Advanced concept and perspectives toward MXenes based energy storage device: Comprehensive review

Advanced concept and perspectives toward MXenes based energy storage device: Comprehensive review

ENDNOTE,  MENDELEY ,  DOWNLOAD PDF

DIO:  https://doi.org/10.1016/j.mtla.2024.102089

Kefayat Ullah¹, Noor Alam¹, Salah Uddin¹ and Won-Chun Oh²*

¹Department of Applied Physical and Material sciences, University of swat, 19120, KPK, Pakistan

²Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungnam 31962, Republic of Korea

 

Abstract:

The hunt for the suitable material to be used as electrode material for Li Ion batteries has increased over the past decades. The emerging of MXenes has been used significantly in the field of batteries. MXenes has been considered as the most suitable 2D materials for battery technologies due to its desirable properties. The production of MXenes has been found to be cheap and scalable as compared to other counterparts such graphene and 2D transition metal chalcogenides. MXenes can be used as electrodes in these batteries owing to its desirable properties such as, outstanding conductivity, hydrophilicity options to intercalate various elements, surface functionalities and various possible structural combinations. Here in we highlighted various compositions as anode/cathode materials and other battery components based MXenes. Specifically, the use of MXenes as host materials for ions as a separator modifier and conductive preservative has been discussed. Also, MXene-based nanostructures are introduced and discussed, focusing on their preparation methods, properties, and applications for energy storage devices.

Keywords: LIBs; MXenes; Anodes; Separator; 2D

 

1. Introduction

Besides graphene, the quickly growing two-dimensional materials have received a lot of interest because of their unique mechanical, electrical, optical, and thermal characteristics that set them apart from their bulk counterparts [1-3]. Recently, novel family of 2D metal nitride, carbide, or carbonitrides layered materials, labeled as MXenes, including MO2C, Ti3C2, Ti2N, Ti3CN, Ti2C, Nb4C3, and so forth, have captured the interest of researchers [4, 5]. MXenes has been identified as a promising new contender that attracts strong interest in energy storage [6, 7], electronics [8, 9], sensors [10, 11], and cancer treatment [12, 13] because of its enormous specific surface area, hydrophilicity, numerous terminal groups, and high electrical conductivity [14]. MXenes are generally denoted by their chemical formula Mn+1XnTx, synthesis approach for etching A-layers selectively from 3D crystalline layered carbides and nitrides, mainly MAX phases (Mn+1AXn) or another layered ceramic precursor involves methods from both the bottom up and the top down [15] resultant multilayer particles are then delaminated into individual MXene sheets or flakes [16] where T is a surface terminal, such as OH, O, or F; n = 1, 2, or 3; M is an early transition metal; X is carbon or nitrogen, or their combination; and A is an element of group 13 or 14 [17]. Over the years there has been an increasing fascination, with MXenes as electrodes in energy storage applications such, as supercapacitors and batteries [18]. The core layer made of metal transition and the surface resembling transition metal oxide, in MXenes create pathways for electron movement which offer numerous sites for chemical reactions [19]. These two advantages working together enable MXenes to achieve charge storage making them suitable, for high speed applications [20, 21]. Additionally by modifying the surface terminations MXenes can be adapted to chemical reactions resulting in increased capacity and added functionality [22, 23]. According to theoretical simulations and calculations, , , , , , and other ions can quickly migrate inside MXenes. It's interesting to mention that when metal ions are intercalated extensively, they can create a layer that leads to an increase, in capacity. On the other hand storage performance is significantly affected by the termination groups, on the surface [24]. The reduced ion diffusion barriers assist in improving the ion batteries' rate performance [25, 26]. Therefore, Metal-oxygen, metal-sulfur, and metal-ion batteries are just a few of the battery systems that have made considerable use of MXenes. MXenes are utilized as coatings, for metal anodes to inhibit dendrite formation [27, 28]. Ever since lithium ion batteries (LIBs) were successfully introduced by SONY to the market in 1991 they have become the choice, for energy storage systems over the few decades [29]. The increasing demand, for high power density and high capacity in Lithium ion batteries (LIBs) is driven by the rapid growth of electric vehicles and portable electronic devices, in today’s industrial landscape. [30]. The large accessible surface areas provided by 2D nanomaterials translate to significantly more electrochemically available sites to attract Li-ions [31]. Thus, by utilizing the 2D electrodes a higher capacity can be achieved. 2D carbonitrides, carbides, and nitrides of transition metals (MXenes) are promising materials to be employed as rechargeable battery electrodes [32] due to their outstanding stability [33, 34] large surface area [35] high electrical/thermal conductivity [36] excellent oxidation resistance [37]. MXenes show extraordinary conductivity (e.g., ≈3250 ± 100 S  for V2CTx, ≈9880 S  for Ti3C2Tx) [38] and greater hydrophilicity (contact angle is ≈21.5°–35°) [39] ensuring aqueous electrolyte wettability and chemical stability [40, 41].

Two-dimensional materials are introduced through various methods, such as etching layers of 3D crystalline materials or delaminating multilayer particles. These two-dimensional materials possess unique mechanical, electrical, optical, and thermal characteristics that distinguish them from their bulk counterparts. Actually, single, multi-layer, or few-layer 2D MXene nanosheets have poor stability in oxygen environment, strong hydrogen bonds and van der Waals interactions between adjacent nanosheets severely restrict further development of pristine MXenes by causing them to collapse and stack, reducing their specific surface area and hampering the electrolyte's penetration and diffusion, which reduces electrochemical performance [42]. Inspired by the graphene's elemental doping process, which has gained popularity in recent years, [43] enhancing pristine MXene's electronic, magnetic, optical, and other properties by elemental doping (MXene composites) techniques has been shown to be a workable strategy [44]. Element doping offers advantages, including the transformation of sites, improving the materials conductivity and converting them into electrochemically active components [45] doping a material to introduce flaws and alter its magnetic characteristics in order to modify its electronic structure[46] causing an effective induction of Fermi energy [47] Changing the chemical bonds or functional groups that promote polysulfide transitions and prevent MXene rearrangement [48] Modifying a material's bandgap and surface chemical characteristics to increase catalytic performance [49, 50]. Also, Introducing spacers between the layers of MXenes has been discovered to be a successful method for improving the electrochemical performance of MXenes to develop a porous structure [51]. The introduction of conductive polymers, nano-carbon, metal oxides, heteroatoms, and TMDs significantly increased the interlayer distance to achieve improved electrochemical performance [52, 53]. Also, a growing number of researchers are focusing on finding ecologically friendly ways to investigate novel MXenes that are fluoride-free [54]. However the high costs of lithium caused by its availability, in nature (for example 0.002% found in the earth’s crust) make it less economically viable for widespread use, on a grid scale level [55]. Consequently, a plethora of innovative rechargeable battery systems, commonly referred to as "beyond-lithium batteries," have been investigated and are demonstrating significant advancement, that depend on elements that are more plentiful, such as aluminum (8.2%), magnesium (2.3%), sodium (2.3%), potassium (2.1%), and zinc (0.0075%) [56]. Importantly, because they are all members of the IA element group and have similar chemical characteristics with lithium, sodium and potassium have garnered increased interest for application in next-generation batteries [57, 58]. Scheme 1 shows a conceptual diagram of a simple MXene-based lithium secondary battery.

 

Scheme 1. Simple secondary battery concept with MXene.

 

 

2. An Overview of MXene

In 2011, By soaking Ti3AlC2 particles at room temperature in hydrofluoric acid (HF), it was shown that the Al layers in a Ti3AlC2 MAX phase could be selectively etched. This finding was made while trying to figure out how to enable the insertion of lithium ions in layered ternary transition-metal carbide/nitride phases or MAX phases [59, 60]. The bulk three-dimensional (3D) crystalline Ti3AlC₂ MAX phase particles underwent this selective etching treatment, which resulted in two-dimensional (2D) sheets of Ti3C₂ layers (MXene), where from the etching medium Ti atoms on the surface are terminated by surface groups, such as –F, –OH, and, –O which in the Ti₃C₂Tx formula are denoted by T [61]. As yet, MAX phases have been converted to the majority of MXenes by delicately etching the Al layers [62]. Layers of an A element bind with layers of Mn+1Xn in ternary nitrides and carbides to form MAX phases [63]. As shown in figure 1(a) M is a transition metal (such as Cr, Zr, Ti, Mo, etc), n value is from 1 to 3. Groups 13–16 of the periodic table, which include Si, Al, Ge, Ga, and so on, are the A elements [64] and X can be either nitrogen, carbon,  or a combination of the two.

Some of the most lately research has demonstrated that transition metals from groups 8–12 (Ir, Au, Fe, Cd, Zn, Cu.) may also form solid solutions or pure A elements that make up the A layer of MAX phases [65]. Ti₃C₂Tx, the first discovered and most popular one among various MXenes which has demonstrated outstanding advantages due to its low open-circuit voltage, low Li-ion diffusion barrier, and large theoretical Li storage capacity [66, 67].  Without any additives, a Ti3C2Tx colloidal solution may be readily built into a flexible, self-supporting Ti₃C₂Tx MXene paper [68]. Crucially, flexible Ti₃C₂Tx MXene paper has outstanding mechanical flexibility and extremely metallic conductivity (about 4000–15,000 S ) [69] which makes it a desirable substitute for metal current collectors [70]. More than 30 MXene members and Over 150 distinct MAX phases were successfully synthesized experimentally to far, and hundreds more are expected to be thermodynamically stable [71]. Though there are rare exceptions, as was previously shown, the majority of MAX phases are the source of MXenes. Despite having a comparable crystal structure to the MAX phase, MO2CTx is the first MXene to be produced from MO2Ga2C, a non-MAX phase [72]. It is more difficult to produce the MO2CTx MXene because both layers of Ga must be selectively etched off. Subsequently, Zr3C2Tx and Hf3C2Tx, two more MXenes, were successfully created from the non-MAX phases as well, namely Zr3Al3C5 and Hf3[Al(Si)]4C6, respectively [73]. As a result, in contrast to other 2D materials, MXenes constitute a large family of materials with a wide range of properties such as electromagnetic interference shielding performance, high electrical conductivity, and volumetric capacitance [74, 75]. Three potential lattice arrangements for the resultant MXene exist since the value of n for MAX phases ranges from 1 to 3: M₂X, M₃X₂, and M₄X₃ as shown in above figure 1(b) [76, 77]. The lighter members of the M₂X family of MXenes often have better gravimetric capacities than the larger formula weight members of M3X2 and M4X3 [78].

Figure 1(a-b): a) Periodic table elements that are known to create the Mn+1AXn phases. b) Analyzed MAX phase structure and related MXenes. Ref [79]. Copyright 2014, Wiley-VCH.

2.1 Synthesis Strategies

The stacking of 2D transition-metal carbide/nitride layers connected by an "A" element explains the structure of the MAX phase [80]. The M–X (covalent/ionic/metallic) bond is comparatively stronger than the M–A (metallic bond) bond in its molecular structure [81]. Because the M-A and M-X have differing strengths, the reactivity of the A layers is greater. Thus, with the right etching reagents (hydrofluoric acid), it should be quite easy to remove the A layers. MXene's properties are dependent on synthesis and processing [82]. The properties of MXenes vary according to the kind of etchant used, its concentration, the duration of the sonication process, and the type of solvent [83]. For example, the capacitance,, and rate performance of free-standing film electrodes, and electrical conductivity are all impacted by the size of the MXene flakes [84]. So synthesis method of MXene has generated extensive interest, and Numerous techniques have been created, which may be categorized into two primary approaches: both bottom-up (like chemical vapor deposition (CVD)) and top-down (like etching from MAX or non-Max precursors) [85]. The synthesis strategy of the article is etching followed by delamination.

2.2 Etching (Removal of “A” Layers)

Many etchants have been created to date. The most widely researched etching etchant is hydrofluoric acid (HF) and the first one to be used to produce MXenes from their equivalent MAX precursors among the many available etchants. Using 50% HF as the etchant, Naguib et al. [86] eliminated Al components from the MAX phase, generating accordion-like Ti₃C₂, as shown in figure 2, between each layer, surface groups generate hydrogen bonds and the van der Waals force [87]. Additional ultrasonic treatment was used to separate the sheets. The -O, -OH, and -F groups easily functionalized the surface of Ti₃C₂ during the etching process, producing T-terminated Ti3C₂Tx having a hydrophilic nature [88]. The exact condition of etching is determined by temperature, reaction time, HF concentration, and the type of MAX phase. For 24 h, 18 h, and 5 h, each gram of Ti₃AlC₂ MAX may be etched at room temperature to 40°C by adding 5, 10, and 30 wt% HF, respectively. [89]. Defining the etching process might include the following reactions [86]:

Ti3AlC2 + 3HF = AlF3 +  H2 + Ti3C2

Ti3C2 + 2HF = Ti3 C2 (F)2 + H2

Ti3C2 + 2H20 = Ti3 C2 (OH)2 +H2

Ti3C2 + 2H20 = Ti3 C2 (0)2 + 2H2

 

Figure 2: Diagrammatic representation of the Ti3AlC2 exfoliation process with HF etchant.[90]

2.2.1 Wet Chemical Routes

Even so, the bond between M-A are naturally less strong than the bonds between M-X in MAX phases even in 2D materials like graphite, they are still much stronger than the van der Waals force between adjacent layers [91]. Before using HF for etching, researchers attempted alternative approaches, but they were unsuccessful. For example, The MAX phases were heated to a high temperature by Barsoum et al. in molten metals (such as molten Al with Ti3SiC2) [92] in molten salts (cryolite) [93] LiF [94] and in vacuum [95]. But again, the high temperatures disrupted the layered structure. Since these high-temperature melt methods did not selectively remove just the A layers, 2D MXenes could not be produced. This led researchers to explore chemical etching methods which could selectively remove the A layers at lower temperatures and preserve the 2D structure of MXenes. It should be noted that powerful etchants like Cl2 gas may damage the 2D MX structure while mild etchants alone cannot fully remove the A layers. Proper control of etchant type and reaction conditions is needed to produce high quality MXenes [96]. In addition to the etchants' chemical activity, the reaction temperature has to be closely examined. Ti2AlC and HF, for instance, would react at 55°C to produce Ti2AlF9 but not Ti2AlC MXene [97]. In order to maximize the etching process and generate high-quality MXenes, a detailed knowledge of the etching processes is necessary [98]. The HF has been used to produce several MXenes since it has been shown to be an efficient MAX etchant as shown in table 1 [99].

Table 1: Different MXenes prepared by HF etching.

S. No.	Origins	MXene		Conditions
			Time (h)		Yield (%)	Ref.
1.	Ti2AIC	Ti2CTx	10		80	[100]
2	Nb4AIC3	Nb4C3Tx	96		77	[101]
3	V4AIC3	V4C3Tx	165		NA	[102]
4	Ti3AICN	Ti3CNTx	18		80	[103]
5	Mo2TiAlC2	Mo2TiC2Tx	48		100	[104]
6	Mo2Ti2AIC3	Mo2Ti2C3Tx	90		100	[104]
7	(Mo2/3Y1/3)2AIC	Mo4/3CTx	6072		NA	[105]
8	(Nb2/3Sc1/3)2AlC	Nb4/3CTx	30		NA	[106]
9	(W2/3Sc1/3)2AlC	W4/3CTx	30		NA	[107]
10	Zr3AI3C5	Zr3C2Tx	60		NA	[108]
11	Hf3[Al (Si)]4C6	Hf3C2Tx	60		NA	[109]
12	V2AIC	V2CTx	90		60	[110]
13	Nb2AIC	Nb2CTx	90		100	[111]
14	Ti2AIN	Ti2NTx	24		NA	[112]
15	Mo2Ga2C	MO2CTx	3		NA	[113]
16	(Ti0.5Nb0.5)2AIC	(Ti0.5Nb0.5)2CTx	28		80	[103]
17	Ti3AIC2	Ti3C2Tx	2		100	[114]
18	(V0.5Cr0.5)3AIC2	(V0.5Cr0.5)3C2Tx	69		NA	[103]
19	Ta4AIC3	Ta4C3Tx	72		90	[103]

 

Primarily based on the MAX particle size and M-A bond strength, the temperature, concentration, and reaction time differ across scenarios. Consequently, to produce high-yield, high-quality, and well-defined MXenes, proper etching conditions are crucial [115, 116]. While HF is a very poisonous and harmful chemical, it can successfully etch MAX phases while preserving its two-dimensional nature. Therefore, other strategies to lessen or even completely minimize the usage of HF have been investigated. Using combinations of several acids and a modest quantity of HF would be an easy way to go. For instance, Ti3AlC2 MAX powder was effectively etched by Dirscoll et al. using a 6:3:1 volumetric ratio of HCl, deionized water, and HF [117, 118]. Compared to the approach that uses HF only, this method significantly decreases the quantity of HF by 90%. Furthermore, at room temperature (25 °C), It is possible to create MXene nanosheets with a high lateral dimension and less defects at a high throughput [119]. Ghidiu et al. devised a milder synthesis approach in 2014 wherein a reaction between HCl and LiF produced the HF acid in situ (where the molar ratio of Ti3AlC2: LiF: HCl is 1:0.5:11.7). By using this technique instead of directly using HF, nanosheets with greater lateral diameters and fewer flaws are produced, in comparison to HF-etched MXenes [99, 119]. Later, by adding more LiF salt and HCl acid, the LiF/HCl pathway was improved [120]. The MILD process, or minimally intensive layer delamination, is the best approach. It's crucial to remember that the MILD approach, which may yield even bigger MXene flakes with less defects than the original HCl/ HF procedure, simply requires manual handshaking and does not need sonication [121]. Figure 3 compares the MILD approach with the LiF/HCl pathway followed by sonication. Similarly, it was shown that Ti₃AlC₂ MAX could be etched under hydrothermal conditions at 180°C without the need of HF using NaBF4/HCl [122].

Figure 3 (a-e): a) A diagrammatic overview of Routes 2 (MILD technique) and 1 (HCL+HF etching followed by sonication). b, c) Ti3C2Tx flakes generated by Route 1 were examined using scanning and transmission electron microscopy (TEM and SEM). d, e) Ti3C2Tx flakes made using Route 2 are characterized using TEM and SEM. Reproduced with permission [98]. Copyright 2021, Wiley-VCH.

In addition to the fluoride salt/acid combinations, Ti₃AlC₂ MAX could also be etched by NH4HF2, a weak acid that is safe for the environment, however it took a lot longer than HF [123]. Compared to the HF etched one (19.8 Å), wider interlayer spacing is shown in the resulting Ti3C2Tx MXene (the c lattice parameter is 24.7 Å). This clearly suggests that TEM and X-ray photoelectron spectroscopy (XPS) studies reveal that the cations (NH4+) and/or NH3 intercalate during the etching process. In addition, NH4F was only used as an etchant in a hydrothermal process to create Ti₃C₂Tx MXene at 150 °C for 24 hours [81]. Furthermore, Mei et al. produced the fluorine-free mesoporous Mo₂C MXene in the phosphoric acid etching solution by using a UV light-induced technique [124]. Recent developments in HF-free approaches for obtaining The field has shown a great deal of interest in and relevance for MXenes with surface terminations other than F. These approaches offer safer alternatives to the traditional HF etching method and open up possibilities for obtaining MXenes with diverse surface terminations, which can significantly affect their properties and electrochemical performance [70]. To achieve high quality MXenes it is crucial to control the etching process and conditions. Typically, weaker etchants, like LiF/HCl, NH4HF2 and NH4F require etching times and/or higher reaction temperatures. When using the etchant to etch MAX phases it is important to adjust the reaction conditions accordingly. Factors such as temperature, fluorine ion concentration, and reaction time reaction, should be modified based on parameters such as, the value of n and the number of M. Generally, when the atomic number of M increases the energy of M-A bonds also increases, necessitating times and/or higher temperatures [125]. More fluorine ions and a longer reaction time are also needed for a higher value of n [116, 126].

2.2.2 Molten Salt Method

Molten salt [127, 128] and halogen-based [129, 130] etching have become more popular than wet-chemical etching in aqueous solutions since these method have generated MXenes with Tx functionalities (-NH, -Cl, -S, -I, -Te, -Se, -Br) besides what is possible with traditional HF-based etching [23]. Due to their abundance, water solubility, and thermal stability, the manufacture of low-cost, high-yield 2D MXene materials is often streamlined by the use of salts [131]. Following melting, the molten salts served as solvents, dissolving solid reactants and solvated ions by intense polarization and quickly transferring reactant species by convection and diffusion [132]. To selectively etch further MAX phases devoid of Al and produce nitride MXenes [133] which are difficult to produce by HF etching [4] molten salt etching is the recommended technique. However, a fluorine-free molten salt etching method has to be created since MXene made with fluoride salts frequently has a fluorine terminal group [134]. Using etching of Lewis acid molten salt, Li et al. synthesized many MXenes (NaCl/KCl/ZnCl2) in 2019. Among these, KCl and NaCl were utilized to reduce the eutectic system's melting point. The displacement interaction in the Al-MAX phases between the Al atomic layer (Ti2AlN, V2AlC Ti3AlC2, and Ti2AlC) and molten zinc chloride (ZnCl2) produced new Zn-MAX phases (Ti2ZnN, V2ZnC, Ti3ZnC2, and Ti2ZnC,) at 550°C. However, the high Lewis acidity of the molten ZnCl2 caused the new MAX phase to delaminate along with the introduction of sufficient ZnCl2 (a molar ratio of Al-MAX/ZnCl2 = 1:6), resulting in the formation of Cl-terminated MXenes  [128]. In 2020, Li et al. proposed a comprehensive Lewis acid etching process [127], which expanded the scope of MAX phase etching beyond only Al-containing phases to include Ga, Zn, and Si (non-Al-MAX phases). Lewis acidic molten salts and MAX phases may be combined, and the mixture could then be heated to 750°C to create MXenes.  CuCl2 molten salt was able to selectively etch a Ti3SiC2 MAX phase, yielding a mixed powder of Ti3C2Tx/Cu. The copper particles were further removed using a solution of ammonium persulfate (APS, (NH4)2S2O8), final result was Ti3C2Tx MXene powder, which had surface-mounted Cl and O terminal groups. The A-site element in the MAX phase may be etched by melted halide with a larger electrochemical redox potential (V against Cl2/Cl−) than one with a lower electrochemical redox potential (V against Cl2/Cl−). Melted halides with a higher electrochemical redox potential (V versus Cl2/Cl−) than a lower one has a greater possibility of etching the A-site element in the MAX phase. This is the etching principle. By using molten salt etching (CuI/CuCl2/CuBr2), Li and associates created the Ti3C2Tx MXene with ternary, binary, and unitary halogen terminals (such as I, Cl, Br, ClBrI, and BrI) [133]. The salt ratio in the etchants may be adjusted to change the ratio of the appropriate halogen-groups.

 Additionally, the one-pot synthesis of both the associated MXenes and MAX phases may be accomplished sustainably and scalable via molten salt synthesis [135, 136]. However, it is still difficult to delaminate MXenes made by molten salt synthesis into single-layer flakes [137].

2.2.3 Electrochemical Method

Electrochemical etching involves applying a potential difference between a MXene sample and a counter/reference electrode in an electrolyte solution. This process can be controlled in terms of voltage, current, time, and temperature. Electrochemical etching can selectively remove specific layers of MXene, leading to tunable properties [138]. This involves applying an electric field to the MXene surface, causing ions to intercalate and alter the surface chemistry. The advantages of this method include the ability to control and tune the surface chemistry of MXenes, as well as the potential for increased charge storage capacity and improved electrode performance. The steps and principles of electrochemical etching involve manipulating the electrochemical potential of the MXene material, and the characteristics of this method include its flexibility and compatibility with various electrolytes [139, 140]. The reactivity differences between the M-C and M-Al bonds are the basis for chemical etching. This states, that because electrochemical etching deals with the actual transfer of charge, it is a reasonable substitute method. In a diluted HCl solution, Al was successfully extracted electrochemically on a porous Ti2AlC electrode by Sun et al. [141]. On Ti2AlC, one may generate a layer of Ti2CTx MXenes. Since no F ions were used in the electrochemical etching process, MXenes containing just -OH, -Cl, and -O. groups were produced, in contrast to chemical etching using HF or LiF/HCl [120]. The over-etching of the MAX phase into carbon generated from carbines, however, may be the cause of the electrochemical etching issues (CDC). To explain the electrochemical etching of Ti2AlC into Ti2CTx and CDC, a core-shell model was put forward. In order to produce MXenes without overloading, it was suggested that etching parameters be properly adjusted. The spectrum of possible MXene compositions and etching procedures has been expanded by electrochemical etching [126].

2.2.4 Other Methods

Selective etching techniques are often used in the fabrication of MXene materials. A few bottom-up techniques that have surfaced recently include chemical vapor deposition (CVD), atomic layer, salt-template, water-free etching, and much more methods as illustrated in figure 4 [142]. For example, Xu et al. used chemical vapor deposition (CVD) to create 2D ultrathin Mo2C crystals [143]. Strong anisotropy and superconducting transitions were seen in the 2D nanostructures at magnetic fields. It was discovered that the thickness of the crystal nanostructures strongly influenced the superconductivity. Other 2D TMC crystal materials could be able to use this CVD method [144]. those created by bottom-up processes, particularly chemical vapor deposition (CVD), have a higher degree of crystalline clarity than those made by selective etching methods. Rather than single-layered MXenes, these techniques have only created multilayer ultrathin films [145]. Halim et al. generated a thin-film Ti3AlC2 MAX phase by depositing components such as C, Al, and Ti onto a sapphire substrate via magnetron sputtering. Furthermore, both non-MAX (Mo2Ga2C) and thin films of the MAX phase (Mo2GaC) may be made using direct magnetron sputtering, and they can subsequently be used to synthesis epitaxial Mo2C films using simple selective chemical etching [142]. When compared to MXene single crystals produced by wet chemical etching techniques, the CVD-grown crystals exhibit a much-reduced fault density. For scaled-up synthesis, the bottom-up approach must be laboriously pursued [81]. With the development of MXenes, various techniques were successfully applied to produce 2D MXenes without the use of dangerous etchants, including UV-induced, electrochemical, chemical vapor

 deposition, physical vapor deposition (PVD), spray drying, and thermal reduction [146, 147]. In table 2, numerous possible synthetic techniques have been covered.

Figure 4: Diagram illustrating various typical MXenes fabrication techniques [148].

2.3 Delamination

The "A" layers are removed one at a time throughout the etching process and are displace with other functional groups. The hydrogen and/or weak van der Waals bonds that occur hold the resultant MXene layers together. Applying further delamination or exfoliation treatment, permits the creation of few-layer or single MXene flakes [70]. Mechanical exfoliation or the intercalation approach may be used to achieve the multi-layered MXenes' delamination. However, it is important to note that mechanical exfoliation can generate MXene layers with strong interlayer connections, making it less effective for achieving intercalation. On the other hand, the intercalation approach specifically focuses on introducing intercalants to promote interlayer spacing and create distinct nanosheets [149]. According to research highly sticky interlayer connections in MXenes generated by mechanical exfoliation render this method ineffective, which are two- to six-fold stronger than those seen in bulk MoS2 and graphite [150]. Furthermore, the length of the sonication process alters the lamellar structure, reduces the size of the MXene sheet, and creates structural flaws [151]. Ion intercalation is proposed as a method for surface modification of MXene materials due to its unique advantages, including charge and small size of ions [152]. This process plays a crucial role in modifying the lattice structure and regulating the surface state of MXene materials, leading to the optimization of their electronic and chemical properties [153]. When using the intercalation process, intercalants are added to the mix to decrease the distance between layers of MXene, lessen their interaction, and promote the creation of distinct nanosheets. This greatly increases numerous surface terminations and the surface area, which are directly connected to the electrically conductive properties of MXene. Both ionic and organic intercalants are commonly used for MXene intercalations. Ionic intercalants include halide salts and metal hydroxides as aqueous solutions [154, 155]. Organic intercalants include tetrabutylammonium hydroxide (TBAOH) [137] isopropyl amine [156] dimethyl sulfoxide (DMSO) [157] tetra propylammonium hydroxide (TPAOH) [158] bovine serum albumin (BSA) [159] and n-butyllithium [160].

Schematic representations illustrating the two chemical modification procedures for the 2D Ti3C2Tx MXenes are shown in figure 5. The sample intercalated with K+ ions was referred to as K+Ti3C2Tx, while the delaminated Ti3C2 was given the name d-Ti3C2. The intercalation tests were conducted using both KOH and KOAc. XRD was used to describe all of the Ti3C2Tx, KOH–Ti3C2, KOAc–Ti3C2, and d-Ti3C2 samples. The Ti3C2Tx MXenes' increasing c-lattice parameter suggests that the interlayer spacings were extended after the intercalation operations. The variation in the c-lattice parameter for KOH and KOAc depended on other variables, such as pH level. For example, the c-lattice parameter of KOH–Ti3C2 was bigger than that of KOAc–Ti3C2, despite the fact that the  -OH ion was smaller than the acetate ion [161].

Figure 5: Diagrams showing the delamination and K+ intercalation processes for the two changes made to the Ti3C2Tx MXenes. Examples of the samples' SEM pictures. Reproduced with permission ref [162]. Copyright © 2014, Elsevier.

 

3. Properties of MXenes

3.1 Structural Properties

Understanding the actions of MXenes requires an in-depth understanding of their distinct atomic structures. After the "A" elements are removed from the MAX phases, the carbon and/or nitrogen (X) atoms and the transition metal (M) atoms combine leaving the overall crystal structure to be of a close-packed hexagonal shape (figure 6) [163]. The sequencing of M atoms within M2X exhibits a notable distinction compared to M3X2 and M4X3. In M2X, the arrangement follows an ABABAB order, while in M3X2 or M4X3, the M atoms adhere to an ABCABC packing scheme [71].

Figure 6: Two-dimensional MXene atomic structures (M2X, M3X2, M4X3, and M5X4) Reproduced with permission [164]. Copyright 2019, Elsevier.

 

To determine the structural properties of the MXenes, modeling modifications, and structural pattern analysis are crucial [165]. Different functionalized surfaces with functionalities including oxygen, hydroxyl, and fluorinated functional groups are created on etching acid. For instance, the largest theoretical metal ion storage capacity (e.g., , , , , , ) is predicted to be exhibited by the -O terminated MXenes [166]. In addition to having a large sodium-ion storage capacity, the S-functionalized Ti3C2 MXene also demonstrates a strong affinity towards polysulfide species, which could be utilized as a prospective Li-SB host [167, 168]. Because of its vulnerability, metals (Pb, Na, Mg, K, Ca, Li) can be used to replace the -OH terminations [169]. The OH-terminated titanium carbide ML MXenes' original structure's modeling of the density functional theory (DFT) really serves as the layered structure's basis. [170]. The XRD data obtained from both the experimentally synthesized and geometry-optimized hydroxylated MXene were highly consistent, exhibiting a close and good relationship with each other. The analysis of the XPS demonstrates the formation of the O, F, and OH functions which further demonstrates the presence of an incomplete or mixed termination at their surface. Additionally, there is a chance that water molecules will intercalate into MXenes' multilayer structure. Following the HF acid etching, the lack of the Ti3AlC2 MAX phase and the elimination of crystallinity are determined by analyzing the XRD pattern [171]. The primary observation of MXenes was its hexagonally tightly packed (HCP) structure [172].

3.2 Electronic Properties

MXenes display several remarkable properties, some of which include electrical properties that are affected by the edge groups attached to their surfaces and their composition [173]. For energy-related applications, MXene electrodes' electrochemical performance is greatly influenced by their electronic characteristics [174]. One of MXenes' most important electronic properties is undoubtedly its great electronic conductivity [175].  metallic behavior is exhibited by vast majority of MXenes that have surface terminations. In order to enhance the metallic conductivity of the MXene, several efforts have been undertaken in recent years. That being said, Ti3C2Tx MXene, which is the very first one to have been discovered remains to be the most conductive of all and also the most investigated one [176]. Similar to MXenes, the free electrons from transition metals drive the metallic conductivity of the solid solutions of MXenes without surface function groups (bare). Because transition metal d electrons have extremely high density of states (DOS) close to the Fermi level, the findings of the DFT calculation results indicate that MXene without surface terminations behaves typically like a metal  [177]. However, a large number of surface terminations has been seen in MXene synthesized through experiments. It is also very challenging to precisely evaluate the type and number of these surface terminations. Moreover, in order to assess the conductivity of MXene monolayer nanosheets, there exists a shortage of modern characterization techniques which makes it difficult to further explore the material's electronic properties. The findings of the HSE06 and Perdew-Burke-Ernzerhof (PBE) techniques, clearly shows that the MAX and Ti2X phases possess metallic properties [178]. At the Fermi level, however, the density of states of Ti2XT2 experiences a notable reduction.

MXene's band structure experiences substantial change with the addition of surface terminations. Surface terminations cause a substantial alteration in MXene's band structure. The peak density of states decreases considerably when the d orbital of the transition metal M and the p orbital of the end group are hybridized. In addition, a band gap may also be created as a result of the elevation of the d orbital above the Fermi level. Furthermore, a higher band gap is typically produced by a higher atomic number of M [179]. Without the termination groups, originally the MXene (Mn+1Xn) and the MAX phase possess metallic characteristics, but they turn into semiconductors when termination groups (Mn+1XnTx) are added to them due to the formation of the band gap [163]. An interesting approach, to impacting the conductivity of MXene is through cation or organic molecule intercalation. By employing this method the resistance of the device can increase significantly by more, than ten times when stacked materials are involved [180]. Above 500 K temperature, several Mo-based MXenes exhibit a notable increase in temperature-dependent electric conductivity [181]. Furthermore, by adjusting the external electric fields and the strain, adjustments can be made to the electronic properties of MXenes [182]. For example, it was demonstrated by Xiao's group that at a particular degree of uniaxial and biaxial strain, Direct to indirect band gap transitions may occur in monolayer Ti2CO2 [183]. The electronic properties are also affected by the "M" in MXenes. For instance, because the band gap of MXenes grows as the atomic number increases, the metal electronegativity weakens with the rising atomic number [184]. To recapitulate, most of the MXenes have outstanding electronic capabilities that contributes to high performance in batteries, which is demonstrated by both the experimental and computational analysis [185].

 

4. Applications in Electrochemical Energy Storage

MXene electrodes have drawn a lot of attention since the discovery of Ti3C2Tx because they could be potentially used in upcoming batteries and other electrochemical energy storage (EES) technologies, such as supercapacitors. Ti3C2Tx MXene is the focus of most studies for LIBs, despite the fact that there exist many EES systems and there's a broad family of MXenes. There has been rapid and significant progress made recently in Research on MXenes advances rapidly forward for a variety of different EES technologies beyond LIBs. Recent discoveries in MXene-based materials such as SIBs, PIBs, Li-SBs and multivalent ion (, , ) batteries for EES other than LIBs have attracted attention of different researcher [186].

4.1 Lithium–Sulfur Batteries (Li-S)

The abundant availability of sulfur and its cost-effectiveness, makes the Li-S stands out as an extremely bright possibility in electrochemical method for storing energy [187]. The Li-S battery system, which consists of an organic liquid electrolyte, A sulfur composite cathode with a lithium metal anode have the potential for extraordinarily high energy density (2567 Wh ) [188] and theoretical specific capacity (1675 mA h ) [189]. Through the formation of soluble polysulfides, these batteries' redox process moves forward [190]. However, two major problems with the Li-S have been the dissolution of polysulfide in the electrolyte and the poor electrical conductivity of sulfur [191]. Accordingly, large surface area, highly conducting 2D MXenes with plenty of functional groups are attractive materials for Li-S, preventing the polysulfide shuttle effect [192] due to their hydrophilic surfaces, which may react chemically with intermediate polysulfides [193, 194]. Notably, it has recently been shown that MXenes with Te, S, Cl, Br, O, NH, and Se terminations can be successfully synthesized [194]. For Na, Ca, K, Li, and Mg intercalation, The lightest MXene Ti2C with S termination has been explored at energy barriers smaller than 0.5 eV [195]. Lithium and sulfur undergo a redox process, which is defined as 16Li + S8 → 8Li2S, to complete the charge/discharge operations as seen, energy storage, and conversion of Li-S as shown in figure 7 [196].

Figure 7 (a-b): (A) An illustration of Lithium and sulfur charge and discharge processes. Reproduced with permission [197] Copyright 2014, American Chemical Society. (B) Li-S discharge and charge curves. Reproduced with permission [198]. Copyright 2019, Wiley-VCH.

4.1.1 MXene Sulfur Host

Among the sulfur hosts for Li-S batteries that show the most promise is considered to be MXenes with extraordinary hydrophilicity and conductivity [199]. MXenes were used in lithium-sulfur battery cathodes for the first time in in 2015 by Liang et al. According to them, the hydroxy groups and the Ti atoms on the surfaces of MXene facilitated substantial chemical adsorption to soluble lithium polysulfides, hence enhancing the battery performance by reducing the shuttle effect [200]. The accordion-like Ti3C2 MXene was formulated by Zhao et al. using HF etching, and then directly coated with sulfur using a melt-diffusion method to create an S/Ti3C2 composite having a sulfur concentration of 57.6 weight percent. This composite demonstrated starting capacity of 1291 mAh g−1 at 200 mA , and after 100 cycles still maintained 970 mAh  [201, 202]. Further exploration and understanding of the structural characteristics of Li2Sx and MXenes is required for advances in Li-S batteries. As illustrated in figure 8(a), Stable MXenes consist of five stacked hexagonal layers (atomic layers O(F)-Ti-O(F)-N-Ti), namely Ti2NF2 and Ti2NO2 [203]. Above the Ti2NF2 and Ti2NO2 hexagonal phases, F and O termination groups are located. Therefore, the most stable configurations are seen in Ti2N-based MXenes [204]. Researches on lithium batteries shows that, S-containing clusters are usually formed by Li ions during the process of discharge. The most stable Li-S structural combination out of a variety of Li-S combinations that is known so far is Li2Sx. Furthermore, the stability of the Li2Sx can be confirmed by figure 8(b) which illustrates that all of the Li2Sx structures are nonlinear and three-dimensional (3D) [205]. Understanding the structure of MXene interaction with Li2Sx requires understanding of the specific structures of MXenes and Li-S ions. Electrostatic forces dominated the strong interaction between Li2Sx and Ti2NO2. Intercalation of the Li2Sx structure occurs within the Ti2NO2 layer during lithiation [206].

Figure 8 (a-b): (a) Ti2N-based MXene monolayers optimized structures (b) Li2Sx species optimized structures. Gray, blue, red, purplish red, yellow, and blue spheres symbolize the atoms N, Ti, O, F, S, and Li, respectively. Reproduced with permission ref [207] Copyright (2019) Elsevier.

 

Furthermore, all-MXene electrodes have been proposed, wherein the two-dimensional delaminated Ti3C2 nanosheet interlayer physically and chemically blocks the transfer of lithium polysulfides LiPSs, whilst the three-dimensional structures alkalized Ti3C2 exposed surface area nanoribbon frameworks with several open macrospores make certain high S loading and speedy ionic diffusion [208]. Additionally, Huang et al. reported the assembly of a laminar MXene-Nafion film via LBL. To generate the 1.0 mm modification layer, MXene sheets were stacked in an orderly fashion with the Nafion inserted evenly into the interlayers. With Nafion and MXene assembled, a physical barrier was provided to the LiPSs which resulted in redox reactions and facilitated rapid ion and electron transit [209].

4..1.2 MXene Composite-Based Sulfur Hosts

4.3.1 MXene–Carbon Composites

In addition to building three-dimensional MXene structures, another effective way to improve the overall efficiency of Li-SBs is to combine MXene with other materials. Porous carbon, graphene, carbon nanotube (CNT) and other carbon-based materials with excellent mechanical strength and good conductivity are popular candidates [210, 211]. The intrinsic advantages of both Carbon and MXenes are combined while constructing the MXene/Carbon composite. Furthermore, for rapid ion transport, The carbons function as a "spacer" to stop the MXene from restacking, and the MXene nanosheets are used to increase the conductivity of the composites for quick electron transit [41]. Moreover, the carbon materials specifically the 2D graphene and the 1D CNTs, offer a multitude of potential for the construction of advanced structures using MXene nanosheets [212]. Consequently, even at high sulfur loading, the sulfur cathode based on MXene/carbon composite may obtain excellent electrochemical performance. The open structures and the excellent conductivity of MXene/CNTs composites lead to both excellent electrochemical performance and high sulfur loading [213]. One way to generate 3D CNTs/MXene composite frameworks is to simply combine 2D MXene nanosheets with 1D CNTs, which then serves as sulfur host. After 1200 cycles at 0.5 C, the as-prepared CNTs/Ti3CN (83 wt% sulfur content), CNTs/Ti2C (83 wt% sulfur content), and CNTs/Ti3C2 (79 wt% sulfur content) all maintained ≈450 mAh  and demonstrated initial capacities above 1200 mAh g−1 at 0.05 C [214, 215]. Because of the strong interaction between LiPSs and Mo atoms, The LiPS shuttle can also be suppressed by Mo2C MXene. A combination of Mo2C and CNTs with high electrical conductivity and an increased surface area of 116.8 cm2 g−1 was produced by adding CNTs to Mo2C MXene, which then was utilized as a sulfur host [216]. The Mo2C-CNTs/S electrode, with 87.1 wt% sulfur content, showed 519 mAh  after 250 cycles at 0.1 C. Its initial capacity was 1235 mAh  with 74.9% retention  [217].

To improve the efficiency of Li-S batteries, rGO and 2D graphene have been coupled with MXenes [218]. Several three-dimensional MXene/graphene composite structures have been designed as sulfur hosts to increase sulfur loading and ion transport [219]. A 3D porous Ti3C2Tx MXene/rGO hybrid aerogel was produced by hydrothermally processing Ti3C2Tx MXene and GO composites and then freeze-drying the resulting material. This aerogel was then used in Li-S batteries to store Li2S6 as a freestanding cathode [220]. The 3D connected MXene/rGO conductive network enables rapid electron transfer and fast Li+ diffusion, with capacities of 1270 mAh g−1 at 0.1 C, 5.27 mAh  with 6 mg sulfur loading, and 0.07% capacity deterioration over 500 cycles at 1 C. Using a simple liquid infiltration–evaporation technique, a novel kind of freestanding three-dimensional porous hybrid aerogel of MXene and graphene was developed. and utilized to load Li2S  [221].

4.1.3 MXene–Metal Oxides Composites

MXene-metal oxide composites are a family of materials that show great promise due to their distinct characteristics and wide range of applications. These composites incorporate the remarkable qualities of MXenes, including their great electrical conductivity, mechanical flexibility, and chemical stability, with the desirable characteristics of metal oxides, such as catalytic activity, high specific surface area, and tunable band gaps. Recent developments in MXene-metal oxide composites have focused on enhancing their performance in several domains, including as electronics, sensing, catalysis, and energy storage [222]. These composites offer enhanced properties and functionalities compared to individual components, opening up new possibilities for advanced materials and devices [223]. Strong chemical interactions can occur between LiPSs and transition metal compounds, such as oxides [211] and transition metal sulfides[224]. Transition metal compounds, when combined with MXenes, may efficiently increase the chemisorption, lowering the 2D MXene nanosheets' tendency to stack again and the shuttle effect using LiPSs. Despite the low electrical conductivity of certain transition metal compounds [211] MXene's metallic conductivity can compensate for it. Therefore, High conductivity composites of MXenes and transition metal compounds are believed to improve sulfur usage and a prospective sulfur host for Li-S batteries with a high capacity to prevent the shuttle effect [225]. MXene-metal oxide composites, including TiO2, SnO2, and NiO, have been synthesized using various methods such as hydrothermal synthesis, sol-gel method, and electrophoretic deposition. These methods allow for the controlled incorporation of metal oxides into MXene structures, resulting in unique composite materials with enhanced properties [226, 227].

4.1.4 MXene–Polymer Composites

The in situ polymerization and ex situ blending processes may be used to mix MXenes with a variety of polymers due to their surface's many active terminations [228, 229]. LiPS's cumulative adsorption on the outside surface increases the likelihood for them to dissolve into the electrolytes [230] even though LiPS's dissolution could be somewhat mitigated by MXenes' strong polar surface. This ultimately causes active materials to leak when the adsorption capacity between outermost LiPS and MXene is not enough to stop LiPS from solvating in organic electrolytes. In this regard, Yang and colleagues have recently proposed a dual polysulfide confinement approach [231] in which sulfur is inserted into Ti3C2Tx's layer space and then covered in a thin layer of polydopamine (PDA). Ti3C2Tx functions as an active 2D surface and a highly conductive skeleton to chemically bond with LiPS. Also, as the XPS measurements showed that the coated PDA layer could both chemically and physically clamp sulfur species in the MXene nanosheets while preventing a direct contact between sulfur and the liquid electrolyte. Additionally, the ternary composite's mechanical flexibility is greatly enhanced by the coated PDA sheath which helps in maintaining the cathode's structural integrity during charge and discharge [199]. Additionally, Ti3C2Tx MXene/polyethyleneimine functionalized carbon nanotube (T@CP) composite was created using a self-assembly technique for use as an anti-fouling separator and sulfur host. This as-prepared T@CP product allows for outstanding electrochemical performance toward the Li-SBs because of its well-organized porosity, dual polarity, high conductivity, and excellent mechanical strength [232]. The as-prepared MXene/polymer composite exhibits exceptional features, including better mechanical properties, enhanced conductivity, and improved thermal stabilities, as a result of the synergistic effect between organic polymer precursors and the inorganic MXene. These properties can be tuned precisely by the types of  MXene and polymers and their composition [233].

4.2 MXenes for Lithium Anode Protection and Separator Modification

It has been shown that altering the separators is a simple and efficient way to stop LiPSs from switching back and forth between the anode and the cathode, which enhances the Li-S batteries' electrochemical performance [234, 235]. Diverse nanostructures of materials based on carbon have been used to alter separators. Li-S batteries were constructed by Dou et al. using an eggshell membrane modified with accordion-like Ti3C2Tx particles as a separator. Ti3AlC2 was etched with an HF solution to produce the accordion-like Ti3C2Tx particles. After 250 cycles at 0.5 C, the resulting Li-S batteries had a 74% retention capacity [236]. Zhang et al. created a 3D separator in Li-S batteries that was modified using Ti3C2Tx aerogel. A cross-linked three-dimensional aerogel, Ti3C2Tx, was created [237]. The high initial discharge capacity of Li-S batteries with Ti3C2Tx aerogel separators is 1487 mAh  at 0.1 C (1C=1672 mAh ) and 670 mAh  at 2 C. With an ultra-low capacity decay rate of 0.037% each cycle, the reversible discharge capacity of 424.3 mAh   at 1 C is maintained for more over 1500 cycles, demonstrating exceptional cycling stability and a long cycle life [238]. Additionally, Zhang et al. modified PP separators by effectively fabricating MXene based (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) Ti3C2Tx–PEDOT: PSS hybrid. PEDOT: PSS in this hybrid may effectively stop Ti3C2Tx nanosheets from restacking and improve Li-S battery electrical conductivity, which in turn promotes quick Li+/electron transit and improves sulfur usage. In the meanwhile, Ti3C2Tx nanosheets are efficiently anchored to polysulfide by the addition of Ti3C2Tx–PEDOT: PSS, which prevents the shuttle effect. Consequently, Li–S cells using modified-separators Ti3C2Tx–PEDOT:PSS have exceptional performance, exhibiting a long cycling stability, a high discharge capacity of 1241.4 mA h  at 0.2C, and a low degradation rate of 0.030% per cycle at 0.5C for 1000 cycles [239]. More research was done by to determine how the MXene modified separator for Li-SBs performed electrochemically in relation to thickness by Han et al. They discovered that as the thickness of the MXene layers on the separator decreases, the lithium-ion diffusion coefficient rises. Particularly at the lowest MXene a thickness of 100 nm, or mass loading of 0.016 mg , the shuttling of LiPS species was effectively suppressed [240]. A novel method for designing and fabricating separators in a variety of industries is provided by a modified material's structural design that anchors LiPSs. Other composites based on MXene that exhibit superior electrochemical performance have also been employed as the interlayer or separator coating between the cathode and the separator [241]. These composites include 3D CNTs/MXene, [242] TiO2/MXene, [243] nitrogen-doped MXene carbon nanosheet-nickel, [244], and Ti3C2Tx nanosheet@Cu/Fe-MOF [245].

The effective functioning of Li-S battery performance is impeded by the dissolved LiPSs causing the Li metal anode to severely corrode. Despite this, the anode component has not garnered as much attention as the cathode designs, which have been the subject of substantial research. Lamellar-structured MXenes have nanoscale gaps that help regulate the deposition of lithium. A flexible Ti3C2 MXene possessing a 950 mAh high reversible capacity was prepared in order to demonstrate this notion [246, 247]. Moreover, as figure 9 illustrates, uniform Li nucleation may be guided by MXene based on Ti3C2Tx (T=−F, −OH) and reduced graphene oxide (rGO), which promotes dendritic-free and extremely reversible Li plating/stripping. For more than a millennia, this Li anode has exceptional cycle stability, even at large area capacities (3 mAh ) and very high rates (10 mA ) [248].

 

 

Figure 9 (a-c): The uniform development of Li and prevention of Li anode corrosion by LiPSs are provided by the Ti3C2Tx/rGO/well-aligned layered structure. a) Concept for moving LiPSs to the anode of lithium from the sulfur cathode. b) LiPSs-induced dendritic development and corrosion of the anode for a bare Li anode. c) Li growth without dendrites and suppression of LiPS for the composite anode of Li-rGO/Ti3C2Tx. Reproduced with the permission ref [248]. Copyright 2019, Wiley-VCH.

4.3 Sodium-Ion Storage (SIBs)

Lithium-ion batteries (LIBs) may be replaced with sodium-ion batteries (SIBs) for grid-scale energy storage. This is because sodium has a similar low potential to lithium, and sodium is also cheaper. This has been supported by research [249, 250]. Similar in energy density to LIBs, sodium-ion batteries have a sufficient redox potential (E0 Na+/Na = 2.71 V vs SHE). Supercapacitors (SIBs) may outperform lithium-ion batteries (LIBs) regarding the storage of electrochemical energy [251].  However, because of the slow kinetics and substantial volume variability in the charge/discharge process of SIBs because of the big Na+ radius. In light of the current situation, the discovery of MXenes with a two-dimensional lamellar structure, adjustable spacing between layers, strong conductivity, and a low barrier for Na+ diffusion presents a promising chance to address the challenges faced by SIBs and enhance their electrochemical capabilities  [252, 253].

When used as anode materials, the electrochemical efficiency of SIBs intimately connected to the surface groups of MXenes [251]. For instance, although the -O and -N groups promote Na+ storage, the and -OH and -F groups raise the Na+ diffusion barrier and decrease MXenes' ability to store Na+. Figure 10 illustrates how Xia et al. [254] demonstrated a modified N surface group on MXenes that improves SIB cycle stability and boosts Na+ storage capacity. In this work, cetyltrimethylammonium bromide (CTAB) is calcined to add N groups while still preserving the structure of MXenes.

Figure 10: Simulation models of Ti3C2 and Ti3C2-N and illustration of the SEI compositions in the Ti3C2-N anode

[254].

Doping MXenes with elements like sulfur and nitrogen has been shown to improve their electrical conductivity and sodium storage capacity. For example, sulfur-doped MXene (Ti3C2S2) demonstrated high sodium storage capacity with low diffusion barriers [255, 256]. Modifying the MXene's interlayer spacing nanosheets can enhance the sodium storage capacity and improve the kinetics of sodiation/desodiation. Metal ion pillaring and intercalation of sulfur atoms have been employed to increase the interlayer spacing, resulting in improved capacity and cycling stability [257, 258]. Also, creating mesoporous or 3D porous MXene structures can enhance the active material utilization and reduce ion diffusion distances. This design approach has shown promising results in improving the capacity and cycle life of sodium-ion batteries based on MXene [259, 260].

4.3.1 MXene-Based Composite

It has been demonstrated that ne effective method for enhancing the electrochemical efficiency of MXene-based anodes for sodium-ion batteries (SIBs) is to combine MXenes with high-capacity anode materials. Significant improvements in battery performance, including as lowering the ion diffusion barrier, using fewer electrolytes, and increasing cycle stability, are made possible by the special shape of MXene-based anodes in SIBs [261]. For instance, a free-standing Na2C6O6/MXene hybrid paper with exceptional flexibility was created by Feng's group [262]. Because of its high theoretical capacity of 501 mAh g−1, Na2C6O6 is a potential anode material in SIBs. But it has problems including low conductivity, quick capacity loss, and electrolyte dissolution. The researchers were able to overcome these obstacles and fully use the high theoretical specific capacity of Na2C6O6 by mixing it with MXenes. The Na2C6O6/MXene flexible anodes demonstrated a remarkable capacity of 231 mAh g−1.  Zhang and coworkers [263] conducted a study where they compounded MXene with red phosphorus through high-energy ball-milling. The red P/Ti3C2Tx composite, as prepared, demonstrated robust sodium storage capabilities. P-O-Ti linkages, which completely combined the conductive MXene material with the high-capacity red phosphorus, are responsible for this. Sodium-ion batteries' electrochemical performance was improved when these two materials were combined through alloying processes. [264]. A composite of Si and MXene self-assembled in Wang's work [265] through electrostatic interaction between Si NPs and FL-MXene that had been treated with acetone. The composite included a well-thought-out FL-Si/MXene structure that combined MXene microspheres and activated Si NPs with controlled interlayer spacing and strong structural stability. Figure 11 illustrates the numerous research that have been published in recent years on anodes for sodium ion storage using materials based on MXene [266].

Figure 11: An overview of notable recent MXene-based anode works in chronological order. Reproduced with the permission of Ref [266] Copyright 2022, The Minerals, Metals & Materials Society.

4.4 Potassium-Ion Storage

Potassium-ion batteries (PIBs) are a promising substitute for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to the suitable potential of potassium (K) metal, which is less than that of Na and close to that of Li. These batteries have higher specific energy, power density, faster ion diffusion, low cost and sustainability, and compatibility with existing infrastructure. They can potentially be integrated into the current lithium-ion battery manufacturing infrastructure with only minor modifications [267, 268].

 K ions have greater ionic radii (0.138 nm) than Li+ (0.076 nm) and Na (0.0106 nm) ions. This means that during the potassiation/depotassium process, there will likely be more severe volume variation, which will restrict the types of materials that may be used for PIB electrodes [261]. Early calculations have demonstrated potential K ion intercalation into conducting 2D MXenes with a substantial interlayer gap, suggesting an appropriate anode material for PIBs with high capacities [269]. Following that, 2D MXene materials containing Nb2CTx, Ti3C2Tx, and Ti3CNTx were experimentally created as components of anode materials for PIBs [270, 271]. When Ti3C2Tx was first investigated by Naguib et al. [272]  as an electrode material for PIBs, the resulting curves showed that after 100 cycles of charge and discharge, the reversible capacity remained at 75 mAh . For instance, Zhang et al. [273] used electrostatic adsorption of Ti3C2Tx with positively charged melamine to create a 3D MXene scaffold. The 3D structured MXene scaffold possesses key characteristics that enhance its efficacy as an anode material for potassium ion batteries (PIBs). Its numerous surface sites and efficient ion transport pathways serve to increase potassium ion storage capability and accelerate dynamics. In electrochemical testing, the 3D Ti3C2Tx MXene exhibited elevated performance, achieving a high specific capacity of 161.4 mAh/g over numerous charge-discharge cycles while maintaining excellent stability. These results indicate that compared to other anode materials explored for PIBs, MXenes may represent a superior option due to their ability to facilitate higher potassium intercalation and faster transfer kinetics, attributes originating from their unique three-dimensional network architecture. PIBs rely heavily on their metal anodes. Nonetheless, the unexpected development of dendrites and it is very challenging to reversibly plate and strip potassium metal during cycling due to the continuous electrolyte consumption. Remarkably, the MXenes might serve as a three-dimensional scaffold to promote consistent K+ deposition, adapt to volumetric variations, and maintain the solid electrolyte interphase (SEI) during cycling. A composite of MXene/carbon nanotube (DN-MXene/CNT) lacking in titanium and containing nitrogen was created by Tang et al. [274] as the 3D skeleton to include K+ metal. Consequently, the K@DN-MXene/CNT electrode displays an outstanding cycle stability and a morphology devoid of dendrites. This work shows that MXenes is an excellent three-dimensional skeleton for boosting K metal stability in batteries. By using a hydrothermal process, TiO2/reduced graphene oxide (rGO) composites based on 2D MXene have been developed by Fang et al. [275] with an enhanced and practical capacitive capacity for LIBs and PiBs. TiO2/rGO, with its long cycle time and enormous capacity, was the cathode of choice for PiBs and LiBs. Rate performance and capacity were enhanced by rGO. Researchers studying batteries across the world have been interested in metal-based anode materials because of their effective conductivity and energy densities. These materials include antimony (Sb), bismuth (Bi), and tin (Sn). Significant volume growth has also been shown to cause fast deterioration. Later, using the electrodeposition approach, Tian et al. successfully developed a glycol-based system including Sb, Bi, and Sn on MXene paper [276]. These materials allow potassium ions to diffuse over short distances, causing volume change in the potassium and depotassium technique. MXene sheets have strong conductivity and efficiency in flexibility [277]. These qualities allowed it to be used as an elastic collector for electronic transmissions and to adjust volume changes during the circulation process.

Multivalent Ion Storage (Mg, Al, Zn)

4.5 Zinc Ion Batteries

Earth-abundant, Low-cost Zn metal has a suitable redox potential (-0.76 V against SHE), ensuring a broad voltage window in aqueous electrolytes [278]. An aqueous zinc ion battery is made up of a mild electrolyte, a cathode, and zinc foil anode  (for example, 2 M ZnSO4) that is both safe and environmentally friendly [279]. Because of their inherent wettability with aqueous electrolytes, for ZIB electrode designs, 2D MXene nanosheets with a conductive core and hydrophilic surface show potential. Although ZIBs' energy densities are not equal to those of LIBs, they could be helpful in situations where extreme safety is required because they don't have the burning risk that comes with organic electrolyte-based MIBs.  

The materials used in traditional ZIB battery cathodes include those based on manganese (Mn), vanadium (V), and Prussian blue equivalents. Hybridizations using nanocarbon or MXene nanosheet materials are also possible [280, 281].  Electrostatic host material Zn2+ ion interaction is one of the most challenging problems for aqueous ZIB cathodes [282]. For aqueous ZIBs to integrate with MnO2, a variety of carbon-based compounds have been used as a conducting medium [283]. 2D MXenes exhibit high density, exceptional hydrophilicity, rich surface functionality, and strong metallic conductivity (4 g ) [284]. It was recently suggested by Minjie Shi et al. [282] that the 3D micro-flower-like structure, when cycled with highly electroactive MnO2, not only offers exceptional structural stability but also enhances electrical conductivity. With a remarkable rate capacity and a high reversible capacity of 301.2 mAh , the 3D Ti3C2Tx@MnO2 micro-flower exhibits outstanding cycling stability over 2000 cycles. When cycling ZIBs using V2CTx MXene as the cathode material, Li et al. saw an extraordinary performance boost [285]. This is because, during exfoliation and cycling, V2CTx oxidizes into V2O5, yielding an energy density of 386.2 Wh Kand a capacity of 508 mAh . By using highly stabilized ZMO nanoparticles (ZMO@Ti3C2Tx) as a cathode for aqueous ZIBs (less expensive than pristine ZMO-based cathode material). This is because, during exfoliation and cycling, V2CTx oxidizes into V2O5, yielding an energy density of 386.2 Wh Kand a capacity of 508 mAh . By using highly stabilized ZMO nanoparticles (ZMO@Ti3C2Tx) as a cathode for aqueous ZIBs (less expensive than pristine ZMO-based cathode material). Minjie Shi et al. [286] were able to prevent irreversible scaffold degradation caused by high conductivity and ZMO side reactions. This resulted in a comparatively high cycling stability and reversible capacity over 5000 cycles. The electrochemical activity of zinc ion storage energy may be improved by structurally altering MXene. Ti2CTX preintercalated with Sn⁴⁺ and with a notably larger interlayer gap is arranged vertically above carbon spheres [287]. This structure increases ion transport and improves diffusion kinetics, resulting in increased electrochemical storage of zinc ions. The Sn⁴⁺-Ti2CTX/C electrode has remarkable cycling stability in zinc-ion capacitors. MXene is commonly utilized in zinc-ion capacitors (particularly flexible ones). However, the types of ion storage and storage processes require additional investigation, and electrochemical performance must be improved by structural design [288]. Zinc-ion batteries have been made using a variety of materials, including organic compounds, V-based materials, Mn-based materials, and Prussian blue analogues, but each has drawbacks. Making full use of MXene's structure and physicochemical properties is one potential technique for overcoming these active materials' inherent weaknesses. Combining different active materials can improve their electrochemical characteristics and provide complementing advantages. Additionally, a complete investigation and understanding of the energy storage and action processes is imperative for the advancement of zinc-ion energy storage and the utilization of MXenes [288].

4.6 Magnesium-Ion Storage

The promise of higher volumetric energy densities than LIBs makes multivalent ion batteries  enticing—like Al³⁺, Zn²⁺, Mg²⁺, and Ca²⁺ [289, 290]. Magnesium is a viable contender due to its abundant resources (104 times Li), large theoretical capacity (2 219 mAh g^-1), and low redox potential (−2.37 V vs. SHE). Mg ions' bivalence makes them a suitable anode material, with an ultra-high volumetric capacity of 3,833 mAh cm⁻³ [291]. There has been a lot of interest in rechargeable magnesium-ion batteries (MIBs) with metal anodes made of magnesium (Mg) since the pioneering work of Aurbach et al. [292]. The lack of dendritic development, the cheaper cost resulting from the earthly abundance of magnesium, and the ability to handle the metal in an ambient environment make MIBs safer than LIBs [293]. On the other hand, slow ion diffusion kinetics and strong ion-electrode interactions are caused by the divalent charge of Mg²⁺ ions. Today, MIBs continue to face challenges related to the absence of stable electrolytes and appropriate cathodes that permit quick diffusion/intercalation of Mg²⁺ ions [294]. A theoretical capacity of up to 1050 mAh g^-1 is possible due to the formation of bilayers and trilayers by Mg²⁺ ions on the MXene surface, as shown by previous simulations [295]. For this reason, there have been many efforts to use MXene-based materials for MIBs. For instance, the performance of novel Ti3C2Tx was poor, whereas Xu et al. showed the potential of CTAB intercalated Ti3C2Tx MXene for Mg-ion storage [296]. The Ti3C2Tx/CTAB electrode, in its prepared state, demonstrated exceptional rate capability and a high capacity of 300 mAh cm−3 at 50 mA g^-1. The DFT results showed that the greatly increased performance was due to a lower diffusion barrier (approximately 0.19 eV) of Mg²⁺ ions on the MXene surface. In order to enable MoS2 composite for MIBs, the same group also synthesized Ti3C2Tx MXene [297].  A petal-like MoS2/MXene hybrid was produced by vertically loading MoS2 nanosheets on Ti3C2Tx MXene nanosheets by a hydrothermal reaction between Na2MoO4 and NH2CSNH2 at 210 °C. Notably, the electrical conductivity of MoS2 was significantly enhanced for high-capacity use by the presence of MXene. As a result, at 5 mA g^-1, 165 mAh g^-1was achieved, a much higher value than the 62 mAh g^-1of pure MoS2 anode [297]. Additionally, enhanced performance was achieved by preparing VS4 anchored on Ti3C2 MXene as a high-performance cathode material for magnesium ion batteries [298]. While the MoS2/MXene hybrid with vertically loaded model is a promising demonstration in the early research stage of MIBs, the performance of the magnesium storage is not very satisfactory.

4.7 Aluminum-Ion Storage

AIBs offer excellent mass-specific capacity (2980 mAh g^-1) and volumetric capacity (8406 mAh cm⁻³), great safety, and affordability making them the most promising energy storage solution. The upper crust of the earth contains large amounts of aluminum elements (around 8%) [299]. However, several challenges persist regarding the use of AIBs, including the difficult-to-synthesize high-specific capacity cathode materials, short cycle life, and limited electrolyte substitutes. Thus, the primary objective in the development of AIBs systems is to design and manufacture cathode materials with high specific capacity and cycling stability. As a common 2D material, MXenes show great potential as high-capacity cathode materials in AIBs. Beidaghi et al. [110] for example, used few-layer V2CTx MXene as the cathode material for AIBs and demonstrated a high capacity of 300 mAh g−1. In order to manufacture a single-layered Nb2CTx MXene for use as the cathode material in AIBs, Tu's group[300] produced a first charge capacity of 275 mAh g^-1. MXene@N-doped carbon@ Ni0.6C00.4S composite was created by Zheng et al.[299] to serve as the cathode material for AIBs. MXene served as the substrate for the Ni0.6C00.4S solid solution. The strong MXene support layer enhances the composite's chemical stability and structural integrity in addition to providing a quicker ion and electron transmission channel. The anode demonstrated a high capacity of 481.2 mAh g^-1 with the help of MXene. According to these studies, MXenes have the potential to function as efficient cathode materials for AIBs. The aforementioned studies demonstrate that MXenes is a promising substrate material in addition to being a superior cathode material for high-capacity AIBs. When used as a substrate, MXenes may enhance the overall characteristics of composite materials and assist active compounds in reaching their full potential.

4.8 Anode Protection for Metal Batteries

Energy storage systems with a greater energy density are now needed for electric cars and portable gadgets. Because metal anodes (such as Zn, K, Li, and Na) have large theoretical specific capacities, metal-air, metal-sulfur, metal-CO2, and metal-oxygen batteries — exhibit energy densities much greater than LIBs. However, the high reactivity, irregular dendritic growth, safety concerns, low long-term durability, and substantial volumetric changes during cycling of metal anodes hinder their development [301]. As a result, a great deal of research has been done to address these issues, with metal anode shielding emerging as one successful strategy. In order to protect metal anodes against uneven dendritic growth, three techniques have been developed: i) modifying metal anodes (e.g., controlling electrochemical processes to control metal deposition, or regulating metal diffusion to capture metals in porous hosts); ii) covering the metal anodes with an artificial SEI film; iii) electrolyte engineering to produce stable and safe metal anodes [302]. Three-dimensional porous current collectors with large surface areas and lithophilic coatings can normalize lithium-ion flux to suppress dendrite formation. Separator modifications like surface coatings with nanoparticles or multilayer films offer enhanced ionic conductivity and mechanical strength while hindering dendrites and side reactions. Electrolyte additives such as halogen salts, water, nitrates, and fluoroethylene carbonate form stable solid electrolyte interphase layers that guide uniform lithium deposition. Developing high conductivity solid-state electrolytes remains challenging but offers excellent safety through suppressing dendrites and side reactions [303]. Certain advantageous properties of materials based on MXene are helpful for effective metal anode protection. For example, large interlayer spacing may significantly reduce volumetric changes during metal plating and stripping; homogeneous nucleation and growth of metals on MXene surfaces is induced by the 2D structure; and rapid electronic and ionic transport is made possible by MXenes' high conductivity and hydrophilicity. Furthermore, at the anode/electrolyte interface, the fluorine termination may result in the development of homogeneous, dense, and long-lasting SEI films that are dominated by LiF salt [304]. As a result, enormous efforts have been made to use MXenes to safeguard metal anodes.

4.9 Lithium Metal Anode

MXenes are potential materials for lithium metal anodes due to their large interlayer spacing and electrochemical activity. Unlike graphite anodes, MXenes can readily intercalate lithium ions with minimal lattice distortion. Their theoretical capacities are higher than graphite and they have slightly higher lithiation potentials, which may help reduce lithium dendrite formation. Dendrites pose significant safety hazards as they can cause short circuits. MXenes aim to suppress dendrites through various strategies [247]. Artificial solid electrolyte interfaces can be constructed on MXene surfaces through inorganic or organic coatings to facilitate uniform lithium-ion transport. Three-dimensional porous current collectors provide large surface areas and lithiophilic coatings to homogenize lithium plating. Separator modifications like surface coatings or multilayer designs enhance ionic conductivity and mechanical strength while hindering dendrites. Electrolyte additives such as salts or polymers form stable solid electrolyte interphase layers that guide uniform lithium deposition. Delamination and porous architectures increase interlayer spacing for facile lithium intercalation. Carbon and metal oxide composites utilize pseudo capacitance and conductive networks for high capacities. Perpendicular MXene arrays (MXene–Li arrays) have been shown to have ordered interspaces at both the nanometer and micrometer scales [305]. This architecture reduces the "lightning rod effect" and volume changes during plating, leading to dendrite-free lithium deposition with high-rate capabilities. The tunable interlayer spacing of MXenes also allows compositing with high-capacity Si for lithium metal anodes. In addition to MXene/rGO aerogel [306] and MXene/graphene framework, [307] additional 3D scaffolds for Li metal anodes, including thin lithium electrodes with interlayer calation [308] were intended for lithium metal batteries without dendrites [309]. Qian et al. inhibited the formation of lithium dendrites by inducing isotropic lithium nucleation and growth on MXene films using an amorphous liquid metal nucleation seed [310]. Overall, the sensible microscale design of MXene-based anodes holds promise for reliable lithium metal batteries.

5. Other Metal Anodes (Zn, K, Na)

MXene-based Materials are also being utilized to stop the formation of dendrites in metal anodes other than lithium metal anode. A Sn2+ pillared Ti3C2 MXene scaffold (CT-Sn(II)@Ti3C2), for example, has been reported by Li et al. as a stable matrix for sodium metal anodes free of dendrites with great performance [311]. Dendrite development on the favorable locations may be efficiently prevented by the sodiophilic seeds' (Sn2+) intercalation into the MXene layers. In the meanwhile, additional sodium ions may be accommodated and volumetric change can be mitigated by the enlarged interlayer distance. In order to create a dendritic-free K-S battery anode, Wang et al. developed a free-standing MXene/CNT host for potassium metal (K@DN-MXene/CNT) that is nitrogen-containing and defect-rich [274]. During plating/stripping, the highly conductive scaffold's quick K⁺ diffusion and electronic transport decreased the local current density and created a uniform ionic flow. The ability of the 3D scaffold to cause homogenous nucleation and, therefore, a uniform distribution of potassium was confirmed by both theoretical calculations and experimental findings. Interestingly, the flexible and free-standing Ti3C2Tx MXene@Zn paper has been shown by Qian et al. to have potential use as a matrix for lithium metal batteries. that are nonaqueous and those that are aqueous [40]. In the first instance, the MXene@Zn films' strong conductivity and excellent hydrophilicity made it possible for the reversible and stable zinc plating/stripping procedure. In contrast, zinc from the MXene@Zn host acted as the nucleation agent in the latter case due to the Zn–Li alloy interaction.

As can be seen from the above data, Materials based on MXene have demonstrated tremendous promise in shielding metal anodes from dendritic development because of their many surfaces functional groups, especially the -F, large interlayer spacing, and high conductivity. It should be highlighted, nonetheless, that further research is needed in this area since we're still in the early stages of using MXenes for metal anode protection.

 

5.1 Conclusion and Future Perspectives

MXenes have shown great potential owing to their qualities, as anode materials for different metal-ion batteries such as wide interlayer spacing, high electrical conductivity, and surface functionalities. As an anode material for Li-ion batteries, MXenes can intercalate Li ions with minimal lattice distortion and offer higher capacities than graphite. However, the challenge remains in obtaining high power and energy densities due to their pseudocapacitive behavior. MXenes have also been investigated for Na-ion and K-ion batteries with theoretical capacities over 200 mAh/g in some cases. However, experimental capacities are lower due to challenges associated with intercalating larger ions. To further improve performance, MXenes have been developed as composites with various metal oxides, sulfides, and silicon to take advantage of their synergistic effects. This allows much higher capacities and electrochemical utilization compared to pristine MXenes or other components alone. 3D nanostructures have also shown benefits by providing shorter ion diffusion paths and preventing flake restacking. Protecting MXenes from oxidation is also critical to achieving long-term cycling stability in practical applications. Still, further work is needed to optimize MXenes composition and surface functionality to promote discrete redox peaks suitable for battery operation. Developing controlled 3D architectures while maintaining high mass loadings will be important to fully realize rate and capacity benefits. Understanding capacity-enhancing effects like interfacial storage is also important. Finally, protecting MXenes from oxidation through carbon coatings or novel synthesis requires more thorough characterization and cycle-life testing. Some perspectives and outlooks for future study are presented as below in figure 12.

Figure 12:  Schematic showing the future possible directions.

A deeper understanding of the in-situ growth mechanisms from nucleation to formation is needed. Theoretical studies could provide insights into these mechanisms to guide experimental work. Advanced characterization techniques may help analyze the growth processes and role of composites in batteries. Surface chemistry changes before and after growth are currently unknown but likely impact performance. Developing green and low-cost synthesis methods applicable to in situ growth is important for scalability. Larger scale testing under practical conditions is needed beyond coin cells. Finally, application of in situ MXene composites in novel battery systems like aluminum, magnesium and iron ion batteries holds promise but has not been extensively explored yet. Overall, broadening the scope of MXenes and growth conditions analyzed, deeper mechanistic understanding, and moving towards applied testing and synthesis approaches are highlighted as important future directions for the field.

 

Author Contributions: K. Ullah: Study design, data collection, data analysis, data interpretation, and original draft preparation; N. Alam: conceptualization, methodology, study design, data collection, data analysis, data interpretation, reviewing and editing; S. Udin: formal analysis, data collection, and data analysis; W.C. Oh: formal analysis, final draft revision.  All authors have read and agreed to the published version of the manuscript.

 

Funding: This research received no external funding.

Data Availability Statement: Not applicable.

Conflicts of Interest: The authors declare no conflict of interest.

 

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