Formation method of iron-based polyphosphate sodium ion battery cell

Abstract

The invention discloses a formation method of an iron-based polyphosphate sodium ion battery cell, and belongs to the technical field of sodium ion batteries. The method comprises the following steps: S1, injecting liquid, sealing, and putting into a cabinet for formation; s2, a formation stage: accumulatively charging in the three stages until the cut-off capacity is 74-82% SOC to obtain a formed battery; s3, aging stage: standing and aging at normal temperature; s4, capacity grading stage: charging to 3.6 V at constant current and constant voltage, charging to cut-off current at constant voltage, and discharging to 1.5 V at constant current; and S5, a circulation stage: charging and discharging at constant power, wherein the voltage window is 1.5-3.6 V. According to the method, a negative pressure environment and a clamp are not needed, the production process is simplified, the production cost is reduced, the trend of characteristic optimization of a current sodium-ion battery material is met through a reasonable three-section charging strategy, and particularly, in a sodium-electricity composite pyrophosphate material, the electrochemical performance of the sodium-electricity composite pyrophosphate material is improved by activating inert components in the material step by step.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium ion batteries, and in particular relates to a formation method of an iron-based polyphosphate sodium ion battery core. Background Art

[0002] Sodium-ion batteries have great application prospects in the field of energy storage due to their advantages such as abundant sodium resources, low cost, high safety, high rate, and low temperature performance. Battery formation is the process of charging a newly produced battery for the first time, which is crucial to the cycle life of the battery. The lack of a suitable corresponding formation process will bring serious disadvantages to the subsequent cycle performance of sodium-ion batteries, resulting in a cycle drop.

[0003] Patent document CN116565358A discloses a sodium ion battery formation method and a sodium ion battery, specifically a formation method of a layered oxide type sodium ion soft pack battery, which mainly adopts a pre-formation-step charging formation method. After the battery cell is filled with liquid, it is first charged to 33%-35% SOC in three steps of 0.02C-0.03C, 0.04C-0.06C, and 0.08C-0.1C to obtain a pre-formed battery cell. Secondly, a constant current and constant voltage charging is performed using a current of 0.2C-0.25C, with a cut-off current of 0.05C, and the battery is charged to 100% SOC, corresponding to an upper cut-off voltage V1. After the formation process is completed, a current of 0.1C-0.2C is finally used to discharge to a lower cut-off voltage V2, and finally the battery is charged to 50% SOC at any current. The invention adopts a three-step stepped formation process, which does not require complicated process steps. Compared with other methods such as high-temperature fixture formation process, one-step formation process, multi-step formation process, etc., the present invention takes into account the battery film formation quality and shortens the process time to achieve excellent performance.

[0004] Iron-based polyphosphate sodium ion battery cell is a sodium ion battery cell based on iron-based polyanion positive electrode material. This type of material has received widespread attention due to its low cost, stable structure, high safety and excellent cycle performance, and has important application prospects in the field of sodium ion batteries. Iron-based polyphosphate positive electrode materials mainly include iron-based phosphates, iron-based pyrophosphates, etc., among which sodium-based composite sodium iron pyrophosphate (NFPP) has attracted much attention due to its stable crystal structure and faster sodium ion diffusion rate. However, the electronic conductivity of this type of material is poor, and the operating voltage is lower than that of other phosphates, which limits its wide application. Through a reasonable chemical formation process, its electrochemical performance can be effectively improved. The method described in CN116565358A is only applicable to sodium ion layered oxide batteries with a voltage window of 2.0V-4.2V, and has certain limitations for sodium-based composite sodium iron pyrophosphate (NFPP) batteries with a voltage window of 1.5V-3.6V; in addition, the chemical formation scheme of the invention is relatively cumbersome, which not only increases equipment and labor costs in the actual production line landing, but also greatly increases the process error rate. Summary of the invention

[0005] The object of the present invention is to provide a method for forming an iron-based polyphosphate sodium ion battery cell to solve the problem of poor performance of sodium-based composite sodium iron pyrophosphate batteries.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0008] S1: Seal the battery after injection, and put it on the cabinet for formation after the electrolyte is fully infiltrated;

[0009] S2: Formation stage: cumulative charging to a cut-off capacity of 74%-82% SOC (state of charge), to obtain a formed battery;

[0010] S3: Aging stage: static aging at room temperature (25℃-30℃);

[0011] S4: Capacity division stage: the aged battery is charged to 3.6V with constant current and constant voltage, then charged to the cut-off current with constant voltage, and then discharged to 1.5V with constant current;

[0012] S5: Cycle stage: constant power charging and discharging, voltage window is 1.5V-3.6V.

[0013] As a further solution of the present invention, the cutoff capacity in the S2 formation stage is 76% SOC.

[0014] As a further solution of the present invention, the S2 formation stage adopts three-stage gradient charging steps, specifically:

[0015] The first stage of formation: the first constant current charge is I1 = 0.02C for 1h-2h;

[0016] The second stage of formation: the second constant current charge is I2 = 0.04C for 1h-4h;

[0017] The third stage of formation: the third constant current charging is performed at I3=0.1C for 5h-7h.

[0018] As a further solution of the present invention, the S2 formation stage adopts three-stage gradient charging steps, specifically:

[0019] The first stage of formation: the first constant current charge is I1 = 0.02C for 1h;

[0020] The second stage of formation: the second constant current charge is I2 = 0.04C for 1h;

[0021] The third stage of formation: the third constant current charging is performed at I3=0.1C for 7h.

[0022] As a further solution of the present invention, the standing time at room temperature in the S3 aging stage is 22h-26h.

[0023] As a further solution of the present invention, the charging current in the constant current and constant voltage charging stage in S4 is 0.4C-0.6C.

[0024] As a further solution of the present invention, the cutoff current in S4 is 0.05C.

[0025] As a further solution of the present invention, the discharge current in the constant current discharge stage in S4 is 0.4C-0.6C.

[0026] As a further solution of the present invention, the constant current and constant voltage charging current in S4 is equal to the constant current and constant voltage discharging current.

[0027] As a further solution of the present invention, the constant power in S5 is 0.5P.

[0028] As a further solution of the present invention, no negative pressure or additional clamps are required during the formation stage, the volume separation stage and the circulation stage.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. The present invention provides a formation method for an iron-based polyphosphate sodium ion battery cell. In the formation stage, the battery cell is charged at three different rates of 0.02C, 0.04C and 0.1C for 1h-2h, 1h-4h and 5h-7h respectively, and is cumulatively charged to 74%-82% SOC to obtain a formed battery. By adopting three-stage charging and gradually increasing the charging rate, the irreversible loss of active substances in the battery during the first charge and discharge process can be effectively reduced, thereby improving the first coulomb efficiency of the battery. On the other hand, charging at different rates is helpful to better activate battery materials, especially in the low rate stage, which helps to form a stable SEI film, reduce internal resistance, improve its electrochemical performance, and reduce stress concentration inside the battery, thereby improving the overall performance and life of the battery.

[0031] 2. The present invention adopts the same current to perform constant current and constant voltage charging and discharging in the capacity division stage, which can more accurately control the charging and discharging process of the battery, avoid overcharging or over-discharging, and help determine the actual capacity of the battery, thereby extending the service life of the battery.

[0032] 3. The present invention does not require a negative pressure environment and a fixture, simplifies the production process, reduces production costs, and improves the performance and life of the battery through a reasonable charging and discharging strategy. This three-stage charging strategy is also in line with the current trend of optimizing the properties of sodium-ion battery materials, especially in sodium-ion composite pyrophosphate materials, by gradually activating the inert components in the material to improve its electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described below in conjunction with the accompanying drawings.

[0034] Figure 1 is the first cycle charge and discharge curve of the battery in Examples 1 to 4 of the present invention;

[0035] Figure 2 is a relationship diagram between the capacity retention rate and the number of cycles of the batteries of Examples 1 to 4 of the present invention;

[0036] Figure 3 It is a schematic diagram of the negative electrode interface of the battery of Example 1 of the present invention in a fully charged state after 32 cycles. DETAILED DESCRIPTION

[0037] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] In the following embodiments and comparative examples, NFPP laminated square shell batteries are used, and the manufacturing process of the batteries includes the following steps:

[0039] Step 1: Prepare positive electrode sheet: Disperse composite sodium iron pyrophosphate Na4Fe3P2O7(PO4)2 material, carbon black SP, carbon tube CNT, and polyvinylidene fluoride PVDF in N-methylpyrrolidone (NMP) at a ratio of 94:2:1:3 to obtain positive electrode slurry; apply the prepared positive electrode slurry evenly on the front and back of carbon-coated aluminum foil with the same coating amount, dry, roll, slit, and die-cut to obtain positive electrode sheet;

[0040] Step 2: Prepare negative electrode sheets: evenly disperse hard carbon material, carbon black SP, carbon tube CNT, and sodium carboxymethyl cellulose (CMC-Na) in pure water at a ratio of 94:2:1:3 to obtain negative electrode slurry; evenly apply the prepared negative electrode slurry on the front and back of aluminum foil at a coating amount of NP ratio of 1.8, dry, roll, slit, and die-cut to obtain negative electrode sheets;

[0041] Step 3: Diaphragm / electrolyte: Commonly purchased commercially available coated diaphragms and electrolytes suitable for NFPP system batteries;

[0042] Step 4: Assemble the battery, assemble the positive and negative electrodes and the separator prepared above into a stacked body by Z-shaped stacking, weld the pole ears, package them in a short-blade aluminum shell (shell size: L364×W118×H22.1mm), bake the pole group, and finally inject liquid.

[0043] Step 5: Test the cabinet for chemical composition.

[0044] Example 1

[0045] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0046] S1: The prepared NFPP laminated square shell battery is filled with liquid and sealed, and after being fully infiltrated with the electrolyte, it is placed in a test cabinet for formation;

[0047] S2: Formation stage: Three-stage gradient charging steps are adopted, specifically:

[0048] The first stage of formation: the first constant current charge is I1 = 0.02C for 1h;

[0049] The second stage of formation: the second constant current charge is I2 = 0.04C for 1h;

[0050] The third stage of formation: the third constant current charging is 7h at I3=0.1C;

[0051] The battery is charged cumulatively until the cut-off capacity is 76% SOC to obtain a formed battery;

[0052] S3: Aging stage: aging at room temperature for 24 hours;

[0053] S4: Capacity division stage: The aged battery is charged with a constant current and constant voltage at a charging current of 0.5C until it reaches 3.6V, and then charged with a constant voltage until the cut-off current reaches 0.05C. The discharge stage is discharged with a constant current of 0.5C until it reaches 1.5V.

[0054] S5: Cycle stage: charge and discharge at a constant power of 0.5P, with a voltage window of 1.5V-3.6V;

[0055] During the above volume separation stage and circulation stage, no negative pressure is required and no additional clamps are required.

[0056] Example 2

[0057] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0058] S1: The prepared NFPP laminated square shell battery is filled with liquid and sealed, and after being fully infiltrated with the electrolyte, it is placed in a test cabinet for formation;

[0059] S2: Formation stage: Three-stage gradient charging steps are adopted, specifically:

[0060] The first stage of formation: the first constant current charge is I1 = 0.02C for 2h;

[0061] The second stage of formation: the second constant current charge is I2 = 0.04C for 2h;

[0062] The third stage of formation: the third constant current charging is 7h at I3=0.1C;

[0063] The battery is charged cumulatively until the cut-off capacity is 82% SOC to obtain a formed battery;

[0064] S3: Aging stage: aging at room temperature for 24 hours;

[0065] S4: Capacity division stage: The aged battery is charged with a constant current and constant voltage at a charging current of 0.5C until it reaches 3.6V, and then charged with a constant voltage until the cut-off current reaches 0.05C. The discharge stage is discharged with a constant current of 0.5C until it reaches 1.5V.

[0066] S5: Cycle stage: charge and discharge at a constant power of 0.5P, with a voltage window of 1.5V-3.6V;

[0067] During the above volume separation stage and circulation stage, no negative pressure is required and no additional clamps are required.

[0068] Example 3

[0069] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0070] S1: The prepared NFPP laminated square shell battery is filled with liquid and sealed, and after being fully infiltrated with the electrolyte, it is placed in a test cabinet for formation;

[0071] S2: Formation stage: Three-stage gradient charging steps are adopted, specifically:

[0072] The first stage of formation: the first constant current charge is I1 = 0.02C for 2h;

[0073] The second stage of formation: the second constant current charging is 4h at I2=0.04C;

[0074] The third stage of formation: the third constant current charging is 6h at I3=0.1C;

[0075] The battery is charged cumulatively until the cut-off capacity is 80% SOC to obtain a formed battery;

[0076] S3: Aging stage: aging at room temperature for 24 hours;

[0077] S4: Capacity division stage: The aged battery is charged with a constant current and constant voltage at a charging current of 0.5C until it reaches 3.6V, and then charged with a constant voltage until the cut-off current reaches 0.05C. The discharge stage is discharged with a constant current of 0.5C until it reaches 1.5V.

[0078] S5: Cycle stage: charge and discharge at a constant power of 0.5P, with a voltage window of 1.5V-3.6V;

[0079] During the above volume separation stage and circulation stage, no negative pressure is required and no additional clamps are required.

[0080] Example 4

[0081] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0082] S1: The prepared NFPP laminated square shell battery is filled with liquid and sealed, and after being fully infiltrated with the electrolyte, it is placed in a test cabinet for formation;

[0083] S2: Formation stage: Three-stage gradient charging steps are adopted, specifically:

[0084] The first stage of formation: the first constant current charge is I1 = 0.02C for 1h;

[0085] The second stage of formation: the second constant current charge is I2 = 0.04C for 3h;

[0086] The third stage of formation: the third constant current charging is 6h at I3=0.1C;

[0087] The battery is charged cumulatively until the cut-off capacity is 74% SOC to obtain a formed battery;

[0088] S3: Aging stage: aging at room temperature for 24 hours;

[0089] S4: Capacity division stage: The aged battery is charged with a constant current and constant voltage at a charging current of 0.5C until it reaches 3.6V, and then charged with a constant voltage until the cut-off current reaches 0.05C. The discharge stage is discharged with a constant current of 0.5C until it reaches 1.5V.

[0090] S5: Cycle stage: charge and discharge at a constant power of 0.5P, with a voltage window of 1.5V-3.6V;

[0091] During the above volume separation stage and circulation stage, no negative pressure is required and no additional clamps are required.

[0092] Comparative Example 1

[0093] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0094] S1: The prepared NFPP laminated square shell battery is filled with liquid and sealed, and after being fully infiltrated with the electrolyte, it is placed in a test cabinet for formation;

[0095] S2: Formation stage: Three-stage gradient charging steps are adopted, specifically:

[0096] The first stage of formation: the first constant current charge is I1 = 0.02C for 1h;

[0097] The second stage of formation: the second constant current charge is I2 = 0.04C for 2h;

[0098] The third stage of formation: the third constant current charging is 6h at I3=0.1C;

[0099] The battery is charged cumulatively until the cut-off capacity is 70% SOC to obtain a formed battery;

[0100] S3: Aging stage: aging at room temperature for 24 hours;

[0101] S4: Capacity division stage: The aged battery is charged with a constant current and constant voltage at a charging current of 0.5C until it reaches 3.6V, and then charged with a constant voltage until the cut-off current reaches 0.05C. The discharge stage is discharged with a constant current of 0.5C until it reaches 1.5V.

[0102] S5: Cycle stage: charge and discharge at a constant power of 0.5P, with a voltage window of 1.5V-3.6V;

[0103] During the above volume separation stage and circulation stage, no negative pressure is required and no additional clamps are required.

[0104] Comparative Example 2

[0105] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0106] S1: The prepared NFPP laminated square shell battery is filled with liquid and sealed, and after being fully infiltrated with the electrolyte, it is placed in a test cabinet for formation;

[0107] S2: Formation stage: Three-stage gradient charging steps are adopted, specifically:

[0108] The first stage of formation: the first constant current charge is I1 = 0.02C for 2h;

[0109] The second stage of formation: the second constant current charge is I2 = 0.04C for 2h;

[0110] The third stage of formation: the third constant current charging is 7h at I3=0.1C;

[0111] The battery is charged cumulatively until the cut-off capacity is 84% ​​SOC to obtain a formed battery;

[0112] S3: Aging stage: aging at room temperature for 24 hours;

[0113] S4: Capacity division stage: The aged battery is charged with a constant current and constant voltage at a charging current of 0.5C until it reaches 3.6V, and then charged with a constant voltage until the cut-off current reaches 0.05C. The discharge stage is discharged with a constant current of 0.5C until it reaches 1.5V.

[0114] S5: Cycle stage: charge and discharge at a constant power of 0.5P, with a voltage window of 1.5V-3.6V;

[0115] During the above volume separation stage and circulation stage, no negative pressure is required and no additional clamps are required.

[0116] Comparative Example 3

[0117] A method for forming an iron-based polyphosphate sodium ion battery cell comprises the following steps:

[0118] S1: The prepared NFPP laminated square shell battery is filled with liquid and sealed, and after being fully infiltrated with the electrolyte, it is placed in a test cabinet for formation;

[0119] S2: formation stage: constant current charging is adopted, specifically: constant current charging is carried out at I constant = 0.06C until the cut-off capacity is 76% SOC, and the formed battery is obtained;

[0120] S3: Aging stage: aging at room temperature for 24 hours;

[0121] S4: Capacity division stage: The aged battery is charged with a constant current and constant voltage at a charging current of 0.5C until it reaches 3.6V, and then charged with a constant voltage until the cut-off current reaches 0.05C. The discharge stage is discharged with a constant current of 0.5C until it reaches 1.5V.

[0122] S5: Cycle stage: charge and discharge at a constant power of 0.5P, with a voltage window of 1.5V-3.6V;

[0123] During the above volume separation stage and circulation stage, no negative pressure is required and no additional clamps are required.

[0124] The performance tests were performed on the batteries after capacity conversion in Examples 1 to 4 and Comparative Examples 1 to 3. The test results of the first cycle charge and discharge capacity and the first coulomb efficiency (first efficiency) of the batteries in each Example and Comparative Example are shown in Table 1.

[0125] Table 1

[0126] SOC Charging capacity / Ah Discharge capacity / Ah First effect / % Example 1 76％ 68.9 56.2 81.6 Example 2 80％ 67.9 54.4 80.2 Example 3 82％ 68.7 55.2 80.3 Example 4 74％ 67.3 54.3 80.7 Comparative Example 1 70％ 64.2 48.7 75.9 Comparative Example 2 84％ 69.0 54.4 78.8 Comparative Example 3 76％ 68.7 50.1 72.9

[0127] As can be seen from Table 1, by comparing Example 1 and Comparative Examples 1-2, by changing the SOC in the formation stage, too large or too small SOC will affect the first effect; by comparing Example 1 and Example 3, the first effect can be effectively improved by gradually increasing the charging current in three stages.

[0128] Among them, the first cycle charge and discharge curves of the batteries in Examples 1 to 4 are as follows: Figure 1 As shown, from Figure 1 It can be seen from the relevant data in Table 1 that the capacity of Example 1 is the highest, which can preliminarily indicate that under the same test conditions, the formation scheme of 76% SOC achieves the best film forming performance for the battery.

[0129] The battery of Example 1 to Example 4 was subjected to cycle performance test, and the relationship between capacity retention rate and cycle number is shown in the figure below: Figure 2 As shown, from Figure 2 It can be seen that the battery of Example 1 shows a relatively excellent effect in capacity retention in the first 32 cycles, which further proves that the battery forms a stable SEI film in the film formation stage. Figure 3 As shown, from Figure 3 It can be seen that the interface of the battery negative electrode sheet is normal, there is no sodium precipitation and the diaphragm has no discoloration, indicating that the formation stage adopts a three-stage gradient charging step, specifically: the first stage formation: the first constant current charging at I1 = 0.02C for 1h; the second stage formation: the second constant current charging at I2 = 0.04C for 1h; the third stage formation: the third constant current charging at I3 = 0.1C for 7h; the cumulative charging to the cut-off capacity of 76% SOC is the most preferred solution of the present invention.

[0130] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0131] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for forming an iron-based polyphosphate sodium ion battery cell, characterized in that: The following steps are involved: S1: Seal the battery after injection, and put it on the cabinet for formation after the electrolyte is fully infiltrated; S2: Formation stage: cumulative charging to a cut-off capacity of 74%-82% SOC to obtain a formed battery; S3: Aging stage: aging at room temperature; S4: Capacity division stage: the aged battery is charged to 3.6V with constant current and constant voltage, then charged to the cut-off current with constant voltage, and then discharged to 1.5V with constant current; S5: Cycle stage: constant power charging and discharging, voltage window is 1.5V-3.6V.

2. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: The cut-off capacity in the S2 formation stage is 76% SOC.

3. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: The S2 formation stage adopts three-stage gradient charging steps, specifically: The first stage of formation: the first constant current charge is I1 = 0.02C for 1h-2h; The second stage of formation: the second constant current charge is I2 = 0.04C for 1h-4h; The third stage of formation: the third constant current charging is performed at I3=0.1C for 5h-7h.

4. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 3, characterized in that: The S2 formation stage adopts three-stage gradient charging steps, specifically: The first stage of formation: the first constant current charge is I1 = 0.02C for 1h; The second stage of formation: the second constant current charging is I2 = 0.04C for 1h; The third stage of formation: the third constant current charging is performed at I3=0.1C for 7h.

5. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: The standing time at room temperature in the S3 aging stage is 22h-26h.

6. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: The charging current in the constant current and constant voltage charging stage in S4 is 0.4C-0.6C.

7. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: The cut-off current in S4 is 0.05C.

8. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: The discharge current in the constant current discharge stage in S4 is 0.4C-0.6C.

9. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: The constant power in S5 is 0.5P.

10. The method for forming an iron-based polyphosphate sodium ion battery cell according to claim 1, characterized in that: No negative pressure or additional clamps are required during the formation stage, the volume separation stage and the circulation stage.

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