Working electrode, battery cell for observation, optical microscope device, and method for manufacturing working electrode

Abstract

The purpose of the present invention is to provide a working electrode that enables operando visualization of a battery reaction at the pressing interface inside a battery cell in an embodiment. A working electrode of a secondary battery for observation, characterized in that said working electrode comprises a support substrate and an electrically conductive film provided on the support substrate, and the working electrode is light-transmissive.

Description

Working electrode, observation battery cell, optical microscope apparatus, and method for manufacturing the working electrode

[0001] This disclosure relates to a working electrode, an observation battery cell, an optical microscope apparatus, and a method for manufacturing a working electrode.

[0002] Metal-based secondary batteries and anodeless secondary batteries are attracting attention as technologies that can achieve higher energy densities compared to conventional secondary batteries. In these batteries, the deposition and dissolution of metal on the negative electrode surface are the basis of operation, and a detailed understanding of this reaction process is essential for optimizing battery performance and evaluating safety.

[0003] Direct observation using an optical microscope has been widely used as an operand for investigating such battery reactions. Optical microscopes allow for non-destructive observation during battery operation and are a method that makes it easy to intuitively understand the reality of the phenomenon. In addition, other optical measurement techniques have been used, such as surface plasmon resonance spectroscopy, as disclosed in Patent Document 1, Non-Patent Documents 4 and 5, which is useful as a non-contact optical sensing technology with excellent sensitivity to the phenomenon. Furthermore, scanning electron microscopes and transmission electron microscopes have been used for observing the deposited metal itself, and observations with high resolution have been performed (Non-Patent Documents 6 and 7).

[0004] Japanese Patent Publication No. JP2022-070122A

[0005] J. Power Sources 43-44 (1993) 27. J. Power Sources 196 (2011) 6663. Electrochim. Acta 136 (2014) 529. J. Am. Chem. Soc. 143 (2021) 11160. Int. J. Electrochem. Sci. 19 (2024) 100448. J. Electrochem. Soc. 162 (2015) A7067. Nano Lett. 15 (2015) 2168.

[0006] Optical microscopy offers advantages over surface plasmon resonance spectroscopy and electron microscopy, such as ease of observation and intuitive understanding of phenomena. The ability to observe liquid electrolytes is a particular advantage. However, low spatial resolution and poor image quality are generally considered drawbacks. Furthermore, applying optical microscopy to operando observation of batteries presents a more fundamental problem: direct visualization of compressed interfaces within the battery is difficult.

[0007] Figure 1A shows an example of a conceptual diagram for observing the deposition and dissolution processes of metals using an optical microscope, as has been attempted in the past. The battery cell consists of a working electrode 10P made of copper or the like as a current collector foil, a counter electrode 30P made of base metal foil, and an electrolyte 20P filled between the two.

[0008] For example, in order to visualize the deposition and dissolution of precipitates 15P (e.g., alkali metals) deposited on the working electrode 10P, it is necessary to secure an optical path through the optical lens 90P inside the cell, as shown by the dotted line in Figure 1A. For this reason, conventional technology has produced prototype battery cells with a viewing hole of a predetermined diameter in the counter electrode 30P (Non-Patent Documents 1 and 2). However, with this method, the current density distribution differs between directly below the counter electrode 30P and the location of the viewing hole, which can lead to differences in the behavior of metal deposition and dissolution between the visualization range and other regions, making accurate evaluation difficult.

[0009] Furthermore, the cell configuration shown in Figure 1A differs from that of a real battery. Specifically, the counter electrode 30P and the working electrode 10P are not pressed together via a separator. For this reason, it is difficult to visualize the essential physicochemical reactions inside the battery in a way that faithfully reproduces the actual battery usage environment using conventional optical microscope observation setups.

[0010] Figure 1B shows an example of a battery configuration equipped with a pressing mechanism 40P to more closely resemble realistic battery usage conditions. In this configuration, the counter electrode 30P and the working electrode 10P are pressed against each other via a separator 201P impregnated with electrolyte. Thus, the battery reaction concerning the precipitate 15P at the interface inside the battery with surface pressure is more important information, and direct observation of it is required.

[0011] However, in the configuration shown in Figure 1B, the separator 201P, counter electrode 30P, and pressing mechanism 40P obstruct microscopic observation with the optical lens 90P, making it impossible. Practical cells are basically sealed with the working electrode and counter electrode facing each other. In other words, it is fundamentally difficult to visualize the inside of such a sealed cell with an optical microscope.

[0012] This disclosure provides a method for directly observing electrochemical reactions inside a battery in a practical cell configuration as described above, and a working electrode capable of realizing this. This makes it possible to directly visualize and understand physicochemical reactions, such as metal deposition and dissolution processes and associated side reactions, inside an operating battery cell. Furthermore, observation is possible not only for the dissolution and deposition of base metals, but also for reactions of other active materials, such as alkali metal insertion and desorption into carbon-based negative electrode materials, such as graphite, and the operation of oxide-based positive electrode materials.

[0013] The purpose of this disclosure is to provide a working electrode that allows for the operand visualization of the battery reaction at the pressure interface inside a battery cell in a practical application.

[0014] The working electrode of the present disclosure is a working electrode for an observation secondary battery, comprising a support substrate and an electrically conductive film provided on the support substrate. Furthermore, the working electrode is characterized by having light transmittance and electrochemical stability.

[0015] According to this disclosure, it becomes possible to directly visualize, operandially, the processes of metal deposition, dissolution, and various side reactions inside a battery under realistic operating conditions, particularly at the sealed interface where the counter electrode and working electrode are pressed together.

[0016] Figure 1A is a conceptual diagram of optical microscope observation when a peephole is provided in a conventional battery cell. Figure 1B is a conceptual diagram of optical microscope observation when a pressing device is provided in a conventional battery cell. Figure 2A is a conceptual diagram showing a method for optically visualizing the pressing interface inside a battery cell. Figure 2B is a conceptual diagram showing a method for optically visualizing the pressing interface inside a battery cell according to the present disclosure. Figure 3 is a diagram showing the working electrode according to the first embodiment. (a) is a top view, (b) is a cross-sectional view, and (c) is a bottom view. Figure 4 is a graph showing the wavelength dependence of the optical transmittance characteristics of a working electrode equipped with copper thin films of different thicknesses (0 nm, 10 nm, 20 nm, 50 nm). (a) shows the wavelength dependence of the relative optical transmittance characteristics of the copper thin film. (b) shows the relative optical transmittance characteristics of the working electrode equipped with copper thin films. Figure 5 is a surface plasmon resonance spectrum showing the wavelength dependence of the reflection intensity of a working electrode equipped with copper thin films of different thicknesses (10 nm, 20 nm, 50 nm). (a) is a spectrum showing the wavelength dependence of the reflection intensity of the working electrode in P-polarized light. (b) is a spectrum showing the wavelength dependence of the reflectance ratio of P-polarized light (R = P / S) to the reflection intensity of S-polarized light. Figure 6 is a graph showing the wavelength dependence of the spectral characteristics of the reflectance ratio (R = P / S) in a working electrode equipped with a 50 nm copper thin film. Figure 7 is a graph showing the change in wavelength-average transmittance (%) and surface plasmon resonance (SPR) relative reflectance dip depth (%) for copper thin films of different thicknesses (0 nm, 10 nm, 20 nm, 50 nm). Figure 8 is a diagram of a working electrode according to a first modification of the first embodiment. (a) is a top view, (b) is a cross-sectional view, and (c) is a bottom view. Figure 9 is a diagram of a working electrode according to a second modification of the first embodiment. (a) is a top view, (b) is a cross-sectional view, and (c) is a bottom view. Figure 10 is a diagram of a working electrode according to a third modification of the first embodiment. (a) is a top view, (b) is a cross-sectional view, and (c) is a bottom view. Figure 11 is a diagram showing a working electrode according to a fourth modification of the first embodiment. (a) is a top view, (b) is a cross-sectional view, and (c) is a bottom view. Figure 12 is a cross-sectional view of an observation battery cell according to the second embodiment. Figure 13 is a configuration diagram of an optical microscope apparatus according to the third embodiment. Figure 14 is a photograph showing the appearance of a working electrode comprising copper thin films of different thicknesses (10 nm, 20 nm, 50 nm).Figure 15 shows the process of forming a metal thin film by mask deposition as a method for forming a working electrode film. (a) is a photograph showing the appearance with the mask used in the deposition process applied. (b) is a photograph showing the appearance of the working electrode with the metal film after the mask has been removed. Figure 16 is a photograph showing the appearance of a working electrode with copper thin films of different thicknesses (10 nm, 20 nm, 50 nm) deposited by the mask deposition method. Figure 17 shows an example of an observation cell. Figure 18 shows a series of optical microscope images showing the deposition and dissolution process of metallic lithium in the central part of a working electrode with a 10 nm copper thin film. (a) shows the time series change of the charging (deposition) process, and (b) shows the time series change of the discharge (dissolution) process. Figure 19 is a diagram comparing the state of a working electrode with a 10 nm copper thin film before and after the metallic lithium deposition and dissolution cycle. Figure 20 shows the results of optical microscope observation of the metallic lithium deposition and dissolution process when using an ionic liquid electrolyte, where (a) shows the charging (deposition) process and (b) shows the discharge (dissolution) process. Figure 21 shows the results of optical microscope observation of the central part of a working electrode equipped with a 10 nm copper thin film during the deposition and dissolution process of metallic sodium, where (a) shows the charging (deposition) process and (b) shows the discharge (dissolution) process. Figure 22 is a high-magnification (×1000) observation image of metallic sodium precipitates at the outer edge of the central part of a working electrode equipped with a 10 nm copper thin film. Figure 23 is a diagram showing a typical potential profile of a charge-discharge test using a concentrated electrolyte. Figure 24 is an optical microscope image of the central part of a working electrode equipped with a 10 nm copper thin film, taken after charging at 40 mC. (a) is a series of optical microscope images obtained using a dilute electrolyte (LiFSA:DMC = 1:10). (b) is a series of optical microscope images obtained using a concentrated electrolyte solution (LiFSA:DMC = 1:1.1). Figure 25 is 8.0 mA cm. －２The images show optical microscope images of the central part of the working electrode equipped with a 10 nm copper thin film and changes in the amount of deposited electricity, taken under the charging conditions. (a) is a series of optical microscope images obtained using a dilute electrolyte (LiFSA:DMC = 1:10). (b) is a series of optical microscope images obtained using a concentrated electrolyte (LiFSA:DMC = 1:1.1). (c-1) is a graph showing the change in the amount of deposited electricity obtained using a dilute electrolyte (LiFSA:DMC = 1:10). (c-2) is a graph showing the change in the amount of deposited electricity obtained using a concentrated electrolyte (LiFSA:DMC = 1:1.1). Figure 26 shows gas generation in a dilute electrolyte. (a) is an optical microscope image of the inside of the battery cell. (b) is the potential profile corresponding to the numbers in the image in (a). Figure 27 is a schematic diagram of the effect of gas generation in a dilute electrolyte on the metallic lithium deposition morphology. (a) shows the surface of the electrode collector in the electrolyte. (b) is a magnified view of the lithium mass. Figure 28 is a conceptual diagram of surface electrical resistance measurement according to Example 2. Figure 29 is the transmittance spectrum against the thickness of each electrically conductive film according to Example 2. Figure 30 is a graph showing the film thickness dependence of relative transmittance (based on the support substrate) and surface electrical resistance in a fully deposited sample according to Example 2. Figure 31 is a conceptual diagram of a mask deposition pattern according to Example 3. Figure 32 is a comparative plot of surface electrical resistance measurements at various electrically conductive film thicknesses for a fully deposited sample and a mask deposition pattern sample according to Example 3. (a) is when the maximum value of the vertical axis scale is 80 Ω, and (b) is when the maximum value of the vertical axis scale is 800 Ω. Figure 33 is the optical transmission spectrum of various support substrates according to Example 4. Figure 34 is the surface morphology observation results of various support substrates according to Example 5. (a) is a surface topography image measured by AFM. (b) is a representative surface profile obtained from each image. (c) is an enlarged view of the vertical axis of the surface profile (b). Figure 35 shows the film thickness measurement results of electrically conductive films deposited on a sapphire glass substrate and a slide glass substrate according to Example 5. (a) shows the correspondence between the deposition mask pattern and the measurement location. (b) shows the height image including the boundary between the electrically conductive film portion and the support substrate portion, and its surface line profile.Figure 36 shows the AFM measurement results of the electrically conductive film surface deposited on a sapphire glass substrate and a slide glass substrate according to Example 5. (a) is the surface relief image, and (b) is a typical line profile. Figure 37 shows the observation results of the deposition and dissolution cycle of metallic lithium using a test cell with an electrically conductive film with a thickness of 15 nm deposited on a sapphire substrate as the working electrode, according to Example 6. (a) is the potential profile of lithium deposition and dissolution in the second cycle. (b) shows the appearance of the electrodes corresponding to each point in the potential profile. Figure 38 is a schematic diagram of a battery fabricated using a coated positive electrode according to Example 7. (a) is the case when a dry coated electrode is used. (b) is the case when a coated electrode impregnated with electrolyte beforehand is used. Figure 39 shows LiNi. ０．５ Mn １．５ O ４ This shows the charging and discharging process in an anodeless battery configuration using a coated positive electrode as the counter electrode. (a) is a typical voltage profile obtained after the first few cycles. (b) are optical photographs of the electrically conductive film observed at points A to F on the voltage profile. Figure 40 shows spectral images of the electrode during the metallic lithium deposition process, taken with a hyperspectral camera according to Example 8. (a) is a normal optical image without any analysis processing. (b) is a spectral image reconstructed using the open-circuit (OCV) spectrum as a reference. Figure 41 shows the analysis results of spectral imaging during the lithium deposition and dissolution cycles according to Example 8. (a) is a spectral image of the electrode from before lithium deposition to after lithium dissolution. (b) is the average spectrum corresponding to each image, analyzed in the rectangular region in the figure. Figure 42 shows LiNi according to Example 8. ０．５ Mn １．５ O ４ This is an example of spectral imaging in an anodeless battery configuration for the counter electrode. (a) is the charge / discharge profile. (b) is the spectral image of the electrode corresponding to each point. Figure 43 is an enlarged view of the analysis image acquired at the end of the discharge profile J according to Example 8.

[0017] The inventors of this application have devised a technique that allows the deposition and dissolution processes of base metals at the pressure interface inside a battery to be observed in an operand state using an optical microscope, by modifying the structure of the working electrode. This will be described in detail below with reference to Figures 2A and 2B.

[0018] Figure 2A is a schematic example showing the observation of the pressure interface inside a battery cell from the working electrode side. As shown in Figure 2A, a typical battery cell has a sealed structure, and the working electrode 10 (current collector foil) itself acts as a factor that obstructs light transmission, making it difficult to optically observe the inside of the battery, especially the pressure interface indicated by the black arrow.

[0019] Generally, metal foil with a thickness of several micrometers or more is used for the current collector foil of the working electrode, and naturally, light cannot pass through it. For this reason, observation with an optical microscope is physically impossible in the example shown in Figure 2A.

[0020] Here, the inventors of the present invention conceived of reducing the thickness of the working electrode 10 (current collector foil) to a thickness that allows light to pass through. Figure 2B is a conceptual diagram showing a method for optically visualizing the pressing interface inside a battery cell according to the present disclosure, based on this concept.

[0021] As shown in Figure 2B, this disclosure employs a configuration in which an electrically conductive film 12 is deposited on a transparent support substrate 11, and this electrically conductive film 12 functions as an working electrode 10 (current collector foil). With this configuration, light can pass through the transparent support substrate 11 and also through the electrically conductive film 12, reaching the interface between the separator 20 and the electrically conductive film 12, as indicated by the black arrow.

[0022] This allows for the induction of electrochemical reactions in the battery cell while simultaneously observing the deposition and dissolution of metal at the pressure interface between the working electrode 10 and the separator 20 in real time using an optical microscope. This enables operand-based analysis of the reaction behavior inside the battery.

[0023] (First Embodiment: Working Electrode) The working electrode 10 according to the first embodiment will be described with reference to Figure 3. Figure 3(a) is a top view of the working electrode 10 according to the first embodiment, Figure 3(b) is a cross-sectional view along line AB in Figure 3(a), and Figure 3(c) is a bottom view of the working electrode 10.

[0024] (Working electrode 10) The working electrode 10 is an electrode in an electrochemical cell where electrons are exchanged during the charging and discharging process, and an electrochemical reaction proceeds. During charging (reduction reaction), ions in the electrolyte are deposited as metals or compounds on the electrically conductive film 12 of the working electrode 10 by receiving electrons. During discharging (oxidation reaction), electrons are released to the electrically conductive film 12 of the working electrode 10, and the deposited metals, etc., dissolve into the electrolyte. The working electrode 10 comprises a support substrate 11 and an electrically conductive film 12.

[0025] (Support Substrate 11) The support substrate 11 is a substrate made of a light-transmitting material that mechanically supports the electrically conductive film 12 of the working electrode 10. The support substrate 11 has a first surface 111 on which the electrically conductive film 12 is provided, and a second surface 112 which is the opposite surface. The first surface 111 is the surface facing the counter electrode. The second surface 112 is the observation surface facing the objective lens of the optical microscope.

[0026] (Planar shape of support substrate 11) Examples of planar shapes of the support substrate 11 include, but are not limited to, the circular shape shown in Figure 3(a), as well as squares, rectangles, polygons, etc. The shape can be selected in consideration of the efficiency of optical observation, compatibility with the structure of the battery cell, and the uniformity of the pressing interface. For example, a circular substrate has high compatibility with the objective lens of an optical microscope and makes it easy to adjust the focus for optical observation. Also, because the stress distribution from the surroundings tends to be uniform in a circular shape, it is particularly suitable for observing pressing interfaces where uniform pressure needs to be applied. The planar shape of the support substrate 11 should be appropriately optimized according to the purpose of optical observation and the design requirements of the battery cell, and is not limited to a specific shape.

[0027] As shown in Figure 3(a), the planar size of the support substrate 11 is set to be larger than the planar size of the electrically conductive film 12. This is because the electrically conductive film 12 is very thin and cannot maintain a plate-like state on its own, so it is mechanically supported by the support substrate 11 to facilitate handling. Note that the planar size of the support substrate 11 may be the same as the planar size of the electrically conductive film 12.

[0028] (Mechanical strength of the support substrate 11) Since the electrically conductive film 12 alone lacks sufficient mechanical strength, the electrically conductive film 12 as the working electrode 10 needs to be formed on the support substrate 11. The support substrate 11 is required to stably support the electrically conductive film 12 and prevent the electrically conductive film 12 from peeling off. Furthermore, depending on the actual usage conditions, the observation battery cell is assumed to be observed under pressure, so it is preferable that the support substrate 11 has the strength to withstand a pressure of at least 100 kPa. When applied to solid batteries, further pressure is required, and the support substrate 11 needs to have the strength to withstand a pressure of several tens of MPa.

[0029] (Optical properties of the support substrate 11) It is desirable that the support substrate 11 has transparency suitable for optical observation. Specifically, it is preferable that the transmittance of the support substrate 11 be 70% or more, more preferably 75% or more, and more preferably 80% or more. In addition, it is even more preferable that the transmittance be 85% or more, and most preferably 90% or more. There is no particular upper limit set for the transmittance. The transmittance may also change depending on the surface roughness of the support substrate 11. Therefore, the surface of the support substrate 11 is required to have sufficient flatness to ensure optical transmittance.

[0030] Furthermore, in order to prevent unwanted reflections on the second surface 112 of the support substrate 11, an anti-reflective treatment may be applied to the second surface 112, which is the observation surface. An example of an anti-reflective treatment is MgF. ２It includes, but is not limited to, single-layer coating using [the above-mentioned material], and multi-layer coating in which a plurality of thin films with different refractive indices are stacked to reduce broadband reflection. Since the reflection loss of observation light can be reduced, the contrast of images during optical microscope observation is improved, making it possible to capture fine structures and changes more clearly. Furthermore, as long as the transmittance is not impaired, any interface treatment for improving the adhesion between the electrically conductive film 12 and the support substrate 11 may be applied on the first surface 111.

[0031] (Material of Support Substrate 11) The material used for the support substrate 11 is a material having sufficient optical transparency, and is a plastic material, glass material, or transparent ceramic material. A plastic material mainly refers to an organic resin containing a carbon compound as a main component, which is excellent in light weight and moldability and has light transmittance. A glass material refers to silicon dioxide (SiO ２ ) as its main component, and is an inorganic material with an amorphous structure obtained by adding other metal oxides and the like, followed by melting and quenching. A transparent ceramic material is an inorganic material that has high mechanical strength, heat resistance, and exhibits light transmittance.

[0032] Examples of plastic materials include, but are not limited to, acrylic (PMMA), polyethylene terephthalate (PET), polycarbonate (PC), polyethylene naphthalate (PEN), cycloolefin polymer / cycloolefin copolymer (COP / COC), polystyrene (PS), polyethersulfone (PES), and polyetherimide (PEI).

[0033] Examples of glass materials include, but are not limited to, quartz glass, soda-lime glass, borosilicate glass, aluminosilicate glass, alkali-free glass, special glass for optics (optical glass), high-pressure resistant transparent glass (soda-lime glass, low-expansion borosilicate glass, quartz glass), and the like. Considering mechanical strength and chemical stability against electrolytes, the material of the support substrate 11 is preferably a glass material.

[0034] Examples of transparent ceramic materials include sapphire glass and alumina (Al ２ O ３ ), titanium dioxide (TiO ２ ), calcium fluoride (CaF ２ ), yttrium oxide (Y ２ O ３ ), magnesium oxide (MgO), scandium oxide (Sc ２ O ３ ), yttrium aluminum garnet (YAG), spinel (MgAl ２ O ４ ), Zirconia (ZrO ２ ), barium magnesium tantalate (Ba(Mg,Ta)O ３ These include, but are not limited to, the following: ) etc. These transparent ceramic materials have excellent high pressure resistance, high strength, and chemical resistance, and are applicable to specific applications because they maintain stable optical properties, especially in high-temperature environments and harsh chemical environments. Also, transparent materials used in infrared spectroscopy (IR), such as alkali halides (lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (FK), sodium chloride (NaCl), potassium bromide (KBr), potassium iodide (KI)), as well as zinc selenide (ZnSe) and barium fluoride (BaF ２ By using various materials such as ), it is possible to perform various spectroscopic measurements using infrared light and spectroscopic imaging within the electrode surface.

[0035] (Refractive index of support substrate 11) When the support substrate 11 is made of a glass-based material, the refractive index of the glass does not particularly affect the optical observation when performed from a direction perpendicular to the glass surface, so there is no restriction on the refractive index.

[0036] (Electrical conductivity of the support substrate 11) If the support substrate 11 itself is electrically conductive, it will contribute to the electrochemical reaction as the working electrode, making it impossible to accurately reproduce the battery reaction. For example, indium, tin, and oxide-based transparent conductive films (ITO glass) are optically transparent, but they are electrically conductive and can contribute to the electrochemical reaction. In that case, physicochemical side reactions such as alloying and reductive corrosion may occur in the support substrate 11 itself. This is a particularly significant and serious problem when dealing with lithium-based batteries. Therefore, it is desirable that the support substrate 11 be an insulator that does not have electrical conductivity.

[0037] (Thickness of the support substrate 11) The thickness of the support substrate 11 is determined based on the working distance of the optical microscope used, its mechanical strength, and its optical transmittance. Here, working distance refers to the distance from the tip of the objective lens to the surface (or interface) of the sample being observed in an optical microscope, with the object being observed within the focal point. It is desirable that the thickness of the support substrate 11 be thinner than this range. Also, since optical transmittance decreases with increasing thickness of the support substrate, a thinner support substrate is desirable from this viewpoint as well.

[0038] However, if it is too thin, the mechanical strength will decrease, and it will lack the durability to withstand the pressure of the counter electrode. When using ordinary glass-based materials, the thickness of the support substrate 11 is 50 mm or less, more preferably 20 mm or less, and even more preferably 10 mm or less. There is no particular lower limit to the thickness as long as the mechanical strength can be ensured.

[0039] (Surface treatment of the support substrate 11) In the surface treatment of the support substrate 11, any polishing scratches that occur after processing may be treated with an optical treatment using a transparent coating agent for the purpose of improving optical properties and ensuring surface smoothness. Specifically, by applying an anti-reflective coating or an optical correction coating, it is possible to improve transmittance and suppress the scattering of unwanted light.

[0040] Examples of transparent coating agents include, but are not limited to, silica-based coatings, fluorine-based coatings, polymer-based coatings, or hybrid materials thereof. The coating agent improves the surface durability and chemical resistance of the support substrate 11 and prevents degradation due to contact with electrolytes. Furthermore, depending on the specific application, the surface functionality can be optimized by applying a coating that imparts hydrophilicity or hydrophobicity.

[0041] (Transmitted Light Wavelength Dependence of Support Substrate 11) The support substrate 11 can be made of an optical component that has a specific wavelength dependence on transmitted light. For example, by providing a bandpass filter function that selectively transmits or absorbs light in a specific wavelength range, it is possible to improve the contrast of the object being observed.

[0042] Furthermore, by employing UV-transmitting glass that transmits short wavelengths (ultraviolet light), colored glass that transmits long wavelengths (infrared light), or glass with optical coatings that absorb specific wavelengths, it becomes possible to optimize the optical properties according to the observation application. In addition, by incorporating polarizing filters that control phase differences and special interference films that take into account the effects of surface plasmon resonance, high-precision optical observation can also be achieved.

[0043] (Electrically Conductive Film 12) The electrically conductive film 12 functions as a current collector at the pressing interface inside the battery cell, and in the case of an anodeless battery, it also functions as the negative electrode. Furthermore, it refers to a planar structure that combines light transmittance and electrical conductivity. The electrically conductive film 12 is provided on the first surface 111 of the support substrate 11 and has an electrode surface 12S that is in contact with the electrolyte. During battery charging, ions in the electrolyte (e.g., Li) ＋ These substances (etc.) accept electrons and are deposited on the electrode surface 12S as metallic lithium or compounds (decomposition film or deposits of the electrolyte). During discharge, the reverse reaction occurs, for example, metallic lithium is converted back into Li ＋ It returns to the electrolyte as an ion.

[0044] (Material for the electrically conductive film 12) The type of metal used as the material for the electrically conductive film 12 is not particularly limited, but a material with high conductivity and appropriate corrosion resistance is desirable. Examples of materials for the electrically conductive film 12 include, but are not limited to, gold (Au), silver (Ag), aluminum (Al), tungsten (W), nickel (Ni), stainless steel (SUS), copper (Cu), iron (Fe), platinum (Pt), and combinations thereof. These metals can form a uniform conductive film on the substrate surface by applying a film deposition method such as sputtering or thermal deposition. Furthermore, it is possible to improve flexibility and chemical resistance by using conductive carbon (carbon black, graphite, carbon nanotubes, graphene, etc.).

[0045] Other material examples for the electrically conductive film 12 include, but are not limited to, transparent conductive oxides (TCOs) such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), zinc oxide (ZnO), and indium gallium zinc oxide (IGZO). These are particularly suitable for applications where conductivity must be imparted while ensuring visible light transmittance, and can be deposited by sputtering or other methods.

[0046] Furthermore, other material examples for the electrically conductive film 12 include conductive polymers and conductive organic compound films. For example, conductive polymers such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PEDOT) can be formed by chemical or electrochemical polymerization, providing electrically conductive films with excellent flexibility.

[0047] The electrically conductive film 12 may be made by laminating or compounding multiple materials, as long as optical transparency is not impaired, and may be made by using the above-mentioned materials alone.

[0048] (Light transmittance of the electrically conductive film 12) The electrically conductive film 12 is required to be thin enough to have sufficient light transmittance in order to observe the pressing interface inside the battery. Light transmitted through the support substrate 11 and the electrically conductive film 12 reaches the electrode surface 12S and deeper, making it possible to observe electrochemical reactions inside the battery, such as metal deposition and dissolution on the electrode surface 12S, or gas generation, without any omissions.

[0049] (Electrical conductivity of the electrically conductive film 12) The electrically conductive film 12 needs to have sufficient electrical conductivity in order to allow the induced electrochemical reaction to proceed properly. If the thin film is too thin, the increase in electrical resistance may affect the battery characteristics, so it is important to form it with an appropriate thickness.

[0050] The surface resistance of the electrically conductive film 12 is preferably 5 kΩ or less, more preferably 2 kΩ or less, and more preferably 1 kΩ or less. There is no particular lower limit set for the resistance value.

[0051] (Alloying resistance of the electrically conductive film 12) When observing the deposition of highly reactive alkali metals such as metallic lithium, it is desirable that the electrically conductive film 12 be made of a material that does not undergo alloying reactions with these metals. Specifically, copper, nickel, tungsten, stainless steel, etc., can be used as the electrically conductive film 12, and an appropriate material can be selected depending on the battery system.

[0052] (Oxidation resistance of the electrically conductive film 12) When handling metal thin films in the atmosphere, it is necessary to select a material with high oxidation resistance in order to minimize the effects of spontaneous oxidation. Copper, in particular, is a useful material in this disclosure because it has high electrical conductivity, good oxidation resistance, and high stability against metallic lithium.

[0053] (Thickness of the electrically conductive film 12) The electrically conductive film 12 is formed with an appropriate thickness in order to maintain electrochemical properties while ensuring light transmittance. If the electrically conductive film 12 is made of copper, for example, the practical thickness is 5 nm to 50 nm, and more preferably 10 nm to 30 nm. If it is too thin, the effect of surface oxidation will be greater and the electrical conductivity may decrease, so adjusting the thickness is important.

[0054] (Coating on the electrically conductive film 12) The surface 12S of the electrically conductive film 12 that faces the counter electrode may be partially coated with an insulating coating. This insulating coating can contribute to controlling the electrochemical properties in a specific region. An example of an insulating coating is silica (SiO ２ ), alumina (Al ２ O ３ ), silicon nitride (Si ３ N ４ These include, but are not limited to, inorganic insulating films such as ), polyimide, polytetrafluoroethylene (PTFE), and polystyrene (PS).

[0055] (Working electrode: Comparison with surface plasmon resonance spectroscopy) Surface plasmon resonance spectroscopy is a known operando analysis method that uses a thin metal film formed on a transparent substrate as a current collector foil (Patent Document 1, Non-Patent Documents 4 and 5). The structure of the working electrode in this application is similar to that, but the choice of metal film type and thickness differs. First, for reasons described later, it is difficult to make the metal film thickness less than about 50 nm in surface plasmon resonance spectroscopy. Furthermore, the metal species that have plasmon resonance activity are limited and cannot be applied to a variety of metal species. In addition, the optical path that satisfies the plasmon resonance conditions is limited, making the construction of a dedicated optical system essential. Furthermore, since this method is based on spectroscopic analysis, it is difficult to directly acquire spatial information. Thus, although the structure of the working electrode is similar between surface plasmon resonance spectroscopy and the present invention, the working electrode of the present invention is more useful in terms of the type and thickness of the electrically conductive film and the applicability to optical observation using a microscope.

[0056] <Transmittance Characteristics of Working Electrode 10> The transmittance of the working electrode 10 will be explained using Figure 4. Figure 4 is a graph showing the wavelength dependence of the optical transmittance characteristics of the working electrode 10, which is equipped with copper thin films (electrically conductive films 12) of different thicknesses (0 nm, 10 nm, 20 nm, 50 nm). Figure 4(a) shows the wavelength dependence of the relative optical transmittance characteristics of the copper thin films. Figure 4(b) shows the relative optical transmittance characteristics of the working electrode 10 equipped with copper thin films.

[0057] (Conditions for manufacturing the working electrode 10) As the support substrate 11, a high refractive index substrate (S-TIH11, manufactured by Ohara, refractive index n) ｄ A ratio of 1.78-1.79 was used. Its thickness was 2.0 mm. The thickness of the copper (Cu) thin film as the electrically conductive film 12 was evaluated under conditions of 10 nm, 20 nm, and 50 nm.

[0058] (Transmittance Measurement Conditions) A fiber-optic multi-channel spectrometer "SPELEC1050" (manufactured by DropSense-Metrohm) was used to measure transmittance. A high-intensity halogen lamp (LS-1, manufactured by Ocean Optics) was used as the light source. Data was acquired by taking 20 cycles with an exposure time of 1 ms, and the average value was used.

[0059] (Transmittance Measurement Method) First, the light source and the spectrometer were placed in a straight line, and light from the light source was directly introduced into the spectrometer. The spectral intensity of the light measured in this state (hereinafter referred to as "light transmitted through air") is defined as I(air). Next, the glass substrate was placed in the optical path so that the optical axis and the glass plane were perpendicular. The spectral intensity of the light measured in this state (hereinafter referred to as "light transmitted through air and glass") is defined as I(air + glass).

[0060] Furthermore, a glass substrate on which a copper thin film with a thickness of x nm is formed is similarly placed on the optical path, and the spectral intensity of the measured light (hereinafter referred to as "transmitted light through air, glass, and copper thin film") is defined as I(air + glass + x Cu).

[0061] Subsequently, the relative transmittance (%) spectrum of each component was calculated by determining the ratio of each spectral intensity. For example, by using I(air + glass) as a reference and calculating the ratio with I(air + glass + x Cu), it is possible to estimate the relative transmittance (%) of a copper thin film (film thickness x nm). Here, when x = 0 nm (no deposition), it represents the transmittance of a vacuum, and that transmittance is 100%.

[0062] Furthermore, by calculating the ratio of I(air + glass + x Cu) to I(air) using I(air) as a reference, the relative transmittance of the entire working electrode, including the copper thin film and the glass substrate, can be estimated.

[0063] (Evaluation Results) Figure 4(a) shows the relative transmittance (%) spectra of each component obtained by the above method. The vertical axis represents relative transmittance (%), the horizontal axis represents wavelength (nm), and the film thickness conditions are 0 nm, 10 nm, 20 nm, and 50 nm.

[0064] When the thickness of the copper thin film is 0 nm (i.e., vacuum), its relative transmittance is approximately 100% across the entire wavelength range. This represents the transmittance in a vacuum (a state without the copper thin film), and is a result consistent with theory.

[0065] When the copper thin film thickness was 10 nm, it showed a maximum transmittance of 77% at a wavelength of 610 nm, and a transmittance of 55% or more in other wavelength ranges. The existence of a peak transmittance wavelength is due to the characteristic color of copper (reddish-brown), indicating that light in a specific wavelength range is more easily transmitted.

[0066] When the copper thin film thickness is 20 nm, the transmittance decreases overall, with a maximum transmittance of approximately 50%. While there is a peak around a wavelength of 600 nm, the overall reduction in light transmission is due to the effect of the film thickness.

[0067] For a copper thin film with a thickness of 50 nm, the transmittance decreases further, reaching a maximum of about 12%. It exhibits a transmittance of less than 10% across almost all wavelength ranges. This indicates that as the thickness increases, light absorption and scattering increase, leading to a decrease in light transmittance.

[0068] Figure 4(b) shows the relative transmittance (%) spectrum of the entire working electrode including the glass substrate. The vertical axis represents transmittance (Transmittance, %), the horizontal axis represents wavelength (Wavelength, nm), and the film thickness conditions are 0 nm, 10 nm, 20 nm, and 50 nm.

[0069] When the thickness of the copper thin film is 0 nm (i.e., vacuum), it shows the transmittance of the glass substrate itself. It shows a transmittance of approximately 80%, independent of wavelength. This means that the light transmittance of the glass substrate (S-TIH11) is approximately 0.8 (80%).

[0070] When the thickness of the copper thin film is 10 nm, the transmittance of the entire working electrode, i.e., the copper thin film and the glass substrate, is approximately 60%. This matches the result of multiplying the transmittance of the copper film alone by the transmittance of the glass substrate (80%).

[0071] When the thickness of the copper thin film is 20 nm, the transmittance decreases to about 40%. This is consistent with the result of multiplying the transmittance of the copper film itself (50%) by the transmittance of the glass substrate (80%).

[0072] When the thickness of the copper thin film is 50 nm, the transmittance is less than 10%. Since the transmittance of the copper film itself with a thickness of 50 nm is approximately 12%, when multiplied by the transmittance of the glass substrate (80%), the transmittance becomes approximately 9.6%. This is in good agreement with the measured results.

[0073] As described above, the relative transmittance of the entire working electrode matches the value obtained by multiplying the relative transmittance of the copper thin film shown in Figure 4(a) by the relative transmittance of the glass substrate. In fact, it was confirmed that the relative transmittance spectrum of the entire working electrode, Figure 4(b), agrees well with the result of multiplying the spectrum of the copper thin film alone, Figure 4(a), by 0.8.

[0074] (Surface Plasmon Resonance Spectroscopy of the Working Electrode) The technique of depositing a thin metal film onto a glass substrate and using it as a working electrode has also been applied to other optical measurement techniques, such as electrochemical surface plasmon resonance spectroscopy. However, the optimal thickness of the metal film used differs significantly from that of the present invention. Using Figure 5, the surface plasmon resonance spectra of working electrodes equipped with copper thin films of different thicknesses (10 nm, 20 nm, and 50 nm) will be explained. The measurement conditions, measurement methods, and obtained results will be described below.

[0075] (Measurement Conditions) In this evaluation, a fiber-optic multi-channel spectrometer "SPELEC1050" (manufactured by DropSense-Metrohm) was used as the spectrometer. A high-intensity halogen lamp (LS-1, manufactured by Ocean Optics) was used as the light source. As for data acquisition conditions, 20 cycles were acquired with an exposure time of 1 ms, and the average value of these acquired data was used.

[0076] (Measurement Method) First, a measurement cell was assembled with the copper thin film facing inward, and the inside of the cell was filled with propylene carbonate (PC) solvent. Furthermore, an equilateral triangular prism and a glass surface were optically connected via refractive index matching oil.

[0077] White light from a light source was introduced to the glass surface so that both the angle of incidence and the angle of reflection were 60 degrees. In this case, the incident light underwent total internal reflection at the interface between the glass and the copper thin film, which caused surface plasmon resonance.

[0078] In particular, by incident only on P-polarized light (polarization capable of exciting surface plasmons), light corresponding to the plasmon resonance wavelength is strongly absorbed, and the intensity of the reflected light is specifically attenuated. Utilizing this phenomenon, the intensity spectrum of the reflected P-polarized light was obtained by spectral analysis, and the state of surface plasmon resonance was evaluated.

[0079] On the other hand, in the case of S-polarization (polarization orthogonal to P-polarization), surface plasmons cannot be excited, so no wavelength-specific change is observed in the reflected light intensity. Utilizing this characteristic, the relative reflectance R = P / S was determined by calculating the ratio of the reflected light intensity of S-polarization to that of P-polarization, using S-polarization as a reference.

[0080] Relative reflectance R (%) is a more accurate indicator for evaluating the wavelength dependence of surface plasmon resonance, and the spectral data obtained as a result of the measurement allows for highly accurate analysis of the characteristics of the working electrode.

[0081] (Evaluation Results) Figure 5(a) shows the spectrum of the wavelength dependence of the reflection intensity of the working electrode in P-polarized light. The vertical axis represents the reflection intensity (Counts / 10 ４ The horizontal axis represents wavelength (nm), and the film thickness conditions are 10 nm, 20 nm, and 50 nm.

[0082] When the deposition film thickness is 10 nm, the light intensity is generally higher compared to other film thicknesses, but a clear dip (drop in reflected light intensity) due to plasmon resonance cannot be observed.

[0083] When the deposition film thickness is 20 nm, a decrease in reflectivity can be observed, but the dip originating from surface plasmon resonance remains unclear. Although the effect of surface plasmon resonance is greater than at 10 nm, it is still not clear.

[0084] When the vapor deposition film thickness was 50 nm, a significant attenuation of reflected light intensity was observed at a wavelength of approximately 650 nm. This decrease in reflected light intensity, known as a "dip," is due to strong surface plasmon resonance originating from copper.

[0085] Generally, the optimal film thickness for surface plasmon resonance of copper is known to be around 50 nm, and in this embodiment, results consistent with this theoretical value were obtained. On the other hand, when the thickness of the copper thin film was 20 nm and 10 nm, no clear dips in reflected light intensity were observed.

[0086] Furthermore, the reflected light intensity itself is also affected by the sensitivity characteristics of the spectrometer used. For example, the "kink" observed around 780 nm corresponds to the switching point between the short-wavelength and long-wavelength spectrometers, reflecting the difference in sensitivity correction between the spectrometers. To remove artifacts caused by such sensitivity characteristics, the relative reflectance R = P / S (%) was calculated, and the results are shown in Figure 5(b).

[0087] Figure 5(b) shows the spectrum of relative reflectance (%) obtained by taking the ratio of the reflectance intensity of P-polarized light to the reflectance intensity of S-polarized light as a reference. The vertical axis represents relative reflectance (Reflectance, %), the horizontal axis represents wavelength (Wavelength, nm), and the film thickness conditions are 10 nm, 20 nm, and 50 nm.

[0088] When the deposition film thickness is 10 nm, the reflectivity remains at approximately 90% or higher. A slight attenuation is observed around a wavelength of 650 nm, but the change is not significant enough to be considered a dip. Due to the thin film thickness, the effect of surface plasmon resonance is almost negligible.

[0089] Similarly, when the deposition film thickness is 20 nm, the effect of surface plasmon resonance is hardly observed.

[0090] When the deposition film thickness is 50 nm, the reflectivity drops sharply to 2% around 650 nm, showing a very clear dip, confirming that surface plasmon resonance is strongly expressed. This indicates that a film thickness of 50 nm is the optimal thickness for most effectively exciting surface plasmon resonance.

[0091] Therefore, it is clear that copper thin films with thicknesses of 20 nm and 10 nm are not suitable for highly sensitive analytical applications utilizing surface plasmon resonance. In this embodiment, a copper thin film with a thickness of 50 nm exhibits the most effective surface plasmon resonance characteristics and can be concluded to be the optimal condition from the viewpoint of analytical accuracy and sensitivity.

[0092] Figure 6 shows the spectrum relating to the wavelength dependence of the relative reflectance R = P / S (%) when the thickness of the copper (Cu) vapor-deposited film is 50 nm. This spectrum is obtained from the ratio of the reflected light intensities of P-polarized and S-polarized light and is used to evaluate optical properties, particularly those originating from surface plasmon resonance, with high accuracy.

[0093] In the spectrum shown in Figure 6, a "dip" is observed around a wavelength of approximately 650 nm, where the relative reflectance R (%) decreases significantly. This dip is caused by a surface plasmon resonance phenomenon induced by the interaction between the copper thin film and light.

[0094] The solid black line in Figure 6 represents the baseline, showing the reference reflectance when unaffected by surface plasmon resonance. The baseline gradually increases with wavelength, reflecting the background component originating from the characteristics of the light source and spectrometer. The minimum dip is observed around 650 nm, with a reflectance decrease of approximately 65.6% from the baseline. This "dip depth" is an important indicator of the suitability of the surface plasmon resonance conditions. The center wavelength of the dip (approximately 650 nm) is the surface plasmon resonance wavelength, which is strongly influenced by the optical properties of the copper thin film surface, particularly the complex dielectric constant at the interface between the copper thin film surface and the solvent.

[0095] Table 1 shows the evaluation of the optical properties and surface plasmon resonance properties of copper thin films with respect to film thickness.

[0096]

[0097] Table 1 shows that there is an inverse correlation between the thickness of the copper vapor-deposited film and the dip depth of the surface plasmon resonance spectrum. In Figure 7, the data from Table 1 are plotted on the same graph. Figure 7 is a graph showing the change in wavelength-average transmittance (%) and surface plasmon resonance (SPR) relative reflectance dip depth (%) for copper thin films of different thicknesses (0 nm, 10 nm, 20 nm, and 50 nm). In Figure 7, the horizontal axis plots the copper thin film thickness (nm), the left vertical axis plots the wavelength-average transmittance (%), and the right vertical axis plots the SPR dip depth (attenuation of relative reflectance, %).

[0098] Regarding the wavelength-average transmittance shown on the left vertical axis, circles represent the case of copper thin film "Cu" alone, and triangles represent the case of "Cu + glass" (working electrode), which is a combination of copper thin film and glass substrate. It can be seen that in both cases, the light transmittance decreases significantly as the film thickness increases. Specifically, when the film thickness is 0 nm (i.e., no copper deposition), the transmittance is 100% for copper only and about 84% when a glass substrate is included, but when the film thickness reaches 50 nm, it decreases to about 7% and 5.9%, respectively.

[0099] On the other hand, the square marks shown on the right vertical axis represent the SPR dip depth (attenuation of relative reflectance, %). This parameter indicates the appropriateness of the surface plasmon resonance phenomenon excited by the interaction between the copper thin film and incident light (P-polarized light), and a larger dip depth indicates a more pronounced effect. As can be seen from Figure 7, the dip depth is small when the film thickness is 10 nm and 20 nm (approximately 6.7% and 13.3%, respectively), while it reaches approximately 65.6% when the film thickness reaches 50 nm. In other words, it was confirmed that the strongest SPR resonance is obtained by setting the copper thin film thickness to approximately 50 nm.

[0100] Based on the above, it is considered difficult to design a copper thin film thickness that satisfies both light transmittance and surface plasmon resonance conditions by controlling the film thickness. If transmittance is prioritized, the film thickness must be kept to around 10-20 nm, but in this case, it does not meet the appropriate conditions for surface plasmon resonance. Furthermore, at a film thickness of 50 nm, which is the appropriate condition for surface plasmon resonance, light transmittance cannot be ensured. The figure shows approximate curves estimated based on each measurement. For example, as shown by the dotted arrow in the figure, in order to ensure a relative reflectance dip depth of 50% for surface plasmon resonance, a film thickness of 43 nm or more is required, but the wavelength-average transmittance at that time is expected to be less than 10%.

[0101] From the measurement results of the transmittance of the working electrode 10, it was found that the optimal metal film thickness for surface plasmon resonance and the optimal metal film thickness for the optically transmitting working electrode in the present invention are contradictory. Therefore, the working electrodes commonly used in surface plasmon resonance are unsuitable for the purpose of the present invention, and in this respect, the optically transmitting working electrode of the present invention is novel.

[0102] (Modification 1 of the First Embodiment) Referring to Figure 8, the working electrode 10A according to Modification 1 of the First Embodiment will be described. Figure 8(a) is a top view of the working electrode 10A according to Modification 1, Figure 8(b) is a cross-sectional view along line AB in Figure 8(a), and Figure 8(c) is a bottom view of the working electrode 10A. The working electrode 10A is characterized in that the support substrate 11A has a "support region S" and an "observation region O" which have different thicknesses.

[0103] (Support substrate 11A) The support substrate 11A comprises a support region S and an observation region O located inside the support region S. The support region S corresponds to the outer periphery of the support substrate 11A, and part or all of it is a region that is sandwiched by the holder. The observation region O is located inside the support region S and is a field of view region for observing an object by transmitting light. That is, the support region S can be made thicker to ensure the mechanical strength of the working electrode, and the observation region O can be made thinner to improve the optical transmittance of the working electrode.

[0104] (Support area S of support substrate 11A) A portion of the support substrate 11A needs to have sufficient strength to withstand the pressure of the counter electrode, and therefore a certain thickness (rigidity) is required. Accordingly, the thickness of the support area S is set to be thicker than the thickness of the observation area O. Examples of the planar shape of the support area S include, but is not limited to, the circular shape shown in Figure 8(a), as well as squares, rectangles, polygons, etc.

[0105] (Observation area O of support substrate 11A) In the observation area O of the support substrate 11A, the thickness is designed to be thinner than that of the support area S in order to further improve the transparency of the observation field of view. As shown in Figure 8(a), the planar size of the observation area O of the support substrate 11A is set to be smaller than the planar size of the electrically conductive film 12A. Examples of the planar shape of the observation area O include, but are not limited to, the circular shape shown in Figure 8(a), as well as squares, rectangles, polygons, etc.

[0106] In the modified working electrode 10A, the support substrate 11A is divided into a "support region S" and an "observation region O," and a configuration is adopted in which the thickness differs according to each region. The support region S is outside the observation field of view and has sufficient thickness to ensure a predetermined rigidity that can withstand pressure from the counter electrode. On the other hand, the observation region O is located inside the support region S and has the minimum necessary thickness to enhance transparency. As a result, it is possible to achieve both the rigidity of the entire substrate and high light transmittance of the observation region.

[0107] (Modification 2 of the First Embodiment) Referring to Figure 9, the working electrode 10B according to Modification 2 of the First Embodiment will be described. Figure 9(a) is a top view of the working electrode 10B according to Modification 2, Figure 9(b) is a cross-sectional view along line AB in Figure 9(a), and Figure 9(c) is a bottom view of the working electrode 10B. The working electrode 10B according to Modification 2 comprises an electrically conductive film 12B provided on a support substrate 11B, a peripheral portion 120B, a central portion 121B located inside the peripheral portion 120B, and an electrically conductive portion 122B connecting the peripheral portion 120B and the central portion 121B, and the peripheral portion 120B, the central portion 121B and the electrically conductive portion 122B are electrically connected.

[0108] (Peripheral portion 120B) The peripheral portion 120B contacts the collecting electrode for applying a potential to the electrically conductive film 12B according to the charge and discharge. Examples of the planar shape of the peripheral portion 120B include, but are not limited to, the circular annular shape illustrated in Figure 9(a), a rectangular frame shape, a polygonal frame shape, and even a mesh shape. It may also have a shape corresponding to the planar shape of the support substrate 11B. The peripheral portion 120B corresponds to the area in contact with the holder and is therefore not intended for observation. The outer circumference of the peripheral portion 120B may also coincide with the outer circumference of the support substrate 11B.

[0109] (Central section 121B) The central section 121B is an observation area for observing the galvanic reaction occurring on the surface of the central section 121B. Examples of the planar shape of the central section 121B include, but are not limited to, a circle as illustrated in Figure 9(a), a rectangle, a polygon, etc.

[0110] (Electrically Conductive Section 122B) The electrically conductive section 122B is a connecting section for supplying the potential applied to the peripheral section 120B to the central section 121B. The width of the electrically conductive section 122B can be set to be smaller than the diameter of the central section 121B. As shown in Figure 9(c), the peripheral section 120B and the central section 121B are electrically connected by one electrically conductive section 122B, but multiple electrically conductive sections 122B may be provided and connected by them. In order to limit the occurrence of metal deposition and dissolution reactions to the central section 121B, an insulating layer may be formed on the electrically conductive section 122B to prevent contact with the electrolyte and thus prevent metal deposition and dissolution reactions from occurring. The insulating layer can be formed, for example, by partial mask deposition using a mask that exposes only the electrically conductive section 122B.

[0111] According to the working electrode 10B of the modified example 2 of the first embodiment, it is possible not only to observe the deposition and dissolution behavior of lithium in the central portion 121B, but also to investigate how lithium deposition and dissolution occur at the edges of the central portion 121B. Furthermore, since the peripheral portion 120B is designated as a "region not for observation" and the central portion 121B as the "region for observation," the area to be observed becomes clear, making it easier to identify the target area when inspecting and evaluating with a microscope. In particular, under observation conditions at high magnification, the field of view inevitably becomes narrower, so by designing an observation section 121B that is limited to the field of view observable with a microscope, it becomes possible to observe the entire phenomenon in detail and without omission. It should be noted that in order to design an observation section that is limited to the observation field of view in this way, it is important that the support substrate 11 is an electrical insulator.

[0112] (Modification 3 of the First Embodiment) Referring to Figure 10, the working electrode 10D according to Modification 3 of the First Embodiment will be described. Figure 10(a) is a top view of the working electrode 10D according to Modification 3, Figure 10(b) is a cross-sectional view along line AB in Figure 10(a), and Figure 10(c) is a bottom view of the working electrode 10D. The support substrate 11D of the working electrode 10D according to Modification 4 has a bottom surface as an observation area O on the second surface 112 of the support substrate 11D and is provided with a recess 1121.

[0113] (Recess 1121) Recess 1121 refers to a recess provided on the second surface 112 of the support substrate 11D. The thickness of the observation area O corresponding to the bottom of the recess is smaller than that of the area without a recess. By making the support substrate 11D thinner in the observation area O, the distance that light travels through the support substrate 11D is shortened, and the amount of light absorbed decreases, so that the transmittance of the support substrate 11D in the observation area O can be substantially increased compared to the area without a recess.

[0114] According to Modification 3 of the First Embodiment, the structure is simpler compared to the tapered shape, thus enabling high mass production.

[0115] Furthermore, the light transmittance in the observation area O can be increased. By providing the recess 1121, the distance that light travels through the support substrate 11D in the observation area O is shortened. As a result, light absorption by the substrate material is suppressed, and the light transmittance in the observation area O is improved. This improvement in transmittance can contribute to achieving clearer and more sensitive observation and measurement in the observation device.

[0116] Therefore, according to Modification 3, it is possible to achieve both ease of molding and improved optical properties, and to simultaneously improve the efficiency of the manufacturing process.

[0117] (Modification 4 of the First Embodiment) Referring to Figure 11, the working electrode 10C according to Modification 4 of the First Embodiment will be described. Figure 11(a) is a top view of the working electrode 10C according to Modification 4, Figure 11(b) is a cross-sectional view along line AB in Figure 11(a), and Figure 11(c) is a bottom view of the working electrode 10C. The support substrate 11C of the working electrode 10C according to Modification 3 has a bottom surface as an observation area O on the second surface 112 of the support substrate 11C and is provided with a tapered recess 1121.

[0118] (Tapered recess 1121) The tapered recess 1121 refers to a recess having a shape in which the cross-sectional area changes continuously or in steps toward the observation area O, which is the bottom of the recess, from the second surface 112 of the support substrate 11C. For example, this includes a shape in which the opening of the recess is wide and gradually narrows toward the bottom, or a shape formed in the form of a frustocone or a frustocone.

[0119] According to Modification 4 of the First Embodiment, the tapered recess 1121 ensures a smooth positional relationship between the lens and the support substrate 11C during high-magnification observation with an optical microscope in the observation area O, reducing the risk of the lens colliding with or interfering with the support substrate 11C, and enabling stable focusing. Furthermore, since the thickness of the support substrate in the observation area can be reduced, it also has the advantage of improving light transmission. Modification 4 of the First Embodiment is advantageous, for example, when it is desired to capture an object in three dimensions by irradiating it with light from an oblique direction using the side illumination method.

[0120] The configuration of the electrically conductive film 12C in Modification 4 is the same as that of the electrically conductive film 12B in Modification 2, so its explanation will be omitted.

[0121] (Second Embodiment: Observation Battery Cell) An observation battery cell 80 according to the second embodiment will be described with reference to Figure 12. Figure 12 is a cross-sectional view of the observation battery cell 80 according to the second embodiment.

[0122] (Observation battery cell 80) An observation battery cell 80 is an observation battery cell having a structure that allows optical observation of the metal deposition and dissolution processes at the interface between the separator and the metal thin film inside the battery.

[0123] The observation battery cell 80 allows for the reproduction of battery reactions under conditions close to those of actual battery use, and furthermore, it enables direct observation of its interior, particularly the interface where the counter electrode and working electrode are pressed together. In other words, it has a configuration including a working electrode 10, a counter electrode 30, and a separator 20 containing electrolyte, and charge-discharge tests can be performed in this state.

[0124] Furthermore, the observation battery cell 80 can apply appropriate surface pressure between the working electrode 10 and the counter electrode 30, thereby reproducing the pressing interface inside the battery. By incorporating a pressing mechanism, a constant pressure is maintained, creating an environment close to the actual operating conditions inside the battery.

[0125] The observation battery cell 80 is equipped with a current collector and external connection terminals for supplying current to the working electrode and counter electrode, and can be used to perform charge-discharge tests and electrochemical measurements.

[0126] The observation battery cell 80 comprises an working electrode 10, an working electrode holder 15, a separator 20, a sealing material 25, a counter electrode 30, a counter electrode plate 35, a pressing mechanism 40, a main body 50, a guide part 60, a cover 70, a first terminal 151, and a second terminal 351.

[0127] (Working electrode 10) The working electrode 10 is the same as the working electrode in the first embodiment and its modified form, and comprises a support substrate 11 and an electrically conductive film 12. Details of the working electrode 10 are omitted.

[0128] (Working electrode holder 15) The working electrode holder 15 is a holder on which the working electrode 10 is placed and on which a potential is applied to the electrically conductive film 12 of the working electrode 10.

[0129] The working electrode holder 15 includes an outer bottom surface that contacts the main body 50, an outer surface that partially contacts the main body 50, an inner bottom surface and an inner surface for housing and placing the working electrode 10, and an upper surface that contacts the cover 70. The working electrode holder 15 may also include a first terminal 151 for applying a potential to the electrically conductive film 12 of the working electrode 10. The potential applied to the working electrode holder 15 is transmitted to the electrically conductive film 12 by contact between the electrically conductive film 12 of the working electrode 10 and the inner bottom surface of the working electrode holder 15.

[0130] Examples of materials for the working electrode holder 15 include, but are not limited to, metal materials such as SUS, zinc, titanium, copper, and aluminum alloys, and other conductive resins. These materials are selected according to the operating environment because they provide stable contact with the electrically conductive film 12 of the working electrode 10 and are excellent in terms of corrosion resistance and mechanical strength. SUS (stainless steel) is particularly preferred because it is less prone to corrosion even at the contact point with the electrolyte and also has high mechanical strength.

[0131] (Separator 20) The separator 20 is a component that contains an electrolyte, physically separates the working electrode 10 and the counter electrode 30, and electrically insulates the working electrode 10 and the counter electrode 30.

[0132] For example, when using a liquid electrolyte, examples of separator 20 include, but are not limited to, microporous films or nonwoven fabrics containing one or more of materials such as polyethylene, polypropylene, ethylene-propylene copolymer, glass fiber, ceramic particles, and cellulose. Furthermore, there are no particular restrictions on the thickness of the separator 20, but in a typical battery pack, it is usually desirable to have a thickness in the range of 10 μm to 200 μm, more preferably in the range of 10 μm to 100 μm.

[0133] As for the electrolyte, it is possible to select one that is suitable for the battery system being used. For example, in relatively stable aqueous metal batteries, an aqueous electrolyte can be used. Examples of aqueous electrolytes include aqueous solutions of various metal salts, and those containing these together with sulfuric acid, hydrochloric acid, or alkaline aqueous solutions (e.g., potassium hydroxide, sodium carbonate, etc.), and these are not particularly limited.

[0134] When using alkali metals (e.g., lithium, sodium, potassium, etc.), alkaline earth metals (e.g., magnesium, calcium), and other polyvalent metals (e.g., aluminum, zinc, iron), it is preferable to use various electrolytes as described below, depending on the potential window (range of usable potential) and reactivity.

[0135] Organic electrolyte: LiPF ６ LiBF ４ LiClO ４Fluorosulfonylimide salts (LiFSA, NaFSA, KFSI, etc.), trifluoromethanesulfonimide salts (LiTFSA, NaTFSA, KTFSI, etc.), Mg(TFSI)2, Ca(TFSI)2, MgCl ２ CaCl ２ , AlCl ３ ZnCl ２ FeCl ３ Organic electrolytes obtained by dissolving electrolyte salts such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or other organic carbonates, sulfones, ethers, nitriles, or any mixture thereof in organic solvents.

[0136] Molten salt electrolytes: Fluorosulfonylimide salts such as LiFSA, NaFSA, KFSI, or trifluoromethanesulfonimide salts such as LiTFSA, NaTFSA, KTFSI, Mg(TFSI) ２ Ca(TFSI) ２ MgCl ２ CaCl ２ AlCl ３ ZnCl ２ , FeCl ３ Molten salts of the above salts, or any mixture thereof. Furthermore, molten salt electrolytes obtained by dissolving the above salts in an ionic liquid composed of cations and anions, such as EMI-TFSA (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonylimide)), Py13-TFSA (N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonimide)), Py14-TFSA (N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonimide)).

[0137] Polymer electrolytes: Polyethylene oxide (PEO) or polypropylene oxide (PPO) with added supporting salts.

[0138] Gel electrolyte: A gel-like electrolyte composed of a polymer substrate (e.g., polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), etc.) and an organic electrolyte.

[0139] Regarding lithium-ion batteries, LiPF is used as a support salt. ６ LiFSA (lithium bis(fluorosulfonyl)imide), LiTFSA (lithium bis(trifluoromethanesulfonimide)), etc., can be used, and there are no particular restrictions as long as the electrolyte has sufficient dissociation ability. Furthermore, it is preferable to use an organic solvent or ionic liquid that sufficiently dissolves the supporting salt and exhibits excellent ionic conductivity as the solvent. Specific organic solvents include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). These organic solvents can be used individually or in combination.

[0140] Furthermore, as ionic liquids, those composed of cations and anions, such as EMI-FSA (1-ethyl-3-methylimidazolium bis(fluorosulfonylimide)), EMI-TFSA (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonylimide)), PP13-TFSA (N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonylimide)), and Py14-TFSA (N-methyl-N-butylpyridinium bis(trifluoromethanesulfonylimide)), can be used.

[0141] Regarding sodium-based batteries, NaPF is used as the supporting salt. ６ NaFSA, NaTFSA, and the like can be used. In addition, other electrolyte systems include concentrated electrolyte systems, dilute electrolyte systems, and solvated ionic liquid systems, which are widely used for research and development and specific applications. In this disclosure, these various electrolyte systems can also be comprehensively applied.

[0142] Furthermore, electrolytes are not limited to liquids; gel-like semi-solid electrolytes, sulfide-based solid electrolytes, oxide-based solid electrolytes, and even mixtures thereof can be used. Specifically, gel electrolytes include, for example, polymer materials such as polyvinylidene difluoride (PVdF), polyacrylonitrile (PAN), polyethylene oxide (PEO), and polymethyl methacrylate (PMMA), which are impregnated with organic electrolytes such as ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC). In addition, gel-like electrolytes can be based on cross-linked polymers, thermoplastic elastomers, or inorganic-organic hybrid materials using a sol-gel process, thereby allowing for adjustment of the balance between flexibility, mechanical strength, and ionic conductivity.

[0143] As solid electrolytes, sulfide-based solid electrolytes, oxide-based solid electrolytes, chloride-based solid electrolytes, and organic solid electrolytes can be used, but are not limited to these. As sulfide-based solid electrolytes, Li, which has lithium ion conductivity, can be used. １０ GeP ２ S １２ (LGPS), Li ７ P ３ S １１ Li ３ PS ４、 Li ６ PS ５ Phosphorus sulfide materials such as Cl, Li ４ SiO ４ -Li ３ PO ４ -Li ２ S-type glass solid electrolyte, or Li ３ PS ４ Examples include, but are not limited to, glass ceramic materials based on these materials. These sulfide-based materials exhibit high ionic conductivity at room temperature and are suitably used in all-solid-state batteries and high-safety batteries.

[0144] Examples of oxide-based solid electrolytes include, but are not limited to, materials having perovskite, garnet, and NASICON structures. Specifically, LLZO is used as a garnet-based material, and LATP and LAGP are used as NASICON structure materials. LLTO is a preferred perovskite-based material. These oxide-based electrolytes exhibit excellent heat resistance and chemical stability, making them particularly useful in high-temperature environments and large-scale battery systems.

[0145] As a chloride-based solid electrolyte, Li ３ InCl ６ Ya Li ３ YCl ６ Examples of lithium chlorides include, but are not limited to, those listed above. Examples of organic solid electrolytes include, but are not limited to, polyethers and polymer electrolytes based on ionic liquids.

[0146] Furthermore, in this disclosure, it is possible to use any combination of gel electrolytes, solid electrolytes, or mixed systems of these with liquid electrolytes, and by optimizing the properties of each electrolyte, it is possible to flexibly design according to the desired battery characteristics (e.g., ionic conductivity, safety, mechanical strength, environmental resistance, etc.).

[0147] (Sealing material 25) The sealing material 25 is a component that seals the battery reaction field from the outside and prevents the electrolyte contained in the separator 20 from coming into contact with the working electrode holder 15. Specifically, the sealing material 25 functions as a physical barrier between the separator 20 and the working electrode holder 15. This prevents corrosion of the working electrode holder 15 by the electrolyte. Also, since the battery reaction field is isolated from the atmosphere, the cell can be handled in the atmosphere. Furthermore, its insulating properties prevent electrical short circuits between the inside and outside of the cell. Examples of sealing material 25 include, but are not limited to, silicone rubber, fluororubber (FKM), perfluoroelastomer rubber, ethylene propylene rubber (EPDM), and polytetrafluoroethylene (PTFE). All of these materials have high chemical resistance and corrosion resistance to the electrolyte and can effectively suppress corrosion and contamination of the working electrode holder 15.

[0148] (Counter electrode 30) The counter electrode 30 is an electrode in the observation battery cell 80 that is paired with the working electrode 10 and is responsible for reactions that complement and equilibrium the electrochemical reaction proceeding at the working electrode 10. Various metal foils or composite coated electrodes can be used as the counter electrode in this disclosure.

[0149] While there are no particular limitations on the base metal foil, alkali metal foils such as metallic lithium foil, metallic sodium foil, and metallic potassium foil can be used. These metal foils possess high electrochemical activity and excellent ionic conductivity, which can improve the stability and efficiency of the counter electrode.

[0150] Furthermore, there are no particular restrictions on the composite coating electrode material, as long as it contains alkali metals and the alkali metals can be extracted by electrochemical methods. For example, lithium cobalt oxide (LiCoO) can be used as the positive electrode material for lithium-ion batteries. ２ ) Electrode, Lithium manganese (LiMn ２ O ４ ) Electrode, lithium nickel manganese (LiNi ０．５ Mn １．５ O ４ ) Electrodes, lithium cobalt-nickel-manganese ternary (NCM) electrodes, lithium iron phosphate (LiFePO) ４ Polyanionic cathode materials such as ) can be used.

[0151] Furthermore, iron-based, manganese-iron-based, nickel-manganese-based, polyanionic phosphate-based, and Prussian complex-based materials can be used as counter electrode materials for sodium-ion battery systems. This means that the present disclosure is applicable not only to lithium-ion batteries but also to other alkali metal-ion batteries such as sodium-ion batteries, giving it broad applicability.

[0152] (Counter electrode plate 35) The counter electrode plate 35 is a base that supports the observation battery cell 80 and is a plate material that supplies potential to the counter electrode 30. The counter electrode plate 35 may be provided with a second terminal 351 for connecting to a device that supplies potential to the counter electrode 30. Examples of materials for the counter electrode plate 35 include metal materials such as SUS or aluminum alloy, conductive resin, etc., but are not limited to these.

[0153] (Pressing mechanism 40) The pressing mechanism 40 is a mechanism that presses the counter electrode 30 and the working electrode 10 via the separator 20 in order to reproduce the actual usage conditions inside the battery. There are no particular restrictions on the pressing mechanism itself, but specifically, a spring of a certain strength or a screw-type mechanism can be used. The pressing mechanism is preferably one that can uniformly pressurize the counter electrode and the working electrode and that can specify the area pressure.

[0154] Examples of the pressing mechanism 40 include, but are not limited to, a spring mechanism using a coil spring or leaf spring, a screw-type mechanism using a bolt and nut, and a cam mechanism that allows the pressing force to be adjusted by rotation. In the example shown in Figure 12, the pressing mechanism 40 comprises a coil spring 41 mounted on the counter electrode plate 35 and an electrode retainer 42 mounted on the coil spring and in contact with the counter electrode 30.

[0155] The pressing force of the pressed counter electrode 30 is applied to the working electrode 10 via the separator 20. This makes it possible to observe the interface between the electrically conductive film 12 of the working electrode 10 and the separator 20 under a pressed state corresponding to the actual usage conditions.

[0156] Furthermore, in cases where high surface pressure is required, such as in solid-state batteries, a high-strength screw-type pressing mechanism can be used. Examples of high-strength screw-type pressing mechanisms include, but are not limited to, high-precision tightening mechanisms using reinforced bolts and nuts, and mechanisms incorporating compression disc springs.

[0157] (Main body 50) The main body 50 is a housing member for housing the guide 60, pressing mechanism 40, counter electrode 30, separator 20, and working electrode 10, and plays a role in ensuring the structural stability and airtightness of the battery cell. The main body 50 also has the function of holding each component inside the battery in the correct position and protecting it from external physical shocks and environmental changes.

[0158] The bottom surface of the main body 50 is in direct contact with the counter electrode plate 35, stabilizing the position of the counter electrode plate 35. The inner surface of the main body 50 is the part that contacts the guide portion 60, providing stable support for the guide portion 60. The mounting surface of the main body 50 is the surface for positioning the working electrode holder 15, which holds the working electrode 10, in the correct position. Guides or stoppers may be provided on the mounting surface of the main body 50 to prevent misalignment or tilting of the working electrode holder 15. Examples of materials for the main body 50 include, but are not limited to, highly airtight metals such as stainless steel, and equally airtight resins such as PEEK (polyether ether ketone) resin, PPS (polyphenylene sulfide), PVDF (polyvinylidene fluoride), and other resin materials with excellent chemical resistance and heat resistance. These metal and resin materials deform and deteriorate little even when exposed to various electrolytes and gases inside the battery, allowing for stable use over a long period of time.

[0159] (Guide section 60) The guide section 60 is a cylindrical member provided between the separator 20 and the counter electrode plate 35, and has a space inside for housing the pressing mechanism 40 and the counter electrode 30. Examples of materials for the guide section 60 include, but are not limited to, Teflon®, PEEK resin, PPS, PVDF and other resins with excellent chemical resistance, and ceramics.

[0160] (Cover 70) The cover 70 is a component that has a hole for use as an observation window for the optical microscope, covers the working electrode holder 15 mounted on the main body 50 from above, receives the pressing force from the pressing mechanism 40, and protects the environment inside the cell from the outside. The material of the cover 70 is the same as that of the main body 50.

[0161] (Heating and Cooling Device) The observation battery cell 80 may be equipped with a temperature sensor, heating and cooling means, and temperature control means. This is to reproduce the actual operating temperature environment of the battery and to conduct tests under harsh conditions.

[0162] Examples of temperature sensors include, but are not limited to, thermocouples, platinum resistance thermometers (PT100), thermistors, and non-contact infrared temperature sensors. There may be a single temperature sensor or multiple temperature sensors.

[0163] The heating means may be, for example, a resistance heating element, but is not limited thereto. The heating and cooling means may be, for example, a Peltier element, but is not limited thereto.

[0164] The temperature control means includes a control mechanism to operate the heating / cooling means based on measurement data from a temperature sensor and maintain the set temperature. Specifically, methods such as PID (proportional-integral-derivative) control, feedback control, and programmed temperature control can be employed. Furthermore, the temperature control means can be configured using a microcontroller (MCU), PLC (programmable logic controller), or a dedicated temperature controller, allowing for manual or automatic temperature adjustment. In addition, by incorporating a data logging function, the history of temperature changes can be recorded and their correlation with battery behavior can be analyzed.

[0165] (Reference Electrode) The observation battery cell 80 may be equipped with a reference electrode. A reference electrode is an electrode used to measure the potential within an electrochemical cell, and refers to an electrode whose potential is known or designed to be kept stable. The purpose of providing a reference electrode is to stably measure the electrode potential in an electrochemical reaction and to perform accurate potential control. In particular, in battery charge / discharge tests and observation of electrode reactions, more accurate data can be obtained by measuring the potential of the working electrode in relation to a reference potential rather than as a relative value with respect to the counter electrode.

[0166] Examples of reference electrodes include standard hydrogen electrodes (SHE), silver / silver chloride electrodes (Ag / AgCl), saturated calomel electrodes (SCE), and lithium metal electrodes (Li / Li ＋ These are examples, but are not limited to these. They are selected depending on the electrolyte used and the measurement environment. For example, in the measurement of lithium-ion batteries, lithium metal is commonly used as the reference electrode.

[0167] According to the observation battery cell 80 of the second embodiment, it is possible to reproduce a pressing state close to that of actual battery use, and to optically observe the metal deposition and dissolution processes at the interface in that state. In particular, in the metal ion deposition and dissolution processes, it is possible to capture in real time the growth process of precipitates (e.g., dendrites), the formation of electrode surface films accompanying the decomposition of electrolytes, and changes in surface morphology during dissolution. This makes it possible to assess the risk of dendrite shorting, which is related to the safety of batteries, and to analyze the degradation mechanism of electrode materials.

[0168] The observation battery cell 80 maintains airtightness through the cover 70 and sealing material 25. This prevents evaporation of the electrolyte and provides a stable measurement environment unaffected by external environmental factors (humidity, oxygen, carbon dioxide, etc.).

[0169] The observation battery cell 80 is equipped with an observation window (hole) in the cover 70, enabling observation using an optical microscope with reflected light. This allows for real-time visualization of minute dynamic changes in metal deposition and dissolution at the electrode interface. Digital analysis of the optical microscope images enables quantitative evaluation of the precipitate growth rate, particle size, and distribution.

[0170] In evaluating new electrode and electrolyte materials, the electrochemical performance of synthesized materials can be verified in actual cell structures. This is expected to improve the accuracy of evaluation during the material development stage and shorten the development cycle.

[0171] By changing the structure of the pressing mechanism 40 and the guide section 60, it is possible to simulate cell operation under different pressure conditions. This makes it possible to evaluate the optimal pressure conditions for commercial battery design and the resistance to changes in internal cell pressure.

[0172] The observation battery cell 80 can simultaneously perform optical and electrochemical evaluations while reproducing the actual internal state of a battery. This can greatly contribute to the evaluation of new materials, improvement of cell design, and strengthening of safety testing in battery development. In particular, the ability to observe the interface under pressure conditions close to actual usage conditions provides valuable insights that cannot be obtained with other evaluation methods, and can accelerate the evolution of battery technology. Specifically, it can visualize gas generation inside the battery, reaction heterogeneity due to gas retention at the pressed interface, and resulting charge / discharge abnormalities.

[0173] (Third Embodiment: Optical Microscope Observation Device 900) Using Figure 13, an optical microscope observation device 900 capable of operando observation according to the third embodiment will be described. Operando observation is a method of observing and measuring physicochemical reactions and material behavior in real time under actual operating or reaction conditions to analyze structural changes, chemical properties, and reaction behavior. In this embodiment, operando observation is performed by the optical microscope observation device 900 and the image processing device 200, and the electrochemical measuring device 300 induces physicochemical reactions in the object of observation.

[0174] (Optical Microscope Observation Device 900) The optical microscope observation device 900 can be various microscopes, such as optical microscopes including monocular microscopes and binocular stereo microscopes, or digital microscopes. Furthermore, as illumination methods for the object to be observed in this device, examples include reflected illumination, where illumination light is shone perpendicularly from the lens direction; side illumination, where illumination is shone obliquely to the object to be observed; and uniform illumination, which uses ring illumination. In Figure 13, a monocular microscope with reflected illumination is used for explanation, but it is not limited to this, and other configurations can be applied, such as a binocular microscope employing side illumination, a digital microscope employing uniform illumination, or a hybrid microscope combining multiple illumination methods. The optical microscope observation device 900 employs reflected illumination and includes multiple objective lenses 90, a revolving nosepiece 91, a half mirror 92, a main body 93, a camera 94, a light source 95, and a stage 96.

[0175] (Multiple Objective Lenses 90) The multiple objective lenses 90 are a group of lenses for magnifying and observing the interface between the electrically conductive film 12 and the separator 20 of the observation battery cell 80, and each has a different magnification and numerical aperture. This allows the user to select the optimal magnification according to the purpose of observation. There are no particular restrictions on the objective lenses 90 as long as they have a wide working distance and sufficient resolution. Lenses with too high a magnification are not very suitable because they have a short working distance and a narrow observation range. The magnification of the lenses is preferably several times to several thousand times, more preferably several tens to several hundred times.

[0176] (Revolver 91) The revolver 91 is a rotating holder for switching between multiple objective lenses 90. By rotating the revolver 91, one of the objective lenses 90 can be precisely positioned on the optical axis.

[0177] (Half-mirror 92) The half-mirror 92 is an optical element for splitting or combining the optical path that guides illumination light from the light source 95 to the observation battery cell 80 and the optical path that guides observation light generated by reflection, transmission, or fluorescence from the sample to the camera 94. It is designed so that the reflectance and transmittance are in a predetermined ratio and is generally also called a beam splitter.

[0178] In this embodiment, the half-mirror 92 is positioned at the point where the optical path on the light source side 95 and the observation optical path merge, enabling a configuration in which illumination and observation are performed coaxially. An optical filter may be provided between the half-mirror 92 and the light source 95.

[0179] (Main body 93) The main body 93 is a structure that holds the main frame, drive mechanism, and optical system of the optical microscope observation device 900, and is responsible for positioning and stabilizing each component. In order to adjust the focus by controlling the height of the stage 96 or the objective lens 90, it may be equipped with a focusing mechanism (coarse adjustment knob, fine adjustment knob, and a Z-axis control mechanism using an electric motor). It may also be equipped with auxiliary lenses, etc.

[0180] (Camera 94) Camera 94 is an image acquisition device for capturing observation light guided through the half mirror 92. Camera 94 is equipped with, for example, a CCD (Charge Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor) as an image sensor. Images acquired by camera 94 are recorded and analyzed as digital data and may be transmitted to the image processing device 200.

[0181] Alternatively, an eyepiece can be installed instead of the camera 94, in which case the operator can directly observe the light being observed. Using an eyepiece allows for real-time monitoring of changes in the observed object, which is particularly useful in situations requiring quick decision-making or fine adjustments. However, for recording and analysis purposes, it is more appropriate to use the camera 94 to acquire digital image data and transmit it to the image processing device 200.

[0182] In Figure 13, the output of camera 94 is connected to the image processing device 200, but it may also be connected to a display device (not shown). In this case, the image acquired by camera 94 can be displayed on the display device in real time, allowing the operator to immediately check the status of the object being observed. Direct monitoring is particularly effective in situations requiring quick judgment and adjustment. Examples of display devices include, but are not limited to, liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs).

[0183] (Light Source 95) The light source 95 is a device that supplies light to illuminate the observation battery cell 80, which is the object of observation. Any white light or monochromatic light of any wavelength can be used as the light for optical illumination. High-brightness LEDs, halogen light sources, mercury lamp light sources, xenon lamp light sources, laser light sources, and ultraviolet light sources can also be used. In this embodiment, the illumination light from the light source 95 irradiates the observation battery cell 80 via the half mirror 92, and the reflected light is guided to the camera 94, resulting in incident illumination. The light intensity and illumination mode can be switched according to the purpose of observation and the properties of the sample.

[0184] (Illumination Methods) For microscopic observation, various illumination methods can be used, such as coaxial reflected light illumination, side illumination, diffuse illumination, total illumination, unidirectional illumination, polarized light illumination, bright-field illumination, and dark-field illumination. Furthermore, by placing phase plates or polarizers in the optical path, the contrast of reflected light can be enhanced. Additionally, by using an optical system with Schlieren lenses, density differences and changes in refractive index of the object being observed inside the cell can be emphasized and visualized. This allows for clear visualization of, for example, gas generation from the electrolyte.

[0185] (Optical Observation Techniques) In addition to bright-field and dark-field observation techniques, phase contrast and Schlieren methods may also be applied as optical techniques for observation. Phase contrast is a technique that visualizes differences in refractive index distribution within a transparent sample as contrast, and is suitable for detailed observation of concentration changes in electrolytes and minute changes on the electrode surface. On the other hand, Schlieren is effective for visualizing fluid flow and gas generation in electrolytes because it can detect minute changes in refractive index with high sensitivity. By applying these techniques, it is possible to accurately analyze interfacial phenomena and concentration changes that are difficult to capture with conventional optical observation techniques, and this can contribute to elucidating the detailed mechanisms of electrochemical reactions. Furthermore, scanning interference microscopy is an optical technique that can visualize changes in interference intensity within the electrode surface by scanning in a direction perpendicular to the surface while changing the focus. By using this technique, it is possible to visualize the modulation of interference intensity inside the battery, from the counter electrode and separator to the working electrode, in the in-plane and layer cross-sectional directions. This makes it possible to visualize in three dimensions physical anomalies that cause changes in interference intensity, such as gas generation inside the battery, deposition of metallic lithium, and degradation of the electrolyte.

[0186] (High-resolution observation) To perform high-resolution observation, a high-magnification optical system can be used. Furthermore, to achieve detailed observation beyond the limits of optical magnification, it is also possible to use digital zoom imaging with a high-resolution image sensor.

[0187] High resolution refers to an image sensor with 1 million pixels (1 megapixel) or more. For more detailed observation, it is desirable to use a high-resolution CMOS / CCD image sensor with 5 million pixels (5 megapixels) or more, or even 10 million pixels (10 megapixels) or more. By using a digital camera or CMOS / CCD image sensor equipped with a high-resolution element, high-definition magnified observation independent of optical magnification becomes possible, allowing for detailed analysis of changes in microstructure and local reactions on electrode surfaces.

[0188] (High-speed shooting) High-speed shooting refers to shooting at a frame rate exceeding the normal video shooting frame rate of 30 frames per second (fps). In particular, for dynamic analysis of electrochemical reactions, a frame rate of 100 fps or higher, and even 1000 fps or higher, is desirable.

[0189] To perform high-speed imaging without compromising image quality and to ensure sufficient light during high-speed imaging, it is desirable to use a highly sensitive CMOS / CCD sensor. For example, using a back-illuminated CMOS sensor or an electron-multiplier CCD (EMCCD) can reduce noise during high-speed imaging while maintaining high resolution. Since a large amount of data is generated in a short time during high-speed imaging, it is desirable to use a camera equipped with an interface that enables high-speed data transfer (such as USB 3.0, PCIe, or CameraLink). Furthermore, it is possible to apply a system equipped with a high-performance image processing processor (GPU or FPGA) for real-time image processing. In high-speed imaging, the exposure time is short, so it is important to ensure sufficient illumination. For example, using high-brightness LEDs, laser light sources, or strobe illumination makes it possible to maintain a high signal-to-noise ratio even under short exposure times.

[0190] (Spectroscopic analysis using a hyperspectral camera) Spectroscopic analysis using a hyperspectral camera is a technique that acquires spectral data over a wide wavelength range for each imaging pixel of an object and analyzes detailed spectral information in two dimensions. This makes it possible to evaluate the chemical composition, crystal structure, oxidation-reduction state, and surface changes of a substance non-destructively and non-contact. Hyperspectral cameras can measure absorption, reflection, and scattering characteristics in specific wavelength ranges such as visible light (400-700 nm), near-infrared (NIR, 700-2500 nm), and mid-infrared (MIR, 2500-5000 nm) with high resolution, so they can visualize chemical changes in electrode surfaces and electrolytes in real time and be used for detailed analysis of electrochemical reactions.

[0191] By capturing spectral changes associated with metal deposition and oxidation-reduction reactions, the spatial distribution of the chemical composition on the electrode surface can be visualized. For example, in lithium-ion battery electrodes, changes in lithium concentration and the oxidation-reduction state of the active material can be analyzed in real time. In metal secondary batteries, it is also possible to visualize the processes of metal deposition and dissolution, as well as the types of inorganic or organic surface coatings and their spatial distribution within the electrode surface associated with the decomposition of the electrolyte.

[0192] By analyzing the absorption spectrum for a specific wavelength range, the behavior of electrolytes and by-products (Li) at electrode surfaces and interfaces can be determined. ２ CO ３ It can detect the formation of (such as SEI films). This makes it possible to evaluate the degradation mechanism and reaction heterogeneity of batteries.

[0193] By using a hyperspectral camera equipped with high-speed imaging capabilities, it is possible to analyze the temporal changes in electrochemical reactions and capture in detail the progress of ion diffusion and interfacial reactions on electrode surfaces.

[0194] For example, it is possible to observe in real time the phase change of electrode materials during the charging and discharging process, as well as the growth process of deposited metals.

[0195] (Stage 96) Stage 96 is a support base for holding the observation battery cell 80 and moving it in the X, Y, and Z directions as needed. Stage 96 may be equipped with, for example, an X-Y-Z movement mechanism. A micrometer head or a stepping motor is provided to control minute amounts of movement, thereby making it easy to align the observation battery cell 80 and change the field of view.

[0196] The stage 96 may include a holder for fixing the observation battery cell 80. The holder is designed to fit the shape and size of the observation battery cell 80, and it is desirable to include a fine adjustment mechanism or fixing clamp, especially when high-precision observation is required. Examples of methods for fixing the holder include, but are not limited to, mechanical clamping mechanisms, spring-type holding mechanisms, and magnetic fixing mechanisms.

[0197] If it is necessary to observe the temperature dependence of the observation battery cell 80, the stage 96 can incorporate a heater or Peltier element that can maintain the stage at a constant temperature.

[0198] (Electrochemical Measurement Device 300) The electrochemical measurement device 300 is a device that applies electrochemical stimuli (voltage, current, etc.) to the observation battery cell 80 and measures and analyzes its response (potential, current, DC resistance, AC impedance, amperometry, voltammetry, Coulomb efficiency, etc.). By combining the electrochemical measurement device 300 with the observation battery cell 80, it is possible to grasp in real time what kind of electrochemical response the sample in the observation battery cell 80 shows under operando observation and perform correlation analysis with observation images acquired by the image processing device 200, etc. An example of the electrochemical measurement device 300 is described below.

[0199] (Example 1 of electrochemical measuring device 300: charge / discharge test device) The charge / discharge test device 330 is a device for performing constant current measurement (Constant Current, CC), constant voltage measurement (Constant Voltage, CV), and pulse measurement on a battery cell. This device preferably has the following functions.

[0200] Charge / Discharge Cycle Test: A function to evaluate the capacity retention rate, charge / discharge charge ratio (Coulomb efficiency), charge / discharge energy ratio (round-trip efficiency), open-circuit voltage characteristics, and cycle life.

[0201] Safety evaluation: Functions to monitor changes in battery characteristics under conditions of overcharging, over-discharging, rapid charging and discharging, and abnormal temperature environments.

[0202] (Example 2 of electrochemical measuring device 300: Continuous electrochemical measurement) Continuous electrochemical measurement is a method of applying a constant current or potential over a long period of time and acquiring the response in real time. This makes it possible to continuously measure the progress of chemical reactions on the electrode surface and the charge / discharge characteristics of a battery. The following methods can be applied as specific measurement methods.

[0203] Constant current measurement (Galvanostatic experiments): By applying a constant current to the cell and measuring the change in potential (cell voltage) during that time, the changes in electrode potential (cell voltage) over time and the progress of the battery reaction are analyzed.

[0204] Potentiometric experiments: By fixing the cell potential to an arbitrary value or changing it over time in an arbitrary pattern, and measuring the current response during that time, the rate of electrode reaction and charge transfer characteristics are evaluated.

[0205] Linear Sweep Voltametry (LSV): This method analyzes the onset potential and reaction rate of redox reactions by sweeping the potential at a constant rate and measuring the current response.

[0206] Cyclic voltammetry (CV): This method involves periodically sweeping the electrical potential in the positive and negative directions to evaluate the reversibility of redox reactions and electrode activity.

[0207] (Example 3 of electrochemical measuring device 300: Intermittent electrochemical measurement) Intermittent electrochemical measurement is a method that involves repeatedly applying and stopping current or potential based on a fixed period or external trigger, and analyzing the response. This allows for highly accurate measurement of time-dependent electrode reactions and diffusion phenomena. The following methods can be applied as specific measurement methods.

[0208] Differential Pulse Voltammetry (DPV) and Square Wave Voltammetry (SWV): These methods apply a pulsed potential and detect weak current changes with high sensitivity to perform quantitative analysis of reactive species on the electrode surface.

[0209] AC impedance measurement (Electrochemical Impedance Spectroscopy, EIS): By applying an AC voltage with a small amplitude of approximately 10 mV or less to a cell at various AC frequencies and analyzing the current response for each frequency component, the reaction resistance and diffusion characteristics of the electrode interface are analyzed in detail, and the internal resistance and degradation process of the battery are evaluated.

[0210] Intermittent charge / discharge measurement (pulse charge / discharge test): By stopping the application of current or potential at regular intervals and measuring the potential change in an open circuit state, the internal resistance and diffusion coefficient are analyzed.

[0211] (Example 4 of electrochemical measuring device 300: Continuous and intermittent electrochemical measurement) By combining both continuous and intermittent measurements, it is possible to evaluate electrochemical properties in more detail. For example, the following measurement methods can be considered.

[0212] Interleaved EIS (Interleaved Electrochemical Impedance Spectroscopy): EIS measurements are inserted during the charge-discharge cycle to track electrode degradation processes and changes in interfacial resistance in real time.

[0213] Combined measurement method: After performing cyclic voltammetry within a specific potential range, constant current measurements or pulse measurements are applied to analyze the detailed mechanism of the electrode reaction.

[0214] (Image Processing Device 200) The image processing device 200 is a device that processes images captured by the camera 94. Specifically, it analyzes and corrects image data acquired by the acquisition unit 202 using the image processing unit 203, and performs feature extraction and identification processing using algorithms such as machine learning and deep learning in the learning unit 204 as needed. It also exchanges data with external devices and servers via the communication unit 205 and stores image data and processing results in the storage unit 206. Furthermore, it can receive user instructions via the input unit 207 and perform operations such as visualizing the processing results on the display unit 208.

[0215] The image processing device 200 includes a processing unit 201, a communication unit 205, a storage unit 206, an input unit 207, and a display unit 208.

[0216] (Processing Unit 201) The processing unit 201 is the module responsible for the central computational processing of the image processing device 200. It has multiple submodules inside and analyzes the image data obtained through the acquisition unit 202 from multiple perspectives. Specifically, it oversees the main logic executed by the image processing device 200, such as filtering and feature extraction in the image processing unit 203 and the construction and inference of machine learning models in the learning unit 204.

[0217] The processing unit 201 can have a multi-core configuration in terms of hardware, including a CPU (Central Processing Unit), GPU (Graphics Processing Unit), and DSP (Digital Signal Processor), enabling high-speed and parallel computation. In terms of software, it can be configured to run on an OS that incorporates image analysis algorithms and machine learning frameworks.

[0218] The processing unit 201 comprises an acquisition unit 202, an image processing unit 203, and a learning unit 204.

[0219] (Acquisition Unit 202) The acquisition unit 202 is responsible for receiving image data to be processed by the image processing device 200. The acquisition unit 202 acquires image data from the camera 94 in real time or at a constant frame rate. USB or the like can be used as an interface. The acquisition unit 202 may have the function of importing image files (JPEG, PNG, TIFF, RAW, etc.) and video files (MP4, AVI, MPEG, etc.). The acquisition unit 202 may have a buffer area for temporarily storing the acquired image data.

[0220] (Image Processing Unit 203) The image processing unit 203 is a module that analyzes and processes image data from the acquisition unit 202 using various algorithms. The image processing unit 203 can perform preprocessing such as noise reduction (filtering), white balance adjustment, brightness / contrast correction, and geometry correction (lens distortion correction, etc.). The image processing unit 203 can perform feature extraction processing such as edge detection, corner detection, and feature extraction. The image processing unit 203 can perform quantitative evaluation of images such as pixel intensity analysis, particle counting, area / perimeter measurement, and shape recognition. The results from the image processing unit 203 are ultimately used as input for further analysis via the learning unit 204, the display unit 208, or external devices.

[0221] (Learning Unit 204) The learning unit 204 is a module that uses algorithms such as machine learning and deep learning to acquire higher-order information from image data. This enables not only feature extraction through simple image processing, but also more advanced analysis such as pattern recognition, anomaly detection, and time-series data analysis.

[0222] The learning unit 204 can use a pre-trained model, but it can also continuously learn using new data as needed. In this case, the model's accuracy can be improved by reflecting new data obtained during operation.

[0223] The output of the learning unit 204 is provided in various forms, such as anomaly detection results, classification labels, and quantified evaluation indicators. For example, the state of battery cells can be classified into categories such as "normal," "slightly degraded," and "severely degraded," or specific features (such as surface metal deposition density and deposition distribution) can be quantitatively evaluated. This makes it possible to judge the state of the observed object more objectively.

[0224] (Communication Unit 205) The communication unit 205 is the interface through which the image processing device 200 exchanges data with external devices and networks. The communication unit 205 sends and receives image data and model information with servers and cloud services via wired LAN (Ethernet) or wireless LAN (Wi-Fi).

[0225] (Storage Unit 206) The storage unit 206 is a storage device for holding internal data of the image processing device 200. The storage unit 206 stores the original image captured by the camera 94, the processed image generated by the image processing unit 203 and the learning unit 204, and the analysis results. The storage unit 206 also stores the learning data created by the learning unit 204. The storage unit 206 may be an HDD (hard disk drive), SSD (solid state drive), DRAM, flash memory, etc., but is not limited to these.

[0226] (Input Unit 207) The input unit 207 is an interface for the user to perform various operations and give instructions to the image processing device 200. The input unit 207 is, for example, a physical operating device such as a keyboard, mouse, or touch panel, but is not limited to these.

[0227] (Display Unit 208) The display unit 208 is a display device for presenting the results output by the image processing device 200 to the user. The display unit 208 is, for example, an LCD (liquid crystal display) or an organic EL display, but is not limited to these.

[0228] (Data Recording) Image data from the optical microscope observation device 900 and charge / discharge data (values ​​such as current and voltage) from the electrochemical measuring device 300 are recorded at the same time. This allows charge / discharge measurement data and corresponding image data to be recorded in perfect synchronization. Specifically, by providing a control circuit that unifies the data acquisition timing of both devices based on a common clock signal, it becomes possible to perfectly synchronize charge / discharge measurement data and corresponding image data. Furthermore, it is desirable that the data recording system be equipped with a processor with high-speed data processing capabilities and a mechanism that can adjust the frame acquisition timing of the optical microscope observation device 900 and the measurement interval of the electrochemical measuring device 300 in real time. This makes it possible to accurately record and analyze minute structural changes and interfacial reactions that occur during charging and discharging, while perfectly correlating them with electrochemical behavior. The time resolution of the data recording is preferably 30 ms, more preferably 10 ms. For video recording, a video rate of about 30 frames / s is desirable.

[0229] According to the third embodiment, by combining the optical microscope observation device 900 and the electrochemical measurement device 300, it becomes possible to observe and analyze electrochemical reactions within a battery cell in real time. This allows for dynamic evaluation of changes in electrode materials, reaction non-uniformity, and degradation behavior during the charge-discharge process. By examining the correlation between images observed by the optical microscope observation device 900 and measurement results obtained by the electrochemical measurement device 300, it is possible to understand the details of the electrochemical response corresponding to events inside the cell. This correlation analysis makes it possible to quantitatively evaluate, for example, structural changes on the electrode surface, metal deposition / dissolution behavior, and film formation / peeling behavior under specific potential and current conditions, and to analyze in detail their impact on the battery's performance and lifespan. In particular, it is possible to optically observe dendrite growth, crack formation in active materials, gas generation associated with electrolyte decomposition, and the progress of interfacial reactions, and quantitatively analyze their impact on charge-discharge characteristics.

[0230] The following describes the specific configuration and evaluation results of the embodiments of this disclosure. While this embodiment focuses on lithium-based metal secondary batteries, which exhibit particularly excellent performance among batteries using base metals, the scope of this disclosure is not limited to these and is applicable to other metal secondary batteries. Examples of other metal secondary batteries include sodium-based metal secondary batteries and potassium-based metal secondary batteries. These batteries are known to be less constrained by lithium resources and exhibit advantageous characteristics in specific applications. Magnesium-based and zinc-based metal secondary batteries are also examples of batteries to which the technology of this disclosure can be applied, each possessing features such as high theoretical capacity and improved safety. Therefore, this disclosure is not limited to lithium-based metal secondary batteries and is applicable to these other metal secondary batteries as well.

[0231] <1. Fabrication of electrically conductive current collector film> A current collector film was fabricated by depositing copper (electrically conductive film) on a transparent glass substrate (support substrate) with a diameter of 32 mm and a thickness of 2 mm using a vacuum deposition method with resistance heating. A vacuum deposition apparatus (JEE-420, JEOL) was used for film fabrication, and a tantalum foil with a thickness of 30 μm was used as the heating boat. A predetermined amount of copper wire was placed on the heating boat, and the copper was evaporated by electric heating and deposited onto the glass substrate. The vacuum level at the start of deposition was 3.0 × 10⁻⁶. －３ It was set to Pa or lower.

[0232] Since the thickness of the copper deposition film varies depending on the deposition distance and the amount of copper wire used, the deposition film thickness at predetermined distances and amounts was evaluated using a film thickness gauge and a scanning electron microscope. Based on these results, copper thin films of the desired thickness were reproducibly fabricated by adjusting the amount of copper wire used while keeping the deposition distance constant.

[0233] <1.1 Thickness Dependence of Copper Current Collector Film> Figure 14 shows the appearance of an electrical conductive film 12 with deposition film thicknesses of 10 nm, 20 nm, and 50 nm formed on the entire surface of a support substrate 11, as imaged under indoor lighting conditions. From Figure 14, it can be seen that in the case of a deposition film thickness of 50 nm, the grid pattern of the underlying substrate is hardly transmitted under indoor lighting conditions. This does not satisfy the transmittance required for optical observation in this disclosure. For example, it shows that observation using only indoor light without using the light source 95 of the optical microscope observation device 900 in Figure 13 is insufficient.

[0234] On the other hand, when the vapor deposition film thickness is 20 nm, the underlying grid is more visible than when the grid is 50 nm, but the transparency is insufficient.

[0235] In contrast, when the vapor deposition film thickness is 10 nm, the grid pattern of the underlying surface is sufficiently visible, making it a suitable film thickness for optical observation under indoor lighting conditions. Therefore, it is clear that when the film thickness is 50 nm, the transmittance is insufficient and it is unsuitable for optical microscopy observation. Accordingly, for the metal current collector film of the present invention, a film thickness of 10 nm or less is desirable for optical microscopy observation.

[0236] <1.2 Mask Deposition> Mask deposition was performed to specify the area of ​​the copper current collector film 12 to an arbitrary size. The method is shown in Figure 15. First, as shown in Figure 15(a), copper deposition was performed with a sticker backing 17, which had been pre-processed to an arbitrary shape, attached to the surface of the glass substrate. After deposition, the sticker backing 17 was removed, and as shown in Figure 15(b), a current collector film 12 having a copper thin film formed to the desired shape was produced. The current collector film 12, made of a copper thin film, comprises a peripheral portion 120, a central portion 121 located inside the peripheral portion 120, and an electrically conductive portion 122 connecting the peripheral portion 120 and the central portion 121, and the peripheral portion 120, the central portion 121, and the electrically conductive portion 122 are electrically connected.

[0237] In the design of the observation cell, the area in contact with the electrolyte is the region inside the black dotted line in Figure 15(b), and the peripheral portion 120 of the copper thin film formed on the edge of the glass substrate does not directly contribute to the electrochemical reaction. However, the peripheral portion 120 contributes well to the electrical contact of the copper current collector film by being connected to the external electrode.

[0238] The region where the electrochemical reaction actually takes place is limited to the circular central portion 121 formed in the center of the glass substrate 11, and in this embodiment, its diameter is set to 6 mm. Although the copper thin film of the electrically conductive portion 122 comes into contact with the electrolyte, for reasons to be described later, the region that actually participates in the deposition of the base metal is limited, and its influence is considered to be negligible in this embodiment. It is also possible to coat the surface of the electrically conductive portion 122 with an insulating layer by some method to avoid contact with the electrolyte.

[0239] Figure 16 shows current collector films 12 made of copper thin films fabricated by mask deposition, arranged for deposition thicknesses of 50 nm, 20 nm, and 10 nm. It was confirmed that the mask deposition method is effective for all film thicknesses, and that copper current collector films 12 of any specified size can be fabricated. Furthermore, it was revealed that a deposition thickness of 10 nm or less provides sufficient transparency.

[0240] <1.3 Measurement of DC Resistance of Copper Current Collector Film> To evaluate whether the fabricated copper current collector film functions properly, its in-plane electrical resistance was measured and compared. Specifically, working electrodes 10 equipped with copper current collector films 12 of various thicknesses were assembled into a predetermined electrochemical cell, and the DC resistance was measured using a simple tester while the cell was short-circuited. The measurement results are shown in Table 2.

[0241]

[0242] The measurement results, as shown in Figure 14, confirmed that when copper was deposited over the entire glass substrate, the resistance was low, around a few ohms, regardless of the film thickness, indicating good electrical conductivity. On the other hand, as shown in Figure 16, when copper was deposited partially using a mask, the resistance increased to several hundred ohms when the film thickness reached 10 nm, and in the highest case, it reached about 1 kΩ. From this, it can be inferred that the thinner the copper thin film, the more difficult it becomes to secure conductive paths in the planar direction.

[0243] If the film thickness is 10 nm or less, the resistance is expected to reach 1 kΩ or more. In this case, it may be difficult to ensure sufficient conductivity as a copper current collector film. From the results of this embodiment, it was determined that a film thickness of about 10 nm is the optimal thickness in terms of both transmittance and conductivity during optical observation.

[0244] <2. Optical Microscope Observation of Metal Deposition and Dissolution> In this example, the deposition and dissolution behavior of metallic lithium and metallic sodium, which are alkali metals with particularly high reactivity among base metals, was investigated by optical microscope observation. In particular, since metallic lithium readily forms alloys with many metals, it is necessary to experimentally confirm whether the 10 nm thick copper current collector film used in this example functions appropriately in electrochemical measurements and optical microscope observation. On the other hand, for base metals other than those mentioned above, since their reactivity is lower than that of metallic lithium and metallic sodium, it is easy to assume that deposition and dissolution will proceed without problems on the copper current collector film fabricated in this example.

[0245] <2.1 Deposition and Dissolution of Metallic Lithium - Organic Electrolyte> To observe the deposition and dissolution processes of metallic lithium, optical microscopy was performed using a commonly used organic electrolyte. The cell configuration in this example is shown in Figure 17, reproducing a normal coin cell structure, and the other observation conditions are as follows. A metallic lithium foil with a diameter of 12 mm and a thickness of 400 μm was used as the counter electrode, and a copper thin film with a thickness of 10 nm and a diameter of 6 mm was used as the collecting electrode. A polypropylene film with a thickness of 25 μm was used as the separator, and 1 M LiPF was used as the electrolyte. ６A solution was used, prepared by dissolving the substance in a mixed solvent of EC (ethylene carbonate) / DEC (diethyl carbonate) = 1.0. For optical microscopy observation, a binocular stereomicroscope with a magnification of 20x was used, and the charge / discharge conditions were set to a current density of 1.0 mAcm². －２ Charging (metallic lithium deposition) was performed for 60 seconds (based on copper current collector area), and then discharging (metallic lithium dissolution) was carried out at the same current density.

[0246] The deposition and dissolution reactions of metallic lithium also occur in the electrically conductive portion 122 facing the metallic lithium counter electrode, as indicated by the black dotted line in Figure 17. The area of ​​the electrically conductive portion 122 is approximately 6 mm². ２ In contrast, the total area of ​​the copper current collector film is approximately 28.3 mm². ２ Therefore, the influence of the electrically conductive portion 122 on the total reaction area contributing to the deposition and dissolution of metallic lithium is less than 20%, and in this embodiment, its influence is considered to be limited.

[0247] Figure 18 shows the changes in the electrode state during the charging and discharging process at the central portion 121 of the copper current collector film 10, in chronological order. In Figure 18, the circular portion corresponds to the central portion 121 of the copper current collector film 10. As shown in the upper part (a) of Figure 18, at the start of charging (metallic lithium deposition), the light transmittance of the copper current collector film is high, and the surface morphology of the metallic lithium foil of the counter electrode can be clearly observed through the separator.

[0248] After 10 seconds of charging, the surface of the counter electrode's metallic lithium foil became invisible, and a decrease in the overall transparency of the copper current collector film was observed. This is thought to be because metallic lithium deposited on the counter electrode surface of the copper current collector film, obstructing the transmission of light.

[0249] Furthermore, when the charging time reached 60 seconds, it was confirmed that the entire copper current collector film was covered with uniformly deposited metallic lithium, and light transmission was completely lost.

[0250] Next, in the discharge (metallic lithium dissolution) process shown in the lower panel (b) of Figure 18, a change in the permeability of the copper current collector film began to be observed approximately 30 seconds after the start of the discharge, and after approximately 40 seconds, an island-like contrast appeared near the right side of the copper current collector film. In this region, the surface of the counter electrode's metallic lithium foil was visible, suggesting that the deposited metallic lithium had dissolved and been removed from the surface of the copper current collector film.

[0251] Further discharge allowed the permeability of the copper current collector film to recover overall, and it was confirmed that by the end of the discharge (metallic lithium dissolution), it had returned to almost the same state as at the start of the charge (metallic lithium deposition). These results clearly demonstrate that in this embodiment, the deposition and dissolution of metallic lithium proceed reversibly.

[0252] It is generally known that metallic lithium and copper do not readily undergo alloying reactions. In this example, to confirm whether the effect of this reaction could be ignored in a 10 nm thick copper current collector film, the cell was disassembled after the test and the copper current collector film was compared to its condition before the test. The results are shown in Figure 19.

[0253] Comparing the copper current collector film before the test (left side of Figure 19) with the copper current collector film after the test (right side of Figure 19), no significant change in the appearance of the copper current collector film was observed. Therefore, this embodiment demonstrates that the effect of the alloying reaction of the copper current collector film due to the deposition of metallic lithium is negligible, or if it is present, it is extremely limited.

[0254] <2.2 Deposition and Dissolution of Metallic Lithium - Ionic Liquid Electrolyte> Next, the following shows the results of observations using an optical microscope regarding the deposition and dissolution of metallic lithium when an ionic liquid is used as the electrolyte instead of a conventional organic solvent. The configuration of the experimental apparatus (cell) in this example is the same as that of the sample investigated using the organic electrolyte above, and the test conditions are as follows. A metallic lithium foil with a diameter of 12 mm and a thickness of 400 μm was used as the counter electrode, and a copper thin film with a diameter of 6 mm and a thickness of 10 nm was used as the collecting electrode. A glass film with a thickness of 40 μm was used as the separator, and a solution of 10 wt% LiTFSA dissolved in PP13TFSA was used as the electrolyte. A digital microscope with a magnification of ×30 was used for optical microscope observation, and the charge / discharge conditions were a current density of 0.5 mAcm². －２ Charging was performed for 60 seconds (based on copper current collector area), followed by discharging at the same current density.

[0255] Figure 20 shows optical microscope images of the deposition and dissolution process of metallic lithium in the central part 121 of the copper current collector film when using an ionic liquid electrolyte (10 wt% LiTFSA in PP13TFSA). From the observation results, it was confirmed that the deposition of metallic lithium showed a more homogeneous planar distribution compared to when an organic electrolyte was used. In addition, it was observed that the color of the copper current collector film changed slightly to reddish-brown during charging, but it reversibly returned to the original color of the copper current collector film upon discharge. This change in color is presumed to be due to a change in optical properties caused by the deposited metallic lithium, rather than a change in the color of the copper film itself.

[0256] These results demonstrate that by using a copper current collector film with a thickness of 10 nm, it is possible to optically observe the deposition and dissolution behavior of metallic lithium in various electrolyte environments, including ionic liquids.

[0257] <2.3 Deposition and Dissolution of Metallic Sodium> Metallic sodium, like metallic lithium, is a highly reactive metal, and it is important to visualize its deposition and dissolution processes using an optical microscope. In this example, an evaluation using an optical microscope was performed to observe the deposition and dissolution of metallic sodium. The configuration of the experimental apparatus (cell) used was the same as that of the sample investigated using the organic electrolyte described above, and the test conditions were as follows: A metallic sodium foil with a diameter of 12 mm and a thickness of 400 μm was used as the counter electrode, and a copper thin film with a diameter of 6 mm and a thickness of 10 nm was used as the current collector electrode. A polypropylene film with a thickness of 25 μm was used as the separator, and 1 M NaPF was used as the electrolyte. ６ A solution was used, in which the substance was dissolved in a mixed solvent with an EC / DEC ratio of 1.0. For optical microscopy observation, a digital microscope with a magnification of 100 was used, and the charge / discharge conditions were set to a current density of 0.5 mA / cm². －２ Charging was performed for 60 seconds (based on copper current collector area), followed by discharging at the same current density.

[0258] Figure 21 shows the organic electrolyte (1 M NaPF ６ The following are optical microscope images of the deposition and dissolution processes of metallic sodium when using EC / DEC = 1.0. As shown in Figure 21, bright, island-like contrasts were observed scattered in various parts of the central portion 121 of the copper current collector film as charging progressed, and their number increased as charging progressed. This is thought to be because the metallic sodium deposited on the surface of the copper current collector film locally changed the reflectivity of light. When discharge was then performed, these island-like bright contrasts gradually decreased, and it was observed that the device eventually returned to its pre-charging state in a reversible manner.

[0259] Furthermore, Figure 22 shows the high-magnification (×1000) observation results of metallic sodium deposits on the outer edge of the central portion 121 of the copper current collector film after repeating this charge-discharge cycle multiple times. From the observation results, it became clear that the metallic sodium deposits grew in a dendritic manner from the edge of the copper current collector film outward. In addition, the base of the dendritic region was significantly brighter than other regions, suggesting that the localized light reflection intensity increased due to the deposition of metallic sodium. In this way, by observing the edge portion of the copper current collector film in detail, it is possible to capture the growth morphology of the metallic deposits from a virtual cross-sectional direction. From these results, it is demonstrated that the process of deposition and dissolution of metallic sodium can be analyzed in detail optically by this disclosure.

[0260] <2.3 Application to Concentrated Electrolyte Systems> Electrolytes prepared by dissolving LiFSA salts in dimethyl carbonate (DMC) as a solvent are considered promising as electrolytes for metallic lithium batteries, but it is known that their properties vary greatly depending on the salt concentration. In particular, a significant difference in charge-discharge efficiency is observed between an electrolyte prepared with a molar composition ratio of LiFSA:DMC = 1:10 (hereinafter referred to as a dilute electrolyte) and an electrolyte prepared with the same composition ratio of LiFSA:DMC = 1:1.1 (hereinafter referred to as a concentrated electrolyte). Generally, when a dilute electrolyte is used, the charge-discharge efficiency (amount of electricity discharged / amount of electricity charged) is low. That is, the amount of electricity charged can be irreversibly consumed during the charge-discharge process. Furthermore, it is known that the deposition of metallic lithium occurs coarsely and heterogeneously. On the other hand, it has been confirmed that concentrated electrolytes result in relatively high charge-discharge efficiency (less irreversible consumption of charged electricity) and dense, homogeneous deposition of metallic lithium. These findings have been revealed through analysis of voltage profiles in conventional charge-discharge tests and observation of the morphology of deposited lithium after disassembling the cell, but observations have not yet been sufficiently conducted with the cell assembled and in operation (operand conditions). Therefore, in this disclosure, the differences in charge-discharge behavior when using these two types of electrolytes were investigated in detail using optical microscopy observations.

[0261] <2.3.1 Experimental Method> To conduct this study, an aramid-coated polypropylene film (thickness: 40 μm) was used as a separator, a 12 mm diameter metallic lithium foil was used as the counter electrode, and a 6 mm diameter copper thin film with a thickness of 10 nm was used as the copper current collector to fabricate an optical observation battery cell. The pressing condition between the electrodes was set to 1 kg-force (approximately 87 kPa) within a 12 mm diameter range. This was the same surface pressure condition as that of a conventional coin battery cell.

[0262] For optical microscope observation, a digital microscope with a magnification of ×30 was used, and observation of the entire surface of the copper current collector was performed under epi-illumination coaxial illumination. In the charge-discharge test, the charge capacity was fixed at 40 mC (40 mAs), and the current densities were 0.5, 1.0, 2.0, 4.0, 8.0, 16.0 (mA cm －２ ) and constant current charge-discharge was performed while increasing the current density stepwise. In addition, the discharge cut-off potential was set to 2.0 V (vs. metallic lithium counter electrode), and a fixed potential discharge at 2.0 V was performed for 10 seconds between each current density step. A representative charge-discharge potential profile is shown in Figure 23.

[0263] <2.3.2 Experimental Result 1 (Appearance of electrodes after 40 mC charging)> For the electrode state when 40 mC charging was performed at each current density, comparison was performed using two different types of electrolytes, and the results are shown in Figure 24.

[0264] Figure 24(a) shows the results when a dilute electrolyte was used. In the initial state (Pristine), the central portion 121 of the copper current collector has high permeability, and the surface structure of the metallic lithium foil disposed as the counter electrode can be observed through it. From this state to 0 V vs. Li counter electrode, when charging was performed, it was confirmed that the permeability of the copper film decreased slightly. Since deposition of metallic lithium does not occur at this potential, it is inferred that the decrease in permeability is due to the effect of some surface film formed by decomposition of the electrolyte. Further, at 0.5 mA cm －２ when charging to 40 mC at the current density, the permeability of the copper film almost disappeared. This is considered to be the result of the progress of metallic lithium deposition.

[0265] Even at the point of complete charging with the same amount of electricity equivalent to 40 mC, changes in the gloss of the copper film surface occur depending on the current density conditions. In particular, at 4.0 mA cm －２ At the above current densities, a bright contrast was observed. The reason for this change in contrast of the copper film is not clear, but considering the observation conditions using coaxial incident illumination, it is possible that the amount of diffusely reflected light increased due to the deposited metallic lithium. In other words, the deposition morphology of metallic lithium is dependent on current density, and it is suggested that metallic lithium deposited under high current density conditions has strong light diffusivity.

[0266] Furthermore, observation of the deposition behavior of metallic lithium across the entire electrode clearly revealed the presence of island-like regions where deposition did not occur (non-lithium deposition regions). This is thought to be because gas generated during the reductive decomposition of the electrolyte accumulated at the pressure interface, thereby inhibiting the deposition of metallic lithium in certain areas. In addition, a tendency was observed for the area of ​​the non-lithium deposition regions to expand as the current density increased. This is presumed to be due to the more pronounced decomposition of the electrolyte under high current densities, resulting in increased gas generation.

[0267] From the above results, it became clear that when a dilute electrolyte is used, the reduction stability is low, gas is generated due to the reductive decomposition of the electrolyte during the charging process, and furthermore, the generated gas accumulates at the pressurized interface, causing non-uniform deposition of metallic lithium on the electrode surface. These findings cannot be clearly understood without directly observing the operating state of the pressed battery interface with operands, and therefore represent an important technical feature of this disclosure.

[0268] A similar experiment was conducted using a concentrated electrolyte, and the results are shown in Figure 24(b). No significant change in the permeability of the copper film was observed in the initial state (Pristine) and during the charging process up to the 0 V vs. Li counter electrode. The surface of the lithium metal foil of the counter electrode could be observed through the film, confirming that the permeability of the copper film is maintained even near the 0 V vs. Li counter electrode in the concentrated electrolyte. This suggests that the film formed by the concentrated electrolyte is thinner or has higher light transmittance compared to the dilute electrolyte. Furthermore, 0.5 mAcm －２ Even when charging at 40 mC with a current density, the surface of the counter electrode's metallic lithium foil is partially visible, suggesting that the deposited metallic lithium may not significantly contribute to the absorption, scattering, or reflection of transmitted light.

[0269] Furthermore, while a similar trend to that of dilute electrolytes is observed even when the current density is increased, a notable difference is the absence of gas generation. In other words, it was revealed that concentrated electrolytes have higher reduction stability than dilute electrolytes, and gas generation at the pressure interface is suppressed.

[0270] <2.3.3 Experimental Results 2 (8.0 mA cm) －２ (Changes in electrode configuration during charging) > Refer to Figure 25, current density 8.0 mAcm² －２ This section describes the deposition of metallic lithium under the following conditions. Note that the electric charge Δ0 mC refers to a current density of 8.0 mAcm². －２ This corresponds to the point in time when the deposition of metallic lithium begins (nucleation time) under these conditions.

[0271] Observations were made to compare the state of the electrodes corresponding to each subsequent amount of deposited electricity. Figures 25(a) and 25(b) show optical microscope images of the electrodes when using dilute and concentrated electrolytes. In Figures 25(c-1) and 25(c-2), the vertical axis is voltage (V), and the negative values ​​represent the potential fluctuations during metal deposition.

[0272] In Figure 25(a), after Δ0 mC, which is the point at which metallic lithium deposition begins (nucleation point) in the central part 121 of the copper current collector film, it was observed that the bright area on the electrode surface expanded as deposition progressed after nucleation. Several factors can be considered for this change. For example, it is known that the surface plasmon resonance wavelength of metallic lithium is about 710 nm, and it is possible that the selective absorption of red light at this wavelength resulted in the reflection of its complementary color, blue light, which became visually prominent as a change in light and dark contrast.

[0273] Furthermore, the deposition morphology of metallic lithium differed significantly depending on the type of electrolyte. For example, in Figure 25(a), where a dilute electrolyte was used, variations in color intensity were observed even in areas other than the bubble generation region of the electrode surface. On the other hand, in Figure 25(b), where a concentrated electrolyte was used, the significant variations in color intensity observed with the dilute electrolyte were not observed. This indicates that when a concentrated electrolyte is used, not only is gas generation suppressed, but a more uniform deposition behavior is reproduced at each stage from nucleation to nucleus growth.

[0274] <Gas Generation in Dilute Electrolytes> According to the present invention, various reactions inside the battery can be directly observed using operands. This allows for detailed investigation of, for example, the process of gas generation in dilute electrolytes. Figure 26(a) shows the current density of 0.5 mA cm² when using a dilute electrolyte. －２This shows the internal state of the electrode during the charging (metallic lithium deposition) and discharging (metallic lithium dissolution) processes. Each number in these photographs corresponds to points 1 to 6 in the charge-discharge profile shown in Figure 26(b). First, at point 1, the permeability of the copper thin film in the corresponding micrograph is high, suggesting that no film has formed at this point. Next, at point 2, the formation of extremely fine bubbles was observed in the area indicated by the black arrow. This point corresponds to 0 V relative to the lithium counter electrode. In other words, gas generation in the dilute electrolyte is thought to occur at a potential higher than the metallic lithium deposition potential, i.e., even before lithium deposition. Furthermore, at point 3, that is, the stage when metallic lithium deposition begins (nucleation occurs), the bubbles have spread to the region enclosed by the ellipse. The spread of the bubbles further extends significantly to the region enclosed by the ellipse at points 4 and 5. And even at point 6, the end of discharge (metallic lithium dissolution), they remain irreversibly in the region enclosed by the ellipse. Incidentally, while gas generation itself has been observed using existing optical microscopy techniques with open observation cells as shown in Figure 1A, there are no examples of visualizing gas generation under pressure from the counter electrode via a separator. Unlike interfaces under open conditions, gas generation at interfaces under surface pressure promotes the significant spread and retention of bubbles in the planar direction of the electrode, thus having a significant impact on the subsequent battery reaction. In other words, this result has only been revealed by this technology, which can visualize the inside of a battery while reproducing the actual battery configuration.

[0275] <Lithium Deposition Mechanism in Dilute Electrolytes> As described above, gas generation in dilute electrolytes can occur even before the deposition of lithium metal. In other words, during the charging process, gas generation and metallic lithium deposition can compete from the initial stages. Therefore, a portion or most of the charged electricity is irreversibly consumed by gas generation. This is considered to be the cause of the low charge-discharge efficiency. Furthermore, the competition between gas generation and metallic lithium deposition that occurs from the initial stages of charging is thought to result in a very sparse lithium deposition pattern on the copper electrode surface, as shown in Figure 27(a), for example. As shown in Figure 26, gas generation occurs from the stage prior to lithium deposition. Especially at the interface under surface pressure, there is no escape route for the generated minute bubbles, so it is thought that the surface of the copper electrode is covered with gas bubbles before lithium deposition. In this case, metallic lithium is forced to precipitate from the gaps in the bubbles covering the copper electrode surface. Therefore, adhesion of the deposited metallic lithium to the current collector film surface and densification by bonding between lithium clumps are hindered, resulting in an extremely coarse physical contact between them. Therefore, it is thought that during the discharge (metallic lithium dissolution) process, a large amount of metallic lithium clumps that have lost electrical connection with the electrode surface are generated. The generation of such so-called dead lithium (metallic lithium that has no electrical contact with the electrode surface and cannot react further) causes an irreversible loss of charged electricity, in addition to gas generation.

[0276] As explained in Figure 26, gas generation in dilute electrolytes arises from the 0 V vs. Li counter electrode potential. This means that gas can also be generated on the surface of metallic lithium in dilute electrolytes. This is shown in Figure 27(b). This figure schematically shows an enlarged view of the area enclosed by the black frame in Figure 27(a). The deposited lithium clumps are aggregates of even smaller lithium particles. Therefore, tiny gas bubbles are generated at the interface between the surface of the lithium particles and the electrolyte, hindering close contact between lithium particles and inhibiting the densification of the metallic lithium clumps themselves. In other words, at a more microscopic level, the metallic lithium clumps themselves are thought to be in a more sparse state. By visualizing the gas generation inside the battery cell in detail in this way, we can gain a deeper understanding of the characteristics and causes of metallic lithium deposition in dilute electrolytes.

[0277] In the previous examples, we described examples of observing the deposition and dissolution processes of metallic lithium and metallic sodium, as well as gas generation and film formation associated with the decomposition of electrolytes. However, the optical microscopy observation technique of the present invention is not limited to these and can be applied to other metal electrode materials and electrolyte systems. For example, by observing the deposition and dissolution of polyvalent metals such as zinc, magnesium, aluminum, and calcium, it can be applied to the development of next-generation secondary batteries and electrolytic capacitors. Furthermore, by visualizing the reaction processes of photoelectrochemical materials and the growth behavior of solid electrolyte interface (SEI) films formed on electrode surfaces, it is expected to contribute to the development of interface control technology for energy storage devices.

[0278] (Example of observation target A: Metal deposition / dissolution and electrolyte reaction) The deposition and dissolution processes of alkali metals (lithium, sodium, potassium) and polyvalent metals (zinc, magnesium, aluminum, calcium) can be visualized. This makes it possible to analyze in detail the behavior of metal electrodes in batteries and electrolytic capacitors.

[0279] Furthermore, it is possible to observe in real time the formation of films and gas generation associated with the decomposition of the electrolyte. For example, by visualizing the formation of solid electrolyte interface (SEI) films in lithium-ion batteries and the deposition of insoluble by-products in magnesium secondary batteries, it is possible to clarify the stability of the electrolyte and the mechanisms of electrode surface reactions. In addition, it is possible to observe the growth of dendrites (dendritic projections) of metallic lithium and sodium and apply this to research aimed at reducing the risk of short circuits.

[0280] (Example of observation target B: Electrochemical response of thin-film active material) Active material refers to a substance in an electrochemical device that is directly involved in the transfer of charge and chemical reactions, and is responsible for the storage and release of energy. Various active materials can be coated as thin films on a current collector film, and the electrode reaction can be observed. This makes it possible to directly analyze the electrochemical response and charge / discharge characteristics of the active material.

[0281] Techniques such as spin coaters, sputter coaters, vacuum deposition apparatuses, and electrochemical film formation methods can be used for thin film coating, enabling precise control of film thickness and composition. For example, lithium iron phosphate (LiFePO ４ ) and lithium nickelate (LiNiO ２ ), lithium manganate (LiMn ２ O ４ ), lithium cobaltate (LiCoO ２ ) and other cathode materials for lithium-ion batteries, as well as anode materials such as graphite, hard carbon, tin oxide (SnO ２ ), silicon (Si), and silicon oxide (SiO) can be formed into thin films to analyze their electrochemical behavior.

[0282] Furthermore, in order to evaluate the influence of coating layers on the electrode surface (binders, conductive additives, solid electrolyte layers, etc.), multilayer structures combining different thin film materials can be fabricated to compare electrochemical properties.

[0283] (Example of observation object C: Photoelectrochemical response and solar cell materials) By using a photoelectrochemical material as an active material, it is possible to record the electrochemical response to light irradiation and the photochemical response to electrochemical operations.

[0284] Specifically, observation of photoelectrochemical reactions using photocatalyst materials such as titanium oxide (TiO ２ ) and zinc oxide (ZnO) is possible, which can support research on improving the efficiency of water splitting reactions and carbon dioxide reduction reactions.

[0285] Furthermore, by applying materials for dye-sensitized solar cells (DSSC), organic thin-film solar cells (OPV), and perovskite solar cells and observing the charge transfer process under light irradiation, it is possible to analyze the power generation mechanism of solar cells and the behavior of charge accumulation at electrode interfaces.

[0286] (Example of observed subject D: Solid electrolytes and all-solid-state battery materials) This technology can also be applied to the development of materials for solid electrolytes and all-solid-state batteries. Examples of solid electrolyte materials include oxide-based (LLZO), sulfide-based (LGPS), and polymer-based (PEO-LiTFSI). Controlling grain boundaries and crystal structure is important to improve the ion conductivity characteristics of solid electrolytes, and this technology makes it possible to visualize ion diffusion behavior and interfacial reactions. Furthermore, it is possible to analyze in detail the interfacial reactions between electrodes and solid electrolytes during the operation of all-solid-state batteries, the effect of suppressing lithium dendrite growth, and the electrode expansion and contraction behavior.

[0287] (Applications to Other Optical Technologies) This specification has mainly described optical observation techniques using white visible light, along with examples of the working electrode. However, the property of the working electrode to transmit light is not limited to white visible light. For example, by using various transparent materials used in infrared spectroscopy as the substrate 11, in-plane infrared spectroscopy and infrared spectroscopic imaging of the target interface can be performed. In particular, the confocal method is effective for in-plane imaging, where the technical advantages of this working electrode, such as its high flatness and ease of determining the focal position of the interface to be observed, become even more pronounced. Furthermore, it can be similarly extended to Raman spectroscopy, confocal Raman spectroscopic imaging, and other techniques, not just infrared light. Thus, this working electrode, which has optical transparency and can visualize the interface inside a battery, can be applied to a wide range of optical science and technology, such as spectroscopy using various types of light and confocal imaging. In addition, scanning interference microscopy is a method that scans the differences in optical interference intensity within the electrode surface in a direction perpendicular to the surface while changing the focus and creating an image. This makes it possible to visualize the inside of a battery in three dimensions. Unlike the currently dominant X-ray CT method, this method allows for internal observation of batteries while reducing time and data costs, and also minimizing beam damage. Thus, the cell configuration described in this disclosure, which uses light-transmitting electrodes, also enables the introduction of both basic and advanced optical technologies.

[0288] <Example 2: Film Thickness Dependence of Surface Electrical Resistance and Transmittance> In this example, copper thin films of various thicknesses were deposited on a support substrate, and the relationship between the film thickness and physical properties of the electrically conductive film was investigated by measuring its surface electrical resistance and transmittance.

[0289] <2.1 Preparation of the Support Substrate> A high refractive index glass (2 mm thick, 32 mm in diameter, manufactured by Optocube Co., Ltd.) was used as the support substrate. Both sides of the support substrate were washed with a neutral detergent, and then thoroughly washed with distilled water. After that, water droplets were blown off both sides with an air blower and air-dried, and then dried in a dryer set to 60°C or higher.

[0290] <2.2 Vacuum Deposition of Electrically Conductive Films> The copper thin film, used as an electrically conductive film, was deposited using a vacuum deposition apparatus (JEE-420, JEOL). A tantalum foil (manufactured by Nilaco) with a thickness of 0.03 mm was used as a heating boat, and copper wire (manufactured by Nilaco) with a diameter of 0.3 mm was cut to a predetermined length and placed on the heating boat. The vacuum gauge reading was 2 × 10⁻⁶ －３ Once the pressure reached below Pa, the tantalum boat was heated by applying an electric current to gradually evaporate the copper. After waiting about 5 minutes after deposition, the bell jar was opened to the atmosphere and the sample was removed.

[0291] <2.3 Surface Electrical Resistance Measurement> Figure 28 is a conceptual diagram of surface electrical resistance measurement. Figure 28 shows a schematic diagram of a mask-deposited electrode as an example, but the measurement target is not limited to this. The upper part of Figure 28 shows a cross-sectional view of the measurement cell, and the lower part shows a top view. A support substrate 11 on which an electrically conductive film 12 has been deposited was assembled into the measurement cell, and the two-terminal resistance was measured with a digital multimeter (DMM). The measurement sample was assembled into the working electrode current collector holder 13, and the top cover was tightened and fixed with screws. The probe of the tester was brought into contact with the center part of the deposited electrically conductive film 12 (marked with an "x" in the figure) and the terminal part of the working electrode current collector holder 13 (marked with an "x" in the figure). The resistance value varied slightly depending on the contact pressure of the probe, but the measurement value stabilized when pressed with a force above a certain level. At least three measurement points were secured within a range of a few mm from the center part of the electrically conductive film 12, and the average of these was calculated as the surface electrical resistance.

[0292] <2.4 Relative Transmittance Measurement> The wavelength transmittance of the sample was measured using a spectrometer (SPELEC1050, Metrohm). The wavelength transmittance of the sample (the electrically conductive film itself) was recorded using the transmittance of the support substrate (without copper deposition) as a reference.

[0293] <2.5 Measurement Results> Figure 29 shows the transmittance spectra of each electrically conductive film thickness in full-surface deposition. Figure 29 shows the transmission spectra of samples obtained when the electrically conductive film thickness was 10 nm, 15 nm, 20 nm, 25 nm, and 30 nm. The vertical axis is the relative transmittance (T%) relative to the support substrate (glass substrate), and the horizontal axis is the wavelength (nm). Each transmission spectrum has a maximum value around 600 nm, which depends on the light transmission characteristics of the copper thin film itself. Furthermore, a tendency for transmittance to decrease as the film thickness increases was observed. In addition, from the measurement data of these transmittance spectra, the maximum transmittance at wavelength (T%) was determined. ＭＡＸ ) and wavelength-average transmittance (T%) in the interval from 450 nm to 900 nm. ４５０－９００ ) was calculated.

[0294] Calculated maximum wavelength transmittance (T%) ＭＡＸ ) and wavelength average transmittance (T% ４５０－９００ The results, along with the measured surface electrical resistance (Ω), are summarized in Table 3.

[0295]

[0296] Table 3 shows that the transmittance decreases as the thickness of the electrically conductive film 12 increases. On the other hand, the surface electrical resistance decreases with increasing thickness. From these relationships, it is possible to select a thickness for the electrically conductive film 12 that balances both light transmittance and electrical conductivity.

[0297] <2.6 Film Thickness Dependence of Transmittance and Surface Resistance> Figure 30 is a graph showing the film thickness dependence of relative transmittance (relative to the support substrate) and surface resistance in a fully deposited sample. In Figure 30, the transmittance and surface resistance values ​​from Table 3 are plotted on the same graph. The horizontal axis is copper deposition film thickness (nm), the left vertical axis is surface resistance (Ω), and the right vertical axis is relative transmittance (T% vs glass substrate). The circles (○) in the figure represent the maximum wavelength transmittance (T%ＭＡＸ ), the diamond shape (◇) indicates the average wavelength transmittance (T%). ４５０－９００ The square (□) indicates the surface electrical resistance (Ω), respectively. Transmittance (T%) and surface electrical resistance (Ω) change with respect to film thickness, and it was confirmed that the changes are particularly significant at thicknesses of 20 nm or less.

[0298] Generally, the permeability property T of a substance follows the Lambert-Beer law shown below. Here, α and β are constants, and the film thickness is x. According to the above law, the transmittance decreases exponentially with respect to the film thickness x. In Figure 30, the solid lines shown on the transmittance plot are exponential approximation curves with respect to film thickness. It can be said that there is a high correlation between the measured transmittance data and the exponential approximation.

[0299] On the other hand, the surface electrical resistance decreased in correlation with the film thickness. Here, the resistance R of the conductor is expressed as follows using the measured length L and cross-sectional area S. The resistivity ρ is material-specific, and the length L depends on the shape of the measurement cell. When only the film thickness is changed, these can all be considered constants, so ultimately the electrical resistance R is inversely proportional to the cross-sectional area. In full-surface deposition, assuming that copper is uniformly deposited across the entire support substrate, the cross-sectional area of ​​the electrically conductive film is proportional to the film thickness x, so the resulting surface electrical resistance is inversely proportional to the deposited film thickness. For example, using appropriate constants a, b, and c, the relationship between resistance R and film thickness x is assumed to be as follows. The dotted approximation curve in Figure 30 shows an example where (a, b, c) = (128.6, -2.18, -2.36) in the above equation. The correlation between the measurement data points and the approximation curve is high, and it can be understood that the reduction in resistance value with increasing film thickness gradually plateaus.

[0300] <2.7 Selection of Optimal Film Thickness> Transmittance and surface electrical resistance change significantly at film thicknesses of 20 nm or less. In particular, at film thicknesses of 10 nm or less, transmittance is good, but the surface electrical resistance increases sharply, making it unsuitable for use as a working electrode. At film thicknesses of 20 nm or more, the surface electrical resistance is 5 Ω or less, but the wavelength-average transmittance falls below 40%, making it difficult to obtain good light transmission characteristics. Therefore, by setting the optimal film thickness to 15 nm, a light-transmitting working electrode with a balanced transmittance and electrical resistance can be obtained.

[0301] In actual measurements, a film thickness of 15 nm showed a wavelength-average transmittance of 46% or higher and a surface electrical resistance of 6 Ω or less. Considering the reproducibility of the experiment and the yield during manufacturing, a film thickness of approximately 15 nm can be said to be relatively easy to use.

[0302] According to Example 2, the relationship between the film thickness, transmittance, and surface electrical resistance of an electrically conductive film can be quantitatively determined. It was found that the transmittance decreases exponentially with respect to the film thickness according to the Lambert-Beer law, and the surface electrical resistance decreases inversely with respect to the film thickness. From these relationships, 15 nm can be selected as the optimal film thickness for achieving both light transmittance and electrical conductivity. An electrically conductive film with a film thickness of 15 nm has a wavelength-average transmittance of 46% or more and a surface electrical resistance of 6 Ω or less, and functions well as a light-transmitting working electrode suitable for optical microscope observation.

[0303] <Example 3: Measurement of Surface Electrical Resistance of Mask Deposition Electrode> In this example, we verified that the surface electrical resistance of an electrically conductive film changes depending on the mask deposition pattern. Below, the surface electrical resistance values ​​in the mask deposition pattern are compared with those in the case of full-surface deposition.

[0304] <3.1 Mask Deposition Pattern> Figure 31 is a conceptual diagram of the mask deposition pattern in this embodiment. In Figure 31, the gray area represents the deposited electrically conductive film 12, and the white area represents the surface of the support substrate 11. A glass plate with a diameter of φ = 32 mm was used as the support substrate 11, and the electrically conductive film 12 was deposited on it. The electrically conductive film 12 consists of a central circular portion (φ = 6 mm), an outer ring portion (φ = 22 mm), and a conductive path with a width of 2 mm connecting them. The measured surface electrical resistance in this embodiment is limited to the shape shown in Figure 31, but the mask deposition pattern itself can be arbitrarily designed by those skilled in the art.

[0305] <3.2 Measurement of Surface Electrical Resistance> The electrical resistance value was measured in the same manner as in Figure 28 of Example 2, with the sample assembled in the working electrode current collector holder 13 and the top cover tightened with screws. As shown in Figure 28, the entire surface of the electrically conductive film 12 on the outer circumference of the support substrate 11 makes good contact with the working electrode current collector holder. For the measurement, the probe of a digital multimeter was brought into contact with the terminal portion of the working electrode holder 13 and the central portion of the 6 mm diameter electrically conductive film, and after leaving it undisturbed until the value stabilized, the resistance value was recorded. The measurement was performed a total of three times at arbitrary locations, and the average value was calculated.

[0306] <3.3 Measurement Results> The measured surface electrical resistance values ​​obtained using the measurement method shown in Figure 28 are shown in Table 4 and Figure 32, respectively. It was confirmed that the surface electrical resistance value did not exceed 1 kΩ for any of the measured samples.

[0307]

[0308] Figure 32 is a comparative plot of surface electrical resistance measurements for various conductive film thicknesses of a fully deposited sample and a mask-deposited pattern sample in this embodiment. The horizontal axis represents copper deposition film thickness (nm), and the vertical axis represents surface electrical resistance (Ω). In the figure, squares (□) represent fully deposited samples, and triangles (△) represent mask-deposited samples. Figure 32(a) shows the case where the maximum value on the vertical axis scale is 80 Ω, and Figure 32(b) shows the case where the maximum value on the vertical axis scale is 800 Ω. From Figure 32(b), it was confirmed that for extremely thin films with a film thickness of about 5 nm, the surface electrical resistance of the mask-deposited sample rises to nearly 700 Ω.

[0309] <3.4 Comparison of Full-surface Deposition and Mask Deposition> In the mask-deposition sample in this embodiment, the surface electrical resistance is higher compared to the full-surface deposition case. Compared to full-surface deposition, in the case of mask deposition, the conductive path is limited to a width of approximately 2 mm, as shown in Figure 31, which is thought to result in a higher resistance. In the range of deposition film thickness of 15 nm or more, the ratio of resistance between full-surface deposition and mask deposition was approximately 4 to 6 times. On the other hand, when the deposition film thickness is 10 nm or less, the value increases sharply to a maximum of 50 times or more. When the conductive path is limited, the increase in surface electrical resistance with decreasing deposition film thickness is thought to become more pronounced. Therefore, in the case of the mask deposition pattern in this embodiment, a film thickness of up to 15 nm is considered to be relatively good.

[0310] <3.5 Transmittance of Mask Deposition Pattern> Since the wavelength-average transmittance depends only on the deposition film thickness and not the planar pattern of the deposition, the same results as full-surface deposition can be obtained regardless of the mask deposition pattern. The wavelength-average transmittance at a film thickness of 15 nm is 40% or more, and it functions well as an optical transmission working electrode.

[0311] According to Example 3, the surface electrical resistance characteristics of an electrically conductive film in a mask deposition pattern can be determined. In a mask deposition pattern, the conductive paths are limited, so the surface electrical resistance increases compared to full deposition. However, in the range of film thickness of 15 nm or more, the increase in resistance is limited to about 4 to 6 times. Furthermore, since the transmittance depends only on the deposition film thickness, the same light transmission characteristics as full deposition can be obtained even with a mask deposition pattern. Therefore, by employing a mask deposition pattern with a film thickness of 15 nm, it is possible to realize a light-transmitting working electrode with a surface electrical resistance of 1 kΩ or less and a wavelength-average transmittance of 40% or more.

[0312] <Example 4: Optical transmission characteristics of the support substrate> In this example, various support substrates were prepared, and a support substrate suitable for forming an electrically conductive film was selected by comparing their optical transmission characteristics and surface morphology.

[0313] <4.1 Preparation of Support Substrates> In addition to the high refractive index glass substrate used as a support substrate, three other types of support substrates that are relatively easy to obtain were added, and the transmittance spectra of a total of four types of support substrates were measured. [A] High refractive index glass S-TIH (manufactured by Ohara Co., Ltd.): Thickness t = 2.0 mm [B] Synthetic quartz glass (manufactured by NOMUKI LLC): Thickness t = 1.0 mm [C] Sapphire glass (manufactured by NOMUKI LLC): Thickness t = 1.0 mm [D] Slide glass (purchased from AS ONE): Thickness t = 1.2 mm

[0314] Both sides of the various support substrates described above were washed with a neutral detergent, and then thoroughly rinsed with distilled water. After that, water droplets were blown off both sides with an air blower and air-dried, and then dried in a dryer set to 60°C or higher.

[0315] <4.2 Relative Transmittance Measurement> The wavelength transmittance of the sample was measured using a spectrometer (SPELEC1050, Metrohm). The transmittance without the support substrate (air) was used as a reference, and the wavelength transmittance spectrum of the sample (the support substrate itself) was recorded.

[0316] <4.3 Measurement Results of Transmittance Spectra> Figure 33 shows the optical transmission spectra of various support substrates. The horizontal axis is wavelength (nm), and the vertical axis is relative transmittance (T% vs. air) relative to air. Figure 33 shows the obtained optical transmission spectra of various support substrates. The dotted line in the figure represents air, [A] represents high refractive index glass, [B] represents synthetic quartz glass, [C] represents sapphire glass, and [D] represents slide glass. The transmittance of air as a reference is 100% regardless of wavelength. [A] High refractive index glass shows a sharp decrease in transmittance in the short wavelength range below 400 nm, but shows a transmittance of approximately 80% or more in the visible wavelength range (450 nm to 900 nm). [C] Sapphire glass similarly showed a transmittance of 80% or more in the visible wavelength range. [B] Synthetic quartz glass and [D] slide glass were confirmed to show a transmittance of 90% or more in the same visible wavelength range. Furthermore, the wavelength-average transmittance (T%) in the visible range for each support substrate. ４５０－９００ The results are summarized in Table 5.

[0317]

[0318] Table 5 shows that synthetic quartz glass has the highest optical transmission properties. Slide glass has the next highest transmittance. The transmittance of high refractive index glass and sapphire glass is similar in measurements, but the thickness of the high refractive index glass is 2 mm, which is twice that of the sapphire glass. Therefore, the transmittance per unit thickness is considered to be lowest for sapphire glass. However, all support substrates have a transmittance of 80% or more, which is sufficient for use as a support substrate for light transmission.

[0319] According to Example 4, the optical transmission characteristics of various support substrates were quantitatively compared, and guidelines for selecting a support substrate 11 suitable for the deposition of an electrically conductive film 12 were obtained. Synthetic quartz glass had the highest wavelength-average transmittance of 94.29%, making it the best in terms of optical transmittance. Slide glass showed a transmittance of 91.12% and has the advantages of being readily available and low-cost. High refractive index glass and sapphire glass have transmittances of approximately 80% or more, and have sufficient characteristics as support substrates 11 for light transmission. Sapphire glass in particular has high mechanical strength and excellent chemical stability, making it suitable for applications where durability is required. Therefore, in the light transmission electrode 10 of this disclosure, synthetic quartz glass can be selected when optical characteristics are important, slide glass when cost is important, and sapphire glass when mechanical strength is important, depending on the purpose.

[0320] <Example 5: Observation of surface morphology of an electrically conductive film (Cu) with a thickness of 15 nm> In this example, the surface morphology of an electrically conductive film with a deposition thickness equivalent to 15 nm was investigated using an atomic force microscope (AFM).

[0321] <5.1 Preparation of Support Substrates> First, several support substrates were prepared as bases for depositing the electrically conductive film, and their arithmetic surface roughness (Ra) was measured. The prepared support substrates were as follows: [A] High refractive index glass S-TIH (manufactured by Ohara Corporation) [B] Synthetic quartz glass (manufactured by NOMUKI LLC) [C] Sapphire glass (manufactured by NOMUKI LLC) [D] Microscope slide (purchased from AS ONE)

[0322] Both sides of the various support substrates described above were washed with a neutral detergent, and then thoroughly rinsed with distilled water. After that, water droplets were blown off both sides with an air blower and air-dried, and then dried in a dryer set to 60°C or higher.

[0323] <5.2 Vacuum Deposition of Electrically Conductive Films> The copper thin film, used as an electrically conductive film, was deposited using a vacuum deposition apparatus (JEE-420, JEOL). A tantalum foil (manufactured by Nilaco) with a thickness of 0.03 mm was used as a heating boat, and copper wire (manufactured by Nilaco) with a diameter of 0.3 mm was cut to a predetermined length and placed on the heating boat. The vacuum gauge reading was 2 × 10⁻⁶ －３ Once the pressure reached below Pa, the tantalum boat was heated by applying an electric current to gradually evaporate the copper. After waiting about 5 minutes after deposition, the bell jar was opened to the atmosphere and the sample was removed.

[0324] <5.3 AFM Measurement Method> An AFM (atomic force microscope) was used, specifically a Bruker Fast Scan, and the surface morphology was evaluated using Peak Force Tapping Mode. A Bruker Scan Systo-Air cantilever (spring constant k = 0.4 Nm-1) was used, and measurements were performed with a Setpoint of 5 nN, Amplitude of 50 nm, and Frequency of 1 kHz.

[0325] <5.4 Quantitative Evaluation of Surface Roughness> The arithmetic mean roughness (Ra) was calculated from the height image of the surface irregularities of the support substrate measured by AFM. Here, the arithmetic mean roughness (Ra) is obtained by assigning the absolute integral value of the height profile curve z = f(x), extracted along a straight line of a certain length l, to the length of that straight line, and is expressed by the following formula.

[0326] The more irregularities there are per unit length l, and the greater the degree of each individual irregularity, the larger the integral value becomes. Therefore, a larger Ra value indicates that the surface irregularities are more pronounced. The actual Ra value was calculated using Nanoscope Analysis software from Bruker.

[0327] <5.5 Measurement Results of Surface Morphology of Support Substrates> Figure 34 shows the surface morphology observation results of various support substrates. Figure 34(a) is a surface topography image measured by AFM, showing four types: [A] high refractive index glass, [B] synthetic quartz glass, [C] sapphire glass, and [D] slide glass. The height scale of all images is standardized from -3 nm to 3 nm, and the scale bar in the planar direction is 200 nm. Figure 34(b) is a representative surface profile obtained from each image, with the horizontal axis representing position x (nm) and the vertical axis representing height z (pm). [D] Slide glass was found to have a large topography amplitude of approximately ±3000 pm, and was confirmed to be significantly rougher than the other substrates. Figure 34(c) is an enlarged view of the vertical axis of the surface profile (b), showing three types: [A] high refractive index glass, [B] synthetic quartz glass, and [C] sapphire glass. The vertical axis scale is ±600 pm, and it was confirmed that the [C] sapphire glass had the smallest surface irregularities.

[0328] Figure 34 shows that the slide glass [D] exhibited the largest variation in surface topography, while the sapphire glass [C] showed the smallest variation in surface topography and was found to be the flattest. The amplitude of the surface topography of the slide glass [D] was large, on the order of several nanometers, and it was confirmed that the surface topography decreased in the order of [A] high refractive index glass, [B] synthetic quartz glass, and [C] sapphire glass. The surface of the sapphire glass [C], which had the smallest surface topography, was found to have only surface topography of 200 pm or less.

[0329] Table 6 summarizes the results of the arithmetic mean roughness (Ra) analysis performed on AFM surface texture images.

[0330]

[0331] The arithmetic mean roughness of the glass slide is about an order of magnitude larger than that of other support substrates, suggesting low flatness. The arithmetic mean roughness of the sapphire glass, which has the highest flatness, is 0.10 nm, making it extremely flat as an optical support substrate for film deposition.

[0332] <5.6 Confirmation of the film thickness of the electrically conductive film (Cu film)> Next, an electrically conductive film with a target thickness of 15 nm was fabricated by vacuum deposition. Furthermore, its actual film thickness was evaluated by line profile measurement using AFM.

[0333] Figure 35 shows the measurement results of the film thickness of the electrically conductive film 12 deposited on a sapphire glass substrate and a slide glass substrate as support substrates. Figure 35(a) shows the correspondence between the deposition mask pattern and the measurement location. The left figure is a conceptual diagram of the mask deposition pattern, where the gray area represents the electrically conductive film (Cu) and the white area represents the support substrate. The right figure is a magnified view of the area near the boundary between the electrically conductive film (Cu) and the support substrate. The difference in height at this boundary was measured to obtain the actual film thickness measurement. Figure 35(b) shows the height image including the boundary between the electrically conductive film (Cu) and the support substrate in [C] sapphire glass and [D] slide glass, and their surface line profiles. The scale bar in the planar direction of the height image is 1 μm, and the height scale of the image is standardized from -5 nm to 5 nm. The arrows in the figure indicate the measurement direction of the line profile crossing the boundary. The horizontal axis of the line profile is position x (nm), and the vertical axis is height z (nm), with [C] sapphire glass shown by a black line and [D] slide glass shown by a gray line. It was confirmed that the height difference between the electrically conductive film and the support substrate was approximately 15 nm.

[0334] <5.7 AFM Observation of Surface Morphology of Electrically Conductive Films> Next, we investigated the influence of the flatness of the support substrate on the flatness of the electrically conductive film itself. We chose [C] sapphire glass, which had the highest flatness, and [D] slide glass, which had the lowest flatness, and compared the surface morphology of electrically conductive films fabricated using both as support substrates.

[0335] Figure 36 shows the AFM measurement results of the surfaces of electrically conductive films (Cu) deposited on a sapphire glass substrate (C) and a glass slide substrate (D). Figure 36(a) is a topographic image, with the left figure showing the electrically conductive film on the sapphire glass substrate (C) (Ra = 0.428 nm) and the right figure showing the electrically conductive film on the glass slide substrate (D) (Ra = 1.60 nm). The scale bar in the planar direction is 200 nm, and the height scale is standardized from -5 nm to 5 nm. Figure 36(b) is a typical line profile, with the horizontal axis representing position x (nm) and the vertical axis representing height z (nm). The electrically conductive film 12 on the sapphire glass substrate (C) is shown by a black line, and the electrically conductive film 12 on the glass slide substrate (D) is shown by a gray line. [D] The electrically conductive film 12 on the slide glass substrate had a large amplitude of irregularities of approximately ±6 nm, and it was confirmed that its surface roughness was significantly greater than that of the electrically conductive film 12 on the sapphire glass substrate. Here, the arithmetic surface roughness of the surfaces of the respective support substrates 11 is Ra = 0.10 nm (sapphire glass) and Ra = 0.81 nm (slide glass), so it can be understood that the roughness of the support substrate surface directly affects the surface roughness of the electrically conductive film (Cu). From the above, it is considered that sapphire glass is suitable as a support substrate for forming an electrically conductive film (Cu) from the viewpoint of surface roughness.

[0336] <5.8 Optimization of Support Substrate> Sapphire glass substrates are more expensive than ordinary glass substrates, but they are ideal as optical support substrates because they are less prone to cracking and scratching. They are also chemically stable and have high resistance to various acids and alkalis. For example, the deposited electrically conductive film (Cu) can be easily dissolved and removed by washing with 10% nitric acid, but the sapphire substrate itself is not affected by this level of acid. Therefore, after dissolving the electrically conductive film in nitric acid, the substrate can be reused as a support substrate after thorough washing with distilled water.

[0337] Furthermore, because sapphire glass itself has high mechanical strength, it is possible to reduce the thickness of the support substrate. In fact, sapphire glass has more than 10 times the bending strength of synthetic quartz. In Example 2, a high refractive index glass substrate with a thickness of about 2 mm is used, but with sapphire glass, due to its high strength, the thickness can be reduced to less than 1 mm. This allows the working distance of the microscope to be shortened, making it possible to observe at higher magnifications. In addition, optical transmission loss due to the support substrate itself can be reduced.

[0338] According to Example 5, the surface morphology of an electrically conductive film (Cu) with a thickness of 15 nm can be quantitatively evaluated. Line profile measurement using AFM confirmed that the thickness of the deposited electrically conductive film was 15 nm as intended. It was also revealed that the surface roughness of the support substrate directly affects the surface roughness of the electrically conductive film. The arithmetic surface roughness of the electrically conductive film deposited on a sapphire glass substrate was Ra = 0.428 nm, which is higher flatness than that of a slide glass substrate (Ra = 1.60 nm). In addition to its high flatness, the sapphire glass substrate has high mechanical strength and is chemically stable, making it possible to use it repeatedly as a support substrate. Furthermore, by reducing the thickness to 1 mm or less, the working distance of the microscope can be shortened, enabling high-magnification observation. Therefore, in this disclosure, a person skilled in the art can freely select and use a sapphire substrate in terms of strength, a synthetic quartz substrate in terms of optical properties, and a commercially available glass substrate in terms of cost, depending on the purpose.

[0339] <Example 6: Operational Verification of a Light-Transmitting Working Electrode Using a Sapphire Glass Substrate> In this example, we verified whether it was possible to observe the deposition and dissolution of lithium metal using a light-transmitting working electrode having a 15 nm thick electrically conductive film (Cu) deposited on a sapphire glass substrate.

[0340] <6.1 Experimental Conditions> Based on the previous examples, it was considered that a thickness of approximately 15 nm for the electrically conductive film is optimal. Furthermore, considering flatness and mechanical strength, sapphire glass was considered suitable as the glass plate to be used as the support substrate. On the other hand, as shown in Table 5 of Example 4, the optical transmission characteristics of sapphire glass are not superior to those of various glass substrates. Therefore, in this example, we confirmed whether the desired experiment was possible using a light-transmitting working electrode having an electrically conductive film with a thickness of 15 nm deposited on a sapphire glass substrate with a thickness of 1 mm.

[0341] <6.2 Configuration of the Electrochemical Cell> The configuration of the electrochemical cell in this embodiment is as follows: Counter electrode: Metallic lithium foil with copper foil (Li 100 μm on Cu 10 μm), φ = 12 mm Working electrode: Cu (φ = 6 mm, t = 15 nm) on sapphire glass (φ = 32 mm, t = 1 mm) Separator: Polypropylene, 40 μm thickness Electrolyte: 1.0 M LiPF ６ in EC / DEC=1 / 1

[0342] <6.3 Vacuum Deposition of Electrically Conductive Film> A sapphire glass substrate, thoroughly washed with distilled water, was air-dried with a blower and then further dried in a dryer. Subsequently, an electrically conductive film (15 nm thick) of the mask pattern was deposited using a vacuum deposition apparatus.

[0343] <6.4 Assembly of Test Cells> The cells were assembled in a dry chamber (dew point below -50°C). First, the separator was impregnated with electrolyte and attached to the working electrode surface, taking care to prevent air bubbles from entering. Next, the counter electrode, a metallic lithium foil, was placed on top and pressurized with an electrode holder and a spring coil. The entire cell was secured with screws, sealed, and then transported to the atmosphere.

[0344] <6.5 Electrochemical Measurement and Observation> The electrochemical measuring device (CompactStat. h, IVIUM) was connected to the assembled cell, and the reaction of metallic lithium deposition and dissolution was repeatedly performed at a constant current. The current density was i = 1.0 mAcm based on the working pole area. －２ The working electrode during the electrochemical operation was recorded as a video using a simple digital microscope.

[0345] <6.6 Observation Results> Figure 37 shows the observation results of the deposition and dissolution cycle of metallic lithium using a test cell with a 15 nm thick electrically conductive film (Cu) deposited on a sapphire substrate as the working electrode. Figure 37(a) shows the voltage profile of typical lithium deposition and dissolution in the second cycle. The horizontal axis represents the amount of electricity (mCcm). －２ The vertical axis represents voltage (V vs LiCE). The current density is i = 1.0 mAcm². －２ The electrolyte is 1.0 M LiPF ６ The voltage in EC / DEC is 1. Points A to C in the figure represent the lithium deposition process, and points C to F represent the lithium dissolution process. The amount of electricity in lithium deposition and dissolution is almost equal, confirming high reversibility. Figure 37(b) shows optical photographs of the electrodes corresponding to each point A to F in the voltage profile. The upper panel shows the lithium deposition process (A to C), and the lower panel shows the lithium dissolution process (D to F). The scale bar is 2 mm.

[0346] Figure 37(b) shows the electrode configurations corresponding to each point in the voltage profile in Figure 37(a), indicated by corresponding letters. Before the cycle (A: 0 mC cm) －２ At this point, several scratches and protrusions were observed on the surface of the counter electrode's metallic lithium foil. Lithium deposition began (B: 6 mC cm －２ At this point, significant lithium deposition was observed along the scratches on the counter electrode. The contrast of the working electrode changed clearly in response to the amount of electricity, and the changes in the electrode due to the deposition of metallic lithium were clearly observed.

[0347] Next, during the dissolution process of metallic lithium (from D to E), it was observed that the deposited metallic lithium was removed from the electrode, returning to the original electrical conductive film's contrast.

[0348] According to Example 6, the deposition and dissolution processes of metallic lithium can be optically observed using a light-transmitting working electrode made of a 15 nm thick electrically conductive film (Cu) deposited on a sapphire glass substrate. The amount of electricity for lithium deposition and dissolution was almost equal, confirming high reversibility. The contrast of the working electrode changed clearly in response to the amount of electricity, making it possible to record the changes in the electrode associated with the deposition and dissolution of metallic lithium as a video. From the above, it was confirmed that the desired experiment can be performed using a 15 nm thick electrically conductive film deposited on a sapphire glass substrate.

[0349] <Example 7: Development of an anodeless battery system using positive electrode active material> In this example, a 15 nm thick electrically conductive film (Cu) deposited on a sapphire substrate as a support substrate is used as the light-transmitting working electrode, and LiNi is used as the counter electrode. ０．５ Mn １．５ O ４ We fabricated an anodeless battery using a coated electrode with a positive electrode active material and verified whether the processes of metallic lithium deposition and dissolution during its charge and discharge cycles could be observed.

[0350] <7.1 Overview of Anodeless Batteries> In Example 6, a test cell using metallic lithium foil as the counter electrode was fabricated, demonstrating that the deposition and dissolution of metallic lithium on the working electrode could be visualized. On the other hand, in more practical batteries, a coated electrode with a positive electrode active material is used as the counter electrode. Batteries consisting of copper foil and a positive electrode active material are known as anodeless batteries. During the charging process, lithium ions released from the positive electrode are deposited as metal on the copper foil, and the reverse reaction occurs during the discharging process. Since only the deposition and dissolution of metallic lithium on the copper foil are repeated during the charging and discharging process, and a negative electrode active material is not required during battery manufacturing, a battery with lower cost and higher energy density can be expected.

[0351] In anodeless batteries, problems such as short circuits caused by heterogeneous lithium deposition on the copper foil and low Coulomb efficiency are considered, and there is a need to clarify the causes of these issues.

[0352] <7.2 Configuration of the electrochemical cell> The configuration of the electrochemical cell in this embodiment is as follows: Counter electrode: LiNi ０．５ Mn １．５ O４ Composite coating electrode cathode coating electrode: φ=12 mm Working electrode: Cu (φ=6 mm, t=15 nm) on sapphire glass (φ=32 mm, t=1 mm) Separator: Polypropylene separator (thickness 25 μm) Electrolyte: 1.0 M LiPF ６ in EC / DMC=3 / 7

[0353] <7.3 Fabrication of Cathode Coated Electrode> The positive electrode active material is a spinel-type lithium nickel manganese oxide (LNMO:LiNi) known to operate in the 5V class. ０．５ Mn １．５ O ４ A conductive additive (manufactured by Nippon Chemical Industrial Co., Ltd.) was used. Acetylene black (AB, HS-100, manufactured by Denka Co., Ltd.) was used as a conductive additive, and polyvinylidene difluoride (PVdF, L9130, manufactured by Kureha Corporation) was used as a binder. The above materials were mixed in a weight ratio of active material / AB / PVdF = 90 / 5 / 5, and a slurry was prepared by adding N-methyl-2-pyrrolidone solvent (manufactured by Kishida Chemical Co., Ltd.). The prepared slurry was coated onto a 20 μm thick aluminum foil and vacuum dried at 120°C. The dried coated electrode was punched out to a diameter of φ = 12 mm and used as the counter electrode.

[0354] <7.4 Cell Prototype> Coated electrodes have many voids and easily absorb electrolyte. Therefore, it is necessary to thoroughly impregnate the positive electrode (counter electrode) with electrolyte before cell prototyping.

[0355] Figure 38 is a schematic diagram of a battery prototyped using a coated positive electrode. In the figure, the components are arranged from top to bottom in the order of positive electrode (counter electrode), separator (electrolyte), and Cu thin film. The left side shows the state before assembly, and the right side shows the state after assembly. Figure 38(a) shows the case when a dry coated electrode is used, and after assembly, the electrolyte in the separator is absorbed by the coated positive electrode, resulting in an electrolyte deficiency in the separator (shaded area in the figure). Figure 38(b) shows the case when a coated electrode that has been pre-impregnated with electrolyte is used, and after assembly, the electrolyte impregnated in the separator is retained, so a good battery can be prototyped.

[0356] In untreated coated positive electrodes, the electrolyte impregnated into the separator is absorbed by the positive electrode, resulting in an electrolyte deficiency at the interface, making it impossible to prototype a good battery. On the other hand, if the coated positive electrode is impregnated with electrolyte beforehand, the electrolyte is retained within the separator even after cell fabrication, allowing for the prototype production of a good battery. In this embodiment, coated positive electrodes punched to a diameter of 12 mm were immersed in electrolyte for more than 10 minutes beforehand, and then the cells were assembled.

[0357] <7.5 Electrochemical Measurement and Observation> The electrochemical measuring device (CompactStat. h, IVIUM) was connected to the assembled cell, and the reaction of metallic lithium deposition and dissolution was repeatedly performed at a constant current. The current density was i = 0.2 mAcm based on the working pole area. －２ The following steps were taken: The device was repeatedly charged and discharged several times beforehand to confirm stable behavior before observation with a digital microscope. The operating voltage range was set from 1.0 V to 5.0 V.

[0358] <7.6 Observation Results> Figure 39 shows LiNi ０．５ Mn １．５ O ４ This shows the charge and discharge behavior of an anodeless battery configuration using a coated positive electrode as the counter electrode. Figure 39(a) is a typical voltage profile obtained after the first few cycles. During the charging process, a clear voltage flat region above 4 V can be observed, suggesting stable charging (lithium desorption) of the positive electrode and lithium deposition on the working electrode (electrically conductive film). Similarly, during the discharge process, a significant voltage flat region around 4 V can be observed, suggesting lithium dissolution from the working electrode (electrically conductive film) and lithium absorption into the positive electrode. The electrical efficiency during the charge-discharge cycle was over 90%, indicating relatively good cycle behavior for an anodeless battery.

[0359] Figure 39(b) shows the working electrode obtained at points A through F on the voltage profile in Figure 39(a), indicated by corresponding letters. During the charging process, an increase in contrast originating from metallic lithium deposition was observed from near the center of the electrode. However, this appearance was not homogeneous, and it was observed that it gradually spread from the peripheral to the central part of the electrically conductive film. Unlike the case where the counter electrode is metallic lithium foil, this behavior is thought to reflect the reaction distribution of the positive electrode coating electrode itself.

[0360] During the discharge process, the contrast of metallic lithium deposition gradually disappeared, and eventually, almost the same contrast was obtained in A (before charging) and F (after discharge).

[0361] Example 7 demonstrates that the deposition and dissolution processes of metallic lithium can be visualized even in an anodeless battery configuration with a coated positive electrode as the counter electrode. The electrical efficiency in the charge-discharge cycle was over 90%, indicating relatively good cycle behavior for an anodeless battery. During the charging process, a gradual spread of contrast originating from metallic lithium deposition was observed from the peripheral to the central part of the electrically conductive film, which is thought to reflect the reaction distribution of the coated positive electrode itself. Therefore, by using the light-transmitting working electrode of this disclosure, it becomes possible to optically observe the deposition and dissolution behavior of metallic lithium in an anodeless battery, contributing to the elucidation of the cause of heterogeneous lithium deposition, which is a problem in anodeless batteries.

[0362] <Example 8: Application of Spectroscopic Imaging> In this example, the changes in the electrodes in this disclosure are captured with a hyperspectral camera, and an example of applying spectroscopic imaging to operando observation is shown.

[0363] <8.1 Overview of Spectral Imaging> Spectral imaging refers to a technique of recording spectra at each pixel of an acquired image. The basic data of spectral imaging has spectral information at each pixel. Therefore, by analyzing this spectrum at each pixel and reconstructing it as an image, even weak changes that cannot be identified by the naked eye can be clearly visualized. Hyperspectral cameras finely disperse and capture the light emitted from an object for each wavelength, and can record the light for each pixel, and are commonly used in spectral imaging.

[0364] <8.2 Spectral Imaging Using Metallic Lithium Foil as the Counter Electrode> The configuration of the electrochemical cell in this example is as follows. Counter electrode: metallic lithium foil on copper foil (Li 100 µm on Cu 10 µm), φ=12 mm Working electrode: Cu (φ=6 mm, t=15 nm) on sapphire glass (φ=32 mm, t=1 mm) Separator: made of polypropylene, 40 µm thick Electrolyte: 1.0 M LiPF6 in EC / DEC=1 / 1

[0365] <8.3 Vacuum Vapor Deposition of Electrically Conductive Film> A sapphire glass substrate sufficiently washed with distilled water was air-dried with a blower, and further dried with a dryer. Thereafter, a mask-patterned electrically conductive film (15 nm thick) was formed by a vacuum vapor deposition apparatus.

[0366] <8.4 Assembly of Test Cell> Cell assembly was performed in a dry chamber (dew point -50 °C or lower). First, the separator was impregnated with the electrolyte and attached to the surface of the working electrode so that no air bubbles entered. Next, the counter electrode metallic lithium foil was placed thereon, and pressurized with an electrode holder and a spring coil. The entire cell was fixed with screws, sealed, and then transported into the atmosphere.

[0367] <8.5 Electrochemical Measurement and Observation> An electrochemical measurement device (CompactStat.h, IVIUM) was connected to the assembled cell, and the current density i=1.0 mAcm －２The operation was performed with a constant current (based on the working electrode area). Charging (deposition of metallic lithium onto the working electrode) was performed from the open circuit voltage (OCV) to 0 V vs. Li+ / Li potential, followed by a 60-second pause. Thereafter, current was applied at the lithium deposition potential for 2 seconds, followed by a 60-second pause. The above operation was repeated to achieve a lithium deposition amount of 2 mA s cm －２ (2 mC cm －２ ) increment, and spectral images were acquired during each pause. The lithium dissolution process was performed in increments of 2 mC cm －２ in the same manner as described above, and spectral images were similarly acquired during each pause.

[0368] <8.6 Acquisition of Spectral Images> Spectral images were obtained using a commercially available hyperspectral camera (NH-1, manufactured by eba japan). The spectral wavelength range was from 400 nm to 1000 nm, and the spectral wavelength resolution was 5 nm. Spectral images were acquired at a pixel count of 640×480 pixels, and the acquisition time for one image was 25 seconds.

[0369] <8.7 Observation Results of Metallic Lithium Deposition Process> FIG. 40 is a spectral image of an electrode during the metallic lithium deposition process, captured with a hyperspectral camera. FIG. 40(a) is a normal optical image that has not been subjected to analysis processing. A change in electrode contrast was confirmed from the open circuit state (OCV) up to metallic lithium deposition corresponding to 8 mC cm －２ . However, since the contrast of the electrically conductive film itself cannot be ignored, it has not been possible to clearly visualize the in-plane state of the electrode for minute metallic lithium deposition, for example, deposition amounts of 6 mC cm －２ or less.

[0370] On the other hand, Figure 40(b) shows the intensity ratio calculation images of the spectra of each image, analyzed using the OCV image spectrum as a reference. Naturally, the intensity ratio calculation of the OCV is a ratio to itself, so in principle, there is no contrast. At 0V vs. Li+ / Li potential, metallic lithium deposition does not occur, but reduction of the electrically conductive film and formation of a surface film can occur. However, the intensity ratio calculation image at 0V vs. Li+ / Li potential does not show a significant change in contrast. In other words, the effects of these side reactions can be said to be almost negligible.

[0371] The amount of metallic lithium deposited is 2 mC cm. －２ The image shows almost no change compared to the image of 0V vs. Li+ / Li potential, but 4 mCcm －２ A clear change in contrast was observed in the image. This change was even more pronounced compared to the normal optical image shown in Figure 40(a). Thus, by performing spectral analysis using the initial OCV spectral image as a reference, it becomes possible to more clearly understand the changes in the electrode due to metallic lithium deposition.

[0372] Here, 1 mC cm －２ The amount of electricity corresponds to approximately 1.35 nm in the average thickness of metallic lithium. Therefore, 4 mCcm －２ This corresponds to an average film thickness of approximately 5.4 nm. Such extremely small amounts of metallic lithium deposition are difficult to detect even with advanced microscopy and X-ray techniques. In other words, by tracking changes in the visible spectrum, it is possible to understand the very early stages of metallic lithium deposition in detail.

[0373] <8.8 Spectroscopic Imaging of Lithium Deposition and Dissolution Cycles> Figure 41 shows the results of the spectral imaging analysis during the lithium deposition and dissolution cycles. Figure 41(a) is a spectral image of the electrode from before lithium deposition to after lithium dissolution, with the open-circuit spectrum used as a reference for intensity ratio calculation. Figure 41(b) shows the average spectrum corresponding to each image, analyzed in the rectangular region in the figure, with the blue light component at wavelength 480 nm and the red light component at 700 nm indicated by black dotted lines.

[0374] The changes in the electrode during lithium deposition and dissolution processes were clearly visualized. The electrode after lithium dissolution showed a more pronounced contrast in density compared to before lithium deposition. This is thought to be due to the lithium remaining on the electrode surface.

[0375] It was observed that the intensity of the blue component at a wavelength of around 480 nm decreased significantly during the lithium deposition process compared to before lithium deposition. This is thought to be a result of strong scattering or absorption of blue light during lithium deposition. Furthermore, it was observed that the intensity of the red component at a wavelength of around 700 nm decreased significantly during the lithium dissolution process. Unlike the lithium deposition process, it is thought that red light is strongly scattered or absorbed during the lithium dissolution process. After lithium dissolution, the intensity of the red component decreases compared to before lithium deposition, which is thought to be because the metallic lithium remaining on the electrode surface absorbs or scatters red light.

[0376] <8.9 Mechanism of Spectral Change> During the lithium deposition process, lithium ions in the electrolyte become metallized, generating nuclei of metallic lithium smaller than the wavelength of light on the electrode surface. Generally, the scattering intensity of light is known to depend on the particle size, and Rayleigh scattering occurs for particles sufficiently smaller than the wavelength of light. Since the intensity of Rayleigh scattering is inversely proportional to the fourth power of the wavelength, shorter wavelength light (blue light) is scattered more strongly. This is thought to be the reason why the intensity of the blue light component decreases during the lithium deposition process. On the other hand, the dissolution of metallic lithium proceeds more heterogeneously than deposition, resulting in porous formation of coarse metallic lithium particles. It is thought that these lithium clumps scatter or absorb red light, resulting in a spectrum with a significantly reduced red light component.

[0377] <8.10 Spectroscopic imaging with a coated cathode electrode as the counter electrode> Next, LiNi ０．５ Mn １．５ O ４ Spectroscopic imaging was performed on an anodeless battery configuration with a composite coated cathode as the counter electrode. The configuration of the electrochemical cell in this example is as follows: Counter electrode: LiNi ０．５ Mn １．５ O ４Composite coating electrode cathode coating electrode: φ=12 mm Working electrode: Cu (φ=6 mm, t=15 nm) on sapphire glass (φ=32 mm, t=1 mm) Separator: Polypropylene separator (thickness 25 μm) Electrolyte: 1.0 M LiPF ６ in EC / DMC=3 / 7

[0378] <8.11 Electrochemical Measurement and Spectroscopic Image Acquisition> The electrochemical measurement device (CompactStat.h, IVIUM) and the assembled cell were connected, and the current density i = 0.1 mAcm² was measured. －２ Charging (deposition of metallic lithium onto the working electrode) and discharging (dissolution of metallic lithium from the working electrode) were performed using a constant current based on the working electrode area. Spectroscopic images were acquired using a commercially available hyperspectral camera (NH-1, eba japan). The spectral wavelength range was 400 nm to 1000 nm, and the spectral wavelength resolution was 5 nm. Spectroscopic images were acquired at a resolution of 640 × 480 pixels, and the acquisition time per image was 25 seconds.

[0379] <8.12 Observation results in anodeless battery configuration> Figure 42 shows LiNi ０．５ Mn １．５ O ４ This is an example of spectral imaging in an anodeless battery configuration for the counter electrode. Figure 42(a) shows the charge / discharge profile, and Figure 42(b) shows the spectral image of the electrode corresponding to each point.

[0380] Figure 42(a) shows LiNi at a cell voltage of approximately 4.7 V to 4.8 V. ０．５ Mn １．５ O ４ A potential flat region originating from the electrochemical reaction was observed. Furthermore, Figure 42(b) shows the state of the electrodes acquired from point A to J during the charging and discharging processes.

[0381] At the start of charging (lithium deposition) (point A), no lithium deposition has occurred, so the electrode contrast is homogeneous. To make lithium deposition easier to see compared to the initial state, the images from points B to F were analyzed using the spectrum of point A as a reference. At point C, where a voltage flat area begins to appear, significant deposition of metallic lithium was observed at the edge of the electrode. Furthermore, as charging progressed from point D to E, heterogeneous island-like deposition of metallic lithium was observed across the entire working electrode. Electrical charge: 0.43 mAhcm －２ At point F, where the battery was fully charged, metallic lithium dendrites extending radially from the edge of the electrode were clearly observed.

[0382] Next, to make the changes due to lithium dissolution more visible, the images from point G to I in the discharge process were analyzed using the spectrum of point F as a reference. As the discharge progressed from point G to I, a dark contrast was observed spreading from the edge of the deposited metallic lithium. This is because the image intensity of the corresponding area decreases compared to point F as the metallic lithium dissolves. In other words, by tracking this dark contrast characteristic, the dissolution behavior of metallic lithium can be understood. At point I, at the end of the discharge, it was confirmed that most of the deposited metallic lithium had dissolved, but a bright contrast remained in some areas. This is thought to be dead lithium that remained dissolved on the electrode surface.

[0383] <8.13 Dissolution Behavior of Dendrite-Shaped Lithium> Figure 43 is a magnified view of the analysis image acquired at point J, the end of the discharge profile. It was confirmed that the contrast of the dendritic lithium extending from the edge of the electrode was slightly brighter compared to the island-shaped deposited lithium. This indicates that more dendritic lithium remained dissolved. On the other hand, it was confirmed that the contrast of the base portion of the dendritic lithium (the portion in contact with the edge of the electrode) was uniquely dark. This means that more of the base portion of the dendrite-deposited lithium dissolved. In other words, it is thought that more of the dendritic lithium remained dissolved because it lost electrical contact with the electrode.

[0384] According to Example 8, spectral imaging using a hyperspectral camera allows for more detailed visualization of the deposition and dissolution processes of metallic lithium. By performing spectral analysis using the open-circuit spectral image as a reference, even minute metallic lithium deposition, which is difficult to identify with conventional optical images, can be clearly detected. 1 mCcm －２ The amount of electricity deposited corresponds to an average thickness of approximately 1.35 nm of metallic lithium, and detecting such an extremely minute amount of deposition is difficult even with advanced microscopy and X-ray techniques. Furthermore, characteristic spectral changes were observed, with a decrease in the blue light component during the lithium deposition process and a decrease in the red light component during the lithium dissolution process. These changes are thought to be due to Rayleigh scattering during lithium deposition and light scattering by coarse particles during lithium dissolution. In addition, in the anodeless battery configuration, the process of the base portion of the dendrite-like lithium preferentially dissolving, resulting in the loss of electrical contact with the electrode and its remaining as dead lithium, could be visualized. From the above, it was demonstrated that spectroscopic imaging is an effective method for understanding the deposition and dissolution behavior of metallic lithium in detail.

[0385] All disclosures in the specification, drawings, and abstract contained in the Japanese application No. 2025-056480, filed on 28 March 2025, are incorporated herein by reference.

[0386] 10, 10A, 10B, 10C, 10D Working electrode 10P Working electrode 11, 11A, 11B, 11C, 11D 12, 12A, 12B, 12C, 12D Electrically conductive film 12S Electrode surface 13 Working electrode current collector holder 15 Working electrode holder 15P Deposit 17 Seal backing 20 Separator 20P Electrolyte 201P Separator 25 Sealing material 30 Counter electrode 30P Counter electrode 35 Counter electrode plate 40 Pressing mechanism 40P Pressing mechanism 41 Coil spring 42 Electrode holder 50 Main body 60 Guide part 70 Cover 77 Maximum value 80 Battery cell for observation 90 Objective lens 90P Optical lens 91 Revolver 92 Half mirror 93 Main unit 94 Camera 95 Light source 96 Stage 111 First surface 112 Second surface 120, 120B Peripheral parts 121, 121B Central parts 122, 122B Electrical conduction part 151 First terminal 200 Image processing device 201 Processing unit 202 Acquisition unit 203 Image processing unit 204 Learning unit 205 Communication unit 206 Storage unit 207 Input unit 208 Display unit 300 Electrochemical measuring device 330 Charge / discharge testing device 351 Second terminal 1121 Recess

Claims

1. An working electrode for an observation battery, wherein the working electrode comprises a support substrate and an electrically conductive film provided on the support substrate, and the working electrode is characterized by being light-transmitting.

2. The working electrode according to claim 1, characterized in that the thickness of the electrically conductive film is 50 nm or less.

3. The working electrode according to claim 1, characterized in that the resistance of the electrically conductive film is 1000 Ω or less.

4. The working electrode according to claim 1, characterized in that the transmittance of the working electrode is 5.9% or more.

5. The working electrode according to claim 1, characterized in that the electrically conductive film contains copper, nickel, tungsten, or stainless steel.

6. The working electrode according to claim 1, characterized in that the thickness of the support substrate is constant.

7. The working electrode according to claim 1, wherein the support substrate comprises an observation region and a support region, and the thickness of the support region is greater than the thickness of the observation region.

8. The working electrode according to claim 1, wherein the electrically conductive film comprises a peripheral portion, a central portion located inside the peripheral portion, and an electrically conductive portion connecting the peripheral portion and the central portion, and is electrically connected to the peripheral portion, the central portion and the electrically conductive portion.

9. The working electrode according to claim 8, characterized in that the support substrate has a tapered recess, and the recess is provided according to the position of the central portion.

10. The working electrode according to claim 8, wherein the support substrate comprises an observation area and a support area, the thickness of the support area is greater than the thickness of the observation area, and the observation area is provided according to the position of the central part.

11. An observation battery cell comprising: a working electrode according to any one of claims 1 to 10; a counter electrode; a separator provided between the working electrode and the counter electrode and capable of holding an electrolyte; and a pressing mechanism configured to press the separator against the working electrode.

12. An optical microscope apparatus comprising a device for inducing an electrochemical reaction in the observation battery cell described in claim 11.

13. A method for manufacturing a working electrode for an observation secondary battery, comprising the steps of: preparing a support substrate; and providing an electrically conductive film on the support substrate, wherein the working electrode is characterized by being light-transmitting.

14. The method for manufacturing a working electrode according to claim 13, wherein the step of providing the electrically conductive film includes a step of vacuum depositing an electrically conductive material on the support substrate with a mask attached.

15. The working electrode according to claim 1, wherein the surface electrical resistance of the electrically conductive film is 6 Ω or less.

16. The working electrode according to claim 1, wherein the wavelength-average transmittance of the electrically conductive film is 46% or more.

17. The working electrode according to claim 1, wherein the surface electrical resistance of the electrically conductive film is 1 kΩ or less, and the wavelength-average transmittance is 40% or more.

18. The working electrode according to claim 1, wherein the wavelength-average transmittance of the support substrate is 80% or more.

19. The observation battery cell according to claim 11, wherein the counter electrode is a coated electrode containing a positive electrode active material.

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