Synthesis of TiO₂@C nanospheres with excellent electrochemical properties

Abstract

TiO₂@C nanospheres are synthesized through annealing a mixture of tetrabutyl titanate and oleic acid, as the titanium and carbon source, respectively. The as-synthesized TiO₂@C with higher carbon content presents a superior specific capacity (286.5 mA h g⁻¹ at 100 mA g⁻¹ after 200 cycles) and higher rate performance (148 mA h g⁻¹ at 500 mA g⁻¹), when tested as the anode material for lithium ion batteries. The as-synthesized TiO₂@C nanospheres are promising anode materials for next-generation LIBs.

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1. Introduction

Nowadays, there is a hot spot in exploring green, low-cost and enduring energy storage systems to prevent global environmental contamination and resource depletion.^1,2 On account of the high energy density and long cycle, lithium-ion batteries (LIBs) have become one of the most promising batteries. It is well-recognized that anode materials play an incredibly important role in LIBs.^3–7 As an anode material in LIBs, graphite is commercially applied due to its good cycling and low cost.⁵ However, the performance has been limited by its poor rate capability due to its low Li diffusion efficiency.^8,9 Thus, some metal oxides or metal oxide based compositions with carbon or carbon nanotubes (CNTs), such as Fe₂O₃,¹⁰ SnO₂,¹¹ Co₃O₄,¹² MnO,¹³ MoO₂/CNTs,¹⁴ Cr₂O₃,¹⁵ TiO₂,¹⁶ etc., have attracted increasing attention. Wei et al. reported that solvothermally synthesized SnO₂ hollow nanospheres exhibited a capacity of 509 mA h g⁻¹ at 0.5 C.¹¹ Cao and his co-workers reported that MnO@C octahedra could realize a capacity of 503 mA h g⁻¹ at 100 mA g⁻¹.¹³

Among these metal oxides, TiO₂ has been received attention as an anode material owing to some advantages including low cost, simple preparation and high security. Besides, TiO₂ possesses superior structural stability owing to its small volume variation with Li ion insertion/extraction processes.^16,17 Four kinds of TiO₂, anatase, rutile, brookite and TiO₂–B, have been studied as anode materials for LIBs.^18,19 Among them, anatase TiO₂ has been regarded as one of the most potential anode materials because of its high theoretical capacity (about 335.5 mA h g⁻¹). Simultaneously, it can be optimized by various synthetic methods. Moreover, TiO₂ shows a considerable improvement in safety due to its high voltage plateau at about 1.7 V versus Li⁺/Li, which could efficaciously avoid the formation of solid electrolyte interphase (SEI) layers and lithium plating on the electrode.²⁰ Nevertheless, the poor ion and electronic conductivity of TiO₂ make it a challenge to present superior rate capability, which is one of important properties related to LIBs.^21–23 Thus, it is of particular importance to seek suitable methods for relieving damages derived from poor electronic and ion conductivity. Fabrication of various nanostructures is a well-acknowledged approach to improving the electronic conductivity and Li⁺ diffusivity in battery materials. Porous nanostructures can not only shorten the length of Li⁺ diffusion in the solid phase, but also enhance the electrolyte's immersion, so nanostructured porous TiO₂ has become one of the most popular nanostructured anode materials. Hence, researchers have been devoted to synthesizing mesoporous TiO₂ for LIBs.^24–26 Meanwhile, researchers found that it could boost the electronic conductivity by combining TiO₂ with highly conductive materials, such as carbon, CNT and graphene, and hence to improve the electrode performance of TiO₂.^27–29 However, TiO₂ nanoparticles on the CNT or graphene surface would still suffer from decomposition and the furious volumetric expansion during the charge/discharge process. As a result, these anode materials would present a gradual weakening cycle performance. Another way to boost rate capability is the carbon coating on TiO₂ nanostructures, which can effectively maintain structure stability.³⁰

Herein, we reported a facile and simple method for synthesizing the anatase TiO₂ coated with carbon (TiO₂@C) by using oleic acid as the carbon source. Compared with general glucose as carbon source, oleic acid possesses carboxyl groups,^30–32 which can be steadily attached to the surface of TiO₂. After annealed at a high temperature in the Ar atmosphere, TiO₂ was coated with thin carbon layer in its surface, which could effectively improve the electrochemical performance of the sample. In this work, to explore effect in electrochemical performance by different coating carbon content, we synthesized two samples by adding different amount of oleic acid. We found that the TiO₂@C with high content of carbon presented superior property not only in cycling performance but also rate performance during the whole cycling process. The as-synthesized TiO₂@C with high content of carbon still maintains a discharge capacity of 286.5 mA h g⁻¹ at 100 mA g⁻¹ after 200 cycles. Besides, the as-synthesized TiO₂@C with high content of carbon also indicates an excellent rate capability of 240, 205, 148 mA h g⁻¹, at 100, 200, 500 mA g⁻¹, respectively. Compared with some previous literatures on TiO₂,^33–35 the as-synthesized TiO₂@C samples in our work has some advantages: our samples show a higher discharge capacity than that of the TiO₂@C samples synthesized by Guo group with a discharge capacity of 236 mA h g⁻¹, although it indicated a superior rate capability than that of TiO₂@C here; our samples show a longer cycle with a relatively stable capacity compared with the TiO₂@C prepared by Yang et al.³⁰

2. Experimental section

2.1 Materials

Tetrabutyl titanate (Ti(OC₄H₉)₄, TBT, 98%) was purchased from Sinopharm Chemical Reagent Co., Ltd, China. Oleic acid (C₁₈H₃₄O₂) was received from Tianjin Fengchuan Chemical Reagent technologies Co., Ltd, China. Ammonia solution (NH₄OH, 25%) was obtained from Tianjin Damao Chemical reagent Factory, China. All the chemicals of analytical grade were used as-received without any further purification.

2.2 Synthesis of TiO₂@C

In a typical synthetic process, 3 mL of TBT and 1 mL of oleic acid were first mixed uniformly. Then, 1 mL of ammonia solution was added into the mixture under stirring drop by drop. After that, the mixture was put into a quartz tube and annealed for 2 h at the temperature of 700 °C at a heating rate of 3 °C min⁻¹ in argon atmosphere. Finally, the sample (TiO₂@C-1) was obtained after cooling to room temperature. Likewise, TiO₂@C-2 was obtained by changing the volume of oleic acid to 0.5 mL, while the amount of other reagents kept the same.

2.3 Characterization

Scanning electron microscopy (SEM), transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) were performed to explore the morphology and structure of samples on a Hitachi S-4800 and a JEOL2010, respectively. To characterize the components of the as-synthesized samples, X-ray diffraction (XRD) characterization was performed by a Rigaku D max-2500. The carbon content of samples was calculated using Thermogravimetric analysis (TGA, Setaram 018124) at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C in air. The Brunauer–Emmett–Teller (BET) measurements were performed to evaluate the pore diameter distribution and specific surface area of two samples by Gemini VII2390 Automated Physisorption Analyzer.

2.4 Electrochemical measurements

The electrodes were prepared by mixing TiO₂@C with acetylene carbon black and carboxyl methyl cellulose (CMC), in a weight ratio of 8 : 1 : 1, respectively. Deionized water and ethyl alcohol were used as the mixed solvents for the mixture to form an electrode slurry. Then the flurry was uniformly coated on the copper foil, dried at 80 °C for 24 h, and cut into pole piece. The obtained pole pieces were dried again at 80 °C for 12 h in vacuum oven. The as-made working electrode were integrated into two-electrode CR 2032 coin-type cells for electrochemical measurements. Li metal and porous polypropylene film worked as the counter and reference electrode and the separator, respectively, and the electrolyte consisted of 1 M LiPF₆, ethylene carbonate and diethyl carbonate in a 1 : 1 : 1 by volume. An automatic battery tester system (Land®, China) was used to measure the obtained battery's properties. Galvanostatic charge–discharge was measured in the voltage range of 0–3 V, with a current density of 100 mA g⁻¹, as well as specific capacity at various current densities.

3. Results and discussions

The X-ray diffraction (XRD) characterization was employed to confirm the structures and phase of the as-synthesized TiO₂@C. As shown in Fig. 1a, all the diffraction peaks of the as-synthesized TiO₂@C samples could be attributed to the anatase structure of TiO₂ (PDF #21-1272). TGA was applied to calculate the carbon content of the TiO₂@C.^36,37 As shown in Fig. 1b, the carbon contents of the TiO₂@C-1 and TiO₂@C-2 were estimated to be 21% and 12.5 wt%, respectively.

Fig. 1 (a) XRD patterns of TiO₂@C-1 and TiO₂@C-2; (b) TGA curve of TiO₂@C-1 and TiO₂@C-2.

The SEM, TEM, HR-TEM were performed to explore the morphology and structure of the as-prepared TiO₂@C-1 and TiO₂@C-2 samples.³⁸ As shown in Fig. 2a, b, e and f, both of TiO₂@C-1 and TiO₂@C-2 are composed of spheres analogue, and it could be seen that the diameter of TiO₂@C-1 (1000 nm) is larger than that of TiO₂@C-2 (500 nm), respectively. In TEM images (Fig. 2c, d, g and h), one can find that carbon layer can be seen to be coated in the surface of TiO₂ nanospheres. In Fig. 2d and h, it can be clearly seen that there is a lattice distance of 0.33 nm between layers corresponding to the (101) plane of TiO₂.

Fig. 2 (a and b) SEM images, (c) TEM image and (d) HR-TEM image of TiO₂@C-1; (e and f) SEM images, (g) TEM image and (h) HR-TEM image of TiO₂@C-2.

To explore the pore diameter distribution and specific surface area of the TiO₂@C-1 and TiO₂@C-2, the BET analysis was characterized. As shown in Fig. 3b, an emblematic hysteresis loop is observed in the TiO₂@C-2 sample, while the hysteresis loop is very narrow in the TiO₂@C-1 (Fig. 3a), which indicates that the TiO₂@C-2 sample owns relatively higher specific surface area. TiO₂@C-1 and TiO₂@C-2 reveal the specific surface areas of 2.7964 m² g⁻¹ and 30.4168 m² g⁻¹, respectively. As shown in the inset of Fig. 3a and b, TiO₂@C-1 exhibits a larger pore diameter than TiO₂@C-2.

Fig. 3 N₂ adsorption–desorption isotherm of (a) TiO₂@C-1 and (b) TiO₂@C-2 (inset of each plot: pore size distribution).

To study the electrochemical performance of the TiO₂@C-1 and TiO₂@C-2, constant current charge–discharge testing was examined. Fig. 4a and b show the charge–discharge voltage profiles of the TiO₂@C-1 and TiO₂@C-2 for 1^st, 2^nd, 3^rd cycles in 0.01–3 V voltage range at 100 mA g⁻¹, respectively. However, it is obvious that a great loss of initial capacity in the 1^st cycle for both the TiO₂@C-1 and TiO₂@C-2. As shown in Fig. 4a and b, the TiO₂@C-1 and TiO₂@C-2 electrodes show the initial discharge/charge capacities of 485/226 and 168/77 mA h g⁻¹, respectively, with a coulombic efficiency (the ratio of charge capacity to discharge capacity) of 46.6% and 45.8%, respectively. The sharp capacity decrease in the initial cycle is mainly attributed to the irreversible process of Li ions inserting into the TiO₂ lattice³⁴ and the formation of a SEI layer along with the irreversible consumption of electrode materials.^14,39 Besides, the coulombic efficiencies in the second and third cycle are almost 100%, which indicates that the electrochemical process is quite stable during the later discharge/charge reactions. Fig. 4c shows the cycling performance and coulombic efficiency of the TiO₂@C-1 and TiO₂@C-2 electrodes. Obviously, the TiO₂@C-1 electrode presents a higher specific capacity than the TiO₂@C-2 electrode. The initial discharge capacities of the TiO₂@C-1 and TiO₂@C-2 electrodes at 100 mA g⁻¹ are 485.6, 168.3 mA h g⁻¹, dropping to 240.0, 98.5 mA h g⁻¹ in the second cycle, respectively. After 200 cycles, the discharge capacities of TiO₂@C-1 and TiO₂@C-2 electrodes still remain at 286.5, 211.6 mA h g⁻¹, respectively. The coulombic efficiency of the TiO₂@C-1 and TiO₂@C-2 electrodes are both around 100% during the whole cycle periods, showing stably reversible lithium diffusion kinetics. The discharge capacities drop from the first cycle to the second cycle, while then slowly increase and reach about 280, 200 mA h g⁻¹ for TiO₂@C-1 and TiO₂@C-2, respectively. As shown in Fig. 4d, when the current density increases from 100 mA g⁻¹ to 1000 mA g⁻¹ and decreases back to 100 mA g⁻¹, the TiO₂@C-1 presents the higher average discharge capacities of 240, 205, 148, 95 and 275 mA h g⁻¹, compared with the TiO₂@C-2 electrodes of 120, 88, 41, 21 and 123 mA h g⁻¹, at 100, 200, 500, 1000 and 100 mA g⁻¹, respectively. Obviously, the TiO₂@C-1 electrode indicates excellent rate performance, remaining a discharge capacity of 148 mA h g⁻¹ even when the current density reaches 500 mA g⁻¹. As can be seen in Fig. 4c and d, the TiO₂@C-1 electrode presents a much higher specific capacity and rate performance than the TiO₂@C-2 electrode. The better electrochemical performance of TiO₂@C-1 are possibly attributed to the higher carbon content. A higher carbon content means that the surface of TiO₂ crystal is coated by more carbon, which contributes to a more stable structure that can effectively restrain from dramatic volumetric change. Besides, more coating carbon can facilitate the electronic conductivity of the TiO₂ electrode, which results in a higher electrochemical activity.

Fig. 4 Discharge–charge profiles of (a) TiO₂@C-1 and (b) TiO₂@C-2 at the 100 mA g⁻¹; (c) cycling performance and coulombic efficiency of TiO₂@C-1 and TiO₂@C-2 at 100 mA g⁻¹; (d) rate performance of TiO₂@C-1 and TiO₂@C-2 at 100 mA g⁻¹, 200 mA g⁻¹, 500 mA g⁻¹, 1000 mA g⁻¹ and 100 mA g⁻¹, respectively.

4. Conclusions

In summary, we have successfully synthesized TiO₂@C with different carbon content by a facile and simple method. The as-synthesized TiO₂@C samples are composed of nanospheres analogue with different diameters. TiO₂@C-1 (with more carbon content) reveals a much higher specific capacity and rate performance than TiO₂@C-2 as the anode material for lithium ion batteries. The TiO₂@C-1 electrode maintains a discharge capacities of 286.5 mA h g⁻¹ at 100 mA g⁻¹ after 200 cycles, and the discharge capacity can remain at 148 mA h g⁻¹ even at 500 mA g⁻¹. The as-synthesized TiO₂@C nanospheres are promising anode materials for next-generation LIBs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51302079 and 21401140).

Notes and references

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