A high-voltage resistant electrolyte and a lithium-ion battery

Abstract

The present invention provides a high-voltage resistant electrolyte and a lithium-ion battery, relating to the technical field of lithium-ion batteries. A high-voltage resistant electrolyte includes a lithium salt, a solvent, and an additive. The additive includes a negative electrode film-forming additive and a high-voltage resistant additive. The solvent includes a fluorinated solvent and other solvents. The fluorinated solvent is selected from one or more of fluoroethylene carbonate, fluoromethyl ethyl carbonate, and diethyl fluorocarbonate. The battery prepared with the high-voltage resistant electrolyte provided by the present invention still has a relatively high discharge energy under high voltage conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a high-voltage resistant electrolyte and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries are widely used in the fields of 3C consumer electronic products such as mobile phones and laptop computers due to their advantages of high specific energy, no memory effect, and long cycle life. In addition, with the rapid development of new energy vehicles, the application of lithium-ion batteries in the fields of power and energy storage will also become more and more common. With the increase in the cruising range of electric vehicles, the requirement for the energy density of power batteries is also getting higher and higher, and increasing the working voltage of lithium-ion batteries is one of the important ways to increase the battery energy density.

[0003] At present, a variety of high-voltage cathode materials have been developed, but conventional electrolytes are prone to side reactions with the surface of cathode materials under high voltage, which affects the performance of high-voltage cathode materials and greatly limits the application of high-voltage lithium-ion batteries. Therefore, it is particularly important to develop a new type of electrolyte that matches high-voltage cathode materials. Using nickel cobalt manganese as the cathode material can effectively enable the cathode material to release more lithium ions, thereby providing instantaneous high power and releasing more energy, which is an excellent choice for devices that need to reduce the load and discharge at high power. Currently, carbonate organic compounds are mainly used as electrolyte solvents in commercial electrolytes. The electrolyte system composed of conventional carbonate solvents and lithium hexafluorophosphate will undergo oxidative decomposition above 4.2V (vs. Li / Li + ), which will lead to a decline in the performance of the entire lithium battery system. Therefore, developing high-voltage resistant electrolytes is the only way to improve the battery energy density. Increasing the oxidative decomposition potential of the electrolyte solvent is the most direct method to improve the stability of the electrolyte under high voltage conditions. Summary of the Invention

[0004] Based on the technical problems existing in the background art, the present invention provides an electrolyte based on a nickel cobalt manganese ternary cathode material of the six-series and composed of a fluorinated solvent, a first additive that helps form a film on the negative electrode, and a high-voltage resistant second additive to meet the above usage scenarios. The lithium-ion battery prepared by using the above-specified cathode material and electrolyte can effectively solve many problems of existing lithium-ion batteries in the market, such as insufficient discharge power under high voltage conditions, gas generation due to electrolyte decomposition under high voltage, and irreversible capacity loss.

[0005] A high-voltage resistant electrolyte proposed by the present invention includes a lithium salt, a solvent, and an additive.

[0006] Preferably, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0007] Preferably, the concentration of the lithium salt is 0.5 to 1.5 M.

[0008] Preferably, the mass of the lithium salt accounts for 8% to 12.5% of the total mass of the high-voltage resistant electrolyte.

[0009] The type, mass range, and molar concentration range of the lithium salt are appropriately selected to provide an appropriate amount of lithium source, which synergistically acts with the solvent to improve the high-voltage resistance performance.

[0010] Preferably, the solvent includes a fluorinated solvent and other solvents.

[0011] Controlling the type of solvent containing fluorine helps to control the oxidative decomposition of organic solvents under high-voltage conditions, and carbonate esters in conventional electrolytes are irreversibly decomposed within this voltage range.

[0012] Preferably, the mass of the solvent accounts for 65% to 80% of the total mass of the high-voltage resistant electrolyte.

[0013] More preferably, the mass ratio of the fluorinated solvent to other solvents is (20 - 30):(70 - 90).

[0014] More preferably, the fluorinated solvent is selected from one or more of fluorinated ethylene carbonate, fluorinated ethyl methyl carbonate, and fluorinated diethyl carbonate.

[0015] Fluorine atoms have strong electronegativity and weak polarity. Fluorine substitution of hydrogen will effectively increase the oxidation decomposition potential of the solvent. The oxidation decomposition potential of fluorinated solvents is higher than that of conventional carbonate solvents, so that the electrolyte is not decomposed at high working potentials, thus ensuring the stable operation of a higher working voltage battery system. Based on this, the operation of high-voltage batteries can be realized, and the energy density of the battery can be increased.

[0016] More preferably, the other solvents are selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propyl acetate, ethyl propionate, and ethyl acetate.

[0017] The role of the solvent is to dissolve the solute in the electrolyte, control the lithium ion conductivity, and reduce the gas generation at high temperatures and the solidification at low temperatures.

[0018] Preferably, the additive includes a negative electrode film-forming additive and a high-voltage resistant additive.

[0019] More preferably, the mass of the negative electrode film-forming additive accounts for 0.1% to 5% of the total mass of the high-voltage resistant electrolyte.

[0020] More preferably, the mass of the high-voltage resistant additive accounts for 0.1% to 5% of the total mass of the high-voltage resistant electrolyte.

[0021] The quality of the negative electrode film-forming additive and the high-voltage resistant additive helps to reduce the dissolution of the positive and negative active materials at the later stage of long-term cycling within a certain range, and at the same time avoids the deterioration of electrical performance caused by the increase in interfacial impedance due to the formation of an overly thick interfacial film caused by excessive additives.

[0022] More preferably, the negative electrode film-forming additive is selected from one or more of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, ethylene sulfate, tris(trimethylsilyl) borate, trimethyl phosphate, dimethyl methylphosphonate, hexamethyldisilazane, methyl dimethanesulfonate, 1,3-propane sultone, ethylene sulfate, ethylene sulfite, and ethylene carbonate.

[0023] Selecting the type of negative electrode film-forming additive helps to form and repair an effective interfacial impedance film, which is beneficial to the insertion and extraction of lithium ions on the negative electrode surface.

[0024] More preferably, the high-voltage resistant additive includes the following structural formula compounds Wherein R 1 , R 2 , R 3 Independently selected from one or more of H, C1-C12 alkyl, C1-C12 alkoxy, amino, and heterocyclic groups.

[0025] The function of the high-voltage resistant additive is to optimize the interface between the electrode and the electrolyte, reduce the direct contact between the positive and negative active material ends and the organic electrolyte under high-voltage conditions, and additionally reduce the dissolution of the active material under high-voltage conditions, improving the electrochemical performance and stability of the battery.

[0026] More preferably, the heteroatom of the heterocyclic group is selected from one or more of oxygen and nitrogen.

[0027] The present invention also provides a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, a separator, and further including the above-mentioned electrolyte.

[0028] Preferably, the positive electrode material in the positive electrode sheet is a nickel-cobalt-manganese ternary positive electrode material, and the nickel content is greater than 60% and less than 75%.

[0029] The nickel content within a certain range can effectively improve the high-voltage performance of the battery and at the same time effectively reduce the dissolution of transition active metals.

[0030] Preferably, the negative electrode material in the negative electrode sheet is one or more of carbon-containing materials, silicon, silicon-carbon, silicon-oxygen, ferrites, nitrides, and alloy materials.

[0031] The beneficial effects of the present invention are as follows:

[0032] The high-voltage resistant electrolyte provided by the present invention is applied to nickel-cobalt-manganese lithium-ion batteries, which helps to improve the battery energy density. Since nickel-cobalt-manganese materials have high specific capacity and have great application potential, but the working voltage of nickel-cobalt-manganese materials is relatively high, carbonate solvents are difficult to meet the requirements, and problems such as unstable material structure, voltage platform drop, and transition metal dissolution during the cycle limit its application. However, fluorine atoms have strong electronegativity and weak polarity, and fluorine substitution for hydrogen will effectively increase the oxidative decomposition potential of the solvent. The present invention uses fluorinated carbonates with higher oxidative decomposition potential than conventional carbonate solvents, so that the electrolyte is not decomposed at a high working potential, thereby ensuring the stable operation of a higher working voltage battery system. Based on this, the operation of high-voltage batteries can be achieved and the battery energy density can be improved. The "high-voltage resistant electrolyte" described in the present invention refers to: the voltage reaches 4.45V. The high-voltage performance of the ternary lithium-ion battery injected with the electrolyte is improved, which helps to improve the energy density of the lithium-ion battery, while reducing the capacity loss caused by irreversible oxidation of the solvent under high voltage conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The discharge curves of lithium-ion batteries prepared with the electrolytes provided in the embodiments and comparative examples of the present invention. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is described in detail through specific embodiments.

[0035] Unless otherwise specified, the materials, reagents, etc. used in the following examples and comparative examples can be obtained from commercial sources.

[0036] Example 1

[0037] Preparation of electrolyte: In an inert atmosphere glove box with water content and oxygen content less than 5ppm, dimethyl carbonate (DMC), ethyl methyl fluorocarbonate (FEMC), ethyl acetate (EA), and ethyl propionate (EP) are prepared into a solvent in a mass ratio of 15:25:20:40, and the high-voltage additive N-tert-butyloxyhydroxypyrrolidine-2-boric acid (Compound 1) is slowly added, and then lithium salt (lithium hexafluorophosphate and lithium bistrifluoromethanesulfonyl imide are mixed in a mass ratio of 1:1) is added, stirred until dissolved, and then the negative electrode film-forming additive is added, and it is completely dissolved after being fully stirred to obtain an electrolyte. The negative electrode film-forming additive is composed of vinylene carbonate, 4-methyl vinyl sulfate, 4-ethyl vinyl sulfate, and 4-propyl vinyl sulfate in a mass ratio of 1:0.25:0.25:0.5. The mass fractions of lithium salt, solvent, negative electrode film-forming additive, and high-voltage additive in the electrolyte are 12%, 85%, 2%, and 1%, respectively.

[0038] The structural formula of N-tert-butyloxyhydroxypyrrolidine-2-boronic acid (CAS: 149682-75-7) is as follows:

[0039]

[0040] Example 2

[0041] Preparation of electrolyte: In an inert atmosphere glove box with water content and oxygen content both lower than 5 ppm, dimethyl carbonate (DMC), ethyl methyl carbonate (FEMC), ethyl acetate (EA), and ethyl propionate (EP) were formulated into an organic solvent according to a mass ratio of 15:25:20:40. A high-pressure resistant additive (R)-N-tert-butoxycarbonylpyrrolidine-2-boronic acid (Compound 2) was slowly added, and then a lithium salt (a mixture of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide in a mass ratio of 1:1) was added. After stirring until dissolved, a negative electrode film-forming additive was added, and after sufficient stirring, it was completely dissolved to obtain the electrolyte. The negative electrode film-forming additive consists of vinylene carbonate, 4-methyl vinyl sulfate, 4-ethyl vinyl sulfate, and 4-propyl vinyl sulfate in a mass ratio of 1:0.25:0.25:0.5. The mass fractions of the lithium salt, solvent, negative electrode film-forming additive, and high-pressure resistant additive in the electrolyte are 12%, 85%, 2%, and 1% respectively.

[0042] (R)-N-tert-butoxycarbonylpyrrolidine-2-boronic acid (CAS: 149716-78-9) has the following structural formula:

[0043]

[0044] Example 3

[0045] Preparation of electrolyte: In an inert atmosphere glove box with water content and oxygen content both lower than 5 ppm, dimethyl carbonate (DMC), diethyl carbonate (FDEC), ethyl acetate (EA), and ethyl propionate (EP) were formulated into an organic solvent according to a mass ratio of 15:25:20:40. A high-pressure resistant additive (S)-N-tert-butoxycarbonylpyrrolidine-2-boronic acid (Compound 3) was slowly added, and then a lithium salt (a mixture of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide in a mass ratio of 1:1) was added. After stirring until dissolved, a negative electrode film-forming additive was added, and after sufficient stirring, it was completely dissolved to obtain the electrolyte. The negative electrode film-forming additive consists of vinylene carbonate, 4-methyl vinyl sulfate, 4-ethyl vinyl sulfate, and 4-propyl vinyl sulfate in a mass ratio of 1:0.25:0.25:0.5. The mass fractions of the lithium salt, solvent, negative electrode film-forming additive, and high-pressure resistant additive in the electrolyte are 12%, 85%, 2%, and 1% respectively.

[0046] (S)-N-tert-butoxycarbonylpyrrolidine-2-boronic acid (CAS: 149716-79-0) has the following structural formula:

[0047]

[0048] Example 4

[0049] Preparation of electrolyte: In an inert atmosphere glove box with water content and oxygen content both below 5 ppm, dimethyl carbonate (DMC), fluoro dimethyl carbonate (FDMC), ethyl acetate (EA), and ethyl propionate (EP) were formulated into an organic solvent according to a mass ratio of 15:25:20:40. A high-pressure resistant additive, (3-(tert-butoxycarbonyl)-3-azabicyclo[3.1.0]hexan-2-yl)boronic acid (Compound 4), was slowly added, and then a lithium salt (a mixture of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide in a mass ratio of 1:1) was added. After stirring until dissolved, a negative electrode film-forming additive was added, and after sufficient stirring, it was completely dissolved to obtain the electrolyte. The negative electrode film-forming additive consists of vinylene carbonate, 4-methyl vinyl sulfate, 4-ethyl vinyl sulfate, and 4-propyl vinyl sulfate in a mass ratio of 1:0.25:0.25:0.5. The mass fractions of the lithium salt, solvent, negative electrode film-forming additive, and high-pressure resistant additive in the electrolyte are 12%, 85%, 2%, and 1% respectively.

[0050] (3-(tert-butoxycarbonyl)-3-azabicyclo[3.1.0]hexan-2-yl)boronic acid (CAS: 936551-51-8) has the following structural formula:

[0051]

[0052] Comparative Example 1

[0053] Preparation of electrolyte: In an inert atmosphere glove box with water content and oxygen content both below 5 ppm, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethyl acetate (EA), and ethyl propionate (EP) were formulated into a solvent according to a mass ratio of 15:25:20:40. A mixture of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide in a mass ratio of 1:1 was used as the lithium salt, and the lithium salt was added to the solvent and stirred until dissolved to obtain the electrolyte. The mass fractions of the lithium salt and solvent in the electrolyte are 15% and 85% respectively.

[0054] The electrolytes of Examples 1 to 4 were assembled with the positive electrode sheet, negative electrode sheet, and separator into lithium-ion batteries, which specifically included: mixing the active material of the positive electrode material (nickel-cobalt-manganese of the six series), conductive agent acetylene black, and binder polyvinylidene fluoride in a mass ratio of 95:2.5:2.5, then adding N-methylpyrrolidone, and fully stirring to obtain a uniform slurry after being evenly mixed. The slurry was evenly coated on an aluminum foil with a thickness of 15 μm, and the positive electrode sheet was obtained after drying; a soft-pack laminated battery was made in a dry environment with the dew point temperature controlled below -40°C. The prepared positive electrode sheet, separator, and graphite negative electrode sheet were stacked in sequence to ensure that the separator completely separated the positive and negative electrode sheets, and the aluminum-plastic film was used for encapsulation and welding the electrode tabs to form a battery to be filled with electrolyte. Before filling, the moisture content of the battery was baked to below 300 ppm, and then the above lithium-ion battery electrolyte was injected. After sealing and standing still, an air bag was reserved, and the lithium-ion battery was completed.

[0055] The electrolyte of Comparative Example 1 was assembled with the positive electrode sheet, negative electrode sheet, and separator into lithium-ion batteries, which specifically included: mixing the active material of the positive electrode material (lithium iron phosphate), conductive agent acetylene black, and binder polyvinylidene fluoride in a mass ratio of 95:2.5:2.5, then adding N-methylpyrrolidone, and fully stirring to obtain a uniform slurry after being evenly mixed. The slurry was evenly coated on an aluminum foil with a thickness of 15 μm, and the positive electrode sheet was obtained after drying; a soft-pack laminated battery was made in a dry environment with the dew point temperature controlled below -40°C. The prepared positive electrode sheet, separator, and graphite negative electrode sheet were stacked in sequence to ensure that the separator completely separated the positive and negative electrode sheets, and the aluminum-plastic film was used for encapsulation and welding the electrode tabs to form a battery to be filled with electrolyte. Before filling, the moisture content of the battery was baked to below 300 ppm, and then the above lithium-ion battery electrolyte was injected. After sealing and standing still, an air bag was reserved, and the lithium-ion battery was completed.

[0056] The performance of the batteries prepared in the examples and comparative examples of the present invention was tested. High-voltage condition discharge capacity test: The lithium-ion battery was left standing at room temperature for 6 minutes, and then it was charged at a constant current of 2.4 A and a constant voltage of 4.45 V until the cut-off current was 0.12 A. Then, it was discharged at a constant current of 0.2C0 rate to 2.8 V, and the change of capacity with voltage was recorded to obtain the discharge curve as Figure 1 shown. The above test steps were repeated (the average value was recorded for multiple groups of tests).

[0057] From Figure 1 the data, it can be seen that the electrolyte prepared by the present invention can effectively improve the battery voltage and discharge capacity, and can stably discharge up to 4.45 V at most. There is no "diving" capacity decay, that is, the solvent does not undergo oxidative decomposition. Under the same conditions, in Comparative Example 1, even when a high-voltage discharge is provided, it will still rapidly decay to 3.5 V and slowly decrease, and only a weak amount of electricity is discharged within the set voltage range. The discharge energy (voltage * capacity) in Comparative Example 1 is five times smaller than that in the examples.

[0058] In summary, the battery prepared with the high-voltage resistant electrolyte provided by the present invention still has a high discharge energy under high voltage conditions.

[0059] As described above, only the preferred specific embodiments of the present invention are provided, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, should be covered within the protection scope of the present invention.

Claims

1. A high voltage resistant electrolyte, characterized in that: It includes lithium salt, solvent and additives, wherein the additives include negative electrode film-forming additives and high-voltage resistant additives; the high-voltage resistant additives include the following structural formula compounds , wherein R1 is tert-butoxy, R2 and R3 are H atoms; the solvent includes a fluorinated solvent and other solvents; the fluorinated solvent is selected from one or more of fluoroethylene carbonate, fluoroethyl methyl carbonate, and fluorodiethyl carbonate.

2. The high voltage resistant electrolyte according to claim 1, characterized in that: The mass of the solvent accounts for 65% to 80% of the total mass of the high-voltage electrolyte.

3. The high voltage resistant electrolyte according to claim 1, characterized in that: The other solvents are selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propyl acetate, ethyl propionate and ethyl acetate.

4. The high voltage resistant electrolyte according to claim 1, characterized in that: The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorooxalate)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

5. The high voltage resistant electrolyte according to claim 1, characterized in that: The mass of the negative electrode film-forming additive accounts for 0.1% to 5% of the total mass of the high-voltage electrolyte; the mass of the high-voltage additive accounts for 0.1% to 5% of the total mass of the high-voltage electrolyte.

6. The high voltage resistant electrolyte according to claim 1, characterized in that: The negative electrode film-forming additive is selected from one or more of vinylene carbonate, vinyl carbonate, vinyl sulfate, tris(trimethylsilyl)borate, trimethyl phosphate, dimethyl methylphosphonate, hexamethyldisilazane, methyl disulfonate, 1,3-propane sultone, and vinyl sulfite.

7. A lithium ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet and a separator, and also comprises the electrolyte according to any one of claims 1 to 6.

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