Self-discharge abnormal battery cell sorting and screening method and system

Abstract

This invention discloses a method and system for capacity screening of self-discharge abnormal battery cells, relating to the field of lithium-ion battery technology. The method includes: after the initial charge-discharge cycle, capacity screening is performed based on the obtained initial capacity; batteries that pass the capacity screening are left to stand at a set temperature for a set time, and voltage decay is monitored; batteries with voltage decay rates less than or equal to a set decay rate threshold are subjected to multi-SOC point internal resistance testing for internal resistance change rate screening; batteries that pass the internal resistance change rate screening undergo pre-stabilization charge-discharge cycles, followed by standard charge-discharge testing, and the final capacity is recorded. The capacity decay rate is calculated based on the initial capacity, and batteries whose final capacity and capacity decay rate both meet the set requirements are selected. This improves the storage performance of lithium-ion batteries and enhances the consistency and safety of battery cell modules.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method and system for screening cells with abnormal self-discharge. Background Technology

[0002] In the lithium-ion battery production process, capacity grading is the last critical step before the battery leaves the factory. By conducting charge and discharge tests and classifying and screening the cells, classifying the cells into grades, and selecting cells with qualified capacity and excellent performance consistency for grouping, it is the core link to ensure the quality of battery products.

[0003] However, traditional capacity grading methods only focus on basic parameters such as capacity and voltage, and cannot effectively screen out cells with normal initial charge and discharge capacity but whose capacity drops drastically after storage due to unstable electrolyte interface or internal micro-short circuits, affecting the consistency and safety of the entire pack. Furthermore, they ignore the characteristics of internal resistance changes of cells under different SOC (State of Charge), misjudging cells with unstable interfaces and abnormal internal resistance changes as qualified products. Finally, traditional capacity grading processes cannot accelerate the stabilization of the cell's SEI (Solid Electrolyte Interphase) film, and the cell performance is prone to large fluctuations after several charge and discharge cycles or subsequent storage. Summary of the Invention

[0004] To address the aforementioned issues, this invention proposes a method and system for screening cells with abnormal self-discharge, thereby improving the storage performance of lithium-ion batteries and enhancing the consistency and safety of cell modules.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for screening cells with abnormal self-discharge, comprising: After the initial charge and discharge of the battery, capacity screening is performed based on the obtained initial capacity. Batteries that have passed capacity screening are left to stand at a set temperature for a set period of time, and voltage decay is monitored. Batteries with voltage decay rates less than or equal to a set decay rate threshold are screened by internal resistance change rate after multi-SOC point internal resistance testing. After pre-stabilizing charge-discharge cycles, the batteries selected by the internal resistance change rate are subjected to standard charge-discharge tests again. The final capacity is recorded, and the capacity decay rate is calculated in combination with the initial capacity. Batteries whose final capacity and capacity decay rate both meet the set requirements are selected.

[0006] As an alternative implementation, the initial charge-discharge process includes: performing a charge-discharge test on the battery at a first temperature for at least one standard cycle of constant current and constant voltage charging and constant current discharging.

[0007] As an alternative implementation method, the capacity screening process includes: after the charge-discharge test is completed, the discharge capacity is used as the initial capacity, and batteries with initial capacities within a set first range are selected to enter the next screening stage.

[0008] As an alternative implementation method, a standard cycle of charge and discharge test is as follows: charge with a constant current of 1 / 3C until the battery voltage reaches the preset charging voltage, then switch to constant voltage charging mode, keep the voltage constant, and continue charging until the charging current is less than or equal to the set value, then stop charging; after charging stops, let it stand for a set time, and after the standing time ends, discharge with a constant current of 1 / 3C until the battery voltage drops to the set discharge voltage, then stop discharging and record the discharge capacity.

[0009] As an optional implementation method, the process of monitoring voltage decay includes: placing the battery that has passed the capacity screening at a second temperature for a set time, measuring the voltage at the beginning of the resting period and the voltage at the end of the resting period, and calculating the voltage decay rate; wherein the value of the second temperature is in the range of 45℃~60℃.

[0010] As an optional implementation method, the process of multi-SOC point internal resistance testing includes: performing AC internal resistance or DC internal resistance testing at different SOC points under a set third temperature, testing the internal resistance of the battery at low SOC points and high SOC points, calculating the rate of change of internal resistance of the battery at low SOC points and high SOC points, and screening out batteries whose rate of change of internal resistance is within a set second range B0; wherein, the value range of low SOC point is 20%~40%, and the value range of high SOC point is 80%~100%.

[0011] As an optional implementation, the pre-stabilized charge-discharge cycle process includes: performing a set number of pre-stabilized charge-discharge cycles at a set fourth temperature, including: charging the battery to a preset SOC value with current I1 and maintaining the voltage until the current is cut off, resting for a second time, and then discharging to a low SOC point with current I2; wherein, the fourth temperature is greater than the temperature of the initial charge-discharge stage, current I1 is greater than or equal to current I2, the preset SOC value ranges from 70% to 90%, and the low SOC point ranges from 20% to 40%.

[0012] As an alternative implementation method, after selecting batteries whose final capacity and capacity decay rate both meet the set requirements, they are grouped according to their final capacity, voltage decay rate, and internal resistance change rate. Batteries whose voltage decay rate difference, internal resistance change rate difference, and final capacity difference all meet the set threshold requirements are grouped into the same battery pack.

[0013] Secondly, the present invention provides a capacity screening system for cells with abnormal self-discharge, comprising: The capacity screening module is configured to perform capacity screening based on the initial capacity obtained after the battery's initial charge and discharge. The voltage screening module is configured to allow batteries that have passed capacity screening to stand at a set temperature for a set time and monitor voltage decay. The internal resistance screening module is configured to screen batteries with voltage decay rates less than or equal to a set decay rate threshold by performing internal resistance change rate testing at multiple SOC points. The testing module is configured to perform a pre-stabilization charge-discharge cycle on batteries that have passed the internal resistance change rate screening, and then perform a standard charge-discharge test again to record the final capacity. The module also calculates the capacity decay rate based on the initial capacity and selects batteries whose final capacity and capacity decay rate both meet the set requirements.

[0014] Thirdly, the present invention provides a lithium-ion battery, characterized in that the lithium-ion battery is screened using the self-discharge abnormal cell capacity screening method of the first aspect.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention proposes a method and system for screening cells with abnormal self-discharge. The system designs a complete process including initial capacity screening, static voltage decay rate monitoring, multi-SOC point dynamic internal resistance change rate screening, pre-stabilization cycling, and capacity verification. By screening multiple parameters such as self-discharge, cell capacity, and SOC internal resistance change characteristics, cells with hidden defects (e.g., interface instability, internal micro-short circuits) that meet initial capacity requirements are eliminated. This reduces the probability of overall pack performance degradation and increased safety risks due to individual cell failures, improving the reliability of cell storage performance and ensuring highly consistent behavior of individual cells during long-term storage and cyclic use, significantly extending the overall lifespan of the battery pack. Furthermore, since unstable cells have been screened out, the cells tend to stabilize after high-temperature aging, further improving the cell capacity recovery rate.

[0016] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0018] Figure 1 This is a flowchart of the self-discharge abnormal cell capacity screening method provided in Embodiment 1 of the present invention. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0020] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0021] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form as well. Furthermore, it should be understood that the terms “comprising” and “including”, and any variations thereof, are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0022] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0023] As described in the background technology, capacity grading is the last critical process before batteries leave the factory during lithium-ion battery production. Its core function is to conduct systematic charge and discharge tests and classify and screen the cells, classify the cells into grades, and finally select cells with qualified capacity and excellent performance consistency for grouping. This directly determines the subsequent reliability, cycle life and safety performance of the battery pack, and is a core link to ensure the quality of battery products.

[0024] However, traditional capacity testing methods have significant technical shortcomings, making it difficult to meet the stringent requirements of high-end battery products for performance stability and safety, as follows: Firstly, traditional capacity grading only focuses on basic electrical performance parameters such as capacity and voltage, lacking the ability to predict the long-term storage stability of cells. It cannot effectively identify cells with potential problems such as "initial charge and discharge capacity meeting the standard, but serious self-discharge phenomena caused by latent defects such as unstable electrolyte interface and internal micro-short circuits during later storage, leading to a sharp drop in capacity." When such cells flow into the battery pack, they will seriously damage the consistency of individual cells in the battery pack, not only shortening the overall life of the battery pack, but also potentially causing safety risks such as thermal runaway.

[0025] Secondly, the traditional capacity grading process ignores the internal resistance variation characteristics of cells under different SOCs. Cells with unstable interfaces and abnormal internal resistance changes with SOC are easily misjudged as qualified products. Such cells experience drastic performance fluctuations in subsequent charge and discharge cycles, further exacerbating the imbalance in battery pack matching.

[0026] Third, the traditional capacity testing process does not include an accelerated stabilization step for the SEI film of the battery cell. After the battery cell leaves the factory, it needs to undergo multiple charge-discharge cycles or long-term storage before the SEI film can gradually stabilize. During this process, the performance of the battery cell is prone to large fluctuations, affecting the initial user experience and long-term reliability of the battery pack.

[0027] To improve the storage performance of battery cells leaving the factory and solve the problem of poor performance consistency among individual cells within the whole package, this invention provides a method for screening battery cells with abnormal self-discharge. By screening based on cell capacity, voltage decay rate, and internal resistance change rate, and by pre-stabilizing the cells, the method significantly improves the consistency of storage performance and enhances storage performance, thereby achieving precision, comprehensiveness, and stability in battery cell capacity screening.

[0028] Example 1 like Figure 1 As shown, this embodiment provides a method for screening cells with abnormal self-discharge, including: After the initial charge and discharge of the battery, capacity screening is performed based on the obtained initial capacity. Batteries that have passed capacity screening are left to stand at a set temperature for a set period of time, and voltage decay is monitored. Batteries with voltage decay rates less than or equal to a set decay rate threshold are screened by internal resistance change rate after multi-SOC point internal resistance testing. After pre-stabilizing charge-discharge cycles, the batteries selected by the internal resistance change rate are subjected to standard charge-discharge tests again. The final capacity is recorded, and the capacity decay rate is calculated in combination with the initial capacity. Batteries whose final capacity and capacity decay rate both meet the set requirements are selected.

[0029] The core of the self-discharge abnormal cell screening method for improving the storage performance of lithium-ion power batteries described in this embodiment lies in the screening of three dimensions: capacity, internal resistance, and voltage, and a stabilization process.

[0030] Specifically, it includes: S1: Preprocessing, including: The cells to be screened are placed at a standard ambient temperature (23±2℃) for 2-4 hours to eliminate temperature differences and voltage fluctuations caused during cell transportation and storage. Then, a constant current and constant voltage charging method is used to charge the battery cell to a medium SOC (40%-60% recommended, 50% preferred). After charging is completed, the battery cell is left to stand at the above-mentioned standard ambient temperature for 48 hours to allow the electrolyte distribution inside the cell to be uniform and the electrode interface to stabilize, providing a stable reference state for subsequent initial charge and discharge and various tests.

[0031] For example, after formation, the battery is charged to 50% SOC at 0.15C at 25°C and left to stand for 48 hours.

[0032] S2: Initial capacity screening is performed through the first charge and discharge cycle.

[0033] Specifically: The battery that has undergone the pretreatment in step S1 is placed in a set first temperature environment T1. After the battery temperature stabilizes, a constant current and constant voltage charging and constant current discharging test is performed for at least one standard cycle. After the charge-discharge test is completed, the discharge capacity is recorded and defined as the initial capacity C0. Based on the initial capacity, select batteries whose initial capacity is within the preset range A0, remove batteries whose initial capacity C0 exceeds this range, and retain the batteries with qualified capacity to enter the screening stage in step S3.

[0034] As an alternative implementation method, the first temperature T1 is set to 25°C, and the temperature fluctuation can be controlled within ±1°C.

[0035] As an alternative implementation, the preset range A0 is set to 68.53Ah~71.47Ah, and batteries with an initial capacity C0 between 68.53Ah and 71.47Ah are selected; it is understood that this range includes the values ​​of 68.53Ah and 71.47Ah.

[0036] As one alternative implementation method, the specific procedure for a standard charge-discharge test cycle is as follows: First, the battery cell is charged with a constant current of 1 / 3C until the battery cell voltage reaches the preset charging voltage. Then, the constant voltage charging mode is switched to maintain the voltage and continue charging until the charging current is less than or equal to the set value, at which point charging stops. After charging stops, allow the battery cell to rest for a set time to eliminate the polarization phenomenon generated during charging and discharging, so that the internal state of the battery cell tends to stabilize. After the settling period, the cells are discharged at a constant current of 1 / 3C until the cell voltage drops to the set discharge voltage. Then the discharge is stopped, the discharge capacity is recorded, and batteries with a discharge capacity between 68.53Ah and 71.47Ah are selected.

[0037] For example, first charge the cell with a constant current of 1 / 3C until it reaches 3.65V, then switch to constant voltage charging mode, keep the voltage at 3.65V, and continue charging until the charging current is ≤0.05C, then stop charging. After charging stops, let the cell rest for 5 minutes to eliminate the polarization phenomenon generated during charging and discharging, so that the internal state of the cell tends to be stable. After resting, discharge the cell with a constant current of 1 / 3C until the cell voltage drops to 2.0V, then stop discharging.

[0038] The core purpose of this step is to accurately obtain the initial capacity of the battery cells through standardized initial charge and discharge tests and complete the initial screening, eliminating cells with seriously substandard capacity or obvious manufacturing defects. This lays the foundation for subsequent refined screening (self-discharge, internal resistance testing, etc.), reduces invalid testing steps, and improves the overall screening efficiency.

[0039] Its advantage lies in using standardized charging and discharging parameters to ensure the accuracy and consistency of initial capacity test results, avoid screening errors caused by differences in test conditions, and eliminate obviously unqualified cells in advance, reducing the manpower and time costs of subsequent screening processes.

[0040] S3: Static voltage decay rate screening (self-discharge and interface stability screening).

[0041] Specifically: (1) Preparation: Place the qualified batteries that have completed the initial capacity screening in step S2 into the constant temperature test chamber one by one to ensure that the cells are arranged neatly and without stacking, so as to avoid uneven temperature conduction between cells affecting the test results.

[0042] (2) Temperature control: Set the second temperature T2 of the constant temperature test chamber, start the constant temperature test chamber, and after the temperature inside the chamber stabilizes at the second temperature T2, maintain the constant temperature state until the end of this step. Monitor the temperature inside the chamber in real time throughout the process to ensure temperature stability.

[0043] T2 can be selected within the range of 45℃ to 60℃, such as 45℃, 50℃, etc., with temperature fluctuation controlled within ±1℃.

[0044] (3) Settling and voltage monitoring: The battery is settling at the second temperature T2 for a first time Δt1. At the beginning of the settling (error ≤ 10s), the initial voltage V1 of each cell is measured using a high-precision voltage tester (accuracy ≥ 0.001V), and the corresponding cell number and V1 value are recorded to ensure one-to-one correspondence. At the end of the settling (error ≤ 10s), the same high-precision voltage tester is used to measure the final voltage V2 of each cell in the same test method, and the corresponding cell number and V2 value are recorded again.

[0045] The first duration Δt1 can be selected according to actual production needs, with a recommended range of 24h~48h, preferably 48h. During the resting period, ensure that the test environment is free from electromagnetic interference and vibration.

[0046] (4) Voltage decay rate calculation and screening: According to the formula ΔVrate = (V1 - V2) / Δt1, calculate the voltage decay rate (unit: V / h) of each cell one by one, where V1 and V2 are in V and Δt1 is in h; preset decay rate threshold K1 (can be set according to cell specifications and quality requirements, it is recommended that the decay rate threshold K1≤0.005V / h), screen out batteries with voltage decay rate ΔVrate less than or equal to the decay rate threshold K1, and remove batteries with ΔVrate greater than K1.

[0047] (5) Finally, the selected batteries are taken out and placed at a standard ambient temperature (23±2℃) for 30 minutes to allow the battery temperature to return to room temperature, so as to avoid the high temperature state from affecting the accuracy of the subsequent S4 internal resistance test. Then, the S4 internal resistance change rate screening process is carried out.

[0048] For example, qualified batteries that have completed the initial capacity screening in step S2 are placed in a constant temperature test chamber at 45°C for 48 hours. The voltage V1 at the beginning of the standing period and the voltage V2 at the end of the standing period are measured respectively. The voltage decay rate ΔVrate = (V1 -V2) / Δt1 is calculated, and batteries with a voltage decay rate of less than or equal to 100mV are selected.

[0049] Batteries with excessively rapid voltage decay usually indicate a severe self-discharge point or an unstable SEI film. During long-term storage, such batteries may experience a sharp decline in capacity and significant performance fluctuations due to increased self-discharge and SEI film damage, and may even lead to safety risks such as internal short circuits.

[0050] The core purpose of this step is to accurately screen the self-discharge characteristics and SEI film stability of the battery cells through static placement and voltage monitoring under high-temperature accelerated conditions, and to eliminate battery cells with hidden defects such as serious self-discharge points, unstable SEI films, or internal micro-short circuits in advance, so as to prevent such potentially hazardous battery cells from entering subsequent stages and ensure the safety and consistency of the battery pack during long-term storage.

[0051] Its core advantage lies in utilizing the high-temperature acceleration effect to rapidly amplify the hidden defects of the battery cell (high temperature accelerates unstable side reactions and micro dendrite growth), shorten the screening cycle, and improve the identification accuracy of potentially hazardous battery cells; at the same time, by quantifying the voltage decay rate, the screening standard is standardized and quantifiable, avoiding subjective judgment errors, and high-risk battery cells are eliminated in the early stage, further reducing the cost and risk of subsequent screening and grouping.

[0052] S4: Screening for dynamic internal resistance change rate (interface health screening) through multi-SOC point internal resistance testing.

[0053] For the batteries screened through step S3, AC internal resistance or DC internal resistance tests are performed at different State of Charge (SOC) points under the set third temperature T3. The internal resistance of the batteries is tested at low SOC (20%~40%) and high SOC (80%~100%), and the rate of change of internal resistance Rrate at low SOC and high SOC is calculated. Batteries with an internal resistance change rate within the preset range B0 are selected.

[0054] Specifically: (1) Preliminary preparation and temperature stabilization: Place the qualified batteries that have passed the static voltage decay rate screening in step S3 into the set third temperature T3 (it is recommended to be the same as the first temperature T1, i.e. 25℃, and the temperature fluctuation should be strictly controlled within ±1℃) environment and let them stand for 30 minutes to ensure that the cell temperature is stable, eliminate the internal polarization phenomenon, and avoid the influence of temperature fluctuation or internal instability on the accuracy of internal resistance test; at the same time, check the internal resistance test equipment (it is recommended to use a high-precision DC internal resistance tester with an accuracy ≥1mΩ), calibrate the equipment parameters, ensure that the test data is accurate and reliable, and firmly connect the cell to the test equipment to avoid test errors caused by poor contact.

[0055] (2) SOC adjustment and low SOC internal resistance test: First, discharge the cell with a constant current of 1 / 3C until the cell SOC reaches the set low SOC range (20%~40%, 30% SOC recommended). After the discharge, let the cell stand at 25℃ for 1 hour to make the electrolyte distribution inside the cell uniform and the electrode interface tend to be stable, eliminating the polarization phenomenon generated during the discharge process. After the stand, discharge with a constant current of 2C for 10s (the discharge time is controlled within 10±0.5s to avoid the discharge time being too long and affecting the cell state). Measure the DC internal resistance of the cell at this time with a high-precision DC internal resistance tester and record it as the low SOC internal resistance R1. At the same time, record the corresponding cell number to ensure that the parameters correspond one-to-one with the cell.

[0056] (3) SOC adjustment and high SOC internal resistance test: After completing the low SOC internal resistance test, charge the cell with a constant current of 1 / 3C until the cell SOC reaches the high SOC range (80%~100%, 90% SOC recommended). After charging, let it stand for 1 hour in a 25℃ environment to eliminate the polarization phenomenon during the charging process and ensure the stability of the cell's internal state. After standing, use the same equipment and the same test method as the low SOC internal resistance test to measure the DC internal resistance of the cell at this time and record it as the high SOC internal resistance R2. Check the cell number again to avoid parameter confusion.

[0057] (4) Calculation and screening of internal resistance change rate: According to the formula Rrate=(R2-R1) / R1×100%, calculate the internal resistance change rate (unit: %) of each cell one by one, where R1 is the low SOC internal resistance and R2 is the high SOC internal resistance. Two decimal places are retained in the calculation process to ensure the calculation accuracy. The preset range of internal resistance change rate B0 is set (which can be set according to the cell specifications and quality requirements, and the recommended range is 15%≤Rrate≤30%). Cells with internal resistance change rate Rrate within the preset range B0 are screened out, and cells with Rrate<15% or Rrate>30% are removed.

[0058] (5) Finally, the qualified cells selected in this step are kept at room temperature of 25°C and then enter the high temperature pre-stabilization treatment step S5. At the same time, the R1, R2 and Rrate data of each cell are retained for subsequent grouping and quality traceability. For example: Cool the battery to 25℃. First, discharge it to 30% SOC with a 1 / 3C current (let it rest for 1 hour), then discharge it at 2C for 10 seconds and measure the DC internal resistance R1. Next, charge it to 90% SOC with a 1 / 3C current (let it rest for 1 hour) and measure the DC internal resistance R2. Calculate Rrate = (R2 - R1) / R1 * 100%. Select batteries with Rrate ≤ 30% and a range of 15% ≤ Rrate ≤ 30%.

[0059] A healthy battery should exhibit a reasonable and consistent trend in internal resistance as state of charge (SOC) increases: at low SOC, the lithium-ion concentration inside the cell is low, resulting in relatively high internal resistance; at high SOC, the lithium-ion concentration increases, leading to a relatively lower internal resistance, thus the rate of change in internal resistance should be within a reasonable range. If the rate of change in internal resistance is abnormally low (<15%), it may indicate a poor conductive network within the cell, poor lithium-ion insertion / extraction kinetics, and performance fluctuations during subsequent charging and discharging. If the rate of change in internal resistance is abnormally high (>30%), it may indicate defects at the electrode interface, instability in the SEI film, and problems such as capacity decay and accelerated self-discharge during long-term use.

[0060] This step involves testing the internal resistance of the battery cell at multiple SOC points and calculating the rate of change. The internal resistance of the cell is tested at different SOCs (low SOC, high SOC), and the rate of change of internal resistance is calculated to reflect the internal health of the battery. Cells with defects in the electrode interface, poor lithium-ion insertion / extraction kinetics, or poor conductive network are eliminated. This overcomes the limitations of traditional single SOC point internal resistance measurement and provides more comprehensive cell performance data for subsequent screening and grouping, further improving the accuracy of cell screening and the consistency of battery packs.

[0061] Its core advantage lies in the fact that, compared with the internal resistance measurement at a single SOC point, the internal resistance change rate at multiple SOC points can more comprehensively and accurately reflect the internal structural state of the cell. It can effectively identify weak cells that have qualified initial capacity and normal self-discharge but have hidden defects. Moreover, the testing process is standardized and quantifiable, making it suitable for industrial mass production. In this embodiment, the above-mentioned solution improves cell reliability through multi-parameter collaborative screening. Specifically, by performing multi-dimensional detection and screening on cell self-discharge characteristics, capacity stability, and SOC internal resistance change characteristics, cells with potential hidden defects (such as interface instability or internal micro-short circuits) that are initially qualified in capacity are accurately eliminated. This reduces the probability of overall pack performance degradation and increased safety risks due to the failure of individual cells from the source, ensuring that the behavior of individual cells in the battery pack is highly consistent during long-term storage and cyclic use, and significantly extending the overall service life of the battery pack.

[0062] S5: High-temperature pre-stabilization treatment (active aging and screening).

[0063] For the batteries selected in step S4, a set number of pre-stabilization charge-discharge cycles are performed at a set fourth temperature T4. The cycle includes: charging the battery to a preset SOC value S0 with current I1 and maintaining the voltage until the current is cut off, resting for a second time Δt2, and then discharging to a low SOC point with current I2. Wherein, T4>T1, 0.3C≤I1, I2≤1C, 70%≤S0≤90%.

[0064] Specifically: (1) Preliminary preparation: The qualified batteries selected in step S4 are placed one by one into the constant temperature test chamber. They are arranged neatly, without stacking or contact, to avoid uneven temperature conduction or short circuit risk between cells. At the same time, it is ensured that the connection between the cells and the test equipment is firm to ensure the stability of the charging and discharging process.

[0065] (2) Temperature setting and stabilization: Set the fourth temperature T4 of the constant temperature test chamber. T4 is required to be greater than T1 (T1 is 25℃, so T4 must be greater than 25℃). T4 is selected according to production needs. The recommended range is 45℃~60℃, with 45℃ being preferred. Temperature fluctuation is controlled within ±1℃. Start the constant temperature test chamber and keep it at a constant temperature until all cycles of this step are completed. Monitor the temperature inside the chamber in real time throughout the process and record the temperature data every 30 minutes to ensure that the temperature is constant.

[0066] (3) Cyclic parameter setting: Determine the number of pre-stabilized charge and discharge cycles, recommended 2 to 3 cycles (preferably 3 cycles). During the cycle, the parameters should meet the following requirements: the charge and discharge currents I1 and I2 should meet 0.3C≤I1, I2≤1C, and it is recommended to use 1 / 3C (to keep the charge and discharge current consistent and reduce the impact of parameter differences on cell performance); the preset charging termination SOC value S0 should meet 70%≤S0≤90%, and it is preferred to use 80% SOC; the low SOC discharge termination point is recommended to be set to 20% SOC (to match the key points of multi-SOC internal resistance test and lay the foundation for subsequent tests); the second duration Δt2 (resting time) is recommended to be 12 hours to ensure that the electrolyte inside the cell is fully diffused and the interface state tends to be stable.

[0067] The fourth step is the cyclic operation process (taking the optimized parameters as an example): each cycle is executed in the order of "charging-constant voltage-resting-discharging".

[0068] Specifically: First, charge the cell with a constant current I1 of 1 / 3C until the cell's SOC reaches 80% (S0); then switch to constant voltage charging mode, keep the current voltage constant, and continue charging until the charging current drops to the cutoff current (recommended ≤0.05C, consistent with the constant voltage cutoff current to ensure parameter consistency), and stop charging; after charging, let the cell stand at a constant temperature of 45℃ for 12 hours (Δt2). During the standing period, avoid electromagnetic interference, vibration, etc., to ensure the stability of the cell's internal state; after standing, discharge the cell with a constant current I2 of 1 / 3C until the cell's SOC drops to 20% (low SOC point), and stop discharging to complete a complete pre-stabilization cycle.

[0069] Step 5, Cyclic process monitoring: During each charge and discharge cycle, the voltage, current, SOC changes and charge and discharge time of the battery cell are recorded in real time. The focus is on monitoring whether the battery cell exhibits abnormal conditions such as a drop in capacity (capacity decay exceeding 5% in a single cycle) or a sharp increase in internal resistance (an increase of more than 10% compared to the internal resistance after S4 screening). If any abnormality is found, the battery cell is immediately marked as unqualified and removed.

[0070] The sixth and final step is to remove the cells that have passed the pre-stabilization charge-discharge cycle (without abnormalities) after completing the preset number of cycles. Place them at a standard ambient temperature (23±2℃) for 1 hour to allow the cell temperature to return to room temperature and the internal state to stabilize before proceeding to the final screening stage.

[0071] For example, place the battery in a 45°C chamber and perform the following operation for 3 cycles: charge at 1 / 3C to 80% SOC, maintain constant voltage until current cutoff, and let stand at 45°C for 12 hours; then discharge at 1 / 3C to 20% SOC.

[0072] The core objective of this step is to achieve proactive accelerated stabilization of the SEI film in the battery cell, secondary screening for latent defects, and release of trace gases through shallow charge-discharge cycles in a high-temperature environment. This further eliminates weaker, potentially defective cells and optimizes the internal structure of the cells, providing stable and consistent cells for final screening and subsequent battery assembly. This fundamentally improves the long-term reliability and cycle life of the battery pack. Through the dual effects of proactive optimization and secondary screening, it actively improves the cell interface performance and releases residual gases, while accurately identifying weaker cells not exposed in the first three rounds of screening. This prevents such cells from entering the final assembly process. Furthermore, it requires no additional complex equipment and can seamlessly integrate with existing screening processes, balancing optimization effectiveness and production efficiency.

[0073] Specifically: Accelerating SEI membrane stability: High-temperature environment (T4) combined with shallow charge and discharge cycle effectively accelerates the decomposition and recombination of unstable SEI membrane components inside the cell, promoting the formation of a denser and more stable solid electrolyte interface membrane. This avoids performance fluctuations and capacity decay caused by SEI membrane damage during subsequent use of the cell, significantly improving the long-term stability of the cell.

[0074] Secondary screening of potentially defective cells: Although some cells have passed the screening in steps S2-S4, they may have potential structural weaknesses (such as micro-defects at the interface or micro-short circuits inside). During high-temperature cycling, these defects will be amplified, manifesting as a drop in capacity or a sharp increase in internal resistance. These defects can be accurately removed through real-time monitoring, further improving the screening accuracy.

[0075] Releasing residual internal gases: Trace amounts of gas may remain during the early formation process of the battery cell. The synergistic effect of high temperature environment and charge-discharge cycle can further drive away and disperse these trace gases, avoid gas accumulation leading to poor contact at the electrode interface, stabilize the internal structure of the battery cell, and reduce performance degradation or safety risks caused by gas problems in subsequent use.

[0076] In summary, this embodiment adds a pre-stabilization charge-discharge cycle step to the screening process, which can effectively accelerate the formation and stabilization of the SEI film in the battery cell, avoiding the performance fluctuation problem caused by the unstable SEI film after traditional capacity grading. At the same time, combined with high-temperature static aging and multiple rounds of screening, the performance of the screened battery cells tends to stabilize in advance, further improving the capacity recovery rate of the battery cells, ensuring that the battery cells can achieve stable performance after leaving the factory, and improving the initial user experience and long-term consistency of battery products.

[0077] S6: Final capacity and self-discharge matching (consistent grouping).

[0078] After completing the screening in step S5, the batteries are cooled to T1 and subjected to standard charge-discharge tests again. Their final capacity C1 is recorded, and the capacity decay rate is calculated. Batteries with a final capacity C1 that meets the requirements and a capacity decay rate below the threshold K2 are selected and matched with the screening parameters of S3 and S4 for grouping within the same battery pack. The rigorously screened batteries are then grouped based on their final stable capacity, self-discharge rate, and internal resistance characteristics to improve the performance consistency of each individual cell within the entire pack.

[0079] Specifically: (1) Pre-cooling and stabilization: Take out the qualified battery cell that has passed the high temperature pre-stabilization treatment in step S5, place it in the set first temperature T1 (25℃, temperature fluctuation controlled within ±1℃) environment, let it cool naturally and stand for 1.5~2h to ensure that the battery cell temperature is completely stable at T1, the internal electrolyte is evenly distributed and the polarization phenomenon is completely eliminated, so as to avoid deviation in the final charge and discharge test results due to high temperature or internal instability.

[0080] (2) Final standard charge and discharge test: Repeat the standard charge and discharge test procedure of step S2 once for the cell after cooling and stabilization (to ensure the consistency of test conditions and improve data comparability).

[0081] The specific process is as follows: Charge the battery cell with a constant current of 1 / 3C until the cell voltage reaches 3.65V; then switch to constant voltage charging mode, maintaining the voltage at 3.65V, and continue charging until the charging current is ≤0.05C, then stop charging; after charging stops, let the battery cell rest for 5 minutes to eliminate the polarization phenomenon generated during charging and discharging; after resting, discharge the battery cell with a constant current of 1 / 3C until the cell voltage drops to 2.0V, then stop discharging. Record the discharge capacity of the battery cell, and define this discharge capacity as the final capacity C1 of the battery cell.

[0082] (3) Calculation and screening of capacity decay rate: Calculate the capacity decay rate (unit: %) according to the formula (C0 - C1) / C0, where C0 is the initial capacity and C1 is the final capacity of this test; preset the capacity decay rate threshold K2, and it is recommended that K2 < 1.0%, and set the qualified range of the final capacity C1 (it is recommended to be consistent with the qualified range of the initial capacity, i.e. 68.53Ah~71.47Ah), screen out the cells whose final capacity C1 is within the qualified range and whose capacity decay rate is less than the threshold K2, and remove the cells whose C1 is outside the range or whose decay rate is ≥ K2.

[0083] (4) Multi-parameter matching and grouping: The qualified cells that have passed this screening will have their key parameters from the previous screening retrieved: voltage decay rate ΔVrate (self-discharge characteristic) and internal resistance change rate Rrate (internal resistance characteristic). Combined with the final capacity C1 obtained this time, multi-dimensional parameter matching will be performed.

[0084] The matching principle is as follows: Select cells with a ΔVrate difference ≤ 0.001V / h, a Rrate difference ≤ 5%, and a C1 difference ≤ 0.5Ah, and incorporate them into the same battery pack to ensure that the self-discharge characteristics, internal resistance characteristics, and capacity characteristics of all individual cells in the same battery pack are highly consistent.

[0085] (5) After the grouping is completed, each group of cells is numbered and recorded, and all screening parameters (C0, C1, capacity decay rate, ΔVrate, Rrate) are retained to facilitate subsequent product traceability and quality control. At this point, the entire self-discharge abnormal cell capacity screening process is completed.

[0086] This step, as the final stage of the entire capacity assessment and screening process, aims to achieve multi-dimensional parameter matching and grouping of battery cells by combining the final capacity verification and capacity decay rate screening with the self-discharge rate and internal resistance characteristics screened in the previous stage. This maximizes the performance consistency of each individual cell in the same battery pack, ensuring the long-term cycle life, charge and discharge efficiency, and safety performance of the battery pack from the end, and avoiding performance shortcomings of the entire pack due to differences in individual cell parameters.

[0087] By combining verification screening with multi-parameter matching, the system performs final performance verification on the pre-screened cells, eliminating those with excessive performance degradation later. It also solves the problem of traditional matching focusing only on a single capacity parameter through precise multi-parameter matching. This significantly reduces the balancing pressure and safety risks caused by poor consistency of individual cells during battery pack use, while also improving the overall performance ceiling of the battery pack.

[0088] This embodiment addresses the pain points of traditional capacity grading methods by combining its own multi-step, multi-parameter screening technology to achieve precise, comprehensive, and stable cell capacity grading and screening. This reduces production costs and improves production efficiency. By accurately screening and eliminating potentially problematic cells, it reduces the probability of rework and repair after subsequent battery pack assembly, and lowers the risk of batch product scrap due to cell failure. At the same time, the pre-stabilization stage completes SEI film stabilization in advance, shortening the performance stabilization cycle after the cells leave the factory, indirectly improving the efficiency of the entire battery production process, and providing support for enterprises to reduce production costs and enhance product competitiveness.

[0089] Verification example.

[0090] The following is an example of the processing steps for a 70Ah prismatic battery cell based on an iron-based system: (1) Pretreatment: Charge the formed battery at 25°C with a current of 0.15C to 50% SOC and let it stand for 48 hours.

[0091] (2) Initial charge and discharge and initial capacity screening: At 25℃, charge at a constant current of 1 / 3C to 3.65V, maintain constant voltage until current ≤0.05C, let stand for 5 minutes, and then discharge at a constant current of 1 / 3C to 2.0V. Record the discharge capacity C0, and screen batteries with C0 between 68.53Ah and 71.47Ah.

[0092] (3) Static voltage decay rate screening: Place the battery in a 45℃ incubator for 48 hours. Record the initial voltage V1 and the final voltage V2. Calculate the voltage decay ΔVrate. Screen batteries with ΔVrate ≤ 100mV.

[0093] (4) Screening based on dynamic internal resistance change rate: Cool the battery to 25℃. First, discharge it to 30% SOC with a 1 / 3C current (let it stand for 1 hour), then discharge it at 2C for 10 seconds and measure the DC internal resistance R1. Then charge it to 90% SOC with a 1 / 3C current (let it stand for 1 hour) and measure the DC internal resistance R2. Calculate Rrate = (R2 - R1) / R1 * 100%. Screen out batteries with Rrate ≤ 30% of their rated internal resistance.

[0094] (5) High temperature pre-stabilization treatment: Place the battery in a 45°C chamber and perform the following operations for 3 cycles: charge at 1 / 3C to 80% SOC, maintain constant voltage until current cutoff, and stand at 45°C for 12 hours; then discharge at 1 / 3C to 20% SOC.

[0095] (6) Final capacity and self-discharge matching: Cool the battery to 25°C and repeat the charge-discharge test of S1 once, and record the final capacity C1. Calculate the capacity decay (C0-C1) / C0, and screen batteries with a decay rate <1.0%. Finally, batteries with similar parameters ΔVrate, Rrate and C1 are grouped into the same battery pack.

[0096] Comparative Example 1.

[0097] The only differences from the verification example are steps (1) and (2). Batteries with capacities between 68.53Ah and 71.47Ah are selected for grouping.

[0098] Comparative Example 2.

[0099] The processing steps (1)-(4) and (6) in the verification example are performed, except that the shallow charge and discharge process in step (5) is not performed.

[0100] Ten cells each from the verification example, comparative example 1, and comparative example 2 were taken and subjected to a 30-day storage performance test at 45℃. The data were collected and the comparison results are shown in Table 1.

[0101] Table 1. Verification examples and comparative examples of storage performance;

[0102] As shown in Table 1, the capacity recovery rates of the ten cells in the verification example are concentrated between 96% and 97%, while the capacity recovery rates of Comparative Example 1 are scattered between 93% and 96%. The tests demonstrate that the method of this embodiment can effectively screen out batteries with consistent and excellent storage performance. Comparative Example 2's capacity recovery rate is concentrated between 94.5% and 96%, a decrease of 0.5% to 1.5% compared to the verification examples. The method of this embodiment performs high-temperature pre-stabilization treatment and active aging during production, accelerating the stabilization of the interface structure and effectively improving storage performance.

[0103] Example 2 This embodiment provides a capacity assessment and screening system for cells with abnormal self-discharge, including: The capacity screening module is configured to perform capacity screening based on the initial capacity obtained after the battery's initial charge and discharge. The voltage screening module is configured to allow batteries that have passed capacity screening to stand at a set temperature for a set time and monitor voltage decay. The internal resistance screening module is configured to screen batteries with voltage decay rates less than or equal to a set decay rate threshold by performing internal resistance change rate testing at multiple SOC points. The testing module is configured to perform a pre-stabilization charge-discharge cycle on batteries that have passed the internal resistance change rate screening, and then perform a standard charge-discharge test again to record the final capacity. The module also calculates the capacity decay rate based on the initial capacity and selects batteries whose final capacity and capacity decay rate both meet the set requirements.

[0104] It should be noted that the above modules correspond to the steps described in Embodiment 1, and the examples and application scenarios implemented by the above modules and the corresponding steps are the same, but are not limited to the content disclosed in Embodiment 1. It should also be noted that the above modules, as part of the system, can be executed in a computer system such as a set of computer-executable instructions.

[0105] In further embodiments, a lithium-ion battery is also provided, which is screened using the capacity screening method for cells with abnormal self-discharge described in Embodiment 1.

[0106] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for screening cells with abnormal self-discharge, characterized in that, include: After the initial charge and discharge of the battery, capacity screening is performed based on the obtained initial capacity. Batteries that have passed capacity screening are left to stand at a set temperature for a set period of time, and voltage decay is monitored. Batteries with voltage decay rates less than or equal to a set decay rate threshold are screened by internal resistance change rate after multi-SOC point internal resistance testing. After pre-stabilizing charge-discharge cycles, the batteries selected by the internal resistance change rate are subjected to standard charge-discharge tests again. The final capacity is recorded, and the capacity decay rate is calculated in combination with the initial capacity. Batteries whose final capacity and capacity decay rate both meet the set requirements are selected.

2. The method for screening cells with abnormal self-discharge as described in claim 1, characterized in that, The initial charge and discharge process includes: performing a charge and discharge test on the battery at a first temperature for at least one standard cycle of constant current and constant voltage charging and constant current discharging.

3. The method for screening cells with abnormal self-discharge as described in claim 2, characterized in that, The capacity screening process includes: after the charge and discharge test is completed, the discharge capacity is used as the initial capacity, and batteries with initial capacities within the set first range are selected to enter the next screening stage.

4. The method for screening cells with abnormal self-discharge as described in claim 2, characterized in that, The standard charge-discharge test procedure is as follows: charge at a constant current of 1 / 3C until the battery voltage reaches the preset charging voltage, then switch to constant voltage charging mode, keep the voltage constant, and continue charging until the charging current is less than or equal to the set value, then stop charging; after charging stops, let it stand for a set time, and after standing, discharge at a constant current of 1 / 3C until the battery voltage drops to the set discharge voltage, then stop discharging and record the discharge capacity.

5. The method for screening cells with abnormal self-discharge as described in claim 1, characterized in that, The process of monitoring voltage decay includes: placing the batteries that have passed capacity screening at a second temperature for a set time, measuring the voltage at the beginning and end of the resting period, and calculating the voltage decay rate; wherein the value of the second temperature ranges from 45℃ to 60℃.

6. The method for screening cells with abnormal self-discharge as described in claim 1, characterized in that, The process of multi-SOC point internal resistance testing includes: conducting AC or DC internal resistance tests at different SOC points under a set third temperature, testing the internal resistance of the battery at low and high SOC points, calculating the rate of change of internal resistance at low and high SOC points, and screening out batteries whose rate of change of internal resistance is within a set second range B0; wherein, the value range of low SOC point is 20%~40%, and the value range of high SOC point is 80%~100%.

7. The method for screening cells with abnormal self-discharge as described in claim 1, characterized in that, The pre-stabilization charge-discharge cycle process includes: performing a set number of pre-stabilization charge-discharge cycles at a set fourth temperature, including: charging the battery to a preset SOC value with current I1 and maintaining a constant voltage until the current is cut off, resting for a second duration, and then discharging to a low SOC point with current I2; wherein, the fourth temperature is greater than the temperature of the initial charge-discharge stage, current I1 is greater than or equal to current I2, the preset SOC value ranges from 70% to 90%, and the low SOC point ranges from 20% to 40%.

8. The method for screening cells with abnormal self-discharge as described in claim 1, characterized in that, After selecting batteries whose final capacity and capacity decay rate both meet the set requirements, they are grouped according to their final capacity, voltage decay rate, and internal resistance change rate. Batteries whose voltage decay rate difference, internal resistance change rate difference, and final capacity difference all meet the set threshold requirements are grouped into the same battery pack.

9. A capacity-based screening system for cells with abnormal self-discharge, characterized in that, include: The capacity screening module is configured to perform capacity screening based on the initial capacity obtained after the battery's initial charge and discharge. The voltage screening module is configured to allow batteries that have passed capacity screening to stand at a set temperature for a set time and monitor voltage decay. The internal resistance screening module is configured to screen batteries with voltage decay rates less than or equal to a set decay rate threshold by performing internal resistance change rate testing at multiple SOC points. The testing module is configured to perform a pre-stabilization charge-discharge cycle on batteries that have passed the internal resistance change rate screening, and then perform a standard charge-discharge test again to record the final capacity. The module also calculates the capacity decay rate based on the initial capacity and selects batteries whose final capacity and capacity decay rate both meet the set requirements.

10. A lithium-ion battery, characterized in that, The lithium-ion battery is screened using the capacity screening method for cells with abnormal self-discharge as described in any one of claims 1-8.

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