Method for testing content of lithium element in negative electrode SEI film

Abstract

The invention belongs to the field of lithium ion batteries, and particularly provides a method for testing the content of a lithium element in a negative electrode SEI film. According to the test method, the lithium content of the SEI in the negative pole piece is obtained by deducting the lithium content of the residual lithium salt and the content of the active lithium from the total lithium content of the negative pole piece. According to the testing method, lithium salt and active lithium left on the pole piece by the electrolyte are considered, the wettability difference of the pole piece and the dynamic difference of the active material can be eliminated, the sensitivity and precision of testing the lithium content in the SEI film are improved, and the difference of the lithium content of the SEI films of different samples can be distinguished.

Description

Technical Field

[0001] This application belongs to the field of lithium-ion batteries, and particularly relates to a method for testing the lithium element content in the negative electrode SEI film. Background Art

[0002] The SEI film is a passivation film spontaneously formed on the surface of the negative electrode during the first charge and discharge process of a lithium-ion battery. It is composed of the reduction decomposition products of the electrolyte, and its main function is to prevent the electrolyte from further reacting with the negative electrode material while allowing lithium ions to pass through. The properties of the SEI film directly affect the Coulomb efficiency, cycle stability, and safety of the battery, and it is an important research object for battery performance optimization. The lithium content in the SEI film is an important indicator for evaluating its key properties such as ionic conductivity, chemical stability, passivation effect, as well as film thickness and uniformity. However, the lithium content in the SEI film is relatively low, and high-sensitivity and high-precision testing methods are required to accurately measure it. Moreover, due to the differences in the wettability of the electrolyte and the electrode sheet and the kinetics of the electrode active material, there are slight differences in the lithium content in the SEI film of different battery samples, and more high-precision and high-sensitivity measurement methods are needed.

[0003] Currently, the methods for determining the lithium content in the SEI film include X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Although these techniques can provide detailed elemental information, quantitative analysis usually requires standard samples, and the testing process may damage the samples. The SEI film is thin, and it is difficult to obtain standard samples, resulting in the inability to accurately obtain the lithium content in the SEI film. Nuclear magnetic resonance (NMR) is also an optional method that can provide information on the chemical environment of lithium. However, NMR has high requirements for the sample amount and purity, and relatively low sensitivity. Although neutron diffraction technology has high sensitivity to lithium, a light element, it requires large equipment, complex sample preparation, and limited resolution. Chemical titration or potentiometric titration methods are widely used for their simple operation and low cost, but their sensitivity and precision are relatively low.

[0004] Therefore, the current methods for testing the lithium content in the SEI film have low sensitivity and precision, and cannot exclude the differences in electrode sheet wettability and active material kinetics, making it impossible to compare the differences in the lithium content in the SEI film of different samples. Therefore, there is an urgent need to develop a method for detecting the lithium content in a low-concentration SEI film with high sensitivity, high precision, and relatively low cost. Summary of the Invention

[0005] In view of the above problems existing in the prior art, this application provides a method for testing the lithium element content in the negative electrode SEI film to improve the accuracy and precision of measuring the lithium content in the negative electrode SEI film.

[0006] To achieve the above object, this application provides a method for testing the lithium element content in the negative electrode SEI film, including the following steps:

[0007] (1) Disassemble a lithium-ion battery without a formed SEI film and obtain the negative electrode sheet. Immerse the negative electrode sheet with an organic solvent, test the mass of lithium element in the negative electrode sheet after immersion, and obtain the mass m of lithium element contained in the lithium salt in the negative electrode sheet. 1 ;

[0008] (2) Charge or discharge the lithium-ion battery after the SEI film is formed, then disassemble the lithium-ion battery and obtain the negative electrode sheet. Immerse the negative electrode sheet with an organic solvent, test the mass of lithium element in the negative electrode sheet after immersion, and obtain the total mass m of lithium element in the negative electrode sheet. 0 ;

[0009] (3) Assemble the negative electrode sheet after immersion in step (2) with a lithium metal sheet into a button cell, test the actual capacity C of the button cell. 1 , and calculate the mass m of active lithium element in the negative electrode sheet according to the following formula. 2 ,

[0010]

[0011] Among them, m 2 is the mass of active lithium element in the negative electrode sheet, with the unit of mg; C 1 is the actual capacity of the button cell, with the unit of mAh; C 0 is the theoretical specific capacity of graphite, with the unit of mAh / g; M 0 is the molar mass of graphite, with the unit of g / mol; M 1 is the molar mass of lithium element, with the unit of g / mol;

[0012] (4) Calculate the mass m of lithium element in the negative electrode SEI film and its mass percentage content w according to the following formula.

[0013] m = m 0 - m 1 - m 2 (Equation 2),

[0014]

[0015] Among them, m is the mass of lithium element in the SEI film, with the unit of mg; w is the mass percentage content of lithium element in the SEI film, with the unit %; m 1 is the mass of lithium element contained in the lithium salt in the negative electrode sheet, with the unit of mg; m 2 is the mass of active lithium element in the negative electrode sheet, with the unit of mg; m 0 is the total mass of lithium element in the negative electrode sheet, with the unit of mg.

[0016] Before the battery is formed, no SEI film is formed in the negative electrode sheet. The electrolyte infiltrates the negative electrode sheet and lithium salts remain on it. After the battery is formed, an SEI film is formed in the negative electrode sheet. At this time, the total lithium content on the negative electrode sheet includes the lithium content of the residual lithium salts, the lithium content constituting the SEI, and the active lithium content. In the method of the present application, by testing the lithium content of the negative electrode sheet in a lithium-ion battery without an SEI film formed, the lithium content of the residual lithium salts is obtained; by disassembling the SEI film-containing battery after charging or discharging to take the negative electrode sheet and forming a button battery with a lithium metal sheet, testing the actual capacity of the button battery, and converting the actual capacity of the button battery to obtain the content of active lithium in the battery. Finally, the lithium content of the residual lithium salts and the content of active lithium are deducted from the total lithium content on the negative electrode sheet to obtain the lithium content of the SEI film in the actual negative electrode sheet. The method of the present application takes into account the lithium salts and active lithium remaining in the electrolyte on the electrode sheet, can exclude the differences in electrode sheet wettability and active material kinetics, improve the sensitivity and accuracy of testing the lithium content in the SEI film, and can distinguish the differences in the lithium content of the SEI films of different samples.

[0017] In some embodiments, in steps (1) and (2), each of the organic solvents is independently selected from at least one of dimethyl carbonate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, or diethyl carbonate.

[0018] In some embodiments, in steps (1) and (2), the impregnation time is independently selected from 10 min to 60 min, and the number of impregnation times is independently selected from 1 to 3 times.

[0019] In some embodiments, in step (2), the lithium-ion battery after forming the SEI film includes a lithium-ion battery that has been formed.

[0020] In some embodiments, in step (2), the lithium-ion battery after forming the SEI film further includes at least one of a lithium-ion battery that has undergone capacity testing, a lithium-ion battery that has been stored, or a lithium-ion battery that has been cycled.

[0021] In some embodiments, in step (2), the negative electrode sheet in the lithium-ion battery after forming the SEI film includes a current collector and a negative electrode active material layer loaded on the current collector. The negative electrode active material layer includes graphite, and the current collector includes copper foil.

[0022] In some embodiments, in steps (1) and (2), the method for testing the mass of lithium element in the negative electrode sheet after impregnation includes an inductively coupled plasma analysis method.

[0023] In some embodiments, in step (2), the discharging process is as follows:

[0024] (2-1) Leave the lithium-ion battery after forming the SEI film standing for 2 min to 10 min;

[0025] (2-2) Then discharge at a current rate of 0.05C to 0.1C until V 1 ;

[0026] (2-3) Leave it standing for 2 min to 10 min;

[0027] (2-4) Discharge at a current rate of 0.005C to 0.01C until V 2 ;

[0028] (2-5) Repeat the process of steps (2-3) and (2-4) 1 to 5 times.

[0029] In some embodiments, the V 1 and V 2 are the lower voltage values of the lithium-ion battery after forming the SEI film.

[0030] In some embodiments, in step (2), the charging process is as follows:

[0031] (2-1) Leave the lithium-ion battery after forming the SEI film standing for 2 min to 10 min;

[0032] (2-2) Discharge at a current rate of 0.05C to 0.1C until the lower limit voltage designed for the lithium-ion battery after forming the SEI film;

[0033] (2-3) Charge at a current rate of 0.05C to 0.1C until the voltage value V 3 ;

[0034] (2-4) Leave it standing for 2 min to 10 min;

[0035] (2-5) Charge at a current rate of 0.005C to 0.01C until the voltage value V 4 ;

[0036] (2-6) Leave it standing for 1 min to 5 min.

[0037] In some embodiments, in step (3), the process of testing the actual capacity C 1 of the button cell is as follows: Leave the button cell standing for 2 min to 10 min, charge it at a constant current I to the cut-off voltage, leave it standing for 2 min to 10 min, and record the curve of the battery voltage changing with the charging time t, and obtain the actual capacity C 1 of the button cell according to the product of the charging current I and the charging time t.

[0038] Advantages of the present application: In the test method of the present application, the lithium content in the SEI of the actual negative electrode sheet is obtained by deducting the lithium content of the residual lithium salt and the content of active lithium from the total lithium content on the negative electrode sheet. The test method of the present application takes into account the lithium salt and active lithium remaining in the electrolyte on the electrode sheet, can exclude the differences in electrode sheet wettability and active material kinetics, improve the sensitivity and accuracy of testing the lithium content in the SEI film, and can distinguish the differences in the lithium content of the SEI films of different samples. Detailed implementation manners

[0039] To better illustrate the purpose, technical solution and advantages of the present application, the technical solution of the present application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The related embodiments described herein are illustrative in nature and are used to provide a basic understanding of the present application. And the embodiments of the present application should not be construed as a limitation of the present application.

[0040] For the sake of brevity, only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recited. In addition, each individually disclosed point or single numerical value itself can be used as a lower limit or an upper limit and combined with any other point or single numerical value or combined with other lower limits or upper limits to form a range not explicitly recited.

[0041] In the description herein, unless otherwise specified, "above" and "below" include the number itself. Unless otherwise specified, the terms used in the present application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in the present application can be measured by various measurement methods commonly used in the art. In the description herein, a list of items connected by the terms "at least one of", "at least one item of", "at least one of", "at least one kind of" or other similar terms may mean any combination of the listed items.

[0042] In order to improve the accuracy and precision of measuring the lithium content in the negative electrode SEI film. The present application provides a test method for the lithium element content in the negative electrode SEI film, including the following steps:

[0043] (1) Disassemble a lithium-ion battery without an SEI film formed and obtain the negative electrode sheet, impregnate the negative electrode sheet with an organic solvent, and test the mass of lithium element in the impregnated negative electrode sheet to obtain the mass m of lithium element contained in the lithium salt in the negative electrode sheet 1 ;

[0044] (2) Charge or discharge the lithium-ion battery after the SEI film is formed, then disassemble the lithium-ion battery and obtain the negative electrode sheet. Immerse the negative electrode sheet with an organic solvent, test the mass of lithium element in the negative electrode sheet after immersion, and obtain the total mass m of lithium element in the negative electrode sheet. 0 ;

[0045] (3) Then assemble the negative electrode sheet after immersion in step (2) with a lithium metal sheet into a button cell, and test the actual capacity C of the button cell. 1 , and calculate the mass m of the active lithium element in the negative electrode sheet according to the following formula. 2 ,

[0046]

[0047] where m 2 is the mass of the active lithium element in the negative electrode sheet, with the unit of mg; C 1 is the actual capacity of the button cell, with the unit of mAh; C 0 is the theoretical specific capacity of graphite, with the unit of mAh / g; M 0 is the molar mass of graphite, with the unit of g / mol; M 1 is the molar mass of lithium element, with the unit of g / mol.

[0048] (4) Calculate the mass m and mass percentage content w of lithium element in the negative electrode SEI film according to the following formula.

[0049] m = m 0 - m 1 - m 2 (Equation 2).

[0050]

[0051] where m is the mass of lithium element in the SEI film, with the unit of mg; w is the mass percentage content of lithium element in the SEI film, with the unit of %; m 1 is the mass of lithium element contained in the lithium salt in the negative electrode sheet, with the unit of mg; m 2 is the mass of the active lithium element in the negative electrode sheet, with the unit of mg; m 0 is the total mass of lithium element in the negative electrode sheet, with the unit of mg.

[0052] Before the battery formation, no SEI film is formed on the negative electrode sheet. The electrolyte infiltrates the negative electrode sheet and lithium salts remain on it. After the battery formation, an SEI film is formed on the negative electrode sheet. At this time, the total lithium content on the negative electrode sheet includes the lithium content of the residual lithium salts, the lithium content constituting the SEI, and the active lithium content. In the method of the present application, by testing the lithium content of the negative electrode sheet in a lithium-ion battery without an SEI film formed, the lithium content of the residual lithium salts is obtained; by disassembling a lithium-ion battery with an SEI film after charging or discharging to take the negative electrode sheet and forming a button battery with a lithium metal sheet, testing the actual capacity of the button battery, and calculating the content of active lithium in the battery through the actual capacity of the button battery. Finally, the lithium content of the residual lithium salts and the active lithium content are deducted from the total lithium content on the negative electrode sheet to obtain the lithium content of the SEI in the actual negative electrode sheet. The method of the present application takes into account the lithium salts and active lithium remaining in the electrolyte on the electrode sheet, can exclude the differences in electrode sheet wettability and active material kinetics, improve the sensitivity and accuracy of testing the lithium content in the SEI film, and can distinguish the differences in the lithium content of the SEI films of different samples.

[0053] In some embodiments, in steps (1) and (2), the organic solvent includes an ester organic solvent.

[0054] In some embodiments, in steps (1) and (2), the organic solvents are each independently selected from at least one of dimethyl carbonate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, or diethyl carbonate.

[0055] Using an organic solution, especially an ester organic solvent, to impregnate and clean the negative electrode sheet can remove some of the residual lithium salts on it from the negative electrode sheet. However, since the electrode sheet is generally a porous material, even after impregnation and cleaning with an organic solvent, a small amount of lithium salts will still remain, forming residual lithium salts. Measuring the lithium content of this part of the lithium salts can deduct it from the negative electrode sheet. This avoids the situation in the conventional method of testing the SEI film where this part of the lithium content is not deducted, resulting in the measured lithium content of the SEI being higher than the actual lithium content of the SEI.

[0056] In some embodiments, in steps (1) and (2), the impregnation time is each independently selected from 10 min to 60 min. Specifically, it can be 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, or the range composed of any two of these values.

[0057] In some embodiments, the number of impregnations is each independently selected from 1 to 3 times. Specifically, it can be 1 time, 2 times, or 3 times.

[0058] In some embodiments, in step (2), the lithium-ion battery after forming the SEI film includes a lithium-ion battery that has undergone formation.

[0059] Generally, when the battery is formed, the SEI film is formed on the negative electrode. Therefore, the method of the present application is applicable to the formed lithium-ion battery.

[0060] The SEI film formed during battery formation exists during the capacity test, storage, or cycling state after formation. Therefore, the method of the present application for measuring the SEI film can be applicable to batteries in different states.

[0061] In some embodiments, the formation conditions are as follows: the current rate is 0.5C to 1.5C, the temperature is 40°C to 48°C, and the pressure is 0.8MP to 1.5MP. In some embodiments, the conditions for the capacity test are: the current rate is 0.5C to 1.5C, and the test is conducted at room temperature.

[0062] In some embodiments, the storage conditions are: storing at a voltage state of 55% to 65% SOC for 0.5 years to 3 years.

[0063] In some embodiments, in step (2), the negative electrode plate in the lithium-ion battery after forming the SEI film includes a current collector and a negative electrode active material layer loaded on the current collector, and the negative electrode active material layer includes graphite.

[0064] In some embodiments, the current collector includes a copper foil.

[0065] When the current collector in the battery negative electrode plate is a copper foil, at a test voltage above 3.6V, the copper foil of the current collector will be corroded, resulting in battery short circuit, causing the battery voltage to drop. If the capacity is tested, part of the capacity comes from the dissolution of the copper foil, resulting in a deviation in the measured capacity of the active material layer in the negative electrode plate. Therefore, when disassembling the lithium-ion battery and assembling the obtained negative electrode plate with a lithium metal sheet into a button battery to test its actual capacity, the actual capacity should be the capacity below 3.6V to avoid deviation in the measured actual capacity of the button battery due to dissolution of the copper foil current collector caused by too high voltage, resulting in inaccurate measurement of the active lithium content.

[0066] In some embodiments, in steps (1) and (2), the method for testing the mass of lithium element in the impregnated negative electrode plate includes inductively coupled plasma analysis method (ICP).

[0067] Compared with other methods, using the ICP method to test the lithium dissolved from the solvent-impregnated negative electrode plate is more convenient, with higher sensitivity and accuracy.

[0068] In some embodiments, in step (2), the discharging process is as follows:

[0069] (2-1) Let the lithium-ion battery after forming the SEI film stand for 2 min to 10 min;

[0070] (2-2) Then discharge at a current rate of 0.05C to 0.1C until the voltage reaches V 1 ;

[0071] (2-3) Let it stand for 2 min to 10 min;

[0072] (2-4) Discharge at a current rate of 0.005C to 0.01C until the voltage reaches V 2 ;

[0073] (2-5) Repeat the process of steps (2-3) and (2-4) 1 to 5 times.

[0074] Discharging at a small current rate multiple times can make the battery discharge fully and improve the accuracy of SEI film measurement.

[0075] In some embodiments, the V 1 is less than or equal to the lower limit voltage of the lithium-ion battery after forming the SEI film.

[0076] In some embodiments, the V 2 is less than or equal to the lower limit voltage of the lithium-ion battery after forming the SEI film.

[0077] Different types of lithium-ion batteries have different lower limit voltages. For example, for a lithium-ion battery with lithium cobaltate active material, its lower limit voltage is 2.5V to 3.0V.

[0078] In some embodiments, the V 1 and V 2 are each independently selected from 2.5V to 3V. Specifically, it can be 2.5V, 2.6V, 2.7V, 2.8V, 2.9V, 3.0V, or a range composed of any two of these values.

[0079] In some embodiments, in step (2), the discharge is a full discharge.

[0080] In some embodiments, in step (2), the discharge is a constant current discharge.

[0081] The states of the SEI film in different discharge states have slight differences. The method of the present application can test the SEI film in different discharge states and has high universality.

[0082] In some embodiments, in step (2), the charging process is as follows:

[0083] (2-1) Let the lithium-ion battery after forming the SEI film stand for 2 min to 10 min;

[0084] (2-2)Discharge the lithium-ion battery designed after forming the SEI film to the lower limit voltage at a current rate of 0.05C to 0.1C;

[0085] (2-3)Charge to a voltage value V at a current rate of 0.05C to 0.1C 3 ;

[0086] (2-4)Let it stand for 2 min to 10 min;

[0087] (2-5)Charge to a voltage value V at a current rate of 0.005C to 0.01C 4 ;

[0088] (2-6)Let it stand for 1 min to 5 min.

[0089] In some embodiments, the V 3 is less than or equal to the upper limit voltage of the lithium-ion battery after forming the SEI film.

[0090] In some embodiments, the V 4 is less than or equal to the upper limit voltage of the lithium-ion battery after forming the SEI film.

[0091] In some embodiments, in step (3), the process for testing the actual capacity C 1 of the button cell is as follows: Let the button cell stand for 2 min to 10 min, charge it to the cut-off voltage at a constant current charging current I, let it stand for 2 min to 10 min, and record the curve of the battery voltage changing with the charging time t, and obtain the actual capacity C 1 of the button cell according to the product of the charging current I and the charging time t.

[0092] In some embodiments, the negative electrode of the button cell is a lithium metal sheet, and the positive electrode of the button cell is graphite.

[0093] In some embodiments, the I is 8 μA to 12 μA. Specifically, it can be 8 μA, 9 μA, 10 μA, 11 μA, 12 μA, or the range composed of any two of these values.

[0094] In some embodiments, the cut-off voltage is 3.5V to 4.5V. Specifically, it can be 3.5V, 3.6V, 3.7V, 3.8V, 3.9V, 4V, 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, or the range composed of any two of these values.

[0095] In some embodiments, when the current collector in the negative electrode sheet of the lithium-ion battery after forming the SEI film is a copper foil, the cut-off voltage is 3.55 to 3.6V. Specifically, it can be 3.55V, 3.56V, 3.57V, 3.58V, 3.59V, 3.6V, or the range composed of any two of these values.

[0096] In some embodiments, the actual capacity C of the coin cell 1 is selected from the capacity of the charging process of the coin cell below 3.6V.

[0097] To better illustrate the purpose, technical solution and advantages of the present application, the present application will be further described below through specific comparative examples and examples.

[0098] Example 1

[0099] A method for testing the lithium content of an SEI film includes the following steps:

[0100] In this embodiment, the tested cell sample is a wound soft lithium cobaltate (using aluminum foil as the positive current collector) / graphite (using copper foil as the negative current collector) wrapped lithium-ion battery, with a 1C capacity of 5920 mAh. Under the condition of full discharge after formation to be evaluated, the lithium content in the SEI film in this case is to be tested. The battery testing device is a conventional charge-discharge instrument, and in this embodiment, a Neware 5V / 6A charge test device is used.

[0101] (1) Obtain 6 packaged cells. Among them, 3 cells are left standing at room temperature for 24h without formation (without forming the SEI film), and are named group A; the other 3 cells are left standing at room temperature for 24h and then subjected to formation treatment, with a formation current of 1C, a formation temperature of 45°C, and a formation pressure of 1MPa, and are named group B, and an SEI film is formed on the negative electrode sheet of the cells.

[0102] (2) Disassemble the cells in group A in a drying room, take the negative electrode sheet, impregnate and clean it with dimethyl carbonate (DMC) for 30min, repeat the impregnation and cleaning 3 times, and then take the impregnated negative electrode sheet for ICP testing to test the mass of lithium element in it, and obtain the mass m of lithium element contained in the lithium salt in the negative electrode sheet 1 = 0.01mg, which is the lithium remaining as the lithium salt residue;

[0103] (3) Discharge the cells in group B. Discharge conditions: S1) The cells are left standing for 5min; S2) Constant current discharge at a current rate of 0.1C until the lower limit voltage of the cells is 3V; S3) The cells are left standing for 5min; S4) Constant current discharge at a current rate of 0.01C until the lower limit voltage of the cells is 3V; S5) Repeat the process of steps S3) and S4) in sequence 3 times;

[0104] (4) Disassemble the battery cells in Group B in a drying room, take the negative electrode plates, impregnate and clean them with DMC for 30 min. After repeating the impregnation and cleaning three times, take the impregnated negative electrode plates for ICP testing to measure the mass of lithium element in them, and obtain the total mass m of lithium element in the negative electrode plates. 0 = 0.21 mg, which is the sum of the lithium content m 1 remaining as lithium salt, the content m of active lithium not released due to polarization in the negative electrode 2 and the irreversible lithium content m in the SEI film.

[0105] (5) Assemble the negative electrode plates of the battery cells in Group B obtained by disassembling, impregnating and cleaning in step (4) with lithium sheets into coin cells; after leaving the coin cells standing for 4 h, test their actual capacity C 1 , and the conditions during the testing process are as follows: 1) Stand for 5 min; 2) Constant current charge to 4 V at a current of 10 μA; 3) Stand for 5 min; and record the curve of the battery voltage changing with the charging time t, and obtain the actual capacity C of the coin cell according to the product of the charging current I and the charging time t. 1 The test results show that during the process of charging to 4 V, around 1 - 1.5 h of charging (voltage above 3.6 V), the battery starts to drop voltage. It is speculated that the corrosion of the negative electrode current collector Cu causes the battery to short - circuit, resulting in a decrease in the battery voltage, and part of the capacity comes from the contribution of the dissolution of Cu. Therefore, the capacity below 3.6 V should be selected, and C 1 = 0.0087 mAh.

[0106] Convert the capacity C 1 of the coin cell and the mass m 2 of active lithium according to the following formula:

[0107]

[0108] where m 2 is the mass of active lithium element in the negative electrode plate, with the unit of mg; C 1 is the actual capacity of the coin cell, with the unit of mAh; C 0 is the theoretical specific capacity of graphite, with the unit of mAh / g; M 0 is the molar mass of graphite, with the unit of g / mol; M 1 is the molar mass of lithium element, with the unit of g / mol; 6 represents that every 6 carbon atoms correspond to 1 lithium ion (LiC 6 ); among them, M 1 = 6.941 g / mol, M 0 = 12.01 g / mol, C 0 = 372 mAh / g;

[0109] (6) Calculate the mass \(m\) and mass percentage content \(w(\%)\) of lithium element in the SEI film according to the following formula:

[0110] m = m 0 -m 1 -m 2 (Formula 2),

[0111]

[0112] where \(m\) is the mass of lithium element in the SEI film, with the unit of mg; \(w\) is the mass percentage content (\%) of lithium element in the SEI film; \(m\) 1 is the mass of lithium element contained in the lithium salt in the negative electrode tab, with the unit of mg; \(m\) 2 is the mass of active lithium element in the negative electrode tab, with the unit of mg; \(m\) 0 is the total mass of lithium element in the negative electrode tab, with the unit of mg.

[0113] Among them, the mass of lithium element in the SEI film of the negative electrode tab of the battery cell in this embodiment

[0114] m = 0.21 - 0.01 - 0.0023 = 0.1977 mg, accounting for the proportion in the measured lithium content

[0115]

[0116] Example 2

[0117] The difference between this embodiment and Example 1 is that the state of the battery is different. This embodiment uses the battery after capacity, and the rest are the same. The specific steps are as follows:

[0118] (1) Obtain 6 packaged battery cells. Among them, 3 battery cells are left standing at room temperature for 24 h without formation (without forming the SEI film), and are named Group A; the other 3 battery cells are left standing at room temperature for 24 h and then subjected to formation treatment. The formation current is 1C, the formation temperature is 45 °C, and the formation pressure is 1 MPa. Then, 1 capacity test is carried out. The capacity current is 1C, and the capacity temperature is 25 °C, and they are named Group B;

[0119] (2) Disassemble the battery cells in Group A in the drying room, take the negative electrode tab, immerse and clean it with dimethyl carbonate (DMC) for 30 min. After repeating the immersion and cleaning 3 times, take the immersed negative electrode tab for ICP test to measure the mass of lithium element in it, and obtain the mass \(m\) 1 = 0.01 mg of lithium element contained in the lithium salt as the residual lithium of the lithium salt;

[0120] (3)Discharge the battery cells in Group B under the following conditions: S1) Let the battery cells stand still for 5 minutes; S2) Constant current discharge at a current rate of 0.1C until the lower limit voltage of the battery cells reaches 3V; S3) Let the battery cells stand still for 5 minutes; S4) Constant current discharge at a current rate of 0.01C until the lower limit voltage of the battery cells reaches 3V; S5) Repeat the processes of steps S3) and S4) in sequence 3 times.

[0121] (4)Disassemble the battery cells in Group B in a drying room, take the negative electrode plates, immerse and clean them with DMC for 30 minutes. After repeating the immersion and cleaning 3 times, take the impregnated negative electrode plates for ICP testing to measure the mass of lithium element in them, and obtain the total mass m of lithium element in the negative electrode plates. 0 = 0.22mg, which is the sum of the lithium content m 1 remaining as lithium salt, the content m of active lithium not released due to polarization in the negative electrode 2 and the irreversible lithium content m in the SEI film.

[0122] (5)Assemble the negative electrode plates of the battery cells in Group B obtained by disassembling, immersing and cleaning in step (4) with lithium sheets into coin cells; after letting the coin cells stand still for 4 hours, test their actual capacity C 1 , and the conditions during the testing process are as follows: 1) Let it stand still for 5 minutes; 2) Constant current charge at 10 μA until the voltage reaches 4V; 3) Let it stand still for 5 minutes; and record the curve of the battery voltage changing with the charging time t, and obtain the actual capacity C of the coin cell according to the product of the charging current I and the charging time t. 1 Select the capacity part below 3.6V for the capacity, C 1 = 0.0084 mAh.

[0123] Convert the capacity C of the coin cell 1 and the mass m of active lithium according to the following formula: 2

[0124]

[0125] where m 2 is the mass of active lithium element in the negative electrode plate, with the unit of mg; C 1 is the actual capacity of the coin cell, with the unit of mAh; C 0 is the theoretical specific capacity of graphite, with the unit of mAh / g; M 0 is the molar mass of graphite, with the unit of g / mol; M 1 is the molar mass of lithium element, with the unit of g / mol; 6 represents that every 6 carbon atoms correspond to 1 lithium ion (LiC 6 ); among them, M 1 = 6.941 g / mol, M 0 = 12.01 g / mol, C0 = 372 mAh / g;

[0126] (6) Calculate the mass m and mass percentage w(%) of lithium element in the SEI film according to the following formula:

[0127] m = m 0 - m 1 - m 2 (Equation 2),

[0128]

[0129] where m is the mass of lithium element in the SEI film, with the unit of mg; w is the mass percentage (%) of lithium element in the SEI film; m 1 is the mass of lithium element contained in the lithium salt in the negative electrode tab, with the unit of mg; m 2 is the mass of active lithium element in the negative electrode tab, with the unit of mg; m 0 is the total mass of lithium element in the negative electrode tab, with the unit of mg.

[0130] The mass m of lithium element in the SEI film of the negative electrode tab of the cell in this example is 0.22 - 0.01 - 0.0022 = 0.2078 mg, accounting for the proportion in the measured lithium content

[0131] Example 3

[0132] The difference between this example and Example 1 is that the states of the batteries are different. This example uses batteries stored at room temperature for one year, and the rest are the same. The specific steps are as follows:

[0133] (1) Obtain 6 packaged cells. Among them, 3 cells are left standing at room temperature for 24 h without formation (without forming an SEI film), and are named Group A; the other 3 cells are left standing at room temperature for 24 h and then subjected to formation treatment. The formation current is 1C, the formation temperature is 45 °C, and the formation pressure is 1 MPa. Then, a capacity test is carried out once more. The capacity current is 1C, the capacity temperature is 25 °C, and finally, they are stored in a conventional environment at a voltage state of 60% SOC for 1 year, and are named Group B;

[0134] (2) Disassemble the cells in Group A in a drying room, take the negative electrode tabs, impregnate and clean them with dimethyl carbonate (DMC) for 30 min, repeat the impregnation and cleaning 3 times, and then take the impregnated negative electrode tabs for ICP testing to measure the mass of lithium element, and obtain the mass m 1 = 0.01 mg of lithium element contained in the lithium salt in the negative electrode tab as the residual lithium of the lithium salt;

[0135] (3) Perform a discharge treatment on the battery cells in Group B. Discharge conditions: S1) Let the battery cells stand still for 5 min; S2) Constant current discharge at a current rate of 0.1C until the lower limit voltage of the battery cells reaches 3V; S3) Let the battery cells stand still for 5 min; S4) Constant current discharge at a current rate of 0.01C until the lower limit voltage of the battery cells reaches 3V; S5) Repeat the processes of steps S3) and S4) in sequence 3 times.

[0136] (4) Disassemble the battery cells in Group B in a drying room, take the negative electrode plates, immerse and clean them with DMC for 30 min. After repeating the immersion and cleaning 3 times, take the immersed negative electrode plates for ICP testing to measure the mass of lithium element in them, and obtain the total mass m of lithium element in the negative electrode plates. 0 = 0.24 mg, which is the sum of the lithium content m 1 remaining as lithium salt, the content m 2 of active lithium not released due to polarization in the negative electrode, and the irreversible lithium content m in the SEI film.

[0137] (5) Assemble the negative electrode plates of the battery cells in Group B obtained by disassembling, immersing, and cleaning in step (4) with lithium sheets into a button cell; after letting the button cell stand still for 4 h, test its actual capacity C 1 . The conditions during the testing process are as follows: 1) Let it stand still for 5 min; 2) Constant current charge at 10 μA until the voltage reaches 4V; 3) Let it stand still for 5 min; and record the curve of the battery voltage changing with the charging time t. Obtain the actual capacity C of the button cell according to the product of the charging current I and the charging time t. 1 Select the capacity part below 3.6V for the capacity, C 1 = 0.0084 mAh.

[0138] Convert the capacity C of the button cell 1 and the mass m of active lithium according to the following formula: 2

[0139]

[0140] where, m 2 is the mass of active lithium element in the negative electrode plate, with the unit of mg; C 1 is the actual capacity of the button cell, with the unit of mAh; C 0 is the theoretical specific capacity of graphite, with the unit of mAh / g; M 0 is the molar mass of graphite, with the unit of g / mol; M 1 is the molar mass of lithium element, with the unit of g / mol; 6 represents that every 6 carbon atoms correspond to 1 lithium ion (LiC 6 ); among them, M 1 = 6.941 g / mol, M 0 = 12.01 g / mol, C0 = 372 mAh / g;

[0141] (6) Calculate the mass m of lithium element in the SEI film and its mass percentage content w(%) according to the following formula:

[0142] m = m 0 - m 1 - m 2 (Formula 2),

[0143]

[0144] where m is the mass of lithium element in the SEI film, with the unit of mg; w is the mass percentage content (%) of lithium element in the SEI film; m 1 is the mass of lithium element contained in the lithium salt in the negative electrode tab, with the unit of mg; m 2 is the mass of active lithium element in the negative electrode tab, with the unit of mg; m 0 is the total mass of lithium element in the negative electrode tab, with the unit of mg.

[0145] Among them, the mass m of lithium element in the SEI film of the negative electrode tab of the battery cell in this example = 0.24 - 0.01 - 0.0019 = 0.2281 mg, accounting for the proportion in the measured lithium content

[0146] Examples 4 - 7

[0147] The difference between Examples 4 - 7 and Example 1 is that the current and voltage of the discharge process in step (3) are different, as shown in Table 1 for details, and the rest are the same.

[0148] Table 1

[0149]

[0150] Example 8

[0151] The difference between Example 4 and Example 1 is that this example adopts a charging process, specifically as follows:

[0152] In this example, the tested battery cell sample is a wound soft lithium cobaltate (aluminum foil) / graphite (copper foil) package lithium-ion battery, with a 1C capacity of 5920 mAh;

[0153] (1) Obtain 6 packaged battery cells. Among them, 3 battery cells are left standing at room temperature for 24 h without formation (without forming an SEI film), and are named Group A; the other 3 battery cells are left standing at room temperature for 24 h and then subjected to formation treatment, with a formation current of 1C, a formation temperature of 45 °C, and a formation pressure of 1 MPa, and are named Group B, and an SEI film is formed on the negative electrode tab of the battery cell;

[0154] (2) Disassemble the battery cells in Group A in a drying room, take the negative electrode plates, immerse and clean them with dimethyl carbonate (DMC) for 30 minutes, repeat the immersion and cleaning 3 times, then take the immersed negative electrode plates for ICP testing to measure the mass of lithium element in them, and obtain the mass m of lithium element contained in the lithium salt in the negative electrode plate. 1 = 0.01 mg, which is the lithium remaining as lithium salt residue;

[0155] (3) Charge the battery cells in Group B under the following charging conditions: S1) Let the battery cells stand for 5 minutes; S2) Constant current discharge at a rate of 0.1C until 3V; S3) Constant current charge at a rate of 0.1C until 4.45V; S4) Let it stand for 5 minutes; S5) Constant current charge at a rate of 0.01C until 4.45V; S6) Let it stand for 5 minutes.

[0156] (4) Disassemble the battery cells in Group B in a drying room, take the negative electrode plates, immerse and clean them with DMC for 30 minutes, repeat the immersion and cleaning 3 times, then take the immersed negative electrode plates for ICP testing to measure the mass of lithium element in them, and obtain the total mass m of lithium element in the negative electrode plate. 0 = 1.33 mg, which is the total content m of lithium remaining as lithium salt residue, 1 the content m of active lithium not released due to polarization in the negative electrode, 2 and the irreversible lithium content m in the SEI film;

[0157] (5) Assemble the negative electrode plates of the battery cells in Group B obtained by disassembling and immersing and cleaning in step (4) with lithium sheets into coin cells; after letting the coin cells stand for 4 hours, test their actual capacity C 1 , and the conditions during the testing process are as follows: 1) Let it stand for 5 minutes; 2) Constant current charge at 10 μA until 4V; 3) Let it stand for 5 minutes; and record the curve of the battery voltage changing with the charging time t, and obtain the actual capacity C of the coin cell according to the product of the charging current I and the charging time t. 1 ; Select the capacity part below 3.6V for the capacity, C 1 = 4.12 mAh;

[0158] Convert the coin cell capacity C 1 and the mass m of active lithium according to the following formula: 2 Conversion:

[0159]

[0160] where m 2 is the mass of active lithium element in the negative electrode plate, with the unit of mg; C 1 is the actual capacity of the coin cell, with the unit of mAh; C 0 is the theoretical specific capacity of graphite, with the unit of mAh / g; M 0is the molar mass of graphite, with the unit of g / mol; M 1 is the molar mass of lithium element, with the unit of g / mol; 6 represents that every 6 carbon atoms correspond to 1 lithium ion (LiC 6 ); where M 1 = 6.941 g / mol, M 0 = 12.01 g / mol, C 0 = 372 mAh / g;

[0161] (6) Calculate the mass m of lithium element in the SEI film and its mass percentage w (%) according to the following formula:

[0162] m = m 0 - m 1 - m 2 (Formula 2),

[0163]

[0164] where m is the mass of lithium element in the SEI film, with the unit of mg; w is the mass percentage (%) of lithium element in the SEI film; m 1 is the mass of lithium element contained in the lithium salt in the negative electrode tab, with the unit of mg; m 2 is the mass of active lithium element in the negative electrode tab, with the unit of mg; m 0 is the total mass of lithium element in the negative electrode tab, with the unit of mg. Among them, in this embodiment, the mass m of lithium element in the SEI film of the negative electrode tab of the battery cell is 1.33 - 0.01 - 1.0668 = 0.2532 mg, accounting for the proportion in the measured lithium content

[0165] Examples 9 - 10

[0166] The difference between Examples 9 - 10 and Example 8 is that the current and voltage of the charging process in step (3) are different, as shown in Table 2 for details, and the rest are the same.

[0167] Table 2

[0168]

[0169] As can be seen from the above Examples 1 - 10, the method of the present application is suitable for batteries in different states such as after formation, after storage, and after capacity; and it can be seen that there are slight differences in the lithium content in the SEI film of the battery under different states. In addition, there are also slight differences in the lithium content in the SEI film under different discharge or charging processes. It can be seen that the method of the present application takes into account the lithium salt and active lithium remaining on the electrode tab by the electrolyte, can exclude the differences in electrode tab wettability and active material kinetics, improve the sensitivity and accuracy of testing the lithium content in the SEI film, and can distinguish the differences in the lithium content of the SEI film of different samples.

Claims

1. A method for testing the lithium content in a negative electrode SEI film, characterized in that: The steps include: (1) disassembling a lithium-ion battery without forming a SEI film and obtaining a negative electrode plate, impregnating the negative electrode plate with an organic solvent, testing the mass of lithium in the negative electrode plate after impregnation, and obtaining the mass m1 of lithium contained in the lithium salt in the negative electrode plate; (2) charging or discharging the lithium-ion battery after the SEI film is formed, disassembling the lithium-ion battery and obtaining the negative electrode plate, impregnating the negative electrode plate with an organic solvent, testing the mass of lithium in the impregnated negative electrode plate, and obtaining the total mass m0 of lithium in the negative electrode plate; (3) The negative electrode plate impregnated in step (2) is assembled with a lithium metal plate into a button battery, and the actual capacity C1 of the button battery is tested. The mass m2 of the active lithium element in the negative electrode plate is calculated according to the following formula: Wherein, m2 is the mass of active lithium element in the negative electrode plate, in mg; C1 is the actual capacity of the button battery, in mAh; C0 is the theoretical specific capacity of graphite, in mAh / g; M0 is the molar mass of graphite, in g / mol; M1 is the molar mass of lithium element, in g / mol; (4) The mass m and mass percentage w of lithium in the negative electrode SEI film are calculated according to the following formula: m=m0-m1-m2 (Formula 2), Among them, m is the mass of lithium element in SEI film, unit is mg; w is the mass percentage of lithium element in SEI film, unit is %; m1 is the mass of lithium element contained in lithium salt in negative electrode plate, unit is mg; m2 is the mass of active lithium element in negative electrode plate, unit is mg; m0 is the total mass of lithium element in negative electrode plate, unit is mg.

2. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In steps (1) and (2), the organic solvent is independently selected from at least one of dimethyl carbonate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate or diethyl carbonate.

3. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In steps (1) and (2), the immersion time is independently selected from 10 min to 60 min, and the immersion times are independently selected from 1 time to 3 times.

4. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In step (2), the lithium-ion battery after forming the SEI film includes a lithium-ion battery that has been formed.

5. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In steps (1) and (2), the method for testing the quality of lithium elements in the negative electrode plate after impregnation includes an inductively coupled plasma analysis method.

6. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In step (2), the discharge process is as follows: (2-1) the lithium-ion battery after forming the SEI film is allowed to stand for 2 to 10 minutes; (2-2) the battery is discharged to V1 at a current rate of 0.05C to 0.1C; (2-3) the battery is allowed to stand for 2 to 10 minutes; (2-4) the battery is discharged to V2 at a current rate of 0.005C to 0.01C; (2-5) the steps (2-3) and (2-4) are repeated 1 to 5 times.

7. The method for testing the lithium content in the negative electrode SEI film according to claim 6, characterized in that: The V1 and V2 are the lower limit voltage values ​​of the lithium ion battery after the SEI film is formed.

8. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In step (2), the charging process is as follows: (2-1) allowing the lithium ion battery after the SEI film is formed to stand for 2 to 10 minutes; (2-2) discharging at a current rate of 0.05C to 0.1C to the lower limit voltage of the lithium ion battery after the SEI film is formed; (2-3) charging at a current rate of 0.05C to 0.1C to a voltage value of V3; (2-4) allowing the battery to stand for 2 to 10 minutes; (2-5) charging at a current rate of 0.005C to 0.01C to a voltage value of V4; (2-6) allowing the battery to stand for 1 to 5 minutes.

9. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In step (2), the negative electrode plate in the lithium ion battery after the SEI film is formed includes a current collector and a negative electrode active material layer loaded on the current collector, the negative electrode active material layer includes graphite, and the current collector includes copper foil.

10. The method for testing the lithium content in the negative electrode SEI film according to claim 1, characterized in that: In step (3), the process of testing the actual capacity C1 of the button battery is as follows: the button battery is allowed to stand for 2 to 10 minutes, charged to a cut-off voltage at a constant current I, allowed to stand for 2 to 10 minutes, and a curve of the battery voltage changing with the charging time t is recorded, and the actual capacity C1 of the button battery is obtained based on the product of the charging current I and the charging time t.

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