Screening method for self-discharge performance of lithium iron phosphate battery

Abstract

The invention relates to the field of lithium ion battery preparation, and discloses a method for screening the self-discharge performance of a lithium iron phosphate battery, which comprises the following steps of: performing discharge operation on a single lithium battery to empty the electric quantity of the single lithium battery; charging the single lithium battery to a state of charge of 3%-10% of the nominal capacity of the single lithium battery by using a current of 0.1 C to 0.3 C; placing the charged single lithium battery at room temperature for a first preset time T1, and measuring the voltage V1 of the single lithium battery; standing the single lithium battery with the measured voltage V1 at room temperature for a second preset time T2, and measuring the voltage V2 of the single lithium battery; calculating a K value of the single lithium battery according to K = (V1-V2) / (T2-T1); and comparing the K value with a preset screening standard, and rejecting the single batteries which do not accord with the standard.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery manufacturing, and more particularly to a method for screening the self-discharge performance of lithium iron phosphate batteries. Background Technology

[0002] In the production of lithium-ion batteries, especially lithium iron phosphate (LiFePO4) batteries, variations in individual cell performance are inevitable due to fluctuations in equipment precision, raw material batches, testing methods, and production environment. The quality and consistency of individual cell performance directly determine the overall performance, cycle life, and safety reliability of the final battery pack. Therefore, employing appropriate self-discharge screening methods to eliminate cells with poor consistency or defects before sorting and grouping is a crucial step in the manufacturing process.

[0003] Currently, the industry commonly uses a room-temperature half-state resting test method to screen lithium iron phosphate batteries for self-discharge performance (usually characterized by the K-value). This method typically involves charging the battery to a moderate state of charge (e.g., 30%-80% State of Charge, or SOC), then leaving it at room temperature for an extended period (approximately 20 days). The K-value is calculated by measuring the voltage drop, and the batteries are then screened accordingly.

[0004] However, during the research process of this invention, the inventors discovered the following defects in the existing technical methods: First, lithium iron phosphate materials inherently possess a very flat voltage plateau. Especially within the broad range of 30% to 80% SOC, the relationship between the battery's terminal voltage and SOC approximates a straight line, with minimal voltage variation. In this "voltage plateau region," the voltage change signal caused by minute differences in self-discharge is extremely weak, much like the imperceptible slight slope on a flat road. This results in insufficient sensitivity of the method, making it difficult to effectively eliminate "edge" batteries with slightly higher self-discharge rates than the standard but potentially poor consistency, thus posing a potential risk to the overall quality of the battery pack.

[0005] Secondly, in order to capture sufficiently distinguishable voltage differences, existing methods have to rely on extremely long static screening cycles (approximately 20 days). This significantly extends the product manufacturing cycle, leading to high work-in-process inventory, severely impacting capital turnover efficiency, and putting enormous pressure on production cost control and customer delivery time responsiveness.

[0006] In summary, there is an urgent need in this field for a self-discharge screening method that can overcome the voltage plateau characteristics of lithium iron phosphate batteries and combine short cycle time with high precision, so as to improve production efficiency and reduce overall costs while ensuring battery quality. Summary of the Invention

[0007] One of the objectives of this invention is to provide a screening method for lithium iron phosphate batteries that can overcome the voltage plateau characteristics of lithium iron phosphate batteries and has both short cycle time and high precision in terms of self-discharge performance, so as to improve production efficiency and reduce overall costs while ensuring battery quality.

[0008] In a first aspect, the present invention provides a method for screening the self-discharge performance of lithium iron phosphate batteries, comprising the following: Perform a discharge operation on a single lithium battery to deplete its charge; The single lithium battery cell is charged to 3% to 10% of its nominal capacity using a current of 0.1C to 0.3C. After the charged single lithium battery is left at room temperature for a first predetermined time T1, its voltage V1 is measured. After measuring the voltage V1, the single lithium battery is left at room temperature for a second predetermined time T2, and then its voltage V2 is measured. The K value of the single lithium battery cell is calculated using K=(V1-V2) / (T2-T1); Compare the K value with the preset screening criteria, and eliminate individual cells that do not meet the criteria.

[0009] Optionally, the step of performing a discharge operation to deplete its charge includes, The single lithium battery cell is discharged to 2.0V at a current of 0.1C to 0.3C and left to stand for 3 to 10 minutes; The single lithium battery is discharged to 2.0V with a current of 0.005C to 0.02C.

[0010] Optionally, the single lithium battery cell is charged to 6% of its nominal capacity using a current of 0.2C.

[0011] Optionally, the room temperature is 25°C to 28°C.

[0012] Optionally, the first predetermined time T1 is 20 to 28 hours.

[0013] Optionally, the first predetermined time T1 is 24 hours.

[0014] Optionally, the second predetermined time T2 is 100 to 140 hours.

[0015] Optionally, the second predetermined time T2 is 120 hours.

[0016] Optionally, the values ​​of voltage V1 and voltage V2 are both in the range of 3090mV to 3240mV.

[0017] Optionally, the preset screening criterion is that the K value is between 0mV / h and 0.06mV / h.

[0018] As can be seen from the above, comparing Example 1 with the two control examples, it can be clearly concluded that "low SOC" is an important factor in achieving high-sensitivity screening; Examples 3, 1, 4, and 2 form an effective gradient of SOC from 3% to 10%, and their effects are significantly better than the control examples; Examples 2, 1, 5, and 6 demonstrate multiple effective combinations within the range of core parameters (SOC, current, time). This collectively proves that the range of technical parameters to be protected is a continuous, complete, and robust effective range, rather than individual point values; the embodiments of the present invention all achieve a false negative rate (all below 1%) far lower than that of traditional methods (3.2%) within a very short period (5-6 days), demonstrating the significant progress and practicality of the present invention in both improving production efficiency and ensuring product quality. Attached Figure Description

[0019] The accompanying drawings, which are provided to further illustrate the invention and form part of this application, do not constitute an undue limitation of the invention.

[0020] Figure 1 This is a flowchart illustrating a method for screening the self-discharge performance of a lithium iron phosphate battery according to Embodiment 1 of the present invention. Detailed Implementation

[0021] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0022] Examples of embodiments of the present invention described in detail below are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout.

[0023] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, but should not be construed as limiting the invention. In the description of the invention, it should be understood that terms such as "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the invention.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0025] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0026] This invention depletes the battery through deep discharge and then charges it to a low SOC (e.g., 3%-10%). By utilizing the characteristic that voltage changes with capacity within the low SOC range, the voltage difference between batteries with inconsistent self-discharge is significantly amplified, thereby achieving high-precision screening in a short time.

[0027] Example 1: Preferred embodiment of the present invention.

[0028] See Figure 1 As shown, the steps are detailed below: Take the same batch of lithium iron phosphate cells with a capacity of 100Ah.

[0029] Discharge steps: First, discharge the battery to 2.0V with a constant current of 0.2C.

[0030] Then, leave the battery at room temperature for 5 minutes.

[0031] Finally, discharge the battery again to 2.0V with a small current of 0.01C to ensure that the charge is fully discharged.

[0032] Charging steps: Use a 0.2C current to charge the discharged battery to 6% of its nominal capacity (SOC).

[0033] First settling and measurement: Place the batteries charged to 6% SOC in a constant temperature environment of 25℃-28℃ for a predetermined settling time T1=24 hours. After the settling period, immediately measure and record the open circuit voltage V1 of each battery.

[0034] At this point, the V1 of a normal battery should fall within the range of 3150mV±60mV.

[0035] Second settling and measurement: After completing the V1 measurement, the batteries continued to be settling in a constant temperature environment of 25℃-28℃ for a second predetermined time T2=120 hours. After the settling period, the open circuit voltage V2 of each battery was immediately measured and recorded.

[0036] Calculation and Screening: Calculate the K value for each battery using the formula K12=(V1-V2) / (T2-T1). Set the screening criterion as K12≤0.06mV / h. Batteries with a K value greater than 0.06mV / h are considered to have excessive self-discharge and are therefore rejected.

[0037] In this embodiment, the K-value distribution range (0.01~0.15mV / h, standard deviation 0.025mV / h) is ideal, clearly distinguishing performance differences. It exhibits the lowest false negative rate, at only 0.5%, achieving optimal results. This demonstrates that 6% SOC combined with a 0.2C current and a 6-day cycle achieves the best balance between screening efficiency and accuracy.

[0038] Test results show that the solution described in this embodiment can effectively detect batteries with slight inconsistencies in self-discharge, significantly improve the consistency of the battery pack after screening, and greatly shorten the production cycle.

[0039] Example 2: Preferred embodiment of the present invention.

[0040] The difference between this embodiment and Embodiment 1 lies in the following steps: During the charging process, the charging SOC is adjusted to 5%.

[0041] In the first settling and measurement step, the first settling time T1 is adjusted to 20 hours.

[0042] In the second settling and measurement step, the second settling time T2 was adjusted to 110 hours.

[0043] In this embodiment, the K value ranges from 0.014 to 0.145 mV / h (standard deviation 0.023 mV / h). The false negative rate after aging verification is 0.7%. This result indicates that reasonable adjustments to core parameters such as SOC and settling time can still maintain excellent screening performance under the concept of this invention.

[0044] Example 3: Minimum SOC value (3% SOC) scheme.

[0045] The steps of this embodiment are described in detail below: The discharge procedure is the same as in Example 1.

[0046] Charging procedure: Use a 0.2C current to charge the battery to 3% of its nominal capacity (SOC).

[0047] Settling and measurement: First settling time T1 = 24 hours, second settling time T2 = 120 hours, ambient temperature 25-28℃.

[0048] Calculation and screening: The calculation method and screening criteria (K12≤0.06mV / h) for the K value are the same as in Example 1.

[0049] In this embodiment, the K value distribution ranged from 0.015 to 0.16 mV / h (standard deviation 0.026 mV / h), indicating good separation. The false negative rate after aging validation was 0.8%. This result demonstrates that when the SOC is set to 3%, the scheme can still achieve effective screening with high sensitivity and low false negative rate within 6 days.

[0050] Test results show that batteries with abnormal self-discharge can be effectively screened out within a total cycle of 6 days. Although the absolute value of voltage V1 is low, the signal strength of the voltage difference (V1-V2) is sufficient for accurate calculation.

[0051] Example 4: Maximum SOC (10% SOC) scheme.

[0052] The steps of this embodiment are described in detail below: Charging procedure: Use a 0.2C current to charge the battery to 10% of its nominal capacity (SOC).

[0053] The remaining steps are the same as in Example 1.

[0054] In this embodiment, the K value ranges from 0.02 to 0.12 mV / h (standard deviation 0.020 mV / h). The false negative rate after aging validation is 0.7%. The data shows that when the SOC is 10% of the upper limit of this range, the screening effect of the scheme within 6 days is still significantly better than all control examples, verifying the effectiveness and practicality of the upper limit of this parameter.

[0055] Test results show that at 10% SOC, although the voltage sensitivity is slightly lower than at 6% SOC, the voltage change signal is still very significant compared to Control Example 2 (40% SOC) because it is still in the non-plateau region of low SOC. The test results confirm that this scheme has a significantly higher screening accuracy than the traditional method over a 6-day period.

[0056] Example 5: Minimum charging current terminal value (0.1C) scheme.

[0057] The steps of this embodiment are described in detail below: Charging procedure: Use a 0.1C current to charge the battery to 6% of its nominal capacity (SOC).

[0058] The remaining steps are the same as in Example 1.

[0059] In this embodiment, after charging with a 0.1C current, the K value distribution ranged from 0.012 to 0.14 mV / h (standard deviation 0.024 mV / h), and the false negative rate during aging verification was 0.6%. This indicates that even with a charging current at the lower limit of this range, excellent screening sensitivity and accuracy can still be maintained within a 6-day cycle, demonstrating the tolerance of the solution to changes in charging current parameters.

[0060] Test results show that although charging with a 0.1C current takes slightly longer, the charging process is more stable, which helps stabilize the battery voltage. After the static test, the calculated K-value data showed good consistency, and the screening effect was comparable to that of Example 1.

[0061] Example 6: Shortest settling period scheme (T1=20h, T2=100h) The steps of this embodiment are described in detail below: The first settling time, T1, is 20 hours.

[0062] The second settling time, T2, is 100 hours.

[0063] The total settling period is 5 days.

[0064] The rest is the same as in Example 1.

[0065] In this embodiment, after compressing the total cycle to 5 days, the K value distribution range still reaches 0.015~0.13mV / h (standard deviation 0.022mV / h), and the aging verification false negative rate is 0.9%. Although the false negative rate is slightly higher than that of the 6-day cycle, it is far lower than the 3.2% of the traditional 20-day scheme, successfully achieving the best trade-off between efficiency and accuracy.

[0066] Test results show that even with the total cycle shortened to 5 days, the calculated K value can still clearly distinguish between good and defective products due to the strong voltage change signal at low SOC. Although the screening accuracy is slightly lower than the 6-day cycle in Example 1, it is far superior to the traditional 20-day scheme, demonstrating the technical effectiveness within a shorter settling time range.

[0067] Compare with Example 1: Traditional semi-electric static solution.

[0068] The steps in this comparative example are detailed below: The same batch of lithium iron phosphate cells were charged to 50% SOC at a current of 0.2C.

[0069] Place the battery in a constant temperature environment of 25°C and leave it to stand continuously for 480 hours (20 days).

[0070] Measure the voltage at the beginning and end of the settling period and calculate the K value.

[0071] In this comparative example, the K-value distribution of the batteries under this method ranged from 0.02 to 0.08 mV / h, showing a highly concentrated distribution (standard deviation of 0.012 mV / h). When the same batch of batteries underwent high-temperature aging verification at 85°C for 7 days, the false negative rate was as high as 3.2%. This indicates that the traditional method has an extremely long cycle and insufficient sensitivity due to its location in the voltage plateau region, resulting in the inability to effectively eliminate a large number of batteries with poor consistency.

[0072] Compare with Example 2: High SOC screening scheme.

[0073] The steps of this embodiment are described in detail below: The same two-step discharge method as in Example 1 was used to deplete the battery and a 6-day resting period was followed.

[0074] Charge the battery to 40% SOC (entering the voltage plateau region) using a 0.2C current.

[0075] Under an environment of 25℃-28℃, let it stand for T1=24 hours and measure V1; then let it stand for T2=120 hours and measure V2.

[0076] Calculate the K value and then filter it.

[0077] In this comparative example, although the total settling time was the same as in Example 1 (6 days), the battery voltage change was very small at this SOC. The calculated K-value data were highly concentrated, with an extremely narrow distribution range (0.03~0.05mV / h) and a very small standard deviation (~0.005mV / h). The K-values ​​of good and defective products overlapped significantly, making it impossible to set an effective screening threshold. Aging verification showed that this scheme failed to achieve effective screening. This comparative example demonstrates that, without adhering to the "low SOC" core of this invention, the scheme completely fails even with the same settling time.

[0078] To more systematically demonstrate the technical effects of the present invention, the core parameters and corresponding quantitative effect indicators of each of Examples 1-6 and Comparative Examples 1 and 2 are summarized in Table 1 below. All tests were conducted based on the same batch of batteries to ensure the fairness of the comparison.

[0079] Table 1: plan Key parameters (SOC / charging current / total cycles) K value distribution range (mV / h) Standard deviation of K value distribution (mV / h) Missed detection rate (%) (after aging verification at 85°C for 7 days) Screening period (days) Compare with Example 1 50% SOC / 0.2C / 20 days 0.02~0.08 0.012 3.2% 20 Compare with Example 2 40% SOC / 0.2C / 6 days 0.03~0.05 ~0.005 Ineffective screening 6 Example 1 6% SOC / 0.2C / 6 days 0.01~0.15 0.025 0.5% 6 Example 2 5% SOC / 0.2C / 5.4 days 0.014~0.145 0.023 0.7% 5.4 Example 3 3% SOC / 0.2C / 6 days 0.015~0.16 0.026 0.8% 6 Example 4 10% SOC / 0.2C / 6 days 0.02~0.12 0.020 0.7% 6 Example 5 6% SOC / 0.1C / 6 days 0.012~0.14 0.024 0.6% 6 Example 6 6% SOC / 0.2C / 5 days 0.015~0.13 0.022 0.9% 5 .

[0080] As can be seen from the above, by comparing Example 1 with the two control examples, it can be clearly concluded that "low SOC" is an important factor in achieving high-sensitivity screening. Examples 3, 1, 4, and 2 formed an effective gradient of SOC from 3% to 10%, and their effects were significantly better than the control example. Examples 2, 1, 5, and 6 demonstrate a variety of effective combinations within the core parameters (SOC, current, time). This collectively demonstrates that the range of technical parameters to be protected is a continuous, complete, and robust effective range, rather than individual point values. All embodiments of the present invention achieve a false negative rate (less than 1%) that is far lower than that of traditional methods (3.2%) within a very short period of time (5-6 days). The present invention demonstrates great progress and practicality in both improving production efficiency and ensuring product quality.

[0081] The embodiments described above do not constitute a limitation on the scope of protection of this technical solution. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the above embodiments should be included within the scope of protection of this technical solution.

Claims

1. A method for screening the self-discharge performance of lithium iron phosphate batteries, characterized in that, Including the following: Perform a discharge operation on a single lithium battery to deplete its charge; The single lithium battery cell is charged to 3% to 10% of its nominal capacity using a current of 0.1C to 0.3C. After the charged single lithium battery is left at room temperature for a first predetermined time T1, its voltage V1 is measured. After measuring the voltage V1, the single lithium battery is left at room temperature for a second predetermined time T2, and then its voltage V2 is measured. The K value of the single lithium battery cell is calculated using K=(V1-V2) / (T2-T1); Compare the K value with the preset screening criteria, and eliminate individual cells that do not meet the criteria.

2. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 1, characterized in that, The step of performing a discharge operation to empty its charge includes... The single lithium battery cell is discharged to 2.0V at a current of 0.1C to 0.3C and left to stand for 3 to 10 minutes; The single lithium battery is discharged to 2.0V with a current of 0.005C to 0.02C.

3. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 1, characterized in that, The single lithium battery cell is charged to 6% of its nominal capacity using a 0.2C current.

4. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 1, characterized in that, The room temperature is between 25°C and 28°C.

5. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 1, characterized in that, The first predetermined time T1 is 20 to 28 hours.

6. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 5, characterized in that, The first predetermined time T1 is 24 hours.

7. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 5, characterized in that, The second predetermined time T2 is 100 to 140 hours.

8. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 5, characterized in that, The second predetermined time T2 is 120 hours.

9. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 5, characterized in that, The values ​​of voltages V1 and V2 are both in the range of 3090mV to 3240mV.

10. The method for screening the self-discharge performance of lithium iron phosphate batteries according to claim 5, characterized in that, The preset screening criterion is that the K value is between 0mV / h and 0.06mV / h.

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