MXenes for advanced energy storage: A computational perspective

Abstract

The urgent demand for sustainable energy solutions has accelerated the development of rechargeable batteries, where the design of high-performance electrode materials plays a pivotal role. Since their discovery in 2011, two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have garnered significant attention owing to their excellent electrical conductivity, high mechanical strength, tailorable surface chemistry, and large specific surface area with abundant active sites. These unique properties render MXenes highly promising for electrochemical energy storage, particularly in lithium−ion, lithium−sulfur, and lithium−oxygen batteries, as well as emerging non-lithium−ion battery systems. However, their practical implementation is hindered by critical challenges including structural restacking, poor stability, and unfavorable surface terminations. In this review, we focus on a computational perspective for the rational design and performance optimization of MXene-based electrodes. Density functional theory (DFT) calculations have yielded fundamental insights into the intrinsic properties, modification mechanisms, and electrochemical behaviors of MXenes. Additionally, machine learning has enabled high-throughput screening and predictive modeling of their structure−property relationships. More importantly, this review not only summarizes the theoretical advances in MXenes for energy storage applications but also extracts actionable theoretical insights and performance prediction principles. These efforts provide a comprehensive theoretical reference for computational studies on MXenes, allowing researchers to efficiently grasp the research landscape, core mechanisms, and modification strategies. Furthermore, this review offers targeted guidance for experimental investigations by facilitating the optimization of modification schemes, the selection of suitable battery systems, and the reduction of trial-and-error in experimental design.

Graphical abstract

Keywords

MXenes / energy storage / density functional theory / machine learning / rechargeable batteries

1 Introduction

Environmental pollution and ecological challenges posed by conventional fossil fuels have necessitated a strategic transition of energy infrastructure toward renewable sources, which is recognized as an inevitable trend for sustainable development in contemporary society [1−3]. Consequently, the efficient utilization of green energy imposes greater demands on energy storage and conversion technologies [4]. Owing to the relatively high energy density and cycling efficiency, rechargeable batteries demonstrate significant potential in the field of electrochemical energy storage, encompassing lithium-ion batteries (LIBs), lithium−sulfur (Li−S) batteries, lithium−oxygen (Li−O₂) batteries, as well as non-lithium-ion battery systems [5−7]. The rapid development of commercial applications such as portable electronic devices, new-energy vehicles, and smart grid systems has intensified the demand for rechargeable batteries with higher energy density, improved cycling stability, and enhanced safety. Consequently, the design and development of high-performance electrode materials have emerged as a critical focus for advancing current research in electrochemical energy storage.

Since the first synthesis of Ti₃C₂ in 2011 [8], two-dimensional (2D) transition metal carbides and nitrides (defined as MXenes) have garnered widespread research attention due to their distinctive combination of properties, including excellent metallic conductivity, remarkable mechanical performance, and a high specific surface area with abundant active sites [9]. MXenes possess the general chemical formula M_n+1X_nT_x (n = 1−4, x ≤ 2), where M represents an early transition metal [green elements in Fig. 1(a)], X denotes C and/or N, and, more recently, O [10, 11] [gray elements in Fig. 1(a)], while T signifies surface termination groups [marked in orange in Fig. 1(a)]. The synthesis process of MXenes is illustrated in Fig. 1(b). MXenes are derived from MAX phase precursors [where A represents an A-group element, red-labeled in Fig. 1(a)], characterized by alternating MX layers and A atomic layers. In MAX phases, M−A bonds (metallic bonding) are weaker than M−X bonds (covalent/ionic bonding), while A-layer elements exhibit higher reactivity than the M_n+1X_n framework. Consequently, the A layers can be selectively removed from MAX precursors through chemical etching, yielding multilayer MXenes [12, 13]. Subsequent exfoliation allows these multilayer MXenes to be delaminated into single- to few-layer sheets. During this process, dangling bonds are generated on the M-layer surface and are rapidly saturated through reactions with environmental termination groups (T_x) [14]. It has been demonstrated that regulating the surface termination groups can chemically modify MXenes, thereby endowing them with diverse physicochemical properties to meet the demands of various applications.

Benefiting from the aforementioned advantages, MXenes have demonstrated broad application potential in cutting-edge fields such as electrochemical energy storage [15−17], sensing [18], catalysis [19−21], and biomedicine [22, 23], as evidenced by the remarkable surge in MXene-related publications [Fig. 1(c)]. Among these applications, energy storage accounts for a substantial proportion, particularly in rechargeable batteries, with the number of publications on MXenes in battery applications steadily increasing (based on the keywords “MXene” and “batter*” in the Web of Science database). Meanwhile, the network visualization map [Fig. 1(d)] highlights MXene materials as a major research hotspot among numerous keywords in battery research [24]. With the continuous discovery of new MXenes, these materials are expected to serve as promising candidates for advancing energy storage technologies. However, their practical applications still face serious challenges that need to be addressed, including instability, low capacity, interlayer restacking during charge-discharge cycles, and undesirable surface functional groups [25−27]. To enhance their performance as electrode materials, extensive efforts have been devoted to developing targeted modification strategies [28−32]: (i) Surface functionalization engineering, (ii) Heteroatom doping engineering, (iii) Defect engineering, (iv) Interlayer intercalation engineering, and (v) Heterogeneous structure engineering.

It is widely recognized that battery performance arises from the complex interplay among multiple components. However, conventional experimental optimization is often time-consuming, repetitive, and costly, which limits the ability to tailor MXene properties for specific applications. Density functional theory (DFT) calculations offer a powerful alternative, providing valuable insights into the physicochemical properties, adsorption behaviors, and modification mechanisms of MXenes, all of which are closely correlated with their electrochemical performance. Furthermore, combining DFT with machine learning (ML) enables both high-throughput screening and the efficient prediction of MXenes with desired electrochemical properties by revealing their underlying design principles.

While numerous reviews have been published on the progress and challenges of MXenes in battery applications, most have primarily focused on experimental findings [24, 33]. In contrast, this review presents a comprehensive overview from a computational perspective, systematically summarizing recent advances in MXenes as electrode materials, and further providing actionable design guidance through the establishment of structure-property-performance relationships derived from DFT and ML studies. The discussion emphasizes the rational structure design, detailed analysis of structural characteristics, and exploration of energy storage mechanisms and performance. Specifically, this review outlines modification strategies to enhance MXene electrodes, examines their applications in LIBs, Li−S, Li−O₂, and non-lithium-ion batteries, and highlights the role of ML in guiding MXene research. Finally, future prospects and challenges for MXenes as electrode materials are summarized to inspire further exploration of high-performance electrochemical energy-storage systems.

2 Strategies to enhance the performance of MXene-based electrode materials

Over the past decade, MXenes have garnered significant research interest and experienced rapid development, demonstrating considerable potential in rechargeable battery applications. Nonetheless, the intrinsic limitations of pristine MXenes as electrode materials severely hinder their practical implementation in high-performance systems [34−36]. Analogous to other 2D materials, the high surface chemical reactivity of MXenes makes them prone to restacking, which poses a significant challenge for energy storage applications. Notably, the interlayer interactions in MXenes involve not only van der Waals forces but also hydrogen-bonding networks formed between neighboring –OH and –O/–F terminations. The synergistic effects of these interactions drive the restacking of MXene sheets, which markedly decreases the accessible surface area, impedes ion transport kinetics, and induces severe volume variations. In addition, MXenes are highly susceptible to oxidation under aqueous environments, potentially triggering structural collapse and performance degradation, while unfavorable surface terminations further restrict their capacity [37, 38]. Overall, developing effective modification strategies is of paramount importance for improving the electrochemical performance of MXene materials [39].

2.1 Surface functionalization engineering

As investigations into MXenes have deepened, it has been found that the surface functional groups of MXenes, prepared through distinct etching methods and post-treatment processes, differ significantly. These variations can markedly affect their hydrophilicity, electronic structure, and electrochemical behavior. To date, a variety of approaches have been employed to modify the surface of MXenes, resulting in a wide spectrum of terminations, including −OH, −NH, −F, −Cl, −Br, −O, −S, −Se, −Te, and −SO₃H [40, 41]. These differences in surface terminations directly modulate the charge storage characteristics of MXenes. Even when governed by the same intercalation mechanism, materials may exhibit different theoretical specific capacities. For example, MXenes synthesized via the conventional HF solution etching method typically contain abundant −F, −O, and −OH functional groups [42, 43]. Among them, the −F and −OH terminations are electrochemically inert, contributing little to the overall capacity while potentially impeding ion transport and suppressing redox reactions, thereby reducing electrochemical efficiency. In contrast, O functionalized surfaces can enhance the hydrophilicity of MXenes, facilitating more efficient ion transport and storage [44]. Typically, three possible termination configurations can be identified when theoretically investigating surface functional groups, as presented in Fig. 2(a) (taking the –F termination as an example) [45]. According to theoretical research summarized by Tang et al. [45] and Xie et al. [46], the specific capacities of bare Ti₃C₂ and MXenes terminated with −O, −F, and −OH groups are 320, 268, 130, and 67 mA·h·g⁻¹, respectively [Fig. 2(b)]. Moreover, the Li⁺ diffusion ability follows the order of Ti₃C₂ > Ti₃C₂O₂ > Ti₃C₂F₂ > Ti₃C₂(OH)₂. These results indicate that −O terminations deliver the highest double-layer lithium storage capacity among the three functionalized Ti₃C₂ variants, while −F and −OH groups suppress Li⁺ adsorption and hinder diffusion kinetics. Consequently, the experimentally observed capacity of Ti₃C₂ is often only about half of its theoretical value. This pronounced discrepancy can mainly be attributed to idealized theoretical models that assume a termination-free Ti₃C₂ structure, while in practice the formation of −F and −OH terminations during synthesis is unavoidable and severely limits the achievable capacity [47]. To mitigate the detrimental effects of unfavorable surface terminations (−OH and −F), inexpensive alkaline reagents such as LiOH, KOH, Mg(OH)₂, and NaOH are often employed for the post‑treatment of MXenes [48]. This strategy enables the replacement of −F terminations with −OH groups and charge‑compensating cations (e.g., −Li, −K, −Mg, and −Na). Subsequent annealing can further convert −OH groups into −O terminations, resulting in MXene surfaces predominantly decorated with −O and cationic species. This surface reconstruction not only enhances the mechanical robustness of MXenes but also significantly improves their electrochemical performance, in good agreement with theoretical predictions.

In addition, the electrical conductivity of MXenes is closely associated with their surface terminations. The transfer of electrons from the transition metal layers to the negatively charged terminations induces charge redistribution and alters transport characteristics, thereby regulating the overall conductivity of MXene [49]. Moreover, the incorporation of surface functional groups causes a downward shift of the Fermi level, which consequently diminishes the electrical conductivity compared with pristine MXenes [50, 51]. Li et al. [52] investigated the electronic properties of chalcogenized-Ti₃C₂ (Ti₃C₂T₂, T = O, S, Se, and Te) MXenes. Notably, the calculated partial density of states (PDOS), as illustrated in Fig. 2(c), reveals that chalcogenation disrupts the intrinsic magnetism of pristine Ti₃C₂, resulting in a non-magnetic ground state. However, this modification does not alter the metallic nature of Ti₃C₂, as evidenced by the pronounced electronic states located near Fermi level. Such metallic behavior enables a high density of charge carriers, endowing these materials with promising potential as electrode candidates for ion batteries.

2.2 Heteroatom doping engineering

Inspired by the diverse doping strategies employed in graphene, heteroatom doping has been proven to be an effective approach for optimizing the properties of pristine MXenes. By introducing local lattice distortions and modulating the atomic coordination environment, this strategy enables structural tailoring at the atomic scale, thereby refining the electronic structure and enhancing charge transport characteristics. As a result, synergistic improvements in electrical conductivity, ion transport kinetics, and overall electrochemical performance can be achieved. Heteroatom doping, encompassing both nonmetallic and metallic dopants, typically occurs at three sites within MXenes: the M, X, and T sites. Owing to the compositional diversity and stoichiometric flexibility inherent in MXene structures, such doping modifications offer precise control over their physicochemical properties.

2.2.1 Metallic element doping

Metal element doping in MXenes generally refers to the substitution of transition metals (e.g., Mo, V, Cr) at the M sites. Since MXenes inherently contain transition metals, introducing additional transition metal dopants is considered one of the most effective strategies to enhance their properties. It is widely recognized that the unique properties of MXenes primarily originate from the partially occupied d orbitals of transition metals. These d orbitals not only govern the electronic structure and the density of states near Fermi level, but also play a crucial role in determining the electrical conductivity and transport characteristics of the material [53−55]. For instance, computational studies by Khazaei et al. [56] reveal that, within the same group of transition metals (Ti, Zr, and Hf), the band gap of the corresponding MXenes increases with the atomic number, exhibiting the trend Hf₂CO₂ > Zr₂CO₂ > Ti₂CO₂ [Fig. 3(a)]. In 2019, Kuznetsov et al. [57] introduced Co into Mo₂CT_x [Fig. 3(b)], with extended X‑ray absorption fine structure (EXAFS) analysis confirming that the Co atoms occupy Mo lattice positions. This arrangement provides isolated Co centers without any detectable formation of other cobalt‑containing phases, thereby successfully extending MXenes toward mid-to-late transition metal systems and further expanding the compositional space of the MXene family. Recently, Li et al. [58] conducted a systematic investigation into the effect of transition-metal doping Ti₂CO₂ (TM-Ti₂CO₂) on the lithium storage properties. The incorporation of eight distinct dopants (Sc, V, Cr, Mn, Fe, Co, Ni, and Cu) induces variations in the d-band center, which, as illustrated in Fig. 3(c), exhibit an approximately linear relationship with the Li adsorption energy (E_ads). In contrast to the pristine Ti₂CO₂, doping with Sc, Cr, and Mn atoms leads to an increase in the migration energy barrier, whereas the incorporation of V atoms effectively reduces the barrier to 0.11 eV [Fig. 3(d)]. Combined with a suitable open-circuit voltage (OCV, typically within 0–1.0 V for metal-ion batteries, where lower values are typically associated with higher output voltage and improved energy density), V-Ti₂CO₂ exhibits outstanding comprehensive performance, featuring an OCV of 0.66 V and a high theoretical capacity of 765.3 mA·h·g⁻¹. More recently, high-entropy MXenes (HE-MXenes) have emerged as a new class of 2D materials, attracting increasing attention owing to their multi-component tunability and the unique physicochemical properties derived from high-entropy effects. The first HE-MXene, (Ti_1/5V_1/5Zr_1/5Nb_1/5Ta_1/5)₂C, was introduced by Du et al. [59] in 2021. This HE-MXene is synthesized by selectively etching a novel HE-MAX phase, (Ti_1/5V_1/5Zr_1/5Nb_1/5Ta_1/5)₂AlC [Fig. 3(e)], in which five size-compatible and infusible transition metal elements render the MXene atomic layers highly stable. Moreover, the resultant HE-MXene atomic layers exhibit pronounced lattice distortions, inducing significant mechanical strain. Such strain effectively regulates the nucleation and uniform growth of dendrite-free lithium on HE-MXene, enabling stable cycling performance for up to 1200 h, as illustrated in Figs. 3(f) and (g). This breakthrough confirms the outstanding electrochemical properties of HE-MXene and provides valuable guidance for their rational design and optimization toward energy storage applications.

2.2.2 Non-metallic element doping

According to prior studies, heteroatoms such as N, O, S, or P can be doped at all three sites (M, X, T), whereas halogens are typically substituted at T sites [60]. Among the various non-metallic dopants, nitrogen is one of the most extensively employed elements due to its pronounced effect on enhancing material performance. In the case of M₃C₂ MXenes, substitution of 50% of the carbon sites with nitrogen atoms results in the formation of hetero-N-MXenes (transition metal carbonitrides, M₃CN). Owing to the higher electronegativity of nitrogen, the resulting M−N bonds are stronger than M−C bonds, endowing the material with superior physical and electronic properties compared with pristine M₃C₂ MXenes. Zhang et al. [61] systematically investigated the electrochemical behavior of Ti₃CNO₂, Ti₃C₂O₂, and Ti₃N₂O₂ [Fig. 4(a)] as anode materials for sodium-ion batteries (SIBs). It was found that the O−Ti−N interaction is stronger than the O−Ti−C interaction, resulting in tighter stacking in Ti₃CNO₂ and Ti₃N₂O₂, along with a larger interlayer spacing compared with Ti₃C₂O₂, which is favorable for the sodiation/desodiation process. As illustrated in Fig. 4(b), relatively low diffusion energy barriers were observed for Ti₃CNO₂ and Ti₃N₂O₂, with values of 0.123 and 0.04 eV, respectively, corroborated by the experimentally measured rate performance. Notably, Fig. 4(c) shows that Ti₃CNT_x exhibits superior cycling stability compared with Ti₃C₂T_x after 200 cycles, underscoring its promise as a high-performance anode material for sodium-ion batteries. Furthermore, Lu et al. [62] conducted a comprehensive investigation into the potential mechanisms of N doping, focusing particularly on the positions of the dopants and their influence on the electronic properties of MXenes. Through comparison of the formation energies of all possible N arrangements in Ti₃C₂T_x (T = O, F, and OH), three energetically most favorable doping configurations were identified: lattice substitution of C atoms (LS), substitution of –OH functional groups (FS), and surface adsorption at –O terminations (SA) [Fig. 4(d)]. These configurations were respectively realized using the following synthesis strategies [Fig. 4(e)]: (i) in situ etching of the MAX phase Ti₃AlCN [63], (ii) hydrothermal treatment of Ti₃C₂ MXene with urea [60], and (iii) cold plasma treatment of Ti₃C₂ MXene under pure N₂ [64]. Each doping mechanism exerts a distinct influence on the structural stability and electrochemical properties of MXenes. Motivated by the promising features of N-doping reported earlier, Ahmed et al. [65] further explored the regulatory effect of nitrogen concentration on Ti₂C. The calculation results revealed that with increasing nitrogen concentration, a significant enhancement in the density of states near Fermi level is observed. This enhancement signifies an increase in carrier concentration and electron mobility, which consequently improves electrical conductivity and modifies electronic transport behavior.

2.3 Defect engineering

In 2D materials, intrinsic defects are almost inevitable during experimental synthesis. Generally, intrinsic defects in MXenes can be categorized into vacancy defects and active edge defects [66]. Vacancy defects originate from the absence of certain atoms in the MXene lattice, primarily including vacancies at the X site and M site vacancies, while active edge defects are associated with the boundaries or pore structures of the nanosheets. The presence of these defects alters the surrounding atomic arrangement and coordination environment, endowing the materials with novel physical properties and functionalities. Carbon vacancies, commonly observed in MAX phases, are frequently inherited by the corresponding MXene derivatives. Hu et al. [67] employed first-principles calculations to explore the effects of introducing carbon vacancies on the structure and physicochemical properties of Ti₂CT₂, with the model of carbon-vacant (labeled as V_C) Ti₂CO₂ presented in Fig. 5(a). In contrast to other prototypical 2D materials such as graphene and MoS₂, Ti₂CT₂ exhibits a relatively low vacancy formation energy, implying a tendency for defect generation [Fig. 5(b)]. Subsequent lattice dynamics analysis combined with ab initio molecular dynamics (AIMD) simulations confirmed that V_C-Ti₂CO₂ remains both thermodynamically and dynamically stable. More importantly, the introduction of carbon vacancies enhances the electronic conductivity and flexibility of Ti₂CT₂, which benefits its performance in energy storage devices. Wu et al. [68] systematically investigated the formation of single M vacancies (defined as V_M) in M₂C MXenes (M = Sc, Ti, V, Zr, Nb, Mo, Hf, Ta, and W) and their effects on Li adsorption and diffusion. Their results indicate that, among all the examined M₂C systems, Mo₂C is the most favorable for V_M formation, exhibiting the lowest vacancy formation energy of 0.96 eV [Fig. 5(c)]. Although the relatively high diffusion barriers of 0.93−0.94 eV hinder Li escape from V_M sites and may negatively affect Li deintercalation, Li diffusion preferentially occurs at the flat surface, with much lower isotropic barriers of 0.096−0.115 eV. In contrast, migration into vacancy sites requires overcoming significantly higher barriers of 0.258−0.321 eV [Fig. 5(d)]. Overall, the vacancy-induced potential trap has a limited effect on Li diffusion and thus exerts minimal influence on the rate capability of LIBs. Moreover, the presence of V_M reduces the effective molecular weight of the host lattice, resulting in an enhanced theoretical capacity of 542 mA·h·g⁻¹ compared with 526 mA·h·g⁻¹ for pristine Mo₂C.

Although defects can increase the surface activity of MXenes, they simultaneously make the materials more prone to oxidation and corrosion in electrolytes. In particular, excessive defects may compromise their oxidation stability, which necessitates effective suppression and mitigation strategies. By tuning the concentration of the etchant (HF produced from the reaction of LiF and HCl) during synthesis, Li et al. [69] successfully achieved controllable preparation of Ti vacancies in MXenes [Fig. 5(e)]. An increase in the Ti vacancy count was observed with higher HF concentrations, which aligns well with the statistical analysis [Fig. 5(f)]. Furthermore, Ibragimova et al. [70] systematically investigated the effects of mixed surface terminations and chemical environments (pH and electrode potential) on defect formation. The results indicated that Ti vacancies are more stable on bare or OH-terminated surfaces, while C and N vacancies preferentially form on O-terminated surfaces. The computational scheme employs chemical potentials derived from Pourbaix diagrams [Fig. 5(g)] to construct formation energy maps [Fig. 5(h)] that are explicitly dependent on electrode potential and pH. These findings enable the prediction of conditions that favor extensive vacancy formation, offering valuable guidance for future experimental efforts: not only in suppressing excessive defect generation but also in forecasting synthesis parameters for achieving desired defect concentrations.

2.4 Interlayer intercalation engineering

In layered MXenes, the interlayer spacing exerts a significant influence on their electrochemical performance. By introducing appropriate intercalation species, such as organic molecules and ions, into multilayer MXenes, targeted modulation of their properties can be achieved to meet diverse application demands. These intercalants not only establish a supporting framework that prevents restacking and enlarges the interlayer spacing, thereby improving cycling stability, but also weaken the interlayer interactions, which promotes faster ion diffusion kinetics and ultimately enhances electrochemical performance.

Metal cation intercalation is a vital strategy for regulating the interlayer environment of MXenes at the atomic scale. Among various intercalants, metal cations possess unique advantages due to their charge and inherently small size, enabling precise tuning of the interlayer spacing. The pioneering study by Lukatskaya et al. [71] revealed that multiple cations, including Li⁺, Na⁺, Mg²⁺, K⁺, NH₄⁺, and Al³⁺, can spontaneously intercalate into Ti₃C₂, highlighting new opportunities for developing improved intercalation electrodes in rechargeable batteries. As illustrated in Fig. 6(a), Li et al. [72] provided a comprehensive summary of the key factors governing ion intercalation in MXenes. Specifically, a smaller effective ionic size and higher valence state facilitate greater ion accommodation within the interlayer space. Additionally, the type of MXene is closely related to ion intercalation. Variations in chemical composition lead to different electronic distributions, interlayer interaction strengths, and lattice parameters, all of which ultimately affect the E_ads and diffusion barrier of the intercalated ions. Beyond these intrinsic factors, the solvation/desolvation effects of ions render the choice of solvent a critical factor in determining ion transport and intercalation kinetics. In a study by Subramanyan et al. [73], Na⁺ intercalation in V₄C₃T_x MXene was found to proceed more efficiently in ester-based electrolytes, where the diffusion kinetics are faster compared with those in the tetraethylene glycol dimethyl ether/tetraglyme (TEGDME)-based electrolytes. Recently, Yin et al. [74] conducted a systematic study on the intercalation of diverse metal cations (including Li⁺, Na⁺, K⁺, Mn²⁺, Zn²⁺, Mg²⁺, and Al³⁺) into MXenes and their influence on electrochemical performance. As shown in Figs. 6(b) and (c), cations in Ti₃C₂T_x can occupy Top, Hollow, or Mid sites, with the preferred site varying across different ion species according to formation energy calculations. Notably, Mn²⁺-intercalated Ti₃C₂T_x exhibits superior cycling stability and overall electrochemical performance compared with the pristine structure.

Although metal cation intercalation is a conventional strategy for tuning the interlayer spacing of MXenes, the limited expansion achievable due to the relatively small ionic radius often fails to meet the demands of advanced applications. To overcome this limitation, researchers have been exploring alternative intercalants capable of providing larger interlayer distances. Notably, incorporating nonmetallic ions and organic molecules into MXene interlayers has been recognized as an effective means to enlarge the interlayer spacing and broaden the possibilities for structural engineering. As reported by Ji et al. [25], the interlayer spacing of Ti₃C₂T_x increased from 1.03 to 1.45 nm, and that of Mo₂TiC₂T_x from 1.26 to 2.14 nm [Fig. 6(d)], through the intercalation of n-hexylamine/N-methylformamide (HA/NMF) molecules. A quantitative correlation exists between the intercalated molecules and the resulting interlayer spacing. Arole et al. [75] tuned the interlayer spacing of multilayer Ti₃C₂T_z (ML-Ti₃C₂T_z) by introducing intercalants of various sizes, including lithium chloride (LiCl), sodium chloride (NaCl), urea (CH₄N₂O), dimethyl sulfoxide (DMSO), and tetrabutylammonium hydroxide (TBAOH). X-ray diffraction (XRD) patterns of intercalated ML-Ti₃C₂T_z were systematically collected to investigate the structural evolution induced by intercalation [Fig. 6(e)]. The XRD analysis reveals that intercalation with different species leads to a leftward shift of the (002) peak of ML- Ti₃C₂T_z, indicating an increase in the interlayer spacing. Owing to its relatively large molecular size, TBAOH causes the (002) peak of ML-Ti₃C₂T_z to shift from 7.2° to 5.1°. As depicted in Fig. 6(f), the linear fit (R² = 0.9028) indicates a strong positive correlation between intercalant size and interlayer spacing. This implies that the interlayer spacing in ML-Ti₃C₂T_z can be accurately predicted based on the size of the intercalants, thereby enabling the rational design of MXene properties for diverse applications. To gain deeper insights into the role of guest molecules in intercalation and chemical modification, Wei et al. [76] investigated the interaction mechanism between [Emim]⁺ and bilayer MXene. Based on the appropriate size and electronic potential maps of [Emim]⁺ [Fig. 6(g)], it was found to intercalate into Ti₂CO₂ bilayers and exhibit a stable adsorption configuration, maintaining an interlayer distance of 8.1 Å. Furthermore, quantitative Bader charge analysis revealed that [Emim]⁺ acquires 0.054 e⁻ from the Ti₂CO₂ bilayer. This mild charge transfer suggests that the insertion of [Emim]⁺ does not drastically distort the MXene bilayer, allowing it to maintain a stable bilayer distance. Meanwhile, diffusion kinetics studies reveal that the diffusion barriers of various metal ions within the pillared structures formed by guest molecule intercalation are significantly lower than those in the pristine Ti₂CO₂ bilayer. This improvement is primarily due to the guest molecules effectively supporting the interlayer spacing and weakening the binding energy of MI. Consequently, the facilitated ion migration leads to enhanced diffusion kinetics and improved rate capability of the battery.

2.5 Heterogeneous structure engineering

The emerging class of van der Waals (vdW) heterostructures, composed of diverse 2D materials, is distinguished by its ability to integrate individual advantages and generate synergistic effects. The rational design of MXene-based heterostructures is particularly promising, as it can significantly enhance ion and electron transport within electrodes while maintaining structural stability [35]. Consequently, such architectures offer novel properties and unexpected opportunities for the design and optimization of advanced electrode materials. At present, a wide range of 2D materials, including graphene [77, 78], transition metal dichalcogenides (TMDs) [79, 80], and transition metal oxides (TMOs) [81], have been employed to construct MXene-based heterostructures. These structures have demonstrated outstanding electrochemical performance as electrode materials.

Figure 7(a) presents the schematic structure of the MXene/graphene heterostructure, where Aierken et al. [82] employed theoretical calculations to reveal that the M₂CT₂ (M = Sc, Ti, V; T = OH, O)/graphene heterostructures undergo only slight structural modifications. Moreover, the negative binding energies indicate that these heterostructures are more stable than their separated phases. In particular, Ti₂CO₂/graphene offers a compromise between capacity and kinetics, as it exhibits the lowest diffusion barrier among the considered systems, even lower than that of graphene [Fig. 7(b)]. In the work of Wang et al. [83], ultrathin 2D graphdiyne oxide (GDYO) was innovatively intercalated into Ti₃C₂T_x to construct a sandwich-like heterostructure, leading to a significant expansion of the interlayer spacing. As illustrated in Fig. 7(c), the difference in work functions reveals that electrons were transferred across the interface toward GDYO under the driving force of the potential difference. Figure 7(d) reveals that approximately 0.36 e⁻ is transferred from Ti₃C₂T_x to GDYO, which verifies the occurrence of electronic coupling in this heterostructure. Furthermore, owing to interlayer expansion and structural evolution during the lithiation process, slight redshifts of the D and G bands are observed for Ti₃C₂T_x/GDYO, accompanied by a decrease in the intensities of the G band and alkyne-related peaks [Fig. 7(e)]. During the subsequent charging process, the peak intensities recover to their initial states, demonstrating the excellent electrochemical reversibility and structural stability of Ti₃C₂T_x/GDYO. This unique architecture endows the material with enhanced electronic conductivity and stronger adsorption, while simultaneously accelerating reaction kinetics during cycling. In a recent study, Ji et al. [84] constructed a van der Waals heterostructure composed of VO₂/V₂CO₂. Among the six possible configurations considered, the most stable one is shown in Fig. 7(f). This structure features the shortest interlayer spacing, which strengthens the van der Waals interactions and consequently enhances the structural stability, as indicated by the most negative formation energy. In addition, AIMD simulations performed at 300 K revealed only minor structural fluctuations, while the overall integrity of the heterostructure was preserved. This outstanding thermal stability under room-temperature conditions is of particular significance, as it underpins the long-term operational stability required for practical battery applications. Electronic characteristic analyses confirm the superior metallic nature of the heterostructure, suggesting high carrier density and favorable electrical conductivity. The migration kinetics results [Fig. 7(g)] reveal that the diffusion barriers of Li, Na, and Mg ions within the heterostructure interlayer are relatively low, which contributes to its superior rate capability. As presented in Fig. 7(h), the VO₂/V₂CO₂ heterostructure delivers impressive theoretical capacities, with the Mg-ion system achieving as high as 936 mA·h·g⁻¹, surpassing other 2D materials and underscoring its strong potential as an advanced anode candidate.

3 Applications to electrode materials of rechargeable battery

From portable electronic devices to electric vehicles, rechargeable batteries have become a cornerstone of modern green living and technological progress. However, the limitations of traditional electrode materials in terms of energy density, cycle life, environmental impact, and cost of use are becoming increasingly prominent [85]. Consequently, designing and developing next-generation high-performance electrode materials is an urgent yet challenging task. Benefiting from the distinctive electronic structures and superior physicochemical characteristics, MXene materials have been recognized as highly promising candidates for rechargeable battery electrodes. Additionally, as discussed in the previous sections, various effective modification strategies have further expanded their applicability and significantly enhanced their overall performance. In the following sections, we will discuss the applications of MXene-based materials in different types of rechargeable batteries.

3.1 Lithium-ion batteries (LIBs)

LIBs, as high-performance secondary batteries, currently dominate the commercial market due to their technological maturity. With the continuous upgrading of industrial products, increasing demands have been placed on the safety, energy density, and fast charge-discharge capability of LIBs. An ideal LIB is expected to possess high storage capacity, outstanding cycling stability, and reliable rate performance. Among the numerous influencing factors, the electrochemical behavior is most strongly dictated by the properties of the electrode materials. Despite being the most widely used anode material in LIBs, graphite is constrained by its low theoretical capacity (372 mA·h·g⁻¹) [86] and lithiation potential (<0.2 V) [87], which make it susceptible to lithium dendrite formation and consequently raises safety concerns during cycling. Since the discovery of MXenes, extensive theoretical studies have been devoted to exploring their feasibility as anode materials for LIBs.

The investigation of Li storage performance in Ti₃C₂T_x through theoretical calculations was initiated by Naguib et al. [8], who employed DFT to demonstrate that Ti₃C₂ MXene possesses excellent electronic conductivity and offers a theoretical Li storage capacity of 320 mA·h·g⁻¹, which is close to the value of graphite (372 mA·h·g⁻¹). Building on this foundation, the study by Tang et al. [45] revealed that bare Ti₃C₂ exhibits a remarkably low diffusion barrier of 0.07 eV, significantly lower than that of graphite (0.3 eV), highlighting its potential as an anode material for LIBs. The specific capacity of MXenes is also affected by factors such as the molar mass of their constituent elements and the thickness of the atomic layers. Eames et al. [88] further investigated a broader range of transition-metal-based MXenes as anode materials for LIBs, including Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta. It was found that MXenes containing lighter 3d elements (Sc, Ti, V, Cr) possess larger theoretical capacities compared with those incorporating heavier 4d/5d metals (Zr, Nb, Mo, Hf, Ta), reflecting the inverse relationship with their molar mass. Motivated by the remarkable Li-storage performance of lighter 3d metals (Sc, Ti, V, Cr), Zhao et al. [89] explored a variety of MXene architectures (M₂C, MC₂, M₂N, MN₂) through theoretical screening and prediction, as illustrated in Fig. 8(a). Their findings culminated in the establishment of a universal functional relationship, which offers a broadly applicable principle to guide the development of high-capacity electrode materials. Following systematic screening involving stability, electronic properties, diffusion barriers, OCV calculations, and theoretical capacity evaluations, Ti₂N (975 mA·h·g⁻¹) and V₂N (924 mA·h·g⁻¹) exhibited storage capacities far exceeding those of graphite, confirming their significant potential as anodes for LIBs. Importantly, by combining Figs. 8(b) and (c), a relationship between the d-band center and concentration can be established. According to this correlation, calculating the d-band center alone is sufficient to determine the E_ads of a single Li ion for unknown MXenes, thus enabling predictions of their theoretical capacity. This provides a powerful descriptor to accelerate the discovery of MXene anodes for LIBs.

Meanwhile, various modified MXene materials have been extensively investigated as LIBs electrodes, exhibiting excellent electrochemical performance. However, pristine MXenes are intrinsically unstable and tend to be terminated by functional groups. Among these, O- and S-functionalized MXenes have shown particular promise as electrode materials [90]. Considering the experimentally observed surface inhomogeneity of MXenes, Wei et al. [91] systematically examined the interactions and cooperative effects between O and S mixed terminations on V₂C. Their findings indicate that V₂CT_x maintains good metallic conductivity across different mixing ratios, primarily due to the d orbitals of V atoms. In particular, V₂CO_2/3S_4/3 delivers a low OCV of 0.29 V along with a theoretical capacity of 729 mA·h·g⁻¹, as summarized in the voltage-capacity plot [Fig. 8(d)], underscoring the extraordinary potential of functional groups in tailoring the properties of MXenes. Compared with single-transition-metal MXenes, the emergence of double-transition-metal MXenes (DTMs) alleviates the limitations imposed by the higher molar mass of early transition metals, thereby exhibiting higher theoretical capacity and greater potential for applications in electrochemical energy storage. Zhou et al. [92] reported WCrC and MoWC DTMs by substituting Mo/Cr for W, yielding materials with excellent electronic conductivity [Fig. 8(e)]. Their study demonstrated that the synergistic effect of dual transition metals, combined with relatively low molar mass, not only provided higher Li storage capacities (648.81 mA·h·g⁻¹ for WCrC and 551.82 mA·h·g⁻¹ for MoWC) but also resulted in remarkably low diffusion barriers (0.045 eV for WCrC and 0.046 eV for MoWC), as depicted in Fig. 8(f). Additionally, the discovery that highly electronegative metals in DTMs weaken the metal–carbon bonds, thus potentially enhancing Li adsorption, provides a means to tune the metal-ion adsorption behavior of MXenes and promotes the advancement in MXene surface engineering. Black phosphorus (BP), as indicated by theoretical studies, possesses characteristics beyond the reach of graphene, TMDs, and other 2D materials [93]. Building on this, Saharan et al. [94] designed a stable BP/MXene heterostructure that combines the unique strengths of the two materials while partially mitigating self-restacking issues. It is evident in Fig. 8(g) that BP/Ti₃C₂ and BP/Ti₃C₂N₂ can accommodate up to seven and six complete layers of Li, respectively, which surpasses the theoretical capacity of their individual components as LIBs cathodes. In terms of migration capability, BP/Ti₃C₂ and BP/Ti₃C₂N₂ demonstrate very low diffusion barriers (0.020 eV and 0.2 eV, respectively) for Li ions along the considered pathways [Fig. 8(h)], which is highly beneficial for achieving fast ion transport and excellent rate performance.

3.2 Lithium−sulfur (Li−S) batteries

As potential contenders for next-generation energy storage systems, Li−S batteries have been extensively investigated, primarily owing to their impressive energy density of 2600 Wh·kg⁻¹, high theoretical capacity of up to 1675 mA·h·g⁻¹, and the abundance of sulfur resources [95, 96]. However, the advancement of Li−S batteries has been significantly hampered by several critical challenges, including the low electrical conductivity of sulfur cathodes, the shuttle effect of lithium polysulfides (LiPSs), and sluggish redox kinetics. To overcome these limitations, extensive research has focused on designing advanced cathode materials with both superior electrical conductivity and strong capability to immobilize LiPSs. This dual characteristic is crucial for promoting rapid charge transfer while alleviating the shuttle effect by restricting the dissolution and diffusion of LiPSs. Benefiting from the synergy between high intrinsic conductivity and abundant surface activity, MXenes demonstrate outstanding potential as sulfur hosts in Li−S batteries.

MXenes possess abundant surface functional groups that play a crucial role in anchoring LiPSs and suppressing the shuttle effect. In 2017, Rao et al. [97] systematically investigated the performance of bare Ti₂C and Ti₂C functionalized with F, O, and OH in Li−S battery systems. As shown in the optimized structure of Fig. 9(a), the S atoms in LiPSs are dispersed on the bare Ti₂C surface due to the strong interactions between S and Ti atoms, indicating that bare Ti₂C cannot serve as an ideal anchoring material. In contrast, surface functionalization mitigates the strong Ti–S interactions, enabling efficient trapping of LiPSs without decomposition and underscoring its significance in enhancing the anchoring ability of MXenes. In addition to the common F, O, and OH terminations, Wang et al. [98] further explored the anchoring effects of Ti₃C₂ functionalized with N, O, F, S, and Cl on LiPSs, all of which retained the same metallic conductivity as bare Ti₃C₂ [Fig. 9(b)]. As illustrated in Fig. 9(c), the order of the adsorption capacity of Ti₃C₂T₂ for LiPSs is as follows: S > O > N > F > Cl. Among these, Ti₃C₂S₂ and Ti₃C₂O₂ stand out with significantly enhanced binding strengths, highlighting their potential as competitive sulfur cathode hosts. Compared with conventional single-metal MXenes, the incorporation of multiple metal sites in HE-MXenes triggers a cocktail effect, thereby endowing them with potentially distinctive catalytic properties. Xu et al. [99] designed a TiVNbMoC₃ HE-MXene with excellent structural compatibility as a platform for multi-active-center synergistic engineering in Li−S batteries. Notably, the significant lattice distortion within this configuration triggers a redistribution of surface charge density [Fig. 9(d)]. The study of electronic properties demonstrates that the DOS of TiVNbMoC₃ is located closer to Fermi level than that of Ti₃C₂ and TiNbC, leading to moderate E_ads and superior catalytic activity. In addition, PDOS analysis shows a pronounced shift of the transition-metal d-band centers toward Fermi level, exceeding those of the single-metal counterparts Ti₄C₃, V₄C₃, Nb₄C₃, and Mo₄C₃ MXenes. This observation suggests that the synergistic interaction among multiple active sites in HE-MXene contributes to the optimization of the d-band center [Figs. 9(e) and (f)]. Further studies demonstrated that HE-MXene reduces the decomposition barrier of Li₂S, which in turn facilitates faster LiPSs conversion kinetics during the charge-discharge process [Fig. 9(g)]. Moreover, constructing heterostructures as sulfur cathode hosts is considered an effective strategy, as it not only enhances interlayer structural stability but also strengthens the anchoring of polysulfides, ultimately improving the electrocatalytic performance in Li−S batteries. Ge et al. [100] reported the catalytic effects of four different MXene−graphene heterostructures on the conversion of S₈ to Li₂S during discharge process. The Gibbs free energy change (ΔG) profiles of the sulfur reduction reaction (SRR) are depicted in Fig. 9(h), where the step from Li₂S₄ to Li₂S₂ exhibits the largest positive ΔG barrier, identifying it as the rate-limiting step. More importantly, when compared with the 1.07 eV barrier reported for the rate-limiting step of the SRR on graphene [101], the calculated results demonstrate that these heterostructures exhibit superior catalytic activity toward SRR. Similarly, Li diffusion on the cathode host plays a crucial role in determining the charging dynamics of Li−S batteries. As exhibited in Fig. 9(i), the calculated diffusion energy barriers reveal that Li migration on the Ti₂CS₂-graphene heterostructure possesses a relatively low barrier, which is favorable for enhancing the charging rate. A comprehensive comparison between pristine graphene and MXene materials confirms the synergistic effect arising from their integration. Consequently, under the premise of effectively suppressing the shuttle effect, the Ti₂CS₂-graphene heterostructure demonstrates enhanced catalytic activity toward SRR and significantly improved charge-discharge kinetics.

3.3 Lithium−oxygen (Li−O₂) batteries

Rechargeable Li−O₂ batteries, with an ultrahigh theoretical energy density of 3500 W·h·kg⁻¹ [102], are regarded as strong candidates for next-generation energy storage systems. From an electrochemical perspective, their charge-discharge process primarily involves the reversible formation and decomposition of lithium peroxide (2Li + O₂ + 2e⁻ ⇌ Li₂O₂). However, several challenges hinder the commercialization of Li−O₂ batteries, including high overpotential, slow ORR/OER kinetics, as well as the insulating and insoluble nature of the discharge products (Li₂O₂). Consequently, the exploration of novel oxygen electrode catalysts is of critical importance for enhancing electrochemical properties, reducing the overpotential, and achieving more efficient reaction pathways.

Functionalization has emerged as an effective strategy to improve the catalytic activity of MXene materials. In a recent study, Wei et al. [103] reported a synergistic engineering approach that integrates the physical structure and surface chemistry of MXenes, thereby delivering exceptional bifunctional electrocatalytic activity. This design significantly accelerates ORR/OER kinetics in Li−O₂ batteries, showcasing great potential as a next-generation power source for electric vehicles. In this study, the conventional layered framework of MXenes was reconstructed into hollow spheres and entangled wires. Such a structural transformation substantially increased the surface area and exposed more interlayer sites, thereby facilitating rapid charge transfer during ORR/OER processes. As displayed in Figs. 10(a)−(c), calculation results confirmed that surface terminations (−O/−F/−OH) can markedly reduce the reaction barriers, thereby lowering the ORR/OER overpotentials, suppressing side reactions, and enhancing catalytic kinetics. In particular, O-functionalized Ti₃C₂O₂ exhibited the lowest overpotential compared to bare Ti₃C₂ [Figs. 10(b) and (c)], which enhanced the reversibility of the ORR/OER while ultimately enabled prolonged cycling stability. Zhao et al. [104] employed a CO₂-assisted strategy to embed Se single atoms (Se-SAs) into Ti vacancies on the surface of Ti₃C₂, breaking the structural symmetry of MXene nanosheets and markedly enhancing the catalytic activity of the electrode. The evident charge transfer in Fig. 10(d) highlights strong interactions between Se and Ti. Combined with a low adsorption energy (E_ads = –0.98 eV), these results indicate that Se-SAs act as catalytic centers, enhancing charge transfer and substantially strengthening the intrinsic LiO₂ adsorption ability, which is beneficial for accelerating the ORR in Li−O₂ batteries. In stark contrast to bare Ti₃C₂, SeSA-Ti₃C₂ exhibited ORR and OER overpotentials of 0.29 and 0.59 V, respectively, which are much lower than the overpotentials of 0.54 and 1.06 V for Ti₃C₂ [Fig. 10(e)]. This finding underscores the crucial role of SAs Se active sites in reducing the overpotentials associated with both the formation and decomposition of Li₂O₂. In heterogeneous catalysis, limited adsorption behaviors of reaction intermediates on single-metal MXenes often restrict charge transfer. The introduction of a secondary metal can effectively strengthen intermediate adsorption by modulating the electronic structure of the metal sites within the lattice. For example, Ren et al. [105] innovatively designed a Ce-doped Ti-vacancy-engineered Ti₃C₂T_x bifunctional catalyst [Fig. 10(f)]. Calculations of E_ads in Fig. 10g indicate that the superior adsorption of O₂ and Li₂O₂ on Ce-Ti₃C₂T_x arises from a dual-regulation mechanism of the Ce−C−Ti triatomic bridge active center involving d-band center modulation and orbital hybridization coupling. Furthermore, a notable reduction in overpotentials (0.09 V for ORR and 0.20 V for OER) was achieved in Ce-Ti₃C₂T_x, which verifies the unique superiority of rare-earth doping in tuning the electronic occupancy of transition-metal d orbitals and introduces a new paradigm for the rational design of 2D catalytic materials. Similarly, exceptional electrochemical performance was observed for the bimetallic TiVC MXene synthesized by Liu et al. [106]. Figure 10(h) reveals electrons transfer from V to Ti within the Ti−C−V bonds, resulting in an upward shift of the V d-band center. This change strengthens the adsorption of the LiO₂ intermediate and accelerates the oxygen electrode reaction.

3.4 Non-lithium-ion batteries

The growing demand for large-scale energy storage has intensified reliance on lithium resources, leading to challenges for LIBs, including limited supply, safety hazards, and environmental impact. Given that sodium and potassium belong to the same group as lithium and possess similar physicochemical properties and storage mechanisms, SIBs and potassium-ion batteries (PIBs) are considered promising alternatives [107, 108]. Beyond SIBs and PIBs, multivalent metal ions such as Ca²⁺, Mg²⁺, Al³⁺ and Zn²⁺ are capable of transferring two or three electrons per ion, thereby providing higher energy density. These metal ion batteries (MIBs), characterized by high crustal abundance, low cost, and enhanced safety, are therefore regarded as strong candidates to address the resource and safety constraints posed by LIBs [109–111].

MXenes (M_n+1X_n) exhibit a diverse range of stoichiometries, such as M₂X, M₃X₂, and M₄X₃, offering abundant structural possibilities for the advancement of energy storage devices. A systematic study conducted by Guo et al. [112] examined Ta_n+1C_n (n = 1, 2, and 3) [Fig. 11(a)] as anode candidates for alkali- and alkaline-earth- MIBs (Li, Na, K, Mg, and Ca). The calculated electronic structures demonstrate that Ta_n+1C_n exhibits no gap between the conduction and valence bands, endowing it with excellent metallic character. An extremely low diffusion barrier ensures that alkali-metal ions can readily migrate across the Ta_n+1C_n surface, enabling ultrafast charge-discharge and high performance. As summarized in Fig. 11(b), the theoretical storage capacity decreases with increasing n value and is negatively correlated with the ionic radius of the metal. These findings may provide theoretical guidance for designing novel MXene-based composite energy materials. Doping with other transition-metal elements of relatively low atomic mass, such as Sc, Ti, or Cr, can be regarded as an effective strategy to reduce energy barriers and enhance capacity for future applications and investigations. Bai et al. [113] analyzed Sc-doped (Mo_2/3Sc_1/3)₂C and (Mo_2/3Sc_1/3)₂CT₂ (T = −O, −OH, and −F) double-transition-metal MXenes as high-performance electrode materials for ion batteries [Fig. 11(c)]. According to thermodynamic adsorption and structural dynamics simulations, the strong chemical reactivity between –OH/–F terminations and adsorbates can trigger structural decomposition. For this reason, these terminations were excluded from subsequent investigations of other electrochemical properties for various ion batteries. In particular, (Mo_2/3Sc_1/3)₂C and (Mo_2/3Sc_1/3)₂CO₂ deliver superior metal-ion storage capacity, ideal OCV, and rapid ion-diffusion kinetics [Fig. 11(d)]. Janus materials, characterized by their asymmetric structures, have distinct chemical compositions on their two surfaces, thereby breaking the intrinsic symmetry and endowing the materials with novel properties. To advance research on Janus materials, Qin et al. [114] performed stability calculations, confirming the favorable thermodynamic and dynamic stability of Zr₂CSSe [Fig. 11(e)]. It is noteworthy that the asymmetric surface of the Janus Zr₂CSSe MXene generates a potential difference, creating an internal electric field that significantly improves ion transport and diffusion. As a substrate, Zr₂CSSe is capable of accommodating up to 24 Mg or Ca ions, corresponding to a theoretical capacity of 1052 mA·h·g⁻¹, which is considerably higher than that of traditional 2D materials. As shown in Fig. 11(f), low OCV values are retained even under the highest adsorption concentration, which guarantees the safety of batteries. By elucidating the role of structural asymmetry in modulating surface and interfacial properties, this study provides a framework for designing next-generation materials with tailored electrochemical functions. Currently, research interest has predominantly focused on carbide-related heterostructures, while comparatively fewer efforts have been directed toward nitride-based systems. To fill this gap, Ma et al. [115] proposed three optimal Ti₃N₂T₂ (T = F, O, OH)/VS₂ heterostructures with distinct terminations and systematically clarified the underlying mechanisms of charge transfer and ion diffusion. As seen in Fig. 11(g), the intrinsic metallicity ensures excellent electrical conductivity for all three heterostructures. To elucidate the interaction between the two monolayers within these heterostructures, the CDD and the planar-averaged CDD were investigated [Fig. 11(h)]. A redistribution of charge was observed in the interlayer region, confirming the existence of interaction and interlayer coupling between the monolayers. The electron localization function (ELF) map, presented in Fig. 11(i), revealed more distinct localized features in Ti₃N₂(OH)₂/VS₂, which implies higher charge density and stronger interactions. Taking into account structural integrity, strong adsorption capability, low diffusion barriers, high capacity, and appropriate OCV, Ti₃N₂O₂/VS₂ was proposed as a high-performance anode material for LIBs, SIB, and MIBs.

Herein, we have summarized the key DFT calculation contents and their physicochemical implications discussed in Sections 2 and 3, and systematically compiled them into Table 1 by categorizing based on the intrinsic properties of MXenes and their application characteristics as electrode materials for various rechargeable batteries.

4 Application of machine learning (ML) to MXenes

The tunable surface terminations, versatile X elements (C/N/O), and diverse transition metals collectively endow MXenes with vast theoretical compositional possibilities, giving rise to a large-scale candidate material library. However, traditional trial‑and‑error experimental approaches and first‑principles computational methods are time‑consuming, inefficient, and resource‑intensive. This makes it challenging to design and evaluate MXene performance for specific application scenarios, thereby constraining the progress of MXene research and even the broader field of materials science. Fortunately, driven by rapid advances in computer science, artificial intelligence (AI), and the Internet of Things (IoT), ML is increasingly recognized as a promising platform to support the development of high-performance MXene electrode materials [2, 116]. By combining efficiency with cost-effectiveness, ML techniques have markedly accelerated research progress in materials discovery, property analysis, performance prediction, and inverse design. Iterative improvements in model accuracy are injecting unprecedented momentum into the development of application-oriented MXene electrodes.

4.1 ML predicts the possibility of MXene material synthesis

Due to the substantial time and cost constraints, the synthesizability of all potential MAX phases cannot be practically evaluated using experimental techniques. Consequently, theoretical calculations are widely employed to efficiently screen unexplored MAX phases, circumventing labor-intensive experimental procedures while providing valuable guidance for subsequent experimental realization. Recent studies have indicated that the thermodynamic stability of compounds can be quantified by the relative formation energy (ΔH) [117, 118]. In the study by Huang et al. [119], a ML approach based on a small dataset was proposed to calculate ΔH for double transition metal MAX phases with the formula M'₂M''AC₂ [Fig. 12(a)]. First, a database containing 1320 candidates was constructed by exploring the chemical search space. Subsequently, descriptors derived from elemental information, structural energies, and lattice parameters were generated and selected based on their relative importance. By examining the synergistic effects of key features on ΔH, it was found that both the average electronegativity and the number of unfilled electronic orbitals play a decisive role in determining ΔH. In particular, M'₂M''AC₂ candidates with an average electronegativity below 2.24 and more than five unfilled electronic orbitals are more likely to be experimentally synthesized [Figs. 12(b) and (c)]. Additionally, the feasibility of synthesizing 2D M'₂M''C₂ was evaluated through a combination of static exfoliation energy, integrated crystal orbital Hamilton population (ICOHP), and phonon spectra calculations, leading to the identification of 75 theoretically accessible candidates. This workflow significantly expedites the screening of MXene properties while reducing the computational cost by more than an order of magnitude.

The stability of materials is considered as a fundamental parameter in nearly all studies, since it strongly affects the cycling performance of batteries. For MXenes, whose layered structures are inherently prone to collapse, structural stability assessments are indispensable before moving forward to experimental validation. He et al. [120] employed ML techniques and symbolic regression to evaluate the stability of MXene materials using four different models. The support vector machine algorithm exhibited the highest predictive accuracy in the stability analysis of MXene materials. In addition, symbolic regression has been demonstrated to automatically identify relevant descriptors that govern material stability without relying on prior human-labeled chemical knowledge. Beyond stability prediction, this method provides a rapid and efficient approach for materials classification and the design of novel descriptors, demonstrating broad applicability across diverse materials problems.

4.2 ML predicts the potential of MXenes as electrode materials for energy storage

In rechargeable batteries, the applicability of electrode materials is determined by several essential parameters, among which electrode voltage plays a decisive role. It exerts a direct impact on the energy density, safety, and cycling stability of the device. In the work of Joshi et al. [121], the electrode voltage of materials in a metal-ion battery is predicted using deep neural networks, support vector regression, and kernel ridge regression as ML algorithms, in combination with DFT data extracted from the Materials Project database [Fig. 13(a)]. As shown in Fig. 13(b), comparison between the voltage profiles generated by ML methods and those obtained from DFT calculations reveals that the ML models can reliably reproduce the DFT trends, thereby validating their effectiveness in exploring the voltage characteristics of electrode materials. Additionally, this study developed an online tool that enables voltage prediction for any metal-ion battery electrode material within minutes using minimal input information. Another critical performance criterion for electrode materials is the volume change during charging and discharging. In this regard, Moses et al. [122] proposed a ML model based on deep neural network regression, quantitatively predicting average voltage and associated volume changes. Model performance was assessed using the mean average absolute error derived from ten-fold cross-validation and an independent test set, confirming its strong predictive accuracy. To further evaluate the screening potential of the constructed model beyond the training database, the authors examined its robustness by applying it to the discovery of novel electrode materials for SIBs. This study identified 22 candidate materials characterized by high energy density and minimal volume change, which were found to exhibit promising electrode properties when benchmarked against DFT+U calculations, showcasing highly encouraging agreement. The correlation between MXene composition and electrochemical performance has also attracted considerable research attention. In this context, Li et al. [123] reported the first inverse design of MXene-based battery materials using multi-objective ML, with the workflow illustrated in Fig. 13(c). Within this framework, new categorical descriptors were adopted to classify MXene formulas while predicting multiple target electrochemical properties. Leveraging inverse design, the most suitable MXenes corresponding to specified gravimetric capacity, voltage, and induced charge were identified. Accordingly, the inverse model highlighted Li₂M₂C and Mg₂M₂C (M = Sc, Ti, Cr) as representative candidates meriting in-depth exploration within the specified performance windows.

To enhance the catalytic performance of Li−S battery cathodes, SA modification has emerged as an effective strategy by increasing surface-active sites and optimizing electronic structures. Sun et al. [124] screened 11 sulfur-functionalized MXenes (S-MXenes) and identified three materials (Ti₂CS₂, Nb₂CS₂, Ta₂CS₂) that stand out with superior stability and strong LiPSs adsorption, which help suppress the shuttle effect. Building on these three identified S-MXenes, further SA modification with ten transition metals yielded 73 stable SA-S-MXenes. To elucidate how the intrinsic adsorption features of LiPSs affect catalytic behavior, ML was applied to predict the E_ads of 1965 possible configurations. The Pearson correlation heatmap [Fig. 13(d)] reveals that the six selected descriptors exhibit low linear correlation coefficients, indicating their independence and suitability for ML modeling. As shown in Fig. 13(e), the trained model achieves high accuracy in predicting E_ads, with an R² value of 0.88. Feature importance analysis [Fig. 13(f)] indicates that the electronegativity of the SA exerts the greatest influence on E_ads. This study not only greatly accelerates the screening of efficient catalysts but also provides valuable guidance for refining LiPSs adsorption models and selecting high-performance catalysts for Li−S batteries.

5 Summary and outlook

From the perspective of theoretical calculations, this review systematically summarizes the progress and future directions of MXene-based electrode materials for rechargeable batteries. DFT calculations, as a powerful approach for understanding material properties and modification mechanisms, have been widely utilized to guide MXene structural design and optimize electrochemical performance. These calculations not only uncover the intrinsic mechanisms of modification but also provide theoretical support for performance enhancement. When further combined with ML, the construction of efficient descriptors and accurate prediction of structure-property relationships significantly accelerate the design and screening of novel MXene electrodes. This combination offers innovative strategies to overcome existing performance bottlenecks and facilitates broader applications in energy storage technologies.

Despite significant progress in the theoretical investigation of MXene-based electrode materials, several critical challenges remain unresolved. To date, more than 100 MXenes have been predicted through theoretical calculations, whereas only about 40 have been successfully synthesized experimentally [24], indicating a substantial gap between computational predictions and experimental realization. Future efforts should focus on developing more efficient and controllable synthesis methods while strengthening the integration of theory with experiment. This approach will enable the practical synthesis and application of a broader range of theoretically predicted MXenes. In addition, although current theoretical studies have significantly contributed to our understanding of structural stability, thermodynamic properties, and electrochemical behavior, they are predominantly based on idealized and simplified models that often fail to fully capture the complexity of real experimental conditions. For example, theoretical calculations often assume that MXenes are ideal, defect-free crystalline structures with homogeneously distributed surface terminations, typically focusing on static or near-equilibrium electrochemical reaction environments. These assumptions, however, deviate greatly from realistic experimental conditions. In practical systems, MXenes inevitably contain intrinsic defects, such as vacancies and interlayer distortions, while the types and distributions of surface functional groups are inherently heterogeneous. Moreover, battery operation involves highly complex dynamic processes, including solid−liquid interfacial evolution, interactions between electrolytes and MXene surfaces, and the continuous formation and migration of by-products such as polysulfides or dendrites during charge-discharge cycling [125–127]. Therefore, integrating experimentally relevant factors, including defect structures, electrolyte solvation effects, and dynamic electrochemical processes, into theoretical models, along with establishing quantitative links between computational predictions and experimental observations, remains a key challenge and an important research direction for the development of MXene-based rechargeable batteries. This will lead to more accurate predictions of MXene performance and provide effective guidance for their accelerated deployment in high-performance rechargeable batteries.

In summary, the deep integration of theoretical simulations, artificial intelligence, and advanced characterization techniques is projected to facilitate the rapid discovery and rational screening of novel MXene materials. This progress will be particularly driven under the joint impetus of high-throughput computation and ML. The advancement will not only foster the development of multifunctional electrode materials with high energy density and long cycle stability but will also establish a robust theoretical foundation and design paradigm for the development of next-generation, efficient, safe, and sustainable energy storage systems.

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