A formation method for reducing battery DCR

Abstract

The present invention relates to the field of battery technology and proposes a formation method for reducing the DCR of a battery. The formation method comprises the following steps: injecting an electrolyte into a battery cell and performing formation using a three-step constant current charging method: a first constant current charging method: a current magnification I1 of 0.01C to 0.05C, and a charging time t1 of 50 minutes or less; a second constant current charging method: a current magnification I2 of 0.05C to 0.2C, and a charging time t2 of 60 minutes or less; and a third constant current charging method: a current magnification I3 of 0.2C to 0.5C, and a charging time t3 of 100 minutes or less. The high current formation SOC is ≤35%, and the total formation SOC is ≤40%. When the formation current is high, the SEI film structure becomes looser, the corresponding thickness increases, and more irreversible reactions occur. When the formation current is low, the SEI film molecules are more easily stacked in an orderly manner, the structure becomes denser, and the corresponding thickness decreases. The present invention controls the total formation SOC and the high-current formation SOC, which can effectively reduce the battery DCR and improve the battery rate performance and cycle stability; at the same time, it can also reduce the time and power consumption required for the formation process, saving equipment and production costs.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a formation method for reducing the DCR of a battery. Background Art

[0002] In recent years, the application of lithium-ion batteries in energy storage power stations, power tools, electric vehicles and other fields has become increasingly widespread, and has become one of the important ways to cope with the global energy crisis and environmental pressure. The production process of lithium-ion batteries is complex, among which formation is one of the key processes, which has a vital impact on battery performance. Formation is the first charging process of the battery after injection and aging. In essence, it is the process of forming a solid electrolyte interface (SEI) film. The SEI film plays a key role in stabilizing the electrode active materials and preventing the electrolyte from continuously undergoing irreversible chemical reactions with the active substances. A stable SEI film can ensure that the lithium-ion battery maintains good electrochemical performance during multiple charge and discharge cycles, and directly affects the battery's performance indicators such as capacity, cycle life, voltage platform and rate.

[0003] However, the formation process of the SEI film is very complex and is affected by many factors. The formation process directly affects the film formation quality of the SEI film, which in turn determines the electrical performance of the battery cell, including the DC internal resistance (DCR). The traditional view is that low current formation is conducive to the formation of the SEI film, but this method is time-consuming, reduces production efficiency, and increases costs. Therefore, exploring an efficient formation method that can effectively reduce the DCR of the battery is of great significance to improving corporate production efficiency and product quality. The optimized formation process can not only improve the electrode structure and interface and reduce the interfacial impedance, but also form a more stable SEI layer, reduce unnecessary side reactions, and fully activate the electrode active material, thereby reducing the internal resistance of the battery, especially the DCR. This method will help improve the overall performance and energy efficiency of the battery to meet the growing market demand.

[0004] Existing lithium-ion battery formation methods have a number of limitations and are difficult to effectively reduce the direct current internal resistance (DCR) of the battery. For example, the "pyramid-type" formation method disclosed in CN117855615A can improve battery performance, but it is mainly applicable to specific material systems and has limited effect in reducing DCR. The soft-pack lithium-ion battery formation method disclosed in CN201210218854.8 is only applicable to soft-pack batteries, which is difficult to fully activate the active materials inside the battery, and there are problems such as uneven gas production and long standing time. These methods often lead to higher battery DCR test results, affecting charge and discharge efficiency, safety, service life and fast charging performance. Therefore, it is urgent to develop a new formation method that can effectively reduce DCR, is applicable to different material systems and application scenarios, and takes into account production efficiency and cost control at the same time to improve the overall performance of the battery. Summary of the Invention

[0005] In view of this, the present invention proposes a formation method that is applicable to a variety of batteries and can reduce the battery DCR and improve the battery cycle performance.

[0006] The technical solution of the present invention is achieved as follows: On the one hand, the present invention provides a formation method for reducing the DCR of a battery, comprising the following steps:

[0007] Inject electrolyte into the battery cell and use a three-step constant current charging method to form:

[0008] The first step is constant current charging: the current rate I1 is 0.01C~0.05C, and the charging time t1 is ≤50min;

[0009] The second step is constant current charging: the current rate I2 is 0.05C~0.2C, and the charging time t2 is ≤60min;

[0010] The third step is constant current charging: the current rate I3 is 0.2C~0.5C, and the charging time t3 is ≤100min;

[0011] Among them, the high current formation SOC = I3×t3 / 60≤35%, and the total formation SOC = (I1×t1+I2×t2+I3×t3) / 60≤40%.

[0012] On the basis of the above technical solution, preferably, the battery cell is formed at a temperature of 40 to 50° C. and a pressure of -42 to -48 KPa.

[0013] On the basis of the above technical solution, preferably, before the first constant current charging and after each constant current charging is completed, the battery is left on hold for 1-2 minutes.

[0014] On the basis of the above technical solution, preferably, the temperature during storage is 40 to 50° C. and the pressure is -42 to -48 KPa.

[0015] On the basis of the above technical solution, preferably, in step S2, the protection voltage is 3-4V during each constant current charging.

[0016] On the basis of the above technical solution, preferably, during the first constant current charging step, the current rate I1 is 0.015C and the charging time t1 is 40 minutes.

[0017] On the basis of the above technical solution, preferably, during the second constant current charging, the current rate I2 is 0.1C and the charging time t2 is 48 minutes.

[0018] On the basis of the above technical solution, preferably, during the third step of constant current charging, the current rate I3 is 0.33C and the charging time t3 is 50 min.

[0019] On the basis of the above technical solution, preferably, the positive electrode material of the battery cell is one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide, and the negative electrode material is graphite.

[0020] On the basis of the above technical solution, preferably, the battery cell is a soft-pack battery cell or an aluminum shell battery cell.

[0021] The present invention provides a formation method for reducing battery DCR, which has the following advantages over the prior art:

[0022] (1) When the SEI film is formed at a high current, the structure is looser, the corresponding thickness is larger, and there are more irreversible reactions; when the SEI film is formed at a low current, the molecules are more likely to stack in an orderly manner, the structure is denser, and the corresponding thickness is smaller. A total formation SOC ≤ 40% and a high current formation SOC ≤ 35% can effectively reduce the battery DCR, improve the battery rate performance and cycle stability. At the same time, it can also reduce the time and power required for the low current formation process, saving equipment and production costs.

[0023] (2) The present invention provides a formation method for improving the battery rate performance and cycle stability for lithium iron phosphate, lithium manganese iron phosphate, and ternary lithium battery systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 This is a graph showing the change in battery cycle capacity after formation. DETAILED DESCRIPTION

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

[0027] The formation of the SEI film is a complex process with many influencing factors. The formation process directly affects the quality of the SEI film and determines the electrical performance of the battery cell. Three factors generally influence SEI film formation: time, current, and temperature.

[0028] Formation temperature: This primarily affects the formation effect by influencing the viscosity and conductivity of the electrolyte and the ion diffusion rate of the electrode material. Generally, the higher the formation temperature, the lower the electrolyte viscosity, the higher the electrolyte conductivity, and the faster the ion diffusion rate of the electrode material. Consequently, the polarization decreases and the formation effect improves. However, excessively high temperatures can damage the structure of the already formed SEI film and alter its composition. Furthermore, since the electrolyte is an organic solvent solution, excessively high temperatures can accelerate the volatilization of low-boiling-point components in the electrolyte, affecting the formation effect.

[0029] Formation current: When the current is large, the two-electron reaction is more likely to occur, that is, the reaction can only occur when two electrons participate at the same time, and it is easier to generate inorganic lithium salt components. At this time, the SEI film molecules are more likely to stack in a disorderly manner, its structure is looser, the corresponding thickness is larger, and there are more irreversible reactions; when the current is small, the single-electron reaction is more likely to occur, that is, the reaction can occur with the participation of only one electron, and it is easier to generate organic lithium salt components. At this time, the SEI film molecules are more likely to stack in an orderly manner, its structure is denser, the corresponding thickness is smaller, and there are fewer irreversible reactions.

[0030] Formation time: The formation time is determined by the formation current and the termination voltage. When the formation current is small, the SEI film formed is more stable and has better cycle performance, so the formation time is relatively long. From the perspective of enterprise production, the time cost needs to be comprehensively considered.

[0031] The present invention achieves the purpose of reducing the DCR of the battery by adjusting the formation current, time and temperature. Specifically, the formation method of reducing the DCR of the battery of the present invention comprises the following steps:

[0032] S1, prepare the battery cells to be formed

[0033] The battery cells can be any of lithium iron phosphate / graphite, lithium manganese iron phosphate / graphite, or lithium nickel cobalt manganese oxide / graphite, and can be packaged in either a soft pack or a hard case. The battery cell sample preparation process is identical to that of standard batteries, including mixing, coating, rolling, winding / stacking, liquid injection, and high-temperature aging, all the way to the formation step.

[0034] S2, Formation: Electrolyte is injected into the cell and three constant current charges are performed to form the cell. If the cell is a soft pack, the electrolyte is injected into the cell, the air bag is sealed, and then the cell is formed.

[0035] Because the SEI film structure is looser when the current is large, the corresponding thickness is larger, and there are more irreversible reactions, which will increase the battery DCR; when the current is small, the SEI film molecules are more likely to stack in an orderly manner, and its structure is denser, the corresponding thickness is smaller, and there are fewer irreversible reactions, so the battery DCR will decrease accordingly. Therefore, the current and corresponding time should meet the requirements of the following table.

[0036]

[0037] The high-current SEI film structure is looser, and the corresponding thickness is larger, which will lead to an increase in the membrane resistance and thus increase the battery DCR. Therefore, the total formation (three total formations) SOC and high-current formation (third constant current charging) SOC should be controlled to reduce the battery DCR.

[0038] Total chemical formation SOC = (I1×t1+I2×t2+I3×t3) / 60×100%≤40%.

[0039] High current formation SOC=I3×t3 / 60×100%≤35%.

[0040] If the total formation SOC and high current formation SOC are too high, it will have an adverse effect on the battery's DC internal resistance (DCR), cycle stability and rate performance. Specifically:

[0041] During the third step and throughout the formation process, excessive SOC leads to excessive current during SEI film formation, resulting in a looser and more uneven SEI film structure. The increased SEI film thickness and loose structure increase the battery's internal resistance. This increases the battery's voltage drop during charge and discharge, leading to a decrease in overall performance.

[0042] High SOC can cause excessive irreversible reactions in electrode materials, leading to changes in the material structure and loss. This can accelerate capacity decay and shorten the battery's cycle life over multiple charge and discharge cycles. This is because excessive irreversible reactions can degrade the electrode material and destabilize the SEI film.

[0043] Due to the increased DCR, the battery is more susceptible to overheating during high-rate discharge, which limits its performance under high-current loads. High-rate performance degrades, including an inability to provide sufficient current output during rapid charge and discharge. Furthermore, the battery's energy efficiency decreases, potentially making it unsuitable for high-power applications.

[0044] The temperature and pressure during formation can also have adverse effects on battery performance, specifically:

[0045] Impact on Battery DCR: Excessively high temperatures can accelerate electrolyte decomposition and side reactions, leading to uneven or excessively thick SEI film formation and increased battery internal resistance. Excessively low temperatures can cause slow and incomplete SEI film formation, increasing battery internal resistance. Excessively high or low pressures can affect electrolyte wettability and contact with electrode materials, resulting in suboptimal SEI film formation and increased internal resistance.

[0046] Impact on Cycling Stability: Excessively high temperatures accelerate the degradation of electrode materials and the decomposition of the electrolyte, leading to faster battery capacity decay and shortened cycle life. Excessively low temperatures can lead to SEI membrane instability, increasing irreversible capacity loss during cycling. Improper pressure can lead to uneven electrolyte distribution, affecting the long-term stability of the battery.

[0047] Impact on rate performance: Excessively high temperatures may cause the battery to overheat during high-rate discharge, affecting battery safety and performance. Excessively low temperatures may limit the migration of lithium ions, reducing the battery's rate performance. Improper pressure may affect the fluidity of the electrolyte and the reactivity of the electrodes, reducing the battery's performance at high rates.

[0048] In summary, factors such as current, time, temperature, and pressure need to be precisely controlled during the formation process to ensure ideal SEI formation, thereby optimizing the battery's DC internal resistance, cycle stability, and rate performance. Adjusting one factor often affects the others, so comprehensive consideration and optimization are necessary.

[0049] Example 1

[0050] The formation method for reducing the DCR of a battery in this embodiment comprises the following steps:

[0051] S1, prepare the battery cell to be formed: the battery cell is a 120Ah lithium iron phosphate square aluminum shell battery cell. The battery cell sample preparation process is normal, including mixing, coating, rolling, winding / stacking, liquid injection, high temperature aging and other processes are exactly the same as normal battery cells until the formation process.

[0052] S2, formation: The formation process is shown in Table 1. After the formation is completed, normal aging, sealing, and capacity separation processes are carried out to obtain the finished battery cells.

[0053] Table 1 Example 1 Formation process

[0054]

[0055] Example 2

[0056] Example 2 is compared with Example 1, with the addition of a battery cell and a formation method. The battery cell is a 120Ah lithium iron manganese phosphate square aluminum shell battery cell. The formation process is shown in Table 2.

[0057] Table 2 Example 2 Formation Process

[0058]

[0059] Example 3

[0060] Compared with Example 1, Example 3 has additional features in terms of the battery cell and the formation method. The battery cell is a 120Ah lithium nickel cobalt manganese oxide soft-pack battery cell. The electrolyte is injected into the soft-pack battery cell, and then the airbag bag is sealed, and the formation is carried out according to the process shown in Table 3.

[0061] Table 3 Formation process of Example 3

[0062]

[0063]

[0064] Comparative Example 1

[0065] Comparative Example 1 is different from Example 1 in terms of the formation method, as shown in Table 4:

[0066] Table 4 Comparative Example 1 Formation Process

[0067]

[0068] Comparative Example 2

[0069] Comparative Example 2 is different from Example 1 in the formation method, as shown in Table 5:

[0070] Table 5 Comparative Example 2 Formation Process

[0071]

[0072] Comparative Example 3

[0073] Comparative Example 3 is different from Example 1 in the formation method, as shown in Table 6:

[0074] Table 6 Comparative Example 3 Formation Process

[0075]

[0076] Comparative Example 4

[0077] Comparative Example 4 is different from Example 1 in the formation method, as shown in Table 7:

[0078] Table 7 Comparative Example 4 Formation Process

[0079]

[0080] Comparative Example 5

[0081] Comparative Example 5 is different from Example 1 in the formation method, as shown in Table 8:

[0082] Table 8 Comparative Example 5 Formation Process

[0083]

[0084] The DCR of the finished cells after formation of the embodiment and the comparative example was tested under the conditions of 25° C., 50% SOC 2C constant current discharge for 10 s. The results are shown in Table 9.

[0085] Table 9 Comparison of formation effects

[0086] High current SOC / % Total chemical composition SOC / % Formation time / min 2Pcs battery DCR value Example 1 27.50 36.50 138 0.7783 / 0.7740 Example 2 33.33 38.72 143 0.7884 / 0.7869 Example 3 21.67 37.50 155 0.7813 / 0.7828 Comparative Example 1 27.50 41.50 158 0.8175 / 0.8147 Comparative Example 2 35.75 39.58 79 0.8198 / 0.8211 Comparative Example 3 38.50 47.50 158 0.8222 / 0.8259 Comparative Example 4 38.50 51.60 169 0.8311 / 0.8307 Comparative Example 5 38.50 60.85 171 0.9156 / 0.9048

[0087] As shown in Table 9, the DCR values ​​of the embodiment formation methods are all lower than those of the comparative examples, among which Example 1 has the best effect and the shortest formation time. As shown in Comparative Examples 1-5, when the constant current charging current and / or charging time exceed the limits of the present invention, the battery DCR is higher than that of the embodiment of the present invention. If the high current formation SOC is less than 35% and the total formation SOC is greater than 40%, or if the high current formation is greater than 35% and the total formation is less than 40%, the battery DCR will increase, indicating that the battery DCR can only be reduced when the high current formation SOC and the total formation SOC meet the conditions at the same time.

[0088] Figure 1 Figure 2 shows the capacity retention decay curves at 25°C and 1C / 1C cycles for different formation methods. The capacity retention of the examples is significantly higher than that of the comparative examples. This shows that controlling the total formation SOC and high-current formation SOC to reduce the battery's DCR can effectively lower the battery's internal resistance and improve its rate performance and cycling stability.

[0089] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A formation method for reducing battery DCR, characterized in that: The following steps are involved: Inject electrolyte into the battery cell and use a three-step constant current charging method to form: The first step is constant current charging: the current rate I1 is 0.01C~0.05C, and the charging time t1 is ≤50min; The second step is constant current charging: the current rate I2 is 0.05C~0.2C, and the charging time t2 is ≤60min; The third step is constant current charging: the current rate I3 is 0.2C~0.5C, and the charging time t3 is ≤100min; Among them, the high current formation SOC = I3×t3 / 60≤35%, and the total formation SOC = (I1×t1+I2×t2+I3×t3) / 60≤40%.

2. A formation method for reducing battery DCR according to claim 1, characterized in that: The battery cell is formed at a temperature of 40 to 50°C and a pressure of -42 to -48KPa.

3. A formation method for reducing battery DCR according to claim 1, characterized in that: Before the first step of constant current charging and after each step of constant current charging is completed, leave it alone for 1-2 minutes.

4. A formation method for reducing battery DCR according to claim 3, characterized in that: The temperature when shelved is 40-50°C and the pressure is -42-48Kpa.

5. A formation method for reducing battery DCR according to claim 1, characterized in that: The protection voltage is 3-4V during each constant current charging.

6. A formation method for reducing battery DCR according to claim 1, characterized in that: In the first step of constant current charging, the current rate I1 is 0.015C and the charging time t1 is 40 minutes.

7. A formation method for reducing battery DCR according to claim 6, characterized in that: In the second step of constant current charging, the current rate I2 is 0.1C and the charging time t2 is 48 minutes.

8. A formation method for reducing battery DCR according to claim 7, characterized in that: In the third step of constant current charging, the current rate I3 is 0.33C and the charging time t3 is 50min.

9. A formation method for reducing battery DCR according to claim 1, characterized in that: The positive electrode material of the battery cell is one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide, and the negative electrode material is graphite.

10. A formation method for reducing battery DCR according to claim 1, characterized in that: The battery core is a soft-pack battery core or an aluminum shell battery core.

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