A lithium iron phosphate battery formation method

Abstract

The present invention relates to a lithium iron phosphate battery formation method, comprising the following steps: (1) adding a gas flowmeter to a formation vacuum pipeline; (2) taking a lithium iron phosphate battery to be formed after liquid injection, forming it according to the following staged formation steps, and recording the gas flow of each stage, the staged formation steps are: S1: high temperature shelving for a period of time; S2: 0.02-0.05C constant current to 2.5V; S3: shelving for a period of time; S4: 0.05-0.2C constant current to 3.3V; S5: shelving for a period of time; S6: 0.2-0.5C constant current to 3.5V; S7: shelving for a period of time; (3) setting different vacuum degrees in different formation stages according to the gas flow rate of step (2), and obtaining formation steps with different vacuum degrees in different stages. The lithium iron phosphate battery formation method of the present invention ensures that the formation gas is completely extracted while reducing the amount of the extracted electrolyte, thereby ensuring the formation effect.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to a formation method of a lithium iron phosphate battery. Background Art

[0002] Lithium batteries are generally made of a coil core consisting of a positive electrode sheet, a negative electrode sheet, and a separator, and an electrolyte is loaded into a shell. The formation of lithium-ion batteries is an important step in the manufacture of lithium batteries, and the formation method has a vital impact on the performance of lithium batteries. Lithium batteries activate their internal positive and negative electrode materials through formation, and at the same time form corresponding solid electrolyte films on the surface of the positive and negative electrode materials, thereby improving the battery's charge and discharge performance, self-discharge, cycle and other electrical properties.

[0003] Gas will be generated during the formation process. If it is not extracted in time, bubbles will be directly stored between the lithium battery diaphragm and the pole piece, resulting in insufficient formation, black spots on the pole piece, etc., thus affecting the battery quality. Therefore, the commonly used formation method for lithium batteries is currently high-temperature negative pressure formation. However, in the high-temperature negative pressure formation method, the gas cannot be extracted when the vacuum degree is set low, and when the vacuum degree is set high, a large amount of electrolyte will be brought out, resulting in a waste of resources. In severe cases, the pole piece is not well wetted and the formation is insufficient due to the low electrolyte content, which seriously affects the electrical performance. Summary of the invention

[0004] The object of the present invention is to provide a lithium iron phosphate battery formation method, which ensures that the formation gas is completely extracted while reducing the amount of the extracted electrolyte, thereby ensuring the formation effect.

[0005] The technical solution adopted by the present invention to solve the above problem is: a lithium iron phosphate battery formation method, comprising the following steps: Add a gas flow meter to the formation vacuum pipeline; Take the lithium iron phosphate battery to be formed after the injection, and form it according to the following staged formation steps, and record the gas flow rate of each stage. The staged formation steps are: S1: leave at high temperature for a period of time; S2: 0.02-0.05C constant current to 2.5V; S3: put aside for a while; S4: 0.05-0.2C constant current to 3.3V; S5: Put it aside for a while; S6: 0.2-0.5C constant current to 3.5V; S7: Put it aside for a while; (3) According to the gas flow rate in step (2), different vacuum degrees are set at different formation stages to obtain formation steps with different vacuum degrees at different stages, and lithium iron phosphate battery formation is performed under this formation step; wherein the vacuum degree at each stage ensures that gas can be removed and no electrolyte is seen to be extracted.

[0006] Preferably, the gas flow meter range shown in step (1) is 0-100 ml.

[0007] Preferably, in step (2), the high temperature in step S1 is 40-85° C., and the holding time is 30 min.

[0008] Preferably, the holding time in S3, S5 and S7 in step (2) is 5-15 minutes.

[0009] Preferably, the vacuum degree in step S1 is set to -70 to -85 KPa. In step S1, the battery is mainly heated, and charging and formation are not performed, and no gas is generated. However, in order to remove the residual gas in the injection stage, the vacuum degree can be set higher, such as -70 to -85 KPa, so that the gas can be removed without extracting the electrolyte.

[0010] Preferably, the vacuum degree in step S2 is set to -30 to -50 KPa. In the S2 stage, due to the small current charging, the electrolyte solvent begins to undergo a reduction reaction, and the solid electrolyte membrane begins to form on the surface of the negative electrode material. The amount of gas generated as observed by the flow meter is small. The vacuum degree is set to -30 to -50 KPa. Within this pressure range, the gas can be discharged and no electrolyte is seen to be extracted.

[0011] Preferably, the vacuum degree in steps S3 and S5 is set to -50 to -70 KPa. Charging and formation are not performed in the S3 holding stage. However, due to the partial reaction of the polarization effect of the battery electrolyte, a small amount of gas continues to be generated as observed by the flow meter. The vacuum degree is set to -50 to -70 KPa. Within this pressure range, the gas can be discharged without the electrolyte being extracted.

[0012] Preferably, the vacuum degree in step S4 is set to -50 to -60 Kpa. In the S4 stage, due to the increase of formation current, the reduction reaction of the electrolyte solvent is intense, and the solid electrolyte membrane is mainly formed in this stage. The amount of gas generated is observed by the flow meter to be large, and the vacuum degree is set to -50 to -60 Kpa. Within this pressure range, the gas can be discharged and the electrolyte is not seen to be extracted. Note that the reaction produces more gas in this stage, but the vacuum degree cannot be less than -60 Kpa, otherwise the electrolyte will be brought out together.

[0013] Preferably, the vacuum degree in step S5 is set to -50 to -70 KPa, and charging formation is not performed during the S5 standby stage. However, due to the partial reaction of the polarization effect of the battery electrolyte, a small amount of gas continues to be generated as observed by the flow meter. The vacuum degree is set to -50 to -70 KPa, and the gas can be discharged within this pressure range without the electrolyte being extracted.

[0014] Preferably, the vacuum degree in step S6 is set to -60 to -70 KPa. Although the formation current is further increased in the S6 stage, it is already the late stage of the electrolyte solvent reduction reaction, the reaction is not intense, the solid electrolyte membrane is further improved, and the amount of gas generated as observed by the flow meter is small. The vacuum degree is set to -60 to -70 KPa. Within this pressure range, the gas can be discharged and no electrolyte is seen to be extracted.

[0015] Preferably, the vacuum degree in step S7 is set to -70 to -85 KPa, and no charging and formation is performed during the S7 holding stage. The battery electrolyte is basically finished, the solid electrolyte membrane is completed, and basically no gas is generated as observed by the flow meter. In order to further eliminate possible residual gas, the vacuum degree is set to -70 to -85 KPa. Within this pressure range, the gas can be discharged and no electrolyte is seen to be extracted.

[0016] Compared with the prior art, the advantages of the present invention are: The lithium iron phosphate battery formation method of the present invention ensures that the formation gas is completely extracted while reducing the amount of the extracted electrolyte, thereby ensuring the formation effect. The lithium iron phosphate battery will not produce black spots on the pole pieces, bubbles in the diaphragm and the pole pieces due to the failure to extract the gas in time, causing problems such as increased internal resistance of the battery and capacity loss. It will not cause waste of resources due to too much electrolyte being extracted or insufficient battery infiltration resulting in insufficient formation, thereby achieving cost reduction and optimization and improvement of battery performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a comparison chart of the cycle retention rates of lithium iron phosphate batteries after formation in Example 1, Example 2 and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0018] The present invention is further described in detail below with reference to the accompanying drawings. Example 1

[0019] A lithium iron phosphate battery formation method comprises the following steps: Add a gas flow meter in the vacuum pipeline, the gas flow meter range is 0-100ml; Take 5 lithium iron phosphate 48173170-140Ah batteries that have been filled with liquid (the amount of liquid filled is controlled at 600±5g) and are to be formed according to the following staged formation steps, and record the gas flow rate of each stage. The staged formation steps are: S1: High temperature 45℃ for 30min; S2: 0.02-0.05C constant current to 2.5V; S3: stand for 5 minutes; S4: 0.05-0.2C constant current to 3.3V; S5: stand for 5 minutes; S6: 0.2-0.5C constant current to 3.5V; S7: Leave for 10 minutes.

[0020] The gas flow rate of each stage is as follows: The S1 step does not perform charging and formation, so no gas is generated. At this time, the gas flow meter displays 0ml, but in order to exclude the residual gas in the injection stage, the vacuum is set higher to -75Kpa. No electrolyte is extracted under this vacuum degree; the S2 step has a small current and a small gas production. The flow meter gas display fluctuates between 3-6ml / min. The vacuum degree is -30kpa. At this time, the gas is extracted and no electrolyte is taken out; the S3 step is a shelving stage and no charging and formation is performed. However, due to the partial reaction of the battery electrolyte polarization effect, a small amount of gas will continue to be generated according to the flow meter. The flow meter displays fluctuations between 0-2ml / min. The vacuum degree is set to -55Kpa. Under this vacuum, the gas can be excluded and no electrolyte is seen to be extracted; in the S4 stage, due to the increase in the formation current, the electrolyte solvent reduction reaction is intense, the flow meter displays fluctuations between 5-10ml / min, and the vacuum degree is set to -50Kpa a. Under this vacuum, gas can be removed and no electrolyte is seen to be extracted; step S5 is a standby stage in which charging and formation are not performed, but due to the partial reaction of the battery electrolyte polarization effect has not completely stopped, a small amount of gas will continue to be produced as observed by the flow meter, and the flow meter shows fluctuations between 0-2ml / min. The vacuum degree is set to -55Kpa, and gas can be removed under this vacuum and no electrolyte is seen to be extracted; step S6, although the formation current is further increased, it is already the late stage of the electrolyte solvent reduction reaction, the reaction is not intense, and the gas production is reduced. The flow meter shows fluctuations between 0-3ml / min, and the vacuum degree is set to -60Kpa, and gas can be removed under this vacuum and no electrolyte is seen to be extracted; step S7 is a standby stage in which charging and formation are not performed, and the battery electrolyte has basically ended, and the flow meter shows 0ml. In order to further eliminate possible residual gas, the vacuum degree is set to -75Kpa, and gas can be removed under this vacuum and no electrolyte is seen to be extracted.

[0021] (3) According to the gas flow rate in step (2), different vacuum degrees are set at different formation stages to obtain formation steps with different vacuum degrees at different stages, specifically: S1: High temperature 45℃ for 30min, vacuum degree -75Kpa; S2: 0.02-0.05C constant current to 2.5V, vacuum degree -30Kpa; S3: stand for 5 minutes, vacuum degree -55Kpa; S4: 0.05-0.2C constant current to 3.3V, vacuum degree -50Kpa; S5: stand for 5 minutes, vacuum degree -55Kpa; S6: 0.2-0.5C constant current to 3.5V, vacuum degree -60Kpa; S7: Leave for 10 minutes, vacuum degree -75Kpa. Example 2

[0022] A lithium iron phosphate battery formation method comprises the following steps: Add a gas flow meter in the vacuum pipeline, the gas flow meter range is 0-100ml; Take 5 lithium iron phosphate 48173170-140Ah batteries that have been filled with liquid (the amount of liquid filled is controlled at 600±5g) and are to be formed according to the following staged formation steps, and record the gas flow rate of each stage. The staged formation steps are: S1: High temperature 45℃ for 30min; S2: 0.02-0.05C constant current to 2.5V; S3: stand for 5 minutes; S4: 0.05-0.2C constant current to 3.3V; S5: stand for 5 minutes; S6: 0.2-0.5C constant current to 3.5V; S7: Leave for 10 minutes.

[0023] The gas flow rate of each stage is as follows: In the S1 step, the flowmeter shows 0ml, and the vacuum is set higher to -75Kpa. No electrolyte is extracted under this vacuum. In the S2 step, the flowmeter shows a fluctuation between 3-6ml / min, and the vacuum is -50kpa. At this time, the gas is extracted and no electrolyte is taken out. In the S3 step, the flowmeter shows a fluctuation between 0-2ml / min, and the vacuum is set to -55Kpa. Under this vacuum, the gas can be removed and no electrolyte is extracted. In the S4 stage, the flowmeter shows a fluctuation between 5-10ml / min, and the vacuum is set The vacuum degree is -60Kpa, under which gas can be removed and no electrolyte is seen to be extracted; the flow meter of step S5 shows fluctuations between 0-2ml / min, and the vacuum degree is set to -55Kpa, under which gas can be removed and no electrolyte is seen to be extracted; the flow meter of step S6 shows fluctuations between 0-3ml / min, and the vacuum degree is set to -70Kpa, under which gas can be removed and no electrolyte is seen to be extracted; the flow meter of step S7 shows 0ml, and the vacuum degree is set to -75Kpa, under which gas can be removed and no electrolyte is seen to be extracted.

[0024] (3) According to the gas flow rate in step (2), different vacuum degrees are set at different formation stages to obtain formation steps with different vacuum degrees at different stages, specifically: S1: High temperature 45℃ for 30min, vacuum degree -75Kpa; S2: 0.02-0.05C constant current to 2.5V, vacuum degree -50Kpa; S3: stand for 5 minutes, vacuum degree -55Kpa; S4: 0.05-0.2C constant current to 3.3V, vacuum degree -60Kpa; S5: stand for 5 minutes, vacuum degree -55Kpa; S6: 0.2-0.5C constant current to 3.5V, vacuum degree -70Kpa; S7: Leave for 10 minutes, vacuum degree -75Kpa.

[0025] Comparative Example 1 Take 5 lithium iron phosphate 48173170-140Ah batteries to be formed after injection, and control the injection amount to 600±5g. Compared with Example 1-2, steps (1) and (2) are omitted, and the third step is directly carried out. The same vacuum degree is used in each stage, and the following formation steps are directly taken respectively: S1: High temperature 45℃ for 30min, vacuum degree -75Kpa; S2: 0.02-0.05C constant current to 2.5V, vacuum degree -75Kpa; S3: Stand for 5 minutes, vacuum degree -75Kpa; S4: 0.05-0.2C constant current to 3.3V, vacuum degree -75Kpa; S5: stand for 5 minutes, vacuum degree -75Kpa; S6: 0.2-0.5C constant current to 3.5V, vacuum degree -75Kpa; S7: Stand for 10 minutes, vacuum degree -75Kpa; Comparative Example 2 Take 5 lithium iron phosphate 48173170-140Ah batteries to be formed after injection, and control the injection amount to 600±5g. Compared with Example 1-2, steps (1) and (2) are omitted, and the third step is directly carried out. The same vacuum degree is used in each stage, and the following formation steps are directly taken respectively: S1: High temperature 45℃ for 30min, vacuum degree -45Kpa; S2: 0.02-0.05C constant current to 2.5V, vacuum degree -45Kpa; S3: stand for 5 minutes, vacuum degree -45Kpa; S4: 0.05-0.2C constant current to 3.3V, vacuum degree -45Kpa; S5: stand for 5 minutes, vacuum degree -45Kpa; S6: 0.2-0.5C constant current to 3.5V, vacuum degree -45Kpa; S7: Stand for 10 minutes, vacuum degree -45Kpa; The lithium iron phosphate batteries of Example 1-2 and Comparative Example 1-2 were weighed to confirm the amount of electrolyte extracted, two of them were disassembled to observe the state of the pole pieces, and the batteries of the above examples and comparative examples were aged at 45°C for 24 hours, and the electrical properties (battery capacity, battery DC internal resistance DCR, cycle performance) of the batteries were tested. The results are as follows: Figure 1 And as shown in Table 1.

[0026] Table 1 Performance test results of lithium iron phosphate battery after formation

[0027] As can be seen from Table 1, the amount of electrolyte extracted in Example 1 and Example 2 during the formation process is less than 5g within the specification range. After the battery is disassembled, the interface of the negative electrode plate is golden and there is no lithium precipitation. The capacity and resistance of the two embodiments are similar, meeting the design requirements; in Comparative Example 1, a fixed high vacuum is used in the entire formation stage, and the amount of electrolyte extracted seriously exceeds the standard. After the battery is disassembled, it is found that there is lithium precipitation at the interface of the plate, which is mainly because too much electrolyte is extracted, resulting in poor plate infiltration and insufficient formation, and the internal resistance of the battery increases, and it is also because the battery ion conductivity is insufficient after the amount of electrolyte is small, resulting in an increase in internal resistance; in Comparative Example 2, a fixed low vacuum is used in the entire formation stage, and the amount of electrolyte extracted is the smallest, but because its vacuum is too small, the battery formation gas is not discharged in time, resulting in bubbles between the diaphragm and the plate, causing wrinkles on the positive and negative plates, and poor contact between the two. The battery is disassembled and it is found that there are black spots on the electrode interface, and the initial capacity of the battery is much lower than that of the embodiment, and it is also because lithium ions are not embedded in the negative electrode, so the capacity will be low during discharge.

[0028] Figure 1 The figure is a comparison chart of the cycle retention rate of lithium iron phosphate batteries after formation in Example 1, Example 2 and Comparative Example 1. Since the initial capacity of Comparative Example 2 does not meet the specification requirements, no cycle test was performed. Figure 1 It can be seen that the capacity retention rates of Example 1 and Example 2 after 1000 cycles are 92.06% and 92.07% respectively; the capacity retention rate of Comparative Example 1 after 1000 cycles is 84.49%, which is mainly because too much electrolyte is extracted, resulting in insufficient formation and increased internal resistance of the battery. During the cycle, lithium ions are continuously deposited, resulting in excessive consumption of lithium ions, thereby reducing the capacity retention rate faster.

[0029] It can be seen from the above results that the capacity, internal resistance, cycle performance and other properties of the lithium battery formed by the method of the present invention are better than those of the conventional negative pressure formation method.

[0030] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A lithium iron phosphate battery formation method, characterized in that: The following steps are involved: Add a gas flow meter to the formation vacuum pipeline; Take the lithium iron phosphate battery to be formed after the injection, and form it according to the following staged formation steps, and record the gas flow rate of each stage. The staged formation steps are: S1: leave at high temperature for a period of time; S2: 0.02-0.05C constant current to 2.5V; S3: put aside for a while; S4: 0.05-0.2C constant current to 3.3V; S5: Put it aside for a while; S6: 0.2-0.5C constant current to 3.5V; S7: Put it aside for a while; (3) According to the gas flow rate in step (2), different vacuum degrees are set at different formation stages to obtain formation steps with different vacuum degrees at different stages, and lithium iron phosphate battery formation is performed under this formation step; wherein the vacuum degree at each stage ensures that gas can be removed and no electrolyte is seen to be extracted.

2. The lithium iron phosphate battery formation method according to claim 1, characterized in that: The gas flow meter range shown in step (1) is 0-100 ml.

3. The lithium iron phosphate battery formation method according to claim 1, characterized in that: In step (2), the high temperature in step S1 is 40-85° C. and the holding time is 30 min.

4. The lithium iron phosphate battery formation method according to claim 1, characterized in that: In step (2), the holding time in S3, S5 and S7 is 5-15 minutes.

5. The lithium iron phosphate battery formation method according to claim 1, characterized in that: In step S1, the vacuum degree is set to -70 to -85 KPa.

6. The lithium iron phosphate battery formation method according to claim 1, characterized in that: In step S2, the vacuum degree is set to -30 to -50 Kpa.

7. The lithium iron phosphate battery formation method according to claim 1, characterized in that: The vacuum degree in steps S3 and S5 is set to -50 to -70 KPa.

8. The lithium iron phosphate battery formation method according to claim 1, characterized in that: In step S4, the vacuum degree is set to -50 to -60 Kpa.

9. The lithium iron phosphate battery formation method according to claim 1, characterized in that: In step S6, the vacuum degree is set to -60 to -70 KPa.

10. The lithium iron phosphate battery formation method according to claim 1, characterized in that: In step S7, the vacuum degree is set to -70 to -85 KPa.

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