MXenes have attracted intensive research attention owing to its unique two-dimensional layered structure, high specific surface area, excellent conductivity, superior surface hydrophilicity and chemical stability. In recent years, selectively etching the A element layers from MAX phases by fluoride-containing etchants (HF, LiF-HCl, etc) is a common method to prepare multilayer MXenes with plentiful surface terminations. Due to the pollution problems of fluoride-containing etchants, at present, many studies have been reported on the use of more green and environmentally friendly fluorine-free etchants (NaOH, ZnCl2, etc) to etch MAX phases. The properties of MXenes are closely related to its structure. Additionally, it is found that the preparation methods have great impacts on the layer spacing and surface terminations of MXenes, consequently affecting its performance. Hence, this paper summarizes and compares the research progress of the preparation strategies, layer spacing and surface terminations regulation of MXenes. Then the applications of MXenes in electrochemical energy storage are outlined. Finally, the challenges and prospects for the future development of MXenes are also proposed.
Contents
1 Introduction
2 Preparation of MXenes
2.1 Preparation of MXenes by fluoride-containing etchants
2.2 Preparation of MXenes by fluoride-free etchants
3 Structure regulating of MXenes
3.1 Interlayer spacing regulating of MXenes
3.2 Surface terminations controlling of MXenes
4 The applications of MXenes in electrochemical energy storage
Keke Guan, Wen Lei, Zhaoming Tong, Haipeng Liu, Haijun Zhang.
Synthesis, Structure Regulating and the Applications in Electrochemical Energy Storage of MXenes.
Progress in Chemistry, 2022, 34(3): 665-682 DOI:10.7536/PC210301
Since Geim et al. First prepared graphene by mechanical exfoliation in 2004, two-dimensional materials have attracted wide attention of researchers at home and abroad because of their unique structure and excellent physical and chemical properties, which are completely different from bulk materials[1⇓⇓⇓⇓~6]. Up to now, more than 100 kinds of two-dimensional materials have been successfully prepared and reported.Uch as carbon nitride, black phosphorus, layered double hydroxides, transition-metal oxides such as δ-MnO2, MoO3, transition-metal sulfides such as MoS2, MoSe2,WS2, WSe2) and transition metal carbides, nitrides and carbonitrides (e.g., Ti3C2Tx,Nb4C3Tx 、V2CTx 、Ti2NTx 、UTi3CNTxMXenes for short), etc[7,8][9][10][11,12][13][14⇓⇓⇓~18]. Because of its unique physical and chemical properties, MXenes has shown broad application prospects in electrochemical storage, catalysis, hydrogen storage, adsorption, photothermal conversion, sensors and biomedical fields[19⇓⇓~22].
The general chemical formula of MXenes is XnTx, where M represents a transition metal, X is C, N, or CN, n = 1, 2, and 3,Tx represents surface terminal groups — O, — OH, and — F, etc. (Figure 1A)[22⇓~24]. Since Gogotsi et al discovered and reported Ti3C2TxMXene in 2011, more than 30 kinds of MXenes with clear structures and properties have been prepared[22⇓⇓~25].
Due to the high cost and low yield of direct synthesis of MXenes by chemical vapor deposition (CVD) and atomic layer deposition (ALD), researchers prefer to selectively etch the A-layer atoms in the MAX phase precursor to prepare MXenes[25,26][23]. The results show that the MAX phase belongs to the close-packed hexagonal system, in which the A-layer atoms are intercalated into the alternating layers formed by M and X, and the M-X bonds have mixed characteristics of covalent, metallic and ionic bonds with high bonding strength[23,24]. However, the M-a bond is mainly metallic bond, and the bonding strength is relatively weak[24]. Therefore, taking advantage of the difference in the bonding strength of the M — a bond and the M — X bond, the a layer can be selectively etched by chemical means without breaking the M — X bond (fig. 1b)[21,22,24].
The MXenes obtained after selective etching are generally multilayer two-dimensional materials, and the multilayer MXenes can be peeled off by simple ultrasonic treatment to obtain single-layer or few-layer MXenes nanosheets. The interlayer spacing of MXenes materials selectively etched by different preparation methods is slightly different. Researchers usually adjust the interlayer spacing by introducing intercalation agents and constructing three-dimensional structures. A larger interlayer spacing can not only improve the specific surface area of MXene, increase the active sites, but also broaden the ion transport channels, thereby affecting its electrochemical performance[27,28]. In addition, the surface of Xn obtained after etching has high chemical activity, which can easily combine with the functional groups in the etchant to form MXenes ( XnTx) with surface groups. Different etching environments will lead to different types and quantities of groups on the surface of MXenes, which will affect the surface hydrophilicity, ion adsorption and diffusion rate of MXenes materials. In addition, the existence of these surface groups can also affect the band structure, conductivity and work function of MXenes[27⇓~29].
At present, most of the reviews on MXenes focus on their preparation and application, but few on their structural regulation. In this paper, the preparation methods of MXenes in the literature are summarized and compared, the latest research progress in the regulation of interlayer spacing and surface groups of MXenes is summarized, and the effects of these regulation methods on the structure of MXenes are compared. At the same time, the application of MXenes in electrochemical energy storage is also summarized. Finally, the current challenges of MXenes materials are analyzed, and the future development direction is prospected.
2 Preparation of MXenes
The commonly used preparation methods of MXenes are mainly divided into two kinds: one is the etching method with fluorine-containing etchant; The other is the fluorine-free etchant etching method, and the common process conditions for preparing MXenes by different etching methods are shown in Table 1.
2.1 Preparing MXenes by fluorine-containing etchant
Fluorine-containing reagent etching is to use the action of fluoride ions to achieve selective etching. Commonly used etchants include HF solution, fluoride salt and strong acid mixture, fluoride hydride solution and molten fluoride salt.
In 2011, Gogotsi et al. First reported the successful etching and preparation of Ti3C2TxMXene at room temperature using 50 wt% HF solution (Figure 2a)[23]. In general, the Ti3C2TxMXene prepared by etching the Ti3AlC2MAX phase with HF solution is accordion-like (fig. 2b, C). Because of its simple operation and high repeatability, this method is widely used to prepare various MXenes materials.Uch as Ti2C, V2C, Nb2C, Ti3CN,V4C3, Nb4C3, and Ta4C3 (Table 1)[30][31][32][30][40][41][30]. The process conditions (mainly including HF concentration, etching time and temperature) for the preparation of MXenes by HF etching are largely dependent on the structure, atomic bonding and particle size of the MAX phase[56]. The results show that the concentration of HF solution, etching temperature and etching time are different for each MXene[23,31]. The larger the atomic number of M and the larger the value of n in the XnTx of , the higher the concentration of HF solution or the longer the etching time is required[56,57]. On the one hand, the bond energy of M-a in the MAX phase is different due to the different atomic number of M. The larger the atomic number of M is, the larger the bond energy of M-A is, and the longer time or higher HF concentration is needed[24]. For example, the Ti-Al and Nb-Al bond energies in Ti2AlC and Nb2AlC are 0.98 eV and 1.21 eV, respectively, and therefore, etching of the Al layer in Nb2AlC requires a longer time or a higher HF concentration than that in Ti2AlC[58][30,32]. On the other hand, the larger the value of n in the XnTx, the better the stability of MAX, and the longer time or higher HF concentration is required[24]. Therefore, the Ti2C MXene can be obtained by etching the Ti2AlC MAX with 10 wt% HF solution for 10 H, while the Ti3C2TxMXene can be prepared by etching the Ti3AlC2MAX with the same conditions for at least 24 H[30][35].
In addition, the researchers also applied this selective etching method to some layered compounds with similar structure to MAX phase, and successfully prepared MXenes[33,36,37]. For example, the researchers used 48 wt% HF solution to etch the Ga layer in the Mo2Ga2C precursor, and successfully prepared Mo2CTxMXene at 140 ℃ for 96 H[34].
Although the HF etching method has the advantages of simple process and strong versatility, it is found that the crystal structure of MXenes will be destroyed by HF if the etching time exceeds 24 H with high concentration of HF solution, and some physical and chemical properties of the prepared MXenes will also be degraded[59]. In addition, HF solution is toxic and harmful to human body and environment, so it is not suitable for large-scale production.
In order to solve the problem of HF etching,Researchers also proposed to use fluoride salts (LiF, NaF, CaF2, KF and NaBF4, etc.) And strong acids (HCl and H2SO4) to mix and react in situ to produce HF.Fluoride hydride solutions (such as NaHF2, KHF2, and NH4HF2) and molten fluoride salts. Gogotsi et al. Etched the Ti3AlC2MAX phase using two different paths with LiF and HCl as etchants, and prepared a high-quality monolayer Ti3C2TxMXene sheet (Figure 2D)[45]. It is found that the prepared Ti3C2Tx sheet not only exhibits high conductivity and field-effect electron mobility, but also has high stability, and can maintain high conductivity even if exposed to air for more than 24 H. Peng et al. Used NaBF4 and HCl as etchants to etch Ti3AlC2 and Nb2AlC at 180 ℃ for 16 H and 20 H, respectively, by hydrothermal method, and prepared Ti3C2 and Nb2C MXene[32]. The results show that compared with the traditional etching of HF solution, the etching method of in-situ synthesis of HF by fluoride salt and strong acid not only removes the Al layer atoms to a higher degree, but also has a milder etching condition.
Feng et al. used NaHF2, KHF2, and NH4HF2 aqueous solutions as etchants, respectively, to selectively etch the A layer of the Ti3AlC2MAX phase (Figure 2e), and studied the effects of different etchants, etching temperatures, and times on the physical and chemical properties of the prepared Ti3C2TxMXene[49]. The results show that when NaHF2 and KHF2 are used as etchants, the Ti3C2TxMXene can be prepared at least at 60 ℃ for 8 H. When NH4HF2 is used as the etchant, the Ti3C2TxMXene can be etched at 60 ℃ for only 2 H.
At present, most of the preparation methods of MXenes mainly use water as the solvent, and the intercalation water and — O/— OH groups are inevitably introduced in the preparation process, which greatly limits the direct use of MXenes in water-sensitive nanocomposites and energy storage electrode materials[60]. In order to solve this problem, Barsoum et al. Used organic solvents (propylene carbonate, methyl pyrrolidone, acetonitrile, etc.) As solvents and NH4HF2 as etchant to selectively etch the Ti3AlC2MAX phase and prepare water-free intercalated high-fluorine Ti3C2TxMXene[61]. The results show that the solvent can be recovered and reused when the low-boiling organic solvent (such as acetonitrile) is used as the solvent for etching, but the reaction mechanism of this method is not clear and needs further study.
In 2011, Barsoum et al. Reported that the selective etching of A-layer atoms in MAX phase could be achieved by using high-temperature molten salt (LiF), and successfully synthesized Ti-C-O-F phase nanomaterials[62]. Inspired by this, Gogotsi et Al. used the molten fluoride salt etching method with KF (59 wt%), LiF (29 wt%) and NaF (12 wt%) as the mixed molten salt to selectively etch the Al layer in the Ti4AlN3 only by holding at 550 ℃ for 30 min, and realized the preparation of two-dimensional transition metal nitride Ti4N3TxMXene for the first time (Figure 2 f)[51]. However, at present, this method is only suitable for the preparation of Ti4N3TxMXene, and whether it can be used for the preparation of other MXenes materials is still unknown. At the same time, under high temperature conditions, the corrosion of fluoride salt to equipment is more serious, which also restricts the large-scale promotion of this preparation method.
To sum up, although HF etching can effectively remove the A layer in the MAX phase, it is a common method to prepare MXenes. However, this method has the disadvantages of low safety and difficult control of etching strength. Fluoride salt and strong acid mixture, fluoride hydride solution and molten fluoride salt etching methods are relatively mild, but they still use fluorine-containing etchants, which are not green and environmentally friendly[32].
2.2 Preparation of MXenes by Fluorine-Free Etchant
Compared with fluorine-containing etching methods, alkali etching, electrochemical etching and Lewis acid molten salt etching are the most studied etching methods in recent years, which have mild etching conditions, green fluorine-free, strong safety and easy control.
In 2018, Li et al. First proposed and successfully prepared high-purity Ti3C2TxMXene by hydrothermal assisted NaOH etching[52]. The results show that the hydrothermal temperature and the concentration of sodium hydroxide are the two key factors for the preparation of MXenes. When the hydrothermal temperature is 270 ° C and the NaOH concentration is 27.5 mol/L, Ti3C2TxMXene with a purity of 92 wt% can be prepared (Fig. 3).
Prior to this, the researchers tried to selectively remove the A-layer elements in the MAX phase using NaOH and KOH, but failed[63,64]. The fundamental reason is that the preparation of MXenes by alkali etching requires high temperature and alkali concentration, because the oxide or hydroxide of Al is inevitably formed in the process of alkali etching.The reaction between Al and OH- is an endothermic process. The lower temperature will hinder the dissolution of Al oxides or hydroxides into OH- to form soluble Al (oh) , and then hinder the further etching of Al atoms. At high temperature, the low concentration of NaOH solution can dissolve the oxide or hydroxide of Al, but the high water content can oxidize the produced MXene into oxidized derivatives such as Na2Ti3O7 and Na2Ti5O11. However, at high temperature, the use of high-concentration NaOH solution can dissolve the oxide or hydroxide of Al, which hinders the etching of Al, and promote the synthesis of MXene, while avoiding the further oxidation of the generated MXene to oxidized derivatives such as Na2Ti3O7 and Na2Ti5O11. Although this etching method can be used to prepare MXenes in a fluorine-free environment, it has only been confirmed to be suitable for etching the Ti3AlC2MAX phase with Al atoms in the a layer, and the etching of other MAX phases has not been involved, which still needs further study.
Based on the fact that the nature of selective chemical etching of the MAX phase is an electrochemical process in which electrons of A-layer elements are transferred to other elements, Gogotsi et al. used electrochemical etching to selectively etch away M and A atoms in the Ti3AlC2, Ti2AlC, and Ti3SiC2MAX phases at room temperature, and prepared carbide-derived carbon (CDC)[65]. On this basis, Sun et al. First prepared MXenes by electrochemical etching method using Ti2AlC as anode and HCl solution as electrolyte[66]. The results show that the etched product has a three-layer core-shell structure, which is CDC, MXenes and MAX phases from outside to inside (Figure 4A). According to this result, they divided the electrochemical etching process into two stages. In the first stage, Al atoms in the outermost Ti2AlC MAX are selectively etched to form Ti2CTxMXene. The reaction in the etching process is shown in Formula 1.
In the second stage, with the extension of the etching time, the Ti atoms in the outermost Ti2CTxMXene are also etched to form a CDC protective layer to prevent the inner MAX layer from being further etched, and finally a core-shell structure is formed, so that the purity of the MXenes prepared by the method is low.
In order to solve this problem, researchers have successively optimized the process conditions such as voltage, electrolyte and etching time to speed up the etching of MAX and control the formation of CDC. Yang et al. used Ti3AlC2 as an anode for electrochemical etching in 1 M ammonium chloride (NH4Cl) and 0.2 mol/L tetramethylammonium hydroxide (TMA · OH) electrolyte (Figure 4 B)[53]. By strictly controlling the process conditions such as pulse voltage and etching time, the Ti3C2TxMXene with a purity of up to 90% was successfully prepared. It was found that the Al layer in the anodic Ti3AlC2 was rapidly etched by the Cl-, and the Ti — Al bond was broken. Tetramethylammonium hydroxide (TMA · OH) as an intercalation agent can open the edge of the anodic Ti3AlC2, promote the Cl- into the interior of the material and further etch the Al atoms. However, the tetramethylammonium hydroxide (TMA · OH) intercalation agent is toxic and harmful to people and the environment. For this reason, Hao et al. successfully prepared Ti2CTxMXene by thermally assisted electrochemical etching method using HCl solution as electrolyte, and extended this method to V2CTx and Cr2CTxMXene which are difficult to prepare by other methods, but the purity of the prepared MXenes is still low[67].
In summary, the electrochemical etching method does not require dangerous fluorine-containing agents, and is a green and safe method for preparing MXenes. However, this method also has two disadvantages: (1) the process is complex and the etching conditions are strict; (2) With the extension of etching time, the metal atoms on the outermost layer of MXenes are easily over-etched to form CDC, which reduces the purity of MXenes and limits its large-scale application.
In 2017, Fashandi et al. Successfully prepared A new MAX phase with noble metal Au or Ir at the A-site according to the different chemical activities of metals[68]. Wang et Al. Also prepared Ti3SnC2MAX by the replacement reaction of Al atoms and SnO2 in Ti3AlC2MAX[69]. Inspired by the above studies, researchers believe that the replacement reaction of A-site elements in MAX phase is similar to the selective etching mechanism of A-site elements, both of which modify the A-site atoms in MAX phase. Therefore, Huang et al. successfully prepared the Ti3ZnC2MAX phase by using the displacement reaction between the Ti3AlC2MAX phase and the ZnCl2 Lewis acid molten salt[54]. It was found that with the further increase of the amount of ZnCl2, Ti3ZnC2 could be further converted into Ti3C2Cl2MXene (Figure 5), and the fluorine-free preparation of MXene materials with surface groups of Cl was realized for the first time. In the etching process, the Zn2+, as a Lewis acid, plays a role similar to that of a H+ in HF, and the Cl- is equivalent to the coordination of a F- and an M atom, and the reactions occurring in the etching process are shown in formulas 2 to 8.
Ti3AlC2 + 1.5ZnCl2 = Ti3ZnC2 + 0.5Zn + AlCl3 ↑
Ti3AlC2 + 1.5ZnCl2 = Ti3C2 + 1.5Zn + AlCl3 ↑
Ti3C2(s) + Zn = Ti3ZnC2(s)
Ti3ZnC2 + ZnCl2 = Ti3C2Cl2+ 2Zn
Ti3ZnC2 + Zn2+ = Ti3C2 + 2Zn2+
Ti3C2 + 2Cl- = Ti3C2Cl2 + 2e-
Zn2+ + 2e- = Zn
Subsequently, they extended this method to the M2AX phase.The replacement reactions of Ti2AlC, Ti2AlN and V2AlC MAX phases with ZnCl2 were used to prepare Ti2ZnC,A Ti2ZnN and a V2ZnC MAX phase,As the amount of ZnCl2 increased, Ti2CCl2MXene was finally prepared. Recently, they have extended this etching method to different Lewis acid chloride molten salts (ZnCl2, FeCl2, CuCl2, CdCl2, NiCl2 and AgCl) and a wider range of MAX phases (such as Al, Zn, Si and Ga).The map of redox potential/Gibbs free energy of substitution reaction between cation and element A in high temperature molten salt environment was also drawn.According to the electrochemical oxidation-reduction potential of the A-site element in the MAX phase and the Lewis acid salt in the chloride, the electron gaining ability of the A-site element from the Lewis acid salt is judged, so that a proper molten salt system is selected to react with the MAX phase[55]. For example, the strength of Ti — Si bond in Ti3SiC2MAX is higher than that of Ti — Al bond. In general, the Ti3SiC2MAX phase is not selected as the precursor for the preparation of MXenes. However, in the CuCl2 molten salt at 700 ℃, the redox potential of Cu/Cu2+ is − 0.43 eV, while the redox potential of Si/Si4+ is − 1.38 eV, and the ionic Cu2+ in the molten salt can easily oxidize Si atoms to Si4+.And then combine with that Cl- to form SiCl4 gas to escape from the Ti3C2 lay, and the Cu2+ is reduced into a simple substance of Cu and is removed by an ammonium sulfate solution to finally obtain the Ti3C2TxMXene with the chlorine group on the surface. Compared with other green fluorine-free etching methods, this method is not only simple and easy to operate, but also can be used to prepare a variety of MXenes, which is expected to become a general method for preparing MXenes.
To sum up, compared with fluorine-containing etchant (Table 2), the preparation of MXenes by fluorine-free etching method not only greatly improves the safety of the experimental process, but also reduces the difficulty and cost of waste liquid treatment, which has very important research significance and application value. However, the research on green fluorine-free etching is still in its infancy. Therefore, there are not only opportunities but also challenges to find and develop new environmentally friendly general etching methods.
3 Structural Regulation of MXenes
In recent years, with the in-depth study of MXenes, researchers have found that the interlayer spacing and surface groups of MXenes have a great impact on their physical and chemical properties. By adjusting the interlayer spacing and surface groups of MXene, we can change its structure and electronic properties, and then design MXenes materials to meet the needs of various applications.
3.1 Interlayer spacing control of MXenes
In general, the interlayer spacing of MXenes prepared by different etching methods is different, as shown in Table 3. Taking the most studied Ti3C2TxMXene as an example, the larger the lattice parameter C, the larger the interlayer spacing of Ti3C2TxMXene, which can not only improve the specific surface area of MXene, increase the active sites, but also broaden the ion transmission channels, thereby improving its electrochemical performance[27,28]. Therefore, researchers usually introduce different intercalation agents to increase the interlayer spacing.
At present, there are mainly two kinds of intercalation agents reported: one is polar organic molecules, such as hydrazine (N2H4·H2O), urea, dimethylsulfoxide (DMSO), dimethylformamide (DMF) and isopropylamine[70,71][70][70][70][72]; The other is large-sized organic base molecules, such as tetrabutylammonium hydroxide (TBA · OH), tetramethylammonium hydroxide (TMA · OH) and cetyltrimethylammonium bromide (CTAB), which can be spontaneously inserted into the interlayer of MXenes (Fig. 6a) to expand the interlayer spacing[73][74][75]. In addition to the intercalation of organic solvents,Yet another class of intercalants are metal cations (such as Li+, Na+, K+, Mg2+,Ca2+, Co2+, Sn4+, etc.) And ammonium ion () solution,Because the surface of MXene prepared after selective etching has terminal groups such as — O, — OH and — F, which are electronegative, these cations can be spontaneously inserted into the interlayer of MXenes under electrostatic action (Fig. 6B) to expand the interlayer spacing[76⇓⇓~79]. The comparison of the lattice constant C of the Ti3C2TxMXene adjusted by different intercalation agents is shown in Table 3. After the polar organic molecules (N2H4·H2O, urea, DMSO and DMF) are used for intercalation, the lattice constant C of the Ti3C2Tx can reach above 25 to 25 Å; Among them, the lattice parameter of Ti3C2Tx prepared by using DMSO as intercalation agent is as high as 35.0404 Å, which also indicates that DMSO has better intercalation effect than other polar organic molecules[70]. When preparing Ti3C2TxMXenes by electrochemical etching, researchers add tetramethylammonium hydroxide (TMA · OH) as intercalation agent in the electrolyte to expand the interlayer spacing of MXene, promote the Cl- in the electrolyte to enter the material and further etch the Al atoms in it[53]. Although MXenes with large interlayer spacing can be prepared by using various polar organic molecules and large-size organic base molecules as intercalation agents, these intercalation agents have a disadvantage, that is, their toxicity will cause great harm to human body and the environment. However, MXenes prepared by in-situ HF etching with strong acid and fluoride generally do not require additional intercalation agents, because cations (Li+ and Na+, etc.) In the solution will spontaneously intercalate into the layers of MXenes during the etching process, resulting in a larger interlayer spacing and a lattice parameter C as high as 27 ~ 28 28 Å[80,81]. MXenes fabricated by NH4HF2 etching also do not require the addition of intercalation agents, and can be spontaneously intercalated into the interlayer of MXenes during the etching process, resulting in a high lattice parameter C of 24.99 Å[49,50]. Similarly, when alkali etching and Lewis acid molten salt etching are used, no intercalation agent is needed, and the cation of the MXenes can be spontaneously inserted into the interlayer of the MXenes to expand the interlayer spacing[52,54].
Although metal cation (such as Li+, Na+, K+, Mg2+, Ca2+,Co2+, Sn4+, and the like) and ammonium ions () as intercalation agents to regulate the interlayer spacing of the MXenes is slightly smaller than that of organic polar molecules and large-size organic base molecules,However, the metal cation and ammonium ion intercalation method is green and safe. In addition to the spontaneous intercalation of cations, different cations interact differently with MXenes, and the researchers further adjusted the interlayer spacing by replacing the intercalated ions by ion exchange (Fig. 6C)[79].
In addition to the introduction of organic solvents and cations, the researchers also introduced polymers, carbon nanotubes and nanoparticles as interlayer supports to increase the interlayer spacing of MXenes[82⇓~84]. Gogotsi et al. Prepared a sandwich-like MXene/carbon nanotube free-standing film by layer-by-layer assembly method. After the introduction of carbon nanotubes, the interlayer spacing of MXene increased from 1.21 nm to 1.42 nm[82]. In addition, Xu et al. Regulated the interlayer spacing of MXenes by natural sedimentation method and construction of three-dimensional structure[85⇓~87]. They prepared flexible MXene membranes by natural sedimentation method. Compared with the MXene membranes prepared by vacuum filtration method, the thickness of the membranes prepared by natural sedimentation method with the same concentration and volume of MXene suspension increased by 22%, and the corresponding interlayer spacing increased from 14.0606 Å/12.2020 Å to 14.7676 Å/12.5656 Å[85]. They also prepared a three-dimensional MXene/carbon nanotube self-supporting film by in-situ ice template method (Fig. 6d), and then freeze-dried it. During the drying process, the water molecules between the layers of MXene formed an ice template in situ, which increased the interlayer spacing of MXene.The introduction of carbon nanotubes can further introduce more water molecules between the layers of MXene, thus forming more ice templates and finally forming a three-dimensional porous structure, and the interlayer spacing of MXene in the corresponding three-dimensional structure is increased to 1.44 nm[86]. In addition, they used the self-polymerization of dopamine hydrochloride to coat the surface of the two-dimensional MXene nanosheets with a polydopamine layer, and then carbonized the polydopamine by high temperature heat treatment to construct a three-dimensional structure of C-coated MXene. Compared with the two-dimensional MXene layered structure, the interlayer spacing of MXene in the three-dimensional structure was increased from 12.33 Å to 14.22 Å, and the specific surface area and pore volume were also significantly improved[87].
In general, the interlayer spacing of two-dimensional MXenes materials is an important part of the regulation of their structure.By expanding the interlayer spacing of MXenes, the stacking of MXenes nanosheets can be prevented, the specific surface area and active sites can be increased, and the diffusion and transport of ions can be promoted, thereby improving the performance of the materials. In the research process, different interlayer spacing control methods can be selected according to the needs of practical applications to meet different application requirements.
3.2 MXenes surface group regulation
Usually, in the etching process, after the A-site elements in the MAX phase are etched away, the Ti atoms in the outer layer will become extremely unstable and easily adsorb the functional groups in the solution, thus forming MXenes with groups on the surface[88,89]. The surface groups of MXenes were quantitatively analyzed by nuclear magnetic resonance (NMR) spectroscopy, and the type and number of surface groups were found to be highly dependent on the synthesis method[90]. The surface of MXenes prepared by traditional fluorine-containing solution and fluoride salt etching method is rich in mixed groups such as -F, -O and -OH[23,46,89]. Although MXenes prepared by hydrothermal assisted alkali etching method does not contain — F group, the use of aqueous solution will introduce — O and — OH surface groups[52]. The electrochemical etching method can introduce groups such as -Cl, -O, -OH and the like[53]; MXenes containing a mixture of — Cl and — O groups can be obtained by Lewis acid molten salt etching[54,55].
In recent years, a large number of studies have shown that the surface groups of MXenes have a great impact on their structure, and then affect their electronic, optical, mechanical, thermal and electrochemical properties[81,91⇓ ~93]. For example, Hu et al. Revealed the thermodynamic stability sequence of MXenes with different groups through Bader charge analysis and thermodynamic calculation, and the results are :Ti3C2O2>Ti3C2F2>Ti3C2(OH)2>Ti3C2H2>Ti3C2 as follows, which indicates that MXenes with — O groups have higher thermodynamic stability[92]. Fu et al. Used density functional theory (DFT) to study the effects of different groups (-Cl, -F, -O and -OH) on the mechanical properties, chemical stability and electronic structure of MXenes[93]. The calculated results show that the MXenes containing — O group exhibit excellent stability and high mechanical strength. Xie et al. Used MXenes containing — O groups in lithium-ion batteries, and found that the — O groups on the surface of MXenes can absorb additional lithium ions, so that it has a higher lithium ion storage capacity[94]. However, when Tang et al used MXenes containing -F and -OH groups in lithium-ion batteries, they found that -F and -OH groups were not conducive to ion transport and redox reactions, and inhibited the ability of MXenes to store charges[95]. Meng et al. studied the characteristics of Ti3C2S2MXenes by DFT calculation, and the calculation results showed that MXenes with — S group exhibited excellent conductivity, high structural stability, low diffusion barrier and high charge storage capacity compared with Ti3C2O2MXenes with — O group[96]. In addition, researchers also believe that the surface groups of MXene directly affect the adsorption energy between metal ions and MXenes layers, thereby affecting its energy storage performance. Compared with MXenes with — O group, MXenes without surface group showed higher capacity and ion mobility[97]. Therefore, in order to meet the practical application requirements, the researchers used a variety of subsequent treatment processes to modify the surface groups of MXenes (Table 4), to regulate the type and number of groups, thereby improving its performance[98⇓~100].
At present, alkalization and vacuum annealing are the two most commonly used methods to modify the surface groups of MXenes. Li et al. First treated the Ti3C2TxMXene” prepared by etching HF solution with KOH solution, and replaced the -F group on the surface of MXenes with -OH group[101]; Then it was heat-treated in argon atmosphere at 400 ℃, which greatly reduced the concentration of surface groups of MXenes, exposed more Ti atoms, and finally increased the number of active sites. In addition, some studies have shown that the hydrogen ions on the -OH groups on the surface of MXenes can become the exchange sites of cations, which is conducive to the exchange of cations[102]. Lu et al. First immersed the V2CTxMXene prepared by the "HF method of in-situ synthesis of NaF and HCl" in a potassium hydroxide solution for alkalization intercalation, on the one hand, to promote the conversion of — F groups on the surface of V2CTx to — OH groups, on the other hand, to replace H atoms on — OH groups with K+, so as to realize the intercalation of K+ and expand the interlayer space of V2CTxMXene[78]; And then immersing in CaCl2 solution to realize the embedding of the Ca2+ through ion exchange.
In addition, Chen et al. Used n-butyl lithium to treat MXenes prepared by fluorine-containing solution etching method, and compared with alkali (KOH, NaOH, and LiOH) treatment methods (Figure 7A)[46]. It was found that n-butyl lithium could successfully remove most of the — F groups on the surface of MXenes and generate abundant — O groups. The first-principles calculation results of Xie et Al. Show that the reaction between MXenes containing oxygen groups and Mg, Ca or Al can produce MXenes without groups and metal oxides (Fig. 7B), which provides a theoretical basis for the preparation of MXenes without groups[97]. Recently, Talapin et al. First prepared MXenes by Lewis acid etching, and then eliminated or replaced the groups on the surface of MXenes with new groups in inorganic molten salts, and finally realized the modification of — O, — NH, — S, — Cl, — Br, — Se and — Te groups on the surface of MXenes[103]. MXenes with different groups exhibit unique structural and electronic properties. For example, MXenes with halide groups, especially those with — Br groups, can effectively participate in the exchange of halide ions and other groups, thus controlling the surface chemistry, structure and properties of MXenes materials. In addition, compared with the M — F and M — OH bonds formed by the — F and — OH groups and M layer atoms (Ti, Mo, Nb, V) in MXenes, the M — Cl and M — Br bonds are weaker, which makes the subsequent surface modification easier. This strategy is a major breakthrough in the regulation of MXenes surface groups, and opens up new design ideas and application fields for MXenes materials.
In a word, MXenes materials with specific structures can be prepared by designing and regulating the surface groups of MXenes to meet the needs of different application fields. However, it is found that there are still a small amount of — F and — OH groups on the surface of MXenes after alkalization, heat treatment and other subsequent processes, which is difficult to completely eliminate and prepare MXenes with only a single group or no group on the surface. The inorganic molten salt method can not only prepare MXenes with — O, — NH, — S, — Cl and — Br groups, but also prepare MXenes without groups, so it has important research value and is expected to become the most promising method to control the surface groups of MXenes.
4 Application of MXenes in Electrochemical Energy Storage
With the rapid development of economy and the accelerated pace of industrialization, people's demand for energy is also increasing dramatically, and the development of safe, reliable and environmentally friendly energy storage devices is still an important challenge facing mankind. Among many energy storage technologies, electrochemical energy storage technology plays an important role in the rapid development of energy storage devices, which is considered to be the most ideal way to develop the next generation of efficient and clean energy devices, and the development of electrode materials with high capacity, high energy density and long life is the key to obtain high-performance energy storage devices. At present, electrochemical energy storage devices mainly include supercapacitors and rechargeable secondary batteries (lithium-ion batteries and non-lithium-ion batteries). In recent years, due to its unique two-dimensional structure, excellent electronic conductivity, low ion diffusion energy barrier, adjustable interlayer spacing and surface groups, MXenes has shown broad application prospects not only as active materials of electrode materials in electrochemical energy storage devices, but also as conductive binders[106⇓⇓⇓⇓⇓⇓⇓⇓~115].
4.1 Supercapacitor
Supercapacitor, also known as electrochemical capacitor, is a kind of energy storage device with high power density, fast charge and discharge ability and long cycle life. However, compared with lithium-ion batteries, the capacity of supercapacitors is low, which greatly limits their application range. Because of its unique layered structure and abundant surface groups, MXenes materials will show intercalation pseudocapacitive behavior when used as active materials for supercapacitors, which is conducive to the improvement of its energy density and capacitance[27,46,101,111,112]. Therefore, in recent years, more and more studies have been carried out to improve the electrochemical performance of supercapacitors by regulating the interlayer spacing and surface groups of MXenes.
Xu et al. Used a hydrothermal method to embed ethylenediamine (EDA), an organic macromolecule, into Ti3C2TxMXene nanosheets through N-Ti coordination bonds, thus obtaining Ti3C2TxMXene with enlarged interlayer spacing[111]. Compared with the pristine Ti3C2Tx, the Ti3C2TxMXene with optimized interlayer spacing exhibited higher specific surface area, abundant mesopores, and higher crystallinity. Then, the material is used as an electrode of a supercapacitor, and the specific capacitance is as high as 486.2 F·g-1(1312.7 F·cm-3) at a scanning rate of 2 mV·s-1 in 1 mol/L H2SO4 electrolyte, and the specific capacitance can still be as high as 249.4 F·g-1(673.4 F·cm-3) at a scanning rate of 100 mV·s-1; In addition, when 3 mol/L H2SO4 was used as the electrolyte, the capacitance of the capacitor could still maintain 89.7% after 10 000 charge-discharge cycles at a current density up to 10 A·g-1. The reason for the high specific capacitance, rate capability and cycle stability of the material may be that the large-sized organic molecules are inserted into the interlayer of the Ti3C2TxMXene, so that the interlayer distance and the specific surface area of the material are increased,Not only the storage capacity of ions in the electrolyte is improved, but also the larger specific surface area is beneficial to forming a sufficient electric double layer at the interface between the electrode and the electrolyte, and/or performing a sufficient faradic redox reaction on the surface to store energy. Li et al. treated the Ti3C2TxMXene with KOH solution and then heat treated at 400 ℃ in argon atmosphere, which not only realized the intercalation of K+ and increased the interlayer spacing of the Ti3C2TxMXene from 9.66 Å to 12.55 Å, but also removed the — OH and — F surface groups (Fig. 8 a)[101]. It was found that the Ti3C2TxMXene modified by K+ intercalation and surface groups exhibited a mass capacitance of up to 500 F·g-1 at a scan rate of 1 mV·s-1, which was much higher than that of the original Ti3C2TxMXene (Figure 8 B); After 10 000 cycles, the capacitance of the capacitor can still be maintained at more than 99% (Fig. 8d), and the intercalation pseudocapacitance (304 F·g-1) contributed by the capacitor is about 3 times of the original Ti3C2Tx(104 F·g-1) (Fig. 8C). These excellent electrochemical performances are attributed to the increased interlayer spacing of Ti3C2TxMXene and the lower surface group concentration on the MXene layer, which allows more active sites (Ti atoms) to be exposed to the electrolyte, resulting in more redox reactions and more pseudocapacitance.
In addition, due to the unique two-dimensional structure, excellent conductivity and good hydrophilicity of MXene, Xu et al. Mixed the suspension of two-dimensional Ti3C2TxMXene nanosheets with activated carbon particles uniformly, and then prepared a self-supporting flexible film directly as the electrode of supercapacitor by vacuum filtration[113]. In this flexible electrode, the Ti3C2TxMXene can not only provide excellent mechanical flexibility, but also act as a conductive binder to wrap the activated carbon particles between its layers, and at the same time increase its interlayer spacing, thus forming a three-dimensional conductive network structure under the synergistic effect of the two. The structure exhibits a large specific surface area and abundant microporous structure, which promotes the transport of electrons and ions and electrolyte penetration, and is conducive to improving the capacitance and rate performance of supercapacitors. The results show that the flexible electrode exhibits a specific capacitance of up to 126 F·g-1 at a current density of 0.1 A·g-1 and a capacity retention of 57.9% at a current density of 100 A·g-1 in the organic electrolyte. This study also further expands the application of MXenes in the field of electrochemistry.
4.2 Lithium-ion battery
Lithium-ion batteries have been widely used due to their high capacity, excellent cycle stability and environmental friendliness. Graphite has been commercialized as an anode material, but it still can not meet the needs of large-scale high energy density and power density lithium-ion batteries in the future. MXenes have attracted much attention in recent years due to their tunable layered structure and abundant surface groups, which can promote the intercalation of Li+ in the MXenes layer, thereby improving the lithium storage capacity[114⇓~116].
Zhao et al. Immersed the multilayer Nb2CTx prepared by etching HF solution in LiOH solution to make the Li+ spontaneously intercalate into the interlayer of Nb2CTxMXene to expand the interlayer spacing, and then dried and heat-treated at 400 ℃ in H2/Ar atmosphere to remove most of the — F and — OH groups and increase the content of — O groups[116]. The Nb2CTx modified by the interlayer spacing and the surface groups is used as an electrode material of a lithium ion battery, shows a specific capacity of 448 mAh·g-1 at a current density of 0.05 A·g-1, shows excellent rate capability at different current densities,And at a high current density of 2 A·g-1, it can still maintain a high reversible capacity of 75% after 2000 cycles, far exceeding most of the pristine MXenes and carbon-based materials reported in the literature. Luo et al. Immersed the multilayer Ti3C2 prepared after etching in HF solution in LiOH solution for alkaline intercalation, on the one hand, the interlayer distance of Ti3C2TxMXene was expanded through the embedding of Li+, on the other hand, the conversion of — F group to — OH group on the surface of Ti3C2 was promoted, and the hydrogen ion on — OH group could become the exchange site of cation[77][102]. Then, under the action of PVP, the Ti3C2Tx after the alkalization intercalation treatment is immersed into the SnCl4 solution, and through the interaction of ion exchange and static electricity,One part of the Sn4+ is adsorbed on the surface of the Ti3C2Tx, and the other part of the Sn4+ enters the interlayer of the Ti3C2Tx to form a "pillar", which further enlarges the interlayer spacing of the Ti3C2TxMXene (Fig. 9a). When the PVP-Sn(Ⅳ)@Ti3C2 composite is used as the anode material of a lithium ion battery, the reversible capacity is as high as 1375 mAh·cm-3(635 mAh·g-1) at a current density of 216.5 mA·cm-3(100 mA·g-1) (Fig. 9 B), which is not only much higher than the 550 mAh·cm-3 of the graphite anode, but also maintains a high capacity after 50 cycles (Fig. 9 C); Even at a high current density of 6495 mA·cm-3(3 A·g-1), the material can maintain a specific capacity up to 504.5 mAh·cm-3(233 mAh·g-1) (Fig. 9d). Researchers have also applied this ion-exchange method to the regulation of interlayer spacing and surface groups of other MXenes, and V2CTxMXene with optimized interlayer spacing and surface groups also shows high capacity, excellent rate performance and cycle stability when used as anode materials for lithium-ion batteries[78].
4.3 Other metal ion batteries
In recent years, with the rapid increase in the use of lithium-ion batteries, lithium resources continue to be consumed and the cost is rising, while non-lithium metal ion batteries such as other alkali metal ions (Na+ and K+) and multivalent metal ions (Mg2+, Ca2+ and Al3+) have attracted wide attention of researchers because of their abundant resources and low cost[117⇓~119]. However, a large number of studies have shown that the current commercial electrode materials for lithium-ion batteries are not suitable for non-lithium metal ion batteries. Therefore, in order to achieve large-scale application of non-lithium ion batteries, it is necessary to develop new electrode materials that can quickly and reversibly intercalate/deintercalate ions and have high capacity[106].
As a new type of two-dimensional layered material, MXenes can not only embed ions of different sizes between its layers, but also adsorb ions of different sizes on its surface, which can provide a large number of storage active sites for ions and ultimately improve the energy storage effect[120⇓⇓⇓~124].
Xie et al. used first-principles to simulate the storage of Na+, K+, Mg2+, Ca2+ and Al3+ of MXene nanosheets, and the results showed that both MXenes with — O groups and MXenes without surface groups were suitable for electrode materials of non-lithium ion batteries[97]. Luo et al. pretreated the Ti3C2TxMXene with CTAB to increase its interlayer spacing, and then mixed it with S simple substance and heat treated it at 450 ℃, so that S atoms were successfully intercalated into the interlayer of Ti3C2 through Ti — S bonds, and prepared the Ti3C2MXene of S atom intercalation (Fig. 10a), which was used as the anode material of sodium-ion battery[125]. The results show that the electrode has a capacity of up to 550 mAh·g-1 at a current density of 0.1 A·g-1 (Figure 10 B), shows good rate capability at different current densities (Figure 10 C), and has a capacity of up to 120 mAh·g-1 at a current density of 15 A·g-1; Even after 5000 cycles at a current density of 10 A·g-1, it still showed good stability (Fig. 10d). Combined with the DFT calculation results, the researchers attributed the excellent electrochemical performance of the material to the fact that the intercalation of S atoms not only enlarged the interlayer spacing of Ti3C2TxMXene, but also provided abundant surface groups, which in turn increased the storage active sites of sodium ions and accelerated the storage kinetics.
Xu et al. Used Ti3C2TxMXene as a multifunctional conductive binder, mixed it with hard carbon particles, and successfully prepared a flexible electrode by vacuum filtration, and directly used it as an anode material for sodium/potassium ion batteries to study its sodium and potassium storage performance[126]. The results show that the prepared flexible electrode exhibits high sodium/potassium storage capacity, excellent cycling and rate performance, and achieves reversible capacities up to 368.5 mAh·g-1 and 280.6 mAh·g-1 in sodium-ion and potassium-ion batteries, respectively, at a current density of 30 mA·g-1, which is much higher than that of the traditional hard carbon electrode with PVDF as the binder. The capacity of the sodium-ion battery is basically not attenuated after 1500 cycles at the current density of 200 mA·g-1; Even at the current density of 500 mA·g-1, the potassium storage capacity can still be maintained at 102.2 mAh·g-1. The excellent electrochemical performance is mainly attributed to the fact that the two-dimensional MXene layers are built into a three-dimensional conductive network, and the hard carbon particles are embedded into the three-dimensional conductive network as interlayer supports, which not only can increase the interlayer spacing of MXene and inhibit its stacking,But also can effectively stabilize the electrode structure, and the stable structure formed by the synergistic effect of the two can effectively relieve the volume expansion of the hard carbon material in the process of storing sodium/potassium and obviously improve the performance of storing sodium/potassium. Xu et al. used Ti3C2TxMXene treated with cationic surfactant CTAB as cathode material for magnesium-ion batteries, which exhibited a volumetric specific capacity up to 300 mAh·cm-3 and excellent rate capability at a current density of 50 mA·g-1, much higher than Ti3C2TxMXene without CTAB treatment[127]. DFT calculations were used to reveal the mechanism of magnesium storage in Ti3C2TxMXene after CTAB treatment. The reason is that the CTA+ of spontaneous intercalation reduces the diffusion barrier of Mg2+ on the surface of MXene, improves the reversible intercalation/deintercalation of Mg2+ between MXene layers, and finally significantly improves the storage capacity of Mg2+.
In conclusion, the regulation of the interlayer spacing and surface groups of MXenes plays a crucial role in their structure and performance, and the regulation of the interlayer spacing and surface groups of MXenes can meet the needs of specific applications. At present, MXenes has shown excellent electrochemical performance in the research of supercapacitors and lithium-ion batteries, but there is still a long way to go for large-scale application. The research in other metal ion batteries is still in the initial stage, and the energy storage mechanism is still in the theoretical simulation stage, which needs further exploration and research.
5 Conclusion and prospect
As a research hotspot in recent years, MXenes materials have achieved creative research results in many fields, such as energy storage, catalysis, electromagnetic shielding and sensors. In this paper, we reviewed the preparation methods and structural regulation methods of MXenes, and obtained the following conclusions.
The traditional etching method of fluorine-containing solution and fluoride salt has low safety and high risk. Alkali etching method requires high hydrothermal temperature, high concentration of alkali and special experimental equipment, which has certain risks and limitations. The process of the electrochemical etching method has strict requirements and is not easy to control, the prepared MXenes are easy to be over-etched to form CDC, and the purity of the MXenes is low; The Lewis acid molten salt etching method prepares MXenes through the replacement reaction of A-site elements with the etchant, while M and X elements do not react with the etchant, which not only improves the chemical safety of the experimental process, but also improves the purity of MXenes, and is expected to become a general method for preparing MXenes in the future.
By adjusting the interlayer spacing of MXenes, the macrostructure of MXenes can be controlled, the stacking of MXene nanosheets can be prevented, the specific surface area and active sites can be increased, and the transport of ions can be promoted. Compared with the intercalation of organic molecules, the intercalation of metal cations and ammonium ions is a simple, green and safe method, which can simplify the preparation process of MXnens and save the cost. The introduction of interlayer support and the construction of three-dimensional structure can significantly regulate the interlayer spacing of MXnens, which will become an important research direction for the structural regulation of MXenes.
The microstructure of MXenes can be changed by adjusting the surface groups of MXenes, so as to meet the needs of different application fields. The treatment of molten inorganic salts can modify some specific terminal groups on the surface of MXenes, which will be the research focus of modified MXenes materials in the future.
The modulation of the interlayer spacing and surface functional groups of MXenes has a great impact on its performance, and MXenes with the modulation of the interlayer spacing and surface functional groups show excellent electrochemical performance in the fields of supercapacitors, lithium-ion batteries and non-lithium-ion batteries.
Although the research on MXenes has achieved remarkable results and the material has broad prospects for development, there are still some challenges and problems.
At present, 155 kinds of MAX phases have been reported, but only more than 30 kinds of MXenes have been successfully prepared, and most of them are prepared by etching the MAX phase with the A-site element of Al. The preparation of MXenes corresponding to the MAX phase with the A-site element of other elements is still the difficulty of current research, and the etching method and mechanism need to be explored. In addition, the reported preparation methods of MXenes are laboratory-scale, and the efficient, green and large-scale preparation methods need to be further explored.
The mechanism of metal cations and ammonium ions as intercalation agents to modify MXenes is still not perfect, especially the interaction mechanism between intercalation ions and terminal groups on the surface of MXenes, which still needs further exploration. In addition, because different cations are spontaneously intercalated into the interlayer of MXenes, they play different roles in regulating the interlayer spacing, thus affecting the structure of MXenes and ultimately affecting its performance. However, at present, according to the required performance, the intercalation agent is determined by theoretical simulation, and then the intercalation agent is selected for practical experiments.
The surface groups of MXenes have a great influence on their physical and chemical properties, but the research on the surface terminal groups of MXenes on their physical and chemical properties is still in the initial stage, especially the research on the single group and non-functional group modified MXenes is mostly in the theoretical stage, and there are few experimental studies.
MXenes has been widely used in supercapacitors and lithium-ion batteries, but it has not yet been commercialized on a large scale, while the research on other lithium-ion batteries is still in the initial stage, and the energy storage mechanism is still in the stage of theoretical calculation and prediction, which still needs further exploration and research.
Therefore, based on the above challenges and problems, we believe that the main research directions of two-dimensional MXenes materials in the future are as follows.
Based on the current theoretical research, a new process for preparing MXenes by Lewis molten salt etching, which is suitable for large-scale production, is discussed. At the same time, we will continue to look for new green, safe, fluorine-free and universal etching methods, and strive to achieve the etching of non-Al element MAX phase at A-site, and develop and prepare new MXenes materials.
To further improve the mechanism study of MXenes modified by cations as intercalation agents, the appropriate intercalation agents were determined by theoretical simulation to meet the needs of specific application areas.
Combined with theoretical simulation and practical experiments, the internal mechanism of the inorganic molten salt method for regulating the terminal groups on the surface of MXenes is further studied, new methods for regulating the terminal groups on the surface of MXenes are continuously searched, MXenes materials with specific surface groups and without surface group modification are synthesized, and the application range of the materials is broadened.
Further explore other methods to modify and optimize MXenes materials, and further study the energy storage mechanism of MXenes in various electrochemical energy storage systems, providing experimental and theoretical basis for its large-scale energy storage applications.
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