MXenes for Rechargeable Batteries Beyond the Lithium-Ion

Recent insights on MXene-based architectures for monitoring and sensing of gaseous pollutants: A review

Epidemiology and public health concerns have primarily relied on the accurate control of gas pollutants, requiring highly efficient gas sensor devices for detecting hazardous gases. Despite the dedication of many efforts in this era, the precise, continuous scrutiny of gases remains elusive for appropriate gas selectivity, prompt response and recovery time, proper repeatability, as well as low cost. Accordingly, nanostructured architectural sensing cues have received enormous attention toward versatile detection and sensing procedures. As a representational nanostructure, the MXene family has been widely introduced to tailor and augment sensor patterns by providing large surface area, tunable surface chemistry, superior electrical conductivity, chemical stability, compatibility with flexible substrates, and potential for multifunctionality. Additionally, they could be synthesized in various formations of film and layered designs, fibrous membranes, and gel-like structures, creating synergetic effects that can provide superior gas-sensing performance. Herein, the synthesis and benefits of MXene nanosheets as gas-sensitive materials, in tandem with the past-to-present progress of MXene-based gas sensors in the formation of films, fibrous, and gel-like configurations, are comprehensively reviewed. As an in-depth reference, the present overview could shed light on further advancing gas sensor architectures developed based on MXene structures.

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.

Rechargeable Seawater-Based Chloride-Ion Batteries Enabled by Covalent Surface Chemistry in MXenes

Rechargeable aqueous chloride-ion batteries (ACIBs) using Cl- ions as charge carriers represent a promising energy-storage technology, especially when natural seawater is introduced as the electrolyte, which can bring remarkable advantages in terms of cost-effectiveness, safety, and environmental sustainability. However, the implementation of this technology is hindered by the scarcity of electrodes capable of reversible chloride-anion storage. Here, we show that a Ti3C2Clx MXene with Cl surface terminations enables reversible Cl- ion storage in aqueous electrolytes. Further, we developed seawater-based ACIBs that show a high specific capacity and an exceptionally long lifespan (40000 cycles, more than 1 year) in natural seawater electrolyte. The pouch-type cells achieve a high energy density (50 Wh Lcell-1) and maintain stable performance across a broad temperature range (-20 to 50 °C). Our investigations reveal that the covalent interaction between Cl- ions and Cl-terminated MXene facilitates Cl- ion intercalation into the MXene interlayer, promoting rapid ion migration with a low energy barrier (0.10 eV). Moreover, this MXene variant also enables the reversible storage of Br- ions in an aqueous electrolyte with a long cycle life. This study may advance the design of anion storage electrodes and enable the development of sustainable aqueous batteries.

Self-Thermal Management in Filtered Selenium-Terminated MXene Films for Flexible Safe Batteries

Li-ion batteries with superior interior thermal management are crucial to prevent thermal runaway and ensure safe, long-lasting operation at high temperatures or during rapid discharging and charging. Typically, such thermal management is achieved by focusing on the separator and electrolyte. Here, the study introduces a Se-terminated MXene free-standing electrode with exceptional electrical conductivity and low infrared emissivity, synergistically combining high-rate capacity with reduced heat radiation for safe, large, and fast Li+ storage. This is achieved through a one-step organic Lewis acid-assisted gas-phase reaction and vacuum filtration. The Se-terminated Nb2Se2C outperformed conventional disordered O/OH/F-terminated materials, enhancing Li+-storage capacity by ≈1.5 times in the fifth cycle (221 mAh·g-1 at 1 A·g-1) and improving mid-infrared adsorption with low thermal radiation. These benefits result from its superior electrical conductivity, excellent structural stability, and high permittivity in the infrared region. Calculations further reveal that increased permittivity and conductivity along the z-direction can reduce heat radiation from electrodes. This work highlights the potential of surface groups-terminated layered material-based free-standing flexible electrodes with self-thermal management ability for safe, fast energy storage.

Molten Salt Derived MXenes: Synthesis and Applications

About one decade after the first report on MXenes, these 2D early transition metal carbides or nitrides have become among the best-performing materials in electrode applications related to electrical energy storage devices and power-to-fuels conversion. MXenes are obtained by a top-down approach starting from the appropriate 3D MAX phase that undergoes etching of the A-site metal. Initial etching procedures are based on the use of concentrated HF or the in situ generation of this highly corrosive and poisonous reagent. Etching of the MAX phase is one of the major hurdles limiting the progress of the field. The present review summarizes an alternative, universal, and easily scalable etching procedure based on treating the MAX precursor with a Lewis acid molten salt. The review starts with presenting the current state of the art of the molten salt etching procedure to obtain or modify MXene, followed by a summary of the applications of these MXene samples. The aim of the review is to show the versatility and advantages of molten salt etching in terms of general applicability, control of the surface terminal groups, and uniform deposition of metal nanoparticles, among other features of the procedure.

Recent Advances in Non-Ti MXenes: Synthesis, Properties, and Novel Applications

One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium-based MXenes, with more than 70% of publication-related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M-based MXenes (M-MXenes), where M stands for non-Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non-Ti MXene outperform standard Ti-MXene in several applications. There is many advancement in top-down as well as bottom-up production of MXenes family members, which allows for exact control of the M-characteristics MXene NMs to contain cutting-edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M-MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group-(III-VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non-Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.

Effects of interlayer space engineering and surface modification on the charge storage mechanisms of MXene nanomaterials: A review on recent developments

Two-dimensional MXenes were discovered in 2011 and, because of their outstanding properties, have attracted significant attention as electrode materials for supercapacitors, rechargeable batteries, and hybrid energy storage devices. Numerous studies were dedicated to identifying feasible charge storage mechanisms in MXenes and investigating the effects of structural and superficial properties on the corresponding mechanisms. The results clarify that interlayer distance and surface termination groups in MXenes significantly determine the deliverable energy and power density in respective energy storage devices. Additionally, due to van der Waals interactions, adjacent MXene sheets tend to aggregate and restack during electrode preparation or charge and discharge cycling, reducing the MXene interlayer distance and deteriorating its energy storage ability. In this review, we first summarize the different charge storage mechanisms applicable to MXenes in different energy storage devices and describe the effect of interlayer spacing and surface termination groups. Then, different interlayer space engineering methods are reviewed in terms of materials and procedures, and their impact on the electrochemical behavior and restacking tendency of MXene is described.

MXene materials in electrochemical energy storage systems

MXenes, due to their unique geometric structure, rich elemental composition, and intrinsic physicochemical properties, have multi-functional applications. In the field of electrochemical energy storage, MXenes can be used as active components, conductive agents, supports, and catalysts in ion-intercalated batteries, metal-sulfur batteries, and supercapacitors. The electrochemical performance of MXene materials is closely related to their distinctive physical and chemical properties, which depend on their geometry, surface functional groups, and elemental composition. How to regulate MXene materials to optimize electrochemical functions is a key scientific challenge. Herein, we correlated the function of MXene materials with their interlayer structure, surface functional groups, and specific catalytic sites, analyzed the electrochemical function of MXene materials, and showed how to design the electrochemical function of MXene materials based on ion/electron transport. Additionally, this feature article provides an outlook on the opportunities and challenges for MXenes, offering theoretical and technical guidance on using MXene materials in energy storage systems.

Synergistic integration of MXene nanostructures into electrospun fibers for advanced biomedical engineering applications

MXene-based architectures have paved the way in various fields, particularly in healthcare area, owing to their remarkable physiochemical and electromagnetic characteristics. Moreover, the modification of MXene structures and their combination with polymeric networks have gained considerable prominence to further develop their features. The combination of electrospun fibers with MXenes would be promising in this regard since electrospinning is a well-established technique that is now being directed toward commercial biomedical applications. The introduction of MXenes into electrospun fibrous frameworks has highlighted outcomes in various biomedical applications, including cancer therapy, controlled drug delivery, antimicrobial targets, sensors, and tissue engineering. Correspondingly, this review describes the employed strategies for the preparation of electrospun configurations in tandem with MXene nanostructures with remarkable characteristics. Next, the advantages of MXene-decorated electrospun fibers for use in biomedical applications are comprehensively discussed. According to the investigations, rich surface functional groups, hydrophilicity, large surface area, photothermal features, and antimicrobial and antibacterial activities of MXenes could synergize the performance of electrospun layers to engineer versatile biomedical targets. Moreover, the future of this path is clarified to combat the challenges related to the electrospun fibers decorated with MXene nanosheets.

Tuning the Surface Stability and Li/Na Storage of MXenes by Controlling the Surface Termination Coverage

2D transition metal carbides and/or nitrides, MXenes, are a class of widely studied materials with great potential for energy storage applications. The control of surface chemistry is an effective approach for preparing novel MXenes and modifying their electrochemical properties. However, an in-depth and systematic atomic-scale study of the effect of surface termination on MXene stability and electrochemical performance is scarce and thus is highly desired. Here, through high-throughput first-principles calculations, 28 stable chalcogen-functionalized M2CTz (M = V, Nb, and Ta, T = S, Se, and Te) under different chemical environments are identified. The reduction of termination coverage improves electrical conductivity but weakens in-plane stiffness. Intriguingly, based on charge transfer mechanism, the diffusion barrier of lithium/sodium atoms on the M2CTz exhibits a volcano-like relationship with termination coverage, and the ion diffusion channel formed in half termination coverage greatly accelerates lithium ion diffusion and returns to or exceeds sodium ion diffusion rate at full termination coverage. V2CSe2/Nb2CSz not only displays the large lithium/sodium capacity (592/409-466 mAhg-1) but also exhibits low barrier energy and open-circuit voltage, suggesting a promising candidate anode material for lithium/sodium-ion batteries. These findings provide insights into the design and fabrication of MXenes and tuning the electrochemical performance of MXenes by controlling termination coverage.

Nitrogen-Doped Graphene-Like Carbon Intercalated MXene Heterostructure Electrodes for Enhanced Sodium- and Lithium-Ion Storage

MXene is investigated as an electrode material for different energy storage systems due to layered structures and metal-like electrical conductivity. Experimental results show MXenes possess excellent cycling performance as anode materials, especially at large current densities. However, the reversible capacity is relatively low, which is a significant barrier to meeting the demands of industrial applications. This work synthesizes N-doped graphene-like carbon (NGC) intercalated Ti3C2Tx (NGC-Ti3C2Tx) van der Waals heterostructure by an in situ method. The as-prepared NGC-Ti3C2Tx van der Waals heterostructure is employed as sodium-ion and lithium-ion battery electrodes. For sodium-ion batteries, a reversible specific capacity of 305 mAh g-1 is achieved at a specific current of 20 mA g-1, 2.3 times higher than that of Ti3C2Tx. For lithium-ion batteries, a reversible capacity of 400 mAh g-1 at a specific current of 20 mA g-1 is 1.5 times higher than that of Ti3C2Tx. Both sodium-ion and lithium-ion batteries made from NGC-Ti3C2Tx shows high cycling stability. The theoretical calculations also verify the remarkable improvement in battery capacity within the NGC-Ti3C2O2 system, attributed to the additional adsorption of working ions at the edge states of NGC. This work offers an innovative way to synthesize a new van der Waals heterostructure and provides a new route to improve the electrochemical performance significantly.

Phase transition and electrochemical properties of S-functionalized MXene anodes for Li-ion batteries: a first-principles investigation

The advancement of anode materials for achieving high energy storage is a crucial topic for high-performance Li-ion batteries (LIBs). Here, first-principles calculations were used to conduct a thorough and systematic investigation into lithium storage properties of MXenes with new S functional groups as LIB anode materials. Density of states, diffusion energy barriers, open circuit voltages and storage capacities were calculated to comprehensively evaluate the lithium storage properties of S-functionalized MXenes. Based on the computational results, Ti2CS2 and V2CS2 were selected as excellent candidates from ten M2CS2 MXenes. The diffusion energy barriers of M2CS2 within the range of 0.26-0.32 eV are lower than those of M2CO2 and M2CF2, indicating that M2CS2 anodes exhibit faster charge/discharge rates. By examining the stable crystal structures and comparing atomic positions before and after Li adsorptions, structural phase transitions during Li-ion adsorptions could happen for nearly all M2CS2 MXenes. The phase transitions predicted were directly observed using ab initio molecular dynamic simulations. The cycle stability, storage capacity and other lithium storage properties were enhanced by the reversible structural phase transition.

Dual Redox Reaction Sites for Pseudocapacitance Based on Ti and -P Functional Groups of Ti3C2PBrx MXene

MXenes have extensive applications due to their different properties determined by intrinsic structures and various functional groups. Exploring different functional groups of MXenes leads to improved performance or potential applications. In this work, we prepared new Ti3C2PBrx (x=0.4-0.6) MXene with phosphorus functional groups (-P) through a two-step gas-phase reaction. The acquisition of -P is achieved by replacing bromine functional groups (-Br) of Ti3C2Br2 in the phosphorus vapor. After -Br is replaced with -P, Ti3C2PBrx MXene shows an improved areal capacitance (360 mF cm-2) at 20 mV s-1 compared with Ti3C2Br2 MXene (102 mF cm-2). At a current density of 5 mA cm-2 after 10000 cycles, the capacitance retention of Ti3C2PBrx MXene has not decreased. The pseudocapacitive enhancement mechanism has been discovered based on the dual redox sites of the functional groups -P and Ti.

Advanced Insights on MXenes: Categories, Properties, Synthesis, and Applications in Alkali Metal Ion Batteries

The development and optimization of promising anode material for next-generation alkali metal ion batteries are significant for clean energy evolution. 2D MXenes have drawn extensive attention in electrochemical energy storage applications, due to their multiple advantages including excellent conductivity, robust mechanical properties, hydrophilicity of its functional terminations, and outstanding electrochemical storage capability. In this review, the categories, properties, and synthesis methods of MXenes are first outlined. Furthermore, the latest research and progress of MXenes and their composites in alkali metal ion storage are also summarized comprehensively. A special emphasis is placed on MXenes and their hybrids, ranging from material design and fabrication to fundamental understanding of the alkali ion storage mechanisms to battery performance optimization strategies. Lastly, the challenges and personal perspectives of the future research of MXenes and their composites for energy storage are presented.

Application Scopes of Miniaturized MXene-Functionalized Electrospun Nanofibers-Based Electrochemical Energy Devices

Exploring combinatorial materials, as well as rational device configuration design, are assumed to be the key strategies for deploying versatile electrochemical devices. MXene sheets have revealed a high hydrophilic surface with proper mechanical and electrical characteristics, rendering them supreme additive candidates to integrate in electrospun electrochemical power tools. The synergetic effects of MXene 2D layers with the nanofibrous networks can boost actuator responsive ability, battery capacity retention, fuel cell stability, sensor sensitivity, and supercapacitor areal capacitance. Their superior mechanical features can be endowed to the electrospun layers through the embedding of the MXene additive. In this review, the preparation and inherent features of the MXene configurations are briefly evaluated. The fabrication and overall performance of the MXene-loaded nanofibers applicable in electrochemical actuators, batteries, fuel cells, sensors, and supercapacitors are comprehensively figured out. Eventually, an outlook on the future development of MXene-based electrospun composites is presented. A substantial focus has been devoted to date to engineering conjugated MXene and electrospun fibrous frames. The potential performance of the MXene-decorated nanofibers presents a bright future of nanoengineering toward technological growth. Meanwhile, a balance between the pros and cons of the synthesized MXene composite layers is worthwhile to consider in the future.

An acetate electrolyte for enhanced pseudocapacitve capacity in aqueous ammonium ion batteries

Ammonium ion batteries are promising for energy storage with the merits of low cost, inherent security, environmental friendliness, and excellent electrochemical properties. Unfortunately, the lack of anode materials restricts their development. Herein, we utilized density functional theory calculations to explore the V2CTx MXene as a promising anode with a low working potential. V2CTx MXene demonstrates pseudocapacitive behavior for ammonium ion storage, delivering a high specific capacity of 115.9 mAh g-1 at 1 A g-1 and excellent capacity retention of 100% after 5000 cycles at 5 A g-1. In-situ electrochemical quartz crystal microbalance measurement verifies a two-step electrochemical process of this unique pseudocapacitive storage behavior in the ammonium acetate electrolyte. Theoretical simulation reveals reversible electron transfer reactions with [NH4+(HAc)3]···O coordination bonds, resulting in a superior ammonium ion storage capacity. The generality of this acetate ion enhancement effect is also confirmed in the MoS2-based ammonium-ion battery system. These findings open a new door to realizing high capacity on ammonium ion storage through acetate ion enhancement, breaking the capacity limitations of both Faradaic and non-Faradaic energy storage.

The heterointerface effect to boost the catalytic performance of single atom catalysts for sulfur conversion in lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are considered as one of the promising next-generation energy storage devices due to their characteristics of high energy density and low cost. However, the shuttle effect and sluggish conversion of lithium polysulfide (LiPs) have hindered their commercial applications. To address these issues, in our previous works, we have screened several highly efficient single atom catalysts (SACs) (MN4@G, M = V, Mo and W) with atomically dispersed transition metal atoms supported by nitrogen doped graphene based on high throughput calculations. Nevertheless, they still suffer from low loading of metal centers and unsatisfactory capability for accelerating the reaction kinetics. To tackle such problems, based on first-principles calculations, we systematically investigated the heterointerface effect on the catalytic performance of such three MN4@G toward sulfur conversion upon forming heterostructures with 5 typical two-dimensional materials of TiS2, C3N4, BN, graphene and reduced graphene oxide. Guided by efficient descriptors proposed in our previous work, we screened VN4@G/TiS2, MoN4@G/TiS2 and WN4@G/TiS2 possessing low Li2S decomposition barriers of 0.54, 0.44 and 0.41 eV, respectively. They also possess enhanced capabilities for catalyzing the sulfur reduction reaction as well as stabilizing soluble LiPs. More interestingly, the heterointerface can enhance the capability of the carbon atoms far away from the metal centers for trapping LiPs. This work shows that introducing a heterointerface is a promising strategy to boost the performance of SACs in Li-S batteries.

Properties of Layered TMDC Superlattices for Electrodes in Li-Ion and Mg-Ion Batteries

In this work, we present a first-principles investigation of the properties of superlattices made from transition metal dichalcogenides for use as electrodes in lithium-ion and magnesium-ion batteries. From a study of 50 pairings, we show that, in general, the volumetric expansion, intercalation voltages, and thermodynamic stability of vdW superlattice structures can be well approximated with the average value of the equivalent property for the component layers. We also found that the band gap can be reduced, improving the conductivity. Thus, we conclude that superlattice construction can be used to improve material properties through the tuning of intercalation voltages toward specific values and by increasing the stability of conversion-susceptible materials. For example, we demonstrate how pairing SnS2 with systems such as MoS2 can change it from a conversion to an intercalation material, thus opening it up for use in intercalation electrodes.

Pillar-Structured Ti3 C2 Tx MXene with Engineered Interlayer Spacing for High-Performance Magnesium Batteries

Two-dimentional (2D) Ti3 C2 Tx MXene has attracted significant attention in non-lithium-ion batteries due to its excellent electrical conductivity, high volumetric capacity, and ability to accommodate intercalants. Rechargeable magnesium batteries with Mg metal anodes are noted for their high theoretical energy density, potential safety, earth abundance, dendrite-free Mg2+ plating/stripping mechanism on the anode side, and low cost. Nevertheless, owing to the large polarity of divalent Mg2+ ions, the insertion of Mg2+ into the MXene layers suffers from sluggish kinetics, limiting the performance for storage of Mg2+ ions. Herein, a simple self-assembly strategy is demonstrated to achieve high magnesium ion storage capability with pillar-structured Ti3 C2 Tx MXene by intercalating a hyperbranched polyethylene ionomer containing quaternary ammonium ions. The ionomer intercalation/modification leads to the expansion of interlayer spacing of the MXene and, meanwhile, improves its affinity to low-polarity THF-based electrolyte. The delaminated ionomer-modified MXene shows significantly improved electrochemical performance as a cathode material for Mg batteries. It shows a promising cycling stability with a capacity retention of 86% after 400 cycles at 200 mA g-1 , as well as outstanding high-rate performance with a capacity of 110 mAh g-1 retained at 1,000 mA g-1 relative to 213 mAh g-1 at 20 mA g-1 .

Gas-phase synthesis of Ti2CCl2 enables an efficient catalyst for lithium-sulfur batteries

MXenes have aroused intensive enthusiasm because of their exotic properties and promising applications. However, to date, they are usually synthesized by etching technologies. Developing synthetic technologies provides more opportunities for innovation and may extend unexplored applications. Here, we report a bottom-up gas-phase synthesis of Cl-terminated MXene (Ti2CCl2). The gas-phase synthesis endows Ti2CCl2 with unique surface chemistry, high phase purity, and excellent metallic conductivity, which can be used to accelerate polysulfide conversion kinetics and dramatically prolong the cyclability of Li-S batteries. In-depth mechanistic analysis deciphers the origin of the formation of Ti2CCl2 and offers a paradigm for tuning MXene chemical vapor deposition. In brief, the gas-phase synthesis transforms the synthesis of MXenes and unlocks the hardly achieved potentials of MXenes.

Improved ion adsorption capacities and diffusion dynamics in surface anchored MoS2⊥Mo4/3B2 and MoS2⊥Mo4/3B2O2 heterostructures as anodes for alkaline metal-ion batteries

First-principles calculations were performed to analyze the atomic structures and electrochemical energy storage properties of novel MoS2⊥boridene heterostructures by anchoring MoS2 nanoflakes on Mo4/3B2 and Mo4/3B2O2 monolayers. Both thermodynamic and thermal stabilities of each heterostructure were thoroughly evaluated from the obtained binding energies and through first-principles molecular dynamics simulations at room temperature, confirming the high formability of the heterostructures. The electrochemical properties of MoS2⊥Mo4/3B2 and MoS2⊥Mo4/3B2O2 heterostructures were investigated for their potential use as anodes for alkaline metal ion batteries (Li+, Na+ and K+). It was revealed that Li+ and Na+ can form multiple stable full adsorption layers on both heterostructures, while K+ forms only a single full adsorption layer. The presence of a negative electron cloud (NEC) contributes to the stabilization of a multi-layer adsorption mechanism. For all investigated alkaline metal ions, the predicted ion diffusion dynamics are relatively sluggish for the adsorbates in the first full adsorption layer on MoS2⊥boridene heterostructures due the relatively large migration energies (>0.50 eV), compared to those of second or third full adsorption layers (<0.30 eV). MoS2⊥Mo4/3B2O2 exhibited higher onset and mean open circuit voltages as anodes for alkaline metal-ion batteries than MoS2⊥Mo4/3B2 hybrids because of enhanced interactions between the adsorbate and the Mo4/3B2O2 monolayer with the presence of O-terminations. Tailoring the size and horizontal spacing between two neighboring MoS2 nano-flakes in heterostructures led to high theoretical capacities for LIBs (531 mA h g-1), SIBs (300 mA h g-1) and PIBs (131 mA h g-1) in the current study.

Multi-Ion Engineering Strategies toward High Performance Aqueous Zinc-Based Batteries

As alternatives to batteries with organic electrolytes, aqueous zinc-based batteries (AZBs) have been intensively studied. However, the sluggish kinetics, side reactions, structural collapse, and dissolution of the cathode severely compromise the commercialization of AZBs. Among various strategies to accelerate their practical applications, multi-ion engineering shows great feasibility to maintain the original structure of the cathode and provide sufficient energy density for high-performance AZBs. Though multi-ion engineering strategies could solve most of the problems encountered by AZBs and show great potential in achieving practical AZBs, the comprehensive summaries of the batteries undergo electrochemical reactions involving more than one charge carrier is still in deficiency. The ambiguous nomenclature and classification are becoming the fountainhead of confusion and chaos. In this circumstance, this review overviews all the battery configurations and the corresponding reaction mechanisms are investigated in the multi-ion engineering of aqueous zinc-based batteries. By combing through all the reported works, this is the first to nomenclate the different configurations according to the reaction mechanisms of the additional ions, laying the foundation for future unified discussions. The performance enhancement, fundamental challenges, and future developing direction of multi-ion strategies are accordingly proposed, aiming to further accelerate the pace to achieve the commercialization of AZBs with high performance.

MXene-Based Current Collectors for Advanced Rechargeable Batteries

As an indispensable component of rechargeable batteries, the current collector plays a crucial role in supporting the electrode materials and collecting the accumulated electrical energy. However, some key issues, like uneven resources, high weight percentage, electrolytic corrosion, and high-voltage instability, cannot meet the growing need for rechargeable batteries. In recent years, MXene-based current collectors have achieved considerable achievements due to its unique structure, large surface area, and high conductivity. The related research has increased significantly. Nonetheless, a comprehensive review of this area is seldom. Herein the applications and progress of MXene in current collector are systematically summarized and discussed. Meanwhile, some challenges and future directions are presented.

Recent advances and promise of MXene-based composites as electrode materials for sodium-ion and potassium-ion batteries

With the increasing demand for sustainable energy and concerns about the scarcity of lithium resources, sodium and potassium ion batteries have emerged as promising alternative energy storage technologies. MXene, as a novel two-dimensional material, possesses exceptional electrical conductivity, high surface area, and tunable structural features that make it an ideal candidate for high-performance electrode materials. However, its limited theoretical capacity hinders its widespread application. To overcome this limitation, MXene has been combined with other materials through synergistic effects between different components to enhance the overall electrochemical performance and expand its application in sodium/potassium ion batteries. Recently, substantial advancements have been realized in the exploration of MXene-based composites as energy storage materials, encompassing their synthesis, design, and the comprehension of charge storage mechanisms. This paper aims to propose a comprehensive summary of the latest developments in MXene-based composites as electrode materials for sodium ion batteries and potassium ion batteries, with a particular emphasis on the enhanced physicochemical properties resulting from composite formation. Moreover, the challenges faced by MXene materials in sodium ion batteries and potassium ion batteries are thoroughly discussed, and future research directions to further advance this field are proposed.

One-Step Method for Fabricating Janus Aramid Nanofiber/MXene Nanocomposite Films with Improved Joule Heating and Thermal Camouflage Properties

The integration of ultraflexible and mechanically robust films with electric heaters and camouflage technology provides a promising platform for the development of wearable devices, especially for aerospace and military applications. Herein, we present a facile and efficient one-step vacuum-assisted filtration method for fabricating Janus films based on aramid nanofibers (ANF) and Ti3C2Tx (MXene). The ANF/MXene nanocomposite film exhibits remarkable properties, including high conductivity (23809.5 S/m), excellent mechanical strength (102.54 MPa), and outstanding thermal stability (575 °C). Most notably, the Janus ANF/MXene composite film demonstrates superior Joule heating performance with a low driving voltage (1-5 V), high heating temperature (30-276 °C), and rapid response time (within 5 s). Additionally, the film exhibits effective thermal camouflage (72 °C for objects with temperatures above 163 °C) and excellent electromagnetic interference shielding properties (SSE/t = 32475.6 dB cm2/g). These results demonstrate that Janus ANF/MXene films possess a unique combination of thermal camouflage, Joule heating, and electromagnetic interference shielding properties, making them highly promising for wearable devices, high-performance electrical heating, infrared stealth, and security protection applications.

Phase Engineering and Synchrotron-Based Study on Two-Dimensional Energy Nanomaterials

In recent years, there has been significant interest in the development of two-dimensional (2D) nanomaterials with unique physicochemical properties for various energy applications. These properties are often derived from the phase structures established through a range of physical and chemical design strategies. A concrete analysis of the phase structures and real reaction mechanisms of 2D energy nanomaterials requires advanced characterization methods that offer valuable information as much as possible. Here, we present a comprehensive review on the phase engineering of typical 2D nanomaterials with the focus of synchrotron radiation characterizations. In particular, the intrinsic defects, atomic doping, intercalation, and heterogeneous interfaces on 2D nanomaterials are introduced, together with their applications in energy-related fields. Among them, synchrotron-based multiple spectroscopic techniques are emphasized to reveal their intrinsic phases and structures. More importantly, various in situ methods are employed to provide deep insights into their structural evolutions under working conditions or reaction processes of 2D energy nanomaterials. Finally, conclusions and research perspectives on the future outlook for the further development of 2D energy nanomaterials and synchrotron radiation light sources and integrated techniques are discussed.

Anion Storage of MXenes

Recently, anion storage materials have gained significant attention owing to the widened cell voltage and additional anion storing capacity for a large energy density. MXenes are considered as the emerging anion storing materials owing to their sufficient interlayer spacing, rich surface chemistries, tunable structures, remarkable electrochemical properties, and mechanical integrity. Herein, a comprehensive review on the anion storage of MXenes covering their anion storage mechanism and state-of-the-art chemical strategies for the improved anion storage performances is reported. The recent progress of MXenes on aluminum ion batteries, metal halogen batteries, halogen ion batteries, and electrochemical electrode deionization is addressed. The scientific and technical challenges and the research direction into the anion storage of MXenes are also addressed and finally the authors' perspective on anion storage of MXenes is provided. Therefore, this review offers an insight into the rational design of MXenes for anion storage materials and the correlation of surface chemistries and structural modifications with anion storage properties for the applications into electrochemical energy storage and water purification.

Interfacial Designs of MXenes for Mild Aqueous Zinc-Ion Storage

Limited Li resources, high cost, and safety risks of using organic electrolytes have stimulated a strong motivation to develop non-Li aqueous batteries. Aqueous Zn-ion storage (ZIS) devices offer low-cost and high-safety solutions. However, their practical applications are at the moment restricted by their short cycle life arising mainly from irreversible electrochemical side reactions and processes at the interfaces. This review sums up the capability of using 2D MXenes to increase the reversibility at the interface, assist the charge transfer process, and thereby improve the performance of ZIS. First, they discuss the ZIS mechanism and irreversibility of typical electrode materials in mild aqueous electrolytes. Then, applications of MXenes in different ZIS components are highlighted, including as electrodes for Zn2+ intercalation, protective layers of Zn anode, hosts for Zn deposition, substrates, and separators. Finally, perspectives are put forward on further optimizing MXenes to improve the ZIS performance.

Vertically aligned MoS2 nanosheets on monodisperse MXene as electrolyte-philic cathodes for zinc ion batteries with enhanced capacity

Zinc ion batteries (ZIBs) have attracted extensive attention for their high safety and environmentally friendly nature, and considerable theoretical capacities. Due to its unique two-dimensional layered structure and high theoretical specific capacities, molybdenum disulfide (MoS₂) presents as a promising cathode material for ZIBs. Nevertheless, the low electrical conductivity and poor hydrophilicity of MoS₂ limits its wide application in ZIBs. In this work, MoS₂/Ti₃C₂T_x composites are effectively constructed using a one-step hydrothermal method, where two-dimensional MoS₂ nanosheets are vertically grown on monodisperse Ti₃C₂T_x MXene layers. Contributing to the high ionic conductivity and good hydrophilicity of Ti₃C₂T_x, MoS₂/Ti₃C₂T_x composites possess improved electrolyte-philic and conductive properties, leading to a reduced volume expansion effect of MoS₂ and accelerated Zn²⁺ reaction kinetics. As a result, MoS₂/Ti₃C₂T_x composites exhibit high voltage (1.6 V) and excellent discharge specific capacity of 277.8 mA h g^-1 at 0.1 A g^-1, as well as cycle stability as cathode materials for ZIBs. This work provides an effective strategy for developing cathode materials with high specific capacity and stable structure.

Effective Screening Descriptor for MXenes to Enhance Sulfur Reduction in Lithium-Sulfur Batteries

MXenes, two-dimensional transition (2D) metal carbides/nitrides, have shown promise as cathodic catalysts for accelerating the conversion of lithium polysulfides (LiPSs) in lithium-sulfur (Li-S) batteries due to their diverse redox-active sites and rapid electron transfer. However, efficiently screening the optimal cathodic catalysts out of thousands of MXenes is challenging. To address this, we developed a model that accurately predicts the thermodynamic energy barrier of the rate-limiting step in Li-S batteries. Our model relates the local chemical reactivity of the MXene sites to the p-band center of the terminations and the electronegativity of subsurface transition metals. The accuracy of the model was verified through density functional theory calculations and contrast experiments in pure and Zn-doping MXenes qualitatively. By utilizing this model, we screened a large library of MXenes (27 types of five-atom-layer MXenes) and identified Ti₂CS₂, Mo₂CS₂, and W₂CS₂ as potential cathodic catalysts for Li-S batteries.

Lithium storage performance enhanced by lithiation-induced structural phase transitions of fluorinated MXenes

Structural phase transitions in electrode materials of Li-ion batteries (LIBs) often occur along with Li-ion extraction/intercalation during charge and discharge processes. Lithiation-induced phase transition behaviors of two-dimensional fluorinated MXenes were investigated systematically by first-principles density functional calculations. The calculated results show that fluorine atoms in the nine MXenes studied moved from the FCC site (or HCP site for Ta₂CF₂) to the TOP site during Li adsorption. Further all the predicted phase transitions were confirmed by ab initio molecular dynamic simulations. The band structure, density of state, diffusion energy barrier, average voltage and storage capacity were calculated to evaluate the lithium storage properties of fluorinated MXenes, which revealed that V₂CF₂ and Ti₂CF₂ are the optimal candidates for LIB electrode materials. The structural phase transition led to improvements in the cycle stability, storage capacity, average voltage, and other lithium storage properties of the fluorinated MXenes.

Fundamental Understanding and Optimization Strategies for Dual-Ion Batteries: A Review

There has been increasing demand for high-energy density and long-cycle life rechargeable batteries to satisfy the ever-growing requirements for next-generation energy storage systems. Among all available candidates, dual-ion batteries (DIBs) have drawn tremendous attention in the past few years from both academic and industrial battery communities because of their fascinating advantages of high working voltage, excellent safety, and environmental friendliness. However, the dynamic imbalance between the electrodes and the mismatch of traditional electrolyte systems remain elusive. To fully employ the advantages of DIBs, the overall optimization of anode materials, cathode materials, and compatible electrolyte systems is urgently needed. Here, we review the development history and the reaction mechanisms involved in DIBs. Afterward, the optimization strategies toward DIB materials and electrolytes are highlighted. In addition, their energy-related applications are also provided. Lastly, the research challenges and possible development directions of DIBs are outlined.

In-situ decoration of tin sulfide on Niobium carbide MXene with robust electronic coupling for improved sodium storage performance

Tin sulfide (SnS) has been considered as one of the most promising sodium storage materials because of its excellent electrochemical activity, low cost, and low-dimensional structure. However, owing to the serious volume change upon discharging/charging and poor electronic conductivity, the SnS-based electrodes often suffer from electrode pulverization and sluggish reaction kinetics, thus resulting in serious capacity fading and degraded rate capability. In this work, SnS nanoparticles uniformly distributed on the surface of the layered Niobium carbide MXene (SnS/Nb₂CT_x) were fabricated through a facile solvothermal approach followed by calcination, endowing the SnS/Nb₂CT_x with a three-dimensional interconnected framework as well as fast charge transfer. Benefitting from the excellent electronic/ionic conductivity, efficient buffering matrix, abundant active sites, and high sodium storage activity inherited from the structure design, the robust electronic coupling between SnS nanoparticle and Nb₂CT_x MXene results in excellent electrochemical output, which demonstrates superior reversible capacities of 479.6 (0.1 A/g up to 100 cycles) and 278.9 mAh/g (0.5 A/g up to 500 cycles) upon sodium storage, respectively. The excellent electrochemical performance manifests the promise of the combination of metal sulfides with Nb₂CT_x MXene to fabricate high-performance electrodes for sodium storage.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Two-dimensional polymer nanosheets as a high-performance organic anode for sodium-ion batteries

Organic compounds have become a potentially important choice for a new generation of energy-storage electrode materials due to their designability, flexibility, green sustainability, and abundance. However, the applications of organic electrode materials are still limited because of their dissolution in electrolytes and low electrical conductivity, which in turn cause poor cycling stability. Here, for the first time, we report 2-amino-4-thiazole-acetic acid (ATA) and its sodium salt, sodium 2-amino-4-thiazol-derived polymer (PATANa), as an anode. The PATANa showed a two-dimensional (2D) nanosheet structure, offering a larger contact area with the electrolyte and a shorter ion-migration path, which improved the ion-diffusion kinetics. The polymer showed excellent cycling stability and outstanding rate capability when tested as an anode for sodium-ion batteries (SIBs). It could deliver a high reversible specific capacity of 303 mA h g^-1 at 100 mA g^-1 for 100 cycles and maintain a high discharge capacity of 190 mA h g^-1 after 1000 long cycle numbers even at a high current density of 1000 mA g^-1. This approach of salinizing the polymer opens a new way to develop anode materials for sodium-ion batteries.

Computational determination of a graphene-like TiB4 monolayer for metal-ion batteries and a nitrogen reduction electrocatalyst

As an emerging two-dimensional (2D) material, the TiB₄ monolayer possesses intrinsic advantages in electrochemical applications owing to its graphene-like structure and metallic characteristics. In this work, we performed density functional calculations to investigate the electrochemical properties of the TiB₄ monolayer as an anode material for Li/Na/K ion batteries and as an electrocatalyst for the nitrogen reduction reaction (NRR). Our investigation reveals that Li/Na/K ions could be steadily adsorbed on the TiB₄ monolayer with moderate adsorption energies, and tended to diffuse along two adjacent C-sites with lower energy barriers (0.231/0.094/0.067 eV for Li/Na/K ions) compared to the currently reported transition-metal boride monolayers. Furthermore, a N₂ molecule can be spontaneously captured by the TiB₄ monolayer with a negative Gibbs free energy (-0.925 eV and -0.326 eV for end-on and side-on adsorptions, respectively), hence provoking a conversion into NH₃ along the most efficient reaction pathway (i.e., N₂* → N₂H* → HNNH* → H₂NNH* → H₃NNH* → NH* → NH₂* → NH₃*). In the hydrogenation process, the TiB₄ monolayer exhibits much higher catalytic activity for the NRR as compared with other electrocatalysts, which should be attributed to the spontaneous achievement (ΔG < 0) at all hydrogenation reaction steps except the potential-determining step. Moreover, the TiB₄ monolayer exhibits higher selectivity toward the NRR than the hydrogen evolution reaction. Our work advances the mechanistic understanding on the electrochemical properties of the TiB₄ monolayer as an anode material for metal-ion batteries and as a NRR electrocatalyst, and provides significant guidance for developing high-performance multifunctional 2D materials.

Rationalizing Functionalized MXenes as Effective Anchor Materials for Lithium-Sulfur Batteries via First-Principles Calculations

The practical application of Li-S batteries has been greatly hindered by severe shuttle effects and sluggish kinetics. Anchoring soluble lithium polysulfides (LiPSs) onto host materials by chemisorption is an effective strategy for extending battery life. In this work, we performed systematic density functional theory calculations to evaluate the anchoring performance of O/F-covered MXene (M₂TC₂) in lithium-sulfur batteries. Our results indicate that the moderate anchoring strength (∼2.5 eV), outstanding sulfur reduction performance (U_L > -0.6 V), and low lithium ion diffusion barrier (<0.2 eV) of Mo₂CF₂ and V₂CF₂ make them promising host materials for LiPSs. We further revealed the determinants of the strength of binding of LiPSs to M₂CT₂. On the basis of the strong correlation among Q_M, χ_O/F, and E_a, we established a "structure-property" equation to reveal the active origin of M₂CT₂. We expect that the framework established in this work will accelerate the development of Li-S batteries.

Revealing the electronic, optical and photocatalytic properties of PN-M2CO2 (P = Al, Ga; M = Ti, Zr, Hf) heterostructures

Using DFT, the electronic structure, optical, and photocatalytic properties of PN (P = Ga, Al) and M2CO2 (M = Ti, Zr, Hf) monolayers and their PN-M2CO2 van der Waals heterostructures (vdWHs) are investigated. Optimized lattice parameters, bond length, bandgap, conduction and valence band edges show the potential of PN (P = Ga, Al) and M2CO2 (M = Ti, Zr, Hf) monolayers in photocatalytic applications, and the application of the present approach to combine these monolayers and form vdWHs for efficient electronic, optoelectronic and photocatalytic applications is shown. Based on the same hexagonal symmetry and experimentally achievable lattice mismatch of PN (P = Ga, Al) with M2CO2 (M = Ti, Zr, Hf) monolayers, we have fabricated PN-M2CO2 vdWHs. Binding energies, interlayer distance and AIMD calculations show the stability of PN-M2CO2 vdWHs and demonstrate that these materials can be easily fabricated experimentally. The calculated electronic band structures show that all the PN-M2CO2 vdWHs are indirect bandgap semiconductors. Type-II[-I] band alignment is obtained for GaN(AlN)-Ti2CO2[GaN(AlN)-Zr2CO2 and GaN(AlN)-Hf2CO2] vdWHs. PN-Ti2CO2 (PN-Zr2CO2) vdWHs with a PN(Zr2CO2) monolayer have greater potential than a Ti2CO2(PN) monolayer, indicating that charge is transfer from the Ti2CO2(PN) to PN(Zr2CO2) monolayer, while the potential drop separates charge carriers (electron and holes) at the interface. The work function and effective mass of the carriers of PN-M2CO2 vdWHs are also calculated and presented. A red (blue) shift is observed in the position of excitonic peaks from AlN to GaN in PN-Ti2CO2 and PN-Hf2CO2 (PN-Zr2CO2) vdWHs, while significant absorption for photon energies above 2 eV for AlN-Zr2CO2, GaN-Ti2CO2 and PN-Hf2CO2, give them good optical profiles. The calculated photocatalytic properties demonstrate that PN-M2CO2 (P = Al, Ga; M = Ti, Zr, Hf) vdWHs are the best candidates for photocatalytic water splitting.

Fabrication of highly efficient Rh-doped cobalt-nickel-layered double hydroxide/MXene-based electrocatalyst with rich oxygen vacancies for hydrogen evolution

The development of nonprecious metal catalysts for producing hydrogen from economical alkaline water electrolysis that is both stable and efficient is crucial but remains challenging. In this study, Rh-doped cobalt-nickel-layered double hydroxide (CoNi LDH) nanosheet arrays with abundant oxygen vacancies (Ov) in-situ grown on Ti₃C₂T_x MXene nanosheets (Rh-CoNi LDH/MXene) were successfully fabricated. The synthesized Rh-CoNi LDH/MXene exhibited excellent long-term stability and a low overpotential of 74.6 ± 0.4 mV at -10 mA cm^-2 for hydrogen evolution reaction (HER) owing to its optimized electronic structure. Experimental results and density functional theory calculations revealed that the incorporation of Rh dopant and Ov into CoNi LDH and the coupling interface between Rh-CoNi LDH and MXene optimized the hydrogen adsorption energy, which accelerated the hydrogen evolution kinetics, thereby accelerating the overall alkaline HER process. This work presents a promising strategy for designing and synthesizing highly efficient electrocatalysts for electrochemical energy conversion devices.

Hybrid Nano-Phase Ion/Electron Dual Pathways of Nickel/Cobalt-Boride Cathodes Boosting Intercalation Kinetics for Alkaline Batteries

Nickel-based hydroxides and their derivatives exhibit relatively low capacities and unsatisfactory durability as cathode materials for rechargeable alkaline batteries. In this work, a hybrid NiCo-B nanosheet cathode, integrating electrolyte ion-shuttling channels and electron-transferring networks into a metal-organic framework (MOF), was devised delicately. In the structure, the hybrid ion/electron dual pathways were constructed by NiCo-MOF frameworks and NiCo-B interpenetration networks. It revealed that nano-phase electron-transferring pathways in the MOF obviously boosted ion intercalation kinetics. The as-obtained hybrid NiCo-B nanosheets as cathode materials exhibited reversible capacity as high as 280 mA h g^-1 at a current density of 1 A g^-1 and excellent rate capability with a capacity retention of 78% from 1 to 10 A g^-1. After 2000 charge/discharge cycles at 4 A g^-1, the capacity still remained at 94% of the initial one. A full battery assembled with a hybrid NiCo-B cathode and a Fe₂O₃ anode showed a high capacity of 250 mA h g^-1 and a considerable stability of 89% after 1000 cycles. Ragone plots indicated the highest energy density of 409 W h kg^-1 and the lowest power density of 1.5 kW kg^-1, outperforming other aqueous batteries. It revealed that a syngenetic structure of ion/electron hybrid dual pathways integrated into an MOF could be a potential strategy to optimize ion intercalation electrode materials for alkaline batteries.

Recent advances in epitaxial heterostructures for electrochemical applications

Construction of epitaxial heterostructures is crucial for boosting the electrochemical properties of various materials, however a review dedicated to this attractive topic is still lacking. In this Minireview, a timely summary on the achievements of epitaxial heterostructure design for electrochemical applications is provided. We first introduce the synthesis strategies to provide fundamental understanding on how to create epitaxial interfaces between different components. Secondly, the superiorities of epitaxial heterostructures in electrocatalysis, supercapacitors and batteries are highlighted with the underlying structure-property relationship elucidated. Finally, a discussion on the challenges and future prospects of this field is presented.

High-Yield Ti3 C2 Tx MXene-MoS2 Integrated Circuits

It is very challenging to employ solution-processed conducting films in large-area ultrathin nanoelectronics. Here, spray-coated Ti₃ C₂ T_x MXene films as metal contacts are successfully integrated into sub-10 nm gate oxide 2D MoS₂ transistor circuits. Ti₃ C₂ T_x films are spray coated on glass substrates followed by vacuum annealing. Compared to the as-prepared sample, vacuum annealed films exhibit a higher conductivity (≈11 000 S cm^-1 ) and a lower work function (≈4.5 eV). Besides, the annealed Ti₃ C₂ T_x film can be patterned through a standard cleanroom process without peeling off. The annealed Ti₃ C₂ T_x film shows a better band alignment for n-type transport in MoS₂ channel with small work function mismatch of 0.06 eV. The MoS₂ film can be uniformly transferred on the patterned Ti₃ C₂ T_x surface and then readily processed through the cleanroom process. A large-area array of Ti₃ C₂ T_x MXene-MoS₂ transistors is fabricated using different dielectric thicknesses and semiconducting channel sizes. High yield and stable performance for these transistor arrays even with an 8 nm-thick dielectric layer are demonstrated. Besides, several circuits are demonstrated, including rectifiers, negative-channel metal-oxide-semiconductor (NMOS) inverters, and voltage-shift NMOS inverters. Overall, this work indicates the tremendous potential for solution-processed Ti₃ C₂ T_x MXene films in large-area 2D nanoelectronics.

Functional MXene-Based Materials for Next-Generation Rechargeable Batteries

MXenes are seen as an exceptional candidate to reshape the future of energy with their viable surface chemistry, ultrathin 2D structure, and excellent electronic conductivity. The extensive research efforts bring about rapid expansion of the MXene families with enriched functionalities, which significantly boost performance of the existing energy-storage devices. In this review, the strategies that are developed to functionalize the MXene-based materials, including tailoring their microstructure by ions/molecules/polymers-initiated interaction or self-assembly, surface/interface engineering with dopants or functional groups, constructing heterostructures from MXenes with various materials, and transforming them into a series of derivatives inheriting the merits of the MXene precursors are highlighted. Their applications in emerging battery technologies are demonstrated and discussed. With delicate functionalization and structural engineering, MXene-based electrode materials exhibit improved specific capacity and rate capability, and their presence further suppresses and even eliminates dendrite formation on the metal anodes, which lengthens the lifespan of the rechargeable batteries. Meanwhile, MXenes serve as additives for electrolytes, separators, and current collectors. Finally, some future directions worth of exploration to address the remaining challenging issues of MXene-based materials and achieve the next-generation high-power and low-cost rechargeable batteries are proposed.

Exploring 2D Energy Storage Materials: Advances in Structure, Synthesis, Optimization Strategies, and Applications for Monovalent and Multivalent Metal-Ion Hybrid Capacitors

The design and development of advanced energy storage devices with good energy/power densities and remarkable cycle life has long been a research hotspot. Metal-ion hybrid capacitors (MHCs) are considered as emerging and highly prospective candidates deriving from the integrated merits of metal-ion batteries with high energy density and supercapacitors with excellent power output and cycling stability. The realization of high-performance MHCs needs to conquer the inevitable imbalance in reaction kinetics between anode and cathode with different energy storage mechanisms. Featured by large specific surface area, short ion diffusion distance, ameliorated in-plane charge transport kinetics, and tunable surface and/or interlayer structures, 2D nanomaterials provide a promising platform for manufacturing battery-type electrodes with improved rate capability and capacitor-type electrodes with high capacity. In this article, the fundamental science of 2D nanomaterials and MHCs is first presented in detail, and then the performance optimization strategies from electrodes and electrolytes of MHCs are summarized. Next, the most recent progress in the application of 2D nanomaterials in monovalent and multivalent MHCs is dealt with. Furthermore, the energy storage mechanism of 2D electrode materials is deeply explored by advanced characterization techniques. Finally, the opportunities and challenges of 2D nanomaterials-based MHCs are prospected.

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.

Nb2CTx MXene Cathode for High-Capacity Rechargeable Aluminum Batteries with Prolonged Cycle Lifetime

Aluminum-ion batteries have garnered significant interest as a potentially safer and cheaper replacement for conventional lithium-ion batteries, offering a shorter charging time and denser storage capacity. Nonetheless, the progress in this field is considerably hampered by the limited availability of suitable cathode materials that can sustain the reversible intercalation of Al³⁺/[AlCl₄]^- ions, particularly after long cycles. Herein, we demonstrate that rechargeable Al batteries embedded with two-dimensional (2D) Nb₂CT_x MXene as a cathode material exhibit excellent capacity and exceptional long cyclic performance. We have successfully improved the initial electrochemical performance of Nb₂CT_x MXene after being properly delaminated to a single-layered microstructure and subjected to a post-synthesis calcining treatment. Compared to pristine Nb₂CT_x MXene, the Al battery embedded with the calcined Nb₂CT_x MXene cathode has, respectively, retained high capacities of 108 and 80 mAh g^-1 after 500 cycles at current densities of 0.2 and 0.5 A g^-1 in a wide voltage window (0.1-2.4 V). Noteworthily, the cyclic lifetime of Nb₂CT_x MXene was extended from ∼300 to >500 times after calcination. We reveal that attaining Nb₂CT_x nanosheets with a controllable d-spacing has promoted the migration of the [AlCl₄]^- and Al³⁺ ions in the MXene interlayers, leading to enhanced charge storage. Furthermore, we found out that the formation of niobium oxides and amorphous carbon after calcination probably benefits the electrochemical performance of Nb₂CT_x MXene electrode in Al batteries.

Ti2CT2 MXene as Anodes for Metal Ion Batteries: From Monolayer to Bilayer to Pillar Structure

The electrochemical performances of Ti₂CT₂ (T = F, O, and OH) MXenes with different layer structures (monolayer, bilayer, and pillared structures) as anodes for mono-/multivalent metal ion (Li⁺, Na⁺, Mg²⁺, and Al³⁺) batteries (MIBs) were studied via first-principles simulations. First, metal ions (MIs) adsorbed on Ti₂CT₂ monolayers were investigated to reveal the influence of MXene terminated groups on MIB performance. This indicated that O-terminated MXenes would be more suitable as electrodes. In particular, the theoretical capacity of Mg²⁺ on a Ti₂CO₂ monolayer could be more than 1500 mA h g^-1. Then, MIs intercalated into MXene bilayers were considered to better understand the charging/discharging mechanism. In a Ti₂CO₂ bilayer with larger interlayer spacing, monovalent MIs and Mg²⁺ could form a multilayer accompanied by drastic expansion/contraction of the electrode, which still needs to be solved. Finally, imidazolium-based ionic liquids were used to preintercalate into MXene due to the matching size of the imidazolium cation, which effectively improved MXene stability and inhibited the self-stacking of layered MXenes. Our research would be helpful for theoretically regulating MXene functional groups and adjusting the interlayer spacing of MXenes via selecting guest molecules for designing MIBs and other energy storage devices.