Method for evaluating lithium precipitation state of lithium battery negative electrode

Abstract

The application provides a kind of lithium battery negative pole lithium precipitation state evaluation method, belongs to lithium battery technical field.The evaluation method provided by the embodiment of the application evaluates the negative pole lithium precipitation state by the content of iron phosphide in lithium iron phosphate positive pole piece, the higher the content of iron phosphide in lithium iron phosphate positive pole piece, the more serious the corresponding negative pole piece lithium precipitation, on the contrary, the lower the content of iron phosphide in lithium iron phosphate positive pole piece, the lighter the corresponding negative pole piece lithium precipitation state.The evaluation method of the embodiment of the application can be used as a failure analysis method to analyze and evaluate materials, can explore the influence of different discharge rates, discharge power, discharge current, discharge temperature, clamp pressure and other factors on the negative pole lithium precipitation state, or explore the lithium precipitation limit, and can also evaluate the lithium precipitation uniformity of different positions of the same negative pole piece, to provide technical support for battery research and development and process.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a method for evaluating the lithium plating state of the negative electrode of a lithium battery. Background Technology

[0002] With the widespread application of lithium batteries, various high-capacity, high-rate lithium batteries have been launched, and manufacturers and customers are increasingly demanding higher safety standards for lithium batteries. The national standard GB / T36276-2018, "Lithium-ion Batteries for Electric Energy Storage," also clearly specifies the charge / discharge rate requirements. As the charge / discharge rate increases, the internal resistance and polarization of lithium batteries increase under high current, causing lithium ions to accumulate on the negative electrode surface, posing a certain risk of lithium plating.

[0003] Currently, the industry commonly uses the following methods to evaluate the lithium plating status of the negative electrode:

[0004] The three-electrode method utilizes an electrochemical system consisting of three electrodes: a reference electrode, a counter electrode, and a working electrode. High-precision electrochemical workstations are used for analysis employing methods such as cyclic voltammetry, linear sweep voltammetry, potential step method, and impedance spectroscopy. However, this method has drawbacks including high precision of the testing equipment, demanding requirements for the testing personnel, easy oxidation of the copper wire, and the complexity of fabricating the three electrodes.

[0005] Industrial CT method: This method uses industrial CT scanners to scan lithium batteries and generate tomographic images, analyzing the internal structure of the sample. It plays an important role in understanding material defects and properties, and it is a non-destructive analysis method as it does not require disassembling the battery. However, industrial CT equipment is very expensive.

[0006] Patent application CN114544793A discloses a quantitative detection method for the amount of lithium plating on the negative electrode of a lithium-ion battery. This method involves disassembling the lithium-ion battery in an inert atmosphere with a water oxygen content ≤1ppm. The resulting negative electrode sheet is placed in a reactor equipped with a temperature sensor, a pressure sensor, a liquid injection pipe, and a gas emission pipe. After sealing the reactor, it is removed from the inert atmosphere, and a reaction solution is injected into the reactor through the liquid injection pipe, immersing the negative electrode sheet in the reaction solution. The reaction is then ultrasonically controlled. The amount of hydrogen gas generated is measured, and the amount of lithium plating on the negative electrode sheet is calculated based on this amount of hydrogen gas. However, this detection method poses a safety risk due to the hydrogen gas generated in a confined space.

[0007] Patent application CN113093029A discloses a method for testing the lithium deposition amount of a lithium-ion battery negative electrode based on ICP. This method involves scraping off powder from a fully charged negative electrode and digesting it, then using ICP-OES for quantitative testing. The calculated value is compared with the sample's constant current charging capacity to obtain the lithium element content corresponding to the constant current capacity per unit Ah. However, this method does not consider the influence of lithium salts in the electrolyte, nor does it account for the impact of lithium oxidation in air on the sample's proportion, leading to biased test results. Furthermore, a fully charged negative electrode poses a significant safety hazard. Summary of the Invention

[0008] This invention aims to at least partially address one of the technical problems in related technologies. To this end, embodiments of this invention propose a method for evaluating the lithium plating state of the negative electrode of a lithium battery.

[0009] In a first aspect, embodiments of the present invention provide a method for evaluating the lithium plating state of a lithium battery negative electrode, comprising the following steps: S101. Discharge the lithium iron phosphate battery cell and disassemble it to obtain the positive electrode sheet. Soak the positive electrode sheet in an organic solvent multiple times and then dry it to obtain the lithium iron phosphate positive electrode sheet. S201. Take a sample with mass m from the lithium iron phosphate positive electrode sheet, immerse the sample in water and let it stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S301. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S401. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S501. Use ICP-OES to test the concentration of Fe in solution b. Fe（b） ; S601. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Where, ω Fe The larger the value of ω, the more lithium is deposited at the negative electrode, and vice versa. Fe The smaller the value, the less lithium is deposited on the negative electrode.

[0010] The advantages and technical effects of the evaluation method in this embodiment of the invention are as follows: (1) The evaluation method provided in this embodiment of the invention evaluates the lithium plating state of the negative electrode by the iron phosphide content in the lithium iron phosphate positive electrode. The higher the iron phosphide content in the lithium iron phosphate positive electrode, the more serious the lithium plating of the corresponding negative electrode. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode, the lighter the lithium plating state of the corresponding negative electrode.

[0011] (2) The evaluation method of the present invention can be used as a failure analysis means for material analysis and evaluation. It can explore the influence of different discharge rates, discharge power, discharge current, discharge temperature, clamp pressure and other factors on the lithium plating state of the negative electrode, and provide technical support for battery research and development and process.

[0012] (3) The evaluation method provided in the embodiments of the present invention does not require analysis of the negative electrode, and has low risk and high accuracy.

[0013] (4) The evaluation method provided in the embodiments of the present invention has low requirements for equipment and personnel, the equipment is inexpensive, the process time is short, and the cost performance is higher.

[0014] Optionally, in step S201, the time for standing at room temperature is 1 to 60 minutes.

[0015] Optionally, in step S301, the time for standing at room temperature is 10~240 minutes.

[0016] Optionally, in step S401, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

[0017] Optionally, in step S601, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

[0018] In some embodiments, step S501 further includes the following step: using ICP-oes to test the concentration c of Li element in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate.

[0019] In some embodiments, when it is necessary to compare the negative electrode lithium plating state of different lithium iron phosphate cells, different lithium iron phosphate cells are used in step S101, and other conditions are consistent with steps S101 to S601.

[0020] In some embodiments, when it is necessary to compare the negative electrode lithium plating state of the same lithium iron phosphate cell under different discharge conditions, in step S101, multiple lithium iron phosphate cells from the same batch are discharged, and the discharge conditions of the multiple lithium iron phosphate cells are controlled to be single-factor variation. The discharge conditions include at least one of discharge rate, discharge power, discharge current, discharge temperature and clamp pressure, and other conditions are consistent with steps S101 to S601.

[0021] Secondly, embodiments of the present invention provide a method for evaluating the lithium plating state of a lithium battery negative electrode, comprising the following steps: S102. After discharging the lithium iron phosphate battery cell, charge it for the Nth cycle at the same rate and then disassemble it to obtain the first positive electrode. Soak the first positive electrode multiple times in an organic solvent and then dry it to obtain the first lithium iron phosphate positive electrode. After discharging another lithium iron phosphate cell from the same batch, it was charged and discharged at the same rate for the Nth cycle and then disassembled to obtain a second positive electrode. The second positive electrode was soaked in an organic solvent multiple times and then dried to obtain a second lithium iron phosphate positive electrode. After discharging another lithium iron phosphate cell from the same batch, it was charged at the same rate for the N+xth cycle and then disassembled to obtain the third positive electrode. The third positive electrode was soaked in an organic solvent multiple times and then dried to obtain the third lithium iron phosphate positive electrode. After discharging the same batch of lithium iron phosphate cells, charge and discharge them at the same rate for N+x cycles, then disassemble them to obtain the fourth positive electrode. Soak the fourth positive electrode in an organic solvent multiple times and then dry it to obtain the fourth lithium iron phosphate positive electrode. Where N is greater than or equal to 1, and x is greater than or equal to 1; S202. Take samples of mass m from the first lithium iron phosphate positive electrode, the second lithium iron phosphate positive electrode, the third lithium iron phosphate positive electrode, and the fourth lithium iron phosphate positive electrode respectively, immerse the samples in water and let them stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S302. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S402. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S502. Use ICP-OES to test the concentration of Fe in solution b. Fe（b） ; S602. Based on the concentration c of Fe element in solution b.Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Among them, if the iron phosphide content ω in the first lithium iron phosphate positive electrode sheet Fe The content of iron phosphide in the lithium iron phosphate cathode is equal to ω. Fe Furthermore, the iron phosphide content ω in the second lithium iron phosphate positive electrode sheet Fe The content of iron phosphide in the lithium iron phosphate cathode is equal to ω. Fe This indicates that lithium is not deposited at the negative electrode. If the iron phosphide content ω in the first lithium iron phosphate positive electrode is... Fe The iron phosphide content ω is less than that in the lithium iron phosphate cathode sheet. Fe Or the iron phosphide content ω in the lithium iron phosphate positive electrode. Fe The iron phosphide content ω is less than that in the lithium iron phosphate cathode sheet. Fe This indicates lithium plating at the negative electrode.

[0022] The advantages and technical effects of the evaluation method in this embodiment of the invention are as follows: (1) The evaluation method provided in this embodiment of the invention evaluates the lithium plating state of the negative electrode by the iron phosphide content in the lithium iron phosphate positive electrode. The higher the iron phosphide content in the lithium iron phosphate positive electrode, the more serious the lithium plating of the corresponding negative electrode. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode, the lighter the lithium plating state of the corresponding negative electrode.

[0023] (2) The evaluation method of the present invention can be used as a failure analysis means for material analysis and evaluation, and can explore the lithium plating limit, providing technical support for battery research and development and process.

[0024] Optionally, in step S202, the time for standing at room temperature is 1 to 60 minutes.

[0025] Optionally, in step S302, the time for standing at room temperature is 10~240 minutes.

[0026] Optionally, in step S402, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

[0027] Optionally, in step S602, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

[0028] In some embodiments, step S502 further includes the following step: using ICP-oes to test the concentration c of Li element in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate.

[0029] Thirdly, embodiments of the present invention provide a method for evaluating the lithium plating state of a lithium battery negative electrode, comprising the following steps: S103. Discharge the lithium iron phosphate battery cell and disassemble it. Take the positive electrode sheet, soak the positive electrode sheet in an organic solvent multiple times and then dry it to obtain the lithium iron phosphate positive electrode sheet. S203. Take samples with a mass of m from different positions on the lithium iron phosphate positive electrode sheet, immerse the samples in water and let them stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S303. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S403. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S503. Use ICP-OES to test the concentration of Fe in solution b. Fe（b） ; S603. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Among them, if the iron phosphide content ω in the lithium iron phosphate positive electrode sheet measured at different locations is... Fe If the relative standard deviation is within 5%, it indicates that the lithium plating on the negative electrode is consistent; conversely, if the iron phosphide content ω in the lithium iron phosphate positive electrode sheet measured at different locations is greater than or equal to 5%, it indicates that the lithium plating on the negative electrode is consistent. Fe If the relative standard deviation is greater than 5%, it indicates that the lithium plating on the negative electrode is inconsistent.

[0030] The advantages and technical effects of the evaluation method in this embodiment of the invention are as follows: (1) The evaluation method provided in this embodiment of the invention evaluates the lithium plating state of the negative electrode by the iron phosphide content in the lithium iron phosphate positive electrode. The higher the iron phosphide content in the lithium iron phosphate positive electrode, the more serious the lithium plating of the corresponding negative electrode. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode, the lighter the lithium plating state of the corresponding negative electrode.

[0031] (2) The evaluation method of the present invention can be used as a failure analysis means to perform material analysis and evaluation, evaluate the uniformity of lithium plating at different positions of the same negative electrode, and provide technical support for battery research and development and process.

[0032] Optionally, in step S203, the time for standing at room temperature is 1 to 60 minutes.

[0033] Optionally, in step S303, the time for standing at room temperature is 10~240 minutes.

[0034] Optionally, in step S403, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

[0035] Optionally, in step S603, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

[0036] In some embodiments, step S503 further includes the following step: using ICP-oes to test the concentration c of Li element in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate. Detailed Implementation

[0037] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0038] In a first aspect, embodiments of the present invention provide a method for evaluating the lithium plating state of a lithium battery negative electrode, comprising the following steps: S101. Discharge the lithium iron phosphate battery cell and disassemble it to obtain the positive electrode sheet. Soak the positive electrode sheet in an organic solvent multiple times and then dry it to obtain the lithium iron phosphate positive electrode sheet. S201. Take a sample with mass m from the lithium iron phosphate positive electrode sheet, immerse the sample in water and let it stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S301. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S401. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S501. Use ICP-OES to test the concentration of Fe in solution b. Fe（b） ; S601. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Where, ω Fe The larger the value of ω, the more lithium is deposited at the negative electrode, and vice versa. Fe The smaller the value, the less lithium is deposited on the negative electrode.

[0039] The evaluation method provided in this invention assesses the lithium plating state of the negative electrode by measuring the iron phosphide content in the lithium iron phosphate positive electrode. A higher iron phosphide content in the lithium iron phosphate positive electrode indicates more severe lithium plating on the corresponding negative electrode, and vice versa. This evaluation method can be used as a failure analysis tool for material analysis and evaluation. It can explore the influence of different discharge rates, discharge power, discharge current, discharge temperature, and fixture pressure on the lithium plating state of the negative electrode, providing technical support for battery research and development and processes.

[0040] The evaluation method provided in this invention assesses the lithium plating state of the negative electrode by evaluating the difference in lithium ion concentration in the positive electrode. However, after the lithium ions return to the positive electrode, the lithium concentration ratio is 3~4%, which fluctuates greatly when using an ICP-oes test instrument. Therefore, the content of iron phosphide or carbon (which is present in PVDF in the positive electrode) in the positive electrode sheet is selected for evaluation. Iron phosphide or carbon can increase the conductivity of the positive electrode sheet. The difference in conductivity of the positive electrode sheet will lead to different degrees of redox in the positive electrode material. Carbon is difficult to collect and quantify. Therefore, the content of iron phosphide in the positive electrode sheet is used to quantitatively evaluate the lithium plating state of the negative electrode.

[0041] It should be understood that the test object in step S101 of the evaluation method of this embodiment can be any kind of lithium iron phosphate battery cell in the prior art, such as undoped lithium iron phosphate battery cell, doped lithium iron phosphate battery cell, carbon-free lithium iron phosphate battery cell, carbon-coated lithium iron phosphate battery cell, etc.

[0042] In step S101 of the evaluation method of this embodiment of the invention, the lithium iron phosphate battery cell is discharged and then disassembled. Discharging allows the lithium stored at the negative electrode to be transferred to the positive electrode, while some dead lithium, lithium compounds, and lithium dendrites that cannot be transferred are referred to as lithium plating. Fully discharging the lithium iron phosphate battery cell in step S101 is crucial, as it helps subsequent steps to confirm the amount of lithium plating.

[0043] Optionally, in step S101, the organic solvent is at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), propylene carbonate (EC), and ethyl methyl carbonate (EMC). All of the organic solvents listed above can completely dissolve the lithium salts remaining in the electrolyte on the disassembled positive electrode.

[0044] In step S101 of the evaluation method of this embodiment of the invention, "soaking multiple times and then drying" refers to soaking in an organic solvent, then replacing the solvent with a new one, and soaking again, repeating this process several times before drying for later use. The purpose of multiple soakings is to completely and thoroughly wash away the lithium salts remaining in the electrolyte within the disassembled positive electrode sheet.

[0045] Optionally, in step S201, the time for standing at room temperature is 1 to 60 minutes, such as 1 minute, 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, 60 minutes, etc.

[0046] In step S301 of the evaluation method of this embodiment of the invention, lithium iron phosphate (undoped or doped lithium iron phosphate) reacts with hydrochloric acid at room temperature, while iron phosphide, binder, and optionally carbon do not react with hydrochloric acid. Therefore, step S301 of the evaluation method of this embodiment of the invention utilizes this difference to dissolve the lithium iron phosphate in the positive electrode active material layer in hydrochloric acid, and then separates it with the filtrate after filtration. The iron phosphide, binder, and optionally carbon in the positive electrode active material layer then form the residual sheet-like material. It should be noted that the iron phosphide and optionally carbon do not scatter into powder under the action of the binder, and still maintain the sheet-like structure of the positive electrode sheet.

[0047] Optionally, in step S301, when the mass fraction of the hydrochloric acid is 34-36%, the ratio between the mass of the lithium iron phosphate positive electrode sheet and the volume of the hydrochloric acid is 2 g / (10-30) mL, for example, 2 g / 10 mL, 2 g / 15 mL, 2 g / 20 mL, 2 g / 25 mL, 2 g / 30 mL, etc. This ratio ensures that the hydrochloric acid is in excess, completely dissolving the lithium iron phosphate in the positive electrode active material layer.

[0048] Optionally, in step S301, the time for standing at room temperature is 10~240 min, for example 10 min, 50 min, 100 min, 150 min, 200 min, 240 min, etc. Within the above soaking time range, it can be ensured that lithium iron phosphate is completely dissolved in hydrochloric acid solution.

[0049] In step S401 of the evaluation method of this embodiment of the invention, perchloric acid is used to digest the residual flaky material at high temperature. Ferric phosphide reacts with perchloric acid to produce phosphoric acid and ferric chloride, which dissolve in the digestion solution. Optionally, carbon dioxide reacts with perchloric acid to form carbon dioxide, and the binder (e.g., PVDF) reacts with perchloric acid to produce water, carbon dioxide, hydrofluoric acid and gas, hydrochloric acid, etc. Therefore, step S401 of the evaluation method of this embodiment of the invention utilizes the above differences to dissolve ferric phosphide in the digestion solution, resulting in a digestion solution containing Fe and P elements.

[0050] Optionally, in step S401, the temperature of the digestion reaction is 200~400℃, such as 200℃, 250℃, 300℃, 350℃, 400℃, etc., and the time of the digestion reaction is 10~60min, such as 10min, 20min, 30min, 40min, 50min, 60min, etc. These temperature and time conditions help to completely digest the residual flaky material, thereby ensuring the accuracy of the test results.

[0051] The evaluation method of this invention assesses the volume V of solution b in step S401. （b） There are no special requirements; it can be of any volume.

[0052] In step S501 of the evaluation method of this embodiment of the invention, the concentration c of Li element in solution a is determined by ICP-oes test. Li（a） Specifically, this may include the following steps: preparing a series of standard solutions containing Li element with varying concentrations; measuring the light intensity of the standard solutions using an inductively coupled plasma atomic emission spectrometer (ICP-AES); establishing a standard curve between Li element concentration and light intensity; and fitting the Li element concentration-light intensity standard curve equation; then measuring the light intensity of solution a using an ICP-AES; and obtaining the Li element concentration c in solution a using the Li element concentration-light intensity standard curve equation.Li（a） .

[0053] In step S501 of the evaluation method of this embodiment, the concentration of Fe element c in solution b is determined by ICP-oes test. Fe（b） Specifically, this may include the following steps: First, use an inductively coupled plasma optical emission spectrometer (ICP-OES) to measure a series of standard solutions containing Fe at different concentrations to obtain the light intensity corresponding to different Fe concentrations. Then, a computer system records and calculates the linear relationship between Fe concentration and light intensity, establishes a standard curve between Fe concentration and light intensity, and fits the Fe concentration-light intensity standard curve equation. Next, test the light intensity of solution b using ICP-OES, and linearly deduce the Fe concentration c in solution b from the measured Fe light intensity. Fe（b） .

[0054] In step S601 of the evaluation method of this embodiment of the invention, due to iron phosphide (Fe x For P, 1 ≤ x < 3, x is neither an integer nor fixed, therefore ω can be used. Fe To characterize the iron phosphide content in the lithium iron phosphate cathode, it is no longer necessary to calculate ω. FexP .

[0055] Optionally, in step S601, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

[0056] In some embodiments, step S501 further includes the following step: using ICP-oes to test the concentration c of Li element in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate. If c Li（b） Not equal to 0, ω Fe The test results may be inaccurate. The specific parameters of steps S101 and S201 can be adjusted, and the test can be repeated until ω...Li It is 0.

[0057] In some embodiments, when it is necessary to compare the negative electrode lithium plating state of different lithium iron phosphate cells, different lithium iron phosphate cells are used in step S101, and other conditions are consistent with steps S101 to S601.

[0058] In some embodiments, when it is necessary to compare the negative electrode lithium plating state of the same lithium iron phosphate cell under different discharge conditions, in step S101, multiple lithium iron phosphate cells from the same batch are discharged, and the discharge conditions of the multiple lithium iron phosphate cells are controlled to be single-factor variation. The discharge conditions include at least one of discharge rate, discharge current, discharge temperature and clamp pressure, and other conditions are consistent with steps S101 to S601.

[0059] In addition, step S401 may also include the following step: adjusting the volume of the filtrate to obtain a volume of V. （a） Solution a; Step S501 may further include the following step: using ICP-oes to test the concentration c of Li element in solution a. Li（a） Step S601 may further include the following step: based on the concentration c of Li element in solution a Li（a） The residual lithium content ω in the lithium iron phosphate positive electrode was calculated. Li Optionally, in step S601, ω Li =m Li（a） / m=c Li（a） V （a） M Li / m, where M Li ρ represents the molar mass of Li, expressed in g / mol. It can be used to assess the proportion of lithium iron phosphate in the active material layer, or to determine the representativeness of a mass sample (m).

[0060] Secondly, embodiments of the present invention provide a method for evaluating the lithium plating state of a lithium battery negative electrode, comprising the following steps: S102. After discharging the lithium iron phosphate battery cell, charge it for the Nth cycle at the same rate and then disassemble it to obtain the first positive electrode. Soak the first positive electrode multiple times in an organic solvent and then dry it to obtain the first lithium iron phosphate positive electrode. After discharging another lithium iron phosphate cell from the same batch, it was charged and discharged at the same rate for the Nth cycle and then disassembled to obtain a second positive electrode. The second positive electrode was soaked in an organic solvent multiple times and then dried to obtain a second lithium iron phosphate positive electrode. After discharging another lithium iron phosphate cell from the same batch, it was charged at the same rate for the N+xth cycle and then disassembled to obtain the third positive electrode. The third positive electrode was soaked in an organic solvent multiple times and then dried to obtain the third lithium iron phosphate positive electrode. After discharging the same batch of lithium iron phosphate cells, charge and discharge them at the same rate for N+x cycles, then disassemble them to obtain the fourth positive electrode. Soak the fourth positive electrode in an organic solvent multiple times and then dry it to obtain the fourth lithium iron phosphate positive electrode. Where N is greater than or equal to 1, and x is greater than or equal to 1; S202. Take samples of mass m from the first lithium iron phosphate positive electrode, the second lithium iron phosphate positive electrode, the third lithium iron phosphate positive electrode, and the fourth lithium iron phosphate positive electrode respectively, immerse the samples in water and let them stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S302. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S402. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （a） Solution b; S502. Use ICP-OES to test the concentration of Fe in solution b. Fe（b） ; S602. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Among them, if the iron phosphide content ω in the first lithium iron phosphate positive electrode sheet Fe The content of iron phosphide in the lithium iron phosphate cathode is equal to ω. Fe Furthermore, the iron phosphide content ω in the second lithium iron phosphate positive electrode sheet Fe The content of iron phosphide in the lithium iron phosphate cathode is equal to ω. Fe This indicates that lithium is not deposited at the negative electrode. If the iron phosphide content ω in the first lithium iron phosphate positive electrode is... Fe The iron phosphide content ω is less than that in the lithium iron phosphate cathode sheet. Fe Or the iron phosphide content ω in the lithium iron phosphate positive electrode. Fe The iron phosphide content ω is less than that in the lithium iron phosphate cathode sheet. Fe This indicates lithium plating at the negative electrode.

[0061] The evaluation method provided in this invention assesses the lithium plating state of the negative electrode by measuring the iron phosphide content in the lithium iron phosphate positive electrode. A higher iron phosphide content in the lithium iron phosphate positive electrode indicates more severe lithium plating on the corresponding negative electrode, and vice versa. This evaluation method can be used as a failure analysis tool for material analysis and evaluation, exploring the lithium plating limit and providing technical support for battery research and development.

[0062] Optionally, in step S102, the organic solvent is at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), propylene carbonate (EC), ethyl methyl carbonate (EMC), etc.

[0063] Optionally, in step S202, the time for standing at room temperature is 1 to 60 minutes.

[0064] Optionally, in step S302, the time for standing at room temperature is 10~240 minutes.

[0065] Optionally, in step S402, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

[0066] Optionally, in step S602, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

[0067] In some embodiments, step S502 further includes the following step: using ICP-oes to test the concentration c of Li element in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate.

[0068] In addition, step S402 may also include the following step: adjusting the volume of the filtrate to obtain a volume of V. （a）Solution a; Step S502 may further include the following step: using ICP-oes to test the concentration c of Li element in solution a. Li（a） Step S602 may further include the following step: based on the concentration c of Li element in solution a Li（a） The residual lithium content ω in the lithium iron phosphate positive electrode was calculated. Li Optionally, in step S601, ω Li =m Li（a） / m=c Li（a） V （a） M Li / m, where M Li ρ represents the molar mass of Li, expressed in g / mol. It can be used to assess the proportion of lithium iron phosphate in the active material layer, or to determine the representativeness of a mass sample (m).

[0069] Thirdly, embodiments of the present invention provide a method for evaluating the lithium plating state of a lithium battery negative electrode, comprising the following steps: S103. Discharge the lithium iron phosphate battery cell and disassemble it. Take the positive electrode sheet, soak the positive electrode sheet in an organic solvent multiple times and then dry it to obtain the lithium iron phosphate positive electrode sheet. S203. Take samples with a mass of m from different positions on the lithium iron phosphate positive electrode sheet, immerse the samples in water and let them stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S303. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S403. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S503. Use ICP-OES to test the concentration of Fe in solution b. Fe（a） ; S603. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Among them, if the iron phosphide content ω in the lithium iron phosphate positive electrode sheet measured at different locations is... Fe If the relative standard deviation is within 5%, it indicates that the lithium plating on the negative electrode is consistent; conversely, if the iron phosphide content ω in the lithium iron phosphate positive electrode sheet measured at different locations is greater than or equal to 5%, it indicates that the lithium plating on the negative electrode is consistent. Fe If the relative standard deviation is greater than 5%, it indicates that the lithium plating on the negative electrode is inconsistent.

[0070] The evaluation method provided in this invention assesses the lithium plating state of the negative electrode by measuring the iron phosphide content in the lithium iron phosphate positive electrode. A higher iron phosphide content in the lithium iron phosphate positive electrode indicates more severe lithium plating on the corresponding negative electrode, and vice versa. This evaluation method can be used as a failure analysis tool for material analysis and evaluation, assessing the uniformity of lithium plating at different locations on the same negative electrode, thus providing technical support for battery research and development and processes.

[0071] Optionally, in step S101, the organic solvent is at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), propylene carbonate (EC), methyl ethyl carbonate (EMC), etc.

[0072] Optionally, in step S203, the time for standing at room temperature is 1 to 60 minutes.

[0073] Optionally, in step S303, the time for standing at room temperature is 10~240 minutes.

[0074] Optionally, in step S403, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

[0075] Optionally, in step S603, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

[0076] In some embodiments, step S503 further includes the following step: using ICP-oes to test the concentration c of Li element in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate.

[0077] In addition, step S403 may also include the following step: adjusting the volume of the filtrate to obtain a volume of V. （a）Solution a; Step S503 may further include the following step: using ICP-oes to test the concentration c of Li element in solution a. Li（a） Step S603 may further include the following step: based on the concentration c of Li element in solution a Li（a） The residual lithium content ω in the lithium iron phosphate positive electrode was calculated. Li Optionally, in step S603, ω Li =m Li（a） / m=c Li（a） V （a） M Li / m, where M Li ρ represents the molar mass of Li, expressed in g / mol. It can be used to assess the proportion of lithium iron phosphate in the active material layer, or to determine the representativeness of a mass sample (m).

[0078] The present invention will now be described in detail with reference to the embodiments.

[0079] Example 1 (Normal Evaluation Process) 1. Discharge the same batch of lithium iron phosphate cells at the same cycle temperature using 1C, 1.5C, 2C, 2.5C, 3C, and 4C respectively. Then disassemble the cells, take any part of the positive electrode sheet, soak it in DMC, change the solution and soak it several times, and then dry it at 80℃ for later use.

[0080] 2. Weigh out the same mass of positive electrode sheets as samples from step 1, and record the mass as m1~m6 (g). Place them in beakers 1~6 respectively, add 20mL of ultrapure water, immerse them for 30min, then take them out and rinse off the aluminum foil.

[0081] 3. Add 20 mL of hydrochloric acid (GR grade, mass fraction 34-36%) to beakers 1-6 from step 2 respectively. After standing at room temperature for 30 min, transfer the solution and make up to a final volume of V. （a） Solution a is 100 mL and is numbered a1~a6.

[0082] 4. Add 10 mL of perchloric acid (GR grade, mass fraction 70~72%) to the beaker from step 3, heat on a plate heater for 30 min, then cool and bring to a final volume of V. （b） Solution b is 100 mL in volume and is numbered b1 to b6.

[0083] 5. Use ICP-OES to test the Li element concentration in solution a and c respectively. Li（a） The concentration of Li element in solution b is c Li（b） The Fe element concentration in solution b is c Fe（b） .

[0084] 6. Based on the concentration c of Li element in solution a Li（a） The residual lithium content ω in the lithium iron phosphate positive electrode was calculated. Li , ω Li =(m Li（a） / m) 100% = (c Li（a） V （a） M Fe / m) 100%, of which M Fe This represents the molar mass of Fe, expressed in g / mol.

[0085] Based on the concentration of Fe in solution b, c Fe（b） The percentage of iron phosphide content in the mass of the lithium iron phosphate cathode was calculated as ω. Fe , ω Fe =(m Fe（b） / m) 100% = (c Fe（b） V （b） M Fe / m) 100%, of which M Fe This represents the molar mass of Fe, expressed in g / mol.

[0086] 7. Iron phosphide content ω in lithium iron phosphate cathode sheet Fe The higher the concentration of iron phosphate (FPP) in the positive electrode, the more lithium is deposited on the negative electrode. Fe The lower the value, the less lithium is deposited on the negative electrode.

[0087] Table 1. Test results of Example 1

[0088] As shown in Table 1, the Li content in solution a decreases with increasing discharge current, while the Li content in solution b is close to 0. No lithium iron phosphate affects the Fe content in solution b. Within the same batch of lithium iron phosphate cells, the proportion of iron phosphide in the positive electrode increases with increasing charge / discharge current. This means that the higher the iron phosphide content in the positive electrode, the more lithium ions are difficult to transfer from the negative electrode to the positive electrode, resulting in more lithium deposition at the negative electrode.

[0089] Example 2 (Finding the lithium plating window, i.e., the state in which the Li content in the positive electrode is the same before and after charging and discharging, that is, the iron phosphide content remains unchanged) 1. Discharge the same batch of lithium iron phosphate cells at the same cycle temperature using 1C and 3C, and then charge and discharge them using the same rate. Disassemble the cells before and after discharge cycles, take any part of the positive electrode sheet, soak it in DEC, change the solution and soak it several times, and then dry it at 80℃ for later use.

[0090] 2. Weigh out the same mass of positive electrode sheets as in step 1, and record the mass as m7~m10 (g). Place them in beakers 7~10 respectively, add 20mL of ultrapure water, immerse them for 30min, then take them out and rinse off the aluminum foil.

[0091] 3. Add 20 mL of hydrochloric acid (GR grade, mass fraction 34-36%) to beakers 7-10 from step 2, let stand at room temperature for 30 min, then transfer the solution and make up to a volume of V. （a） Solution a, numbered a7~a10.

[0092] 4. Add 10 mL of perchloric acid (GR grade, mass fraction 70~72%) to the beaker from step 3, heat on a plate heater for 30 min, cool and dilute to volume to obtain solution b, labeled b7~b10.

[0093] 5. Use ICP-OES to test the Li element concentration in solution b. Li（b） The Fe element concentration in solution b is c Fe（b） .

[0094] 6. Based on the concentration of Fe in solution b, c Fe（b） The percentage of iron phosphide content in the mass of the lithium iron phosphate cathode was calculated as ω. Fe , ω Fe =(m Fe（b） / m) 100% = (c Fe（b） V （b） M Fe / m) 100%, of which M Fe This represents the molar mass of Fe, expressed in g / mol.

[0095] 7. Iron phosphide content ω in lithium iron phosphate cathode sheet Fe If the discharge cycles are consistent, then there is no lithium plating on the negative electrode. If the iron phosphate content ω in the positive electrode of the previous cycle is... Fe The iron phosphide content ω in the lithium iron phosphate positive electrode sheet is less than that in the subsequent cycle. Fe Then lithium will be deposited at the negative electrode.

[0096] Table 2. Test results of Example 2

[0097] As can be seen from the data in Table 2, when using a high current of 1C for charging and discharging, the iron phosphate content ω in the lithium iron phosphate positive electrode sheet of the previous and subsequent cycles... Fe If the values ​​are consistent, then there is no lithium plating on the negative electrode; when using a high-current 3C charge-discharge cycle, the iron phosphate content ω in the lithium iron phosphate positive electrode from the previous cycle... Fe The iron phosphide content ω in the lithium iron phosphate positive electrode sheet is less than that in the subsequent cycle. Fe This indicates that some lithium ions cannot return from the negative electrode to the positive electrode, meaning lithium plating has occurred. Therefore, 1C to 3C encompasses the lithium plating window of this lithium iron phosphate positive electrode. Further trials using the evaluation method from Example 2 can be conducted within the 1C to 3C range to find a more precise lithium plating window.

[0098] Example 3 (Evaluation of lithium plating status at different locations on the same cell electrode, consistency evaluation) 1. Discharge the same batch of lithium iron phosphate cells at the same 3C cycle temperature, then disassemble the cells, take any part of the positive electrode sheet, soak it in DMC, change the solution and soak it several times, then dry it at 80℃ for later use.

[0099] 2. Weigh out 6 groups of positive electrode sheets of the same mass but different positions from step 1 as samples, and record the mass as m1~m6 (g). Place them in beakers 1~6 respectively, add 20mL of ultrapure water and immerse them for 30min. Then take them out and rinse off the aluminum foil.

[0100] 3. Add 20 mL of hydrochloric acid (GR grade, mass fraction 34-36%) to beakers 1-6 from step 2 respectively. After standing at room temperature for 30 min, transfer the solution and make up to a final volume of V. （a） Solution a is 100 mL and is numbered a1~a6.

[0101] 4. Add 10 mL of perchloric acid (GR grade, mass fraction 70~72%) to the beaker from step 3, heat on a plate heater for 30 min, then cool and bring to a final volume of V. （b） Solution b is 100 mL in volume and is numbered b1 to b6.

[0102] 5. Use ICP-OES to test the Li element concentration in solution a and c respectively. Li（a） The concentration of Li element in solution b is c Li（b） The Fe element concentration in solution b is c Fe（b） .

[0103] 6. Based on the concentration of Fe in solution b, c Fe（b） The percentage of iron phosphide content in the mass of the lithium iron phosphate cathode was calculated as ω. Fe , ωFe =(m Fe（b） / m) 100% = (c Fe（b） V （b） M Fe / m) 100%, of which M Fe This represents the molar mass of Fe, expressed in g / mol.

[0104] 7. Pay attention to the iron phosphide content (ω) in several sets of lithium iron phosphate cathode plates. Fe If the relative standard deviation is within 5%, the state of the negative electrode is consistent, and the lithium deposition on the negative electrode is consistent; if it is greater than 5%, the state of the negative electrode is inconsistent, and the lithium deposition on the negative electrode is inconsistent.

[0105] Table 3. Test results of Example 3

[0106] As can be seen from the data in Table 3, the consistency of the negative electrode sheet of this cell is 1.8% under a current of 3C, and the lithium plating state of the negative electrode sheet is consistent.

[0107] Comparative Example 1 (CN113093029A, Lithium content measured by electrode scraping) 1. Take the lithium iron phosphate battery cell of Example 3, take negative electrode sheets with a length of 2~10cm respectively, fully charge them at 0.1C or 0.2C and then disassemble them to form test sample a and test sample b. Test sample b is soaked in DMC, the solution is changed and soaked several times, and then dried at 80°C for later use.

[0108] 2. Take 2.0g of each of the test sample a and reference sample b obtained in the first step and scrape off the active material. Place the scraped-off active material in different beakers, weigh them, and record the mass of the active material.

[0109] 3. Add 6 mL of hydrochloric acid (GR grade, mass fraction 34~36%), 3 mL of nitric acid (GR grade, mass fraction 64~66%), and 30 mL of deionized water to a beaker and heat to digest the solution. After cooling, filter and make up to volume.

[0110] 4. The lithium content of the digested sample was determined. The lithium content of the solution after volume adjustment was determined by inductively coupled plasma atomic emission spectrometry. Then the mass of lithium in the solution after volume adjustment was calculated and the results were recorded as X1 and X2 (g).

[0111] The test results are as follows: Table 4. Test results of Comparative Example 1

[0112] Comparing data X1 in Table 4 with that in Example 3, not soaking in organic solvent leads to a higher lithium content test value. This is due to the residual lithium hexafluorophosphate electrolyte on the negative electrode, which may even exceed the total lithium content of the active material. Comparing data X2 with that in Example 3, soaking in organic solvent leads to a lower lithium content test value. This is because during the cleaning of the electrolyte on the negative electrode with organic solvent, the elemental lithium on the negative electrode reacts with the water in the organic solvent, resulting in loss and thus a lower test result. Therefore, the method of testing the lithium content from the negative electrode and comparing it with the lithium ions transferred during constant current charging at the positive electrode has limitations and requires further investigation.

[0113] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0114] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for evaluating the lithium plating state of a lithium battery negative electrode, characterized in that, Includes the following steps: S101. Discharge the lithium iron phosphate battery cell and disassemble it to obtain the positive electrode sheet. Soak the positive electrode sheet in an organic solvent multiple times and then dry it to obtain the lithium iron phosphate positive electrode sheet. S201. Take a sample with mass m from the lithium iron phosphate positive electrode sheet, immerse the sample in water and let it stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S301. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S401. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S501. Use ICP-OES to test the concentration of Fe in solution b. Fe ; S601. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Where, ω Fe The larger the value of ω, the more lithium is deposited at the negative electrode, and vice versa. Fe The smaller the value, the less lithium is deposited on the negative electrode.

2. The evaluation method according to claim 1, characterized in that, In step S201, the time for standing at room temperature is 1 to 60 minutes.

3. The evaluation method according to claim 1, characterized in that, In step S301, the time for standing at room temperature is 10~240 minutes.

4. The evaluation method according to claim 1, characterized in that, In step S401, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

5. The evaluation method according to claim 1, characterized in that, In step S601, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

6. The evaluation method according to claim 1, characterized in that, Step S501 further includes the following step: using ICP-oes to test the concentration of Li element c in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate.

7. The evaluation method according to any one of claims 1 to 6, characterized in that, When it is necessary to compare the negative electrode lithium plating state of different lithium iron phosphate cells, different lithium iron phosphate cells are used in step S101, and other conditions are consistent with steps S101 to S601. When it is necessary to compare the negative electrode lithium plating state of the same lithium iron phosphate cell under different discharge conditions, multiple lithium iron phosphate cells from the same batch are discharged in step S101, and the discharge conditions of multiple lithium iron phosphate cells are controlled to be single-factor variation. The discharge conditions include at least one of discharge rate, discharge power, discharge current, discharge temperature and clamp pressure, and other conditions are consistent with steps S101 to S601.

8. A method for evaluating the lithium plating state of a lithium battery negative electrode, characterized in that, Includes the following steps: S102. After discharging the lithium iron phosphate battery cell, charge it for the Nth cycle at the same rate and then disassemble it to obtain the first positive electrode. Soak the first positive electrode multiple times in an organic solvent and then dry it to obtain the first lithium iron phosphate positive electrode. After discharging another lithium iron phosphate cell from the same batch, it was charged and discharged at the same rate for the Nth cycle and then disassembled to obtain a second positive electrode. The second positive electrode was soaked in an organic solvent multiple times and then dried to obtain a second lithium iron phosphate positive electrode. After discharging another lithium iron phosphate cell from the same batch, it was charged at the same rate for the N+xth cycle and then disassembled to obtain the third positive electrode. The third positive electrode was soaked in an organic solvent multiple times and then dried to obtain the third lithium iron phosphate positive electrode. After discharging the same batch of lithium iron phosphate cells, charge and discharge them at the same rate for N+x cycles, then disassemble them to obtain the fourth positive electrode. Soak the fourth positive electrode in an organic solvent multiple times and then dry it to obtain the fourth lithium iron phosphate positive electrode. Where N is greater than or equal to 1, and x is greater than or equal to 1; S202. Take samples of mass m from the first lithium iron phosphate positive electrode, the second lithium iron phosphate positive electrode, the third lithium iron phosphate positive electrode, and the fourth lithium iron phosphate positive electrode respectively, immerse the samples in water and let them stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S302. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S402. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S502. Use ICP-OES to test the concentration of Fe in solution b. Fe（b） ; S602. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Among them, if the iron phosphide content ω in the first lithium iron phosphate positive electrode sheet Fe The content of iron phosphide in the lithium iron phosphate cathode is equal to ω. Fe Furthermore, the iron phosphide content ω in the second lithium iron phosphate positive electrode sheet Fe The content of iron phosphide in the lithium iron phosphate cathode is equal to ω. Fe This indicates that lithium is not deposited at the negative electrode. If the iron phosphide content ω in the first lithium iron phosphate positive electrode is... Fe The iron phosphide content ω is less than that in the lithium iron phosphate cathode sheet. Fe Or the iron phosphide content ω in the lithium iron phosphate positive electrode. Fe The iron phosphide content ω is less than that in the lithium iron phosphate cathode sheet. Fe This indicates lithium plating at the negative electrode.

9. The evaluation method according to claim 8, characterized in that, In step S202, the time for standing at room temperature is 1 to 60 minutes.

10. The evaluation method according to claim 8, characterized in that, In step S302, the time for standing at room temperature is 10~240 minutes.

11. The evaluation method according to claim 8, characterized in that, In step S402, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

12. The evaluation method according to claim 8, characterized in that, In step S602, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

13. The evaluation method according to claim 8, characterized in that, Step S502 further includes the following step: using ICP-oes to test the concentration of Li element c in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate.

14. A method for evaluating the lithium plating state of a lithium battery negative electrode, characterized in that, Includes the following steps: S103. Discharge the lithium iron phosphate battery cell and disassemble it. Take the positive electrode sheet, soak the positive electrode sheet in an organic solvent multiple times and then dry it to obtain the lithium iron phosphate positive electrode sheet. S203. Take samples with a mass of m from different positions on the lithium iron phosphate positive electrode sheet, immerse the samples in water and let them stand at room temperature to separate the positive electrode current collector from the positive electrode active material layer. S303. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain residual flake material and filtrate; S403. The residual flaky material is mixed with perchloric acid and then subjected to a digestion reaction under heating conditions to obtain a digestion solution; then the digestion solution is cooled and diluted to a final volume of V. （b） Solution b; S503. Use ICP-OES to test the concentration of Fe in solution b. Fe（b） ; S603. Based on the concentration c of Fe element in solution b. Fe（b） The iron phosphide content ω in the lithium iron phosphate positive electrode was calculated. Fe ; Among them, if the iron phosphide content ω in the lithium iron phosphate positive electrode sheet measured at different locations is... Fe If the relative standard deviation is within 5%, it indicates that the lithium plating on the negative electrode is consistent; conversely, if the iron phosphide content ω in the lithium iron phosphate positive electrode sheet measured at different locations is greater than or equal to 5%, it indicates that the lithium plating on the negative electrode is consistent. Fe If the relative standard deviation is greater than 5%, it indicates that the lithium plating on the negative electrode is inconsistent.

15. The evaluation method according to claim 14, characterized in that, In step S203, the time for standing at room temperature is 1 to 60 minutes.

16. The evaluation method according to claim 14, characterized in that, In step S303, the time for standing at room temperature is 10~240 minutes.

17. The evaluation method according to claim 14, characterized in that, In step S403, the temperature of the digestion reaction is 200~400℃, and the time of the digestion reaction is 10~60min.

18. The evaluation method according to claim 14, characterized in that, In step S603, ω Fe =m Fe（b） / m=c Fe（b） V （b） M Fe / m, where M Fe This represents the molar mass of Fe, expressed in g / mol.

19. The evaluation method according to claim 14, characterized in that, Step S503 further includes the following step: using ICP-oes to test the concentration of Li element c in solution b. Li（b） According to the concentration c of Li element in solution b Li（b） Determine ω Fe Whether the test results are affected by lithium iron phosphate; where, when c Li（b） When ω is 0, it indicates that solution b does not contain Li element. Fe The test results are not affected by lithium iron phosphate; conversely, when c Li（b） When ω is not 0, it indicates that solution b contains Li element. Fe The test results are affected by lithium iron phosphate.

Images