TiO2 nanotubes manufactured by anodization of Ti thin films for on-chip Li-ion 2D microbatteries

Electrochimica Acta 54 (2009) 4262–4268

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TiO2 nanotubes manufactured by anodization of Ti thin films for on-chip Li-ion 2D microbatteries Gregorio F. Ortiz a,b,∗ , Ilie Hanzu a , Philippe Knauth a , Pedro Lavela b , José L. Tirado b , Thierry Djenizian a a

University of Aix-Marseille I, II, III - CNRS, Laboratoire Chimie Provence (UMR 6264), Electrochemistry of Materials Research Group, Centre Saint-Jérôme, F-13397 Marseille Cedex 20, France Laboratorio de Química Inorgánica, Universidad de Córdoba, Edificio Marie Curie, Campus de Rabanales, 14071 Córdoba, Spain

b

a r t i c l e

i n f o

Article history: Received 31 October 2008 Received in revised form 22 January 2009 Accepted 25 February 2009 Available online 9 March 2009 Keywords: Anodization Self-organization Titania nanotubes Li-ion microbatteries Anode materials

a b s t r a c t The use of self-organized TiO2 nanotube arrays electrochemically grown onto Si is investigated for the fabrication of an alternative electrode dedicated to on-chip Li-ion 2D microbatteries. Discharge/charge curves and cycling performance are studied in lithium-anode electrochemical test cells for both amorphous and crystalline titania nanotubes. At 5 ␮A cm−2 amorphous TiO2 nanotube layers onto Si deliver a maximum areal capacity of 89 ␮Ah cm−2 in the first reversible discharge and 56 ␮Ah cm−2 over 50 cycles. We demonstrate that these nanostructured thin film electrodes showing such electrochemical performances are compatible with IC technology. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Electrochemical energy storage devices with high energy and power density have been widely studied because of their high capability in powering electric vehicles and portable electronic devices. Among the batteries of commercial application that are on the market (lead acid, nickel metal hydride and nickel cadmium batteries), lithium-ion batteries (such as graphite/liquid electrolyte/LiCoO2 ) present important advantages, such as high energy storage capacity, reduced size and weight and low toxicity. Nevertheless, work is still to be done to improve their benefits and the performances of the electrodes and the electrolyte [1–4]. Nowadays, the improvement of carbon-based materials, LiCoO2 , or the replacement of these electrode materials by others with different composition and structure are subjects of study. The key resides in the use of nanostructured materials that can improve the properties for lithium insertion and extraction [3]. TiO2 is a potential alternative for the graphite anode, because of higher voltage operation (ca. 1.75 V vs. Li+ /Li redox couple). This eliminates the risk of overcharging that may lead to growth of metallic lithium dendrites. Hence, the irreversible capacity due to

∗ Corresponding author at: University of Aix-Marseille I, II, III - CNRS, Laboratoire Chimie Provence (UMR 6264), Electrochemistry of Materials Research Group, Centre Saint-Jérôme, F-13397 Marseille Cedex 20, France. Tel.: +33 491637074; fax: +33 491637111. E-mail address: [email protected] (G.F. Ortiz). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.085

formation of the passive layer can be avoided, as the passive layer (SEI) would be very thin or inexistent. Without the passive layer, the overall electrode kinetics should be improved as there are no diffusion penalties introduced by a thick solid SEI. On the other hand, spinel-type LiMn2 O4 and olivine-type LiFePO4 are most promising cathode materials up to now [5–7] and 5 V materials have emerged as a different option, such as LiM0.5 Mn1.5 O4 (M = Ni, Co) and LiCoPO4 [8–11]. The high working potential of these materials makes possible a combination with a high voltage anode material, such as TiO2 and/or Li4 Ti5 O12 [12–14]. A cell built according to this concept belongs to the category of third generation Li-ion batteries. The properties of TiO2 electrodes may improve dramatically when the material is porous because of a large surface area. Due to exciting properties offered by self-organaized TiO2 nanotube (ntTiO2 ) layers such as porosity and many potential applications, the preparation of this kind of nanostructured titania by electrochemical techniques has attracted the interest of researchers [15–21]. An early report [22] demonstrated that the anodization of Ti foils in fluoride-containing medium leads to porous nanostructured titania. Since Macklin and Neat [23] reported the high capacity and reversibility of lithium insertion into titanium oxide electrodes in lithium batteries, a considerable research effort has been aimed at optimising and understanding the titanium oxide anode [24]. The aim of this work is to show that self-organized TiO2 nanotube layers electrochemically grown onto Si can be used as potential electrode for on-chip Li-ion 2D microbatteries. For this, we

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Fig. 1. Schematic representation of ntTiO2 grown onto two different substrates: (a) silicon and (b) Ti foil.

propose to study and compare the electrochemical performances of self-organized TiO2 nanotubes obtained from Ti foils (Refs. [16,18] provide examples about the fabrication process) and Ti thin films deposited onto Si substrates according to a procedure described in Ref. [25]. Layers of nanotubular titania (ntTiO2 ) are directly produced by anodization of Ti in fluoride-containing medium and without the use of any template. We have demonstrated that this kind of nanotubular morphology can be beneficial for an extra lithium accommodation into the structure apart from the interstitial and octahedral sites and allows the manufacture of electrodes without the use of additives and binders [26] in agreement with the study by Liu et al. [27]. The use of TiO2 reduces the overall cell voltage but it is reported to provide cells with enhanced safety, low self-discharge, good capacity retention on cycling and high power due to the possibility to be used in the nanoscale form. In addition, TiO2 is chemically stable, economically competitive, non-toxic, and an environmental “White Knight”. 2. Experimental Self-organized TiO2 nanotube layers with the same thickness were manufactured by anodization of two different Ti substrates (see Fig. 1). First, Ti thin films with a thickness of 1.92 ␮m were deposited by cathodic sputtering using a D.C. triode system onto Si substrates engraved from p-type Si(1 0 0) wafers with a resistivity of 1–10  cm [Wafer World, Inc.], following a similar procedure given in Ref. [25]. The second Ti substrate consisted of commercial Ti foils from Sigma Aldrich with a thickness of 250 ␮m and 99.7% purity. Anodization experiments were carried out by applying a constant voltage of 20 V during 20 min in a conventional two electrode cell. The working electrode consisted of titanium (Ti films sputtered onto Si or alternatively commercial Ti foil), while a platinum grid served as a counter electrode. Both electrodes were kept in a distance of 3 cm and the electrolyte was a solution of 1 M H3 PO4 , 1 M NaOH and 0.5 wt% HF. Prior to anodization, Ti was cleaned by sonicating in acetone, isopropanol and methanol during 30 min and afterwards the substrates were rinsed with distilled water and dried in Ar flow. To ensure good electronic contact of the electrodes, a

fast drying silver paint [Ted Pella, Inc.] was used. Optional thermal treatment at 450 ◦ C in air was performed during 3 h. X-ray-diffraction (XRD) patterns were recorded at room temperature using a Siemens D5000 diffractometer with Cu K␣ radiation (1.5406 Å). Scanning electron microscopy (SEM) images were obtained in a Philips XL-30 FEG SEM. The electrochemical performance of the ntTiO2 layers was studied by experiments in Li/LiPF6 (EC:DEC)/ntTiO2 cells. The electrolyte was embedded in a Whatman glass microfiber acting as a separator and the current collector for the ntTiO2 was a copper foil (99.99% purity). For these experiments no additives such as poly(vinyl difluoride), that acts as binder agent, and carbon black (conductive agent) were used as it has been previously described [26]. The electrodes were dried at 60 ◦ C under dynamic vacuum (1–3 mbar) during 16 h using BÜCHI Glass Oven B-585 and BÜCHI Vacuum controller V-800, respectively. Assembling of the cells was performed in a glove box filled with purified argon in which moisture content and oxygen level were less than 2 ppm. Lithium cells were galvanostatically cycled using an Arbin potentiostat/galvanostat multichannel system. For the discharge/charge reaction a constant current density of 5 (C/8) and 100 ␮A cm−2 (2.5C) was applied to the assembled cells in the range between 2.6 and 1.0 V. C/n rate refers to the intensity needed to react one Li per formula unit after “n” hours. 3. Results and discussion 3.1. Growth of ntTiO2 : self-organization mechanism and sample characterization During anodization at 20 V in 1 M H3 PO4 + 1 M NaOH + 0.5 wt% HF solution, the current response of the cell as function of the time is shown in Fig. 2. After applying the high potential, the current drops quickly in the first seconds and a compact oxide layer on Ti surface is formed. Then, the current density increases (2.5 min), because the pores are soaking through the barrier oxide layer exposing the titanium to the anodization solution, and afterwards decreases with time due to the increase of the diffusion length for ionic species in the electrolyte [28]. The nanotubes grow as a result of a competition

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between electrochemical oxide formation and chemical dissolution of TiO2 due to the action of fluoride ions according to the following reactions: Ti + 2H2 O → TiO2 + 4H+ + 4e− +

−

TiO2 + 4H + 6F → TiF6

2−

(1)

+ 2H2 O

(2)

SEM pictures of the resulting titanium dioxide nanotubes are shown in Fig. 3. From top view (Fig. 3a and c) and cross section (Fig. 3b and d) images the ntTiO2 present a tube diameter that varies from about 50 to about 200 nm and about 600 nm of tube length. The thickness of nanotubes can be estimated from reaction (1) and using Faraday’s law: L = QM(Fnı)

−1

(3) (C cm−2 ),

Fig. 2. Current–time curves for the 20 min of anodization at 20 V in 1 M H3 PO4 + 1 M NaOH + 0.5 wt% HF solution. Dotted line corresponds to Ti film deposed onto Si substrate, and solid line to Ti film.

M the molecular where Q is the circulated charge weight of the oxide (79.9 g mol−1 ), F is the Faraday constant (96500 C equiv−1 ), n is the number of electrons involved in the reaction and ı is the density of TiO2 (3.8–4.1 g cm−3 ). Therefore, the theoretical thickness of the nanotube layers should be 3500 nm, leading to an anodization efficiency of about 20%. This relative low value can be explained by the competition between reactions (1) and (2), i.e., between growth and dissolution of the nanotube layer [16].

Fig. 3. SEM images of TiO2 nanotube layers after anodization (20 V—20 min) in a fluoride-containing electrolyte: (a) top view, and (b) cross-section of the amorphous TiO2 nanotubes layers onto Si Substrate; (c) top view, and (d) cross-section of the amorphous TiO2 nanotubes layers onto Ti foil substrate; (e) top-view SEM image of ntTiO2 after annealing at 450 ◦ C.

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Fig. 4. XRD patterns of: (a) as-prepared and (b) after heat treatment at 450 ◦ C of TiO2 nanotube layers onto Si substrate; (c) as-prepared and (d) obtained at 450 ◦ C of TiO2 nanotube layers onto Ti foil substrate. Samples obtained by a simple anodization under a potential of 20 V during 20 min.

Fig. 4 shows the XRD pattern of ntTiO2 layers onto Si substrate and Ti foil that were obtained at room temperature (R.T) and after thermal treatment at 450 ◦ C. Independently of the employed substrate, the ntTiO2 nanotubes obtained at R.T are X-ray amorphous (Fig. 4a and c). The Ti layer deposed on Si substrate was identified as hexagonally close-packed ␣-Ti (JCPDS file no.: 44-1294). On the other hand, for the commercial Ti foils all the peaks can be indexed as Ti (JCPDS file no. 5-682). A thermal treatment of the samples at 450 ◦ C during 3 h in air leads to a partial conversion into anatase. In Fig. 4(b and d) two new and low intense peaks at about 23.4◦ and 48◦ turn up; they correspond to (1 0 1) and (2 0 0) reflections of anatase TiO2 structure (JCPDS file no. 21-1272), respectively. Peaks of Ti are still present after growing the ntTiO2 onto Si because a layer of Ti (1.25 ␮m in thickness) remains after anodization (see SEM image in Ref. [25]). Therefore, only 35% of the initial Ti layer is converted into ntTiO2 . This remaining Ti layer is not electroactive versus lithium as it is discussed later. After the thermal treatment, the nanotube morphology remains basically unmodified as can be seen from Fig. 3e. 3.2. Electrochemical behaviour of samples in Li cell Because of its relevance to batteries, the mechanism of lithium insertion into anatase TiO2 has been extensively studied. The electrochemical insertion/extraction of Li is believed to be driven by the accumulation of electrons in TiO2 electrodes in contact with Li+ containing electrolytes, and the overall cell reaction can be written

as: TiO2 + xLi+ + xe− ↔ Lix TiO2

(4)

The crystalline structure of anatase is tetragonal (s.g. I41 /amd) contains distorted TiO6 octahedra, which define a series of octahedral and tetrahedral vacant sites. These sites allow lithium uptake of 0.5 Li per formula unit, corresponding to a theoretical capacity of 168 mAh g−1 [29]. A two-phase mechanism has been suggested to describe the electrochemical insertion of lithium into anatase, involving equilibrium between Li-poor (tetragonal) and Li-rich (orthorhombic) phases [30]. The latter phase results from a structural distortion caused by a cooperative Jahn–Teller effect, as the incoming electrons increase the d electron density in localized Ti-d orbitals above a critical intercalation concentration. In the following, all capacity values (Table 1) are expressed per electrode area to allow better comparison with literature data for thin film microbatteries. In Fig. 5a and b are represented the galvanostatic discharge/charge curves versus composition of the electrodes on Si substrates using a current of 5 ␮A cm−2 (C/8). The crystallized electrode exhibits a pseudo-plateau at 1.75 V during the discharge. During the charge a potential plateau at 1.95 V is visible. These two potential plateaus (Fig. 5a) correspond to Li+ insertion and deinsertion from interstitial and octahedral sites of crystalline anatase TiO2 nanotubes. A total capacity of 165 ␮Ah cm−2 was obtained in the first discharge, which includes reversible and irreversible reactions. The first reversible capacity obtained in the second discharge was 76 ␮Ah cm−2 . For amorphous nanotubes, no

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Table 1 Reversible and irreversible areal capacities and efficiency on cycling of the ntTiO2 layers manufactured onto Si substrates and Ti foil used as electrodes in experimental test cells. Electrode

Kinetic (␮A cm−2 )

Total capacity (␮Ah cm−2 )

Irreversible capacity (␮Ah cm−2 )

1st reversible capacity (␮Ah cm−2 )

50th reversible capacity (␮Ah cm−2 )

Efficiency (%)

Amorphous ntTiO2 on Si

5 100

196 45

107 19

89 26

56 17

63 65

Crystalline ntTiO2 on Si

5 100

165 34

89 10

76 24

40 23

53 96

Amorphous ntTiO2 on Ti foil

5 100

129 41

76 18

53 23

37 15

70 65

Crystalline ntTiO2 on Ti foil

5 100

83 39

34 13

49 26

29 20

60 77

Fig. 5. Galvanostaic discharge/charge curves during the first 20 cycles of ntTiO2 nanotube layers obtained onto Si substrates: (a) after thermal treatment at 450 ◦ C, and (b) as-prepared. (c) Evolution of its areal capacity as a function of cycle number using a kinetic of 5 (C/8) and 100 (2.5C) ␮A cm−2 .

Fig. 6. Galvanostaic discharge/charge curves during the first 20 cycles of ntTiO2 nanotube layers obtained onto Ti foils: (a) after thermal treatment at 450 ◦ C, and (b) as-prepared. (c) Evolution of its areal capacity as a function of cycle number using a kinetic of 5 (C/8) and 100 (2.5C) ␮A cm−2 .

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Fig. 7. Ex situ SEM images of ntTiO2 nanotube layers stopped at 2.6 V in the charged state and after cycle number 50: (a and c) top-view, and (b and d) cross-section of samples onto Si and Ti foil substrates, respectively.

plateau is observed during the discharge/charge (Fig. 5b). Only one pseudo-plateau at about 1.15 V contributes to a large irreversible capacity. The total capacity in the first discharge (196 ␮Ah cm−2 ) and the first reversible capacity (89 ␮Ah cm−2 ) are higher than for crystalline structure. From these results, the irreversible capacity is strongly dependent on the structure of ntTiO2 . The relatively high irreversible capacity can be attributed to different phenomena. First, the irreversible reaction of Li+ with adsorbed water molecules onto the ntTiO2 electrode [31]. Second, the formation of a very thin disordered layer at the electrode surface that may appear on both amorphous and crystalline electrodes [26,32]. Water is certainly still present in some proportion in annealed samples, because the annealing temperature is not high enough to completely remove strongly chemisorbed water [26] or bound water [33]. The difference in the irreversible capacity can be explained by the fact that an annealing treatment removes structural and chemical defects in the amorphous phase that act as Li+ ion traps [26]. A cycling life study at two different kinetics was carried out (Fig. 5c). The Li intercalation can be viewed as a filling of titanium d levels by neglecting the influence of Li levels. The partially filled titanium levels are the t2g set in an ideal octahedral coordination. Such behaviour is expected for compounds which are able to incorporate Li easily and reversibly [34]. At 5 ␮A cm−2 the capacity for amorphous ntTiO2 onto Si is about 56 ␮Ah cm−2 after 50 cycles leading to an efficiency of about 63%. The ntTiO2 annealed at 450 ◦ C presented a maximum capacity of 76 ␮Ah cm−2 and after 50 cycles 40 ␮Ah cm−2 resulting in an efficiency of about 53%. These results suggest that amorphous nanotubes can accommodate extra Li into its structure as compared with heated sample. Part of this enhancement is due to the amorphous structure of titania nanotubes, which facilitates an extra lithium insertion. Other possible reasons for the high experimental capacity values are the high surface area and highly organized 1D structure of titania nanotube layers. When a rate of 100 ␮A cm−2 (2.5C) is used the reversible and irreversible capacities are lower than at a rate of 5 ␮A cm−2 but the capacity

retention for crystalline is around 96% (open and triangle-shaped symbols in Fig. 5c and Table 1). Compared with other thin layer-based electrodes that have been reported in literature (e.g., MoS2 -based planar Li-ion battery [35], and titania films [32]) ntTiO2 thin films supported onto Si substrate show an areal capacity of 56 ␮Ah cm−2 after 50 cycles, suggesting that this electrode is compatible with IC technology and is a potential candidate for the fabrication of on-chip 2D microbatteries. Similar behaviour was found in the galvanostatic curves of ntTiO2 samples manufactured on commercial Ti foil (Fig. 6a and b). The main difference between amorphous and crystalline nanotubes resides in the observation of a voltage plateau during the discharge (1.75 V) and charge (1.95 V) of the cell during the first 20 cycles. Under low kinetics the amount of lithium inserted into amorphous (53 ␮Ah cm−2 ) is higher than in crystalline ntTiO2 (49 ␮Ah cm−2 ), and they exhibit after 50 cycles an efficiency of 70 and 60%, respectively. A cycling life study of the electrodes under fast kinetics (100 ␮A cm−2 , 2.5C) show that capacity retention is better in crystalline (77%) than amorphous ntTiO2 (65%), but the capacity value is about 20 ␮Ah cm−2 (Fig. 6 and Table 1). The better electrochemical performances exhibited by the ntTiO2 layers grown onto Si substrate is attributed solely to the presence of the nanotubes. According to literature, silicon and titanium cannot react with Li+ between 2.6 and 1.0 V. Indeed, Si is only electroactive in the region that ranges between 0.45 and 0.01 V versus lithium [36,37] forming LiSi and Li3.6 Si alloys during the discharge of the cell. Ti metal is not electroactive versus Li+ and do not form alloys as corroborated by other authors who have also used Ti foils for the production of TiO2 nanotubes [27,32]. Therefore, the better capacities obtained from ntTiO2 layers formed onto Si are attributed to nature of Ti thin film. Compared to ntTiO2 layers grown from mechanically polished Ti foils, anodization of titanium thin film obtained by physical vapour deposition (PVD) process leads to the formation of highly flat ntTiO2 layer. Thus, we assume that the contact between the ntTiO2 layers with the electrolyte is drastically improved.

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3.3. SEM study of cycled electrodes The structural rigidity of nanostructured materials is a positive property in order to achieve extended cycle life in advanced electrodes. To assess the stability of the self-assembled titania nanotubes, ex situ SEM images of cycled electrodes after 50 cycles at 2.6 V charge voltage are collected in Fig. 7. For the sample supported onto a Si substrate, the characteristic nanotube geometry – about 600 nm nanotube length (cross-sectional view), and between 50 and 200 nm diameter (top view) – is preserved. However, the geometry of the ntTiO2 manufactured on Ti foil is affected after reaction with lithium (Fig. 7c and d). These analyses demonstrate the robustness of the nanotube morphology for those samples grown onto Si substrate, which survive after repeated cycling. 4. Conclusions Self-organized TiO2 nanotubes were obtained by a powder-free fabrication method based on the electrochemical anodization of titanium foil or titanium sputtered onto a silicon substrate. The TiO2 nanotubes prepared onto Si substrate had an inner diameter between 50 and 200 nm and about 600 nm length. This method ensures good electrical contact between titania nanotube layers and the current collector. Also, the resulting array shows a special mechanical stability and buffers possible volume changes arising from the structural tetragonal–orthorhombic conversion of anatase, or possible expansion on insertion in amorphous titania. Lithium test cells using self-organized TiO2 nanotubes as active electrode material deliver reversible areal capacity after 50 cycles of about 56 ␮Ah cm−2 leading to an efficiency of 63%. It can be noticed that crystalline ntTiO2 onto Si shows the best efficiency (96%) at 100 ␮A cm−2 although areal capacity is relatively low. The analysis of cycled electrodes suggests that the nanotube morphology persists on cycling. These results suggest that the highly flat ntTiO2 layer supported onto Si substrate can be used as an alternative electrode for rechargeable on-chip 2D Li-ion microbatteries. Another observed advantage is that ntTiO2 avoids the potential risk of lithium electrodeposition.

fully acknowledged. We thank A. Tonetto and A. Garnier for helpful assistance in SEM and XRD measurements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

Acknowledgements G.F. Ortiz is indebted to the Ministerio de Educación y Ciencia through the Programa José Castillejo and Junta de Andalucía (contract no. FQM 1447). The financial support by ANR Programme Blanc (LIBAN project) and European Research Institute ALISTORE is grate-

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