Apparatus for evaluating the electrical conductivity distribution of electrodes and method for evaluating the electrical conductivity distribution of electrodes

Abstract

This invention provides an evaluation device for the electrical conductivity distribution of electrodes that can quickly and accurately visualize the electrical conductivity distribution of electrodes. [Solution] An apparatus 100 for evaluating the electrical conductivity distribution of an electrode, comprising: an electron beam irradiation system configured to irradiate an electrode with an electron beam 300 accelerated at a predetermined acceleration voltage; a first detector 107 for detecting secondary electrons; a second detector 106 for detecting backscattered electrons; and a control device 108 configured to provide a secondary electron image from the detected secondary electrons and a backscattered electron image from the detected backscattered electrons, wherein the predetermined acceleration voltage is 20V or more and 2kV or less.

Description

[Technical Field]

[0001] This disclosure relates to an apparatus for evaluating the electrical conductivity distribution of electrodes and a method for evaluating the electrical conductivity distribution of electrodes. [Background technology]

[0002] In composite-coated electrodes commonly used in lithium-ion batteries, the extent to which the battery active material in the composite contributes to the battery reaction is crucial. This quality is determined by the degree of electrical contact between the active material and the current collector foil, i.e., the distribution of electrical conductivity. Thus, it is necessary to evaluate the electrical properties of composite-coated electrodes. A basic method for electrode evaluation is to actually assemble a battery cell and perform various battery tests. This is an effective method for confirming the actual operation of the electrodes. On the other hand, prototyping lithium-ion battery cells, for example, requires a highly sophisticated non-aqueous environment, and various processes and assembly work are necessary to actually produce the cells, making the sophistication of equipment and the complexity of the work unavoidable. In particular, prototyping product-level battery cells such as cylindrical, prismatic, and laminated types requires enormous costs, even more so than prototyping coin cells at the laboratory level.

[0003] Furthermore, the information obtained from battery testing is the electrochemical average information of the entire battery, such as the cell potential profile and cell resistance values. It is fundamentally difficult to obtain localized information, such as the electrical properties of each active material in the composite coated electrode. Moreover, the information obtained from battery test data includes a great deal of information other than the quality of the electrode itself, such as the active material and the electrolyte. In general, it can be said that it is difficult to purely evaluate the electrical properties of the electrode itself through battery testing.

[0004] Scanning Spreading Resistance Microscopy (SSRM) is used as a method for locally evaluating the electrical properties of electrodes. This method involves tracing the electrode surface with an electrically conductive probe sharpened to the nanometer level and imaging the resistance value obtained. By applying a constant voltage between the probe and the current collector foil and imaging the value of the current flowing between the electrode surface and the probe point by point, the local electrical resistance value of the electrode surface can be visualized as a distribution map. In principle, this method makes it possible to directly evaluate the electrical conductivity of the sample and to quantitatively understand the electrical conductivity distribution of the electrode. However, obtaining reliable images requires measurement techniques such as slowing down the scanning speed and improving the flatness of the sample, making it difficult to perform rapid and simple evaluations. In typical SSRM measurements, the scanning range is several hundred μm, and it takes approximately several tens of minutes to acquire one image. Therefore, if we consider evaluating an electrode several centimeters square without any omissions, it would theoretically take about 100 days. Since practical electrodes range in size from tens of centimeters to several meters, it is impractical to evaluate the electrical conductivity distribution of electrodes using SSRM alone. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Probing electrical degradation of cathode materials for lithium-ion batteries with nanoscale resolution SY Park et al., Nano Energy 49 (2018) 1-6. [Non-Patent Document 2] Visualizing Local Electrical Properties of Composite Electrodes in Sulfide All-Solid-State Batteries by Scanning Probe Microscopy M. Otoyama et al., J. Phys. Chem. C 125 (2021) 2841-2849. Summary of the Invention Problems to be Solved by the Invention

[0006] With conventional evaluation methods, it is difficult to rapidly and widely evaluate the electrical conductivity distribution of electrodes, leading to a decrease in the efficiency of product quality inspection. To solve this problem, the present invention aims to provide a technique for rapidly visualizing the electrical conductivity distribution of electrodes using an electron beam and an automatic quality inspection apparatus applying the same.

[0007] The present disclosure aims to provide an evaluation apparatus for the electrical conductivity distribution of an electrode that can rapidly and highly accurately visualize the electrical conductivity distribution of the electrode. Means for Solving the Problems

[0008] To achieve the above object, an evaluation apparatus for the electrical conductivity distribution of an electrode in the present disclosure includes an electron beam irradiation system configured to irradiate the electrode with an electron beam accelerated at a predetermined acceleration voltage, a first detection device for detecting secondary electrons, a second detection device for detecting reflected electrons, and a control device configured to provide a secondary electron image from the detected secondary electrons and a reflected electron image from the detected reflected electrons, wherein the predetermined acceleration voltage is 20 V or more and 2 kV or less. Effects of the Invention

[0009] According to the evaluation apparatus for the electrical conductivity distribution of an electrode of the present disclosure, the electrical conductivity distribution of the electrode can be rapidly and highly accurately visualized. Brief Description of the Drawings

[0010] [Figure 1A] This is a schematic diagram illustrating the principle of backscattered electron emission when an electron beam is irradiated onto a group of particles with varying degrees of electrical conduction pathway quality. [Figure 1B] This is a schematic diagram illustrating the principle of secondary electron emission when electrically conductive and insulating particles are irradiated with an electron beam. [Figure 2] This is a diagram showing the configuration of an apparatus for evaluating the electrical conductivity distribution of electrodes according to an embodiment. [Figure 3] This is a diagram showing the configuration of a sample feeding device according to an embodiment. [Figure 4A] This is a schematic diagram showing the arrangement of the LTO working electrode and counter electrode and their current flux in a lithium storage reaction. [Figure 4B] This is a photograph of a sample electrode after disassembling a battery cell following a lithium storage reaction. [Figure 5A] This graph shows the charge-discharge curves when preparing sample electrodes using different current densities. [Figure 5B] These are photographs of the reaction results of sample electrodes prepared with different current densities. [Figure 5C] This graph shows the cell voltage profile against the reaction diameter. [Figure 6A] This figure shows a secondary electron image acquired at an extremely low acceleration voltage (500V). [Figure 6B] This figure shows the acceleration voltage dependence of secondary electron image contrast acquired under different acceleration voltage conditions (500V, 1kV, 2kV, 5kV). [Figure 7A] This figure shows secondary electron images and high-angle backscattered electron images acquired at extremely low acceleration (500V). [Figure 7B] This figure shows the dependence of the contrast of high-angle backscattered electron images on the acceleration voltage. [Modes for carrying out the invention]

[0011] The apparatus for evaluating the electrical conductivity distribution of electrodes according to this disclosure will be described below with reference to the drawings. While preferred embodiments of the apparatus for evaluating the electrical conductivity distribution of electrodes according to this disclosure will be described below, the apparatus is not limited thereto, and various modifications and improvements are possible based on the knowledge of those skilled in the art.

[0012] (Visualization principle) When imaging a sample using an electron beam, there are mainly two types of information sources. One is secondary electrons (SE), which are electrons emitted from the sample after receiving a portion of the energy of the incident electrons. The energy of secondary electrons is usually 10 eV or less, which is much lower than the energy of the incident electrons (generally several keV or more). Therefore, secondary electrons are thought to originate from electrons in the sample itself. The other is backscattered electrons (BSE), which are electrons emitted when a portion of the incident electrons are back-elastically scattered by atoms in the sample, and their energy is equal to the energy of the incident electrons. In electron beam imaging, these two types of electrons are detected by detectors in different energy bands, and images based on each type of electron are obtained. Therefore, secondary electron images and backscattered electron images are basically acquired simultaneously.

[0013] In general electron beam imaging, the acceleration voltage of the incident electrons is often set to several kV or higher. This is because a higher acceleration voltage results in a shorter electron beam wavelength, improving the spatial resolution of the image. Additionally, deeper electron penetration from the sample surface into the interior increases the emission of secondary and backscattered electrons, resulting in a clearer and brighter image.

[0014] On the other hand, under such accelerating voltage conditions, incident electrons penetrate deep into the sample, so the information obtained is primarily about the sample itself. For example, in backscattered electron images, a contrast in brightness corresponding to the atomic number can be obtained. However, the distribution of electrical conductivity within the electrode often depends not on the properties of the active material itself, but on physical conditions such as the electrical contact state between the material and the current collector foil. Therefore, it is necessary to propose new imaging conditions specifically for acquiring physical information.

[0015] In this disclosure, the inventors have found that extremely low acceleration voltages are effective in acquiring physical information. The principle and experimental results are described below. First, when the acceleration voltage is reduced to several hundred volts, the penetration depth of incident electrons is limited to a very shallow region of the surface, approximately 10 nm from the sample surface. As a result, the emission region of secondary electrons and backscattered electrons is also limited to a very shallow region of the surface. At this time, the yield of secondary electrons and backscattered electrons obtained is greatly influenced by the electrical properties of the sample surface.

[0016] Furthermore, "extremely low acceleration voltage" refers to acceleration voltages in the range of 2kV to 20V. However, this also includes cases where the voltage range is 1kV to 50V or higher, and 500V to 100V or higher.

[0017] The reason for setting the acceleration voltage to 2kV or less is that at acceleration voltages higher than 2kV, incident electrons penetrate deeply into the sample, resulting in a more prominent display of internal sample information while detailed surface information is obscured. At low acceleration voltages of 2kV or less, the electron penetration depth is limited to a few nanometers to tens of nanometers, allowing for information limited to the sample surface and its vicinity, resulting in significantly higher surface sensitivity.

[0018] Next, the reason for setting the acceleration voltage to 20V or higher is that below 20V, the energy of the incident electrons is very low, resulting in weak interaction with the sample, which can reduce the yield of electrons and potentially lead to blurry images. By ensuring an acceleration voltage of 20V or higher, electrons with the necessary energy interact with the sample, allowing for an appropriate yield of secondary electrons and backscattered electrons. Furthermore, extremely low acceleration voltages require specialized designs for the electron optics and detectors, which can make implementation difficult. Setting the acceleration voltage to 20V or higher resolves these instrument configuration issues, enabling stable measurements. With an acceleration voltage of 20V or higher, sufficient image brightness can be obtained, allowing for surface-sensitive analysis.

[0019] (The principle of backscattered electron emission) Figure 1A is a schematic diagram showing the difference in backscattered electron yield when electrons are irradiated at extremely low acceleration onto a group of particles with different electrical conduction paths to the current collector foil. Here, "backscattered electrons" refer to electrons that are reflected from the sample with almost no energy lost after undergoing backelastic scattering with atomic nuclei at the sample surface of the incident electrons.

[0020] Referring to Figure 1A, the yield of backscattered electrons emitted from a group of particles 11 with good surface electrical conduction paths and particles 12 with lost surface electrical conduction paths, which are arranged on the current collector foil 10, will be explained when they are irradiated with an electron beam at an extremely low acceleration. Normally, in composite electrodes, the electrical contact between each particle is good, and a surface electrical conduction path to the current collector foil 10 is ensured through the contact between particles. However, if the physical contact between particles is insufficient, or if a resistive material such as a binder is interposed, the surface electrical conduction path may be lost in some particles. As will be described later, the yield of backscattered electrons differs between particles with sufficiently ensured surface electrical conduction paths and electrically floating particles, even under the same electron beam irradiation conditions. This phenomenon makes it possible to precisely evaluate the quality of electrically deactivated active material particles, and by extension, the electrical conductivity distribution of the entire electrode, by performing electron beam imaging under extremely low acceleration voltages.

[0021] (A group of particles with good surface electrical conduction pathways 11) An electron beam is irradiated at extremely low acceleration onto a group of particles 11 that have good electrical conduction paths to the current collector foil. At extremely low acceleration, most of the incident electrons are allowed to penetrate only to the vicinity of the outer surface of the particles. In the case of a group of particles with good surface electrical conduction paths, the electrons that penetrate to the vicinity of the surface travel along the surface of the particles and quickly flow to the current collector foil 10.

[0022] At extremely low acceleration voltages (e.g., around several hundred eV), the penetration depth of incident electrons is limited to only a few nanometers to about 10 nanometers from the particle surface. Therefore, the influence of the surface electrical conduction path of each particle becomes more pronounced. In the case of particles with good electrical conduction paths, most of the incident electrons flow through the conductive path on the particle surface to the current collector foil 10, so electrons do not accumulate on the surface of the particle group 11 and cause charging. As a result, the yield of backscattered electrons obtained from the particle group 11 with good surface electrical conduction paths decreases. Thus, when the surface electrical conduction path of a particle is good, most of the incident electrons flow to the current collector foil via surface conduction, so the amount of backscattered electrons emitted is relatively small.

[0023] (Particles with insufficient surface electrical conduction pathways 12) Now consider irradiating particle 12, which has insufficient surface electrical conduction pathways, with an electron beam at extremely low acceleration. Particle 12 does not have surface electrical conduction pathways that extend to the current collector foil, unlike particle group 11. Therefore, electrons that penetrate near the particle surface cannot flow out to the current collector foil 10 by surface conduction. As a result, most of the incident electrons are reflected on the spot, and consequently the yield of backscattered electrons increases. Furthermore, due to the accumulation of negative charge on the surface of particle 12, which has insufficient surface conduction pathways, newly incident electrons may also be electrostatically repelled. Consequently, most of the incident electrons are reflected at the particle surface, and the yield of backscattered electrons increases.

[0024] Thus, the yield of backscattered electrons at extremely low acceleration is strongly influenced by the presence or absence of surface electrical conduction paths. Here, the influence of the electrical conductivity of the particles themselves can be almost ignored. In other words, it does not depend on whether the particles themselves are conductors or insulators. Rather, it can be said that the yield of backscattered electrons is determined by the quality of the electrical contact between particles and the surface electrical conduction paths that extend to the current collector foil.

[0025] As described above, based on the brightness contrast of the backscattered electron image obtained when an extremely low-acceleration electron beam is irradiated, it becomes possible to locally image the electrical contact conditions of each particle group on the current collector foil 10, that is, the degree of quality of the electrical conduction path and its distribution.

[0026] Furthermore, the uniform presence of surface electrical conduction paths for each particle on the current collector foil 10 enhances the overall utilization efficiency of the electrode and improves the overall performance of the battery. This is because the electrical resistance of the electrode is kept low, and the entire electrode can react uniformly, thus improving charge and discharge efficiency. By visualizing the quality of the local electrical conduction paths for each electrode particle through backscattered electron imaging, electrode non-uniformity and local abnormalities can be identified, which can be used for quality evaluation of coated electrodes and optimization of the manufacturing process.

[0027] (Principle of secondary electron emission) Figure 1B is a schematic diagram showing the difference in secondary electron yield for electrically conductive and insulating particles relative to incident electrons. Here, "secondary electrons" refer to low-energy electrons emitted from the sample in response to incident electrons.

[0028] Using Figure 1B, we will explain how the yield of secondary electrons emitted from electrically conductive particles and insulating particles differs when irradiated with an electron beam at extremely low acceleration.

[0029] (Electrically conductive particles) When electrons are irradiated at extremely low acceleration, secondary electron emission is limited to the region near the surface of the sample. Therefore, if secondary electron emission continues, the electrons on the sample surface become depleted, and eventually the yield of secondary electrons decreases. However, if the sample has good electrical conductivity, the electrons that form secondary electrons are efficiently transported from deep within the sample and rapidly supplied to the surface. In other words, even if secondary electrons are emitted from near the surface, electrons are quickly supplied from within the particles to compensate for the electron deficiency on the surface. As a result, the secondary electron yield remains consistently high, and consequently, a high-brightness secondary electron image is obtained.

[0030] (Insulating particles) When electrons are irradiated at extremely low acceleration, secondary electrons are mainly emitted from the very shallow regions of the sample surface, resulting in a localized electron deficiency at the surface. Therefore, in order to maintain the yield of secondary electrons, it is necessary to supply electrons from deeper within the sample. In the case of insulating particles, the low electrical conductivity of the material itself slows down electron transport from the depths to the surface, and it is not able to adequately compensate for the electron deficiency at the surface. As a result, the surface of insulating particles instantly becomes electron-deficient, and the yield of secondary electrons decreases. Consequently, the brightness of the secondary electron image decreases, and it is imaged as a low-contrast image with lower brightness compared to electrically conductive particles.

[0031] As described above, we propose that, based on the emission principle of backscattered electrons and secondary electrons by extremely low-acceleration electron beams, it is possible to obtain information regarding the presence or absence of surface electrical conduction pathways in particles within electrodes, as well as information regarding the electrical conductivity of the material itself.

[0032] (Backscattered electron image and secondary electron image) By utilizing both backscattered electron images and secondary electron images, complex electrode structures, such as coated electrodes in lithium-ion batteries, can be effectively analyzed, including both their surface electrical properties and internal structure, such as the electrical conduction pathways of each active material particle. Each image provides different information, and combining them allows for a more detailed and comprehensive evaluation.

[0033] Not only at extremely low acceleration, but also when the acceleration voltage is several kV or higher, backscattered electron images can mainly reflect differences in the atomic number of the material. The yield of backscattered electrons also varies depending on the material and density of the sample. In particular, materials with high atomic numbers and high density emit more backscattered electrons, resulting in higher contrast brightness.

[0034] Regardless of whether the acceleration voltage is extremely low or above, secondary electron images primarily provide information about the surface morphology of the sample. By combining this with extremely low-acceleration observation, it is possible to obtain images that simultaneously reflect information about the surface microstructure and the electrical conductivity of the material.

[0035] Backscattered electron imaging allows for the simultaneous observation of physical properties such as atomic number, density, material, and electrical contact conditions of each active material particle within the electrode, as well as the surface microstructure and electrical conductivity obtained from secondary electron imaging. This enables comprehensive acquisition of multifaceted information about the electrode sample. Therefore, by integrating and analyzing this information, detailed material evaluations that cannot be obtained with a single method can be performed, improving evaluation accuracy.

[0036] (Evaluation device 100 for the electrical conductivity distribution of electrodes) Figure 2 shows a diagram of the configuration of the electrode electrical conductivity distribution evaluation device 100 according to the embodiment. The electrode electrical conductivity distribution evaluation device 100 (hereinafter sometimes referred to as the electrode electrical conductivity evaluation device 100 or evaluation device 100) may utilize the basic functions of a general-purpose scanning electron microscope (SEM) for acquiring secondary electron images (SE images) and backscattered electron images (BSE images). However, the design philosophy of the evaluation device 100 according to this embodiment differs from conventional scanning electron microscopes (SEMs) in the following respects. The evaluation device 100 does not aim for high-resolution observation of fine structures pursued by microscopes, but rather focuses on low-magnification and rapid imaging over a wide area. Conventional SEMs require high resolution to observe structures on the scale of several nanometers, but the evaluation device 100 aims to scan a wide area on the order of several millimeters at low resolution and quickly visualize the electrical conductivity distribution of electrodes. In particular, for samples with particle sizes reaching tens of micrometers, such as coated electrodes, it is necessary to understand the overall distribution rather than observing fine local details. This evaluation device 100 maintains the basic functions of electron beam imaging but is optimized for applications different from conventional SEMs, and specifically has the following features.

[0037] (Scan range) While conventional SEMs perform observations at high magnification and in a narrow field of view, the evaluation device 100 according to this embodiment can rapidly scan a wide area of ​​several millimeters square. This makes it possible to detect uneven or non-uniform conductivity throughout the entire sample in a short time.

[0038] (Low magnification / low resolution) The evaluation apparatus 100 according to this embodiment is intentionally designed with low resolution and is not intended for detailed observation of microstructures, but rather focuses on efficiently understanding the electrical conductivity distribution of the electrodes. For example, in coated electrodes of lithium-ion batteries where the particle size reaches several tens of micrometers, it is more important to evaluate the uniformity of the entire particle than to analyze local details.

[0039] (Rapid data acquisition) While conventional SEMs require a long time to acquire detailed images, the evaluation device 100 according to this embodiment can capture a wide area at low magnification in one go, significantly improving throughput. For example, while conventional microscopes sometimes required several hours to image the entire sample, this device can complete the process in just a few minutes.

[0040] (Use of low acceleration voltage) Conventional SEMs have focused on using high acceleration voltages for high-resolution observation. The evaluation device 100 according to this embodiment is configured to efficiently evaluate the electrical conductivity distribution of electrodes by employing a low acceleration voltage instead of the high acceleration voltage used for high-resolution observation.

[0041] (fully automated) As described above, fully automated operation of the device is desirable for rapidly scanning a wide area and screening the quality of the electrical conductivity of electrodes. In normal microscopic observation, an appropriate field of view is found from a wide area, and imaging is performed after beam adjustment and focus adjustment. This is an essential operation for microscale observation, but this device does not necessarily require such operations. In other words, beam adjustment and focus adjustment can be performed with an accuracy that the device can automatically achieve, and there is no need to find a specific field of view that is easy to see. That is, the entire series of operations from sample feeding to imaging, data acquisition, and analysis can be achieved unmanned and fully automated. The operator only needs to set the coated electrode in the sample loader and press the execute button.

[0042] As shown in Figure 2, the electrode electrical conductivity distribution evaluation device 100 comprises an electron gun 101, a focusing electron lens 102, a deflector 103, an electron lens 104, a sample holder 105, a backscattered electron detector 106, a secondary electron detector 107, a control device 108, an input device 109, a display device 110, a memory 111, and a communication device 112.

[0043] (Electron gun 101) The electron gun 101 is a device for generating an electron beam. The electron beam 300, accelerated by the acceleration voltage, is focused by the focusing electron lens 102. The "acceleration voltage" refers to the voltage applied to impart energy to the electron beam 300 generated from the electron gun 101, specifically the voltage applied between the cathode and anode of the electron gun 101. This acceleration voltage accelerates the electrons to a high-energy state, which are then sent to the focusing electron lens 102. The electron gun 101 can be of either thermionic emission or field emission type, for example, and is selected according to the application. The generated electron beam is given energy by the acceleration voltage and sent to the focusing electron lens 102. The acceleration voltage is preferably 20V or more for 2kV or less, 50V or more for 1kV or less, and 100V or more for 500V or less. The electron beam irradiation current value can be arbitrarily set according to the user, for example, 2 × 10⁻⁶ -12 From A, 1 x 10 -4The irradiation current can be set within the range of A. A higher irradiation current increases the yield of secondary and backscattered electrons (described later), improving the quality of the resulting image, but also makes surface contamination and irradiation damage associated with scanning more pronounced. Conversely, a lower irradiation current reduces these problems, but degrades image quality. Therefore, it is desirable to perform the scan using the minimum irradiation current value that guarantees image quality sufficient for satisfactory analysis.

[0044] (Focusing electron lens 102) The focusing electron lens 102 is a device that focuses the electron beam generated from the electron gun 101 to a single point. The focusing electron lens 102 uses electromagnetic lenses and electrostatic lenses and plays a role in precisely controlling the electron beam and irradiating it accurately towards the object to be observed on the sample holder 105. To optimize focusing performance, the focusing electron lens 102 has a function to appropriately adjust the focus of the electron beam, and the focal length changes according to the observation magnification and resolution. Especially during high-magnification observation, focusing accuracy is important for accurately displaying the fine structure of the sample surface, and the adjustment can be performed automatically or manually. Furthermore, the focusing electron lens 102 is designed to maintain high resolution even at low acceleration voltages.

[0045] (Deflector 103) The deflector 103 is a device for controlling the direction of the electron beam. The electron beam needs to be irradiated to different positions on the sample holder 105, and the deflector 103 deflects the electron beam for this purpose. The deflector 103 is usually composed of an electromagnetic coil and can precisely move the beam to any position on the sample. The deflector 103 scans the electron beam at high speed and with high precision, making it possible to efficiently observe the electrical conductivity distribution of the entire sample. In addition, the deflector 103 can control the speed and direction of the beam movement in real time, so it is appropriately adjusted according to the observation magnification and the size of the sample. As a result, the electron beam is scanned uniformly over the entire target area of ​​the sample, and a high-resolution image is obtained. It should be noted that, unlike a microscope, this device does not necessarily require observation of fine structures. For example, in the case of coated electrodes of a typical lithium-ion battery, the particle size of the active material reaches several tens of micrometers. Therefore, in order to visualize such large particles without fail, there is no need to design a deflection system specifically for fine structure analysis and local observation. Rather, a deflection system with a larger scanning range, such as scanning a few millimeters square at low magnification, is preferable.

[0046] (Electronic lens 104) The electron lens 104, acting as the objective lens, is the final lens for precisely irradiating the focused electron beam onto the sample surface. The electron lens 104 finely adjusts the distance to the sample and the beam's focus, controlling the electron beam to precisely concentrate on the observation target area on the sample surface. Especially during high-magnification observation, the performance of this lens greatly affects the image resolution and accuracy, so the electron lens 104 incorporates a high-precision adjustment mechanism. Unlike a microscope, this device does not necessarily require the observation of fine structures. For example, an optical lens system capable of imaging even at low magnifications of several tens to several hundreds of times is required.

[0047] Furthermore, the electron lens 104 not only focuses the beam but also controls the beam irradiation angle and beam spot size. This control allows for observation of the sample's surface structure under optimal conditions, enabling accurate evaluation of electrical conductivity distribution and fine physical properties. In addition, the electron lens 104 enables high-resolution observation at low acceleration voltages, providing functionality for detailed observation.

[0048] (Sample holder 105) The sample holder 105 is a device for fixing the sample to be observed and accurately maintaining the irradiation position of the electron beam. The sample holder 105 is designed to accommodate samples of various sizes and shapes and is equipped with a movable stage that allows for fine adjustments. The movable stage can be finely adjusted in the up, down, left, right, and even front and back directions, allowing for precise setting of the sample position. In addition, the sample holder 105 is equipped with a mechanism to adjust the working distance in order to maintain an appropriate distance (working distance) between the electron beam and the sample surface.

[0049] Furthermore, the sample holder 105 is made of a highly electrically conductive material to electrically ground the sample and prevent charge buildup in the sample. The sample holder 105 can also be modified to accommodate specifications for observing the behavior of the sample under temperature changes by adding heating and cooling functions.

[0050] (Reflected electron detector 106) The backscattered electron detector 106 is a device that detects electrons (backscattered electrons) reflected by an electron beam irradiated onto the sample surface. Backscattered electrons produce signals that depend on the atomic number and density of the sample, providing information about the composition and internal structure of the material. In particular, backscattered electron images (BSE images) are effective in identifying different materials and structures within a sample because they enhance the atomic number contrast of the sample.

[0051] The backscattered electron detector 106 is installed near the sample to capture backscattered electrons emitted from the sample with high precision. The detector clearly senses backscattered electrons and images the non-uniformity of the electrical conductivity distribution throughout the sample in real time. This visualizes the microstructure on the sample surface and inside, providing important information for evaluating the electrical conductivity distribution of the electrodes.

[0052] Furthermore, the backscattered electron detector 106 is linked to a signal processing system that efficiently processes the detected signals, and has the function of rapidly analyzing the obtained data and generating precise images. The signal processing system digitizes the acquired backscattered electron signals and generates high-contrast images based on the material composition and surface condition of the sample.

[0053] Backscattered electron detectors are installed near the sample and at a position corresponding to the reflection angle of the electron beam in order to optimally capture backscattered electrons emitted from the sample surface. Multiple detectors may be combined to efficiently detect high-angle backscattered electrons (HA-BSE) and low-angle backscattered electrons (LA-BSE). High-sensitivity silicon semiconductor detectors are used to precisely detect weak backscattered electron signals. This allows for detailed evaluation of subtle differences in material composition and internal structure.

[0054] The detection sensitivity of the backscattered electron detector 106 should be 10% or more, 20% or more, or 30% or more compared to the detection sensitivity of a backscattered electron detector in a standard SEM. This is because, while a standard SEM emphasizes high-resolution observation for fine local observation, the evaluation device 100 according to this embodiment is intended for rapid observation of a wide area and therefore focuses on stable detection over a wide region.

[0055] (Secondary electron detector 107) The secondary electron detector 107 is a device that detects low-energy secondary electrons emitted from the sample surface. Secondary electrons mainly originate near the sample surface and provide detailed information about the surface structure and morphology of the sample. Secondary electron images (SE images) are very useful for evaluating the surface condition of a sample because they visualize fine surface irregularities. In the case of extremely low-acceleration observation, it is also possible to observe the electrical conductivity of the sample.

[0056] The secondary electron detector 107 is designed to efficiently capture low-energy secondary electrons generated by the electron beam irradiated onto the sample, enabling real-time analysis of the fine structure and localized distribution of electrical conductivity on the sample surface. Furthermore, since the amount of secondary electrons emitted is influenced by the electrical conductivity of the sample itself, it is possible to evaluate the quality of materials such as the distribution of different resistance components in the composite electrode and the homogeneity of the electrode with high precision.

[0057] The secondary electron detector 107 is often used in conjunction with the backscattered electron detector 106 to accurately observe the detailed shape and electrical conductivity distribution of the surface. Combining the images from both detectors allows for a deeper understanding of the sample's characteristics. In particular, the secondary electron image provides surface-specific information and plays an important role in comprehensively evaluating the sample's condition.

[0058] The detection sensitivity of the secondary electron detector 107 should be 10% or more, 20% or more, or 30% or more compared to the detection sensitivity of a secondary electron detector in a standard SEM. While a standard SEM emphasizes high-resolution observation for fine local observation, the evaluation device 100 according to this embodiment is intended for rapid observation of a wide area and focuses on stable detection over a wide region.

[0059] (Control device 108) The control device 108 comprises a device control unit 108A, an image processing unit 108B, and an image analysis unit C, and is configured to comprehensively control and manage the operation of the entire electrode electrical conductivity evaluation device 100.

[0060] (Device control unit 108A) The device control unit 108A is the main control unit for controlling the operation of the entire hardware of the electrode electrical conductivity evaluation device 100. The controlled objects include the main components such as the electron gun, focusing electron lens, deflector, sample holder, backscattered electron detector, and secondary electron detector.

[0061] The device control unit 108A is configured to control, for example, the acceleration voltage of the electron beam, the filament current, the alignment of the electron beam, and so on, with respect to the electron gun 101.

[0062] The device control unit 108A is configured to control the focusing electron lens 102, including the lens current, beam spot size, focus adjustment, aperture size, convergence angle, alignment correction, and the like.

[0063] The device control unit 108A is configured to control the beam deflection angle, beam scanning speed, beam scanning range, beam position correction, electron beam scanning method, etc., with respect to the deflector 103.

[0064] The device control unit 108A is configured to control the lens current, focal length, defocus amount, convergence angle, aperture size, beam spot size, beam shift, etc., with respect to the electron lens 104.

[0065] The device control unit 108A is configured to control the X-axis position, Y-axis position, Z-axis position, rotation angle, tilt angle, stage speed, automatic positioning, temperature control, etc., with respect to the movable stage of the sample holder 105.

[0066] The device control unit 108A is configured to control the sensitivity of the backscattered electron detector 106 and the secondary electron detector 107, the signal gain of the detected backscattered electron signal, the signal processing mode, the position of the detector, the filter setting of the detected backscattered electron signal, the detection area, and the operating mode of the detector.

[0067] (Image processing unit 108B) The image processing unit 108B converts the detection signals detected by the backscattered electron detector 106 and the secondary electron detector 107 into digital signals and generates images of the sample's surface shape and electrical conductivity distribution based on these signals.

[0068] The image processing unit 108B converts analog signals from the backscattered electron detector and secondary electron detector into digital signals using an A / D converter. The image processing unit 108B maps the signals detected at each point scanned by the electron beam to the corresponding pixels in the image. The brightness (luminance) of each pixel is assigned based on the detected signal intensity. The beam position when the electron beam scans the surface to be measured is known and correlated with the detected signal, so the operation of the scan coil and signal acquisition are synchronized. For example, the image processing unit 108B filters and removes noise contained in the acquired digital signal and adjusts the image contrast based on the strength of the detected signal.

[0069] The image processing unit 108B assembles a pixel array based on the signal across the entire beam scan range to complete an image representing the overall structure of the sample. The brightness and color of each pixel are determined, and the image processing unit 108B generates the final image and displays it on the display device 110.

[0070] The image processing unit 108B may perform the processes of forming the BSE image and the SE image in parallel. For example, the image processing unit 108B may be configured to receive the BSE signal and the SE signal simultaneously and process each in a separate processor. This enables parallel processing. The image processing unit 108B may also include software for processing the BSE image and the SE image separately.

[0071] (Image analysis department 108C) The image analysis unit 108C is configured to analyze the obtained BSE image and / or SE image. For example, if particles that have lost their surface electrical conduction path (particles that are electrically floating away from the current collector foil) are imaged in the BSE image as high-brightness contrast areas, the image analysis unit 108C counts these particles from the BSE image and determines their frequency and density (particles / mm²) within the observation field of view (moving image). 2 The system may automatically analyze whether these elements are present and provide quality based on the resulting counts as a quality control result.

[0072] The image analysis unit 108C performs multiple image processing steps to analyze the backscattered electron image (BSE image) and secondary electron image (SE image) obtained in real time. First, it automatically detects areas in the BSE image where particles that have lost their surface electrical conduction pathways are represented as high-brightness contrast. A specific image processing algorithm is used for this detection. Specifically, the brightness level of each pixel is analyzed on a pixel-by-pixel basis, and regions exceeding a preset threshold are identified. By setting multiple thresholds, hierarchical extraction according to brightness becomes possible.

[0073] Next, the image analysis unit 108C individually recognizes the particles belonging to the extracted high-luminance regions. In this process, the particles are separated using contour extraction algorithms and region segmentation methods, and their shape and size are identified. This ensures that the particles are accurately identified, and then each particle is counted. Based on the particle shape information, shape features (e.g., ratio of major axis to minor axis, peripheral length, shape skewness, etc.) are also analyzed to identify the type of particle. The counting results are performed for the entire image frame or for specific sub-regions, allowing for an understanding of the local density distribution of particles.

[0074] Next, the image analysis unit 108C calculates the total number of counted particles in the area (mm²) of the observation field. 2 By dividing by the unit, the particle density per unit area (particles / mm³) can be calculated. 2The density is calculated. Furthermore, the positional information of the particles is analyzed simultaneously, and the spatial distribution of density is obtained based on this. For example, regions where particles are locally concentrated and regions showing a non-uniform distribution can be identified. This process makes it possible to precisely analyze the non-uniformity of the electrical conductivity distribution within the electrode.

[0075] Furthermore, to achieve high-precision processing in real time, the image analysis unit 108C may be equipped with a parallel processing image processing engine. The image processing engine can perform particle detection, shape analysis, and counting in real time for each frame of acquired BSE and SE images, and by utilizing multiple threads and processor cores, it can perform multiple processes simultaneously at high speed. This also enables the analysis of video data and the tracking of dynamic regions.

[0076] Furthermore, the obtained particle frequency and density data are statistically processed and compared against established quality control standards. The statistical processing considers not only the number of particles but also their size distribution, shape distribution, and density distribution, automatically detecting outliers and trends. Based on these analysis results, the image analysis unit 108C automatically determines whether there are any problems with the electrode quality and outputs the results as a report. Additionally, the analysis data is fed back to the quality control system as needed and used to control the manufacturing line. In this way, real-time, highly accurate electrode quality evaluation is achieved.

[0077] Furthermore, the image analysis unit 108C can detect the number of non-electrically conductive particles on the electrode based on one of (a) to (d).

[0078] (a) Contrasts of backscattered electron image and secondary electron image The brightness difference of each pixel in the backscattered electron image and secondary electron image is calculated, and the brightness difference (contrast difference) is obtained. Regions where the contrast difference is greater than a predetermined threshold are automatically identified, and these are estimated to be the corresponding particles. For example, based on the contrast difference of backscattered electrons, groups of particles with lost surface conductive paths are identified. For example, a contour extraction algorithm is applied to separate and count the detected corresponding particles. The number of detected corresponding particles per unit area is calculated to obtain a density distribution.

[0079] (b) Low-luminance regions of the secondary electron image For example, contour extraction and region segmentation algorithms are used to identify non-conductive particles corresponding to low-luminance regions, and their shape and number are counted. The number of detected non-conductive particles per unit area is calculated to obtain a density distribution.

[0080] (c) High-luminance contrast of backscattered electron image High-brightness regions are automatically detected from backscattered electron images. These regions are highly likely to represent non-conductive particles that have lost their conductive pathways. Contour extraction is performed on the high-brightness regions, and each particle is separated and counted. The position and density distribution of the particles are analyzed to visualize the non-uniformity of the local electrical conductivity distribution.

[0081] (d) Brightness distribution of backscattered electron image and secondary electron image The brightness distribution of each image is analyzed to identify regions with large brightness differences. These regions with large differences suggest the presence of the relevant particles. By setting multiple thresholds and hierarchically extracting particles corresponding to different brightness levels, differences in the electrical properties of the particle groups are distinguished. The shape and number of detected particles are counted, and the density distribution is analyzed to precisely evaluate the electrical conductivity distribution of the entire sample.

[0082] Regarding the difference between (a) the contrast difference between the backscattered electron image and the secondary electron image, and (d) the brightness distribution of the backscattered electron image and the secondary electron image, both are methods for detecting non-conductive particles based on image comparison, but their approaches and focus differ. (a) is an approach that detects non-conductive particles based on the direct contrast difference between each pixel and quickly identifies the number of particles, emphasizing the comparison of each individual pixel. (d) analyzes the overall brightness distribution of the backscattered electron image and the secondary electron image and extracts particles hierarchically using multiple thresholds, enabling a more detailed analysis of the distribution of electrical conductivity and non-conductivity.

[0083] (Input device 109) The input device 109 is a device used to input control values ​​and settings for the electrode electrical conductivity evaluation device 100, and functions as a user interface. Specifically, it includes physical operating devices such as a keyboard, mouse, touch panel, trackpad, joystick, or touchscreen. Through these input devices, the user can operate the electrode electrical conductivity evaluation device 100, such as setting electron beam parameters, starting and stopping image analysis, specifying the observation area, and checking the analysis results. In addition, it is used to input various commands related to the operation of the system, such as changing setting values ​​and thresholds, fine-tuning observation conditions, and giving control instructions for real-time data.

[0084] (Display device 110) The display device 110 is a device for displaying BSE images (backscattered electron images) and SE images (secondary electron images) acquired by the electrode electrical conductivity evaluation device 100, and is configured to provide information visually as a user interface. The display device 110 is, for example, a display device using a liquid crystal display (LCD), an organic light-emitting diode (OLED), a plasma display, or other high-resolution display technology.

[0085] (Memory 111) Memory 111 is a storage device for temporarily or permanently storing data and programs necessary for the operation of the electrode electrical conductivity evaluation device 100. It stores acquired BSE and SE image data, analysis results, setting information, control parameters, etc. Memory 111 includes high-speed access storage such as RAM, flash memory, and SSD, enabling efficient data reading and writing to support real-time image processing and analysis. Past measurement data and analysis history can also be stored.

[0086] (Communication device 112) The communication device 112 is an interface for connecting to external devices via wired or wireless connections and for sending and receiving data. This allows the control device 108 to share data with other computers, servers, cloud services, or other devices on the network, enabling remote control and monitoring, transmission of data analysis results, and software updates.

[0087] In the electrode electrical conductivity evaluation device 100, the image acquisition speed is preferably 30 frames / second or more, 20 frames / second or more, or 10 frames / second or more. Furthermore, it is desirable that the upper limit of the image acquisition speed be 60 frames / second and the lower limit be 5 frames / second. If the image acquisition speed is less than 5 frames / second, the scan speed is too slow, making efficient measurement difficult, and it becomes impossible to evaluate a wide range of electrodes in a short time. In particular, when performing large-scale processing in industrial applications, a low speed may increase processing time and reduce productivity. Also, if the image acquisition speed exceeds 60 frames / second, the electron beam irradiation time becomes too short, making it impossible to detect a sufficient signal, which may reduce the image resolution and reliability. For this reason, in order to achieve high evaluation efficiency while maintaining appropriate image quality, it is preferable to set the image acquisition speed within the range of 5 frames / second to 60 frames / second. Within this range, the image acquisition speed can be flexibly adjusted according to the characteristics of the sample, the purpose of measurement, and the performance of the device, making it possible to optimize the balance between scan accuracy and processing efficiency.

[0088] The electrode electrical conductivity evaluation device 100 may include an energy dispersive spectroscopy (EDS) or wavelength dispersive spectroscopy (WDS) X-ray spectrometer.

[0089] The spatial resolution of an image is defined as the shortest distance between two approaching objects that can be accurately identified as two distinct points. This is determined based on the quality of the electron beam and the number of pixels in the recorded image (number of mapping pixels). In other words, the shorter the distance between objects that can be identified as two distinct points, the higher the spatial resolution. In the electrode electrical conductivity evaluation device 100, the spatial resolutions of the BSE image and SE image are, for example, 500 nm or less, 100 nm or less, and 50 nm or less. Furthermore, it is desirable that the upper limit of the high resolution be up to 10 nm and the lower limit be up to 1000 nm (1 μm). If the spatial resolution exceeds 1000 nm, it becomes impossible to recognize fine electrical conductivity distributions on the sample surface or internal structure, or localized abnormalities in the electrode, as an image, making it difficult to properly evaluate localized non-uniformity or defects in the electrode. In particular, when dealing with electrodes that require high quality, such as coated electrodes for lithium-ion batteries, low spatial resolution may hinder the evaluation of performance and quality. On the other hand, if the resolution is less than 10 nm, it becomes necessary to increase the sophistication of the device configuration and the precision of each component, which increases the manufacturing cost of the device and tends to lead to disadvantages such as a decrease in the processing speed of image analysis. Also, at extremely high resolution, the scan range becomes small, making it difficult to efficiently evaluate a wide range of electrode surfaces. Therefore, it is preferable to set the resolution of BSE and SE images within the range of 10 nm to 1000 nm. By adjusting the resolution within this range, it becomes possible to perform appropriate evaluation according to the characteristics of the sample, the purpose of measurement, and the performance of the device, and the balance between the accuracy and efficiency of the analysis can be optimized.

[0090] The minimum observation magnification is, for example, 10x, 50x, and 100x, while the maximum is 100,000x, 10,000x, and 1,000x. At observation magnifications below 10x, the spatial resolution is low, and the reliability of electrode quality evaluation decreases. At observation magnifications exceeding 100,000x, detailed observation of local microstructures becomes possible, but the observation range becomes extremely narrow. Therefore, it is preferable to set the observation magnification range between 10x and 100,000x. Within this range, it is possible to efficiently and accurately evaluate everything from the macroscopic structure of a wide electrode surface to minute electrical conductivity heterogeneities and defects.

[0091] (Sample feeding device 500) The electrode electrical conductivity distribution evaluation apparatus 100 shown in Figure 2 can be equipped with a sample feeding device 500 having a roll-to-roll mechanism. Figure 3 is a schematic diagram of the sample feeding device 500. The sample feeding device 500 has a roll 502 on which the collector electrode 501 is wound. The collector electrode 501 is a current collector foil supplied as a sample and is set on the roll 502. It is also possible to configure the collector electrode 501 to be placed on a belt and wound onto the roll 502 together with the belt.

[0092] The sample feeding device 500 is equipped with a drive mechanism for rotating the roll 502, which is driven by, for example, an electric motor (not shown). It may also be equipped with a roller guide system for feeding the sample with constant tension.

[0093] The drive mechanism for rotating the roll 502 is controlled by the control device 108 shown in Figure 2. The control device 108 is configured to synchronize the scanning speed of the backscattered electron image (BSE image) and secondary electron image (SE image) with the rotation speed of the roll 502 (the amount of movement of the collector electrode 501). This enables both intermittent scanning, where the roll 502 is rotated to move the measurement target area to the next area after scanning and image acquisition are complete for the measurement target area of ​​the collector electrode 501, and continuous scanning, where scanning and rotation of the roll 502 are performed simultaneously for the measurement target area.

[0094] The scan speed and the amount of movement of the roll 502 are adjusted in real time by the control device 108. The scan speed is set arbitrarily based on, for example, the image resolution, acceleration voltage, sample characteristics, and the performance of the device. The control device 108 adjusts the amount of rotation of the roll 502 according to the scan speed, setting it so that the amount of movement is neither excessive nor insufficient. This ensures that backscattered electron images and secondary electron images acquired in each region are captured continuously, preventing image overlap or gaps. Furthermore, because the roll 502 moves in a constant step according to the electron beam irradiation area and images are acquired continuously, the electrical conductivity distribution of electrodes over a wide area can be efficiently evaluated.

[0095] The control device 108 employs an algorithm for precisely controlling the roll 502 after the scanning of backscattered electron images (BSE images) and secondary electron images (SE images) is complete. Specifically, it accurately determines the width (scan size) of each scan area and controls the amount of rotation of the roll 502 based on this. This control ensures that the edges of each scan area precisely coincide with adjacent areas, preventing excessive or insufficient movement.

[0096] To prevent image duplication, an algorithm is employed that creates a small overlap in the scan range of adjacent areas and integrates and corrects the data in the overlapping area during post-processing. This ensures continuity and generates a seamless image even if minute positional shifts occur, by using the image data in the overlapping area complementaryly.

[0097] The system constantly monitors the movement of roll 502 and corrects the scan speed and roll movement in real time if even a slight deviation is detected. This feedback control allows for fine adjustments to the movement amount each time a gap is likely to occur, ensuring continuous scanning without any gaps.

[0098] By using a high-precision encoder to position the roll 502, the amount of movement can be precisely measured, enabling accurate positioning for each scan. The encoder tracks the position of the roll 502 in real time, and if a slight misalignment is detected, the control device 108 immediately corrects it, preventing image overlap and gaps.

[0099] This method employs a technique that overlaps portions of adjacent scan areas during scanning. The overlapping portions are later integrated by software in the image analysis unit 108C, optimizing the connections between each region to generate a seamless, continuous image with virtually invisible seams. This technique absorbs slight errors and variations in movement, preventing image loss or gaps.

[0100] Images acquired in each region are automatically integrated and processed by software into a seamless sequence of images. This integration process optimizes positional misalignment and image continuity between regions. Finally, the control device 108 displays the continuous electrical conductivity evaluation results based on the acquired backscattered electron and secondary electron images on the display device 110 in real time. This not only allows for seamless electrical conductivity evaluation over a wide area, even for long collecting electrodes 501, but also improves the accuracy and efficiency of the evaluation. The observable sample size should be at least 20 cm square, 10 cm square, and 5 cm square.

[0101] A continuous secondary electron image refers to a series of images of secondary electrons acquired by a continuous scan of the electron beam over the object being measured. Similarly, a continuous backscattered electron image refers to a series of images of backscattered electrons obtained by a continuous scan of the electron beam over the object being measured.

[0102] (Examples) To confirm the performance of the electrode electrical conductivity evaluation device 100 of the present invention, experimental verification was performed. As an example, lithium titanate (Li4Ti5O) was used. 12 (LT-106) Coated electrodes using active material were used as the measurement target and evaluation was performed.

[0103] (LTO working electrode 21) First, commercially available lithium titanate (Li4Ti5O 12 A coated electrode with LT-106 as the active material is prepared as the working electrode. In this embodiment, in order to investigate the contrast of the active material alone, the electrically conductive additive (carbon material such as acetylene black) added to the composite material was not added, and a coated electrode with only the active material was prepared as the measurement target. This is to enable a clearer and more reliable demonstration of the basic principle by imaging the electrode with only the active material.

[0104] The prepared LT-106 powder and polyvinylidene fluoride (PvDF) were mixed in a weight ratio of 95 wt%:5 wt%. This mixture was dispersed in n-methylpyrrolidone solvent to prepare a slurry. The prepared slurry mixture was coated onto aluminum foil and then dried overnight in a vacuum environment to remove moisture and solvent. After that, it was rolled using a roll press to create a coated electrode.

[0105] The coated electrode was punched out to a diameter of φ12 mm to prepare the LTO working electrode 21.

[0106] (Opposite pole 22) The counter electrode for the LTO working electrode 21 is a metallic lithium foil punched to a diameter of φ5 mm with a thickness of approximately 400 μm. The metallic lithium foil used as the counter electrode supplies lithium ions to the LTO working electrode 21.

[0107] (Porous polypropylene film separator 23) The porous polypropylene film separator 23 functions as an insulating material to physically isolate the positive and negative electrodes inside the battery and prevent short circuits. On the other hand, since it is necessary to allow ions such as lithium ions to move freely between the electrodes, the separator has a porous structure with fine pores. The thickness in the example is 40 μm.

[0108] (electrolyte 24) The electrolyte 24 is a medium for lithium ions to move from the counter electrode 22 to the LTO working electrode 21. The electrolyte 24 is a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1:1, and dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1 M [1 mol(dm) -3 .

[0109] (Assembly of battery cell) The LTO working electrode 21 and the counter electrode 22 were opposed to each other through a porous polypropylene film separator 23, and the cell was constructed by filling with the electrolyte 24. The assembly of the cell was carried out in a dry chamber with a dew point of -80 °C. The cell was assembled in a properly controlled environment to prevent the mixing of moisture and impurities and to cause an accurate electrochemical reaction. This is particularly because in the case of a lithium-ion battery, if there is moisture, it may react with the electrolyte to generate gas and deteriorate the battery performance.

[0110] (Partial lithium intercalation reaction into the LTO working electrode 21) For the cell composed of the LTO working electrode 21, the counter electrode 22, the porous polypropylene film separator 23, and the electrolyte 24, it is operated at a constant current from the open-circuit voltage to 1 V vs. Li + / Li potential. An operation of lowering the potential at a constant current from the initial state (open circuit) of the cell to 1 V is performed. Here, the "open-circuit voltage" is the initial electromotive force generated between these electrodes in a state where no current is flowing between the LTO working electrode 21 and the counter electrode 22. It indicates the natural potential difference between the LTO working electrode 21 and the counter electrode 22 and reflects the internal chemical energy state. The "1 V vs. Li + / Li potential" indicates the voltage when lithium ions are used as the reference electrode. Here, "Li + / Li" is the standard potential of lithium and indicates that it is treated based on metallic lithium. In a lithium-ion battery, the potential of the electrode is evaluated in comparison with the potential of Li + / Li (lithium ion / lithium metal). The "1 V vs. Li +"Li potential" refers to the point at which the electrode potential reaches 1V relative to lithium metal. This operation is performed to evaluate the behavior of electrode materials until they reach a specific potential during the charge-discharge process. In particular, this potential indicates material properties related to the insertion and removal of lithium. Constant current operation is a method of changing the potential between electrodes while applying a constant current. In the evaluation of lithium-ion batteries and electrode materials, the behavior of the electrodes and the change in potential are investigated by applying a constant current during the charge-discharge process. This method is used to evaluate the ability of electrode materials to accept and release lithium ions (i.e., charge-discharge performance). By operating with a constant current, the current applied to the sample is kept constant, making it possible to analyze the material properties while observing the change in potential.

[0111] Open circuit voltage: 1V vs. Li + By operating with a constant current up to / Li, it is possible to evaluate the behavior of lithium ions when they are inserted into or removed from the electrode, as well as their reaction characteristics within that potential range. Furthermore, it is possible to measure the amount of lithium (i.e., charging capacity) stored in the electrode material until it reaches 1V.

[0112] This is to facilitate the insertion of lithium ions from the counter electrode 22 into the LTO working electrode 21 (lithium intercalation reaction). By performing a constant current operation to advance the reaction while keeping the current constant, the supply of lithium ions is controlled, enabling uniform insertion of lithium ions into the working electrode 21.

[0113] Because the area of ​​the Li foil counter electrode is smaller than that of the LTO working electrode, a partial lithium intercalation reaction proceeds in the LTO working electrode 21 under constant current operation. Here, partial lithium intercalation reaction refers to the significant intercalation of lithium ions not in the entire LTO working electrode 21, but in a part of it, particularly in the region facing the Li foil counter electrode. This is because the lithium ion flux is concentrated directly beneath the counter electrode 22, preventing the reaction from spreading throughout the entire LTO working electrode 21. This phenomenon arises because the diameter of the counter electrode 22 is smaller than the diameter of the LTO working electrode 21, and the distance between the two electrodes is short, only a few tens of micrometers.

[0114] A schematic diagram of the lithium storage reaction and the battery cell after disassembly are shown in Figures 4A and 4B.

[0115] As shown in Figure 4A, the LTO working electrode 21 faces the counter electrode 22 via a porous polypropylene film separator 23. Because the distance is short, approximately 40 μm (the thickness of the separator), the flux of lithium ions and reaction current from the counter electrode 22 is localized directly beneath the counter electrode 22, as shown in Figure 4A. Figure 4A demonstrates that the lithium storage reaction in the LTO working electrode 21 is limited by the diameter of the counter electrode 22, and the inflow of lithium ions is concentrated directly beneath the counter electrode 22. Therefore, the lithium storage region within the LTO working electrode 21 is limited to its center, clearly separating the lithium-ion-storable portion from the unstorable portion. This partially lithium-storage electrode, subjected to this localized lithium storage reaction, is important for observing the reaction region and comparing the electrochemical and structural properties of the reacted / unreacted portions.

[0116] Figure 4B shows a current density of 10 mA / cm². 2 This is an example of a photograph of a cell after disassembly, which was reacted with the area current density set to (based on metallic lithium foil). As is clear from Figure 4B, only the central part of the white LTO working electrode 21 has turned black. This part corresponds to the part that was facing the counter electrode 22, which is metallic lithium foil. This suggests that the lithium storage reaction was localized to only the central part corresponding to the counter electrode 22.

[0117] (Current density dependence of sample electrode) The partial lithium storage reaction using battery cells is thought to exhibit different behaviors depending on the current density conditions during constant current operation. Therefore, electrodes were fabricated under various current density conditions, and the results are shown in Figures 5A, 5B, and 5C.

[0118] Figure 5A shows the current densities (0.1, 0.5, 5, 10, 20, 50 mA / cm²). 2These are the charge-discharge curves of the six sample electrodes created according to the following. The vertical axis represents the potential (Voltage, V vs. Li). + The horizontal axis represents the reaction capacity (mAh / g) and shows the reaction profile of the electrode. Here, the current density (mA / cm²) is shown. 2 The area of ​​the cell is based on the area of ​​the counter electrode 22, which is a metallic lithium foil (φ5mm). The current density J is calculated by J = I / A, where I is the current (mA) flowing through the cell and A is the area of ​​the counter electrode 22 (metallic lithium foil).

[0119] Generally, in a charge-discharge curve, a wider flat potential region indicates that the battery voltage remains constant for a longer period. This means that the reaction of the electrode material is stable, enabling a stable power supply. Furthermore, a high reaction capacity (mAh / g) is desirable. This represents the amount of charge that can be stored per gram of electrode material; a higher value means that more energy can be stored. The further the horizontal axis of the charge-discharge curve extends to the right, the higher the capacity.

[0120] As shown in Figure 5A, 0.1 mA / cm 2 The curve for sample electrode 1, in which lithium was absorbed into the LTO working electrode 21 at a current density of 1.55V vs. Li, is compared to other samples. + A significant potential flat region is observed near / Li. The potential flatness around this potential indicates that lithium is efficiently absorbed by sample electrode 1. Furthermore, the reaction capacity reaches 150 mAh / g. This is due to lithium titanate (Li4Ti5O). 12This amount corresponds to approximately 86% of the theoretical capacity of 175 mAh / g. The reason why a higher capacity is obtained compared to other sample electrodes is that lithium ions are efficiently absorbed into the LTO material of sample electrode 1, and performance close to the theoretical capacity is achieved. Sufficient reaction capacity is obtained despite the counter electrode 22 (φ5 mm) being smaller than the LTO working electrode 21 (φ12 mm). Even when the working electrode is larger than the counter electrode, the lithium ion flux spreads not only directly beneath the counter electrode but also around it, indicating that lithium ions are sufficiently supplied to the working electrode overall. 0.1 mA / cm 2 In sample electrode 1, which was prepared with such a low current density, lithium ions were slowly and uniformly inserted into the LTO working electrode 21. As a result, the reaction proceeded uniformly throughout, and it is believed that a higher capacity was obtained compared to other sample electrodes.

[0121] As shown in Figure 5A, 0.5 mA / cm 2 The curve of sample electrode 2, in which lithium was absorbed into the LTO working electrode 21 in the sample obtained at the current density, shows a smaller reaction capacity and a shorter flat region compared to sample electrode 1. This is thought to be because, as the current density increases, the insertion rate of lithium ions increases, and lithium ions are not uniformly distributed in the LTO working electrode 21.

[0122] As shown in Figure 5A, 5 mA / cm 2 In the sample obtained at the given current density, the curve for sample electrode 3, in which lithium was absorbed into the LTO working electrode 21, shows a further decrease in reaction capacity and a shortened flat region on the low-capacity side compared to sample electrode 1 and sample electrode 2. This is thought to be because, as the current density increases, the insertion rate of lithium ions becomes even faster, preventing lithium from being uniformly distributed throughout the electrode material.

[0123] As shown in Figure 5A, 10 mA / cm 2 and 20mA / cm 2The curves for sample electrodes 4 and 5, in which lithium was absorbed into the LTO working electrode 21, obtained by current density analysis, show a narrower potential flat region and a smaller reaction capacity compared to sample electrodes 1, 2, and 3. This is thought to be because, as the current density increases, the lithium ion flux becomes more localized to the counter electrode region, resulting in a more partial electrode reaction.

[0124] As shown in Figure 5A, 50 mA / cm 2 In the sample obtained at the current density, the curve of sample electrode 6, in which lithium was absorbed into the LTO working electrode 21, shows a significantly lower reaction capacity compared to sample electrodes 1 to 5, and a stable, flat region of potential is no longer observed.

[0125] Figure 5B shows the current densities (0.1, 0.5, 5, 10, 20, 50 mA / cm²). 2 This shows an example of photographs of six sample electrodes 1-6, which were prepared according to the instructions.

[0126] As shown in Figure 5B, 0.1 mA / cm 2 In the sample electrode 1 obtained under the current density conditions of 0.5 mA / cm², the lithium ion flow is slow, and the lithium ions diffuse uniformly throughout the electrode material. As a result, the sample electrode 1 is discolored black overall, indicating that lithium intercalation is occurring uniformly throughout the electrode. The movement and reaction of lithium ions are proceeding sufficiently, and the entire electrode material is being effectively utilized. Under low current density conditions, lithium ions spread not only directly beneath the counter electrode (lithium foil) 22 but also to its periphery, and the reaction occurs throughout the entire electrode. Therefore, the reaction of lithium ions is not limited to the counter electrode area but can spread over a wide area, indicating that a homogeneous reaction is occurring throughout the electrode. 2 In sample electrode 2 under the same current density conditions, lithium is absorbed across the entire surface of the LTO working electrode 21, similar to sample electrode 1, but to a slightly reduced degree.

[0127] As shown in Figure 5B, the current density is 5 mA / cm². 2 Sample electrode 3,10 mA / cm 2Sample electrode: 4.20 mA / cm² 2 Regarding sample electrode 5, as the current density increases further, lithium storage becomes increasingly localized directly beneath the counter electrode 22. Under these conditions, lithium ions are supplied at a faster rate than sample electrodes 1 and 2, so the reaction does not occur throughout the entire electrode, and lithium storage concentrates in a specific area. At high current densities, the flow (flux) of lithium ions is concentrated directly beneath the counter electrode 22, and horizontal diffusion hardly proceeds, so the lithium storage reaction is localized only directly beneath the counter electrode. Therefore, the entire electrode cannot participate in the reaction, and the reaction capacity decreases compared to constant current density conditions. This is consistent with the results in the graph in Figure 5A.

[0128] These results show that current density has a significant impact on lithium storage within the electrodes of lithium-ion batteries. At low current densities, the entire electrode reacts uniformly, achieving the battery's ideal performance. However, at high current densities, the reaction becomes localized, and the entire electrode is not effectively utilized.

[0129] Figure 5C is a graph showing the cell voltage profile against reaction diameter.

[0130] The reaction diameter 2r (mm) of the sample electrode can be estimated from its reaction capacity as follows: If the theoretically obtainable capacity when the entire electrode reacts completely is Q (mAh), then its size is determined by the theoretical capacity of the active material C (mAh / g), the radius of the electrode R (mm), and the area gravimetric density d (g / mm²) of the electrode layer. 2 ) is expressed by the following formula:

[0131]

number

[0132] Next, if the actual electrode reaction radius is r (mm), the electrode reaction capacity q (mAh) can be expressed as follows.

[0133]

number

[0134] By solving the two equations above simultaneously, the electrode reaction radius r (mm) can be expressed by the following equation.

[0135]

number

[0136] In the experiment, the radius of the lithium titanate electrode was r=6 (mm), and its theoretical electrode capacity was Q=0.43 (mAh). Therefore, when the voltage profile in Figure 5A is redrawn with the electrode reaction diameter 2r (mm) on the horizontal axis, it becomes as shown in Figure 5C.

[0137] Figure 5C is a graph showing the extent of the reaction region at sample electrodes 1 to 6. The horizontal axis represents the reaction diameter 2r (in mm), and the vertical axis is the same as in Figure 5A. Each curve represents a different current density (0.1, 0.5, 5, 10, 20, 50 mA / cm²). 2 The following are the voltage profiles against the reaction diameter for sample electrodes 1 to 6 prepared under the conditions shown in Figure 4C. Here, the diameter (φ5 mm) of the counter electrode 22, which is a metallic lithium foil, is shown by a black dotted line. From this, it can be seen that a change in the profile is observed around a reaction diameter of approximately φ5 mm for each current density operation. In particular, at a current density of 10 mA / cm², 2 and 20mA / cm 2 The current profile is remarkable, clearly showing that the reaction terminates around a reaction diameter of approximately φ5 mm. In other words, these current densities are considered optimal conditions for reproducing the partial lithium storage reaction. From the perspective of reaction efficiency, 10 mA / cm² was particularly chosen in this experiment. 2 We determined that this was the most optimal option and decided to use it as the sample for the following SEM observations.

[0138] (Extremely low-acceleration SEM observation of sample electrodes: An example of a secondary electron image) The following shows an example of observation of the reaction / unreacted interface of a partial lithium storage electrode using a scanning electron microscope (SEM, Regulus 8220, Hitachi High-Tech). The beam emission current was approximately 20 μA during the observation. The acceleration voltage was set as appropriate. Figure 6A shows a current density of 10 mA / cm². 2 Figure 6B shows a secondary electron image obtained by irradiating the sample electrode 4, which was prepared in [location], with an electron beam at an extremely low acceleration voltage of 500V. Figure 6B shows secondary electron images obtained by irradiating the same scan area of ​​the sample electrode 4 with electron beams at acceleration voltages of 500V, 1kV, 2kV, and 5kV. In order to prevent deterioration of the sample, observations were performed without exposure to the atmosphere using an air-shielded holder.

[0139] Figure 6A shows a secondary electron image of sample electrode 4 observed at low magnification (approximately ×400). An optical microscope image of the corresponding sample electrode 4 is inserted in the upper left of Figure 6A. As is clear from Figure 6A, region A is the unreacted region where lithium has not been absorbed into the LTO working electrode 21. Region B is the reacted region where lithium has been absorbed into the LTO working electrode 21. In the secondary electron image, a clear contrast of light and dark is shown between the unreacted region A and the reacted region B, corresponding to the optical microscope image. This demonstrates that it is possible to distinguish between the unreacted region A and the reacted region B using the secondary electron image.

[0140] Lithium titanate (Li4Ti5O) is used to form the LTO working electrode 21 through lithium storage reactions. 12 When ) takes in lithium ions, Li7Ti5O 12 It changes to Li4Ti5O. This reaction proceeds through the intercalation process of lithium ions. 12 This is due to lithium ions being incorporated into the crystal structure of the material. Specifically, lithium ions become embedded in the crystal lattice of lithium titanate, resulting in Li4Ti5O 12 Li7Ti5O 12 This changes to [a different form]. This reaction occurs during the charging process in lithium-ion batteries with LTO as the negative electrode.

[0141] Lithium titanate (Li4Ti5O) in unreacted region A 12 The electronic conductivity of ) and lithium titanate (Li7Ti5O) after the lithium storage reaction in reaction region B. 12 The electronic conductivity of ) is significantly different. Li4Ti5O 12 The particles are 10 -13 ~10 -8 Scm -1 In contrast, Li7Ti5O 12 The particle is 10 -2 ~100cm -1 Therefore, the lithium titanate particles in the unreacted region A are insulating materials, while the particles in the reacted region B are good electrical conductors.

[0142] Incidentally, as explained using Figure 1B, the amount of secondary electrons emitted by electrically conductive materials is significantly greater than that of insulating materials. As shown in Figure 6A, the unreacted region A, consisting of insulating material particles, is dark, while the reacted region B, consisting of electrically conductive material particles, has a higher brightness contrast than the unreacted region A, indicating that a large amount of secondary electrons were emitted from the reacted region B. In other words, this demonstrates that the emission of secondary electrons is actually visualized based on the principle of secondary electron emission shown in Figure 1B.

[0143] Figure 6B shows secondary electron images obtained by irradiating the same scan area with electron beams at acceleration voltages of 500V, 1kV, 2kV, and 5kV using sample electrode 4. The contrast difference between the secondary electron images in the reacted region B and the unreacted region A is most pronounced in the extremely low-acceleration observation at an acceleration voltage of 500V, and it can be seen that the contrast difference is lost as the acceleration voltage increases. In particular, at an acceleration voltage of around 5kV, it was confirmed that the distinction between the two almost disappears, and only a simple shape image remains. These effects clearly suggest that observation using electron beams at extremely low acceleration voltages is especially important for visualizing electrical conductivity contrast. In other words, secondary electron emission from the very shallow surface of the particles is more important for the visualization technique according to this embodiment. Furthermore, the results in Figure 6B are in good agreement with the visualization principle shown in Figure 1B.

[0144] (SEM observation of sample electrodes at extremely low acceleration: an example of a backscattered electron image) Next, we will explain the backscattered electron image acquired simultaneously with the secondary electron image. Figures 7A and 7B show a side-by-side comparison of images of secondary electrons and backscattered electrons acquired under extremely low acceleration (500V) in the same field of view. Figures 7A and 7B show the results of comparing secondary electron images (SE images) and high-angle backscattered electron images (HA-BSE images) acquired under extremely low acceleration conditions of 500V in the same field of view.

[0145] Figure 7A compares the secondary electron image and the high-angle backscattered electron image, confirming that different contrasts are obtained. Specifically, the secondary electron image (SE image) shows a contrast of light and dark between reacted and unreacted particles, while the backscattered electron image (HA-BSE image) shows scattered particles with remarkable brightness, independent of these. Here, the positions of these particles with remarkable brightness are marked with arrows in the backscattered electron image (HA-BSE image), and these are superimposed on the SE image. The arrows in the figure indicate particles with high brightness in the unreacted region and particles with high brightness in the reacted region. For example, focusing on the particles marked with arrows in the lower right of the secondary electron image (SE image) and the high-angle backscattered electron image (HA-BSE image), it can be seen that the brightness of these particles in the secondary electron image (SE image) is remarkably dark, even though it is in the reacted region, and is no different from that of the unreacted region. In other words, these particles are considered to be particles that remain unreacted in the lithium storage reaction region. There are several possible reasons why these particles remain unreacted, but the most straightforward reason is the lack of surface electrical conduction pathways as electrode particles. In other words, they are electrically isolated from the current collector foil and are therefore thought to have been left unreacted during the battery reaction.

[0146] Based on the backscattered electron emission principle shown in Figure 1A, particles lacking an electrical conduction path to the current collector foil are expected to produce a higher yield of backscattered electrons, resulting in a higher brightness contrast in the backscattered electron image (HA-BSE image). This is consistent with the results in Figure 7A. Incidentally, in the backscattered electron image (HA-BSE image), there is almost no contrast difference between the reacted and unreacted regions. This means that differences in the electrical conductivity of the material are hardly reflected in the backscattered electron image. Unlike secondary electrons, the source of backscattered electrons is the incident electrons themselves, so they are not significantly affected by the electrical conductivity of the material itself. It is thought that only particles that have lost their surface electrical conduction path leading to the current collector foil are being visualized.

[0147] Furthermore, as shown in Figure 7B, the contrast change when the acceleration voltage is increased is shown. Similar to the case of secondary electrons mentioned earlier, the characteristic contrast of the backscattered electron image is lost with increasing acceleration voltage, and above 5kV, no significant differences between particles can be observed. In other words, it is thought that the presence or absence of surface electrical conduction paths has a significant effect when the penetration depth of incident electrons is extremely shallow. This also highlights the importance of extremely low-acceleration observation.

[0148] The electrode electrical conductivity distribution evaluation device described herein is based on a novel visualization technology utilizing electron beams, enabling high-precision and rapid evaluation of the local electrical conductivity distribution within an electrode. This device differs from conventional scanning spreading resistance microscopy (SSRM) in that it performs evaluation without physical contact, visualizing electrical conductivity without direct contact with the sample by irradiating it with an electron beam. As a result, the image acquisition speed is significantly improved compared to conventional technology, enabling high-resolution observation at television rates (tens of frames per second), for example. Furthermore, the image acquisition range can be extended to several millimeters or more, making it possible to perform evaluations of practical electrode sizes, which were difficult with conventional nanoscale evaluation technology, in a short amount of time.

[0149] Furthermore, this device employs a Roll-to-Roll mechanism for sample introduction, enabling continuous evaluation. Acquired image data is automatically analyzed, allowing for real-time detection and evaluation of non-uniformity of electrical conductivity and localized defects. This eliminates the need for manual operation, offering the advantage of fully automated electrode quality evaluation. These features enable efficient and rapid evaluation of the wide-ranging electrical properties of electrodes, a process that previously required considerable time. Additionally, as part of quality control in the manufacturing process, it enables real-time and highly accurate evaluation, significantly contributing to improved product yield and reduced manufacturing costs. [Explanation of Symbols]

[0150] 10 Current collector foil 11. A group of particles with good surface electrical conduction pathways. 12. Particles with insufficient surface electrical conduction pathways 21 LTO working electrode 22 Opposite Poles 23 Porous polypropylene film separator 100 Electrode Electrical Conductivity Distribution Evaluation Device 101 Electron Gun 102 Focusing electron lens 103 Deflector 104 Electronic Lens 105 Sample holder 106 Backscattered electron detector 107 Secondary electron detector 108 Control device 108A Device Control Unit 108B Image Processing Unit 108C Image analysis section 109 Input device 110 Display device 111 memory 112 Communication equipment 500 Sample dispensing device 501 Collector electrode 502 rolls

Claims

1. A device for evaluating the distribution of electrical conductivity of electrodes, An electron beam irradiation system configured to irradiate the electrode with an electron beam accelerated at a predetermined acceleration voltage, A first detector for detecting secondary electrons, A second detector for detecting backscattered electrons, The system comprises a control unit configured to provide a secondary electron image from the detected secondary electrons and a backscattered electron image from the detected backscattered electrons, An apparatus for evaluating the electrical conductivity distribution of electrodes, characterized in that the predetermined acceleration voltage is 20V or more and 2kV or less.

2. The apparatus for evaluating the electrical conductivity distribution of an electrode according to claim 1, characterized in that the acceleration voltage is 50V or more and 1kV or less.

3. The apparatus for evaluating the electrical conductivity distribution of electrodes according to claim 2, characterized in that the acceleration voltage is 100V or more and 500V or less.

4. The control unit, The system includes an image analysis unit configured to analyze the secondary electron image and the backscattered electron image, The device for evaluating the electrical conductivity distribution of an electrode according to claim 1, characterized in that the image analysis unit is configured to evaluate the electrical conductivity of the electrode by analyzing the secondary electron image and the backscattered electron image.

5. The device for evaluating the electrical conductivity distribution of an electrode according to claim 4, characterized in that the image analysis unit detects the number of non-electrically conductive particles of the electrode based on one of the following (a) to (d). (a) Contrast luminance of the backscattered electron image and the secondary electron image (b) Low-luminance region of the secondary electron image (c) High-luminance contrast of the backscattered electron image (d) Brightness distribution of the backscattered electron image and the secondary electron image

6. The electrode electrical conductivity distribution evaluation apparatus according to claim 5, characterized in that the image analysis unit is configured to generate a quality report of the electrode based on the number.

7. The system further includes a feeding device configured to feed out roll-shaped electrodes, The apparatus for evaluating the electrical conductivity distribution of an electrode according to claim 1, characterized in that the control unit is configured to provide a continuous secondary electron image and a continuous backscattered electron image according to the amount of feed of the roll-shaped electrode.

8. A method for evaluating the electrical conductivity of electrodes, The steps include: irradiating the electrode with an electron beam accelerated at a predetermined acceleration voltage; The step of detecting secondary electrons, The steps include detecting backscattered electrons, The steps include generating a secondary electron image from the detected secondary electrons and generating a backscattered electron image from the detected backscattered electrons, A method for evaluating the electrical conductivity distribution of an electrode, characterized in that the predetermined acceleration voltage is 20V or more and 2kV or less.

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