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Orientation alignment of epitaxial LiCoO2 thin films on vicinal SrTiO3 (100) substrates
Journal of Power Sources 325 (2016) 306e310
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Orientation alignment of epitaxial LiCoO2 thin films on vicinal SrTiO3 (100) substrates Kazunori Nishio a, 1, Tsuyoshi Ohnishi a, b, c, Kazutaka Mitsuishi a, Narumi Ohta a, c, Ken Watanabe c, Kazunori Takada a, b, c, * a b c
Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan International Center for Materials Nanoarchitectonics (MANA), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan Environment and Energy Materials Division, National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan
h i g h l i g h t s LiCoO2 epitaxial thin films are synthesized on vicinal SrTiO3 (100) substrates. The (104)-oriented films exhibit single-domain structure on the vicinal substrates. The orientation alignment is explainable by step-flow growth mechanism. The influence of domain boundaries on electrode performance is discussed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2016 Received in revised form 1 June 2016 Accepted 3 June 2016 Available online 16 June 2016
LiCoO2 is epitaxially grown on SrTiO3 (100) substrates with (104) orientation. Because the LiCoO2 film is grown with its c-axis parallel to four equivalent 〈111〉 axes of the SrTiO3, the (104)-oriented film exhibits four-domain structure on the SrTiO3 (100) substrate. Introducing off-cut angle to the substrate surface breaks the equivalency between the four 〈111〉 axes of the SrTiO3 substrate to induce preferential growth of specific orientation with the c-axis in a descending direction of off-cut surface. Increasing off-cut angle and lowering deposition rate promote the preferential growth, because they facilitate step-flow growth mode, and finally align the c-axes in the domains completely into one 〈111〉 direction of the SrTiO3 substrate. The LiCoO2 film delivers a discharge capacity of 90 mAh g1 at a low discharge rate of 0.01 C, and 25% of capacity is kept even at a high rate of discharge with 100 C. © 2016 Elsevier B.V. All rights reserved.
Keywords: Epitaxial film Vicinal substrate Pulsed laser deposition Lithium cobalt oxide Thin-film battery
1. Introduction Studies on thin-film batteries started in 1980s [1], and they are growing in importance with the births of new micro-devices, e.g. nonvolatile memories, micro electro mechanical systems (MEMS), etc. Another importance of thin-film systems is originated from their simple geometry. Because electrode/electrolyte interfaces and ion-conduction paths can be simplified in the geometry, thin-film systems are beneficial for fundamental research. Especially,
* Corresponding author. National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan. E-mail address: [email protected] (K. Takada). 1 Present address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980e8577, Japan. http://dx.doi.org/10.1016/j.jpowsour.2016.06.015 0378-7753/© 2016 Elsevier B.V. All rights reserved.
recent materials research has provided new materials with excellent bulk properties. They have often replaced their bulk with their interfaces that govern the battery performance and initiated many studies on interface, where planar interfaces and one-dimensional ionic conduction in the simplified geometry are useful. Although the electrochemical systems become to have simplified geometry in thin-film systems, more efforts are necessary to gain basic insight into intrinsic interface properties. Grain boundaries affect the ionic conduction, and the interfaces are formed between uncontrolled crystal faces, as far as the films are polycrystalline. Epitaxial films are anticipated to provide ideal interfaces for the fundamental research on the interfaces due to their specified surface. However, even epitaxial films do not always show intrinsic nature of the materials. For example, they often have multi-domain structure, whereas a recent computational study predicts that twin domain boundaries existing in epitaxial films
K. Nishio et al. / Journal of Power Sources 325 (2016) 306e310
affect ion transport in the films [2]. Therefore, ultimate goal is to form these materials in monolithic single-crystal films without domain boundaries, which is equivalent to bulk single crystal, although it is challenging task. Our previous study on epitaxial growth of LiCoO2 has revealed that LiCoO2 is epitaxially grown with (104), (018), and (001) orientations on SrTiO3 (100), (110), and (111) substrates, respectively [3]. In the study, we demonstrated electrode properties of the (104)-oriented film, because CoO2 layers in the film are the most upright and thus offer the most accessible interlayers for lithium ions. However, the film has four-domain structures, in which domain boundaries may affect the electrode properties. Vicinal substrates, surface of which is at off-cut angle to a specific crystal plane, are sometimes utilized in order to improve the crystallinity of epitaxial thin films [4,5], because they are generally believed to enhance step flow growth of thin films and control domain structure. In this study, (104) oriented LiCoO2 epitaxial thin films are grown on vicinal SrTiO3 (100) substrates to promote preferential formation of specific domains.
2. Experimental Epitaxial LiCoO2 thin films were grown by pulsed laser deposition (PLD). A sintered Li1$2CoO2þd pellet (TOSHIMA Manufacturing Co., Ltd.) was used as the PLD target. A KrF excimer laser (COMPex Pro 50, COHERENT, Inc., l ¼ 248 nm) was operated at a repetition rate of 2 Hz or 40 Hz for the target ablation. Laser spot size on the target was 0.031 cm2, and the laser fluence was 0.65 J cm2. The incident angle of the excimer laser on the target surface was 30 , and the targetesubstrate distance was set at 58 mm. Oxygen pressure in the chamber and substrate temperature were set at 0.1 Pa and 725 C, respectively. Detailed information regarding the selection of target composition, laser fluence, and oxygen pressure can be found in Refs. [6,7]. Films were deposited on non-doped SrTiO3 single crystal substrates, which were exactly-cut SrTiO3 (100) substrates and vicinal substrates with 3 off-cut toward in-plane [001] direction, and those with 2 and 4 off-cut toward in-plane [011] direction from (100) plane as illustrated in Fig. 1 in order to investigate the effects of the off-cut directions and off-cut angles from (100) plane of SrTiO3 on the film structure. A vicinal 3 at% La-doped SrTiO3 (La:SrTiO3) single crystal substrate with 5 off-cut toward in-plane [011] direction was used in place of the non-doped insulating ones in order to investigate the electrode properties of the LiCoO2 film grown on the vicinal substrate, because the La-doping provides electronic conduction to the substrates and makes them act as current collectors in the electrochemical measurement. All the substrates were made by SHINKOSHA Co., Ltd., and the substrate size is typically 10 mm in diameter, 10 mm 10 mm, or 7.5 mm 7.5 mm with 0.5 mm in thickness. An X-ray diffractometer (XRD, SmartLab, Rigaku Corp.)
Fig. 1. Schematics of SrTiO3 single crystal substrates with “Exactly-cut”, off-cut toward [001] direction, and off-cut toward [011] direction.
307
equipped with a rotating Cu target was used for crystallographic and thickness analyses of obtained thin films, which include inplane pole figure measurement [8]. The pole figures were taken for reflections of LiCoO2 003, as well as LiCoO2 104, 110 and SrTiO3 100, 110, and 111 with a step of 1.0 , where non-monochromated Xray was used to obtain high diffraction intensity. Prior to the pole figure measurement, asymmetric diffraction measurements were performed with 2-bounce Ge 220 monochromated CuKa1 radiation in order to determine the 2q value precisely for each reflection. Transmission electron microscopy (TEM) was employed to investigate the structural features of the films. Cross-sectional samples were made by wedge polishing. Bright field (BF) and dark-field (DF) transmission electron micrographs were taken on using a JEOL ARM-200F microscope at 200 kV acceleration voltage. Electrode performance of the obtained films was investigated in an all-solid-state cell. Powder Li3.25Ge0.25P0$75S4 (thio-LISICON [9]) was used as the solid electrolyte. An IneLi alloy was selected as the counter electrode and formed by attaching a piece of lithium (2 mg) to an indium foil (60 mg). The surface of LiCoO2 film was coated with a 20-nm-thick Li3PO4 layer to reduce the interfacial resistance to be low enough for the electrochemical measurement [10,11] The Li3PO4-coated LiCoO2 film on the La:SrTiO3 substrate and the IneLi alloy were attached to respective sides of a thio-LISICON layer (150 mg) and they were pressed together at around 500 MPa to form a three-layered pellet with a 10 mm diameter. The cell was charged and discharged at room temperature using a potentio-galvanostats (VSP, Bio-Logic SAS, and PS-08, Toho Technical Research Co., Ltd.). Because the electrode potential of the IneLi alloy counter electrode is 0.62 V vs. Liþ/Li, the upper cutoff voltage was set at 3.58 V in order to charge the epitaxial LiCoO2 thin film up to 4.2 V vs. Liþ/Li. The charging rate was fixed at 0.01 C, while the discharging rate was varied from 0.01 C to 100 C, where the rate of 137 mA g1 was defined as 1 C. The specific capacity was estimated based on the film thickness determined by X-ray reflectivity measurement and the theoretical density of 5.0 g cm3. 3. Results and discussion Our previous study, in which exactly-cut SrTiO3 substrates were used, has revealed that LiCoO2 epitaxial thin films grow on SrTiO3 (100) surface with (104) orientation. The films exhibit four-domain structure, in which the four kinds of domains whose c-axes of LiCoO2 are parallel to the four equivalent 〈111〉 directions of SrTiO3 substrates appear with equal probability [3]. The LiCoO2 films in this study are also (104) oriented regardless of the exactly-cut or vicinal SrTiO3 (100) substrates; however, the four kinds of domains do not appear with equal probability on the vicinal substrates. Fig. 2 shows X-ray pole figures for 003 reflections of the LiCoO2 films, and Fig. 3 indicates azimuthal phi scan curves (a ¼ 36 and phi ¼ b, a: angle from substrate surface) extracted from the pole figures with off-cut angle adjustment. The LiCoO2 film grown on the exactly-cut SrTiO3 (100) substrate gives four poles in 〈111〉 directions with an equal intensity, as indicated in Fig. 2a and the bottom curve in Fig. 3, which support that the four kinds of domains appear with the equal probability. Such domain structure is dramatically changed on the vicinal SrTiO3 (100) substrates, while the out-of-plane (104) orientation is preserved. The off-cut toward [001] direction prefers two orientations out of the four, as can be recognized in the pole figure in Fig. 2c and the second curve from the bottom in Fig. 3. The XRD data obtained from the LiCoO2 thin film grown on a 3 off-cut substrate toward [001] direction at a laser repetition rate of 40 Hz clearly indicate that the off-cut promotes two orientations with the c-axis parallel to ½111 and ½111 directions of the SrTiO3 substrate, i.e. in a descending direction of in-plane off-cut direction (arrows in Fig. 1)
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Fig. 2. LiCoO2 003 pole figures of LiCoO2 thin films on various SrTiO3 (100) substrates. Substrate information, used laser repetition rate, and the film thickness are labeled on each data. The data on the exactly-cut substrate in Fig. 2a are referred from the previous report [K. Nishio, T. Ohnishi, K. Akatsuka, K. Takada, J. Power Sources 247 (2014) 687e691.]. Although the previous deposition conditions are somewhat different from the present ones, we have confirmed that the thin films with the symmetric four orientations of LiCoO2 caxis are obtained on the exactly-cut SrTiO3 (100) substrates regardless of the deposition conditions.
whereas suppresses the other two orientations. Other pole figures and phi scan curves for the films grown on the off-cut substrates toward in-plane [011] direction also support that c-axes are preferentially oriented in the descending direction. In the respective pole figures and corresponding curves, the highest intensity is found in the [111] direction of the SrTiO3 substrate, which indicates the LiCoO2 films are preferentially grown with their c-axes parallel to the [111] direction, i.e. with their c-axes leaning toward the descending direction. In addition, it is clear from the comparison between the pole figures in Fig. 2d and e that increasing cut-off angle from 2 to 4 strengthens the preferential orientation. LiCoO2 domains with their c-axes parallel to ½111 of the SrTiO3 (hereinafter ½111 domains) appear with the smallest fraction accompanied by ½111 and ½111 domains with the second smallest and equal fractions on the 2 cut-off substrate, when the laser repetition rate is 40 Hz. On the other hand, only [111] domains survive on the 4 cut-off substrate, as shown in Fig. 2e and the second curve in Fig. 3, suggesting almost perfect single orientation. In addition to the increasing off-cut angle, lowering laser repetition rate enhances the preferential orientation. Minor domains observed on the substrate with 2 off-cut toward [011] disappear, when the laser repetition rate is reduced from 40 to 2 Hz. These behaviors can be understood by the step-flow thin film growth manner. A (104)-oriented film grown on an exactly-cut substrate has the four-domain structure, because the four kinds of domains grow with their c-axes parallel to the four 〈111〉 directions of the SrTiO3 substrates with the same probability, and the four 〈111〉 directions are equivalent against the substrate surface.
On the other hand, the off-cut introduces steps along a specified direction and breaks the equivalency in the substrate surface. However, the off-cut does not change the four-domain structure, unless the domains grow from the step edges, because the introduced steps do not influence nuclei formed on the terraces. For the preferential orientation, the ablated species landing on the substrate surface to be adatoms must diffuse on the terraces and reach the step edges before nucleation. Employment of the off-cut substrates in this study changes the orientation structure, which evidences that the films grow in step-flow growth mode. Important parameters that govern the step-flow mode are the substrate temperature and the off-cut angle. The step-flow growth of high melting point oxide thin films takes place usually when the growth temperature is very high, e.g. over 1000 C, where the migration length, which is defined as the average length between the deposited point and the nucleation (or growth) point for the adatoms on the growth surface, exceeds an average width of atomic terraces on a step-and-terrace substrate surface [12]. On the other hand, the substrate temperature of the present growth conditions is only 725 C, and the step-flow growth will not take place on the wide terraced exactly-cut substrates. The narrow terraces on the vicinal substrates are considered to compensate for the relatively low substrate temperature. The average terrace width is determined by the off-cut angle of the substrate and the step height; it becomes infinite when off-cut angle is zero, i.e. the surface is parallel to a specific crystallographic plane (ideal exactly-cut), and it shortens as the off-cut angle enlarges. In case of SrTiO3 (100) surface, since the step height is a unit cell length of 0.3905 nm, the average terrace width can be
K. Nishio et al. / Journal of Power Sources 325 (2016) 306e310
Thickness normalized intensity (kcps/nm)
20
15
2º off toward [011], 2 Hz 4º off toward [011], 40 Hz 2º off toward [011], 40 Hz 3º off toward [001], 40 Hz Exactly-cut, 10 Hz
10
SrTiO3 direction 5
[111]
[111]
[111]
[111]
180 Phi (°)
270
360
0
0
90
Fig. 3. Azimuthal phi scans of LiCoO2 003 reflection for LiCoO2 thin films. The diffraction intensity is extracted from in-plane pole figures shown in Fig. 2, and the curves are offset for clarity.
calculated by 0.3905 nm/tanq, where q is the off-cut angle and thus varies from 11.2 nm to 5.6 nm with increasing off-cut angle from 2 to 4 in the present film growth. Comparison between the pole figures in Fig. 2d and e clearly demonstrates that the orientation is more unified on the substrate with the higher off-cut angle, which should be attributed to the promoted step-flow growth. Another factor that influences the migration length is the density of the adatoms, which is correlated to the super-saturation of deposited species. When the surface density of the adatoms increases, they will have more chances to collide with each other and nucleate on the terrace before reaching the specific growth sites on
309
the surface such as the step edges and kinks. The time scale of migration process also changes as a function of temperature, ranging from the order of second to several tens of seconds at lower temperatures such as around 800 C [12]. The laser repetition rates in this study give shorter laser pulse intervals than the above time scale: even when the laser repetition rate decreases from 40 Hz to 2 Hz, the laser pulse interval elongates from 0.025 s only to 0.5 s. Under this condition, new species ablated by a laser pulse reach the substrate before that supplied by the previous pulse nucleate. Therefore, changing repetition rate varies the adatom density, and thus they influence the domain structure. In fact, decreasing the repetition rate enhances the single c-axis orientation of LiCoO2, as can be recognized in Figs. 2 and 3, which is due to the lowering adatom density. Fig. 4 shows cross-sectional DF TEM images (a and c) of the LiCoO2 thin films observed parallel to the [011] direction of SrTiO3 substrates. A typical electron diffraction pattern from a part of LiCoO2 thin film taken in the same direction is shown in Fig. 4b. These TEM images are composed of overlapping two DF images, which are projected through the LiCoO2 003 diffraction spots indicated by red and green arrows in Fig. 4b. They are colored in red and green, respectively, and overlap in Fig. 4a and c, and thus the red and green parts in the images correspond to the LiCoO2 with their c-axes parallel to the SrTiO3 ½111 and ½111 directions, respectively. On the exactly-cut SrTiO3 (100) substrate (Fig. 4a), columnar LiCoO2 domains are clearly visible with the lateral size less than 100 nm. Since the c-axes of LiCoO2 are aligned in the four equivalent SrTiO3 〈111〉 directions, as indicated by the phi scan result in Fig. 3, non-colored regions in Fig. 4a correspond to LiCoO2 domains with c-axis parallel to either [111] or ½111 directions of SrTiO3, both of which do not appear in the DF observation along this direction. The colored and non-colored regions are found to be roughly fiftyfifty when the large area DF images are analyzed, because the caxes are evenly distributed to the four 〈111〉 directions. Fig. 4c shows the combined DF images of the LiCoO2 film grown on the vicinal SrTiO3 (100) substrate with 4 off-cut toward inplane [011] direction. The phi scan result in Fig. 3 shows high diffraction intensity in the SrTiO3 [111] direction, which indicates that the LiCoO2 should have single orientation of c-axis. However, it also has weak peaks in the other 〈111〉 directions, suggesting that the film includes trace amounts of domains with different orientations. The minor domains are clearly seen in the DF images, because the DF observation parallel to the off-cut direction puts its finger on ½111 and ½111 domains. As expected from the phi scan
Fig. 4. Cross-sectional TEM images of LiCoO2 thin films grown on SrTiO3 (100) substrates (a, c) and a typical electron diffraction pattern from the thin film part (b). The substrates are exactly-cut (a) and vicinal toward [011] direction with a 4 off-cut angle (c). Observation direction is parallel to the [011] direction of SrTiO3, which is the off-cut direction for (c).
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result, the overlapping images are mostly non-colored, because the majority is [111] domains. On the other hand, the minor ½111 and ½111 domains are observed as columns with lateral size again less than 100 nm in the image. Although the c-axes of (104) oriented LiCoO2 have been successfully aligned with a high level on the vicinal SrTiO3 (100), the high rate capability has not been significantly improved. Fig. 5 shows discharge curves for a (104) oriented LiCoO2 film with the thickness of 140 nm grown on a La:SrTiO3 (100) substrate with 5 off-cut toward in-plane [011] direction. Prior to the cell construction, nearly perfect single c-axis orientation is confirmed by the pole figure measurement. Although the capacity is not high at a low discharge rate of 0.01 C, 25% of capacity is kept even at a high rate of discharge with 100 C. However, this rate capability is not so high as that of our previous (104) oriented 77-nm-thick LiCoO2 film grown on an exactly-cut substrate with the four-domain structure [3], where 35% of capacity at 0.01 C is kept at 100 C discharge. Although the present LiCoO2 film is almost twice thicker than the previous one, it is likely that aligning the c-axis of LiCoO2 does not improve the rate capability significantly. Considering the ineffectiveness of aligning the c-axis of (104) oriented LiCoO2 thin films in improving the rate capability, one possible reason is the structural features of the films, if other resistive components do not predominate. Such a situation is conceivable in this case, because the solid electrolyte is an oxide: cathode/sulfide electrolyte interfaces have been often reported to be highly-resistive to be rate-determining [10], whereas those are not resistive, when the electrolytes are oxides [13]. Although it has not been clear at the present stage whether the rate-determining step is inside the film, it is worth investigating ideality of the films. As shown in Fig. 4a, the films are composed of the columnar domains with different c-axes orientation on the exactly-cut substrates. On the off-cut substrates toward in-plane [011] direction, again the columnar domains with the similar size are grown, as observed in Fig. 4c. Although the colored columns are minority with different c-axes orientation and the BF images are unclear enough to determine the respective [111] domains comprising a majority of the film, intervals between the minor domains found in Fig. 4c suggest that the majority domains have almost the same size as that on the exactly-cut substrate. Therefore, it should be concluded that the films grown on off-cut substrates include domain boundaries with the same density as that on exactly-cut ones, where discontinuity of the CoO2 layers including antiphase boundaries observed in Ref. [14] can take place. They can be the reason for the unimproved rate capability in spite of the much higher degree of the c-axis alignment on the off-cut substrates. In general, columnar growth of thin films is originated from lattice mismatch. The mismatch with SrTiO3 is relatively small as ca. 2.6% (LiCoO2 is larger); however, relationship between the in-plane lattice constants and the film thickness indicates that the crystal
Fig. 5. Discharge curves for an all-solid-state battery using a single domain (104)oriented LiCoO2 epitaxial thin film on 5 off-cut La:SrTiO3 (100) substrate toward [011] direction. Discharge rates for the presented curves are 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, and 100 C.
lattice of (104) oriented LiCoO2 film tends to be relaxed at rather small thickness (less than 50 nm) at the present growth conditions and promote the columnar growth. Therefore, conductive substrates with better lattice matching will be necessary in order to avoid the relaxation of LiCoO2 lattice and the resultant columnar growth. They are considered to provide large monolithic LiCoO2 domains, where high rate capability is expected. 4. Conclusions In this study, LiCoO2 films are epitaxially grown on vicinal SrTiO3 (100) substrates. The off-cut angle makes four 〈111〉 directions in the SrTiO3 substrates inequivalent, which promotes specific orientations in the four-domain structure on exactly-cut substrates. High off-cut along [011] direction and film growth in step-flow growth mode aligns c-axes of the LiCoO2 in a SrTiO3 [111] direction. However, the alignment of the c-axes does not affect the electrode properties of the LiCoO2 films probably due to the presence of domain boundaries, because the films still have columnar structure. Acknowledgements This work was partially supported by MEXT Program for Development of Environmental Technology using Nanotechnology, the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, and JSPS KAKENHI Grant Number 25420724 and 25289253, Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). KM is grateful to Ms. Akiko Matsuo and Ms. Yumi Fujioka for TEM sample preparations. References [1] K. Kanehori, K. Matsumoto, K. Miyauchi, T. Kudo, Thin film solid electrolyte and its application to secondary lithium cell, Solid State Ionics 9 & 10 (1983) 1445e1448. [2] H. Moriwake, A. Kuwabata, C.A.J. Fisher, R. Huang, T. Hitosugi, Y.H. Ikuhara, H. Oki, Y. Ikuhara, First-principles calculations of lithium-ion migration at a coherent grain boundary in a cathode material, LiCoO2, Adv. Mater. 25 (2013) 618e622. [3] K. Nishio, T. Ohnishi, K. Akatsuka, K. Takada, Crystal orientation of epitaxial LiCoO2 films grown on SrTiO3 substrates, J. Power Sources 247 (2014) 687e691. [4] Q. Gan, R.A. Rao, C.B. Eom, Control of the growth and domain structure of epitaxial SrRuO3 thin films by vicinal (001) SrTiO3 substrates, Appl. Phys. Lett. 70 (1997) 1962e1964. [5] T. Ohnishi, K. Mitsuishi, K. Nishio, K. Takada, Epitaxy of Li3xLa2/3xTiO3 films and the influence of La ordering on Li-ion conduction, Chem. Mater. 27 (2015) 1233e1241. [6] T. Ohnishi, B.T. Hang, X. Xu, M. Osada, K. Takada, Quality control of epitaxial LiCoO2 thin films grown by pulsed laser deposition, J. Mater. Res. 25 (2010) 1886e1889. [7] T. Ohnishi, K. Takada, High-rate growth of high-crystallinity LiCoO2 epitaxial thin films by pulsed laser deposition, Appl. Phys. Express 5 (2012) 055502. [8] K. Nagao, E. Kagami, X-ray thin film measurement techniques VII. Pole figure measurement, Rigaku J. 27 (2011) 6e14. [9] R. Kanno, M. Murayama, Lithium ionic conductor thio-LISICON, J. Electrochem. Soc. 148 (2001) A742eA746. [10] N. Ohta, K. Takada, L.Q. Zhang, R.Z. Ma, M. Osada, T. Sasaki, Enhancement of the high-rate capability of solid-state lithium batteries by nanoscale interfacial modification, Adv. Mater. 18 (2006) 2226e2229. [11] X.X. Xu, K. Takada, K. Fukuda, T. Ohnishi, K. Akatsuka, M. Osada, B.T. Hang, K. Kumagai, T. Sekiguchi, T. Sasaki, Tantalum oxide nanomesh as self-standing one nanometre thick electrolyte, Energy Environ. Sci. 4 (2011) 3509e3512. [12] M. Lippmaa, N. Nakagawa, T. Kinoshita, T. Furumochi, M. Kawasaki, H. Koinuma, Growth dynamics of oxide thin films at temperature above 1000 C, Physica C 335 (2000) 196e200. [13] M. Haruta, S. Shiraki, T. Suzuki, A. Kumatani, T. Ohsawa, T. Takagi, R. Shimizu, T. Hitosugi, Negligible “negative space-charge layer effects” at oxideelectrolyte/electrode interfaces of thin-film batteries, Nano Lett. 15 (2015) 1498. [14] S.J. Zheng, C.A.J. Fisher, T. Hitosugi, A. Kumatani, S. Shiraki, Y.H. Ikuhara, A. Kuwabata, H. Moriwake, H. Oki, Y. Ikuhara, Antiphase inversion domains in lithium cobaltite thin films deposited on single-crystal sapphire substrates, Acta Mater. 61 (2013) 7671e7678.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Orientation alignment of epitaxial LiCoO2 thin films on vicinal SrTiO3 (100) substrates Kazunori Nishio a, 1, Tsuyoshi Ohnishi a, b, c, Kazutaka Mitsuishi a, Narumi Ohta a, c, Ken Watanabe c, Kazunori Takada a, b, c, * a b c
Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan International Center for Materials Nanoarchitectonics (MANA), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan Environment and Energy Materials Division, National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan
h i g h l i g h t s LiCoO2 epitaxial thin films are synthesized on vicinal SrTiO3 (100) substrates. The (104)-oriented films exhibit single-domain structure on the vicinal substrates. The orientation alignment is explainable by step-flow growth mechanism. The influence of domain boundaries on electrode performance is discussed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2016 Received in revised form 1 June 2016 Accepted 3 June 2016 Available online 16 June 2016
LiCoO2 is epitaxially grown on SrTiO3 (100) substrates with (104) orientation. Because the LiCoO2 film is grown with its c-axis parallel to four equivalent 〈111〉 axes of the SrTiO3, the (104)-oriented film exhibits four-domain structure on the SrTiO3 (100) substrate. Introducing off-cut angle to the substrate surface breaks the equivalency between the four 〈111〉 axes of the SrTiO3 substrate to induce preferential growth of specific orientation with the c-axis in a descending direction of off-cut surface. Increasing off-cut angle and lowering deposition rate promote the preferential growth, because they facilitate step-flow growth mode, and finally align the c-axes in the domains completely into one 〈111〉 direction of the SrTiO3 substrate. The LiCoO2 film delivers a discharge capacity of 90 mAh g1 at a low discharge rate of 0.01 C, and 25% of capacity is kept even at a high rate of discharge with 100 C. © 2016 Elsevier B.V. All rights reserved.
Keywords: Epitaxial film Vicinal substrate Pulsed laser deposition Lithium cobalt oxide Thin-film battery
1. Introduction Studies on thin-film batteries started in 1980s [1], and they are growing in importance with the births of new micro-devices, e.g. nonvolatile memories, micro electro mechanical systems (MEMS), etc. Another importance of thin-film systems is originated from their simple geometry. Because electrode/electrolyte interfaces and ion-conduction paths can be simplified in the geometry, thin-film systems are beneficial for fundamental research. Especially,
* Corresponding author. National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan. E-mail address: [email protected] (K. Takada). 1 Present address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980e8577, Japan. http://dx.doi.org/10.1016/j.jpowsour.2016.06.015 0378-7753/© 2016 Elsevier B.V. All rights reserved.
recent materials research has provided new materials with excellent bulk properties. They have often replaced their bulk with their interfaces that govern the battery performance and initiated many studies on interface, where planar interfaces and one-dimensional ionic conduction in the simplified geometry are useful. Although the electrochemical systems become to have simplified geometry in thin-film systems, more efforts are necessary to gain basic insight into intrinsic interface properties. Grain boundaries affect the ionic conduction, and the interfaces are formed between uncontrolled crystal faces, as far as the films are polycrystalline. Epitaxial films are anticipated to provide ideal interfaces for the fundamental research on the interfaces due to their specified surface. However, even epitaxial films do not always show intrinsic nature of the materials. For example, they often have multi-domain structure, whereas a recent computational study predicts that twin domain boundaries existing in epitaxial films
K. Nishio et al. / Journal of Power Sources 325 (2016) 306e310
affect ion transport in the films [2]. Therefore, ultimate goal is to form these materials in monolithic single-crystal films without domain boundaries, which is equivalent to bulk single crystal, although it is challenging task. Our previous study on epitaxial growth of LiCoO2 has revealed that LiCoO2 is epitaxially grown with (104), (018), and (001) orientations on SrTiO3 (100), (110), and (111) substrates, respectively [3]. In the study, we demonstrated electrode properties of the (104)-oriented film, because CoO2 layers in the film are the most upright and thus offer the most accessible interlayers for lithium ions. However, the film has four-domain structures, in which domain boundaries may affect the electrode properties. Vicinal substrates, surface of which is at off-cut angle to a specific crystal plane, are sometimes utilized in order to improve the crystallinity of epitaxial thin films [4,5], because they are generally believed to enhance step flow growth of thin films and control domain structure. In this study, (104) oriented LiCoO2 epitaxial thin films are grown on vicinal SrTiO3 (100) substrates to promote preferential formation of specific domains.
2. Experimental Epitaxial LiCoO2 thin films were grown by pulsed laser deposition (PLD). A sintered Li1$2CoO2þd pellet (TOSHIMA Manufacturing Co., Ltd.) was used as the PLD target. A KrF excimer laser (COMPex Pro 50, COHERENT, Inc., l ¼ 248 nm) was operated at a repetition rate of 2 Hz or 40 Hz for the target ablation. Laser spot size on the target was 0.031 cm2, and the laser fluence was 0.65 J cm2. The incident angle of the excimer laser on the target surface was 30 , and the targetesubstrate distance was set at 58 mm. Oxygen pressure in the chamber and substrate temperature were set at 0.1 Pa and 725 C, respectively. Detailed information regarding the selection of target composition, laser fluence, and oxygen pressure can be found in Refs. [6,7]. Films were deposited on non-doped SrTiO3 single crystal substrates, which were exactly-cut SrTiO3 (100) substrates and vicinal substrates with 3 off-cut toward in-plane [001] direction, and those with 2 and 4 off-cut toward in-plane [011] direction from (100) plane as illustrated in Fig. 1 in order to investigate the effects of the off-cut directions and off-cut angles from (100) plane of SrTiO3 on the film structure. A vicinal 3 at% La-doped SrTiO3 (La:SrTiO3) single crystal substrate with 5 off-cut toward in-plane [011] direction was used in place of the non-doped insulating ones in order to investigate the electrode properties of the LiCoO2 film grown on the vicinal substrate, because the La-doping provides electronic conduction to the substrates and makes them act as current collectors in the electrochemical measurement. All the substrates were made by SHINKOSHA Co., Ltd., and the substrate size is typically 10 mm in diameter, 10 mm 10 mm, or 7.5 mm 7.5 mm with 0.5 mm in thickness. An X-ray diffractometer (XRD, SmartLab, Rigaku Corp.)
Fig. 1. Schematics of SrTiO3 single crystal substrates with “Exactly-cut”, off-cut toward [001] direction, and off-cut toward [011] direction.
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equipped with a rotating Cu target was used for crystallographic and thickness analyses of obtained thin films, which include inplane pole figure measurement [8]. The pole figures were taken for reflections of LiCoO2 003, as well as LiCoO2 104, 110 and SrTiO3 100, 110, and 111 with a step of 1.0 , where non-monochromated Xray was used to obtain high diffraction intensity. Prior to the pole figure measurement, asymmetric diffraction measurements were performed with 2-bounce Ge 220 monochromated CuKa1 radiation in order to determine the 2q value precisely for each reflection. Transmission electron microscopy (TEM) was employed to investigate the structural features of the films. Cross-sectional samples were made by wedge polishing. Bright field (BF) and dark-field (DF) transmission electron micrographs were taken on using a JEOL ARM-200F microscope at 200 kV acceleration voltage. Electrode performance of the obtained films was investigated in an all-solid-state cell. Powder Li3.25Ge0.25P0$75S4 (thio-LISICON [9]) was used as the solid electrolyte. An IneLi alloy was selected as the counter electrode and formed by attaching a piece of lithium (2 mg) to an indium foil (60 mg). The surface of LiCoO2 film was coated with a 20-nm-thick Li3PO4 layer to reduce the interfacial resistance to be low enough for the electrochemical measurement [10,11] The Li3PO4-coated LiCoO2 film on the La:SrTiO3 substrate and the IneLi alloy were attached to respective sides of a thio-LISICON layer (150 mg) and they were pressed together at around 500 MPa to form a three-layered pellet with a 10 mm diameter. The cell was charged and discharged at room temperature using a potentio-galvanostats (VSP, Bio-Logic SAS, and PS-08, Toho Technical Research Co., Ltd.). Because the electrode potential of the IneLi alloy counter electrode is 0.62 V vs. Liþ/Li, the upper cutoff voltage was set at 3.58 V in order to charge the epitaxial LiCoO2 thin film up to 4.2 V vs. Liþ/Li. The charging rate was fixed at 0.01 C, while the discharging rate was varied from 0.01 C to 100 C, where the rate of 137 mA g1 was defined as 1 C. The specific capacity was estimated based on the film thickness determined by X-ray reflectivity measurement and the theoretical density of 5.0 g cm3. 3. Results and discussion Our previous study, in which exactly-cut SrTiO3 substrates were used, has revealed that LiCoO2 epitaxial thin films grow on SrTiO3 (100) surface with (104) orientation. The films exhibit four-domain structure, in which the four kinds of domains whose c-axes of LiCoO2 are parallel to the four equivalent 〈111〉 directions of SrTiO3 substrates appear with equal probability [3]. The LiCoO2 films in this study are also (104) oriented regardless of the exactly-cut or vicinal SrTiO3 (100) substrates; however, the four kinds of domains do not appear with equal probability on the vicinal substrates. Fig. 2 shows X-ray pole figures for 003 reflections of the LiCoO2 films, and Fig. 3 indicates azimuthal phi scan curves (a ¼ 36 and phi ¼ b, a: angle from substrate surface) extracted from the pole figures with off-cut angle adjustment. The LiCoO2 film grown on the exactly-cut SrTiO3 (100) substrate gives four poles in 〈111〉 directions with an equal intensity, as indicated in Fig. 2a and the bottom curve in Fig. 3, which support that the four kinds of domains appear with the equal probability. Such domain structure is dramatically changed on the vicinal SrTiO3 (100) substrates, while the out-of-plane (104) orientation is preserved. The off-cut toward [001] direction prefers two orientations out of the four, as can be recognized in the pole figure in Fig. 2c and the second curve from the bottom in Fig. 3. The XRD data obtained from the LiCoO2 thin film grown on a 3 off-cut substrate toward [001] direction at a laser repetition rate of 40 Hz clearly indicate that the off-cut promotes two orientations with the c-axis parallel to ½111 and ½111 directions of the SrTiO3 substrate, i.e. in a descending direction of in-plane off-cut direction (arrows in Fig. 1)
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Fig. 2. LiCoO2 003 pole figures of LiCoO2 thin films on various SrTiO3 (100) substrates. Substrate information, used laser repetition rate, and the film thickness are labeled on each data. The data on the exactly-cut substrate in Fig. 2a are referred from the previous report [K. Nishio, T. Ohnishi, K. Akatsuka, K. Takada, J. Power Sources 247 (2014) 687e691.]. Although the previous deposition conditions are somewhat different from the present ones, we have confirmed that the thin films with the symmetric four orientations of LiCoO2 caxis are obtained on the exactly-cut SrTiO3 (100) substrates regardless of the deposition conditions.
whereas suppresses the other two orientations. Other pole figures and phi scan curves for the films grown on the off-cut substrates toward in-plane [011] direction also support that c-axes are preferentially oriented in the descending direction. In the respective pole figures and corresponding curves, the highest intensity is found in the [111] direction of the SrTiO3 substrate, which indicates the LiCoO2 films are preferentially grown with their c-axes parallel to the [111] direction, i.e. with their c-axes leaning toward the descending direction. In addition, it is clear from the comparison between the pole figures in Fig. 2d and e that increasing cut-off angle from 2 to 4 strengthens the preferential orientation. LiCoO2 domains with their c-axes parallel to ½111 of the SrTiO3 (hereinafter ½111 domains) appear with the smallest fraction accompanied by ½111 and ½111 domains with the second smallest and equal fractions on the 2 cut-off substrate, when the laser repetition rate is 40 Hz. On the other hand, only [111] domains survive on the 4 cut-off substrate, as shown in Fig. 2e and the second curve in Fig. 3, suggesting almost perfect single orientation. In addition to the increasing off-cut angle, lowering laser repetition rate enhances the preferential orientation. Minor domains observed on the substrate with 2 off-cut toward [011] disappear, when the laser repetition rate is reduced from 40 to 2 Hz. These behaviors can be understood by the step-flow thin film growth manner. A (104)-oriented film grown on an exactly-cut substrate has the four-domain structure, because the four kinds of domains grow with their c-axes parallel to the four 〈111〉 directions of the SrTiO3 substrates with the same probability, and the four 〈111〉 directions are equivalent against the substrate surface.
On the other hand, the off-cut introduces steps along a specified direction and breaks the equivalency in the substrate surface. However, the off-cut does not change the four-domain structure, unless the domains grow from the step edges, because the introduced steps do not influence nuclei formed on the terraces. For the preferential orientation, the ablated species landing on the substrate surface to be adatoms must diffuse on the terraces and reach the step edges before nucleation. Employment of the off-cut substrates in this study changes the orientation structure, which evidences that the films grow in step-flow growth mode. Important parameters that govern the step-flow mode are the substrate temperature and the off-cut angle. The step-flow growth of high melting point oxide thin films takes place usually when the growth temperature is very high, e.g. over 1000 C, where the migration length, which is defined as the average length between the deposited point and the nucleation (or growth) point for the adatoms on the growth surface, exceeds an average width of atomic terraces on a step-and-terrace substrate surface [12]. On the other hand, the substrate temperature of the present growth conditions is only 725 C, and the step-flow growth will not take place on the wide terraced exactly-cut substrates. The narrow terraces on the vicinal substrates are considered to compensate for the relatively low substrate temperature. The average terrace width is determined by the off-cut angle of the substrate and the step height; it becomes infinite when off-cut angle is zero, i.e. the surface is parallel to a specific crystallographic plane (ideal exactly-cut), and it shortens as the off-cut angle enlarges. In case of SrTiO3 (100) surface, since the step height is a unit cell length of 0.3905 nm, the average terrace width can be
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Thickness normalized intensity (kcps/nm)
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SrTiO3 direction 5
[111]
[111]
[111]
[111]
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0
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Fig. 3. Azimuthal phi scans of LiCoO2 003 reflection for LiCoO2 thin films. The diffraction intensity is extracted from in-plane pole figures shown in Fig. 2, and the curves are offset for clarity.
calculated by 0.3905 nm/tanq, where q is the off-cut angle and thus varies from 11.2 nm to 5.6 nm with increasing off-cut angle from 2 to 4 in the present film growth. Comparison between the pole figures in Fig. 2d and e clearly demonstrates that the orientation is more unified on the substrate with the higher off-cut angle, which should be attributed to the promoted step-flow growth. Another factor that influences the migration length is the density of the adatoms, which is correlated to the super-saturation of deposited species. When the surface density of the adatoms increases, they will have more chances to collide with each other and nucleate on the terrace before reaching the specific growth sites on
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the surface such as the step edges and kinks. The time scale of migration process also changes as a function of temperature, ranging from the order of second to several tens of seconds at lower temperatures such as around 800 C [12]. The laser repetition rates in this study give shorter laser pulse intervals than the above time scale: even when the laser repetition rate decreases from 40 Hz to 2 Hz, the laser pulse interval elongates from 0.025 s only to 0.5 s. Under this condition, new species ablated by a laser pulse reach the substrate before that supplied by the previous pulse nucleate. Therefore, changing repetition rate varies the adatom density, and thus they influence the domain structure. In fact, decreasing the repetition rate enhances the single c-axis orientation of LiCoO2, as can be recognized in Figs. 2 and 3, which is due to the lowering adatom density. Fig. 4 shows cross-sectional DF TEM images (a and c) of the LiCoO2 thin films observed parallel to the [011] direction of SrTiO3 substrates. A typical electron diffraction pattern from a part of LiCoO2 thin film taken in the same direction is shown in Fig. 4b. These TEM images are composed of overlapping two DF images, which are projected through the LiCoO2 003 diffraction spots indicated by red and green arrows in Fig. 4b. They are colored in red and green, respectively, and overlap in Fig. 4a and c, and thus the red and green parts in the images correspond to the LiCoO2 with their c-axes parallel to the SrTiO3 ½111 and ½111 directions, respectively. On the exactly-cut SrTiO3 (100) substrate (Fig. 4a), columnar LiCoO2 domains are clearly visible with the lateral size less than 100 nm. Since the c-axes of LiCoO2 are aligned in the four equivalent SrTiO3 〈111〉 directions, as indicated by the phi scan result in Fig. 3, non-colored regions in Fig. 4a correspond to LiCoO2 domains with c-axis parallel to either [111] or ½111 directions of SrTiO3, both of which do not appear in the DF observation along this direction. The colored and non-colored regions are found to be roughly fiftyfifty when the large area DF images are analyzed, because the caxes are evenly distributed to the four 〈111〉 directions. Fig. 4c shows the combined DF images of the LiCoO2 film grown on the vicinal SrTiO3 (100) substrate with 4 off-cut toward inplane [011] direction. The phi scan result in Fig. 3 shows high diffraction intensity in the SrTiO3 [111] direction, which indicates that the LiCoO2 should have single orientation of c-axis. However, it also has weak peaks in the other 〈111〉 directions, suggesting that the film includes trace amounts of domains with different orientations. The minor domains are clearly seen in the DF images, because the DF observation parallel to the off-cut direction puts its finger on ½111 and ½111 domains. As expected from the phi scan
Fig. 4. Cross-sectional TEM images of LiCoO2 thin films grown on SrTiO3 (100) substrates (a, c) and a typical electron diffraction pattern from the thin film part (b). The substrates are exactly-cut (a) and vicinal toward [011] direction with a 4 off-cut angle (c). Observation direction is parallel to the [011] direction of SrTiO3, which is the off-cut direction for (c).
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result, the overlapping images are mostly non-colored, because the majority is [111] domains. On the other hand, the minor ½111 and ½111 domains are observed as columns with lateral size again less than 100 nm in the image. Although the c-axes of (104) oriented LiCoO2 have been successfully aligned with a high level on the vicinal SrTiO3 (100), the high rate capability has not been significantly improved. Fig. 5 shows discharge curves for a (104) oriented LiCoO2 film with the thickness of 140 nm grown on a La:SrTiO3 (100) substrate with 5 off-cut toward in-plane [011] direction. Prior to the cell construction, nearly perfect single c-axis orientation is confirmed by the pole figure measurement. Although the capacity is not high at a low discharge rate of 0.01 C, 25% of capacity is kept even at a high rate of discharge with 100 C. However, this rate capability is not so high as that of our previous (104) oriented 77-nm-thick LiCoO2 film grown on an exactly-cut substrate with the four-domain structure [3], where 35% of capacity at 0.01 C is kept at 100 C discharge. Although the present LiCoO2 film is almost twice thicker than the previous one, it is likely that aligning the c-axis of LiCoO2 does not improve the rate capability significantly. Considering the ineffectiveness of aligning the c-axis of (104) oriented LiCoO2 thin films in improving the rate capability, one possible reason is the structural features of the films, if other resistive components do not predominate. Such a situation is conceivable in this case, because the solid electrolyte is an oxide: cathode/sulfide electrolyte interfaces have been often reported to be highly-resistive to be rate-determining [10], whereas those are not resistive, when the electrolytes are oxides [13]. Although it has not been clear at the present stage whether the rate-determining step is inside the film, it is worth investigating ideality of the films. As shown in Fig. 4a, the films are composed of the columnar domains with different c-axes orientation on the exactly-cut substrates. On the off-cut substrates toward in-plane [011] direction, again the columnar domains with the similar size are grown, as observed in Fig. 4c. Although the colored columns are minority with different c-axes orientation and the BF images are unclear enough to determine the respective [111] domains comprising a majority of the film, intervals between the minor domains found in Fig. 4c suggest that the majority domains have almost the same size as that on the exactly-cut substrate. Therefore, it should be concluded that the films grown on off-cut substrates include domain boundaries with the same density as that on exactly-cut ones, where discontinuity of the CoO2 layers including antiphase boundaries observed in Ref. [14] can take place. They can be the reason for the unimproved rate capability in spite of the much higher degree of the c-axis alignment on the off-cut substrates. In general, columnar growth of thin films is originated from lattice mismatch. The mismatch with SrTiO3 is relatively small as ca. 2.6% (LiCoO2 is larger); however, relationship between the in-plane lattice constants and the film thickness indicates that the crystal
Fig. 5. Discharge curves for an all-solid-state battery using a single domain (104)oriented LiCoO2 epitaxial thin film on 5 off-cut La:SrTiO3 (100) substrate toward [011] direction. Discharge rates for the presented curves are 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, and 100 C.
lattice of (104) oriented LiCoO2 film tends to be relaxed at rather small thickness (less than 50 nm) at the present growth conditions and promote the columnar growth. Therefore, conductive substrates with better lattice matching will be necessary in order to avoid the relaxation of LiCoO2 lattice and the resultant columnar growth. They are considered to provide large monolithic LiCoO2 domains, where high rate capability is expected. 4. Conclusions In this study, LiCoO2 films are epitaxially grown on vicinal SrTiO3 (100) substrates. The off-cut angle makes four 〈111〉 directions in the SrTiO3 substrates inequivalent, which promotes specific orientations in the four-domain structure on exactly-cut substrates. High off-cut along [011] direction and film growth in step-flow growth mode aligns c-axes of the LiCoO2 in a SrTiO3 [111] direction. However, the alignment of the c-axes does not affect the electrode properties of the LiCoO2 films probably due to the presence of domain boundaries, because the films still have columnar structure. Acknowledgements This work was partially supported by MEXT Program for Development of Environmental Technology using Nanotechnology, the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, and JSPS KAKENHI Grant Number 25420724 and 25289253, Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). KM is grateful to Ms. Akiko Matsuo and Ms. Yumi Fujioka for TEM sample preparations. References [1] K. Kanehori, K. Matsumoto, K. Miyauchi, T. Kudo, Thin film solid electrolyte and its application to secondary lithium cell, Solid State Ionics 9 & 10 (1983) 1445e1448. [2] H. Moriwake, A. Kuwabata, C.A.J. Fisher, R. Huang, T. Hitosugi, Y.H. Ikuhara, H. Oki, Y. Ikuhara, First-principles calculations of lithium-ion migration at a coherent grain boundary in a cathode material, LiCoO2, Adv. Mater. 25 (2013) 618e622. [3] K. Nishio, T. Ohnishi, K. Akatsuka, K. Takada, Crystal orientation of epitaxial LiCoO2 films grown on SrTiO3 substrates, J. Power Sources 247 (2014) 687e691. [4] Q. Gan, R.A. Rao, C.B. Eom, Control of the growth and domain structure of epitaxial SrRuO3 thin films by vicinal (001) SrTiO3 substrates, Appl. Phys. Lett. 70 (1997) 1962e1964. [5] T. Ohnishi, K. Mitsuishi, K. Nishio, K. Takada, Epitaxy of Li3xLa2/3xTiO3 films and the influence of La ordering on Li-ion conduction, Chem. Mater. 27 (2015) 1233e1241. [6] T. Ohnishi, B.T. Hang, X. Xu, M. Osada, K. Takada, Quality control of epitaxial LiCoO2 thin films grown by pulsed laser deposition, J. Mater. Res. 25 (2010) 1886e1889. [7] T. Ohnishi, K. Takada, High-rate growth of high-crystallinity LiCoO2 epitaxial thin films by pulsed laser deposition, Appl. Phys. Express 5 (2012) 055502. [8] K. Nagao, E. Kagami, X-ray thin film measurement techniques VII. Pole figure measurement, Rigaku J. 27 (2011) 6e14. [9] R. Kanno, M. Murayama, Lithium ionic conductor thio-LISICON, J. Electrochem. Soc. 148 (2001) A742eA746. [10] N. Ohta, K. Takada, L.Q. Zhang, R.Z. Ma, M. Osada, T. Sasaki, Enhancement of the high-rate capability of solid-state lithium batteries by nanoscale interfacial modification, Adv. Mater. 18 (2006) 2226e2229. [11] X.X. Xu, K. Takada, K. Fukuda, T. Ohnishi, K. Akatsuka, M. Osada, B.T. Hang, K. Kumagai, T. Sekiguchi, T. Sasaki, Tantalum oxide nanomesh as self-standing one nanometre thick electrolyte, Energy Environ. Sci. 4 (2011) 3509e3512. [12] M. Lippmaa, N. Nakagawa, T. Kinoshita, T. Furumochi, M. Kawasaki, H. Koinuma, Growth dynamics of oxide thin films at temperature above 1000 C, Physica C 335 (2000) 196e200. [13] M. Haruta, S. Shiraki, T. Suzuki, A. Kumatani, T. Ohsawa, T. Takagi, R. Shimizu, T. Hitosugi, Negligible “negative space-charge layer effects” at oxideelectrolyte/electrode interfaces of thin-film batteries, Nano Lett. 15 (2015) 1498. [14] S.J. Zheng, C.A.J. Fisher, T. Hitosugi, A. Kumatani, S. Shiraki, Y.H. Ikuhara, A. Kuwabata, H. Moriwake, H. Oki, Y. Ikuhara, Antiphase inversion domains in lithium cobaltite thin films deposited on single-crystal sapphire substrates, Acta Mater. 61 (2013) 7671e7678.