Sodium-ion battery formation process and sodium-ion battery

Abstract

The invention relates to the technical field of sodium-ion batteries, and particularly discloses a sodium-ion battery formation process and a sodium-ion battery, the formation process comprises the following operation steps: reverse charging formation: performing reverse charging on the sodium-ion battery after liquid injection and shelving, injecting non-aqueous electrolyte into the sodium-ion battery, the non-aqueous electrolyte contains a first additive of which the reduction potential is greater than or equal to 0.8 V vs Na / Na < + >; and forward charging formation: performing forward charging formation on the battery subjected to the reverse charging formation, wherein the sodium ion battery is prepared by adopting the formation process. According to the invention, the first additive is added into the electrolyte, and the reverse charging formation is carried out before the forward charging formation, so that the first additive forms an interface passivation layer on the surface of the positive electrode, the film forming quality of the positive electrode is remarkably improved, the reconstruction of the surface of the positive electrode during circulation under high voltage is inhibited, and the strain and cracks are reduced; therefore, the cycle performance of the battery is remarkably improved, and the impedance growth rate in the cycle process is reduced.

Description

Technical Field

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

[0002] Compared to lithium-ion batteries, sodium-ion batteries, with their greater resource abundance, offer significant advantages in safety, high- and low-temperature performance, and rate capability. They are expected to enter commercialization as a complement to lithium-ion batteries, filling market niches in A00-class power and energy storage applications. To increase the energy density of sodium-ion batteries, existing technologies employ high operating voltages. By increasing the average operating voltage of sodium-ion batteries, the battery's energy output can be increased, thereby improving the battery's energy density. However, at higher charge cut-off voltages, the positive electrode surface passivation layer can break down, and the electrolyte undergoes irreversible oxidative decomposition on the positive electrode surface, leading to gas generation, battery swelling, increased interfacial impedance, and, in turn, severe degradation of the positive electrode material's cycling performance. Therefore, improving the stability of the positive electrode interface passivation layer is imperative. In existing technologies, the formation steps of sodium-ion batteries are similar to those of lithium-ion batteries, involving a series of alternating pulse charges or stepped pulse charges. This allows sodium ions at the electrode / electrolyte interface to react with specific additives in the electrolyte, forming a thin passivation layer, or SEI, covering the carbon electrode surface. However, conventional formation processes can specifically add specific additives to the electrolyte to cause the specific additives to undergo a reduction reaction at the negative electrode, thereby achieving the purpose of designing and regulating the composition of the negative electrode SEI film and improving the quality of the negative electrode SEI film. However, it is impossible to design and regulate the passivation layer on the positive electrode surface, and therefore it is impossible to improve the electrochemical performance of sodium ion batteries under high voltage.

[0003] For this purpose, this application is filed. Summary of the Invention

[0004] In response to the problems mentioned in the background technology, the present invention discloses a sodium ion battery formation process and a sodium ion battery.

[0005] The present invention is achieved through the following technical solutions:

[0006] A sodium ion battery formation process comprises the following steps:

[0007] S1. Reverse charging and formation: reverse charging the sodium ion battery after injection and shelving, wherein the sodium ion battery is injected with a non-aqueous electrolyte containing a first additive, and the reduction potential of the first additive is ≥0.8Vv Na / Na + ;

[0008] S2, forward charge formation: forward charge formation is performed on the battery after reverse charge formation.

[0009] The sodium ion battery formation process proposed in the present invention is based on the existing technology. Before the forward charge formation, reverse charge formation is first performed, so that the positive electrode is at a low potential. The first additive in the electrolyte undergoes a reduction reaction on the positive electrode surface, thereby forming a low-impedance, ion-conductive interface passivation layer on the positive electrode surface. This significantly improves the positive electrode film quality, inhibits the reconstruction of the positive electrode surface during cycling at high voltage, reduces the formation of strain and cracks, and then significantly improves the battery's cycle performance and reduces the impedance growth rate during the cycle. Moreover, the formation process is compatible with existing sodium ion battery generation processes, is low-cost, easy to implement, and suitable for industrialization.

[0010] In this embodiment, in S1, the charge capacity is used as the charge cutoff condition, the reverse charge current is 0.01-1C, and the reverse charge cutoff capacity is 1%-20% of the battery capacity. In S2, forward charging is performed as a normal operation: first, the battery is placed at 10-75°C for 1-240 hours, then charged at 0.05-0.2C to 10%-100% SOC, and finally charged at 0.1-0.5C to 100% SOC before ending.

[0011] In this embodiment, in S1 , the reverse charging current is 0.05-0.5C, preferably 0.1C; the reverse charging cut-off capacity is 2%-10% of the battery capacity, preferably 5%.

[0012] During reverse charge formation, under the same reverse charge cut-off capacity, if the reverse charge current used is too small, the reverse charge formation time will be too long, affecting the battery production capacity, and it may not be possible to polarize the positive electrode to a suitable reduction potential, affecting the reduction of the first additive, and then failing to generate a passivation layer that meets the requirements on the positive electrode interface; if the reverse charge current used is too large, although the positive electrode can be polarized to a suitable reduction potential, the reverse charge formation ends in a very short time, resulting in the first additive being unable to fully react at the positive electrode interface to form a positive electrode interface passivation layer; therefore, the reverse charge cut-off capacity and the reverse charge current need to meet specific conditions to ensure that the first additive forms a positive electrode interface passivation layer that meets the requirements on the positive electrode surface.

[0013] During reverse charging, under the same reverse charging current, if the reverse charging cut-off capacity is too small, the positive electrode interface passivation layer will be too thin and unable to play an effective protective role. If the reverse charging cut-off capacity is too large, the positive electrode interface passivation layer will be too thick. The adhesion between the excessively thick positive electrode interface passivation layer and the positive electrode will deteriorate, and there is a risk of cracking, peeling, or even falling off in the future.

[0014] Through creative work, the present applicant found that: under the premise of consistent addition amount of the first additive, a positive electrode interface passivation layer that meets the requirements can be obtained when the reverse charge cut-off capacity is 1%-20% of the battery capacity and the reverse charge current is 0.01-1C. When the reverse charge cut-off capacity is 2%-10% of the battery capacity and the reverse charge current is 0.05-0.5C, the obtained positive electrode interface passivation layer has good performance. When the reverse charge cut-off capacity is 5% of the battery capacity and the reverse charge current is 0.1C, the obtained positive electrode interface passivation layer has the best performance, with good density, toughness and appropriate thickness.

[0015] Preferably, the first additive comprises a reduction potential ≥ 0.8V vs Na / Na + Organic additives, reduction potential ≥ 0.8V vs Na / Na + At least one of the inorganic additives, the first additive is preferably a combination of the organic additive and the inorganic additive.

[0016] The combined use of organic and inorganic additives can further improve the quality of the positive electrode interface passivation layer during reverse charging, making it denser and more flexible, thereby more significantly improving the battery cycle performance and effectively reducing the impedance growth during the cycle.

[0017] In this embodiment, when the composition is added, the amount of the inorganic additive added to the non-aqueous electrolyte is a, 0.1wt%≤a≤3wt%, preferably 0.3wt%≤a≤1.5wt%;

[0018] In this embodiment, when the composition is added, the amount of the organic additive added to the non-aqueous electrolyte is b, 0.1 wt% ≤ b ≤ 5 wt%, preferably 0.5 wt% ≤ b ≤ 3 wt%. Preferably, the amounts of the inorganic additive and the organic additive added satisfy the following conditions: 0.5 wt% ≤ a + b ≤ 8 wt%, where a / (a + b) ≥ 10%.

[0019] Preferably, the organic additive is selected from at least one of carbonates, sulfonates, sulfates, sulfites, and acid anhydrides. The carbonate is preferably at least one of fluoroethylene carbonate (FEC), difluoroethylene carbonate, and trifluoromethylethylene carbonate. The sulfonate is preferably at least one of methylene methanedisulfonate and 1,3-propylene sultone (PST). The sulfate is at least one of vinyl sulfate and propylene sulfate. The sulfite is ethylene sulfite (ETAS). The acid anhydride is preferably at least one of maleic anhydride (MAH), succinic anhydride, and sulfobenzoic anhydride.

[0020] The inorganic additive is selected from at least one of sodium bis(fluorosulfonyl)imide (NaFSI), sodium perchlorate (NaClO4), sodium difluorooxalatoborate (NaDFOB), and sodium bis(oxalatoborate) (NaBOB).

[0021] Preferably, the non-aqueous electrolyte further contains a second additive, the reduction potential of the second additive is less than 0.8V vs Na / Na + .

[0022] The second additive does not participate in the positive electrode film formation reaction during reverse charge formation. During forward charge formation, it participates in the negative electrode film formation, making the negative electrode SEI film dense. A high-quality SEI film can effectively avoid side reactions between the electrolyte and the negative electrode material and effectively improve the formation interface state. The first and second additives work together to help maximize the battery's capacity retention rate and reduce the impedance growth rate and volume expansion rate.

[0023] Preferably, the second additive is selected from at least one of sodium difluorophosphate (NaDFP), sodium tetrafluoroborate (NaBF4), 1,3-propane sultone (PS), and vinylene carbonate (VC). The addition amount of the second additive in the non-aqueous electrolyte is c, 0.1wt%≤c≤3wt%, preferably 0.5wt%≤c≤2wt%, and the addition amounts of the organic additive, the inorganic additive, and the second additive satisfy: 1wt%≤a+b+c≤10wt%, a+b / (a+b+c)≥50%.

[0024] Preferably, the non-aqueous electrolyte further comprises a non-aqueous solvent and a sodium salt; the non-aqueous solvent comprises a cyclic ester and a chain ester, the cyclic ester is selected from at least one of ethylene carbonate, propylene carbonate, and γ-butyrolactone, and the chain ester is selected from at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate; the sodium salt is selected from at least one of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluoroarsenate (NaAsF6), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium trifluoromethanesulfonate (NaOTF), and the concentration of the sodium salt in the non-aqueous electrolyte is 0.2-1.5 mol / L.

[0025] On the other hand, the present invention provides a sodium ion battery produced using the above-mentioned formation process.

[0026] In terms of battery shape, the sodium ion battery can be any one of a cylindrical battery, a soft-pack battery, and a square battery; in terms of battery type, the sodium ion battery can be any one of a liquid battery, a semi-solid battery, a quasi-solid-state battery, and an all-solid-state battery.

[0027] The sodium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode active material disposed thereon. The positive electrode current collector is made of aluminum foil, and the positive electrode active material is a layered metal oxide. The positive electrode of a sodium-ion battery using layered metal oxide as the active positive electrode material can withstand a certain degree of reverse charging. The negative electrode includes a negative electrode current collector and a negative electrode active material disposed thereon. The negative electrode current collector is made of aluminum foil, and the negative electrode active material is at least one of natural graphite, artificial graphite, soft carbon, hard carbon, lithium titanate, silicon, and a silicon-carbon alloy. The separator is any one of PE, PP, and PE / PP.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) By adding the first additive and performing reverse charge formation, a film is pre-formed on the positive electrode side by reverse charging, which effectively improves the positive electrode film quality, inhibits the reconstruction of the positive electrode surface during cycling at high voltage, reduces the formation of strain and cracks, and then significantly improves the cycle performance of the battery and reduces the impedance growth rate during the cycle process;

[0030] (2) By adding the second additive, the membrane components of the negative electrode film are optimized, making it denser and tougher, enhancing the protection of the negative electrode side, further improving the battery cycle performance, and reducing the impedance growth rate and volume expansion rate during the cycle;

[0031] (3) The sodium ion battery formation process proposed in the present invention is compatible with the existing sodium ion battery production process, and can achieve significant improvements in the cycle performance and impedance growth rate of sodium ion batteries with low-cost operations. It is low-cost, easy to implement, and suitable for industrialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 These are the charging DQ / DV curves of NaDFOB, FEC, and PS.

[0034] Figure 245°C cycle performance test diagram of the sodium ion batteries obtained in Example 1, Example 10, Comparative Example 1, and Comparative Example 5.

[0035] Figure 3 These are photos of the volume growth rates of Example 5, Example 10, Comparative Example 1, and Comparative Example 5 tested by the drainage method. DETAILED DESCRIPTION

[0036] In order to facilitate understanding of the present invention, the present invention will be described in more detail below, and embodiments of the present invention are given, but the scope of the present invention is not limited thereby.

[0037] All chemical substances involved in the raw materials in the following examples and comparative examples were commercially available. The experimental apparatus involved in the following examples and comparative examples are all conventional apparatuses in the art. Technical details not described in detail in the following examples and comparative examples are all conventional techniques in the art.

[0038] The preparation method of the positive electrode sheet involved in the following embodiments is as follows: the positive electrode active material, the conductive agent Super P and the binder PVDF are uniformly dispersed in the solvent N-methylpyrrolidone in a mass ratio of 98:0.6:1.4 to obtain a positive electrode slurry, and then the positive electrode slurry is coated on the positive electrode current collector aluminum foil, dried and pressed to obtain a positive electrode sheet; wherein the positive electrode active material is specifically selected from NaCu 0.2 Fe 0.3 Mn 0.5 O2 (existing technology, for specific preparation methods, please refer to Chinese invention patent application No. 201410549896.9 and authorization announcement No. CN 104795552 B).

[0039] The preparation method of the negative electrode plate involved in the following embodiments is as follows: the negative electrode active material hard carbon, the conductive agent SuperP, the sodium carboxymethyl cellulose powder CMC and the styrene-butadiene rubber SBR are uniformly dispersed in the solvent N-methylpyrrolidone according to a mass ratio of 94.3:1.4:1.6:2.7 to obtain a negative electrode slurry, and then the negative electrode slurry is coated on the negative electrode current collector aluminum foil, dried and pressed to obtain a negative electrode plate.

[0040] The sodium ion battery assembly operation involved in the following embodiments is as follows: the positive electrode sheet, the polyethylene separator, and the negative electrode sheet are stacked in order, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wound to obtain a bare battery cell, and the bare battery cell is placed in an aluminum-plastic film outer packaging. The first electrolyte is injected as required and reverse charging and forward charging are performed.

[0041] The specific operation of the forward charge formation in the following examples and comparative examples is as follows: first, at 25°C and 10 kg / cm 2Place under pressure for 10 hours, then charge at a constant current of 0.1C to 3V, and finally charge at a constant current of 0.2C to 3.5V.

[0042] The preparation method of electrolyte A in the following examples and comparative examples is as follows: ethylene carbonate EC and ethyl methyl carbonate EMC are mixed in a mass ratio of EC:EMC=1:1, and purified by molecular sieve to remove impurities and water to obtain a mixed organic solvent; at room temperature, sodium hexafluorophosphate NaPF6, sodium difluorooxalatoborate NaDFOB, and fluoroethylene carbonate FEC are added to the mixed organic solvent, dissolved and stirred uniformly; in electrolyte A, the molar concentration of NaPF6 is 1 mol / L, and the mass percentages of NaDFOB and FEC are 0.5wt% and 1.5wt%, respectively. The preparation methods of electrolyte B, electrolyte C, electrolyte D, electrolyte E, electrolyte F, electrolyte G, electrolyte H, electrolyte I, electrolyte J, electrolyte K, electrolyte L, electrolyte M, electrolyte N, electrolyte O, electrolyte P, electrolyte Q, electrolyte R, electrolyte S, and electrolyte T are similar to those of electrolyte A, but the additives and addition amounts in the electrolytes are different. The reduction potential of the first additive selected is ≥0.8V vs Na / Na + The reduction potential of the selected second additive is less than 0.8V vs Na / Na + , where the specific reduction potentials of NaDFOB, FEC, and PS are as follows: Figure 1 shown.

[0043] Table 1

[0044]

[0045]

[0046] Example 1

[0047] A sodium ion battery formation process comprises the following steps:

[0048] S1. Reverse charging formation: The sodium ion battery after injection and shelving is reverse charged, with the charging capacity as the charging cutoff condition, the reverse charging current is 0.05C, the reverse charging cutoff capacity is 1% of the battery capacity, and the sodium ion battery is injected with electrolyte A;

[0049] S2, forward charge formation: forward charge formation is performed on the battery after reverse charge formation.

[0050] Example 2

[0051] Compared with Example 1, the reverse charge cutoff capacity is adjusted from 1% of the battery capacity to 5%, and the rest are the same as Example 1.

[0052] Example 3

[0053] Compared with Example 1, the reverse charge cutoff capacity is adjusted from 1% of the battery capacity to 10%, and the rest is the same as Example 1.

[0054] Example 4

[0055] Compared with Example 1, the reverse charging current is adjusted from 0.05C to 0.1C, and the rest are the same as Example 1.

[0056] Example 5

[0057] Compared with Example 2, the reverse charging current is adjusted from 0.05C to 0.1C, and the rest are the same as Example 2.

[0058] Example 6

[0059] Compared with Example 3, the reverse charging current is adjusted from 0.05C to 0.1C, and the rest are the same as Example 3.

[0060] Example 7

[0061] Compared with Example 1, the reverse charging current is adjusted from 0.05C to 0.5C, and the rest are the same as Example 1.

[0062] Example 8

[0063] Compared with Example 2, the reverse charging current is adjusted from 0.05C to 0.5C, and the rest are the same as Example 2.

[0064] Example 9

[0065] Compared with Example 3, the reverse charging current is adjusted from 0.05C to 0.5C, and the rest are the same as Example 3.

[0066] Example 10

[0067] Compared with Example 5, the electrolyte A was adjusted to electrolyte F, and the rest were the same as Example 5.

[0068] Example 11

[0069] Compared with Example 5, the reverse charging current is adjusted from 0.1C to 0.01C, and the rest are the same as Example 5.

[0070] Example 12

[0071] Compared with Example 5, the reverse charging current is adjusted from 0.1C to 0.03C, and the rest are the same as Example 5.

[0072] Example 13

[0073] Compared with Example 5, the reverse charging current is adjusted from 0.1C to 0.8C, and the rest are the same as Example 5.

[0074] Example 14

[0075] Compared with Example 5, the reverse charging current is adjusted from 0.1C to 1C, and the rest are the same as Example 5.

[0076] Example 15

[0077] Compared with Example 5, the reverse charge cutoff capacity is adjusted from 5% of the battery capacity to 2%, and the rest is the same as Example 5.

[0078] Example 16

[0079] Compared with Example 5, the reverse charge cutoff capacity is adjusted from 5% of the battery capacity to 8%, and the rest are the same as Example 5.

[0080] Example 17

[0081] Compared with Example 5, the reverse charge cutoff capacity is adjusted from 5% of the battery capacity to 15%, and the rest is the same as Example 5.

[0082] Example 18

[0083] Compared with Example 5, the reverse charge cutoff capacity is adjusted from 5% of the battery capacity to 20%, and the rest is the same as Example 5.

[0084] Example 19

[0085] The electrolyte A in Example 10 was adjusted to electrolyte G, and the rest were the same as in Example 10.

[0086] Example 20

[0087] The electrolyte A in Example 10 was adjusted to electrolyte H, and the rest were the same as in Example 10.

[0088] Example 21

[0089] The electrolyte A in Example 10 was adjusted to electrolyte I, and the rest were the same as in Example 10.

[0090] Example 22

[0091] The electrolyte A in Example 10 was adjusted to electrolyte J, and the rest were the same as in Example 10.

[0092] Example 23

[0093] The electrolyte A in Example 10 was adjusted to electrolyte K, and the rest was the same as Example 10.

[0094] Example 24

[0095] The electrolyte A in Example 10 was adjusted to electrolyte L, and the rest were the same as in Example 10.

[0096] Example 25

[0097] The electrolyte A in Example 10 was adjusted to electrolyte M, and the rest was the same as Example 10.

[0098] Example 26

[0099] The electrolyte A in Example 10 was adjusted to electrolyte N, and the rest was the same as Example 10.

[0100] Example 27

[0101] The electrolyte A in Example 10 was adjusted to electrolyte O, and the rest were the same as in Example 10.

[0102] Example 28

[0103] The electrolyte A in Example 10 was adjusted to electrolyte P, and the rest were the same as in Example 10.

[0104] Example 29

[0105] The electrolyte A in Example 10 was adjusted to electrolyte Q, and the rest were the same as in Example 10.

[0106] Example 30

[0107] The electrolyte A in Example 10 was adjusted to electrolyte R, and the rest were the same as in Example 10.

[0108] Example 31

[0109] The electrolyte A in Example 10 was adjusted to electrolyte S, and the rest were the same as in Example 10.

[0110] Example 32

[0111] The electrolyte A in Example 10 was adjusted to electrolyte T, and the rest were the same as in Example 10.

[0112] Example 33

[0113] The electrolyte A in Example 10 was adjusted to electrolyte U, and the rest were the same as in Example 10.

[0114] Example 34

[0115] The electrolyte A in Example 10 was adjusted to electrolyte V, and the rest were the same as in Example 10.

[0116] Example 35

[0117] The electrolyte A in Example 10 was adjusted to electrolyte W, and the rest were the same as in Example 10.

[0118] Comparative Example 1

[0119] Compared with Example 1, reverse charge formation is not performed, and the rest is the same as Example 1.

[0120] Comparative Example 2

[0121] Compared with Example 2, the reverse charging current is adjusted from 0.05C to 0.005C, and the rest are the same as Example 2.

[0122] Comparative Example 3

[0123] Compared with Example 2, the reverse charging current is adjusted from 0.05C to 1.5C, and the rest are the same as Example 2.

[0124] Comparative Example 4

[0125] Compared with Example 4, the reverse charge cutoff capacity is adjusted from 1% of the battery capacity to 25%, and the rest is the same as Example 1.

[0126] Comparative Example 5

[0127] Compared with Example 5, electrolyte A was adjusted to electrolyte B, and the rest were the same as Example 5.

[0128] Comparative Example 6

[0129] Compared with Example 5, the electrolyte A was adjusted to electrolyte C, and the rest were the same as Example 5.

[0130] Comparative Example 7

[0131] Compared with Example 5, the electrolyte A was adjusted to electrolyte D, and the rest were the same as Example 5.

[0132] Comparative Example 8

[0133] Compared with Example 9, the electrolyte A was adjusted to electrolyte E, and the rest was the same as Example 9.

[0134] Comparative Example 9

[0135] Compared with Example 4, the reverse charge cut-off capacity is adjusted from 1% of the battery capacity to 0.05%, and the rest is the same as Example 4.

[0136] The sodium ion batteries prepared in each embodiment and comparative example were subjected to a 45°C cycle performance test: the battery was placed in a 45°C oven, the charge and discharge voltage was set to 2-4.15V, and 1C current constant voltage charging and constant voltage discharge were performed for 600 cycles of charge / discharge. The impedance growth rate, capacity retention rate, and volume growth rate of the battery after 600 cycles at 45°C were calculated. Impedance growth rate (%) = [(impedance after cycle - impedance before cycle) / impedance before cycle] × 100%, capacity retention rate = discharge capacity after cycle / discharge capacity before cycle × 100%, volume growth rate = [(volume after cycle - volume before cycle) / volume before cycle] × 100%. The specific test results are shown in Table 2 (the reverse charge current is referred to as the reverse charge current, and the reverse charge cut-off capacity is referred to as the reverse charge cut-off capacity).

[0137] Table 2

[0138]

[0139]

[0140]

[0141] As shown in Table 1, Table 2, Figure 2 、 Figure 3 Commonly shown:

[0142] From the comparison of the test results of Examples 1-9, 11-18 and Comparative Example 1, it can be seen that: for the same electrolyte A, compared with the simple forward charge formation, the reverse charge formation is combined with the forward charge formation. After 600 cycles of the sodium ion battery, the capacity retention rate is increased from 63.5% to 94.1%, and the impedance growth rate and volume growth rate are significantly reduced to varying degrees. In Comparative Example 1, which does not undergo reverse charge formation, a new passivation layer is continuously generated at the positive electrode interface during the battery cycle at a high cut-off voltage, resulting in an irreversible loss of active sodium. The electrolyte is continuously consumed and continuously generates gas, ultimately showing a low capacity retention rate, a high impedance growth rate, and a high volume growth rate. In the above-mentioned embodiment in which reverse charge is performed, a film is pre-formed on the positive electrode side during reverse charge formation to avoid the loss of active sodium during circulation at high voltage, improve the cycle performance of the battery, and ultimately show a high capacity retention rate, a low impedance growth rate, and a low volume growth rate.

[0143] From the comparison of the test results of Example 11, Example 12, Example 2, Example 5, Example 8, Example 13, and Example 14 with Comparative Example 2 and Comparative Example 3, it can be seen that: under the premise that the electrolyte and the reverse charge cut-off capacity are the same, if the reverse charge current is too low or too high, a positive electrode passivation layer that meets the requirements cannot be obtained, and then the performance of the sodium ion battery cannot be significantly improved. Within the scope defined in this application, when the reverse charge current is 0.01, 0.03, 0.05, 0.1, 0.5, 0.8, and 1C, respectively, the capacity retention rate is 75.2%, 78.8%, 80.5%, 93.5%, 77.4%, 76.5%, and 74.8%, respectively; the impedance growth rate is 97.6%, 88.9%, 80.8%, 57.5%, 93.7%, 94.8%, and 99.6%, respectively; and the volume growth rate is 35.7%, 35.4%, 35%, 5.7%, 21.2%, and 22. 4%, 23.5%, the capacity retention rate shows a trend of first increasing and then decreasing with the increase of reverse charging current, the impedance growth rate shows a trend of first decreasing and then increasing with the increase of reverse charging current, and the volume growth rate shows a trend of first decreasing and then increasing with the increase of reverse charging current. The preferred reverse charging current is 0.05-0.5C, and 0.1C is most preferred. Under this reverse charging current, the positive electrode is polarized to a lower potential, and the additive in the electrolyte A undergoes a reduction reaction on the surface of the positive electrode to form a positive electrode passivation layer with oxidation resistance, low impedance and flexibility, which significantly improves the performance of the sodium ion battery.

[0144] From the comparison of the test results of Example 4, Example 15, Example 5, Example 16, Example 6, Example 17, and Example 18 with Comparative Example 4 and Comparative Example 9, it can be seen that: under the premise that the electrolyte and reverse charge current are the same, if the reverse charge cut-off capacity is too large, the additive will be consumed in large quantities during the reverse charge process, which is not conducive to the film formation of the positive charge formation at the negative electrode and the subsequent battery performance. If the reverse charge cut-off capacity is too small, the positive electrode interface passivation layer will be too thin, and it will not be able to play an effective protective role, and then the performance of the sodium ion battery will not be significantly improved. Within the scope defined in this application, when the reverse charge cut-off capacity is 1%, 2%, 5%, 8%, 10%, 15%, and 20% of the battery capacity, the corresponding capacity retention rates are 86.1%, 89.7%, 93.5%, 90.3%, 83.9%, 81.4%, and 80.4%, respectively, and the corresponding internal resistance growth rates are 73.1%, 60.7%, 57.5%, 61.8%, 78.3%, 94.7%, and 99.7%, respectively. %, and the corresponding volume growth rates are 15.5%, 11.4%, 5.7%, 7.4%, 9.3%, 12.8% and 27.3% respectively. The capacity retention rate shows a trend of first increasing and then decreasing with the increase of the reverse charge cut-off capacity. The impedance growth rate shows a trend of first decreasing and then increasing with the increase of the reverse charge cut-off capacity. The volume growth rate shows a trend of first decreasing and then increasing with the increase of the reverse charge current. The preferred reverse charge cut-off capacity is 2%-10%, and 5% is most preferred.

[0145] Under optimal reverse charge current and reverse charge cutoff capacity conditions, we further explored the impact of the electrolyte on battery performance. As shown in the test results of Examples 10, 19, and 32, when NaDFOB and FEC are used as the first additive, PS is used as the second additive, and the FEC and PS addition amounts are consistent, when the NaDFOB addition amount is between 0.1wt% and 3wt%, the capacity retention rate increases first and then decreases with increasing NaDFOB addition, the impedance growth rate decreases first and then increases with increasing NaDFOB addition, and the volume growth rate decreases first and then increases with increasing NaDFOB addition. The preferred NaDFOB addition amount is 0.3wt% to 1.5wt%, with 0.5wt% being the most preferred. Similar trends are observed with the FEC and PS addition amounts, with the preferred FEC addition amount being 0.5wt% to 3wt%, with 1.5wt% being the most preferred, and the preferred PS addition amount being 0.5wt% to 2wt%, with 1wt% being the most preferred.

[0146] As shown in the test results of Examples 5 and 19: when the amount of NaDFOB added is less than 10% of the total amount of NaDFOB and FEC added, the battery's capacity retention and impedance growth both show obvious performance degradation. This is because too little NaDFOB addition will deteriorate the film formation quality during reverse charging and normal formation, resulting in a decrease in battery performance; when the amount of NaDFOB added is greater than or equal to 10% of the total amount of NaDFOB and FEC added, it helps to obtain a high-quality positive electrode interface film, thereby improving battery performance.

[0147] As shown in the test results of Example 5 and Example 32: when the sum of the added amounts of NaDFOB and FEC is less than 50% of the total added amount of NaDFOB, FEC, and PS, the capacity retention, impedance growth rate, and volume growth rate of the battery are all deteriorated. This is because the added amounts of NaDFOB and FEC are crucial for the positive electrode film formation during the reverse charge stage. When the sum of the added amounts of NaDFOB and FEC is greater than or equal to 50% of the total added amount of NaDFOB, FEC, and PS, the quality of the positive electrode interface film formed is better and the protection of the positive electrode is more effective.

[0148] From the comparison between Example 5 and Comparative Example 5, it can be seen that when NaDFOB and FEC are added to the electrolyte, the positive electrode surface generates Na x PO y F z , Na3PO4, Na2O, NaF, etc., which inhibits the reconstruction of the positive electrode surface under high voltage, reduces strain and cracks, and has high ionic conductivity; when NaDFOB and FEC are not added to the electrolyte, the capacity retention rate after reverse charging is greatly reduced, the impedance growth rate is greatly increased, and the volume growth rate is also greatly increased. This is mainly because when the electrolyte does not contain the above-mentioned additives, the solvent and main salt in the electrolyte react on the positive electrode surface after reverse charging to form a high-impedance interface passivation layer, which cannot effectively protect the positive electrode.

[0149] From the comparison between Example 5 and Comparative Examples 6-7, it can be seen that when only NaDFOB or only FEC is added to the electrolyte, the capacity retention rate of the battery after reverse charging is significantly lower than when both are added at the same time, and the impedance growth rate and volume growth rate are significantly higher than when both are added at the same time. It is obvious that NaDFOB and FEC have a synergistic effect. When added simultaneously, the quality of the positive electrode passivation layer is better after reverse charging, and the improvement in battery performance is more obvious.

[0150] From the comparison between Example 9 and Comparative Example 8, it can be seen that the electrolyte E injected into the battery of Comparative Example 8 contains only PS with a reduction potential less than 0.8V. Even if a larger reverse charge current is used during reverse charge formation, the reduction potential of PS cannot be reached, making it impossible to form a film on the positive electrode surface, and then the composition of the positive electrode interface layer cannot be controlled, which ultimately leads to the deterioration of the performance of the sodium ion battery at high voltage.

[0151] From the comparison of Example 5 and Example 10, it can be seen that: on the basis of containing the first additive, the addition of the second additive with a low reduction potential can further improve the capacity retention rate of the sodium ion battery and reduce the volume growth rate. The first additive regulates the composition of the interfacial passivation layer generated on the surface of the positive electrode during the reverse charge process, thereby improving the quality of the positive electrode interfacial passivation layer; the second additive with a low reduction potential does not participate in the positive electrode film formation reaction during the reverse charge process, is retained, and plays a positive role in the subsequent forward formation and battery cycling process.

[0152] Compared with the prior art, the present invention adds a first additive and performs reverse charging and formation, and pre-forms a film on the positive electrode side by reverse charging, thereby effectively improving the quality of the positive electrode film formation, inhibiting the reconstruction of the positive electrode surface during cycling at high voltage, reducing the formation of strain and cracks, and then significantly improving the cycle performance of the battery and reducing the impedance growth rate during the cycle; by adding a second additive, the film components of the negative electrode film are optimized, making it denser and tougher, enhancing the protective effect of the negative electrode side, further improving the battery cycle performance, and reducing the impedance growth during the cycle; the entire formation process is low-cost, easy to implement, and suitable for industrialization.

[0153] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A sodium ion battery formation process, characterized in that: The steps are as follows: S1. Reverse charging and formation: reverse charging the sodium ion battery after injection and shelving, wherein the sodium ion battery is injected with a non-aqueous electrolyte containing a first additive, and the reduction potential of the first additive is ≥0.8VvNa / Na + ; S2, forward charge formation: forward charge formation is performed on the battery after reverse charge formation.

2. The sodium ion battery formation process according to claim 1, wherein: In S1, the charging capacity is used as the charging cutoff condition, the reverse charging current is 0.01-1C, and the reverse charging cutoff capacity is 1%-20% of the battery capacity.

3. The sodium ion battery formation process according to claim 1, wherein: In S1, the reverse charging current is 0.05-0.5C, preferably 0.1C; the reverse charging cut-off capacity is 2%-10% of the battery capacity, preferably 5%.

4. The sodium ion battery formation process according to claim 1, wherein: The first additive includes a reduction potential ≥ 0.8V vs Na / Na + Organic additives, reduction potential ≥ 0.8V vs Na / Na + at least one of the inorganic additives; the first additive is preferably a combination of the organic additive and the inorganic additive, wherein: the addition amount of the inorganic additive in the non-aqueous electrolyte is a, 0.1wt%≤a≤3wt%, preferably 0.3wt%≤a≤1.5wt%; the addition amount of the organic additive in the non-aqueous electrolyte is b, 0.1wt%≤b≤5wt%, preferably 0.5wt%≤b≤3wt%.

5. The sodium ion battery formation process according to claim 4, wherein: The addition amounts of the inorganic additive and the organic additive satisfy: 0.5 wt%≤a+b≤8 wt%, wherein a / (a+b)≥10%.

6. The sodium ion battery formation process according to claim 4, wherein: The organic additive is selected from at least one of carbonates, sulfonates, sulfates, sulfites, phosphates, and acid anhydrides. The carbonate is preferably at least one of fluoroethylene carbonate, difluoroethylene carbonate, trifluoromethylethylene carbonate, fluoromethylethyl carbonate, and vinylethylene carbonate. The sulfonate is preferably at least one of methylene methanedisulfonate and 1,3-propylene sultone. The sulfate is vinyl sulfate, the sulfite is vinyl sulfite, the phosphate is preferably at least one of trifluoroethyl vinyl phosphate and tris(trifluoroethyl)phosphate, and the acid anhydride is preferably at least one of maleic anhydride, succinic anhydride, and sulfobenzoic anhydride. The inorganic additive is selected from at least one of sodium bis(fluorosulfonyl)imide, sodium perchlorate, sodium difluorooxalatoborate, and sodium bisoxalatoborate.

7. The sodium ion battery formation process according to any one of claims 1 to 6, characterized in that: The non-aqueous electrolyte further contains a second additive, the reduction potential of which is less than 0.8 V vs Na / Na + .

8. The sodium ion battery formation process according to claim 7, wherein: The second additive is selected from at least one of sodium difluorophosphate, sodium tetrafluoroborate, 1,3-propane sultone, and vinylene carbonate. The addition amount of the second additive in the non-aqueous electrolyte is c, 0.1wt%≤c≤3wt%, preferably 0.5wt%≤c≤2wt%, and the addition amounts of the organic additive, the inorganic additive, and the second additive satisfy: 1wt%≤a+b+c≤10wt%, a+b / (a+b+c)≥50%.

9. The sodium ion battery formation process according to any one of claims 1 to 6, characterized in that: The non-aqueous electrolyte further includes a non-aqueous solvent and a sodium salt; The non-aqueous solvent includes a cyclic ester and a chain ester, the cyclic ester is selected from at least one of ethylene carbonate, propylene carbonate, and γ-butyrolactone, and the chain ester is selected from at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate; The sodium salt is selected from at least one of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide and sodium trifluoromethanesulfonate, and the concentration of the sodium salt in the non-aqueous electrolyte is 0.2-1.5 mol / L.

10. A sodium ion battery, characterized in that: The product is prepared by the chemical conversion process according to any one of claims 1 to 9.

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