Electrolyte, secondary injection high-safety lithium ion battery based on electrolyte and preparation method of secondary injection high-safety lithium ion battery

Abstract

The invention discloses an electrolyte, a secondary injection high-safety lithium ion battery based on the electrolyte and a preparation method of the secondary injection high-safety lithium ion battery. In the preparation process of the lithium ion battery, two times of liquid injection are adopted, a solvent of an electrolyte used in the first time of liquid injection comprises a carboxylic ester solvent, a lithium salt is a mixed lithium salt of LiPF6 and LiFSI, the second time of liquid injection is carried out after formation, and the used electrolyte is formed by adding at least one cyclic sulfonate or sulfate compound into a first injection of electrolyte. The mass of the LiFSI in the electrolyte accounts for XL% of the total mass of the electrolyte, the mass of the carboxylic ester accounts for YE% of the total mass of the electrolyte, the mass of the cyclic sulfate and the cyclic sulfonate accounts for ZS% of the total mass of the electrolyte, and XL / YE is larger than or equal to 0.1 ZS. The electrolyte has the advantages that the high-power characteristic of the battery is ensured by using high-content LiFSI and the carboxylic ester solvent, and meanwhile, the electrolyte also has high thermal safety.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to an electrolyte that can maintain both the power performance and safety performance of the battery, a secondary-injection high-safety lithium-ion battery based on the electrolyte, and a preparation method thereof. Background Art Lithium-ion batteries have the advantages of high energy density, long cycle life, and no memory effect, and are widely studied and applied. Currently, the commercial power batteries are mainly lithium iron phosphate (LiFePO4) and ternary (NCM) batteries. Among them, lithium iron phosphate (LiFePO4) power batteries are widely used in the field of electric vehicles, especially in mid- to low-end electric vehicles and buses, due to their high safety, long cycle life, and relatively low cost. However, there is a large gap in the energy density between lithium iron phosphate (LiFePO4) power batteries and ternary (NCM) power batteries. Therefore, it has become a consensus in the industry to develop fast charging technology for lithium iron phosphate (LiFePO4) power batteries to make up for their disadvantage of low energy density.

[0002] The main current ways to improve the fast charging performance of lithium iron phosphate (LiFePO4) power batteries include using low-viscosity solvent carboxylates such as ethyl acetate EA, methyl acetate MA, and ethyl propionate MP; and improving the amount of electrolyte additive vinylene carbonate VC used by the method of secondary injection of the electrolyte to improve the interfacial stability of the negative electrode and reduce the increase in DCR during battery cycling. In addition, in terms of lithium salts, lithium bis(fluorosulfonyl)imide LiFSI with stronger dissociation ability and higher conductivity is used to partially replace the existing lithium hexafluorophosphate LiPF6, and as the replacement ratio increases, the conductivity of the electrolyte also increases accordingly, and the fast charging performance of the battery increases significantly. However, after the content of LiFSI increases to 60% of the total amount of lithium salts, the thermal safety performance of the battery cells drops severely, and the battery cells cannot pass the national standard thermal box test. In view of this, it is urgent to develop a method that can maintain the safety performance of the battery while using a high proportion of LiFSI lithium salts. Summary of the Invention

[0003] Aiming at the defects existing in the prior art, the purpose of the present invention is to provide an electrolyte that can maintain both the power performance of lithium iron phosphate (LiFePO4) power batteries and extend the calendar life of the battery, and a secondary-injection high-safety lithium-ion battery based on the electrolyte.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions: In the first aspect, the present invention provides an electrolyte.

[0005] The electrolyte is used for secondary injection of a lithium-ion battery; the electrolyte includes a first-injection electrolyte and a second-injection electrolyte; The first injection electrolyte is used for the first injection and formation of the lithium-ion battery, and the second injection electrolyte is used for the second injection of the lithium-ion battery; The first injection electrolyte includes a first lithium salt and a first solvent; The first lithium salt is a mixed lithium salt of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), wherein the mass percentage of lithium bis(fluorosulfonyl)imide in the mixed lithium salt is ≥60%; The first solvent includes a carboxylic ester solvent; The second injection electrolyte includes a second lithium salt and a second additive; The second lithium salt is a mixed lithium salt of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI); The second additive includes at least one of cyclic sulfate and cyclic sulfonate.

[0006] The electrolyte provided by the present invention ensures the high-power characteristics of the battery and also has high thermal safety by using a high content of LiFSI, carboxylic ester solvents, and adding cyclic sulfate or cyclic sulfonate additives to the second injection electrolyte.

[0007] Further, the mass percentage of lithium bis(fluorosulfonyl)imide in the first lithium salt in the mixed lithium salt is 60%-85%; specifically, such as 65%, 70%, 75%, 80%, etc.

[0008] Further, the carboxylic ester solvent is selected from at least one of methyl formate (MF), methyl acetate (MA), ethyl propionate (EP), ethyl acetate (EA), and propyl propionate; preferably at least one of methyl formate (MF), methyl acetate (MA), and ethyl propionate (EP).

[0009] Further, in the first injection electrolyte, the mass percentage of the carboxylic ester solvent in the total mass of the electrolyte is ≥30%, preferably, the mass percentage of the carboxylic ester solvent in the total mass of the electrolyte is 30%-50%, specifically, such as 35%, 40%, 45%.

[0010] Further, the first solvent further includes a carbonate solvent, and the carbonate solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC).

[0011] Furthermore, the first solvent further includes one or several of tetrahydrofuran (THF), acetonitrile, propionitrile, and succinonitrile.

[0012] Preferably, the first solvent is a combination of at least one of ethyl acetate and ethyl propionate and at least one of ethylene methyl carbonate, ethylene carbonate, and diethyl carbonate.

[0013] According to an embodiment of the present invention, the first solvent is selected from the combination of ethylene carbonate (EC), ethylene methyl carbonate (EMC), and ethyl acetate (EA), and the mass ratio of the three is 3:3:4 in sequence.

[0014] Furthermore, in the second additive of the second-injection electrolyte, the cyclic sulfate and cyclic sulfonate are selected from any one or more of pentaerythritol dicyclic sulfate TDT, 4,4'-bis-1,2-oxathiane, 2,2,2',2'-tetraoxide Bi-PS, bis(ethylene sulfate) Bi-DTD, and ethylene carbonate mannitol sulfate EC-DTD; The structural formula thereof is any one or more of the following formulas I - IV:

[0015] As an improvement of the lithium-ion battery with secondary injection according to the present invention, the sum of the mass of one or several combinations of the additive cyclic sulfate and cyclic sulfonate and the lithium salt LiFSI in the two injections accounts for 10% - 20% of the total mass of the electrolyte (the sum of the mass of the first-injection electrolyte and the second-injection electrolyte), specifically such as 10%, 11%, 15%, 20%.

[0016] Furthermore, in the second-injection electrolyte, the proportion of LiFSI in the second lithium salt is ≥60% of the mass of the second lithium salt.

[0017] Furthermore, the total mass of the first lithium salt and the second lithium salt accounts for 10 - 25% of the total mass of the electrolyte (the sum of the mass of the first-injection electrolyte and the second-injection electrolyte), specifically such as 10%, 13%, 15%, 20%, 25%.

[0018] Furthermore, in the second additive of the second-injection electrolyte, the mass of the cyclic sulfate and cyclic sulfonate additives accounts for Zs% of the total mass of the electrolyte (the sum of the mass of the first-injection electrolyte and the second-injection electrolyte), and the range of Zs% is 2% - 5%, specifically such as 2%, 2.5%, 3%, 3.5%.

[0019] Furthermore, the second-injection electrolyte further includes a second solvent, and the second solvent includes a carboxylic acid ester solvent.

[0020] Furthermore, the carboxylic acid ester solvent is selected from at least one of methyl formate (MF), methyl acetate (MA), ethyl propionate (EP), ethyl acetate (EA), and propyl propionate; preferably at least one of methyl formate (MF), methyl acetate (MA), and ethyl propionate (EP).

[0021] Further, the second solvent further includes one or more of tetrahydrofuran (THF), acetonitrile, propionitrile, and succinonitrile.

[0022] According to an embodiment of the present invention, the composition of the second solvent in the two-injection electrolyte is the same as that of the first solvent in the one-injection electrolyte.

[0023] Further, in the electrolyte, the mass ratio of lithium bis(fluorosulfonyl)imide to the total mass of the electrolyte is X L %, the mass ratio of the carboxylic ester solvent (total carboxylic ester in the one-injection and two-injection electrolytes) to the total mass of the electrolyte is Y E %, and the mass ratio of cyclic sulfate and cyclic sulfonate in the second additive to the total mass of the electrolyte is Z S %, and the three satisfy X L / Y E ≥0.1Z S .

[0024] Further, the one-injection electrolyte and / or the two-injection electrolyte further includes other functional additives.

[0025] Further, the other functional additives are selected from any one or more of cycling additives, low-temperature additives, high-temperature additives, flame retardant additives, and overcharge prevention additives; preferably, the other functional additives are selected from functional additives such as vinylene carbonate VC, fluoroethylene carbonate FEC, tris(trimethylsilyl) phosphate TMSP, tris(trimethylsilyl) borate TMSB, lithium difluorophosphate LiPOF2, lithium difluorooxalate borate LiODFB, and lithium bis(oxalato)borate LiBOB.

[0026] Further, the mass of the other functional additives accounts for 0.1-5 wt% of the mass of the one-injection electrolyte, specifically 0.1 wt%, 1 wt%, 3 wt%, or 5 wt%.

[0027] Further, the mass of the other functional additives accounts for 0.1-5 wt% of the mass of the two-injection electrolyte, specifically 0.1 wt%, 1 wt%, 3 wt%, or 5 wt%.

[0028] Further, the mass ratio of the one-injection electrolyte to the two-injection electrolyte is 9:1-5:1, specifically 9:1, 7:1, or 5:1.

[0029] In a second aspect, the present invention provides a lithium-ion battery.

[0030] The lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; the electrolyte is the electrolyte described in the first aspect of the present invention, and the electrolyte is filled by two-injection method.

[0031] Further, the positive electrode sheet includes a positive electrode active material, and preferably the positive electrode active material is selected from lithium iron phosphate and / or lithium iron manganese phosphate.

[0032] Further, the negative electrode sheet includes a negative electrode active material, and preferably the negative electrode active material is selected from any one or more of natural graphite, artificial graphite, silicon, and lithium titanate.

[0033] Further, the separator may be a polyethylene film coated with a ceramic separator.

[0034] In a third aspect, the present invention provides a method for injecting an electrolyte solution of a lithium-ion battery.

[0035] The method for injecting an electrolyte solution of a lithium-ion battery provided by the present invention includes the following steps: Performing a first injection on the lithium-ion battery using the first injection electrolyte solution; sequentially performing encapsulation, standing, and formation on the lithium-ion battery after the first injection; Performing a second injection on the lithium-ion battery after formation using the second injection electrolyte solution; Sequentially performing aging, secondary encapsulation, and grading on the lithium-ion battery after the second injection.

[0036] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention can not only use a high proportion of LiFSI lithium salt, but also by adding a cyclic sulfate or cyclic sulfonate additive to the second injection electrolyte solution, not only does not increase the impedance of the battery and maintains the power characteristics of the battery; moreover, it also prevents the safety hazards brought by using a high content of LiFSI. By using this electrolyte solution, in addition to maintaining the power performance of the lithium-ion battery, the battery also has good safety performance.

[0037] The present invention solves the problem of using an electrolyte containing a high addition amount of LiFSI in a lithium-ion battery. Through the first liquid injection (a common electrolyte without cyclic sulfate and cyclic sulfonate additives) and formation, a good SEI is formed on the surface of the graphite negative electrode, effectively isolating the direct contact between the electrolyte and the negative electrode material. Through the second liquid injection, an electrolyte containing a cyclic sulfate or cyclic sulfonate additive is added. At this time, due to the presence of SEI, the reduction reaction of the cyclic sulfate or cyclic sulfonate additive on the surface of the graphite negative electrode is prevented, so that the impedance of the battery will not increase, and thus the battery can retain good power performance. In addition, since the added cyclic sulfate or cyclic sulfonate additive will form an SEI film rich in a large amount of Li2SO3 or Li2SO4 on the surface of the graphite negative electrode during the heating process of the battery, and they both have good thermal stability, they can prevent the contact between LiFSI and the graphite negative electrode, completely inhibiting the heat generation caused by the reduction decomposition of LiFSI, thereby greatly improving the thermal safety performance of the battery.

[0038] In addition, the method of the present invention is simple and easy to implement. By adding a second liquid injection process and the use of specific additives on the basis of the existing process, the production of a high-safety lithium-ion battery with an electrolyte containing a high LiFSI addition amount can be realized. Detailed implementation manners

[0039] The present invention will be further described in detail below in conjunction with the specific implementation manners. The provided embodiments are only for clarifying the present invention, rather than limiting the scope of the present invention. The following provided embodiments can be used as a guide for those of ordinary skill in the art to make further improvements, and do not limit the present invention in any way.

[0040] The experimental methods in the following embodiments are all conventional methods unless otherwise specified, and are carried out according to the techniques or conditions described in the literature in this field or according to the product instructions. The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels unless otherwise specified.

[0041] The test methods adopted in the following embodiments are as follows: 1. Cell hot box test At room temperature, fix the contact of the thermocouple on the large surface of the battery and place it in a hot box. Set the heating rate of the oven to 5°C / min. After the oven temperature reaches 130°C, keep it warm for 30 min. During this period, record the thermocouple values and the breakage and explosion times of the cell thermal runaway. If the cell catches fire and explodes, it is judged as not passing; if it does not catch fire and explode, it is judged as passing.

[0042] 2. Battery internal resistance test Discharge the lithium-ion battery at a constant current of 1C until the cut-off voltage of 2.5V. After leaving it at rest at 25°C for 5h, connect the test fixture of the AC impedance analyzer to the positive and negative electrodes of the lithium battery to ensure firm connection and good contact. Set the test frequency between 1Hz and 100kHz, set the polarization voltage amplitude between 10mV and 100mV, start the test, and record the ACR value.

[0043] 3. Test for battery cycle capacity retention rate Under the condition of 25°C, charge the lithium-ion battery at a constant current of 1C to 3.65V, then charge it at a constant voltage of 3.65V until the current is 0.05C, and then discharge it at a constant current of 1C to 2.5V. After 1000 charge-discharge cycles, calculate the capacity retention rate after the 1000th cycle at 25°C according to the following formula: Discharge capacity after the 1000th cycle / Discharge capacity of the first cycle × 100%.

[0044] Under the condition of 45°C, charge the lithium-ion battery at a constant current of 2C to 3.65V, then charge it at a constant voltage of 3.65V until the current is 0.05C, and then discharge it at a constant current of 2C to 2.5V. After 500 charge-discharge cycles, calculate the capacity retention rate after the 500th cycle at 45°C according to the following formula: Discharge capacity after the 500th cycle / Discharge capacity of the first cycle × 100%.

[0045] 4. Test for battery storage capacity retention rate at 60°C Charge the battery at a constant current of 0.5C to 3.65V at 25°C, then charge it at a constant voltage of 3.65V until the current is 0.05C, and record the capacity at this time as Q1; then place the battery in a 60°C constant temperature oven. After 28 days, test to charge the battery at a constant current of 0.5C to 3.65V at 25°C, then charge it at a constant voltage of 3.65V until the current is 0.05C, and record the capacity at this time as Q2. The calculation formula for the capacity retention rate of the battery stored at 60°C for 28 days is: (Q1 - Q2) / Q1 × 100%.

[0046] The following will illustrate the beneficial technical effects of the present application in combination with specific examples and comparative examples.

[0047] Example 1 Preparation of electrolyte: A first electrolyte solution is prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and ethyl acetate (EA) in a mass ratio of 3:3:4 (mass percentages are 25 wt%, 25 wt% and 34 wt% respectively). After mixing, 4 wt% of lithium hexafluorophosphate (LiPF6) and 9 wt% of lithium bis(fluorosulfonyl)imide (LiFSI) (based on the total mass of the electrolyte solution), 2 wt% of vinylene carbonate VC (based on the total mass of the electrolyte solution), 0.5 wt% of tris(trimethylsilyl) phosphate TMSP (based on the total mass of the electrolyte solution) and 0.5 wt% of lithium difluoro(oxalato)borate LiODFB (based on the total mass of the electrolyte solution) are added.

[0048] A second electrolyte solution is prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and ethyl acetate (EA) in a weight ratio of 3:3:4 (weight ratios are 19 wt%, 19 wt% and 28 wt% respectively). After mixing, 4 wt% of lithium hexafluorophosphate (LiPF6) and 9 wt% of lithium bis(fluorosulfonyl)imide (LiFSI) (based on the total mass of the electrolyte solution), 20 wt% of pentaerythritol dicyclic sulfate TDT (based on the total mass of the electrolyte solution), 0.5 wt% of tris(trimethylsilyl) phosphate TMSP (based on the total mass of the electrolyte solution) and 0.5 wt% of lithium difluoro(oxalato)borate LiODFB (based on the total mass of the electrolyte solution) are added.

[0049] The mass ratio of the first electrolyte solution to the second electrolyte solution is 9:1. The specific formulations of the first and second electrolyte solutions are shown in Table 1 below.

[0050] Table 1

[0051] Preparation of the positive electrode sheet: Lithium iron phosphate material LiFePO4, conductive agent Super P, binder PVDF and carbon nanotubes (CNT) are mixed uniformly in a mass ratio of 97.5:0.5:1:1 to form a lithium-ion battery positive electrode slurry with a certain viscosity, which is coated on aluminum foil used as a current collector, and the coating amount is 360 g / m 2 , dried at 85 °C and then cold-pressed; then cut into strips and slices, and then dried in vacuum at 85 °C for 24 h to make a lithium-ion battery positive electrode sheet that meets the requirements.

[0052] Preparation of the negative electrode sheet: Artificial graphite, conductive agent Super P, thickener CMC, and binder SBR (styrene-butadiene rubber latex) are made into a slurry in a mass ratio of 95:1.5:1.0:2.5, mixed evenly, and the prepared slurry is coated on both sides of copper foil, dried and roll-pressed to obtain a negative electrode sheet, and then dried in vacuum at 85 °C for 24 h to make a lithium-ion battery negative electrode sheet that meets the requirements.

[0053] Preparation of Lithium-Ion Battery: The positive electrode sheet, negative electrode sheet, and separator (polyethylene film coated with ceramic separator) prepared according to the above process are made into a prismatic lithium-ion battery with a thickness of 50 mm, a width of 90 mm, and a length of 200 mm through a winding process. The capacity is 54 Ah. It is vacuum baked at 85 °C for 48 hours, and the above electrolyte is injected. The amount of the first injection of electrolyte is 175 g. After encapsulation, standing, formation, secondary injection (the amount of the second injection of electrolyte is 19.5 g), aging, secondary encapsulation, and grading, the production of a 53 Ah prismatic lithium-ion battery is completed.

[0054] In Examples 2 to 9 and Comparative Examples 1 to 4, the specific ratios and types of substances in the electrolyte were changed. Among them, PS is 1,3-propane sultone, and DTD is vinylene sulfate. Lithium-ion batteries were obtained with reference to the preparation method of Example 1. The electrolyte formulations are shown in Table 2 below.

[0055] Table 2

[0056] Note: In Examples 6 - 9 in the above table, as the amount of cyclic sulfonic acid / sulfate additive or LiFSI increases, the amount of carbonate solvents decreases accordingly. In Comparative Example 3, as the amount of cyclic sulfonic acid / sulfate additive decreases, the amount of carbonate solvents increases accordingly. In Comparative Example 3, as the amount of cyclic sulfonic acid / sulfate additive increases, the amount of carbonate solvents decreases accordingly.

[0057] The 54 Ah prismatic lithium-ion batteries assembled in Examples 1 - 9 and Comparative Examples 1 - 4 above were respectively tested for cyclic electrical performance and high-temperature safety performance. The specific test methods are shown in Table 3 below.

[0058] Table 3

[0059] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention and without unnecessary experiments, the present invention can be implemented within a relatively wide range under equivalent parameters, concentrations, and conditions. Although specific examples of the present invention are given, it should be understood that the present invention can be further improved. In short, according to the principle of the present invention, this application intends to cover any modifications, uses, or improvements to the present invention, including those that depart from the scope disclosed in this application but are made using conventional techniques known in the art. Some basic features can be applied according to the scope of the following appended claims.

Claims

1. An electrolyte for secondary injection in a lithium-ion battery; the electrolyte includes a first-injection electrolyte and a second-injection electrolyte; The first-injection electrolyte includes a first lithium salt and a first solvent; The first lithium salt is a mixed lithium salt of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, wherein, The mass percentage of lithium bis(fluorosulfonyl)imide in the mixed lithium salt ≥ 60%; The first solvent includes a carboxylate solvent; The second-injection electrolyte includes a second lithium salt and a second additive; The second lithium salt is a mixed lithium salt of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide; The second additive includes at least one of a cyclic sulfate and a cyclic sulfonate; 2. The electrolyte according to claim 1, wherein: The carboxylate solvent is selected from at least one of ethyl acetate, methyl formate, methyl acetate, ethyl propionate, and propyl propionate; And / or, in the first-injection electrolyte, the mass of the carboxylate solvent accounts for ≥ 30% of the total mass of the electrolyte.

3. The electrolyte according to claim 1, wherein: The first solvent further includes a carbonate solvent, and the carbonate solvent is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propyl carbonate.

4. The electrolyte according to claim 1 or 2, characterized in that: The cyclic sulfate and cyclic sulfonate in the second additive are selected from any one or more of pentaerythritol dicyclic sulfate TDT, 2,2,2',2'-tetraoxo-4,4'-bi-1,2-oxathiolane Bi-PS, ethylene bisulfate, and mannitol ethylene carbonate sulfate; And / or, the total mass of the cyclic sulfate and cyclic sulfonate in the second additive accounts for Zs% of the total mass of the electrolyte, and the range of Zs% is 2% - 5%.

5. The electrolyte according to claim 1 or 2, characterized in that: The mass percentage of lithium bis(fluorosulfonyl)imide in the second lithium salt ≥ 60% of the mass of the second lithium salt.

6. The electrolyte according to claim 1 or 2, characterized in that: The total mass of the first lithium salt and the second lithium salt accounts for 10 - 25% of the total mass of the electrolyte.

7. The electrolyte according to claim 1 or 2, characterized in that: The sum of the mass of lithium bis(fluorosulfonyl)imide in the first lithium salt and the second lithium salt and the mass of the cyclic sulfate and cyclic sulfonate in the second additive accounts for 10% - 30% of the total mass of the electrolyte.

8. The electrolyte according to claim 1, wherein: The second-injection electrolyte further includes a second solvent, and the second solvent includes a carboxylate solvent; the carboxylate solvent is selected from at least one of ethyl acetate, methyl formate, methyl acetate, ethyl propionate, and propyl propionate.

9. The electrolyte according to claim 1 or 8, characterized in that: In the electrolyte, the proportion of lithium bis(fluorosulfonyl)imide by mass in the total mass of the electrolyte is X L %, the proportion of the carboxylic ester solvent by mass in the total mass of the electrolyte is Y E %, and the proportion of cyclic sulfate and cyclic sulfonate in the second additive by mass in the total mass of the electrolyte is Z S %, and the three satisfy X L / Y E ≥0.1Z S .

10. The electrolyte according to claim 1 or 2, characterized in that: The first-injection electrolyte and / or the second-injection electrolyte further includes other functional additives; the other functional additives are selected from any one or more of a cycle additive, a low-temperature additive, a high-temperature additive, a flame retardant additive, and an overcharge prevention additive; The mass of the other functional additives accounts for 0.1 - 5 wt% of the mass of the first-injection electrolyte; The mass of the other functional additives accounts for 0.1 - 5 wt% of the mass of the second-injection electrolyte.

11. The electrolyte according to claim 10, characterized in that: The other functional additives are selected from any one or more of vinylene carbonate VC, fluoroethylene carbonate FEC, tris(trimethylsilyl) phosphate TMSP, tris(trimethylsilyl) borate TMSB, lithium difluorophosphate LiPO2F2, lithium difluorooxalate borate LiODFB, and lithium bis(oxalato)borate LiBOB.

12. The electrolyte according to claim 1 or 2, characterized in that: The mass ratio of the first-injection electrolyte to the second-injection electrolyte is 9:1 - 5:

1.

13. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; characterized in that: The electrolyte is the electrolyte according to any one of claims 1 - 12.

14. A method for injecting electrolyte into a lithium-ion battery, comprising the following steps: Performing a first injection of electrolyte into the lithium-ion battery using the first-injection electrolyte described in any one of claims 1-12; Successively encapsulating, standing, and forming the lithium-ion battery after the first injection; Performing a second injection of electrolyte into the formed lithium-ion battery using the second-injection electrolyte described in any one of claims 1-12; Successively aging, secondarily encapsulating, and grading the lithium-ion battery after the second injection.

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