Lithium iron phosphate battery capacity grading charging and discharging method and system and lithium iron phosphate battery

Abstract

The invention discloses a capacity grading charging and discharging method for a lithium iron phosphate battery, which comprises the following steps of: charging the battery with 0.5 C constant current to 3.65 V after secondary liquid injection and liquid supplement, then charging the battery with the constant voltage of 3.65 V to 0.05 C cut-off current, and standing for 5 minutes; then, 0.5 C constant current is used for discharging to 2.5 V, standing is carried out for 5 min, and the voltage rises again; then the voltage is raised to 2.5 V with the constant current of 0.2-0.3 C, standing is carried out for 5 min, and the voltage rises again; then placing to 2.5 V at a constant current of 0.05-0.1 C, and standing for 5 minutes; and finally, 0.5 C constant current is used for charging until 15% of electric quantity is maintained. Wherein the constant current in the step (2), the step (3) and the step (4) needs to be gradually reduced, namely the current in the step (4) is greater than the current in the step (3) and is less than the current in the step (2). The invention mainly aims to ensure that the electric quantity of a single battery after capacity grading tends to be consistent, improve the voltage consistency, ensure the authenticity of self-discharge data and facilitate the removal of abnormal batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium iron phosphate battery manufacturing, and specifically relates to a lithium iron phosphate battery capacity-dividing charging and discharging method and system, and a lithium iron phosphate battery. Background Art

[0002] Currently, lithium iron phosphate batteries have been used in many fields, including energy storage devices and electric vehicles. They have a long cycle life, a stable cathode material that does not decompose, and a safety that is unmatched by other cathode materials.

[0003] In the manufacturing process of lithium iron phosphate batteries, capacity separation is a very important process. The lithium battery cells produced will not be sold immediately. The basic performance data of each battery will be obtained through capacity separation and post-process testing.

[0004] The existing capacity-differentiated charging and discharging process is affected by the ambient temperature of the capacity-differentiated environment, resulting in incomplete discharge. After recharging to the specified SOC, the charge of a single battery is not uniform, affecting the consistency of subsequent voltage tests. The benchmark for the K value test is the battery voltage, and the self-discharge data will also deviate. Summary of the Invention

[0005] One of the objectives of the present invention is to develop a method for charging and discharging lithium iron phosphate batteries with divided capacity, which can ensure that the charge of a single battery after capacity division tends to be consistent, improve voltage consistency, ensure the authenticity of self-discharge data, and facilitate the elimination of abnormal batteries.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0007] A lithium iron phosphate battery capacity-dividing charging and discharging method adopts the following steps:

[0008] (1) After the battery is refilled, first charge it to 3.65V with a constant current of 0.5C, then charge it to a cut-off current of 0.05C at a constant voltage of 3.65V, and leave it for 5 minutes;

[0009] (2) Then use 0.5C constant current to lower the voltage to 2.5V, leave it for 5 minutes, and the voltage will rise again;

[0010] (3) Then use a constant current of 0.2-0.3C to lower the voltage to 2.5V, leave it for 5 minutes, and the voltage will rise again;

[0011] (4) Then use a constant current of 0.05-0.1C to discharge the voltage to 2.5V and leave it for 5 minutes;

[0012] (5) Finally, charge the battery with a constant current of 0.5C to maintain 15% of its capacity.

[0013] The constant currents in steps (2), (3) and (4) must be guaranteed to decrease, that is, the current in step (4) < the current in step (3) < the current in step (2).

[0014] Furthermore, the positive electrode material of the lithium iron phosphate battery is lithium iron phosphate, and the negative electrode material of the battery is graphite.

[0015] Furthermore, the current of the constant current charging module described in step (1), step (2) and step (5) is consistent and greater than or equal to 90A.

[0016] The capacity division method described in the present invention is to ensure the consistency of the charge of a single battery after capacity division. This method first uses a constant current and constant voltage method to fully charge the lithium battery, and then uses a 0.5C constant current discharge to empty the battery. This step can obtain the capacity data of each single battery. However, on the manufacturing production line, the temperature of the capacity division charging and discharging mechanism cabinet will vary due to the different locations. As is well known, a high ambient temperature will lead to increased ion activity, which may slightly increase the capacity; a low temperature will cause the migration of lithium ions to slow down, the viscosity of the electrolyte to increase, and the internal resistance to increase, resulting in a decrease in discharge capacity and the inability to fully discharge the battery charge; among them, the problem of discharge capacity offset can be balanced using a temperature compensation formula, but inconsistent residual charge will lead to inconsistent charge of a single battery after subsequent recharging to a specified SOC. Adding two steps of small current discharge can completely discharge the remaining charge, so that the charge of a single battery after recharging the SOC tends to be consistent.

[0017] Another object of the present invention is to provide a lithium iron phosphate battery, comprising: a high-precision charge and discharge module (current ≥ 90A); a voltage / current monitoring device; and a temperature control system to ensure that the temperature of the divided volume environment is stable at 20-30°C.

[0018] Another object of the present invention is to provide a lithium iron phosphate battery that uses a capacity-dividing charge and discharge method to perform capacity calibration, so that the SOC consistency deviation of the battery after capacity division is ≤±1%.

[0019] Beneficial effects of the present invention:

[0020] Traditional capacity-dividing solutions are significantly affected by ambient temperature and are prone to incomplete discharge in high and low temperature environments, resulting in large SOC deviations and inconsistent voltage references after recharging. This invention completely discharges the battery, eliminating the interference of residual charge on subsequent SOC calibration. This improves voltage consistency and makes K-value testing more reliable. Multi-step discharge reduces polarization, improves energy efficiency, and extends cycle life.

[0021] In summary, the equipment has the advantages of good consistency, high energy efficiency and long service life, and is particularly suitable for the field of lithium iron phosphate battery production technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0023] Figure 1 This is a comparison diagram of the voltage distribution of the lithium iron phosphate batteries corresponding to Example 1, Comparative Example 1 and Comparative Example 2 of the present invention after capacity division is completed and left standing at room temperature for three days.

[0024] Figure 2 This is the voltage difference between the PACK modules corresponding to Example 1 and Comparative Example 1 of the present invention after being discharged to 2.8V.

[0025] Figure 3 The voltage difference between the PACK modules corresponding to Example 1 and Comparative Example 1 of the present invention after being discharged to 2.5V is shown. DETAILED DESCRIPTION

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0027] Example 1

[0028] A method for charging and discharging lithium iron phosphate batteries with different capacities, a square aluminum shell battery with a nominal capacity of 280Ah;

[0029] Use the following steps:

[0030] (1) Place the lithium iron phosphate battery after the secondary filling and rehydration on the capacity distribution cabinet for 2 minutes;

[0031] (2) 0.5C (140A) constant current charging to 3.65V; constant voltage 3.65V charging to 0.05C (14A) cut-off current;

[0032] (3) Set aside for 5 minutes;

[0033] (4) 0.5C (140A) constant current to 2.5V;

[0034] (5) After 5 minutes of suspension, the voltage will recover;

[0035] (6) 0.2C (56A) constant current to 2.5V;

[0036] (7) After 5 minutes of suspension, the voltage will rise again;

[0037] (8) 0.1C (28A) constant current to 2.5V;

[0038] (9) Set aside for 5 minutes;

[0039] (10) 0.5C (140A) constant current charging to maintain 15% of the capacity;

[0040] (11) The process is completed.

[0041] Comparative Example 1

[0042] A method for charging and discharging lithium iron phosphate batteries with different capacities, a square aluminum shell battery with a nominal capacity of 280Ah;

[0043] Use the following steps:

[0044] (1) Place the lithium iron phosphate battery after the secondary filling and rehydration on the capacity distribution cabinet for 2 minutes;

[0045] (2) 0.5C (140A) constant current charging to 3.65V; constant voltage 3.65V charging to 0.05C (14A) cut-off current;

[0046] (3) Set aside for 5 minutes;

[0047] (4) 0.5C (140A) constant current to 2.5V;

[0048] (5) Set aside for 5 minutes;

[0049] (6) 0.5C (140A) constant current charging to maintain 15% of the capacity;

[0050] (7) The process is completed.

[0051] Comparative Example 2

[0052] A method for charging and discharging lithium iron phosphate batteries with different capacities, a square aluminum shell battery with a nominal capacity of 280Ah;

[0053] Use the following steps:

[0054] (1) Place the lithium iron phosphate battery after the secondary filling and rehydration on the capacity distribution cabinet for 2 minutes;

[0055] (2) 0.5C (140A) constant current charging to 3.65V; constant voltage 3.65V charging to 0.05C (14A) cut-off current;

[0056] (3) Set aside for 5 minutes;

[0057] (4) 0.5C (140A) constant current to 2.5V;

[0058] (5) After 5 minutes of suspension, the voltage will recover;

[0059] (6) 0.2C (56A) constant current to 2.5V;

[0060] (7) Set aside for 5 minutes;

[0061] (8) 0.5C (140A) constant current charging to maintain 15% of the capacity;

[0062] (9) The process is completed.

[0063] from Figure 1 It can be seen that the measured ranges of the battery voltages of the corresponding processes of Example 1, Comparative Example 1, and Comparative Example 2 are as follows. Comparative Example 1 does not add any form of discharge after 0.5C constant current discharge, and the voltage range is relatively dispersed, with a voltage difference range of up to 7.89mV; Comparative Example 2 adds a step of low current discharge after 0.5C constant current discharge, and the voltage range is narrowed compared to Comparative Example 1, with a voltage difference range of 5.53mV; while the measured voltage data of this patent stabilizes the voltage difference at 1.06mV, significantly narrowing the voltage range and improving voltage consistency.

[0064] from Figure 2 、 Figure 3 It can be seen that the discharge voltage difference range of the PACK module after using two different processes, Example 1 and Comparative Example 1. In Comparative Example 1, whether discharged to 2.8V or 2.5V, the terminal voltage difference is relatively discrete; after the patent is discharged to 2.8V, the average voltage difference is 98mV, which is nearly 50mV less than that of Comparative Example 1, and after the patent is discharged to 2.5V, the average voltage difference is 254mV, which is nearly 80mV less than that of Comparative Example 1, and the consistency of the terminal voltage difference is significantly improved. (The capacity division process corresponding to containers 1 to 23 is Comparative Example 1, and the capacity division process corresponding to containers 24 to 28 is Example 1).

[0065] In the description of the present invention, it should be understood that the terms "front and back", "left and right", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the equipment or components referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they cannot be understood as limitations on the invention.

[0066] Of course, in this technical solution, those skilled in the art should understand that the term "one" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element may be one, while in another embodiment, the number of the element may be multiple, and the term "one" should not be understood as a limitation on the quantity.

[0067] The above description in conjunction with the accompanying drawings is only a preferred embodiment of the present invention, but the present invention is not limited to the above embodiment. It should be pointed out that for those skilled in the art, various modifications and improvements can be made without departing from the structure of the present invention. These should also be regarded as the scope of protection of the present invention and will not affect the effect and practicality of the implementation of the present invention.

Claims

1. A method for charging and discharging a lithium iron phosphate battery by varying the capacity, characterized in that: The following steps are involved: (1) After the secondary injection and rehydration, the lithium iron phosphate battery was charged to 3.65V using a constant current of 0.5C, then charged to a cut-off current of 0.05C using a constant voltage of 3.65V, and then allowed to stand for 5 minutes; (2) Discharge at a constant current of 0.5C to 2.5V, let it stand for 5 minutes, and the voltage will rise again; (3) Discharge at a constant current of 0.2-0.3C to 2.5V, let it stand for 5 minutes, and the voltage will rise again; (4) Discharge at a constant current of 0.05-0.1C to 2.5V and let it stand for 5 minutes; (5) Charge at a constant current of 0.5C until the battery SOC reaches 15%; The discharge currents of steps (2), (3) and (4) satisfy the decreasing relationship: (0.05-0.1)C<(0.2-0.3)C<0.5C.

2. A lithium iron phosphate battery capacity-dividing charging and discharging method according to claim 1, characterized in that: The positive electrode active material of the lithium iron phosphate battery is lithium iron phosphate, and the negative electrode active material is graphite.

3. A lithium iron phosphate battery capacity-dividing charging and discharging method according to claim 1, characterized in that: The current of the constant current charging module in steps (1), (2), and (5) is ≥90A, and the charging current in each step remains consistent.

4. A lithium iron phosphate battery capacity-divided charging and discharging method according to claim 1, characterized in that: The discharge cut-off voltage in steps (2), (3) and (4) is 2.5V, and the battery is discharged.

5. A lithium iron phosphate battery capacity-divided charging and discharging method according to claim 1, characterized in that: After charging to 15% SOC in step (5), the battery enters a resting state or a subsequent testing process for K value (self-discharge rate) detection.

6. A lithium iron phosphate battery capacity division system, characterized in that: The charging and discharging method according to any one of claims 1 to 5 comprises: a high-precision charging and discharging module (current ≥ 90A); a voltage / current monitoring device; and a temperature control system, wherein the divided volume environment temperature is stabilized at 20 to 30°C.

7. A lithium iron phosphate battery, characterized in that: The capacity calibration is performed using the capacity-dividing charge and discharge method according to any one of claims 1 to 5, and the SOC consistency deviation of the battery after capacity division is ≤±1%.

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