Stepped capacity grading method for lithium battery

Abstract

The invention relates to a stepped capacity grading method for a lithium battery. The method comprises the following steps: standing the lithium battery after liquid injection; charging to a first charging cut-off voltage at a first multiplying power constant current, and standing; carrying out constant-current and constant-voltage charging at a second multiplying power to a second charging cut-off voltage, and standing; wherein the second multiplying power is smaller than the first multiplying power; carrying out constant-current discharge at a third multiplying power to a first discharge cut-off voltage, and standing; carrying out constant-current discharge at a fourth multiplying power to a second discharge cut-off voltage, and standing; and carrying out constant-current charging at a fifth multiplying power to a second charging cut-off voltage, standing, and ending. The problems of interface lithium precipitation and battery heating in the charging process can be effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a lithium battery step-by-step capacity division method. Background Art

[0002] During the production of lithium batteries, the capacity of lithium batteries needs to be divided. The capacity division process is to obtain capacity, voltage and other data through the charge and discharge of lithium batteries, which can distinguish the performance of lithium batteries. The size of the charge and discharge current determines the efficiency of lithium battery production. The greater the charge and discharge rate, the shorter the capacity division time. However, too high a charge rate will cause the negative electrode to not have time to embed lithium ions, and then lithium deposition will occur at the negative electrode interface. Lithium deposition not only affects the capacity of the lithium battery, but also its cycle life.

[0003] The existing capacity division process basically adopts high-rate current constant current constant voltage charging, directly charging to the maximum voltage limit, and then discharging with high-rate current constant current to the lower voltage limit, and then using low-rate current to complete the bottom end. However, long-term high-rate current charging will prevent the lithium ions in the positive electrode from being embedded in the negative electrode in time, and precipitate on the surface of the negative electrode, affecting the discharge capacity and cycle life of the battery, and there is also a risk of thermal runaway during the charging process. Summary of the invention

[0004] Based on this, the purpose of the present invention is to provide a lithium battery step-by-step capacity division method, which can effectively improve the problem of lithium deposition on the interface and heating of the battery during the charging process.

[0005] To achieve the above object, the present invention adopts the following technical solutions: The present invention first provides a lithium battery step-by-step capacity division method, which comprises the following steps: S1. Let the lithium battery with liquid injection stand still; S2. Charge at a first rate constant current to a first charge cut-off voltage and leave to stand; S3. charging at a second rate with constant current and constant voltage to a second charging cut-off voltage, and standing; wherein the second rate is less than the first rate; S4. Discharge at a third rate constant current to the first discharge cut-off voltage and leave it alone; S5. Discharge at a fourth rate constant current to the second discharge cut-off voltage and leave to stand; S6. Charge at the fifth rate constant current to the second charge cut-off voltage, let stand, and end.

[0006] As a further improvement of the above solution of the present invention, the first charging cut-off voltage is greater than the second charging cut-off voltage.

[0007] As a further improvement of the above solution of the present invention, the first charging cut-off voltage is 3.4-3.5V, and the second charging cut-off voltage is 3.65V.

[0008] As a further improvement to the above solution of the present invention, in step S2, the first rate is 0.3 - 1C rate, and the constant current charging time is controlled within 80 - 267 min; and / or, in step S3, the second rate is 0.1 - 0.3C rate, and the constant current and constant voltage charging time is controlled within 20 - 60 min.

[0009] As a further improvement to the above solution of the present invention, the first discharge cut-off voltage is equal to the second discharge cut-off voltage.

[0010] As a further improvement to the above solution of the present invention, both the first discharge cut-off voltage and the second discharge cut-off voltage are 2.0 - 2.5V.

[0011] As a further improvement to the above solution of the present invention, in step S4, the third rate is 0.3 - 1C rate, and the constant current discharge time is controlled within 100 - 333 min.

[0012] As a further improvement to the above solution of the present invention, in step S5, the fourth rate is 0.1 - 0.3C rate, and the constant current discharge time is controlled within 20 - 60 min.

[0013] As a further improvement to the above solution of the present invention, in step S6, the fifth rate is 0.2 - 0.5C rate, and the constant current charging time is controlled within 21 - 51 min.

[0014] As a further improvement to the above solution of the present invention, in step S1, the standing time is 5 - 10 min; and / or, in step S2, the standing time is 5 - 10 min; and / or, in step S3, the standing time is 30 - 60 min; and / or, in step S4, the standing time is 30 - 60 min; and / or, in step S5, the standing time is 5 - 10 min; and / or, in step S2, the standing time is 3 - 10 min.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The stepwise grading charge method provided by the present invention splits the first-step high-rate charging in the prior art into two steps. In the first step, the upper voltage limit is reduced, which is equivalent to shortening the charging duration at a high-rate current. In the second step, charging is changed to a low-rate current until the original upper voltage limit is reached, which can effectively improve the problems of interfacial lithium deposition and battery heating during the charging process. The improvement of interfacial lithium deposition is because the charging is switched to a low rate in the later stage of charging: during the charging process, lithium ions are removed from the positive electrode and embedded in the negative electrode. If the charging rate is small, the embedding rate of lithium ions is slow, which helps the lithium ions to be evenly distributed on the surface of the negative electrode, reducing the accumulation on the surface of the negative electrode, and thus reducing the occurrence of lithium deposition; when the charging rate is high, the diffusion rate of lithium ions may not be able to keep up with the charging rate, resulting in the accumulation of lithium ions on the surface of the negative electrode and forming lithium deposition, while low-rate charging can reduce the diffusion resistance of lithium ions, allowing lithium ions to have enough time to diffuse into the interior of the negative electrode and reducing surface lithium deposition; the lithium deposition on the negative electrode usually occurs when the potential of the negative electrode is lower than 0V and the lithium concentration is saturated. Low-rate charging can reduce the concentration difference and polarization effect on the surface of the negative electrode, thus reducing the risk of lithium deposition; the composition and concentration of the electrolyte also have an impact on lithium deposition. Low-rate charging can reduce the side reactions in the electrolyte, reduce the consumption and decomposition of the electrolyte, and thus improve the lithium deposition phenomenon. Description of the Drawings

[0016] Figure 1 It is a flowchart of a lithium battery stepwise grading method proposed in an embodiment of the present invention; Figure 2 It is a schematic diagram of the lithium deposition and non-lithium deposition conditions of lithium batteries in the comparative example and the embodiment. Detailed Embodiments

[0017] For the convenience of understanding the present invention, the present invention will be described more comprehensively below in combination with specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0019] Combined with Figure 1 , this embodiment provides a lithium battery stepwise grading method, which includes the following steps: S1. Let the lithium battery after liquid injection stand still.

[0020] The standing duration is 5 - 10 min, preferably 5 min.

[0021] S2. Constant current charge at the first rate until the first charge cut-off voltage, and then let it stand still.

[0022] The first rate is 0.3 - 1C rate, and the constant current charge time is controlled within 80 - 267 min; preferably 0.5C, 160 min. The first charge cut-off voltage is 3.4 - 3.5V, preferably 3.5V. The standing still time is 5 - 10 min, preferably 5 min.

[0023] S3. Constant current and constant voltage charge at the second rate until the second charge cut-off voltage, and then let it stand still; wherein, the second rate is less than the first rate.

[0024] The second rate is 0.1 - 0.3C rate, and the constant current and constant voltage charge time is controlled within 20 - 60 min; preferably 0.2C, 30 min. The second charge cut-off voltage is 3.65V. The standing still time is 30 - 60 min, preferably 30 min.

[0025] S4. Constant current discharge at the third rate until the first discharge cut-off voltage, and then let it stand still.

[0026] The third rate is 0.3 - 1C rate, and the constant current discharge time is controlled within 100 - 333 min; preferably 0.5C, 200 min. The first discharge cut-off voltage is 2.0 - 2.5V, preferably 2.0V. The standing still time is 30 - 60 min, preferably 30 min.

[0027] S5. Constant current discharge at the fourth rate until the second discharge cut-off voltage, and then let it stand still.

[0028] The fourth rate is 0.1 - 0.3C rate, and the constant current discharge time is controlled within 20 - 60 min; preferably 0.2C, 30 min. The second discharge cut-off voltage is 2.0 - 2.5V, preferably 2.0V. The standing still time is 5 - 10 min, preferably 5 min.

[0029] S6. Constant current charge at the fifth rate until the second charge cut-off voltage, and then let it stand still, and end.

[0030] The fifth rate is 0.2 - 0.5C rate, and the constant current charge time is controlled within 21 - 51 min; preferably 0.3C, 34 min. The second charge cut-off voltage is 3.65V. The standing still time is 3 - 10 min, preferably 3 min.

[0031] Next, a lithium iron phosphate system lithium battery is used to further illustrate this embodiment.

[0032] Embodiment This embodiment proposes a stepped grading method for a lithium battery, which includes the following steps: S1. Let the lithium iron phosphate system lithium battery after liquid injection stand still for 5 min; S2. Charge at a constant current of 0.5C to the first charging cut-off voltage of 3.5V for a limited time of 160 minutes; after charging is completed, let it stand for 5 minutes; S3. Charge at a constant current and constant voltage of 0.2C to the second charging cut-off voltage of 3.65V, with a current limit of 2000mA for a limited time of 30 minutes; after charging is completed, let it stand for 30 minutes; where the second rate is less than the first rate; S4. Discharge at a constant current of 0.5C to the first discharge cut-off voltage of 2.0V for a limited time of 200 minutes; after discharging is completed, let it stand for 30 minutes; S5. Discharge at a constant current of 0.2C to the second discharge cut-off voltage of 2.0V for a limited time of 30 minutes; after discharging is completed, let it stand for 5 minutes; S6. Charge at a constant current of 0.3C to the second charging cut-off voltage of 3.65V for a limited time of 34 minutes; after charging is completed, let it stand for 3 minutes to end.

[0033] Comparative Example S1. Let the lithium iron phosphate system lithium battery after liquid injection stand for 5 minutes; S2. Charge at a constant current of 0.5C to the cut-off voltage of 3.65V, with a current limit of 5000mA for a limited time of 160 minutes; after charging is completed, let it stand for 30 minutes; S3. Discharge at a constant current of 0.5C to the first discharge cut-off voltage of 2.0V for a limited time of 200 minutes; after discharging is completed, let it stand for 30 minutes; S4. Discharge at a constant current of 0.2C to the second discharge cut-off voltage of 2.0V for a limited time of 30 minutes; after discharging is completed, let it stand for 5 minutes; S5. Charge at a constant current of 0.3C to the second charging cut-off voltage of 3.65V for a limited time of 34 minutes; after charging is completed, let it stand for 3 minutes to end.

[0034] It should be noted that in the examples and comparative examples, except for the different grading processes, the environmental temperature, humidity, the type of grading cabinet used, the cell system, the manufacturing process, etc. are all kept the same.

[0035] As Figure 2 shown, it is a schematic diagram of the lithium deposition and non-lithium deposition situations of the batteries in the comparative example and the examples; Figure 2 a, Figure 2 b are respectively schematic diagrams of the lithium deposition situations of the two winding cores of the battery in the comparative example. The grayish-white areas circled in red in the figure are lithium deposition; Figure 2 c, Figure 2 d are respectively schematic diagrams of the non-lithium deposition situations of the two winding cores of the battery in the examples; through comparison, it can be seen that the grading method of the lithium battery in the examples of the present application can improve the lithium deposition problem and improve the grading effect and performance of the battery.

[0036] The stepwise grading charge method of this embodiment is applicable to batteries such as ternary, lithium iron (manganese) phosphate in soft-pack or square-shell form.

[0037] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0038] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A lithium battery step-by-step capacity division method, characterized in that: It includes the following steps: S1. Let the lithium battery with liquid injection stand still; S2. Charge at a first rate constant current to a first charge cut-off voltage and leave to stand; S3. charging at a second rate with constant current and constant voltage to a second charging cut-off voltage, and standing; wherein the second rate is less than the first rate; S4. Discharge at a third rate constant current to the first discharge cut-off voltage and leave it alone; S5. Discharge at a fourth rate constant current to the second discharge cut-off voltage and leave to stand; S6. Charge at the fifth rate constant current to the second charge cut-off voltage, let stand, and end.

2. The lithium battery step-by-step capacity division method according to claim 1, characterized in that: The first charge cut-off voltage is greater than the second charge cut-off voltage.

3. The lithium battery step-by-step capacity division method according to claim 2, characterized in that: The first charge cut-off voltage is 3.4-3.5V, and the second charge cut-off voltage is 3.65V.

4. The lithium battery step-by-step capacity division method according to claim 1, characterized in that: In step S2, the first rate is 0.3-1C rate, and the constant current charging time is controlled within 80-267 min; and / or, in step S3, the second rate is 0.1-0.3C rate, and the constant current and constant voltage charging time is controlled within 20-60 min.

5. The lithium battery step-by-step capacity division method according to claim 1, characterized in that: The first discharge cut-off voltage is equal to the second discharge cut-off voltage.

6. The lithium battery step-by-step capacity division method according to claim 5, characterized in that: The first discharge cut-off voltage and the second discharge cut-off voltage are both 2.0-2.5V.

7. The lithium battery step-by-step capacity division method according to claim 1, characterized in that: In step S4, the third rate is 0.3-1C rate, and the constant current discharge time is controlled within 100-333 minutes.

8. The lithium battery step-by-step capacity division method according to claim 1, characterized in that: In step S5, the fourth rate is 0.1-0.3C rate, and the constant current discharge time is controlled within 20-60 minutes.

9. The lithium battery step-by-step capacity division method according to claim 1, characterized in that: In step S6, the fifth rate is 0.2-0.5C rate, and the constant current charging time is controlled at 21-51 min.

10. The lithium battery step-by-step capacity division method according to claim 1, characterized in that: In step S1, the standing time is 5-10 min; and / or, in step S2, the standing time is 5-10 min; and / or, in step S3, the standing time is 30-60 min; and / or, in step S4, the standing time is 30-60 min; and / or, in step S5, the standing time is 5-10 min; and / or, in step S2, the standing time is 3-10 min.

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