A method for analyzing capacity fading based on a three-electrode lithium ion battery

Abstract

The application discloses a method for analyzing capacity attenuation based on a three-electrode lithium ion battery, which can perform nondestructive detection on the lithium ion battery, judges the cause of capacity attenuation of the lithium ion battery by analyzing a discharge voltage curve V-Q graph and a voltage differential curve dV / dQ-Q graph under different cycle numbers in cycle aging without disassembling the battery, and then can propose the root cause of battery failure and provide a basis and direction for performance improvement of the battery in the future.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for analyzing capacity decay in three-electrode lithium-ion batteries. Background Technology

[0002] Environmental pollution and energy depletion are two major problems facing humanity. To address these dual pressures, most countries worldwide have prioritized the development of electric vehicles as a crucial means of energy conservation and emission reduction in recent years. The power battery is one of the core components of an electric vehicle, and its performance directly impacts the overall performance of the vehicle. To ensure the driving and safety performance of electric vehicles, the lithium-ion batteries must be replaced after a certain mileage. In the battery industry, battery capacity is typically used as an indicator of whether a battery is ready for retirement. However, lithium-ion batteries experience continuous capacity decay during use. Therefore, researching and analyzing the mechanism of lithium-ion battery capacity decay is crucial.

[0003] The capacity decay of lithium-ion batteries can be caused by the loss of active materials in the positive electrode, the loss of active materials in the negative electrode, or the loss of usable lithium ions. Different positive and negative electrode materials, different cycle conditions, and different environmental conditions lead to different mechanisms of capacity decay.

[0004] Research on battery capacity degradation mechanisms often requires disassembling the battery and using methods such as XRD (X-ray Diffraction) and SEM (Scanning Electron Microscopy) to analyze changes in the positive and negative electrodes, thereby determining the degradation mechanism. However, for batteries used in actual electric vehicles, such damaging methods are completely impractical. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a method for analyzing capacity decay in three-electrode lithium-ion batteries.

[0006] The technical problem to be solved by the present invention is achieved through the following technical solution:

[0007] A method for analyzing capacity decay in a three-electrode lithium-ion battery includes the following steps:

[0008] S1. Provides a three-electrode lithium-ion battery;

[0009] S2. Perform a cycle aging test on the three-electrode lithium-ion battery. During the cycle aging test, at certain intervals, perform battery capacity tests on the three-electrode lithium-ion battery at different cycle counts. The battery capacity test includes: a. constant current charging to a set charging cutoff voltage; b. constant voltage charging to a set current; c. constant current discharge after resting; e. During the battery capacity test, collect battery capacity Q data and voltage V data of the three-electrode lithium-ion battery at a preset frequency during the discharge process, wherein the voltage V data includes the full cell voltage V0. F Data, positive voltage V P Data and negative voltage V N data;

[0010] S3. After the three-electrode lithium-ion battery has been cycle-aged to the specified battery life end condition, stop the cycle aging test;

[0011] S4. Plot the discharge voltage curves VQ diagrams for different cycle numbers in the same coordinate system. The discharge voltage curves VQ diagrams include the full battery voltage curve V. F -Q, Positive voltage curve V P -Q and negative electrode voltage curve V N -Q;

[0012] S5. Based on the discharge voltage curve VQ diagram, analyze the reasons for the capacity decay of the lithium-ion battery.

[0013] Furthermore, based on the discharge voltage curve VQ diagram, the specific reasons for the capacity decay of lithium-ion batteries include:

[0014] If the full cell voltage curve V F In the -Q diagram, the discharge is cut off at the positive electrode voltage at the end of the discharge period, indicating that the main reason for the capacity decay of the lithium-ion battery is the loss of the positive electrode active material inside the lithium-ion battery.

[0015] If the full cell voltage curve V F In the -Q diagram, the battery is cut off at the negative electrode voltage at the end of the discharge period. It is determined that the main reason for the capacity decay of the lithium-ion battery is the loss of the negative electrode active material inside the lithium-ion battery or the loss of the available lithium ions in the battery.

[0016] Among them, the positive voltage curve V P If -Q shows a significant decrease at the end of the discharge period, then the full cell voltage curve V is determined. F In the -Q diagram, the discharge is cut off at the positive terminal voltage at the end of the discharge period; otherwise, it is determined that the discharge is cut off at the negative terminal voltage at the end of the discharge period.

[0017] Furthermore, if the full-cell voltage curve V F In the -Q diagram, if the discharge is cut off at the negative electrode voltage at the end of the discharge phase, it also includes:

[0018] The collected battery capacity Q data and voltage V data are differentiated to obtain dQ and dV;

[0019] The collected battery capacity Q data is used as the X-axis data, and the dV / dQ ratio is used as the Y-axis data. After filtering the dV / dQ ratio using a Gaussian filtering method, the voltage differential curve dV / dQ-Q plot is obtained; wherein, the voltage differential curve dV / dQ-Q plot includes the full battery voltage differential curve dV F / dQ-Q, positive voltage differential curve dV P / dQ-Q and the differential curve of the negative electrode voltage dV N / dQ-Q;

[0020] The differential curve dV of the negative electrode voltage obtained from battery capacity tests at different cycle numbers. N Align / dQ-Q to the right in the same coordinate system and analyze the reasons for the capacity decay of lithium-ion batteries;

[0021] If the right halves of the differential curves of the negative electrode voltages basically overlap, then the main reason for the capacity decay of the lithium-ion battery is the loss of usable lithium ions in the lithium-ion battery; otherwise, the main reason for the capacity decay of the lithium-ion battery is the loss of the active material of the negative electrode inside the lithium-ion battery.

[0022] Furthermore, the battery capacity test includes the following steps:

[0023] a. Under a constant temperature environment of 25℃±2℃, charge at a constant current of unit current I until the charging cutoff voltage V1;

[0024] b. Charge the three-electrode lithium-ion battery with constant voltage V1 until the current drops to 0.05I and then stop. After charging, let it stand for 60 minutes.

[0025] c. Discharge at a constant current of unit current I until the discharge cutoff voltage V2, and let stand for 60 minutes after the discharge is completed;

[0026] d. Calculate the discharge capacity;

[0027] e. Repeat steps a to d. When the range of three consecutive test results is less than 3% of the rated value, end the test early and take the average of the last three results.

[0028] f. During the battery capacity test, the battery capacity Q data and voltage V data of the three-electrode lithium-ion battery are collected at a preset frequency during the discharge process, wherein the voltage V data includes the full-cell voltage V. F Data, positive voltage V P Data and negative voltage V N data.

[0029] Furthermore, the unit current I is 0.2C, the charging cutoff voltage V1 is 4.2V, and the preset frequency is 10s-30s.

[0030] Furthermore, the cyclic aging test is a high-temperature accelerated cycling experiment, specifically:

[0031] (1) Under a constant temperature environment of 45℃±2℃, the three-electrode lithium-ion battery is charged to 4.2V under a constant current of 0.5C, and then charged under a constant voltage until the current of the three-electrode lithium-ion battery drops to 0.01Cn; (2) After standing for 1 hour, it is discharged under a constant current of 1C until the voltage drops to 2.75V, and then left to stand for 1 hour; (3) Calculate the discharge capacity; (4) Repeat steps (1)-(3) for a cycle.

[0032] Furthermore, the cyclic aging test is a high-rate charge-discharge cycle.

[0033] Furthermore, in step S3, the three-electrode lithium-ion battery is cycle-aged until the discharge capacity relative to the initial discharge capacity decreases to 80%, at which point the cycle aging test is stopped.

[0034] The present invention has the following beneficial effects:

[0035] This invention provides a method for analyzing capacity decay in three-electrode lithium-ion batteries. It can perform non-destructive testing on lithium-ion batteries. Without disassembling the battery, by analyzing the discharge voltage curve (VQ diagram) and voltage differential curve (dV / dQ-Q diagram) at different cycles during cyclic aging, the cause of capacity decay in lithium-ion batteries can be determined. This allows for the identification of the root cause of battery failure and provides a basis and direction for future battery performance improvements. Attached Figure Description

[0036] Figure 1 This is a discharge voltage curve VQ diagram of a different number of cycles according to an embodiment of the present invention;

[0037] Figure 2 This is a voltage differential curve dV / dQ-Q plot of a certain number of cycles according to an embodiment of the present invention;

[0038] Figure 3 This is the voltage differential curve dV under different cycle numbers according to an embodiment of the present invention. N / dQ-Q graph. Detailed Implementation

[0039] A method for analyzing capacity decay in a three-electrode lithium-ion battery includes the following steps:

[0040] S1. Provides a three-electrode lithium-ion battery;

[0041] In this invention, the structure of the three-electrode lithium-ion battery is not specifically limited, as it can be known to those skilled in the art through technical manuals or conventional experimental methods. For example, the three-electrode lithium-ion battery can be a three-electrode battery disclosed in CN204130649U, including an upper cover, a lower cover that snaps onto the upper cover, and a positive electrode, a negative electrode, a separator, an electrolyte, and a reference electrode disposed within the upper and lower covers, wherein the reference electrode is a micron-sized platinum or copper wire with a pre-plated lithium layer; the three-electrode lithium-ion battery can also be a three-electrode device for a lithium-ion battery disclosed in CN202949008U, where the length and width of the positive and negative electrodes are both 1–3 cm; the reference electrode is a lithium metal sheet with a diameter of 10–20 mm and a thickness of 0.2 cm; and the tabs of the positive, negative, and reference electrodes extend from different sides of the casing. It is understood that the three-electrode lithium-ion battery of this invention includes, but is not limited to, the structures listed above, and can also be other structures not listed in this embodiment but well known to those skilled in the art.

[0042] S2. Perform a cycle aging test on the three-electrode lithium-ion battery. During the cycle aging test, at certain intervals, test the battery capacity of the three-electrode lithium-ion battery at different cycle numbers.

[0043] The battery capacity test includes: a. constant current charging to a set charging cutoff voltage; b. constant voltage charging to a set current; c. constant current discharge after resting; e. during the battery capacity test, collecting battery capacity Q data and voltage V data of the three-electrode lithium-ion battery at a preset frequency during the discharge process, wherein the voltage V data includes the full cell voltage V0. F Data, positive voltage V P Data and negative voltage V N data;

[0044] More specifically, the battery capacity test includes the following steps:

[0045] a. Under a constant temperature environment of 25℃±2℃, charge at a constant current of unit current I until the charging cutoff voltage V1;

[0046] b. Charge the three-electrode lithium-ion battery with constant voltage V1 until the current drops to 0.05I and then stop. After charging, let it stand for 60 minutes.

[0047] c. Discharge at a constant current of unit current I until the discharge cutoff voltage V2, and let stand for 60 minutes after the discharge is completed;

[0048] d. Calculate the discharge capacity;

[0049] e. Repeat steps a to d. When the range of three consecutive test results is less than 3% of the rated value, end the test early and take the average of the last three results.

[0050] f. During the battery capacity test, the battery capacity Q data and voltage V data of the three-electrode lithium-ion battery are collected at a preset frequency during the discharge process, wherein the voltage V data includes the full-cell voltage V. F Data, positive voltage V P Data and negative voltage V N Data. Preferably, the unit current I is 0.2C, and the charging cutoff voltage V1 is 4.2V.

[0051] Preferably, the preset frequency is 10s-30s. If the preset frequency is too short, the data volume will be too large; if the preset frequency is too long, the phase transition process cannot be captured, and the analysis will be meaningless.

[0052] In this invention, the cyclic aging test can be a high-temperature accelerated cycle experiment or a high-rate charge-discharge cycle. It is understood that the cyclic aging test of this invention includes, but is not limited to, the test methods listed above, or other cyclic aging test methods not listed in this embodiment but well known to those skilled in the art.

[0053] In this invention, the device for collecting battery capacity Q and voltage V data of a three-electrode lithium-ion battery during discharge is not specifically limited; conventional devices used in the prior art can be employed. Preferably, an Arbin array is used to connect the positive and negative electrodes of the three-electrode lithium-ion battery for capacity testing, while an Agilent array is used to collect the voltage on one electrode relative to a reference electrode. Thus, the full battery voltage V... F Data, positive voltage V P Data and negative voltage V N The data can be obtained simultaneously.

[0054] It is understood that the electrodes of a three-electrode lithium-ion battery are the positive electrode, the negative electrode, and the reference electrode. In this invention, the full cell voltage V... F This refers to the voltage data between the positive and negative terminals; positive voltage V P This refers to the voltage data between the positive electrode and the reference electrode; the negative electrode voltage V. N This refers to the voltage data between the negative electrode and the reference electrode.

[0055] Lithium-ion batteries are chemical batteries, and electrochemical reactions occur inside them constantly during charging and discharging. This requires a method for in-situ detection of the chemical reactions inside the battery. This invention is based on the capacity decay analysis of a three-electrode lithium-ion battery. By introducing a reference electrode, in-situ monitoring of the battery's internal structure can be performed, which can help researchers better understand the electrochemical reactions that occur at the positive and negative electrodes during charging and discharging.

[0056] S3. After the three-electrode lithium-ion battery has been cycle-aged to the specified battery life end condition, stop the cycle aging test;

[0057] Preferably, the three-electrode lithium-ion battery is cycle-aged until the discharge capacity relative to the initial discharge capacity decreases to 80%, at which point the cycle aging test is stopped.

[0058] S4. Plot the discharge voltage curves VQ diagrams for different cycle numbers in the same coordinate system. The discharge voltage curves VQ diagrams include the full battery voltage curve V. F -Q, Positive voltage curve V P -Q and negative electrode voltage curve V N -Q;

[0059] The discharge voltage curve VQ graph uses voltage V as the vertical axis and battery capacity Q as the horizontal axis.

[0060] S5. Based on the discharge voltage curve VQ diagram, analyze the reasons for the capacity decay of the lithium-ion battery.

[0061] If the full cell voltage curve V F In the -Q diagram, the discharge is cut off at the positive electrode voltage at the end of the discharge period, indicating that the main reason for the capacity decay of the lithium-ion battery is the loss of the positive electrode active material inside the lithium-ion battery.

[0062] If the full cell voltage curve V F In the -Q diagram, the battery cuts off at the negative electrode voltage at the end of the discharge phase. This indicates that the main reason for the capacity decay of the lithium-ion battery is the loss of the internal negative electrode active material or the loss of usable lithium ions. Further analysis is needed at this point. The specific analysis method is as follows:

[0063] The collected battery capacity Q data and voltage V data are differentiated to obtain dQ and dV;

[0064] The collected battery capacity Q data is used as the X-axis data, and the dV / dQ ratio is used as the Y-axis data. After filtering the dV / dQ ratio using a Gaussian filtering method, the voltage differential curve dV / dQ-Q plot is obtained; wherein, the voltage differential curve dV / dQ-Q plot includes the full battery voltage differential curve dV F / dQ-Q, positive voltage differential curve dV P / dQ-Q and the differential curve of the negative electrode voltage dV N / dQ-Q;

[0065] The differential curve dV of the negative electrode voltage obtained from battery capacity tests at different cycle numbers. N Align / dQ-Q to the right in the same coordinate system and analyze the reasons for the capacity decay of lithium-ion batteries;

[0066] If the right halves of the differential curves of the negative electrode voltages basically overlap, then the main reason for the capacity decay of the lithium-ion battery is the loss of usable lithium ions in the lithium-ion battery; otherwise, the main reason for the capacity decay of the lithium-ion battery is the loss of the active material of the negative electrode inside the lithium-ion battery.

[0067] Lithium-ion batteries can be simplified into a continuous cycle of charging, resting, and discharging during use. During charging, lithium ions are released from the positive electrode, pass through the separator, and gain electrons on the surface of the negative electrode. During discharging, lithium ions are released from the negative electrode, pass through the separator, and embed into the positive electrode. This invention focuses on the discharge process of a lithium-ion battery. When lithium ions are released from the negative electrode and embed into the positive electrode, and the positive electrode, as the host structure for lithium ions, has no extra vacancy sites for embedding, according to the Gibbs law, the rate of decrease in the positive electrode potential will accelerate, thus causing the full cell voltage to reach the discharge cutoff voltage. In this invention, the positive electrode voltage curve V... P If -Q shows a significant decrease at the end of the discharge period, i.e., the positive electrode voltage curve V P -Q exhibits a small tail at the end of discharge, indicating a significant drop in voltage relative to the plateau. The appearance of this voltage tail suggests that the positive electrode active material no longer has a host structure suitable for lithium-ion intercalation. The greater the drop in the voltage tail, the more the full cell voltage is affected by the positive electrode voltage. Therefore, the full cell voltage curve V is used to determine... F In the -Q diagram, the discharge is cut off at the positive electrode voltage at the end of the discharge period; otherwise, it is determined that the discharge is cut off at the negative electrode voltage at the end of the discharge period. As active lithium ions are released from the negative electrode, the negative electrode voltage plateau will gradually rise, and the rate of increase of the negative electrode voltage curve will accelerate at the end of the discharge. The negative electrode voltage cutoff described in this invention needs to be referenced to the positive electrode voltage curve and the specific value of the negative electrode potential during the initial battery capacity calibration.

[0068] The "active material" mentioned in this invention refers to the material in the positive and negative electrodes of a lithium-ion battery that participates in the lithium-ion insertion / extraction reaction during the charging and discharging process. Commonly used positive electrode active materials include lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium manganese oxide (LMO), and nickel-cobalt-manganese ternary materials (NCM) or nickel-cobalt-aluminum ternary materials (NCA), etc. Commonly used negative electrode active materials include graphite and lithium titanate, etc.

[0069] The present invention will now be described in detail with reference to the embodiments. These embodiments are merely preferred implementations of the present invention and are not intended to limit the present invention.

[0070] A method for analyzing capacity decay in a three-electrode lithium-ion battery includes the following steps:

[0071] S1. Provide a three-electrode lithium-ion battery, wherein the three-electrode lithium-ion battery adopts a ternary cylindrical lithium-ion battery Li[Ni0.8Co0.1Mn0.1]O2 with a nominal capacity of 2.75Ah;

[0072] S2. Perform a cycle aging test on the three-electrode lithium-ion battery. During the cycle aging test, at 100-cycle intervals, test the battery capacity of the three-electrode lithium-ion battery at different cycle numbers.

[0073] The battery capacity test includes the following steps:

[0074] a. Under a constant temperature environment of 25℃±2℃, charge at a constant current of 0.2C until the charging cutoff voltage of 4.2V;

[0075] b. Charge the three-electrode lithium-ion battery at a constant voltage of 4.2V until the current drops to 0.01C and then stop. After charging, let it stand for 60 minutes.

[0076] c. Discharge at a constant current of 0.2C until the discharge cutoff voltage is reached, and let stand for 60 minutes after the discharge is completed;

[0077] d. Calculate the discharge capacity;

[0078] e. Repeat steps a to d. When the range of three consecutive test results is less than 3% of the rated value, end the test early and take the average of the last three results.

[0079] f. During the battery capacity test, the battery capacity Q data and voltage V data of the three-electrode lithium-ion battery are collected at a preset frequency of 10s during constant current discharge. The voltage V data includes the full-cell voltage Vc. F Data, positive voltage V P Data and negative voltage V N data;

[0080] S3. Cyclic aging of the three-electrode lithium-ion battery is stopped when the discharge capacity relative to the initial discharge capacity decreases to 80%.

[0081] S4. Plot the discharge voltage curves VQ diagrams for different cycle numbers in the same coordinate system. The discharge voltage curves VQ diagrams include the full battery voltage curve V. F -Q, Positive voltage curve V P-Q and negative electrode voltage curve V N -Q; such as Figure 1 As shown;

[0082] S5. Based on the discharge voltage curve VQ diagram, analyze the reasons for the capacity decay of the lithium-ion battery:

[0083] If the full cell voltage curve V F In the -Q diagram, the positive electrode voltage is cut off at the end of the discharge period, indicating that the reason for the capacity decay of the lithium-ion battery is the loss of the positive electrode active material inside the lithium-ion battery.

[0084] If the full cell voltage curve V F In the -Q diagram, the battery is cut off at the negative electrode voltage at the end of the discharge period. It is determined that the reason for the capacity decay of the lithium-ion battery is the loss of the negative electrode active material inside the lithium-ion battery or the loss of the available lithium ions in the battery.

[0085] Among them, the positive voltage curve V P If -Q shows a significant decrease at the end of the discharge period, then the full cell voltage curve V is determined. F In the -Q diagram, the discharge is cut off at the positive terminal voltage at the end of the discharge period; otherwise, it is determined that the discharge is cut off at the negative terminal voltage at the end of the discharge period.

[0086] If the full cell voltage curve V F In the -Q plot, where the discharge is cut off at the negative electrode voltage at the end, the process also includes: differentiating the collected battery capacity Q data and voltage V data to obtain dQ and dV; using the collected battery capacity Q data as the X-axis data and the ratio of dV / dQ as the Y-axis data, and filtering the dV / dQ ratio using a Gaussian filtering method to obtain the voltage differential curve dV / dQ-Q plot, as shown below. Figure 2 As shown; wherein, the voltage differential curve dV / dQ-Q diagram includes the full-cell voltage differential curve dV F / dQ-Q, positive voltage differential curve dV P / dQ-Q and the differential curve of the negative electrode voltage dV N / dQ-Q; see also Figure 3 The differential curve dV of the negative electrode voltage obtained from battery capacity tests at different cycle numbers is used. N Align / dQ-Q to the right in the same coordinate system and analyze the reasons for the capacity decay of lithium-ion batteries; if the right half of the differential curves of each negative electrode voltage basically overlap, then it is determined that the reason for the capacity decay of the lithium-ion battery is the loss of usable lithium ions in the lithium-ion battery; otherwise, it is determined that the reason for the capacity decay of the lithium-ion battery is the loss of active material of the negative electrode inside the lithium-ion battery.

[0087] The cycle aging test is as follows: (1) Under a constant temperature environment of 45℃±2℃, the three-electrode lithium-ion battery is charged to 4.2V at a constant current of 0.5C, and then charged at a constant voltage until the current of the three-electrode lithium-ion battery drops to 0.01C; (2) After standing for 1 hour, it is discharged at a constant current of 1C until the voltage drops to 2.75V, and then left to stand for 1 hour; (3) Calculate the discharge capacity; (4) Repeat steps (1)-(3) to perform the cycle test.

[0088] exist Figure 1 As can be seen from the data, in the initial stage of cycle aging, the full cell voltage curve V... F The -Q diagram shows a cutoff voltage at the negative electrode, gradually transitioning to a cutoff voltage at the positive electrode during cyclic aging. This indicates that at the beginning of cyclic aging, the capacity decay of a lithium-ion battery is due to the loss of the negative electrode active material or the loss of usable lithium ions. However, during cyclic aging, the positive electrode active material cycles faster, thus its loss gradually becomes the cause of capacity decay.

[0089] exist Figure 3 As can be seen from the curves, the right half of the differential curves of each negative electrode voltage basically overlaps, so it can be determined that the reason for the capacity decay of the lithium-ion battery is the loss of usable lithium ions in the lithium-ion battery.

[0090] Therefore, it can be concluded that the initial capacity decay of this lithium-ion battery during cycle aging is mainly due to the loss of usable lithium ions. In the later stages of cycle aging, the main cause of capacity decay is the loss of the positive electrode active material, possibly due to a phase change in the structure of the positive electrode active material.

[0091] The above embodiments merely illustrate the implementation of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention. Any technical solutions obtained by adopting equivalent substitutions or equivalent transformations should fall within the protection scope of the present invention.

Claims

1. A method for analyzing capacity fade based on a three-electrode lithium-ion battery, characterized in that, The method comprises the following steps: S1. providing a three-electrode lithium ion battery; S2. performing a cycle aging test on the three-electrode lithium ion battery, and performing a battery capacity test under different cycle numbers on the three-electrode lithium ion battery at intervals during the cycle aging test; wherein the battery capacity test comprises: a. constant current charging to a set charging cut-off voltage; b. constant voltage charging to a set current; c. constant current discharging after standing; e. during the battery capacity test, collecting battery capacity Q data and voltage V data of the three-electrode lithium ion battery during discharging at a preset frequency, wherein the voltage V data comprises full battery voltage VF data, positive electrode voltage VP data and negative electrode voltage VN data; S3. stopping the cycle aging test after the three-electrode lithium ion battery is cycled to a specified battery life cut-off condition; S4. drawing a discharging voltage curve V-Q graph under different cycle numbers in the same coordinate system, wherein the discharging voltage curve V-Q graph comprises a full battery voltage curve VF-Q, a positive electrode voltage curve VP-Q and a negative electrode voltage curve VN-Q; S5. analyzing the reason for lithium ion battery capacity attenuation according to the discharging voltage curve V-Q graph; aligning the negative electrode voltage differential curves dVN / dQ-Q obtained in the battery capacity test under different cycle numbers right in the same coordinate system, and analyzing the reason for lithium ion battery capacity attenuation: if the right half parts of the negative electrode voltage differential curves substantially coincide, it is judged that the main reason for the lithium ion battery capacity attenuation is loss of available lithium ions in the lithium ion battery, otherwise it is judged that the main reason for the lithium ion battery capacity attenuation is loss of internal negative electrode active material in the lithium ion battery; According to the discharging voltage curve V-Q graph, the reason for lithium ion battery capacity attenuation is analyzed in detail as follows: if the full battery voltage curve VF-Q graph ends discharging at the positive electrode voltage, it is judged that the main reason for the lithium ion battery capacity attenuation is loss of internal positive electrode active material in the lithium ion battery; if the full battery voltage curve VF-Q graph ends discharging at the negative electrode voltage, it is judged that the main reason for the lithium ion battery capacity attenuation is loss of internal negative electrode active material in the lithium ion battery or loss of available lithium ions in the battery; if the positive electrode voltage curve VP-Q decreases obviously at the end of discharging, it is judged that the full battery voltage curve VF-Q graph ends discharging at the positive electrode voltage, otherwise it is judged that the full battery voltage curve VF-Q graph ends discharging at the negative electrode voltage; if the full battery voltage curve VF-Q graph ends discharging at the negative electrode voltage, it further comprises: differential processing the collected battery capacity Q data and voltage V data to obtain dQ and dV; The collected battery capacity Q data is taken as X-axis data, the ratio of dV / dQ is taken as Y-axis data, the ratio of dV / dQ is filtered by using a Gaussian filtering data processing method, and a voltage differential curve dV / dQ-Q graph is obtained; wherein the voltage differential curve dV / dQ-Q graph includes a full battery voltage differential curve dVF / dQ-Q, a positive electrode voltage differential curve dVP / dQ-Q, and a negative electrode voltage differential curve dVN / dQ-Q.

2. The method for analyzing capacity fade based on three-electrode lithium-ion battery of claim 1, wherein, The battery capacity test comprises the following steps: a. Constant current charging to the charging cutoff voltage V1 at a unit current I in a constant temperature environment of 25°C±2°C; b. Constant voltage charging of the three-electrode lithium ion battery at a constant voltage V1 until the current drops to 0.05I cutoff, and resting for 60 min after charging is completed; c. Constant current discharging to the discharging cutoff voltage V2 at a unit current I, and resting for 60 min after discharging is completed; d. Calculate the discharge capacity; e. Repeat steps a-d, when the range of the results of three consecutive tests is less than 3% of the rated value, end the test in advance, and take the average of the last three results; f. In the battery capacity test process, collect the battery capacity Q data and voltage V data of the three-electrode lithium ion battery during discharging at a preset frequency, wherein the voltage V data includes full battery voltage VF data, positive electrode voltage VP data, and negative electrode voltage VN data.

3. The method for analyzing capacity fade based on a three-electrode lithium-ion battery of claim 2, wherein, The unit current I is 0.2C, the charging cutoff voltage V1 is 4.2V; and the preset frequency is 10s-30s.

4. The method for analyzing capacity fade based on three-electrode lithium-ion battery of claim 1, wherein, The cycle aging test is a high-temperature accelerated cycle experiment, specifically: (1) Constant current charging of the three-electrode lithium ion battery to 4.2V at a constant current of 0.5C in a constant temperature environment of 45°C±2°C, and then constant voltage charging is performed until the current of the three-electrode lithium ion battery drops to 0.01Cn; (2) After resting for 1 hour, constant current discharging at a constant current of 1C until the voltage drops to 2.75V, and resting for 1 hour; (3) Calculate the discharge capacity; (4) Repeat steps (1)-(3) for cycling.

5. The method for analyzing capacity fade based on three-electrode lithium-ion battery of claim 1, wherein, The cycle aging test is a high-rate charge-discharge cycle.

6. The method for analyzing capacity fade based on three-electrode lithium-ion battery of claim 1, wherein, In step S3, the three-electrode lithium ion battery is cycled to a discharge capacity retention rate of 80% relative to the initial discharge capacity, and the cycle aging test is stopped. In step S3, the three-electrode lithium ion battery is cycled to a discharge capacity retention rate of 80% relative to the initial discharge capacity, and the cycle aging test is stopped.

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