Adjustable MXene‐Based Materials in Metal‐Ion Batteries: Progress, Prospects, and Challenges

Rechargeable metal‐ion batteries (MIBs) beyond lithium‐ion batteries based on Na, K, Mg, and Al metal electrodes which are earth‐abundant and low‐cost have been developed for large‐scale energy storage systems. MXenes, a type of transition metal carbides, nitrides, and carbonitrides, are discovered as electrodes for MIBs owing to their distinctive properties of large‐scale ultrathin conductive 2D structures, adjustable surface functional groups, regulable interlayer spacing, and high specific surface area. Herein, the properties of MXenes are summarized and the recent progress on MXene‐based materials for diversified MIBs (Li, Na, K, Mg, and Al‐ion batteries) is introduced. The main focus is on the synthesis and applications of MXene‐based materials in MIBs, and their roles in electrochemical reactions. Further examples are provided to demonstrate the significant function of MXene‐based composite as active materials, substrates, collectors, and precursors in different MIBs, highlighting the enormous potential of MXene‐based materials to construct advanced electrodes. This review expects to offer a deep understanding of the relationship between the electrochemical performance and the MXene‐based electrodes, which will promote more novel and creative breakthroughs in the MXenes‐based electrodes for MIBs.


Introduction
[3] As a response, the goal of global carbon peak and carbon neutrality has been established, and more attention is focusing on green energy like wind, tidal, solar energy, etc. [4] Simultaneously, due to renewable energy's intermittency, electrochemical energy storage devices (EEDs) are urgently needed to efficiently store and utilize them. [5]Among the EEDs, lithium-ion batteries (LIBs) have been occupying the market since the 21st century as the power source for powering up consumable electronics, electric vehicles, etc.However, the future large-scale implementation of LIBs is impeded by the limited Li resource, energy density, cost, and safety concerns. [6]n this case, exploring alternative energy storage systems with high energy density and earth-abundant is particularly urgent. [7]Hence, rechargeable metal-ion batteries (MIBs) based on sodium, potassium, magnesium, and aluminum metal electrodes which are earth-abundant and low-cost have been developed. [8,9]In general, high-performance battery electrodes should have suitable electron/ion conduction and a stable structure to accommodate the volume fluctuation in the repeated charging/discharging cycles and excellent electrochemical performance. [10]16] Over the past years, great efforts have been made to research 2D materials as electrodes for MIBs owing to their high surface area, 2D structure, high flexibility, extraordinary mechanical strength, and outstanding electrochemical properties. [17,18][24] Benefiting from these advantages, plentiful novel functional MXenes and MXene-based materials emerge, providing new approaches for high-performing energy storage applications. [25]Figure 1 shows the number of published works based on MXene and energy storage drastically increasing year-by-year, confirming the importance of MXenes and MXene-based materials in scientific research and the development of energy storage applications. [26]So far, many MXene materials with 2D/3D structures, functional surface groups, enlarged interlayer spacing, doped atoms, or MXene-based composites have been well developed as electrode materials in MIBs, as shown in Figure 2.These data demonstrate that MXene plays an important role in MIBs.[29] Hence, the fundamentals of MXenes such as basic properties, working mechanisms, and kind of applications should be studied clearly.Therefore, a timely review of the latest progress and challenges in MXenes research is urgently needed to streamline work and point out the direction.
In this review, we focus on the fabrication and applications of MXenes, MXene-based, and MXene-derived materials in different MIBs, from classical LIBs to novel SIBs, PIBs, MgIBs, and AIBs, in particular concerning their roles in electrochemical reactions. [30]Initially, a brief introduction and overview of the properties of MXenes are presented.Then highlight the recent progress by using MXene-based materials for different MIBs.Finally, the challenges for the development of MXenes as advanced electrodes are elaborated.

MXenes Synthesis
MXenes is a 2D compound with a general formula of M nþ1 X n T x , in which M is a transition metal (Sc, Ti, V, Cr, Mo, Hf, Nb, Ta, W, etc.); X is one or both of the C and N element; T is the surface functional group such as -F, -O, -Cl, -OH, etc. [31] MXenes can generally be fabricated through the "top-down" and "bottomup" strategies.The "top-down" approach is based on the exfoliation of bulk MAX phases into few-layer or monolayer sheets by selective etching of group A elements.The etching process could be classified into fluorine-containing and fluoride-free approaches.The fluorine-containing etching process which is associated with HF or HF-forming or HF-containing solutions is the primary method for synthesizing of MXenes.For example, Gogotsi et al. [32] first fabricated Ti 3 C 2 T x MXene by etching Al from Ti 3 AlC 2 in a concentrated HF solution.Ghidiu et al. [33] used a solution containing LiF and HCl to etch Ti 3 AlC 2 for synthesizing Ti 3 C 2 T x , which shows a larger interlayer spacing of %40 Å.Although fluorine-containing etching processes have been widely applied in the production of MXenes and achieved many successes, requires handling of harmful HF hinders further application.Therefore, fluoride-free etching approaches have been developed to synthesize MXenes.Li et al. [34] investigated an HF-free of the alkali-assisted hydrothermal method to fabricate accordion-like Ti 3 C 2 T x .During the etching process, the Al layer reacted with OH À and formed soluble Al(OH) 4À , which gives a much safer procedure for MXenes preparation.MXenes fabricated through the "top-down" strategies whatever fluorine-containing and fluoride-free approaches are inevitably defective, and the morphology and layers are uncontrollable.Unlike the "top-down" approach, the "bottom-up" strategy represents a controllable path for obtaining a few layers of MXene epitaxial film.For example, Wang et al. [35] reported a direct synthetic route for the atom-economic synthesis of MXenes by chemical vapor deposition (CVD) at 950 °C, which shows outstanding electrochemical performance in LIBs.Furthermore, a series of efficient MXene etchants of Lewis acidic molten salts have been designed, such as ZnCl 2 , CdBr 2, and CuCl 2 . [36,37]In this strategy, metal ions act as etchants to etch the A-layer in the MAX phase, resulting in a series of MXenes materials with controllable morphology and surface functional groups.For example, Yang et al. [38] developed a molten salt method to prepare Zn atoms doped MXenes using ZnCl 2 at 550 °C, which showed unique properties to induce lithium deposition.Although various synthesis strategies for MXenes with controllable morphology and properties have been developed, the yield of MXenes from these preparation methods is mostly produced in very small quantities, which is not conducive to the large-scale application of MXenes materials.Therefore, further exploration is needed for new synthesis strategies to expand the preparation scale of MXenes.

Properties of MXenes
As a rising star, MXenes have many characteristics like high conductivity, larger surface area, hydrophilic surface, functionalize ability, and outstanding electrochemical properties, enabling them highly ideal electrode materials for energy storage systems. [39,40]The properties of electrodes are significant to their electrochemical performance, therefore, fully studying the basic properties of MXenes is of great significance for enhancing their electrochemical performance. [31,41,42]In this section, the basic properties of MXenes such as structural stability, surface groups, interlayer spacing, and electronic properties are summarized, and it will be helpful to fully understand MXenes and the preparation of MXene-based composite in the future.

Structure Properties
The atomic structures should be well explored due to the structural stability of MXenes being highly related to their atomic structures.MXenes have a hexagonal close-packed (hcp) crystal structure which is similar to their corresponding MAX precursors. [32,43]Importantly, the presence of surface groups such as -F, -O, -Cl, -OH, etc. plays a crucial role in adjusting the properties of MXenes. [44]For example, the capacity of MXenes with -O surface groups is much higher than that with -F/-OH in MIBs when serving as electrodes, because the -O surface groups could provide additional active sites to store Li þ , Na þ , K þ , Mg 2þ , Al 3þ , etc. Therefore, the electrochemical performance of MXenes could be enhanced by adjusting the type of surface groups.Notably, the kinds of surface groups are highly related to the fabrication procedure and the type of etchants.For instance, the fluorine-containing methods usually produce the -F, -O, -OH, and the ZnCl 2 etchants always bring about the -Cl surface groups. [45]The MXenes surface functional groups can also be regulated by posttreatment such as displacement reaction, calcination, and reaction with alkali metal hydroxide.Therefore, it is important to choose a suitable synthesis procedure or etchants to prepare MXenes for specific energy storage applications. [46]xcept for the surface groups, the interlayer spacing of MXenes is another important parameter for highly efficient energy storage.In general, the interlayer spacing is highly related to the n value in M nþ1 X n T x , which expands with the increase of the n.For example, the interlayer spacing of Ti 3 C 2 T x and Nb 4 C 3 T x is 20.51 and 30.47 Å, respectively, showing larger interlayer spacing than that of Ti 2 CT x (15.04 Å) and Nb 2 CT x (22.34 Å).Importantly, the interlayer spacing could be expanded by adopting different intercalant varieties, surface group regulation, heteroatom doping, and building 3D structures.The enlarged interlayer spacing could improve the structural adjustability, expose more active sites, broaden the ion transfer channels, and increase the capacity of MXene materials, which are significant for improving the electrochemical performance of MXenebased electrodes. [47]

Electronic Properties
Both theoretical studies and experimental analysis confirm that the electronic properties severely affect the electrochemical performance of MXenes electrodes in MIBs.The electronic properties of MXenes are highly related to their component, structure, surface groups, and interlayer chemistry. [48]DFT calculations prove that most pure MXenes have metallic band structures that show a high electron density near the Fermi level.While the surface groups on MXenes can change the DOS and Fermi levels, making MXenes into semiconductors.The published literature has shown that surface functional groups seriously affect the electronic structure of MXenes, due to the transfer of electrons from transition metals to electronegative surface functional groups.For instance, -OH and -F are expected to have a similar impact on the electronic structure of MXenes, as they can only receive one electron from the MXenes' surface.In contrast, -O may have a different effect as it needs two electrons from the surface of MXenes. [31]Therefore, the introduction of surface groups may enhance the structural stability and also change the electronic properties of MXenes.For example, Ti 3 C 2 (OH) 2 , Ti 2 CO 2 , Zr 2 CO 2 , and Hf 2 CO 2 , etc., exhibit semiconducting properties. [49]n addition, the electronic properties of MXenes could be adjusted by regulating the strain and external electric fields. [50]or instance, Xiao's group proved that the bandgap of monolayer Ti 2 CO 2 could turn indirect into direct ones in a certain degree of biaxial strain and uniaxial. [51]The "M" in MXenes also affects the electronic properties.For example, due to the metal electronegativity weakening with the atomic number increasing, the bandgap of MXenes increases with the atomic number increasing. [52]To sum up, both the computational and experimental analysis prove that the majority of MXenes possess excellent electronic properties which are favorable for high-performance MIBs. [53]. Applications in Different MIBs

Lithium-Ion Batteries
Due to the peculiarities of high specific capacity and energy density, cycling stability, and no memory effect, LIBs have been dominating the market since the 21st century as the power source for powering up consumable electronics and electric vehicles, etc. [54] However, with the increased needs of society, LIBs cannot meet the future requirements of high energy density for long-mile electric vehicles and large-scale energy storage systems. [55]As one of the most important parts of LIBs, the property of electrodes plays a crucial role in the upgrade of high energy density LIBs.In the past years, great efforts have been devoted to designing and fabricating novel electrode materials and structures with ultrahigh Li þ storage capacities and superb ion transportation ability.Therefore, numerous advanced materials and structures have been prepared, greatly promoting the development of LIBs.Particularly, MXenes have been widely used in high energy density LIBs, owing to their peculiarities of high surface area, fast ion transfer pathways, and short interaction distance between the charge carriers and ions. [56]herefore, extensive MXenes and MXene-based composites with diverse structures and physicochemical properties have been developed, to serve as electrodes in LIBs.

MXenes
MXenes possess the merits of high theoretical capacity (320 mAh g À1 in Ti 3 C 2 ), [32] low operating voltage, and low Li þ diffusion barrier (0.07 eV on Ti 3 C 2 ) compared with graphite (0.3 eV), making them the most promising anode materials for the high-performance LIBs. [57]Naguib et al. [58] first implemented the application of MXenes as electrodes in LIBs.Then, Zhou et al. [59] theoretically demonstrated the feasibility of MXenes as electrodes for LIBs through DFT calculations.After these two groundbreaking studies, extensive experimental and computational studies have been conducted on MXene electrodes for rechargeable LIBs.In the past decade, many types of MXenes were exploited and synthesized as anode materials in high-performance LIBs.For example, Yanwei Ma's group [40] reported accordion-like Ti 3 C 2 T x MXene as the anodes in LIBs with a high capacity of 236 mAh g À1 after 1600 cycles, confirming the outstanding cycle stability of MXenes.Lin et al. [5] explored a one-pot fabrication method to synthesize MXenes by combining the MS 3 strategy and Lewis acid etching method, as shown in Figure 3a.This strategy is simplified and the time required for the preparation of MXenes is reduced.Importantly, the MXenes obtained by this facile method delivered a high capacity of 280 mAh g À1 (Figure 3b-e), which will pave the way for simplifying the MXenes synthesis procedure and MXenes applied in high energy density batteries.
Notably, the Li þ storage ability of MXenes is highly correlated to the chemical components, surface terminations, type of doped atoms, and structures.By and large, an MXene with a light heavier formula weight like M 2 C tends to deliver a higher capacity compared with the heavier formula weight MXenes of M 3 C 2 and M 4 C 3 , due to the formed inactive MC layer.Except for formula weight, the types of transition metals are highly related to the Li þ storage capacity of MXenes.For example, Gogotsi et al. [60] synthesized two new MXenes of Nb 2 C and V 2 C in 2013, which showed a high capacity of 260 and 170 mAh g À1 at 1 C, respectively, when served as an anode in LIBs.In addition, the Ti 2 C only delivered a low capacity of 110 mAh g À1 at the same experimental conditions with Nb 2 C and V 2 C anodes in LIBs.Moreover, the surface terminations "T x ", which are derived from the etching procedure, highly affect the electrochemical performance of the MXenes anodes in LIBs.Both theoretical and experimental analyses confirmed that the surface terminations on MXenes like -F and -OH impede Li þ transfer and reduce Li storage ability and -O could provide additional Li þ storage sites. [16,59,61]Therefore, the surface terminations "T x " should take into special consideration to improve the Li þ storage and transmission when MXenes serve as anodes in LIBs.
Heteroatoms like N, O, B, S, or P doping could improve surface chemistries, ensure adequate interface interactions of electrode-electrolytes, and enhance the ion and electron transfer ability of electrode materials.Therefore, many types of doping heteroatoms are introduced in MXenes-based electrode materials to boost their electrochemical performance. [62]For instance, Yang's group [63] developed a facile hydrothermal method to synthesize the N-doped Nb 2 CT x MXenes as the anode materials in LIBs.The doping of N atoms enlarges the interlayer spacing of Nb 2 CT x MXenes from m 22.32 to 34.78 Å and increases the conductivity and surface area.Consequently, the N-doped Nb 2 CT x MXenes anode delivered a high capacity of 360 mAh g À1 at 0.2 C compared with Nb 2 CT x MXenes (190 mAh g À1 ).Zhang et al. [64] reported an N, S codoped V 2 CT x MXene (N-S-VCT) serves as an anode material for LIBs, prepared via an easy one-pot calcination procedure with thiourea.The N-S-VCT delivered a high capacity of 590 mAh g À1 , due to the introduction of N, S doping heteroatoms expanded interlayer spacing, and improved charge transfer ability of V 2 CT x MXene.The abovepublished works prove that heteroatom doping is a simple and effective strategy to enhance the performance of MXenes.In general, a certain amount of heteroatom doping could improve the electrochemical performance of MXenes anodes, but, what is the perfect heteroatom doping content lack research.In addition, how to continuously increase the content of heteroatoms is the main difficulty faced by the doping strategy.
In a batter battery, the electrode structure plays a key role in superior electrochemical performance.Therefore, designing and synthesizing the structure of the MXenes electrode is vital to improve the Li þ transfer ability and storage capacity.To this end, many strategies have been designed to synthesize the unique 1D, 2D, and 3D structures for MXenes-based electrodes in LIBs.For instance, Zhao et al. [65] reported a flexible 3D porous MXene foam as anode material in LIBs, enabling an excellent electrochemical performance.The unique 3D porous MXene framework was synthesized with S as a sacrificial template and then removed the sulfur particles by sublimating at 300 °C.Consequently, the 3D porous MXene framework possesses sufficient S-doped functional groups which are beneficial for enhancing ion transfer and improving the electrolyte wettability.
Recently, previous works prove that the Li storage capability of 2D Ti 3 C 2 T x MXene anode is highly related to their interlayer spacing.For example, Zhang et al. [66] reported a general strategy to enlarge the interlayer spacing by Lewis-basic halides.The interlayer spacing is enlarged by the synergistic effect of the surface functional group substitution and cations intercalation (Na þ and K þ ) in Lewis-basic halides.Consequently, the enlarged spacing MXenes delivered a high capacity of 229 mAh g À1 , which is almost two times higher than the original MXenes.This work demonstrates a general strategy for tuning the interlayer spacing of various MXenes, which is significant in enhancing the electrochemical performance of MXenes-based electrodes.Song's group [10] developed a substitution and intercalation method to enlarge the interlayer spacing of V 2 C MXenes, which realized outstanding Li þ storage ability as anode materials in LIBs.This work proves that expanding the interlayer spacing of MXene could improve the Li þ storage capacity of MXene-based electrodes in LIBs.In addition, MXene could as conductive binders for both cathode and anode in LIBs, which was proved by Nicolosi's group [67] in 2023, as shown in Figure 3f.The cathodes and anodes in this work are prepared directly by slurry-casting of MXene aqueous inks, which form an integrated network due to the outstanding toughness and strength properties of MXene nanosheets.To better understand the viscosity of the MXenes/NMP ink, the rheological behaviors were evaluated.The results prove that the MXenes/NMP ink viscosity is much higher than that of the CB/PVDF/NMP system.The viscoelastic properties were also detected to give insights into the structural integrity of electrodes, which confirm the higher G 0 and G 00 of MXenes/NMP ink with active materials.Consequently, the slurry with MXenes/NMP ink and active materials shows no cracks upon drying.Therefore, the full battery with MXenes as binders delivered a high capacity of 154 mAh g À1 and no capacity decay after 200 cycles, as shown in Figure 3g-i.
Although MXenes possess many merits of high theoretical capacity, low operating voltage, and low Li þ diffusion barrier as anode materials for LIBs, the Li-ion storage in MXenes shows a slopy discharge curve without a voltage plateau, which means the inferior energy density compared with a battery material that has a plateau.Therefore, there is still a long way to go for the practical application of MXenes as LIBs anode materials, and perhaps MXene-based composite materials or as collectors are the future of MXenes.For example, Wang et al. [68] developed a free-standing Ti 3 C 2 T x film as the conductive current collector for LIBs.In this work, free-standing Ti 3 C 2 T x film replaces Cu x MXene electrode at various potentials.Reproduced with permission. [5]Copyright 2021, Nature Publishing Group.f ) Schematic illustration and SEM images for the MXene binders in the cathode and anode.g-i) Electrochemical characterizations of LTO/MX anode.Reproduced with permission. [67]opyright 2023, Wiley-VCH.
and Al current collector to support anode and cathode materials of multilayer Ti 3 C 2 T x and commercial LiFePO 4 .The freestanding Ti 3 C 2 T x film shows a low density of 3 g cm À3 compared with the Cu (9 g cm À3 ) and Al (2.7 g cm À3 ) current collector, which is critical to reducing the total weight and increasing the energy density of LIBs.As a result, the LiFePO 4 electrode delivers a high capacity of 122 mAh g À1 even at 8.7 mg cm À3 .This work demonstrates that MXenes could as outstanding current collectors in lightweight energy-storage devices.

MXene-Based Composite
Generally, the electrochemical performance of individual material is restricted by its intrinsic properties, for example, the single Ti 3 C 2 Mxenes material tends to agglomerate due to the van der Waals force and has poor cycle stability when used as an anode in a battery.Therefore, integrating MXenes with other novel nano or micron materials is an efficient approach to cover its shortage and improve the electrochemical performance.For example, combining MXenes with highcapacity electrode materials that suffer undesirable cycle stability issues could improve the composite electrode electrochemical performance.Because MXenes can accommodate the volumetric expansions and enhance the conductivity, giving full play to the high-capacity characters of MXene-based composite electrodes.
Carbon materials like CNTs, CNFs, and graphene et al. possess the features of high conductivity, surface area, and lightweight, making them better electrode materials in LIBs.For example, Guo et al. [69] fabricated 1D N-doped CNFs by electrospinning, using MXene flakes and Fe 3 O 4 hollow nanospheres as raw material.In this 1D structure, the CNFs could provide fast electrons and ion pathways due to the tight coupling of MXene and Fe 3 O 4 .Consequently, the Fe 3 O 4 @MXene/CNFs electrodes delivered a high capacity of 1786 mAh g À1 , demonstrating a feasible strategy for the fabrication of highperformance LIBs anodes.Interestingly, Ahn's group [70] prepared hollow 1D MXene@carbon nanofibers as efficient Li þ storage anodes in LIBs.The hollow 1D MXene@carbon nanofibers with tailored surface groups and special freestanding structures serve as a binder-free anode in LIBs and show outstanding electrochemical performance.In another work, Cao's group. [71]developed an effective method to synthesize the CNTs@Ti 3 C 2 T x composite as anode materials in LIBs.They used melamine as a carbon source and Co 2þ as the catalyst to in situ growth CNTs as conductive bridges which connect the MXenes nanosheets and form an impressive 3D structure.In this 3D composite, the growth of 1D CNTs enlarges the interlayer spacing of MXenes, and improves the integrality of the MXene nanosheets due to the conductive CNTs bridges, enabling enhanced cycle stability and capacity of the electrodes.Consequently, the unique 3D structure consists of high-conductivity CNTs and MXene nanosheets delivering a high capacity of 491 mAh g À1 in LIBs.Furthermore, high-conductivity and flexible graphene could also enhance the rate performance of MXene-based electrodes.
In addition to being used directly as an active material, MXene can be used as a substrate to fabricate electrode materials due to its outstanding lamellar structure, sufficient surface functional groups, and high conductivity.As a typical example, N-doped Ti 3 C 2 T x MXene nanosheet-coated silicon suboxide (SiO x , 0 < x < 2) was prepared by Zhang et al. via ball milling and annealing procedures. [72]The composite combines the highcapacity merit of SiO x and conductivity lamellar structure MXene nanosheets, enabling an excellent electrochemical performance of 1.28 mA h cm À2 over 100 cycles.Yang's group [73] fabricated core-shell structures composite of Si@Ti 3 C 2 T x by an interfacial assembly method, as shown in Figure 4a.In the Si@Ti 3 C 2 T x composite, Si nanospheres were synthesized through a magnesiothermic reduction process, and Ti 3 C 2 T x were obtained by the chemical etching procedure.The Ti 3 C 2 T x nanosheets were closely and tightly wrapped Si nanospheres, which enhance the conductivity, structural integrity, and against pulverization of Si nanospheres.Consequently, the Si@Ti 3 C 2 T x electrode showed a much enhanced electrochemical performance of 760 mAh g À1 in LIBs (Figure 4b,c).Furthermore, many types of MXene-based composite ware are designed and prepared as anode materials in LIBs using MXene as a substrate, such as NiCo-LDH/Ti 3 C 2 MXene, [74] MXene-decorated SnS 2 /Sn 3 S 4 , [75] MoO 3Àx @Ti 3 C 2 -MXene, [76] etc.

MXene-Derived Materials
Except for being directly used as active materials and conductive substrates, MXenes could be as the precursor to synthesize kinds of anodes, such as transition metal oxides, nitrides, sulfides, etc. Chen et al. [77] constructed MoS 2 /Mo 2 TiC 2 T x heterostructures as high-performance anode materials in LIBs using Mo 2 TiC 2 T x MXene nanosheets as precursors by in situ sulfidation procedure, as shown in Figure 4d.Both experimental and theoretical results prove that the MoS 2 /Mo 2 TiC 2 T x heterostructures possess high conductivity and excellent Li þ storage ability when used as anode materials in LIBs (Figure 4e-h).Meanwhile, the robust heterointerface between the MoS 2 and Mo 2 TiC 2 T x MXenes which are derived from the in situ sulfidation procedure ensures fast electron transmit pathways, which is the key to improving the redox reaction kinetics in LIBs.This work demonstrates a universal strategy for preparing MXene-based heterostructures anode materials for LIBs, as well as other energy storage systems.
In conclusion, the electrochemical performance of MXenes or MXene-based anode materials in LIBs is highly related to surface terminations, interlayer spacing, components, and structures.Therefore, the performance of LIBs could improve by adjusting the key parameters of MXenes.We believe that the potential of MXenes has not yet been fully exploited and MXene-based composite will play a more critical role in future LIBs.

Sodium-Ion Batteries (SIBs)
SIBs, as a promising system, show great potential to replace LIBs for grid-scale energy storage, due to the analogous low potential of Na to Li, and the low cost of Na. [78][79][80] However, due to the large radius of Na þ , SIBs have sluggish kinetics and serious volume fluctuation in the charge/discharge process.Under the circumstances, the emergence of MXenes with 2D lamellar structure, tunable interlayer spacing, high conductivity, and low diffusion Na þ barrier gives an excellent opportunity to handle the problems of SIBs and improve their electrochemical performance. [24,81,82]

MXenes
To better understand the Na þ storage capability and diffusion kinetics of MXenes, DFT calculation is adopted to clarify the diffusion barriers, open-circuit voltage (OCV), and theoretical capacities for different types of MXenes in SIBs. [83]Yang's group [84] predicted that the TiC 3 monolayer possesses the highest theoretical capacities of 1278 mAh g À1 when used as an anode in SIB.Surprisingly, the TiC 3 monolayer still maintains a stable structure and high conductivity even after adsorbing two layers of Na atoms, enabling outstanding cycle stability during the battery cycle.This work also proved that surface functionalization of -O could increase the theoretical capacity and rate ability of TiC 3 monolayer, due to the improved Na absorption capability.Furthermore, many other types of MXenes with monolayers have been simulated by DFT calculations, such as V 2 C, [85] V 3 C 2 , [86] Mn 2 C, [81] and Ti 3 C 2 , [87] could be the high-capacity anode materials in SIBs owing to the low Na þ diffusion barrier and high OCV.
The surface groups of MXenes are highly related to the electrochemical performance of SIBs when served as anode materials. [88]For example, the -F and -OH groups will increase the Na þ diffusion barrier and decrease the Na þ storage capacity of MXenes, but the -N and -O groups are favorable to the Na þ storage in MXenes.Xia et al. [89] reported a tailored N surface group on MXenes which enhances the Na þ storage ability and improves the cycle stability of SIBs, as shown in Figure 5a.In this work, the N groups are introduced by calcining the cetyltrimethylammonium bromide (CTAB), which not only brings the N source but also protects the structure of MXenes.Experimental results prove that the introduction of N groups could enhance the stability of SEI, which is favorable to increasing the Na þ transfer rate at the electrode/electrolyte interface, enhancing the Na þ storage ability, and improving the cycle stability.And DFT calculation indicates that the N groups could narrow the bandgap, improve the sodiophilicity of MXenes, and reduce the Na þ diffusion barrier.Therefore, the MXenes with N surface group endow a high capacity of 220 mAh g À1 , rate ability of 90 mAh g À1 at 5 A g À1 , and excellent cycle stability (80.9% over .Reproduced with permission. [73]Copyright 2020, the American Chemical Society.d) Schematic diagram and SEM images for the fabrication of MoS 2 @MXene composite.e-h) Electrochemical performance of MoS 2 /Mo 2 TiC 2 T x heterostructures for LIBs.Reproduced with permission. [77]Copyright 2018, Wiley-VCH.
5000 cycles) at À25 °C (Figure 5b-e).Feng's group [78] reported a kind of constructed redox-active phosphorus-oxygen surface group on Nb 4 C 3 MXenes (PO 2 -Nb 4 C 3 ), which enables a remarkable specific capacity for Na þ storage, as shown in Figure 5f-l.The functional PO 2 group was introduced through a targeted terminal conversion strategy of calcining the Nb 4 C 3 T x and black phosphorus hybrid membrane, which boosts the Na þ storage capacity, enriches carrier density, improves conductivity, offers extra redox-active sites, increases Na þ diffusion rate, and buffers internal stress during Na þ intercalation/deintercalation.Consequently, the PO 2 -Nb 4 C 3 anode shows a high capacity of 221 mAh g À1 with cycle stability (Figure 5i-l).In addition, both experimental and computational results prove that the interlayer spacing of MXenes also occupies a significant position in determining the Na þ storage capacity of MXenes in SIBs, especially the larger radius of Na þ than Li þ .Therefore, many strategies have been developed to adjust and enlarge the interlayer spacing of MXenes and MXene-base anodes in SIBs.

MXene-Based Composite
Combining MXenes with high-capacity SIBs anode materials is an effective approach to enhance the electrochemical performance of MXene-based anodes.Furthermore, the outstanding electrode structure is favorable to reducing the ion diffusion barrier, declining the usage of electrolytes, and improving the cycle stability of the battery.Therefore, designing and synthesizing the unique structure of MXene-based anodes in SIBs are crucial strategy.
Recently, many approaches have been proposed to synthesize excellent structures for MXenes-based anodes in SIBs.For example, Feng's group [90] constructed a free-standing Na 2 C 6 O 6 / MXene hybrid paper with superior flexibility, as shown in Figure 6a,b.The Na 2 C 6 O 6 is a promising anode material in SIBs owing to its high theoretical capacity of 501 mAh g À1 but suffers the issues of low conductivity, rapid capacity decay, and dissolution in electrolytes.In this work, combining with MXenes offers a favorable solution to take full advantage of  [89] Copyright 2022, Springer Nature.f ) The schematic illustration for the fabrication of PO 2 -Nb 4 C 3 composite.g,h) Cross-sectional SEM images of the PO 2 -Nb 4 C 3 film.The initial three charging/discharging curves of the i) O-Nb 4 C 3 and j) PO 2 -Nb 4 C 3 electrodes.Cycling performance of the k) O-Nb 4 C 3 and l) PO 2 -Nb 4 C 3 electrodes at 0.1 and 1 A g À1 .Reproduced with permission. [78]Copyright 2022, Wiley-VCH.
the high theoretical specific capacity of Na 2 C 6 O 6 .At last, the flexible Na 2 C 6 O 6 /MXene anodes exhibited a high capacity of 231 mAh g À1 .As a potential anode material, the MoS 2 possesses a higher theoretical capacity of 670 mAh g À1 but suffers the issues of inferior rate ability and fast capacity decay in the batteries, because of the poor conductivity and volume expansion.To this end, Wang et al. [91] constructed a MoS 2 nanosheet anchored on MXenes substrate as anode materials in SIBs, via a simple sulfuration procedure.The high conductivity MXene in the heterostructure could as a framework to support MoS 2 nanosheets and the 2D lamellar structure can enhance the electron transfer and the infiltration with electrolyte, while the small MoS 2 nanosheets can offer sufficient Na þ storage active sites.Meanwhile, the robust heterointerface between the MoS 2 and MXenes ensures fast electron transmit pathways, which is the key to improving the redox reaction kinetics in SIBs.Consequently, the MoS 2 @MXene heterostructure anode delivered a high capacity of 315 mAh g À1 .Cao et al. [92] prepared a nanofiber network skeleton that consists of Fe xÀ1 Se x and 2D MXene as high-performance SIBs and PIBs anode materials, as shown in Figure 6c-f.In the composite, using MXene as support layers to suppress the restacking of Fe xÀ1 Se x sheets, and pyrolysis of the fungus as a nanofiber network skeleton to provide additional electron and ion pathways.As a result, the assembled SIBs using Fe xÀ1 Se x @MXene@nanofiber network skeleton as anode materials exhibited a high capacity of 610.9 mAh g À1 .These works demonstrate that MXenes could be an outstanding substrate in high-performance SIBs systems.

MXene-Derived Materials
MXenes could as precursors to prepare kinds of unique derivatives as electrodes in SIBs.The conversion process is interesting because the formed composite partially retains the 2D ultra-thin properties of the MXenes and generates rich defects/vacancies on the surface, leading to the promoted functionality of the MXene-derived materials.For instance, Tang et al. [93] fabricated an MXene-derived TiS 2 nanosheet by annealing the polyvinylpyrrolidone (PVP) modified Ti 3 C 2 T x MXenes in Ar/ H 2 S atmosphere.The derived carbon-coated TiS 2 nanosheet (C-TiS 2 ) remains the 2D structure of MXenes, enabling a fast Na þ diffusion kinetics and high storage capacity.As a result, the C-TiS 2 anode delivered a high capacity of 448 mA h g À1 at 0.1 A g À1 with good cycling stability in SIBs.Zhong et al. [94] reported an MXene-derived Na 2 Ti 3 O 7 @C anode material in SIBs.The derived Na 2 Ti 3 O 7 @C composite avoids the disadvantage of MXenes being prone to agglomeration and has a crosslinked nanobelt skeleton with appropriate interlayer spacing and high surface area, which is conducive to ion diffusion and electrolyte penetration.Consequently, the Na 2 Ti 3 O 7 @C anode delivered a high capacity of 173 mA h g À1 at 0.2 A g À1 with a cycling life of over 200 cycles.These works demonstrate that transforming MXenes into different derivatives is an effective strategy for improving the electrochemical performance of SIBs.
Analogous to LIBs, Na þ storage capability in SIBs is also affected by surface groups, interlayer spacing, components, and structures of MXenes or MXene-based anodes. [84,95] [90]Copyright 2023, Springer Nature.c) Schematic illustration for the fabrication of the Fe xÀ1 Se x @MXene@nanofiber network.
Compared to Li þ in LIBs, Na þ possesses a lower diffusion rate and sluggish redox reaction kinetics in SIBs, due to the large radius of Na þ . [96]Therefore, to realize a higher energy and power density for SIBs, MXenes or MXene-based materials should have higher quality in SIBs than those of LIBs.In addition, many theoretical simulations about the Na þ storage and diffusion in SIBs are still limited due to the simple model construction or plain interaction between Na atoms and the substrate.However, the reactions at the electrode/electrolyte interface and the active materials, in reality, are rather complicated.Therefore, much effort has to be invested in experimental research and theoretical simulation to enhance the electrochemical performance of SIBs.The electrochemical performance of MXene-based composite in SIBs is summarized in Table 1.

MXenes
PIBs are a promising candidate for future grid-scale energy storage systems owing to their elemental abundance, low cost, high ionic conductivity, and suitable redox potential.In addition, even though potassium possesses analogous chemical chemistry to lithium, graphite can not be used as anode material for PIBs, due to the low theoretical capacity of 279 mAh g À1 and low ion diffusion rate.Therefore, it is significant to develop high-capacity PIBs anode materials with a fast ion diffusion rate.Recently, MXenes material with 2D lamellar structure, large interlayer spacing, and high conductivity has received great attention as anode materials for PIBs.For instance, Zhang et al. [97] prepared a 3D MXene scaffold through electrostatic adsorb Ti 3 C 2 T x with positive-charged melamine followed by a calcination procedure, as shown in Figure 7a.The 3D MXene scaffold enables sufficient surface active sites and fast ion transfer path, which are beneficial to increase the K þ storage capacity and improve the dynamics.Consequently, the 3D Ti 3 C 2 T x MXene delivered an improved electrochemical performance of 161.4 mAh g À1 and outstanding cycle stability.These works demonstrated that MXnens could be a better anode material for PIBs.
The metal anode is a critical part of PIBs.However, the reversible plating and stripping of potassium metal during cycling remain a severe challenge, due to the uncontrollable growth of dendrites and continuous consumption of electrolytes.Interestingly, the MXenes could as a 3D skeleton induce the uniform deposition of K þ , accommodate the volumetric changes, and stabilize the solid electrolyte interphase (SEI) during cycling.For instance, Tang et al. [98] developed a titanium-deficient nitrogen-containing MXene/carbon nanotube (DN-MXene/ CNT) composite as the 3D skeleton to confine potassium metal.As a result, the K@DN-MXene/CNT electrode shows a dendritefree morphology and excellent cycling stability.This work proves that MXenes is a better 3D skeleton to enhance the stability of K metal in the batteries.

MXene-Based Composite
Except for directly as an anode material, MXene can as a substrate to combine any anode materials with high K þ storage capacity like single atoms, [99] metal selenide, [100] sulfide, [101] etc., as active materials for PIBs.For example, Hou's group [102]  constructed a 3D Nb 2 C/rGO materials as high-performance anode material for PIBs by a hydrothermal self-assembly procedure, as shown in Figure 7b.The 3D Nb 2 C/rGO composite integrates the features of a high surface area, high conductivity, and low K þ diffusion energy barrier of Nb 2 C and rGO, which offers fast ion transfer pathways, more active sites, prevented restacking structure, and improved electrolyte accessibility.Furthermore, the unique 3D structure could accommodate the volume fluctuation during the intercalation and extrication process of K þ .As a result, the 3D Nb 2 C/rGO hybrid anode delivered a high capacity of 301.7 mAh g À1 .Xia et al. [103] designed a CVD method to construct nitrogen-doped graphene/ReSe 2 composite on the MXene substrate, thereby synthesizing anode materials of NG/ReSe 2 /MXene heterostructure for PIBs with high K þ storage capacity and outstanding cycle stability (Figure 7c).Compared with prevailing wet-chemistry techniques, the CVD method constructs a clean and tight interface for heterostructures due to high temperature and a gaseous environment.The NG/ReSe 2 /MXene heterostructure prepared by CVD possesses the following properties: 1) high conductivity due to the tight interface connection of ReSe 2 with MXene; 2) stable structure of ReSe 2 nanoparticles during the charging/discharging process since the support of MXene and NG; 3) improved electrolyte penetration due to the integrated 3D structure.Consequently, the NG/ReSe 2 / MXene heterostructure anode delivered a high capacity of 138 mAh g À1 at 10 A g À1 .Cao et al. [104] reported a flexible framework of GNFM as high-performance PIBs anode by integrated graphite nanoflake and MXene, as shown in Figure 7d.The flexible 3D MXene framework with structural stability constructs a fast ion transfer pathway and exhibits much-enhanced K þ storage capacity.These works prove that MXene is a superior substrate for improving the electrochemical performance of PIBs.

MXene-Derived Materials
Except for being directly used as active materials and conductive substrates, MXenes could as the precursor to synthesize kinds of anodes, such as transition metal oxides, nitrides, sulfides, MOF, etc.For instance, Tao et al. [105] fabricated titanium oxynitride NPs/carbon composites for high-performance KIBs anode using Ti 3 C 2 T x as a precursor.This work demonstrated a novel strategy to prepare high-performance PIBs anode materials.Recently, Ti-based polyanionic compounds become a promising anode material for PIBs, due to the inorganic-open structure being Reproduced with permission. [97]Copyright 2022, Wiley-VCH.b) Schematic illustration of the fabrication process of Nb 2 C/rGO.Reproduced with permission. [102]Copyright 2023, Springer Nature.c) Schematic illustration of the fabrication process of NG/ReSe 2 /MXene materials and the corresponding SEM images.Reproduced with permission. [103]opyright 2021, Elsevier.d) The schematic diagram for the preparation of GNFM.Reproduced with permission. [104]Copyright 2021, Wiley-VCH.e) Schematic synthetic process of TOP@C.Reproduced with permission. [106]Copyright 2023, Wiley-VCH.f ) Schematic illustration of the unlocking process of MD-MOF.Reproduced with permission. [107]Copyright 2022, Wiley-VCH.
favorable for ion transfer, relatively low redox potential for K þ , high abundance, cost-effectiveness, structure stability, and environmentally friendly.Therefore, a Ti 3 C 2 T x derived 1D π-Ti 2 O(PO 4 ) 2 was prepared as high-performance PIBs anode material by hydrothermal method, as shown in Figure 7e. [106]oth theoretical and experimental analyses prove that the π-Ti 2 O(PO 4 ) 2 anode possesses an improved K þ storage capacity, large storage tunnels, and lower migration energy during the charging/discharging process.Consequently, the π-Ti 2 O(PO 4 ) 2 anode in PIBs with an intercalation pseudo-capacitance energy storage mechanism delivered a high capacity of 134.5 mAh g À1 .Sun et al. [107] fabricated a nod-unlocked MOF as the anode for high-performance PIBs using MXenes as metal precursors, as shown in Figure 7f.In this work, DFT calculations prove that the unlocked sites could improve the K þ adsorption energy which is favorable to enhance the K þ storage ability.As a result, the NMD-MOF anode delivered a high capacity of 540 mAh g À1 .These works prove that the conversion process could retain the properties of the MXenes and generate rich functional sites on the surface, leading to the electrochemical performance of the MXene-derived materials.
In conclusion, combining MXenes with any other highcapacity electrodes is a better strategy for high-performance PIBs, due to the limited interlayer spacing of MXenes.Designing a novel structure of MXene-based electrodes is a key parameter for enhancing electrochemical performance, such as assembling 2D MXenes into 3D hierarchical frameworks.The electrochemical performance of MXene-based composite in PIBs is summarized in Table 1.

MXenes
As a promising energy storage system, MgIBs possess many peculiarities such as the abundant Mg anode (104 times of Li), dendrite-free features, high theoretical capacity of 2219 mAh g À1 , low redox potential (À2.37 V versus standard hydrogen electrode (SHE)), environmentally benign and high safety.However, MgIBs nowadays still lack adaptive cathode materials that enable fast diffusion/intercalation of Mg 2þ ions and stable electrolytes.Therefore, many types of transition metal oxides, sulfides, and selenides have been constructed as cathode material, significantly expanding the cathode materials system of MgIBs.However, the low conductivity and inferior cycling stability of the aforementioned material systems severely limit the electrochemical performance of MgIBs.In recent years, due to the excellent properties of intrinsic conductive, active surface, and large layer spacing, MXenes show excellent potential as cathode materials in MgIBs, making the MgIBs system full of vitality.For example, Eames et al. [108] using DFT simulations proved that M 2 C MXenes composed of light transition metals with unfunctionalized or -O surface groups possess an ultrahigh theoretic capacity of 400 mAh g À1 for Mg-ion storage.Xie et al. [87] demonstrated that both bare and -O surface groups modified MXenes could be high-performance cathode materials for MgIBs through DFT simulations and experimental analysis, as shown in Figure 8a.

MXene-Based Composite
MXenes are an ideal substrate due to their outstanding lamellar structure, sufficient surface functional groups, and high conductivity.Therefore, many types of MXene-based materials with unique structures were designed and fabricated as cathodes for MgIBs.Liu et al. [109] prepared a unique Ti 3 C 2 /CoSe 2 composite by in situ selenization procedure as high-performance cathode materials for MgIBs, as shown in Figure 8b.In this heterostructure, the MXenes with high conductivity and distinctive 2D planar structure help CoSe 2 nanoparticles to fully liberate the characteristic of high theoretic capacity.Consequently, the Ti 3 C 2 /CoSe 2 cathodes showed a high performance of 75.7 mAh g À1 with cycle stability.Fan's group [110] fabricated an MXene@carbon nanosphere by electrostatic interactions as a cathode for Mg 2þ storage, as shown in Figure 8c.The positively charged carbon nanospheres insert into MXenes effectively enlarge the interlayer spacing and inhibit the restacking of MXene nanoflakes, which could promote ion transfer and improve electrolyte infiltration.This work demonstrated better electrochemical performance of cathode materials with a high capacity of 198.7 mAh g À1 and outstanding cycling stability.
Both experimentally and theoretically prove that MXene-based cathodes can deliver a high capacity for MgIBs.Therefore, studying the mechanism of Mg 2þ storage is of great significance.For instance, Zhang et al. [111] investigated the Mg 2þ storage mechanism in the NiSe 2 -CoSe 2 @TiVCT x cathode, via ex situ XRD, XPS, SEM techniques, and DFT analysis (Figure 8d).This work demonstrated that the NCSe@TiVC cathode did not suffer obvious structural evolution, valences change or any alloying during the charging/discharging process, confirming the outstanding Mg 2þ storage ability of NCSe@TiVC.The DFT simulation results also revealed that the NCSe@TiVC composite could improve the ion diffusion and enhance the Mg 2þ storage capacity.As a result, the NCSe@TiVC cathodes delivered a high capacity of 136 mAh g À1 and better cycle stability over 500 cycles.The Mg 2þ storage mechanism in MXenes or MXene-based composted is more complex than Li þ or Na þ storage, owing to the stronger electrostatic interaction of the Mg 2þ and the cathode.However, the storage mechanism in MgIBs has not been fully understood even though many charge carriers have been identified.
Due to the strong solventization and interactions of Mg 2þ with the cathode severely affecting the development of MgIBs, the concept of a hybrid Mg 2þ /Li þ battery is put forward.Such a hybrid battery includes an Mg anode, a Li þ intercalation cathode, and a Mg 2þ /Li þ dual-salt electrolyte.The hybrid system combines the rapid ion intercalation kinetics, high voltage and capacity of LIB cathodes, and cost-effectiveness, dendrite-free, and high safety of Mg anodes, making it a good choice to replace MgIBs systems.For example, Liu et al. [112] prepared a hybrid Li þ /Mg 2þ battery using the prelithiated V 2 C as the cathode (Figure 8e).During the discharging process, the CV, XPS, and ex situ XRD tests proved that the Mg 2þ dissolved in the electrolyte from Mg foil and Li þ was inserted into the MXenes, while the reversible ions deposition and extraction took place in the charging process.Consequently, the hybrid Mg 2þ /Li þ battery delivered a high electrochemical performance of 230.3 mAh g À1 and opened a new path to develop potential hybrid Mg 2þ /Li þ battery electrodes.The electrochemical performance of MXene-based composite in MgIBs is summarized in Table 1.

MXenes
Due to the high volumetric capacity (8406 mAh cm À3 ) and massspecific capacity (2980 mAh g À1 ), the abundance of aluminum elements in the earth's upper crust (about 8%), high safety, and cost-effectiveness, AIBs becoming the most promising energy storage systems.However, some issues remain for the exploitation of AIBs, such as the high specific capacity cathode materials that cannot be easily synthesized, poor cycle life, and fewer electrolyte alternatives.Therefore, designing and fabricating cathode materials with high specific capacity and cycle stability is the highest priority in AIBs system development.As a typical 2D material, MXenes show great potential as highcapacity cathode materials in AIBs.For instance, Beidaghi et al. [113] directly adopt few-layer V 2 CT x MXene as AIBs cathode materials and showed a high capacity of 300 mAh g À1 .Tu's group [114] prepared a single-layered Nb 2 CT x MXene as cathode materials in AIBs and delivered a first charge capacity of 275 mAh g À1 .These works demonstrated that MXenes could act as high-performance cathode materials in AIBs.

MXene-Based Composite
Besides being used directly as active material in AIBs, MXene can be used as a substrate or framework to support the cathode materials due to its unique 2D structure, sufficient surface groups, and high conductivity.For example, Yao et al. [115] constructed an MXene framework to support ultrafine CoSe 2 nanoparticles as cathode materials in AIBs (Figure 9a-e), which enabled a high capacity of 436 mAh g À1 .In this work, the MXene framework could enhance the integrality and conductivity of the cathodes, which are vital to enhancing the electrochemical performance of AIBs.Zheng et al. [116] prepared an MXene@N-doped carbon@Ni 0.6 Co 0.4 S composite as cathode material for AIBs, with MXene as the substrate to support the Ni 0.6 Co 0.4 S solid solution (Figure 9f-k).The robust MXene support layer not only offers a faster electron and ion transmission channel but also improves the chemical stability and structural integrality of the composite.With the aid of MXene, the anode  [87] Copyright 2014, the American Chemical Society.b) Schematic illustration of the Ti 3 C 2 /CoSe 2 fabrication process.Reproduced with permission. [109]Copyright 2021, Elsevier.c) Schematic illustration of the synthesis process of Ti 3 C2T x @C nanospheres.Reproduced with permission. [110]Copyright 2019, The Royal Society of Chemistry.d) The synthesis process of the TiVCT x and NCSe@TiVC heterostructure.Reproduced with permission. [111]Copyright 2022, Wiley-VCH.e) Schematic illustration of the fabrication procedure for prelithiated MXene.Reproduced with permission. [112]Copyright 2020, Wiley-VCH.
showed a high capacity of 481.2 mAh g À1 .The above works prove that MXenes is not only an excellent cathode material for AIBs with high capacity, but also could be an outstanding substrate material.When the MXenes are used as a substrate, they can help active substances to realize their potential and improve the comprehensive properties of composite materials.
Except for the cathode materials, the electrochemical performance of AIBs is highly correlated to the working mechanism, due to the Al 3 þ or chloroaluminate anions (AlCl 4 À and Al 2 Cl 7 À ) intercalated into the cathode interlayer involving a series of complicated structural and compositional evolution.However, the complicated process of the ion transport and intercalation mechanism in AIBs has not been fully understood.Therefore, the intrinsic ion-transport mechanism between electrolyte and cathode materials needs to be fully investigated.For instance, Lv et al. [117] constructed a V 2 C@Se composite as cathode material to reveal the working mechanism of AIBs (Figure 10a-f ).The MXenes have been proven to be a better cathode material in AIBs and showed capacitor-type behaviors with inconspicuous charge-discharge plateaus.In this work, the authors testify the reversible redox reaction of V 2 C@Se cathodes, which consist of , and Se 2À /Se 2þ in the charge/discharge process by XPS measurement and electrochemical performance analysis.The DFT calculations confirm that the selenium process improves the adsorption, diffusion, and reversible intercalation of [AlCl 4 ] À on the V 2 C.This work demonstrates the V 2 C@Se cathode working mechanism and the intercalation procedure of [AlCl 4 ] À in AIBs.As a result, the V 2 C@Se cathode delivered a high capacity of 402.5 mAh g À1 .Yuan et al. [118] reported a strategy of optimizing the design of cathodes and electrolytes to underlying the mechanism of poor cycle stability and low Coulombic efficiency in AIBs (Figure 10g-n).The Al storage mechanism in CoSe 2 /MXene cathodes was comprehensively analyzed by XRD, TEM, and DFT simulations, which reveal that the behavior of AlCl 4À , Cl À , and Al 3þ contribute a lot to the electrochemical performance of the AIBs.These results could further improve the design of electrode materials and electrolytes.These pioneering works promoted the development of the AIBs system.
The introduction of MXenes enhanced the electrochemical performance of AIBs, owing to their high capacity, conductivity, sufficient surface groups, and unique 2D lamellar structure.However, many problems need to be solved due to the mechanism of AIBs is not clear and more complicated than that of LIBs or SIBs.The intercalation of Al 3þ or [AlCl 4 ] À involves a series of complex cathode structural and compositional evolution, which makes it more difficult to uncover the reaction mechanism of AIBs.Fortunately, with the development of technology, more and more advanced characterization and in situ detection techniques are being developed, which provides a guarantee for exploring the working mechanism of AIBs.The electrochemical performance of MXene-based composite in AIBs is summarized in Table 1.
development of CIBs has been slow in recent years due to the structure of reported cathode materials cannot withstand the insertion and deinsertion of Ca 2þ during cycling.Under the circumstances, the emergence of MXenes with 2D lamellar structure, tunable interlayer spacing, and high conductivity gives an excellent opportunity to handle the problems of CIBs and improve their electrochemical performance.For instance, Xie et al. [87] demonstrated that Ti 2 CO 2 MXenes could be highperformance cathode materials for MgIBs through DFT simulations and showed a high theoretical Ca 2þ storage capacity of 487 mAh g À1 .Demiroglu et al. [119] adopted first-principles methods to analyze the adsorption and diffusion of Ca atoms on Ti 2 CO 2 MXenes/graphene surfaces, proved excellent Ca 2þ storage performance.Although theoretical simulation confirms the great potential of MXenes for Ca 2þ storage, there is still a lack of experimental results to demonstrate the feasibility of MXenes for Ca 2þ storage in CIBs.

Conclusion and Perspectives
MXene-based composites attract much attention owing to their extensive chemical and structural diversity.In this review, we have summarized the properties of MXenes and introduced the recent progress of MXene-based materials in LIBs, SIBs, PIBs, MgIBs, and AIBs.The main focus is on the synthesis and applications of MXene-based materials in MIBs, and their roles in electrochemical reactions.Further examples were provided to demonstrate the significant role of MXenes as active materials, substrates, collectors, and even precursors in different MIBs, highlighting the enormous potential of MXene-based materials to construct advanced electrodes.Although significant progress has been achieved, there are still several unresolved issues with MXenes and MXene-based composites for different types of MgIBs to date: 1) Synthesis of MXenes.The electrochemical performance of MXenes is highly dependent on the layer structure, interlayer spacing, and surface groups.In addition, the chemical components, pores, and morphology also affect the electrochemical performance of MXenes.Therefore, improving metal ion storage capacity could be achieved through controllable synthesis of MXenes.However, the present fabrication methods for MXenes from corresponding MAX precursors generally involve fluorine-containing etchants, which inevitably introduce harmful surface functional groups and are harmful to human beings.Recently, F-free and bottom-up strategies for controllable synthesis of a few layers of MXene film like chemical CVD, atomic layer deposition, and Lewis acid molten salts have attracted more attention.Although various synthesis strategies for MXenes with controllable morphology and properties have been developed, the yield of MXenes from these preparation methods is mostly produced in very small quantities, which is not conducive to the large-scale application of Reproduced with permission. [117]Copyright 2022, Elsevier.g) Schematic illustration of the discharging process in AIBs and theoretical simulation of AlCl 4À , Cl À , and Al performance of CoSe 2 @TiO 2 /Ti 3 C 2 and control samples at 1.6 A g À1 and rate performance.Reproduced with permission. [118]Copyright 2023, Wiley-VCH.
MXenes materials.Therefore, novel synthetic methods for MXenes are highly needed and remain challenging;

Figure 1 .
Figure 1.Statistics of publications based on MXenes from 1 January 2014 to 1 July 2023 by searching "MXene" and "Energy storage" as "topic" in the website of Web of Science.

Figure 3 .
Figure 3. a) Schematic diagram for the synthesis of Ti 3 C 2 T x MXene and corresponding TEM images.b) CV curves of Ti 3 C 2 T x and T 2 CT x MXene anodes.c) Specific capacity comparison of Ti 3 C 2 T x and T 2 CT x MXenes.d) Charge/discharge curves of the Ti 2 CT x MXene.e) EIS measurements of the Ti 2 CTx MXene electrode at various potentials.Reproduced with permission.[5]Copyright 2021, Nature Publishing Group.f ) Schematic illustration and SEM images for the MXene binders in the cathode and anode.g-i) Electrochemical characterizations of LTO/MX anode.Reproduced with permission.[67]Copyright 2023, Wiley-VCH.

Figure 4 .
Figure 4. a) Schematic illustration for the fabrication of Si@Ti 3 C 2 T x materials and the corresponding SEM and TEM images.b,c) Electrochemical performance of the different electrodes at 0.2 and 1 A g À1 .Reproduced with permission.[73]Copyright 2020, the American Chemical Society.d) Schematic diagram and SEM images for the fabrication of MoS 2 @MXene composite.e-h) Electrochemical performance of MoS 2 /Mo 2 TiC 2 T x heterostructures for LIBs.Reproduced with permission.[77]Copyright 2018, Wiley-VCH.

Figure 5 .
Figure 5. a) Simulation models of Ti 3 C 2 and Ti 3 C 2 -N and illustration of the SEI compositions in the Ti 3 C 2 -N anode.b-e) Electrochemical performance of Ti 3 C 2 and Ti 3 C 2 -N funct anodes in Na-ion half-cells and Ti 3 C 2 -N funct //NVPF full-cell.Reproduced with permission. [89]Copyright 2022, Springer Nature.f ) The schematic illustration for the fabrication of PO 2 -Nb 4 C 3 composite.g,h) Cross-sectional SEM images of the PO 2 -Nb 4 C 3 film.The initial three charging/discharging curves of the i) O-Nb 4 C 3 and j) PO 2 -Nb 4 C 3 electrodes.Cycling performance of the k) O-Nb 4 C 3 and l) PO 2 -Nb 4 C 3 electrodes at 0.1 and 1 A g À1.Reproduced with permission.[78]Copyright 2022, Wiley-VCH.

Figure 6 .
Figure 6.a,b) Schematic illustration of the synthesis and corresponding electrochemical performance of Na 2 C 6 O 6 /MXene paper.Reproduced with permission.[90]Copyright 2023, Springer Nature.c) Schematic illustration for the fabrication of the Fe xÀ1 Se x @MXene@nanofiber network.d,e) Cyclic and the rate performance of the Fe xÀ1 Se x @MXene@nanofiber network.f ) CV curves of different electrodes.Reproduced with permission.[92]Copyright 2021, Wiley-VCH.

Figure 10 .
Figure 10.a) Diagrammatic sketch of V 2 C@Se synthesis process.c) XPS spectrum of Al 2p, Cl 2p, C 1s, and Se 3d at different voltages which are shown in (b).d,f ) The adsorption energies of [AlCl 4 ] À and Al 3 þ on V 2 CO and V 2 CSe.e) Schematic illustration of the V 2 C@Se cathode in the discharging process.

2 )
Working mechanism of MXenes in MIBs.The working mechanism of MXene-based composites in multivalent MIBs needs to be further explored because of the unclear transfer and intercalation of multivalent ions in batteries.For example, the intercalation of Al 3þ or [AlCl 4 ] À involves a series of complex cathode structural and compositional evolution, which makes it more difficult to uncover the reaction mechanism of AIBs.Therefore, advanced in situ or ex situ detection techniques such as ex situ SEM, in situ electrochemical Raman, electrochemical quartz crystal microbalances, and neutron diffraction analysis should be further explored to reveal MXene-based composites in terms of composition and structure changes during the charge/discharge process; and 3) Practical applications of MXenes in MIBs.MXene-based composite greatly enhanced the electrochemical performance of MIBs, however, they were obtained mainly based on coin-type cells, far from practical application.First, MXene materials are very sensitive to oxygen and water, making the layer structure prone to collapse.Second, there are many scientific and technical issues of MXene-based materials in MIBs that remain unsolved.Third, the working mechanism of MXene-based electrodes in MIBs is still unclear.Therefore, the practical applications of MXene-based materials in MIBs still have a long way to go.Fortunately, the fastdeveloping nanotechnologies have delivered optimistic waves that MXene-based structures could realize promising electrodes in MIBs to enable practical applications.In summary, significant achievements of MXene-based materials have been made in the field of MIBs in the past decades.However, MXene-based electrodes in PIBs, MgIBs, and AIBs remain challenging due to the large ionic radius and strong interaction with the host material compared with LIBs and SIBs.Therefore, further research is still needed for the targeted design of the MXene-based composites and structure to adapt the application of the multivalent-ion systems.We look forward to more novel and creative breakthroughs in the MXenes-based electrodes for MIBs shortly.