A method for low-temperature charging of a lithium-ion battery cell

Abstract

The present invention discloses a method for low-temperature charging of a lithium-ion battery cell, which overcomes the problems of low charging efficiency and certain low-temperature charging safety risks during low-temperature charging of a lithium-ion battery cell in the prior art. The method includes the following steps: S1: Define different temperature charging windows and the charge-discharge energy efficiency of the battery cell at different temperatures; S2: Establish the corresponding relationship between the constant-current pulse charge-discharge energy efficiency and the low-temperature safe charging usage boundary of the battery cell; S3: Use positive and negative power pulses for charge and discharge until the charging is completed. It can more accurately define and identify the reserved charging safety boundary, improve the charging efficiency, and ensure charging safety.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery charging, and particularly relates to a method for charging a lithium-ion battery cell at low temperature. Background Art

[0002] Lithium-ion battery cells have the advantages of high voltage, high specific energy, long charge and discharge life, no memory effect, and environmental friendliness, and are widely used in various fields such as industrial production and daily life. With the increasingly wide application of lithium-ion battery cells, the requirements for the charge and discharge performance of lithium-ion battery cells at low temperature by customers at the user end are also getting higher and higher. However, the dynamic performance of lithium-ion battery cells at low temperature is poor, the viscosity of the electrolyte increases, the conductivity decreases, and the ion migration rate decreases. These factors lead to poor performance of lithium-ion battery cells at low temperature and a high risk of lithium plating.

[0003] Existing technologies for low-temperature charging basically adopt intermittent charging and rest to eliminate polarization, and constant-current positive and negative pulses to eliminate polarization to obtain an effect of improving the low-temperature charging capacity; or methods such as external heating and pulse heating of the battery cell are used to increase the internal temperature of the battery cell, and the temperature of the battery cell is detected to define the charging regime under this temperature condition. However, whether it is external heating or internal heating, there are problems regarding the uniformity of the internal temperature of the battery cell, especially for large battery cells, and whether the temperature at the monitoring point represents the overall state of the battery cell. It is impossible to accurately define the electrochemical dynamic state inside the battery cell and the lithium plating boundary at the negative electrode; the charging efficiency is low and there is a certain safety risk for low-temperature charging. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problems of low charging efficiency and certain safety risks for low-temperature charging of lithium-ion battery cells in the prior art, and provide a method for charging a lithium-ion battery cell at low temperature, which has higher accuracy compared with the existing method that simply relies on single-point temperature detection for characterization, can more accurately define and identify the reserved charging safety boundary, improve the charging efficiency, and ensure charging safety.

[0005] In order to achieve the above purpose, the present invention adopts the following technical solutions: A method for charging a lithium-ion battery cell at low temperature, comprising the following steps:

[0006] S1: Define different temperature charging windows and the charge and discharge energy efficiency of the battery cell at different temperatures;

[0007] S2: Establish the corresponding relationship between the charge and discharge energy efficiency of constant-current pulses and the low-temperature safe charging use boundary of the battery cell;

[0008] S3: Use positive and negative power pulses for charge and discharge until the charging is completed.

[0009] The present invention establishes the corresponding relationship between the energy efficiency of constant-current pulse charge and discharge and the low-temperature safe charging operation boundary of the battery cell, and corrects the error caused by simply using temperature detection to characterize the charging boundary. Compared with the existing method that simply relies on single-point temperature detection for characterization, it has higher accuracy, can more accurately define and identify the reserved charging safety boundary, improve the charging efficiency, and ensure charging safety. At the same time, the present invention uses positive and negative power pulse charge and discharge for self-heating. Compared with the existing constant-current positive and negative pulses, the charge and discharge current can be adjusted within a certain range according to the degree of low-temperature polarization; the risk of lithium plating is reduced and the efficiency is improved.

[0010] Preferably, step S1 is further expressed as:

[0011] S1.1: Measure the negative electrode potential of the battery cell at different gradient temperatures, different gradient constant-current modes, and different gradient SOCs using a conventional three-electrode test, and define the charging window with these three parameters.

[0012] S1.2: Keep the battery cell stable at different gradient temperatures, fix the positive and negative constant-current pulse charge and discharge modes, and calculate the energy efficiency under this condition to define the charge and discharge energy efficiency at different temperature gradients.

[0013] When measuring the negative electrode potential, use the negative electrode potential = 0 as a reference.

[0014] Preferably, in step S2, it is specifically shown as:

[0015] S2.1: Measure the negative electrode potential under different temperature conditions and different constant-current charging modes, and define the safe charging boundary.

[0016] S2.2: Determine the low-temperature safe charging boundary under the charging mode.

[0017] S2.3: When the temperature is the low-temperature safe charging boundary, measure the decay curve of the charge and discharge energy efficiency with the cycle life under the fixed constant-current pulse mode as the judgment boundary model.

[0018] Establish the corresponding relationship between the energy efficiency of constant-current pulse charge and discharge and the low-temperature safe charging operation boundary of the battery cell, and correct the error caused by simply using temperature detection to characterize the charging boundary. Among them, the error: Conventional temperature detection is single-point local temperature measurement, which has a certain error; the actual charging safety boundary can be accurately identified through energy efficiency judgment. Correction: That is, it is identified and feedback whether it is within the safe charging boundary under the charging mode by whether the pulse energy efficiency is below the energy efficiency curve.

[0019] Preferably, in step S2.1, the safe charging boundary is that the negative electrode potential is greater than 0; in step S2.2, the low-temperature safe charging boundary is the temperature when the negative electrode potential is 0.

[0020] When the negative electrode potential is greater than 0, it is the safe charging boundary; the temperature when the negative electrode potential is 0 is the low-temperature boundary for safe charging under the charging mode. In actual application, if the energy efficiency detected by the constant current pulse is below the decay curve, it is determined that the charging is below the safe boundary line.

[0021] Preferably, in the step S3, the specific expression of using positive and negative power pulses for charge and discharge is as follows:

[0022] S3.1: Using positive and negative power pulses to perform charge and discharge in stages, which altogether includes three stages;

[0023] S3.2: Determine the charging time and charging capacity for each stage;

[0024] S3.3: Cycle through the three stages until the charging is completed.

[0025] The charge and discharge with positive and negative power pulses improves the low-temperature self-heating efficiency of the battery cell. Compared with the existing constant current positive and negative pulses, the charge and discharge current can be adjusted within a certain range according to the degree of low-temperature polarization; the risk of lithium plating is reduced and the efficiency is improved.

[0026] Preferably, the three stages specifically include:

[0027] The first stage: Charge at power P1 for t1 seconds and discharge at power nP1 for (t1 / n * x) seconds, with a total charge of a1% - a2% SOC. Here, n represents a natural number from 1 to 10, and x represents the percentage of single-pulse discharge.

[0028] The second stage: Perform positive and negative charge and discharge with a constant current I1 pulse for t seconds, calculate the energy efficiency; correct the low-temperature boundary for safe charging using the energy efficiency of the battery cell, adjust the charging current I2, and charge for t2 seconds, with a total charge of a1% - a2% SOC;

[0029] The third stage: Perform positive and negative charge and discharge with a constant current I1 pulse for t seconds, calculate the energy efficiency; correct the low-temperature boundary for safe charging using the energy efficiency of the battery cell, adjust the charging current I2, and charge for t3 seconds, with a total charge of a1% - a2% SOC.

[0030] The first stage is a low-rate charge, and a large rate is used to discharge a part (discharge 30% - 50% of the charged electricity) to heat up the battery cell. P1 = 0.1C0 * OCV, and x represents the percentage of single-pulse discharge, which can be taken as 0.3 - 0.5.

[0031] Preferably, in the second stage and the third stage, it further includes:

[0032] Determine whether the fixed-mode charging is within the safety margin according to the energy efficiency; according to the correspondence between the low-temperature charging / energy efficiency of the battery cell, adjust the actual charging current to within the defined safety margin, and correspond the charging current to the pre-set mode proximity current, with a total charging of a1% - a2% SOC.

[0033] Preset the mode current, that is, the stepped current, adjust the actual charging current to within the defined safety margin, and correspond the part of the actual charging current within the safety margin to the preset mode current.

[0034] Preferably, the step S3 further includes: during the charging process, calculate the change in energy efficiency using intermittent constant-current pulses, where:

[0035] Energy efficiency = pulse discharge energy / pulse charging energy.

[0036] Calculating the change in energy efficiency using intermittent constant-current pulses during the charging process can accurately define the actual dynamic state inside the battery cell, optimize the defined charging current, improve the charging efficiency, and ensure charging safety.

[0037] Therefore, the present invention has the following beneficial effects: 1. It can improve the self-heating efficiency of the battery cell and dynamically adjust the charging system; 2. It can accurately characterize the actual dynamic state inside the battery cell and correct the charging capacity determination error caused by simple temperature detection, especially the error caused by the local temperature difference of large battery cells; 3. It can improve the low-temperature charging efficiency and charging safety. Description of the Drawings

[0038] Figure 1 It is a specific operation flowchart of the method of the present invention. Detailed Embodiments

[0039] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments:

[0040] As Figure 1In the illustrated embodiment, a low-temperature charging method for a lithium-ion battery cell can be seen. Its operation process is as follows: Step 1, define charging windows at different temperatures and charge-discharge energy efficiencies of the battery cell at different temperatures; Step 2, establish the corresponding relationship between the constant-current pulse charge-discharge energy efficiency and the low-temperature safe charging usage boundary of the battery cell; Step 3, use positive and negative power pulses for charge and discharge until the charging is completed. The present invention establishes the corresponding relationship between the constant-current pulse charge-discharge energy efficiency and the low-temperature safe charging usage boundary of the battery cell, and corrects the error caused by simply using temperature detection to characterize the charging boundary. Compared with the existing method that simply relies on single-point temperature detection for characterization, it has higher accuracy, can more accurately define and identify the reserved charging safety boundary, improve the charging efficiency, and ensure charging safety. At the same time, the present invention uses positive and negative power pulse charge and discharge for self-heating. Compared with the existing constant-current positive and negative pulses, the charge and discharge current can be adjusted floatingly within a certain range according to the degree of low-temperature polarization; the risk of lithium plating is reduced and the efficiency is improved.

[0041] Specifically manifested as:

[0042] The first step: Define charging windows at different temperatures and charge-discharge energy efficiencies of the battery cell at different temperatures.

[0043] Use a conventional three-electrode to test the negative electrode potential of the battery cell at different temperature gradients, different constant-current regimes, and different SOC gradients. Taking the negative electrode potential = 0 as a reference, define the charging window with these three parameters; keep the battery cell stable at different temperature gradients, fix the positive and negative constant-current pulse charge-discharge regimes, and calculate the energy efficiency under this condition to define the charge-discharge energy efficiency at different temperature gradients.

[0044] The second step: Establish the corresponding relationship between the constant-current pulse charge-discharge energy efficiency and the low-temperature safe charging usage boundary of the battery cell.

[0045] Test the negative electrode potential under different temperature conditions and different constant-current charging regimes, and define the negative electrode potential > 0 as the safe charging boundary.

[0046] Determine the low-temperature safe charging boundary under the charging regime. When the negative electrode potential is 0, this temperature is the low-temperature safe charging boundary under the charging regime.

[0047] Test the decay curve of the charge-discharge energy efficiency with the life under the fixed constant-current pulse regime at this temperature as the judgment boundary model. In practical applications, when the constant-current pulse detects that the energy efficiency is below the decay curve, it is determined that the charging is below the safe boundary line.

[0048] Establish the corresponding relationship between the energy efficiency of constant-current pulse charge and discharge and the low-temperature safe charging operation boundary of the battery cell, and correct the error caused by using only temperature detection to characterize the charging boundary. Error: Conventional temperature detection is single-point local temperature measurement, which has certain errors; the actual charging safety boundary can be accurately identified through energy efficiency determination. Correction: That is, it is identified through whether the pulse energy efficiency is below the energy efficiency curve and whether the feedback is within the safe charging boundary under the charging mode.

[0049] The third step: Use positive and negative power pulses for charge and discharge until the charging is completed.

[0050] Use positive and negative power pulses to perform charge and discharge in stages to improve the low-temperature self-heating efficiency of the battery cell. It includes three stages in total; determine the charging time and charging capacity for each stage.

[0051] Among them:

[0052] The first stage: Charge at power P1 for t1 seconds and discharge at power nP1 for (t1 / n * x) seconds, with a total charge of 3% - 8% SOC. Among them, P1 = 0.1C0 * OCV, n represents natural numbers 1 - 10, x represents the percentage of single-pulse discharge, and x = 0.3 - 0.5.

[0053] Charge at a low rate and discharge a part at a high rate (discharge 30% - 50% of the charged electricity) to heat up the battery cell.

[0054] The second stage: Perform positive and negative charge and discharge of constant current I1 for t seconds, and calculate the energy efficiency; use the energy efficiency of the battery cell to correct the low-temperature safety charging boundary, adjust the charging current I2, and charge for t2 seconds, with a total charge of 3% - 8% SOC. Determine whether the fixed-mode charging is within the safe boundary based on the energy efficiency; according to the correspondence between the low-temperature charging / energy efficiency of the battery cell, adjust the actual charging current to within the defined safe boundary, and correspond the charging current to the current close to the preset mode, with a total charge of 3% - 8% SOC.

[0055] The third stage: Perform positive and negative charge and discharge of constant current I1 for t seconds, and calculate the energy efficiency; use the energy efficiency of the battery cell to correct the low-temperature safety charging boundary, adjust the charging current I2, and charge for t3 seconds, with a total charge of 3% - 8% SOC. Determine whether the fixed-mode charging is within the safe boundary based on the energy efficiency; according to the correspondence between the low-temperature charging / energy efficiency of the battery cell, adjust the actual charging current to within the defined safe boundary, and correspond the charging current to the current close to the preset mode, with a total charge of 3% - 8% SOC.

[0056] During the charging process, use intermittent constant-current pulses to calculate the change in energy efficiency. Among them:

[0057] Energy efficiency = Pulse discharge energy / Pulse charge energy. Accurately define the actual dynamic state inside the battery cell, optimize the defined charging current, improve the charging efficiency and ensure charging safety.

[0058] Repeat the three charging stages until charging is completed.

[0059] Through this application, the self-heating efficiency of the battery cell is improved, and the charging system can be dynamically adjusted. At the same time, it can accurately characterize the actual dynamic state inside the battery cell and correct the charging capacity determination error caused by the simple temperature detection determination, especially the error caused by the local temperature difference of large battery cells, improving the low-temperature charging efficiency and charging safety.

[0060] The above embodiments are only a preferred solution of the present invention, and do not impose any form of limitation on the present invention. There are other variations and modifications without exceeding the technical solutions described in the claims.

Claims

1. A low-temperature charging method for a lithium-ion battery cell, characterized in that, It includes the following steps: S1: Define the charging energy efficiency of the battery cell under different temperature charging windows and fixed positive and negative constant current pulse charge and discharge systems at different temperatures. S2: Establish the corresponding relationship between the constant current pulse charge and discharge energy efficiency and the low-temperature safe charging usage boundary of the battery cell, including testing the negative electrode potential under different temperature conditions and different constant current charging systems, defining the safe charging boundary, and determining the low-temperature safe charging boundary under the charging system. S3: Use positive and negative power pulses for charge and discharge until charging is completed.

2. The low-temperature charging method of a lithium-ion battery cell according to claim 1, characterized in that, The step S1 is further expressed as: S1.1: Use a conventional three-electrode to test the negative electrode potential of the battery cell at different gradient temperatures, different gradient constant current systems, and different gradient SOCs, and define the charging window with these three parameters. S1.2: Keep the battery cell stable at different gradient temperatures, fix the positive and negative constant current pulse charge and discharge systems, calculate the energy efficiency under this condition, and define the charge and discharge energy efficiency at different temperature gradients.

3. A method for low-temperature charging of a lithium-ion battery cell according to claim 1 or 2, characterized in that, In the step S2, specifically: When the test temperature is the low-temperature safe charging boundary, test the decay curve of the charge and discharge energy efficiency with the life under the fixed constant current pulse system as the judgment boundary model.

4. A low-temperature charging method for a lithium-ion battery cell according to claim 3, characterized in that, The safe charging boundary is that the negative electrode potential is greater than 0; the low-temperature safe charging boundary is the temperature when the negative electrode potential is 0.

5. A method for low-temperature charging of a lithium-ion battery cell according to claim 1, characterized in that, In the step S3, using positive and negative power pulses for charge and discharge is specifically expressed as: S3.1: Use positive and negative power pulses for charge and discharge in stages, which altogether include three stages. S3.2: Determine the charging time and charging capacity for each stage. S3.3: Cycle through the three stages until charging is completed.

6. A method for low-temperature charging of a lithium-ion battery cell according to claim 5, characterized in that, The three stages specifically include: The first stage: Charge at power P1 for t1 seconds, discharge at power nP1 for (t1 / n * x) seconds, and the overall charge is a1% - a2% SOC, where n represents a natural number from 1 to 10, and x represents the percentage of single-pulse discharge. The second stage: Charge and discharge in positive and negative directions with a constant current I1 for t seconds, and calculate the energy efficiency; use the energy efficiency of the battery cell to correct the low-temperature safe charging boundary, adjust the charging current to I2, and charge for t2 seconds, with the overall charge being a1% - a2% SOC. The third stage: Charge and discharge in positive and negative directions with a constant current I1 for t seconds, and calculate the energy efficiency; use the energy efficiency of the battery cell to correct the low-temperature safe charging boundary, adjust the charging current to I2, and charge for t3 seconds, with the overall charge being a1% - a2% SOC.

7. A method for low-temperature charging of a lithium-ion battery cell according to claim 6, characterized in that, In the second stage and the third stage, it also includes: Judge whether the fixed-system charging is within the safe boundary according to the energy efficiency; adjust the actual charging current to within the defined safe boundary according to the correspondence between the low-temperature charging / energy efficiency of the battery cell, and correspond the charging current to the pre-set system-approaching current, with the overall charge being a1% - a2% SOC.

8. A method for low-temperature charging of a lithium-ion battery cell according to claim 1, characterized in that, The step S3 also includes: During the charging process, calculate the change in energy efficiency using intermittent constant current pulses, where: Energy efficiency = pulse discharge energy / pulse charging energy.

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