Method for evaluating lithium precipitation state of negative electrode of lithium battery

Abstract

The invention provides a method for evaluating the lithium precipitation state of a lithium battery negative electrode, and belongs to the technical field of lithium batteries. According to the evaluation method provided by the embodiment of the invention, the lithium precipitation state of the negative electrode is evaluated according to the iron phosphide content in the lithium iron phosphate positive pole piece, the higher the iron phosphide content in the lithium iron phosphate positive pole piece is, the more serious the lithium precipitation of the corresponding negative pole piece is, otherwise, the lower the iron phosphide content in the lithium iron phosphate positive pole piece is, the lighter the lithium precipitation state of the corresponding negative pole piece is. The evaluation method provided by the embodiment of the invention can be used as a failure analysis means for material analysis and evaluation, and can be used for exploring the influence of different factors such as discharge multiplying power, discharge power, discharge current, discharge temperature, clamp pressure and the like on the lithium precipitation state of the negative electrode, or exploring the lithium precipitation limit; and the lithium precipitation uniformity of different positions of the same negative plate can be evaluated, and technical support is provided for battery research and development and process.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a method for evaluating the lithium deposition state of a negative electrode of a lithium battery. Background Art

[0002] With the widespread use of lithium batteries, a variety of high-capacity and high-rate lithium batteries have been introduced, leading to increasing safety requirements from manufacturers and customers. The national standard GB / T36276-2018, "Lithium-ion Batteries for Power Energy Storage," also specifies charge and discharge rate requirements. As the charge and discharge rate increases, the internal resistance and polarization of lithium batteries increase under high currents, causing lithium ions to accumulate on the negative electrode surface, posing a certain risk of lithium plating.

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

[0004] The three-electrode method uses three electrodes to form an electrochemical system: a reference electrode, a counter electrode, and a working electrode. Cyclic voltammetry, linear sweep voltammetry, potential step method, and impedance spectroscopy are performed using a high-precision electrochemical workstation. However, this method has drawbacks such as high-precision test equipment and personnel requirements, the copper wire is easily oxidized, and the three-electrode fabrication process is complex.

[0005] Industrial CT scans lithium batteries to generate cross-sectional images, analyzing the internal structure of the sample. This is crucial for analyzing material defects and performance, and it requires no disassembly of the battery, making it a non-destructive analysis. However, industrial CT equipment is prohibitively expensive.

[0006] Patent application CN114544793A discloses a method for quantitatively detecting the amount of lithium deposited in the negative electrode of a lithium-ion battery. This method involves disassembling the lithium-ion battery in an inert atmosphere with a water and oxygen content of ≤1 ppm. The resulting negative electrode sheet is then placed in a reactor equipped with a temperature sensor, a pressure sensor, a liquid injection tube, and a gas discharge pipe. The reactor is sealed and removed from the inert atmosphere. A reaction solution is injected into the reactor through the liquid injection tube, submerging the negative electrode sheet. Ultrasound is then applied. The amount of hydrogen generated by the reaction is measured, and the amount of lithium deposited in the negative electrode sheet is calculated based on this hydrogen. However, this method poses a safety risk when the hydrogen generated in a confined space is present.

[0007] Patent application CN113093029A discloses a method for measuring the amount of lithium deposited in the negative electrode of a lithium-ion battery based on the ICP method. This method involves scraping powder from a fully charged negative electrode and then digesting it. ICP-oes is then used for quantitative testing. The calculated value is then combined with the sample's constant current charge capacity to determine the lithium content per unit Ah of constant current capacity. However, this method does not account for the effects of lithium salts in the electrolyte or the influence of oxidation of lithium in air on the sample's proportion, resulting in biased test results. Furthermore, fully charged negative electrodes are more dangerous. Summary of the Invention

[0008] The present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, an embodiment of the present invention provides a method for evaluating the lithium deposition state of a negative electrode of a lithium battery.

[0009] In a first aspect, an embodiment of the present invention provides a method for evaluating the lithium deposition state of a negative electrode of a lithium battery, comprising the following steps:

[0010] S101. The lithium iron phosphate battery cell is disassembled after being discharged to obtain a positive electrode sheet, and the positive electrode sheet is soaked with an organic solvent several times and then dried to obtain a lithium iron phosphate positive electrode sheet;

[0011] S201. Take a sample of mass m from the lithium iron phosphate positive electrode sheet and immerse the sample in water and allow it to stand at room temperature to separate the positive current collector from the positive active material layer;

[0012] S301. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain a residual flaky substance and a filtrate;

[0013] S401. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b) Solution b;

[0014] S501. Use ICP-oes to test the concentration of Fe in solution b. Fe(b) ;

[0015] S601. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ;

[0016] Among them, ω Fe The larger the value, the more lithium is deposited on the negative electrode. On the contrary, Fe The smaller it is, the less lithium is deposited on the negative electrode.

[0017] The advantages and technical effects brought by the evaluation method of the embodiment of the present invention are:

[0018] (1) The evaluation method provided in the embodiment of the present invention evaluates the lithium deposition state of the negative electrode by the iron phosphide content in the lithium iron phosphate positive electrode sheet. The higher the iron phosphide content in the lithium iron phosphate positive electrode sheet, the more serious the lithium deposition of the corresponding negative electrode sheet. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode sheet, the lighter the lithium deposition state of the corresponding negative electrode sheet.

[0019] (2) The evaluation method of the embodiment of the present invention can be used as a failure analysis method to perform material analysis and evaluation, and 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, providing technical support for battery research and development and process.

[0020] (3) The evaluation method provided by the embodiment of the present invention does not require analysis of the negative electrode sheet, and has low risk and high accuracy.

[0021] (4) The evaluation method provided by the embodiment of the present invention has low requirements on equipment and personnel, low equipment cost, short process time, and higher cost performance.

[0022] Optionally, in step S201, the time of standing at room temperature is 1 to 60 minutes.

[0023] Optionally, in step S301, the time of standing at room temperature is 10 to 240 minutes.

[0024] Optionally, in step S401, the temperature of the digestion reaction is 200-400° C., and the time of the digestion reaction is 10-60 minutes.

[0025] Optionally, in step S601, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

[0026] In some embodiments, step S501 further includes the following steps: using ICP-oes to measure the concentration c of the Li element in the solution b. Li(b) ; According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate.

[0027] In some embodiments, when it is necessary to compare the lithium deposition states of the negative electrodes of different lithium iron phosphate battery cells, different lithium iron phosphate battery cells are used in step S101 , and other conditions remain consistent with steps S101 to S601 .

[0028] In some embodiments, when it is necessary to compare the negative electrode lithium plating state of the same lithium iron phosphate battery cell under different discharge conditions, multiple lithium iron phosphate battery cells from the same batch are used for discharge in step S101, and the discharge conditions of the multiple lithium iron phosphate battery cells are controlled to be a single-factor change, and the discharge conditions include at least one of the discharge rate, discharge power, discharge current, discharge temperature and clamp pressure, and other conditions are consistent with steps S101 to S601.

[0029] In a second aspect, an embodiment of the present invention provides a method for evaluating the lithium deposition state of a lithium battery negative electrode, comprising the following steps:

[0030] S102. After discharging the lithium iron phosphate battery cell, charging it for N cycles at the same rate and then disassembling it to obtain a first positive electrode sheet. The first positive electrode sheet is soaked in an organic solvent several times and then dried to obtain a first lithium iron phosphate positive electrode sheet.

[0031] Discharging another lithium iron phosphate battery cell from the same batch, performing charge and discharge for N cycles at the same rate, and then disassembling the cell to obtain a second positive electrode sheet; soaking the second positive electrode sheet in an organic solvent for multiple times and then drying the sheet to obtain a second lithium iron phosphate positive electrode sheet;

[0032] Discharging another lithium iron phosphate battery cell from the same batch, charging it at the same rate for N+x cycles, and then disassembling it to obtain a third positive electrode sheet; soaking the third positive electrode sheet in an organic solvent for multiple times and then drying it to obtain a third lithium iron phosphate positive electrode sheet;

[0033] Discharging another lithium iron phosphate battery cell from the same batch and performing charge and discharge for N+x cycles at the same rate, and then disassembling the battery cell to obtain a fourth positive electrode sheet; soaking the fourth positive electrode sheet in an organic solvent for multiple times and then drying the resulting sheet to obtain a fourth lithium iron phosphate positive electrode sheet;

[0034] Where N is greater than or equal to 1, x is greater than or equal to 1;

[0035] S202. Take samples of mass m from the first, second, third, and fourth lithium iron phosphate positive electrode sheets, respectively, and immerse the samples in water and allow them to stand at room temperature to separate the positive current collector from the positive active material layer.

[0036] S302. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain a residual flaky substance and a filtrate;

[0037] S402. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b)Solution b;

[0038] S502. Use ICP-oes to test the concentration of Fe in solution b Fe(b) ;

[0039] S602. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ;

[0040] Among them, if the content of iron phosphide in the first lithium iron phosphate positive electrode plate is ω Fe Equal to the content of iron phosphide in the third lithium iron phosphate positive electrode plate ω Fe , and the content of iron phosphide in the second lithium iron phosphate positive electrode sheet ω Fe Equal to the content of iron phosphide in the fourth lithium iron phosphate positive electrode plate ω Fe , it means that the negative electrode does not precipitate lithium. If the content of iron phosphide ωFe in the first lithium iron phosphate positive electrode is less than the content of iron phosphide ωFe in the third lithium iron phosphate positive electrode Fe , or the content of iron phosphide in the second lithium iron phosphate positive electrode sheet ω Fe Equal to the content of iron phosphide in the fourth lithium iron phosphate positive electrode plate ω Fe , which indicates that lithium is deposited on the negative electrode.

[0041] The advantages and technical effects brought by the evaluation method of the embodiment of the present invention are:

[0042] (1) The evaluation method provided in the embodiment of the present invention evaluates the lithium deposition state of the negative electrode by the iron phosphide content in the lithium iron phosphate positive electrode sheet. The higher the iron phosphide content in the lithium iron phosphate positive electrode sheet, the more serious the lithium deposition of the corresponding negative electrode sheet. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode sheet, the lighter the lithium deposition state of the corresponding negative electrode sheet.

[0043] (2) The evaluation method of the embodiment of the present invention can be used as a failure analysis method to perform material analysis and evaluation, explore the boundaries of lithium plating, and provide technical support for battery research and development and technology.

[0044] Optionally, in step S202, the time of standing at room temperature is 1 to 60 minutes.

[0045] Optionally, in step S302, the time of standing at room temperature is 10 to 240 minutes.

[0046] Optionally, in step S402, the temperature of the digestion reaction is 200-400°C, and the time of the digestion reaction is 10-60 minutes.

[0047] Optionally, in step S602, ω Fe =m Fe(b) / m=cFe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

[0048] In some embodiments, step S502 further includes the following steps: using ICP-oes to measure the concentration c of the Li element in the solution b. Li(b) ; According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate.

[0049] In a third aspect, an embodiment of the present invention provides a method for evaluating the lithium deposition state of a lithium battery negative electrode, comprising the following steps:

[0050] S103. The lithium iron phosphate battery is disassembled after being discharged, and the positive electrode sheet is taken, and the positive electrode sheet is soaked with an organic solvent several times and then dried to obtain a lithium iron phosphate positive electrode sheet;

[0051] S203. Samples of mass m are taken from different positions on the lithium iron phosphate positive electrode sheet, and the samples are immersed in water and allowed to stand at room temperature to separate the positive current collector from the positive active material layer;

[0052] S303. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain a residual flaky substance and a filtrate;

[0053] S403. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b) Solution b;

[0054] S503. Use ICP-oes to test the concentration of Fe in solution b c Fe(b) ;

[0055] S603. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ;

[0056] Wherein, if the content of iron phosphide in the lithium iron phosphate positive electrode sheet measured at different positions is ωFe If the relative standard deviation is within 5%, it means that the negative electrode lithium deposition is consistent; on the contrary, if the content of iron phosphide in the lithium iron phosphate positive electrode piece measured at different positions is ω Fe If the relative standard deviation is greater than 5%, it means that the lithium deposition on the negative electrode is inconsistent.

[0057] The advantages and technical effects brought by the evaluation method of the embodiment of the present invention are:

[0058] (1) The evaluation method provided in the embodiment of the present invention evaluates the lithium deposition state of the negative electrode by the iron phosphide content in the lithium iron phosphate positive electrode sheet. The higher the iron phosphide content in the lithium iron phosphate positive electrode sheet, the more serious the lithium deposition of the corresponding negative electrode sheet. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode sheet, the lighter the lithium deposition state of the corresponding negative electrode sheet.

[0059] (2) The evaluation method of the embodiment of the present invention can be used as a failure analysis method to perform material analysis and evaluation, and to evaluate the uniformity of lithium deposition at different positions of the same negative electrode sheet, thereby providing technical support for battery research and development and process.

[0060] Optionally, in step S203, the time of standing at room temperature is 1 to 60 minutes.

[0061] Optionally, in step S303, the time of standing at room temperature is 10 to 240 minutes.

[0062] Optionally, in step S403, the temperature of the digestion reaction is 200-400° C., and the time of the digestion reaction is 10-60 minutes.

[0063] Optionally, in step S603, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

[0064] In some embodiments, step S503 further includes the following steps: using ICP-oes to measure the concentration c of the Li element in the solution b. Li(b) ; According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate. DETAILED DESCRIPTION

[0065] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0066] In a first aspect, an embodiment of the present invention provides a method for evaluating the lithium deposition state of a negative electrode of a lithium battery, comprising the following steps:

[0067] S101. The lithium iron phosphate battery cell is disassembled after being discharged to obtain a positive electrode sheet, and the positive electrode sheet is soaked with an organic solvent several times and then dried to obtain a lithium iron phosphate positive electrode sheet;

[0068] S201. Take a sample of mass m from the lithium iron phosphate positive electrode sheet and immerse the sample in water and allow it to stand at room temperature to separate the positive current collector from the positive active material layer;

[0069] S301. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain a residual flaky substance and a filtrate;

[0070] S401. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b) Solution b;

[0071] S501. Use ICP-oes to test the concentration of Fe in solution b. Fe(b) ;

[0072] S601. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ;

[0073] Among them, ω Fe The larger the value, the more lithium is deposited on the negative electrode. On the contrary, Fe The smaller it is, the less lithium is deposited on the negative electrode.

[0074] The evaluation method provided in the embodiment of the present invention evaluates the state of lithium deposition at the negative electrode by using the iron phosphide content in the lithium iron phosphate positive electrode sheet. The higher the iron phosphide content in the lithium iron phosphate positive electrode sheet, the more severe the lithium deposition at the corresponding negative electrode sheet. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode sheet, the milder the lithium deposition at the corresponding negative electrode sheet. The evaluation method of the embodiment of the present invention can be used as a failure analysis tool for material analysis and evaluation. It can explore the impact of different discharge rates, discharge power, discharge current, discharge temperature, fixture pressure and other factors on the lithium deposition state of the negative electrode, providing technical support for battery research and development and process technology.

[0075] The evaluation method provided in the embodiment of the present invention evaluates the state of lithium plating at the negative electrode, and evaluates the deactivated lithium ions from the difference in lithium ion concentration in the positive electrode. However, the lithium concentration after the positive electrode lithium ions return is 3-4%, and the fluctuation is large when using an ICP-oes test instrument. Therefore, the content of iron phosphide or carbon (there is C 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 the conductivity of the positive electrode sheet will lead to different degrees of redox of the positive electrode material. Carbon is difficult to collect and quantify. Therefore, the iron phosphide content in the positive electrode sheet is used to quantitatively evaluate the state of lithium plating at the negative electrode.

[0076] It should be understood that the test object in step S101 of the evaluation method of an embodiment of the present invention can be any type of lithium iron phosphate battery cell in the prior art, for example, it can be an undoped lithium iron phosphate battery cell, a doped lithium iron phosphate battery cell, a carbon-free lithium iron phosphate battery cell, a carbon-coated lithium iron phosphate battery cell, etc.

[0077] In step S101 of the evaluation method of this embodiment of the present invention, the lithium iron phosphate cell is discharged and then disassembled. Discharging allows lithium stored in the negative electrode to transfer to the positive electrode. The remaining dead lithium, lithium compounds, and lithium dendrites that cannot be transferred are lithium deposits. Fully discharging the lithium iron phosphate cell in step S101 is crucial to help determine the amount of lithium deposits in subsequent steps.

[0078] Optionally, in step S101, 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. The organic solvents listed above can completely dissolve the lithium salt in the electrolyte remaining on the disassembled positive electrode sheet.

[0079] In step S101 of the evaluation method of the present embodiment, multiple soaking followed by drying refers to soaking in an organic solvent, replacing the solvent with a new one, and then soaking several times before drying for later use. The multiple soaking is performed to completely and thoroughly remove the lithium salts remaining in the electrolyte within the disassembled positive electrode sheet.

[0080] Optionally, in step S201, the standing time at room temperature is 1 to 60 min, for example, 1 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, etc.

[0081] In step S301 of the evaluation method of an embodiment of the present invention, lithium iron phosphate (undoped lithium iron phosphate 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 an embodiment of the present invention can use the above distinction to dissolve the lithium iron phosphate in the positive electrode active material layer in hydrochloric acid, and then separate it with the filtrate after filtration, while the iron phosphide, binder, and optionally carbon in the positive electrode active material layer constitute residual flaky material. It should be noted that the iron phosphide and optionally carbon will not be scattered into powder under the action of the binder, and can still maintain the flaky structure of the positive electrode sheet.

[0082] 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 2g / (10-30)mL, for example, 2g / 10mL, 2g / 15mL, 2g / 20mL, 2g / 25mL, 2g / 30mL, etc. The above ratio can ensure that the hydrochloric acid is excessive and can completely dissolve the lithium iron phosphate in the positive electrode active material layer.

[0083] Optionally, in step S301 , the standing time at room temperature is 10 to 240 minutes, for example, 10 minutes, 50 minutes, 100 minutes, 150 minutes, 200 minutes, 240 minutes, etc. Within the above soaking time range, the lithium iron phosphate can be completely dissolved in the hydrochloric acid solution.

[0084] In step S401 of the evaluation method according to an embodiment of the present invention, perchloric acid is used to digest the residual flaky material at high temperature. The iron phosphide reacts with the perchloric acid to produce phosphoric acid and ferric chloride, which dissolve in the digestion solution. Optionally, carbon reacts with the perchloric acid to produce carbon dioxide. The binder (e.g., PVDF) reacts with the perchloric acid to produce water, carbon dioxide, hydrofluoric acid and gas, hydrochloric acid, etc. Therefore, step S401 of the evaluation method according to an embodiment of the present invention utilizes the above distinction to dissolve the iron phosphide in the digestion solution, resulting in a digestion solution containing the elements Fe and P.

[0085] Optionally, in step S401, the digestion reaction temperature is 200-400° C., for example, 200° C., 250° C., 300° C., 350° C., 400° C., etc., and the digestion reaction time is 10-60 min., for example, 10 min., 20 min., 30 min., 40 min., 50 min., 60 min., etc. The above temperature and time conditions help to completely digest the residual flaky material, thereby ensuring the accuracy of the test results.

[0086] The evaluation method of the embodiment of the present invention is to determine the volume V of the solution b in step S401. (b) There are no special requirements and it can be any size.

[0087] In step S501 of the evaluation method of the embodiment of the present invention, the concentration c of the Li element in the solution a is tested by ICP-oes. Li(a) , specifically, the following steps may be included: preparing a series of standard solutions containing the Li element with gradient concentrations, using an inductively coupled plasma emission spectrometer to test the light intensity of the standard solutions, establishing a standard curve between the Li element concentration and the light intensity, and fitting to obtain a Li element concentration-light intensity standard curve equation; then using an inductively coupled plasma emission spectrometer to test the light intensity of the solution a, and obtaining the Li element concentration c in the solution a according to the Li element concentration-light intensity standard curve equation. Li(a) .

[0088] In step S501 of the evaluation method of the embodiment of the present invention, the concentration c of the Fe element in the solution b is tested using ICP-oes. Fe(b) Specifically, the method may include the following steps: first, using an inductively coupled plasma optical emission spectrometer (ICP-oes) to measure a series of standard solutions containing the Fe element with gradient concentrations to obtain the light intensity corresponding to the Fe element at different concentrations; then, using a computer system to record and calculate the linear relationship between the Fe element concentration and the light intensity, establish a standard curve between the Fe element concentration and the light intensity, and obtain a Fe element concentration-light intensity standard curve equation by fitting; and then, using the ICP-oes to test the light intensity of solution b, and linearly inferring the concentration c of the Fe element in solution b from the measured Fe element light intensity. Fe(b) .

[0089] In the evaluation method step S601 of the embodiment of the present invention, due to the iron phosphide (Fe x P, 1≤x<3) where x is not an integer and not fixed, so ω can be used Fe To characterize the content of iron phosphide in the lithium iron phosphate positive electrode, there is no need to calculate ωFexP.

[0090] Optionally, in step S601, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

[0091] In some embodiments, step S501 further includes the following steps: using ICP-oes to measure the concentration c of the Li element in the solution b. Li(b) ; According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when cLi(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate. If c Li(b) Not 0, ω Fe The test result may be inaccurate. You can adjust the specific parameters of steps S101 and S201 and retest until ω is correct. Li is 0.

[0092] In some embodiments, when it is necessary to compare the lithium deposition states of the negative electrodes of different lithium iron phosphate battery cells, different lithium iron phosphate battery cells are used in step S101 , and other conditions remain consistent with steps S101 to S601 .

[0093] In some embodiments, when it is necessary to compare the negative electrode lithium plating state of the same lithium iron phosphate battery cell under different discharge conditions, multiple lithium iron phosphate battery cells from the same batch are used for discharge in step S101, and the discharge conditions of the multiple lithium iron phosphate battery cells are controlled to be a single factor change, and the discharge conditions include at least one of the discharge rate, discharge current, discharge temperature and clamp pressure, and other conditions are consistent with steps S101 to S601.

[0094] In addition, step S401 may further include the following steps: fixing the volume of the filtrate to obtain a volume of V (a) Step S501 may further comprise the following steps: using ICP-oes to test the concentration of Li element in solution a c Li(a) Step S601 may further include the following steps: according to the concentration c of the Li element in the solution a Li(a) Calculate the residual lithium content ω in the lithium iron phosphate positive electrode sheet Li Optionally, in step S601, ω Li =m Li(a) / m=c Li(a) *V (a) *M Li / m, where M Li is the molar mass of Li, in g / mol. It can be used to assess the proportion of lithium iron phosphate in the active material layer, or the representativeness of the sample mass m.

[0095] In a second aspect, an embodiment of the present invention provides a method for evaluating the lithium deposition state of a lithium battery negative electrode, comprising the following steps:

[0096] S102. After discharging the lithium iron phosphate battery cell, charging it for N cycles at the same rate and then disassembling it to obtain a first positive electrode sheet. The first positive electrode sheet is soaked in an organic solvent several times and then dried to obtain a first lithium iron phosphate positive electrode sheet.

[0097] Discharging another lithium iron phosphate battery cell from the same batch, performing charge and discharge for N cycles at the same rate, and then disassembling the cell to obtain a second positive electrode sheet; soaking the second positive electrode sheet in an organic solvent for multiple times and then drying the sheet to obtain a second lithium iron phosphate positive electrode sheet;

[0098] Discharging another lithium iron phosphate battery cell from the same batch, charging it at the same rate for N+x cycles, and then disassembling it to obtain a third positive electrode sheet; soaking the third positive electrode sheet in an organic solvent for multiple times and then drying it to obtain a third lithium iron phosphate positive electrode sheet;

[0099] Discharging another lithium iron phosphate battery cell from the same batch and performing charge and discharge for N+x cycles at the same rate, and then disassembling the battery cell to obtain a fourth positive electrode sheet; soaking the fourth positive electrode sheet in an organic solvent for multiple times and then drying the resulting sheet to obtain a fourth lithium iron phosphate positive electrode sheet;

[0100] Where N is greater than or equal to 1, x is greater than or equal to 1;

[0101] S202. Take samples of mass m from the first, second, third, and fourth lithium iron phosphate positive electrode sheets, respectively, and immerse the samples in water and allow them to stand at room temperature to separate the positive current collector from the positive active material layer.

[0102] S302. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain a residual flaky substance and a filtrate;

[0103] S402. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (a) Solution b;

[0104] S502. Use ICP-oes to test the concentration of Fe in solution b Fe(b) ;

[0105] S602. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ;

[0106] Among them, if the content of iron phosphide in the first lithium iron phosphate positive electrode plate is ω FeEqual to the content of iron phosphide in the third lithium iron phosphate positive electrode plate ω Fe , and the content of iron phosphide in the second lithium iron phosphate positive electrode sheet ω Fe Equal to the content of iron phosphide in the fourth lithium iron phosphate positive electrode plate ω Fe , it means that the negative electrode does not precipitate lithium. If the content of iron phosphide ωFe in the first lithium iron phosphate positive electrode is less than the content of iron phosphide ωFe in the third lithium iron phosphate positive electrode Fe , or the content of iron phosphide in the second lithium iron phosphate positive electrode sheet ω Fe Equal to the content of iron phosphide in the fourth lithium iron phosphate positive electrode plate ω Fe , which indicates that lithium is deposited on the negative electrode.

[0107] The evaluation method provided in the embodiment of the present invention evaluates the lithium deposition state of the negative electrode by the iron phosphide content in the lithium iron phosphate positive electrode sheet. The higher the iron phosphide content in the lithium iron phosphate positive electrode sheet, the more severe the lithium deposition of the corresponding negative electrode sheet. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode sheet, the milder the lithium deposition state of the corresponding negative electrode sheet. The evaluation method of the embodiment of the present invention can be used as a failure analysis tool for material analysis and evaluation, and can explore the boundaries of lithium deposition, providing technical support for battery research and development and process technology.

[0108] 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.

[0109] Optionally, in step S202, the time of standing at room temperature is 1 to 60 minutes.

[0110] Optionally, in step S302, the time of standing at room temperature is 10 to 240 minutes.

[0111] Optionally, in step S402, the temperature of the digestion reaction is 200-400°C, and the time of the digestion reaction is 10-60 minutes.

[0112] Optionally, in step S602, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

[0113] In some embodiments, step S502 further includes the following steps: using ICP-oes to measure the concentration c of the Li element in the solution b. Li(b) ; According to the concentration c of Li element in the solution b Li(b) JudgmentFe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate.

[0114] In addition, step S402 may further include the following steps: fixing the volume of the filtrate to obtain a volume of V (a) Step S502 may further comprise the following steps: using ICP-oes to test the concentration of Li element in solution a c Li(a) Step S602 may further include the following steps: according to the concentration c of the Li element in the solution a Li(a) Calculate the residual lithium content ω in the lithium iron phosphate positive electrode sheet Li Optionally, in step S601, ω Li =m Li(a) / m=c Li(a) *V (a) *M Li / m, where M Li is the molar mass of Li, in g / mol. It can be used to assess the proportion of lithium iron phosphate in the active material layer, or the representativeness of the sample mass m.

[0115] In a third aspect, an embodiment of the present invention provides a method for evaluating the lithium deposition state of a lithium battery negative electrode, comprising the following steps:

[0116] S103. The lithium iron phosphate battery is disassembled after being discharged, and the positive electrode sheet is taken, and the positive electrode sheet is soaked with an organic solvent several times and then dried to obtain a lithium iron phosphate positive electrode sheet;

[0117] S203. Samples of mass m are taken from different positions on the lithium iron phosphate positive electrode sheet, and the samples are immersed in water and allowed to stand at room temperature to separate the positive current collector from the positive active material layer;

[0118] S303. The positive electrode active material layer is immersed in hydrochloric acid and allowed to stand at room temperature, and then filtered to obtain a residual flaky substance and a filtrate;

[0119] S403. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b) Solution b;

[0120] S503. Use ICP-oes to test the concentration of Fe in solution b cFe(a) ;

[0121] S603. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ;

[0122] Wherein, if the content of iron phosphide in the lithium iron phosphate positive electrode sheet measured at different positions is ω Fe If the relative standard deviation is within 5%, it means that the negative electrode lithium deposition is consistent; on the contrary, if the content of iron phosphide in the lithium iron phosphate positive electrode piece measured at different positions is ω Fe If the relative standard deviation is greater than 5%, it means that the lithium deposition on the negative electrode is inconsistent.

[0123] The evaluation method provided in the embodiment of the present invention evaluates the lithium deposition state of the negative electrode by using the iron phosphide content in the lithium iron phosphate positive electrode sheet. The higher the iron phosphide content in the lithium iron phosphate positive electrode sheet, the more severe the lithium deposition of the corresponding negative electrode sheet. Conversely, the lower the iron phosphide content in the lithium iron phosphate positive electrode sheet, the milder the lithium deposition state of the corresponding negative electrode sheet. The evaluation method of the embodiment of the present invention can be used as a failure analysis tool for material analysis and evaluation, and can evaluate the uniformity of lithium deposition at different locations on the same negative electrode sheet, providing technical support for battery research and development and process technology.

[0124] Optionally, in step S101, 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.

[0125] Optionally, in step S203, the time of standing at room temperature is 1 to 60 minutes.

[0126] Optionally, in step S303, the time of standing at room temperature is 10 to 240 minutes.

[0127] Optionally, in step S403, the temperature of the digestion reaction is 200-400° C., and the time of the digestion reaction is 10-60 minutes.

[0128] Optionally, in step S603, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

[0129] In some embodiments, step S503 further includes the following steps: using ICP-oes to measure the concentration c of the Li element in the solution b. Li(b); According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate.

[0130] In addition, step S403 may further include the following steps: fixing the volume of the filtrate to obtain a volume of V (a) Step S503 may also include the following steps: using ICP-oes test solution a Li element concentration c Li(a) Step S603 may further include the following steps: according to the concentration c of the Li element in the solution a Li(a) Calculate the residual lithium content ω in the lithium iron phosphate positive electrode sheet Li Optionally, in step S603, ω Li =m Li(a) / m=c Li(a) *V (a) *M Li / m, where M Li is the molar mass of Li, in g / mol. It can be used to assess the proportion of lithium iron phosphate in the active material layer, or the representativeness of the sample mass m.

[0131] The present invention is described in detail below with reference to the embodiments.

[0132] Example 1 (normal evaluation process)

[0133] 1. Discharge the same batch of lithium iron phosphate batteries at the same cycle temperature using 1C, 1.5C, 2C, 2.5C, 3C, and 4C respectively. Then disassemble the batteries, take any part of the positive electrode, soak it in DMC, change the solution, soak it several times, and then dry it at 80℃ for later use.

[0134] 2. Weigh the same mass of the positive electrode sheet as in step 1 as a sample, and record the mass as m1~m6 (g) respectively. Place them in beakers 1~6 respectively, add 20mL of ultrapure water and immerse them for 30 minutes, then take out and rinse the detached aluminum foil.

[0135] 3. Add 20 mL of hydrochloric acid (GR grade, mass fraction 34-36%) to each of beakers 1 to 6 in step 2, let stand at room temperature for 30 minutes, and then transfer the solution to a fixed volume of V. (a) 100mL of solution a, numbered a1 to a6.

[0136] 4. Add 10 mL of perchloric acid (GR grade, mass fraction 70-72%) to the beaker in step 3 and heat on a flat plate heater for 30 minutes. Then cool and adjust the volume to V. (b) 100mL of solution b, numbered b1 to b6.

[0137] 5. Use ICP-oes to test the Li element concentration c in solution a Li(a) , Li element concentration c in solution b Li(b) , Fe element concentration c in solution b Fe(b) .

[0138] 6. According to the concentration c of Li element in solution a Li(a) Calculate the residual lithium content ω in the lithium iron phosphate positive electrode sheet Li ,

[0139] ω Li =(m Li(a) / m)*100%=(c Li(a) *V (a) *M Fe / m)*100%, where M Fe is the molar mass of Fe in g / mol.

[0140] According to the concentration c of Fe element in solution b Fe(b) Calculate the proportion of iron phosphide content in the mass of lithium iron phosphate positive electrode sheet ω Fe ,

[0141] ω Fe =(m Fe(b) / m)*100%=(c Fe(b) *V (b) *M Fe / m)*100%, where M Fe is the molar mass of Fe in g / mol.

[0142] 7. Iron phosphide content in lithium iron phosphate positive electrode sheet ω Fe The higher the value, the more lithium is deposited on the negative electrode. The iron phosphide content in the lithium iron phosphate positive electrode sheet is ω Fe The lower it is, the less lithium is deposited on the negative electrode.

[0143] Table 1. Test results of Example 1

[0144]

[0145] The data in Table 1 shows that the Li content in solution a decreases with increasing discharge current, while the Li content in solution b remains close to zero. The absence of lithium iron phosphate (LFP) affects the Fe content in test solution b. For the same batch of LFP cells, the proportion of iron phosphide in the LFP positive electrode increases with increasing charge and discharge current. This means that the higher the iron phosphide content in the LFP 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.

[0146] Example 2 (Searching for the lithium precipitation window, i.e., the state where the Li content in the positive electrode sheet remains the same before and after charge and discharge, i.e., the iron phosphide content remains unchanged)

[0147] 1. Discharge the same batch of lithium iron phosphate batteries at 1C and 3C at the same cycle temperature, and then charge and discharge them at the same rate. Disassemble the batteries before and after the cycle. Take any part of the positive electrode and soak it in DEC, then change the solution and soak it several times, and then dry it at 80℃ for use.

[0148] 2. Weigh the same mass of positive electrode sheets as in step 1, record the masses as m7~m10 (g), place them in beakers 7~10 respectively, add 20mL of ultrapure water and immerse them for 30 minutes, then take out and rinse the detached aluminum foil.

[0149] 3. Add 20 mL of hydrochloric acid (GR grade, mass fraction 34-36%) to each of beakers 7-10 in step 2, let it stand at room temperature for 30 minutes, and then transfer the solution to a volume of V. (a) Solution a, numbered a7 to a10.

[0150] 4. Add 10 mL of perchloric acid (GR grade, mass fraction 70-72%) to the beaker in step 3 and heat on a flat plate heater for 30 min. Then cool and dilute to the volume to obtain solution b, numbered b7-b10.

[0151] 5. Use ICP-oes to test the Li element concentration c in solution b Li(b) , Fe element concentration c in solution b Fe(b) .

[0152] 6. According to the concentration of Fe element c in solution b Fe(b) Calculate the proportion of iron phosphide content in the mass of lithium iron phosphate positive electrode sheet ω Fe ,

[0153] ω Fe =(m Fe(b) / m)*100%=(c Fe(b) *V (b) *M Fe / m)*100%, where M Fe is the molar mass of Fe in g / mol.

[0154] 7. Iron phosphide content in lithium iron phosphate positive electrode sheet ω Fe If the discharge cycle is the same before and after, there is no lithium precipitation at the negative electrode. If the iron phosphide content in the lithium iron phosphate positive electrode of the previous cycle is ω Fe Less than the iron phosphide content in the lithium iron phosphate positive electrode sheet in the next cycle ω Fe , then lithium is deposited on the negative electrode.

[0155] Table 2. Test results of Example 2

[0156]

[0157] It can be seen from the data in Table 2 that when a large current of 1C is used for charge and discharge, the iron phosphide content in the lithium iron phosphate positive electrode sheet in the previous cycle and the next cycle is ω Fe If the charge and discharge current is high at 3C, the iron phosphide content in the lithium iron phosphate positive electrode of the previous cycle is ω Fe Less than the iron phosphide content in the lithium iron phosphate positive electrode sheet in the next cycle ω Fe , indicating that some lithium ions are unable to return from the negative electrode to the positive electrode, indicating that lithium deposition has occurred. Therefore, 1C to 3C include the lithium deposition window of this lithium iron phosphate positive electrode. The evaluation method of Example 2 can be used to further try between 1C and 3C to find a more accurate lithium deposition window.

[0158] Example 3 (Lithium Plating Status Evaluation at Different Positions of the Same Cell Electrode, Consistency Evaluation)

[0159] 1. Discharge the same batch of lithium iron phosphate batteries at 3C at the same cycle temperature, then disassemble the batteries, take any part of the positive electrode plate, soak it in DMC, change the solution, soak it several times, and then dry it at 80℃ for later use.

[0160] 2. Weigh 6 groups of positive electrode sheets with the same mass but at different positions in step 1 as samples, and record their masses as m1 to m6 (g), respectively. Place them in beakers 1 to 6, add 20 mL of ultrapure water, and immerse them in water for 30 minutes. Then take out and rinse the detached aluminum foil.

[0161] 3. Add 20 mL of hydrochloric acid (GR grade, mass fraction 34-36%) to each of beakers 1 to 6 in step 2, let stand at room temperature for 30 minutes, and then transfer the solution to a fixed volume of V. (a) 100mL of solution a, numbered a1 to a6.

[0162] 4. Add 10 mL of perchloric acid (GR grade, mass fraction 70-72%) to the beaker in step 3 and heat on a flat plate heater for 30 minutes. Then cool and adjust the volume to V. (b) 100mL of solution b, numbered b1 to b6.

[0163] 5. Use ICP-oes to test the Li element concentration c in solution a Li(a) , Li element concentration c in solution b Li(b) , Fe element concentration c in solution b Fe(b) .

[0164] 6. According to the concentration of Fe element c in solution b Fe(b) Calculate the proportion of iron phosphide content in the mass of lithium iron phosphate positive electrode sheet ω Fe ,

[0165] ω Fe =(m Fe(b) / m)*100%=(c Fe(b) *V (b) *M Fe / m)*100%, where M Fe is the molar mass of Fe in g / mol.

[0166] 7. Pay attention to the iron phosphide content in several groups of lithium iron phosphate positive electrode sheets Fe If the relative standard deviation is within 5%, the negative electrode sheet state is consistent and the negative electrode lithium deposition is consistent; if it is greater than 5%, the negative electrode sheet state is inconsistent and the negative electrode lithium deposition is inconsistent.

[0167] Table 3. Test results of Example 3

[0168]

[0169] It can be seen from the data in Table 3 that the consistency of the negative electrode sheet of this battery cell is 1.8% at a current of 3C, and the lithium deposition state of the negative electrode sheet is consistent.

[0170] Comparative Example 1 (CN113093029A, Lithium Content Measurement by Powder Scraping of Electrode)

[0171] 1. Take the lithium iron phosphate battery cell of Example 3, take 2 to 10 cm long negative electrode sheets respectively, fully charge them at 0.1C or 0.2C, and then disassemble them to form test samples a and b. Test sample b is soaked in DMC, the solution is changed, and then soaked several times, and then dried at 80°C for use.

[0172] 2. Take 2.0 g of each of the test sample a and reference sample b obtained in the first step and scrape them into powder. Place the scraped active substances in different beakers, weigh them, and record the mass of the active substances.

[0173] 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 it. After cooling, filter and adjust the volume.

[0174] 4. Determine the lithium content of the sample after digestion treatment, and use inductively coupled plasma emission spectrometry to determine the lithium content of the solution after volume adjustment. Then calculate the mass of lithium in the solution after volume adjustment, and record the results as X1 and X2 (g).

[0175] The test results are as follows:

[0176] Table 4. Test results of Comparative Example 1

[0177]

[0178] Comparing data X1 in Table 4 with Example 3, the lack of organic solvent immersion results in a higher lithium content test value. This is due to the influence of residual lithium hexafluorophosphate (LiPF6) in the electrolyte on the negative electrode plate, which can even exceed the total lithium content of the active material. Compared with Example 3, data X2, immersion in an organic solvent results in a lower lithium content test value. This is because when the organic solvent is used to clean the negative electrode plate electrolyte, the negative electrode lithium reacts with the water in the organic solvent, resulting in loss of the negative electrode lithium, which in turn leads to a lower test result. Therefore, the method of measuring lithium content from the negative electrode plate and comparing it with lithium ions transferred during constant current charging at the positive electrode has flaws and requires further investigation.

[0179] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0180] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A method for evaluating the lithium deposition state of a lithium battery negative electrode, characterized in that: The following steps are involved: S101. The lithium iron phosphate battery cell is disassembled after being discharged to obtain a positive electrode sheet, and the positive electrode sheet is soaked with an organic solvent several times and then dried to obtain a lithium iron phosphate positive electrode sheet; S201. Take a sample of mass m from the lithium iron phosphate positive electrode sheet and immerse the sample in water and allow it to stand at room temperature to separate the positive current collector from the positive 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 a residual flaky substance and a filtrate; S401. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b) Solution b; S501. Use ICP-oes to test the concentration of Fe in solution b. Fe ; S601. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ; Among them, ω Fe The larger the value, the more lithium is deposited on the negative electrode. On the contrary, Fe The smaller it is, the less lithium is deposited on the negative electrode.

2. The evaluation method according to claim 1, wherein: In step S201, the time of standing at room temperature is 1 to 60 minutes; Optionally, in step S301, the time of standing at room temperature is 10 to 240 minutes; Optionally, in step S401, the temperature of the digestion reaction is 200-400° C., and the time of the digestion reaction is 10-60 minutes; Optionally, in step S601, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

3. The evaluation method according to claim 1, wherein: Step S501 also includes the following steps: using ICP-oes to test the concentration c of the Li element in the solution b Li(b) ; According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate.

4. The evaluation method according to any one of claims 1 to 3, characterized in that: When it is necessary to compare the negative electrode lithium deposition states of different lithium iron phosphate battery cells, different lithium iron phosphate battery 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 deposition states of the same lithium iron phosphate battery cell under different discharge conditions, multiple lithium iron phosphate battery cells of the same batch are used for discharge in step S101, and the discharge conditions of the multiple lithium iron phosphate battery cells are controlled to be single-factor changes, and the discharge conditions include at least one of the discharge rate, discharge power, discharge current, discharge temperature and clamp pressure, and other conditions are consistent with steps S101 to S601.

5. A method for evaluating the lithium deposition state of a lithium battery negative electrode, characterized in that: The following steps are involved: S102. After discharging the lithium iron phosphate battery cell, charging it for N cycles at the same rate and then disassembling it to obtain a first positive electrode sheet. The first positive electrode sheet is soaked in an organic solvent several times and then dried to obtain a first lithium iron phosphate positive electrode sheet. Discharging another lithium iron phosphate battery cell from the same batch, performing charge and discharge for N cycles at the same rate, and then disassembling the cell to obtain a second positive electrode sheet; soaking the second positive electrode sheet in an organic solvent for multiple times and then drying the sheet to obtain a second lithium iron phosphate positive electrode sheet; Discharging another lithium iron phosphate battery cell from the same batch, charging it at the same rate for N+x cycles, and then disassembling it to obtain a third positive electrode sheet; soaking the third positive electrode sheet in an organic solvent for multiple times and then drying it to obtain a third lithium iron phosphate positive electrode sheet; Discharging another lithium iron phosphate battery cell from the same batch and performing charge and discharge for N+x cycles at the same rate, and then disassembling the battery cell to obtain a fourth positive electrode sheet; soaking the fourth positive electrode sheet in an organic solvent for multiple times and then drying the resulting sheet to obtain a fourth lithium iron phosphate positive electrode sheet; Where N is greater than or equal to 1, x is greater than or equal to 1; S202. Take samples of mass m from the first, second, third, and fourth lithium iron phosphate positive electrode sheets, respectively, and immerse the samples in water and allow them to stand at room temperature to separate the positive current collector from the positive 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 a residual flaky substance and a filtrate; S402. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b) Solution b; S502. Use ICP-oes to test the concentration of Fe in solution b Fe(b) ; S602. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ; Among them, if the content of iron phosphide in the first lithium iron phosphate positive electrode plate is ω Fe Equal to the content of iron phosphide in the third lithium iron phosphate positive electrode plate ω Fe , and the content of iron phosphide in the second lithium iron phosphate positive electrode sheet ω Fe Equal to the content of iron phosphide in the fourth lithium iron phosphate positive electrode plate ω Fe , it means that the negative electrode does not precipitate lithium. If the content of iron phosphide ωFe in the first lithium iron phosphate positive electrode is less than the content of iron phosphide ωFe in the third lithium iron phosphate positive electrode Fe , or the content of iron phosphide in the second lithium iron phosphate positive electrode sheet ω Fe Equal to the content of iron phosphide in the fourth lithium iron phosphate positive electrode plate ω Fe , which indicates that lithium is deposited on the negative electrode.

6. The evaluation method according to claim 5, characterized in that In step S202, the time of standing at room temperature is 1 to 60 minutes; Optionally, in step S302, the time of standing at room temperature is 10 to 240 minutes; Optionally, in step S402, the temperature of the digestion reaction is 200-400°C, and the time of the digestion reaction is 10-60 minutes; Optionally, in step S602, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

7. The evaluation method according to claim 5, characterized in that Step S502 also includes the following steps: using ICP-oes to test the concentration c of the Li element in the solution b. Li(b) ; According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate.

8. A method for evaluating the lithium deposition state of a lithium battery negative electrode, characterized in that: The following steps are involved: S103. The lithium iron phosphate battery is disassembled after being discharged, and the positive electrode sheet is taken, and the positive electrode sheet is soaked with an organic solvent several times and then dried to obtain a lithium iron phosphate positive electrode sheet; S203. Samples of mass m are taken from different positions on the lithium iron phosphate positive electrode sheet, and the samples are immersed in water and allowed to stand at room temperature to separate the positive current collector from the positive 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 a residual flaky substance and a filtrate; S403. After the residual flaky material is mixed with perchloric acid, a digestion reaction is carried out under heating conditions to obtain a digestion solution; the digestion solution is then cooled and constant to obtain a volume of V (b) Solution b; S503. Use ICP-oes to test the concentration of Fe in solution b c Fe(b) ; S603. According to the concentration c of Fe element in the solution b Fe(b) Calculate the content of iron phosphide in the lithium iron phosphate positive electrode plate ω Fe ; Wherein, if the content of iron phosphide in the lithium iron phosphate positive electrode sheet measured at different positions is ω Fe If the relative standard deviation is within 5%, it means that the negative electrode lithium deposition is consistent; on the contrary, if the content of iron phosphide in the lithium iron phosphate positive electrode piece measured at different positions is ω Fe If the relative standard deviation is greater than 5%, it means that the lithium deposition on the negative electrode is inconsistent.

9. The evaluation method according to claim 8, characterized in that In step S203, the time of standing at room temperature is 1 to 60 minutes; Optionally, in step S303, the time of standing at room temperature is 10 to 240 minutes; Optionally, in step S403, the temperature of the digestion reaction is 200-400° C., and the time of the digestion reaction is 10-60 minutes; Optionally, in step S603, ω Fe =m Fe(b) / m=c Fe(b) *V (b) *M Fe / m, where M Fe is the molar mass of Fe in g / mol.

10. The evaluation method according to claim 8, characterized in that Step S503 also includes the following steps: using ICP-oes to test the concentration c of the Li element in the solution b. Li(b) ; According to the concentration c of Li element in the solution b Li(b) Judgment Fe Whether the test results are affected by lithium iron phosphate; Among them, when c Li(b) When it is 0, it means that there is no Li element in the solution b, ω Fe The test results are not affected by lithium iron phosphate; on the contrary, when c Li(b) When it is not 0, it means that the solution b contains Li element, ω Fe The test results are affected by lithium iron phosphate.

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