Lithium iron phosphate cell formation method

Abstract

The invention provides a formation method of a lithium iron phosphate cell, which comprises the following steps: pretreatment before formation in a first stage, cell pre-formation in a second stage, cell formation aging in a third stage and cell formation full charge in a fourth stage, the cell pre-formation in the second stage adopts step constant current charging to 60%-75% SOC, the step constant current charging is divided into three steps, the first-step constant-current charging current is larger than or equal to the second-step constant-current charging current and smaller than or equal to the third-step constant-current charging current. The SEI film formed by the method is stable and compact.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and in particular to a method for forming a lithium iron phosphate battery cell. Background Art

[0002] As a new generation of green, high-energy and environmentally friendly batteries, lithium-ion batteries are widely used in life and production, and have become the focus of development of secondary batteries. In the production process of lithium-ion batteries, formation is an extremely important step, which mainly includes pre-formation, formation aging, and formation full charging after the battery cell is filled with liquid. This process will activate the active substances in the battery and activate the lithium-ion battery; at the same time, the electrolyte solvent and lithium salt will react with each other, forming a solid electrolyte interface (SEI) film at the negative electrode of the lithium-ion battery. This film will prevent the further occurrence of side reactions, thereby reducing the loss of lithium content in the lithium-ion battery. Therefore, it has an important impact on the initial capacity loss, internal resistance, cycle life, low temperature performance, and high temperature storage of lithium-ion batteries.

[0003] Different formation processes form different SEI film states, and different SEI film states have different effects on battery performance. Therefore, different formation processes have different effects on lithium-ion battery performance. Different formation processes mainly include different formation charging currents, different formation charging SOC (State of Charge, battery state of charge, also called remaining power), different formation aging time and temperature, etc.

[0004] If the pre-formation charging current is too large, the battery cell will produce gas too quickly, resulting in a large amount of gas inside the electrode, affecting the interface flatness and causing cycle diving; if the pre-formation charging SOC is too low, the SEI film is not dense enough, resulting in performance degradation; if the pre-formation charging SOC is too high, the SEI film will be too thick, the first efficiency of the battery cell will be reduced, and the irreversible capacity loss will increase; if the aging time is too low, the SEI film will be unstable, resulting in battery cell performance degradation; if the formation temperature is too low, the electrolyte viscosity will increase, the lithium ion diffusion rate will decrease, and the composition and distribution of the SEI film will be uneven, resulting in battery cell performance degradation; if the formation temperature is too high, the decomposition degree of the electrolyte components will be aggravated, and some components of the SEI film will decompose, resulting in increased irreversible capacity loss of the battery cell. Therefore, it is necessary to develop a formation process that can form a stable and appropriately dense SEI with low irreversible capacity of the battery cell. Summary of the invention

[0005] The inventors found that the prior art (CN111106405A) discloses a formation method for a square aluminum shell lithium iron phosphate battery for energy storage. The method adopts a high-temperature negative pressure constant power formation method to improve the SEI film formation stability of the energy storage lithium iron phosphate battery. However, due to the continuous increase in voltage during the constant power charging process, the current tends to gradually decrease, and the gas production is the largest in the initial stage of formation. This method has a large current and violent gas production in the initial stage, and there is a risk of poor lithium insertion and a long process time; the prior art (CN114388865A) discloses a formation method, which adopts constant current charging pre-formation, but its cut-off SOC is 28.0%-32.0% which is low, and the formed SOC is not dense enough.

[0006] The present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, one object of the present invention is to provide a formation process that can form a stable SEI with suitable density while having a low irreversible capacity of the battery cell.

[0007] In one aspect of the present invention, the present invention proposes a formation method for lithium iron phosphate batteries. According to an embodiment of the present invention, the formation method of the lithium iron phosphate battery includes: a first stage pre-formation pretreatment, a second stage battery pre-formation, a third stage battery formation aging and a fourth stage battery formation full charge, the second stage battery pre-formation adopts a step constant current charge to 60%-75% SOC, wherein the step constant current charge is divided into three steps, the first step constant current charge current ≤ the second step constant current charge current ≤ the third step constant current charge current. The method requires a short time and forms a dense SIE film. In addition, the fourth stage battery formation full charge adopts a large current constant current constant voltage full charge, shortens the process time, and gradually increases the current so that the formed SEI film satisfies the transition from dense to loose, and the SIE film finally formed satisfies both dense and stable and has a high ion mobility, and the battery has excellent low temperature performance, high temperature storage and cycle performance. According to an embodiment of the present invention, the above-mentioned formation method of lithium iron phosphate batteries can also further include at least one of the following attached technical features:

[0008] According to an embodiment of the present invention, the first stage pre-formation pretreatment is performed in the following manner:

[0009] 1) Primary filling of the battery cell;

[0010] 2) After the injection, the injection port of the lithium iron phosphate battery is plugged with glue nails, and the battery is placed in a high temperature room for high temperature immersion.

[0011] According to an embodiment of the present invention, the temperature of the high temperature room is 45±3°C.

[0012] According to an embodiment of the present invention, the infiltration time is 12-48 hours.

[0013] According to an embodiment of the present invention, the second-stage cell pre-forming is performed in the following manner:

[0014] 1) The product after pretreatment before the first stage of formation is vacuumed;

[0015] 2) Carry out step-by-step constant current charging according to the pre-forming process.

[0016] According to an embodiment of the present invention, the vacuum value of the vacuum treatment is -85±5Kpa.

[0017] According to an embodiment of the present invention, the temperature of the vacuum treatment is 45±3°C.

[0018] According to an embodiment of the present invention, the temperature of the battery cell in the third stage of battery cell formation aging is 45±3°C.

[0019] According to an embodiment of the present invention, the time of the battery cell formation aging in the third stage is 24±2h.

[0020] According to an embodiment of the present invention, the first constant current charging current is 0.05-0.1 C. Under this charging current, a dense SEI film is formed, and a large amount of gas is generated and discharged under negative pressure.

[0021] According to an embodiment of the present invention, the second step constant current charging current is 0.1-0.2 C. Under this condition, the SEI film is continuously generated and thickened, and increasing the current can save time compared to the first stage.

[0022] According to an embodiment of the present invention, the third step constant current charging current is 0.2-0.3 C. The corresponding SOC needs to be cut off in the range of 60-75% SOC. Under this condition, the SEI film formed is stable and dense.

[0023] According to an embodiment of the present invention, the step constant current charging is divided into three steps, and the interval between each step is 5-10 minutes.

[0024] According to an embodiment of the present invention, the fourth stage of cell formation and full charging is performed by constant current and constant voltage full charging.

[0025] According to an embodiment of the present invention, the charging rate of the constant current and constant voltage full charge is 0.5-1C.

[0026] According to an embodiment of the present invention, the cut-off voltage of the constant current and constant voltage full charging is 3.65V.

[0027] According to an embodiment of the present invention, the cut-off current of the constant current and constant voltage full charging is 0.05C.

[0028] According to an embodiment of the present invention, the ambient temperature for fully charging the battery cell in the fourth stage is 20-40°C.

[0029] According to an embodiment of the present invention, since the SEI film formed by pre-forming is too dense and will affect the transmission of lithium ions during the reaction, it is necessary to gradually increase the current so that the formed SEI film satisfies the transition from dense to loose, and a large current can save time.

[0030] In another aspect of the present invention, the present invention further provides a lithium iron phosphate battery cell. According to an embodiment of the present invention, the lithium iron phosphate battery cell is prepared by the method described above.

[0031] In another aspect of the present invention, the present invention further provides a lithium battery. According to an embodiment of the present invention, the lithium battery comprises the lithium iron phosphate battery cell described above.

[0032] In another aspect of the present invention, a formation method of a lithium iron phosphate battery cell is proposed. According to an embodiment of the present invention, the formation method of the lithium iron phosphate battery cell is divided into four stages to form the lithium ion battery, the first stage is pre-treatment before formation; the second stage is pre-formation of the battery cell, the third stage is battery cell formation aging, and the fourth stage is battery cell formation full charge. The specific operation steps are as follows:

[0033] Phase 1:

[0034] S1: One-time filling of the battery cell;

[0035] S2: After the injection, the injection port of the lithium iron phosphate battery is plugged with a rubber nail, and the battery is placed in a high temperature room for high temperature immersion, with an ambient temperature of 45±3℃ and a standing time of 12-48h;

[0036] Phase 2:

[0037] S3: Remove the plastic nails from the pre-treated battery cells and place them in the forming cabinet to evacuate the battery cells. The vacuum value is -85±5KPa and the ambient temperature is 45±3℃.

[0038] S4: Perform stepwise constant current charging according to the pre-forming step;

[0039] Phase 3:

[0040] S5: Battery cell 45±3℃, stand for 24±2h;

[0041] Phase 4:

[0042] S6: Secondary injection;

[0043] S7: battery cell filling port sealing;

[0044] S8: Constant current and constant voltage full charge.

[0045] According to an embodiment of the present invention, the above-mentioned lithium iron phosphate battery cell formation method may further include at least one of the following additional technical features:

[0046] According to an embodiment of the present invention, the second stage of pre-charging the battery cells is divided into three steps.

[0047] According to an embodiment of the present invention, the first constant current charging current in the second stage of the pre-formation of the battery cell is 0.05-0.1 C. Under this charging current, a dense SEI film is formed, and a large amount of gas is generated and discharged under negative pressure.

[0048] According to an embodiment of the present invention, the second constant current charging current in the second stage of the pre-formation of the battery cell is 0.1-0.2 C. Under this condition, the SEI film is continuously generated and thickened, and increasing the current compared to the first stage can save time.

[0049] According to an embodiment of the present invention, the third step of the constant current charging current in the second stage of the battery cell pre-formation is 0.2-0.3C, and the corresponding SOC needs to be cut off in the range of 60-75% SOC. Under this condition, the SEI film formed is stable and dense. According to an embodiment of the present invention, the fourth stage of the battery cell formation is full charge with constant current and constant voltage, with a charging rate of 0.5-1C, a cut-off voltage of 3.65V, a cut-off current of 0.05C, and an ambient temperature of 20-40°C. Since the SEI film formed by the pre-formation is too dense, it will affect the transmission of lithium ions during the reaction process. Therefore, it is necessary to gradually increase the current so that the formed SEI film meets the transition from dense to loose, and at the same time, a large current can save time.

[0050] According to the embodiments of the present invention, the present invention has at least one of the following advantages and technical effects:

[0051] 1) In the method according to the embodiment of the present invention, the cell is pre-charged to 60%-75% SOC by step constant current charging, the process time is short, and a dense SIE film is formed;

[0052] 2) In the method according to the embodiment of the present invention, the battery cell is fully charged and formed by large current constant current constant voltage full charging, which shortens the process time, and gradually increases the current so that the formed SEI film satisfies the transition from dense to loose. The SIE film finally formed satisfies the requirements of density and stability while having a high ion mobility, and the battery cell has excellent low temperature performance, high temperature storage, and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0054] Figure 1 It is a flow chart of a formation method of a lithium iron phosphate battery cell provided by an embodiment of the present invention;

[0055] Figure 2 It is a pre-forming process step diagram provided by an embodiment of the present invention;

[0056] Figure 3 It is a schematic diagram of high temperature cycle performance test results of a lithium ion battery formed by a formation method according to an embodiment of the present invention and a lithium ion battery formed by a formation method according to the prior art;

[0057] Figure 4 It is a schematic diagram of low temperature discharge performance test results of a lithium ion battery formed by a formation method according to an embodiment of the present invention and a lithium ion battery formed by a formation method according to the prior art;

[0058] Figure 5 It is a schematic diagram of storage performance test results of a lithium ion battery formed by the formation method of the embodiment of the present invention and a lithium ion battery formed by the formation method of the prior art. DETAILED DESCRIPTION

[0059] The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0060] Example

[0061] Phase 1:

[0062] S1: One-time filling of the battery cell;

[0063] S2: After the injection, the injection port of the lithium iron phosphate battery is plugged with a rubber nail, and the battery is placed in a high temperature room for high temperature immersion, with an ambient temperature of 45±3℃ and a static time of 24h;

[0064] Phase 2:

[0065] S3: Remove the plastic nails from the pre-treated battery cells and place them in the forming cabinet to evacuate the battery cells. The vacuum value is -85±5KPa and the ambient temperature is 45±3℃.

[0066] S4: Set the pre-forming steps, step 1: let stand for 5 minutes; step 2: 0.05C constant current charge for 60 minutes; step 3: let stand for 10 minutes; step 4: 0.1C constant current charge for 240 minutes; step 5: let stand for 5 minutes; step 6: 0.3C constant current charge for 50 minutes; step 7: let stand for 5 minutes;

[0067] Phase 3:

[0068] S5: Battery cell 45±3℃, stand for 24±2h;

[0069] Phase 4:

[0070] S6: Secondary injection;

[0071] S7: battery cell filling port sealing;

[0072] S8: Full charge at 1C constant current and constant voltage at 25°C, cut-off voltage 3.65V, cut-off current 0.05C;

[0073] Figure 3 The figure is a schematic diagram of the high temperature cycle performance test results of the lithium ion battery formed by the formation method of the embodiment of the present invention and the lithium ion battery formed by the formation method of the prior art, wherein the more cycles at the same capacity retention rate, the better the cell cycle performance of the lithium ion battery. Figure 3 It can be seen that under the same temperature conditions (taking the test condition of 45 degrees Celsius as an example), the number of cycles of the lithium ion battery (embodiment) formed by the embodiment of the present invention is greater than the number of cycles of the lithium ion battery (comparative example) formed by the prior art formation method. Therefore, the high temperature cycle performance of the lithium ion battery formed by the formation method of the embodiment of the present invention is better than the cycle performance of the lithium ion battery formed by the prior art formation method.

[0074] Figure 4 The figure is a schematic diagram of the low temperature discharge performance test results of the lithium ion battery formed by the formation method of the embodiment of the present invention and the lithium ion battery formed by the formation method of the prior art, wherein the higher the capacity retention rate and the average discharge voltage of the lithium ion battery, the better the low temperature discharge performance of the lithium ion battery. Figure 4 The horizontal axis represents the experimental group, including the embodiment (lithium ion battery formed by the formation method of the embodiment of the present invention) and the comparative example (lithium ion battery formed by the formation method of the prior art), and the vertical axis represents the capacity retention rate & average voltage. Figure 5 It can be seen that under the same temperature and rate conditions (taking the test condition of -20°C & 1C as an example), the capacity retention rate and average voltage of the lithium ion battery formed by the embodiment of the present invention are higher than those of the prior art formation method. Therefore, the low-temperature discharge performance of the lithium ion battery formed by the formation method of the embodiment of the present invention is better than the low-temperature discharge performance of the lithium ion battery formed by the prior art formation method.

[0075] Figure 5 The present invention provides a schematic diagram of storage performance test results of a lithium-ion battery formed by the formation method of the present invention and a lithium-ion battery formed by the prior art formation method, wherein the higher the capacity retention rate of the lithium-ion battery, the lower the self-discharge rate of the lithium-ion battery; the higher the capacity recovery rate of the lithium-ion battery, the better the cycle performance of the lithium-ion battery. Figure 5The horizontal axis represents the experimental group, including the embodiment (lithium ion battery formed by the formation method of the embodiment of the present invention) and the comparative example (lithium ion battery formed by the formation method of the prior art), and the vertical axis represents the capacity retention rate & capacity recovery rate. Figure 5 It can be seen that under the same temperature and time conditions (taking the test conditions of 45°C and 30d as an example), the capacity retention rate and capacity recovery rate of the lithium ion battery formed by the embodiment of the present invention are higher than those of the prior art formation method. Therefore, the storage performance of the lithium ion battery formed by the formation method of the embodiment of the present invention is better than the storage performance of the lithium ion battery formed by the prior art formation method.

[0076] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0077] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for forming a lithium iron phosphate battery cell, characterized in that: include: The first stage is pre-formation pretreatment, the second stage is cell pre-formation, the third stage is cell formation aging and the fourth stage is cell formation full charge. The second stage cell pre-formation adopts step constant current charging to 60%-75% SOC, wherein the step constant current charging is divided into three steps, the first step constant current charging current ≤ the second step constant current charging current ≤ the third step constant current charging current.

2. The method for forming a lithium iron phosphate battery cell according to claim 1, characterized in that: The first stage pretreatment before formation is carried out in the following manner: 1) Primary filling of the battery cell; 2) After the injection, the injection port of the lithium iron phosphate battery is plugged with glue nails, and the battery is placed in a high temperature room for high temperature immersion; Optionally, the temperature of the high temperature room is 45±3°C; Optionally, the infiltration time is 12-48 hours.

3. The formation method of lithium iron phosphate battery according to claim 1, characterized in that: The second stage of cell pre-forming is carried out in the following manner: 1) The product after pretreatment before the first stage of formation is vacuumed; 2) Carry out stepwise constant current charging according to the pre-forming process; Optionally, the vacuum value of the vacuum treatment is -85±5Kpa; Optionally, the temperature of the vacuum treatment is 45±3°C.

4. The formation method of lithium iron phosphate battery according to claim 1, characterized in that: The temperature of the battery cell in the third stage of battery cell formation aging is 45±3°C; Optionally, the time of the third stage battery cell formation aging is 24±2h.

5. The formation method of lithium iron phosphate battery according to claim 1, characterized in that: The first step constant current charging current is 0.05-0.1C; Optionally, the second step constant current charging current is 0.1-0.2C; Optionally, the third step constant current charging current is 0.2-0.3C.

6. The formation method of lithium iron phosphate battery according to claim 1, characterized in that: The step constant current charging is divided into three steps, with an interval of 5-10 minutes between each step.

7. The formation method of lithium iron phosphate battery according to claim 1, characterized in that: The fourth stage of cell formation and full charging is performed by constant current and constant voltage full charging.

8. The method for forming a lithium iron phosphate battery cell according to claim 7, characterized in that: The charging rate of the constant current and constant voltage full charge is 0.5-1C; Optionally, the cut-off voltage of the constant current and constant voltage full charge is 3.65V; Optionally, the cut-off current of the constant current and constant voltage full charge is 0.05C; Optionally, the ambient temperature of the fourth stage when the battery cell is fully charged is 20-40°C.

9. A lithium iron phosphate battery cell, characterized in that: The lithium iron phosphate battery cell is prepared by the method according to any one of claims 1 to 8.

10. A lithium battery, characterized in that: The lithium battery comprises the lithium iron phosphate battery cell as described in claim 9.

Images