A lithium-ion battery

Abstract

The application discloses a lithium ion battery. It comprises a positive electrode sheet, a negative electrode sheet, a diaphragm and an electrolyte; wherein the positive electrode sheet comprises a positive electrode active material, the positive electrode active material is lithium iron phosphate; the electrolyte comprises a fluorinated ether solvent, a cyclic carbonate and a vinylene carbonate; the fluorinated ether solvent is a compound shown in formula I; R2 and R3 are each independently selected from an alkylene or a fluorinated alkylene, and the total number of carbon atoms of R2 and R3 is 4, the total number of fluorine atoms in R2 and R3 is m, m>0; R1 and R4 are each independently methyl or fluorinated methyl, and the total number of fluorine atoms in R1 and R4 is n, n>1; and m+n=4; the content of the cyclic carbonate is 0.5wt%-7wt%, the percentage is the percentage of the mass of the cyclic carbonate in the mass of the electrolyte. The lithium ion battery provided by the application has good high-temperature cycle performance.

Description

TECHNICAL FIELD

[0001] The present application relates to a lithium ion battery. BACKGROUND

[0002] The lithium iron phosphate battery is currently the mainstream of passenger car batteries. The lithium iron phosphate battery for vehicles has the intrinsic defects of large impedance and easy lithium precipitation of the negative electrode at high rate charging, which is one of the main reasons limiting its further development in the market. One solution is to introduce natural graphite into the negative electrode. Natural graphite has high porosity, high graphitization degree and high compaction density, and is the most suitable large-capacity carbon negative electrode material for fast charging. It can also rely on its compaction and capacity advantages to improve the overall energy density of the battery, thereby extending the endurance of the electric vehicle.

[0003] However, due to the highly unstable chemical-electrochemical properties of natural graphite, the initial efficiency of the battery will be significantly reduced, and the cycle storage performance will be deteriorated. The conventional method is to use more stable solvents and more negative electrode film-forming additives. However, both of these methods will significantly deteriorate ion conduction and battery DCR, thereby significantly affecting the fast charging performance of the battery and reducing the service life of the battery.

[0004] In order to solve the problem of instability of the natural graphite negative electrode, one technical route is to use a reduction-resistant solvent such as DME, which does not form a film on the negative electrode, thus avoiding the problem of negative electrode impedance caused by the formation of a solid electrolyte layer. However, DME has poor solubility for lithium salts, low ionic conductivity, and poor oxidation resistance, making it difficult to maintain stability even in a lithium iron phosphate system. In order to solve the problem of oxidation resistance, the ether solvent needs to be fluorinated, but this will greatly reduce the solubility of common lithium salts, making the electrolyte unusable. Therefore, other substances need to be added to the ether electrolyte to improve the solubility of lithium salts, and the added substances will cause additional positive and negative electrode side reactions, making the electrolyte window narrower and reducing the high-temperature cycle performance of the lithium ion battery. SUMMARY

[0005] The present application is mainly to overcome the defect that the negative electrode of the lithium ion battery using an ether solvent electrolyte in the prior art is prone to side reactions, resulting in poor high-temperature cycle performance, and provides a lithium ion battery. The lithium ion battery provided by the present application has good high-temperature cycle performance.

[0006] The lithium ion battery provided by the present application comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; wherein the positive electrode sheet comprises a positive electrode active material, and the positive electrode active material is lithium iron phosphate;

[0007] The electrolyte comprises a fluorinated ether solvent, a cyclic carbonate and a vinylene carbonate; the fluorinated ether solvent is a compound represented by the following formula I:

[0008] Formula I,

[0009] R2 and R3 are each independently selected from alkylene or fluoroalkylene, and the total number of carbon atoms of R2 and R3 is 4, and the total number of fluorine atoms in R2 and R3 is m, m≥0;

[0010] R1 and R4 are each independently methyl or fluoromethyl, and the total number of fluorine atoms in R1 and R4 is n, n≥1; and, m+n=4;

[0011] The content of the cyclic carbonate is 0.5wt%-7wt%, the percentage being the percentage of the mass of the cyclic carbonate in the mass of the electrolyte.

[0012] On the basis of common general knowledge in the art, the above-mentioned preferred conditions can be combined arbitrarily, i.e. to obtain each preferred example of the present application.

[0013] The reagents and raw materials used in the present application are commercially available.

[0014] The positive progress effect of the present application is that:

[0015] The lithium ion battery prepared by using the electrolyte including the fluoroether solvent with specific structure, cyclic carbonate and vinylene carbonate can effectively reduce the occurrence of negative electrode side reactions, and realize higher high-temperature cycle life. DETAILED DESCRIPTION

[0016] Lithium ion battery

[0017] The lithium ion battery provided by the present application includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; wherein the positive electrode sheet includes a positive electrode active material, and the positive electrode active material is lithium iron phosphate;

[0018] The electrolyte includes a fluoroether solvent, a cyclic carbonate and a vinylene carbonate; the fluoroether solvent is a compound shown in the following formula I:

[0019] Formula I,

[0020] R2 and R3 are each independently selected from alkylene or fluoroalkylene, and the total number of carbon atoms of R2 and R3 is 4, and the total number of fluorine atoms in R2 and R3 is m, m≥0;

[0021] R1 and R4 are each independently methyl or fluoromethyl, and the total number of fluorine atoms in R1 and R4 is n, n≥1; and, m+n=4;

[0022] The content of the cyclic carbonate is 0.5wt%-7wt%, the percentage being the percentage of the mass of the cyclic carbonate in the mass of the electrolyte.

[0023] In the present application, preferably, the content of the cyclic carbonate is 0.5wt%-5wt%, the percentage being the percentage of the mass of the cyclic carbonate in the mass of the electrolyte.

[0024] In the present application, the total number of fluorine atoms in R2and R3is preferably m≥1.

[0025] In the present application, the compound of formula I is selected from at least one of the following compounds:

[0026] 、 、

[0027] 、 、

[0028] 、 、

[0029] and .

[0030] In the present application, the content of the fluoroether solvent is 80wt%-90wt%, the percentage being the percentage of the mass of the fluoroether solvent in the mass of the electrolyte.

[0031] In some specific embodiments, the content of the fluoroether solvent is 80.00wt%, 82.00wt%, 84.00wt%, 84.50wt%, 85.00wt%, 85.50wt%, 86.00wt%, 86.50wt%, 86.50wt% or 86.80wt%.

[0032] In some specific embodiments, the kind of the fluoroether solvent is compound 1, and the content of the fluoroether solvent is 80wt%-87wt%.

[0033] In some specific embodiments, the kind of the fluoroether solvent is compound 2, and the content of the fluoroether solvent is 85wt%.

[0034] In some specific embodiments, the kind of the fluoroether solvent is compound 1 and compound 2, the content of the compound 1 is 6wt%, and the content of the compound 2 is 80wt%.

[0035] In some specific embodiments, the kind of the fluoroether solvent is compound 1 and compound 2, the content of the compound 1 is 40wt%, and the content of the compound 2 is 46wt%.

[0036] In some embodiments, the fluorinated ether solvent is a mixture of Compound 1 and Compound 2, wherein Compound 1 is present in an amount of 60 wt% and Compound 2 is present in an amount of 26 wt%.

[0037] In some embodiments, the fluorinated ether solvent is a mixture of Compound 1 and Compound 2, wherein Compound 1 is present in an amount of 80 wt% and Compound 2 is present in an amount of 6 wt%.

[0038] In some embodiments, the electrolyte further comprises a cyclic carbonate. In some embodiments, the cyclic carbonate comprises one or more of ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC).

[0039] In some embodiments, the electrolyte further comprises a cyclic carbonate. In some embodiments, the cyclic carbonate comprises one or more of ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC).

[0040] In some embodiments, the cyclic carbonate is present in an amount of 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, or 7 wt%.

[0041] In some embodiments, the electrolyte further comprises vinylene carbonate (VC) in an amount of 0.2 wt% to 2 wt%, where the percentage is the mass of the vinylene carbonate divided by the mass of the electrolyte.

[0042] In some embodiments, the vinylene carbonate is present in an amount of 0.2 wt%, 0.5 wt%, 1 wt%, 2 wt%, or 2.5 wt%.

[0043] In some embodiments, the electrolyte further comprises thiophene in an amount of 0.2 wt% to 2 wt%, such as 0.3 wt%, 0.5 wt%, or 1.8 wt%, where the percentage is the mass of the thiophene divided by the mass of the electrolyte.

[0044] In some embodiments, the electrolyte further comprises vinyl sulfate (DTD) in an amount of 0.1 wt% to 1.5 wt%, such as 1 wt%, where the percentage is the mass of the vinyl sulfates divided by the mass of the electrolyte.

[0045] In some embodiments, the electrolyte comprises vinylene carbonate and thiophene, wherein the vinylene carbonate is present in an amount of 1 wt% and the thiophene is present in an amount of 1 wt%.

[0046] In some embodiments, the electrolyte comprises vinylene carbonate and vinyl sulfates, wherein the vinylene carbonate is present in an amount of 1 wt% and the vinyl sulfates are present in an amount of 1 wt%.

[0047] In some embodiments, the electrolyte comprises vinylene carbonate, thiophene and vinylsulfate, the content of the vinylene carbonate is 0.2wt%, the content of the thiophene is 0.3wt%, and the content of the vinylsulfate is 1wt%.

[0048] In some embodiments, the electrolyte comprises vinylene carbonate, thiophene and vinylsulfate, the content of the vinylene carbonate is 0.5wt%, the content of the thiophene is 0.5wt%, and the content of the vinylsulfate is 1wt%.

[0049] In some embodiments, the electrolyte comprises vinylene carbonate, thiophene and vinylsulfate, the content of the vinylene carbonate is 0.2wt%, the content of the thiophene is 1.8wt%, and the content of the vinylsulfate is 1wt%.

[0050] In the present application, the negative electrode sheet comprises a negative electrode active material layer and a negative electrode current collector, and the negative electrode active material layer comprises natural graphite.

[0051] In the present application, the electrolyte further comprises a lithium salt, the lithium salt comprises one or more of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methylsulfate and lithium trifluoromethylsulfate; the content of the lithium salt is 9wt%-17wt%, for example, 12wt%, the percentage is the percentage of the mass of the lithium salt in the mass of the electrolyte.

[0052] The present application is further illustrated by the following examples, but the present application is not limited to the scope of the examples. The experimental methods in the following examples, if not otherwise specified, are selected according to the conventional methods and conditions, or according to the instructions of the commercial products.

[0053] The structures of the compounds involved in the examples and comparative examples are shown in the following table:

[0054]

[0055] The above compounds are commercially available or synthesized by the conventional methods in the art.

[0056] Example 1

[0057] (1) Preparation of the electrolyte:

[0058] The ether solvent was prepared by mixing the components in the proportions shown in Table 1 in an argon atmosphere glove box with a water content of <10 ppm, and then mixing the cyclic carbonate with the ether solvent in a certain proportion to form an organic solvent. A certain amount of additive and LiPF6 were mixed with the aforementioned organic solvent to obtain an electrolyte. The content of LiPF6 in the obtained electrolyte was 12 wt%, and the content of each component was the weight percentage calculated based on the total weight of the electrolyte;

[0059] (2) Preparation of the positive electrode sheet

[0060] LiFePO4 was used as the positive electrode active material, polyvinylidene fluoride was used as the binder, and Super P was used as the conductive agent, and they were mixed in a weight ratio of 97:1:2, N-methyl pyrrolidone (NMP) was added, and the system was stirred to be uniform and transparent under the action of a vacuum stirrer to obtain a positive electrode slurry; the positive electrode slurry was uniformly coated on an aluminum foil; the aluminum foil was dried in an oven after being air-dried at room temperature, and then cold-pressed and cut to obtain a positive electrode sheet.

[0061] (3) Preparation of the negative electrode sheet

[0062] Graphite was used as the negative electrode active material, Super P was used as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) was used as the thickening agent, and styrene-butadiene rubber (SBR) was used as the binder, and they were mixed in a mass ratio of 96:1:1:2, deionized water was added, and a negative electrode slurry was obtained under the action of a vacuum stirrer; the negative electrode slurry was uniformly coated on a negative electrode current collector copper foil; the copper foil was dried in an oven after being air-dried at room temperature, and then cold-pressed and cut to obtain a negative electrode sheet.

[0063] (4) Preparation of the separator:

[0064] A polypropylene film was used as the separator.

[0065] (5) Preparation of the lithium ion battery:

[0066] The aforementioned positive electrode sheet, separator, and negative electrode sheet were stacked in sequence, with the separator between the positive electrode and the negative electrode to play a separating role. Then, an aluminum plastic film was wrapped outside, transferred to a vacuum oven for drying at 120°C, and then 3.0 g / Ah of the electrolyte prepared above was injected, followed by sealing, electrolyte formation, and finally a lithium ion battery with a capacity of 1 Ah was prepared.

[0067] The types and amounts of raw materials and specific process parameters in the preparation process of the lithium ion batteries in Examples 1-25 and Comparative Examples 1-6 are shown in Table 1:

[0068] Table 1

[0069]

[0070] Effect examples

[0071] High temperature cycle performance test

[0072] The battery was cycled in the oven at 45℃ in the range of 2.0V-3.8V, specifically, the battery was charged from 2.0V to 3.8V at a current of 1C, then charged to 0.05C at constant voltage, discharged from 3.8V to 2.0V at a current of 1C, then discharged to 0.05C at constant voltage, the above process was counted as one cycle of charge and discharge, the discharge capacity of each cycle was recorded, when the battery capacity reached 80% of the first cycle capacity, the test was ended, the cycle number was recorded as the high temperature cycle performance effect data, and recorded in Table 1.

[0073] The lithium ion battery provided by the application has good high temperature cycle performance, in some specific embodiments, it can reach 573 cycles and above when cycled to 80% SOC at 45℃, and in some preferred embodiments, it can reach 1037 cycles and above when cycled to 80% SOC at 45℃.

[0074] In Examples 1-5, the use of different contents of fluorinated ether solvents and cyclic esters in the electrolyte was verified, and according to the effect data of the 45℃ cycle number, it can be seen that within a certain range, the addition of cyclic esters is beneficial to improve the high temperature cycle performance of lithium ion batteries. In the system in which fluorinated ether is used as the main solvent, appropriate EC can participate in the formation of the electrode interface protection film, and at the same time, the occurrence of side reactions in the cycle process is inhibited, and the high temperature cycle performance of the battery is improved.

[0075] In Examples 6-7, the mixed solvent of the fluorinated ether solvent and dimethyl ether of the application was verified, and according to the effect data of the 45℃ cycle number, it can be seen that within a certain range, the higher the content of the fluorinated ether solvent, the more beneficial to improve the high temperature cycle performance of the lithium ion battery.

[0076] In Examples 8-12, the influence of the type of different fluorinated ether solvents on the high temperature cycle performance was verified, and it can be seen that the use of the electrolyte of compound 2 can make the lithium ion battery have good high temperature cycle performance.

[0077] In Examples 13-17, electrolytes with different VC contents were verified, and it can be seen that within a certain range, the higher the content of VC, the more beneficial to improve the high temperature cycle performance of the lithium ion battery. With the addition of VC, the negative electrode protection is appropriate first and excessive later, and the cycle life first rises and then falls.

[0078] In Examples 18-22, the influence of the mixing of VC and other additives in a certain proportion on the high temperature cycle performance was verified, and the addition of DTD or thiophene also has an influence on the high temperature cycle performance, but good cycle performance can be obtained.

[0079] Examples 23 and 24 verify that different kinds of cyclic carbonates, in combination with other features, can also achieve good cycle performance.

[0080] Example 25 verifies that artificial graphite as a negative active material, in combination with other features, can also achieve good cycle performance.

[0081] Comparative Example 1 differs from Example 2 in that no cyclic carbonate is added in Comparative Example 1; Comparative Example 2 differs from Example 2 in that no VC is added in Comparative Example 2, and the cycle number at 45°C is somewhat reduced.

[0082] Comparative Example 3 differs from Example 2 in that no fluorinated ether solvent of the present application is added in Comparative Example 3, and the cycle number at 45°C is significantly reduced.

[0083] Comparative Example 4 also adds a fluorinated ether solvent, but its structure is different from the fluorinated ether solvent of the present application, and it cannot widen the electrochemical reaction window of the electrolyte while maintaining the appropriate solubility of the lithium salt in the electrolyte, and the cycle number at 45°C is somewhat different from the present application.

[0084] Comparative Example 5 does not use the electrolyte system of the present application, even though it has the same additives as the present application, and the high-temperature cycle performance is significantly reduced.

[0085] Comparative Example 6 uses an electrolyte commonly used in the prior art, and does not add VC or other additives, and the high-temperature cycle performance is very poor.

[0086] The above-described examples are only some of the embodiments of the present application, and facilitate those skilled in the art to understand and use the present application. Obviously, any skilled person in the art can make slight modifications or changes to the present embodiments without creative labor and apply them to other embodiments. Therefore, the present application is not limited to the above-described examples, and any equivalent changes, simple modifications and modifications within the scope of the present application still fall within the scope of the present application.

Claims

1. A lithium-ion battery, characterized by, It includes a positive electrode, a negative electrode, a separator, and an electrolyte; wherein the positive electrode includes a positive electrode active material, and the positive electrode active material is lithium iron phosphate; The negative electrode sheet includes a negative electrode active material layer and a negative electrode current collector, wherein the negative electrode active material layer includes natural graphite; The electrolyte comprises fluorinated ether solvents, cyclic carbonates, and vinylene carbonate; the fluorinated ether solvents are compounds represented by Formula I below: Formula I, R2 and R3 are each independently selected from alkylene or fluoroalkylene, and the total number of carbon atoms in R2 and R3 is 4, and the total number of fluorine atoms in R2 and R3 is m, where m≥0; R1 and R4 are each independently methyl or fluoromethyl, and the total number of fluorine atoms in R1 and R4 is n, n≥1; and m+n=4; The content of the cyclic carbonate is 0.5wt%-5wt%, and the percentage is the percentage of the mass of the cyclic carbonate to the mass of the electrolyte; The content of the fluorinated ether solvent is 80wt%-90wt%, and the percentage is the percentage of the mass of the fluorinated ether solvent to the mass of the electrolyte; The content of vinylene carbonate is 0.2wt%-2wt%, and the percentage is the percentage of the mass of vinylene carbonate to the mass of the electrolyte.

2. The lithium-ion battery of claim 1, wherein, The compound represented by Formula I is selected from at least one of the following compounds: 、 、 Compound 1 Compound 2 、 、 Compound 3 Compound 4 、 、 Compound 5 Compound 6 or , Compound 7. Compound 8.

3. The lithium-ion battery as described in claim 1, characterized in that, The cyclic carbonates include one or more of ethylene carbonate, fluoroethylene carbonate, and propylene carbonate.

4. The lithium-ion battery as described in claim 1, characterized in that, The cyclic carbonate includes ethylene carbonate, and the content of ethylene carbonate is 1wt%-5wt%, where the percentage is the mass of ethylene carbonate relative to the mass of the electrolyte.

5. The lithium-ion battery as described in claim 1, characterized in that, The electrolyte also includes thiophene, the content of which is 0.2wt%-2wt%, and the percentage is the percentage of the mass of thiophene to the mass of the electrolyte.

6. The lithium-ion battery as described in claim 1, characterized in that, The electrolyte also includes vinyl sulfate, the content of which is 0.1wt%-1.5wt%, and the percentage is the mass of vinyl sulfate as a percentage of the mass of the electrolyte.

7. The lithium-ion battery as described in claim 1, characterized in that, The electrolyte also includes lithium salts, which include one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methanesulfonate, and lithium trifluoromethylsulfonate; the content of the lithium salt is 9wt%-17wt%, where the percentage is the mass of the lithium salt relative to the mass of the electrolyte.

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