Pseudocapacitive Lithium-Ion Storage in Oriented Anatase TiO2

Article pubs.acs.org/JPCC

Pseudocapacitive Lithium-Ion Storage in Oriented Anatase TiO2 Nanotube Arrays Kai Zhu,* Qing Wang, Jae-Hun Kim, Ahmad A. Pesaran, and Arthur J. Frank* Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States ABSTRACT: We report on the synthesis and electrochemical properties of oriented anatase TiO2 nanotube (NT) arrays as electrodes for Li-ion batteries. The TiO2 NT electrodes displayed both pseudocapacitive Li+ storage associated with the NT surface and the Li+ storage within the bulk material. The relative contribution of the pseudocapacitive and bulk storages depends strongly on the scan rate. While the charges are stored primarily in the bulk at low scan rates (≪1 mV/s), the surface storage dominates the total storage capacity at higher scan rates (>1 mV/s). The storage capacity of the NT electrodes as a function of charge/discharge rates showed no dependence on the NT film thickness, suggesting that the Li+ insertion/extraction processes occur homogeneously across the entire length of NT arrays. These results indicated that the electron conduction along the NT walls and the ion conduction within the electrolyte do not cause significant hindering of the charge/discharge kinetics for NT electrode architectures. As a result of the surface pseudocapacitive storage, the reversible Li+ storage capacities for TiO2 NT electrodes were higher than the theoretical storage capacity for bulk anatase TiO2 materials.

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INTRODUCTION During the past two decades, titanium dioxide has been studied extensively as an electrode material for Li-ion batteries resulting from its good rate capability,1−9 capacity retention,1−4,7−11 low cost, and low toxicity. Most of this research has centered on randomly packed mesoporous films comprised of TiO2 nanoparticles, nanotubes (NTs), and/or nanowires. Recently, oriented anatase TiO2 NT arrays, prepared by a simple electrochemical anodization technique,12,13 have attracted a lot of attention for a variety applications, such as photoelectrochemical solar cells,14−19 gas sensors,20,21 H2 generation,22−24 and electrochromic devices.25,26 Electrodes consisting of oriented NT arrays also have several features that are suitable for Li-ion battery applications. The oriented pore structure of NT arrays is expected to facilitate one-dimensional electronic/ionic conduction and to accommodate volume changes during charge/discharge cycling. The large interior and exterior surfaces of NT walls are accessible to Li+ in the electrolyte, leading to a small Li+ insertion/extraction current density per surface area. The thin wall thickness provides short solid-state Li+ diffusion pathways and high tolerance to the structural changes during repeated Li+ charge/discharge processes. Despite these potential advantages associated with oriented NT arrays, there are only a few papers on the subject of using oriented anatase TiO2 NTs as electrode materials for Li-ion batteries.27,28 Important issues such as the interfacial contribution29−34 to the Li+ storage, charge/discharge kinetics, and the effect of NT morphology (e.g., NT film thickness) on the electrode characteristics have not been fully investigated. In this paper, we present results of our investigation on the electrochemical properties of oriented anatase TiO2 NT arrays © 2012 American Chemical Society

as electrode materials for Li-ion batteries. Galvanostatic charge/ discharge measurements showed that the Li+ storage capacities of TiO2 NT electrodes at relatively low charge/discharge rates were higher than the theoretical value for the bulk anatase TiO2 materials. Cyclic voltammograms (CVs) indicated that NT electrodes have both pseudocapacitive Li+ storage associated with the NT surface and the Li+ storage within the bulk material. The relative contribution of the pseudocapacitive and bulk storages depends strongly on the scan rate. Electrochemical characteristics of the NT electrodes showed virtually no dependence on the NT film thickness. The Li+ insertion/ extraction processes were found to occur homogeneously across the entire length of NT arrays. The implications of these results on the electronic and ionic conductions associated with the TiO2 NT electrodes are discussed.

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EXPERIMENTAL SECTION Aligned TiO2 NT arrays were prepared by electrochemically anodizing a Ti foil (Aldrich, 0.25 mm, 99.7% purity) in a twoelectrode cell, which contained a Pt counter electrode and 0.5 wt % NH4F (Aldrich, 99.99% purity) in glycerol (Alfa Aesar, 99% purity).16 The Ti foils were anodized at 20 V for 6−46 h at room temperature to produce NT arrays with lengths varying from 0.6 to 3.8 μm unless otherwise stated. In general, electrochemical anodization can be used to produce NTs with a length ranging from a few hundred nanometers to more than Received: February 26, 2012 Revised: April 7, 2012 Published: May 25, 2012 11895

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1000 μm.35 After electrochemical anodization, the as-anodized NT films were first cleaned with water, then soaked sequentially in the bath of 20/80, 40/60, 60/40, 80/20, and 100/0 vol% ethanol/water for 5 min each, and finally dried using a supercritical CO2 drying apparatus.36 After the postgrowth cleaning and drying treatments, the as-deposited NT arrays were annealed for 1 h in air at 400 °C (ramp rate 2 °C/min). Annealing NTs under this condition transforms the asdeposited amorphous titanium oxide phase to the crystalline anatase TiO2 phase, and no amorphous phase is present in the annealed NTs.37 The structural properties of NT films were characterized by scanning electron microscopy (SEM, JEOL JSM-7000F) and X-ray diffraction (XRD, SCINTAG DMS2000). Cyclic voltammetry and galvanostatic charge/discharge cycling were conducted at room temperature with a conventional three-electrode glass cell using an EG&G potentiostat/ galvonostat (model 263). The working electrode was made from annealed TiO2 NTs on a Ti foil without extra conducting additives and binders. Lithium foils (Alfa Aesar, 99.9% purity) were used as both the counter and the reference electrodes. The electrolyte was 1 M solution of LiPF6 in a 1:1 v/v mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (Ferro Corporation). The potential window for electrochemical studies was set between 1.4 and 3.0 V vs Li/Li+. All electrochemical characterizations were conducted in an argonfilled glovebox (VAC model EE-493) with moisture and oxygen levels below 0.5 ppm.

deposited NT arrays from amorphous materials to polycrystalline anatase TiO2, consistent with previous reports.16,38,39 The average crystallite size was around 30 nm as determined by applying Scherrer equation40 to the anatase (101) diffraction peak. Figure 2 shows the first five cycles of galvanostatic charge/ discharge curves conducted at 40 mA/g, which is approximately

Figure 2. First five cycles of the charge/discharge curves for the TiO2 NT electrode. The charge/discharge current was held at 40 mA/g.

equivalent to a C/4 rate for anatase TiO2 materials (where 1 C is defined as the current required to fully charge/discharge the electrode material in 1 h; 1 C ≈ 168 mA/g for the bulk anatase TiO2 materials).41,42 The discharge curve, corresponding to the Li+ insertion process, can be divided into three consecutive potential regions. The first region, where the potential decreased rapidly and monotonously from 3 to about 1.75 V (vs Li/Li+), corresponds to the formation of a solid solution domain (LixTiO2 with x up to 0.15) associated with the initial Li+ insertion process.43 In the second region, the potential of the TiO2 NT electrode approximately reaches a plateau at 1.75 V. This potential plateau signifies the biphasic region (i.e., coexistence of TiO2 and Li0.5TiO2), which is typical for Li insertion into anatase TiO2 electrodes.41 However, in contrast to the usually observed flat potential for an anatase TiO2 electrode in the second potential region, the TiO2 NT electrode displayed a slight decrease of potential with increasing discharge capacity. In the third region from 1.75 to 1.4 V, the potential of the NT electrode decreases linearly with the increasing capacity from about 120 to 200−250 mAh/g, signifying a characteristic pseudocapacitive behavior. Recent studies showed for anatase TiO2 nanoparticle films that this third potential region would become longer at the expense of a shorter second (plateau) region, primarily because of the large surface area associated with the small dimensions of nanoparticles.34,44 Similarly, the charge curves can also be divided into three consecutive potential regions, corresponding to the reverse processes (i.e., Li+ extraction) of the three regions of the discharge curves. In the charge process, the plateau potential during Li+ extraction (charge) process is reached at about 1.95 V vs Li/Li+, which is typical for the Li+ extraction from anatase TiO2 electrodes.41 Figure 3 shows the typical galvanostatic cycling performance of a TiO2 NT electrode at various charge/discharge rates; the film thickness of the TiO2 NT electrode was 1.7 μm. The initial discharge and charge capacities were about 250 and 190 mAh/ g, respectively. There are significant irreversible capacity losses during the initial several cycles, especially during the first cycle

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RESULTS AND DISCUSSION Figure 1 shows the typical surface and cross-sectional (inset) SEM images of TiO2 NT films annealed in air at 400 °C for 1 h.

Figure 1. Surface and cross-sectional (inset) SEM images of annealed TiO2 NT arrays.

NTs were closely packed in an approximately hexagonal symmetry. Analysis of these SEM images showed that the respective averages of the NT pore diameters, wall thicknesses, and center-to-center NT distances were 43, 12, and 73 nm. On the basis of a simple geometric consideration,16 we estimated a film porosity of 55%, an intertube spacing of 6 nm, and a NT outer diameter of 67 nm. The aspect ratio (defined as the ratio of the NT length to the NT outer diameter) of these NTs ranged approximately from 10 to 60. XRD measurements (data not shown) indicated that annealing transforms the as11896

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the cycling rate was increased from 0.2 to 7 C. In general, the decrease in the Li+ storage capacities at faster charge/discharge rates is attributable to the kinetic limitations inherent in one or more of the four basic steps associated with the Li+ insertion/ extraction processes: (i) electron transport within the electrode material, (ii) ion diffusion in the electrolyte, (iii) charge transfer at the electrode/electrolyte interface, and (iv) solid-state Li+ diffusion in the electrode material.45,46 The observed lack of the thickness dependence of the charge capacity for TiO2 NT electrodes implies that the electron transport across the NT electrode is not the rate-limiting step of the Li+ insertion/ extraction processes even when no extra conducting additives (e.g., carbon black) are used. This observation also indicates that the ion conduction within the NT pores (or between the NTs) is fast enough when compared to the charge/discharge rate used in this study. Thus, these results suggest that for the TiO2 NT electrodes the electron conduction within NTs and ion conduction in the electrolyte do not cause significant hindering of the charge/discharge kinetics. Figure 5a shows typical CVs of the TiO2 NT electrode measured at scan rates from 0.05 to 1.3 mV/s. The CV curve at the slowest scan rate (0.05 mV/s) shows a pair of cathodic/ anodic peaks at 1.75 and 1.95 V, which correspond, respectively, to the characteristic Li+ insertion/extraction potentials for the anatase TiO2 materials.41 The positions of the cathodic/anodic peaks agree well with the discharge/charge potential plateaus of galvanostatic measurements (Figure 2). The intensities of both cathodic (discharge) and anodic (charge) currents increased significantly at higher scan rates over the entire potential window. Figure 5b shows the dependence of the peak discharge current Ip on the scan rate v. Similar scan rate dependence is observed for the charge current. The best fit of the data to an apparent power-law dependence yields Ip ∝ v0.7. The apparent exponent value of 0.7 could presumably be attributed to a mixed process involving both the intercalation of Li+ in the bulk of TiO2 lattice and the pseudocapacitive storage of Li+ associated with the TiO2 NT surface. In general, the expression for a mixed Li+ storage process is given by42,47

Figure 3. Cycling performance of a TiO2 NT electrode at various charge/discharge rates as indicated.

at the lowest rate (C/4). The irreversible capacity loss is usually attributed to the trapping of Li+ at the defect sites of anatase TiO2 nanostructures and/or the irreversible reaction of Li+ with adsorbed water molecules.8,27,28,33 We expect no significant capacitance loss associated with the SEI formation because of the high operating potential (>1.4 V vs Li/Li+) for anatase TiO2 materials. The specific capacities for both discharge and charge processes were stabilized after a few cycles. The Coulombic efficiency at the fifth charge/discharge cycle increased approximately from 92% at a C/4 rate to >99% at rates greater than 1 C. These results indicate that anodically prepared TiO2 NT arrays are mechanically stable for accommodating structural changes during the repeated Li+ insertion/extraction processes. Moreover, no morphological changes of the NT arrays were observed (SEM image not shown) after >50 charge/discharge cycles, suggesting that the NT electrodes are structurally stable for Li+ storage, a requirement for having good cycling stability. Figure 4 shows the rate capability of the charge capacity for TiO2 NT electrodes with different film thickness (0.6−3.8 μm).

Ip = C1v + C2v1/2

(1)

The first term (C1v) on the right corresponds to the pseudocapacitive current (Ic), which has a linear dependence on the scan rate (as indicated by the upper dashed line in Figure 5b); the parameter C1 is determined by the product of AC, where A is the total electrode surface area and C is the amount of pseudocapacitive storage (capacitance) per electrode surface area. The second term (C2v1/2) corresponds to the (bulk) diffusion current (Id), which has a square-root dependence on the scan rate (as indicated by the lower dashed line in Figure 5b); the parameter C2 is determined by the product of 0.4958 nFAc(DαnF/RT)1/2, where n is the number of electrons, F is the Faraday constant, c is the maximum concentration of Li+ (or Ti3+) in the lattice, D is the solid-state diffusion coefficient of Li+ in the TiO2 electrode, α is the transfer coefficient, R is the ideal gas constant, and T is the temperature.48 The apparent power-law dependence of Ip on the scan rate is determined by the relative contribution of (bulk) diffusive and (surface) pseudocapacitive currents to the total Li+ storage current. If the total current were dominated by the Li+ intercalation into the bulk of TiO2 lattice, the exponent value would be 0.5; if, however, the total current were primarily

Figure 4. Effect of film thickness on the charge capacity of TiO2 NT electrodes as a function of the cycling rate. The specific capacities are obtained from the fifth charge/discharge cycle for each rate.

The capacity values were extracted from the fifth charge/ discharge cycle for each rate. For a given charge/discharge rate, there is essentially no difference in the capacity among NT electrodes with different film thicknesses, suggesting that the Li+ insertion/extraction processes occur homogeneously across the entire length of NT films. The capacity for all NT electrodes changed approximately from 190 to 50 mAh/g when 11897

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Figure 5. (a) CVs and (b) the peak discharge current of the TiO2 NT electrode measured at various scan rates (from 0.05 to 1.3 mV/s).

controlled by the pseudocapacitive Li+ storage at or near the surface of TiO2 electrode, the exponent value would be unity. The relative contribution of Id and Ic depends on C1, C2, and the scan rate. The best fit of the data to eq 1 (as indicated by the solid line in Figure 5b) yields C1 = 0.059 ± 0.012 and C2 = 0.0017 ± 0.0002. From these parameters, the respective values of Ic and Id at various scan rates can be determined, allowing for the evaluation of the relative contribution of bulk and surface storages of Li+ for the TiO2 NT electrodes. Figure 6 shows the calculated surface pseudocapacitive current (Ic = C1v) and bulk intercalation current (Id = C2v1/2)

TiO2 electrodes at relatively low rates (Figures 3 and 4). The surface storage of Li+ in TiO2 nanostructured electrodes has recently received some attention both experimentally and theoretically.49−51 It was shown for nanosized anatase TiO2 particles that a new stable crystal phase Li1TiO2 formed within a region of around 3−4 nm near the surface of TiO2 particles.50,51 Presumably, the formation of Li1TiO2 near the TiO2 NT surface accounts for the observed pseudocapacitive Li+ storage effect as well as the improved Li+ storage capacity in this study. It is noteworthy that in the near-surface region, where the pseudocapacitive reaction is dominant, the bulk insertion reaction may still exist. In principle, the contribution of the surface pseudocapacitive storage should increase with the surface area of NT electrodes. Thus, varying key morphological parameters (e.g., NT pore diameter and wall thickness) is expected to affect the pseudocapacitive properties of TiO2 NT electrodes. These studies will be useful for elucidating the morphological effects on Li-ion storage for TiO2 NT electrodes.

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CONCLUSIONS We investigated the electrochemical properties of the oriented anatase TiO2 NT arrays as Li-ion battery electrodes by galvanostatic method and cyclic voltammetry. The TiO2 NT arrays were prepared by electrochemical anodizing Ti foils followed by thermal annealing. Galvanostatic measurements showed that the Li+ storage capacity is higher for the NT electrode than the theoretical value for the bulk anatase TiO2 materials. The scan-rate dependence of the CVs revealed that the NT electrodes have both pseudocapacitive Li+ storage near the NT surface and diffusive Li+ storage within the bulk material. The relative contributions of surface pseudocapacitive and bulk storages depend strongly on the scan rate. The surface pseudocapacitive storage dominates the total storage capacity at scan rates above 1 mV/s. The rate capabilities of the NT electrodes were found to have no dependence on the NT film thickness, suggesting that the Li+ insertion/extraction processes occur homogeneously across the entire length of NT arrays. The observed lack of film thickness dependence of the charge capacity implies that for TiO2 NT electrodes both electron conduction along the NT walls and ion conduction in the electrolyte do not cause significant hindering of the charge/ discharge kinetics.

Figure 6. Calculated surface pseudocapacitive and bulk intercalation discharge currents.

as a function of the scan rate. At the lowest scan rate (0.05 mV/ s), the relative ratio of Ic:Id is approximately 1:4, indicating that the bulk Li+ storage dominates the total Li+ storage for TiO2 NT electrodes. Because of the respective linear scan-rate dependence for the pseudocapacitive current and the squareroot scan-rate dependence for the bulk intercalation current, the relative contribution of the pseudocapacitive storage to the total Li+ storage (eq 1) increases significantly at higher scan rate. At the scan rate of 1 mV/s, the pseudocapacitive storage capacity is approximately equal to the bulk storage capacity. With a further increase of the scan rate, the pseudocapacitive storage eventually dominates the total storage capacity. It is worth noting that the scan rate of 1 mV/s corresponds to a time of approximately 0.9 h for completing a full charge/ discharge cycle. The storage of Li+ associated with the surface along with the storage in the bulk TiO2 could explain the improved Li+ storage capacity (ca. 190 mAh/g) for TiO2 NTs than the theoretical capacity (ca. 168 mAh/g41,42) for the bulk

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AUTHOR INFORMATION

Corresponding Author

*Tel: +1-303-384-6353; Fax:+1-303-384-6150. E-mail: Kai. [email protected] (K.Z.) or [email protected] (A.J.F.). 11898

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy/ National Renewable Energy Laboratory's Laboratory Directed Research and Development (LDRD) program under Contract No. DE-AC36-08GO28308.

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