Method for determining the threshold point of lithium deposition in batteries

Abstract

The present invention discloses a method for determining a lithium deposition threshold point of a battery, comprising: performing a first discharge operation on a three-electrode battery cell to discharge the battery cell to an empty state; performing a first charge operation on the battery cell after the first discharge operation to charge the battery cell to a fully charged state; performing a second discharge operation on the battery cell after the first charge operation to discharge the battery cell to an empty state to obtain a first capacity of the battery cell; performing a second charge operation on the battery cell after the second discharge operation to charge the battery cell to a state in which the battery cell has a second capacity; performing positive and negative lithium plating on reference electrodes of the battery cell after the second charge operation, respectively; performing a third discharge operation on the battery cell after the lithium plating to an empty state; performing a third charge operation on the battery cell after the third discharge operation to charge the battery cell to a state in which the battery cell has a third capacity, recording the real-time potential of the first negative electrode and the first real-time capacity of the battery cell in the third charge operation, and taking the capacity when the real-time potential of the first negative electrode is 0 as the lithium deposition threshold point.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a method for determining a lithium deposition threshold point of a battery. Background Art

[0002] In the existing technology, the aging state of lithium-ion batteries varies during different cycles, and it is impossible to accurately and effectively judge the aging state of the battery cells at different stages. Judging by physical detection of expansion or electrochemical signal attenuation after battery aging will lead to inaccuracy because the battery may have failed, thereby posing a safety risk.

[0003] However, existing technologies are unable to accurately identify the lithium plating points of battery cells. Summary of the Invention

[0004] The main purpose of the present invention is to propose a method for determining the lithium deposition threshold point of a battery, aiming to solve the problem that the existing technology cannot accurately identify the lithium deposition point of the battery cell.

[0005] To achieve the above objectives, the present invention proposes a method for determining a battery lithium deposition threshold point, the method comprising:

[0006] Performing a first discharge operation on the three-electrode battery cell to discharge the battery cell to an empty state;

[0007] performing a first charging operation on the battery cell after the first discharging operation is completed, so as to charge the battery cell to a fully charged state;

[0008] performing a second discharging operation on the battery cell after the first charging operation is completed, so as to discharge the battery cell to an empty state and obtain a first capacity of the battery cell;

[0009] performing a second charging operation on the battery cell after the second discharging operation, so as to charge the battery cell to a state where the power reaches a second capacity;

[0010] Performing positive and negative lithium plating on the reference electrodes of the battery cell after the second charging operation;

[0011] Performing a third discharge operation on the battery cell after lithium plating to an empty state;

[0012] performing a third charging operation on the battery cell after the third discharging operation to charge the battery cell to a state of a third capacity, recording the real-time potential of the first negative electrode of the battery cell and the first real-time capacity of the battery cell during the third charging operation, and taking the capacity when the real-time potential of the first negative electrode is 0 as the lithium precipitation threshold point;

[0013] The second capacity is smaller than the first capacity, and the third capacity is larger than the first capacity.

[0014] Preferably, the second charging operation includes:

[0015] Performing constant current charging on the battery cell at a first preset current;

[0016] The first preset current is 0.1C to 0.5C.

[0017] Preferably, the second capacity is greater than or equal to 30% of the first capacity and less than or equal to 90% of the first capacity.

[0018] Preferably, after performing a second discharge operation on the battery cell after completing the first charge operation and before performing a second charge operation on the battery cell after completing the second discharge operation, the determining method further includes:

[0019] After completing the second discharging operation, the battery cell is left for a first preset time;

[0020] The first preset time is 5 minutes to 30 minutes.

[0021] Preferably, the current of the positive electrode lithium plating and the negative electrode lithium plating are both greater than 0 mA and less than or equal to 1 mA, and the duration of the positive electrode lithium plating and the negative electrode lithium plating are both 2 hours to 4 hours.

[0022] Preferably, after performing a second charging operation on the battery cell after completing the second discharging operation, and before performing positive electrode lithium plating and negative electrode lithium plating on the reference electrode of the battery cell after completing the second charging operation, the determining method further includes:

[0023] After completing the second charging operation, the battery cell is left for a second preset time;

[0024] The second preset time is 5 minutes to 30 minutes.

[0025] Preferably, performing a third charging operation after completing the third discharging operation to charge the battery cell to a state of a third capacity, and recording the real-time potential of the first negative electrode of the battery cell and the first real-time capacity of the battery cell during the third charging operation includes:

[0026] After completing the third discharge operation, the battery cell is charged with a second preset constant current, so that the battery cell is charged to a state where the power is a third capacity;

[0027] recording the real-time potential of the first negative electrode of the battery cell and the first real-time capacity of the battery cell during the third charging operation at a first preset frequency;

[0028] The second preset current is 0.01C to 0.03C, and the first preset frequency is 1 time / s to 10 times / s.

[0029] Preferably, the third capacity is 110% to 130% of the first capacity.

[0030] Preferably, the performing a first discharge operation on the three-electrode battery cell includes: performing a constant current discharge on the three-electrode battery cell at a third preset current;

[0031] The performing a first charging operation on the battery cell after the first discharging operation includes: performing constant current and constant voltage charging on the battery cell after the first discharging operation at a fourth preset current;

[0032] The performing a second discharging operation on the battery cell after the first charging operation includes: performing a constant current discharge on the battery cell after the first charging operation at a fifth preset current;

[0033] The performing a third discharge operation on the battery cell after the lithium plating is completed comprises: performing a constant current discharge on the battery cell after the lithium plating is completed at a sixth preset current;

[0034] The third preset current is 0.1C to 0.5C, the fourth preset current is 0.4C to 0.6C, the fifth preset current is 0.1C to 0.5C, and the sixth preset current is 0.1C to 0.5C.

[0035] Preferably, before performing the first discharge operation on the three-electrode battery cell, the determination method further comprises: placing the three-electrode battery cell on hold for a third preset time;

[0036] After the first discharge operation is performed on the three-electrode battery cell and before the first charge operation is performed on the battery cell after the first discharge operation, the determination method further includes: placing the battery cell after the first discharge operation for a fourth preset time;

[0037] After performing the first charging operation on the battery cell after the first discharging operation and before performing the second discharging operation on the battery cell after the first charging operation, the determining method further includes: placing the battery cell after the first charging operation on hold for a fifth preset time;

[0038] After the reference electrodes of the battery cell after the second charging operation are respectively subjected to positive and negative lithium plating, and before the battery cell after the lithium plating is subjected to a third discharge operation to an empty state, the determining method further comprises: placing the battery cell after the lithium plating for a sixth preset time;

[0039] The third preset time is 5 minutes to 30 minutes, the fourth preset time is 5 minutes to 30 minutes, the fifth preset time is 5 minutes to 30 minutes, and the sixth preset time is 5 minutes to 30 minutes.

[0040] The beneficial effects of the technical solution of the present invention are: the determination method of the present invention is accurate and reliable, and can realize the non-destructive lithium deposition warning of the battery, help to obtain the lithium deposition threshold capacity of the aged battery cell in the actual cycle process, and at the same time obtain the N / P value of the battery cell after aging, which can mitigate the risk of lithium deposition, improve the cycle life of the battery cell, and prevent the battery cell from failing too quickly. The present invention adopts a micro-overcharge and small current mechanism to make the entire charging process tend to be non-polarized, which is conducive to accurately obtaining the lithium deposition threshold point and reducing the risk of overcharging; the capacity when the negative electrode potential of the three-electrode battery cell is 0 is used as the lithium deposition threshold point, combined with the decreasing trend of the negative electrode potential of the three-electrode battery cell, the lithium deposition point of the battery cell can be accurately identified. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A flow chart of a method for determining a lithium deposition threshold point according to an embodiment of the present invention;

[0042] Figure 2 dV / dQ curves of the electrodes and the whole battery according to the embodiment of the present invention. DETAILED DESCRIPTION

[0043] The following will be combined with the accompanying drawings to clearly and completely describe the solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0044] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, top, bottom, side, etc.) are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0045] It should also be noted that when an element is referred to as being "fixed on" or "disposed on" another element, it may be directly on the other element or there may be an intermediate element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an intermediate element.

[0046] In addition, the descriptions involving "first", "second", etc. in the embodiments of the present invention are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features.

[0047] In view of the technical defects existing in the related art, an embodiment of the present invention provides a method for determining the threshold point of lithium deposition in a battery, referring to Figure 1 , the determination method includes:

[0048] Step S100: performing a first discharge operation on the three-electrode cell to discharge the cell to an empty state;

[0049] Step S200: performing a first charging operation on the battery cell after the first discharging operation, so as to charge the battery cell to a fully charged state;

[0050] Step S300: performing a second discharging operation on the battery cell after the first charging operation, so as to discharge the battery cell to an empty state and obtain a first capacity of the battery cell;

[0051] Step S400: performing a second charging operation on the battery cell after the second discharging operation, so as to charge the battery cell to a second capacity;

[0052] Step S500, performing positive lithium plating and negative lithium plating on the reference electrode of the battery cell after completing the second charging operation;

[0053] Step S600, performing a third discharge operation on the battery cell after lithium plating to an empty state;

[0054] Step S700: performing a third charging operation on the battery cell after the third discharging operation, so as to charge the battery cell to a state of a third capacity, recording the real-time potential of the first negative electrode of the battery cell and the first real-time capacity of the battery cell during the third charging operation, and taking the real-time potential of the first negative electrode as 0 as a lithium deposition threshold point;

[0055] The second capacity is smaller than the first capacity, and the third capacity is larger than the first capacity.

[0056] In this embodiment, the above method can realize the non-destructive lithium deposition warning of the battery, help obtain the lithium deposition threshold capacity of the aged battery cell during the actual cycle process, and at the same time obtain the N / P value of the battery cell after aging, which can mitigate the risk of lithium deposition, improve the cycle life of the battery cell, and prevent the battery cell from failing too quickly. The present invention adopts a micro-overcharge and low-current mechanism to make the entire charging process tend to be non-polarized, which is conducive to accurately obtaining the lithium deposition threshold point and reducing the overcharge risk; the capacity when the negative electrode potential of the three-electrode battery cell is 0 is used as the lithium deposition threshold point, combined with the decreasing trend of the negative electrode potential of the three-electrode battery cell, the lithium deposition point of the battery cell can be accurately identified.

[0057] Specifically, the lithium plating threshold point determination method of the present application can be used for batteries with different specifications of positive active materials, and the normal operating voltage ranges corresponding to batteries with different specifications of positive active materials are also different. The normal operating voltage range of the battery in the present application is 2.5V-4.75V. Empty is the termination voltage when the battery is discharged, and full is the termination voltage when the battery is charged. Since the battery cells in the present application are battery cells in different aging stages, it is necessary to discharge the battery cells to empty first to ensure the accuracy of the measured capacity, and then the first capacity of the battery cell can be obtained by charging the battery cell to full charge and then discharging it to empty.

[0058] In some embodiments, the three-electrode battery cell includes a positive electrode, a negative electrode, a reference electrode, and an electrolyte.

[0059] In this embodiment, the battery is configured as a three-electrode cell structure, and a reference electrode is introduced to monitor changes in the internal potential of the battery in real time, thereby providing more accurate information about the battery status.

[0060] In some embodiments, in addition to recording the real-time potential of the first negative electrode of the battery cell and the first real-time capacity of the battery cell in the third charging operation, the full electrode potential of the battery cell may also be recorded.

[0061] In some embodiments, the second charging operation includes:

[0062] Performing constant current charging on the battery cell at a first preset current;

[0063] The first preset current is 0.1C to 0.5C.

[0064] In some embodiments, the second capacity is greater than or equal to 30% of the first capacity and less than or equal to 90% of the first capacity. Preferably, the second capacity is 50% of the first capacity.

[0065] Specifically, Li +It migrates from the positive electrode to the negative electrode. If it is in the high SOC range (such as above 90%), the battery cell is in a high voltage state. Although lithium plating can still be completed with a small current, the positive electrode will de-Li under this condition. + It is more likely to trigger irreversible structural phase changes, resulting in material loss, as well as negative effects such as SEI consumption on the negative electrode surface and electrolyte consumption. Therefore, it is necessary to avoid lithium plating operations at high SOC, and therefore the battery cell charging capacity is limited to the above range.

[0066] In some embodiments, after performing a second discharge operation on the battery cell after the first charge operation is completed and before performing a second charge operation on the battery cell after the second discharge operation is completed, the determining method further includes:

[0067] After completing the second discharging operation, the battery cell is left for a first preset time;

[0068] The first preset time is 5 minutes to 30 minutes.

[0069] In some embodiments, the current of the positive electrode lithium plating and the negative electrode lithium plating are both greater than 0 mA and less than or equal to 1 mA, preferably 20 μA; the time of the positive electrode lithium plating and the negative electrode lithium plating are both 2h to 4h, preferably 3h.

[0070] Specifically, taking the battery cell as a three-electrode battery cell as an example, the specific steps of lithium plating the battery cell include: respectively holding the battery cell tabs that have been placed in the three electrodes and measuring the voltage at both ends of the multimeter to see if it is normal, adjusting the battery cell SOC not to a high SOC state (>90%), because the potential of the copper wire is higher than the negative electrode corresponding to the Ni tab, it is necessary to prioritize lithium plating on the positive electrode, the positive electrode clamps the Al tab, the negative electrode clamps the reference electrode, the current selected for lithium plating is 20μA, the lithium plating time must not be less than 2h, and the use of a small current for lithium plating is more uniform and dense, thereby making the potential of the reference electrode more stable. After the lithium plating is completed, the potential of the copper wire attached to the lithium drops to a potential lower than the potential corresponding to the Ni tab. In order to ensure that the surface of the copper wire lithium plating is denser and more uniform, the negative electrode needs to be lithium plated after the positive electrode lithium plating is completed, the positive electrode clamps the Ni tab, the negative electrode clamps the reference electrode, the lithium plating time and current are the same as the positive electrode lithium plating, and after the lithium plating is completed, the voltage is monitored to see if it is within the normal range. Furthermore, during the lithium plating process, the potential and capacity of the battery cell need to be recorded according to a second preset frequency to determine whether the battery cell is in a normal state. Preferably, the second preset frequency is 10s / point.

[0071] In some embodiments, after performing a second charging operation on the battery cell after completing the second discharging operation, and before performing positive electrode lithium plating and negative electrode lithium plating on the reference electrode of the battery cell after completing the second charging operation, the determining method further includes:

[0072] After completing the second charging operation, the battery cell is left for a second preset time;

[0073] The second preset time is 5 minutes to 30 minutes.

[0074] In some embodiments, performing a third charging operation after completing the third discharging operation to charge the battery cell to a state of a third capacity, and recording the real-time potential of the first negative electrode of the battery cell and the first real-time capacity of the battery cell during the third charging operation includes:

[0075] After completing the third discharge operation, the battery cell is charged with a second preset constant current, so that the battery cell is charged to a state where the power is a third capacity;

[0076] recording the real-time potential of the first negative electrode of the battery cell and the first real-time capacity of the battery cell during the third charging operation at a first preset frequency;

[0077] The second preset current is 0.01C to 0.03C, preferably 0.02C, and the first preset frequency is 1 time / s to 10 times / s, preferably 1 time / s.

[0078] In some embodiments, the third capacity is 110% to 130% of the first capacity.

[0079] Specifically, the present application charges the capacity of the battery cell to 110% to 130% of the first capacity through a small current micro-overcharge mechanism, which can make the entire charging process approach a non-polarization process. Polarization refers to the phenomenon that the charge distribution inside the battery is uneven due to the mismatch between the electrochemical reaction rate and the charge transfer rate during the battery charging and discharging process. Polarization will cause a voltage drop inside the battery, affecting the charging efficiency and performance stability of the battery. Therefore, the present application uses a small current to charge to the process state to make the charging process of the battery cell approach a non-polarization process, and the real-time potential and real-time capacity of the battery cell during the charging process are recorded to facilitate the accurate judgment of the lithium deposition threshold point of the battery cell, and the lithium deposition threshold capacity of the aged battery cell can be obtained in the actual cycle process. At the same time, the N / P value (negative electrode positive electrode capacity ratio) after aging can be obtained. Its key role is to provide a better solution for designers to prevent the battery cell from failing too quickly. While ensuring that the battery is intact, the corresponding lithium deposition capacity is captured at the same time, which can alleviate the risk of lithium deposition, improve the cycle life of the battery cell and other industry pain points. Furthermore, by setting the first preset frequency to 1 time / s to 10 times / s, the phase change peak boundary of the dV / dQ curve drawn in the subsequent calculation of the N / P value can be clear and free of burrs, thereby achieving higher accuracy.

[0080] In some embodiments, performing the first discharge operation on the three-electrode battery cell includes: performing a constant current discharge on the three-electrode battery cell at a third preset current;

[0081] The performing a first charging operation on the battery cell after the first discharging operation includes: performing constant current and constant voltage charging on the battery cell after the first discharging operation at a fourth preset current;

[0082] The performing a second discharging operation on the battery cell after the first charging operation includes: performing a constant current discharge on the battery cell after the first charging operation at a fifth preset current;

[0083] The performing a third discharge operation on the battery cell after the lithium plating is completed comprises: performing a constant current discharge on the battery cell after the lithium plating is completed at a sixth preset current;

[0084] The third preset current is 0.1C to 0.5C, the fourth preset current is 0.4C to 0.6C, the fifth preset current is 0.1C to 0.5C, and the sixth preset current is 0.1C to 0.5C.

[0085] Specifically, since the battery cell has been charged to 50% of the first capacity before lithium plating, the battery cell also needs to be discharged to empty, so as to facilitate the subsequent small current micro-overcharging operation.

[0086] In some embodiments, before performing the first discharge operation on the three-electrode battery cell, the determination method further includes: placing the three-electrode battery cell on hold for a third preset time;

[0087] After the first discharge operation is performed on the three-electrode battery cell and before the first charge operation is performed on the battery cell after the first discharge operation, the determination method further includes: placing the battery cell after the first discharge operation for a fourth preset time;

[0088] After performing the first charging operation on the battery cell after the first discharging operation and before performing the second discharging operation on the battery cell after the first charging operation, the determining method further includes: placing the battery cell after the first charging operation on hold for a fifth preset time;

[0089] After the reference electrodes of the battery cell after the second charging operation are respectively subjected to positive and negative lithium plating, and before the battery cell after the lithium plating is subjected to a third discharge operation to an empty state, the determining method further comprises: placing the battery cell after the lithium plating for a sixth preset time;

[0090] The third preset time is 5 minutes to 30 minutes, the fourth preset time is 5 minutes to 30 minutes, the fifth preset time is 5 minutes to 30 minutes, and the sixth preset time is 5 minutes to 30 minutes.

[0091] In the embodiments of the present application, during the charge and discharge process, the concentration of a certain ion near the electrode changes due to the electrode reaction, and the diffusion rate of the ions in the bulk solution cannot keep up with this change, resulting in a concentration gradient between the concentration of the solution near the electrode and the bulk solution. The change in electrode potential caused by this concentration difference is called concentration polarization. Therefore, the shelving operations in this application are all aimed at eliminating this concentration polarization, reducing the polarization voltage, restoring the equilibrium potential, and making the electrode exhibit the best performance curve during discharge, thereby facilitating more accurate data obtained in subsequent steps.

[0092] In some embodiments, the positive electrode of the three-electrode battery cell includes a positive electrode active material, and the positive electrode active material is selected from at least one of lithium cobalt oxide, sodium-containing lithium cobalt oxide, nickel-cobalt-manganese ternary material, nickel-cobalt-aluminum ternary material, lithium iron phosphate, lithium-rich manganese-based material, lithium iron manganese phosphate, lithium titanate, lithium nickel manganese oxide, lithium nickel oxide, lithium manganese oxide, and nickel-manganese binary material; the negative electrode material of the three-electrode battery cell includes graphite.

[0093] In some embodiments, after obtaining the lithium deposition threshold point, the method further includes:

[0094] The effectiveness of the lithium deposition threshold point is judged.

[0095] Specifically, since the aging state of the battery cell cannot be determined, the accuracy of the lithium deposition threshold point cannot be known, and therefore the effectiveness of the obtained lithium deposition threshold point needs to be judged.

[0096] In some embodiments, determining the effectiveness of the lithium deposition threshold point includes:

[0097] Obtaining a charging curve of the three electrodes according to the recorded first real-time voltage and first real-time capacity;

[0098] Differentiating the charging curve to obtain a differential curve with the SOC value on the horizontal axis and dV / dQ on the vertical axis;

[0099] Obtaining the negative electrode capacity and the positive electrode capacity of the three-electrode battery cell according to the differential curve, calculating the ratio of the negative electrode capacity to the positive electrode capacity to obtain the N / P value of the three-electrode battery cell after aging;

[0100] Comparing the N / P value of the three-electrode battery cell after aging with the experimental simulation value to determine the validity of the lithium deposition threshold point;

[0101] The experimental simulation value is the N / P value obtained by testing when the positive electrode, the negative electrode and the electrolyte of the three-electrode battery cell are assembled into a button battery.

[0102] In some embodiments, comparing the N / P value of the three-electrode battery cell after aging with an experimental simulation value to determine the validity of the lithium deposition threshold point includes:

[0103] When the error between the N / P value of the three-electrode battery cell after aging and the experimental simulation value is within a preset range, the lithium deposition threshold point is effective;

[0104] And / or, when the error between the N / P value of the three-electrode battery cell after aging and the experimental simulation value is not within a preset range, the lithium deposition threshold point is not valid.

[0105] In some embodiments, the preset range is -0.5% to 0.5%.

[0106] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be further described in detail below in conjunction with specific implementation methods, but the implementation methods of the present invention are not limited thereto.

[0107] Example 1

[0108] The battery cell is a three-electrode battery cell, wherein the positive electrode active material of the three-electrode battery cell is lithium cobalt oxide, and the negative electrode active material is graphite.

[0109] A method for determining a battery lithium deposition threshold point, comprising:

[0110] Step S100: Leave the battery cell for 5 minutes and discharge it at a constant current of 0.2C until it is empty.

[0111] Step S200: Leave the battery cell for 5 minutes and charge it to full power at 0.5C constant current and constant voltage;

[0112] Step S300: The battery cell is left for 5 minutes and discharged at a constant current of 0.2C until it is empty, thereby obtaining a first capacity Q1 of the battery cell;

[0113] Step S400 , placing the battery cell for 5 minutes and charging the battery cell to a second capacity Q2 at a constant current of 0.2C, where Q2 = 50% of Q1;

[0114] Step S500: The battery cell is placed aside for 5 minutes, and the positive electrode and negative electrode of the reference electrode of the battery cell are plated with lithium in sequence at a lithium plating current of 20 μA, with the lithium plating time being 3 hours for the positive electrode and 3 hours for the negative electrode, and the battery cell is placed aside for 10 minutes after the lithium plating;

[0115] Step S600: The battery cell is left for 5 minutes, and the battery cell is discharged again at a constant current of 0.2C until it is empty;

[0116] Step S700, the battery cell is placed for 5 minutes, and the battery cell is charged to a third capacity Q3 at a constant current of 0.02C, Q3=120% Q1, and the first real-time potential and the first real-time capacity of the battery cell during the charging process are recorded at a frequency of 1 time / s, and the negative electrode potential is 0 as the lithium deposition threshold point, and the SOC value and capacity corresponding to the lithium deposition threshold point are recorded.

[0117] like Figure 2 As shown, the charging curve and dV / dQ curve of the battery cell in Example 1 are shown, wherein: Figure 2 The circled portion in (a) is the lithium deposition area of ​​the battery cell. Figure 2 The marked point in (b) is the capacity corresponding to the negative electrode lithium deposition threshold point. Figure 2 (d) is Figure 2 The enlarged view of the box part (c) Figure 2 Point C in (d) is the capacity Q corresponding to the negative electrode lithium deposition threshold point C , draw a straight line parallel to the X axis from point C to the Y axis, and mark the adjacent intersection of this straight line and the dV / dQ curve as point A. Point A is the capacity Q of the positive electrode when it is at the lithium deposition threshold point. A , the N / P value of the battery cell = Q C / Q A .

[0118] Accuracy Verification: The accuracy of the method was verified by measuring the N / P ratio of fresh button-type batteries of the same system. Because button-type batteries eliminate the influence of the consistency of pouch cells, the test results are more accurate and have reference value.

[0119] Button cell battery preparation operation

[0120] 1. Pole sheet production: The formula must be determined before pole sheet production. The cathode and anode sheets obtained in this process are linked to the final N / P value of the button battery. The positive pole sheet contains active substances, conductive carbon black, PVDF, and NMP, among other chemicals; the negative pole sheet mixing material contains active substances, CMC, SBR, and other chemicals. After processing through stirring, coating, baking, and roller pressing, the final pole sheet morphology should meet the requirements of uniform color, uniform thickness, no obvious scratches, no particulate matter, no powder loss, and no exposed bottom. The button battery uses the same positive and negative active materials as the three-electrode cell in Example 1.

[0121] 2. Sample Selection: Weigh the electrode using an analytical balance and record the raw data. Calculate the N / P ratio, assuming the cathode and anode are equal in area. This N / P ratio corresponds to the design value in the examples. Take three parallel samples of the selected electrode and punch them into discs using a punching machine. After punching, dry the electrode at 60°C for 2 hours and weigh it. The weight should be close to the disc weight for assembly.

[0122] 3. Button Cell Assembly: Assemble button cells while maintaining a water / oxygen concentration of less than 0.1ppm in the glove box. Six parallel samples of button cells at different N / P ratios must be assembled simultaneously. Prepare the required quantities of steel casings, separators, electrolyte, and other auxiliary materials in advance. Place the negative electrode casing on a sheet of cleanroom paper and place the spring clip opening downward into the negative electrode casing. Place a gasket (smooth side facing up) over the spring clip. Place the positive electrode casing on a sheet of cleanroom paper and place the electrode sheet, coated side up, in the center of the positive electrode casing. Use a pipette to drip 50μL of electrolyte into the positive electrode casing. Place the separator over the electrode sheet and adjust it to completely cover the positive electrode casing. Drip an equal amount of electrolyte onto the separator. Place the negative electrode sheet, coated area downward, onto the separator. Then, flip the positive electrode casing upside down and align it with the negative electrode casing for assembly. Using insulated tweezers, place the assembled button cell, negative electrode facing upward, into a hydraulic sealer. Place cleanroom paper under the seal to absorb the electrolyte. Once sealed, assembly is complete. The assembled button cell was placed in a dust-free bag and transferred outside the glove box. After recording its internal resistance, it was left to stand at room temperature for >6 h.

[0123] 4. Button battery test:

[0124] Step 1: Set aside for 5 minutes;

[0125] Step 2: Charge at 0.5C constant current and constant voltage until fully charged;

[0126] Step 3: Set aside for 5 minutes;

[0127] Step 4: Discharge at a constant current of 0.04C until the battery is empty. Set the sampling interval to 1s / point and record the capacity as Q4.

[0128] Step 5: Set aside for 5 minutes;

[0129] Step 6: Charge at 0.02C constant current and constant voltage to 110% Q4, set the sampling interval to 1s / point, and record the capacity corresponding to the lithium deposition threshold point;

[0130] Step 7: Let stand for 5 minutes.

[0131] Based on the button cell test results, we created a charging curve and a dV / dQ curve to calculate the N / P ratio, which served as the N / P experimental simulation value. Four sets of parallel samples were prepared. The results are shown in Table 1.

[0132] Table 1 Test results of N / P value of battery cells in Example

[0133] Sample 1 Sample 2 Sample 3 Sample 4 N / P design value 1.065 1.054 1.044 1.026 N / P experimental simulation value 1.062 1.057 1.042 1.032 N / P measured value 1.058 1.052 1.039 1.028

[0134] Note: The N / P design values ​​in Table 1 are given values ​​of the cell design parameters (if laser drilling technology is added to the cell, the actual N / P value has been taken into account), the N / P experimental simulation values ​​are the actual N / P values ​​determined by testing when the positive and negative electrodes and the electrolyte of the cell are assembled into a button cell, and the N / P measured values ​​are the N / P values ​​obtained by measuring the negative electrode potential of the three-electrode cell and analyzing the lithium precipitation threshold point through differential curve analysis using the method of Example 1.

[0135] As can be seen from Table 1, the errors between all the measured values ​​of Example 1 and the experimental simulation values ​​of the power-off are controlled within 0.5%, which proves that the method of the present application is effective and the errors are all within the allowable range. The lithium deposition threshold points determined are accurate and effective. Therefore, the method of the present invention can realize the non-destructive lithium deposition early warning of the battery, help to obtain the lithium deposition threshold capacity of the aged battery cell during the actual cycle process, and at the same time obtain the N / P value of the battery cell after aging, which can mitigate the risk of lithium deposition, improve the cycle life of the battery cell, and prevent the battery cell from failing too quickly.

[0136] The above description is only a partial or preferred embodiment of the present invention. Neither the text nor the drawings can limit the scope of protection of the present invention. Any equivalent structural transformation made by using the contents of the present invention specification and drawings under the overall concept of the present invention, or direct / indirect application in other related technical fields, is included in the scope of protection of the present invention.

Claims

1. A method for determining a battery lithium deposition threshold point, characterized in that: include: Performing a first discharge operation on the three-electrode battery cell to discharge the battery cell to an empty state; performing a first charging operation on the battery cell after the first discharging operation is completed, so as to charge the battery cell to a fully charged state; performing a second discharging operation on the battery cell after the first charging operation is completed, so as to discharge the battery cell to an empty state and obtain a first capacity of the battery cell; performing a second charging operation on the battery cell after the second discharging operation, so as to charge the battery cell to a state where the power reaches a second capacity; Performing positive and negative lithium plating on the reference electrodes of the battery cell after the second charging operation; Performing a third discharge operation on the battery cell after lithium plating to an empty state; performing a third charging operation on the battery cell after the third discharging operation, wherein the third charging operation is constant current charging with a second preset current, so that the battery cell is charged to a state of a third capacity; Recording the first negative electrode real-time potential and the first real-time capacity of the battery cell in the third charging operation at a first preset frequency, wherein the second preset current is 0.01C to 0.03C, the first preset frequency is 1 time / s to 10 times / s, and the capacity when the first negative electrode real-time potential is 0 is used as the lithium deposition threshold point; After obtaining the lithium deposition threshold point, the method further includes: judging the validity of the lithium deposition threshold point; The determining of the validity of the lithium deposition threshold point includes: Obtaining a charging curve of the three electrodes according to the recorded first real-time voltage and first real-time capacity; differentiating the charging curve to obtain a differential curve with an SOC value as the horizontal coordinate and a dV / dQ as the vertical coordinate; obtaining a negative electrode capacity and a positive electrode capacity of the three-electrode battery cell according to the differential curve, calculating a ratio of the negative electrode capacity to the positive electrode capacity to obtain an N / P value of the three-electrode battery cell after aging; comparing the N / P value of the three-electrode battery cell after aging with an experimental simulation value to determine the validity of the lithium deposition threshold point; When the error between the N / P value of the three-electrode battery cell after aging and the experimental simulation value is within a preset range, the lithium deposition threshold point is valid; and / or when the error between the N / P value of the three-electrode battery cell after aging and the experimental simulation value is not within a preset range, the lithium deposition threshold point is not valid; the preset range is -0.5% to 0.5%; The second capacity is smaller than the first capacity, and the third capacity is larger than the first capacity.

2. The method for determining the battery lithium deposition threshold point according to claim 1, wherein: The second charging operation includes: Performing constant current charging on the battery cell at a first preset current; The first preset current is 0.1C to 0.5C.

3. The method for determining the battery lithium deposition threshold point according to claim 1 or 2, wherein: The second capacity is greater than or equal to 30% and less than or equal to 90% of the first capacity.

4. The method for determining the battery lithium deposition threshold point according to claim 1, wherein: After performing a second discharge operation on the battery cell after the first charge operation is completed, and before performing a second charge operation on the battery cell after the second discharge operation is completed, the determining method further includes: After completing the second discharging operation, the battery cell is left for a first preset time; The first preset time is 5 minutes to 30 minutes.

5. The method for determining the battery lithium deposition threshold point according to claim 1, wherein: The currents of the positive electrode lithium plating and the negative electrode lithium plating are both greater than 0 mA and less than or equal to 1 mA, and the durations of the positive electrode lithium plating and the negative electrode lithium plating are both 2 hours to 4 hours.

6. The method for determining the battery lithium deposition threshold point according to claim 1 or 5, characterized in that: After performing a second charging operation on the battery cell after completing the second discharging operation, and before performing positive electrode lithium plating and negative electrode lithium plating on the reference electrode of the battery cell after completing the second charging operation, the determining method further includes: After completing the second charging operation, the battery cell is left for a second preset time; The second preset time is 5 minutes to 30 minutes.

7. The method for determining the battery lithium deposition threshold point according to claim 1, wherein: The third capacity is 110% to 130% of the first capacity.

8. The method for determining the battery lithium deposition threshold point according to claim 1, wherein: The performing a first discharge operation on the three-electrode battery cell includes: performing a constant current discharge on the three-electrode battery cell at a third preset current; The performing a first charging operation on the battery cell after the first discharging operation includes: performing constant current and constant voltage charging on the battery cell after the first discharging operation at a fourth preset current; The performing a second discharging operation on the battery cell after the first charging operation includes: performing a constant current discharge on the battery cell after the first charging operation at a fifth preset current; The performing a third discharge operation on the battery cell after the lithium plating is completed comprises: performing a constant current discharge on the battery cell after the lithium plating is completed at a sixth preset current; The third preset current is 0.1C to 0.5C, the fourth preset current is 0.4C to 0.6C, the fifth preset current is 0.1C to 0.5C, and the sixth preset current is 0.1C to 0.5C.

9. The method for determining the battery lithium deposition threshold point according to claim 1 or 8, characterized in that: Before performing the first discharge operation on the three-electrode battery cell, the determination method further includes: placing the three-electrode battery cell on hold for a third preset time; After the first discharge operation is performed on the three-electrode battery cell and before the first charge operation is performed on the battery cell after the first discharge operation, the determination method further includes: placing the battery cell after the first discharge operation for a fourth preset time; After performing the first charging operation on the battery cell after the first discharging operation and before performing the second discharging operation on the battery cell after the first charging operation, the determining method further includes: placing the battery cell after the first charging operation on hold for a fifth preset time; After the reference electrodes of the battery cell after the second charging operation are respectively subjected to positive and negative lithium plating, and before the battery cell after the lithium plating is subjected to a third discharge operation to an empty state, the determining method further comprises: placing the battery cell after the lithium plating for a sixth preset time; The third preset time is 5 minutes to 30 minutes, the fourth preset time is 5 minutes to 30 minutes, the fifth preset time is 5 minutes to 30 minutes, and the sixth preset time is 5 minutes to 30 minutes.

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