An electrolyte system for constructing a sodium-ion battery gradient SEI film and a liquid injection method thereof

Abstract

The application discloses an electrolyte system for constructing a sodium ion battery gradient SEI film and a liquid injection method thereof, relates to the technical field of sodium ion batteries, and discloses the electrolyte system, which comprises a sodium salt ether first electrolyte containing a film-forming additive, a sodium salt ether second electrolyte and a sodium salt ester third electrolyte containing a film-forming additive, and the concentration gradient ratio of the sodium salt is controlled. The application adopts a three-step liquid injection and a staged formation process to sequentially construct a gradient SEI film with inorganic passivation, ion conduction and high-voltage stability, overcomes the defects of single ether electrolyte, such as high-voltage decomposition and ester electrolyte SEI loose reconstruction, enables the battery to stably work in a voltage interval of 1.5-3.95V, significantly improves the first coulomb efficiency and long cycle performance, and the process is suitable for mass production.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and more specifically to an electrolyte system for constructing a gradient SEI film in a sodium-ion battery and its injection method. Background Technology

[0002] Existing sodium electrolytes face significant technical challenges: (1) Ether electrolytes (such as DME / THF-based) can form a thin and dense SEI film on the surface of the negative electrode. The film thickness can be controlled within 3 nm. However, the voltage window of this type of electrolyte is narrow, usually not exceeding 3.8 V. Under high voltage window, the electrolyte is prone to decomposition. At the same time, the side reaction between ether electrolytes and high voltage positive electrodes (such as O3-type layered oxides) is more serious, which greatly limits the practical application of ether electrolytes in high voltage sodium-ion battery systems.

[0003] (2) Although ester electrolytes (such as EC / DEC-based electrolytes) have a wide voltage window and can withstand voltage up to 4.2V and above, which is suitable for high voltage positive electrode operation requirements, such electrolytes are prone to forming a porous and loose SEI film on the negative electrode surface, with a film thickness of more than 10nm. This porous and loose SEI film is prone to repeated reconstruction during battery charge and discharge cycles, which leads to loss of active sodium and rapid decay of battery capacity.

[0004] In summary, traditional single-type electrolyte systems cannot simultaneously ensure both SEI membrane quality and high-voltage operational stability, making it difficult to meet the industrial application requirements of high-voltage, long-life sodium-ion batteries. Summary of the Invention

[0005] Therefore, the purpose of this invention is to provide a method for injecting electrolyte in stages for sodium-ion batteries and a compatible electrolyte system thereof.

[0006] The objective of this invention is achieved through the following technical solution: <First Aspect> An electrolyte system for constructing a gradient SEI film in a sodium-ion battery comprises three parts: a first electrolyte, a second electrolyte, and a third electrolyte, wherein... The first electrolyte is an ether electrolyte A containing sodium salt A and film-forming additive A; The second electrolyte is an ether electrolyte B containing sodium salt B; The third electrolyte is an ester electrolyte containing sodium salt C and film-forming additive C.

[0007] As one implementation, the concentration M1 of sodium salt A in the first electrolyte is 0.8~1.5 mol / L, for example 1 mol / L.

[0008] As one implementation, the concentration M2 of sodium salt B in the second electrolyte is 0.2~0.5 mol / L, for example 0.3 mol / L.

[0009] As one embodiment, the concentration of sodium salt C in the third electrolyte is 0.8~1.5 mol / L, for example 1.0 mol / L.

[0010] As one implementation scheme, the ratio of M1 to M2 is not less than 3:1 and not more than 7.5:1; for example, 3:1 or 5:1.

[0011] As one embodiment, sodium salt A, sodium salt B, and sodium salt C are each independently selected from one or more of NaPF6, NaClO4, NaFSI, and NaTFSI, such as NaPF6 and NaFSI.

[0012] As one implementation, the solvent A of the first electrolyte and the solvent B of the second electrolyte are each independently selected from ether solvents.

[0013] As one implementation, solvent C in the third electrolyte is an ester solvent.

[0014] As one embodiment, solvent A and solvent B are each independently selected from one or more of DME, DOL, DEGDME, EGDEE, and THF.

[0015] As one embodiment, the solvent C is selected from one or more of EC, PC, DEC, EMC, and DMC.

[0016] As one implementation, solvent A is a mixture of at least two ether solvents.

[0017] As one embodiment, solvent A is selected from a mixture of at least two of DME, DOL, DEGDME, EGDEE, or THF.

[0018] In some embodiments, solvent A is DME / DOL.

[0019] In some embodiments, solvent A is DME and DOL in a volume ratio of 3:1.

[0020] In some embodiments, solvent B is DME.

[0021] As one embodiment, the solvent C is a mixture of at least two ester solvents.

[0022] As one embodiment, the solvent C is selected from one or more of EC / EMC, PC / DMC, and EC / PC.

[0023] As one embodiment, the film-forming additive A is selected from one or more of FEC, SN, TMSP, and DMS, such as FEC.

[0024] As one embodiment, the film-forming additive A has a mass fraction of 2-5 wt.% in the first electrolyte.

[0025] As one embodiment, the film-forming additive C is VC and / or ES, such as VC.

[0026] As one embodiment, the film-forming additive C has a mass fraction of 1~3 wt.% in the third electrolyte.

[0027] As one implementation, the electrolyte system is used in the segmented electrolyte injection and multiple formation processes of sodium-ion batteries.

[0028] As one implementation, the segmented electrolyte injection involves injecting the electrolyte in segments, first ethers and then esters.

[0029] <Second aspect> The above-mentioned electrolyte system injection method in sodium-ion batteries includes the following steps: S. The first electrolyte, the second electrolyte, and the third electrolyte are injected into the battery sequentially and independently.

[0030] As one implementation method, the injection method includes the following steps: S1. Inject the first electrolyte into the battery and perform a first formation. S2. Remove the free electrolyte after the first formation; S3. Inject the second electrolyte and perform secondary formation; S4. Remove the free electrolyte after secondary formation; S5. Inject the third electrolyte and perform three formation processes; S6. Perform capacity testing on the battery.

[0031] As one implementation, the injection volume of the first electrolyte is 35-45% of the total injection volume, for example, 40%; the injection volume of the second electrolyte is 18%-22% of the total injection volume, for example, 20%; the injection volume of the third electrolyte is 35-45% of the total injection volume, for example, 40%; and the total injection volume is the sum of the injection volumes of the first electrolyte, the second electrolyte, and the third electrolyte.

[0032] In some embodiments, the injection volume of the first electrolyte is 40% of the total injection volume; the injection volume of the second electrolyte is 20% of the total injection volume; the injection volume of the third electrolyte is 40% of the total injection volume; and the total injection volume is the sum of the injection volumes of the first electrolyte, the second electrolyte, and the third electrolyte.

[0033] As one implementation, in step S2, the amount of free electrolyte removed is 20% to 35% of the amount of the first electrolyte injected.

[0034] In some embodiments, in step S2, the amount of free electrolyte removed is 28% to 29% of the amount of the first electrolyte injected.

[0035] As one implementation, in step S4, the amount of free electrolyte removed is 7% to 30% of the amount of the second electrolyte injected.

[0036] In some embodiments, in step S4, the amount of free electrolyte removed is 14% to 15% of the amount of the second electrolyte injected.

[0037] As one implementation scheme, in step S1, after the first electrolyte is injected into the battery, it is placed at 40~50 ℃ and left to stand for 12~24 h.

[0038] As one implementation scheme, in step S3, after the second electrolyte is injected into the battery, it is placed at 40~50 ℃ and left to stand for 2~3 hours.

[0039] As one implementation scheme, in step S5, after the third electrolyte is injected into the battery, it is placed at 40~50℃ and left to stand for 2~3 hours.

[0040] As one implementation, the primary formation is charged to 2.5V using a constant current of 0.05-0.2C.

[0041] As one implementation, the secondary formation is charged to 3.5V using a constant current of 0.05-0.2C.

[0042] As one implementation, the third formation is charged to 4V using a constant current of 0.05-0.2C.

[0043] As one implementation, the capacity gradation involves constant current discharge to 1.5V with a current of 0.05-0.2C; and finally constant current charging to 3V with a current of 0.05-0.2C.

[0044] As one implementation, removing the free electrolyte after formation refers to slightly tilting the formed battery and vacuuming it.

[0045] As one implementation, the slight tilting refers to tilting the battery stably at a tilt angle of 40° to 60° for 1 minute; then tilting it stably at a tilt angle of 60° to 75° for 30 seconds to slowly remove the free electrolyte after formation.

[0046] As one implementation, the vacuum suction uses a low vacuum negative pressure of -30~-40kPa, suctions for 30s, pauses for 5s, and repeats twice.

[0047] As one implementation, the first electrolyte, the second electrolyte, and the third electrolyte are injected into the battery under a protective atmosphere.

[0048] <Third aspect> A sodium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the surface of the negative electrode has a gradient SEI film.

[0049] As one implementation, the gradient SEI membrane is prepared using the aforementioned staged electrolyte injection method.

[0050] As one implementation, the gradient SEI film includes an inorganic SEI layer, an organic SEI layer, and a high-pressure stabilizing layer arranged sequentially.

[0051] As one implementation, the positive electrode sheet is prepared by coating a positive active slurry onto an aluminum foil, followed by drying, rolling, and stamping.

[0052] As one implementation, the positive electrode active slurry is NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 It is composed of O2, PVDF and super-P dispersed and mixed in NMP.

[0053] In some embodiments, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The mass ratio of O2, PVDF and super-P is 9.5 : 0.2 : 0.3.

[0054] In some embodiments, the double-sided areal density of the positive electrode coating is 350 g / m². 2 The compaction density of the positive electrode sheet is 3.1 g / cm³. 3 , As one embodiment, the negative electrode sheet is prepared by coating a negative electrode active slurry onto an aluminum foil or copper foil, followed by drying, rolling, and stamping.

[0055] In some embodiments, the negative electrode active slurry is formed by dispersing and mixing hard carbon, SBR, CMC and super-P in water.

[0056] In some embodiments, the mass ratio of hard carbon, SBR, CMC and super-P is 9.5 : 0.15 : 0.15 : 0.2.

[0057] In some embodiments, the double-sided areal density of the negative electrode coating is 165 g / m². 2 The compaction density of the negative electrode sheet is 0.95 g / cm³. 3 .

[0058] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention proposes a staged electrolyte injection process of first using ethers and then esters. At the same time, by controlling the sodium salt concentration gradient ratio in the first two steps (preferably sodium salt concentration ratio ≥3:1 and ≤7.5), an SEI film with functional gradients of inorganic passivation, ionic conductivity and high voltage stability can be constructed in an orderly manner through a three-step electrolyte injection process. By optimizing the SEI film forming components, the stability of the SEI film is effectively improved, and the electrochemical performance of the battery is significantly improved.

[0059] (2) The present invention utilizes high-concentration sodium salt to form an inorganic SEI layer with low electronic conductivity, which can effectively slow down the occurrence of side reactions during battery charging and discharging. It also utilizes low-concentration sodium salt to form an inorganic SEI layer with high ion diffusion, thereby improving the battery dynamic performance.

[0060] (3) This invention effectively improves the uniformity of electrolyte wetting inside the battery by organically combining a staged electrolyte strategy with a staged formation process, making the electrode interface film more uniform and dense. Under the combined effect of optimizing the interface structure, suppressing side reactions, and improving ion conductivity, while taking into account the excellent SEI film formation quality, it significantly expands the stable operating voltage range of the battery to 1.5~3.95V, effectively improving the battery's initial coulombic efficiency and long-term cycle stability. Detailed Implementation

[0061] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0062] To facilitate understanding, the abbreviations mentioned below will be explained first: NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3O2: Layered sodium nickel iron manganese oxide; PVDF: Polyvinylidene fluoride; super-P: Conductive carbon black; NMP: N-methylpyrrolidone; SBR: Styrene-butadiene rubber; CMC: Sodium carboxymethyl cellulose; SEI: Solid electrolyte interfacial membrane; NaPF6: Sodium hexafluorophosphate; NaClO4: Sodium perchlorate; NaFSI: Sodium bis(fluorosulfonyl)imide; NaTFSI: Sodium bis(trifluoromethanesulfonyl)imide; DME: Diethylene glycol dimethyl ether; DOL: 1,3-dioxolane; DEGDME: Diethylene glycol dimethyl ether; EGDEE: Diethylene glycol diethyl ether; THF: Tetrahydrofuran; VC: Ethylene carbonate; FEC: Fluorinated ethylene carbonate; SN: Succinate; TMSP: Trimethylsilyl phosphate; EC: Ethyl carbonate; PC: Propylene carbonate; DEC: Diethyl carbonate; EMC: Ethyl methyl carbonate; DMC: Dimethyl carbonate; ES: Ethyl sulfite; DMS: Vinyl sulfate.

[0063] This specific embodiment provides a staged electrolyte injection method for sodium-ion batteries and its compatible electrolyte system, specifically using a stacked soft-pack battery as an example. Process parameters, control conditions, and other auxiliary operation requirements not explicitly mentioned are all conventional technical means well known to those skilled in the art.

[0064] First, prepare the stacked soft-pack battery. The steps are as follows: Preparation of positive electrode NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, PVDF, and super-P were dispersed in N-methylpyrrolidone (NMP) at a mass ratio of 9.5:0.2:0.3 to prepare a positive electrode active slurry. This slurry was then coated onto a 13 μm thick aluminum foil. After drying, rolling, and stamping, the positive electrode sheet was obtained. The double-sided areal density of the positive electrode sheet coating was 350 g / m². 2 The compaction density of the positive electrode sheet is 3.1 g / cm³. 3 The positive electrode sheet is 170mm long and 80mm wide.

[0065] The thickness of the positive electrode current collector is 6-20μm, for example, it can be 6μm, 10μm, 13μm, 15μm or 20μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable. The positive electrode current collector can be aluminum foil or carbon-coated aluminum foil.

[0066] Preparation of negative electrode Hard carbon, SBR, CMC, and super-P were dispersed in water at a mass ratio of 9.5:0.15:0.15:0.2 to prepare a negative electrode active slurry. This slurry was then coated onto a 13 μm thick aluminum foil or a 6 μm thick copper foil. After drying, rolling, and stamping, the negative electrode sheet was obtained. The areal density of the coated negative electrode sheet on both sides was 165 g / m². 2 The compaction density of the negative electrode sheet is 0.95 g / cm³. 3 ; The thickness of the negative electrode current collector is 4-15μm, such as 5μm, 10μm, 13μm or 15μm, but not limited to the listed values. Other unlisted values ​​within the range are also applicable. The negative electrode current collector can be any one of copper foil, aluminum foil, carbon-coated copper foil or carbon-coated aluminum foil.

[0067] The negative electrode sheet after die-cutting has a length of 172mm and a width of 82mm, both of which are 2mm larger than the length and width of the positive electrode after die-cutting.

[0068] As an example, both the positive and negative current collectors are made of carbon-coated aluminum foil.

[0069] When die-cutting the positive and negative electrode sheets, the length and width of the die-cut negative electrode are 1-4mm larger than those of the die-cut positive electrode. For example, they can be 1mm, 2mm, 3mm or 4mm, but are not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0070] Battery manufacturing Fifteen positive electrode sheets and sixteen negative electrode sheets are selected and assembled with a separator to form a stacked soft-pack battery cell. The cell is then packaged, a liquid injection port is reserved, and vacuum dried to complete the battery preparation. Electrolyte is injected into the battery and it is then packaged. The sodium-ion battery after liquid injection is then formed and capacity tested to obtain a sodium-ion battery. This invention provides an electrolyte system with stepwise ether / ester solvent separation and sodium salt concentration gradient control, and provides an implementation method for staged electrolyte injection. The first step uses a relatively high concentration of sodium salt electrolyte and adds film-forming additives to prepare an inorganic-dominated SEI film. The second step uses a lower concentration of sodium salt electrolyte to reduce the sodium ion concentration in the system, inhibiting the growth of the inorganic layer and constructing an organic-dominated flexible SEI film. The third step uses a conventional sodium salt concentration ester electrolyte, combined with an organic layer-directing film-forming additive, to strengthen the organic phase SEI film, passivate the surface, prevent continuous electrolyte decomposition, and ensure cycle stability by relying on the wide voltage window of the ester electrolyte.

[0071] The process of the present invention will be described in detail below with reference to specific embodiments.

[0072] In this invention, the total electrolyte injection volume for a single battery is set to 35g. The staged electrolyte injection process for sodium-ion batteries involves the following steps: S1, One-time injection and one-time formation S11, Single injection: In an argon atmosphere, the first electrolyte is injected into the battery through a pre-reserved injection port.

[0073] The first electrolyte is an ether-based electrolyte, with sodium salt as the solute and a mixed ether solvent as the solvent, and film-forming additives are added.

[0074] The sodium salt in the first electrolyte can be one or more of NaPF6, NaClO4, NaFSI, and NaTFSI, with a concentration of 0.8~1.5 mol / L.

[0075] The ether solvent in the first electrolyte can be selected from one or more of DME, DOL, DEGDME, EGDEE, and THF. Preferably, at least two ether solvents are mixed together.

[0076] The film-forming additive in the first electrolyte can be one or more of FEC, SN, TMSP, and DMS, with a mass fraction of 2-5 wt% (based on the total mass of the first electrolyte).

[0077] The injection volume is 14g per injection.

[0078] After one injection, place the battery at 45°C and let it stand for 12-24 hours.

[0079] S12, One-time transformation: Charge to 2.5V at a constant current of 0.1C.

[0080] S2. Remove the free electrolyte after primary formation. In an argon atmosphere, the battery that has completed one formation is slightly tilted, then vacuumed to remove the free electrolyte, and then a second electrolyte, which is an ether-based electrolyte, is injected into the battery.

[0081] Slight tilting refers to tilting the battery steadily at a tilt angle of 40°~60° for 1 minute; then tilting it steadily at a tilt angle of 60°~75° for 30 seconds to slowly remove the free electrolyte after the first formation.

[0082] Vacuum suction is performed using a low vacuum pressure of -30 to -40 kPa, with suction for 30 seconds, pause for 5 seconds, and repeated twice.

[0083] S3, Secondary Injection and Secondary Formation S31, Secondary Injection: The solute in the second electrolyte is a sodium salt, which can be one or more of NaPF6, NaClO4, NaFSI, and NaTFSI, with a concentration of 0.2~0.5 mol / L.

[0084] The solvent in the second electrolyte is an ether solvent, which can be one or more of DME, DOL, DEGDME, EGDEE, and THF.

[0085] The second injection volume is 7g.

[0086] After the second electrolyte injection, the battery was placed at 45°C and left to stand for 2 hours.

[0087] S32, Secondary transformation: Charge at a constant current of 0.1C until it reaches 3.5V.

[0088] S4. Remove the free electrolyte after secondary formation. In an argon atmosphere, the battery that has completed the second formation is slightly tilted, then vacuumed to remove the free electrolyte, and then a third electrolyte, which is an ester electrolyte, is injected into the battery.

[0089] Slight tilting refers to tilting the battery steadily at a tilt angle of 40°~60° for 1 minute; then tilting it steadily at a tilt angle of 60°~75° for 30 seconds to slowly remove the free electrolyte after secondary formation.

[0090] Vacuum suction is performed using a low vacuum pressure of -30 to -40 kPa, with suction for 30 seconds, pause for 5 seconds, and repeated twice.

[0091] S5, three injections and three formations S51, Three injections: In an argon atmosphere, a third electrolyte is injected into the battery that has completed secondary formation using a normal pressure and gentle perfusion method. The third electrolyte is an ester electrolyte with added film-forming additives.

[0092] The solute in the third electrolyte is a sodium salt, which can be one or more of NaPF6, NaClO4, NaFSI, and NaTFSI, with a concentration of 0.8~1.5 mol / L.

[0093] The solvent in the third electrolyte is an ester solvent, which can be one or more of EC, PC, DEC, EMC, and DMC, and more preferably one or more of EC / EMC, PC / DMC, and EC / PC.

[0094] The film-forming additives in the third electrolyte can be selected from VC and / or ES, with a mass fraction of 1~3 wt% (based on the total mass of the third electrolyte).

[0095] The total volume of the three injections was 14g.

[0096] After three electrolyte injections, the battery was placed at 45°C and left to stand for 2 hours.

[0097] S52, Tertiary Transformation: Three-stage formation: charging to 4V with a constant current of 0.1C; S6, Capacity Divided Capacity testing: Discharge to 1.5V with a constant current of 0.1C, and finally charge to 3V with a constant current of 0.1C.

[0098] Example The process parameters are shown in Table 1.

[0099] Table 1

[0100] The final electrolyte prepared in each embodiment is approximately 30g.

[0101] Comparative Example Comparative Example 1 uses a single full electrolyte injection, single formation and capacity testing, eliminating the step-by-step process of the second and third electrolytes, and does not use a multi-stage formation method, but instead uses a conventional constant current direct charging method.

[0102] Comparative Example 2 employs a two-step electrolyte injection and two-step formation and volume separation process, eliminating the third electrolyte step and increasing the proportion of secondary electrolyte injection.

[0103] Comparative Example 3 employs a two-step electrolyte injection and two-step formation and volume separation process, omitting the second electrolyte step and adjusting the electrolyte injection ratio.

[0104] The details are shown in Table 2.

[0105] Table 2

[0106] Detection and Analysis Test instrument: Xinwei CT-4000-5V6A, Shenzhen Xinwei Electronics Co., Ltd.

[0107] Electrochemical performance tests were conducted on the batteries prepared in the examples and comparative examples. The cycle cutoff voltage range was set to 1.5~3.95V, and the charge / discharge current was 0.5C for cyclic charge / discharge.

[0108] The initial coulomb efficiency is the initial discharge capacity divided by the sum of the three formation charge capacities.

[0109] Cyclic performance is the ratio of capacity after 1000 cycles to initial capacity.

[0110] Table 3

[0111] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. An electrolyte system for constructing a gradient SEI film in a sodium-ion battery, characterized in that, It includes a first electrolyte, a second electrolyte, and a third electrolyte, wherein, The first electrolyte is an ether electrolyte A containing sodium salt A and film-forming additive A; The second electrolyte is an ether electrolyte B containing sodium salt B; The third electrolyte is an ester electrolyte containing sodium salt C and film-forming additive C; The concentration of sodium salt A in the first electrolyte is M1, which is 0.8~1.5 mol / L; the concentration of sodium salt B in the second electrolyte is M2, which is 0.2~0.5 mol / L; and the concentration of sodium salt C in the third electrolyte is 0.8~1.5 mol / L. The solvent A of the first electrolyte and the solvent B of the second electrolyte are each independently selected from ether solvents; The solvent C of the third electrolyte is an ester solvent.

2. The electrolyte system according to claim 1, characterized in that, The ratio of M1 to M2 shall not be less than 3:1 and not more than 7.5:

1.

3. The electrolyte system according to claim 1, characterized in that, It also includes at least one of the following technical features: A1. The sodium salts A, B, and C are each independently selected from one or more of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI). B1. Solvent A and solvent B are each independently selected from one or more of ethylene glycol dimethyl ether (DME), 1,3-dioxolane (DOL), diethylene glycol dimethyl ether (DEGDME), ethylene glycol diethyl ether (EGDEE), and tetrahydrofuran (THF). C1, the solvent C is selected from one or more of ethylene carbonate EC, propylene carbonate PC, diethyl carbonate DEC, ethyl methyl carbonate EMC, and dimethyl carbonate DMC.

4. The electrolyte system according to claim 1, characterized in that, The film-forming additive A is selected from one or more of fluoroethylene carbonate (FEC), succinate (SN), trimethylsilyl phosphate (TMSP), and ethylene sulfate (DMS).

5. The electrolyte system according to claim 1, characterized in that, The film-forming additive C is selected from vinylene carbonate VC and / or vinyl sulfite ES.

6. A method for injecting electrolyte into a sodium-ion battery, characterized in that, The electrolyte injection method uses the electrolyte system as described in any one of claims 1-5 for electrolyte injection, and the electrolyte injection method includes the following steps: S. The first electrolyte, the second electrolyte, and the third electrolyte are injected into the battery sequentially and independently.

7. The injection method according to claim 6, characterized in that, Step S specifically includes the following process: S1. Inject the first electrolyte into the battery and perform a first formation; S2. Remove the free electrolyte after the first formation; S3. Inject the second electrolyte and perform secondary formation; S4. Remove the free electrolyte after secondary formation; S5. Inject the third electrolyte and perform three formation processes; S6. Perform capacity testing on the battery.

8. The injection method according to claim 6, characterized in that, The injection volume of the first electrolyte is 35% to 45% of the total injection volume; the injection volume of the second electrolyte is 18% to 22% of the total injection volume; the injection volume of the third electrolyte is 35% to 45% of the total injection volume; and the total injection volume is the sum of the injection volumes of the first electrolyte, the second electrolyte, and the third electrolyte.

9. The injection method according to claim 7, characterized in that, It also includes at least one of the following technical features: A2. The primary formation is performed by constant current charging to 2.5V, with a current of 0.05-0.2C. B2. The secondary formation is charged to 3.5V using a constant current, with a current of 0.05-0.2C. C2. The three-stage formation is charged to 4V using a constant current, with a current of 0.05-0.2C. D2. The capacity grading is performed by constant current discharge to 1.5V, with a current of 0.05-0.2C; and finally, constant current charging to 3V, with a current of 0.05-0.2C.

10. A sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, characterized in that, The negative electrode sheet has a gradient SEI film on its surface, and the gradient SEI film is prepared by the liquid injection method according to any one of claims 6 to 9.

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