Formation method of sodium ion battery

Abstract

The invention discloses a formation method of a sodium ion battery, and relates to the technical field of sodium ion batteries. The time for reaching the SEI film-forming potential is shortened by adopting the first-section high-current charging formation breaking through the conventional method; two-stage low-current charging formation is carried out, so that an electrolyte / negative electrode interface tends to generate a single-electron reaction, and an SEI film rich in organic components is generated and is used for stabilizing inorganic SEI components generated by first-stage high-current charging, so that SEI is uniform and compact, and the cycle life of the battery is not damaged; the second-stage low-current charging voltage is within the SEI film-forming voltage range, the excessive loss of active sodium ions can be effectively avoided through low-current charging, and the first effect is reduced; and large current adopted in the third stage can be used for quickly charging to enable the battery to reach a cut-off condition, so that the formation time is shortened, and the formation charging time is remarkably shortened in the whole formation process. Tests show that the formation method can improve the first efficiency and the capacity grading capacity of the sodium-ion battery, and has no negative influence on the cycle life.

Description

Technical Field

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

[0002] The production process of sodium-ion batteries is similar to that of lithium-ion batteries, both of which include pole piece manufacturing (pulping-coating-rolling, etc.) and battery assembly (slitting-packaging-liquid injection-formation-capacity division, etc.). Therefore, in the formation process of sodium-ion batteries, the formation system of lithium-ion batteries is used. Currently, the more representative ones are the following two formation methods:

[0003] (1) Adopting a small current step-by-step formation method, such as the sodium ion battery formation process in patent number CN118054108A: first use 0.01C~0.03C constant current to charge to 5~10% SOC and let it stand, then use 0.02C~0.05C constant current to charge to 15%~30% and let it stand, and then use a step-by-step current to charge to a capacity of 100% SOC. The above-mentioned similar small current step-by-step formation process takes a long time in the formation stage. Excluding the time spent on the battery after liquid injection and the battery aging after formation, the charging step alone takes at least 14 hours, the battery production cycle is long, and the energy consumption is increased. The small current step-by-step formation makes the surface of the hard carbon of the negative electrode of the sodium ion battery form a dense and uniform SEI film, which is beneficial to improve the cycle performance of the sodium ion battery to a certain extent. However, during the formation charging stage, the integrity of the SEI film generated by the corresponding small current charging in the process of embedding (adsorbing) the hard carbon of the negative electrode will be destroyed. In the subsequent process, the current increases step by step, continues to react with the electrolyte, repairs and reforms the SEI film, passivates the negative electrode, and improves the cycle life of the battery. In the process of SEI film destruction and reformation, the limited sodium ions provided by the positive electrode material during the charging process are consumed excessively, resulting in lower subsequent capacity and lower initial efficiency of the battery.

[0004] (2) In view of the characteristics of the hard carbon of the negative electrode commonly used in sodium ion batteries, patent CN202310721355.9 proposes that a large current is used for formation charging in the first stage to reach the SEI film formation potential, and then the current is gradually reduced and shallowly cycled for 3-4 times within the SEI film formation potential range, and finally charged to the cut-off potential to complete the formation. The above-mentioned method of charging with a large current in the first stage, supplemented by a small current shallow cycle, improves the initial discharge capacity and efficiency. However, the ultra-high initial efficiency shown in the embodiments of the patent, reaching more than 95%, is because the sodium ion battery is achieved under the cycle of the third step and subsequent steps. The first efficiency value obtained by the conventional first efficiency calculation method has a large deviation from the actual value and is not comparable. In addition, the nested multi-step cycle process and the pressing force adjustment process in the patent also increase the difficulty of operation and the formation time to a certain extent. The formation charge and discharge time requires at least 12h. In patent CN202310721355.9, the first charging current is 1C-2C, and the charging cut-off voltage is 3.0-3.4V. Using the charging cut-off voltage as the process jump condition is not conducive to the adaptation of different sodium battery material systems, resulting in uneven sodium insertion and sodium precipitation in the first stage of sodium ion battery charging. If the high current charging capacity of the first stage exceeds the sodium insertion capacity of the hard carbon "slope" zone, the subsequent insertion link requires a high energy barrier, and the sodium ion insertion is slow, which is very likely to cause uneven sodium insertion.

[0005] Therefore, there is an urgent need to develop a formation process with a short formation time, which can improve the initial efficiency and fractional capacity of sodium-ion batteries without negatively affecting the cycle life.

[0006] In view of this, the present invention is proposed. Summary of the invention

[0007] The object of the present invention is to provide a sodium ion battery formation method, aiming to shorten the sodium ion battery formation process time, improve the sodium ion battery initial efficiency and fractional capacity, and have no negative impact on the cycle life.

[0008] The present invention is achieved in that:

[0009] In a first aspect, the present invention provides a formation method for a sodium ion battery, comprising sequentially performing a first stage, a second stage and a third stage:

[0010] The first stage: the sodium ion battery is charged with a constant current of I1 for the first time to 20%-40% of the SOC of the sodium ion battery;

[0011] The second stage: charging with constant current I2 to 50%-70% of the sodium ion battery SOC, and then leaving it alone;

[0012] The third stage: charging with constant current and constant voltage of current I3 to the cut-off voltage and cut-off current of the sodium ion battery, and the formation process is completed;

[0013] Among them, I3≥I1>I2, and the current range of I1 is 0.1C-0.2C.

[0014] In an optional implementation, the current range of I2 is 0.02C-0.05C, and the current range of I3 is 0.2C-0.5C.

[0015] In an optional embodiment, the cut-off SOC capacity of the first stage is determined according to the slope region capacity of the negative electrode material used, and the cut-off SOC capacity is ≤ (slope region specific capacity×mass of negative electrode material for battery).

[0016] In an optional embodiment, the negative electrode of the sodium ion battery adopts a hard carbon material, and the cut-off SOC capacity is ≤ (specific capacity of the hard carbon slope region × mass of the hard carbon used in the battery).

[0017] In an optional implementation, the cut-off SOC of the second stage differs from the cut-off SOC of the first stage by less than 30%, and the cut-off SOC difference is proportional to the current of the second stage.

[0018] In an optional embodiment, the cut-off SOC of the second stage differs from the cut-off SOC of the first stage by less than 20%.

[0019] In an optional implementation, the cut-off voltage in the third stage is 3.9V-3.95V, and the cut-off current is 0.02C-0.05C.

[0020] In an optional embodiment, the holding time of the second stage is 12h-24h.

[0021] In an optional embodiment, the formation temperature is controlled to be 40° C.-50° C., and the formation pressure is controlled to be 0.4 MPa-0.6 MPa.

[0022] In an optional embodiment, the positive electrode material of the sodium ion battery is selected from at least one of a layered oxide positive electrode material, a polyanion positive electrode material and a Prussian blue positive electrode material.

[0023] The present invention has the following beneficial effects: adopt the first section of high current charging formation that breaks through the routine, shorten the time to reach SEI film potential; two sections of low current charging formation, make electrolyte / negative electrode interface tend to have single electron reaction, generate SEI film rich in organic components, for stabilizing the inorganic SEI components generated by the first section of high current charging, make SEI uniform, compact, no damage to the cycle life of battery; two sections of low current charging voltage within the SEI film forming voltage range, low current charging can effectively avoid excessive loss of active sodium ions, bring the first effect reduction; the high current used in the third stage can be fast charged to make the battery reach the cut-off condition, shorten the formation time, and in the whole formation process, the formation charging time is significantly shortened (such as the longest only 12h). After testing, the formation method provided by the present invention is used to form and can improve the first efficiency and fractional capacity of sodium ion battery, and has no negative impact on cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0025] Figure 1 is the charge and discharge curve of hard carbon;

[0026] Figure 2 The graph is a test result of the cycle performance of the sodium ion battery obtained in the embodiment and the comparative example. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0028] The present invention provides a sodium ion battery formation method, comprising sequentially performing a first stage, a second stage and a third stage, as follows:

[0029] S1. Phase 1

[0030] The sodium ion battery entering the formation process is charged with a constant current I1 for the first time to 20%-40% of the sodium ion battery SOC, and the current I1 is controlled to be 0.1C-0.2C. Compared with the existing technology, the first stage of high current charging formation that breaks through the convention is adopted to shorten the time to reach the SEI film formation potential.

[0031] Specifically, the value of the current I1 can be 0.10C, 0.13C, 0.15C, 0.18C, 0.20C, etc., and the constant current is charged to 20%, 25%, 30%, 35%, 40%, etc. of the SOC of the sodium ion battery.

[0032] "SOC" refers to the state of charge of the battery, that is, an indicator used to quantify the remaining capacity of the battery. The 100% SOC corresponding capacity in this article is the design capacity of the sodium-ion battery, and the remaining percentage SOC capacity is calculated based on the corresponding percentage. For example, if the design capacity is 3Ah, the 40% SOC charging capacity is 1.2Ah. The cut-off SOC capacity of the first stage is determined according to the slope zone capacity of the negative electrode material used, and the cut-off SOC capacity is ≤ (slope zone specific capacity × mass of negative electrode material for the battery). It can effectively avoid the formation problem caused by the charging capacity exceeding the sodium embedding capacity in the "slope" zone during the first stage of high current charging.

[0033] In some embodiments, the negative electrode of the sodium ion battery uses hard carbon material, and the cut-off SOC capacity is ≤ (hard carbon slope area specific capacity × battery hard carbon mass). Figure 1 As shown in the figure, the first current size and charging cut-off capacity are determined according to the capacity of the hard carbon "slope zone" of the negative electrode of the sodium ion battery, which can effectively avoid the formation problem caused by the charging capacity exceeding the sodium embedding capacity of the hard carbon "slope" zone during the first large current charging. Specifically, for a specific type of sodium ion battery, the hard carbon slope zone specific capacity is a constant value.

[0034] It should be noted that for the button-type half-cell of the hard carbon material of the negative electrode of the sodium ion battery, its discharge curve has a capacity "slope zone". In this zone, the sodium ion migration rate is fast and the rate performance is excellent, which can support high current charging and discharging. For sodium ion batteries, the positive electrode is de-sodiumed in the first stage of the formation process, and the hard carbon of the negative electrode is in the sodium embedding stage of the "slope zone". The large current formation charging will not cause problems such as uneven sodium embedding and sodium precipitation. The research results show that the sodium embedding mechanism of the hard carbon negative electrode includes "intercalation-pore filling", "adsorption-intercalation", "adsorption-intercalation-pore filling", etc. Under the adsorption mechanism, the energy barrier for sodium ions to embed into hard carbon is low. At the same time, the capacity of the hard carbon "slope zone" is higher than that of the graphite negative electrode, resulting in the potential of sodium ions embedded in hard carbon being lower than the decomposition potential of the electrolyte film-forming additive. At this stage, the battery polarization is small, the negative electrode potential does not reach the decomposition potential of the film-forming additive, and the SEI film formation is incomplete. Therefore, the use of a large current in the first stage of charging can cause the battery to produce a large polarization, and the electrode interface quickly reaches the film-forming potential. Subsequently, a small current is used at this potential to form a uniform and dense SEI film, shortening the formation time.

[0035] In some embodiments, the formation temperature is controlled to be 40°C-50°C, such as 40°C, 43°C, 45°C, 48°C, 50°C, etc.; the formation pressure is 0.4MPa-0.6MPa, such as 0.4MPa, 0.5MPa, 0.6MPa, etc. The medium temperature pressurized formation process is more suitable for the formation method provided in the embodiments of the present invention.

[0036] In some embodiments, the positive electrode material of the sodium ion battery is selected from at least one of a layered oxide positive electrode material, a polyanion positive electrode material and a Prussian blue positive electrode material. The positive electrode material of the sodium ion battery can be any one or more of the above.

[0037] S2, the second stage

[0038] The sodium-ion battery is charged with a constant current of I2 to 50%-70% of the SOC, and then set aside. The current range of I2 is 0.02C-0.05C. The second stage of low-current charging formation makes the electrolyte / negative electrode interface tend to undergo a single-electron reaction, generating a SEI film rich in organic components, which is used to stabilize the inorganic SEI components generated by the first stage of high-current charging, making the SEI uniform and dense, and not damaging the cycle life of the battery. The second stage of low-current charging voltage is within the SEI film formation voltage range, and low-current charging can effectively avoid excessive loss of active sodium ions, resulting in a reduction in the first effect.

[0039] Specifically, the value of the current I2 can be 0.02C, 0.03C, 0.04C, 0.05C, etc., and the constant current is charged to 50%, 55%, 60%, 65%, 70%, etc. of the SOC of the sodium ion battery.

[0040] In some embodiments, the second stage of the shelf time is 12 hours to 24 hours, such as 12 hours, 15 hours, 18 hours, 20 hours, 22 hours, 24 hours, etc. Long-term shelf aging further stabilizes the SEI and replaces the subsequent separate aging process.

[0041] In some embodiments, the cut-off SOC of the second stage differs from the cut-off SOC of the first stage by less than 30%, and the cut-off SOC difference is proportional to the current of the second stage to avoid reducing the formation efficiency. Specifically, the cut-off SOC of the second stage can be 30% greater than the cut-off SOC of the first stage, or 30% less, within this range.

[0042] In a preferred embodiment, the cut-off SOC of the second stage differs from the cut-off SOC of the first stage by less than 20%. The cut-off SOC of the second stage may be 20% greater than the cut-off SOC of the first stage, or 20% less, within this range.

[0043] S3, the third stage

[0044] The formation process is completed by charging with constant current and constant voltage with current I3 to the cut-off voltage and cut-off current of the sodium ion battery. The constant current and constant voltage charging method is to first charge with constant current to the cut-off voltage of the battery, then maintain the voltage constant, and gradually reduce the current to charge to the cut-off current, so both the cut-off voltage and the cut-off current need to be reached. Among them, I3≥I1>I2, the current range of I3 is 0.2C-0.5C, and the large current used in the third section can charge quickly to make the battery reach the cut-off condition, shortening the formation time. In the entire formation process, the longest formation charging time is only 12h.

[0045] Specifically, the charging current I3 in the third stage can be 0.2C, 0.3C, 0.4C, 0.5C, etc.

[0046] In some embodiments, the cut-off voltage in the third stage is 3.9V-3.95V, such as 3.90V, 3.91V, 3.92V, 3.93V, 3.94V, 3.95V, etc.; the cut-off current is 0.02C-0.05C, such as 0.02C, 0.03C, 0.04C, 0.05C, etc.

[0047] In summary, the formation method provided by the embodiment of the present invention has the following advantages: (1) high current formation is used for the first stage of charging of sodium ion batteries, saving process time; (2) the capacity of the hard carbon "slope zone" assists in determining the SOC state of the first stage of charging, avoiding the situation in which the charging capacity exceeds the capacity of the hard carbon slope zone during the high current charging stage, resulting in poor formation; (3) low current formation is used for SEI generation and reforming of sodium ion batteries, improving the stability of the passivation film and lossless cycle and rate performance.

[0048] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0049] Example 1

[0050] This embodiment provides a sodium ion battery formation method, the steps are as follows:

[0051] (1) Provide batteries to be formed

[0052] Sodium-ion battery layered oxides (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3O2) positive electrode sheet, hard carbon negative electrode sheet (N / P is 1.2, N / P ratio is the ratio of negative electrode capacity (Negative) to positive electrode capacity (Positive) when designing a specific type of sodium ion battery) and PE ceramic diaphragm are assembled into a 1.6Ah soft pack battery cell. After 48 hours of vacuum baking, the battery cell has qualified moisture (negative electrode ≤ 250ppm). Then inject the liquid in a humidity-controlled environment (glove box or a drying room with a dew point below -45°C, the same below), and the injection volume is 4mL / Ah. After the injection, the battery is left to soak at room temperature for 24 hours before entering the formation process. The electrolyte composition: 1 mol / L NaPF6 organic solution, the solvent is DMC, EC and EMC (DMC:EC:EMC=1:1:1Vol%), the additive is FEC (5% Vol%), and the electrolyte in the other embodiments and comparative examples is the same.

[0053] (2)Chemification

[0054] The formation process is as follows: During the formation process, a pressure of 0.5MPa is applied and the temperature is set at 45°C. Charge for 3 hours with a current of 0.1C (about 160mA) to reach 30% SOC; then charge for 6 hours with a current of 0.05C (about 80mA) to reach 60% SOC; leave for 24 hours; after leaving, charge to 3.95V with a constant current and constant voltage of 0.2C (about 320mA) at a cut-off current of 0.05C, and the formation is completed. Then, degas and separate the volume at room temperature. The total charging time for formation is about 11 hours.

[0055] Among them, the specific capacity of the hard carbon slope zone × the mass of hard carbon used in the battery = 88.6mAh / g × 8.238g ≈ 729.8868mAh, the SOC capacity at the end of the first stage is 480mAh < 729.88mAh; the SOC capacity at the end of the second stage is 960mA.

[0056] Example 2

[0057] This embodiment provides a sodium ion battery formation method, the steps are as follows:

[0058] (1) Provide batteries to be formed

[0059] Sodium-ion battery layered oxides (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2) positive electrode, hard carbon negative electrode (N / P is 1.15) and PE ceramic diaphragm are assembled into a 1.1Ah soft pack battery. After 48 hours of vacuum baking, the battery has qualified moisture (negative electrode ≤ 250ppm). Then, the battery is injected with liquid in a humidity-controlled environment, with an injection volume of 4mL / Ah. After the battery is left to soak at room temperature for 24 hours, it enters the formation process.

[0060] (2)Chemification

[0061] The formation process is as follows: during the formation process, a pressure of 0.5MPa is applied and the temperature is set at 45°C. Charge for 4 hours at a current of 0.1C (about 110mA) to reach 40% SOC; then charge for 6 hours at a current of 0.05C (about 55mA) to reach 70% SOC; leave for 24 hours; after leaving, charge to 3.95V at a constant current and constant voltage of 0.2C (about 220mA) with a cut-off current of 0.02C, and the formation is complete. Then, degas and separate the volume at room temperature. The total charging time for formation is about 12 hours.

[0062] Among them, the specific capacity of the hard carbon slope zone × the mass of hard carbon used in the battery = 90.2mAh / g × 5.49g ≈ 495.20mAh, the SOC capacity at the end of the first stage is 440mAh < 495.20mAh; the SOC capacity at the end of the second stage is 770mAh.

[0063] Comparative Example 1

[0064] This comparative example provides a sodium ion battery formation method, the steps are as follows:

[0065] (1) Provide batteries to be formed

[0066] Sodium-ion battery layered oxides (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2) positive electrode, hard carbon negative electrode (N / P is 1.2) and PE ceramic diaphragm are assembled into 1.6Ah soft pack battery. After 48 hours of vacuum baking, the battery has qualified moisture (negative electrode ≤250ppm). Then, the battery is injected with liquid in a humidity-controlled environment, with an injection volume of 4mL / Ah. After the battery is left to soak at room temperature for 24 hours, it enters the formation process.

[0067] Note: The specific operation of step (1) is the same as that of Example 1.

[0068] (2)Chemification

[0069] The formation process is as follows: During the formation process, a pressure of 0.5MPa is applied and the temperature is set at 45°C. Charge for 6 hours at 0.05C (about 80mA) to reach 30% SOC; then charge for 3 hours at 0.1C (about 160mA) to reach 60% SOC; leave for 24 hours; after leaving, charge to 3.95V at 0.2C (about 320mA) with constant current and constant voltage, and cut off the current at 0.05C to complete the formation. Then, degas and separate the volume at room temperature. The formation time is about 11 hours.

[0070] Comparative Example 2

[0071] This comparative example provides a sodium ion battery formation method, the steps are as follows:

[0072] (1) Provide batteries to be formed

[0073] Sodium-ion battery layered oxides (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2) positive electrode, hard carbon negative electrode (N / P is 1.15) and PE ceramic diaphragm are assembled into a 1.1Ah soft pack battery. After 48 hours of vacuum baking, the battery has qualified moisture (negative electrode ≤ 250ppm). Then, the battery is injected with liquid in a humidity-controlled environment, with an injection volume of 4mL / Ah. After the battery is left to soak at room temperature for 24 hours, it enters the formation process.

[0074] Note: The specific operation of step (1) is the same as that of Example 2.

[0075] (2)Chemification

[0076] The formation process is as follows: During the formation process, a pressure of 0.5MPa is applied and the temperature is set at 45°C. Charge for 6 hours at 0.05C (about 55mA) to reach 30% SOC; then charge for 4 hours at 0.1C (about 110mA) to reach 70% SOC; leave for 24 hours; after leaving, charge to 3.95V at 0.2C (about 220mA) with constant current and constant voltage, and cut off the current at 0.02C to complete the formation. Then, degas and separate the volume at room temperature. The formation time is about 12 hours.

[0077] Comparative Example 3

[0078] This comparative example provides a sodium ion battery formation method, the steps are as follows:

[0079] (1) Provide batteries to be formed

[0080] Sodium-ion battery layered oxides (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2) positive electrode, hard carbon negative electrode (N / P is 1.15) and PE ceramic diaphragm are assembled into a 1.1Ah soft pack battery. After 48 hours of vacuum baking, the battery has qualified moisture (negative electrode ≤ 250ppm). Then, the battery is injected with liquid in a humidity-controlled environment, with an injection volume of 4mL / Ah. After the battery is left to soak at room temperature for 24 hours, it enters the formation process.

[0081] Note: The specific operation of step (1) is the same as that of Example 2.

[0082] (2)Chemification

[0083] The formation process is as follows: During the formation process, a pressure of 0.5MPa is applied and the temperature is set at 45°C. A current of 0.02C (about 22mA) is used to charge to 3.2V; then a current of 0.05C (about 55mA) is used to charge to 3.4V; then a current of 0.1C (about 110mA) is used to charge to 3.7V; finally, a constant current and voltage of 0.2C (about 220mA) is used to charge to 3.95V, and the cut-off current is 0.02C. The formation time is about 40 hours.

[0084] Comparative Example 4

[0085] This comparative example provides a sodium ion battery formation method, the steps are as follows:

[0086] (1) Provide batteries to be formed

[0087] The specific operation of step (1) is the same as that of Example 2.

[0088] (2)Chemification

[0089] The only difference from Example 2 is that the currents in the first and second stages are both 0.05C, and the current in the third stage is 0.2C, so that the formation charging current is I3>I2=I1. The formation time is about 20 hours.

[0090] In each case of the embodiment and comparative example, after the formation process, the capacity was divided by charging and discharging at 0.2C, and the capacity is shown in Table 1.

[0091] Table 1 Capacity, initial efficiency and cycle test results of the batteries obtained by the embodiment and comparative example

[0092]

[0093] The capacity division and first effect of the embodiment are slightly higher than those of the step current formation. (Example 2 and Comparative Example 3). At the same time, the capacity division result of the large current first stage formation process of Example 1 is also higher than the conventional small current first stage formation result of Comparative Example 1. The formation time of Example 2 is shortened by 28 hours compared with the step current formation time of Comparative Example 3, and shortened by 8 hours compared with the conventional small current formation time.

[0094] Figure 2 The 1C / 1C long cycle results of Example 1 and Comparative Example 1 show that the high current first stage formation process of Example 1 has no damage to the cycle performance and is slightly better than the conventional low current formation cycle of Comparative Example 1.

[0095] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A sodium ion battery formation method, characterized in that: It includes the first, second and third stages in sequence: The first stage: the sodium ion battery is charged with a constant current of I1 for the first time to 20%-40% of the SOC of the sodium ion battery; The second stage: charging with constant current I2 to 50%-70% of the sodium ion battery SOC, and then leaving it alone; The third stage: charging with constant current and constant voltage of current I3 to the cut-off voltage and cut-off current of the sodium ion battery, and the formation process is completed; Among them, I3≥I1>I2, and the current range of I1 is 0.1C-0.2C.

2. The chemical formation method according to claim 1, characterized in that: The current range of I2 is 0.02C-0.05C, and the current range of I3 is 0.2C-0.5C.

3. The chemical formation method according to claim 1, characterized in that: The cut-off SOC capacity of the first stage is determined according to the slope region capacity of the negative electrode material used, and the cut-off SOC capacity is ≤ (slope region specific capacity×mass of negative electrode material for battery).

4. The chemical formation method according to claim 3, characterized in that: The negative electrode of the sodium ion battery adopts hard carbon material, and the cut-off SOC capacity is ≤ (hard carbon slope area specific capacity×the mass of hard carbon used in the battery).

5. The chemical formation method according to claim 1, characterized in that: The cut-off SOC of the second stage differs from the cut-off SOC of the first stage by less than 30%, and the cut-off SOC difference is proportional to the current of the second stage.

6. The chemical formation method according to claim 5, characterized in that: The cut-off SOC of the second stage differs from the cut-off SOC of the first stage by less than 20%.

7. The chemical formation method according to claim 1, characterized in that: In the third stage, the cut-off voltage is 3.9V-3.95V, and the cut-off current is 0.02C-0.05C.

8. The chemical formation method according to claim 1, characterized in that: The holding time of the second stage is 12h-24h.

9. The chemical formation method according to claim 1, characterized in that: The formation temperature is controlled at 40°C-50°C and the formation pressure is controlled at 0.4MPa-0.6MPa.

10. The chemical formation method according to claim 1, characterized in that: The positive electrode material of the sodium ion battery is selected from at least one of a layered oxide positive electrode material, a polyanion positive electrode material and a Prussian blue positive electrode material.

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