Strong Sulfur Binding with Conducting Magneli-Phase TinO2n−1Nanomaterials for Improving Lithium−Sulfur BatteriesXinyong Tao,†,‡,⊥ Jianguo Wang,†,⊥ Zhuogao Ying,†,⊥ Qiuxia Cai,† Guangyuan Zheng,§,‡ Yongping Gan,†

Hui Huang,† Yang Xia,† Chu Liang,† Wenkui Zhang,*,† and Yi Cui*,‡,∥

†College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, China‡Department of Materials Science and Engineering, §Department of Chemical Engineering, Stanford University, Stanford, California94305, United States∥Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park,California 94025, United States

*S Supporting Information

ABSTRACT: Lithium−sulfur batteries show fascinatingpotential for advanced energy storage systems due to theirhigh specific capacity, low-cost, and environmental benignity.However, the shuttle effect and the uncontrollable depositionof lithium sulfide species result in poor cycling performanceand low Coulombic efficiency. Despite the recent success intrapping soluble polysulfides via porous matrix and chemicalbinding, the important mechanism of such controllabledeposition of sulfur species has not been well understood.Herein, we discovered that conductive Magneli phase Ti4O7 is highly effective matrix to bind with sulfur species. Compared withthe TiO2−S, the Ti4O7−S cathodes exhibit higher reversible capacity and improved cycling performance. It delivers high specificcapacities at various C-rates (1342, 1044, and 623 mAh g−1 at 0.02, 0.1, and 0.5 C, respectively) and remarkable capacityretention of 99% (100 cycles at 0.1 C). The superior properties of Ti4O7−S are attributed to the strong adsorption of sulfurspecies on the low-coordinated Ti sites of Ti4O7 as revealed by density functional theory calculations and confirmed throughexperimental characterizations. Our study demonstrates the importance of surface coordination environment for stronglyinfluencing the S-species binding. These findings can be also applicable to numerous other metal oxide materials.

KEYWORDS: Lithium Sulfur Battery, Magneli Phase, Ti4O7, Strong Adsorption, Nanoparticles

The lithium−sulfur (Li−S) battery have attracted consid-erable attention owing to its high theoretical capacity of

1672 mAh g−1 (3467 mAh cm−3),1−4 based on the reaction: S8+ 16Li ↔ 8Li2S. Upon discharging, S8 will go through a seriesof soluble intermediate higher-order polysulfides (Li2S8, Li2S6,and Li2S4) and transform into solid Li2S2 and Li2S.

5 During asubsequent charging, Li2S2 and Li2S should be reversiblyconverted to S via the soluble polysulfide intermediates. Thediffusion of the polysulfides into the electrolyte will result incapacity decay, low Coulombic efficiency (CE) and theuncontrollable deposition of Li2S2, Li2S, and sulfur.6−8

In order to solve these challenges, there have been recentdevelopments on advanced sulfur cathode structures, as well asnew electrolytes, separators and binders, to enhance the cyclingstability of Li−S batteries. In general, the new sulfur cathodematerials can be grouped into three categories: (1) carbon−sulfur nanocomposites to encapsulate S species usingmesoporous carbon,4,9 graphene,10−13 carbon nanotubes,14−16

carbon fibers,17,18 carbon nanotiles,19 and so on; (2) polymer−sulfur composites to enhance S binding by adding poly-pyrrole,20 polyaniline,21,22 and polyvinylpyrrolidone;23 (3)oxide−sulfur composites by adding SiO2,

24 TiO2,25,26 Al2O3,

27

La2O3,28 and Mg0.6Ni0.4O

29 additives or constructing TiO2

coating.30 Different from the conventional carbon materialsfor trapping the polysulfides via physical absorption, oxideshave chemical bonding with the polysulfide species, which isfavorable for restraining the shuttle effect.6 Our recent work hasproved that the polar surface such as polymer modifiedcarbon31 or tin-doped indium oxide6 can enhance the redoxkinetics of polysulfides, mediate the deposition of polysulfides,and facilitate the growth of LixS on the electrode. Despite theseexciting progresses, there are some questions that need to beaddressed: (1) the detailed bonding mechanism of sulfurspecies on the surface of oxides is still unknown; (2) theadvanced sulfur cathodes usually involve complex manufactur-ing processes; (3) the specific volumetric capacity of thecathode composite is relatively low due to the porosity ofinactive ingredient such as mesoporous carbon.Herein, Magneli phase TinO2n−1(3 < n < 10),32,33 a kind of

highly conductive oxide, was selected to fabricate high-

Received: June 20, 2014Revised: July 28, 2014

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performance TinO2n−1-modified sulfur (TinO2n−1−S) cathodecomposites. Due to the narrow band gap, TinO2n−1 showsunique physical, chemical, and optical properties that havepotential application in photocatalysis, photovoltaics, fuel cells,information storage, and energy conversion devices.34−36 Thestructure of TinO2n−1 was solved by Magneli in the 1950s,32,33

which is similar to chains structure of edge-shared TiO6octahedra in rutile. The difference is that there is shearingevery nth octahedron due to the reduction of Ti and theresulting oxygen vacancies in TinO2n−1.

32−34 Figure 1 showsstructures of the most stable surfaces of one typical Magneliphase Ti4O7 and rutile TiO2. The coordination numbers ofsurface Ti on Ti4O7(1−20) are 4(Ti4c), 5(Ti5c), and 6(Ti6c), inwhich the ratio of Ti6c is only 37.5%. That is, 62.5% of surface

Ti atoms have unsaturated chemical bonding, whereas on thesurface layer of TiO2(110), the ratio of Ti5c and Ti6c is 1:1. Inaddition, the ensemble of low coordinated Ti (for exampleTi5c) on Ti4O7(1−20) are arranged in step sites (highlighted bythe blue line), which are easily accessible to forming externalchemical interaction. In comparison, the trough line Ti5c onTiO2(110) are in the terrace sites. The structure characteristicsof Ti4O7(1−20) may lead to strong binding with sulfur andlithium sulfur species.In this work, the TinO2n−1−S composite cathode is prepared

with a facile and cost-effective method of sublimation anddeposition of S into TinO2n−1 nanomaterials (see SupportingInformation for detail procedures). Moreover, transmissionelectron microscopy (TEM), X-ray photoelectron spectroscopy

Figure 1. Schematics of (a) Magneli phase Ti4O7(1−20) and (b) the rutile TiO2(110) surfaces. Legend: gray, Ti; pink, O.

Figure 2. Structure and morphology characterization of titanium oxides. (a), (d), and (g) are XRD patterns of rutile TiO2, Magneli phase Ti6O11 andTi4O7, respectively. The inset is the corresponding stacking of the oxygen octahedral. (b), (e), and (h) are representative TEM images of rutile TiO2,Ti6O11, and Ti4O7, respectively. The inset is the corresponding digital photo, revealing the color change. (c), (f), and (i) are representative highresolution transmission electron microscope (HRTEM) images of TiO2, Ti6O11, and Ti4O7, respectively. The insets of (c), (f), and (i) show the fastFourier transform (FFT) diffraction pattern.

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(XPS), in combination with first-principles density functiontheory (DFT) calculations, were used to study the bondingmechanism of polysulfides with oxides. Compared with TiO2,Magneli phase TinO2n−1 shows highly effective binding with Sspecies, resulting in outstanding electrochemical performanceof TinO2n−1−S composites.We have developed the synthesis for three types of TiOx

nanomaterials in this work. Figure 2a shows the X-raydiffraction (XRD) pattern of TiO2 nanotube sample obtainedby facile hydrothermal treatment of TiO2 particles in NaOHsolution (See Supporting Information). All the diffraction peakscan be assigned to rutile TiO2 nanotubes (JCPDS No. 76-1940). TEM image in Figure 2b indicates abundant nanotubeswith diameters ranging from 6 to 25 nm. HRTEM image(Figure 2c) and the corresponding FFT diffraction pattern(inset of Figure 2c) reveal that the TiO2 nanotubes are singlecrystalline with inner diameter around 3 nm. After a 4 hhydrogen reduction at 950 °C of the as-prepared TiO2

nanotubes, deep purple powder (inset of Figure 2e) can beobtained. XRD (Figure 2d) analysis indicates that the rutilephase is converted into high purity Magneli phase Ti6O11

(JCPDS No. 500788). The TEM image in Figure 2e and theHRTEM image in Figure 2f prove that the TiO2 nanotubestransform into crystalline Ti6O11 nanowires. As the reductiontemperature was increased to 1050 °C, deep black powders(inset of Figure 2h) can be obtained. The XRD peaks (Figure2g) are indexed to the Magneli phase Ti4O7 (JCPDS No. 50-0787). TEM image (Figure 2h) shows abundant nanoparticleswith diameters from 80 to 400 nm, suggesting that there isobvious morphology evolution from TiO2 to Ti4O7, which canbe attributed to the rearrangement of lattice derived fromoxygen release during the reduction process. HRTEMobservation (Figure 2i) combined with FFT diffraction pattern(inset of Figure 2i) suggest the single crystalline nature of theTi4O7 nanoparticles. These structure analyses demonstrate that

the structure of Magneli crystalline TinO2n−1 can be controlledby changing reduction temperatures. The inset in Figure 2a, d,and g show the stacking of the oxygen octahedra containing theTi cations in rutile TiO2, Magneli phase Ti6O11 and Ti4O7,respectively. The numbers 1, 2, 3, 4, 5, and 6 refer to the cationsites. Different from the rutile TiO2 with only edge-sharedoctahedron strings connected to each other via shared corners(inset in Figure 2a), Ti4O7 and Ti6O11 have face-sharing Ti2O9

dimers connected with shared corner and edges every fourthand sixth octahedral,36 which result from the partial reductionof titanium and the migration of vacancies. The reductiondegree enable fine-tuning of the electrical conductivity, showingeither semiconductor (Ti6O11) or metallic (Ti4O7) behavior atroom temperature.37 Many researches proved that the surfaceTi atoms and oxygen vacancies on defective titanium oxideswere absorb sites for the oxide or oxygen species.38,39 Becausesulfur and oxygen belong to the same group in periodic table,they have similar electron configurations and share some similarchemical bonding properties. Therefore, it is expected that theMagneli phase titanium oxides can be used for the fabrication ofhigh performance sulfur cathode materials.Facile dispersing and thermal diffusion of S into titanium

oxide nanomaterials (see Supporting Information) was used tofabricate the composite cathodes. Figure 3 shows theelectrochemical performance of composite cathodes based ondifferent titanium oxides. The sulfur content is 74.3, 64.5, and64.2 wt % for TiO2−S, Ti6O11−S, and Ti4O7 composites,respectively. The mass loading of the composite electroderanges from 1 to 3 mg cm−2. The electrolyte is 1 MLiN(CF3SO2)2 in a mixed solvent of 1,3-dioxolane (DOL)and dimethoxymethane without LiNO3 additive. All capacitieswere calculated based on the sulfur mass. Figure 3a shows thefirst cycle charge−discharge curves of cathodes at a current rateof 0.02 C (1 C = 1672 mA g−1). All charge−discharge profilesshow two typical plateaus, corresponding to the formation of

Figure 3. Electrochemical properties of composite cathodes. (a) Initial charge−discharge curves of the cathode with different titanium oxides at a0.02 C rate. (b) Cycling performance and the related CE of the electrodes for 100 cycles at 0.1 C rate. (c) Rate performance of the Ti4O7−Scomposite. (d) Nyquits plots for the three samples with different titanium oxides.

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Figure 4. TEM and EDS characterization of Ti4O7−S before and after cycling. (a) TEM image, (b) STEM image, (c) Ti elemental mapping, and (d)S elemental mapping of the Ti4O7−S composite before cycling. (e) TEM image, (f) STEM image, (g) Ti maps, and (h) S maps of Ti4O7−S after 100cycles. The particle in (g) and (h) is selected from the rectangular region of (f).

Figure 5. XPS spectra of titanium oxide modified composites. (a) Ti 2p and (b) S 2p XPS spectra of the Ti4O7−S electrode discharged to 1.5 V. (c)Ti 2p and (d) S 2p XPS spectra of Ti4O7−S electrode charged to 2.4 V. (e) S 2p XPS spectra of the mixture of TiO2 and Li2Sx.

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high-order polysulfides at 2.3 V and low-order polysulfides at2.1 V, respectively.5,19 The third discharge plateau of the TiO2−S sample at 1.75 V can be attributed to TiO2.

40 Obviousovercharge and a discharge capacity of 824 mAh g−1 with CE of78% can be found in the TiO2−S sample. The lower CE resultsfrom the significant polysulfide dissolution, causing the loss of Smaterials and shuttle effect. Without LiNO3 additive, the Limetal surface is not passivated well. However, both Ti6O11−Sand Ti4O7−S exhibit higher discharge capacity of 1108 and1342 mAh g−1, respectively. The Coulombic efficiency at thefirst cycle of Ti6O11−S and Ti4O7−S were increased to 91% and96%. Figure 3b shows the cycling stability and the related CE ofthe electrodes a 0.1 C rate for 100 cycles. The initial dischargecapacity of TiO2−S, Ti6O11−S, and Ti4O7−S are 701, 713, and1044 mAh g−1, respectively. The capacity retention of TiO2−Sis only 58% after 100 cycles. Remarkably, the Ti6O11−S andTi4O7−S show much better capacity retention of 89% and 99%,respectively. Among the evaluated cathodes, Ti4O7−S cathodeexhibits the best cycling stability and highest Coulombicefficiency. Prolonged cycling for 250 cycles at 0.5 C rate wasalso used to further evaluate the Ti4O7−S sample (SupportingInformation Figure S1). The initial discharge capacity is 623mAh g−1 and the capacity retains as high as 604 mAh g−1 after250 cycles, indicating that the capacity decay is only 0.012% percycle. Figure 3c reveals the rate performance of the Ti4O7−Scomposite. The discharge capacities steadily changed alongwith the increase of the current rate from 0.1 to 1 C. Theaverage reversible capacity at 1 C rate is 527 mAh g−1. Whenthe current density was back to 0.1 C, the capacity can berecovered to the original 0.1 C level after 10 cycles, suggestingthe reliability and stability of the Ti4O7−S structure. It is worthmentioning that the Ti4O7−S composite has a highercompacted density of 1.84 g cm−3, which is close to thedensity of bulk S (1.96 g cm−3). Compared with the normal C−S composites,9 the Ti4O7−S composite shows a highervolumetric capacity of 1580 mAh cm−3 (0.02 C) Figure 3dshows Nyquits plots for the three samples with differenttitanium oxides. The semicircle in high-frequency region andthe line in the low frequency region can be ascribed to thecharge-transfer resistance of electrochemical reaction and thediffusion-controlled Warburg impedance, respectively.41 It canbe found that the Ti4O7−S sample has the minimum charge-transfer resistance compared with Ti6O11−S and TiO2−S,which can be attributed to the higher conductivity of Ti4O7.

To further demonstrate the role of the Ti4O7, TEM and EDSanalysis were performed on the sample before and after cycling.Figure 4a is a typical TEM image of the Ti4O7−S beforecycling. Our previous research proved that taking a high qualitybright field TEM image of a pristine S microparticle was verydifficult due to its flash vaporization within 0.5−2 s underelectron beam irradiation (EBI).42 However, we can obtain adistinct bright field TEM image (Figure 4a) of the Ti4O7−Sunder high energy EBI (300 keV). No obvious shrink wasobserved after 2 min EBI (Figure 4b). The elemental mapping(Figure 4c and d) reveal that several Ti4O7 nanoparticles aremounted in the S microparticle. During the high energy EBI,inelastic scattering will cause the heating effect.42 In addition,the displacement of atoms caused by the incident electrons andthe charging of the specimen system result in the flashvaporization of sulfur.42 The stability of Ti4O7−S under EBImay be ascribed to the improved conductivity and strongbonding effect of Ti4O7. Figure 4e shows the bright field TEMimage of Ti4O7−S sample after 100 cycles. Figure 4f is thecorresponding STEM image, showing that Ti4O7 particles arewell dispersed in the composite electrode. Figure 4g and hdepict the Ti and S mapping around the Ti4O7 nanoparticleshowed in the rectangular region of Figure 4f. Abundant Ssignal can be detected around the Ti4O7 nanoparticle (Figure4h). It is notable that the rich S area overlays very well with theTi map, indicating that Ti4O7 is favorable for the adsorptionand selective deposition of the sulfur species.To further demonstrate the interaction of LixS and titanium

oxide, we use XPS to study the titanium oxide modifiedcomposites. Because Li2Sx species are sensitive to ambient air,all samples were transferred to the XPS instrument from theAr-filled glovebox using a specialized chamber. Figure 5a and bshow the Ti 2p and S 2p XPS spectra of the Ti4O7−S electrodedischarged to 1.5 V, respectively. Figure 5a shows two obviouscharacteristic peaks of Ti4O7 at 459 and 464.6 eV.35 The smallpeak at 456.7 eV reveals Ti−S binding in the Ti4O7−S.

43,44 Inthe XPS S 2p spectra, various sulfur bonds can be observed.The peaks in the range of 169−172 eV and 163−165 eV inFigure 5b are assigned to electrolyte and Li2Sx species,45

respectively. The peaks around 162 eV can be attributed to theoverlap of both S−Ti44 and Li−S binding.45 In addition, thepeaks around 164.8 eV correspond to S−S bond from Li2Sx.

46

Figure 5 c and d reveal the Ti 2p and S 2p XPS spectra of theTi4O7−S electrode charged to 2.4 V, respectively. The increase

Figure 6. DFT analysis of the adsorption of S species on Ti4O7(1−20) and TiO2(110) surfaces. (a), (b), (d), and (e) are the optimized geometriesfor Sx on Ti4O7(1−20), Li2Sx on Ti4O7(1−20), Sx on TiO2(110), and Li2Sx on TiO2(110), respectively. (c) Adsorption energies of most stablestructures of sulfur species. Legend: gray, Ti; pink, O; yellow, S; purple, Li.

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in peak intensity ratio of polysulfides (S−S bonds) toelectrolyte suggests that low order polysulfides are oxidizedduring the charging process. In addition, similar characteristicpeaks of Ti−S bonds can also be detected in the chargedTi4O7−S electrode (Figure 5c and d). Therefore, Ti−S bondsexist in both charging and discharging process, which must bebenefit for the adsorption of polysulfides and deposition of Sspecies. Figure 5e shows the S 2p XPS spectra of the mixture ofTiO2 and Li2Sx. It is notable that there are two strong peaksaround 167 eV, which can be indexed to the S−O binding.46

However, no obvious S−O binding can be detected in thecharged and discharged Ti4O7−S samples, suggesting that theremay be different adsorption behaviors for TiO2 and Ti4O7.In order to reveal the different performances of Magneli

phase and rutile TiO2 in lithium−sulfur batteries, the bondingproperties of Sx (x = 1, 2, and 4) and Li2Sx (x = 1, 2 and 4) onTi4O7(1−20) and TiO2(110) were studied by DFT calcu-lations. Figure 6 shows the optimized geometries andadsorption energies of most stable structures of these species.First, the bonding sites of sulfur on Ti4O7(1−20) andTiO2(110) are dramatically different. On Ti4O7(1−20), thelow coordinated Ti (Ti4c and Ti5c) are the most favorableadsorption sites for small sulfur clusters (Figure 6a). On TiO2

(110), it is found that sulfur is strongly bonding with twobridging oxygen atoms, which leads to the formation of SO2

like species (Figure 6d). This result can be supported by theXPS analysis in Figure 5e. Second, the adsorption strengths ofsulfur clusters Sx (x = 1, 2, and 4) on Ti4O7(1−20) are nearlythe same (Figure 6c), which leads to the easily formation ofsulfur clusters on Ti4O7. However, on TiO2(110), theadsorption of S1 is much stronger than S2 and S4 (Figure 6c).Small sulfur clusters on TiO2(110) are thermodynamically lessstable than sulfur monomer, which has ability to form SO2-likebonding, but small sulfur clusters do not. It is concluded thatthe low coordinated Ti on titanium oxide can stabilize thesulfur clusters, whereas oxygen rich titanium oxide can stabilizemetal (for example Au) clusters47,48 due to the formation ofstrong chemical bonding. Finally, the adsorption of Li2 onSx_Ti4O7(1−20) shows similar trend as Sx that all with strongadsorption (Figure 6c). Taking Sx_TiO2(110) as the referencesystem, the adsorption of Li2 is also very strong (Figure 6c).These DFT-based simulation results, combined with the XPS,TEM, and EDS analysis, prove that the remarkable electro-chemical performance of Ti4O7−S composites must derivefrom both the high conductivity and the unique surfacechemical properties of Ti4O7.In summary, we have fabricated high-performance sulfur

composite cathodes based on Magneli phase Ti4O7, which actsas a nearly ideal matrix because of its high electricalconductivity and unique strong sulfur binding. Comparedwith the TiO2−S counterpart, the Ti4O7−S cathodes exhibithigher reversible capacity and significantly improved cyclingperformance. DFT calculations and experimental results provethat the surface of Ti4O7 with abundant low coordinated Tisites is favorable for the adsorption and selective deposition ofthe sulfur species. We believe that our findings can begeneralized to other kinds of Magneli phase metal oxides(VnO2n−1, WnO2n−1, MonO2n−1, etc.) and some other matrixmaterials with abundant low coordinated metal sites on surface,which will be studied in the future.

■ ASSOCIATED CONTENT

*S Supporting InformationDetailed description of the experimental procedures andcalculations. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*W. Zhang. E-mail: msechem@zjut.edu.cn.*Y. Cui. E-mail: yicui@stanford.edu.

Author Contributions⊥These authors contributed equally.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Y.C. acknowledges the support from the Assistant Secretary forEnergy Efficiency and Renewable Energy, Office of VehicleTechnologies of the U. S. Department of Energy. Weacknowledge support from National Natural Science Founda-tion of China (Grants 51002138, 51172205, 21136001, and91334013), the Natural Science Foundation of ZhejiangProvince (Grants LR13E020002 and LY13E020010) and the973 project (2013CB733501).

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Nano Letters Letter

dx.doi.org/10.1021/nl502331f | Nano Lett. XXXX, XXX, XXX−XXXG