Fluorinated ester-fluorinated ether-ionic liquid mixed high-entropy electrolyte, preparation method thereof and lithium metal secondary battery

Abstract

The invention relates to a fluorinated ester-fluorinated ether-ionic liquid mixed high-entropy electrolyte, a preparation method thereof and a lithium metal secondary battery, and belongs to the technical field of lithium metal secondary batteries. The electrolyte comprises a lithium salt, a fluorinated ester organic solvent as shown in a structural formula (I), a fluorinated ether organic solvent as shown in a structural formula (II) and an ionic liquid as shown in a structural formula (III), by utilizing the high dielectric constant of the fluorinated ester organic solvent of the structural formula (I), the electric field intensity of an electrolyte interface is reduced, and the concentration polarization of lithium ions is relieved; a thinner SEI film is generated by using a fluorinated ether organic solvent of a structural formula (II), so that a solid-phase transmission path is reduced; a lithium-phobic protective layer is formed by utilizing symmetrically extending alkyl chain modified ILs cations of the ionic liquid of the structural formula (III), so that uniform deposition of lithium is realized. By virtue of the advantages, the lithium metal secondary battery with high safety, high energy efficiency, high rate capability and high stability can be designed and developed.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium metal secondary batteries, and in particular to a fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, a preparation method thereof, and a lithium metal secondary battery. Background Art

[0002] In recent years, with the popularity of portable electronic devices, the development of power tools and electric vehicles, lithium secondary batteries as a new generation of high-energy power sources have received widespread attention. Compared with fuel cell electric vehicles, the slow charging and limited energy density of electric vehicles are the main causes of range anxiety. Lithium metal has a high energy density (3860mAh g -1 ) and extremely low electrochemical potential (-3.04V vs standard hydrogen electrode) are considered to be the next generation material that can replace graphite negative electrode. At present, the positive electrode materials of commercial lithium-ion batteries mainly include lithium manganese oxide, lithium cobalt oxide, ternary materials, lithium iron phosphate, etc.

[0003] However, with the development and popularization of lithium metal batteries with higher voltage, faster charging rate and wider operating temperature, conventional electrolytes will not only undergo oxidation and decomposition reactions themselves, but also undergo oxidation and decomposition on the surface of the positive electrode materials. Positive electrode materials containing transition metal elements play a catalytic role, thereby promoting the oxidation of the electrolyte. In addition, during the normal operation of lithium metal secondary batteries, the metal lithium negative electrode is also very prone to volume expansion and rupture, and the electrolyte undergoes a series of adverse reactions on the negative electrode material, continuously generating deposits, which increases the thickness of the solid electrolyte interface layer, ultimately leading to an increase in the internal resistance of the battery, damage to the cycle performance, and shortening the battery life.

[0004] Chinese patent CN110176630B discloses an electrolyte and an electrochemical device using the same, wherein the electrolyte comprises a sulfone compound, a phosphorus-containing compound and an additive A, wherein the additive A comprises a compound of a benzene ring, a fluoroether, a fluoroketone or a fluorosiloxane. The invention focuses on improving the conductivity of the electrolyte and does not consider the problem of slow interface / solid phase mass transfer.

[0005] In related technologies, in order to stabilize the interfacial side reactions and reduce the growth of lithium dendrites, the use of high-concentration or locally concentrated ester electrolytes can improve the fast charging electrochemical performance to a certain extent (ACS Energy Lett. 2022, 7, 3826-3834). However, the high viscosity of high-concentration electrolytes leads to a decrease in carrier mobility; the diluent used for local concentration has a low dielectric constant (Angew. Chem. Int. Ed. 2021, 60, 11442–11447) and causes a sudden increase in the electric field strength at the electrolyte interface; at the same time, both have defects such as high cost that make them difficult to popularize and apply.

[0006] Fast charging is charging to 80% of the state of charge within 5-15 minutes, and the process is accompanied by high current density, resulting in Li + Concentration polarization, repeated solid electrolyte interface (SEI) breakage / repair and uncontrolled growth of lithium dendrites. Therefore, it is of great significance to develop an electrolyte for lithium metal batteries suitable for fast charging. Summary of the invention

[0007] Based on this, the object of the present invention is to provide a fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte and a preparation method thereof and a lithium metal secondary battery. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte can act on the negative electrode of the lithium metal secondary battery, so that the lithium metal secondary battery has good room temperature cycle performance, high temperature cycle performance and high temperature storage performance at a fast charging rate (such as 5C).

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] The first aspect of the present invention is to provide a fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, comprising a lithium salt, a fluorinated ester organic solvent, a fluorinated ether organic solvent and an ionic liquid; wherein:

[0010] The fluorinated ester organic solvent includes an ester compound represented by structural formula (I);

[0011]

[0012] In the structural formula (I), R1 and R2 are each independently selected from one of trifluoromethyl, difluoromethyl, monofluoromethyl, and a fluorinated methyl or fluorinated methylene group containing 1 to 2 carbon chain extensions;

[0013] The fluorinated ether organic solvent includes an ether compound represented by structural formula (II);

[0014]

[0015] In the structural formula (II), R1 and R2 are each independently selected from one of trifluoromethyl, difluoromethyl, monofluoromethyl and a fluorinated methylene group containing 1-2 carbon chain extensions;

[0016] The ionic liquid includes an ionic liquid represented by structural formula (III);

[0017]

[0018] In the structural formula (III), R1 is selected from one of isopropyl, 3-methylenepentane, 4-methyleneheptane and 5-methylenenonane groups.

[0019] In order to solve the problem of slow kinetics of rate-determining steps such as desolvation and solid-phase mass transfer in the lithium ion transport process of lithium metal secondary batteries under high current density of fast charging, the present invention provides a fluorinated ester-fluorinated ether-ionic liquid mixed high-entropy electrolyte that can form an aggregate solvation sheath to accelerate the desolvation process.

[0020] The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention comprises a lithium salt, a fluorinated ester organic solvent of structural formula (I), a fluorinated ether organic solvent of structural formula (II), and an ionic liquid of structural formula (III); the high dielectric constant of the fluorinated ester organic solvent of structural formula (I) is utilized to reduce the electric field intensity of the electrolyte interface and alleviate the polarization of lithium ion concentration; the fluorinated ether organic solvent of structural formula (II) is utilized to generate a thinner SEI film and reduce the solid phase transmission path; the ILs cation modified by the symmetrically extended alkyl chain of the ionic liquid of structural formula (III) (that is, R1 in structural formula (III), isopropyl, 3-methylenepentane, 4-methyleneheptane, 5-methylenenonane groups are symmetrically extended alkyl chains) forms a lithium-phobic protective layer, thereby achieving uniform deposition of lithium.

[0021] As a preferred embodiment, the ester compound represented by the structural formula (I) is selected from at least one of the ester compounds of formula (I-1), formula (I-2), formula (I-3), formula (I-4), formula (I-5), and formula (1-6);

[0022]

[0023]

[0024] As a preferred solution, the ether compound represented by structural formula (II) is at least one selected from the ether compounds of formula (II-1), formula (II-2), and formula (II-3);

[0025]

[0026] From the above content, it can be seen that in the ionic liquid represented by structural formula (III), R1 is selected from one of isopropyl, 3-methylenepentane, 4-methyleneheptane, and 5-methylenenonane groups. That is, the ionic liquid represented by structural formula (III) is selected from at least one of the ionic liquids of formula (III-1), formula (III-2), formula (III-3), and formula (III-4);

[0027]

[0028] As a preferred embodiment, the lithium salt is selected from at least one of lithium dioxalatoborate, lithium difluorooxalatoborate, lithium difluorophosphate, lithium difluorobisoxalatophosphate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethylsulfonate, lithium bis(fluorosulfonyl imide) and lithium bis(trifluoromethylsulfonyl imide).

[0029] As a preferred solution, the mass of the lithium salt accounts for 5-25% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, specifically 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable. More preferably, the mass of the lithium salt accounts for 10-20% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte.

[0030] As a preferred solution, the mass of the fluorinated ester organic solvent accounts for 20-40% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, and more preferably 25-35%; the mass of the fluorinated ether organic solvent accounts for 20-40% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, and more preferably 25-35%. As a preferred solution, the mass ratio of the fluorinated ester organic solvent to the fluorinated ether organic solvent is preferably 1:1.

[0031] As a preferred solution, the mass of the ionic liquid accounts for 15%-30% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte.

[0032] As a preferred solution, the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention further includes an auxiliary agent, and the auxiliary agent is selected from at least one of vinylene carbonate (VC), 1,3-propane sultone (PS), 1,4-butane sultone (BS), acrylic acid lactone (RPS), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), and lithium difluorophosphate (LiPO2F2). The addition of the auxiliary agent can further improve the performance of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, and improve the cycle performance and high temperature storage performance of the lithium metal secondary battery.

[0033] As a preferred solution, the mass of the auxiliary agent accounts for 0.1-8.0% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, and specifically may be but not limited to 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 4%, 4.5%, 5%, 5.5%, 6.0%, 7%, 8%. More preferably, the mass of the auxiliary agent accounts for 0.5-1% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte.

[0034] The second aspect of the present invention is to provide a method for preparing any of the above-mentioned fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolytes, comprising the following steps: mixing the substances uniformly under the protection of an inert atmosphere to obtain the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte. The preparation method is simple and easy to operate.

[0035] As a preferred solution, the inert atmosphere may be a conventional argon atmosphere or a nitrogen atmosphere.

[0036] The third aspect of the present invention is to provide a lithium metal secondary battery (lithium ion battery), including a positive electrode, a negative electrode, and any of the above-mentioned fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolytes. The lithium metal secondary battery (lithium ion battery) has good room temperature cycle performance, high temperature cycle performance and high temperature storage performance under high voltage (such as 5C), and can still achieve good room temperature cycle performance, high temperature cycle performance and high temperature storage performance when the fastest charging rate is 10C.

[0037] As a preferred solution, the active material of the positive electrode includes LiFePO4, LiCoO2 and Li (1+a) Ni x Co y M z N 1-x-y-z O 2+b At least one of Li (1+a) Ni x Co y M z N 1-x-y-z O 2+b In the formula, M is Mn or Al, N is any one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, Ba, V and Ti, -0.10≤a≤0.50, 0 <x<1,0<y<1,0<z<1,0.7<x+y+z≤1,-0.05≤b≤0.10。

[0038] Commonly used negative electrode materials are at least one of artificial graphite, natural graphite, Si and its alloys, Sn and its alloys, metallic lithium and its alloys, transition metal oxides and lithium titanate. Among these negative electrode materials, the lithium metal negative electrode has a higher theoretical specific capacity (mAh / g), but it is very easy to generate lithium dendrites during the cycle process, resulting in a decrease in the battery cycle stability. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention can especially overcome the above-mentioned defects caused by the lithium metal negative electrode. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention can form a solid electrolyte interface film rich in inorganic substances on the lithium metal negative electrode to protect the lithium metal negative electrode.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] (1) The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention adopts the fluorinated ester organic solvent shown in structural formula (I), and utilizes the high dielectric constant of the fluorinated ester organic solvent of structural formula (I) to reduce the electric field strength of the electrolyte interface and alleviate the polarization of lithium ion concentration. The fluorinated ether organic solvent of structural formula (II) can generate a thinner SEI film and reduce the solid phase transmission path. The ionic liquid shown in structural formula (III) is adopted, and the ILs cation modified by the symmetrically extended alkyl chain of the ionic liquid of structural formula (III) is used to form a lithium-phobic protective layer, thereby achieving uniform deposition of lithium. The lithium metal secondary battery (lithium ion battery) using the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention can form an SEI film with excellent mechanical properties at the interface of the negative electrode material during the formation stage of the lithium metal secondary battery. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention provides a design idea for the design and development of lithium metal secondary batteries with high safety, high energy efficiency, high rate performance and high stability.

[0041] (2) The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of the present invention preferably uses a fluorinated ester organic solvent containing a difluoro substituent (-CHF2) and a fluorinated ether organic solvent containing a difluoro substituent (-CHF2). Compared with the trifluoro substituent (-CHF3), the difluoro substituent (-CHF2) can occupy more local electrons, further improving the ionic conductivity of the electrolyte without reducing the oxidative stability. A strong and low-impedance SEI film can be formed at the electrode / electrolyte interface, avoiding the formation of an SEI film with high impedance and loose structure due to other functional groups. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] For better understanding and implementation, the present invention is described in detail below with reference to the accompanying drawings.

[0043] Figure 1 Impedance diagrams of lithium metal secondary batteries prepared with the electrolytes of Example 20 and Comparative Example 1;

[0044] Figure 2 In the figure, a is the SEM of the lithium metal secondary battery prepared with the electrolyte of Comparative Example 1, and b is the SEM of the lithium metal secondary battery prepared with the electrolyte of Example 20.

[0045] Figure 3 Coulomb efficiency diagram of Li||Cu batteries prepared with the electrolytes of Example 20, Comparative Example 1, Comparative Example 3, and Comparative Example 5;

[0046] Figure 4 This is a long cycle performance diagram of the Li||Li symmetric battery prepared with the electrolyte of Example 20 and Comparative Example 1. DETAILED DESCRIPTION

[0047] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0048] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0049] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0050] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

[0051] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0052] In order to better illustrate the purpose, technical scheme and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the following implementation method is a further explanation of the present invention and should not be used as a limitation of the present invention.

[0053] Example 1

[0054] The present embodiment provides a fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, comprising a lithium salt, a fluorinated ester organic solvent, a fluorinated ether organic solvent, an ionic liquid and an additive.

[0055] Wherein, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium difluorooxalatoborate (LiDFOB); the fluorinated ester organic solvent is an ester compound shown in structural formula (I); the fluorinated ether organic solvent is an ether compound shown in structural formula (II); the ionic liquid is a compound shown in structural formula (III); and the auxiliary agent is fluoroethylene carbonate (FEC). Specifically, the specific composition and dosage of each component can be found in Tables 1 and 3.

[0056] This embodiment also provides a method for preparing a fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, comprising the following steps: under experimental conditions of water content <1ppm, oxygen content <1ppm, and inert atmosphere protection, according to the component ratio in Table 1, a fluorinated ester organic solvent and a fluorinated ether organic solvent are mixed, and then a lithium salt (i.e., lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate) is added, and after the lithium salt is completely dissolved, an additive and an ionic liquid are added and mixed evenly.

[0057] The water content of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte obtained in this example is below 20 ppm.

[0058] Example 2-24

[0059] Examples 2-24 respectively provide a fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, including a lithium salt, a fluorinated ester organic solvent, a fluorinated ether organic solvent, an ionic liquid and an additive.

[0060] Wherein, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium difluorooxalatoborate (LiDFOB); the fluorinated ester organic solvent is an ester compound shown in structural formula (I); the fluorinated ether organic solvent is an ether compound shown in structural formula (II); the ionic liquid is a compound shown in structural formula (III); and the auxiliary agent is fluoroethylene carbonate (FEC). Specifically, the specific composition and dosage of each component in the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 2-24 are shown in Tables 1 and 3.

[0061] The preparation method of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 2-24 is the same as that of Example 1.

[0062] Comparative Example 1-2

[0063] Comparative Examples 1-2 respectively provide an electrolyte solution including a lithium salt, an ester organic solvent, an ether organic solvent and an additive.

[0064] Wherein, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium difluorooxalatoborate (LiDFOB); the ester organic solvent is an ester compound shown in structural formula (I), but does not contain fluorine; the ether organic solvent is an ether compound shown in structural formula (II), but does not contain fluorine; and the auxiliary agent is fluoroethylene carbonate (FEC). Specifically, the specific composition and dosage of each component in the electrolyte of Comparative Examples 1-2 are shown in Tables 2 and 3.

[0065] The preparation method of the electrolyte of Comparative Example 1-2 is substantially the same as that of Example 1, except that no ionic liquid is added.

[0066] Comparative Examples 3-10

[0067] Comparative Examples 3-10 respectively provide an electrolyte solution including a lithium salt, an ester organic solvent, an ether organic solvent, an ionic liquid and an additive.

[0068] Wherein, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium difluorooxalatoborate (LiDFOB); the ester organic solvent is an ester compound shown in structural formula (I), but does not contain fluorine; the ether organic solvent is an ether compound shown in structural formula (II), but does not contain fluorine; the ionic liquid is a compound shown in structural formula (III); and the auxiliary agent is fluoroethylene carbonate (FEC). Specifically, the specific composition and dosage of each component in the electrolyte of Comparative Examples 3-10 are shown in Tables 2 and 3.

[0069] The preparation method of the electrolyte of Comparative Examples 3-10 is the same as that of Example 1.

[0070] Table 1 Compositions of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24

[0071]

[0072]

[0073] Table 2 Composition of the electrolyte of Comparative Examples 1-10

[0074]

[0075]

[0076] Table 3 Structural formula of each compound in Table 1 and Table 2

[0077]

[0078]

[0079] Examples 25-48, Comparative Examples 11-20

[0080] Examples 25-48 and Comparative Examples 11-20 respectively provide a lithium metal secondary battery. The lithium metal secondary batteries of Examples 25-48 are respectively prepared from the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24; the lithium metal secondary batteries of Comparative Examples 11-20 are respectively prepared from the electrolytes of Comparative Examples 1-10.

[0081] The preparation methods of the lithium metal secondary batteries of Examples 25-48 and the lithium metal secondary batteries of Comparative Examples 11-20 are as follows:

[0082] The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24 and the electrolyte of Comparative Examples 1-10 were respectively injected into button lithium metal batteries with lithium iron phosphate (LFP) as the positive electrode material and lithium metal as the negative electrode material, and charged and discharged at a constant current of 0.1C for 3 times, then charged to 4.3V at a constant current of 0.5C, and discharged to 2.8V at a constant current of 5C.

[0083] Performance Testing

[0084] The lithium metal secondary batteries prepared in Examples 25-48 and Comparative Examples 11-20 were subjected to room temperature rapid charge cycle test, high temperature cycle test, and high temperature storage test respectively according to the following test conditions. The test results are shown in Table 4. The test items are as follows:

[0085] (1) Normal temperature cycle performance test:

[0086] Under normal temperature (25°C) storage conditions:

[0087] The lithium metal secondary battery was charged to 4V at 5C constant current and constant voltage, with a cut-off current of 0.02C, and then discharged to 2.4V at 0.5C constant current. The 5C charge / 5C discharge cycle was repeated 200 times to evaluate the cycle performance. The cycle performance was calculated by the capacity retention rate of the following formula:

[0088] Capacity retention rate (%) = (discharge capacity at the 200th cycle / initial discharge capacity) × second discharge capacity.

[0089] (2) High temperature cycle performance test:

[0090] Under high temperature (45°C) storage conditions:

[0091] The lithium metal secondary battery was charged to 4V at 5C constant current and constant voltage, with a cut-off current of 0.02C, and then discharged to 2.8V at 5C constant current, and the 5C charge / 5C discharge cycle was repeated 100 times to evaluate the cycle performance. The cycle performance was calculated by the capacity retention rate of the following formula.

[0092] Capacity retention rate (%) = (discharge capacity at the 100th cycle / initial discharge capacity) × second discharge capacity.

[0093] (3) High temperature storage performance test:

[0094] The lithium metal secondary battery was charged to 4V at 5C constant current and constant voltage, stored at 45°C for 14 days, and then discharged to 2.8V at 5C constant current to measure the capacity recovery rate of the battery. The capacity recovery rate was calculated by the following formula.

[0095] Capacity recovery rate (%) = recovery capacity / initial capacity × initial capacity.

[0096] The performance test results of the lithium metal secondary batteries of Examples 25-48 and Comparative Examples 11-20 are shown in Table 4:

[0097] Table 4 Performance test results of lithium metal secondary batteries of Examples 25-48 and Comparative Examples 11-20

[0098]

[0099]

[0100] As can be seen from Table 4, the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolytes of Examples 1-24 of the present invention corresponding to the lithium metal secondary batteries of Examples 25-48 are significantly improved in room temperature cycle performance, high temperature storage performance, and high temperature cycle performance compared with the lithium metal secondary batteries of Comparative Examples 11-20 corresponding to the electrolytes of Comparative Examples 1-10.

[0101] This is because in the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24, the high dielectric constant of the fluorinated ester organic solvent shown in structural formula (I) is utilized to increase the dielectric constant of the electrolyte, reduce the electric field strength at the electrolyte interface, and alleviate the polarization of lithium ion concentration; the fluorinated ether organic solvent shown in structural formula (II) is utilized to reduce the high viscosity brought by the fluorinated ester organic solvent shown in structural formula (I), and is conducive to the formation of a thinner and anion-rich inorganic SEI film, reducing the solid phase transmission path; the alkyl chain-modified ILs cation of the ionic liquid shown in structural formula (III) is utilized to form a lithium-phobic protective layer on the surface of lithium metal, effectively preventing the irregular growth of negative electrode lithium dendrites and inducing uniform lithium deposition, thereby achieving uniform lithium deposition. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24 corresponds to the lithium metal secondary batteries of Examples 25-48, which can form a SEI film with excellent mechanical properties at the interface of the negative electrode material during the formation stage, and can alleviate the irregular growth of lithium dendrites at the lithium metal negative electrode during the charging process, thereby improving the room temperature cycle performance, high temperature cycle performance and high temperature storage performance of the battery.

[0102] In addition, from the above data, it can be seen that the lithium metal secondary battery of Example 44 prepared by the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 has excellent room temperature cycle performance, high temperature cycle performance and high temperature storage performance. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 uses a compound of structural formula (I-5) as a fluorinated ester organic solvent and a compound of structural formula (II-2) as a fluorinated ether organic solvent. Compared with the compounds of other structural formulas mentioned in the electrolytes of other embodiments of the present invention, the compounds of structural formula (I-5) and structural formula (II-2) contain a difluoro substituent (-CHF2), and the fluorinated ester organic solvent and the fluorinated ether organic solvent containing a difluoro substituent (-CHF2) have high ionic conductivity, low and stable overpotential, and high coulomb efficiency. A strong and low-impedance SEI film can be formed at the electrode / electrolyte interface, avoiding other functional groups to form a SEI film with large impedance and loose structure. Therefore, the lithium metal secondary battery of Example 44 prepared using the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 containing a fluorinated ester organic solvent containing a difluoro substituent (-CHF2) and a fluorinated ether organic solvent exhibits superior room temperature cycle performance, high temperature cycle performance and high temperature storage performance.

[0103] In the electrolytes of Comparative Examples 1-10, the compound represented by structural formula (Ⅰ-7) or the compound represented by structural formula (Ⅰ-8) is used as the ester organic solvent, and the compound represented by structural formula (Ⅱ-4) is used as the ether organic solvent, and both solvents do not contain fluorinated structures. The normal temperature cycle performance, high temperature cycle performance and high temperature storage performance of the lithium metal secondary batteries of Comparative Examples 11-20 prepared corresponding to the electrolytes of Comparative Examples 1-10 are significantly low. Since no ionic liquid is added to the electrolytes of Comparative Examples 1 and 2, the high temperature cycle performance of the corresponding lithium metal secondary batteries of Comparative Examples 11 and 12 is particularly low.

[0104] See also Figure 1 From the impedance diagrams of the lithium metal secondary battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 (i.e., the lithium metal secondary battery of Example 44) and the lithium metal secondary battery prepared with the electrolyte of Comparative Example 1 (i.e., the lithium metal secondary battery of Comparative Example 11), it can be seen that the lithium metal secondary battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 has smaller interface impedance and charge transfer impedance, and faster interface dynamics.

[0105] See also Figure 2From the SEM image of the lithium metal secondary battery prepared with the electrolyte of Comparative Example 1 (i.e., the lithium metal secondary battery of Comparative Example 11) and the SEM image of the lithium metal secondary battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 (i.e., the lithium metal secondary battery of Example 44), it can be seen that the growth of lithium dendrites of the lithium metal secondary battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 is controlled and the lithium deposition is more uniform.

[0106] Examples 49-72, Comparative Examples 21-30

[0107] Examples 49-72 and Comparative Examples 21-30 respectively provide a Li||Cu battery. The Li||Cu batteries of Examples 49-72 are respectively prepared from the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24; the Li||Cu batteries of Comparative Examples 21-30 are respectively prepared from the electrolytes of Comparative Examples 1-10.

[0108] The preparation method of the Li||Cu battery of Examples 49-72 and Comparative Examples 21-30 is:

[0109] The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24 and the electrolyte of Comparative Examples 1-10 were respectively injected into button lithium metal batteries with a copper sheet as a positive electrode and lithium metal as a negative electrode.

[0110] The battery is charged at a constant current until the cut-off voltage reaches 0.5V, and the discharge time is fixed at 1 hour. The current density is calculated by the electrode area. -2 The current density and 5 mAh cm -2 The capacity of the battery was used for long-term cycling. The battery coulombic efficiency (CE) is used to analyze the reversibility of Li+ during the cycling process. The cycle performance was evaluated by repeating the cycle 100 times. The cycle performance was calculated by the coulombic efficiency of the following formula.

[0111] Coulomb efficiency (%) = (charge capacity at the 100th cycle / discharge capacity at the 100th cycle) × discharge capacity.

[0112] The test results are shown in Table 5:

[0113] Table 5 Performance test results of Li||Cu batteries of Examples 49-72 and Comparative Examples 21-30

[0114]

[0115]

[0116] It can be seen from Table 5 that the Li||Cu batteries of Examples 49-72 prepared from the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolytes of Examples 1-24 of the present invention have lower first deposition nucleation overvoltage and higher coulombic efficiency than the Li||Cu batteries of Comparative Examples 21-30 prepared from the electrolytes of Comparative Examples 1-10.

[0117] See also Figure 3 From the coulombic efficiency diagrams of the Li||Cu battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 (i.e., the Li||Cu battery of Example 67) and the lithium metal secondary battery prepared with the electrolyte of Comparative Example 1 (i.e., the Li||Cu battery of Comparative Example 21), it can be seen that the Li||Cu battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 has better coulombic efficiency and higher Li utilization.

[0118] Examples 73-96, Comparative Examples 31-40

[0119] Examples 73-96 and Comparative Examples 31-40 respectively provide a Li||Li symmetric battery. The Li||Li symmetric batteries of Examples 73-96 are respectively prepared from the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24; the Li||Li symmetric batteries of Comparative Examples 31-40 are respectively prepared from the electrolytes of Comparative Examples 1-10.

[0120] The preparation method of the Li||Li symmetric battery of Examples 73-96 and the Li||Li symmetric battery of Comparative Examples 31-40 is:

[0121] The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Examples 1-24 and the electrolyte of Comparative Examples 1-10 were respectively injected into symmetric lithium metal batteries with lithium metal as the positive electrode and lithium metal as the negative electrode to assemble Li||Li symmetric batteries. The button cell model was CR2032 and the separator was a polypropylene microporous membrane Celgard 2400.

[0122] The Li||Li symmetrical batteries of Examples 73-96 and the Li||Li symmetrical batteries of Comparative Examples 31-40 were respectively subjected to 5 mA cm -2 The current density and 5 mAh cm -2 The test results are shown in Table 6:

[0123] Table 6 Performance test results of Li||Li symmetric batteries of Examples 73-96 and Comparative Examples 31-40

[0124]

[0125]

[0126] It can be seen from Table 6 that the Li||Li symmetric batteries of Examples 73-96 prepared from the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolytes of Examples 1-24 of the present invention have lower polarization voltage and longer cycle life than the Li||Li symmetric batteries of Comparative Examples 31-40 prepared from the electrolytes of Comparative Examples 1-10.

[0127] See also Figure 4 From the long cycle performance diagrams of the Li||Li symmetric battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 (i.e., the Li||Li symmetric battery of Example 91) and the Li||Li symmetric battery prepared with the electrolyte of Comparative Example 1 (i.e., the Li||Li symmetric battery of Comparative Example 31), it can be seen that the lithium metal secondary battery prepared with the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte of Example 20 has a higher capacitance retention rate.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte, characterized in that: Including lithium salts, fluorinated ester organic solvents, fluorinated ether organic solvents and ionic liquids; wherein, The fluorinated ester organic solvent includes an ester compound represented by structural formula (I); In the structural formula (I), R1 and R2 are each independently selected from one of trifluoromethyl, difluoromethyl, monofluoromethyl, and a fluorinated methyl or fluorinated methylene group containing 1 to 2 carbon chain extensions; The fluorinated ether organic solvent includes an ether compound represented by structural formula (II); In the structural formula (II), R1 and R2 are each independently selected from one of trifluoromethyl, difluoromethyl, monofluoromethyl and a fluorinated methylene group containing 1-2 carbon chain extensions; The ionic liquid includes an ionic liquid represented by structural formula (III); In the structural formula (III), R1 is selected from one of isopropyl, 3-methylenepentane, 4-methyleneheptane and 5-methylenenonane groups.

2. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte according to claim 1, characterized in that: The ester compound represented by structural formula (I) is at least one selected from the group consisting of ester compounds of formula (I-1), formula (I-2), formula (I-3), formula (I-4), formula (I-5), and formula (1-6); 3. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte according to claim 1, characterized in that: The ether compound represented by structural formula (II) is at least one selected from the ether compounds of formula (II-1), formula (II-2), and formula (II-3); 4. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte according to claim 1, characterized in that: The lithium salt is selected from at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorophosphate, lithium difluorobis(oxalatophosphate), lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl imide) and lithium bis(trifluoromethylsulfonyl imide).

5. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte according to claim 1, characterized in that: The mass of the lithium salt accounts for 5-25% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte; the mass of the fluorinated ester organic solvent accounts for 20-40% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte; the mass of the fluorinated ether organic solvent accounts for 20-40% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte; the mass of the ionic liquid accounts for 15-30% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte.

6. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte according to claim 1, characterized in that: Also includes adjuvants.

7. The fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte according to claim 6, characterized in that: The auxiliary agent is selected from at least one of vinylene carbonate, 1,3-propane sultone, 1,4-butane sultone, acrylic acid lactone, vinyl ethylene carbonate, fluoroethylene carbonate, and lithium difluorophosphate; the mass of the auxiliary agent accounts for 0.1-8.0% of the total mass of the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte.

8. A method for preparing a fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte as claimed in any one of claims 1 to 7, characterized in that: The following steps are involved: The substances are mixed uniformly under the protection of an inert atmosphere to obtain the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte.

9. A lithium metal secondary battery, characterized in that: It comprises the fluorinated ester-fluorinated ether-ionic liquid mixed high entropy electrolyte as described in any one of claims 1 to 7.

10. The lithium metal secondary battery according to claim 9, characterized in that: Also includes a positive electrode and a negative electrode; the active material of the positive electrode includes LiFePO4, LiCoO2 and Li (1+a) Ni x Co y M z N 1-x-y-z O 2+b , at least one of which, M is Mn or Al, N is any one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, Ba, V and Ti, -0.10≤a≤0.50, 0 <x<1,0<y<1,0<z<1,0.7<x+y+z≤1,-0.05≤b≤0.10。

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