Atomically Well-Ordered Structure at Solid Electrolyte and Electrode

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Atomically Well-Ordered Structure at Solid Electrolyte and Electrode Interface Reduces the Interfacial Resistance Susumu Shiraki,*,†,‡,# Tetsuroh Shirasawa,*,§,∥,# Tohru Suzuki,‡ Hideyuki Kawasoko,‡ Ryota Shimizu,‡,∥,⊥ and Taro Hitosugi‡,⊥ †

Department of Applied Chemistry, Nippon Institute of Technology, Saitama 345-8501, Japan Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai, Miyagi 980-8577, Japan § National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ∥ JST, PRESTO, Kawaguchi, Saitama 332-0012, Japan ⊥ School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo 152-8552, Japan

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‡

S Supporting Information *

ABSTRACT: Using synchrotron surface X-ray diffraction, we investigated the atomic structures of the interfaces of a solid electrolyte (Li3PO4) and electrode (LiCoO2). We prepared two types of interfaces with high and low interface resistances; the low-resistance interface exhibited a flat and well-ordered atomic arrangement at the electrode surface, whereas the highresistance interface showed a disordered interface. These results indicate that the crystallinity of LiCoO2 at the interface has a significant impact on interface resistance. Furthermore, we reveal that the migration of Li ions along the interface and into grain boundaries and antiphase domain boundaries is a critical factor reducing interface resistance. KEYWORDS: X-ray crystal truncation rod scattering, thin film, all-solid-state battery, solid-electrolyte/electrode interface, LiCoO2, Li3PO4, interface resistance interfaces.11−14 In previous work, we reported a very low solidelectrolyte/electrode interface resistance of 8.6 Ω cm2 in thinfilm batteries consisting of Li3PO4−xNx (LiPON) and LiCoO2.13 Surprisingly, this value is much lower than that of batteries using a liquid electrolyte, and we claimed that the key to obtaining a low interface resistance is to reduce sputtering damage at the solid-electrolyte/electrode interface. The atomic structures of the interface, however, had not yet been clarified. Such microscopic origins of the interface resistances are a topic of great importance in the development of solid-state Li batteries, motivating us to investigate the difference in the interfacial structures between high- and low-resistance interfaces. X-ray crystal truncation rod (CTR) scattering analysis, a kind of surface X-ray diffraction (SXRD), is one of the most powerful nondestructive techniques to investigate the atomic structures of buried interfaces. When the penetration depth of the incident X-ray is greater than the thickness of a crystalline thin film, the atomic positions in the thin film and the interface can be determined by analyzing the CTR scattering profiles.15 In most cases, X-rays can reach the buried interfaces without

1. INTRODUCTION Reducing electrical resistance at the interface of a solidelectrolyte and electrode (solid-electrolyte/electrode) is crucial for the development of fast charge/discharge solid-state Li batteries. Although extensive efforts have been made to reduce the resistance values across the interfaces, the origin of the resistance has been in fierce debate. Many different origins have been proposed, such as the formation of space charge layers, voids, and reaction layers at interfaces.1−3 However, scientific understanding of the origins is still unsatisfactory. Because it is very difficult to investigate the relation between the atomic structures and ionic conductivity across buried interfaces in practical solid-state batteries, atomic-scale understanding of the interfaces has been limited. The difficulty lies in the fact that solid-state Li batteries consist of aggregations of grains, hindering the evaluation of reaction areas, crystal orientations, grain structures, and interface atomic structures. Accordingly, it is highly desirable to reveal the relation between atomic-scale Li-ion pathways and interface resistance. Epitaxial thin films and their heterostructures offer an ideal platform to define Li-ion pathways and to study the structural origin of interface resistance. Recent progress in thin-film deposition techniques of electrodes and electrolytes4−10 has opened a path to quantitatively study the interface resistance values using solid-state thin-film batteries with controlled © XXXX American Chemical Society

Received: May 29, 2018 Accepted: November 5, 2018

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DOI: 10.1021/acsami.8b08926 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

deposition was held at 1 × 10−2 Torr, and the repetition rates were varied: 5 and 20 Hz. A typical thickness of Li3PO4 layers was about 200 nm. The deposition rates of Li3PO4 were 105 and 450 nm/h at the repetition rate of 5 and 20 Hz, respectively. We hereafter refer to samples with Li3PO4 deposited at 5 and 20 Hz as 5 Hz-Li3PO4 and 20 Hz-Li3PO4, respectively. Finally, Li metal films were deposited using a thermal evaporation method. Cyclic voltammetry (CV) measurements were carried out in the voltage range between 3.0 and 4.3 V at a scan rate of 0.1 mV/s. The impedance spectra were obtained by applying an AC voltage with a peak-to-peak amplitude of 100 mV at frequencies ranging from 0.1 Hz to 1 MHz. The thin-film batteries were never exposed to air throughout their fabrication and electrochemical evaluation, and thus, the electrolyte/electrode interfaces were free from contamination.16 To investigate the structures of 5 Hz- and 20 Hz-Li3PO4 batteries, we first measured CTR scattering profiles of a bare (Li3PO4-free) LiCoO2 thin film on the Al2O3(0001) substrate and then analyzed the atomic structure of the LiCoO2/Al2O3-substrate interface. To synthesize a flat LiCoO2 thin film, the substrate temperature was maintained at room temperature during the deposition of LiCoO2. After the deposition, the LiCoO2 film was annealed at 650 °C in an oxygen atmosphere. The thickness of the bare LiCoO2 film was 5 nm, and it was capped with thermally evaporated amorphous silicon to protect the LiCoO2 film from degradation in ambient air. The nominal thickness of the silicon layer was 50 nm. We also prepared Li3PO4/LiCoO2/Al2O3(0001) samples, following the recipes shown above, for synchrotron X-ray CTR scattering analysis of the solidelectrolyte/electrode interface. The thicknesses of the LiCoO2 and Li3PO4 films were 10 and 50 nm, respectively. The CTR scattering measurements were performed using X-rays of 11 keV at BL-4C and BL-18B in The High Energy Accelerator Research Organization (KEK)Photon Factory (PF), Tsukuba, Japan. Integrated scattering intensity at each point in the reciprocal space was measured along the specular CTR by the rocking-scan method using a point detector. The measured intensities were corrected for scattering geometry and active sample area. The specular CTR was measured as a function of the Xray glancing angle (1−33°). The resulting X-ray penetration depth is more than 1 μm, which is much larger than the thickness of LiCoO2 and Li3PO4 films. Accordingly, it is possible to analyze the structure of LiCoO2 and the LiCoO2/Al2O3(0001) interface. During the course of the measurements, the CTR intensities did not change in time, indicating that irradiation damage is negligible.

destructing the structures, which is a significant advantage over other techniques such as transmission electron microscopy. In this letter, we compare the atomic structures of two solidelectrolyte/electrode interfaces with high and low interface resistances using a CTR scattering analysis. We fabricated Li/ Li3PO4/LiCoO2 thin-film Li batteries with high and low interface resistances by changing the repetition rate of the excimer laser in a pulsed laser deposition (PLD) processes. We stress here that the Li-ion pathway is defined in this thin-film battery. Using CTR scattering, we revealed that the lowresistance interface shows flat and atomically well-ordered structures at the LiCoO2 surface, whereas the high-resistance interface showed a disordered LiCoO2 surface. This result indicates that migration of Li ions along the Li3PO4/LiCoO2 interface and into grain boundaries or antiphase domain boundaries in LiCoO2 is the limiting step for ionic conduction between the solid electrolyte and electrode, thereby increasing the Li3PO4/LiCoO2 interface resistance values.

2. EXPERIMENTAL SECTION We fabricated all-solid-state thin-film batteries consisting of LiCoO2, Li3PO4, and Li, as shown in the inset of Figure 1a. First, a current

3. RESULTS AND DISCUSSION We fabricated all-solid-state thin-film batteries consisting of LiCoO2 (positive electrode), Li3PO4 (solid electrolyte), and Li (negative electrode). In the Li3PO4 deposition processes, we changed the repetition rate of the excimer laser (5 and 20 Hz) and obtained two types of Li3PO4 films with different interface resistance values. The 5 Hz-Li3PO4 thin-film battery exhibited good battery performance with a low interface resistance. The sharp peaks in the CV curves (Figure 1a: the oxidative reaction peaks at 3.91, 4.06, and 4.17 V in the charging process, and the reductive ones at 3.90, 4.05, and 4.16 V in the discharging process) indicate the smooth extraction and insertion of Li ions in LiCoO2.6,7,11,12 The ratio of the oxidative/reductive peak current at 3.90−3.91 V is nearly one (0.97), and the peak separations between the oxidative/reductive reactions are very small, only 0.01 V. In addition, the CV curves from first to third cycles almost overlap, indicating superior stability and repeatability in battery operations. The charge/discharge curves show stable reactions with discharge capacities higher than 120 mA h/g (Figure 1b). In contrast to the stable operation of the 5 Hz-Li3PO4 batteries, the 20 Hz-Li3PO4 batteries exhibited unstable charge and discharge reactions. In the CV curves shown in Figure 1c, the reaction current decreases with the cycle, indicating

Figure 1. Electrochemical properties of all-solid-state thin-film batteries of Li/Li3PO4/LiCoO2. The Li3PO4 films were deposited at a repetition rate of 5 (a,b) and 20 Hz (c,d). (a,c) Cyclic voltammogram at a scan rate of 0.1 mV/s. (b,d) Charge and discharge curves at the current of 14 nA (1 C rate). The inset in (a) shows a schematic cross-sectional view of a fabricated thin-film battery. The thicknesses of the LiCoO2 and Li3PO4 films were 100 and 200 nm, respectively. collector composed of an Au film was deposited on unheated Al2O3(0001) single-crystal substrates. Next, a LiCoO2 thin film was deposited using a Li-rich Li1.2CoO2 target to compensate for the loss of Li during deposition. A KrF excimer laser (wavelength: 248 nm) was used to irradiate the target at a repetition rate of 5 Hz. The fluence at the target was 1.0 J/cm2. During deposition, the oxygen partial pressure was kept at 1 × 10−3 Torr, and the substrate temperature was maintained at 330 °C. The LiCoO2 films were typically 100 nm in thickness, and LiCoO2 was a (0001)-oriented film (LiCoO2(0001)//Al2O3(0001)) in the hexagonal setting. The LiCoO2 films have grain boundaries perpendicular to the film surface, with a typical grain size of ∼300 nm.16 Although the LiCoO2 was (0001) orientated in out-of-plane direction, the grains were randomly oriented in the in-plane direction. After cooling the samples to room temperature, two types of Li3PO4 films with different interface resistance values were deposited using a PLD system with an ArF excimer laser (wavelength: 192 nm). The Ar partial pressure during B

DOI: 10.1021/acsami.8b08926 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

values related to the Li3PO4 thin films are 125 and 4.72 pF for 5 Hz- and 20 Hz-Li3PO4, respectively. The poor electrochemical properties of the 20 Hz-Li3PO4 batteries are attributed to both low ionic conductivity of Li3PO4 and the large interface resistance. The interface resistance between Li and 20 Hz-Li3PO4 may be different from that of Li/5 HzLi3PO4 interface. However, it is reasonable to consider that both interface resistance is quite low because, to our knowledge, there has been no report on a high Li/Li3PO4 interface resistance. The confirmation of the low Li/Li3PO4 interface resistance is underway. Consequently, we were able to prepare interfaces with small (5 Hz) and large (20 Hz) interface resistance. These results demonstrate that the resistance at the electrolyte/electrode interface depends not only on electrochemical properties of electrolyte and electrode materials but also on the fabrication processes. The difference in the interface resistance values leads us to consider the microscopic conduction path of Li ions at the electrolyte/electrode interface. In the discharge process, Li ions in Li3PO4 would first migrate through the solid electrolyte to the LiCoO2 surface (Figure 3a). Because the LiCoO2 film is

degradation during cycling. The charge and discharge capacities in Figure 1d are much smaller than those found in Figure 1b. Next, we evaluated the interface resistance using impedance spectroscopy and confirmed that 5 Hz-Li3PO4 batteries show smaller interface resistances than that of 20 Hz-Li3PO4. We analyzed the Nyquist plot measured at a potential of 4.3 V with the assumption of two semicircles. In the case of the 5 HzLi3PO4 batteries, the curve fitting analysis showed two semicircles with resistances of 22.5 and 2.8 kΩ observed in the high (104 to 106 Hz) and low-frequency regions (102 to 103 Hz), respectively (Figure 2a). In addition, a 45° inclined

Figure 2. Impedance spectra of thin-film batteries of Li/Li3PO4/ LiCoO2 measured at a potential of 4.3 V. The Li3PO4 films were deposited at repetition rates of 5 (a) and 20 Hz (b).

straight line followed by a vertical line at frequency region of 100 to 101 Hz can be interpreted as a finite-length Warburg Impedance. The semicircle in the high-frequency region (104 to 106 Hz) originates from the resistance of Li3PO4 and indicates an ionic conductivity of 5.1 × 10−7 S/cm, which is consistent with earlier reports.12,13 A semicircle observed at the low-frequency region should be attributed to the interface resistance at the Li3PO4/LiCoO2 interface because the semicircle was observed only in the potentials at 4.3 V, at which the electrochemical reaction occurs.12 We note that there are at least two interfaces in the device: the Li3PO4/ LiCoO2 and Li/Li3PO4 interfaces, but the interface resistance at the Li/Li3PO4 interface is too small to be characterized.17 The Li3PO4/LiCoO2 interface resistance, ∼5.5 Ω cm2, is comparable to the value reported for the sputter-deposited films (∼10 Ω cm2)13 and smaller than that of Li-ion batteries using liquid electrolytes.11 In contrast, the Nyquist plots of the batteries with 20 HzLi3PO4 show a remarkable increase of resistance (Figure 2b), as compared to those of 5 Hz-Li3PO4. Although an arc corresponding to interface resistance is not clearly visible, a resistance value of ∼180 Ω cm2 is determined by a curve fitting assuming resistance component at around 103 Hz. This resistance value is more than a factor of about 33 and larger than that observed in the 5 Hz-Li3PO4/LiCoO2 interface (Figure 2a). The capacitance values related to the interface are 113 and 34.7 nF for 5 Hz- and 20 Hz-Li3PO4 batteries, respectively. In addition, the ionic conductivity of 20 HzLi3PO4 was estimated to be 1.02 × 10−8 S/cm, which is around 50 times smaller than that of 5 Hz-Li3PO4. The capacitance

Figure 3. Conduction path of Li ions at the solid-electrolyte/ electrode interface. In the discharge process, Li ions migrate through the solid electrolyte to the interface. (a) Because the LiCoO2 film is (0001)-oriented, the migration of Li ions into LiCoO2 is hindered by the CoO2 layers aligned parallel to the substrate surface. Therefore, Li ions migrate laterally on the surface of LiCoO2 and finally diffuse into grain boundaries or antiphase domain boundaries. (b) In the case of the disordered LiCoO2 surface, the diffusion of Li ions along the surface and into the grain boundary is restricted, resulting in a high interface resistance value.

(0001)-oriented, the direct migration of Li ions into LiCoO2 is hindered by the CoO2 layers aligned parallel to the substrate surface. Therefore, Li ions migrate laterally on the surface of LiCoO2 and then diffuse into grain boundaries, and finally, the Li ions are intercalated between CoO2 layers. In case of the (0001)-oriented epitaxial LiCoO2 films, antiphase domain boundaries could also contribute to the diffusion of Li ions.5,18,19 Accordingly, the origin of the interface resistance should be in the limiting steps of migration along the surface or C

DOI: 10.1021/acsami.8b08926 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

of the LiCoO2 layers are 21 and 22 for the high-resistance and low-resistance interface samples, respectively. The structural analyses also revealed that the interfacial LiCoO2 layer of the high-resistance interface is disordered, whereas the low-resistance interface shows flat and atomically well-ordered structures at the LiCoO2 surface. To evaluate the disorder of atomic arrangements at the interface and in bulk, we analyzed the CTR profiles (Figure 4a) using the following three parameters. The first two parameters are the atomic layer widths of the interfacial LiCoO2 layer (usurf) and interior bulk LiCoO2 (ubulk). These two values represent the positional fluctuation of the atoms in an atomic layer due to thermal vibration and static positional inhomogeneity and are altogether expressed in the form of the conventional Debye− Waller factor. In the present case, static inhomogeneity is dominant. The last parameter represents the thickness distribution of the LiCoO2 films, in which the coverage of the Nth LiCoO2 layer (counted from the bottom) is expressed as 1 − β(Ntot − N), where β denotes a coverage factor (0 ≤ β ≤ 1) and Ntot is the total number of LiCoO2 layers. The fitted values of the structural parameters are summarized in Table 1.

migration into or through the grain boundaries and antiphase domain boundaries. To clarify the structural origin of the difference in the interface resistance values, we measured CTR scattering profiles of the two types of Li3PO4/LiCoO2 interface. An inset in Figure 4a shows a schematic cross-sectional view of

Table 1. Values of the Structural Parameters Obtained by CTR Scattering Analysis 5 Hz-Li3PO4/LiCoO2 20 Hz-Li3PO4/LiCoO2

Figure 4. (a) Specular X-ray CTR scattering profiles of the lowresistance (5 Hz-Li3PO4/LiCoO2) (red) and high-resistance (20 HzLi3PO4/LiCoO2) (blue) interfaces. The diffraction index L is based on the lattice constant of LiCoO2, c = 14.0516 Å.20 The solid lines are calculated profiles for the optimized structural models. The inset in (a) shows a schematic cross-sectional view of Li3PO4/LiCoO2 samples. The thicknesses of the LiCoO2 and Li3PO4 films were 10 and 50 nm, respectively, and no Li metal film was deposited on them. (b) Electron density profiles of the LiCoO2 layers in the lowresistance film (red) and high-resistance interfaces (blue). The dashed line is the profile of the Si-capped LiCoO2 film.

ubulk (Å)

usurf (Å)

β

0.4 ± 0.3 0.6 ± 0.5

0.5 ± 0.4 1.9 ± 0.9

0.66 ± 0.08 0.71 ± 0.10

For other effective parameters, such as the atomic structure of LiCoO2/Al2O3-substrate interface and the population ratio of the antiphase domains in the LiCoO2 film,5 we used values deduced from the analysis of bare LiCoO2 (see the Supporting Information for details). The evaluated electron density profiles (solid lines) of the low- and high-resistance interfaces are shown in Figure 4b. The profile of the bare LiCoO2 film is also shown for reference (dashed line). In the profiles, each sharp peak corresponds to an atomic layer of Li, Co, and O. In the case of high-resistance interface, the profile at a depth of 0 Å shows a very broad peak with no sharp features, indicating that the surface of the LiCoO2 film loses its crystallinity. In contrast, the LiCoO2 film surface of the low-resistance interface exhibits sharp peaks corresponding to the Co and O layers, representing the high crystallinity of the LiCoO2 surface at this interface. In the case of the low-resistance interface, usurf and ubulk have similar values, within error (Table 1), indicating that the surface of LiCoO2 films maintains crystallinity even after the growth of the 5 Hz-Li3PO4 film. The similarity in the usurf and ubulk is consistent with those of the bare LiCoO2 films. In contrast, the high-resistance interface shows a large usurf value of 1.9 ± 0.9 Å. This value exceeds the largest interlayer spacing of 1.4 Å in the LiCoO2 crystal, showing that the surface of the LiCoO2 film completely loses its crystallinity. This dead layer hardly contributes to the thickness fringes of CTR scattering profile, so that the thickness of the LiCoO2 film estimated from the thickness fringe periodicity (Figure 4a) appears to be smaller than the actual thickness by one LiCoO2 layer. These results indicate that the surface LiCoO2 layer is damaged during the growth of the 20 Hz-Li3PO4 film, which results in the high-resistance interface. Considering the Li-ion conduction pathways (Figure 3a), the above results strongly suggest that the disorder at the

Li3PO4/LiCoO2 samples. The thicknesses of the LiCoO2 and Li3PO4 films were 10 and 50 nm, respectively, and no Li metal film was deposited on them. Figure 4a shows the CTR scattering profiles of the low-resistance (5 Hz-Li3PO4/ LiCoO2) interface (red circles) and high-resistance (20 HzLi3PO4/LiCoO2) interface (blue triangles). The abscissa is the Miller index L along the surface normal direction, which is defined with respect to the lattice constant of LiCoO2, c = 14.0516 Å.20 The profiles show the LiCoO2 0003 Bragg peaks and thickness fringes, indicating that the LiCoO2 films maintain high crystallinity. Note that Li3PO4 does not contribute to the CTR intensity because the Li3PO4 films are in the amorphous phase. Here, we notice two differences in the two CTR profiles in Figure 4a. One is that the high-resistance interface shows a rapid fall in the intensity at regions higher than the 0003 Bragg peak (L > 3) compared with the low-resistance interface. This result indicates that the LiCoO2 of the high-resistance interface is less crystalline than the LiCoO2 in the low-resistance interface. The other difference is the periodicity of the thickness fringe; the periodicity of the low-resistance interface is slightly larger than that of the high-resistance interface, indicating that the number of crystalline LiCoO2 layers is smaller in the high-resistance interface. The estimated numbers D

DOI: 10.1021/acsami.8b08926 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces LiCoO2 surface intensely affects the interface resistance, that is, the degradation of crystallinity at the surface of LiCoO2 (electrolyte/electrode interface) leads to high interface resistance. As stated earlier, the high migration barrier on surfaces, and into and through grain boundaries or antiphase domain boundaries, should have a significant impact on increasing interface resistance. Because the difference in the crystallinity of the interior LiCoO2 is negligible in the two samples, the migration through those boundaries can be ruled out. Accordingly, the result indicates that migration of Li ions along the Li3PO4/LiCoO2 interface and into those boundaries in LiCoO2 is the limiting process for ionic conduction at the interface (Figure 3b). We speculate that similar damage at LiCoO2 surfaces was also present in the thin-film batteries fabricated using sputter-deposited LiPON;13 the LiCoO2 surface was damaged by high-energy O- and N-ion bombardment. Our structural analysis revealed that even a single dead LiCoO2 layer at the topmost surface of the LiCoO2 electrode acts as a diffusion barrier (Figure 3b). Hence, controlled fabrication of the electrolyte/electrode interface is crucial to obtain low interface resistance in solid-state Li batteries. To further understand the migration of Li ions at the interfaces in detail, theory and multiscale computation simulations are needed.21−24

ACKNOWLEDGMENTS

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REFERENCES

(1) Takada, K. Progress and Prospective of Solid-State Lithium Batteries. Acta Mater. 2013, 61, 759−770. (2) Ohta, N.; Takada, K.; Zhang, L.; Ma, R.; Osada, M.; Sasaki, T. Enhancement of the High-Rate Capability of Solid-State Lithium Batteries by Nanoscale Interfacial Modification. Adv. Mater. 2006, 18, 2226−2229. (3) Yada, C.; Ohmori, A.; Ide, K.; Yamasaki, H.; Kato, T.; Saito, T.; Sagane, F.; Iriyama, Y. Dielectric Modification of 5V-Class Cathodes for High-Voltage All-Solid-State Lithium Batteries. Adv. Energy Mater. 2014, 4, 1301416. (4) Tsuruhama, T.; Hitosugi, T.; Oki, H.; Hirose, Y.; Hasegawa, T. Preparation of Layered-Rhombohedral LiCoO2 Epitaxial Thin Films Using Pulsed Laser Deposition. Appl. Phys. Express 2009, 2, 085502. (5) Zheng, S. J.; Fisher, C. A. J.; Hitosugi, T.; Kumatani, A.; Shiraki, S.; Ikuhara, Y. H.; Kuwabara, A.; Moriwake, H.; Oki, H.; Ikuhara, Y. Antiphase Inversion Domains in Lithium Cobaltite Thin Films Deposited on Single-Crystal Sapphire Substrates. Acta Mater. 2013, 61, 7671−7678. (6) Shiraki, S.; Oki, H.; Takagi, Y.; Suzuki, T.; Kumatani, A.; Shimizu, R.; Haruta, M.; Ohsawa, T.; Sato, Y.; Ikuhara, Y.; Hitosugi, T. Fabrication of All-Solid-State Battery Using Epitaxial LiCoO2 Thin Films. J. Power Sources 2014, 267, 881−887. (7) Shiraki, S.; Takagi, Y.; Shimizu, R.; Suzuki, T.; Haruta, M.; Sato, Y.; Ikuhara, Y.; Hitosugi, T. Orientation Control of LiCoO2 Epitaxial Thin Films on Metal Substrates. Thin Solid Films 2016, 600, 175− 178. (8) Sonoyama, N.; Iwase, K.; Takatsuka, H.; Matsumura, T.; Imanishi, N.; Takeda, Y.; Kanno, R. Electrochemistry of LiMn2O4 Epitaxial Films Deposited on Various Single Crystal Substrates. J. Power Sources 2009, 189, 561−565. (9) Kumatani, A.; Ohsawa, T.; Shimizu, R.; Takagi, Y.; Shiraki, S.; Hitosugi, T. Growth Processes of Lithium Titanate Thin Films Deposited by Using Pulsed Laser Deposition. Appl. Phys. Lett. 2012, 101, 123103. (10) Kumatani, A.; Shiraki, S.; Takagi, Y.; Suzuki, T.; Ohsawa, T.; Gao, X.; Ikuhara, Y.; Hitosugi, T. Epitaxial Growth of Li4Ti5O12 Thin Films Using RF Magnetron Sputtering. Jpn. J. Appl. Phys. 2014, 53, 058001. (11) Iriyama, Y.; Kako, T.; Yada, C.; Abe, T.; Ogumi, Z. Reduction of Charge Transfer Resistance at the Lithium Phosphorus Oxynitride/ Lithium Cobalt Oxide Interface by Thermal Treatment. J. Power Sources 2005, 146, 745−748.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08926. Experimental and calculated specular CTR scattering profile of the LiCoO2 film covered with an amorphous Si film; impedance spectra of 5 Hz-Li3PO4 batteries measured at the potentials between 3.83 and 4.30 V; and equivalent circuit model for the analysis of impedance spectra (PDF)

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This study was supported by the “Applied and Practical LiB Development for Automobile and Multiple Application” project of the New Energy and Industrial Technology Development Organization (NEDO), Private University Research Branding Project from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Advanced Low Carbon Technology Research and Development Program (ALCA), PREST (JPMJPR13C5), and CREST of the Japan Science and Technology Agency (JST). The authors also acknowledge the support of Toyota Motor Corporation. T.S., S.S., and T.H. acknowledge a Grant-in-Aid for Scientific Research (no. 26105008, no. 25390072, no. 26106502, no. 26108702, no. 26246022, no. 26610092, and no. 16H03864) from MEXT, Japan. The synchrotron radiation experiments were performed at PF with the approval of Photon Factory Program Advisory Committee Proposals no. 2014G155 and no. 2015G661. The authors thank Professor Munekazu Motoyama, Professor Akichika Kumatani, and Professor Masakazu Hatura for helpful discussions about the interpretation of impedance spectra.

4. CONCLUSIONS We investigated the atomic structures of the electrolyte/ electrode interfaces of solid-state thin-film Li batteries with different electrolyte/electrode interface resistance values. The CTR scattering profile reveals that the surface roughness of LiCoO2 is larger in high interface resistance films. The origin of the high interface resistance is a dead layer which loses its crystallinity at the topmost surface of LiCoO2 electrode. The key to obtaining a low resistive electrolyte/electrode interface is to maintain high crystallinity at the surface of the electrode materials.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.S.). *E-mail: [email protected] (T.S.). ORCID

Susumu Shiraki: 0000-0001-9111-6233 Tetsuroh Shirasawa: 0000-0001-5519-6977 Hideyuki Kawasoko: 0000-0001-9069-3784 Taro Hitosugi: 0000-0002-7795-0683 Author Contributions #

S.S and T.S. authors contributed equally to this work.

Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acsami.8b08926 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b08926 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX