High-voltage-resistant electrolyte and lithium ion battery

Abstract

The invention provides a high-voltage-resistant electrolyte and a lithium ion battery, and relates to the technical field of lithium ion batteries. The high-voltage-resistant electrolyte comprises a lithium salt, a solvent and an additive. The additive comprises a negative electrode film-forming additive and a high-pressure-resistant additive. The solvent comprises a fluorinated solvent and other solvents. The fluorinated solvent is selected from one or more of fluoroethylene carbonate, fluoromethyl ethyl carbonate and fluorodiethyl carbonate. The battery prepared from the high-voltage-resistant electrolyte provided by the invention still has relatively high discharge energy under the condition of high voltage.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in 3C consumer electronic products such as mobile phones and laptops due to their 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 become more and more common. As the mileage of electric vehicles increases, the energy density requirements for power batteries are also getting higher and higher, and increasing the working voltage of lithium-ion batteries is one of the important ways to increase the energy density of batteries.

[0003] At present, a variety of high-voltage positive electrode materials have been developed, but conventional electrolytes are prone to side reactions with the surface of the positive electrode materials under high voltage, affecting the performance of the high-voltage positive electrode materials and greatly limiting the application of high-voltage lithium-ion batteries. Therefore, it is particularly important to develop new electrolytes that match high-voltage positive electrode materials. Using nickel, cobalt and manganese as positive electrode materials can effectively allow the positive electrode materials to release more lithium ions, thereby providing instantaneous high power and releasing more energy. It is an excellent choice for equipment that needs to reduce loads while discharging at high power. Currently, commercial electrolytes mainly use carbonate organics as electrolyte solvents. The electrolyte system composed of conventional carbonate solvents and lithium hexafluorophosphate is 4.2V (vs. Li / Li + ) will cause oxidative decomposition, resulting in a decrease in the performance of the entire lithium battery system. Therefore, developing high-voltage resistant electrolytes is the only way to increase battery energy density. Increasing the oxidative decomposition potential of electrolyte solvents is the most direct way to improve the stability of electrolytes under high voltage conditions. Summary of the invention

[0004] Based on the technical problems existing in the background technology, the present invention provides an electrolyte based on a six-series nickel-cobalt-manganese ternary positive electrode material and a fluorinated solvent, a first additive that helps the negative electrode film formation, and a second additive that is resistant to high voltage, to meet the above usage scenarios. The lithium-ion battery prepared using the positive electrode material and electrolyte of this specification can effectively solve the problems of insufficient discharge power under high-voltage conditions, high voltage causing electrolyte decomposition and gas production, and irreversible capacity loss of many lithium-ion batteries on the existing market.

[0005] The invention provides a high-voltage resistant electrolyte comprising 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(oxalatoborate), lithium difluorooxalatoborate, lithium bis(fluorosulfonyl imide), and lithium bis(trifluoromethanesulfonyl imide).

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

[0008] Preferably, the mass of the lithium salt accounts for 8% to 12.5% ​​of the total mass of the high-voltage 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 and to work synergistically with the solvent to enhance high-voltage resistance.

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

[0011] Controlling the type of solvent containing fluorine helps to control the oxidative decomposition of organic solvents under high voltage conditions. The carbonates in conventional electrolytes cannot be reversibly 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 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 fluoroethylene carbonate, fluoroethyl methyl carbonate, and fluorodiethyl carbonate.

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

[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 conductivity of lithium ions, and reduce gas production under high temperature conditions and solidification under low temperature conditions.

[0018] Preferably, the additives include negative electrode film-forming additives and high-voltage resistant additives.

[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 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 negative electrode film-forming additives and high-voltage resistant additives within a certain range helps to reduce the dissolution of active materials at the positive and negative ends in the late stage of long-term cycling, while avoiding the degradation of electrical performance caused by increased interface impedance due to the formation of too thick an interface film due to excessive additives.

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

[0024] More preferably, the high pressure resistant additive comprises a compound of the following structural formula: Wherein R1, R2, and R3 are independently selected from one or more of H, C1-C12 alkyl, C1-C12 alkoxy, amine, and heterocyclic group.

[0025] The role of high-voltage resistant additives 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 reduce the dissolution of active materials under high-voltage conditions, thereby 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, comprising a positive electrode sheet, a negative electrode sheet, a separator, and 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] Nickel content within a certain range can effectively improve the high voltage performance of the battery while effectively reducing 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, ferrite, nitride and alloy materials.

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

[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 less than 5ppm, dimethyl carbonate (DMC), ethyl methyl fluorocarbonate (FEMC), ethyl acetate (EA), and ethyl propionate (EP) are prepared into an organic solvent in a mass ratio of 15:25:20:40, and a high-voltage additive (R)-N-tert-butyloxycarbonylpyrrolidine-2-boric acid (Compound 2) is slowly added, and then a lithium salt (lithium hexafluorophosphate and lithium bistrifluoromethanesulfonyl imide are mixed in a mass ratio of 1:1) is added, stirred until dissolved, and then a negative electrode film-forming additive is added, and it is fully stirred and completely dissolved 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 resistant additive in the electrolyte are 12%, 85%, 2% and 1% respectively.

[0042] (R)-N-tert-Butyloxycarbonylpyrrolidine-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 less than 5ppm, dimethyl carbonate (DMC), diethyl fluorocarbonate (FDEC), ethyl acetate (EA), and ethyl propionate (EP) are prepared into an organic solvent in a mass ratio of 15:25:20:40, and a high-voltage additive (S)-N-tert-butyloxycarbonylpyrrolidine-2-boric acid (Compound 3) is slowly added, and then a lithium salt (lithium hexafluorophosphate and lithium bistrifluoromethanesulfonyl imide are mixed in a mass ratio of 1:1) is added, stirred until dissolved, and then a negative electrode film-forming additive is added, and it is fully stirred and completely dissolved 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 resistant additive in the electrolyte are 12%, 85%, 2% and 1% respectively.

[0046] (S)-N-tert-Butyloxycarbonylpyrrolidine-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 a water content and an oxygen content of less than 5 ppm, dimethyl carbonate (DMC), dimethyl fluorocarbonate (FDMC), ethyl acetate (EA), and ethyl propionate (EP) are prepared into an organic solvent in a mass ratio of 15:25:20:40, and a high-voltage additive (3-(tert-butyloxycarbonyl)-3-azabicyclo[3.1.0]hexane-2-yl)boric acid (compound 4) is slowly added, and then a lithium salt (lithium hexafluorophosphate and lithium bistrifluoromethanesulfonyl imide are mixed in a mass ratio of 1:1) is added, stirred until dissolved, and then a negative electrode film-forming additive is added, and it is fully stirred and completely dissolved 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 resistant additive in the electrolyte are 12%, 85%, 2% and 1% respectively.

[0050] (3-(tert-Butyloxycarbonyl)-3-azabicyclo[3.1.0]hexane-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 less than 5ppm, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethyl acetate (EA), and ethyl propionate (EP) are prepared into a solvent in a mass ratio of 15:25:20:40, lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide are mixed into a lithium salt in a mass ratio of 1:1, and the lithium salt is added into the solvent and stirred until dissolved to obtain an electrolyte. The mass fractions of lithium salt and solvent in the electrolyte are 15% and 85%, respectively.

[0054] The electrolytes of Examples 1 to 4 are assembled with the positive electrode sheet, the negative electrode sheet, and the separator into a lithium ion battery, specifically comprising: mixing the positive electrode material active substance (hexa-nickel-cobalt-manganese), the conductive agent acetylene black, and the binder polytetrafluoroethylene in a mass ratio of 95:2.5:2.5, then adding N-methylpyrrolidone, stirring thoroughly to obtain a uniform slurry, coating the slurry evenly on a 15 um thick aluminum foil, and drying to obtain a positive electrode sheet; making a soft-pack laminated battery in a dry environment with a dew point temperature controlled below -40°C, stacking the prepared positive electrode sheet, the separator, and the graphite negative electrode sheet in sequence to ensure that the separator completely separates the positive and negative electrode sheets, and packaging and welding the pole ears with an aluminum-plastic film to form a battery to be injected, baking the battery moisture content to below 300 ppm before injection, and then injecting the above lithium ion battery electrolyte, sealing and letting it stand and retaining the air bag, and the lithium ion battery is completed.

[0055] The electrolyte of comparative example 1 is assembled with the positive electrode sheet, the negative electrode sheet and the separator into a lithium ion battery, specifically comprising: mixing the positive electrode material active substance (lithium iron phosphate), the conductive agent acetylene black and the binder polytetrafluoroethylene in a mass ratio of 95:2.5:2.5, then adding N-methylpyrrolidone, stirring evenly to obtain a uniform slurry, coating the slurry evenly on a 15um thick aluminum foil, and drying to obtain the positive electrode sheet; making a soft-pack laminated battery in a dry environment with a dew point temperature controlled below -40°C, stacking the prepared positive electrode sheet, separator and graphite negative electrode sheet in sequence to ensure that the separator completely separates the positive and negative electrode sheets, and packaging and welding the pole ears with aluminum-plastic film to form a battery to be injected, baking the battery moisture content to below 300ppm before injection, and then injecting the above lithium ion battery electrolyte, sealing and letting it stand and retaining the air bag, and the lithium ion battery is completed.

[0056] The performance of the batteries prepared in the embodiments of the present invention and the comparative examples was tested. The capacity was discharged under high-voltage conditions: the lithium-ion battery was left to stand at room temperature for 6 minutes, then charged to 4.45V at a constant current and constant voltage of 2.4A, with a cut-off current of 0.12A, and then discharged to 2.8V at a constant current of 0.2C0, and the capacity was recorded as a function of voltage. The discharge curve was obtained as shown in FIG. Figure 1 As shown, the above test steps are repeated (average values ​​are recorded for multiple groups of tests).

[0057] Depend on Figure 1 It can be seen from the data that the electrolyte prepared by the present invention can effectively improve the battery voltage and discharge capacity, and can reach a maximum of 4.45V stable discharge, and there is no "diving" capacity decay, that is, the solvent does not undergo oxidative decomposition. Under the same conditions, in Comparative Example 1, even if a high voltage discharge is provided, it will still decay rapidly to 3.5V and slowly decrease, and only a weak amount of electricity is released within the set voltage range. The discharge energy (voltage*capacity) in Comparative Example 1 is five times smaller than that in the embodiment.

[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] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A high voltage resistant electrolyte, characterized in that: It includes lithium salt, solvent, and additives. 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, R2, and R3 are independently selected from one or more of H, C1-C12 alkyl, C1-C12 alkoxy, amine, and heterocyclic group.

2. The high voltage resistant electrolyte according to claim 1, characterized in that: 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.

3. 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.

4. The high voltage resistant electrolyte according to claim 2, 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.

5. 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.

6. 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.

7. 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, fluoroethylene carbonate, vinyl sulfate, tris(trimethylsilyl)borate, trimethyl phosphate, dimethyl methylphosphonate, hexamethyldisilazane, methyl disulfonate, 1,3-propane sultone, vinyl sulfate, vinyl sulfite, and ethylene carbonate.

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

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