Elsevier

Journal of Power Sources

Volume 302, 20 January 2016, Pages 259-265
Journal of Power Sources

Carbon-wrapped TiO2 nanocubes exposed with (001) active facets for high-rate and long-life lithium-ion batteries

https://doi.org/10.1016/j.jpowsour.2015.10.072Get rights and content

Highlights

  • Show a novel method to synthesis TiO2/carbon nanostructured composites (T/CNC) exposed (001) active facet as anode material.

  • At 0.59 C (100 mA g−1), the remained capacity after 90 cycles is superior to the previously reported results.

  • At 15 C (2550 mA g−1), a high reversible discharge capacity of 126.7 mA h g−1 is obtained up to 500 cycles.

  • The unique nanostructure demonstrates excellent rate performance and long-term cycling stability.

Abstract

A novel method is developed to synthesis TiO2/carbon nanostructured composites (T/CNC) as high-performance anode materials for Li-ion batteries using resorcinol/formaldehyde (RF) as the carbon source. With assistance of hydrofluoric acid (HF) as a shape-controlling agent, TiO2 nanocubes exposed with (001) active facets can be obtained. The T/CNC sample shows higher BET surface area (358 m2 g−1) than the previous materials reported before. When evaluated for lithium storage properties, the T/CNC is able to deliver a capacity of 340.4 mA h g−1 after 90 cycles at 0.59 C, which is superior to those reported in previous work. At a high rate of 15 C, a high reversible discharge capacity of 126.7 mA h g−1 is obtained up to 500 cycles and demonstrates excellent rate performance and long-term cycling stability.

Introduction

With the development of portable electronics, electric vehicles (EVs) and hybrid electric vehicles (HEVs), there is increased demand for lithium ion batteries (LIBs) with better performances for higher-energy storage density, safety and long cycle life [1], [2], [3], [3](a), [3](b). Among the myriad electrode materials that have been studied, titanium dioxide (TiO2) has been intensively studied as one of the most prominent anode materials because of the greatly improved safety and high rate capability [4].

However, the low ionic diffusivity and electrical conductivity lower its reversible capacity and limit its rate performance, which hinders its practical application [5], [6]. Enormous attempts have been explored within roughly three strategies, tuning the shape at the nanoscale (nanosheet, nanotube, hollow sphere, nanocage, etc) [7], [8], [9], [10], element doping (nickel, boron, etc) [11], [12] as well as compositing with carbonaceous materials [13], [14], [15], [16].

Recently, the facet orientation of nanocrystalline particles has been shown to be critical in determining the performance of lithium battery electrodes. Special crystal facets can play the role of easy access for lithium ions, while single-crystalline structure can offer clear passages for lithium ion transfer. Many works focus on layered cathode materials have demonstrated that materials expose active facets can significantly improve the electrochemical performance [17], [18], [19]. To date, however, a few works on anode materials with active surface planes have been focus on. Therefore, to design and synthesis anode materials with active facets exposure remains highly desired.

Anatase titanium dioxide (TiO2) with exposed (001) active planes has been found to manifest enhanced lithium storage. The (001) surface has lower barriers of both electron migration and lithium ion insertion across this facet in comparison with other crystal facet [20], [21], [22]. Nevertheless, the (001) facets of anatase TiO2 have higher surface energy than (101) facets through calculation [23]. Those high-energy (001) facets are more likely to form during the earliest stages of crystal growth and quickly eliminated during further growth. Thus, growth of TiO2 crystals with the (001) active facets exposure is extremely difficult. Lou et al. [23] reported TiO2 hollow spheres with large amount of exposed (001) facets for lithium storage retained a discharge capacity of 148 mA h g−1 after 200 cycles at the rate of 1 C. Despite the progress that has been made, there are still some shortcomings for lithium storage such as poor rate capacity and poor cycle stability. So there is still room to improve its electrochemical properties. To the best of our knowledge, there are no reports on the synthesis of carbon-wrapped TiO2 nanocubes with (001) active facet exposing as anode materials.

Inspired by the above elaboration, herein we developed a novel approach for the preparation of TiO2/carbon nanostructured composites (T/CNC). Resorcinol/formaldehyde (RF) as the carbon source, hydrofluoric acid (HF) as a shape-controlling agent, after hydrothermal reaction and calcination, which were assembled into carbon wrapped single crystal TiO2 nanocubes exposed with (001) active facet. The T/CNC exhibits higher BET surface area than the previous materials reported before and manifests excellent electrochemical lithium storage properties with high capacity, outstanding rate capacity and good cycling stability. A superior capacity up to 483.5 mA h g−1 is achieved at a current rate of 0.59 C (1C = 170 mA g−1) for the first cycle. At a high current rate of 15 C (2550 mA g−1), a high reversible discharge capacity of 126.7 mA h g−1 is obtained after 500 charge–discharge cycles.

Section snippets

Synthesis of resorcinol/formaldehyde (RF) sphere

The method is similar as the work our group reported before [24]. RF spheres were synthesized in basic solution at room temperature using ethanol and water as cosolvents, cetyltrimethyl ammonium bromide (CTAB) as surfactant, resorcinol/formaldehyde (RF) as precursors, and ammonia as catalyst. Typically, ammonia aqueous solution (NH3·H2O, 0.2 mL) and resorcinol (0.2 g) were first mixed with a solution containing 8 mL anhydrous ethanol (C2H5OH) and 20 mL deionized water (H2O). After the mixture

Results and discussion

Field-emission scanning electron microscopy (FESEM) is used to determine the morphology of the T/CNC sample (Fig. 1a–b). In the FESEM image, we can observe that carbon layers wrapped the TiO2 nanocubes. These also can be confirmed from the transmission electron microscopy (TEM) images (Fig. 2a–b). The TiO2 nanocubes are regular and around 90 nm (Fig. 1b in the white box). The structures of the T/CNC sample are further elucidated by TEM. When during the hydrothermal reaction without carbon

Conclusions

In summary, TiO2/carbon nanostructured composites (T/CNC) exposed (001) active facet are successfully synthesized by a novel method. In virtue of the unique structural features, these T/CNC materials are achieved to deliver a high lithium storage capacity up to 483.5 mA h g−1 at the current rate of 0.59 C for the first cycle, remarkable rate performance (126.7 mA h g−1 after 500 cycles at a current density of 15 C) and long-term cycling stability as anodes for LIBs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21371023) and Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20101101110026).

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