Combinatorial Synthesis of Epitaxial LiCoO2 Thin ... - ACS Publications

Apr 28, 2016 - Shingo Maruyama, ... Department of Pure and Applied Chemistry, Tokyo University of Science, Yamazaki 2641, Noda, Chiba 278-8510, Japan...
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Combinatorial Synthesis of Epitaxial LiCoO2 Thin Films on SrTiO3(001) via On-Substrate Sintering of Li2CO3 and CoO by Pulsed Laser Deposition Shingo Maruyama,† Osamu Kubokawa,‡ Kohei Nanbu,§ Kenjiro Fujimoto,§ and Yuji Matsumoto*,†,‡ †

Department of Applied Chemistry, Tohoku University, 6-6-07 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan Materials and Structure Laboratory, Tokyo Institute of Technology, 8259 Nagatsuta-cho, Aoba-ku, Yokohama, Kanagawa 226-8503, Japan § Department of Pure and Applied Chemistry, Tokyo University of Science, Yamazaki 2641, Noda, Chiba 278-8510, Japan ‡

S Supporting Information *

ABSTRACT: High-quality single-phase epitaxial LiCoO2 thin films are synthesized on 0.5 wt % Nb-doped SrTiO3(001) substrates by nanoscale alternate deposition of Li2CO3 and CoO as Li and Co sources, respectively, using a combinatorial pulsed laser deposition (PLD) technique. The formation of LiCoO2 thin films from these two sources results from the sintering reaction between Li2CO3 and CoO, which is commonly used in a bulk ceramics process, but simultaneously takes place on the substrate during the deposition at a temperature of 550 °C. Electrochemical characterization reveals that the charge/discharge property of LiCoO2 thin films as a cathode is severely sensitive to the nominal Li:Co composition ratio. The best-quality film shows an excellent discharge capacity comparable with the characteristic capacity of LiCoO2. KEYWORDS: LiCoO2, epitaxial thin film, pulsed laser deposition, composition spread

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materials, precision tuning of the film composition in solid solution systems, or fabricating super lattice structures, etc. In view of the flexibility in the composition control by the nanoscale alternate deposition, it would be natural to apply this technique to synthesize a LCO thin film from two different oxide sources of, for example, Li2O and CoO/Co3O4, instead of from a single target of LCO. However, one problem is that Li2O is not suitable for a starting target material because it is ready to react with moisture in air.14 In this study, we demonstrate the PLD synthesis of LiCoO2 thin films from Li2CO3 and CoO targets, where the air stable Li2CO3 and CoO are expected to react together on a substrate, accompanied by generating CO2 gas, as is similar to the so-called sintering process for powder ceramics synthesis. Efficiently screened the mixing ratio dependence of Li2CO3 and CoO with an aid of the continuous composition spread technique15 high-quality LCO thin films have been successfully fabricated enough to exhibit excellent charge/discharge properties comparable with those characteristic of LiCoO2.

INTRODUCTION LiCoO2 (LCO) is one of the major cathode materials used for rechargeable Li+ ion batteries. To understand the intrinsic behavior of Li+ ions in this material during the charge/discharge process, it would be better to employ high-quality epitaxial thin film LCO as a cathode electrode. Pulsed laser deposition (PLD) is a popular technique to grow various high-quality oxide thin films because it is, in many cases, believed that the obtained film would retain almost the same composition as that of the target used in PLD. However, growing an exactly stoichiometric LCO thin film is not so easy by PLD when a single LCO target is used. The deposited Li species would be easily evaporated from the film due to their high vapor pressures, and/or the Li species are scattered by oxygen gas molecules more easily than Co species in the plume,1,2 both resulting in a significant compositional deviation from the original target. Therefore, to compensate for such a Li deficiency in the LCO film, Li-excess Li1+xCoO2 targets, the values of x in which are unfortunately different among research groups, have been used in most of the previous PLD growths of LCO thin films.1,3−13 Since the PLD technique enables researchers to precisely control the deposition on a nanometer scale, a nanoscale alternate deposition of the multiple targets, which are selected to contain the elements one aims to control as is desired in the thin film, has been widely employed in the PLD thin film growth for impurity doping into mother oxide semiconductor © XXXX American Chemical Society

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EXPERIMENTAL SECTION First, a 200 nm-thick Li2CO3−CoO composition spread thin film was grown on a 0.5 wt % Nb-doped SrTiO3(001) Received: March 3, 2016 Revised: April 18, 2016

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DOI: 10.1021/acscombsci.6b00027 ACS Comb. Sci. XXXX, XXX, XXX−XXX

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Figure 1. (a, b) Schematic illustration of the Li2CO3−CoO composition spread thin film and its optical microscope image (the white dotted lines indicate the edge of substrate clamps and the red crossed lines are the scale bar). (c) XRF Co Kα intensity variation with the sample position X. (d) Out-of-plane XRD image plot. (e, f) XRD peak intensities of LCO(104) and Co3O4(400) normalized by that of STO(002) and their d-spacing values plotted against the sample position X. (g) Raman peak intensity variations of LCO (∼593 cm−1) and Co3O4 (∼689 cm−1) with the sample position X.

Araldite epoxy resin so that the exposed area to the electrolyte was limited to the thin film surface. The potential scan rate in cyclic voltanmmetry was fixed at 0.1 mV/s.

(Nb:STO) substrate by alternate deposition of Li2CO3 and CoO using PLD with a combinatorial moving mask. For electrochemical characterizations, uniform LCO thin films were also grown at deposition ratios of Li2CO3 and CoO near the optimized conditions figured out by the composition-spread experiment. PLD was carried out using a KrF excimer laser (248 nm, 10 Hz, 1.5 J/cm2), which was focused on either tablet of a nonsintered Li2CO3 or a sintered CoO. The growth temperature and oxygen partial pressure were fixed for all the experiments at 550 °C and 200 mTorr, respectively. The growth temperature was set to the value such that the XRD peak intensity of Li2CO3(00l) became maximum for a single deposition of Li2CO3, which was determined by the temperature gradient experiment16 (see Supporting Information Figure S1). Deposition rates of Li2CO3 and CoO were ∼0.06 and ∼0.007 nm/pulse, respectively, measured prior to the LCO thin film growth. X-ray fluorescence (XRF) analysis was used to examine the Co Kα intensity variation in the composition spread thin film. The epitaxial structure of the thin films was characterized by X-ray diffraction (XRD) with Cu Kα1 beam. Surface morphology was observed by an atomic force microscope (AFM) and a scanning electron microscope (SEM). Raman spectroscopy measurements were performed with an incident laser of 532 nm by NRS-5100 (JASCO). Electrochemical measurements were carried out at room temperature in a three-electrode sealed beaker cell that was prepared in an Ar-filled glovebox. The electrolyte was 1 M LiClO4/EC:DEC(1:1 vol %). Li foils were used as counter and reference electrodes, respectively. For preparing a thin-film working electrode, the back side of the Nb:STO substrate was scratched, on which a thin Ga−In eutectic layer was applied. Subsequently, a Ni wire was affixed with a Ag paint on the Ga− In layer. The sides and back of the electrode were coated by

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RESULTS AND DISCUSSION Figure 1a,b shows a schematic illustration of the Li2CO3−CoO composition spread thin film and its optical microscope image. The XRF intensity of the Co Kα (Figure 1c) almost linearly increased with the CoO thickness ratio, indicating that the composition spread was successfully prepared as designed. Note that the Co Kα signal was slightly detected even in the sample region where only Li2CO3 had to be deposited in the design. The scattering of the ablated species in the high oxygen deposition pressure might lead to not negligible deposition on the shielded region under the mask which is located at a ∼1 mm distance from the substrate. Figure 1d shows the out-ofplane XRD intensity mapping of the Li2CO3−CoO composition spread thin film (the blighter color represents strong intensity). The LCO(104) peak was observed in the Li2CO3rich region, while the Co3O4(400) peak was observed in the CoO-rich region. The growth of (104)-oriented LCO on STO(001) is consistent with the previous results.4,5 The appearance of a highly oxidized Co3O4 phase in spite of using the CoO target is reasonable for the present growth temperature and oxygen partial pressure according to the Ellingham diagram.17 The intensities of these peaks normalized by the substrate peak intensity are plotted against the sample position in Figure 1e. The LCO(104) peak took a maximum intensity with no detectable Co3O4(400) peak at a sample position X of 5 mm, which well coincides with the sample position where the Raman peak intensity of LCO18 (∼593 cm−1) becomes maximum as shown in Figure 1g. Raman spectra also showed the absence of Co3O4- and graphite-related B

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Figure 2. (a−j) Set of SEM and AFM images (inset, 5 μm × 5 μm) for each sample position of X, whose nominal composition ratio of Li2CO3:CoO (vol) is indicated in the bracket. (k) A plot of AFM RMS roughness values for the Li2CO3−CoO composition spread thin film against sample position X.

bands at this position (see Supporting Information Figure S2 for the details of the Raman spectra). These results indicate that a single-phase (104)-oriented LCO thin film can be grown by the alternate deposition of two targets: Li2CO3 and CoO. During this process, the following sintering reaction is expected to take place on the substrate.

quality single-phase LCO as determined by XRD as well as Raman spectroscopy. On the basis of these results in the composition spread experiment, we fabricated three uniform LCO thin films deposited with different deposition ratios of Li2CO3 and CoO near the optimized conditions for electrochemical characterizations. In Figure 3a−c the XRD intensity and its d104 value of LCO(104) as well as the AFM RMS roughness for these uniform LCO thin films are plotted against the nominal atomic ratio Co/(Li + Co) by colored filled circles, respectively. All the uniform LCO thin films showed a dominant (104)-oriented epitaxial growth on the Nb:STO(001) substrate, as was confirmed by the out-of-plane XRD patterns, while the Lirich samples showed a small peak of LCO(003) (see Supporting Information Figure S3). The data points of the composition spread thin film shown in Figure 1 were also replotted in Figure 3 by the black filled circles. These plots for the three uniform thin films show a similar trend to that for the composition spread thin film; i.e., the sample showing the strongest LCO(104) peak intensity has the most flat surface, and its d104 value increases with the Co/(Li + Co) ratio. However, the nominal Co/(Li + Co) ratio that gives the bestquality single-phase LCO is very close to 50% in the experiment for uniform LCO thin films, while it is shifted to the Li-rich side in the composition-spread experiment. This discrepancy might be explained by the enhanced re-evaporation effect of Li species from the thin film because the surface temperature should be increased due to the reflection of heating radiation from the mask in the composition-spread experiment.

2Li 2CO3 + 4CoO + O2 → 4LiCoO2 + 2CO2 ↑

The out-of-plane d-spacings of LCO(104) and Co3O4(400) are shown in Figure 1f. The d-spacing of LCO(104) gradually becomes larger with increasing the Co ratio and is very close to the bulk value at around the sample position that gives the strongest LCO(104) peak intensity. The trend in the d-spacing of LCO, i.e., Li-poor states having smaller lattice constants, is consistent with the previous results.19,20 Surface morphology of the Li2CO3−CoO composition spread thin film was investigated by SEM and AFM (Figure 2a−j). The relatively large precipitates imaged in darker contrast than the surrounding film surface in the SEM increase with Li2CO3 composition. In general, a lighter atomic weight composite will be imaged with a lower brightness in SEM, and these precipitates thus can be attributed to Li2CO3 crystals, which is supported by the fact that the Li2CO3(002) peak was observed in the Li2CO3-rich region (not shown). On the other hand, in the Co-rich region, some square-shape grains, which can be assigned to the Co3O4 spinel crystals, are observed. In between those relatively rough surfaces in Li2CO3- and CoOrich regions, the most flat film surface (RMS roughness = ∼0.83 nm as shown in Figure 2k) is found at a sample position X of 5 mm which also well coincides with the position for the bestC

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Figure 3. Nominal atomic ratio of Co/(Li + Co) dependence of (a) XRD intensity ratio of LCO(104) to STO(002), (b) the d-spacing value of LCO(104), and (c) AFM RMS roughness, for the Li2CO3− CoO composition spread thin film (black filled circle) and uniform thin films (red, blue, and green filled circle).

Figure 4a shows an out-of-plane XRD pattern of the uniform LCO thin film deposited at a Li/Co ratio of 1.04 (the red filled circle shown in Figure 3) and the growth of a single-phase (104)-oriented LCO thin film without any detectable impurity phases such as Co3O4 was confirmed. The epitaxial relationship was determined from the Φ scans recorded for LCO(003) and STO(111) reflections, as shown in Figure 4b. There are four peaks found both for LCO(003) and STO(111), indicating that the LCO thin film was grown epitaxially on the STO(001) substrate with a relationship of STO⟨111⟩//LCO⟨003⟩. The surface morphology of the film (Figure 4c,d) seems to be similar to that at the sample position of X = 6 (69:31) in the composition spread film. Figure 5a shows a comparison of cyclic voltammograms taken at a scan rate of 0.1 mV/s for the uniform LCO thin films deposited at Li:Co ratios of 51:49 (red), 57:43 (blue), and 64:36 (green), respectively. Although these thin films include only the LCO phase as indicated by their XRD patterns, the relatively Li-rich LCO thin film (deposited at the Li:Co = 64:36) exhibits no clear oxidation and reduction peaks even when higher potentials are applied up to 4.5 V versus Li/Li+ in the second sweep. On the other hand, the film deposited at the Li:Co = 57:43 has a pair of small oxidation and reduction peaks at 3.92 and 3.90 V versus Li/Li+, respectively. Since the currents for a bare Nb:STO substrate were much smaller than those for the LCO/Nb:STO (see Supporting Information Figure S4), all the peaks found in Figure 5a should come from the delithiation and lithiation of the LCO thin films. In contrast, the film deposited at the Li:Co = 51:49 shows a pair of sharp peaks of delithiation (3.99 V) and lithiation (3.86 V) with several small peaks at the higher potentials due to the phase transitions.19 The shape of CV is very close to those of the epitaxial LCO thin films grown on a SrRuO3 buffered STO(001) substrate by PLD using a Li1.4CoO2 target.5 The charge/discharge curves deduced from the cyclic voltammograms by integrating the currents (Figure 5a) are shown in Figure 5b for the two films exhibiting the clear delithiation/lithiation peaks in CV. The

Figure 4. (a) Out-of-plane XRD pattern, (b) Φ scans recorded for LCO(003) and STO(111) reflections, (c) SEM, and (d) AFM images of the uniform LCO thin film deposited at the nominal Li:Co ratio of 51:49 denoted as red circle in Figure 3.

discharge capacity of the films deposited at the Li:Co = 57:43 was ∼40 mAh/g, which is much smaller than the practical specific capacity of LCO (∼137 mAh/g, LixCoO2 (0.5 ≤ x ≤ 1) cycled between 3 and 4.2 V). On the other hand, the capacity of the films deposited at the Li:Co = 51:49 reached as large as ∼180 mAh/g. This value is larger than the practical specific capacity, which might be due to the higher upper cutoff potential (4.4 V)21 and/or the underestimation of the film thickness in this experiment. The observed good capacity in this film indicates that the carbon in the carbonate would be easily eliminated by evaporating as CO2 during the sintering process and hence did not give a significant negative effect on the electrochemical property of LCO thin film. Note that the relatively low coulomb efficiency (∼70% for the Li:Co = 51:49 sample) might be due to the inclusion of a small amount of strained Li2Co2O4 phase, which is difficult to distinguish from LCO in XRD, but suppresses the first cycle coulomb efficiency.22 From these results, we stress that the electrochemical properties of our epitaxial LCO thin films are sensitive to the nominal deposited composition of Li:Co, exhibiting a clearer electrochemical delithiation/lithiation behavior when D

DOI: 10.1021/acscombsci.6b00027 ACS Comb. Sci. XXXX, XXX, XXX−XXX

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sintering reaction of Li carbonate and cobalt oxide took place on the substrate during the deposition. The combinatorial thin film technique allowed us to systematically investigate the Li2CO3−CoO compositional dependence of the crystalline phase and morphology. The experiment of the Li2CO3−CoO composition spread thin film revealed that a very narrow composition region could give the single-phase, high-quality LCO thin film. The nominal composition of Li and Co, which is controlled by changing the laser pulse ratio of Li2CO3 and CoO, has a significant impact on the charge/discharge property of LCO thin films. The LCO thin film grown almost at a stoichiometric Li:Co ratio exhibited a good charge/discharge property comparable to the characteristic LCO capacity. The present results indicate that the Li carbonate target can be a better Li source in PLD for precision tuning the stoichiometry of LCO thin films, instead of Li-excess Li1+xCoO2 targets. Furthermore, this may not be a particular case, but would hold true when not only other carbonate salts but also nitrate or sulfate salts are used as a target, as in the bulk sintering process. Similarly, the use of these salt targets will be useful to compensate a poor species in PLD grown films, whose composition does not retain the composition of a single target due to, for example, its easily scattering in the plume or volatile nature in the deposition atmosphere.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 5. (a) Cyclic voltammograms and (b) charge−discharge curves of the uniform LCO thin films deposited at different nominal Li:Co ratios.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscombsci.6b00027. XRD patterns of a Li2CO3 thin film grown with a temperature gradient and uniform LCO thin films, Raman spectra of the composition spread thin film, and CV of the bare substrate (PDF)

the nominal Li:Co ratio is close to 50:50, i.e., stoichiometric. Assuming that the stoichiometric LCO has the best electrochemical property, this result implies that Li species will not reevaporate from the thin film once LCO is formed on the substrate at the present growth temperature and thus supports the previous argument: the scattering of Li species in the plume is the main reason for the Li loss during the PLD of Li containing oxides.1,2 The small capacity in the relatively Li-rich LCO thin films (Li:Co = 64:36 and 57:43) as compared with that in the almost stoichiometric film can be understand as follows. It is well-known that overdischarged (overlithiated) LCO undergoes significant capacity losses, accompanied by its irreversible phase tranformation.23,24 Although the as-deposited Li-rich LCO is not exactly the same as the electrochemically Lirich LCO, it might be reasonable that the excess Li and/or Li2CO3 in the as-deposited LCO thin film causes some structural disorders or small inclusions of Li2CO3 and Li-oxide phases, even if they are not detectable in XRD, and consequently degrades the electrochemical properties. It should be noted that a small inclusion of the (003)-oriented LCO phase in the Li-rich films (see XRD patterns in Supporting Information Figure S3) would not be a major explanation of the observed suppression of delithiation/lithiation because even polycrystalline LCO thin films grown by PLD have good electrochemical properties, as reported in the previous literature.7−13,25

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-22-795-7266. Fax: +81-22-795-7268. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS We have demonstrated the PLD synthesis of a high-quality LCO thin film with good electrochemical properties by the alternate deposition of nonsintered Li carbonate as a Li source and sintered cobalt oxide as a Co source. In this process, the E

DOI: 10.1021/acscombsci.6b00027 ACS Comb. Sci. XXXX, XXX, XXX−XXX

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ACS Combinatorial Science (6) 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. (7) Striebel, K. A.; Deng, C. Z.; Wen, S. J.; Cairns, E. J. Electrochemical Behavior of LiMn2O4 and LiCoO2 Thin Films Produced with Pulsed Laser Deposition. J. Electrochem. Soc. 1996, 143, 1821. (8) Lee, J.; Lee, S.; Baik, H.; Lee, H.; Jang, S.; Lee, S. Substrate Effect on the Microstructure and Electrochemical Properties in the Deposition of a Thin Film LiCoO2 Electrode. Electrochem. SolidState Lett. 1999, 2, 512. (9) Perkins, J. D.; Bahn, C. S.; Parilla, P. A.; McGraw, J. M.; Fu, M. L.; Duncan, M.; Yu, H.; Ginley, D. S. LiCoO2 and LiCo1−xAlxO2 Thin Film Cathodes Grown by Pulsed Laser Ablation. J. Power Sources 1999, 81−82, 675−679. (10) Tang, S. B.; Lu, L.; Lai, M. O. Characterization of a LiCoO2 Thin Film Cathode Grown by Pulsed Laser Deposition. Philos. Mag. 2005, 85, 2831−2842. (11) Xia, H.; Lu, L. Texture Effect on the Electrochemical Properties of LiCoO2 Thin Films Prepared by PLD. Electrochim. Acta 2007, 52, 7014−7021. (12) Yamada, I.; Iriyama, Y.; Abe, T.; Ogumi, Z. Lithium-Ion Transfer on a LixCoO2 Thin Film Electrode Prepared by Pulsed Laser Deposition-Effect of Orientation-. J. Power Sources 2007, 172, 933− 937. (13) Iriyama, Y.; Inaba, M.; Abe, T.; Ogumi, Z. Preparation of c-Axis Oriented Thin Films of LiCoO2 by Pulsed Laser Deposition and Their Electrochemical Properties. J. Power Sources 2001, 94, 175−182. (14) Sifuentes, A.; Stowe, A. C.; Smyrl, N. Determination of the Role of Li2O on the Corrosion of Lithium Hydride. J. Alloys Compd. 2013, 580, S271−S273. (15) Koinuma, H.; Takeuchi, I. Combinatorial Solid-State Chemistry of Inorganic Materials. Nat. Mater. 2004, 3, 429−438. (16) Koida, T.; Komiyama, D.; Koinuma, H.; Ohtani, M.; Lippmaa, M.; Kawasaki, M. Temperature-Gradient Epitaxy under in Situ Growth Mode Diagnostics by Scanning Reflection High-Energy Electron Diffraction. Appl. Phys. Lett. 2002, 80, 565−567. (17) Swalin, R. A. Thermodynamics of Solids; Wiley-VCH, 1962. (18) Huang, W.; Frech, R. Vibrational Spectroscopic and Electrochemical Studies of the Low and High Temperature Phases of LiCo1−xMxO2 (M = Ni or Ti). Solid State Ionics 1996, 86−88, 395− 400. (19) Reimers, J. N.; Dahn, J. R. Electrochemical and In Situ X-Ray Diffraction Studies of Lithium Intercalation in LixCoO2. J. Electrochem. Soc. 1992, 139, 2091. (20) Khatun, F.; Gafur, M. A.; Ali, M. S.; Islam, M. S.; Sarker, M. A. R.; Road, K. Impact of Lithium Composition on Structural, Electronic and Optical Properties of Lithium Cobaltite Prepared by Solid-State Reaction. J. Sci. Res. 2014, 6, 217−231. (21) Xia, H.; Lu, L.; Meng, Y. S.; Ceder, G. Phase Transitions and High-Voltage Electrochemical Behavior of LiCoO2 Thin Films Grown by Pulsed Laser Deposition. J. Electrochem. Soc. 2007, 154, A337. (22) Kan, Y.; Hu, Y.; Ren, Y.; Bareño, J.; Bloom, I.; Sun, Y.-K.; Amine, K.; Chen, Z. Differentiating Allotropic LiCoO2/Li2Co2O4: A Structural and Electrochemical Study. J. Power Sources 2014, 271, 97− 103. (23) Maleki, H.; Howard, J. N. Effects of Overdischarge on Performance and Thermal Stability of a Li-Ion Cell. J. Power Sources 2006, 160, 1395−1402. (24) Shu, J.; Shui, M.; Huang, F.; Ren, Y.; Wang, Q.; Xu, D.; Hou, L. A New Look at Lithium Cobalt Oxide in a Broad Voltage Range for Lithium-Ion Batteries. J. Phys. Chem. C 2010, 114, 3323−3328. (25) Julien, C.; Camacho-Lopez, M. A.; Escobar-Alarcon, L.; HaroPoniatowski, E. Fabrication of LiCoO2 Thin-Film Cathodes for Rechargeable Lithium Microbatteries. Mater. Chem. Phys. 2001, 68, 210−216.

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DOI: 10.1021/acscombsci.6b00027 ACS Comb. Sci. XXXX, XXX, XXX−XXX