Headway towards contemporary 2D MXene-based hybrid electrodes for alkali-ion batteries

Abstract

Recent studies have revealed that the current consumption rate of fossil fuels may lead to their exhaustion in the coming years. Due to the depletion of non-renewable energy sources and the fitness of renewable energy sources, energy storage and management can be viewed as a prime requisite. The inception of advancements in energy storage techniques and rechargeable batteries has eased the problem to a certain extent. With highly exceptional properties such as inherent volumetric capacitance, outstanding conductivity, durability, low energy barriers for metal ion diffusion and the presence of surface functional groups, MXenes have entered the field of energy storage. MXenes are 2-D layers of transition metal carbides or nitrides, synthesized by the selective etching of aluminium from layered MAX phases. The large interlayer spaces for ion intercalation permits alkali ions in these layers, revealing unexpected properties that can be exploited for energy storage in alkali-ion batteries (AIBs) such as lithium/sodium/potassium/magnesium/calcium ion batteries. Electrochemical energy storage can also be visualised as a clean energy storage system. This review covers the synthesis of MXenes, their eminent properties and the fabrication of MXene-based nanomaterials and hybrids for electrochemical energy storage in various alkali-ion batteries. Finally, possible challenges are highlighted for the future exploration of the research possibilities.

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Helen Treasa Mathew

Ms Helen Treasa Mathew received her bachelors’ and masters’ degree from Mahatma Gandhi University and Kannur University Kerala, India in 2018 and 2020, respectively. She has worked as project trainee in ESMB Division of CSIR-NPL, New Delhi. Currently, she is pursuing PhD from the Department of Nanoscience and Technology, Central University of Jharkhand, India. Her research interest is in the synthesis and modification of MXene-based hybrid materials for multifarious applications.

Kumar Abhisek

Mr Kumar Abhisek obtained his masters’ degree from NIST, Berhampur, Odisha, India in 2020, and pursuing PhD in Department of Nanoscience and Technology, Central University of Jharkhand, India. He is currently working on the development of transition metal dichalcogenide-based multifunctional hybrid materials for environmental applications.

Shashikant Shivaji Vhatkar

Mr Shashikant Shivaji Vhatkar is pursuing his PhD in the Department of Nanoscience and technology from Central University of Jharkhand, Ranchi, India. He received his masters’ from Central University of Tamil Nadu, India majoring in materials science in 2018. He has been working on development and electrochemical study of polyindole-based hybrid electrode materials for energy storage applications.

Ramesh Oraon

Dr Ramesh Oraon is presently working as an assistant professor in Department of Nanoscience and technology (now Dept. of Chemistry) since 2018. He obtained his bachelors’ from St. Xavier's college, Ranchi, Jharkhand, India in 2010. He received his masters and PhD from IIT-ISM Dhanbad in 2012 and 2017 respectively. He has been actively involved in developing hybrid electrode materials and their electrochemical study for energy storage and environmental applications.

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1. Introduction

Sustainable energy utilisation is vital since the consumption of resources for our present needs should be established without compromising the needs of future generations. Maximising the efficiency of the energy storage devices and the subsequent utilisation of best available technologies to harvest renewable energy would lead to fully sustainable global energy systems.^1–3 In addition to sustainability, efficient energy storage systems can pave the way towards the reduced usage of fossil fuels thereby remediating environmental pollution.^4–6 Such a scenario can be achieved through suitable electrochemical energy storage devices.^7,8 Among the many such devices, lithium-ion batteries (LIBs), with high energy density, limited self-discharge, long wheel-of-life and environmental amiability have become more popular.^9,10 The reduced availability of naturally occurring Li and economic feasibility hinder the further enhancement of these batteries to a certain extent.^11,12 However, an alternative is needed to meet the requirements for suitable storage devices. Sodium and potassium have similar properties as Li and could easily alleviate the disadvantages due to their natural abundance and lower supply cost. Both sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) suffer from low reversible capacity and poor cycling stability due to low open circuit voltages and large ionic radii, making it harder to find a suitable host.^13,14 Multivalent alkali metal-ion batteries such as magnesium-ion batteries (MIBs) and calcium-ion batteries (CIBs) have been advantageous due to their low price, abundant resources and high safety applications.^11,15,16 Moreover, all alkali metal batteries are susceptible to dendrite growth, which will adversely intensify their electrochemical performance and cause a safety hazard.^17,18 To satisfy the requirements, electrode materials are expected to have desirable electron or ion conduction, physicochemical properties, excellent electrochemical performance and structural stability during repeated charge/discharge cycles.^19,20 To meet these requirements, diligent efforts have been made to investigate and establish suitable nanomaterials for advanced electrodes in alkali-ion batteries.

Interestingly as a solution to all the above-mentioned problems, MXenes, a new family of compounds, have emerged, in which alkali and non-alkali ions can be intercalated. MXenes are a 2D group of early transition metal carbides and/or carbonitrides, produced by the etching out of the A layers from MAX phases.²¹ The MAX phases are composed of an early transition metal M, a group 13 or 14 element denoted as A and C and/or N denoted as X, forming M_n+1AX_n, where n = 1, 2, or 3. Among the different family groups of MAX phases, more than 130 possess the MX₆ octahedral structure.²² At the initial stage, almost 50 MAX phases were discovered.²³ The number of M layers found between A layers differs from one type to another, namely the 211, 312 or 413 phases.²⁴ In 2014, B. Ansori et al. synthesised a new chemically ordered ternary carbide, Mo₂TiAlC₂, in which Ti layers are sandwiched between two Mo layers.²⁵ Later, in 2017, Liu et al. developed other ordered MAX phases (Cr_2/3Ti_1/3)₃AlC₂ and (Cr_5/8Ti_3/8)₄AlC₃.²⁶ These were out-of-the-plane or o-MAX 312 or 412 phases. However, the 2D Mo_1.33C sheets, which possessed high conductivity and volumetric capacitance, were in-plane ordered or i-MAX 211 phase.²⁷ Bond structures of the MAX phases are strong enough so that they cannot be broken by any mechanical means like stress or shear. Yet, the differences in the relative strengths of the M–A and M–X bonds permit the selective etching of A layers from M–A bonds without disturbing the M–X bonds.²⁸ Thereby, 2D MXenes are characterised by the formula M_n+1X_nT_x, where T represents the functional groups getting adsorbed at the surface of MXenes and x means the number of surface functionalities.²⁹ Alterations made in the interlayer spacing of MXenes, without disrupting the layered structure, promote the synthesis of MXenes-based composites with outstanding performance. In addition to that, the properties such as electronegativity, hydrophilicity and surface structure tunability of the adsorbed terminations show up without affecting the conductivity.³⁰

According to theoretical calculations MXenes and MXene-based electrodes, possess low ion-diffusion barriers, specific capacity, cyclability and appreciable conductivity. This in turn enhances the electrochemical performance of these alkali-ion batteries.^31–33 Furthermore, MXene-based materials that inhibit the growth of metal dendrites also encourage research enthusiasm towards the manufacture of high-performance electrodes. The incorporation of MXenes with heteroatoms, CNTs or other nanostructures forms MXene-based hybrids that enhance the electrochemical performance of the cathode/anode materials. It was shown that alkalization transforms MXenes into 3D nanoribbons and they have been applied to PIBs, SIBs and LSBs to obtain dendrite-free electrode materials.

The development of suitable composites can elevate their exploration in different fields as depicted in Fig. 1. By inducing suitable morphological modification and with appropriate functionalisation, MXenes work as an intriguing interface in various sensing applications since they possess strong metallic conductivity, hydrophilicity, low diffusion barrier, high ion transport characteristics and large surface areas.³⁴ They are utilised in physical sensing, which includes the sensing of stress, strain or pressure, and chemical sensing, which comprises gas sensing as well as biosensing.³⁵ Wang et al. developed hybrid MXene/ZnO nanorods for NO₂ sensing, which exhibited higher sensing capability than the pristine ZnO rods at ambient temperature.³⁶

Fig. 1 MXene-based hybrid electrode materials and their diverse applications. This figure has been reproduced/adapted from ref. 37 with permission from Elsevier Inc.

Ultra-thin, flexible MXene-based materials can be utilised for electromagnetic interference (EMI) shielding, owing to their exceptional conductivity, lightweight nature, easy producibility, as well as large surface area.³⁸ Apart from these applications, they are employed in supercapacitors because they have extraordinary physical and electrochemical characteristics and their composition can be tuned accordingly.³⁹ Interestingly, MXene-based hybrid materials are used as photocatalysts as they might enable quick photoinduced separation of charge carriers in photocatalytic activity and offer many surface functionalities for materials used for light-harvesting, ensuring the possibility of achieving high photoconversion efficiencies.⁴⁰ 2D MXene-based nanomaterials have a myriad of very desirable characteristics, including rich surface functionalities, excellent conductivity, changeable bandgap architectures, hydrophobicity, a sizable surface area and thermal stability. When used with various sophisticated oxidation techniques for the breakdown of hazardous contaminants, MBNs function as an exemplifiable alternative.⁴¹ Additionally, there is considerable interest in investigating the potential of MXenes to be introduced at the interfaces of biological systems due to their dimensionality, large effective surface area, planar shape and surface chemistry, along with favourable physicochemical features.⁴²

In this review, we initially focus on various bottom-up and top-down synthesis methods for MXenes and their outstanding properties that make them dominant in the field of energy storage applications. This review highlights the development of different electrodes that may use MXenes, heteroatom-doped MXenes, or MXene-based hybrids/composites as hosts to enhance the performance of lithium-ion batteries and non-lithium-ion batteries. Also, we cover the synthesis of high-performance lithium-based batteries like lithium–metal batteries (LMBs), lithium–air batteries (LABs) and lithium–sulphur batteries (LSBs). Finally, we set forth the hurdles and challenges for the synthesis of dendrite-free, high-capacity MXene-based electrodes and unveil the possible horizons for MXene materials for future research.

2. Synthesis of MXenes

MXenes are derived by the selective etching of definite atomic layers of the precursor MAX phase, which are a group of ternary carbides and nitrides possessing a double transition metal structure. The wedging of the A layers between the M and X layers produces a structure with weak M–A bonds and strong M–X bonds.^28,43 Therefore, utmost care should be taken for the selection of an appropriate, simplistic and harmless method of synthesis that would retain the M–X bonds. This choice is significant because the properties of MXenes are greatly influenced by the etching environments during synthesis.⁴⁴ The comparative studies of different MXene-based electrodes reveal that different surface functional groups affect the performance to an extent, as they can be tuned to obtain desired properties. MXenes with surface terminations, bromine and iodine significantly increase the electrochemical performance of the AIBs as compared to chlorine.⁴⁵ Unfortunately, the majority of the currently employed synthetic methods yield MXenes that are surface halogenated with Cl and F. In addition to that, the morphology, internal defects and yields of the synthesised MXenes are strongly influenced by experimental factors like the concentration of the etchant, temperature, and time taken for synthesis, etc. Nan et al. reported that these defects in MXenes tend to react with active cations, which may further lead to batteries with poor first coulombic efficiency.⁴⁶ Hence, before the detailed analysis, sufficient knowledge of the synthesis of MXenes is requisite. In general, MXenes can be synthesised either by top-down or bottom-up techniques (Table 1).

Table 1 A glimpse at the MXenes synthesised to date

Precursors	MXenes synthesised	Etchants used	Synthesis parameters		Ref.
			Temperature (°C)	Reaction time (h)
Ti2AlC	Ti2CTx	10% HF	RT	10	47
V2AlC	V2CTx	50% HF	RT	90	48
Nb2AlC	Nb2CTx	50% HF	RT	90	49
Ti2AlN	Ti2NTx	5% HF	RT	24	50
Mo2Ga2C	Mo2CTx	50% HF	50	3	51
(Ti0.5Nb0.5)2AlC	(Ti0.5Nb0.5)2CTx	51% HF	RT	28	52
Ti3AlC2	Ti3C2Tx	50% HF	RT	2	53
(V0.5Cr0.5)3AlC2	(V0.5Cr0.5)3C2Tx	50% HF	RT	69	52
Ta4AlC3	Ta4C3Tx	50% HF	RT	72	52
Nb4AlC3	Nb4C3Tx	50% HF	RT	96	54
V4AlC3	V4C3Tx	40% HF	RT	165	55
Ti3AlCN	Ti3CNTx	30% HF	RT	18	52
Mo2TiAlC2	Mo2TiC2Tx	50% HF	RT	48	56
Mo2Ti2AlC3	Mo2Ti2C3Tx	50% HF	55	90	57
(Mo2/3Y1/3)2AlC	Mo4/3CTx	48% HF	RT	60	58
(Mo2/3Y1/3)2AlC	Mo4/3CTx	10% HF	RT	72	58
(Nb2/3Sc1/3)2AlC	Nb4/3CTx	48% HF	RT	30	59
(W2/3Sc1/3)2AlC	W4/3CTx	48% HF	RT	30	60
Zr3Al3C5	Zr3C2Tx	50% HF	RT	60	61
Hf3[Al(Si)]4C6	Hf3C2Tx	35% HF	RT	60	62
Ti2AlC	Ti2CTx	0.9 M LiF + 6 M HCl	40	15	63
Mo2Ga2C	Mo2CTx	3 M LiF + 12 M HCl	35	384	64
V2AlC	V2CTx	2 g LiF + 40 M HCl	90	48	65
Ti3AlC2	Ti3C2Tx	3 M LiF + 6 M HCl	40	45	66
Ti3AlCN	Ti3CNTx	0.66 g LiF + 6 M HCl	35	12	67
Cr2TiAlC2	Cr2TiC2Tx	5 M LiF + 6 M HCl	55	42	56
(Nb0.8Zr0.2)4AlC3	(Nb0.8Zr0.2)4C3Tx	LiF + 12 M HCl	50	168	68
(W2/3Sc1/3)2AlC	W4/3CTx	4 g LiF + 12 M HCl	35	48	60
Ti3AlC2	Ti3C2Tx	1 M NH4HF2	80	12	69
Ti3AlC2	Ti3C2Tx	NH3F	150	24	70
Ti4AlN3	Ti4N3Tx	59% KF + 29% LiF + 12% NaF	550	0.5	71

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2.1 Top-down techniques

These techniques involve the depilation of the precursor MAX phases to form MXenes.⁷² The precursor for this method is the Al-containing MAX phases that have given rise to more than 30 MXenes to date and they inherit the structural ordered features of their corresponding MAX phases.²⁸ After the selection of a suitable precursor, it is exposed to etching, which can be done by using fluoride-carrying aqueous solutions, molten-salt etching, or by alkaline/hydrothermal treatments. High-temperature etching can give rise to 3-D phases, and hence low-temperature methods are promoted for the formation of 2D structures.⁷³ Once etching is done, due to the large numbers of cations and organic molecules present within these multilayers, suitable molecules have to be brought in to separate them into single nanosheets. These by-products are washed away by the process of sonication or shaking with liquid molecules. If it is done by HCl/LiF, direct sonication cannot be applied.^74,75 The most commonly used polar organic molecule for exfoliating Ti₃AlC₂ is dimethyl sulfoxide (DMSO).^76,77 The position of (0002) peaks in the XRD pattern represents the interlayer d-spacing of 9.8 Å. The co-instantaneous intercalation of DMSO and water results in the inflation of the d-spacing to about 22.4 Å.⁷⁷ This sonication process can also be carried out with a solution of tetrabutylammonium hydroxide (TBAOH).⁷⁸ The intercalation of TBA ions into the layers will be enhancing the interlayer distance greatly and therefore, this method is most commonly used for exfoliation purposes. Fig. 2(A) and (B) illustrate the SEM and (C) TEM micrographs obtained during the top-down synthesis scheme of Ti₃C₂T_x MXene from the Ti₃AlC₂ MAX phase as shown Fig. 2(D).

Fig. 2 SEM micrographs of (A) the Ti₃AlC₂ MAX phase and (B) Ti₃C₂T_x multi-layered MXene. (C) The TEM micrograph of the overlapping MXene layers. This figure has been reproduced/adapted from ref. 79 with permission from Elsevier Inc. (D) A schematic diagram of the etching and delamination of Ti₃C₂T_x MXene. This figure has been reproduced/adapted from ref. 80 from Chemical Society Reviews 2020.

2.1.1 HF Etching. This is the conventional or oldest method of MXene synthesis, where the powdered MAX phase is submerged in a specifically concentrated aqueous HF solution at a predetermined temperature for a particular period.⁸¹ Selective etching can give rise to multi-layered MXenes, which on sonication yield the desired MXene nanosheets.⁸² The concentration of the etchant used, time taken for etching and reaction temperature vary according to the type of metal in the MAX phase, as there is an increase in bond energy with the increase in the atomic number;^21,46 the type of MXene to be synthesised can also alter the etching conditions. Studies have revealed that it is comparatively easier to synthesise transition metal carbides than nitrides.⁸³ Not all MXenes can be produced by HF etching; however, Ta₄C₃T_x, TiNbCT_x, Ti₃CN_xT_x, Nb₄C₃T_x, (Nb_0.8Zr_0.2)₄C₃T_x, etc., have been successfully prepared by this method.^{56,64,81,84,85} Because the process of HF etching is exothermic, precautions must be taken to control the reaction parameters to avoid hazardous circumstances. On taking an account of the issue, the direct usage of HF can be duplicated by the use of a mixture of fluoride salts and strong acid, which locally forms the HF required for etching.⁸⁶ Michael Ghidiu et al. reported the synthesis of MXene with the help of the milder LiF and HCl. The fascinating achievement was the formation of single- or few-layered MXene nanosheets with large interlayer spacing and no interaction, induced by the spontaneous intercalation of cations in metal fluorides and water molecules into the layers during etching, without any additional intercalating reagent.⁶⁶ Another method for alleviating the rigorous behaviour of HF is with the aid of HF-containing NH₄HF₂. It was used in etching epitaxially developed Ti₃AlC₂ by Halim et al. in 2014 to obtain MXenes with larger interlayer spacing due to the presence of NH₃ and NH₄⁺ intercalated between the layers.⁶⁹ This result is advantageous for the delamination of multilayer MXenes and the introduction of metal ions like Li⁺, Na⁺, K⁺, etc. into these layers.⁸⁷ Simon et al. experimented to proclaim that the intercalation of surfactants could control the interlayer spacing, which would, in turn, improve their electrochemical performance.⁸⁸Fig. 3 represents an illustration where MXene foam is generated by the LiF/HCl etching of the Ti₃AlC₂ MAX phase.

Fig. 3 Schematic representation of the fabrication of a flexible and hydrophobic MXene foam. The image was adapted/reproduced from ref. 89 with permission from Advanced Materials 2017.

2.1.2 Other top-down methods. Methods for the synthesis of fluorine-free MXenes include molten salt etching, alkali etching assisted by high-temperature, and electrochemical etching.^46,86 Even though fluorine has a high affinity towards the A layers of the MAX phase, higher concentrations reduce the MXenes’ conductivity, thereby restricting application in energy storage.⁹⁰ MXenes prepared by molten salt etching have proved to be efficient in manufacturing negative-electrode materials with a high rating for electrochemical energy storage.⁹¹ One of the viable methods of preparing MXenes deprived of fluorine is by Lewis acid etching. Li et al. used ZnCl₂ as a Lewis acid to prepare MXenes that are surface terminated with chlorine from Al-layered MAX phases (Ti₃AlC₂, Ti₂AlC, Ti₂AlN and V₂AlC) in an argon environment at 550 °C for 5 hours.^92,93 The quantity of ZnCl₂ should be taken in excess so that the formation of the Zn MAX phase is prevented. However, Zn by-products will be formed and this can be removed by treatment with hydrochloric acid. Later, in 2020, Youbing Li and co-workers synthesised MXenes from Al-containing and Si, Zn, and Ga-containing MAX phases.⁹⁴ CuCl₂ molten salt was used to etch Ti₃SiC₂ at 750 °C to obtain the corresponding MXene and the Cu formed as a by-product was washed out by (NH₄)₂S₂O₈ solution. This increases the electrochemical performance of the MXene as they provide terminations of O and Cl at the surface. Later, O, NH, S, Cl, Se, Br and Te-terminated MXenes were produced by treating them with molten inorganic salts.⁹⁵

Another method used to prepare fluorine-free MXenes is by treating the MAX phases with an alkali solution at a high temperature. If the reaction is conducted without high-temperature conditions, etched Al forms an aluminum oxide or hydroxide layer around the MAX phases, prohibiting further reactions. Li et al. described the etching of Ti₃AlC₂ with a 27.5 M NaOH solution at 270 °C.⁹³ Ti₃C₂T_x electrodes made free of fluorine showed a higher gravimetric capacitance (314 F g⁻¹), which was 214% more than that made out of HF-etched MXenes.⁹⁶

Electrochemical etching is another method for synthesising MXenes without the utilisation of toxic substances. However, the reaction parameters and requirements must be strictly met to prevent excessive etching. Over-etching can cause the development of a carbon layer over the MAX phase that prevents subsequent etching reactions.⁹⁷ Yang et al. prepared MXenes through a controlled lithiation-alloying expansion micro explosion mechanism.⁹⁸ In this process, Ti₃C₂T_x MXene layers can be synthesised after lithium intercalation and sonication in water. Lithium intercalation processes give rise to 100% volume expansion. This Al–Li alloy immersed in water reacts to form hydrogen gas by a micro-explosion process, which leads to the formation of exfoliated MXenes nanosheets. The exfoliating process is performed in water, which automatically avoids the usage of toxic etchants.

2.2 Bottom-up synthesis techniques

There are various MXenes with certain stoichiometries that cannot be synthesised through selective etching and they can utilise this method of synthesis. Ultra-thin 2D films with high crystalline quality can be fabricated using bottom-up methods like chemical vapour deposition, the template method, and plasma-enhanced pulsed laser deposition.⁷⁹ Single layers are not formed through these methods; instead, thin films of few layers are prepared. Also, the films prepared by this method possess superconductive transitions within.⁵⁹

2.2.1 Chemical vapour deposition (CVD). Even though the notable hurdles put forward in HF etching were overcome by the subsequent substitutes, a more suitable and sustainable method called chemical vapour deposition was introduced. Researchers prefer this method because it can regulate the material thickness, which was found to be of great importance.^87,99 Larger-sized MXenes with fewer functional defects can be synthesised by this method. Xu et al. used a copper/transition bilayer metal foil for growing α-Mo₂C crystals by this method.¹⁰⁰ Mo₂C crystals were formed by heating the stack to 1085 °C in hydrogen and then using the methane flow rate. The Cu layer acts as a catalyst to form carbon from methane and also channels the diffusion of Mo atoms to liquid Cu, which eventually forms ultrathin layers. Although these layers were stable and exhibited superconducting transitions, they were a combination of 6 ultrathin layers and not a single MXene nanosheet. The thickness of the Mo₂C crystals was found to be altered by altering the thickness of Cu foil and methane concentration.¹⁰¹ Polygonal crystals were obtained after the alterations. Wang et al. synthesised Ta-compounds using CVD by heating the tantalum-copper bilayer with the precursor (C₂H₂, B powder, and NH₃).¹⁰² The copper foil was kept in the solid-state and ultrathin TiC, TiN, and TiBr were prepared by using boron instead of ammonia. The thickness of 2D crystals was controlled below the melting temperature of copper.

Although it produces high-quality films, this technique is not suitable for the synthesis of MXenes as they produce thin layers rather than a single layer. Additionally, high production costs and low yields also set forth a hurdle for its further application.

2.2.2 Other bottom-up methods. Increasing the yield of production was the core concern in moving from CVD to other techniques. An alternative method used for the preparation of high-yield 2D transition metal carbides and nitrides is the template technique. This method uses 2D transition metal oxide nanosheets, which are further carbonized or nitrified. The structure of the synthesised carbides and nitrides depends upon the type of metal oxide used. This method converts 2D MoO₃ nanosheets by heating ammonia at 800 °C to make 2D hexagonal molybdenum nitride (MoN) nanosheets.¹⁰³ These layered 2D MoN nanosheets possess a lateral size of 20–30 μm and a thickness of 5–40 nm, which is thicker than that produced earlier. Another bottom-up method for a similar preparation is the plasma-enhanced pulsed laser deposition method (PEPLD). This method is the integration of chemical vapour deposition and pulsed laser deposition methods. Zang et al. successfully developed continuous Mo₂C ultrathin single-crystalline films by plasma-enhanced pulsed laser deposition.¹⁰⁴ Methane is used as the carbon source; the substrate used is sapphire and is heated at a temperature of 700 °C. The films formed by PEPLD exhibited low crystalline quality and an increased number of stacking faults prevailed in the films. Advanced measures have to be taken for the improvement of the bottom-up synthesis method of MXenes to make the process more sustainable, non-toxic and productive (Table 2).

Table 2 Advantages and disadvantages of different MXene synthetic methods¹⁰⁵

Type of method	Advantages	Disadvantages
HF etching	• Convenient and easy method	• The reaction is highly exothermic and may invite hazardous situations.
	• Produces high-quality MXene layers	• HF can be corrosive/poisonous.
	• Retains the surface properties.
HCl + fluoride salt etching	• Less toxic than HF	• Time-consuming multi-step synthesis
	• Enhances the cation intercalation
Molten salt etching	• Fast processing time	• Highly energy-consuming process
	• Produces MXenes of limited stability	• Produces MXenes of poor crystallinity and purity.
		• A large number of vacancies and surface faults.
CVD technique	• Produces high-quality films.	• High production cost and low yields.
	• Regulates the material thickness.	• Produce thin layers rather than a single layer.
	• Sustainable method.
PEPLD technique	• Produces films that are ultra-thin and continuous.	• Low crystalline quality.
		• A large number of stacking defects.

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3. Characteristic features of MXenes

MXenes, with their versatile physical, chemical, electrical, and magnetic properties, have attracted many researchers to explore the scope for their employment in various applications. As we have already observed, the synthesis environment, the chemistry of the etchant used, the properties exhibited by the corresponding MAX phases and surface terminations are the general factors that influence the properties of MXenes.^106,107 The 2D morphology, ion mobility and electronic transport features make it convenient to fabricate electrodes with expected performances for energy storage. Studies have revealed that Density Function Theory (DFT) has helped to unveil the chemical and physical properties of MXenes.⁵⁶ According to DFT, Sc-based MXenes are more semiconductor-like with a free carrier density of 8 ± 3 × 10²¹ cm⁻³, mobility of 0.7 ± 0.2 cm² V⁻¹ s⁻¹, an electrical conductance of 9.5 × 10⁴ S cm⁻¹ and Hall carrier mobility of 54.58 cm² V⁻¹ s⁻¹.^108,109 The first principal study also manifested that Ti-based, alkali-adsorbed MXenes like Ti₃C₂T_x and Ti₂CO₂ were metallic.^110,111

3.1 Mechanical properties

MXenes exhibit superior flexibility and high mechanical strength. However, the mechanical properties can vary with the number of layers denoted by the letter n in the structural formula of the surface-modified MXenes. M₂X is stronger and more rigid as compared to M₃X and M₄X MXenes as stated by DFT and molecular dynamics.^112,113 The elasticity, optical response, and crystal structure also show strong reliability on the surface terminated groups. Due to the smaller lattice constants, the elastic stiffness of the O-terminated MXenes is predicted to be much higher than F/OH terminated MXenes.¹¹⁴ Guo et al. explained that surface functional groups form a buffer layer when Ti₂C is exposed to tensile deformation, thereby increasing its resistance towards strain and slowing down the fracture of Ti layers.¹¹⁵ Kurtogolo et al. explained the smaller modulus of MXene with the help of first principles theory, while Lei et al. presented a decline in the surface modulus and enhancement in the flexibility of MXene as compared with graphene.^112,116 Recent studies have revealed that the formation of composites of MXenes like Ti₃C₂T_x–PVA, Ti₃C₂T_x–PVA can enhance the mechanical strength and elasticity even more.^70,117

3.2 Thermal stability

In MXene-based nanomaterials, thermal stability can be viewed as a crucial element as it directly influences the properties, thereby restricting their applications.¹¹⁸ The type of highly active transition metal, etching agent, surface functionalisation, etc. can affect the stability of MXenes. According to Seredych et al., the thermal stability of Ti₃C₂T_x MXene was comparatively superior to Nb₂CT_x and Mo₂CT_x MXene and can be improved using weak etchants.¹¹⁹ Absorbed functional groups establish a negative charge on the surface of MXenes and their edges will be positively charged.¹²⁰ Hence the oxidation process starts from the edge and grazes into the entire MXene nanosheets. Barsoum et al. declared that polyanions adsorbed at the edge of MXenes nanosheets diminish the oxidation tendency of nanosheets, even if the MXenes are exposed to aerated water for several weeks.^121–123 Because oxidation can deteriorate the properties of MXenes, the proper choice of synthesis methods, and regulating the storage environment and edge capping methods can improve the stability and performance of O-terminated MXenes.¹²⁴

3.3 Adsorption sites and surface functionalisation

There are four prevailing adsorption sites for the functional groups in the MXenes.¹²⁵ Firstly, the functional groups can be found on the top of the hollow sites above the metal M ions. Secondly, the functional group can be attached at the top of the hollow sites above the X ions. Thirdly, the functional groups can be directly located on the top of the transition metal (M) ions. Lastly, some of the functional groups located on the top of the hollow sites above transition metal (M) ions, with other functional groups can be located on the hollow sites of X ions. A comparative study regarding the binding energy of the surfactants O, OH, and F on 2D MXene surfaces was done by Ashton et al.¹²⁶ A study was conducted to understand the thermodynamic stability of these compounds and their variation in chemical composition. On referring to the thermodynamic stability of all 54 MXene compounds with M_n+1X_nO₂ (M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta; X = C, N; n = 1, 2, 3), 38 were found to have formation energies of 200 meV per atom. Among these 38 compounds, only six are assumed to possess formation energies below 100 meV per atom. However, Sc-based MXenes are highly stable because their surfaces are terminated with F. They can easily produce freestanding single layers that result in the easiest exfoliation.

3.4 Electric and magnetic properties

MXenes are known to be versatile as they exhibit dynamic electrical properties ranging from insulation in the bulk to metallicity, semiconductivity and even superconductivity, which are strongly influenced by surface functionalisation, the precursor MAX phase and etching process.^95,127 The studies on this interesting behaviour by Bansori et al. confirmed that the molybdenum-containing MXene, Mo₂TiC₂T_x with OH termination possesses high conductivity, though Ti₃C₂T_x shows metallicity. The synthesis method can also alter the electrical conductivity of the MXenes. The usage of mild etchants and proper delamination can give rise to MXenes with larger-sized flakes and fewer defects, which will automatically increase conductivity.¹²⁸ According to first principles calculations, alteration of Fermi levels by surface terminations can change the conductivity of MXenes.¹²⁹ Surface terminations induce changes in the work functions of the MXenes. Recent researchers established that OH-terminated MXenes exhibit very low work functions (1.6–2.8 eV), O-terminated ones increase the work function, whereas F-terminated ones increase or decrease it based on the type of material used.¹³⁰ Increased electronic transport and conductivity can help MXenes to be deployed in the manufacture of electrode materials for various alkali-ion and metal-ion batteries.

The strong covalent bonding between metal, X element and the terminal group makes pure MXenes non-magnetic. However, when exposed to mechanical strain, the magnetic moments of such non-magnetic MXenes show a transition from zero magnetism to ferromagnetism.^131,132 MXenes can also be combined with suitable compounds to form composites with even better conductivity and magnetoresistance can act as a frontier to the further exploration of MXene application in the field of electrochemical energy storage.

4. MXenes in alkali-ion batteries

MXenes with their prominent electrochemical performance can be used for preparing electrodes for alkali-ion batteries. Lithium–ion batteries have become popular in the field of electrochemical energy storage with their high energy density, reliability and cyclability. However, their natural abundance and similar properties to Li have helped Na and K to get employed in energy storage applications. An inevitable challenge to be faced for the large-scale production of SIBs and PIBs is the requirement for electrode materials with large intercalation spaces due to their larger ionic radii. Studies have revealed that surface-functionalized MXenes that can form well-defined composites can meet the requirements for the manufacture of SIBs with advanced storage capacities and low diffusion barriers for sodium.^31,32 Because of the radius of the potassium ion, PIBs can be intercalated within MXenes to form composites to achieve the expected performance.³³ The proper design of electrode materials can improve their storage capacity, electronic transport, and efficiency to improve the performance of LIBs as well as non-LIBs. Fig. 4 illustrates the schematic representation of the measures that can be adopted to enhance the rate of performance in non-lithium AIBs.

Fig. 4 A pictorial representation of the various methods that can deploy MXenes in non-lithium alkali-ion batteries to enhance their performance rates. The image was adapted/reproduced from ref. 133 with permission from Advanced Materials 2021.

4.1 MXenes in lithium-ion batteries

Lithium-ion batteries with their appreciable electrochemical properties can be deemed as the most convincing option for energy storage in portable devices, electric vehicles and forthcoming storage devices.¹³⁴ During the initial phase, the conventional C/LiCoO₂ electrode used in LIBs, due to the low ability of graphite for intercalation, high cost, toxic nature of Co and Li_1−xCoO₂ phase thermal stability, was displaced by the C/LiFePO₄ configuration. Although the latter solved the affordability and environmental toxicity issues, the energy density was diminished due to reduced operating voltage.¹³⁵ The advent of MXenes paved the way towards the manufacture of advanced hybrid electrodes that exhibited outstanding stability, cyclability and rate performance.¹³⁶ The storage of lithium-ion MXene-based electrodes is greatly influenced by chemical components, surface terminations, doped atoms, and porous structures.¹³⁷ Theoretically, it was found that gravimetric capacities vary inversely with the formula weight of the MXenes.¹³⁸ It was experimentally shown that Ti₂C exhibited a capacity of 160 mA h g⁻¹, whereas Ti₃C₂ possessed 110 mA h g⁻¹ due to the formation of the inactive TiC layer in Ti₃C₂.¹³⁹ However, V₂C displayed a higher capacity of 280 mA h g⁻¹ than Nb₂C and Ti₂C, indicating the influence of the type of transition metal present, as well as preparation environments.⁴⁹ The surfactants like F and OH choke Li⁺ transportation and inhibit the storage of Li⁺ in Ti₃C₂ and V₂C, whereas O-functionalized MXenes boost Li⁺ storage capacity.^140,141 Once we practically formed the pure MXene anodes, we came to know that the reversible capacity was comparatively lower than predicted. A 2D Hf₃C₂T_z MXene was made by introducing silicon to intensify the Al (Si)–C bond and reduce the adhesive energy of the Hf–C bond, exposing a reversible gravimetric capacity of 146 mA h g⁻¹ at a current density of 200 mA g⁻¹ at the end of 200 cycles.⁶²

The introduction of a heteroatom can also alter the properties of MXenes. The intercalation of Co²⁺ ions into multi-layered V₂C MXenes yielded a widened interlayer spacing (9.52 Å) due to the strong V–O–Co bond formation. This method achieved an excellent storage capacity of 1117.3 mA h g⁻¹ at 0.1 A g⁻¹ and a very long cycling life of over 15 000 times.¹⁴² The characteristic features of this electrode material for electrochemical storage are shown in Fig. 5. Because there is a reduction in the contents of heteroatoms in the synthesised MXenes, various improvements can be made to make it impressive. Moreover, the improvement of ion diffusion kinetics within the electrode material is also achieved by the pore engineering of MXene-based electrodes. A freestanding MXene 3D macroporous foam was derived from the sulphur-template method to exhibit a high capacity of 455.5 mA h g⁻¹ at 50 mA g⁻¹, a high rate capacity of 101 mA h g⁻¹ at 18 A g⁻¹ and very long cyclic stability.¹⁴³ This method can improve the energy density of LIBs, by restricting the usage of Cu/Al current collectors within it.

Fig. 5 The performance of the multi-layered V₂C MXene with the intercalation of Co²⁺ ions. (a) Specific capacity curves of the V₂C electrode at different scan rates. Inset: The change in log(i) vs. log(v). (b) Rate performance of V₂C and V₂C interlayer engineered electrodes. (c) A comparative analysis of galvanostatic charge and discharge curves under different current densities V₂C and Co²⁺ interlayer-engineered V₂C electrodes. (d) A comparison of the EIS spectra of V₂C and Co²⁺ interlayer-engineered V₂C electrodes. Inset: Impedance phase angle vs. frequency. The images were adapted/reproduced from ref. 142 with permission from Advanced Materials 2018.

Another way to improve the existing properties and to hamper the hurdles put forth is by the fabrication of MXene-based hybrids. Compositing 2D MXenes with 0D materials like nanodots, nanospheres and nanoparticles to form LIB electrodes enhances the properties. Herein, the 2D MXene nanosheets remediate the volumetric variance that occurs in charge/discharge cycles and 0D materials restrict the restacking of these layers. This increases the ion diffusion kinetics and storage. The facial CO₂ oxidation of Nb₄C₃T_x and Ti₃C₂T_x gives rise to new 0D hybrids Nb₂O₅@Nb₄C₃T_x and TiO₂@Ti₃C₂T_x, respectively. Both composites express higher lithium storage performance when compared to bare MXenes due to the intimate coupling of interfaces.¹⁴⁴ Further strategies have to be invented to obtain highly coupled interfaces in MXene/0D hybrids for electrodes with better performance. In the sandwich model the 0D materials, MXene layers, and coating layers are assembled, to accommodate their surface pulverization. The fabrication of PVP–Sn(IV)@Ti₃C₂ sandwiched hybrids can be done by the method of PVP (polyvinyl pyrrolidone)-assisted liquid-phase immersion of Sn nanoparticles into the MXene matrix, delivering an appreciable reversible capacity of 1375 mA h cm⁻² and high cycling stability after 50 cycles.¹⁴⁵

The integration of 2D MXenes with 1D nanomaterials like nanofibers, nanowires, nanotubes, nanobelts, nanoribbons, or nanorods, forms MXene/1D hybrids that are either vertically or horizontally loaded. The vertically loaded CNTs and CNFs with MXene nanosheets enhance the overall electrode conductance and rate performance of LIBs.^146,147 Similarly, non-carbon oxides can also be introduced to obtain high transport kinetics and cycling stability with excellent capacity retention.¹⁴⁸ Horizontally loaded TiO₂ nanorods and SnO₂ nanowires in Ti₃C₂ not only offer a short lithium diffusion length and additional active sites in the electrode material but also prevent the aggregation of the nanosheets.¹⁴⁹ As a result, a maximum capacity of 138 mA h g⁻¹ was achieved at a current density of 2000 mA g⁻¹, claiming its importance in making anode materials for LIBs with excellent power density and energy density.

The horizontal loading of 2D nanomaterials with 2D MXene nanosheets produces improved gravimetric capacity, whereas high-rate anodes with excellent transport kinetics are achieved by vertical loading. The 2D MoS₂-on-MXene anode displayed an extraordinary charge capacity of 509 mA h g⁻¹ at 0.1 A g⁻¹ after 100 cycles, which is much higher than bare Mo₂TiC₂T_x with a 52 mA h g⁻¹ capacity.¹⁵⁰ A facile hydrothermal method was employed in the vertical loading of thin SnO_x nanosheets (∼5 nm) uniformly into the layers and surfaces of the Ti₃C₂ matrix to form SnO_x@Ti₃C₂ composites.¹⁵¹ They provided an enhanced capacity of 450 mA h g⁻¹ at 0.2 A g⁻¹ over 250 cycles, revealing the possibilities of further exploring these methods (Table 3).

Table 3 A brief outlook on the electrochemical performances of MXenes and MXene-based materials for lithium-ion batteries

Electrode material	Current rate (mA g^−1)	Reversible capacity (mA h g^−1)	Cycle number	Capacity after cycle (mA h g^−1)	Ref.
Ti2CTx	0.04C	225	80	110	152
Ti3C2Tx	260	123.6	100	118.7	153
Nb4C3	100	310	100	380	154
Sn@V2C	100	1284.6	90	1262.9	155
Ag/Ti3C2	1C	330	5000	310	156
CNTs@Ti3C2	1000	479	250	445	146
Mo2CTx/CNT	5000	560	1000	250	64
Nb2CTx@CNT	2.5C	420	300	430	157
LiMn2O4/Ti3C2Tx	1C	—	200	114.1	158
CNF/Ti3C2	1C	320	2900	320S	147
BPQDs/Ti3C2	1000	170	2400	520	159
TiO2(nanorod)/Ti3C2Tx	500	204	200	209	149
SnO2(nanowire)/Ti3C2Tx	1000	560	500	530	149
Fe3O4@Ti3C2	1C	336.8	1000	747.4	160
Si@SiOx@C/Ti3C2Tx	0.2C	1674	200	1547	161
Na0.55Mn1.4Ti0.6O4/Ti3C2Tx	5000	440	5000	350.68	162
Bi2MoO6/MXene	100	—	200	692	163
MoS2/Mo2TiC2Tx	100	646	100	509	150
(Ti3C2)/rGo	50	335.5	1000	212.5	164
δ-MoN	0.05C	336	200	320	103

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There are a few other lithium-based batteries, such as lithium–metal batteries, lithium–air batteries and lithium sulphur batteries, which are expected to overrule the area of high-performance batteries.

4.1.1 Lithium–metal batteries. Lithium metal can be conceded as a perfect anode material for lithium-based batteries because of its extremely low potential and high theoretical specific capacity.¹⁶⁵ lithium–metal batteries have attained great popularity in energy storage applications due to their very high theoretical specific capacity of 3860 mA h g⁻¹, very low negative potential (−3.04 V vs. the standard hydrogen electrode), low density of 0.534 g cm⁻³ and the outstanding material flexibility of Li metal. Therefore, they are considered to be the most relevant high-energy density next-generation batteries along with lithium–sulphur batteries, lithium–air batteries and lithium-ion batteries.^86,166 Although this can give rise to lithium batteries with high operating voltage and enhanced energy density, the commercialisation of the Li metal anode is not well appreciated. A serious challenge faced is dendrite formation on the electrode surface, perforating the diaphragm, which may cause a short circuit and even lead to hazardous situations.¹⁶⁷ Lithium dendrites can be easily peeled off to form “dead lithium” with hindered electrochemical activity, further rapidly diminishing the reversible capacity. However, the large variation in volume, inevitable dendrite formation, short life span low electrode conductivity, and fluctuating solid electrolyte interphase (SEI) films can be named as the major challenges that forestall their commercial applications.^168,169 Therefore, Li metal anodes with large current density (>5 mA cm⁻²) and high Li loading (>90%), which are dendrite-free, are requisite for high-performance batteries. This can be accomplished by the proper engineering of the metal anodes and by selecting/modifying a suitable electrolyte. Wu et al. devised a dendrite-free Li metal anode by designing a light-weight framework of MXene and graphene.¹⁶⁵ This highly conductive and lithiophilic 3D network enabled the suppression of Li dendrite formation, escalation of storage capacity and compensation of volume changes. Another remarkable feature was the record lifespan of the electrode (2700 h); it showcased stability after 230 cycles at 20 mA cm⁻², which is an exceptionally high current density. Zhang et al. developed 3D porous MXene aerogels displaying high electron conductivity, Li⁺ transport kinetics, and plentiful Li nucleation sites, which act as scaffolds for the Li metal electrode.¹⁶⁸ They ensure the uniform nucleation of the metal ion in the presence of the oxygen-rich surface functionalisation on the MXene sheet. A notable advantage of MXene aerogels is the networked interspaces that help to cushion volume changes and accomplish high lithium loading. Along with all these features, the cross-linked structure of MXene sheets establish a channel for the fast transportation of ions and hence, produced appreciably high cycling stability, even up to a current density of 10 mA cm⁻². In 2019, Yang et al. contrived a perpendicular MXene–Li array with tunable MXene walls to achieve a dendrite-free, high-rate electrode material for LMBs.¹⁶⁹ This array was comprised of fixed periodic nanoscale and microscale interspaces, facilitating the fast-paced Li⁺ transfer while stripping/plating and the homogenization of the electric field, respectively. This electrode resulted a low potential of 25 mV, outstanding capacity of 2056 mA h g⁻¹, and appreciable cycle stability up to 1700 h.

Another way to obtain high-capacity, dendrite-free, long-living electrode materials is by the optimisation of the electrolyte. MXene-based nanocomposite polymer electrolytes (CPEs) can be synthesised for high-performance solid-state LMBs by an aqueous solution blending method.¹⁷⁰ The MXene-based CPEs (MCPEs) fabrication is done by the uniform dispersion of Ti₃C₂T_x flakes into PEO₂₀–LiTFSI (poly (ethylene oxide)/lithium bis(trifluoromethyl sulphonyl)imide) complex. The ionic conductivity of the electrolyte is improved with the introduction of these 2D flakes into PEO, as it encourages segmental motion and obstructs the crystallisation of PEO. It was experimentally derived that CPE with 3.6 wt% MXene exhibited the highest ionic conductivity (2.2 × 10⁻⁵ S m⁻¹ at 28 °C) at room temperature and the MCPE with 1.5 wt% MXene possessed high-rate capability and stability in comparison to the latest CPELMB. This indicates that MXenes are best compared to other 0D/1D nanofillers, and are armed to enhance the performance of electrolytes for LMBs. Both methods proclaim the significant role played by MXenes in the development and promotion of highly efficient electrode materials for LMBs. Diverse methods of synthesis have resulted in compounds with diverse properties and features. Hence, more methods have to be introduced to make it feasible for the commercialisation of LMBs.

4.1.2 Lithium–sulphur batteries. Lithium–sulphur batteries have attained much attention due to the high capacity, high energy density, simple configuration, and environmentally amiable nature of sulphur materials.¹⁷¹ Unlike lithium–metal batteries, sulphur is the cathode material of LSBs, which undergoes multi-electron reactions during charging and discharging cycles. However, further applications are restrained for certain reasons. During the charge/discharge cycles, lithium polysulphide intermediates are generated, which rapidly decreases the overall performance of the battery.¹⁷² The dissolution of polysulphide intermediates as well as sulphur in non-aqueous solvents like DMSO, THF, dioxolane and glyme, may lead to its reaction with lithium, which induces the formation of an insulating layer on the surface of the lithium anode, eventually generating severe polarization.¹⁷³ Efforts have been made to engineer and select suitable electrode materials to absorb the polysulphides.^174–176 Sulphur is also an insulator that exhibits very low conductivity.¹⁷⁷ Additionally, the conversion of S into Li₂S gives rise to a volume expansion of 80% and is not favourable for retaining the electrode stability. MXenes with their extraordinary properties have channelled a new area for the manufacture of high-capacity Li-based electrode materials. It was also observed that the engineering of MXene nanoribbons effectively improved their electrochemical performance. Dong et al. used Ti₃C₂ nanoribbons as the sulphur host to attain the expected rate performance and durability.¹⁷⁸ The a-Ti₃C₂-S/d-Ti₃C₂/PP electrode was prepared by the application of Ti₃C₂ nanosheets on the polypropylene separator as an interlayer and it hampered the shuttle effect of lithium polysulfide through chemical absorption and physical blocking. This electrode material exhibited a high reversible capacity of 1062 mA h g⁻¹ at 0.2C and an enhanced capacity of 632 mA h g⁻¹ for up to 50 cycles at 0.5C, which is remarkable when compared with conventional a-Ti₃C₂-S on an Al current collector. Heteroatom doping can be performed to enhance the cyclic stability and rate capability of LSBs. Bao et al. designed N-doped MXene nanosheets by an annealing process, which showed a crumpled morphology as a host material for sulphur.¹⁷⁷ N-Doped MXene nanosheets were experimentally found to have strong physicochemical adsorption capacities. This facilitates the effective absorption of polysulphides and encourages sulphur loading of 5.1 mg cm⁻² over a wide area. The crumpled morphology of N-Ti₃C₂T_x/S achieved the expected electrochemical properties with a high reversible capacity of 1144 mA h g⁻¹ at 0.2C and the cycling stability was extended to 610 mA h g⁻¹ for up to 1000 cycles at 2C. LSBs with the modified Li-metal anode and sulfurized polyacrylonitrile (S@PAN) cathode can produce high capacity and promise a high performance.¹⁷⁹ Also, Mo₂CT_x integrated with MXene nanosheets can form S@Mo₂CT_x, which enhances the ion transport kinetics due to high conductivity and a low diffusion barrier.¹⁸⁰ This material displays a high reversible capacity (918 mA h g⁻¹) at 1C and enhanced cycling stability. Yao et al. used first-principles calculations to study S-functionalized Ti₂N (Ti₂NS₂) as the host materials for LSB cathodes and it was found to enhance the conductivity and hinder the shuttle effect.¹⁸¹ 3D-structured MXene nanosheets can also alleviate the inherent problem of LSBs by absorbing the discharged substances within their porous structure and thereby increasing their conductivity and ion transport.^182,183 The selection of different materials manifested different properties for the synthesised electrode material. A suitable combination of materials/compounds helps in the fabrication of electrode materials with desired properties to make them popular among the manufacturers.

4.1.3 Lithium–air batteries. LABs or lithium–air batteries have acquired great popularity in recent days due to their high energy density (10 times LIBs) and power density and hence they can be employed in future technology for long-range electric vehicles (>500 km).¹⁸⁴ The general composition of LABs is a metallic Li anode and a porous cathode (oxygen being the active material) with suitable Li-containing electrolytes that can be aqueous, non-aqueous, hybrid or solid electrolytes.¹⁸⁵ The electrochemical interaction between O₂ and Li⁺ in LABs results in the formation of Li₂O₂ during the discharge process and is stored within the cathode. Hence, this electrode plays a crucial role in influencing the storage capability of LABs and the reversibility of Li-ions. Even though they are viewed as the future generation of environmentally amiable rechargeable batteries, their low capacity, low rate capability, large voltage gaps between charging/discharging cycles, and poor cycle life hinder their practical applications.^186,187 Apart from all the problems mentioned, the lethargic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the critical factors restricting the further development of LABs.¹⁸⁸ It reflects the priority of an efficient catalytic system to improve the reaction rate, thereby enhancing the performance rate and cycle life of LABs. Owing to their large surface area, high electrical conductivity, porous structure and less expensive nature, carbon materials have acquired great popularity in the manufacturing of electrodes in LABs. Irrespective of these advantages, carbon electrodes are generally corrosive and may induce associated reactions and therefore reduce the reaction efficiency of the cathode and the lifetime of the battery.¹⁸⁹ Recent studies revealed that MXenes can emerge as a promising cathode material for LABs due to their structural stability, highly appreciable surface area and outstanding electron/ion conductivity.¹⁹⁰ The combination of a high-yielding catalyst and MXenes can enhance the performance of the electrode materials of LABs. Recently, Zheng et al. synthesised a V-TiO₂/Ti₃C₂T_x composite by an ethanol-thermal method that involves the in situ growth of high-efficiency catalytic TiO₂ nanoparticles with surplus oxygen vacancies on Ti₃C₂T_x nanosheets.¹⁹¹ This system possessed certain advantages that encourage its application as a LAB electrode material. TiO₂ nanoparticles are tiny and are fixed evenly on multilayer MXene nanosheets, which enhance the stability and make active materials attainable. Another important feature is that MXene can stabilize oxygen vacancies, thus contributing to the improvement of electrons and Li⁺ transfer kinetics, as they are the active sites for cathodic catalysing reactions. In addition, the multilayer structure of this composite expedites the penetration of the electrolyte and hinders the electrode volume variation caused as a result of the deposition and decomposition of Li₂O₂. As a result, the V-TiO₂/Ti₃C₂T_x composite when employed as the cathode material displayed a high specific capacity of 11 487 at 100 mA g⁻¹, low overpotential of 0.21 V and appreciable cyclic stability where 93% round-trip efficiency was maintained even after 200 cycles. The dendrite formation hinders the popularisation of LAB. An important way to improve the energy efficiency and block redox shuttling, capacity fading, and electrolyte degradation, is by the use of redox mediators that will decompose the products discharged.¹⁹² Oxygen can lead to the corrosive effects of Li and stabilizing agents like LiNO₃ can be used to improve the battery life as it can act as a passivation interface between anodes.¹⁹³ The Ti₂C MXene with different multifunctional groups can give rise to high-performance LABs with appreciable energy density.¹⁹⁴ However, highly efficient LABs are still under research interest and improvements have to be made to promote them in the market (Table 4).

Table 4 A summary of the electrochemical performances of MXene-based materials for LABs and LSBs

Electrode material	Current rate (mA g^−1)	Cycle number	Capacity after cycle (mA h g^−1)	Rate performance (mA h g^−1) at current rate (mA g^−1)	Ref.
Lithium–sulphur batteries
MSC-2	167.5(0.1C)	200	946.7	502.3 at 2C	195
Ti3C2Tx/S	1675 (1C)	1500	940	1075 at 2C	196
Co-CNT@MXene/S	0.5C	200	600	789.29 at 0.5C	197
a-Ti3C2-S/d-Ti3C2/PP	2C	200	50.4%	288 at 10C	178
S@N-PC/Ti3C2	1C	800	527	595 at 2C	198
Ti3C2Tx(4h)-GN	2C	1000	–	663 at 2C	199
S@SA-Zn-MXene	1C	400	706	517 at 2C	200
Ti3C2Tx/RGO/S	0.5C	300	878.4	750 at 0.5C	201
S@TiO2/Ti2C	2C	200	464	317.7 at 5C	202
N-Ti3C2Tx/S	2C	1000	610	770 at 2C	177
MPP separators	0.5C	500	550	743.7 at 1C	203
CMP separator	1C	600	614	728 at 2C	204
50% S@ Ti3C2Tx	2C	175	1170	1161 at 2C	205
N-Ti3C2/C@PP separator	0.5C	500	716	675 at 2C	206
79S/CNT-Ti3C2	0.5C	1200	450	510 at 0.5C	172
MX-NF/PP	1C	1000	645	794 at 3C	207
S@V2C–Li/C	0.5C	500	600	400 at 5C	208
T@CP separator	0.25C	200	1100	950 at 2.5C	209
MXene/ESM separator	0.5C	250	877	948 at 1C	210
MXene/1T-2H MoS2-C-S	0.5C	300	799.3	677.2 at 2C	211
Ti3C2Tx@AlF3/Ni(OH)2	1C	1000	435	635 at 4C	212
N-RGO/Ti3C2Tx	0.5C	200	850	1180 at 0.1C	213
MPF13-550/PP	0.2C	200	721	593 at 2C	214
nMOF-867/Ti3C2Tx	1C	1000	801	581 at 4C	215
Graphene/MXene fibre	1C	1000	—	733.3 at 2C	216
S/MXene-CS2/IPA	1C	1000	522.7	1474.5 at 0.1C	217
MXene/CNC/S	0.1C	100	823.8	630.5 at 4C	218
3D S-CNT@MXene	4C	150	656.3	557.3 at 8C	219
Lithium–air batteries
CoO/Ti3C2Tx	100	160	—	8560 at 500	220
V–TiO2/Ti3C2Tx	200	100	1000	1.61 V at 800	191
NiO/Ti3C2	500	90	500	5790 at 500	221
LaSrCoO/Ti3C2Tx	500	80	1000	1.29 V at 1000	222
Nb2C MXene	3000	130	—	—	223

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4.2 MXenes in sodium-ion batteries

Sodium-ion batteries can be viewed as a strong alternative to LIBs with their cost-effectiveness and improved performance rates. However, the large ionic radii of Na⁺ ions demand electrode materials with large interlayer spacing for their integration. MXenes and their composites that accommodate ions of different sizes can therefore be employed as host materials in SIBs.¹³⁶ Along with surface tunability and mechanical flexibility, MXenes can provide a range of working potentials, enabling them to be used as either anodes or cathodes.^224,225 Surface-functionalised MXenes are favourable for the insertion/de-insertion of Na⁺ ions and show low diffusion lengths. When the Ti₃C₂T_x MXene is used as the substrate, the resulting anode exhibits appealing rate performance and cyclic stability.^226,227 Later, Yu et al. theoretically calculated Na⁺ storage capacities of 413.0 mA h g⁻¹ for interlayer expanded bare Ti₃C₂, 367.7 mA h g⁻¹ for O-terminated Ti₃C₂, and 151.2 mA h g⁻¹ for F-terminated Ti₃C₂, which occur within negligible volume changes (−0.5% to +1.6%) during intercalation/deintercalation.²²⁸ The doping of heteroatoms is a plausible method for adjusting the structural parameters of MXenes. It was estimated theoretically that a sulphur-doped MXene (Ti₃C₂S₂) exhibited appreciable conductivity and sodium storage capacity and a low diffusion barrier.²²⁹ S-Doped Ti₃C₂T_x, due to their enlarged multilayered morphology and surface-induced capacitance, provided a storage capacity of 183.2 mA h g⁻¹ at 100 mA g⁻¹ after 100 cycles, an unexpected rate capacity of 113.9 mA h g⁻¹ at 4000 mA g⁻¹ and outstanding cycling stability for 2000 cycles, surpassing all Ti₃C₂T_x anodes reported up to that time (<100 mA h g⁻¹).²³⁰ Moreover, with nitrogen being highly electronegative, Ti₃CN has emerged as a promising anode material in SIBs, producing a large capacity of 98.9 mA h g⁻¹ at 500 mA g⁻¹, which is 1.65 times the capacity of the Ti₃C₂ anode.²³¹ Although these results proclaim the improvement in the capacity and rate capability achieved in heteroatom doping, the MXene matrix is comprised of quite low doping contents.

Another way of achieving a high capacity of Na storage, increased rate capacity and ion kinetics is by establishing a pillaring effect, taking the electrostatic interaction of negatively charged MXenes and metal ions into account. Luo et al. formed pillared MXene sheets (Na–Ti₃C₂) with sufficiently enlarged interlayer spacing. The material exhibited an increased reversible capacity of 175 mA h g⁻¹ at 0.1 A g⁻¹, which is approximately 170% of the original value, and cycling stability for 2000 cycles at 2.0 A g⁻¹. There was a critical increase in the number of active sites and a decrease in the Na⁺ diffusion barrier post-Na⁺ pillaring as compared to that of Ti₃C₂, Li–Ti₃C₂, and K–Ti₃C₂.²³² Even though the nanoengineering of MXene layers has shown a rapid rise in the required properties, the precise tuning of interlayer spaces has not been achieved in all methods.

The introduction of porous structures can also be done to tune the transport properties. The porous Ti₃C₂T_x electrode possessed capacities of 166 mA h g⁻¹ at 1 A g⁻¹ and 124 mA h g⁻¹ at 10 A g⁻¹. The electrode also delivered a long cycle life of up to 1000 cycles with negligible capacity decay at 1 A g⁻¹.²³³ The performance results of these electrodes are displayed in Fig. 6. The enhancement of ion transfer between electrodes can be accomplished by employing PMMA spheres as pore-forming templates. The 2D MXene nanosheets were fully covered by the PMMA spheres and were thermally treated and filtered in a vacuum to obtain flexible, impressively conductive 3D freestanding porous MXene films. When used as SIB anodes, these films, showed enhanced capacity performances (rate capability and cycling stability).²³⁴

Fig. 6 p-Ti₃C₂T_x electrodes and half-cell tests: (a) charge/discharge curves of the p-Ti₃C₂T_x electrodes at 100 mA g⁻¹. (b) Charge capacity and coulombic efficiency vs. the cycle number of the as-prepared p-Ti₃C₂T_x, multi-layered Ti₃C₂T_x, and restacked Ti₃C₂T_x at 100 mA g⁻¹, as well as the coulombic efficiency vs. cycle number for p-Ti₃C₂T_x. (c) The performance rate of p-Ti₃C₂T_x. (d) A comparative study of the rate capability of p-Ti₃C₂T_x with some other conventional anode materials. The image was adapted/reproduced from ref. 234 with permission from Advanced Materials 2018.

The fabrication of MXene-based hybrid materials or composites can give rise to high-capacity active materials for high-rated SIBs. An encapsulated TiO₂@Ti₃C₂T_x structure was synthesised by exploiting the electrostatic interactions between MXenes and TiO₂ mono-dispersed nanospheres that delivered a reversible capacity of 116 mA h g⁻¹ at 960 mA g⁻¹ for up to 5000 cycles.²³⁵ Even though metal sulphide anodes possess high specific capacities, bulk materials hinder their cyclability. Therefore, a critical examination has to be done on nanodots with a small diameter (<10 nm) to fully utilise the intrinsic capacity. A composite FeS₂@MXene synthesised by the chemical assembly of Ti₃C₂T_x nanosheets, iron hydroxide, followed by sulphurization resulted in a capacity of 563 mA h g⁻¹ at 100 mA g⁻¹.²³⁶ Hence, uniform distribution and the incorporation of sulphides in MXenes enhanced the performance rate of SIB electrodes.

The restacking of MXene nanosheets can be effectively blocked by 1D hybrid materials and can achieve electrodes with high volumetric capacity with the architecture of a horizontally loaded model. MXene/CNTs films were manufactured by electrostatically combining negatively charged Ti₃C₂ flakes and positively charged CNTs with cetyltrimethyl ammonium bromide modification and then by filtration. This can be directly used as an anode for SIBs and they provide a volumetric capacity of 421 mA h cm⁻³ at 20 mA g⁻¹ current density.²³⁷ Similarly, 2D materials can also be loaded horizontally onto MXene nanosheets to get MXene/2D hybrid electrodes. One drawback of this fabrication is that the horizontally loaded model has low ion conductivity. The synthesis of MoS₂/Ti₃C₂T_x composites alleviated this problem and enhanced the ion kinetics in MXene layers by raising the interlayer space and enhancing its specific capacity. When substituted as the anode in SIBs they obtained a specific capacity of 250.9 mA h g⁻¹ in 100 cycles along with an extraordinary rate performance of 162.7 mA h g⁻¹ at 1 A g⁻¹.²³⁸ However, the poor cyclic stability can be overpowered by engineering a sandwich model to include an outer carbon layer.

4.2.1 Sodium-metal batteries. Another means of sodium storage can be achieved by sodium-metal anodes in sodium-metal batteries (SMBs). Although there is the uncontrollable and uneven formation of dendrites, MXenes have eased their way towards energy storage applications. The host material is to be taken in such a way that it suppresses the metal dendrite formation and optimises the infinite volume expansion and maintains the desired electrochemical performance. Pillared MXenes help in the enhancement of electrochemical performance and rate capability. In addition to the nucleation of Na into the large interlayer space of Sn²⁺-pillared Ti₃C₂MXene, the uniform deposition of Na can also be achieved. This pillared SMB electrode yields high current density (up to 10 mA cm⁻²) and high areal capacity (up to 5 mA h cm⁻²) for up to 500 cycles. Sodium-metal batteries show higher electrochemical performance.²³⁹

On reviewing the recent advancements, it was obvious that the electrochemical performance of SIBs is regulated by various factors that include the type of MXenes used, space for intercalation, surface specification, reaction environment and electrode surface properties. Being larger, the diffusion of sodium ions takes more time than Li-ions. Reasonable cycle stability and energy density of SIBs can be achieved by increasing the interlayer spacing, the formation of composites or generating pillared structures. All these strategies can be adopted to improve the electrochemical performance and rate capacity. However, the low capacity and poor rate capability of anodes are still hurdles to the commercial production of SIBs. The architecture of good performance and environmentally friendly MXene-based electrode materials for SIBs opens up a large area for research opportunities.

4.3 MXenes in potassium-ion batteries

PIBs are generally known as the cheapest alkali-ion batteries due to the natural abundance of potassium. Although they possess similar properties to Li and a low potential (−2.93 V), the large ionic radius of potassium (0.138 nm) is the first hurdle to finding a suitable host material for electrodes.²⁴⁰ The first MXene experimentally found to be suitable to intercalate K⁺ ions was Ti₃CNT_z.²⁴¹ This resulted in an extraordinary reversible capacity of 75 mA h g⁻¹, after 100 charge–discharge cycles at 20 mA g⁻¹. Interlayer spacing, surface imperfections and the number of layers can be adjusted to improve the accommodation of K⁺ ions to obtain electrodes with appreciable cyclic and storage capacity. The SEM micrograph of this Ti₃CNT_z, specific capacity vs. number of cycles at 20 mA g⁻¹ and the structure of Ti₃CNO₂ before and after electrochemical potassiation and de-potassiation are shown in Fig. 7(a)–(c), respectively.

Fig. 7 (a) SEM Micrograph of Ti₃CNT_z. (b) Specific capacity vs. number of cycles at 20 mA g⁻¹. (c) Schematic representation of Ti₃CNO₂ before and after electrochemical potassiation and de-potassiation. The image was adapted/reproduced from ref. 241 with permission from Chemical Communications 2017.

Ming et al. tried to enhance the K⁺ storage performance by using the porous K-V₂C nanosheets, formed by the subsequent acid/alkali treatment.²⁴² This brings in a 195 mA h g⁻¹ capacity at 50 mA g⁻¹, and better rate performance. Anodes made out of alkalised-Ti₃C₂ MXene, with its better structure stabilisation, reaction kinetics, 3D porous structure and narrowed networking nanoribbons, delivered a high capacity of 78 mA h g⁻¹ at 200 mA g⁻¹ while incorporated in PIBs.²⁴³ The capacity can be made even higher with the synthesis of MXene-based derivatives. Dong et al. derived K₂Ti₄O₉ ultrathin nanoribbons by the concurrent oxidation and alkalization of Ti₃C₂ MXene and obtained a capacity of 151 mA h g⁻¹ at 50 mA g⁻¹ and 900 times the stable cyclability as compared to normal MXenes.²⁴⁴ Su et al. synthesised TiO₂-RP/CN nanofibers from red phosphorus and TiO₂ to obtain a potential anodic material for excellent storage performance for KIBs. In addition to the derivatives, MXene-based hybrids/composites can also increase the capacity and rate performance of electrode materials. A vertically aligned sandwich model of carbon-coated MoSe₂/MXene hybrid nanosheets exhibited high conductivity and durability as a superior electrode material for PIBs.⁷⁵ The vertical alignment prevented the severe stacking of MXene nanosheets, improved its structural stability, and the reinforcement of the composite structure by the carbon layer enhanced its electronic conductivity. As a result, MoSe₂/MXene@C acquired a reversible capacity of 355 mA h g⁻¹ at 200 mA g⁻¹ and magnificent rate performance of 183 mA h g⁻¹ at 10.0 mA g⁻¹, even after 100 cycles, making it the most advantageous electrode material for PIBs as compared to the conventional MXenes. Another method of producing high-density PIBs is by using MXene/K hybrid anodes. A defect-rich MXene (Ti_3−xCNT_y) established the freestanding scaffold as a metal K host that controls nucleation to make it free of dendrites.²⁴⁶ These DN MXenes in the scaffold are generally potassium-philic, thereby encouraging the K nucleation during cycling, which strictly prevents the direct plating of K outside of the scaffold to obtain a dendrite-free morphology, high coulombic efficiency of 98.6% and a long cycle life up to 300 h. They can also be employed in potassium-sulphur anodes to get better performance. A very recent finding in 2021 was the formation of microorganism-derived MXene@N-doped CNF(carbon nanofibre) anode materials that provided high-performance PIBs.²⁴⁷ In this method of bio-adsorption, Aspergillus niger biofungal nanoribbons were the substrate and Ti₃C₂T_x nanosheets were assembled on them to obtain a 2D/1D heterostructure. This MXene-based N-doped CNF structure with sufficient fully opened pores, eases the ion kinetics and gives a high reversible capacity and stability.

4.4 MXenes in magnesium-ion batteries

MIBs have drawn considerable research attention due to their benign and hazardless nature, despite several challenges. Their low cost and abundant sources also inflate the curiosity for further explorations to employ them in large-scale renewable energy storage applications. The divalent nature of Mg²⁺ ions comes with a reduction in transport kinetics that eventually dampens the reversible capacity and output power of the electrodes.²⁴⁸ Research strategies implemented to develop required cathode materials initially ended in polyanions, chalcogenides and Chevrel phase cathodes. However more suitable materials have to be investigated to ensure high capacity and stability during the charge/discharge cycles and rate performance. MXenes were theoretically found to be suitable for the manufacturing of the electrodes for MIBs, as they can ensure stable multilayer intercalation.³² Djire et al. found that nanolayered Ti₂NT_x, with a high multilayer adsorption rate for Mg²⁺ ions on Ti₂NT_x nanosheets, exhibited exemplary reversible redox peaks and a wide working voltage.²⁴⁹ By the end of 1000 cycles, they successfully reproduced a capacitance of 200 F g⁻¹ at 500 mA g⁻¹ that increased up to 160% of its initial value, when placed in a safe and hazardless Mg-ion electrolyte. The introduction of Mg²⁺ ions from a magnesium electrolyte into the 3D microporous Ti₃C₂T_x MXene sheets was done to increase the reversible capacity and to test the performance of MIB cathodes.²⁵⁰ The Mg_0.21Ti₃C₂T_x cathode displayed plausible cycling stabilities of 210 mA h g⁻¹ at 0.5C, 140 mA h g⁻¹ at 1C, 55 mA h g⁻¹ at 5C and appreciable rate performance. The large irreversible capacity explained by the redox reaction mechanism should be kept increasing to prevent capacity decay. MXene-based nanocomposites can also increase the durability and storage capacity of the electrodes. Another method used for making cathodes with high-rate performance and cyclability is the electrostatic incorporation of negatively charged MXene nanosheets and space-consuming positively charged carbonaceous nanospheres. Liu et al. successfully synthesized the Ti₃C₂T_x@C cathode to achieve a notable reversible specific capacity of 198.7 mA h g⁻¹ at 10 mA g⁻¹ and rate capacity of 123.3 mA h g⁻¹ at 200 mA g⁻¹.²⁵¹ These hybrid materials are characterised by improved cyclic stability with 85% capacity retention after 400 cycles. This method of synthesis forbids MXene layer restacking and widens the interlayer spacing to foster ion-transport kinetics and permeation of the electrolyte. Similarly, a durable V₂CT_x@C hybrid was also derived by this method and found to have a storage capacity of 75.4 mA h g⁻¹ at 10 mA g⁻¹. Xu et al. vertically loaded MoS₂ between Ti₃C₂T_x nanosheets, using a single-step hydrothermal method to obtain a petal-like MoS₂/MXene composite.²⁵² Even though this hybrid gives an appreciable reversible capacity of 165 mA h g⁻¹ at 50 mA g⁻¹ and high rate performance of 93 mA h g⁻¹ at 200 mA g⁻¹, it cannot be considered for commercialisation. A very recent study devised the synthesis of the PI@Ti₃C₂T_x composite through electrostatic self-assembly within a mild reaction environment that hindered the oxide formation of Ti₃C₂T_x.²⁵³ The 3D composite showed a high tapped density (∼1.12 g cm⁻³), amplified electrical conductivity (∼3.26 × 10⁻² S cm⁻¹), supreme storage of aqueous Mg²⁺, eminent rate capability and durability. All these methods open up the possibility for further research for forming commercially viable and exploitable electrode materials for MIBs.

4.5 MXenes in calcium-ion batteries

Calcium is the fifth most abundant element in nature. With their attractive reduction potential (−2.87 vs. NHE) and reduced polarizability, the ions are expected to have high reaction speed and power output.²⁵⁴ However, suitable host materials are required for the fast integration and de-insertion of Ca²⁺ ions. MXene nanosheets, owing to their high conductivity, surface functionalisation and intercalation volume, serve as a substantial host for manufacturing cathodes for CIBs. Xie et al. predicted that MXene nanosheets functionalised with O can easily incorporate Ca²⁺ ions within the layers.³² Through theoretical simulation, it was found that Ti₂CO₂ has a notable storage capacity of 487 mA h g⁻¹ for Ca²⁺, as compared to the monovalent Na⁺ and K⁺ ions, with capacities of 288 and 264 mA h g⁻¹, respectively. Since the smallest Ca²⁺ ions show negligible interactions with each other on intercalation with Ti₃C₂, more ions can be adsorbed and a storage capacity of 319.8 mA h g⁻¹ is gained. This indicates that the adsorption energy decreases as there is an increase in the number of ions adsorbed.³¹ Meanwhile, the integration of Ca²⁺ ions in Ti₂CO₂/graphene and V₂CO₂/graphene heterostructures produced reliable storage capability and stability. However, the theoretical predictions and simulations are meaningless if the electrodes are not practically synthesised. Considering the manufacturing techniques of LIBs, SIBs, MIBs and PIBs, further studies can be done to discover a feasible, stable and efficient electrode material for CIBs. Porous nanomaterials can also be attributed to these applications as they can accommodate large-sized ions (Tables 5–7).

Table 5 A brief outlook on the superior electrochemical performances of MXenes and MXene-based materials for non-lithium batteries

Electrode material	Current rate (mA g^−1)	Cycle number	Capacity after cycle (mA h g^−1)	Rate performance (mA h g^−1) at current rate (mA g^−1)	Ref.
Sodium-ion batteries
Ti3C2/NiCoP	1000	2000	261.7	416.9 at 100	255
a-Ti3C2	200	500	50	168 at 20	243
c-Ti3C2Tx	20	50	246	120 at 500	256
p-Ti3C2Tx	1000	1000	189	166 at 1000	233
f-Ti3C2Tx_DMSO	1000	1500	76	110 at 2000	257
2D MXene/SnS2	100	125	120	78 at 2000	258
VO2/MXene	100	200	280.9	206 at 1600	259
Sulphur-decorated Ti3C2	2000	1000	135	136.6 at 5000	260
3D V2CTx	500	1000	310	170 at 5000	234
PDDA-BP/Ti3C2	100	2000	658	461 at 2000	261
Sb2O3/Ti3C2Tx	100	100	472	295 at 2000	262
Bi2S3/MXene	500	250	155	168 at 5000	263
NaTi1.5O8.3	200	150	130	101 at 2000	244
Ti3CN	500	500	60	98.8 at 500	231
MoSe2/MXene	2000	400	384	250 at 10 000	264
NiCoS4@ReS2	1000	500	396	297 at 3000	265
rGO/p-Ti3C2Tx	1000	1000	84.8%	280 at 100	266
ZnSe@NCNF	10 000	1700	197.3	–	267
CoSe2@CNT	2000	200	400	280 at 2000	268
Ti3C2Tx/graphene	2500	100	220	600 at 50	269
Na0.23TiO2/Ti3C2	2000	4000	56	178 at 5000	270
Ti3C2-Sb2S3	200	300	475	410 at 1000	271
Potassium-ion batteries
M-KTO, K2Ti4O9	50	900	151	88 at 300	244
a-Ti3C2	200	500	42	60 at 300	243
MoSe2/MXene@C	100	300	317	183 at 10 000	245
PDDA-NPCN/Ti3C2	1000	2000	252.2	254.8 at 1000	272
MXene/MoS2	200	50	145.5	168.2 at 500	273
(MoS2/N, P-rGO)	2000	7000	236.6	224.9 at 20 000	274
Ti3C2Tx @Sb	500	500	79.1435	270 at 500	275
SnS2 NSs/MXene	500	200	206.1	342.4 at 50	276
Fex−1Sex/MXene/FCR	100	80	449.3	15.3 at 1000	277
MXene-bonded HC	50	100	210	102.2 at 500	278
BPE@V2CTx	2000	3000	261	229.6 at 2000	279
NG/ReSe2/MXene	5000	300	90	137.5 at 10 000	280
MXene@NCNF	1000	1000	201.5	—	247
Magnesium-ion batteries
Ti3C2Tx@C	50	400	140.6	123.3 at 200	251
3D Mg0.21Ti3C2Tx	100	60	50	55 at 500	250
MnO2/MXene–Ti3C2	50	100	105	21 at 500	281
Ti3C2Tx/CTAB	200	250	135	32 at 2000	282
MoS2/MXene	50	50	108	93 at 200	252
d-Ti3C2Tx/CNT(Hybrid Mg^2+/Li^+ batteries)					283

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Table 6 A summary of the electrochemical performances of MXene-based materials for alkali metal batteries

Electrode material	Cyclic performance	Rate performance	Ref.
Lithium–metal batteries
PA-MXene-Li	Lifespan of 900 cycles at 1 mA cm^−2	∼0.4 V at 15 mA cm^−2	284
MXene-MF-Li	Lifespan of 3800 h at 10 mA cm^−2	∼0.085 V at 50 mA cm^−2	285
MXene@CNF/Li	Lifespan over 1300 h at 0.5 mA cm^−2	—	286
Zn/MXene films	Lifespan of 1200 h at 1 mA cm^−2 and capacity 1 mA h cm^−2	0.14 V at 8 mA cm^−2	287
MXene@NPSi@C	Lifespan over 800 cycles at 2 mA cm^−2	—	288
MXene–Li arrays	2500 cycles at 20 mA cm^−2	−25 mV at 20 mA cm^−2	169
Ti3C2/rGO scaffolds	350 cycles at 10 mA cm^−2 and capacity 1 mA h cm^−2	42 mV at 10 mA cm^−2	168
3DP-MXene arrays-Li	Lifespan of 1200 h at 1.0 mA cm^−2	∼100 mV at 20 mA cm^−2	289
Laser-MXene-50	1500 h (750 cycles) at 1 mA cm^−2	—	290
MXene/liquid-metal film	Lifespan over 350 h at 0.5 mA cm^−2	78.4 mV at 1.2 mA cm^−2	291
3D MXene/graphene	230 cycles at 20 mA cm^−2	∼13 mV at 3 mA cm^−2	165
Ti3C2Tx@Zn@Li	Stable over 600 cycles at 1 mA cm^−2	—	292
C/TiO2@Li||LiFePO4	Stable over 250 cycles at 1C	59 mA h g^−1 even at 3.0C	293
Li TiO2-Li3N-C	Lifespan of 2000 h at 1 mA cm^−2	30 mV at 1 mA cm^−2	294
MXene/AgNW scaffold	3000 h lifespan at 20 mA cm^−2	—	295
Sodium metal batteries
h-Ti3C2Tx/CNT	Lifespan over 4000 h at 1.0 mA cm^−2 and capacity 1.0 mA h cm^−2	46 mV at 5 mA cm^−2	296
Na-Ti3C2Tx-CC	Lifespan over 300 h at 3 mA cm^−2 and capacity 1.0 mA h cm^−2	∼0.35 V at 9 mA cm^−2	297
CT-Sn(II)@Ti3C2	Lifespan over 500 cycles at 4 mA cm^−2	40 mV at 4 mA cm^−2	239
C-NTO-3/Na	Lifespan over 400 cycles at 3 mA cm^−2 and capacity 3 mA h cm^−2	—	298
MXene-MF-Na	720 h at 10 mA cm^−2	∼15 mV at 20 mA cm^−2	285
Potassium metal batteries
MXene-MF-K	800 h at 5 mA cm^−2	—	285
K@DN-MXene/CNT	300 h at 0.5 mA cm^−2	0.40 V at 2 mA cm^−2	246
a-Ti3C2	700 h at 10 mA cm^−2	—	299
K-ACM	230 h at 1 mA cm^−2	≈0.31 V at 5 mA cm^−2	300

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Table 7 A comparative study on the advantages and disadvantages of using MXene in various alkaline batteries

Type of battery	Advantages	Ref.	Disadvantages	Ref.
Lithium-ion batteries	• Being flexible, they stay as a conductive scaffold as well as a multifunctional binder for other materials.	44	• MXenes are highly expensive and denser
	• Build strong connectivity within the composites.	301	• Misinterpretation of the performance data
	• Continuous variation in the oxidation state of transition metals can boost specific capacity.	302	• MXenes are oxidised quickly, thus limiting commercialisation.	44
	• Consistent and rapid lithium storage
Lithium–sulphur batteries	• Entrap the long-chain and short-chain polysulphides with suitable termination.	133	• If MXene is used in excess (>10% of total sulphur), it lowers the energy density
	• Suppression of the shuttle phenomenon	303	• A dark inert condition for both storage and operation is compulsorily maintained to retain the inherent properties of MXenes.	304
	• Enhances the charge storage capacity as well as the life cycle.
	• Improved utilization of the sulphur due to rapid charge transfer ability.
Lithium-air batteries	• Improved overall rate performance.	190	• Although DFT studies have been reported for LABs, only a small number of MXenes candidates are being exploited.	305
	• Enable to sketch the way of Li2O2 formation.
	• Enhance the rate of ion/electron migration.
Lithium–metal batteries	• Mitigate dendrite formation.	306	• Aggregation of MXene intersheets during plating as well as stripping of Li metal diminishes the lithiophilic interfaces and intercalation space.	307
	• Compensate volume changes		• The brittle nature of 3 D MXenes cannot tolerate the strain generated by Li plating, thereby it reduces stability as well as reversibility
	• Increased lifespan of the electrode.
	• Li metal can be nucleated and grown uniformly	286
Sodium-ion batteries	• Lower diffusion activation energy is established	308	• MXene matrix offers lower doping contents.	231
	• Enhances transport kinetics and performance rate	309	• De-sodiation capability is lower for pure MXene than other alloying materials	310
	• Uniform distribution is ensured by their increased surface area.	233	• Pure MXenes produce low reversible capacity	309
	• Porous MXenes improve transport characteristics.
Sodium–metal batteries	• MXene composites prevent the formation of dead Na	19	• The direct inclination of Na onto the MXene matrix surface may give rise to dendrite formation sites.	239
	• The pillar effect can enhance the accommodation of more Na.	239
Potassium-ion batteries	• Enhanced electric conductivity		• Poor potassium storage capability
	• Lower K^+ diffusion barrier	308	• Expeditious capacity degradation	308
Magnesium-ion batteries	• Increase durability and storage capacity.	250	• The occurrence of a positive charge on the MXene surface causes repulsion with Mg^2+ ions that eventually affects desolvation.
	• Enhance electrolytic permeability.	251	• Fluorine and hydroxyl terminations affect average voltages.	311

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5. Conclusions

This review has mainly focused on the recent innovations made in the field of energy storage using MXenes and MXene-based composites in various alkali-ion batteries, including LIBs, SIBs, PIBs, MIBs and CIBs. On evaluating heteroatom integration, the loaded model, encapsulated model, sandwiched model and cost-effective metal host electrodes, we encountered great potential and good performances of MXenes and their hybrids, making them efficient and suitable for energy storage. We also observed that MXenes can be substituted as conductive substrates, and even current collectors in various MIBs, which could increase the density of the electrode material and the maximum usage of the material. In spite of these advantages, MXene-based electrodes still face limitations that prevent their application in large-scale production. All these controlling factors are to be evaluated for further exploration.

(1) Synthesis methods and reaction environments can affect the expected electrochemical performance and yield of the electrodes made out of MXenes and their derivatives, along with the type of transition material used, interlayer spacing, and surface functional groups (O, OH, F, etc.). It was also observed that the surface/bulk morphology of the nanosheets greatly affected the rate and storage capacity of the electrodes. Certain surface functional groups can either enhance or hinder the electrode performance. Since F and Cl terminations decrease the rate capacity of metal-ion batteries, synthetic methods have to be adopted in such a way that it prevents the functionalisation with these elements. However currently used top-down methods retain these elements within them. In addition, the HF-involved etching methods are considered to be environmentally hazardous. Interestingly, Lewis acid etching and bottom-up methods like chemical vapour deposition (CVD) may promote the achievement of high-purity MXenes devoid of these surfactants. The decline in the yield, number of layers and high synthesis cost limit the large-scale application of CVD. Further research can be conducted to make it economically feasible and efficient, as pure MXenes can be employed as current collectors in metal-ion batteries, superconductors and separation processes.

(2) The use of nanospheres, porous/pillared structures and doping of heteroatoms can also increase the capacity and rate even after many charge/discharge cycles. The heteroatom introduction provides excellent storage capacity and ion diffusion kinetics in LIBs and SIBs, as it increases the interlayer spacing significantly. The compositing of 2D MXene nanosheets with 0D/1D/2D nanomaterials forms hybrids that redeem the volumetric variance that occurs in charge/discharge cycles and restrict the MXene layer restacking to enhance the increase in the ion diffusion kinetics and storage. The porous MXenes, when tested in SIBs, exhibited dominant capacity, rate capability and stability in comparison with the non-pillared materials, and reserved 98.5% capacity within the 50th and 100th cycles. This promises the requirement of pillaring techniques applied to MXenes, providing a new approach to optimizing their properties for a range of applications, including energy storage, conversion, catalysis, and gas separations. Even though MXenes were initially introduced for making electrode materials, eventually, their modification helped them to prevent dendrite formation, form substrates for intercalation, block the MXene restacking, provide active sites and encourage diffusion kinetics.

(3) On considering the rechargeable non-Li-ion batteries, we came to realise that even though theoretical predictions have offered a bright hope for SIB application, experimental results proposed low capacity and poor rate capability for the existing anodes synthesised to date. Initial studies advocated that the introduction of carbon materials like CNTs could develop free-standing electrodes, though low reversible capacity remained an obstacle for future developments. Even though multivalent ion batteries like MIBs are an appealing alternative due to their abundance, low dendrite electrodes, and high volumetric energy capacity, their low operation voltage and high ion-migration barrier restrain further applications.

(4) Other Li-based batteries like LSBs, LMBs and LABs were able to modify the electrochemical performance, energy density and capacity in comparison with the prevailing LIBs. A magnificent incident happened in the design of a 3D conductive Ti₃C₂T_x MXene–melamine foam (MXene–MF) by Shi et al., which turned out to be a universal host material for Na, K and Li.²⁸⁵ Their works substantiated the effectiveness of MXenes in hindering the growth of metal dendrites within the electrode material. However, ideal anode/cathode materials that can be exploited as commercial electrodes for Li-based batteries are still residing in the green room.

(5) Lastly, the applications of MXene-based electrodes should be analysed with supreme importance. MXenes can be employed as conductive additives, 2D conductive substrates in electrode materials that possess high capacity, and as a protective layer on metal anodes in various MIBs. The MXene-based fibres can find applications in electrically operated vehicles. Moreover, they are incorporated in fibre-based batteries to increase conductivity, flexibility and lower the internal resistance to obtain highly efficient and cost-effective batteries. Since these fibres can be moulded and framed into different shapes and sizes, they can be used in wearable electronics and flexible devices.

Recently, a lot of bio-derived MXenes and MXene-based hybrids have attained high popularity since they possess appreciable capacity and stability. The nanoengineering of various microstructures such as 3D hybrids and sandwich-like structures should be done to produce favourable specific surface area, pore sizes, and pore size distribution. The nanostructures synthesised artificially have great potential for solving some pivotal issues that may occur during cycling, thereby enhancing the reversible capacity and cycle life of the batteries. However, the manufacture of high-capacity MXene-based nanostructures/nanocomposites with high retention capacity is still in the dormant stage. Increased research in this field can only enhance their properties to ensure their further practical applications. The hydrophilicity of MXene layers can also be studied in depth to incorporate that feature in AIBs to achieve versatile properties. Theoretical predictions made in the case of divalent alkali-ion batteries should be discovered in the coming years. In looking forward, MXene-based nanomaterials can be used to make smart AIBs that may possess various properties, opening the gateway towards the manufacture of next-generation batteries, electrochemical energy vehicles and wearable devices.

Author contributions

Helen Treasa Mathew: Conceptualization, investigation, writing-original draft preparation, writing-review and editing. Kumar Abhisek: writing-review and editing, visualization. Shashikant Shivaji Vhatkar: writing-review and editing, visualization. Ramesh Oraon: funding acquisition, resources, supervision, validation, writing-review and editing.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

Dr R. O. is grateful to DST (EEQ/2018/000595) and UGC BSR Start Up grant (F.30-462/2019 BSR) for financial assistance under the projects. H. T. M. and K. A. is thankful for university research fellowship, central university of Jharkhand. S. S. V. is also thankful for project assistant fellowship funded by DST/EEQ project (EEQ/2018/000595).

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