Method for detecting manganese and iron elements of lithium manganese iron phosphate

Abstract

The application discloses a manganese-iron element detection method of lithium manganese iron phosphate, and belongs to the technical field of batteries. The manganese-iron element detection method of lithium manganese iron phosphate comprises the following steps: providing a slice containing lithium manganese iron phosphate; obtaining an element distribution graph of manganese and iron elements of the lithium manganese iron phosphate in the slice; generating a gray scale graph according to pixel values corresponding to each pixel point in the element distribution graph; converting the gray scale graph into an element distribution intensity curve graph, and obtaining the manganese-iron ratio and manganese-iron element distribution uniformity of the lithium manganese iron phosphate in the slice through the element distribution intensity curve graph. The manganese-iron ratio and manganese-iron element distribution uniformity of the lithium manganese iron phosphate can be quantitatively calculated, and an accurate and efficient method is provided for researching the influence of the Mn / Fe element distribution uniformity in the lithium manganese iron phosphate material particles on performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a method for detecting manganese and iron elements in lithium manganese iron phosphate. Background Technology

[0002] Since its commercialization, rechargeable batteries have undergone continuous iterations and updates in materials and technology, expanding from their initial widespread application in the 3C digital field to today's electric vehicles and energy storage fields.

[0003] Lithium manganese iron phosphate (LFP) cathode materials combine the high safety and low cost of LFP with the high power of ternary materials. In recent years, the resurgence of LFP cathodes has spurred research into the application of LFP materials, which are considered promising replacements for LFP cathodes as the next generation of cathode materials for large-scale applications, possessing a broad market prospect. Current research has found that under extreme cycling or near the end of its lifespan, LFP materials may exhibit uneven distribution of the originally uniform Mn / Fe elements, or even separate from the original single-phase LFP material into a two-phase material of LFP and LMP. At this point, the lithium storage mechanism of the material may change from single-phase solid solution to two-phase separation, thereby accelerating battery degradation and triggering BMS system alarms.

[0004] Therefore, this application is hereby submitted. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a method for detecting manganese and iron elements in lithium manganese iron phosphate. This application can quantitatively calculate the manganese-iron ratio and the uniformity of manganese and iron element distribution in lithium manganese iron phosphate.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for detecting manganese and iron elements in lithium manganese iron phosphate, comprising the following steps:

[0008] Provide slices containing lithium manganese iron phosphate;

[0009] Obtain the elemental distribution map of manganese and iron in lithium manganese iron phosphate slices;

[0010] Generate a grayscale image based on the pixel values ​​corresponding to each pixel in the element distribution map;

[0011] The grayscale image is converted into an elemental distribution intensity curve, and the manganese-iron ratio and uniformity of manganese-iron element distribution in the slice are obtained from the elemental distribution intensity curve.

[0012] As an embodiment of this application, the thickness of the slice is 50-150 nm.

[0013] As an implementation of this application, the slice containing lithium manganese iron phosphate is obtained by cutting the positive electrode sheet using a focused ion beam device.

[0014] As an implementation of this application, a target lithium manganese iron phosphate particle in the positive electrode sheet is selected by an electron beam, and the positive electrode sheet is cut by an ion beam to obtain the slice containing lithium manganese iron phosphate.

[0015] As an implementation scheme of this application, the elemental distribution map of manganese and iron elements in lithium manganese iron phosphate in the slice was obtained by using an EDS elemental distribution analyzer.

[0016] As an embodiment of this application, the grayscale image has 8 to 16 bits of pixels.

[0017] As an implementation scheme of this application, converting a grayscale image into an element distribution intensity curve specifically involves: setting a threshold based on the pixel values ​​of the grayscale image, and performing binarization processing on the pixel values ​​of the grayscale image based on the threshold to convert it into an element distribution intensity curve.

[0018] As an implementation of this application, pixels with pixel values ​​greater than a threshold in the grayscale image are considered valid pixels.

[0019] As an implementation of this application, pixels in the grayscale image that are ≤ a threshold are considered invalid pixels.

[0020] As an implementation scheme of this application, setting a threshold based on the pixel value of the grayscale image specifically involves: traversing the pixel value of each pixel in the grayscale image, calculating the pixel value difference between each adjacent pixel, determining two target pixel values ​​based on the maximum difference between adjacent pixels, and using the smaller pixel value among the target pixel values ​​as the threshold.

[0021] The beneficial effects of this invention are as follows: This application obtains the elemental distribution map of manganese and iron elements in sliced ​​lithium manganese iron phosphate, generates a grayscale image based on the pixel value corresponding to each pixel point in the elemental distribution map, and then converts the grayscale image into an elemental distribution intensity curve. Finally, based on the elemental distribution intensity curve, the manganese-iron ratio and the uniformity of manganese and iron element distribution in lithium manganese iron phosphate can be quantitatively calculated, providing an accurate and efficient method for studying the influence of the uniformity of Mn / Fe element distribution inside lithium manganese iron phosphate material particles on performance. Attached Figure Description

[0022] Figure 1 SEM images of the preparation process of the electrode containing lithium manganese iron phosphate in Example 1:

[0023] (a) Lithium iron manganese phosphate particles are sliced ​​and adhered to a mechanical needle;

[0024] (b) Extract the slice;

[0025] (c) Slicing;

[0026] (d) Transfer the slice to the sample stage;

[0027] (e) Connect the slice to the sample stage;

[0028] (f) The connection between the ion beam cutting robot and the slice;

[0029] (g) Slicing and grinding;

[0030] (h) Slicing, polishing, and impurity removal;

[0031] (i) Ultrathin section.

[0032] Figure 2 This is an elemental distribution diagram of manganese and iron in lithium manganese iron phosphate in the slice of Example 1.

[0033] Figure 3 This is a grayscale image of Example 1.

[0034] Figure 4 This is a collection image of effective pixels from Example 1.

[0035] Figure 5 This is a graph showing the element intensity distribution of Example 1.

[0036] Figure 6 The elemental distribution diagram of manganese and iron in lithium manganese iron phosphate slices from Example 2.

[0037] Figure 7 This is a grayscale image of Example 2.

[0038] Figure 8 This is a collection image of effective pixels in Example 2.

[0039] Figure 9 This is a graph showing the elemental intensity distribution of Example 2. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0041] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0042] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0043] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0044] This application provides a method for detecting manganese and iron elements in lithium manganese iron phosphate, including the following steps:

[0045] Provide slices containing lithium manganese iron phosphate;

[0046] Obtain the elemental distribution map of manganese and iron in lithium manganese iron phosphate slices;

[0047] Generate a grayscale image based on the pixel values ​​corresponding to each pixel in the element distribution map;

[0048] The grayscale image is converted into an elemental distribution intensity curve, and the manganese-iron ratio and uniformity of manganese-iron element distribution in the slice are obtained from the elemental distribution intensity curve.

[0049] This application obtains the elemental distribution map of manganese and iron in sliced ​​lithium manganese iron phosphate, generates a grayscale image based on the pixel value corresponding to each pixel in the elemental distribution map, and then converts the grayscale image into an elemental distribution intensity curve. Finally, based on the elemental distribution intensity curve, the manganese-iron ratio and the uniformity of manganese and iron element distribution in lithium manganese iron phosphate can be quantitatively calculated. This provides an accurate and efficient method for studying the influence of the uniformity of Mn / Fe element distribution inside lithium manganese iron phosphate material particles on performance.

[0050] The method described in this application can be used to analyze the manganese-iron ratio and the uniformity of manganese-iron element distribution of lithium manganese iron phosphate in the positive electrode of a secondary battery before and after cycling. Based on the analysis results, the elemental changes and elemental distribution of lithium manganese iron phosphate in the positive electrode of the secondary battery can be determined.

[0051] When analyzing the manganese-iron ratio and the uniformity of manganese-iron element distribution of lithium manganese iron phosphate on the positive electrode of the secondary battery before and after cycling, the analytical methods must be consistent.

[0052] In one embodiment, the lithium manganese iron phosphate chip is derived from a secondary battery.

[0053] The method for preparing the slices containing lithium manganese iron phosphate is as follows:

[0054] Disassemble the secondary battery to obtain the positive electrode sheet; prepare the positive electrode sheet into slices containing lithium manganese iron phosphate.

[0055] In one embodiment, the thickness of the slice is 50 to 150 nm, for example, it can be a range of 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm or any combination thereof. Controlling the thickness of the slice to 50 to 150 nm can eliminate the influence of the thickness effect on the strength of manganese and iron elements in lithium manganese iron phosphate, which is convenient for subsequent characterization.

[0056] In one embodiment, the slice containing lithium manganese iron phosphate is obtained by cutting the positive electrode sheet using a focused ion beam device.

[0057] In one embodiment, a target lithium manganese iron phosphate particle in the positive electrode is selected by an electron beam, and the positive electrode is cut by an ion beam to obtain a slice containing lithium manganese iron phosphate.

[0058] In one embodiment, the elemental distribution map of manganese and iron in lithium manganese iron phosphate in the slice is obtained by using an EDS elemental distribution analyzer.

[0059] In one embodiment, the grayscale image has 8 to 16 bits of pixels.

[0060] Taking a grayscale image with 8 bits as an example, in 8-bit mode, there are a total of 2 8 =256 color levels, meaning the pixel value range covers 0 to 255, which is sufficient to subdivide the common color composition. This step converts the pixels that make up the image into their corresponding 8-bit values ​​(pixel values) for display. Obtain the corresponding 8-bit values.

[0061] In one embodiment, converting a grayscale image into an element distribution intensity curve specifically involves setting a threshold based on the pixel values ​​of the grayscale image, binarizing the pixel values ​​of the grayscale image based on the threshold, and converting it into an element distribution intensity curve.

[0062] In one embodiment, pixels in the grayscale image with pixel values ​​greater than a threshold are considered valid pixels.

[0063] In one implementation, pixels in the grayscale image that are ≤ a threshold are considered invalid pixels.

[0064] Among them, when the 8-bit value is less than or equal to the threshold, it is defined as an invalid signal point. At this time, it is converted into a binary value of 0, thereby further converting the 8-bit mode into a binary black and white mode, that is, the valid pixel point is determined to be black (corresponding to value 1), and the invalid pixel point is white (corresponding to value 0), thus selecting the valid pixel point.

[0065] In one implementation, setting the threshold based on the pixel values ​​of the grayscale image specifically involves: traversing the pixel values ​​of each pixel in the grayscale image, calculating the pixel value difference between each adjacent pixel, determining two target pixel values ​​based on the maximum difference between adjacent pixels, and using the smaller pixel value among the target pixel values ​​as the threshold.

[0066] The present application is further illustrated below with specific embodiments:

[0067] Example 1

[0068] A method for detecting manganese and iron elements in lithium manganese iron phosphate, comprising the following steps:

[0069] (1) Disassemble a secondary battery that has not undergone cycle aging and is at 50% SOC in a room with controlled room temperature and humidity (humidity less than 10%). After separating the negative electrode and the separator, take out the corresponding positive electrode and soak it in DMC organic solvent for 20 minutes. After drying, cut it into 3*3cm pieces. 2 .

[0070] (2) Transfer the dried positive electrode sample to the focused ion beam equipment, such as... Figure 1 As shown, the target lithium manganese iron phosphate particles are selected using an electron beam, and then the particles are cut into slices and thinned using an ion beam (current 1A). The sample is extracted by a robotic arm and attached to the transmission electron microscope sample holder. The sample is cleaned with a small current (50μA) until a slice containing lithium manganese iron phosphate with a thickness of 100nm is obtained.

[0071] (3) The slices were placed in a transmission electron microscope (TEM) using an EDS elemental distribution analyzer. The corresponding lithium manganese iron phosphate (LMP) particles were located using normal TEM mode. After selecting the LMP particles, the image was switched to STEM mode. At this point, the scale precisely included the single LMP particle, avoiding interference from other particles. The magnification was set to a 50 nm scale. After eliminating spherical aberration and astigmatism interference, the LMP particles were focused to obtain a high-magnification STEM image of the corresponding particles. EDS was then used to scan and collect Mn / Fe elements from the target LMP particles, ultimately obtaining the image as shown below. Figure 2 The elemental distribution of manganese and iron in lithium manganese iron phosphate in the slice shown.

[0072] (4) Based on the pixel values ​​corresponding to each pixel in the element distribution map, use image processing software to convert them into an 8-bit grayscale mode, generating a model like... Figure 3The grayscale image shown;

[0073] (5) Traverse the pixel value of each pixel in the grayscale image, calculate the pixel value difference of each adjacent pixel, determine two target pixel values ​​based on the maximum difference between adjacent pixels, and use the smaller pixel value among the target pixel values ​​as the threshold. In this embodiment, the threshold value is set to 49. The pixel value of the grayscale image is binarized according to the threshold. When the 8-bit value is less than or equal to 49, it is defined as an invalid signal point. At this time, it is converted into a binary value of 0, thereby further converting the 8-bit mode into a binary black and white mode. That is, the valid pixel point is determined to be black (corresponding to value 1), and the invalid pixel point is white (corresponding to value 0), thereby selecting the valid pixel point.

[0074] (5) Figure 4 As shown, a specific region (dashed box) within the particle is selected. Using the contour image function of image processing software, the corresponding region's image is analyzed by collecting effective pixels. Higher element distribution intensity corresponds to a denser distribution of effective pixels. The selected region is then divided into 1000 equal parts at a fixed height, thus obtaining the corresponding Mn / Fe element pixels in each horizontal section. Pixel intensity is plotted on the vertical axis, and horizontal position on the horizontal axis, resulting in a horizontal element intensity distribution curve. Figure 5 After reading the intensity value, the average value can be used to calculate that the Mn / Fe ratio inside the particle is 1.30, and the manganese and iron elements are evenly distributed.

[0075] Example 2

[0076] A method for detecting manganese and iron elements in lithium manganese iron phosphate, comprising the following steps:

[0077] (1) Disassemble a secondary battery that has undergone cycle aging and has a capacity retention rate of 80% SOH and 50% SOC in a room with controlled room temperature and humidity (humidity less than 10%). After separating the negative electrode and the separator, take out the corresponding positive electrode and soak it in DMC organic solvent for 20 minutes. After drying, cut it into 3*3cm pieces. 2 .

[0078] (2) Transfer the dried positive electrode sample to the focused ion beam device, select the target lithium manganese iron phosphate particles using the electron beam, and then use the ion beam (current 1A) to cut the particles into slices and thin them. The robot arm extracts the sample and attaches it to the transmission electron microscope sample holder. The sample is cleaned with a small current (50μA) until a slice containing lithium manganese iron phosphate with a thickness of 100nm is obtained.

[0079] (3) The slices were placed in a transmission electron microscope (TEM) using an EDS elemental distribution analyzer. The corresponding lithium manganese iron phosphate (LMP) particles were located using normal TEM mode. After selecting the LMP particles, the image was switched to STEM mode. At this point, the scale precisely included the single LMP particle, avoiding interference from other particles. The magnification was set to a 50 nm scale. After eliminating spherical aberration and astigmatism interference, the LMP particles were focused to obtain a high-magnification STEM image of the corresponding particles. EDS was then used to scan and collect Mn / Fe elements from the target LMP particles, ultimately obtaining the image as shown below. Figure 6 The elemental distribution of manganese and iron in lithium manganese iron phosphate in the slice shown.

[0080] (4) Based on the pixel values ​​corresponding to each pixel in the element distribution map, use image processing software to convert them into an 8-bit grayscale mode, generating a model like... Figure 7 The grayscale image shown;

[0081] (5) Similar to Example 1, 49 is selected as the threshold. The pixel values ​​of the grayscale image are binarized according to the threshold. When the 8-bit value is less than or equal to 49, it is defined as an invalid signal point. At this time, it is converted into a binary value of 0, thereby further converting the 8-bit mode into a binary black and white mode. That is, the valid pixel point is determined to be black (corresponding to value 1), and the invalid pixel point is white (corresponding to value 0), thereby selecting the valid pixel point.

[0082] (5) Figure 8 As shown, a specific region (dashed box) within the particle is selected. Using the contour image function of image processing software, the corresponding region's image is analyzed by collecting effective pixels. Higher element distribution intensity corresponds to a denser distribution of effective pixels. The selected region is then divided into 1000 equal parts at a fixed height, thus obtaining the corresponding Mn / Fe element pixels in each horizontal section. Pixel intensity is plotted on the vertical axis, and horizontal position on the horizontal axis, resulting in a horizontal element intensity distribution curve. Figure 9 After reading the values, the average of the two segments was used to calculate the Mn / Fe ratio on the left, which was 1.28, very close to that of Example 1. The Mn / Fe ratio on the right was 1.08, indicating a significant decrease in the proportion of Mn, which may be related to the dissolution of Mn during the cycling process. Furthermore, the X-axis can be used to precisely locate the position of change. It can be seen that a decrease in elemental intensity occurs at approximately 380° horizontally (the overall horizontal plane is divided into 1000 equal parts), indicating a significant loss of Mn to the right of the 380° horizontal position of the particles.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for detecting manganese and iron elements in lithium manganese iron phosphate, characterized in that, Includes the following steps: Provide slices containing lithium manganese iron phosphate; Obtain the elemental distribution map of manganese and iron in lithium manganese iron phosphate slices; Generate a grayscale image based on the pixel values ​​corresponding to each pixel in the element distribution map; The grayscale image is converted into an elemental distribution intensity curve. The manganese-iron ratio and the uniformity of manganese-iron element distribution in the slice are obtained from the elemental distribution intensity curve. The specific steps to convert a grayscale image into an element distribution intensity curve are as follows: set a threshold based on the pixel values ​​of the grayscale image, and then perform binarization on the pixel values ​​of the grayscale image based on the threshold to convert it into an element distribution intensity curve. Pixels in the grayscale image whose pixel value is greater than the threshold are considered valid pixels. Setting a threshold based on the pixel values ​​of a grayscale image involves: traversing the pixel values ​​of each pixel in the grayscale image, calculating the difference in pixel values ​​between adjacent pixels, determining two target pixel values ​​based on the maximum difference between adjacent pixels, and using the smaller of the target pixel values ​​as the threshold.

2. The method for detecting manganese and iron elements in lithium manganese iron phosphate according to claim 1, characterized in that, The thickness of the slice is 50~150nm.

3. The method for detecting manganese and iron elements in lithium manganese iron phosphate according to claim 1, characterized in that, The slices containing lithium manganese iron phosphate are obtained by cutting the positive electrode sheet using a focused ion beam device.

4. The method for detecting manganese and iron elements in lithium manganese iron phosphate according to claim 3, characterized in that, The target lithium manganese iron phosphate particles in the positive electrode are selected by electron beam, and the positive electrode is cut by ion beam to obtain the slice containing lithium manganese iron phosphate.

5. The method for detecting manganese and iron elements in lithium manganese iron phosphate according to claim 1, characterized in that, The elemental distribution maps of manganese and iron in lithium manganese iron phosphate slices were obtained using an EDS elemental distribution analyzer.

6. The method for detecting manganese and iron elements in lithium manganese iron phosphate according to claim 1, characterized in that, The grayscale image has 8-16 bits of pixels.

7. The method for detecting manganese and iron elements in lithium manganese iron phosphate according to claim 1, characterized in that, Pixels in the grayscale image that are ≤ a threshold are considered invalid pixels.

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