FULL PAPER DOI: 10.1002/asia.201301183

Compositing Amorphous TiO2 with N-Doped Carbon as High-Rate Anode Materials for Lithium-Ion Batteries Ying Xiao, Changwen Hu, and Minhua Cao*[a] Abstract: Compositing amorphous TiO2 with nitrogen-doped carbon through TiN bonding to form an amorphous TiO2/N-doped carbon hybrid (denoted a-TiO2/CN) has been achieved by a two-step hydrothermal– calcining method with hydrazine hydrate as an inhibitor and nitrogen source. The resultant a-TiO2/CN hybrid has a surface area as high as

108 m2 g1 and, when used as an anode material, exhibits a capacity as high as 290.0 mA h g1 at a current rate of 1 C and a reversible capacity over 156 mA h g1 at a current rate of 10 C Keywords: amorphous materials · doping · electrochemistry · lithium · titanium

after 100 cycles; these results are better than those found in most reports on crystalline TiO2. This superior electrochemical performance could be ascribed to a combined effect of several factors, including the amorphous nature, porous structure, high surface area, and N-doped carbon.

Introduction Nowadays, owing to their low cost, absence of memory effect, high electrical conductivity, and long cycling life, lithium-ion batteries (LIBs) have become a crucial power source for electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs).[1–4] Additionally, they have been widely used in portable devices, such as laptops, portable electronics, and digital cameras. Although graphite is the most popular anode material for LIBs, there are still several drawbacks, including poor rate performance, low capacity, structural deformation, and a tendency to lose internal electrical connectivity; these drawbacks greatly restrict its application in wider areas.[3, 5] Since pioneering work by Tarascon et al. in 2000,[6] transition-metal oxides have attracted much attention as high-capacity anode materials for next-generation LIBs.[7] Among all transition-metal oxides, TiO2 is considered to be one of the most promising alternatives to conventional graphite anodes because it possesses excellent rechargeable ability and improved safety, which arises from its higher lithium-insertion potential over conventional graphite anodes.[8] From a structural viewpoint, TiO2 is beneficial for lithium storage because the three-dimensional framework constructed by octahedral TiO6 sharing corners and edges could leave favorable empty sites available for lithium insertion (Figure 1 a).[7, 9]

Figure 1. a) Crystal structure of anatase TiO2. b) XRD patterns of the precursor and the a-TiO2/CN hybrid; inset gives the energy-dispersive spectroscopy (EDS) results for the a-TiO2/CN hybrid.

Therefore, various kinds of TiO2, including rutile, anatase, brookite, and amorphous phase, have been investigated for LIBs;[8–10] reports related to amorphous TiO2 are relatively few. However, it has homogeneous volume expansion and contraction during the lithium insertion and desertion process. Especially for particles, this homogeneous effect in the coexistence region of the phases within different lithium concentrations with the same particles could effectively avoid particle pulverization.[11–13] However, the poor conductivity of TiO2 results in great capacity fading and a poor rate capability, which is unfavorable for its large-scale application. According to reported in the literature,[14–17] tuning of the conductivity of TiO2 by doping is a positive route to enhance its conductivity and subsequent excellent electrochemical performance. For instance, Zheng et al. reported CN codoped mesoporous TiO2, which exhibited a high capacity of about 272 mA h g1 at a current density of 0.1 C, with a surface area of 44.29 m2 g1 through the hydrolysis of

[a] Dr. Y. Xiao, Prof. C. Hu, Prof. M. Cao Key Laboratory of Cluster Science, Ministry of Education of China Department of Chemistry, Beijing Institute of Technology Beijing 100081 (P.R. China) Fax: (+ 86) 10-68918572 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301183.

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tetra-n-butyl titanate in ethanol with the assistance of urea.[14] Liu et al. reported anatase TiO2 crystals doped with nitrogen and sulfur, and the resultant material showed a superior rate capability.[15] Mesoporous fluorine-doped TiO2 was synthesized by a unique, low-temperature, urea-assisted, hydrothermal synthesis and it exhibited a reversible capacity of 157 mA h g1 after 100 cycles at 0.5 C.[16] Furthermore, designing a porous composite structure of TiO2 with carbon is an effective strategy to improve the electrochemical behavior of TiO2 anodes. First, the porous structure could effectively shorten the diffusion pathway of lithium ions and electrons, increase the high contact area between electrolyte and electrode, and provide good accommodation of strain during cycling.[14–18] Second, compositing materials with carbon can improve the electrochemical conductivity and release the volume expansion efficiently, and subsequently, improve the cycling stability.[1, 2] In particular, nitrogen doping can generate more defects in the materials and provide more active sites for lithium-ion storage sites.[14–16, 19] However, most previous reports involved tedious processes and rigid conditions, such as assistance from templates, high temperatures, and multistep reactions, which are unfavorable for scale up. Herein, we introduce a facile strategy to realize the compositing of amorphous TiO2 with nitrogen-doped carbon (abbreviated as a-TiO2/CN hybrid). Hydrazine hydrate used in our method, on one hand, serves as a nitrogen source for nitrogen doping and, on the other hand, inhibits the crystallinity of TiO2 to form an amorphous structure. Nitrogen doping of carbon generally requires a higher temperature, although the higher temperature, in turn, easily leads to the formation of crystalline TiO2. These two aspects, therefore, are very difficult to consider simultaneously, but was successfully realized herein. The resultant a-TiO2/CN hybrid has a high nitrogen content and high surface area of 108 m2 g1. This unique structure is expected to exhibit a high-rate lithium storage performance. When used as anode materials, the a-TiO2/CN hybrid can maintain a specific capacity as high as 290.0 mA h g1 at a high current rate of 1 C and a reversible capacity over 156 mA h g1 at a current rate of 10 C after 100 cycles; these values are better than most of those reported for crystalline TiO2. This result also further confirms superior structural advantages in lithium storage.

Figure 2. FE-SEM (a,b) and TEM (c,d) images of the as-prepared aTiO2/CN hybrid. HRTEM image (e) and the corresponding selectedarea electron diffraction (SAED) pattern (f) of the a-TiO2/CN hybrid.

comes from the substrate and platinum is used to increase the conductivity when carrying out the SEM measurement). Figure 2 a and b shows field-emission scanning electron microscopy (FE-SEM) images of the a-TiO2/CN hybrid. It can be clearly seen that the sample is composed of spherical particles with diameters in the range of 20–50 nm. TEM images with different magnifications further confirm the spherical particles (Figure 2 c and d), in agreement with the results from SEM analysis. High-resolution TEM (HRTEM) images show that the texture of the particles is very homogenous and no lattice fringes can be detected (Figure 2 e). In addition, the SAED pattern displays a very ambiguous halo (Figure 2 f). Therefore, HRTEM and SEAD both confirm the amorphous nature of the TiO2/CN hybrid. Moreover, surface-scanning element mapping of the EDS results clearly proves the existence and homogeneous distribution of the titanium, oxygen, carbon, and nitrogen elements (Figure 3 a). The carbon, hydrogen, and nitrogen content of the annealed sample was 24.80, 1.60, and 8.87 %, respectively, as determined by CHN elemental analysis. Therefore, the TiO2 content was evaluated to be 64.73 %. Additionally, thermogravimetric (TG) analysis also further confirmed this value (Figure S1 in the Supporting Information). The Raman spectrum also demonstrated the existence of carbon in this sample (Figure 3 b). The intensity ratio of the D and G bands (ID/ IG) was calculated to be 1.06; this indicated that more defects were generated on the surface of the sample.[19] Furthermore, the N2 adsorption–desorption measurement (Figure 3 c and inset) for the as-synthesized a-TiO2/CN hybrid gave a microporous structure and a high BET surface area of 108.5 m2 g1, which may mainly have resulted from amorphous TiO2 and carbon. It should be noted that the high surface area and porous structure would be beneficial for the electrochemical performance of the hybrid and could provide a high electrolyte/electrode contact area; thus facilitating transport of lithium ions across the electrolyte/electrode interface.[17–19]

Results and Discussion The crystal structure of the precursor and annealed sample (a-TiO2/CN hybrid) was studied by means of XRD. As shown in Figure 1 b, for both samples, only a plain pattern with a broad peak between 20 and 308 is observed and no diffraction peaks corresponding to the crystalline phase are detected; this indicates the amorphous structure of the precursor and the a-TiO2/CN hybrid. EDS analysis (inset in Figure 1 b) of the annealed sample confirmed the coexistence of titanium, oxygen, carbon, and nitrogen (silicon

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Figure 3. a) The selected FE-SEM area for element mapping and the corresponding element mapping images for Ti, O, C, N, respectively. b) The Raman spectrum of the a-TiO2/CN hybrid. c) Nitrogen adsorption–desorption isotherms of the a-TiO2/CN hybrid and the pore size distribution (inset).

To further investigate the surface composition of the a-TiO2/ CN hybrid, X-ray photoelectron spectroscopy (XPS) was carried out. As shown in Figure 4 a, the peaks of titanium, oxygen, carbon, and nitrogen were detected in the survey XPS spectrum, which was in good agreement with the abovementioned EDS results. The high-resolution Ti 2p spectrum exhibits two peaks at 463.9 and 458.2 eV, which correspond to Ti 2p1/2 and Ti 2p3/2 of TiO2, respectively (Figure 4 b). Moreover, a peak separation of 5.7 eV is observed between the two Ti 2p peaks, which is consistent with values reported in the literature.[20] The XPS peak of C 1s can be divided into four peaks, which are centered at 284.5, 285.5, 286.0, and 288.5 eV and represent sp3Csp3C, Nsp2C, CO C, and COTi, respectively.[21] Moreover, the N 1s peak can be divided into two peaks centered at 399.9 and 398.4 eV, which can be identified as two types of nitrogen species (pyrrolic and pyridinic) on nitro-

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gen-doped carbon (Figure 4 d). Additionally, the amount of the latter calculated is 54.2 % of the total nitrogen atoms. According to reports in the literature, pyridinic nitrogen-doped carbon is more favorable than pyrrolic nitrogen-doped for lithium storage.[19, 22] In addition, it should be noted that the introduction of nitrogen species played an important role in the formation of the a-TiO2/CN hybrid because crystalline TiO2 was obtained in the absence of N2H4 in our experiments (Figure S2 in the Supporting Information). This result can be ascribed to the strong interaction between titanium and nitrogen, which effectively restrains the crystallinity of TiO2. This strong interaction

Figure 4. a) The survey XPS spectrum of the a-TiO2/CN hybrid and high-resolution XPS spectra of Ti (b), C (c), and N (d). The high-resolution XPS spectra of the a-TiO2/CN hybrid after etching for N 1s (e) and Ti 2p (f).

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can be further confirmed by XPS of the a-TiO2/CN hybrid after etching treatment conducted three times. Compared with that of the unetched sample, the etched sample exhibited an evident N 1s peak at 396.8 eV, which could be attributed to the OTiN bond (Figure 4 e).[23] Moreover, for Ti 2p, a new peak at 456.4 eV was also observed after etching, which could be ascribed to the TiN bond (Figure 4 f).[14] In addition, FTIR spectra of the as-obtained a-TiO2/CN hybrid, the crystalline TiO2/C hybrid, and TiO2 were compared, as shown in Figure 5. It can be clearly seen that com-

Figure 5. FTIR spectra of samples of a-TiO2/CN, TiO2/C, and TiO2.

pared with those of the TiO2/C and TiO2 samples, the TiO stretching of the a-TiO2/CN sample exhibits an apparent redshift from 467.2 to 545.8 cm1. According to reports in the literature,[24] the redshifting phenomenon can be ascribed to amorphization of the sample. Therefore, the amorphous nature and nitrogen doping of the a-TiO2/CN hybrid are expected to have an important effect on the electrochemical performance of the hybrid. The electrochemical performance of the a-TiO2/CN hybrid was evaluated as an anode material in LIBs. Figure 6 a shows the charge–discharge voltage profiles cycled at a current rate of 1 C (1 C = 305.4 mA g1, which is calculated based on the capacity of the C and TiO2) over the potential window of 0.01–3.00 V versus Li + /Li. Usually the commonly used potential window is 0.01–3.00 V for carbon materials and 1.00–3.00 V for TiO2 ; therefore, to more effectively achieve the synergistic effect between carbon and TiO2 in this hybrid, we used a voltage window of 0.01–3.00 V.[25] The initial discharge and charge specific capacities are 871.8 and 483.3 mA h g1, respectively, in which the large initial discharge capacity can be attributed to the formation of solid electrolyte interface (SEI) films on the surface of the electrode owing to electrolyte decomposition and some side reactions; this is similar to most reports on LIBs.[1–3, 26] Although a large irreversible capacity loss is observed in the first cycle, the reversible capacity is still as high as 541.1 mA h g1 in the second cycle and in subsequent cycles it exhibits very slight fading. Unlike previous reports that crystalline bulk TiO2 exhibits plateaus in its charge–discharge profiles, which is an indication of a two-phase intercalation process in TiO2,[27] no clear plateau was detected for the a-TiO2/CN electrode; this may be attributed to the dif-

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Figure 6. Electrochemical properties of the as-prepared a-TiO2/CN hybrid for lithium storage. a) The charge–discharge profiles at a current density of 1 C. b) The discharge capacity versus cycle number of the aTiO2/CN hybrid at 1 and 10 C, and that of TiO2/C and TiO2 cycled at 10 C. c) Rate performance of the a-TiO2/CN hybrid, TiO2/C, and TiO2 at varied rates from 1 to 20 C.

ferent thermodynamics of the insertion reactions.[28, 29] According to previous reports, particle size induced solid solution behavior leads to the sloped charge–discharge potential curves and this character is believed to be beneficial for delivering high lithium storage capacities. Therefore, the asprepared a-TiO2/CN hybrid can be expected to achieve a high capacity delivery and enhanced rate capability. Figure 6 b displays the cycling performance of the a-TiO2/ CN composite at 1 and 10 C, which is compared with the TiO2/C hybrid and crystalline TiO2 samples at 10 C. The TiO2/C hybrid was synthesized through a method similar to that used for the a-TiO2/CN hybrid, but in the absence of hydrazine. TiO2 was obtained without the use of hydrazine and ascorbic acid. Compared with the a-TiO2/CN hybrid, TiO2/C and TiO2 both display crystalline states and spherical

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morphologies (Figure S1 in the Supporting Information). As shown in Figure 6 b, the a-TiO2/CN hybrid showed reversible capacities of up to 290.0 mA h g1 (1 C), which corresponded to 99.1 % Coulombic efficiency, and 156 mA h g1 (10 C) after 100 cycles, which was far higher than that of TiO2/C (83.4 mA h g1) and TiO2 (26.6 mA h g1) at 10 C after 100 cycles. The rate capability plays a significant role in practical applications, such as in EVs and portable power tools. Therefore, we investigated the rate performance of the materials at various rates (from 1 to 20 C). As long as the rate rolls back to 1 C, the capacity of the a-TiO2/CN hybrid can recover to the original value; this indicates an excellent capacity capability. Compared with the TiO2/C hybrid and pure TiO2, the TiO2/CN hybrid displays a superior rate capability and larger capacity under the same testing conditions. From the above results, compared with crystalline TiO2, the high capacity and rate capability as well as excellent cycling performance of the a-TiO2/CN hybrid could be attributed to its special composition and microstructural features, that is, the amorphous nature, the porous structure, high surface area, and nitrogen-doped carbon. First, the homogeneous volume expansion and contraction of amorphous TiO2 result in little particle pulverization during cycling.[30] Second, the small size of the nanoparticles combined with the porous property and large surface area are beneficial for easy electrolyte immersion and diffusion, which can provide short path lengths with less resistance for both lithium ions and electron transport within the electrolyte; meanwhile, the hybrid possesses a low surface energy, which results in less self-aggregation during the charge–discharge process.[14–18] Third, nitrogen doping generates more defects, which may serve as additional active sites for lithium storage.[16, 19] Finally, carbon has good conduction and elasticity to accommodate the volume strain effectively during the charge–discharge process. All of these advantages lead to excellent electrochemical performance of the hybrid.

Experimental Section Synthesis of a-TiO2/CN Hybrid All chemicals used were of analytical grade used without further purification. In a typical synthesis, ascorbic acid (1 mmol) was dissolved in ethanol (50 mL). After stirring for 20 min, TiCl4 (0.1 mL) was added to this solution to form a yellow solution. N2H4 (2 mL) was then added dropwise. The as-formed dark yellow solution was transferred into an 80 mL Teflon-lined stainless-steel autoclave, sealed, and maintained at 120 8C for 24 h to yield a precursor. Subsequently, the precursor was freezedried overnight and annealed in a N2 atmosphere at 500 8C for 2 h at a heating rate of 5 8C min1. For comparison, TiO2/C hybrid and pure TiO2 were also prepared by means of a similar method. TiO2/C was obtained in the absence of N2H4 and pure TiO2 was formed without the addition of ascorbic acid and N2H4, while other conditions were kept constant in a typical experiment. Characterization The composition and phase purity of the as-synthesized samples were analyzed by powder XRD with CuKa (l = 1.54178 ) incident radiation by using a Shimadzu XRD-6000 instrument operated at 40 kV voltage and 50 mA current. XRD patterns were recorded from 10 to 808 (2q) with a scanning step of 58 min1. The sizes and morphologies of the resulting products were studied by using a H-8100 transmission electron microscope operating at 200 kV accelerating voltage. FE-SEM and EDS element mapping images of the sample were recorded on a Hitachi S-4800 SEM unit. Raman spectra were recorded on an Invia Raman spectrometer, with an excitation laser wavelength of 514.5 nm. The gas flow and etching duration are 20 sccm and 3 min, respectively. The carbon content was determined by using an elemental analyzer (Vario EI) with the combustion method. XPS results were recorded on an ESCALAB 250 spectrometer (Perkin–Elmer) to characterize the surface composition. The depth profile was realized by combing Ar-ion sputter etching with the XPS technique by using a 3 keV Ar-ion beam to provide a sample current of 3 mA and an etching time of 30 s each time. FTIR spectra were recorded on a Nicolet 170SXFTIR spectrometer (400–4000 cm1). The BET surface area of as-synthesized samples was measured by using a Belsorpmax surface area detecting instrument by N2 physisorption at 77 K. TG analysis of the composite was carried out with a DTG-60AH instrument with a heating rate of 20 8C min1 from 25 to 700 8C. Electrochemical Measurements The electrochemical performance was tested by means of coin-type LIB cells (2025) assembled in an argon-filled glove box. For anode preparation, a mixture of active material, carbon black, and polyvinylidene fluoride (PVDF) binder with a weight ratio of 80:10:10 was dispersed in Nmethylpyrrolidone (NMP) solution, and the resultant slurry was then uniformly pasted on a Cu foil current collector. A typical electrode was dried at 120 8C for 24 h under vacuum before being assembled into coin cells in an argon-filled glove box. A Celgard 2400 microporous polypropylene membrane was used as the separator and Li foil was used as the counter electrode. The nonaqueous electrolyte used was 1 m LiPF6 dissolved in a mixture (1:1:1, in wt %) of ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC). Galvanostatic cycling experiments of the cells were performed on a LAND CT2001A battery test system in the voltage range of 0.01–3.00 V versus Li + /Li at room temperature. The 1 C value used in our experiments was calculated based on the theoretical capacity of the a-TiO2/CN hybrid, which was calculated by using Equation (1):

Conclusion Amorphous TiO2/CN hybrid was synthesized by using hydrazine hydrate as an inhibitor and nitrogen source. The resultant a-TiO2/CN hybrid possessed a porous structure with a high surface area of 108 m2 g1. When used as an anode materials, the a-TiO2/CN hybrid could maintain a specific capacity as high as 290.0 mA h g1 at a current rate of 1 C and a reversible capacity of over 156 mA h g1 at a current rate of 10 C after 100 cycles; these values are better than most of those reported for crystalline TiO2. The amorphous nature, porous structure, high surface area, and nitrogen-doped carbon were expected to be responsible for the excellent lithium storage performance. This work provides some insight into the design of a composite of an amorphous component with nitrogen-doped carbon for LIBs.

CTiO2 =NC ¼ CC  mass%C þ CTiO2  mass%TiO2 ¼ 370  24:80 % þ 330  64:73 % ¼ 305:369

ð1Þ

 305:4 mA h g1

Thus, 1 C corresponds to 305.4 mA g1.

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Acknowledgements

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This work was financially supported by the National Natural Science Foundation of China (21173021, 21231002, 21276026, 21271023, 91022006, and 20973023) and the 111 Project (B07012).

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Received: August 31, 2013 Revised: September 27, 2013 Published online: November 4, 2013

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Compositing amorphous TiO2 with N-doped carbon as high-rate anode materials for lithium-ion batteries.

Compositing amorphous TiO2 with nitrogen-doped carbon through Ti-N bonding to form an amorphous TiO2/N-doped carbon hybrid (denoted a-TiO2/C-N) has be...
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