Evaluation method for coated active material particles

Abstract

To provide a new method for evaluating the coating state of coated active material particles. [Solution] A method for evaluating coated active material particles, comprising active material particles and a coating layer covering the surface of the active material particles, comprising: applying a negative voltage to the coated active material particles and acquiring secondary electron images and backscattered electron images of the coated active material particles using a SEM; superimposing the secondary electron images and backscattered electron images to acquire a composite image; and identifying the coated active material particles into coated active material particles with high coverage and coated active material particles with low coverage based on the brightness of the composite image.

Description

Technical Field

[0001] The present disclosure relates to a method for evaluating coated active material particles.

Background Art

[0002] JP 2023-136230 A (Patent Document 1) discloses a method of acquiring an SEM image of coated active material particles in a state where a negative voltage is applied to the coated active material particles, detecting the presence or absence of a coating layer on the surface of the active material particles based on contrast in the SEM image, and evaluating the coating state.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For the purpose of improving the performance of secondary batteries, coated active material particles in which a coating layer is formed on the surface of active material particles (positive electrode active material particles or negative electrode active material particles) have been developed. Regarding such coated active material particles, a method for confirming the coating state (coating ratio) of the surface of the coated active material particles as described in Patent Document 1 is known. However, in the method described in Patent Document 1, the coating state may not be accurately evaluated depending on the materials of the active material particles and the coating layer, the thickness of the coating layer, and the like.

[0005] An object of the present disclosure is to provide a new method for evaluating the coating state of coated active material particles.

Means for Solving the Problems

[0006] Hereinafter, the technical configuration and operational effects of the present disclosure will be described. However, the operational mechanism includes estimation. The operational mechanism does not limit the technical scope of the present disclosure.

[0007] [1] A method for evaluating coated active material particles, comprising active material particles and a coating layer covering the surface of the active material particles, With a negative voltage applied to the coated active material particles, secondary electron images and backscattered electron images of the coated active material particles are obtained using a scanning electron microscope (SEM). A composite image is obtained by superimposing the secondary electron image and the backscattered electron image. An evaluation method comprising distinguishing the coated active material particles into coated active material particles with high coverage and coated active material particles with low coverage based on the brightness of the composite image.

[0008] By performing SEM (scanning electron microscope) observation in a retarding mode with a negative voltage (retarding voltage) applied, it becomes possible to obtain precise information about the surface of the coated active material particles while lowering the voltage of primary electrons irradiated onto the coated active material particles (irradiation voltage). In addition, secondary electron images and backscattered electron images contain information such as the composition of the active material particles, the surface topography of the active material particles, and the potential. Furthermore, by using a composite image obtained by superimposing the secondary electron image and backscattered electron image, differences in brightness between coated active material particles are more easily generated. Since the difference in brightness has a high correlation with the coating state of the coated active material particles, it is expected that the identification of the coating state of each coated active material particle will become easier.

[0009] [2] The evaluation method according to [1], wherein the acquisition of the secondary electron image and the backscattered electron image and the acquisition of the composite image are performed simultaneously.

[0010] If secondary electron images and backscattered electron images are not acquired simultaneously with the acquisition of a combined image, there is a risk of misalignment of the observation position or contamination. Therefore, by acquiring secondary electron images and backscattered electron images simultaneously with the acquisition of a combined image, it becomes possible to more accurately identify the coating state of the coated active material particles.

[0011] [3] The evaluation method according to [1] or [2], wherein the coated active material particles are analyzed by EDX or EPMA.

[0012] By using EDX (Energy Dispersive X-ray Spectroscopy) or EPMA (Electron Probe Microanalysis), qualitative and quantitative analysis of the elements constituting the coating layer becomes possible. Therefore, in addition to the visual information obtained from the composite image, it becomes possible to acquire even more detailed information.

[0013] [4] The coating layer is a carbon layer, The evaluation method according to any one of [1] to [3], wherein the thickness of the carbon layer is 1 nm or less.

[0014] Conventionally, the coating state of the carbon layer covering the surface of coated active material particles was confirmed using TEM (transmission electron microscope) or EDX. However, when the carbon layer was thin, it was difficult to confirm the coating state using TEM or EDX. This evaluation method makes it possible to identify the coating state even when the coating layer is a carbon layer and the carbon layer is as thin as 1 nm or less.

[0015] [5] The evaluation method according to any one of [1] to [4], wherein the maximum Ferret diameter of the active material particles is 10 μm or more.

[0016] When the maximum Ferret diameter of the active material particles (secondary particles) is 10 μm or more, the formation of the coating layer becomes easier.

[0017] One embodiment of the present disclosure (which may be abbreviated as "this embodiment") is described below. However, this embodiment does not limit the technical scope of the present disclosure. This embodiment is illustrative in all respects. This embodiment is non-restrictive. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the claims. For example, it is intended from the outset that any configuration may be extracted from this embodiment and combined in any way. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a schematic diagram illustrating the evaluation method of this embodiment (a method using low-acceleration images in retarding mode). [Figure 2] FIG. 2 is a schematic diagram for explaining a conventional evaluation method (a method using low-acceleration images in a normal mode). [Figure 3] FIG. 3 is a schematic diagram for explaining the positions of a detector and acquirable information in a retarding mode and a normal mode. [Figure 4] FIG. 4 is a conceptual diagram of a composite image of a secondary electron image and a backscattered electron image of Sample 1 in an example. MODE FOR CARRYING OUT THE INVENTION

[0019] <Method for Evaluating Coated Active Material Particles> The method for evaluating coated active material particles in the present embodiment includes: acquiring a secondary electron image and a backscattered electron image of the coated active material particles using an SEM in a state where a negative voltage (retarding voltage) is applied to the coated active material particles (retarding mode); superimposing the secondary electron image and the backscattered electron image to obtain a composite image; and distinguishing the coated active material particles into coated active material particles with a high coverage rate and coated active material particles with a low coverage rate based on the luminance of the composite image.

[0020] The coated active material particles include active material particles and a coating layer that coats the surfaces of the active material particles. The active material particles are, for example, positive electrode active material particles or negative electrode active material particles for batteries. The present evaluation method can be suitably used when the active material particles are positive electrode active material particles.

[0021] When the active material particles are positive electrode active material particles, examples of the positive electrode active material particles include lithium nickel composite oxide (LNO), lithium cobalt composite oxide (LCO), lithium manganese composite oxide (LMO), olivine-type compounds, and the like. Examples of the olivine-type compounds include lithium iron phosphate (LFP), lithium manganese phosphate (LMP), lithium manganese iron phosphate (LMFP), and the like.

[0022] When the active material particles are negative electrode active material particles, examples of negative electrode active material particles include graphite, soft carbon, hard carbon, silicon, silicon oxide, silicon-based alloys, tin, tin oxide, tin-based alloys, and lithium titanate (Li4Ti5O4). 12 Examples include:

[0023] The maximum Ferret diameter of the active material particles is, for example, 10 μm or more. When the maximum Ferret diameter of the active material particles is 10 μm or more, the formation of the coating layer becomes easier.

[0024] The surface of the active material particles is covered by a coating layer, at least partially. The entire surface of the active material particles may also be covered by the coating layer.

[0025] The coating layer may be, for example, a carbon layer. The carbon layer is a layer containing a carbon material. The carbon material may be derived from, for example, sugars. When the coating layer is a carbon layer, it is expected that the coated active material particles will be imparted with electronic conductivity.

[0026] The thickness of the coating layer is, for example, 10 nm or less. A coating layer thickness of 10 nm or less is expected to improve the performance of secondary batteries containing coating active material particles. The coating layer thickness may also be 5 nm or less, 3 nm or less, or 1 nm or less. If the coating layer thickness is 1 nm or less, further performance improvement of secondary batteries containing coating active material particles is expected.

[0027] Coated active material particles can be prepared by coating active material particles with a coating layer using various known coating methods.

[0028] The "retarding mode" is a mode in which a negative voltage (retarding voltage) is applied to the sample being observed (coated active material particles) to slow down the primary electron beam just before it reaches the sample. The method of acquiring an image (electron image) in "retarding mode" is also called the retarding method (deceleration method). In the retarding method, the electron beam (primary electrons) accelerated by the electron gun is slowed down by electrostatic decomposition (deceleration electrolysis) just before the sample (for example, between the sample and the beam column) (see Figure 1).

[0029] In normal mode, when performing image observation with a low acceleration voltage, lowering the acceleration voltage of the primary electrons (i.e., the irradiation voltage) reduces the amount of primary electrons, thus lowering the sensitivity (see Figure 2). In contrast, with the retarding method, the primary electron beam is decelerated just before the sample to lower the irradiation voltage, eliminating the need to lower the acceleration voltage of the primary electrons, and thus maintaining the irradiation amount of primary electrons (see Figure 1). As a result, the retarding method makes it possible to detect precise information on the surface of coated active material particles without reducing sensitivity.

[0030] In this embodiment, secondary electron images and backscattered electron images of coated active material particles are acquired using a SEM by retarding. Figure 3 is a schematic diagram illustrating the detectors and the information obtainable from each detector in normal mode and retarding mode. In normal mode, backscattered electrons HA-BSE are detected as signal electrons in the Top detector (T detector). HA-BSE contains information such as the composition and crystalline state of the sample. In the Upper detector (U detector), backscattered electrons LA-BSE and secondary electrons SE(U) are detected as signal electrons. LA-BSE contains information such as the composition and surface topography of the sample, while SE(U) contains information such as the outermost surface and potential of the sample. When a Lower detector (L detector) is provided, secondary electrons SE(L) are detected as signal electrons in the L detector. SE(L) contains information such as the surface topography of the sample.

[0031] On the other hand, in retarding mode, the T detector further detects backscattered electrons HA-BSE and secondary electrons SE(T) as signal electrons. SE(T) contains information about the outermost surface of the sample. In this way, retarding makes it possible to obtain more information.

[0032] Furthermore, while it is possible to obtain the above-mentioned information even in normal mode, when performing image observation with a low acceleration voltage in normal mode, a lot of information is picked up about the active material particles inside the coated active material particles, and accurate information about the coating state cannot be obtained. Therefore, retarding makes it possible to obtain more accurate information about the coating state.

[0033] In this embodiment, a composite image is obtained by superimposing a secondary electron image and a backscattered electron image. By using a composite image, differences in brightness between each coated active material particle are more easily generated. Since the difference in brightness is highly correlated with the coating state of the coated active material particles, it is expected that the identification of the coating state of each coated active material particle will become easier.

[0034] Furthermore, it is preferable that the acquisition of secondary electron images and backscattered electron images, and the acquisition of a composite image, be performed simultaneously. If the acquisition of secondary electron images and backscattered electron images, and the acquisition of a composite image are not performed simultaneously, there is a risk of misalignment of the observation position or contamination. Therefore, by performing the acquisition of secondary electron images and backscattered electron images, and the acquisition of a composite image simultaneously, it becomes possible to more accurately identify the coating state of the coated active material particles.

[0035] When obtaining an electron image, the irradiation voltage (acceleration voltage and retarding voltage) of primary electrons is preferably adjusted so that the contrast difference between the active material particles and the coating layer is sufficiently large. Although it varies depending on the type of active material particles and coating layer, the irradiation voltage is preferably 0.1 to 1.0 kV, and more preferably 0.2 to 0.5 kV. If the irradiation voltage exceeds 1.0 kV, information from the inside as well as the surface of the coated active material particles will be picked up, making it difficult to accurately measure the coating state. If the irradiation voltage is less than 0.1 kV, the amount of primary electrons irradiated onto the coated active material particles decreases, and the amount of secondary electrons detected also decreases, resulting in a decrease in measurement sensitivity.

[0036] The irradiation voltage is basically "[primary electron acceleration voltage] - [retarding voltage]", and the irradiation voltage can be adjusted by controlling the primary electron acceleration voltage and retarding voltage.

[0037] The acceleration voltage is not particularly limited, but is preferably 2kV or less in order to minimize damage to the sample (coated active material particles). The retarding voltage only needs to be lower than the acceleration voltage, for example, 1.9kV or less.

[0038] In this embodiment, coated active material particles are distinguished into highly coated active material particles and low coated active material particles based on the brightness of the composite image. For example, the coating state of each coated active material particle may be evaluated based on an evaluation criterion in which coated active material particles whose brightness of the composite image is above a predetermined threshold are considered to have "high coating," and coated active material particles whose brightness is below the threshold are considered to have "low coating." As a specific example, the obtained composite image is converted to grayscale and given 256 gradations. Among the 256 gradations, a higher gradation number means that it is whiter (coated layer present), and a lower gradation number means that it is blacker (no coating). In this case, highly coated active material particles will have more white areas, and low coated active material particles will have more black areas. In this way, the coating level of each coated active material particle can be visually identified.

[0039] In this embodiment, the coated active material particles may be analyzed by EDX or EPMA. Using EDX or EPMA enables qualitative and quantitative analysis of the elements constituting the coating layer. Therefore, in addition to the visual information obtained from the composite image, it becomes possible to obtain more detailed information.

[0040] EDX and EPMA are methods for measuring elemental amounts by irradiating with an electron beam and measuring characteristic X-rays. However, EPMA spectrally analyzes the signal based on the wavelength of the characteristic X-rays, resulting in superior detection sensitivity compared to EDX. Therefore, using EPMA enables more accurate quantitative analysis.

[0041] The SEM used in this embodiment is not particularly limited and may be, for example, an SEM using a thermionic emission electron gun, or an SEM using a field emission electron gun (FE-SEM: field emission scanning electron microscope). When using an FE-SEM, the coated active material particles can be evaluated more precisely.

[0042] In principle, it is conceivable to evaluate coated active material particles using a method that combines SEM observation with analytical methods such as EDX, without employing retarding. This method utilizes elemental analysis results obtained by EDX, etc. However, obtaining information on the outermost surface of coated active material particles using EDX, etc., and obtaining an elemental mapping image typically takes more than an hour for even just one field of view. Furthermore, obtaining overall information on the raw material powder consisting of coated active material particles requires measurement of numerous fields of view, thus requiring an enormous amount of time for measurement. Therefore, evaluating coated active material particles using such a method is not practical. It is also conceivable to analyze only a specific point for each particle, but obtaining overall information would require measuring a large number of particles, which would also require an enormous amount of time for measurement.

[0043] In contrast, the evaluation method for coated active material particles according to this embodiment is simple and requires a short observation time, allowing for rapid evaluation of the coating state of the active material particles. Therefore, by using the evaluation method of this embodiment, it is possible to easily perform quality evaluations (inspections) of coated active material particles used, for example, as raw materials for electrodes in secondary batteries.

[0044] <Method for manufacturing coated active material particles> In the method for producing coated active material particles of this embodiment, coated active material particles are produced, which include active material particles and a coating layer that covers at least a portion of the surface of the active material particles.

[0045] The method for producing coated active material particles according to this embodiment includes at least a coating step of obtaining coated active material particles by forming a coating layer on the surface of the active material particles, and an evaluation step of evaluating the coating state of the coated active material particles obtained in the coating step using the evaluation method described above.

[0046] According to the method for producing coated active material particles of this embodiment, the coating state of the coated active material particles can be easily evaluated by the evaluation method described above. Therefore, it is possible to obtain coated active material particles with a high coating rate using the information from the evaluation results of the coating state.

[0047] The method for producing coated active material particles in this embodiment may include, for example, a selection step in which powder raw materials consisting of coated active material particles have a ratio of coated active material particles with a low coating rate that exceeds a predetermined threshold, based on the evaluation method described above. By such a step, it is possible to provide powder raw materials containing coated active material particles with a high coating rate.

[0048] Furthermore, for example, the production conditions for coated active material particles may be adjusted by feeding back the evaluation results so that the proportion of coated active material particles with low coverage is reduced, thereby obtaining a raw material powder containing coated active material particles with high coverage. [Examples]

[0049] <Preparation> As coated active material particles, coated cathode active material particles (Sample 1) having a carbon layer on the surface of LMFP were prepared. Sample 1 was granulated by mixing a carbon material with primary particles having a Ferret diameter of 100 nm or less. When Sample 1 was observed with a TEM, the thickness of the carbon layer could not be measured. Therefore, it is considered that the thickness of the carbon layer is 1 nm or less.

[0050] <Observation> Sample 1 was scattered onto a conductive tape. Secondary electron images and backscattered electron images of Sample 1 were acquired using an FE-SEM (Hitachi High-Tech Corporation: SU8230) under the following conditions. In addition, a composite image of the secondary electron image and backscattered electron image was acquired simultaneously with the acquisition of the secondary electron image and backscattered electron image.

[0051] (conditions) Mode: Retarding mode Irradiation voltage: 0.2kV Acceleration voltage: 1.7kV Retarding voltage: 1.5kV

[0052] (Image analysis) The resulting composite image was converted to grayscale, and the coated active material particles were evaluated using 256 gradations. In the composite image, a higher gradation number indicates whiter areas (with a coating layer), while a lower gradation number indicates darker areas (without a coating layer). In other words, coated active material particles with high coating density will have more white areas, while coated active material particles with low coating density will have more dark areas.

[0053] Figure 4 shows a conceptual diagram of the composite image of sample 1. In Figure 4, coated active material particle A has many white areas, while coated active material particle B has many black areas. That is, it can be seen that coated active material particle A with a high coating rate and coated active material particle B with a low coating rate are distinguishable.

[0054] (analysis) The mass fraction of the carbon layer in coated active material particle A and coated active material particle B was measured by EDX. The results showed that coated active material particle A had a higher mass fraction of the carbon layer compared to coated active material particle B. Therefore, it can be seen that the results from image analysis and the results from EDX analysis are correlated.

[0055] Thus, by using the evaluation method disclosed herein, it becomes possible to distinguish coated active material particles into coated active material particles with high coverage and coated active material particles with low coverage. In particular, this evaluation method can distinguish coated active material particles containing a carbon layer with a thickness of 1 nm or less, which was difficult to distinguish using conventional methods.

Claims

1. A method for evaluating coated active material particles, comprising active material particles and a coating layer covering the surface of the active material particles, With a negative voltage applied to the coated active material particles, secondary electron images and backscattered electron images of the coated active material particles are acquired using a SEM. A composite image is obtained by superimposing the secondary electron image and the backscattered electron image. An evaluation method comprising distinguishing the coated active material particles into coated active material particles with high coverage and coated active material particles with low coverage based on the brightness of the composite image.

2. The evaluation method according to claim 1, wherein the acquisition of the secondary electron image and the backscattered electron image and the acquisition of the composite image are performed simultaneously.

3. The evaluation method according to claim 1 or 2, comprising analyzing the coated active material particles by EDX or EPMA.

4. The aforementioned coating layer is a carbon layer, The evaluation method according to claim 1 or 2, wherein the thickness of the carbon layer is 1 nm or less.

5. The evaluation method according to claim 1 or 2, wherein the maximum Ferre diameter of the active material particles is 10 μm or more.

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