Enhancing MXene Materials for Electrochemical Energy Storage Through Functionalization Strategies

Functionalization Strategies of MXene Architectures for Electrochemical Energy Storage Applications

MXenes, an innovative class of two-dimensional materials, have attracted considerable interest for their potential in electrochemical energy storage due to their high specific surface area, adjustable surface functional groups, excellent electrical conductivity, and mechanical stability. However, their practical use in energy storage devices faces challenges, including the tendency to stack in layered structures, surface degradation, and limited ion diffusion properties. To enhance the performance of MXenes, functionalization has emerged as a critical strategy. By modifying surface functional groups, introducing various doping elements, and integrating MXenes with other materials, researchers have significantly improved their electrical conductivity, chemical stability, ion transport properties, and mechanical strength.

This review provides a comprehensive overview of MXene materials, categorizing them while highlighting their advantages in electrochemical energy storage applications. It examines recent advancements in the preparation of MXenes and optimized synthesis strategies. Detailed discussions focus on the functionalization of MXenes and their applications in energy storage devices, including supercapacitors, lithium-ion batteries, and sodium-ion batteries. The review concludes with a summary of the practical applications of MXenes and outlines future research directions to guide advancements in the energy storage field.

1. Introduction

Two-dimensional (2D) materials consist of single or few atomic layers and display remarkable physical, chemical, and mechanical properties. These unique characteristics have garnered significant attention across various fields, including electronics, energy storage, catalysis, sensors, and flexible electronics. Prominent examples of 2D materials include graphene, transition metal dichalcogenides (TMDs), and black phosphorus. Among them, MXenes have gained considerable research focus due to their multifunctional attributes. MXenes are defined as transition metal carbides, nitrides, and their compounds, featuring a distinctive 2D layered structure composed of alternating layers of transition metal atoms and carbon or nitrogen atoms. This architecture endows MXenes with a high specific surface area, superior electrical conductivity, favorable mechanical strength, and remarkable electrochemical activity, making them ideally suited for electrochemical energy storage applications.

Nevertheless, the performance of MXenes in real-world electrochemical energy storage devices is hindered by several limitations. The layered structure is susceptible to stacking and aggregation during charge and discharge cycles, reducing the available specific surface area. Additionally, the presence of functional groups like oxides and fluorides can lead to unstable reactions with electrolytes, causing surface degradation over time and impacting cycling stability. Furthermore, ion diffusion rates within MXenes are constrained by interlayer spacing and surface morphology, which may deteriorate electrochemical performance under high-rate charging and discharging conditions. Addressing these challenges is essential for optimizing the electrochemical performance of MXenes in energy storage applications.

In recent years, the functionalization of MXene materials has provided significant opportunities for enhancing their applications. By altering surface chemistry, doping with foreign elements, and fabricating compounds, the electrical, chemical, and mechanical properties of MXenes have been effectively enhanced. Modulating surface functional groups can reduce the self-stacking phenomenon and improve the surface activity of MXenes. Elemental doping, such as with nitrogen, sulfur, and phosphorus, or integrating with carbon-based materials and transition metal oxides, can boost electronic conductivity and improve interface stability and ionic transport. Moreover, adjusting interlayer spacing or constructing heterojunction structures can further enhance electrochemical performance by optimizing the “space charge effect” at interfaces.

While existing reviews primarily focus on preparation methods or performance in specific applications of MXenes, there is a lack of systematic investigations into the functionalization strategies that enhance their performance in electrochemical energy storage. This review aims to bridge this gap by summarizing recent advancements in the functionalization of MXene materials for energy storage, providing a detailed analysis of performance improvements.

2. Fundamentals of MXene Materials

2.1. Composition and Structure of MXenes

MXenes are classified as 2D materials composed of transition metal carbides, nitrides, or carbonitrides, typically following the formula Mn+1XnTx. Here, M represents transition metal elements such as Ti, V, Cr, Mo, etc.; X denotes C or N; and Tx refers to surface functional groups like -OH, -O, or -F. MXenes are synthesized through the selective etching of the A layer from the MAX phase, forming a 2D structure. In the MAX phase, M and X atoms are covalently and ionically bonded to create a hexagonal crystal structure, with alternating layers of M and X separated by the A layer. Appropriate etching conditions allow for the selective removal of the A layer while retaining the M-X layers, resulting in a 2D structure composed of transition metals and carbon or nitrogen atoms.

2.2. Advantages of MXenes

The layered architecture of MXenes provides several distinct advantages. Specifically, their large specific surface area offers numerous active sites for ion storage and electron conduction. This characteristic significantly enhances the electrode–electrolyte interface, improving charge storage efficiency and energy density while facilitating rapid ion transport, reducing diffusion distances, and enabling high power density and fast charge–discharge capabilities. Additionally, MXenes exhibit exceptional electrical conductivity, attributed to the high electron density near the Fermi level and weak van der Waals forces that allow for free electron migration within their layers.

MXenes are also characterized by strong M-N and M-C bonds, contributing to their mechanical stability. Compared to their parent MAX phases, MXenes exhibit higher strength and durability due to the accumulation of electron density between layers following the removal of the A layer. Furthermore, their tunable surface chemistry allows for modifications that adjust their electronic structure and electrochemical properties, making them adaptable for various energy storage applications.

3. Synthesis Methods of MXenes

The primary methods for synthesizing MXenes include fluoride-containing methods, fluoride-free methods, and molten salt methods. The fluoride-containing method, such as hydrofluoric acid (HF) etching, selectively removes the aluminum layer from the MAX phase, yielding highly conductive MXenes. However, this method poses safety concerns due to the corrosive nature of HF. Fluoride-free methods utilize alternative reagents to mitigate these risks and have gained popularity as safer options for synthesizing MXenes.

4. Issues with MXenes

Despite their promising electrochemical properties, MXenes face critical challenges that hinder their practical implementation. These include susceptibility to oxidation, limitations in ion diffusion and charge transfer, structural instability during cycling, electrolyte compatibility, limited conductivity, and low synthesis quality and yield.

5. Functional Modification of MXenes and Their Applications

To overcome the challenges faced by MXenes in electrochemical energy storage, various functionalization approaches have been proposed, including surface chemical modification, doping, intercalation, and composite creation. These strategies aim to enhance the electrochemical performance, stability, and versatility of MXenes, thus expanding their applicability in next-generation energy storage technologies.

6. Conclusions and Perspectives

In conclusion, MXenes exhibit immense potential for electrochemical energy storage applications due to their unique structure, excellent conductivity, high surface area, and tunable surface chemistry. Although challenges such as self-stacking, structural instability, and scalability of functionalization methods remain, significant progress in enhancing MXene performance through functionalization strategies has been made. Future research should target scalable, green synthesis methods, investigate the stability of functionalized MXenes, and explore their multifunctional applications to fully realize their potential in energy storage technologies.