MXene-Based Materials for Multivalent Metal-Ion Batteries

Abstract

Multivalent metal ion (Mg²⁺, Zn²⁺, Ca²⁺, and Al³⁺) batteries (MMIBs) emerged as promising technologies for large-scale energy storage systems in recent years due to the abundant metal reserves in the Earth’s crust and potentially low cost. However, the lack of high-performance electrode materials is still the main obstacle to the development of MMIBs. As a newly large family of two-dimensional transition metal carbides, nitrides, and carbonitrides, MXenes have attracted growing focus in the energy storage field because of their large specific surface area, excellent conductivity, tunable interlayer spaces, and compositional diversity. In particular, the multifunctional chemistry and superior hydrophilicity enable MXenes to serve not only as electrode materials but also as important functional components for heterojunction composite electrodes. Herein, the advances of MXene-based materials since its discovery for MMIBs are summarized, with an emphasis on the rational design and controllable synthesis of MXenes. More importantly, the fundamental understanding of the relationship between the morphology, structure, and function of MXenes is highlighted. Finally, the existing challenges and future research directions on MXene-based materials toward MMIBs application are critically discussed and prospected.

1. Introduction

Driven by the global carbon peaking and carbon neutrality goals, more and more attention has been focused on abundant and clean renewable energy sources, such as wind energy and solar energy [1,2]. However, these energy resources suffer from intermittent volatility; thus, they cannot meet the stability and reliability requirements of the rigid grid [3]. The lithium-ion battery is widely accepted as the suitable energy storage device to bridge renewable energy power generation to the power grid due to its high-energy density, long lifetime, and fast response capability, but it is plagued by increasing costs and safety issues [4]. In this regard, exploring alternative electrochemical energy storage technologies is particularly urgent. Magnesium, zinc, calcium, and aluminum metals are all Earth-abundant elements, and their redox reactions are always accompanied by a two or three electron transfer; hence, batteries based on these metal anodes in theory can offer much higher capacity than lithium-ion batteries [5,6]. Inspired by the above advantages, the research of multivalent metal ion (Mg²⁺, Zn²⁺, Ca²⁺, and Al³⁺) batteries (MMIBs) has flourished. However, the development of MMIBs is hindered by several critical issues. Firstly, the large radius of hydrated ions slows the solid-phase ion diffusion kinetics of carriers in the electrode material, resulting in severe attenuation of battery performance [7]. Secondly, the high electrostatic repulsion between the inserted cations and the host structure often leads to drastic structural deterioration and capacity decay [8]. Therefore, on the basis of good conductivity, small loss and easy modeling of general electrode materials, MMIB electrode materials also need to have a wide ion diffusion channel and sufficient structural stability to adapt to the enhanced electrostatic effect.

Two-dimensional (2D) materials have exotic electronic, chemical, mechanical, and optical properties that distinguish them from three-dimensional (3D) materials [9]. These properties are mainly derived from the thin layer structure of atoms, with strong intralayer bonds and weak interlaminar bonds. Their relatively large layer spacings allow for rapid ionic (de)intercalating, while the highly anisotropic 2D structure imparts the ability to transfer charges [10]. At the same time, two-dimensional (2D) materials provide a platform that allows the creation of heterogeneous structures with a variety of properties [11]. Therefore, synthesizing 2D materials containing various components can give us access to a wider range of properties and technical applications. As a newly large family of 2D transition metal carbides, nitrides, and carbonitrides, MXenes have attracted growing focus because of their large and adjustable layer spacing, rich specific surface area, good bending property, high electrical conductivity, hydrophilicity and rich surface functional groups (-F, -OH, -O, etc.) [12,13]. Naturally, since MXene was discovered by Yury Gogotsi’s group in 2011 [14], it has been applied in various fields such as sensing, photocatalysis, electrocatalysis, and energy. In terms of energy storage, Gogotsi unfolded the lithium storage activity of MXene through DFT calculations at the beginning of its discovery. Because both Na and K have similar structures and chemical properties to Li, much extensive research has been made on MXene for alkali metal battery applications [15].

To date, many review papers have summarized the advances of MXene in alkali metal-ion batteries [16]. However, MMIBs undergo quite different redox chemistry from alkali metal-ion batteries; hence, the role of MXenes in MMIBs will be different from that in alkali metal-ion batteries. Unfortunately, the study of MXene for MMIBs is still in its infancy, and the compatibility between MXenes and MMIBs is being investigated. Therefore, it is urgent to summarize the latest research progress of functionalized MXene for MMIBs. Herein, this review will first briefly introduce the synthesis and properties of MXene materials. Then, the application of MXenes in MMIBs is categorically overviewed separately in terms of magnesium ion battery, aluminum ion battery, calcium ion battery, and zinc-ion battery. Finally, the advantages, the remained challenges, and the future direction toward the practical applications of MXenes in MMIBs are deeply discussed.

2. MXene

MXene is a class of compounds with a two-dimensional layered structure. Its general formula can be written as M_n+1X_nT_x, in which M refers to some transition metal elements, such as Sc, Ti, V, Cr, Mo, Hf, Nb, Ta, W, etc. More in-depth studies show that M can be expanded to binary or even multiple metallic elements. X contains the C and/or N element (Figure 1a); T is the surface terminal functional group, generally -F, -OH, -O, etc., which is closely related to the synthesis method [17]. Among them, the early n value is 1~3 [18], and the discovery of Mo₄VC₄T_x extends the scope of research to n = 4 [19]. Later, with the diversification of synthesis mechanisms, the discovery of Mo_1.33CT_x [20], V_2−xC (x ≤ 0.05) [21], and solid solution structures [22] greatly enriched the members of the MXene family (Figure 1b).

2.1. Synthesis of MXene

The most ordinary method for preparing MXene is etching the MAX precursor. MAX [23] can be expressed as M_n+1AX_n (n ranges from 1 to 4), where M and X are the same compositions as mentioned above; A is mainly from the elements of the main group 13–15, such as Si, Al, Ge, Sn, etc. (Figure 1b). The MAX phase exists as three different types of unit cells with a six-square tightly packed structure of the space group P63/mmc. The A atomic layer in the MAX phase is sandwiched between the densely packed M layers, and the octahedral position is occupied by the X atom. In accordance with the crystal structure of MAX, M_n+1AX_n is also recognized with a layered structure, in which the two-dimensional M_n+1X_n layer is connected by the A layer. M-A is a metal bond, while M-X has both covalent and ionic bond properties, which are more stable than the M-A bond [24]. This feature makes it possible to remove the A atoms from the MAX phase to obtain MXene.

2.1.1. Fluorine-Containing Etching Method

• HF Acid Etching

Unlike the van der Waals forces between black phosphorus [25], the strong metal bonds make directional removal of the A atomic layer still challenging. Until 2011, Gogotsi et al. [14] successfully prepared accordion-like multilayer Ti₃C₂T_x by chemical etching the Al atomic layers in Ti₃AlC₂ with HF acid, which opened the prelude of etching the A-layer structure from the MAX phase by fluorine-containing etchants (Figure 2a). Thereafter, lots of MXenes have successfully synthesized with different M elements (such as (V_0.5Cr_0.5)₃C₂, Nb₂C, V₂C, TiNbC), different X elements (such as Ti₃CN_x) and different compositions (such as Ti₂C, Ta₄C₃) by HF acid etching [18,26], which promotes the universality of HF acid etching in MXene synthesis [14].

Taking the synthesis of Ti₃AlC₂ as an example, the reaction route of HF acid etching is as follows:

Ti₃AlC₂ + 3 HF → AlF₃ + 3/2 H₂ + Ti₃C₂

(1)

Ti₃C₂ + 2 H₂O → Ti₃C₂(OH)₂ + H₂

(2)

Ti₃C₂ + 2 HF → Ti₃C₂F₂ + H₂

(3)

After the reaction of HF acid with the Al layer, each M_n+1X_n group was exposed to two highly active Ti atomic sites, which spontaneously combined with -OH and -F in the solution, so the surface terminal of the HF acid etching method is randomly distributed in -OH and -F groups. As for HF etching, the HF acid concentration, etching time, and etching temperature need to be appropriately adjusted for different precursors [27,28,29]. Excessive etching time and HF acid concentration will cause defects in the synthesized MXene and even lead to the complete collapse of the structure. Conversely, insufficient etching time or HF acid concentration results in incomplete etching.

• In Situ Generation of HF

Although HF acid etching methods are easy to operate and have a wide range of applications, hydrogen fluoride and its aqueous solutions are toxic and harmful. To avoid the use of HF acid, a method of in situ generating HF acids by strong acids (HCl/H₂SO₄) and fluorine-containing salts (LiF/NaF/KF/CsF/CaF₂) has emerged. In 2014, Gogotsi et al. [30] successfully prepared Ti₃C₂T_x by etching Ti₃AlC₂ at 40 °C for 45 h with HCl/LiF. Liu et al. [31] etched V₂AlC with NaF/HCl at 90 °C for more than 48 h to obtain V₂C. This synthesis method is milder than HF acid etching, thus greatly reducing the defect content [32]. The prepared MXene is rich in the -F/-OH terminal group, which is easy to form hydrogen bonds, showing hydrophilicity and negative electricity. Due to the spontaneous intercalation of metal cations and H₂O molecules, the prepared MXene exhibits a larger layer spacing and fewer layers than that obtained by the HF acid preparation method, eliminating the subsequent peeling step.

• Other Fluoride Salts

In addition to the (in situ) HF acid method, some fluoride salts and their mixtures can also be used for etching purposes to avoid the use of strong acids. Gogotsi et al. [33] prepared Ti₃C₂ by etching 1 M NH₄HF₂ at room temperature, and the embedding of NH₄⁺ increased the layer spacing from 18.6 Å to 24.7 Å. Nitride MXene has higher conductivity than carbide, but because of the stronger Al bonding force compared to Ti_n+1AlC_n in the Ti_n+1AlN_n phase which is soluble in HF solution, the traditional synthesis method is not suitable for nitride MXene. Gogotsi et al. [34] mixed Ti₄AlN₃ powder with molten fluorine salts (59 wt% KF, 29 wt% LiF, 12 wt% NaF) and heated at 550 °C for 30 min under argon gas protection to synthesize Ti₄N₃T_x (Figure 2b), opening up the research field of nitride MXene.

MXene prepared by chemical etching is usually multilayered, it is usually followed by a peeling procedure for better application (Figure 2c). In addition to the metal cation [35] spontaneously interpolated in the etching above, the layer spacing can also be extended by dimethyl sulfoxide (DMSO) [36], tetrabutylammonium hydroxide (TBAOH) [37], isopropylamine [38], tetramethylammonium hydroxide (TMAOH) [39], the water molecule (H₂O), cetyltrimethylammonium bromide (CTAB) [40], etc., and subsequent sonication can mechanically peel off the MXene to form a few-layer suspension.

Although the fluorine-containing etching method can well break the M-A bond to complete the synthesis of a variety of MXene materials, the MXene terminal group etched by the fluorine-containing method is single and the fluoride pollutes the environment, which is also very harmful to the human body. Therefore, it is urgent to find a feasible fluorine-free etching method to prepare MXene.

Figure 2. (a) The history of MXene synthesis [41]. (a) Reproduced with permission from the American Chemical Society. (b) Schematic diagram of molten fluorine salt etching Ti₄N₃T_x [34]. (b) Reproduced with permission from RSC Pub. (c) Nb₂CT_x MXene delamination process [38]. (c) Reproduced with permission from WILEY-VCH.

Figure 2. (a) The history of MXene synthesis [41]. (a) Reproduced with permission from the American Chemical Society. (b) Schematic diagram of molten fluorine salt etching Ti₄N₃T_x [34]. (b) Reproduced with permission from RSC Pub. (c) Nb₂CT_x MXene delamination process [38]. (c) Reproduced with permission from WILEY-VCH.

2.1.2. Fluorine-Free Etching Method

Although the fluorine-containing etching method can break the M-A bond to synthesize a variety of MXene materials, the MXene terminal group etched by the fluorine-containing method is single. Therefore, with the amphoteric elements characteristics of Al in the middle of Ti₃AlC₂, Li et al. [42] designed a fluorine-free NaOH-assisted hydrothermal method etching Ti₃AlC₂ to prepare Ti₃C₂T_x (T=OH, O). This method does not produce -F termination groups, but there are special application conditions: (1) alkali concentration and hydrothermal temperature should be screened according to the precursor; (2) Phase A in the original MAX phase needs to be amphoteric/acidic atoms. In 2018, Yang et al. [43] demonstrated a high-efficiency fluorine-free etching method based on Ti₃AlC₂ anodic corrosion in binary aqueous electrolyte (1 M NH₄Cl +0.2 M TMAOH). During this process, NH₄⁺ is constantly intercalated to ensure an ongoing reaction, resulting in more than 90% of the single and double layers of MXene. Sun et al. [44] prepared monolayer and few-layer fluorine-free Ti₃C₂T_x (T=O or OH) by intercalation-alloying-expansion-micro-explosion mechanism. Huang et al. [45] first discovered that Lewis acid molten salts undergo a displacement reaction through redox to produce MXene structures, and Figure 3a shows the etching route using CuCl₂ as an example. The capacity of Lewis acids to extract electrons from element A in the MAX phase depends on their respective electrochemical redox potentials in halide fusion. According to thermodynamic calculations (Figure 3b) [46], A atoms in the MAX phase can be engraved by means of molten Lewis acid that has a higher redox potential. Therefore, their group was successful in extending this synthesis strategy to various Lewis acid molten salts (CuCl₂, ZnCl₂, FeCl₂, and AgCl) and more MAX family members with different A elements (Zn, Al, Ga, Si). This method also enables the successful preparation of MXene for specific groups (O, S, Se, Te, Cl, Br, I, NH) [47] by displacement and elimination reactions (Figure 3c,d), extending the boundaries of MAX precursors and providing significant opportunities to tune the surface chemistry of Mxene.

2.1.3. Non-Etching Methods

For the method of preparing MXene by means of the etching method, the etching conditions (etchant concentration, temperature, time, etc.) are greatly dependent on the strength of the M-A bond and n value. The prepared MXene is often inevitably defective, and the topography and number of layers are uncontrollable. Dimoulas et al. [48] and Zhao et al. [49] synthesized high-purity bare Mo₂C without terminal groups in one step by means of the chemical vapor deposition method (CVD), which can regulate the size and morphology of materials through synthetic parameters. However, whether other MXene materials can be synthesized through this method is still unknown.

Table 1. Summary of etching methods.

Etching Method		Surface Terminals	Ref.
Fluorine-containing etching	HF	-F, -OH, -O	[14]
	HCl+LiF/NaF/KF	-F, -OH, -O, -Cl	[30,31]
	NH4HF2		[33]
	LiF+NaF+KF	-F, -OH, -O	[34]
Fluorine-free etching	NaOH	-OH, -O	[42]
	Electrochemistry (NH4Cl+TMAOH)	-OH, -O	[43]
	Intercalation-alloying-expansion-micro-explosion	-OH, -O	[44]
	Lewis acid molten salt	-O, -NH, -S, -Cl, -Se, -Br, -Te	[46,47]
Non-etching method	CVD	/	[48,49]

summarizes the reagents and surface terminals in different preparation methods of MXene. Through comparison, it is found that although the fluorine-containing etching method has many unavoidable shortcomings, it is still the most popular and widely used etching method. In addition, researchers are also accelerating the research of fluorine-free preparation methods, which not only abandon toxic fluoride reagents but also greatly enrich the surface functional groups of MXene. All in all, each preparation method has its own advantages and disadvantages, so it is necessary to choose the appropriate etching method according to the desired performance of the material.

2.2. Properties of MXene

To select MXene synthesis methods according to the purpose of the experiment, understanding the influence of MXene composition and structure on properties can help us better design experiments.

2.2.1. Structural Stability

Considering the application of a material, including laboratory and industrial applications, the first thing that must be considered is the stability of the material, which is the cornerstone of the performance and application of the material in all aspects. MXene prepared by chemical etching is very similar to its parent MAX. MXene has a hexagonal dense structure in which X atoms are filled as solid solutions to octahedral interstitial sites in the tightly packed structure of M atoms. M₂X-M₂X crystals are typically packed hexagonal stacks with high stability [50]. Rosen et al. [51] experimentally confirmed that Mo₂CT_x has a stable, tightly packed hexagonal structure. In particular, Ti₃C₂ also has a densely packed hexagonal structure, so it is the first to discover and the most studied MXene material. The M₄X₃-M₄X₃ structure and the M₃X₂-M₃X₂ structure are typical face-centered cubic crystal structures, making these two materials unstable [52]. This structure can be stabilized by introducing another transition metal element, M’ (such as Ti) [53].

2.2.2. Environmental Stability

The bare MXene surface has abundant reactive sites and excellent conductivity due to the presence of transition metal-free electrons, but this also brings challenges to the long-term stable existence of MXene in the environment. Zhang et al. [54] exposed the Ti₃C₂T_x colloidal solution to air, which was completely oxidized to TiO₂ in 30 days. The stability of MXene materials under water and oxygen conditions is a key factor restricting their development, and the introduction of surface terminals, surface modification, or the construction of MXene composites to occupy the active site can effectively slow down the oxidation rate and improve the stability of MXene. For example, in Ti₃C₂T_x, the -O-containing MXene is more stable than the -F/-OH containing group [55].

2.2.3. Interlayer Stability

In MXene, the layers rely on the van der Waals force between -F/-O and the hydrogen bonds between -F/-O and -OH to link, and the strength of the hydrogen bonds between layers is determined by the number and distribution of -OH and -F/-O on the sheet [56]. The weak interlayer adhesion is conducive to peeling by various intercalators. However, it generates stack tendency due to its large surface energy, leading to hydrophilicity and conductivity attenuation in the practical application. Currently, stacking between layers can be avoided by increasing layer spacing, building three-dimensional (3D) network structures, or building composite materials. Layer spacing is influenced by a combination of surface termination, synthesis method, precursor, n value, and intercalator. The originator of MXene [14] proposed that the geometric optimization of hydroxylated and fluorinated structures led to the expansion of the original Ti₃AlC₂ lattice by 5% and 16%, respectively. Conversely, simply removing the Al atom without being replaced by a functional group would result in a 19% contraction of the structure. Therefore, the presence of surface terminals will occupy the exposed Ti atomic active site, increasing the stability of the material. At the same time, the tendency to stack can be suppressed by reducing the surface contact with the material. The vacuum filtration step of MXene during preparation is subjected to atmospheric pressure, resulting in reduced layer spacing and increased stacking. Starting from the material synthesis method, Xu et al. [57] replaced vacuum filtration with natural sedimentation, and the prepared layer spacing changed from the original 14.06 Å to 14.76 Å, showing good stability (Figure 4e). In addition, the layer spacing of MXene materials can also be adjusted by intercalation reaction or by regulating the precursor MAX phase. Typical intercalators have been described in the previous synthesis section and will not be repeated here. Zhou et al. [58] also obtained an increased layer spacing by etching the precursor V₄AlC₃ after ball milling (c value changed from 2.534 to 3.210 nm, Figure 4f). At the same time, some researchers have shown that for the same type of MXene, the larger the n value, the larger the layer spacing [5]. The increased layer spacing not only contributes to the stability of MXene materials but also exposes more active sites, broadens ion transport channels, and further activates more potential of MXene.

The construction of a three-dimensional network structure can effectively avoid stacking between layers and build an ion transport network, so as to better utilize the high conductivity of MXene material, and also has a miraculous effect in alleviating some high-volume strain materials. The template method and freeze-drying method are common methods for constructing 3D network structures. Xu et al. [59] in situ grew S particles on the surface of Ti₃C₂ by chemical reduction, after which the S particles were removed as templates at a high temperature of 300 °C to obtain flexible MXene foam with a well-developed 3D structure (Figure 4g). Gogotsi and colleagues [60] prepared porous Ti₃C₂ (p-Ti₃C₂) by rapid and simple chemical etching using in situ oxidation TiO₂ as a template at room temperature (Figure 4h), which is used as an electrode material for lithium-ion batteries with long lifetime and excellent rate performance (330 mAh g⁻¹ at 10 C). Yang et al. [61] freeze-dried the Ti₃C₂T_x/CNT nanosheet colloidal solution, the specific procedure is illustrated in Figure 4a, and the SEM of the resulting parallel arrangement of Ti₃C₂T_x/CNT (PA-MXene/CNT) structure is depicted in Figure 4b–d. It can be seen that numerous pores are formed between the nanosheets, avoiding the stacking of Ti₃C₂T_x nanosheets. Because of the large specific surface area and rich active sites of MXene, it is easy to connect with many electronic active materials by ultrasound, hydrothermal and other methods. The introduction of composite materials is often accompanied by an increase in layer spacing, and they can also be seen as special intercalators in this regard. Specific applications are presented in “Applications in Multivalent Metal-Ion Batteries (MMIBs)”.

Figure 4. (a) PA-MXene/CNT preparation scheme using unidirectional freeze-drying; (b–d) SEM image of PA-MXene/CNT [61]. (a–d) Reproduced with permission from WILEY-VCH. (e) Comparison of MXene films naturally sedimented and conventional filtered under vacuum [57]. (e) Reproduced with permission from Springer Nature. (f) Schematic showing the two different processes of V₄C₃T_x MXene [58]. (f) Reproduced with permission from ELSEVIER BV. (g) Diagram showing the chemical engraving of Ti₃C₂T_x flakes to produce a porous structure [60]. (g) Reproduced with permission from John Wiley and Sons. (h) Diagram of the porous, free-standing, and flexible 3D MXene foam prepared with an S template [59]. (h) Reproduced with permission from WILEY VCH.

Figure 4. (a) PA-MXene/CNT preparation scheme using unidirectional freeze-drying; (b–d) SEM image of PA-MXene/CNT [61]. (a–d) Reproduced with permission from WILEY-VCH. (e) Comparison of MXene films naturally sedimented and conventional filtered under vacuum [57]. (e) Reproduced with permission from Springer Nature. (f) Schematic showing the two different processes of V₄C₃T_x MXene [58]. (f) Reproduced with permission from ELSEVIER BV. (g) Diagram showing the chemical engraving of Ti₃C₂T_x flakes to produce a porous structure [60]. (g) Reproduced with permission from John Wiley and Sons. (h) Diagram of the porous, free-standing, and flexible 3D MXene foam prepared with an S template [59]. (h) Reproduced with permission from WILEY VCH.

2.2.4. Mechanical Properties

MXene inevitably carries terminal groups T_x (-F, -OH, -O) during synthesis, which is conducive to stabilizing bare MXene materials. It can also avoid the collapse of the atomic layer on the surface of bare MXene materials due to tensile strain, improving the mechanical flexibility of materials [62]. It is therefore urgent to examine the impact of terminal groups on performance. Many studies have found that the terminal group greatly reduces the elastic coefficient of intrinsic MXene [63,64], but the elastic coefficient is still higher than the original MAX phase and other two-dimensional materials (such as MoS₂) excluding graphene [65]. The elastic coefficient relationship of typical terminal groups is -O > -F > -OH, which is proved by nanoindentation experiments to measure Young’s modulus of Ti₃C₂T_x [66]. In addition, the presence of surface terminal groups can also greatly enhance the bending properties of MXene, making MXene materials show great application prospects in composite structures and flexible electronics.

2.2.5. Electronic Performance

The introduction of surface terminals can improve MXene stability but may come with a certain sacrifice in electronic performance (Figure 5). Theoretical calculations suggest that bare MXene exhibits metallic properties [67], although most bare MXenes have not been experimentally synthesized. The introduction of terminal groups turns MXene into a semiconductor or even a topological insulator. DFT calculations show that most M₂CT_x compounds with functional groups on their surfaces are semiconductors with bandgap values that typically increment according to the atomic number of “M” and with the order of -O > -F > -OH with the terminal group [68,69]. For example, after converting Sc₂CT₂ (T=F, OH, O), Ti₂CO₂, Zr₂CO_2, and Hf₂CO₂ into semiconductors, the band gap of Sc₂CT₂ (T=F, OH, O) is 1.03, 0.45 and 1.8 eV, respectively, and the band gap values of Ti₂CO₂, Zr₂CO_2, and Hf₂CO₂ are 0.24 eV, 0.88 eV, and 1.0 eV, respectively [70]. Further, electronic performance is also related to M and X types. “M” is Cr, W, and Mo, which may be topological insulators [71], while Ti₃C₂, Ti₄C₃, V₂C, and Nb₄C₃ maintain metallic properties [72]. It was also found that nitride and carbonitride MXenes have higher metallicity than carbide MXene due to the fact that nitrogen has more electrons [73].

In addition to theoretical studies, there are also studies that measure the resistivity of some MXenes experimentally. We summarize the results in

Table 2. Resistivity of some MXenes and graphene.

Name	Sheet Resistance, Ω sq−1	Resistivity, Ω m	Ref.
Ti2C	339	0.068	[18]
TiNbC	171	0.052
Ti3CNx	125	0.037
Ta4C3	104	0.021
Ti3C2	22	0.005
Mo1.33C	/	0.033	[20]
Mo2C	/	0.6
graphene	350	/	[74]

. It can be seen that the sheet resistance of some MXenes is in the same order of magnitude as graphene, reflecting excellent conductivity.

2.2.6. Surface Terminals

In addition to affecting the stability of MXene materials, surface terminals of special groups have some specific uses. The capacity of the -O terminal is significantly higher than that of -F/-OH in energy storage performance [75]. -S terminal has an adsorption effect on dissolved polysulfides and the cycling performance of the battery can be obvious when used in Li-S batteries [76]. The tunable fit of surface terminal groups begins in the material synthesis phase. When etching using fluorine-containing methods, the terminals are mainly -O, -OH, and -F [77]. -F content also depends on the type of etchant used. For example, the HF acid etching -F terminal is four times LiF+HCl [77]. Fully chlorinated MXene (T=Cl) is obtained using ZnCl₂ etching [46]. Secondly, the MXene terminal group can also be changed by post-treatment such as displacement reaction, annealing treatment, chemical method, or reaction with alkali metal hydroxide. Kamysbayev et al. [47] successfully prepared Ti₃C₂T_x containing S, Se, Te, Cl, Br, and NH terminals by displacement reaction, and a variety of groups brought different stability and electronic properties from traditional -F/-O/-OH. However, the discovery time of these groups is short, so the current research on their performance is not in-depth enough. Persson et al. [78] completed the transition from -F to -O by thermal annealing at 750 °C, and Xie et al. [55] completed the transition from -OH to -O. Although the -O band gap is slightly higher than -F, the stability of the -O-terminal group is greatly improved, and it has been proven to have the highest storage capacity, so the O-terminal group may be advantageous in energy storage batteries. The post-treatment method also expands the scope of the application of fluorine-containing etching.

However, MXene also has some challenges. As we mentioned earlier, The M₄X₃-M₄X₃ structure and the M₃X₂-M₃X₂ belong to unstable face-centered cubic structures. However, some of these structures MXene exist (Zr₃C₂, Hf₃C₂, V₄C₃, Nb₄C₃, Ta₄C₃), while others do not exist with this chemical formula at all (V₃C₂, Mo₃C₂, Cr₃C₂, Mo₄C₃, Cr₄C₃). In addition, M₂C-M₂C MXene is more active than other types and is susceptible to environmental oxidation. The stability of N-based MXene is lower than its carbide counterpart. Therefore, when using M₂C-M₂C or N-based MXene, strict oxidation resistance or rational utilization of oxidation is necessary.

Therefore, choosing the right MXene and appropriate etching method is primarily for the experiment. If used as battery electrode materials, Ti₃C₂, V₂C, Nb₂C, etc. with good conductivity are preferred. Once the type and number of layers are determined, MXene is more of a surface chemical. Terminal groups can improve the stability of the material, improve layer spacing, and affect electronic and mechanical properties. The modification of the terminal group by the post-treatment method widens the access channels of the individual terminals. In addition, the trend of MXene being stackable can be improved by many treatments, offering more potential for MXene applications in the energy storage area [79].

3. Applications in Multivalent Metal-Ion Batteries (MMIBs)

The MMIBs currently studied mainly include Mg²⁺, Al³⁺, Ca²⁺, and Zn²⁺ batteries. Because of their abundant reserves and significantly lower vivacity than lithium, the corresponding metal is commonly used as an anode in MMIBs [80,81,82]. However, due to the high charge and large hydration radius of polyvalent ions, the ion has a strong binding force with electronic materials and slow diffusion kinetics, which is not conducive to the reversibility of the battery [83]. Therefore, new challenges are posed to the electrode materials of MMIBs.

3.1. Magnesium Ion Batteries (MIBs)

Magnesium [84] is a reactive metal with a density of 1.74 g cm⁻³, a high theoretical volumetric energy density (∼3833 mAh cm⁻³), rich reserves, low price (1/24 of Li), good thermal and electrical conductivity, and low potential (Mg²⁺/Mg redox potential −2.37 V). In the periodic table, magnesium is diagonal to lithium and has similar chemical properties. Therefore, the research of MIBs as an alternative to lithium-ion batteries is of great significance for the exploration and utilization of future energy.

However, MIBs lack suitable cathode materials with high ion diffusion/intercalation rates [85] hindered by the common problems of polyvalent ions. It is expected that MXene will store magnesium because of its wide and adjustable layer spacing, high conductivity, and abundant surfactant sites. Based on theoretical calculations, Ti₂CO₂ has a theoretical magnesium storage capacity of 570 mAh g⁻¹ (comparable to the alloy anode) [55], demonstrating the magnesium storage activity of MXene material. Xie et al. [55] calculated the reaction mechanism of Mg²⁺ in MXene materials. When MXene is used as a cathode material for MIBs, Mg²⁺ will first be adsorbed to negatively charged O-MXene through electrostatic interaction due to negative surface adsorption energy. Herein, because of the positive adsorption energy of OH-MXene and the instability of F-MXene, these two are not considered. Because of the small repulsion of Mg²⁺ and the transition metal atoms in MXene and the strong shielding of negative electron clouds, Mg²⁺ is the only polyvalent metal ion that can form two intact adsorption layers. After that, the lower adsorbed Mg²⁺ is intercalated/deintercalated between the MXene layers as the reaction progressed, and the upper adsorbed Mg²⁺ is released/stored by similar deposition/dissolution mechanisms, which greatly increases the magnesium storage capacity of MXene. It is worth noting that Mg²⁺ on the adsorption layer does not form a passivation layer like magnesium metal during deposition, which is conducive to the reaction. Finally, in the presence of Mg, O-MXene undergoes a transformation reaction to decompose into bare MXene and magnesium oxide. Although the ion diffusion of bare MXene is better than that of O-MXene, in fact, due to the lack of support of stable electrolyte, the final conversion reaction has not been experimentally demonstrated, and even the dissolution/deposition reaction of the second adsorption layer does not necessarily occur. It is inevitable that the prepared MXene material contains terminals, and the polyvalent metal-ion diffusion performance in T_x-MXene is far inferior to that of alkali metal ions, which make the rate of performance slightly worse. Therefore, promoting Mg²⁺ intercalation/deintercalation by improving the adsorption and diffusion processes is a key driver for enhancing the utilization of MXene materials.

Xu et al. [40] demonstrated the ability of CTAB intercalated Ti₃C₂T_x (Ti₃C₂T_x/CTAB) for the storage of magnesium ions, whereas the original Ti₃C₂T_x did not function well (Figure 6a). The main performance improvement factor is the reduction of the diffusion barrier of Mg²⁺ on the surface of Ti₃C₂T_x/CTAB (about 0.19 eV, Figure 6c–h), which endows the prepared Ti₃C₂T_x/CTAB cathode an elevated capacity of 300 mAh cm⁻³ at 50 mA g⁻¹ (Figure 6b). Additionally, the intercalation of CTAB prevents Ti₃C₂T_x from self-stacking, improving the overall stability. Besides that, the pre-intercalated layers of Mg²⁺ [86], K⁺ [87], and nanocarbon materials [88] can also realize the magnesium storage properties of MXene materials.

Apart from being directly used as an active electrode material, MXene is an excellent substrate material due to its good laminated structure and electrical conductivity. The combination with magnesium cathode active material can stimulate the maximum potential for both of them. As a typical magnesium storage cathode material, MoS₂ is plagued by low capacity and poor cycle stability. Xu et al. [89] introduced Ti₃C₂T_x as an “enhancer” into MoS₂ by in situ hydrothermal method to prepare the MoS₂/Ti₃C₂T_x composite structure. MXene adsorbed Mo⁴⁺ as a nucleation site, and the flaky MoS₂ gradually grew to obtain thin petal-like MoS₂/Ti₃C₂T_x (Figure 7a–d). This particular topography provides more exposed active sites for Mg²⁺ storage while providing more channels for improving Mg²⁺ diffusion kinetics. In addition, MXene’s excellent electrical conductivity gives the composite excellent electron transport properties. At the same time, MoS₂ between MXene layers further inhibits the stacking of MXene materials, and the confinement effect of MXene structure synergistically alleviates the volume expansion during cycling, jointly maintaining the stability of the composite material. For these reasons, MoS₂/Ti₃C₂T_x composite reached a reinforced capacity of 165 mAh g⁻¹ at 50 mA g⁻¹, which was much higher than other MoS₂ (Figure 7e,f). Li et al. [90] synthesized TiS₂/Ti₃C₂ composites by hydrothermal assembly using Ti₃C₂ MXene as an ideal substrate. Compared with pure TiS₂ cathode material, TiS₂/Ti₃C₂ has a cumulative capacity of 97 mAh g⁻¹ for MIBs at 50 mA g⁻¹. Liu et al. [91] prepared Ti₃C₂/CoSe₂ heterojunctions by in situ growth of MOF(ZIF-67) structure on Ti₃C₂ combined with selenization (Figure 7g). Similar to the mechanism proposed by Xu et al. [89], Ti₃C₂/CoSe₂ as a cathode material for MIBs showed excellent cycle life (>500 cycles) and excellent rate performance (75.7 mA h g⁻¹ at 1000 mA g⁻¹, Figure 7h,i). During the in situ process, MXene is more susceptible to interference from external unstable factors (such as high temperature) to accelerate the oxidation process. Zhu et al. [92] prepared VS₄@Ti₃C₂/C by adding glucose to inhibit the oxidation of Ti₃C₂ by providing a reducing environment with the in situ synthetic of VS₄@Ti₃C₂. Glucose is anchored on the surface of Ti₃C₂ by forming a hydrogen bond with the oxygen-containing terminal, and finally forms a carbon film uniformly coated on the surface of Ti₃C₂ through pyrolysis, which not only achieves the purpose of inhibiting oxidation but also increases the overall conductivity of the system. VS₄@Ti₃C₂/C as a cathode has a smaller impedance than VS₄, showing a capacity of 492 mAh g⁻¹ at 50 mA g⁻¹, as well as over 900 cycles stability and excellent rate performance. Nevertheless, the in situ generation method is highly susceptible to oxidation of MXene, so the development of simpler composite combination methods is also a favorable solution. Yang et al. [93] prepared Li₄Ti₅O₁₂ nanosheets/d-Ti₃C₂ flexible film(LTO NSs@d-Ti₃C₂) by simple ultrasonic mixed filtration, which perfectly utilized the excellent kinetic properties, abundant active sites, and mutually supportive stability provided by the composite structure to achieve a self-supporting and non-current collecting flexible electrode.

In composite materials, the proportion between the constituents is vital. Too little MXene leads to excessive filling between MXene layers, which has a negative impact on ion transport; too much MXene challenges the uniformity of combinations. Li et al. [94] demonstrated that in MnO₂/V₂C composites with MnO₂ content of 20%, MnO₂ and V₂C achieve complementary advantages, where the uniform combinations facilitate the mobility of ions. Therefore, for the composite of different materials with MXene, the first thing to prove is the feasibility of compounding, in other words, to consider the stability after compounding. Followed immediately, the optimal compound ratio needed to be determined according to the actual situation.

It can be found that the preparation of MXene composites is an important part of MXene applications. Therefore, here is a brief summary of composite synthesis methods. In addition to the in situ generation method similar to MoS₂/Ti₃C₂T_x [89], in situ conversion (partial oxidation [95], vulcanization [96] or selenization [97]) and ex situ methods (electrostatic action or van der Waals self-assembly by ball milling [98], ultrasound [93], spray [99,100], chemical vapor deposition [101], etc.) can also successfully prepare MXene composites.

In order to increase MXene’s magnesium storage capacity, the researchers also turned to magnesium-lithium hybrid batteries (HMLBs). In such batteries, the cathode material needs to have good dual-ion (Mg²⁺ and Li⁺) compatibility. The storability of MXene to single ions (Li⁺, Mg²⁺), respectively, makes it a favorable candidate for HMLB electrode materials. Gogotsi et al. [102] report a free-standing flexible Ti₃C₂T_x/carbon nanotube (Ti₃C₂T_x/CNT) composite “paper” HMLB electrode that provides only 105 mAh g⁻¹ reversible capacity at 10 mA g⁻¹. Therefore, the researchers hope to increase the layer spacing by selecting other intercalators or compounding some “pillar effect” materials to achieve high-capacity HMLBs. Prelithiated-V₂C [103] as a cathode can achieve a fascinating reversible capacity of about 230 mA h g⁻¹ at a current density of 0.02 A g⁻¹. The increased interlayer spacing influenced by Li⁺ pre-embedding unlocked additional ion diffusion channels, which provides insight into the rapid reaction kinetics and high Mg²⁺ storage capacity of MXene. Similarly, Li et al. [104] obtained Ti₃C₂T_x with different layer spacing through different alkyl chain surfactant intercalation layers, indicating that Mg²⁺ intercalating/de-intercalating is only possible if the layer spacing is greater than 1.83 nm. They also pointed out that the N element in the CTAB is tightly bound to Ti₃C₂T_x, which can be regarded as N doping, further neutralizing the electronegativity of Ti₃C₂T_x so that Ti₃C₂/CTAB exhibits a reversible capacity of 115.9 mA h g⁻¹ at 0.1 A g⁻¹ in an HMLB. This method has also been experimentally proven to be applied to Mo₂C MXene [102].

At present, the application of MXene in MIBs mainly focuses on the cathode. The biggest challenge of MXene when storing Mg²⁺ independently is the slow Mg²⁺ diffusion, so various intercalation methods are often used to improve MXene’s Mg²⁺ storage performance, which is also reflected in HMLBs. Some studies on transition metal chalcogenides as cathodes for MIBs have been added to Table 3. Compared with other cathode materials, the magnesium storage capacity of Ti₃C₂T_x/CTAB can be compared with some classical MIBs cathode materials (50 mAh g⁻¹ for V₂O₅ [105], 130 mAh g⁻¹ for Mo₆S₈ [106], 85 mAh g⁻¹ for MnO₂ [107]). In composites, with the addition of MXene-Ti₃C₂, the storage capacity of magnesium ions of MoS₂ and TiS₂ is improved. MXene-Ti₃C₂ nanosheets with good electrical conductivity provide a looser structure and more active sites for the entry and exit of magnesium ions. Of course, intercalation and composite can cooperate to achieve a win-win situation, so we briefly summarize some composite synthesis methods above.

3.2. Aluminum Ion Batteries (AIBs)

Aluminum [108] is the most abundant metal element in Earth’s crust. The specific volume capacity of aluminum metal is 8046 mAh cm⁻³, which is up to four times that of lithium metal, and its mass-specific capacity is 2980 mAh g⁻¹, which is also relatively high. In addition, the low cost and environmental friendliness advantages of AIBs further promote its research progress. However, the stable cycle of Al³⁺ is closely related to the electrolyte. It was found that only the ionic electrolyte (anhydrous aluminum chloride (AlCl₃) + 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl)) can ensure the dissolution/deposition of the aluminum metal anode, in which the carriers exist in the form of AlCl₄⁻ or Al₂Cl₇⁻ [109]. Therefore, the main research is divided into two categories: one focuses on AIBs where Al³⁺ is extracted from cathode materials, and the other focuses on the two-ion reaction of AlCl₄⁻ or Al₂Cl₇⁻ as carriers at the cathode. At present, the transition metal chalcogenides (TMCs) are mainly used as cathode materials for AIBs, but their extraction process of Al³⁺ is the result of the combined action of intercalation and phase transition, thus resulting in poor cycle stability [110]. This is because of the difficulty of extracting Al³⁺ from the electrolyte and the strong interaction between Al³⁺ and the electrode material that the performance of AIBs cathode materials still needs to be further improved.

Similar to the storage mechanism of Mg²⁺ in MXene, the adsorption energy of Al³⁺ on O-MXene is slightly higher than that of Mg²⁺ because of the strong repulsion between Al³⁺ and the transition metal atoms. As a result, during the adsorption process, the adsorption layer only has a 2/3 coverage, so the intercalation/deintercalation storage mechanism of Al³⁺ is revealed [55]. However, thanks to the charge contribution of each atom (1.09 e⁻/Al, higher than 0.76 e⁻/Mg, 0.79 e⁻/Ca), MXene still has a high aluminum storage capacity. For instance, theoretical calculations show that Ti₂CO₂ has a theoretical aluminum storage capacity of 552 mAh g⁻¹ [55].

Beidaghi et al. [111] confirmed that Al³⁺ can be reversibly inserted into the V₂CT_x host. With the help of TBAOH, the expanded layer spacing improved electrochemical performance (Figure 8a,b). The optimized MXene provides a high capacity of over 300 mAh g⁻¹ at of 100 mA g⁻¹, along with an approximately 50% capacity remaining after 100 cycles at 200 mA g⁻¹ (Figure 8c–e). The embedding mechanism is revealed as follows: during the discharge process, Al³⁺ generated by dissociation of Al₂Cl₇⁻ at the electrode/electrolyte interface is inserted into the MXene interlaminar region. At the same time, the anode undergoes the process of Al³⁺ generated by Al dissolution contacting AlCl₄⁻ to form Al₂Cl₇⁻. The charging process is the opposite. Although continuous capacity deterioration was observed, the specific capacity and cycling performance reported here were the best at that time. With the deepening of the research of AIBs, Lee et al. [112] found that the Fe₂CS₂ obtained by replacing Ti with a late transition metal Fe and replacing O-terminal groups with S-terminal groups has higher capacity and kinetic properties than Ti₂CO₂. This is attributed to the redistribution of charge under the action of Fe and -S, along with the reduction of the Al³⁺ intercalation barrier by -S.

Recently, Li et al. [113] found that Ti₃C₂ possesses a 455.5 mAh g⁻¹ capacity at 100 mA g⁻¹ when used as a cathode in AIBs, but the stability is extremely poor due to the adsorption of polychloride anions by the -OH and -F terminals. Therefore, they improve the performance by building composite materials. They first ultrasonicated the mixture of CTAB and Ti₃C₂ to enlarge the layer spacing in Ti₃C₂ (Ti₃C₂@CTAB). Then, after being selenized, selenium was deposited between layers to prepare Ti₃C₂@CTAB-Se [113]. Through the intercalation of CTAB and selenium, Ti₃C₂@CTAB-Se electrode demonstrates 583.7 mAh g⁻¹ discharge capacity at 100 mA g⁻¹, and still 132.6 mAh g⁻¹ maintained after 400 cycles. It is worth noting that this cathode composite relies on AlCl₄⁻ to achieve charge storage/release, in which selenium can not only improve the surface adsorption of AlCl₄⁻ but also improve the overall capacity as an active material. Li et al. [114] synthesized D-Ti₃C₂T_x@S@TiO₂ with -S terminal by in situ deposition S into the DMSO stripped Ti₃C₂T_x layers and then high-temperature oxidation method (Figure 8f), whose working mechanism is also AlCl₄⁻ intercalation/deintercalation when used as the cathode of AIBs (Figure 8g). Unlike in the previous study, the residual S between the layers will form polysulfides dissolved in the electrolyte during the electrochemical activation process at a small current density and are not used as active materials. The D-Ti₃C₂T_x@S@TiO₂ shows a discharge capacity of 151.3 mAh g⁻¹ after 120 cycles (72.3% retention), which is about 80.0 mAh g⁻¹ higher than Ti₃C₂T_x in AIBs (Figure 8h). The improvement is mainly due to the following reasons: the -S terminal generated by high temperatures avoids the capacity decay caused by the irreversible reaction of -OH and AlCl₄⁻, and the TiO₂ generated by partial oxidation attached to the surface to maintain the overall frame to further increase the stability of MXene material. It can be seen that scientists have noticed the exploration of MXene in AIBs. However, both bare and modified MXene have the problem of rapid deterioration of capacity, and the mechanism needs to be further explained to better guide the design of cathode materials for AIBs.

Figure 8. (a) Schematic diagram of TBAOH intercalated ML-V₂CT_x for interlayer expansion; (b) XRD patterns of ML-V₂CT_x and TBAOH treated ML-V₂CT_x, showing an increased interlayer spacing of about 5.73 Å after TBAOH treatment; (c) Charge/discharge curves of TBAOH treated ML-V₂CT_x for the first five cycles; (d) Cycling performance of TBAOH treated ML-V₂CT_x cathode over 100 cycles at 200 mA g⁻¹; (e) Rate performance of a TBAOH treated ML-V₂CT_x cathode [111]. (a–e) Reproduced with permission from ACS Nano. (f) Schematic diagram of the synthesis process of D-Ti₃C₂T_x@S@TiO₂; (g) The working mechanism of Ti₃C₂T_x as a cathode material for AIBs; (h) Cycling performances of D-Ti₃C₂T_x@S@TiO₂ [114]. (f–h) Reproduced with permission from RSC Pub.

Figure 8. (a) Schematic diagram of TBAOH intercalated ML-V₂CT_x for interlayer expansion; (b) XRD patterns of ML-V₂CT_x and TBAOH treated ML-V₂CT_x, showing an increased interlayer spacing of about 5.73 Å after TBAOH treatment; (c) Charge/discharge curves of TBAOH treated ML-V₂CT_x for the first five cycles; (d) Cycling performance of TBAOH treated ML-V₂CT_x cathode over 100 cycles at 200 mA g⁻¹; (e) Rate performance of a TBAOH treated ML-V₂CT_x cathode [111]. (a–e) Reproduced with permission from ACS Nano. (f) Schematic diagram of the synthesis process of D-Ti₃C₂T_x@S@TiO₂; (g) The working mechanism of Ti₃C₂T_x as a cathode material for AIBs; (h) Cycling performances of D-Ti₃C₂T_x@S@TiO₂ [114]. (f–h) Reproduced with permission from RSC Pub.

3.3. Calcium Ion Batteries (CIBs)

Ca²⁺ carries two units of positive charge like Mg²⁺, and Ca is the fifth most abundant element in Earth’s crust, with low polarization and low reduction potential (−2.87 V), making it a promising energy system [115]. However, CIBs have developed slowly since their discovery. In the 1990s, Aurbach et al. [116] found that the dissolution/deposition of Ca²⁺ on calcium/precious metal electrodes is impossible due to the fact that the formed surface film has no Ca²⁺ transport properties. In 2016, Ponrouch et al. [117] achieved reversible dissolution/deposition of Ca²⁺ in Ca(TFSI)₂, Ca(ClO₄)₂, or Ca(BF₄)₂ electrolytes using EC: PC as a solvent, that CIBs research showed signs of recovery. Therefore, the research of CIBs is in an embryonic state, and the report of MXene for CIBs is only in the theoretical research stage as well. Because the electronic shielding layer of Ca²⁺ is not as strong as Mg²⁺, Ca²⁺ partially forms a second adsorption layer on the basis of forming at least one stable adsorption layer on the surface of MXene [55]. Through theoretical calculations, it is shown that Ti₂CO₂ has a theoretical calcium storage capacity of 487 mAh g⁻¹, which is slightly lower than Mg²⁺ since the increased relative atomic mass, but it can still be comparable to defective graphene materials [55]. Later, Dunda et al. [118] calculated that the V₃C₂/graphene composite structure has a calcium storage capacity of 598.63 mAh g⁻¹. Demiroglu et al. [119] calculated that the theoretical calcium storage capacities of V₂CO₂/graphene and Ti₂CO₂/graphene were 411.31 mAh g⁻¹ and 416.39 mAh g⁻¹, respectively, which were significantly higher than their theoretical lithium storage capacity. Further altering the surface terminals, V₂CSe₂ [120] exhibited a maximum theoretical capacity of 394.12 mA h g⁻¹ for Ca²⁺ storage.

The applications of MXene in MIBs, AIBs, and CIBs are summarized in Table 3. It can see that although MXenes’ ability to store calcium has been theoretically demonstrated, there is still a certain distance from practical application considering the adaptation of electrode materials to electrolytes.

Table 3. Applications of MXene and MXene-based composites in MIBs/HMLBs, AIBs, CIBs.

Material		Cycling Performance			Rate Performance Capacity (mAh g−1)	Ref.
		Ion Storage System	Current Density (mA g−1)	Initial Capacity (mAh g−1)
MXene	Ti2CO2	MIB	/	570	/	[55]
	Ti3C2Tx/CTAB	MIB	50	100	42 at 1 A g−1	[40]
	Mg0.21Ti3C2Tx	MIB	50	210	55 at 0.5 A g−1	[86]
	Ti3C2Tx@C	MIB	10	198.7	123.3 at 0.2A g−1	[88]
	Ti3C2Tx/CNT	HMLB	10	105	∼50 at 1 A g−1	[102]
	Prelithiated-V2C	HMLB	20	230.3	260.7 at 1 A g−1	[103]
	Ti3C2/CTAB	HMLB	100	119.5	100.5 at 2 A g−1	[104]
	Ti2CO2	AIB	/	552	/	[55]
	V2CTx	AIB	100	162	∼150 at 0.3 A g−1	[111]
	Fe2CS2	AIB	/	642	/	[112]
	Ti2CO2	CIB	/	487	/	[55]
	V3C2/graphene	CIB	/	598.63	/	[118]
	V2CO2/graphene	CIB	/	411.31	/	[119]
	Ti2CO2/graphene	CIB	/	416.39	/	[119]
	V2CSe2	CIB	/	394.12	/	[120]
MXene-based composites	MoS2/Ti3C2Tx	MIB	50	165	93 at 0.2A g−1	[89]
	TiS2/Ti3C2	MIB	50	97	/	[90]
	Ti3C2/CoSe2	MIB	20	114.5	75.7 at 1A g−1	[91]
	VS4@Ti3C2/C	MIB	50	492	129 at 1A g−1	[92]
	LTO NSs@d-Ti3C2	MIB	20	320	42 at 0.3A g−1	[93]
	MnO2/V2C	MIB	100	130	25 at 0.5A g−1	[94]
	Ti3C2@CTAB-Se	AIB	100	583.7	68.1 at 0.3A g−1	[113]
	D-Ti3C2Tx@S@TiO2	AIB	100	209.2	51.8 at 0.5A g−1	[114]
Classic cathode material	MoS2	MIB	50	62	/	[89]
	TiS2	MIB	50	58	/	[90]

Table 3. Applications of MXene and MXene-based composites in MIBs/HMLBs, AIBs, CIBs.

Material		Cycling Performance			Rate Performance Capacity (mAh g−1)	Ref.
		Ion Storage System	Current Density (mA g−1)	Initial Capacity (mAh g−1)
MXene	Ti2CO2	MIB	/	570	/	[55]
	Ti3C2Tx/CTAB	MIB	50	100	42 at 1 A g−1	[40]
	Mg0.21Ti3C2Tx	MIB	50	210	55 at 0.5 A g−1	[86]
	Ti3C2Tx@C	MIB	10	198.7	123.3 at 0.2A g−1	[88]
	Ti3C2Tx/CNT	HMLB	10	105	∼50 at 1 A g−1	[102]
	Prelithiated-V2C	HMLB	20	230.3	260.7 at 1 A g−1	[103]
	Ti3C2/CTAB	HMLB	100	119.5	100.5 at 2 A g−1	[104]
	Ti2CO2	AIB	/	552	/	[55]
	V2CTx	AIB	100	162	∼150 at 0.3 A g−1	[111]
	Fe2CS2	AIB	/	642	/	[112]
	Ti2CO2	CIB	/	487	/	[55]
	V3C2/graphene	CIB	/	598.63	/	[118]
	V2CO2/graphene	CIB	/	411.31	/	[119]
	Ti2CO2/graphene	CIB	/	416.39	/	[119]
	V2CSe2	CIB	/	394.12	/	[120]
MXene-based composites	MoS2/Ti3C2Tx	MIB	50	165	93 at 0.2A g−1	[89]
	TiS2/Ti3C2	MIB	50	97	/	[90]
	Ti3C2/CoSe2	MIB	20	114.5	75.7 at 1A g−1	[91]
	VS4@Ti3C2/C	MIB	50	492	129 at 1A g−1	[92]
	LTO NSs@d-Ti3C2	MIB	20	320	42 at 0.3A g−1	[93]
	MnO2/V2C	MIB	100	130	25 at 0.5A g−1	[94]
	Ti3C2@CTAB-Se	AIB	100	583.7	68.1 at 0.3A g−1	[113]
	D-Ti3C2Tx@S@TiO2	AIB	100	209.2	51.8 at 0.5A g−1	[114]
Classic cathode material	MoS2	MIB	50	62	/	[89]
	TiS2	MIB	50	58	/	[90]

3.4. Zinc-Ion Batteries (ZIBs)

3.4.1. Anode

Compared to other polyvalent ions, the high redox potential (−0.76 V) of Zn makes it possible to use water as an electrolyte solvent, greatly enhancing the safety of the battery [121]. As the hydrophilicity of MXene can promote the penetration of aqueous electrolytes, the application of MXene in aqueous ZIBs far exceeds other MMIBs. However, the commonly used zinc anode is prone to generate dendrites similar to Li during the dissolution/deposition in aqueous electrolytes and suffered from side reactions such as corrosion and hydrogen evolution, which seriously affects the safety and lifespan of the ZIBs [122]. The essence of dendrite formation lies in the uneven zinc nucleation site, and the “top effect” aggravates this phenomenon into a vicious circle [123]. At present, surface modification, structural design, alloyed anode, intercalated anode, and electrolyte modification methods have been used to improve the surface uniformity of zinc anode [122]. As a category of two-dimensional layered materials with excellent conductivity, hydrophilicity, and extensive specific surface area, MXene has the hope of promoting uniform zinc deposition in all these aspects.

The structural design of zinc anode is one of the effective methods to reduce dendrite growth. In aqueous ZIBs, the purpose of structural optimization is to alleviate the formation of zinc dendrites by reducing the local current density and further achieving the homogenization of the surface electric field [124]. The most common method is to load Zn onto a highly conductive substrate by electrodeposition to form a composite material. MXene’s high conductivity is considered a good conductive substrate. Tian et al. [125] prepared a Ti₃C₂T_x@Zn paper electrode by electrochemically depositing Zn onto Ti₃C₂T_x, which replaced Zn as the anode of ZIBs, showing a much better cycling performance. Good electrolyte infiltration of Ti₃C₂T_x@Zn guaranteed by Ti₃C₂T_x and fast electron transport channels and ion diffusion channels provided by the layered composite structures endow Ti₃C₂T_x@Zn with uniform charge distributions (Figure 9a,j). When 1 mAh cm⁻² is deposited at 1 mA cm⁻², the surface remains smooth and dendritic free (Figure 9b–i). On this basis, shifting the 2D structure to 3D can further promote the uniform Zn²⁺ concentration on the Zn surface by shortening the ion migration pathway. Chen et al. [126] first prepared a 3D porous structure (MGA) by freeze-drying MXene (Ti₃C₂T_x) and graphene aerogel and then used it as an electrodeposition matrix to prepare MGA@Zn anodes. Thanks to the formation of ZnF₂-rich solid electrolyte interface (SEI) induced by the -F terminal and inhibited side reactions by composite structure, MGA@Zn has a flat zinc deposition layer even at 10 mA cm⁻². Zinc powder with a large specific surface area and abundant electroactive sites has been widely used as the anode of ZIBs [127]. However, the increase of the surface area aggravates the thermodynamic and kinetic instability of zinc in the weak acidic electrolytes, which will produce a violent hydrogen evolution and corrosion reaction, so zinc powder is usually mixed with other materials to make a zinc powder composite anode. Zhi et al. [128] obtained the Ti₃C₂T_x@Zn electrode by Ti₃C₂T_x wrapping zinc powder through a solution mixing method.

Zinc surface modification is another strategy to improve the performance of zinc anodes. The aim is to achieve uniform zinc nucleation and a flat deposition layer on the surface of the zinc foil by constructing artificial interface layers [129]. Niu et al. [130] coated Ti₃C₂T_x on the surface of zinc foil(MZn) by self-assembly for the first time, inducing a homogeneous zinc deposition process. After this, various modified Mxene as a surface modification layer began to emerge. Zhi et al. [131] proposed a rare halogen-containing terminal Mxene as an artificial interface layer to regulate zinc deposition, and prepared (Ti₃C₂Cl₂, Ti₃C₂Br₂, and Ti₃C₂I₂)@Zn composite anodes. Ti₃C₂@Zn heterogeneous structure, coupled with ion adsorption with orderly regulation of halogen terminals, jointly promote uniform deposition of zinc planes (Figure 9k,l). It is worth noting that good results can only be achieved when the lattice is matched between Mxene and metal anode, such as Ti₃C₂Cl₂@Zn is the best performance here. It can cycle stably for 840 h at 2 mA cm⁻² at a capacity of 1 mAh cm⁻², and the polarization voltages are only 103 mV at 10 mA cm⁻²(Figure 9m,n). The emergence of this study has given researchers more inspiration. The zinc-ion full battery assembled with Zn/Co co-doped MnO/C (ZnCo-MnO/C) as the cathode, and Ti₃C₂Cl₂@Zn as the anode attain at least 3000 cycles at 3.0 A g⁻¹, showing good stability [132]. Qian et al. [133] constructed a composite protective film from MXene composite materials. First, S-loaded Ti₃C₂T_x (S@Ti₃C₂T_x) is combined with Zn, and then the S-doped Ti₃C₂T_x and ZnS composite film are obtained by high-temperature treatment, which is anchored on the zinc surface (S/Ti₃C₂T_x@ZnS@Zn) as a protective layer. S-Ti₃C₂T_x can effectively instruct uniform zinc deposition by uniform surface electric field distribution, reducing local current density, and mitigating volume changes during charge and discharge. The function of ZnS is to accelerate Zn²⁺ migration, further promote the homogenization of Zn²⁺ concentration, and inhibit side reactions (such as passivation, corrosion, H₂ precipitation, etc.). The synergy between them additionally improves the stability of the surface protection film. This anode with a multifunctional composite interface coating is assembled into a symmetrical battery with good rate performance and cycle stability (up to 1600 h at 0.5 mA cm⁻² under 0.5 mAh cm⁻²). Feng et al. [134] designed a flexible artificial protection interface based on self-assembled Ti₃C₂T_x nanosheets and chitosan (MX/CS) through a simple blade casting strategy, which achieved high reversibility of zinc metal anodes.

The zinc-preferred nucleation sites and uniformly interface electric field provided by the alloyed anode [127] make it a practical and effective solution for dendrite-free ZIBs. However, the violent volume expansion of such materials in the process of alloying/dealloying can easily lead to the crushing and collapse of the material structure, resulting in a sharp attenuation of material capacity. Qian et al. [135] obtained the Ti₃C₂T_x@Sb-300 composite backbone by annealing the product acquired through the substitution reaction of SbCl₃ with Ti₃C₂T_x@Zn. The volume expansion during cycling is effectively mitigated by the introduction of flexible Ti₃C₂T_x, which additionally increases the overall integrity and conductivity of the material. The original Sb electrode exhibits rapid capacity degradation, with only 136 mA h g⁻¹ capacity left after 100 cycles. Conversely, with similar initial capacities, the Ti₃C₂T_x@Sb-300 maintained 299.6 mAh g⁻¹ even after 200 cycles, indicating a significant improvement in cycle performance.

The Zn²⁺ insertion/extraction potential of intercalation anodes is generally higher than that of zinc deposition/dissolution potential, which can effectively avoid dendrite problems. MXene is often used as a modified material to compound with other intercalation anodes to improve the performance of the original material. (NH₄)₂V₁₀O₂₅·8H₂O@Ti₃C₂T_x (NHVO@Ti₃C₂T_x) [136] and Ti₃C₂T_x-TiS₂ [137] are practically used in “rocking chair” ZIBs by Yuan’s group and Ni’s group. Ti₃C₂T_x’s conductive structure and stabilizing effect on the material give the composite a competitive rate performance and excellent long-cycle stability.

Zn²⁺ is carried by electrolyte, so the significance of the electrolyte is obvious. Thus, using of electrolyte additives to control the deposition/dissolution behavior of Zn²⁺ is also an irreplaceable strategy. Ti₃C₂T_x [138] was first used as an electrolyte additive to guide uniform Zn deposition by controlling the nucleation and growth process of Zn. Ti₃C₂T_x adsorbed on Zn foils, inducing uniform initial Zn deposition by providing abundant nucleation –O terminals and subsequently participating in the formation of an intact solid electrolyte interface (SEI) film. At the same time, ion transport can be accelerated to reduce the Zn²⁺ concentration gradient at the electrode/electrolyte interface. The symmetrical battery containing Ti₃C₂T_x is stable to 1180 cycles at 2 mA cm⁻² under 1 mAh cm⁻² capacity (only 118 cycles without Ti₃C₂T_x). Because of the problems in the aqueous solutions mentioned above, alternative non-liquid electrolytes (hydrogels and solid electrolytes, etc.) have been widely experimented with for ZIBs. Due to the low water content of the non-liquid electrolyte, the adverse side reactions (corrosion and passivation) caused by the presence of H₂O molecules were avoided effectively. The key to high-performance non-liquid electrolytes is the conductive property, so high-ionic conductivity additives are often needed. Zhi et al. [139] added Ti₃C₂T_x of surface-grafted polymethyl acrylate (Ti₃C₂T_x-g-PMA) to the polyvinylidene fluoride-hexafluoropropylene (PVHF) solid polymer electrolyte (SPE), and the grafting of PMA was to improve the compatibility of PVHF with Ti₃C₂T_x. Ti₃C₂T_x greatly improves the ion conduction of SPE while reducing the crystallinity of the polymer. The SPE added by Ti₃C₂T_x can achieve a Zn²⁺ ion conductivity of 2.69 × 10⁻⁴ S cm⁻¹ at room temperature, and reversible dendrite-free plating/stripping for over 1000 h.

Because of MXene’s large specific surface area and high conductivity, it can reduce the local current density as an electrodeposition substrate or electrolyte additive, make the electric field on the surface of Zn more uniform, and achieve dendrite-free zinc deposition. Its abundant surface activity makes it easy to be perfectly compatible with other materials. When it is used as a composite material for surface modification, accommodation of alloyed anodes, and construction of embedded anodes, it can achieve a more uniform Zn²⁺ distribution, giving the battery excellent zinc deposition stability.

Figure 9. Schematic diagram of Zn deposition on the (a) Zn foil and (j) Ti₃C₂T_x MXene@Zn paper; SEM images of Zn deposition after plating capacity up to (b,f) 1, (c,g) 10, (d,h) 20 mAh cm⁻² on Zn foil and Ti₃C₂Tx MXene@Zn paper at 1 mA cm⁻², respectively; Cross-sectional SEM images of (e) Zn foil and (i) Ti₃C₂T_x MXene@Zn paper corresponding to d and h, respectively [125]. (a–j) Reproduced with permission from ACS Nano. (k) Schematic illustration of the Zn deposition process on Ti₃C₂Cl₂; (l) SEM images and EDX mapping of the Ti₃C₂Cl₂-Zn electrode after cycling with a flat and smooth surface; (m) Long cycling performance of symmetric batteries; (n) Comparison of rate performance of bared Zn metal and Ti₃C₂Cl₂–Zn symmetric batteries with a capacity of 1 mAh cm⁻² [131]. (k–n) Reproduced with permission from ACS Nano.

Figure 9. Schematic diagram of Zn deposition on the (a) Zn foil and (j) Ti₃C₂T_x MXene@Zn paper; SEM images of Zn deposition after plating capacity up to (b,f) 1, (c,g) 10, (d,h) 20 mAh cm⁻² on Zn foil and Ti₃C₂Tx MXene@Zn paper at 1 mA cm⁻², respectively; Cross-sectional SEM images of (e) Zn foil and (i) Ti₃C₂T_x MXene@Zn paper corresponding to d and h, respectively [125]. (a–j) Reproduced with permission from ACS Nano. (k) Schematic illustration of the Zn deposition process on Ti₃C₂Cl₂; (l) SEM images and EDX mapping of the Ti₃C₂Cl₂-Zn electrode after cycling with a flat and smooth surface; (m) Long cycling performance of symmetric batteries; (n) Comparison of rate performance of bared Zn metal and Ti₃C₂Cl₂–Zn symmetric batteries with a capacity of 1 mAh cm⁻² [131]. (k–n) Reproduced with permission from ACS Nano.

3.4.2. Capacitive Electrode

In addition to solving the problem of zinc anodes, the researchers also explored MXene’s own zinc storage activity. Zeng’s group [140] used MnO₂–CNTs as the cathode and Ti₃C₂T_x as the anode for charge/discharge tests, demonstrating the energy storage characteristics of hybrid capacitors. Furthermore, MXene is widely used in the field of flexible electronics because of its flexibility. Therefore, they further assembled δ-MnO₂ on carbon cloth (δ-MnO₂@CAC) and Ti₃C₂T_x on cotton cloth (MXene@COC) to prepare flexible zinc-ion hybrid capacitors(ZIHCs) [141]. The Ti₃C₂T_x, which exhibits the capacitance effect of zinc storage, keeps the power density of hybrid capacitors at a high level, even when used as cathodes in ZIHCs. Wang et al. [142] manufactured Ti₃C₂-Zn hybrid capacitors by combining Ti₃C₂ and Zn. To take the conductivity of MXene to the next level, N-doped Ti₃C₂ (N-Ti₃C₂) [143] was synthesized by a one-step hydrothermal method using urea as a N source. Compared with the C element, the additional electrons of N increase the conductivity of the material and increase the repulsion between layers, which inhibits the self-stacking trend of MXene material and gives the material good stability. The N-Ti₃C₂/Zn hybrid capacitors prepared with N-Ti₃C₂ as the cathode, Zn as the anode, and ZnSO₄ as the electrolyte had a discharge capacitance of 247.9 F g⁻¹ and an energy density of 45.54 Wh kg⁻¹ at 0.1 A g⁻¹. In addition to heteroatom doping, mixing with carbon materials is also a commonly used way to improve conductivity. Wang et al. [144] prepared Ti₃C₂T_x-reduced graphene oxide (Ti₃C₂T_x-rGO) as a cathode in ZIHCs, improving the conductivity of MXene by adding carbon materials. Ti₃C₂T_x-rGO shows a high capacitance of 128.6 F g⁻¹ at 0.4 A g⁻¹. Song et al. [145] prepared a DV₂C@CNT cathode by adding carbon nanotubes (CNTs), which exhibit a high capacity of 190.2 F g⁻¹ at 0.5 A g⁻¹ with excellent rate performance and durability. Maughan et al. [146] prepared pillared Ti₃C₂ by in situ embedding CTAB, which showed higher capacity when assembled with a zinc anode to form a hybrid capacitor. The main reason is that the embedding CTAB increases the layer spacing of Ti₃C₂, which brings more adsorption site exposure. Fan et al. [147] also prepared Ti₃C₂-Zn hybrid capacitors, but he proposed for the first time that 3D printing technology improved the topography of Ti₃C₂, providing optimized carrier transport, simple electrolyte permeation, and sufficient porosity. The application of 3D printing technology in MXene provides the possibility for more MXene shapes. Although these hybrid capacitors have excellent performance, the application of zinc anodes still faces dendrite risks. Therefore, referring to the method mentioned above, Li et al. [148] showed a cycle life of more than 10,000 times at 3.3 A g⁻¹ by alkalizing MXene-Zn composite (AMX-Zn) as the anode matched with the porous carbon as the cathode. Zhi et al. [149] used electrodeposited Zn@Ti₃C₂ anodes to improve the zinc anode cycling problem in the Ti₃C₂-Zn system.

3.4.3. Composite Redox Cathode

It can be seen that MXene’s storage of Zn²⁺ exhibits capacitive behavior, and the early application of MXene to ZIBs redox cathodes was achieved by constructing composite materials with known ZIBs active materials. In the composite system, the high conductivity of MXene can ensure the rapid transfer of electrons in the electrode and improve the rate characteristics and utilization rate of the active material. The active material is uniformly loaded on the MXene matrix, and the structural stability is effectively improved, which is conducive to improving cycling performance.

• Manganese-Based Material/MXene

Most of the cathode materials of ZIBs belong to transition metal oxides, such as redox materials based on manganese (Mn⁴⁺/Mn²⁺) and vanadium (such as V⁵⁺/V⁴⁺ in V₂O₅). Manganese oxides always suffer severe structural transformations, which will lead to large volume expansions and structural collapse during cycling. MXene can stabilize the structure by providing a stable support framework. Yan et al. [100] prepared 3D Ti₃C₂T_x@MnO₂ composites by vapor spray drying a mixed solution of Ti₃C₂T_x and manganese nitrate. Ti₃C₂T_x protects MnO₂ inside and forms a surface fold spherical shape under the impact of spray hot air flow, effectively constructing a solid and conductive 3D microflower structure (Figure 10a–c). The 3D structure improves the wettability of the electrolyte, and the high conductivity of Ti₃C₂T_x builds a three-dimensional conductive network, which greatly improves the dynamic properties of the material (Figure 10d,e). At the same time, the special 3D structure avoids the self-stacking of Ti₃C₂T_x in the cycle, and the material stability is improved. When used as ZIBs cathode, Ti₃C₂T_x@MnO₂ microflowers demonstrate a high reversible capacity (∼301.2 mA h g⁻¹ at 100 mA g⁻¹), excellent rate capability and excellent cycle stability over 2000 cycles (Figure 10f,g). Luo et al. [150] obtained MnO_x@Ti₃C₂T_x composite materials by uniformly growing manganese oxide (MnO_x) on Ti₃C₂T_x two-dimensional surface by hydrothermal method. The MnO_x@Ti₃C₂T_x composite retains Ti₃C₂T_x’s layered structure, similar to a parallel circuit. This parallel circuit distributes the current evenly in the layers to increase the upper limit of the electrode’s current tolerance, resulting in excellent zinc storage at high current densities. When the current density increases from 0.1 A g⁻¹ to 10 A g⁻¹, the capacity retention rate of the MnO_x@Ti₃C₂T_x cathode is 50%. However, the specific capacity is only 82.5 mAh g⁻¹ at 0.1 A g⁻¹. In addition to manganese oxides, ZnMn₂O₄(ZMO), a zinc-rich cathode that realizes charge storage by reversible with MnO₂, has been widely studied. However, during the cycle of ZMO, it is plagued by structural degradation (agglomeration and dissolution) and side reactions (ZnO generation), resulting in serious capacity loss during long cycles. Peng et al. [151] successfully synthesized ZMO@Ti₃C₂T_x composite structures by freeze-drying them after using the hydrothermal method. Ti₃C₂T_x inhibits the structural degradation and side reactions of ZMO while improving its kinetic performance and realizing long-cycle performance (92.4% capacity retention after 5000 cycles). In addition to compounding with Ti-based MXene, Zhu et al. [152] also uniformly anchored MnO₂ to K-embedded V₂CT_x (K-V₂C) by hydrothermal method. The K-V₂C@MnO₂ material remains 119.2 mAh g⁻¹ capacity in long-term cycles (10,000 cycles) even at 10 A g⁻¹.

• Vanadium-Based Materials/MXene

Similar to manganese oxide materials, vanadium oxide materials have poor intrinsic conductivity and vanadium dissolution problems, resulting in suboptimal cycle life and rate performance. As the first discovered MXene material, Ti₃C₂T_x was first used to study the composite with V-based zinc storage active materials. Pan et al. [153] prepared V₂O₅·nH₂O/Ti₃C₂T_x (VOM) 3D flower-like composites by one-step hydrothermal method and freeze-drying. H₂O embedding V₂O₅ and 3D structures provide rich active sites, laying the foundation for the realization of high capacity [154]. The excellent kinetic properties provided by Ti₃C₂T_x confer excellent rate properties on the composite cathode material (225 mAh g⁻¹ at 2 A g⁻¹). The alleviation of V₂O₅ deformation during cycling by the 3D structure was synergistic with the stabilization of the composite structure, achieving 60 mAh g⁻¹ capacity, which remained after 500 cycles at 1 A g⁻¹. At the same time, they also paid attention to the ratio of composites, and the lowest charge transfer impedance can be achieved with sufficient active sites at 20 wt% Ti₃C₂T_x. In addition, the high average valence state (4.67) of mixed V⁵⁺/V ⁴⁺ in H₂V₃O₈ can accommodate many inserted Zn²⁺. Wu et al. [155] constructed H₂V₃O₈/MXene composites by one-step hydrothermal method and freeze-drying. The composite structure provides a large number of feasible paths for the diffusion of Zn²⁺ and H₂O, and in synergy with the high conductivity of MXene, the composite exhibits 365 mAh g⁻¹ at 0.2 A g⁻¹ (306 mAh g⁻¹ for H₂V₃O₈), higher rate performance of 73 mAh g⁻¹ at 20 A g⁻¹ (24 mAh g⁻¹ for H₂V₃O₈), and better cycling performance up to 5600 cycles at 5 A g⁻¹ (only 900 cycles for H₂V₃O₈). However, Wei et al. [156] synthesized a flexible, independent VO₂/Ti₃C₂T_x hybrid film with a three-dimensional interleaved network using only ultrasound methods. Unlike constructing the 3D structure, Guo et al. [157] designed the Ti₃C₂T_x MXene layer on the surface of the V₂O₅ nanoplate (VPMX) by van der Waals self-assembly method under ultrasound to inhibit vanadium dissolution in the electrochemical process, thereby greatly improving its zinc-ion storage performance.

With the deepening of MXene research, the research of V-matrix active materials and V-based MXene composites began to appear. Interestingly, because it contains the same metallic elements, V-based MXene can be used not only as a frame structure but also as a precursor material. MXene’s easy oxidation allows it to partially or completely oxidize to form vanadium oxide active materials. The first to exploit the instability of MXene materials in ZIBs was Zhi et al. [158], who found a large capacity increase during cycling when they used V₂CT_x as the cathode of aqueous zinc/lithium-ion hybrid batteries. The co-embedding of Li⁺/Zn²⁺ increases the layer spacing, which in turn peels off V₂CT_x, exposing more active sites and exposing V to the electrolyte. During the cycling process, V₂CT_x is partially oxidized to V₂O₅, which is equivalent to in situ conversion to generate V₂O₅/V₂CT_x, greatly improving the capacity and rate performance of the material (Figure 10h,i). This discovery has inspired researchers, and a large number of research protocols have emerged to activate the zinc storage performance of V-based MXene in recent years. Chou et al. [159] achieved the transition from V²⁺/V³⁺ to V⁴⁺/V⁵⁺ on the surface of V₂CT_x by precisely controlling the voltage (1.8V) and activation time (2h), which is a process of electrochemical in situ oxidation to form a VO_x/V₂CT_x composite structure. The activated VO_x/V₂CT_x cathode showed a high capacity (423.5 mAh g⁻¹ at 1 A g⁻¹) and excellent rate performance (358 mAh g⁻¹ at 30 A g⁻¹). Later, Narayanasamy et al. [160] generated V₂O₅@V₂C by partially oxidizing V₂CT_x through controlling hydrothermal conditions (180 °C, 12 h), achieving high-performance Zn²⁺ storage. Tian et al. [161] used V₂CT_x as a precursor to oxidize V₂CT_x by one-step annealing to prepare a porous two-dimensional V₂O₅ material. V₂O₅ inherits the V₂CT_x accordion-like structure and has good structural stability, which guarantees fast charge transport and reversible transport of zinc ions. In addition, Qin et al. [162] achieved the oxidation of V₂CT_x using a high temperature during in situ generation of HF. The V₂O_x@V₂CT_x was directly synthesized by etching V₂AlC at 90 °C by NaF+HCl.

Chen et al. [95] successfully synthesized VO₂@V₂CO_x by combining hydrothermal and electrochemical oxidation. They first prepared VO₂@V₂C heterostructures by hydrothermal reactions of V₂C with H₂O₂ mixtures. Compared with traditional methods, the shorter length of the prepared VO₂ nanorods shortens the diffusion path. Coupled with the good conductivity and wettability of the V₂C frame, the VO₂@V₂C rate performance and stability are improved, but the performance in terms of capacity is not satisfactory. Therefore, they prepared a VO₂@V₂CO_x by increasing the electrode working voltage activation V₂C with reference to the electrochemical activation method, and the capacity was greatly improved while maintaining the rate of performance (327 mAh g⁻¹ at 5 A g⁻¹).

When the V₂CT_x is oxidized, the ion intercalation can also be performed synchronously, so as to combine the advantages of the two modification methods to obtain a high zinc storage active cathode material. Shen et al. [163] soaked V₂CT_x powder in NaOH solution to perform Na⁺ intercalation of V₂CT_x MXene to increase the layer spacing. Afterwards, the V₂CT_x of the Na⁺ intercalation was hydrothermally treated in ZnCl₂ and H₂O₂ solution, and Zn²⁺ intercalation and the oxidation of V₂CT_x were realized simultaneously. The generated Zn_xV₂O₅ (ZVO) nanoribbons are uniformly distributed on the surface of V₂CT_x, and Zn_xV₂O₅∙nH₂O-V₂CT_x (VC-ZVO) hybrid materials are obtained with high conductivity and large lattice channels. Electrochemical kinetic tests show that the pre-embedded Zn²⁺ effectively increases the lattice spacing of vanadium oxides, while the V⁴⁺ generated by in situ oxidation can provide more d-electrons, thereby reducing the activation energy of charge transfer at the electrode/electrolyte interface. In addition, the charge redistribution at the heterogeneous interface between ZVO and V₂CT_x weakens the electrostatic interaction of Zn²⁺, which promotes its diffusion. Therefore, the VC-ZVO electrode shows amazing electrochemical performance, and 96.4% capacity retention can be achieved after 8000 cycles at 10 A g⁻¹. Using a similar method, the same group synthesized pre-embedded composites Mn_xV₁₀O₂₄·nH₂O@V₂CT_x (MVO@VC), Li_xV₂O₅·H₂O@V₂CT_x (LVO@VC) and Al_xV₂O₅· H₂O@V₂CT_x [164]. As cathode materials for ZIBs, they exhibit improved performance.

In the process of studying the electrochemical in situ oxidation of V₂CT_x, Zhi et al. also found that the XRD peak strength of the residual precursor V₂AlC weakened and disappeared during the cycling process. Therefore, they replaced the cathode material with V₂AlC in the same electrolyte system (21 M LiTFSI + 1 M Zn (OTf)₂) [165], with zinc metal as anode assembled into ZIB [166]. During the cycle, V₂AlC is in situ etched into V₂CT_x MXene by a fluorine-rich electrolyte inside the battery. This method is a one-step green etching, peeling method without any acid/alkali to prepare MXene. ZIB always works normally during the etching process and exhibits a gradually increasing specific capacity with the formation of MXene and the oxidation product V₂O₅.

Figure 10. (a) Schematic of 3D Ti₃C₂T_x@MnO₂ microflower synthesis progress. (b,c) SEM images of 3D Ti₃C₂T_x@MnO₂ microflowers with different magnifications. (d) Contact angles of droplets of electrolyte on the surface of MnO₂ and 3D Ti₃C₂T_x@MnO₂ microflower. (e) Comparison of EIS for MnO₂ and 3D Ti₃C₂T_x@MnO₂ microflower cathodes in aqueous ZIBs. (f) Long-term cycling stability at 500 mA g⁻¹; (g) rate performance of 3D Ti₃C₂T_x@MnO₂ microflowers in aqueous ZIBs [100]. (a–g) Reproduced with permission from the Royal Society of Chemistry. (h) Schematic diagram of possible structural changes in the V₂CT_x cathode during cycling. (i) Long-term cycling performance of V₂CT_X cathode at 10 A g⁻¹ [158]. (h,i) Reproduced with permission from ACS Nano.

Figure 10. (a) Schematic of 3D Ti₃C₂T_x@MnO₂ microflower synthesis progress. (b,c) SEM images of 3D Ti₃C₂T_x@MnO₂ microflowers with different magnifications. (d) Contact angles of droplets of electrolyte on the surface of MnO₂ and 3D Ti₃C₂T_x@MnO₂ microflower. (e) Comparison of EIS for MnO₂ and 3D Ti₃C₂T_x@MnO₂ microflower cathodes in aqueous ZIBs. (f) Long-term cycling stability at 500 mA g⁻¹; (g) rate performance of 3D Ti₃C₂T_x@MnO₂ microflowers in aqueous ZIBs [100]. (a–g) Reproduced with permission from the Royal Society of Chemistry. (h) Schematic diagram of possible structural changes in the V₂CT_x cathode during cycling. (i) Long-term cycling performance of V₂CT_X cathode at 10 A g⁻¹ [158]. (h,i) Reproduced with permission from ACS Nano.

• Other Categories Materials/MXene

In addition to Mn-based and V-based cathode materials, MXene materials can also be compounded with other categories of ZIBs cathode materials. Layered transition metal chalcogenides (TMCs) are also potential candidates for ZIBs cathode materials, and low conductivity and collapse-prone layered structures are the main factors plaguing the cycling performance of TMCs. Long et al. [167] synthesized 1 T-MoS₂/Ti₃C₂ heterojunctions by in situ hydrothermal method (Figure 11a), and used them as ZIBs cathode material to demonstrate high capacity, rate performance (284.3 mAh g⁻¹ at 0.10 A g⁻¹ and 105.2 mAh g⁻¹ at 10.00 A g⁻¹) and cycle stability (93.2% capacity retention after 3000 cycles, Figure 11b), all higher than pristine 1 T-MoS₂. The elevated capacity is due to the richer ion storage space caused by the expansion layers of 1 T-MoS₂. Excellent rate capability thanks to ultrafast electron and ion transport from Ti₃C₂. The outstanding long-term cycle stability is attributed to the synergy effect of 1 T-MoS₂ and Ti₃C₂ in 3D interconnected networks. Gao et al. [168] also applied MXene to the MoO₃ cathode with high capacity and low conductivity to prepare MoO_3−x/Ti₃C₂T_x composites. MoO_3−x/Ti₃C₂T_x, as the ZIBs cathode, provides an ultra-large capacity of 369.8 mAh g⁻¹ at 0.20 A g⁻¹, maintaining 46.7% capacity after 1600 cycles. Inspired by in situ oxidation of V-based MXene to produce vanadium oxides, Sun et al. [97] proposed a gentle one-step surface selenization strategy, which has successfully synthesized VSe₂@V₂CT_x, TiSe₂@Ti₃C₂T_x, and NbSe₂@Nb₂CT_x. MXene’s surface metal atoms are converted into high-capacity transition metal selenides (TMSe₂), while MXene’s inner layer is deliberately preserved to improve overall conductivity and structural stability. Typically, VSe₂@V₂CT_x cathode provides a high-rate capability (132.7 mA h g⁻¹ at 2.0 A g⁻¹) and long-term cycling capability (93.1% capacity retention at 2.0 A g⁻¹ after 200 cycles). Similarly, many cathode materials suffer from low conductivity, including two-dimensional layered double hydroxides (LDHs). Han et al. [169] obtained Co-doped NiMn-LDH@V₂CT_x (CNMV) by a one-step hydrothermal method. V₂CT_x as a frame structure improves the overall conductivity, and the specific capacity of CNMV as ZIBs cathode material is 322.7 mAh g⁻¹ after 100 cycles at 0.2 A g⁻¹. In addition to compounding with highly conductive materials, finding active materials with high intrinsic conductivity is also an option. Studies have found that many conductive polymers exhibit energy storage activity. Polyaniline (PANI) has been applied to ZIBs as a typical conductive polymer, but volumetric strain and protonation hinder its stable cycling. Huang et al. [170] used sulfonated group grafting Ti₃C₂T_x MXene (S-Ti₃C₂T_x) with high conductivity, good flexibility to composite with PANI. S-Ti₃C₂T_x continuously contributes to protons and locally delivers high acidity, keeping the polymer at a low pH, effectively inhibiting the deprotonation of PANI (Figure 11c). Consequently, high discharge capacity (262 mAh g⁻¹ at 0.5 A g⁻¹), excellent rate performance (160 mAh g⁻¹ at 15 A g⁻¹, Figure 11d) and cycle stability (more than 5000 cycles) are simultaneously demonstrated on the S-Ti₃C₂T_x-enhanced PANI cathode.

3.4.4. Redox Cathode

Previous studies have shown that MXene itself exhibits capacitive behavior for Zn²⁺ storage. However, inspired by the conversion of V-based MXene, changes in voltage and surface terminal groups may stimulate the redox behavior of MXene. Nb₂CT_x/Zn battery exhibits capacitive behavior when the voltage is lower than 2.0 V, and Zhi et al. successfully activated the Nb₂CT_x zinc storage voltage platform at 1.55 V by extending the voltage window from 2.0 V to 2.4 V [171]. The results of molecular dynamics simulation tell us that sufficient energy can be assimilated to pierce the embedding barrier when the voltage reaches a certain value. Zn²⁺ is further embedded between layers from simple surface adsorption to exhibit a redox plateau. They also increased the voltage to activate the redox zinc storage behavior of Ti₃C₂T_x, which is expected to be applied to more MXene materials. Because the activation voltage is much higher than the electrochemical window of water (1.23 V), the implementation of this method requires the guarantee of high salt concentration.

The above research on multivalent ion batteries basically focuses on M materials in MXene, and little is known about X materials, let alone T_x, which makes the rich surface chemistry of MXene not reflected in multivalent ion batteries. In 2021, Zhi et al. [172] first studied the effect of terminal groups on the zinc storage performance of Ti₃C₂, and successfully activated the redox activity of Ti₃C₂ by changing the terminal group. As mentioned above, Ti₃C₂Cl₂ and Ti₃C₂(OF) exhibit zinc storage capacitance behavior without an obvious plateau. Varying their terminal groups, Ti₃C₂Br₂ and Ti₃C₂I₂ successfully exhibited different discharge plates with capacities of 97.6 and 135 mAh g⁻¹, respectively (Figure 12). Taking Ti₃C₂Br₂ as an example to illustrate its zinc storage mechanism, there is a 2 Br⁻ −2 e⁻↔ 2 Br⁰ redox reaction during the electrode charge/discharge process, followed by the following reaction Br⁻ + (n − 1) Br⁰ ↔ Brⁿ⁻ (3 ≤ n ≤ 5), and finally Zn²⁺ achieves deintercalation/intercalation. The success of this experiment provides strong proof of the surface chemistry of MXene and will greatly increase the confidence of researchers in the exploration of MXene surface chemistry. The applications of MXene in ZIBs are summarized in

Table 4. Applications of MXene and MXene-based composites in ZIBs.

Application	Material	Cycling Performance			Rate Performance Capacity (mAh g−1) [Overpotential of Zn Deposition (mV)]	Ref.
		Current Density (A g−1)	Capacity Retention (mAh g−1)	CycleNumber
Anode *	Ti3C2Tx@Zn	1	1	300h	∼83 at 1 mA cm−2	[125]
	MGA@Zn	10	1	1050h	∼64 at 10 mA cm−2	[126]
	Ti3C2Tx@Zn	1	0.5	200h	30 at 1 mA cm−2	[128]
	MZn	0.2	0.2	800h	112 at 5 mA cm−2	[130]
	Ti3C2Cl2@Zn	2	1	840h	103 at 10 mA cm−2	[131]
	S/Ti3C2Tx@ZnS@Zn	0.5	0.5	1600h	142 at 10 mA cm−2	[133]
	MX/CS	1	1	2100h	~300 at 10 mA cm−2	[134]
	Ti3C2Tx@Sb-300	0.5	0.5	1000h	~300 at 10 mA cm−2	[135]
	Ti3C2Tx additive	2	1	1180	~160 at 4 mA cm−2	[138]
	Ti3C2Tx-g-PMA	0.1	0.1	1200h	/	[139]
	NHVO@Ti3C2Tx	500	331.8	500	135.1 at 5 A g1	[136]
	Ti3C2Tx-TiS2	125.7	129.6	/	97.5 at 1.2566 A g1	[137]
MXene-based composites	3D Ti3C2Tx@MnO2	0.5	237	2000	202.2 at 2 A g−1	[100]
	MnOx@Ti3C2Tx	5	50	400	~35 at 10 A g−1	[150]
	ZMO@Ti3C2Tx	1	159.5	5000	84.5 at 4 A g−1	[151]
	K-V2C@MnO2	10	119.2	10,000	87.7 at 15 A g−1	[152]
	V2O5·nH2O/Ti3C2Tx	0.1	223	50	225 at 2 A g−1	[153]
	H2V3O8/MXene	5	159.3	5600	73.1 at 20 A g−1	[155]
	VO2/Ti3C2Tx	2	126.6	7000	94.05 at 5 A g−1	[156]
	VPMX	10	∼99.5%	5000	243.6 at 5 A g−1	[157]
	V2CTx	10	202	16,000	141.8 at 20 A g−1	[158]
	VOx/V2CTx	30	283.7	2000	358 at 30 A g−1	[159]
	V2O5@V2C	4	252.3	2000	290 at 4 A g−1	[160]
	Derivative V2O5	2	279	3500	250.4 at 8 A g−1	[161]
	V2Ox@V2CTx	1	87.74	200	84 at 2 A g−1	[162]
	VO2@V2COx	5	327	1500	327 at 5 A g−1	[95]
	VC-ZVO	10	223.9	8000	184.3 at 20 A g−1	[163]
	MVO@VC	10	287.6	25,000	235.5 at 22 A g−1	[164]
	V2AlC	5	240	1500	97.5 at 64 A g−1	[166]
	1 T-MoS2/Ti3C2	1	177	3000	105.2 at 10 A g−1	[167]
	MoO3−x/Ti3C2Tx	4	51.65	1600	110.6 at 4 A g−1	[168]
	VSe2@V2CTx	2	158.1	600	132.7 at 2 A g−1	[97]
	CNMV	0.2	322.7	100	51.4 at 5 A g−1	[169]
	S-Ti3C2Tx/PANI	15	103	5000	160 at 15 A g−1	[170]
Redox cathode	Nb2CTx	1	121	1800	77 at 6 A g−1	[171]
	Ti3C2Br2	4	80%	1000	67.2 at 15 A g−1	[172]
	Ti3C2I2	4	80%	700	101 at 15 A g−1	[172]
Classic cathode material	MnO2	0.5	~75	2000	125.4 at 2 A g−1	[100]
	V2O5·nH2O	0.1	142	50	2 at 2 A g−1	[153]
	1 T-MoS2	1	~100	1000	78.5 at 10 A g−1	[167]

* When used as an anode, the current density unit is mA cm⁻², and the capacity unit is mAh cm⁻².

.

In summary, MXene is used independently as an electrode material for MMIBs with the following challenges: slope curves that only reflect capacitive effects, such as the zinc storage process of Ti₃C₂; the low diffusivity brought by the large radius of polyvalent ions, which is most obvious in MIBs; and the problem of the collapse of the structure during cycling caused by the high charge of carriers, which is most obvious in AIBs. Nevertheless, the application of MXene in MMIBs is developing rapidly. The researchers not only explored the multivalent ion storage activity of MXene itself but also utilized the high conductivity and surface activity of MXene as a substrate material and artificial interface layer to achieve excellent cycling and rate performance. Although many excellent results have been achieved, they are generally concentrated in several classes of MXene materials, such as Ti-based (Ti₃C₂, Ti₂C) and V-based (V₂C). The calcium storage activity of MXene is only at the theoretical level, and there is no experimental study to prove it. Research on the surface chemistry of MXene by changing terminals is also limited, as is topography.

4. Conclusions

Herein, we first introduce the synthesis methods of MXene materials, including fluorine-containing and fluorine-free etching methods as well as non-etching methods. Generally, the subsequent peeling process is closely followed, and the obtained few-layer MXene not only avoids agglomeration but also provides more active sites. The stability, mechanical and electronic properties of MXene were then analyzed. In order to make the MXene material stable, its surface-active sites should be protected as much as possible, and secondly, interlayer stacking can be avoided by expanding layer spacing, constructing three-dimensional structures, and compounding with other materials. The excellent elastic coefficient is a strong contender for flexible electrodes, and the desired electronic properties can also be achieved through the design of terminal groups. Finally, we focus on practical applications in MMIBs. Because of its large specific surface area, hydrophilicity, and high conductivity, it is used as an electrode coating and electrolyte additive, greatly enriching the diversity of MXene applications. We found that MXene has polyvalent ion storage activity, but has the characteristics of low capacity and poor cycling performance. One feasible approach is to activate MXene energy storage activity by electrochemistry or surface terminal groups, and another method is to use intercalated substances or to construct composite materials with other active materials to achieve high capacity, exceptional rate performance, and long-cycle life.

5. Outlooks

The research on MXene materials is rich and diverse, but there is still a long way to go to meet the requirements of industrial production. There are still a number of issues that deserve our attention and need to be addressed urgently.

(1) The stability of MXene material. MXene materials are very sensitive to oxygen and water. The high surface activity makes the layered structure of MXene prone to collapse, and it is important to stabilize MXene materials in practical applications.

(2) Synthesis of MXene material. So far, more than 30 kinds of MXene materials have been discovered, but there are more than 70 MAX precursors. Each MXene has its own unique etching parameters, so matching the synthesis conditions for each MXene is essential to broaden the MXene family.

(3) MXene species. The application of MXene is still only in the common Ti₃C₂, V₂C, etc., but different components of MXene may bring unexpected results. For example, nitrogen-doped carbides and nitride MXene exhibit higher electrical conductivity, while there is little research on nitride MXene due to synthesis difficulty. Therefore, putting more types of MXene into practice is a major topic for subsequent research, in which the issue of cost should also be considered.

(4) Terminal group of MXene material. MXene material has rich surface chemistry, which can change the terminal group to change its properties, thereby broadening the application scenarios. Although MXene materials of various terminal groups such as NH, S, Cl, Se, Br, Te, etc. can be prepared by post-treatment methods, it is still very rare to apply them to practice. The general lack of attention to the terminal groups on the surface undoubtedly deprives MXene materials of a significant advantage.

(5) Composite structure. Because of its high surface activity and high electrical conductivity, MXene is often used to build composite materials with other materials. However, whether all materials can be combined and whether they can have a favorable effect on the performance after compounding should be summarized in this regard. The optimal composite ratio should also be discussed.

(6) Application in MMIBs. Because of the inherent problem of multivalent ions as carriers, there is still a huge space for exploration in the application of MXene in MMIBs. For example, the application of MXene in CIBs is still in the theoretical research stage, and experiments are urgently needed to prove it. The second adsorption layer of MIBs can exhibit dissolution/deposition behavior, and how to release the capacity of this part is also a question worth considering. The problem of actual carriers in AIBs is also worth studying. Whether the method of activating more redox activity of ZIBs can be extended to other materials or other fields will also be an interesting research topic.

(7) MXene exhibits the characteristics of pseudocapacitance in the storage of MMIBs, and although it has high conductivity, its low capacity cannot meet the needs of high-energy devices. Fortunately, composites partially compensate for these shortcomings, but they are still a long way from practical applications. Commercial LiFeO₄ materials can still maintain 90% capacity after more than 6000 cycles at 1 C, while MXene is rarely achieved even based on composite materials. However, because MMIBs are different from LIBs, this comparison is not convincing. Therefore, determining the application scenarios and evaluation criteria of MMIBs is convenient for more objective evaluations. Another limitation is that the amount of material used in the lab is small and the cost is often rarely considered. In this regard, titanium-based MXene is the most attractive because precursors (i.e., Ti, Al, C) are relatively abundant and inexpensive.

To date, there have been many fantastic research works on the application of MXenes in MMIBs. It is believed that the application of MXene will be extended to the fields of supercapacitors and catalysis on the basis of these achievements.