Method for optimizing and inhibiting sodium precipitation of sodium ion soft package battery core hard carbon by external pressure

Abstract

The invention discloses a method for optimizing and inhibiting sodium precipitation of sodium ion soft package battery core hard carbon by external pressure, and belongs to the field of new energy batteries, and the method comprises the following steps: S1, manufacturing a battery core; s2, a charging system is set, and charging multiplying power in different ranges is set; s3, pressurizing and charging, applying pressure to the battery cell, and charging by adopting the charging system until the battery cell is fully charged; and S4, critical pressure is determined, after charging is finished, the battery cell is disassembled, and the minimum pressure when hard carbon does not separate out sodium is analyzed. According to the method, a critical pressure point for inhibiting sodium precipitation of the hard carbon negative electrode is formulated according to different charging multiplying powers; through external pressure optimization, a pressure critical point which is suitable for different charging scenes and inhibits sodium precipitation of a hard carbon negative electrode is formulated, and another basis is provided for group packaging pressure selection of sodium ion soft package cells.

Description

Technical Field

[0001] The present invention relates to a method for optimizing and suppressing sodium deposition on the hard carbon negative electrode of a sodium-ion soft-pack battery cell, and more specifically, it relates to a method for optimizing and suppressing sodium deposition on the hard carbon of a sodium-ion soft-pack battery cell by applying an external pressure. Background Art

[0002] Currently, the negative electrodes used in sodium-ion batteries are usually hard carbon materials. To ensure the safety of the battery pack, when soft-pack battery cells are assembled and encapsulated, a certain external pressure needs to be applied to fix them. Since the outer packaging of the soft-pack battery cell is an aluminum-plastic composite film and lacks the protection of a rigid housing, the external pressure will affect the internal electrodes of the battery cell, the interface structure, the solubility of the gas generated during the charge and discharge process, and the physical and chemical properties of the separator. These effects may change the sodium deposition situation of the hard carbon negative electrode in the battery cell. However, in different usage scenarios, there is no suitable strategy for determining the appropriate external pressure to suppress sodium deposition on the hard carbon negative electrode.

[0003] For example: Chinese Patent Publication No. CN116825960A, publication date September 29, 2023, invention title "A Modified Carbon Negative Electrode Sheet for Suppressing Sodium Deposition, Its Preparation Method and Application". This application discloses a modified carbon negative electrode sheet for suppressing sodium deposition. This solution mainly coats a functional coating on the modified carbon negative electrode sheet to modify the carbon negative electrode or perform bulk modification. When this modified carbon negative electrode sheet is applied to a sodium-ion battery, it can effectively suppress the sodium deposition phenomenon on the negative electrode during the charge and discharge process of the sodium-ion battery, improve the rate performance and cycle stability of the carbon negative electrode. However, this method does not disclose the influence of external pressure on sodium deposition on the negative electrode of a sodium-ion battery, nor an optimized solution for suppressing sodium deposition on the negative electrode of a sodium-ion battery. Summary of the Invention

[0004] The present invention overcomes the risk of sodium deposition in the hard carbon negative electrode material of sodium-ion batteries and provides a method for optimizing and suppressing sodium deposition on the hard carbon of a sodium-ion soft-pack battery cell by applying an external pressure. This method determines the critical pressure points for suppressing sodium deposition on the hard carbon negative electrode for different charging rates.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions: A method for optimizing and suppressing sodium deposition on the hard carbon of a sodium-ion soft-pack battery cell by applying an external pressure, including the following steps, S1, battery cell manufacturing; S2, charging regime setting, setting different ranges of charging rates; S3, pressurized charging, applying an external pressure to the battery cell and charging it using the charging regime until the battery cell is fully charged; S4, critical pressure determination, after charging is completed, disassemble the battery cell and analyze the minimum pressure when sodium is not deposited on the hard carbon.

[0006] This solution charges the sodium-ion soft-pack battery cell by applying pressure to its exterior, thereby suppressing sodium deposition on the hard carbon negative electrode and improving the electrical performance of the sodium-ion battery. When charging the battery cell, it needs to be charged to a fully charged state for testing. During the pressure application process, the external pressure applied to the battery cell needs to be continuously increased until no sodium deposition occurs on the negative electrode of the sodium-ion battery, that is, the critical pressure at which no sodium deposition occurs on the negative electrode of the sodium-ion battery can be obtained.

[0007] Preferably, in step S1, hard carbon from different sources is used as the negative electrode, and any one of layered oxides, polyanion compounds, Prussian blue, or Prussian white is used as the positive electrode material to assemble a sodium-ion soft-pack battery cell. Hard carbon is the negative electrode of the sodium-ion battery, and the positive electrode materials of sodium-ion batteries usually adopt layered oxides, polyanion compounds, Prussian blue, or Prussian white.

[0008] Preferably, the charging rates include a primary charging rate, and the primary charging rate is less than or equal to 0.5C; a secondary charging rate, and the secondary charging rate is greater than 0.5C and less than or equal to 1C; a tertiary charging rate, and the tertiary charging rates are 2C, 3C, and 4C.

[0009] The primary charging rate is a low charging rate, including 0.01C, 0.02C, 0.04C, 0.05C, 0.06C, 0.08C, 0.1C, 0.2C, 0.3C, 0.4C, and 0.5C; the secondary charging rate is a medium charging rate, between 0.5C and 1C; the tertiary charging rate is a fast charging rate, including 2C, 3C, and 4C; the three charging rates correspond to different charging speeds. The greater the charging rate, the faster the charging speed. Therefore, when charging with different charging rates, the sodium deposition situation on the negative electrode of the sodium-ion battery is different, and different degrees of pressure need to be applied.

[0010] Preferably, when charging at the primary rate, the battery cell is directly charged to a fully charged state at any rate within the range. The external critical pressure needs to be measured when the battery cell is charged to a fully charged state, so as to obtain the external critical pressure for suppressing sodium deposition. Specifically, the primary charging rates include 0.01C, 0.02C, 0.04C, 0.05C, 0.06C, 0.08C, 0.1C, 0.2C, 0.3C, 0.4C, and 0.5C, and the soft-pack battery cell is charged to a fully charged state at any of the above charging rates to improve the reliability of the results.

[0011] Preferably, when charging at the secondary charging rate and the tertiary charging rate, a stepped charging strategy is implemented, and the battery cell is charged to a fully charged state. When charging and testing at the secondary charging rate and the tertiary charging rate, it is also not possible to directly charge at the maximum charging rate. A stepped charging strategy with a low charging rate transition is also required to improve the accuracy of the critical external pressure.

[0012] Preferably, the stepped charging strategy is as follows: First, charge at the primary charging rate until the negative electrode potential reaches 0V, and then use the secondary charging rate or the tertiary charging rate to charge the battery cell to a fully charged state. For hard carbon negative electrode materials, during the charging process, when the sodium insertion potential is close to the precipitation potential of metallic sodium, 0V (vs. Na + / Na), there is a risk of sodium precipitation. Therefore, it is necessary to first charge at a low rate until the negative electrode potential reaches 0V (vs. Na + / Na) corresponding terminal voltage, and then use the secondary rate or the tertiary rate to charge to the fully charged state to test the critical external pressure for suppressing sodium precipitation; here, 0V is the potential of the negative electrode relative to the reference electrode, and the terminal voltage is the voltage between the positive and negative electrodes of the battery cell.

[0013] Preferably, the hard carbon negative electrode is prepared using biomass-based, polymer-based, resin-based, and coal-based precursors.

[0014] Preferably, when manufacturing the battery cell, multiple hard carbon negative electrode materials are stacked. After stacking, the tab is welded, and the liquid injection and heat sealing are carried out in a vacuum glove box. Stack the hard carbon negative electrode materials and then form a battery cell together with the positive electrode material. Then, the battery cell is sealed with an aluminum-plastic film and a liquid injection port is left for injecting the electrolyte. Finally, the battery cell is completely heat-sealed. The heat sealing of the battery cell needs to be carried out in a vacuum glove box to ensure the electrical performance of the battery cell. After the above heat sealing, the battery cell can also be secondarily heat-sealed to improve the sealing effect and prevent electrolyte leakage.

[0015] Preferably, when charging under pressure, the battery cell is placed between the clamping plates using a tooling fixture, and different pressures are applied. Applying pressure to the battery cell with the clamping plates can fix the clamping plates, and at the same time, the force is more uniform and it is easy to change the pressure.

[0016] Preferably, when using the primary charging rate, the external pressure is not less than 2.1 MPa; when using the secondary charging rate, the external pressure is not less than 4 MPa; when using the tertiary charging rate, the external pressure is not less than 6 MPa.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) By optimizing the external pressure, the pressure critical points for suppressing sodium deposition on the hard carbon negative electrode under different charging scenarios are determined, providing another basis for the selection of the pressure for the packaging of sodium-ion soft-pack battery cells; (2) The test method is simple, reliable, easy to implement, and it is easy to implement a relatively large number of comparison schemes. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of the tooling fixture and the battery cell of the present invention.

[0019] Figure 2 It is a schematic diagram of the analysis of sodium deposition on the hard carbon negative electrode under different pressures during the first-stage charging rate charging of the present invention.

[0020] Figure 3 It is a schematic diagram of the analysis of sodium deposition on the hard carbon negative electrode under different pressures during the second-stage charging rate charging of the present invention.

[0021] Figure 4 It is a schematic diagram of the analysis of sodium deposition on the hard carbon negative electrode under different pressures during the third-stage charging rate charging of the present invention.

[0022] In the figure: 1. Tooling fixture, 2. Soft-pack battery cell, 3. Tab. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] The technical solutions of the present invention will be further specifically described below through specific embodiments in conjunction with the drawings.

[0024] Example 1: As Figure 1 and Figure 2 shown, a method for optimizing the external pressure to suppress sodium deposition on the hard carbon of sodium-ion soft-pack battery cells, and the critical point of the external pressure for suppressing sodium deposition on the hard carbon negative electrode at the first charging rate is determined. The specific steps are as follows.

[0025] Fabrication of experimental soft-pack battery cells: First, fix the positive electrode material of the sodium-ion battery, then stack 8 pieces of hard carbon negative electrode materials and 7 pieces of positive electrodes, weld the tabs and attach high-temperature glue after stacking, and place them in an aluminum-plastic film to form a battery cell. Then, use a heat-sealing machine to heat-seal the battery cell. During the heat-sealing operation, first heat-seal one side and the top surface of the battery cell, then inject the electrolyte into the battery cell from the unsealed surface inside the glove box. After the injection is completed, continue to heat-seal the injection end of the battery cell inside the glove box. Leave it for an appropriate time to allow the electrolyte to fully infiltrate the electrode sheets to ensure the electrical performance of the battery cell. The inside of the glove box is in a vacuum state, and the soft-pack battery cell is fabricated in a vacuum state to ensure the electrical performance of the battery cell and improve the accuracy of the test results. Finally, transfer the soft-pack battery cell out of the glove box and use a heat-sealing machine to perform secondary heat-sealing on the soft-pack battery cell to improve the sealing effect of the soft-pack battery cell and prevent the leakage of the electrolyte. After heat-sealing, transfer the soft-pack battery cell to an incubator at 45 °C for aging.

[0026] Among them, the hard carbon negative electrode of the soft-pack battery cell is hard carbon prepared from biomass-based, polymer-based, resin-based, and coal-based precursors, and the positive electrode material is any one of layered oxides, polyanion compounds, Prussian blue, or Prussian white. Each material is tested to make the experimental results more representative and make the final critical pressure generally applicable.

[0027] Charging regime setting: Set the first-stage charging rate to charge the soft-pack battery cell. The first-stage charging rate is a charging rate less than or equal to 0.5C; in this embodiment, the battery cell is first formed, and then the soft-pack battery cell is charged to a full charge state using a charging rate of 0.5C.

[0028] Pressurized charging and critical pressure determination: As Figure 2 shown, prepare several groups of the above-prepared soft-pack battery cells, place the soft-pack battery cells between the tooling jigs respectively, make the tooling jigs apply different pressures to each group of soft-pack battery cells, start increasing the pressure from 0 MPa, and gradually increase the applied pressure in a gradient of 0.5 MPa; then charge each group of the above battery cells using the first-stage charging rate. When charging using the first-stage charging rate,. After charging is completed, disassemble the soft-pack battery cell in the glove box, and analyze the degree of sodium deposition on the hard carbon negative electrode according to the disassembly results. The results are shown in Table 1 below. External pressure (MPa) Sodium separation situation 0 General 0.5 General 1.0 General 1.5 Slight 2.0 Slight 2.5 None Table 1

[0029] It can be seen from Table 1 that under the charging conditions of the first-stage charging rate, when the applied pressure on the soft-pack battery cell is 2.0 MPa, there is still a slight sodium deposition phenomenon on the hard carbon negative electrode, and when the applied pressure is 2.5 MPa, there is no sodium deposition phenomenon on the hard carbon negative electrode. Therefore, the critical point of the applied pressure to inhibit sodium deposition on the hard carbon negative electrode is between 2.0 MPa and 2.5 MPa.

[0030] Based on the above results, further repeat the above steps for determining the critical pressure between 2.0 MPa and 2.5 MPa of the applied pressure. Prepare several groups of the above-prepared soft-pack battery cells, place the soft-pack battery cells between the tooling jigs respectively, make the tooling jigs apply different pressures to each group of soft-pack battery cells, start increasing the pressure from 2.1 MPa, and gradually increase the applied pressure in a gradient of 0.1 MPa; then charge each group of the above battery cells using the first-stage charging rate. When charging using the first-stage charging rate, test using charging rates of 0.01C, 0.02C, 0.04C, 0.05C, 0.06C, 0.08C, 0.1C, 0.2C, 0.3C, 0.4C, and 0.5C, and charge the soft-pack battery cell to a full charge state using any of the above charging rates. After charging is completed, disassemble the soft-pack battery cell in the glove box, and analyze the degree of sodium deposition on the hard carbon negative electrode according to the disassembly results. The results are shown in Table 2 below. External pressure (MPa) Sodium separation situation 2.1 None 2.2 None 2.3 None 2.4 None Table 2

[0031] As can be seen from Table 1 and Table 2, the critical point of the external pressure for suppressing sodium deposition on the hard carbon negative electrode is between 2.0 MPa and 2.1 MPa. To more accurately determine the critical pressure, the above steps for determining the critical pressure are repeated between 2.0 MPa and 2.1 MPa of the external pressure. However, in practical applications, in order to avoid the occurrence of sodium deposition phenomenon, the applied pressure should be greater than the theoretical critical pressure. Therefore, 2.1 MPa can be directly used as the critical pressure for its actual use. Selecting this pressure as the critical pressure under the first-level charge rate condition can effectively suppress sodium deposition on the hard carbon negative electrode and ensure the electrical performance of the sodium-ion battery. On the other hand, if a larger external pressure is adopted, it is easy to cause deformation of the soft-pack battery cell and affect the electrical performance of the sodium-ion battery.

[0032] Figure 2 Only the result diagrams under partial external pressures are shown, where a is the sodium deposition situation on the hard carbon negative electrode under 0 MPa external pressure, b is the sodium deposition situation on the hard carbon negative electrode under 0.5 MPa external pressure, c is the sodium deposition situation on the hard carbon negative electrode under 1.0 MPa external pressure, and d is the sodium deposition situation on the hard carbon negative electrode under 2.1 MPa external pressure. Among them, in each group of result diagrams, the one in the upper right corner is the positive electrode material, and the remaining 8 are hard carbon negative electrode materials.

[0033] Example 2: As Figure 1 and Figure 3 shown, a method for optimizing the external pressure to suppress sodium deposition on hard carbon in a sodium-ion soft-pack battery cell. The determination of the critical point of the external pressure for suppressing sodium deposition on the hard carbon negative electrode at the second-level charge rate is as follows.

[0034] Fabrication of the experimental soft-pack battery cell: First, fix the positive electrode material of the sodium-ion battery, then stack 8 pieces of hard carbon negative electrode materials and 7 pieces of positive electrodes, weld the tab and paste the high-temperature adhesive after stacking, and put them into the aluminum plastic film to form the battery cell, and then heat-seal the battery cell with a heat-sealing machine; during the heat-sealing operation, first heat-seal one side and the top surface of the battery cell, then inject the electrolyte from the unsealed surface of the battery cell in the glove box, and after the injection is completed, continue to heat-seal the injection end of the battery cell in the glove box; place it for an appropriate time to allow the electrolyte to fully infiltrate the electrode sheet and ensure the electrical performance of the battery cell. The inside of the glove box is in a vacuum state, and the soft-pack battery cell is fabricated in a vacuum state to ensure the electrical performance of the battery cell and improve the accuracy of the test results. Finally, transfer the soft-pack battery cell out of the glove box and perform secondary heat-sealing on the soft-pack battery cell with a heat-sealing machine to improve the sealing effect of the soft-pack battery cell and avoid electrolyte leakage. After heat-sealing, transfer the soft-pack battery cell to a constant temperature oven at 45 °C for aging.

[0035] Among them, the hard carbon negative electrode of the soft-pack battery cell is hard carbon prepared from biomass-based, polymer-based, resin-based, and coal-based precursors, and the positive electrode material is any one of layered oxides, polyanion compounds, Prussian blue, or Prussian white. Each material is tested to make the experimental results more representative and make the final critical pressure generally applicable.

[0036] Charging regime setting: The soft-pack battery cell is charged with a two-stage charging rate. The two-stage charging rate is a charging rate greater than 0.5C and less than or equal to 1C; in this embodiment, the battery cell is first formed, and then the soft-pack battery cell is charged to a full charge state using a charging rate of 1C.

[0037] Critical pressure determination: As Figure 3 shown, prepare several groups of the above-prepared soft-pack battery cells, place the soft-pack battery cells between the tooling jigs respectively, and make the tooling jigs apply different pressures to each group of soft-pack battery cells. As can be seen from Embodiment 1, under the condition of the first-stage charging rate, when the external pressure is 2.0 MPa, there is still a slight sodium precipitation phenomenon. Therefore, under the second-stage charging rate, there will also be a sodium precipitation phenomenon when the external pressure is 2.0 MPa. Therefore, in this embodiment, the external pressure can start from 2.5 MPa and gradually increase the external pressure in a gradient of 0.5 MPa; then charge each group of the above battery cells using the second-stage charging rate. When charging using the second-stage charging rate, a stepped charging strategy is first required. Specifically, first charge at the first-stage charging rate until the negative electrode potential reaches 0 V, and then charge the battery cell to the full charge state using the second-stage charging rate condition. For the hard carbon negative electrode material, during the charging process, when the sodium insertion potential is close to the sodium precipitation potential of metallic sodium, 0 V (vs. Na + / Na), there is a risk of sodium precipitation. Therefore, it is necessary to first charge at a small rate until the negative electrode potential reaches 0 V (vs. Na + / Na) corresponding terminal voltage, and then charge to the full charge state using the second-stage rate to test the external critical pressure for suppressing sodium precipitation; 0 V here is the potential of the negative electrode relative to the reference electrode, and the terminal voltage is the voltage between the positive and negative electrodes of the battery cell; then increase the charging rate to 1C to charge the soft-pack battery cell to the full charge state. After charging, disassemble the soft-pack battery cell in the glove box, and analyze the degree of sodium precipitation of the hard carbon negative electrode according to the disassembly results. The results are shown in Table 3 below. External pressure (MPa) Sodium separation situation 2.5 General 3.0 General 3.0 Slight 3.5 Slight 4.0 None 4.5 None Table 3

[0038] It can be obtained from Table 3 that under the charging condition of the second-stage charging rate, when the external pressure of the soft-pack battery cell is 3.5 MPa, the hard carbon negative electrode still has a slight sodium precipitation phenomenon, and when the external pressure is 4.0 MPa, the hard carbon negative electrode has no sodium precipitation phenomenon. Therefore, the critical point of the external pressure for suppressing sodium precipitation of the hard carbon negative electrode is between 3.5 MPa and 4.0 MPa.

[0039] On the basis of the above results, repeat the steps for determining the critical pressure between an external pressure of 3.5 MPa and 4.0 MPa. Prepare several groups of the above-prepared soft-pack battery cells, place the soft-pack battery cells between the tooling jigs respectively, apply different pressures to each group of soft-pack battery cells by the tooling jigs, start increasing the pressure from 3.5 MPa, and gradually increase the external pressure in a gradient of 0.1 MPa; then charge each group of the above battery cells at a two-stage charging rate. When charging at a two-stage charging rate, first a step charging strategy needs to be adopted, and then the charging rate is increased to 1C to charge the soft-pack battery cells to a full charge state. After the charging is completed, disassemble the soft-pack battery cells in the glove box, and analyze the degree of sodium precipitation on the hard carbon negative electrode according to the disassembly results. The results are shown in Table 4 below. External pressure (MPa) Sodium separation situation 3.6 Slight 3.7 Slight 3.8 Slight 3.9 Slight Table 4

[0040] As can be seen from Table 3 and Table 4, the critical point of the external pressure for suppressing sodium precipitation on the hard carbon negative electrode is between 3.9 MPa and 4.0 MPa. If a more precise critical pressure needs to be determined, repeat the above steps for determining the critical pressure between an external pressure of 3.9 MPa and 4.0 MPa. However, in practical applications, in order to avoid the occurrence of sodium precipitation, the applied pressure should be greater than the theoretical critical pressure. Therefore, 4.0 MPa can be directly used as the actual critical pressure for use. Selecting this pressure as the critical pressure under the two-stage charging rate condition can, on the one hand, effectively suppress sodium precipitation on the hard carbon negative electrode and ensure the electrical performance of the sodium-ion battery; on the other hand, if a larger external pressure is adopted, it is easy to cause deformation of the soft-pack battery cells and affect the electrical performance of the sodium-ion battery.

[0041] Figure 3 Only the result diagrams under partial external pressures are shown. Among them, e is the sodium precipitation situation on the hard carbon negative electrode under an external pressure of 2.5 MPa, f is the sodium precipitation situation on the hard carbon negative electrode under an external pressure of 3.0 MPa, and g is the sodium precipitation situation on the hard carbon negative electrode under an external pressure of 4.0 MPa. Among them, in each group of result diagrams, the one in the upper right corner is the positive electrode material, and the remaining 8 are hard carbon negative electrode materials.

[0042] Example 3: A method for optimizing the external pressure to suppress sodium precipitation on the hard carbon of a sodium-ion soft-pack battery cell as shown in Figure 1 and Figure 4 shown. Determine the critical point of the external pressure for suppressing sodium precipitation on the hard carbon negative electrode at a three-stage charging rate. The specific steps are as follows.

[0043] Fabrication of experimental soft-pack battery cells: First, fix the positive electrode material of the sodium-ion battery. Then, stack 8 pieces of hard carbon negative electrode material and 7 pieces of positive electrode. After stacking, weld the tab and apply high-temperature adhesive, and place it in an aluminum-plastic film to form a battery cell. Then, use a heat-sealing machine to heat-seal the battery cell. During the heat-sealing operation, first heat-seal one side and the top surface of the battery cell, and then inject electrolyte into the battery cell from the unsealed side inside the glove box. After the injection is completed, continue to heat-seal the injection end of the battery cell inside the glove box. Leave it for an appropriate time to allow the electrolyte to fully infiltrate the electrode sheets and ensure the electrical performance of the battery cell. The inside of the glove box is in a vacuum state, and the soft-pack battery cell is fabricated in a vacuum state to ensure the electrical performance of the battery cell and improve the accuracy of the test results. Finally, transfer the soft-pack battery cell out of the glove box and use a heat-sealing machine to perform secondary heat-sealing on the soft-pack battery cell to improve the sealing effect of the soft-pack battery cell and prevent electrolyte leakage. After heat-sealing, transfer the soft-pack battery cell to an incubator at 45 °C for aging.

[0044] Among them, the hard carbon negative electrode of the soft-pack battery cell is hard carbon prepared from biomass-based, polymer-based, resin-based, and coal-based precursors, and the positive electrode material is any one of layered oxides, polyanion compounds, Prussian blue, or Prussian white. Each material is tested to make the experimental results more representative and make the final critical pressure generally applicable.

[0045] Charging regime setting: Set a two-stage charging rate to charge the soft-pack battery cell. The two-stage charging rates are 2C, 3C, and 4C charging rates. In this embodiment, first, the battery cell is formed, and then the soft-pack battery cell is charged to a full charge state using a 4C charging rate.

[0046] Critical pressure determination: As Figure 4 shown, prepare several groups of the above-prepared soft-pack battery cells, and place the soft-pack battery cells between the tooling fixtures respectively, so that the tooling fixtures apply different pressures to each group of soft-pack battery cells. As known from Example 2, under the condition of the two-stage charging rate, when the external pressure is 3.9 MPa, there is still a slight sodium precipitation phenomenon. Therefore, under the three-stage charging rate, there will also be a sodium precipitation phenomenon when the external pressure is 3.9 MPa. Therefore, in this embodiment, the external pressure can start from 4.0 MPa and gradually increase the external pressure in a gradient of 0.5 MPa. Then, charge each group of the above battery cells using a three-stage charging rate. When charging using a three-stage charging rate, first, a stepped charging strategy is required. Specifically, first charge at a first-stage charging rate until the negative electrode potential reaches 0 V, and then charge the battery cell to a full charge state using the three-stage charging rate conditions. For the hard carbon negative electrode material, during the charging process, when the sodium insertion potential is close to the sodium precipitation potential of metallic sodium, 0 V (vs. Na + / Na), there is a risk of sodium precipitation. Therefore, it is necessary to first charge at a small rate until the negative electrode potential reaches 0 V (vs. Na +The terminal voltage corresponding to (Na⁺ / Na), and then charge it to the fully charged state at three times the rate to test the external critical pressure for suppressing sodium deposition; here, 0V is the potential of the negative electrode relative to the reference electrode, and the terminal voltage is the voltage between the positive and negative electrodes of the battery cell; then gradually increase the charging rate to 2C, 3C, and finally charge the soft-pack battery cell to the fully charged state at a charging rate of 4C. After charging, disassemble the soft-pack battery cell in the glove box, and analyze the degree of sodium deposition on the hard carbon negative electrode according to the disassembly results. The results are shown in Table 5 below. External pressure (MPa) Sodium separation situation 4.0 Severe 4.5 General 5.0 General 5.5 Slight 6.0 None 6.5 None Table 5

[0047] It can be seen from Table 5 that under the charging conditions of three charging rates, when the external pressure of the soft-pack battery cell is 5.5 MPa, there is still a slight sodium deposition phenomenon on the hard carbon negative electrode. When the external pressure is 6.0 MPa, there is no sodium deposition phenomenon on the hard carbon negative electrode. Therefore, the critical point of the external pressure for suppressing sodium deposition on the hard carbon negative electrode is between 5.5 MPa and 6.0 MPa.

[0048] Based on the above results, further repeat the steps for determining the critical pressure between 5.5 MPa and 6.0 MPa. Prepare several groups of the above-prepared soft-pack battery cells, place each soft-pack battery cell between the tooling jigs respectively, and make the tooling jigs apply different pressures to each group of soft-pack battery cells. The pressure starts from 5.5 MPa and increases gradually in a gradient of 0.1 MPa; then charge each group of the above battery cells using three charging rates. When charging using three charging rates, first, a stepped charging strategy is required, and then gradually increase the charging rate to 2C, 3C, and finally charge the soft-pack battery cell to the fully charged state at a charging rate of 4C. After charging, disassemble the soft-pack battery cell in the glove box, and analyze the degree of sodium deposition on the hard carbon negative electrode according to the disassembly results. The results are shown in Table 6 below. External pressure (MPa) Sodium separation situation 5.6 Slight 5.7 Slight 5.8 Slight 5.9 Slight Table 6

[0049] From Table 5, Table 6, and Figure 4 It can be known that the critical point of the external pressure for suppressing sodium deposition on the hard carbon negative electrode is between 5.9 MPa and 6.0 MPa. To more accurately determine the critical pressure, repeat the above steps for determining the critical pressure between 5.9 MPa and 6.0 MPa. However, in practical applications, to avoid the occurrence of sodium deposition, the applied pressure should be greater than the theoretical critical pressure. Therefore, 6.0 MPa can be directly used as the actual critical pressure. Selecting this pressure as the critical pressure under the condition of three charging rates can, on the one hand, effectively suppress sodium deposition on the hard carbon negative electrode and ensure the electrical performance of the sodium-ion battery; on the other hand, if a larger external pressure is used, it is easy to cause deformation of the soft-pack battery cell and affect the electrical performance of the sodium-ion battery.

[0050] Figure 4 Only the result diagrams under partial applied pressures are shown. Among them, h is the sodium precipitation situation of the hard carbon negative electrode under an applied pressure of 4.0 MPa, i is the sodium precipitation situation of the hard carbon negative electrode under an applied pressure of 5.0 MPa, and j is the sodium precipitation situation of the hard carbon negative electrode under an applied pressure of 6.0 MPa. Among them, in each group of result diagrams, the one in the upper right corner is the positive electrode material, and the remaining 8 are hard carbon negative electrode materials.

Claims

1. A method for optimizing the suppression of sodium precipitation in hard carbon of sodium-ion soft-packaged battery cells by applying external pressure, characterized in that, It includes the following steps: S1, cell fabrication; S2, charging regime setting, setting charging rates in different ranges; S3, pressure-assisted charging, applying an external pressure to the cell and charging it using the said charging regime until the cell is fully charged; S4, critical pressure determination, after charging is completed, disassembling the cell and analyzing the minimum pressure when no sodium is separated from the hard carbon.

2. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, In the said step S1, hard carbon is used as the negative electrode, and any one of layered oxides, polyanion compounds, Prussian blue or Prussian white is matched as the positive electrode material to assemble a sodium-ion soft-pack cell.

3. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, The said charging rates include a primary charging rate, and the primary charging rate is less than or equal to 0.5C; a secondary charging rate, and the secondary charging rate is greater than 0.5C and less than or equal to 1C; a tertiary charging rate, and the tertiary charging rates are 2C, 3C and 4C.

4. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, When charging at the said primary rate, directly charge the cell to the fully charged state at any rate within the range.

5. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, When charging at the said secondary charging rate and the tertiary charging rate, implement a stepped charging strategy and charge the cell to the fully charged state.

6. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, The step charging strategy is as follows: first, charge at the first-stage charging rate until the negative electrode potential reaches 0 V (vs. Na + / Na), and then charge the battery cell to the full charge state using the second-stage charging rate or the third-stage charging rate.

7. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, The hard carbon negative electrode is prepared using biomass-based, polymer-based, resin-based and coal-based materials as precursors.

8. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, During cell fabrication, stack multiple hard carbon negative electrode materials, after stacking is completed, weld the tabs, and perform liquid injection and heat sealing in a vacuum glove box.

9. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, During pressure-assisted charging, use a tooling fixture to place the cell between the clamping plates and apply different pressures.

10. A method for optimizing the suppression of sodium precipitation in hard carbon of a sodium-ion soft-pack battery cell by applying external pressure, characterized in that, When using the primary charging rate, the external pressure is not less than 2.1 MPa; when using the secondary charging rate, the external pressure is not less than 4 MPa; when using the tertiary charging rate, the external pressure is not less than 6 MPa.

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