A quick formation process for lithium iron phosphate-graphite system battery cell
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- 天能新能源(湖州)有限公司
- Filing Date
- 2026-04-15
- Publication Date
- 2026-08-07
AI Technical Summary
然而，该工艺技术充电耗时需要4~12h，长时间占用化成柜导致产能受限，并且过长低倍率工序会导致膜层阻抗增大，从而对电池倍率性能等有潜在的负面影响
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of lithium battery technology, specifically relating to an improved rapid formation process for lithium iron phosphate-graphite system cells. Background Technology
[0002] In the manufacturing process of lithium-ion batteries, the formation process is the first charging stage after electrolyte injection. The purpose of formation is to generate a solid electrolyte interphase (SEI) film on the negative electrode surface and activate the active materials of both the positive and negative electrodes. An ideal SEI film structure possesses electronic insulation properties, effectively preventing direct contact between the electrolyte and graphite and avoiding continuous electrolyte decomposition. Furthermore, it exhibits excellent lithium-ion transport performance, enabling the battery to operate normally during charging and discharging. Additionally, an ideal SEI film structure should also possess sufficient mechanical strength and good stability.
[0003] The process conditions of the formation process can significantly affect the properties of the SEI film, and thus the performance of the battery cell. Current formation technologies mainly use low current density process conditions to generate a dense and highly stable SEI film structure. However, this process technology requires 4 to 12 hours of charging, which occupies the formation cabinet for a long time, resulting in limited production capacity. Furthermore, excessively long low-rate processes can lead to increased film impedance, which may have a potential negative impact on battery rate performance. Summary of the Invention
[0004] Based on the problems mentioned in the background technology, this invention proposes a method to improve the formation efficiency of lithium iron phosphate-graphite system battery cells. Targeting the characteristics of the system and the formation principle, a strategy of initial nucleation of the SEI film with low current density and repair of the SEI film with high current density is adopted. Furthermore, the matching of the formation charging regime with temperature and electrolyte additives is improved. While ensuring the stability of the SEI film and the electrochemical performance of the battery cell, the formation time is shortened and the formation efficiency is significantly improved.
[0005] This invention is achieved through the following technical solution:
[0006] A rapid formation process for lithium iron phosphate-graphite battery cells includes the following steps:
[0007] S1. Assemble the battery cell and inject electrolyte to obtain a newly manufactured lithium iron phosphate-graphite battery cell;
[0008] S2. Let the newly prepared lithium iron phosphate-graphite battery cell stand for 12~36 hours;
[0009] S3. Charge at a constant current of 0.01~0.05C for 10~30 minutes, with a cutoff voltage of 3.45~3.60V;
[0010] S4. Charge at a constant current of 0.50C for 70~90 minutes, with a cutoff voltage of 3.50~3.70V;
[0011] S5, stationary battery cell.
[0012] This invention proposes a rapid formation process for lithium iron phosphate-graphite battery cells. The post-electrolyte settling step promotes full electrolyte wetting of the electrodes, ensuring sufficient contact between the electrodes and the active material surface, providing a good substrate for the formation of a uniform and complete SEI film. Step S3 employs a low-current slow charge to maintain the film-forming side reactions on the negative electrode surface at a low rate, ensuring the formation of uniform film-forming nuclei on the graphite surface, and limiting the voltage to no more than 3.50V to prevent solvent molecules from embedding into the graphite crystals and suppress lithium plating side reactions. Step S4 employs a high-current fast charge close to actual operating conditions to further improve the SEI film structure and complete the activation of the positive and negative electrode materials.
[0013] Preferably, step S3 includes:
[0014] S3.1, Charge at a constant current of 0.01C for 5~15 minutes, with a cutoff voltage of 3.45~3.50V;
[0015] S3.2. Charge at a constant current of 0.05C for 5~15 minutes, with a cutoff voltage of 3.45~3.60V.
[0016] Preferably, the formation temperature in S3 to S4 is 35°C to 50°C.
[0017] The formation temperature selected in this scheme is higher than that in conventional technologies, which effectively promotes the formation reaction of the SEI film. Furthermore, the high temperature and low polarization conditions can suppress the lithium plating side reaction that may be caused by subsequent high-rate charging steps. This scheme also controls the upper limit of the formation temperature to avoid the aggravation of side reactions under higher temperature conditions, thereby improving the first coulombic efficiency of the cell and reducing the internal resistance of the cell.
[0018] Preferably, in step S5, the formation temperature is maintained and the mixture is left to stand for 1 to 10 minutes.
[0019] After charging is complete, the lithium-ion concentration gradient quickly returns to equilibrium, while the formation and maturation of the SEI film continue. S5 enables these active materials to quickly complete the final surface repair and charge relaxation in situ, providing the SEI film with repair time, filling in the minor damage generated during charging, and improving the integrity and stability of the film.
[0020] Preferably, the switching time interval between S3.1 and S3.2 is ≤2 min.
[0021] Shortening the waiting time between the first and second stages in S3 prevents the lithium-ion concentration gradient at the electrode interface from completely disappearing during the switching gap, avoiding the deintercalation of lithium ions already embedded on the electrode surface due to the disappearance of the electric field, reducing repeated formation and damage of the SEI film, and ensuring the continuity of the SEI film structure. Rapid switching makes the current change during charging more stable, reducing local overpotential fluctuations caused by current interruption and suppressing the risk of lithium dendrite precipitation. This scheme maintains the high reactivity of the electrode surface and the continuity of the overpotential history, allowing the 0.05C stage to more effectively inherit the film formation effect of 0.01C.
[0022] Preferably, the battery cell in S1 is a pouch cell, and the battery cell pressure in S3 is maintained at 0.1~0.3MPa.
[0023] For pouch cells, higher charging rates in the later stages can easily cause expansion. Applying positive pressure ensures tight contact between the electrodes, suppressing volume expansion during the formation of the SEI film on the negative electrode, preventing poor contact between particles, and simultaneously promoting the uniformity and density of the SEI film, while also quickly expelling gas. The pressure also promotes electrolyte wetting in the electrode pores, compensating for insufficient wetting during static S2 loading, resulting in higher utilization of active materials. Closer contact between the electrodes, separator, and electrolyte reduces interfacial gaps and improves ion conduction efficiency.
[0024] Preferably, the battery cell in S1 is an aluminum-cased battery cell, and the battery cell pressure is maintained at a negative pressure of 10~40kPa in S3.
[0025] For aluminum-cased battery cells, negative pressure allows the electrolyte to penetrate deeper into the electrodes more quickly under the pressure difference, improving wetting efficiency. It also promotes tighter adhesion between the electrode plates and the separator, reducing interfacial resistance. High-temperature environments cause internal gas expansion, while negative pressure allows these gases to be expelled promptly and absorbs gases generated during the formation process. This eliminates gas expansion, improves the consistency of SEI film formation, and ensures the dimensional stability of the battery cell.
[0026] Preferably, the electrolyte components include: an EC / DEC / DMC solution, wherein the mass ratio of EC, DEC, and DMC is 1:(0.8~1.2):(0.8~1.2); LiPF6, with a concentration of 0.8~1.2 mol / L in the electrolyte; vinylene carbonate (VC), with a concentration of 1.0~5.0 wt% in the electrolyte; and fluoroethylene carbonate (FEC), with a concentration of 2.0~6.0 wt% in the electrolyte.
[0027] Vinylene carbonate forms an SEI film primarily composed of polymer (polyvinyl carbonate) on the negative electrode surface, improving the mechanical strength and flexibility of the SEI and preventing capacity decay due to rupture during cycling. Fluorinated vinyl carbonate, with its fluorine atoms, further reduces the internal resistance of the SEI film and improves its protection of the aluminum current collector, thereby enhancing high-temperature stability.
[0028] Preferably, the electrolyte in S1 includes a tetralithium salt of 1,2-vinyl diphosphonate.
[0029] The decomposition potential of 1,2-vinyl diphosphonate tetralithium salt (Li-VDPA) is lower than that of conventional solvent components, enabling preferential electrochemical reduction reactions during the early stages of formation. From a molecular structure perspective, its phosphonate groups (-PO4Li2) can chemically adsorb onto the graphite anode surface, constructing a stable phosphorus-oxygen bond (PO) network structure during film formation. This structure exhibits high chemical stability and mechanical strength, effectively suppressing the thermal and electrochemical decomposition of the SEI film during cycling. Simultaneously, the vinyl groups (-CH=CH2) in the molecular chain endow the SEI film with organic polymer properties, significantly improving its flexibility and interfacial adhesion, buffering the mechanical stress generated by the volume changes of the graphite anode during charge and discharge, and reducing the continuous consumption of active lithium caused by SEI film rupture and regrowth. This organic-inorganic hybrid structure achieves a balance between SEI film stability and flexibility, improving the long-cycle performance of the electrode.
[0030] Preferably, the mass of the 1,2-vinyl diphosphonate tetralithium salt added to the electrolyte is 1 to 5% of the mass of LiPF6.
[0031] Preferably, the preparation method of the tetralithium 1,2-vinyl diphosphonate includes:
[0032] Add 50-150 parts by mass of 1,2-vinyldiphosphonic acid to 1800-2200 parts by mass of deionized water and stir for 25-35 minutes until completely dissolved. At 20-30°C, slowly add 130-160 parts by mass of 5-15 wt% LiOH·H2O aqueous solution to the solution, and stir the reaction for 1.5-2.5 hours after the addition is complete. Filter the reaction solution through a 0.22 μm filter membrane to remove insoluble matter. Evaporate the filtrate under reduced pressure at 55-65°C until it becomes viscous. Add 700-900 parts by mass of anhydrous methanol and stir to precipitate crystals. Filter and collect the solid phase. Dry the solid phase under vacuum at 75-85°C for 10-14 hours to obtain tetralithium 1,2-vinyldiphosphonic acid.
[0033] The present invention also includes a lithium iron phosphate-graphite system battery cell, the formation method of which includes the rapid formation process of lithium iron phosphate-graphite system battery cells described in any of the preceding claims.
[0034] Combining the aforementioned rapid formation process, the manufactured battery cells possess both the advantage of a short formation cycle and excellent cycle stability and rate performance. The adaptability to different processes also meets the needs of battery cells with different structures, such as pouch cells and aluminum-cased cells, improving product applicability. Attached Figure Description
[0035] Figure 1 The image shows the SEM characterization of the negative electrode (graphite electrode sheet) after disassembly following the capacity testing and production line completion (after cell fabrication) in Example 1.
[0036] Figure 2 The image shows the SEM characterization of the negative electrode (graphite electrode sheet) after disassembly following the capacity testing and production line completion (after the cell fabrication is finished) in Example 2.
[0037] Figure 3 For Comparative Example 1, after capacity testing (cell fabrication completed), the negative electrode (graphite electrode sheet) was disassembled and displayed using SEM.
[0038] Figure 4 For Comparative Example 2, after capacity testing (cell fabrication completed), the negative electrode (graphite electrode sheet) was disassembled and displayed using SEM.
[0039] Figure 5 For Comparative Example 3, after capacity testing (cell fabrication completed), the negative electrode (graphite electrode sheet) was disassembled and displayed using SEM.
[0040] Figure 6 For Comparative Example 4, after capacity testing (cell fabrication completed), the negative electrode (graphite electrode sheet) was disassembled and displayed using SEM.
[0041] Figure 7 The figures show the results of room temperature cycling for the examples and comparative examples. Detailed Implementation
[0042] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.
[0043] The steps and methods involved in the embodiments and comparative examples are described below.
[0044] Cell fabrication: A 5.5Ah soft-pack cell with lithium iron phosphate (LiFePO4) as the positive electrode active material and artificial graphite as the negative electrode active material is prepared. The cell is fabricated using a series of conventional lithium-ion battery manufacturing processes, including slurry preparation, coating, rolling, die-cutting, stacking, assembly, and electrolyte injection.
[0045] Electrolyte preparation: Based on LiPF6-EC / DEC / DMC, film-forming additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added, along with tetralithium 1,2-vinyl diphosphonate. The electrolyte injection coefficient was between 4.2 and 5 g / Ah. After injection, the electrolyte was immersed in a 45°C oven for 24 hours.
[0046] The positive electrode active material of the battery cell is lithium iron phosphate, and the positive electrode material contains 92 wt% LiFePO4, 5 wt% carbon black as a conductive agent, and 3 wt% PVDF as a binder.
[0047] The active material of the negative electrode of the battery cell is artificial graphite. The negative electrode ingredients consist of 92 wt% artificial graphite, 5 wt% conductive agent, and 3 wt% binder.
[0048] The electrolyte is composed of LiPF6 dissolved in EC / DEC / DMC, wherein the concentration of LiPF6 is 0.9 mol / L, and the electrolyte contains film-forming additives in the following proportions: 3.5 wt% vinylene carbonate, 4.5 wt% fluoroethylene carbonate, and 1-5% by mass of tetralithium 1,2-vinyl diphosphonate.
[0049] The mass ratio of EC, DEC, and DMC in the electrolyte is 1:1:1.
[0050] Preparation method of tetralithium 1,2-vinyldiphosphonate:
[0051] 100g of 1,2-vinyldiphosphonic acid was added to 2000g of deionized water and stirred for 30 minutes until completely dissolved. At 25°C, 145g of a 10% (w / w) LiOH·H2O aqueous solution was slowly added dropwise to the solution, and the reaction was stirred for 2 hours after the addition was completed. The reaction solution was filtered through a 0.22μm filter membrane to remove insoluble matter. The filtrate was evaporated under reduced pressure at 60°C until it became viscous. 800g of anhydrous methanol was added and stirred to induce crystallization. The crystals were collected by filtration and a white solid was collected. The white solid was placed in a vacuum drying oven and dried at 80°C for 12 hours to obtain tetralithium 1,2-vinyldiphosphonic acid.
[0052] Example 1
[0053] This embodiment provides a rapid formation process for lithium iron phosphate-graphite system cells, the steps of which are as follows.
[0054] Following the aforementioned steps and methods, the battery cell is assembled and an electrolyte is injected, the electrolyte of which does not include 1,2-vinyl diphosphonate tetralithium salt.
[0055] Turn on the formation equipment, set the temperature to 45℃, and preheat the instrument for 10 minutes.
[0056] The battery is placed in the formation cabinet, the pressure is set to 0.3MPa, and the cell is preheated for 15 minutes;
[0057] First stage: Charge at a constant current of 0.01C for 10 minutes, with a charging cutoff voltage of 3.45V;
[0058] Second stage: Switch to 0.05C constant current charging for 10 minutes, cutoff voltage 3.45V;
[0059] Third stage: Switch to 0.50C constant current charging for 83 minutes, maintaining a cutoff voltage of 3.65V;
[0060] After stopping charging, keep the formation temperature at rest for 2 minutes to finish.
[0061] Example 2
[0062] The difference between this embodiment and Example 1 is that the electrolyte includes tetralithium 1,2-vinyl diphosphonate. The formation process is the same as in Example 1.
[0063] Comparative Example 1
[0064] In this comparative example, the electrolyte did not contain tetralithium 1,2-vinyldiphosphonate.
[0065] Turn on the formation equipment, set the temperature to 45℃, and preheat the instrument for 10 minutes.
[0066] The battery is placed in the formation cabinet, the pressure is set to 0.3MPa, and the cell is preheated for 15 minutes;
[0067] First stage: Charge at a constant current of 0.01C for 10 minutes, with a charging cutoff voltage of 3.45V;
[0068] Second stage: Switch to 0.05C constant current charging for 60 minutes, cutoff voltage 3.45V;
[0069] Third stage: Switch to 0.50C constant current charging for 78 minutes, maintaining a cutoff voltage of 3.65V;
[0070] After stopping charging, keep the formation temperature at rest for 2 minutes to finish.
[0071] Comparative Example 2
[0072] In this comparative example, the electrolyte did not contain tetralithium 1,2-vinyldiphosphonate.
[0073] Turn on the formation equipment, set the temperature to 45℃, and preheat the instrument for 10 minutes.
[0074] The battery is placed in the formation cabinet, the pressure is set to 0.3MPa, and the cell is preheated for 15 minutes;
[0075] First stage: Charge at a constant current of 0.01C for 10 minutes, with a charging cutoff voltage of 3.45V;
[0076] Second stage: Switch to 0.05C constant current charging for 240 minutes, cutoff voltage 3.45V;
[0077] Third stage: Switch to 0.50C constant current charging for 60 minutes, maintaining a cutoff voltage of 3.65V;
[0078] After stopping charging, keep the formation temperature at rest for 2 minutes to finish.
[0079] Comparative Example 3
[0080] In this comparative example, the electrolyte did not contain tetralithium 1,2-vinyldiphosphonate.
[0081] Turn on the formation equipment, set the temperature to 45℃, and preheat the instrument for 10 minutes.
[0082] The battery is placed in the formation cabinet, the pressure is set to 0.3MPa, and the cell is preheated for 15 minutes;
[0083] First stage: constant current charging at 0.05C for 36 minutes, charging cut-off voltage 3.45V;
[0084] Second stage: Switch to 0.10C constant current charging for 220 minutes, cut-off voltage 3.65V;
[0085] After stopping charging, keep the formation temperature at rest for 2 minutes to finish.
[0086] Comparative Example 4
[0087] In this comparative example, the electrolyte did not contain tetralithium 1,2-vinyldiphosphonate.
[0088] Turn on the formation equipment, set the temperature to 45℃, and preheat the instrument for 10 minutes.
[0089] The battery is placed in the formation cabinet, the pressure is set to 0.3MPa, and the cell is preheated for 15 minutes;
[0090] First stage: constant current charging at 0.05C for 36 minutes, charging cut-off voltage 3.45V;
[0091] Second stage: Switch to 0.10C constant current charging for 580 minutes, cutoff voltage 3.65V;
[0092] After stopping charging, keep the formation temperature at rest for 2 minutes to finish.
[0093] The key electrochemical performance of the examples and comparative examples was tested, and the results are shown in Table 1.
[0094] Table 1 Results of Key Electrochemical Performance Tests Initial charge capacity / (Ah) 5.8789 5.8871 5.9265 5.9134 5.6469 5.7969 Initial charge gram capacity / (mAh / g) 159.73 159.95 161.02 160.66 153.42 157.50 Initial discharge capacity / (Ah) 5.2399 5.2521 5.2338 5.2322 5.0629 5.0051 Initial discharge gram capacity / (mAh / g) 142.37 142.70 142.20 142.16 137.56 135.99 Initial coulombic efficiency 89.13% 89.21% 88.31% 88.48% 89.66% 86.34% Ohmic resistance after sorting / (mΩ) 3.64 3.43 3.59 3.56 3.73 3.62 K value / (mV / h) 0.0167 0.0146 0.0146 0.0292 0.0271 0.0104 50% SOC HPPC discharge resistance / (mΩ) 16.10 15.89 16.17 15.92 16.40 16.18 50% SOC HPPC charge resistance / (mΩ) 16.48 16.25 16.60 16.34 16.90 16.62 0.5P energy efficiency 94.86% 94.90% 94.85% 94.84% 94.90% 94.88% Formation step length / (min) 120 120 165 327 273 633
[0095] As shown in Table 1, both Example 1 (without 1,2-vinyl diphosphonate tetralithium salt) and Example 2 (containing 1,2-vinyl diphosphonate tetralithium salt) adopted a three-stage short-time rapid formation process with a total time of only 120 min. Compared with the traditional long-time formation of 165-633 min in Comparative Examples 1-4, the time was significantly shortened, only 1 / 5 to 1 / 2 of the traditional process, which greatly improved production efficiency.
[0096] In terms of electrochemical performance, Examples 1 and 2 showed the best overall performance. Appropriately increasing the formation charge rate under high-temperature conditions had no negative impact on the key electrochemical performance of the cell. Specifically, the initial charge capacity (5.8789 Ah, 5.8871 Ah) and specific charge capacity (159.73 mAh / g, 159.95 mAh / g) of Examples 1 and 2 were slightly lower than those of Comparative Examples 1 and 2, but their initial discharge capacity (5.2399 Ah, 5.2521 Ah) and specific discharge capacity (159.73 mAh / g, 159.95 mAh / g) were significantly higher. The 42.37mAh / g and 142.70mAh / g values are superior to all comparative examples, especially significantly higher than the corresponding values of comparative examples 3 and 4; the initial coulombic efficiency reaches 89.13% and 89.21%, higher than comparative examples 1, 2, and 4, and only slightly lower than comparative example 3, which incurred an ultra-long formation time; the 0.5P charge-discharge energy efficiency is approximately 94.90%, similar to the results of other comparative examples; the K-value test results also meet the commercial soft-pack battery standard (≤0.03mV / h); the AC internal resistance after capacity grading and 50% The DC internal resistance results under SOC conditions are comparable to those of the comparative examples. In Example 2, the introduction of tetralithium 1,2-vinyl diphosphonate, whose carbon-carbon double bonds and phosphonate groups have low least occupied molecular orbital energy levels, allows it to preferentially bind to active sites on the graphite surface. During the formation and charging process, it decomposes preferentially on the graphite surface compared to the solvent, leading to polymerization and deposition on the graphite surface to form a uniform and dense SEI film. Simultaneously, tetralithium 1,2-vinyl diphosphonate can synergistically interact with the film-forming additives vinyl carbonate (VC) and fluoroethylene carbonate (FEC) in the electrolyte, further optimizing the SEI film formation efficiency and quality. Therefore, the ohmic and DC internal resistance test results of Example 2 are relatively lower. Furthermore, the performance differences between Examples 1 and 2 are small, indicating that this rapid formation process itself already possesses strong advantages. The addition of tetralithium 1,2-vinyl diphosphonate can further optimize film formation properties and reduce the internal impedance of the battery cell.
[0097] Figure 7 Based on the results of ambient temperature cycling, Examples 1 and 2 showed the best cycle capacity retention, which was significantly better than Comparative Examples 3 and 4.
[0098] In summary, the rapid formation process for lithium iron phosphate-graphite system cells provided in this experiment (three-stage stepped current, 120-minute short formation time) significantly shortens the formation time and improves production efficiency without sacrificing, and may even optimize, the core electrochemical performance of the cells. Furthermore, this improved process is fully compatible with existing mass production equipment and possesses outstanding industrial application value. The addition of tetralithium 1,2-vinyl diphosphonate can serve as an auxiliary means for further optimization. Compared with traditional long-time formation methods, this method significantly reduces the time, significantly improves production efficiency, maintains electrochemical performance, and even surpasses some indicators.
Claims
1. A rapid formation process for lithium iron phosphate-graphite system battery cells, characterized in that, Includes the following steps: S1. Assemble the battery cell and inject electrolyte to obtain a newly manufactured lithium iron phosphate-graphite battery cell; S2. Let the newly prepared lithium iron phosphate-graphite battery cell stand for 12~36 hours; S3. Charge at a constant current of 0.01~0.05C for 10~30 minutes, with a cutoff voltage of 3.45~3.60V; S4. Charge at a constant current of 0.50C for 70~90 minutes, with a cutoff voltage of 3.50~3.70V; S5, stationary battery cell.
2. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 1, characterized in that, Step S3 includes: S3.1, Charge at a constant current of 0.01C for 5~15 minutes, with a cutoff voltage of 3.45~3.50V; S3.
2. Charge at a constant current of 0.05C for 5~15 minutes, with a cutoff voltage of 3.45~3.60V.
3. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 1, characterized in that, In S3~S4, the formation temperature is 35℃~50℃.
4. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 1, characterized in that, In step S5, the formation temperature is maintained, and the mixture is left to stand for 1 to 10 minutes.
5. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 2, characterized in that, The switching time interval between S3.1 and S3.2 is ≤2 min.
6. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 1, characterized in that, The battery cell in S1 is a pouch cell, and the battery cell pressure is maintained at 0.1-0.3 MPa in S3-S4.
7. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 1, characterized in that, The battery cell in S1 is an aluminum-cased battery cell, and the battery cell pressure is maintained at a negative pressure of 10-40 kPa in S3-S4.
8. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 1, characterized in that, The electrolyte in S1 includes a tetralithium salt of 1,2-vinyl diphosphonate.
9. The rapid formation process for lithium iron phosphate-graphite system cells according to claim 8, characterized in that, The preparation method of the tetralithium 1,2-vinyl diphosphonate includes: Add 50-150 parts by mass of 1,2-vinyldiphosphonic acid to 1800-2200 parts by mass of deionized water and stir for 25-35 minutes until completely dissolved. At 20-30°C, slowly add 130-160 parts by mass of 5-15 wt% LiOH·H2O aqueous solution to the solution, and stir the reaction for 1.5-2.5 hours after the addition is complete. Filter the reaction solution through a 0.22 μm filter membrane to remove insoluble matter. Evaporate the filtrate under reduced pressure at 55-65°C until it becomes viscous. Add 700-900 parts by mass of anhydrous methanol and stir to precipitate crystals. Filter and collect the solid phase. Dry the solid phase under vacuum at 75-85°C for 10-14 hours to obtain tetralithium 1,2-vinyldiphosphonic acid.
10. A lithium iron phosphate-graphite battery cell, characterized in that, Its formation method includes the rapid formation process of lithium iron phosphate-graphite system battery cells as described in any one of claims 1 to 9.