Novel Synthesis of Red Phosphorus Nanodot/Ti3C2Tx MXenes from Low-Cost Ti3SiC2 MAX Phases for Superior Lithium- and Sodium-Ion Batteries

MXene-based anode materials for high performance sodium-ion batteries

As an emerging class of layered transition metal carbides/nitrides/carbon-nitrides, MXenes have been one of the most investigated anode subcategories for sodium ion batteries (SIBs), due to their unique layered structure, metal-like conductivity, large specific surface area and tunable surface groups. In particular, different MAX precursors and synthetic routes will lead to MXenes with different structural and electrochemical properties, which actually gives MXenes unlimited scope for development. In this feature article, we systematically present the recent advances in the methods and synthetic routes of MXenes, together with their impact on the properties of MXenes and also the advantages and disadvantages. Subsequently, the sodium storage mechanisms of MXenes are summarized, as well as the recent research progress and strategies to improve the sodium storage performance. Finally, the main challenges currently facing MXenes and the opportunities in improving the performance of SIBs are pointed out.

Recent Advances and Strategies in MXene-Based Electrodes for Supercapacitors: Applications, Challenges and Future Prospects

With the growing demand for technologies to sustain high energy consumption, supercapacitors are gaining prominence as efficient energy storage solutions beyond conventional batteries. MXene-based electrodes have gained recognition as a promising material for supercapacitor applications because of their superior electrical conductivity, extensive surface area, and chemical stability. This review provides a comprehensive analysis of the recent progress and strategies in the development of MXene-based electrodes for supercapacitors. It covers various synthesis methods, characterization techniques, and performance parameters of these electrodes. The review also highlights the current challenges and limitations, including scalability and stability issues, and suggests potential solutions. The future outlooks and directions for further research in this field are also discussed, including the creation of new synthesis methods and the exploration of novel applications. The aim of the review is to offer a current and up-to-date understanding of the state-of-the-art in MXene-based electrodes for supercapacitors and to stimulate further research in the field.

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.

Defect Engineering in Biomedical Sciences

With the promotion of nanochemistry research, large numbers of nanomaterials have been applied in vivo to produce desirable cytotoxic substances in response to endogenous or exogenous stimuli for achieving disease-specific therapy. However, the performance of nanomaterials is a critical issue that is difficult to improve and optimize under biological conditions. Defect-engineered nanoparticles have become the most researched hot materials in biomedical applications recently due to their excellent physicochemical properties, such as optical properties and redox reaction capabilities. Importantly, the properties of nanomaterials can be easily adjusted by regulating the type and concentration of defects in the nanoparticles without requiring other complex designs. Therefore, this tutorial review focuses on biomedical defect engineering and briefly discusses defect classification, introduction strategies, and characterization techniques. Several representative defective nanomaterials are especially discussed in order to reveal the relationship between defects and properties. A series of disease treatment strategies based on defective engineered nanomaterials are summarized. By summarizing the design and application of defective engineered nanomaterials, a simple but effective methodology is provided for researchers to design and improve the therapeutic effects of nanomaterial-based therapeutic platforms from a materials science perspective.

MXene-Based Nanocomposites for Piezoelectric and Triboelectric Energy Harvesting Applications

Due to its superior advantages in terms of electronegativity, metallic conductivity, mechanical flexibility, customizable surface chemistry, etc., 2D MXenes for nanogenerators have demonstrated significant progress. In order to push scientific design strategies for the practical application of nanogenerators from the viewpoints of the basic aspect and recent advancements, this systematic review covers the most recent developments of MXenes for nanogenerators in its first section. In the second section, the importance of renewable energy and an introduction to nanogenerators, major classifications, and their working principles are discussed. At the end of this section, various materials used for energy harvesting and frequent combos of MXene with other active materials are described in detail together with the essential framework of nanogenerators. In the third, fourth, and fifth sections, the materials used for nanogenerators, MXene synthesis along with its properties, and MXene nanocomposites with polymeric materials are discussed in detail with the recent progress and challenges for their use in nanogenerator applications. In the sixth section, a thorough discussion of the design strategies and internal improvement mechanisms of MXenes and the composite materials for nanogenerators with 3D printing technologies are presented. Finally, we summarize the key points discussed throughout this review and discuss some thoughts on potential approaches for nanocomposite materials based on MXenes that could be used in nanogenerators for better performance.

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.

Conductive Iodine-Doped Red Phosphorus Enabled Dendrite-Free Lithium Deposition on MXene Matrix

The highest theoretical capacity and lowest redox potential of lithium metal make lithium-based batteries the "holy grail" of the next-generation batteries. However, the uncontrollable dendrite growth and infinite volume change of lithium seriously hinder the real-world implementation of lithium-based batteries. Herein, a flexible MXene@iodine-doped red phosphorus (MXene@RP) paper with iodine-doped red phosphorous particles evenly distributed on the surface and interlayer of MXene matrix is designed by a simple vapor condensation reduction approach. The MXene@RP paper can be used as an efficient matrix to enable dendrite-free lithium deposition. On the one hand, the iodine doping alleviates the low conductivity shortcoming of red phosphorus, making it facilitate homogeneous lithium nucleation, thus promoting uniform lithium deposition and suppressing dendrite growth. On the other hand, the unique layered structure of conductive MXene paper provides ion transport channels and free spaces for lithium loading, alleviating the volume change induced structural damage. As a result, the MXene@RP paper with preloaded lithium exhibits long-term cycling stability. Particularly, a full cell based on Li-MXene@RP anode can maintain 81.4% of the initial capacity after 600 cycles at 4 C. The MXene@RP-based anode increases the potential applications of MXene and provides a guide for the design of dendrite-free lithium hosts.

Room-Temperature Assembled MXene-Based Aerogels for High Mass-Loading Sodium-Ion Storage

Low-temperature assembly of MXene nanosheets into three-dimensional (3D) robust aerogels addresses the crucial stability concern of the nano-building blocks during the fabrication process, which is of key importance for transforming the fascinating properties at the nanoscale into the macroscopic scale for practical applications. Herein, suitable cross-linking agents (amino-propyltriethoxysilane, Mn²⁺, Fe²⁺, Zn²⁺, and Co²⁺) as interfacial mediators to engineer the interlayer interactions are reported to realize the graphene oxide (GO)-assisted assembly of Ti₃C₂T_x MXene aerogel at room temperature. This elaborate aerogel construction not only suppresses the oxidation degradation of Ti₃C₂T_x but also generates porous aerogels with a high Ti₃C₂T_x content (87 wt%) and robustness, thereby guaranteeing the functional accessibility of Ti₃C₂T_x nanosheets and operational reliability as integrated functional materials. In combination with a further sulfur modification, the Ti₃C₂T_x aerogel electrode shows promising electrochemical performances as the freestanding anode for sodium-ion storage. Even at an ultrahigh loading mass of 12.3 mg cm^-2, a pronounced areal capacity of 1.26 mAh cm^-2 at a current density of 0.1 A g^-1 has been achieved, which is of practical significance. This work conceptually suggests a new way to exert the utmost surface functionalities of MXenes in 3D monolithic form and can be an inspiring scaffold to promote the application of MXenes in different areas.

Hierarchical MXene/transition metal chalcogenide heterostructures for electrochemical energy storage and conversion

MXenes have gained rapidly increasing attention owing to their two-dimensional (2D) layered structures and unique mechanical and physicochemical properties. However, MXenes have some intrinsic limitations (e.g., the restacking tendency of the 2D structure) that hinder their practical applications. Transition metal chalcogenide (TMC) materials such as SnS, NiS, MoS₂, FeS₂, and NiSe₂ have attracted much interest for energy storage and conversion by virture of their earth-abundance, low costs, moderate overpotentials, and unique layered structures. Nonetheless, the intrinsic poor electronic conductivity and huge volume change of TMC materials during the alkali metal-ion intercalation/deintercalation process cause fast capacity fading and poor-rate and poor-cycling performances. Constructing heterostructures based on metallic conductive MXenes and highly electrochemically active TMCs is a promising and effective strategy to solve these problems and enhance the electrochemical performances. This review highlights and discusses the recent research development of MXenes and hierarchical MXene/TMC heterostructures, with a focus on the synthesis strategies, surface/heterointerface engineering, and potential applications for lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, supercapacitors, electrocatalysis, and photocatalysis. The critical challenges and perspectives of the future development of MXenes and hierarchical MXene/TMC heterostructures for electrochemical energy storage and conversion are forecasted.

2D MXene Nanomaterials for Versatile Biomedical Applications: Current Trends and Future Prospects

Research on 2D nanomaterials is still in its early stages. Most studies have focused on elucidating the unique properties of the materials, whereas only few reports have described the biomedical applications of 2D nanomaterials. Recently, important questions about the interaction of 2D MXene nanomaterials with biological components have been raised. 2D MXenes are monolayer atomic nanosheets derived from MAX phase ceramics. As a new type of inorganic nanosystems, they are being widely used in biology and biomedicine. This review introduces the latest developments in 2D MXenes for the most advanced biomedical applications, including preparation and surface modification strategies, treatment modes, drug delivery, antibacterial activity, bioimaging, sensing, and biocompatibility. Besides, this review also discusses the current development trends and prospects of 2D inorganic nanosheets for further clinical applications. These emerging 2D inorganic MXenes will play an important role in next-generation cancer treatments.

Delamination and Engineered Interlayers of Ti3C2 MXenes using Phosphorous Vapor toward Flame-Retardant Epoxy Nanocomposites

As recently created inorganic nanosheet materials, more and more light has been shed on MXenes, which have emerged as a hotspot of intensive investigations. The simple exfoliation method for MXenes attracts numerous studies to pay efforts on. Compared with the extensive research about ultrasonication and mechanical milling, gas-assisted exfoliation has never been carried out for MXenes. Meanwhile, MXene-based nanocomposites are always prepared after exfoliation step by step. In this work, a facile way to fabricate a few-layered Ti₃C₂ MXene delaminated using phosphorous vapor evolved from commercial red phosphorous (RP) is put forward. The vapor deposits on the surface of Ti₃C₂ and also partially intercalates into the interlayers to obtain a novel two-dimensional RP/Ti₃C₂ nanocomposite directly. The P element strongly connects with the substrate by a covalent bond that improves the safety problems for RP during storage and usage. Due to the versatile feature of MXenes, the nanocomposite has the potential to be applied in a variety of fields. Herein, it is employed as a flame retardant for epoxide resin and effectively reduces fire disaster. The one-step exfoliation plus nanocomposite fabrication provides a more feasible way for the practical application of MXenes.

MXene-Based Materials for Electrochemical Sodium-Ion Storage

Advanced architecture and rational design of electrode materials for electrochemical sodium-ion storage are well developed by researchers worldwide. MXene-based materials are considered as one of the most potential electrode materials for sodium-ion-based devices, such as sodium-ion batteries (SIBs), sodium-sulfur batteries (SSBs), and sodium-ion capacitors (SICs), because of the excellent physicochemical characteristics of MXenes. Here, in this review, the recent research work and progress, both theoretical and experimental, on MXene-based materials including pure MXenes and MXene-based composites in application of SIBs, SSBs, and SICs are comprehensively summarized. The sodium storage mechanisms and the effective methods to enhance the electrochemical performance are also discussed. Finally, the current critical challenges and future research directions on the development of these MXene-based materials for electrochemical sodium-ion storage are presented.

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.

Green, Template-Less Synthesis of Honeycomb-like Porous Micron-Sized Red Phosphorus for High-Performance Lithium Storage

Large-volume-expansion-induced material pulverization severely limits the electrochemical performance of high-capacity red phosphorus (RP) in alkali-ion batteries. Honeycomb-like porous materials can effectively solve the issues due to their abundant interconnected pore structures. Nevertheless, it is difficult and greatly challenging to fabricate a honeycomb-like porous RP that has not yet been fabricated via chemical synthesis. Herein, we successfully fabricate a honeycomb-like porous micron-sized red phosphorus (HPRP) with a controlled pore structure via a large-scale green and template-less hydrothermal strategy. It is demonstrated that dissolved oxygen in the solution can accelerate the destruction of P9 cages of RP, thus forming abundant active defects with a faster reaction rate, so the fast corrosion forms the honeycomb-like porous structure. Owing to the free volume, interconnected porous structure, and strong robustness, the optimized HPRP-36 can mitigate drastic volume variation and prevent pulverization during cycling resulting in tiny particle-level outward expansion, demonstrated by in situ TEM and ex situ SEM analysis. Thus, the HPRP-36 anode delivers a large reversible capacity (2587.4 mAh g^-1 at 0.05 A g^-1) and long-cycling stability with over 500 cycles (∼81.9% capacity retention at 0.5 A g^-1) in lithium-ion batteries. This generally scalable, green strategy and deep insights provide a good entry point in designing honeycomb-like porous micron-sized materials for high-performance electrochemical energy storage and conversion.

Recent Developments of Nanomaterials and Nanostructures for High-Rate Lithium Ion Batteries

Lithium ion batteries have been considered as a promising energy-storage solution, the performance of which depends on the electrochemical properties of each component, including cathode, anode, electrolyte and separator. Currently, fast charging is becoming an attractive research field due to the widespread application of batteries in electric vehicles, which are designated to replace conventional diesel automobiles in the future. In these batteries, rate capability, which is closely linked to the topology and morphology of electrode materials, is one of the determining parameters of interest. It has been revealed that nanotechnology is an exceptional tool in designing and preparing cathodes and anodes with outstanding electrochemical kinetics due to the well-known nanosizing effect. Nevertheless, the negative effects of applying nanomaterials in electrodes sometimes outweigh the benefits. To better understand the exact function of nanostructures in solid-state electrodes, herein, a comprehensive review is provided beginning with the fundamental theory of lithium ion transport in solids, which is then followed by a detailed analysis of several major factors affecting the migration of lithium ions in solid-state electrodes. The latest developments in characterisation techniques, based on either electrochemical or radiology methodologies, are covered as well. In addition, state-of-the-art research findings are provided to illustrate the effect of nanomaterials and nanostructures in promoting the rate performance of lithium ion batteries. Finally, several challenges and shortcomings of applying nanotechnology in fabricating high-rate lithium ion batteries are summarised.

Fast and Universal Solution-Phase Flocculation Strategy for Scalable Synthesis of Various Few-Layered MXene Powders

MXenes have gained great attention in various fields because of their fascinating properties; however, the preparation of few-layered MXene powders is still limited by serious restacking of MXene nanosheets. Herein, for the first time, we have demonstrated an effective ammonium ion route to fundamentally address restacking and aggregation of the MXene nanosheets, using a solution-phase flocculation method (NH⁴⁺ method and modified NH⁴⁺ method) for large-scale preparation of few-layered Ti₃C₂T_x MXene powders in large quantities. The as-prepared few-layered MXene nanosheet powders show large size in the ab plane without the restacking phenomenon even at scanning electron microscopy measurements of 400× magnification, demonstrating the effectiveness of the proposed method. The method is also suitable for large-scale synthesis of other few-layered MXene powders, including Nb₄C₃T_x, V₂CT_x, Nb₂CT_x, etc., providing a general approach for the preparation of various few-layered MXene nanosheet powders, which represents a significant result for the development of MXenes.

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.