Method for evaluating structural stability of sodium-ion battery cathode material

Abstract

The application relates to the technical field of sodium ion battery detection, and particularly discloses a method for evaluating the structural stability of a positive electrode material of a sodium ion battery, which comprises the following steps: preparing a plurality of sodium ion batteries of a layered oxygen positive electrode-hard carbon negative electrode system by using a layered oxygen positive electrode material to be evaluated; installing and fixing the sodium ion batteries in an in-situ expansion force tool; performing small-current charging and discharging on the sodium ion batteries at a constant temperature, and synchronously collecting voltage, capacity and expansion force data of each sodium ion battery in the charging and discharging process; and comparing and evaluating the structural stability of the positive electrode material in each sodium ion battery according to an expansion force and deformation force formula. The in-situ expansion force test tool is used to detect the change rule of the expansion force of the battery assembled by the layered oxygen positive electrode and the hard carbon negative electrode material of the sodium ion battery in the charging and discharging process, the deformation expansion rate of the sodium ion battery in the charging and discharging process is obtained according to the expansion and contraction behavior of the layered oxygen positive electrode material in the sodium ion intercalation and deintercalation process, and the structural stability of the layered oxygen positive electrode material is evaluated.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery testing technology, and specifically to a method for evaluating the structural stability of sodium-ion battery cathode materials. Background Technology

[0002] Driven by both the global goal of carbon neutrality and the accelerated transformation of the energy structure, sodium-ion batteries, with their advantages of abundant and widely distributed sodium resources, low cost, and excellent safety and rate performance, have become a key technology for replacing lithium-ion batteries, showing broad industrial prospects in electric vehicles, distributed energy storage, and smart grids. As the core of sodium-ion batteries, the cathode material directly determines the battery's energy density, cycle life, and safety. Among them, layered oxide (layered oxygen) cathode materials, with a crystal structure similar to that of ternary lithium-ion cathodes and the general chemical formula NaxMO2 (where M is a transition metal such as Fe, Co, Mn, or Ni), possess advantages such as high specific capacity, high energy density, and strong compatibility with existing lithium-ion battery production lines, making them the mainstream choice for sodium-ion battery cathodes.

[0003] During charge-discharge cycles, the repeated insertion and extraction of sodium ions in laminar oxide cathode materials can lead to problems such as lattice distortion, cation mixing, irreversible phase transitions, and oxygen release, resulting in material structure degradation and battery performance decline. This has become a core bottleneck restricting its industrialization. Accurate detection of structural stability is a prerequisite for material modification optimization, performance evaluation, and industrial application. The core requirement is to comprehensively capture the changes in bulk structure, microstructure, and interface states of the material throughout the entire charge-discharge cycle, providing a scientific basis for material design and process improvement, while simultaneously meeting the performance testing requirements of relevant standards such as GB / T 36943 for sodium-ion cathode materials.

[0004] Currently, the industry's testing methods for the structural stability of layered oxide cathode materials are mainly divided into two categories: offline testing and long-cycle cycle testing. Offline testing primarily uses X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS). These methods can detect lattice parameters, microstructure, elemental valence states, and the amount of transition metal dissolution. XRD is commonly used to analyze phase transition processes such as O3→P3, SEM / TEM can observe the generation and propagation of microcracks in particles, and ICP-MS can accurately quantify the degree of transition metal dissolution. Long-cycle cycle testing typically involves performing long-term charge-discharge cycles on coin cells or full cells until the battery capacity decays to 80% of its initial capacity. Reverse coin cell tests are used to determine the capacity loss of the cathode material throughout the entire charge-discharge process, and the capacity decay rate of the cathode material in the battery characterizes its performance. In addition, the intrinsic physicochemical properties of the cathode material can be characterized through battery impedance and physical characterization, thereby evaluating its structural stability.

[0005] However, existing testing methods have significant limitations: offline testing requires disassembling and sampling the battery, making it impossible to capture the dynamic evolution of the material structure during charging and discharging in real time. Furthermore, the sampling process can easily cause secondary damage to the material structure, leading to discrepancies between the test results and the material's actual working state. Traditional in-situ testing methods, such as in-situ XRD, can achieve real-time monitoring of phase transition processes, but they struggle to simultaneously capture the coordinated changes in microstructure and elemental valence states. Moreover, the testing equipment is complex, cumbersome to operate, and has a long testing cycle, making it unsuitable for the rapid testing needs of large-scale production. While long-cycle cyclic testing can reflect the long-term performance of materials, its lengthy testing cycle also makes it difficult to meet the rapid testing requirements of large-scale production.

[0006] Based on this technical background, this invention studies a method for evaluating the structural stability of sodium-ion battery cathode materials, a preparation method, and applications, as well as sodium-ion batteries. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for evaluating the structural stability of cathode materials in sodium-ion batteries. By using an in-situ expansion force testing fixture, the method detects the change in expansion force during the charging and discharging process of a battery assembled with a sodium-ion layered oxygen cathode material and a hard carbon anode material. Based on the expansion and contraction behavior of the layered oxygen cathode material during sodium ion insertion / extraction and the structural stability of the hard carbon material during sodium storage / extraction, the deformation expansion rate of the sodium-ion battery during charging and discharging is obtained to evaluate the structural stability of the layered oxygen cathode material.

[0008] To achieve the above objectives, the present invention provides a method for evaluating the structural stability of a sodium-ion battery cathode material, comprising: Multiple sodium-ion batteries with layered oxygen cathode-hard carbon anode systems were prepared using the layered oxygen cathode material to be evaluated. Each sodium-ion battery is installed and fixed in the in-situ expansion force fixture; Each sodium-ion battery was charged and discharged at a constant temperature with a small current, and the voltage, capacity and expansion force data of each sodium-ion battery were collected simultaneously during the charging and discharging process. The structural stability of the cathode material in various sodium-ion batteries was evaluated by comparing the formulas for expansion force and deformation force.

[0009] The beneficial effects of this invention include: (1) The method for evaluating the structural stability of sodium-ion battery cathode material proposed in this invention uses an in-situ expansion force testing fixture to detect the change law of expansion force of a battery assembled with sodium-ion battery layered oxygen cathode material and hard carbon anode material during charging and discharging. Based on the expansion and contraction behavior of sodium-ion battery layered oxygen cathode material during sodium ion insertion and extraction process and the structural stability of hard carbon material during sodium storage and extraction process, the deformation expansion rate of sodium-ion battery during charging and discharging process is obtained to evaluate the structural stability of layered oxygen cathode material.

[0010] (2) The method for evaluating the structural stability of sodium-ion battery cathode materials proposed in this invention adopts in-situ expansion force testing technology. Through the comprehensive evaluation system of the structural stability of layered oxide cathode materials (in-situ expansion force testing fixture), it effectively solves the technical limitations of existing detection methods, such as the inability to capture the dynamic evolution of material structure in real time, long detection cycle, large result deviation, and difficulty in adapting to the testing needs of large-scale production. It provides reliable technical support for the modification, optimization, performance evaluation and industrial application of layered oxide cathode materials.

[0011] (3) The method for evaluating the structural stability of sodium-ion battery cathode materials proposed in this invention uses a simple and reliable detection device that is easy to operate, convenient to process data, and has a short testing cycle. Based on the electrochemical characteristics and structural evolution law of sodium-ion layered oxide cathode materials, key external characteristic parameters related to the structural stability of the cathode material are extracted, which can realize a rapid and effective evaluation of the structural stability of layered oxide cathode materials.

[0012] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0013] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings.

[0014] Figure 1 This is a flowchart illustrating the method for evaluating the structural stability of sodium-ion battery cathode materials proposed in this invention.

[0015] Figure 2 This is a flowchart illustrating a specific implementation of the method for evaluating the structural stability of sodium-ion battery cathode materials proposed in this invention.

[0016] Figure 3 This is a schematic diagram of the expansion and contraction mechanism of the oxygen-layer cathode material during the charging and discharging process in the method for evaluating the structural stability of sodium-ion battery cathode materials proposed in this invention.

[0017] Figure 4This is a schematic diagram of the in-situ expansion force testing fixture structure used in the comprehensive evaluation system for assessing the structural stability of sodium-ion battery cathode materials proposed in this invention.

[0018] Figure 5 This is a schematic diagram of the expansion force curve during the charging process of a sodium-ion battery, which is a specific embodiment of the method for evaluating the structural stability of the cathode material of a sodium-ion battery proposed in this invention.

[0019] Figure 6 This is a schematic diagram comparing the cycle verification curves of sodium-ion batteries in various embodiments of the present invention.

[0020] Explanation of reference numerals in the attached figures: 1-Test fixture clamp, 2-Fixing screw, 3-Fixing nut, 4-Pressure sensor, 5-Positive electrode of sodium-ion battery under test, 6-Negative electrode of sodium-ion battery under test, 7-Charge-discharge test equipment, 8-Control computer. Detailed Implementation

[0021] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0022] This invention provides a method for evaluating the structural stability of cathode materials in sodium-ion batteries, such as... Figure 1 As shown, it includes: Multiple sodium-ion batteries with layered oxygen cathode-hard carbon anode systems were prepared using the layered oxygen cathode material to be evaluated. Each sodium-ion battery is installed and fixed in the in-situ expansion force fixture; Each sodium-ion battery was charged and discharged at a constant temperature with a small current, and the voltage, capacity and expansion force data of each sodium-ion battery were collected simultaneously during the charging and discharging process. The structural stability of the cathode material in various sodium-ion batteries was evaluated by comparing the formulas for expansion force and deformation force.

[0023] In this invention, an in-situ expansion force testing fixture is used to detect the change law of expansion force in a sodium-ion battery assembled with a layered oxygen cathode and a hard carbon anode material during charging and discharging. Based on the expansion and contraction behavior of the layered oxygen cathode material during sodium ion insertion and extraction in the sodium-ion battery structure and the structural stability of the hard carbon material during sodium storage and extraction, the deformation expansion rate of the sodium-ion battery during charging and discharging is obtained to evaluate the structural stability of the layered oxygen cathode material.

[0024] According to the present invention, the preparation of a sodium-ion battery using a multi-layered oxygen cathode-hard carbon anode system using the cathode material to be evaluated includes: Under the same conditions, sodium-ion batteries of the same size and capacity with multiple layered oxygen cathode-hard carbon anode systems were prepared using the cathode material to be evaluated.

[0025] According to the present invention, the chemical formula of the laminar oxide cathode material is NaxMO2, wherein M is at least one of Fe, Mn, Ni and Co; The negative electrode material of sodium-ion batteries is hard carbon material.

[0026] In this invention, during the development of sodium-ion batteries, layered oxygen cathode materials are the earliest and most mature system studied among sodium-ion battery cathode materials, and their general chemical formula can be represented as Na. x MO2 (where M is a transition metal such as Fe, Mn, Ni, or Co); the structure of the layered oxygen cathode material is similar to that of ternary materials in lithium-ion batteries. Its sodium ion insertion / extraction mechanism during charging and discharging is similar to the lithium ion insertion / extraction mechanism and deformation law of lithium-ion ternary materials. The mechanism diagram is shown below. Figure 3 During the charging process (sodium ions are released from the oxygen-coated cathode material), the interlayer spacing in the oxygen-coated cathode structure first expands and then contracts. During the discharging process (sodium ions are inserted into the oxygen-coated cathode material), the interlayer spacing in the oxygen-coated cathode structure also exhibits the same phenomenon of first expanding and then contracting. Based on the expansion law of the oxygen-coated cathode-hard carbon, by analyzing the in-situ expansion law of the battery during charging and discharging, a method for evaluating the structural stability of the oxygen-coated cathode material in sodium-ion batteries is established by analyzing the in-situ expansion force data.

[0027] According to the present invention, the initial adjustment pressure of the in-situ expansion force tool is 50 kPa-1 MPa; The constant temperature range is -30 to 60℃; The small current is 0.04-0.1C.

[0028] According to the present invention, the sampling time length of synchronous acquisition is 1s-10s.

[0029] According to the present invention, the expansion force data is the pressure sensor value collected by the pressure sensor of the in-situ expansion force tooling.

[0030] This invention applies to sodium-ion batteries with a laminar oxide positive electrode and a hard carbon negative electrode. The applicable temperature range is -30-60℃. It utilizes a low-rate charge / discharge current of 0.04C-0.1C and employs in-situ expansion force testing fixtures, such as... Figure 4 As shown, during the charging and discharging process, the mechanical force (expansion force) data of the battery is collected. The charging data collection time is 1s-10s, and the mechanical data collection time is 1s-10s.

[0031] In this invention, the in-situ expansion force testing fixture includes a testing fixture clamp 1, a fixing screw 2, a fixing nut 3, a pressure sensor 4, a positive electrode 5 and a negative electrode 6 of the sodium-ion battery under test, a charge-discharge testing device 7, and a control computer 8. The sodium-ion battery under test and the pressure sensor 4 are fixed by the testing fixture clamp 1, the fixing screw 2, and the fixing nut 3. The positive electrode 5 and the negative electrode 6 of the sodium-ion battery under test are connected to the charge-discharge testing device 7. The voltage and capacity data of the sodium-ion battery under test are transmitted to the control computer 8 through the charge-discharge testing device 7, and the expansion force data are transmitted to the control computer 8 through the pressure sensor 4.

[0032] According to the present invention, the formulas for expansion force and deformation force are as follows: B1 = A1 - A2; B2 = A3 - A2; C1% = B2 / B1; D1 = (A1 - A2) / SOC1; D2 = (A3 - A2) / (1 - SOC1); Where A1 is the pressure sensor value when the sodium-ion battery's state of charge is 0%, A2 is the maximum pressure sensor value during the charging process, A3 is the pressure sensor value when the sodium-ion battery's state of charge is 100%, and SOC1 is the battery's state of charge when the pressure sensor value is at its maximum during the charging process.

[0033] According to the present invention, in the formulas for expansion force and deformation force, B1 is the maximum pressure change of the positive electrode of the sodium-ion battery, B2 is the pressure contraction at the charging end of the positive electrode of the sodium-ion battery, C1% is the percentage change of the positive electrode pressure at the charging end of the sodium-ion battery, D1 is the rate of change of the positive electrode pressure of the sodium-ion battery, and D2 is the rate of change of the positive electrode pressure at the charging end of the sodium-ion battery.

[0034] In this invention, in-situ expansion force testing technology is adopted. Through the comprehensive evaluation system for the structural stability of laminar oxide cathode materials (in-situ expansion force testing fixture), the technical limitations of existing testing methods, such as the inability to capture the dynamic evolution of material structure in real time, long testing cycle, large result deviation, and difficulty in adapting to the testing needs of large-scale production, are effectively solved. This provides reliable technical support for the modification, optimization, performance evaluation, and industrial application of laminar oxide cathode materials.

[0035] According to the present invention, the structural stability of the cathode material in various sodium-ion batteries is evaluated by comparing the expansion force and deformation force formulas, including: Comparing the B1, C1%, and D1 of various sodium-ion batteries with the same initial pressure, size, and capacity, the sodium-ion battery with smaller B1, C1%, and D1 corresponds to a sodium-ion battery with better structural stability of the layered oxygen cathode material.

[0036] According to the present invention, the method of evaluating the structural stability of the cathode material in various sodium-ion batteries by comparing the expansion force and deformation force formulas further includes: Comparing the B2, C1%, and D2 of various sodium-ion batteries with the same initial pressure, size, and capacity, the sodium-ion battery with smaller B2, C1%, and D2 values ​​has better structural stability of the layered oxygen cathode material and better high-voltage performance.

[0037] The detection device used in this invention has a simple structure, reliable operation, convenient operation, easy data processing, and short testing cycle. Based on the electrochemical characteristics and structural evolution law of sodium ion layered oxide cathode material, key external characteristic parameters related to the structural stability of the cathode material are extracted, which can realize rapid and effective evaluation of the structural stability of layered oxide cathode material.

[0038] The present invention will be described in more detail below through embodiments.

[0039] Example 1

[0040] This embodiment provides a method for evaluating the structural stability of cathode materials in sodium-ion batteries, such as... Figure 2 As shown, it includes: a) The positive electrode material to be evaluated, a, was made into positive electrode sheets under the same conditions and assembled with a hard carbon negative electrode into sodium-ion pouch batteries of the same size and capacity. b) At a constant temperature of 25°C, the sodium-ion soft-pack battery is fixed in the expansion force fixture, and the initial pressure is adjusted to 50 kPa. c) Perform low-rate (0.05C) charge-discharge tests on sodium-ion pouch batteries at a constant temperature of 25°C; the voltage range is 2.0-4.0V; collect voltage and capacity data during the charge-discharge process, and the sampling time is 1s; d) Synchronously collect pressure sensor data of sodium-ion pouch battery during charging and discharging, with a detection sampling time of 1 second. The expansion force curve of sodium-ion battery during charging is shown in the figure below. Figure 5 As shown, B1 represents the maximum expansion force of the sodium-ion pouch battery during charging and discharging, and B2 represents the minimum expansion force of the sodium-ion pouch battery during charging and discharging. e) Obtain the maximum positive electrode pressure change B1, the positive electrode charging end pressure contraction B2, the positive electrode charging end pressure change percentage C1%, the positive electrode pressure change rate D1, and the positive electrode charging end pressure change rate D2 according to the following formulas: B1 = A1 - A2; B2 = A3 - A2; C1% = B2 / B1; D1 = (A1 - A2) / SOC1; D2 = (A3 - A2) / (1 - SOC1); Wherein, A1 represents the pressure sensor value when the state of charge of the sodium-ion pouch battery is 0%; A2 represents the maximum value of the pressure sensor value of the sodium-ion pouch battery during charging; A3 represents the pouch battery when the state of charge of the sodium-ion pouch battery is 100%; SOC1 represents the state of charge of the sodium-ion pouch battery when the pressure sensor value of the sodium-ion pouch battery is at its maximum during charging; the evaluation parameters of the positive electrode material of the sodium-ion pouch battery are obtained, and the parameters are shown in Table 1 and... Figure 6 .

[0041] Example 2

[0042] This embodiment differs from Embodiment 1 in that it uses different batches of the same positive electrode material b from the same manufacturer; otherwise, it is the same as Embodiment 1. Evaluation parameters for the sodium-ion pouch battery positive electrode material were obtained, as shown in Table 1. Room temperature 1C / 1C cycle testing was used to verify the battery's cycle capacity retention and the stability of the battery's positive electrode material. Figure 6 As shown.

[0043] Example 3

[0044] This embodiment differs from Embodiment 1 in that it uses the cathode material c to be evaluated, which has undergone coating modification to optimize material stability. The rest is the same as in Embodiment 1; evaluation parameters for the sodium-ion pouch battery cathode material are obtained, as shown in Table 1. Room temperature 1C / 1C cycle testing is used to verify the battery's cycle capacity retention and the stability of the battery cathode material. Figure 6 As shown.

[0045] Example 4

[0046] This embodiment differs from Embodiment 1 in that it uses the cathode material d to be evaluated, which has undergone doping modification to optimize material stability; otherwise, it is the same as Embodiment 1. Evaluation parameters for the sodium-ion battery cathode material were obtained, as shown in Table 1. The battery cycle capacity retention rate was verified using a room temperature 1C / 1C cycle test to verify the stability of the battery cathode material. Figure 6 As shown.

[0047] Example 5

[0048] This embodiment differs from Embodiment 1 in that it uses an unmodified cathode material e from a different manufacturer than that used in Embodiment 1; otherwise, it is the same as in Embodiment 1. Evaluation parameters for the sodium-ion battery cathode material were obtained, as shown in Table 1. Room temperature 1C / 1C cycle testing was used to verify the battery's cycle capacity retention and the stability of the cathode material. Figure 6 As shown.

[0049] Example 6

[0050] This embodiment differs from Embodiment 1 in that it uses the same cathode material f from Embodiment 5, which has undergone doping modification to optimize material stability. The rest is the same as in Embodiment 1. Evaluation parameters for the sodium-ion battery cathode material are obtained, as shown in Table 1. Room temperature 1C / 1C cycle testing is used to verify the battery's cycle capacity retention and the stability of the cathode material. Figure 6 As shown.

[0051] Comparative Example 1

[0052] The difference between this comparative example and Example 1 is that the initial pressure used was 1 MPa, while the rest was the same as in Example 1; the evaluation parameters of the sodium-ion battery cathode material were obtained, and the parameters are shown in Table 1.

[0053] Comparative Example 2

[0054] The difference between this comparative example and Example 1 is that the charge / discharge rate used is 0.1C, while the rest is the same as in Example 1; the evaluation parameters of the sodium-ion battery cathode material were obtained, and the parameters are shown in Table 1.

[0055] The test results of Examples 1 and 2 show that the stability of sodium-ion layered oxygen cathode materials from different batches of the same manufacturer is similar, and no significant differences were found in the cycle tests. The test results of Examples 1, 3, and 4 show that the structural stability of the sodium-ion battery layered oxygen cathode material is significantly improved after coating and doping modification, and the cycle tests show that the lifespan of the sodium-ion battery is improved after coating and doping modification. The test results of Examples 1 and 5 show that the stability of sodium-ion layered oxygen cathode materials from different manufacturers is significantly different, and the cycle tests show that the lifespan of sodium-ion batteries is significantly different after coating and doping modification. The test results of Examples 5 and 6 show that the structural stability of the sodium-ion battery layered oxygen cathode material is significantly improved after coating and doping modification, and the cycle tests show that the lifespan of the sodium-ion battery is improved after coating and doping modification. The test results are consistent with the implementation... The test results of Examples 1, 3, and 4 are consistent. A comprehensive comparison of C1%, D1, and D2 in the test results of the examples shows the following stability: Example 4 > Example 3 > Example 2 ≈ Example 1 > Example 6 > Example 5. Simultaneously, the battery cycle verification results show the following battery life for the sodium-ion layer oxygen cathode material: Example 4 > Example 3 > Example 2 ≈ Example 1 > Example 6 > Example 5. The stability test results are basically consistent with the cycle test results. This proves that the present invention can quickly and effectively assess the structural stability differences of sodium-ion battery cathode materials by comparing the changing trend of the in-situ expansion force of the soft-pack battery and the evaluation parameters. Similarly, the evaluation results of Example 1, Comparative Example 1, and Comparative Example 2 show that using a smaller charging current and a smaller initial pressure can make the sodium-ion layer oxygen cathode material more stable during application.

[0056] In summary, the embodiments of the present invention can quickly and efficiently evaluate the structural stability of sodium-ion layered oxide cathode materials by in-situ detection of the expansion force change trend and law of the layered oxide cathode sodium-ion soft-pack battery during charge and discharge cycles. Moreover, the detection device is simple in structure and easy to implement.

[0057] Table 1. Comparison of evaluation parameters between Examples 1-6 and Comparative Examples 1-2

[0058] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for evaluating the structural stability of a sodium-ion battery cathode material, characterized in that, include: Multiple sodium-ion batteries with layered oxygen cathode-hard carbon anode systems were prepared using the layered oxygen cathode material to be evaluated. Each sodium-ion battery is installed and fixed in the in-situ expansion force fixture; Each sodium-ion battery was charged and discharged at a constant temperature with a small current, and the voltage, capacity and expansion force data of each sodium-ion battery were collected simultaneously during the charging and discharging process. The structural stability of the cathode material in various sodium-ion batteries was evaluated by comparing the formulas for expansion force and deformation force.

2. The method according to claim 1, characterized in that, Sodium-ion batteries using multiple layered oxygen cathode-hard carbon anode systems prepared with the cathode materials to be evaluated include: Under the same conditions, sodium-ion batteries of the same size and capacity with multiple layered oxygen cathode-hard carbon anode systems were prepared using the cathode material to be evaluated.

3. The method according to claim 1, characterized in that, The chemical formula of the layered oxygen cathode material is NaxMO2, wherein M is at least one of Fe, Mn, Ni and Co; The negative electrode material of the sodium-ion battery is hard carbon material.

4. The method according to claim 1, characterized in that, The initial adjustment pressure of the in-situ expansion force tool is 50 kPa-1 MPa; The constant temperature is -30 to 60°C; The small current is 0.04-0.1C.

5. The method according to claim 1, characterized in that, The sampling time length of the synchronous acquisition is 1s-10s.

6. The method according to claim 1, characterized in that, The expansion force data is the pressure sensor value collected by the pressure sensor of the in-situ expansion force tooling.

7. The method according to claim 6, characterized in that, The formulas for the expansion force and deformation force are as follows: B1 = A1 - A2; B2 = A3 - A2; C1% = B2 / B1; D1 = (A1 - A2) / SOC1; D2 = (A3 - A2) / (1 - SOC1); Wherein, A1 is the pressure sensor value when the state of charge of the sodium-ion battery is 0%, A2 is the maximum value of the pressure sensor value during the charging process of the sodium-ion battery, A3 is the pressure sensor value when the state of charge of the sodium-ion battery is 100%, and SOC1 is the state of charge of the battery when the pressure sensor value is at its maximum value during the charging process.

8. The method according to claim 7, characterized in that, In the formulas for expansion force and deformation force, B1 is the maximum pressure change of the positive electrode of the sodium-ion battery, B2 is the pressure contraction at the charging end of the positive electrode of the sodium-ion battery, C1% is the percentage change of the positive electrode pressure at the charging end of the sodium-ion battery, D1 is the rate of change of the positive electrode pressure of the sodium-ion battery, and D2 is the rate of change of the positive electrode pressure at the charging end of the sodium-ion battery.

9. The method according to claim 8, characterized in that, The structural stability of cathode materials in various sodium-ion batteries is evaluated based on a comparison of the expansion force and deformation force formulas, including: Comparing the B1, C1%, and D1 of various sodium-ion batteries with the same initial pressure, size, and capacity, the sodium-ion battery with smaller B1, C1%, and D1 corresponds to a sodium-ion battery with better structural stability of the layered oxygen cathode material.

10. The method according to claim 8, characterized in that, The evaluation of the structural stability of cathode materials in various sodium-ion batteries based on the formulas for expansion force and deformation force also includes: Comparing the B2, C1%, and D2 of various sodium-ion batteries with the same initial pressure, size, and capacity, the sodium-ion battery with smaller B2, C1%, and D2 values ​​has better structural stability of the layered oxygen cathode material and better high-voltage performance.

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