Electrolyte, negative-electrode-free lithium metal battery and electric equipment

Abstract

The invention relates to the technical field of batteries, in particular to an electrolyte, a cathode-free lithium metal battery and electric equipment. The electrolyte comprises a lithium salt, a solvent and a diluent, the solvent comprises at least two of cyclic ether solvents; the diluents comprise at least two of fluorobenzene diluents. According to the electrolyte disclosed by the invention, various cyclic ether solvents and various fluorobenzene diluents are mixed, so that the entropy value of the electrolyte is increased, the disorder of the electrolyte is improved, the formation of the weak solvated electrolyte is promoted, the stability of an electrode interface is favorably improved, and the ion transference number and the ion conductivity of the electrolyte are improved. The electrolyte provided by the invention is suitable for the negative-electrode-free lithium metal battery, and the prepared negative-electrode-free lithium metal battery has high coulombic efficiency and excellent cycle performance and rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to an electrolyte, a negative electrode-free lithium metal battery and electrical equipment. Background Art

[0002] As a high-energy-density battery, lithium metal batteries are widely used in electric vehicles, energy storage systems, etc. However, lithium metal batteries have serious safety issues during the cycle process, such as the growth of lithium dendrites, the instability of lithium metal anodes, and severe volume expansion during the cycle. These problems seriously limit the large-scale commercial application of lithium metal batteries.

[0003] Anode-free lithium metal batteries do not use lithium metal in the battery manufacturing process, which greatly reduces safety risks, while improving the simplicity of battery manufacturing and reducing battery assembly costs. During the first charge of anode-free lithium metal batteries, lithium ions are released from the positive electrode material and then directly deposited on the negative electrode current collector. However, for anode-free lithium metal batteries, due to the absence of excess lithium, the battery capacity usually has the problem of rapid capacity decay.

[0004] Developing an electrolyte suitable for anode-free lithium metal batteries and capable of improving their cycle stability is of great significance to the development of anode-free lithium metal batteries.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] The purpose of the present invention is to provide an electrolyte, a negative electrode-free lithium metal battery and an electrical device. The electrolyte of the present invention can achieve a certain electrochemical stability at the electrode interface and has high ionic conductivity, and can improve the coulombic efficiency, cycle performance and rate performance of the negative electrode-free lithium metal battery containing the electrolyte.

[0007] In order to achieve the above-mentioned object of the present invention, the present invention provides an electrolyte, including a lithium salt, a solvent and a diluent;

[0008] The solvent includes at least two cyclic ether solvents; and the diluent includes at least two fluorobenzene diluents.

[0009] In a specific embodiment of the present invention, the cyclic ether solvent includes at least one of the structures shown in Formula I:

[0010]

[0011] Wherein, X is selected from one of a methylene group and an oxygen atom, R1 is selected from one of H, a methyl group, and a substituted or unsubstituted C1-C5 alkoxy group, and at least one of X and R1 contains oxygen.

[0012] In a specific embodiment of the present invention, the fluorobenzene diluent includes at least one of the structures shown in Formula II:

[0013]

[0014] Wherein, R2 is selected from one of substituted or unsubstituted C1-C5 alkyl groups; 0≤m≤6, 0≤n≤6, m+n≥1, and when m=0, R2 contains at least one fluorine atom.

[0015] In a specific embodiment of the present invention, the electrolyte further includes an additive, and the additive includes at least one of triethyl phosphate, ethoxy (pentafluoro) cyclotriphosphazene, hexafluorocyclotriphosphazene, pentafluoro (phenoxy) cyclotriphosphazene, tris (2,2,2-trifluoroethyl) phosphite and tris (trimethylsilane) phosphite.

[0016] In a specific embodiment of the present invention, the mass ratio of the solvent to the diluent is 1:(0.5-7).

[0017] In a specific embodiment of the present invention, the cyclic ether solvent includes at least one of cyclopentyl methyl ether, 2-methoxy-1,3-dioxolane, 2-ethoxy-1,3-dioxolane, 1,3-dioxolane and 2-(2-methoxyethoxy)-1,3-dioxolane.

[0018] In a specific embodiment of the present invention, the fluorobenzene diluent includes at least one of trifluoromethoxybenzene, fluorobenzene, 1,2-fluorobenzene, 1,3-fluorobenzene and 2,3-difluorophenyl ether.

[0019] In a specific embodiment of the present invention, in the electrolyte, the mass fraction of the cyclic ether solvent with a relative molecular mass ≤ 95 is greater than 9%.

[0020] In a specific embodiment of the present invention, in the electrolyte, the cyclic ether solvent includes a first solvent, a second solvent and a third solvent; the first solvent is a cyclic ether solvent with a relative molecular mass ≤ 95; the second solvent is a cyclic ether solvent with a relative molecular mass > 95, and the second solvent has a structure as shown in the following formula I1; the third solvent is a cyclic ether solvent with a relative molecular mass > 95, and the third solvent has a structure as shown in the following formula I2;

[0021]

[0022] In a specific embodiment of the present invention, the mass fractions of the first solvent, the second solvent and the third solvent are 10% to 19%, 5% to 15% and 5% to 15% respectively.

[0023] In a specific embodiment of the present invention, the diluent includes trifluoromethoxybenzene and 1,2-fluorobenzene. Further, the mass ratio of the trifluoromethoxybenzene to the 1,2-fluorobenzene is 1:(0.5-2).

[0024] In a specific embodiment of the present invention, in the electrolyte, the mass fraction of the lithium salt is 10% to 20%.

[0025] In a specific embodiment of the present invention, the lithium salt includes at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide salt, lithium bis(trifluoromethylsulfonyl)imide, lithium tetrafluorooxalatephosphate, lithium bis(oxalatodifluorophosphate) and lithium hexafluorophosphate.

[0026] In a specific embodiment of the present invention, the ion transfer number of the electrolyte is ≥0.3.

[0027] In a specific embodiment of the present invention, the ionic conductivity of the electrolyte is ≥3mS / cm.

[0028] Another aspect of the present invention provides a negative electrode-free lithium metal battery, comprising any one of the above-mentioned electrolytes.

[0029] In a specific embodiment of the present invention, the battery comprises a negative electrode current collector.

[0030] Another aspect of the present invention provides an electrical device comprising any one of the negative electrode-free lithium metal batteries described above.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The electrolyte of the present invention increases the entropy value of the electrolyte, improves the disorder of the electrolyte, promotes the formation of a weakly solvated electrolyte, helps to improve the stability of the electrode interface, and improves the ion migration number and ion conductivity of the electrolyte by mixing multiple cyclic ether solvents and multiple fluorobenzene diluents;

[0033] (2) The present invention further adjusts the number of components of the solvent and the diluent to increase the solubility and stability of the lithium salt, thereby providing sufficient lithium salt electrolyte for the negative electrode-free lithium metal battery during a long cycle;

[0034] (3) The negative electrode-free lithium metal battery using the electrolyte of the present invention has high coulombic efficiency, excellent cycle performance and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] Figure 1 The cycle life of the electrolytes of Examples 5, 7 and 8 of the present invention in a negative electrode-free lithium metal battery;

[0037] Figure 2 The cycle life of the electrolytes of Example 9 and Comparative Example 1 of the present invention in a negative electrode-free lithium metal battery;

[0038] Figure 3 It is the coulombic efficiency of the electrolyte of Example 9 of the present invention and Comparative Example 1 in the negative electrode-free lithium metal battery. DETAILED DESCRIPTION

[0039] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer is not specified in the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0040] With the development of electric vehicles and rechargeable mobile devices, the research on secondary batteries, as a representative of the new energy field, has also developed rapidly. Secondary batteries are small in size and weight, have high specific energy, and have broad application prospects.

[0041] The study demonstrates that solvation is crucial for the stability of liquid electrolytes in lithium metal batteries. Through molecular design and the use of highly fluorinated co-solvents, Li + Weakly solvated electrolytes with a solvation structure can achieve electrochemical stability at the interface between the high-voltage positive electrode and the lithium metal negative electrode, thereby extending the battery cycle life. However, such electrolytes often have problems such as low ionic conductivity, which limits the high-rate cycling capability of the battery.

[0042] Based on this, the present invention provides an electrolyte, including a lithium salt, a solvent and a diluent;

[0043] The solvent includes at least two of cyclic ether solvents; and the diluent includes at least two of fluorobenzene diluents.

[0044] In the electrolyte of the present invention, a cyclic ether solvent and a fluorobenzene diluent are used, and the solvation entropy and the solvation structure are adjusted by adjusting the component number of the solvent and the diluent, so as to improve the stability of the electrode interface and the sample conductivity of the electrolyte. At the same time, the boiling points of the cyclic ether solvent and the fluorobenzene diluent are relatively high, which further improves the stability of the electrolyte.

[0045] In some embodiments of the present invention, the cyclic ether solvent includes at least one of the structures shown in Formula I:

[0046]

[0047] Wherein, X is selected from one of a methylene group and an oxygen atom, R1 is selected from one of H, a methyl group, and a substituted or unsubstituted C1-C5 alkoxy group, and at least one of X and R1 contains oxygen.

[0048] In the present invention, "substitution" means that a hydrogen atom bonded to a carbon atom of a compound is replaced by another substituent. The substituted position is not limited as long as it is a position where a hydrogen atom can be substituted, that is, a position where a substituent can be substituted. When there are more than two substitutions, the two or more substituents may be the same or different from each other.

[0049] "Substituted or unsubstituted" means substituted by one or more substituents selected from a halogen group, a cyano group, an alkyl group, a cycloalkyl group, an alkoxy group and the like, or having no substituent.

[0050] In some embodiments of the present invention, the fluorobenzene diluent comprises at least one of the structures shown in Formula II:

[0051]

[0052] Wherein, R2 is selected from one of substituted or unsubstituted C1-C5 alkyl groups; 0≤m≤6, 0≤n≤6, m+n≥1, and when m=0, R2 contains at least one fluorine atom.

[0053] In the present invention, m and n represent the number of substitutions of the substituent -F and the substituent -OR2 in the structure shown in Formula II, respectively.

[0054] In some embodiments of the present invention, R2 is selected from substituted or unsubstituted methyl, ethyl, propyl, isopropyl, etc. The substituents in R2 include but are not limited to fluorine atoms.

[0055] In some embodiments of the present invention, the cyclic ether solvent includes at least one of cyclopentyl methyl ether, 2-methoxy-1,3-dioxolane, 2-ethoxy-1,3-dioxolane, 1,3-dioxolane and 2-(2-methoxyethoxy)-1,3-dioxolane. In the solvent of the present invention, the ether bond and the lithium ions in the lithium salt can form a stable solvation structure, thereby stabilizing the lithium negative electrode interface; at the same time, the formed SEI film has a low affinity for lithium ions, inhibiting dendrite growth, thereby improving the cycle stability of the negative electrode-free lithium metal battery. In addition, the cyclic structure in the solvent is more conducive to improving the ionic conductivity and ion migration number of the electrolyte, further improving the rate performance.

[0056] In some embodiments of the present invention, the fluorobenzene diluent includes at least one of trifluoromethoxybenzene, fluorobenzene, 1,2-fluorobenzene, 1,3-fluorobenzene and 2,3-difluorophenyl ether. The present invention uses a fluorobenzene diluent based on increasing the number of diluent components, on the one hand, utilizing its higher boiling point, and on the other hand, increasing the fluorination degree of the electrolyte, improving the compatibility of the electrolyte with the positive and negative electrodes, and thus improving the cycle performance of the negative electrode-free lithium metal battery.

[0057] In some embodiments of the present invention, the electrolyte further comprises an additive, and the additive comprises at least one of triethyl phosphate, ethoxy (pentafluoro) cyclotriphosphazene, hexafluoro cyclotriphosphazene, pentafluoro (phenoxy) cyclotriphosphazene, tris (2,2,2-trifluoroethyl) phosphite and tris (trimethylsilane) phosphite. In the electrolyte of the present invention, additives can be selectively added according to actual needs. The introduction of the above-mentioned additives can further improve the high temperature performance of the electrolyte.

[0058] In some embodiments of the present invention, the mass fraction of the additive in the electrolyte is 2% to 8%. Studies have found that when the amount of the additive in the electrolyte is within the above range, it is more helpful to improve the safety of the electrolyte while ensuring the ionic conductivity and ion migration number of the electrolyte. For example, in different embodiments, the mass fraction of the additive in the electrolyte can be 2%, 3%, 4%, 5%, 6%, 7%, 8% or a range consisting of any two of them.

[0059] In some embodiments of the present invention, the mass ratio of the solvent to the diluent is 1:(0.5-7). For example, in different embodiments, the mass ratio of the solvent to the diluent can be 1:0.5, 1:0.6, 1:0.8, 1:1, 1:2, 1:4, 1:5, 1:6, 1:7 or a range consisting of any two of them. The mass ratio of the solvent to the diluent within the above range is more conducive to improving the solubility of the lithium salt in the electrolyte, and taking into account the improvement of the ion migration number, ion conductivity and coulombic efficiency.

[0060] In some embodiments of the present invention, in the electrolyte, the mass fraction of the cyclic ether solvent with a relative molecular mass of ≤95 is greater than 9%. For example, in the electrolyte, the mass fraction of 1,3-dioxolane is greater than 9%. Here, the cyclic ether solvent with a relative molecular mass of ≤95 may include one cyclic ether solvent with a relative molecular mass of ≤95, or may be a compound of two or more cyclic ether solvents with a relative molecular mass of ≤95.

[0061] The study found that in the electrolyte system of the present invention, when the corresponding number of components of solvents and diluents are used, if the mass fraction of the cyclic ether solvent with a relative molecular mass of ≤95 contained in the electrolyte does not exceed 9%, the electrolyte cannot be dissolved. When the mass fraction of the cyclic ether solvent with a relative molecular mass of ≤95 in the electrolyte meets the above conditions, the solubility of the electrolyte can be guaranteed, while improving the performance such as ionic conductivity.

[0062] In some embodiments of the present invention, in the electrolyte, the cyclic ether solvent includes a first solvent, a second solvent and a third solvent; the first solvent is a cyclic ether solvent with a relative molecular mass ≤ 95; the second solvent is a cyclic ether solvent with a relative molecular mass > 95, and the second solvent has a structure as shown in the following formula I1; the third solvent is a cyclic ether solvent with a relative molecular mass > 95, and the third solvent has a structure as shown in the following formula I2;

[0063]

[0064] In some embodiments of the present invention, the mass fractions of the first solvent, the second solvent and the third solvent are 10% to 19%, 5% to 15% and 5% to 15%, respectively.

[0065] For example, when the first solvent includes 1,3-dioxolane and 1,4-dioxane, the second solvent includes cyclopentyl methyl ether and 2-methyl-1,4-dioxane, and the third solvent includes 2-methoxy-1,3-dioxolane, the mass fractions of 1,3-dioxolane, cyclopentyl methyl ether and 2-methoxy-1,3-dioxolane in the electrolyte are 10% to 19%, 5% to 15% and 5% to 15%, respectively.

[0066] For example, in different embodiments, in the electrolyte, the mass fraction of cyclopentyl methyl ether can be 5%, 8%, 10%, 12%, 15% or a range consisting of any two thereof; the mass fraction of 1,3-dioxolane can be 10%, 12%, 15%, 17%, 19% or a range consisting of any two thereof; the mass fraction of 2-methoxy-1,3-dioxolane can be 5%, 8%, 10%, 12%, 15% or a range consisting of any two thereof; in the electrolyte, the mass fraction of each solvent is within the above range, which can ensure the dissolution of the electrolyte while improving the ionic conductivity, ion migration number and coulombic efficiency of the negative electrode-free lithium metal battery containing it.

[0067] In some embodiments of the present invention, the diluent includes trifluoromethoxybenzene and 1,2-fluorobenzene.

[0068] By using the above two diluents in combination, without changing the solvation structure of the electrolyte, it is helpful to reduce the viscosity of the electrolyte, improve the wettability, etc., which helps to improve the ion conductivity and ion migration number, thereby improving the cycle performance of the negative electrode-free lithium metal battery containing it.

[0069] In some embodiments of the present invention, the mass ratio of trifluoromethoxybenzene to 1,2-fluorobenzene is 1:(0.5-2).

[0070] For example, in different embodiments, the mass ratio of trifluoromethoxybenzene to 1,2-fluorobenzene can be 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2 or a range consisting of any two of them.

[0071] In some embodiments of the present invention, the mass fraction of lithium salt in the electrolyte is 10% to 20%.

[0072] For example, in different embodiments, the mass fraction of lithium salt in the electrolyte can be 10%, 12%, 15%, 16%, 18%, 20% or any two of them. The present invention helps to increase the concentration of lithium salt in the electrolyte by optimizing the number and type of solvent and diluent components, and provides sufficient lithium salt electrolyte for the long cycle process of the negative electrode-free lithium metal battery; at the same time, the concentration of lithium salt is adjusted within the above range to improve the ion migration number, ion conductivity and coulomb efficiency of the negative electrode-free lithium metal battery containing it.

[0073] In some embodiments of the present invention, the lithium salt includes at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide salt, lithium bis(trifluoromethylsulfonyl)imide, lithium tetrafluorooxalatophosphate, lithium bis(oxalatodifluorophosphate) and lithium hexafluorophosphate.

[0074] For example, the lithium salt may include at least two types, such as lithium bis(fluorosulfonyl)imide salt and lithium bis(trifluoromethylsulfonyl)imide, but is not limited thereto.

[0075] In some embodiments of the present invention, the electrolyte includes the following components by mass percentage: 10% to 20% of lithium salt, 10% to 54% of solvent and 30% to 70% of diluent.

[0076] For example, in different embodiments, in the electrolyte, the amounts of each component, by mass percentage, may be as follows:

[0077] The mass fraction of the lithium salt may be 10%, 12%, 15%, 16%, 18%, 20% or a range consisting of any two thereof;

[0078] The mass fraction of the solvent may be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 54% or a range consisting of any two thereof;

[0079] The mass fraction of the diluent can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or a range consisting of any two thereof.

[0080] In some embodiments of the present invention, the ion transfer number of the electrolyte is ≥ 0.3.

[0081] In some embodiments of the present invention, the ionic conductivity of the electrolyte is ≥ 3 mS / cm.

[0082] The present invention significantly improves the ion migration number and ion conductivity of the electrolyte while improving the stability of the electrode interface by adjusting the number and type of solvent and diluent components in the electrolyte, so that the negative electrode-free lithium metal battery containing it has high coulomb efficiency, excellent cycle performance and rate performance. For example, in different embodiments, the ion migration number of the electrolyte can be 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4 or a range consisting of any two thereof; the ion conductivity of the electrolyte can be 3mS / cm, 3.01mS / cm, 3.1mS / cm, 3.2mS / cm, 3.3mS / cm, 3.4mS / cm, 3.5mS / cm, 3.6mS / cm, 3.7mS / cm, 3.8mS / cm, 3.9mS / cm, 4mS / cm or a range consisting of any two thereof. Among them, the ion migration number and ion conductivity correspond to the condition of 25℃.

[0083] The electrolyte of the present invention can be prepared by the following method: at room temperature, in a container, lithium salt, solvent and diluent are mixed and stirred uniformly to obtain the electrolyte.

[0084] Another aspect of the present invention provides a negative electrode-free lithium metal battery, comprising any one of the above electrolytes. The negative electrode-free lithium metal battery of the present invention has excellent cycle performance and rate performance.

[0085] In some embodiments of the invention, a battery includes a negative electrode current collector.

[0086] Among them, the negative electrode current collector may include conventional copper foil, stainless steel foil, titanium foil, etc., and a conventional modification layer may be set on the surface of the aforementioned negative electrode current collector. The function of the modification layer may include but is not limited to promoting the adsorption, nucleation and deposition of lithium ions.

[0087] Generally, a non-negative-electrode lithium metal battery includes a positive electrode sheet, a negative electrode current collector, an electrolyte, and a separator. During the first charge of a non-negative-electrode lithium metal battery, lithium ions are released from the positive electrode material and then directly deposited on the negative electrode current collector. The separator is set between the positive electrode sheet and the negative electrode current collector, mainly to prevent short circuits, while allowing ions to pass through.

[0088] In some embodiments of the present invention, the positive electrode plate includes a positive electrode current collector, and the positive electrode current collector can be a metal foil, a foam metal, or a composite current collector. For example, as the metal foil, stainless steel, copper, aluminum, nickel, etc. can be used. The composite current collector can include a polymer material base layer and a metal layer. The foam metal can be foam nickel, foam copper, foam aluminum, foam alloy, etc.

[0089] In some embodiments of the present invention, the positive electrode plate may further include a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, the specific type of the positive electrode active material is not limited, and active materials known in the art that can be used for battery positive electrodes can be used, and those skilled in the art can select according to actual needs.

[0090] In some embodiments of the present invention, the charge and discharge rate of the battery can be controlled between 0.1C and 3C.

[0091] The present invention has no particular limitation on the shape of the battery cell, which may be cylindrical, square or any other shape.

[0092] Another aspect of the present invention provides an electric device, comprising any one of the above-mentioned negative electrode-free lithium metal batteries. The battery can be used as a power source for the electric device, or as an energy storage unit for the electric device. Specifically, the electric device may include but is not limited to mobile devices (such as mobile phones, laptops), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, electric bicycles), ships and satellites, and energy storage systems.

[0093] Example 1

[0094] This embodiment provides an electrolyte, including 10% lithium salt, 20% solvent and 70% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide; the solvent is cyclopentyl methyl ether and 1,3-dioxolane in a mass ratio of 10%:10%; and the diluent is trifluoromethoxybenzene and 1,3-fluorobenzene in a mass ratio of 35%:35%.

[0095] The preparation method of the electrolyte comprises: fully stirring the lithium salt, the solvent and the diluent in proportion to obtain a highly dispersed and clear electrolyte.

[0096] Example 2

[0097] This embodiment provides an electrolyte, including 10% lithium salt, 20% solvent and 70% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 6%:4%; the solvent is cyclopentyl methyl ether and 1,3-dioxolane in a mass ratio of 10%:10%; and the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 35%:35%.

[0098] The preparation method of the electrolyte is the same as that of Example 1.

[0099] Example 3

[0100] This embodiment provides an electrolyte, including 10% lithium salt, 20% solvent and 70% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 6%:4%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 6%:7%:7%; the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 35%:35%.

[0101] The preparation method of the electrolyte is the same as that of Example 1.

[0102] Example 4

[0103] This embodiment provides an electrolyte, including 10% lithium salt, 20% solvent and 70% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 6%:4%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 6%:7%:7%; the diluent is trifluoromethoxybenzene, 2,3-difluorophenylethyl ether and 1,3-fluorobenzene in a mass ratio of 20%:25%:25%.

[0104] The preparation method of the electrolyte is the same as that of Example 1.

[0105] Example 5

[0106] This embodiment provides an electrolyte, including 16% lithium salt, 14% solvent and 70% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 9.6%:6.4%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 4.2%:4.9%:4.9%; the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 35%:35%.

[0107] The preparation method of the electrolyte is the same as that of Example 1.

[0108] Example 6

[0109] This embodiment provides an electrolyte, including 20% ​​lithium salt, 10% solvent and 70% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 12%:8%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 3%:3.5%:3.5%; the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 35%:35%.

[0110] The preparation method of the electrolyte is the same as that of Example 1.

[0111] Example 7

[0112] This embodiment provides an electrolyte, including 16% lithium salt, 34% solvent and 50% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 9.6%:6.4%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 10.2%:11.9%:11.9%; the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 25%:25%.

[0113] The preparation method of the electrolyte is the same as that of Example 1.

[0114] Example 8

[0115] This embodiment provides an electrolyte, including 16% lithium salt, 54% solvent and 30% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 9.6%:6.4%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 16.2%:18.9%:18.9%; the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 15%:15%.

[0116] The preparation method of the electrolyte is the same as that of Example 1.

[0117] Example 9

[0118] This embodiment provides an electrolyte, including 16% lithium salt, 30% solvent, 50% diluent and 4% flame retardant additive ethoxy (pentafluoro) cyclotriphosphazene, wherein the lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 9.6%:6.4%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 9%:10.5%:10.5%; the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 25%:25%.

[0119] The preparation method of the electrolyte is similar to that of Example 1, and the flame retardant additive is added and stirred.

[0120] Example 10

[0121] This embodiment provides an electrolyte and a preparation method thereof, with reference to Example 7, except that the solvent composition is different.

[0122] In the electrolyte of this embodiment, the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 5%:19%:10%.

[0123] Embodiment 11

[0124] This embodiment provides an electrolyte and a preparation method thereof, with reference to Example 7, except that the solvent composition is different.

[0125] In the electrolyte of this embodiment, the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 15%:9%:10%.

[0126] Example 12

[0127] This embodiment provides an electrolyte and a preparation method thereof, with reference to Example 7, except that the solvent composition is different.

[0128] In the electrolyte of this embodiment, the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 10%:9%:15%.

[0129] Embodiment 13

[0130] This embodiment provides an electrolyte, including 8% lithium salt, 66% solvent and 26% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 5%:3%; the solvent is cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane in a mass ratio of 20%:26%:20%; and the diluent is trifluoromethoxybenzene and 2,3-difluorophenyl ether in a mass ratio of 13%:13%.

[0131] The preparation method of the electrolyte is the same as that of Example 1.

[0132] Comparative Example 1

[0133] Comparative Example 1 provides an electrolyte solution, comprising 10% lithium salt, 20% solvent and 70% diluent by mass, wherein the lithium salt is lithium bis(fluorosulfonyl)imide, the solvent is cyclopentyl methyl ether and the diluent is 1,3-fluorobenzene.

[0134] The preparation method of the electrolyte is the same as that of Example 1.

[0135] Comparative Example 2

[0136] Comparative Example 2 provides an electrolyte solution, comprising 10% lithium salt, 20% solvent and 70% diluent by mass, wherein the lithium salt is lithium bis(fluorosulfonyl)imide; the solvent is cyclopentyl methyl ether; and the diluent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0137] The preparation method of the electrolyte is the same as that of Example 1.

[0138] Comparative Example 3

[0139] Comparative Example 3 provides an electrolyte, comprising 10% lithium salt, 20% solvent and 70% diluent by mass percentage. The lithium salt is lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethyl)sulfonylimide in a mass ratio of 6%:4%; the solvent is DME (ethylene glycol dimethyl ether) and DEE (ethylene glycol diethyl ether) in a mass ratio of 10%:10%; and the diluent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether in a mass ratio of 35%:35%.

[0140] The preparation method of the electrolyte is the same as that of Example 1.

[0141] Experimental example

[0142] (1) Ionic conductivity of the electrolyte

[0143] The room temperature ionic conductivity of the electrolytes prepared in different embodiments and comparative examples was tested respectively. The electrolytes were poured into beakers and tested for ionic conductivity using a DDS-308F conductivity meter. Each electrolyte was measured three times to obtain an average value. The test results are shown in Table 1.

[0144] (2) Ion migration number

[0145] The electrolytes, diaphragms, and working electrodes and counter electrodes prepared in different embodiments and comparative examples were respectively used to assemble CR2032 button batteries; the lithium metal electrode and diaphragm were assembled into a lithium-lithium symmetric battery in a glove box filled with argon protection, and the electrolyte of the embodiment or comparative example was dripped on both sides of the diaphragm to test the ion migration number; the test method of the ion migration number is: the assembled lithium-lithium symmetric battery is first measured for its electrochemical impedance spectrum (general conditions are 100kHz~0.1Hz, voltage disturbance 10mV), and then a DC polarization test is performed under the conditions of: voltage 10mV, time 300s, and then an AC impedance test is performed again after completion, and the lithium ion migration number is calculated by the following formula:

[0146] t + =I ss (ΔV-R0 I0) / I0(ΔV-R ss I ss )

[0147] Where R0 and R ss are the initial and steady-state interface impedances (obtained from the impedance spectrum by equivalent circuit fitting), I0 and I ss is the initial and steady-state current in the chronoamperometry, and ΔV is the potential difference in the chronoamperometry.

[0148] The test results are shown in Table 1.

[0149] (3) Coulomb efficiency

[0150] The copper foil, the diaphragm and the metallic lithium were assembled into a lithium copper half-cell in a glove box protected by argon, and the electrolytes prepared in different embodiments and comparative examples were dripped on both sides of the diaphragm. The assembled button cell was tested for coulombic efficiency, and the amount of deposited lithium metal was 7.422 mAh. The charge and discharge cycle was carried out at a rate of 3.33C, and the current density was 1.284 mAh / cm 2 ; The test results are shown in Table 1.

[0151] Table 1 Test results corresponding to different electrolytes

[0152]

[0153] Combined with Examples 1 to 4, it can be seen that at the same lithium salt concentration, the electrolyte obtained by using two lithium salts, three solvents and two diluents in Example 3 has the best corresponding ion conductivity, ion migration number and coulomb efficiency. Specifically, compared with Example 1, Example 2 increases the number of lithium salt components, Example 3 further increases the number of solvent components on the basis of Example 2, and Example 4 further increases the number of diluent components on the basis of Example 3. By comparing Example 1 with Examples 2 to 4, it can be seen that when the number of components of the electrolyte increases, the entropy value of the electrolyte increases, the disorder of the electrolyte is improved, the formation of a weakly solvated electrolyte is promoted, and it is helpful to improve the stability of the electrode interface, and the ion migration number and ion conductivity of the electrolyte are improved. By comparing Example 3 and Example 4, it can be seen that the appropriately increased number of components of the solvent and diluent is more conducive to increasing the solubility and stability of the electrolyte. If the number of components is too large, the stability of the electrolyte system becomes low.

[0154] In combination with Example 3 and Examples 5 to 6, it can be seen that on the basis of using the same lithium salt, solvent and diluent components, the concentration of the lithium salt is changed. The results show that when the mass fraction of the lithium salt is 16%, the ion migration number, ion conductivity and coulomb efficiency reach 0.36, 3.82 mS / cm and 99.76% respectively.

[0155] In combination with Example 5 and Examples 7 to 8, it can be seen that when the mass fraction of the lithium salt remains unchanged and the mass fraction of the diluent is 50% (ie, Example 7), the corresponding ion migration number, ion conductivity and coulomb efficiency are further improved, reaching 0.38, 3.90 mS / cm and 99.85%, respectively.

[0156] Combining Example 7 and Example 9, it can be seen that further introducing a certain amount of flame retardant additives into the system of Example 7 can further increase the safety of the electrolyte and basically will not cause a significant negative impact on the ion conductivity, ion migration number and coulomb efficiency.

[0157] Embodiments 10 to 12 regulate the proportion of multiple solvents used in the electrolyte on the basis of Embodiment 7. The study found that when the mass fraction of 1,3-dioxolane in the electrolyte is 9%, the electrolyte cannot dissolve whether the content of cyclopentyl methyl ether is increased or the content of 2-methoxy-1,3-dioxolane is increased; when the mass fraction of 1,3-dioxolane in the electrolyte is increased to 19%, when the content of cyclopentyl methyl ether is reduced to 5% or the content of 2-methoxy-1,3-dioxolane is reduced to 5%, various properties of the electrolyte decrease; it can be seen that in the electrolyte system of the present invention, the optimal mass fraction ranges of cyclopentyl methyl ether, 1,3-dioxolane and 2-methoxy-1,3-dioxolane are 5% to 15%, 10% to 19% and 5% to 15%, respectively.

[0158] Example 13 reduces the lithium salt concentration to below 10%, and it is found that the ionic conductivity and ion migration number of the electrolyte are significantly reduced, which not only reduces the high-rate charge and discharge performance of the battery, but also greatly shortens the cycle life of the lithium battery due to excessive consumption of lithium salt during long-term cycles.

[0159] To further demonstrate the effect of the embodiment, we first assembled the full battery of Embodiments 5, 7 and 8, using NCM811 for the positive electrode and copper foil for the negative electrode, assembled Ah-level soft-pack batteries, and tested the electrochemical performance of the soft-pack batteries at a rate of 0.2C charge and 1C discharge. The test results are shown in Figure 1 In the negative electrode-free lithium metal battery, when the electrolyte of Example 7 is used, when the capacity retention rate is 80%, the battery cycle life is still 250 cycles, and can support a discharge rate of 1C; however, when the diluent ratio is 30% or 70%, when the capacity retention rate is 80%, the cycle life is 130 cycles and 160 cycles respectively, so the cycle life effect of Example 7 is the best. This is because the above-mentioned Example 7 greatly improves the electrolyte ion conductivity and ion migration number, increases the ability of lithium ions, and further increases the cycle life of the battery.

[0160] On this basis, we used the electrolytes of Example 9 and Comparative Example 1 to assemble full batteries for testing. The positive electrode used NCM811 and the negative electrode used copper foil to assemble Ah-level soft-pack batteries, and the electrochemical performance of the soft-pack batteries was tested at a rate of 0.2C charge and 1C discharge. The test results are shown in Figure 2 and Figure 3 . As can be seen from the figure, in a negative electrode-free lithium metal battery, using the electrolyte of Example 9, when the capacity retention rate is 80%, the battery cycle life is increased from 100 cycles to 250 cycles, and can still support a discharge rate of 1C. It is further explained that the electrolyte of the present invention is more helpful in regulating the electrolyte solvation structure to promote the formation of more anion-derived uniform SEI, and on the other hand, it can improve the solubility of lithium salts and can fully provide lithium salt electrolytes in long cycles.

[0161] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrolyte, characterized in that: including lithium salts, solvents and diluents; The solvent includes at least two cyclic ether solvents; and the diluent includes at least two fluorobenzene diluents.

2. The electrolyte according to claim 1, characterized in that The cyclic ether solvent includes at least one of the structures shown in Formula I: Wherein, X is selected from one of a methylene group and an oxygen atom, R1 is selected from one of H, a methyl group, and a substituted or unsubstituted C1-C5 alkoxy group, and at least one of X and R1 contains oxygen; The fluorobenzene diluent includes at least one of the structures shown in Formula II: Wherein, R2 is selected from one of substituted or unsubstituted C1-C5 alkyl groups; 0≤m≤6, 0≤n≤6, m+n≥1, and when m=0, R2 contains at least one fluorine atom.

3. The electrolyte according to claim 1, characterized in that The mass ratio of the solvent to the diluent is 1:(0.5-7).

4. The electrolyte according to claim 1, characterized in that The cyclic ether solvent includes at least one of cyclopentyl methyl ether, 2-methoxy-1,3-dioxolane, 2-ethoxy-1,3-dioxolane, 1,3-dioxolane and 2-(2-methoxyethoxy)-1,3-dioxolane; The fluorobenzene diluent includes at least one of trifluoromethoxybenzene, fluorobenzene, 1,2-fluorobenzene, 1,3-fluorobenzene and 2,3-difluorophenyl ether.

5. The electrolyte according to claim 1, characterized in that In the electrolyte, the mass fraction of the cyclic ether solvent with a relative molecular mass of ≤95 is greater than 9%; Preferably, in the electrolyte, the cyclic ether solvent includes a first solvent, a second solvent and a third solvent; the first solvent is a cyclic ether solvent with a relative molecular mass ≤ 95; the second solvent is a cyclic ether solvent with a relative molecular mass > 95, and the second solvent has a structure as shown in the following formula I1; the third solvent is a cyclic ether solvent with a relative molecular mass > 95, and the third solvent has a structure as shown in the following formula I2; Preferably, the mass fractions of the first solvent, the second solvent and the third solvent are 10% to 19%, 5% to 15% and 5% to 15% respectively.

6. The electrolyte according to claim 1, characterized in that The diluent includes trifluoromethoxybenzene and 1,2-fluorobenzene; Preferably, the mass ratio of the trifluoromethoxybenzene to the 1,2-fluorobenzene is 1:(0.5-2).

7. The electrolyte according to claim 1, characterized in that In the electrolyte, the mass fraction of the lithium salt is 10% to 20%; Preferably, the lithium salt includes at least one of lithium dioxalatoborate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium tetrafluorooxalatophosphate, lithium dioxalatodifluorophosphate and lithium hexafluorophosphate; Preferably, the electrolyte further comprises an additive, wherein the additive comprises at least one of triethyl phosphate, ethoxy (pentafluoro) cyclotriphosphazene, hexafluorocyclotriphosphazene, pentafluoro (phenoxy) cyclotriphosphazene, tris (2,2,2-trifluoroethyl) phosphite and tris (trimethylsilane) phosphite.

8. The electrolyte according to claim 1, characterized in that Having at least one of the following characteristics: (1) The ion transfer number of the electrolyte is ≥ 0.3; (2) The ionic conductivity of the electrolyte is ≥3 mS / cm.

9. A negative electrode-free lithium metal battery, characterized in that: An electrolyte comprising the electrolyte according to any one of claims 1 to 8; Preferably, the battery comprises a negative electrode current collector.

10. An electrical device, characterized in that: Including the negative electrode-free lithium metal battery as described in claim 9.

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