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Polymer-Promoted Synthesis of Porous TiO Nanofibers Decorated with NDoped Carbon by Mechanical Stirring for High-Performance Li-Ion Storage Ming Luo, Xiao Yu, Wenxia Zhao, Ruimei Xu, Yong Liu, and Hui Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10437 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Applied Materials & Interfaces

Polymer-Promoted

Synthesis

of

Porous

TiO2

Nanofibers Decorated with N-Doped Carbon by Mechanical Stirring for High-Performance Li-Ion Storage †§

†§

†§

Ming Luo, , Xiao Yu, , Wenxia Zhao,‡ Ruimei Xu,‡ Yong Liu *, , and Hui Shen †

§

School of Materials Science and Engineering, State Key Laboratory of Optoelectronic Materials

and Technologies, ‡Instrumental Analysis & Research Center, and §Institute for Solar Energy System, Sun Yat-sen University, Guangzhou 510275, China KEYWORDS: Li-ion batteries, TiO2/C nanofiber, mechanical stirring, polymer, porous structure

ABSTRACT Extensive efforts have been devoted to developing simple, low-cost and high production yield methods to prepare hybrid materials with desired structural features for high-performance lithium storage. Here, a novel strategy is reported for fabricating the porous TiO2 nanofibers decorated with N-doped carbon (TiO2/C nanofibers) by a combination of mechanical stirring and the addition of a polymer in a beaker at ambient temperature, followed by calcination. The

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mechanical stirring process can provide homogeneous mixing of reactants in solution, while the polymer acts not only as a structure-directing agent for fabricating one-dimensional nanofibers but also as the carbon and nitrogen sources to generate N-doped carbon framework and porous structures. The TiO2/C nanofibers have average diameters of 500 nm and lengths up to 65 um and are further composed of intercrossed TiO2 nanocrystals with sizes of 8 nm, with micropores centered at 1.5 nm and mesopores at 3-6 nm. The TiO2/C electrodes demonstrated high reversible capacities (368 mAh g-1 at 0.25 C after 200 cycles and 176 mAh g-1 at 10 C over 2000 cycles) with good cycling and an excellent rate capability (97 mAh g-1 at 20 C).

1. INTRODUCTION To date, Li-ion batteries (LIBs) have been widely used in portable electronic products, and their applications have gradually extended to powering hybrid electric vehicles (HEVs) and electric vehicles (EVs). TiO2 materials have been consider as promising alternative anode materials for LIBs due to their high abundance, non-toxicity, cost efficiency, chemical stability and environmental friendliness.1-3 In general, TiO2 has four main crystalline polymorphs including rutile, brookite, anatase and bronze (TiO2(B)).4,5 Among them, anatase TiO2 have been widely studied as anode materials in LIBs.6 However, the bulk anatase TiO2 has the inherent low conductivity and poor ion diffusion rate, which can only uptake 0.5 Li+ per Ti atom to form orthorhombic Li0.5TiO2, corresponding to a theoretical specific capacity of 168 mAh g-1. Recent studies show that the nanostructuring of anatase TiO2 may shorten Li+ ion diffusion length and increase electrode-electrolyte contact area, giving the cubic LiTiO2 with a higher lithium storage capacity of 336 mAh g-1.7 For example, Wagemaker et al. reported that 7 nm anatase nanoparticles were completely converted to the LiTiO2 phase to reach its theoretical capacity.8


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Shin et al. showed that pseudocapacitive interfacial storage might occur on the surface of the anatase TiO2 nanoparticles, which delivered 302 mAh g-1 and 229 mAh g-1 of fully reversible discharge capacity at charge/discharge rates of 1 C and 5 C after 100 cycles, respectively.9 However, practical application of anatase TiO2 in LIBs has still been hampered by its low reversible capacity, poor rate capability and bad cycling stability. Great efforts have been made to search for simple, low-cost, and innovative synthesis strategies for high-performance electrode structures with excellent electronic conductivity and diffusivity of Li+ ion. First, the ability to synthesize one-dimensional (1D) nanostructures such as nanowires, nanorods, nanofibers and nanotubes with high aspect ratios is highly desirable for TiO2 applications in LIBs since 1D nanostructures can provide the efficient electron transport pathways and shortened solid-phase ion diffusion length along the longitudinal and radial directions, respectively. To date, many synthetic methods, such as the hydrothermal method, electrochemical anodization, chemical vapor deposition, template-assisted method, and electrospinning method have been developed to prepare 1D anatase TiO2 nanostructures.5 However, most of these methods have shortcomings, such as tedious production procedures, low production rates, high energy costs and difficulty scaling up, which have limited their practical applications. For example, the alkaline hydrothermal synthesis of 1D TiO2 nanostructures needs to be conducted in a stainless-steel vessel with the participation of highly concentrated NaOH or KOH under high temperature and pressure conditions, and the process requires at least a threestep complex procedure including the growth of sodium titanate, ion exchange and a calcination procedure to transform sodium titanate to protonated bititanate and finally to TiO2.10,11 The template-assisted method requires complex multi-step procedures including template fabrication (e.g., anodic aluminum oxide nanoporous membrane), the deposition of a TiO2 precursor into the


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template, and chemical etching to remove the template.12 In addition, chemical vapor deposition, electrochemical anodization and electrospinning are mainly limited by low mass production rates and high energy consumption.13-15 Second, to improve the conductivity of TiO2 electrodes, most researchers have focused on coating TiO2 materials with a carbon shell or graphene,1,16,17 but that method requires an extra carbon encapsulating process, and the outer carbon layer also limits the diffusion of Li ions into the host materials.18 Third, the reduction of the TiO2 particle size or the rational design of pore structures in the electrode can provide large surface areas and shorten the solid-state ion diffusion length to achieve near-theoretical capacities (335 mAh g-1 for LiTiO2).79,19-22

However, nanoparticle aggregation during cycling and the interrupted electron transport

within the electrode limits its cycle life and rate capability.23 Recent studies have shown that the construction of hierarchically porous architectures can not only supply a high ion-accessible surface area by micropores, but also enable fast ion transport through mesopores.24 Based on the above analysis, the development of simple ways to directly fabricate 1D TiO2 nanostructures with rational carbon framework design and reasonable pore size distribution for achieving excellent electrochemical performance is desirable. Herein, we demonstrate a simple, innovative, low-cost, high-yield strategy for preparing the porous TiO2 nanofibers decorated with N-doped carbon (TiO2/C nanofibers) with combined mechanical stirring and addition of a polymer. The TiO2 nanocrystals with average sizes of 8 nm and carbon framework are well intercrossed with each other, constructing high aspect ratio nanofibers with diameters of 500 nm and lengths up to 65 µm. The calcination process pyrolyzes and carbonizes the polymer, which produce N-doped carbon framework and hybrid of micropores (1.5 nm) and mesopores (3-6 nm). The small size of the TiO2 nanocrystals and micro/mesopores shorten the lithium ion diffusion distance and increase the interfacial area


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between the electrode and electrolyte, while N-doped carbon framework in the nanofibers provides efficient electron conductive pathways, resulting in a high reversible capacity and an improved rate capability and cycling stability. The TiO2/C electrodes demonstrated high reversible capacities (368 mAh g-1 at 0.25 C after 200 cycles and 176 mAh g-1 at 10 C over 2000 cycles) with good cycling and an excellent rate capability (97 mAh g-1 at 20 C, 1 C=335 mAh g1

).

2. EXPERIMENTAL SECTION 2.1. Fabrication of TiO2/C Nanofibers and TiO2 Nanofibers. First, 3 ml of Polyethyleneimine (PEI, M.W. 600, 99%, Aladdin) was added to 60 ml of deionized water in an open beaker and stirred for 5 min at room temperature (25 °C). Then, 0.03 g of titanium(IV) oxysulfate-sulfuric acid hydrate (TiOSO4·xH2SO4·xH2O, 93%, Aladdin reagent) was added to the above solution followed by vigorous stirring using a magnetic stirring apparatus (DF-101S, Gongyi Yuhua Instrument Co., Ltd) for 5 h at ambient temperature. The white solid was collected by filtering. Finally, the obtained solid was calcined at 600 °C for 2 h under an argon environment for the TiO2/C nanofibers and calcined at 600 °C for 2 h in air for the TiO2 nanofibers. 2.2. Material Characterization. The morphology of the sample was examined by field emission scanning electron microscopy (FESEM, Quanta 400, FEI Company and JSM-6330F). X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (D/Max-IIIA, Rigaku, Japan) with Cu Kα radiation (λ=1.5418 Å). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) patterns were obtained using a transmission electron microscope (FEI Tecnai


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G2 F20) with an acceleration voltage of 200 kV. Thermogravimetry (TG) curves were obtained on a thermogravimetric analyzer (TG209, Netzsch Instruments, Germany) at temperatures from 30 to 800 °C in air at a heating rate of 10 °C min-1. N2 adsorption and desorption isotherms were carried out using a N2 adsorption-desorption apparatus at 77 K (NOVA1200 instrument, Quantachrome Corporation). Raman spectra were carried out using a confocal Raman microscope (Horiba Jobin Yvon XploRa) at a wavelength of 532 nm for the excitation laser. Xray photoelectron spectroscopy (XPS) was carried out by Thermo ESCALAB 250 spectrometer. Infrared (IR) spectra was carried out by fourier transform infrared spectrometer (Perkin Elmer Spectrum). 2.3. Electrochemical Characterization. For fabricating the working electrodes, a slurry was prepared by homogeneously mixing 80 wt% active material (e.g., TiO2/C nanofibers or TiO2 nanofibers), 10 wt% carbon black (CB), and 10 wt% polyvinylidene difluoride (PVDF) in Nmethyl-2-pyrrolidone (NMP), which was uniformly coated onto a Cu foil with an active material mass loading of approximately 1.8-2.3 mg cm-2 and then dried for 12 h at 120 °C under vacuum. For comparison, TiO2/C nanofiber electrode with weight ratio of TiO2/C:CB:PVDF=88.1:1.9:10 and CB electrode with weight ratio of CB:PVDF=90:10 were also fabricated. CR 2032-type halfcells were assembled in an argon-filled glovebox (Mikarouna) with moisture and oxygen concentrations below 0.1 ppm; the half-cells contained the working electrode, Li metal foil as the counter electrode, a microporous polypropylene membrane (Celgard 3400) as the separator, and 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) (1:1 vol%) as the electrolyte. The cells were cycled galvanostatically in a voltage range of 0.01-3.0 V using a battery tester (Arbin Instruments, BT2043) at various current rates, where 1 C=335 mAh g-1. The cyclic


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voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (Autolab pgstat302n). 3. RESULTS AND DISCUSSION

Figure 1. (a) Low magnification, (b) side-view, and (c) cross-sectional SEM images of asprepared TiO(OH)2/PEI nanofibers; (d) low magnification, (e) side-view, and (f) cross-sectional SEM images of TiO2/C nanofibers; (g) low magnification, (h) side-view, and (i) cross-sectional SEM images of TiO2 nanofibers. Figure 1a-c shows field emission scanning electron microscopy (FESEM) images of TiO(OH)2/PEI nanofibers with smooth surfaces obtained after directly stirring the mixture of


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TiOSO4·xH2SO4·xH2O and PEI in water. PEI is a polymer with a repeating unit composed of an amine group and two carbon aliphatic CH2CH2 spacers; hydrolysis of the titanium precursor lead to the formation of the hydrated titanium complex TiO(OH)2, and the -OH groups of TiO(OH)2 are absorbed onto the NH2 groups of PEI, resulting in the formation of TiO(OH)2/PEI nanofibers.25,26 Therefore, PEI, which can not only act as the carbon and nitrogen sources, but also structure-directing agent to fabricate one-dimensional TiO2 structures, while the mechanical stirring process can provide homogeneous mixing of reactants in solution.27 Infrared spectra (Figure S1) show that absorption peak at approximately 1630 cm-1 can be ascribed to the bending vibration of the -OH group in the sample, while the absorption peaks in the interval range from 400 cm-1 to 750 cm-1 were associated with the stretching and bending vibrations of the Ti-O atomic bond on the surface.25 In addition, the peaks at 1590 cm-1 and 1507 cm-1 are due to asymmetric and symmetric NH2 stretching vibrations, while peaks at 2947, 2837, and 1310 cm-1 are ascribed to CH2 vibrations, and the band at 3358 cm-1 is due to N-H stretching. The peaks at 1085 cm-1 and 783 cm-1 are also derived from PEI.26,28 Large-scale, uniform and monodisperse TiO2/C nanofibers (Figure 1d-f) and TiO2 nanofibers (Figure 1g-i) can be obtained after TiO(OH)2/PEI nanofibers were calcined at 600 °C under the argon and air atmosphere, respectively. The TiO2/C nanofibers have mean diameter approximately 500 nm and length of 65 µm, with a high aspect ratio of 130, exhibiting almost the same size as the TiO(OH)2/PEI nanofibers and TiO2 nanofibers. No obvious pores are observed in TiO(OH)2/PEI nanofibers (Figure 1b,c). However, small nanocrystals and pores were formed in the TiO2/C nanofibers due to the pyrolysis and carbonation of PEI after calcination in argon (Figure 1e,f), while large ones were generated in TiO2 nanofibers because of the combustion of PEI and further growth of crystals after calcination in air (Figure 1h,i).


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Figure 2a shows X-ray diffraction (XRD) patterns of TiO(OH)2/PEI nanofibers, TiO2/C nanofibers and TiO2 nanofibers. The TiO(OH)2/PEI nanofibers demonstrate obvious broad peaks with amorphous characteristics. After heat treatment at 600 °C, all the diffraction peaks of the TiO2/C nanofibers and TiO2 nanofibers became sharp and the peaks match well with anatase TiO2 (JCPDS No. 84-1285). In both TiO2/C and TiO2 nanofibers, the rutile phase does not exist since residual SO42- ions absorbed on the surface of hydrated titanium dioxide hinders the transformation of anatase into rutile.25 In addition, in the XRD pattern of the TiO2/C nanofibers, a broad peak appears at approximately 2θ=20°-30°, corresponding to amorphous carbon contained in the TiO2/C nanofibers, and the broad peak overlaps with the (101) peak of anatase at 2θ=25°. Raman spectra were further conducted to confirm the XRD results. As shown in Figure 2b, five typical characteristic peaks at 148, 196, 400, 508 and 621 cm-1 were observed for the TiO2/C and TiO2 nanofibers, corresponding to the Eg, Eg (2), B1g, A1g and Eg (3) vibration modes of anatase TiO2, respectively.29,30 Furthermore, two characteristic peaks, representing the D- and G-bands, were observed at 1350 and 1600 cm-1 for the TiO2/C nanofibers, reflecting the degree of disorder of the crystal structure and scattering E2g order vibration mode of atoms in rings and chains, respectively.31 The ID/IG ratio is calculated as 1.12, indicating the highly disordered structure of amorphous carbon in TiO2/C nanofibers. Thermal gravimetric analysis (TGA) of TiO2/C and TiO2 nanofibers are shown in Figure 2c, and only one distinct weight loss step corresponding to the oxidation of carbon is observed in the range of 450-570 °C, and the amount of carbon contained in TiO2/C nanofibers is approximately 9.2%. Figure 2d shows the N2 adsorption-desorption isotherms of TiO2/C nanofibers and TiO2 nanofibers and their corresponding pore size distribution curves, respectively. The isotherms of both the TiO2/C nanofibers and the TiO2 nanofibers are type IV curves with a H2 hysteresis loop, indicating the


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Figure 2. (a) XRD patterns of as-prepared TiO(OH)2/PEI, TiO2/C and TiO2 nanofibers; (b) Raman spectra and (c) TG curves of TiO2/C and TiO2 nanofibers; (d) Nitrogen adsorptiondesorption isotherms for TiO2/C and TiO2 nanofibers; Inset in (d) shows the corresponding pore size distribution. existence of mesopores or micropores.32 The specific surface area of TiO2/C nanofibers and TiO2 nanofibers, as calculated by the Brunauer-Emmett-Teller (BET) method, are 128.89 and 89.97 m2 g-1, respectively. In addition, the inset in Figure 2d exhibits the narrow pore size distribution of TiO2/C nanofibers centered at 1.5 nm and approximately 3-6 nm. However, a shift in the peak positions toward larger pore sizes between 4 and 11 nm was observed in the TiO2 nanofibers. These results are consistent with scanning electron microscopy (SEM) observations. The pyrolysis of polyethylenimine under an Ar atmosphere introduced a great number of the


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interstices among TiO2 crystallites and carbon nanograins in the TiO2/C nanofibers, resulting in the formation of many micro and mesopores; meanwhile, the growth of TiO2 crystals were confined within the carbon matrix, leading to the formation of TiO2 nanocrystals with small sizes. The decreased size of the pores and TiO2 nanocrystals would help accommodate more lithium ions and shorten the diffusion length of Li-ion batteries. Furthermore, TiO2 nanofibers were obtained after calcination under air environments, and the combustion of carbon in the fibers leads to the formation of large mesopores and a reduction in surface area. Because PEI was removed by combustion at approximately 570 °C without confinement of carbon skeleton, the TiO2 nanocrystals became further interconnected and grew larger. The morphology and crystal structure of TiO2/C nanofibers were further characterized by Transmission electron microscopy (TEM) technologies (Figure 3). The elemental composition and corresponding spatial distribution of the TiO2/C nanofibers are shown in Figure 3a-e, and Ti, O, C and N elements are uniformly distributed over the entire TiO2/C nanofibers. The inset of Figure 3f is the selected area electron diffraction (SAED) patterns of the corresponding TiO2/C nanofibers, and the diffraction rings are indexed to the (101), (004), (200), (211) and (204) planes of the anatase phase demonstrating its polycrystalline nature. As shown from the magnified images from Figure 3g and Figure 3h, the TiO2/C nanofibers consists of numerous ultrasmall TiO2 nanocrystallites with average sizes of 8 nm and carbon framework. Furthermore, the carbon framework are continuous as shaded with yellow color, which are due to the crisscross of PEI and TiO(OH)2 during stirring process. Figure 3i,j are magnified high-resolution transmission electron microscopy (HRTEM) images of TiO2 nanocrystal and carbon framwok, corresponding to regions I and II outlined by the dashed lined in Figure 3g. According to previous studies, the small size of the anatase nanocrystallites can shorten the Li-ion diffusion


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Figure 3. (a) STEM image of a single representative TiO2/C nanofibers and the corresponding elemental mapping of (b) Ti, (c) O and (d) C and (e) N elements; (f) TEM image and SAED (inset), and (g) and (h) high-resolution TEM images of a TiO2/C nanofibers; (i) and (j) magnified HRTEM images corresponding to regions I, II outlined by the dashed line in (g). length and enhance the overall storage performance.8 In addition, the carbon framework is continuous, which facilitates charge transfer; the formation of defects such as edges and micropores in carbon framework can also provide numerous active sites for pseudocapacitive interfacial storage.31 However, for the TiO2 nanofibers shown in Figure S2, the size of the TiO2 nanocrystals is up to approximately 35 nm, and the pores formed among the TiO2 nanocrystals also become larger than those in the TiO2/C nanofibers. Figure S3 shows the X-ray photoelectron spectroscopy (XPS) spectrum of TiO2/C nanofibers,


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indicating the existence of Ti, O, C, and N in the samples, which is consistent with the element mapping of the STEM results. The high-resolution Ti 2p spectrum (Figure S3b) can be divided into two peaks with binding energies of 459.0 eV for Ti 2p3/2 and 464.7 eV for Ti 2p1/2, respectively.33,34 The fine C 1s spectrum (Figure S3c) can be deconvoluted into four peaks at approximately 284.6, 285.3, 286.0 and 288.4 eV, respectively, which are characteristic of sp2 hybridized carbon, C=N, C-N and O=C-O, respectively.34-38 In the high-resolution N 1s spectrum (Figure S3d), the two peaks with binding energies of 398.5 eV and 400.5 eV are assigned to C-N and C=N, respectively.39 Thus, N species are doped into carbon framework, which can further improve the conductivity of TiO2/C nanofibers.31,40,41 The weight content of nitrogen is calculated to be 2.5 wt% according to the XPS measurements. Thereafter,

the

electrochemical

performances

of

TiO2/C

nanofiber

electrode

(TiO2/C:CB:PVDF=80:10:10) and TiO2 nanofiber electrode (TiO2:CB:PVDF=80:10:10) were investigated. For comparison, TiO2/C nanofiber electrode (TiO2/C:CB:PVDF=88.1:1.9:10) and CB electrode (CB:PVDF=90:10) were also tested under the same conditions. Figure 4a and Figure S4 show the CV curves of the TiO2/C nanofiber electrode (80:10:10) and TiO2 nanofiber electrode (80:10:10) at a scan rate of 0.1 mV s-1 in voltage window of 0.01 to 3.0 V. For both the TiO2/C nanofibers and TiO2 nanofibers, the pair of peaks at 1.72 V in the cathodic scan and 2.0 V in the anodic scan that appear in the CV curves are characteristic of Li+ intercalation and extraction in the TiO2 lattice; reduction peaks at 0.74 V were observed in the first cathodic sweep and disappeared in the following scans, which can be ascribed to the irreversible decomposition of electrolyte and the formation of a solid electrolyte interphase (SEI). However, TiO2/C nanofibers demonstrate a broad peak at 1.4 V and an increase in the curvilinear integral area in subsequent cathodic scans, which may be due to pseudocapacitive lithium storage on the surface


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Figure 4. (a) Cyclic voltammograms of the TiO2/C nanofibers (80:10:10) at a scan rate of 0.1 mV s-1; (b) cyclic voltammograms of TiO2/C nanofibers (80:10:10) at different scan rates from 0.1 to 1 mV s-1 and (c) the relationship between the log-peak current and the log-sweep rate; (d) galvanostatic charge-discharge curves of the obtained TiO2/C nanofibers (80:10:10) cycled at 0.1 C; (e) galvanostatic cycling performance of the TiO2/C and TiO2 nanofibers (80:10:10) cycled at 0.25 C; (f) The rate performance, (g) long-term cycling performance at 10 C and (h) capacity retention at increasing rates of the TiO2/C nanofibers (80:10:10), TiO2/C nanofibers (88.1:1.9:10) and TiO2 nanofibers (80:10:10).


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and at the interface of the electrode nanostructure.42 Therefore, the CV shape of the TiO2/C nanofibers demonstrates the combined characteristics of pseudocapacitive charge storage behavior and the bulk diffusion of lithium in the TiO2 lattice. From the second scan onwards, the CV curves are well overlapped, indicating reversible electrochemical reactions in the TiO2/C nanofiber electrode. As the scan rate of CV measurements increases from 0.1 to 1 mV s-1, the peak shape of the CV curves for TiO2/C nanofibers (80:10:10) was well maintained (Figure 4b). According to previous studies, the peak current (i) and the scan rate (v) followed the relationship i=aVb, and the b-value can be calculated from the slope of the log(v)-log(i) plots.43 A b-value equal to 0.5 indicates total diffusion-controlled behavior in the electrode, whereas a b-value equal to 1.0 indicates that a capacitive process occurs in the electrode. Figure 4c shows that the logarithm for the peak current of the TiO2/C nanofibers (80:10:10) varies linearly with the scan rate, and b-values of 0.86 and 0.77 were obtained for cathodic and anodic peaks, respectively, which indicates both bulk diffusion of lithium in the TiO2 lattice and interfacial pseudocapacitive lithium storage contribute to the total lithium storage.19,42-45 Figure 4d and Figure S5 show the discharge and charge voltage profiles of the TiO2/C nanofiber electrode (80:10:10) and TiO2 nanofiber electrode (80:10:10) at a current rate of 0.1 C over a potential range of 0.01-3 V. The TiO2/C nanofiber electrode delivered a high discharge capacity of 567 mAh g-1 in the first cycle and a reversible discharge capacity of 410 mAh g-1 at second cycle and 408 mAh g-1 at fifth cycle, respectively. In contrast, the TiO2 nanofiber electrode demonstrated only a discharge capacity of 323 mAh g-1 in the first cycle and a reversible discharge capacity of 216 mAh g-1 at second cycle and 200 mAh g-1 at the fifth cycle, respectively. The curve shape of the voltage profiles is associated with the particle size and pore size distribution.22 The TiO2 nanofiber electrode demonstrate a relatively long plateau region at 1.7 V for Li+ intercalation and


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extraction in the TiO2 lattice. Whereas, the TiO2/C nanofibers consist of TiO2 nanocrystals with small size of less than 8 nm, abundant micropores and mesopores, whose voltage plateau region almost disappeared.22,32 Furthermore, the high reversible capacity of TiO2/C nanofiber electrode mainly derived from the sloping curve below 1.4 V, which is associated with the pseudocapacitance and consistent with the CV analysis.7,32,46,47 Figure 4e shows the cycling performances of TiO2/C nanofibers (80:10:10) and TiO2 nanofibers (80:10:10) at 0.25 C. TiO2 nanofibers delivered a discharge capacity of 174 mAh g-1 after 200 cycles, while TiO2/C nanofibers demonstrated significantly improved lithium storage capabilities and delivered a discharge capacity of 368 mAh g-1. The rate capability and cycling performance of the TiO2/C nanofiber electrode (80:10:10) were also investigated, as shown in Figure 4f,g. The TiO2/C nanofibers (80:10:10) delivered high discharge capacities of 401, 380, 354, 313, 254, 204, 156 and 97 mAh g-1 when the current rate changed stepwise from 0.1 C to 0.25 C, 0.5 C, 1 C, 2.5 C, 5 C, 10 C and 20 C, respectively. When the current rate was reduced from 20 C to 0.1 C, it still recovered to 391 mAh g-1 (with 97.5% of its initial value), indicating a good rate capability and excellent charge transfer kinetics within the TiO2/C nanofibers. Furthermore, TiO2/C nanofiber electrode (80:10:10), delivered a discharge capacity as high as 176 mAh g-1 at a high rate of 10 C over 2000 cycles. The discharge capacity retention, cycling stability and rate capability of the TiO2/C nanofiber electrodes reported here are competitive with those of previously reported TiO2 materials with different nanostructures and TiO2 composites containing graphene, carbon nanotubes and carbon layers (Table S1). Moreover, the discharge capacities of TiO2/C nanofibers (80:10:10) at both 0.1 C and 0.25 C are larger than the theoretical capacity of TiO2 (335 mAh g1

). In our studies, TiO2/C nanofibers consists of the TiO2 nanocrystals with sizes of less than 8

nm, which can shorten the solid-state ion diffusion length;8 they also provide abundant


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microspores and mesopores with large contacted surface area for the interfacial accumulation of Li+.2 The large excess capacity may be ascribed to a combination of bulk diffusion of lithium intercalation in TiO2 lattice and pseudocapacitive charge storage, which is verified by the CV measurements. In addition, the N-doped carbon framework can further improve the conductivity and bind with TiO2 nanocrystals together in TiO2/C nanofibers, which can improve the reversible capability, rate capability and cycling stability of TiO2/C elctrodes.31,48 Another issue to consider is that the pure anatase TiO2 has the intrinsic poor electric conductivity, which severely decreases the discharge capacity and rate capability of the electrode. A common approach is to introduce carbon black as the electronic conductor for the enhancement of electrode conductivity. Thus, we investigated the electrochemical performance of TiO2/C nanofiber electrode (88.1:1.9:10), TiO2 nanofiber electrode (80:10:10) and CB electrode (90:10), which aim to elucidate influence of carbon framework contained in TiO2/C nanofiber and carbon black on their electrochemical performance (Fig. S6). The TiO2/C nanofibers contained about 9.2% of carbon components, and the total carbon content are the same in both TiO2/C nanofiber electrode (88.1:1.9:10) and TiO2 nanofiber electrode (80:10:10). Whereas, TiO2/C nanofiber electrode (88.1:1.9:10) exhibit much better rate capability and cycling stability than TiO2 nanofiber electrode (80:10:10) since carbon black additives do not improve the electronic conductivity within TiO2 nanofibers by simply mixing them with active materials in electrodes. Moreover, TiO2 nanofiber electrode (80:10:10) show much better rate capability and cycling performance than TiO2/C nanofiber electrode (88.1:1.9:10) because the carbon black contained in TiO2/C nanofiber electrode (88.1:1.9:10) stands at just 1.9%, which can not sufficiently transmit electrons in the electrode. Impressively, TiO2/C nanofiber electrode (80:10:10) show much better rate capabilities at high rates compared to TiO2/C nanofiber


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electrode (88.1:1.9:10) and TiO2 nanofiber electrode (80:10:10), as shown in Figure 4h. At high rates of 5 C, 10 C and 20 C, TiO2/C nanofiber electrode (80:10:10) retain 50.8%, 38.9% and 24.1% of its capacity at 0.1 C, respectively, while such retention ratios are only 39.4%, 24.2% and 14.3% for TiO2/C nanofiber electrode (88.1:1.9:10), and only 23.5%, 15.3% and 11.0% for TiO2 nanofiber electrode (80:10:10). Moreover, the carbon black in electrode (90:10) deliver a capacity of only 60 mAh g-1 at 10 C, which contribute very little Li+ ion storage capacity because the ratio of carbon black in the TiO2 electrode is less than 10%. Therefore, the enhanced electrochemical performance of TiO2/C nanofiber electrode (80:10:10) is mainly due to a synergistic effect from carbon framework contained in TiO2/C nanofiber and carbon black in the electrode. Future studies should focus on optimizing the ratio of carbon component in TiO2/C nanofibers to improve their electrochemical performance. To further investigate the structural stabilizing effects of the electrode architecture on the electrochemical performance, we carried out postmortem studies of the TiO2/C nanofiber electrode (80:10:10) after cycling at a current rate of 10 C over 2000 cycles. Apparently, the SEM (Figure S7) and TEM image (Figure 5a) shows that TiO2/C nanofiber architectures were well maintained and a polymeric gel-like film with a thickness of approximately 36 nm was formed on the surface of the TiO2/C nanofibers,. Compared to the structure of uncycled TiO2/C nanofibers (Figure 3g,h), the hybrid structure of carbon framework and TiO2 nanocrystals were reconstructed in cycled TiO2/C nanofibers after repeated Li+ insertion/exaction,18 but both of them are still in close contacts to maintain the stability of 1D nanofiber structures. In addition, the SEM and EDX mapping images shown in Figure 5d-h confirm that the Ti, O and N elements are still uniformly distributed within the TiO2/C nanofibers. Meanwhile, the radius of the C region is approximately 33 nm larger than that of the Ti, O and N region, confirming the


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Figure 5. (a) TEM image and SAED (inset) of TiO2/C nanofibers (80:10:10) after 2000 discharge/charge cycles at 10 C; The magnified HRTEM images of (b) TiO2 and (c) ploymeric gel-like film corresponding to regions I and II outlined by the dashed line in (a); (d)-(h) SEM and EDX mapping images of TiO2/C nanofibers (80:10:10) after 2000 discharge/charge cycles at 10C. formation of a polymeric gel-like film on the TiO2/C nanofibers. In contrast, the TiO2 nanofibers (80:10:10) completely pulverized into irregular particles after being cycled at 10 C over 100 cycles (Figure S8). Thus, the long cycling stability, the high capacity and the excellent rate capability of TiO2/C nanofibers could be ascribed to their stable 1D nanofiber structures with hybrid structure of TiO2 nanocrystals and carbon framework in close contacts, enabling efficient electron transport and short ion diffusion lengths. Figure S9 shows the Nyquist plots of a TiO2/C


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nanofiber electrode (80:10:10) and a TiO2 nanofiber electrode (80:10:10) after 2000 cycles at 10 C. An equivalent circuit was used to fit the Nyquist plots to further investigate the kinetic parameters of the nanofiber electrode. The charge transfer resistance Rct of the TiO2/C nanofiber electrode is 8.5 Ω, which is much lower than that of the TiO2 nanofiber electrode (19.9 Ω) because the TiO2/C nanofiber electrode possesses a one-dimensional structure and interconnected carbon framework, which lead to improved charge transfer. 4.CONCLUSIONS In summary, we have presented the synthesis of TiO2/C nanofibers in an atmospheric vessel at room temperature through the combination of mechanical stirring and addition of a polymer. PEI can not only serve as a structure-directing agent to fabricate a one-dimensional TiO2/C structure during the stirring process but also act as a carbon and nitrogen source and a micro/meso- pore-forming agent during the annealing process. The excellent electrochemical performance, including high reversible capacity, excellent cycling performance, and superior rate capability was obtained for TiO2/C nanofibers due to their unique architecture. On one hand, the small size of TiO2 nanocrystals can shorten the lithium ion diffusion distance; on the other hand, micro- and mesopores act as “cavities” to contribute additional pseudocapacitive charge storage. Lastly, N-doped carbon framework in the nanofibers provide efficient electron conductive pathways. The synthetic methodology we have developed is very simple, low cost, high yield and scalable and may be applied to all other metal oxide nanofibers.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXX FTIR spectra of the TiO(OH)2/PEI nanofibers; TEM images, CV curves and galvanostatic charge-discharge curves of TiO2 nanofibers; XPS spectra of the TiO2/C nanofibers; SEM images of TiO2/C and TiO2 nanofibers after being cycled; cycling performance of the CB electrode; Nyquist plots and the equivalent circuit; a comparison list of electrochemical performance (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Yong Liu: 0000-0002-0357-9906 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No.51772337), Fund for Fostering Talents in State Key Laboratory of Optoelectronic Materials and Technologies, China (No. OEMT-2017-ZY-09) and the Fundamental Research Funds for the Central Universities, China (No. 16lgjc60).


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