Method for evaluating overcharge resistance of lithium iron phosphate cathode material

Abstract

The present application relates to the technical field of battery positive electrode material detection and evaluation, and particularly relates to a lithium iron phosphate positive electrode material overcharge resistance performance evaluation method. The method comprises the following steps: preparing a battery by using a lithium iron phosphate positive electrode material, wherein the battery is a soft package battery or / and a button battery; performing overcharge test on the battery, and recording the overcharge time of the battery from the deep delithiation state to the preset cut-off condition; disassembling the battery, and detecting the negative electrode iron content; when the overcharge time is less than or equal to T and the negative electrode iron content is less than or equal to C, it is determined that the overcharge resistance performance of the lithium iron phosphate positive electrode material is qualified; T is a preset overcharge time threshold value, and C is a preset iron element content threshold value. The present application at least describes the occurrence degree of the side reaction by the side reaction overcharge time and the negative electrode iron content, and can comprehensively, accurately and unambiguously evaluate the overcharge resistance performance of the lithium iron phosphate positive electrode material.

Description

Technical Field

[0001] This invention relates to the field of battery cathode material testing and evaluation technology, and in particular to a method for evaluating the overcharge resistance of lithium iron phosphate cathode materials. Background Technology

[0002] Lithium-ion batteries are increasingly favored due to their higher energy density, longer cycle life, and environmental friendliness, and are appearing more and more frequently in our daily lives. Their safety performance is also becoming increasingly important. In recent years, with the increasing number of cases of illegal use of lithium batteries, the safety performance of lithium batteries, especially their performance under overcharge conditions, has become particularly crucial for their application. In overcharge incidents, the positive electrode, as the high-voltage side, is subjected to a more extreme electrochemical environment, placing particularly stringent requirements on the performance of materials. Lithium iron phosphate cathode materials, as the mainstream cathode material today, are primarily evaluated based on particle size, conductivity, main element content, carbon content, common impurity element content, magnetic material content, and discharge capacity at 0.1C and 1C. This characterization only focuses on the material's conventional properties and lacks characterization of its overcharge resistance.

[0003] The patent document with publication number CN 120065029 A discloses a rapid evaluation method for battery performance. After overcharge testing of the battery under test and the reference battery, the battery performance is evaluated by the capacity retention rate of the battery under test and the reference battery. When the difference in capacity retention rate is small, the performance of the lithium iron phosphate battery is evaluated a second time by the Fe content and / or Li content in the negative electrode of the battery under test and the reference battery.

[0004] The evaluation method in this comparative document focuses on capacity retention rate, and only uses element content as an auxiliary evaluation when the capacity retention rate cannot show differences. The evaluation logic is rather one-sided. Moreover, the capacity retention rate reflects the final comprehensive result of system function degradation, which is affected by many factors. Using it as the main indicator may lead to the wrong selection of materials with latent defects, and it is easily affected by fluctuations in battery manufacturing. It is not suitable for the fine screening of the overcharge resistance performance of cathode materials.

[0005] Meanwhile, in the comparative document, the overcharge test involves charging the battery from its normal voltage range (2.5–3.65 V) to 4.9–5.3 V at a constant current of 0.5–1C, and then maintaining this constant voltage for 0.5–1 hours. This is a direct overcharge from the discharge state, and the overcharge duration includes a significant amount of normal lithium desorption time and the initial overcharge period, which cannot reflect the pure side reaction kinetics. Summary of the Invention

[0006] The present invention aims to solve the above problems by providing a method with strong anti-interference ability that can comprehensively and accurately evaluate the overcharge resistance of lithium iron phosphate cathode materials.

[0007] The concept of this invention is as follows: Overcharge thermal runaway mainly originates from heat generated by side reactions. Large battery cells, due to their high energy density, are more sensitive to the occurrence of side reactions. Their excessive volume prevents the heat generated by side reactions from being released, causing it to accumulate inside the cell and ultimately leading to overcharge thermal failure. Therefore, evaluating the overcharge resistance of a lithium iron phosphate cathode material is an evaluation of the extent of side reactions occurring under high-voltage overcharge conditions.

[0008] Based on this, this invention considers evaluating the overcharge resistance of lithium iron phosphate cathode materials using three data points: the duration of the side reaction (overcharge time from the deep delithiation state), the total gas production during the side reaction process, and the iron content on the negative electrode side after the side reaction. Among these, the iron content on the negative electrode, as a characterization of the stability of the cathode material, is also a major influencing factor on the difference in gas production due to side reactions under overcharge conditions. The total gas production reflects the degree of decomposition of the electrolyte due to side reactions. The overcharge time, as a comprehensive indicator of the degree of battery side reactions, is used to determine whether the side reactions are within an acceptable range. Through this three-dimensional evaluation system, the degree of side reactions, i.e., the overcharge resistance, can be comprehensively, accurately, and unambiguously evaluated.

[0009] Furthermore, the overcharge time and negative electrode iron content exhibit the same performance in both pouch and button cells, without significant differences due to the different battery types. However, the testing scale for button cells is relatively smaller than that for pouch cells, so the overcharge resistance of lithium iron phosphate cathode materials can be preliminarily evaluated through overcharge testing of button cells.

[0010] Therefore, the technical solution to the problem of this invention is to provide a method for evaluating the overcharge resistance of lithium iron phosphate cathode materials, comprising the following steps:

[0011] S1. A battery is made from lithium iron phosphate cathode material, wherein the battery is a pouch cell and / or a button cell;

[0012] S21. Charge the battery with a constant current to a first voltage, and charge it with a constant voltage at the first voltage until the current drops below 50 μA. After resting, the battery reaches a deep delithiation state. Record the overcharge time taken for the battery to reach the preset cutoff condition from the start of the deep delithiation state.

[0013] S22. Disassemble the battery and test the iron content of the negative electrode;

[0014] S3. When the overcharge time is ≤ T and the negative electrode iron content is ≤ C, the overcharge resistance of the lithium iron phosphate cathode material is deemed to be qualified;

[0015] Where T is the preset overcharge time threshold and C is the preset iron content threshold.

[0016] In some embodiments, when the lithium iron phosphate cathode material is made into a pouch battery, step S21 further includes the following steps: recording the total gas production of the battery from the start of the deep delithiation state, during the overcharge process when the preset cutoff condition is reached, and during the resting period of 1.5 to 2.5 hours after overcharging; at this time, when the overcharge time, total gas production, and negative electrode iron content in step S3 simultaneously meet the requirements, the overcharge resistance performance of the lithium iron phosphate cathode material is determined to be qualified.

[0017] Based on this, the present invention can perform a preliminary evaluation using only overcharge time and negative electrode iron content. A more refined evaluation can be achieved by combining overcharge time and negative electrode iron content with total gas production: while overcharge time and negative electrode iron content can initially reflect overcharge resistance, they cannot reflect the overcharge process itself. For example, two batteries with the same overcharge time and negative electrode iron content may have different degrees of severity and safety during overcharge, requiring further differentiation of their overcharge resistance. The present invention provides total gas production data through a pouch cell, reflecting the severity of side reactions and the response of the pressure relief mechanism during overcharge. This data is independent of overcharge time and negative electrode iron content, allowing identification of batteries that, while intrinsically stable and exhibiting good macroscopic performance, suffer from severe side reactions. This enables further refined grading of overcharge resistance, resulting in a more comprehensive and accurate evaluation.

[0018] In step S1 The method of manufacturing button cells is not limited. As a means of evaluating positive electrode materials, and as a preferred method of this invention, lithium iron phosphate positive electrode material is used as the positive electrode, lithium metal sheet is used as the negative electrode, and electrolyte specifications are used as the basic electrolyte.

[0019] There are no restrictions on the method of manufacturing pouch batteries. As a means of evaluating positive electrode materials, and as a preferred method of this invention, lithium iron phosphate positive electrode material is used as the positive electrode, graphite as the negative electrode, and electrolyte specifications are used as the basic electrolyte.

[0020] As a preferred embodiment of the present invention, after the button cell or pouch cell is formed and tested for capacity, the cell is cycled at 0.05–0.2 C for one cycle. After activation, the cell is directly overcharged at 0.2–0.8 C. This pre-cycling ensures that all tested cells reach the same initial state, guaranteeing the reliability of the evaluation results.

[0021] In step S21 First, the battery is brought to a deep delithiation state. It should be understood that the overcharge time starting from the deep delithiation state in this invention is different from the overcharge time in conventional overcharge tests in the prior art: the conventional overcharge time in the prior art includes the normal delithiation stage, the residual lithium removal and side reaction initiation stage, and the side reaction stage; while the overcharge time obtained by this invention after battery pretreatment is only the side reaction stage, directly reflecting the side reaction kinetics.

[0022] The overcharge time measurement in this invention measures the battery's ability to maintain the side reaction current under extreme voltage. When the cathode material interface is stable and has strong overcharge resistance, the side reaction is difficult to sustain, the current decays rapidly, and the voltage is forced to rise rapidly under constant current conditions, resulting in a short overcharge time. When the interface is unstable and the catalytic activity is strong, the side reaction continuously consumes current, and the voltage rises slowly, resulting in a long overcharge time. This means that in step S3 of this invention, the overcharge test is passed when the overcharge time of the lithium iron phosphate cathode material under test is less than a threshold.

[0023] As a preferred embodiment of the present invention, the first current for pretreatment is 0.2 to 0.8 C, for example, it can be 0.2 C, 0.3 C, 0.4 C, 0.5 C, 0.6 C, 0.7 C, or 0.8 C.

[0024] As a preferred embodiment of the present invention, the first voltage for pretreatment is 3.5 to 4.0 V, for example, it can be 3.5 V, 3.6 V, 3.7 V, 3.8 V, 3.9 V, or 4.0 V.

[0025] As a preferred embodiment of the present invention, the pretreatment settling time is 2 to 8 minutes, for example, it can be 2 minutes, 3 minutes, 4 minutes, 5 minutes, 6 minutes, 7 minutes, or 8 minutes.

[0026] In some preferred embodiments, the battery is charged with a constant current of 0.5 C to 3.75 V, and then charged with a constant voltage at 3.75 V until the current drops below 50 μA. After resting for 5 minutes, it reaches a deep delithiation state. By using constant current and constant voltage to push the battery into a deep delithiation state, it is ensured that the subsequent overcharge time measurement is a pure side reaction kinetic signal.

[0027] Then, the overcharge time taken for the battery to reach the preset cutoff condition from the deep delithiation state is recorded.

[0028] The overcharge test has no restrictions on the preset cutoff conditions. For example, it can set the charging termination voltage or add an overcharge duration such as 4000 s.

[0029] As a preferred embodiment of the present invention, the preset cutoff condition is: the battery reaches a second voltage; the second voltage is 4.8 to 6.0 V, for example, it can be 4.8V, 4.9V, 5.0V, 5.1V, 5.2V, 5.3V, 5.4V, 5.5V, 5.6V, 5.7V, 5.8V, 5.9V, or 6.0V.

[0030] As a preferred embodiment of the present invention, starting from the deep delithiation state, the device is charged at a constant current of 0.2 to 0.8 C until the preset cutoff condition is reached. For example, this could be 0.2 C, 0.3 C, 0.4 C, 0.5 C, 0.6 C, 0.7 C, or 0.8 C.

[0031] In some preferred embodiments, the overcharge time of the battery from a deep delithiation state to 5.5 V at a constant current of 0.2 to 0.8 C is recorded.

[0032] In the testing of pouch batteries, it is also necessary to record the total gas production during overcharging and during the 1.5–2.5 hours of resting after overcharging. Preferably, the total gas production during overcharging and during the 2 hours of resting after overcharging is recorded.

[0033] In step S22 The iron content of the negative electrode is obtained by detecting the iron content in the negative electrode.

[0034] As a preferred embodiment of the present invention, in the coin cell, the negative electrode acid hydrolysis solution is subjected to ICP testing, and the ratio of the product of the ICP test value and the constant volume of the solution to the positive electrode active material is used to characterize the iron content of the negative electrode. Since the negative electrode sample size in a coin cell is small and prone to error, while the mass of the positive electrode active material is accurately recorded during battery preparation, the present invention uses the amount of negative electrode iron deposited per unit mass of positive electrode active material as the negative electrode iron content. This indicator directly reflects the iron dissolution tendency of the positive electrode material under overcharge conditions and is suitable for relative comparisons between different positive electrode materials.

[0035] As a preferred embodiment of the present invention, in the pouch battery, the acid hydrolysis solution of the negative electrode powder is subjected to ICP testing, and the ratio of the product of the ICP test value and the fixed volume of the solution to the negative electrode powder is used to characterize the iron content of the negative electrode.

[0036] In step S3 Overcharge resistance can be evaluated using two or three indicators simultaneously, and there are various evaluation methods.

[0037] In some embodiments, the battery is a button cell;

[0038] Step S3 includes the following steps: when the overcharge time of the coin cell is ≤ T1 and the negative electrode iron content is ≤ C1, the overcharge test is passed; when the overcharge time of the coin cell is > T1 and the negative electrode iron content is > C1, the overcharge test is failed; when the overcharge time of the coin cell is > T1 and the negative electrode iron content is ≤ C1, or when the overcharge time of the coin cell is ≤ T1 and the negative electrode iron content is > C1, a retest is performed; when the overcharge time of the coin cell in the retest is ≤ T1 and the negative electrode iron content is ≤ C1, the overcharge test is passed; when any one of these conditions is not met, the overcharge test is failed; wherein, T1 and C1 are determined by the test results of standard lithium iron phosphate cathode materials, or by experimental data and statistical analysis.

[0039] In other embodiments, the battery is a pouch cell battery;

[0040] Step S21 further includes the following steps: recording the total gas production of the battery from the start of the deep delithiation state, during the overcharge process when the preset cutoff condition is reached, and during the resting period of 1.5 to 2.5 hours after overcharging;

[0041] Step S3 includes the following steps: when the overcharge time of the soft-pack battery is ≤ T2, the negative electrode iron content is ≤ C2, and the gas production is ≤ V2, the overcharge test is passed; when any one of these conditions is not met, the overcharge test is not passed; wherein, T2, C2, and V2 are determined by the test results of standard lithium iron phosphate cathode materials, or by experimental data and statistical analysis.

[0042] In some embodiments, the lithium iron phosphate cathode material is first made into a coin cell. When the overcharge time of the coin cell is ≤ T1 and the iron content of the negative electrode is ≤ C1, the overcharge test is passed; when the overcharge time of the coin cell is > T1 and the iron content of the negative electrode is > C1, the overcharge test is not passed.

[0043] When the overcharge time of the coin cell is greater than T1 and the iron content of the negative electrode is less than or equal to C1, or when the overcharge time of the coin cell is less than or equal to T1 and the iron content of the negative electrode is greater than C1, the lithium iron phosphate cathode material is made into a soft-pack battery and retested.

[0044] The overcharge test is passed when the overcharge time of the soft-pack battery is ≤T2, the negative electrode iron content is ≤C2, and the gas production is ≤V2; the overcharge test is failed when any one of these conditions is not met.

[0045] Among them, T1, T2, C1, C2, and V2 are determined by the test results of standard lithium iron phosphate cathode materials, or by experimental data and statistical analysis.

[0046] As a preferred embodiment of the present invention, lithium iron phosphate cathode sheets covering different performance levels are selected and tested according to steps S1, S21, and S22 to obtain data. The boundary points of T1, T2, C1, C2, and V2 are determined by statistical methods. The statistical methods are not limited and can be failure mode-based quantile methods, cluster analysis, etc.

[0047] In other implementations, overcharge resistance performance can be ranked based on whether the overcharge test is passed or not, allowing for further refined evaluation.

[0048] As a preferred embodiment of the present invention, overcharge time is the primary evaluation criterion, while the negative electrode iron content and / or the gas generation during overcharge of the soft pack are only used as screening indicators.

[0049] The beneficial effects of this invention are:

[0050] 1. This invention evaluates the overcharge resistance of lithium iron phosphate cathode materials using three data points: overcharge time of side reactions, total gas production during overcharge, and iron content in the negative electrode. The iron content in the negative electrode, as a characterization of the cathode material's stability, is also a major influencing factor on the difference in gas production due to side reactions under overcharge conditions. The total gas production reflects the degree of decomposition of the electrolyte due to side reactions. The overcharge time, as a comprehensive indicator of the extent of battery side reactions, is used to determine whether the side reactions are within an acceptable range. This three-dimensional evaluation system allows for a comprehensive, accurate, and unambiguous evaluation of overcharge resistance.

[0051] 2. In some implementations, the overcharge resistance of lithium iron phosphate cathode materials is first evaluated by overcharge testing of button cells, and then further differentiated by overcharge testing of pouch cells, thereby improving the precision of the overcharge resistance evaluation. Attached Figure Description

[0052] Figure 1 This is a flowchart for evaluating the overcharge resistance of lithium iron phosphate cathode materials.

[0053] Figure 2 These are the overcharge voltage-time curves of coin cells made from lithium iron phosphate cathode materials A, B-1, B-2, C, D, and E in Example 1. Detailed Implementation

[0054] The following are specific embodiments of the present invention, and the technical solution of the present invention will be further described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0055] Example 1

[0056] Coin cells were fabricated using lithium iron phosphate cathode materials A, B-1, B-2, C, D, and E as cathode plates, under identical conditions, and then tested and evaluated. Material property and electrical tests were conducted at room temperature (25±2 ℃), and included the following steps:

[0057] S1. Using lithium iron phosphate cathode materials A, B-1, B-2, C, D, and E as cathodes, lithium metal sheets as anodes, a 1 mol / L LiPF6 ethylene carbonate / dimethyl carbonate (volume ratio 1:1) solution as electrolyte, and a polypropylene microporous membrane as separator, a coin cell was prepared. The cell was first cyclically charged at 0.1 C for one cycle, and then cyclically charged at 0.5 C for one cycle.

[0058] S21. Perform an overcharge test on the battery: First, charge the battery to 3.75 V at a current of 0.5 C, then charge it at a constant potential of 3.75 V until the current is <50 μA, and let it stand for 5 min. Then charge the battery to 5.5 V at a current of 0.5 C, and record the overcharge duration.

[0059] S22. Disassemble the overcharged coin cell, remove the lithium negative electrode and separator, add 20 mL of deionized water. The deionized water reacts with the lithium to generate hydrogen gas. After the reaction is complete, add 10 mL of 36% hydrochloric acid. Stir and acidify at room temperature (25°C) for 10 min. Then add water to make up the volume to 100 mL, filter the solution, and perform ICP testing. Calculate the iron content of the negative electrode of the overcharged coin cell using the following formula.

[0060] .

[0061] S3. In the early stage, the overcharge time threshold T1 of the button cell was determined to be 1350 s and the negative electrode iron content threshold C1 was determined to be 600 ppm through statistical methods. The test results and overcharge evaluation of each lithium iron phosphate cathode material are shown in Table 1 below.

[0062] Table 1.

[0063]

[0064] Table 1 shows that lithium iron phosphate cathode materials A, B-2, C, and E have qualified overcharge resistance. Based on this, using overcharge time as the main evaluation condition (the less the better) and iron content of the anode as the screening index, it can be further concluded that the overcharge resistance performance is: A > C > B-2 > E.

[0065] Example 2

[0066] Example 2 is basically the same as Example 1, except that: lithium iron phosphate cathode materials A, B-1, C, D and E are used as cathode plates, and other conditions are the same, and soft-pack batteries are made for testing and evaluation.

[0067] Includes the following steps:

[0068] S1. Using lithium iron phosphate cathode materials A, B-1, C, D, and E as positive electrode sheets, graphite as the negative electrode sheet, a 1 mol / L LiPF6 ethylene carbonate / dimethyl carbonate (volume ratio 1:1) solution as the electrolyte, and a polypropylene microporous membrane as the battery separator, a 2 Ah pouch battery was prepared. After the pouch battery underwent formation and capacity testing, it was first cycled once at a 0.1 C circuit and then once at a 0.5 C current.

[0069] S21. Overcharge test of the battery: Place the prepared pouch battery into the IEST GVM2200 for charging test. First, charge the battery to 3.75 V with a 0.5 C current, then charge it at a constant potential of 3.75 V until the current is <50 μA, and let it stand for 5 minutes. Then charge the battery to 5.5 V with a 0.5 C current, and record the gas production and overcharge duration. After that, let the battery stand for two hours, and continue to record the total gas production of the pouch battery.

[0070] S22. Disassemble the overcharged pouch battery. Clean the negative electrode sheet with dimethyl carbonate, dry it, and weigh it to obtain its mass m1. Then, immerse the negative electrode sheet in 20 mL of deionized water until the graphite peels off. Remove the negative electrode film, take out the current collector copper foil, wipe it clean, and weigh it to obtain its mass m2. Subtract m2 from m1 to obtain the mass of the negative electrode powder. Add the negative electrode powder and deionized water to 10 mL of 36% hydrochloric acid. Stir and acidify at room temperature (25 ℃) for 30 min. Then, add water to make up to 100 mL, filter the solution, and perform ICP testing. Calculate the iron content of the negative electrode after overcharging using the following formula.

[0071] .

[0072] S3. In the early stage, the overcharge time threshold T2 of the soft pack battery was determined to be 800 s, the negative electrode iron content threshold C2 was 400 ppm, and the gas production threshold V2 was 10 mL using statistical methods. The test results and overcharge evaluation of each lithium iron phosphate cathode material are shown in Table 2 below.

[0073] Table 2.

[0074]

[0075] As shown in Table 2, the overcharge resistance of lithium iron phosphate cathode materials A, C, and E is still qualified, while the overcharge resistance of lithium iron phosphate cathode material D is still unqualified. Lithium iron phosphate cathode material B-1, which needs to be retested, passed the test and its overcharge resistance is qualified.

[0076] Meanwhile, it was found that in pouch cells, lithium iron phosphate cathode material B-1 performed better than E. This may be because in button cells, lithium iron phosphate cathode material B-1 has a longer overcharge time due to the smaller side reaction current rather than the continuous side reaction, while in pouch cells, its advantages of low side reaction activity and low gas production are truly reflected.

[0077] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for evaluating the overcharge resistance of lithium iron phosphate cathode materials, characterized in that, Includes the following steps: S1. A battery is made from lithium iron phosphate cathode material, wherein the battery is a pouch cell and / or a button cell; S21. Charge the battery with a constant current to a first voltage, and charge it with a constant voltage at the first voltage until the current drops below 50 μA. After resting, the battery reaches a deep delithiation state. Record the overcharge time taken for the battery to reach the preset cutoff condition from the start of the deep delithiation state. S22. Disassemble the battery and test the iron content of the negative electrode; S3. When the overcharge time is ≤ T and the negative electrode iron content is ≤ C, the overcharge resistance of the lithium iron phosphate cathode material is deemed to be qualified; Where T is the preset overcharge time threshold and C is the preset iron content threshold.

2. The method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1, characterized in that: The preset cutoff condition is: the battery reaches a second voltage; the second voltage is 4.8 to 6.0 V.

3. A method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1 or 2, characterized in that: In step S21, the first current is 0.2 to 0.8 C, the first voltage is 3.5 to 4.0 V, and the settling time is 2 to 8 min.

4. A method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1 or 2, characterized in that: In step S21, starting from the deep delithiation state, the battery is charged at a constant current of 0.2 to 0.8 C until the preset cutoff condition is met.

5. The method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1, characterized in that: Step S3 includes the following steps: When the overcharge time of the button cell is ≤ T1 and the iron content of the negative electrode is ≤ C1, the overcharge test is passed. If the overcharge time of the button cell is greater than T1 and the iron content of the negative electrode is greater than C1, the overcharge test will not be passed. When the overcharge time of the button cell is greater than T1 and the iron content of the negative electrode is less than or equal to C1, or when the overcharge time of the button cell is less than or equal to T1 and the iron content of the negative electrode is greater than C1, a retest should be conducted or the cell should be made into a soft-pack cell for retesting. T1 and C1 are determined by the test results of standard lithium iron phosphate cathode materials, or by experimental data and statistical analysis.

6. A method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1 or 5, characterized in that: When the battery is a pouch battery; Step S21 further includes the following steps: recording the total gas production of the battery from the start of the deep delithiation state, during the overcharge process when the preset cutoff condition is reached, and during the resting period of 1.5 to 2.5 hours after overcharging; Step S3 includes the following steps: when the overcharge time of the soft-pack battery is ≤ T2, the negative electrode iron content is ≤ C2, and the gas production is ≤ V2, the overcharge test is passed; when any one of these conditions is not met, the overcharge test is not passed; wherein, T2, C2, and V2 are determined by the test results of standard lithium iron phosphate cathode materials, or by experimental data and statistical analysis.

7. The method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1, characterized in that: In step S1, after the battery is manufactured, it is cyclically charged at 0.05 to 0.2 C for at least one cycle before proceeding to step S21.

8. The method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1, characterized in that: In the coin cell, the negative electrode acid hydrolysis solution is subjected to ICP testing, and the ratio of the product of the ICP test value and the constant volume of the solution to the positive electrode active material is used to characterize the iron content of the negative electrode.

9. The method for evaluating the overcharge resistance of lithium iron phosphate cathode material according to claim 1, characterized in that: In the pouch cell, the acid hydrolysis solution of the negative electrode powder is subjected to ICP testing, and the ratio of the product of the ICP test value and the constant volume of the solution to the negative electrode powder is used to characterize the iron content of the negative electrode.

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