Formation method of lithium manganese iron phosphate battery

Abstract

The invention discloses a formation method of a lithium manganese iron phosphate battery, and relates to the technical field of lithium ion batteries. The charging process of the lithium manganese iron phosphate battery formation process is divided into three stages, the first stage and the second stage adopt stepped small current with a specific range and a proper difference value to perform constant-current charging, and the third stage adopts large charging current to perform constant-current and constant-voltage full charging, so that on the premise that the structural stability of an SEI film is not damaged, the service life of the SEI film is prolonged, and the service life of the SEI film is prolonged. And the cycle performance, high and low temperature discharge performance and charge capacity of the battery are comprehensively improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a formation method for a lithium manganese iron phosphate battery. Background Technology

[0002] In the manufacturing process of lithium-ion batteries, the formation process is a crucial step. This step includes pre-formation after electrolyte filling, formation aging, and formation full charge. These processes activate the active materials in the battery, thus activating the lithium-ion battery. Simultaneously, lithium salts undergo side reactions with the electrolyte solvent, forming a dense solid electrolyte interphase (SEI) film at the negative electrode of the lithium-ion battery. This film inhibits further side reactions, thereby reducing the loss of lithium content in the lithium-ion battery.

[0003] Lithium manganese iron phosphate (LMFP) cathode material is obtained by doping lithium iron phosphate (LFP) cathode material with manganese metal. Because Mn... 2 + / Mn 3+ The presence of redox active pairs results in an average voltage plateau of 4.3V, with an energy density exceeding that of LFP by 20%. Therefore, the emergence of lithium manganese iron phosphate (LFP) cathode materials further enhances the energy density advantage of olivine-structured cathode materials. A significant difference between LFP and LFP cathodes lies in the particle size, which is an order of magnitude smaller (average particle size of LFP is 200–400 nm). This results in numerous pores between primary and secondary particles, leading to higher water content and greater difficulty in drying. This also causes more significant side reactions during electrolyte contact, posing a significant challenge to the stability of the formed SEI film and thus a greater impact on lithium deposition on the negative electrode. Because LFP has a wider operating voltage range and a smaller particle size, its larger specific surface area results in greater electrolyte consumption and more significant side reactions. The generated HF repeatedly damages the SEI film. Simultaneously, the higher charging cutoff voltage requires the SEI film to possess better mechanical elasticity and stability to ensure the free movement of active lithium during charge-discharge cycles. To better address these issues, it is necessary to develop a formation process suitable for LFP materials. Summary of the Invention

[0004] Based on the technical problems existing in the background technology, the present invention proposes a formation method for lithium manganese iron phosphate batteries. The SEI film formed by this method is dense and stable, and has good lithium-ion transport performance, thereby effectively improving the discharge specific capacity, cycle performance and high and low temperature performance of lithium manganese iron phosphate batteries.

[0005] The present invention proposes a formation method for lithium manganese iron phosphate batteries, comprising the following steps:

[0006] S1. The lithium iron phosphate soft-pack battery cell is injected with electrolyte and soaked.

[0007] S2. The battery cell processed in S1 is first charged at 42~48℃ with a first charging current to 5%~20% SOC, and then charged with a second charging current to 45%~60% SOC. The first charging current is 0.02~0.05C, the second charging current is 0.05~0.08C, and the second charging current minus the first charging current is ≥0.03C.

[0008] S3. Allow the cells treated in S2 to age at 42~48℃ for 10~14 hours.

[0009] S4. Charge the battery cells treated in S3 at 20~40℃ with a constant current and constant voltage at a charging current of 0.3~0.5C until they reach 100% SOC.

[0010] This invention divides the charging process of lithium manganese iron phosphate battery formation into three stages. The first stage uses a constant current charging current of 0.02~0.05C to charge the battery to 5%~20% SOC of the battery capacity, which is used to form an initial solid electrolyte interphase (SEI) film on the negative electrode surface. The second stage uses a second charging current slightly higher than the first stage at 0.05~0.08C to charge the battery to 45%~60% SOC of the battery capacity, and controls the difference between the current in the second stage and the current in the first stage to be ≥0.03C. This stage is used to promote the formation of an interface structure with high ionic conductivity based on the initial SEI film. The third stage uses a large charging current of 0.3~0.5C for constant current and constant voltage full charging. Through a stepped initial small current charging step with a suitable difference, a well-covered basic SEI film is formed, reducing the loss of active lithium. The subsequent moderate current charging step optimizes the ion transport characteristics of the SEI film and reduces the interface impedance. Ultimately, this method comprehensively improves the battery's cycle performance, high and low temperature discharge performance, and charging capacity without compromising the stability of the SEI film structure.

[0011] Preferably, in step S1, after the electrolyte injection is completed and before immersion, the battery cell undergoes vacuum sealing to retain the gas bag. Retaining the gas bag allows for the storage of gases generated during the formation and capacity testing process, preventing damage to the electrodes inside the battery cell. The gas bag is removed after formation and capacity testing are completed.

[0012] In S1, the impregnation process parameters are conventionally selected and can be adjusted according to actual needs. Preferably, in S1, the impregnation is carried out at room temperature and pressure, and the impregnation time is 24~48 hours.

[0013] Preferably, in S1, the injection is performed at normal temperature and pressure.

[0014] Preferably, in step S1, after impregnation, foam is applied to both sides of the battery cell. The present invention does not specifically limit the parameters of the foam. Preferably, the foam has a single-sided thickness of 3-5 mm and is made of polyethylene.

[0015] Preferably, S2 is performed under a pressure of 500~800Kgf applied to the battery cell.

[0016] Preferably, S4 is performed under a pressure of 500~800Kgf applied to the battery cell.

[0017] Pressure can be applied to the battery cell using conventional methods, such as applying pressure through a clamp.

[0018] This invention applies higher pressure during the first and second stages of low-current gradient charging, which enhances the interfacial contact between the electrode and the separator, significantly reduces interfacial contact resistance, and promotes the formation of a uniform and dense SEI film, thus significantly improving cycle performance and safety. It also allows gases generated during the cell formation process to enter a retained gas bag, preventing gas accumulation within the cell that could lead to electrode-separation, effectively improving the cell's physical stability and electrochemical performance. Furthermore, the double-sided foam covering of the cell protects it from damage under the applied higher pressure and ensures consistent stress throughout the pouch cell, resulting in a uniform and well-defined electrode interface, thereby reducing interfacial contact resistance, improving charge-discharge uniformity, and enhancing rate and cycle performance.

[0019] Preferably, in step S2, there is a 5-10 minute rest period between the first constant current charging and the second constant current charging to ensure the electrolyte inside the cell is balanced and the initial state is stable.

[0020] Preferably, in S2, a rest period of 5-10 minutes is included before the first constant current charging, which allows for cell concentration polarization relaxation and redistribution, optimizes the film formation process, and facilitates gas pre-discharge.

[0021] Preferably, in step S2, after the second constant current charging, there is a rest period of 5 to 10 minutes, which can eliminate cell polarization and improve cell stability.

[0022] Preferably, in S2, the temperature of the resting period is 42~48°C.

[0023] Preferably, in step S3, the static aging is carried out under normal pressure.

[0024] Preferably, in S4, the cutoff voltage of the constant current and constant voltage charging is 4.3V, and the cutoff current is 0.05C.

[0025] The present invention also proposes a lithium manganese iron phosphate battery, wherein the preparation method of the lithium manganese iron phosphate battery includes the formation method described above.

[0026] The beneficial effects of this invention are as follows:

[0027] This invention divides the charging process of lithium manganese iron phosphate battery formation into three stages. The first and second stages employ a stepped, low-current constant-current charging within a specific range with suitable differences. The third stage uses a large charging current for constant-current and constant-voltage full charging. The initial low-current charging step within a specific range with suitable differences forms a well-covered basic SEI film, reducing active lithium loss. Subsequent moderate-current charging steps optimize the ion transport characteristics of the SEI film and reduce interfacial impedance. Ultimately, this method comprehensively improves the battery's cycle performance, high and low temperature discharge performance, and charging capacity without compromising the structural stability of the SEI film. Attached Figure Description

[0028] Figure 1 The images show the interface diagrams of the negative electrode sheet of the lithium manganese iron phosphate battery cell formed using the formation methods of Example 1 and Comparative Example 1, after non-cycle and high-temperature cycle.

[0029] Figure 2 The results show the high-temperature cycling performance of lithium manganese iron phosphate cells formed using the formation methods of Example 1 and Comparative Example 1. Detailed Implementation

[0030] The technical solution of the present invention will now be described in detail through specific embodiments.

[0031] Example 1

[0032] A formation method for a lithium manganese iron phosphate battery is as follows:

[0033] S1. The lithium iron phosphate soft-pack battery cell is injected with electrolyte at room temperature of 25°C. After the electrolyte injection is completed, the battery cell is vacuum sealed, retaining the air bag. Then, it is soaked at room temperature of 25°C for 48 hours. After the soaking is completed, polyethylene foam with a thickness of 5mm is covered on both sides of the battery cell. The electrolyte injection and soaking are carried out under normal pressure.

[0034] S2. Place the battery cell processed in S1 into a fixture, set the temperature inside the fixture to 45°C, and apply a pressure of 800 kgf through the fixture; let it rest for 10 minutes, then perform a first constant current charge with the first charging current to 19.8% SOC, let it rest for 5 minutes, then perform a second constant current charge with the second charging current to 46.3% SOC, and let it rest for 5 minutes. The first charging current is 0.05C and the second charging current is 0.08C.

[0035] S3. The battery cells treated in S2 are left to age at 45°C and normal pressure for 12 hours.

[0036] S4. Place the battery cell processed in S3 into the fixture, set the temperature inside the fixture to 25℃, and apply a pressure of 800Kgf through the fixture; charge it to 100% SOC with a constant current and constant voltage at a charging current of 0.33C, with a cutoff voltage of 4.3V and a cutoff current of 0.05C.

[0037] Assembly process of S1 lithium iron phosphate manganese phosphate soft-pack battery cells:

[0038] Positive electrode preparation: LMFP positive electrode material (Mn:Fe content of 6:4) is mixed with conductive agent and dispersant PVP (polyvinylpyrrolidone) and PVDF (polyvinylidene fluoride) adhesive to form a slurry. The obtained positive electrode slurry is coated on the substrate aluminum foil, and then rolled and slit to obtain the positive electrode sheet. The conductive agent is composed of conductive carbon black and single-walled carbon nanotubes in a mass ratio of 1:6. The mass ratio of LMFP positive electrode material to conductive agent, dispersant PVP and PVDF is 85:13:1:2.

[0039] Negative electrode preparation: Graphite and conductive agent are mixed with SBR (styrene-butadiene rubber) adhesive according to the designed ratio to form a slurry. The resulting negative electrode slurry is coated on a copper foil substrate and then rolled and slit to obtain the negative electrode sheet. The conductive agent is composed of conductive carbon black and single-walled carbon nanotubes in a mass ratio of 1:10. The mass ratio of graphite to conductive agent and SBR is 20:1:1.

[0040] Separator: Made of PE, with double-sided alumina ceramic coating.

[0041] Electrolyte: In a glove box where the H2O and O2 contents are both less than 0.01 ppm, EC, EMC, DEC, and DMC are mixed uniformly in a mass ratio of 30.38:69.04:0.26:0.21 to obtain a mixed solvent. LiPF6, LiPO2F2, LiDFOB, LiBOB, and LiFSI are then added to the mixed solvent to obtain the electrolyte; the concentration of LiPF6 is 1.25 mol·L⁻¹. -1 The concentration of LiPO2F2 was 0.02 mol·L. -1 The concentration of LiDFOB was 0.03 mol·L⁻¹. -1 The concentration of LiBOB was 0.12 mol·L⁻¹. -1 The concentration of LiFSI was 0.06 mol·L⁻¹. -1 .

[0042] The positive and negative electrode sheets and the separator are placed into an integrated stacking machine. They are stacked in the order of negative electrode sheet-separator-positive electrode sheet-separator-negative electrode sheet to form a soft-pack battery cell with 9 positive electrode sheets + 10 negative electrode sheets. Then, the cells are placed in an aluminum-plastic film, and the tabs are welded, the top and side are sealed, and the cells are baked until the internal moisture content is less than 300 ppm.

[0043] Example 2

[0044] A formation method for a lithium manganese iron phosphate battery is as follows:

[0045] S1. The lithium iron phosphate soft-pack battery cell is injected with electrolyte at room temperature of 25°C. After the electrolyte injection is completed, the battery cell is vacuum sealed, retaining the air bag. Then, it is soaked at room temperature of 25°C for 24 hours. After the soaking is completed, polyethylene foam with a single-sided thickness of 5mm is covered on both sides of the battery cell. The electrolyte injection and soaking are carried out under normal pressure.

[0046] S2. Place the battery cell processed in S1 into a fixture, set the temperature inside the fixture to 42℃, and apply a pressure of 800Kgf through the fixture; let it rest for 10 minutes, then perform a first constant current charge with the first charging current to 8.4% SOC, let it rest for another 10 minutes, then perform a second constant current charge with the second charging current to 47.1% SOC, and let it rest for another 10 minutes. The first charging current is 0.02C and the second charging current is 0.05C.

[0047] S3. The battery cells treated in S2 are left to age at 42°C and normal pressure for 10 hours.

[0048] S4. Place the battery cell processed in S3 into the fixture, set the temperature inside the fixture to 20℃, and apply a pressure of 800Kgf through the fixture; charge it to 100% SOC with a constant current and constant voltage at a charging current of 0.3C, with a cutoff voltage of 4.3V and a cutoff current of 0.05C.

[0049] The assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1 is the same as in Example 1.

[0050] Example 3

[0051] A formation method for a lithium manganese iron phosphate battery is as follows:

[0052] S1. The lithium iron phosphate soft-pack battery cell is injected with electrolyte at room temperature of 25°C. After the electrolyte injection is completed, the battery cell is vacuum sealed, retaining the air bag. Then, it is soaked at room temperature of 25°C for 36 hours. After the soaking is completed, polyethylene foam with a single-sided thickness of 3mm is covered on both sides of the battery cell. The electrolyte injection and soaking are carried out under normal pressure.

[0053] S2. Place the battery cell processed in S1 into a fixture, set the temperature inside the fixture to 48°C, and apply a pressure of 500 kgf through the fixture; let it rest for 5 minutes, then perform a first constant current charge with the first charging current to 8.4% SOC, let it rest for 5 minutes, then perform a second constant current charge with the second charging current to 47.5% SOC, and let it rest for 5 minutes. The first charging current is 0.03C and the second charging current is 0.07C.

[0054] S3. The battery cells treated in S2 are left to age at 48°C and normal pressure for 14 hours.

[0055] S4. Place the battery cell processed in S3 into a fixture, set the temperature inside the fixture to 40℃, and apply a pressure of 500Kgf through the fixture; charge it to 100% SOC with a constant current and constant voltage at a charging current of 0.5C, with a cutoff voltage of 4.3V and a cutoff current of 0.05C.

[0056] The assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1 is the same as in Example 1.

[0057] Comparative Example 1

[0058] The only difference between Comparative Example 1 and Example 1 is that in S2, the first charging current is 0.01C and the second charging current is 0.05C.

[0059] The remaining formation steps and parameters, as well as the assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1, are the same as in Example 1.

[0060] Comparative Example 2

[0061] The only difference between Comparative Example 2 and Example 1 is that in S2, the first charging current is 0.05C and the second charging current is 0.05C.

[0062] The remaining formation steps and parameters, as well as the assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1, are the same as in Example 1.

[0063] Comparative Example 3

[0064] The only difference between Comparative Example 3 and Example 1 is that in S2, the first charging current is 0.01C and the second charging current is 0.08C.

[0065] The remaining formation steps and parameters, as well as the assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1, are the same as in Example 1.

[0066] Comparative Example 4

[0067] The only difference between Comparative Example 4 and Example 1 is that in S2, the first charging current is 0.05C and the second charging current is 0.1C.

[0068] The remaining formation steps and parameters, as well as the assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1, are the same as in Example 1.

[0069] Comparative Example 5

[0070] The only difference between Comparative Example 5 and Example 1 is that in S2, the second constant current charging is performed to 40% SOC.

[0071] The remaining formation steps and parameters, as well as the assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1, are the same as in Example 1.

[0072] Comparative Example 6

[0073] The only difference between Comparative Example 6 and Example 1 is that in S2, the second constant current charging is performed to 65% SOC.

[0074] The remaining formation steps and parameters, as well as the assembly method of the lithium manganese iron phosphate soft-pack battery cell in S1, are the same as in Example 1.

[0075] Test case

[0076] The lithium manganese iron phosphate cells formed using the formation methods of Example 1 and Comparative Example 1 were disassembled, and the interface diagrams of the resulting negative electrode sheets are shown below. Figure 1 As shown in a and b. From Figure 1 As can be clearly seen in Figures a and b, the electrode formed using the formation method of Comparative Example 1 exhibits obvious lithium plating on its surface, along with numerous bubbles formed from the gas generated during the formation process. In contrast, the electrode formed using the formation method of Example 1 of this invention has a smooth surface with a metallic luster, indicating good lithium ion transport during formation. Furthermore, the gas generated during formation is discharged into the pre-reserved gas bag, and there are no obvious bubbles on the electrode surface. The lithium manganese iron phosphate cells formed using the methods of Example 1 and Comparative Example 1 were respectively cycled 800 times at a high temperature of 45°C. After cycling, the cells were disassembled, and the interface diagrams of the resulting negative electrode are shown below. Figure 1 As shown in c and d. From Figure 1 As can be seen from images c and d, lithium plating on the electrode surface in c is severe, and the accumulation of a large amount of gas causes separation between the positive and negative electrodes and the separator, preventing normal lithium-ion transport. This results in increased internal resistance of the cell, significant loss of active lithium, and severe bulging of the cell. In contrast, fewer gas bubbles are visible on the electrode surface in image d, and the cell does not exhibit overheating or bulging, representing a significant improvement over image c. Through the above comparison, the lithium manganese iron phosphate cell formed using the formation method of this invention exhibits excellent lithium-ion transport performance, reduced internal resistance, and good performance in high-temperature cycling tests. This is because the lithium manganese iron phosphate cell formed using the formation method of this invention not only forms a dense and stable SEI film but also significantly improves lithium-ion transport.

[0077] Figure 2 The results show the high-temperature cycle performance of lithium manganese iron phosphate cells formed using the formation methods of Example 1 and Comparative Example 1. The test conditions were: high temperature 45°C and clamp pressure 1.5 N·m. The results show that the cells formed according to the present invention have a higher capacity retention rate under the high temperature condition of 45°C.

[0078] The formation data of lithium manganese iron phosphate cells formed using the formation processes of Examples 1-3 and Comparative Examples 1-6 were analyzed respectively. The formation data results are shown in Table 1:

[0079] Table 1

[0080]

[0081] As can be seen from the data in Table 1, the charging capacity and discharging capacity of the battery cell are improved by using the formation method described in this invention, indicating that this formation step promotes the transport of lithium ions.

[0082] Cyclic performance tests were conducted on lithium manganese iron phosphate cells formed using the formation methods of Examples 1-3 and Comparative Examples 1-6, respectively. The test methods were as follows: Test conditions: high temperature 45℃, clamp pressure 1.5 N·m; Test process: (1) rest for 30 min (2) constant current charging with 1C current to 2.75V (3) constant current and constant voltage charging with 1C current to 4.3V, cut-off current 0.05C (4) rest for 30 min (5) constant current discharge with 1C current to 2.75V (6) repeat the above process for 1500 cycles. The test results are shown in Table 2.

[0083] Table 2

[0084]

[0085] The data in Table 2 shows that the cells formed using the above-mentioned formation method of the present invention have the largest charge-discharge capacity in the first cycle. At the same time, the DCR test of the formed cells shows that the resistance is the lowest, and the capacity retention rate is also the highest after 800 cycles at 1C. This indicates that the SEI film formed on the electrode surface of the cells formed using the above-mentioned formation method of the present invention is more dense and stable, the cell resistance is reduced, the cycle performance is significantly improved, the lithium-ion transport rate is faster, and the cell capacity is also improved.

[0086] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A formation method for a lithium manganese iron phosphate battery, characterized in that, Includes the following steps: S1. The lithium iron phosphate soft-pack battery cell is injected with electrolyte and soaked. S2. The battery cell processed in S1 is first charged at 42~48℃ with a first charging current to 5%~20% SOC, and then charged with a second charging current to 45%~60% SOC. The first charging current is 0.02~0.05C, the second charging current is 0.05~0.08C, and the second charging current minus the first charging current is ≥0.03C. S3. Allow the cells treated in S2 to age at 42~48℃ for 10~14 hours. S4. Charge the battery cells treated in S3 at 20~40℃ with a constant current and constant voltage at a charging current of 0.3~0.5C until they reach 100% SOC.

2. The formation method of lithium manganese iron phosphate battery according to claim 1, characterized in that, In S1, after the battery cell is injected with liquid and before it is soaked, the battery cell is further subjected to vacuum sealing treatment to retain the air bag.

3. The formation method of lithium manganese iron phosphate battery according to claim 1, characterized in that, In S1, the impregnation is carried out at normal temperature and pressure, and the impregnation time is 24~48h.

4. The formation method of lithium manganese iron phosphate battery according to claim 1, characterized in that, In S1, after impregnation, double-sided foam covering of the battery cell is also included.

5. The formation method of a lithium manganese iron phosphate battery according to claim 1, characterized in that, S2 is performed under a pressure of 500~800Kgf applied to the battery cell; S4 is performed under a pressure of 500~800Kgf applied to the battery cell.

6. The formation method of a lithium manganese iron phosphate battery according to claim 1, characterized in that, In S2, there is a 5-10 minute rest period between the first constant current charging and the second constant current charging.

7. The formation method of a lithium manganese iron phosphate battery according to claim 1, characterized in that, In S2, a rest period of 5 to 10 minutes is included before the first constant current charging.

8. The formation method of a lithium manganese iron phosphate battery according to claim 1, characterized in that, In S2, after the second constant current charging, there is a rest period of 5 to 10 minutes.

9. The formation method of a lithium manganese iron phosphate battery according to claim 1, characterized in that, In S4, the cutoff voltage of the constant current and constant voltage charging is 4.3V, and the cutoff current is 0.05C.

10. A lithium manganese iron phosphate battery, characterized in that, The method for preparing the lithium manganese iron phosphate battery includes the formation method as described in any one of claims 1 to 9.

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