Formation method for reducing DCR of battery

Abstract

The invention relates to the technical field of batteries, and provides a formation method for reducing the DCR of a battery, and the formation method comprises the following steps: injecting an electrolyte into a battery cell, and carrying out formation by adopting a three-step constant current charging mode: first-step constant current charging: the current rate I1 is 0.01-0.05 C, and the charging time t1 is less than or equal to 50min; in the second step, the current multiplying power I2 is 0.05 C to 0.2 C, and the charging time t2 is smaller than or equal to 60 min; the third step of constant-current charging, wherein the current multiplying power I3 is 0.2 C-0.5 C, and the charging time t3 is smaller than or equal to 100 min; wherein the large-current formation SOC is smaller than or equal to 35%, and the total formation SOC is smaller than or equal to 40%. When large current is formed, the SEI film structure is looser, the corresponding thickness is larger, and more irreversible reactions exist; during low-current formation, SEI film molecules are easier to accumulate in order, the structure is more compact, and the corresponding thickness is smaller. The total formation SOC and the large-current formation SOC are controlled, the DCR of the battery can be effectively reduced, and the rate capability and the cycling stability of the battery are improved; and meanwhile, the time and the power consumption required by the formation process can be reduced, and the equipment and production cost is saved.

Description

Technical Field

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

[0002] In recent years, lithium-ion batteries have been increasingly used in energy storage power stations, power tools, electric vehicles and other fields, and have 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 crucial impact on battery performance. Formation is the first charging process of the battery after injection and aging, which is essentially 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 continuing to undergo irreversible chemical reactions with the active substances. A stable SEI film can ensure that lithium-ion batteries maintain good electrochemical properties 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 complicated and is affected by many factors. The formation process directly affects the film quality of the SEI film, which in turn determines the electrical properties 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 the production efficiency and product quality of the enterprise. The optimized formation process can not only improve the electrode structure and interface, reduce the interface impedance, but also form a more stable SEI layer, reduce unnecessary side reactions, and fully activate the electrode active substances, 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] There are many limitations in the existing lithium-ion battery formation method, and it is 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 substances 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, so as to improve the overall performance of the battery. Summary of the invention

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

[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 three-step constant current charging to form:

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

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

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

[0011] Among them, the large current formation SOC = I3×t3 / 60≤35%, and the total formation SOC = (I1×t1+I2×t2+I3×t3)≤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, it is left alone 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, in the first step of constant current charging, the current rate I1 is 0.015C and the charging time t1 is 40min.

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

[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 50min.

[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 iron manganese 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 formation method of reducing the DCR of a battery of the present invention has the following beneficial effects compared with 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, it is easier for the SEI film molecules to stack in an orderly manner, its structure is denser, and the corresponding thickness is smaller. The total formation SOC ≤ 40% and the 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 consumption 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 iron manganese 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 drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 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 described embodiments 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 creative work are within the scope of protection of the present invention.

[0027] The formation process of SEI film is very complicated and has many influencing factors. Among them, the formation process directly affects the quality of SEI film and determines the electrical performance of the battery. There are usually three factors that affect SEI film formation: time, current, and temperature.

[0028] Formation temperature: It mainly affects the formation effect by affecting the viscosity and conductivity of the electrolyte and the diffusion rate of the ions of the electrode material. Generally speaking, the higher the formation temperature, the lower the viscosity of the electrolyte, the higher the conductivity of the electrolyte, and the faster the diffusion rate of the ions of the electrode material. Therefore, the smaller the polarization, the better the formation effect. However, when the temperature is too high, the structure of the SEI film that has been formed will be destroyed and its components will be changed; at the same time, the electrolyte is an organic solvent solution, and too high a temperature will accelerate the volatilization rate of the low-boiling point components in the electrolyte, affecting the formation effect.

[0029] Formation current: When the current is large, two-electron reactions are more likely to occur, that is, two electrons are required to participate in the reaction 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 be randomly stacked, and its structure is looser, the corresponding thickness is larger, and there are more irreversible reactions; when the current is small, single-electron reactions are more likely to occur, that is, reactions that only require the participation of one electron, and it is easier to generate organic lithium salt components. At this time, the SEI film molecules are more likely to be orderly stacked, and 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. The time taken for formation 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 cell can be any one of lithium iron phosphate / graphite, lithium iron manganese phosphate / graphite, and lithium nickel cobalt manganese oxide / graphite, and the battery cell packaging method can be soft pack or hard shell. The battery cell sample preparation process is normal, and other processes such as mixing, coating, rolling, winding / stacking, liquid injection, high temperature aging, etc. are exactly the same as normal batteries until the formation process.

[0034] S2, formation: inject electrolyte into the battery cell and use three constant current charging methods to form it. If it is a soft pack battery cell, inject electrolyte into the soft pack battery cell, seal the airbag bag, and then form it.

[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 easier to stack in an orderly manner, and its structure is denser, the corresponding thickness is smaller, and there are fewer irreversible reactions. The battery DCR will decrease accordingly, so 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 SOC and high current 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] In the third step and the entire formation process, too high SOC will cause excessive current when the SEI film is formed, resulting in a looser and more uneven structure of the SEI film. The thickness of the SEI film increases and the structure becomes loose, which increases the internal resistance of the battery. This means that the voltage drop of the battery during charging and discharging increases, resulting in a decrease in overall performance.

[0042] High SOC may cause excessive irreversible reactions in electrode materials, resulting in changes and loss of material structure. The battery may experience faster capacity decay and shorter cycle life during multiple charge and discharge cycles. This is because excessive irreversible reactions will cause degradation of electrode materials and instability of the SEI film.

[0043] Due to the increase in DCR, the battery is more likely to overheat during high-rate discharge, which limits the battery's performance under high current loads. High-rate performance is degraded, including the inability to provide sufficient current output during rapid charge and discharge. In addition, the battery's energy efficiency is reduced, which may lead to failure to meet demand in high-power applications.

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

[0045] Impact on battery DCR: Too high temperature may accelerate the decomposition of the electrolyte and the occurrence of side reactions, resulting in uneven or excessively thick SEI film formation, thereby increasing the internal resistance of the battery. Too low temperature may cause the SEI film to form slowly and incompletely, increasing the internal resistance of the battery. Too high or too low pressure may affect the wettability of the electrolyte and the contact of the electrode material, resulting in unsatisfactory SEI film formation and increased internal resistance.

[0046] Impact on cycle stability: Too high temperature will accelerate the degradation of electrode materials and the decomposition of electrolyte, resulting in faster battery capacity decay and shortened cycle life. Too low temperature may cause SEI membrane instability and increase irreversible capacity loss of battery during the cycle. Improper pressure may lead to uneven distribution of electrolyte, affecting the long-term stability of the battery.

[0047] Impact on rate performance: Too high a temperature may cause the battery to overheat during high-rate discharge, affecting the safety and performance of the battery. Too low a temperature may limit the migration speed of lithium ions and reduce the rate performance of the battery. Improper pressure may affect the fluidity of the electrolyte and the reactivity of the electrode, reducing the performance of the battery 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 the ideal formation of the SEI film, thereby optimizing the battery's DC internal resistance, cycle stability, and rate performance. Adjusting one of these factors usually affects the other factors, so comprehensive consideration and optimization are required.

[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 carried out normally, including mixing, coating, rolling, winding / stacking, liquid injection, high temperature aging and other processes are exactly the same as normal batteries until the formation process.

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

[0053] Table 1 Example 1 Formation process

[0054]

[0055] Example 2

[0056] Compared with Example 1, Example 2 has additional features in terms of the battery cell and the formation method, and the battery cell is a 120Ah lithium manganese iron 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 that the battery cell and the formation method are supplemented. The battery cell is a 120Ah nickel cobalt lithium 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 performed according to the process shown in Table 3.

[0061] Table 3 Example 3 Formation Process

[0062]

[0063]

[0064] Comparative Example 1

[0065] Comparative Example 1 is compared with Example 1, and the formation method is different, 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 terms of the chemical 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 compared with Example 1, and the formation method is different, 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 terms of the chemical 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 compared with Example 1, and the formation method is different, 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 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 formation methods of the embodiments are 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 embodiments of the present invention. If the high current formation SOC is less than 35%, the total formation SOC is greater than 40%, or 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 The 25°C, 1C / 1C cycle capacity retention rate attenuation curves of different formation methods show that the capacity retention rate of the embodiment is significantly higher than that of the comparative example. It can be seen that controlling the total formation SOC and high current formation SOC to reduce the battery DCR can effectively reduce the battery internal resistance and improve the battery rate performance and cycle 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 protection scope 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 three-step constant current charging to form: The first step is constant current charging: the current rate I1 is 0.01C~0.05C, and the charging time t1≤50min; The second step is constant current charging: the current rate I2 is 0.05C~0.2C, and the charging time t2≤60min; The third step is constant current charging: the current rate I3 is 0.2C~0.5C, and the charging time t3≤100min; Among them, the large current formation SOC = I3×t3 / 60≤35%, and the total formation SOC = (I1×t1+I2×t2+I3×t3)≤40%.

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

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

7. A formation method for reducing battery DCR as claimed in 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 48min.

8. A formation method for reducing battery DCR as claimed in 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 as claimed in claim 1, characterized in that: The positive electrode material of the battery cell is one of lithium iron phosphate, lithium iron manganese 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 as claimed in claim 1, characterized in that: The battery core is a soft-pack battery core or an aluminum shell battery core.

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