Method for evaluating structural stability of lithium iron phosphate electrode based on pressure effect of button cell and application of method

Abstract

The invention discloses a method for evaluating the structural stability of a lithium iron phosphate electrode based on the pressure effect of a button cell and application of the method, the test method comprises the following steps: assembling samples to be tested into two groups of button cells with different internal pressures, respectively carrying out charge and discharge tests, recording and calculating the specific discharge capacity under 1C multiplying power and the 3.2 V voltage platform proportion, and determining the stability of the lithium iron phosphate electrode structure according to the specific discharge capacity and the 3.2 V voltage platform proportion. And then calculating the absolute values delta 1C and delta r3.2 V of the difference value of the average values of the two groups of button cells, and evaluating the structural stability of the lithium iron phosphate electrode based on the delta 1C and the delta r3.2 V. It is found for the first time that in tests of button cells with different internal pressures and different models, particularly, the significant difference existing in electrical properties under the multiplying power of 1C or above is closely related to the particle morphology (especially the content of large-size framework particles) of the material, the stability of the electrode material when the electrode material is pressed can be reflected, the concept of the'pressure sensitive coefficient 'is put forward and verified, and the potential safety hazard of the button cells is eliminated. The process adaptability of the material in large-scale production is effectively predicted, and a new optimization direction is pointed out for material research and development.

Description

Technical Field

[0001] This invention relates to a method for testing the performance of lithium iron phosphate electrode materials, and more particularly to a method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become an important cathode material in the fields of power batteries and energy storage batteries due to its advantages such as high safety, long cycle life, and relatively low cost. However, its intrinsic low electronic conductivity and slow lithium-ion diffusion rate limit its high-rate performance. To improve its performance, coin cells are the standard tool for evaluating electrochemical performance in material research and development and quality control. Currently, the industry mainly uses conventional indicators such as powder resistivity and coin cell performance under single test conditions to initially screen materials. Materials that meet the initial screening criteria are further processed into full-cell batteries for processing performance and electrical performance evaluation. Laboratories generally use coin cell cases of specifications such as 2430 (diameter 24mm, height 3.0mm) or 2016 (diameter 20mm, height 1.6mm) for testing, focusing on the absolute performance of the material under single test conditions (such as specific capacity and rate performance). However, the performance of coin cells of different specifications varies, but the test results of different specifications of battery cases are generally considered equivalent or normal measurement errors. Furthermore, conductivity testing of materials typically focuses on the intrinsic electronic conductivity (powder resistivity). However, the electrical performance of batteries made from powder materials is affected not only by the powder resistivity but also by the battery structure. The battery structure must withstand significant internal pressure, which influences carrier concentration and mobility, thus affecting battery performance. Therefore, focusing solely on the powder resistivity does not fully represent the performance of electrode materials in real-world applications. Early and comprehensive analysis of material performance, assessment of material stability within the battery electrode conductive network, and evaluation of material sensitivity to internal battery pressure, as well as the impact of rate performance on internal battery pressure, are all goals that researchers have consistently pursued to provide forward-looking guidance for material design and production. Summary of the Invention

[0003] Objective: This invention addresses the significant differences in electrical performance (e.g., discharge specific capacity, voltage plateau percentage) of different lithium iron phosphate (LFP) cathode materials under varying internal pressures in coin cell tests, particularly at rates of 1C and above. These differences vary by sample. To fully investigate the material performance information implied by these differences, rather than viewing the data in isolation or attributing them to experimental errors, this invention aims to provide an early and comprehensive evaluation of LFP cathode material performance. This will predict the material's pressure sensitivity and conductive network stability in practical battery applications, effectively distinguishing material quality and providing a basis for material selection. Furthermore, it aims to fully leverage the performance of cathode materials by providing a method for evaluating the structural stability of LFP electrodes based on the pressure effect of coin cells. Another objective of this invention is to provide the application of the above method in the performance research of LFP cathode materials.

[0004] Technical solution: The present invention provides a method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells, comprising the following steps:

[0005] The lithium iron phosphate samples to be tested were assembled into two sets of coin cells with different internal pressures, and charge-discharge tests were performed on them respectively. The discharge specific capacity at 1C rate and the percentage of the 3.2V voltage plateau were recorded.

[0006] Calculate the average performance discharge specific capacity and the percentage of the 3.2V voltage plateau for each group of coin cells at a 1C rate;

[0007] Calculate the absolute value of the difference between the average values ​​of the discharge specific capacity and the percentage of the 3.2V voltage plateau for the two sets of coin cells at 1C rate performance. The calculation formula is as follows:

[0008] |Δ1C|=|1C1-1C2|;

[0009] |Δr 3.2v |=|r 3.2V1 -r 3.2V2 |;

[0010] Based on |Δ1C| and |Δr 3.2V |Evaluate the structural stability of lithium iron phosphate electrodes;

[0011] Among them, the average discharge specific capacity and 3.2V voltage plateau percentage of the first group of coin cells at 1C rate are 1C1 and r. 3.2V1 The second group of coin cells, at a 1C rate, had an average discharge specific capacity and a 3.2V voltage plateau percentage of 1C2 and r. 3.2V2 .

[0012] Furthermore, the two sets of button cells with different internal pressures are selected as 2016 button cells and 2430 button cells, or 2016 button cells and 2032 button cells. At least five button cells of the same model with the same internal pressure are assembled respectively. By testing the same sample with multiple button cells, the test error is avoided, thereby improving the accuracy of the difference between different models of button cells.

[0013] Furthermore, the smaller the absolute value of the difference between the average values ​​of the discharge specific capacity and the 3.2V voltage plateau percentage of the different coin cell battery packs at 1C rate, the better the stability of the lithium iron phosphate electrode structure. If |Δ1C| is less than or equal to 3 mAH / g and |Δr 3.2V A pressure sensitivity of 3% or less indicates that the lithium iron phosphate electrode material is insensitive to pressure. The internal pressure of 2016 coin cells is higher than that of 2430 or 2032 coin cells. Under this higher internal pressure, the contact between particles is more stable, resulting in smaller differences in battery performance between materials with different particle morphologies or intrinsic electronic conductivity. However, the internal pressure of 2430 or 2032 coin cells is lower, leading to unstable particle contact and thus larger differences in battery performance between materials with different particle morphologies or intrinsic electronic conductivity. The differences in test data between different coin cell models reflect the changes in the material due to internal battery pressure, thus demonstrating the stability of the electrode material structure under pressure.

[0014] Furthermore, we define |Δ1C| and |Δr|. 3.2V | represents the "pressure sensitivity coefficient", based on |Δ1C| and |Δr|. 3.2V |And determine the lithium iron phosphate electrode structure based on powder resistivity: if |Δ1C| is less than or equal to 3 mAH / g and |Δr| 3.2V If |Δ1C| is less than or equal to 3% and the powder resistivity is less than 10 Ω·cm, then it is an intrinsic electronic conductivity type; if |Δ1C| is less than or equal to 3 mAH / g and |Δr| is less than or equal to 3 mAH / g, then it is an intrinsic electronic conductivity type. 3.2V If the resistivity of the powder is less than or equal to 3% and the resistivity is greater than or equal to 10 Ω·cm, it is classified as a physical framework type. Intrinsically electronically conductive materials exhibit good conductivity under different pressures due to their superior electronic conductivity, and are not sensitive to pressure. Physical framework types contain many large framework particles (large secondary particles formed by the agglomeration of primary particles, such as secondary spherical particles or irregular secondary particle agglomerates). Although the intrinsic electronic conductivity of these materials is poor, the large particles provide support, resulting in a relatively stable conductive network under different pressures, and the electrode structure is also not sensitive to pressure.

[0015] Furthermore, the assembled coin cells need to be left to stand for at least 3 hours before testing. Both types of cells require the addition of sufficient electrolyte and a sufficient time to allow for thorough internal wetting, eliminating differences caused by variations in electrolyte wetting.

[0016] Furthermore, the assembled coin cells were subjected to charge-discharge tests at 25±0.5℃.

[0017] Furthermore, the ratio of active material: conductive agent: binder in multiple coin cells is 90:5:5. The coin cells are assembled in an argon-filled glove box, with a lithium metal sheet as the counter electrode. The assembly sequence is: positive electrode shell → 1-2 drops of electrolyte → positive electrode sheet → 4-5 drops of electrolyte → separator → 1-2 drops of electrolyte → lithium sheet → nickel mesh → negative electrode shell. An electric sealing machine is used for sealing, with a pressure of 600-700 kg and a time of 3-5 seconds. During assembly, ensure sufficient electrolyte immersion; the added electrolyte should fill the battery casing and be immersed for a sufficient time, with any excess electrolyte overflowing during sealing.

[0018] This invention also provides the application of the above-mentioned method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells in the performance research of lithium iron phosphate cathode materials.

[0019] Beneficial Effects: Compared with the prior art, this invention has the following significant advantages: 1. It is the first to discover that the significant differences in electrical performance (such as discharge specific capacity and voltage plateau ratio) of coin cells under different internal pressures and of different models are closely related to the particle morphology of the material (especially the content of large-size skeleton particles) and can reflect the stability of the electrode material under pressure; 2. It proposes and verifies the |Δ1C| and |Δr of different models of coin cells. 3.2V This study presents a novel testing method for evaluating electrode material stability using powder resistivity. It reveals and quantifies the relationship between electrode material stability and particle morphology (especially the content of large-size framework particles), providing a more accurate reflection of material stability compared to traditional testing methods that only focus on intrinsic electronic conductivity. Furthermore, it offers a new method for dynamically evaluating electrode structure stability, which is unavailable through conventional static tests such as powder resistivity and specific surface area. This method can effectively predict the process adaptability of materials in large-scale production. Finally, it points to a new optimization direction for material development. By constructing a particle size distribution containing physical framework particles (increasing D90), a low-Δ value and high-stability electrode structure can be obtained, thereby improving product consistency and yield. Finally, this method is based on conventional coin cell testing, requires no special or expensive equipment, is simple and easy to implement, and has low cost, making it highly suitable for widespread application in material development, quality control, and incoming inspection. Attached Figure Description

[0020] Figure 1These are SEM images of samples 1-5 in a specific embodiment of the present invention;

[0021] Figure 2 The image shows the 1C discharge curves of samples 1-5 in a specific embodiment of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to specific embodiments. Several representative lithium iron phosphate samples are selected, with 2016 coin cells and 2430 coin cells as examples for relevant explanation.

[0023] Table 1 Physicochemical parameters of lithium iron phosphate samples

[0024]

[0025] The method for evaluating the electrode structure stability of the above three lithium iron phosphate samples based on the pressure effect of coin cells includes the following steps:

[0026] The powder resistivity, D90, and particle micromorphology of the lithium iron phosphate samples were tested. See Table 1 for details. Figure 1 The powder resistivity test method is as follows: 2g of sample is weighed into a mold with a diameter of 13mm, and the resistivity of the powder is tested using a four-probe tester.

[0027] Five sets of 2016 and 2430 coin cells were assembled from the lithium iron phosphate samples to be tested. The three samples were prepared into slurries according to the ratio of active material: conductive agent (Super P): binder (PVDF) = 90:5:5, coated on aluminum foil, and punched into electrode sheets with a diameter of 14 mm (2016 cell) and (2430 cell). The coin cells were assembled in an argon glove box. The counter electrode was a lithium metal sheet. The assembly sequence of the cells was: positive electrode shell → 1-2 drops of electrolyte → positive electrode sheet → 4-5 drops of electrolyte → separator → 1-2 drops of electrolyte → lithium sheet → nickel mesh → negative electrode shell. The cells were sealed using an electric sealing machine at a pressure of 650 kg for 3 seconds.

[0028] Electrical performance testing: After the manufactured batteries were left to stand for 3 hours, charge-discharge tests were conducted at 25°C, with a voltage range of 2-3.75V. The specific capacity at 0.1C discharge, the specific capacity at 1C discharge, and the percentage of the 3.2V plateau at 1C discharge were recorded. See Table 2 for details. Figure 2 .

[0029] The average values ​​of the performance discharge specific capacity and the percentage of the 3.2V voltage plateau at 1C rate were calculated for five 2016 coin cell packs and five 2430 coin cell packs of the same sample, respectively. See Table 2 for details.

[0030] Calculate the differences in average performance discharge specific capacity and the percentage of the 3.2V voltage plateau between the 2016 coin cell battery pack and the 2430 coin cell battery pack at 1C rate: |Δ1C| and |Δr|. 3.2V See Table 2 for details, based on |Δ1C| and |Δr|. 3.2V |Evaluate the structural stability of lithium iron phosphate electrodes.

[0031] Table 2 Electrical performance test results and Δ value calculation

[0032]

[0033] Comparing samples 1-3, Table 2 shows the |Δ1C| and |Δr| of sample 1. 3.2V The |Δ1C| and |Δr values ​​are the largest, and the 1C rate performance is significantly affected by the different internal pressures of different battery models. Meanwhile, the |Δ1C| and |Δr values ​​for samples 2 and 3 are significantly different. 3.2V With a smaller value, the 1C rate performance and the proportion of the 3.2V platform are basically unaffected by the internal pressure of different battery models, resulting in more stable performance.

[0034] Combination Figures 1-2 As shown in Table 1-2, the morphology of samples 2 and 1 is basically micron or submicron particles and agglomerates. However, the powder resistivity of sample 2 (6.8 Ω·cm) is smaller than that of sample 1 (11.1 Ω·cm). The |Δ1C| and |Δr| of sample 2 are also different. 3.2V The smaller size compared to sample 1 is mainly attributed to the better intrinsic electronic conductivity of sample 2. Under different pressures, the rate performance of the battery showed no significant difference. Figures 1-2 As shown in Table 1-2, although the resistivity of sample 1 powder (11.1 Ω·cm) is smaller than that of sample 3 powder (64.9 Ω·cm), its |Δ1C| and |Δr| are significantly different. 3.2V The rate performance of sample 3 is greater than that of sample 3, mainly because sample 3 has a physical framework structure composed of more large secondary spherical particles. Therefore, the rate performance of sample 3 is dominated by the physical framework structure rather than the intrinsic conductivity. Its D90 is also larger, and its electrode structure is extremely stable, almost unaffected by changes in external pressure. Through the above comparison, it can be concluded that the rate performance of a sample in the battery structure cannot be judged solely by the powder resistivity. |Δ1C| and |Δr 3.2V The relationship between resistivity and powder resistivity is not necessarily negative; it is also closely related to the morphology and particle size distribution of the sample.

[0035] The DCIR of 2016 coin cells and 2430 coin cells of samples 1, 2, and 3 were tested respectively, as detailed in Table 2. The test results verified |Δ1C| and |Δr|. 3.2VThe physical properties of the materials exhibited are as follows: The DCIR of Sample 1 in the 2430 cell is greater than that in the 2016 coin cell, indicating that the internal resistance of Sample 1 is greater than that of the 2016 coin cell under lower battery pressure. This suggests that Sample 1 is more sensitive to different internal pressures of the two cell types, and there is a significant difference in internal resistance under different pressures. Based on the high intrinsic electronic conductivity or the presence of a physical framework structure, the internal resistances of Samples 2 and 3 are basically the same in the two cell types, indicating that Samples 2 and 3 are not sensitive to different internal pressures of the two cell types, and there is no significant difference in internal resistance under different pressures.

[0036] Furthermore, by comparing samples 1 and 2 with samples 4 and 5, samples 1, 2, 4, and 5 have similar morphologies and small particle sizes (D90), but the resistivity of powders in samples 1 and 4 is greater than 10 Ω·cm, and their |Δ1C| is greater than 3 mAH / g and |Δr| is greater than 3 mAH / g. 3.2V The value is greater than 3%, indicating a pressure-sensitive material. Samples 2 and 5, however, have a powder resistivity less than 10 Ω·cm, and their |Δ1C| is less than 3 mAH / g and |Δr| is also less than 3 mAH / g. 3.2V With a conductivity of less than 3%, it is an intrinsically electronically conductive material, and is less affected by the internal pressure of the battery.

[0037] Based on |Δ1C| and |Δr 3.2V The type of lithium iron phosphate electrode material is determined by combining powder resistivity with |Δ1C| and |Δr|. 3.2V |Determination of powder resistivity: If |Δ1C| is less than or equal to 3 mAH / g and |Δr| 3.2V If |Δ1C| is less than or equal to 3% and the powder resistivity is less than 10 Ω·cm, then it is an intrinsic electronic conductivity type; if |Δ1C| is less than or equal to 3 mAH / g and |Δr ..., then it is an intrinsic electronic conductivity type. 3.2V If the resistivity is less than or equal to 3% and the powder resistivity is greater than or equal to 10 Ω·cm, it is classified as a physical framework type. Using the above method, samples 1 and 4 can be determined to be "pressure-sensitive" structures. Their performance is highly dependent on manufacturing processes (such as internal battery pressure). When used in the production of all-electric batteries, they require high internal pressure and must be adjusted to a suitable pressure range to achieve the actual rate performance of the battery. This process is demanding, and improper pressure adjustment can lead to the material not achieving its actual rate performance or poor electrical performance consistency between batteries due to pressure differences. Samples 2, 3, and 5 are "pressure-insensitive" structures with stable performance. When used in the production of all-electric batteries, they are suitable for applications where internal pressure requirements are not high and actual rate performance can be achieved over a wider pressure range. This improves the consistency between batteries and is suitable for fields requiring high rate performance consistency. This provides crucial scientific basis for material selection and design. When designing materials, by improving the intrinsic electronic conductivity of the material and combining this with improving the particle framework, pressure-insensitive materials can be prepared, thereby improving the pressure resistance, stability, and cycle rate performance of electrode materials.

Claims

1. A method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells, characterized in that, Includes the following steps: The lithium iron phosphate samples to be tested were assembled into two sets of coin cells with different internal pressures, and charge-discharge tests were performed on them respectively. The discharge specific capacity at 1C rate and the percentage of the 3.2V voltage plateau were recorded. Calculate the average performance discharge specific capacity and the percentage of the 3.2V voltage plateau for each group of coin cells at a 1C rate; Calculate the absolute value of the difference between the average values ​​of the discharge specific capacity and the percentage of the 3.2V voltage plateau for the two sets of coin cells at 1C rate performance. The calculation formula is as follows: |Δ1C|=|1C1-1C2|; |Δr 3.2v |=|r 3.2V1 -r 3.2V2 |; Based on |Δ1C| and |Δr 3.2V |Evaluate the structural stability of lithium iron phosphate electrodes; Among them, the average discharge specific capacity and 3.2V voltage plateau percentage of the first group of coin cells at 1C rate are 1C1 and r. 3.2V1 The second group of coin cells, at a 1C rate, had an average discharge specific capacity and a 3.2V voltage plateau percentage of 1C2 and r. 3.2V2 .

2. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 1, characterized in that, The two sets of button cells with different internal pressures can be selected as 2016 button cells and 2430 button cells, or 2016 button cells and 2032 button cells.

3. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 1, characterized in that, If |Δ1C| is less than or equal to 3 mAH / g and |Δr| 3.2V | If the pressure is less than or equal to 3%, the lithium iron phosphate electrode material is determined to be insensitive to pressure.

4. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 3, characterized in that, Based on |Δ1C| and |Δr 3.2V |And determine the lithium iron phosphate electrode structure based on powder resistivity: if |Δ1C| is less than or equal to 3 mAH / g and |Δr| 3.2V If |Δ1C| is less than or equal to 3% and the powder resistivity is less than 10 Ω·cm, then it is an intrinsic electronic conductivity type; if |Δ1C| is less than or equal to 3 mAH / g and |Δr| is less than or equal to 3 mAH / g, then it is an intrinsic electronic conductivity type. 3.2V If the content is less than or equal to 3% and the powder resistivity is greater than or equal to 10Ω·cm, then it is a physical skeleton type.

5. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 1, characterized in that, At least five button cells of the same model are assembled.

6. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 5, characterized in that, Multiple assembled button cells need to be left to stand for at least 3 hours before testing.

7. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 6, characterized in that, The assembled coin cells were subjected to charge-discharge tests at 25±0.5℃.

8. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 1, characterized in that, In multiple coin cells, the ratio of active material: conductive agent: binder is 90:5:

5.

9. The method for evaluating the structural stability of lithium iron phosphate electrodes based on the pressure effect of coin cells according to claim 1, characterized in that, Assemble button cells in an argon glove box. The counter electrode is a lithium metal sheet. The assembly sequence of the battery is: positive electrode shell → 1-2 drops of electrolyte → positive electrode sheet → 4-5 drops of electrolyte → separator → 1-2 drops of electrolyte → lithium sheet → nickel mesh → negative electrode shell. Use an electric sealing machine to seal the cells at a pressure of 600-700 kg for 3-5 seconds.

10. The application of the method for evaluating the structural stability of lithium iron phosphate electrode based on the pressure effect of coin cell as described in claim 1 in the performance research of lithium iron phosphate cathode materials.

Images