One-step synthesis of Nb-doped TiO₂ rod@Nb₂O₅ nanosheet core–shell heterostructures for stable high-performance lithium-ion batteries

Abstract

One of the main drawbacks with applying the most promising electrode materials in lithium-ion batteries is their intrinsic poor electronic conductivity. Many approaches which include mixing them with good electrical conductors have been developed to solve this problem, yet they do not improve the lattice electronic conductivity within the crystal. Here we present a one-step hydrothermal route for fabricating the core–shell heterostructures through sequential growth of the core and shell materials. The cores are Nb-doped TiO₂ rods with improved conductivity that can deliver high capacity, yet the Nb₂O₅ nanosheets shells can act as a protective layer to prevent electrode dissolution and maintain the electrode integrity over long term cycles and contribute additional lithium storage capacity. When employed as an anode material in lithium-ion batteries, it exhibits an outstanding cycle stability of 189 mA h g⁻¹ at 170 mA g⁻¹ for more than 700 cycles. This work provides an important strategy to improve the conductivity of the host and construct a protective layer by simply adding a guest ion in reaction.

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1. Introduction

As a low cost, environmentally friendly material, TiO₂ has been extensively investigated because of its wide potential applications in photocatalysis, photovoltaics, and energy storage.^1–3 The safety issue is one of major technological barriers that hinder the large-scale applications of lithium-ion batteries (LIBs) in electric vehicles and renewable energy storage. The cause has been widely attributed to the decomposition of the solid electrolyte interphase (SEI) film and subsequent continuously exothermal reaction between the lithiated graphite and electrolyte for commercial LIBs using graphite as the anode.⁴ In this regard, TiO₂-related materials have been considered as a class of potential alternatives to graphite due to their safe lithiation potential (1.5–1.8 V vs. Li/Li⁺), negligible volume change during Li⁺ ion insertion/extraction (<4%) and relatively high theoretical capacity of 335 mA h g⁻¹.⁵ One-dimensional TiO₂ nanostructures such as nanowires, nanorod, and tubular structures have always been preferred architectures since they can provide direct electron transport pathways.⁶ However, the intrinsic poor electronic conductivity of TiO₂ material itself leads to the poor rate capability and great degradation of capacity upon prolonged cycling, which severely limits its practical application.⁷ One possible way to mitigate these problems is to surround TiO₂ materials with good electronic conductors such as graphene,⁸ carbon nanotube,⁹ Ag,¹⁰ and conducting polymers.¹¹ However, the obtained electrochemical performance is still far from what is expected since these approaches do not improve the lattice electronic conductivity within crystal. Furthermore, most of these additional conductive additives such as Ag and conducting polymers do not contribute to the capacity of LIBs and require additional processing.¹² Previous studies show that Nb-doped anatase TiO₂ is an indium-free transparent conductive oxide (TCO) alternatives with metallic property.^13,14 Recently, Nb-doped anatase TiO₂ has also attracted great scientific interest as a potential anode material for LIBs due to their improved conductivity.^15,16 Furthermore, an emerging concept of core–shell heterostructures is also attracting increasing interest due to their unique properties and wide applications in p–n junction photovoltaic devices,¹⁷ photocatalysis,¹⁸ and energy storage.¹⁹ Considering their applications in LIBs, coating a protecting layer on the active material surface will effectively reduce the direct contact between electrodes to prevent electrode dissolution and maintain the electrode integrity, thus improving the cyclic stability.²⁰ However, most of them are fabricated by the complicated multistep fabrication procedures and difficult to realize due to the lattice mismatch and lack of appropriate synthesis methods.^18,19 Therefore, it is important to develop an novel strategy to improve the conductivity of host and construct a protective layer by simply adding a guest ion in reaction.

Here, we report one-step hydrothermal route to synthesize the core–shell heterostructures through sequential growth of Nb-doped TiO₂ rod core and Nb₂O₅ nanosheet shell. The Nb-doped TiO₂ core can greatly improve the intrinsic conductivity of TiO₂, which would favour the charge transfer and electrochemical performance. While the Nb₂O₅ shell can act as a protective layer to accommodate the volume change of the electrode and maintain the electrode integrity during cycles. Different from the metal coating, the Nb₂O₅ material can also contribute additional lithium storage capacity, which has also been extensively investigated as a promising anode material for high-power LIBs because of its safe operating voltage window in the range of 1.0–3.0 V (vs. Li/Li⁺) and high theoretical capacity (200 mA h g⁻¹).^21–24 As a comparison, we also fabricated the bare TiO₂ rods without addition of Nb precursor in the reaction. When tested as anode materials for LIBs, Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures electrode exhibited a high reversible capacity and long cycling stability with a discharge capacities of 189 mA h g⁻¹ at 170 mA g⁻¹ over 700 cycles, which is much better than that of bare TiO₂ rod electrode, showing a strong synergistic effects from Nb-doped TiO₂ core and Nb₂O₅ shell.

2. Experimental section

2.1 Synthesis of hierarchically Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures

To synthesize hierarchically mesoporous TiO₂@Nb₂O₅ core–shell heterostructures, 0.3 g hexamethylenetetramine (A.R., Kermel) was dissolved in 60 mL of ammonia solution under vigorous stirring for 10 min. Then, 0.3 g titanium(IV) oxysulfate–sulfuric acid hydrate (93%, Aladdin) and 0.1 g NbCl₅ (A.R. Aladdin) was added to this solution sequentially and stirred for 30 min. The homogeneously mixed solution was then fed into a 100 mL Teflon-lined autoclave and hydrothermally heated at 150 °C for 12 h. Effects of hydrothermal reaction time on the morphologies were also studied at 150 °C for 1–9 h. The synthetic procedure of bare TiO₂ rod was similar to that of Nb-doped TiO₂@Nb₂O₅ heterostructures except without the addition of NbCl₅ precursor. Then, the autoclaves were cooled to room temperature. The obtained samples were filtered, washed with ethanol and deionized water for several times, and dried at 60 °C for 24 h to remove absorbed water. Finally, the obtained Nb-doped TiO₂@Nb₂O₅ and bare TiO₂ rod samples were calcined at 580 °C for 1 h before they were investigated as anode material in lithium-ion batteries (LIBs).

2.2 Characterization

The surface morphologies of samples were examined by using a thermal field emission environment scanning electron microscope (FESEM, Quanta 400). X-ray diffraction (XRD) measurements were done using Rigaku instrument (D/Max-IIIA, 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) were recorded to further investigate the morphology and crystal structure with a high resolution transmission electron microscope (FEI Tecnai G2 F20) operated at 200 kV. Four-point conductivity measurements were carried out using the van der Pauw method on discs of ∼15 mm diameter lapped to ∼80 μm thickness using a CRESBOS resistivity measuring instrument (NAPSON, Japan).

2.3 The electrochemical measurements

The electrochemical measurements were carried out using standard CR2032-type coin cells with pure lithium metal serving as both the counter and reference electrodes at room temperature. The slurry was fabricated by mixing 80% active material (e.g., bare TiO₂ rods, annealed Nb-doped TiO₂@Nb₂O₅ heterostructures via hydrothermal reaction for 12 h), 10% conductive carbon black (super-p-Li), and 10% polyvinylidene difluoride (PVDF, Aldrich) using N-methyl-2-pyrrolidone (NMP) as the solvent and then pasted on pure Cu foil to prepare the working electrodes. The electrolyte was a solution of 1.0 M LiPF₆ dissolved in a mixture of ethylene carbonate-diethyl carbonate (1 : 1 by weight). A celgard 3400 microporous polymer membrane was used as a separator. The cells was assembled in an argon-filled glove box (Mikrouna) with moisture and oxygen concentrations below 0.1 ppm. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on a CHI760C electrochemical workstation. CV tests were carried out at a scanning rate of 0.1 mV s⁻¹ over the potential range from 3.0 to 1.0 V. EIS were recorded by applying a sine wave with amplitude of 5.0 mV over the frequency range from 100 kHz to 0.01 Hz. The galvanostatic charge/discharge cycling measurements were conducted with a NEWARE battery testing system with a voltage window of 1–3.0 V at different current densities.

3. Results and discussion

Fig. 1a–c show the scanning electron microscopy (SEM) images of post-annealed Nb-doped TiO₂ rod@Nb₂O₅ nanosheet core–shell heterostructures after being annealed at 580 °C for 1 h. As shown in Fig. 1a, large scale Nb-doped TiO₂@Nb₂O₅ heterostructures with diameters of 1–2 μm and length up to 150 μm were successfully synthesized using one-step hydrothermal route. It can be further confirmed by Fig. 1b and c and S1† that the inner core is mesoporous Nb-doped TiO₂ rods, whereas the shell consists of Nb₂O₅ nanosheets with thickness of about 10 nm.

Fig. 1 FESEM images of post-annealed Nb-doped TiO₂@Nb₂O₅ samples; (a) low-magnification SEM images; (b) cross-sectional FESEM image and (c) side view FESEM image of single Nb-doped TiO₂@Nb₂O₅ rod.

To further understand the growth mechanism, we carried out other two experiments. Firstly, when there was no addition of Nb precursor in the reaction keeping other conditions unchanged, only bare TiO₂ rods were obtained (Fig. S2†).

Secondly, we also examined the effect of reaction time on morphology evolution and dopant concentration. As shown in Fig. 2, the morphologies of products collected at an early stage were the rod shape with smooth surface despite undergoing 6 h of reaction. As the reaction proceeded to 9 h, the TiO₂ rod began to couple with Nb₂O₅ nanosheets and then the core–shell heterostructures formed. It can also be seen from time-dependent EDX measurements that Nb content is 0.67 atom%, 2.36 atom% and 8.49 atom% for samples obtained at reaction time of 1 h, 6 h and 12 h, respectively (Fig. S3†). This result demonstrates that Nb doping content in TiO₂ rod increases with the increased reaction time for the samples obtained before 6 h, and the extra amount of Nb elements in sample obtained at 12 h reaction time is ascribed to the Nb₂O₅ nanosheet anchoring on TiO₂ rods, which is also consistent with above SEM observations. Therefore, we conclude that these well-defined core–shell heterostructures are obtained by one-step hydrothermal process through sequential growth of Nb-doped TiO₂ rod core and then Nb₂O₅ nanosheet shell. Fig. 3 shows the X-ray diffraction (XRD) patterns of post-annealed samples corresponding to bare TiO₂ rod shown in Fig. S2† and Nb-doped TiO₂@Nb₂O₅ heterostructures in Fig. 2, respectively. It can be seen from the Fig. 3a that the bare TiO₂ rod sample shows a pure anatase phase, whereas the coexistence of the anatase TiO₂ (JCPDF no. 71-1168) and T-Nb₂O₅ (Nb_16.8O₄₂, JCPDF no. 71-0336) phase occurs in all Nb-doped TiO₂@Nb₂O₅ heterostructures samples collected at different hydrothermal reaction time (1, 3, 6, 9 and 12 h). In addition, Fig. 3b shows that the position of the (101) peak shifts to smaller 2θ angle before 6 h reaction time. This can be explained that the substitution of Nb⁵⁺ ion with the larger radius (0.064 nm) for Ti⁴⁺ (0.061 nm) ion in TiO₂ rods leads to the increase of the d-spacing according to the Bragg equation 2d sin θ = kλ.^14,25,26 Besides, the peak positions of samples is relatively stable before 6 h, which is consistent with the morphology evolution in Fig. 2a–f. In this stage, Nb impurity atoms were introduced to TiO₂ crystal structure, but the morphologies of products were still the rod shape without formation of Nb₂O₅ nanosheet coating. When reaction time is increased from 9 h to 12 h, Nb₂O₅ nanosheets have grown on the Nb-doped TiO₂ core to form core–shell heterostructures (see Fig. 2g–j). The lattice mismatch between TiO₂ core surface and Nb₂O₅ nanosheets would introduce much more disorder that makes XRD peaks broader and even splitting at 12 h reaction time. With regard to the splitting peaks at 12 h, one peak appears almost at the same position to the Nb-doped TiO₂@Nb₂O₅ samples collected at reaction time of 1–6 h which can be ascribed to the doping of Nb in TiO₂ core. Another peak located at smaller 2θ angle may be assigned to disorder caused by the lattice mismatch between TiO₂ core and Nb₂O₅ shell. Fig. S4† shows XPS spectra for the TiO₂@Nb₂O₅ heterostructures and bare TiO₂ rods. As shown in Fig. S4a,† the binding energy of Ti 2p_3/2 for bare TiO₂ and TiO₂@Nb₂O₅ heterostructures are 458.7 and 458.85 eV, respectively, which agree well with the values for Ti⁴⁺ (459.3 eV).^14,26,27 Besides, the high-resolution Ti 2p spectra of TiO₂@Nb₂O₅ heterostructures exhibited a ca. 0.15 eV shift to higher binding energy compared with bare TiO₂ rods, which indicated a modified local electronic structure of Ti cations, likely due to the lattice mismatch binding between TiO₂ microrods and Nb₂O₅ nanosheets. As can be seen in Fig. S4b,† the peak positions of Nb 3d are located at the binding energies of 207.2 and 209.9 eV for the TiO₂@Nb₂O₅ heterostructures, which are consistent with the binding energy values for Nb 3d_5/2 (207.28 eV) and Nb 3d_3/2 (210.08 eV) for Nb oxides, indicating only peaks associated with Nb⁵⁺ appear in TiO₂@Nb₂O₅ heterostructuress.²³ In order to determine the local elemental composition and distribution of the Nb-doped TiO₂@Nb₂O₅ heterostructures, the element mapping and EDX spectrum were carried out. As shown in Fig. S5,† the Ti, Nb, and O elements are uniformly distributed in the whole Nb-doped TiO₂@Nb₂O₅ heterostructures, which corresponds to the observation of SEM images (Fig. 1).

Fig. 2 FESEM images showing the effect of growth time on morphologies of TiO₂@Nb₂O₅ products. The as-synthesized samples were obtained by hydrothermal treatment at 150 °C for (a) and (b) 1 h, (c) and (d) 3 h, (e) and (f) 6 h, (g) and (h) 9 h, (i) and (j) 12 h, respectively.

Fig. 3 XRD patterns of post-annealed bare TiO₂ rods and Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures collected at different hydrothermal reaction time after calcination at 580 °C for 1 h. Notes: small peaks marked with asterisk (*) represent Nb₂O₅ phase.

Fig. 4a and b clearly show that Nb-doped TiO₂ rod core is highly porous, composed of nanocrystals with 10–15 nm in size, which is strongly coupled with ultrathin Nb₂O₅ nanosheets. High resolution TEM images (Fig. 4c) and selective area electron diffraction (SAED) patterns (Fig. 4d) depict that the core–shell heterostructures have both anatase phase and T-Nb₂O₅ phase, which is consistent with XRD results. As shown from high resolution TEM images (Fig. 4c) and corresponded ball-and-stick model (Fig. 4f), each adjacent {3,8,0}_Nb2O5 planes inclined with an angle of 30° to the normal of interface, that is d′ = d_{3,8,0}/cos(30°) = 0.353 nm, coincides regularly with a lattice distance given by each consecutive {1,0,1}_TiO2 spacing, that is d₍₁₀₁₎ = 0.354 nm. The interfacial lattice mismatch can be calculated as 0.3% [(0.354 − 0.353)/0.353 = 0.3%]. According to the calculated angle between {3,8,0}_Nb2O5 and {1,8,0}_Nb2O5, a straightforward relationship of planar epitaxy should exist in the direction orthogonal to the bonding interface, along which a lattice distance d = 0.354 nm of the {1,0,1}_TiO2 planes equals to a sum of the two consecutive {3,8,0}_Nb2O5 planes (d = 0.179 nm) with a 1.12% misfit. The observed lattice distortions in the connecting region between Nb-doped TiO₂ core and Nb₂O₅ shell is consistent with the peak splitting in XRD spectra of Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures collected at 12 h. In addition, the above observation can be supported by the diffractogram in Fig. 4d, it can be seen that the diffraction pot of {3,8,0}_Nb2O5 plane and {1,0,1}_TiO2 plane are almost in a line.

Fig. 4 (a and b) TEM, (c) HRTEM image and (d) SAED pattern of post-annealed Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures; (e) schematic and (f) structural model of Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures.

Fig. 5 shows the CV curves of the Nb-doped TiO₂@Nb₂O₅ electrode. The strong cathodic and anodic peaks were observed at 1.69 V and 2.05 V, which are ascribed to the lithium insertion/desertion processes in the anatase framework, respectively.²⁸ In addition, the existence of other minor cathodic peaks at 1.32 V and anodic peaks at 1.63 V are connected to the contribution of small amount of Nb₂O₅ in electrodes.^21,29

Fig. 5 Cyclic voltammograms of Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures electrode.

Fig. 6a and b show charge–discharge voltage profiles of the Nb-doped TiO₂@Nb₂O₅ electrode and bare TiO₂ electrode at a current rate of 0.5C (1C = 335 mA g⁻¹), respectively. Obviously, the Nb-doped TiO₂@Nb₂O₅ electrode exhibits much higher reversible capability than the bare TiO₂ electrode. It can be seen from Fig. 6a that the voltage plateaus around 1.95 and 1.73 V were observed for Nb-doped TiO₂@Nb₂O₅ electrode, which is consistent with CV observations. In addition, this unique electrode exhibits initial discharge capacity of 225 mA h g⁻¹ and subsequent charge capacity of 202 mA h g⁻¹, respectively, which indicate the high coulombic efficiency of 89.6%. After the second cycle, such electrode can also preserve relatively high electrochemical lithium storage performance and excellent cycling stability. For example, it can still deliver discharge/charge capacities of 229/203 mA h g⁻¹, 217/198 mA h g⁻¹, 220/205 mA h g⁻¹, 211/198 mA h g⁻¹, 212/199 mA h g⁻¹ in the 2^nd, 10^th, 30^th, 50^th, 100^th cycle, respectively. The cycle performance for the Nb-doped TiO₂@Nb₂O₅ and bare TiO₂ rod electrodes is given in Fig. 6c. Apparently, the Nb-doped TiO₂@Nb₂O₅ electrode demonstrated the higher discharge capacity and better cycling stability than the bare TiO₂ rod electrode. Even after 700 cycles, this Nb-doped TiO₂@Nb₂O₅ electrode could still deliver a reversible capacity of 189 mA h g⁻¹, maintaining 84% of the initial discharge capacity, which is much higher that of the bare TiO₂ nanorod (148 mA h g⁻¹). It can also be seen from Fig. S6† that aggregated Nb₂O₅ nanosheet clusters were obtained without the addition of Ti precursor while keeping other synthetic conditions unchanged. When used as anodes, Nb₂O₅ materials can deliver its initial capacity up to 208 mA h g⁻¹ in the first discharge cycle, but it demonstrates relatively poor cycling stability due to the dense aggregation of nanosheets which greatly prohibit the electrolyte from penetrating into them. By anchoring Nb₂O₅ nanosheets on Nb-doped TiO₂ rods to form heterostructures, the TiO₂ rod can both provide a support for anchoring Nb₂O₅ nanosheets and hinder them agglomeration, and thus keeping their large active contact area between the electrode and electrolyte. Therefore, the Nb₂O₅ nanosheets can contribute to the additional lithium storage capacity, and the Nb-doped TiO₂ rod can both improve electronic conductivity in electrode and maintain the cycling stability, resulting in a synergistic effect for the enhanced electrochemical performance. Fig. 6d compares the rate performance of Nb-doped TiO₂@Nb₂O₅ and bare TiO₂ rods at the current rates of 0.25–10C. Obviously, Nb-doped TiO₂@Nb₂O₅ electrode demonstrates much better rate capability than the bare TiO₂ rod electrode does. The Nb-doped TiO₂@Nb₂O₅ electrode could deliver high discharge capacity of 243, 219, 161, 145 and 113 mA h g⁻¹ when the current density changed stepwise from 0.25 to 0.5, 2.5, 5 and 10C, respectively. More importantly, when the current rate was reduced back to 0.25C from a high current rate of 10C, a capacity of 242 mA h g⁻¹ can still be delivered, demonstrating the great potential as the high rate anode materials. The discharge capacity retention, cycling stability and rate capability for Nb-doped TiO₂@Nb₂O₅ electrodes reported here are superior to those of most of the previously reported one-dimensional anatase TiO₂ nanostructures³⁰ and even those mixing with Ag, Au, carbon nanotube, and graphene conductive composite^8–10 as well as Nb₂O₅ materials²⁹ obtained while being tested under similar conditions.

Fig. 6 Galvanostatic charge–discharge curves of LIBs made of (a) Nb-doped TiO₂@Nb₂O₅ heterostructures and (b) bare TiO₂ rods cycled at 0.5C; (c) the cycling performance and (d) rate capability of Nb-doped TiO₂@Nb₂O₅ and bare TiO₂ electrodes.

As discussed in previous studies,^13,14 the Nb-doped TiO₂ core would lead to a sharp increase in electrical conductivity of whole TiO₂@Nb₂O₅ coaxial cable electrode, and this was proven by electrochemical impedance spectra and four-point conductivity measurements. The electrochemical impedance spectroscopy (EIS) analysis was then carried out to elucidate the kinetic process of electron transfer and lithium ion diffusion of the electrodes. Fig. 7a illustrates the Nyquist plots of LIBs made of Nb-doped TiO₂@Nb₂O₅ composite and bare TiO₂ electrode after 100 cycles at a current rate of 1 A g⁻¹. In accordance with the previous studies,^31–33 the semicircle diameter of Nb-doped TiO₂@Nb₂O₅ composite electrode is much smaller than that of bare TiO₂ nanorod electrodes before and after cycling, demonstrating Nb-doped TiO₂@Nb₂O₅ composite possessed the good electron conductivity during cycles. An equivalent circuit was used to fit the obtained Nyquist plots to investigate their kinetic parameters in detail, as shown in Fig. 7c, and the resultant values are enlisted in Table S1.† In this model, R_e is associated with the combined resistance of electrolyte and cell components. R_(sf+ct) represents the surface film (sf) and charge transfer resistance (ct), and CPE_(sf+dl) depicted the surface film and double layer (dl) capacitance. R_b and CPE_b refer to the bulk (b) resistance and bulk capacitance. Furthermore, W_s and C_i described the Warburg impedance and intercalation capacitance, respectively. According to the fitted values shown in Table S1,† R_(sf+ct) of the fresh and cycled Nb-doped TiO₂@Nb₂O₅ electrodes are 8.2 Ω and 4.1 Ω, respectively, which are much smaller than those of the bare TiO₂ nanorod electrodes (21.7 Ω and 14.3 Ω). The reduced R_(sf+ct) of Nb-doped TiO₂@Nb₂O₅ electrode can be attributed to the enhanced electronic conductivity originated from Nb doping, which might be the key factors for the improved electrochemical performance of TiO₂@Nb₂O₅ electrode during the cycle processes.^8,31 The improved electrical conductivity of the TiO₂@Nb₂O₅ electrode can be further verified by using four-point conductivity measurements, as shown Fig. 7d. It can be seen that the lattice electronic conductivity of the Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures is more than 5000 times higher than that of the bare TiO₂ rods, reaching values of 10⁻⁷ S cm⁻¹ at room temperature. Moreover, it is also noted from Fig. 7a and b and Table S1† that the diameter of these semicircles and R_(sf+ct) in both Nb-doped TiO₂@Nb₂O₅ and bare TiO₂ electrodes become smaller after 100 cycles, which might be due to the enhanced wetting between electrolyte and electrode after cycling promote the charge transfer.^34,35

Fig. 7 Nyquist plots of Nb-doped TiO₂@Nb₂O₅ electrode (a) after 100 galvanostatic discharging/charging cycles and (b) before cycling; (c) equivalent circuit used to fit the experimental data; (d) the lattice electronic conductivity measured by four-point van der Pauw methods using 20 Nb-doped TiO₂@Nb₂O₅ and 20 bare TiO₂ samples, respectively.

Fig. S7† shows the nitrogen adsorption–desorption isotherms (77 K) and its corresponding pore size distribution curves of the calcined Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures. The Barrett–Joyner–Halenda (BJH) pore size distribution shown in the inset demonstrates a narrow pore size distribution centered at 25.0 nm, suggesting the mesoporous characteristics of sample. The specific surface area of the Nb-doped TiO₂@Nb₂O₅ sample, as calculated by the Brunauer–Emmett–Teller (BET) method, is only 26 m² g⁻¹, which is much lower than that of the previously reported calcined TiO₂ samples (122 m² g⁻¹).³⁶ Therefore, the enhanced electrochemical performance is mainly due to a synergistic effect from Nb-doped TiO₂ core and Nb₂O₅ shell.

4. Conclusions

In summary, we have demonstrated a conceptually novel Nb-doped TiO₂@Nb₂O₅ core–shell heterostructures, which can be obtained by one-step hydrothermal synthesis through sequential growth of core and shell structures. This strategy is very simple yet interesting compared to previous multi-step fabrication procedures for core–shell structures. The Nb-doped TiO₂@Nb₂O₅ electrode exhibits enhanced electrical conductivity, high reversible capacity, long cycle life and excellent rate capability, which are attributed as follows: firstly, the core material is Nb-doped TiO₂ rod, which improves conductivity and favours Li ion insertion and extraction. Secondly, the Nb₂O₅ nanosheets shell not only contributes additional lithium storage capacity but also shortens the diffusion paths of Li⁺ ions. Thirdly, the voids surrounded by the nanosheets can accommodate the volume change, and thus effectively mitigates the stress and protects TiO₂ rods from pulverization during the discharge/charge process. We believe the present work not only enriches the fabrication method of core–shell heterostructures materials but also expands their more applications in catalysis, sensors, photovoltaics, and so on.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51272294), National Basic Research Program of China (No. 2012CB933704), and Program for Changjiang Scholars Innovative Research Team in University (No. IRT13042) and Guangzhou collaborative innovation MajorProject of producing, teaching and researching (No. 201508010011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra28065f

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