Formation method of sodium ion battery

Abstract

The invention discloses a formation method of a sodium ion battery, which comprises the following steps: S1, performing constant current I1 charging formation on the sodium ion battery after liquid injection and standing at preset temperature and pressure, and charging to cut-off capacity Q; s2, discharging the battery at a constant current I2 to a cut-off voltage V1 during normal use; s3, the discharge current is adjusted, and the small current I3 is adopted to continuously discharge the battery to the cut-off voltage V2; s4, placing for a certain time t; s5, the steps S3-S4 are repeated for N times, and N is equal to 1-5; s6, the battery is subjected to air exhaust and sealing; and S7, charging the battery at the current I3 to a charging state required by the battery. According to the invention, gas molecules in an adsorption state can be removed, gas generation expansion during overdischarge of the battery is inhibited, and a stable SEI membrane can be constructed, so that the cycle life of the sodium ion battery can be prolonged.

Description

Technical Field

[0001] The invention relates to a sodium ion battery, and in particular to a formation method of a sodium ion battery. Background Art

[0002] In recent years, with the rapid development of domestic new energy vehicles and energy storage batteries, the demand for lithium-ion batteries has exploded. Due to the sharp increase in demand for upstream raw materials, the price of raw materials such as lithium carbonate has continued to rise. Due to the relatively lagging development of lithium mines and the scarcity of lithium mine resources, the cost of battery cells has remained high. In this context, sodium-ion batteries have received widespread attention and have ushered in an opportunity for rapid development.

[0003] The working principle of sodium-ion batteries is similar to that of lithium-ion batteries. They use the reversible insertion and deinsertion of sodium ions between the positive and negative electrodes to achieve energy storage and release. Due to the wide range of sodium resources, the potential low-cost expectations, and the use of non-graphitized carbon materials with large interlayer spacing (such as hard carbon) in the negative electrode, it can achieve fast charging and discharging rate performance and a wide operating temperature range, and has excellent safety performance. These advantages make it have broad application prospects in the fields of new energy storage and small power.

[0004] However, during the use of batteries, over-discharge scenarios often occur. Sodium-ion batteries have unique advantages over lithium-ion batteries in terms of over-discharge resistance, but they are still prone to produce gas when over-discharged, mainly due to the release of gas adsorbed on the hard carbon of the negative electrode. The generation of gas can cause the battery to swell, and in severe cases it can damage the external seals, causing battery leakage and posing a safety hazard. In addition, after over-discharge, the battery is further charged, and the gas is prone to uneven adsorption or reduction on the negative electrode side, resulting in a decrease in the battery cycle performance.

[0005] At the same time, formation is one of the important links in the production of lithium-ion and sodium-ion batteries, affecting the performance of sodium-ion batteries such as the initial capacity, charge and discharge efficiency, and cycle life. During the formation process of sodium-ion batteries, gas is formed along with the negative electrode SEI film. In order to eliminate the gas generated by these reduction reactions, the prior art either sets a negative pressure environment to exhaust during the formation process (as shown in Chinese patent CN202411038260.8), or performs exhaust after formation (such as shown in Chinese patents CN202210455617.7 and CN202310715632.5).

[0006] Although the above-mentioned existing technical means can significantly eliminate the gas in the free state, the gas molecules in the adsorbed state cannot be eliminated. Therefore, the sodium ion battery still has the above-mentioned problems that occur during over-discharge, so the formation process of the sodium ion battery still needs to be further optimized. Summary of the invention

[0007] The object of the present invention is to provide a formation method for a sodium ion battery, which can remove gas molecules in an adsorbed state, inhibit the problem of gas expansion during overdischarge of the battery, and construct a stable SEI film, thereby extending the cycle life of the sodium ion battery.

[0008] In order to achieve the above purpose, the following technical solutions are adopted:

[0009] A formation method for a sodium ion battery comprises the following steps:

[0010] S1: The sodium ion battery after injection and standing is charged at a preset temperature and pressure with a constant current I1 to a cut-off capacity Q;

[0011] S2: Discharge the battery with a constant current I2 to the cut-off voltage V1 during normal use;

[0012] S3: Adjust the discharge current and use a small current I3 to continue discharging the battery to the cut-off voltage V2;

[0013] S4: put aside for a certain time t;

[0014] S5: Repeat steps S3-S4 N times, where N=1-5;

[0015] S6: evacuate and seal the battery;

[0016] S7: Charge the battery with the current I3 to the required charging state of the battery.

[0017] Furthermore, in S1, the temperature is between room temperature and 90° C., and the pressure is between 0.02 MPa and 3 MPa.

[0018] Furthermore, in S1, the cut-off capacity Q is set to 10%-50% of the capacity of the negative electrode of the battery.

[0019] Furthermore, the current I1 and the current I2 are between 0.1-2C.

[0020] Furthermore, in S2, the cut-off voltage V1 is determined according to the battery material system used.

[0021] Furthermore, in S3, the current I3 is between 0.01C and 0.1C.

[0022] Furthermore, in S3, 0≤cut-off voltage V2<cut-off voltage V1.

[0023] Furthermore, in S4, t is between 1 and 30 min.

[0024] Furthermore, the positive electrode active material of the sodium ion single cell is a material that can reversibly embed and de-insert sodium ions.

[0025] Furthermore, the negative electrode active material of the sodium ion single cell is a soft or hard carbon material.

[0026] By adopting the above scheme, the beneficial effects of the present invention are:

[0027] 1) Compared with the existing formation process, the present invention controls the cut-off capacity in the first step of charging, which can prevent the generated gas from being reduced or adsorbed in the high SOC state during the formation process, thereby avoiding the problem of aggravating the non-uniformity of the SEI film and the difficulty of gas desorption;

[0028] 2) The present invention adds a small current over-discharge process, which can desorb the gas generated in the formation process and circulate it multiple times to make the gas desorption more thorough. At the same time, the over-discharge oxidizes and decomposes the unstable components in the SEI film to form more inorganic components, thereby constructing an inorganic-rich SEI film on the surface of the negative electrode. The inorganic SEI film has high ion conductivity, high mechanical strength and good stability, which greatly improves the cycle stability of the negative electrode;

[0029] 3) The sodium ion battery provided by the present invention does not produce gas when over-discharged, thereby improving safety performance. At the same time, under normal use, its cycle performance is also significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of the present invention;

[0031] Figure 2 The graphs are the cycle life curves of the battery cells of Example 1 and Comparative Example 1 of the present invention;

[0032] Figure 3 The graphs of the cycle life of the battery cells of Example 5 and Comparative Example 2 of the present invention are as follows;

[0033] Figure 4 The graph shows the cycle life of the battery cells of Example 7 of the present invention and Comparative Example 3. DETAILED DESCRIPTION

[0034] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0035] Reference Figures 1 to 4 As shown, the present invention provides a formation method of a sodium ion battery, comprising the following steps:

[0036] S1: The sodium ion battery after injection and standing is charged and formed at a constant current I1 at a preset temperature and pressure until it reaches a cut-off capacity Q.

[0037] In this step, the temperature value and the pressure value are not limited. In a preferred embodiment, the temperature is set between room temperature and 90 °C, and the pressure is set between 0.02 Mpa and 3 Mpa. At the same time, the cut-off capacity Q is set to 10%-50% of the negative electrode capacity of the battery. If the charging capacity is too low, it is not sufficient to generate a stable SEI film, while if the charging capacity is too high, it takes a long time, and the gas generated during the SEI film formation process will be reduced or adsorbed, affecting the battery performance.

[0038] S2: Discharge the battery at a constant current I2 to the cut-off voltage V1 during normal use.

[0039] In this step, the current values I1 and I2 are not limited. In a preferred embodiment, they can be set between 0.1-2C, and variable current gradient charging can also be performed. At the same time, the cut-off voltage V1 is not limited and is determined according to the battery material system used. For example, for a sodium-ion battery with a layered oxide cathode material as the cathode and hard carbon as the anode, V1 is between 1.5-2.0V; if a sodium-ion battery with sodium iron phosphate pyrophosphate NFPP as the cathode and hard carbon as the anode, then V1 is between 1.5-2.0V; if a sodium-ion battery with sodium iron sulfate NFS as the cathode and hard carbon as the anode, then V1 is between 2.5-2.8V.

[0040] S3: Adjust the discharge current and continue to discharge the battery to the cut-off voltage V2 using a small current I3.

[0041] In this step, the current I3 is between 0.01C-0.1C, and the cut-off voltage V2 satisfies 0≤V2<V1, which is determined according to the battery material system used. If the cut-off voltage is too high and close to the voltage during normal use, the gas desorption is incomplete. When using aluminum foil as the negative electrode, it can be discharged to 0V. In a preferred embodiment, V2 is between 0.0-1.0V.

[0042] S4: Leave it for a certain period of time t.

[0043] In this step, the leaving time t is not restricted and depends on the actual production needs. In a better embodiment, t is between 1-30 minutes.

[0044] S5: Repeat steps S3-S4 N times, where N = 1-5.

[0045] In this step, a method of discharging multiple times in a cycle is adopted, and the purpose is to completely desorb the gas. The number of cycles is at least once. If the number of times is too many, it will affect the production efficiency.

[0046] S6: Evacuate and seal the battery.

[0047] S7: Charge the battery to the required charging state with current I3.

[0048] In this step, the current I3 and the charging state are not limited. The charging conditions can be the same as or different from those in step S1.

[0049] At the same time, in one embodiment, the positive electrode active material of the sodium ion monomer battery is a material that can reversibly embed and deintercalate sodium ions, which may specifically include vanadium-based, iron-based polyanion positive electrode materials containing sodium elements, Prussian white, Prussian blue positive electrode materials or layered oxide positive electrode materials, etc.; and the negative electrode active material of the sodium ion monomer battery is carbon materials such as soft and hard carbon.

[0050] The following is a comparative description with specific embodiments:

[0051] Embodiment 1:

[0052] Using layered ternary material NaNi1 / 3Fe1 / 3Mn1 / 3O2 (NFM) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium-ion battery monomer with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 4.0-1.5V; after baking and injection processes, the sodium-ion battery monomer is formed using a charge and discharge formation cabinet, and the specific steps are as follows:

[0053] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.5Mpa, the first charging current is set to 0.5C (current 2.5A), and the sodium ion battery is charged with a constant current, and the charging cut-off capacity Q1 = 2.4AH;

[0054] 2) Discharge the battery cell at a current of 1C to a cut-off voltage of 1.5V;

[0055] 3) Discharge the battery cell at a current of 0.05C to a cut-off voltage of 0.5V;

[0056] 4) Leave for 10 minutes;

[0057] 5) Repeat steps 3-4 twice;

[0058] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0059] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0060] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as sealing, aging and capacity separation are completed.

[0061] Comparative Example 1:

[0062] It is prepared using the same materials and the same process as Example 1, except that steps 3 to 5 of the above-mentioned formation process are omitted.

[0063] Embodiment 2:

[0064] Using layered ternary material NaNi1 / 3Fe1 / 3Mn1 / 3O2 (NFM) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium-ion battery monomer with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 4.0-1.5V; after baking and liquid injection processes, the sodium-ion battery monomer is formed in a charge and discharge formation cabinet, and the specific steps are as follows:

[0065] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.5Mpa, the first charging current is set to 0.5C (current 2.5A), and the sodium ion battery is charged with a constant current, and the charging cut-off capacity Q1 = 5AH;

[0066] 2) Discharge the battery cell at a current of 1C to a cut-off voltage of 1.5V;

[0067] 3) Discharge the battery cell at a current of 0.05C to a cut-off voltage of 0.5V;

[0068] 4) Leave for 10 minutes;

[0069] 5) Repeat steps 3-4 twice;

[0070] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0071] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0072] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as aging and capacity separation are completed.

[0073] Embodiment 3:

[0074] Using layered ternary material NaNi1 / 3Fe1 / 3Mn1 / 3O2 (NFM) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium-ion battery monomer with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 4.0-1.5V; after baking and liquid injection processes, the sodium-ion battery monomer is formed in a charge and discharge formation cabinet, and the specific steps are as follows:

[0075] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.5Mpa, the first charging current is set to 0.5C (current 2.5A), and the sodium ion battery is charged with a constant current, and the charging cut-off capacity Q1 = 0.5AH;

[0076] 2) Discharge the battery cell at a current of 1C to a cut-off voltage of 1.5V;

[0077] 3) Discharge the battery cell at a current of 0.05C to a cut-off voltage of 0.5V;

[0078] 4) Leave for 10 minutes;

[0079] 5) Repeat steps 3-4 twice;

[0080] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0081] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0082] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as aging and capacity separation are completed.

[0083] Embodiment 4:

[0084] Using layered ternary material NaNi1 / 3Fe1 / 3Mn1 / 3O2 (NFM) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium-ion battery monomer with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 4.0-1.5V; after baking and liquid injection processes, the sodium-ion battery monomer is formed in a charge and discharge formation cabinet, and the specific steps are as follows:

[0085] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.5Mpa, and the first charging current is set to 0.5C (current 2.5A), which performs constant current charging on the sodium ion battery, and the charging cut-off capacity Q1 = 2.4AH;

[0086] 2) Discharge the battery cell at a current of 1C to a cut-off voltage of 1.5V;

[0087] 3) Discharge the battery cell at a current of 0.02C to a cut-off voltage of 0.0v;

[0088] 4) Leave for 10 minutes;

[0089] 5) Repeat steps 3-4 twice;

[0090] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0091] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0092] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as aging and capacity separation are completed.

[0093] Embodiment 5:

[0094] Using sodium sulfate (NFS) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium-ion battery cell with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 4.2-2.6V; after the baking and liquid injection process, the sodium-ion battery cell is formed in a charge and discharge formation cabinet, and the specific steps are as follows:

[0095] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.3Mpa, the first charging current is set to 1C (current 5A), and the sodium ion battery is charged with a constant current, and the charging cut-off capacity Q1 = 3AH;

[0096] 2) Discharge the battery cell at a current of 1C to a cut-off voltage of 2.6V;

[0097] 3) Discharge the battery cell at a current of 0.05C to a cut-off voltage of 1.5V;

[0098] 4) Leave for 10 minutes;

[0099] 5) Repeat steps 3-4 once;

[0100] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0101] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0102] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as aging and capacity separation are completed.

[0103] Comparative Example 2:

[0104] It is prepared using the same materials and the same process as Example 5, except that steps 3 to 5 in the above-mentioned formation process are omitted.

[0105] Embodiment 6:

[0106] Using sodium sulfate (NFS) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium-ion battery cell with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 4.2-2.6V; after baking and the injection process, the sodium-ion battery cell is formed in a charge and discharge formation cabinet, and the specific steps are as follows:

[0107] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.3Mpa, the charging current is set to 1C (current 5A), and the sodium ion battery is charged with a constant current, and the charging cut-off capacity Q1 = 3AH;

[0108] 2) Discharge the battery cell at a current of 1C to a cut-off voltage of 2.6V;

[0109] 3) Discharge the battery cell at a current of 0.05C to a cut-off voltage of 1.5V;

[0110] 4) Leave for 10 minutes;

[0111] 5) Repeat steps 3-4 3 times;

[0112] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0113] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0114] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as aging and capacity separation are completed.

[0115] Embodiment 7:

[0116] Using phosphoric acid-sodium iron pyrophosphate (NFPP) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium ion battery monomer with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 3.45-1.5V; after the baking and liquid injection process, the sodium ion battery monomer is formed in a charge and discharge formation cabinet, and the specific steps are as follows:

[0117] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.6Mpa, the charging current is set to 1C (current 5A), and the sodium ion battery is charged with a constant current, and the charging cut-off capacity Q1 = 4AH;

[0118] 2) Discharge the battery cell at a current of 1C to a cut-off voltage of 1.5V;

[0119] 3) Discharge the battery cell at a current of 0.05C to a cut-off voltage of 0.5V;

[0120] 4) Leave for 10 minutes;

[0121] 5) Repeat steps 3-4 twice;

[0122] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0123] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0124] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as aging and capacity separation are completed.

[0125] Comparative Example 3:

[0126] It is prepared using the same materials and the same process as Example 7, except that steps 3 to 5 of the above-mentioned formation process are omitted.

[0127] Embodiment 8:

[0128] Using phosphoric acid-sodium iron pyrophosphate (NFPP) as the positive electrode and hard carbon as the negative electrode, a soft-pack sodium ion battery monomer with a designed capacity of 5Ah is prepared, wherein the capacity of the negative electrode is 6Ah (based on the gram capacity of the negative electrode active material * the weight of the active material), and the battery uses a charge and discharge range of 3.45-1.5V; after the baking and liquid injection process, the sodium ion battery monomer is formed in a charge and discharge formation cabinet, and the specific steps are as follows:

[0129] 1) The battery cell is placed on the cabinet, the formation temperature is set to 45°C, the formation pressure is set to 0.6Mpa, the charging current is set to 1C (current 5A), and the sodium ion battery is charged with a constant current, and the charging cut-off capacity Q1 = 4AH;

[0130] 2) Discharge the battery cell at a current of 0.5C to a cut-off voltage of 1.5V;

[0131] 3) Discharge the battery cell at a current of 0.02C to a cut-off voltage of 0.5V;

[0132] 4) Leave for 20 minutes;

[0133] 5) Repeat steps 3-4 twice;

[0134] 6) Remove the battery cells from the cabinet and evacuate and seal the above battery cells;

[0135] 7) Put the battery cells in the cabinet and charge the battery at 0.5C (2.5A) until 50% capacity is reached.

[0136] After the formation is completed, the preparation process of the sodium ion battery is completed according to the existing technology, and the subsequent preparation processes such as aging and capacity separation are completed.

[0137] The following method was used to test Examples 1-8 and the sodium ion batteries obtained in Comparative Examples 1-3 for over-discharge test and cycle test. The test results are shown in Table 1. The specific test method is as follows:

[0138] 1) After fully charging the battery with a current of 0.5C, discharge it at 0.5C to the lower limit voltage of normal use; measure the thickness of the battery, and then continue to discharge it at 0.1C to 0.5V; after that, measure the thickness of the battery at the same position;

[0139] 2) Perform charge and discharge tests at a current of 0.5C within the normal operating voltage range of the battery cell, and record the battery capacity retention rate after 500 cycles.

[0140]

[0141] Table 1 Test results of examples and comparative examples

[0142] It can be seen from Table 1 that the sodium ion battery prepared by the present invention has significantly improved over-discharge resistance after the formation process of the present invention, which is specifically manifested in that the thickness expansion rate is small, which is much lower than that of the comparative example; at the same time, during over-discharge, the gas generated during the adsorption process of the hard carbon negative electrode of the battery cell of the comparative example during the first charging process cannot be desorbed under normal discharge conditions, while in the battery in the embodiment of the present invention, during the formation process, the gas generated by the formation can be completely discharged through deep over-discharge, and the unstable organic components in the SEI film can be oxidized and decomposed at the same time, and after charging again, the SEI film can be further repaired to improve the stability of the SEI film, thereby improving the battery cycle life.

[0143] At the same time, in Examples 3, 4, 5, 6, 7, and 8 in Table 1, the lower the over-discharge voltage, the more over-discharge times, the smaller the over-discharge current, and the more thorough the over-discharge degree, the more conducive to the discharge of gas, the improvement of the over-discharge resistance of the battery cell, the reduction of the expansion rate of the battery cell during over-discharge, and the formation of a stable SEI film and the improvement of the cycle performance; in addition, it can be seen that by controlling the capacity of the first charge, the discharge of over-discharge gas can be better regulated, thereby regulating the expansion rate and cycle life of the battery cell during over-discharge. In addition, from Figure 2-3 It can also be clearly seen that the cycle life of the sodium ion battery after the formation process of the present invention is significantly improved.

[0144] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A sodium ion battery formation method, characterized in that: The steps include: S1: The sodium ion battery after injection and standing is charged at a preset temperature and pressure with a constant current I1 to a cut-off capacity Q; S2: Discharge the battery with a constant current I2 to the cut-off voltage V1 during normal use; S3: Adjust the discharge current and use a small current I3 to continue discharging the battery to the cut-off voltage V2; S4: put aside for a certain time t; S5: Repeat steps S3-S4 N times, where N=1-5; S6: evacuate and seal the battery; S7: Charge the battery with the current I3 to the required charging state of the battery.

2. The sodium ion battery formation method according to claim 1, characterized in that: In S1, the temperature is between room temperature and 90°C, and the pressure is between 0.02Mpa and 3Mpa.

3. The sodium ion battery formation method according to claim 1, characterized in that: In S1, the cut-off capacity Q is set to 10%-50% of the capacity of the negative electrode of the battery.

4. The sodium ion battery formation method according to claim 1, characterized in that: The current I1 and the current I2 are between 0.1-2C.

5. The sodium ion battery formation method according to claim 1, characterized in that: In S2, the cut-off voltage V1 is determined according to the battery material system used.

6. The sodium ion battery formation method according to claim 1, characterized in that: In the S3, the current I3 is between 0.01C and 0.1C.

7. The sodium ion battery formation method according to claim 1, characterized in that: In S3, 0≤cutoff voltage V2<cutoff voltage V1.

8. The sodium ion battery formation method according to claim 1, characterized in that: In the S4, t is between 1 and 30 min.

9. The sodium ion battery formation method according to any one of claims 1 to 8, characterized in that: The positive electrode active material of a sodium ion single cell is a material that can reversibly insert and remove sodium ions.

10. The sodium ion battery formation method according to claim 9, characterized in that: The negative electrode active materials of sodium ion single cells are soft and hard carbon materials.

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