Lithium ion battery and preparation method thereof

Abstract

The invention provides a lithium ion battery and a preparation method thereof. The preparation method comprises a liquid injection process of injecting an electrolyte into a dry battery cell, and the liquid injection process comprises the following steps: injecting a first electrolyte into the dry battery cell to obtain a first battery cell; charging the first battery cell to obtain a second battery cell; injecting a second electrolyte into the second battery cell to obtain a third battery cell, thereby obtaining the lithium ion battery, both the first electrolyte and the second electrolyte contain a fluoroethylene carbonate additive and a vinylene carbonate additive. The mass ratio of fluoroethylene carbonate (FEC) to vinylene carbonate (VC) in the electrolyte for primary and secondary liquid injection is accurately controlled by cooperating with the ratio of the electrolyte for primary and secondary liquid injection, so that the technical effects of improving the rate capability and cycling stability of the lithium ion battery and prolonging the service life of the battery are realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, and in particular, to a lithium ion battery and a preparation method thereof. BACKGROUND

[0002] The core of a lithium ion battery lies in its internal electrochemical reactions, and the electrolyte is the medium for this reaction, directly affecting the performance and safety of the battery. The electrolyte contains solvents, lithium salts, and additives, among which the solvents provide the path for lithium ion transport, the lithium salts ensure the conduction of electric charge, and the additives play a key role in the process of forming a solid electrolyte interface (SEI) film, which can effectively prevent the decomposition of the electrolyte and reduce the occurrence of side reactions, thereby prolonging the service life of the battery and improving its performance.

[0003] The secondary injection technology, i.e., injecting electrolyte again after the initial injection of the battery and the initial formation (pre-lithiation process aimed at forming the initial SEI film), aims to supplement the additives consumed by the initial formation, further optimize the SEI film structure, and improve the overall performance of the battery. Although this method has shown its superiority under laboratory conditions, in commercial production, how to accurately control the ratio of the two injections, the amount of additives, and the concentration of lithium salts to balance the cost and performance remains a challenge.

[0004] Therefore, how to provide a secondary injection method that better considers the interaction between components in the electrolyte, thereby establishing a more stable SEI film in the lithium ion battery, and further more significantly improving the cycle stability of the obtained lithium ion battery, is one of the important technical problems to be solved in the field. SUMMARY

[0005] The main purpose of the present application is to provide a lithium ion battery and a preparation method thereof to solve the problem of poor cycle stability of the lithium ion battery in the prior art due to the formation of an unsatisfactory SEI film.

[0006] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a lithium-ion battery, comprising an electrolyte injection process of injecting an electrolyte into a dry cell, the electrolyte injection process comprising: step S1, injecting a first electrolyte into a dry cell to obtain a first cell; the first cell being charged to obtain a second cell; step S2, injecting a second electrolyte into the second cell to obtain a third cell, thereby obtaining a lithium-ion battery; both the first electrolyte and the second electrolyte contain fluoroethylene carbonate additives and vinylene carbonate additives; the total weight content of fluoroethylene carbonate additives and vinylene carbonate additives in the first electrolyte is denoted as a; the total weight content of fluoroethylene carbonate additives and vinylene carbonate additives in the second electrolyte is denoted as b; taking the total weight of the first electrolyte and the second electrolyte as 1, the weight content of the first electrolyte is denoted as x; (x-0.5) / 30≤a≤(x-0.5) / 5, (1.2-x) / 10≤b≤2(1.2-x) / 3.

[0007] Further, (x-0.5) / 20≤a≤(x-0.5) / 6, (1.2-x) / 8≤b≤(1.2-x) / 2; preferably, in the first electrolyte, the weight ratio of vinylene carbonate additive to fluoroethylene carbonate additive is (0.3~4.0):(0.4~1.5); in the second electrolyte, the weight ratio of vinylene carbonate additive to fluoroethylene carbonate additive is (4~15):(0.5~7).

[0008] Furthermore, the first electrolyte also includes a first lithium salt, and the molar concentration of the first lithium salt in the first electrolyte is denoted as c mol / L; the second electrolyte also includes a second lithium salt, and the molar concentration of the second lithium salt in the second electrolyte is denoted as d mol / L; 2x-0.6≤c≤3x-0.5, 1.3-x≤d≤1.8-x; preferably, 2x-0.5≤c≤3x-0.8, 1.4-x≤d≤1.7-x.

[0009] Furthermore, the first lithium salt and the second lithium salt are each independently selected from one or more of lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(fluorooxalate-borate); preferably, the first lithium salt and the second lithium salt are each independently lithium hexafluorophosphate and / or lithium bis(fluorosulfonyl)imide.

[0010] Furthermore, 0.7≤x≤0.95, 0.01≤a≤0.15, 0.01≤b≤0.3, 0.8≤c≤2.0, and 0.4≤d≤1.0.

[0011] Furthermore, the first electrolyte also includes a first solvent, and the second electrolyte also includes a second solvent, wherein the first solvent and the second solvent are each independently selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; preferably, both the first solvent and the second solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0012] Further, the charging process includes the following sequential steps: a first charging stage, charging at a first current rate to 10% SOC to 30% SOC; a second charging stage, charging at a second current rate to 60% SOC to 80% SOC; preferably, the first current rate is 0.5C to 3C; and / or, the second current rate is 0.1C to 0.5C.

[0013] Furthermore, the injection process is carried out at 25℃~60℃ and 0.1MPa~1MPa.

[0014] A second aspect of the present invention provides a lithium-ion battery prepared by the above-described method for preparing lithium-ion batteries.

[0015] Furthermore, the lithium-ion battery also includes a positive electrode and a negative electrode. The positive electrode includes a positive active material, and the negative electrode includes a negative active material. The positive active material is selected from lithium transition metal oxides, preferably LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li... 1+y Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4 and Li2Mn 1-x One or more of O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, and F, 0≤y<0.2, 0≤x<1; and / or, the negative electrode active material is selected from one or more of graphite and silicon-carbon composite materials, preferably graphite.

[0016] By applying the technical solution of this invention, the mass ratio of fluoroethylene carbonate (FEC) to vinylene carbonate (VC) in the primary and secondary electrolytes is precisely controlled through coordinated ratio of primary and secondary electrolytes. This achieves the goal of optimizing the solid electrolyte interface film formation process, thereby improving the rate performance and cycle stability of lithium-ion batteries and extending battery life. Detailed Implementation

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0018] As described in the background section, existing lithium-ion batteries suffer from poor cycle stability due to an imperfect SEI film formation. To solve the above-mentioned technical problems, the first aspect of the present invention provides a method for preparing a lithium-ion battery, including an electrolyte injection process of injecting electrolyte into a dry cell. The electrolyte injection process includes: step S1, injecting a first electrolyte into a dry cell to obtain a first cell; the first cell is charged to obtain a second cell; step S2, injecting a second electrolyte into the second cell to obtain a third cell, thereby obtaining a lithium-ion battery; both the first electrolyte and the second electrolyte contain fluoroethylene carbonate additives and vinylene carbonate additives; the total weight content of fluoroethylene carbonate additives and vinylene carbonate additives in the first electrolyte is denoted as a; the total weight content of fluoroethylene carbonate additives and vinylene carbonate additives in the second electrolyte is denoted as b; taking the total weight of the first electrolyte and the second electrolyte as 1, the weight content of the first electrolyte is denoted as x; (x-0.5) / 30≤a≤(x-0.5) / 5, (1.2-x) / 10≤b≤2(1.2-x) / 3.

[0019] This invention effectively improves the cycle stability and rate performance of lithium-ion batteries obtained through electrolyte injection by dynamically adjusting the content of additives in the first and second electrolyte injection processes and establishing a quantitative relationship between the additive content and the electrolyte injection ratio. Specifically:

[0020] First, a first electrolyte (containing FEC and VC additives, with a total weight content of 'a') is injected into the unfilled dry cell. During subsequent charging, the additives are decomposed to form an SEI film. When (x-0.5) / 30 ≤ a ≤ (x-0.5) / 5, the SEI film formed during the initial formation is sufficiently dense to suppress electrolyte decomposition and direct reactions between the positive and negative electrode active materials, while also avoiding excessive thickness that could affect lithium-ion transport efficiency. Subsequently, a second electrolyte is injected, with a total FEC and VC content of 'b'. When the condition (1.2-x) / 10 ≤ b ≤ 2(1.2-x) / 3 is met, the SEI film structure can be further optimized, replenishing any additives potentially consumed during the first charging process. This ensures that sufficient additives remain in the second electrolyte injection after formation to participate in the repair and enhancement of the SEI film, addressing structural changes and electrochemical stresses during subsequent charge-discharge cycles.

[0021] More importantly, the two relationships mentioned above, (x-0.5) / 30≤a≤(x-0.5) / 5 and (1.2-x) / 10≤b≤2(1.2-x) / 3, are essentially a dynamic adjustment mechanism provided by this invention. This mechanism aims to optimize the usage of FEC and VC based on the relative ratio of the first and second electrolyte injections, i.e., the magnitude of x. This ensures that a stable SEI film can be constructed in different electrolyte injection schemes, ultimately significantly improving the cycle stability and rate performance of the battery.

[0022] Furthermore, to more precisely control the SEI film formation process, ensuring that the SEI film does not become excessively thick due to excessive additives, affecting lithium-ion transport efficiency, nor does it become unstable due to insufficient additive content failing to effectively suppress electrolyte decomposition, a preferred ratio is (x-0.5) / 20≤a≤(x-0.5) / 6. Simultaneously, a preferred ratio is (1.2-x) / 8≤b≤(1.2-x) / 2 to better address the SEI film repair issue after secondary electrolyte injection, enabling the battery to maintain a high capacity retention rate even after multiple charge-discharge cycles, ultimately significantly extending battery life.

[0023] In several typical implementations, in the first electrolyte, the weight ratio of vinylene carbonate additive to fluoroethylene carbonate additive is (0.3~4.0):(0.4~1.5), more preferably (1~2):1; in the second electrolyte, the weight ratio of vinylene carbonate additive to fluoroethylene carbonate additive is (4~15):(0.5~7), more preferably (4~7):1. Because FEC decomposes at low temperatures to form a fluoride SEI film, it is more suitable for the first electrolyte injection stage to ensure the initial low-temperature performance of the battery. VC, on the other hand, tends to decompose at high temperatures to generate a carbide SEI film, making it more suitable for the second electrolyte injection stage, enhancing the high-temperature stability and improving cycle performance of the battery. Based on this, by gradually optimizing the relative proportions of the two additives in the electrolytes used for the two electrolyte injections, the uniform growth of the SEI film is more effectively promoted, reducing the film property bias caused by excessive enrichment of one component, and enabling the resulting battery to maintain excellent electrochemical behavior over a wider temperature range.

[0024] In practical applications, the weight ratio of vinylene carbonate additive to fluoroethylene carbonate additive in the first electrolyte can specifically be 0.5:1.5, 1:1, 1.2:1, 1.5:1, 2:1, 2.5:0.5, 0.3:0.9, or any range of two of the above values. Similarly, the weight ratio of vinylene carbonate additive to fluoroethylene carbonate additive in the second electrolyte can specifically be 15:6, 11:4, 4:1, 13:2, 8:1, 13:1, or any range of two of the above values.

[0025] Furthermore, the first electrolyte also includes a first lithium salt, and the molar concentration of the first lithium salt in the first electrolyte is denoted as c mol / L; the second electrolyte also includes a second lithium salt, and the molar concentration of the second lithium salt in the second electrolyte is denoted as d mol / L; 2x-0.6≤c≤3x-0.5, 1.3-x≤d≤1.8-x. By combining this with the relationship between a, b, and x, adjusting the lithium salt concentration in the two electrolytes can further ensure that the resulting lithium-ion battery has suitable conductivity at different electrolyte filling stages, more reliably supporting efficient lithium-ion transport. Based on this, to promote a more uniform distribution of the electrolyte inside the battery, improve lithium-ion transport efficiency, and more effectively maintain charge balance, reducing possible lithium plating side reactions and battery performance degradation caused by charge imbalance, the following values ​​are further preferred: 2x-0.5≤c≤3x-0.8, 1.4-x≤d≤1.7-x.

[0026] Specifically, the first lithium salt and the second lithium salt are each independently selected from one or more of lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(fluorooxalate-borate). Preferably, the first lithium salt and the second lithium salt are each independently lithium hexafluorophosphate and / or lithium bis(fluorosulfonyl)imide. In order to better adapt the two additive systems and obtain lithium-ion batteries with better cycle performance, the first lithium salt is preferably a mixed salt formed by lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, and the molar ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide in the mixed salt is (7~8):4 (specifically 7:4, 7.5:4 or 8:4), and the second lithium salt is preferably lithium hexafluorophosphate.

[0027] In several typical embodiments, to more effectively improve the performance and reliability of the resulting lithium-ion battery, the following values ​​are more precisely and specifically preferred: 0.7≤x≤0.95, 0.01≤a≤0.15, 0.01≤b≤0.3, 0.8≤c≤2.0, and 0.4≤d≤1.0. However, through extensive experimentation, the inventors have further preferred the following values ​​in several more typical embodiments: 0.8≤x≤0.90, 0.02≤a≤0.04, 0.10≤b≤0.20, 1.0≤c≤1.2, and 0.5≤d≤0.8. The setting of 0.8≤x≤0.90 ensures that the initial electrolyte injection provides sufficient electrolyte to form the basic SEI film, while leaving more room for secondary electrolyte injection to supplement and optimize the SEI film. By adjusting the total content of FEC and VC in the first and second electrolytes, it helps to enable additives to participate more effectively in the formation and repair of the SEI film during the initial formation and secondary electrolyte injection stages of the battery. The preferred range of 0.02 ≤ a ≤ 0.04 promotes high-quality initial SEI film formation, while the preferred range of 0.10 ≤ b ≤ 0.20 allows for more timely repair and enhancement of the SEI film during subsequent cycles, thus more effectively addressing volume changes and electrochemical stresses during charging and discharging. c, within the range of 1.0 to 1.2 M, provides the battery with more ideal conductivity, promoting efficient lithium-ion transport while reducing side reactions. d, within the range of 0.5 to 0.8 M, provides sufficient lithium salt for secondary electrolyte injection, thereby more effectively promoting SEI film repair while maintaining good electrolyte conductivity. Ultimately, this results in a lithium-ion battery exhibiting higher cycle safety.

[0028] In practical applications, the first electrolyte further includes a first solvent, and the second electrolyte further includes a second solvent. The first and second solvents are each independently selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Preferably, both the first and second solvents include ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. More preferably, both the first and second solvents are mixed solvents formed from ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, with a volume ratio of ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate of 3:(1~1.5):3:3. In the above electrolyte solvent system, the components can better coordinate, thereby forming a more stable SEI film and significantly improving the cycle life of the resulting battery.

[0029] After obtaining the first cell, the preferred charging process includes the following sequential steps: a first charging stage, charging at a first current rate to 10%~30% SOC; and a second charging stage, charging at a second current rate to 60%~80% SOC. Preferably, the first current rate is 0.5C~3C; and / or the second current rate is 0.1C~0.5C, to more effectively promote the uniform deposition of FEC and VC on the electrode surface, forming a denser SEI film rich in inorganic components, thereby more effectively preventing further decomposition of the electrolyte while ensuring smooth lithium-ion transport. Ultimately, this significantly improves the cycle stability of the resulting lithium-ion battery.

[0030] In several typical embodiments, the electrolyte injection process is carried out at 25°C to 60°C and 0.1 MPa to 1 MPa. By optimizing the temperature and pressure range during the electrolyte injection process, the composition of the first and second electrolytes can be adapted, thereby improving the tightness of the contact between the electrolyte system formed by the two and the electrode, accelerating the reaction of additives on the electrode surface, forming a more stable SEI film in a shorter time, and thus significantly improving the cycle stability of the resulting lithium-ion battery.

[0031] A second aspect of the present invention provides a lithium-ion battery prepared by the aforementioned method. The electrolyte injection method of the present invention, through a two-stage precise electrolyte injection and dynamic additive adjustment strategy, endows the prepared lithium-ion battery with significant performance advantages. First, the separate implementation of the primary and secondary electrolyte injections, combined with the differentiated content (a, b) of additives FEC (fluoroethylene carbonate) and VC (ethylene carbonate) in the two stages, effectively optimizes the formation of the solid electrolyte interface (SEI) film. During the primary electrolyte injection and formation process, the SEI film is initially constructed, forming a stable protective layer rich in inorganic components. Subsequently, the secondary electrolyte injection replenishes the necessary additives, further consolidating the SEI film, enhancing its durability under high-rate charge-discharge conditions, and avoiding the capacity decay problem commonly encountered during battery aging.

[0032] Furthermore, the lithium-ion battery also includes a positive electrode and a negative electrode. The positive electrode includes a positive active material, and the negative electrode includes a negative active material. The positive active material is selected from lithium transition metal oxides, and the negative active material is selected from one or more of graphite and silicon-carbon composite materials. More specifically, in order to better adapt to the above-mentioned secondary electrolyte injection process, better coordinate with the electrolyte system constructed by the two electrolyte injections, and more significantly optimize the various performance characteristics of the lithium-ion battery, the preferred positive active material is LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li... 1+y Mn 1-x M xO2, LiCo 1- x M x O2, LiFe 1-x M x PO4 and Li2Mn 1-x One or more of O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, and F, 0≤y<0.2, 0≤x<1; and / or, the negative electrode active material is selected from one or more of graphite and silicon-carbon composite materials; preferably, the negative electrode active material is graphite.

[0033] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0034] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0035] Example 1

[0036] A method for preparing a lithium-ion battery:

[0037] Preparation of dry battery cells:

[0038] Preparation method of positive electrode sheet: Positive electrode material LiFePO4, binder polyvinylidene fluoride, and conductive agent acetylene black are mixed in a ratio of 95:2:3. Then, N-methylpyrrolidone (NMP) is added and stirred until the mixture is homogeneous to prepare the positive electrode slurry. This positive electrode slurry is uniformly coated onto aluminum foil, then dried, and subsequently subjected to cold pressing and other processes to finally produce the positive electrode sheet.

[0039] Preparation method of negative electrode sheet: The negative electrode active material graphite, conductive agent acetylene black, binder styrene-butadiene rubber and thickener sodium carboxymethyl cellulose are dispersed in a solvent in a mass ratio of 96:2:1:1, stirred evenly to obtain a slurry, coated on the negative electrode current collector, and then dried and cold pressed to obtain the negative electrode sheet.

[0040] Cell assembly: The positive and negative electrode sheets and separators after vacuum drying are cut, die-cut, and slit to obtain dry cells without liquid filling.

[0041] For the electrolyte injection process of the obtained dry cell:

[0042] (1) At 45℃ and 0.8MPa, a first electrolyte was injected into the dry cell to obtain a first cell. The first electrolyte used EC:PC:DMC:EMC = 3:1:3:3 as a solvent, which included 1.1M of mixed lithium salt, in which LiPF6:LiFSI = 7:4 (molar ratio), and the first electrolyte included 1wt% VC and 1wt% FEC (i.e., 2% VC+FEC, a = 0.02). The obtained first cell was first charged at a constant current of 2C to 30% SOC, and then charged at a constant current of 0.3C to 60% SOC to obtain a second cell.

[0043] (2) A second electrolyte is injected into the obtained second cell to obtain a third cell. The second electrolyte uses EC:PC:DMC:EMC = 3:1:3:3 (weight ratio) as a solvent, including 0.8M LiPF6 lithium salt, and the first electrolyte includes 13wt% VC and 2wt% FEC (i.e., 15% VC + FEC, b = 0.15). The third cell is assembled into a square aluminum-cased battery to obtain the lithium-ion battery.

[0044] During the above-mentioned electrolyte injection process, the weight content of the first electrolyte in the total electrolyte is 80%, i.e., x=0.8.

[0045] The above content parameters are shown in Table 1.

[0046] Examples 2 to 8

[0047] The only difference between Examples 2 to 8 and Example 1 is that the amount of each component used in the injection process is different, as shown in Table 1.

[0048] Example 9

[0049] A method for preparing a lithium-ion battery:

[0050] The only difference between this embodiment and Embodiment 1 is that in the first electrolyte, only 1.1M LiPF6 is used as the first lithium salt.

[0051] Example 10

[0052] A method for preparing a lithium-ion battery:

[0053] The only difference between this embodiment and Embodiment 1 is that, in the second electrolyte, LiFSI of equimolar concentration is used instead of LiPF6 as the second lithium salt.

[0054] Example 11

[0055] A method for preparing a lithium-ion battery:

[0056] The only difference between this embodiment and Embodiment 1 is that DMC was not added to either the first electrolyte or the second electrolyte.

[0057] Example 12

[0058] A method for preparing a lithium-ion battery:

[0059] The only difference between this embodiment and Embodiment 1 is that the ratio of EC:PC:DMC:EMC is changed to 3:2:3:3 in both the first and second electrolytes.

[0060] Example 13

[0061] A method for preparing a lithium-ion battery:

[0062] The only difference between this embodiment and Embodiment 1 is that the charging process is changed to: first, constant current charging at 0.1C to 5% SOC, and then constant current charging at 0.8C to 90% SOC to obtain the second battery cell.

[0063] Example 14

[0064] A method for preparing a lithium-ion battery:

[0065] The only difference between this embodiment and Embodiment 1 is that the charging process is changed to: first, constant current charging at 4C to 40% SOC, and then constant current charging at 0.3C to 50% SOC to obtain the second battery cell.

[0066] Comparative Examples 1 to 2

[0067] The only difference between Comparative Examples 1 and 2 and Example 1 is that the amount of each component used during the injection process is different, as shown in Table 1.

[0068] Comparative Example 3

[0069] A method for preparing a lithium-ion battery:

[0070] The only difference between this comparative example and Example 1 is that the second electrolyte in Example 1 is used as the electrolyte for the first injection; the first electrolyte is injected during the second injection, thus obtaining a lithium-ion battery.

[0071] Table 1

[0072]

[0073] Performance testing methods

[0074] Rate performance test: The battery samples obtained from each example and comparative example were charged at 25°C with a constant current and constant voltage of 0.5C, with a cutoff voltage of 4.2V and a cutoff current of 0.05C. They were then discharged at a constant current of 0.5C to 2.5V, and this cycle was repeated 3 times. The discharge capacity of the last cycle was taken as the initial capacity and recorded as Q0. Then, the battery was charged again with a constant current and constant voltage of 0.5C, with a cutoff voltage of 4.2V and a cutoff current of 0.05C. After being fully charged, the battery was discharged at a rate of 2C and the discharge capacity was recorded as Q1. The discharge capacity retention rate was tested as Q1 / Q0×100%. The test results are shown in Table 2.

[0075] Cyclic performance test:

[0076] (1) Room temperature cycling test: After the battery samples obtained from each example and the comparative example were placed at 25°C for 1 hour, they were subjected to full charge and discharge cycles. The capacity retention rate was recorded after 500 and 1000 cycles. The test results are shown in Table 2. Full charge: The battery was charged with a constant current and constant voltage at a current of 0.5C, with a cutoff voltage of 4.2V and a cutoff current of 0.05C; Full discharge: The battery was discharged with a constant current at a current of 0.5C to 2.5V.

[0077] (2) High-temperature cycling test: After each battery sample was placed at 45℃ for 1 hour, it was subjected to full charge and discharge cycles. The capacity retention rate was recorded after 500 and 1000 cycles. The test results are shown in Table 2. Among them, full charge: the battery was charged with constant current and constant voltage at a current of 0.5C, with a cutoff voltage of 4.2V and a cutoff current of 0.05C; full discharge: the battery was discharged with constant current at a current of 0.5C to 2.5V.

[0078] Table 2

[0079]

[0080] As can be seen from the above description, the embodiments of the present invention, through a two-stage precise liquid injection and dynamic additive adjustment strategy, endow the prepared lithium-ion battery with significant performance advantages. The resulting lithium-ion battery exhibits superior durability under high-rate charge-discharge conditions and good cycle stability over a wide temperature range.

[0081] Specifically, based on the data in the table above:

[0082] Comparing Examples 1 and 2, it can be seen that when the ratio of the first and second electrolyte injections is determined, and the content of fluoroethylene carbonate (FEC), the content of vinylene carbonate (VC), and the lithium salt concentration in the electrolyte are within the optimal concentration range, it can be ensured that there are enough VC, FEC additives and a higher lithium salt in the first electrolyte injection to form a stable SEI film rich in inorganic matter. Then, as much VC and FEC additives as possible can be distributed to the electrolyte of the second electrolyte injection after formation, thereby significantly improving the cycle stability of the battery in the later stage.

[0083] Comparing Examples 1 and 2 with Examples 3 to 7, it can be seen that when the VC, FEC and lithium salt contents are optimized, the rate and cycle performance of the resulting battery can be improved more significantly.

[0084] Comparing Example 8 with Examples 1 and 2, it can be seen that when the lithium salt content is further optimized, the electrolyte can form a more stable SEI film rich in inorganic matter during the electrolyte injection process, thereby more effectively improving the rate performance and cycle performance of the battery.

[0085] Comparing Examples 9 and 10 with Example 1, it can be seen that by optimizing the types of lithium salts in the electrolytes used for the two injections, the two additive systems can be better adapted to each other, resulting in lithium-ion batteries with superior cycle performance.

[0086] Comparing Examples 11 and 12 with Example 1, it can be seen that by optimizing the solvent system in the electrolyte used for the two injections, better coordination between the components can be promoted, thereby forming a more stable SEI film and further improving the cycle performance of the resulting battery.

[0087] Comparing Examples 13 and 14 with Example 1, it can be seen that after obtaining the first dry cell, by optimizing the charging conditions, the uniform deposition of FEC and VC on the electrode surface can be promoted more effectively, forming a denser SEI film rich in inorganic components, so as to more effectively prevent further decomposition of the electrolyte and significantly improve the cycle stability of the obtained lithium-ion battery.

[0088] As can be seen from Comparative Examples 1 and 2, when the content of VC and FEC is too low or too high, a dense SEI cannot be formed and the volume expansion cannot be restrained in the later cycle, which will cause the battery life to decline or increase the impedance, and deteriorate the rate performance and cycle performance.

[0089] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing a lithium ion battery, comprising an injection process of injecting an electrolyte into a dry cell, characterized in that, The liquid injection process comprises: Step S1, injecting a first electrolyte into the dry battery cell to obtain a first battery cell; the first battery cell is subjected to a charging process to obtain a second battery cell; Step S2, injecting a second electrolyte into the second battery cell to obtain a third battery cell, and further obtaining the lithium ion battery; The first electrolyte and the second electrolyte each contain a fluoroethylene carbonate additive and a vinylene carbonate additive; let the total weight content of the fluoroethylene carbonate additive and the vinylene carbonate additive in the first electrolyte be a; let the total weight content of the fluoroethylene carbonate additive and the vinylene carbonate additive in the second electrolyte be b; The weight content of the first electrolyte is x, based on the total weight of the first electrolyte and the second electrolyte being 1; (x-0.5) / 30≤a≤(x-0.5) / 5, (1.2-x) / 10≤b≤2(1.2-x) / 3.

2. The method for preparing a lithium-ion battery according to claim 1, characterized in that, (x-0.5) / 20≤a≤(x-0.5) / 6, (1.2-x) / 8≤b≤(1.2-x) / 2; Preferably, the weight ratio of the vinylene carbonate additive to the fluoroethylene carbonate additive in the first electrolyte is (0.3-4.0):(0.4-1.5); the weight ratio of the vinylene carbonate additive to the fluoroethylene carbonate additive in the second electrolyte is (4-15):(0.5-7).

3. The preparation method of the lithium ion battery according to claim 1 or 2, characterized in that, The first electrolyte further comprises a first lithium salt, and let the molar concentration of the first lithium salt in the first electrolyte be c mol / L; the second electrolyte further comprises a second lithium salt, and let the molar concentration of the second lithium salt in the second electrolyte be d mol / L; 2x-0.6≤c≤3x-0.5, 1.3-x≤d≤1.8-x; Preferably, 2x-0.5≤c≤3x-0.8, 1.4-x≤d≤1.7-x.

4. The method for preparing a lithium-ion battery according to claim 3, characterized in that, The first lithium salt and the second lithium salt are each independently selected from one or more of lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethylsulfonylimide, and lithium bisfluoroxalate borate; Preferably, the first lithium salt and the second lithium salt are each independently lithium hexafluorophosphate and / or lithium bisfluorosulfonylimide.

5. The method of producing a lithium-ion battery according to any one of claims 1 to 4, characterized in that, 0.7≤x≤0.95, 0.01≤a≤0.15, 0.01≤b≤0.3, 0.8≤c≤2.0, 0.4≤d≤1.

0.

6. The method of producing a lithium-ion battery according to any one of claims 1 to 5, wherein The first electrolyte further comprises a first solvent, the second electrolyte further comprises a second solvent, and the first solvent and the second solvent are each independently selected from one or more of vinylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate; Preferably, the first solvent and the second solvent each comprise the vinylene carbonate, the propylene carbonate, the dimethyl carbonate, and the methyl ethyl carbonate.

7. The method of producing a lithium-ion battery according to any one of claims 1 to 6, characterized in that, The charging process comprises the following steps in sequence: a first charging stage, constant current charging to 10% SOC~30% SOC at a first current multiple; a second charging stage, constant current charging to 60% SOC~80% SOC at a second current multiple; Preferably, the first current multiple is 0.5C~3C; and / or, the second current multiple is 0.1C~0.5C.

8. The method of producing a lithium-ion battery according to any one of claims 1 to 7, characterized in that, The liquid injection process is carried out at 25℃~60℃, 0.1MPa~1MPa.

9. A lithium-ion battery, characterized by The lithium ion battery is prepared by the preparation method of the lithium ion battery according to any one of claims 1 to 8.

10. The lithium-ion battery of claim 9, wherein, The lithium ion battery further comprises a positive electrode sheet and a negative electrode sheet, the positive electrode sheet comprising a positive electrode active material, and the negative electrode sheet comprising a negative electrode active material; The positive electrode active material is selected from lithium transition metal oxides, preferably LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+y Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4 and Li2Mn 1-x One or more of O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, and F, 0≤y<0.2, 0≤x<1; and / or, The negative electrode active material is selected from one or more of graphite and silicon-carbon composite material, preferably graphite.

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