Research progress of MXenes and layered double hydroxides for supercapacitors

Abstract

As a new type of 2D materials, MXene has the features of good conductivity and high specific surface area. Layered double hydroxide (LDH) is often used in the field of supercapacitors owing to its special layered structure, strong adjustability, and large specific capacitance. In recent years, many researchers have combined MXenes and LDHs to prepare electrode materials with high capacitance performance and studied their preparation methods, morphology, and electrochemical properties. This study summarizes the fabrication methods of MXenes and LDHs, as well as the recent progress of MXene/LDH composites in the field of supercapacitors in recent years. Moreover, the corresponding study direction on supercapacitors has been prospected. This study aims to illustrate the application of MXene/LDH composites for supercapacitors and hopes to offer guidance for further research on the basis of these promising materials.

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Zhongtai Lin

Zhongtai Lin is pursuing his master's degree at the Shandong University of Science and Technology, majoring in materials and chemical engineering.

Xue Li

Xue Li is pursuing her master's degree at the Shandong University of Science and Technology, majoring in materials and chemical engineering.

Yong Ma

Yong Ma received his Ph.D. in Northwestern Polytechnical University in 2017 and is currently an academic associate professor at the Shandong University of Science and Technology in Qingdao, China.

Tingxi Li

Tingxi Li received his Ph.D. in Yamagata University in 2002 and is currently a professor at the Shandong University of Science and Technology in Qingdao, China.

Zhanhu Guo

Zhanhu Guo earned his Ph.D. degree in chemical engineering from Louisiana State University (LSU) in 2005. He received three-year postdoctoral training in Mechanical and Aerospace Engineering at the University of California Los Angeles (UCLA). He is a Professor of Mechanical & Construction Engineering at Northumbria University Newcastle, UK. Professor Guo is a life-time fellow of Indian Chemical Society (India), a Fellow of the Royal Society of Chemistry (RSC, UK), a Fellow of Engineered Science Society (FESS, USA), and a Fellow of Institute of Materials, Minerals & Mining (FIMMM). His current research interests are in the areas of multifunctional nanocomposites research and education.

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1. Introduction

In recent years, supercapacitors have been favored because of their high power density, stable cycling performance, adjustable operating voltage, and wide operating temperature.^1–6 One of the key technologies in supercapacitor research and development is the selection and use of electrode materials.^7–11 Future supercapacitors possessing excellent energy and power density will depend on materials that store charges through fast redox reactions.^12–15 Therefore, 2D materials are suitable candidates for advanced supercapacitor electrode materials.^16–23

Owing to their good electrical conductivity (10⁴ S cm⁻¹), mechanical properties (Young's modulus of 330 GPa), and specific capacitance (1500 F cm⁻³),^24–29 MXenes have a very broad prospect in the application of electromagnetic shielding, microwave absorption, smart fabrics, and capacitive electrodes.^30–35 The precursor for MXene is MAX.^36–41Fig. 1a shows MAX elements, where M, A, and X are metal elements from a transition group, III or IV group, and a C or N, respectively.^31,42–46 The general chemical formula of MXene is M_n+1X_nT_x, in which T denotes functional groups of –OH, O²⁻, or F⁻ linked on its surface.^47–51 Mashtalir et al.⁵² isolated monolayer MXene flakes by intercalating large organic molecules and layering agents, which opened the door to explore the true 2D properties of MXenes. Naguib et al.⁵³ also found that ordered double transition metal carbides, such as Mo₂TiC₂T_x, Cr₂TiC₂T_x, and Mo₂Ti₂C₃T_x, have added more than 25 members to the Mxene family.^54–60 At present, more than 70 kinds of Mxene precursors have been explored, but the most widely studied is still the Ti₃C₂T_x material.^40,61–68

Fig. 1 (a) MAX elements in the periodic table; (b) histogram of the number of papers published on Web of Science in recent years on MXene, LDH, and their complexes in the field of supercapacitors and other fields.

MXenes are widely used as electrodes for supercapacitors owing to their excellent physicochemical properties.^69–74 Since Lukatskaya et al.⁷⁵ first proposed that cations of Na⁺, K⁺, NH₄⁺, Mg²⁺, and Al³⁺ can spontaneously intercalate and desorb between Ti₃C₂T_x MXene layers, various supercapacitors based on MXene have been developed. Electrode materials, such as clay-like Ti₃C₂T_x MXene,⁷⁶ macroporous Ti₃C₂T_x MXene,^18,77 PPy/Ti₃C₂T_x,⁷⁸ MXene/CNT,⁷⁹ and MXene/LDH⁸⁰ have also been extensively synthesized, and studied, showing huge application prospects in energy storage.^81–88

Recently, layered bimetallic hydroxides (LDHs) have received an upsurge of study at around the globe owing to their adjustable composition, structure, and morphology.^87,89–94 With their unique layered structure and anion exchange properties,^17,95–98 LDHs have been widely used in catalysis, adsorption, pharmacy, photochemistry, electrochemical energy storage equipment, and other fields.^99–103 LDHs have the general structural formula:¹⁰⁴ M_1−x²⁺M_x³⁺(OH)₂A_x/nⁿ⁻¹·mH₂O, where M²⁺ and M³⁺ represent divalent (e.g. M = Ni, Co, Cu, Mg, etc.) and trivalent (e.g. M = Al, Fe, Mn, Cr, etc.) metal ions, respectively.^105–107 A represents the interlayer anion for separation (e.g. CO₃²⁻, NO₃⁻, Cl⁻, etc.), which is used to balance the medium positive charge of LDHs main laminate.^108–111 LDHs with different physicochemical properties are obtained by varying the molar ratios of M²⁺/M³⁺,^112–114 the properties of metal cations, the type of interlayer anions, and other conditions.^115–120

LDHs have attracted extensive and in-depth research attention in the field of electrochemistry.^39,121–126 Sarfraz et al.¹²⁷ summarized and discussed the preparation method of LDHs and their latest progress in supercapacitors. Zhao et al.¹²⁸ focused on summarizing the relationship between various LDH composites and material structures and electrochemical properties, and the corresponding charge storage mechanism. Yan et al.¹²⁹ Briefly introduced the preparation method, structural design, chemical modification, effective methods to improve the performance of supercapacitors, and future development of NiMn-LDH.

Although LDH materials have become an attractive candidate for ultra-high energy storage capacitor materials due to outstanding theoretical capacitance and low cost,^130–136 the original LDH nanostructures have serious defects, including weak stability of the microstructure, poor conductivity, limited ion/electron transfer rate, surface re-stacking hinders the accessibility of the structure, and poor magnification performance and cycle stability in practical applications.^39,137–140 Surprisingly, LDH materials can better interact with high-performance 2D materials through the –OH group on the surface to form solid nanocomposites.^141–145 Through synergetic heterostructures, the structural characteristics of LDHs and MXenes are complementary, which has the hope to improve the shortcomings of LDHs and the relatively poor capacitance of MXene and realize the application of excellent electrode materials as supercapacitors.^146–149Fig. 1b is a bar chart of the number of studies on MXene, LDH, and their composites in the field of supercapacitors and other fields published on the Web of Science. One can find that the number of studies published each year is on the rise, indicating that more and more people have begun to pay attention to these materials, as well as their superior properties and broad prospects in related fields. In recent years, Zhang et al.⁹⁵ summarized the stripping methods of LDHs, Rohit et al.¹⁵⁰ summarized the application of electrodeposited LDH in environment and energy, Kosnan et al.¹⁵¹ summarized the preparation of MXene and MoS₂, and Luo et al.³⁶ summarized the composite and research progress of MXene and conductive polymer, however, the review of MXene/LDH composites is very rare, therefore, here, various preparation methods of MXene and LDH materials and the research progress of their composites are reviewed in detail.

2. Preparation of MXenes

2.1 HF etching

The MAX phase can generally be exfoliated into the corresponding MXene by HF solution. Taking Ti₃AlC₂ as an example, in 2011 Naguib et al.¹⁵² placed the layered precursor Ti₃AlC₂ in HF for the first time to weaken the M–A metal bond and keep the M–X bond strong. Fig. 2a is a schematic diagram of the process of Ti₃AlC₂ being corroded by HF. HF preferentially selects weak Ti–Al bond, so that the Ti–C layer is gradually divided to generate MXenes phase materials. At the same time, the peeling of the Al atomic layer exposes the Ti element in the Ti–C layer, which combines with the functional groups of –OH and F⁻ rich in the solution to form complexes. The prepared Ti₃C₂T_x presents an accordion-like layered stacking structure (as shown in Fig. 2b) and is hard to prepare single-layer layers of low-defect Ti₃C₂T_x. TEM images of exfoliated MXene nanosheets are shown in Fig. 2c and d. Reactions that may occur during the reaction include:¹⁵³

Ti₃AlC₂ + 3HF = AlF₃ + 3/2H₂ + Ti₃C₂ (1)

Ti₃C₂ + 2H₂O = Ti₃C₂(OH)₂ + H₂ (2)

Ti₃C₂ + 2HF = Ti₃C₂F₂ + H₂. (3)

Fig. 2 (a) Schematic diagram of corrosion behavior of Ti₃AlC₂; (b) SEM image of multi-layer Ti₃C₂T_x in the shape of the accordion; (c and d) TEM images of exfoliated MXene nanosheets.¹⁵⁶ © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei. (e and f) SEM images of blocky Ti₃AlC₂ and layered Ti₃C₂T_x; TEM images of (g) exfoliated single-layer Ti₃C₂T_x MXene and (h) multi-layer Ti₃C₂ after ultrasonic treatment (side views).¹⁵⁴ © 2013 Elsevier B.V. (i) Synthesis and structure diagram of Ti₃C₂T_x; (j and k) schematic diagram of multi-layer Ti₃C₂ after HF treatment.¹⁵⁵ © 2015 American Chemical Society.

Mashtalir et al.¹⁵⁴ studied the kinetic control process of Al etching using 50 wt% HF, which shows that increasing the immersion temperature, increasing the reaction time, and reducing the initial maximum particle size is conducive to the rapid phase transition from bulk Ti₃AlC₂ to Ti₃C₂T_x (Fig. 2e and f). Besides, the TEM images shown in Fig. 2g and h confirm the successful preparation of exfoliated single-layer Ti₃C₂T_x MXene. Wang et al.¹⁵⁵ synthesized Ti₃C₂T_x MXene by etching Ti₃AlC₂ powder in the presence of 50 wt% HF, resulting in the periphery of MXene with –OH or –F surface groups. After HF treatment, accordion-like morphology is obtained (Fig. 2j and k). For the surface structure of single-layer Ti₃C₂T_x, it is proved on the atomic scale that after the HF treatment, the functional groups would tend to be distributed on the surface of Ti atoms (Fig. 2i).

2.2 In situ generated HF etching

Etching with HF is a facile and versatile method to prepare MXenes, but HF is highly toxic and dangerous, and the synthesized MXenes may generate a large number of defects. In the later stage of preparation, macromolecular intercalation technology needs to be introduced, so the hydrolysis of hydrofluoride salt or the method of in situ etching of the precursor with fluoride salt and acid has emerged (as shown in Fig. 3a).^157–159 Instead of adding HF directly, LiF and high-concentration HCl are used to generate HF to corrode the A layer through the metathesis reaction.^84,160 It is found that HF synthesized in situ by applying the mixture of fluoride salts (LiF, NaF, CaF₂, etc.) and HCl or H₂SO₄ is much milder than the pure HF added directly, and the obtained MXenes flakes have better etching properties including smaller lateral dimensions and fewer defects.

Fig. 3 (a) Synthesis of Ti₃C₂T_x MXene flakes by etching Ti₃AlC₂ MAX particles with LiF and HCl.¹⁵⁹ © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhm. (b) SEM image of the internal thin film, showing the lamellar structure; (c and d) TEM images of single-layer and double-layer MXene edges.¹⁶⁰ © 2014 Macmillan Publishers Limited. (e) SEM images of Ti₃AlC₂; SEM images of the products etched using (f) NaHF₂, (g) KHF₂, and (h) NH₄HF₂.¹⁵⁷ © 2016 Elsevier Ltd. (i) TEM image of Ti₃C₂T_x MXene wafer, inset showing diffraction patterns for selected area.³⁰ © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei. (j and k) SEM images of F-Ti₃C₂ intercalated by N₂H₄·H₂O and DMF, respectively.⁵² © 2013 Macmillan Publishers Limited. (l) TEM image of d-Nb₂CT_x slice.¹⁶³ © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhm.

In 2014, Ghidiu et al.¹⁶⁰ used the mixture of HCl and LiF to etch Ti₃AlC₂ at 40 °C for 45 h, and successfully prepared a Ti₃C₂ 2D layered structure material with excellent performance. Fig. 3b is an SEM image of the internal thin film, showing the lamellar structure. Fig. 3c and d are TEM images of monolayer and bilayer slices, respectively. The larger interlayer spacing has the potential to obtain more electrochemically active surfaces, providing faster electrolyte ion diffusion channels. The principle is shown below:

2Ti₃AlC₂ + 6LiF + 6HCl = 2Ti₃C₂ + Li₃AlF₆ + AlF₃ + 3LiCl + 3H₂. (4)

The obtained samples possess few defects, and the sheet size can reach several hundreds of nanometers (as shown in Fig. 3i). The single-layer or few-layer (thickness ≈ 3 μm) Ti₃C₂T_x can be obtained without ultrasonic and intercalation exfoliation. In addition, due to the entry of Li⁺, the interlayer spacing between Ti₃C₂T_x sheets is increased, making it hydrophilic.¹⁶¹ Although this method has a mild reaction, it is necessary to strictly control the relative ratio of LiF and HCl, etching time, and etching temperature during the preparation process, and it will contain many unetched MAX phases and surface groups such as –OH and –F.

Similarly, the mixtures of H₂SO₄ or HCl containing NaF, KF, CsF, [(C₄H₉)₄NF], and CaF₂ are used as etchants. Halim et al.¹⁶² used NH₄HF₂ to etch the MAX phase, and the corresponding reactions that occur are:

2Ti₃AlC₂ + 6NH₄HF₂ = 2(NH₄)₃AlF₆ + 3H₂ + 2Ti₃C₂ (5)

Ti₃C₂ + aNH₄HF₂ + bH₂O = (NH₃)_c(NH₄)_dTi₃C₂(OH)_xF_y. (6)

This process is slow, and the exfoliation and the intercalation proceed at the same time. Therefore, a more uniform atomic layer distribution of Ti₃C₂T_x was obtained. In 2017, Feng et al.¹⁵⁷ etched the MAX phase with NaHF, KHF, and NH₄HF₂. At the same time, due to the insertion of ions into the middle of the MXene lamellae, the lamellar gap increased, and MXene with a larger interlayer gap was obtained. Fig. 3e, f, and h show the SEM images of Ti₃AlC₂ and MXene etched by NaHF₂, KHF₂, and NH₄HF₂, respectively. Compared with the HF corrosion method, this method is both safe and effective, and the prepared MXene is of better quality and has greater advantages in energy storage and electrical properties.

In addition, in 2013, Gogotsi et al.⁵² first used dimethyl sulfoxide (DMSO) as an intercalant to separate stacked Ti₃C₂ sheets, and then separated multilayer MXenes into monolayer or few-layer sheet structures by ultrasonic treatment. However, DMSO has a high boiling point, which may remain in the solution and be difficult to remove, replacing the active groups on the periphery of the MXene phase, and making the sheets stick together. After that, they proposed improved intercalating agents, tetrabutylammonium hydroxide (TBAOH), choline hydroxide, n-butylamine, etc. to separate Ti₃CN, V₂C, and Nb₂C flakes.^53,163Fig. 3j and k are the SEM images of F-Ti₃C₂ intercalated by N₂H₄·H₂O (HM) and DMF, respectively, showing obvious macropores. Fig. 3l shows the TEM image of the d-Nb₂CT_x slice. However, these intercalators are commonly just suitable for particular MXene phases, and single-layer flake structures have not yet been obtained.

2.3 Alkaline etching

For avoiding the employment of hazardous fluoride salts and HF, new synthetic routes have been explored. Because of the strong binding between alkali and Al elements, it is theoretically feasible to prepare MXenes from the MAX phase as an etchant. Therefore, MXenes can be synthesized from the MAX phase using an alkali-based etching method. In Fig. 4a, Ti₃AlC₂ reacted in the presence of NaOH aqueous solution under various conditions is shown. Route a shows that aluminum hydroxide (oxide) hinders the aluminum extraction process at low temperatures. Route b exhibits that some aluminum hydroxide dissolves in NaOH at high temperatures and lower NaOH concentrations, but high water content causes MXene to oxidize and generate NTOs. Route c displays that high temperature and high NaOH concentration help to dissolve aluminum hydroxide in NaOH.¹⁶⁴ Chemical reactions between Ti₃AlC₂ and NaOH aqueous solution are as follows:¹⁵⁸

Ti₃AlC₂ + OH⁻ + 5H₂O = Ti₃C₂(OH)₂ + Al(OH)₄⁻ + 5/2H₂ (7)

Ti₃AlC₂ + OH⁻ + 5H₂O = Ti₃C₂O₂ + Al(OH)₄⁻ + 7/2H₂. (8)

Fig. 4 (a) Reaction of Ti₃AlC₂ and NaOH aqueous solution under different conditions; (b) SEM, (c) TEM, and (d) HAADF-STEM of Ti₃C₂T_x flakes.¹⁵⁸ © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Schematic diagram of etching and peeling of Ti₃AlC₂ in KOH solution; SEM images of untreated Ti₃AlC₂ (f) and KOH treated Ti₃AlC₂ (g); (h and i) SEM and (j) TEM images of exfoliated nanosheets.¹⁶⁶ © 2017 American Chemical Society.

Thermochemical reactions of NaOH aqueous solution with Al oxide/hydroxide:¹⁵⁸

Al(OH)₃ + OH⁻ = Al(OH)₄⁻ (9)

AlO(OH)(γ-AlO(OH)) + OH⁻ + H₂O = Al(OH)₄⁻ (10)

AlO(OH) (α-AlO(OH)) + OH⁻ + H₂O = Al(OH)₄⁻. (11)

Li et al. reported a hydrothermal process to produce Ti₃C₂T_x with the addition of NaOH.¹⁵⁸ The outcomes indicate that at 270 °C, Al in Ti₃AlC₂ can be successfully and selectively removed in 27.5 M NaOH solution. SEM and TEM images (Fig. 4b–d) exhibit that the prepared Ti₃C₂T_x has a compact layered structure. In addition, Xie et al.¹⁶⁵ successively treated Ti₃AlC₂ powder in 1 M NaOH solution and 1 M H₂SO₄ at 80 °C for 2 h to obtain Ti₃C₂T_x MXene. Li et al.¹⁶⁶ treated Ti₃AlC₂ in high concentrations of KOH and found that –OH groups replaced the Al layer. Fully layered Ti₃C₂(OH)₂ nanosheets were obtained by washing (Fig. 4e). After reacting with KOH solution for 6 h, the product was fanned out, but the particle profile of the precursor was basically maintained (Fig. 4f–i). Further etching of residual Al atoms could result in complete delamination, as shown in SEM images of Fig. 4h and i. One can observe the unorderly stacked nanosheets. Fig. 4j is a TEM image of exfoliated Ti₃C₂ nanosheets. Intercalation without fluorine functional groups and cations results in a larger interlayer distance, which showed higher capacitance and better rate capability of Ti₃C₂T_x in comparison with that prepared by HF etching.¹⁶⁷

2.4 Electrochemical etching

The operating conditions of the above methods are relatively harsh, which are dangerous, and limited. Electrochemical etching is milder since it is carried out in fluorine-free electrolytes. Chloride ion (Cl–) has a strong binding ability with Al and can destroy the Ti–Al bond to produce Ti₃C₂T_x MXenes without any fluorine terminal. The subsequent opening of grain boundaries is conducive to further penetration of Cl- and the intercalation of other substances in the electrolyte. Yang et al.¹⁶⁸ reported an anodic corrosion process to peel Ti₃AlC₂. The electrolyte is a mixture of 1.0 M NH₄Cl and 0.2 M TMAOH (Fig. 5a). In the etching process, Cl- and Al have a strong binding ability, which breaks the Ti–Al bond. Moreover, the existence of NH₄OH opens the edge etching of Ti₃AlC₂ and promotes deep etching under the surface. Fig. 5b shows SEM and HR-TEM images of Ti₃AlC₂ and Ti₃C₂T_x. Fig. 5c and d are SEM images of monolithic Ti₃C₂T_x. It can be seen that after electrochemical etching, the multilayer Ti₃C₂T_x powder remains tightly stacked, similar to the dense layered massive Ti₃AlC₂, showing no accordion-like structure. Nevertheless, due to the destruction of the Al–Ti bond, the etched powder can be separated into thin layers by ultrasound.

Fig. 5 (a) Schematic of etching and layering process for Ti₃C₂T_x; (b) SEM and HR-TEM images of Ti₃AlC₂ and Ti₃C₂T_x, respectively; (c and d) SEM images of monolithic Ti₃C₂T_x.¹⁶⁸ © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhem. (e) Electroetching mechanism of Ti₂AlC in HCl electrolyte; (f) SEM image of Cr₂CT_x with flower-like structure.¹⁶⁹ © 2019 American Chemical Society.

Pang et al.¹⁶⁹ studied a universal method to synthesize Ti-based MXenes through a thermal-assisted electrochemical etching by adding dilute HCl solution, and extended the etching strategy of V₂CT_x and Cr₂CT_x. Fig. 5e exhibits the corresponding mechanism. Fig. 5f is an SEM image of Cr₂CT_x with a flower-like structure. After ultrasound-assisted liquid stripping and purification, the densely layered MXene is separated into a large number of sheets with transverse sizes greater than 1 μm and thickness of about 5–80 nm, which are assembled into flower structure Cr₂CT_x. The electrochemical etching method is an ideal method for preparing MXene. However, due to the lack of previous research, this method still has many shortcomings.

2.5 Lewis acid molten salt etching

Whether it is prepared by HF direct etching method or fluorine salt plus acid in situ etching method, it is more or less harmful to the environment and experimenters. The preparation of MXene via etching of the precursor in the presence of molten salt gradually ranks among the mainstream preparation methods because it can reduce the damage of layered MXene in the experimental process and is friendly to the environment. Based on the principle of Lewis alkali reaction, the molten salt method uses the cations of the molten salt to capture the A layer of the MAX phase, so that the A layer is oxidized and removed to obtain the MXene lamellar structure. In this method, the solid salt must be heated to the molten state before the reaction can be carried out, so the requirement of the reaction temperature is very high. However, above 800 °C, MXene can phase change to form a 3D crystalline structure,¹⁷⁰ so the reaction temperature of this method is generally controlled below 800 °C.

Urbankowski et al.¹⁷¹ synthesized Ti₄N₃/MXene by etching Al atoms in the Ti₄AlN₃ precursor with molten fluoride salts (KF, NaF, and LiF) in an argon environment at 550 °C, and then used (C₄H₉)₄NOH (TBAOH) as the intercalation agent to obtain the stratification of MXene (Fig. 6a). SEM images also show the successful preparation of Ti₄N₃ with a typical accordion-like structure (Fig. 6b). Li et al.¹⁷² proposed a Lewis acid molten salt preparation method to prepare MXene. At 750 °C, taking the synthesis of Ti₃C₂T_x by Ti₃SiC₂ and CuCl₂ as an example (Fig. 6c), the principles are as follows:

Ti₃SiC₂ + 2CuCl₂ = Ti₃C₂ + SiCl₄ + 2Cu (12)

Ti₃C₂ + CuCl₂ = Ti₃C₂Cl₂ + Cu. (13)

Fig. 6 (a) Schematic of preparation of Ti₄N₃T_x with adding molten salt at 550 °C, and TBAOH as intercalator; (b) SEM image of the processed Ti₄N₃T_x.¹⁷¹ © 2016 The Royal Society of Chemistry. (c) Schematic of preparation of Ti₃C₂T_x MXene; (d) SEM image of MS-Ti₃C₂T_x MXene; (e) SEM image of MXene materials prepared with different molten salts.¹⁷² © 2021 Springer Nature Limited. (f) The surface reaction of MXene in molten inorganic salt; (g) image after multi-layer Ti₃C₂T_n MXene layering.¹⁷³ © 2020 The American Association for the Advancement of Science. The formation of nanolaminates after ion irradiation analysed by HRTEM: (h) stacked nanolaminates and (i) localized cluster of nanolaminates.¹⁷⁴ © 2020 Elsevier B.V.

They also found that etching the Ti₃AlC₂ MAX phase with the addition of molten ZnCl₂ and other Lewis acid salts above 500 °C can produce Ti₃C₂Cl₂ MXenes containing Cl surface terminals.¹⁷²Fig. 6d shows an SEM image of the prepared MS-Ti₃C₂T_x MXene. By increasing the ratio of MAX : ZnCl₂, Ti₃ZnC₂ is further changed into Ti₃C₂Cl₂ MXene, which eliminates unnecessary oxidation and hydrolysis in the presence of etching molten salts. Moreover, a general way to synthesize MXene in the presence of Lewis acid molten salt is proposed. Fig. 6e shows SEM images of the MXene material finally obtained from the stripping system determined according to the mapping map. Vladislav Kamysbayev et al.¹⁷³ fabricated a variety of MXenes containing Cl or Br surface terminals such as Ti₃C₂Cl₂ in CdCl₂ and Ti₃C₂Br₂ in CdBr₂. Fig. 6f exhibits the surface reaction of MXenes in molten inorganic salts. Fig. 6g displays an SEM image of Ti₃C₂T_n MXene after delamination.

The preparation of MXene materials through the Lewis acid molten salt stripping route obviously increases security and lowers the difficulty to treat waste liquid. However, it has not been widely used because of the harsh reaction conditions of high reaction temperature and low etching rate. Recently, Horak et al.¹⁷⁴ prepared MXene thin films by low-energy ion beam sputtering, annealing, and strong ion beam irradiation. Fig. 6h shows the HAADF-STEM image, presenting the layered structure composed of stacked crystalline flakes with smooth surfaces. In addition, argon ion irradiation produces defects in MXene materials, such as nanoplate precipitation (Fig. 6i). However, at present, it is only a preliminary preparation process, and there are still existing problems such as high equipment requirements and difficult processes.

2.6 Chemical vapor deposition

Compared with the above strategy involving chemical etching to extract the “A” layer from MAX, researchers also studied the preparation of MXenes by chemical vapor deposition (CVD), which is usually used to prepare ultra-thin materials with a thickness of nano level.¹⁷⁵ The specific step of the CVD method is to gasify various raw materials into the system for chemical reaction, and then deposit them on the crystal surface preset in the reaction chamber to obtain a film, or the gaseous raw materials react directly with the crystal and deposit them on the surface to form a film. When preparing MXene by the CVD method, the suitable C source or N source is gasified at high temperature and reacted on the surface of transition metal foil to obtain ultra-thin layers with nano thickness. The obtained MXene crystal structure and surface group distribution are highly controllable, with few crystal defects and excellent electrical conductivity.^176,177

Halim et al.¹⁶² prepared sputter-deposited epitaxial Ti₃AlC₂ films in ultra-high vacuum systems by DC magnetron sputtering. Epitaxial Ti₃C₂T_x films can be prepared by etching epitaxial Ti₃AlC₂ films at room temperature in HF or NH₄HF₂ solutions. As shown in Fig. 7a–c, STEM images of Ti₃AlC₂, Ti₃C₂T_x after HF etching, and Ti₃C₂T_x after NH₄HF₂ etching are separately shown. The results show that due to the intercalation of NH₃ and NH₄⁺, the lattice parameter c (∼25 Å) of Ti₃C₂T_x thin films etched with NH₄HF₂ is 25% larger than that etched with HF, and has higher transparency and resistivity. Furkan Turker et al.¹⁷⁸ used methane as the C source and carried out CVD with Mo foil placed on copper foil at 1090 °C to obtain 2D ultra-thin Mo₂C material. In Fig. 7d and e, SEM images of Mo₂C are shown. Thin crystals with small planes are observed, which proved the successful growth of Mo₂C. Thirumal et al.¹⁷⁹ used C₂H₄ as a carbon source to grow MXene on carbon nanotubes (CNTs) at a fixed temperature of 700 °C. Fig. 7f is the preparation flow chart. SEM images of Fig. 7g–i display the grown Ti₃C₂T_x CNTs hybrid composite. However, the MXene material prepared by the CVD method is a large-size single layer, which is not easy to stack into a loose structure. Its own lattice gap is small, and ions cannot be embedded, so it is difficult to obtain intercalation capacitance.¹⁷⁷ Moreover, the periphery groups of MXene materials prepared via this method are mainly –CH₃ or –N, the electric double layer effect is not obvious, the contribution to ion adsorption is low, and the electrochemical performance is not prominent.¹⁷⁶

Fig. 7 STEM images of the cross-section of (a) Ti₃AlC₂, (b) Ti₃C₂T_x, and (c) Ti₃C₂T_x IC films grown on the substrate. The illustration shows the SAED images of the substrate and film. (d and e) SEM images of Mo₂C crystal grown on copper foil.¹⁷⁸ © 2020 The American Ceramic Society. (f) Flow chart of synthesis of Ti₃C₂T_x carbon nanotube hybrid by CVD technology; (g–i) SEM images of Ti₃C₂T_x grown on carbon nanotubes.¹⁷⁹ © 2021 Elsevier Ltd.

Moreover, the electrochemical performances of MXene prepared using different preparation methods have been summarized in recent years in the form of a table. Table 1 shows the performance of different MXene materials in the three-electrode test, and Table 2 shows the two-electrode electrochemical performance of the assembled devices.

Table 1 Electrochemical performance characterization of MXene by using a three-electrode test system in recent years

Electrode	Electrolyte	Potential window/V	Condition	Capacitance	Cyclic performance	Ref.
Ti3C2Tx-10	1 M H2SO4	−0.4–0.1	1 A g^−1	372 F g^−1	4 A g^−1, 5000, 95%	180
Mo2CTx	1 M H2SO4	−0.3–0.3	0.3 A g^−1	79.14 F g^−1	5 A g^−1, 5000, 98%	181
C@Mxene	3 M H2SO4	−0.4–0.6	1 A g^−1	447.67 F g^−1	—	182
N, O co-doped C@Ti3C2	6 M KOH	−1.0 to −0.2	1 A g^−1	250.6 F g^−1	5 A g^−1, 5000, 94%	183
Ti3C2Tx-DC	1 M KCl	−0.8–0	0.5 A g^−1	353 F g^−1	1 A g^−1, 10 000, 99%	184
Mo1.33CTz MXene	1 M H2SO4	−0.3–0.3	0.5 A g^−1	436 F g^−1	10 A g^−1, 10 000, 98%	185
MXene hydrogel	—	−1.1 to −0.1	5 A g^−1	370 F g^−1	1000 mV s^−1, 10 000, 98%	186
MXene-NPO	1 M KOH	0.1–0.5	0.5 A g^−1	639 C g^−1	10 A g^−1, 10 000, 85%	187
V2CTx	1 M H2SO4	−0.8 to −0.3	0.5 A g^−1	171.4 F g^−1	2 A g^−1, 5000, 89.1%	188
MXene films	3 M H2SO4	−0.5–0.3	0.5 A g^−1	351 F g^−1	5 A g^−1, 10 000, 90.5%	189
Ti3C2Tx/N-doped OMC	3 M KOH	−1.0–0	1 A g^−1	329 F g^−1	100 mV s^−1, 10 000, 97%	190
V4C3Tx	1 M H2SO4	−0.25–0.2	2 mV s^−1	209 F g^−1	10 A g^−1, 10 000, 97.23%	191
MXene/C	1 M H2SO4	−0.6–0.2	2 mV s^−1	403 mF cm^−2	2500 cycles, 90%	192
V4C3	1 M H2SO4	−0.4–0.4	5 mV s^−1	330 F g^−1	5 mV s^−1, 3000, 90%	193
MXene film	1 M Na2SO4	−0.7 to −0.3	5 mV s^−1	1225 μF cm^−2	60 μA cm^−2, 10 000, 85%	194
MoS2–Ti3C2Tx	1 M H2SO4	0–0.5	5 mV s^−1	405.7 F g^−1	10 A g^−1, 10 000, 82%	195
CNF/MXene@SnS2	1 M H2SO4	−0.2–0.4	10 mV s^−1	190 F g^−1	5 A g^−1, 5000, 87.4%	196
d-Ti3C2	1 M KOH	−1.0 to −0.4	20 mV s^−1	134 F g^−1	100 mV s^−1, 5000, 94%	197
MXene/CNF	1 M H2SO4	0–0.7	300 mV s^−1	90 F g^−1	100 mV s^−1, 10 000, 98%	198
NH3–V4C3Tx	1 M H2SO4	−0.3–0.3	10 mV s^−1	210 F g^−1	10 A g^−1, 10 000, 96.3%	199

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Table 2 Electrochemical performance characterization of the devices based on MXene using a two-electrode test system in recent years

Devices	Electrolyte	Potential window/V	Capacitance	Energy density	Power density	Cyclic performance	Ref.
Ti3C2Tx-10//Ti3C2Tx-10	PVA/H2SO4	0–1.0	0.5 A g^−1, 151 F g^−1	4.7 W h kg^−1	242 W kg^−1	2 A g^−1, 4000, 85%	180
Mo2CTx//Mo2CTx	1 M H2SO4	0–0.6	0.2 A g^−1, 64.74 F g^−1	16 W h L^−1	1449.1 W L^−1	1 A g^−1, 10 000, 89.2%	181
C@MXene//C@MXene	3 M H2SO4	−0.8–0.8	1 A g^−1, 90.8 F g^−1	303.91 W h g^−1	11.92 W g^−1	10 A g^−1, 4000, 80.1%	182
AT2-NH3//AT2-NH3	6 M KOH	0–1.2	1 A g^−1, 53.8 F g^−1	10.8 W h kg^−1	600 W kg^−1	2 A g^−1, 5000, 90%	183
MXene-NPO//PPD-rGO	PVA/H2SO4	0–1.5	1 A g^−1, 187 F g^−1	72.6 W h kg^−1	932 W kg^−1	5 A g^−1, 10 000, 94%	187
MoS2–Ti3C2Tx//MoS2–Ti3C2Tx	—	0–0.6	0.5 A g^−1, 115.2 F g^−1	5.1 W h kg^−1	298 W kg^−1	5 A g^−1, 10 000, 72.3%	195
CNF/MXene@SnS2//CNF/MXene@SnS2	PVA/H2SO4	0–0.6	1 mA cm^−2, 205 mF cm^−2	6.7 μW h cm^−2	1206 μW cm^−2	10 mA cm^−2, 4000, 91.5%	196

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3. Preparation of LDHs

3.1 Coprecipitation method

Coprecipitation is the most traditional way to fabricate LDHs. The universal method is to mix the solution of metal ions (M²⁺/M³⁺) with a precipitant (such as OH⁻, CO₃²⁻) at an appropriate pH,²⁰⁰ and then conduct hydrothermal treatment. LDHs with different particle sizes, morphology, and crystallinity are synthesized by controlling the reaction time, concentration, temperature, and reactants.^201,202

Woo et al.²⁰³ synthesized a new phase of ZnCo-LDHs for the first time by coprecipitation reaction with hydrogen peroxide as the oxidant. SEM images of Fig. 8a and b show a sheet shape, and the primary particle size is hundreds of nanometers. A stable colloidal suspension of ZnCo-LDH was successfully prepared by dispersing ZnCo-LDH-NO₃⁻ powder in formamide. A very thin ZnCo-LDH sheet is displayed in the HR-TEM image of Fig. 8c. The corresponding SAED showed an obvious hexagonal pattern, meaning that the pattern of LDH after peeling does not change. Djebbi et al.²⁰⁴ prepared MgAl-LDH and ZnAl-LDH nanosheets using the same method, and the morphologies of the product are shown in Fig. 8d and e. The particle sizes of them are about 50–200 nm and 100–500 nm, respectively. After testing, it was found that the former had better electrochemical properties than the later, which proves that the change of divalent metal cations plays a key decisive in the resulting electrochemical performances. Zhang et al.²⁰⁵ deposited NiCo LDH on rGO sheet by the deposition method. With the progress of the reaction, staggered NiCo LDH nanosheets gradually appeared (Fig. 8f and g). The TEM image in Fig. 8h shows that the entire surface of rGO is uniformly coated with staggered NiCo-LDH of about 2.4 nm thickness. In the EDS mapping shown in Fig. 8i, the uniform distribution of Ni, Co, C and O was found, demonstrating the successful preparation.

Fig. 8 (a and b) SEM images of ZnCo-LDH; (c) TEM image and SAED of ZnCo-LDH nanosheets.²⁰³ © 2011 The Royal Society of Chemistry. (d and e) SEM images of MgAl-LDH and ZnAl-LDH.²⁰⁴ © 2016 Elsevier B.V. (f and g) SEM images of NiCo-LDH/rGO; (h and i) TEM image and EDS mapping of NiCo-LDH/rGO.²⁰⁵ © 2020 Elsevier Inc.

3.2. Solvent/hydrothermal method

The solvent/hydrothermal method is the most common synthesis method of LDHs. It has the advantages of simple and easy operation, environmental protection, and cleaning in the synthesis process. Because of the easy regulation of the reaction time and temperature, LDHs fabricated by solvent/hydrothermal method commonly possess the features of good crystallinity, homogeneous size, and uniform morphology.^206–208 Rao et al.²⁰⁹ prepared MgAl LDH by a simple method of urea hydrolysis and hydrothermal synthesis, as shown in Fig. 9a. The sample is hexagonal and its size is less than 1 mm. Compared with the conventional coprecipitation method, the product did not need repeated washing and alkali removal.

Fig. 9 (a) SEM image of MgAl LDH.²⁰⁹ © 2004 Elsevier Ltd. (b) SEM image of NiMn LDHs; (c) HAADF-STEM image of NiMn LDHs; (d) corresponding EDS mapping analysis image.²⁰⁸ © 2018 Elsevier Ltd. (e) SEM image of Ni₄Yb(OH)₁₀NO₃·3H₂O nanosheets.²¹⁰ © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (f) Preparation mechanism of NiAl-LDH; (g) SEM image of NiAl-LDH; (h) side view of NiAl-LDH.²¹¹ © 2013 Elsevier B.V. (i) SEM image of LDH/CSs composite.²¹² © 2017 The Nonferrous Metals Society of China. Published by Elsevier Ltd. (j) SEM images of NiCu-LDH material grown uniformly on carbon fibers; (k) locally enlarged FESEM image.²¹³ © 2017 Science China Press and Springer-Verlag GmbH Germany; (l and m) SEM images of NiCo LDH samples on the surface of textile.²¹⁴ © 2018 Elsevier B.V.

Wang et al.²⁰⁸ fabricated NiMn LDHs via a simple one-step hydrothermal method, and adjusted their interlayer space by keeping the mole number of urea less than the stoichiometric ratio and controlling the reaction time. Fig. 9b shows an SEM image of NiMn LDHs, which have a flower-like morphology with layered nanosheets and slight aggregation. The HAADF-STEM image and the corresponding EDS spectra (Fig. 9c and d) clearly confirm the well-defined morphology of NiMn LDHs, in which Mn, Ni, C, and N are uniformly distributed throughout the LDHs. The electrochemical tests indicated the specific capacitance of NiMn LDHs at 1 A g⁻¹ of 846.5 C g⁻¹. Duan et al. synthesized thin-layer hexagonal structure Ni₄Yb(OH)₁₀NO₃·3H₂O nanosheets by the solvothermal method.²¹⁰ The product morphology is shown in Fig. 9e, which is hexagonal nanosheets with a diagonal length of about 1.5 μm. Moreover, it was found that the sample possesses remarkable energy storage properties and excellent cycle stability. At 1 A g⁻¹, it can show 1482.1 F g⁻¹ specific capacitance, and can still maintain 71.8% capacitance after 2000 cycles.

3.3 In situ growth method

The in situ growth method involves placing the conductive base material as a template into the metal salt solution, and these metal ions grow LDHs on their surface. The most commonly used conductive substrates are carbon cloth or nickel foam, which can increase the conductivity of LDHs. Wang et al.²¹¹ successfully produced NiAl-LDH onto nickel foam via the in situ method (Fig. 9f). The periphery of nickel foam is covered by petal-like NiAl-LDH with 200 nm thickness (Fig. 9g and h). Concurrently, these NiAl-LDH sheets are arranged perpendicular to the substrate, forming a porous nanowall structure and leaving huge pores.

Huang et al.²¹² fabricated CoAl-LDH on the periphery of carbon spheres (CSs), as shown in Fig. 9i. It is proved that the prepared composites having large specific surface areas increase the contact between active materials and electrolytes. Through electrochemical tests, it was found that at 1 A g⁻¹ and 10 A g⁻¹, the composites separately showed specific capacitances of 1198 F g⁻¹ and 920 F g⁻¹. Wang et al.²¹³ synthesized NiCu LDH by applying a one-step solvothermal method on carbon fiber cloth (CFC). After the reaction, the CFC was uniformly coated with an active material porous layer (Fig. 9j). SEM image demonstrates the hierarchical porous layer composed of ultrathin nanosheets (Fig. 9k). Through the tests, one can view that the sample displays excellent electrochemical properties. Jeong et al.²¹⁴ prepared NiCo-LDHs by a solvothermal method on nickel-coated fabric. It was found that the entire surface was decorated with vertical NiCo-LDH (Fig. 9l). Fig. 9m proves that NiCo-LDH nanosheets are uniformly arrayed on the surface of the textile. Since the nanosheets form a rich pore structure, it obviously reduces the ion diffusion distance and enables the electrolyte to easily penetrate into the internal electrode.

3.4 Electrochemical deposition method

The electrode prepared by electrochemical deposition has the advantage of directly fabricating the electrode in one step, which simplifies the electrode preparation process. At the same time, the material prepared by electrochemical deposition has a high specific capacitance. Therefore, this method is one of the most active ways to fabricate materials as electrodes at present. During the preparation process, by selecting appropriate electrode materials and electrolytes, and adjusting the electrode potential and current to control the direction and speed of the reaction, the required substances are produced on the electrode. Han et al.²¹⁵ prepared a 3D NiCo-LDH network on activated carbon cloth (ACC) having high-quality loading through electrochemical deposition. In the SEM image shown in Fig. 10a, the nanosheets form a 3D network structure with a uniform distribution on the carbon fiber surface. The SEM image of Fig. 10b presents that these nanosheets are uniformly covered on ACC. The asymmetric supercapacitor (ASC) based on NiCo-LDH@ACC serving as a positive electrode exhibits power density and energy density of 2.24 mW h cm⁻² and 3.71 mW cm⁻². Fig. 10c exhibits the illustration of assembling NiCo-LDH@ACC.

Fig. 10 (a and b) SEM images of NiCo(NA)-LDH@ACC; (c) synthesis and assembly of NiCo-LDH@ACC ASC.²¹⁵ © 2021 Elsevier Ltd. (d) Schematic of the preparation of NiCo-LDH/Co(OH)₂@PPY and corresponding (e and f) SEM images.²¹⁶ © 2021 Elsevier B.V. (g) The synthesis route of ZnO@NiCo-LDH; (h) FESEM image of ZnO@NiCo-LDH300s.²¹⁷ © 2021 Informa UK Limited, trading as Taylor & Francis Group.

Yao et al.²¹⁶ prepared NiCo LDH/Co(OH)₂@PPY electrode by −2.0 V electrochemical deposition. The formation process is displayed in Fig. 10d. Through the adjustment of time and voltage, the obtained material presents a cauliflower-like nanostructure (Fig. 10e and f), which is conducive to charge transfer and electrolyte penetration. Chen et al.²¹⁷ synthesized ZnO@NiCo-LDH composites on the surface of carbon cloth (CC) through the hydrothermal method and electrochemical deposition (Fig. 10g). The influences of the reaction time on the resulting electrochemical performances are researched in detail. The outcomes indicate that the NiCo-LDH film with a deposition time of 300 s can form an interconnected network morphology (Fig. 10h). This electrode possesses good area capacitance (667.3 mF cm⁻² at 1 mA cm⁻²) and cycle stability (86.78% capacitance retention after 2000 cycles). In addition, the ASC based on this electrode displays the energy density of 0.044 mW h cm⁻² at the power density of 1.17 mW cm⁻².

Furthermore, the electrochemical performances of LDH prepared via different methods are summarized in recent years in the form of a table. Table 3 shows the performance of different LDH materials in the three-electrode test, and Table 4 shows the two-electrode electrochemical properties of the assembled device.

Table 3 Electrochemical performance characterization of LDHs by using a three-electrode test system in recent years

Electrode	Electrolyte	Potential window/V	Condition	Capacitance	Cyclic performance	Ref.
Mn–Ni LDO-C	1 M KOH	0–0.6	1 mA cm^−2	2.36 C g^−1	2 mA cm^−2, 5000, 92.1%	218
ZnNiFe-LDH@Cu(OH)2/CF	6 M KOH	0.15–0.45	3 mA cm^−2	6100 mF cm^−1	30 mA cm^−2, 5000, 88.1%	219
P-Ni(OH)2@Co(OH)2/NF	1 M LiOH	0–0.5	1 mA cm^−2	2.36 C cm^−2	20 mA cm^−2, 10 000, 91%	220
ZIF-8-C@NiAl-LDH	—	0–0.5	5 mV s^−1	1403 F g^−1	4 A g^−1, 1000, 77%	221
Ni-foam@Cu–Al LDH/g-C3N4	6 M KOH	−0.1–0.5	0.4 A g^−1	838.8 F g^−1	50 mV s^−1, 5000, 92.71%	222
NiCoZn-LDH@PANI	3 M KOH	−0.1–0.4	1 A g^−1	1749 F g^−1	10 A g^−1, 40 000, 89%	223
NiCo2Al-LDH/N-GO	2 M KOH	0–0.6	1 A g^−1	1136.6 F g^−1	10 A g^−1, 5000, 90.1%	224
CoMn LDHs/NF	1 M LiOH	−0.2–0.45	1 A g^−1	1409 F g^−1	1 A g^−1, 3000, 93.2%	225
Co, Mn-LDH@NF	3 M KOH	0–0.4	1 A g^−1	1140 F g^−1	1 A g^−1, 3000, 86.5%	226
NiMnCr-LDHs@CS/NF	2 M KOH	0–0.5	3 A g^−1	569 C g^−1	20 A g^−1, 10 000, 76%	227
NiCo2S4/Ni–Co LDH	3 M KOH	0–0.5	1 A g^−1	1765 F g^−1	5 A g^−1, 2000, 56%	228
CoFe-ONP/LDH/MLG	2 M KOH	0–0.4	1 A g^−1	882.5 F g^−1	1 A g^−1, 5000, 67.1%	229
NiCo/NiMn-LDH	6 M KOH	0–0.4	1 A g^−1	2950 F g^−1	10 A g^−1, 10 000, 79%	230
Co-LDH/G2	1 M KOH	0–0.45	2 A g^−1	400 F g^−1	2 A g^−1, 10 000, 90.4%	231
NiCo2O4@Co–Fe LDH	2 M KOH	0–0.4	1 A g^−1	1557.5 F g^−1	—	232
Ni–Al LDH/NNDG	6 M KOH	0–0.5	1 A g^−1	975 F g^−1	5 A g^−1, 5000, 86%	233
Co3O4@CoNi-LDH	2 M KOH	0–0.5	0.5 A g^−1	2676.9 F g^−1	30 A g^−1, 10 000, 67%	234
Ni–Co LDH/CFC	1 M KOH	0–0.55	1 A g^−1	1540 F g^−1	10 A g^−1, 5000, 84.6%	235
NiCo-LDH/NF	2 M KOH	0–0.5	1 A g^−1	1198 F g^−1	5 A g^−1, 2000, 81.6%	236
NiFe-LDH/NF	1 M KOH	0–0.4	5 A g^−1	2708 F g^−1	10 A g^−1, 500, 42.6%	237

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Table 4 Electrochemical performance characterization of the devices based on LDHs by using a two-electrode test system in recent years

Device	Electrolyte	Potential window/V	Capacitance	Energy density	Power density	Cyclic performance	Ref.
Mn–Ni LDO-C//AC	1 M KOH	0–2.0	1 A g^−1, 563.2 C g^−1	78.2 W h kg^−1	499.7 W kg^−1	2 A g^−1, 5000, 85.3%	218
NiCoZn-LDH@PANI//AC	KOH	0–1.6	1 A g^−1, 97.0 F g^−1	37.2 W h kg^−1	362 W kg^−1	5 A g^−1, 30 000, 90%	223
NiCo2Al-LDH/N-GO//AC	PVA/KOH	0–1.6	1 A g^−1, 80.2 F g^−1	42.25 W h k^−1	800 W kg^−1	10 A g^−1, 10 000, 88.9%	224
CoMn LDHs/NF//AC	1 M LiOH	0–1.5	1 A g^−1, 118 F g^−1	34.5 W h kg^−1	362.3 W kg^−1	5 A g^−1, 3000, 83.7%	225
ZnNiFe-LDH@Cu(OH)2/CF//AC	PVA/KOH	0–1.0	0.4 A g^−1, 88 F g^−1	44 W h kg^−1	720 W kg^−1	5 A g^−1, 5000, 83.4%	219
P-Ni(OH)2@Co(OH)2/NF//Fe2O3/CC	1 M LiOH	0–1.6	1 mA cm^−2, 4.4 C cm^−2	0.21 mW h cm^−2	16 mW cm^−2	20 mA cm^−2, 5000, 81%	220
NiMnCr LDHs@CS/NF//FeOOH/NF	2 M KOH	0–1.7	1 A g^−1, 167.4 F g^−1	48 W h kg^−1	402.7 W kg^−1	5 A g^−1, 10 000, 83%	227
NiCo/NiMn-LDH//AC	—	0–1.6	1 A g^−1, 185.1 F g^−1	45.16 W h kg^−1	1600 W kg^−1	10 A g^−1, 10 000, 82.2%	230
NiCo2O4@Co-Fe LDH//AC	2 M KOH	0–1.7	1 A g^−1, 64.76 F g^−1	28.94 W h kg^−1	950 W kg^−1	2 A g^−1, 5000, 76.9%	232
Ni–Al LDH/NNDG//AC	PVA/KOH	0–1.6	2.5 A g^−1, 166 F g^−1	46 W h kg^−1	1991 W kg^−1	10 A g^−1, 10 000, 95%	233
Co3O4@CoNi-LDH//AC	PVA/KOH	0–1.6	1 A g^−1, 195.9 F g^−1	61.23 W h kg^−1	750 W kg^−1	10 A g^−1, 5000, 99%	234
Ni–Co LDH/CFC//rGO/NF	1 M KOH	0–1.6	1 A g^−1, 104.1 F g^−1	37 W h kg^−1	800 W kg^−1	10 A g^−1, 10 000, 84.6%	235
NiCo-LDH/NF//AC	2 M KOH	0–1.6	1 A g^−1, 81.8 F g^−1	29.1 W h kg^−1	903.1 W kg^−1	2 A g^−1, 10 000, 88.3%	236
NiFe-LDH//AC	1 M KOH	0–1.6	1 A g^−1, 141.3 F g^−1	50.2 W h kg^−1	800 W kg^−1	4 A g^−1, 2000, 65%	237
CoSx/Ni–Co LDH//AC	6 M KOH	0–1.6	1 A g^−1, 100 F g^−1	35.8 W h kg^−1	800 W kg^−1	5 A g^−1, 10 000, 94.5%	238

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4. Research progress of MXenes and LDHs composites for supercapacitors

MXenes have typical 2D layered structure characteristics, high specific surface area, good electrical conductivity as well as abundant surface groups, which can be used as matrix materials.²³⁹ LDHs with high aspect area are potentially excellent materials for supercapacitors, but their low conductivity and agglomeration behavior seriously influence the improvement of electrochemical performance.²⁴⁰ Therefore, the fabrication of LDHs on the surface of MXenes is hoped to prepare electrode materials with high performance. The following describes the recent progress of LDH/MXene in the field of supercapacitors, mainly including the preparation and performances of the electrode materials and the assembled devices.

Li et al.²⁴¹ prepared NiFe LDH/Ti₃C₂ MXene composite by a hydrothermal method, as shown in Fig. 11a, in which NiFe LDH nanosheets were regularly and vertically arrayed on the surface of MXene nanosheets (Fig. 11b and c). Besides, the element mapping certifies the uniform distribution of Ti, Ni, and Fe (Fig. 11d). The CV tests of the prepared electrode were performed (Fig. 11e), and the peaks are related to the reduction of Ni²⁺, Fe³⁺ (ref. 242 and 243) and the oxidation of Ni, Fe.^244–246 All curves remained unchanged, manifesting excellent reversibility. Fig. 11f exhibits GCD curves, and discharge and charge potential platforms are in line with those of CV curves. Moreover, the initial discharge capacitance at 0.1 A g⁻¹ is 1376.4 mA h g⁻¹. Fig. 11g shows the corresponding cycle stability for 200 cycles at 0.1 A g⁻¹. In the case of 1 A g⁻¹, the composite shows 726.1 mA h g⁻¹ capacitance (Fig. 11h). The excellent properties benefit from MXene providing a conductive framework as well as NiFe LDH exposing the active surface. Moreover, the assembled device verifies the practical application of the electrode (Fig. 11i). It can be found that CV curves do not possess a perfect rectangle owing to the existence of two energy storage mechanisms (Fig. 11j). GCD curves present almost the same linear shapes and there is no IR drop (Fig. 11k), meaning remarkable reversibility. At a power density of 1158 W kg⁻¹, the device has an energy density of 53 W h kg⁻¹. In addition, after 500 cycles, the capacitance is 46.7 mA h g⁻¹, and the coulombic efficiency at 1 A g⁻¹ is 96% (Fig. 11l), which implies good cycle stability.

Fig. 11 (a) Schematic of fabrication of NiFe-LDH/MXene composite; (b and c) SEM images and (d) elemental mapping; (e and f) CV and GCD curves. Cycling capacitance at (g) 0.1 A g⁻¹ and (h) 1 A g⁻¹; (i) schematic of the NiFe-LDH/MXene//AC and corresponding (j) CV curves, (k) GCD profiles and (l) cycle stability.²⁴¹ © 2021, American Chemical Society.

Wang et al.²⁴⁷ synthesized intercalated LDH MXene LDH (LM-4) by coprecipitation. NiMn-LDH nanosheets are grown on the periphery of the Ti₃C₂ sheet (Fig. 12a). The TEM image also confirms the successful preparation of NiMn-LDH nanosheets (Fig. 12b). The existence of MXene increases the conductivity (Fig. 12c), and LDH uniformly covering the surface of MXene exhibits more available active sites. The electron coupling between different components increases the cycling stability (Fig. 12d). All-solid-state asymmetric supercapacitor based on LM-4 working as the positive electrode was constructed (Fig. 12h). Fig. 12e shows the CV curves of the device with varied voltage windows in the range of 1.3–1.7 V. The typical symmetric GCD curves illustrate the excellent electrochemical reversibility at 1 to 10 A g⁻¹ (Fig. 12f). The cyclic stability of the device was further studied (Fig. 12g). The device exhibits 90.3% capacitance retention after 5000 cycles and an energy density of 44.7 W h kg⁻¹ at a power density of 800 W kg⁻¹.

Fig. 12 (a) SEM and (b) TEM images of MXene@NiMn-LDH; (c) Nyquist plots and (d) cyclic performance at 6 A g⁻¹ of NiMn-LDH and LM-4; (e) CV curves at 50 mV s⁻¹, (f) GCD curve and (g) cycle performance at 6 A g⁻¹ of LM-4//AC; (h) specific capacitance curves at different current densities, illustration is a schematic diagram of an asymmetric device.²⁴⁷ © 2020 Elsevier B.V.

Guo et al.²⁴⁸ used 3D foam nickel (NF) and Ti₃C₂T_x layers as substrates and prepared NiAl-LDH composites by the immersion hydrothermal method (Fig. 13a–c). The NiAl-LDH array on the substrate has stronger electrochemical performance because of the synergy effect of different parts. CV and GCD tests were performed by the employment of a three-electrode system (Fig. 13d and e). There is little change in shape for CV curves, suggesting good reversibility of the redox reaction. There are two platforms for GCD curves ascribed to the oxidation–reduction reaction of the composite, illustrating the pseudo-capacitance characteristics. Besides, the ASC device is assembled with NiAl-LDH/MXene and AC, whose voltage window reached 1.7 V (Fig. 13f). Fig. 13g and h show the corresponding CV and GCD curves. The almost unchanged shape for CV curves suggests the device possesses impressive fast charge and discharge characteristics. On the basis of GCD curves, the specific capacitance of NiAl-LDH/MXene//AC at 1 A g⁻¹ is about 50 F g⁻¹. After 5000 cycles, the retention rate can reach 89.1% (Fig. 13i). It is calculated that the device has an energy density of 27.6 W h kg⁻¹ at a power density of 255 W kg⁻¹.

Fig. 13 (a) Roadmap for preparing NiAl LDH/MXene on foam nickel; (b and c) SEM images of NiAl-LDH/MXene composite; (d) CV curves at different scanning rates; (e) GCD curves at different current densities; CV curves of NiAl-LDH/MXene//AC (f) under different voltage windows and (g) at different scanning rates; (h) GCD curves of NiAl-LDH/MXene//AC at different current densities; (i) cycle performance and coulombic efficiency of NiAl-LDH/MXene//AC at 3 A g⁻¹.²⁴⁸ © 2022 The author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

Chen et al.²⁴⁹ prepared intercalated MXene/GO/NiMn-LDH (MGL) by coprecipitation (Fig. 14a–c). The sheet-like LDH could increase the specific capacitance. In addition, Go acted as a thin layer, covering the periphery of the complex to accelerate charge transfer. The electrochemical properties of the samples were investigated in detail in a 6 M KOH electrolyte. Under the same scanning rate (Fig. 14d), the current density of MGL is the highest, declaring that the introduction of Go and MXene greatly improves the electrochemical properties of LDH. GCD curves of MGL exhibit a much longer time than that of other materials at the same current density (Fig. 14e). From the EIS results shown in Fig. 14f, it is noteworthy that the excellent capacitance retention of the composite is attributed to LDH with good conductivity and MXene with low resistance. The cyclic stability further proves the improvement of electrochemical performance (Fig. 14g). An asymmetric device is prepared with MGL as the positive electrode. CV curves of MGL//AC in the range of 1.4–2.0 V are shown (Fig. 14h). Further measurements proceeded at 0–1.8 V (Fig. 14i), and CV curves with olive shape are shown from 10 mV s⁻¹ to 100 mV s⁻¹. The symmetric GCD diagram shows that the device has excellent reversibility at 1–20 A g⁻¹ (Fig. 14j). In Fig. 14k, the device still has a 94.7% capacitance retention rate after 4000 cycles and has an energy density of 55.3 W h kg⁻¹ at a power density of 800 W kg⁻¹.

Fig. 14 (a) Schematic diagram of preparing MGL composite; (b) SEM image of MXene/GO/NiMn-LDH; (c) element mapping of Ti, C, F, Ni, Mn, and O; (d) CV curves at 20 mV s⁻¹; (e) GCD curves at 1 A g⁻¹; (f) Nyquist plots of different electrodes; (g) cycle test at 5 A g⁻¹ for 4000 cycles; (h) CV curves of MGL//AC in different scanning potential windows at 100 mV s⁻¹; (i) CV curves of MGL//AC at different scanning rates; (j) GCD curves of MGL//AC at different current densities; (k) cycle test of MGL//AC at 5 A g⁻¹.²⁴⁹ © 2021 Elsevier Ltd.

Wang et al.²⁵⁰ uniformly grown NiCoAl-LDH nanosheets on MXene sheets by the hydrothermal method to form NiCoAl-LDH/V₄C₃T_x heterostructures (Fig. 15a and b). In SEM and TEM images of Fig. 15b and e, it can be readily viewed that the NiCoAl-LDH nanosheets are regularly grown on MXene sheets forming a porous network. CV measurements were performed on different electrodes at 10 mV s⁻¹ (Fig. 15f). For all three samples, two obvious Faraday redox peaks having relatively large potential separation can be seen, showing typical cell type behavior.²⁵¹ At 10 mV s⁻¹, the CV area of the NiCoAl-LDH/V₄C₃T_x heterostructure is the largest, illustrating the best specific capacitance. GCD tests were carried out at 2 A g⁻¹ (Fig. 15g). The platform also proved the battery type behavior of the composite. In order to prove the practical application of the heterostructure, the supercapacitor was produced (Fig. 15h). The voltage window of the device was investigated by CV curves (Fig. 15i), and it can be seen that the appropriate voltage window was 0–1.6 V. The GCD curves of the NiCoAl-LDH/V₄C₃T_x//AC hybrid device were collected under different current densities (Fig. 15j). Fig. 15k displays the capacitance performance of the device, which presents a high specific capacitance of 194 F g⁻¹ at 1 A g⁻¹ and 102 F g⁻¹ at 20 A g⁻¹. Fig. 15l exhibits the cycle test of the device for 10 000 cycles within 0–1.6 V at 20 A g⁻¹, showing 98% capacitance retention rate after 10 000 cycles, and 96% coulombic efficiency (Fig. 15l).

Fig. 15 (a) Schematic of synthesis of NiCoAl-LDH/V₄C₃T_x composite; (b–d) FE-SEM image of NiCoAl-LDH/V₄C₃T_x; (e) TEM image of NiCoAl-LDH/V₄C₃T_x; (f) CV curves of different samples at 10 mV s⁻¹; (g) GCD testing of different samples at 2 A g⁻¹; (h) schematic diagram of NiCoAl-LDH/V₄C₃T_x//AC hybrid supercapacitor; (i) CV curves of the device under different operating voltage windows; (j) GCD curves of the device at various current densities; (k) specific capacitance of the device under different discharge current densities; (l) cycle performance of the device at 20 A g⁻¹.²⁵⁰ © 2019 The Royal Society of Chemistry.

Zhou et al.²⁴⁰ fabricated low-layer MXene with dimethyl sulfoxide, and then NiFe-LDH was deposited on its surface using a hydrothermal method to obtain the NiF-LDH/MXene composite (Fig. 16a and b).^237,252 The Al element in Ti₃AlC₂ was selectively etched by HF to obtain a multilayer MXene (Fig. 16c). Fig. 16d shows a TEM image of MXene. Fig. 16e and f shows SEM images of NiFe-LDH/MXene composites. Adding MXene to LDH solves the problem of LDH agglomeration. In addition, LDH grown on MXene suppresses the aggregation and self-overlap between MXene sheets caused by the van der Waals force, further increasing the layer spacing of MXene. The three-electrode tests were performed in 1 M KOH solution. Compared with MXene and LDH, NiFe-LDH/MXene composite showed higher peak redox current (Fig. 16g), indicating better capacitance characteristics. The nonlinear discharge curves of the composite showed typical pseudo capacitance behavior (Fig. 16h), in accordance with the CV results. Compared with MXene and LDH, NiFe LDH/MXene had a longer discharge time and has optimal energy storage properties. Fig. 16i exhibits Nyquist curves of different samples. It was found that the internal resistance of the prepared composite is less than that of pure LDH, which proves the addition of MXene provides more conductive transmission paths for electron transmission. Besides, it can be seen that the slope of the composite is larger than that of LDH, showing ideal capacitance characteristics. Fig. 16j shows the CV curves of the NiFe-LDH/MXene//AC device. Fig. 16k shows the GCD curves of the device under various current densities. When the current density was 1 A g⁻¹, the specific capacitance of the device reached 135.7 F g⁻¹. It can also be seen that the power density was 758.27 W kg⁻¹ when the energy density was 42.4 W h kg⁻¹. In Fig. 16l, the electrochemical property of NiFe-LDH/MXene composites improved during the first 400 cycles, which may be due to sufficient electrolyte penetration and electrode activation.²⁵³ Furthermore, 84% specific capacitance remained after 1000 cycles, proving that this 2D interconnected porous structure effectively prevented structural variation and collapse of LDH during the electrochemical reaction, which is important for the stability of the electrode.

Fig. 16 (a) Schematic of s-MXene synthesis; (b) schematic of preparation of NiFe-LDH/MXene composite; (c) SEM image of m-MXene; (d) TEM image of s-MXene; (e and f) SEM images of NiFe-LDH/MXene composites; (g) CV curves at 5 mV s⁻¹, (h) GCD curves at 1 A g⁻¹ and (i) Nyquist curves of different samples; (j) CV curves of NiFe-LDH/MXene based asymmetric SC (ASC); (k) GCD curves of ACS; (l) cycle performances of NiFe-LDH/MXene//AC.²⁴⁰ © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd.

NiMn-LDH nanosheets were deposited on single-layer MXene through in situ synthesis (Fig. 17a and b).²⁵⁴ The existence of chemical bonding between LDH and MXene favors accelerated ion diffusion and electron transfer of the composite. Fig. 17c and d shows the EIS curves and GCD curves of different samples at 2 A g⁻¹, it can be seen that the NiMn-LDH/MXene (2 : 1) composite exhibits the smallest impedance and the longest charge and discharge time, whose electrochemical performance is superior to other materials. Fig. 17e and f displays the CV and GCD curves of the best sample. Through calculation, the specific capacitance at 0.5 A g⁻¹ was found to be 1575 F g⁻¹. Fig. 17g presents 10 000 charge–discharge cycles at 5 A g⁻¹, and a 90.3% capacitance retention rate was achieved. Fig. 17h is a voltage window diagram of the ASC device. One can see that the voltage can be expanded to 1.5 V. Fig. 17i displays GCD curves under 1.5 V. Fig. 17j is a cycle performance test chart, and in the case of 2 A g⁻¹, 91.8% capacitance retention rate was obtained after 10 000 cycles. At the same time, AC//NiMn-LDH/MXene (2 : 1) ASC devices show an energy density of 126 W h kg⁻¹ at a power density of 0.74 kW kg⁻¹ and a lower energy density of 97 W h kg⁻¹ at a power density of 3.3 kW kg⁻¹.

Fig. 17 (a) Schematic of preparing NiMn-LDH/MXene; (b) SEM images of NiMn-LDH/MXene (2 : 1); (c) Nyquist plots of different samples and (d) GCD curves at 2 A g⁻¹ of different samples; (e) CV curves at 5–25 mV s⁻¹, (f) GCD curves at 0.5–20 A g⁻¹ and (g) cling performance over 10 000 cycles at 5 A g⁻¹ of NiMn-LDH/MXene (2 : 1); (h) CV curves in different voltage windows at 5 mV s⁻¹, (i) GCD curves at 1.5 V and (j) cycling stability of AC//NiMn-LDH/MXene (2 : 1) ASC device at 2 A g⁻¹.²⁵⁴ © 2020 American Chemical Society.

Zhang et al.²⁵⁵ synthesized CoNiMn-LDH/V₂CT_x composite through the electrostatic cooperative assembly by a hydrothermal method, as exhibited in Fig. 18a. From Fig. 18b, one can find that a nanoflower-like morphology was obtained in which LDH nanoflowers are tightly coated on the surface of MXene nanosheets. TEM images in Fig. 18c and d testify that the inter-plane spacing is very consistent with the unique plane of LDH, also proving the successful preparation of the composite. From Fig. 18e, it can be viewed that the composite exhibits a large specific surface area characteristic with uniform distribution of V, C, Ni, Mn, and Co elements. In the 6 M KOH solution, the electrochemical performances are evaluated with three-electrode configurations. In Fig. 18f, two redox peaks of CV curves are seen at 10 mV s⁻¹, indicating that the reversible Faraday reaction happens. Of particular attention is that the composite electrode exhibits the largest CV integration area because of the metal ion doping effect, which suggests that the Faraday reaction speed is fast and the specific capacitance is high. In Fig. 18g, the discharge time of the composite electrode is the longest, indicating that its specific capacitance is the highest. The specific capacitances of V₂CT_x, NiMn-LDH/V₂CT_x, and CoNiMn-LDH/V₂CT_x at 1 A g⁻¹ are 38, 591, and 1005 F g⁻¹ (Fig. 18h). In the CV curves of Fig. 18i, the anode and cathode peaks separately move towards more anode and more cathode directions with the increase of scanning rate, implying the improvement of ion and electron transfer rates. An ASC device was fabricated in 6 M KOH electrolyte using CoNiMn-LDH/V₂CT_x as the cathode and V₂CT_x MXene as the anode (Fig. 18j). In Fig. 18k, because of the obvious polarization behavior in 1.6 V voltage window, 1.4 V was chosen as the optimal value. In Fig. 18l and m, CV and the GCD curves of the ASC device show a combined shape of an electric double-layer capacitance and pseudocapacitance. At 2 A g⁻¹, the specific capacitance was 112 F g⁻¹ after 3000 cycles, meaning that the device had excellent cycle stability (Fig. 18n).

Fig. 18 (a) Schematic of preparation of CoNiMn-LDH/V₂CT_x composite; (b) SEM and (c and d) TEM images of CoNiMn-LDH/V₂CT_x; (e) elemental mapping of V, C, Mn, Ni, and Co; (f) CV curves at 10 mV s⁻¹; (g) GCD curves at 1 A g⁻¹ and (h) rate capability of different composites; (i) CV curves of the CoNiMn-LDH/V₂CT_x electrode; (j) schematic of preparation of CoNiMn-LDH/V₂CT_x//V₂C ASC device; (k) CV curves of the ASC at 50 mV s⁻¹; (l) CV curves of ASC at different scan rates; (m) GCD curves of the ASC at various scan rates; (n) cycling performances of ASC at 2 A g⁻¹ after 3000 cycles.²⁵⁵ © 2021 Elsevier B.V.

Niu et al.²⁵⁶ prepared CoM (M = Fe, Mn)/MXene layered composite via the hydrothermal method (Fig. 19a), demonstrated that the properties of the single metal compound are adjusted by the introduction of other metal ions, as well as studying the influence of various metal ions. It was found that the presentation of Fe element greatly increases the specific capacitance (Fig. 19f), while the introduction of Mn ions reduces the ion diffusion resistance (Fig. 19g). The specific capacitance of the prepared CoFe-LDH/MXene (CF-MX10) was 664 F g⁻¹ at 1 A g⁻¹, which is higher than that of CoMn-LDH/MXene (CM-MX10, 524 F g⁻¹) and CO(OH)₂/MXene (C-MX10, 226 F g⁻¹). The high specific capacitance corresponds to the strong cooperation effect of different components, which greatly improves the conductivity and electrochemical stability. Fig. 19b–d shows SEM images of CF-MX10 composites. It can be observed that a large number of CoFe-LDH nanosheets are coated on the periphery of the MXene sheet, forming a layered structure (Fig. 19c), giving rise to excellent ionic/electronic conductivity and high energy storage properties. SEM image (Fig. 19d) presents that the structure of CoFe-LDH nanosheets is almost the same as a triangle with a needle-like structure at the top. Therefore, the layered morphology has lots of edges and points, resulting in forming a large number of active sites. Fig. 19h displays CV curves of the electrode, which has two pairs of obvious peaks, suggesting that the capacitance is dominated by the Faraday redox reaction. In addition, the almost symmetrical shape of the GCD curves (Fig. 19i) indicates that the electrode has significant reversibility. Fig. 19j exhibits CV curves under various voltage windows. Nevertheless, in the case of 2.0 V potential, the curve is obviously polarized, meaning that the oxygen evolution behavior is serious. Hence, 0–1.8 V was chosen as the voltage window for the asymmetric device. Fig. 19k displays CV curves of CF-MX10//AC ASC in 0–1.8 V. Fig. 19l exhibits GCD curves of CF-MX10//AC. When the power density was 450 W kg⁻¹, the energy density of ASC was 18.5 W h kg⁻¹. When the energy density was 4.5 W h kg⁻¹, the power density of ASC was 9000 W kg⁻¹. In addition, Fig. 19m proves the cycle test of the device. One can see at 10 A g⁻¹, after 8000 cycles, a 95% specific capacitance retention rate was achieved. These results indicated that the device had excellent stability.

Fig. 19 (a) Schematic diagram of fabricating CoFe-MXene and CoMn-MXene layered composites; (b–d) SEM and (e) TEM images of CF-MX10; (f) EIS curves of CF-MX10, CM-MX10, and C-MX10; (g) specific capacitance bar chart; (h and i) CV and GCD curves of CF-MX10 electrode; (j) CV curves under various voltage windows; (k) CV curves of ASC at various scan rates; (l) GCD curves of ASC at various current densities; (m) cyclic stability test at 10 A g⁻¹.²⁵⁶ © 2021 Elsevier B.V.

Fig. 20a exhibits a process for preparing a positive electrode of Ti₃C₂T_x@LDH-Ag. Fig. 20b and c are SEM images of Ti₃C₂T_x and NiCoAl-LDH, respectively. Fig. 20d is an SEM image of Ti₃C₂T_x MXene/NiCoAl-LDH, in which the NiCoAl-LDH nanosheets are evenly covered on Ti₃C₂T_x. To demonstrate the effectiveness of multicomponent composites, electrochemical performances are measured at the same conditions including CV, GCD, and rate plots (Fig. 20e–g). For improving the energy density, Zhang et al.²⁵⁷ proposed an in-plane hybrid supercapacitor (IHSC) using a special positive electrode on the fabric (Fig. 20h). By combining NiCoAl-LDH with high capacitance, Ti₃C₂T_x MXene with good conductivity as the skeleton and Ag nanowires, the battery type electrode achieved an excellent capacitance of 592 C g⁻¹ at 1 A g⁻¹, having a cycle life of more than 10 000 times. Fig. 20i exhibits GCD curves of the IHSC device based on KOH gel on the fabric, which can still maintain a substantially triangular shape. The surface capacitances calculated from the discharge curves are separately 64, 55, 43, 33, 24, and 17 mF cm⁻² at 0.5, 1, 1.5, 2, 3, and 4 mA cm⁻² (Fig. 20j). After 10 000 cycles at 2 mA cm⁻², 80% of the initial capacitance was maintained (Fig. 20k). A bending test was carried out to prove the flexibility, and Fig. 20l shows photos of the devices encapsulated by transparent tape in different bending cases. The bending angle is from 0° to 150°. Fig. 20m shows that bending deformation does not influence the shape and the area of the CV curve. In order to further evaluate the flexibility of the manufactured device, fatigue resistance tests were performed, and the obtained GCD curves under different bending cycles in 1000 cycles are shown in Fig. 20o. With 1000 bending cycles, the capacitance retention rate remained at 83%, although the capacitance retention rate rapidly decreased in the first 100 bending cycles.

Fig. 20 (a) Schematic of preparing Ti₃C₂T_x@LDH and Ti₃C₂T_x@LDH-Ag composite ink; (b) SEM image of multilayer Ti₃C₂T_x and (c) SEM image of NiCoAl-LDH; (d) SEM image of Ti₃C₂T_x@LDH; (e) CV curve at 5 mV s⁻¹; (f) GCD curve at 1 A g⁻¹; (g) capacitance performance of different materials; (h) manufacturing scheme of IHSC devices; (i) GCD curve of IHSC at 0.25 to 4.0 mA cm⁻²; (j) the change of area capacitance with discharge current; (k) cycling property of IHSC at 2 mA cm⁻²; (l) images of IHSC in different bending states; (m) CV curves of IHSC under various bending states at 100 mV s⁻¹; (n) surface capacitance and coulombic efficiency under different bending states; (o) the relationship between the capacitance retention and the number of bends of IHSC devices.²⁵⁷ © 2022 Elsevier B.V.

Moreover, the specific capacitance and the resistance of MXene, LDH, and their composite materials at the same current density were compared, and the results are presented as bar graphs, as shown in Fig. 21. The left figure shows that the specific capacitance of composite materials is higher than that of pure MXene and LDH materials, indicating that the synergistic effect of the two can improve the specific capacitance. The right figure displays that the resistance of the composite material is between that of MXene and LDH, proving that the combination of MXene and LDH can reduce the resistance of LDH.

Fig. 21 Bar graphs of the comparison for the resistance and a specific capacitance of MXene, LDH, and their composites at the same current density.

Furthermore, the electrochemical performances of MXene/LDH prepared by different preparation methods are summarized in recent years in the form of a table. Table 5 shows the performance of different MXene/LDH materials in the three-electrode test, and Table 6 shows the two-electrode electrochemical properties of the assembled device.

Table 5 Electrochemical performance characterization of MXene/LDH composites by using a three-electrode test system in recent years

Electrode	Electrolyte	Potential window/V	Condition	Capacitance	Cyclic performance	Ref.
NiAl-LDH/MXene	6 M KOH	0–0.55	1 A g^−1	1061 F g^−1	4 A g^−1, 4000, 70%	80
NiFe-LDH/MXene	1 M KOH	0–0.37	1 A g^−1	720.2 F g^−1	—	240
Ni–Mn LDH@MXene	6 M KOH	0–0.5	1 A g^−1	179 mA h g^−1	6 A g^−1, 5000, 79.1%	247
NiAl-LDH/MXene	2 M KOH	0–0.45	1 A g^−1	1600 F g^−1	10A g^−1, 3000, 78%	248
MXene/GO/LDH	6 M KOH	0–0.35	1 A g^−1	241.9 mA h g^−1	5 A g^−1, 4000, 90.9%	249
NiCoAl-LDH/V4C3Tx	1 M KOH	−0.1–0.45	1 A g^−1	627 C g^−1	20 A g^−1, 3000, 82.6%	250
Co-NiMn-LDH/V4C3Tx	6 M KOH	−0.1–0.35	1 A g^−1	1005 F g^−1	—	255
CoFe-LDH/MXene	1 M KOH	−0.1–0.4	0.5 A g^−1	808 F cm^−2	10 A g^−1, 8000, 80%	256
Ti3C2Tx@NiCoAl LDH-Ag	1 M KOH	0–0.5	1 A g^−1	592 C g^−1	10 A g^−1, 10 000, 88%	257
NiCo-LDHs/MXene	1 M (NH4)2SO4	−0.2–0.6	0.5 A g^−1	1207 F g^−1	0.5 A g^−1, 5000, 93%	258
NiCo-LDH/Ti3C2	6 M KOH	0–0.6	2 A g^−1	984 F g^−1	30 A g^−1, 5000, 76%	259
NiMn-LDH/MXene	6 M KOH	0–0.45	0.5 A g^−1	1575 F g^−1	5 A g^−1, 10 000, 90.3%	254
Ti3C2/Ni–Co–Al-LDH	1 M KOH	0–0.6	1 A g^−1	748.2 F g^−1	2 A g^−1, 10 000, 78.7%	260
NiCoFe-LDH/Ti3C2	1 M KOH	0–0.3	1 A g^−1	1990 F g^−1	10 A g^−1, 5000, 84.8%	261
NiCo2-LDHs@MXene/rGO	2 M KOH	0–0.45	1 A g^−1	332.3 mA h g^−1	5 A g^−1, 5000, 87.5%	262
MXene/CoAl-LDH-80%	3 M KOH	0–0.5	1 A g^−1	1930 F g^−1	—	263

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Table 6 Electrochemical performance characterization of devices based on MXene/LDH composites by using a two-electrode test system in recent years

Device	Electrolyte	Potential window/V	Capacitance	Energy density	Power density	Cycling stability	Ref.
NiFe-LDH/MXene//AC	—	0.2–1.5	1 A g^−1, 135.7 F g^−1	42.4 W h kg^−1	758.2 W kg^−1	1 A g^−1, 1000, 84%	240
MXene@Ni–Mn LDH//AC	PVA/KOH	0–1.6	1 A g^−1, 56 mA h g^−1	44.7 W h kg^−1	800 W kg^−1	6 A g^−1, 5000, 90.3%	247
NiAl-LDH/MXene//AC	2 M KOH	0–1.7	3 A g^−1, 37 F g^−1	27.6 W h kg^−1	255 W kg^−1	3 A g^−1, 5000, 89.1%	248
MXene/GO/LDH//AC	PVA/KOH	0–1.6	1 A g^−1, 69.1 mA h g^−1	55.3 W h kg^−1	800 W kg^−1	5 A g^−1, 4000, 94.7%	249
NiCoAl-LDH/V4C3Tx//AC	1 M KOH	0–1.6	1 A g^−1, 194 F g^−1	71.7 W h kg^−1	830 W kg^−1	20 A g^−1, 10 000, 98%	250
CoNiMn/V4C3Tx//V4C3Tx	6 M KOH	0–1.4	1 A g^−1, 110 F g^−1	30.16 W h kg^−1	700 W kg^−1	2 A g^−1, 3000, 95.7%	255
CoFe-LDH/MXene//AC	1 M KOH	0–1.8	0.5 A g^−1, 295 F g^−1	18.5 W h kg^−1	450 W kg^−1	10 A g^−1, 8000, 95%	256
Ti3C2Tx@NiCoAl LDH-Ag//AC	PVA/KOH	0–1.5	0.5 mA cm^−2, 64 mF cm^−2	22.18 μW h cm^−2	3.0 mW cm^−2	2 mA cm^−2, 10 000, 80%	257
MXene-LDH@NF//MWCNT@NF	PVA/KOH	0–1.6	—	36.7 W h kg^−1	1440 W kg^−1	—	259
NiMn-LDH/MXene//AC	6 M KOH	0–1.5	0.5 A g^−1, 169.8 F g^−1	126 W h kg^−1	740 W kg^−1	2 A g^−1, 10 000, 91.8%	254
Ti3C2/Ni–Co–Al-LDH//AC	PVA/KOH	0–1.6	0.5 A g^−1, 128.89 F g^−1	45.8 W h kg^−1	346 W kg^−1	5 A g^−1, 10 000, 97.8%	260
NiCoFe-LDH/Ti3C2//RGO	1 M KOH	0–1.55	—	54.4 W h kg^−1	895.1 W kg^−1	5 A g^−1, 5000, 80.2%	261
NiCo2-LDHs@MXene/rGO//MXene/rGO	2 M KOH	0–1.4	1 A g^−1, 240 F g^−1	65.3 W h kg^−1	700 W kg^−1	5 A g^−1, 10 000, 92.8%	262
MXene/CoAl-LDH-80%//MXene/GO-5%	3 M KOH	0.4–1.6	1 A g^−1, 112 C g^−1	30.9 W h kg^−1	992.6 W kg^−1	30 A g^−1, 30 000, 94.4%	263

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5. Summary

Here, the fabrication methods of MXene and LDH and their composites, as well as the latest progress and achievements in the application frontier of supercapacitors are reviewed. MXene has become an inevitable hotspot in the field of energy storage, especially in the field of supercapacitors in the past decade. Its simple synthesis strategy, unique combination, and adjustable properties of specific applications provide support for it. However, the urgent need for the enhancement of structure and morphology has realized the idea of the introduction of complementary materials. LDHs provide enhanced electrochemical activity and high specific surface area, avoid the problem of re-blockage, and enhance conductivity. In addition, the structures of these heterostructures play a synergistic interface function and induce a separate energy storage contribution while improving the overall interface structure and electrochemical stability. In addition, each MXene nanocomposite has different 2D heterostructures and special advantages. Its composition needs to be optimized to achieve strong supercapacitor performance.

At last, the real potential coming from 2D materials and heterostructures is large enough, which provides a huge base to understand the interaction dynamics and the energy storage of other 2D materials such as H-BN, MOFs, and covalent organic skeletons. MXene is used to produce heterostructures for driving towards one of the optimal electrode materials. The MXene/LDH heterostructures can be transformed into supercapacitor devices, for instance, rugged wires, flexible and micro-supercapacitors for portable and wearable electronic devices, with excellent device compatibility Hence, MXene/LDH heterostructures are expected to become electrode materials for the next generation of advanced energy storage applications.

Author contributions

Zhongtai Lin: conception and designed the work, writing-original draft, and editing. Xue Li: methodology, data curation. Hao Zhang, Ben Bin Xu: formal analysis, investigation. Priyanka Wasnik, Handong Li, Man Vir Singh: software, supervision. Yong Ma, Tingxi Li, Zhanhu Guo: reviewing and editing.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Acknowledgements

We gratefully appreciate the support of the Natural Science Foundation of Shandong (ZR2019BB063).

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