High-performance lithium storage of Ti³⁺-doped anatase TiO₂@C composite spheres

Abstract

Ti³⁺-Doped anatase TiO₂@C composite spheres (TD-TiO₂@C) are synthesized for the first time by the solvothermal method, followed by calcination using as-prepared anatase/TiO₂-B hybrid TiO₂ spheres and glycerol solution in alcohol as reactants. TD-TiO₂@C spheres exhibit excellent electrochemical performance as anode materials for lithium ion batteries (LIBs). The electrodes not only exhibit a superior capacity of 244.80 mA h g⁻¹ at 1C (0.168 A g⁻¹) after 100 cycles, but also show an eminent rate capability. TD-TiO₂@C spheres retain discharge capacities of 171.78, 143.45, 119.17, 105.82 mA h g⁻¹ after 500 cycles at 5C, 10C, 20C, 30C, holding a capacity retention as high as 91.0%, 84.2%, 82.1%, 82.0% when compared with discharge capacities at the 10th cycle, respectively. The improved electrochemical properties are mainly due to the synergistic effects of carbon coating and Ti³⁺ doping, which can enhance significantly the inherent electronic conductivity of TiO₂. Therefore, TD-TiO₂@C can be an attractive candidate for anode materials in LIBs, also with great promise.

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1. Introduction

Lithium-ion batteries with superior electrochemical properties (e.g., large energy density, good cycle performance and eco-friendliness) compared to conventional batteries, have attracted great attention and have been considered to have great value in theoretical studies and commercial applications.^1,2 Although LIBs are widely used for cell phones, notebook computers, electronic electric vehicles and even hybrid electric vehicles, their anode materials’ electrochemical properties and safety need to be improved further.^2,3 So, LIBs have continuously attracted lots of research in the design of satisfactory electrodes to meet the requirements of high energy/power density, long lifespan and safety.

TiO₂ with different polymorphs (rutile, anatase, brookite, etc.) has been studied consumingly in recent years for its outstanding properties like minimal toxicity, low cost, nonpolluting nature, high discharge potential and most importantly, inherent chemical stability, which is derived from TiO₂’s negligible volume expansion during the Li-ion insertion/extraction processes.^4–7 Nevertheless, the widespread practical application of TiO₂ as anodes in LIBs is seriously impeded by its inferior rate performance resulting from both extremely poor electronic conductivity (10⁻¹³ S cm⁻¹) from the huge band gap (3.0 eV for anatase and 3.2 eV for rutile) and sluggish lithium ion diffusion (10⁻¹⁰ to 10⁻¹⁷ cm² s⁻¹).^8–11 Many researches have been devoted to overcome these intrinsic drawbacks.

In recent years, more efforts have been devoted to enhance significantly the inherent electronic conductivity through metal or non-metal doping, such as B,¹² N,¹³ Zn¹⁴ and Sn¹⁵ doping, which can offer more open channels in a particular direction for Li⁺ diffusion owing to the slight modification in the TiO₂ lattice. Recently, it has been found that introducing Ti³⁺ or oxygen vacancies into TiO₂, usually displaying a black or dark color, can markedly narrow the band gap to about 1.54 eV and thus enhance the inherent electronic conductivity significantly, which has drawn much interest, especially in photocatalysis.^16–18 Since S. Myung et al. reported the ultra high cycling rates of black anatase titania via a solution evaporation process, the application of Ti³⁺-doped TiO₂ for lithium storage has been studied very recently.¹⁹ It was revealed that the presence of Ti³⁺ narrowed the inherent high band gap energy to a semiconductor level, which was 1.8 eV, resulting in a very high electrical conductivity of 8 × 10⁻² S cm⁻¹.¹⁹ However, in order to produce Ti³⁺ or oxygen deficiency into bulk TiO₂, harsh conditions, tedious experimental steps, and hazardous operation are usually required, which limits the practical utilization. In most reports, heat-treating TiO₂ for several hours in a reducing atmosphere, such as H₂, Ar/H₂ or CO, was usually used to prepare Ti³⁺ doped TiO₂.^{17,18,20–23} Although this method is effective, the need for expensive reducing atmospheres limits the large scale production of black TiO₂. Other novel solution processes were also successfully carried out to prepare black Ti³⁺ doped TiO₂. For example, S. Myung et al.,¹⁹ and Y. Ren et al.²⁴ reported a solution evaporation process or a solvothermal process to prepare black anatase TiO₂ using an amount of HF solution and TiCl₄ solution as reactants. However, the requirements of corrosive HF and noxious TiCl₄ are unsafe.

As we know, interconnecting electrode materials with electronic conductive additives (such as carbon) is a highly efficient way to enhance the electron transport of a material. Combining with CNTs,²⁵ carbon hollow spheres,²⁶ porous carbons,²⁷ etc. to form TiO₂/C composites is an effective method to improve the electrochemical properties and has been extensively investigated. Nevertheless, Ti³⁺ doping combined with carbon coating to improve the electrochemical properties of TiO₂ hasn’t yet been reported.

In this work, Ti³⁺-doped anatase TiO₂@C composite spheres (TD-TiO₂@C) have been firstly synthesized by the solvothermal method followed by calcination using as-prepared anatase/TiO₂-B hybrid TiO₂ spheres and glycerol solution in alcohol as reactants. Comparing with the conventional doping method, this special method not only introduces Ti³⁺ into the bulk TiO₂, but also produces a uniform carbon network. It decreases the band gap and enhances the inherent electronic conductivity of anatase TiO₂ significantly. Here, the TD-TiO₂@C spheres show excellent electrochemical performance as anode materials for LIBs.

2. Experimental

2.1. Synthesis of anatase–TiO₂-B spheres

The anatase–TiO₂-B spheres were prepared via a solvothermal process. This method is similar to our group’s and Yan’s work.²⁸ The final products were white anatase/TiO₂-B hybrid TiO₂ (named as W-TiO₂).

2.2. Synthesis of TD-TiO₂@C spheres

1 g anatase–TiO₂-B precursor was added into a mixture of 15 mL glycerol and 45 mL alcohol with stirring for 2 h. Afterwards, the mixture was transferred into a Teflon-lined stainless steel autoclave (80 mL), which was subsequently heated at 180 °C for 24 h. The precipitate was rinsed with distilled water and alcohol, then dried in air at 60 °C, and a creamy-white powder was obtained. The creamy-white precursor was annealed in a tubular furnace at 470 °C for 4 h in Ar to obtain black Ti³⁺-doped anatase TiO₂@C composite spheres (TD-TiO₂@C).

2.3. Structure and morphology characterization

X-Ray photoelectron spectroscopy (XPS ESCALAB 250 Xi Thermo Fisher Scientific, America) was performed to examine the composition of the surface materials. The structures of the as-synthesized samples were characterized by X-ray diffraction (XRD). XRD data were obtained by using a Rigaku D/MAX-2500 powder diffractometer with graphite monochromatic Cu Kα radiation (λ = 0.15418 nm). It operated at a scan rate of 5° min⁻¹ in the 2θ range of 5–90°. Scanning electron microscope (SEM) images of the samples were collected using a JEOL JSM-6610 scanning electron microscope. Besides, high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction were carried out using a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV. Raman spectra were conducted on a Horiba–Jobin–Yvon–Labor Raman HR-800 spectrometer with an argon ion laser of 523 nm. The specific surface areas (S_BET) of the powders were evaluated following the multipoint Brunauer–Emmett–Teller (BET) equation and Barrett–Joyner–Halenda (BJH) method. The electronic conductivity of the powders was evaluated with an RTS-8 four point probe tester.

2.4. Electrochemical characterization

The W-TiO₂ and TD-TiO₂@C working electrodes for lithium cells were fabricated from the as-synthesized samples, carbon black, and polyvinylidene fluoride (PVDF) binder with a weight ratio of 80 : 10 : 10 in N-methyl pyrrolidinone, which were then pasted onto copper foil followed by drying under vacuum at 110 °C for 12 h. The average mass loading of the active material was about 1.6 mg cm⁻². The testing cells were assembled with the working electrode thus fabricated, metallic lithium anode, Celgard 2300 film separator and LiPF₆, (at a concentration of 1 M) in ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte (v/v = 1 : 1). The testing cells were assembled in an argon-filled glove box, where the water and oxygen concentrations were kept less than 1 ppm.

All the cells were allowed to age for one night before testing. The charge–discharge cycle tests of cells (168 mA g⁻¹, assumed to be at a 1C rate) were run at different current densities between 1.4 and 2.4 V. Cyclic voltammetry (CV) tests and EIS experiments were performed on a Zahner Zennium electrochemical workstation. CV tests were carried out at various cycles with a scan rate of 0.1 mV s⁻¹ with the potential interval 1.4–2.4 V (vs. Li⁺/Li). The ac perturbation signal was ±5 mV and the frequency range was from 10 mHz to 100 kHz. All the tests were performed at room temperature.

3. Results and discussion

3.1. Characterization of TD-TiO₂@C

The crystal structures of W-TiO₂ and TD-TiO₂@C were analyzed through XRD as shown in Fig. 1. The visual photographs of these samples are shown in the inset, where the TD-TiO₂@C showed a dark black color. Most of diffraction peaks of W-TiO₂ can be indexed well, based on an anatase phase corresponding to JCPDS no. 21-1272. Besides, a minor peak at 29.34° is obviously observed, which can be identified as the monoclinic TiO₂(B) phase (JCPDS no. 35-0088). So, W-TiO₂ can be indicated as anatase/TiO₂-B hybrid TiO₂. The hydrothermal treatment of anatase TiO₂ in NaOH solution would facilitate its phase transformation to TiO₂-B.^28–30 Based on the nitrogen adsorption–desorption isotherms (Fig. S1†), the anatase/TiO₂-B hybrid TiO₂ (W-TiO₂) shows a much larger BET surface area than anatase TiO₂ prepared without NaOH solution treatment, which were 130.7321 m² g⁻¹ and 94.7824 m² g⁻¹, respectively. Besides, it has also been proved that anatase/TiO₂-B hybrid TiO₂ shows a much better chemical reactivity than pure anatase TiO₂,³⁰ which maybe is beneficial for the formation of Ti³⁺ during the following processes. After undergoing solvothermal treatment with glycerol solution in alcohol, the white anatase/TiO₂-B hybrid TiO₂ transformed to a creamy-white powder, which can be identified as titanium glycerolate (TiGly) according its XRD pattern shown in Fig. S2.† All the diffraction peaks of TD-TiO₂@C can be indexed well based on anatase TiO₂ (JCPDS no. 21-1272). The peak of TiO₂-B disappeared after solvothermal treatment with glycerol and the following calcination, which may be due to the instability of TiO₂-B at high temperature. TiO₂-B transfers to anatase TiO₂ close to 500 °C.³¹ It is noteworthy that the (101) diffraction peak of DT-TiO₂@C shifts to lower angles and the width of (101) plane diffraction peak becomes broader as compared with W-TiO₂.

Fig. 1 XRD patterns of W-TiO₂ and TD-TiO₂@C (the inset is a visual photographs of the two samples).

Fig. 2 displays the X-ray photoelectron spectra (XPS) of W-TiO₂ and TD-TiO₂@C. The survey scan of XPS (Fig. 2a) shows a typical TiO₂ spectrum and again confirms the absence of impurities. The Ti 2p core-level XPS spectra were measured in order to obtain detailed information on the chemical state of the titanium ion in the W-TiO₂ and TD-TiO₂@C. As shown in Fig. 2b, there are two strong peaks at 458.6 and 464.4 eV in the spectra of W-TiO₂, corresponding to the typical 2p_3/2 and 2p_1/2 core levels of Ti⁴⁺, respectively. In contrast, as displayed in Fig. 2c, two major peaks at 458 and 463.5 eV were observed in the spectra of TD-TiO₂@C, assigned to the 2p_3/2 and 2p_1/2 core levels of Ti⁴⁺, whereas the two shoulder peaks at 456.4 and 461.8 eV are attributed to the 2p_3/2 and 2p_1/2 core levels of Ti³⁺, respectively, revealing the existence of Ti³⁺ on TiO₂, which is in good accordance with previous reports on Ti³⁺ doped TiO₂.³² It is obvious that the typical Ti⁴⁺ 2p_3/2 and Ti⁴⁺ 2p_1/2 peaks of TD-TiO₂@C shift to low binding energy compared with W-TiO₂, demonstrating the existence of Ti³⁺ on TiO₂ again.^32,33 Since the radius of Ti⁴⁺ (0.606 Å) is almost the same as Ti³⁺ (0.670 Å), the Ti³⁺ ions can enter into the crystal structure of the anatase titanium dioxide and locate at the interstices or occupy some of the lattice sites of TiO₂.^34,35 For anatase TiO₂, Ti³⁺ doping would induce the XRD diffraction peaks’ intensities to decrease and the width of (101) plane diffraction peak would become broader, which is in good agreement with the XRD pattern of TD-TiO₂@C shown in Fig. 1.³⁵

Fig. 2 (a) The XPS survey scans of W-TiO₂ and TD-TiO₂@C. (b) Ti 2p XPS spectrum of W-TiO₂. (c) Ti 2p XPS spectrum of TD-TiO₂@C. (d) Raman spectra of W-TiO₂ and TD-TiO₂@C (the inset is the Raman spectra of W-TiO₂ and TD-TiO₂@C between a smaller range of Raman shift).

In addition, Raman spectra (Fig. 2d) also confirmed the presence of oxygen vacancies due to the existence of Ti³⁺. It is well known that the pure anatase TiO₂ displays 6 typical peaks around 144, 197, 639, 399, 519 and 513 cm⁻¹, corresponding to the E(1)_g, E(2)_g, E(3)_g, B(1)_1g, B(2)_2g and A_1g modes.^36,37 As shown in Fig. 2d, the W-TiO₂ and the fresh TD-TiO₂@C exhibited a principal peak around 145 and 153 cm⁻¹, respectively. According to the Raman spectra, TD-TiO₂@C has a positive shift of the principal peak, which is characteristic of oxygen vacancies.³⁸ These results clearly demonstrated that the oxygen vacancies were produced due to the replacement of Ti⁴⁺ with Ti³⁺ in the TiO₂ lattice. This is in a good agreement with XPS spectra.

Besides, although no characteristic peak of carbon can be observed in the XRD pattern of TD-TiO₂@C (Fig. 1), its Raman spectrum proves clearly the existence of carbon. The two peaks at around 1355 cm⁻¹ and 1597 cm⁻¹ (the inset of Fig. 2d), indicates that the carbon is partially graphitized with some defects and disorders. The G-band signifies the in-plane vibration of sp² carbon atoms, while the D-band originates from the carbon defects.^39,40 The calculated peak intensity ratio (I_D/I_G) is a useful index for comparing the degree of crystallinity of various carbon materials, i.e., a smaller value of the I_D/I_G ratio reflects a higher degree of ordering in the carbon material, which is related to the higher electronic conductivity.⁴¹ As shown in Fig. 2d, a small value of the I_D/I_G ratio of TD-TiO₂@C is close to 0.81, suggesting that in the TD-TiO₂@C sample has ordered graphitic carbon, and the amount of ordered graphitic carbon in the TD-TiO₂@C sample is more than that of the disordered carbon. These results demonstrate that the TD-TiO₂@C sample will have good conductivity. Certainly, the G-band and D-band are nonexistent in the Raman spectrum of W-TiO₂. The conductivities of TD-TiO₂@C and W-TiO₂@C determined using the four point probe tester at 5 MPa (the sheet thickness is 1.5 mm, radius is 10 mm), are 8.918 × 10⁻⁶ S cm⁻¹ and 3.234 × 10⁻⁷ S cm⁻¹, respectively. It is obvious that TD-TiO₂@C shows much higher electronic conductivity than of W-TiO₂@C because of the tight carbon coating. Besides, it is interesting to note that W-TiO₂@C shows a relatively higher electronic conductivity than some related reports in previous work,^8–11 which can be attributed to the special bicrystalline structure.

As shown in Fig. 3a and b, the SEM image of W-TiO₂ reveals the distribution of inhomogeneous and irregular spheres with a relatively smooth surface and diameters ranging from 200 to 500 nm. After glycerol treatment, the TD-TiO₂@C (Fig. 3c and d) get smaller and quite regular, with wrinkled skin. The surface of the particles is changed from smooth to rough, which is clearly visible and shows the adoption of random orientations.

Fig. 3 SEM images of W-TiO₂ (a and b) and TD-TiO₂@C (c and d).

In order to further investigate the morphology and structure of the final products, TEM and HRTEM images of W-TiO₂ spheres and TD-TiO₂@C spheres are shown in Fig. 4. In the TEM image, it can be found that both samples are composed of tiny nanoparticles. This special structure can increase the diffusion path and provide highly efficient solid-state diffusion of Li⁺. Compared with W-TiO₂, with diameters ranging from 15 to 20 nm, the TD-TiO₂@C spheres composed with nanosized particles show better regularity and smaller size with diameters ranging from 10 to 12 nm. This is in good agreement with the results of nitrogen adsorption–desorption isotherms. From Fig. S1,† the specific surface area of TD-TiO₂@C is 182.6725 m² g⁻¹, which is much larger than that of W-TiO₂ (130.7321 m² g⁻¹). HRTEM analysis is employed to determine the crystal facets. Fig. 4d and h clearly show the lattice fringes with d-spacing values of 0.34–0.35 nm corresponding to the (101) planes of TiO₂, while an amorphous carbon layer is observed over the TD-TiO₂@C surface, exactly as attested by the Raman spectra. The homogeneously dispersed carbon not only sheathes the single TiO₂ particles but also combines all individual TiO₂ particles in a stable union.

Fig. 4 TEM images of W-TiO₂ (a–c) and TD-TiO₂@C (e–g). HRTEM images of W-TiO₂ (d) and TD-TiO₂@C (h).

Based on the above analysis, the synthetic mechanism of TD-TiO₂@C is shown in Fig. 5. Here, the solvothermal process converts the TiO₂ and glycerol to titanium glycerolate (TiGly). In the process of calcination, glycerol was employed to reduce Ti⁴⁺ into Ti³⁺ and to avoid the oxidation of Ti³⁺,^42,43 and was regarded as the source of the carbon. The formation of the carbon is due to the high temperature carbonization of the oligomer.

Fig. 5 Schematic illustration for the formation of the TD-TiO₂@C spheres’ microstructure.

3.2. Electrochemical analysis of TD-TiO₂@C

Fig. 6 shows the charge/discharge performance of W-TiO₂ and TD-TiO₂@C. Fig. 6a shows the cycle performance and charge/discharge profiles (insets) of W-TiO₂. W-TiO₂ shows initial discharge capacities of 175.37 and 127.71 mA h g⁻¹ at 1C and 0.5C, respectively. As shown in Fig. 6b, the cycling performances of TD-TiO₂@C at the rates of 0.5C and 1C are depicted. TD-TiO₂@C shows initial discharge capacities of 371.9, 362.2 mA h g⁻¹ at 0.5C and 1C, respectively. It is easily observed that after modification by carbon and Ti³⁺, TD-TiO₂@C exhibits a much higher capacity, which is 2–3 times higher than that of W-TiO₂. TD-TiO₂@C shows a relative large irreversible capacity loss on the first cycle. The coulombic efficiency of TD-TiO₂@C on the first cycle is 78.2% at 0.5C, which is lower than that of W-TiO₂. This may be due to the following reasons. First, TD-TiO₂@C has a larger specific surface area than W-TiO₂, which contributes to the improvement of the capacity of electrode, but could be detrimental for the coulombic efficiency. The formation of heterojunction interfaces involving electronic conductive additives can induce carrier trapping centers and hamper the reversible lithiation/de-lithiation process.^44,45 Second, Fransson et al.⁴⁶ reported that the carbon shows a tremendous uptake of Li⁺, resulting that the lithium ions intercalated in the material cannot deintercalate promptly. So, the amorphous carbon layer of TD-TiO₂@C could also induce the uptake phenomenon, therefore the coulombic efficiency is low in the first cycle.

Fig. 6 (a) Charge (hollow) and discharge (solid) capacity versus cycle number and charge/discharge profiles (insets) for (a) W-TiO₂, and (b) TD-TiO₂@C at current densities of 0.5C and 1C (1C = 168 mA g⁻¹) in the range of 1.4–2.4 V in Li half-cells. (c) The rate capabilities of W-TiO₂ and TD-TiO₂@C. (d) Charge–discharge capacities of TD-TiO₂@C over 500 cycles at 5C, 10C, 20C, 30C in Li half-cells.

To compare with the rate performance of TD-TiO₂@C and W-TiO₂, these two samples were cycled at various rates (from 0.5C to 30C) after cycling for 100 cycles at 0.5C, as shown in Fig. 6c. W-TiO₂ delivers a discharge capacity of 138.8 mA h g⁻¹ (0.5C), 129.9 mA h g⁻¹ (1C), 110.4 mA h g⁻¹ (2C), 96.9 mA h g⁻¹ (5C), 76.3 mA h g⁻¹ (10C), 57.6 mA h g⁻¹ (20C) and 40.7 mA h g⁻¹ (30C). TD-TiO₂@C shows 265.1 mA h g⁻¹ (0.5C), 219.3 mA h g⁻¹ (1C), 193.9 mA h g⁻¹ (2C), 178.8 mA h g⁻¹ (5C), 147.7 mA h g⁻¹ (10C), 129.1 mA h g⁻¹ (20C) and 109.5 mA h g⁻¹ (30C), which is far better than the electrochemical performance of W-TiO₂. The results reveal that Ti³⁺ self-doping and carbon modification can enhance the electrochemical performance of TiO₂ remarkably, which can be attributed to the enhanced electrical conductivity. The introduction of Ti³⁺ species enables fast electron transfer.¹⁹ According to the report of Wang et al.,⁴⁷ the presence of abundant defects makes the c axis of anatase shrink significantly, whereas the a and b axes elongated slightly, this widens the valence band top edge. So, the defects of TiO₂ switch TiO₂ conductivity from n-type to p-type with a higher charge mobility. Carbon combines all individual TiO₂ nano-particles in stable spherical unions, which form a good network of electrically conductive paths among the TiO₂ particles. So, the TiO₂ active material can get electrons from all directions and be fully utilized for lithium ion insertion and extraction reactions, which can improve lithium storage properties with enhanced rate capability. Besides, a larger capacity could be obtained at high rates if the particle diameter of the electrode material can be made smaller. It has been reported that during the charge/discharge process, the effective diffusion length for Li⁺ ion is 24–240 nm at a rate of 1C, and 3.2–32 nm at 60C.⁴⁸ The average diameter of the well-dispersed TD-TiO₂@C was 200–500 nm, over 240 nm, but the the whole TD-TiO₂@C consisted of ultrafine nanoparticles of about 10–12 nm; this special structure can increase the diffusion path and provide highly efficient solid-state diffusion of Li⁺, and could also shorten remarkably the Li⁺ diffusion length, especially at high rates. Besides, the spaces between the nanoparticles were beneficial for Li⁺ diffusion.

To evaluate the cyclability of TD-TiO₂@C at high rate, the cells were tested at a high rate. TD-TiO₂@C electrode was sequentially discharged–charged at 5C to 30C over 500 cycles (as shown in Fig. 6d). It is found that the coulombic efficiency increases upon cycling; it reaches nearly 100% after 10 cycles. TD-TiO₂@C shows discharge capacities of 188.54, 170.2, 145.19, and 121.9 mA h g⁻¹ at the 10th cycle at 5C, 10C, 20C, and 30C, respectively. After 500 cycles, the retained discharge capacities are 171.78, 143.45, 119.17, and 100.02 mA h g⁻¹, holding a capacity retention as high as 91%, 84.2%, 82.1%, and 82% compared with the capacities at the 10th cycle, respectively. These results dramatically show that TD-TiO₂@C can be widely employed in research and development for the improvement of LIBs because of its excellent electrochemical performance.

Fig. 7 shows cyclic voltammetry (CV) profiles of W-TiO₂ and TD-TiO₂@C spheres between 1.4 and 2.4 V at a scanning rate of 0.1 mV s⁻¹. As shown in Fig. 7a, W-TiO₂ shows a pair of oxidation/reduction peaks (A) at 2.07/1.64 V, corresponding to the extraction and insertion of lithium ions in anatase TiO₂; the other reduction and oxidation potentials of the remaining two pairs of weak peaks, denoted as S1 and S2, are 1.50 V/1.54 V and 1.50 V/1.60 V vs. Li⁺/Li, respectively. These other two pairs (S1, S2) reflect the characteristic pseudo-capacitive behavior of lithium storage in TiO₂-B.^49,50 The pseudo-capacitive behavior derives from the unique sites and energetics of lithium absorption and diffusion in the TiO₂-B structure, which have been studied by theoretical methods.^51,52 As shown in Fig. 7b, the voltammogram of TD-TiO₂@C, there is a pair of clearly defined reduction/oxidation peaks, observed at 1.70 V (reduction peak) and 2.00 V (oxidation peak), which can be attributed to the discharge and charge processes of the Li⁺ intercalation/deintercalation of the anatase framework. As shown in the Figure, after ten cycles, the consistency of the TD-TiO₂@C voltammetry curves was better than W-TiO₂; that is say, the stability of TD-TiO₂@C is better than W-TiO₂. According to the second cycles, the value of the reduction and oxidation potentials’ difference for TD-TiO₂@C is 0.32 V, which is much smaller than that of W-TiO₂ (0.4 V). This obviously demonstrates that TD-TiO₂@C has better reversibility.

Fig. 7 Cyclic voltammograms of (a) W-TiO₂ and (b) TD-TiO₂@C at a scan rate of 0.1 mV s⁻¹ between 1.4 and 2.4 V (vs. Li⁺/Li).

In order to compare and gain insight into the further changes of the electrode microstructure during cycling in the TD-TiO₂@C and W-TiO₂ battery, electrochemical impedance spectroscopy (EIS) was conducted; these were tested at different cycles at the rate of 5C and charged to the same voltage of 2.4 V. The shapes of the Nyquist plots for each cycle are similar. It is evident from Fig. 8 that there is a sloping line in the low frequency region, also one semicircle was observed in the high frequency region for all of the Nyquist plots. Nyquist plots are fitted using the simplified equivalent circuit model (Fig. 8c). Table 1 shows the fitted impedance data. The fitting patterns show that fitting data are in good agreement with experimental data, as shown in Fig. 8a and b. The equivalent circuit model includes R_s at the small intercept of the Z′ axis, showing the internal resistance of the electrode and electrolyte in the battery, a constant phase element (CPE) associated with the surface resistance of the electrode, while the semicircle in the high frequency region corresponding to R_ct represents the lithium charge transfer resistance on the interface of the electrode electrolyte. The sloping line portion is assigned to the Warburg impedance (W₁), which is attributed to the diffusion of lithium ion into the bulk of the electrode materials. It is explicit from Table 1 that the R_s and R_ct of TD-TiO₂@C are much smaller than those of W-TiO₂, indicating that the TD-TiO₂@C has higher conductivity and faster lithium ion diffusion than W-TiO₂. The R_ct of W-TiO₂ is 56.88 Ω after one cycle, and this value increases to 122.40 Ω after 500 cycles. However, the R_ct of the TD-TiO₂@C is 59.90 Ω after one cycle, and this value only increases to 90.30 Ω after 500 cycles. As everyone knows, a lower growth rate of impedance during cycling represents lower polarization, which indicates good cycling behavior. The result is attributed to the higher electronic conductivity resulting from the presence of Ti³⁺ and the unique structures of TD-TiO₂@C, enabling much faster charge transfer and transport at the electrode/electrolyte interface. These results are in good agreement with the excellent electrochemical performance of TD-TiO₂@C nanocomposites.

Fig. 8 Three-dimensional Nyquist plots measured for (a) W-TiO₂ and (b) TD-TiO₂@C after cycling for different cycles at 5C in Li half-cells. (c) The equivalent circuit model.

Table 1 R_s and R_ct values of W-TiO₂ and TD-TiO₂@C nanocomposites after different cycles

	Rs/Ω						Rct/Ω
	1st	100th	200th	300th	400th	500th	1st	100th	200th	300th	400th	500th
TD-TiO2@C	1.15	1.52	1.95	2.16	2.17	2.31	59.9	60.88	66.91	70.07	84.25	90.3
W-TiO2	2.66	2.79	2.83	2.85	3.1	3.33	56.88	58.05	69.07	102.7	108.5	122.4

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4. Conclusion

Ti³⁺-Doped anatase TiO₂@C composite spheres (TD-TiO₂@C) presenting a dark color were successfully prepared by the solvothermal method followed by calcination. TD-TiO₂@C demonstrates remarkable electrochemical performance as the anode material for LIBs. The electrode exhibits a capacitance of 244.8 mA h g⁻¹ at 1C (0.168 A g⁻¹) after 100 cycles, which is far superior to the white TiO₂, and shows a good rate capability. TD-TiO₂@C retains the discharge capacities of 171.78, 143.45, 119.17 and 105.82 mA h g⁻¹ after 500 cycles at 5C, 10C, 20C and 30C, holding a capacity retention of as high as 91.0%, 84.2%, 82.1% and 82.0% when compared with discharge capacities at the 10th cycle, respectively. The considerably enhanced durable high-rate performance is attributed to the improved electronic conductivity resulting from the introduction of Ti³⁺ and carbon into the TD-TiO₂@C composites. It is anticipated that the Ti³⁺-doped anatase TiO₂@C composite spheres can be employed as promising candidates for anode materials for fast and durable LIBs.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Grant No. 51672234), the Research Foundation of Education Bureau of Hunan Province (Grant No. 15B229), the Research Foundation for Hunan Youth Outstanding People from Hunan Provincial Science and Technology Department (2015RS4030), the Hunan Provincial Natural Science Foundation of China (Grant No. 14JJ6017), and the Program for Innovative Research Cultivation Team in University of the Ministry of Education of China (1337304).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22105j

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