structural Engineering of Carbon Host Derived from Organic Pigment toward Physicochemically Confinement and Efficient Conversion of Polysulfide for Lithium-Sulfur Batteries

Lithium-Sulfur Batteries (LSBs) have attracted significant attention as promising next-generation energy storage systems. However, the commercial viability of LSBs have been hindered due to lithium polysulfides (LiPSs) shuttle effect, resulting in poor cycling stability and low sulfur utilization. To address this issue, herein, the study prepares a sulfur host consisting of micro/mesopore-enriched activated carbonaceous materials with ultrahigh surface area using organic pigment via facile one-step activation. By varying the proportion of chemical agent, the pore size and volume of the activated carbonaceous materials are manipulated and their capabilities on the mitigation of LiPSs shuttle effect are investigated. Through the electrochemical measurements and spectroscopic analysis, it is verified that structural engineering of carbon hosts plays a pivotal role in effective physical confinement of LiPSs, leading to the mitigation of LiPSs shuttle effect and sulfur utilization. Additionally, nitrogen and oxygen-containing functional groups originated from PR show electrocatalytic activation sites, facilitating LiPSs conversion kinetics. The approach can reveal that rational design of carbon microstructures can improve trapping and suppression of LiPSs and shuttle effect, enhancing electrochemical performance of LSBs.

Recent advances of MXene@MOF composites for catalytic water splitting and wastewater treatment approaches

MXenes are a group of 2D material which have been derived from the layered transition metal nitrides and carbides and have the characteristics like electrical conductivity, high surface area and variable surface chemical composition. Self-assembly of clusters/metal ions and organic linkers forms metal organic framework (MOF). Their advantages of ultrahigh porosity, highly exposed active sites and many pore architectures have garnered them a lot of attention. But poor conductivity and instability plague several conventional MOF. To address the issue, MOF can be linked with MXenes that have rich surface functional groups and excellent electrical conductivity. In this review, different etching methods for exfoliation of MXene along with the synthesis methods of MXene/MOF composites are reviewed, including hydrothermal method, solvothermal method, in-situ growth method, and self-assembly method. Moreover, application of these MXene/MOF composites for catalytic water splitting and wastewater treatment were also discussed in details. In addition to increasing a single MOF conductivity and stability, MXenes can add a variety of new features, such the template effect. Due to these benefits, MXene/MOF composites can be effectively used in several applications, including photocatalytic/electrocatalytic water splitting, adsorption and degradation of pollutants from wastewater. Finally, the authors explored the current challenges and the future opportunities to improve the efficiency of MXene/MOF composites.

Rationally designing a Ti3C2Tx/CNTs-Co9S8 heterostructure as a sulfur host with multi-functionality for high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have been regarded as potential next-generation batteries owing to their ultrahigh theoretical capacity and abundance of sulfur. However, polysulfide shuttling, poor electronic conductivity, and severe volume expansion limit their commercial prospects. In this work, we rationally constructed a 3D porous Ti₃C₂T_x/CNTs-Co₉S₈ heterostructure derived from a zeolite imidazole framework (ZIF)/Ti₃C₂T_x MXene composite via carbonization and subsequent sulfidation. In this 3D porous Ti₃C₂T_x/CNTs-Co₉S₈ heterostructure, the 3D porous Ti₃C₂T_x MXene structure can provide facilitated ion and electron transport, good structural stability, and polar bonds to anchor sulfur and polysulfides. The formed CNTs can enhance ion diffusion and electron transport. The Co₉S₈ nanoparticles can accelerate the conversion reaction of polysulfides to Li₂S, which can further prevent polysulfide shuttling. The 3D porous structure can buffer the electrode volume change upon cycling. This rationally designed Ti₃C₂T_x/CNTs-Co₉S₈/S cathode exhibits a high initial capacity of 1389.8 mA h g^-1 at 0.1C, good cyclic stability (730.7 mA h g^-1 at 0.2C after 100 cycles), and excellent rate capacities (530.7 mA h g^-1 at 1C). When the S loading was 2.5 mg cm^-2, the Ti₃C₂T_x/CNTs-Co₉S₈/S cathode still exhibited a reversible capacity of 472.8 mA h g^-1 at 0.5C after 300 cycles.

A Ti3C2Tx-Based Composite as Separator Coating for Stable Li-S Batteries

The nitrogen-doped MXene carbon nanosheet-nickel (N-M@CNi) powder was successfully prepared by a combined process of electrostatic attraction and annealing strategy, and then applied as the separator coating in lithium-sulfur batteries. The morphology and structure of the N-M@CNi were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectrum, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption-desorption method. The strong LiPS adsorption ability and high conductivity are associated with the N-doped carbon nanosheet-Ni modified surface. The modified separator offers the cathode of Li-S cell with greater sulfur utilization, better high-rate adaptability, and more stable cycling performance compared with the pristine separator. At 0.2 C the cell with N-M@CNi separator delivers an initial capacity of 1309 mAh g^-1. More importantly, the N-M@CNi separator is able to handle a cathode with 3.18 mg cm^-2 sulfur loading, delivering a capacity decay rate of 0.043% with a high capacity retention of 95.8%. Therefore, this work may provide a feasible approach to separator modification materials towards improved Li-S cells with improved stability.

Emerging MXene@Metal-Organic Framework Hybrids: Design Strategies toward Versatile Applications

Rapid progress on developing smart materials and design of hybrids is motivated by pressing challenges associated with energy crisis and environmental remediation. While emergence of versatile classes of nanomaterials has been fascinating, the real excitement lies in the design of hybrid materials with tunable properties. Metal-organic frameworks (MOFs) are the key materials for gas sorption and electrochemical applications, but their sustainability is challenged by limited chemical stability, poor electrical conductivity, and intricate, inaccessible pores. Despite tremendous efforts towards improving the stability of MOF materials, little progress has made researchers inclined toward developing hybrid materials. MXenes, a family of two-dimensional transition-metal carbides, nitrides and carbonitrides, are known for their compositional versatility and formation of a range of structures with rich surface chemistry. Hybridization of MOFs with functional layered MXene materials may be beneficial if the host structure provides appropriate interactions for stabilizing and improving the desired properties. Recent efforts have focused on integrating Ti₃C₂T_x and V₂CT_x MXenes with MOFs to result in hybrid materials with augmented electrochemical and physicochemical properties, widening the scope for emerging applications. This review discusses the potential design strategies of MXene@MOF hybrids, attributes of tunable properties in the resulting hybrids, and their applications in water treatment, sensing, electrochemical energy storage, smart textiles, and electrocatalysis. Comprehensive discussions on the recent efforts on rapidly evolving MXene@MOF materials for various applications and potential future directions are highlighted.

Amorphous CoP nanoparticle composites with nitrogen-doped hollow carbon nanospheres for synergetic anchoring and catalytic conversion of polysulfides in Li-S batteries

The commercial viability of Li-S batteries was obstructed by short cycle life and poor capability owing to slow redox kinetics and polysulfide shuttle effect. To tackle these challenges, the amorphous CoP anchored on N-doped carbon nanospheres with hollow porous structures (CoP/HCS) has been synthesized as a superior sulfur host via a facial pyrolysis approach. The debilitating effect would be hampered during the cycling processing resulting from two reasons:(1) the powerful chemical anchoring between unsaturated Co and Li-polysulfides, (2) the remarkable adaption of volume variation originating from the hollow porous architectures. The amorphous CoP nanoparticles not only catalyze the transformation of lithium polysulfides as electrocatalyst, but also acquired a high sulfur loading as sulfur host materials. More importantly, the synergistic incorporation of CoP and HCS improved the inherit low conductivity by anchoring on the N-doped carbon hollow, thus leading to excellent performance for Li-S batteries. Benefiting from these advantages, the amorphous CoP/HCS-based sulfur electrodes exhibited outstanding rate performance (685.6 mAh g^-1 at 3C), excellent long-cycling stability with a low capacity decay of only 0.03% per cycle over 1000 cycles at 2C, and a high areal capacity of 5.16 mAh cm^-2 under high sulfur loading.

Performance Study of MXene/Carbon Nanotube Composites for Current Collector- and Binder-Free Mg-S Batteries

The realization of sustainable and cheap Mg-S batteries depends on significant improvements in cycling stability. Building on the immense research on cathode optimization from Li-S batteries, for the first time a beneficial role of MXenes for Mg-S batteries is reported. Through a facile, low-temperature vacuum-filtration technique, several novel current collector- and binder-free cathode films were developed, with either dipenthamethylene thiuram tetrasulfide (PMTT) or S8 nanoparticles as the source of redox-active sulfur. The importance of combining MXene with a high surface area co-host material, such as carbon nanotubes, was demonstrated. A positive effect of MXenes on the average voltage and reduced self-discharge was also discovered. Ascribed to the rich polar surface chemistry of Ti3 C2 Tx MXene, an almost doubling of the discharge capacity (530 vs. 290 mA h g-1 ) was achieved by using MXene as a polysulfide-confining interlayer, obtaining a capacity retention of 83 % after 25 cycles.

Hetero-MXenes: Theory, Synthesis, and Emerging Applications

Since their discovery in 2011, MXenes (abbreviation for transition metal carbides, nitrides, and carbonitrides) have emerged as a rising star in the family of 2D materials owing to their unique properties. Although the primary research interest is still focused on pristine MXenes and their composites, much attention has in recent years been paid also to MXenes with diverse compositions. To this end, this work offers a comprehensive overview of the progress on compositional engineering of MXenes in terms of doping and substituting from theoretical predictions to experimental investigations. Synthesis and properties are briefly introduced for pristine MXenes and then reviewed for hetero-MXenes. Theoretical calculations regarding the doping/substituting at M, X, and T sites in MXenes and the role of vacancies are summarized. After discussing the synthesis of hetero-MXenes with metal/nonmetal (N, S, P) elements by in situ and ex situ strategies, the focus turns to their emerging applications in various fields such as energy storage, electrocatalysts, and sensors. Finally, challenges and prospects of hetero-MXenes are addressed. It is anticipated that this review will be beneficial to bridge the gap between predictions and experiments as well as to guide the future design of hetero-MXenes with high performance.

Incorporation ZnS quantum dots into carbon nanotubes for high-performance lithium-sulfur batteries

Constructing sulfur hosts with high electronic conductivity, large void space, strong chemisorption, and rapid redox kinetics is critically important for their practical applications in lithium-sulfur batteries (LSBs). Herein, by coupling ZnS quantum dots (QDs) with carbon nanotubes (CNTs), one multifunctional sulfur host CNT/ZnS-QDs is designed via a facile one-step hydrothermal method. SEM and TEM analyses reveal that small ZnS-QDs (<5 nm) are uniformly anchored on the CNT surface as well as encapsulated into CNT channels. This special architecture ensures sulfur direct contacting with highly conductive CNTs; meanwhile, the catalytic effect of anchored ZnS-QDs improves the chemisorption and confinement to polysulfides. Benefiting from these merits, when used as sulfur hosts, this special architecture manifests a high specific capacity, superior rate capability, and long-term cycling stability. The ZnS-QDs dependent electrochemical performance is also evaluated by adjusting the mass ratio of ZnS-QDs, and the host of CNT/ZnS-QDs 27% owns the optimal cell performance. The specific capacity decreases from 1051 mAh g^-1 at 0.2 C to 544 mAh g^-1 at 2.0 C, showing rate capability much higher than CNT/S and other CNT/ZnS-QDs/S samples. After 150 cycles, the cyclic capacity at 0.5 C exhibits a slow reduction from 1051 mAh g^-1 to 771 mAh g^-1, showing a high retention of 73.4% with a coulombic efficiency of over 99%. The electrochemical impedance spectroscopy analyses demonstrate that this special architecture juggles high conductivity and excellent confinement of polysulfides, which can significantly suppress the notorious shuttle effect and accelerate the redox kinetics. The strategy in this study provides a feasible approach to design efficient sulfur hosts for realizing practically usable LSBs.

CO2-Oxidized Ti3C2Tx-MXenes Components for Lithium-Sulfur Batteries: Suppressing the Shuttle Phenomenon through Physical and Chemical Adsorption

Lithium-sulfur (Li-S) batteries are one of the main challenges facing Li-ion technology because the insulating nature of sulfur and the shuttle phenomenon of dissolved lithium polysulfides (LPSs) in liquid electrolytes result in critical problems, including low Coulombic efficiency, loss of active material, and rapid capacity decay. Here, we oxidized delaminated transition metal carbides (MXenes) using CO₂ (Oxi-d-MXenes) and used them as both cathode electrode with sulfur and modified separator coated onto the glass fiber without a conductive material and binder to suppress the diffusion of LPSs. Oxi-d-MXenes annealed at 900 °C using CO₂ gas formed perfectly converted rutile-TiO₂ nanocrystalline particles on their two-dimensional sheets. Li-S batteries fabricated with the Oxi-d-MXenes cathode and the Oxi-d-MXenes-modified separator exhibited high Coulombic efficiency (nearly 99%) and retained a capacity of about 900 mAh g^-1 after 300 cycles at a current density of 1C. These results were attributed to the chemical and physical adsorption between the Oxi-d-MXenes and the LPSs. Our results imply that Oxi-d-MXenes prepared by the CO₂ treatment exhibit physical and electrochemical properties that enhance the performance of Li-S batteries.

Recent advances in MXenes and their composites in lithium/sodium batteries from the viewpoints of components and interlayer engineering

An up-to-date review about MXenes based on their distinguishing properties, namely, large interlayer spacing and rich surface chemistry.