Oxygen-deficient anatase TiO₂@C nanospindles with pseudocapacitive contribution for enhancing lithium storage

Abstract

Utilising pseudocapacitive lithium storage is one of the most effective methods to improve the rate performance. In this work, hydrogenated anatase TiO₂ nanospindles with a carbon coating (H-TiO₂@C) are synthesized using a facile hydrothermal process, followed by an annealing treatment of 506 epoxy resin coated TiO₂ nanospindles in a H₂/Ar atmosphere. The hydrogenation process can generate rich oxygen vacancies on the outermost surface of TiO₂ nanospindles. When used as an anode material for LIBs, the H-TiO₂@C electrode exhibits excellent cycling ability and rate performance. It delivers a high reversible capacity of 310 mA h g⁻¹ at 0.1 A g⁻¹ and 126 mA h g⁻¹ at 1 A g⁻¹. The pseudocapacitive contribution is as high as 63.9% as revealed by cyclic voltammetry. Such an excellent electrochemical performance can be ascribed to the synergistic effect of oxygen vacancies and the carbon coating for enhancing the pseudocapacitive effect. This work may provide a new strategy to effectively improve the pseudocapacitive properties of metal oxides.

---

1. Introduction

Lithium-ion batteries (LIBs) have attracted great attention in a wide range of applications from small portable electronics to electric vehicles (EVs)/hybrid electric vehicles (HEVs).^1–5 The electrode behavior involves two types of Li ion storage mechanisms: diffusion-controlled insertion, conversion, or alloying process; surface occurred capacitive process.^6,7 The interfacial lithium storage is a faradaic charge-transfer reaction occurring at the surface or near-surface,^8–11 and it is also called the pseudocapacitive effect. The pseudocapacitive contribution plays a significant role when the material is close to the nanoscale for metal oxides or sulfides.¹² They include intrinsic pseudocapacitive materials (e.g., T-Nb₂O₅, RuO₂ and MnO₂)^11,13,14 and extrinsic pseudocapacitive materials (i.e., MoO₂, V₂O₅, TiO₂, SnS, MoS₂, etc.).^8,15–17

Anatase TiO₂ has a high voltage plateau of above 1.5 V vs. Li, which can afford LIBs with good safety by eliminating the electroplating of lithium dendrites.^18–20 Moreover, it also possesses a high theoretical capacity of 335 mA h g⁻¹ and shows a good structural stability during the charge/discharge process.^21–25 Both the capacitive effect and the lithium insertion process contribute to the whole charge storage in anatase TiO₂ effectively. However, the poor ion and electron transport in TiO₂ greatly restricts its electrochemical performance including cycling life, capacity and rate capability.²⁶ To address this issue, many efforts have been made. One of the feasible approaches is to introduce a carbon coating layer to enhance the ionic and electronic conductivity and maintain the structural stability, which can greatly enhance the Li-ion de/intercalation behavior in TiO₂ electrodes.^27–30 Additionally, fabricating low-dimensional nanostructures (i.e., nanoparticles, nanowires, nanorods and nanotubes) can shorten the Li-ion diffusion distance and enlarge the electrode/electrolyte contact area to enhance Li-ion diffusivity.^29,31–34 It is also well recognized that nanostructures can induce a significant improvement of the pseudocapacitive effect in lithium storage.^9,10 Wang et al. reported that nanostructured anatase TiO₂ with a size of sub-10 nm possessed a large pseudocapacitive effect, which resulted in a faster charge/discharge performance.³⁵ Hao et al. demonstrated anatase TiO₂ nanosheets with a superior cyclability and rate capability due to the diffusive and pseudocapacitive Li⁺ storage in the bulk and at the surface, respectively. The oriented anatase TiO₂ arrays also exhibited obvious pseudocapacitive Li⁺ storage.³⁶

The introduction of oxygen vacancies into electrode materials can also enhance the pseudocapacitive charge storage performance. The oxygen vacancies of MoO_3−x were reported to speed up charge storage.³⁷ Hydrogenated TiO₂ (H-TiO₂) could improve the ionic and electrical conductivity and simultaneously enhance the lithium storage capability. This is ascribed to the formation of some oxygen vacancies and trivalent titanium (Ti³⁺).^38–41 In this work, hydrogenated anatase TiO₂ nanospindles with a carbon coating (H-TiO₂@C nanospindles) were synthesized using a facile hydrothermal process coupled with an annealing process in a H₂/Ar atmosphere with 506 epoxy resin embedded TiO₂ nanospindles. The H-TiO₂@C nanospindles exhibited a favorable performance in terms of cycling stability and rate capability, with a reversible capacity of 310 mA h g⁻¹ at 0.1 A g⁻¹ and 126 mA h g⁻¹ at 1 A g⁻¹. A dominating capacitive contribution of 63.9% to the Li-ion storage was revealed by CV tests. The introduction of oxygen vacancies and the carbon coating accounted for enhancing the pseudocapacitive effect.

2. Experimental section

2.1 Synthesis of TiO₂, H-TiO₂, TiO₂@C and H-TiO₂@C samples

In a typical synthesis, 2 mmol titanium tetraisopropoxide (TBOT) was firstly dissolved in 25 ml distilled water, followed by the addition of 8 mmol Li₂SO₄ and 4 ml hydrogen peroxide (H₂O₂, 30%) into above solution. And then, the solution was transferred into a 40 ml Teflon-lined autoclave after being stirred for 2 h at room temperature and treated at 120 °C for 12 h. Finally, the resulting TiO₂ was washed with distilled water and ethanol thoroughly and then dried in an oven at 60 °C.

The as-obtained TiO₂ was further immersed in 506 epoxy resin and kept at 80 °C for 24 h. Then, the mixture was centrifuged to obtain TiO₂@506 epoxy resin. The H-TiO₂@C could be obtained through annealing the TiO₂@506 epoxy resin in a 5%-H₂/Ar atmosphere at 750 °C for 4 h. For comparison, the TiO₂ was treated under the same conditions to obtain H-TiO₂. The TiO₂@506 was treated in an Ar atmosphere to generate TiO₂@C. In addition, pure TiO₂ was treated in air at 750 °C for 4 h.

2.2 Characterization

The morphology of the samples was analyzed by SEM (Hitachi-S4800) with an accelerating voltage of 15 kV and TEM (JEOL JEM-3010) performed at 200 kV. The crystal structure was characterized by XRD (Rigaku D/MAX-2500) at 50 kV and 250 mA with Cu Kα radiation. To confirm the carbon content of the samples, a thermogravimetric analysis (TGA, Setaram 018124) was carried out from room temperature to 800 °C in air with a heating rate of 10 °C min⁻¹. The surface area and pore diameter distribution were evaluated by Brunauer–Emmett–Teller (BET) measurements using an Autosorb-iQ (Quantachrome Instruments). The bonding nature and chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 spectrometer).

2.3 Electrochemical testing

The working electrodes were prepared with a mixture of active material (80 wt%), acetylene carbon black (10 wt%) and carboxyl methyl cellulose (CMC) (10 wt%) which was dispersed in a mixed solvent of distilled water and ethanol to form a homogeneous slurry. Then, the slurry was uniformly coated onto a piece of Cu foil and subsequently dried at 60 °C for 24 h to form the working electrode. The average active material loading was controlled to be 1.0–1.5 mg cm⁻².

For half-cell tests, two-electrode cells were made with the working electrode, Li metal counter electrode, porous polypropylene separator, and 1 M LiPF₆ in a 1 : 1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte. Galvanostatic charge–discharge testing was performed on a Neware battery test system (CT-3008) over a voltage window of 0.01–3 V. Cyclic voltammetry measurements (CV) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical workstation (PARSTAT 2273 electrochemical measurement).

3. Results and discussion

The crystal structure of the as-obtained samples was determined by XRD (Fig. 1a). All the samples TiO₂, H-TiO₂, TiO₂@C and H-TiO₂@C present well-corresponding diffraction peaks of anatase TiO₂ (JCPDS card no. 21-1272). This indicates that these samples have the same crystal anatase structure, even after being treated with different methods. TGA analysis was carried out to confirm the carbon content of H-TiO₂@C and TiO₂@C. TiO₂ demonstrated a weight loss of 6% in air until 800 °C, which can be attributed to the removal of adsorbed water. For H-TiO₂@C and TiO₂@C, their carbon contents were estimated to be 10 wt% and 12 wt%, respectively, which rule out the loss of adsorbed water and systematic errors. For H-TiO₂, a bigger weight loss occurred, compared to the as-treated TiO₂ in air, which might be originated from the additional adsorbed n-butyl alcohol from the reaction system.

Fig. 1 (a) XRD patterns and (b) TGA curves of the as-prepared TiO₂, H-TiO₂, TiO₂@C and H-TiO₂@C nanospindles.

The morphology of the as-obtained TiO₂, H-TiO₂, TiO₂@C and H-TiO₂@C was characterized by SEM, TEM and HR-TEM (Fig. 2 and S1†). TiO₂ nanospindles were successfully obtained by a hydrothermal method, and these nanospindles had an average diameter of ∼50 nm and length of 100–150 nm (Fig. 2a–d). As shown in high-magnification TEM images (Fig. 2e–h), these nanospindles displayed a clear spindle-like nanostructure with an aspect ratio of ∼3. High-resolution TEM (HR-TEM) images (Fig. 2i–l and S1a–d†) revealed lattice spacings of 0.17 nm and 0.24 nm corresponding to the (105) and (103) planes of TiO₂, respectively. Additionally, distinct carbon layers could be observed on TiO₂@C and H-TiO₂@C nanospindles (Fig. 2k and l).

Fig. 2 (a–d) SEM images, (e–h) TEM images and (i–l) HR-TEM images of TiO₂ (a, e, i), H-TiO₂ (b, f, j), TiO₂@C (c, g, k) and H-TiO₂@C (d, h, l) nanospindles.

XPS measurements were carried out to investigate the bonding nature and chemical states of the samples. Fig. 3a displays the whole XPS spectrum, confirming the existence of Ti, O or C elements in these samples. The Ti 2p spectrum of H-TiO₂@C (Fig. 3b) exhibits two peaks, located at 464.90 eV and 459.19 eV respectively corresponding to Ti 2p_1/2 and Ti 2p_3/2. After deconvolution, three peaks at 464.49 eV, 458.76 eV and 460.41 eV are observed, which are assigned to Ti⁴⁺ 2p_1/2, Ti⁴⁺ 2p_3/2 and Ti³⁺ 2p_1/2, respectively.⁴² This proves the existence of Ti³⁺ after hydrogenation. Additionally, the peak centered at 460.41 eV is associated with Ti–C bonds.⁴³ The Ti 2p spectra of TiO₂@C, H-TiO₂ and TiO₂ are shown in Fig. S2a–c.† After fitting, no peak that can be ascribed to Ti³⁺ was presented for TiO₂@C and TiO₂. The Ti³⁺ 2p_1/2 peak of H-TiO₂ was located at 460.70 eV.⁴² The presence of Ti³⁺ could be associated with the formation of O_v (oxygen vacancies) after hydrogenation treatment.⁴¹ In the O 1s XPS spectrum of H-TiO₂@C, the peaks at 530.02 eV and 532.26 eV can be ascribed to Ti–O bonds and C–O bonds (Fig. 3c) and are associated with oxygen vacancy defects and the lattice oxygen species O²⁻.^32,44 The O 1s XPS spectra of TiO₂@C, H-TiO₂ and TiO₂ are shown in Fig. S2d–f.† The peak at 531.7 eV is associated with the carbonyl groups (C=O) of H-TiO₂, which may be due to the decomposition of TBOT during the reaction process.⁴⁴ Additionally, the C 1s spectra of H-TiO₂@C in Fig. 3d were fitted into three peaks, including C=C (284.0 eV), hydroxyl (C–O, 284.91 eV) and O–C=O (288.43 eV).^6,44 The C 1s spectra of TiO₂@C and H-TiO₂ are shown in Fig. S2g and h.† Here, it should be pointed out that the C 1s spectrum of H-TiO₂ is associated with n-butyl alcohol, generated from the hydrolysis of TBOT.

Fig. 3 (a) XPS spectra of the H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ samples; (b–d) Ti 2p, O 1s and C 1s spectra of the H-TiO₂@C.

We also studied the formation mechanism of TiO₂ nanospindles. Fig. S3† displays the SEM images of the samples synthesized at different reaction times. When the reaction time is 1 h (Fig. S3a†), unshaped particles were formed. Over a reaction time of 6 h, some spindle-like nanostructures were formed but with obvious agglomeration. By further prolonging the reaction time to 12 h (Fig. S3f†), uniform nanospindles were formed. The SEM images of samples synthesized over reaction times of 3 h, 8 h and 10 h can further provide evidence for the formation of nanospindles, as shown in Fig. S3.† The XRD patterns of the samples obtained at different reaction stages (1 h, 3 h, 6 h and 12 h) are also given in Fig. S4.† Based on the evolution process of TiO₂ nanocrystals, a possible formation process of anatase TiO₂ nanospindles is proposed in Scheme 1. TBOT was firstly hydrolyzed in the presence of Li₂SO₄ and H₂O₂ to form unshaped particles via a fast nucleation process. These unshaped particles evolved into spindle-like nanostructures when the reaction proceeded, which might be ascribed to the sufficient reaction of H₂O₂ and Li₂SO₄ to combine with the facets between two TiO₂ nuclei.⁴⁵ Uniform spindle-shaped particles were finally formed via the Ostwald ripening process.⁴⁶ After a further hydrogenation and carbon coating process in an Ar/H₂ atmosphere, H-TiO₂@C was obtained and it still retained a nanospindle morphology. In contrast to the TiO₂ nanoparticles, spindle-like nanostructures have much higher electron transfer and less aggregation,⁴⁷ which are suitable as promising anode materials for LIBs.

Scheme 1 The schematic growth process of the anatase TiO₂ and H-TiO₂@C nanospindles.

Moreover, the effect of the reaction temperature on the final product was also studied as shown in Fig. S5.† When the reaction temperature was set to 140 °C while keeping other conditions the same, a spindle-like structure with obvious agglomeration was formed. At a low reaction temperature (100 °C), large, aggregated spindle-shaped nanoparticles were obtained; however, the growth of uniform nanospindles was achieved at 120 °C. In order to explore the influence of Li₂SO₄ on the growth of anatase TiO₂, different ratios of Li₂SO₄ to titanium tetraisopropoxide (i.e., Li : Ti = 6 : 1, 2 : 1 and 0 : 1) were chosen. As shown in Fig. S6,† the main peaks in all the XRD patterns can be well corresponded to the standard card (JCPDS card no. 21-1272) for anatase TiO₂, indicating that Li₂SO₄ did not show an obvious effect on the crystal phase. However, Li₂SO₄ was found to play an essential role in directing the growth of TiO₂ nanospindles. As shown in Fig. S6a,† no nanospindle could be formed without adding Li₂SO₄. Additionally, the size of spindle-like particles changed with the different amounts of Li₂SO₄ (Fig. S7†). These results clearly demonstrate that Li₂SO₄ can effectively tune the growth of TiO₂. This might be attributed to the influence of SO₄²⁻ anions on crystal nucleation.⁴⁸ Because the effects from the concentration of Ti sources, pH of solutions, amine ligands, and H₂O₂ had already been reported,^45,49,50 we will not discuss the influence of H₂O₂ on the morphology of the final product.

Brunauer–Emmett–Teller (BET) analysis was used to determine the specific surface area of the H-TiO₂@C, TiO₂@C and H-TiO₂. As shown in Fig. S8 and Table S1,† the H-TiO₂@C sample possessed a smaller pore size and a comparatively higher specific surface area of 136.93 m² g⁻¹, whereas the specific surface areas of TiO₂@C and H-TiO₂ nanospindles were much smaller, 86.15 m² g⁻¹ and 51.90 m² g⁻¹, respectively. The increase in the specific area and the decrease in the pore size after carbon coating and hydrogenation may be ascribed to a synergistic effect of the shrinkage of the carbon skeleton and weak lattice expansion of TiO₂ nanocrystals.^51,52 These H-TiO₂@C nanospindles are expected to have excellent performance in Li-ion storage.

The H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ nanospindles were assembled into a Li half-cell to estimate their Li-storage capability over an extended potential range of 0.01 to 3.00 V. Here, the enlargement of the potential window could lead to lithium intercalation and de-intercalation in the carbon phase and ultimately result in a higher reversible capacity.⁵³ The charge–discharge voltage profiles of the H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ for the 1^st, 5^th, 10^th, 50^th and 100^th cycles at 0.1 A g⁻¹ (∼0.3C, where current for 1C was 336 mA g⁻¹) are shown in Fig. 4a and S9.† Generally, the decomposition of the electrolyte and the formation of the SEI film lead to an irreversible capacity in the first cycle for anode materials.^54–57 The H-TiO₂@C electrode exhibited a discharge capacity of 500 mA h g⁻¹ and 385 mA h g⁻¹ in the second and the tenth cycle, a capacity much higher than the theoretical value (336 mA h g⁻¹). Such a high capacity may be attributed to the defects in TiO₂ nanospindles resulted from oxygen vacancies and Ti³⁺ after hydrogenation treatment. The potential plateaus approximately around 1.75 V and 2 V indicate a reversible intercalation of Li ions into the anatase TiO₂ nanospindles for all electrodes. Fig. 4b shows the cycling stability of H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ electrodes at 0.1 A g⁻¹, which displayed an initial discharge capacity of 797 mA h g⁻¹, 404 mA h g⁻¹, 281 mA h g⁻¹ and 299 mA h g⁻¹, which dropped to 500 mA h g⁻¹, 300 mA h g⁻¹, 219 mA h g⁻¹ and 205 mA h g⁻¹ in the second cycle, respectively. After 150 cycles, the discharge capacities of H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ electrodes remained at about 310 mA h g⁻¹, 150 mA h g⁻¹, 115 mA h g⁻¹ and 103 mA h g⁻¹, with a high coulombic efficiency of 101%, 105%, 98% and 101%, respectively. In order to explore the structural change of these electrodes after 150 cycles, their morphologies were investigated. The H-TiO₂@C still maintained its structural integrity to some extent (Fig. S10†), while the other materials could not keep their spindle-like morphology. The cycling performance of H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ was also investigated at a high current density of 1 A g⁻¹ (Fig. 4c). The discharge capacity of H-TiO₂@C still remained at 126 mA h g⁻¹ after 200 cycles, in contrast to 81 mA h g⁻¹, 50 mA h g⁻¹ and 45 mA h g⁻¹ for TiO₂@C, H-TiO₂ and TiO₂, respectively. During the cycling (Fig. 4b and c) process, there is an obvious capacity decay in the first 25 cycles for H-TiO₂@C, which might be related to the irreversibly inserted lithium from the formation of irreversible nanostructures and crystalline grains and amorphous regions.^58–60 These results clearly demonstrate that H-TiO₂@C possesses the best electrochemical properties. As shown in Fig. 4d, the charge capacity of the H-TiO₂@C electrode was 410, 280, 191, 108 and 50 mA h g⁻¹ at 0.3C (0.1 A g⁻¹), 0.5C (0.168 A g⁻¹), 1C (0.34 A g⁻¹), 2C (0.68 A g⁻¹) and 5C (0.168 A g⁻¹), respectively. When the discharge rate was reversed to 0.3C, a discharge capacity of 309 mA h g⁻¹ was recovered, which is better than those of TiO₂@C (207 mA h g⁻¹), H-TiO₂ (161 mA h g⁻¹) and TiO₂ (54 mA h g⁻¹) electrodes. The H-TiO₂@C electrode had the best cycling performance and rate performance among them. The improved performance could possibly be ascribed to the oxygen vacancies introduced by hydrogenation and the carbon coating. The carbon coating can provide H-TiO₂ with a more stable framework that effectively restrains the expansion of volume, showing a volume stability.^61,62 Moreover, the carbon layer can promote the electrical conductivity of the H-TiO₂ electrode, leading to a better electrochemical activity.⁵² When compared with reported TiO₂-based materials for LIBs, the H-TiO₂@C shows a better performance. For comparison, the cycling and the rate performance of TiO₂@C,¹⁸ rGO/TiO₂,²⁹ H-TiO₂,³⁴ TiO₂@C,⁶³ 3D C@TiO₂,⁶⁴ TiO₂@GO⁶⁵ and TiO₂/TiO₂(B)⁶⁶ are given in Table S2.†

Fig. 4 (a) Charge/discharge curves of H-TiO₂@C electrodes at 0.1 A g⁻¹; (b, c) cycling performance and coulombic efficiency of H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ electrodes at 0.1 A g⁻¹ (b) and at 1 A g⁻¹ (c); (d) rate performance of H-TiO₂@C, TiO₂@C, TiO₂ and H-TiO₂.

To study the charge storage in the different samples, CV measurements were carried out. For H-TiO₂@C (Fig. 5a), the cathodic/anodic peaks appeared at ca. 2.12 V and 1.75 V at 0.2 mV s⁻¹, representing the Li⁺ insertion/extraction behavior of anatase H-TiO₂@C. The CV curves of TiO₂@C, H-TiO₂ and TiO₂ electrodes are shown in Fig. S11a, S12a and S13.† The area under the CV curves is the whole charge storage arising from faradaic and non-faradaic processes. It is well known that the faradaic reaction comprises a redox pseudocapacitive process occurring at the surface of the materials and a diffusion-controlled process in the bulk fast ion intercalation.^8,26,67 By using the various scan rate CV curves, we can quantify the capacitive effect and the diffusion-controlled Li⁺ intercalation effect on the total lithium storage according to the followed equations:^8,13,67

i(V) = k₁v + k₂v^1/2 (1)

For analytical purposes, the above equation can also be reformulated as:

i(V)/v^1/2 = k₁v^1/2 + k₂ (2)

In eqn (1), i(V), k₁v and k₂v^1/2 correspond to the whole current, current from the surface capacitive effects, and current from the diffusion-controlled Li⁺ intercalation process, respectively. Therefore, it is the key to determine k₁ and k₂ values. In Fig. 5b, S11b and S12b,† we plotted i(V)/v^1/2versus v^1/2 at various potentials during the charge/discharge process from 0.2 mV s⁻¹ to 1 mV s⁻¹, where we can get the slope (k₁) and intercept (k₂) from the straight lines after linear fitting. This enables us to distinguish the currents arising from Li⁺ insertion and those arising from capacitive processes. Fig. 5c shows the potential profile for the current from capacitive effects (red region) in H-TiO₂@C at 1 mV s⁻¹, and the capacitive contribution was 63.9%, whereas the TiO₂@C electrode (Fig. S11c†) showed a capacitive contribution of 58.1%. With increasing scan rate, the current contribution from the surface capacitive effect (k₁v) increases. Thus, the pseudocapacitive contribution for H-TiO₂@C and TiO₂@C enlarged to 81% vs. 72.9% at 2 mV s⁻¹ (Fig. 5d and S11d†). The pseudocapacitive contributions are smaller at lower sweep rates (Fig. 5d and S11d†). They were 48.5%, 50%, 54.7%, and 58.5% at 0.2, 0.4, 0.6 and 0.8 mV s⁻¹ for H-TiO₂@C, and 34.8%, 44.5%, 50.6%, and 53.6% for TiO₂@C, respectively. The pseudocapacitive contribution for the H-TiO₂ is 56.6% at 1 mV s⁻¹ (Fig. S12c†). This demonstrates that oxygen vacancies could enhance the pseudocapacitive charge storage performances and the carbon coating could improve the poor Li ion and electron transport, resulting in good performance in lithium storage. Electrochemical impedance spectra (EIS) were recorded to demonstrate that the hydrogen reduction and the carbon coating could increase the electrical conductivity and keep the surface stability of TiO₂, which is shown in Fig. 6 and the equivalent circuit model is also presented. Nyquist plots display a depressed semicircle in the high- and medium-frequency regions and an inclined line in low-frequency regions, corresponding to the ohmic resistance (R_b), charge–discharge resistance (R_ct) and Warburg impedance of diffusive resistance (W), respectively.^8,44,57 Additionally, the CPE (constant phase element) is also included within the fabricated model. For the charge transfer kinetics, R_ct is a key indicator for the electrodes. Obviously, it can be seen that the anatase H-TiO₂@C electrode exhibits a much smaller R_ct than TiO₂@C, H-TiO₂ and TiO₂ electrodes, which would be beneficial to the Li-ion transport during the Li⁺ insertion/extraction process.

Fig. 5 Kinetic analysis of H-TiO₂@C: (a) CV curves at different sweep rates. (b) i(V)/v^1/2versus v^1/2 at various potentials during the charge/discharge process from 0.2 mV s⁻¹ to 1 mV s⁻¹. (c) Capacitive current contribution (red region) to the charge storage at 1 mV s⁻¹. (d) The contribution ratio of pseudocapacitive and diffusion-controlled current at different scan rates.

Fig. 6 EIS of H-TiO₂@C, TiO₂@C, H-TiO₂ and TiO₂ electrodes before cycling. Inset: equivalent circuit model.

4. Conclusion

In conclusion, we have fabricated oxygen-deficient H-TiO₂@C nanospindles by hydrothermal–coating–annealing processes. This material exhibits an exceptional performance: a remarkably high discharge capacity of 310 mA h g⁻¹ after 150 cycles at 0.1 A g⁻¹; a high discharge capacity of 126 mA h g⁻¹ after 200 cycles at 1 A g⁻¹. Such performance can be ascribed to a dominating capacitive contribution (63.9% at 1 mV s⁻¹) due to the carbon coating and abundant oxygen vacancies on the material surface. This work may provide an effective method to improve the pseudocapacitive properties of other metal oxides.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51302079 and 11675051), the National Natural Science Foundation of Hunan Province (Grant No. 2017JJ1008) and the Shenzhen Technical Plan Project (KQJSCX20160226191136).

References

1. J. T. Xu, Y. H. Dou, Z. X. Wei, J. M. Ma, Y. H. Deng, Y. T. Li, H. K. Liu and S. X. Dou, Recent progress in graphite intercalation compounds for rechargeable metal (Li, Na, K, Al)-ion batteries, Adv. Sci., 2017, 4, 1700146.
2. L. Mei, J. T. Xu, Z. X. Wei, H. Liu, Y. T. Li, J. M. Ma and S. X. Dou, Chevrel phase Mo₆T₈ (T = S, Se) as electrodes for advanced energy storage, Small, 2017, 13, 1701441.
3. J. T. Xu, J. M. Ma, Q. H. Fan, S. J. Guo and S. X. Dou, Recent progress in the design of advanced cathode materials and battery models for high-performance lithium-X (X = O₂, S, Se, Te, I₂, Br₂) batteries, Adv. Mater., 2017, 1606454.
4. X. C. Duan, J. T. Xu, Z. X. Wei, J. M. Ma, S. J. Guo, H. K. Liu and S. X. Dou, Atomically thin transition-metal dichalcogenides for electrocatalysis and energy storage, Small Methods, 2017, 1, 1700156.
5. X. Q. Zhang, X. B. Cheng and Q. Zhang, Nanostructured energy materials for electrochemical energy conversion and storage: A review, J. Energy Chem., 2016, 25, 967–984.
6. D. L. Chao, P. Liang, Z. Chen, L. Y. Bai, H. Shen, X. X. Liu, X. H. Xia, Y. L. Zhao, S. V. Savilov, J. Y. Lin and Z. X. Shen, Pseudocapacitive Na-ion storage boosts high rate and areal capacity of self-branched 2D layered metal chalcogenide nanoarrays, ACS Nano, 2016, 10, 10211–10219.
7. V. Augustyn, J. Come, M. A. Lowe, J. W. Kim, P.-L. Taberna, S. H. Tolbert, H. D. Abruña, P. Simon and B. Dunn, High-rate electrochemical energy storage through Li⁺ intercalation pseudocapacitance, Nat. Mater., 2013, 12, 518.
8. B. Hao, Y. Yan, X. B. Wang and G. Chen, Synthesis of anatase TiO₂ nanosheets with enhanced pseudocapacitive contribution for fast lithium storage, ACS Appl. Mater. Interfaces, 2013, 5, 6285–6291.
9. J. Kang, S.-H. Wei, K. Zhu and Y.-H. Kim, First-principles theory of electrochemical capacitance of nanostructured materials: Dipole-assisted subsurface intercalation of lithium in pseudocapacitive TiO₂ anatase nanosheets, J. Phys. Chem. C, 2011, 115, 4909–4915.
10. D. L. Chao, C. R. Zhu, P. H. Yang, X. H. Xia, J. L. Liu, J. Wang, X. F. Fan, S. V. Savilov, J. Y. Lin, H. J. Fan and Z. X. Shen, Array of nanosheets render ultrafast and high-capacity Na-ion storage by tunable pseudocapacitance, Nat. Commun., 2016, 7, 12122.
11. A. A. Lobinsky and V. P. Tolstoy, Synthesis of γ-MnOOH nanorods by successive ionic layer deposition method and their capacitive performance, J. Energy Chem., 2017, 26, 336–339.
12. J. Wang, S. Y. Dong, B. Ding, Y. Wang, X. D. Hao, H. Dou, Y. Y. Xia and X. G. Zhang, Pseudocapacitive materials for electrochemical capacitors: from rational synthesis to capacitance optimization, Natl. Sci. Rev., 2016, 4, 71–90.
13. V. Augustyn, P. Simon and B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci., 2014, 7, 1597–1614.
14. L. P. Kong, C. F. Zhang, J. T. Wang, W. M. Qiao, L. C. Ling and D. H. Long, Free-standing T-Nb₂O₅/graphene composite papers with ultrahigh gravimetric/volumetric capacitance for Li-ion intercalation pseudocapacitor, ACS Nano, 2015, 9, 11200–11208.
15. C. T. Zhao, C. Yu, M. D. Zhang, H. W. Huang, S. F. Li, X. T. Han, Z. B. Liu, J. Yang, W. Xiao, J. N. Liang, X. L. Sun and J. S. Qiu, Ultrafine MoO₂ carbon microstructures enable ultralong-life power-type sodium ion storage by enhanced pseudocapacitance, Adv. Energy Mater., 2017, 1602880.
16. Z. Y. Le, F. Liu, P. Nie, X. R. Li, X. Y. Liu, Z. F. Bian, G. Chen, H. B. Wu and Y. F. Lu, Pseudocapacitive sodium storage in mesoporous single-crystal-like TiO₂–graphene nanocomposite enables high-performance sodium-ion capacitors, ACS Nano, 2017, 11, 2952–2960.
17. J. B. Cook, H. S. Kim, Y. Yan, J. S. Ko, S. Robbennolt, B. Dunn and S. H. Tolbert, Mesoporous MoS₂ as a transition metal dichalcogenide exhibiting pseudocapacitive Li and Na-ion charge storage, Adv. Energy Mater., 2016, 6, 1501937.
18. F. Yang, Y. X. Zhu, X. Li, C. Lai, W. Guo and J. M. Ma, Crystalline TiO₂@C nanosheet anode with enhanced rate capability for lithium-ion batteries, RSC Adv., 2015, 5, 98717–98720.
19. Y. Yeo, J.-W. Jung, K. Park and I.-D. Kim, Graphene-wrapped anatase TiO₂ nanofibers as high-rate and long-cycle-life anode material for sodium ion batteries, Sci. Rep., 2015, 5, 13862.
20. H. Hu, L. Yu, X. Gao, Z. Lin and X. W. Lou, Hierarchical tubular structures constructed from ultrathin TiO₂ (B) nanosheets for highly reversible lithium storage, Energy Environ. Sci., 2015, 8, 1480–1483.
21. Y. Wu, X. W. Liu, Z. Z. Yang, L. Gu and Y. Yu, Nitrogen-doped ordered mesoporous anatase TiO₂ nanofibers as anode materials for high performance sodium-ion batteries, Small, 2016, 12, 3522–3529.
22. U. Lafont, D. Carta, G. Mountjoy, A. V. Chadwick and E. Kelder, In situ structural changes upon electrochemical lithium insertion in nanosized anatase TiO₂, J. Phys. Chem. C, 2009, 114, 1372–1378.
23. B. M. Feng, H. X. Wang, Y. Q. Zhang, X. Y. Shan, M. Liu, F. Li, J. H. Guo, J. Feng and H. T. Fang, Free-standing hybrid film of less defective graphene coated with mesoporous TiO₂ for lithium ion batteries with fast charging/discharging capabilities, 2D Mater., 2016, 4, 015011.
24. D. McNulty, E. Carroll and C. O'Dwyer, Adv. Energy Mater., 2017, 1602291,  DOI:10.1002/aenm.201602291.
25. G. Q. Zhang, H. B. Wu, T. Song, U. Paik and X. W. Lou, TiO₂ hollow spheres composed of highly crystalline nanocrystals exhibit superior lithium storage properties, Angew. Chem., Int. Ed., 2014, 53, 12590–12593.
26. J. Y. Shin, D. Samuelis and J. Maier, Sustained lithium-storage performance of hierarchical, nanoporous anatase TiO₂ at high rates: emphasis on interfacial storage phenomena, Adv. Funct. Mater., 2011, 21, 3464–3472.
27. S. Qiu, L. F. Xiao, X. P. Ai, H. X. Yang and Y. L. Cao, Yolk–shell TiO₂@C nanocomposite as high-performance anode material for sodium-ion batteries, ACS Appl. Mater. Interfaces, 2016, 9, 345–353.
28. R. Qing, L. Liu, H. Kim and W. M. Sigmund, Electronic property dependence of electrochemical performance for TiO₂/CNT core–shell nanofibers in lithium ion batteries, Electrochim. Acta, 2015, 180, 295–306.
29. P. Zheng, T. Liu, Y. Su, L. F. Zhang and S. W. Guo, TiO₂ nanotubes wrapped with reduced graphene oxide as a high-performance anode material for lithium-ion batteries, Sci. Rep., 2016, 6, 36580.
30. Y. Xiong, J. F. Qian, Y. L. Cao, X. P. Ai and H. X. Yang, Electrospun TiO₂/C nanofibers as a high-capacity and cycle-stable anode for sodium-ion batteries, ACS Appl. Mater. Interfaces, 2016, 8, 16684–16689.
31. H. Q. Liu, K. Z. Cao, X. H. Xu, L. F. Jiao, Y. J. Wang and H. T. Yuan, Ultrasmall TiO₂ nanoparticles in situ growth on graphene hybrid as superior anode material for sodium/lithium ion batteries, ACS Appl. Mater. Interfaces, 2015, 7, 11239–11245.
32. Y. Q. Ge, H. Jiang, J. D. Zhu, Y. Lu, C. Chen, Y. Hu, Y. P. Qiu and X. W. Zhang, High cyclability of carbon-coated TiO₂ nanoparticles as anode for sodium-ion batteries, Electrochim. Acta, 2015, 157, 142–148.
33. H. W. Tian, K. Shen, X. Y. Hu, L. Qiao and W. T. Zheng, N, S co-doped graphene quantum dots–graphene–TiO₂ nanotubes composite with enhanced photocatalytic activity, J. Alloys Compd., 2017, 691, 369–377.
34. X. Li, J. B. Zhao, S. G. Sun, L. Huang, Z. P. Qiu, P. Dong and Y. J. Zhang, The application of plasma treatment for Ti³⁺ modified TiO₂ nanowires film electrode with enhanced lithium-storage properties, Electrochim. Acta, 2016, 211, 395–403.
35. J. Wang, J. Polleux, J. Lim and B. Dunn, Pseudocapacitive contributions to electrochemical energy storage in TiO₂ (anatase) nanoparticles, J. Phys. Chem. C, 2007, 111, 14925–14931.
36. K. Zhu, Q. Wang, J.-H. Kim, A. A. Pesaran and A. J. Frank, Pseudocapacitive lithium-ion storage in oriented anatase TiO₂ nanotube arrays, J. Phys. Chem. C, 2012, 116, 11895–11899.
37. H.-S. Kim, J. B. Cook, H. Lin, J. S. Ko, S. H. Tolbert, V. Ozolins and B. Dunn, Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO_3−x, Nat. Mater., 2017, 16, 454–460.
38. J. C. Huo, Y. J. Hu, H. Jiang and C. Z. Li, In situ surface hydrogenation synthesis of Ti³⁺ self-doped TiO₂ with enhanced visible light photoactivity, Nanoscale, 2014, 6, 9078–9084.
39. W. Zhou, W. Li, J.-Q. Wang, Y. Qu, Y. Yang, Y. Xie, K. F. Zhang, L. Wang, H. G. Fu and D. Y. Zhao, Ordered mesoporous black TiO₂ as highly efficient hydrogen evolution photocatalyst, J. Am. Chem. Soc., 2014, 136, 9280–9283.
40. J.-Y. Shin, J. H. Joo, D. Samuelis and J. Maier, Oxygen-deficient TiO_2−δ nanoparticles via hydrogen reduction for high rate capability lithium batteries, Chem. Mater., 2012, 24, 543–551.
41. X. Y. Zhang and Z. Y. Chen, Enhanced photoelectrochemical performance of the hierarchical micro/nano-structured TiO₂ mesoporous spheres with oxygen vacancies via hydrogenation, RSC Adv., 2015, 5, 9482–9488.
42. J. W. Zhang, L. L. Zhang, J. W. Zhang, Z. J. Zhang and Z. S. Wu, Effect of surface/bulk oxygen vacancies on the structure and electrochemical performance of TiO₂ nanoparticles, J. Alloys Compd., 2015, 642, 28–33.
43. H.-K. Kim, D. Mhamane, M.-S. Kim, H.-K. Roh, V. Aravindan, S. Madhavi, K. C. Roh and K.-B. Kim, TiO₂–reduced graphene oxide nanocomposites by microwave-assisted forced hydrolysis as excellent insertion anode for Li-ion battery and capacitor, J. Power Sources, 2016, 327, 171–177.
44. F. Li, J. Jiang, X. Wang, F. Liu, J. Wang, Y. Chen, H. Lin and S. Han, Assembly of TiO₂/graphene with macroporous 3D network framework as an advanced anode material for Li-ion batteries, RSC Adv., 2016, 6, 3335–3340.
45. L. Song, Y. Zhang, X. Wu, Z. G. Liao, Q. F. Yue, F. Y. Qu and X. Zhang, One-step synthesis of crystalline anatase TiO₂ nanospindles and investigation on their photocatalytic performance, Mater. Lett., 2013, 100, 198–200.
46. L. Wang, J. M. Ma, L. C. Chen, Z. Xu and T. H. Wang, Tailoring the subunits of α-Fe₂O₃ nanoplates for optimizing electrochemical performance, Electrochim. Acta, 2013, 113, 194–199.
47. H. T. Wu, J. Fan, E. Z. Liu, X. Y. Hu, Y. N. Ma, X. Fan, Y. Y. Li and C. N. Tang, Facile hydrothermal synthesis of TiO₂ nanospindles–reduced graphene oxide composite with an enhanced photocatalytic activity, J. Alloys Compd., 2015, 623, 298–303.
48. X. L. Pan, Y. Y. Zeng, Z. Gao, S. K. Xie, F. Xiao and L. J. Liu, Effects of Li₂SO₄·H₂O amounts on morphologies of hydrothermal synthesized LiMnPO₄ cathodes, RSC Adv., 2016, 6, 103232–103237.
49. Z. Zhao, J. Xu, C. Shang, R. Ye and Y. Wang, Dealloying-driven synthesis of sea-urchin like titanate nanowires and hierarchically porous anatase TiO₂ nanospindles with enhanced photocatalytic performance, Corros. Sci., 2015, 98, 651–660.
50. Y. C. Qiu, K. Y. Yan, S. H. Yang, L. M. Jin, H. Deng and W. S. Li, Synthesis of size-tunable anatase TiO₂ nanospindles and their assembly into anatase@titanium oxynitride/titanium nitride–graphene nanocomposites for rechargeable lithium ion batteries with high cycling performance, ACS Nano, 2010, 4, 6515–6526.
51. H. Zou, K. Yan, Y. Cong, X. Li, J. Zhang, Z. Cui, Z. Dong, G. Yuan and Y. Li, Synthesis of hierarchical porous carbon–TiO₂ composites as anode materials for high performance lithium ion batteries, Res. Chem. Intermed., 2017, 43, 2891–2904.
52. J. Wang, L. Shen, P. Nie, G. Xu, B. Ding, S. Fang, H. Dou and X. Zhang, Synthesis of hydrogenated TiO₂–reduced-graphene oxide nanocomposites and their application in high rate lithium ion batteries, J. Mater. Chem. A, 2014, 2, 9150–9155.
53. R. F. Wu, S. Y. Shen, G. F. Xia, F. J. Zhu, C. M. Lastoskie and J. L. Zhang, Soft-templated self-assembly of mesoporous anatase TiO₂/carbon composite nanospheres for high-performance lithium ion batteries, ACS Appl. Mater. Interfaces, 2016, 8, 19968–19978.
54. H. T. Sun, G. Q. Xin, T. Hu, M. P. Yu, D. L. Shao, X. Sun and J. Lian, High-rate lithiation-induced reactivation of mesoporous hollow spheres for long-lived lithium-ion batteries, Nat. Commun., 2014, 5, 4526.
55. J. J. Liang, X. Gao, J. Guo, C. M. Chen, K. Fan and J. M. Ma, Electrospun MoO₂@NC nanofibers with excellent Li⁺/Na⁺ storage for dual applications, Sci. China Mater., 2018, 61, 30–38.
56. Y. Cai, J. M. Ma and T. H. Wang, Hydrothermal synthesis of α-Ni(OH)₂ and its conversion to NiO with electrochemical properties, J. Alloys Compd., 2014, 582, 328–333.
57. C. Y. Cui, X. Li, Z. Hu, J. T. Xu, H. K. Liu and J. M. Ma, Growth of MoS₂@C nanobowls as a lithium-ion battery anode material, RSC Adv., 2015, 5, 92506–92514.
58. X. Y. Yu, H. B. Wu, L. Yu, F. X. Ma and X. W. Lou, Rutile TiO₂ submicroboxes with superior lithium storage properties, Angew. Chem., Int. Ed., 2015, 54, 4001–4004.
59. X. Li, G. Wu, X. Liu, W. Li and M. Li, Orderly integration of porous TiO₂ (B) nanosheets into bunchy hierarchical structure for high-rate and ultralong-lifespan lithium-ion batteries, Nano Energy, 2017, 31, 1–8.
60. B. Y. Guan, L. Yu, J. Li and X. W. Lou, A universal cooperative assembly-directed method for coating of mesoporous TiO₂ nanoshells with enhanced lithium storage properties, Sci. Adv., 2016, 2, e1501554.
61. J. J. Liang, C. C. Yuan, H. H. Li, K. Fan, Z. X. Wei, H. Q. Sun and J. M. Ma, Growth of SnO₂ nanoflowers on N-doped carbon nanofibers as anode for Li- and Na-ion batteries, Nano-Micro Lett., 2018, 10, 21.
62. J. J. Liang, X. Gao, J. Guo, C. M. Chen, K. Fan and J. M. Ma, Electrospun MoO₂@NC nanofibers with excellent Li⁺/Na⁺ storage for dual applications, Sci. China Mater., 2018, 61, 30–38.
63. J. Ding, X. Gao, L. M. Cha, M. Q. Cai and J. M. Ma, Synthesis of TiO₂@C nanospheres with excellent electrochemical properties, RSC Adv., 2016, 6, 108310–108314.
64. X. H. Wang, C. Guan, L. M. Sun, R. A. Susantyoko, H. J. Fan and Q. Zhang, Highly stable and flexible Li-ion battery anodes based on TiO₂ coated 3D carbon nanostructures, J. Mater. Chem. A, 2015, 3, 15394–15398.
65. T. F. Zhou, Y. Zheng, H. Gao, S. D. Min, S. A. Li, H. K. Liu and Z. P. Guo, Surface engineering and design strategy for surface-amorphized TiO₂@graphene hybrids for high power Li-ion battery electrodes, Adv. Sci., 2015, 2, 1500027.
66. H. Wei, E. F. Rodriguez, A. F. Hollenkamp, A. I. Bhatt, D. H. Chen and R. A. Caruso, High reversible pseudocapacity in mesoporous yolk–shell anatase TiO₂/TiO₂ (B) microspheres used as anodes for Li-ion batteries, Adv. Funct. Mater., 2017, 27, 1703270.
67. X. H. Xia, D. L. Chao, Y. Q. Zhang, J. Y. Zhan, Y. Zhong, X. L. Wang, Y. D. Wang, Z. X. Shen, J. P. Tu and H. J. Fan, Generic synthesis of carbon nanotube branches on metal oxide arrays exhibiting stable high-rate and long-cycle sodium-ion storage, Small, 2016, 12, 3048–3058.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta11301c

‡ X. Deng and Z. Wei contributed equally to this work.

---