Electrolyte additive and preparation method thereof, electrolyte and lithium ion battery

Abstract

The present invention provides an electrolyte additive and a preparation method thereof, an electrolyte and a lithium ion battery. The S atoms in the structure of the electrolyte additive of the present invention have low electronegativity and low binding energy with lithium ions, which can reduce the desolvation energy of lithium ions and significantly improve the rate performance and cycle life of the battery at low temperatures. Moreover, the O atoms in its structure can significantly improve the solubility of the additive to compensate for the defect of poor solubility caused by the S atoms. In addition, the additive contains unsaturated bonds, which can form a dense SEI film containing S, which is beneficial to the migration of lithium ions at the electrode / electrolyte interface at low temperatures, and the SEI film containing S can effectively inhibit the generation of lithium dendrites at low temperatures, further improving the rate performance and cycle life of the battery at low temperatures.

Description

Technical Field

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

[0002] Since the beginning of the 21st century, lithium-ion batteries have developed rapidly, and lithium batteries with higher energy density have become the future trend. While pursuing lithium-ion batteries with higher energy density, the requirements for safety performance have become increasingly higher.

[0003] Lithium-ion batteries are composed of a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte is the "blood" of the battery and an important carrier for transporting lithium ions inside the battery. During the charge and discharge process of the battery, the lithium-ion electrolyte acts as a medium to achieve reversible shuttling between the positive and negative electrodes. However, close contact between the electrodes (especially the positive electrode) and the electrolyte will trigger a series of side reactions, which is one of the important reasons for the battery capacity decay and structural degradation, thereby affecting the actual performance of lithium-ion batteries. In particular, commonly used electrolytes are easily oxidized and decomposed under high voltage / high temperature, making the formed solid electrolyte interface (SEI) film unstable. By adding functional film-forming additives to the electrolyte, a stable positive electrode-electrolyte interface (CEI) film and SEI film can be constructed between the positive / negative electrode and the electrolyte, which can effectively improve the electrochemical performance of lithium-ion batteries.

[0004] Currently, the mainstream positive electrode film-forming additives used in lithium-ion electrolytes can be divided into four types, including inorganic solid additives, electrochemical oxidation polymerization additives, phosphate additives and fluorinated organic additives. Inorganic solid additives have poor solubility in the electrolyte, which will have a negative impact on the conductivity of the electrolyte and cause uneven dispersion in practical applications; electrochemical oxidation polymerization additives are prone to self-discharge. If the addition amount is too small, the electrolyte and the positive electrode cannot be effectively isolated. If the addition amount is too large, the impedance will be too large; phosphate additives and fluorinated organic additives have good application prospects because they can form a relatively stable CEI film, which isolates the positive electrode material and the electrolyte. However, the existing phosphate additives and fluorinated organic additives have single functions and cannot well take into account the improvement of the cycle performance of lithium-ion batteries. Therefore, there is an urgent need to develop a universal multifunctional film-forming additive. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide an electrolyte additive and a preparation method thereof, an electrolyte and a lithium ion battery. The S atoms in the structure of the electrolyte additive of the present invention have a small electronegativity and a small binding energy with lithium ions, which can reduce the desolvation energy of lithium ions and significantly improve the rate performance and cycle life of the battery at low temperatures. And the O atoms in its structure can significantly increase the solubility of the additive to make up for the defect of poor solubility caused by the S atoms. In addition, the additive contains unsaturated bonds, which can form a dense SEI film containing S, which is beneficial to the migration of lithium ions at the electrode / electrolyte interface at low temperatures, and the SEI film containing S can effectively inhibit the generation of lithium dendrites at low temperatures, further improving the rate performance and cycle life of the battery at low temperatures.

[0006] To achieve this object, the present invention adopts the following technical solutions:

[0007] The present invention provides an electrolyte comprising a lithium salt, a solvent and a first electrolyte additive of formula (1);

[0008]

[0009] The solvent is a mixed solvent of two or more carbonate solvents or ether solvents.

[0010] The solvent is selected from any one or more of dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, propyl propionate, and ethylene glycol dimethyl ether.

[0011] Preferably, the solvent includes tetrahydrofuran, tetrahydropyran and tetrahydrothiophene;

[0012] The mass ratio of tetrahydrofuran, tetrahydropyran and tetrahydrothiophene is (20-60):(10-40):(10-40).

[0013] The first electrolyte additive is independently selected from any one of the electrolyte additives described above or the electrolyte additive prepared according to any one of the preparation methods described above.

[0014] Preferably, the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium perchlorate, lithium nitrate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), lithium difluorooxalatoborate or lithium difluorobis(oxalatophosphate).

[0015] Preferably, the electrolyte further comprises a second electrolyte additive;

[0016] The second electrolyte additive is selected from vinylene carbonate, vinyl ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinyl sulfate, vinyl bissulfate, propylene sulfate, 1,3-propane sultone, 1,3-propylene sultone, 1,4-butane sultone, 2,4-butane sultone, phenyl methanesulfonate, methylene methanedisulfonate, N-phenylbis(trifluoromethanesulfonyl)imide, triallyl phosphate, tripropargyl phosphate, tris(trifluoromethanesulfonyl)imide ... Any one or more of (trimethylsilyl) phosphate, trimethyl phosphite, ethyl diprop-2-ynyl phosphate, tetramethylmethylene diphosphate, (2-allylphenoxy) trimethylsilane, 1,2-bis(cyanoethoxy)ethane, tris(trimethylsilyl) borate, 1,3,5-triallyl isocyanurate, isocyanoethyl methacrylate, hexamethylene diisocyanate, p-phenylene diisocyanate or 2,4-toluene diisocyanate.

[0017] Preferably, the solvent accounts for 72-91% by mass of the electrolyte, the sum of the first electrolyte additive and the second electrolyte additive accounts for 0.1-2% by mass of the electrolyte, and the lithium salt accounts for 8-25% by mass of the electrolyte.

[0018] In a fourth aspect, the present invention provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte; the electrolyte is the electrolyte described in any one of the above technical solutions.

[0019] Preferably, the active material of the positive electrode is selected from any one of lithium cobalt oxide, ternary nickel cobalt manganese lithium, lithium nickel manganese oxide, lithium iron phosphate or lithium manganese iron phosphate;

[0020] The active material of the negative electrode is selected from any one of artificial graphite, natural graphite, lithium titanate, metallic lithium, silicon-carbon composite material or silicon oxide;

[0021] The diaphragm is selected from a polypropylene diaphragm or a polyethylene diaphragm.

[0022] Preferably, the charge and discharge voltage of the lithium-ion battery is 2.5 to 3.65V.

[0023] The S atoms in the electrolyte additive structure of the present invention have low electronegativity and low binding energy with lithium ions, which can reduce the desolvation energy of lithium ions and significantly improve the rate performance and cycle life of the battery at low temperatures. The O atoms in its structure can significantly increase the solubility of the additive to compensate for the poor solubility caused by the S atoms. In addition, the additive contains unsaturated bonds, which can form a dense SEI film containing S, which is beneficial to the migration of lithium ions at the electrode / electrolyte interface at low temperatures. The SEI film containing S can effectively inhibit the generation of lithium dendrites at low temperatures, further improving the rate performance and cycle life of the battery at low temperatures. DETAILED DESCRIPTION

[0024] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0025] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items.

[0026] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0027] Furthermore, the terms “first,” “second,” etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance.

[0028] The present invention also provides an electrolyte comprising a lithium salt, a solvent and a first electrolyte additive of formula (1);

[0029]

[0030] The lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium perchlorate, lithium nitrate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), lithium difluorooxalatoborate or lithium difluorobis(oxalatophosphate).

[0031] Preferably, the lithium salt is selected from any two or more of lithium hexafluorophosphate, lithium perchlorate, lithium nitrate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl imide), lithium bis(fluorosulfonyl imide), lithium bis(oxalatoborate), lithium difluorooxalatoborate or lithium difluorobis(oxalatophosphate); more preferably, lithium hexafluorophosphate and lithium bis(fluorosulfonyl imide).

[0032] In an embodiment of the present invention, lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide may be mixed in a weight percentage of 3.0% and 8.0%; lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide may be mixed in a weight percentage of 5.0% and 5.0%; lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide may be mixed in a weight percentage of 3.0% and 15.0%.

[0033] The electrolyte further includes a second electrolyte additive; the second electrolyte additive is selected from vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, vinyl bissulfate, propylene sulfate, 1,3-propane sultone, 1,3-propylene sultone, 1,4-butane sultone, 2,4-butane sultone, phenyl methanesulfonate, methylene methanedisulfonate, N-phenylbis(trifluoromethanesulfonyl)imide, triallyl phosphate, phosphoric acid Any one or more of tripropargyl ester, tris(trimethylsilyl) phosphate, trimethyl phosphite, ethyl diprop-2-ynyl phosphate, tetramethylmethylene diphosphate, (2-allylphenoxy)trimethylsilane, 1,2-bis(cyanoethoxy)ethane, tris(trimethylsilyl) borate, 1,3,5-triallyl isocyanurate, isocyanoethyl methacrylate, hexamethylene diisocyanate, p-phenylene diisocyanate or 2,4-toluene diisocyanate.

[0034] In some embodiments of the present invention, the lithium salt accounts for 8% to 25% by mass of the electrolyte; preferably 9% to 23%; more preferably 10% to 20% by mass.

[0035] The mass content of the solvent in the electrolyte is 72% to 91%, preferably 75% to 90%, and more preferably 77% to 89%.

[0036] In some embodiments of the present invention, the sum of the first electrolyte additive and the second electrolyte additive accounts for 0.1% to 2% by mass of the electrolyte; preferably 0.5% to 2% by mass.

[0037] In some embodiments of the present invention, the electrolyte is prepared in an argon glove box, preferably by uniformly mixing the organic solvent, lithium salt, and electrolyte additives. More preferably, the preparation is carried out in an argon glove box with a water and oxygen content of ≤0.1 ppm, specifically comprising the following steps:

[0038] In an argon-filled glove box (moisture content <0.1ppm), the solvent was stirred and placed in a -20°C refrigerator for 30 minutes. The first electrolyte additive in the formulated amount was added, followed by the electrolyte salt. After stirring and complete dissolution, the second additive was added and stirred for 10 to 15 minutes to obtain the low-temperature electrolyte.

[0039] The solvent of the present invention is a mixed solvent of two or more carbonate solvents or ether solvents, and the solvent is selected from any one or more of dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, propyl propionate, and ethylene glycol dimethyl ether.

[0040] The solvents of the present invention may also include tetrahydrofuran, tetrahydropyran and tetrahydrothiophene;

[0041] The mass ratio of tetrahydrofuran, tetrahydropyran and tetrahydrothiophene is (20-60):(10-40):(10-40).

[0042] Quantum chemical calculations indicate that the desolvation energy of the tetrahydrofuran, tetrahydropyran, and tetrahydrothiophene mixture of the present invention is low when used as a solvent, and no prior art reports have been found to explore this mixed solvent. Furthermore, adding an ester structure to the mixed solvent can further reduce the desolvation energy and increase the solubility of the lithium salt.

[0043] The present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and the electrolyte described in the above technical solution. The positive electrode material is selected from any one of lithium cobalt oxide, ternary nickel-cobalt-manganese lithium, lithium nickel-manganese oxide, lithium iron phosphate, or lithium iron manganese phosphate, with lithium iron phosphate being preferred. The negative electrode active material is preferably selected from any one of artificial graphite, natural graphite, lithium titanate, metallic lithium, a silicon-carbon composite material, or silicon monoxide, with artificial graphite being preferred. The separator is selected from either a polypropylene separator or a polyethylene separator, with polyethylene being preferred.

[0044] The lithium ion battery made of the electrolyte of the present invention has a charge and discharge voltage of 2.5 to 3.65V, and significantly improves the rate performance and cycle life of the battery at low temperatures.

[0045] In some embodiments, the positive electrode is prepared by adding a positive electrode active material LiFePO4, a conductive agent carbon black, and a binder polyvinylidene fluoride (PVDF) to a solvent N-methylpyrrolidone, wherein the mass ratio of the positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) is 90:5:5, and mixing them uniformly to obtain a positive electrode slurry; the positive electrode slurry is uniformly coated on a positive electrode current collector Al foil with a thickness of 13 μm, and then rolled after drying to obtain a positive electrode;

[0046] Preparation of the negative electrode: Adding artificial graphite (a negative electrode active material), carbon black (a conductive agent), sodium carboxymethyl cellulose (CMC) (a thickener), and SBR (styrene-butadiene rubber) (a binder) to deionized water (a solvent) in a mass ratio of 90:4:3:3, and mixing to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on a negative electrode current collector Cu foil with a thickness of 8 μm, dried, and then roll-pressed to obtain a negative electrode;

[0047] Battery cell assembly: The positive electrode, separator and negative electrode are stacked to form a battery cell assembly and placed into the shell;

[0048] Injection of electrolyte: inject the above-mentioned low-temperature electrolyte into the battery shell to obtain the low-temperature lithium-ion battery.

[0049] To further illustrate the present invention, the following examples are provided for detailed description. In the following examples of the present invention, if specific conditions are not specified, the experiments were performed according to conventional conditions or the conditions recommended by the manufacturer. Reagents or instruments used, if the manufacturer is not specified, are commercially available conventional products.

[0050] Example 1

[0051] Synthesis of additives

[0052] Take 5g of formula C-1.1 and add it to a flask with a condenser, then add formula C-2.1 and sodium hydroxide (the molar ratio of C-1.1, C-2.1 and sodium hydroxide is 1:1.2:2), and heat at 80°C for 5h, during which water vapor is continuously discharged. After the reaction is completed, pour the product into a separatory funnel for separation, take out the product layer, and then purify it by column chromatography with dichloromethane as the eluent. After purification, rotary evaporation is used to remove the solvent to obtain the pure product formula (1) with a yield of 92.8%.

[0053]

[0054] The product obtained in Example 1 was analyzed by mass spectrometry and the results were HRMS (ESI): calcd.forC 11 H 14 O3S,226.07,found 226.65.

[0055] The low-temperature electrolyte for a lithium-ion battery of this embodiment is composed of an organic solvent, an electrolyte salt, and an additive. The mass percentages of the electrolyte salts lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide mixed lithium salt in the low-temperature electrolyte are 3.0% and 8.0%, respectively. The mass percentages of the additives in the low-temperature electrolyte are 0.8%, and the remainder is a mixed solvent. The mixed solvent is a mixture of dimethyl carbonate and ethylene carbonate in a volume ratio of 2:1.

[0056] The additive is a mixture of formula (1) and fluoroethylene carbonate in a mass ratio of 3:5.

[0057]

[0058] The preparation method of the low-temperature electrolyte for lithium-ion batteries of this embodiment is as follows:

[0059] In an argon-filled glove box (moisture content <0.1ppm), the mixed solvent was stirred for 15 minutes and then placed in a -20°C refrigerator for cooling for 30 minutes. The formulated electrolyte salts of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide were added. After the electrolyte salts were completely dissolved, the formulated amounts of formula (1) and fluoroethylene carbonate were added to the system, and the mixture was stirred for another 15 minutes to obtain the low-temperature electrolyte.

[0060] The low-temperature lithium-ion battery of this embodiment includes a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte is the above-mentioned low-temperature electrolyte; the positive electrode active material used in the positive electrode is LiFePO4; and the negative electrode active material used in the negative electrode is artificial graphite.

[0061] The preparation method of the low-temperature lithium-ion battery of this embodiment is as follows:

[0062] Preparation of the positive electrode: The positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) were added to the solvent N-methylpyrrolidone. The mass ratio of the positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) was 90:5:5, and the mixture was evenly mixed to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on a 13 μm thick positive electrode current collector Al foil, dried, and then roll-pressed to obtain a positive electrode.

[0063] Preparation of the negative electrode: Adding artificial graphite (a negative electrode active material), carbon black (a conductive agent), sodium carboxymethyl cellulose (CMC) (a thickener), and SBR (styrene-butadiene rubber) (a binder) to deionized water (a solvent) in a mass ratio of 90:4:3:3, and mixing to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on a negative electrode current collector Cu foil with a thickness of 8 μm, dried, and then roll-pressed to obtain a negative electrode;

[0064] Battery cell assembly: The positive electrode, separator and negative electrode are stacked to form a battery cell assembly and placed into the shell;

[0065] Injection of electrolyte: inject the above-mentioned low-temperature electrolyte into the battery shell to obtain the low-temperature lithium-ion battery.

[0066] Example 2

[0067] Take 5g of formula C-1.1 and add it to a flask with a condenser, then add formula C-2.1 and sodium hydroxide (the molar ratio of C-1.1, C-2.1 and sodium hydroxide is 1:1.2:2.6), and heat at 70°C for 7h, during which water vapor is continuously discharged. After the reaction is completed, pour the product into a separatory funnel for separation, take out the product layer, and then purify it by column chromatography with dichloromethane as the eluent. After purification, rotary evaporation is performed to remove the solvent. After purification, the pure product formula (1) is obtained with a yield of 91.5%.

[0068]

[0069] The product obtained in Example 1 was analyzed by mass spectrometry and the results were HRMS (ESI): calcd.forC 11 H 14 O3S,226.07,found 226.65.

[0070] The low-temperature electrolyte for the lithium-ion battery of this embodiment is the same as that of Example 1.

[0071] The low-temperature lithium-ion battery of this embodiment includes a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte is the above-mentioned low-temperature electrolyte; the rest is the same as in Example 1.

[0072] Example 3

[0073] Take 5g of formula C-1.1 and add it to a flask with a condenser, then add formula C-2.1 and sodium hydroxide (the molar ratio of C-1.1, C-2.1 and sodium hydroxide is 1:1.2:2.8), heat at 60°C for 8h, and continuously discharge water vapor during the reaction. After the reaction is completed, pour the product into a separatory funnel for separation, take out the product layer, and then purify it by column chromatography with dichloromethane as the eluent. After purification, rotary evaporation is performed to remove the solvent. After purification, the pure product formula (1) is obtained with a yield of 90.3%.

[0074]

[0075] The product obtained in Example 1 was analyzed by mass spectrometry and the results were HRMS (ESI): calcd.forC 11 H 14 O3S,226.07,found 226.65.

[0076] The low-temperature electrolyte for the lithium-ion battery of this embodiment is the same as that of Example 1.

[0077] The low-temperature lithium-ion battery of this embodiment includes a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte is the above-mentioned low-temperature electrolyte; the rest is the same as in Example 1.

[0078] Table 1 is a statistical table of process parameters of Examples 1 to 3 of the present invention.

[0079] Table 1

[0080]

[0081] Comparative Example 1

[0082] The electrolyte of this comparative example was added with additive formula D-1, whose weight ratio accounted for 0.5% of the total electrolyte. The structural formula of D-1 is as follows:

[0083]

[0084] The usage of LiPF6, ethylene carbonate, propylene carbonate, diethyl carbonate and propyl propionate in the electrolyte are 15.42%, 21.79%, 9.89%, 22.40% and 30.50% of the total mass of the electrolyte respectively. Through orthogonal experiments, the experimenters found that the electrolyte under this ratio has the best electrochemical performance in the following battery system.

[0085] In an argon-filled glove box (moisture content <0.1ppm), take the formulated amount of ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate, stir for 15 minutes, and then put into a -20°C refrigerator to cool for 30 minutes. After the formulated amount of LiPF6 is completely dissolved, add the formulated amount of D-1 to the system and stir for another 15 minutes to obtain a low-temperature electrolyte.

[0086] The preparation method of the low-temperature lithium-ion battery of this comparative example is as follows:

[0087] Preparation of the positive electrode: The positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) were added to the solvent N-methylpyrrolidone. The mass ratio of the positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) was 90:5:5, and the mixture was evenly mixed to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on a 13 μm thick positive electrode current collector Al foil, dried, and then roll-pressed to obtain a positive electrode.

[0088] Preparation of the negative electrode: Adding artificial graphite (a negative electrode active material), carbon black (a conductive agent), sodium carboxymethyl cellulose (CMC) (a thickener), and SBR (styrene-butadiene rubber) (a binder) to deionized water (a solvent) in a mass ratio of 90:4:3:3, and mixing to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on a negative electrode current collector Cu foil with a thickness of 8 μm, dried, and then roll-pressed to obtain a negative electrode;

[0089] Battery cell assembly: The positive electrode, separator and negative electrode are stacked to form a battery cell assembly and placed into the shell;

[0090] Injection of electrolyte: Inject the above-mentioned low-temperature electrolyte into the battery shell to obtain a low-temperature lithium-ion battery.

[0091] The battery of Experimental Comparative Example 1 was prepared by the method of Reference Example 1.

[0092] Comparative Example 2

[0093] The electrolyte of this comparative example was added with additive formula D-2, whose weight ratio accounted for 0.5% of the total electrolyte. The structural formula of D-2 is as follows:

[0094]

[0095] The electrolyte solution contains a solvent (ethylene glycol dimethyl ether) and additive D-2 in a volume ratio of 4.5:1, dissolving 0.75 mol of lithium bis(fluorosulfonyl)imide (LiFSI) and 5% by weight of lithium nitrate. Through orthogonal experiments, the researchers found that this electrolyte solution exhibited optimal electrochemical performance in the following battery systems.

[0096] In an argon-filled glove box (moisture content <0.1ppm), take the formulated amount of ethylene glycol dimethyl ether, stir for 15 minutes, and then cool it in a -20°C refrigerator for 30 minutes. After the formulated amount of lithium nitrate is completely dissolved, add the formulated amount of D-2 to the system and stir for another 15 minutes to obtain a low-temperature electrolyte.

[0097] The preparation method of the low-temperature lithium-ion battery of this comparative example is as follows:

[0098] Preparation of the positive electrode: The positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) were added to the solvent N-methylpyrrolidone. The mass ratio of the positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) was 90:5:5, and the mixture was evenly mixed to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on a 13 μm thick positive electrode current collector Al foil, dried, and then roll-pressed to obtain a positive electrode.

[0099] Preparation of the negative electrode: Adding artificial graphite (a negative electrode active material), carbon black (a conductive agent), sodium carboxymethyl cellulose (CMC) (a thickener), and SBR (styrene-butadiene rubber) (a binder) to deionized water (a solvent) in a mass ratio of 90:4:3:3, and mixing to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on a negative electrode current collector Cu foil with a thickness of 8 μm, dried, and then roll-pressed to obtain a negative electrode;

[0100] Battery cell assembly: The positive electrode, separator and negative electrode are stacked to form a battery cell assembly and placed into the shell;

[0101] Injection of electrolyte: Inject the above-mentioned low-temperature electrolyte into the battery shell to obtain a low-temperature lithium-ion battery.

[0102] The electrochemical properties of the low-temperature lithium-ion batteries prepared in Examples 1 to 3 and the comparative example at -20°C are shown in Table 2. Table 2 is a table of the electrochemical properties of the low-temperature lithium-ion batteries prepared in Examples 1 to 3 and the comparative example at -20°C:

[0103] Table 2

[0104]

[0105]

[0106] As can be seen from the above table, the low-temperature electrochemical performance of Examples 1 to 3 is better than that of Comparative Examples 1 and 2. Since the additives in the examples all have an ester structure, the solubility of the additives can be effectively improved, thereby increasing the upper limit of the addition amount. The S atoms in their structures have a small electronegativity and a small binding energy with lithium ions, which can reduce the desolvation energy of lithium ions and significantly improve the rate performance and cycle life of the battery at low temperatures. In addition, the examples use cyclic ether solvents with low desolvation energy, while Comparative Example 1 uses an ester solvent that is difficult to desolvate, which is not conducive to low-temperature cycle performance. The additives in the examples have a small electronegativity of the S atoms in their structures and a small binding energy with lithium ions, which can reduce the desolvation energy of lithium ions and significantly improve the rate performance and cycle life of the battery at low temperatures. In addition, the examples use mixed cyclic ether solvents with low desolvation energy, including tetrahydrothiophene. Solvents containing S are easier to desolvate. However, Comparative Example 2 uses a single ethylene glycol dimethyl ether that is difficult to desolvate as a solvent, and has poor reduction resistance, which is not conducive to low-temperature cycle performance. Therefore, the low-temperature electrochemical performance of Examples 1 to 8 is better than that of Comparative Examples 1 and 2.

[0107] Through quantum chemical calculations, we discovered that a mixture of tetrahydrofuran, tetrahydropyran, and tetrahydrothiophene has a low desolvation energy when used as a solvent. Furthermore, adding an ester structure to this solvent mixture can further reduce the desolvation energy and improve lithium salt solubility. Compared to most current low-temperature electrolytes that use additives to form an SEI film to improve low-temperature performance, reducing the desolvation energy is a more effective approach. The following are relevant examples.

[0108] Example 4

[0109] The low-temperature electrolyte for a lithium-ion battery of this embodiment is composed of a weak solvating solvent, an electrolyte salt, and an additive. The mass percentages of the electrolyte salts lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide mixed lithium salt in the low-temperature electrolyte are 3.0% and 8.0%, respectively. The mass percentage of the additives in the low-temperature electrolyte is 0.8%, and the balance is an organic solvent.

[0110] The weak solvating solvent is composed of the following components in volume percentages: 58.2% tetrahydrofuran, 20% tetrahydropyran, and 10% tetrahydrothiophene;

[0111] The additive is a mixture of formula (1) and fluoroethylene carbonate in a mass ratio of 3:5.

[0112]

[0113] The preparation method of the low-temperature electrolyte for lithium-ion batteries of this embodiment is as follows:

[0114] In an argon-filled glove box (moisture content <0.1ppm), a mixture of tetrahydrofuran, tetrahydropyran, and tetrahydrothiophene in a formulated amount was stirred for 15 minutes and then placed in a -20°C refrigerator for cooling for 30 minutes. The formulated amounts of electrolyte salts lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide were added. After the electrolyte salts were completely dissolved, the formulated amounts of formula (1) and fluoroethylene carbonate were added to the system, and the mixture was stirred for another 15 minutes to obtain the low-temperature electrolyte.

[0115] The lithium-ion battery of this embodiment includes a positive electrode, a negative electrode, a separator and an electrolyte. The electrolyte is the above-mentioned low-temperature electrolyte. The positive electrode active material used in the positive electrode is LiFePO4; the negative electrode active material used in the negative electrode is artificial graphite.

[0116] The preparation method of the low-temperature lithium-ion battery of this embodiment is as follows:

[0117] Preparation of the positive electrode: The positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) were added to the solvent N-methylpyrrolidone. The mass ratio of the positive electrode active material LiFePO4, the conductive agent carbon black, and the binder polyvinylidene fluoride (PVDF) was 90:5:5, and the mixture was evenly mixed to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on a 13 μm thick positive electrode current collector Al foil, dried, and then roll-pressed to obtain a positive electrode.

[0118] Preparation of the negative electrode: Adding artificial graphite (a negative electrode active material), carbon black (a conductive agent), sodium carboxymethyl cellulose (CMC) (a thickener), and SBR (styrene-butadiene rubber) (a binder) to deionized water (a solvent) in a mass ratio of 90:4:3:3, and mixing to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on a negative electrode current collector Cu foil with a thickness of 8 μm, dried, and then roll-pressed to obtain a negative electrode;

[0119] Battery cell assembly: The positive electrode, separator and negative electrode are stacked to form a battery cell assembly and placed into the shell;

[0120] Injection of electrolyte: inject the above-mentioned low-temperature electrolyte into the battery shell to obtain the low-temperature lithium-ion battery.

[0121] Table 3 is a comparison of the electrochemical performance of the low-temperature lithium-ion batteries prepared in Example 4 and the comparative example at -20°C:

[0122] Table 3

[0123]

[0124]

[0125] The additive in Example 4 has an ester structure, which can effectively improve the solubility of the additive, thereby increasing the upper limit of the addition amount. The S atom in its structure has a small electronegativity and a small binding energy with lithium ions, which can reduce the desolvation energy of lithium ions and significantly improve the rate performance and cycle life of the battery at low temperatures. In addition, a mixed solvent with a lower desolvation energy is used in Example 4, while Comparative Example 1 uses an ester solvent that is difficult to desolvate, which is not conducive to low-temperature cycle performance.

[0126] The additive in Example 4 has a low electronegativity of the S atom in its structure and a low binding energy with lithium ions, which can reduce the desolvation energy of lithium ions and significantly improve the rate performance and cycle life of the battery at low temperatures. In addition, Example 4 uses a mixed cyclic ether solvent with low desolvation energy, including tetrahydrothiophene. S-containing solvents are easier to desolvate. However, Comparative Example 2 uses a single ethylene glycol dimethyl ether that is difficult to desolvate and has poor reduction resistance, which is not conducive to low-temperature cycling performance.

[0127] The above description of the disclosed embodiments will enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is to be construed in the widest manner consistent with the principles and novel features disclosed herein.

Claims

1. An electrolyte, characterized in that: A first electrolyte additive comprising a lithium salt, a solvent and formula (1); 。 2. The electrolyte according to claim 1, characterized in that The lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium perchlorate, lithium nitrate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), lithium difluorooxalatoborate or lithium difluorobis(oxalatophosphate).

3. The electrolyte according to claim 1, characterized in that The solvent is selected from any one or more of dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, propyl propionate, and ethylene glycol dimethyl ether.

4. The electrolyte according to claim 1, characterized in that The electrolyte further includes a second electrolyte additive; The second electrolyte additive is selected from vinylene carbonate, vinyl ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinyl sulfate, vinyl bissulfate, propylene sulfate, 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, 2,4-butane sultone, phenyl methanesulfonate, methylene methanedisulfonate, N-phenylbis(trifluoromethanesulfonyl)imide, triallyl phosphate, tripropargyl phosphate, tris(trifluoromethanesulfonyl)imide ... Any one or more of (trimethylsilyl) phosphate, trimethyl phosphite, ethyl diprop-2-ynyl phosphate, tetramethylmethylene diphosphate, (2-allylphenoxy)trimethylsilane, 1,2-bis(cyanoethoxy)ethane, tris(trimethylsilyl)borate, 1,3,5-triallyl isocyanurate, isocyanoethyl methacrylate, hexamethylene diisocyanate, p-phenylene diisocyanate, or 2,4-toluene diisocyanate.

5. The electrolyte according to claim 4, characterized in that The solvent accounts for 72-91% by mass of the electrolyte, the sum of the first electrolyte additive and the second electrolyte additive accounts for 0.1-3% by mass of the electrolyte, and the lithium salt accounts for 8-25% by mass of the electrolyte.

6. The electrolyte according to claim 1, characterized in that The solvents include tetrahydrofuran, tetrahydropyran and tetrahydrothiophene; The mass ratio of tetrahydrofuran, tetrahydropyran and tetrahydrothiophene is (20-60):(10-40):(10-40).

7. A lithium-ion battery, characterized in that: Including positive electrode, negative electrode, separator and electrolyte; The electrolyte is the electrolyte according to claim 1; The active material of the positive electrode is selected from any one of lithium cobalt oxide, ternary nickel cobalt manganese lithium, lithium nickel manganese oxide, lithium iron phosphate or lithium manganese iron phosphate; The active material of the negative electrode is selected from any one of artificial graphite, natural graphite, lithium titanate, metallic lithium, silicon-carbon composite material or silicon oxide; The diaphragm is selected from a polypropylene diaphragm or a polyethylene diaphragm.

8. The lithium-ion battery according to claim 7, characterized in that The charge and discharge voltage of the lithium-ion battery is 2.5-3.65V.

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