Hollow Core–Shell Mesoporous TiO2 Spheres for Lithium Ion Storage

ARTICLE pubs.acs.org/JPCC

Hollow Core
Shell Mesoporous TiO2 Spheres for Lithium Ion Storage Sukeun Yoon and Arumugam Manthiram* Electrochemical Energy Laboratory, Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States

bS Supporting Information ABSTRACT: Hollow core
shell mesoporous TiO2 spheres have been synthesized by a hydrothermal reaction without surfactant, followed by firing at 500 °C. X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), and Barrett
Joyner
Halenda (BJH) data reveal a mesoporous and nanosize building-block morphology with large surface area. The samples have an average pore size, surface area, and total pore volume of 8 nm, 131 m2/g, and 0.2 cm3/g, respectively. The hollow core
shell mesoporous TiO2 spheres exhibit a high capacity of >200 mAh/g with good cyclability and high rate capability as the mesoporous, nanosize building-block morphology of TiO2 spheres provides good electrical contact, accommodates the strain smoothly, and facilitates facile lithium-ion diffusion.

’ INTRODUCTION Crystalline TiO2 (titania) has attracted much attention in recent years because of its excellent physicochemical properties and potential application in lithium-ion batteries.1
4 In the TiO2 structure, the TiO6 octahedra share vertices and edges to build a three-dimensional framework, leaving favorable empty sites available for lithium insertion.1 Accordingly, TiO2 can accommodate theoretically one lithium per formula unit as LixTiO2 (0 e x e 1), involving the Ti4þ/3þ redox couple. TiO2 offers several attractive features such as a low volume change (∼4%) during the charge
discharge process, low production cost, low toxicity, and the ability to be prepared as nanotubes, nanowires, nanoparticles, and mesoporous structures.5
11 Moreover, with an operating voltage well above that of Liþ/Li and less surface reactivity with the electrolyte, it offers better safety than graphite. Although the working potential of TiO2 is high for a negative electrode, its electrochemical stability in common electrolytes and the absence of formation of harmful solid-electrolyte interfacial (SEI) layers result in better overcharge protection and safety.12 Various crystalline phases such as rutile, anatase, brookite, and TiO2(B) have been identified for crystalline TiO2. The rutile phase is the thermodynamically most stable under standard conditions. In the case of micrometer-sized rutile particles, only a small amount of lithium (0.1
0.25 per formula) could be inserted into the lattice.4,13 The brookite phase is the least stable.14 TiO2(B) is monoclinic, and the kinetics of reaction of microfibrous TiO2(B) with lithium ion has recently been reported to be controlled by a pseudocapacitive faradaic process.2 The anatase phase is metastable, but it is generally considered to be the most electroactive lithium intercalation host among the various polymorphs of TiO2. However, the anatase phase, with an average lithium insertion/extraction potential of ∼1.8 V, accommodates only ∼0.5 Li per formula unit at room temperature.3,9,10,12 r 2011 American Chemical Society

In this regard, nanoporous structures with a large active surface area are appealing as they can offer fast lithium insertion/extraction kinetics with reduced diffusion pathways for electronic and ionic transport, resulting in excellent power density.15
17 Accordingly, we present here a facile synthesis of hollow core
shell mesoporous TiO2 spheres that exhibit high capacity with good cycling performance and high rate capability. The hollow core
shell mesoporous TiO2 spheres are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), BET surface area measurement, and electrochemical charge
discharge measurements including impedance analysis.

’ EXPERIMENTAL SECTION The hollow core
shell mesoporous TiO2 spheres were obtained by a hydrothermal reaction without any surfactants. In a typical experiment, 10 mmol of glucose (C6H12O6, Fisher Sci.) and 30 mmol of urea ((NH2)2CO, EM) were dissolved in 10 mL of deionized water (DI water) and 25 mL of ethanol under mild stirring. A 1 mmol sample of titanium tetrachloride (TiCl4, Acros) was then added into this solution, and the colorless, transparent solution obtained was transferred into a Teflon-lined autoclave and heated at 180 °C for 24 h with a heating/cooling rate of 2 deg/min. The ammonia produced from urea controls the pH of the reaction medium and the rate of hydrolysis of TiCl4 at T > 80 °C. The resulting slurry was then filtered and washed with DI water before drying in a vacuum oven. The powder was finally heated at 500 °C in air for 5 h. The phase analysis of the synthesized samples was performed with a Phillips X-ray diffractometer and Cu KR radiation. Thermogravimetric analysis (TGA) and differential scanning calorimetry Received: December 28, 2010 Revised: April 9, 2011 Published: April 22, 2011 9410

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Figure 1. XRD patterns of the hollow core
shell mesoporous TiO2 spheres obtained by the hydrothermal reaction, before and after firing at 500 and 800 °C.

(DSC) were carried out in air at a heating/cooling rate of 5 deg/ min with a Perkin-Elmer series 7 Thermal Analysis System. The specific surface areas were evaluated with a Quantachrome analyzer based on the Brunauer
Emmett
Teller (BET) multipoint method from the results of N2 physisorption at 77 K. Adsorption
desorption isotherm measurements were used to determine the porosity and the pore size distribution using the Barrett
Joyner
Halenda (BJH) method. The morphology, microstructure, and composition of the synthesized powders were examined with a Hitachi S-5500 scanning transmission electron microscope (STEM) and JEOL 2010F transmission electron microscope (TEM). The electrodes for the electrochemical evaluation were prepared by mixing 70 wt % active material (TiO2) powder, 15 wt % carbon black (Super P) as a conducting agent, and 15 wt % polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidone (NMP) as a binder to form a slurry, followed by coating on a copper foil, pressing, and drying at 120 °C for 3 h under vacuum. The CR2032 coin cells were assembled in an Ar-filled glovebox with use of Celgard polypropylene as a separator, lithium foil as the counter electrode, and 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 v/v) as the electrolyte. The charge
discharge experiments were performed galvanostatically at a constant current density of 30 mA/g of active material within the voltage range of 1
3 V vs Li/Liþ. The electrochemical impedance spectroscopic analysis (EIS) was carried out with Solartron SI1260 equipment by applying a 10 mV amplitude signal in the frequency range of 10 kHz to 0.001 Hz. In the EIS measurements, the hollow core
shell mesoporous TiO2 spheres with an active material content of ∼1.8 mg served as the working electrode and lithium foil served as the counter and reference electrodes. The impedance response was measured after a different number of charge
discharge cycles (after 1 and 20 cycles) at 3 V vs Li/Liþ.

’ RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the hollow core
shell mesoporous TiO2 spheres prepared by the hydrothermal reaction, followed by heat treatment. All the reflections before and after firing at 500 °C could be indexed based on the anatase TiO2 phase (JCPD No. 84-1286). No peaks corresponding to carbon are seen due to its amorphous nature or removal as CO2. The

Figure 2. (a) TGA and (b) DSC plots of hollow core
shell TiO2 spheres before and after firing at 500 °C.

mean crystallite size D101, calculated based on the (101) reflection using Scherer’s equation, is ∼10 nm for the 500 °C sample. For a comparison, the crystallite size of a commercial anatase TiO2 (Alfa Aesar) is ∼38 nm. In contrast, the sample fired at 800 °C shows sharp diffraction peaks that could be indexed based on the rutile TiO2 phase (JCPD No. 87-0920). The 800 °C sample also has a larger crystallite size of 23 nm. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) plots recorded in air with a heating rate of 5 deg/min are compared in Figure 2. The as-prepared sample shows a weight loss of ∼30% and two exothermic peaks below 500 °C, corresponding to the removal of both water and carbon from the sample as CO2. On the other hand, the sample already fired at 500 °C does not show any weight loss or exothermic peak up to 700 °C, indicating that all the carbon has been removed from the sample during the firing in air at 500 °C. The STEM images shown in Figure 3 for the 500 °C sample reveal an average particle size of ∼1 μm and a homogeneous distribution of Ti (green color) and O (red color) in the hollow core
shell mesoporous TiO2 sphere. The selected individual single particle (Figure 3c,d) clearly reveals a rough surface and the existence of a large number of mesopores on the sphere surface. The mesopores have a 3 dimensionally interconnected framework that is built by the nanosize TiO2 building-block particles. The nanosize building-block morphology as shown by the SEM images is beneficial to reduce the diffusion pathways for ionic and electronic transport and increase the power density.10 Figure S1 in the Supporting Information compares the morphology of the TiO2 samples before and after firing at 800 and 1100 °C. The SEM images show spherical particles before firing, and the rough surface and the nanosize building-block morphology disappear after firing. Figure 4 shows the TEM images of the hollow core
shell mesoporous TiO2 spheres obtained after firing at 500 °C along 9411

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Figure 3. (a
c) SEM images of the hollow core
shell mesoporous TiO2 spheres at different magnifications and (d) elemental distribution. The scale bars in panels a
c refer to 5 μm, 4 μm, and 200 nm, respectively. The green and red colors in panel d refer, to Ti and O respectively.

with the selected area diffraction (SAD). As seen, the core
shell and the mesopore are uniformly dispersed in the whole sphere, and each nanoparticle has an average size of ∼30 nm. The SAD pattern shows a set of sharp spots corresponding to (101), (004), and (200) planes of the anatase phase, which is consistent with the XRD data in Figure 1. To have a better characterization of the porous nature of the hollow core
shell mesoporous TiO2 spheres obtained after firing at 500 °C, N2 adsorption
desorption isotherms were collected as shown in Figure 5. The BJH pore size distribution obtained from the adsorption
desorption isotherms indicates the highly mesoporous nature of the synthesized TiO2 spheres. The TiO2 spheres show a nitrogen adsorbed volume of P/Po = 0.53 and an average pore size of 8 nm. The BET surface area and the total pore volume of the sample were found to be 131 m2/g and 0.2 cm3/g, respectively. The high BET surface area and large porosity illustrate the mesoporous structure of the sample. These results demonstrate 3 dimensionally interconnected TiO2 spheres and a hollow core
shell mesoporous structure. A series of experiments (Figure 6) were conducted to determine the optimum conditions in the hydrothermal reaction for realizing the hollow core
shell mesoporous TiO2 spheres. During the hydrothermal process, hydrolysis of TiCl4 and carbonization of glucose occur simultaneously. First, TiCl4 forms a complex [Ti(OH)nClm]2- (n þ m = 6),18,19 which on hydrolysis produces TiO2 embryos that act as seed for the growth of TiO2. The linkage between the [Ti(OH)4Cl2]2
complex and TiO2 embryos takes place by a dehydration reaction.20 Consequently, the connection of TiO2 nanoparticles and/or formation of mesopores occur in the TiO2 nanopaticles. Also, the hydrolysis and ethanolysis of TiCl4 occur within the surrounding of glucose dehydration and carbonization, so the TiO2 particles formed

thereupon serve as the nucleation sites for amorphous carbon deposition. The amorphous carbon surface is hydrophilic because of the presence of —OH or dCdO groups, so the characteristics of the TiO2 particles are strongly dependent on the solvent properties.21 Under the condition of high water content, the TiO2 particles are well solubilized, so aggregation is limited. The particle aggregation increases with increasing ethanol content, and under the right mix of ethanol and water, the particles aggregate into mesospheres exclusively. During the firing in air, the carbon in the mesospheres is oxidized to CO2, and the diffusion of CO2 toward the outside causes a compression of the TiO2 particles at the surface region inside a shell. The shell and core continue to anneal in the process and form a thermally stable structure. This progression leads to the formation of a hollow core
shell structure consisting entirely of aggregates of mesoporous TiO2 particles. On the basis of the data in Figure 6 and electrochemical performance data (see below), a mixture consisting of 10 mL of water and 25 mL of ethanol was identified as the optimum reaction medium. Figure 7 shows the first and second discharge
charge profiles and differential capacity plots (DCPs) of the hollow core
shell mesoporous TiO2 spheres obtained after firing at 500 °C. The sample exhibits a first discharge and charge capacities values of 230 and 202 mAh/g, respectively, implying an initial Coulombic efficiency of 88%. The voltage profile exhibits a monotonic decrease from 2.5 to 1.75 V corresponding to the storage of 0.1 lithium during the first discharge, which is close to the value expected for a monolayer of lithium on the surface of the TiO2 spheres. It is a common phenomenon related to the small crystallite size of nanosized materials.3,22 The DCP in Figure 7b displays features characteristic of the voltage plateaus during discharge and charge at around 1.75 and 1.9 V, respectively, which corresponds 9412

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Figure 4. (a, b) TEM, (c) HRTEM, and (d) fast Fourier transform images over selected regions of the hollow core
shell mesoporous TiO2 spheres. The scale bars in panels a
c refer to 200, 50, and 5 nm, respectively.

Figure 5. Nitrogen adsorption
desorption isotherm and BJH pore size distribution plot (inset) of the hollow core
shell mesoporous TiO2 spheres.

to the known biphasic process (Li-poor phase and Li-rich phase) of lithium insertion/extraction into/from anatase TiO2. In this operating voltage window, ∼0.7 lithium is inserted into the hollow core
shell mesoporous TiO2 spheres. Anatase has a tetragonal structure (space group I41/amd) with the Ti4þ ions at the 4a octahedral sites and the O2
ions at the 8e sites. The lithium ions are inserted into the empty 4b octahedral sites during discharge, and they are mobile along the 3-dimensional channels formed by the 4b sites.23 While anatase is known to insert generally only about 0.5 lithium, the mesoporous TiO2

spheres obtained by the hydrothermal process described here insert ∼0.7 lithium, increasing the specific capacity. Figure 8a compares the cyclability of the hollow core
shell mesoporous TiO2 spheres (obtained after firing at 500 °C) between 1 and 3 V at a constant current of 30 mA/g (C/10 rate). While commercial anatase exhibits a drastic capacity fade within 20 cycles, the TiO2 samples synthesized by the hydrothermal reaction show good cyclability. Particularly, the sample prepared in a mixture consisting of 10 mL of water and 25 mL of ethanol exhibits excellent cyclability, retaining 94% of the capacity after 100 cycles. A smooth accommodation of the strains during the lithium insertion process by the mesoporous and nanosize building-block morphology leads to much better cyclability. We also compared the cyclability of the hollow core
shell mesoporous TiO2 obtained after firing at 500 °C and the asprepared TiO2
C spheres. As seen in Figure S2 in the Supporting Information, the cyclabilities of the two samples do not differ significantly, except for a slightly larger irreversible capacity loss in the first cycle for the TiO2
C sample. This may be due to the poor crystallinity of the sample and the presence of —OH or dCdO groups from residual solvents on the particle surface. Figure 8b compares the rate capabilities of the hollow core
shell mesoporous TiO2 spheres with that of commercial anatase at various C rates from 0.1C to 5C rates. The hollow core
shell mesoporous TiO2 spheres exhibit excellent rate capability. Especially, it retains a high capacity of ∼172 and 170 mAh/g at 3C and 5C rates, respectively, with stable cycling. 9413

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Figure 6. SEM images of the products obtained in a reaction medium containing various amounts of water and ethanol: (a) 35 mL of water and 0 mL of ethanol, (b) 17.5 mL of water and 17.5 mL of ethanol, (c) 10 mL of water and 25 mL of ethanol, and (d) 5 mL of water and 30 mL of ethanol. The scale bar in each Figure refers to 5 μm.

Figure 7. (a) Discharge
charge profiles and (b) differential capacity plots (DCP) of the hollow core
shell mesoporous TiO2 spheres.

Figure 8. Comparison of the (a) cycling performances and (b) capacity retention at various C rates, illustrating the rate capability of the hollow core
shell mesoporous TiO2 spheres with those of commercial anatase TiO2.

To gain further insight on electrochemical performances, EIS measurements were carried out at 3 V vs Li/Liþ with the hollow core
shell mesoporous TiO2 spheres sample (obtained after firing at 500 °C) and commercial anatase after the 1st and 20th cycles (Figure 9) and they were fitted with use of the Zview software. The EIS data were analyzed based on an equivalent

circuit given in Figure 9.24 In general, the EIS spectrum can be divided into three frequency regions: low, medium to low, and high frequency regions, which correspond to cell geometric capacitance, charge transfer reaction, and lithium-ion diffusion through the surface layer, respectively. The EIS spectra of the 9414

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’ CONCLUSIONS Hollow core
shell mesoporous TiO2 spheres synthesized by a hydrothermal reaction followed by firing at 500 °C have been investigated as an anode material for lithium-ion batteries. Characterization data collected with XRD, STEM, TEM, and BET reveal a mesoporous and nanosize building-block morphology with large surface area. The hollow core
shell mesoporous TiO2 spheres exhibit high capacity (>200 mAh/g) with excellent cycle life and rate capability. The mesoporous TiO2 spheres with a small crystallite size and large surface area provide good electrical contact and accommodate smoothly the strain occurring during the charge
discharge process, resulting in facile lithium-ion diffusion and superior electrochemical performance. ’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images of TiO2 spheres (Figure S1) and comparison of the discharge
charge profiles and cycling performances of TiO2 spheres (Figure S2). This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (512) 471-1791. Fax: (512) 471-7681. E-mail: rmanth@ mail.utexas.edu. Figure 9. (a) Equivalent circuit and electrochemical impedance spectra (EIS) after the 1st and 20th cycles of (b) commercial anatase TiO2 and (c) the hollow core
shell mesoporous TiO2 spheres.

commercial anatase recorded after the 1st and 20th cycles in Figure 9a consist of two semicircles and a line. The small diameter of the first semicircle (at high frequency region) is a measure of the surface layer resistance Rs, which is ascribed to lithium-ion diffusion through the surface layer, and the diameter of the second semicircle (at medium-low frequency region) is a measure of the charge transfer resistance Rct, which is related to the contact between the particles or between the electrode and the electrolyte. The sloping line is related to lithium-ion diffusion in the bulk of the active material. The commercial anatase TiO2 after the 1st and 20th cycles show surface resistances of 65.5 and 93 Ω, respectively, and a charge transfer resistance of 128 and 179 Ω, respectively, indicating an increase in Rct as the electrode is cycled, possibly due to the breaking of the interparticle contact. The EIS spectra of the hollow core
shell mesoporous TiO2 spheres, on the other hand, consist of three semicircles and a line (Figure 9b). The small diameters of the first and second semicircles (at high frequency region) are a measure of the native film resistance25 and the surface layer resistance Rs, respectively, which are ascribed to the formation of complex native passivation film on the particle surface and lithium-ion diffusion through the surface layer. The hollow core
shell mesoporous TiO2 spheres after the 1st and 20th cycles show native film resistances of 7.47 and 7.55 Ω, respectively, surface resistances of 16.96 and 19.3 Ω, respectively, and charge transfer resistances of 17.9 and 24.5 Ω, respectively. Both the Rs and Rct values increase only slightly from 1st to 20th cycle due to good electrical contact among the particles as well as between the current-collector and particles due to the mesoporous and nanosize building-block morphology.

’ ACKNOWLEDGMENT This work was supported by the Department of Energy Office of Basic Energy Science grant no. DE-SC0005397and Welch Foundation grant F-1254. One of the authors (S.Y.) acknowledges the Korea Research Foundation Grant funded by the Korean Government [KRF-2008-357-D00065]. ’ REFERENCES (1) Kuhn, A.; Amandi, R.; García-Alvarado, F. J. Power Sources 2001, 92, 221. (2) Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. Angew. Chem., Int. Ed. 2004, 43, 2286. (3) Sudant, G.; Baudrin, E.; Larcher, D.; Tarascon, J.-M. J. Mater. Chem. 2005, 15, 1263. (4) Reddy, M. A.; Kishore, M. S.; Pralong, V.; Caignaert, V.; Varadaraju, U. V.; Raveau, B. Electrochem. Commun. 2006, 8, 1299. (5) Kyriaki, E. K.; Xenophon, E. V. J. Phys. Chem. 1993, 97, 1184. (6) Lakshmi, B. B.; Patrissi, C. J.; Martin, C. R. Chem. Mater. 1997, 9, 2544. (7) Wang, C. C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113. (8) Cozzoli, P. D.; Korrowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (9) Gao, X. P.; Lan, Y.; Zhu, H. Y.; Liu, J. W.; Ge, Y. P.; Wu, F.; Song, D. Y. Electrochem. Solid-State Lett. 2005, 8, A26. (10) Guo, Y.-G.; Hu, Y.-S.; Maier, J. Chem. Commun. 2006, 2783. (11) Yu, J.; Guo, H.; Davis, S. A.; Mann, S. Adv. Funct. Mater. 2006, 16, 2035. (12) Wagemaker, M.; Kearley, G. J.; Well, A. A.; van; Mutka, H.; Mulder, F. M. J. Am. Chem. Soc. 2003, 125, 840. (13) Bai, X.; Xie, B.; Pan, N.; Wang, X.; Wang, H. J. Solid State Chem. 2008, 181, 450. (14) Yu, J. C.; Zhang, L.; Yu, J. Chem. Mater. 2002, 14, 4647. (15) Davis, M. E. Nature 2002, 417, 813. (16) Goltner, C. G.; Smarsly, B.; Berton, B.; Antonietti, M. Chem. Mater. 2001, 13, 1617. 9415

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