ALD TiO2 coated flower-like MoS2 nanosheets on carbon cloth as sodium ion battery anode with enhanced cycling stability and rate capability

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1 Article Subscriber access provided by NEW YORK UNIV ALD TiO coated flower-like MoS nanosheets on carbon cloth as sodium ion battery anode with enhanced cycling stability and rate capability Weina Ren, Weiwei Zhou, Haifeng Zhang, and Chuanwei Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript DOI: 0.0/acsami.b Publication Date (Web): Dec Downloaded from on December, Just Accepted Just Accepted manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides Just Accepted as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. Just Accepted manuscripts appear in full in PDF format accompanied by an HTML abstract. Just Accepted manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI ). Just Accepted is an optional service offered to authors. Therefore, the Just Accepted Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these Just Accepted manuscripts. is published by the American Chemical Society. Sixteenth Street N.W., Washington, DC 0 Published by American Chemical Society. Copyright American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

2 Page of 0 ALD TiO coated flower-like MoS nanosheets on carbon cloth as sodium ion battery anode with enhanced cycling stability and rate capability Weina Ren a, Weiwei Zhou b, Haifeng Zhang a, Chuanwei Cheng a* a Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 00, P.R. China b School of Materials Science and Engineering, Harbin Institue of Technology at Weihai, Weihai,, P. R. China cwcheng@tongji.edu.cn Abstract In this paper, we report the fabrication of D flower-like MoS nanosheets arrays on carbon cloth as a binder-free anode for sodium ion battery. Ultrathin and conformal TiO layer are used to modify the surface of MoS by atomic layer deposition. The electrochemical performance measurements demonstrate that the ALD TiO layer can improve the cycling stability and rate capability of MoS. The MoS nanosheets with 0. nm TiO coating electrode shows the highest initial discharge capacity of ma h g - at 0 ma g -, which is increased by % compared with that of bare MoS. After cycles, the capacity retention rates of the TiO coated MoS achieves.% of its second cycle s capacity at 0 ma h g - in contrast to that of % of pure MoS. Furthermore, the mechanism behind the experimental results are revealed by ex-situ scanning electron microscope (SEM), X-ray powder diffraction (XRD) and electrochemical impedance spectroscopy (EIS) characterizations, which confirm that the ultrathin TiO modifications can prevent the structural degradation and the formation of SEI film of MoS electrode. Keywords: Sodium ion battery; Anode; Atomic layer deposition; Flower-like MoS nanosheets; TiO ;

3 Page of 0. Introduction Na-ion batteries (NIBs) have received an increasing attention as promising alternatives to Li-ion batteries (LIB) due to the low cost, natural abundance and a similar redox potential to lithium of sodium element. - However, the selection and exploring of suitable electrode materials for Na-ion batteries is still a great challenge due to that the Na + has larger radius of (.0 Å) than that of Li + (0. Å), leading to a higher volume expansion and lower energy densities. Currently, the investigated anode materials for sodium ion batteries include carbon based materials, - alloy-based materials, -0 metal oxides - and transition metal sulfides -. Among them, MoS, as a member of layer transition metal dichalcogenides, is a promising candidate anode because of its graphite-like structure with large layer spacing of 0. nm as well as high theoretical capacities (0 ma h g - ). However, the poor cycling stability and rates capability arising from the low electronic transmission rate and aggregation tendency plus the dissolution of reaction intermediates (Na S) during charging/discharging remain the stumbling blocks on the road of development MoS anode. - To address these problems, one effective approach is preparation of, ultra-thin and expanded spacing MoS and combined with high conductivity materials like carbon, -. Besides, deposition of a passivation layer at the electrode/electrolyte interface is another effective strategy to prevent the structure degradation and repeated formation of SEI films, which have been widely adopted in LIBs research, and also NIBs anodes,. To this end, atomic layer deposition shows great potential for thin film deposition and electrode surface modification due to its advantages in excellent shape retention and thickness controllability. - Herein, we report the preparation of ultrathin TiO modified D flower-like MoS nanosheets arrays on flexible carbon cloth by a hydrothermal method and a subsequent ALD process for the first time. As a binder-free anode for sodium ion battery, such an electrode design is favorable to improve the specific capacity, cycling stability and rates performance. First, the D flower-like nanosheets arrays directly supported on current collector with large surface area are facilitating the electron and

4 Page of 0 ion diffusion and transportation (Scheme ). Second, the TiO layer is stable enough during sodiation/desodiation cycles, which can act as a satisfactory passivation and buffer layer of MoS to prevent the electrode corrosion and collapse. Moreover, the synergistic effect between the MoS and TiO is conducive to improve the electrode capacity and cycling stability. By testing them as NIBs anodes, enhanced capacity, cycling stability and rates capability are demonstrated by ALD TiO modification on MoS nanosheets surface.. Experimental Section. Samples preparation Synthesis of D MoS nanosheets on carbon cloth: First, the carbon cloth was cleaned in acetone, ethanol and deionized water by ultrasonication for minutes each, respectively, and then soaked in nitric acid with a concentration of % for hours to improve its surface hydrophilicity. Then the growth of MoS nanosheets on carbon cloth was accomplished by a hydrothermal method. In a typical process: mg sodium molybdate (NaMoO.H O) and 0 mg of thiourea (CH N S) were dissolved in ml of deionized water and stirred for minutes until clear, and then the above solution was transferred to a ml sealed Teflon-lined stainless steel autoclave with one piece of the treated carbon cloth placed inside. The temperature was kept at 0 C by an electric oven and maintained for h. After that, the sample was taken out and washed with deionized water repeatedly, then dried in an oven at 0 C. ALD of ultrathin TiO layer on MoS nanosheets: The ultrathin TiO layer coated on the MoS surface with different thickness was conducted by a Picosun R-0 ALD system. During the deposition process, TiCl and H O were used as the precursors of Ti and O, respectively. The TiO was deposited at C with the speed of ~ 0. Å/cycle and the thickness of the TiO layer can be effectively controlled through different cycles settings: cycles, cycles and cycles were used to deposit about 0. nm, nm and nm, respectively.. Material characterizations

5 Page of 0 The microscopic structure and morphology of the as-synthesized MoS nanosheets and TiO coated MoS nanosheets were recorded by field-emission scanning electron microscope (FE-SEM, FEI Sirion0), high-resolution transmission electron microscope (HR-TEM, JEM-0F) and selected area diffraction patterns. The crystal structure, composition and valence state of the samples were characterized by X-ray powder diffraction (XRD, Bruker D-Advance) and X-ray photoelectron spectra (XPS, Kratos Axis Ultra DLD).. Electrochemical Measurements Electrochemical measurements of all the active materials were carried out by using CR coin-type cells. MoS and MoS /TiO nanosheets on carbon cloth were directly used as binder-free anodes, and sodium foil (J&K Scientific) was used as the cathode for sodium ion battery. A microporous glass fiber membrane (Whatman) and M solution of NaClO in ethylene carbonate (EC) and dimethyl carbonate (DMC) (: by weight) with vol% fluoroethylene carbonate (FEC)were employed as the separator and electrolyte for the cells, respectively. The carbon s density is. mg cm - and the average loading density of these active materials was about mg cm -. The galvanostatic charge discharge cycles at different current densities and the rates performance were tested in a multi-channel battery tester (Neware Co., China) with a voltage of 0.0~. V at room temperature. The cyclic voltammetry measurements were performed on a CHID electrochemical workstation.. Results and Discussions. Morphology and structure characterizations The morphology and structure of the as-obtained MoS nanosheets and TiO coated ones were characterized by SEM and XRD. The low magnification SEM image in Figure a shows that spherical MoS structures with textured surfaces are almost uniformly wrapped around each carbon fiber. From a closer view in Figure b,it can be seen that the flower-like MoS structures are assembled of several thin nanosheets. The diameter of each flower is about µm. After the ALD of nm TiO coating, the morphology doesn t change much, as shown in Figure c, which is attributed to the

6 Page of 0 characteristics of ultrathin TiO layer and ultra-uniform deposition characteristics of ALD. The SEM images of MoS nanosheets coated with 0. nm and nm TiO are also provided in Figure S in Supporting Information. The EDX elemental mapping (Figure S (b)-(e)) images obtained from the MoS - nm TiO sample indicate the well distribution of the Mo, S, Ti and O elements. The phase purity and crystallinity of the as-prepared samples were further demonstrated by XRD. As displayed in Figure d, all the diffraction peaks are matched with the hexagonal phase of H-MoS (JCPDS No. -)., The peaks at.,,, and are respectively indexed to (00), (00), (0), (00) and (0) planes of the single phase MoS, and the peak at comes from the carbon cloth substrate. Especially, the (00) peak at. indicates the stacked layered structure of the MoS nanosheets. Noting that we haven t observe the characteristic peaks of TiO in the TiO coated sample, which is due to the amorphous structure of TiO deposited at. In order to explore more detailed structural information of the MoS nanosheets and MoS -TiO composites, TEM and HR-TEM observations were performed. As shown in Figure a, the ultrathin MoS nanosheets are interlaced with each other to form the flowers, which is consistent with the SEM observations. From the HR-TEM image of the pure MoS nanosheets in Figure b, we can easily observe the lamellar structure on the edge of MoS nanosheet with a thickness of about nm, which is corresponding to ~ S-Mo-S layers with an interlayer spacing of about 0. nm. The thicknesses of MoS nanosheets are between ~0 nm, which can be clearly observed from the TEM image in Figure S. The corresponding SAED patterns in Figure c demonstrate that the MoS nanosheets are polycrystalline. The diffraction rings are accurately indexed to (0), (0), (00), and (00) planes of H-MoS. The HR-TEM picture of the MoS with nm TiO coating was shown in Figure d, which confirms that the amorphous TiO layer is uniformly coated the outside of the multilayer structure, and the thickness is precisely controlled at about nm by ALD. The composition and chemical state of the TiO coated MoS sample was further investigated by XPS. The high-resolution XPS spectra in Figure show a narrow range scans for the peaks of the four elements. The high-resolution Mo d XPS

7 Page of 0 spectrum in Figure a show two peaks with binding energies of. and. ev, which are corresponding to the Mo + d / and Mo + d /, respectively. The peak at ev is attributed to the Mo + d, which might be due to the oxidation of Mo + in the atmosphere. The S s peak is detected at. ev. The S - p / and S - p / peaks in Figure b are centered at. ev and. ev, respectively., As show in Figure c, there are two peaks at. and., which can be attributed to Ti p / and Ti p /, respectively. The O s XPS spectrum was shown in Figure d, besides the O s peaks at ev attributed to the Ti O Ti bond, the peak at.0 ev, could be assigned to the adsorbed water.. Electrochemical Performance The electrochemical performance of as-synthesized MoS nanosheets and TiO coated ones with different thickness were evaluated by tested them as sodium ion battery anodes. First, we carried out the cyclic voltammetry measurements for each electrode at a scan rate of 0. mv s - between 0.00 V and. V. In the first cycle as shown in Figure a, the curves of all the electrodes show typical peaks of MoS in the reduction and oxidation reactions. The first reduction peak at~. V attributed to the insertion of Na + into MoS, forming Na x MoS are observed in pure MoS, 0. nm and nm TiO coated MoS electrodes. Noting that this peak intensity decrease with the increase of TiO layer thickness until disappearing in the nm TiO deposited MoS electrode, which might be due to that thick TiO layer would prevent the sodium ion to diffuse and react with the MoS in a timely manner. The following reduction peaks of all the electrodes at ~0. V and ~0. V are relevant to the further insertion of Na ions in MoS., The peak under 0. V in the deep cathodic process corresponds to the subsequent conversion reaction from Na x MoS to Mo. The corresponding oxidation peaks at about 0. V,. V and. V are also observed. In the second cycle in Figure b of CV curves, the reduction peaks at ~0. V and ~0. V are reduced and change to higher potential of 0. V and. V, respectively. One peak at. V is attributed to the intercalation reaction of Na + ions, and the other one at 0. V is assigned to the conversion reaction. The redox peaks tend to keep stable in the following four cycles as displayed in Figure S, which indicates the stable and reversible process of

8 Page of 0 sodiation/desodiation reactions. - During the oxidation reduction process, the typical peaks of TiO are not observed, which may be due to the ultra-thin thickness of TiO layer. Figure a shows the charge-discharge voltage-capacity curves of all the samples for the first cycle at the current density of 0 ma g - and a potential window of 0.0 V~. V. The three voltage plateaus located at 0. V, 0. V and 0. V~0.0 V are observed in the four electrodes, which are consistent with the CV analysis in Figure a. In order to prove the stable cycling performance of the TiO modified electrode, the galvanostatic charge-discharge test was continued until cycles under the same condition, which was demonstrated in Figure b. According to the order of TiO thickness from thin to thick, the initial discharge & charge capacities of each electrode are. ma h g - &. ma h g - (0. nm), 0 ma h g - &. ma h g - ( nm) and. ma h g - &. ma h g - ( nm), respectively. Compared with that of the pure MoS (0 ma h g - &. ma h g - ), the initial discharge capacity of the electrodes modified by TiO was significantly improved, and the initial coulombic efficiency (CE) has also been increased from % to %. After cycles, the discharge specific capacity of MoS is. ma h g -, which is only % of its second cycle s capacity. Meanwhile, the capacity retention rate of the composite electrodes with 0. nm, nm and nm TiO coating are.%,.% and %, respectively. This performance improvement in TiO coated MoS might be attributed to the synergistic effect of the composite materials and the passivation nature of TiO layer with functions of preventing pulverization of MoS caused by the volume change during charging/discharging process, as well as protecting the MoS from corrosion and preventing the formation of SEI film. Figure c exhibits the rates performance of the samples, the MoS nanosheets electrodes with ultrathin TiO modification show outstanding capacity retention at various current densities from to 00 ma g -. Even at a high rate of 00 ma g -, their capacities retention could be still preserved at more than %, which corresponds to the capacities of ma h g - for MoS -0. nm TiO, ma h g - for MoS - nm TiO and ma h g - for MoS - nm TiO, respectively. When the

9 Page of 0 current returns to ma g -, the TiO coated MoS electrodes showed the specific capacity of 0 ma h g -, ma h g - and ma h g -, respectively. Their capacity recovery rates are as high as % or more, while the pure MoS is only %. The capacity and cycling performance of the TiO coated MoS electrodes in our case are superior to most of the previous reported MoS based composite electrodes, as summarized in Table S in Supporting information. In order to verify the superiority of TiO coated MoS electrode, the long-term stability of the samples was further evaluated at a high current density of 0 ma g - as shown in Figure d. After 0 cycles, the reversible capacity of MoS - nm TiO and MoS - nm TiO electrodes could be still maintained at and ma h g -, respectively, and the corresponding capacity retention were % and %. These capacities retention are far higher than that of bare and 0. nm TiO coated MoS electrodes, which are only % and %. From the view of this, the cycle stability of MoS -0. nm TiO electrode at high rate is not ideal. Such a similar poor cyclic performance with MoS might be due to that the 0. nm TiO layer is too thin to adapt to the rapid volume change during sodium ion intercalation/deintercalation, leading to severe structural damage and sulfur dissolution. Taking into account the capacity performance and the cycle stability, in can be concluded that the comprehensive performance of MoS - nm TiO electrode is the best among the three electrodes with different thickness of TiO.. Mechanism Investigation The mechanism behind the high rate performance and exceptional cycling stability of the MoS /TiO anode is also investigated. The morphology structure of electrode material after cycling can directly reflect its structural stability. Figure a-d present the SEM images of MoS and MoS coated with 0. nm, nm, and nm TiO electrodes after cycles at 0 ma g -. For the pure MoS sample, the flower-like structure is completely disappeared after several cycles due to the serious collapse and agglomeration problems. While for the TiO coated ones, the flower-like structures are kept well, demonstrating the protective effect of the TiO layer. The TEM images of bare MoS and TiO coated MoS samples after charging-discharging cycles were

10 Page of 0 also shown in Figure S. No SEI films were observed in the TiO coated electrode in contrast to that of bare MoS one, indicating that the TiO layer modification can effectively prevent the formation of SEI films. Based on the above discussions, the high capacity retention characteristic of the ALD TiO coated MoS anodes might be attributed to the function of ultrathin TiO layer that can buffer the volume change of MoS during charging/discharging process and act as a protective barrier layer between the anode/electrolyte interface to prevent the formation of SEI film,as a result of keeping the structural integrity. Furthermore, the ex-situ XRD patterns of all the electrodes after 00 cycles at 0 ma g - were measured to study the structure and phase change after sodium ion intercalation/de-intercalation process. As shown in Figure, from the bottom to up, the diffraction patterns correspond to the pure MoS (I), MoS -0. nm TiO (II), MoS - nm TiO (III), MoS - nm TiO (IV), respectively. After several cycles, two new additional peaks at. and. are clearly observed in the TiO coated samples, which correspond to the estimated lattice spacings of. and. Å, respectively. The dual relationship between these two lattice spacings demonstrates that a new lamellar structure with an enlarged interlayer spacing was formed during the process of Na + insertion and extraction. And the diffraction peaks of composite material at.,. and. indexed to the (00), (00) and (0) planes of MoS were also observed. These results suggest that the TiO layer can protect the crystal structure of MoS very well. However, the peak of corresponding (00) plane doesn t appear in the pure MoS electrode, which indicates that the layered structure was severely damaged during the cycles without the protection of TiO. The XRD results are consistent with the analyses of SEM in Figure. To understand why the MoS -0. nm TiO electrode and MoS - nm TiO electrode exhibit different long cycling stability at varied current density, impedance measurements of the two samples were carried out after cycles at a constant current density of 0 ma g - and after 00 cycles at a constant current density of 0 ma g -. As show in Figure a and b, all the Nyquist plots were consisted of two semicircles at high and medium frequencies and straight sloping lines at low

11 Page 0 of 0 frequencies. It well known that the intercept of the Z axis in the high frequency corresponds to the bulk resistance of electrolyte, separator and electrodes, the high frequency semicircle corresponds to the resistance of SEI film, the medium frequency semicircle corresponds to the charge-transfer resistance, and the low frequency sloping line is associated with the diffusion of sodium ions in the active material. - The corresponding Nyquist plots was fitted by an equivalent circuit model as shown in Figure S. As shown in Figure a, it can be seen that the semicircles at high and medium frequencies of MoS -0. nm TiO electrode are smaller than that of the MoS - nm TiO electrode, which reveals that the MoS -0. nm TiO electrode possesses the lower interfacial resistance after cycles at 0 ma g -. Obviously, when the current density changes to 0 ma g - as shown in Figure b, the interface resistance of MoS -0. nm TiO electrode is much larger than that of MoS - nm TiO electrode, which is arising from the collapse of structures and the absence of electrical contact. Figure c and d show the Z -ω / (ω =πf) curves in the low frequency region, and the low slope indicates good sodium ion kinetics in the electrode materials.,, They clearly show that the sodium ion kinetics of MoS -0. nm TiO was better than that of MoS - nm TiO after cycles at 0 ma g - and worse than it after cycles at 0 ma g -. This result is consistent with the phenomenon of the above long-term galvanostatic charge-discharge test, the lower interface resistance and better sodium ion kinetics are the main factors leading to the excellent cycling stability.. Conclusions In conclusion, D flower-like MoS nanosheets arrays on carbon cloth have been fabricated by a facile and scalable hydrothermal route as binder-free anode for sodium ion batteries. The cycling stability and rate capability of MoS nanosheets electrodes were improved greatly by ALD deposited conformal TiO layer. After cycles at 0 ma g -, a remarkable capacity retention characteristic (.% of second capacity) was demonstrated for the TiO coated electrode. Such an excellent electrochemical performance is ascribed to functional TiO layer that can keep the structure integrity of MoS nanosheets as well as prevent the SEI layer formation. The ex-situ SEM and

12 Page of 0 XRD results revealed that the TiO layer is able to improve the crystallinity and D structure retention of MoS nanosheets during cycling process, and the EIS tests showed the relatively thick TiO layer can reduce the resistance of SEI film and the electron-transfer, and promote the diffusion efficiency of the sodium ion at a higher current density. Our study provides a valuable reference for the electrode design and surface engineering towards high performance sodium ion batteries with long cycling life and high rate capability. Supporting Information SEM images of D MoS nanosheets coated with 0. nm and nm TiO layer; EDX mapping of MoS - nm TiO ; TEM images of D MoS nanosheets; CV curves of MoS, MoS -0. nm TiO, MoS - nm TiO and MoS - nm TiO at a scanning rate of 0. mv s - in the first cycles; TEM images of MoS nanosheets and MoS - nm TiO after cycles; The equivalent circuit diagram of the Nyquist plots; The table of comparison of electrochemical performance. Acknowledgements This work was financially supported by Program (Grant no.cb0) and the National Natural Science Foundation of China (Grant no. ). References () Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S., Research Development on Sodium-Ion Batteries. Chem. Rev.,, -. () Pan, H.; Hu, Y.-S.; Chen, L., Room-Temperature Stationary Sodium-Ion Batteries for Large-Scale Electric Energy Storage. Energy Environ. Sci.,, -. () Wang, L.; Lu, Y.; Liu, J.; Xu, M.; Cheng, J.; Zhang, D.; Goodenough, J. B., A Superior Low-Cost Cathode for a Na-Ion Battery. Angew. Chem. Int. Ed.,, -. () Dahbi, M.; Yabuuchi, N.; Kubota, K.; Tokiwa, K.; Komaba, S., Negative Electrodes for Na-Ion Batteries. Phys. Chem. Chem. Phys.,, 0-. () Wen, Y.; He, K.; Zhu, Y.; Han, F.; Xu, Y.; Matsuda, I.; Ishii, Y.; Cumings, J.; Wang, C., Expanded Graphite as Superior Anode for Sodium-Ion Batteries. Nat. Commun.,,. () Sun, J.; Lee, H.-W.; Pasta, M.; Yuan, H.; Zheng, G.; Sun, Y.; Li, Y.; Cui, Y., A Phosphorene-Graphene Hybrid Material as a High-Capacity Anode for Sodium-Ion Batteries. Nature Nanotech., 0, 0-. () Kong, D.; Cheng, C.; Wang, Y.; Liu, B.; Huang, Z.; Yang, H. Y., Seed-Assisted Growth of Α-Fe O Nanorod Arrays on Reduced Graphene Oxide: A Superior Anode for High-Performance Li-Ion and Na-Ion Batteries. J. Mater. Chem. A,, 00-.

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16 Page of 0 Figure captions: Scheme Schematic diagram for the description of Na + insertion and e - diffusion processes. Figure SEM images of (a, b) D MoS nanosheets; (c) MoS nanosheets coated with nm TiO ; (d) XRD patterns of MoS and TiO coated MoS nanosheets. Figure (a) TEM image, (b) HRTEM image and (c) SAED patterns of MoS nanosheets; (d) HRTEM images of MoS - nm TiO. Figure High-resolution XPS spectra of the (a) Mo d peak, (b) S p peak, (c) Ti p peak and (d) O s peak, respectively. Figure (a) the first cycle and (b) the second cycle of CV curves of MoS, MoS -0. nm TiO, MoS - nm TiO and MoS - nm TiO electrodes measured at a scanning rate of 0. mv s -. Figure (a) Charge discharge profiles of initial cycles at 0 ma g - ; (b) Cycling performances at 0 ma g - ; (c) The rate of electrode at various current densities of, 00, 0, 0 and 00 ma g -, respectively; (d) Cycling performances at 0 ma g - for the electrodes of MoS, MoS -0. nm TiO, MoS - nm TiO and MoS - nm TiO. Figure SEM images of (a) D MoS nanosheets; (b) coated with 0. nm TiO ; (c) coated with nm TiO ; (d) coated with nm TiO ; after cycles at a current density of 0 ma g -. Figure XRD patterns of MoS nanosheets (I) and coated with 0. nm (II), nm (III) and nm (IV) TiO after cycles at a current density of 0 ma g -. Figure Nyquist plots of MoS -0. nm TiO and MoS - nm TiO electrodes: (a) after cycles at 0 ma g - and (b) after 00 cycles at 0 ma g -. ) Linear fits in low frequency region of the Nyquist plots (c) after cycles at 0 ma g - ; (d) after 00 cycles at 0 ma g -.

17 Page of 0 Scheme Schematic diagram for the description of Na + insertion and e - diffusion processes.

18 Page of 0 Figure SEM images of (a, b) D MoS nanosheets; (c) MoS nanosheets coated with nm TiO ; (d) XRD patterns of MoS and TiO coated MoS nanosheets.

19 0 Figure (a) TEM image, (b) HRTEM image and (c) SAED patterns of MoS nanosheets; (d) HRTEM images of MoS- nm TiO. Page of

20 Page of 0 Figure High-resolution XPS spectra of the (a) Mo d peak, (b) S p peak, (c) Ti p peak and (d) O s peak, respectively.

21 Page of 0 Figure (a) the first cycle and (b) the second cycle of CV curves of MoS, MoS -0. nm TiO, MoS - nm TiO and MoS - nm TiO electrodes measured at a scanning rate of 0. mv s -.

22 Page of 0 Figure (a) Charge discharge profiles of initial cycles at 0 ma g - ; (b) Cycling performances at 0 ma g - ; (c) The rate of electrode at various current densities of, 00, 0, 0 and 00 ma g -, respectively; (d) Cycling performances at 0 ma g - for the electrodes of MoS, MoS -0. nm TiO, MoS - nm TiO and MoS - nm TiO.

23 0 Figure SEM images of (a) D MoS nanosheets; (b) coated with 0. nm TiO; (c) coated with nm TiO; (d) coated with nm TiO; after cycles at a current density of 0 ma g-. Page of

24 Page of 0 Figure XRD patterns of MoS nanosheets (I) and coated with 0. nm (II), nm (III) and nm (IV) TiO after cycles at a current density of 0 ma g -.

25 Page of 0 Figure Nyquist plots of MoS -0. nm TiO and MoS - nm TiO electrodes: (a) after cycles at 0 ma g - and (b) after 00 cycles at 0 ma g -. ) Linear fits in low frequency region of the Nyquist plots (c) after cycles at 0 ma g - ; (d) after 00 cycles at 0 ma g -.

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