A segmented temperature-variable formation method for lithium iron phosphate square shell battery cell

Abstract

The application discloses a segmented variable-temperature formation method for lithium iron phosphate square shell batteries. The method comprises the following steps: battery pretreatment: placing the LPF square shell battery after liquid injection in a sealed formation cavity and standing under vacuum conditions; segmented variable-temperature formation: sequentially performing normal-temperature pre-charging in a first temperature interval, medium-temperature activation in a second temperature interval and low-temperature gradient formation in a third temperature interval; charging the LPF square shell battery in a multi-stage variable-current charging mode in the low-temperature gradient formation stage; wherein the third temperature interval < 0 DEG C < the first temperature interval < the second temperature interval; dynamic pressure control: applying dynamic pressure varying with the formation stage and / or battery voltage during the pretreatment and the segmented variable-temperature formation; low-temperature aging after formation: performing aging treatment under the third temperature interval and pressure maintaining conditions after the segmented variable-temperature formation, and then heating to the first temperature interval. The application effectively improves the low-temperature discharge capacity and rate performance of the LFP square shell battery.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and more specifically, relates to a segmented temperature-varying formation method for lithium iron phosphate prismatic cells. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) cells have been widely used in new energy vehicles and energy storage due to their advantages such as long cycle life, high safety, and low cost. However, the performance of LFP cells deteriorates sharply at low temperatures, manifesting as significant capacity decay, increased polarization voltage, and poor rate performance. At low temperatures, electrolyte viscosity increases and ionic conductivity decreases, significantly reducing the migration rate of lithium ions in the electrolyte and their diffusion rate in the electrode materials. Simultaneously, the impedance of the solid electrolyte interphase (SEI) film on the negative electrode surface increases, reducing the conduction rate of lithium ions across the SEI film, ultimately leading to intensified charge-discharge polarization and severe capacity decay. These problems severely restrict the commercial application of LFP cells in cold regions.

[0003] The formation process is one of the core steps in lithium-ion battery cell manufacturing. During formation, lithium ions form an SEI film on the surface of the negative electrode graphite. The quality of the SEI film, including its density, uniformity, thickness, and impedance characteristics, directly determines the low-temperature performance, cycle life, and safety of the battery cell. Parameters such as formation temperature, formation current, cutoff voltage, and environmental pressure are the core process parameters affecting the quality of the SEI film.

[0004] Currently, the existing LFP cell formation process has the following shortcomings: Firstly, the formation temperature control strategy is simplistic. Existing formation processes mostly employ either constant-temperature formation throughout or single-temperature-zone formation. While high-temperature formation can improve the electrochemical reaction rate and SEI film formation rate, the resulting SEI film is porous and unstable, prone to cracking and pulverization at low temperatures. Conversely, while the SEI film formed by low-temperature formation is relatively dense, the reaction rate is slow and the formation time is long. In recent years, some patents have proposed two-stage temperature-controlled formation schemes. For example, patent document CN121885809A discloses a technical solution where the first-stage charging environment temperature is lower than the second-stage charging environment temperature; and patent document CN119447538B discloses a process combining low-temperature, low-current pre-treatment with subsequent segmented formation. However, these existing solutions are still limited to a simple combination of two-stage temperature control or low-temperature pre-treatment followed by temperature increase, lacking multi-stage temperature gradient designs for different growth stages of the SEI film (initial nucleation, densification growth, and surface passivation), making it difficult to balance the density, flexibility, and formation efficiency of the SEI film.

[0005] Secondly, the formation current mode is singular. Existing formation methods mostly employ fixed single-stage constant current charging or simple step-by-step incremental charging. A single current mode or a simple incremental mode makes it difficult to achieve multiple objectives such as uniform nucleation, dense growth, and surface passivation at different stages of SEI film formation. Especially at low temperatures, a single current mode can easily lead to local concentration polarization and lithium deposition, resulting in an inhomogeneous SEI film structure and high interfacial impedance.

[0006] Third, there is a lack of pressure adaptation design specifically for the rigid structure of LFP prismatic cells. The rigid casing of prismatic cells makes it difficult to dynamically adjust the interfacial contact state between the electrodes and the separator during formation, resulting in uneven wetting of the electrodes and electrolyte. At low temperatures, electrode shrinkage further exacerbates problems such as interfacial delamination and increased voids, severely affecting the consistency of the SEI film and the low-temperature rate performance of the cell. Although some existing technologies propose variable pressure formation (such as the simultaneous adjustment of temperature and pressure in polymer lithium-ion battery formation methods), they fail to provide a systematic pressure adaptation solution for the rigid structural characteristics and low-temperature shrinkage problem of LFP prismatic cells.

[0007] Fourth, there is a lack of post-formation treatment processes. After the existing formation process is completed, the process usually proceeds directly to the capacity separation process. There is a lack of treatment to eliminate the interfacial stress of the SEI film under low-temperature conditions and to passivate the surface active sites, which makes the SEI film prone to structural degradation during subsequent cycles.

[0008] Therefore, designing a formation method specifically adapted to improve low-temperature performance for LFP prismatic cells, and constructing a high-quality, low-impedance SEI film by optimizing the formation temperature, current, pressure, and timing, thereby enhancing the low-temperature ion conductivity and interface stability of the cell, has become an urgent technical problem to be solved. Summary of the Invention

[0009] The purpose of this invention is to propose a segmented temperature-varying formation method for lithium iron phosphate (LFP) prismatic cells, addressing the problems of insufficient SEI film density, high low-temperature impedance, and poor electrode interface contact caused by mismatched formation processes in existing LFP prismatic cells, resulting in severe low-temperature discharge capacity decay and poor rate performance. This method achieves SEI film densification, low-impedance growth, and improved electrode interface stability, thereby significantly improving the low-temperature discharge capacity and rate performance of LFP prismatic cells.

[0010] To achieve the above objectives, this invention proposes a segmented temperature-varying formation method for lithium iron phosphate prismatic cells, comprising: Cell pretreatment: The lithium iron phosphate square-shell cell after liquid injection is placed in a sealed formation chamber and left to stand under vacuum conditions; Segmented temperature-dependent formation: The lithium iron phosphate prismatic cell is sequentially pre-charged at room temperature in the first temperature range, activated at medium temperature in the second temperature range, and formed at low temperature in the third temperature range; in the low temperature gradient formation stage, the lithium iron phosphate prismatic cell is charged using a multi-stage variable current charging mode. Dynamic pressure control: During the pretreatment and segmented temperature-varying formation process, dynamic pressure is applied to the lithium iron phosphate prismatic cell as the formation stage and / or cell voltage changes. Low-temperature aging after formation: After the segmented temperature-varying formation is completed, aging treatment is carried out under the third temperature range and pressure holding conditions, and then the temperature is raised to the first temperature range. Among them, the third temperature range is <0℃ < the first temperature range < the second temperature range.

[0011] Optionally, the step of settling the lithium iron phosphate prismatic cell under vacuum conditions includes: The dew point of the sealed formation chamber is controlled to be ≤-40℃ and the oxygen content to be ≤100ppm. The chamber is then left to stand for 8 to 12 hours at 25±2℃ and a vacuum of -0.08 to -0.09MPa.

[0012] Optionally, the segmented temperature variation specifically includes: During the ambient temperature pre-charging stage, the temperature of the lithium iron phosphate square-shell cell is controlled within the first temperature range, and the lithium iron phosphate square-shell cell is charged to 3.2V using a constant current of 0.02 to 0.03C for 1 to 2 hours. During the medium-temperature activation stage, the temperature of the lithium iron phosphate square-shell cell is raised to the second temperature range at a temperature rise rate of 0.5 to 1℃ / min, and the lithium iron phosphate square-shell cell is charged to 3.4V using a constant current of 0.05 to 0.08C for 2 to 3 hours. In the low-temperature gradient formation stage, the temperature of the lithium iron phosphate prismatic cell is cooled to the third temperature range at a temperature drop rate of 0.3 to 0.5 °C / min, and the lithium iron phosphate prismatic cell is charged using a multi-stage converter charging mode.

[0013] Optionally, the multi-stage converter charging mode includes: In the first sub-stage, the lithium iron phosphate square-shell cell is charged to 3.45V using a constant current of 0.03 to 0.05C for 1.5 to 2.5 hours. In the second sub-stage, the lithium iron phosphate square-shell cell is charged to 3.5V using a constant current of 0.08 to 0.1C for 1 to 2 hours. In the third sub-stage, the lithium iron phosphate prismatic cell is micro-charged to a preset capacity using a current of 0.02–0.03C for a charging time of 0.5–1 hour.

[0014] Optionally, the dynamic pressurization control specifically includes: During the cell pretreatment and room temperature pre-charging stages, a pressure of 0.1–0.2 MPa is applied. During the intermediate-temperature activation stage, the applied pressure is 0.3–0.4 MPa; During the low-temperature gradient formation stage, the applied pressure is 0.25 to 0.35 MPa, and decreases by 0.01 MPa for every 0.1 V increase in cell voltage.

[0015] Optionally, the aging process includes: After the segmented temperature-controlled formation is completed, the temperature of the lithium iron phosphate square-shell cell is maintained at -18 to -22°C, the pressure at 0.2 to 0.25 MPa, and the aging time is 4 to 6 hours. After the aging process is completed, the temperature of the lithium iron phosphate square-shell cell is raised to the first temperature range at a heating rate of 0.5 to 1℃ / min, and then depressurized to atmospheric pressure; after depressurization, vacuum degassing is performed under vacuum conditions.

[0016] Optionally, the first temperature range is 23 to 27°C; the second temperature range is 35 to 40°C; and the third temperature range is -18 to -22°C.

[0017] Optionally, the preset capacity is 8% to 10% of the design capacity.

[0018] Optionally, it also includes post-processing steps after charging each sub-stage: After the first sub-stage charging is completed, the cell is left to stand for 0.5 to 1.0 hours in the third temperature range, and then discharged for 10 to 15 minutes using a current of 0.01 to 0.02C. After the second sub-stage charging is completed, the cell is first left to stand in the third temperature range for 0.5 to 1.0 hours, and then discharged with a current of 0.01 to 0.02C for 10 to 15 minutes. After the third sub-stage charging is completed, the battery is first left to stand in the third temperature range for 0.5 to 1.0 hours, and then discharged with a current of 0.01 to 0.02C for 10 to 15 minutes.

[0019] The beneficial effects of this invention are as follows: By adopting a three-stage variable-temperature formation strategy of "room temperature pre-charging → medium-temperature activation → low-temperature gradient formation," the SEI film undergoes gradient optimization in three stages under different temperature conditions: initial nucleation, densification growth, and low-temperature stabilization. The room temperature pre-charging stage initiates initial SEI film formation under mild conditions, avoiding the loose film layer caused by direct low-temperature formation. The medium-temperature activation stage increases the reaction rate to promote the densification growth of the SEI film. The low-temperature gradient formation stage utilizes the dense and stable growth characteristics of the SEI film at low temperatures, ultimately forming a high-quality SEI film with both density and flexibility, effectively solving the problems of loose high-temperature film formation and low low-temperature film formation efficiency in traditional single-temperature formation. Furthermore, by employing a multi-stage variable-current charging mode in the low-temperature gradient formation stage, a non-monotonic variable-current strategy of "low-current nucleation → medium-current thickening → micro-current passivation" is used to match the current requirements of different growth stages of the SEI film, resulting in a uniform and dense SEI film structure. This avoids interfacial resistance caused by local concentration polarization and lithium deposition under single-current or monotonically increasing current modes. To address the issue of excessively high pressure, dynamic pressure is applied throughout the formation process, linked to the formation stage and cell voltage. Higher pressure is used during room temperature pre-charging and medium-temperature activation to ensure tight adhesion between the electrode and separator, improving electrolyte wetting uniformity. During the low-temperature gradient formation stage, pressure is gradually reduced as voltage increases to accommodate electrode shrinkage at low temperatures, avoiding interface delamination and increased voids caused by the rigid prismatic structure, thus ensuring the stability and consistency of the SEI film growth interface. Low-temperature aging is performed directly in the third temperature range after low-temperature formation, immediately stabilizing the SEI film structure and eliminating residual interface stress after formation, avoiding structural degradation of the SEI film in subsequent cycles, which is common in traditional processes where direct entry into the capacity testing phase is avoided. Through the synergistic effect of segmented temperature variation, multi-stage current conversion, dynamic pressure application, and low-temperature aging, a low-impedance, high-stability SEI film is constructed on the negative electrode surface of the LFP prismatic cell, significantly improving the cell's low-temperature ion conductivity and interface stability, thereby effectively enhancing the low-temperature discharge capacity and rate performance of the LFP prismatic cell.

[0020] The system of the present invention has other features and advantages that will be apparent from or will be set forth in detail in the accompanying drawings and following detailed description, which together serve to explain the particular principles of the invention. Attached Figure Description

[0021] The above and other objects, features and advantages of the present invention will become more apparent from the accompanying drawings, in which like reference numerals generally denote like parts.

[0022] Figure 1A flowchart of the steps of the segmented temperature-varying formation method for lithium iron phosphate prismatic cells according to the present invention is shown.

[0023] Figure 2 A flowchart of the segmented temperature-varying formation according to the present invention is shown.

[0024] Figure 3 A flowchart of the multi-stage converter charging mode according to the present invention is shown. Detailed Implementation

[0025] The invention will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0026] like Figure 1 As shown, a segmented variable-temperature formation method for a lithium iron phosphate prismatic cell according to the present invention includes: S1. Cell pretreatment: The lithium iron phosphate square cell after liquid injection is placed in a sealed formation chamber and left to stand under vacuum conditions. In this step, the static treatment of the lithium iron phosphate prismatic cell under vacuum conditions includes: The dew point of the sealed formation chamber is controlled to be ≤-40℃ and the oxygen content to be ≤100ppm. The chamber is then allowed to stand for 8 to 12 hours at 25±2℃ and a vacuum of -0.08 to -0.09MPa.

[0027] Specifically, the internal structure of a lithium iron phosphate (LFP) prismatic cell after electrolyte injection is not in an ideal wetting state. After the electrolyte is injected into the cell casing, it needs to fully penetrate the pores of the positive and negative electrode sheets and the microporous structure of the separator. However, the electrode coating contains a large number of micropores filled with air, and the microstructure of the electrode material surface makes it difficult for the electrolyte to spread quickly under surface tension. If the formation process is carried out directly in this state, the areas on the electrode surface that are not completely covered by the electrolyte will become "dry areas." A uniform SEI film cannot be formed in these areas, leading to excessively high local current density, uneven lithium ion embedding, and problems such as lithium plating and black spots. Therefore, a settling treatment must be performed before formation to allow the electrolyte to fully wet the electrode sheets.

[0028] Applying vacuum conditions is a core method for improving impregnation efficiency and quality. Its mechanism mainly manifests in three aspects: First, vacuuming removes air from between electrode coating particles and within pores, reducing gas phase resistance at the solid-gas-liquid interface and allowing the electrolyte to directly contact the electrode surface. Second, the pressure difference between the inside and outside of the cell under vacuum increases the gap between the cell casing and the core, and allows gas in the gas bag to slowly escape, making it easier for the electrolyte to flow into the cell to impregnate the diaphragm and electrode. Third, vacuuming helps to remove gas generated during impregnation in a timely manner, preventing air bubbles from remaining in the electrode pores and hindering further electrolyte penetration. Cells treated with vacuum before electrolyte injection can achieve maximum impregnation within hours, while those without vacuum treatment may require several days to achieve sufficient impregnation.

[0029] The settling stage requires strict control of several process parameters to ensure effective wetting. Specifically, the injected lithium iron phosphate prismatic cells must be placed in a sealed formation chamber, with the chamber's dew point controlled to ≤-40℃ to isolate moisture and the oxygen content ≤100ppm to prevent oxidation. The cells are then settling for 8–12 hours at a room temperature of 25±2℃ and a vacuum of -0.08 to -0.09MPa. The control of dew point and oxygen content is crucial to prevent side reactions between moisture and oxygen and the electrolyte or electrode materials during wetting, which could affect the SEI film quality. The room temperature of 25±2℃ ensures suitable fluidity of the electrolyte for wetting while avoiding potential side reactions at high temperatures. The vacuum of -0.08 to -0.09MPa ensures sufficient removal of gas from the cell's interior. The 8–12 hour settling time provides ample time for the electrolyte to fully penetrate the electrode pores.

[0030] Through the aforementioned static treatment under vacuum conditions, the electrolyte can fully wet the positive and negative electrode plates and the separator, eliminating "dry areas" on the electrode surface and ensuring that lithium ions can be uniformly inserted and extracted across the entire electrode surface during subsequent formation processes. Sufficient wetting also effectively avoids defects such as excessive local current density, lithium deposition, and uneven SEI film caused by insufficient wetting, providing uniform interface conditions for the formation of a high-quality SEI film in subsequent segmented temperature-varying formation processes.

[0031] S2. Segmented temperature formation: The lithium iron phosphate prismatic cells are sequentially pre-charged at room temperature in the first temperature range, activated at medium temperature in the second temperature range, and formed at low temperature in the third temperature range. During the low temperature gradient formation stage, the lithium iron phosphate prismatic cells are charged using a multi-stage current conversion charging mode. The third temperature range is <0℃ < the first temperature range < the second temperature range.

[0032] Specifically, segmented variable-temperature formation is the core technical approach of this invention. Its design logic lies in the fact that the formation of the SEI film on the graphite anode surface is not completed in a single stage, but rather involves a gradual process of "initial nucleation → densification growth → surface passivation." A single temperature or single-current strategy cannot simultaneously meet the differentiated requirements of the SEI film for reaction kinetics and film structure at different growth stages: high temperatures can increase the reaction rate, but the resulting SEI film is loose and unstable; low temperatures can form a dense and stable SEI film, but the reaction rate is too slow and the formation efficiency is low. This invention employs a three-stage variable-temperature strategy of "room temperature pre-charging → medium-temperature activation → low-temperature gradient formation," combined with a multi-stage current-switching charging mode at the low-temperature stage, to match optimal formation conditions for each growth stage of the SEI film, thereby constructing a high-quality SEI film with characteristics of density, flexibility, and low impedance. The core temperature relationship in the entire segmented variable-temperature formation process is: third temperature range < 0℃ < first temperature range < second temperature range.

[0033] like Figure 2 As shown, in this step, the segmented temperature variation specifically includes: During the room temperature pre-charge stage, the temperature of the lithium iron phosphate square cell is controlled in the first temperature range, and the lithium iron phosphate square cell is charged to 3.2V using a constant current of 0.02 to 0.03C for 1 to 2 hours. During the medium-temperature activation stage, the temperature of the lithium iron phosphate square cell is raised to the second temperature range at a temperature rise rate of 0.5 to 1℃ / min, and the lithium iron phosphate square cell is charged to 3.4V using a constant current of 0.05 to 0.08C for 2 to 3 hours. During the low-temperature gradient formation stage, the temperature of the lithium iron phosphate prismatic cell is cooled to the third temperature range at a temperature drop rate of 0.3 to 0.5℃ / min, and the lithium iron phosphate prismatic cell is charged using a multi-stage converter charging mode.

[0034] Specifically, during the room-temperature pre-charging stage, the temperature of the lithium iron phosphate (LFP) prismatic cell is controlled within the first temperature range, and the LFP prismatic cell is charged to 3.2V using a constant current of 0.02–0.03C for 1–2 hours. This stage is the initiation stage of the entire formation process, and its core objective is "gentle initiation," triggering uniform nucleation of the SEI film under relatively mild electrochemical reaction conditions. Within the first temperature range (e.g., 23–27°C), the electrolyte has suitable ionic conductivity and moderate lithium-ion transport resistance; combined with a small current density of 0.02–0.03C, this ensures that the lithium-ion insertion rate on the negative electrode surface is sufficiently slow, allowing the SEI film to nucleate uniformly on the graphite particle surface rather than growing in localized clusters, laying the structural foundation for subsequent SEI film densification.

[0035] During the intermediate-temperature activation stage, the temperature of the lithium iron phosphate (LFP) prismatic cell is increased to the second temperature range at a rate of 0.5–1 °C / min. The cell is then charged to 3.4V using a constant current of 0.05–0.08 C for 2–3 hours. After initial nucleation at room temperature, the temperature needs to be increased to accelerate the electrochemical reaction kinetics and promote further densification of the SEI film. The uniform temperature control ensures a uniform temperature field within the cell, preventing uneven SEI film thickness caused by excessively high local temperatures. At intermediate temperatures (e.g., 35–40 °C), the ionic conductivity of the electrolyte is further improved, and the lithium-ion migration rate is accelerated, which is beneficial for the continuous and dense growth of the SEI film based on existing nucleation sites. This maintains a high reaction rate to ensure formation efficiency while avoiding the problems of unstable SEI film structure and easy cracking and pulverization at high temperatures.

[0036] In the low-temperature gradient formation stage, the temperature of the lithium iron phosphate prismatic cell is cooled to the third temperature range (e.g., -18 to -22℃) at a cooling rate of 0.3–0.5℃ / min. This uniform cooling rate ensures a smooth transition from the high-temperature to the low-temperature region, avoiding thermal stress concentration and structural defects in the film layer caused by sudden temperature drops, and ensuring the structural integrity of the SEI film during the cooling process. After cooling to the third temperature range, a multi-stage current-conversion charging mode is entered, employing a non-monotonic current-conversion strategy of "low-current nucleation → medium-current thickening → micro-current passivation" to complete the final growth and stabilization of the SEI film in a low-temperature environment. Through this three-stage temperature-variable and multi-stage current-conversion formation strategy, the SEI film gradually forms along a complete temperature path of "mild nucleation → accelerated growth → low-temperature stabilization," ultimately obtaining a dense, stable, and low-impedance high-quality SEI film.

[0037] like Figure 3 As shown, in this step, the multi-stage converter charging mode includes: In the first sub-stage, the lithium iron phosphate square-shell cells are charged to 3.45V using a constant current of 0.03–0.05C for 1.5–2.5 hours. The second sub-stage uses a constant current of 0.08–0.1C to charge the lithium iron phosphate square-shell cell to 3.5V for 1–2 hours. In the third sub-stage, a micro-current supplementary charge of 0.02–0.03C is used to charge the lithium iron phosphate square-shell cell to the preset capacity, with a charging time of 0.5–1 hour.

[0038] Specifically, the multi-stage variable-current charging mode is the core charging strategy of this invention in the low-temperature gradient formation stage. Its design logic lies in the fact that the current density requirements of the SEI film differ significantly at different growth stages. The nucleation stage requires a low current density to ensure uniform nucleation, the thickening stage requires an appropriately increased current density to improve efficiency, and the surface passivation stage requires an extremely low current density to reduce interface impedance. A single constant current or a simple monotonically increasing current cannot simultaneously meet these differentiated requirements. This invention employs a non-monotonic variable-current strategy of "low-current nucleation → medium-current thickening → micro-current passivation" in the low-temperature gradient formation stage. Through the current gradient changes of three sub-stages, it precisely matches the optimal current density requirements of the SEI film at different growth stages.

[0039] The first sub-stage involves constant-current charging of the lithium iron phosphate prismatic cell to 3.45V using a current of 0.03–0.05C for 1.5–2.5 hours. This stage is crucial for constructing the SEI film base layer under low-temperature conditions. The low current density of 0.03–0.05C ensures a sufficiently slow lithium-ion insertion rate on the negative electrode surface, allowing the SEI film to nucleate and grow uniformly and densely on the graphite particle surface. This avoids the risk of local concentration polarization and lithium deposition caused by excessively rapid lithium-ion insertion at high current densities. Low-current charging is particularly important in the low-temperature environment of -18 to -22°C. At low temperatures, the migration rate of lithium ions in the electrolyte and the diffusion rate in the electrode material are significantly reduced. If the current density is too high, the lithium-ion insertion rate on the negative electrode surface will exceed its diffusion rate, leading to uneven deposition of lithium ions on the graphite particle surface, forming lithium dendrites or an uneven SEI film. By charging with a small current of 0.03 to 0.05C, it is ensured that the base layer of the SEI film can be densely and uniformly covered on the surface of graphite particles under low temperature conditions, providing a structural basis for the subsequent thickening of the SEI film.

[0040] The second sub-stage involves constant-current charging of the lithium iron phosphate prismatic cell to 3.5V using a current of 0.08–0.1C for 1–2 hours. This stage is crucial for the thickening and densification of the SEI film. After the formation of a uniform and dense basic SEI film in the first sub-stage, appropriately increasing the current density to 0.08–0.1C accelerates the electrochemical reaction rate, improves formation efficiency, and shortens formation time. Furthermore, the higher current density helps the SEI film thicken further based on existing nucleation sites, resulting in a denser and more complete film layer. At low temperatures of -18 to -22°C, the current density of 0.08–0.1C remains within the safe window for lithium-ion intercalation, effectively improving the thickness and density of the SEI film without causing concentration polarization or lithium deposition due to excessively rapid lithium-ion intercalation. Charging to a cutoff voltage of 3.5V in this sub-stage ensures that the SEI film thickening process is completed within a suitable potential window, preventing excessive decomposition of the electrolyte on the SEI film surface due to excessively high cutoff voltage.

[0041] The third sub-stage uses a current of 0.02–0.03C to perform micro-current supplementary charging on the lithium iron phosphate prismatic cells to the preset capacity, with a charging time of 0.5–1 hour. This stage is the passivation stage of the active sites on the SEI film surface and is also the final stage of the entire formation process. After the SEI film thickening and densification is completed in the second sub-stage, some active sites may still exist on the SEI film surface. These active sites are prone to side reactions with the electrolyte during subsequent cycles, leading to continuous thickening of the SEI film and a continuous increase in impedance. Using an extremely low current density (0.02–0.03C) for micro-current supplementary charging results in an extremely slow lithium-ion insertion rate, enabling lithium ions to be inserted uniformly and slowly onto the SEI film surface, thereby effectively passivating the active sites on the SEI film surface and reducing interfacial impedance. Charging to the preset capacity (8%–10% of the design capacity) instead of a fixed cutoff voltage ensures that different cells complete the SEI film passivation treatment under the same charging state, avoiding overcharging or undercharging of some cells due to inconsistencies between cells. The charging time of 0.5 to 1 hour ensures sufficient microcurrent supplementation charging, allowing the active sites on the SEI film surface to be fully passivated.

[0042] The current variation in the three sub-stages exhibits a non-monotonic pattern of "low→high→low," the technical significance of which lies in the following: the low current in the first sub-stage ensures uniform and dense growth of the SEI film base layer, avoiding lithium deposition at low temperatures; the medium current in the second sub-stage accelerates the thickening and densification of the SEI film on the base layer, improving formation efficiency; and the micro current in the third sub-stage passivates the active sites on the SEI film surface, reducing interfacial impedance. If a monotonically increasing current pattern (e.g., low→medium→high) is used, the high current in the third sub-stage will prevent the effective passivation of the active sites on the SEI film surface, leading to increased interfacial impedance; if a constant current is used, it is impossible to achieve the multiple objectives of uniform nucleation, dense growth, and surface passivation at different growth stages of the SEI film. This invention, through a non-monotonic current variation strategy of "low→high→low," precisely matches the differentiated current density requirements of the SEI film at different growth stages, enabling the SEI film to achieve optimal performance in multiple dimensions such as density, uniformity, thickness, and impedance characteristics.

[0043] In this step, the preset capacity is 8% to 10% of the design capacity.

[0044] In this step, the first temperature range is 23 to 27°C; the second temperature range is 35 to 40°C; and the third temperature range is -18 to -22°C.

[0045] This step also includes post-processing steps after charging each sub-stage: After the first sub-stage charging is completed, the battery cell is left to stand for 0.5 to 1.0 hours in the third temperature range, and then discharged for 10 to 15 minutes using a current of 0.01 to 0.02C. After the second sub-stage charging is completed, the battery cell is left to stand for 0.5 to 1.0 hours in the third temperature range, and then discharged for 10 to 15 minutes using a current of 0.01 to 0.02C. After the third sub-stage charging is completed, the battery is left to stand for 0.5 to 1.0 hours in the third temperature range, and then discharged for 10 to 15 minutes using a current of 0.01 to 0.02C.

[0046] Specifically, a "resting-discharging" post-processing procedure is implemented after each sub-stage charging is completed. This means that after charging, the cell is first allowed to rest for 0.5–1.0 hours within the third temperature range, and then discharged for 10–15 minutes using a current of 0.01–0.02C. This process is not simply waiting and discharging, but a precise post-processing step designed to optimize the state of the SEI film after its formation, based on electrochemical mechanisms. Resting and micro-discharging act on different stages after SEI film formation. Resting addresses the polarization problem accumulated during charging, while micro-discharging addresses the activity problem on the SEI film surface. Both work together and are indispensable, forming a complete process chain for SEI film interface modification.

[0047] The core function of the settling step is to eliminate polarization and stabilize the SEI film structure. During charging, lithium ions are extracted from the positive electrode under the drive of the electric field, pass through the electrolyte and SEI film, and embed themselves between the graphite layers of the negative electrode. In this non-equilibrium process, the concentration gradient of lithium ions in the electrolyte, the charge transfer resistance at the SEI film interface, and the uneven distribution of lithium ion concentration inside the graphite particles collectively lead to the accumulation of electrochemical polarization and concentration polarization. When the charging current is cut off, the cell remains in a non-equilibrium state. During the settling period, the polarization gradually disappears, and the electrode potential recovers from the deviation value in the polarized state to the equilibrium electrode potential. Specifically, lithium ions continue to diffuse into the graphite particles under the drive of the concentration gradient, making the distribution of lithium ions in the electrode material tend to be uniform; the charge transfer process at the SEI film interface tends to be balanced under the condition of no external current; at the same time, the SEI film undergoes structural rearrangement and compositional ordering after its formation. If the discharge step is initiated before polarization is eliminated, residual polarization will lead to an unstable voltage plateau in the initial stage of discharge, affecting the accurate assessment of the SEI film interface state and the consistency of subsequent processes. Resting promotes the relaxation and redistribution of internal stress, helping to reduce the risk of electrode micro-region deformation and core twisting. Performing the resting step within the third temperature range of -18 to -22°C has the advantage of higher structural stability of the SEI film at low temperatures, and is more conducive to the ordered arrangement and structural stability of the SEI film under resting conditions without applied current.

[0048] The core function of the micro-discharge step is to passivate the active sites on the SEI film surface. The formation of the SEI film is essentially an electrochemical reduction process, where electrolyte components gain electrons on the negative electrode surface and are reduced, forming a passivation layer covering the graphite particles. During charging, the formation of the SEI film is not completely synchronous across the entire cell surface; the growth progress and density of the SEI film vary in different regions. After charging, some "active sites" may still exist on the SEI film surface—areas not yet fully covered by the SEI film or where the SEI film is not dense enough. These active sites have high reactivity and are prone to continuous side reactions with the electrolyte during subsequent cycles, leading to continuous thickening of the SEI film, a continuous increase in impedance, and increased irreversible lithium loss. The technical significance of using a micro-current of 0.01–0.02C for 10–15 minutes of discharge is that a small amount of charge is extracted from the cell through an extremely small discharge current, allowing lithium ions to escape from the graphite layers of the negative electrode and return to the positive electrode. This process allows the still-unstable active sites on the SEI film surface to undergo electrochemical passivation under the influence of microcurrent. Specifically, through controlled, minute-level lithium ion removal, the SEI film surface reaches a more stable chemical state, reducing the probability of side reactions with the electrolyte in subsequent cycles. Simultaneously, microcurrent discharge can further verify the structural stability of the SEI film during the initial lithium removal process. If defects exist in the SEI film structure, abnormal voltage fluctuations may occur during microdischarge, thus providing an early basis for judging the formation quality.

[0049] If micro-discharge is performed immediately after charging (skipping the resting period), the discharge occurs under conditions of significant concentration polarization and electrochemical polarization within the cell. The distribution of lithium ions within the graphite particles is not yet uniform, and the SEI film structure is not yet fully stable. Applying a discharge current under these conditions will result in significant differences in the rate at which lithium ions escape from the graphite interlayers across different regions of the particle, leading to uneven local current density. This could cause uneven mechanical stress and structural damage to the not-yet-fully-stable SEI film. The sequence of resting first, followed by discharge, ensures that the cell is in a state of fully depolarized and stable SEI film structure at the start of discharge. This allows the micro-current discharge to act uniformly and controllably across the entire SEI film surface, achieving effective passivation of active sites without causing additional damage to the SEI film.

[0050] A settling time of 0.5–1.0 h is sufficient to ensure that concentration polarization and electrochemical polarization within the cell are fully eliminated, stabilizing the electrode potential; at the same time, it avoids a decrease in formation efficiency due to excessively long settling times. The micro-discharge current setting of 0.01–0.02 C and the duration of 10–15 min ensures that the discharge is sufficient to passivate the active sites on the SEI film surface, while preventing excessive SEI film shrinkage or structural damage due to excessive discharge depth.

[0051] Through the above-mentioned "static-discharge" post-treatment process, the SEI film underwent a complete optimized path of "polarization elimination → structural stabilization → surface passivation" after formation. The interfacial impedance was effectively reduced, and the structural and chemical stability of the SEI film were significantly improved, providing a more stable SEI film foundation for the subsequent low-temperature aging process after formation.

[0052] S3. Dynamic pressure control: During the pretreatment and segmented temperature-controlled formation process, dynamic pressure is applied to the lithium iron phosphate prismatic cells as the formation stage and / or cell voltage changes. In this step, dynamic pressurization control specifically includes: During the cell pretreatment and room temperature pre-charging stages, the applied pressure is 0.1–0.2 MPa; During the intermediate-temperature activation stage, the applied pressure is 0.3–0.4 MPa; During the low-temperature gradient formation stage, the applied pressure is 0.25–0.35 MPa, and decreases by 0.01 MPa for every 0.1 V increase in cell voltage.

[0053] Specifically, during the formation process, complex electrochemical reactions continuously occur inside the cell, accompanied by the growth of the SEI film, gas generation, and microscopic changes in electrode volume. For prismatic cells, their rigid casing cannot automatically adapt to changes in electrode thickness through the flexible deformation of the aluminum-plastic film, as is possible with pouch cells. Without dynamic adjustment of external pressure, the interfacial contact state between the electrode and the separator will continuously deteriorate during the formation process, directly affecting the uniformity and consistency of SEI film growth. Pressure can affect various performance characteristics of lithium batteries through interfacial interactions. This invention applies dynamic pressure to the cell throughout the pretreatment and segmented temperature-controlled formation process, linked to the formation stage and cell voltage. Through a dual control strategy of "segmented voltage variation + voltage-linked regulation," it ensures that the electrode and separator maintain optimal interfacial contact at different formation stages.

[0054] During the cell pretreatment and room temperature pre-charging stages, a pressure of 0.1–0.2 MPa is applied. The core task of this stage is to promote the full wetting of the electrode by the electrolyte and the initial nucleation of the SEI film. The initial pressure of 0.1–0.2 MPa is a moderate clamping force, which serves two purposes: First, it gently presses the wound or stacked electrode and the separator together with appropriate mechanical pressure, eliminating the macroscopic gap between the electrode and the separator, allowing the electrolyte to penetrate more smoothly into the micropores of the electrode and the pores of the separator under capillary action, thus improving the uniformity of wetting. Second, this pressure value should not be too high, to avoid over-compacting the electrode before the SEI film has formed, which would obstruct the flow channels of the electrolyte inside the electrode and affect the wetting effect. During the low-current charging process of room temperature pre-charging, the moderate pressure ensures that the SEI film can nucleate uniformly on the electrode surface, avoiding uneven local current density distribution caused by poor interfacial contact.

[0055] During the intermediate-temperature activation stage, the pressure is increased to 0.3–0.4 MPa. As the formation process enters the intermediate-temperature activation stage, the internal temperature of the cell rises to 35–40°C, the electrochemical reaction rate accelerates, and the SEI film enters the densification growth stage. Simultaneously, the amount of gas generated during the formation process increases. Increasing the pressure to 0.3–0.4 MPa at this stage has several technical significances: First, the higher pressure allows for tighter bonding between the electrode and separator, effectively compressing the microscopic gaps between the electrodes, reducing the lithium-ion transport path length at the electrode-separator interface, lowering interfacial contact resistance, and improving ion conduction efficiency. Second, the increased pressure helps to expel the gas generated during the formation process from the electrode-separator interface, preventing the accumulation of bubbles at the interface to form "dry areas," thus preventing SEI film defects and black spots. Third, the higher pressure ensures a tighter and more uniform bond between the electrode and separator, which is beneficial for the uniform distribution of electrons in the electrode plane, ensuring that the SEI film can cover the entire electrode surface during the densification growth stage.

[0056] During the low-temperature gradient formation stage, the applied pressure is 0.25–0.35 MPa, decreasing by 0.01 MPa for every 0.1V increase in cell voltage. This stage is the essence of the dynamic pressure control in this invention, and its design fully considers the special mechanical behavior of the prismatic cell in a low-temperature environment. When the temperature drops from the mid-temperature range of 35–40°C to the low-temperature range of -18–-22°C, the electrode material, separator, and electrolyte all undergo varying degrees of thermal shrinkage. The rigid shell of the prismatic cell restricts the thermal shrinkage of the electrode in the in-plane direction, but the shrinkage of the electrode in the thickness direction still exists, leading to a decrease in the interfacial pressure between the electrode and the separator, and even microscopic interfacial delamination and increased porosity. If the high pressure of 0.3–0.4 MPa from the mid-temperature activation stage is maintained during this stage, stress concentration at the interface due to electrode shrinkage may occur, causing mechanical damage to the SEI film.

[0057] The dual pressure regulation strategy employed in this invention effectively solves the aforementioned problems. First, the pressure is moderately reduced from 0.3–0.4 MPa to 0.25–0.35 MPa to accommodate the thickness reduction caused by electrode shrinkage at low temperatures, avoiding excessive pressure that could generate excessive compressive stress at the shrinkage interface and prevent the SEI film from cracking or delaminating under mechanical stress. Second, a voltage-linked regulation mechanism is adopted, where the pressure decreases by 0.01 MPa for every 0.1V increase in cell voltage. The design logic is as follows: during the low-temperature gradient formation stage, as charging progresses, the cell voltage continuously increases (gradually from 3.45V to 3.5V and eventually approaches the preset capacity). Lithium ions continuously intercalate between the graphite layers of the negative electrode, causing the negative electrode to expand. This increases the interlayer spacing of the graphite after lithium intercalation, resulting in increased electrode thickness. This lithium intercalation expansion and low-temperature shrinkage create opposing forces: low temperature causes electrode shrinkage, while lithium intercalation causes electrode expansion. By implementing voltage-linked regulation—with each 0.1V increase in voltage resulting in a 0.01MPa decrease in pressure—dynamic compensation is achieved for the two opposing effects mentioned above: when the voltage is low (lithium intercalation expansion is small, and low-temperature contraction is dominant), a higher pressure is maintained to compensate for the decrease in interfacial pressure caused by low-temperature contraction; as the voltage increases (lithium intercalation expansion gradually increases), the pressure is gradually reduced to avoid excessive interfacial pressure caused by the superposition of lithium intercalation expansion and external pressure. This mechanism ensures that the interfacial contact pressure between the electrode and the separator remains within the optimal range throughout the entire low-temperature gradient formation process, avoiding interfacial delamination and increased porosity.

[0058] The synergistic design of segmented pressurization and voltage-linked regulation fundamentally solves the interface contact problem caused by the rigid structure of prismatic cells. Using constant pressure cannot simultaneously meet the differentiated interface pressure requirements of different formation stages. The pretreatment and room-temperature pre-charge stages require moderate pressure to promote wetting and uniform nucleation; the intermediate-temperature activation stage requires higher pressure to reduce interface impedance and expel gases; and the low-temperature gradient formation stage requires dynamic voltage regulation to adapt to the low-temperature shrinkage and lithium intercalation expansion of the electrode. The dynamic pressurization control of this invention, through the organic combination of a three-stage pressure gradient and voltage-linked regulation, maintains a high degree of consistency and stability in the interface contact state between the electrode and the separator throughout the entire formation process. This provides an optimal mechanical environment for the uniform and dense growth of the SEI film, effectively avoiding the poor interface contact problem caused by the rigid structure of prismatic cells.

[0059] S4. Low-temperature aging after formation: After the segmented temperature change formation is completed, aging treatment is carried out in the third temperature range and under pressure holding conditions, and then the temperature is raised to the first temperature range. In this step, the aging process includes: After the segmented temperature-controlled formation is completed, the temperature of the lithium iron phosphate square-shell cell is maintained at -18 to -22℃, the pressure at 0.2 to 0.25 MPa, and the aging time is 4 to 6 hours. After the aging process is completed, the temperature of the lithium iron phosphate square cell is raised to the first temperature range at a heating rate of 0.5 to 1℃ / min, and then depressurized to atmospheric pressure; after depressurization, vacuum degassing is performed under vacuum conditions.

[0060] Specifically, low-temperature aging after formation is the final step in the formation process of this invention. Its design logic is as follows: After low-temperature gradient formation, the cell has just completed the final growth and stabilization of the SEI film, but the SEI film structure has not yet fully relaxed. Internal stress remains at the interface due to lithium-ion insertion / extraction and SEI film formation during the low-temperature formation process. If the cell is immediately heated to room temperature or directly enters the capacity testing process at this point, the sudden temperature change may cause microcracks in the SEI film due to differences in thermal expansion coefficients. The residual stress at the interface is gradually released in subsequent cycles, accelerating the structural deterioration of the SEI film. This invention, after segmented temperature-varying formation, directly performs low-temperature aging treatment within the third temperature range, allowing the SEI film to fully relax and stabilize its structure in an environment consistent with its formation temperature, thereby eliminating residual stress at the interface and improving the long-term structural stability of the SEI film.

[0061] The aging process specifically includes: after the segmented temperature-controlled formation, maintaining the temperature of the lithium iron phosphate prismatic cell at -18 to -22°C and the pressure at 0.2 to 0.25 MPa for 4 to 6 hours. The temperature setting in this step is completely consistent with the third temperature range (-18 to -22°C). Its technical significance lies in the fact that the SEI film has just completed its growth at the low temperature of -18 to -22°C, and the molecular arrangement and microstructure within the film layer have not yet reached thermodynamic equilibrium. If the temperature is changed at this time, the differences in the thermal expansion coefficients of the SEI film components will generate internal stress, potentially leading to micro-cracks or interface delamination within the film layer. Maintaining aging within the third temperature range allows the SEI film to fully relax under conditions without external current, further homogenizing lithium ions within the graphite layers, achieving ordered arrangement of the organic and inorganic components within the SEI film, and releasing and eliminating residual stress at the interface through microstructural rearrangement. The pressure was set at 0.2–0.25 MPa, slightly lower than the initial pressure of the low-temperature gradient formation stage (0.25–0.35 MPa), and comparable to the pressure level at the end of the voltage-linked regulation process. This ensured stable interfacial contact between the electrode and the separator during aging, preventing mechanical compressive stress damage to the SEI membrane due to excessive pressure, and avoiding interface relaxation due to excessively low pressure. The aging time of 4–6 hours ensured sufficient time for the SEI membrane to complete structural relaxation and internal stress elimination, while also considering the overall efficiency of the formation process.

[0062] After the aging process, the temperature of the lithium iron phosphate prismatic cell is raised to the first temperature range at a heating rate of 0.5–1℃ / min, and then depressurized to atmospheric pressure. This heating rate of 0.5–1℃ / min is consistent with the heating rate during the medium-temperature activation stage, ensuring a stable and controllable temperature change process and preventing micro-cracks in the SEI film due to thermal stress caused by excessively rapid heating. After reaching the first temperature range (23–27℃), the cell is returned to room temperature. Having undergone a complete temperature cycle from room temperature → medium temperature → low temperature → room temperature, the SEI film's structure has been sufficiently stabilized during the low-temperature aging process, and no new internal stress will be generated due to temperature changes after returning to room temperature. The pressure relief to atmospheric pressure operation is performed after the heating process is complete. Its technical significance lies in the fact that the cell remains pressurized (0.1–0.4 MPa) throughout the formation process to ensure interfacial contact between the electrode and the separator. After formation, the pressure has fulfilled its technological purpose, and the chamber pressure needs to be restored to atmospheric pressure to facilitate subsequent vacuum degassing and unloading operations. The pressure relief process should be carried out after the temperature has reached room temperature to avoid deformation of the cell casing or damage to its seal due to the pressure difference between the inside and outside of the casing caused by pressure relief at low temperatures.

[0063] After depressurization, vacuum degassing is performed under vacuum conditions. During the formation process, while the electrolyte undergoes a reduction decomposition reaction on the negative electrode surface to form an SEI film, a small amount of side reaction gases (mainly C2H4, CO, CO2, etc.) are inevitably generated. Some of these gases are squeezed out of the electrode interface and discharged through the exhaust system of the formation chamber during dynamic pressurization, but some gases remain dissolved in the electrolyte or remain in the tiny gaps inside the cell. If these residual gases are not removed in time after formation, they will occupy the effective space inside the cell, affecting the tightness of the adhesion between the electrode and the separator, and may cause problems such as local gas expansion and poor interface contact during subsequent cycles. Vacuum degassing applies a vacuum condition (negative pressure environment) in a closed formation chamber, causing the gases dissolved in the electrolyte to precipitate from the liquid phase under reduced pressure, while simultaneously extracting the gases remaining in the gaps inside the cell, thereby reducing the gas content inside the cell and ensuring that the interface between the electrode and the separator remains tightly adhered during subsequent cycles. Vacuum degassing is a standard procedure after the formation of lithium-ion cells and before packaging. Its connection with the preceding steps of this invention is as follows: After low-temperature aging and temperature-heating depressurization, the SEI film of the cell has been fully stabilized, the contact state of the electrode interface has been fixed, and the temperature has returned to room temperature. At this time, vacuum degassing can remove the side reaction gases generated during the formation process from the cell to the maximum extent, avoiding the adverse effects of gas residue on subsequent cycle performance.

[0064] Through the above-mentioned low-temperature aging treatment after formation, the SEI film has undergone a complete post-treatment path of "structural relaxation → internal stress elimination → stable heating → vacuum degassing" after low-temperature formation. Its structural stability and chemical stability have been significantly improved, the residual stress at the interface has been effectively eliminated, and the side reaction gases have been fully removed. This effectively reduces the risk of structural degradation and impedance growth of the SEI film in subsequent cycles, and provides a guarantee for the long-term low-temperature cycling stability of LFP prismatic cells.

[0065] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the invention. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0066] Example 1

[0067] This embodiment provides a segmented variable-temperature formation method for lithium iron phosphate prismatic cells, including: A 100Ah lithium iron phosphate prismatic cell (positive electrode: lithium iron phosphate, negative electrode: graphite, separator: PP / PE composite separator) was selected, with an electrolyte containing 1.0% FEC + 0.5% VC. The injected cell was placed in a sealed formation chamber, with the chamber dew point controlled at ≤-45℃ and oxygen content at ≤80ppm, and allowed to stand for 10 hours at 25℃ and a vacuum of -0.085MPa.

[0068] After the settling period, segmented temperature-dependent formation is performed. Room temperature pre-charge stage: At 25℃, charge to 3.2V with a constant current of 0.025C for 1.5 hours. Medium temperature activation stage: Heat to 38℃ at a rate of 0.8℃ / min, and charge to 3.4V with a constant current of 0.06C for 2.5 hours. Low temperature gradient formation stage: Cool to -20℃ at a rate of 0.4℃ / min, entering a multi-stage variable current charging mode.

[0069] The multi-stage converter charging mode is as follows: The first sub-stage involves constant current charging at 0.04C to 3.45V for 2 hours, followed by resting at -20℃ for 0.8 hours. The second sub-stage involves constant current charging at 0.09C to 3.5V for 1.5 hours, followed by resting at -20℃ for 0.8 hours. The third sub-stage involves micro-current supplementary charging at 0.025C to the preset capacity (9% of the design capacity, i.e., 9Ah) for 0.8 hours. After each sub-stage charging and resting period, a 0.015C current is used for discharging for 12 minutes.

[0070] Dynamic pressure is applied throughout the formation process: 0.15 MPa is applied during the cell pretreatment and room temperature pre-charge stages; 0.35 MPa is applied during the intermediate temperature activation stage; and 0.3 MPa is applied initially during the low temperature gradient formation stage, decreasing by 0.01 MPa for every 0.1 V increase in cell voltage.

[0071] After the low-temperature formation was completed, the material was aged at -20℃ and 0.22MPa for 24 hours. Then, the temperature was increased to 25℃ at a rate of 0.8℃ / min, the pressure was released to atmospheric pressure, and then vacuum degassing was performed at -0.085MPa for 45 minutes to complete the formation.

[0072] In comparison, this embodiment also includes a comparative example, using a traditional high-temperature formation method: the injected lithium iron phosphate square-shell cell was immersed in a 45°C environment for 10 hours, during which it was charged to 3.6V at a constant current of 0.1C and then left to stand for 2 hours; during the high-temperature formation stage, it was charged at a constant current of 0.05C for 2 hours, and then charged at a constant current of 0.1C for 2 hours; a constant pressure of 0.35MPa was applied throughout the process; after formation, it was aged at 45°C for 24 hours.

[0073] The battery cells prepared in this embodiment and the comparative embodiment were subjected to performance tests under the same conditions. The results showed that the battery cell in this embodiment had a discharge capacity of 68.9 Ah at 0.3C at -20℃, with a capacity retention rate of 69.9%; after 50 cycles at 0.2C at -20℃, the capacity retention rate was 75.1%; the low-temperature impedance at -20℃ of the comparative embodiment battery cell was 38% higher than that of the battery cell in this embodiment.

[0074] The test results above show that the formation method of this embodiment can significantly improve the low-temperature performance of lithium iron phosphate prismatic cells. It is significantly better than the traditional high-temperature formation process in terms of low-temperature discharge capacity, capacity retention rate and low-temperature impedance at -20℃, and effectively solves the technical problem of insufficient low-temperature performance caused by the traditional formation process.

[0075] Example 2

[0076] This embodiment provides a segmented variable-temperature formation method for lithium iron phosphate prismatic cells, including: A 100Ah lithium iron phosphate prismatic cell (positive electrode: lithium iron phosphate, negative electrode: graphite, separator: PP / PE composite separator) was selected, with an electrolyte containing 1.0% FEC + 0.5% VC. The injected cell was placed in a sealed formation chamber, with the chamber dew point controlled at ≤-40℃ and oxygen content at ≤100ppm, and allowed to stand for 8 hours at 23℃ and a vacuum of -0.08MPa.

[0077] After the settling period, segmented temperature-dependent formation is performed. Room temperature pre-charge stage: At 23℃, charge to 3.2V with a constant current of 0.02C for 2 hours. Medium temperature activation stage: Heat to 35℃ at a rate of 0.5℃ / min, and charge to 3.4V with a constant current of 0.05C for 3 hours. Low temperature gradient formation stage: Cool to -18℃ at a rate of 0.3℃ / min, entering a multi-stage variable current charging mode.

[0078] The multi-stage converter charging mode is as follows: The first sub-stage involves constant current charging at 0.03C to 3.45V for 2.5 hours, followed by resting at -18℃ for 0.5 hours. The second sub-stage involves constant current charging at 0.08C to 3.5V for 2 hours, followed by resting at -18℃ for 0.5 hours. The third sub-stage involves micro-current supplementary charging at 0.02C to the preset capacity (8% of the design capacity, i.e., 8Ah) for 1 hour. After each sub-stage charging and resting period, a 0.01C current is used for discharging for 15 minutes.

[0079] Dynamic pressure is applied throughout the formation process: 0.1 MPa is applied during the cell pretreatment and room temperature pre-charge stages; 0.3 MPa is applied during the intermediate temperature activation stage; and 0.25 MPa is applied initially during the low temperature gradient formation stage, decreasing by 0.01 MPa for every 0.1 V increase in cell voltage.

[0080] After the low-temperature formation was completed, the material was aged at -18℃ and 0.2MPa for 4 hours. Then, the temperature was increased to 23℃ at a rate of 0.5℃ / min, the pressure was released to atmospheric pressure, and then vacuum degassing was performed to complete the formation.

[0081] The battery cell prepared in this embodiment was compared with the comparative example described in Example 1 under the same conditions. The results showed that the battery cell of Example 2 had significantly better 0.3C discharge capacity and capacity retention rate than the comparative example at -20℃, and its low-temperature impedance at -20℃ was significantly lower than that of the comparative example. Both this embodiment and Example 1 achieved significantly better low-temperature performance improvement than the traditional high-temperature formation process under different process parameters.

[0082] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A segmented temperature-varying formation method for lithium iron phosphate prismatic cells, characterized in that, include: Cell pretreatment: The lithium iron phosphate square-shell cell after liquid injection is placed in a sealed formation chamber and left to stand under vacuum conditions; Segmented temperature-dependent formation: The lithium iron phosphate prismatic cell is sequentially pre-charged at room temperature in the first temperature range, activated at medium temperature in the second temperature range, and formed at low temperature in the third temperature range; in the low temperature gradient formation stage, the lithium iron phosphate prismatic cell is charged using a multi-stage variable current charging mode. Dynamic pressure control: During the pretreatment and segmented temperature-varying formation process, dynamic pressure is applied to the lithium iron phosphate prismatic cell as the formation stage and / or cell voltage changes. Low-temperature aging after formation: After the segmented temperature-varying formation is completed, aging treatment is carried out under the third temperature range and pressure holding conditions, and then the temperature is raised to the first temperature range. Among them, the third temperature range is <0℃ < the first temperature range < the second temperature range.

2. The segmented temperature-varying formation method for lithium iron phosphate prismatic cells according to claim 1, characterized in that, The step of settling the lithium iron phosphate prismatic cell under vacuum conditions includes: The dew point of the sealed formation chamber is controlled to be ≤-40℃ and the oxygen content to be ≤100ppm. The chamber is then left to stand for 8 to 12 hours at 25±2℃ and a vacuum of -0.08 to -0.09MPa.

3. The segmented temperature-varying formation method for lithium iron phosphate prismatic cells according to claim 1, characterized in that, The segmented temperature variation specifically includes: During the ambient temperature pre-charging stage, the temperature of the lithium iron phosphate square-shell cell is controlled within the first temperature range, and the lithium iron phosphate square-shell cell is charged to 3.2V using a constant current of 0.02 to 0.03C for 1 to 2 hours. During the medium-temperature activation stage, the temperature of the lithium iron phosphate square-shell cell is raised to the second temperature range at a temperature rise rate of 0.5 to 1℃ / min, and the lithium iron phosphate square-shell cell is charged to 3.4V using a constant current of 0.05 to 0.08C for 2 to 3 hours. In the low-temperature gradient formation stage, the temperature of the lithium iron phosphate prismatic cell is cooled to the third temperature range at a temperature drop rate of 0.3 to 0.5 °C / min, and the lithium iron phosphate prismatic cell is charged using a multi-stage converter charging mode.

4. The segmented temperature-varying formation method for lithium iron phosphate prismatic cells according to claim 3, characterized in that, The multi-stage converter charging mode includes: In the first sub-stage, the lithium iron phosphate square-shell cell is charged to 3.45V using a constant current of 0.03 to 0.05C for 1.5 to 2.5 hours. In the second sub-stage, the lithium iron phosphate square-shell cell is charged to 3.5V using a constant current of 0.08 to 0.1C for 1 to 2 hours. In the third sub-stage, the lithium iron phosphate prismatic cell is micro-charged to a preset capacity using a current of 0.02–0.03C for a charging time of 0.5–1 hour.

5. The segmented temperature-varying formation method for lithium iron phosphate prismatic cells according to claim 1, characterized in that, The dynamic pressurization control specifically includes: During the cell pretreatment and room temperature pre-charging stages, a pressure of 0.1–0.2 MPa is applied. During the intermediate-temperature activation stage, the applied pressure is 0.3–0.4 MPa; During the low-temperature gradient formation stage, the applied pressure is 0.25 to 0.35 MPa, and decreases by 0.01 MPa for every 0.1 V increase in cell voltage.

6. The segmented temperature-varying formation method for lithium iron phosphate prismatic cells according to claim 1, characterized in that, The aging process includes: After the segmented temperature-controlled formation is completed, the temperature of the lithium iron phosphate square-shell cell is maintained at -18 to -22°C, the pressure at 0.2 to 0.25 MPa, and the aging time is 4 to 6 hours. After the aging process is completed, the temperature of the lithium iron phosphate square-shell cell is raised to the first temperature range at a heating rate of 0.5 to 1℃ / min, and then depressurized to atmospheric pressure; after depressurization, vacuum degassing is performed under vacuum conditions.

7. The formation method for improving the low-temperature performance of lithium iron phosphate prismatic cells according to claim 1 or 3, characterized in that, The first temperature range is 23 to 27°C; the second temperature range is 35 to 40°C; and the third temperature range is -18 to -22°C.

8. The formation method for improving the low-temperature performance of lithium iron phosphate prismatic cells according to claim 4, characterized in that, The preset capacity is 8% to 10% of the design capacity.

9. The segmented temperature-varying formation method for lithium iron phosphate prismatic cells according to claim 4, characterized in that, It also includes post-processing steps after charging each sub-stage: After the first sub-stage charging is completed, the cell is left to stand for 0.5 to 1.0 hours in the third temperature range, and then discharged for 10 to 15 minutes using a current of 0.01 to 0.02C. After the second sub-stage charging is completed, the cell is first left to stand in the third temperature range for 0.5 to 1.0 hours, and then discharged with a current of 0.01 to 0.02C for 10 to 15 minutes. After the third sub-stage charging is completed, the battery is first left to stand in the third temperature range for 0.5 to 1.0 hours, and then discharged with a current of 0.01 to 0.02C for 10 to 15 minutes.

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