Method for detecting gram volume of lithium battery negative plate

Abstract

The invention relates to the technical field of lithium batteries, in particular to a gram volume detection method of a lithium battery negative plate. The method comprises the following steps: taking a lithium battery to be detected, discharging to cut-off voltage, disassembling the battery, and obtaining a negative plate; treating the negative plate by adopting a solvent; the discharging at least comprises the following steps: discharging to a cut-off voltage at a current of (0.1-1) C, and then discharging to the cut-off voltage at a current of (0.01-0.1) C; the solvent is selected from alcohol solvents; taking the negative pole piece, removing the active coating on the single surface of the negative pole piece, punching the negative pole piece into a button pole piece, and measuring the mass m of the active coating on the button pole piece; after the button pole pieces are assembled into a button cell, charging and discharging the button cell, and measuring the charging capacity C for charging; the gram capacity of the negative plate is calculated according to the formula of Ccharge / (m * a); a is the content of the main material in the active coating of the negative plate. According to the invention, the stepped decreasing type low-current discharge and alcohol solvent cleaning treatment modes are adopted at the same time, so that the accuracy of the gram capacity detection result of the negative plate can be effectively and synergistically improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a method for detecting the gram capacity of a negative electrode sheet of a lithium battery. Background Art

[0002] In the analysis of lithium-ion battery electrode layers, it is necessary to perform charge-off production and testing on the disassembled positive and negative electrodes in order to obtain the charge-off capacity (or gram capacity) data of the electrode to characterize the attenuation and failure of the positive and negative electrode materials. Currently, the charge-off production and testing of the positive electrode can obtain reliable results, but there are many problems in the charge-off production and testing of the negative electrode, and it is basically impossible to obtain reliable charge-off capacity (or gram capacity) test results of the electrode.

[0003] The patent document with publication number CN107192908A discloses a method for testing the gram capacity of the pole piece material after the disassembly of a lithium-ion battery. The specific steps are as follows: removing the dressing on one side of the disassembled pole piece: applying the wiping liquid evenly on the side of the pole piece where the dressing needs to be removed, and then wiping it with dust-free paper until there is no dressing on this side of the pole piece; removing the dressing on both sides of the disassembled pole piece: applying the wiping liquid evenly on both sides of the pole piece, and then wiping it with dust-free paper until there is no dressing on both sides of the pole piece; The electrode with one side of the dressing removed and the electrode with both sides of the dressing removed are dried in the sun until the wiping liquid on the surface of the electrode evaporates completely; the electrode with the clean side of the dressing removed and the electrode with both sides of the dressing removed are sliced ​​into circular electrode pieces, and the cut circular electrode pieces are weighed with a balance, and the mass of the electrode with one side of the dressing removed is recorded as M1, and the mass of the electrode with both sides of the dressing removed is recorded as M2; the electrode with one side of the dressing removed is assembled into a button battery; the assembled button battery is charged and discharged with a small current to test the electrode charging capacity Q 充 , and the discharge capacity Q 放 The mass of the electrode dressing material is corrected according to the charge and discharge capacity, and the button battery is subjected to charge and discharge tests at different rates to obtain the gram capacity value of the electrode dressing material.

[0004] The patent document with publication number CN113777491A discloses a method for detecting the gram capacity of the negative electrode material of a failed lithium-ion battery, comprising the following steps: disassembling the failed lithium-ion battery and taking out the negative electrode sheet; dripping and wiping the solvent on the non-edge part of the negative electrode sheet and wiping it open to obtain the negative electrode sheet on one side of the dripping area and on both sides of the surrounding area; punching the electrode sheet to obtain a single-sided negative electrode unit sheet; assembling the negative electrode unit sheet into a buckle and testing its gram capacity.

[0005] It can be seen that in the prior art, when testing the gram capacity of the negative electrode sheet, the negative electrode sheet is obtained by directly disassembling the battery. The disassembled negative electrode sheet of the lithium-ion battery contains lithium salts and the SEI film produced by chemical formation, as well as byproducts produced by the reduction of the electrolyte on the negative electrode surface during the charge and discharge process. These impurities will affect the buckle impedance of the negative electrode sheet, thereby significantly affecting the test accuracy of the gram capacity of the negative electrode sheet. Even though some prior arts have adopted a cleaning step, the SEI film and the byproducts produced during the charge and discharge process can exist relatively stably in common electrolyte solvents such as DMC / EMC / DEC and cannot be effectively removed. Summary of the invention

[0006] The present invention aims to solve the above-mentioned problem and provide a method for detecting the gram capacity of a negative electrode sheet of a lithium battery with high accuracy.

[0007] The technical solution to the problem of the present invention is to provide a method for detecting the gram capacity of a negative electrode sheet of a lithium battery, comprising the following steps: S1. After discharging the lithium battery to be tested to the cut-off voltage, disassembling the battery to obtain the negative electrode sheet; and treating the negative electrode sheet with a solvent; The discharge comprises at least the following steps: firstly discharging with a current of (0.1-1)C to a cut-off voltage, and then discharging with a current of (0.01-0.1)C to a cut-off voltage; The solvent is selected from alcohol solvents; S2. Take the negative electrode sheet, remove the active coating on one side, and punch it into a button-type electrode sheet, and measure the mass m of the active coating on the button-type electrode sheet; S3. After assembling the button-type electrode into a button-type battery, charging and discharging the button-type battery to measure the charging capacity C 充电 ; S4. Calculate the gram capacity of the negative electrode = C 充电 / (m*a); where a is the content of the main material in the active coating of the negative electrode sheet.

[0008] In the present application, in step S1, a small step-by-step decreasing current is first used to discharge to a lower voltage so that the lithium ions in the negative electrode sheet of the battery cell are completely released and returned to the positive electrode, thereby reducing the reaction of the disassembled negative electrode sheet with moisture or oxygen in the air. The negative electrode sheet is then treated with an alcohol solvent to completely remove the residual lithium salt, SEI film, and reaction by-products on the negative electrode sheet, thereby reducing the buckling resistance of the negative electrode sheet, thereby significantly improving the accuracy of the negative electrode sheet buckling gram capacity test.

[0009] Among them, the discharge of small current with step-by-step current decrease can reduce the volume expansion and contraction during the insertion and extraction of lithium ions, ensure the structural stability of the negative electrode sheet, and reduce the cracks and pores caused by the volume expansion and contraction of the negative electrode sheet. The structurally stable negative electrode sheet is conducive to the penetration of alcohol solvents with better polarity and less susceptible to steric hindrance due to its uniform and dense structure, so as to improve the comprehensiveness of alcohol solvent treatment of impurities; the reduction of cracks and pores reduces the impurity retention points on the negative electrode sheet, avoids the diffusion, accumulation, and re-deposition of impurities at the retention points during the alcohol solvent treatment process, and ensures the removal effect of impurities.

[0010] Step S1 In the method, the selection of the lithium battery to be tested is not limited. For example, the positive electrode of the lithium battery to be tested may include at least one of lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium titanate; the negative electrode may include at least one of graphite, carbon, and silicon. As a preferred embodiment of the present invention, the lithium battery to be tested uses graphite as the negative electrode active material. The detection method of the present application has a particularly high detection accuracy for the gram capacity of the negative electrode sheet using graphite as the negative electrode active material.

[0011] When discharging the lithium battery to be tested, at least the steps of (0.1~1)C current discharge and (0.01~0.1)C current discharge are included. When discharging to the cut-off voltage with a current of (0.1~1)C, for example, the current can be 0.1C, 0.2C, 0.3C, 0.4C, 0.5C, 0.6C, 0.7C, 0.8C, 0.9C, 1C. When discharging to the cut-off voltage with a current of (0.01~0.1)C, for example, the current can be 0.01C, 0.02C, 0.03C, 0.04C, 0.05C, 0.06C, 0.07C, 0.08C, 0.09C, 0.1C.

[0012] As a preferred embodiment of the present invention, the battery is first discharged to the cut-off voltage with a current of (0.2-0.5) C, and then discharged to the cut-off voltage with a current of (0.01-0.02) C. Further preferred, the battery is first discharged to the cut-off voltage with a current of 0.33 C, and then discharged to the cut-off voltage with a current of 0.02 C.

[0013] In some embodiments, one or two or more intermediate current discharge steps are included between the two current discharge steps, the current size of the intermediate current discharge step is between the two currents, and the current of the intermediate current discharge step is reduced successively; for example, first discharge to the cut-off voltage with a current of 0.33C, then discharge to the cut-off voltage with a current of 0.1C, and finally discharge to the cut-off voltage with a current of 0.02C. In parallel, in some embodiments, before the (0.1~1)C current discharge step, one or two or more higher current discharge steps are included, the current of the higher current discharge step is higher than (0.1~1)C, and the current of the higher current discharge step is reduced successively; for example, first discharge to the cut-off voltage with a current of 0.8C, then discharge to the cut-off voltage with a current of 0.33C, and finally discharge to the cut-off voltage with a current of 0.02C. In parallel, in some embodiments, after the (0.01~0.1)C current discharge step, one or two or more lower current discharge steps are also included, and the current of the lower current discharge step is lower than (0.01~0.1)C, and the current of the lower current discharge step decreases successively; for example, first discharge with a current of 0.33C to the cut-off voltage, then discharge with a current of 0.02C to the cut-off voltage, and finally discharge with a current of 0.01C to the cut-off voltage.

[0014] After the discharge is completed, the battery to be tested is disassembled. As a preferred embodiment of the present invention, the battery is disassembled in an environment with a humidity not higher than 10% to avoid the reaction of environmental humidity and oxygen with the negative electrode sheet.

[0015] After disassembling the battery to obtain the negative electrode sheet, there are many choices of alcohol solvents for treating the negative electrode sheet. For example, the alcohol solvent can be methanol, ethanol, ethylene glycol, propanol, tert-butyl alcohol, isobutyl alcohol, etc. As a preferred embodiment of the present invention, the alcohol solvent is selected from at least one of methanol, ethanol, ethylene glycol, and propanol. Low molecular weight alcohol solvents are more likely to penetrate the negative electrode sheet through the tiny pores of the material. Further preferably, the alcohol solvent is ethanol.

[0016] The alcohol solvent is not limited to the treatment method of the negative electrode sheet. For example, it can be impregnation, rinsing, etc. As a preferred embodiment of the present invention, after the negative electrode sheet is immersed in the solvent for 20 to 40 minutes, the negative electrode sheet is taken out and rinsed with the solvent. The impregnation allows the solvent to completely penetrate into the negative electrode sheet and dissolve impurities, and the impurities are removed in time by rinsing. Among them, the immersion time can be 20min, 21 min, 22 min, 23 min, 24min, 25 min, 26 min, 27 min, 28min, 29min, 30 min, 31 min, 32min, 33 min, 34 min, 35 min, 36min, 37 min, 38 min, 39 min, 40min. The immersion time should not be too long or too short. Too short may lead to incomplete removal of impurities and thus affect the buckle impedance of the negative electrode sheet. Too long may cause the active coating on the negative electrode sheet to fall off, affecting the subsequent weighing calculation results.

[0017] Step S2 In the present invention, the method of removing the active coating is not limited, as long as no other impurities are introduced during the removal process. As a preferred method of the present invention, the surface solvent of the cleaned negative electrode sheet is absorbed with dust-free paper, the negative electrode sheet is placed on a clean glass plate, and the surface of the negative electrode sheet is gently wiped with dust-free paper dipped in pure water until the smooth copper foil is exposed. The area of ​​the wiped single-sided electrode sheet is preferably enough to flush 3 to 10 button battery electrodes.

[0018] The method of measuring the mass m of the active coating on the button-type electrode is not limited, as long as the accuracy of the detection is ensured. As a preferred embodiment of the present invention, the mass m is measured by the following steps: take two negative electrode sheets, remove the active coating on only one side of one sheet, punch out the button-type electrode sheet, weigh it after drying, and the mass is m1; remove the active coating on both sides of the other sheet, punch out the copper foil sheet, weigh it after drying, and the mass is m2; m = m1-m2.

[0019] The button-type electrode requires one side to be a smooth copper foil and the other side to be a negative electrode active material coating, the copper foil is free of dirt, the active material coating is free of cracks and powder. The copper foil requires both sides to be smooth copper foil, the copper foil surface is free of dirt, and the edges are free of burrs.

[0020] As a further preferred embodiment of the present invention, the button-type electrode and the copper foil are dried in the same environment.

[0021] As a further preference of the present invention, during drying, the button-type electrode and copper foil are laid flat on dust-free paper with the copper foil side of the button-type electrode facing upward, and the button-type electrode, copper foil and dust-free paper are clamped by two glass plates from top to bottom, placed in a blast drying oven, and dried at appropriate temperature and time.

[0022] As a further preference of the present invention, the drying temperature is 60-90°C, and the drying time is 12-48 h. The appropriate drying temperature and drying time can ensure the drying effect without damaging the button-type electrode and the copper foil. For example, the drying temperature can be 60°C, 63°C, 65°C, 67°C, 70°C, 73°C, 75°C, 77°C, 80°C, 73°C, 85°C, 87°C, 90°C; the drying time can be 12 h, 15 h, 18 h, 21 h, 24 h, 27 h, 30 h, 33 h, 36 h, 39 h, 42 h, 45 h, 48 h.

[0023] Step S3 In the embodiment, the method of assembling into a button battery is not limited. For example, the button battery can be assembled with a button electrode sheet as the positive electrode, a metal lithium sheet as the negative electrode, and a base film as the separator.

[0024] As a preferred embodiment of the present invention, the assembled button cell is left standing at room temperature for 3 to 12 hours before the charging and discharging is performed. By standing, the battery can gradually restore its internal stable state, thereby more accurately reflecting the actual voltage and capacity of the battery. For example, the standing time can be 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours.

[0025] As a preferred embodiment of the present invention, charging and discharging at least comprises the following steps: S31. (0.09~0.15) C 理论 Discharge to 0.001~0.005 V, S32. (0.04~0.08) C 理论 Discharge to 0.001~0.005 V, S33. (0.01~0.03) C 理论 Discharge to 0.001~0.005 V; S34. After standing for 20-40 min, the 理论 Charge to cut-off voltage; Among them, C 理论 is the theoretical capacity of the button battery.

[0026] In this embodiment, first, the theoretical capacity of the button battery is estimated, and then the charge and discharge schedule is determined based on the theoretical capacity, so that the obtained charging capacity is more accurate.

[0027] Among them, the theoretical capacity C 理论=m*a*S, a is the content of the active main material in the negative electrode sheet, and S is the theoretical gram capacity of the negative electrode sheet. In some embodiments, when the main material in the negative electrode sheet is graphite, the commercial graphite negative electrode gram capacity is S=340~360mAh / g, a=0.95~0.97. 理论 Only used to determine the charge and discharge system, S changes in the range of 340~360 to C 理论 The value of a has little effect on C. 理论 The value of has little effect, so in some embodiments, S=350mAh / g, a=0.96, that is, C 理论 =m*0.96*350.

[0028] Second, during the charging and discharging process, the current is reduced at least three times in sequence, which can reduce the impact of the internal polarization of the battery on the actual capacity of the battery and the impact of the discharge temperature on the battery performance, ensuring the stability of the battery structure while ensuring complete discharge, thereby further improving the accuracy of the detection of the charging capacitor. For example, in step S31, the discharge current can be 0.09 C 理论 , 0.1C 理论 , 0.11C 理论 , 0.12C 理论 , 0.13C 理论 , 0.14 C 理论 , 0.15C 理论 , can be discharged to 0.001 V, 0.002 V, 0.003 V, 0.004 V, 0.005 V; in step S32, the discharge current can be 0.04C 理论 , 0.05 C 理论 , 0.06 C 理论 , 0.07 C 理论 , 0.08 C 理论 , can be discharged to 0.001 V, 0.002 V, 0.003 V, 0.004 V, 0.005 V; in step S33, the discharge current can be 0.01 C 理论 , 0.02 C 理论 , 0.03 C 理论, can be discharged to 0.001 V, 0.002 V, 0.003 V, 0.004 V, 0.005 V. During the charging process, standing for a period of time before charging helps the battery to reach a more stable state before charging, which helps to more accurately evaluate the charging capacity of the battery. For example, in step S34, the standing time can be 20 min, 22 min, 24 min, 26 min, 28 min, 30 min, 32 min, 34 min, 36 min, 38 min, 40 min, and the charging current can be 0.09 C 理论 , 0.1 C 理论 , 0.11 C 理论 , 0.12 C 理论 , 0.13 C 理论 , 0.14 C 理论 , 0.15 C 理论 .

[0029] As a preferred embodiment of the present invention, the charging and discharging comprises at least the following steps: S31. 理论 Discharge to 0.003 V, S32. at 0.05C 理论 Discharge to 0.003 V, S33. at 0.02C 理论 Discharge to 0.003 V; S34. After standing for 30 min, discharge at 0.1C 理论 Charge to cut-off voltage.

[0030] In some embodiments, during the discharge process, one, two or more discharge steps may be added to ensure that the discharge current decreases. 理论 Discharge to 0.003 V at 0.08 C 理论 Discharge to 0.003 V at 0.05C 理论 Discharge to 0.003 V at 0.02C 理论 Discharge to 0.003 V; after standing for 30 min, at 0.1C 理论 Charge to cut-off voltage.

[0031] As a preferred embodiment of the present invention, the charging and discharging is performed at least twice. For example, the discharge-charging step can be cycled for 2, 3, 4, etc. times.

[0032] Step S4 In the calculation, after obtaining the charge capacity, the gram capacity of the negative electrode can be calculated = C 充电 / (m*a); where a is the content of the main material in the active coating of the negative electrode sheet. The content of the main material in the negative electrode sheet is generally 0.95~0.97. If the formula is known, the actual main material content is substituted into the calculation; if the formula is unknown, 0.96 can be directly substituted for calculation, and the deviation is small.

[0033] Beneficial effects of the present invention: 1. In the present application, by adjusting the discharge steps of the lithium battery to be tested and the processing steps of the negative electrode sheet after disassembly, the lithium ions in the negative electrode sheet are completely released and returned to the positive electrode, thereby reducing the reaction of the negative electrode sheet after disassembly with moisture or oxygen in the air and the residual impurities on the negative electrode sheet, thereby effectively improving the accuracy of the negative electrode sheet gram capacity test.

[0034] 2. In some embodiments, the charge and discharge regime is determined by estimating the theoretical capacity of the button cell, and a discharge regime with at least three current decreases and charging after standing is adopted to further improve the detection accuracy of the charge capacity, thereby improving the accuracy of the negative electrode sheet gram capacity test. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 1 is a SEM image of the negative electrode sheets obtained in Example 1, Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0036] The following are specific embodiments of the present invention, and further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0037] Example 1 A method for detecting the gram capacity of a lithium battery negative electrode sheet comprises the following steps: S1. Take the 86Ah lithium iron phosphate battery to be tested (graphite as the negative electrode active material), first discharge it at 0.33C (current is 0.33*86 A) to a cut-off voltage of 2V, and then discharge it at 0.02C (current is 0.02*86 A) to a cut-off voltage of 2V. After discharge, disassemble the battery in an environment with a humidity of less than 10% to obtain an appropriate amount of negative electrode sheets; completely immerse the obtained negative electrode sheets in anhydrous ethanol; after 30 minutes, take out the negative electrode sheets and rinse the front and back of the negative electrode sheets with a spray bottle filled with anhydrous ethanol.

[0038] The obtained negative electrode sheet is shown in Figure 1 In the rightmost small picture, it can be clearly observed that the anhydrous ethanol cleaning treatment of the negative electrode is more thorough.

[0039] S2. Take two cleaned negative electrode sheets, use dust-free paper to absorb the surface solvent of one sheet, place the negative electrode sheet on a clean glass plate, and gently wipe the surface of the negative electrode sheet with dust-free paper dipped in pure water until the smooth copper foil is exposed. The area of ​​the wiped single-sided electrode sheet should be large enough to punch out 10 button-type battery electrodes. Sandwich the prepared single-sided negative electrode sheet between two weighing papers, and use button-type battery punching equipment to punch out (punching diameter 12 mm), and punch out 10 button-type electrodes.

[0040] Another sample was wiped clean with dust-free paper and pure water on both sides of the negative electrode to expose the smooth copper foil on both sides. The prepared copper foil was sandwiched between two weighing papers and punched with a button cell punching device (punching diameter 12 mm), and 10 copper foils were punched.

[0041] Lay the punched button-type electrode and copper foil flat on the dust-free paper, with the copper foil side of the button-type electrode facing up. Use two glass plates to clamp the button-type electrode, copper foil and dust-free paper from top to bottom, place in a forced air drying oven, set to 70℃, and dry for 24 hours.

[0042] The dried button-type electrode and copper foil were weighed separately. The average mass of the button-type electrode was m1=14.6 mg, the average mass of the copper foil was m2=4.58 mg, and the mass of the active coating on the button-type electrode was m=14.6-4.58=10.02 mg=0.01002 g.

[0043] S3. Assemble the button-type electrodes into a button-type battery, let it stand at room temperature for 3 hours, and then charge and discharge it. The capacity of the button-type battery is C 理论 Estimated as C 理论 =0.01002*0.96*350=3.367 mAh.

[0044] Charge and discharge: 0.1C 理论 (0.3367mA) current discharge to 0.003V, 0.05C 理论 (0.1684mA) current discharge to 0.003V, 0.02C 理论 (0.0673mA) current discharge to 0.003V; stand for 30min, 0.1C 理论 (0.3367 mA) current to charge to 2 V. The cycle was repeated 2 times.

[0045] Measured charging capacity C 充电 =3.368 mAh.

[0046] S4. Calculate the gram capacity of the negative electrode = C 充电 / (m*a)=3.368 / (0.01002*0.96)=350.1 mAh / g.

[0047] Example 2 This embodiment is basically the same as Embodiment 1, and the only difference is that in step S1, ethylene glycol is used as the solvent.

[0048] Specifically, in step S1, after the discharge is completed, the battery is disassembled in an environment with a humidity of less than 10% to obtain an appropriate amount of negative electrode sheets; the obtained negative electrode sheets are completely immersed in ethylene glycol; after 30 minutes, the negative electrode sheets are taken out and the front and back sides of the negative electrode sheets are rinsed with a spray bottle filled with ethylene glycol.

[0049] Example 3 This embodiment is basically the same as Embodiment 1, and the only difference is that in step S1, the immersion time is different.

[0050] Specifically, in step S1, after the discharge is completed, the battery is disassembled in an environment with a humidity of less than 10% to obtain an appropriate amount of negative electrode sheets; the obtained negative electrode sheets are completely immersed in anhydrous ethanol; after 60 minutes, the negative electrode sheets are taken out and the front and back sides of the negative electrode sheets are rinsed with a spray bottle filled with anhydrous ethanol.

[0051] Example 4 This embodiment is basically the same as the first embodiment, and the only difference is that in step S1, the discharge system is different.

[0052] Specifically, in step S1, a 86Ah lithium iron phosphate battery to be tested is first discharged at 0.6C to a cut-off voltage of 2V, and then discharged at 0.03C to a cut-off voltage of 2V.

[0053] Example 5 This embodiment is basically the same as the first embodiment, and the only difference is that in step S1, the discharge system is different.

[0054] Specifically, in step S1, a 86Ah lithium iron phosphate battery to be tested is first discharged at 0.1C to a cut-off voltage of 2V, and then discharged at 0.01C to a cut-off voltage of 2V.

[0055] Example 6 This embodiment is basically the same as Embodiment 1, and the only difference is that in step S2, the drying conditions are different.

[0056] Specifically, in step S2, the punched button-type electrode and copper foil are laid flat on the dust-free paper with the copper foil side of the button-type electrode facing upward, and the button-type electrode, copper foil and dust-free paper are clamped by two glass plates from top to bottom, and placed in a blast drying oven, set at 100°C, and dried for 24 hours.

[0057] Example 7 This embodiment is basically the same as the embodiment 1, and the difference is only that in step S3, the charging and discharging system is different.

[0058] Specifically: S3. Assemble the button-type electrodes into a button-type battery, let it stand for 3 hours at room temperature, and then charge and discharge it. The charge and discharge is: 0.1C 理论 Current discharge to 0.003 V; stand for 30 min, 0.1C 理论 The current was charged to 2 V. The cycle was performed 2 times.

[0059] Example 8 This embodiment is basically the same as the embodiment 1, and the difference is only that in step S3, the charging and discharging system is different.

[0060] Specifically: S3. Assemble the button-type electrodes into a button-type battery, let it stand for 3 hours at room temperature, and then charge and discharge it. The charge and discharge is: 0.05C 理论 Current discharge to 0.003V, 0.05mA current discharge to 0.003V, 0.05C 理论 Current charges to 2V; then 0.1C 理论 Current discharge to 0.003V, 0.05mA current discharge to 0.003V, 0.1C 理论 The current is charged to 2V.

[0061] Example 9 This embodiment is basically the same as the embodiment 1, and the difference is only that in step S3, the charging and discharging system is different.

[0062] Specifically: S3. Assemble the button-type electrodes into a button-type battery, let it stand for 3 hours at room temperature, and then charge and discharge it. The charge and discharge is: 0.1C 理论 Current discharge to 0.003V, 0.05C 理论 Current discharge to 0.003V, 0.02C 理论 Discharge current to 0.003V, stand for 30min, 0.1C 理论 The current charges to 2 V. This is done only once.

[0063] Example 10 This embodiment is basically the same as the embodiment 1, and the difference is only that in step S3, the charging and discharging system is different.

[0064] Specifically: S3. Assemble the button-type electrodes into a button-type battery, let it stand for 3 hours at room temperature, and then charge and discharge it. The charge and discharge is: 0.1C 理论 Current discharge to 0.003V, 0.05C 理论 Current discharge to 0.003V, 0.02C 理论 Current discharge to 0.003V, immediately 0.1C 理论 The current was charged to 2 V. The cycle was repeated 2 times.

[0065] Comparative Example 1 This comparative example is basically the same as Example 1, and the only difference is that in step S1, the negative electrode sheet is not cleaned.

[0066] Specifically, in step S1, after the discharge is completed, the battery is disassembled in an environment with a humidity of less than 10% to obtain an appropriate amount of negative electrode sheets without cleaning.

[0067] The obtained negative electrode sheet is shown in Figure 1 In the leftmost picture, impurities can be observed remaining on the negative electrode.

[0068] Comparative Example 2 This comparative example is basically the same as Example 1, and the only difference is that in step S1, dimethyl carbonate is used as the solvent.

[0069] Specifically, in step S1, after the discharge is completed, the battery is disassembled in an environment with a humidity of less than 10%, an appropriate amount of negative electrode sheets are obtained, and the obtained negative electrode sheets are completely immersed in DMC; after 30 minutes, the negative electrode sheets are taken out and the front and back sides of the negative electrode sheets are rinsed with a spray bottle containing DMC.

[0070] The obtained negative electrode sheet is shown in Figure 1 The middle small picture, compared with Example 1 and Comparative Example 1, shows that the cleaning effect is not as good as the cleaning effect of using anhydrous ethanol in Example 1.

[0071] Comparative Example 3 This comparative example is basically the same as Example 1, and the only difference is that in step S1, the discharge system is different.

[0072] Specifically, in step S1, a 86Ah lithium iron phosphate battery to be tested is taken and discharged at only 0.33C to a cut-off voltage of 2V.

[0073] Comparative Example 4 This comparative example is basically the same as Example 1, and the only difference is that in step S1, the discharge system is different.

[0074] Specifically, in step S1, a 86Ah lithium iron phosphate battery to be tested is taken and discharged at only 0.02C to a cut-off voltage of 2V.

[0075] The charge capacity and gram capacity data obtained in the examples and comparative examples are shown in Table 1 below.

[0076] Table 1. As shown in Table 1, by comparing Example 1 with Comparative Examples 1 to 4, it can be seen that the present application simultaneously adopts a step-by-step decreasing small current discharge and an alcohol solvent cleaning treatment method, which can effectively and synergistically improve the accuracy of the negative electrode sheet gram capacity detection results. This may be because the step-by-step decreasing small current discharge ensures the structural stability of the negative electrode sheet, is conducive to the penetration of alcohol solvents, and improves the removal effect of impurities on the negative electrode sheet, thereby ensuring the negative electrode sheet buckle impedance, thereby improving the accuracy of the negative electrode sheet buckle gram capacity test. If a non-alcohol solvent is used as in Comparative Example 2, it may not be able to penetrate and dissolve impurities well on the negative electrode sheet due to its polarity and solubility problems. If a single current discharge is used as in Comparative Examples 3 and 4, whether the current is large or small, it may affect the structural stability of the negative electrode sheet, and form cracks and pores, affecting the subsequent treatment effect of the alcohol solvent.

[0077] In addition, it can be seen from the comparison within the embodiment that, on the basis of using step-by-step decreasing small current discharge and alcohol solvent cleaning at the same time, the selection of alcohol solvents, suitable treatment methods, discharge systems, drying conditions, and charge-discharge systems can further improve the accuracy of the negative electrode sheet gram capacity detection results. For example, it can be seen from the comparison of Example 1 and Example 2 and 3 that ethanol has a better graphite negative electrode sheet treatment effect than ethylene glycol and a reasonable immersion time, so as to further improve the accuracy of the negative electrode sheet gram capacity detection results. It can be seen from the comparison of Example 1 and Example 4 and 5 that the discharge system of 0.33C first and then 0.02C has a suitable difference between the two currents, which makes the stability of the graphite negative electrode sheet better, so as to further improve the accuracy of the negative electrode sheet gram capacity detection results. It can be seen from the comparison of Example 1 and Example 6 that the appropriate drying temperature and time can reduce the damage to the graphite negative electrode sheet under the condition of ensuring drying, so as to further improve the accuracy of the negative electrode sheet gram capacity detection results. Comparing Example 1 with Examples 7 and 8, it can be seen that the three-current decreasing discharge system can ensure the stability of the battery structure while ensuring complete discharge, so as to further improve the accuracy of the negative electrode sheet capacity detection result. Comparing Example 1 with Examples 9 and 10, it can be seen that the three-cycle current decreasing discharge-charge twice, and the static treatment between discharge and charge, help the battery reach a more stable state before charging, so as to more accurately evaluate the battery's charge capacity.

[0078] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A method for detecting the gram capacity of a lithium battery negative electrode sheet, characterized in that: The following steps are involved: S1. Take the lithium battery to be tested and discharge it to the cut-off voltage, disassemble the battery and obtain the negative electrode sheet; and treating the negative electrode sheet with a solvent; The discharge comprises at least the following steps: firstly discharging with a current of (0.1-1)C to a cut-off voltage, and then discharging with a current of (0.01-0.1)C to a cut-off voltage; The solvent is selected from alcohol solvents; S2. Take the negative electrode sheet, remove the active coating on one side, and punch it into a button-type electrode sheet, and measure the mass m of the active coating on the button-type electrode sheet; S3. After assembling the button-type electrode into a button-type battery, charging and discharging the button-type battery to measure the charging capacity C 充电 ; S4. Calculate the gram capacity of the negative electrode = C 充电 / (m*a); where a is the content of the main material in the active coating of the negative electrode sheet.

2. The method for detecting gram capacity of a negative electrode sheet of a lithium battery according to claim 1, characterized in that: The alcohol solvent is selected from at least one of methanol, ethanol, ethylene glycol and propanol.

3. The method for detecting gram capacity of a negative electrode sheet of a lithium battery according to claim 1, characterized in that: In step S1, after the negative electrode sheet is immersed in the solvent for 20 to 40 minutes, the negative electrode sheet is taken out and rinsed with the solvent.

4. The method for detecting gram capacity of a lithium battery negative electrode sheet according to claim 1, characterized in that: In step S1, discharging includes at least the following steps: first discharging with a current of (0.2-0.5) C to a cut-off voltage, and then discharging with a current of (0.01-0.02) C to a cut-off voltage.

5. The method for detecting gram capacity of a negative electrode sheet of a lithium battery according to claim 1, characterized in that: In step S3, charging and discharging at least includes the following steps: S31. (0.09~0.15) C 理论 Discharge to 0.001~0.005 V, S32. (0.04~0.08) C 理论 Discharge to 0.001~0.005 V, S33. (0.01~0.03) C 理论 Discharge to 0.001~0.005 V; S34. After standing for 20-40 min, the 理论 Charge to cut-off voltage; Among them, C 理论 is the theoretical capacity of the button battery.

6. A method for detecting gram capacity of a lithium battery negative electrode sheet according to claim 1 or 5, characterized in that: In step S3, the charging and discharging is performed at least twice.

7. The method for detecting gram capacity of a negative electrode sheet of a lithium battery according to claim 1, characterized in that: In step S3, the assembled button battery is left to stand at room temperature for 3 to 12 hours before the charging and discharging is performed.

8. The method for detecting gram capacity of a negative electrode sheet of a lithium battery according to claim 1, characterized in that: In step S1, the battery is disassembled in an environment with a humidity not higher than 10%.

9. The method for detecting gram capacity of a negative electrode sheet of a lithium battery according to claim 1, characterized in that: In step S2, the mass m of the active coating on the button-type electrode sheet is measured by the following steps: take two portions of the negative electrode sheet, remove the active coating on only one side of one portion, punch out the button-type electrode sheet, and weigh it after drying, with a mass of m1; remove the active coating on both sides of the other portion, punch out a copper foil sheet, and weigh it after drying, with a mass of m2; m= m1-m2.

10. The method for detecting gram capacity of a negative electrode sheet of a lithium battery according to claim 9, characterized in that: The drying temperature is 60~90℃ and the drying time is 12~48 h.

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