Elsevier

Journal of Power Sources

Volume 281, 1 May 2015, Pages 362-369
Journal of Power Sources

Carbon-coated anatase titania as a high rate anode for lithium batteries

https://doi.org/10.1016/j.jpowsour.2015.02.011Get rights and content

Highlights

  • Nanostructured TiO2 nanorods and nanowires are synthesized via a hydrothermal reaction.

  • Carbon coating enhances electric conductivity of nanostructured TiO2 to ∼10−1 S cm−1.

  • Conducting carbon layer greatly improves electrochemical property of nanostructured TiO2.

Abstract

Anatase titania nanorods/nanowires, and TiO2(B) are synthesized via a hydrothermal reaction of commercial TiO2 (P-25) in strong alkaline environment. Surfaces of these products are modified by carbon to improve the electrical conductivity through carbonization of pitch as the carbon source at 700 °C for 2 h in an Ar atmosphere. Even after carbon coating, the resultants exhibit the same crystal structure and morphology as confirmed by Rietveld refinement of x-ray diffraction data and transmission electron microscopic observation that the images display thin carbon coating layers on the surfaces of anatase nanorods and nanowires. Although the bare and carbon-coated anatase TiO2 nanorods exhibit stable cycling performance, the high rate performance is highly dependent on the presence of carbon because of high electrical conductivity, ∼10−1 S cm−1, enabling Li+ ion storage even at 30 °C (9.9 A g−1) approximately 100 mAh (g-TiO2)−1 for the carbon-coated anatase TiO2 nanorods. Besides, the bare and carbon-coated anatase TiO2 nanowires show poor electrode performances due to their large particle size and high crystallinity causing Li+ insertion into the host structure difficult. It is believed that the conducting carbon coating layers greatly improves the electrochemical property through the improved electrical conductivity and shortened diffusion path.

Introduction

Lithium-ion batteries (LIBs) are applicable to various electronic devices. In particular, a combination of high power and safety is essential to rechargeable LIBs for energy storage system (ESS), electric vehicles (EVs) and hybrid electric vehicles (HEVs) [1]. Commercial LIBs commonly use graphite as the anode material. The graphite electrode, however, has several disadvantages such as its electrical disconnection, structural deformation and initial loss of capacity [2], [3], [4]. To solve the above-mentioned demerits, transition metal oxides have been widely investigated by numerous research groups. Among them, TiO2 has been paid much attention as anode materials for LIBs because it operates in the voltage range of 1.6–1.9 V vs. Li/Li+ accompanied by insertion and extraction of Li+ ion [5]. These characteristics render it a potential anode material for high power lithium ion batteries, avoiding the necessity of a passivation layer at the contact to the liquid electrolyte [5], [6], [7], [8].

Interestingly, TiO2 has various polymorphs such as rutile [9], brookite [10], anatase [11], [12], [13], TiO2(B) [14], etc. Under standard conditions, rutile is the thermodynamically most stable structure of TiO2, and is also the most common natural form [2]. In particular, anatase phase is capable of electrochemical Li+ ion storage showing a voltage plateau (1.6–1.9 V vs. Li/Li+), where the lithium intercalation is controlled by the diffusion of lithium ion. Also, nanostructured anatase TiO2 are believed as promising materials for high rate Li+ storage because of their large specific surface area and suitable structure for lithium intercalation [1], [4], [11], [15], [16], [17], [18]. TiO2(B) is more open structure than rutile, anatase or brookite, with significant voids and continuous channels that are capable of intercalation [14].

In this work, anatase TiO2 nanorods/nanowires and TiO2(B) nanowires are synthesized via a hydrothermal reaction in strong alkaline environment. Also, we attempt to carbon coating on the surface of nanostructured particles to improve their poor electric conductivity. The produced TiO2 are tested for Li+ insertion or extraction as high power anode materials.

Section snippets

Synthesis

To obtain nanorods and nanowires-shaped of TiO2, TiO2 powders (0.2 g, P-25, Degussa) were dispersed in 10 M NaOH aqueous solution (40 ml), and then transferred to a Teflon-lined stainless steel autoclave (50 ml). The reaction time was set for 11 h for nanorods and 48 h for nanowires at 170 °C [19]. After the hydrothermal reactions the white precipitates were washed by distilled water till pH 7 and were dried at 80 °C overnight in air. The dried powders, presumably Na2Ti3O7 nanorods and

Results and discussion

According to our previous report [19], the formation of anatase nanorods and nanowires is greatly dependent on reaction time under the hydrothermal environment, so that longer reaction leads to the formation of nanowires through merging of smaller nanorods. The starting TiO2 particles dissolve in the NaOH solution during the hydrothermal reaction that breaks the Ti–O bond from TiO2. This leads to immediate formation of low crystalline Na2Ti3O7 lamellar sheet. Since the bond strength of Na–O is

Conclusion

Anatase TiO2 nanorods and nanowires are successfully synthesized via a hydrothermal reaction. Carbon coating on the nanostructured materials is also attempted by carbonization of pitch in Ar atmosphere. The carbon coating does not affect structural and morphological changes but enables enhancement of electric conductivity significantly as high as 1.8 × 10−1 S cm−1 from 6.9 × 10−6 S cm−1 for nanorods. As a result, the carbon-coated anatase TiO2 nanorods show outstanding cycle performance and

Acknowledgments

This research was partly supported by a grant from the National Research Foundation of Korea by the Korean government (MEST) (NRF-2009-C1AAA001-0093307). This work was also supported by MKE/KEIT (10041856 and 10041094). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2014R1A2A1A11051197).

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