Rapid charging method of lithium iron phosphate battery and testing method of cycle life of lithium iron phosphate battery

Abstract

The invention relates to the technical field of lithium batteries, in particular to a rapid charging method of a lithium iron phosphate battery and a cycle life testing method, and the rapid charging method comprises the following steps: carrying out at least one section of constant current charging on the lithium iron phosphate battery; and stopping charging the lithium iron phosphate battery when the current of the tail-section constant current charging is not greater than the set current and the cut-off voltage of the constant current charging is not less than the set cut-off voltage. According to the rapid charging method provided by the invention, the lithium iron phosphate battery is subjected to multi-section constant-current charging, so that the charging time of the lithium iron phosphate battery is shortened, the rapid charging capability of the lithium iron phosphate battery is improved, and the technical problem of inaccurate SOC estimation of the lithium iron phosphate battery system by a traditional rapid charging method is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a fast charging method for a lithium iron phosphate battery and a test method for the cycle life. Background Art

[0002] Based on the characteristics of lithium-ion batteries such as high energy density, long cycle life, no memory effect and green environmental protection, it stands out from many energy storage devices and is widely used. However, with the iterative upgrades and intelligent development of various power terminals, as well as the continuous improvement of user experience requirements, improving the endurance of equipment has become an urgent problem to be solved. As we all know, as a key technical approach to solving the above problems, improving the energy density of single cells and increasing the charging speed of batteries have once become research hotspots for scientific research and technical personnel. Specifically for lithium iron phosphate system batteries, if fast charging can be achieved, this will largely make up for the shortcomings of insufficient single-time endurance due to the material's own specific energy characteristics.

[0003] The survey found that there are currently few patent applications related to effectively solving the technical problems of fast charging of lithium iron phosphate batteries, and the technology is relatively scarce. In a method for formulating a step-charging strategy for lithium iron phosphate batteries that takes into account the cycle life, the Chinese invention patent with publication number CN114117733A discloses that before the fast charging method is determined, the battery needs to be pre-tested and the SOC interval divided. The process is relatively cumbersome and is also a means that is widely used in current charging method research. In the fast charging method based on temperature and charging power disclosed by the Chinese invention patent with publication number CN102856604A, a fast charging strategy for battery modules is mainly described, but its maximum charging current is only 1C. The Chinese invention patent with publication number CN107240937A mainly reports a charging method and device that focuses on considering the impact of temperature on the battery during the charging process. Its technology can adjust the charging current in real time according to the change of battery temperature, ensuring the safety of battery use and extending the service life of the battery. The Chinese invention patents with publication numbers CN112583066A, CN113964401A, CN109586367A, etc. are all based on the research on charging strategy formulation methods or specific charging methods under the premise of improving battery cycle life. The fast charging technology of the battery is not the focus of their application. The charging rates involved in the corresponding technical solutions are not high, and they cannot truly achieve the purpose of fast charging of the battery.

[0004] In addition, for the fast charging method of lithium iron phosphate batteries, the industry currently adopts several SOC ranges divided from 0% SOC to 100% SOC, using a larger current to charge in the low SOC range and a smaller current to charge in the high SOC range. However, due to the unique discharge platform characteristics of lithium iron phosphate batteries, the SOC estimation accuracy is not high, especially after the vehicle has been used for a long time, the SOC estimation may have a large error.

[0005] Based on this technical background, the present invention proposes a fast charging method for a lithium iron phosphate battery and a test method for the cycle life. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention proposes a fast charging method for lithium iron phosphate batteries and a test method for the cycle life. The fast charging method shortens the charging time of the lithium iron phosphate battery and improves the fast charging capability of the lithium iron phosphate battery by performing multi-stage constant current charging on the lithium iron phosphate battery, thereby solving the technical problem of inaccurate SOC estimation of the lithium iron phosphate battery system by traditional fast charging methods.

[0007] In order to achieve the above object, the first aspect of the present invention provides a method for fast charging of a lithium iron phosphate battery, the method comprising the following steps:

[0008] Performing at least one constant current charge on the lithium iron phosphate battery;

[0009] When the current of the final constant current charging is not greater than the set current, and the cut-off voltage of the constant current charging is not less than the set cut-off voltage, the charging of the lithium iron phosphate battery is stopped.

[0010] A second aspect of the present invention provides a method for testing the cycle life of a lithium iron phosphate battery that is fast charged using the above method, the testing method comprising the following steps:

[0011] 1) placing the fully charged lithium iron phosphate battery at rest for the first time to obtain a resting lithium iron phosphate battery;

[0012] 2) discharging the stationary lithium iron phosphate battery at a constant current and then allowing it to stand for a second time to obtain a discharged lithium iron phosphate battery;

[0013] 3) Record the discharge capacity of the lithium iron phosphate battery and calculate the capacity retention rate;

[0014] If the capacity retention rate is not less than 80%, repeat steps 1)-3) until the capacity retention rate is less than 80%, and record the number of cycles, which is the cycle life of the lithium iron phosphate battery.

[0015] The technical effects of the present invention include:

[0016] (1) The fast charging method of the lithium iron phosphate battery proposed in the present invention shortens the charging time of the lithium iron phosphate battery by performing multi-stage constant current charging on the lithium iron phosphate battery, improves the fast charging capability of the lithium iron phosphate battery, and solves the technical problem of inaccurate SOC estimation of the lithium iron phosphate battery system by the traditional fast charging method.

[0017] (2) The fast charging method for lithium iron phosphate batteries proposed in the present invention uses the cut-off voltage of each constant current charging stage as the cut-off condition. It does not require the SOC calculation and charge interval division steps involved in the research process of conventional fast charging methods, thus avoiding the technical difficulty of inaccurate SOC estimation of lithium iron phosphate battery systems and simplifying the charging process.

[0018] (3) The fast charging method of the lithium iron phosphate battery proposed in the present invention adopts a step-down current charging strategy, that is, first adopting a high current constant current charging, and then gradually switching to a low current constant current charging, thereby shortening the charging time and alleviating the trend of increasing ion concentration on the negative electrode surface caused by continuous high current charging, reducing the possibility of lithium lateral deposition on the negative electrode, and effectively ensuring the cycle life of the battery cell.

[0019] (4) The fast charging method for lithium iron phosphate batteries proposed in the present invention controls the charging time of the lithium iron phosphate battery in the charge range of 20% to 90% SOC to within 30 minutes, and the maximum charging rate current reaches 3C. Under the premise of achieving fast charging of the lithium iron phosphate battery, the cycle characteristics of the battery are ensured at the same time.

[0020] (5) The fast charging method of the lithium iron phosphate battery proposed in the present invention is verified by a three-electrode lithium iron phosphate verification battery, and the method has strong universality.

[0021] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention in conjunction with the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0023] Figure 1 The figure is a schematic flow chart of the rapid charging method for lithium iron phosphate battery proposed in the present invention.

[0024] Figure 2 The present invention is a flowchart of a specific implementation method of the rapid charging method of the lithium iron phosphate battery proposed in the present invention.

[0025] Figure 3 This is a schematic diagram of a full battery charging process curve in the fast charging method for a lithium iron phosphate battery proposed in Example 1 of the present invention.

[0026] Figure 4 This is a schematic diagram of a full battery charging process curve in the fast charging method for a lithium iron phosphate battery proposed in Example 2 of the present invention.

[0027] Figure 5 This is a schematic diagram of the full battery cycle performance at 25±3°C in the fast charging method for a lithium iron phosphate battery proposed in Example 1 of the present invention.

[0028] Figure 6 This is a schematic diagram of the 25±3°C cycle performance of a full battery in the fast charging method for a lithium iron phosphate battery proposed in Example 2 of the present invention.

[0029] Description of reference numerals:

[0030] I1-the current of the first constant current charging, I2-the current of the second constant current charging, I n-1 - The current of the constant current charging of the n-1th stage, I n - the current of the last constant current charging, V1- the cut-off voltage of the first constant current charging, V2- the cut-off voltage of the second constant current charging, V n-1 - The cut-off voltage of the n-1th constant current charging, V n -The cut-off voltage of the final constant current charge, Voltage, Current, Time, Capacity retention, Cycle number. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0032] In the present invention, unless otherwise specified, directional words such as "upper" and "lower" generally refer to the upper and lower parts of a device in normal use. Figure 1 In the drawing direction, "inside" and "outside" refer to the outline of the device. In addition, the terms "first, second, third" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first, second, third" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0033] The present invention provides a method for quickly charging a lithium iron phosphate battery, such as Figure 1 As shown, the method comprises the following steps:

[0034] Performing at least one period of constant current charging on the lithium iron phosphate battery;

[0035] When the current of the final constant current charging is not greater than the set current, and the cut-off voltage of the constant current charging is not less than the set cut-off voltage, the charging of the lithium iron phosphate battery is stopped.

[0036] In the present invention, by performing multi-stage constant current charging on the lithium iron phosphate battery, the charging time of the lithium iron phosphate battery is shortened, the fast charging capability of the lithium iron phosphate battery is improved, and the technical problem of inaccurate SOC estimation of the lithium iron phosphate battery system by the traditional fast charging method is solved.

[0037] According to the present invention, when the lithium iron phosphate battery is subjected to at least one stage of constant current charging, except for the first stage, the current of each stage of constant current charging is less than the current of the previous stage of constant current charging, and the cut-off voltage of the constant current charging is not less than the cut-off voltage of the previous stage of constant current charging;

[0038] Set the current to 0.05C-0.3C;

[0039] Set the cut-off voltage to 3.55V-3.65V.

[0040] According to the present invention, the lithium iron phosphate battery is charged in sequence according to the current and cut-off voltage of each constant current charging section. When the measured cut-off voltage of a certain constant current charging section is less than the cut-off voltage of the constant current charging section, the constant current charging of the section is repeated until the measured cut-off voltage of the constant current charging section is not less than the cut-off voltage of the constant current charging section.

[0041] Preferably, except for the first section, the current of each constant current charging section is 0.2 to 0.8 times the current of the previous constant current charging section;

[0042] The current of the first constant current charging is not less than 1C, and the current of the last constant current charging is not greater than the set current.

[0043] Preferably, when the current of a certain constant current charging section is not less than 1C, the cut-off voltage of the constant current charging section is not greater than 3.52V;

[0044] When the current of a certain constant current charging section is less than 1C and greater than the set current, the cut-off voltage of the constant current charging section is not greater than 3.55V;

[0045] When the current of a certain constant current charging section is not greater than the set current, the cut-off voltage of the constant current charging section is not greater than the set cut-off voltage.

[0046] In the present invention, a step-down current charging strategy is adopted, that is, high-current constant-current charging is first adopted, and then gradually switched to low-current constant-current charging, which shortens the charging time and alleviates the trend of increasing ion concentration on the negative electrode surface caused by continuous high-current charging, reduces the possibility of lithium lateral deposition on the negative electrode, and effectively ensures the cycle life of the battery cell.

[0047] According to the present invention, after each constant current charging is completed, the next constant current charging is performed after standing for 5-15 seconds;

[0048] The fast charging temperature of lithium iron phosphate battery is 20-50℃, the charging range is 20%-90% SOC, and the charging time is no more than 30 minutes.

[0049] In the present invention, the fast charging method of lithium iron phosphate battery can be implemented by the following technical scheme: Figure 2 As shown, a lithium iron phosphate battery is selected, and a constant current of I1 is used to charge to V1, and then the battery is left standing for 10 seconds. A constant current of I2 is used to charge to V2, and then the battery is left standing for 10 seconds. A constant current of I3 is used to charge to V3, and then the battery is left standing for 10 seconds. A constant current of I4 is used to charge to V4, and then the battery is left standing for 10 seconds. And so on. Finally, a constant current of I n Constant current charging to V n , the battery charging process is completed. The voltage of each stage above is V1≤V2≤V3≤V4≤......≤V n , current I1>I2>I3>I4>.......>I n ; The maximum current of I1 is 3C, I n When the current is ≤0.1C; when the current is ≥1C, the corresponding charging cut-off voltage is ≤3.52V; when the current is less than 1C and greater than 0.1C, the corresponding charging cut-off voltage is ≤3.55V; when the current is ≤0.1C, the corresponding charging cut-off voltage is ≤3.65V.

[0050] According to the present invention, before the lithium iron phosphate battery is quickly charged, the charging interval is not calculated and divided.

[0051] In the present invention, the cut-off voltage of each constant current charging stage is used as the cut-off condition, and there is no need to perform the SOC calculation and charge interval division steps involved in the research process of the conventional fast charging method, thereby avoiding the technical difficulty of inaccurate SOC estimation of the lithium iron phosphate battery system and simplifying the charging process.

[0052] In the present invention, the charging time of the lithium iron phosphate battery in the SOC charge range of 20% to 90% is controlled within 30 minutes, and the maximum charging rate current reaches 3C, which ensures the cycle characteristics of the battery while achieving fast charging of the lithium iron phosphate battery.

[0053] Preferably, the lithium iron phosphate battery is a three-electrode lithium iron phosphate verification battery or a two-electrode lithium iron phosphate working battery.

[0054] In the present invention, the three-electrode lithium iron phosphate battery is used for verification, and the method has strong universality.

[0055] The present invention also provides a method for testing the cycle life of a lithium iron phosphate battery that is quickly charged using the above method, and the testing method comprises the following steps:

[0056] 1) placing the fully charged lithium iron phosphate battery at rest for the first time to obtain a resting lithium iron phosphate battery;

[0057] 2) discharging the stationary lithium iron phosphate battery at a constant current and then allowing it to stand for a second time to obtain a discharged lithium iron phosphate battery;

[0058] 3) Record the discharge capacity of the lithium iron phosphate battery and calculate the capacity retention rate;

[0059] If the capacity retention rate is not less than 80%, repeat steps 1)-3) until the capacity retention rate is less than 80%, and record the number of cycles, which is the cycle life of the lithium iron phosphate battery.

[0060] According to the present invention, the first standing time is 5-15min;

[0061] The second standing time is 5-15 minutes;

[0062] The constant current discharge current is 0.5-1.5C, and the discharge cut-off voltage is 2.0-3.0V.

[0063] The present invention will be described in more detail below through specific embodiments.

[0064] Example 1

[0065] like Figure 1 , Figure 2 As shown, this embodiment provides a fast charging method for a lithium iron phosphate battery, and the implementation steps are as follows:

[0066] Among them, the positive electrode of the sample battery cell is lithium iron phosphate, and the negative electrode is graphite;

[0067] The specific charging strategy of this embodiment is shown in Table 1. The battery cells with qualified indicators are selected and placed under 20-50°C, and charged to 3.50V with 3C constant current, left to stand for 10s, charged to 3.50V with 2C constant current, left to stand for 10s, charged to 3.50V with 1C constant current, left to stand for 10s, charged to 3.50V with 0.5C constant current, and finally charged to 3.65V with 0.1C constant current to complete the battery charging process;

[0068] In order to fully determine the feasibility of the above fast charging method, the battery fully charged by the above method was left to stand for 10 minutes, then discharged at a constant current of 1C to 2.5V, and left to stand for 10 minutes. The discharge capacity was recorded and the capacity retention rate was calculated. If the capacity retention rate was ≥80%, the above charging and discharging steps were repeated to examine the battery cycle life.

[0069] like Figure 3As shown in the figure, it is a typical charging process curve under the above charging strategy. Analysis shows that the charging time of the sample battery in the 20% to 90% SOC charging range is about 27 minutes, and the charging time in the 0% to 80% SOC charging range is about 24.8 minutes. Figure 5 As shown, under this fast charging strategy, the battery cycle trend is stable and the performance is good. The capacity retention rate after 500 cycles can reach 97.29%. The specific test results are shown in Table 1.

[0070] Example 2

[0071] like Figure 1 , Figure 2 As shown, a fast charging method for a lithium iron phosphate battery, the implementation steps are as follows:

[0072] Among them, the positive electrode of the sample battery cell is lithium iron phosphate, and the negative electrode is graphite;

[0073] The specific charging strategy of this embodiment is shown in Table 1. The battery cells with qualified indicators are selected and placed under 20-50°C, charged to 3.50V with 2C constant current, left to stand for 10s, charged to 3.50V with 1C constant current, left to stand for 10s, charged to 3.50V with 0.5C constant current, and finally charged to 3.65V with 0.1C constant current to complete the battery charging process;

[0074] In order to fully determine the feasibility of the above fast charging method, the battery fully charged by the above method was left to stand for 10 minutes, then discharged at a constant current of 1C to 2.5V, and left to stand for 10 minutes. The discharge capacity was recorded and the capacity retention rate was calculated. If the capacity retention rate was ≥80%, the above charging and discharging steps were repeated to examine the battery cycle life.

[0075] like Figure 4 As shown in the figure, it is a typical charging process curve under the above charging strategy. Analysis shows that the charging time of the sample battery in the 20% to 90% SOC charging range is about 26.3 minutes, and the charging time in the 0% to 80% SOC charging range is about 26.2 minutes. Figure 6 As shown in Table 1, the battery cycle trend is stable and the performance is good under this fast charging strategy. The capacity retention rate after 500 cycles can reach 95.91%. The specific test results are shown in Table 1.

[0076] Example 3

[0077] like Figure 1 , Figure 2 As shown, a fast charging method for a lithium iron phosphate battery, the implementation steps are as follows:

[0078] Among them, the positive electrode of the sample battery cell is lithium iron phosphate, and the negative electrode is graphite;

[0079] Select qualified batteries with all indicators and place them at 20-50℃, use 1.8C constant current to charge to 3.50V, let it stand for 10s, use 1C constant current to charge to 3.50V, let it stand for 10s, use 0.5C constant current to charge to 3.50V, and finally use 0.1C constant current to charge to 3.65V to complete the battery charging process;

[0080] In order to fully determine the feasibility of the above fast charging method, the battery fully charged by the above method was left to stand for 10 minutes, then discharged at a constant current of 1C to 2.5V, and left to stand for 10 minutes. The discharge capacity was recorded and the capacity retention rate was calculated. If the capacity retention rate was ≥80%, the above charging and discharging steps were repeated to examine the battery cycle life.

[0081] As shown in Table 1, the charging time of the sample battery in the 20% to 90% SOC charge range is approximately 28.4 minutes, and the charging time in the 0% to 80% SOC charge range is approximately 27.8 minutes, which is slightly longer than the time required for the first two strategies.

[0082] The embodiments of the present invention realize fast charging of lithium iron phosphate batteries, and control the charging time of the lithium iron phosphate batteries in the charge range of 20% to 90% SOC to within 30 minutes. The battery cycle performance is guaranteed to a certain extent, and the technical problem of inaccurate SOC estimation of lithium iron phosphate battery systems by traditional fast charging methods is solved.

[0083] Table 1 Summary of specific charging strategies and corresponding test results adopted in various embodiments of the present invention

[0084]

[0085] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A fast charging method for a lithium iron phosphate battery, characterized in that: The method comprises the following steps: Performing at least one constant current charge on the lithium iron phosphate battery; When the current of the final constant current charging is not greater than the set current, and the cut-off voltage of the constant current charging is not less than the set cut-off voltage, the charging of the lithium iron phosphate battery is stopped.

2. The method according to claim 1, characterized in that When the lithium iron phosphate battery is subjected to at least one section of constant current charging, except for the first section, the current of each section of constant current charging is less than the current of the previous section of constant current charging, and the cut-off voltage of the constant current charging is not less than the cut-off voltage of the previous section of constant current charging; The set current is 0.05C-0.3C; The set cut-off voltage is 3.55V-3.65V.

3. The method according to claim 2, characterized in that The lithium iron phosphate battery is charged in sequence according to the current and cut-off voltage of each constant current charging section. When the measured cut-off voltage of a certain constant current charging section is less than the cut-off voltage of the constant current charging section, the constant current charging of the section is repeated until the measured cut-off voltage of the constant current charging section is not less than the cut-off voltage of the constant current charging section.

4. The method according to claim 3, characterized in that Except for the first section, the current of each constant current charging section is 0.2 to 0.8 times the current of the previous constant current charging section; The current of the first constant current charging is not less than 1C, and the current of the last constant current charging is not greater than the set current.

5. The method according to claim 4, characterized in that When the current of a certain constant current charging section is not less than 1C, the cut-off voltage of the constant current charging section is not greater than 3.52V; When the current of a certain constant current charging section is less than 1C and greater than the set current, the cut-off voltage of the constant current charging section is not greater than 3.55V; When the current of a certain constant current charging section is not greater than the set current, the cut-off voltage of the constant current charging section is not greater than the set cut-off voltage.

6. The method according to claim 1, characterized in that After each constant current charging is completed, wait for 5-15 seconds before proceeding to the next constant current charging; The rapid charging temperature of the lithium iron phosphate battery is 20-50° C., the charging range is 20%-90% SOC, and the charging time is no more than 30 minutes.

7. The method according to claim 6, characterized in that Before the lithium iron phosphate battery is quickly charged, the charging interval is not calculated and divided.

8. The method according to claim 1, characterized in that: The lithium iron phosphate battery is a three-electrode lithium iron phosphate verification battery or a two-electrode lithium iron phosphate working battery.

9. A method for testing the cycle life of a lithium iron phosphate battery that is quickly charged using the method described in any one of claims 1 to 8, characterized in that: The testing method comprises the following steps: 1) placing the fully charged lithium iron phosphate battery at rest for the first time to obtain a resting lithium iron phosphate battery; 2) discharging the stationary lithium iron phosphate battery at a constant current and then allowing it to stand for a second time to obtain a discharged lithium iron phosphate battery; 3) Record the discharge capacity of the lithium iron phosphate battery and calculate the capacity retention rate; If the capacity retention rate is not less than 80%, repeat steps 1)-3) until the capacity retention rate is less than 80%, and record the number of cycles, which is the cycle life of the lithium iron phosphate battery.

10. The method according to claim 9, characterized in that The first standing time is 5-15 minutes; The second standing time is 5-15min; The constant current discharge current is 0.5-1.5C, and the discharge cut-off voltage is 2.0-3.0V.

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