Enhanced Rate Capabilities of Nanobrookite with Electronically

Nov 14, 2008 - Du-Hee Lee, Dong-Wan Kim,* and Jae-Gwan Park. Nano-Science Research DiVision, Korea Institute of Science and Technology, Seoul ...
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CRYSTAL GROWTH & DESIGN

Enhanced Rate Capabilities of Nanobrookite with Electronically Conducting MWCNT Networks Du-Hee Lee, Dong-Wan Kim,* and Jae-Gwan Park Nano-Science Research DiVision, Korea Institute of Science and Technology, Seoul 136-791, Korea

2008 VOL. 8, NO. 12 4506–4510

ReceiVed May 9, 2008; ReVised Manuscript ReceiVed August 4, 2008

ABSTRACT: Nanostructuring of intercalating electrode materials is a promising strategy in advanced lithium ion batteries with advantages of fast rate capabilities, high energy density, and excellent cycle life. However, the interparticle contact resistance caused by the easy aggregation of nanomaterials limits the electronic conduction paths and thereby reduces the power density. Herein, nanocomposites were formed by synthesizing brookite-type, TiO2 nanoparticles attached to carbon nanotubes with surfaces that had been functionalized with a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The specific capacity of the nanobrookite composite incorporating carbon nanotubes was estimated to be approximately 3-fold greater than that of pure nanobrookite mechanically mixed with conventional carbon black. Furthermore, this composite electrode offers an outstanding rate capability with good capacity retention via the combined benefits of the decrease in interparticle contact resistance and one-dimensional (1D) electron transport of carbon nanotubes. Introduction Extensive research has been conducted to develop better lithium ion batteries with high power, large capacity, and long cycle life, because of their potential use in powering electric vehicles and portable electronic devices.1 The design of nanostructured materials has led to multiple advances in the performance of lithium ion batteries by providing shorter path lengths for both electronic and Li ionic transport, higher electrode/electrolyte contact area, and better accommodation of the strain of Li ion insertion/extraction.2 Nanostructured TiO2 has attracted attention for application in energy conversion/ storage devices,3 especially for those with high charge/discharge current rates such as high-power lithium ion batteries because the low voltage cutoff (1 V) avoids electrolyte reduction and the formation of any passivation layer.4 Readily available, metastable, anatase-type TiO2 nanostructures have been considered a good candidate as a lithium ion battery electrode among several TiO2 polymorphs, including the thermodynamically most stable phase, rutile.5 In addition, TiO2 (B) nanostructures are excellent intercalation hosts for lithium.4a,6 Although micrometer-sized rutile can only accommodate a negligible amount of Li ions at room temperature, high reversible capacity and excellent high-rate performance have been observed for nanometer-sized rutile.7 On the other hand, the lithium electroactivity of metastable, brookite-type TiO2 has been poorly characterized, largely because of the difficulties encountered in obtaining its pure form. The lithium insertion into brookite has recently been demonstrated.8 A significant impact on the electrochemical performance of nanoparticles cannot be fully realized because, in practice, nanoparticle-based electrodes rarely show the expected kinetic supremacy due to the so-called problem of “electronic wiring.”9 Mechanical mixing of the active materials with carbon black, which is commonly used as a conducting additive, is not a proper solution because nanoparticles tend to attract each other and the resulting increased interparticle contact resistance limits the electronic conduction paths to the current collector.9,10 In this communication, we propose the assembly of phase-pure brookite nanoparticles without appreciable particle aggregation * Corresponding author. Tel: 82 2 958 5473. Fax: 82-2-958-5489. E-mail: [email protected].

to the multiwalled carbon nanotubes (MWCNTs), the surfaces of which are functionalized using a cationic surfactant, cetyltrimethylammonium bromide (CTAB). We demonstrate the ability of this nanocomposite, designed to include onedimensional electronic conductive paths, to decrease the interparticle contact resistance, thereby delivering large reversible capacity along with fast kinetic performance. These capabilities render the nanocomposite a promising electrode for high-power lithium ion batteries. Experimental Section To prepare MWCNT/brookite composites, we dispersed 200 mg L-1 of synthetic MWCNT (CNT Co., Ltd.) in deionized water in the presence of 2 mg L-1 of CTAB (Aldrich) first and then sonicated the solution for 3 h. After complete dispersion of MWCNTs, 0.015 M titanium trichloride (TiCl3, Alfa Aesar, 20% in 3% hydrochloric acid) and 0.5 M urea ((NH2)2CO, Alfa Aesar, 99.3%) were subsequently added to the solution at room temperature. The solution pH was monitored with increasing temperature at each reaction time. The solution was refluxed at 100 °C for 7 h under stirring for complete precipitation, followed by centrifuging and washing (four times with deionized water and once with anhydrous ethanol) repetitively. The precipitated powders were then long-time dried at 100 °C for over 5 days in a vacuum oven until completely dehydrated. For the electrochemical evaluation of the MWCNT/brookite composite powders, positive electrodes were prepared by mixing powders (∼2 mg) with the Kynar 2801 (PVdF-HFP) binder (the mass percentage was controlled to be 85:15). For comparison, electrodes using pure brookite powders were also prepared by mechanically mixing together 1-3 mg of powder, binder, and Super P (MMM Carbon, Brussels, Belgium) carbon black as a conducting additive at a mass percentage of 65:15:20. Swageloktype cells using Li metal foil, which were used as the negative electrode, and a separator film of Celgard 2400 were assembled and saturated with the liquid electrolyte, 1 M LiPF6, in ethylene carbonate and dimethyl carbonate (1:1 by volume, Techno Semichem Co., Ltd., Korea). The assembled cells were galvanostatically cycled between 2.5 and 1.0 V using a Solartron 1280C electrochemical test system and automatic battery cycler (WBCS 3000, WonATech, Korea). The powders were characterized using X-ray powder diffraction (XRD: M18XHF, Macscience Instruments, Japan) and high-resolution transmission electron microscopy (HRTEM: Tecnai G2, FEI Hong Kong Co., Ltd.).

Results and Discussion Similar to the chemical route introduced by Li et al., the present approach to brookite nanocrystals proceeds through a

10.1021/cg800481a CCC: $40.75  2008 American Chemical Society Published on Web 11/14/2008

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Figure 2. TEM images of MWCNT/brookite nanocomposites.

Figure 1. (a) Synthetic steps involved in the preparation of brookite and the correspondence of pH with increasing solution temperature. (b) Typical TEM image of brookite nanopowders dried at 100 °C. (c, d) SAD pattern and lattice fringe of the nanocrystals indicating brookite structure, respectively.

urea-mediated precipitation method.11 Figure 1a shows the synthetic stages involved in the brookite preparation. The pH of the starting solution was 2.2 at room temperature and slightly decreased to 0.8 at 70 °C, indicating a strongly acidic condition due to the dissociation of HCl in the TiCl3 source (20% TiCl3 in 3% HCl). It has been reported that a blue coloration of solution is accompanied by the presence of Ti3+ species with distorted octahedral shape.12 A dark purple color of solution is based on the hydrolysis of urea [(NH2)2CO + 4H2O f CO2 + 2NH3 · H2O f CO2 + 2NH4+ + 2OH- ] and the subsequent formation of titanium(III) hydroxides by the ion exchange reaction, soon after a temperature of ∼100 °C is reached. Later, the pH sharply increased from ∼1 to 6 while the temperature leveled off at ∼100 °C, after which white powder started to precipitate through the oxidation reaction. The powders obtained were composed of nanocrystallites of average size 10-20 nm, although the primary particles were considerably overlapped and formed large aggregates, as shown in Figure 1b. These powders were indexed as a phase-pure brookite (Figure 1c) having high crystallinity (Figure 1d) without any other TiO2 polymorphs such as anatase or rutile. Recent studies by Tomita et al. have described the preparation of brookite through a soft chemistry route involving an intermediate phase of chains of TiO6 octahedra.13 Pottier et al. reported that low solution pH and Cl- ion concentration were a possible reaction pathway for brookite formation from its intermediate complex.14 Therefore, it was believed that an appropriate concentration of chloride ions in strongly acidic aqueous solution containing Ti3+ ions should favor the formation of brookite in this study. To realize the MWCNT/brookite composites, we first investigated the dispersion behavior of commercially produced, long MWCNTs, which have diameters ranging from 20 to 30 nm and lengths on the order of several micrometers, in aqueous solution after 3 h of sonication. Most MWCNTs easily settled

down in the suspension without a surfactant (see the Supporting Information, Figure S1) In contrast, the homogeneous MWCNT dispersion in the suspension with an appropriate amount of CTAB was maintained even after 2 days. The MWCNT stability was believed to be significantly enhanced by the addition of the cationic surfactant, CTAB, due to the adsorption of CTA+ on the surface of the MWCNT and the resultant electrostatic repulsion between the individual MWCNTs.15 After suspending the MWCNTs, brookite was synthesized via urea-mediated homogeneous precipitation, resulting in hybrid MWCNT/ brookite nanocomposites. Single particles or small clusters of nanobrookite were distributed without any appreciable agglomeration, as observed in typical transmission electron microscopy (TEM) images of the MWCNT/brookite composites (Figure 2). This observation confirmed the important role played by CTAB in anchoring the TiO2 particles. MWCNTs positively charged by the functionalization of the CTA+ group were surrounded by OH- generated during the urea hydrolysis. Afterward, Ti3+ ions in solution reacted with OH- at the MWCNT surface and yielded nanobrookite crystallines by the ion exchange and oxidation reactions, as aforementioned. The brookite crystallites seen in Figure 2 had an average diameter of 7 nm, according to Scherrer’s equation, which was smaller than the values obtained from pure brookite due to homogeneous nucleation on the MWCNT surface. The Li electroactivity for the pure nanobrookite and MWCNT/ brookite nanocomposites is shown in Figure 3a. Thermogravimetric (TG) analysis, used for exact determination of the weight percentages of the MWCNT and brookite in the composites, indicated that 12-16 wt % of the MWCNTs was associated with brookite. For comparison, 20 wt % of Super P carbon black was mechanically mixed with nanobrookite as a conducting additive to prepare the electrodes of pure brookite (hereafter designated as “pure brookite”). Each sample/Li half-cell was cycled at a rate of C/5 (here, a rate of C/1 was defined as the current required to fully discharge TiO2 in 1 h on the basis of the reaction, TiO2 + 0.5Li f Li0.5TiO2, which corresponds to a capacity of 168 mA h g-1).16 Reversible capacities of pure brookite were clearly observed at room temperature, thereby confirming the electrochemical activity of brookite. In brookite, the TiO6 octahedra share both edges and corners, forming an orthorhombic structure.17 It is believed that the atom-free channels observed in brookite along the [001] direction enable lithium mobility into the structure (Figure 3a, inset). The pure brookite exhibited an initial discharge capacity of 284 mA h g-1 but the capacity subsequently faded rapidly to 53 mA h g-1 after 50 cycles. This fast capacity decay was attributed to

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Figure 4. (a) Charge-discharge curves (every 10th cycle) of the MWCNT/brookite cycled between 2.5 and 1.0 V at a rate of C/5. (b) Comparison of cycling behavior at 10th cycle for the pure brookite and MWCNT/brookite at a rate of C/5 showing the difference in polarization.

Figure 3. (a) Variation of the discharge-charge specific capacity versus the cycle number at a rate of C/5 for the pure brookite and MWCNT/ brookite prepared using various amounts of CTAB. The inset shows the crystal structure of brookite using 3D networks of TiO6 octahedra. (b) HRTEM image of fully lithiated brookite in MWCNT/brookite composite electrodes (taken after five cycles) and the FFTs calculated using selected regions of each nanocrystal.

considerable aggregation of nanobrookite, which increased the interparticle contact resistance. However, MWCNT/brookite composite has a superior reversible capacity and capacity retention. The influence of the CTAB concentration on the electrochemical performance is also presented in Figure 3a, which shows that the specific capacity was lowered by the increased CTAB concentration. We found that the excess addition of CTAB (more than the MWCNTs and a CTAB ratio of 1:0.01 by mass, that is, a CTAB concentration of 2 mg L-1) impeded the stabilization of the MWCNT suspension. In this case, however, the specific capacity and its retention upon cycling were also enhanced in comparison with the pure brookite, even though the particle size of the precipitated brookite nanocrystalline in the composites (see the Supporting Information, c and d in Figure S2) were similar to or larger than that of pure brookite. To further elucidate in the Li ion insertion reaction into the brookite, we performed HRTEM observation of the MWCNT/brookite samples in the discharge state. Figure 3b (and the Supporting Information, Figure S3) is a typical, bright-field image of brookite in the composite (at ∼1.0 V). The morphology of the brookite nanocrystals was preserved without any surface amorphous character.7 Addition-

ally, the fast Fourier transforms (FFTs) realized from each particle (insets in Figure 3b) corresponded to the brookite structure. The ex situ XRD patterns of the MWCNT/brookite that were fully (de)lithiated after 5 cycles, as shown in the Supporting Information, Figure S4a, confirmed the stability of the brookite structure upon cycling and the absence of any formation of new Li-intercalated phase such as LiTiO2.7,18 Although further research is needed, the peak shift was negligible in both the discharge and charge states (see the Supporting Information, Figure S4b), confirming the absence of any significant dimensional change of the brookite, which indicates a lower-strain process. As described earlier, the MWCNT/brookite composite (the MWCNT-to-CTAB mass ratio was controlled at 1:0.01 to optimize the electrochemical performance) achieved remarkable high-capacity delivery and stable cyclability. We confirmed the high discharge capacity of 151 mA h g-1, corresponding to Li0.45TiO2, even after 100 cycles at a rate of C/5, as shown in Figure 4a. The contribution of the MWCNT incorporation in the composite electrode was supported by the typical charge/discharge profiles of pure brookite and MWCNT/brookite (Figure 4b). In the MWCNT/ brookite, extended plateaus were observed and the cell polarization between the charge and discharge plateaus was reduced to ∼70 mV from 150 mV for the pure brookite. Similar behavior was observed in LiFePO4 after RuO2 coating and in anatase TiO2 or Li4Ti5O12 after carbon coating.9,19 For comparison, we dispersed spherical Super P carbon in CTAB-containing solvent and then prepared Super P/brookite nanocomposite through the same synthetic process of MWCNT/brookite composite as described in the Experimental Section. Although Super P/brookite composite showed larger capacity than pure brookite (mechanically mixed with

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electrode-electrolyte contact area by the optimum realization of nanobrookite without appreciable aggregations onto MWCNTs. Conclusions

Figure 5. (a) Rate capabilities and Coulombic efficiency for the pure brookite and MWCNT/brookite at different C rates. (b) Schematic model comparing the electronic transport in pure brookite mechanically mixed with Super P carbon black and MWCNT/brookite.

same amount of Super P), we confirmed the best reversible capacity and the lowest cell polarization in MWCNT/brookite composite (Figure S5). From these results, it is believed that 1D nature of the MWCNTs can offer much easier conductive pathways from electrode (nanobrookite particles) to the copper current collector. This decreased polarization was expected to improve the storage reaction kinetics in the MWCNT/brookite. To demonstrate the rate capability of MWCNT/brookite, the cells were first cycled at a rate of 1C and after every 10 cycles, the rate was increased in stages to 40C (Figure 5a). MWCNT/brookite exhibited much better capacity at high rates than the corresponding capacity achieved under identical conditions for pure brookite. At rates of 3, 10, 20, and 40C, the MWCNT/brookite delivered specific capacities of 170, 132, 100, and 51 mA h g-1, respectively, maintaining an excellent Coulombic efficiency of more than 98% (i.e., a high ratio of extractable-to-inserted Li). This rate performance at such high rates was superior to any of the various kinds of TiO2 nanostructures such as anatase nanoparticle/nanotubes, TiO2-B nanowires, and rutile nanoparticles.4,6,7,10,20 TiO2 has poor intrinsic electronic conductivity, which renders its electrochemical performance particularly sensitive to the distribution of conducting additives such as carbon black. However, it is difficult to mix such conducting additives homogeneously with nanomaterials that are easily aggregated, leading to high interparticle contact resistance, as shown in Figure 5b. On the other hand, the incorporation of MWCNTs in MWCNT/brookite a sufficient degree of electronic connection from a current collector to each host material (brookite), thereby ensuring efficient electron wiring. Therefore, the high rate capability of MWCNT/brookite resulted from the advantageous electron transport by long, one-dimensional (1D) MWCNT, as well as from the fast Li ion transport related to the higher

In summary, we demonstrated the successful formation of a nanocomposite electrode by synthesizing nanobrookite using a urea-mediated precipitation process attached to MWCNTs whose surfaces had been functionalized with a cationic surfactant, CTAB. CTAB thus served as the linker between MWCNTs and 7 nm sized brookite nanoparticles by electrostatic interaction in aqueous solution. Because of the presence of an oxygen octahedral vacancy that exhibits large channels, brookite favored the insertion of lithium but showed poor electrochemical performance at room temperature because of their high aggregation and resultant interparticle resistance. MWCNT/ brookite composites exhibited improved reversible capacity and excellent cycle life because of the better electronic/ionic transport characteristics provided by the 1D path of MWCNTs and the optimized distribution of nanobrookite on the MWCNTs. In line with previous reports on nanostructured TiO2, the MWCNT/brookite studied here offered outstanding rate capability performance. This proposed methodology could potentially be applied to other nanoparticle systems with the prospect of rendering them excellent host materials for high-power Li ion batteries. Supporting Information Available: Images showing dispersion of MWCNTs in aqueous solution with/without CTAB (2 mg L-1); XRD patterns of pure brookite and MWCNT/CTAB prepared using MWCNTs and a CTAB ratio of 1:0.01, 1:0.1, and 1:1 by mass; HRTEM image of fully lithiated brookite in composite electrodes (taken after five cycles); FFT calculated using selected regions of a nanocrystal; ex situ XRD patterns of fully lithiated and delithiated brookite in MWCNT/brookite composite electrodes taken after five cycles; cycling behavior at 10th cycle for the Super P/brookite at a rate of C/5 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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