A lithium battery and a pre-charging method of a series battery pack thereof

Abstract

This invention discloses a pre-charging method for a lithium battery and its series-connected battery pack. A small-current charge forms a stable SEI film on the negative electrode surface, improving cycle life. A subsequent large-current charge enhances charging efficiency. During the large-current charge, the charging current is adjusted according to the battery voltage to adapt to the battery's charging capacity, minimizing damage to battery materials. A final small-current constant-voltage charge prevents lithium plating on the battery electrodes. When charging the series-connected battery pack, the large-current stage current is calculated based on the maximum voltage of each individual cell, improving cell consistency and reducing damage caused by overcharging due to inconsistencies, thus extending the lifespan of the series-connected battery pack.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery charging, specifically relating to a pre-charging method for a lithium battery and its series-connected battery pack. Background Technology

[0002] Lithium-ion batteries are rechargeable batteries with advantages such as high energy density, long lifespan, high rated voltage, low self-discharge rate, and environmental friendliness. They are widely used in new energy vehicles, grid energy storage, and digital products. The market demand for new energy products is increasing, placing higher demands on battery manufacturers. While ensuring product quality, manufacturers must also improve battery production efficiency and increase production volume. This requires optimizing existing production processes. Pre-charging, as a crucial step in lithium battery manufacturing, directly affects the electrochemical performance and lifespan of the battery. Typically, the pre-charging process takes a considerable amount of time, extending the lithium battery production cycle. Shortening the pre-charging time without affecting battery performance will effectively improve battery production efficiency.

[0003] During the initial charge, a solid electrolyte interphase (SEI) film forms on the surface of the battery's negative electrode. This SEI film is a passivation film, insoluble in organic solvents, and can exist stably in organic solvents, effectively preventing the co-intercalation of solvent molecules. This avoids damage to the electrode material caused by solvent molecule co-intercalation, greatly improving battery life and cycle performance. Therefore, the formation of a stable SEI film is crucial to battery performance. During SEI formation, high-current formation results in a more porous SEI film, while low-current formation results in a denser SEI film. Under high-current charging, lithium plating can occur on the negative electrode, so it is essential to prevent lithium plating during pre-charging. Shortening the pre-charging time without affecting the density of the SEI film or preventing lithium plating would greatly benefit battery production efficiency.

[0004] Currently, commonly used lithium battery pre-charging methods typically include constant current pre-charging mode, constant current equalization charging mode, constant voltage drop current charging mode, constant current trickle charging mode, and combinations of the above pre-charging methods. These methods involve changing the current and voltage parameters during the pre-charging process to complete the pre-charging of the battery. For example, Chinese invention patent application number 201710053448.3, entitled "A Formation Method for a Lithium-ion Battery," discloses a pre-charging method for lithium batteries: a combination of constant current charging, constant voltage charging, and constant current discharging steps is used to obtain a formed lithium-ion battery. This method is divided into pre-formation and main formation steps. In the initial SEI film formation stage, low-pressure trickle segmented formation is used to prevent lithium-ion co-intercalation, increase the "growth" time of the dense components of the SEI film, and form a dense SEI film on the surface of the negative electrode material. Chinese invention patent application number 201410621731.8, entitled "A Charging Method for Lithium-ion Battery Packs," discloses a charging method for lithium-ion battery packs, comprising four stages: low-current constant-current pre-charging, high-current constant-current equalization charging, constant voltage drop current mode, and constant-current trickle charging. Through these four stages, the differences between individual battery cells are gradually reduced. By extending the charging time throughout the entire charging process, the differences between individual cells are minimized, resulting in a more fully charged battery and a longer battery pack lifespan. This charging method can improve the consistency of individual battery performance within the battery pack and extend battery lifespan; however, the long charging time is not conducive to improving battery production efficiency.

[0005] In the battery production process, ensuring product performance while shortening the battery pre-charge time is an important direction for improving battery production efficiency. Summary of the Invention

[0006] To address the shortcomings of the prior art, this invention provides a lithium battery pre-charging method. First, the battery is charged at a constant current I0 until the voltage reaches U1, and then allowed to rest for 3-6 minutes. Then, starting from U1, charging is initiated using I1 as the initial current. During charging, the battery voltage U2 is sampled at a period of 0.1-0.5 seconds. Whenever the difference between two consecutive voltage samples is 0.03-0.05V, the current is adjusted to I2, and charging continues until U2 equals the target charging voltage U0 of the lithium battery. Here, I2 = I1 - 3 × (U2 / U0) × I0, where U2 is the larger of the two consecutive voltage samples. Finally, the lithium battery is charged at a constant voltage using current I3 and voltage U0, with a cutoff current of 0.01-0.05C, completing the pre-charging of the lithium battery.

[0007] This invention also provides a pre-charging method for a lithium battery series battery pack. First, the series battery pack is charged with a constant current I0 until the average voltage of each individual cell in the series battery pack reaches voltage U1, and then left to stand for 3-6 minutes. Then, starting from voltage U1, charging is performed using current I1 as the initial current. During the charging process, the voltage data of each individual cell in the series battery pack is sampled at a period of 0.1-0.5 seconds. Whenever the difference between two consecutive voltage data samples of an individual cell is 0.03-0.05V, the current is adjusted to current I5, and charging continues until the minimum voltage of each individual cell in the series battery pack is taken as the target charging voltage value U0 for that individual cell. Here, I5 = I1 - 3 × (U3 / U0) × I0, and U3 is the maximum voltage of each individual cell in the series battery pack during each voltage data sampling. Finally, the series battery pack is charged with a constant voltage using current I3 and voltage U0, with a cutoff current of 0.01-0.05C, completing the pre-charging of the series battery pack.

[0008] In this invention, a small-current charge forms a stable SEI film on the negative electrode surface, improving cycle life. A subsequent large-current charge further enhances charging efficiency. During the large-current charge, the charging current is adjusted according to the battery voltage to adapt to the battery's charging capacity, minimizing damage to battery materials. Finally, a small-current constant-voltage charge prevents lithium plating on the battery electrodes. When charging a series-connected battery pack, the large-current phase is calculated based on the maximum voltage of each individual cell, improving cell consistency and reducing damage caused by overcharging due to inconsistencies, thus extending the lifespan of the series-connected battery pack.

[0009] The technical effects to be achieved by this invention are accomplished through the following solutions:

[0010] This invention provides a method for precharging a lithium battery, characterized by comprising the following steps:

[0011] Step 1: Take the dried and stored lithium battery, charge it with a constant current I0 until the voltage is U1, and then let it stand.

[0012] Step 2: Starting from voltage U1, use current I1 as the starting current for charging. During the charging process, sample the lithium battery voltage U2 data. Whenever the difference between two consecutive sampled voltage data is equal to the set value ΔU, adjust the current to current I2 and continue charging until U2 is equal to the target charging voltage value U0 of the lithium battery.

[0013] Step 3: Charge the lithium battery with a constant voltage of current I3 and voltage U0, and cut off the current I4 to complete the pre-charging of the lithium battery.

[0014] Furthermore, the current I0 mentioned in step one is 0.1-0.4C. Step one uses a small current to charge the lithium battery, forming a dense SEI film on the surface of the battery's negative electrode, thus improving the battery's cycle life.

[0015] Furthermore, the settling time mentioned in step one is 3-6 minutes.

[0016] Furthermore, the current I1 mentioned in step two is 0.5-2.5C.

[0017] Furthermore, the sampling period for the voltage U2 data in step two is 0.1-0.5 seconds. Periodic sampling of the battery voltage data can reflect the actual situation inside the lithium battery during charging. Preferably, the sampling period is 0.2-0.3 seconds. If the sampling period is too short, too many samples will be taken, resulting in higher costs. At the same time, if the current adjustment frequency is too fast, it will place higher demands on the charging equipment and make the process difficult to control. If the sampling period is too long, it will not objectively reflect the actual situation inside the battery, reducing charging efficiency.

[0018] Furthermore, the set value △U mentioned in step two is 0.03-0.05V.

[0019] Furthermore, in step two, the current I2 = I1 - 3 × (U2 / U0) × I0; where U2 is the larger value among the two consecutively sampled voltage data.

[0020] Furthermore, the current I3 in step three is 0.1-0.4C; the cutoff current I4 is 0.01-0.05C.

[0021] Furthermore, the difference (U0-U1) between the target charging voltage U0 and the voltage U1 is 0.5-1.8V.

[0022] This invention also provides a pre-charging method for a lithium battery series battery pack, which uses the current I0, current I1, voltage U0, voltage U1, set value ΔU, current I3, cutoff current I4, and voltage data sampling period parameters from the above-mentioned lithium battery pre-charging method to pre-charge the lithium battery series battery pack, including the following steps:

[0023] S01, the series battery pack is charged with constant current I0 until the average voltage of the individual cells in the series battery pack is voltage U1, and then left to stand for 3-6 minutes.

[0024] S02, starting from voltage U1, use current I1 as the starting current for charging. During the charging process, the voltage data of individual cells in the series battery pack are sampled. Whenever the difference between two consecutive voltage data samples of an individual cell is a set value △U, the current is adjusted to current I5, and charging continues until the minimum voltage of the individual cell in the series battery pack is taken as the target charging voltage value U0 of the individual cell.

[0025] S03, the series battery pack is charged with constant voltage using current I3 and voltage U0, and the current I4 is cut off to complete the pre-charging of the series battery pack.

[0026] The current I5 = I1 - 3 × (U3 / U0) × I0, where U3 is the maximum value of the voltage of a single cell in the series battery pack each time voltage data is sampled.

[0027] Lithium-ion batteries have relatively low individual cell voltages. In practical applications, multiple lithium-ion batteries are connected in series to obtain the required voltage. This series-connection method allows for selection of the number of batteries to meet different voltage demands, expanding the battery's application range. However, during the charging process of series-connected batteries, due to the significant differences between individual cells, directly using constant voltage or constant current charging methods can lead to increasing inconsistencies between individual cells, resulting in some cells being fully charged while others are not, thus shortening the battery's lifespan. In this invention, during the charging process of the series-connected battery pack, the charging current of the series-connected battery pack is calculated based on the highest voltage value of each individual cell during charging. By using an appropriate voltage sampling period, the current value is adjusted to further reduce the differences between individual cells, improve the consistency of the series-connected battery pack, and extend its lifespan.

[0028] The present invention has the following advantages:

[0029] 1. In this invention, by sampling the voltage data of the rechargeable battery and selecting an appropriate sampling period and voltage data difference, the actual internal condition of the rechargeable battery is objectively reflected, reducing damage to the battery materials during charging; the charging parameters conform to the actual performance of the battery, giving full play to the charging performance of the lithium battery.

[0030] 2. The present invention employs a three-stage charging method consisting of constant current with low current, variable current, and constant voltage with low current, which can form a dense SEI film on the surface of the battery negative electrode, thereby improving the cycle life of the battery; effectively shortening the charging time and improving the charging efficiency of the lithium battery; and also preventing the problem of lithium plating on the battery electrode.

[0031] 3. The charging method for series-connected battery packs in this invention can improve the consistency of individual cells within the series-connected battery pack and extend the service life of the series-connected battery pack. Detailed Implementation

[0032] The present invention will now be described in detail with reference to the embodiments.

[0033] In this embodiment, an 18650 lithium battery was used for the experiment. The battery capacity was 2400mAh, and the positive electrode surface density was 440±8.8g / m³. 2 The negative electrode surface density is 219.6 ± 3.9 g / m³. 2 The separator thickness is 20 μm, and the liquid injection volume is 5.25 ± 0.2 g. In this embodiment, the 18650 lithium battery is first pre-charged using the pre-charging method of the lithium battery in this invention, and the charging time is recorded. Then, the cycle performance of the lithium battery at room temperature (25 ± 3 °C) and high temperature (45 ± 3 °C) is tested respectively.

[0034] Example 1

[0035] Lithium battery pre-charge:

[0036] Step 1: Take the dried and stored lithium battery, charge it with a constant current of 0.2C until the voltage reaches 3.5V, and then let it stand for 3 minutes.

[0037] Step 2: Starting from a voltage of 3.5V, charge the lithium battery using a current of 1.0C as the initial current. During the charging process, sample the lithium battery voltage U2 data with a sampling period of 0.2s. Whenever the difference between two consecutive sampled voltage data is a set value of 0.03V, adjust the current to I2 and continue charging until U2 equals the target charging voltage value of the lithium battery, 4.2V; I2 = 1 - 3 × (U2 / 4.2) × 0.2; where U2 is the larger value among the two consecutive sampled voltage data.

[0038] Step 3: Perform constant voltage charging on the lithium battery at a current of 0.3C and a voltage of 4.2V, and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0039] Example 2

[0040] Lithium battery pre-charge:

[0041] Step 1: Take the dried and stored lithium battery, charge it with a constant current of 0.3C until the voltage reaches 3.5V, and then let it stand for 3 minutes;

[0042] Step 2: Starting from a voltage of 3.5V, charge the lithium battery using a current of 1.2C as the initial current. During the charging process, sample the lithium battery voltage U2 data with a sampling period of 0.2s. Whenever the difference between two consecutive sampled voltage data is 0.05V, adjust the current to I2 and continue charging until U2 equals the target charging voltage value of the lithium battery, 4.2V; I2 = 1 - 3 × (U2 / 4.2) × 0.3; where U2 is the larger value among the two consecutive sampled voltage data.

[0043] Step 3: Perform constant voltage charging on the lithium battery at a current of 0.2C and a voltage of 4.2V, and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0044] Example 3

[0045] Lithium battery pre-charge:

[0046] Step 1: Take the dried and stored lithium battery, charge it with a constant current of 0.4C until the voltage reaches 3.2V, and then let it stand for 3 minutes.

[0047] Step 2: Starting from a voltage of 3.2V, charge the lithium battery using a current of 1.5C as the initial current. During the charging process, sample the lithium battery voltage U2 data with a sampling period of 0.2s. Whenever the difference between two consecutive sampled voltage data is a set value of 0.04V, adjust the current to current I2 and continue charging until U2 equals the target charging voltage value of the lithium battery, 4.2V; I2 = 1 - 3 × (U2 / 4.2) × 0.4; where U2 is the larger value among the two consecutive sampled voltage data.

[0048] Step 3: Perform constant voltage charging on the lithium battery at a current of 0.1C and a voltage of 4.2V, and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0049] Example 4

[0050] Lithium battery pre-charge:

[0051] Step 1: Take the dried and stored lithium battery, charge it with a constant current of 0.2C until the voltage reaches 3.0V, and then let it stand for 3 minutes;

[0052] Step 2: Starting from a voltage of 3.0V, charge the lithium battery using a current of 0.8C as the initial current. During the charging process, sample the lithium battery voltage U2 data with a sampling period of 0.2s. Whenever the difference between two consecutive sampled voltage data is a set value of 0.04V, adjust the current to current I2 and continue charging until U2 equals the target charging voltage value of the lithium battery, 4.2V; I2 = 1 - 3 × (U2 / 4.2) × 0.2; where U2 is the larger value among the two consecutive sampled voltage data.

[0053] Step 3: Perform constant voltage charging on the lithium battery at a current of 0.4C and a voltage of 4.2V, and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0054] Example 5

[0055] Lithium battery pre-charge:

[0056] Step 1: Take the dried and stored lithium battery, charge it with a constant current of 0.2C until the voltage reaches 3.5V, and then let it stand for 3 minutes.

[0057] Step 2: Starting from a voltage of 3.5V, charge the lithium battery using a current of 1.8C as the initial current. During the charging process, sample the lithium battery voltage U2 data with a sampling period of 0.2s. Whenever the difference between two consecutive sampled voltage data is 0.05V, adjust the current to I2 and continue charging until U2 equals the target charging voltage value of the lithium battery, 4.2V; I2 = 1 - 3 × (U2 / 4.2) × 0.2; where U2 is the larger value among the two consecutive sampled voltage data.

[0058] Step 3: Perform constant voltage charging on the lithium battery at a current of 0.3C and a voltage of 4.2V, and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0059] Example 6

[0060] Lithium battery pre-charge:

[0061] Step 1: Take the dried and stored lithium battery, charge it with a constant current of 0.3C until the voltage reaches 3.2V, and then let it stand for 3 minutes;

[0062] Step 2: Starting from a voltage of 3.2V, charge the lithium battery using a current of 2.0C as the initial current. During the charging process, sample the lithium battery voltage U2 data with a sampling period of 0.2s. Whenever the difference between two consecutive sampled voltage data is a set value of 0.03V, adjust the current to I2 and continue charging until U2 equals the target charging voltage value of the lithium battery, 4.2V; I2 = 1 - 3 × (U2 / 4.2) × 0.3; where U2 is the larger value among the two consecutive sampled voltage data.

[0063] Step 3: Perform constant voltage charging on the lithium battery at a current of 0.3C and a voltage of 4.2V, and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0064] Comparative Example 1

[0065] Lithium battery pre-charge:

[0066] Take the dried and stored lithium battery and charge it with a constant current of 0.2C until the battery voltage reaches 4.2V. Then switch to constant voltage charging at 4.2V and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0067] Comparative Example 2

[0068] Lithium battery pre-charge:

[0069] Take the dried and stored lithium battery and charge it with a constant current of 0.3C until the battery voltage reaches 4.2V. Then switch to constant voltage charging at 4.2V and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0070] Comparative Example 3

[0071] Lithium battery pre-charge:

[0072] Take the dried and stored lithium battery and charge it with a constant current of 0.4C until the battery voltage reaches 4.2V. Then switch to constant voltage charging at 4.2V and cut off the current at 0.01C to complete the pre-charging of the lithium battery.

[0073] In the examples and comparative examples, the charging time, room temperature cycling performance, and high temperature cycling performance of the lithium batteries are shown in the table below. In the table, the test conditions for room temperature cycling performance are 500 cycles at 25°C; the test conditions for high temperature cycling performance are 400 cycles at 45°C.

[0074]

[0075] As shown in the table, the lithium batteries in the examples and comparative examples exhibit good cycle performance at both high and low temperatures, with a capacity retention rate exceeding 94% at room temperature and exceeding 90% at high temperature. The charging method used in the comparative examples was a low-current constant-current pre-charging method, which resulted in good cycle performance for the lithium batteries. Furthermore, the lithium batteries pre-charged using the method described in this invention also exhibit good cycle performance, demonstrating that the pre-charging method in this invention can maintain the cyclic performance of the lithium batteries.

[0076] As can be seen from the data in the table, the lithium battery pre-charging method of this invention significantly shortens the pre-charging time by about 60 minutes and significantly improves the charging efficiency.

[0077] Example 7

[0078] Pre-charging of lithium-ion battery series packs:

[0079] S01, the series battery pack is charged with a constant current of 0.2C until the average voltage of the individual cells in the series battery pack is 3.5V, and then left to stand for 3 minutes;

[0080] S02, starting from a voltage of 3.5V, charging is initiated with a current of 1.0C. During charging, the voltage data of each individual cell in the series-connected battery pack is sampled at a sampling period of 0.2s. Whenever the difference between two consecutive voltage data samples of an individual cell is 0.03V, the current is adjusted to current I5, and charging continues until the minimum voltage of the individual cell in the series-connected battery pack is 4.2V. The current I5 = I1 - 3 × (U3 / 4.2) × 0.2, where U3 is the maximum voltage of the individual cell in the series-connected battery pack at each voltage data sampling.

[0081] S03, the series battery pack is charged at a constant voltage of 0.3C and 4.2V, and the cut-off current is 0.01C, thus completing the pre-charging of the series battery pack.

[0082] Example 8

[0083] Pre-charging of lithium-ion battery series packs:

[0084] S01, the series battery pack is charged with a constant current of 0.4C until the average voltage of the individual cells in the series battery pack is 3.2V, and then left to stand for 3 minutes;

[0085] S02, starting from a voltage of 3.2V, charging is initiated with a current of 1.5C. During charging, the voltage data of each individual cell in the series-connected battery pack is sampled at a sampling period of 0.2s. Whenever the difference between two consecutive voltage data samples of an individual cell is 0.04V, the current is adjusted to current I5, and charging continues until the minimum voltage of the individual cell in the series-connected battery pack reaches 4.2V. The current I5 = I1 - 3 × (U3 / 4.2) × 0.4, where U3 is the maximum voltage of the individual cell in the series-connected battery pack at each voltage data sampling.

[0086] S03, the series battery pack is charged at a constant voltage of 0.1C and 4.2V, and the current is cut off at 0.01C to complete the pre-charging of the series battery pack.

[0087] In summary, the lithium battery pre-charging method of this invention can shorten the pre-charging time, improve charging efficiency, and increase battery production efficiency while maintaining the cyclic performance of lithium batteries. The lithium battery series pack pre-charging method of this invention can effectively improve the consistency of individual battery cells within the pack and extend the lifespan of the series pack.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. Although the embodiments of the present invention have been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the embodiments of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A pre-charging method for a lithium battery series battery pack, characterized in that, The process includes the following steps: S01, constant current charging of the series battery pack using current I0 until the average voltage of the individual cells in the series battery pack is voltage U1, then resting for 3-6 minutes; S02, starting from voltage U1, charging using current I1 as the initial current, sampling the voltage data of the individual cells in the series battery pack during the charging process, and adjusting the current to current I5 whenever the difference between two consecutive voltage data samples of an individual cell is a set value ΔU, continuing charging until the minimum voltage of the individual cell in the series battery pack is taken as the target charging voltage value U0 of the individual cell; S03, constant voltage charging of the series battery pack using current I3 and voltage U0, cutting off current I4 to complete the pre-charging of the series battery pack; the current I5 = I1 - 3 × (U3 / U0) × I0, where U3 is the maximum value of the individual cell voltage in the series battery pack each time the voltage data is sampled; The current I0 is 0.1-0.4C, the current I1 is 0.5-2.5C, the sampling period for voltage U2 is 0.1-0.5s, the current I3 is 0.1-0.4C, the cutoff current I4 is 0.01-0.05C, the difference between the target charging voltage U0 and the voltage U1 (U0-U1) is 0.5-1.8V, and the set value ΔU is 0.03-0.05V.

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