TiO2 nanocrystalline-assembled mesoporous nanosphere as high-performance anode for lithium-ion batteries

TiO2 nanocrystalline-assembled mesoporous nanosphere as high-performance anode for lithium-ion batteries

Accepted Manuscript TiO2 nanocrystalline-assembled mesoporous nanosphere as high-performance anode for lithium-ion batteries Y. Wu, Y.F. Yuan, F. Chen...

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Accepted Manuscript TiO2 nanocrystalline-assembled mesoporous nanosphere as high-performance anode for lithium-ion batteries Y. Wu, Y.F. Yuan, F. Chen, M. Zhu, G.C. Cai, S.M. Yin, J.L. Yang, S.Y. Guo PII: DOI: Reference:

S0167-577X(18)32053-6 https://doi.org/10.1016/j.matlet.2018.12.085 MLBLUE 25487

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

15 November 2018 13 December 2018 16 December 2018

Please cite this article as: Y. Wu, Y.F. Yuan, F. Chen, M. Zhu, G.C. Cai, S.M. Yin, J.L. Yang, S.Y. Guo, TiO2 nanocrystalline-assembled mesoporous nanosphere as high-performance anode for lithium-ion batteries, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.12.085

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TiO2 nanocrystalline-assembled mesoporous nanosphere as highperformance anode for lithium-ion batteries Y. Wu, Y.F. Yuan, F. Chen, M. Zhu, G.C. Cai, S.M. Yin, J.L. Yang, S.Y. Guo College of Machinery and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China

Abstract: Through a facile chemical route, TiO2 nanosphere with the size of around 230 nm is assembled by TiO2 nanocrystalline (about 10 nm) and possess abundant mesopores throughout the entire nanosphere, forming TiO2 nanocrystalline-assembled mesoporous nanosphere. The small size of TiO2 nanocrystalline and large specific surface area (153 m2 g-1) of the mesoporous nanosphere significantly boost reaction kinetics of TiO2, and enhance the bulk intercalation and the interfacial storage, which remarkably increases lithium storage capacity of TiO2. Average discharge capacity over 100 cycles at 1C reaches 219.2 mAh g-1, demonstrating the superiority of this material structure and its potential in lithium ion batteries. Key words: TiO2; Nanosphere; Porous materials; Energy storage and conversion 1. Introduction Lithium ion batteries (LIBs) as the main power sources have been widely applied for portable electronic devices, electric vehicles and other fields [1]. Recently, TiO2 have attracted intense interest as a promising LIBs anode candidate owing to the superior safety, environmental friendliness, low cost and abundance [2]. Nevertheless, TiO2-based electrodes usually suffer from the large polarization at high charge/discharge rates due to the sluggish Li+ diffusion and poor electron transport, which greatly limits practical application of TiO2 in LIBs. Nanosizing TiO2 can significantly enhance electrode reaction kinetics of TiO2 during lithiation/delithiation processes and increase lithium storage capacity of TiO2. Various low

Corresponding author. Tel.: +86-571-8684-3343

E-mail address: [email protected] (Y. F. Yuan) 1

dimensional TiO2 nanomaterials, e.g. nanorod [3], nanotube [4], nanosheet [5], have been synthesized to improve LIB performance. Nevertheless, the nanomaterials aggregate especially seriously during the synthetic and electrochemical test processes due to their especially high surface activity. To overcome this aggregation shortcoming, the porous and hollow TiO2 nanomaterials have been developed to take advantages of structural superiorities of nanosized building units and submicronsized assemblies. Walnut-like porous core/shell TiO2 [6], mesoporous anatase TiO2 [email protected] [7], TiO2 hollow nanostructures [8] exhibit the improved lithium storage performances. Therefore, nanosizing TiO2 combined the mesoporous or hollow structure is one effective way for the development of high-performance TiO2 anodes. Herein, we make use of hydrolysis polymerization reaction of titanium isopropoxide (TIP) and pore-creating effect of hexadecylamine (HDA) to synthesize TiO2 nanocrystalline-assembled mesoporous nanospheres (TiO2 NMNs). As anode material for LIBs, TiO2 NMNs show superior lithium storage performances, demonstrating the superiority of this material structure. 2. Experimental All chemicals are used as-received without any further purification. 0.1 g HDA is dissolved into 10 ml ethanol by ultrasonication for 10 min. 0.2 ml ammonia (NH3 H2O) is then added to the solution under stirring. Afterwards 0.1 ml TIP is slowly dropwise added to the dispersion under vigorously stirring. After reaction for 15 min, the product of hydrous titania/HDA nanospheres is collected by centrifugation, washed with ethanol three times, and dried at 80 °C for 12 h. 50 mg hydrous titania/HDA nanospheres are dispersed in a mixture of 40 ml ethanol and 20 ml H2O. The mixture is sealed in a Teflon-lined autoclave (100 ml in capacity) and heated at 160 °C for 16 h. The resulting TiO2 nanospheres are collected by centrifugation, washed with water three times, and dried at 80 °C for 12 h. 2

The as-prepared products are characterized with powder X-ray diffraction (XRD ARLXTRA), scanning electron microscopy (SEM, vltra55), and transmission electron microscopy (TEM, JEM2100). The specific surface areas and the porosity are calculated by Brunauer-Emmett-Teller (BET) analysis based on N2 adsorption-desorption isotherms measurements (3Flex). Electrochemical experiments are performed with CR2025 coin-type cells assembled in a glovebox with lithium foil as the counter/reference electrode and Celgard 2400 as the separator. The working electrodes with mass loading of around 0.8-1 mg are prepared by dispersing active material, acetylene carbon black, polyvinylidene fluoride (PVDF) at a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone. The slurry is uniformly plastered onto Cu foil and then dried at 100 °C for 12 h under vacuum. The electrolyte is 1 M LiPF6 with the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v). The galvanostatic discharge/charge measurements are performed through Battery Testing System (Neware) at room temperature. Cyclic voltammetry (CV) is studied on an electrochemical workstation (PARSTAT 2273).

3. Results and discussion In Fig. 1a and b, hydrous titania/HDA nanospheres as the precursor exhibit perfect sphere morphology, smooth surface and uniform size with about 230 nm. Hydrous titania and HDA totally coalesce together and form inorganic-organic composites due to hydrogen-bonding interaction between the amino group of HDA molecule and hydrous titania. The solvothermal treatment at 160 °C removes the organic species of HDA and simultaneously crystallize hydrous titania to TiO2 nanocrystalline. Therefore, the products become very rough; and the surface displays numerous nanocrystalline; but the morphology still retains the sphere with the size of about 230 nm; the crack doesn’t occur (Fig. 1c and d). The high-magnification TEM image (Fig. 1e) distinctly exhibits three features of TiO2 NMN: nanocrystalline, mesopore and the assembling structure. The size of TiO2 3

nanocrystalline is around 10 nm. The assembling structure contains abundant mesopores. These endow TiO2 with high electrochemical activity, good accommodation capability to volume variation, and good accessibility for electrolyte. In HRTEM image (Fig. 1f), the products exhibit clear crystal lattice with interplanar spacing of 0.352 nm, corresponding to (101) crystal plane of TiO2, confirming TiO2 nature of the nanocrystalline, indicating the availability of the hydrothermal crystallization reaction. The crystal phase of the products is further affirmed by XRD, as shown in Fig. 1g. All diffraction peaks can be well indexed to single pure phase of anatase TiO2 (JCPDS Card No. 04-0477). The dwarf and broadened diffraction peaks indicate small size of TiO2. Calculated through the Scherrer equation, the average crystallite size is 12.8 nm, which coincides well with TEM result. The nanocrystalline and mesopore structure can endow TiO2 with large specific surface area, which can be confirmed by BET characterization. Nitrogen adsorptiondesorption isotherms together with pore size distributions of TiO2 NMNs have been measured. Fig. 1h shows a typical IV isotherm with a distinct hysteresis loop, which is the typical characteristic of mesoporous materials. Based on Barrett-Joyner-Halenda model, the pore size of TiO2 NMNs ranges mostly from 4 to 20 nm, and the main pore size is 13.5 nm, confirming the mesoporous structure of TiO2 NMNs (Fig. 1i). The specific surface area and the pore volume of TiO2 NMNs are 153 m2 g-1 and 0.487 cm3 g-1. The great specific surface area is favorable to efficient contact between TiO2 and electrolyte as well as providing more active sites, thus improving lithium storage performance of TiO2. We next evaluate LIB performance of TiO2 NMNs in the voltage range of 1-3 V vs Li/Li+. Fig. 2a displays cycling performance and coulombic efficiencies of TiO2 NMNs at 1C (1C= 168 mA g1).

The activation at 0.3C for 6 cycles is performed before the long-term cycle test. The discharge

capacity at the 1st cycle is 258.6 mAh g-1 with coulombic efficiency (CE) of 99.6%, then declines 4

slowly. At the 100th cycle, the discharge capacity is 194.1 mAh g-1 with CE of 99.8%, presenting a capacity retention ratio of 75% with respect to the first discharge. The average discharge capacity over 100 cycles is 219.2 mAh g-1, which is superior or comparable to 151 mAh g-1 of walnut-like porous core/shell TiO2 at the 100th cycle at 1C [6], 175 mAh g-1 of TiO2/graphene at the 100th cycle at 1C [9], 175 mAh g-1 of TiO2-rGO hollow spheres at the 100th cycle at 188 mA g-1 [10], 170 mAh g-1 of urchin-like [email protected] composite at the 100th cycle at 1C [11], demonstrating the superiority of this nanocrystalline-assembled mesoporous nanosphere structure. Fig. 2b presents the first three galvanostatic discharge/charge profiles. The discharge/charge profiles nearly overlap, reflecting superior cycling stability. The discharge profiles exhibit a remarkable horizontal voltage plateau at 1.77 V, corresponding to Li+ intercalation storage in TiO2. The end capacity of the discharge plateaus reaches 156 mAh g-1, which is 92.8% of expected bulk limitation (168 mAh g-1) of intercalation reaction of anatase TiO2, reflecting the most volume of TiO2 nanocrystalline is available for the (de)lithiation reactions. The terminal voltage slope appears below the voltage plateau, corresponding to Li+ pseudo-capacitive (interfacial) storage. At 1.5 V, a distinct pseudo-plateau can be seen. The pseudo-capacitive storage capacity reaches 103 mAh g-1, which is 39.8% of the total capacity. This indicates the small size and large specific surface area of TiO2 nanocrystalline play a big role in the discharge process. The charge profiles present a corresponding pseudo-plateau at 1.73 V and a distinct delithiation-plateau at 1.9 V. Fig. 2c depicts CV curves in the first three cycles at a scan rate of 0.2 mV s-1. CV curves display two pairs of redox peaks, coinciding well with the discharge/charge profiles. Note that the redox peaks at 1.47 and 1.79 V reflect the pseudocapacitive lithium storage behavior of TiO2. Fig. 2d exhibits the Nyquist plots of TiO2 NMNs before the first discharge and after 5 discharge-charge cycles. After 5 cycles, the semicircle in the high frequency region enlarges slightly, which means 5

the discharge-charge reactions only lead to limited increase in charge transfer resistance of TiO2 NMNs. The sloping line after 5 cycles is basically parallel to that before the first discharge, which indicates TiO2 NMNs always maintain stable structure favorable to Li+ diffusion. The outstanding electrochemical performance of TiO2 NMNs should be attributed to the unique nanocrystalline-assembled mesopore nanosphere structure. The small size of TiO2 nanocrystalline shortens Li diffusion path length. The mesoporous nanosphere brings about a great specific surface area, providing more active sites. These beneficial aspects significantly boost reaction kinetics of TiO2. Meanwhile, the small crystallite size and the great specific surface area remarkably enhance both the bulk intercalation and the interfacial storage, which is the main reason for high specific capacity of TiO2 NMNs. Moreover, the abundant mesopores are favorable to buffering the volume variation of TiO2 nanocrystalline during (de)lithiation and improving the cycling stability of TiO2.

4. Conclusion We have successfully prepared TiO2 nanocrystalline-assembled mesoporous nanosphere. As anode material for LIBs, TiO2 NMNs present the enhanced Li storage capabilities with high specific capacity and good cycling performance, which is mainly ascribed to the small size of TiO2 nanocrystalline and the great specific surface area of the mesopore nanosphere. This work offers a new high-performance TiO2-based anode material for LIBs. Acknowledgments This work was funded by 521 talents-cultivated projects of Zhejiang Sci-Tech University; National Natural Science Foundation of China (No. 51302247 and 51602286); Natural Science Foundation of Zhejiang province (No. LQ18E050010 and LY18E010004). References [1] R.C. Xu, X.H. Xin, S.Z. Zhang, D. Xie, X.L. Wang, J.P. Tu, Electrochim. Acta 284 (2018) 1776

187. [2] J.S. Chen, Z.Y. Wang, X.C. Dong, P. Chen. X.W. Lou, Nanoscale 3 (2011) 2158-2161. [3] C. Yang, S.J. Liao, W. Shi, Y.D. Wu, R.H. Zhang, S.L. Leng, RSC Adv. 7 (2017) 10885-10890. [4] J.Y. Liu, P. Zhou, T.L. Han, J.R. Huang, J.H. Liu, J.J. Li, P.V. Braun, Mater. Lett. 219 (2018) 12-15. [5] Y.Z. Wang, S.Y. Duan, Z.F. Tian, Y. Shen, M.J. Xie, X.K. Guo, X.F. Guo, J. Mater. Chem. A 5 (2017) 6047-6051. [6] Y. Cai, H.E. Wang, X. Zhao, F. Huang, C. Wang, Z. Deng, Y. Li, G.Z. Cao, B. L. Su, ACS Appl. Mater. Interfaces 9 (2017) 10652-10663. [7] S.X. Yu, L.W. Yang, Y. Tian, P. Yang, F. Jiang, S.W. Hu, X.L. Wei, J.X. Zhong, J. Mater. Chem. A 1 (2013) 12750-12758 [8] Y. Li, J.D. Luo, X.Y. Hu, X.F. Wang, J.C. Liang, K.F. Yu, J. Alloy. Compd. 651 (2015) 685689. [9] X. Xin, X.F. Zhou, J.H. Wu, X.Y. Yao, Z.P. Liu, ACS Nano 6 (2012) 11035-11043. [10] A. Mondal, S. Maiti, K. Singha, S. Mahanty, A. B. Panda, J. Mater. Chem. A 5 (2017) 2385323862. [11] Y.L. Xing, S.B. Wang, B.Z. Fang, G. Song, D.P. Wilkinson, S.C. Zhang, J. Power Sources 385 (2018) 10-17. Figure Captions Fig. 1 (a) SEM and (b) TEM images of hydrous titania/HDA nanospheres; (c) SEM, (d) TEM, (e) high-magnification TEM and (f) HRTEM images of TiO2 NMNs; (g) XRD pattern of TiO2 NMNs; (h) Nitrogen adsorption–desorption isotherm and (i) pore size distribution curve of TiO2 NMNs. Fig. 2 (a) Cycling performance and (b) the first three discharge/charge curves of TiO2 NMNs in the 7

voltage range of 1-3 V (vs. Li/Li+) at current rate of 1 C; (c) Cyclic voltammograms at a scanning rate of 0.2 mV s-1; (d) Electrochemical impedance spectra of TiO2 NMNs.

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Highlights ► TiO2 nanocrystalline-assembled mesoporous nanosphere ► High specific capacity and good cycling stability ► Small size of nanocrystalline, large specific surface area of mesoporous nanosphere ► Enhanced bulk intercalation and the interfacial storage

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