Two-dimensional MXenes for lithium-sulfur batteries

Rechargeable lithium-sulfur (Li-S) batteries have attracted significant research attention due to their high capacity and energy density. However, their commercial applications are still hindered by challenges such as the shuttle effect of soluble lithium sulfide species, the insulating nature of sulfur, and the fast capacity decay of the electrodes. Various efforts are devoted to address these problems through questing more conductive hosts with abundant polysulfide chemisorption sites, as well as modifying the separators to physically/chemically retard the polysulfides migration. Two dimensional transition metal car-bides, carbonitrides and nitrides, so-called MXenes, are ideal for confining the polysulfides shuttling effects due to their high conductivity, layered structure as well as rich surface terminations. As such, MXenes have thus been widely studied in Li-S batteries, focusing on the conductive sulfur hosts, polysulfides interfaces, and separators. Therefore, in this review, we summarize the significant progresses regarding the design of multifunctional MXene-based Li-S batteries and discuss the solutions for improving electrochemical performances in detail. In addition, challenges and perspectives of MXenes for Li-S batteries are also outlined.


| INTRODUCTION
2][3][4] Comparing to Li-ion batteries and other metal-sulfur batteries, lithiumsulfur (Li-S) battery is considered as one of the promising electrical energy storage systems owing to the natural abundance of sulfur, high theoretical specific capacity (1675 mA h g −1 ) and energy density (~2670 W h kg −1 ), [5][6][7][8] as shown in Figure 1A.However, the soluble long chain lithium polysulfides (Li 2 S n , 4 ≤ n ≤ 8) produced in the discharge process disassociate into electrolyte and shuttle between cathode and anode (Figure 1B), typically known as the shuttling effect. 6,7This effect results in active material loss, causing rapid capacity decay, low cyclic performance and Coulombic efficiency.The Li 2 S n also reacts with the Li anode and forms an insulating layer, leading to serious polarization.In the meanwhile, the insulating nature of sulfur greatly lowers down the redox reaction Chuanfang (John) Zhang and Linfan Cui contributed equally.kinetics, thus reducing the rate capabilities and sulfur utilization.All of these abovementioned issues urgently require effective conductive hosts or separators to suppress the polysulfide migration process.In general, methods can be classified as physically confinements and chemically adsorption of the Li 2 S n . 70][11][12] A conductive polymer matrix can also help to encapsulate sulfur in the cathode and achieve good capacity and cycling performance. 12,13Graphene oxides with abundant surface functional groups, as well as the hydrophilic metal oxides on the other hand, trap the polysulfide diffusion through a chemical adsorption mechanism. 14,15ince the discovery of Ti 3 C 2 T x in 2011, 16 transition metal carbides and nitrides, so-called MXenes, have attracted extensive attention and exhibited excellent performances in energy, catalysis, optoelectronics, biomedical, environment, sensors, electromagnetic fields and so on (Figure 1C). 17MXenes with formula M n + 1 X n T x (n = 1-3) are usually derived from the parental MAX phases by selectively etching the A layer (where M represents a transition metal, A represents the group 13 or 14 elements in the periodic table and X represents carbon or nitrogen, T x stands for the surface terminations). 15,16,18,19Thus, the obtained MXenes possess a unique layered structure.After etching, the surface of MXenes is terminated with various functional groups such as hydroxyl ( OH), oxygen ( O), chlorine ( Cl) and fluorine ( F), leading to a good hydrophilicity in MXenes which is in sharp contrast with hydrophobic graphene nanosheets. 18,20Beyond the excellent electronic conductivity, MXenes have also exhibited impressive performances in many areas, especially in energy storage. 21hus, the past few years have witnessed the rise of MXenes, best evidenced by the ever-increasing number of publications on this new class of two-dimensional (2D) wonder materials (Figure 1D).Particular attention should be paid to the application of MXenes in Li-S batteries.The surface groups on MXenes (especially hydroxyl groups) are highly affinitive to polysulfides and can spontaneously attract them without additional surface modifications.Moreover, the highly conductive core (Ti C Ti bonds) can greatly facilitate the charge transfer kinetics, allowing great enhancement of sulfur utilization and cell rate handling as a result. 22,23Said otherwise, using MXenes as sulfur conductive host and/or modified separator can dramatically boost the long-term cycling and capacity of the assembled Li-S cells.
As a matter of fact, publications on the MXenes for Li-S batteries also increase apparently, suggesting this field gets more and more research attention (Figure 1E).As such, it is important to summarize the significant progresses covering the topics like conductive hosts, high sulfur loading, modified separators, as well as the theoretical simulations and calculations, and so forth.Herein, we review the recent progresses of MXenes for Li-S batteries.We especially focus on the structural design of the MXene-S electrode, as well as solutions for boosting the electrode/device performances.We also revisit the theoretical calculations based on density-functional theories (DFT) to simulate the interactions between MXenes and polysulfides.Finally, we present the challenge of MXenes for the Li-S batteries and outlook the future possible solutions to these challenges.

| MXENE: SYNTHESIS AND PROPERTIES 2.1 | Synthesis of MXenes
As the first discovered MXene, Ti 3 C 2 T x is produced by a wet-chemical etching process from MAX phase (Figure 2A). 16,18,19,21,24,25In this method, the etchant, typically aqueous hydrofluoric acid (HF), selectively etches away the A atomic layer from the MAX precursor as the M-A bond is more chemically active than the M-X bond.After etching, the solids are repeatedly washed with deionized water followed by centrifugation till the pH of the supernatant reaches 4-6, and multilayered (m-) Ti 3 C 2 T x MXene is obtained.Similarly, by adjusting the HF etching conditions such as HF concentration, etching time and temperature, other types of MXenes, such as Ti 2 CT x , Ti 3 CNT x , Nb 2 CT x and V 2 CT x have also been synthesized. 16,26,27n order to obtain individual 2D MXene nanosheets, delamination of the m-Ti 3 C 2 T x is necessary.This can be done by intercalating polar organic molecules such as hydrazine, urea, dimethyl sulfoxide (DMSO) and isopropylamine into the m-Ti 3 C 2 T x , followed by sonication to delaminate the sheets. 28,29In order to increase the yield of delaminated nanosheets, organic intercalants with large cation size, such as tetrabutylammonium hydroxide (TBAOH), choline hydroxide or n-butylamine, are used to give large amounts of delaminated nanosheet solutions, as shown in Figure 2B,C. 30After mechanical vibration or sonication in water, single or few-layer MXene solutions can be collected.
Unlike the HF acid etching, using lithium fluoride (LiF) and hydrochloric acid (HCl) mixture to produce HF in-situ can effectively etch away the A layer and result in Ti 3 C 2 T x MXene with Li ion (Li + ) pre-intercalated in the layered solids.Consequently, by repeated ion exchanging (through DI-water washing) and manual shaking/sonication, delaminated nanosheets, with predominantly monolayered flakes enriched in the solution, can be effectively prepared. 21,32Compared to the nanosheets produced via direct HF etching, the flakes prepared from the LiF-HCl route possess a cleaner surface with much less defects, and thus termed as minimally intensive layer delamination (MILD) route. 33,34Ti 3 C 2 T x MXene can also be produced upon etching in the ammonium bifluoride salt (NH 4 HF 2 ). 35Similarly, using molten salts, that is, KF, LiF, NaF is another route to effectively etch away the A layer and produce good quality MXenes. 36evertheless, the abovementioned etching-delamination routes produce Ti 3 C 2 T x nanosheet solutions at a low yield, typically <20%.To boost the yield of nanosheets, other novel methods such as electro-chemical etching and microwave-assisted delamination strategies have been developed. 37,38During the electro-chemical etching process, Ti 3 AlC 2 can be etched in the Cl − containing electrolyte under a low potential as Cl − has a strong binding capability with Al.The microwave-assisted delamination strategy utilizes an agitation-intercalation-exfoliation process with organic solvent/ionic liquid media to break the interactions among the Ti 3 C 2 T x nanosheets.
In particular, through employing a hydrothermalassisted interaction strategy, the yield of Ti 3 C 2 T x sheets can achieve 74% (Figure 3A). 39In this process, the intercalation process is facilitated under the hydrothermal conditions, thus promoting the delamination yield.On the other side, as the HF acid is highly corrosive, other safer strategies, such as alkali-etching method has also been emerged to obtain Ti 3 C 2 T x (Figure 3B). 40hile the wet-chemistry method is able to efficiently synthesize carbide MXenes, it is quite challenging to prepare nitride MXenes using this method.Instead, nitride MXenes can be fabricated by high temperature etching of the MAX phase.A typical example is the synthesis of Ti 4 N 3 T x , which was obtained by etching the Ti 4 AlN 3 MAX phase in a molten fluoride salt mixture under 550 C in argon atmosphere (Figure 3C). 36By ammonization of the carbide MXene (ie, Mo 2 CT x and V 2 CT x ) at 600 C, corresponding Mo 2 NT x and V 2 NT x (mixed with cubic VN) MXenes can be effectively prepared. 42Nevertheless, the production yield of nitride MXenes is low based on the ammonization of carbide MXene route.Said otherwise, more efficient routes to the nitride MXene synthesis are greatly in need to continuously expand the MXene family.
In addition, carbides with double or triple A layers have also been selected as precursors to synthesize MXenes. 43,44Mo 2 CT x can be prepared from Mo 2 Ga 2 C by etching two A-element layers while Zr 3 C 2 T x is synthesized from Zr 3 Al 3 C 5 by etching Al 3 C 3 . 44On the other hand, molybdenum carbide with large lateral size and less defects can also be prepared through a chemical vapor deposition (CVD) method. 41As shown in Figure 3D, ultrathin α-Mo 2 C has been produced through CVD on a copper foil which was placed on a molybdenum foil.During the F I G U R E 3 A, Schematic of hydrothermal-assisted intercalation strategy to synthesize Ti 3 C 2 T x .Reproduced with permission. 39Copyright 2019, American Chemical Society.B, Schematic of fluorine-free etching method to synthesize MXene via the reaction between Ti 3 AlC 2 and NaOH water solution under different conditions.Reproduced with permission. 40Copyright 2018, Wiley-VCH.C, Synthesis process of Ti 4 N 3 T x using a high temperature etching method.Reproduced with permission. 36Copyright 2016, The Royal Society of Chemistry.D, Schematic of the synthesis process of α-Mo 2 C crystals via CVD method.Reproduced with permission. 41Copyright 2015, Wiley-VCH reaction, the Mo atoms diffuse to the top Cu surface and combine with the decomposed carbon atoms, forming α-Mo 2 C nuclei which grow continuously along the epitaxial direction.As a result, the α-Mo 2 C flakes are of high quality, allowing the investigation of their intrinsic properties such as electrical, mechanical and optoelectronic properties. 41,45

| Properties of MXenes
MXene crystals possess a hexagonal close-packed stacking structure where M atoms are closely packed and X atoms fill the octahedral interstitial sites. 21The MXene family usually has three packing configurations, that is, M 2 X, M 3 X 2 and M 4 X 3 with the corresponding single sheets of 3, 5, 7 layers, rendering the great diversity of the MXene family (Figure 4A).Recently, Anasori et al reported the ordered double transition metal MXenes, where one (or two) transition metal layer(s) is (are) sandwiched between the second transition metal layers (as shown in Figure 4B). 25Such an atomic structure is totally different from the solid-state solutions MXenes, where the different metal atoms are distributed randomly.The discovery of the ordered double transition metal MXenes have greatly enriched the MXene family, opening up new possibilities in tuning the band gap, work function, and optoelectronic properties of MXenes.
Typically, MXenes synthesized via HF or HFcontaining solution possess abundant hydrophilic surface functional groups ( OH, O, and F).In order to modify the surface groups, especially to produce MXenes with reduced surface functionalities, annealing MXenes under argon (Ar) has been proved to be quite effective. 46For example, by annealing the Ti 3 C 2 T x MXene in Ar at 500 C, most of the surface groups can be removed.It is quite challenging to produce MXenes with one specific surface group.Nevertheless, many studies assume that MXene possess only one type of surface groups, which greatly simplify the simulation process.For instance, one can predict the locations of the hydroxyl groups by assuming Ti 3 C 2 T x MXene is terminated with OH groups only.As shown in Figure 4C, three possible configurations are predicted to exist according to the principle of energetically favorable termination orientations. 47In configuration X, OH groups are positioned above the hallow sites between the neighboring C atoms.In configuration Y, OH groups are located above the C atoms toward both sides of the Ti 3 C 2 layers.Configuration Z is defined as one side of the Ti 3 C 2 is in configuration X and the other side is in configuration Y.Other possible configurations are unstable and tend to transform into these three configurations. 48Moreover, configuration X is found to have the lowest energy, suggesting the highest structure stability.In other words, the OH groups are more likely to arrange in configuration X in Ti 3 C 2 T x MXene.
In practice, MXene is terminated with mixed types of groups (OH, O, and F terminations), rendering the DFT calculations much more complex. 28Furthermore, water molecules are typically trapped among the layers, 49 which for sure have a great influence on the stability, conductivity and capacitance in the energy storage device.In other words, understanding the surface groups (including distribution and arrangement of surface terminations) as well as the trapped water (and the water removal if needed) are useful for both theoretical studies and the resultant MXene properties.This is especially true for some applications that require MXenes with specific properties, which are typically obtained by engineering the surface functionalities and interlayer chemistries.
Unlike 2D graphene material, delaminated MXenes with single or few layer flakes are hydrophilic, and thus can form a stable dispersion in aqueous media without the addition of surfactant.In addition, due to the match of the Hansen solubility parameters, MXene nanosheets can be dispersed in a range of organic solvents, forming stable MXene organic solutions without the necessity of a binary solvent. 50We note this is a great advantage over graphene, allowing facile solution processing of MXene dispersions into any items or composites, and opening up great opportunities in thin-film coating, inkjet/extrusion printing, and many other applications. 51When dispersed in aqueous media, MXene nanosheets are vulnerable to oxidation by dissolved oxygen and water, highlighting the proper storage of the delaminated nanosheets solution.Zhang et al revealed that by isolating the solution from the dissolved oxygen through Ar-filling hematic bottles, which were placed in a low-temperature environment, the shelf-life of Ti 3 C 2 T x and Ti 2 CT x aqueous solutions were greatly extended. 52Recently, Mochalin et al suggested that water is more pronounced in oxidizing MXene nanosheets; the solution's stability was much improved when dispersing the nanosheets in isopropanol. 53Actually, by dispersing Ti 3 C 2 T x nanosheets in organic solvents like N-Methyl-2-Pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), the dispersions are stable over 12 months, showcasing the long-term stability. 54ccording to DFT calculations, the electronic properties of Ti 3 C 2 T x layers are surface functionalities dependent.The theoretical calculations shows that bare Ti 3 C 2 is a metallic conductor, which switches to a semiconductor as the surface is terminated by OH or F (bandgaps are 0.05 eV and 0.1 eV for Ti 3 C 2 [OH] 2 and Ti 3 C 2 F 2 , respectively), as shown in Figure 5A. 16  Reproduced with permission. 16Copyright 2011, Wiley-VCH.B, Band structures near the Fermi level for (I) Reproduced with permission. 48Copyright 2012, American Chemical Society.C, Optical images of Ti 3 C 2 T x films on glass (I) and polyester (II) substrates, respectively (scale bars are 1 cm).(III) UV-vis spectra of Ti 3 C 2 T x films with different thicknesses.Reproduced with permission. 63Copyright 2016, Wiley-VCH O-terminated MXenes are also predicted to be semiconducting. 55Thus, the electronic properties of MXenes such as the bandgaps can be tuned by varying surface functional groups.The locations of surface functionalities also influence the band gap structures of MXenes. 48or instance, Ti 3 C 2 F 2 and Ti 3 C 2 (OH) 2 in the configuration Y (Figure 4C) are predicted to be metallic.In contrast, Ti 3 C 2 F 2 and Ti 3 C 2 (OH) 2 in configuration X and Z are shown to be semiconductors with narrow bandgaps (X-Ti 3 C 2 F 2 :0.04 eV, Z-Ti 3 C 2 F 2 :0.03 eV, X-Ti 3 C 2 (OH) 2 :0.05 eV, Z-Ti 3 C 2 (OH) 2 :0.07 eV) (Figure 5B).There are also some MXene phases with heavier transition metals such as chromium, molybdenum and tungsten that are predicted to be topological insulators. 24,56,57he electrical conductivity of Ti 3 C 2 T x heavily depends on the preparation method.][60] Thanks to the strong M-X bonds, MXenes are predicted to possess enhanced mechanical properties compared to their parental MAX phases. 61Moreover, M 2 X MXenes are predicted to be stiffer and stronger than their M 3 X 2 and M 4 X 3 counterparts based on the DFT and molecular dynamics results.The elastic modulus of single Ti 3 C 2 T x sheet terminated with -OH is calculated to be ~300 GPa along the basal plane. 16When well dispersed in a polymer matrix, the mechanical strength of Ti 3 C 2 T x layers can be further reinforced by compositing with various polymers, 18 opening up vast opportunities in fabricating items with strong mechanical properties.Other properties like magnetic properties have been predicted for some MXenes with a bare surface while the magnetism disappears once the surface is terminated by the functionalities.Exceptions are Cr 2 C and Cr 2 N MXenes, which are predicted to retain magnetic properties in the terminated state at nearly room temperature. 55The MXene thin films are transparent and Ti 3 C 2 T x monolayer transmits >97% visible light which can be tuned by chemical and electrochemical intercalation of cations (Figure 5C). 60,62,63

| Polysulfide anchoring behavior of MXenes
Considering the notorious polysulfide shuttling effect, an ideal sulfur host should be conductive and possesses a strong affinity to Li 2 S n so as to suppress the migration kinetics of the latter.2D MXenes, especially titanium carbide MXenes, have demonstrated high electronic conductivity and strong polysulfide adsorption capabilities, thus have been received substantial research attention for Li-S application.
Nazar's group reported Ti 2 CT x MXene as sulfur host for the first time to entrap polysulfides and achieved good capacity retention. 22Interaction and chemisorption mechanism of polysulfides with the active species (mainly Ti sites and OH surface groups) are postulated.As shown in Figure 6A, polysulfides are firstly chemisorbed on the MXene surface and undergo redox reactions with the OH terminations, forming thiosulfate groups.Those surface thiosulfate groups further react with the soluble polysulfides to form polythionates and eventually insoluble "lower" polysulfides. 23,64Such a polysulfide entrapping mechanism is similar to that of MnO 2 and graphene oxide. 64The cleavage of surface OH groups exposes Ti atoms with unoccupied orbitals, which greatly facilitate the chemisorption of electronegative polysulfide ions and form Ti S bonds with high binding energy via Lewis acidbase interaction. 22Thus, Ti-based MXenes with dual-mode entrapping behaviors have exhibited excellent performance in suppressing the polysulfides dissociation and migration back to the electrolyte, resulting in MXenebased Li-S batteries with stable cycling life.
In addition, an ideal host should possess a relatively mild adsorption strength with Li 2 S n intermediates so that the conversion kinetics to the low-order Li 2 S n is facilitated without apparent Li 2 S n intermediate dissolution.
Otherwise, if the adsorption strength is too strong, the high-order Li 2 S n tends to decompose and then S atoms are easy to diffuse into electrolyte.
Taking Ti 2 C as an example, when S atoms disperse on the surface of bare Ti 2 C, the distance between S atoms and Ti atoms is short (ie, close contact), thus the S Ti bonds are so strong that the bare Ti atomic layer can further "grab" sulfur atoms by breaking the Li S bonds in Li 2 S n and form Ti S bonds.Once the Ti S bonds are formed, sulfur atoms are unable to dissociate from the Ti S bonds and form Li 2 S n again by combining with Li + (Figure 6B).Although the interaction between Ti and S becomes weaker if the surface is terminated with functional groups, the surface functionalized groups also can interact with Li 2 S n intermediates and may help to strike a balance between the interaction strength and structural intactness of the Li 2 S n .
Most theoretical studies on Ti-based MXenes  69 On the O-functionalized surface, Li 2 S n tends to be oxidized to neutral S which exhibits slight solubility.For the F-functionalized Ti 2 C, it exhibits strong interaction with Li 2 S n intermediates.
Figure 6C(I) shows the binding energy of Li 2 S n on various functionalized Ti 2 C MXene. 65The differences of the binding energy could be originated from the Coulomb interactions between Li 2 S n and MXenes (Figure 6CII).Bare MXenes possess strong Coulombic interactions and thus the induced Ti-S interactions are strong.When the Ti 2 C surface is terminated with functional groups, Ti-S interactions will be reduced by the repulsive force from negatively charged atoms (O/F to S). Li atoms also play a role in the Li 2 S n binding.Although the interaction between Li and O/F could enhance the binding of Li 2 S n , these groups lead to a larger Ti-S distance and weaken the Li 2 S n binding effect. 65This is why the binding energy of Li 2 S n on bare MXenes is much stronger than that of functionalized MXenes.Since the O atoms in Ti 2 C(OH) 2 are passivated with H atoms, the repulsive forces caused by negatively charged O atoms are reduced. 65The positively charged H atoms can enhance the attraction for S atoms.As a result, firm interactions between Li 2 S n and Ti 2 C(OH) 2 are obtained.
Some functional groups such as S and Cl can also be modified on the MXene surface to further suppress Li 2 S n shuttling.S terminated Ti 2 C is calculated to possess a higher efficiency to restrain the polysulfide shuttle as its binding energy is higher than that of O/F terminated Reproduced with permission. 68Copyright 2019, Elsevier Ti 2 C. S terminated Ti 3 C 2 T x also exhibits the highest adsorption strength when compared with pristine Ti 3 C 2 T x (T x = OH, O, F) (Figure 6D). 66,70he theoretical analysis performed by Zhao et al indicates that Ti 3 C 2 (OH) 2 becomes unstable after absorption of Li 2 S n species; H atoms spontaneously migrate to S atoms of the Li 2 S n while the remained O atoms interact with Li atoms, leading to the breakage of the Li 2 S n species eventually. 72While the F-functionalized Ti 2 C showcases weak binding energy of Li 2 S n , the Ti 2 CO 2 and Ti 3 C 2 O 2 MXenes exhibit similar binding energies for Li 2 S n compared to that between Li 2 S n and electrolyte.As a result, oxygenterminated MXenes are more efficient in chemisorption of the polysulfides. 72Nevertheless, these DFT calculations were performed on the interactions between polysulfides and MXenes terminated with only one type of functional group, which is different from the real case where different types of functional groups may coexist on the MXenes, and may change the polysulfides suppression mechanism. 71he interaction behavior between Li 2 S n and other MXenes has also been theoretically studied which may guide future rational selection of MXenes to act as sulfur hosts for Li-S batteries. 67,72Fan's group investigated the Li 2 S n anchoring behavior of five O-functionalized MXenes (M 3 C 2 O 2 , M = Cr, V, Nb, Hf and Zr) through DFT calculations. 67The study suggests that all the selected MXenes possess higher binding energies with Li 2 S n than that with organic electrolytes (Figure 6EI), among which Cr 3 C 2 O 2 MXene shows the strongest anchoring effect toward Li 2 S n based on a lattice constant-dependent anchoring effect. 67he interactions with Li 2 S n mainly originate from the Li-O covalent bonds (Figure 6EII).When the lattice constant increases, the Li-O bond length extends and then structural distortion happens which weakens the interaction between Li 2 S n and M 3 C 2 O 2 , reducing the binding energy. 674][75][76] Titanium nitride based MXenes are also potential conductive sulfur hosts with quite promising performance.Specifically, the anchoring mechanism of O-and F-functionalized Ti 2 N has been investigated. 68ased on the charge transfer analysis, chemical interactions are confirmed between Li 2 S n and Ti 2 NO 2 , leading to a large adsorption energy (Figure 6F).For the Ti 2 NF 2 , the contribution to interactions is mainly van der Waals (vdW) interaction with less charge transfer from Li 2 S n to Ti 2 FO 2 .

| Catalytic behavior of MXenes in Li-S batteries
Apart from the Li 2 S n shuttling effect, the final discharged product Li 2 S with high decomposition energy would also compromise the electrochemical performances such as high overpotential and low rate capability.To this end, materials with high catalytic capability can effectively reduce the overpotential, promote the charge transfer kinetics process and improve the rate performance as a result.The 2D MXenes with various surface terminations exhibit unique catalytic capabilities and thus can be employed to address these issues. 66,77,78Surface-terminated Ti 3 C 2 T x typically demonstrates strong interactions with Li 2 S n while possessing a low Li 2 S decomposition barrier. 66 Appropriate functional group vacancies could promote the interactions with Li 2 S n species. 70For example, Ti 3 C 2 Cl 2 exhibits poor adsorption capability and electrode reaction kinetic as the vdW interaction with Li 2 S n species dominates the lithiation process.However, when Cl vacancies form on the Ti 3 C 2 Cl 2 surface, the chemical interactions with Li 2 S n could be increased, leading to improved Li 2 S n adsorption capacity and further lowering down the decomposition energy of Li 2 S due to the presence of Cl vacancies (Figure 7C,D).Thus, surface modification is an efficient method to adjust the electrocatalysis behavior and to improve the electrode kinetics for boosting the electrochemical performances of Li-S batteries.Besides that, the total number of atomic layers of carbide-based MXenes also has an effect on catalytic ability. 79s a crucial aspect for Li 2 S n nucleation and decomposition, fast Li + diffusivity can assist the electrochemical process as well.Calculations on Ti 3 C 2 T x demonstrate much lower Li + diffusion barriers than that for graphene (Figure 7DIII-DVII). 66V-based MXenes with different surface terminations have also been studied, revealing that V 2 CS 2 has a lower energy barrier for Li + diffusion, facilitating the conversion of Li 2 S n into low order Li 2 S (Figure 7E). 7873,78 We note that the good preservation of metallic host even after chemisorption of Li 2 S n is beneficial for both the continuous redox reactions and the enhancement of active S utilization.
Based on the above discussions, MXenes possess substantial advantages in Li-S batteries due to their unique properties including surface chemistry, metallic conductivity and ultrathin nanosheet morphology, and so forth.
Understanding the Li 2 S n anchoring and catalytic behaviors of MXenes are important for designing highperformance Li-S batteries, which will be discussed below.

| Design of MXenes-based sulfur host in Li-S cathode
Introducing functional additives and constructing advanced structures for sulfur hosts are desirable strategies to entrap Li 2 S n in the cathode and suppress the shuttle problem.Thanks to the strong interaction with polysulfides, MXenes with excellent electronic conductivity have been developed as highly effective sulfur cathode host materials, achieving improved cycling performance. 23,802][83][84] Beside the utilization as sulfur cathode, MXenes can also be used to fabricate Ti 3 C 2 T x /Li 2 S cathodes to further improve the Li 2 S n trapping ability. 85elaminated MXene sheets are easily restacked due to the vdW interactions which may hinder the chemical interaction with polysulfides.To improve the MXene nanosheet utilization, one desirable strategy is to develop MXene based hybrids through introducing spacers among  66 Copyright 2019, American Chemical Society.E, The Li 2 S decomposition paths and Li + diffusion on (I) V 2 CS 2 , (II) V 2 CO 2 and (III) V 2 C, and energy profiles.Reproduced with permission. 78Copyright 2019, The Royal Society of Chemistry the MXene layers.Nazar's group reported a porous and conductive Ti 3 C 2 T x /carbon nanotube (CNT) structure as sulfur host. 23The CNTs dispersed among the MXene sheets effectively suppress the nanosheets restacking, and thus achieve a high surface area (MXene/CNT: 350 m 2 g −1 , exceeding the sum surface area of MXene and CNT) which is beneficial for both physically confinement of Li 2 S n and enhanced utilization of chemisorptive MXene polar sites.Interconnecting CNTs in the MXene can maintain high electrical conductivity and facilitate the electron transport.Carbon fibers (CF) 86 and reduced graphene oxide (rGO) 87 are typically used to hybridize with MXenes so as to alleviate the MXene nanosheet restacking phenomenon (Figure 8A).The alternative strategy is to construct three-dimensional (3D) MXene architectures by introducing Ti 3 C 2 T x sheets into porous skeletons as polysulfide reservoir to improve the utilization of the Ti 3 C 2 T x sheets.For example, the Ti 3 C 2 T x /mesoporous carbon matrix exhibits a higher surface area of 1531.9 m 2 g −1 and pore volume of 0.577 cm 3 g −1 than those of restacked Ti 3 C 2 T x /mesoporous carbon mixtures, suggesting the important role of the 3D matrix. 88Besides the mesoporous carbon, MXene/graphene aerogel also provides large spaces for encapsulating sulfur and accommodating the volumetric expansion of the cathode 89,90 (Figure 8B,C).
Utilizing conductive carbon materials as additives can effectively improve the electrochemical performances of MXene/S cathode.However, the weak affinity among different host materials increases the interfacial transfer resistance and slows down the reaction kinetics of sulfur species.To improve the affinity to sulfur, a crumpled nitrogen-doped Ti 3 C 2 T x (N-Ti 3 C 2 T x ) nanosheet with welldefined porous structure was developed by Wang's group, as shown in Figure 9A(I). 91This optimized MXene structure possesses a 10 times enhanced surface area (385.4 m 2 g −1 ) and large pore volume (0.342 cm 3 g −1 ) compared with those of the mechanically mixed Ti 3 C 2 T x (surface area: 30 m 2 g −1 , pore volume: 0.0321 cm 3 g −1 , Figure 9A[II]).Such nitrogen-doped MXenes also showcase a greater capability to adsorb polysulfides than that of pure MXenes, demonstrating an improving reversible capacity of 950 mAh g −1 after 200 cycles at 0.2C (Figure 9A[III]).Beyond nitrogen doping to promote the surface polarity, the redox reactivity of sodium polysulfides can also be enhanced by incorporating sulfur surface groups on MXenes to boost the performance of Na-S batteries. 92In another case, Ti 3 C 2 T x nanodots as spacer were integrated among the Ti 3 C 2 T x nanosheets (TCD-TCS), as shown in Figure 9B(I). 93Such a nanostructure exposes more surface terminations and provides large amounts of active sites to trap the Li 2 S n , thus boosting the overall performances of the electrode (Figure 9BII-IV).
Materials with polar sites such as TiO 2 , polydopamine (PDA), MnO 2 , and MoS 2 have also been composited with MXenes as sulfur hosts to increase the active polysulfide entrapping sites and to enhance the Li 2 S n immobilization F I G U R E 8 Various designs to prevent MXenes from restacking.A, Schematic illustration of Ti 3 C 2 T x /rGO fabrication.Reproduced with permission. 87Copyright 2017, Wiley-VCH.B, Synthesis procedure of the Ti 3 C 2 T x /mesoporous carbon (Meso-C) composites.Reproduced with permission. 88Copyright 2016, Wiley-VCH.C, Preparation process of the Ti 3 C 2 T x /rGO aerogel electrodes.Reproduced with permission. 895][96][97][98][99] Figure 10 summarizes the detailed materials synthesis strategies, including oxide loading, freeze-drying, polymer coating, pumping, solvothermal and hydrothermal, and so forth.The as-obtained Ti 3 C 2 T x based heterostructures well preserve the 2D layered geometry and alleviate the nanosheet restacking, greatly opening up more polysulfide entrapping sites.
For instance, MXene decorated with TiO 2 quantum dots (QDs) as sulfur host showcases quite promising performance for achieving fast and stable Li-S batteries. 98ompared with a MXene/S cathode which displayed 308 mAh g −1 , TiO 2 QDs@ MXene/S delivered a much higher capacity of 680 mAh g −1 at 2C (1C = 1675 mA g −1 ) after 500 cycles (Figure 11A), due to the stronger adsorption ability of Li 2 S n species in the latter. 98Similarly, another polar hybrid sulfur host was achieved by confining conductive MXenes into 1T-2H MoS 2 -nitrogendoped carbon (Ti 3 C 2 T x /1T-2H MoS 2 -C) composites, demonstrating a higher initial capacity of 1194.7 mAh g −1 at 0.1C than that of MXene/S electrode (845.1mAhg −1 ) (Figure 11B).After cycling, the black surface of Ti 3 C 2 T x /1T-2H MoS 2 -C cathode without obvious sulfur spots, coupled with the flat, shinning Li metal surface, suggests that the Li 2 S n shuttling has been effectively suppressed and the smooth deposition of Li during upon repeated charge-discharge processes. 99hile titanium carbides have been widely studied as sulfur host, other types of MXenes, with different  93 Copyright 2019, American Chemical Society electronic conductivity, surface chemistry and other properties, certainly deserve to be explored for the Li-S battery applications.For instance, Mo 2 CT x MXene was confirmed with good capability in immobilizing polysulfides due to the Lewis acid-base interaction from the Mo atoms with Li 2 S n . 100By interweaving with CNTs, a highly conductive Mo 2 CT x MXene-CNT hybrid was developed as sulfur host, as shown in Figure 12A.The Mo 2 CT x /CNTs electrode delivered excellent electrochemical performances in terms of high capacity, good rate capability and high initial reversible capacity at various sulfur loading (1314 mAh g −1 at 1.8 mg cm −2 sulfur loading, 959 mAh g −1 at 5.6 mg cm −2 sulfur loading) (Figure 12A). 100][103] We note that metal carbides play important roles in enhancing the polysulfide anchoring, improving the electrocatalytic kinetics and accelerating the polysulfide conversion into lower-order Li 2 S (Figure 12D).Similarly, tungsten carbide (W 2 C) was reported as an efficient sulfur host with stable cycling for Li-S batteries, which could be ascribed to the enhanced adsorption and catalytic sites on W 2 C. For example, decorating W 2 C nanoparticles on the F I G U R E 1 0 A, Schematic illustration of the Ti 3 C 2 T x /TiO 2 /S composite synthesis.Reproduced with permission. 94Copyright 2019, Elsevier.B, Schematic illustration shows the synthesis processes of Ti 3 C 2 T x /MnO 2 /S composites.Reproduced with permission. 95Copyright 2019, American Chemical Society.C, Schematic illustration of Ti 3 C 2 T x /PDA/S fabrication.Reproduced with permission. 96Copyright 2019, Elsevier.D, Schematic illustration of the synthesis process of TiO 2 /Ti 2 C/S composites.Reproduced with permission. 97Copyright 2019, Elsevier CNF (W 2 C NPs/CNFs) and loading W 2 C nanoclusters on the nitrogen-phosphorous co-doped carbon matrix (W 2 C/ N/P-rGO) (Figure 12EI) resulted in composites with enhanced polysulfides interaction and improved electrochemical kinetics. 102,104The W 2 C/N/P-rGO composites delivered high reversible capacities under different sulfur loadings (Figure 12EII,III). 104

| MXene as separator in Li-S battery
Another important aspect of MXenes in Li-S battery is the employment as separator.As we know, Li 2 S n diffusion not only causes active mass loss, but also results in unfavorable reactions with the Li anode, causing significant polarization. 86][107][108] For example, when inserting a CNT freestanding interlayer between MXene/S cathode and separator, the separator almost remained its original color after cycling and achieved better electrochemical performances compared to the cell without CNT interlayer (the color of the separator changed to a light yellowish color after cycling), indicating the inserted CNT membrane efficiently block the migration of Li 2 S n . 109nspired by this design, Wang et al modified the separator by coating a MXene thin film onto the battery  99 Copyright 2018, Wiley-VCH Celgard separator, in which the coated conductive MXene film acts as a second current collector to reduce the electron transfer resistance as well as suppresses the polysulfide shuttling effects, 110 as shown in Figure 13A.Consequently, Li-S batteries with a Ti 3 C 2 T xmodified separator exhibit better cycling retention and reversibility than that of the pristine battery. 110Based on the preliminary research on the MXene for modifying separators, extensive studies have been devoted to optimizing the structures such as Ti 3 C 2 T x /GF, 111 Ti 3 C 2 T x /eggshell membrane (ESM), 112 Ti 3 C 2 T x /CNT, 113 N-Ti 3 C 2 T x /C 114 and Ti 3 C 2 T x /Nafion 115 (Figure 13B-E).MXene-modified separators also facilitate good areal capacity under high sulfur loading.For a Ti 3 C 2 T x /Nafion modified polypropylene (PP) separator, the Li-S cell achieved a reversible areal capacity of 5 mAh cm −2 with sulfur loading of 6.0 mg cm −2 . 115When loading an impressively high amount of sulfur (10 mg cm −2 ), very high areal capacity of 6.3 mAh cm −2 was achieved with the N-Ti 3 C 2 T x /C coated PP separator. 114t should be noted that the coated MXene-based composites usually have a dense laminar structure which may hinder the effective infiltration of electrolyte.Meanwhile, insulating Li 2 S/Li 2 S 2 products keep accumulating on the separator, leading to increased interfacial charge transfer resistance and sluggish ion transport kinetics.As a result, serious polarization and capacity decay especially under high sulfur loading or at high rate may occur.In addition, the MXene nanosheets stacking behavior (ordered vs random) is also important for  102 Copyright 2018, American Chemical Society.C, Schematic illustration of the preparation strategy for MoC 2 /C/S composites.Reproduced with permission. 103Copyright 2019, Elsevier.D, Schematic illustration of the metal carbide/CNF hybrid electrodes to improve the performance of Li-S batteries.Reproduced with permission. 102Copyright 2018, American Chemical Society.E, (I) Schematic illustration of the preparation strategy of W 2 C/N/P-rGO/S composites.(II, III) Rate ability of W 2 C/N/P-rGO/S cathode under different sulfur loadings.Reproduced with permission. 104Copyright 2019, Wiley-VCH achieving high Li-S performances.In any case, the modified layer partially suppresses the diffusion of Li 2 S n species and inhibits Li + mobility to some extent, highlighting the importance of optimization on the MXene membrane thickness. 116

| Comprehensive Li-S cell design based on MXenes
To further improve the Li-S battery performance, some strategies are proposed by comprehensively designing MXene-based cathodes and separators.For instance, Wu et al reported the employment of Ti 3 C 2 T x MXene for hosting sulfur and modifying the separator. 117By using 3D alkalized Ti 3 C 2 T x MXene nanoribbon as sulfur host, as well as a 2D Ti 3 C 2 T x nanosheet coated on the separator, a high S loading coupled with high reversible capacity (1062 mAh g −1 at 0.2C) and energy density (833 Wh kg −1 ) were achieved, as shown in Figure 14A.Very recently, an antifouling separator of self-assembled Ti 3 C 2 T x / CNT-polyethyleneimine (Ti 3 C 2 T x /CNT-P) composite was reported (Figure 14BI). 118The nanohybrid based separator possesses well-developed 3D conductive channels and good affinity to polysulfides.These advantages from the hybrid membrane allow rapid electrolyte transport as well as fully utilization of active species.As a result of the comprehensive cell design, the pouch cell demonstrated excellent rate performance (1110, 1035 and 950 mAh g −1 at 0.5C, 1C and 2.5C, respectively) and stable cycling performance under high sulfur loading of 5.8 mg cm −2 (Figure 14BII,BIII). 118ombining the advantages of Li 2 S n immobilization and high electrocatalytic properties, a multifunctional Ti 3 C 2 T x /TiO 2 heterostructure was prepared by a controlled oxidation process of MXenes. 119The in-situ formation of Ti 3 C 2 T x /TiO 2 heterostructure with stronger Li 2 S n chemisorption capability ensures smooth Li 2 S n diffusion to the MXene and then accelerate the conversion to low-order Li 2 S 1-2 (Figure 14CI).The Ti 3 C 2 T x /TiO 2 could also act as interlayer for the Li-S pouch cell with stable cycling, ultralow capacity decay rate (0.028% per cycle over 1000 cycles at 2C) and excellent capacity retention (93% after 200 cycles) even at a high sulfur loading of 5.1 mg cm −2 (Figure 14CII). 1191][122] Therefore, utilizing a highly conductive host is quite necessary so as to maintain good electron transport paths after loading a large amount of insulating sulfur.Constructing 3D microporous channels to form MXene foam, as well as increasing interlayer spacing by pillaring the nanosheets in order to increase surface area, are promising routes for achieving high sulfur loading electrodes. 23,86,89,93,99,114,115,118For example, the self-supported Ti 3 C 2 T x foam was able to uniformly accommodate a large amount of sulfur due to the well-defined porous structure.Consequently, the cell still maintained 689.7 mAh g −1 after 1000 cycles with sulfur loading of 5.1 mg cm −2 (Figure 15A). 123ecently, Wang et al designed a flower-like porous Ti 3 C 2 T x nanomesh, in which well-aligned conductive Ti 3 C 2 T x nanosheets provide abundant active sites to entrap polysulfides, allowing rapid electron/ion transport and electrolyte penetration for fast redox reactions (Figure 15B). 124Based on this structure, the sulfur mass loading was improved to 10.5 mg cm −2 , resulting in high areal and volumetric capacities (10.04 mAh cm −2 and 2009 mAh cm −3 ) (Figure 15C).The abovementioned Ti 3 C 2 T x nanodots in nanosheets electrode configuration (TCD-TCS) demonstrated the highest sulfur loading (13.8 mg cm −2 ) among the reports, leading to an areal capacity of 13.7 mAh cm −2 that is three times higher than that of commercialized Li-ion batteries (~4 mAh cm −2 , Figure 15D). 93

| Novel design for MXene-based Li-S battery
To match the trends of the fast-growing portable and wearable devices, power sources should also be flexible and robust enough.This requires the sulfur electrodes to be highly resilient while exhibiting excellent capacities upon bending or twisting.To address these challenging issues, a general strategy is to embed sulfur into a conductive matrix with impressive mechanical properties. 125As detailed previously, 2D Ti 3 C 2 T x MXenes possess metallic conductivity and the nanosheets showcase an ultrahigh  124 Copyright 2019, American Chemical Society.D, (I) Schematic of Ti 3 C 2 T x TCD-TCS/S composites, (II) cycling performance of Ti 3 C 2 T x TCD-TCS electrodes under sulfur loading of 13.8 mg cm −2 at 0.05C.Reproduced with permission. 93opyright 2019, American Chemical Society tensile strength of 300 MGa. 126Thus, delaminated Ti 3 C 2 T x MXene nanosheets can be used as conductive binder and backbone for flexible sulfur cathode.For example, a free-standing Ti 3 C 2 T x electrode with ultramicroporous channels was employed to encapsulate small sulfur molecules (Ti 3 C 2 T x -UMC/S 2-4 ), achieving both excellent flexibility and high charge-storage performances. 127The MXene/1T-2H MoS 2 -C electrode also showcases excellent mechanical properties, which can withstand repeated bending without compromising the electrochemical performances, demonstrating its potential applications in the flexible Li-S punch cell. 98o further improve the electrical, mechanical and electrochemical performances of MXene-S electrodes, Zhang  130 Copyright 2019, Wiley-VCH et al reported an aqueous Ti 3 C 2 T x MXene-S composite ink, by which electrodes with high S content (70%) were quickly fabricated through a commercialized slurry casting technique. 128Thanks to the high concentration and viscosity from the composite ink, the as-formed Ti 3 C 2 T x -S freestanding electrodes were highly conductive and flexible that was achieved in the absence of additional conductive agents and polymeric binders (Figure 16AI-III).The architecture based on the highly conductive network ensures intimate contact between sulfur species and Ti 3 C 2 T x .In addition, a complex sulfate layer can be in situ formed through the interactions between the OH surface groups and Li 2 S n species.Combined with Lewis acid-base interaction between Ti atoms and Li 2 S n , the Li-S batteries exhibited excellent electrochemical performances including high capacity of 1244 to 1350 mAh g −1 and impressive cycling stability (0.035-0.048% capacity loss per cycle), showing great promise for applications in wearable device (Figure 16AIV-VI). 128Very recently, Zhang et al further extended the applications of the viscous MXene aqueous inks, including constructing high capacity MXene-silicon anodes 129 and additive-free printing of micro-supercapacitor. 54 Furthermore, Zhang et al prepared MXene/S conductive paper as electrodes by filtration the viscous MXene ink, followed by S impregnation, as demonstrated in Figure 16BI. 130Such robust electrodes showcased excellent long-term cycling performance with a capacity decay rate of 0.014% per cycle, which has greatly surpassed all other MXene-S electrodes in terms of cycling stability, 130 as shown in Table 2.A prototype full cell based on MXene/S paper and lithium foil is demonstrated in Figure 16BII-BIV, implying the promising applications in next-generation wearable, portable electronics.

| SUMMARY AND OUTLOOK
In this work, we summarize the applications of MXenes for Li-S batteries.Due to their exotic properties, such as abundant surface terminated groups, excellent electronic conductivity and mechanical strength, and so forth, the laminated MXenes possess many unique advantages over other 2D materials and are quite promising for constructing high-performance electrodes.Specifically, (a) the unique layered structure endows MXene nanosheet with sufficient contact area for sulfur/sulfides, and provides good mechanical strength to withstand the stress induced by large volume expansion of sulfur; (b) the metallic conductivity facilitates the electron transport kinetics across the electrode/electrolyte interface, alleviates electrode polarizations and facilitates high rate responses even at a high sulfur loading; (c) MXenes can entrap the soluble Li 2 S n though a strong Ti-S interaction to suppress the shuttling effect.Meanwhile, MXene surface functional groups also have a strong chemisorption to Li 2 S n , which efficiently reduces the active material loss and maintain high capacities after long-term cycling; (d) MXenes with specific surface groups and configurations show a low Li 2 S decomposition energy barrier and fast Li + diffusivity, leading to good catalytic performance and facilitating the electrochemical redox reactions.
Based on these merits, MXenes have been devoted as both sulfur hosts and modified separators.The electrochemical performances of MXene-based Li-S batteries can be improved mainly through the materials and cell design.For instance, increasing the active interaction sites to strength Li 2 S n immobilization, introducing spacers to suppress the nanosheet restacking and constructing 3D frameworks to enhance sulfur loading, and so forth.We believe the electrochemical performance of MXene-based electrodes/devices can be further improved through the following categories.
First, the most common way to synthesize MXenes is by etching of the MAX phase.As a result, functional surface groups are inevitably terminated on the MXenes.According to the theoretical analysis, MXene surface groups play an important role in confining Li 2 S n .The interaction mode of Ti-S is influenced by the functional groups like O, OH, F, S, and so forth.On the one hand, reducing the amount of these terminations is beneficial to form strong Ti S bonds.On the other side, Li 2 S n species are prefer to interact with MXenes terminated with a specific type of functional groups.Thus, engineering MXene surface chemistry is crucial in order to achieve electrodes with a good affinity to Li 2 S n species.
Second, although the electrochemical performances of Li-S batteries have been efficiently improved by introducing MXenes, some of the properties are achieved at a low sulfur loading or tap density.Thus, the areal and volumetric capacities are typically low at the current stage.Future studies should pay attention to the construction of high S loading electrodes with a high tap density.As such, MXene-based Li-S batteries with superior properties are expected.Third, in line with the fast development of portable and wearable electronic devices, the power sources should also be lightweight and flexible enough (even foldable) without compromising their charge-storage performances.To this end, exploring facile and low-cost techniques to construct electrodes with robust structure and high mass loading are meaningful for new type MXene-based Li-S batteries and beyond.
Finally, as a new class of 2D wonder materials, MXenes are not just Ti 3 C 2 T x .To date, more than 30 MXene phases have been reported with more predicted to exist.There are certainly enough incentives to explore other MXene members for high-performance Li-S batteries.Moreover, through tuning the physical and chemical properties of existing MXenes under the guide of theoretical simulations, there is certainly much room available for the reported MXene-S systems to further boost their electrochemical performances in Li-S batteries.We firmly believe that, through the collective efforts from all over the world, MXene-S based Li-S batteries will finally come to our daily life, just give it enough time.

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I G U R E 1 A, Comparison of energy densities of various metal-sulfur batteries.B, Schematic illustration of working mechanism of Li-S batteries.C, Explored applications of MXenes.Images are from the internet.D, The number of publications on MXene and E, MXene-based Li-S batteries (Source: Web of Science) Some

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I G U R E 5 A, Calculated band structure of single-layer MXene with OH and F surface terminations and no termination (Ti 3 C 2 ).

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I G U R E 6 A, Schematic of two-step interactions between OH decorated MXene phase and Li 2 S n .Reproduced with permission. 22Copyright 2017, Wiley-VCH.B, The shortest distances from Li 2 S n to Ti 2 C and functionalized Ti 2 C. Values are taken from reference 65.C, (I) Binding energies of Li 2 S n on functionalized Ti 2 C without electrolytes.(II) Scheme of charged atoms of Li 2 S n and Ti 2 C. "+": electropositive atoms; "−": electronegative atoms; Ti: green balls; S: yellow balls; Li: purple balls; O or F: red balls in pink area; and H: white balls in lightblue area.Reproduced with permission. 65Copyright 2017, Elsevier.D, Adsorption energies of S 8 and Li 2 S n on Ti 3 C 2 T 2 (T = N, O, S, F, Cl).Reproduced with permission. 66Copyright 2019, American Chemical Society.E, (I) Binding energies of Li 2 S n as a function of the lattice constants of M 3 C 2 O 2 (M = Cr, V, Ti, Nb, Hf and Zr).(II) Differential charge densities between Li 2 S 8 and various M 3 C 2 O 2 surfaces.Blue and red regions indicate charge accumulation and depletion, respectively.Reproduced with permission. 67Copyright 2019, The Royal Society of Chemistry.F, Comparison of adsorption energies for Li 2 S n species on nitrogen-based MXene, carbide-based MXenes and electrolytes.
Direct comparisons of Li 2 S decomposition barriers on the different surface-terminated MXenes are shown in Figure 7A.Unlike Ti 3 C 2 N 2 , Ti 3 C 2 F 2 and Ti 3 C 2 Cl 2 , the Ti 3 C 2 S 2 and Ti 3 C 2 O 2 MXenes showcase much reduced Li 2 S decomposition barriers, which facilitate the decomposition of Li 2 S, shorten the Li 2 S n accumulation time in the cathode and further suppress the Li 2 S n dissolution.Ti 3 C 2 S 2 can also maintain good catalytic capability even at high sulfur loading (Figure 7B).

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I G U R E 7 A, Li 2 S, Li 2 S 6 decomposition barriers on Ti 3 C 2 T 2 (T = N, O, S, F, Cl).Values are taken from reference 66. B, Decomposition barriers of bare Li 2 S and Li 2 S on the Ti 3 C 2 S 2 with different sulfur loadings.C, (I) Adsorption energies, (II) ratio of vdW interaction of S 8 and Li 2 S n on Ti 3 C 2 Cl 2 with Cl vacancy (Ti 3 C 2 Cl 2 − x [x = 1/16]), electron localization functions of the (110) slice of (III) Ti 3 C 2 Cl 2 − x and (IV) Ti 3 C 2 Cl 2 .D, Energy profiles for the decomposition of (I) Li 2 S and (II) Li 2 S 6 on Ti 3 C 2 Cl 2-x .(III-VII) Energy profiles for Li + diffusion on Ti 3 C 2 T 2 (T = O, S, N, F, Cl).Reproduced with permission.

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I G U R E 9 A, (I) Schematic illustration of synthesis process of the crumpled N-Ti 3 C 2 T x /S electrodes.(II) N 2 adsorption-desorption isotherm curves of crumpled N-Ti 3 C 2 T x nanosheets.(III) Cycling performances of crumpled N-Ti 3 C 2 T x /S electrode and mixed-Ti 3 C 2 T x /S electrodes.Reproduced with permission. 91Copyright 2018, Wiley-VCH.B, (I) Schematic illustration for the preparation of TCD-TCS/S composites.(II-IV) Electrochemical performances of TCD-TCS/S electrodes.Reproduced with permission.

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I G U R E 1 1 A, (I) Schematic illustration of the preparation process of Ti 3 C 2 T x /TiO 2 QD sulfur cathodes, (II) Cycle behaviors of Ti 3 C 2 T x /TiO 2 QDs/S and Ti 3 C 2 T x /S cathodes.Reproduced with permission. 98Copyright 2018, Wiley-VCH.B, (I) Schematic illustration of the fabrication process of Ti 3 C 2 T x /1T-2H MoS 2 -C composites; (II) Cycling performance and coulombic efficiency of Ti 3 C 2 T x /1T-2H MoS 2 -C sulfur cathodes, inset: image of Ti 3 C 2 T x /1T-2H MoS 2 -C sulfur cathode after 300 cycles.Reproduced with permission.

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I G U R E 1 2 A, Schematic diagram of the growth routine of Mo 2 CT x /CNT composites, and initial charging/discharging curves of Mo 2 CT x /CNT cells under various sulfur loadings.Reproduced with permission. 100Copyright 2018, Wiley-VCH.B, Schematic illustration of the metal carbide (Mo 2 C and W 2 C)/CNF fabrication.Reproduced with permission.

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I G U R E 1 4 A, (I) Schematic of the fabrication process of flexible and integrated alkalized Ti 3 C 2 T x MXene nanoribbon(Ti 3 C 2 T x MNR)/ S-Ti 3 C 2 T x /PP electrode for Li-S batteries.(II) Rate capability of Ti 3 C 2 T x MNR/S-Ti 3 C 2 T x /PP electrode, (III) power density and energy density of Ti 3 C 2 T x MNR/S-Ti 3 C 2 T x /PP and other compared electrodes, inset is an LED array panel powered by two Li-S coin cells.Reproduced with permission. 117Copyright 2018, American Chemical Society.B, (I) Schematic illustration of preparing Ti 3 C 2 T x /CNT-P for cathode host and modified separator.(II) Rate capability of Ti 3 C 2 T x /CNT-P electrode with modified separator measured at different current densities, (III) cycling performances of the Ti 3 C 2 T x /CNT-P electrode with Ti 3 C 2 T x /CNT-P separator and bare GF separator under sulfur loading of 5.8 mg cm −2 .Reproduced with permission. 118Copyright 2019, Elsevier.C, (I) Schematic illustration of Li 2 S n anchoring and conversion process on the Ti 3 C 2 T x /TiO 2 heterostructure.(II) Cycling performance of Li-S cells upon Ti 3 C 2 T x /TiO 2 heterostructure.Reproduced with permission. 119Copyright 2019, Wiley-VCH F I G U R E 1 5 A, Schematic illustration of Ti 3 C 2 T x foam/S in the charge/discharge process.Reproduced with permission. 123Copyright 2018, The Royal Society of Chemistry.B, Schematic illustration of synthesizing flower-like Ti 3 C 2 T x and the formation mechanism of the nanomeshes in the Ti 3 C 2 T x nanosheet.C, Areal and volumetric capacities at 1/30C for the flower-like Ti 3 C 2 T x electrode under various sulfur loadings.Reproduced with permission.

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I G U R E 1 6 A, (I-III) Images of Ti 3 C 2 T x /S ink with high viscosity which can be written on Celgard membrane and coated on Al foil.(IV) Cycling performance and (V) Coulombic efficiency of Ti 3 C 2 T x /S electrode at 0.2C.(VI) Schematic demonstration of Ti 3 C 2 T x entrapping the polysulfides by forming a sulfate complex protective barrier.Reproduced with permission. 128Copyright 2018, Wiley-VCH.B, (I) Schematic fabrication process of the Ti 3 C 2 T x /S paper.(II, III) Galvanostatic charge-discharge profiles and (IV) application of the pouch cells (Ti 3 C 2 T x /S paper//Li foil) under flat and bent states.Reproduced with permission.
65,[69][70][71][72]mainly focus on the interaction behavior of Li 2 S n and Ti 2 CT x with various surface functional groups.DFT calculations indicate that the OH-terminated Ti 2 C (Ti 2 C (OH) 2 ) distort the long chain Li 2 S n .By contrast, the Li 2 S n molecular configuration is maintained when it is absorbed on the surface of Ti 2 CO 2 and Ti 2 CF 2 , indicating that OH-terminated Ti 2 C possesses stronger attractions for Li 2 S n compared to that of Ti 2 CO 2 and Ti 2 CF 2 .Actually, O and F functionalized Ti 2 C MXenes show different suppressing mechanisms.
Table 1 summarizes the areal capacities of MXene-based Li-S batteries with high sulfur loadings.
T A B L E 1 Summary of areal capacity and sulfur loading of reported MXene based Li-S batteries a Carbon fiber (CF).b Carbon nanotube (CNT).c Polyethyleneimine (P).d Ti 3 C 2 T x nanodots/nanosheets (TCD-TCS).