TiO 2 hollow spheres on reduced graphene oxide with high rate performance as anodes for lithium-ion batteries †

* a Anatase TiO 2 anchored on graphene oxide (GO) can be synthesized through a one-step hydrothermal method. The as-formed nanohybrid has a unique hollow structure and a large surface area. More importantly, compared to the pristine TiO 2 counterpart, TiO 2 @RGO composite materials as anodes in lithium-ion batteries have demonstrated a uniform and highly crystallized morphology and exhibited excellent cycling stability and rate capability of 352 mA h g (cid:1) 1 at 0.5C and 223 mA h g (cid:1) 1 at 5C after 100 cycles, indicating that the TiO 2 @RGO nanocomposite has promise in advanced Li-ion batteries. The improvement of electrochemical performance is assigned to the enhanced conductivity in the presence of GO in the TiO 2 @RGO nanocomposite, the anatase and TiO 2 – B mixed crystal phase of the hollow sphere TiO 2 @RGO nanocomposite, the small size of TiO 2 particles in the nanocomposite, and the enlarged electrode/electrolyte contact area, leading to more active sites in TiO 2 @RGO.


Introduction
5][6] In addition, owing to the higher Li-ion intercalation potential (1.5-1.8V versus Li + /Li) of TiO 2 , the generation of lithium dendrites could be suppressed.Its reaction process can be described properly with the following equation: TiO 2 + xLi + + xe À / 2Li x TiO 2 (0 # x # 0.5).8][9] Meanwhile, the reunion and decomposition of TiO 2 during charge-discharge cycles can result in the reduction of active sites and the degradation of specic capacity, which denitely hinder the application of TiO 2 in LIB anode materials. 10In order to improve electrical and ionic conductivities, adding some conductive materials, for example, carbon nanomaterials, metals, metal oxides and polymers, is a common practice.In this way, we can shorten the transmission distance and decrease the resistance of intrinsic electrons and lithium ions.At same time, different structures of TiO 2 , such as 1D nanowires or nanotubes, 2D nanosheets and even complicated 3D structures, can get higher capacities and high rate capabilities in the electrodes of lithium ions batteries based on following reason: higher surface area which is bene-cial to increase the contact area between electrolyte and electrode, resulting in an excellent cycling performance of electrode materials.
5][16] Li et al. adopted the hydrothermal method to fabricate mesoporous anatase TiO 2 nanospheres anchoring onto graphene sheets, which demonstrated a reversible capacity of 200 mA h g À1 at the rate of 1C aer 100 cycles. 17hen et al. used hydrothermal process to construct mesoporous structure TiO 2 /RGO hybrid materials, which possessed an excellent capacity retention about 260 mA h g À1 at 1.2C aer 400 cycles. 18However, most of their experimental process are quiet complex and are not easily operated in the practical application.Therefore, exploring a simple, facile and easy-to-handle method to synthesize well-dispersed TiO 2 /GO hybrid anode materials which possessed properties of structural stability and great performance is particularly important.
In this paper, we propose a simple strategy to fabricate nanostructured hollow spheres TiO 2 @RGO anode materials which make nanocrystalline TiO 2 grown in situ on graphene via a facile template-free one-step followed by calcinations.
Compared with the reported hydrothermal-solvothermal routes, the current experiment schemes were performed without addition of any surfactants.It only use tetrabutyl titanate(Ti(OR) 4 ), LiOH and GO as the raw materials.What is novel about our work is that there are few literatures on the synthesizing nanostructured hollow spheres TiO 2 @RGO electrode used TBT as precursor via template-free one-step in situ method which was deployed in LIBs.It delivers the characteristics of hollow structures with large surface area and highly mesoporous structures.We signicantly improve the electrochemical performance of anode materials and preparation procedures of TiO 2 @RGO composite materials.As a result, the excellent electrochemical performance of as-prepared TiO 2 @-RGO electrode materials shows its great potential values in advanced Li-ion batteries under a high-rate current density.Scheme 1 illustrates the synthesis process of the TiO 2 @RGO nanocomposite.

Experiment
Preparation of GO Graphene oxide was prepared via the oxidation of graphene by applying improved Hummers' method. 44eparation of nanostructure TiO 2 @RGOcomposites In a typical synthetic procedure, 50 mg GO was dissolved in 30 mL solution containing 5.0 g LiOH, followed by ultrasound for 2 h.Then, 2 mL tetrabutyl titanate (TBT) was dispersed into the above solution drop by drop along with vigorous agitation for 30 min and ultrasounded for another two hours.Thereaer, the mixture was placed into a 50 mL Teon-lined autoclave and maintained at 130 C for 48 h.Aer naturally cooled to room temperature, the obtained products were soaked in 0.1 M H 2 SO 4 solution for some time, centrifuged and washed several times by distilled water and ethanol until the pH value was close to 7, then dried at 60 C in oven for 24 h and annealed at 450 C for 4 h under nitrogen to get the nal product, namely, the TiO 2reduced graphene oxide (termed as TiO 2 @RGO) nanocomposite.The fabrication of the pure TiO 2 nanoparticles, a similar process was proceeded without the addition of GO in the original solution.

Material characterizations
The X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advance X-ray Diffractometer.Raman spectroscopy (Renishaw, 633 nm excitation) was used to characterize the interaction among of TiO 2 and GO.Structure and morphology of TiO 2 @RGO-Nd nanohybrid, the TiO 2 @RGO and the pure TiO 2 were observed by a transmission electron microscope (TEM; JSM-2100F) and a Cold Field Scanning electron microscope (SEM; JSM-7500F).Nitrogen adsorption/desorption tests were conducted at Brunauer-Emmett-Teller (BET) measurements (JW-BK222 surface area analyzer).

Electrochemical measurements
The electrochemical measurements were carried out with twoelectrode CR2025 coin cells.The working electrode was prepared by mixing 80 wt% active material (TiO 2 @RGO), 10 wt% carbon black and 10 wt% polyvinylidene uoride (PVDF).Then the mixture was dissolved in N-methyl-2pyrrolidone under stirring.The slurry was coated onto a Cu foil and dried at 120 C for 12 h.The loading mass of active material was approximately 0.43 mg cm À2 .The electrochemical measurements were carried out with two-electrode CR2025 coin cells.The lithium foil was used as the counter and reference electrodes, Celgard 2300 as the separator and 1 M LiPF 6 as the electrolyte.Constant current charge-discharge tests were carried out on a Land Battery Measurement System (CT2001A) at various current densities with a cut off voltage of 1-2.5 V at room temperature.Cyclic voltammograms (CV) at a scan rate of 0.2 mV s À1 and electrochemical impedance spectra over a frequency range of 100 kHz to 0.1 Hz were measured on a CHI660E electrochemical work station at room temperature.
However, the diffraction peaks of the TiO 2 in the TiO 2 @RGO nanocomposite become broader and lower, demonstrating a small crystallite size.Meanwhile, the XRD spectrums of hybrid nanocomposite and pure TiO 2 all display a typical TiO 2 -B diffraction peaks (space group C2/m, JCPD no.35-0088) at 14.3 , 25.0 , 28.7 , 44.7 and 58.4 corresponding to the (001), ( 110), (002), (À511), (À421) reections, respectively. 22The skeleton of the TiO 2 -B structure is consisted of corrugated sheets of edge and corner-sharing TiO 6 octahedra are linked by bridging oxygen atoms, formed a three-dimensional network.This nanostructure is more open than those of rutile and anatase, which makes the material an effective host for lithium ions storage. 23,24One diffraction peak centered at 2q ¼ 11.41 corresponds to the (001) reection of GO with an interlayer spacing of 7.75 Å. 25 The peak disappeared in the following compound process, indicating that the GO nanosheets have been completely reduced to RGO nanosheets using the current hydrothermal treatment.In the same position, the TiO 2 @RGO nanocomposite don't have the characteristic peaks of GO, showing that the TiO 2 uniformly dispersed on the GO nanosheets.Fig. 1b shows the Raman spectra of GO before and aer deposition of TiO 2 .The GO exhibited Raman shis at approximately 1354 and 1570 cm À1 corresponding to the D and G bands, respectively.Aer heat-treatment at 450 C, the D and G bands of TiO 2 @RGO located at about 1358 and 1594 cm À1 .The intensity of D band to G band (I D /I G ) for the TiO 2 @RGO (1.06) is a bit higher than that of GO (0.85).The intensity of D band to G band (I D /I G ) for the TiO 2 @RGO (1.06) is a bit higher than that of GO (0.85), indicating the reduction of GO in the TiO 2 @RGO nanocomposite.
When TBT was added into GO sheets, the TiO 2 nanoparticles homogeneously anchored on RGO nanosheets.The morphology of TiO 2 @RGO hybrid electrode material, GO and pure TiO 2 were observed via scanning electron microscopy (SEM).It can be clearly seen that GO has a wrinkled ake-like structure in Fig. 2a  and b, which is in accorded with the results in the recent article. 45,46As shown in the Fig. 3a and c, the TiO 2 were regularly spherical.Fig. 3c shows that the TiO 2 nanospheres are welldispersed and uniformly attached on the surface of the RGO nanosheets.The TiO 2 exhibited good dispersibility in the pure phase and composite electrode materials.To further explore the microstructure of the materials, the products were investigated by transmission electron microscopy (TEM).It can be seen that the TiO 2 exhibits a hollow sphere structure with uniform particle size in Fig. 3b.The hollow structure of the TiO 2 nanocomposite remained unchanged as shown in Fig. 3d.There is no structure collapse before and aer the composite.The hollow structure can be seen clearly in Fig. 3b and d.The wall thickness of TiO 2 hollow nanostructures is about 2-4 nm.The TiO 2 possesses nearly spherical structure with the size of particle about 10 nm and deliver good features, such as more stable, active site distribution more uniform, high specic surface area, wall thickness in 2-4 nm, which are contribute to the ions and electrons transfer, benecial to the improvement of the capacity.The combination of TiO 2 materials with such structure advantages and graphene materials will greatly increase the performance of TiO 2 as anode material.The selected area electronic diffraction nanostructure pattern (SAED) of the hollow spheres TiO 2 @RGO nanostructure delivers a set of diffraction rings of the anatase and TiO 2 -B structure of the TiO 2 , indicating the polycrystalline nature of the products, which can be clearly assigned to the anatase TiO 2 and TiO 2 -B, respectively.This result is consistent with the XRD results.As shown in Fig. 3d, the TiO 2 @RGO composites, in which the hollow spheres were uniformly and well dispersedly attached on the surface of RGO sheets.Fig. 3e is the High-Resolution TEM(HRTEM) image of TiO 2 @RGO.As shown in Fig. 3e, the  lattice fringes with the spaces of 0.35 nm and 0.62 nm correspond to the (101) and (001) of anatase TiO 2 and TiO 2 -B, respectively.Elemental mapping and EDX spectrum for TiO 2 @RGO nanocomposite are also shown in Fig. S1 (in the ESI †).The corresponding element mapping images (Fig. S1 †) of C, Ti, and O in the TiO 2 @RGO nanocomposite overlapped with each other, indicating the uniform distribution of these elements and the nanohollow sphere TiO 2 are evenly distributed across the surfaces of rGO.The corresponding EDX of the selected area is also depicted in Fig. S1e, † which agrees well with the mapping results, except for the Si peaks that originated from silicon substrate.Based on the EDS elements mapping analysis, we conrm that it has a unique hollow structure.
Nitrogen isotherm adsorption-desorption curves with the pore size distributions of the pristine TiO 2 and TiO 2 @RGO are presented in Fig. 4a and b.For TiO 2 @RGO, a type IV isotherm is observed, which is characteristic of mesoporous materials. 49ased on the Barrett-Joyner-Halenda (BJH) equation, the main pore size (inset in Fig. 4b) in TiO 2 @RGO is 8.5 nm less than 11.6 nm of the pure TiO 2 (inset in Fig. 4a).It is in agreement with the pore size determined from the TEM images and the results described in the XRD, which further conrms a uniform pore size distribution.Compared to the specic surface area (105.5 m 2 g À1 ) of the TiO 2 , the specic surface area of TiO 2 @RGO is 145.8 m 2 g À1 .The larger surface area of TiO 2 @RGO is conductive to increasing contact area of electrode-electrolyte and can improve diffusion of electrolyte ions.
The electrochemical performance of the prepared pure TiO 2 and TiO 2 @RGO materials were investigated as electrode for LIBs.Fig. 5a and b shows the discharge-charge curves of the pure TiO 2 and TiO 2 @RGO at a rate of 0.5C (1C ¼ 168 mA h g À1 ) in the potential window of 1.0-2.5 V versus Li + /Li in the 1st, 2nd, 10th, 50th and 100th.The initial discharge-charge capacities of the pure TiO 2 were 570/399 mA h g À1 , and subsequently deceased to just 223 mA h g À1 aer 100 cycles.As a comparison, the TiO 2 @RGO hybrid nanocomposite obtained higher capacities with 634/388 mA h g À1 at the rst cycle.The columbic efficiency of the pure TiO 2 and TiO 2 @RGO is comparatively low in the rst cycle, which can be attributed to the existence of irreversible Li trapped sites. 26There are two obvious plateaus at 1.75 V and 1.95 V, corresponding to Li + insertion and Li + extraction between the TiO 2 nanoparticles and Li 0.5 TiO 2 , which meet the characteristics of anatase TiO 2 lithiation. 47,48anwhile, it is obvious to see that there are huge differences of plateau voltage between pure TiO 2 and TiO 2 @RGO hybrid electrode, indicating that the TiO 2 @RGO nanocomposites possessed excellent reaction kinetics and low polarization.Compared with TiO 2 @RGO electrode, the redox potential plateaus of pure TiO 2 is getting smaller and smaller until it disappeared in the end because of its bigger size and inherently bad electronic conductivity.Fig. 5c and d depicts the cycling performance of the pristine TiO 2 and TiO 2 @RGO electrode at a rate of 0.5C and 5C.Just as showed in Fig. 5d, the TiO 2 @RGO hybrid composite displays excellent reversible capacity and cycle characteristics.The results demonstrated that the TiO 2 @RGO electrode obtained high reversible capacity of 352 mA h g À1 (0.5C) and 223 mA h g À1 (5C) up to 100 cycles, which were higher than that of anode materials composed of anatase TiO 2 nanospheres/hollow structures, rutile TiO 2 nanorods/porous microspheres/nanoparticles, etc. [27][28][29][30][31][32] At the same time, the initial discharge-charge capacities of pure anatase TiO 2 were 570/399 mA h g À1 , which demonstrate that the pure phase TiO 2 has high specic capacity.The rst discharge capacity of TiO 2 is about 570 mA h g À1 , a value much higher than the theoretical capacity of TiO 2 .It can be explained that the specic surface area of pure TiO 2 is 105.5 m 2 g À1 and the specic surface area of TiO 2 @rGO is more than 145.8 m 2 g À1 .The large specic surface area plays an important role in the consumption of lithium ion at initial discharge process.At the same time, the formation of the SEI lm will consume a large amount of lithium ion.The reasons for the above statement will lead to the increase of capacity.Subsequently, discharge capacities of pure anatase TiO 2 quickly decreased to 129 mA h g À1 at 5C and about 223 mA h g À1 at 0.5C (aer 100 cycles) accompanied by capacities rose and downed which could be attributed to the activation and the structure collapsed of active materials. 33Meanwhile, on the other hand, we can see that the pure TiO 2 electrode has the potential value based on its high initial capacities.Compared with pure TiO 2 electrode, TiO 2 @RGO shown a higher rate performance and ultra long  cycle life for lithium storage, which could be attributed to its unique hollow nanostructure, relatively large surface areas (the particle size of sample is about 8.5 nm).Also, RGO nanosheets can benet to restraining grain growth and inhabit anatase TiO 2 aggregation.Fig. 6 shows the rate performance of the pure TiO 2 and TiO 2 @RGO hybrid composites at different rates.When cycled at the rate of 0.5, 1, 2 and 5C, the reversible discharge capacities of the pure TiO 2 were 263, 229, 198, and 147 mA h g À1 , respectively.Compared with pure TiO 2 electrode, the TiO 2 @RGO hybrid composites owned higher reversible capacities of 585, 509, 434, 354 mA h g À1 , respectively.Especially, there are no obvious capacity recession when the rate returned to 2, 1 and 0.5C, which demonstrated that TiO 2 @RGO hybrid composites possess excellent electrochemical properties even at high rates due to the superiority of taken experimental methods and unique features of hybrid nanostructure.The performance of TiO 2 @RGO hybrid composites were superior to other the report results. 34,35As a typical example, Lee et al. fabricated TiO 2 nanorod arrays on RGO as LIB anodes, and the capacity was only 94 mA h g À1 at 5C less than the capacity of 354 mA h g À1 at 5C in this article. 36In order to verify the characteristics of the synthesized electrode materials, the electrochemical performance of the nanohybrid was analyzed by the charge/discharge cycle at a high rate.As shown in Fig. 6b, the charge-discharge curve was well tted and the capacity remained at 217 mA h g À1 at a rate of 10C aer 2000 cycles, indicating a good capacity retention, high specic capacity, good cycle reversibility and stability of the material.
In order to further explore the synergistic effects of RGO nanosheets and the pristine TiO 2 , the CV curve was tested for the rst three cycles of the Li-ion half-cell at a voltage range of 1-2.5 V vs. Li/Li + at a scanning speed of 0.1 mV s À1 .As shown in the gure, hybrid TiO 2 @RGO spectrums have three pairs of obvious oxidation (insertion) and deoxidization (extraction) peaks, with the rst red-ox peaks of 1.49/1.54,1.57/1.68and 1.73/1.98V, respectively.The second and third cycles of red-ox peaks correspond to 1.49/1.54,1.57/1.68,1.73/1.98V.The redox peaks appearing on the rst cycle are shied relative to the second and third cycle, which is due to the initial decomposition of the electrolyte in the hybrid electrode and the formation of the mesophase of the solid electrolyte interphase (SEI). 37,38he pair of cathodic/anodic peaks at 1.73/1.98V correspond to the anatase TiO 2 electrode.The relatively weak peaks at 1.49/ 1.54 and 1.57/1.68V correspond to the red-ox peaks for Li/Li + embedded/out in the TiO 2 -B, indicating the content of Li/Li + is relatively less than that of anatase TiO 2 .It agrees with the results of XRD analysis.The CV curve of the rst cycle and the next two cycles CV spectrums suggest the rst irreversible charge-discharge capacity of hybrid electrode and lost capacity (Fig. 7).The second and third CV curves are well overlapped, which indicate that the hybrid electrode has excellent cycle performance, high reversibility and good capacity retention.
To gain insight into the remarkable rate performance of the TiO 2 @RGO composites, electrochemical impedance spectroscopy (EIS) was performed on the cells consisting of pure TiO 2 and TiO 2 @RGO composites as the working electrode vs. Li without discharge and charge cycles before cycles.Fig. 8, the Nyquist plots display an inclined line in the low-frequency range and only one semicircle in the high-frequency range.The inclining line and high frequency oblate semicircle correspond to the Li + diffusion process and charge-transfer impedance in the electrode/electrolyte interface, respectively. 39ompared with pure TiO 2 (132 U), TiO 2 @RGO nanostructure has a smaller resistance value of 69.2 U that can effectively improve the electron transport.
The enhanced lithium storage performance could be assigned to the following reasons.On the one hand, the fabrication of the anatase TiO 2 itself has a unique hollow crystal nanostructure that facilitate the Li ion diffusion owing to the   open channels and the shortened transport pathways, a small grain size (less than 10 nm) and a large surface that which benets to the contact area between the electrode and the electrolyte.On the other hand, the functional active groups on the surface of RGO nanosheets, such as -OH, -COOH, are conductive to adhesion of the hollow anatase TiO 2 which can reduce the aggregation of TiO 2 and make the TiO 2 obtain a good dispersibility.At the same time, it benets the combination of RGO nanosheets and the hollow anatase TiO 2 that can achieve a more stable hybrid nanostructure composite.The aggregation of TiO 2 and stacking of GO can be effectively prevented, leading to good cycling performance.1][42][43] The synergistic effect of nanostructured TiO 2 and RGO is a main factor for the superior rate capability and excellent cycle ability of the TiO 2 @RGO nanohybrid.

Conclusions
We have developed a facile in situ hydrothermal synthesis approach to construct a hollow TiO 2 @RGO nanohybrid composite.The combination of the TiO 2 has an open crystal structure, a small particle size and a large surface.Compared with the pristine TiO 2 and other TiO 2 @RGO nanocomposites prepared through hydrothermal/solvothermal approaches, the as-prepared TiO 2 @RGO nanocomposite shown signicantly enhanced lithium storage performance, much higher reversible capacity, and better cycling stability.More importantly, the asprepared TiO 2 @RGO nanocomposite still showed a higher reversible capacity for 220 mA h g À1 at 5C current aer 100 cycles.The simple and steerable synthesis, excellent electrochemical performance, and the hollow sphere structure could also provide a new idea for developing higherperformance lithium storage electrodes and other energy storage applications.

Fig. 2
Fig. 2 SEM and TEM images of bare GO.

Fig. 4
Fig. 4 Nitrogen isotherm adsorption-desorption curves of (a) the pure TiO 2 and (b) TiO 2 @RGO.The inset in (a) and (b) show pore-size distributions of the pure TiO 2 and TiO 2 @RGO.

Fig. 6
Fig.6(a) Rate performance of TiO 2 @RGO nanocomposite and pure TiO 2 at various current densities between 0.5C and 5C.(b) Durability test at high 5C and 10C currents of the TiO 2 @RGO at 5C and 10C.

Fig. 8
Fig.8Nyquist plots for the EIS of TiO 2 @RGO and pure TiO 2 electrodes before cycling.