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Understanding the Behavior and Mechanism of OxygenDeficient Anatase TiO2 toward Sodium Storage Weigang Wang, Meng Wu, Peng Han, Yu Liu, Liang He, Qinghong Huang, Jing Wang, Wensheng Yan, Lijun Fu, and Yuping Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19288 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018
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ACS Applied Materials & Interfaces
Understanding the Behavior and Mechanism of Oxygen-Deficient Anatase TiO2 toward Sodium Storage
Weigang Wanga,#, Meng Wuc,# , Peng Hand, Yu Liua, Liang Hea, Qinghong Huanga, Jing Wanga, Wensheng Yane, Lijun Fua,b* and Yuping Wu a,b* aState
Key Laboratory of Materials-Oriented Chemical Engineering, College of Energy Science and Technology and Institute of Advanced Materials , Nanjing Tech University, No. 30, Puzhu Road (S), Nanjing, Jiangsu, 211800, China bSouth China Normal University, No. 55, West Zhongshan Road, Tianhe District, Guangzhou, Guangdong, 510631, China cFujian Provincial Key Laboratory of Semiconductors and Applications, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen, 361005, China dCapital Normal University, 05 West Third Ring Road North, Haidian District, Beijing, 100048 China eNational Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, 230029, China *Corresponding
authors. E-mail addresses:
[email protected] (L.Fu);
[email protected] (Y. Wu) #The authors contribute equally to this work. KEYWORDS: Storage mechanism; Titanium dioxide; Graphene; Oxygen-deficient; Local and electronic structures
ABSTRACT:
TiO2 has drawn increasing research attention as negative electrode material in sodium ion battery because of its natural abundance, low cost, nontoxicity and facile preparation. Despite of tremendous studies carried out, the sodium storage mechanism is still under discussion, and the electronic and local structures of TiO2 during sodiation/desodiation process are not well understood either. Herein, we reported a mechanism study of graphene supported oxygen-deficient anatase TiO2 nanotubes 1
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(nanowires) as the negative electrode material for sodium ion batteries. Different from the previous reports, the insertion/extraction of Na+ ions leads to almost no changes of titanium valence state, but a charge redistribution of O 2p orbitals which alters the hybridization between O 2p and Ti 3d states, suggested by the combined electrochemical and X-ray spectroscopic study. Both the electronic and local structures of TiO2 during the reversible sodiation/desodiation process are revealed from the Ti L-edge and O K-edge spectra. This detailed study would shed light on the material design and structural optimization of TiO2 as energy storage material in different systems.
1. INTRODUCTION
Lithium-ion batteries (LIBs) are regarded as one of the most popular electrochemical energy storage systems1 and have been widely used in portable electronic equipment, electric vehicles and smart grid storage, due to its efficient energy storage and environment benignity.2 With the continuous expansions of lithium-ion battery applications, the demand for lithium has increased dramatically.3, 4 Since the global lithium reserves are limited and unevenly distributed, the security and cost issues would hinder the development of lithium ion battery market in the future from a long-term perspective.5, 6 In this context, sodium ion batteries (NIBs) have attracted great attention as an alternative since sodium is rich in the earth crust and uniformly distributed.7, 2
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8
In
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ACS Applied Materials & Interfaces
addition, the redox potential of sodium metal is only a bit higher (-2.7 V vs. SHE) than that of lithium (-3.0 V vs. SHE). Therefore, many negative electrode materials of lithium ion batteries can be used in sodium ion batteries, such as carbon-based materials (hard carbon,9 graphene10), MoS2,11 Sn-based materials12 (SnSx, SnOx),13 alloying materials14,15,16 (Sb, Sn, and P) and so on. TiO2 is one of the promising negative electrode materials for NIBs, because of its natural abundance, low cost, easy preparation and non-toxicity.17 However, TiO2 is a semiconductor, which exhibits poor electronic conductivity, resulted in poor electron transfer during the electrochemical reaction.18 This behavior is similar with a majority group of electrode materials, thus TiO2 has been regarded as a typical electrode material for fundamental battery research.5, 19, 20 TiO2 exhibits different crystal polymorphs, including anatase,21 rutile,22 TiO2-(B)23 and so on, among which, anatase TiO2 is most widely studied as negative electrode for sodium ion batteries. Compared with other crystal forms, anatase TiO2 presents relatively low energy barrier during sodiation process and stable structure during cycling.24 Though low energy barrier, the diffusion of sodium ions in anatase TiO2 is still sluggish, due to the larger ionic radius of sodium ion. Both the poor electronic (as aforementioned) and ionic conductivities lead to slow kinetics of reversible sodium storage in anatase TiO2. To address this problem, many efforts have been made, including design of nanostructured anatase TiO2,25,
26
incorporation of electronic
conductive materials,27 and defect introduction, etc.19 Among them, defect introduction via generating oxygen vacancy and introducing heteroatoms, has been 3
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proved to be an effective strategy to improve the sodium storage performance, since the electronic conductivity28, 29 and ionic diffusion kinetics can be enhanced.30 The electrochemical performance of anatase TiO2 as negative electrode has been improved efficiently via the aforementioned modification methods, yet challenges remain in the aspects of fundamental understanding and its real application. Different sodation processes have been reported in literatures,31 including insertion reaction accompanied with a redox reaction of Ti4+/Ti3+,32 conversion reaction during which anatase TiO2 turned to sodium titanate phase, metallic titanium and NaO2 upon sodiation,33 and pseudocapacitive storage during which the reversible surface reaction is significant.34 In addition, the first coulombic efficiency of anatase TiO2 is quite low, ranging from 20% to 50%,31, 35, 36 which might be ascribed to the decomposition of electrolyte and irreversible Na trap in the anatase TiO2 host. Despite of some fundamental investigations carried out,3,
27, 37
the electronic and local structures of
TiO2 during sodiation/desodiaiton process are still not well understood yet, which is essential to modify the TiO2 material and further improve its electrochemical performance. In this study, oxygen-deficient anatase TiO2 nanotubes (wires)/reduced graphene oxide (rGO) composites (GTNs-OD) were prepared and used for the mechanism study as negative electrode material for Na-ion batteries. The electronic and local structures of TiO2 during sodiation/desodiaiton process are studied in detail. Investigations via high resolution transmission electron microscope (HRTEM), in situ X-ray diffraction (XRD) indicate that an insertion reaction occurs during the sodiation 4
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ACS Applied Materials & Interfaces
process of GTNs-OD. Different from the previous reports, the valence state of titanium barely varies during the charge and discharge processes. In addition, both the local
valence
and
electronic
structures
of
TiO2
during
the
reversible
sodiation/desodiation process are revealed based on the analyses of X-ray absorption spectra (XAS).
2. EXPERIMENTAL SECTIONS 2.1 Materials and reagents All chemical reagents in this work including commercial anatase TiO2 (
