Synergistic effects from graphene oxide nanosheets and TiO₂ hierarchical structures enable robust and resilient electrodes for high-performance lithium-ion batteries

Abstract

We present a facile one-step route for successfully fabrication of a flowerlike rutile/anatase TiO₂ hierarchical structure decorated with graphene oxide (GO). The primary oriented TiO₂ nanorods, with diameter of 10–20 nm, were closely wrapped with GO nanosheets to build secondary hierarchical architecture with a size of 1.07 μm, resulting in a compact, mechanically robust and stable structure, which is completely different from the TiO₂ nanoparticle/graphene composite obtained by using graphene as precursor under the same conditions. When used as an anode materials for lithium-ion batteries (LIBs), it exhibited a high reversible capacity and long cycling stability with high discharge capacities of 211 mA h g⁻¹ at 1C over 500 cycles and 130 mA h g⁻¹ at 5C after 1000 cycles. To better understand what underlying factors lead this TiO₂@GO hierarchical structure to achieve its excellent electrochemical performance, we have synthesized a series of bare TiO₂ hierarchical structures with systematic phase evolution changing from rutile to rutile/anatase and anatase. The effect of crystal phase on the electrochemical performance is then discussed, and the anatase TiO₂ electrode exhibited the best electrochemical performance among the bare TiO₂ electrodes. However, the rutile/anatase TiO₂@GO hierarchical structure demonstrated much better performance than the anatase TiO₂ electrode, which is ascribed to the synergistic effects from the TiO₂ hierarchical structure and GO nanosheets. This study will further guide the fabrication of functional nanocomposites by applying the “two is better than one” strategy.

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1 Introduction

Lithium-ion batteries (LIBs) have obtained commercial success in portable electronic devices and are considered the most promising large-scale power sources for electric and hybrid electric vehicles. However, the safety issue is the major technological barrier that has inhibited the development of LIBs in electric vehicles operated at high charge/discharge rates since the breakdown of the solid electrolyte interphase (SEI) layer and exothermic reaction between the lithiated carbon and electrolyte may occur for current commercial LIBs using graphite as the anode.^1–3 Another limitation is that graphite suffers greatly from repeated volume expansion and shrinkage during Li-ion intercalation and extraction, which fatigues the graphite particles. As a consequence, the particles break apart, leading to a loss in electrical contact between the cracking particles.⁴

TiO₂ material has received extensive attention due to its potential applications in photocatalysis, sensors, photovoltaic devices, and energy storage. Within the field of LIB research and development, TiO₂ material has been particularly investigated as a safe alternative candidate to the current graphite anodes since it operates at a relatively high lithium insertion/extraction voltage (1.5–1.8 V vs. Li/Li⁺), which can efficiently avoid electroplating of lithium during the cycling processes.^5,6 In addition, TiO₂ is an abundant, low cost and environmentally benign material with relatively high theoretical capacity of 167 mA h g⁻¹.^7,8 Besides, it demonstrates a negligible volume change (<4%) and excellent structure stability during lithium ion intercalation/deintercalation processes that can ensure long cycle life.⁹

It is widely accepted that the electrochemical performance of LIBs intimately depends on engineering materials to desired morphologies, sizes and crystalline phase. Of the manifold of synthesizable crystalline TiO₂ polymorphs, mainly the three crystal types: rutile, anatase and brookite are found in nature.¹⁰ Among them, the investigation on TiO₂ brookite is limited due to its rareness and difficult preparation.¹¹ It is generally accepted that anatase is the most electroactive host for lithium insertion.¹² Recently, a number of reports have supported that nanosized rutile TiO₂ also exhibits enhanced lithium storage performance.^13–16 Unfortunately, there is little agreement on which TiO₂ polymorphs is really the dominant even among the same group since their fabrication methods, synthetic morphologies and particle sizes are completely different.¹⁷ Another important consideration in the use of TiO₂ material is its size. Reducing the material size down to nanoscale level can shorten the lithium ion diffusion path and increase the specific surface area. However, nanostructure materials often tend to aggregate either during synthesis or the charge–discharge process, which not only increase the large contact resistance within electrodes but also decrease the effective surface area contacted with the electrolyte.^3,4 Alternately, it is the best choice to purposely assemble nanosized primary building blocks into desired hierarchical structures. In these hierarchical structures, the secondary micro-sized assemblies effectively avoid aggregation and preserve good structural stability while nanosized building blocks can both provide highly exposed surface area and effectively shorten the lithium ion diffusion length, leading to improved rate capability and cycling performance.^18–20

As a typical two dimensional layered materials, graphene, which is often fabricated by reducing graphene oxide, has been widely investigated as a promising electrode material candidate for LIBs relatively due to its unusual properties such as high surface area and good electrical conductivity.^21–23 However, reduced graphene oxide is usually produced from graphene oxide by using highly toxic hydrazine (N₂H₄).²⁴ In addition, the strong π–π interaction of graphene and their intrinsic incompatibility with inorganic components constitutes a major obstacle for its homogeneous dispersion of nanoparticles onto graphene.⁵ Another drawback is that the connections between the active materials and graphene sheets are commonly via weak van der Waals force which may not endure the structural stress during the charge/discharge process.²⁵ Recently, as the precursor of graphene, graphene oxide (GO), has also attracted great scientific interest as a potential anode material for electrochemical energy storage devices such as LIBs,^26–29 Li/S batteries,^30,31 and supercapacitors^32–36 Moreover, the production of GO does not need a reduction treatment, which has lower cost as compared with that of graphene. In addition, it has good solubility in solvents and thus provides fertile opportunities for the fabrication of GO and nanomaterial hybrid composites owing to its abundant oxygen-containing functional groups.³⁵ Furthermore, the ion conductivity can also be improved due to the interaction between the functional groups of GO and lithium ions.³⁷

Here, we have presented a one-step hydrothermal method for assembling oriented rutile/anatase nanorods with GO nanosheets to build hierarchical structure, which can form a compact, mechanically robust and very stable structure. As a comparison, we also demonstrate the successful fabrication of TiO₂ hierarchical structures with three types of phase such as anatase, rutile/anatase, and rutile by only adjusting the ratio of TiCl₃ solution to formic acid. Then, the effect of crystal phase on their electrochemical performance is discussed under almost the same conditions including structures, sizes and synthetic methods. When tested as anode materials for LIBs, GO nanosheet wrapped TiO₂ hierarchical structure exhibited a high reversible capacity and long cycling stability with a discharge capacities of 211 mA h g⁻¹ at 1C over 500 cycles and 130 mA h g⁻¹ at 5C after 1000 cycles, which is much better than those of bare TiO₂ hierarchical structure electrodes with anatase, rutile/anatase, and rutile phase, showing a strong synergistic effect from TiO₂ hierarchical structure and GO nanosheet.

2 Experimental

2.1 Synthetic procedures

All chemical reagents were commercially available and used without further purification. The preparing procedures can be described as follows:

(1) Synthesis of anatase TiO₂ hierarchical structure: 4.5 mL of titanium(III) chloride solution (15% TiCl₃ basis) was dissolved in 20 mL formic acid under vigorous stirring for 30 min. The homogeneous solution was then transferred into a 100 mL Teflon-lined autoclave. After that, the autoclave was kept at 150 °C for 12 h. Afterward, autoclave was taken out and cooled naturally to room temperature. Then, the white precipitate was collected by centrifugation, and rinsed with deionized water and absolute ethanol several times. Finally, it was dried at 70 °C for 24 h.

(2) Synthesis of rutile TiO₂ hierarchical structure: the fabrication process is similar to that of anatase TiO₂ except that the 0.5 mL of TiCl₃ solution was added into 20 mL formic acid.

(3) Synthesis of rutile/anatase TiO₂ hierarchical structure: the fabrication process is similar to that of anatase TiO₂ except that 2 mL of TiCl₃ solution and 5 mL of deionized water was added into 20 mL formic acid.

(4) Synthesis of rutile/anatase TiO₂@GO hierarchical structure: GO or reduced GO was produced from natural graphite flakes by using a modified Hummers method as described elsewhere.³⁸ Then, the fabrication process of rutile/anatase TiO₂@GO hierarchical structure is similar to that of rutile/anatase TiO₂ except that 2 mL of TiCl₃ solution and 5 mL of GO solution with concentration of 5 g L⁻¹ were added into 20 mL formic acid. As a comparison, the reduced GO solution with the same concentration was also added in the reaction to study the effect of reduced GO on the morphology of the nanocomposite while keeping other conditions same to the fabrication of rutile/anatase@GO sample. Finally, the bare TiO₂ samples were annealed at 400 °C for 3 h in air, and the rutile/anatase TiO₂@GO sample was annealed at 400 °C for 3 h with a stream of 5% hydrogen in nitrogen flowing at 50 sccm (standard cubic centimeter per minute) before they were used as anode materials in LIBs.

2.2 Characterization

Scanning electron microscope (SEM) images and energy-dispersive X-ray spectrometry (EDS) were taken on a thermal field-emission environmental scanning electron microscope (Quanta 400F, FEI/OXFORD/HKL, Dutch). The powder X-ray diffraction (XRD) analysis was performed by a XRD diffractometer (D/max-IIIA, Rigaku, Japan) using Cu-Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) were recorded with a FEI Tecnai-G20 transmission electron microscope operating at 200 kV. GO nanosheet characterization was performed on a NanoScope-IIIa Multimode-Atomic Force Microscopy (AFM, Veeco, USA). Thermogravimetric analysis (TGA) was conducted on a thermogravimetry analyzer (TG-209/Vector-22) in flowed air environment, and the temperature range was from room temperature to 800 °C at a heating rate of 10 °C min⁻¹. Raman spectra were recorded from 100 to 2000 cm⁻¹ using an Almega Dispersive Raman spectroscopy (Renishaw inVia) with an Nd:YAG intracavity doubled laser operating at 514.5 nm.

2.3 Electrochemical measurements

The electrochemical measurements were investigated using a two-electrode configuration assembled in the CR2032-type coin cells. The working electrodes were fabricated by mixing active materials, carbon black, and binder (polyvinylidene difluoride, PVDF) in a weight ratio of 7 : 2 : 1 in N-methyl-2-pyrrolidone (NMP) and pasted on pure Cu foil. The mass loading of active materials was controlled at about 1.98 mg cm⁻². Lithium metal foil was served as both the counter and reference electrodes. The working electrodes and counter electrodes were separated by porous membrane (Celgard 3400), which were soaked with electrolyte containing 1.0 M LiPF₆ in a 1 : 1 (w/w) mixture of ethylene carbonate–diethyl carbonate (Technologies, USA). The electrochemical cells were assembled in an argon-filled glovebox (Mikrouna) with O₂ and H₂O below 0.1 ppm level. The cyclic voltammetry (CV) were carried out on a CHI 760C electrochemical workstation scanned at 0.2 mV s⁻¹ in a voltage window of 1.0–3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were also conducted on the CHI 760C electrochemical workstation by applying a sine wave with an amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz. Galvanostatic charge/discharge test were performed using a NEWARE battery testing system with a voltage window of 1.0–3.0 V at various current densities.

3 Results and discussion

Fig. 1 show the XRD patterns of as synthesized TiO₂ samples obtained at hydrothermal temperature of 150 °C. Obviously, the bare TiO₂ samples with rutile, rutile/anatase and anatase phase are obtained by changing the ratio of TiCl₃ solution to formic acid. In addition, while the synthesized condition is same to that of rutile/anatase TiO₂ sample except that additional GO nanosheets was added in the reaction, the obtained TiO₂@GO sample still shows rutile/anatase phase. No peaks corresponding to the stacking of GO layers are observed in the case of rutile/anatase TiO₂@GO sample, suggesting the notorious aggregation problem of graphene or GO layers doesn't exist in our approach. In addition, the rutile/anatase fractions were determined by Rietveld refinement to 83.5% rutile and 16.5% anatase for rutile/anatase TiO₂ and 96.9% rutile and 3.1% anatase for rutile/anatase TiO₂@GO samples, respectively (see Fig. S1†).

Fig. 1 XRD patterns for the bare TiO₂ samples with (a) rutile (pink line), (b) rutile/anatase (blue line) and (c) anatase TiO₂ phase (red line), and (d) rutile/anatase TiO₂@GO sample (green line), along with the corresponding rutile and anatase TiO₂ reference patterns (asterisk * represents anatase peaks in mixed phases).

GO nanosheets were further characterized by AFM, Raman and TGA measurements. It can be seen from AFM images in Fig. S2a† that curly GO nanosheets have been successfully produced by using modified Hummers methods, which exposed very thin wrinkled paper-like structure on their edges. In addition, the height profile of the AFM image in Fig. S2b† shows that the thickness of GO is about 0.8–1.2 nm, which is corresponded to that of single-layer GO nanosheets. Fig. S2c† further shows the Raman spectra of sample. The peak at 146 cm⁻¹ is assigned to characteristic mode of anatase phase³⁹ while the peaks at around 236, 447, and 612 cm⁻¹ are due to the characteristic modes of rutile.¹⁶ Another two peaks observed at 1353 and 1605 cm⁻¹ can be ascribed to the D and G bands of GO, respectively, and a ratio of the D-band to G-band intensity (I_D/I_G) is approximately 0.91, which is in good agreement with previously reported Raman characteristics of GO.^24,40 The as-synthesized TiO₂@GO composite was then characterized by TG technique, and the weight fraction of GO in the sample is about 7.2 wt% (Fig. S2d†).

Fig. 2 shows typical FESEM images of the bare TiO₂ samples with rutile, rutile/anatase and anatase phases as well as rutile/anatase TiO₂@GO samples at different magnifications, showing that large-scale, monodisperse, and uniform hierarchical structures were obtained. In addition, one can clearly see that the bare TiO₂ products undergo obvious morphological and size changes as their crystal phase change from rutile to rutile/anatase and anatase. As shown in Fig. 2a–c, the rutile samples demonstrate well-defined flowerlike hierarchical architecture with a size of about 1.38 μm, and they are composed of a large amount of radially oriented nanorods. Moving to Fig. 2d–f, we can see that rutile/anatase products still possess the flowerlike hierarchical architecture consisting of oriented nanorods but the diameter of nanorods decreased and the number of nanorods increased as compared with the rutile samples. Accordingly, the size of rutile/anatase hierarchical architecture is decreased to about 0.97 μm. But for anatase products, they completely changed into shapeless nanoparticle aggregates with a size of about 400 nm (Fig. 2g–i). Interestingly, it can be seen from the Fig. 2j–l that rutile/anatase TiO₂@GO hierarchical structures have the almost same morphology with bare rutile/anatase TiO₂ samples and close size of 1.07 μm since they have been synthesized under almost same synthetic condition except that the additional GO was added in the reaction. The presence of GO in the rutile/anatase TiO₂@GO composite can be further confirmed by the EDX mapping and spectrum shown in Fig. S3,† indicating that uniformly dispersed Ti, O and C elements in the rutile/anatase TiO₂@GO hierarchical structure. A weight ratio of about 9.1 wt% GO in the rutile/anatase TiO₂@GO composite was detected, which is very close to TG analysis. As a comparison, the reduced GO was also added in the reaction while keeping other conditions same to the fabrication of rutile/anatase@GO sample. We can see from Fig. S4† that the reduced GO have an obviously different effect on the morphology of the nanocomposites. The rutile/anatase TiO₂ nanoparticles with serious aggregation were deposited on the surface of the reduced GO nanosheets, which is completely different from the rutile/anatase TiO₂@GO hierarchical structure, suggesting their intrinsic incompatibility with TiO₂ constitutes a major obstacle for its homogeneous dispersion of nanoparticles onto graphene.⁵

Fig. 2 FESEM images of the TiO₂ hierarchical structure with different crystal phase (a–c): rutile TiO₂; (d–f): rutile/anatase mixed phase; (g–i) anatase phase; (j–l) rutile/anatase TiO₂@GO composite.

Fig. 3 shows the morphological and structural characterizations of rutile/anatase TiO₂@GO hierarchical structure obtained by TEM technique. It becomes clear that numerous fine oriented nanorods with diameter of about 10–20 nm were wrapped in GO nanosheets to build up the resulting flowerlike hierarchical architecture (Fig. 3a and b). The presence of GO in the TiO₂@GO hierarchical structure can be confirmed by HRTEM (Fig. 3c). Fig. 3d shows the fast Fourier transform (FFT) image corresponded to pink circle on the upper right area in Fig. 3c, which reveal coexistence of diffraction peaks of typical six fold symmetry GO and TiO₂ crystal phase.³⁸ Besides, Fig. 3e is the corresponded Fourier-filtered image taken from the same area that further demonstrates the existence of two-dimensional macromolecular sheet of carbon atoms with honeycomb structure. Here, it can be seen from the selected area electron diffraction (SAED) pattern (Fig. 3f) that two set of diffraction spots are observed. One set of spots corresponding to the crystal lattice of rutile are marked R(111), R(110), and R(210), while those corresponding to the crystal lattice of anatase are indicated A(105), A(220), and A(211), revealing the coexistence of rutile and anatase crystal lattices in these hierarchical structures.

Fig. 3 (a) The typical TEM and (b) magnified TEM images of rutile/anatase TiO₂@GO hierarchical structures. (c) HRTEM image of rutile/anatase TiO₂@GO hierarchical structure. (d) The FFT image and (e) Fourier-filtered image derived from up-right pink circle area in (c). (f) The SAED pattern corresponded to rutile/anatase@GO hierarchical structure in (c).

In order to elucidate the synergistic effects of GO and TiO₂ hierarchical structure on their electrochemical performance, a series of electrochemical tests have been conducted. The rutile, rutile-anatase, anatase TiO₂ and rutile/anatase TiO₂@GO electrodes were firstly investigated by means of cyclic voltammetry (CV). The sharp cathodic/anodic peaks located at 1.70 V/2.10 V, 1.69 V/2.11 V and 1.67/2.15 are observed for rutile (Fig. 4a), rutile/anatase (Fig. 4b) and anatase electrodes (Fig. 4c), respectively, corresponding to the Li-insertion and extraction processes in three types of bare TiO₂ electrodes.⁴¹ As shown in Fig. 4d, cathodic and anodic peaks at 1.68 and 2.12 V are corresponded to Li⁺ insertion/extraction into/from the rutile/anatase mixed phase TiO₂. It is interesting that the CV curve demonstrate a rectangular shape in the range of 1.0–1.7 V, which is characteristic of charging/discharging of supercapacitance derived from the TiO₂ and GO for the rutile/anatase@GO electrodes.⁴²

Fig. 4 Cyclic voltammograms of (a) rutile, (b) rutile/anatase, (c) anatase, and (d) rutile/anatase TiO₂@GO electrodes obtained at scan rate of 0.2 mV s⁻¹.

Fig. S5† shows the charge–discharge profiles of the rutile/anatase TiO₂@GO and anatase electrodes at a rate of 0.5C. In the region I, a fast drop in the potential between 3 V and 1.7 V was observed due to a solid solution insertion process. The region II (plateau) is corresponded to the known biphasic process where Li-poor phase and Li-rich phase coexist. Finally, a sloping region of voltage ranging from 1.7 V to 1 V was observed in region III. Compared to the anatase electrode, the rutile/anatase@GO electrode possess a relatively small plateau region II, yet the main lithium storage region occurred in the long sloping region III can be ascribed to lithium ion insertion into the surface layer of TiO₂ and GO, which is also inconsistent with the observations in CV curves in Fig. 4c and d.⁴² Besides, the rutile/anatase@GO electrode delivers much higher reversible capacity than the anatase electrode, demonstrating that the GO in TiO₂ hierarchical structure play an important role for the improvement of the lithium ion storage.

The cycling performance of rutile, rutile/anatase, anatase and rutile/anatase TiO₂@GO electrodes were tested at a current rate of 1C, 2C, and 5C (1C rate is equal to a current density of 167 mA g⁻¹), respectively, as shown in Fig. 5a–c. For the three types of the bare TiO₂ electrodes, it is clear that anatase electrode presents much better electrochemical performance and cycling stability than the rutile/anatase and rutile ones. For instance, the anatase electrode delivered the discharge capacities of 108 mA h g⁻¹ at 1C over 500 cycles, and 91 mA h g⁻¹ at 2C and 82 mA h g⁻¹ at 5C even after 1000 cycles, respectively. This can be explained that the crystal structure of anatase is composed of empty zigzag channels in the whole framework so that the uptake of Li⁺ ion appears more facile in the anatase TiO₂ lattice than that in rutile TiO₂ lattice.^3,4,44 Surprisingly, with GO nanosheets decorated, the resulting rutile/anatase TiO₂@GO hierarchical structure was seen here to greatly enhance electrochemical cycling performance as compared to rutile, rutile/anatase and anatase TiO₂ electrodes. For example, it can exhibit greatly improved reversible capacity of 211 mA h g⁻¹ at 1C over 500 cycles and 138 mA h g⁻¹ at 2C, and 130 mA h g⁻¹ at 5C even after 1000 cycles, respectively. In addition, the increased and resilient capacity was observed for rutile/anatase@GO electrode during cycles, which is ascribed to a better strain relaxation of hybrid structure and firm contacts of oriented TiO₂ nanorod clusters within hierarchical structure during discharge/charge cycles. In addition, the rutile/anatase TiO₂@GO electrode showed high reversible capacity and longer life cycle performance than the other previously reported TiO₂ based materials such as mesoporous anatase TiO₂,⁴³ Ag or Au metallic-nanoparticle-embedded TiO₂ fibers,⁴⁴ and TiO₂/graphene composite.^21,22,39 The rate capability of anatase and rutile/anatase TiO₂@GO electrodes were compared in current rates of 0.5–10C in Fig. 5d. Obviously, rutile/anatase TiO₂@GO electrode demonstrates better rate capability than that of anatase TiO₂ electrode at all current densities. In the case of rutile/anatase TiO₂@GO electrode, it delivered the specific capacities of 180, 150, 100, 90, and 80 mA h g⁻¹ at current rate of 0.5C, 1C, 2C, 5C, and 10C, respectively. Even after experiencing the large current rate of 10C, the specific capacity can still recovers to 160 mA h g⁻¹ at 1C, demonstrating good recovery efficiency of 106%.

Fig. 5 Cycling performance of rutile, rutile/anatase, anatase and rutile/anatase@GO electrode obtained at different current rates of (a) 1C, (b) 2C, (c) and 5C, respectively. (d) The rate capacity of anatase TiO₂ and rutile/anatase TiO₂@GO electrodes. All measurements were carried out in a voltage window of 1–3 V.

To better understand why rutile/anatase TiO₂@GO electrodes exhibit such a superior electrochemical performance compared to the bare TiO₂ electrodes, the electrochemical impedance spectroscopy (EIS) measurements were performed after completing 100 charge/discharge cycles at a current rate of 1C. As shown in Nyquist plots in Fig. 6, the diameter of the semicircle for rutile/anatase@GO electrode in the high-medium frequency region is much smaller than those of rutile, rutile/anatase and anatase electrodes, suggesting that rutile/anatase TiO₂@GO electrodes maintained best electronic contacts and effective charge transport after repeated Li⁺ ions insertion and extraction process. Besides, the Warburg-type straight lines of all bare TiO₂ electrodes show an small angle of 12–18° to the Z′-axis, yet rutile/anatase TiO₂@GO electrode demonstrate a much larger angle approaching 48° to the Z′-axis, which is characteristic of Li⁺ ion diffusion through the homogeneous carbon materials of the electrodes.⁵ The kinetic parameters of rutile, rutile/anatase, anatase, and rutile/anatase TiO₂@GO electrodes were further investigated by modeling Nyquist plots based on the modified Randles equivalent circuit,⁶ as shown in Fig. S6.† The value of the surface film and charge-transfer resistance R_(sf+ct) for rutile/anatase@GO electrode is 1.5 Ω, which is much lower than those of rutile (20.5 Ω), rutile/anatase (6.0 Ω), and anatase electrodes (4.9 Ω) (Table S1†).

Fig. 6 Nyquist plots of rutile, rutile/anatase, anatase and rutile/anatase TiO₂@GO electrodes.

To further confirm the structure stabilizing effect of robust structure on long life electrochemical performance, we carried out the TEM characterization of rutile/anatase TiO₂@GO hierarchical structure electrodes after completions of 500 charge/discharge cycles at a current rate of 1C. Surprisingly, rutile/anatase TiO₂@GO hierarchical structure can still maintain their initial morphology and crystal structure after completing severe cycles (Fig. 7).

Fig. 7 Morphology and structure characterization of rutile/anatase@GO hierarchical structure after completing 500 charge/discharge cycles at a current rate of 1C. (a) TEM image, (b) magnified TEM image, (c) HRTEM, and (d) SAED.

According to above experimental data, we conclude that the GO nanosheets play a significant role in both the morphology of as-prepared composites and improved electrochemical performance. Firstly, the functional groups on the GO surface can effectively confine TiO₂ nanorods like a spring that keep intimate contact between them and then construct a more stable hierarchical structure. This unique structure can accommodate the volume change of the electrode and maintain the integrity of the electrode during cycles, resulting in the high reversible capability, rate capability and cycling stability. Secondly, it is composed of oriented nanorods, which not only provides high contact area with electrolyte for electrochemical activity but also favors good electron accessibility.³⁰ Thirdly, wetting properties of GO may also influence the penetration of electrolyte into the electrode structure than then improve the ionic conductivity within the electrodes at the same time.³⁷ Finally, the integrated of TiO₂ nanorods in GO may prevent the possible side reactions with the electrolyte.⁴⁵ Therefore, the outstanding electrochemical performance of rutile/anatase TiO₂@GO can be ascribed to the synergistic effect of GO and oriented TiO₂ nanorod in hierarchical structure.

4 Conclusions

In summary, we have demonstrated one-step route for assembling oriented rutile/anatase nanorods with GO nanosheets to build hierarchical structure design, which can form a compact, mechanically robust and very stable structure. The unique structures demonstrate an enhanced lithium-ion storage performance in terms of their specific capacity, rate capability, and cycle stability when they are used as anode material in LIBs. As a comparison, we also have synthesized a series of bare TiO₂ hierarchical structure with systematic phase evolution changing from rutile to rutile/anatase and anatase phases by only controlling the ratio of TiCl₃ solution to formic acid. Accordingly, the effect of crystal phase on their electrochemical performance was evaluated under the almost same conditions including structure, size and synthetic method. For the bare TiO₂ hierarchical structures, the anatase TiO₂ electrode exhibited the better electrochemical performance than rutile/anatase and rutile TiO₂ ones. However, after decoration with GO nanosheets, the rutile/anatase TiO₂@GO hierarchical structure greatly improved lithium ion storage such as high reversible capacity, good rate capability, and superior cycling retention compared to the bare TiO₂ electrodes, which can be ascribed to the synergistic effects of oriented TiO₂ nanorod clusters and GO nanosheets. In addition, our integration of GO in hierarchical structure design strategy may be applied to other electrode materials for the realization of stable and high performance LIBs.

Acknowledgements

This work was financially supported by the Beijing Scientific Research Project for Undergraduate (No. 08150115/163, 25000115006/019).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24131f

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