Study of Surface Reaction of Spinel Li4Ti5O12 during the First Lithium

Jul 27, 2012 - Nobuyuki Serizawa , Kumi Shono , Yo Kobayashi , Hajime Miyashiro , Yasushi Katayama , Takashi Miura. Journal of Power Sources 2015 295,...
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Study of Surface Reaction of Spinel Li4Ti5O12 during the First Lithium Insertion and Extraction Processes Using Atomic Force Microscopy and Analytical Transmission Electron Microscopy Mitsunori Kitta,* Tomoki Akita, Yasushi Maeda, and Masanori Kohyama Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ABSTRACT: Spinel lithium titanate (Li4Ti5O12, LTO) is a promising anode material for a lithium ion battery because of its excellent properties such as high rate charge−discharge capability and life cycle stability, which were understood from the viewpoint of bulk properties such as small lattice volume changes by lithium insertion. However, the detailed surface reaction of lithium insertion and extraction has not yet been studied despite its importance to understand the mechanism of an electrochemical reaction. In this paper, we apply both atomic force microscopy (AFM) and transmission electron microscopy (TEM) to investigate the changes in the atomic and electronic structures of the Li4Ti5O12 surface during the charge−discharged (lithium insertion and extraction) processes. The AFM observation revealed that irreversible structural changes of an atomically flat Li4Ti5O12 surface occurs at the early stage of the first lithium insertion process, which induces the reduction of charge transfer resistance at the electrolyte/Li4Ti5O12 interface. The TEM observation clarified that cubic rock-salt crystal layers with a half lattice size of the original spinel structure are epitaxially formed after the first charge−discharge cycle. Electron energy loss spectroscopy (EELS) observation revealed that the formed surface layer should be α-Li2TiO3. Although the transformation of Li4Ti5O12 to Li7Ti5O12 is well-known as the lithium insertion reaction of the bulk phase, the generation of surface product layers should be inevitable in real charge−discharge processes and may play an effective role in the stable electrode performance as a solid-electrolyte interphase (SEI).

1. INTRODUCTION

lithium-rich phases or surface compound layers as solidelectrolyte interphases (SEI), and so on.6,7 Recently, from electrochemical experiments, it was reported that the surface reaction of Li4Ti5O12 possibly occurs during the first lithium insertion process and that the surface reaction possibly induces the reconstruction of the Li4Ti5O12 surface structure at the Li4Ti5O12/electrolyte interface.8 However, the detailed atomic-structure changes and procedure of the surface reaction of Li4Ti5O12 during the first lithium insertion process are still unclear. It is of great importance to study the details of the surface reaction at the atomic and electronic scales so as to elucidate the charge−discharge mechanism of Li4Ti5O12, which should contribute to the significant improvement of battery performance and to the development of materials design of electrode materials. For this purpose, it is desirable to obtain detailed information on the surface structure and its changes during the charge and discharge processes directly as previously reported.9 Thus in this paper, we apply the scanning probe microscopy (SPM) and transmission electron microscopy (TEM) observations. The SPM such as atomic force microscopy (AFM) allows us to obtain direct images of the electrode-surface structure and morphology at the nano or subnano scale, which cannot be obtained by usual spectroscopic studies.10−13 Indeed, the SPM observation was applied to

Lithium titanium oxide, Li4Ti5O12 (LTO), is one of the most promising candidates for anode materials of lithium ion batteries due to its stability and high reaction rate of lithium insertion and extraction.1,2 Li4Ti5O12 has a spinel structure (space group, Fd3̅m) and is well-known to be transformed to Li7Ti5O12 with an ordered rock-salt structure during the lithium insertion process through a two-phase equilibrium reaction,3−5 revealing the voltage plateaus in the charge and discharge profile. The lattice parameters of Li4Ti5O12 and Li7Ti5O12 are very similar to each other with the difference less than 0.1%.4,5 This minimal lattice change should contribute to the exceptional stability of this material under electrochemical charge− discharge cycles. Studies of crystal structural changes contributed to the understanding of the reaction mechanism in the bulk phase. However, little is known about the electrochemical reaction at the Li4Ti5O12 electrode surface or electrolyte/electrode interface in spite of its importance for the battery performance. The high rate charge−discharge performance of Li4Ti5O12 should also be dominated by atomistic or molecular-level mass transport at the electrode/electrolyte interface. It is crucial to investigate the electrochemical phenomena at the electrode surface or electrolyte/electrode interface such as lithium diffusion in the electrolyte, adsorption of solvated lithium or desalvation on the electrode surface, surface diffusion and reduction of lithium ions on the electrode surface, formation of © 2012 American Chemical Society

Received: May 14, 2012 Revised: July 25, 2012 Published: July 27, 2012 12384

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was dismantled in a dry air filled box, and the film sample was rinsed by DMC solution to wash out the electrolyte from its surface. AFM (NanoNavi-II, SII) was used to evaluate the roughness of reacted sample surfaces by dynamic force mode, and a scanning electron microscope (SEM) (S-5500, Hitachi) was used to analyze the surface composition via energy dispersive X-ray (EDX) spectroscopy measured at 10 kV of electron accelerating voltage. JEM-3000F (JEOL) TEM was used at 200 keV to take the bright field micrographs of the samples. The elemental analysis was performed by EELS using a Gatan imaging filter (Gatan) at 197 keV. The degradation of sample surfaces occurs in lithium inserted samples for the AFM observation during the charge−discharge cycle. Spherical precipitates on the surface were observed when the sample was exposed to air for a long time (about 1 h). We can easily distinguish the precipitate morphology in the AFM image, and we terminated the observation before such precipitates are formed. A thin specimen for TEM observation after the charge−discharge cycle was easily acquired without a destructive method because the Li4Ti5O12 film has thin edges suitable for TEM observation.

graphite negative electrodes14,15 and some transition-metal oxide surfaces16,17 for electrochemical applications, which revealed surface reaction mechanisms in real time. Furthermore, the TEM observation allows us to obtain crosssectional images of the electrode surfaces with atomic structure changes or formation of surface layers or products by the electrochemical reaction. By combining the TEM highresolution observation with electron energy-loss spectroscopy (EELS), we can investigate the atomic and electronic structures, compositions and chemical states at the electrode surface and bulk, as recently performed in electrode materials for lithium ion batteries.18−22 It can be said that the combination between the SPM and TEM observations should provide valuable insights into the surface electrochemical reaction of electrode materials. However, there have been no such combined studies for any electrode materials. This is due to the difficulty in preparing experimental samples, suitable to both the SPM and TEM observations, especially to the SPM observation since the real electrode materials are usually polycrystalline or aggregation of particles, which cannot be used in the SPM observation. Recently, we have developed the scheme to prepare a well-defined Li4Ti5O12(111) surface sample with atomic flatness suitable to the SPM observation by using a conventional solid-state reaction. The formed Li4Ti5O12 sample was proved to show a usual electrochemical reaction in electrochemical experiments.23 In this paper, we perform SPM and TEM observations of well-defined Li4Ti5O12 surface samples from the above viewpoint. We examine the surface structure changes during the first lithium insertion process and after the first charge− discharge cycle and investigate the detailed process of the surface electrochemical reaction.

3. RESULTS AND DISCUSSION 3.1. Surface Morphology Changes of Li4Ti5O12 by the Lithium Insertion and Extraction Reactions. We performed AFM observation of surface morphology changes by the lithium insertion and extraction processes. Figure 1 shows

2. MATERIALS AND METHODS The sample preparation method is the same as our previous study.23 A rutile TiO2(111) wafer of 2 × 2 × 0.5 mm3 with one side polished (purchased from Shinkosha Co., Ltd.) was cleaned in acetone by a ultrasonic cleaner and calcined in air at 1073 K for 12 h to remove possible organic contaminations. Then the wafer was calcined at 1173 K for 24 h in air using an alumina crucible with LiOH·H2O powder (99%, Wako) to produce Li4Ti5O12. After the calcinations, a Li4Ti5O12(111) film of about 10 μm thickness was produced on the TiO2(111) wafer surface with preferential orientation. This film was considered to be synthesized by solid-state reaction of TiO2 with lithium hydroxide, which was volatilized by heating and spread to the TiO2 wafer surface. Electrochemical lithium-ion intercalation experiments were carried out for the formed Li4Ti5O12 films using a test-type cell (HS-cell, Hosen Corp.) with metallic lithium as a counter electrode. A prepared Li4Ti5O12(111) film on a TiO2 wafer with a Cu film as a current collector was used as the positive electrode. The cells were assembled and sealed in a dry air-filled box with lithium foil as the anode and 1 M LiPF6 dissolved in EC/DMC (1:1 by volume) as the electrolyte. A galvanostatic experiment was performed in the voltage range between 1.1 and 2 V versus lithium. We performed charge−discharge experiments at a current density of 20 mA/g, corresponding to about 0.1 C in terms of ideal specific capacity of Li4Ti5O12. This is because the charge−discharge voltage profile and specific capacity depend on a current rate, and the characteristic voltage plateau is reproduced in a low rate as 0.2 C.24 Electrochemical impedance spectroscopy was applied before and after the galvanostatic experiment by using impedance modules. The frequency range was 10 kHz to 10 mHz with an ac signal of 5 mV. An additional dc bias voltage of 1.6 V was applied to compensate for the open circuit voltage. After the electrochemical experiments, the cell

Figure 1. AFM observation of surface morphology changes of Li4Ti5O12 surfaces by the charge−discharge reaction. Topographical AFM images of an as-prepared sample (A) and a sample after five lithium insertion−extraction cycles (B), respectively. The surface profiles of blue and red lines in part C correspond to each line in parts A and B.

the topographical AFM images of an as-prepared Li4Ti5O12 sample (A) and after five cycles of lithium insertion and extraction reactions (B). On the as-prepared sample surface, we can see steps and terraces clearly, where the step height is about 0.5 nm as shown in part C, corresponding to a single step height of the Li4Ti5O12(111) surface.23 However, on the sample surface after the five charge−discharge cycles, we cannot see any clear steps or terraces by remarkably increased roughness of about 10 nm as clearly shown in part C. It can be said that the lithium insertion and extraction reaction changes atomically flat Li4Ti5O12(111) surfaces into remarkably rough surfaces. To examine the compositional changes in the sample surface after the lithium insertion and extraction reactions, we performed EDX analysis by using SEM. Figure 2A shows the 12385

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Figure 3. Potential profile in the first lithium insertion and extraction cycle (discharge and charge cycle in the present cell) of an as-prepared Li4Ti5O12 sample with a flat (111) surface. AFM observations were performed for the samples of various lithium insertion and extraction states from points a to d.

the present experiment, respectively. The points c and d correspond to the full lithium insertion and extraction in the first cycle, respectively. The AFM images in Figure 4 are Figure 2. SEM image (A) and EDX spectrum (B) of the Li4Ti5O12 surface after the initial five cycles of lithium insertion and extraction. The EDX spectrum was obtained in the area of part A at a 10 kV of accelerating voltage.

SEM image of the sample surface after the initial five cycles of lithium insertion and extraction, which resembles the AFM image in Figure 1B. The EDX spectrum in Figure 2B, acquired in the observation area of part A, shows clear peaks at 0.52 and 4.5 keV of the binding energy, which are confirmed as Kα peaks of oxygen and titanium, respectively. No other peaks concerning C, F, or P are observed, at least within the detection limit. This indicates that the surface roughness means surface structural changes of Li4Ti5O12 by the lithium insertion and extraction reactions. Here, the EDX analysis is known to be suitable to detect various elements rather deep in the surface at about the micrometer range, while this does not necessarily mean insensitivity to elements in shallower regions, as with successful EDX analyses of surface products or electrolyte components.25,26 Additionally, we have examined this point by EELS observation of the sample after the first cycle as will be discussed in section 3.5. Also, it was previously reported that no decomposition of the EC/DMC electrolyte occurs in the present experimental condition.8,27,28 3.2. Surface Morphology Changes during the First Lithium Insertion and Extraction Cycle. To investigate the morphology change of the atomically flat Li4Ti5O12(111) surface during the first lithium insertion and extraction cycle, we performed AFM observations at various lithium inserted states from points a to d in Figure 3. The voltage plateaus at around 1.5−1.6 V on the potential profile indicate that the prepared sample is surely active as a battery electrode material. The voltage plateau in the lithium insertion reaction in Figure 3 has a slope in contrast to the characteristic flat plateau at 1.55 V in conventional experiments of Li4Ti5O12 powder samples.24 This phenomenon was also observed for other Li4Ti5O12 single crystal samples,23,29 which is caused by low electron conductivity and flat surface morphology and not necessarily caused by different electrochemical reactions. The points a and b correspond to the exposure 15 and 30 min from the beginning of the lithium insertion process, corresponding to 10% and 20% of the full lithium insertion in

Figure 4. Topographical AFM images of Li4Ti5O12 surfaces at various stages in the lithium insertion and extraction process. Images from parts a to d correspond to each state from points a to d in Figure 3. All images were acquired at the 500 nm × 500 nm scale.

acquired at each point in Figure 3. All the images have features similar to the image of Figure 1B, indicating that the surface roughness is increased by the lithium insertion. The surface roughness can be measured by an arithmetic mean profile deviation (Ra) value. For the samples at the points a, b, c, and d, the Ra values are 1.5, 2.63, 2.36, and 2.32 nm, respectively. Even the sample at point a with 10% of the full lithium insertion has the roughness 50 times larger than the asprepared sample of Ra = 0.03 nm. The surface roughness reveals the maximum at point b and does not show large changes for points c and d. Note that the accuracy of Ra for the surface structural changes of flat crystalline samples does not seriously depend on the substrate or thickness of samples, different from the cases of deposited films on some substrates.30 It is clear that the surface morphology change occurs irreversibly at the early stage of the initial lithium insertion process and that the surface roughness cannot be removed by lithium extraction. 3.3. Surface Morphology Changes in the Early Stage of the First Lithium Insertion Process. Figure 5 shows potential profiles of the lithium insertion process in the initial 12386

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nm, twice as large as the sample at point a. For the surface at point c (Figure 6c), steps and terraces are marginally observed. The roughness is greatly increased as Ra = 0.24 nm, which is 3 times larger than that of the sample at point b. Figure 6d shows the sample at point d with a further lithium insertion reaction after the potential fluctuation. We cannot see clear surface structures, and the roughness reaches an Ra value of 0.6 nm. Note that this value is yet smaller than the value at point a in Figure 3 due to a shorter time from the beginning. Generally, Li4Ti5O12 is regarded as the zero-strain insertion material because of the negligible lattice volume change by the lithium insertion. In spite of this, the present results suggest that substantial structural changes occur at the surface during the first lithium insertion process. In layered compounds as graphite, it is reported that remarkable structural changes occur at the step edges during the lithium insertion process.31 However, the present morphology changes are observed in whole places as steps and terraces on the surface. This is because the spinel structure of Li4Ti5O12 has three-dimensional lithium diffusion passes, different from layered compounds.32 From Figures 3−5, it can be said that significant changes in the surface structure and property occur mainly in the first lithium insertion and extraction cycle. No remarkable potential fluctuation was seen after the second cycle. Thus we investigated the change in the charge transfer resistance at electrolyte/electrode interfaces before and after the first cycle, via ac-impedance measurement of the the Li|electrolyte| Li4Ti5O12 cell. The impedance spectra were acquired with a constant voltage of 1.6 V, slightly higher than the lithium insertion potential of Li4Ti5O12, to prevent the surface morphology change before the first cycle. Results are shown as a Cole−Cole plot in Figure 7. The size of a semicircle was

Figure 5. Cell potential profiles of the lithium insertion process (discharge curves) in the initial five cycles. The first one is a simple magnification of the curve in Figure 3.

five cycles. The profiles except the first cycle are all similar to each other, indicating that the lithium insertion processes from the second cycle are performed rather stably and reproducibly. This also indicates that the first lithium insertion reaction should have quite a different nature, compared to the lithium insertion after the second cycle. In Figure 5, there is a peculiar potential fluctuation in the early stage of the first lithium insertion process, which means the change in the cell internal resistance that contained the bulk or electrode/electrolyte interface resistance during the insertion process, as discussed later. It is quite interesting to examine the correlation between the surface morphology change and the potential fluctuation. Thus as shown in Figure 6, we examined the surface morphology

Figure 7. ac-impedance spectra of a Li/Li4Ti5O12 cell before the first cycle and after the first and fifth cycles.

Figure 6. Topographical AFM images of the sample surfaces acquired from various time points in the early stage of the first lithium insertion process. Images from parts a−d correspond to the samples from points a−d in in Figure 5.

decreased into a half after the first cycle, indicating that the charge transfer resistance of electrode/electrolyte interfaces was reduced into a half after the first cycle. Here, the interface electric resistance measured by the present experiment also contains the resistance at the electrolyte/Li-metal interface, and a small semicircle is observed at the high frequency region (near the origin in Figure 7) only after the cycles, corresponding to an increase in the resistance of the Li/ electrolyte interface.33 It was also reported that the interface resistance of the electrolyte/Li-metal system is rather increased by the charge−discharge cycle due to the formation of passivation films on Li-metal surfaces during the electrochemical reaction. Thus the observed decrease in the interface

changes at each point of potential fluctuation shown in Figure 5. Figure 6a shows the AFM image of the sample at point a in Figure 5, just before the local minimum of the potential profile. There are clear steps and terraces like the as-prepared sample shown in Figure 1A. The roughness on the terraces is rather small with an Ra value of 0.037 nm. Thus the surface morphology change is very small at this point at the beginning of the lithium insertion. Figure 6b shows the AFM image of the sample at point b as the local minimum of the potential profile in Figure 5. We can see clear steps and terraces, while the surface roughness of the terrace part is increased as Ra = 0.07 12387

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fluctuation in the early stage of the lithium insertion reaction really means the progress of the reduction in the internal resistance. Of course, the measured resistance contains the contribution from the bulk region in addition to the surface region. The bulk region is basically common in each cycle, and the potential fluctuation is observed only in the first cycle. Thus the resistance reduction should mean the reduction in the surface (interface) resistance, in accordance with the progress of the surface structural change from point b to point d shown in Figure 6. This is consistent with the results of the impedance measurement. The present surface structural change observed in the early stage of the first lithium insertion process should be promoted by the electrochemical reason. Atomically flat crystal surfaces should have a high internal resistance, due to the lack of reaction sites. Then the surface reconstruction or surface structural change should occur so as to promote the lithium insertion reaction. Of course, there may exist atomistic reconstruction or compositional changes other than the observed morphology changes by AFM. In any case, the present observation of the initial potential fluctuation, the surface structural change, and the reduction in the charge transfer resistance has clarified the importance of the initial lithium insertion process for the electrochemical performance of Li4Ti5O12. 3.4. TEM Observation of the Sample Surface after the First Lithium Insertion and Extraction Cycle. To investigate detailed structural changes at the Li4Ti5O12 surface, we performed high-resolution TEM observations of the sample surface before and after the first lithium insertion−extraction cycle as shown in Figure 9. For the sample surface before the first cycle (Figure 9A), we can see only the lattice fringes of the spinel structure, indicating that the sample has a pure Li4Ti5O12 surface without the formation of any products or defected layers. On the other hand, there exists a relatively thick surface layer with amorphous-like contrasts after the first charge− discharge cycle (Figure 9B)). It is clear that the surface product with rather disordered structures was newly formed by the electrochemical lithium insertion and extraction reaction. In the interface region between the bulk and surface product shown in Figure 9C, there are some lattice fringes, different from the spinel structure in the bulk side. Here, it is well-known that Li4Ti5O12 is transformed to Li7Ti5O12 by lithium insertion. However, the structure of Li7Ti5O12 is difficult to be distinguished from the spinel structure of Li4Ti5O12 by electron diffraction. Thus it seems that the surface product contains some crystal structure different from Li7Ti5O12 or Li4Ti5O12. It is interesting that the lattice fringes in the surface product layer are just parallel to the lattice fringes of the bulk crystal region in Figure 9C. In addition, the spacing of the lattice fringes in the surface product layer is about half of the spacing in the bulk spinel crystal. The spacing along the (111) direction is 0.24 nm and that along the (100) direction is 0.2 nm, in contrast to the values of 0.48 and 0.4 nm in the bulk spinel crystal. The Fourier transform patterns in Figures 9D,E provide more clear information on the crystal structure in the surface product layer. Figure 9D shows the diffraction pattern of the spinel structure with [110] incidence. On the other hand, the Fourier pattern of the crystal in the surface product layer in Figure 9E reveals the bright points corresponding not to {111} but to {222} lattice spacing of the spinel structure. In this way, it can be said that a cubic rock-salt structure with half-size

resistance should be mainly caused by the electrolyte/Li4Ti5O12 interface. Figure 7 also shows the spectrum after the fifth cycle, which is similar to that after the first cycle, indicating no remarkable changes in the interface electrochemical properties after the second cycle as also seen in Figure 5. The reduction in the resistance may be explained by the increase in the specific surface area of the sample due to the increased roughness as shown in Figures 4 and 6. From the AFM images, the surface area of the as-prepared sample is estimated to be 6.31 × 104 nm2 for the 250 × 250 nm2 range and that of the sample after the first cycle to be 6.91 × 104 nm2 for the same scale range. Here the surface area was calculated by summing the areas of elemental triangles made from three adjacent points in the three-dimensional coordinates in topographical AFM data for some specified region. The increase in the surface area is only about 10%, because the increased roughness is only about the nanometer scale. This cannot explain the decrease of one-half in the interface resistance observed by the impedance measurement after the first charge−discharge cycle. Thus it is necessary to analyze further detailed changes in the crystal structure, chemical composition, and electronic structure at the surface. We have performed a more detailed analysis on the change in the surface electrochemical properties during the first lithium insertion process that would cause the potential fluctuation in the early stage of the lithium insertion in Figure 5. Generally, the cell voltage (V) observed in the constant current (I) experiment was expressed as follows, V = E − Ir

(1)

where E is the Nernst potential of this electrochemical system and r is the cell internal resistance. The potential change from the minimum point (point b in Figure 5) to the restored point (part d in Figure 5) should correspond to the decrease in r from point b to point d. In order to examine this, we obtained the voltage profile of as-prepared samples as that for the first cycle in Figure 5 under a different current density of 100 and 200 mA/g, in addition to the original data with 20 mA/g, and examined the internal resistance values at the potential minimum and restored points in each profile by a conventional I−V plot.34 The result is shown in Figure 8, where the slope of each line corresponds to the magnitude of the internal resistance at each state. It can be said that the resistance of the samples at the restored point (line B) is smaller than that at the potential minimum point (line A) by about 30%. This suggests that the potential

Figure 8. I−V profiles at the potential local minimum point (A) and the restored point (B) in the initial lithium insertion curve in Figure 5, obtained by different current densities. 12388

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In order to decide which is really formed, we performed further EELS analysis of the surface layer compared to reference crystals, Li4Ti5O12, β-Li2TiO3, and Ti2O3 as shown in Figure 11. Here, β-Li2TiO3 has a monoclinic C2/c structure

Figure 9. TEM micrographs of the sample surfaces before the first charge−discharge cycle (A) and after the first cycle (B). The interface region between bulk and surface regions indicated by a square in part B is magnified in part C. Parts D and E show the Fourier transform images of the bulk and surface regions, respectively.

periodicity is formed in the surface product layer with the epitaxial relation to the bulk spinel structure. 3.5. EELS Characterization of the Surface Crystal Layer. To examine the chemical composition and electronic structure of the formed crystal in the surface product layer, we performed EELS analysis as shown in Figure 10. There exist clear peaks around 60, 450, and 530 eV, corresponding to Li-K, Ti-L, and O-K edges, respectively. There are no peaks around 686 eV, corresponding to the F-K edge, indicating that the surface crystal is not formed by decomposition or reaction of electrolytes but formed by some electrochemical reaction of Li4Ti5O12 itself. It can be said that the formed crystal consists of Li, Ti, and O in accordance with the EDX spectrum in Figure 2. Thus, LiTiO235 and α-Li2TiO336,37 are the candidates for the formed crystal. These have a cubic rock-salt structure with the lattice parameter of about 4.15 Å, which is half of 8.36 Å for Li4Ti5O12. This is consistent with the TEM observation of the {111} and {100} lattice fringes.

Figure 11. Detailed EELS spectra of Li-K edge (A) and Ti-L edge (B) of the crystal region in the surface product layer compared to the spectra of reference crystal samples, Li4Ti5O12, β-Li2TiO3,38 and Ti2O3. All the spectra are normalized by the intensity of the Ti-M edge peak in part A.

with stable chemical composition.38 For the Li-K edge spectra of Li4Ti5O12, β-Li2TiO3, and the formed crystal in the surface layer (Figure 11A), the peak intensity of the Li-K edge should represent the quantity of lithium in each sample, while it is difficult to determine the absolute quantity due to an unclear thickness and dose amount for each sample. Thus we estimated the ratio of the lithium amount relative to the Ti amount, namely, the Li/Ti composition ratio, via normalizing spectrum peaks by the intensity of Ti-M peak around 47 eV. First, we examined the normalized intensity of the Li-K edge peak at 58.5

Figure 10. EELS spectra of Li-K, Ti-L, O-K, and F-K edge regions for the crystal region in the surface product layer. 12389

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eV for the reference crystal samples of β-Li2TiO3 and Li4Ti5O12. The ratio of the normalized Li-K edge peak intensities of the two crystals is about 2.48, which is rather close to the ratio of the ideal Li/Ti composition ratios for the two crystals, namely, 2.5 from the formal Li/Ti values of 2.0 and 0.8 for β-Li2TiO3 and Li4Ti5O12. Thus, the present method to identify the composition ratio between Li and Ti is rather reliable. Then we have made a similar comparison between the formed crystal in the surface layer and Li4Ti5O12 and between the formed crystal and β-Li2TiO3. The ratio of the normalized Li-K edge peaks is about 2.6 and 1.1 for the two cases, respectively. This means that the Li/Ti value of the formed crystal should be 2.08 by 2.6 × 0.8, or 2.2 by 1.1 × 2.0. From the present argument, it can be said that the Li/Ti value of the formed crystal should be close to 2. This supports α-Li2TiO3 as the formed crystal. As for another candidate LiTiO2, the Li/Ti composition ratio is formally 1.0, which is too far from the present estimation by the normalized Li-K edge peak intensity. The feature of the Ti-L edge spectrum remarkably depends on the valence states of Ti as Ti4+ and Ti3+.39,40 This point should be also used to identify the formed crystal in the surface layer. Figure 11B shows the comparison of the Ti-L edge spectra among the formed crystal, Li4Ti5O12, β-Li2TiO3, and Ti2O3. It is clear that the Ti-L edge spectra of Ti3+ (Ti2O3) and Ti4+ (Li4Ti5O12 and β-Li2TiO3) have different features. The spectrum of Ti4+ has clearly four peaks, whereas that of Ti3+ has two peaks. The spectrum acquired from the formed crystal in the surface layer shows four peaks at the same energy positions as Ti4+ (Li4Ti5O12 and β-Li2TiO3), indicating that the main Ti state should be Ti4+ in the formed crystal. Here, as for LiTiO2 as another candidate, it was examined that the Ti-L edge spectrum has the feature of Ti3+,41 which is not consistent with the present observed spectrum of the formed crystal. In this way, we can conclude that the formed crystal in the surface layer is α-Li2TiO3 within the candidates of cubic rock-salt lithium titanate. Of course, we cannot conclude that all of the formed crystals, including the disordered regions, in the surface layer are α-Li2TiO3, but we can say that α-Li2TiO3 crystals are surely formed epitaxially in the surface product layer after the first charge−discharge cycle. In regards to the reason of the epitaxial growth, first, both the rock-salt and spinel structures belong to a cubic structure, and second, lattice constants of these two crystals have simple integer multiple relationships to each other, which should be beneficial to the epitaxial growth. In regards to the formation mechanism of α-Li2TiO3, the lithium insertion reaction in the bulk Li4Ti5O12 is expressed as follows: Li4Ti5O12 + 3Li+ + 3e− → Li 7Ti5O12

Li 7Ti5O12 + 3Li 2O → 5Li 2TiO3 + 3Li+ + 3e−

In this case, the pre-existence of O2− combined with Li+, expressed as Li2O in eq 4, near the lithiated surface is also essential. The formation of O2‑ from the oxygen molecule in the electrolyte should occur in the lithium insertion process as mentioned above. Then Li2TiO3 is formed associated with the lithium extraction during the lithium extraction process. At the least, the reduction of neutral oxygen molecules is essential to generate a compound with a higher value of the O/Ti ratio as Li2TiO3 compared to Li4Ti5O12 or Li7Ti5O12. Also there are two possibilities of the formation of Li2TiO3 layers in the lithium insertion process and in the extraction process. Of course, at present, we cannot make any definite conclusion on the formation mechanism within the present experimental results. It is interesting to investigate how the surface product layers containing epitaxial α-Li2TiO3 crystal affect the electric transport properties at the electrolyte/electrode interface after the first lithium extraction cycle. As mentioned above, the stable cycle process from the second cycle may be associated with the lithium-ion transport through the formed layer. This suggests the possibility that the formed surface layer may work effectively as a so-called SEI for the electrode performance of Li4Ti5O12, although the detailed analysis of this issue is beyond the scope of the present study. In any case, the surface product layer seems to contribute to the reduction of the charge-transfer resistance as examined by the electrochemical impedance measurement in Figure 7. There is a possibility that the generated surface layer could contribute to the enhanced lithium-ion diffusion at the electrolyte/electrode interface. Actually, it is known that the distribution of Li and Ti ions on cation sites is a random occupation in the rock-salt structure,36,43 which may lead to the increase of lithium-ion diffusion rate because of its site flexibility. In order to clarify the detailed mechanism of the surface structural changes during the first lithium insertion and extraction processes and to clarify the effects of such surface structural changes on the electrochemical properties, we have to add more TEM observations, combined with the SPM observations, especially for the initial structural changes in the early stage of the lithium insertion, all of which will be examined in the near future.

4. CONCLUSION Surface structural changes of the spinel Li4Ti5O12(111) surface, having atomistic flatness originally, during the first lithium insertion process and after the first lithium insertion-extraction cycle were observed by AFM and TEM. The AFM observation revealed that the surface structural changes occur irreversibly in the early stage of the first lithium insertion. There is the correlation between the increase in the surface roughness and the reduction of the electrolyte/electrode interface resistance. In the surface layer produced by the first lithium insertionextraction cycle, the high-resolution TEM observation revealed that cubic rock-salt crystal layers are epitaxially formed on the Li4Ti5O12 surface with a half-size lattice parameter compared to the bulk Li4Ti5O12. The EELS observation clarified that the formed crystal should be α-Li2TiO3. Because of the stable charge−discharge process after the second cycle, there is a possibility that the formed surface layer in the first cycle should be beneficial to stable Li-ion transport at the electrolyte/ electrode interface, just as SEI for Li4Ti5O12.

(2)

This involves the reduction of 60% of Ti4+ into Ti3+, which was previously detected also at the surface.42 As a possible reaction to form α-Li2TiO3 during the lithium insertion process, the following can be considered: Li4Ti5O12 + 6Li+ + 6e− + 3/2O2 → 5Li 2TiO3

(4)

(3)

Here, the presence of neutral oxygen molecules in the electrolyte is essential, and the reduction of oxygen molecules at the electrode surface should occur, because of its higher redox potential than Ti4+. Then the presence of O2− and Li+ should lead to the formation of Li2TiO3. On the other hand, we do not deny the possible formation of α-Li2TiO3 during the lithium extraction process as follows: 12390

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Kentaro Kuratani and Dr. Masaru Yao for electrochemical experiments and valuable discussions. This work was supported by the Japan Society for the Promotion of Science (JSPS Grant-in-Aid for Scientific Research (B) Grant 22360276)

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