Preparation of Highly Oriented Porous LiCoO2 Crystal Films via Li

Subscriber access provided by University of Winnipeg Library

Article

Preparation of Highly Oriented Porous LiCoO2 Crystal Films via Li-Vapor Crystal Growth Method Mitsunori Kitta, and Kentaro Kuratani Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01176 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Preparation of Highly Oriented Porous LiCoO2 Crystal Films via Li-Vapor Crystal Growth Method

Mitsunori Kitta* and Kentaro Kuratani

Research Institute of Electrochemical Energy, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST)

KEYWORDS Li-ion batteries, LiCoO2 electrode, highly oriented porous film, Li-vapor crystal growth

ACS Paragon Plus Environment

1

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

ABSTRACT To construct better Li-ion-based batteries, highly oriented porous LiCoO2 crystal film is urgently needed for an active positive electrode. In this study, we prepared such a crystal film via the Li-vapor crystal growth method, which involves a simple reaction between the CoO substrate and Li-vapor. Highly crystalline LiCoO2 particles 2–3 µm in size were grown on the CoO substrate surface with strong relation of their orientation. The and -oriented LiCoO2 particles, which are preferable for Li-ion conduction, were generated on the CoO(110) substrate. On the CoO(111) substrate, LiCoO2 particles were grown with not only the orientation but also the orientation, suggesting that the crystal growth should follow the three-dimensional structure of the CoO and LiCoO2 lattice. Both the prepared LiCoO2 films from the CoO(110) and CoO(111) substrates exhibited stable and superior electrochemical properties for Li-ion battery cycling, indicating that the films will be useful for high-performance Li-ion-based batteries.


2

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Lithium cobalt oxide (LiCoO2, LCO) has a layered rock-salt-type structure (α-NaFeO2 type) and exhibits comparatively high electronic conductivity among Li transition-metal oxides1– 3;

moreover, its cycle and rate properties are stable. Hence, it is widely used as a cathode material

in Li-ion batteries. However, in LCO, the Li-ion diffusion characteristic depends strongly on the crystal orientation, 4–7 and to design a battery with high input–output properties, it is essential to form a crystalline film with controlled electrode form, particularly the orientation of the crystals. Thus far, LCO crystalline films have been formed by employing various thin-film preparation methods, such as pulsed-laser deposition, 5, 6, 8–17 sputter deposition, 18–22 the sol-gel method, 23–26 spray deposition, 27–29 and chemical vapor deposition.30 Among these, the pulsed-laser deposition method can form LCO crystalline films that maintain an epitaxial orientation relationship with the deposition substrate and has yielded LCO crystalline film with the orientation controlled in various plane orientations of the substrate. However, these conventional thin-film preparation methods have processing disadvantages, such as the need for multiple-step processes, the necessity for expensive and large specialized equipment, and the requirement of special labor and costs. Therefore, even though these methods are appropriate for fundamental research at the laboratory level, they are not suitable for practical processes. Moreover, even if a high-quality crystalline film is formed, if it separates from the current-collecting substrate, the device will no longer function as a battery. Battery materials are particularly prone to volume change during the charge and discharge processes; hence, in the case of crystalline films formed via deposition methods, a basic challenge is to prevent the film from separating from the current-collecting substrate. Furthermore, high-quality thin-films are generally deposited under advanced control on top of a substrate with good flatness. The formed crystalline films are thus fine homogenous


3


Page 4 of 29

single-crystal films; hence, their specific facet areas are small, and it is difficult to mitigate the volume change of the electrode layer during charging and discharging, which indicates that there are many disadvantages for their use as electrodes. Thus, it is believed that the type of crystalline film suitable for use as an electrode is not fine crystalline film, but rather porous crystalline film. The spray deposition method, sol-gel method, etc., have been used to form various porous LCO crystalline films.19, 24, 29, 31, 32 However, these crystalline films are often obtained as polycrystalline films with a disordered orientation; thus, porous LCO crystalline films with controlled orientation have not yet been obtained. We proposed the Li-vapor crystal growth (Li-VCG) method to resolve the aforementioned challenges.33–35 This method is implemented by enclosing a substrate of a transition-metal oxide and a volatile Li-salt source in a crucible and calcining it in the atmosphere, resulting in the growth of a crystalline film of the Li transition-metal oxide on the substrate. This method offers multiple benefits for forming the crystalline film of battery materials, such as a convenient forming process, the ability to obtain highly oriented crystalline films with high crystallinity, and the strength of the bond between the substrate and crystalline film. In this study, we applied the Li-VCG method to the formation of an LCO crystalline film, demonstrating the ability of this method to form a new kind of LCO crystalline film that is both highly oriented and highly porous. It was also confirmed that the crystalline film functioned well as an electrode. The unique form of the crystalline film was considered to be the result of the unique crystal growth mechanism of the Li-VCG method, which differs from the conventional deposition film preparation methods. For clear discussion, we used a CoO single crystal as the growth substrate. However, the substrate is not limited to a bulk single crystal, and the


4

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


convenience of our method would contribute to any practical route of thin-film preparation, as discussed later.

EXPERIMENTAL A cobalt oxide single-crystal substrate (CoO(111) and CoO(110)) was purchased from Crystal Base Co., Ltd. Samples were fabricated by enclosing this substrate (2 × 2 × 0.5 mm3) and 4 mg of volatile Li salt in a 15-mL alumina crucible and calcining it for 15 h in the atmosphere using an electric furnace. The calcination temperature selected was 800 °C, which is generally used in LCO preparation.36 If the calcination temperature falls below 700 °C, LCO with a spinel crystal structure, which differs from the layered rock-salt-type structure, 37–39 is formed. This is known as a “low-temperature phase” and is not desirable as a cathode material, because its electrochemical reaction potential is lower than that of layered rock-salt-type LCO.37–40 In this experiment, lithium hydroxide monohydrate (LiOH･H2O) was used as the volatile Li-salt source. If a halide (LiCl, for example) was used as the Li-salt source in the experiment, coarse single crystallization would occur, 41 which would not be suitable. Figure 1 shows a schematic of the Li-VCG procedure. In the calcination process, LiOH･H2O volatizes, and a crystalline film is formed by the reaction of the vapor of the Li source with the substrate surface. The crystallinity and orientation of the resulting crystalline film sample were evaluated using an X-ray diffractometer (MiniFlex 600, Rigaku) to perform out-of-plane X-ray diffraction (XRD) measurement of the sample. The X-ray source was Cu-Ka, the measurement steps were 0.01°, the measurement range was 10° to 120°, and the scanning speed was 0.1°/min. The morphology of the crystalline film was evaluated using scanning electron microscopy (SEM) with secondary-


5


Page 6 of 29

electron imaging. The observation was conducted using an S-5500 (Hitachi) at an acceleration voltage of 30 kV. The electrochemical properties of the LCO crystalline film were evaluated using an assembled Li-ion battery. The sample was used as the cathode without modification, and no conduction additives, binders, or other additives were used. The anode was a metal Li film with a diameter of 12 mm, and the electrolyte was a 1 M lithium hexafluorophosphate (LiPF6) solution dissolved in a 1:1 mixture of ethylene carbonate and diethyl carbonate (1 M LiPF6 EC/DEC = 1). The test cells (HS-cell, Hohsen Corp.) were used for cell assembly, and a glass filter and a polypropylene separator were inserted between the two electrodes, i.e., the anode and the cathode; they were subsequently saturated with the electrolyte and then sealed. The entire battery cell fabrication process was conducted in a dry-air-filled box at the dew point of -80 °C. The electrochemical properties were evaluated using a charge/discharge unit (BLS, Keisokuki). The voltage cutoff range was 3.1–4.1 V (vs. Li+/Li), and the electric current densities of the charging (Li extraction) were 0.25, 0.5, 1, 2, and 4 mA cm-2. The current density during discharging (Li insertion) was fixed at 0.25 mA cm-2. The area used was the geometrical area of the electrode. The charging and discharging were performed for three cycles at each current density, and the rate characteristics were compared.

RESULTS AND DISCUSSION The out-of-plane XRD profiles of the LCO crystalline films formed on the CoO singlecrystal substrate with the plane orientations of (111) and (110) are shown as red and blue solid lines, respectively, in Figure 2. In the figure, the diffraction profile obtained from the standard LCO powder used is shown by black lines for comparison. Additionally, the peak position of the


6

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diffraction profile of LCO is shown to be indexed. The LCO crystalline film formed on the CoO(111) substrate shows a strong diffraction peak in the and orientations in the out-of-plane direction, but the diffraction profile in other orientations cannot be confirmed. Thus, on the CoO(111) substrate, LCO crystalline films oriented to and formed. Moreover, the crystalline film on the CoO(110) substrate showed strong diffraction peaks in and , but it was impossible to confirm the diffraction profile for other orientations. Therefore, LCO crystalline film with and orientations was formed on the CoO(110) substrate. The LCO crystal orientation depends on the rate of crystal growth. When 8 mg of LiOH.H2O was used for the LCO film preparation, the film orientation was deteriorated, as shown in Figure S1(a) (Supporting Information). The larger amount of the Li volatile source provided highly concentrated Li vapor during the calcination process, inducing quick crystal growth with a random orientation. As the most stable facet index of a normal LCO crystal is (003), 41–43 crystalline films formed on these substrates can be considered to possess a unique orientation. This indicates that the Li-VCG method using the CoO substrate can perform advanced control of the growth orientation of an LCO crystalline film. To confirm the degree of priority of the two kinds of orientations observed on each substrate, the intensity of each diffraction profile with powder diffraction as the standard LCO powder was compared, and the results are presented in Table 1. Via calculations, the orientation ratios in the crystalline film samples according to the integral intensity of each diffraction peak were estimated as I012 / I003 = 0.567 and I110 / I018 = 4.518. For the powder, in all cases, these values were I012 / I003 = 0.111 and I110 / I018 = 1.015. Comparing the orientation ratio in the powder sample indicated that in the crystalline film on CoO(111), the orientation was prioritized over the orientation


7


Page 8 of 29

by a factor of 5.1 or higher, and in the crystalline film on CoO(110), the orientation was prioritized over the orientation by a factor of 4.5 or higher. To evaluate the morphology of the LCO crystalline film formed on the CoO(111) and (110) substrates, SEM observations of the surface of the samples were performed, and the results are shown in Figure 3. The SEM images obtained at a magnification of 5,000× showed that in each crystalline film, plate-like grains with advanced order in both shape and size were systematically configured. According to the results of the XRD profile, these are LCO crystal grains. In the 500× observation images, a white contrast, with island-shaped dark areas within, is observed. Generally, in SEM observations using secondary electrons, under the edge effect, protruding parts are observed as white contrast. Thus, the white contrast in the 500× observation images indicates plate-like grains, and the dark parts indicate flat grains. From the XRD profiles, two kinds of orientation growth were confirmed in each crystalline film, which correspond closely to the two kinds of contrast, i.e., the coexistence of plate-like grains and flat crystals in the 500× images. This is discussed later; the plate-like grains are crystals with or growth in the out-of-plane direction, whereas the flat parts are crystals with or growth. A wide-area SEM image reveals that the range of the white contrast of the plate-like crystals has a higher area ratio than the dark parts of the flat crystals. This trend shows good conformity with the results of the integrated intensity ratio analysis of the XRD peak: the orientation or orientation exists with priority over the orientation and orientation in each crystalline film. As the average size of the LCO plate-like grains constituting the crystalline film was approximately 3 to 4 µm and they had a systematic shape, it was believed that the primary grains with high crystallinity covered the substrate surface. In addition, as the crystal grains existed at intervals of approximately 1–2 µm, this crystalline film is


8

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


presumed to be adequately porous. This confirms that the crystalline film has both porosity and the appropriate crystal orientation. The in-plane orientation of the plate-like grains differed on each substrate; on the CoO(111) substrate, it showed three-fold symmetry intersecting an equilateral triangle shape, and on the CoO(110) substrate, it showed two-fold symmetry intersecting a rhombus. It was also confirmed that the plate-like grains on the CoO(111) substrate grew diagonally toward the substrate, whereas those on the CoO(110) substrate grew vertically toward the substrate. Therefore, the growth of the LCO crystal grains strongly depended on the orientation of the CoO substrate used. Of course, the crystal randomness of the LCO film affected the morphology of the in-plane arrangement, as shown in Figure S1(b) (Supporting Information). To understand the idiosyncratic growth of the LCO crystals on the CoO(111) and CoO(110) substrates, the crystal growth mechanism of the Li-VCG process was considered, as shown in Figure 4. Figure 4(a) shows the results for the growth mechanism of LCO crystals from CoO(111) based on a lattice model of the crystals. The blue, red, and green spheres represent Co, O, and Li atoms, respectively. The figures were rendered using a crystal structure simulation software (VESTA).44 In the case where the upward direction on the screen was considered to be the z-axis direction of the substrate, the orientation of each atom on the CoO(111) substrate appeared as shown in Figure 4(a). In the case where LCO crystals with a layered rock-salt-type structure were formed from CoO crystals with a cubic rock-salt structure, growth based on an orientation relationship, such that the homology of the atom positions is maximized, is the most rational growth mechanism.13 In CoO and LCO, the O atoms have the closest-packed framework, which is analogous to a rock-salt structure; hence, it is believed that the Co and Li are rearranged in a form that maintains the alignment of O atoms, causing crystal growth. Here, in the case


9


Page 10 of 29

where the LCO crystals are arranged in a form that maintains the homology of the O atom framework, two orientations are possible, as shown in the figure. In the arrangement shown on the left, the CoO(111) facet and LCO(003) facet are parallel, and in the arrangement shown on the right, the CoO(-111) facet and LCO(003) facet are parallel. These arrangements both maintain the homology of the O atom framework; hence, it can be presumed that both crystal growths can occur. Therefore, for the CoO(111) substrate, the and diffraction peaks are observed. As described previously, the growth of LCO crystals occurs with CoO crystals. Such a crystal growth process is schematically shown in Figure 4(b). First, at the early stage of the calcination, the vapor of the Li-salt packed in the crucible contacts the substrate surface, and the Li topotactically penetrates the shallow region of the CoO surface. When the (111) substrate was used, within the CoO crystals, there existed a (-111) facet inclined diagonally to the (111) facet parallel to the substrate, as shown in the figure. The penetration to the interior of the CoO lattice of the Li ions advanced, and at the stage where the critical concentration was exceeded, LCO crystal nuclei production occurred in the near-surface region of the substrate. At this time, as shown in Figure 4(a), nuclei formation occurred; thus, the (003) facets of the LCO crystals were parallel to the (111) and (-111) planes of CoO. Moreover, according to the nuclei formed inside the substrate, the crystal growth advanced as Li was supplied from the vapor phase and Co was supplied from within the substrate. An LCO crystalline film was ultimately obtained in the form of a mixture of LCO-oriented crystals grown smoothly and plate-like LCO-oriented crystals grown diagonally. Regarding the abundance ratios of these orientations, as indicated by the XRD results in Figure 2 and the SEM results in Figure 3, the plate-like LCO crystals dominated the flat LCO crystals. This indicates that the crystal growth of the former takes


10

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


priority over that of the latter. In LCO crystals with a layered rock-salt-type structure, material diffusion parallel to the (003) lattice plane is advantageous; 4–7 hence, this crystal growth occurs easily in the direction parallel to the (003) plane. In the aforementioned crystal growth process, Li and Co are supplied vertically to the substrate surface; hence, it is assumed that crystal growth occurs more readily with a higher degree of verticality of the LCO(003) facet to the substrate surface. Therefore, when the Li-VCG process is used, a higher percentage of LCO will presumably be in the form of plate-like grains than flat crystals. As explained previously, the material diffusion in the in-plane direction of the LCO grains was remarkable; hence, the crystal growth occurred in a form that expanded the (003) plane, i.e., the close-packed atomic plane. Therefore, the LCO crystal grains that grew freely had a plate-like crystal shape, where the (003) facet was the main facet.41–43 In the case where LCO crystal grains grow from a CoO substrate, plate-like grains grow in a form such that the (003) facet preferentially expands, but unlike free growth, growth occurs such that the CoO{111} and LCO(003) planes are parallel. Here, among the CoO crystals with cubic rock-salt structures, there are eight facets, which are crystallographically equal to (111) facets. Therefore, it is believed that the plate-like grain growth of LCO reflects the form of the regular octahedron formed by cutting on the eight equal facets existing in CoO. Figure 4(c) shows a model of the form of the growth of the LCO plate-shaped crystals on the CoO(111) substrate. When a regular octahedron was observed from the orientation, the flat (111) facet and diagonal {111} facet were observed with three-fold symmetry. If crystals grow in such a way that these equal {111} facets and LCO (003) facets are parallel, as indicated by the arrow in Figure 4(c), crystals with two kinds of out-of-plane orientations and trigonal symmetry of the in-plane orientation are observed in the SEM image. Generally, with deposition methods, in cases where the (111) facet


11


Page 12 of 29

orientation is used on the growth substrate, only a (003) single-crystal film with a high flatness can be obtained.6, 15, 45, 46 With a deposition method, crystal growth occurs following the ordinality of a two-dimensional atomic array of the topmost surfaces of the substrate; hence, it is believed that only LCO (003) orientation growth can occur. In contrast, in the Li-VCG method, crystal growth can occur such that the LCO(003) facets are parallel to all equal {111} facets inside the substrate; thus, it is possible to achieve crystal growth. For the CoO(110) substrate, the same explanation applies, as described in Figure S2 (Supporting Information). As described above, the crystal growth process via the Li-VCG method differs from the deposition of materials on a substrate surface, in that it is direct crystal growth from inside the substrate. Therefore, the LCO crystal grains that have been grown are each directly connected to the inside of the substrate. This is superior from the perspective of electric current collection and mechanical connectivity with the substrate; thus, this LCO crystalline film can act as an electrode film that does not require a conductive additive or binder, for example. Further, the CoO substrate used to prepare the film acquires electrical conductivity from the calcination process; 47–50 hence, it can be a good current-collector substrate. Therefore, the fabricated sample is considered to be a good electrode. To confirm its electrode properties, a half-cell experiment was performed. Figure 5(a) shows the charge/discharge voltage profile of LCO crystal film samples formed on the CoO(111) and (110) substrates. The measured capacity was slightly different between these two samples, as the amounts of LCO film were not exactly equal. The samples show similar charge/discharge profiles, confirming a flat electrical potential around 3.9– 4.1 V vs. Li+/Li, indicating that they operate adequately as LCO with an α-NaFeO2 layered rocksalt structure. Furthermore, for comparison of the rate properties of the samples, Figure 5(b) shows a plot with the normalized capacity retention rate, assuming the charge capacity under


12

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


charging at a current density of 0.25 mA cm-2 as 100 %, on the vertical axis and the logarithmic scale of each current density on the horizontal axis. We measured the current-rate properties with respect to the charge capacity, as our focus was the effect of the orientation of the CoO substrate on the Li diffusion in the obtained films. The discharging process includes not only Li diffusion in the LCO but also the desolvation of Li at the electrode/electrolyte interface. This desolvation process strongly affects the discharge rate properties. On the other hand, the solvation process in the charging hardly affects the rate properties. Thus, the evaluation of the rate properties in the charging process is more important for evaluating the performance of the electrode. In the figure, at a high current density (4 mA cm-2), the capacity retention rate is CoO(110) > CoO(111). The verticality of the (003) facet on the LCO plate-like grains to the substrate surface was 90° at the LCO orientation and approximately 70° at the LCO orientation. In summary, higher verticality of the (003) facet yields a shorter Li diffusion distance inside the crystals; hence, it is believed that the rate characteristics are improved with the increase of the verticality. The aforementioned trends are consistent with this study. In the current-density range of 0.25–2 mAh cm-2, a capacity retention rate of 70 % or higher was confirmed for all the samples. Therefore, this experiment shows that an LCO crystalline film formed on a CoO substrate with any orientation will have good rate characteristics. Although an amount of cobalt-oxide impurity existed, the film electrodes exhibited high-current rate properties. This indicates that the LCO films have good properties. Of course, the electrochemical properties are affected by the film orientation, and a randomly oriented LCO film could not provide good electrochemical activity, as shown in Figure S1(c) (Supporting Information). As stated previously, an LCO crystalline film formed via the Li-VCG method has platelike crystal grains with high verticality of the LCO(003) crystal facets to the substrate surface.


13


Page 14 of 29

Idiosyncratic crystal growth of this kind can, as previously mentioned, achieve good rate properties regardless of the surface orientation of the substrate used. In practical applications, a CoO bulk single crystal may not suitable as the growth substrate. However, the present Li-VCG method is not limited to a bulk single crystal, and if we choose a thinner substrate, a high-quality LCO film without CoO impurities can be easily acquired, as the thin CoO substrate is completely converted to LCO. If we prepare the CoO thin-film via conventional thin-film preparation methods, it will be a suitable growth substrate for Li-VCG. The preparation of a Li transitionmetal oxide thin-film requires carefulness for Li evaporation in the growth condition. Thus, the cobalt oxide thin-film is easier to prepare than the LCO thin-film, and it can be simply converted into an LCO film via Li-VCG. Our method is convenient and widely applicable, and we believe that it may be useful for practical preparation.

CONCLUSIONS The Li-VCG method with a CoO single-crystal substrate can be used to form LCO crystalline films with both the proper orientation and porosity. The resulting crystalline film consists of LCO crystal grains with a diameter of 2–3 µm, and XRD and SEM observations confirmed that they grow with characteristic out-of-plane and in-plane orientations. The growth orientation of the LCO crystal grains was and on CoO(111) single crystals and and on CoO(110) single crystals. These characteristic orientations can be explained according to the crystal growth mechanism characteristic of the Li-VCG process, in which a CoO crystal lattice is employed as the pattern. The results of battery testing confirmed that the material formed operates as an electrode without modification and without the use of a binder or a


14

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conductive additive. Additionally, samples fabricated from a CoO single-crystal substrate with both orientations showed good rate characteristics. LCO crystalline films formed via the Li-VCG method have many promising merits for use as electrodes in batteries; hence, this method and LCO crystalline films fabricated via this method have potential for use in high-performance Li-ion battery systems.


15


Page 16 of 29

FIGURES

(a) Before

Calcination

Al2O3 crucible

Li-salt vapor

Crystal film formed

Wafer Au plate

After

Li-salt

(b) Before

After

Figure 1. Process overview of Li-VCG with the CoO substrate. (a) Schematic diagrams of the calcination. A thin Au plate was used to prevent the contamination of the bulk wafer by Al3+ from the Al2O3 crucible. (b) Photographs of the samples before and after calcination. The LiOH･H2O granules, which could be seen before, disappeared after calcination, indicating that they were vaporized during the preparation process.


16

16 from CoO(111)

14 12

CoO 222

CoO 220

from CoO(110) LiCoO2 powder

10

CoO 111

8

Co3O4 220

6

0 0 12 024

213

018 110

009 107

105

003

2

104

4 101 006 012

X-ray counts (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


千

Page 17 of 29

0 15

25

35

45

55 2θ (deg)

65

75

85

Figure 2. Out-of-plane XRD profiles of the crystal films formed on CoO(110) and CoO(111), represented by blue and red solid lines, respectively. Commercial LiCoO2 powder diffraction patterns are plotted as black solid lines with the assigned reflection index.


17


Page 18 of 29

Figure 3. Secondary-electron SEM images of the crystal films formed on CoO(111) (upper) and CoO(110) (lower).


18

Page 19 of 29

(a)

CoO(-111) CoO(111)

LiCoO2(003)

LiCoO2(003)

(b) Li-salt vapor

orientation orientation

CoO(111) CoO(-111)

Co

(c)

(-111)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(111)

Figure 4. Crystal growth models of LCO on the CoO substrate according to the Li-VCG procedure. (a) Typical lattice structure of CoO and LCO. The blue, red, and green spheres represent Co, O, and Li atoms, respectively. (b) Schematics of the LCO crystal growth in the CoO substrate. The


19


Page 20 of 29

nucleation of the LCO crystal should occur in the shallow region of the substrate surface, and its (003) plane was parallel to the CoO{111} lattice plane. (c) Comparison of the CoO{111} crystal faceting from the viewpoint of the direction with the growth morphology of LCO crystals.


20

Page 21 of 29

(a)

Voltage (V vs Li+/Li)

4.2

Chg

4

2

1

4

0.5 0.25

from CoO(111)

3.8 3.6 0

Dis

0.1

0.2

0.3

4.2 4

from CoO(110)

3.8 3.6 0

0.1

0.2

Capacity (mAh cm-2)

(b) Capacity retention (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 80 60 40

from CoO(111) from CoO(110)

20 0.25 0.5 1 2 4 -2 Current density (mA cm )

Figure 5. Electrochemical evaluation of the prepared LCO crystal film sample. (a) Charge/discharge profiles of LCO films prepared from CoO(111) and (110) substrates. The numbers beside the profiles indicate the current density (mA cm-2) of the experiment. (b) Charge capacity retention rate properties with various current densities. The absolute charge capacity of each current density was normalized by the capacity of 0.25 mA cm-2 as 100%.


21


Page 22 of 29

TABLES (003)

(012)

(018)

(110)

I012 / I003

I110 / I018

LCO film on CoO(111)

389.34

220.7

-

-

0.567

-

LCO film on CoO(110)

-

-

524.85

2371.91

-

4.518

Powder

3708.38

413.69

755.38

767.29

0.111

1.015

Table 1. Diffraction peak intensities for the powder LCO sample and prepared LCO crystal films.


22

Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information Figure S1 and S2. (PDF)

AUTHOR INFORMATION Corresponding Author Mitsunori Kitta (M. K.) *[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgment The authors thanks C. Fukada (AIST) for beneficial assistance of sample preparation.


23


Page 24 of 29

REFERENCES (1) Ménétrier, M.; Saadoune, I.; Levasseur, S.; Delmas, C. The insulator–metal transition upon lithium deintercalation from LiCoO2: electronic properties and 7Li NMR study. J. Mater. Chem. 1999, 9, 1135-1140. (2) Parka, M.; Zhanga, X.; Chunga, M.; Lessa, G. B.; Sastry, A. M. A review of conduction phenomena in Li-ion batteries. J. Power Souces 2010, 195, 7904-7929. (3) Nitta, N.; Wu, F.; Lee, J. T.; Yushin, G. Li-ion battery materials: present and future. Mater. Today 2015, 18, 252-264. (4) Bouwman, P. J.; Boukamp, B. A.; Bouwmeester, H. J. M.; Notten, P. H. L. Influence of Diffusion Plane Orientation on Electrochemical Properties of Thin Film LiCoO2 Electrodes. J. Electrochem. Soc. 2002, 149, A699-A709. (5) Xia, H.; Lu, L.; Ceder, G. Li diffusion in LiCoO2 thin films prepared by pulsed laser deposition. J. Power Sources 2006. 159, 1422-1427. (6) Hirayama, M.; Sonoyama, N.; Abe, T.; Minoura, M.; Ito, M.; Mori, D.; Yamada, A.; Kanno, R.; Terashima, T.; Takano, M.; Tamura, K.; Mizuki, J. Characterization of electrode/electrolyte interface for lithium batteries using in situ synchrotron X-ray reflectometry—A new experimental technique for LiCoO2 model electrode. J. Power Sources 2007, 168, 493-500. (7) Xie, J.; Imanishi, N.; Matsumura, T.; Hirano, A.; Takeda, Y.; Yamamoto, O. Orientation dependence of Li–ion diffusion kinetics in LiCoO2 thin films prepared by RF magnetron sputtering. Solid State Ion. 2008, 179, 362-370. (8) Antaya, M.; Dahn, J. R.; Preston, J. S.; Rossen, E.; Reimers J. N. Preparation and Characterization of LiCoO2 Thin Films by Laser Ablation Deposition. J. Electrochem. Soc. 1993, 140, 575-578. (9) 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, 18211827. (10) 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, 175182.


24

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Perkins, J. D.; Bahn, C. S.; McGraw, J. M.; Parilla, P. A.; Ginley, D. S. Pulsed Laser Deposition and Characterization of Crystalline Lithium Cobalt Dioxide (LiCoO2) Thin Films. J. Electrochem. Soc. 2001, 148, A1302-A1312. (12) Ohnishi, T.; Takada, K. High-Rate Growth of High-Crystallinity LiCoO2 Epitaxial Thin Films by Pulsed Laser Deposition. Appl. Phys. Express 2012, 5, 055502. (13) Nishio, K.; Ohnishi, T.; Akatsuka, K.; Takada, K. Crystal orientation of epitaxial LiCoO2 films grown on SrTiO3 substrates. J. Power Sources 2014, 247, 687-691. (14) 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. (15) Takeuchi, S.; Tan, H.; Bharathi, K. K.; Stafford, G. R.; Shin, J.; Yasui, S.; Takeuchi, I.; Bendersky, L. A. Epitaxial LiCoO2 Films as a Model System for Fundamental Electrochemical Studies of Positive Electrodes. ACS Appl. Mater. Interfaces 2015, 7, 7901-7911. (16) 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. (17) Okada, K.; Ohnishi, T.; Mitsuishi, K.; Takada, K. Epitaxial growth of LiCoO2 thin films with (001) orientation. AIP Adv. 2017, 7, 115011. (18) Wang, B.; Bates, J. B.; Hart, F. X.; Sales, B. C.; Zuhr, R. A.; Robertson, J. D. Characterization of Thin-Film Rechargeable Lithium Batteries with Lithium Cobalt Oxide Cathodes. J. Electrochem. Soc. 1996, 143, 3203-3213. (19) Zhu, X.; Guo, Z.; Du, G.; Zhang, P.; Liu, H. LiCoO2 cathode thin film fabricated by RF sputtering for lithium ion microbatteries. Surf. Coat. Technol. 2010, 204, 1710-1714. (20) Tintignac, S.; Baddour-Hadjeana, R.; Pereira-Ramos, J.-P.; Salot, R. High performance sputtered LiCoO2 thin films obtained at a moderate annealing treatment combined to a bias effect. Electrochim. Acta 2012, 60, 121-129. (21) Yoon, Y.; Park, C.; Kim, J.; Shin, D. Lattice orientation control of lithium cobalt oxide cathode film for all-solid-state thin film batteries. J. Power Sources 2013, 226, 186-190. (22) Joo, H.; Lee, H.; Cho, G.; Nam, T.; Huh, S.; Choi, B.; Jueong, H.; Noh, J. Influence of the metal-induced crystallization on the structural and electrochemical properties of sputtered LiCoO2 thin films. Thin Solid Films 2017, 641, 53-58.


25


Page 26 of 29

(23) Rho, Y. H.; Kanamura K.; Fujisaki, M.; Hamagami, J.; Suda, S.; Umegaki, T. Preparation of Li4Ti5O12 and LiCoO2 thin film electrodes from precursors obtained by sol–gel method. Solid State Ion. 2002, 151, 151-157. (24) Fonseca, C. P.; Fantini, M. C. A.; Neves, S. Improving the electrochemical properties of porous LiCoO2 films obtained by template synthesis. Thin Solid Films 2005, 488, 68-73. (25) Porthault, H.; Cras, F. L.; Franger, S. Synthesis of LiCoO2 thin films by sol/gel process. J. Power Sources 2010, 195, 6262-6267. (26) Kwon, T.; Ohnishi, T.; Mitsuishi, K.; Ozawa, T. C.; Takada, K. Synthesis of LiCoO2 epitaxial thin films using a solegel method J. Power Sources 2015, 274, 417-423. (27) Chen, C. H.; Buysman, A. A. J.; Kelder, E. M.; Schoonman, J. Fabrication of LiCoO2, thin film cathodes for rechargeable lithium battery by electrostatic spray pyrolysis. Solid State Ion. 1995, 80, 1-4. (28) Zhang, S.; Gu, H.; Pan, H.; Yang, S.; Du, W.; Li, X.; Gao, M.; Liu, Y.; Zhu, M.; Ouyang, L.; Jian, D.; Pan, F. A Novel Strategy to Suppress Capacity and Voltage Fading of Li- and Mn-Rich Layered Oxide Cathode Material for Lithium-Ion Batteries. Adv. Energy Mater. 2017, 7, 1601066. (29) Koike, S.; Tatsumi, K. Preparation and performances of highly porous layered LiCoO2 films for lithium batteries. J. Power Sources 2007, 174, 976-980. (30) Katsui, H.; Goto, T. Epitaxial growth of (104)- and (018)-oriented LiCoO2 films on MgO single crystals prepared by chemical vapor deposition. Surf. Coating. Technol. 2013, 218, 57-61. (31) Pentyala, N.; Guduru, R. K.; Mohanty, P. S. Binder free porous ultrafine/nano structured LiCoO2 cathode from plasma deposited cobalt. Electrochemi. Acta 2011, 56, 9851-9859. (32) Kim, D.-W.; Zettsu, N.; Mizuno, Y.; Teshima, K. Effect of Side-Plane Width on Lithium-Ion Transportation in Additive-Free LiCoO2 Crystal Layer-Based Cathodes for Rechargeable LithiumIon Batteries. J. Phys. Chem. C 2016, 120, 18496-18502. (33) Kitta, M.; Akita. T.; Maeda, Y.; Kohyama, M. Preparation of a spinel Li4Ti5O12 (111) surface from a rutile TiO2 single crystal. Appl. Surf. Sci. 2012, 258, 3147-3151. (34) Kitta, M.; Akita, T.; Kohyama, M. Preparation of a spinel LiMn2O4 single crystal film from a MnO wafer J. Power Sources 2013, 232, 7-11. (35) Kitta, M.; Akita, T.; Maeda, Y.; Tanaka, S.; Kohyama, M. Li-vapor induction growth of single-crystalline Li4Ti5O12 specimen for transmission electron microscopy. Surf. Interface Anal. 2014, 46, 1245–1248.


26

Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Antolini, E. LiCoO2: formation, structure, lithium and oxygen nonstoichiometric, electrochemical behaviour and transport properties. Solid State Ion. 2004, 170, 159-171. (37) Gummow, R. J.; Thackeray, M. M.; David, W. I. F.; Hull, S. Structure and electrochemistry of lithium cobalt oxide synthesised at 400℃. Mater. Res. Bull. 1992, 27, 327-337. (38) Rossen, E.; Reimers, J. N.; Dahn, J. R. Synthesis and electrochemistry of spinel LT-LiCoO2 Solid State Ion. 1993, 62, 53-60. (39) Kang, S. G.; Kang, S. Y.; Ryu, K. S.; Chang, S. H. Electrochemical and structural properties of HT-LiCoO2 and LT-LiCoO2 prepared by the citrate sol-gel method. Solid State Ion. 1999, 120, 155-161. (40) Garcia, B.; Farcy, J.; Pereira-Ramos, J. P. Electrochemical Properties of Low Temperature Crystallized LiCoO2. J. Electrochem. Soc. 1997, 144, 1179-1184. (41) Lin, Q.; Li, Q.; Gray, K. E.; Mitchell, J. F. Vapor Growth and Chemical Delithiation of Stoichiometric LiCoO2 Crystals. Cryst. Growth Des. 2012, 12, 1232-1238. (42) Akimoto, J.; Gotoh, Y.; Oosawa, Y. Synthesis and Structure Refinement of LiCoO2 Single Crystals. J. Solid State Chem. 1998, 141, 298-302. (43) Kramer, D.; Ceder, G. Tailoring the Morphology of LiCoO2: A First Principles Study. Chem. Mater. 2009, 21, 3799-3809. (44) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Cryst. 2011, 44, 1272-1276. (45) Tang, S. B.; Lai, M. O.; Lu, L. Li-ion diffusion in highly (003) oriented LiCoO2 thin film cathode prepared by pulsed laser deposition. J. Alloy. Comp. 2008, 449, 300-303. (46) Li, Z.; Yasui, S.; Takeuchi, S.; Creuziger, A.; Maruyama, S.; Herzing, A. A.; Takeuchi, I.; Bendersky, L. A. Structural study of epitaxial LiCoO2 films grown by pulsed laser deposition on single crystal SrTiO3 substrates. Thin Solid Films 2016, 612, 472-782. (47) Gesmundo, F.; Paladino, U.; Buscaglia, V.; Petot-Ervas, G. Electrical conductivity and defect structure of pure and Li-doped CoO at high temperatures. Mater. Chem. Phys. 1987, 17, 567-576. (48) Borchardt, G.; Kowalski, K.; Nowotny, J.; Rekas, M.; Weppner, W. Thermopower and Electrical Conductivity of Single Crystal and Polycrystalline CoO. J. Europ. Ceram. Soc. 1994, 14, 369-376. (49) Ijjaali, M.; Kowalski, K.; Bak, T.; Dupre, B.; Gleitzer, C.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Electrical Properties of Cr-Doped CoO. Ionics 2001, 7, 351-359.


27


Page 28 of 29

(50) Rao, K. V.; Rao, K. S. Effect of oxygen impurity on the electrical conductivity and dielectric properties of CoO and NiO single crystals. Philos. Mag. 1971, 23, 1053-1060.


28

Page 29 of 29

For Table of Contents Use Only Preparation of Highly Oriented Porous LiCoO2 Crystal Films via Li-Vapor Crystal Growth Method

Mitsunori Kitta* and Kentaro Kuratani

Research Institute of Electrochemical Energy, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST)

*Corresponding; [email protected]

TABLE OF CONTENTS GRAPHIC

CoO substrate

LiCoO2 crystal

On CoO(111)

Calcination

Li-salt vapor

On CoO(110)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Highly oriented porous LiCoO2 crystal films for Li-ion batteries were prepared via the Li-vapor crystal growth method.


29

Preparation of Highly Oriented Porous LiCoO2 Crystal Films via Li

Recommend Documents