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Iron doped cauliflower-like rutile TiO with superior sodium storage properties Hanna He, Dan Sun, Qi Zhang, Fang Fu, Yougen Tang, Jun Guo, Minhua Shao, and Haiyan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15516 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 30, 2017
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Iron doped cauliflower-like rutile TiO2 with superior sodium storage properties Hanna Hea, Dan Suna, Qi Zhang a, Fang Fub, Yougen Tang a, Jun Guoc, Minhua Shaob*, and Haiyan Wanga,b* a
College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, P.R
China. b
Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong c
School of Chemistry and Materials Science, Guizhou Normal University, Guiyang, 550001, P.R
China.
ABSTRACT Developing advanced anodes for sodium ion batteries is still challenging. In this work, Fe-doped threedimensional (3D) cauliflower-like rutile TiO2 was successfully synthesized by a facile hydrolysis method followed by a low temperature annealing process. The influence of Fe content on the structure, morphology and electrochemical performance was systematically investigated. When utilized as sodium ion battery anode, 6.99%-Fe-doped TiO2 exhibited the best electrochemical performance. This sample delivered a very high reversible capacity (327.1 mAh g-1 at 16.8 mA g-1) and superior rate performance (160.5 mAh g-1 at 840 mA g-1), as well as long term cycling stability (no capacity fading at 1680 mA g-1 over 3000 cycles). Density functional theory (DFT) calculations combined with experimental results indicated that the significantly improved sodium storage ability of the Fe-doped sample should be mainly due to the increased oxygen vacancies, narrowed band gap and lowered
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sodiation energy barrier which enabled much higher electronic/ionic conductivities and more favorable sodium ion intercalation into rutile TiO2. KEYWORDS: sodium ion battery ·Fe-doped TiO2 ·oxygen vacancies ·rate performance· density functional theory calculations INTRODUCTION Emerging as a state-of-the-art battery technology, sodium ion battery has drawn increasing attention due to the abundance of sodium reserve and low cost. It can effectively address the issues of lithium source shortage and rising cost for lithium ion batteries.
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Nowadays, tremendous efforts have been
made to develop high performance electrode materials and matched electrolytes for sodium ion batteries. Several types of cathode materials including sodium transition-metal oxides,3-4 polyanionic phosphates5 and NASICON framework compounds6-7 have been investigated. Graphite, the most successfully commercial anode material in lithium ion batteries, exhibits very poor sodium storage ability.8 Alternatively, alloy-type materials (such as Na15Sn4,9 Na3Sb10 and Na3P11) together with conversion-type (such as metal oxides and sulfides12) have been proved as high capacity anodes, but the huge volume change during cycling limits their applications. Therefore, developing a robust and high-capacity anode material is still a big challenge for constructing advanced sodium ion batteries. Titanium dioxides (TiO2) with a network structure formed by the stacking and edge-sharing of TiO6 octahedra, can provide possible interstitial sites for sodium ion accommodation and suitable sized pathways for its diffusion. Recently, TiO2 with various polymorphs (e.g, amorphous TiO2,13 TiO2-B,14 TiO2-H,15 anatase
16
and rutile TiO2 17) have been widely reported as sodium ion battery anode and
exhibited different sodium storage performances due to their different tunnel structures. Among these polymorphs, rutile phase possesses good cycling stability because of its robust structure. However, compared to the most studied anatase TiO2, high sodium storage performance of rutile TiO2 was rarely reported probably due to the kinetically slow ion diffusion in this structure.17 Moreover, its large band gap (3.0 eV) resulted in a very low electronic conductivity, leading to an inferior rate capability.18
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Enormous efforts, including nanostructuring and morphology optimizing,19 conductive carbon coating20 and heteroatoms doping17 have been devoted to solving the shortcomings of rutile TiO2. Among these methods, doping has been expected to be a direct and efficient approach to improve the intrinsic conductivity of TiO2 thus rendering an enhanced sodium storage performance. It is well believed that doping a suitable element with lower valence (e.g., Ni2+,21 Fe3+,22.) into rutile TiO2 could create abundant vacancies in the oxygen sub-lattice thus leading to an enhanced electronic conductivity. Particularly, Fe was demonstrated to be a promising dopant ion due to its nontoxic, natural abundance and the similar ionic radius with Ti.23-24 Fe-doped anatase TiO2 with highly improved lithium storage performance was firstly demonstrated by Shyamal et al.25 Very recently, Lai et al. found that Fe-doped anatase TiO2/C composite exhibited a much higher reversible capacity than pristine one in sodium ion batteries.26 Note that previous publications have not systematically investigated the origins of enhanced Li and Na storage properties upon Fe doping. In addition, it is not clear whether Fe doping has the similar effect on the rutile TiO2. In this work, we focus on improving the performance of rutile TiO2 anode for sodium ion batteries via Fe doping strategy. Density functional theory (DFT) calculation was applied to clarify the reasons of performance improvement. Carbon coating was not considered in this work to avoid any effect from the carbon that would complicate the mechanism analysis. To our knowledge, this is the first paper involving the sodium storage performance of Fe-doped rutile TiO2. By a facile hydrolysis method, we successfully synthesized Fe-doped cauliflower-like rutile TiO2, which exhibited significantly improved sodium storage properties in comparison with the pristine one. A very high discharge capacity of 327.1 mAh g-1 was demonstrated at 16.8 mA g-1 for the sample with 6.99% Fe doping, which is even higher than those of carbon coated rutile TiO2.27 Superior rate capabilities and cycling performance up to 3000 cycles were also obtained. RESULTS AND DISCUSSION
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Fig. 1 XRD patterns (a) and Raman spectra (b) of pristine TiO2 and iron doped TiO2 samples. Crystalline structures of pristine TiO2 and Fe-doped TiO2 samples were characterized by XRD. As shown in Fig.1a, the main diffraction peaks of all samples are almost identical, which can be well indexed to rutile TiO2 phase (JCPDS No.21-1276) with a space group of P42/mnm (136). Notably, no obvious diffraction peaks of Fe-based phases are observed, indicating that Fe was successfully doped into TiO2. It is well known that the ion radii of Ti4+ (0.68 Å) and Fe3+ (0.64 Å) are very close. Thus, Fe ions can be easily inserted into TiO2 crystal and probably occupy some of Ti4+ sites, forming an Fe-Ti solid solution during the sintering process.28-29 Meanwhile, the decrease of the interplanar spacing distance of TiO2 (111) plane (Table S1), as reflected by the shift of (111) diffraction peaks to a higher angle (Fig. S1), are resulted from the Fe replacement to Ti, which is in accord with previous reports.3031
Furthermore, the main diffraction peaks become weaker and broader with increasing the Fe content,
revealing the decreased crystallite size and crystallization of rutile TiO2 after Fe doping. The average crystallite sizes of all samples are calculated by the Scherrer formula and displayed in Table S1. Farhangi et al. reported that Fe doping could decrease the crystallization of TiO2 and also slightly restrain the growth of TiO2 crystallite.32 Raman spectra were used to further analyze the structure of pristine TiO2 and Fe-doped TiO2. Four typical Raman modes of rutile TiO2, i.e., B1g at 144.2 cm-1, Eg at 236.9 cm-1, Eg at 448.0 cm-1 and A1g at 610.6 cm-1, are clearly observed for all samples in Fig.1b.33 It is known that the Eg mode at 448.0
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cm-1 with the highest intensity corresponds to the symmetric stretching vibration of Ti-O-Ti in rutile TiO2.30 Generally, the substitution of Ti by Fe ion would generate oxygen vacancies thus weaken the symmetric stretching vibration of Ti-O-Ti, leading to decreased relative intensity of Eg to A1g. Obviously, the A1g mode shows no shift while the Eg (448.0 cm-1) mode slightly shifts to higher wavenumbers when increasing the Fe content. As suggested, blue shift of Raman spectra (Eg mode) would be caused by the finite size of grains (
