Electrolyte and lithium ion battery containing same

Abstract

The invention provides an electrolyte and a lithium ion battery containing the same. The electrolyte comprises an electrolyte and a solvent; the solvent comprises fluoroethyl acetate and fluoroethylene carbonate. According to the electrolyte provided by the invention, an elastic coating can be formed on a negative electrode, the problem of SEI film rupture caused by volume expansion of a negative electrode material in the charging and discharging process is relieved, and meanwhile, the electrolyte has good oxidation resistance and can be adapted to a lithium ion battery adopting a high-voltage positive electrode material and a silicon-based negative electrode material; and the stability of the battery at high temperature and the cycle performance of the battery at high pressure are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to an electrolyte and a lithium ion battery containing the electrolyte. Background Art

[0002] Lithium-ion batteries have the advantages of high voltage, high energy density and long cycle life. With the increasing popularity of electric vehicles, they have become one of the most widely used secondary batteries. With the rapid development of pure electric vehicles and hybrid electric vehicles, people's requirements for the range, service life and safety of electric vehicles are constantly increasing. High-nickel ternary positive electrode materials with high specific capacity, lithium-rich manganese-based positive electrode materials and high-specific capacity silicon-based negative electrode materials have become important directions for the development of positive and negative electrode materials for high-specific energy batteries.

[0003] In traditional carbonate electrolytes, ternary cathode materials will undergo drastic structural changes and interface side reactions under high temperature and high voltage, which will deteriorate the battery's cycle performance and safety. In addition, silicon-based anode materials will undergo significant volume expansion during the charge and discharge process, causing the SEI film (solid electrolyte interface membrane) to rupture, exposing a fresh interface, thereby continuously consuming electrolyte and deteriorating the battery's cycle performance. In other words, traditional carbonate electrolytes cannot fully adapt to lithium-ion batteries that use high-voltage cathode materials and silicon-based anode materials, and are prone to problems such as cycle diving. Summary of the invention

[0004] In view of the shortcomings of the prior art, the object of the present invention is to provide an electrolyte and a lithium-ion battery containing the same. The electrolyte can form an elastic coating on the negative electrode, alleviating the problem of SEI film rupture caused by the volume expansion of the negative electrode material during the charge and discharge process. At the same time, the electrolyte has good oxidation resistance and can be adapted to lithium-ion batteries using high-voltage positive electrode materials and silicon-based negative electrode materials, thereby improving the stability of the battery at high temperature and the cycle performance under high pressure.

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides an electrolyte solution, the electrolyte solution comprising an electrolyte and a solvent;

[0007] The solvent includes ethyl fluoroacetate and fluoroethylene carbonate.

[0008] The electrolyte provided by the present invention contains ethyl fluoroacetate and fluoroethylene carbonate at the same time, and can form a LiF-rich SEI film at the negative electrode. The fluoroethylene carbonate can promote free radical polymerization and cross-linking of polymer chains through a defluorination reaction mechanism, thereby generating an elastic coating, and the coating can withstand the strain of the LixSiy electrode caused by Li cycling, thereby improving the problem of SEI film rupture caused by volume expansion of silicon-based negative electrode materials during cycling; at the same time, ethyl fluoroacetate can improve the oxidation resistance of the electrolyte, improve the stability of the electrolyte at high temperature and high pressure, thereby improving the stability of the lithium ion battery at high temperature and the cycling performance at high pressure.

[0009] In the present invention, the structural formula of the ethyl fluoroacetate is as follows:

[0010]

[0011] Wherein, R1, R2, R3, R4, R5, and R6 are each independently H or F, and at least one is F.

[0012] The more fluorine content in ethyl fluoroacetate, the better its antioxidant and high temperature resistance, but it will also lead to higher costs and increased battery impedance. (DFEA), (EFA) can well balance the above properties, so the ethyl fluoroacetate in the present invention is preferably DFEA and / or EFA.

[0013] The structural formula of the fluoroethylene carbonate (FEC) is

[0014] In some embodiments of the present invention, the mass percentage of the ethyl fluoroacetate to the solvent is 1-20%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%, etc.

[0015] In some embodiments of the present invention, the mass percentage of the fluoroethylene carbonate to the solvent is 1-15%, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%, etc.

[0016] In the present invention, the contents of ethyl fluoroacetate and fluoroethylene carbonate are kept within the above-mentioned range, which helps to ensure that the lithium ion battery has good high temperature stability and high voltage cycle performance. If the content of fluoroethylene carbonate is too little, its corresponding effect is difficult to play; if the content of fluoroethylene carbonate is too much, it is easy to cause the high temperature stability of the battery to deteriorate. If the content of ethyl fluoroacetate is too little, its corresponding effect is difficult to play; if the content of ethyl fluoroacetate is too much, it is easy to cause the initial film thickness of the SEI film to be large, resulting in a larger DCR (direct current resistance) of the battery.

[0017] In some embodiments of the present invention, the solvent further comprises a residual amount of carbonate.

[0018] In some embodiments of the present invention, the carbonate is selected from one or more of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and propylene carbonate (PC).

[0019] In some embodiments of the present invention, the content of the electrolyte in the electrolyte is 12-15wt%; for example, it can be 12wt%, 12.5wt%, 13wt%, 13.5wt%, 14wt%, 14.5wt% or 15wt%, etc.

[0020] In some embodiments of the present invention, the electrolyte is selected from one or more of LiPF6, LiBF4, LiBOB, LiClO4, LiCF3SO3, LiODFB, LiN(SO2CF3)2, LiN(SO2F)2, LiTFSI, LiFSI, LiHFDF and cyclic imide lithium salts.

[0021] In some embodiments of the present invention, the electrolyte further includes an additive.

[0022] In some embodiments of the present invention, the content of the additive in the electrolyte is 0.1-2wt%; for example, it can be 0.1wt%, 0.2wt%, 0.3wt%, 0.5wt%, 0.8wt%, 1wt%, 1.2wt%, 1.3wt%, 1.5wt%, 1.6wt%, 1.8wt% or 2wt%, etc.

[0023] In some embodiments of the present invention, the additive is selected from one or more of vinyl sulfate (DTD), tripropynyl phosphate (TPP), vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), propane sultone (PS) and lithium difluorophosphate (LiOP2F2).

[0024] In a second aspect, the present invention provides a lithium-ion battery comprising the electrolyte as described in the first aspect.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The electrolyte provided by the present invention contains ethyl fluoroacetate and fluoroethylene carbonate at the same time, and can form a LiF-rich SEI film at the negative electrode. The fluoroethylene carbonate can promote free radical polymerization and cross-linking of polymer chains through a defluorination reaction mechanism, thereby generating an elastic coating, and the coating can withstand the strain of the LixSiy electrode caused by Li circulation, and improve the problem of SEI film rupture caused by volume expansion of silicon-based negative electrode materials during the circulation process; at the same time, ethyl fluoroacetate can improve the oxidation resistance of the electrolyte, and improve the stability of the electrolyte under high temperature and high pressure, so that it can be adapted to lithium ion batteries using high-voltage positive electrode materials and silicon-based negative electrode materials, and improve the stability of the battery at high temperature and the cycle performance under high pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The capacity recovery rate curves of lithium ion batteries using the electrolytes provided in Example 1 and Comparative Example 1 of the present invention after being stored at high temperatures for different periods of time are shown;

[0028] Figure 2 The graph is a cycle capacity retention rate graph of a lithium ion battery using the electrolyte provided in Example 1 and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below with reference to the accompanying drawings and through specific implementations. It should be understood by those skilled in the art that the specific implementations are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0030] Example 1

[0031] This embodiment provides an electrolyte, and the preparation method thereof is as follows:

[0032] The organic solvent and the lithium salt are mixed and stirred evenly to obtain a primary mixed electrolyte; the additive is added to the primary mixed electrolyte and stirred evenly to obtain an electrolyte sample;

[0033] The organic solvent consists of DFEA, FEC, PC and EMC in a mass ratio of 15:10:20:55;

[0034] The lithium salts are LiPF6 and LiFSI, and their contents in the electrolyte are 8wt% and 7wt% respectively;

[0035] The additives are LiOP2F2 and DTD, and their contents in the electrolyte are 0.8wt% and 1wt%, respectively;

[0036] The structural formula of DFEA is

[0037] Example 2

[0038] This embodiment provides an electrolyte, which differs from Embodiment 1 only in that:

[0039] The organic solvent consists of DFEA, FEC, PC and EMC in a mass ratio of 10:10:20:60;

[0040] The lithium salt is LiPF6, and its content in the electrolyte is 13wt%.

[0041] Example 3

[0042] This embodiment provides an electrolyte, which differs from Embodiment 1 only in that:

[0043] The organic solvent consists of DFEA, FEC, PC and EMC in a mass ratio of 20:10:20:50.

[0044] Example 4

[0045] This embodiment provides an electrolyte, which differs from Embodiment 1 only in that:

[0046] The organic solvent consists of EFA, FEC, PC and EMC in a mass ratio of 15:10:20:55;

[0047] The structural formula of EFA is

[0048] Example 5

[0049] This embodiment provides an electrolyte, which differs from Embodiment 1 only in that:

[0050] The organic solvent consists of EFA, FEC, PC and EMC in a mass ratio of 10:10:20:60.

[0051] Example 6

[0052] This embodiment provides an electrolyte, which differs from Embodiment 1 only in that:

[0053] The organic solvent consists of EFA, FEC, PC and EMC in a mass ratio of 20:10:20:50.

[0054] Example 7

[0055] This embodiment provides an electrolyte, which differs from Embodiment 1 only in that:

[0056] The organic solvent consists of DFEA, FEC, PC and EMC in a mass ratio of 1:15:22.4:61.6.

[0057] Example 8

[0058] This embodiment provides an electrolyte, which differs from Embodiment 1 only in that:

[0059] The organic solvent consists of DFEA, FEC, PC and EMC in a mass ratio of 20:1:21:58.

[0060] Comparative Example 1

[0061] This comparative example provides an electrolyte, which differs from Example 1 only in that:

[0062] The organic solvent is composed of FEC, PC and EMC in a mass ratio of 10:24:66.

[0063] Comparative Example 2

[0064] This comparative example provides an electrolyte, which differs from Example 3 only in that:

[0065] The organic solvent is composed of DFEA, PC and EMC in a mass ratio of 20:22.8:57.2.

[0066] Comparative Example 3

[0067] This comparative example provides an electrolyte, which differs from Example 6 only in that:

[0068] The organic solvent is composed of EFA, PC and EMC in a mass ratio of 20:22.8:57.2.

[0069] Preparation of lithium-ion batteries:

[0070] Positive electrode sheet: including positive electrode current collector (aluminum foil) and positive electrode material layer coated thereon. The positive electrode material layer has a ratio of high nickel ternary active material NCM811: SP (conductive carbon black): CNTs (carbon nanotubes): PVDF (polyvinylidene fluoride) = 97.6: 0.8: 0.6: 1.0, with a surface density of 297g / m 2 , compacted density is 3.45g / cm 3 ;

[0071] Negative electrode sheet: including negative electrode current collector (copper foil) and negative electrode material layer coated thereon, the negative electrode material layer ratio is negative electrode active material (10wt% SiO and 90wt% artificial graphite): SP (conductive carbon black): SWCNT (multi-walled carbon nanotubes): CMC (carboxymethyl cellulose): PAA (polyacrylic acid): SBR (styrene-butadiene rubber) = 95.94:1:0.06:0.3:2:0.7, with a surface density of 156g / m 2 , compacted density is 1.55g / cm 3 ;

[0072] The positive electrode sheet, PE separator, and negative electrode sheet were stacked in order, and the electrolytes provided in the above embodiment and comparative example were injected respectively to obtain a soft-pack battery. The positive and negative electrode CB values ​​were 1.11 and the capacity was 10.8Ah.

[0073] Performance Testing:

[0074] High temperature storage performance test: At room temperature, the battery is first charged to 4.25V at a constant current of 1 / 3C, then charged to a termination current of 0.05C at a constant voltage, left for 10 minutes, and then discharged to 2.5V at 1 / 3C to obtain the initial discharge capacity Q1. After storage at 55°C for a period of time, it is discharged to a termination voltage of 2.5V at room temperature at a constant current of 1 / 3C and left for 10 minutes (record the retention capacity); then at room temperature, the battery is first charged to 4.25V at a constant current of 1 / 3C, then charged to a cut-off current of 0.05C at a constant voltage, left for 10 minutes, and then discharged to 2.5V at a constant current of 1 / 3C and left for 10 minutes. The discharge capacity Q2 is recorded, and Q2 / Q1 is the capacity recovery rate. Continue to repeat the above steps to obtain the capacity recovery rate curve of high temperature storage at different times.

[0075] Cycle performance test: The lithium-ion battery was charged to 4.25V at 4C at 25°C, left for 30 minutes, and discharged to 2.5V at 1C. The cycle was repeated until the discharge capacity dropped to 80% of the initial discharge capacity, and the test was stopped to draw a cycle curve.

[0076] The results of the above tests are shown in Table 1 below:

[0077] Table 1

[0078]

[0079]

[0080] The high temperature storage performance test results of Example 1 and Comparative Example 1 are as follows: Figure 1 The cycle performance test results are shown in Figure 2 As shown in Table 1 and Figure 1-2 It can be seen from the test results that, compared with the electrolyte containing only ethyl fluoroacetate or fluoroethylene carbonate (Comparative Examples 1-3), the electrolyte provided by the embodiments of the present invention can significantly improve the high temperature resistance and cycle performance of the lithium ion battery.

[0081] The above description is only a specific embodiment of the present disclosure, so that those skilled in the art can understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to the embodiments described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An electrolyte, characterized in that: The electrolyte comprises an electrolyte and a solvent; The solvent includes ethyl fluoroacetate and fluoroethylene carbonate.

2. The electrolyte according to claim 1, characterized in that The mass percentage of the ethyl fluoroacetate to the solvent is 1-20%.

3. The electrolyte according to claim 1 or 2, characterized in that The mass percentage of the fluoroethylene carbonate in the solvent is 1-15%.

4. The electrolyte according to any one of claims 1 to 3, characterized in that The fluoroethylene carbonate is and / or 5. The electrolyte according to any one of claims 1 to 4, characterized in that The solvent also includes a balance of carbonate.

6. The electrolyte according to any one of claims 1 to 5, characterized in that: The content of the electrolyte in the electrolyte solution is 12-15 wt %.

7. The electrolyte according to any one of claims 1 to 6, characterized in that The electrolyte is selected from one or more of LiPF6, LiBF4, LiBOB, LiClO4, LiCF3SO3, LiODFB, LiN(SO2CF3)2, LiN(SO2F)2, LiTFSI, LiFSI, LiHFDF and cyclic imide lithium salts.

8. The electrolyte according to any one of claims 1 to 7, characterized in that: The electrolyte further comprises an additive, and the additive is selected from one or more of vinyl sulfate, tripropynyl phosphate, vinylene carbonate, tri(trimethylsilyl)phosphate, propane sultone and lithium difluorophosphate.

9. The electrolyte according to claim 8, characterized in that The content of the additive in the electrolyte is 0.1-2 wt %.

10. A lithium ion battery, characterized in that: Comprising the electrolyte as described in any one of claims 1-9.

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