Ultra-thin film electrodes of tetratitanate nanosheets

Solid State Ionics 204-205 (2011) 66–72

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Ultra-thin film electrodes of tetratitanate nanosheets☆ Shinya Suzuki ⁎, Masaru Miyayama Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153–8904, Japan

a r t i c l e

i n f o

Article history: Received 24 June 2011 Received in revised form 28 September 2011 Accepted 29 September 2011 Available online 26 October 2011 Keywords: Nanosheets Tetratitanate Diffusion coefficient Thin film Li-ion batteries

a b s t r a c t The lithium intercalation properties of ultra-thin films of tetratitanate nanosheets were examined to determine the intrinsic electrode performance and the influence of nanosheet layer lamination on the electrode properties. Ultra-thin films of tetratitanate nanosheets were prepared by layer-by-layer assembly directly onto a Pt current collector. The ultra-thin film of tetratitanate nanosheets prepared by single deposition operation exhibited a specific capacity of 0.13 μAh cm− 2, which is a large value corresponding to the reduction and oxidation of approximately 87% of the Ti ions. A change in the rate controlling process from a charge transfer reaction to diffusion was observed by cyclic voltammetry at 50 mV s− 1. The lithium ion diffusion coefficient was determined by analyzing the results computationally with a mathematical model, and it was found to be 3 ×10− 10 cm2 s− 1. The nanosheets exhibited high reactivity in a thin film with an estimated thickness of 9.1 nm. Sufficient electronic conductivity of the electrode, whose thickness is below 10 nm, and sufficient lithium transportation through the electrode contribute to the high reactivity. © 2011 Elsevier B.V. All rights reserved.

1. Introduction There has been a great demand for energy storage devices with high energy and power densities for application to electric/hybrid vehicles. Lithium-ion secondary batteries have a high energy density and good cyclability [1] and are therefore suitable for high-power applications. However, Li-ion batteries have latent problems in high-power applications, namely, a fairly small lithium ion diffusion coefficient in the solid phase. Nanoscale materials with a small diffusion length on the order of nanometers have been widely investigated as electrode materials for high-power Li-ion batteries [2,3]. The electrochemical properties of nanoscale electrode materials are affected by the large surface area. For example, it has been reported that the surface energy in anatase-type TiO2 [4] and olivine LiFePO4 [5] decreases the miscibility gap and stabilizes a solid solution state. Unusual charge/discharge curves have been reported for LiCoO2, with a crystallite size below 30 nm due to the spatial distribution of the energy required for the extraction/ insertion of lithium ions [6]. Nanosheets are two-dimensional nanoscale materials prepared by disintegrating a layered compound into a single layer or several layers [7,8]. These unilamellar or multilamellar crystallites have thicknesses on the order of nanometers, with submicrometer to micrometer lateral dimensions. We previously reported the lithium intercalation properties of octatitanate synthesized by reassembling titanate nanosheets

☆ Contribution to Solid State Ionics. ⁎ Corresponding author at: Miyayama Lab. Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan. Tel.: +81 3 5452 5082; fax: +81 3 5452 5083. E-mail address: [email protected] (S. Suzuki). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.09.025

using a salt deposition reaction with hydrochloric acid [9]. The octatitanate synthesized by reassembly of nanosheets exhibited a larger capacity than conventional octatitanate. In general, nanoscale materials tend to form aggregates, and it is difficult to homogeneously mix nanoscale materials with fine particles of conducting carbon additives. Insufficient electronic conductivity through the electrode restricts the high-rate capability of nanoscale materials and causes the capacity to decrease during discharge/charge cycling [10]. Fast lithium ion transportation through the electrode is also required to sustain a large current and to exhibit a high-rate capability in nanoscale materials. Various materials have been investigated with the goal of producing high-rate electrodes with these merits through microstructural control [11–13]. We have reported the preparation and electrode properties of a porous composite of carbon fiber and nanosheets [14]. Nanosheets are notable building blocks for electrode materials with microstructural controls for high-power Li-ion batteries, since nanosheets are considered to be the minimum units of materials used to express several functions, including electrochemical lithium insertion [15,16]. Nanosheets were obtained as a colloidal suspension, and homogeneous composites of nanosheets and carbon particles were easily prepared from a dispersion of a carbon and nanosheet mixture. The composites with carbon fibers showed an excellent high-rate capability, that is, a large capacity of 190 mAh g− 1 at a low current density of 100 mA g− 1, and a large capacity of 130 mAh g− 1 at a low current density of 10 A g− 1 [14]. The high-rate electrode performance is expected to be increased by the optimization of the electrode's microstructure. It is essential to determine the following in order to optimize the microstructure of a high-rate electrode composed of nanosheets: (i) the intrinsic electrode performance of the nanosheets and (ii) the influence

S. Suzuki, M. Miyayama / Solid State Ionics 204-205 (2011) 66–72

of the nanosheet layer lamination on the electrode properties. These cannot be determined using electrode materials obtained by restacking nanosheets since it is almost impossible to control the layer lamination. In addition to the self-restacking of nanosheets, several methods of stacking nanosheets are already known, for example, the layer-bylayer assembly of nanosheets and cationic polymers [16] and the electrophoretic deposition (EPD) of negatively charged nanosheets [17]. Among these, layer-by-layer assembly is the most suitable method for controlling the number of nanosheet layer laminations. Sasaki and his co-workers have already reported that lithium intercalation properties of ultra-thin films composed of titanate nanosheet [16], but lithium intercalation properties were not sufficiently revealed due to the narrow potential range for electrochemical measurements. In this study, ultra-thin film electrodes of tetratitanate nanosheets were prepared using the layer-by-layer assembly method, and their high-rate electrode properties were examined to reveal their intrinsic properties as intercalation electrodes.

2. Experimental The fibrous tetratitanate hydrate, H2Ti4O9·1.9H2O (Otsuka Chemical), which had a length of 20 μm and a diameter of 200 nm, was used as the starting material. A weighed amount (1.0 g) of the fibrous tetratitanate powder was dispersed in 200 cm3 of an aqueous solution of tetrabutylammonium hydroxide (TBAOH), and the dispersion was shaken for 2 days or 10 days at room temperature [7]. A tetratitanate was reacted with twofold moles of TBAOH. A colloidal suspension of tetratitanate nanosheets was obtained by the exfoliation of tetratitanate by the reaction with TBAOH. Relatively thick nanosheets were separated from the unreacted particles by centrifugal separation at 10,000 rpm, and the supernatant was used in the following procedure. Pt/Ti/SiO2/Si (Pt/Si) substrates were used to fabricate electrodes for electrochemical measurements. Ultra-thin films of tetratitanate nanosheets were prepared by the following layer-by-layer deposition onto the Pt surface. The substrates were precoated with polyethylenimine (PEI) by immersing them in an aqueous solution of PEI at pH 9 for 30 min. The PEI-primed substrates were then immersed in a colloidal suspension of tetratitanate nanosheets at pH 9 with a concentration of 60 mg dm − 3, followed by washing with pure water. These operations were repeated to obtain multilayer thin films [16]. Electrochemical measurements were performed using a beakertype three-electrode cell with lithium strips as the reference and counter electrodes, and the obtained films on Pt/Si substrates used directly as the working electrode. The electrolyte solution was 1 mol dm − 3 lithium perchlorate in propylene carbonate (Kishida Chemical). Cyclic voltammetry tests were performed in a voltage range of 0.8–3.2 V (vs. Li/Li+) using a HAG-5001 potentiostat/galvanostat (Hokuto Denko).

5.0 nm

a

67

3. Results and discussion 3.1. Characterization of tetratitanate nanosheets Fig. 1(a) and 1(c) shows atomic force microscope (AFM) images of the obtained nanosheets deposited on mica substrates. The tetratitanate nanosheets obtained by shaking in an aqueous solution of TBAOH for 2 days are shown in Fig. 1(a). The lighter color clearly shows small flakelets. The lateral dimensions of these particles are approximately 200 nm × 1 μm. Fig. 1(c) shows an AFM image of tetratitanate nanosheets obtained by shaking for 10 days. Relatively small particles with lateral dimensions of approximately 200 nm × 200 nm are observed. Fig. 1(b) and 1(d) shows cross-sectional analyses at the lines indicated in Fig. 1(a) and 1(c), respectively. The average thickness of the observed particles was approximately 1.7 nm. This 1.7-nm thickness suggests that the particles contained 2 oxide layers since the basal spacing of layered titanate hydrate is 1.0 nm, and the thickness of an oxide layer is 0.6 nm [18]. The AFM images clearly show the nature of the nanosheets. Fig. 2(a) and 2(b) shows scanning electron micrograph of the starting fibrous tetratitanate and schematic image of the disintegration of fibrous tetratitanate. Interlayer guest cations and hydrated water molecules are omitted for comfortable viewing. A tetratitanate has a layer structure and one oxide layer consists of liner group of four TiO6 octahedra with overlapping edges. The disintegration of tetratitanate into nanosheets occurs along the bc-plane. The lateral dimension of 200 nm× 1 μm for the obtained tetratitanate nanosheets shown in Fig. 1(a) obviously demonstrates that the nanosheets were formed by the disintegration of the fibrous tetratitanate [9, 17]. The long sides and short sides of the tetratitanate nanosheets are parallel to the b-axis and c-axis of original tetratitanate, respectively. These results also suggest that the exfoliation of the tetratitanate into nanosheets was completed within 2 days. The tetratitanate nanosheets were torn into small particles by further shaking. The nanosheets with smaller dimensions and larger dimensions are hereafter denoted as NS-S and NS-L, respectively. 3.2. Electrode properties of ultra-thin films of tetratitanate nanosheets A ultra-thin film of NS-L (NS-L-1) was fabricated onto a Pt/Si substrate by single depositing operation. The surface coverage of NS-L-1 was determined by SEM observation to be approximately 75%. Fig. 3 shows cyclic voltammograms for the Pt/Si substrate and NS-L-1 fabricated onto the Pt/Si substrate measured at a sweep rate of 10 mV s − 1. It also shows a cyclic voltammogram for fibrous tetratitanate hydrates measured at 0.1 mV s− 1 for comparison. Irreversible reduction current was observed for both Pt/Si substrate and NS-L-1 at first 10 cycles. After that, stable capacity was observed. The cyclic voltammograms shown

5.0 nm

c

0 nm

0 nm

500 mn

500 mn

d

b 1 nm

1 nm

Fig. 1. Atomic force micrographs of (a) NS-L and (c) NS-S deposited on mica substrate. (b) and (d) show cross-sectional analyses at the lines indicated in (a) and (c), respectively.

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a

1 m exfoliation

b

a c

b

tetratitanate

tetratitanatenanosheet

Fig. 2. (a) Scanning electron micrograph of the starting fibrous tetratitanate, and (b) schematic image of the disintegration of fibrous tetratitanate.

þ

H2 Ti4 O9 ·nH2 O þ xLi þ xe←→H2 Lix Ti4 O9 ·nH2 O

ð1Þ

100

10

0

0 NS-L-1 Substrate

-10

H2Ti4O9·1.9H 2O

1

2

3

-100

Current density / mA g-1

Current density / A cm-2

The reaction potential was lower than that of the fibrous tetratitanate hydrates. The change in the reaction potential would be caused by the change in the distortion of the TiO6 octahedra due to the exfoliation into nanosheets [19]. Fig. 4 shows discharge and charge curves for NS-L-1 and Pt/Si substrate measured at a current density of 1 μA cm − 2. This result shows that the charge capacity of NS-L-1 above 2.3 V is caused by Pt/Si substrate, and it is reasonable considering cyclic voltammograms shown in Fig. 3. The discharge and charge curves for NS-L-1 below 1.8 V showed capacitor-like behavior where the potential changes linearly with the amount of stored charge. The reversible capacity caused by reduction and oxidation of tetratitanate nanosheets is estimated to be 0.13 μAh cm − 2. Normalization of the capacity by the weight was impossible due to the smallness of the mass loadings. Then, the

Potential / V (vs. Li/Li+) Fig. 3. Cyclic voltammograms of NS-L-1 measured at a sweep rate of 10 mV s− 1 in comparison with the Pt/Si substrate (10 mV s− 1) and tetratitanate hydrates (0.1 mV s− 1).

reaction formula was estimated as follows. The area density of the Ti ions in tetratitanate nanosheets composed of 2 oxide layers was 3.553 × 10 15 cm − 2 [20]. Consequently, the theoretical capacity was estimated to be 0.1186 μAh cm − 2 for a monolayer thin film of tetratitanate nanosheets with 75% surface coverage, on the assumption that all of the Ti ions were electrochemically reduced and oxidized. The observed capacity was larger than the theoretical capacity without considering the nanosheet overlaps. When the nanosheet overlaps are assumed to be 20% (20% of the surface is covered with 4 oxide layers and 55% is covered with 2 oxide layers), approximately 87% of the Ti ions are reduced and oxidized. This value is much larger than the values reported for conventional TiO2 powders [21] or reassembled nanosheets [9] or 2D nanostructured anatase TiO2 [22, 23]. The high reactivity and charge–discharge voltage profile were very similar to those of carbon-supported stacked TiO2 nanosheets reported by Liu et al. [24]. Nanosheet structure composed of TiO2 causes capacitor-like behavior during charge–discharge test. The fact that almost all of the nanosheets are in contact with both the electrolyte solution and current collector contributes to their high reactivity. Capacity fade per cycle was 0.5%. Cycleability was not good but enough for following examinations. Relatively poor cycleability is due to the electrode structure where the active material is directly deposited onto current collector without binder.

Potential / V (vs. Li/Li+)

here were obtained after 20 cycles. Reversible reduction and oxidation currents were observed for NS-L-1. The peak potentials for the reduction and oxidation currents of NS-L-1 were 1.3 and 1.5 V, respectively. This results exhibit that electrochemical lithium insertion and extraction reactions occur on tetratitanate nanosheets. The reaction would be expressed as follows:

3 Substrate

NS-L-1

2

1 0

0.1

0.2

0.3

Specific capacity / Ah cm-2 Fig. 4. Discharge and charge curves of NS-L-1 obtained at 50th cycle. The current density was 1 μA cm− 2.

S. Suzuki, M. Miyayama / Solid State Ionics 204-205 (2011) 66–72

100 80 60 40 20 0 0 10

NS-S-1 NS-L-1

Current density / µA cm-2

a 10

0 Sweep rate / mV s 2, 3, 5, 10

-10

1

Current density / µA cm-2

200 0 -200

-1

Sweep rate / mV s 200, 300, 500, 1000

-400 1

2

Potential / V (vs. Li/Li+)

Sweep rate / mV

103

s-1

Fig. 5. Normalized capacities of NS-L-1 and NS-S-1 measured at various sweep rates. The capacities were normalized by that at a sweep rate of 1 mV s− 1.

3

Fig. 6. Cyclic voltammograms of NS-L-1 measured at (a) relatively low sweep rates below 10 mV s− 1 and (b) relatively high sweep rates above 200 mV s− 1.

The long side of the rectangular-shaped NS-L was longer than the sides of NS-S, and the diffusion along the long side of NS-L dominates the reaction rate at a relatively high sweep rate. The long sides of NS-L are parallel to the b-axis of original tetratitanate as shown in Fig. 2(b). We recently reported that diffusion of lithium ion along the b-axis in layered alkali tetratitanate A2Ti4O9·nH2O (A= Li, Na) controls the rate of lithium intercalation reaction [26]. Thus, it is reasonable that the lithium diffusion along the long sides of NS-L dominates the reaction rate. 3.3. Mathematical model development The following assumptions are applied when we simulate the lithium insertion/extraction behavior of a platelet particle following the application of a potentiodynamic stimulus [27] (Fig. 8). (a) The one-dimensional diffusion of lithium ions along the sides which are parallel to the original b-axis of tetratitanate dominates the process, and the flux of the lithium ions in the platelet particles follows Fick's second law. (b) The lithium ion diffusion coefficient is constant. (c) A Butler–Volmer type reaction at the short sides of the nanosheets governs the charge-transfer kinetics

103

102

101 NS-S-1 NS-L-1 Simulated NS-L-1

100

102

3

400

100 101

2

Potential / V (vs. Li/Li+)

-1

b

Peak current density / µA cm-2

Normalized capacity / %

Cyclic voltammetry tests were performed at various sweep rates in order to clarify the reaction mechanisms of the ultra-thin film electrodes. The capacities were determined by integrating the oxidation current, and the relationship between the capacity and sweep rate is shown in Fig. 5. Monolayer thin films of NS-S (NS-S-1) were also fabricated onto a Pt/Si substrate. The same measurements were conducted for NS-S-1, and the results are also shown in Fig. 5. The capacity of NS-S-1 was larger than that of NS-L-1. A non-uniform deposition and an overlapping of nanosheets easily occurred in the layer-by-layer assembly when relatively small nanosheets with submicrometer lateral dimensions were used. Non-uniform deposition means that monolayer thin film was obtained in some places on the surface of the substrate as shown in Fig. 1(c), and nanosheets were overlapped to form trilayer or more film in other places. The overlapping of nanosheets is responsible for the large capacity. For comparison, the capacities were normalized by that measured at a relatively low sweep rate of 1 mV s − 1. Both electrodes showed capacity fading with an increase in the sweep rate. The fading rate for NS-L-1 was larger than that for NS-S-1 for sweep rates above 20 mV s − 1. NS-S-1 maintained 39% of its capacity at a high sweep rate of 1 V s − 1. NS-S-1 showed a good high-rate capability. The high-rate capability of NS-L-1 was inferior to that of NS-S-1. To clarify the reason, detailed analyses of the cyclic voltammograms were conducted. Fig. 6(a) and 6(b) shows the cyclic voltammograms for NS-L-1 measured at (a) relatively low sweep rates below 10 mV s − 1 and (b) relatively high sweep rates above 200 mV s − 1. The reduction and oxidation currents increased with an increase in the sweep rate, but there is a difference in the rates of increase for the peak current densities shown in Fig. 6(a) and 6(b). The relationship between the reduction peak current density and sweep rate is replotted as a double logarithmic plot in Fig. 7. The change in the slope is clearly observed in Fig. 7. The slope changed from 0.86 under a relatively low sweep rate below 10 mV s − 1 to 0.52 under a relatively high sweep rate above 200 mV s − 1. When the reaction rate is controlled by the charge transfer reaction between the electrode and electrolyte, the peak current density in the cyclic voltammogram increases in proportion to the sweep rate [25]. In this case, the slope in the double logarithmic plot in Fig. 7 is 1. On the other hand, when the reaction rate is controlled by the diffusion, the peak current density varies as the square root of the sweep rate [21], and the slope in Fig. 7 is 0.5. A change in the rate controlling process from a charge transfer reaction to diffusion was observed by cyclic voltammetry at 50 mV s − 1. The same analyses were conducted for NS-S-1, and these results are also shown in Fig. 7. The slope for NS-S-1 is 0.81 under a wide range of sweep rates, and no obvious change in the slope is observed. These results indicate that the reaction rate for NS-S-1 was not controlled by diffusion, but was controlled by the charge transfer reaction at all of the measured sweep rates, and that the lateral dimensions of the nanosheets caused the difference in the rate-determining process.

69

101

102

103

Sweep rate / mV s-1 Fig. 7. Relationships between the reduction peak current densities of NS-L-1 and NS-S1 in cyclic voltammograms and the sweep rate. The results of a computational simulation with a mathematical model are also indicated (D: 3 × 10− 10 cm2 s− 1, k: 1 × 10− 4 cm5/2 s− 1 mol− 1/2).

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S. Suzuki, M. Miyayama / Solid State Ionics 204-205 (2011) 66–72

b-axis

deposited directly onto the Pt current collector. The open circuit potential, U, as a function of the lithium ion concentration at the edge of the nanosheets, is expressed as [29]:

c-axis

x 0

-L

c

cl

x L

Fig. 8. Schematic image of the reaction model of a nanosheet. Arrows indicate the directions of fluxes.

at the electrode/electrolyte interface. (d) The double layer capacitance at the electrode/electrolyte interface is neglected. (e) Side reactions such as the decomposition of the electrolyte do not occur. (f) The concentration of the lithium ions in the electrolyte is constant. (g) The potential is uniform in the particle. The charged state is employed as an initial condition. For a single nanosheet, the governing equations are: Fick's second law, 2

∂c ∂ c ¼D 2; ∂t ∂x

ð2Þ

where x is the distance from the center of the long side, c is the concentration of lithium ions, and D is the diffusion coefficient of the lithium ions. Initial condition: cðx;0Þ ¼ 0:

ð3Þ

Boundary condition: i ∂c ¼ −D F ∂x ∂c ∂x

j

j

;

ð4Þ

x¼L

¼ 0;

ð5Þ

x¼0

where L is the half length of the long side, and i is the current density. The current density i is assumed to be given by a Butler–Volmer type equation as follows [28].      i ð1−βÞFη βFη 1−β 1−β β ¼ kðcl Þ − exp − ; ðcθ Þ ðcs Þ exp F RT RT

ð6Þ

where k is a reaction rate constant, cl is the lithium ion concentration in the electrolyte solution, cθ is the concentration of vacant sites ready for lithium intercalation at the edge of the nanosheets, cs is the lithium ion concentration at the edge of nanosheets, and β is a symmetry factor. The overpotential, η, is defined as: η ¼ Uapp −U;

ð8Þ

This relational equation is determined to recreate the reduction part of the cyclic voltammogram for NS-L-1 measured at a low sweep rate of 1 mV s − 1, with the assumption that the concentration of lithium intercalation sites in the tetratitanate nanosheets is equal to that of the titanium ions. The applied potential changes linearly with time under the potentiodynamic stimulus, and can be expressed as:

ct

0

  cθ ct þ 0:1096 −0:5 : ct ct þ cθ

L

ct + c

-L

U ¼ 1:298 þ 0:2646 ln

ð7Þ

under the supposition that the potential is uniform, where Uapp is the applied potential and U is the open circuit potential of the tetratitanate nanosheets. This supposition is reasonable since the nanosheets were

Uapp ¼ U0 þ vt;

ð9Þ

where U0 is the initial open circuit potential and v is the sweep rate. The equations above were solved with a partial differential equation solver, PDE2D [30], using the parameters listed in Table 1, and changing the values of D, k, and v to reproduce the change in the rate-determining process shown in Fig. 7. When the value of k changed, the behavior of the peak shift varied in the transition region from charge transfer reaction control to diffusion control. In addition, when the value of D increased, the sweep rate increased where the rate controlling process changed. When 3 × 10 − 10 cm2 s− 1 and 1 × 10− 4 cm5/2 s − 1 mol− 1/2 are used for D and k, respectively, the change in the rate controlling process was well reproduced, and is shown in Fig. 7. The value of k could not be refined due to the failure of assumption (g) under relatively high sweep rates. The reduction peak currents were shown in Fig. 7, and thus the estimated diffusion coefficient is that for lithium insertion. The estimated lithium diffusion coefficient of 3 × 10 − 10 cm 2 s − 1 is reasonable for lithium diffusion in the solid phase. Lithium adsorption onto the surface sites in contact with the electrolyte solution is a possible reaction without diffusion. However, the reaction rate of adsorption is not fast enough to enhance the high-rate capability of NS-L-1 because the reaction rate of NS-L-1 was controlled by diffusion in the solid phase. The lithium diffusion coefficient along the short sides of the nanosheets must have been smaller than 1 × 10− 11 cm2 s − 1 since the diffusion along the short sides was independent of the rate determining process. The sweep rate where the rate determining process changed from a charge transfer reaction to diffusion in NS-S was estimated to be as high as 1300 mV s− 1. This is why NS-S-1 exhibited a good high-rate capability. The lithium ion diffusion coefficient was estimated by a simulation using a mathematical model. The high-rate capability of the electrodes composed of tetratitanate nanosheets with various lateral sizes can be calculated using the diffusion coefficient. 3.4. Electrode properties of multilayer thin films of tetratitanate nanosheets The electrode properties of multilayer thin films of NS-S were examined to determine the effect of layer lamination. Fig. 9(a) and 9(b) shows cross-sectional TEM images of the thin films of NS-S prepared by three repetitions of the layer-by-layer assembly deposition onto a

Table 1 Parameters for mathematical model. β k cl cθ ct L U0

0.5 5/2

−1

/cm s mol /mol dm− 3 −3 /mol dm /mol dm− 3 /m /V

− 1/2

1 × 10− 4 1.0 28.5 (initial value) 0.0 (initial value) 5.0 × 10− 7 2.5

Specific capacity / Ah cm-2

S. Suzuki, M. Miyayama / Solid State Ionics 204-205 (2011) 66–72

Carbon overcoat

Substrate

71

0.8 0.6

NS-S-3

0.4 0.2 NS-L-1

0

0 1 2 3 4 5 6 7 8 9 10 11 12

Number of oxide layers Fig. 11. Relationships between the number of oxide layers and capacity for NS-L-1 and NS-S-3. The broken and dotted lines indicate the theoretical capacities where 100% and 50% of the titanium ions in the films were electrochemically active.

Nanosheets

through an electrode with an extremely small thickness below 10 nm would contribute to the high reactivity [24]. The detailed effects of the layer lamination of nanosheets on their reactivity will be reported in our next paper.

Substrate

Fig. 9. Cross-sectional TEM images of a thin film of NS-S prepared by three repetitions of layer-by-layer assembly deposition onto a mica substrate.

mica substrate. A non-uniformly deposited thin film with an interlayer distance of 0.97 nm was observed. The average thickness of the thin film was estimated to be 9.1 ± 1.8 nm in this observation area by image analysis. This thickness of 9.1 nm corresponds to the lamination of 9.4 oxide layers. The thin film of NS-S (NS-S-3) was prepared by three-time deposition operation onto a Pt/Si substrate, and the electrode properties were examined. Fig. 10 shows the cyclic voltammogram for NS-S-3 measured at a sweep rate of 10 mV s − 1. NS-S-3 also exhibited an almost reversible reaction. The reversible capacity of NS-S-3 was 0.62 μAh cm − 2. The relationship between the number of oxide layers and capacity is shown in Fig. 11, on the assumption that NS-S-3 had the same thickness as the film shown in Fig. 9. The results for NS-L-1 are also shown in Fig. 11. The capacity normalized by the area seems to increase in proportion to the number of oxide layers. The broken and dotted lines indicate the theoretical capacities where 100% and 50% of the titanium ions in the films were electrochemically active. Approximately 50% of the titanium ions have been reported to be electrochemically active in a lithium insertion reaction in conventional TiO2 powders [21] or reassembled nanosheets [9]. The high reactivity of the nanosheets exhibited by NS-L-1 was maintained in NS-S-3. Sufficient electronic conductivity and sufficient lithium transportation

4. Conclusions The electrode properties of ultra-thin films of tetratitanate nanosheets were examined. The ultra-thin film of tetratitanate nanosheets prepared by single deposition operation exhibited a specific capacity of 0.13 μAh cm− 2, which was a large value corresponding to the reduction and oxidization of approximately 87% of the Ti ions. A change in the rate controlling process from a charge transfer reaction to diffusion was observed by cyclic voltammetry at 50 mV s− 1. The lithium ion diffusion coefficient in tetratitanate nanosheets along the b-axis of original tetratitanate was determined by analyzing the results computationally with a mathematical model, and was found to be 3×10− 10 cm2 s− 1. The nanosheets exhibited high reactivity in a multilayer thin film with an estimated thickness of 9.1 nm. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Current density / Acm-2

[11] [12] [13]

20

[14] [15]

0

[16] [17]

-20 1

NS-S-3 Substrate

[18] [19]

3

[20]

2

Potential / V (vs.

Li/Li+)

Fig. 10. Cyclic voltammogram for NS-S-3 measured at a sweep rate of 10 mV s

[21] −1

.

[22]

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