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Insight into the Origin of Capacity Fluctuation of Na2Ti6O13 Anode in Sodium Ion Batteries Chunjin Wu, Zhen-Guo Wu, Xiaobing Zhang, Ranjusha Rajagopalan, Benhe Zhong, Wei Xiang, Mingzhe Chen, Hongtai Li, Tingru Chen, Enhui Wang, Zuguang Yang, and Xiaodong Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11507 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017
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ACS Applied Materials & Interfaces
Chunjin Wu†, Zhen-Guo Wu*,†, Xiaobing Zhang‡, Ranjusha Rajagopalan§ , Benhe Zhong †, Wei Xiang#, Mingzhe Chen§ , Hongtai Li£, Tingru Chen†, Enhui Wang†,§ , Zuguang Yang†, Xiaodong Guo*,† †
School of Chemical Engineering, Sichuan University, Chengdu, 610065, PR China
‡
Chongqing Natural Gas Purification Plant, Petrochina Southwest Oil & Gasfield Company, Chongqing, 401220, PR China §
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollogong, NSW 2522, Australia #
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, PR China £
Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, PR China
KEYWORDS: sodium ion battery, titanium-based anode, capacity fluctuation, pseudocapacitance, metallic titanium. ABSTRACT: The capacity fluctuation phenomenon during cycling, which is closely related with solid electrolyte interphase and plays a key role for the design for advanced electrode, could be frequently observed in the titanium-based anode. However, the underlying reason for capacity fluctuation still remains unclear with rare related reports. Here, the origin of capacity fluctuation is verified with a long-life Na2Ti6O13 anode. The reaction mechanism, structural evolution and reaction kinetics during the reported sodiation/desodiation processes were carefully investigated. The gradually enhanced diffusion controlled contribution resulted in the capacity increasing. And the capacity decay could be ascribed to the irreversible reaction of metallic titanium formation and the increasing potential polarization. It is worth noting that sodium ions seem to partially reduce NTO to metallic state, which is irreversible. The present study can provide more information for the design of advanced Na2Ti6O13 anode.
1.INTRODUCTION The increasing demands for green and sustainable energy sources direct the research to focus on renewable energy storage devices that are inexpensive, better safe and environmentally benign. Lithium-ion batteries (LIBs), which have achieved a great success, could not be the suitable tactics for the large-scale energy storage application, when it comes to the abundance and cost. 1,2 Room temperature sodium-ion batteries (SIBs) have been considered to be a potential candidate, owing to its inexhaustible resources and cheaper price. In the past few years, the cathode materials have attracted more attentions and made a significant development.3,4 As another important part of SIBs, the anode electrodes also need an urgent development. Despite continuous efforts in developing proper new anode electrode materials, such as carbon-based materials, 5-7 alloy materials, 8, 9 metal oxide compounds, 10, 11 metal sulfides materials, 12, 13 and titanium-based compounds, 14-16 the anode electrode materials with better stability, faster sodium insertion/desertion rate, and longer life are still very challenging. As a delegate of sodium titanates, Na2Ti6O13 (NTO) was recently reported as the anode material for SIBs due to its excellent cycling stability and rather low voltage platform. 17,18 During the cycling test, obvious capacity fluctuation could be observed. In general, the behavior of capacity fluctuation could also be observed in the electrode materials such as SnFe3O4@Graphite,19 Na2Ti4O9,20 Na0.23TiO2 and Na0.46TiO2.21 Usually, the reason is associated with formation/dissolution of SEI film and an activation process. Recently, Wu et al.22 and co-workers claimed that the origin of the capacity fluctuation of anatase TiO2 was quite different with the tradi-
tional view. The reason of capacity fluctuation could be ascribed to the disproportionation reaction that sodium ions reduced the anatase TiO2 to form metallic titanium, sodium oxide and amorphous sodium titanate. Similarly, Kim et al,23 also proposed that the disproportionation reaction and the formation of metallic titanium was verified by X-ray absorption spectroscopy. Besides, Kalubarme et al.24 also reported that the capacity fluctuation coincided with the reduction of NiTiO3 to the formation of Ni, Ti and Na2O. Though Cao et al.25 and Zhang et al.26 had observed such obvious capacity fluctuation of NTO, the underlying reaction mechanism of reversible sodium storage and the origin of capacity fluctuation still remain unclear. In the present study, NTO was fabricated with simple solid-state method and served as anode of SIBs. To elaborate the detailed origin of capacity fluctuation, the reaction mechanism, structural evolution and reaction kinetics of NTO electrode material were investigated in-depth. The obtained results indicated that the change of capacitive/diffusion controlled capacity contribution resulted in capacity fluctuation. The results also demonstrated that sodium could partially reduce NTO to metallic state. The irreversible reduction process was associated with the decrease of capacity. 2. EXPERIMENTAL SECTION 2.1 Preparation of NTO All the chemicals used in this study were obtained from Sinopharm and used without further purification. The NTO material was synthesized via simple solid-state method. Where, a stoichiometric proportion of anatase TiO2 and
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Na2CO3 (1:6), was mixed for 2 h with ethanol as solvent. The obtained mixture was dried at 80 ̊C overnight. The dried powder was annealed at 650°C, 750 ̊C, 850°C for 24 h in air, respectively. In order to improve the purity of the NTO materials, the obtained powder was grounded and sintered again using the same temperature program sequence (5°C/min).
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group of C2/m without impurity. In order to further understand the crystal structure characteristics, Rietveld refinement of XRD pattern was carried out (Figure 1a). And the relevant crystal parameters obtained from the refinement analysis were listed in Table S1, which are in good agreement with some previous reports.17 Inductive Coupled Plasma Emission Spectrometer (ICP) analysis was performed to determine the composition of synthesized NTO. The result of ICP measurement showed that the chemical formula of NTO was Na2Ti6.24O13 (Table S2).
2.2 Characterization and Electrochemical Measurements The morphologies and structures of the as-prepared and cycled NTO samples were characterized by field emission scanning electron microscopy (SEM, HITACHI S-4800), transmission electron microscopy (TEM, JEOL 2100F), and powder X-ray diffraction (XRD, Panalytical EMPYREAN) analysis. The XRD data were collected using Cu Kα radiation in the 2θ range of 10-80° and refined by Rietveld method using PDXL software (Rigaku Co., Ltd., PDXL 2.1). X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo ESCALAB 250XI instrument. And Ar+ etching was performed by applying a voltage of 2000 eV for 120 s. Raman measurements (Thermo DXRxi) were conducted with a green semiconductor laser (532 nm). FTIR instrument (Bruker R200-L spectrophotometer) were used to detect bending and stretching vibrations of the band in the different number of cycles.
The morphology of the as-prepared NTO sample was characterized by SEM and TEM analysis. Figure 1b presents the SEM image of well dispersed NTO samples. The morphology shows the homogeneous nature of the sample without any agglomeration. The higher magnification SEM image (Figure S1b) exhibits that the surface of NTO samples could be clean and smooth. The samples show rod-like morphology, with a width of 100200 nm and a length about 0.41 µm, which is in good agreement with the TEM image (Figure 1c). The diameter of as-prepared NTO sample is observed to be much wider than in the reported results,25,27 which may block the bulk diffusion of Na+ and cause pseudocapacitive contribution. The High Resolution TEM (HRTEM) image (Figure 1d) and the corresponding Fast Fourier Transformation (FFT) pattern (inset of Figure 1d) clearly demonstrates the high crystallinity of the NTO sample, with an inter-lattice spacing of 0.296 nm, corresponding to the (-203) plane of NTO.
The electrochemical performances of NTO electrode were evaluated by coin cells (type CR2025) assembled in an argon-filled glove box (O2 and H2O levels
