Shell Nanotubes: Ideal

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New Approach to Create TiO2(B)/Carbon Core/Shell Nanotubes: Ideal Structure for Enhanced Lithium Ion Storage Xiaoyi Zhu, Xianfeng Yang, Chunxiao Lv, Shaojun Guo, Jianjiang Li, Zhanfeng Zheng, Huaiyong Zhu, and Dongjiang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04588 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 8, 2016

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ACS Applied Materials & Interfaces

New Approach to Create TiO2(B)/Carbon Core/Shell Nanotubes: Ideal Structure for Enhanced Lithium Ion Storage Xiaoyi Zhu,†,¶ Xianfeng Yang,‡,¶ Chunxiao Lv,† Shaojun Guo§,*, Jianjiang Li,†,‖ Zhanfeng Zheng, ⊥

†

Huaiyong Zhu,¥ and Dongjiang Yang†,*

School of Environmental Science and Engineering; Collaborative Innovation Center for Marine Biomass

Fibers, Materials and Textiles of Shandong Province, Qingdao University, No. 308, Ningxia Road, Qingdao 266071, China, ‡

Analytical and Testing Centre, South China University of Technology, Guangzhou 510640, China,

§

Department of Materials Science and Engineering, & Department of Energy and Resources Engineering,

College of Engineering, Peking University, Beijing 100871, China. ǁ

Shanghai Key Lab of Electrical Insulation and Thermal Aging, Shanghai jiaotong University, Shanghai, 20024

0, P. R. China ⊥

Institute of Coal Chemistry, CAS, Taiyuan 030001, PR China.

¥

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland

University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia. ¶

These authors contributed equally to this work.

Address correspondence to d. [email protected], [email protected]. KEYWORDS: TiO2(B) nanotube · lithium-ion battery anode · core/shell structure · surface binding · longchain silane

ACS Paragon Plus Environment

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ABSTRACT To achieve uniform carbon coating on TiO2 nanomaterials, high temperature (> 500 °C) annealing treatment is a necessity. However, the annealing treatment inevitably leads to the strong phase transformation from TiO2(B) with high lithium ion storage (LIS) capacity to anatase with low LIS one as well as the damage of nanostructures. Herein, we demonstrate a new approach to create TiO2 (B)/carbon core/shell nanotubes (C@TBNTs) using a long-chain silane polymethylhydrosiloxane (PMHS) to bind the TBNTs by forming Si−O−Ti bonds. The key feature of this work is that the introduction of PMHS onto TBNTs can afford TBNTs with very high thermal stability at higher than 700 °C, and inhibit the phase transformation from TiO2(B) to anatase. Such high thermal property of PMHS-TBNTs make them easily coated with highly graphitic carbon shell via CVD process at 700 °C. The as-prepared C@TBNTs deliver outstanding rate capability and electrochemical stability, i.e. reversible capacity above 250 mAh g-1 at 10 C and a high specific capacity of 479.2 mAh g-1 after 1000 cycles at 1 C. As far as we know, the LIS performance of our sample is the highest among the previously reported TiO2(B) anode materials.


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INTRODUCTION As lithium-ion batteries (LIBs) increase the demand for advanced materials, TiO2 has emerged as a potential anode material for the future LIBs-based electric vehicle and grid storage ascribed to its abundance, low cost, safety, high operating voltage, chemical stability and nontoxicity.1-5 Among different types of TiO2, TiO2(B) can accommodate 1.01 Li+ per Ti, showing a capacity as high as 335 mAh g-1.6-9 It also exhibits more favorable channel structure possessing freely open channels for Li+ transport, and features a pseudo-capacitive Faradic process that is different from the anatase phase (accommodate 0.85 Li+ per Ti), giving rise to Li ion transport into the crystal structure much easier and better charge-discharge capability (Scheme 1a).10-12 These particular chemical properties of TiO2(B) make it the best choice in all the types of TiO2 for achieving highly efficient lithium ion storage. Nanostructuring TiO2(B) is an efficient approach to improve the LIB properties of TiO2 materials because it can rapid electrochemical reactions by means of improving the surface area, and enhance the electrical and ionic conductivity, thus allowing for more efficient lithiation and delithiation.13, 14 In particular, onedimensional (1D) TiO2(B) nanotubes (TBNTs) attract more attention because their 1D hollow structure and large surface area are beneficial for rapid ion/electron transports, improve the surface of electrode and electrolyte, and short Li+ diffusion distance.15 Nevertheless, there are still some challenging issues in the use of TBNTs for LIBs, due to their semiconductor property (relatively poor conductivity). The decoration of transition metal oxide with carbon shell is the most widely used for boosting LIBs performance in terms of capacity, rate performance and cycle life since carbon shell can effectively enhance the electrical conductivity of whole composite materials.16, 17 For instance, a graphitic-like carbon shell has been deposited on anatase or rutile TiO2 nanostructures


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by heat treatment higher than 700 °C to improve their electrochemical performance.18,19 However, the improvement is very limited because of the low lithium ion storage (LIS) of anatase and rutile. Unfortunately, currently it is impossible to introduce carbon shell on high LIS TiO2(B) electrode, because a phase transformation would occur from TiO2(B) to anatase above 500 °C, and further convert to rutile above 700 °C. Also, during the phase transformation, the intense stress variation can destroy the tubular morphology, thus reducing the specific surface area.20 These two challenging issues stimulate the researchers to explore more effective strategy for making stable TiO2(B)/carbon core/shell NTs to achieve more efficient LIS, but is still a great challenge.

Scheme 1. (a) Unit cells of TiO2(B) and anatase with idealized Li+ insertion sites. (b) New approach to make TiO2(B)/carbon core/shell structure for LIBs. Herein, we demonstrate a new approach to create TiO2(B)/carbon core/shell NTs using a long-chain silane polymethylhydrosiloxane (PMHS, −(CH3(H)Si−O)−, MW: 1700-320021) as a


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coupling agent to bind on the wall of TBNTs by forming Si−O−Ti bonds. The key feature of this work is that the introduction of PMHS onto TBNTs can afford TBNTs with very high thermal stability at higher than 700 °C, and inhibit the phase transformation from TiO2(B) to anatase since the long PMHS molecules act as the chains to tightly twin the TBNTs. Such high thermal property of PMHS-TBNTs make them easily coated with highly graphitic carbon shell by using a chemical vapor deposition (CVD) process at 700 °C (Scheme 1b). The as-prepared TiO2(B)/carbon core/shell NTs exhibit the highest specific capacity of 376.9 mAh g-1 at 1 C in all the reported TiO2-based nanomaterials. It also delivers outstanding rate properties and long-term cycle life, i.e. reversible capacity above 250 mAh g-1 at 10 C and the high reversible capacity of 479.2 mAh g-1 at a 1 C up to 1000 cycles. Experimental Section Materials Synthesis. All the agents were used directly without any other retreatment. In a typical procedure of synthesizing H-titanate nantubes (HTNTs), a suspension was prepared by mixing 6 g anatase particles (Aldrich, 325 mesh) and 80 mL 10 M NaOH, and sonicated in an ultrasonic bath for 0.5 h. Then, the dispersed suspension was transferred into a PTFE container equipped with an autoclave outside. The autoclave was kept at 150 °C for 48 h. The precipitate collected was Na2Ti3O7 nanotube. Excess NaOH was removed by washing the precipitate with distilled water. HTNTs was produced by exchanging Na2Ti3O7 nanotube with H+. Then, the HTNTs react with PMHS (molecular weight of 1700～3200) by the reflux reaction as follows: 0.1 g HTNTs was added into a 250 mL round flask, followed by adding 0.1 g PMHS which was dissolved in 100 mL toluene. The mixture for magnetic stirring was maintained for 24 h. At last, the collected product was dried at 80 °C for 24 h.


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Carbon was coated on the nanotubes surface via a CVD process. Briefly, the quartz tube was firstly purged using high purity Ar flow (60 mL min-1). Then, it was heated to 700 °C at a heating rate of 6 °C min-1. Subsequently, the Ar flow (30 mL min-1) used as the carrier gas passed through a solution of toluene for 3 h. At last, the tube was cooled down in Ar flow. To optimize the synthetic conditions, the CVD deposition time were varied from 2 to 4 h while all other parameters remain unchanged. Detailed synthetic conditions are summarized in Table S1. For comparison, TiO2(B) nanotubes named TBNTs were prepared calcinating HTNTs at 450 °C in air. And, anatase TiO2/carbon core/shell nanorods denoted as C@ATNRs were synthesized by CVD treatment of HTNTs without PMHS functionalization at 700 °C. Characterization. The phase structure analysis, morphology, surface chemical constitution, specific surface area of C@TBNTs-X samples were measured by X-ray diffraction (XRD), Raman analysis, water contact angle analysis, X-ray photoelectron spectroscopy (XPS), filedemission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM) and high resolution TEM (HRTEM), respectively. The test details see previously reported.22,23 Electrochemical measurements. The working electrode slurry was mixed with the C@TBNTs samples, polyvinylidene fluoride (PVDF), and acetylene black in 80:10:10 weight ratio using Nmethylpyrrolidone (NMP) as the solvent . The resultant slurry was pasted on to the copper foil substrate to form the working electrode. The details of assembling CR2016 coin-type cells and electrochemical tests have been explained in our previous reported work.22-27

RESULTS AND DISCUSSION


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As shown in Figure 1a, the layered protonated titanate NTs (HTNTs) with protons between the TiO6 octahedra layers were first obtained by the hydrothermal treatment of NaOH solution and TiO2 powder at 150 °C, followed by the functionalization with PHMS molecules via the reflux process in toluene. The long PHMS chains twine along the HTNTs like a rope to firmly bind the HTNTs (PMHS−HTNTs), since the Ti−OH groups on the wall of HTNTs can react with PMHS to form Si−O−Ti bonds with H2 release. This has been proved by Fourier transform infrared spectroscopy (FTIR) spectrum of PMHS−HTNTs (Figure S1) by showing the typical – CH3 stretching vibration band at 2900 cm-1, the Si−CH3 stretching vibration peaks at 1260 and 794 cm-1, the broad peak of –Si–O–Si– at 1100 cm-1, and the stretching vibration peak of Ti–O– Si bonds at 919 cm-1. The hydrophobic nature of the methyl groups makes the water contact angle of the PMHS−HTNTs increase to 97±2° from 60±2° of pristine HTNTs (Figure 1a and Figure S2). The as-obtained PMHS−HTNTs were treated through a CVD process at 700 °C using toluene as carbon source, making the HTNTs transform to TBNTs and simultaneously producing carbon shell on the surface. Different amounts of PMHS were used to modify the TBNTs, leading to a serial of C@TBNTs-X samples (X=10, 20, 30, 40, denotes the mass ratio of PMHS/HTNTs in the reflux system). The thickness of the carbon shell was controlled by adjusting the deposition time. It should be noted that during the formation of highly graphitic carbon shell, the phase conversion of TiO2 from TiO2(B) to anatase, usually occurs at over 400 °C, was significantly inhibited since the edge– and corner–sharing TiO6 octahedra of TiO2(B) phase were firmly fixed by the twining –Si–O–Si– chains. Therefore, the wellmaintained nanotubular morphology of TBNTs coated by carbon shell is the key to achieve highly efficient LIS here.


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Figure 1. (a) Schematic illustration on synthesis procedure for making C@TBNTs using longchain PMHS as the linker and protecting agent. (b) TEM and (c) HRTEM image of C@TBNTs20 NTs. (d) FFT and IFFT image of the selected area. (e) EDS mapping images and (f) elemental line profiles for Ti, O, Si, C of C@TBNTs-20. The morphology and microstructure analysis of C@TBNTs-20 were examined by using TEM and HRTEM. As shown in Figure 1b, C@TBNTs-20 shows uniform 1D NT structure. These core/shell NTs with the outer diameters of ~ 20 nm, and ~ 300 nm in length have a highly oriented growth behavior. As a comparison, the pristine HTNTs were treated at 700 °C by CVD, and only carbon-coated anatase TiO2 nanorods (C@ATNRs) were obtained (Figure S3). Obviously, the long HTNTs were fully converted to short anatase nanorods without the addition of PMHS. Figure 1c displays the HRTEM image of white rectangle in Figure 1b. It can be seen


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that for a single TBNT obtained after 3 h CVD treatment, ~5 nm carbon shell is observed on surface. The Fast Fourier Transform (FFT) and the inverse FFT (IFFT) images of the area marked by two rectangles in Figure 1c were presented in Figure 1d. The d–spacing value of 0.36 nm is attributed to (110) lattice fringe of TiO2(B) phase. The 0.35 nm d–spacing is identified as (011) lattice fringe of trace anatase TiO2 formed at the PMHS unbound area. The trace anatase is beneficial to enhance the LIB performance, because the TiO2(B)/anatase phase interfaces can provide additional LIS.28 The element distribution in C@TBNTs-20 was analyzed by EDS (Figure 1e). It is found that the elements of Ti, O, Si and C exhibit uniformly distribution on the whole NT. To further investigate the element distribution along a single C@TBNTs-20 NT, the elemental line-scan profiles were collected across the radial direction (Figure 1f), showing a typical low plateau in the center area due to the tubular morphology. However, compared with the profiles of Ti, O and Si, the C signal spreads across a broader area, further demonstrating the core/shell structure of C@TBNTs-20.


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Figure 2. (a, b) XRD patterns of C@TBNTs-10, C@TBNTs-20, C@TBNTs-30 and C@TBNTs40. Figure 1b shows the enlarged view. (c, d) Raman spectra of C@TBNTs-20. The phase transition of PMHS–HTNTs to C@TBNTs-X has been proved by X-ray diffraction (XRD) and Raman spectroscopy. XRD patterns of TBNTs-X (Figure 2a), synthesized after 3 h CVD treatment show the typical TiO2(B) diffraction peaks (JCPDS 741940) and weak peaks of anatase phase (JCPDS 084-1285). While under the same condition, the XRD pattern of pristine HTNTs without adding PMHS can only show the characteristic peaks of anatase phase (Figure S4). This indicates that the phase transition of TiO2 from TiO2(B) to anatase has been efficiently inhibited by the PMHS chains. The XRD patterns of C@TBNTs-X at 2θ angle range from 20 to 32o (Figure 2b) shows the ratio of anatase phase to TiO2(B) become lower with increasing the PMHS content. C@TBNTs-40 has the lowest ratio of anatase to TiO2(B). This is due to that more long –Si–O–Si– chains from PMHS firmly bind the HTNTs, making the phase conversion of TiO2 from TiO2(B) more difficult. The C@TBNTs-20 with the optimal phase conversion from TiO2(B) to anatase results in better anatase/TiO2(B) interface for achieving higher additional interfacial LIS.28 The higher diffraction peak of anatase is due to its well-crystallized nature compared with metastable TiO2(B) phase, although only trace anatase phase is observed from the HRTEM image.28-30 Raman spectrum of C@TBNTs-20 (Figure 2c) shows the signals of both TiO2(B) and anatase, being in good agreement with XRD analysis.29 In addition, two prominent G band (1580 cm-1) and D band (1350 cm-1) peaks are observed from the Raman spectrum (Figure 2d), indicating the effective deposition of carbon shell after CVD treatment.31 The specific surface areas of the C@TBNTs-X are in the range of 228.6 to 282.4 m2 g-1, and the corresponding pore volumes ranged from 1.02 to 1.35 cm3 g-1 (Table S2). The


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diffusion distance of Li+ ions and electrons is expected to be shortened due to their high specific area and large pore volume. The C@TBNTs-20 was further investigated by XPS. The peaks of Ti, O, C and a trace amount of Si are observed from the wide XPS spectrum (Figure S5a). Two peaks located at 464.6 eV and 458.9 eV are attributed to Ti 2p3/2 and Ti 2p1/2 , respectively (Figure S5b), corresponding to the Ti–O bond in the TiO2 phase.32-34 C 1s spectrum (Figure S5c) can be divided into four peaks located at 284.6, 285.3, 286.0 and 289.2 eV, which are assigned to sp3C– sp3C, hydrocarbon, disorder carbon or oxidant carbon, respectively.35 XPS spectrum of Si 2p (Figure S5d) shows that the major peak around 101.3 eV is attributed to the (–O)3Si− deriving from (–O)3Si−CH3.36 Energy Dispersive Spectrometer (EDS) results show the mass percent of Si content is 1.51%, 1.98%, 2.52%, and 3.17% for C@TBNTs-10, C@TBNTs-20, C@TBNTs-30, and C@TBNTs-40, respectively.


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Figure 3. Electrochemical performance of C@TBNTs. (a) CVs of C@TBNTs-20 at a scan rate of 0.1 mV s-1, (b) Charge and discharge profile of C@TBNTs-20 at 1 C for the 1st, 2nd, 100th, 500th and 1000th cycles, (c) Rate capability of C@TBNTs-10, C@TBNTs-20, C@TBNTs-30, and C@TBNTs-40, (d) Long-term cycle life and Coulombic efficiency of C@TBNTs-20 at 1 C. (The voltage window extends between 0.01 and 3 V vs Li/Li+ and the specific capacities are calculated by using the mass of composite as the active electrode material). (e) Rate performance


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and (f) Long-term cycle life and Coulombic efficiency of C@TBNTs-20 at 1 C with a window extends from 1 to 3 V vs Li/Li+ for the tests. Cyclic voltammetry (CV) and galvanostatic charge/discharge techniques were carried out in order to evaluate the LIB properties of C@TBNTs-X. The results show that the C@TBNTs-20 electrode with the optimal carbon shell (3 h treatment) displays the best rate performance and highest specific capacity (Figure S7) since too thick carbon film will lead to the slow lithium ion diffusion. CVs of C@TBNTs-20 from 1st to 4th cycle at 0.1 mV s-1 in the voltage window 0.01−3.0 V are shown in Figure 3a. In the first cycle, the two peaks located at 0.012 and 0.14 V are assigned to the Li+ intercalation and deintercalation of the carbon shell.22 The peak at around 0.7 V is ascribed to the decomposition of solvent which formed solid electrolyte interface (SEI) on carbon shell surface. The two S-peaks at 1.45/1.59 V (S1−peak) and 1.51/1.68 V (S2−peak) are attributed to the pseudocapacitive LIS behavior of TiO2(B). Furthermore, two peaks at around 1.7 and 2.1 V are associated with the Ti4+/Ti3+ redox couple during lithium intercalation and deintercalation in the trace amount of anatase lattice.37 There is no significant change in CVs after the initial three cycles, confirming stable storage and release of Li+ in C@TBNTs-20. Figure 3b displays the charge/discharge potential curves of C@TBNTs-20 at 1 C. We can find that the C@TBNTs-20 has typical characteristic of carbon electrode. During the first discharge process, a voltage plateau sloped from 0.75 to 0.20 V caused by the formation of SEI is observed.38 The flat voltage plateau under 0.20 V is due to the insertion of Li+ in carbon shell. Additionally, a plateau at 1.5 V and a sloped region of 1.7 − 1.5 V, associated with the pseudocapacitive process of TiO2(B) and the lithiation of anatase.39 The C@TBNTs-20 electrode exhibits an initial discharge capacity (492.2 mAh g-1) and the charge capacity (376.9 mAh g-1), respectively, corresponding to the Coulombic efficiency of 76.6%. The initial capacity loss is


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mainly ascribed to electrolyte decomposition and the formation of SEI layer. After 500 cycles, the overlapped curves indicate long cycle life. Figure 3c shows the rate capability of the C@TBNTs-10, C@TBNTs-20, C@TBNTs-30 and C@TBNTs-40. Remarkably, the C@TBNTs-20 has the best capacity at the same current density. It can be reversibly charged to 415.8, 376.9, 339.3, 286.1, and 259.8 mAh g-1 at 0.5, 1, 2, 5, and 10 C for 10 cycles. As the current density lowered to 0.2 C, the capacity is regained and even increase to 549.8 mAh g-1. This confirmed that the integrity of C@TBNTs-20 can be well maintained even after undergoing the high-rate charge/discharge cycles. The reason why C@TBNTs-20 exhibits the best capacity in all the investigated C@TBNTs-X is due to the balanced electrical conductivity and the amount of TiO2(B) phase. Figure 3d displays the cycle performance and Coulombic efficiency of C@TBNTs-20 at 1 C in the window between 0.01 and 3 V. It exhibits the superior specific capacity ( 479.2 mAh g-1) over 1000 cycles at 1 C, and maintains a high Coulombic efficiency of above 99 % over 1000 cycles. There is no nanotubular morphology damage on C@TBNTs-20 after 1000 long-term cycling (Figure S6), further revealing its excellent stability for LIBs. The capacity decreases slightly at the initial 50 cycles and then increases slightly and remains stable after 250 cycles, meaning that the electrode possesses superior electrochemical stability and reversibility. The stage of the capacity decrease may be ascribed to the structure re-organization of the carbon coatings.40 In the case of C@TBNTs, the large initial capacity fading may be also attributed to the fact that Lithium react with surface functional group coming from PMHS. In the subsequent cycles, the capacity gradually increased due to an activation process. This because a reversible polymeric gel-like film can be formed from the activated electrolyte degradation and low potential through a so-called “pseudo-capacitance-type behavior”.41 Apart from that, the


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electrode crystallinity could partially lost or even transform to an amorphous structure after initial activation cycles, which can improve the Li+ diffusion kinetics. Moreover, the enhanced cycling stability are probably ascribed to the uniform carbon coating, which can significantly improve the interface of electrode and electrolyte, and greatly reduce the decomposition of electrolyte. The specific capacities for C@TBNTs-20 and pure TBNTs at different rates in the window between 1-3 V are shown in Figure 3e. Clearly, C@TBNTs-20 still delivers good rate capabilities with the charge capacities of 205.8, 189.8, 181.7, 165.3 and 150.3 mAh g-1 at 0.5 C, 1 C, 2 C, 5 C and 10 C, respectively, much better than those of pure TBNTs. The electrode also delivers the good long-term cycle performance. The reversible capacity still remain 192.6 mAh g-1 after 500 cycles (Figure 3f). The Coulombic efficiency is 85.6 % in the first cycle but remains over 99% after 500 cycles, indicating excellent cycling stability. When a wider window (0-3 V) is used for tests, substantial improvements in capacity are realized. This because both TBNTs and carbon are utilized as the active material, allowing for Li+ insertion into or extraction out of amorphous carbon.42


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Figure 4. (a) Cyclic stability of C@TBNTs-20, pure TBNTs and C@ATNRs at 1 C rate, and (b) Comparison of cycle performance of the C@TBNTs-20 electrode with other C@TiO2 electrodes recently reported. The cycle performance of pure TBNTs and C@ATNRs (the sample without PMHS binding) are measured to compare with C@TBNTs-20 (Figure 4a). After 200 cycles at 1C, the overall steady charge capacities are 142.3, 159.1 for C@ATNRs and TBNTs, respectively, much lower than that of C@TBNTs-20 (408.2 mAh g-1). It indicates that coating carbon and adding PMHS are both beneficial to improving electrochemical performance. Figure 4b shows the comparison of C@TBNTs-20 with other C@TiO2 anodes reported recently in the voltage window of both 13 V and 0.01-3 V. Apparently, the C@TBNTs-20 electrode presents much better cycling stability and superior specific capacity.42-50 These in-depth studies and comparisons reveal that the pseudo-capacitive property of TiO2(B) phase, highly conductive carbon shell (Figure S8), and nanotubular morphology with high surface area synergistically contribute to the superior LIB performance. CONCLUSION To summarize, we demonstrate a new approach to make a new class of TiO2(B)/carbon core/shell NTs using the long-chain PMHS as the coupling and protective agent. The key feature for making such structure is that PMHS plays a key role in bounding TBNTs, thus inhibiting the phase transformation from B phase to anatase. Due to the highly graphitic carbon shell, the type B of TiO2 and nanotubular morphology, the as-prepared TiO2(B)/carbon core/shell NTs exhibit high reversible capacity of 376.9 mAh g-1 at 1 C, good rate capability (over 250 mAh g-1 at 10 C) and excellent cycle performance (479.2 mAh g-1 after 1000 cycles). The work provides a new


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structure tuning concept to enhance the LIBs performance of titanium dioxide anode and may open new ways for the design of future high properties LIBs. Acknowledgement. This work is financially supported by the National Natural Science Foundation of China (No. 51473081 and 51503109), Research award fund for outstanding young scientists in Shandong province (Grant no. BS2014CL006), the start-up funding from Peking University and Young Thousand Talented Program. Supporting Information Available: Supporting Figures S1-8, Tables S1-2. FTIR spectra of HTNTs and PMHS−HTNTs (Figure S1); the water contact angle of the PMHS−HTNTs (Figure S2); TEM and HRTEM images (Figure S3) of carbon-coated anatase TiO2 nanorods (HTNTs were treated at 700 °C by CVD); XRD patterns of the sample without adding PMHS (Figure S4), XPS spectra of C@TBNTs-20 (Figure S5); TEM image of C@TBNTs-20 after 1000 cycle at 1C (Figure S6); TG curves and electrochemical performance of C@TBNTs-20 with different CVD time (Figure S7); Nyquist plots of C@TBNTs-X (Figure S8); Synthetic conditions used for the preparation of C@TBNT-X samples (Table S1), and physical characteristics of C@TBNTs-X (Table S2). This material is available free of charge via the internet at http://pubs.acs.org.

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TOC figure


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Shell Nanotubes: Ideal

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