Electrolyte and lithium-ion battery comprising same

Abstract

An electrolyte, which comprises: 20-80 wt% of an ether solvent, 5-20 wt% of a lithium salt, 5-70 wt% of an ionic liquid, 5-30 wt% of an ester solvent, and 0-8 wt% of an additive. The electrolyte system has good compatibility with a metal lithium negative electrode, and the voltage window is relatively high.

Description

Electrolyte and lithium ion battery containing the same Technical Field

[0001] The present invention relates to an electrolyte suitable for a lithium ion battery and a lithium ion battery containing the electrolyte. Background Art

[0002] As society develops, the use of lithium-ion batteries, a type of secondary battery, is increasing. Because the theoretical capacity of lithium metal is approximately ten times that of traditional graphite, using lithium metal directly as the negative electrode offers significant advantages in increasing energy density. Combining a lithium metal negative electrode with a high-voltage positive electrode could potentially yield a battery with an energy density exceeding 400 Wh / kg.

[0003] However, most commercial electrolytes currently use organic carbonate solvents, which have poor stability with lithium metal anodes. This makes the lithium metal anode easily react with the electrolyte, resulting in a large loss of active lithium and continuous consumption of the solvent in the electrolyte, which leads to accelerated decay of the battery cell's cycle capacity retention rate. Due to these reasons, lithium metal anode materials cannot be widely used and promoted. Therefore, it is particularly important to develop electrolyte systems with good compatibility with lithium metal anode materials.

[0004] Furthermore, research has found that while existing electrolytes based on ether solvents have good compatibility with metallic lithium anodes, they have a narrow electrochemical window and low oxidation resistance potential, making them suitable only for low-voltage systems with a maximum voltage below 4.0V and unable to be used with high-voltage cathode materials. This, to a certain extent, limits the improvement of the overall energy density of the battery cell.

[0005] Summary of the Invention

[0006] The present invention aims to construct an electrolyte system with good compatibility with metallic lithium anodes and a high voltage window (>4.2V). On this basis, the present invention provides an electrolyte suitable for lithium-ion batteries, and a lithium-ion battery using the electrolyte.

[0007] A first aspect of the present invention provides an electrolyte comprising:

[0008] 20-80 wt% of ether solvent,

[0009] 5-20wt% lithium salt,

[0010] 5-70 wt% ionic liquid,

[0011] 5 to 30 wt% of an ester solvent, and

[0012] 0 to 8 wt% of additives.

[0013] In some embodiments, the ether solvent is present in an amount of 40-60 wt %. In some embodiments, the lithium salt is present in an amount of 10-18 wt %. In some embodiments, the ionic liquid is present in an amount of 5-40 wt %. In some embodiments, the ester solvent is present in an amount of 10-25 wt %. In some embodiments, the additive is present in an amount of 1-5 wt %.

[0014] In some specific embodiments, the ether solvent is one or more selected from ethylene glycol dimethyl ether (DME), ethylene glycol methyl ethyl ether (EME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), ethylene glycol diethyl ether (DEE), diethylene glycol methyl ethyl ether (DGEME), 1,3-dioxolane (DOL), 1,3-dioxane (1,3-DX), 1,4-dioxane (1,4-DX), 2-methyl-1,4-dioxane (2-Me-1,4-DX), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-Me-THF), and tetrahydropyran (THP).

[0015] In some specific embodiments, the lithium salt is one or more selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorooxalatoborate (LiDFOB) and lithium bis(oxalatoborate) (LiBOB).

[0016] In some specific embodiments, the ionic liquid is one or more selected from 1-butyl-1-propylimidazolium bis(fluorosulfonyl)imide, 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide and 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide.

[0017] In some specific embodiments, the ester solvent is one or more selected from γ-butyrolactone (GBL), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl acetate (PA), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), (2-methoxyethyl) carbonate (BMEC), bisfluoroethylene carbonate (DFEC), fluorinated bis(2,2,2-trifluoroethyl) carbonate (TFEC) and 2,2,2-trifluoroethyl methyl carbonate (FEMC).

[0018] In some specific embodiments, the additive is one or more selected from vinylene carbonate (VC), diethylene sulfate (DTD), 1,3-propylene sultone (PST), lithium nitrate (LiNO3), lithium phosphate (Li3PO4) and lithium borate (Li3BO3).

[0019] In some embodiments, the ether solvent is ethylene glycol dimethyl ether. In some embodiments, the lithium salt is lithium bis(trifluoromethanesulfonyl)imide. In some embodiments, the ionic liquid is 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide. In some embodiments, the ester solvent is fluoroethylene carbonate. In some embodiments, the additive is LiNO3.

[0020] The second aspect of the present invention provides use of the electrolyte according to the first aspect of the present invention in a lithium ion battery.

[0021] A third aspect of the present invention provides a lithium-ion battery, comprising the electrolyte according to the first aspect of the present invention.

[0022] In some embodiments, the lithium-ion battery of the present invention includes metallic lithium as the negative electrode material.

[0023] In some specific embodiments, the positive electrode active material of the lithium ion battery of the present invention is one or more of nickel cobalt manganese, lithium iron phosphate, lithium cobaltate, lithium manganate, lithium nickelate and lithium nickel cobalt manganate, preferably a nickel cobalt manganese ternary positive electrode material, more preferably LiNi 0.8 Co 0.1 Mn 0.1 O2.

[0024] The electrolyte system of the present invention has good compatibility with the metal lithium negative electrode and a high voltage window (anti-oxidation potential) (>4.2V). Therefore, the lithium-ion battery including the electrolyte of the present invention can still maintain a high capacity retention rate after multiple cycles of charge and discharge at a higher voltage. Furthermore, according to some preferred embodiments of the present invention, by selecting preferred electrolyte components and the preferred content of each component, the compatibility of the electrolyte with the metal lithium negative electrode can be further improved, and at the same time, a voltage window of up to 4.5V or more can be obtained, so that the lithium-ion battery including the electrolyte of the present invention can still maintain a capacity retention rate of more than 80% after 100 cycles of charge and discharge at a charging voltage of 4.2V.

[0025] In addition, the present invention also provides a lithium-ion battery using metallic lithium as the negative electrode material and the electrolyte of the present invention. The lithium-ion battery has low cost, simple structure, and is suitable for large-scale production, while also achieving the above-mentioned technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1 shows a cycle capacity diagram of high-voltage lithium batteries according to embodiments 1-8 of the present invention.

[0027] FIG2 shows the ratio of the constant current charging capacity in the 15th cycle to the capacity of the total charging process (constant current charging ratio) during the cycling process of the high-voltage lithium batteries of Examples 1-8 of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below by way of specific embodiments. Unless otherwise specified, the terms used herein have the same meanings as those generally understood by those of ordinary skill in the art. The numerical limits or ranges set forth herein include endpoints, specifically all values ​​and subranges within the numerical limits or ranges.

[0029] Unless otherwise specified, the contents (%) in this application are all weight contents (wt%).

[0030] A first aspect of the present invention provides an electrolyte, comprising:

[0031] 20-80 wt% of ether solvent,

[0032] 5-20wt% lithium salt,

[0033] 5-70 wt% ionic liquid,

[0034] 5 to 30 wt% of an ester solvent, and

[0035] 0 to 8 wt% of additives.

[0036] In the present invention, the ether solvent can be one or more selected from ethylene glycol dimethyl ether (DME), ethylene glycol methyl ethyl ether (EME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), ethylene glycol diethyl ether (DEE), diethylene glycol methyl ethyl ether (DGEME), 1,3-dioxolane (DOL), 1,3-dioxane (1,3-DX), 1,4-dioxane (1,4-DX), 2-methyl-1,4-dioxane (2-Me-1,4-DX), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-Me-THF), and tetrahydropyran (THP). From the perspectives of solvent properties, solubility of lithium salts, further improving the ionic conductivity of the resulting electrolyte, and compatibility with metallic lithium negative electrodes, the ether solvent is preferably DME as a linear ether.

[0037] According to a preferred embodiment of the present invention, the content of the ether solvent is 40-60wt%, more preferably 45-55wt%. Specifically, the content of the ether solvent can be 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, 75wt% or 80wt%, etc.

[0038] In the present invention, the lithium salt can be one or more selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorooxalatoborate (LiDFOB), and lithium dioxalatoborate (LiBOB). Among these lithium salts, the fluoride ion in the anion of LiFSI has a strong electron-withdrawing effect, which makes the lithium in the lithium salt + The dissociation degree is high and the solubility in various electrolyte solvents is good, so that the ionic conductivity of the configured electrolyte is high. Therefore, the lithium salt is preferably LiFSI.

[0039] According to a preferred embodiment of the present invention, the content of the lithium salt is 10-18 wt%, preferably 12-17 wt%. Specifically, the content of the lithium salt can be 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt% or 20 wt%, etc.

[0040] In the present invention, the ionic liquid can be one or more selected from 1-butyl-1-propylimidazolium bis(fluorosulfonyl)imide, 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide. The ionic liquid of the present invention includes but is not limited to ionic liquid anions and cationic group similar substitutes. Examples of ionic liquid anions include but are not limited to Cl - Br - , I - 、BF4 - PF6 - 、N(CF3SO2)2 - and CF3SO3 -Ionic liquid cations include, but are not limited to, organic cations containing nitrogen, sulfur, or phosphorus, such as alkylammonium cations, imidazolium cations, pyridinium cations, and piperidinium cations. Compared to piperidinium ionic liquids, pyrrolyl ionic liquids have smaller cations, more moderate viscosities, and higher ionic conductivity; and compared to imidazolium ionic liquids, pyrrolyl ionic liquids have greater stability toward lithium metal. Therefore, in some embodiments, the ionic liquid is preferably 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide.

[0041] According to a preferred embodiment of the present invention, the content of the ionic liquid is 5 to 40 wt%, preferably 8 to 24 wt%, and more preferably 10 to 20 wt%. Specifically, the content of the ionic liquid can be 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt% or 70 wt%, etc.

[0042] In the present invention, the ester solvent can be selected from γ-butyrolactone (GBL), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl acetate (PA), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), (2-methoxyethyl) carbonate (BMEC), bisfluoroethylene carbonate (DFEC), fluorinated bis (2,2,2-trifluoroethyl) carbonate (TFEC) and 2,2,2-trifluoroethyl methyl carbonate (FEMC) One or more. Considering that FEC can be used as a solvent, it can also be used as a film-forming aid and participate in the construction of the solid electrolyte interface (SEI) of the lithium metal negative electrode to react and generate LiF, LiCO3, LiRO x , which helps to further isolate the electrolyte from the metallic lithium, reduce the side reaction between the electrolyte and the lithium negative electrode, thereby reducing electrolyte consumption and improving the cycle life. Therefore, the ester solvent is preferably FEC.

[0043] According to a preferred embodiment of the present invention, the content of the ester solvent is 10-25 wt%, more preferably 12-20 wt%. Specifically, the content of the ester solvent can be 3 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% or 30 wt%.

[0044] In the present invention, the additive can be one or more selected from vinylene carbonate (VC), diethylene sulfate (DTD), 1,3-propylene sultone (PST), lithium nitrate (LiNO3), lithium phosphate (Li3PO4), and lithium borate (Li3BO3). When used as an additive, lithium nitrate can provide lithium ions on the one hand, and on the other hand, nitrate has a high donor number (DN) and a strong interaction with lithium ions, pushing away solvent molecules and reducing the interaction between solvent molecules and Li + The coordination of NO3 reduces the probability of solvent molecules decomposing into membranes, increases the ion clusters in the primary solvation sheath, and thus forms anion-rich solvation sheaths, such as contact ion pairs (CIPs) and ion aggregates (AGGs); at the same time, NO3 - Decomposes into a film on the surface of the lithium negative electrode, and the generated LiN x O y / LiN x / LiO x It has high ionic conductivity, and the SEI formed has high interfacial energy and interfacial adhesion energy to Li metal, which promotes uniform lithium deposition and is conducive to maintaining stable circulation of lithium metal negative electrode. Therefore, LiNO3 is preferably used as the additive.

[0045] According to a preferred embodiment of the present invention, the content of the additive is 1-7 wt %, specifically, 0 wt %, 1 wt %, 2 wt %, 3 wt %, 4 wt %, 5 wt %, 6 wt %, 7 wt % or 8 wt %.

[0046] The second aspect of the present invention provides use of the electrolyte according to the first aspect of the present invention in a lithium ion battery.

[0047] A third aspect of the present invention provides a lithium-ion battery, comprising the electrolyte according to the first aspect of the present invention.

[0048] In some specific embodiments, the lithium ion battery of the present invention can be prepared by the following steps:

[0049] (1) preparing a positive electrode sheet;

[0050] (2) preparing an electrolyte; and

[0051] (3) Assemble the battery.

[0052] Step (1) and step (3) can be carried out according to the method commonly used in the art for preparing positive electrode sheets and assembling batteries. In some specific embodiments, in the step of preparing the positive electrode sheet, the positive electrode active material, the conductive agent, and the binder are mixed in a predetermined mass ratio and dispersed in an organic solvent to form a positive electrode slurry. The positive electrode slurry is then scraped onto a metal foil, dried, and further vacuum-dried. The positive electrode sheet is then made by rolling and slicing. The positive electrode active material can be any one of nickel cobalt manganese, lithium iron phosphate, lithium cobaltate, lithium manganate, lithium nickelate, and nickel cobalt manganese oxide, preferably a nickel cobalt manganese ternary positive electrode material, more preferably LiNi 0.8 Co 0.1 Mn 0.1 The conductive agent is preferably super P, the binder is preferably PVDF, and the metal foil is preferably aluminum foil.

[0053] In step (2), the electrolyte solution can be obtained by mixing the various components of the electrolyte solution of the first aspect of the present invention in a predetermined ratio and then stirring until the lithium salt is completely dissolved.

[0054] In some embodiments, the button cell positive and negative electrode shells, springs, and gaskets used in assembling the lithium battery are commercially available CR2032, and the metal lithium sheet used as the negative electrode material is a metal lithium sheet with a thickness of 10-500 μm. From the perspective of cost and energy density, 10-50 μm is preferred.

[0055] Example

[0056] The present invention is described in detail below by way of examples, which are not intended to limit the present invention. The experimental methods in the following examples are conventional methods unless otherwise specified. The reagents and devices used in the following examples are all commercially available and are not particularly limited.

[0057] Example 1 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0058] (1) Preparation of positive electrode sheet

[0059] The ternary cathode material (LiNi 0.8 Co 0.1 Mn 0.1 O2), conductive agent Super P, and binder PVDF are mixed in a mass ratio of 8:1:1, dispersed in an organic solvent NMP (N-methylpyrrolidone), and stirred until stable and uniform to form a positive electrode slurry. The positive electrode slurry is scraped onto an aluminum foil with a thickness of 10 μm, dried at 80°C, and then heated to 120°C for further vacuum drying. The positive electrode sheet is then made by rolling and slicing.

[0060] (2) Preparation of electrolyte

[0061] 51.5 wt% of ether solvent DME, 15 wt% of lithium salt LiTFSI, 16 wt% of ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide, 16 wt% of ester solvent FEC and 1.5 wt% of additive LiNO3 were mixed and stirred using a magnetic stirrer until the lithium salt was completely dissolved to obtain an electrolyte.

[0062] (3) Lithium battery preparation

[0063] Under the inert atmosphere of the glove box, the negative electrode shell, spring sheet, gasket, lithium sheet, electrolyte, diaphragm, positive electrode sheet, and positive electrode shell were assembled in sequence, and the button battery was assembled by pressing at 800kPa for 5s.

[0064] Table 1 shows the basic compositions of the electrolytes used in Examples 1 to 8 and Comparative Example 1.

[0065] Example 2 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0066] The steps for preparing the positive electrode sheet and the lithium battery in Example 2 are the same as those in Example 1.

[0067] As shown in Table 1, unlike Example 1, in this example, the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide was reduced to 8 wt %, and the proportion of the ether solvent, a 1:1 (volume ratio) mixture of DME and DOL, was increased to 59.5 wt %. The mass fractions of the remaining components remained unchanged.

[0068] Example 3 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0069] The steps for preparing the positive electrode sheet and the lithium battery in Example 3 are the same as those in Example 1.

[0070] As shown in Table 1, unlike Example 1, the proportion of the ester solvent FEC in this example was reduced to 8 wt %, and the proportion of the ether solvent, a 1:1 (volume ratio) mixed solvent of DME and DOL, was increased to 59.5 wt %. The mass fractions of the other components remained unchanged.

[0071] Example 4 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0072] The steps for preparing the positive electrode sheet and the lithium battery in Example 4 are the same as those in Example 1.

[0073] As shown in Table 1, unlike Example 1, the ionic liquid in this example is 1-butyl-1-propylimidazolium bis(fluorosulfonyl)imide, with the proportion increased to 24 wt %, and the proportion of the ether solvent DME is reduced to 43.5 wt %. The mass fractions of the other components remain unchanged.

[0074] Example 5 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0075] The steps for preparing the positive electrode sheet and the lithium battery in Example 5 are the same as those in Example 1.

[0076] As shown in Table 1, unlike Example 1, the ester solvent in this example is a 1:1 (volume ratio) mixed solvent of EC and FEC, with the ratio increased to 25 wt %, and the ratio of the ether solvent DME is reduced by 42.5 wt %. The mass fractions of the other components remain unchanged.

[0077] Example 6 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0078] The steps for preparing the positive electrode sheet and the lithium battery in Example 6 are the same as those in Example 1.

[0079] As shown in Table 1, the difference from Example 1 is that the proportion of the additive LiNO3 in this example is increased to 2.5 wt%, and the proportion of the ether solvent DME is reduced to 50.5 wt%. The mass fractions of the other components remain unchanged.

[0080] Example 7 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0081] The steps for preparing the positive electrode sheet and the lithium battery in Example 7 are the same as those in Example 1.

[0082] As shown in Table 1, unlike Example 1, the lithium salt in this embodiment uses lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in the same proportion, the proportion of the additive LiNO3 is increased to 4.0 wt%, and the proportion of the ether solvent DME is reduced to 49 wt%. The mass fractions of the remaining components remain unchanged.

[0083] Example 8 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0084] The steps for preparing the positive electrode sheet and the lithium battery in Example 8 are the same as those in Example 1.

[0085] As shown in Table 1, unlike Example 1, the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide and the proportion of the ether solvent DME were reduced to 8 wt % and 47 wt % respectively, while the mass fractions of the other components remained unchanged.

[0086] Example 9 Preparation of a lithium-ion battery using the electrolyte of the present invention

[0087] The steps for preparing the positive electrode sheet and the lithium battery in Example 9 are the same as those in Example 1.

[0088] As shown in Table 1, unlike Example 1, in this example, the proportion of the lithium salt LiTFSI was reduced to 5 wt %, the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide was increased to 50 wt %, the proportion of the ester solvent FEC was increased to 20 wt %, and the proportion of the ether solvent DME was reduced to 23.5 wt %. The mass fractions of the remaining components remained unchanged.

[0089] Comparative Example 1 Preparation of a lithium-ion battery using an electrolyte of the prior art

[0090] The steps of preparing the positive electrode sheet and the lithium battery in Comparative Example 1 are the same as those in Example 1.

[0091] As shown in Table 1, in Comparative Example 1, a lithium battery was prepared using an electrolyte solution having a ratio of ethylene carbonate (EC) to dimethyl carbonate (DMC) of 3:7 and containing 1 mol / L LiPF6 and 2 wt% FEC.

[0092] Comparative Example 2 Preparation of a lithium-ion battery using an electrolyte of the prior art

[0093] The steps for preparing the positive electrode sheet and the lithium battery in Comparative Example 2 are the same as those in Example 1.

[0094] As shown in Table 1, in Comparative Example 2, a lithium battery was prepared using an electrolyte containing 1 mol / L LiFSI and 2 wt% LiNO 3 in a ratio of 1,3-dioxolane (DOL) to 1,2-dimethoxyethane (DME) of 3:7.

[0095] Comparative Example 3 Preparation of a lithium-ion battery using an electrolyte with an excessively low ionic liquid ratio

[0096] The steps for preparing the positive electrode sheet and the lithium battery in Comparative Example 3 are the same as those in Example 1.

[0097] As shown in Table 1, unlike Example 1, the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide in this example was reduced to 2 wt %, and the proportion of the ether solvent DME was increased to 65.5 wt %. The mass fractions of the other components remained unchanged.

[0098] Comparative Example 4 Preparation of a lithium-ion battery using an electrolyte with an excessively low ionic liquid ratio

[0099] The steps for preparing the positive electrode sheet and the lithium battery in Comparative Example 4 are the same as those in Example 1.

[0100] As shown in Table 1, unlike Example 1, this example reduced the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide to 2 wt %, increased the proportion of the ether solvent DME to 67.0 wt %, and did not use any additives. The mass fractions of the remaining components remained unchanged.

[0101] Comparative Example 5 Preparation of a lithium-ion battery using an electrolyte with an excessively low ionic liquid ratio

[0102] The steps for preparing the positive electrode sheet and the lithium battery in Comparative Example 5 are the same as those in Example 1.

[0103] As shown in Table 1, unlike Example 1, in this example, the proportion of the lithium salt LiTFSI was increased to 20 wt %, the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide was decreased to 2 wt %, the proportion of the ester solvent FEC was decreased to 8 wt %, and the proportion of the ether solvent DME was increased to 68.5 wt %. The mass fractions of the remaining components remained unchanged.

[0104] Comparative Example 6 Preparation of a lithium-ion battery using an electrolyte with an excessively low ionic liquid ratio

[0105] The steps for preparing the positive electrode sheet and the lithium battery in Comparative Example 6 are the same as those in Example 1.

[0106] As shown in Table 1, unlike Example 1, in this example, the proportion of the lithium salt LiTFSI was reduced to 12 wt %, the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide was reduced to 2 wt %, the proportion of the ester solvent FEC was increased to 30 wt %, and the proportion of the ether solvent DME was increased to 54.5 wt %. The mass fractions of the remaining components remained unchanged.

[0107] Comparative Example 7 Preparation of a lithium-ion battery using an electrolyte containing an excessively high proportion of ester solvents

[0108] The steps for preparing the positive electrode sheet and the lithium battery in Comparative Example 7 are the same as those in Example 1.

[0109] As shown in Table 1, unlike Example 1, in this example, the proportion of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide was reduced to 8 wt %, the proportion of the ester solvent FEC was increased to 40 wt %, and the proportion of the ether solvent DME was reduced to 23.5 wt %. The mass fractions of the remaining components remained unchanged.

[0110] Table 1

[0111] Performance testing:

[0112] The electrolytes and the lithium-ion batteries prepared using the above-mentioned Examples 1 to 9 and Comparative Examples 1 to 7 were subjected to performance tests, and the test process and method are as follows:

[0113] (1) Oxidation potential test

[0114] Linear sweep voltammetry was used to test the high-voltage resistance of the electrolyte. Under an argon atmosphere, a coin cell was constructed using the electrolyte under test, a stainless steel sheet, and a lithium sheet (the stainless steel sheet served as the working electrode, and the lithium sheet served as the reference and counter electrodes). The electrochemical window was measured on an electrochemical workstation (CHI760D) over a 2.5-5V range at a scan rate of 1mV / s.

[0115] (2) Normal temperature cycle performance test

[0116] A lithium battery using the sample electrolyte was charged at 0.33C constant current and constant voltage to 4.2V at 25°C, and then discharged at 0.5C constant current to 3.0V. This cycle was considered one cycle. Coulombic efficiency was calculated as the ratio of discharge capacity to charge capacity during the same cycle.

[0117] Table 2 shows the antioxidant potentials (in V) of the electrolytes of Examples 1-9 and Comparative Examples 1-7. As shown in Table 2, the electrolytes of the present invention exhibit excellent high-voltage resistance and are suitable for use in high-voltage lithium batteries. As shown in Examples 4-9, when appropriate components and contents are selected, the antioxidant potential of the electrolytes can reach above 4.4 V.

[0118] In contrast, when the ionic liquid content is less than 8 wt% (Comparative Examples 3-5), the voltage window decreases to a certain extent (all below 4.2 V). Therefore, when charging to 4.2 V, the ether solvent in the electrolyte is easily oxidized and decomposed under high voltage, and the battery will be overcharged.

[0119] In addition, in Comparative Examples 6 and 7, the antioxidant potential can reach above 4.20 V due to the increase in the ester solvent content. However, on the other hand, the assembled cells in Comparative Examples 6 and 7 have too high a proportion of ester electrolyte, and the excessively high proportion of solvent molecules affects the Li + Solvation structure, making Li + The reduced surrounding anion content and increased FEC coordination lead to the formation of an SEI on the anode surface, primarily composed of decomposed FEC solvent molecules. This solvation structure results in poor compatibility between the electrolyte and the lithium anode. Furthermore, the decomposition of FEC into a film leads to increased gas production during cycling, causing cell swelling and potentially compromising cell safety.

[0120] Table 2

[0121] Figure 1 shows the cycle capacity diagram of the high-voltage lithium battery using Examples 1 to 8 and Comparative Examples 1 to 2. As shown in Figure 1, since the electrolytes of Examples 1 to 8 have high compatibility with the metal lithium negative electrode material and a high antioxidant potential, the lithium-ion battery including the electrolyte of the present invention can still have a capacity retention rate of more than 70% after 60 charge and discharge cycles. In contrast, the capacity of the lithium-ion battery using the electrolyte of Comparative Example 1 began to decline significantly after 20 cycles, and the capacity decayed to below 70% after about 40 cycles. Analysis shows that the reason for this phenomenon may be that the electrolyte is incompatible with the lithium metal negative electrode material and fails to form a stable SEI film. The lithium-ion battery using the electrolyte of Comparative Example 2 has a narrow electrochemical stability window, and the ether solvent is easily oxidized and decomposed during the cycle, the electrolyte is consumed faster, and the capacity decay is accelerated.

[0122] On the other hand, as shown in FIG1 , by selecting appropriate electrolyte components and contents (Examples 6 to 8), the capacity of the lithium-ion battery using the electrolyte of the present invention can be maintained at above 80% after up to 100 charge and discharge cycles.

[0123] Figure 2 shows the constant current charge ratio at cycle 15 of the high-voltage lithium batteries of Examples 1-8 and Comparative Example 1, and the corresponding specific values ​​are listed in Table 3. Analysis of Table 2 shows that the mixture of ionic liquids and a large proportion of ester solvents improves the overall voltage window of the electrolyte, with the voltage window being >4.2V.

[0124] Table 3

[0125] On the other hand, when the content of the ionic liquid increases within the content range of the present invention, the actual use window of the electrolyte can be widened, and the stability and antioxidant properties of the electrolyte can be improved. When the ionic liquid content is too low, its ionic conductivity increases, the system impedance decreases, and the constant current charge ratio increases, but the system stability weakens. When the ionic liquid content is too high, the system impedance increases significantly and the constant current charge ratio decreases. When the content of the ionic liquid is outside the content range of the present invention, there is an overall poor effect. When the ionic liquid content is too low, the system ionic conductivity increases, its impedance decreases, the constant current charge ratio increases, but the system stability weakens; and when the ionic liquid content is too high, the electrolyte viscosity will be very large, the system impedance increases significantly, the constant current charge ratio decreases, and the infiltration of the battery cell diaphragm and the positive electrode gap is very poor, and the formation, capacity calibration and other processes in the battery cell production cannot be completed. The resulting electrolyte cannot meet the normal circulation of the battery cell.

[0126] On the other hand, ester solvents and ether solvents also reduce the viscosity of the electrolyte to a certain extent and improve the ionic conductivity; secondly, ester solvents participate in the negative electrode SEI film formation process to generate LiF, Li(COOR) xThe main function of ether solvents is to dissolve LiNO3; therefore, when the content of ester solvents is too low, the film formation is insufficient, which affects the battery life; when the content of ester solvents is too high, the content of ether solvents will inevitably decrease, resulting in a decrease in the solubility of the additive LiNO3 or even precipitation; the additives, especially LiNO3, have reduction products such as LiN and LiNO3 with high conductivity and good stability. x Etc., play a key role in the formation of the negative electrode SEI film. When the additive content is reduced, the film formation is insufficient, which affects the battery life. When the LiNO3 content is too much, the viscosity of the electrolyte increases significantly, the system impedance increases, the constant current charge ratio of the battery cell decreases, and the cycle effect gradually decreases.

[0127] The above examples are used to describe exemplary embodiments of the present invention, but the present invention is not limited thereto. It should be understood by those skilled in the art that the above examples are for illustrative purposes only, and the specific embodiments and examples of the present invention should not be construed as limiting the scope of the present invention. The embodiments can be changed and modified within the scope of the present invention, and such changes and modifications should fall within the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that The electrolyte includes: 20-80 wt% of ether solvent, 5-20wt% lithium salt, 5-70 wt% ionic liquid, 5 to 30 wt% of an ester solvent, and 0 to 8 wt% of additives.

2. The electrolyte according to claim 1, characterized in that The content of the ether solvent is 40-60 wt%, and / or the content of the lithium salt is 10-18 wt%, and / or the content of the ionic liquid is 5-40 wt%, and / or the content of the ester solvent is 10-25 wt%, and / or the content of the additive is 1-5 wt%.

3. The electrolyte according to claim 1, characterized in that The ether solvent is one or more selected from ethylene glycol dimethyl ether (DME), ethylene glycol methyl ethyl ether (EME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), ethylene glycol diethyl ether (DEE), diethylene glycol methyl ethyl ether (DGEME), 1,3-dioxolane (DOL), 1,3-dioxane (1,3-DX), 1,4-dioxane (1,4-DX), 2-methyl-1,4-dioxane (2-Me-1,4-DX), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-Me-THF), and tetrahydropyran (THP).

4. The electrolyte according to claim 1, characterized in that The lithium salt is one or more selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorooxalatoborate (LiDFOB) and lithium bis(oxalatoborate) (LiBOB).

5. The electrolyte according to claim 1, characterized in that The ionic liquid is one or more selected from 1-butyl-1-propylimidazolium bis(fluorosulfonyl)imide, 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide and 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide.

6. The electrolyte according to claim 1, characterized in that The ester solvent is one or more selected from γ-butyrolactone (GBL), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl acetate (PA), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), (2-methoxyethyl) carbonate (BMEC), bisfluoroethylene carbonate (DFEC), fluorinated bis(2,2,2-trifluoroethyl) carbonate (TFEC) and 2,2,2-trifluoroethyl methyl carbonate (FEMC).

7. The electrolyte according to claim 1, characterized in that The additive is one or more selected from vinylene carbonate (VC), diethylene sulfate (DTD), 1,3-propylene sultone (PST), lithium nitrate (LiNO3), lithium phosphate (Li3PO4) and lithium borate (Li3BO3).

8. The electrolyte according to any one of claims 1 to 7, characterized in that The ether solvent is ethylene glycol dimethyl ether, and / or the lithium salt is lithium bis(trifluoromethanesulfonyl)imide, and / or the ionic liquid is 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide, and / or the ester solvent is fluoroethylene carbonate, and / or the additive is LiNO3.

9. Use of the electrolyte according to any one of claims 1 to 8 in the preparation of lithium-ion batteries.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the electrolyte according to any one of claims 1 to 8.

11. The lithium-ion battery according to claim 10, wherein: The lithium-ion battery includes metallic lithium as the negative electrode material.

12. The lithium-ion battery according to claim 10 or 11, characterized in that The positive electrode active material of the lithium-ion battery is one or more of nickel cobalt manganese, lithium iron phosphate, lithium cobaltate, lithium manganese oxide, lithium nickelate and lithium nickel cobalt manganese oxide.

13. The lithium-ion battery according to claim 12, characterized in that The positive electrode active material of the lithium ion battery is a nickel-cobalt-manganese ternary positive electrode material.

14. The lithium-ion battery according to claim 13, wherein: The positive electrode active material of the lithium ion battery is LiNi 0.8 Co 0.1 Mn 0.1 O2.

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