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Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors
Author’s Accepted Manuscript Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors Tsukasa Hirayama, Yuka Aizawa, Kazuo Yamamoto, Takeshi Sato, Hidekazu Murata, Ryuji Yoshida, Craig A.J. Fisher, Takehisa Kato, Yasutoshi Iriyama
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To appear in: Ultramicroscopy Received date: 30 August 2016 Revised date: 8 November 2016 Accepted date: 13 November 2016 Cite this article as: Tsukasa Hirayama, Yuka Aizawa, Kazuo Yamamoto, Takeshi Sato, Hidekazu Murata, Ryuji Yoshida, Craig A.J. Fisher, Takehisa Kato and Yasutoshi Iriyama, Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors, Ultramicroscopy, http://dx.doi.org/10.1016/j.ultramic.2016.11.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors Tsukasa Hirayamaa,*, Yuka Aizawaa1, Kazuo Yamamotoa, Takeshi Satoa1, Hidekazu Muratab, Ryuji Yoshidaa, Craig A. J. Fishera, Takehisa Katoc2, Yasutoshi Iriyamac a
Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, Aichi, 456-8587, Japan. b
Faculty of Science and Technology, Meijo University, Shiogamaguchi, Tempaku-ku, Nagoya, Aichi, 468-8502, Japan
1-501
c
Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601, Japan
*Corresponding author: Tsukasa Hirayama, Tel: +81-52-871-3500, Fax: +81-52-871-3599. Email: [email protected]
1 2
now with Hitachi High-Technologies Corporation now with TDK Corporation.
Abstract Advanced techniques for overcoming problems encountered during in situ electron holography experiments in which a voltage is applied to an ionic conductor are reported. The three major problems encountered were 1) electric-field leakage from the specimen and its effect on phase images, 2) high electron conductivity of damage layers formed by the focused ion beam method, and 3) chemical reaction of the specimen with air. The first problem was overcome by comparing experimental phase distributions with simulated images in which three-dimensional leakage fields were taken into account, the second by removing the damage layers using a low-energy narrow Ar ion beam, and the third by developing an air-tight biasing specimen holder. Keywords: electron holography; lithium ion conductor; electric-field leakage; 3D-boundary-charge method; focused ion beam damage layers; chemical stability in air
1. Introduction Of the several extant battery technologies, lithium-ion batteries are the most advanced and practical because they can provide the largest energy storage densities [1-4]. Despite the success of Li-ion batteries, more advanced rechargeable batteries with higher energy densities, lower cost, and greater reliability and safety are urgently required. Commercial Li-ion batteries use a combustible liquid electrolyte, and several undesirable side reactions (dissolution of electrode materials, irreversible decomposition of solvents, etc.) at the electrode/electrolyte interface degrade battery performance. One strategy for developing the next generation of rechargeable batteries is to replace the combustible liquid electrolyte with a non-combustible inorganic solid electrolyte. Use of inorganic solid electrolytes simplifies battery packaging, reduces the side reactions, and minimizes the volume of the electrolyte in the battery cell. Batteries using such an inorganic solid electrolyte are termed all-solid-state Li-ion batteries (ASSLIBs).
For ASSLIBs to become practical, several problems need to be overcome. The main problem is low power density, which is partly due to the low ion conductivity of solid electrolytes. Several new types of solid electrolyte with higher Li-ion conductivities have recently been developed [5-6]. Nevertheless, the performance of all-solid-state batteries is not yet sufficient for use in, for example, commercial electric vehicles. Measuring electric potential distributions inside electrolytes would enable sources of resistance to be identified, because the driving force for ion migration in electrolytes is electric fields. Such data would contribute substantially to the development of more efficient batteries. Electron holography (EH) is a well-known transmission electron microscopy (TEM) technique for observing electric potential distributions in electronic components at the micrometer and nanometer scales [7-12]. In particular, in situ EH is a useful technique for revealing changes in potential distributions within nanoscale devices as they take place [13-16]. We have applied in situ EH to different lithium ion conductors [17-18], but three main problems have hampered progress. The first major problem was electric-field leakage from the specimen. When a voltage is applied to a specimen, electric fields are formed both inside and outside the specimen, with external electric field spreading three dimensionally around the specimen. Since the incident electrons pass through both the thin TEM specimen and these leakage fields, the detected phase image is a projection of the three-dimensional (3D) potential and is not just a representation of the potential distribution inside the specimen [19]. The second problem that needed to be overcome was the formation of damage layers on the surface of the specimen during focused ion beam (FIB) milling. These layers have high electron conductivity, resulting in a large leak current that alters the ion distributions within the electrolyte. The third main problem was chemical reaction of the specimen with air. In the course of our research, we have developed several advanced techniques and pieces of equipment to carry out in situ EH experiments of ion conducting materials successfully. These techniques and equipment are described in more detail in the sections below. Some results demonstrating the effectiveness of these methods are also presented.
2. Three-dimensional electric leakage fields and phase profiles EH enables electromagnetic fields to be observed directly not only inside but also outside the specimen (in the vacuum region). This feature becomes problematic when we observe a specimen that has a voltage applied to it. The effect of electric charging of specimens or optical components of a TEM on phase images was examined in the 1990s by Matteucci et al. [20] and Frost [21]. In this paper we examine the effect of three dimensional leakage fields generated by applying a voltage to a specimen on EH phase profiles. Figure 1 shows a cross-sectional schematic of a thin TEM specimen in which an electrolyte is sandwiched between two metals. In general, metals are good electronic conductors and the electric potential within the same kind of metal is constant. When a voltage is applied, an electric potential distribution is formed both inside and outside the specimen. The curved red broken lines in Fig. 1 represent equipotential lines outside the specimen. If we consider two points P1 and P2, it can be seen that the potential at P1 is higher than that at P2. Since the phase shift of an electron wave is proportional to the integral of the potential along the electron trajectory, a phase slope is detected for an electron wave passing through the metal region, even though the potential inside the metal is flat. Note that the detected phase distributions are projections of the three-dimensional (3D) potential, not just projections of the potential distribution inside the specimen. Our method for accurately determining potential distributions inside a specimen involves comparing the experimentally obtained phase distributions with simulated images generated for different conditions. To do this, many types of models of potential distributions inside the specimen are tested. The electric potential distributions outside the specimen for each of these models are then calculated, and 2D phase images generated. The model that provides the best match to the experimental phase is taken to represent the correct phase distribution within the specimen. When calculating the 3D potential distributions around the model sample, we use the 3D boundary charge method (3D-BCM) [22], which is commonly used for designing electron-optical systems used in transmission electron microscopes. Figure 2 illustrates the principle of the 3D-BCM
method. One surface (S) of the specimen is described in terms of its x, y, z coordinates, and divided into a large number of micro-areas (dS). When the charge density (R) at position vector R on the surface is constant in the corresponding micro-area dS, the potential (V (R0)) at an arbitrary point R0 can be calculated using the equation 𝑉 (𝐑 ) =
∬
(𝐑) |𝐑 𝐑 |
𝑑𝑆,
(1)
where ε0 is the vacuum permittivity. The (R) in each micro-area can be calculated in advance by substituting the given potential value (V(R)) applied to the electrodes. Using this method, a numerical accuracy of about 1 in 1014 can be attained. In our study, the surface plates of the simulation model were divided into more than ten thousands surface elements. After the 3D potential around the model is calculated, the phase map projected onto the x-y plane, (x,y), is calculated by integrating the potential value in the direction of the incident electrons (z direction) using [23]
(x,y) zV(x,y,z)dz E
, (2) where Δϕ is the phase shift measured by EH, λ is the wavelength of electrons, E is a constant value related to the acceleration voltage of the electrons, and V is the electric potential inside and outside the specimen. Test simulations were carried out to better understand the effect of leakage fields on the observed phase distribution. Figure 3(a) shows the configuration, shape and dimensions of the specimen model for the phase simulations. The specimen was modeled as a dielectric sandwiched between two electronically conducting electrodes, such as metal plates, because the electric potential distributions inside a dielectric are easily calculated, while those inside a solid-state ion conductor are unknown. One electrode was grounded, and a voltage of 1 V applied to the opposite electrode. The accelerating voltage of the incident electrons was set at 300 kV. The 3D leakage field was calculated using 3D-BCM, and 2D phase images generated by integrating the electric potential along the electron beam trajectory. Test simulations proved that the definite integral from +100 µm to -100 µm is close to the indefinite integral and provides sufficient accuracy for a discussion of phase distributions in our setup. The
effect of electric fields in the reference-wave region was also taken into account. Figures 3(b) and 3(c) show simulation results under two different conditions, the former in the absence of leakage electric fields, and the latter in the presence of an electric field. The dielectric constant (relative permittivity) of the dielectric was set at 11.6, which is the value for lithium phosphorus oxynitride (Li3.3PO3.8N0.22; LiPON) solid-electrolyte [24], in both cases. These simulations thus generate the phase distribution in a LiPON conductor when Li ions do not diffuse, even if a voltage is applied to the specimen. The phase profile in Fig. 3(b) shows that in the absence of leakage fields, the phase in the conductor regions is flat and that in the dielectric has an almost linear slope, as expected from the standard theory of dielectrics. In contrast, when both the electric potential distribution inside the specimen and the 3D leakage electric field around it are included, the phase lines are curved even in the electrode regions (Fig. 3(c)). This curvature is the effect of the leakage fields. In addition, surprisingly, the phase difference between the two conductor plates in Fig. 3(c) is more than 10 times larger than that in Fig. 3(b). This implies that the phase shift induced in the leakage field is much larger than that in the specimen, and the potential inside the specimen cannot be obtained by simply conducting an EH experiment on its own. We compared the experimental and simulated phase distributions according to the method described above to obtain electric potential distributions inside a lithium phosphorus oxynitride (Li3.3PO3.8N0.22; LiPON) solid electrolyte [19]. A TEM specimen of the configuration shown in Fig. 4 was prepared by FIB milling and narrow Ar-ion beam milling using the techniques described in section 3 and the air-tight biasing TEM holder described in section 4. The thickness of the thinned region was about 100 nm. The glassy-carbon (GC) side was kept grounded, and constant voltages, VCu-Cu, of -2, -1, 1, and 2 V were applied to the Cu electrode on the tungsten (W) side. The thin area enclosed by the dotted box in Fig. 4(a) was observed using an EH TEM (Hitachi HF3300EH) operated at 300 kV under a column vacuum of ~10-5 Pa. Holograms were recorded at each voltage and the phase images were reconstructed by the Fourier transformation method. To measure the potential change when a voltage was applied, the phase distribution of the short-circuited (0 V)
specimen was subtracted from that of the voltage-applied specimen. Electron beam irradiation damage to the specimen was deemed negligible because no difference between TEM images before and after the experiment could be detected, and the changes in phase distribution were consistent with changes in the applied voltages. In the simulations, a model of the Cu/LiPON/Cu sample with the same shape and size as shown in Fig. 4 was constructed. An example of the calculated potential profile inside the Cu/LiPON/Cu model in the direction parallel to the edge of the specimen is displayed in Fig. 5(a). To simplify the simulation, we assumed that the potentials in the Cu electrodes and the LiPON electrolyte were flat and that the electric double layer (EDL) had a linear rather than curved potential slope. The electric potential within the specimen was also assumed to be uniform in the direction of the electron beam. In the first simulation, the potential in the LiPON layer, VLiPON, was set at half the applied voltage VCu-Cu, and the width t of the EDL was set at 25 nm, as shown in Fig. 5(a). These values were then varied until the simulated phase distributions agreed with those obtained from experiment. The 3D potential distribution outside the specimen was calculated by the 3D boundary-charge method (3D-BCM), dividing all the surface plates of the simulation model into 26,050 surface elements, and the same potentials applied across the Cu electrodes, GC, and W components. Finally, 2D phase images were calculated by integrating the 3D potential distribution along the electron trajectory. Integration was carried out from +100 µm to -100 µm as in the case of the simulated profiles shown in Fig. 3. The effect of electric fields in the reference-wave region was also taken into account. Thin black and red lines in Fig. 5 (b) are the reconstructed phase profiles obtained from the dotted box region in Fig. 4. Thick green lines are the simulated phase profiles at different voltages when the potential in Cu and LiPON regions is flat, VLiPON is half of the applied voltage VCu-Cu, and the EDL width is 25 nm. In both the experimental and simulation profiles the phase distributions within Cu regions are sloped, even though flat potentials were assumed in the simulation. The gradient in the experimental profile is thus likely due to electric leakage fields above and beneath the specimen. Sloping of phase distribution curves is also seen in the LiPON region, presumably due to the same effect. It can also be noticed that the
simulated phase lines are slightly above the measured lines, implying that potentials assumed in the simulations (VLiPON) were higher than the actual potentials. Simulations were repeated with lower values of VLiPON until a better match with experiment was obtained. Fig. 5(c) shows a model with a lower VLiPON, and Fig. 5(d) compares the simulated phase profiles obtained using this model with the measured profiles. They are in much better agreement than those in Fig. 5(b). This suggests that the potential is less than half the applied voltage and that a larger potential change occurs at the interface on the high-voltage side, and a smaller change on the low-voltage side compared to the potential in the short-circuited specimen [19]. If the detected potential change around the interface is assumed to originate solely from the distribution of Li ions, Li-ion and Li-vacancy distributions in LiPON can be inferred. The relationship between changes in electric potential and the behavior of Li-ion and ion vacancies, as well as the width of the electric double layers, are discussed in a recent paper [19]. The good agreement between experimental and simulated phase distributions suggests that this method is a useful way of examining charge distributions in solid-state ion conductors. 3. Damage layers formed during FIB milling and narrow Ar-ion beam milling FIB milling is a powerful and widely used method for preparing TEM specimens. However, FIB causes damage to the surface of the specimen because of the high energy of the Ga ion beam used. Earlier work has shown that phase images of semiconductors are improved by in situ annealing [25]. In the case of ion conductors, these damage layers have high electronic conductivity, which interferes with the formation of the “true” potential distribution inside the specimen under an applied voltage. An effective method for removing damage layers using a low-voltage Ar ion beam has been reported, for example, for dopant profiling in compound semiconductor devices [26]. In this method, thin Al foil is attached to a Cu plate using epoxy resin. Next, a small cross-sectional specimen is extracted from the test sample using an FIB micro-sampling technique and fixed to the cross-sections of the thin Al foil by W deposition.
After the specimen is thinned in the FIB system, it is transferred (in air) to an Ar ion milling system (for example, Gatan PIPS Model 691), and both the top and bottom surfaces of the specimen are milled using a low energy Ar ion beam. The thin Al foil serves to prevent re-deposition of matter typically produced by Ar ion beam sputtering over a wide area. It is important that the specimen be small (usually on the scale of a few tens of microns) so as to prevent large-area sputtering by the Ar ion beam. Although clean specimens without damage layers can be prepared using this method, for in situ measurements, micro-sampling is not suitable, as the thin region needs to remain in contact with the specimen bulk to which the voltage is applied across metal electrodes. We therefore developed a method to remove the FIB damage layers from the surfaces of a thin film region still connected to the bulk ceramic and its two metal electrode plates. Figure 6 shows a schematic of the setup used for removing damage layers from our specimens. It should be noted that the FIB damage layers lie only within a small region around the thin film section to be observed, which is usually only several tens of microns or less. In this setup, a tungsten plate with a small hole around 300 ~ 400 m in diameter is placed between the Ar ion gun and the specimen. The damage layers are removed by the low-energy narrow Ar ion beam passing through the small hole in the tungsten plate. This setup enables us to selectively remove the damage layers around the region of observation and minimize the amount of sputter coming from the surrounding material, i.e., the bulk of the dielectric, electrodes and metal coatings on the insulator plates (deposited to prevent electric charging of the plates by the incident electron beam). We used this narrow Ar ion beam technique to prepare a cross-sectional W/Cu/LiPON/Cu/GC specimen thinned with an FIB milling system (Hitachi High-Technologies NB 5000) under a column vacuum of ~10-4 Pa [19]. The DC electron-stream resistance of the whole specimen measured before forming a thin region by FIB was about 10 M. The resistance after FIB milling was about 10 , indicating that the electrical conductivity of the FIB damage layer was very high. However, this resistance returned to about 10 M after the damage layers were removed using the narrow Ar ion beam method with an accelerating voltage of 300 V at room temperature (Gentle Mill Hi, Technoorg Linda Ltd Co., Hungary). The
electronic conductivity of any residual damage layers on the specimen was treated as infinitely small and ignored in the in situ EH experiments and simulations. 4. Reactions with air and the air-tight biasing TEM holder Exposing the specimen to air when transferring it from the FIB system to the Ar ion milling system and from the Ar ion milling system to the EH microscope is problematic because many Li-ion conductors are chemically active, readily reacting with oxygen or water vapor in air. To avoid this problem we developed an air-tight biasing TEM holder that enables the specimen to be kept under vacuum during transfer. This holder is shown schematically in Fig. 7, and contains all the components to be milled using the methods described in the previous section. In this setup, the specimen is electrically connected to two electrodes fixed onto holder plates, and enclosed in an air-tight metal cylinder during transfer. Opening the holder while inserted in the FIB chamber allows a thin observation region to be produced by FIB milling (Fig. 7(a)). To transfer the specimen to the Ar ion mill, the head of the holder is slid into the metal cylinder, sealing it under vacuum (Fig 7(b)). The holder is then removed from the FIB mill and inserted into the Ar ion mill, where the head of the holder is slid out of the metal cylinder, and the specimen milled using the low-energy narrow Ar ion beam method, still under vacuum. After removing the damage layers, the holder is again closed and brought to the EH microscope, where it is inserted, and the head of the holder slid open. Various voltages can then be applied to the specimen using a voltammeter, and in situ EH experiments carried out. The air-tightness of the holder was verified by taking readings from the vacuum gauges attached to the FIB system and the Ar-beam mill. These indicated that the vacuum of around 10-4 Pa remained unchanged when the head of the specimen holder was opened and closed within their respective chambers, confirming that there was essentially no leakage during transfer in. The specimen holder was developed to be general purpose, and can be used for observing a range of different materials and nanodevices in addition to ion conductors in the same way as more conventional specimen
holders. The maximum specimen size the holder can accommodate is about 3 mm. 5. Other problems and their solutions A number of other problems often arise during in situ EH observation. One unavoidable problem is that the observed region of the TEM specimens is a thin film rather than the bulk form of the device. While there is no evidence that the behavior of the thin film is fundamentally different to that of the bulk system, there is also no evidence that it is identical. Observation of thicker specimens using a high voltage electron microscope might be possible, but the electron beam irradiation damage in this case would be more serious. At present, we know of no way to overcome this problem directly. Development of theoretical calculations or computer simulation methods that are able to predict behavior of the bulk system from thin film measurements are a possible way forward. The difference in mean inner potential across the interface between two materials is also a serious problem. For example, in the case of the Cu/LiPON/Cu sample reported here, there is a potential drop at the interface between Cu and LiPON, even when the two Cu electrodes of the specimen are short-circuited. The mean inner potential drop is not important in terms of the observed electrochemical behavior because the potential change when a voltage is applied is the driving force for Li-ion transport. To measure this potential change, we subtracted the phase distribution of the short-circuited (0 V) specimen from that of the specimen with the voltage applied. The reported data thus correspond to the potential change upon application of the voltage rather than the absolute potentials [19]. Another problem that needed to be overcome to perform this study successfully was charging of the electrolyte due to secondary electron emission from the electron beam irradiation. The effect of charging can be canceled out by subtracting the phase distribution of the short-circuited specimen from that of the specimen with a voltage applied to it, which is the method we used. Ideally, however, specimens that do not become electrically charged even under electron beam irradiation should be used. Electron beam damage is another problem that needs to be considered.
EH experiments, however, are usually carried out using a weak electron beam (less than 1/100th of that used for standard low magnification TEM observation) to obtain high spatial coherence of the incident electron wave; consequently specimen damage appeared to be negligible in our case and no difference between the TEM images of the Cu/LiPON/Cu specimen before and after EH measurements was detected. Measured phase changes were also consistent with the changes in applied voltage [19]. Lastly, when the specimen is a polycrystal, its phase image suffers from complicated phase noise produced by electron diffraction, and the quality of the phase images deteriorates as a result. The only way to avoid this problem is to use an amorphous material, as we did in the case of amorphous LiPON [19]. 6. Conclusions Several problems for in situ EH experiments, such as the effect of electric-field leakage from the specimen onto phase images, high electron conductivity of damage layers formed during focused ion beam milling, and chemical reaction of the specimen with air, were described, and the methods we developed for overcoming them, namely comparison of experimental and simulated phase distributions in which three-dimensional leakage fields are taken into account, removal of damage layers using a low-energy narrow Ar ion beam, and development of an air-tight biasing TEM holder, respectively, were presented. These advanced techniques are effective and indispensable for the successful carrying out of in situ EH experiments in which voltage is applied to a solid-state ion conductor.
Acknowledgments Part of this work was supported by the RISING project of the New Energy and Industrial Technology Development Organization (NEDO) in Japan. We thank Prof. Z. Ogumi, Prof. T. Abe, Prof. Y. Uchimoto, and Prof. Y. Ukyo of Kyoto University for their valuable suggestions. We also thank Ms. M. Takatsuji of Japan Fine Ceramics Center for technical assistance.
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Figure captions Fig. 1. Cross-sectional schematic of a thin TEM specimen. The electrolyte is sandwiched between two metals. Curved red broken lines are equipotential lines formed outside the specimen. The potential at P1 is higher than that at P2. Since the phase shift of an electron wave is proportional to the integral of the potential along the electron beam trajectory, a phase slope is observed when the electron wave passes through the metal region, even though the potential inside the metal is flat. Fig. 2. Principle of the 3D boundary charge method (3D-BCM) commonly used for designing electron-optical systems used in transmission electron microscopes. A numerical accuracy of about 1 in 1014 is attained. In our study, the surface plates in the simulation model were divided into more than ten thousand surface elements. Fig. 3. Examples of phase simulations under different conditions. (a) Schematic of the model used in the simulations, in which a dielectric substance is sandwiched between two electronic conductors such as metals. The conductor on one side is grounded, and a voltage of 1 V applied to that on the other side. (b) Simulated phase distribution in the absence of a leakage electric field, showing the flat phase in the conductor regions and almost linear slope in the dielectric region. (c) Simulated phase distribution in the presence of a leakage electric field, showing bending of the phase lines in the conductor regions. The effect of electric fields in the reference-wave region is taken into account in both (b) and (c). The phase difference between the two conductor plates in (c) is more than 10 times larger than that in (b). Fig. 4. Schematic of the specimen used in in situ electron holography experiments. The thin region for observation was prepared by FIB milling, and the damage layers on the surface were carefully removed with a low-energy (0.3 kV) Ar ion beam. Fig. 5. Comparison of experimental and simulated phase distributions. (a) Schematic of potential distribution assuming the potential in LiPON
(VLiPON) is half the applied voltage (VCu-Cu). (b) Experimental and simulated phase distributions obtained using model (a). (c) Potential distribution in which VLiPON is 0.1 to 0.2 V less than 1/2 VCu-Cu. (d) Experimental and simulated phase distributions using model (c). Fig. 6. Schematic of the setup to remove damage layers after FIB milling. A tungsten plate with a small hole at its center was placed between the Ar ion gun and specimen. Damage layers were removed by a low-energy narrow Ar ion beam passing through the hole in the tungsten plate. Fig. 7. Schematic of the air-tight biasing TEM holder. The specimen is electrically connected to two electrodes fixed on holder plates. After a thin observation region is fabricated in the FIB system with the holder held open as shown in (a), the head of the holder is mechanically slid into the metal cylinder to keep the specimen under vacuum, as shown in (b). The holder can then be transferred from the FIB system to the Ar ion milling system and then from the Ar ion beam system to the EH microscope without exposing the specimen to air.
Highlights 1. Phase distributions derived by comparing experimental and simulated measurements. 2. Simulations take into account leakage electric fields. 3. Electric potential distributions inside Li-ion conductors are obtained. 4. FIB damage layers are removed using a low-energy narrow Ar ion beam. 5. An air-tight biasing TEM holder for protecting air-sensitive specimens is reported.
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S0304-3991(16)30335-7 http://dx.doi.org/10.1016/j.ultramic.2016.11.019 ULTRAM12249
To appear in: Ultramicroscopy Received date: 30 August 2016 Revised date: 8 November 2016 Accepted date: 13 November 2016 Cite this article as: Tsukasa Hirayama, Yuka Aizawa, Kazuo Yamamoto, Takeshi Sato, Hidekazu Murata, Ryuji Yoshida, Craig A.J. Fisher, Takehisa Kato and Yasutoshi Iriyama, Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors, Ultramicroscopy, http://dx.doi.org/10.1016/j.ultramic.2016.11.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors Tsukasa Hirayamaa,*, Yuka Aizawaa1, Kazuo Yamamotoa, Takeshi Satoa1, Hidekazu Muratab, Ryuji Yoshidaa, Craig A. J. Fishera, Takehisa Katoc2, Yasutoshi Iriyamac a
Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, Aichi, 456-8587, Japan. b
Faculty of Science and Technology, Meijo University, Shiogamaguchi, Tempaku-ku, Nagoya, Aichi, 468-8502, Japan
1-501
c
Department of Materials, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601, Japan
*Corresponding author: Tsukasa Hirayama, Tel: +81-52-871-3500, Fax: +81-52-871-3599. Email: [email protected]
1 2
now with Hitachi High-Technologies Corporation now with TDK Corporation.
Abstract Advanced techniques for overcoming problems encountered during in situ electron holography experiments in which a voltage is applied to an ionic conductor are reported. The three major problems encountered were 1) electric-field leakage from the specimen and its effect on phase images, 2) high electron conductivity of damage layers formed by the focused ion beam method, and 3) chemical reaction of the specimen with air. The first problem was overcome by comparing experimental phase distributions with simulated images in which three-dimensional leakage fields were taken into account, the second by removing the damage layers using a low-energy narrow Ar ion beam, and the third by developing an air-tight biasing specimen holder. Keywords: electron holography; lithium ion conductor; electric-field leakage; 3D-boundary-charge method; focused ion beam damage layers; chemical stability in air
1. Introduction Of the several extant battery technologies, lithium-ion batteries are the most advanced and practical because they can provide the largest energy storage densities [1-4]. Despite the success of Li-ion batteries, more advanced rechargeable batteries with higher energy densities, lower cost, and greater reliability and safety are urgently required. Commercial Li-ion batteries use a combustible liquid electrolyte, and several undesirable side reactions (dissolution of electrode materials, irreversible decomposition of solvents, etc.) at the electrode/electrolyte interface degrade battery performance. One strategy for developing the next generation of rechargeable batteries is to replace the combustible liquid electrolyte with a non-combustible inorganic solid electrolyte. Use of inorganic solid electrolytes simplifies battery packaging, reduces the side reactions, and minimizes the volume of the electrolyte in the battery cell. Batteries using such an inorganic solid electrolyte are termed all-solid-state Li-ion batteries (ASSLIBs).
For ASSLIBs to become practical, several problems need to be overcome. The main problem is low power density, which is partly due to the low ion conductivity of solid electrolytes. Several new types of solid electrolyte with higher Li-ion conductivities have recently been developed [5-6]. Nevertheless, the performance of all-solid-state batteries is not yet sufficient for use in, for example, commercial electric vehicles. Measuring electric potential distributions inside electrolytes would enable sources of resistance to be identified, because the driving force for ion migration in electrolytes is electric fields. Such data would contribute substantially to the development of more efficient batteries. Electron holography (EH) is a well-known transmission electron microscopy (TEM) technique for observing electric potential distributions in electronic components at the micrometer and nanometer scales [7-12]. In particular, in situ EH is a useful technique for revealing changes in potential distributions within nanoscale devices as they take place [13-16]. We have applied in situ EH to different lithium ion conductors [17-18], but three main problems have hampered progress. The first major problem was electric-field leakage from the specimen. When a voltage is applied to a specimen, electric fields are formed both inside and outside the specimen, with external electric field spreading three dimensionally around the specimen. Since the incident electrons pass through both the thin TEM specimen and these leakage fields, the detected phase image is a projection of the three-dimensional (3D) potential and is not just a representation of the potential distribution inside the specimen [19]. The second problem that needed to be overcome was the formation of damage layers on the surface of the specimen during focused ion beam (FIB) milling. These layers have high electron conductivity, resulting in a large leak current that alters the ion distributions within the electrolyte. The third main problem was chemical reaction of the specimen with air. In the course of our research, we have developed several advanced techniques and pieces of equipment to carry out in situ EH experiments of ion conducting materials successfully. These techniques and equipment are described in more detail in the sections below. Some results demonstrating the effectiveness of these methods are also presented.
2. Three-dimensional electric leakage fields and phase profiles EH enables electromagnetic fields to be observed directly not only inside but also outside the specimen (in the vacuum region). This feature becomes problematic when we observe a specimen that has a voltage applied to it. The effect of electric charging of specimens or optical components of a TEM on phase images was examined in the 1990s by Matteucci et al. [20] and Frost [21]. In this paper we examine the effect of three dimensional leakage fields generated by applying a voltage to a specimen on EH phase profiles. Figure 1 shows a cross-sectional schematic of a thin TEM specimen in which an electrolyte is sandwiched between two metals. In general, metals are good electronic conductors and the electric potential within the same kind of metal is constant. When a voltage is applied, an electric potential distribution is formed both inside and outside the specimen. The curved red broken lines in Fig. 1 represent equipotential lines outside the specimen. If we consider two points P1 and P2, it can be seen that the potential at P1 is higher than that at P2. Since the phase shift of an electron wave is proportional to the integral of the potential along the electron trajectory, a phase slope is detected for an electron wave passing through the metal region, even though the potential inside the metal is flat. Note that the detected phase distributions are projections of the three-dimensional (3D) potential, not just projections of the potential distribution inside the specimen. Our method for accurately determining potential distributions inside a specimen involves comparing the experimentally obtained phase distributions with simulated images generated for different conditions. To do this, many types of models of potential distributions inside the specimen are tested. The electric potential distributions outside the specimen for each of these models are then calculated, and 2D phase images generated. The model that provides the best match to the experimental phase is taken to represent the correct phase distribution within the specimen. When calculating the 3D potential distributions around the model sample, we use the 3D boundary charge method (3D-BCM) [22], which is commonly used for designing electron-optical systems used in transmission electron microscopes. Figure 2 illustrates the principle of the 3D-BCM
method. One surface (S) of the specimen is described in terms of its x, y, z coordinates, and divided into a large number of micro-areas (dS). When the charge density (R) at position vector R on the surface is constant in the corresponding micro-area dS, the potential (V (R0)) at an arbitrary point R0 can be calculated using the equation 𝑉 (𝐑 ) =
∬
(𝐑) |𝐑 𝐑 |
𝑑𝑆,
(1)
where ε0 is the vacuum permittivity. The (R) in each micro-area can be calculated in advance by substituting the given potential value (V(R)) applied to the electrodes. Using this method, a numerical accuracy of about 1 in 1014 can be attained. In our study, the surface plates of the simulation model were divided into more than ten thousands surface elements. After the 3D potential around the model is calculated, the phase map projected onto the x-y plane, (x,y), is calculated by integrating the potential value in the direction of the incident electrons (z direction) using [23]
(x,y) zV(x,y,z)dz E
, (2) where Δϕ is the phase shift measured by EH, λ is the wavelength of electrons, E is a constant value related to the acceleration voltage of the electrons, and V is the electric potential inside and outside the specimen. Test simulations were carried out to better understand the effect of leakage fields on the observed phase distribution. Figure 3(a) shows the configuration, shape and dimensions of the specimen model for the phase simulations. The specimen was modeled as a dielectric sandwiched between two electronically conducting electrodes, such as metal plates, because the electric potential distributions inside a dielectric are easily calculated, while those inside a solid-state ion conductor are unknown. One electrode was grounded, and a voltage of 1 V applied to the opposite electrode. The accelerating voltage of the incident electrons was set at 300 kV. The 3D leakage field was calculated using 3D-BCM, and 2D phase images generated by integrating the electric potential along the electron beam trajectory. Test simulations proved that the definite integral from +100 µm to -100 µm is close to the indefinite integral and provides sufficient accuracy for a discussion of phase distributions in our setup. The
effect of electric fields in the reference-wave region was also taken into account. Figures 3(b) and 3(c) show simulation results under two different conditions, the former in the absence of leakage electric fields, and the latter in the presence of an electric field. The dielectric constant (relative permittivity) of the dielectric was set at 11.6, which is the value for lithium phosphorus oxynitride (Li3.3PO3.8N0.22; LiPON) solid-electrolyte [24], in both cases. These simulations thus generate the phase distribution in a LiPON conductor when Li ions do not diffuse, even if a voltage is applied to the specimen. The phase profile in Fig. 3(b) shows that in the absence of leakage fields, the phase in the conductor regions is flat and that in the dielectric has an almost linear slope, as expected from the standard theory of dielectrics. In contrast, when both the electric potential distribution inside the specimen and the 3D leakage electric field around it are included, the phase lines are curved even in the electrode regions (Fig. 3(c)). This curvature is the effect of the leakage fields. In addition, surprisingly, the phase difference between the two conductor plates in Fig. 3(c) is more than 10 times larger than that in Fig. 3(b). This implies that the phase shift induced in the leakage field is much larger than that in the specimen, and the potential inside the specimen cannot be obtained by simply conducting an EH experiment on its own. We compared the experimental and simulated phase distributions according to the method described above to obtain electric potential distributions inside a lithium phosphorus oxynitride (Li3.3PO3.8N0.22; LiPON) solid electrolyte [19]. A TEM specimen of the configuration shown in Fig. 4 was prepared by FIB milling and narrow Ar-ion beam milling using the techniques described in section 3 and the air-tight biasing TEM holder described in section 4. The thickness of the thinned region was about 100 nm. The glassy-carbon (GC) side was kept grounded, and constant voltages, VCu-Cu, of -2, -1, 1, and 2 V were applied to the Cu electrode on the tungsten (W) side. The thin area enclosed by the dotted box in Fig. 4(a) was observed using an EH TEM (Hitachi HF3300EH) operated at 300 kV under a column vacuum of ~10-5 Pa. Holograms were recorded at each voltage and the phase images were reconstructed by the Fourier transformation method. To measure the potential change when a voltage was applied, the phase distribution of the short-circuited (0 V)
specimen was subtracted from that of the voltage-applied specimen. Electron beam irradiation damage to the specimen was deemed negligible because no difference between TEM images before and after the experiment could be detected, and the changes in phase distribution were consistent with changes in the applied voltages. In the simulations, a model of the Cu/LiPON/Cu sample with the same shape and size as shown in Fig. 4 was constructed. An example of the calculated potential profile inside the Cu/LiPON/Cu model in the direction parallel to the edge of the specimen is displayed in Fig. 5(a). To simplify the simulation, we assumed that the potentials in the Cu electrodes and the LiPON electrolyte were flat and that the electric double layer (EDL) had a linear rather than curved potential slope. The electric potential within the specimen was also assumed to be uniform in the direction of the electron beam. In the first simulation, the potential in the LiPON layer, VLiPON, was set at half the applied voltage VCu-Cu, and the width t of the EDL was set at 25 nm, as shown in Fig. 5(a). These values were then varied until the simulated phase distributions agreed with those obtained from experiment. The 3D potential distribution outside the specimen was calculated by the 3D boundary-charge method (3D-BCM), dividing all the surface plates of the simulation model into 26,050 surface elements, and the same potentials applied across the Cu electrodes, GC, and W components. Finally, 2D phase images were calculated by integrating the 3D potential distribution along the electron trajectory. Integration was carried out from +100 µm to -100 µm as in the case of the simulated profiles shown in Fig. 3. The effect of electric fields in the reference-wave region was also taken into account. Thin black and red lines in Fig. 5 (b) are the reconstructed phase profiles obtained from the dotted box region in Fig. 4. Thick green lines are the simulated phase profiles at different voltages when the potential in Cu and LiPON regions is flat, VLiPON is half of the applied voltage VCu-Cu, and the EDL width is 25 nm. In both the experimental and simulation profiles the phase distributions within Cu regions are sloped, even though flat potentials were assumed in the simulation. The gradient in the experimental profile is thus likely due to electric leakage fields above and beneath the specimen. Sloping of phase distribution curves is also seen in the LiPON region, presumably due to the same effect. It can also be noticed that the
simulated phase lines are slightly above the measured lines, implying that potentials assumed in the simulations (VLiPON) were higher than the actual potentials. Simulations were repeated with lower values of VLiPON until a better match with experiment was obtained. Fig. 5(c) shows a model with a lower VLiPON, and Fig. 5(d) compares the simulated phase profiles obtained using this model with the measured profiles. They are in much better agreement than those in Fig. 5(b). This suggests that the potential is less than half the applied voltage and that a larger potential change occurs at the interface on the high-voltage side, and a smaller change on the low-voltage side compared to the potential in the short-circuited specimen [19]. If the detected potential change around the interface is assumed to originate solely from the distribution of Li ions, Li-ion and Li-vacancy distributions in LiPON can be inferred. The relationship between changes in electric potential and the behavior of Li-ion and ion vacancies, as well as the width of the electric double layers, are discussed in a recent paper [19]. The good agreement between experimental and simulated phase distributions suggests that this method is a useful way of examining charge distributions in solid-state ion conductors. 3. Damage layers formed during FIB milling and narrow Ar-ion beam milling FIB milling is a powerful and widely used method for preparing TEM specimens. However, FIB causes damage to the surface of the specimen because of the high energy of the Ga ion beam used. Earlier work has shown that phase images of semiconductors are improved by in situ annealing [25]. In the case of ion conductors, these damage layers have high electronic conductivity, which interferes with the formation of the “true” potential distribution inside the specimen under an applied voltage. An effective method for removing damage layers using a low-voltage Ar ion beam has been reported, for example, for dopant profiling in compound semiconductor devices [26]. In this method, thin Al foil is attached to a Cu plate using epoxy resin. Next, a small cross-sectional specimen is extracted from the test sample using an FIB micro-sampling technique and fixed to the cross-sections of the thin Al foil by W deposition.
After the specimen is thinned in the FIB system, it is transferred (in air) to an Ar ion milling system (for example, Gatan PIPS Model 691), and both the top and bottom surfaces of the specimen are milled using a low energy Ar ion beam. The thin Al foil serves to prevent re-deposition of matter typically produced by Ar ion beam sputtering over a wide area. It is important that the specimen be small (usually on the scale of a few tens of microns) so as to prevent large-area sputtering by the Ar ion beam. Although clean specimens without damage layers can be prepared using this method, for in situ measurements, micro-sampling is not suitable, as the thin region needs to remain in contact with the specimen bulk to which the voltage is applied across metal electrodes. We therefore developed a method to remove the FIB damage layers from the surfaces of a thin film region still connected to the bulk ceramic and its two metal electrode plates. Figure 6 shows a schematic of the setup used for removing damage layers from our specimens. It should be noted that the FIB damage layers lie only within a small region around the thin film section to be observed, which is usually only several tens of microns or less. In this setup, a tungsten plate with a small hole around 300 ~ 400 m in diameter is placed between the Ar ion gun and the specimen. The damage layers are removed by the low-energy narrow Ar ion beam passing through the small hole in the tungsten plate. This setup enables us to selectively remove the damage layers around the region of observation and minimize the amount of sputter coming from the surrounding material, i.e., the bulk of the dielectric, electrodes and metal coatings on the insulator plates (deposited to prevent electric charging of the plates by the incident electron beam). We used this narrow Ar ion beam technique to prepare a cross-sectional W/Cu/LiPON/Cu/GC specimen thinned with an FIB milling system (Hitachi High-Technologies NB 5000) under a column vacuum of ~10-4 Pa [19]. The DC electron-stream resistance of the whole specimen measured before forming a thin region by FIB was about 10 M. The resistance after FIB milling was about 10 , indicating that the electrical conductivity of the FIB damage layer was very high. However, this resistance returned to about 10 M after the damage layers were removed using the narrow Ar ion beam method with an accelerating voltage of 300 V at room temperature (Gentle Mill Hi, Technoorg Linda Ltd Co., Hungary). The
electronic conductivity of any residual damage layers on the specimen was treated as infinitely small and ignored in the in situ EH experiments and simulations. 4. Reactions with air and the air-tight biasing TEM holder Exposing the specimen to air when transferring it from the FIB system to the Ar ion milling system and from the Ar ion milling system to the EH microscope is problematic because many Li-ion conductors are chemically active, readily reacting with oxygen or water vapor in air. To avoid this problem we developed an air-tight biasing TEM holder that enables the specimen to be kept under vacuum during transfer. This holder is shown schematically in Fig. 7, and contains all the components to be milled using the methods described in the previous section. In this setup, the specimen is electrically connected to two electrodes fixed onto holder plates, and enclosed in an air-tight metal cylinder during transfer. Opening the holder while inserted in the FIB chamber allows a thin observation region to be produced by FIB milling (Fig. 7(a)). To transfer the specimen to the Ar ion mill, the head of the holder is slid into the metal cylinder, sealing it under vacuum (Fig 7(b)). The holder is then removed from the FIB mill and inserted into the Ar ion mill, where the head of the holder is slid out of the metal cylinder, and the specimen milled using the low-energy narrow Ar ion beam method, still under vacuum. After removing the damage layers, the holder is again closed and brought to the EH microscope, where it is inserted, and the head of the holder slid open. Various voltages can then be applied to the specimen using a voltammeter, and in situ EH experiments carried out. The air-tightness of the holder was verified by taking readings from the vacuum gauges attached to the FIB system and the Ar-beam mill. These indicated that the vacuum of around 10-4 Pa remained unchanged when the head of the specimen holder was opened and closed within their respective chambers, confirming that there was essentially no leakage during transfer in. The specimen holder was developed to be general purpose, and can be used for observing a range of different materials and nanodevices in addition to ion conductors in the same way as more conventional specimen
holders. The maximum specimen size the holder can accommodate is about 3 mm. 5. Other problems and their solutions A number of other problems often arise during in situ EH observation. One unavoidable problem is that the observed region of the TEM specimens is a thin film rather than the bulk form of the device. While there is no evidence that the behavior of the thin film is fundamentally different to that of the bulk system, there is also no evidence that it is identical. Observation of thicker specimens using a high voltage electron microscope might be possible, but the electron beam irradiation damage in this case would be more serious. At present, we know of no way to overcome this problem directly. Development of theoretical calculations or computer simulation methods that are able to predict behavior of the bulk system from thin film measurements are a possible way forward. The difference in mean inner potential across the interface between two materials is also a serious problem. For example, in the case of the Cu/LiPON/Cu sample reported here, there is a potential drop at the interface between Cu and LiPON, even when the two Cu electrodes of the specimen are short-circuited. The mean inner potential drop is not important in terms of the observed electrochemical behavior because the potential change when a voltage is applied is the driving force for Li-ion transport. To measure this potential change, we subtracted the phase distribution of the short-circuited (0 V) specimen from that of the specimen with the voltage applied. The reported data thus correspond to the potential change upon application of the voltage rather than the absolute potentials [19]. Another problem that needed to be overcome to perform this study successfully was charging of the electrolyte due to secondary electron emission from the electron beam irradiation. The effect of charging can be canceled out by subtracting the phase distribution of the short-circuited specimen from that of the specimen with a voltage applied to it, which is the method we used. Ideally, however, specimens that do not become electrically charged even under electron beam irradiation should be used. Electron beam damage is another problem that needs to be considered.
EH experiments, however, are usually carried out using a weak electron beam (less than 1/100th of that used for standard low magnification TEM observation) to obtain high spatial coherence of the incident electron wave; consequently specimen damage appeared to be negligible in our case and no difference between the TEM images of the Cu/LiPON/Cu specimen before and after EH measurements was detected. Measured phase changes were also consistent with the changes in applied voltage [19]. Lastly, when the specimen is a polycrystal, its phase image suffers from complicated phase noise produced by electron diffraction, and the quality of the phase images deteriorates as a result. The only way to avoid this problem is to use an amorphous material, as we did in the case of amorphous LiPON [19]. 6. Conclusions Several problems for in situ EH experiments, such as the effect of electric-field leakage from the specimen onto phase images, high electron conductivity of damage layers formed during focused ion beam milling, and chemical reaction of the specimen with air, were described, and the methods we developed for overcoming them, namely comparison of experimental and simulated phase distributions in which three-dimensional leakage fields are taken into account, removal of damage layers using a low-energy narrow Ar ion beam, and development of an air-tight biasing TEM holder, respectively, were presented. These advanced techniques are effective and indispensable for the successful carrying out of in situ EH experiments in which voltage is applied to a solid-state ion conductor.
Acknowledgments Part of this work was supported by the RISING project of the New Energy and Industrial Technology Development Organization (NEDO) in Japan. We thank Prof. Z. Ogumi, Prof. T. Abe, Prof. Y. Uchimoto, and Prof. Y. Ukyo of Kyoto University for their valuable suggestions. We also thank Ms. M. Takatsuji of Japan Fine Ceramics Center for technical assistance.
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Figure captions Fig. 1. Cross-sectional schematic of a thin TEM specimen. The electrolyte is sandwiched between two metals. Curved red broken lines are equipotential lines formed outside the specimen. The potential at P1 is higher than that at P2. Since the phase shift of an electron wave is proportional to the integral of the potential along the electron beam trajectory, a phase slope is observed when the electron wave passes through the metal region, even though the potential inside the metal is flat. Fig. 2. Principle of the 3D boundary charge method (3D-BCM) commonly used for designing electron-optical systems used in transmission electron microscopes. A numerical accuracy of about 1 in 1014 is attained. In our study, the surface plates in the simulation model were divided into more than ten thousand surface elements. Fig. 3. Examples of phase simulations under different conditions. (a) Schematic of the model used in the simulations, in which a dielectric substance is sandwiched between two electronic conductors such as metals. The conductor on one side is grounded, and a voltage of 1 V applied to that on the other side. (b) Simulated phase distribution in the absence of a leakage electric field, showing the flat phase in the conductor regions and almost linear slope in the dielectric region. (c) Simulated phase distribution in the presence of a leakage electric field, showing bending of the phase lines in the conductor regions. The effect of electric fields in the reference-wave region is taken into account in both (b) and (c). The phase difference between the two conductor plates in (c) is more than 10 times larger than that in (b). Fig. 4. Schematic of the specimen used in in situ electron holography experiments. The thin region for observation was prepared by FIB milling, and the damage layers on the surface were carefully removed with a low-energy (0.3 kV) Ar ion beam. Fig. 5. Comparison of experimental and simulated phase distributions. (a) Schematic of potential distribution assuming the potential in LiPON
(VLiPON) is half the applied voltage (VCu-Cu). (b) Experimental and simulated phase distributions obtained using model (a). (c) Potential distribution in which VLiPON is 0.1 to 0.2 V less than 1/2 VCu-Cu. (d) Experimental and simulated phase distributions using model (c). Fig. 6. Schematic of the setup to remove damage layers after FIB milling. A tungsten plate with a small hole at its center was placed between the Ar ion gun and specimen. Damage layers were removed by a low-energy narrow Ar ion beam passing through the hole in the tungsten plate. Fig. 7. Schematic of the air-tight biasing TEM holder. The specimen is electrically connected to two electrodes fixed on holder plates. After a thin observation region is fabricated in the FIB system with the holder held open as shown in (a), the head of the holder is mechanically slid into the metal cylinder to keep the specimen under vacuum, as shown in (b). The holder can then be transferred from the FIB system to the Ar ion milling system and then from the Ar ion beam system to the EH microscope without exposing the specimen to air.
Highlights 1. Phase distributions derived by comparing experimental and simulated measurements. 2. Simulations take into account leakage electric fields. 3. Electric potential distributions inside Li-ion conductors are obtained. 4. FIB damage layers are removed using a low-energy narrow Ar ion beam. 5. An air-tight biasing TEM holder for protecting air-sensitive specimens is reported.
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