DOI: 10.1002/cssc.201500005

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Nanotubular Heterostructure of Tin Dioxide/Titanium Dioxide as a Binder-Free Anode in Lithium-Ion Batteries Myungjun Kim,[a] Joobong Lee,[a] Seonhee Lee,[a] Seongrok Seo,[a] Changdeuck Bae,[a, b] and Hyunjung Shin*[a] Titanium dioxide (TiO2), tin dioxide (SnO2), and heterostructured TiO2/SnO2 nanotube (NT) arrays have been fabricated by template-assisted atomic-layer deposition (ALD) for use as anodes in a lithium-ion battery (LIB). TiO2 NT arrays with 8 nm thick walls showed higher capacity ( … 250 mA h g¢1 after the 50th cycle at a rate of C/10) than the typical theoretical capacity of bulk TiO2 and a radically improved capacity retention property upon cycling. SnO2 NT arrays with different wall thicknesses (8, 10, 13, and 20 nm) were also fabricated and their

electrochemical performances were measured. All of the SnO2 NT arrays showed substantially higher initial irreversible capacity and higher reversible capacity than those of bulk TiO2. Thinner walls of the SnO2 NTs result in better capacity retention. Heterotubular structures of TiO2 (5 nm)/SnO2 (10 nm)/TiO2 (5 nm) were successfully fabricated, and displayed a sufficiently high capacity ( … 300 mA h g¢1 after 50 cycles) with exceptionally improved cycling performance up to the 50th cycle.

Introduction One-dimensional nanomaterials, notably, nanorods, nanowires, and nanotubes (NTs), have received significant attention as advanced functional materials along with their interesting electrical, optical, and electrochemical properties.[1] In lithium-ion batteries (LIBs), nanostructured materials used as electrodes provide new avenues to improve electrochemical performance.[2] The high surface-to-volume ratio, surface activity, short diffusion length, and efficient charge transport of 1D nanomaterials provide a potential engineering solution for high energy density with improved cyclability, as well as a high rate of charging and discharging.[3] Many different metal oxide material systems have been reported as anodes with a higher gravimetric capacity and improved safety to replace graphitic materials (theoretical maximum capacity of 372 mAh g¢1). Idota et al.[4] initiated research efforts for the use of tin-containing oxides as anodes and reported a 50 % increase in the gravimetric capacity compared with graphitic anodes. Subsequently, Courtney et al.[5] reported the chemical reaction of SnO2 with Li by an irreversible conversion reaction, SnO2 + 4 Li!Sn + 2 Li2O, followed by a reversible alloying/dealloying reaction, Sn + xLi$LixSn.[6] The theoretical maximum capacity is 781 mA h g¢1 (or 5400 Ah L¢1, exceeding [a] M. Kim, J. Lee, S. Lee, S. Seo, Dr. C. Bae, Prof. H. Shin Department of Energy Science, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeong gi-do (South Korea) E-mail: [email protected] [b] Dr. C. Bae Integrated Energy Center for Fostering Global Creative Researcher (BK 21 plus), Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeong gi-do (South Korea) This publication is part of a Special Issue on “Sustainable Chemistry at Sungkyunkwan University”. To view the complete issue, visit: http://onlinelibrary.wiley.com/doi/10.1002/cssc.v8.14/issuetoc.

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837 Ah L¢1 for graphite) for SnO2 (the alloy most responsible for the capacity is Li4.4Sn) with the combination of the intercalation, conversion, and alloying/dealloying reactions described above. Despite their high capacity and sufficiently low onset potentials ( … 0.01 V vs. Li + /Li), SnO2 anodes undergo severe pulverization (i.e., mechanical degradation) with a substantial volume change (up to 300 %) and continuous formation of solid–electrolyte interphase (SEI) layers. Therefore, the SnO2 anodes show unacceptable cycling ability, that is, capacity retention property, and innovative strategic ideas are required to achieve improved cycling performance. One of the ideas is to fabricate tin-containing oxide electrodes with mesoporous nanostructures[7] to ensure better structural stability. In particular, 1D nanomaterials,[8] such as nanowires,[9] nanorods,[10] and NTs,[11] have been used to accommodate a large volume change and enhanced charge transport. Another method is to construct a heterostructure of tin-containing oxides with other materials (particularly, carbon[12] and TiO2[13]) as surface coatings to stabilize the SEI and to accommodate the volume expansion of the lithium and tin alloying materials. TiO2 has been proposed as a prospective candidate for the mechanical support of SnO2 through the formation of heterostructures.[14] Nanostructured TiO2 has been extensively studied and has shown its robustness in cycle retention and high power capability because of its low volume change (< 4 %) during the lithiation/delithiation process.[15] However, the maximum theoretical capacity of TiO2 is relatively low ( … 170 mA h g¢1). According to a recent report, nanostructured TiO2 can accept more Li in the structure to form a new Li1TiO2 phase that has the same space group (I41/amd) as anatase, but with the shifted lattice parameters of a (from 3.792 to 4.043 æ) and b (from 9.497 to 8.628 æ).[16] During the formation of Li1TiO2, the maximum capacity reaches about 335 mA h g¢1.

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Full Papers TiO2 nanoparticles that are 7 nm in diameter can host up to 1 mol of Li per TiO2 formula unit, whereas larger particles exhibit a typical maximum capacity. It is speculated that all of the octahedral interstices are occupied by Li + ions and overcome the strong Li +¢Li + repulsive interaction in cases in which a few nanometer-sized materials are used as electrodes. Monodispersed nanotubular TiO2 structures with a precisely controlled wall thickness ( … 5 nm) for use as anodes were fabricated with a maximum capacity of about 330 mA h g¢1. Our research group has proven the “true” nanoscale size effects.[17] Similar to TiO2, nanostructured Sn-containing oxides for use as Figure 1. Galvanostatic charge/discharge curves of the electrodes of the TiO2 anodes showed higher capacity beyond the typical theoretical NT array over the voltage range of 0.7 to 3.0 V at a current rate of C/10. maximum value ( … 783 mA h g¢1). Mesoporous SnO2 anodes Curves of specific capacity versus potential represent the 1st (black in color and denoted #1), 2nd (blue, #2), and 50th (red, #50) charging/discharging have been prepared by two research groups, who reported cacycles. During the discharging cycles, another pseudoplateau was observed, ¢1 [18] pacities of 960 and up to 1000 mA h g . It was revealed that even after reaching the known maximum theoretical capacity of the Li2O phase decomposed to form the SnOx phase with Sn 170 mA h g¢1 with 0.5 mol of Li per formula unit of TiO2, showing a typical upon delithiation, parallel to the dealloying of LixSn. However, plateau at 1.7 V. the Li2O phase is known to be electrochemically inactive, but capable of enhancing cycling characteristics by reducing agglomeration of Sn or the Li¢Sn alloy. After several cycles, it is inevitable that small, active metallic tin aggregates into larger, inactive tin clusters. Dimensioncontrolled nanostructures with nanometer-scale precision will play a key role in improving the cycling performance and capaciFigure 2. a) Rate performances at C/10, about 20C, and back to C/10; and b) an excellent capacity retention propty retention of tin-containing erty up to 50 cycles of the array of TiO2 NT electrode at C/10. oxide anodes in LIBs. Herein, the dimension-controlled nanotubular structures of TiO2, SnO2, TiO2/SnO2 (double wall), and TiO2/SnO2/TiO2 were fabricated by using template-assisted atomic-layer deposition (ALD).[19] ALD is an ideal technique to deposit functional oxide materials as nanostructures, in this case, NTs, with a high aspect ratio (length/diameter) because it uses a self-limiting surface reaction. This method has been reported to be an effective depositing tool for coating the surface of particles, such as LIB electrodes, typically with Al2O3, to improve electrochemical performance and battery safety.[20] A combination of well-engineered porous alumina membranes (PAMs)[21] with Figure 3. Selected-area electron diffraction (SAED) patterns and bright-field TEM images of a) lithiated (discharge) monosized porous channels and and b) delithiated (charge) TiO2 NT electrodes after the 50th cycle. Ex situ TEM observations were performed and a conformal ALD coating of showed no structural disintegration or phase transformation between tetragonal anatase TiO2 (space group of TiO2[22] and SnO2, along with the I41/amd) and orthorhombic lithium titanate (Li0.5TiO2, space group of Imma). ChemSusChem 2015, 8, 2363 – 2371

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Full Papers precise control of the wall thickness, in NTs provides an excellent model system to investigate their intrinsic electrochemical performances as anodes in LIBs on the nanometer scale without any conducting additives and/or binder. The electrochemical properties of the TiO2 and SnO2 NT arrays and the heterotubular structures of TiO2/SnO2 and TiO2/SnO2/TiO2 were also investigated.

Results and Discussion TiO2 NT array anode

measured from C/10 to 20 C and back to C/10 (Figure 2 a). After rapid charging/discharging rates of C/10, 20 C, and back to C/10, no sign of degradation was observed. It is known that the cyclability and/or capacity retention of TiO2 as the anode should be superior due to small structural changes during intercalation and deintercalation of the Li + ions. In this study, the array anodes showed excellent capacity retention up to the 50th cycle (Figure 2 b). Ex situ TEM observation results are shown in Figure 3 a and b. Figure 3 a shows a bright-field TEM image and SAED patterns of a lithiated TiO2 NT. Each SAED pattern was obtained from the region with colored circles, which indicated orthorhombic lithium titanates (JCPDS no. 77-1387). After deintercalation of the Li + ions, the NT transformed into the original tetragonal anatase TiO2 (JCPDS no. 21-1272). These results clearly showed that there was no sign of structural disintegration during cycling. Ex situ high-resolution TEM (HRTEM) images and simulated crystal structures with fast Fourier transform (FFT) patterns at the zone axis of [010] are shown in Figure 4 a and b. The ex situ experimental results indicated the phase transformation between tetragonal anatase TiO2 and or-

Figure 1 shows typical galvanostatic charge/discharge curves of the array anode of TiO2 NTs with an average wall thickness of approximately 8 nm after the 1st, 2nd, and 50th cycles at a current rate of C/10. The first discharge capacity is about 348 mA h g¢1, which corresponds to a lithium concentration of x … 1.00. The reversible capacity is reduced to about 286 mAh g¢1 (x … 0.85) in the second discharging cycle, leaving an irreversible capacity of 62 mAh g¢1. The Coulombic efficiency (CE = charge out/charge in) in the first cycle is about 82 %, which increases to 94 % in the second cycle and stabilizes at about 99 %. Remarkably, the discharging curves clearly represent a “true” nanometer scale effect, accommodating more lithium ions than in the bulk (0.5 mol per formula unit, with a theoretical maximum capacity of … 170 mA h g¢1). The discharging and charging plateaus appeared at about 1.7 and 1.8 V, respectively, as previously reported; this indicated a reversible phase transformation upon the intercalation/deintercalation of the lithium ions. After reaching a capacity of approximately 170 mA h g¢1, which is a known theoretical maximum capacity of TiO2, the discharging curves showed a drop to another pseudoplateau at 1.4 V to extend to an even higher capacity of about 250 mA h g¢1, even after the 50th cycle. Without any conducting binders, the monosized TiO2 NT array anodes in this study provide solid evidence for the nanoscale size effect of increased Li ion uptake compared Figure 4. HR-TEM images with computer-generated FFTs as insets of a) lithiated (discharge) and b) delithiated with that in the bulk. Further(charge) TiO2 NTs with simulated crystal structures and FFT patterns at the [010] zone axis (right-hand side) (Crysmore, the rate performances of talMaker Software Ltd.) The C axis was experimentally obtained as indicated by the colored dotted boxes in a) the array anodes of the NTs were and b), which showed excellent agreement with the simulated results (right-hand images). ChemSusChem 2015, 8, 2363 – 2371

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Full Papers thorhombic lithium titanates with a slight difference in the lattice constant (as indicated in the simulated crystal structures and diffraction patterns with the C axis, i.e., 4.518 æ for lithium titanates and 4.756 æ for anatase TiO2 in Figure 4 a and b, respectively), leading to a small volume change in cycling. The C axis was also experimentally measured to be about 4.50 æ for lithium titanates and about 4.70 æ for anatase TiO2, as shown in Figure 4 a and b, respectively, showing excellent agreement. Therefore, the phase transformation in the TiO2 NTs guaranteed a robust capacity retention property and showed no pulverization.

tained by using the Scherrer equation (d = (kl)/bcos qB), the size of the mean crystallites of SnO2 NT was estimated to be about 10 nm based on the peak from the (110) plane. The results in Figure 5 c also confirm that SnO2 crystallized as rutile structures with crystallites of about 10 nm in diameter. Figure 6 shows cyclic voltammograms of SnO2 NT arrays for the 1st, 2nd, and 3rd cycles at a scan rate of 0.1 mV s¢1 with

SnO2 NT array anode Figure 5 a shows a mosaic of TEM micrographs obtained from a subsequently annealed ALD-grown SnO2 NT with a length of

Figure 6. a) Cyclic voltammetry (CV) curves of the array of SnO2 NTs as anodes with a wall thickness of 10 nm at a scan rate of 0.1 mV s¢1 for the first three cycles. Black: first cycle, red: second cycle, and blue: third cycle. During the first discharging cycle, SnO2 irreversibly decomposed into metallic Sn and Li2O at 0.78 V versus Li + /Li, as denoted by 1, with the following chemical reaction: SnO2 + 4 Li + + 4 e¢ !Sn + 2 Li2O. Partial reversible oxidation/reduction between SnO2 and Sn during the anodic and cathodic reactions was evident at approximately 1.24 V, as denoted by 1’ and 4. The reversible and most responsive reactions for alloying and dealloying occurred at 0.01 and 0.60 V, respectively, and are denoted by 2 and 3.

Figure 5. a) Mosaic of bright-field TEM images of an ALD-grown SnO2 NT; the magnified images obtained from three different regions (as indicated by red boxes) of the NT are given as insets. The NT was 16 mm in length, with an outer diameter of 60 nm and 7 nm wall thickness from the three different regions; this showed excellent conformal coatings of SnO2 in PAM by using ALD. b) XRD result showing that the annealed SnO2 NT crystallized into the rutile structure (JCPDS no. 41-1445) of nanocrystallites with a diameter of approximately 10 nm, as estimated by the Scherrer formula from broadening of the (110) peak. c) SAED pattern confirming that the NT was composed of SnO2 nanocrystallites.

16 mm and an outer diameter of approximately 60 nm. Higher magnification micrographs, given as insets in Figure 5 a, of the three different regions in the SnO2 NT, as indicated by arrows and boxes, confirmed that ALD was an excellent technique for conformal coating with a uniform thickness ( … 7 nm on average) in the wall and aspect ratio ( … 260 from the broken NT in the micrograph). Figure 5 b is the XRD spectrum of the fabricated SnO2 NT with PAM annealed at 500 8C. All of the diffraction peaks are indexed to the rutile structure of SnO2 (JCPDS no. 41-1445) with the space group of P42/mnm (136), except for the peak from the Al substrate. From peak broadening, obChemSusChem 2015, 8, 2363 – 2371

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a cutoff voltage of 0.01–2.5 V versus Li/Li + . The CV behavior is consistent with that reported in the literature.[23] This result indicated that the SnO2 NT array underwent typical electrochemical pathways for Li + ion intercalation and deintercalation. During the cathodic reaction, a relatively sharp peak appeared at 0.78 V, which was due to the initial irreversible decomposition reaction of SnO2 into Sn and Li2O, and the formation of the SEI layer upon lithiation (Figure 6, denoted as 1 in the black solid curve). After the first cycle, this peak nearly disappeared, resulting in a large initial irreversible capacity. The peaks for the partial reversible reactions between SnO2 and Sn became evident at approximately 1.24 V in the cathodic and anodic reactions (Figure 6, denoted by 1’ and 4). Another important pair of current peaks at 0.01 and 0.60 V, denoted by 2 and 3 in Figure 6, respectively, which are attributed to the alloying and dealloying reactions, are known to be highly reversible and the reactions most likely to be responsible for the apparent capacity. Curves of galvanostatic potential versus specific capacity and cycling performances as plots of specific capacity versus number of cycles for an array of SnO2 NTs with different wall thicknesses of 8, 10, 13, and 20 nm, which are precisely controlled by the number of ALD cycles, are shown in Figures 7 a–d. The scan rate of charging/discharging was 0.1 C with a cutoff potential range of 0.01–2.5 V versus Li/Li + . The irreversible 1st discharging capacity is exceptionally large, for example, nearly reaching 2000 mA h g¢1 for the NTs with 8 nm thick walls (Figure 7 a). The large, irreversible first dis-

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Full Papers tion of the capacity. As shown in Figure 7, the thicker-walled NTs showed more severe degradation in capacity. The apparent capacity after the 50th cycle was nearly 0 in the case of the NT array with a 20 nm thick wall. The NT array with the thinnest wall ( … 8 nm) showed the best performance in cycling, and a reversible capacity of approximately 350 mA h g¢1 was obtained after the 50th cycle (Figure 7 a). After the 50th cycle, ex situ TEM observations of NTs with 10 nm thick walls were performed. Figure 8 a is a mosaic of TEM micrographs, showing no structural disintegration of the NT, but the inner empty space remains barely visible (Figure 8 b). The charged (delithiated) SnO2 NT contained a large number of small crystalline metallic Sn particles, shown in black in Figure 8 b. The SAED pattern (Figure 8 c) was analyzed by indexing of metallic tin (JCPDS no. 040673). The lattice image in the HRTEM micrograph, along with FFT analysis (Figure 8 d), confirmed the presence of metallic Sn reduced from SnO2 with the Li2O matrix, which was amorphous. The metallic Sn particle size ranges from 5 to 10 nm in diameter and is embedded in ¢1 Figure 7. Curves of galvanostatic potential [V] versus specific capacity [mAh g ] and plots of specific capacity Li2O. It is well known that this versus number of cycles of the arrays of SnO2 NTs with wall thicknesses of a) 8, b) 10, c) 13, and d) 20 nm over the voltage range of 0.01–2.5 V at a current rate of C/10. Large, irreversible discharging capacities of up to large volume change occurs 2000 mA h g¢1 were observed in all cases. The NT with a wall thickness of 8 nm showed a larger capacity and during the uptake of a large better capacity retention performance compared with the NTs with thicker walls. In the case of the NT with amount of lithium. The volume a 20 nm thick wall, the capacity was nearly 0 in the 50th cycle. change leads to a significant decrease in capacity due to struccharging capacities were observed for all of the cases shown tural instability, and eventually, pulverization; thus limiting the in Figure 7. Another common feature is stabilization upon cycycling performance after only a few cycles. Reducing the metallic tin particles to the nanometer scale and to a wall of a few cling immediately after the first discharge cycle. The reductive decomposition reaction of SnO2 into Sn and Li2O, as shown in nanometers thick does not reduce the volume change, but Figure 6, was assumed to be responsible for the large, irreversidoes facilitate the phase transitions that accompany alloy forble initial capacity and the rapid stabilization immediately after mation and reduce cracking within the electrodes of the NT the first discharge cycle. After the first discharge cycle, typical array. reversible curves were obtained for all of the cases in Figure 7. In this study, each NT, approximately 1010 NTs per cm2, was diHeterostructures of TiO2/SnO2 and TiO2/SnO2/TiO2 NTs rectly connected to the current collectors without any binder or conducting additives. Unfortunately, the reduction of capacity cannot be prevented After cycling, the mechanical instability caused by pulverizaby the nanostructuring strategy alone. A combination of TiO2 tion in the SnO2 electrochemical reaction resulted in a degradaNTs, which showed robust cycling behavior, and SnO2 NTs, with ChemSusChem 2015, 8, 2363 – 2371

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Full Papers above for the individual NT arrays of TiO2 and SnO2 was used. A SEM cross-sectional view and the corresponding energydispersive X-ray spectroscopy (EDX) mapping of Al, O, Sn, and Ti of the double-wall heterostructured TiO2/SnO2 NTs with PAM are shown in Figure 9. The Sn, Ti, and O EDX signals can be found from the top to the bottom of the 50 mm thick PAM, which indicates the successful fabrication of the heterostructured NTs with a nearly 1:1000 aspect ratio. Further structural analysis of the annealed heterostructures was performed by means of HRTEM and XRD (Figure 10). Mosaic bright-field TEM images (Figure 10 a, with SAED results in the inset) revealed the structure of the fabricated and crystallized heterostructures of TiO2 and SnO2 NTs. After subsequent annealing at 500 8C, the rutile structures of the SnO2 NTs and the anatase heterostructures were determined by HRTEM and XRD (Figure 10 b and c, respectively). Notably, peak broadening was observed in the SnO2 layer, whereas sharp peaks were found in Figure 8. a) Mosaic of bright-field TEM images of a delithiated SnO2 NT with a 10 nm thick wall after the 50th the TiO2 layer, which indicated charging cycle. b) Magnified TEM image showing nanocrystallite metallic Sn particles in black, presumably embedelongated grain growth along ded in the matrix of amorphous Li2O. c) SAED pattern indexed with metallic Sn. d) HR-TEM image of the NT in a) showing metallic Sn particles 5–10 nm in diameter with a lattice spacing of 0.21 nm, which is the (220) plane of the axial direction, as reported Sn. The computer-generated FFT pattern in the inset confirmed the presence of the metallic Sn particle. previously.[24] In Figure 10 b, the inner layer with darker contrast is SnO2 with a thickness of 8 nm, a higher initial capacity, could show significantly better electroand the outer layer is TiO2 with a thickness of 10 nm; both of chemical performances as anodes in LIBs. Heterostructures of the structures are crystalline. TiO2 and SnO2 NTs were fabricated by template-assisted ALD, as shown in Scheme 1. Double-walled heterostructures of SnO2/TiO2 NTs were fabricated by sequential ALD coatings of TiO2 and SnO2. The same fabricating strategy as that described

Scheme 1. Schematic illustration of the template-directed ALD process for the fabrication of SnO2/TiO2 heterostructured NT arrays as anodes. Sequential ALD coatings of TiO2 and SnO2 were performed.

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Figure 9. a) Cross-sectional SEM image of an ALD-grown SnO2/TiO2 NT array in PAM, and EDX elemental mapping of Al (b), O (c), Sn (d), and Ti (e). A complete conformal coating of SnO2/TiO2 in a thick ( … 50 mm) PAM was achieved by ALD.

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Full Papers Conclusions

Figure 10. a) Mosaic of low-magnification TEM images of SnO2/TiO2 NT (inset: SAED pattern obtained from the region of the red circle, corresponding to the composite of crystalline TiO2 and SnO2). b) HR-TEM image showing the double walls of the inner SnO2 layer and the outer TiO2 layer; this indicates the measured lattice spacing with anatase TiO2 and the rutile structure of SnO2. c) XRD pattern of the electrode of the SnO2/TiO2 NT array in PAM, showing peaks of crystalline anatase TiO2 and the rutile structure of SnO2 with a peak from the Al substrate. Peak broadening was observed in the SnO2 layer with nanocrystallites, whereas relatively sharp peaks were observed in the TiO2 layer; this indicates that elongated grain growth has occurred.

The gravimetric capacity in both cases (TiO2 (5 nm)/SnO2 (8 nm) and TiO2 (10 nm)/SnO2 (8 nm)) is relatively low ( … 250 mA h g¢1) because of the low capacity of the TiO2 NTs (Figure 11). However, the capacity retention property was remarkably improved after the first discharge cycle in both cases. Finally, we successfully fabricated heterotubular structures of TiO2(5 nm)/SnO2(10 nm)/TiO2(5 nm). The improved electrochemical performances are shown in Figure 12. The first irreversible capacity was about 850 mA h g¢1, and immediately after discharging an improvement in the reversible cycling property was observed with capacity degradation ( … 33.5 % up to the 50th cycle). After the 50th cycle, the capacity was about 300 mA h g¢1. The lithiated composite structure, which consisted of electrochemically active Sn and an inactive Li2O phase, acted as a matrix in which the reduced metallic Sn phases were finely dispersed and prevented the aggregation of Sn.[25] After repeated cycles, the Sn phases surrounded by the Li2O matrix, arising from delithiation of the Li¢Sn alloy, showed a tendency to aggregate into larger clusters.[26] Coarsening of the tin phases after many cycles caused a significant volume change and, eventually, pulverization of the anodes. When inserted between the TiO2 layers, SnO2 showed an improved capacity retention performance because the TiO2 layers effectively protected SnO2 from structural disintegration. The few-nanometer-thick walls also suppressed the aggregation of tin into larger particles, and maintained sufficiently small tin particles along the axial direction; this led to an improvement in the cycling retention.[27] ChemSusChem 2015, 8, 2363 – 2371

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TiO2, SnO2, heterostructured TiO2/SnO2 and TiO2/SnO2/TiO2 NT arrays as binder-free anodes in LIBs were successfully fabricated by using template-assisted ALD, which enabled not only precise structural control, but also surface modification of electrode materials. The ALD-grown TiO2 and SnO2 NT anode showed clear wall-thickness-dependent electrochemical behavior in terms of capacity and/or capacity retention. The thinner wall of the SnO2 NTs showed better performances of capacity retention upon cycling. Furthermore, we fabricated heterostructured TiO2/ SnO2 and TiO2/SnO2/TiO2 NT array electrodes by conducting a sequential ALD process, and we evaluated their electrochemical performances. A sufficiently high capacity with improved cycling performance up to the

Figure 11. a) Cycling performances at a current rate of 100 mA g¢1 of SnO2/ TiO2 NTs with different TiO2 thicknesses (red: 5 nm, blue: 10 nm). After the first discharge cycle, the capacity retention property has been remarkably improved in both cases. b) Cross-sectional SEM image of TiO2 NTs/SnO2 NTs in PAM with a Ti layer after 50th charging/discharging cycles.

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Full Papers Characterization of the materials The morphology and crystalline structure of the ALD-grown electrodes were analyzed by fieldemission (FE) SEM (JEOL, JSM 7600F), differential thermal analysis (DTA; Seiko, TG/DTA 7300), XRD (Rigaku, d-MAX2500), and HR-TEM (JEOL, JEM 2100F, 400 kV). TEM specimens were obtained by wet Figure 12. a) Galvanostatic potential [V] versus specific capacity curves of the array of the heterotubular structure chemical etching of the alumina in PAM, as schematically shown in the inset for the first three cycles at a scan rate of C/10. b) Specific capacity template with a 1 m solution of versus number of cycles upon cycling up to the 50th cycle of the TiO2/SnO2/TiO2 heterostructures as anodes. NaOH for a few hours. After etching the entire membranes, the samples were repeatedly washed and dispersed in DI water. An aqueous suspension of the electro50th cycle was observed. Therefore, surface modification of des was dropped onto a TEM grid. After cycling, the electrodes the electrodes by ALD permitted effective strain relaxation of were disassembled from the coin cells and washed with diethyl the active materials based on alloying/dealloying reactions. carbonate (DEC) followed by drying in an oven.

TiO2/SnO2 and TiO2/SnO2/TiO2 NT-based electrodes provided reversible capacities of 293 and 457 mA h g¢1, respectively. The structural design and surface modification of the electrode materials with precisely controlled dimensions by ALD could be extended to the engineering of other material systems and could provide solutions to overcome the limitations presented in conventional electrode material systems for LIBs.

Experimental Section PAM fabrication In this study, two types of membranes were used. Commercial PAMs (Anodisc 13; thickness, 60 mm; pore diameter, 200 nm) were purchased from Whatman (UK) and homemade PAMs were prepared by a two-step anodization process reported elsewhere.[20] Briefly, high-purity aluminum foils (5N) from Goodfellow Cambridge (UK) were sequentially sonicated in acetone, ethanol, and deionized (DI) water. Then, the foils were electropolished in a solution containing ethanol and perchloric acid (4:1, v/v) at 20 V. The first anodization process was performed in 0.3 m oxalic acid at 10 8C under a constant voltage of 40 V. The alumina films formed were removed by dipping into a mixture of 1.8 wt % H2CrO7 and 6 wt % H3PO4 for several hours. Subsequently, the second anodization process was conducted under anodizing conditions identical to those used in the first process for the desired time.

Materials synthesis ALD reactions of both SnO2 and TiO2 were performed in a commercial showerhead-type reactor (ForAll, Korea) at 175 and 160 8C, respectively. Tetrakis(dimethylamino)tin(IV) (TDMASn; UP chemical, Korea), titanium(IV) isopropoxide (TTIP; UP Chemical, Korea), and water vapor were employed as tin, titanium precursor, and oxygen source, respectively. Ultrahigh purity (5N) Ar gas at a mass flow rate of 200 sccm (cubic centimeters per minute) was used as the carrier and purging gas. On the basis of the growth rates on planar substrates (0.06 nm per cycle for SnO2, and 0.04 nm per cycle for TiO2), the number of ALD cycles was set for the desired NT wall thicknesses. As each ALD process finished, the membranes were annealed at 500 8C for SnO2 and 400 8C for TiO2. ChemSusChem 2015, 8, 2363 – 2371

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Electrochemical measurements Electrochemical measurements were performed by using two-electrode CR2032 coin-type cells consisting of working electrodes (SnO2 NTs, TiO2 NTs, or SnO2/TiO2 NTs) and lithium foil as the counter and reference electrodes. Copper metal as the current collector was deposited onto the active materials in the alumina membrane by thermal evaporation. The electrolyte used was 1 m LiPF6 in a mixture of ethylene carbonate and diethylene carbonate (EC/DEC = 1:1, v/v). The cells were assembled in an argon-filled glove box in which the moisture and oxygen contents were less than 1 ppm. A galvanostatic charge–discharge test was performed by using a Won A Tech WBCS 3000 system and cutoff voltage windows of 0.01–3 V (vs. Li/Li + ) for the SnO2 NT and SnO2/TiO2 NT electrodes and 0.7–3 V (vs. Li/Li + ) for the TiO2 electrode. CV tests were conducted over a potential range of 0.01–3 V at a scan rate of 0.1 mV s¢1. The loading weight of active materials was estimated from actual mass differences before and after ALD coating by using a microbalance with microgram resolution.

Acknowledgements We acknowledge financial support from the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-0028972,2013R1A2A3A01068499 and 2014M3A7B4052201). This study was supported in part by the Agency for Defense Development (ADD) of the Republic of Korea. We also thank Sorae Lee for providing assistance with the experiments. Keywords: electrochemistry · nanotubes · surface analysis · tin · titanium

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