Metal-Organic Framework-Based Sulfur-Loaded Materials

Meng Du,Qing Li,Guangxun Zhang,Feifei Wang,and Huan Pang*

Lithium-sulfur batteries(LSBs)are considered promising new energy storage systems given their outstanding theoretical energy densities.Nevertheless,issues such as low electrical conductivity and severe volume expansion,along with the formation of polysulfides during cycling,restrict their practical applications.To overcome these issues,it is necessary to find suitable and effective sulfur host materials.Metal-organic frameworks(MOFs),which are porous crystalline materials in the bourgeoning developmental stages,have demonstrated enormous potential in LSBs owing to their high porosity and tunable porous structure.Herein,we provide a comprehensive overview of MOF-based sulfur-loaded materials and discuss the charge/discharge mechanisms,strategies of enhancing battery performance,sulfur loading methods,and applications in LSBs.An outlook on future directions,prospects,and possible obstacles for the development of these materials is also provided.

Keywords

lithium-sulfur batteries,metal-organic frameworks,sulfur-loaded materials

1.Introduction

The consumption of nonrenewable fossil fuels,such as coal and oil,and public consciousness regarding the escalating impact of CO2emissions have prompted development of renewable energy devices.Lithium-ion batteries(LIBs),as classic rechargeable battery systems,have become vital energy storage systems in our everyday lives since their commercialization.[1-11]However,LIBs cannot fully satisfy the rapidly growing market demand because the energy density that can be achieved with the current systems is approaching its limit.[12-19]Therefore,development of alternative advanced electrical energy storage devices that can provide high energy densities has become a research hotspot.

Lithium-sulfur batteries(LSBs)are touted as one of the most promising alternatives to fulfill future needs on account of their overwhelming superiority of high theoretical specific capacity(1672 mA h g-1)and energy density(2600 W h kg-1).[20-33]More importantly,compared with traditional LIBs,the existence of extremely abundant sulfur in the Earth’s crust features LSBs as more cost-effective,thereby greatly satisfying the demands of clean and renewable energy development.[34-44]Despite their intriguing virtues,the implementation of high-performance LSBs is still perplexed by three main intractable challenges that arouse unsatisfactory practical capacity and cycle stability.First,the low electrical conductivity of both S(5× 10-30S cm-1at 25°C)and its reduction products(Li2S and Li2S2)led to poor electrochemical activity and limited utilization efficiency of active materials.Second,severe volume expansion(up to 80% )exists in the cathode after lithiation,which results in electrode structural pulverization and thus severe capacity decay.Third,the soluble polysulfides of the cathode easily diffuse to and react with the lithium electrode in the discharge/charge process.[45-50]The resultant “shuttle effect”causes short battery life and potential safety issues.[51-54]

Numerous studies have focused on developing advanced sulfur cathodes to overcome these problems,with focus on preventing shuttling of the soluble polysulfides.Traditional strategies employ porous carbon/sulfur as cathode materials for LSBs.[55-64]Nevertheless,such weak interaction between nonpolar carbon and polar lithium polysulfides(LiPS)is not sufficient to effectively curtail the shuttling of soluble polysulfides,leading to unsatisfactory cycle life.[65-67]To promote the host interaction with LiPS,some polar materials have been recently regarded as cathode materials,including metal oxides,[68,69]sulfides,[70,71]carbides,[72]and nitrides.[47]Although these polar materials provide abundant LiPS adsorption sites,most metal oxides and sulfides possess inherently poor electrical conductivity.In addition,it is difficult to clarify the precise interaction between polysulfide guests and the above material hosts thoroughly due to their low crystallinity,uncertain structures,and vague multistage pore structures.Therefore,the selection of more advanced sulfur-loaded materials is imperative.

Metal-organic frameworks(MOFs),also termed porous coordination polymers,are porous crystalline materials comprising metal sites and organic linkers.[73-85]The metal sites include ions of alkaline earth metals,transition metals,or lanthanides.Multidentate molecules with N-or O-donor atoms(pyridyl,carboxylates,polyamines,etc.)are generally employed as the organic linkers.The surface area of MOFs typically ranges from 1000 to 10 000 m2g-1,and pore size can be tuned as large as 9.8 nm by altering the organic and metal-containing units.[86]By simply regulating the metal species,size,geometry,and functionality,more than 20 000 types of MOFs have been achieved.[87-94]In view of the merits of high porosity and controllable pore structure,MOFs provide enormous opportunities for developing advanced cathode materials that can enhance the capacities and cycling stabilities of LSBs.[95-100]The high porosity allows high sulfur loading,while the soluble polysulfides can be confined by optimizing the size of the pore aperture.The numerous open metal sites along with the functional groups on the organic linkers in MOFs can interact with polysulfides strongly.The high surface area of MOFs provides the abundant exposure of active sites,which prompts this strong interaction and thus impedes the diffusion of polysulfides effectively.Moreover,the combination of MOFs and functional materials exhibits synergistic effect.The doped heteroatoms(e.g.,N,S,and O)and incorporated metal compounds(e.g.,metal oxides and metal sulfides)can provide polar/catalytic sites to chemically anchor polysulfides within the cathodes or catalyze the sulfur reduction/oxidation processes.[101]Extensive efforts have been made to employ MOFs as LSBs cathodes,where reliable results have been achieved,demonstrating their exceptional performance.

In this work,we present a comprehensive introduction of MOF-based sulfur-loaded materials.We first introduce the charge/discharge mechanism of LSBs and then discuss strategies for enhancing the battery performance in detail,including fabricating a suitable pore structure and choosing an appropriate particle size,as well as enhancing the Lewis acid-base interaction between the MOFs and sulfur.The methods of sulfur loading,including physical(melt-diffusion,solvent evaporation)and chemical methods(sulfides conversion),are further elaborated.Finally,the applications of MOF-based sulfur-loaded materials and perspectives for addressing the challenges and the future prospects of MOFs in LSBs are discussed.Previous reviews mainly emphasized the application of MOFs in LSBs.[34,102]Differently,this review gives specific strategies to achieve high-performance LSBs for MOF-based sulfur-loaded materials.Additionally,the description of sulfur loading methods further broadens their large-scale synthesis and application in LSBs.

2.Charge/Discharge Mechanism

A LSB is an electrochemical energy storage device that can store electrical energy in sulfur electrodes.[103-114]A schematic of the components of LSBs and the corresponding charge/discharge processes is presented in Figure 1a.A typical LSB generally comprises a sulfur cathode,a Li metal anode,an organic electrolyte,and a separator.[115-119]During discharge,Li metal at the negative electrode is oxidized to yield electrons and Li+.The generated electrons move toward the positive electrode through the external circuit,while the Li+travels toward the positive electrode through the electrolyte within the battery,thus producing an electric current.By receiving Li+and electrons,sulfur is converted to lithium sulfide.The reactions involved in the process are as follows: negative electrode: Li→Li++e-;positive electrode:S8+16Li++16e-→8Li2S;overall reaction:16Li+S8→8Li2S.The charge process occurs via the reverse reactions.

The mechanism of the electrochemical reaction of LSBs involves solid(cyclo-S8)-liquid(chain-polysulfides(S4-82-)-solid(Li2S2/Li2S)phase changes,as illustrated in Figure 1b.During discharge,S8is reduced to form S82-,S62-,and S42-,successively,generating an upper plateau of approximately 2.4 V,while the second flat plateau of approximately 2.15 V originates from the conversion of S42-to S22-.The first and second processes represent the formation of soluble polysulfides(Li2Sx,4≤x≤8)as well as insoluble Li2S2and Li2S,respectively.Eventually,Li2S is obtained as the end product of the discharge process.In the subsequent charge process,Li2S is converted into S8by forming multiple intermediate polysulfides.

In case of LSBs,different components have different functions and requirements.Sulfur cathode is possibly the most significant component in LSBs as its properties strongly affect the electrochemical performance of the battery.In this respect,a reasonable design of sulfur cathode materials for meliorating the shuttle effect,increasing the electrical conductivity,and limiting the volume variation of sulfur,is imperative.Additionally,metallic Li is the most preferred anode material for LSBs due to its high specific capacity(3860 mA h g-1)and low potential(-3.040 V)in comparison with standard hydrogen electrode(SHE).The separator isolates the cathode from the anode to prevent short circuit,and it also acts as an ion-selective membrane to allow only Li+passing through.The ideal separator should satisfy the requirements of possessing sufficient barrier properties,superior thermal stability,and enough wettability.Electrolytes function as media for ions transfer in batteries.Due to the presence of severe shuttle effect in LSBs,different requirements should be met.Currently,the most commonly used organic solvents are dimethyl ether(DME)and 1,3-dioxolane(DOL),as well as other solvents such as tetraethylene glycol dimethyl ether(TEGDME),polyethylene glycol dimethyl ether(PEGDME),tetrahydrofuran(THF),and ionic liquids.

3.Strategies for Enhancing Battery Performance

To mitigate the issues in LSBs,strategies for enhancing the battery performance are discussed in the following paragraphs,including fabricating a suitable pore structure and choosing an appropriate particle size,along with enhancing the Lewis acid-base interaction between the MOFs and sulfur by introducing functional groups into the organic linkers and exposing the open metal sites(Figure 2).

3.1.Suitable Pore Structure

Rational design of the pore structure is important for improving the performance of an LSB.[101,120-126]Generally,larger pores can store more sulfur,but the open structure is not conducive to confining polysulfides,while smaller pores can effectively impede the shuttle effect of polysulfides,but the sulfur loading is limited.[127-130]S8with 0.68 nm diameter is the major existence of sulfur.Successful infiltration of sulfur requires that the pore size cannot be lower than 0.68 nm.Otherwise,S8hardly introduces into the host cavity and mainly disperses on the host surface,which leads to low sulfur utilization and fast capacity fading.However,overlarge pore size is likely to cause rapid loss of internal sulfur in host and lose the effect of sulfur fixation.Therefore,MOFs with tunable well-defined pore structures and ultrahigh porosities can potentially host large amounts of sulfur in the pores while exhibiting superior abilities to trap the polysulfide intermediates through the apertures.

The sulfur storage performance of MIL-100(Cr),comprising two types of mesoporous cages(～25-29?A)interconnected via microporous pentagonal(～5?A)and hexagonal windows(～9?A),was investigated.[131]By applying the melt-diffusion technique,about 48 wt% sulfur was successfully embedded in the porous MIL-100(Cr).Electrochemical tests indicated that the obtained MIL-100(Cr)/S cathode delivered higher capacity retention than the mesoporous carbon/S cathode.The excellent performance originated from the advantageous cage-like pore structure.Subsequently,benefitting from the appropriate pore space and open Cu(II)sites,HKUST-1 was employed as a sulfur host to fabricate a HKUST-1?S composite.[132]The crystal structure(Figure 3c)confirmed that a large amount of sulfur was impregnated into the pores of HKUST-1.The powerful ability of HKUST-1 to confine sulfur led to outstanding performance of the HKUST-1?S cathode.In addition,four MOFs,ZIF-8,HKUST-1,NH2-MIL-53(Al),and MIL-53(Al),with different diameters of the largest apertures(0.34,0.69,0.75,and 0.85 nm,respectively),were investigated as sulfur hosts for LSBs.[133]In 300 cycles at a constant current density of 0.5 C,the four S/MOF cathodes delivered reversible capacities of 553,286,332,and 347 mA h g-1with average fading rates of 0.08% ,0.11% ,0.14% ,and 0.19% per cycle,respectively(Figure 3a).The results demonstrate that the capacity attenuation was closely associated with the pore sizes of the MOFs,and small apertures that facilitate interactions with the polysulfide anions contributed to better cycling stability.

Figure 3.a)Schematic illustration of the largest apertures,the discharge capacities,and the capacity fading rate at 0.5 C of four MOFs.b)Preparation of S@ZIF-8.c)Crystal architecture of HKUST-1?S;yellow represents sulfur molecules.a)Adapted with permission.[133]Copyright 2014,The Royal Society of Chemistry.b)Adapted with permission.[134]Copyright 2014,Elsevier.c)Adapted with permission.[132]Copyright 2013,American Chemical Society.

Almost at the same time,ZIF-8 with large cage-like pores and small apertures was chosen as a sulfur host for the synthesis of S@ZIF-8,as shown in Figure 3b.[134]Sulfur was encapsulated in the ZIF-8 matrix and restricted in the cage-like pores,thus alleviating the dissolution issue and affording good cyclability in the charge/discharge process.In another report,Li and co-workers fabricated a Cd-based MOF possessing tentacles comprising aromatic rings and employed this MOF as the host species for sulfur impregnation.[135]The channels in this MOF provide a π-π*conjugated matrix for guest molecule trapping and charge transfer,where the open channels insured 72 wt% sulfur loading,and the aromatic tentacles in the semi-open channel enhanced the confinement of the polysulfides.Owing to the superior properties derived from the unique pore structure,the resulting S/MOF cathode exhibited superior electrochemical behavior.

3.2.Lewis Acid-Base Interactions

Lewis acid-base interactions between MOFs and polysulfides were first proposed by Xiao and co-workers.[136]They utilized Ni-MOF as a sulfur host and demonstrated that polysulfides can be anchored within the cathode.The formed Ni-MOF/S composites with 60 wt% sulfur loading exhibited outstanding cycling performance.Besides the pore confining effects of the MOF,the strong Lewis acid-base interaction further mitigated the shuttle effect of the soluble polysulfide intermediates.In this interaction,Ni(II)acting as a soft Lewis acid site in the Ni-MOF tended to bond with the soluble polysulfides that act as a soft Lewis base,effectually trapping the intermediate soluble polysulfides.Density functional theory(DFT)calculations indicated that only one S atom in the polysulfide chain was combined with Ni-MOF(Figure 4e).Moreover,the calculated binding energies showed that longer LiPS chains afforded increasingly stronger interactions(Figure 4d).X-ray photoelectron spectroscopy(XPS)analysis demonstrated that the binding energy of Ni decreased,indicative of interaction between Ni2+and the soluble polysulfides.Inspired by Xiao’s work,Qian and colleagues developed MOF-525 as a sulfur host.[137]Fe3+and Cu2+ions were embedded into the porphyrin moieties through a coordination interaction to provide Lewis acidic sites for trapping sulfur.Compared with MOF-525(2H)having no Lewis acid sites,MOF-525(FeCl)and MOF-525(Cu)with one and two Lewis acid sites,respectively,exhibited significantly enhanced cycling stability.For the two resulting S@MOF-525 materials,S2pshifts of～0.7 eV each were observed,indicating strong interactions with sulfur.Remarkably,MOF-525(Cu)delivered the most outstanding electrochemical performance,attributed to the abundant Lewis acidic sites that can generate strong interactions with the polysulfides,thus retarding migration of the soluble polysulfides.

Figure 4.a)Schematic of interactions between sp2nitrogen atoms and LiPS in nMOF-867,and dissolution of LiPS in nUiO-67.b)Schematic illustration of cages in Mn-CCs-xH2O and Mn-CCs,as well as S@Mn-CCs with S8molecules.c)Color change of nMOF-867 impregnated with Li2S4solution after 120 and 240 min.d)Comparison of binding energy for LiPS with Ni-MOF or CoMOF.e)Schematic illustration of the interaction between Ni-MOF and polysulfides.a,c)Adapted with permission.[138]Copyright 2016,Nature Publishing Group.b)Adapted with permission.[140]Copyright 2019,The Royal Society of Chemistry.d,e)Adapted with permission.[136]Copyright 2014,American Chemical Society.

The strong interaction of the functional heteroatoms,which served as Lewis basic sites,with the LiPS was investigated.As shown in Figure 4a,the Lewis basic sp2nitrogen in the H2BPYDC ligand was embedded in nanosized Zr-MOF(Zr6O4(OH)4(BPYDC)6,BPYDC=2,2′-bipyridine-5,5′-dicarboxylate, known as nMOF-867).[138]Another nanosized MOF(Zr6O4(OH)4(BPDC)6,BPDC=4,4′-biphenyldicarboxylate,abbreviated as nUiO-67)without nitrogen was also studied.Electrochemical tests demonstrated that the discharge capacity of the two MOFs in the first cycle was similar,but nMOF-867/S delivered much higher cycling stability than nUiO-67/S.The enhanced capacity retention resulted from the Lewis basic nitrogen in nMOF-867 that can undergo strong interactions with the Li+of the polysulfides.Additionally,a visualization method was employed to observe the color changes induced by mixing the two MOFs with Li2S4solution.After 240 min,the Li2S4solution containing nMOF-867 became transparent(Figure 4c),while the Li2S4solution with nUiO-67 retained its yellow color.This suggests that nMOF-867 can capture Li2S4and the sp2nitrogen atoms can exert a strong confinement effect on the polysulfides.

In subsequent research,nanosized Cu-MOF (Cu-TDPAT, H6TDPAT=2,4,6-tris(3,5-dicarboxylphenyl-amino)-1,3,5-triazine)was constructed using the H6TDPAT ligand with numerous nitrogen functional groups and paddle-wheel-like Cu2(COO)4with Cu2+open metal sites.[139]The Cu2+ions acted as Lewis acid sites that could bind to the polysulfide anions,and the N atoms in the ligand served as Lewis base sites for coordination with Li+,thereby effectively mitigating loss of the polysulfides.The energy for adsorption of the polysulfides by Cu-TDPAT was calculated,demonstrating that the strongest interaction occurred between Li2S4and Cu-TDPAT.Upon immersion in Li2S4/DME solution,Cu-TDPAT completely adsorbed Li2S4.Furthermore,an obvious shift in the S2ppeak of S@Cu-TDPAT was observed in comparison with the peak of pristine sulfur,thus confirming the good interaction between Cu2+and sulfur.The resultant S@Cu-TDPAT cathode delivered superior cycling performance,which originated from the Lewis base and acid sites in Cu-TDPAT.A 9-manganese node-based MOF[Mn1.125(OH)0.25(H2O)1.75L0.5]n(Mn-CCsxH2O,L=5-(phosphonomethyl)isophthalic acid)with abundant cubic cages was constructed.[140]After dehydration,numerous open Mn sites were generated as Lewis acid sites that could undergo interactions with sulfur(Figure 4b),thus achieving enhanced electrochemical performance.To reveal the influence of the Mn-CCs on the polysulfides,the concentration of sulfur that passed through the separator and reached the Li foil cathode was estimated.A minor amount of sulfur was detected for the S@Mn-CC electrode,indicating that the shuttle issue was more strongly mitigated in S@Mn-CC than in the S/Mn-CCs and S@CNTs.

3.3.Appropriate Particle Size

The particle size of the MOFs also exerts a significant influence on the performance of the LSB.[141-147]A smaller particle size not only promotes electron transport but also provides a larger contact area with the electrolytes and conductive additives.[148-151]However,as the particle size decreases,leaching of the soluble polysulfides is accelerated,which further complicates the situation.

Li et al.discussed the influence of the MOF particle size in detail.[133]ZIF-8 with different particle sizes(150 nm,1 μm,and 3 μm)was synthesized and employed as a host for trapping sulfur.Electrochemical measurements suggested that the cycling stability of the three S/ZIF-8 cathodes at 0.5 C over 100 cycles was similar.The primary difference between the three cathodes was in the reversible capacity,with values of 733,556,491 mA h g-1,respectively(Figure 5c).The results demonstrated that a smaller particle size of the MOF was associated with a higher maximum capacity,attributed to the shorter ion diffusion length,and thus facilitated sulfur utilization.However,as the size of the MOF particles decreased,the voltage hysteresis loop became larger,indicating insufficient electron transport,which might be attributed to fewer contact points with the conductive network.

Figure 5.a)Schematic of S@ZIF-8 particles in the discharge process.b)Cycling performance of ZIF-8 host with distinct particle size(15,70,200,800,2000 nm)at 0.5 C over 250 cycles.c)Schematic of maximum discharge capacity obtained with different particle sizes.d)Statistical results of maximized capacity and decay rate.The “golden size”is the “best”ZIF-8 particle size for achieving maximum capacity with the slowest decay rate.a,b,d)Adapted with permission.[152]Copyright 2015,The Royal Society of Chemistry.c)Adapted with permission.[133]Copyright 2014,The Royal Society of Chemistry.

In another study,the authors further explored the influence of the MOF particle size on the electrochemical performance and provided more insight into this subject.[152]Five ZIF-8 samples with different particle sizes from 15 nm to 2 μm were fabricated and utilized as sulfur hosts.As illustrated in Figure 5b,for the smaller MOF particles,the maximum capacity increased gradually,but the highest capacity retention was delivered with a moderate particle size(～200 nm).The authors proposed a model to demonstrate that MOFs with a suitable particle size were advantageous for achieving excellent sulfur utilization and cycle stability(Figure 5a).For ZIF-8 with a larger particle size,a long time was required for the polysulfides to react with the internal sulfur,thus leading to poor sulfur utilization.Conversely,if the particle size was too small,ZIF-8 possessed a large external surface area and the generated polysulfides readily escaped from the ZIF-8 host,resulting in unsatisfactory cycling stability.This work demonstrated the presence of a“golden size”(Figure 5d).

4.Sulfur Loading Method

Currently,the construction of MOF-based sulfur-loaded materials adopts melt-diffusion method.For other non-MOF materials,sulfide conversion and solvent evaporation methods are also employed to load sulfur.In subsequent research,other methods can be extended to the sulfur impregnation of MOFs,which can promote the abundant fabrication of MOF-based sulfur-loaded materials and their application in LSBs.

4.1.Melt-Diffusion Method

The melt-diffusion method is an extremely simple and effective physical sulfur loading method in which sulfur and the host materials are mixed and reacted at 155 °C.At 155 °C,sulfur exists in the form of a liquid,where the viscosity is lowest,allowing sulfur to easily enter the pore structure of the host material or attach to the surface.Qian and coworkers successfully fabricated HKUST-1?S through melt diffusion.[132]They loaded sublimated sulfur and HKUST-1 into a mortar and ground the mixture into a uniform purple powder in a glovebox.After heating in a glass tube at 155°C for 24 h,a military green colored HKUST-1?S powder was obtained.LSBs prepared using HKUST-1?S as the cathode displayed excellent performances.In another report,active CuBTC(BTC=benzene-1,3,5-tricarboxylate)and sulfur were thoroughly mixed by utilizing a vortexer,then heated at 155°C for 10 h to diffuse the melted sulfur into the CuBTC pores.[153]After cooling,dark green CuBTC@S powder was obtained and the resultant CuBTC@S cathode delivered a high specific capacity.A S@Ni3(HITP)2composite was also constructed by this method.[154]Ni3(HITP)2(HITP=2,3,6,7,10,11-hexaiminotriphenylene)powder and sublimed sulfur were ball-milled and loaded into a sealed glass tube.After heating at 155°C for 12 h,S@Ni3(HITP)2was obtained by ball-milling again.When utilized as the cathode for an LSB,exceptional sulfur utilization and cycling stability were achieved.

4.2.Sulfide Conversion Method

The sulfide conversion method utilizes sulfur-containing precursors(Sx2-,S2O32-,SO2,etc.)to deposit sulfur in a specific area of the host materials through redox reactions.[155-157]Li and co-workers utilized Na2S as a sulfur precursor and Fe(NO3)3as an oxidant to construct 3D porous composites containing sulfur nanoparticles with a unique interconnected hierarchical structure(Figure 6a).[58]The reaction involved is described by Equation(1).They first dissolved NaCl,Na2S,and glucose in deionized water and obtained 3D NaCl-Na2S@glucose after freezedrying.Upon carbonizing,the glucose was converted into graphitic carbon(GC)possessing micro-and mesopores,thus resulting in 3D NaCl-Na2S@GC.When immersed in Fe(NO3)3solution,sulfur nanoparticles were formed and finally filled the micro-and mesopores.LSBs prepared using this composite as a cathode displayed high sulfur utilization,excellent specific capacity, and outstanding cycling durability.

Figure 6.a)Schematic of in situ strategy for the fabrication of 3D porous composites containing sulfur nanoparticles.b)Schematic of synthesis of sulfur-TiO2yolk-shell nanostructure and corresponding characterizations.a)Adapted with permission.[58]Copyright 2016,Nature Publishing Group.b)Adapted with permission.[159]Copyright 2013,Nature Publishing Group.

The use of iron ions as an oxidant to oxidize sulfur-containing precursors was also reported by Zong and colleagues.[158]ZnS spheres were dissolved in acetone solution containing phenol formaldehyde(PF)resin to prepare ZnS@PF resin composites,which were then calcined to form 3D carbon-coated ZnS spheres(ZnS@C).When the ZnS@C composite was immersed in Fe(NO3)3solution,the ZnS→S conversion reaction proceeded as shown in Equation(2).The generated sulfur particles were confined inside the carbon shell.The resultant sulfur-carbon yolk-shell materials delivered excellent electrochemical properties.

In another work,a sulfur-TiO2yolk-shell nanoarchitecture was achieved using sodium thiosulfate as a sulfur precursor.[159]The synthesis and corresponding characterization are shown in Figure 6b.Sodium thiosulfate and hydrochloric acid were first used as raw materials to prepare the sulfur nanoparticles,which were then coated with TiO2and formed sulfur-TiO2nanoparticles with core-shell structures.Finally,sulfur was partially dissolved in toluene to form an empty space between the TiO2shell and sulfur core,leading to the formation of a yolk-shell structure.When utilized as the cathode for LSBs,outstanding capacity retention was achieved.Wang and coworkers[160]fabricated sulfur-carbon composites through in situ sulfur deposition.Conductive carbon black(CCB)was first dissolved in aqueous ethanol by ultrasonication and Na2S4aqueous solution was added to this solution.Subsequently,the as-obtained precipitates(Na2S4/CCB)were fumigated with HCl vapor;the reaction that occurred is described by Equation(3).Afterward,the S-CCB composites were heated to promote the diffusion of sulfur into the CCB pores.Finally,the transformation from interparticle sulfur to absorbed sulfur was achieved with increasing temperature.Electrochemical measurements demonstrated that the S-CCB composite cathodes delivered remarkably high initial capacity and capacity retention.

4.3.Solvent Evaporation Method

Solvent evaporation method usually uses the solubility of sulfur in carbon disulfide(CS2)to form an S/CS2solution,which is mixed with materials and finally achieves complete evaporation of CS2.Su et al.dissolved sulfur into CS2solution and dispersed Na2Fe[Fe(CN)6]in CS2under stirring.[161]Then,CS2was completely evaporated from the mixture at 40°C.The final S@Na2Fe[Fe(CN)6]composites possessed a sulfur loading of up to 82 wt% .When coated with poly(3,4-ethylenedioxythiophene)(PEDOT)uniformly,the S@Na2Fe[Fe(CN)6]@PEDOT electrodes delivered outstanding polysulfides confining ability.In another report,Prussian blue(PB)/CNT nanocomposites were dispersed in 5 mL CS2by sonication,and 65 mg sulfur was dissolved into this solution.[162]The CS2evaporation was performed under stirring at 40°C.The as-obtained S@PB/CNT with 64.4 wt% sulfur content given rise to high energy/rate capacity and cycling performance.

5.Applications of MOF-Based Sulfur-Loaded Materials in LSBs

LSBs have been researched for decades for their ultrahigh energy densities.[163-169]Nevertheless,the latest LSB systems still face many problems that prevent their commercial application.To overcome the biggest problems,the most prevalent strategies involve finding suitable sulfur hosts to curtail the dissolution and diffusion of polysulfides into the electrolyte.[170-180]Owing to their unique chemical environment and advanced pore structure,MOFs have great potential for storing active sulfur,inhibiting shuttling of polysulfides,and mitigating volume change.[181-190]The open metal sites and functional groups in the organic ligands can bind with polysulfides to confine them and prevent shuttling.Moreover,the porous networks of MOFs with high surface areas can efficaciously mitigate the volume variations during battery cycling,while acting as effective electrolyte reservoirs to offer ample routes for ion diffusion.Because of the conversion reactions of sulfur and the intercalation effect of the MOF structure,the sulfur/MOF composite cathode can provide high energy density and exhibit rapid charge/discharge cycling performance.Some MOF-based sulfur-loaded materials employed for LSBs are summarized in Table 1.

Table 1.MOF-based sulfur-loaded materials employed for LSBs.

5.1.Representative MOFs

5.1.1.MIL Series

MIL-100(Cr)with a large pore volume(～1 cm3g-1)and small windows(～5 and～9?A)is expected to be a suitable candidate for sulfur encapsulation.MIL-100(Cr)consisting of chromium metal ions and BTC ligands was first exploited as a MOF host material by Tarascon and co-workers.[131]After sulfur infiltration by melt diffusion,the total pore volume of the obtained MIL-100(Cr)/S@155 decreased,which indicated that most of the pores were filled with sulfur.Moreover,MIL-100(Cr)/S with 50% carbon exhibited a flatter capacity decay,and a capacity of 420 mA h g-1was retained over 60 cycles(Figure 7f).In another study,mesoporous MIL-100(V)was selected as the sulfur host for LSBs.[191]Vanadium ions in various valence states can interact with LiPS to hinder the shuttling of polysulfides.When tested as cathodes for LSBs,S@MIL-100(V)maintained a capacity of～550 mA h g-1over 200 cycles,with 0.17% capacity fading per cycle.Subsequently,Zeng et al.utilized MIL-101(Cr)as a host material owing to its large specific surface area(5000 m2g-1)and pore volume(＞1.6 cm3g-1).[192]Multicore-shell structured composites were also obtained by coating MIL-101(Cr)/S with PEDOT:PSS(polystyrene sulfonic acid).Owing to the benefits of its unique structure and conductive shell,sulfur was highly dispersed in the pores and polysulfide loss was reduced.The sulfur electrodes doped with PEDOT:PSS exhibited enhanced electrochemical performance.A high initial capacity(1439.71 mA h g-1at 0.1 C)and capacity retention rate(99.1% over 192 cycles)were achieved.

Figure 7.a)Evolution of porous architecture with the etching time.b-d)TEM images of solid(ZIF-67),core/shell(ZIF-67-5),and entirely hollow structure(ZIF-67-10),respectively.e)Schematic of sulfur storage mechanism during TA etching.f)Cycling performance of MIL-100(Cr)/S@155 cathode with different content of carbon additives.g)Cycling performance of S@ZIF-8.h)Voltage profiles of S@ZIF-8 composite at 0.5 C.a-e)Adapted with permission.[193]Copyright 2018,Elsevier.f)Adapted with permission.[131]Copyright 2011,American Chemical Society.g)Adapted with permission.[134]Copyright 2014,Elsevier.h)Adapted with permission.[152]Copyright 2015,The Royal Society of Chemistry.

5.1.2.ZIF Series

ZIF-8 is considered an outstanding sulfur host due to its large cage-like pores(11.6?A)and small apertures(3.4?A).Li and colleagues constructed a S/ZIF-8 cathode with long cycle life.[133]With 50 wt% sulfur loading in S/ZIF-8,a discharge capacity in the 1st cycle of 738 mA h g-1was delivered,and a capacity of 553 mA h g-1was maintained after 300 cycles at 0.5 C,corresponding to an average fading rate of 0.08% .Three other MOFs(HKUST-1,NH2-MIL-53,and MIL-53)with different pore sizes were also used for comparison,with decay rates of 0.11% ,0.14% ,and 0.19% per cycle,respectively.These MOFs present a cross section of unique features such as a breathing network with 1D channels(MIL-53),unsaturated metal sites(HKUST-1),and amine functionality(NH2-MIL-53).This research confirmed that small apertures contributed to the affinity for the polysulfide anions and better cycling stability,which provides insight for designing high-performance LSBs.A S@ZIF-8 composite was fabricated and employed as a sulfur cathode host for LSBs.[134]Sulfur molecules were encapsulated in the ZIF-8 matrix and thus alleviated dissolution issues during charge/discharge.Power X-ray diffraction(PXRD)confirmed that the crystal structure of ZIF-8 was intact after sulfur impregnation.The S@ZIF-8 cathode delivered higher capacity(～510 mA h g-1)after 100 cycles than the hand-milled mixture(S/ZIF-8)of sulfur and activated ZIF-8(Figure 7g).

Subsequently,a series of ZIF-8 samples with different particle sizes(15,70,200,800 nm,and 2 μm)was prepared and utilized as S@MOF cathodes.[152]The results showed that 15 nm ZIF-8 afforded the highest sulfur utilization and delivered a maximum capacity of over 950 mA h g-1(Figure 7h).ZIF-8 with a particle size of～200 nm afforded the most excellent capacity retention of 75% over 250 cycles.Ge et al.[193]used tannic acid(TA)to regularize the hydrophilicity and polarity of ZIF-67.After sulfur encapsulation,the formed C-S and Co-S bonds significantly improved confinement of the polysulfides(Figure 7e).Moreover,structural transformation of solid(ZIF-67)to core/shell(ZIF-67-5)and an entirely hollow structure(ZIF-67-10)proceeded in succession based on the etching time(Figure 7a-d).When utilized as host species for LSBs,ZIF-67-5-S afforded the best cycling performance(521 mA h g-1after 500 cycles)and rate capability(510 mA h g-1at 1.6 A g-1).This excellent electrochemical behavior mainly resulted from the following advantages.The core/shell configuration could supply more active sites and abundant suitable space to promote electron and ion transport,thus effectively confining the polysulfides.Moreover,the formed C-S and Co-S bonds strongly improved the ability to encapsulate sulfur.More importantly,the functionalized-OH groups could trap and transform the polysulfides into insoluble polysulfides and thiosulfates,thereby mitigating the loss of soluble polysulfides.

5.1.3.HKUST-1

HKUST-1 with moderate pore spaces and open metal sites exhibited satisfactory ability to confine polysulfides.Qian and co-workers utilized HKUST-1 as a host species to capture sulfur,thereby restricting the shuttling issue.[132]Up to 40 wt% sulfur was embedded in the pores of HKUST-1 to generate HKUST-1?S composites.The corresponding preparation steps are illustrated in Figure 8a.Electrochemical tests demonstrated that the HKUST-1?S electrode afforded more outstanding performance than the composites of mesoporous silica(SBA-15)with oxygenated porous architectures,mesoporous carbon,and MIL-100(Cr).Excellent performance(～500 mA h g-1)was achieved after 170 cycles,which is superior to that of the HKUST-1/S cathode(Figure 8c).SEM observation of the HKUST-1?S electrode after 70 cycles showed no reduction of the particle size of HKUST-1,and PXRD demonstrated that HKUST-1 remained structurally stable in the charge/discharge process.In another work,CuBTC(also known as HKUST-1)with different particle sizes(0.16,1.6,and 5.9 μm)was successfully constructed and used as a sulfur host.[153]To investigate the influence of the particle size on polysulfide dissolution,aliquots of the electrolyte were measured using UVVis absorbance spectroscopy.The results showed that S42-was more strongly retained in 0.16 μm CuBTC than in 5.9 μm CuBTC and S/C.Extended X-ray absorption fine structure(EXAFS)analysis showed that the Cu-S interactions were enhanced as the S content increased.Electrochemical tests showed that the 0.16μm CuBTC@S displayed the most outstanding electrochemical behavior,affording a capacity of 679 mA h g-1at 0.1 C(Figure 8e).This further proves that a high density of Cu-rich surface defects can significantly promote polysulfide uptake,thus enhancing the maximum discharge capacity and cycling stability.

Figure 8.a)Schematic of the fabrication of HKUST-1?S.b)Synthesis of S@MOF-525(M)at 155 °C by melt-diffusion technique.c)Cycling performance of HKUST-1?S and corresponding Coulombic efficiency.d)Cycling performance of three S@MOF-525 cathodes.e)GCD curves of CuBTC@S after the 1st discharge cycle at 0.1 C.a,c)Adapted with permission.[132]Copyright 2013,American Chemical Society.b,d)Adapted with permission.[137]Copyright 2015,American Chemical Society.e)Adapted with permission.[153]Copyright 2018,The Royal Society of Chemistry.

5.1.4.MOF-525 Series

Mixed-MOFs(MMOFs)assembled from metalloporphyrin ligands have been reported as host species.[137]Qian and colleagues constructed three kinds of MMOFs(MOF-525(2H),MOF-525(FeCl),and MOF-525(Cu))and impregnated the pores with sulfur(Figure 8b).The sulfur confinement differed for the various MMOFs due to the different environments at the center of the porphyrin moieties.Rate capability tests demonstrated that S@MOF-525(FeCl)and S@MOF-525(Cu)delivered high reversible capacities and good cycling stability,with capacities of＞400 mA h g-1at 5 C.Due to the combined effects of the two Lewis acid sites,high porosity,and ultrastability,MOF-525(Cu)was the most powerful host material for confining polysulfides,affording a capacity of ～700 mA h g-1(Figure 8d). The electrochemical stability of the S@MOF cathodes was confirmed through SEM analysis of the morphology and the PXRD patterns after 200 cycles.

5.2.Other Metal-Based MOFs

Xiao et al.investigated a Ni-MOF,(Ni6(BTB)4(BP)3,BTB=benzene-1,3,5-tribenzoate and BP=4,4′-bipyridyl)with interwoven mesopores(～2.8 nm)and micropores(～1.4 nm)(Figure 9b).[136]The collaborative combination of the hierarchical porous structure and strong Lewis acid-base interactions between Ni2+and the polysulfides impeded the rapid migration of soluble species from the cathode,thereby greatly improving the cycling stability of the LSBs.When this Ni-MOF was utilized as a sulfur host,the capacity retention could reach 89% at 0.1 C after 100 cycles(Figure 9e).The stability of the metal complexes followed the order:Mn(II)＜Fe(II)＜Co(II)＜Ni(II)＜Cu(II).Another Ni-MOF(Ni3(HITP)2)complex with high conductivity was synthesized and investigated as a sulfur host for LSBs.[154]In this Ni-MOF,the Ni atom was dsp2hybridized to form a 2D conjugated structure.Ni3(HITP)2with a grass-like structure formed by weak metal-metal interaction and powerful π-π conjugation afforded high conductivity(200 S m-1).Owing to its highly porous nature and ability to chemically adsorb polysulfides,the shuttle effect was effectually suppressed.CNTs were also employed as a conductive additive for fabricating the matrix conduction network to achieve enhanced electrochemical properties.The cooperative effect of Ni3(HITP)2and the CNTs on the function of the LSBs is illustrated in Figure 9a.Ni3(HITP)2offered shortrange electron pathways,while the CNTs provided long-range channels for electron and Li+.XPS analysis revealed the existence of Ni-S and NS bonds,thereby confirming the powerful interaction between the LiPS and Ni3(HITP)2.The results demonstrate that the S@Ni3(HITP)2electrode delivered high reversible capacity(848.9 mA h g-1at 0.2 C)and cycling stability(629.6 mA h g-1after 300 cycles at 1 C)(Figure 9g).

Figure 9.a)Schematic of cooperative effect of Ni3(HITP)2and CNTs on properties of LSBs.b)Crystal structure of Ni-MOF with mesopores and micropores.c)Crystal structure of Cu-TDPAT with three types of cages prepared from H6TDPAT and Cu2+ions.d)Comparison of the binding energy of LiPS with Cu-BHT and electrolyte solvents.e)Long-term cycling performance of Ni-MOF/S composites at different current densities.f)Cycling performance of nUiO-67/S and nMOF-867/S at 0.835 A g-1.g)Cycling performance of S@Ni3(HITP)2-CNT electrode at 1 C.h)Cycling performance of S@Cu-TDPAT cathodes at 0.5 C for 300 cycles.i)Cycling stability of 100 nm-size S@Cu-TDPAT electrode at 1 C over 500 cycles.a,g)Adapted with permission.[154]Copyright 2019,WILEYVCH.b,e)Adapted with permission.[136]Copyright 2014,American Chemical Society.c,h,i)Adapted with permission.[139]Copyright 2018,The Royal Society of Chemistry.d)Adapted with permission.[194]Copyright 2018,American Chemical Society.f)Adapted with permission.[138]Copyright 2016,Nature Publishing Group.

For in-depth exploration of the potential of MOF hosts,the pore structure of the MOFs and LSB performance should be synergistically studied.A cage-like Cu-MOF(Cu-TDPAT)with abundant open metal sites and nitrogen functional sites was employed as a sulfur host to confine sulfur/polysulfides(Figure 9c).[139]Three kinds of cages were present in this MOF,including a cage similar to that of HKUST-1,a tetrahedral cage with a size of 14?A,and an octahedral cage with a size of 20.3?A.Benefitting from the strong interactions between Cu2+and the polysulfides,as well as that of the N atoms in the ligand and Li+,Cu-TDPAT demonstrated excellent ability for sulfur confinement.Thermogravimetric analysis showed that the sublimation temperature of sulfur was higher than that of pristine sulfur,indicative of interactions between sulfur and Cu-TDPAT.Moreover,the size of the Cu-TDPAT particles was also successfully tuned.As shown in Figure 9h,100 nm S@Cu-TDPAT delivered outstanding electrochemical performance.Even at 1 C,a stable capacity of～745 mA h g-1was maintained over 500 cycles(Figure 9i).Zhao and colleagues fabricated another Cu-MOF(Cu-BHT,BHT=benzenehexathial)with high electrical conductivity(～1580 S cm-1)via a liquid-liquid interfacial reaction.[194]First-principle calculations indicated that Cu-BHT satisfied the criteria of ideal host materials.Figure 9d shows the binding energies,indicative of a powerful interaction between Cu-BHT and the LiPS,which largely prevents dissolution of the LiPS into the electrolyte.Li2S deposited on Cu-BHT facilitated rapid transformation between Li2S and LiPS and improved utilization of the active materials.

In addition to Ni-and Cu-based MOFs,other metal-based MOFs have been exploited as sulfur hosts for LSBs.A Cd-based MOF([(CH3)2NH2]2[Cd(L)]·5DMF)with tentacles comprising aromatic rings was synthesized from 1,4-bis(3,5-isophthalic acid)naphthalene(H4L)as an organic ligand.[135]The open channels guaranteed a higher sulfur loading,while the semi-open channel insured excellent capacity and cycling stability.When employed as a sulfur cathode,a stable capacity of 799 mA h g-1was maintained over 50 cycles.To further extend the cycle life of the battery,Kang et al.constructed a Zr-based MOF(nMOF-867)with sp2nitrogen in its organic linker.[138]Fouriertransform infrared(FTIR)and XPS characterizations confirmed interaction of the LiPS with the functional heteroatoms, thus preventing dissolution of the LiPS into the electrolyte.When using nMOF-867/S as a cathode material,a high specific energy density of～1700 Wh g-1and low capacity fading rate of～0.027% per cycle over 500 cycles at 0.835 A g-1were achieved(Figure 9f).Subsequently,Feng et al.reported the fabrication of CoMOF with a rough porous surface using cobalt salt and BTC as raw materials.[195]Three CoMOF-S composites(CoMOFS1,CoMOFS2,and CoMOFS3)were obtained,with 39.0,56.6,and 65.6 wt% sulfur,respectively.Electrochemical tests showed that the CoMOFS2 cathode exhibited good capacity recovery(87.18% retention)after 100 cycles.Even at 2 C,the CoMOFS2 electrode retained a capacity of over 600 mA h g-1.The outstanding electrochemical behavior originated from the strong covalent bonds with sulfur and appropriate pore size for sulfur confinement in the MOF structure, which greatly mitigated shuttling of the soluble polysulfides.

Recently,a Mn-based MOF was reported as a sulfur host for LSBs.Zang and co-workers fabricated a 9-Mn node-based MOF(Mn-CCsxH2O)with cubic cages and prominent thermal stability.[140]After dehydration,Mn-CCs possessing empty cubic cages were obtained and applied as cathode host material of LSBs.Profiting from the appropriate pore volume and abundant open metal sites,a high initial specific capacity of 1460 mA h g-1was achieved,with a stable capacity of 990 mA h g-1after 200 cycles.Three transition metal hexaaminobenzene-based coordination polymers(TM-HAB-CPs)with excellent conductivity were also investigated as cathode materials for LSBs.[196]V-HAB-CP exhibited the strongest polysulfide confining ability of the three coordination polymers(V-,Cr-,and Fe-HAB-CPs).DFT calculations showed that V-HAB-CP possessed the most negative adsorption energy for LiPS,strongly curtailing the shuttling of polysulfides.Moreover,the volume variation of V-HAB-CP was quite small(about 3.06% )before and after battery cycling.Li16S8/V-HAB-CP as the electrode reaction product afforded an energy density of 808.465 W h kg-1.

6.Conclusions and Outlook

In the past decade,LSBs have been extensively developed.Although the incredible theoretical specific capacity and energy density make LSBs promising electrochemical energy storage systems,achieving high-performance LSBs is still challenged by major problems,necessitating the selection of suitable and effective sulfur host materials.Loading MOFs with sulfur has emerged as a topical research field and has aroused great interest owing to the follow merits:1)the high porosity insures high loading of sulfur(ordinarily＞50% );2)small pores can restrict the loss of polysulfides from the pores,thus effectively impeding the shuttle effect;3)the porous structure can alleviate volume expansion during lithiation process;and 4)the abundant open metal sites acting as Lewis acid sites and functional groups in the organic linkers serving as Lewis base sites can generate strong interactions with LiPS,thus delivering exceptional polysulfide confining ability.Strategies for enhancing the performance of LSBs were summarized.It can be concluded that MOFs as host materials can afford exceptional performance by selecting an appropriate particle size,constructing a suitable pore structure,and enhancing the Lewis acid-base interaction via introducing functional groups onto the organic linkers and exposing the open metal sites.Sulfur loading could be achieved by melt-diffusion,sulfide conversion,and solvent evaporation.Application of the as-obtained sulfur/MOF composites in LSBs demonstrated their outstanding electrochemical performance.

Although MOFs have provided encouraging results in LSBs,research on MOF-based sulfur-loaded materials is still in its infancy,and thus,future work should focus on this research direction.Currently,the selection of host materials mainly focuses on some representative MOFs(e.g.,ZIF-8,ZIF-67,MIL-53,HKUST-1,MIL-100(Cr),and MIL-101(Cr)).In further research,other MOFs with high chemical and thermal stability should be explored to fabricate advanced sulfur-loaded materials.Considering the published literature,there are still many challenges to overcome.

1 More studies should focus on fabricating MOF materials with more favorable pore structures.MOFs as host materials are conducive to confining polysulfides due to their microporous characteristics.However,due to the low diffusion rate of Li+,the resistance of LSBs may increase,resulting in lower sulfur utilization.The fabrication of more favorable porous structures can afford better polysulfide confining ability and effectually inhibit polysulfide diffusion into the electrolyte.

2 A systematic evaluation of the fundamental electrochemical reaction mechanism in LSBs should be performed as an important undertaking for providing guidance for designing advanced LSBs.Although the basic reaction and corresponding products during charge/discharge have been revealed,the conversion mechanism of LSBs is still not understood deeply.The charge/discharge process is highly complicated,not only involving many complex electrochemical reactions,but also entailing structural and compositional variation of multiple intermediates.During the redox process,more advanced techniques are required to investigate the reaction kinetics of the active materials in LSBs.

3 MOFs with excellent electrical conductivity should be selected as more advanced sulfur host materials.A common disadvantage of directly utilizing MOFs as the sulfur hosts is the poor conductivity.MOFs with good conductivity can heighten the utilization of insulating sulfur during battery cycling and reduce the interface resistance of LSBs.Moreover,combining MOF/sulfur composites with conductive additives,such as carbon materials(e.g.,reduced graphene oxide and carbon nanotubes)and polymers(e.g.,polypyrrole and PEDOT),can alleviate this issue.

4 The addition of inactive substances should be reduced as much as possible.High sulfur loading is the fundamental requirement for realizing high energy density LSBs,while adding more inactive materials can inevitably decrease the mass ratio of the active materials.Therefore, the introduction of inactive materials should be assessed on the premise of inhibiting polysulfide shuttling.Furthermore,the host materials should be precisely designed in terms of porosity and morphology to obtain high sulfur content.

5 Practical technology for manufacturing materials and battery constructions should be further developed.The research on LSBs is driven by their practical value;thus,the results of laboratory experiments should be consistent with the actual application.Although the experimental results for MOFs employed as hosts suggest excellent performance,the practical feasibility of these hosts still requires further confirmation.Although complex operation processes can be easily realized in the laboratory,the harsh experimental conditions make it difficult to achieve large-scale production in actual operation.Therefore,practical,low cost,and environmentally friendly technologies should be further developed.

6 Different experimental environments should be explored to generate unexpected performance.In addition to the materials themselves,the operating conditions are also important factors that can affect the experimental results.Based on previous studies,it can be concluded that the charge-discharge voltage and the amount of electrolyte can affect the performance of LSBs.Choosing appropriate experimental conditions can enhance the stability of MOFs,thereby furnishing excellent performance.

Based on the challenges involved in LSBs,the design of MOF-based sulfur-loaded materials for high-performance LSBs can be realized by optimizing structure,selecting components,and enhancing conductivity(Figure 10).Overall,although the commercialization of LSBs is still hampered by many challenges,MOF-based sulfur-loaded materials are definitely a significant research area.With continuous research efforts,the number of unresolved problems in this field will continue to decrease,and more breakthroughs are bound to be realized in the future.We believe that this review is conducive to the design of more favorable MOF-based sulfur-loaded materials and offers beneficial reference for future developments.

Figure 10.A summary of three aspects to be considered to design MOF-based sulfur-loaded materials for high-performance LSBs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(NSFC-U1904215,and 21671170),Changjiang scholars program of the Ministry of Education(Q2018270),the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions(TAPP),Program for New Century Excellent Talents of the University in China(NCET-13-0645),the Six Talent Plan(2015-XCL-030),and Qinglan Project of Jiangsu and Program for Colleges Natural Science Research in Jiangsu Province(18KJB150036).We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received at the Testing Center of Yangzhou University.

Conflict of Interest

The authors declare no conflict of interest.