Proceedings of 10th Thin Film Materials & Devices Meeting
October 31 - November 2, 2013, Kyoto, Japan
http://www.tfmd.jp/Solid-Liquid Phase Epitaxy of Li4Ti5O12 Thin FilmNing Li1, Takayoshi Katase1, Yanbei Zhu2, Tomonari Umemura3, Takao
Matsumoto4, Yuichi Ikuhara4 and Hiromichi Ohta1
1RIES, Hokkaido Univ., N20W10 Kita, Sapporo 001-0020, Japan
2
AIST, 1-1-1 Umezono, Tsukuba 305-8563, Japan3
School of Life Sci., Tokyo Univ. Pharm. & Life Sci., 1432-1 Horinouchi, Hachioji, Tokyo192-0392, Japan
4Inst. Eng. Innovation, The Univ. of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656,
Japan
Keywords: Li4Ti5O12, Li-ion battery, Thin film, Solid-liquid phase epitaxy, Pulsed laser
deposition
Abstract. Thin film of Li4Ti5O12, a candidate material of anode for next generation Li-ionbattery, was heteroepitaxially grown on SrTiO3 single crystal by the solid-liquid phase
epitaxy method: Amorphous Li-Ti-O thin film, which was deposited by pulsed laser
deposition at room temperature and was covered with LiNO3 molten salt, was heated at 600ºC. Then it was washed with distilled water. The product was polycrystalline epitaxial
Li4Ti5O12 thin film, which was composed of rather small crystallites (~20 nm), with an
epitaxial relationship of (001)[100]Li4Ti5O12 || (001)[100]SrTiO3.

Introduction
Recently, all-solid-state Li-ion batteries (LIBs) have received much attention because thenonflammable nature can improve the safety of LIBs [1]. The key for realization of the
all-solid-state LIBs is to clarify the ion transport mechanism at the interface between solid
electrode and electrolyte [2]. However, it is very difficult to investigate the transportmechanism of Li ion with the powders or ceramics. Single crystal and/or single crystalline
film of solid electrode and electrolyte with atomically flat surface is essentially important toproduce the ideal interfaces, but the fabrication of single crystalline film of Li-containingoxide is extremely difficult by a conventional vapor phase epitaxy method because there-vaporization of Li occurs during film growth due to its extremely high vapor pressure. For
example, in order to supplement the lost Li during the film growth by pulsed laser deposition(PLD) at high temperature, the Li-rich (Li was additionally added) target was selected, but
the composition of Li was very difficult to control [3,4].
Here, we have proposed novel solid-liquid phase epitaxial growth method for the- 13101019-01 -fabrication of Li-containing oxide thin films. The key point of our method is that the surfaceof the film is covered with the Li molten salt during annealing at high temperature to prevent
re-evaporation of Li-species from the film. In the present study, we fabricated Li4Ti5O12 [5]thin film, which is most promising anode materials for the next generation LIBs [6].
Experimental
Figure 1 schematically illustrates the solid-liquid phase epitaxy procedure for the growth ofLi4Ti5O12 epitaxial films. Amorphous Li-Ti-O films were deposited at room temperature (RT)on (001) SrTiO3 single crystal substrates by PLD with KrF excimer laser pulse (λ=248 nm, 20ns, 10 Hz, ~3 J cm2 pulse1) using a dense Li4Ti5O12 ceramic as the target. The resultant film
was covered with LiNO3 powder, and was heated at 600 ºC for 30 min in air in an Al2O3
crucible using electric furnace. During the heating process, the LiNO3 powder melted due tothe low melting point of 261 oC and entirely covered the Li-Ti-O film at 600 oC. Then thefilm was naturally cooled down to RT in the furnace. The annealed film was washed by
distilled water since the film surface was covered with the remaining LiNO3 film.
Crystalline phase and orientation were analyzed by high-resolution X-ray diffraction(HR-XRD, ATX-G, CuKα1, Rigaku Co.) and a conventional transmission electron
microscope (TEM, JEM-2010HC, JEOL Ltd.) and an atomic-resolution scanningtransmission electron microscope (STEM, JEM-ARM200F, JEOL Ltd.). Surface morphologyof the Li4Ti5O12 epitaxial film was observed by a scanning probe microscopy (SPM,Nanocute, Hitachi High-Tech Science Co.).

Fig. 1: Solid-liquid phase epitaxy of Li4Ti5O12 film. Step 1: Amorphous Li-Ti-O films were
deposited at RT on (001) SrTiO3 single crystal substrates by PLD using a dense Li4Ti5O12 ceramic
as the target. Step 2: The resultant film was heated at 600 ºC for 30 min in air with LiNO3 powder,which melted during the heating process, in an Al2O3 crucible using electric furnace. Then the film
was naturally cooled down to room temperature in the furnace. Step 3: The resultant film waswashed by distilled water since the film surface was covered with remaining LiNO3 film.

Results and discussion
Figure 2 summarizes the XRD patterns of the resultant films. Only intense diffraction
peaks of 00l SrTiO3 can be observed in the out-of-plane XRD pattern of as-deposited Li-Ti-O
film [Fig. 2(a)], indicating that the as-deposited film was amorphous. After this amorphousfilm was annealed at 600 ºC in air, 004 TiO2 diffraction peak was observed though Li4Ti5O12- 13101019-02 -diffraction peak could not be seen in theout-of-plane XRD pattern [Fig. 2(b)],suggesting re-vaporization of Li occurred.
In contrast, an intense diffraction peak of004 Li4Ti5O12 was observed after the film washeated with LiNO3 molten salt at 600 ºC [Fig.
2(c)]. At this stage, the actual re-vaporization
temperature of Li in Li-Ti-O film has not been
examined yet, the LiNO3, which melted at low
melting point of 261 oC and entirely covered Fig. 2: Out-of-plane XRD patterns of thethe film, successfully suppressed the Li-Ti-O films [(a) as-deposited, (b) heated at
re-vaporization of Li and enabled the 600ºC without LiNO3, (c) heated at 600ºC with
LiNO3 (solid-liquid phase epitaxy)]. (d) X-ray
crystallization of Li4Ti5O12 phase.
rocking curve of the 004 Li4Ti5O12. FWHMvaluewas ~2 degrees. (e) In-plane XRD pattern
The estimated crystallite size was ~23 nm,of the Li4Ti5O12 film grown by solid-liquid
which was ~1/7 of the film thickness (150 nm). phase epitaxy.
The full width at half maximum (FWHM)
value of the out-of-plane rocking curve for 004 Li4Ti5O12 diffraction is ~2º [Fig. 2(d)]. The
epitaxial orientation was investigated by in-plane XRD measurement [Fig. 2(e)]. Only intense
diffraction peak of 400 Li4Ti5O12 was observed with h00 SrTiO3. The scan of 400 Li4Ti5O12
diffraction [Fig. 2(e) inset] shows four-fold rotational symmetry every 90º originating fromthe cubic symmetry of Li4Ti5O12 lattice. These results prove the heteroepitaxial growth ofLi4Ti5O12 film on SrTiO3 substrate with an epitaxial relationship of(001)[100]Li4Ti5O12||(001)[100]SrTiO3.
The surface morphology of Li4Ti5O12 epitaxial film wasobserved by SPM (Fig. 3). Rather smooth surface (rootmean square roughness, Rrms ~5 nm) with grain structure(grain size 100  200 nm) was seen in the whole area.
The magnified image shows that every grain consisted ofvery small (~20 nm) crystallites, indicating
polycrystalline nature of the film.
In order to further clarify the microstructure of theresultant film, TEM/STEM analyses were carried out (Fig.
4). The plan view TEM bright field image shows that theLi4Ti5O12 film is composed of the small grains with thediameter of ~50 nm [Fig. 4(a)]. In addition, the grainboundaries could be clearly observed [inset of Fig. 4(a)],Fig. 3: Topographic SPM image of theindicating that the Li4Ti5O12 film is polycrystalline. In theLi4Ti5O12 epitaxial film grown by theSTEM bright field image, Li4Ti5O12 lattice (ab plane) is solid-liquid phase epitaxy.

- 13101019-03 -clearly seen [Fig. 4(b)]. However, rather
broad spots are seen in the electron
diffraction pattern [Fig. 4(c)], indicatinglarge twisting of the Li4Ti5O12 grains. Theseresults are consistent with the results ofXRD patterns.
SummaryWe have demonstrated the solid-liquidphase epitaxy of Li4Ti5O12 thin film as anexample. Li4Ti5O12 thin film was
successfully obtained by our solid-liquid
phase epitaxy method using LiNO3 molten
salt though re-vaporization of Li species
occurred when the film was heated without
LiNO3 molten salt. The resultant film was(a)(b)

2 nmTiLiO(c)Fig. 4: TEM/STEM analyses of the Li4Ti5O12
epitaxial film grown by the solid-liquid phase
epitaxy. (a) Plan view TEM bright field image. The
inset shows a magnified view. (b) STEM brightfield image of [100]-oriented Li4Ti5O12 grain. (c)Electron diffraction pattern of [100]-oriented
Li4Ti5O12. Right shows the simulation of (c).

composed of rather small crystallites (~20nm) with an epitaxial relationship of (001)[100]Li4Ti5O12 || (001)[100]SrTiO3.
Acknowledgements
A part of this work is supported by Nano-Macro Materials, Devices, and System Research
Alliance Project. H.O. was supported by MEXT (25246023 and 25106007). Part of thiswork was conducted at the Green Network of Excellence (GRENE) and supported by“Nanotechnology Platform” (project No. 12024046), both sponsored by MEXT, Japan.
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